Organic Coatings : Properties, Selection, And

DATE DUE

OCT 2 9 1974 1

30 501 JOSTEN-S

are used for Organic coatings in the form of paints, varnishes, lacquers, and related products from chil- protective, decorative, and functional purposes on a wide variety of man's products— identification, environ- dren's toys to guided missiles. On rockets and spacecraft, paints provide mental protection, and temperature control.

UNITED STATES DEPARTMENT OF COMMERCE"'"' Alexander B. Trowbridge, Secretary '~~T A NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

A/7

Organic Coatings

Properties, Selection, and Use

Aaron Gene jRobeils

Building Research Division Institute for Applied Technology National Bureau of Standards Washington, D.C. 20234

Building Science Series 7

Issued February 1968

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price $2.50 Abstract

This publication was prepared to fill the need for a comprehensive, unifying treatise in the field of organic coatings. Besides presenting practical information on the properties, selection, and use of organic coatings (and certain inorganic coatings), it provides basic principles in a number of important areas such as polymer structure, coatings formulation, pigment function, use of thinners, coating system compatibility, and theory of corrosion. Each chapter deals with a major area of the coatings field, including types of coatings, properties of synthetic resins, selection of coating systems, storage and safety, application methods, and surface preparation and pretreatment. There is also a consolidating chapter with illustrative examples of solutions to typical coatings problems. Interrelationships among the various areas of information are indicated through appropriate cross-referencing in the text. Specific references to Federal, Military, and other specifications are given where pertinent, and an entire chapter is devoted to a quick guide and summary of Federal specifications for organic coating materials. Finally, a selected bibliography and a comprehensive index are provided. While written primarily to meet the informational needs of the engineer, architect, maintenance superintendent, and responsible coatings procurement officer, the treatise is sufficiently broad in scope to serve as a general manual, a concise text, or a convenient reference source in the field of organic coatings.

Key words: application, coatings, coating systems, corrosion-inhibiting. Federal specifications, fire-retardant, lacquer, latex, multicolor, organic coatings, paint, pigments, polymers, properties, resins, selection, substrates, surface preparation, varnish, water-thinned.

Library of Congress Catalog Card No. 66-61927

II Foreword

Since publication in 1945 of the Paint Manual issued as National Bureau of Standards Building Materials and Structures Report 105, there have been many significant technological advances in the field of organic coatings. Materials and techniques that were nonexistent or only dimly foreseen twenty years ago are now in daily use. Coatings with new and improved properties emerge continually from research laboratories and production lines. The task of choosing wisely from this huge array of materials is a complex and difficult one that requires both a knowledge of what is available and an understanding of where and how these materials and techniques can best be utilized.

The present publication was written to fill this need. Through careful selection and orderly presentation of information, it has been possible to include a broad range of subject matter not previously available within a single volume. Practical information is given on the properties, selection, and application of organic coatings, including methods of adequate surface preparation. Basic principles also are discussed. Thus, a description is given of the polymeric nature of the synthetic resins that are the basis for most of our modern coating systems, and relationships between polymer structure and properties are indicated. A unique feature which should prove useful to those concerned with the procurement or supply of organic coatings for the Government is a quick guide to Federal specifications for these materials in the form of a classified table and a series of summarizing charts. Names and descriptions of some specific proprietary products are included for the convenience of the user. The omission of any particular product does not imply that it is considered unsuitable or unsatisfactory. Conversely, inclusion of a particular proprietary product does not constitute endorsement. Photographs of proprietary instrumenta- tion and apparatus are included for illustrative and educational purposes and do not constitute approval or endorsement of the particular product. Use of the information given herein for advertising or publicity purposes is not authorized. For the architect, engineer, maintenance superintendent, procurement officer, and others responsible for the proper selection and application of organic coating systems, this volume should be useful as a general manual, a unifying text, or a convenient source of authoritative information.

A. V. AsTiN, Director.

—

Preface

Paints of the pigment-in-oil type have long provided man with durable protective and decorative coatings for his houses and other structures. Such paints, improved and embellished with new pigments in a wide variety of colors, are useful even today. In recent years, however, paralleling the tremendous growth of the plastics industry, conventional paints have given way more and more to new and often superior coatings based on the new synthetic resins and the new formulating techniques that have been developed. The list of available and useful film-forming materials is almost as long as the roster of different plastics, resins, and cellulose derivatives. These materials, singly or in physical admixture or chemical combination, have been modified with a wide variety of pigments, plasticizers, extenders, activators, stabilizers, solvents, and diluents to produce an almost endless proliferation of coating materials, each with its own set of useful properties. The information problem confronting those who must choose wisely from this bewildering array is indeed a formidable one. It is a basic purpose of this publication to familiarize responsible personnel in the Federal establishment and elsewhere with the types of coating materials that are available, and to organize the essential information concerning such materials in a manner that will permit the selection and application of an adequate coating system for a particular job without requiring that the individuals responsible be or become expert coatings technologists. The sheer magnitude of the problem precludes a discussion or even merely a listing of every material; nor would such an approach be desirable in anything but an encyclo- pedic treatment of the subject. Instead, the guiding philosophy throughout the preparation of this volume has been to anticipate and to meet the practical informational needs of the engineer, architect, maintenance superintendent, and coatings procurement officer while keeping in mind the need for conciseness and clarity. This volume is not intended to take the place of the many excellent manuals tailored in detail to the specific needs of a specialized Government agency or activity. Rather, it offers a broad survey of the field of organic coatings that will serve to orient the reader and may be useful in indicating areas in which available manuals might advantageously be modified to utilize the newer materials and techniques. The information has been selected and presented in a manner that will provide maximum usefulness when the monograph is used as an ajunct to the handbooks and manuals of particular agencies to provide basic information and practical working details not usually found in the more specialized manuals. The author wishes to express his deep appreciation to Paul T. Howard, Head, Organic Coatings Group, National Bureau of Standards, for his encouragement, critical review, and many constructive suggestions throughout the preparation of this work. Grateful acknowledgment is made to Dr. S. G. Weissberg of this Bureau for his scholarly review of the completed manuscript. Other NBS colleagues to whom the author is indebted are Dr. A, F. Robertson, for his review of the information on fire-retardant coatings; Dr. R. Florin, for reviewing the section on effects of high energy radiation; W. C. Cullen, for his review of the section on bituminous coatings; and Dr. W. W. Walton, for his overall review of the manuscript. The author wishes to thank J. H. Geyer of Amchem Products, Inc., G. R. Hoover of Armco Steel Corporation, and M. B. Roosa of Parker Rust Proof Division, Hooker Chemical Corporation, for their valued review of the material on pretreatment of steel, aluminum, and galvanized surfaces. Appreciation is expressed also to Anthony Gallaccio, Frankford Arsenal, for his constructive review of the section on surface preparation and pretreatment of magnesium and its alloys. Finally, the author's great debt to the literature on organic coatings is humbly acknowledged. It is hoped that this volume will contribute tangibly to the knowledge and perspective so essential for the wise choice and proper application of modern coating systems.

A. G. Roberts. Washington, D.C.

Contents

Page

Abstract ii

Foreword III Preface v

1. Introduction 1 1.1. Scope 1 1.2. Basic objectives and arrangement of information 1 1.3. Structure and use of the index 2 1.4. Historical background ^ 3 1.4.1. Early history of varnish and paint 3 1.4.2. Twentieth-century developments 3

2. Types of coatings 5 2.1. General description and basis for classification 5 2.2. Clear coatings 5 2.2.1. Drying oils 5 2.2.1.1. Linseed 6 2.2.1.2. Tung 6 2.2.1.3. Dehydrated castor 6 2.2.1.4. Perilla, oiticica 6 2.2.1.5. Refined fish oils 6 2.2.1.6. Semi-drying oils 6 2.2.1.7. Non-drying oils 6 2.2.1.8. Tall oil 6

2.2.2. Varnish 6 2.2.3. Spirit varnish 7 2.2.4. Lacquer , 7 2.2.5. Sealer 7 2.2.6. Wax polishes 8 2.2.7. Strippable coatings 8 2.2.8. Synthetic resin coatings 9 2.3. Pigmented coatings 9 2.3.1. Pigments 9 2.3.1.1. Pigment function 9 2.3.1.2. White hiding pigments 9 2.3.1.3. Extender pigments 10 2.3.1.4. Color pigments 10 2.3.1.4.1. Natural 10 2.3.1.4.2. Synthetic inorganic 11 2.3.1.4.3. Synthetic organic 12 2.3.1.5. Corrosion-inhibiting pigments 13 2.3.1.6. Metallic pigments 13 2.3.1.6.1. Aluminum 13 2.3.1.6.2. MetalHc copper and its alloys 13 2.3.1.6.3. Zinc dust 14 2.3.1.6.4. Gold and silver flake 14 2.3.1.6.5. Stainless steel flake 14 2.3.1.6.6. Other metallic pigments 14 2.3.1.7. Black pigments 14 2.3.1.8. Fluorescent pigments 14

2.3.2. Oil paints 15 2.3.2.1. Straight oil paints 15

2.3.2.2. Oleoresinous paints . 15 2.3.3. Pigmented lacquers 15 2.3.4. Enamels 16 2.3.5. Latex paints 16 2.3.5.1. General 16 2.3.5.2. Forerunners 16 2.3.5.3. Uses 17 2.3.5.4. Formulation 17

vn Contents Pae^

2.3.5.5. Film fonnation 18 2.3.5.6. Chemical types 18 2.3.5.6.1. Styrene-butadiene 18 2.3.5.6.2. Polyvinyl acetate 19 2.3.5.6.3. Acrylic 19 2.3.5.6.4. Latex blends and copolymers 19 2.3.6. Water-solution coatings 20 2.3.7. Multicolor finishes 21 2.3.8. Fire-retardant coatings 21 2.3.8.1. General 21 2.3.8.2. Flame-retardant coatings 22 2.3.8.3. Insulative fire-retardant coatings 22 2.3.8.4. Evaluation of fire-retardant coatings 23 2.3.8.5. Selection of fire-retardant coatings systems 23 2.3.8.6. Summary 24 2.3.9. Cold water paints 25 2.3.9.1. Cement-water paints 25 2.3.9.2. Whitewash 25 2.4. Primers 26 2.4.1. Wood primers 26 2.4.2. Metal primers 26 2.4.2.1. Primers for iron and steel 27 2.4.2.2. Primers for galvanized iron and steel 27 2.4.2.3. Primers for aluminum 27 2.4.2.4. Primers for magnesium 27 2.4.3. Masonry primers 28 2.4.4. Primers for plaster and wallboard 28 2.5. Bituminous coatings 29 2.5.1. Hot applied 29 2.5.2. Cut-backs 29 2.5.3. Paints and varnishes 29 2.5.4. Emulsions 29 2.5.5. References 30

3. Properties of synthetic resins for coatings 33 3.1. General nature of synthetic resins 33 3.1.1. Polymer structure and properties 33 3.1.2. Functionality and polymerization mechanisms 34 3.1.2.1. Functionality 34 3.1.2.2. Addition polymerization 34 3.1.2.3. Condensation polymerization 35 3.1.3. Methods of polymerization 35 3.1.3.1. Bulk polymerization 35 3.1.3.2. Solution 35 3.1.3.3. Emulsion 36 3.1.3.4. Suspension 36 3.1.4. Types of polymers 36 3.1.4.1. Homopolymer 36 3.1.4.2. Copolymer 36 3.1.4.3. Block polymer 37 3.1.4.4. Graft polymer 37 3.1.4.5. Stereoregular polymers 37 3.1.5. Classification of synthetic resins 37 3.2. Cellulose derivatives 38 3.2.1. Cellulose esters 38 3.2.1.1. Cellulose nitrate 38 3.2.1.2. Cellulose acetate 39 3.2.1.3. Cellulose propionate 40 3.2.1.4. Cellulose butyrate 40

mi Contents

3.2.2. Cellulose mixed esters 40 3.2.2.1. Cellulose acetate-butyrate 40 Z.I.I.Z. Cellulose acetate-propionate 40

3.2.3. Cellulose ethers 40 3.2.3.1. Ethyl cellulose 40 3.2.3.2. Methyl cellulose 41 3.2.3.3. Carboxymethyl cellulose 41 3.2.3.4. Hydroxyethyl cellulose 41 3.2.3.5. Benzyl cellulose 41

3.3. Phenolic resins 41 3.3.1. Modified phenolic resins, oil-soluble 42 3.3.2. Pure phenolic resins (100% phenolic) 42 3.3.2.1. Oil-soluble, thermoplastic 42 3.3.2.2. Oil-soluble, heat-reactive 42 3.3.2.3. Alcohol-soluble, oil-insoluble, thermosetting 43 3.3.3. Phenolic dispersion resins 43

3.4. Alkyd resins 43 3.4.1. Composition and properties of alkyd resins 43 3.4.1.1. General 43 3.4.1.2. Effect of oil-modification 44 3.4.1.3. Uses 44 3.4.2. Resin-modified alkyds 45 3.4.2.1. Styrenated-alkyds 45 3.4.2.2. Silicone-alkyds 45

3.4.2.3. Other resins with which alkyds may be combined. _ 45 3.4.3. Polyester resins 45

3.5. Amino resins 46 3.5.1. Properties 46 3.5.2. Formation 46 3.5.3. Uses 46 3.5.4. Comparison of urea and raelamine resins 47

3.6. Vinyl resins 47 3.6.1. Polyvinyl acetate 47 3.6.2. Polyvinyl chloride 48 3.6.2.1. General properties 48 3.6.2.2. Plastisols and organosols 48 3.6.3. Polyvinyl chloride-acetate 48 3.6.4. Polyvinyl alcohol 49 3.6.5. Polyvinyl acetals 50 3.6.5.1. Polyvinyl formal 50 3.6.5.2. Polyvinyl acetal 50 3.6.5.3. Polyvinyl butyral 50 3.6.6. Polyvinylidene chloride and its copolymers 51 3.6.6.1. Polyvinylidene chloride 51 3.6.6.2. Polyvinylidene chloride copolymers (general) 51 3.6.6.3. Vinylidene chloride-vinyl chloride copolymers 51 3.6.6.4. Vinylidene chloride-acrylonitrile copolymers 51 3.6.7. Polyvinyl ethers 52

3.7. Acrylic resins 52 3.7.1. Properties 52 3.7.2. Uses 52

3.8. Polystyrene and its copolymers 53 3.8.1. Polystyrene homopolymer 53 3.8.2. Styrene-butadiene copolymers 53 3.8.3. Styrenated oils and alkyds 54 3.8.4. Other styrene derivatives 54 3.8.4.1. Copolymers 54 3.8.4.2. a-Methyl styrene 54 3.8.4.3. Vinyl toluene polymers 54

IX Contents ^as*

3.9. Silicone resins 54 3.9.1. Pure silicone resins 54 3.9.2. Modified silicone resins 55 3.9.2.1. Silicone-alkyds 55 3.9.2.2. Other silicone-modified resins 55 3.10. Epoxy resins 55 3.10.1. Unmodified epoxy baking resins 56 3.10.2. Unmodified epoxy air-dry resins 56 3.10.3. Esterified epoxy resins 56 3.11. Polyamide resins 57 3.11.1. Nylon resins 57 3.11.2. Thermoplastic polyamides 57 3.11.3. Thermosetting polyamides 58 3.11.3.1. Epoxy-polyamides 58 3.12. Polyurethanes 58 3.12.1. Two-component urethane systems 59 3.12.1.1. Prepolymer type 59 3.12.1.2. Polyisocyanate type 59 3.12.2. Single-package urethane systems 59 3.12.2.1. Heat-cured 59

3.12.2.2. Air-cured . 60 3.12.2.3. Moisture-cured 60 3.12.3. Properties and uses of the polyurethanes 60 3.13. Polyethylene 60 3.13.1. Properties and uses of low- and high-density polyethylene 60 3.13.2. Irradiated polyethylene 61 3.13.3. Chlorosulfonated polyethylene 61 3.14. Fluorocarbon resins 62 3.14.1. Polytetrafluoroethylene 62 3.14.2. Polychlorotrifluoroethylene (Kel F) 63 3.15. Synthetic rubbers 64 3.15.1. Polychloroprene (neoprene) 64 3.15.2. Chlorosulfonated polyethylene 65 3.15.3. Styrene-butadiene rubber (SBR) 65 3.15.4. Butadiene-acrylonitrile rubber (nitrile rubber) 65 3.15.5. Isobutylene-isoprene copolymer (butyl rubber) 65 3.15.6. Polysulfide rubber 66 3.15.7. Silicone rubber 66 3.15.8. Polyurethane rubber 66 3.15.9. Chlorinated rubber 66 3.15.10. Polyacrylic ester rubber 67 3.15.11. Cyclized rubber 67 3.16. Water-soluble resins 67 3.16.1. Thermoplastic types 67 3.16.2. Thermosetting types 67 3.17. Miscellaneous resins 68 3.17.1. Resin combinations 68 3.17.2. Resins discussed elsewhere in text 68 3.17.3. Natural resins 68 3.17.4. Furan resins 68 3.17.5. Polypropylene 68 3.17.6. Polycarbonate resin (Lexan) 69

4. Selection of coating systems 71 4.1. Basis for selection 71 4.1.1. Service environment 72 4.1.2. Nature of substrate 72 4.1.3. Basic function.. 73 4.1.4. Appearance 73

X Contents

4.1.4.1. Clarity and whiteness 74 4.1.4.2. Translucent coatings 74 4.1.4.3. Opacity 74 4.1.4.4. Gloss 74 4.1.4.5. Textured coatings 75 4.1.4.5.1. Wrinkle 75 4.1.4.5.2. Crackle 75 4.1.4.5.3. Pebble 75 4.1.4.5.4. Hammertone 75 4.1.4.5.5. Multicolor 75 4.1.5. Application limitations 76 4.1.6. Cost 76 4.2. Coating systems for wood 77 4.2.1. Exterior coatings for wood 77 4.2.2. Interior coatings for wood 78 4.3. Coating systems for metals 79

4.3.1. Coatings for structural steel 1 80 4.3.1.1. Oil-base and oleoresinous systems 80 4.3.1.1.1. Intermediate and finish coats 81 4.3.1.2. Vinyl systems 81 4.3.1.3. Bituminous coatings 82 4.3.1.4. Galvanized steel 82 4.3.2. Industrial coatings for steel 83 4.3.3. Coatings for other metals 83

4.4. Coatings for concrete and masonry 84 4.4.1. Cement-water paints 84 4.4.2. Oleoresinous paints 84 4.4.3. Latex paints 84 4.4.4. Synthetic resin coatings 85

4.5. Coatings for plaster and wallboard 85 4.6. Functional coatings 86 4.6.1. Chemical resistance 86 4.6.1.1. Acid resistance 87 4.6.1.2. Alkali resistance 87 4.6.1.3. Oxidation resistance 87

4.6.1.4. Resistance to organic solvents .. 87 4.6.1.5. Water resistance 87 4.6.2. Heat resistance 88 4.6.3. Abrasion and mar resistance 88 4.6.4. Electrical properties 88 4.6.5. Impermeability 88 4.6.6. Transparency 88 4.6.7. Fire resistance 88 4.6.8. Radiation resistance 88 4.7. Summary 90

5. Storage, safety, and application methods 91 5.1. Storage of coating materials 91 5.2. Safety precautions 91 5.2.1. Toxicity 91 5.2.2. Fire 92 5.2.3. Falls and spillage 93 5.3. Mixing, thinning, and activation 93 5.3.1. Mixing of paints 93 5.3.2. Use of thinners 94 5.3.2.1. Principles of proper thinning 95 5.3.3. Activation of two-package systems 96 5.4. AppUcation methods and equipment 96 5.4.1. Brush application 96

XI Contents ^^^e

5.4.2. Roller application 97 5.4.3. Spray application 98 5.4.3.1. Conventional cold spray 98 5.4.3.2. Airless spray 99 5.4.3.3. Hot spray 100 5.4.3.4. Flame spraying 100 5.4.3.5. Electrostatic spray 100 5.4.3.5.1. Liquid 100 5.4.3.5.2. Airless 101 5.4.3.5.3. Dry 101 5.4.3.6. Dual-gun spraying 101 5.4.4. Dipping and flow coating 102 5.4.4.1. Dipping 102 5.4.4.2. Flow coating 102 5.4.5. Knife and roller coating 102 5.4.5.1. Knife coating 102 5.4.5.2. Roller coating 108 5.4.6. Fluidized bed coating 103

6. Surface preparation and pretreatment 105 6.1. Importance of surface preparation and pretreatment 105 6.2. Preparation of wood surfaces 106 6.2.1. Bare wood 106 6.2.1.1. General 106 6.2.1.2. Exterior wood 106 6.2.1.3. Interior wood 106 6.2.2. Previously painted wood 107 6.3. Preparation and pretreatment of metal surfaces 108 6.3.1. Cleaning of steel (and iron) surfaces 108 6.3.1.1. Solvent cleaning 108 6.3.1.1.1. Wiping, dipping, or spraying 109 6.3.1.1.2. Vapor degreasing 109 6.3.1.2. Steam cleaning 109 6.3.1.3. Emulsion cleaning 109 6.3.1.4. Alkali cleaning 110 6.3.1.5. Acid cleaning or pickling 110 6.3.1.5.1. Phosphoric acid cleaning 110 6.3.1.5.2. Pickling 111 6.3.1.6. Hand cleaning 111 6.3.1.7. Power tool cleaning 112 6.3.1.8. Sandblasting 112 6.3.1.8.1. Sandblasting to "white" metal 112 6.3.1.8.2. Commercial blast cleaning 113 6.3.1.8.3. "Near-white" blast cleaning 113 6.3.1.8.4. "Brush-off" blast cleaning 113 6.3.1.8.5. Wet sandblasting 113 6.3.1.8.6. Dry sandblasting equipment and techniques 113 6.3.1.8.7. Vacuum blast cleaning 113 6.3.1.8.8. Cost and production rate 113 6.3.1.8.9. Safety 114 6.3.1.8.10. After sandblasting 114 6.3.1.9. Flame cleaning 114 6.3.2. Chemical pretreatment of steel (and iron) surfaces 115 6.3.2.1. Cold phosphate pretreatment (phosphoric acid type) 115 6.3.2.2. Cold phosphate pretreatment (zinc phosphate type) 116 6.3.2.3. Hot phosphate pretreatment (zinc phosphate type) 116 6.3.2.4. Hot phosphate pretreatment (iron phosphate type) 117 6.3.2.5. Organic pretreatment coating (wash primer) 117 xn Contents

6.3.2.5J.. Two-component wash primer 117 6.3.2.5.2. Single-package wash primers 118 6.3.3. Galvanized surfaces 118 6.3.3.1. New galvanized surfaces 118 6.3.3.2. Old galvanized surfaces 119 6.3.3.3. Cadmium surfaces 119 6.3.4. Aluminum and its alloys 119 6.3.5. Magnesium and its alloys 120 6.3.5.1. Cleaning 120 6.3.5.1.1. Electrochemical cleaning 120 6.3.5.1.2. Alkaline cleaning 120 6.3.5.1.3. Acid pickling 121 6.3.5.1.4. Mechanical cleaning methods 121 6.3.5.2. Chemical pretreatments 121 6.3.5.2.1. Type I chrome-pickle treatment 121 6.3.5.2.2. Type II sealed chrome-pickle treatment 121 6.3.5.2.3. Type III dichromate treatment 122 6.3.5.2.4. Type IV galvanic anodizing treatment 122 6.3.5.2.5. Type V caustic anodizing treatment. 122 6.3.5.3. Anodic or electrochemical treatments 122 6.3.5.3.1. The HAE process 122 6.3.5.3.2. Acid chromate anodic process 123 6.3.6. Other metals 124 6.3.6.1. General 124 6.3.6.1.1. The cleaning process 124 6.3.6.1.2. Wash primer type of pretreatment... 124 6.3.6.1.3. Proprietary chromate treatments 124 6.3.6.2. Zinc and cadmium 124 6.3.6.3. Copper, brass, and bronze 124 6.3.6.4. Silver 124 6.3.6.5. Tin 124 6.3.6.6. Terneplate 125 6.3.6.7. Lead 125 6.3.6.8. Titanium 125 6.3.6.9. Stainless steel, nickel, and chromium 125

6.4. Preparation of concrete and masonry surfaces 125 6.4.1. Aging and weathering 125 6.4.1.1. Latex (water emulsion) paints 126 6.4.1.2. Cement-water paints 126 6.4.1.3. Alkali-resistant synthetic resin coatings 126 6.4.1.4. Oil-base and oleoresinous paints 126 6.4.2. Cleaning 126 6.4.2.1. Dust, dirt, and loose material 126 6.4.2.2. Form oil 126 6.4.2.3. Efflorescence 127 6.4.2.4. Mildew or mold 127 6.4.3. Patching, grouting, and filling 127 6.4.4. Roughening 127 6.4.5. Pretreatment 127 6.4.6. Conditioning of chalky surfaces 128 6.4.7. Summary 128

6.5. Preparation of plaster and wallboard surfaces 128 6.5.1. Plaster 128 6.5.1.1. New Plaster 128 6.5.1.2. Old plaster 128 6.5.2. WaUboard 129 6.5.2.1. Gypsum wallboard 129 6.5.2.2. Fiberboard, hardboard 129 xm Contents

6.6. Preparation of previously painted surfaces 129 6.6.1. Maintenance painting 129 6.6.2. Previously painted wood surfaces 130 6.6.3. Previously painted metal surfaces 130 6.6.4. Previously painted concrete and masonry 130 6.6.4.1. Latex (emulsion) paint 130 6.6.4.2. Solvent-thinned paints 131 6.6.4.3. Cement-water paints 131 6.6.5. Previously painted plaster and wallboard surfaces 131 6.7. Summary 132

7. Consolidation of coatings information 135 7.1. A glance backward and forward 135 7.2. Supplementary information 135 7.2.1. Painting conditions 135 7.2.2. General coating requirements 136

-• ' 7.2.2.1. Thickness 136 7.2.2.1.1. Spreading rate 136 7.2.2.1.2. Thickness measurement 137 7.2.2.2. Drying time 138 7.2.2.3. Coating system compatibility. 139 7.2.2.4. Workmanship and general appearance 140 7.2.3. Application of corrosion-inhibitive primers 140 7.2.4. Helpful hints 141 7.2.4.1. Wetting properties of common coatings 141 7.2.4.2. Sources of specifications 141 7.2.4.2.1. Federal specifications 141 7.2.4.2.2. Military specifications 141 7.2.4.2.3. Federal test methods 141 7.2.4.2.4. ASTM test methods 141 - 7.2.4.2.5. ASA standards 141

7.2.4.3. Application hints. . 141 7.2.4.3.1. When applying multicoat paint systems 141 7.2.4.3.2. When painting metal 142 7.2.4.3.3. When applying latex paints 142 7.2.4.3.4. Vinyl coating systems 142 7.2.5. Test equipment and test methods 142 7.2.6. Paint film defects and failures 142 7.2.7. Basic information (table) 142

7.3. Use of the text to solve specific coating problems 143 7.3.1. Selection and application of a suitable coating system for new exterior wood siding on a home in an ordinary urban environment 143 7.3.2. Selection and application of a coating system for structural steel to be exposed in a moderate industrial environment 143 7.3.3. Selection and application of a coating system for galvanized steel to be exposed in an ordinary environment 144 7.3.4. Selection of a coating system for the inside of a potable water tank 144 7.3.5. Selection and application of a coating for a cinderblock basement wall 144 7.3.6. Selection and application of a coating system for exterior concrete 144 7.3.7. Coating system for gypsum wallboard (dry wall) 144 7.3.8. Selection and application of a coating system for aluminum exposed to a moderately corrosive atmosphere 145 7.3.9. Coating for wooden gymnasium floor 145 7.3.10. Acid-resistant coating 145 7.3.11. Fire-retardant coating system 145 7.4. Summary and conclusion 145

XIV Contents

8. Summary of Federal specifications for organic coatings.- 149 8.1. Quick guide to Federal specifications for organic coating materials 149 8.1.1. Enamels 149 8.1.2. Lacquers 149 8.1.3. Paints 149 8.1.4. Primers 149 8.1.5. Sealers 150 8.1.6. Stains 150 8.1.7. Varnishes 150 8.1.8. Miscellaneous coatings 150 8.1.9. Accessory materials 150

8.2. Tabular summary of important Federal specifications for organic coatings _ 150 8.2.1. Description and use 150 8.2.2. Quick-glance summary tables 152

9. Selected bibliography , 169 9.1. Specific references 169 9.2. General references 172

10. Index 175

XV List of Illustrarions

Frontispiece. Rocket in flight A

Figure 1.1. Early varnish-making 4

Figure 1.2. Modern phthaHc anhydride plant 4

Figure 2.1. Integrometer increases speed and precision of adhesion measurements 30

Figure 2.2. Radiant panel flame spread apparatus 31

Figure 2.3. Explosion of cellulose nitrate film in storage 31

Figure 2.4. Bubble viscometer 32

Figure 2.5. Washability machine 32

Figure 3.1. Jet abrader 69

Figure 3.2. Parallel-grove adhesion tester 70

Figure 3.3. Infrared spectrophotometer 70

Figure 4.1. Xenon-arc accelerated weathering machine 90

Figure 4.2. Carbon-arc accelerated weathering machine 90

Figure 5.1. Coil-coating inspection station 104

Figure 5.2. Electrostatic spray gun paints chain-link fence 104

Figure 6.1. Glossmeter 132

Figure 6.2. Portable slipperiness tester 133

Figure 6.3. Testing machine for stress-strain measurements 133

FiGtmB 6.4. Spreading grout coat onto concrete surface 134

Figure 6.5. Trowel finishing mortar topping on concrete surface 134

Figure 7.1. Eddy-current non-destructive thickness gage 146

Figure 7.2. Gage for thickness measurement on magnetic substrates 146

Figure 7.3. Wet film thickness gage 147

Figure 7.4. Balanced-beam scrape adhesion tester 147

Figure 7.5. Indentation hardness tester 148

FiGtmE 7.6. Hardness rocker 148

Figure 8.1. , Improved dip-coater 150

Figure 8.2. Modern exposure racks 151

Figure 8.3. Cinder-block test house 168

Figure 8.4. Masonry wall specimen 168

XVI 1. Introduction

1.1. Scope

Surfaces are painted to protect them from apply and maintain these coatings with suffi- harmful agents in their environment, to deco- cient detailed information to do the job in a rate them, or to provide some functional pur- manner that will ensure adequate adhesion and pose such as light diffusion or reflection, heat durability. reflection or absorption, abrasion resistance, The subject matter is not limited to conven- fire retardance, cleanliness, imperviousness, or tional paints but also includes pigmented and identification. Whatever the objective of paint- clear coatings based on a wide variety of cellu- ing, it is successfully achieved only to the extent losic and resinous film-forming materials that that the protective, decorative, or functional have become available in recent years. The coating remains bonded to its substrate and term "paint" is often used in its broadest sense retains its essential properties. To ensure this to denote a liquid material that requires careful selection of the proper coating has the property, when applied in thin system, scrupulous attention to surface prep- a layer, of hardening aration, skilled application, and adequate main- into a continuous film that adheres to the sub- tenance. It is the purpose of this manual to strate; in this sense, any organic coating provide sufficient information relating to these material may be considered a paint. A few requirements to enable those responsible for the inorganic coating materials, such as cement- maintenance of structures and equipment to water paints and phosphate coatings, also have select the proper coating material for a particu- been considered because of their important lar application, and to provide those who must uses in the coatings field.

1.2. Basic Objectives and Arrangement of Information

This volume seeks to provide coverage in ing of the subject that would make repetition depth of the organic coatings field by giving of detail unnecessary. Toward this end, each basic principles and information as well as chapter deals with a major area of the coatings practical working details. Through treatment field. Cross references within the text show of both the fundamental and practical aspects interrelationships among areas of information of the subject, it is hoped that the volume will while serving to avoid repetition and conserve be found useful as a working manual, as a con- space. Specific references to Federal, Military, cise textbook, or as a convenient reference and other specifications are given at appropri- source on the properties, selection, and use of ate places, and an entire chapter has been organic coatings. devoted to a quick guide and summary of It has been neither feasible nor desirable to Federal specifications for organic coating ma- specify a particular coating system for every terials. A consolidating chapter provides sup- application or to give individual instructions, plementary information and furnishes examples without explanatory background, for handling of use of the volume in solving typical coating every situation. Such a presentation, while problems. appropriate and useful for specialized manuals With the wide variety of coating materials of limited scope, would involve almost endless available today, the solution to a particular repetition of detail if applied to the entire field coatings problem is seldom a unique one. This of organic coatings. Instead, the objective has publication has not presumed to make those been to provide an adequate basic understand- decisions which are the prerogative of the

1 responsible agency or coatings official. Instead, In presenting information on the properties, it has sought to provide reliable and compre- selection, and use of organic coatings, the vol- hensive information upon which such decisions ume has endeavored to be a comprehensive might be based. By treating each broad aspect guide rather than a specification. It should be of coatings technology in one unified portion of understood, therefore, that when the general the text, repetition of detail has been kept to or basic information presented herein differs a minimum and it has been possible to broaden from the stipulated 7equirements of a particular the scope of information that could be included. specification or from the specific instructions of Given such information in a well-indexed and the manufacturer of a particular product, the accessible form the reader can readily extract requirements of the specification or the manu- the information he needs and arrange it himself facturer's instructions normally should take in whatever sequence best suits his purpose. precedence.

1.3. Structure and Use of the Index

The paramount objective of any index should aware of it) that a water-soluble type of linseed oil is available; under "Tung oil" he will be di- be to lead the user quickly to all the information rected to information on its use in spar varnishes, that pertains to his problem. Toward this end, in wrinkle finishes, etc. entries have not merely been listed but have It is seen from the preceding example that even been organized under a system of headings and the user with only a general knowledge of clear subheads that not only direct the user to the coatings would be led by the index to all the rele- vant information in the text, through a series of immediate subject matter but also make the expanding steps; and that he would learn about index itself a source of integrated information the basic types of clear coatings and of many that can lead him to pertinent related areas. specific materials along the way. Alternatively, To achieve adequacy of coverage and cross- the user with specific informational needs could enter the index at any stage in the interrelated referencing without undue repetition, a "pyr- chain of entries with equal success. amidal" arrangement of entries has been Example 2: Let us consider briefiy the gen- employed in which the user is directed to succes- eral way in which the index deals with the complex and voluminous subject of surface prep- sively more detailed branches until the subject aration. In this case, it has been desirable and has been fully covered. Two examples will feasible to employ the "pyramidal" index struc- serve to illustrate how this arrangement pro- ture largely under the main heading "Surface vides maximum utility with minimal redund- preparation for painting", and the bulk of the information on this subject is developed here. ancy. It is suggested that the reader actually Major subheads provide references to general follow the examples through the index. principles and methods and to each type of substrate (concrete, metals, wood, etc.), and these categories are further developed through detailed sub-entries. Cross references direct the user to Example 1 : Suppose it is desired to obtain "See also Surface pre-treatment, specific clean- detailed information on clear coatings. One ing method, and specific substrate". The com- might first look for this under the general head- prehensive listing under "Surface preparation" ing "Coatings", with the expectation of finding together with the cross references guide the user "clear" as one of the sub-entries under it. This to all the pertinent information. is actually the way it appears in the index; reference is made to a single set of pages on clear coatings, with no further breakdown at this point (to subdivide the index further at this point Attention is invited to the detailed treatment would make the already long listing under "Coat- given to the subject "Coatings for" in the index. ings" very cumbersome, especially if such sub- While neither the list nor the accompanying dividing were applied generally to the other entries). The main heading, however, directs the page references are exhaustive in nature, they

user to "See also . . . under specific coating type", provide an entree to places in the text which in this instance under "Clear coatings", where he mention coating materials for many specific finds a listing of the various types of clear coat- applications. ings (drying oils, lacquer, varnish, etc.) as well as a general reference to their "durability com- A formal glossary was considered beyond the pared to pigmented" types. Again, the is of the present volume however, the index user scope ; directed to "See also under specific type" if he will often serve in lieu of one to direct the user wishes further details; thus, he can look under to places in the text where paint terms or con- "Drying oils" to find page references to types of drying oils such as boiled or raw oils, or to specific cepts are defined or discussed. oils such as linseed or tung oil. Here, again, the Although practical considerations have nec- is user told to "See . . . under specific oil" where essarily limited the space that could be devoted he is given a final breakdown of detailed informa- to indexing, it may be apparent from the fore- tion; for example, under "Linseed oil" he will find one. references to its use in alkyd resins and in house going that the index is not a perfunctory paints, and he will learn (if he is not already The reader is urged to make full use of it.

2 1.4. Historical Background

1.4.1. Early History of Varnish and Paint 1.4.2. Twentieth-Century Developments

Varnish much like that which we know today As with many of the arts of man, the paint was made by the Egyptians as early as 500 and varnish industry, which had undergone B.C. [1].^ Some of it still endures today as little change for many centuries, felt the impact a coating for their mummy cases. Their var- of the scientific and technological awakening nish was made of resin and oil and evidently ushered in with the twentieth century. New was applied in a viscous, unthinned form with pigments, improved drying oils, cellulosics and a blade or with the fingers. A durable varnish synthetic resins, and a variety of modifying made by cooking amber resin with linseed oil agents began to flow from research laboratories is described in the works of Theophilus in the and production lines in ever-increasing num- tenth century A.D. Theophilus ascribed his bers and became the basis for a never-ending formulas to earlier compilations dating back to stream of new coating materials. the eighth and the first centuries. His varnish A few milestones may be mentioned here. formula remained the standard of the art for Titanium dioxide was developed as a superior another 800 years. Turpentine, though known white pigment with outstanding hiding power since early times, was used for other purposes and durability. Tung oil, when heat-treated (for example, as an embalming fluid) rather at 500 °F (260 °C), acquired excellent prop- than as a varnish component or thinner until erties useful in the manufacture of fast-drying, about the fourteenth or fifteenth century. The durable paints and varnishes. Phenolic resins first technical treatise on the paint and varnish were developed and in combination with tung industry as it is known today was prepared by oil yielded varnishes of exceptional water re- Watin in 1772. He described the formulation sistance and outdoor durability. Linseed and and manufacture of oleoresinous varnishes, soybean oil-modified alkyd resins were used in spirit varnishes, and resin dispersions in tur- the manufacture of fast-drying paints with pentine. His book went through many editions excellent gloss and weathering properties. The with only minor changes and remained the introduction of quick-drying nitrocellulose lac- standard of the varnish-making art until the quers revolutionized the automobile and furni- turn of the twentieth century. ture finishing industries. Baked urea- and Paint in the form of finely divided solid melamine-formaldehyde resins provided dur- particles (pigment) suspended in a fluid me- able, gleaming finishes for automobiles, house- dium has been used since prehistoric times for hold appliances, and table tops. Recent years decorative purposes. Protective and decorative have seen the development of abrasion- and paints using egg yolk or albumen, gum arable, water-resistant vinyls, tough and chemically and beeswax as vehicles or binders were made resistant epoxies, durable acrylics, heat-resist- for many centuries. Pigments often were sus- ant silicones, hard high-gloss polyurethanes, pended in water, with or without binder. For and many others. The advent of water emul- more durable paints, the pigment was sus- sion latex paints and convertible water-solution pended in a varnish vehicle like that described paints has added yet another dimension to the by Theophilus. Straight linseed oil paints, variety, usefulness, and complexity of the or- though slower drying than varnish-base paints, ganic coatings field; the elimination of the were lower in cost and easier to apply, and hazards, inconvenience, and expense of organic found considerable use during the middle cen- solvents in such paints has given tremendous turies. During most of the nineteenth century, impetus to their further development. paints based on white lead-in-oil were widely Most of these new materials are discussed used for both interior and exterior purposes. in the pages which follow. In the interests of For many centuries paint formulation was an unencumbered and concise presentation, free an art that was carefully guarded and handed use is made, throughout this publication, of down from one generation to the next. As the common paint and technical terms, e.g., plasti- paints were made in small batches using crude cizer, polymerization, copolymer, pigment vol- grinding mills and laborious mixing methods, ume concentration (PVC), extender, gloss, they were expensive and available only to the thermoplastic, emulsion, etc. The meaning of affluent segments of society. Not until the many of these terms is made clear through dis- establishment of the first paint and varnish cussion at appropriate places in the text, and factories during the nineteenth century did the Index will serve in lieu of a glossary to paints begin to find their way into general use. direct the reader to them. Standard definitions of many terms relating to paint, varnish, lacquer, and related materials are given in ASTM ' Figrures in brackets indicate the literature references at the end of the text. D16-59 [115].

2. Types of Coatings

2.1. General Description and Basis for Classification

The fundamental constituent of all coating acetate. Or the film-former may be a chem- materials is the film-forming base which at- ically reacted combination, such as a drying taches itself to the substrate to which it is oil-alkyd. It may also be a physical mixture applied, and into which are incorporated all of two or more film-forming ingredients, such the other ingredients that impart to the film as an alkyd-nitrocellulose blend. the special properties required for a particular The film-forming solution constitutes a liquid purpose. In order to adhere to the substrate, vehicle in which insoluble pigment particles the film must be applied in the form of a liquid may be dispersed to give color, opacity, and that wets the substrate (see Chapter 6 on sur- increased outdoor durability to the film. Many face preparation) and then cures to a solid other substances may be added to the vehicle state. Some film-formers, such as linseed and (and if dissolved in it become a part of it) to other drying oils, are liquids of usable viscosity achieve specific properties in the film driers to initially. Others, such as uncured phenolic or — hasten the film curing process, plasticizers to epoxy resins, are viscous liquids which may impart flexibility or moisture resistance or require thinning with suitable solvents and other properties, stabilizers to lessen the dete- diluents before use. Many (e.g., cellulose de- riorative effects of heat and sunlight, fillers and rivatives) are solids in fibrous, granular, or extenders to modify gloss, or improve me- powder form; these must be dissolved, dis- chanical properties or reduce cost, thinners persed, or suspended in a volatile liquid medium (solvents, latent solvents, diluents) to adjust (vehicle) before they can be applied. Uipon viscosity and to control the evaporation rate evaporation of the volatile portion, the solids for proper application and flowout, etc. are deposited in the form of a continuous, adherent, transparent or translucent film. The wide variety of coating materials that The film-forming base (often referred to as are available may be conveniently classified on the binder in pigmented paints) need not be a the basis of their composition, behavior, and simple or single resin. Often it is a copolymer function, as discussed in the sections which (see sec. 3.1.4.2), such as polyvinyl chloride- follow.

2.2. Clear Coatings

2.2.1. Drying Oils brushing and penetrating properties as the raw oils; however, because they have been purified The drying oils used in organic coatings are by chemical and mechanical treatment, their derived chiefly from the nuts or seeds of certain clarity and color are improved and they can be plants and from certain species of fish. Chem- heated to varnish-cooking temperatures with- ically, these are mixed glycerides (glyceryl out the formation of a sludge that would impair esters) of saturated and unsaturated fatty the drying properties and gloss of the varnish acids. They have the property, when exposed film. "Boiled" oils are not actually boiled, but in a thin layer to the atmosphere, of "drying" are drying oils which have been heated with or hardening into a clear or translucent film by lead, cobalt, and manganese compounds to pro- a process of oxidation and polymerization (see duce a solution that will have the necessary sec. 3.1.1). An indication of their drying abil- drying properties; for practical purposes, the ity (chemical unsaturation) is given by their same drying results are achieved by adding Iodine Number.^ When combined with a hard soluble organic salts of these heavy metals to resin, the drying oils impart their inherent the cold untreated oil. Heat-bodied oils are flexibility, elasticity, and durability to the de- refined oils (usually alkali-refined) that have rived film. been held at an elevated temperature (about The drying oils are useful in paints in both 500 to 600 °F [260 to 316 °C]) until suflRcient their raw and variously processed forms. Raw polymerization has taken place to produce the OILS, besides being the least expensive, possess desired viscosity. While such bodied oils lack the best brushability and the greatest penetrat- the easy brushability and excellent penetrating ing ability; however, their flow and leveling ability of raw oil, they possess superior flow properties are inferior to that of the processed and leveling properties, convert to a dry film forms. Refined oils have almost the same more rapidly, and yield films of higher gloss

5 and paler color. Blown oils are those through 2.2.1.6. Semi-drying oils are in a slower dry- which air has been blown at temperatures ing category than the oils just discussed. Typi- around 200 °F (93 °C), resulting in partial cal of these are soybean oil, safflower oil, and oxidation of the oil with a consequent increase tall oil (see sec. 2.2.1.8). Semi-drying oils dry in its viscosity and in its tolerance for water. to soft, tacky films at ordinary temperatures, Blown oils are darker in color and less durable but they may be blended or baked with faster- than the heat-bodied oils, but they have excel- drying oils or hard resins to produce excellent lent flow, leveling, and gloss properties—in coatings. When combined with a hard resin, fact, they are so free-flowing that difficulties the semi-drying oils, like the drying oils, impart may arise from pigment settling in containers their inherent flexibility, elasticity, and durabil- and from a tendency to run or sag when ap- ity to the resultant film. plied. Their brushability, penetrating properties, and drying rate resemble those of the 2.2.1.7. Non-drying vegetable oils, such as heat-bodied oils. cottonseed, cocoanut, castor, corn, and peanut oil should not be confused with the drying and 2.2.1.1. Linseed oil, derived from the seed of semi-drying oils. Although widely used in the the flax plant, is the most important and most organic coatings field, the non-drying oils func- widely used of the drying oils. In addition to tion as plasticizers rather than as film-formers. being the only drying oil which is used extensively by itself as a paint vehicle (in exterior 2.2.1.8. Tall oil is a semi-drying oil derived, house paints), it is an important constituent of unlike the oils already described, as a by- many high-grade varnishes and oleoresinous product of the sulfate wood pulp process used in paints. In various combinations with alkyd paper-making. Acidification of the black liquor resins (see sec. 3.4), it forms one of our most skimmings from the pulping process yields an versatile and useful classes of coating material. incompatible liquid mixture from which, upon Recently, a water-soluble from of linseed oil separation, the tall oil is drawn off. The oil has been developed [121]. consists chiefly of a mixture of resin acids related to abietic acid, and of fatty acids related 2.2.1.2. Tung oil is invaluable in imparting to oleic acid, together with a small proportion fast drying qualities, excellent water resistance, of nonacidic substances. Because of its ready and exterior durability to varnishes. Its drying availability, low cost, and good properties when ability is the result of a highly unsaturated, combined with various resins, it is finding conjugated double-bond molecular structure. steadily increasing use in alkyd resin manu- Its tendency to form wrinkled films when used facture and in combination with other resins alone can be utilized in the manufacture of such as epoxies, phenolics, and styrenated wrinkle finishes (see sec. 4.1.4.4). resins (see Ch. 3).

2.2.1.3. Dehydrated castor oil somewhat 2.2.2. Varnish resembles tung oil in its drying characteristics but forms a softer, more flexible film with some Although a varnish has sometimes been de- tendency (controllable) to develop after-tack. fined as any liquid composition which is con- It has excellent color and color retention prop- verted to a transparent or translucent film erties and is widely used in the paint and after application as a thin layer, the term is varnish industry. most often used in a more specific and descriptive sense to denote a homogeneous mixture of 2.2.1.4. Perilla and oiticica are important resin, drying oil, drier, and solvent which dries plant-derived oils with fast-drying properties. by a combination of evaporation, oxidation, and Oiticica oil has a conjugated double-bond type polymerization to give a transparent or trans- of unsaturation similar to that of tung oil. lucent film. The resin imparts certain desirable properties, such as hardness or faster 2.2.1.5. Refined fish oils have found exten- drying, to the film. Such a varnish is often sive use in the low cost paint field and in the called an oleoresinous or oil varnish. It is formulation of heat-resistant paints for such made by cooking the drying oil and resin applications as smoke-stack exteriors, boiler together until chemical combination and partial fronts, and roofing, where their softness and polymerization have taken place, and then add- elasticity can be used to advantage, and their ing the drier and solvent to the cooled ingredi- tendency to yellow and to develop after-tack ents. The ratio of oil to resin is customarily and undesirable odor is not objectionable. Cur- stated as the "oil length" (number of gallons rent research on fish oils holds a promise of of oil per hundred pounds of resin) of the better utilization of these materials in the varnish. Varnishes are further classified as future. "short oil" varnishes if their oil length is below ^ For a description of Iodine Number and a discussion of its uses 15 gallons, "medium oil" varnishes if the'r oil and limitations, refer to Federal Test Method 5061 of Fed. Std. No. 141 (see general bibliography). length is between 15 and 30 gallons, and "long

6 oil" varnishes if their oil length is over 30 evaporation to form a solid film. Others, in a gallons. Short oil varnishes dry more rapidly yet narrower sense, limit the term solely to than long oil varnishes and form a harder film, solutions of nitrocellulose (plus modifying but the long oil varnishes have greater elas- agents). Such restricted definitions ignore ticity and better exterior durability. the existence of many modern coating materials Interior varnishes often are based on linseed which are, quite properly, called lacquers. For oil in combination with estergum or a maleic- example, wide reference is made in the industry modified rosin resin. Rosin-modified phenolic to vinyl lacquers, acrylic lacquers, butyrate resins form varnishes with fairly good water lacquers, polyurethane lacquers, and others. and alkali resistance, but for exterior durabil- (The long-known "Chinese lacquer" is a differ- ity a long oil length is required. The most ent type of material that dries slowly by ab- durable spar (wood) varnishes for exterior sorption of oxygen in a cool, moist atmosphere. exposure are usually made from tung oil com- It is somewhat toxic even when dry and is little bined with a 100 percent phenolic resin (sub- used in this country.) stituted phenol-formaldehyde, oil soluble—as The definition of a lacquer, to be generally described in section 3.3.2.1). The phenolics, acceptable and useful, should not exclude mod- as a group, yield varnishes of relatively dark ern materials to which the term has been color. Terpene-phenolic resins are lighter in reasonably applied. Present usage strongly color and lower in cost than the comparable suggests that the properties most often asso-

phenolic resins ; they have very good water and ciated with the term "lacquer" by those in the

alkali resistance, but only fair exterior dura- industry are : (1) rapid air-dry by solvent evap- bility. The terpene resins themselves are low oration, (2) ease of application by spraying in cost and make good light-colored interior (although other means, such as brush or dip, varnishes with good color retention and good are not necessarily excluded), (3) solubility of water and alkali resistance. the dried film in the original solvents, making Coumarone-indene resins are thermoplastic it relatively easy to touch up or repair. A resins available in a wide range of hardnesses. lacquer may therefore be appropriately and Although poor in color and outdoor weather- usefully defined as a quick-drying, film-forming ing, they contribute excellent water and alkali solution, especially well-suited for spray appli- resistance and good dielectric properties to cation, which dries by evaporation of its vola- varnishes. tile constituents to form a coating that can be Petroleum hydrocarbon resins are soluble in redissolved by the original solvents. (The drying oils and form varnishes with good latter requirement is met only by thermo- water, alkali, and alcohol resistance, but only plastic resins.)

I fair color. Within the above definition, lacquers may be Natural resins (sec. 3.17.2) such as congo, either clear or pigmented. In general, they are j kauri, pontianak, and batu are often used in set to the touch (tack-free) within a few min-

; varnishes because of their low cost and their utes after application, can be over-coated in ability specific properties such as firm film in a I to impart about a half hour, and dry to a hardness, gloss, and moisture resistance. few hours. The drying process can be con- j So-called alkyd varnishes differ somewhat siderably hastened, and adhesion and durability I

1 from the conventional type of varnish in that often improved, by baking the film at a mod- the drying oil is co-reacted with the resin at erate temperature.

I the time the alkyd resin itself is being formed. Among the most widely used lacquers are I Because of the wide versatility and importance those based on cellulose derivatives in combina- of alkyd resin vehicles, these are discussed tion with a modifying resin and a suitable separately in section 3.4 on Alkyd Resins. plasticizer. The volatile solvent in which these j film-forming ingredients are dissolved is a 2.2.3. Spirit Varnish carefully balanced mixture formulated to give the proper application viscosity and drying A spirit varnish consists of a resin or gum characteristics. A discussion of the principles (e.g., shellac or damar) dissolved in a volatile involved in formulating such a balanced solvent solvent (e.g., alcohol). Such a varnish dries is given in the section on Use of Thinners primarily by evaporation rather than by oxida- (sec. 5.3.2). tion or polymerization. A shellac varnish is covered by Fed. Spec. TT-S-300. 2.2.5. Sealer

2.2.4. Lacquer A sealer is a relatively thin liquid composition that is applied to porous surfaces to pre- Much confusion has resulted from the diver- vent excessive absorption ("suction") of the j sity of meanings given to the term "lacquer." finish coats. Sealers for wood are essentially Some restrict the term to solutions of cellulose thinned "versions of a varnish or a clear lac- derivatives in volatile solvents, which dry by quer; their low viscosity and good aflfinity for I

7 the substrate result in good penetration and organic solvents, such as naptha, turpentine, sealing of the pores, so that good hold-out or mineral spirits, so that the final liquid wax (uniform appearance) of topcoats is obtained. preparation contains both water and organic Sealers for plaster formerly were usually thin solvents. solutions of water-soluble substances such as The organic solvent type of wax polish, par- starch or glue, sometimes containing a small ticularly in paste form, is generally considered amount of pigment to help seal the pores ; such to be the most durable and lustrous type, and sealers are often referred to as "sizes." They the kind best suited for floors, although appli- are quite inferior to the oleoresinous and latex- cation is more difficult and considerable rub- base primer-sealers now available. (See sec. bing is required to bring out the full luster; 2.3.5 on Latex Paints, and sec. 2.4.4 on Primers this type cannot be used on asphalt tile or on for Plaster and Wallboard.) some types of rubber tile, because these mate- In the absence of a sealer on porous surfaces, rials (especially asphalt) are attacked by the the absorption of the finish coat often occurs solvents usually employed. The water-emul- unevenly, resulting in variations in the gloss, sion type of wax preparation must be used on thickness, and durability of the topcoat. In such surfaces. The water-plus-solvent type of the case of pigmented materials, the uneven wax emulsion finds application in furniture absorption also causes variations in hiding and automobile polishes and in emulsion pol- power. In addition, the liquid vehicle may be ishes for vinyl tile and linoleum; the organic selectively absorbed into the substrate, thus solvent helps to clean and brighten the surface increasing the pigment volume concentration and reduces the time required for drying to a in the remaining topcoat material ; this process tack-free state. Care should be taken that reduces gloss and also may leave the pigment such solvent-containing emulsion types are not "starved" in binder and therefore prone to used on solvent-susceptible surfaces such as excessive chalking. asphalt tile. Emulsion type waxes may incorporate a mild abrasive and possibly a soap or 2.2.6. Wax Polishes detergent to provide a cleansing action as well as a waxing operation. The abrasive should Wax polishes, although seldom used alone not be used in furniture polishes, but is useful as a finish, are widely used to protect and give in automobile polishes. luster to other finishes for floors, furniture, automobiles, and appliances because of the rela- 2.2.7. Strippable Coatings tive ease with which they can be applied and renewed. Most wax polishes are mixtures of Strippable coatings are coatings of low ad- waxes of various degrees of hardness in a suit- hesion and considerable toughness designed to able solvent or dispersing agent, together with afford temporary protection against corrosion natural or synthetic resins and other ingredi- and abrasion of metal surfaces, parts, equip- ents to impart specific desirable properties. ment, and structures. They are usually applied Hardness and luster are derived chiefly from as a continuous envelope that is strong enough the carnauba wax content (or other hard waxes to remain intact and in place until the coating such as candelilla, ouricury, esparto, or sugar- is slit and peeled off. Strippable coatings may cane). The semi-hard waxes (e.g., beeswax, be formulated for application by hot dip [133] parafin, montan, ceresin, and oxidized micro- or by spray [134]. The film-forming base is crystalline wax) contribute other properties, usually ethly cellulose, cellulose acetate butyr- such as ease of application, flexibility, and ate, or a copoljmier of vinyl chloride with vinyl lower cost. Shellac and other resins are often acetate or vinylidene chloride (these resins are incorporated into wax floor polishes to reduce described in Chapter 3). In the hot-dip type, slippage, to increase gloss, or to increase dura- the film-forming base is dissolved in a hot bility. Colloidal silica also is effective in im- plasticizer or oil into which the part to be parting anti-slip properties. coated is dipped; setting takes place as soon From the standpoint of proper selection and as the film cools. Spraying types are formu- use, wax polishes are of three basic types: (1) lated with organic solvents in addition to The organic solvent type, in which the mixture plasticizers, and cure by evaporation of solvent. of waxes is dissolved in a volatile medium In both hot-dip and spray types, strippability such as a turpentine-mineral spirits mixture; is achieved by incorporating into the formula- this type may be in either paste or liquid form, tion a suitable proportion of an incompatible depending on the amount of solvent present. plasticizer or oil which will migrate to the (2) The aqueous emulsion type, in which the coating-substrate interface to form a very thin waxes are dispersed in water by use of an liquid layer that prevents the development of emulsifying agent such as morpholine or tri- coating-to-substrate adhesion. ethanolamine to produce a "self-polishing" Strippable coatings are used for the packag- liquid wax preparation. (3) An emulsion type ing and protection of items ranging from small in which the emulsified waxes are diluted with tools and machine parts to large completely

8 assembled machines and equipment such as to which some polyvinylidene chloride has been military aircraft, ship-board gun batteries, and added to promote cobwebbing. The final spray field artillery pieces. Besides use for pack- coating is generally a polyvinyl chloride-acetate aging a variety of small consumer articles type. The cocoon may be slit to permit flushing (jewelry, kitchen gadgets, hardware, etc.), of the interior with dry air and the insertion strippable coatings have become important to of a desiccant or preservative, after which it the industrial builder for the on-site protection is resealed. Major pieces of equipment may of building hardware (ornamental railings, thus be stored for long periods, in readiness grilles, door knobs, and fixtures of brass, cop- for immediate operation as soon as the cocoon per, and aluminum) during the period of is cut away. construction. Small articles are generally coated by the hot dip method. "Mothballing" of the large 2.2.8. Synthetic Resin Coatings items is accomplished by first spraying a fibrous "cobweb" coating onto a framework of Among synthetic resin coatings, the distinc- tape around the equipment to form a "cocoon", tions between the various general types of coat- after which additional strippable coating ma- ings (i.e., whether a varnish, lacquer, enamel, terial is sprayed on until a coating of sufficient etc.) and between clear and pigmented mate- thickness to provide the necessary strength rials are secondary in importance to the nature and sealing properties has been built up. The of the resin or resins employed. The prop- "cocoon" coating is usually a polyvinyl chlo- erties of the main classes of synthetic resins ride-acetate type of high vinyl chloride content. are therefore discussed separately in chapter 3.

2.3. Pigmented Coatings

Pigmented coating materials fall basically ifications for pigments is given in the Index into two classes: (1) those based on clear of Federal Specifications, Standards, and Hand- solution-type vehicles, to which the pigment or books [67]. A comprehensive list of the com- blend of pigments has been added in the form mercially available pigments appears in the of finely divided particles that are essentially Pigment Section of the Raw Materials Index insoluble in the vehicles, and (2) those con- of the National Paint Varnish and Lacquer sisting of a suspension of insoluble, finely Association [2]. divided pigment particles in a resin-water A primary purpose of the pigment, in all emulsion or latex (sec. 2.3.5.1). The latter pigmented coatings, is to hide the substrate type is of comparatively " recent development, by preventing the transmission of light through and many of the principles of formulation are the film to the substrate (and out again). Col- quite different from those employed in conven- ored pigments accomplish this, to a greater or tional solution-type vehicles. Before detailed lesser extent, by absorbing some of the light consideration of pigmented coating materials, rays and reflecting others. White pigments, a description of the pigments themselves is however, absorb relatively little light, so that given. their hiding power depends primarily on their ability to scatter and reflect the incident light. 2.3.1. Pigments This in turn depends on the particle size and size distribution and on the difference in re- 2.3.1.1. Pigment function.—The pigment fractive index between the pigment and the not only adds opacity and a decorative or surrounding medium. The higher the refrac- functional color to the film but also increases tive index of the pigment, the greater is its its durability and protective character by hiding power, other factors such as particle screening out harmful light rays, controlling size and shape being equal. The greatest hid- the transmission of moisture and gases through ing power among available pigments is ex- the film, imparting desirable mechanical prop- hibited by titanium dioxide; the rutile form has erties, and contributing various other proper- a refractive index of 2.76 as compared with ties (e.g., corrosion inhibition, chalking, control an index of about 1.5 for the common paint of gloss), depending on the nature of the pig- vehicles. ment and the concentration in which it is used. The number of available pigments, particu- 2.3.1.2. White hiding pigments.—The num- larly the colored ones, is very large, and it is ber of white pigments having good hiding not a purpose of this manual to list them all. power is quite limited. The chief ones are However, a representative group of the most basic carbonate white lead, basic silicate white important types of pigments are described in lead, basic sulfate white lead, titanium dioxide the following sections. A list of Federal Spec- (both anatase and rutile forms), zinc oxide.

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leaded zinc oxide, zinc sulfide, antimony oxide, the film (e.g., washability and abrasion resist- and lithopone. The latter is a co-precipitated ance) through reinforcement of the film struc- mixture of zinc sulfide and barium sulfate. ture. The above pigments are often referred to as prime pigments. They are relatively high in 2.3.1.4. Color pigments comprise a w^ide cost and are most often used in combination variety of natural and synthetic materials, both with less expensive extender pigments. organic and inorganic. They may conveniently White lead is a very durable pigment which be classified on the basis of their source and contributes to the formation of flexible, durable general nature as: (1) natural pigments, (2) films. It suffers, however, from a tendency to synthetic inorganic pigments, or (3) synthetic darken in industrial atmospheres. Although organic pigments. Only the more important this tendency does not impair the protective and widely used pigments in each class can be character of the film, the decorative properties discussed here. From the standpoint of the suffer. Titanium dioxide and zinc oxide are paint user, the chief properties that govern the widely used in these environments to prolong usefulness of a color pigment are the decorative life of exterior white paints. Color (hue, brightness, and saturation) dioxide is superior to white lead in Titanium Durability (lightfastness, heat resistance, power, whiteness, and brilliance; the hiding chemical resistance—depending on the because of its chalking tendency, anatase form, requirements of the application) imparts self-cleaning properties to white paints Bleeding resistance (in oils and solvents) (not desirable in tints). Since the rutile form Tinting strength is chalk-resistant, the degree of chalking of Hiding power (or, in some instances, titanium dioxide-pigmented paints can be con- transparency) trolled by the ratio of anatase to rutile pigment. Zinc oxide contributes improved mildew re- Properties which are primarily the concern of sistance, increased durability, and other valu- the paint manufacturer, such as ease of grind- able properties to the paint film. ing and oil absorption, will not be considered here.

2.3.1.3. Extender pigments are white pigments of relatively low refractive index ap- 2.3.1.4.1. Natural pigments comprise the in- proaching that of the common paint vehicles, organic earth colors or mined products and a few organic materials so that they have little hiding power at ordinary of vegetable and animal pigment volume concentrations (below the origin. Of chief importance are the iron com- critical PVC). However, they are very useful pounds, composed mainly of iron oxides in in combination with the prime pigments. combination with siliceous material and smaller Among the most important extender pigments percentages of the oxides of manganese, alu- are minum, calcium, and/or magnesium, together with some carbonaceous matter. These ferro- hydrated aluminum silicates (china clays) ferric oxide pigments include the yellow ochers, magnesium silicate (talc) the dark yellow siennas, the brown umbers, silica (usually in diatomaceous form, but the red hematites and burnt siennas, and the sometimes as quartz) black magnetite or magnetic oxide. With the calcium carbonate (natural chalk and exception of magnetite, which is 94-95 percent limestone ["whiting"] , or chemically pre- iron oxide (Fe304), the natural iron oxide pig- cipitated) ments vary widely in composition. The yellow, barium sulfate (natural barytes or pre- brown, and red iron oxides are largely in the cipitated blanc fixe) ferric form, and the ferric oxide (Fe203) con- calcium sulfate tent may range from 11 powdered mica as low as percent in some of the ochers to as high as 80 percent in Although the extender pigments are low in some of the hematites. The iron oxide pig- cost, their use does not necessarily result in an ments lack brilliancy, but they are widely used inferior paint; in fact, the extenders usually for their low cost, good durability, and excel- contribute many valuable properties. They aid lent non-bleeding properties. However, the in producing the proper paint consistency for natural pigments, because of their variable good non-settling characteristics and smooth composition, have been replaced in many ap- application. They help to control the penetration plications by synthetic iron oxide pigments of of priming paints. They are useful for dilut- greater uniformity, strength, and brilliance. ing colored pigments of great tinting strength. The natural organic color pigments are of They assist in obtaining the desired surface only minor importance. Their chief use is in appearance (degree of gloss or flatness) in the stains, artists' colors, and printing inks. In dried film. They may inhibit the tendency to- the vegetable group are Vandyke brown and ward film cracking on aging, and they can im- various precipitated dyes. In the animal group prove various other mechanical properties of are cochineal (carmine) and sepia.

10 2.3.1.42. Synthetic inorganic pigments of yellows and molybdate orange are poor in al- uniform composition and properties and in a kali resistance while the chrome oranges have variety of chemical types have largely replaced poor acid resistance. All except the deep the natural pigments in modern paint tech- chrome oranges tend to darken somewhat on nology. The most important types are dis- exposure. All have excellent bleeding resist- cussed in the follow^ing paragraphs. ance. The deeper chrome oranges of high Synthetic iron oxide pigments of high purity basic lead chromate content have been effec- are available as single oxides or blends of iron tively utilized in metal protective primers. oxides in a range of colors that include yellow, The chrome yellows enjoy wide general use in orange, tan, red, maroon, brown, and black. the coatings field. The shade of the pure colors depends on the Chrome greens are composite pigments con- particle size and shape as well as on compo- sisting of an intimate blend of iron blue and sition. Pure yellow iron oxide is hydrated ferric chrome yellow. By varying the proportions of oxide (Fe203 • H2O) prepared by precipitation yellow and blue, a wide range of hues is ob- and oxidation from solutions of iron salts. tainable. Chrome greens have high tinting Pure red iron oxide is ferric oxide (Fe203) strength and hiding power and very good color obtained by calcining the hydrated form. Pure permanency and outdoor durability. Although -^lack iron oxide is a precipitated ferro-ferric sensitive to alkalis, they are resistant to acids oxide (Fe304) of bluish-black hue. The other and are quite stable on baking. Their cost is iron oxide colors are made by blending yellow, relatively low, and they find wide application red, and black forms. Venetian reds are cal- in the coatings field. cium sulfate base iron oxide pigments contain- Chromium oxides and their hydrates are ing from 25 to 40 percent of red iron oxide. green pigments of exceptional stability and The iron oxide pigments as a group are chemi- color permanence. They are lower in tinting cally inert, nonfading, bake-resistant, non- strength and hiding power than the chrome bleeding, and low in cost. They find wide use green pigments described above, but they resist in applications where durability rather than acids and alkalis and withstand high tempera-

brightness of color is required ; for example, in tures. The pure chromium (chromic) oxide paints for railway freight cars, in industrial will withstand even ceramic heats. Chromium paints, barn paints, floor paints, and metal oxide pigment is relatively low in brightness; protective paints. however, hydrated chromium oxide has good Iron blues are complex ferric-ferrocyanides brightness. Their chief use is in applications known by a variety of names. Chinese blue where their stability to light, heat, and alkali and Milori blue have a greenish tint tone (i.e., justifies their higher cost. the tone perceived when the colored pigment is Ultramarine blue is a dark, brilliant blue added in small quantities to a large quantity pigment with high decorative appeal. It is

of light pigment) ; Prussian blue is reddish in made by calcining mixtures of china clay, soda tint tone. However, in mass tone (where the ash, silica, and sulfur in the presence of a re- colored pigment predominates), Milori is red- ducing agent such as charcoal, pitch, or rosin. dish and Prussian is greenish. The iron blues The calcined material is washed to remove are non-bleeding in oils and solvents but are soluble salts, wet-milled, filtered, dried, and somewhat reactive with certain organic ve- ground into a range of particle sizes yielding hicles. Although stable to acids, they have poor diff'erent shades and tinting strengths. Al- alkali resistance. They may be baked at 350 °F though low in hiding power and tinting (177 °C). Their hiding power and tinting strength, ultramarine blue finds wide use as a strength are excellent, as is their outdoor dura- tinting pigment in interior protective and deco- bility when used full shade. They are rela- rative coatings, where it offers good heat stabil- tively low in cost and are very widely used in ity, non-bleeding character, excellent resistance paints, lacquers, and enamels. to most alkalis (except lime), and extreme Chrornate pigments comprise the chrome lightfastness. It is very sensitive to acids, how- yellows and oranges, and molybdate chrome ever, and since most vehicles develop some acid- orange. Medium chrome yellow is almost pure ity during outdoor exposure, ultramarine blue normal lead chromate; the lighter shades con- is not usually used in exterior coatings. tain lead sulfate. Blends of normal and basic Cadmium yellows and reds are essentially lead chromate yield the chrome oranges, the sulfides (yellow) or sulfo-selenides (red) of higher proportions of basic lead chromate pro- cadmium, usually combined with an inert base ducing the deepest shades. Molybdate chrome of barium sulfate. They are available in a orange is a coprecipitated mixture of lead wide range of colors, from a delicate light chromate, lead sulfate, and lead molybdate; it yellow to a deep maroon. They possess good is a strong, bright orange pigment with excel- color brightness, lightfastness, and resistance lent hiding power. In general, the chromate to acids. Their excellent alkali resistance pigments provide good hiding and durability makes them suitable for use in emulsion paints at relatively low cost; however, the chrome and special alkali-resistant finishes. Although

11 the yellows tend to fade and discolor on outdoor Lithol reds are members of the soluble azo exposure, the reds and maroons exhibit good group of pigments. They range in shade from exterior durability. The cadmium pigments a light yellow red to a deep red, depending on have excellent heat resistance and find im- the metal precipitant used to insolubilize them. portant use in high temperature baking By resination, maroon shades are obtained. enamels. The lithols show good resistance to bleeding in oils, hydrocarbons, and water, but they may 2.3.1.4.3. Synthetic organic pigments com- bleed considerably in lacquer thinners, and prise a very large and important group of their alkali resistance is poor. Although colors that are prepared by the insolubiliza- neither durable nor lightfast, they are the least tion or precipitation of organic dyestuffs. As expensive of the organic red pigments and find with inorganic pigments, particle size and considerable use in interior applications where shape are important factors in determining the durability is unimportant and a strong color is color shade obtained with a particular chemical desired. type, the smaller particles generally producing Hansa yellows (toluidine yellows) are mem- the lighter shades and greater brightnesses. bers of the insoluble azo group. Like the tolui- The most widely used synthetic organic pig- dine reds, they have good acid and alkali resist- ments are discussed in the paragraphs which ance but bleed in organic solvents and oils. follow. They are durable and lightfast in full shade. Copper phthalocyanine blues and greens are Because of their alkali resistance, they are chemically inert pigments of high tinting suitable for use in water emulsion paints, be- strength and excellent lightfastness. They ing superior to the chrome yellows in this re- are highly resistant to acids and alkalis and spect. They are lower in cost than the cadmium are completely free from bleeding in oil and yellows sometimes used in this application. A organic solvents, even at elevated temperatures. growing field of use for these pigments is in toy Although not in themselves readily wetted with enamels where pigments free of toxic metals are water, they may be treated with surface active required. Hansa yellows find wide use in paints, agents to render them easily dispersible for lacquers, and enamels requiring a durable, bril- use in water base paints. Because of their liant yellow pigment. strong, dark mass tone, the phthalocyanines are Benzidine yellows belong to the insoluble azo used chiefly in relatively light tints, yielding group. They are two to three times as strong clean, bright blues and greens of outstanding tinctorially as the Hansa yellows but are some- durability. They have found wide acceptance what less durable. They are more than ten in high quality paints, lacquers, and decorative times as strong as the chrome yellows. Like enamels for exterior exposure. Phthalocya- the toluidines, they have good acid and alkali nine greens differ in composition from the blues resistance but bleed in lacquer solvents. They in that most of the hydrogen atoms present in are of general usefulness in coatings where the blues have been replaced with chlorine strong color and fairly good durability are atoms. The greens are free from the tend- required. encies toward flocculation and crystallization Tungstated and molybdated pigments are exhibited by the blues; however, phthalocya- organo-metallic precipitates of the fugitive but nine blues with good resistance in these re- brilliant basic dyestuffs such as malachite spects have been developed. green, methyl violet, and the rhodamines. Toluidine reds are dyestuffs belonging to the These pigments are strong and brilliant in insoluble azo group of pigments that are inher- color but are not sufficiently lightfast to with- ently insoluble in water without containing a stand exterior exposure. They are fairly stable metal precipitant. A range of light red to me- at moderate baking temperatures and fairly dium red shades is available. The toluidines resistant to acids, but their resistance to al- have good hiding power and excellent bright- kalies is poor, and they bleed in lacquer solvents. ness, durability, and color permanence when Their chief use is in specialty coatings where used full shade (undiluted), but the tints tend permanence is unimportant and a striking to fade. Although resistant to acids and alkalis, decorative efi'ect is desired. They are available they bleed in many organic solvents, fats and in a variety of colors including blue, violet, oils. Toluidine reds find wide use in industrial green, yellow, and red. and decorative enamels where brilliance of shade Indanthrenes are strong, bright pigments and durability are of prime importance. possessing exceptional lightfastness even in Para reds also belong to the insoluable azo light tints. They are non-bleeding in oils and group. Though not as durable or lightfast as solvents and resistant to acids and alkalis. the toluidines, they are lower in cost. Their Their use is limited by their high cost, so that bright color and adequate lightfastness when they are used chiefly for toning and tinting used full shade make them useful in trim other colors. However, indanthrene maroon paints and in porch and deck paints. They has become a preferred pigment for high qual- are not suitable in tints because of fading. ity maroon automotive finishes. The blue also

12 finds use in the automotive field. Indanthrene flake form. The flakes are obtained by rolling greens and yellows are less used. out the metal particles in a ball mill or by ham- mering thin sheets of the metal in a stamp 2.3.1.5. Corrosion-inhibiting pigments.— mill, in either case in the presence of a lubri- A small but important group of pigments cant. They are either leafing or non-leafing, worthy of special mention are those used almost depending on whether or not they have been entirely for their ability to inhibit metallic coated with a leafing agent (usually stearic corrosion. These include red lead (Pb304), acid) and polished during the manufacturing

sublimed blue lead (basic lead sulfate, blue) , cal- process. In a suitable vehicle, the flakes coated cium plumbate, basic lead chromate, zinc chro- with leafing agent float to the surface of the mate (zinc yellow), zinc tetroxychromate, and paint, where they orient and interleave to form

I strontium chromate. Unique among the rust- an almost continuous, bright metallic surface [ inhibiting pigments is a recently marketed that is highly resistant to the passage of mois- basic lead silico-chromate pigment consisting of ture and gasses, in hiding j high power and a shell of basic lead chromate fused to an inert specular light reflectance, and durable on ex- j core of silica. The economy effected by limiting terior exposure. The coarse particle sizes pro- the costly basic lead chromate to the effective, duce the brightest, most highly reflective sur- outer reactive shell, together with the high faces, while the fine particles are preferred in bulking value of the light and inexpensive silica decorative coatings, printing inks, and aerosol core, makes it feasible to incorporate a high paints. volume concentration of this pigment into in- In the non-leafing type, the metal flakes, intermediate and finish coats as well as in pri- stead of floating to the surface and forming an mers, thus providing rust-inhibitive properties interleaved layer, remain suspended through- throughout the entire coating system. out the film. Although a high degree of re- The effectiveness of these corrosion-inhibit- flectance is not possible with this type, a wide ing pigments derives chiefly from their very array of different colored metallic shades (in- limited solubility in water and their ability to cluding hammertone finishes) can be achieved, chemically combine with the metal surface to depending on the particle size and size distri- produce an impervious, insulating film thus bution. The larger particles produce the great- rendering the metal passive. When such pig- est brightness or sparkle, while the fine particles ments are incorporated in a metal priming yield the highest opacity and deepest shades. paint, their limited solubility furnishes a small 2.3.1.6.1. Aluminum is most widely but adequate supply of corrosion-inhibiting the used metallic pigment. It is available both as a pow- ions without leaching enough pigment to render der and as a paste, in a variety of grades ac- the film porous. cording to particle size. Aluminum paints are Zinc dust, when used in a primer in a suffici- lightfast, heat-resistant, water-resistant, and ently high concentration (92 to 95% of the dry durable on outdoor exposure except in acidic or film weight) to ensure particle-to-particle con- alkaline environments. The leafing types are tact in the dry film, is an effective corrosion high in hiding power and in moisture resistance inhibitor for steel surfaces, producing the and provide an effective barrier against the so-called "zinc-rich" paints [128]. It is now transmission of water vapor and gases through a generally accepted view [117, 118] that the the film. Important applications for aluminum zinc serves initially as the sacrificial (anodic) paints include marine coatings, aircraft coat- material in a galvanic reaction in which the ings, solvent tanks, transmission line towers, steel, as cathode, is afforded protection at the highway bridges, furnace fronts, and interior expense of the zinc. The period of cathodic decorative finishes. protection is limited by the build-up of zinc corrosion products at the metal-coating inter- 2.3.1.6.2. Metallic copper and its alloys with face, which stifles the galvanic action; how- aluminum, tin or zinc yield the popular bronze ever, the deposited corrosion products form a finishes that range in shade from a light brass hard, impervious, adherent barrier layer that to a dark antique copper, depending on compo- is combined with both the coating and the sub- sition and on the heat treatment received dur- strate and affords excellent long-term protec- ing manufacture. Care in formulation is tion against corrosion. required to counteract the tendency of the The formulation of metal primers incorporat- copper bronze pigments to retard the drying ing corrosion-inhibiting pigments is discussed of oil, varnish, and other vehicles that cure by in section 2.4.2. Important principles and pre- an oxidative process. These pigments are high cautions which should be borne in mind when in hiding power but tend to discolor on exterior applying corrosion-inhibiting primers are dis- exposure. They are used chiefly in interior cussed in section 7.2.3. decorative finishes, specialty lacquers, printing inks, and anti-fouling paints. The latter often 2.3.1.6. Metallic pigments for use in paints are made with a dendritic, non-leafing type of are, with the exception of zinc dust, usually in copper pigment.

13 2.3.1.6.3. Zinc dust is made by distilling zinc tinting strength, are all stable to light, heat, and condensing it under conditions that pre- acid, alkali, and atmospheric conditions. « vent coalescence. It has very high hiding Vegetable blacks may contain from 50 to 90 j power, being almost completely opaque. It is percent of fixed carbon, depending on the vege- j used chiefly in corrosion-inhibiting paints for table material from which they are made. Ani- ? steel, where it serves initially as the sacrificial mal blacks (boneblack) contain only 10 to 20 anodic material providing cathodic protection percent of carbon, the remainder being chiefly to the steel substrate and later as a protective calcium phosphate. Although they are much barrier (see sec. 2.3.1.5), It is used also in lower in tinting strength than the carbon , paints for galvanized surfaces, where it pro- blacks, they also have a much lower oil-absorp- vides good adhesion. It is not resistant to acids tion, so that larger proportions can be incor- or alkalis. porated into a formulation to produce low sheen black finishes. Boneblacks also find use , 2.3.1.6.4. Gold and silver flake are used prin- in artists colors and in latex paints. Mineral cipally in decorative applications such as sign blacks are sometimes used in place of bone- writing, book-binding, picture frames, and glass black but are more variable in composition and and chinaware. properties. Mineral and vegetable blacks are used mostly to color cement and mortar. 2.3.1.6.5. Stainless steel flake has recently been used as the main pigment, along with some 2.3.1.8. Fluorescent pigments are finding organic pigment, in a metal protective paint increasing use in high visibility paints. Al- reported [3] to have shown excellent durability though formerly too expensive and short-lived in corrosive exterior environments. The effec- for practical use, specially formulated fluores- tiveness of the stainless steel pigment is attrib- cent paints are now available that will retain uted not to rust-inhibitive properties but to its satisfactory color and fluorescent properties own chemical resistance plus its exceptional during a year or more of exterior service in resistance to moisture. The aged film takes on temperate climates. Fluorescent substances the appearance of stainless steel itself. have the property of absorbing energy in one spectral region and re-emitting it in another 2.3.1.6.6. Other metallic pigments used to a spectral region of lower energy (longer wave- limited extent in the coatings field are nickel, length) . A number of organic and metal- tin, and lead. organic dyes yielding a variety of fluorescent colors are available. For high visibility aircraft

2.3.1.7. Black pigments comprise the syn- paints [4, 4a] , a red-orange fluorescent pigment thetic carbons manufactured by various fume has been preferred because this color affords and incomplete combustion processes, natural high contrast with outdoor surroundings and and synthetic graphite, pyrolyzed vegetable is in the spectral region to which the eye is and animal blacks, and the naturally occurring quite sensitive. This pigment absorbs energy in combinations of carbonaceous and siliceous ma- the ultraviolet and blue regions and re-emits it terials known as mineral black. Black iron in the red-orange region. The re-emitted light oxide has already been discussed under Color is added to the light ordinarily reflected from Pigments (sec. 2.3.4). the paint surface, greatly increasing its bril- Carbon black (sometimes called channel liance. The total light radiated and reflected black) is manufactured by the closely-con- from a fluorescent paint may be three to four trolled burning of natural gas. It is the finest times as great as that of the same color with- of the black pigments, the particles being of out the fluorescence. colloidal dimensions. It is the blackest of pig- So-called "daylight" fluorescent paints utilize ments and the highest in tinting strength and chiefly the visible, blue portion of the spectrum opacity, and it finds wide use in the coatings to produce fluorescence, the harmful invisible or field. Other synthetic blacks are lampblack, "black" ultraviolet (which would soon deterio- furnace black, and thermal black. Graphite is rate the fluorescent pigment) being largely a soft crystalline form of carbon that occurs screened out by the use of an ultraviolet ab- in natural deposits of varying carbon content; sorber or inhibitor in the paint formulation. it can also be manufactured by heating coke The initial reduction in brightness from not or charcoal with about three percent of iron utilizing the ultraviolet is offset by a substan- in the high temperature of an electric furnace. tial gain in exterior durability. Compared with other black pigments, graphite Fluorescent pigments are low in opacity. is relatively low in blackness and tinting The fluorescent paints made from them, to be strength and is used chiefly for its protective fully effective, must be applied over a white qualities and its electrical conductivity. The undercoat that will reflect the light which various synthetic blacks (carbon black, lamp- would otherwise be transmitted through the black, etc.), although differing in amount of coating and lost by absorption into the sub- fixed carbon, particle size, oil absorption, and strate [139].

14 :

The use of high visibility, daylight fluores- ing, increased gloss, improved adhesion, and cent paints on some aircraft is believed to have greater durability. The types of resins em- significantly lessened the risk of mid-air col- ployed have been discussed in sec. 2.2.2 on lisions. Other applications include warning Varnish. Of particular importance are oleo- signs for hazardous areas or equipment, explo- resinous paints ultilizing phenolic resins and sives- and fuel-carrying vehicles, advertising alkyd resins. signs, and eye-catching decorative effects both Phenolic 'paints based on an oil-soluble phe- indoors and out, in artificial light as well as nolic resin (section 3.3.2.1) combined with tung daylight. oil have very good water resistance and weather- 2.3.2. Oil Paints ing resistance. Their fast drying and poor wetting characteristics, however, necessitate Oil paints consist basically of pigment parti- very thorough surface preparation to obtain cles dispersed in a drying oil or varnish vehicle, adequate adhesion. Since phenolic paints con- together with thinners to provide a suitable tinue to cure and harden on aging, failure from consistency and evaporation rate for proper a lack of intercoat adhesion can occur if too application, and driers to give the desired cur- much time elapses between coats. Ordinarily, ing characteristics. Oil paints dry by a com- successive coats of phenolic paint should be bination of solvent evaporation, oxidation, and applied within 24 hours after the preceding polymerization. The several types of oil paints coat. This time may be extended somewhat may be distinguished as follows provided a small amount of a strong active solvent is added to the follow-up coat. 2.3.2.1. Straight oil paints are those utiliz- Alkyd paints are among the most versatile ing a drying oil or combination of drying oils and useful materials in the entire field of or- as the nonvolatile vehicle. Of the various dry- ganic coatings. The oil-alkyd vehicle on which ing oils available (see sec. 2.2.1), linseed oil is such paints are based can be varied over a wide by far the most widely used because of its ex- compositional range to give a broad range of cellent brushing characteristics, wetting and coating properties for numerous applications penetrating properties, adhesion, elasticity, in the consumer, manufacturing, and industrial and durability on exterior exposure, as well maintenance fields. Among the valuable prop- as its general availability and relatively low erties imparted to paints by alkyd resins are cost. The linseed oil film is fairly permeable fast dry, hardness, and durability, with excel- to water vapor—a property that is useful for lent chalk resistance, gloss, and color retention. reducing the tendency toward blister formation A comprehensive discussion of alkyd resins is in paints on wood and other porous surfaces, given in section 3.4. but one which limits its usefulnes in applica- Long-oil alkyds of the air-drying type are tions where low permeability and good chemical widely used in high quality interior architec- resistance are required (e.g., an industrial tural paints and enamels. The long-oil alkyds metal paint) . Oil paints are relatively poor are also the basis for durable exterior trim in abrasion resistance, in resistance to water enamels for both wood and metal surfaces. immersion, and in resistance to corrosive (espe- Blister and stain resistant exterior house paints cially alkaline) environments. based on long-oil alkyds plus added bodied oil Both raw and bodied (heat polymerized) are available but have not yet achieved wide linseed oil are used in paints. The raw oil, be- use. Medium-oil alkyds are found in many good cause of its superior wetting ability, is used quality floor, porch and deck enamels. Baking- extensively in metal primers; the bodied oil type medium- and short-oil alkyds, often in is used in finish coats because of its faster dry- combination with urea- or melamine-formalde- ing and better leveling characteristics. High- hyde resins, are the basis for premium coating quality exterior house paints are still based materials of exceptional durability that find largely on the time-tested linseed oil vehicle. wide use in automotive enamels, appliance fin- Pigmentation consists of various combinations ishes, and industrial equipment coatings. The of titanium with lead and/or zinc pigments and uses of alkyd resins are discussed in greater extender pigments. detail in section 3.4.1.3.

2.3.2.2. Oleoresinous paints consist of a 2.3.3. Pigmented Lacquers dispersion of pigment particles in a varnish or oleoresinous vehicle, i.e., a vehicle in which the By dispersing suitable pigments in a lacquer drying oil and resin have been combined by a vehicle, opaque and colored lacquer-type paints cooking process. The addition of the resin re- are obtained which possess the characteristic duces the permeability and increases the chem- sprayability, fast dry, and repairability of clear ical resistance and hardness of the film as lacquers as described in section 2.2.4. Since compared to a straight oil paint. The resin, de- the solids content of lacquer formulations is pending on the choice, may also contribute limited by the viscosity and solubility of the other desirable properties, such as faster dry- film-forming base in the volatile solvent medium,

15 it is necessary to use prime pigments with max- tion. In these coatings the film-forming in- imum hiding power and low oil absorption to gredient has been dissolved in organic solvents achieve good hiding without an undue increase to give a vehicle having a single, continuous in consistency. Extender pigments are not used solution phase. In latex paints, however, the in lacquers, since they increase consistency structure of the system is very different. The and reduce the proportion of film-forming base resinous or film-forming ingredient,^ instead of that can be used, without contributing to hid- being dissolved in a solvent medium, is dis- ing power. persed (in the presence of an emulsifying One of the first and most important pig- agent) in the form of fine particles in a liquid mented lacquers was based on nitrocellulose (water) which is not a solvent for the resm. plasticized with camphor or castor oil. Later, The pigment, also, is suspended in the water the combination of nitrocellulose with an alkyd medium. resin and suitable pigments and plasticizers The use of water as the volatile vehicle in yielded the widely used "Duco"-type automo- latex paints offers many advantages. It is tive finish. After many years nitrocellulose- readily available, inexpensive, noncombustible, alkyd combinations are still the basis for many nontoxic, odorless, and colorless. Storage facil- of our present-day lacquers. Other film-form- ities for a variety of organic solvents are not ing materials used in the formulation of pig- needed. Its evaporation rate is well suited for mented lacquers are cellulose acetate, cellulose brush or roller application. The paint may be acetate butyrate, ethyl cellulose, vinyl resins, applied on damp surfaces or in damp weather and polyurethanes—to name only a few. Very and is not harmed by rain an hour or two after recently, acrylic lacquers having excellent hard- application. Surfaces may usually be recoated ness, durability, and gloss have been developed the same day, or under favorable conditions and are being widely used in the automotive after an hour or two of drying. Brushes, rollers finishing industry. The properties of many of and containers can be cleaned by washing in the resins used in lacquers are given in chap- warm, soapy water after completion of the ter 3 on Properties of Synthetic Resins for work. Coatings. The disadvantages of water as the volatile 2.3.4. Enamels vehicle stem from: its high surface tension which may cause foaming, crawling, or crater-

Enamels are pigmented coating compositions ing of the applied film ; its high heat of vapori- that yield films characterized by smoothness zation which slows evaporation and lengthens and freedom from brush or other tool marks. the flash-off time necessary to avoid blistering Although most enamels are glossy, flat enamels in baked coatings; its essentially fixed boiling also are available. Enamels are usually thought point which makes it difficult to adjust evapora- of as hard coatings. They are often of the bak- tion rate in the manner that is possible with ing type, as in the case of automobile and mixtures of organic solvents; and its suscepti- appliance finishes in which a short-oil alkyd bility to freezing under adverse storage or is combined with a melamine or urea resin to application conditions. produce a hard, durable finish. Floor, porch, and deck enamels are air-drying types that are 2.3.5.2. Forerunners of our modern latex often based on a medium-oil alkyd resin. Ex- paint vehicles were the alkyd resin emulsions terior architectural enamels are usually based that first appeared around 1929, some twenty on long-oil alkyd resins modified with drying years before the advent of commercial latex oils such as linseed, soya, or dehydrated castor. paints. The vehicles of these early resin emul- Pigmented varnishes often fall into the enamel sion paints consisted of a dispersion of a long- classification. Rutile type titanium pigments oil alkyd resin in water in the presence of con- are preferred in exterior enamels because of siderable amounts of casein and other emulsi- their excellent hiding power and resistance to fying agents. Paints made from these vehicles chalking. The pigment volume concentration had a number of faults, including poor emul- (ratio of pigment to binder) of enamels, though sion stability, clogging of paint brushes, poor dependent on the nature of the pigments and water resistance, and a general lack of the extenders employed, is generally lower than outstanding properties usually associated with that of ordinary paints (see sec. 4.1,4.4). alkyd resins. Subsequent developmental work greatly improved the properties of alkyd resin 2.3.5. Latex Paints emulsions, and their usefulness for interior paints was recognized in 1940 by a Federal 2.3.5.1. The various types of pigmented coat- Specification (TT-P-88). This was followed ings that have been discussed thus far, in 1947 by a Federal Specification (TT-P-18) whether oil paint or lacquer or enamel, have con- for an exterior masonry paint based on an sisted basically of insoluble pigment particles dispersed in a homogeneous liquid vehicle con- 1 Contrary to a popular misconception, a latex paint does not necessarily or even usually contain rubber, as will be apparent from sisting primarily of drying oil or resin solu- the discussion of types of latex paints given in subsection 2.3.5.6.

16 alkyd resin emulsion. Alkyd emulsion paints be met either by diluting the paint with water are supplied in the form of a paste that is or, more desirably, by dampening the surface thinned with water before use. Since the long- to be painted. Excessive dilution of the paint oil alkyds on which they are based are liquid itself will break the emulsion. resins that have been only partly polymerized Although the widest and most effective use during manufacture, the paint film laid down of latex paints has been for interior coatings must harden by a process of oxidation and on plaster and wall-board, and for exterior polymerization after the water phase has evap- masonry surfaces such as Wick, cinder block, orated, in the same manner as when deposited stucco, cement, and weathered asbestos shingle, from an organic solvent solution. With the in- some latex paints have been formulated for use troduction of latex paints in 1949 and their as exterior house paints on wood, as metal subequent wide acceptance, interest in resin primers, and as industrial alkali-resistant coat- emulsion paints dwindled. Today, alkyd resin ings. Most latex paints give a flat or matte emulsions find their chief use in blends with finish; however, acceptable gloss and semi- the latexes. gloss types have been developed. Industrial Actually, a latex paint is also a resin emul- baking-type latex finishes are also under devel- sion paint; however, it is a special type that opment. More specific information concerning is characterized by the higher molecular weight, uses of latex paints is given later in this section smaller particle size, and more uniform size in the discussion of the individual chemical distribution of the resin particles as compared types of latexes that are available. to the older, so-called resin emulsion paints. These properties of the latex are achieved by an 2.3.5.4. Formulation.—The film-forming emulsion polymerization process (sec. 3.1.3.3) resin on which latex paints are based is a long- which not only permits better control of the chain polymer of high molecular weight that polymerization than is possible by bulk or solu- has been produced, by an emulsion polymerization methods but also permits the development tion process (section 3.1.3.3), in the form of of a high molecular weight polymer without very small spherical particles (not over 0.2 problems of high viscosity or insolubility. micron nor below 0.01 micron in diameter). Even a small proportion of large particles will 2.3.5.3. Uses.—Latex paints are particularly act as seeds to accelerate coagulation of the well-suited for use on porous, absorbent sur- smaller ones. The emulsified or dispersed state faces such as plaster and masonry. The latex is achieved by surrounding each resin particle particles, although very small, are still large with a film of emulsifying agent. Dispersion compared to the size of a dispersed polymer of the resin particles in the water vehicle con- molecule in solution; hence, they do not pene- stitutes an oil-in-water type emulsion, which trate into porous surfaces to so great an ex- is most stable when both phases are present tent as do solution-type coatings, and their in about equal proportions. Excessive thinning sealing properties are excellent. The inherent, with water breaks the emulsion by reducing good sealing properties of the latexes are uti- the thickness of the emulsifying film around lized to the maximum in the formulation of each particle. At high solids content (about primer-sealers (see sec. 2.4.4) . Although latex 70 percent) , the emulsion inverts to the water- paints may be successfully applied in most in- in-oil type and is no longer thinnable with stances without requiring a primer-sealer, use water. of the latter provides maximum holdout of top- Latex emulsions differ from resin solutions coats and is particularly useful in securing in a number of important respects. In vehicles uniform gloss, hiding, and color over substrates of the resin solution type, the solids content is (especially plaster) of uneven porosity or com- limited by the viscosity and the solubility of position. the resin in the solvents employed. These Latex paints are not alkali-sensitive and may properties in turn depend on the molecular be applied even over unaged (but not fresh, weight of the resin, the viscosity increasing uncured) masonry or plaster surfaces. It and the solubility decreasing with increasing should be noted, however, that when latex molecular weight. Although durability and me- paints are modified with other materials (e.g., chanical properties generally improve with in- an alkyd resin) to achieve some of the advan- creasing molecular weight (increasing polymer tageous properties of the modifier, the alkali chain length), a practical upper limit is im- resistance of the latex may be impaired. Also, posed by the necessity for maintaining a prac- the same non-penetration properties of the tical viscosity and solubility. Latex emulsions latexes which give them their excellent sealing do not suffer from these limitations. Since the properties causes diflRculties when they are latex particles are suspended, not dissolved, in applied over chalky surfaces, because of their the water phase, insolubility at high molecular inability to penetrate the chalk to adhere to weights is a desirable property rather than a the substrate. problem. Moreover, since the latex particles The water demand of masonry surfaces must are not dissolved in the water, they contribute

17 little to the viscosity of the vehicle. Thus latex a spongy or powdery structure rather than a paints are able to utilize the inherent advan- continuous film remains after evaporation of tages of high molecular weight and high solids the water. content without the viscosity and solubility The polymer deformability upon which film limitations imposed upon solution-type vehicles. formation depends has its basis in the physico- chemical structure of the polymer that deter- The formulation of latex paints, however, mines the modulus of elasticity and the glass involves characteristic problems of its own. transition temperature, Te. At temperatures Before a pigment is incorporated into the resin above Te, a rigid polymer acquires rubber-like emulsion, the pigment must itself be dispersed characteristics which permit it to deform appre- in water in the presence of an efficient dispers- ciably under relatively low stresses. The tem- ing agent. Titanium dioxide, particularly the perature at which a latex paint is applied is rutile form, is the preferred white hiding pig- therefore the most important environmental ment. Extender pigments (e.g., calcium car- factor involved in film formation. Application bonate, china clay, mica, etc.) are used to help at low temperatures (below 50 °F or 10 °C) control flow, leveling, film build, and gloss ; ex- should be avoided, since drying and coalescence tenders may also have a pronounced effect on suffer. Porous surfaces require higher tem- film properties such as cleanability, scrub re- peratures for film formation than non-porous sistance and appearance. Good dispersion is surfaces. Film formation is also influenced by essential for high solids content, package sta- the size and the size distribution of the latex bility, freeze-thaw resistance, good flow and particles, the quality of the dispersion (free- leveling with minimum floating and flooding, dom from flocculation) , the pigment volume and for maximum hiding power and color uni- concentration, and the presence of coalescing formity. Thickeners, in the form of water- aids. A small quantity of coalescing agent, in soluble protective colloids such as casein or the form of a high-boiling solvent or plasticizer methyl cellulose, are often necessary to stabilize that acts to soften the polymer spheres during the emulsion and to produce a suitable consist- the final stages of drying, is sometimes incor- ency for good application properties. These porated as an aid in film formation. The time water-soluble colloids are used in minimal required for film formation depends on the rate amounts (or not at all) in exterior formula- of water removal as determined by the tem- tions because they tend to reduce the water perature, the relative humidity, the porosity resistance of the film in which they remain. (water demand) of the substrate, and the film Bacterial contamination during manufacture thickness. The final latex film, while continu- must be guarded against, and a preservative ous and water-resistant, remains permeable to (e.g., one percent of a phenyl mercuric salt) water vapor (i.e., it "breathes") and therefore must be incorporated to inhibit the growth of does not tend to blister when applied over damp bacteria, mildew and mold in the container and surfaces. in the film. An anti-foaming agent must be present to suppress the tendency toward foam- 2.3.5.6. Chemical types.—Three different ing during manufacture, filling of containers, types of latex paints, based on the general and application. The pH of the system is an chemical nature of the polymer employed, are important factor in stability. Styrene-buta- presently in wide use. These are: (1) the diene and acrylic latexes must be maintained on styrene-butadiene type, (2) the polyvinyl ace- the alkaline side; for best results, a pH range tate type, and (3) the acrylic type. The most of 9.0-9.5 is usually maintained. Polyvinyl important properties of each type are briefly acetate latexes are slightly acidic. Finally, considered below. when the paint is applied to a surface, the individual resin particles must somehow fuse to 2.3.5.6.1. Styrene-butadiene latex was the form a continuous film upon evaporation of the first commercially available type, is the lowest water vehicle. in cost, and has the largest market. The chemical unsaturation imparted by the butadiene 2.3.5.5. Film formation results from co- portion of the copolymer allows initial use of alescence of the latex particles under the influ- a soft yet strong film-former to obtain maxi- ence of driving forces generated when the resin mum pigment binding and film continuity, particles are brought into close contact as the while laying down a film which subsequently water evaporates. As the last portions of water hardens rapidly (through a curing process sim- are vaporized, capillary forces act strongly to ilar to that which takes place in oleoresinous deform the latex (polymer) spheres, thus filling paints) to a tough, durable coating. However, the voids left by the vacating water, so that this reactivity also results in yellowing and fusion of the particles takes place and a con- eventual embrittlement on aging. Protective tinuous film is formed. If the polymer is too colloids such as methyl cellulose and casein, de- rigid to deform sufficiently under the forces spite their adverse effect on water resistance, generated, then coalescence does not occur and are usually necessary in styrene-butadiene la-

18 texes to stabilize the emulsion. The latex is minimizes blistering. Like other types of latex, compatible with a wide range of long-oil alkyds, it does not penetrate chalky surfaces sufl[lciently heat-bodied drying oils, and long-oil varnishes. to provide a good bond to the substrate. Al- It may be blended with these (after first though PVA formulations said to adhere to emulsifying the modifier) to obtain a wide chalky surfaces have been offered recently, not variety of end-use properties. Rutile titanium enough experience with these is available at dioxide is the preferred pigment for maximum this writing to reliably assess their effective- hiding and good non-chalking properties. ness. In addition to the usual flat finishes, PVA emulsions for use in gloss and semi-gloss Styrene-butadiene latexes have found wide interior paints have recently been developed. application in flat and low sheen interior paints, These have been formulated at pigment volume exterior masonry paints [5], high-solids tex- concentrations ranging from 10 to 25 percent ture paints> and in primer-sealers. The some- and at a solids content of about 50 percent. what objectionable odor of the wet film dis- appears soon after drying. Specialized uses 2.3.5.6.3. Acrylic latexes are based on a film- include coating the interiors of tank cars carry- forming acrylic copolymer that is tough, ad- ing hot caustic soda, the dip-coating of mattress herent, flexible, color-retentive, and resistant springs, and as a baking-type primer for auto- to ultraviolet light, alkalies, oil, grease, and mobiles. Industrial gloss and semi-gloss paints moisture. The latex emulsion may be formu- have been developed which are suggested for lated into emulsion paints with a minimum of use on metal furniture, automobile interiors, additives, thus preserving all the desirable drums, and motor shells. One of the latest properties of the acrylic resin base. Although developments in styrene-butadiene latexes is the acrylics are highest in cost among the three an industrial type primer in which the re- types of latex available, their excellent combi- sidual chemical unsaturation is utilized to ef- nation of properties continues to win them an fect further polymerization in situ through the increasing share of the latex paint market. In use of catalysts and heat to produce a hard, addition to their excellent durability, they are solvent-resistant film having improved adhesion easily applied, are practically odor-free, dry to metal and excellent salt spray resistance. rapidly, and develop excellent resistance to Suggested end uses for this primer include water within an hour or two after application. automotive parts and bodies, metal furniture, electrical equipment, refrigerators, and air- Acrylic latex paints [7] have shown excellent conditioners. exterior durability in a wide variety of climates, particularly when applied over masonry 2.3.5.6.2, Polyvinyl acetate (PVA) latexes surfaces. Recently, they have been offered also are intermediate in cost and usage among the as exterior house paints on wood surfaces that three main types of latex. Polyvinyl acetate have first been sealed with a zinc-free oil-type itself is usually plasticized in order to yield a primer. Weathered oil paints on exterior wood film having adequate flexibility. For this pur- surfaces also can be successfully recoated with pose it may be internally plasticized (pre- acrylic latex paint. Excellent primer-sealers plasticized) by copolymerization with a suitable for porous surfaces can be formulated from monomer, or it may be externally plasticized acrylic latexes, over which virtually any type (post-plasticized) by blending with 10-15 per- of topcoat may be used, including water paints, cent of a chemical plasticizer such as dibutyl oil paints, enamels, varnishes, and lacquers. A phthalate or a polymeric plasticizer such as an new, thermosetting acrylic emulsion polymer is alkyd resin. Maximum durability is obtained now available which, it is claimed, yields in- either by pre-plasticizing or by the use of poly- dustrial baking finishes of excellent hardness, meric plasticizers that remain in the film impact resistance, adhesion, and durability throughout its life. The chemical plasticizers superior to that of premium melamine-alkyds, gradually volatilize, and their loss eventually while equaling the latter in gloss and stain results in embrittlement of the film. resistance. PVA emulsions are slightly acidic and may cause corrosion of unprotected metallic con- 2.3.5.6.4. Latex blends and copolymers seek- tainers with resultant impairment of emulsion ing to combine the most desirable properties stability by the metallic ions liberated. of each component into a single superior ma- PVA latexes have excellent sealing proper- terial are receiving a great deal of attention ties and find wide use in primer-sealers. They from latex paint manufacturers. Among the have performed well in interior finishes and more promising recent developments reported have found use also in exterior coatings [6] are vinyl-acrylic copolymer dispersions that for masonry, asbestos shingle, and wood such produce films having very good clarity, gloss, as yellow pine and red cedar. The PVA film flexibility, and water resistance. The vinyl- has a relatively high permeability to water acrylic latexes are said to possess high pigment- vapor that permits "breathing" and hence binding ability, excellent thermal and mechani-

19 .

cal stability, insensitiveness to electrolytes, and problem has been to render water-insensitive good film-forming ability at relatively low the film formed from an initially water-soluble temperatures. They have been formulated into resin. In recent years, a number of synthetic, fiat wallpaints, semi-gloss enamels, baking coat- water-soluble resins have been developed which ings, and primer-surfacers. The latter is a have a built-in chemical reactivity that permits

sandable first coat which serves both a priming them to be insolubilized (see sec. 3.16.2) . These and a surface-smoothing function. resins will dissolve in water to form a single- Latex copolymers may be further modified phase solution that is infinitely dilutable with by blending with various compatible resins to water (unlike latex emulsions which are two- obtain other desirable properties, such as im- phase dispersions of resin-in-water that are proved adhesion, greater toughness, or lower capable of only limited dilution). Conversion cost. The formulating possibilities, as with (insolubilization) of the film laid down from conventional resinous coatings, are virtually such a solution is accomplished by a chemical limitless, and the full potential of versatility cross-linking reaction between polar groups in and usefulness of modified latex copolymers the water-soluble molecule and an added re- has only begun to be realized. active resin or drying oil, under the influence of heat, oxidizing agents, and/or driers. Al- 2.3.6. Water-Solution Coatings though some air-drying types of water-solution coatings have been made, baking is generally A natural outgrowth of the wide acceptance required to develop optimum film properties. of latex paints in the consumer field has been Baking schedules of about 30 minutes at 300 °F the increased attention being given by the paint (149 °C) are typical. industry to the development of water-thinnable All the advantages (and disadvantages) of coatings for industrial applications, where the a water vehicle inherent in latex paints (see protection and durability requirements are sec. 2.3.5.1) apply also to the water-solution more severe. In such coating material type. The fire hazard, toxicity, odor, and combining the convenience and safety of latex expense of organic solvents are eliminated paints with the film integrity of solution-type while the advantages of a true solution-type coatings would be highly useful. coating, such as good film clarity, high gloss, The use of water paints goes back to early and excellent film continuity are retained. The civilizations. The ancient Hebrews and Egyp- latter property is reflected in desirable per- tians mixed milk curds and freshly burned lime formance characteristics such as low moisture "on the job" to make a binder (calcium and gas permeability, and excellent water and caseinate) for the earth colors with which they salt spray resistance—properties of particular decorated the interiors of their houses. This importance in coatings for metals. type of painting still persists in some of the The formulation of water-solution type paints rural villages of middle Europe and elsewhere. does not require the use of wetting agents, Colonial Americans often mixed the clabber thickeners, stabilizers, and bactericides such as from skim milk into the whitewash, made of are needed in latex paints. The only ingredi- slaked lime and water, which was their main ents generally necessary are pigment, resin,

coating material ; oil paints, though known and water, and (in some instances) driers. A used during this period, were an expensive small amount of high flash aromatic solvent luxury. Whitewash (see sec. 2.3.9.2) remained or odorless mineral spirits may be added in in favor as both an interior and exterior coat- some cases to function as a defoamer. ing until the latter part of the nineteenth Application of water-solution coatings is century, when powder paints for mixing with usually accomplished by dipping, flow coating, water introduced. were spray, before use were These or conventional cold spray ; however, hot the kalsomines, consisting of glue-bound clay airless (high pressure) spray, and electrostatic and whiting. Although factory-made casein spray also have been employed to some extent. paints in powder form were introduced at about While the number of satisfactory types of the same time, it was not until the 1920's that water-solution coatings presently available is casein paints reached large volume usage. not large, technological progress in this area Later, to improve the water resistance and continues. Durable types have been developed durability of casein paints, they were modified which are reported to combine the outstanding with alkyd emulsions. Soon casein assumed a gloss and color-retention characteristics of a secondary role as emulsifying agent for the nondrying alkyd with the hardness, toughness, alkyd resin in emulsion paints (see sec. 2.3.5.2) and adhesion of a drying-oil alkyd; others are The latter gave way to the latex emulsion reported to possess the hardness, gloss, and paints which, ever since their introduction in durability of the best melamine-alkyds [123] 1949, have dominated the water-based paint (cf. sec. 3.16.2). market. A water-soluble linseed oil that can be for- Meanwhile, the quest for a durable water- mulated into house paints having properties solution type coating has continued. The basic comparable to those obtained with conventional

20 linseed oil also is available [121]. storage; rapid stirring or violent shaking will It is probable that the next few years will alter the physical characteristics of the multi- see the development of a greater variety of color material and should be avoided. water-soluble convertible resins, including sat- Most present-day commercial multicolor fin- isfactory air-drying types for use on wood and ishes are based on cellulose nitrate as the chief other surfaces that do not permit baking. film-forming component; however, finishes based on alkyd, acrylic, vinyl, and other resins 2.3.7. Multicolor Finishes as well as on varnish or oil-base vehicles are possible or available. A Federal specification Multicolor finishes—sometimes called "polka- [10] describes a multicolor lacquer based on a dot paints"—consist essentially of discrete par- blend of cellulose nitrate with an alkyd resin. ticles of viscous coating material suspended in A typical commercial interior type blends cellu- water containing a suitable stabilizing agent. lose nitrate with a rosin ester. Water-soluble The stabilizer is used in an amount sufficient organic polymers (protective colloids) such as to prevent coalesence of the particles but in- methyl cellulose or polyvinyl alcohol are the pre- sufficient to cause their emulsification. A vari- ferred type of stabilizing agent and are effec- ety of colors and particle sizes and shapes can tive at relatively low concentrations (0.3 to be incorporated in a single multicolor finish by 5.0%). Various other materials—organic and first preparing separate dispersions of each inorganic, soluble and insoluble—in finely di- color and then mixing them in the desired provided form may be employed; the insoluble portions. Each color retains its particular iden- types must be used in much greater concentra- tity in the final mixture for the same reasons tion (perhaps as high as 25 percent of the that it remains in discrete particle form in its aqueous phase) to be effective. own individual dispersion. The stabilizing Multicolor finishes are usually applied as agent, when properly chosen and used in the heavily sprayed coats that completely mask the correct concentration, forms a protective film surface. If desired, they may be lightly sprayed around each coating particle which effectively to produce a "spatter" coat over a colored base isolates it from neighboring particles in the coat. They have also been used as a sprayed aqueous medium, irrespective of their color, coat on paper or textiles subsequently drawn provided that certain basic requirements are under a doctor blade to produce a striped effect. fulfilled during manufacturing. The literature The large size and viscous character of the [8, 9] indicates the following to be the most dispersed coating particles in multicolor finishes important factors: permits their application to highly porous sur- (1) The solvents used in the coating mate- faces without excessive penetration. Thus, dis- rial phase must have a low solubility in water, similar adjoining surfaces such as masonry and the water must have a low solubility in the and wood, and rough or imperfect surfaces, can solvents. be masked completely to give a uniform ap- (2) Pigments employed should be non-bleed- pearance. Heavy films, 5 mils or greater in ing either in the solvents used or in water. thickness, can be sprayed in one application (3) The specific gravity of the coating mate- without sagging or wrinkling. Multicolor fin- rial and of the stabilized aqueous medium must ishes can be apphed to damp (but not fresh be closely controlled ; too great a difference will alkaline) surfaces without difficulty because result in coalescence during storage. their porous nature permits substrate moisture (4) Particle size is largely controlled by the to evaporate without blistering the film. No speed of stirring, the size decreasing with in- primer is required for porous surfaces such as creased speed of agitation. The addition of wood, masonry, plaster, wallboard, cloth or wetting agents or an increase in the amount paper. A primer may be useful, however, over of stabilizer also reduces the particle size. The non-absorbent surfaces such as metal. viscosity and the temperature of the coating The limitless color combinations and wide material phase also have an effect, the size of variety of patterns provided by multicolor fin- the particles increasing as the viscosity or tem- ishes, coupled with the good durability exhib- perature-is increased. ited by many of the types now available, have (5) Particle shape depends primarily on the led to their use in continually increasing quan- viscosity of the pigmented coating phase but is tities in both interior and exterior applications also affected by the type and amount of stabi- for hotels, public buildings, industrial plants, lizer employed. Higher viscosities, in addition homes and furniture. to increasing the particle size, yield globules and filaments instead of spheres. 2.3.8. Fire-Retardant Coatings Although the particle size depends strongly on the speed of agitation, the time of agitation 2.3.8.1. The primary function and effective- appears to have relatively little effect provided ness of present fire-retardant coatings is in re- sufficient time for adequate dispersion is al- ducing the surface flammability of substrates lowed. Only gentle stirring is needed after over which they are applied. To accomplish

21 this, a fire-retardant coating must itself be and for purposes of discussion, it is desirable essentially nonflammable (non-flame-propagat- to distinguish between fire-retardant coatings ing). Additionally, it may provide a limited that are merely flame-retardant (subsection degree of shielding and insulation, against the 2.3.8.2) and those which provide the additional heat generated by a fire, thereby delaying the function of heat insulation (section 2.3.8.3), time for the substrate to reach the ignition while bearing in mind that the distinction be- temperature or—in the the case of a non-com- tween the two vanishes, for practical purposes, bustible substrate—the time for the heat to pass after the first 5 to 15 minutes of exposure to a through the substrate to combustibles on the large-scale fire. A discussion of these two other side. types follows. Not even the best fire-retardant coating can be expected, however, to provide protection for 2.3.8.2. Flame-retardant coatings may be long under the onslaught of a full-scale fire, described as those which, though decomposed particularly in a confined area.^ After the few by exposure to fire, serve to retard the spread minutes of initial delay, the chief value of a of flame, although they may not significantly fire-retardant coating is in reducing the rate insulate the substrate from the heat generated. of further flame spread rather than in prevent- They have application in situations where a ing substrate ignition or structural failure in fire of only short duration is to be expected the immediate area under attack by fire. How- (e.g., in an area where only small amounts of ever, while fire-retardant coatings have not combustible material are stored), or to keep a proven capable of adding significantly to the localized ignition (such as that from momen- fire endurance of buildings and other structures tary contact with a torch or heated object) as measured in terms of hours of fire exposure from spreading across the exposed surface. before collapse, the few minutes delay they Flame retardancy in such coatings is achieved afford in the early stages of a fire can be of by the use (to the extent feasible) of non-flame- critical importance to the escape of personnel propagating resins and plasticizers, by the use and to the ability of the fire department to con- of fusible inorganic salts and pigments (e.g., trol the blaze and minimize damage. It has borax) that form an air-excluding glaze on been aptly said, from the viewpoint of the fire- heating, and/or by incorporating special agents fighters who must battle a blaze, that "the first (e.g., brominated compounds) that inhibit the five minutes are worth the next five hours." tendency to ignite. The fire-retardant coatings discussed herein The ignition temperature of a flammable should not be confused with heat-resistant coat- solid is the temperature at which flammable ings (see sec. 3.9.1) of the type employed on gases are generated and ignited. By incorpora- surfaces that become or remain hot in normal ting into the material to be protected chemical service (e.g., stoves, boilers, hot pipe liners, agents that will convert flammable gases to etc.). The effectiveness of the heat-resistant non-flammable ones as they are generated, an coatings depends on their ability to withstand effective degree of flame retardancy can be more or less continuous exposure to elevated achieved. Lewis acids, such as hydrogen hal- service temperatures without igniting and with- ides or phosphorus oxides, are effective agents out undue impairment of their protective and that appear to function in this manner. Al- decorative qualities, rather than on their ability though these cannot be used directly in a paint to withstand direct attack by fire; hence they because of their hazardous, corrosive proper- are not properly classed as fire-retardant. ties, compounds have been developed that are The term "fireproof" is not meaningful when capable of giving off the desired active agent applied to coatings, since it implies a complete upon heating. Among the most successful of and permanent resistance to fire that is not these have been brominated compounds. The possessed by any organic coating material, no bromine can be incorporated directly into the matter how formulated or modified. Reliance molecule of resins such as alkyds and epoxies. upon so-called "fireproof" coatings to the ex- A brominated phenol is compatible with, and clusion of other recognized fire-preventive meas- may be physically blended with, phenolic resins ures and fire-fighting devices can have disas- to render them flame-retardant. Vinyl coat- trous consequences. ings can be rendered self-extinguishing by using As for fire-retardant coatings—these are a a small percentage of a brominated organo- supplement, not a substitute, for adequate struc- phosphate compound, such as tris (dibromo- tural design, good housekeeping, care in han- propyl) phosphate, as part of the plasticizer dling materials, fire extinguishers, sprinkler in the formulation. systems, fire exits, and the fire department. From the standpoint of formulation and use, 2.3.8.3. INSULATIVE FIRE-RETARDANT COATINGS also possessing flame-retardant properties have ^ A fire-retardant material fibers is incorporating chopped glass proven to be the effective type in impeding available which does significantly increase the fire endurance of most wood or steel structures; however, this material is applied at a the progress of a fire. Of these, intumescent thickness (about one-quarter inch) far in excess of that normally designated as a coating. coatings that bubble and swell on heating to

22 form an insulating mat between the fire and tests. Among test methods recognized as use- the substrate have been the most successful. ful by those concerned with problems of fire The blowing agents employed in such coatings protection are the following: are sometimes water-soluble compounds that (1) The standard "tunnel" test developed are susceptible to leaching; the coatings thereby Underwriters Laboratories [11, 12]. fore may be somewhat water-sensitive and may (2) The radiant panel test devised by the properties not retain their full intumescent National Bureau of Standards [13, 14]. after periods of prolonged (particularly ex- (3) The flame spread test described in Fed- terior) service. eral Specification SS-A-118b [15]. The degree of protection afforded by a partic- "cabinet method", in which meas- ular fire-retardant coating depends not only on (4) The urements are made of the weight loss and char its intrinsic fire-retardant properties (i.e., non- flame that con- flammability, intumescence, insulative charac- volume produced by a standard sumes 5 ml of absolute alcohol [16]. ter, strength of charred matrix, etc.) but also on the thickness at which it is applied and on (5) The "stick and wick method", in which the nature of the substrate. On completely the weight loss, rate of vertical flame spread, non-combustible substrates, such as ceramics and char height are measured [17]. and most metals, application of any organic (6) Various test methods developed by com- coating, even a nonflammable one, will increase mercial firms, such as the tests with standard to some extent the rate of fire spread along the bowling pins and with load-bearing steel mem- surface—the rate of spread increasing with bers used by the Fire Research Laboratory of coating thickness, at least within the practical Union Carbide Chemicals Company [18]. gener- range. However, since metal surfaces (7) Full-scale experimental fire tests on ex- ally require such a protective coating to resist pendable buildings or structures. corrosion, the somewhat reduced resistance of Among the test methods (other than full-scale the surface to fire must be accepted. In such tests) cited above, the tunnel test is the most instances, the reduced surface resistance to fire generally accepted means available for evalu- increasing thickness may be offset to some with ating the flame-spread characteristics of build- extent by the greater heat-insulating effective- ing materials. The more recently developed ness of the thicker coating. As already pointed radiant panel test was designed to be less elab- out, the heat barrier furnished by the insulative orate and expensive and has given highly re- type of fire-retardant coating, though only tem- producible results, while appearing to correlate porary, can be of vital importance in the first fairly well with tunnel results. However, the few minutes of a fire. radiant panel test, largely because of its small combustible substrates, application of a On scale, has not yet been widely accepted by code fire-retardant coating of either the flame-re- officials. Full scale fire tests, while highly in- insulating type affords increasingly tardant or formative, are too expensive for routine evalu- fire protection as the coating thickness greater ations. is increased [19] . In fact, any coating that is In interpreting fire tests, it should be borne less flammable than the substrate will afford in mind that different test methods may give degree of protection to that substrate. some diflferent results with different types of mate- This latter fact sometimes results in erroneously rials. For example, a highly reflective coating attributing significant fire-retardant properties (e.g., an aluminum-pigmented material) may to coating material that in reality is contrib- a perform better than a matte coating in the radi- uting only the normal effect of added thickness. ant panel test and yet appear inferior by a Such misevaluation may not only involve the flame-impingement test. Similarly, tunnel test needless expense of paying for a fire-retardancy results may not coincide with results obtained that is not achieved, but can also create a in a load-bearing test or with a test house. dangerous sense of false security about the pro- Nevertheless, such tests are a necessary pre- afforded. tection requisite in the screening of fire-retardant materials. Appropriatelj' applied and wisely in- 2.3.8.4. of fire-retardant coat- Evaluation terpreted, the results obtained are very useful. ings.—Although the ultimate and only fully reliable test for the effectiveness of a particular 2.3.8.5. Selection of fire-retardant coat- fire-retardant coating is its actual service per- ing SYSTEMS.—From the information that has formance, several laboratory and large-scale been presented on the nature and effectiveness test methods have been developed which give a of fire-retardant coatings, a basis for their good indication of the performance that may selection may be given. reasonably be expected from a given material Flame-retardant coatings (and impregnants, under specified conditions. Such methods are with which this volume does not deal) are use- particularly useful in screening out inferior ful in the following situations: materials that do not merit the expense of (1) To prevent the ignition of combustible large-scale tests or the risk of actual service fabric surfaces (e.g., draperies or upholstered

23 — furniture) that may inadvertently come into specifications refer to "fire-retardant" coatings, brief contact with a flame or heated object. many of the coatings may be more aptly de- (2) To avoid the rapid spread of flame along scribed as flame-retardant according to the combustible surfaces such as cellulosic wall- criteria set forth in the preceding paragraphs, board or ceiling materials. since their primary function is to reduce flame spread rather than to insulate the substrate (3) In combustible enclosures where only from the fire. For example, flame-retardant small quantities of flammable materials are coatings of conventional coating thickness present, since in such cases the fire (fuel) is rather than heavy intumescent fire-retardant likely to burn itself out before the blaze becomes coatings are widespread. specified for use on metal bulkheads and overheads of navy ships [19, 20, 21, (4) To reduce the danger of deadly flash 22, 23, 24], where the primary concern is to fires in susceptible public areas such as schools, minimize flame spread via the paint (both di- theatres, night clubs, tents, and exhibit build- rectly and through heated bulkheads) and to ings, where the loss of life very great can be limit the production of toxic decomposition although such coatings should by no means be products in the confined space aboard ship. considered to have entirely eliminated the haz- On the other hand, fire-retardant coatings of ard of fire. the intumescent type [25, 26] have generally Insulative (intumescent) fire-retardant coat- been the most effective and are usually specified ings are useful: (1) Where a brief delay in for wood surfaces, where the combustible nature transmission of heat from a fire to a combust- of the substrate requires the greater protection ible substrate may prevent ignition or serious that is afforded by the insulative type of fire- damage. For example, utility and telephone retardant coating in order to avoid quick igni- poles protected with a weather-resistant coat- tion. ing (applied to glass screening stapled to the pole) have withstood repeated brush and grass 2.3.8.6. In summary, it should be pointed out fires without suffering significant structural that while there exists general agreement as to damage ; such fires usually will burn completely the ability of fire-retardant coatings to impede past a stationary object in two or three minutes, the progress of a fire, there is considerable ques- an interval during which the coating is capable tion as to the extent and effectiveness of the of providing full protection to the structure. protection provided. It must be emphasized that (2) In aircraft, particularly on large com- the protection afforded by even the best fire- partmentalized types. Although the efi'ective- retardant coating is limited to the early stages ness of fire-retardant coatings for aircraft has of a fire. However, to the extent that the delay not been fully evaluated, there is reason to be- afforded enables personnel to escape and fire- lieve that in some kinds of fires aboard aircraft fighting equipment to be brought into action, in flight, personnel might have time to bail out valuable protection is achieved. Once the fire or possibly make a safe forced landing if vital reaches full-scale proportions, no coating will areas of the aircraft could be insulated from the appreciably affect the course of the blaze. heat of the fire for only a few minutes. De- In many applications, flame-retardant and spite the existence of a definite interest in such insulating types may perform equally well. In coatings, their aircraft not use on has been fact, it may often not be known whether a given widespread. fire-retardant paint is one type or the other (3) On ground structures within quick range unless specific information is provided by the of fire-fighting equipment, as at airfields and manufacturer. There are situations, also, in military installations, which may be saved from which a coating of demonstrable fire-retardant severe structural damage if ignition of com- effectiveness may be unsatisfactory for certain bustible substrates can be delayed long enough applications because of deficiencies in conven- for the fire-fighting equipment to be brought tional paint properties such as appearance, dur-

into action. ability and weathering resistance ; or the thick- (4) To provide heat insulation as well as ness required may be too great for some appli- flame retardance on walls and ceilings of cations (as aboard high-speed aircraft, where schools, office buildings, and other heavily popu- weight must be kept to a minimum) . It is lated compartmented buildings, where every probable that the shortcomings of fire-retardant minute of delay in the transmission of heat coatings with respect to some desirable paint from one room to another can be of vital im- properties have been a strong factor in slowing portance in permitting personnel to escape, their widespread adoption. even though the ultimate fate of the structure Although there remains much room for im- itself may be little affected. provement in the formulation and efficacy of A number of Federal and Military Specifi- fire-retardant paints, continuing developmental cations relating to fire- and flame-retardant work is being carried on by a number of paint coatings are available. Although all of these manufacturers, and it is likely that the role of

24 , such paints will become increasingly important ough stirring) during this period to counter- as their effectiveness is increased. act the tendency of the paint to stiffen as a result of its reaction with the water and the loss 2.3.9. Gold Water Paints (Inorganic) of water by evaporation. Concrete and masonry surfaces that are to Water-base paints utilizing organic binders receive cement-water paints should be cleaned of the latex type and water-soluble type were free of dirt, oil, grease, and efflorescence as de- discussed in sections 2.3.5 and 2.3.6, respec- scribed in section 6.4. Since cement-water tively. The present section briefly describes paints are completely resistant to the moisture two other types of cold-water paints which, and alkalinity in concrete, little or no aging of though inorganic or largely inorganic in nature, the surface is required. Previously peinted have important application in the coatings field. concrete or masonry surfaces must be cleaned free of all masonry coatings, whitewash, or 2.3.9.1. Cement-water paints are very use- weak, crumbly cement paint (see sec. 6.6.4.3). ful for concrete and masonry surfaces (see sees. Application of cement-water paints is best

2.4.3 and 4.4) . They are composed chiefly of accomplished with a stiff fiber brush which per- white Portland cement together with some hy- mits the paint to be scrubbed into the voids of drated lime or siliceous aggregate or both, de- the surface. The surface should be uniformly pending on the requirements of the application. damp but not wet with water before the paint Cement-water paints of two types (medium or is applied. The paint should be applied in two high cement content) and two classes (without coats with at least 24 hours allowed between or with siliceous aggregate) are described by coats. The first coat should be dampened with Federal Specification TT-P-21 [32]. The lime water before applying the second coat. Exces- contributes easier brushability and, if not used sively thick or thin coats are undesirable. Thick in excess (not more than 30 percent by weight) coats are somewhat prone to crack or chip assists in obtaining a hard, weather-resistant from the substrate also, it is more difficult to ; coating. The aggregate (usually a white or apply them uniformly. Thin coats lack opacity light-colored silica sand) is very useful in ce- and durability. Too wet a mix will produce ment-water paints that are to be applied to a powdery texture when dry, as well as too thin open-textured masonry surfaces where filling a coating. The proper spreading rate for ce- is needed; two parts by volume of portland ment-water paints depends on the texture of cement to one part of aggregate produces a good the surface; as a rough guide, the spreading fill coat composition. For use as a "grout rate may be about 100 square feet per gallon coat" on very coarse surfaces such as cinder for two coats on smooth masonry and about 50 block, equal parts by volume of cement and sand square feet per gallon for two coats on rough are most effective. Proprietary formulas for masonry. general-purpose cement-water paints often add Proper curing and hardening of cement- small proportions of other ingredients to the water paint into a durable, stress-free coating white Portland cement and lime. For example, that will not be prone to cracking, crumbling, calcium chloride may be added for its hygro- or powdering depends on the continual availa- scopic effect in drawing moisture from the air, bility of adequate moisture for reaction with which promotes proper curing and hardening. the Portland cement, especially during the first Titanium dioxide or zinc sulfide pigments may few days after application. To accomplish the be added to improve the wet opacity, and col- necessary damp curing, the painted surface ored pigments may be added to produce tinted should be wet down with a fog spray two or paints. About one percent of calcium stearate three times a day, starting within 6 to 12 hours may be incorporated to provide some degree of after application of the first coat and continu- water repellency in the dried paint. ing for at least 48 hours after application of The mixture of solid ingredients is furnished the final coat. as a dry powder that needs only to be mixed Properly applied and damp-cured, cement- with water to prepare it for use. The dry ma- water paints provide a durable coating for ex- terial should first be made into a stiff paste by posed concrete or masonry surfaces above or adding small portions of water with continuous below grade, inside or outdoors. stirring, after which additional water should be gradually stirred into the mix until a con- 2.3.9.2. Whitewash, for centuries before sistency resembling that of rich cream is ob- the advent of modern coating systems, was tained (except that a slightly thinner consist- widely used as a readily available, inexpensive, ency is desirable for the first coat applied on general-purpose coating for interior use on open-textured masonry surfaces) . Cement- wood, plaster, concrete, masonry, glass, and water paints normally remain in usable condi- metal. While lacking the durability and the tion for three or four hours after being pre- protective and decorative qualities of many pared for use; however, small additional present-day coatings, whitewash still finds some amounts of water need to be added (with thor- use as a low-cost, easily applied coating for

25 sheds, fences, tree trunks, concrete curbs, gas- added to the mix. For general use on wood, oline station islands, brickwork, cinder block, glass, and metal, the additives most often used and other noncritical applications in mild en- are common salt and calcium chloride, which vironments where its flat, extreme chalky-white are dissolved in the water to be used for diluting appearance gives a clean, pleasing effect. In the original lime paste. The use of calcium particular, whitewash still finds favor as a coat- chloride rather than salt reduces the tendency ing for the interior walls and ceilings of dairy of the whitewash coating to chalk. For use on barns, for farm implements and structures, and concrete and masonry, more satisfactory results as a light- and heat-reflective coating for coal are obtained with a whitewash consisting of tar pitch or asphaltic surfaces to reduce their equal parts by weight of hydrated lime and tendency to soften, sag, wrinkle, or run when white Portland cement (a composition ap- exposed to direct sunlight in hot climates. proaching that of the cement-water paints de- Whitewash in its simplest form is prepared scribed in the preceding section) with sufficient by slaking quicklime or soaking hydrated lime water to produce a mixture having the consist- in water, the latter method being preferable be- ency of heavy cream. Other materials which cause of its greater safety, convenience, and may be added to whitewash for various pur- uniformity of product. Approximately 8 gal- poses include casein plus formaldehyde for in- lons of stiff" lime paste are produced by slaking terior walls of dairy barns, casein plus borax 38 pounds of quicklime with 8 gallons of water, to produce a fire-retardant coating, animal glues or by soaking 50 pounds of hydrated lime in 6 for use on plaster, titanium or zinc sulfide pig- gallons of water. Before use, sufficient water is ments to improve opacity, and iron oxide pig- stirred into the paste to obtain a thin consist- ments for tinted coatings. More detailed infor- ency resembling that of whole milk. To improve mation on whitewash is available in a bulletin durability, other materials are nearly always by the National Lime Association [126].

2.4. Primers

A primer is applied as the first coat of paint larger proportions of raw oil brushing proper- on the substrate (sometimes preconditioned) ties are improved and degree of penetration is to be painted. Its chief function is to provide a increased, but leveling is poorer. Conversely, satisfactory bond between the finish coats and larger proportions of bodied oil improve level- the substrate. The primer may also be called ing properties, but reduce ease of brushing and upon to impart other important properties to penetration. A 1 :1 blend of raw oil with bodied the system, such as corrosion resistance, water oil of consistency approximately "Z" on the resistance, substrate sealing, enamel hold-out, Gardner Scale (equivalent to about 23 poises) and some degree of substrate filling and hiding. has been widely used. The choice of vehicle and the proper pigment- Zinc oxide is sometimes used in the pigment binder ratio are important considerations in ob- portion in place of part or all of the resin in the taining the required degree of wetting, pene- vehicle, although its use in wood primers is con- tration, and adhesion. The formulation is troversial. Chalk-resistant titanium pigment, usually such as to yield a satin surface to which white lead, and extenders usually comprise the topcoats will adhere well pigment volume con- pigment portion. An excellent primer for ex- ; centration is intermediate between that for terior wood is covered by Fed. Spec. TT-P-25 gloss and dead flat finishes. The pigments em- [111]. ployed are similar to those used in finish coats, Exterior oil paints may be made into so- except for the addition of corrosion-inhibiting called "self-primers" by thinning with turpen- pigments in metal primers. tine or mineral spirits and/or by adding raw linseed oil. 2.4.1. Wood Primers Water-based coatings have not, as yet, proven satisfactory as wood primers. However, some Primers for wood that is to withstand ex- water-based paints (e.g., an acrylic or a poly- terior exposure are usually of the oil or oleo- vinyl acetate latex paint) can be successfully resinous type. The primer (or undercoater, as applied on wood that has been primed with an it is often called in wood painting) serves to oil paint. seal the wood pores, so that good hold-out and adhesion of topcoats is obtained. The vehicle is 2.4.2. Primers for Metal usually a blend of heat-bodied and raw or refined oils (see sec. 2.2.1), usually linseed, modi- Primers are nearly always required for ade- fied with a small amount of resin. Brushing quate adhesion of topcoats in coating systems characteristics, control of penetration into the for metals. They may be either air-drying or wood, and leveling of the paint film are largely baking types. Most metals need extra protec- dependent on the ratio of raw to bodied oil. With tion against corrosion which necessitates the

26 incorporation of corrosion-inhibiting pigments types, have been successfully formulated for into the primer. The manner in which such pig- some applications. For example, a styrene- ments function is discussed in section 2.3.1.5. butadiene latex containing iron oxide pigments The choice of pigment depends on the metal to plus china clay and barytes extenders is used be protected. A metal conditioning pre-treat- as the primer on some automobiles. A water- ment (see sec. 6.3.2), such as phosphatizing, solution type primer is available for use under- when used prior to application of the primer, neath a recently-developed, water-solution type greatly improves primer adhesion and corro- industrial topcoat (cf. sec. 2.3.6.). In these sion resistance; under these conditions, corro- types, the water-soluble resin is converted to a sion resulting from scratches or other breaks cross-linked, insoluble coating by baking. in the coating is confined largely to the immediate area of the scratch or break. 2.4.2.2. Primers for galvanized iron and STEEL are most effective when formulated with 2.4.2.1. Primers for iron and steel are high percentages of zinc dust as the chief pig- based on a variety of rust-inhibitive pigments ment [29] . The zinc dust, in addition to aiding in a suitable vehicle, such as linseed oil, an alkyd adhesion, may provide some degree of cathodic resin, or a phenolic resin-drying oil varnish and barrier layer protection to the underlying [27, 28] (also see list in sec. 8.1.4 and summary iron or steel (see sec. 2.3.1.5). charts in sec. 8.2). Raw linseed oil possesses the best penetrating properties and is essential 2.4.2.3. Primers for aluminum are based where complete removal of surface corrosion largely on zinc chromate and zinc tetroxychro- products has not been feasible. Among pigments mate as the corrosion-inhibiting pigments in a often used in primers for iron and steel are red variety of vehicles, including phenolic alkyds, lead, basic lead chromate, blue lead, and zinc styrenated alkyds, and polyvinyl butyral. The yellow, together with iron oxide pigments and latter is combined with phosphoric acid and al- silicate extenders. The time-honored superiority cohol just before use to yield a so-called wash of red lead is believed to result from its ability primer or metal pretreatment coating [30] to combine chemically with the iron or steel which provides both a phosphatizing and a substrate to form a hard, highly adherent, mois- priming action that gives excellent results (see ture-impervious coating that passivates the subsec. 6.3.2.5). A conventional spray-applied, strate against corrosion. The use of a basic lead fast-drying zinc chromate primer [88] or a low silico-chromate pigment to impart corrosion- moisture sensitivity type [54] may be used inhibiting properties not only to primers but [122] over the wash primer to provide an also to intermediate and finish coats is discussed anchor coat for the finish coats. A relatively in section 2.3.1.5. slow drying alkyd-base zinc chromate primer, The choice of pigment and vehicle depends on for brush or spray application on chemically the nature of the application. For outdoor steel cleaned bare metal, is covered by Fed. Spec. structures, primers based largely on red lead TT-P-645 [89] (see summary chart in sec.

[27] in a linseed oil or long oil-alkyd vehicle 8.2) ; this primer is particularly suitable as an are widely used (also see 4.3.1.1). In marine undercoat for alkyd enamels and for marine long oil-phenolic vehicle is often pre- service. Lead-containing pigments are not con- ferred. Under severely corrosive conditions, sidered satisfactory in primers for aluminum vinyl primers [143, 144, 145] in an all-vinyl because of the possibility of a galvanic action coating system give durable service (see sec. between deposited lead and the aluminum 4.3.1.2). Automotive primers often contain which would result in pitting of the surface. iron oxide as the chief pigment in a short or medium oil-alkyd baking vehicle; amino resin- 2.4.2.4. Primers for magnesium must be alkyd vehicles are also used. Zinc-rich paints carefully formulated to obtain adequate adhe- containing high percentages of zinc dust (see sion and corrosion resistance. Zinc chromate sec. 2.3.1.5) in an epoxy, epoxy-polyamide, is the preferred corrosion-inhibiting pigment polyurethane, or other alkali-resistant vehicle [31]. Pigments such as red lead and other are being successfully used as primers for the lead pigments (except lead chromate) that per- undersides of automobiles, for underwater form excellently in primers for steel may pro- structures, and for other types of severe ma- mote the corrosion of magnesium. Acidic ve- rine service where their excellent corrosion-re- hicles may cause gassing at magnesium sur- sistant properties justify their higher cost. In- faces and so destroy adhesion and accelerate dustrial primers for machinery, refrigerators, corrosion. Satisfactory vehicles have been washing machines, and other appliances are based on phenolic-modified alkyd resins [54, generally baking types based on short oil-alkyd 88], phenolic-tung oil varnishes, epoxy baking vehicles or newer vehicles such as epoxy ester- resins, chlorinated rubber, vinyl copolymer modified alkyds. (A discussion of alkyd resins resins, and polyvinyl butyral resin. The latter is given in sec. 3.4). is usually a non-acid type, although a polyvinyl Water-base metal primers, mostly baking butyral wash primer with reduced acid content

27 ,

is sometimes used on adequately pretreated sur- dampen the porous surface and thereby satisfy faces. Whenever magnesium is to receive an its water demand. organic protective coating system, proper sur- Oleoresinous primers and paints for masonry face pretreatment (usually chromating) before surfaces are generally capable of good perform- application of the primer is of great value (see ance only when applied to well-aged, clean, sec. 6.3.5). dry surfaces that can be expected to remain reasonably dry during service. Otherwise, discoloration, blistering, peeling, and other defects 2.4.3. Masonry Primers may occur. Resin-emulsion paints used as masonry are not a well-defined Primers for masonry primers in the past have been largely replaced class specially formulated materials as are of by the more alkali-resistant latex types. How- usually made primers for metal. They are ever a few alkyd resin emulsion paints are simply by addition of an appropriate thinner available [36], and some developmental work to the topcoat paint. Latex-base paints [5, 6, 7] on these materials is continuing. which are now the most widely used type for Cement-water paints [32], despite their low suitable masonry surfaces, are thinned with strength and chalking tendency as often for- with portland amounts of water. Paints made mulated and applied, are used on open-textured lime are also thinned with cement and [32] masonry surfaces because of their low cost and water. Oleoresinous and synthetic resin [33] their complete freedom from difficulties arising types, including the various solvent-thinned from dampness or alkalinity in the substrate. require specific rubber-base products [34, 35], Because they are highly permeable to water usually identified organic solvents which are vapor but are virtually unaffected by water, in specifications for materials on the the and they are suitable for application to interior container labels. As a general rule, ordinary masonry below grade (e.g., basement walls of mineral spirits can be used to thin oleoresinous cinder block, etc.). With proper formulation paints whereas thinners of greater solvent and wet-curing, their durability is quite good. power, such as aromatic and chlorinated hydro- (See sec. 2.3.9.1). aliphatic hydrocar- carbons (along with some Under conditions of prolonged immersion, as diluent), are required for such products bon in swimming pools, primers and paints based as the styrene-butadiene copolymers (non-la- on a chlorinated rubber vehicle [35] or on a tex, synthetic , chlorinated rubber, rubber type) solvent-type styrene-butadiene rubber vehicle and others. have been oflfered and have achieved a consid- Apart from primers made as indicated above, erable measure of success. there are a limited number of products made expressly for priming purposes. For example, 2.4.4. Primers for Plaster and Wallboard special primers have been developed to combat mottling of deep-colored masonry finishes. Unlike masonry primers which usually are Others are designed to alleviate some of the simply thinned topcoat paints, primers or difficulties encountered in the painting of primer-sealers for plaster and wallboard are chalked masonry surfaces. Although removal usually specially formulated materials that dif- of the chalk is the only sure answer (see sec. fer from finish coats in pigment-binder ratio 6.4.6), some benefit can be obtained by the use and vehicle composition. The term "primer- of special oleoresinous anchor coats (e.g., a sealer" emphasizes the important sealing func- phenolic resin-tung oil varnish) use of an , by tion to be performed on porous surfaces (see emulsion type surface conditioner containing sec. 2.3.5.3). Both latex-base and oleoresin- special wetting agents, or by stirring some oil ous [37] types of primer-sealer are available. (e.g., boiled linseed oil) into latex-base paints Either type may be over-coated with virtually just before applying them. any type of finish coat—latex, oil-base, or alkyd. Latex primers and paints are relatively non- A thinned latex-base finish coat paint such as sensitive to water and alkali and provide excel- that described by Federal Specification TT-P- lent sealing of concrete, brick, cinder block, 29 [38] is sometimes used as a primer. stucco, and other porous masonry surfaces. Although the latex-base primer-sealers are They may be applied over damp, relatively un- relatively new materials, they are widely used aged masonry surfaces without difficulty. because of their fast dry, excellent sealing prop- (Fresh, uncured surfaces should not, of course, erties, and superior water and alkali resistance. be painted). Although permeable to water They may be applied on only briefly aged plaster vapor, latex primers aff'ected least are not by passage surfaces ; however, a drying period of at of the vapor through them or through the sur- two weeks is highly desirable. They are par- face on which they are applied. In addition ticularly well-suited for application on gypsum to thinning latex paints with limited amounts wallboard [127] because of their nonsensitivity of water to make them suitable for masonry to the alkalinity in the plastered tape joints priming purposes, it is good practice also to employed and because they do not raise the nap

28 of the outer paper layer as do oleoresinous adhesion typically associated with oil-base types. A phenolic emulsion-type dry wall paints. On inferior plaster having a powdery primer also is available for use on gypsum or chalky surface, oleoresinous primers provide wallboard. better wetting and adhesion than is obtained Oleoresinous primer-sealers, while less re- with the latex types. As always, as much of sistant to water and alkali than the latex types, the chalk as possible should be removed before give satsfactory results when applied to dry, painting. well-aged plaster surfaces (60 to 90 days of Composition-board and other non-alkaline aging are usually required) . The oleoresinous wallboards (see sec. 6.5.2) may be sealed with primer-sealers offer the good brushability, good either the latex or oleoresinous type of primer- flow-out and leveling characteristics, and good sealer with good results.

2.5. Bituminous Coatings

2.5.1. Hot Applied when cut-backs are compounded with suitable mineral fillers, such as asbestos, durable prod- Bituminous coatings based on coal-tar pitch ucts are obtained which are used extensively as or on natural or petroleum-derived asphalt find protective coatings in roofing and waterproof- wide use in underground and underwater appli- ing applications. An asphalt roof coating of cations, in roofing", and for general waterproof- this type is covered by Fed. Spec. SS-A-694 ing and damp-proofing applications on concrete [112]. and metal in buildings, dams, penstocks, reser- voirs, tanks, sewage disposal plants, etc. Their 2.5.3. Bituminous Paints and Varnish usefulness stems chiefly from their virtually complete imperviousness to water and moisture Useful yet relatively inexpensive coatings in the thick coats (3/32 to 1/2 inch) that can are obtained by combining pitch or asphalt be economically applied. As hot melts or hot- with resins and/or drying oils in a suitable sol- applied mastics, they are the most widely used vent medium to yield bituminous paints and coating materials for the protection of buried or varnishes. Such coatings provide good protec- submerged steel pipe and other ferrous metal- tion against industrial and corrosive environ- work. ments at low cost. Although dark in color and Coal-tar pitch enamels and asphalt mastics quick to dull upon weathering, they can main- for hot application are made by compounding tain their protective qualities for long periods the bituminous base material with mineral provided they have been carefully applied (at filler. The mineral filler permits application least three coats) in a manner that ensures of a thicker coating, raises the softening tem- freedom from pinholes and holidays (non-wet- perature, and reduces the tendency of the coat- ted areas). An asphalt varnish of general util- ing to crack and disbond at low temperatures. ity is described by Fed. Spec. TT-V-51 (see

Coal-tar pitch, while superior to asphalt in summary chart in section 8.2) ; while this var- water imperviousness and dielectric strength, nish is particularly suited for painting interior is more susceptible to temperature changes and water and gas pipes, it may also be used out- to the deteriorative effects of atmospheric doors if sheltered from the sun. weathering. Therefore, pitch is the preferred Bituminous paints and varnishes may be ap- bitumen for underground and underwater applied over old oil paints, but oil paints should plications, while asphalt is more widely used not be used over the bituminous types. Oil in above-ground and roofing applications. The paints tend to crack and flake off from bitumi- low temperature working limit for coal-tar nous undercoats because of their inability to coatings has been extended down to about follow the greater thermal expansion and con- —20 °F by incorporating plasticizer into the traction of the bitumen. Furthermore, the dark- pitch to impart some flexibility. colored bituminous material will eventually bleed through an oil or oleoresinous topcoat 2.5.2. Cut-Backs (see sec. 7.2.2.3).

Pitch and asphalt may be "cut back" with 2.5.4. Emulsions suitable coal tar or petroleum hydrocarbon solvents to give coatings that may be readily Bituminous emulsions are available which cold-applied. While the straight "cut-backs" possess many of the desirable properties of both afford little protection because of their suscep- bituminous and water-base coatings. Certain tibility to cracking from hardening and shrink- asphalt and coal tar emulsions are superior in age during atmospheric exposure, they are use- weathering characteristics to their "cut-back" ful as primers on surfaces that are to receive counterparts. In fact, clay type asphalt-base hot melt coatings of the same substance. Also, emulsions, such as that included in Mil. Spec.

29 MIL-R-3472 [113], when used as roof coat- C-15203 [116]. This coating is intended pri- ings, exhibit the best durability among bitumi- marily for atmospheric exposure; it should be nous coatings. Asphalt-base emulsions for use allowed to dry for several months in good as thick protective coatings for metal are cov- weather before immersion in water. ered by ASTM D 1187-56 [114] ) ; the type A material is a quick-setting type for use where 2.5.5. References the coating is to be subjected to water immersion, while the type B material is for use on For a more detailed treatment of the subject surfaces exposed to the weather only. of bituminous materials and coatings, the reader A coal tar emulsion that may be used by itself is referred to the works of Abraham [39] and or as a topcoat over coal tar mastics or coal to the manuals [40, 41] and specifications [42, tar enamel to improve their resistance to sun- 43] of those Departments and Associations in light is covered by Military Specification MIL- which these materials have an important role.

Figure 2.1. The NBS Integrometer (center) increases the speed, ease, and precision of adhesion measurements with the Adherometer (left), by converting the variable stripping force into electric impulses and adding them to give an output that is indicated on the recorder. (Roberts, A. G., and R. S. Pizer, Protective coating adhesion measurement using an electronic averaging device for the Adherometer, ASTM Bulletin No. 221, April 1957, pp. 53-58.)

30 Figure 2.3. Explosion of cellulose nitrate film from decomposition in storage.

31 Figure 2.5. Washability machine for measuring scrub resistance of -paints as described in ASTM Method D 2U86-66T.

32 .

3. Properties of Synthetic Resins for Coatings

3.1. General Nature of Synthetic Resins

Synthetic resins are poljoneric substances of the crystallites (highly ordered regions.) high molecular weight made up of repeating While polymers may be simply characterized molecular structural units, usually in the form as crystalline (ordered) or amorphous (disor- of chains, with a terminating group at each dered) on the basis of whether or not they are chain end. As many as a hundred thousand or capable of yielding definite X-ray difi'raction more repeating units may be combined into a patterns, seldom if ever is a polymer completely single polymer molecule having a molecular one or the other. Instead, highly ordered re- weight in the millions. The most useful high gions, amorphous regions, and regions of in- polymers, however, are those with average termediate order or disorder are randomly in- molecular weights ranging from a few thou- terspersed, and the ratio of crystalline to amor- sand to several hundred thousand, correspond- phous material strongly influences the mechan- ing to a degree of polymerization (average ical properties of the polymer. This is because number of repeating units per chain) ranging the close proximity of the polymer chains in from about a hundred to several thousand. crystalline regions gives rise to large intermo- The polymer chains may be linear, branched, lecular forces which are manifest as greater or crosslinked—or some combination of these stiffness, greater tensile strength, a higher soft- —depending on the functionality and reactivity ening temperature, and increased resistance to of the monomers from which they are formed swelling and solvation in the crystalline poly- and on the manner of polymerization. What- mer as compared to its amorphous counterpart. ever the basic chain structure, the solid poly- Crystallinity in a polymer mass may be in- mer consists of a closely entangled mass of duced or extended by stretching (cold draw- kinked, coiled, sometimes folded, and variously ing.) Stretching an amorphous polymer causes oriented and ordered polymer chains. The phys- an alinement of polymer chains which may pro- ical properties of the bulk polymer are largely duce some oriented crystallites. The drawing determined by the structure of the polymer of a crystalline polymer causes the existing chains, their molecular weight (chain length), crystallites to become oriented along the axis and the nature and magnitude of the intermo- of draw and may form still more crystallites. lecular forces (van der Waals forces) which Orientation substantially increases the tensile are operative between chains in close proxim- strength of the polymer, greatly enhancing its ity. The chemical properties of the polymer de- suitability as a fiber. Typical crystalline poly- pend on the chemical nature, i.e., the reactivity mers are polyethylene, polypropylene, polyvinyl and bond energies, of the linkages between chloride, polyvinylidene chloride, polytetraflu- chain segments, and on the reactivity of side oroethylene, and linear polyamides. Among groups and end groups. typical amorphous polymers are polystyrene, acrylic resins, and copolymers in general (the 3.1.1. Polymer Structure and Prop)erties latter are amorphous because the random and irregular spacing of the component monomeric In linear polymers the molecular chains, al- species prevents the close packing of polymer though closely entangled, are separate enti- chains) ties. Hence, they are relatively free to kink (shorten), uncoil (stretch), or slip past each In crosslinked polymers, the molecular chains other (undergo plastic flow or permanent de- are tied together and their mobility is reduced formation) under the influence of applied by the many primary valence bonds between stresses; they acquire greatly increased mobil- chains. Such polymers are described as ther- ity (soften) as the temperature is raised ; and moset materials and are infusible and insoluble, they can be molecularly dispersed (dissolved) except under chemical attack or degradation. in suitable solvents. Linear polymers, there- It is evident that the degree of stiffness and as- fore, are permanently fusible and soluble, and sociated properties exhibited by a crosslinked are described as thermoplastic materials. (A material will depend on the extent of the cross- notable exception to this generalization is poly- linking. Thus, a material with few cross links tetrafluoroethylene which, though linear and between chains will deform relatively easily. classified as thermoplastic, is neither fusible The cross links serve to limit chain slippage, nor soluble in the ordinary sense.) permitting the material to exhibit rubbery or When the polymer chains have a regular mo- elastic properties; such a material may swell lecular configuration that is conducive to aline- markedly in certain solvents yet not dissolve in ment and close packing, the bulk polymer ex- them. Highly crosslinked materials, on the hibits crystalline properties to a degree deter- other hand, exhibit a high degree of stiffness, mined by the number, size, and arrangement of infusibility, and insolubility.

33 Branched polymers are those in which basic- from the following discussion of the main types ally linear molecular chains acquire long-chain of polymerization. side groups as a result of the random activation of sites along the already polymerized main 3.1.2.2. Addition polymerization.—Bifunc- chain or by the introduction of occasional tri- tionality arising from the opening of a double functional groups during the polymerization bond in an unsaturated monomeric material, process. The side chains are similar in molec- such as ethylene (CHo=CH,^ -CH0-CH2-), ular structure to the parent chain and may grow or styrene (CH=CH2->-CH2-CH2-) , affords to considerable length, with side chains of their own, so that a highly branched structure with polymers properties resembling crosslinked an opportunity for chain growth by a simple may be built up. addition process in which successive monomer radicals become attached end-to-end to form a Crosslinked or thermosetting polymers, be- linear polymer molecule, without the splitting cause of their infusible and insoluble charac- off of a by-product molecule. Occasional ran- ter, are generally superior in overall chemical dom activation of sites along a polymer chain resistance and dimensional stability to the ther- permits the growth of side chains, again by an moplastic types. The thermoplastic polymers, addition process, so that various degrees of however, offer a wide range of properties and branching may occur in otherwise linear poly- a versatility of formulation and application that mers. If the monomer is polyfunctional (func- find wide use in the coatings field. Coatings tionality greater than 2), then the addition re- based on thermosetting resins require heat and/ action will proceed between growing chains as or chemical curing agents to develop their full well as by end-to-end addition, so that a three- crosslinking and cure. Coatings based on ther- dimensional, crosslinked structure will be built moplastic resins are deposited as continuous, up. For example, the tetrafunctional p-divinyl cured films upon evaporation of the solvents in benzene CH=CH2 quickly forms a highly cross- which they are dispersed or dissolved.

Coatings based on drying oils (see section CH=CH2 2.2.1) cure by a combination of oxidation and linked, rigid, insoluble polymer which has little polymerization to form a loosely crosslinked practical use. If, however, a very small quan- and fairlj' flexible structure in which neighbor- tity of p-divinyl benzene (as little as 0.002 per- ing molecules are tied together through the recent) is incorporated into styrene monomer, active unsaturated groups occurring toward the then a lightly crosslinked polymer is formed in middle of the fatty acid radical. This process which the linear polystyrene molecules are tied oils harden is often described by which drying together through an occasional divinyl benzene as auto-oxidative polymerization. cross link. Even with so few cross links, complete dispersion of the polymer is no longer pos- 3.1.2. Functionality and Polymerization sible, so that solvents that would dissolve the Mechanisms linear polystyrene will only swell the crosslinked type. The limited degree of crosslink- 3.1.2.1, Functionality.—The type of struc- ing also raises the heat distortion point of the ture obtained when a given material or group polymer and reduces the tendency toward cold of reactants is polymerized depends on their flow, without seriously affecting the basic de- chemical reactivity and functionality. Func- sirable properties of the material. tionality may be defined as the number of active In addition polymerization the growth of in- sites (primary valence bonds) per molecule dividual chains is propagated by a free radi- available for reaction under the conditions im- cal mechanism and is very rapid—an activated posed. In order for polymerization to take monomer molecule may grow to a terminated place, the functionality of each reacting mole- polymer chain in a small fraction of a second. cule must be at least 2; that is, there must be However, complete conversion of monomer to two or more functional or reactive groups per polymer may require several hours. During molecule. A functionality of 2 results in the this time, monomer and polymer coexist while formation of linear polymers. Branched poly- the monomer material is gradually used up in mers also may be formed when the functional- the production of polymer chains. The molec- ity is essentially 2, through the random activa- ular weight obtained depends on the conditions tion of sites as discussed previously. When of the reaction and on the presence of inhibi- the average functionality of the reactants is tors, monofunctional reactants, and other chain greater than 2 (not necessarily an integer), terminating mechanisms. Careful control of crosslinked polymers are formed. The manner monomer purity, catalyst, and reaction condi- in which functionality determines the type of tions is required in order to obtain a polymer polymer structure obtained will be apparent of sufficient uniformity to be useful.

34 In a bifunctional (linear) polymerization, tively low molecular weight chains will soon there is no propagation between growing build up a three-dimensional structure and lead chains, and high molecular weights of a hun- to gelation and polymer insolubility. dred thousand or greater are readily attained. During the course of a given addition polymer- 3.1.3. Methods of Polymerization ization reaction, the average molecular weight of the polymer chains remains practically the Four different methods of polymerization are same; however, the polymer yield (i.e., the rain general use: bulk, solution, emulsion, and tio of polymer to monomer) continues to in- suspension. Each process is briefly discussed crease, and the viscosity of the monomer-poly- below. mer mixture continues to rise, until all tlie monomer is consumed. Because of the great chain 3.1.3.1. Bulk polymerization.—This is the length of addition-type polymers, the presence simplest type of polymerization process, since of even small amounts of polyfunctional sub- the reaction mixture consists of a single phase stances quickly produces a rigid, crosslinked, with nothing usually added, except catalyst insoluble polymer. or initiator if required. Thus the polymerized product is obtained in an inherently pure chem- 3.1.2.3. Condensation polymerization.— ical state that eliminates the need for subse- Condensation polymers are formed through the quent purification steps. However, the bulk chemical combination of reactive groups of bi- process is difficult to control from the stand- functional or polyfunctional molecules, with the point of obtaining a product of reasonably splitting out of a small molecule, usually water, uniform molecular weight and density. The as a by-product. Either linear or crosslinked reaction mixture soon becomes very viscous, polymers may be formed, depending on whether causing difficulties in agitation and heat re- the functionality of the reacting molecules is 2 moval. Since most polymerizations are exo- or greater than 2. The reactive groups that thermic (addition polymers, in particular, take part in polycondensation reactions are the usually have a high exotherm) , local variations same as those (e.g., hydroxyl, carboxyl, amino, in temperature and extent of reaction occur, etc.) that take part in the ordinary condensa- and the final product may vary considerably tion reactions between small molecules, as for throughout the polymerized mass. The high example when an alcohol and an organic acid viscosities encountered in bulk polymerizations react to form an ester. often limit the degree of conversion that can A polycondensation reaction proceeds at the be obtained within a reasonable length of time. same characteristic rate as the reaction be- The bulk method is used chiefly for cast plastics tween similar groups in a monofunctional re- and for in situ polymerizations in which the action. Unlike addition polymers in which long time element is relatively unimportant, so that chains form rapidly by a free radical propa- a slow but controllable reaction (perhaps at gation mechanism, condensation polymers have low temperature) is feasible. a reaction activation energy that requires time and heat for the formation of long chains. 3.1.3.2. Solution polymerization.—In the The polycondensation reaction proceeds ran- solution process, the monomeric substances domly among all functional groups, whether (with added catalyst or initiator if required) they are on a monomer molecule or on a short are dissolved in an inert liquid which may or or a long polymer chain. That is, reactions may not be a solvent for the polymer. If the between partly grown chains, between a grow- polymer is insoluble in the liquid, it precipitates ing chain and a monomer molecule, and be- out as it forms. Otherwise, when the polymeri- tween two monomer molecules are all equally zation has proceeded to the extent desired, the probable. Consequently, the monomer mole- polymer is recovered by precipitating it with cules are soon consumed in the production of a suitable nonsolvent, or less often by evapor- dimer, trimers, and short polymer chains. The ating off the solvent and residual monomer. reaction mixture now consists largely of low The solution polymerization process is more molecular weight polymer chains which con- easily controlled than the bulk process, and a tinue to grow by combining with each other more uniform product is obtained. The lower for as long as they are permitted, or until the viscosity and better heat transfer properties viscosity of the mixture increases to an extent of the solution medium permit easier agitation that limits the mobility of reactive groups and with better control of temperature and a more becomes the rate-determining factor in further uniform reaction mixture. Although applicable polymerization. to either addition or condensation polymeriza- Even when polycondensation reactions are tions, the solution method is particularly well- forced to completion, molecular weights higher suited for the condensation type of reaction. than 25,000 are seldom obtained. However, if Alkyds, phenolics, epoxies, and many other con- the reacting substances are polyfunctional, densation polymers are prepared by the solu- then cross-linking reactions between even relation method.

35 .

3.1.3.3. Emulsion polymerization.—In the temperatures employed are readily maintained emulsion process, the monomer (which is in- throughout the thoroughly agitated reaction soluble in water) is emulsified in water with mixture, and a very uniform polymer is ob- the aid of suitable emulsifying- agents. The tained. Again, the economy and nonflamma- method is used almost exclusively for addition bility of the water dispersion medium are im- polymerizations, since the moderate or low portant processing advantages. The suspension temperatures employed are unsuitable for most method, like the emulsion method, is used condensation reactions. The polymerization primarily for addition polymerizations. proceeds very rapidly in the presence of an initiator, yielding a high molecular weight 3.1.4. Types of Polymers product in finely divided form. Excellent control of the process is possible, and a very uni- The complexity and diversity of chemical form polymer is obtained. Since the polymer composition and physical structure among the is obtained in the dispersed phase, there is no many synthetic polymers that have become appreciable increase in the viscosity of the re- available has resulted in a growing descriptive action mixture as the polymerization proceeds. nomenclature by which to classify them. Be- Many difficult polymerizations and copolymeri- sides the broad distinctions already made in zations that take place very slowly or not at the preceding sections, such as addition or con- all by bulk or solution methods are readily densation polymers, linear or cross-linked, carried out by the emulsion method. The econ- thermoplastic or thermosetting, etc., we may omy and nonflammability of the aqueous dis- now consider, from the point of view of struc- persion medium are also distinct processing ture, types of addition polymers variously de- advantages. scribed as homopolymers, copolymers, hetero- The very small particle size and good uni- polymers, and stereo-regular polymers. A fa- formity of emulsion p(51ymers permits their use miliarity with the basic structure and proper- (after removal of processing agents where ties of the materials described by these terms necessary) directly in the manufacture of latex will be of value to those in the coatings field paints, plastisols (vinyl dispersion resins), and who are interested in the potential usefulness special molding powders. of recent types of polymers.

3.1.3.4. Suspension polymerization.—This 3.1.4.1. A homopolymer is formed from a is the newest method of polymerization. The single monomer, so that all the structural re- polymer is obtained in the form of tiny beads peating units in every chain are identical. All or pearls that are readily washed free of pro- the polymer chains are therefore similar in cessing contaminations. The suspension poly- chemical composition, but they may vary in merization process is somewhat similar to an chain length and structure (and in the associ- emulsion system in that there is a dispersed ated physical properties), depending on the phase and a dispersion medium, usually water. degree and type of process control that has However, no emulsifying agent is used and an been exercised. Typical homopolymers are emulsion does not exist. Instead, monomer polystyrene and poly (methyl methacrylate) droplets (much larger than those in an emulsion system) are mechanically dispersed in the 3.1.4.2. A copolymer is formed from two water medium by vigorous agitation. The drop- (or more) different monomers. The definition lets remain suspended only so long as the agita- of copolymer sometimes includes the stipula- tion continues. A suitable suspension agent tion that each of the constituent monomers (e.g., an insoluble inorganic salt or oxide, or a must be capable of undergoing polymerization soluble organic polymer) is usually used to by itself. In this school of thought, if one (or prevent the coalescence of partly polymerized more) of the monomeric constituents is itself particles and to help control the particle size incapable of polymerization (although it will developed. polymerize in the presence of the other, true

The mechanism of suspension polymerization monomer) , then the resulting polymer is termed is analogous to that of the bulk method rather a heteropolymer. The author believes that than the emulsion method. Each monomer the distinction made between "copolymer" and droplet or bead acts, in effect, as a small, dis- "heteropolymer" tends to create more confu- crete bulk polymerization system. Hence, only sion than order, and finds that the term "hetero- those substances capable of bulk polymerization polymer" is not favored in the writings of most can be polymerized by the suspension method. leaders in the high polymer field. However, whereas the bulk process is very diffi- In ordinary copolymers, the comonomers are cult to control, excellent control of the sus- combined into the polymer chain randomly pension polymerization process is possible. As rather than in a regular sequence, so that in the emulsion method, there is virtually no different chains vary in chemical composition increase in the viscosity of the reaction mixture as well as in length and structure. Practically, with polymerization. The moderate or low however, it is the average composition that

36 . largely determines the bulk properties of the a few organic solvents and be virtually insoluble polymer produced. The average copolymer in water. The block copolymer of a hydro- composition may be expressed in terms of the phobic and a hydrophilic monomer, on the other over-all ratio of one component to the other, hand, would tend to have a dual character and and it depends on the relative amounts and show solubility in both organic and aqueous reactivities of the comonomers present in the media. Such a block polymer would be expected reaction mixture. To achieve a uniform aver- to make a good emulsifying agent. Block co- age composition throughout a particular copolymers can be prepared either by addition polymerization, it is necessary to add appropri- polymerization (e.g., poly (ethylene-propylene ate amounts of the faster reacting monomer as oxide) ) or by a polycondensation reaction the reaction progresses in order to maintain (e.g., epoxy-phenolic copolymer). the original reaction environment. Through copolymerization it is possible to 3.1.4.4. A GRAFT POLYMER is a Special type of obtain polymers with properties intermediate branched copolymer consisting of a linear main between those of the individual homopolymers. chain of one monomeric repeating unit with Homopolymers that cannot be mechanically side chains of another. It is made by grafting blended because of incompatibility often have side chains composed of one polymeric species compatible monomers that can be successfully onto a main chain of another polymeric species. combined through copolymerization. It should Since each copolymer molecule thus has a dis- be pointed out, however, that the properties tinctly dual structural character, graft poly- of a copolymer may be very different from those mers (like the block polymers described above) of a physical mixture of the two homopolymers tend to exhibit the properties of each polymeric in the same proportion. For instance, a ran- constituent rather than a single set of inter- dom copolymer, under any given set of condi- mediate properties. tions, behaves as a single compositional entity, Graft copolymers are prepared by dissolving whereas each component in the physical mix- the already polymerized main polymer in the ture may "go its own way." monomer to be grafted onto it. The grafting A good example of copolymerization is that reaction usually proceeds by an addition TJrocess of vinyl chloride and vinyl acetate. Vinyl at active sites (double bonds, free radicals, etc.) chloride alone forms a hard, horny, difficultly activated or induced by heat, catalysis, or radia- soluble homopolymer, while vinyl acetate alone tion. Graft polymers that have been success- forms a soft, tacky, easily-soluble homopoly- fully prepared include grafts of vinyl acetate mer. Copolymerization of the two monomers onto poly (vinyl alcohol), polyethylene onto in various ratios yields a series of copolymer poly (vinyl acetate), and styrene onto buta- resins with highly useful intermediate chemical diene-styrene copolymer. and physical properties (see sec. 3.6.3). 3.1.4.5. Stereoregular polymers are a recent development in linear, addition polymers 3.1.4.3. A BLOCK POLYMER is a special type that followed the discovery of solid, hetero- of copolymer in which the different monomeric geneous-type catalysts capable of exerting a repeating units, instead of occurring randomly highly stereospecific effect on the polymeriza- along the polymer chain as in ordinary co- tion of olefin-type monomers, such as ethylene polymers, occur in relatively long sequences and propylene. It is thought that the catalyst, (blocks) of one constituent alternating with through a highly specific selectivity for par- relatively long sequences of the other. The ticular isomeric forms, activates only certain alternating runs or blocks of each repeating sites in the monomer molecule in that unit may be of varying length and are usually a manner leads to an orderly polymerization in which the linked together in linear fashion (although the monomer molecule always adds to the growing possibility of making crosslinked block poly- chain in the same steric configuration. mers, through the use of polyfunctional mono- The resulting polymer chains have a predetermined, mers, exists). regular structure that permits close packing Because they contain long runs of each con- with formation of a high-density, crystalline stituent monomer, block copolymers tend to polymer. Such a polymer possesses a higher exhibit the chemical properties of each con- softening point, greater mechanical strength, stituent rather than a single set of intermediate and greater chemical resistance than the anal- properties as in random copolymers, where the ogous amorphous polymer (see sec. 3.13.1 on closer alternations of the different monomeric Polyethylene) units impart a pseudo-homogeneous character to the polymer. Thus, a random copolymer of 3.1.5. Classification of Synthetic Resins a hydrophobic and a hydrophilic monomer would ordinarily exhibit an intermediate, less- Synthetic resins may be classified in various ened hydrophobic or hydrophilic character, de- ways : according to chemical or structural type, pending on the average monomer ratio ; for manner of formation, properties, applications, example, it might have only limited solubility in or other criteria, depending on the objective to

37 . be served. Categories such as condensation are possible even with only a limited number and addition polymers, thermoplastic and ther- of basic resins. Nevertheless, it is possible to mosetting resins, linear and cross-linked sub- predict much about the properties and potential stances, etc. were discussed in the preceding performance of a given coating material from section. Such information is of basic useful- a knowledge of the properties of the basic in- ness in understanding the nature of synthetic gredients, particularly the film-former, of which resins and the relationships between their com- it is composed. position, structure, and properties. It is neces- The following main classes of synthetic resins sary now to consider these materials from the are discussed in the sections of this chapter des- point of view of their selection for coatings ignated below: applications. For this purpose they will be discussed under headings that refer to the com- Section monly accepted designations for these classes of resins. Broad structural generalizations 3.2. Cellulose derivatives that would lead us, for example, to classify 3.3. Phenolic resins polystyrene, acrylic, and other familiar resins 3.4. Alkyd resins as vinyl polymers will be avoided. 3.5. Amino resins Only the main types of resins that are im- 3.6. Vinyl resins portant or promising for coatings applications 3.7. Acrylic resins can be discussed in the space available. It 3.8. Polystyrene should be borne in mind that, in general, a com- 3.9. Silicone resins bination of resins will yield a material with 3.10. Epoxy resins intermediate or combined properties. These 3.11. Polyamide resins properties may be further altered upon formu- 3.12. Polyurethanes lation of the resin into a coating material. The 3.13. Polyethylene ultimate properties of the coating are deter- 3.14. Fluorocarbon resins mined also by the manner in which the film is 3.15. Synthetic rubbers deposited and cured. Thus an almost infinite 3.16. Water soluble resins variety of coatings formulations and properties 3.17. Miscellaneous resins and information

3.2. Cellulose Derivatives

The chief sources of cellulose for the manu- ity in particular solvents and in their softening facture of "cellulosic" coating materials are temperatures, depending on chemical type, de- wood, cotton, and cotton linters. Although gree of hydroxyl substitution, molecular weight cellulose itself is insoluble except in a few spe- and viscosity. Some are widely soluble in or- cialized reagents (e.g., cuprammonium solu- ganic solvents, others only in water. Softening tion), it can be converted by suitable chemical points may range from about 285 to 570 °F treatment, involving esterification or etherifica- (about 140 to 300 °C). Cellulose nitrate is an tion of the hydroxyl groups with or without exception in that it decomposes (sometimes subsequent hydrolysis, into a variety of useful, with explosive violence) when heated, before soluble cellulose derivatives, including cellulose reaching a melting temperature. esters, cellulose mixed esters, and cellulose The cellulose derivatives, being thermoplas- ethers. Through control of the degree of sub- tic, form films from solution by simple evapora- stitution (esterification or etherification) and of tion of solvent. As a group, they are tough, the processes of hydrolysis and degradation, horny materials that must be plasticized to the chemical composition and the molecular achieve adequate flexibility and adhesion for weight of the cellulose derivative can be con- coating purposes. They are fast drying because trolled to give the desired solubility, viscosity, they release easily the quick-evaporating liq- and mechanical, physical and chemical prop- uids in which they are soluble. It should be noted, diluents higher- erties. however, that both and boiling solvents are used along with the low- The most important and widely used cellulose boiling active solvents in their formulation into derivatives are the esters, particularly cellulose lacquers, to control the evaporation rate and nitrate and cellulose acetate. Cellulose acetate- solution viscosity (see sec. 5.3.2 on Use of butyrate is the most widely used mixed ester Thinners) and finds important use in the coatings field.

Ethyl cellulose is the most widely used of the 3.2.1. Cellulose Esters cellulose ethers. The cellulose derivatives are thermoplastic 3.2.1.1. Cellulose nitrate (or nitrocellu- materials and therefore are permanently solu- lose, as it is often but incorrectly called) is the ble and heat-softenable. The different cellulose oldest of the cellulose derivatives, its prepara- derivatives differ considerably in their solubil- tion having been reported by Bracconot [44]

38 in 1833. It was not until 1923, however, that ture resistance and dimensional stability. It is a cellulose nitrate lacquer was developed with resistant to weak acids and alkalis but is de- the durability and fast-drying qualities that composed by strong ones. revolutionized the automobile finishing indus- Pure, dry cellulose nitrate is a dangerously try by cutting the finishing time from weeks to flammable material. It is therefore handled hours. Shortly thereafter, the furniture indus- and shipped only in a wet state. For shipping, try adopted cellulose nitrate lacquers, and soon it is usually wet with alcohol as 30-35 percent its use had spread to a wide variety of products. of the total weight. After compounding with To this day, as the film-forming base in most other ingredients, the flammability of the final quick-drying lacquers, and as a versatile mate- film is greatly reduced but still high compared rial that is widely compatible with other resins, to other film-forming substances. oils, and plasticizers, cellulose nitrate is exten- Although cellulose nitrate is available in vis- sively used in the coatings field. cosity grades ranging from about 0.05 second Cellulose nitrate is produced by the nitration (20 centipoises) to 1000 seconds or more [45], of purified cellulose with a mixture of nitric the majority of lacquers utilize a 14-second or and sulfuric acids. The degree of nitration ob- a i/o-second material as a good compromise be- tained is determined largely by the ratio of the tween tensile strength and solubility require- mixed acids. The fully nitrated product (con- ments. For airplane dopes, where extra tensile taining 14.14 percent nitrogen) is insoluble and strength and the fabric-tautening property are unstable. Soluble cellulose nitrate for coatings necessary, cellulose nitrate of higher viscosity purposes [45] usually has a nitrogen content (5-20 seconds) has been found to give the best between 11.5 and 12.2 percent. The solubility results, as reflected in Government specifica- also depends on the molecular weight or vis- tions for such materials [45, 46]. cosity of the cellulose ester. Although strength While cellulose nitrate has been combined and durability improve with increasing molecu- with many different resins to produce useful lar weight and increasing viscosity, the solu- coatings, perhaps the most outstanding have bility becomes poorer. To secure the good been those in combination with alkyd and amino solubility and low solution viscosity needed for resins to produce tough, hard, durable lacquers practical high-solids coatings formulations, the capable of withstanding the severe service re- cellulose nitrate as first produced is subjected quirements of automotive, aircraft, and other to controlled degradation by digestion in water industrial finishes. Successful coatings have under heat and pressure until the molecular also been based on blends of cellulose nitrate weight has been reduced to the desired value. with natural resins and drying oils, resin de- Thus the viscosities of commercial cellulose rivatives, phenolic resins, acrylic resins [138], nitrates represent a compromise (as with many certain vinyl resins, and others. Applications processes) between the conflicting requirements include coatings for metal, wood, paper, fab- of product formulation and product perform- rics, leather, and cellophane. These are avail- ance. Both nitrogen content and viscosity must able in a wide variety of colors and surface be controlled within fairly narrow limits to textures. A recent successful application for yield a material that is sliflftciently uniform for cellulose nitrate has been as the film-forming coatings formulation; otherwise, less soluble base in multi-color finishes (see sec. 2.3.7). portions may not dissolve completely, while very low molecular weight portions may reduce 3.2.1.2. Cellulose acetate, although used in tensile strength. To obtain a material of speci- large quantities in the plastics and textiles fied average viscosity, it is common practice to fields, finds only limited use in the coatings blend different batches of cellulose nitrate that field because of its poor solubility in many com- are not too widely apart in this property. monly used organic solvents, its limited com- Cellulose nitrate itself is a tough, horny patibility with plasticizers and resins, and its material that soon discolors, embrittles, and relatively high moisture absorption which re- decomposes on exposure to heat or sunlight. sults in poor dimensional stability. However, However, when compounded with suitable plas- it is resistant to discoloration and degradation ticizers, resins, stabilizers, and pigrnents in a by heat and ultraviolet light, has a high soften- properly formulated solvent vehicle it yields a ing temperature (approximately 446 to 572 °F very wide variety of useful and durable coat- [230-300 °C], depending on the composition ing materials. Cellulose nitrate is soluble in and molecular weight) , and is relatively slow- esters, ketones, nitrohydrocarbons, and ether- burning. alcohols. Alcohols alone swell and soften it but Cellulose acetate is manufactured by acety- do not dissolve it (an exception is a special lating cellulose with acetic anhydride and gla- alcohol-soluble grade containing 10.7 percent cial acetic acid in the presence of sulfuric acid

nitrogen) . It is insoluble in aliphatic and aro- as a catalyst. The fully acetylated material matic hydrocarbons but will tolerate large pro- (cellulose triacetate) thus obtained is virtually portions of these diluents in the presence of insoluble except in chloroform and a very few suitable active solvents. It has very good mois- other solvents. It is therefore partially hydro-

39 lyzed to yield more soluble materials of wider of the most desirable properties of each con- usefulness. Various commercial grades are stituent. The ratio of acetyl to propionyl or available for particular applications. These to butyryl can be varied and the degree of es- differ in degree of hydrolysis (or esterifica- teriflcation (or hydrolysis) can be controlled tion), molecular weight, and viscosity; there- to produce a wide range of materials. As with fore, they differ in solubility, softening point, other polymeric materials, the physical and tensile strength, and other properties. mechanical properties also depend on molecular Solvents useful with cellulose acetate include weight and viscosity. acetone, methyl ethyl ketone, methyl acetate, ethyl acetate, dioxane, and the glycol-ethers. 3.2.2.1. Cellulose acetate-butyrate com- Methyl alcohol and ethyl alcohol are good co- bines the good light and heat resistance of cel- solvents in combination with methylene chlo- lulose acetate with the better moisture resist- ride, ethylene dichloride, nitromethane, and ance and dimensional stability of the butyrate. 1-nitropropane. The number of plasticizers The butyryl radical imparts to the mixed ester useful with cellulose acetate is very limited, greater flexibility, better solubility, and wider and a careful choice for particular applications compatibility than is obtainable with the ace- is necessary. Methyl- and ethyl phthalyl ethyl tate alone. Cellulose acetate-butyrate requires glycollate are generally useful, and a few much less plasticization than cellulose acetate phthalates, phosphates, and citrates are of lim- and is compatible with a much broader range ited usefulness as plasticizers. Among the few of plasticizers and resins. It is soluble in ke- resins with which cellulose acetate is fairly tones, esters, chlorinated hydrocarbons, and compatible are certain phenolics, alkyds, and nitrohydrocarbons. Alcohols and aromatic hy- ester gums. drocarbons are well-tolerated diluents. Be- Cellulose acetate is resistant to everyday cause of its versatility of composition, moisture chemicals such as dilute acids, animal fats, resistance, dimensional stability, and weather- vegetable and mineral oils, gasoline, and ethyl ing resistance, cellulose acetate butyrate finds alcohol. It is decomposed by strong acids and considerable use in both clear and pigmented by alkalis. It is used in lacquers for paper, lacquers for metal, wire, paper, plastics, wood fabrics, wood, metal and wire, particularly and fabrics. Its slow-burning characteristics where good resistance to heat and light is combined with good tensile and tautening prop- required. erties and excellent outdoor durability have been the basis for its use by the Navy and Air 3.2.1.3. Cellulose propionate is prepared Force as a dope [47, 48, 49] for the fabric sur- by the esteriflcation of cellulose with propionic faces of military aircraft (training planes and anhydride, in a manner similar to the prepara- slower transport planes) in place of the hazard- tion of cellulose acetate. The larger propionyl ously flammable cellulose nitrate dope. Cellu- radical yields a softer, lower-melting material lose acetate butyrate has been notably success- than does the acetyl radical. Cellulose propio- ful in the formulation of strippable coatings, nate is more soluble and requires much less hot melt lacquers, and gel lacquers [158]. plasticization than the corresponding acetate. 3.2.2.2. Cellulose acetate-propionate, al- It is tough, shock-resistant, heat-stable, and its utility as a film-forming base has weather-resistant. It has only recently become though study commercially available. Although at present been established through a comprehensive has been used to only a limited extent in used chiefly as a plastics molding powder, it has [50], the coatings field. This is largely because of found some application in strippable coatings the earlier and adequate availability of cellu- (sec. 2.2.7) and gel lacquers [158]. lose acetate-butyrate, and the similarity in properties of the two mixed esters, which has 3.2.1.4. Cellulose butyrate can be made by it to manufacture the acetate- processes analagous to those for the acetate and made uneconomic the propionate. The bulkier butyryl radical propionate on a large scale. results in a still softer, lower-melting, and more 3.2.3. Cellulose Ethers soluble material that is not in itself very useful. However, the properties of the butyryl In general, cellulose ethers are prepared by radical excellent have been utilized to advan- treating alkali cellulose with an appropriate tage in the coatings field by combining butyryl etherifying agent, usually an alkyl chloride, with acetyl in the manner described under under heat and pressure. By control of the Cellulose Mixed Esters. reaction conditions, it is possible to control the degree of substitution and the viscosity, upon 3.2.2. Cellulose Mixed Esters which the physical and chemical properties of the cellulose ether depend. By esterifying cellulose with a mixture of acetic and propionic or butyric anhydrides, 3.2.3.1. Ethyl cellulose finds extensive use mixed esters can be made that combine many in the coatings field [51]. It is tough, flexible

40 .

at low temperatures, and stable to heat, light, emulsion paints and as a thickener in paint and weathering, particularly when properly removers. plasticized and stabilized. Its wide compatibility with plasticizers, resins, gums, and waxes 3.2.3.3. Carboxymethyl cellulose as usu- permits an almost endless variety of useful ally manufactured is a water-soluble cellulose formulations. It is completely compatible with ether that is insoluble in organic solvents. It cellulose nitrate and is often used to augment has poor resistance to acids and alkalis. Its the properties of cellulose nitrate lacquers [52] aqueous solutions are highly viscous, making it Though widely soluble in organic solvents, ethyl a useful thickening and stabilizing agent for cellulose is resistant to alkalis, dilute acids, and latex paints. It finds some use also as a fabric aliphatic hydrocarbons. Although somewhat finishing agent and as a grease-proofing coating permeable to water vapor, it is low in water for paper, leather, and other materials. absorption and has good dimensional stability. Its electrical properties (insulative, dielectric, 3.2.3.4. Hydroxyethyl cellulose, as usu- etc.) are excellent. ally prepared, is a water soluble product. It Ethyl cellulose is used extensively in coatings is very resistant to organic solvents, oils, and for wire, paper, fabric, leather, and plastics. greases. It has fair resistance to dilute acids It is also used in coatings for metal, wood, and alkalis. It finds some use in coatings for glass, and ceramics. Hot-melt strippable coat- paper, fabrics, and leather where a grease- ings (sec. 2.2.7) for metal parts are readily proof or water-soluble coating is desired. Its formulated with ethyl cellulose. chief use in the coatings field is as a protective colloid in latex polymerization and as a thick- 3.2.3.2. Methyl cellulose can be produced ening agent and stabilizer for latex paints. with a wide range of solubility properties, from solubility in alkalis to solubility in organic sol- 3.2.3.5. Benzyl cellulose resembles ethyl vents, depending on the degree of methylation. cellulose in general properties but is softer and However, the usual commercial product has a lower in melting point because of its bulkier degree of substitution of about 2 and is soluble molecular structure. It is widely soluble in in cold water but not in hot. The water soluble organic solvents and is compatible with most product has very poor acid and alkali resistance common resins and plasticizers. However, it but is insoluble in organic solvents and has ex- has found only limited use in the United States, cellent resistance to oils and greases. An im- where ethyl cellulose has met with far more portant use has been as a grease-proof coating favor. The chief uses for benzyl cellulose are for paper. Methyl cellulose is used extensively in chemical-resistant coatings and electrical as a thickening agent and protective colloid in insulating coatings.

3.3. Phenolic Resins

Phenolic resins were the first truly synthetic ing characteristics to these resins also. resins to become commercially available. They Phenolic resins have a yellow color which were introduced to the coatings field by Baeke- tends to darken on aging; hence they are not land about 1909 [53]. Their characteristic fast usually employed in white finishes. The dark- dry, moisture resistance, toughness, and dura- ening tendency is greatest with the resin- bility, and their reasonable cost have led to modified phenolics and least with the para- their wide use in coatings. substituted pure phenolics. In general, phe- Phenolic resins are made by the condensation nolic resins have good resistance to acids and of a phenol with an aldehyde. A variety of to weak bases but poor resistance to strong types is available, depending on the nature and alkalis. For use as coatings under the weakly proportions of the phenol and aldehyde, the alkaline conditions encountered in ordinary en- type of catalyst employed, and the reaction vironments and on common alkaline surfaces conditions. Formaldehyde, because of its high such as plaster or concrete, phenolic resins are reactivity and low cost, is by far the most used very satisfactory. aldehyde. Besides phenol itself, many substi- Phenolic resins used in the coatings field may tuted phenols have been of wide commercial be classified on the basis of their composition importance. Both heat-reactive and non-heat- and properties into the following main cate-

reactive types of phenolic resins are available. gories : Heat-reactivity, with the associated capability (1) Modified phenolic resins, oil-soluble for cross-linking and insolubilization, is favored (2) Pure phenolic resins (100% phenolic) by high proportions of formaldehyde and by a. Oil-soluble, thermoplastic alkaline catalysts. Acid catalysts generally b. Oil-soluble, heat-reactive yield thermoplastic resins (novolacs) that are c. Alcohol-soluble, oil-insoluble, ther- permanently fusible and soluble, although a mosetting high formaldehyde ratio can impart cross-link- (3) Phenolic dispersion resins.

41 Each of the above types is briefly discussed in either thermoplastic or heat-hardening types. the sections which follow. Alternatively, by using other phenols and suitable phenol-formaldehyde ratios and appropriate catalysts, oil-insoluble but alcohol-soluble 3.3.1. Modified Phenolic Resins, Oil-Soluble resins can be formed which are completely thermosetting. The pure phenolic resins as a coatings The first phenolic resins used in the group are characterized by excellent exterior phenol-formaldehyde field were of the modified durability and outstanding water, solvent, and or its glyc- type, the usual modifier being rosin chemical resistance. Although better than the (ester The modification was eryl ester gum). modified phenolics in color stability, they still phenol-formaldehyde necessary to render the tend to yellow and hence are not usually used oils. The use of these resin soluble in drying in white finishes. Another limitation is their phenolic resins in varnishes in place modified relatively poor adhesion; this is usually over- maleic of the natural resins, ester gum, and come by combining them with small portions of other resins used up to that time resulted in a modified phenolic. Drying properties range properties varnishes with much faster drying from slow to very fast, with most of the pure alkali resist- and greatly improved water and phenolics in a fast-drying category. ance and durability. Other phenolic resins oil-soluble by modification are those rendered 3.3.2.1. Oil-soluble, thermoplastic, pure 3,5-xylenol. It may be based on m-cresol and phenolic resins are made by condensing ap- rule, it is the meta-substituted noted that, as a proximately equimolar proportions of a para- that must be modified to phenol derivatives substituted phenol with formaldehyde in the render oil-soluble. them presence of an acid catalyst. This is the type highly resistant and durable Though not as of phenolic resin most used in air-drying var- the pure phenolic resins that were developed as nishes and oleoresinous paints. Such a resin is phenolic resins are lower in cost, later, modified covered by Fed. Spec. TT-R-271 [119]. Such better adhesion properties, and are very have phenolic varnishes [135, 137] and paints have important coating appli- well-suited for many excellent outdoor durability and are used ex- are widely used for interior cations. They tensively in structural and marine applications drying, water and applications where fast such as bridges, locks and dams, and storage durability are essential, alkali resistance, and tanks. They find use also as exterior trim floor varnishes. as in furniture finishes and paints, automotive primers, and appliance coat- primer-sealers for plaster and They make good ings. Some of the finest exterior spar var- considerable use in long-oil wood. They find nishes are made with this oil-soluble, thermo- such as spar varnishes, and exterior coatings plastic type of pure phenolic resin. (For more fast- porch and deck enamels, and in short-oil, information on phenolic paints and varnishes, traffic paints. As already mentioned, drying, see sec. 2.3.2.2.) modified phenolics are not ordinarily used in white finishes because of their strong yellowing 3.3.2.2. Oil-soluble, heat-reactive, pure tendency. Their electrical properties (insula- phenolic resins are made by condensing a para- tive, dielectric, etc.) are poor. substituted phenol with formaldehyde in the In addition to being very useful in them- presence of an alkaline catalyst, using a higher selves, modified phenolic resins are of general proportion of formaldehyde (about 2 moles of usefulness in upgrading the drying speed, formaldehyde to one mole of substituted phe- water resistance, and durability of lower cost nol) than is used in making the thermoplastic coating materials. Also, they are frequently resins. The condensation reaction is inter- used to improve the adhesion properties of the rupted at an intermediate stage at which the otherwise superior pure phenolic resins. resinous condensate is still oil-soluble though reactive. The partly polymerized resin is sub- 3.3.2. Pure Phenolic Resins (100% Phenolic) sequently heat-reacted with drying oils to produce varnishes with very rapid bodying proper- Pure phenolic resins (or 100% phenolic res- ties. Short-oil industrial baking varnishes of ins, as they are often called) are among the this type make excellent insulating and impreg- most chemical-resistant and durable resins in nating varnishes for electrical equipment be- the coatings field. They have been available cause of their fast, deep-curing characteristics since about 1928. The oil-soluble types are and their excellent insulative and dielectric made by the condensation of para-substituted properties. Even small proportions of an oil- phenols with formaldehyde. Occupation of the soluble, heat-reactive type of pure phenolic highly reactive para position by the substituent resin have a marked effect in increasing the group reduces the reactivity (and crosslinking drying speed, water resistance, hardness, and tendency) of the molecule sufficiently to render durability of other, lower grade resin formula- it soluble in drying oils without rosin modifications. This "upgrading" function is an impor- tion. These oil-soluble phenolic resins may be tant use for this type of phenolic resin.

42 ,

3.3.2.3. Alcohol-soluble, oil-insoluble, vents, foodstuffs, and chemicals. They find THERMOSETTING, pure phenolic resins are based wide use on military equipment [125, 136] such on phenols other than the oil-soluble para- as ammunition boxes, aircraft parts, water and substituted types. The condensation reaction gasoline tanks, and food containers. They are between the oil-insoluble phenol (usually a used for steel structures and fabricated parts meta-substituted phenol or phenol itself) and subject to severe atmospheric exposure. And formaldehyde is carried out in alkaline solution they have proved durable as coatings for sub- at a fairly high formaldehyde ratio that im- merged steel parts and for buried iron pipe. parts a high degree of reactivity to the resulting resin. The reaction is stopped at an inter- 3.3.3. Phenolic Dispersion Resins mediate stage of polymerization while the resin is still alcohol-soluble and reactive. The par- Phenolic dispersion resins are highly polym- tially polymerized resin is supplied as a solution erized phenolic resin-oil complexes dispersed in in an organic solvent. The applied coating, volatile organic solvents. They are non-oxidiz- when heated, converts to a completely polym- ing and dry quickly by evaporation of solvent erized, thermoset, insoluble resin film that is to form highly weather-resistant films that have highly resistant to corrosion, abrasion, and a high degree of impermeability to moisture, chemical attack. excellent resistance to water and dilute alkali, In resistance and durability, the alcohol- good adhesion to metals, and good resistance to soluble, thermosetting phenolic resins are supe- strong solvents such as alcohols, ketones, esters, rior to other types of pure phenolic resin. The and ethers. Because of their resistance to such solvents, they make good non-lifting primers completely thermoset resin is highly resistant under lacquers based on vinyls cellulose to organic solvents, organic acids, mineral acids and on derivatives. Although useable alone, they are (except strong oxidizing acids such as nitric) usually blended with an alkyd or other varnish and atmospheric corrosion. Special types re- resin. The military specification for zinc chro- sistant even to strong alkali also are available. mate primer for aircraft use [54] calls for a The thermosetting phenolics may be cured at vehicle based on a phenolic dispersion resin room temperature by adding amine-type or blended with an alkyd resin. Phenolic disper- other alkaline catalysts; however, maximum sion resins find use in quick-drying industrial chemical and solvent resistance is obtained with primers and undercoats, as shop coats for struc- heat curing. tural and fabricated steel parts, in traffic paints Extensive use is made of the thermosetting where they contribute fast dry and durability, pure phenolic resins in coatings for the insides in deck and floor coatings, and in sealers for of cans, drums, and tanks carrying organic sol- wood, paper, and wallboard.

3.4. Alkyd Resins

Alkyds are the most important and most oils (see sec. 2.2.1). Pentaerythritol is the widely used resins in the coatings field. In ver- most important of the lesser used polyhydric satility of formulation and application, in over- alcohols ; it yields resins that are faster setting all economy, and in general durability, they are and more durable than glyceryl alkyds, but unexcelled. They have been aptly character- which are more expensive to manufacture and ized as "the work-horse of the coatings indus- formulate. Isophthalic, maleic, sebacic, and try". Alkyd resins have been available com- other polybasic acids also find use in alkyd resin mercially since 1927. manufacture. Long chain dibasic acids, such as sebacic or adipic acids, yield soft resins that 3.4.1. Composition and Properties of make good polymeric plasticizers for harder Alkyd Resins materials. The unmodified glyceryl phthalate resin is a 3.4.1.1. An alkyd is a synthetic resin made hard, brittle solid of only limited solubility. by the esterification of a polyhydric alcohol Through reaction with fatty acids or oils at the with a polybasic acid. Although a wide variety time the alkyd resin itself is being formed in of resinous products of polyhydric alcohols and the cooking kettle, a series of modified alkyd polybasic acids is included in the general term resins having a wide range of specific desired "alkyd", the most important group (and the properties intermediate between those of the type generally meant when speaking, without resin and the oil can be produced. The resin other qualification, of an alkyd resin) are the can be prepared by either bulk or solution po- oil-modified glyceryl phthalate resins. These lymerization methods (see sec. 3.1.3) . In either are the reaction products of glycerol and case the cooking or refluxing is continued until phthalic anhydride (phthalic acid), modified the viscosity and acid number of the reaction with either drying, semi-drying, or non-drying mixture reach a predetermined value (when the

43 resin is close to the gel stage), whereupon the widely used in alkyd resin modification because mixture is quickly cooled to stop the reaction. of its good non-yellowing properties, ready The resin is usually supplied as a solution in an availability, and low cost. Good brushability, organic solvent. Long-oil alkyd resins are surface wetting and penetration, flexibility, soluble in mineral spirits; short-oil alkyds re- and elasticity are favored by a long oil length. quire stronger solvents such as xylene or other Short-oil alkyds have the best baking proper- aromatic solvents ; the unmodified resin is sol- ties, greatest hardness and chemical resistance, uble only in strong solvents such as ketones highest gloss and chalking resistance, and best and esters and other oxygenated solvents. Sev- general durability. Intermediate properties for eral types of long- and medium-oil alkyd resins specific applications may be obtained with me- are covered by Fed. Spec. TT-R-266 [120]. dium oil lengths. Solubility in polar solvents (e.g., ketones and esters) and compatibility Oil-alkyds may be further modified with a with polar resins (e.g., cellulose nitrate and variety of other resins to achieve certain prop- urea-formaldehyde) is greatest at the higher erties. In general, most of these doubly-modi- alkyd ratios (short oil length). With increas- fied resins (amino resins excepted) tend to have ing oil length, the solubility and compatibility a darker color than the straight oil-alkyd. with polar solvents and resins decreases, and Some resins can be incorporated into the alkyd the alkyd resin becomes increasingly compat- during the original cooking process ; for exam- ible with less polar resins (e.g., polystyrene and ple, a modified natural resin like congo ester to chlorinated rubber) and increasingly soluble impart faster drying properties, or a rosin- in nonpolar solvents, first in aromatic hydro- modified phenolic resin to improve water and carbons such as xylene and finally even in ali- alkali resistance—although the exterior dura- phatic hydrocarbons such as mineral spirits. bility may be lessened by such modification. Or An alkyd need not be greater in oil length than the alkyd, particularly non-drying types, may 65 gallons, since beyond this point the modified be physically blended with tough resins such as alkyd becomes freely compatible with drying a vinyl chloride-acetate [55] or cellulose nitrate oils, and oil can be added as required for par- [52] to plasticize them and to improve their ticular applications. Long-oil alkyd resins for gloss and chalk resistance. Alkyds may also metal or wood priming paints often have some be chemically combined with reactive monomers raw linseed mixed in to promote good wetting such as styrene or a silicone to obtain copoly- and penetration. mers with intermediate properties. Non-drying (non-oxidizing) alkyds can be cured to 3.4.1.3. Uses of alkyd resins.—Huge quan- exceptionally durable films by baking with tities of oil-modified glyceryl phthalate alkyds resins (sec. such as urea- or mela- amino 3.5) are used in interior and exterior enamels, archi- air-drying alkyds re- mine formaldehyde. The tectural paints, wall paints, automotive finishes, quire use metallic driers as in ordi- the of used appliance enamels, industrial finishes, and ma- nary drying oil paints in order to dry properly rine coatings. Long-oil, air-drying alkyds have of time. within a reasonable length very good elasticity and exterior durability, making them suitable for architectural trim 3.4.1.2. Effect of oil-modification.—Oil- paints (trim and trellis paints), garden furni- modified alkyd resins may be either air-drying ture coatings, porch and deck enamels, and au- or baking types, drying or non-drying (oxidiz- tomotive refinishing enamels. In marine appli- ing yellowing non-yellow- cations, alkyd finishes are often used above the or non-oxidizing) , or ing, weather- or chemical-resistant, flexible or water line for their good durability, while a hard, etc., depending on the type of oil and the phenolic varnish is used below the water line oil length. Drying, semi-drying, and non-dry- because of its better water resistance. Between ing oils (see sec. 2.2.1) may be used, singly or the high and low water lines a vinyl-alkyd coat- together, in either long, medium or short oil ing (sec. 4.3.1.2) gives good service. The long- lengths. The best air-drying properties are oil alkyds also make excellent interior wall obtained with long oil alkyds made with good paints, either flat or gloss, that brush on easily drying oils such as linseed and dehydrated cas- and provide a long-lasting finish. Pentaeryth- tor, although air-drying alkyds of medium oil ritol type alkyds have fast air-drying charac- length also are available. The palest color and teristics despite a long oil length; although best color-retention are obtained with non-dry- more expensive than glyceryl phthalate alkyds, ing oils such as cocoanut and castor; however, they are more durable and make excellent white these non-drying alkyds must be blended or architectural trim paints and marine enamels. baked with a film-forming resin (e.g., ethyl cel- Medium-oil and short-oil alkyds are often lulose or urea-formaldehyde) in order to form combined with other resins to make fast air- a suitable film. Excellent intermediate drying drying or baking-type colored finishes for toys, and color-retention properties are achieved by tools, and sundry items. Resin-and-oil modified use of a semi-drying oil such as soybean or alkyds find use in corrosion resistant primers safflower soybean oil, in particular, is very for metal ; for example, a phenolic-modified oil- ;

44 alkyd employing zinc yellow pigment is the 3.4.2.2. SILICONE-ALKYDS are copolymerized basis for Fed. Spec. TT-P-666 zinc chromate silicone and alkyd resins which combine the primer [88] for use on aluminum and magne- excellent heat resistance, water and chemical sium surfaces. Phenolic-modified alkyds are resistance, and electrical properties of the sili- also used in interior finishes requiring improved cone resins with the good brushability, gloss, alkali (soap) resistance. hardness, and durability of the alkyd resins

Automotive finishes have progressed from the (also see sec. 3.9.2.1) . Silicone-alkyds are used early alkyd-modified cellulose nitrate finishes to in high-grade heat-resistant baked coatings for the highly durable present-day melamine-alkyd stoves, boilers, machinery, and electrical equip- finishes in which a short-oil alkyd is combined ment. Because of their good water, alkali, and with about 30 percent of melamine-formalde- chemical resistance, they are also used to ad- hyde resin to produce a fast-baking enamel with vantage in coatings for washing machines, in- excellent hardness, gloss, color retention, and dustrial equipment, and kitchen equipment. exterior durability. White baking enamels for Air-drying silicone-alkyds for industrial main- kitchen cabinets, refrigerators, and other appli- tenance paints also have been developed. ances are often based on short-oil, soybean alkyds combined with urea- or melamine-for- 3.4.2.3. Other resins with which alkyds maldehyde resin. For maximum whiteness and MAY BE CHEMICALLY COMBINED (in addition to non-yellowing with heat or aging, non-oxidizing the amino, phenolic, and other resins already alkyds modified with non-drying oils, such as mentioned) include epoxies," acrylics, and poly- cocoanut or castor oil, are employed. Non- urethanes. The use of such resin combinations oxidizing alkyds in combination with melamine- is just beginning to be exploited for coatings formaldehyde resin make good heat-resistant purposes. In general, the properties of the dual finishes for lighting fixtures and stove enamels. resin may be expected to be a combination of, The non-drying alkyds are also used as plasti- or intermediate between, those of the individual cizers in cellulose nitrate lacquers for furniture, constituents (see sec. 8.1.4.2). aircraft machinery, and tools. automobiles, [56] ,

3.4.3. Polyester Resins 3.4.2. Resin-Modified Alkyds

Although in a general sense all alkyd resins 3.4.2.1. Styrenated-alkyds are being used are polyesters, since they are the product of an effectively in very fast air-drying and baking esterification reaction between a polyhydric al- finishes. The resin is prepared either by sty- cohol and a polybasic acid, the term "polyester" renating the oil with which the alkyd is to be is commonly used in a narrower sense to denote reacted, or by direct styrenation of the oil- a linear-type alkyd possessing carbon-to-carbon alkyd; in either case, the reaction is with sty- double-bond unsaturation in the polymer chain. polystyrene resin, besides rene monomer. The These unsaturated polyesters may be cross- being relatively low in cost, contributes its own linked by reaction with a monomer such as good water and alkali resistance and good color styrene or diallyl phthalate (often with a per- retention properties to the styrenated-alkyd. oxide added also) to form an insoluble and in- However, styrenated-alkyds are not as durable fusible resin, without the formation of a by- on exterior exposure as the pure oil-modified product during the curing reaction. Although alkyds, and the sensitiveness of polystyrene to the chief use for the unsaturated polyester res- oils and hydrocarbon solvents also manifests it- ins in the past has been in the low-pressure self to some extent in the modified alkyd. The laminating field, the polyesters are now among hydrocarbon sensitivity can be reduced, in the the most promising of the newer resins finding case of baking finishes, by adding an amino application in the coatings field as attractive resin. Air-drying styrenated-alkyds usually and durable finishes for concrete, masonry, contain metallic driers. With high pigment wood, plaster, wallboard, and metal. On porous content and a solvent such as xylene, they will or rough surfaces (all the aforementioned ex- dry hard in about 10 minutes to a lusterless fin- cept metal), reliance is placed primarily on mechanical adhesion ("bite" or penetration) ish. For best flow in high gloss finishes, the rather than on specific adhesion (chemical at- styrenated-alkyd is dissolved in the slower- traction) to keep the coating in place. The good evaporating mineral spirits, and it dries hard specific adhesion needed on smooth, nonporous in 3 to 6 hours. Styrenated-alkyds are used in surfaces of metal has been obtained by combin- fast-drying finishes for wood and metal furni- ing the polyester with an isocyanate polymer ture, household implements, toys, and interior (see sec. 3.12.1.2) in addition to a styrene mon- applications requiring resistance to alkaline omer hardening agent. Thorough surface prep- cleaners (e.g., washing machine coatings) but aration based on the principles given in chapter not subject to contact with oils and greases. 6 and in accordance with the manufacturers in- Best resistance is obtained with baking finishes structions for filling, cleaning, sealing, and incorporating amino resins. priming is very important for the successful

45 application of polyester coatings, and well- is accomplished by chopping and blowing glass trained experienced personnel are required. roving into a layer of wet coating by means of Since the uncured polyester resin can be made an electrical or air-driven chopper and then in a liquid form suitable for application with- rolling in the fibers with another layer of coat- out the addition of volatile solvents or thinners, ing material. Appearance is enhanced by de- a 100 percent solids coating system capable of positing colored tracer threads along with the high film build (10 to 20 mils per coat) is pos- regular glass roving. Finally, a topcoat of sible, and tile-like polyester coatings up to 40 polyester resin may be applied to provide a mils in overall thickness are available for heavy smooth finish. For the glass fabric reinforce- duty applications. The hardening agent is ment, the fabric is laid on the primed, dry sur- mixed into the polyester resin just before it is face, all wrinkles are worked out, and the fabric applied. Curing proceeds throughout the coat- is saturated with the polyester applied by brush ing by an exothermic, chemical crosslinking or roller to wet the fabric but not fill the weave. reaction at a rate dependent primarily on the After the saturation coat is cured, it is sanded coating temperature and on the type and con- lightly and a heavy topcoat of polyester resin centration of hardening agent. A hard film is is applied to complete the coating system. The obtained in a few hours. The thickness of the glass fabric reinforcement imparts maximum coating affects the cure indirectly through its strength to the coating system. effect on the quantity of heat released and re- Uses for polyester resin coatings include hard sultant temperature, and through its effect on lustrous clear finishes for furniture, water- and the relative amount of hardener lost by vola- weather-resistant finishes for boats, durable tilization at the surface, as determined by the and attractively colored coatings for the exte- ratio of surface area to coating volume. On the riors of concrete or masonry buildings, tough basis of these factors, a coating thickness of 6 impact- and abrasion-resistant coatings for cor- mils is considered to be the minimum necessary ridor walls and floors of schools and hospitals, to ensure adequate cure. wear- and wash-resistant sanitary coatings for The tile-like polyester coatings are hard, non- cafeteria and hospital kitchens and for dust- porous, durable coatings capable of withstand- free rooms, decontaminable and radiation-re- ing dilute alkalies and acids, organic solvents, sistant coatings for walls and floors of atomic hot water, and a high order of nuclear radiation facilities, hot water- and radiation-resistant (see sec. 4.6.8), as well as mechanical abuse, coatings for canal walls and floors of nuclear impact, and abrasion. The mechanical strength, reactors, tough wear-resistant coatings for toughness, and flexibility of polyester coatings stairways, bowling alleys, and gymnasium can be greatly increased by incorporating glass floors, coatings for swimming pools, and indus- fibers or by laminating glass fabric into the trial coatings for metal structures and equip- coating system. The glass fiber reinforcement ment.

3.5. Amino Resins

The amino resins (or aminoplasts, as they are the resin solids, although it may be as low as sometimes called) that are of chief importance 5 to 10 percent in some automobile finishes are the urea- and the melamine-formaldehyde and as high as 50 percent in some low-bake compounds. The urea resins have been avail- furniture finishes. able since 1930 and the melamine resins since 1940. 3.5.2. Formation The formation of amino resins proceeds in 3.5.1. Properties three stages. In the first stage methylol amine monomers are formed by reaction of urea or The amino resins are thermosetting materials melamine with formaldehyde under mildly alka- that readily form hard, brittle, colorless, chemi- line conditions. In the second or intermediate cally resistant films on baking. Because of stage, these monomers condense to form linear their brittlenes adhesion used , and poor when water-soluble polymers which are partially > alone, they require proper plasticization for use etherified with alcohols (usually butanol) as the in coatings. This is usually accomplished by polymerization proceeds, to confer organic-sol- combining them with short- or medium-oil vent solubility. In the third and final stage, alkyds. amino-alkyd blend provides an The the resin is converted by heat to a cross-linked, excellent combination of coating properties in thermoset, infusible, insoluble resin. which the adhesion, toughness, gloss, and durability of the alkyd are complemented by the 3.5.3. Uses baking speed, hardness, color retention, and chemical resistance of the amino component. The excellent combination of coating proper- The amino resin is used as the minor component ties (see above) possessed by the urea and alkyd resins has led to their wide use, and usually constitutes from 15 to 35 percent of melamine ; singly or together, in high-grade, fast-baking, applications, are more chemical-resistant, have durable, mar-resistant, chemically resistant fin- superior hardness and mar resistance, and ishes for automobiles, refrigerators, washing possess better exterior durability. Melamine machines, stoves, electrical appliances, hospital resins, though capable of very fast cures at high equipment, counter tops, furniture, and other temperatures without color impairment, may industrial and household applications. also be cured quite rapidly at lower temperatures and therefore are very adaptable to infra- 3.5.4. Comparison of Urea and red baking schedules. Because of their greater Melamine Resins functionality and faster cure, the melamine resins are generally used in lower proportions than Although there are many similarities in the the urea resins. In general, then, it may be preparation, properties, and uses of urea and stated that the urea resins yield coatings hav- melamine resins, there are also important dif- ing an excellent combination of color retention, ferences which influence the selection of one chemical resistance, and durability properties (or combination of both) for a or the other a at relatively low cost. The melamines, though particular application. The urea-formaldehyde considerably higher in cost, extend the range of resins are lower in cost, have better adhesion desirable properties still further and are used properties, are more amenable to the repair of to particular advantage in applications requir- coating defects, and yield (in combination with ing very fast cure, prolonged resistance to ele- an alkyd) excellent white enamels that are as vated temperatures, superior alkali resistance, good or better in color retention than the mela- maximum hardness, and outdoor durability. mine-alkyds at moderate baking temperatures As a single (below 300 °F [149 °C]) and in indoor service example, a urea-alkyd produces a gleam- at moderate temperatures. The melamine- ing white refrigerator enamel of excellent dura- alkyds, although considerably more expensive, bility at relatively low cost; on the other hand, are much faster-curing, undergo less shrinkage for a heat-resistant stove enamel or an alkali- during cure, have better color and gloss reten- resistant washing machine finish, the melamine- tion on overbaking and in elevated temperature alkyd would be the preferred coating material.

3.6. Vinyl Resins

In a broad structural sense, all resins derived 3.6.1. Polyvinyl Acetate from the vinyl radical (CH2=CH-) may be Polyvinyl acetate is produced either as a classified as vinyl resins. However, such a granular resin or as a dispersion in water broad classification would include many types (latex). The latex was discussed in section of resins that are better known by more famil- 2.3.5 on Latex Paints. The granular resin is

! iar individual designations such as polyethyl- usually prepared by solution polymerization; , ene, polystyrene, acrylic, and others. The term the vinyl acetate monomer is heated, in a sol- "vinyl resin" is applied here in the narrower I vent such as benzene or toluene, in the presence I sense commonly employed in the plastics and of a peroxide catalyst. The resulting polymer coatings field. Thus, vinyl resins include chiefly I is a colorless, odorless, tasteless, nontoxic resin the polymers and copolymers of vinyl acetate, j that is widely soluble in most of the common

1 vinyl chloride, and vinylidene chloride, and the organic solvents such as the lower ketones, polyvinyl acetals and polyvinyl alcohol that are esters, methanol, higher alcohols containing derived from polyvinyl acetate. j some water, aldehydes, aliphatic acids, aromatic Vinyl resins are thermoplastic, addition poly- and chlorinated hydrocarbons, and nitrohydro- ! mers that may be prepared by solution, suspen- carbons. Polyvinyl acetate is less soluble or ' sion, emulsion, or bulk polymerization methods insoluble in the higher homologs. It is insolu- (see sec. 3.1.3). Structurally, they may be ble in aliphatic hydrocarbons, in animal and thought of as substituted ethylenes in which vegetable fats and oils, and in water but it one or two hydrogen atoms have been replaced — swells and loses strength upon prolonged im- by a chlorine atom, an acetate radical, an hy- mersion in water. Polyvinyl acetate is com- droxide group, or an acetal ring structure. In patible with many of the common ester-type properties, they range from soft, tacky, widely plasticizers such as dibutyl phthalate (but not soluble polymers to horny, difficultly sol- tough, dioctyl phthalate), phosphates, abietates, and uble ones, depending on their composition, mo- glycolates. Other materials with which it is lecular weight, and molecular structure. Since partially or completely compatible include cel- first achieving commercial importance in the lulose nitrate, cellulose ethers, chlorinated rub- early 1930's, their use has grown steadily, and ber, some phenolic resins, and a few acrylic vinyl resins constitute one of the most versatile resins and natural resins. and widely used classes of resins in the coatings Polyvinyl acetate is light-stable, transparent field. to solar ultraviolet and visible light, abrasion-

47 resistant, flexible, and very resistant to outdoor stabilized. The degradation reaction produces aging. However, it softens at temperatures HCl which catalyzes further degradation unless above 150 °F (66 °C), swells and weakens upon the acid is removed as it forms. Fortunately, a prolonged immersion in water, is hydrolyzed by large number of very effective heat and light strong acids or alkalis, and dissolved by many stabilizers for polyvinyl chloride have been de- organic solvents. Its many desirable proper- veloped. These function as HCl acceptors or ties, coupled with good ahesion and ease of for- neutralizers, ultraviolet absorbers, and antioxi- mulation and application, have led to its exten- dants to impart excellent stability to the resin. sive use in the coatings field, as well as in the The general-purpose polyvinyl chloride resin related field of adhesives. The latex form has finds wide use in high-grade electrical insulat- found wide acceptance in primer-sealers, inte- ing coatings for wire and cable; it combines rior wall finishes, and exterior masonry paints. excellent dielectric strength with flexibility and In emulsion or solution form, polyvinyl acetate toughness, resistance to moisture and chemicals, is used as a water- and grease-resistant coating and fire resistance. The plasticized resin may on paper, fabrics, and leather. Such coatings also be calendered onto paper or fabric to can be easily heat-sealed or solvent-sealed if provide tough, water-proof and grease-proof desired, and they are widely used in food pack- coatings however, polyvinyl chloride surface ; aging. Lacquers based on polyvinyl acetate coatings are most conveniently applied in the solutions find use as decorative and protective form of plastisols or organosals (see below). coatings for paper, wood, and metal. Suitably The use of plasticized polyvinyl chloride film formulated PVA coatings have shown very good and sheeting for shower curtains, inflatables, adhesion, durability, and corrosion resistance upholstery, etc., is so extensive as to merit when applied to metals. mention even in a discussion of coatings. In addition to its compatibility with other resins as noted above, vinyl acetate can be co- 3.6.2.2. Plastisols and organosols.—Plasti- polymerized with various vinyl derivatives such sols are dispersions of flnely divided polyvinyl as vinyl chloride, styrene, and acrylics to yield chloride resins (dispersion resins) in suitable other materials with very desirable properties. liquid plasticizers. The dispersion resins are powders of very fine particle size (0.05 to 1.0 3.6.2. Polyvinyl Chloride micron in diameter) made by emulsion polymerization. Among suitable plasticizers are com- 3.6.2.1. General properties. —Polyvinyl pounds such as dioctyl phthalate, tricresyl chloride is a tough, horny, transparent, thermo- phosphate, other alkyl and aryl phthalates and plastic resin having high inherent strength and phosphates, adipates, sebacates, and azelates. excellent chemical resistance. Its properties Plastisols may be applied in a thick coat, by stem in part from its crystalline nature. It is dipping or knife-coating, to paper, cloth, wood, highly resistant to strong acids and alkalis, and metal. The deposited coating is then fused water, alcohols, aliphatic hydrocarbons, fats, by flash-heating at 300-350 °F (149-177 °C) to yield a tough, durable, chemically resistant film. and oils—but it is swelled by aromatic hydrocarbons. It is soluble in only a few special Organosols are similar to plastisols, except that types of solvent, chiefly unsaturated ketones they contain some organic solvent to facilitate (e.g., mesityl oxide and isophorone) and ali- application of the coating for purposes requir- cyclic compounds (e.g., cyclohexanone and tet- ing a lower viscosity. Upon evaporation of the organic solvent, the deposited coating is fused rahydrofuran) . It is soluble also in nitroben- zene and in dimethyl formamide. Because of by flash-heating just as in the case of the plas- this very limited solubility in organic solvents, tisol. Organosols and plastisols are used for polyvinyl chloride is not widely used in sol- coatings on lawn furniture, office equipment, vent-type coatings; however, in the form of a industrial equipment, and home appliances. plastisol or organosol (discussed below) that They give excellent service in such severe appli- is readily applied by dipping or knife-coating cations as coatings for plating baths, chemical methods, it finds extensive use in severe service tanks, and dish-washer racks. applications. Aside from the external plasticization of poly- plastisols Polyvinyl chloride is made chiefly by suspen- vinyl chloride as in and organosols, sion or emulsion polymerization. The resin is the resin can be modified for use in solvent-type available as a general-purpose granular white coatings by copolymerization with vinyl acetate powder, as a fine-particle paste dispersion for to provide permanent internal plasticization, as use in plastisols and organosols, and as a water- described in the following section. base latex. In each case, the key to its effective use is suitable plasticization and stabilization. 3.6.3. Polyvinyl Chloride-Acetate The unmodified resin, though tough and chemically resistant, is not flexible enough for coat- Polyvinyl chloride-acetate is made by the co- ings use unless plasticized, and it is degraded polymerization of vinyl chloride and vinyl ace- by heat and by ultraviolet light unless properly tate, the chloride constituting from 85 to 97

48 .

percent of the copolymer. Solution or suspen- ishes; textured finishes for high-quality radio sion polymerization methods are usually em- and amplifier cabinets, which impart a furni- ployed. The vinyl chloride-acetate copolymers ture-like appearance to the metal cabinets, have retain much of the toughness and chemical re- become popular. Other uses include coatings sistance of polyvinyl chloride while partaking for garden furniture, metal signs, hospital and of the greater flexibility and solubility of poly- kitchen furniture that must withstand wear vinyl acetate. The copolymers are easily for- and frequent washing, refrigerator parts, col- mulated into solution-type coating materials for lapsible tube coatings, perspiration-resistant either air-drying or baking applications. A few articles, and specially-formulated strippable j types are made as dispersion resins for use in coatings. The combination of toughness and plastisols and organosols (see previous section) resistance to weather, water, and chemicals Like the parent polyvinyl chloride, the copoly- possessed by the vinyl-chloride acetate copolymers must be stabilized against the deteriorat- mers makes them valuable coatings for use in ing effects of heat and ultraviolet light. Plasti- durable, corrosion-resistant coating systems for cizers, pigments, and fillers are incorporated as severe industrial and marine applications (see

required for particular coatings applications. sec. 4.3.1.2) ; for example, the coating system Polyvinyl chloride-acetate copolymers are sol- selected for protecting the locks of the St. uble in ketones, esters, ethers, and chlorinated Lawrence Seaway is based on vinyl copolymer hydrocarbons. Aromatic hydrocarbons such as resins. toluene and xylene are well-tolerated diluents 3.6.4. Polyvinyl Alcohol and are nearly always part of the composite i solvent formulation. In addition to their avail- Since vinyl alcohol does not exist in mono- ability in a range of chloride-to-acetate ratios meric form, polyvinyl alcohol cannot be pre-

1 chlo- and a variety of molecular weights, vinyl pared by direct polymerization. Instead, it is ride-acetate resins may be copolymerized with obtained by the hydrolysis of polyvinyl acetate a small proportion (about 1%) of maleic resin in an essentially water-free alcohol medium to impart improved adhesion of air-drying coat- (usually methanol) in the presence of small ings to smooth metal surfaces. Vinyl copoly- amounts of acid or alkaline catalysts. The mers without the maleic resin must be baked to properties of the final resin depend largely on develop good ahesion to smooth metals, but they the degree of hydrolysis and on the molecular adhere well to porous surfaces of paper, fabric, weight. With increasing hydroxyl content (in- leather, and concrete. creasing polar character) the polymer becomes Most unmodified vinyl chloride-acetate co- less hydrophobic and more hydrophilic. When polymers are poorly compatible with other res- about 65 percent of the original acetate groups ins. However, when modified by incorporating have been hydrolyzed (replaced by hydroxyl hydroxyl groups, the copolymer becomes com- groups), the product loses its solubility in or- patible with various alkyd, phenolic, amino, and ganic solvents and becomes soluble in cold other resins. Compatibility with nitrocellulose I water but precipitates on heating. With fur- ' can be obtained with copolymers of high acetate ther hydrolysis, when about 80 to 90 percent content. |j of the acetate groups have been replaced with i Vinyl chloride-acetate copolymer coatings are hydroxyl groups, the product is soluble in both j odorless, tasteless, and nontoxic (unless ren- cold and hot water. When the hydrolysis is ingredients) ii dered otherwise by modifying virtually complete (95 percent or more of the ! The coatings are tough and flexible, resistant to acetate groups replaced), the polyvinyl alcohol

I water and chemicals, non-flammable, abrasion- will no longer dissolve in cold water; but if resistant, weather-resistant. I and dissolved in hot waterp-it will remain in solution on cooling. Uses. Polyvinyl chloride-acetate resins find Coatings deposited from solutions of poly- wide and important use in the coatings field. vinyl alcohol in water are tough and flexible and

; Their lack of taste, odor, or toxicity makes them highly I'esistant to water-insoluble organic sol- well suited for coating food and beverage con- vents such as aromatic, aliphatic, and chlori- tainers. Their flexibility and adhesion permit nated hydrocarbons, and vegetable and animal their use on flat sheet metal that must with- fats and oils. Polyvinyl alcohol coatings are stand subsequent deep drawing or stamping highly impermeable to most gases, have high I operations. Their chemical resistance makes tensile strength, and are odorless and tasteless. them suitable as coatings for the insides of Polyvinyl alcohol resins can be applied to drums and tanks and pipes that must carry cor- paper, fabrics, and leather to provide a highly

I rosive chemicals. Their alkali resistance per- grease- and gas-resistant coating. In combina- mits their use over concrete, plaster, and alka- tion with a water-resistant film or coating, an ] line wallboards without special precautions, effective barrier against water, gases, greases, Wrinkle and textured coatings based on these and most organic solvents can be obtained. j i resins can be applied to paper, cloth, wood, and Polyvinyl alcohol is a versatile emulsifying metal to produce durable attractive fin- agent and finds use as an emulsifier, thickener. j and

49 . and stabilizer in emulsion polymerizations and coatings for fabrics and, in combination with latex paints. cellulose nitrate, in interior coatings for house- The flexibility of polyvinyl alcohol films and hold articles. coatings can be increased by incorporating water-soluble plasticizers such as glycerol, the 3.6.5.3. Polyvinyl butyral is best known lower glycols, and various amines. A number for its wide use as the clear, tough interlayer of processes have been patented for insolubil- in safety glass. However, it also performs ex- izing polyvinyl alcohol coatings through metal cellently in the coatings field and has proved complexing or through reaction with other res- particularly useful as a base for wash primers ins such as phenolics, melamines, and diisocya- (see sec. 6.3.2.5) and metal conditioners. As nates. A weather-resistant polyvinyl alcohol made commercially, polyvinyl butyral resins water-base coating has been described [57] contain a considerable percentage of hydroxyl which is formulated so as to form insoluble groups (about 7 percent) which impart a hy- acetals (without baking) that resist solution in drophilic character to the resin and contribute water while retaining good resistance against to its excellent specific adhesion to nonporous penetration by gases and by organic solvents surfaces such as glass, metal, and phenolic plas- and vapors. tics. Polyvinyl butyral coatings also have very good adhesion to porous surfaces such as wood, 3.6.5. Polyvinyl Acetals paper, and fabrics. Baking is not required to develop good adhesion. Polyvinyl acetals are the products of the con- Polyvinyl butyral is soluble in non-anhy- densation reaction alcohol between polyvinyl drous alcohols, glycol ethers, and some esters and aldehydes. The aldehydes of commercial and ketones. Water is an effective latent sol- importance for this use are formaldehyde, acet- vent ; in fact, 3 to 8 percent must be present to aldehyde, and butyraldehyde. The reaction can achieve good solvent properties. Mixed thin- be controlled to yield acetals having various ners containing about 40 percent alcohol with ratios acetal of acetate, hydroxyl, and groups. esters, ketones, aromatic hydrocarbons, and a In general, solubility decreases while viscosity small amount of water are excellent solvents strength increasing of and increase with degree for use in coatings formulations based on poly- acetalization. resins are usually supplied The vinyl butyral. Although flexible enough for as powders which with may be compounded many coatings applications without added plas- plasticizers and other resins to produce tough, ticizer, polyvinyl butyral resins may be com- elastic, water-insoluble coatings. pounded with various chemical plasticizers, such as trioctyl phosphate, triethylene glycol di 3.6.5.1. Polyvinyl is the toughest of formal (2-ethylbutyrate) , and dibutyl sebacate, to in- the vinyl resins. It has a tensile strength of crease their flexibility and to decrease their about 20,000 psi, a softening temperature of viscosity in hot melt applications (e.g., in high- about 375 °F (191 °C) , and a water absorption speed book-binding) of about 1 percent. It is unaffected by aliphatic Polyvinyl butyral is compatible with phenol-, hydrocarbons, gasoline, oils, fats, waxes, and urea-, and melamine-formaldehyde resins and alkalies, but it is soluble in chlorinated hydro- can be cured or cross-linked by heating with carbons, dioxane, phenol, and aliphatic and these resins. Air-dry cure and crosslinking cyclic ethers. It has excellent abrasion resist- can be accomplished by the addition of glyoxal. ance and electrical properties, and a chief use Crosslinked polyvinyl butyral coatings are has been in combination with phenolic resins as tough, heat-resistant, and solvent-resistant; a tough, flexible, electrical insulating coating they have been used for gasoline tank linings, for wire. It has also been used as the inner can coatings, laboratory equipment, and as wa- gasoline-resistant liner of self-sealing "bullet- terproof cloth coatings replacing rubberized proof" gasoline tanks, as an abrasion-resistant fabrics. coating for wooden propellers of aircraft, for An effective knot sealer (WP-578) developed can linings, and in metal primers. by the Western Pine Association is based on a combination of polyvinyl butyral with a phe- 3.6.5.2. Polyvinyl acetal is a slightly yel- nolic resin in ethanol (95%) solution [149]. lowish, odorless and tasteless resin that is The sealer may also be applied over asphaltic widely soluble in ketones, esters, alcohols, and materials to prevent bleeding into enamel top- chlorinated hydrocarbons, but insoluble in aro- coats. Polyvinyl butyral may be used as a heat- matic and aliphatic hydrocarbons, oils, and sealable coating for foil and other packaging water. Its tensile strength (about 8000 psi) materials. Durable wood finishes can be made and its softening temperature (about 320 °F) from combinations of polyvinyl butyral with [160 °C] are considerably below that of poly- shellac or with phenolic resins. The most im- vinyl formal, and it has a somewhat higher portant application of polyvinyl butyral in the water absorption (about 2 percent) . Although coatings field, however, has been as the base used chiefly in adhesives, it finds limited use in for corrosion-inhibiting metal primers (wash

50 primers) utilizing chromate pigments for cor- as water-impermeable or chemically resistant rosion inhibition, and phosphorc acid to achieve as the solution-type coatings, largely because metal conditioning and cross-linking of the of the water-sensitive emulsifiers, stabilizers coating [30]. Such wash primers have ex- and other additives that remain in the cured cellent adhesion to metal substrates and to vir- latex film. tually any type of topcoat lacquer or paint (except a few types of nitrocellulose and vinyl 3.6.6.3. Vinylidene chloride-vinyl chlo- lacquers) . Excellent all-vinyl-base coating sys- ride copolymers possess their maximum solu- tems for severe marine applications employ a bility, compatibility, and flexibility when they polyvinyl butyral wash primer as the first coat contain approximately equal proportions of applied to the metal (see sec. 4.3.1.2). vinyl and vinylidene groups. Tensile strength and softening temperature are higher when

3.6.6. Polyvinylidene Chloride and its either monomeric group predominates. Color Copolymers stability is better with higher vinyl chloride content; as with straight vinyl chloride resins,

3.6.6.1. Polyvinylidene chloride is a tough, the copolymer resins require heat and ultravio- nonflammable, crystalline resin characterized let stabilizers for best results. A typical co- by extremely low water vapor permeability and polymer of medium or somewhat higher vinyl absorption, and outstanding resistance to oils, chloride content is soluble in ketones, cyclic greases, acids, alkalis (except strong ammo- ethers, and amides ; it has a high tolerance for aromatic hydrocarbon diluents and exhibits nium hydroxide) , and most organic solvents and reagents. It is soluble only in cyclic ethers good compatibility with the common vinyl plas- and cyclic ketones, but is attacked and weak- ticizers, as well as with many alkyds and a few ened by halogens and strong organic amines. other resins; it has excellent resistance to wa- Because of its extremely limited solubility and ter, chemicals, and many organic solvents, incompatibility with other materials, polyvinyl- dicating a potential usefulness for maintenance idene chloride itself has not in the past been coatings and marine finishes. A copolymer of high vinylidene chloride content is available used in the coatings field ; however, a polyvinylidene chloride latex for coatings applications which is soluble in aromatic hydrocarbons and requiring exceptional water and chemical re- in mixtures of aromatic hydrocarbons with sistance is now available. When polyvinylidene ketones, and which is compatible with many chloride is internally plasticized by copolymer- common vinyl plasticizers and a few other ization with vinyl chloride or acrylonitrile, very resins. Another type, a latex, does not form useful coatings resins are obtained. films alone but is compatible with a number of water dispersable materials and oleoresinous 3.6.6.2. Polyvinylidene chloride copoly- and alkyd resins to which it contributes toughness and water resistance. In general, vinyli- mers (general) . The copolymers (commonly known as saran resins in the trade) have a dene chloride-vinyl chloride copolymers are less crystalline and less closely packed struc- used in applications requiring exceptional re- ture than the polyvinylidene chloride homo- sistance to water, acids, alkalis, and many polymer because of the irregularities intro- solvents, and a high degree of impermeability duced into the polymer chains by the random to water vapor and gases. presence of the copolymerized vinyl chloride or acrylonitrile groups. The reduced crystallinity 3.6.6.4. Vinylidene chloride-acrylonitrile of the copolymers increases their solubility and copolymers of high vinylidene chloride content compatibility and permits their use in solution- are available as solution or emulsion types. The type coatings and in plasticized latexes. It is resin is soluble in methyl ethyl ketone and in of general interest to note that the irregular various cyclic and unsaturated ketones and will arrangement of vinyl and vinylidene groups in tolerate limited amounts of diluents. It has the copolymer chains produces a fairly soluble suflficient flexibility to permit its use without and compatible resin even though both polyvinyl plasticization in coatings where maximum chloride and polyvinylidene chloride are them- chemical and solvent resistance are needed. selves only narrowly soluble and compatible Where greater flexibility is required, the resin materials (cf. discussion of crystallinity in sec. may be plasticized with various phthalates, 3.1.1. A range of properties is achieved by phosphates, glycolates, or sebacates. Although varying the copolymer ratio and system. In vinylidene chloride-acrylonitrile copolymers general, commercial polyvinylidene chloride- have a limited compatibility with a few other acrylonitrile copolymers for coatings contain resins, they are usually used alone because of a low proportion of acrylonitrile, whereas poly- the better chemical resistance of the unmodi- vinylidene chloride-vinyl chloride copolymers fied resin. Coatings based on these resins find may vary in composition from low to high use in the packaging industry where they are ratios of vinyl chloride. The copolymer la- applied to paper, fabrics, and metal foil to texes, although convenient to use, are not quite provide heat-sealable, chemical-resistant.

51 grease-proof, and water-proof coatings. Other are linear, thermoplastic, tacky polymers that uses include cap linings, coatings for the in- are widely soluble in organic solvents and wide- terior of gasoline storage tanks, drum coat- ly compatible with other resins. The vinyl ings, chemical-resistant maintenance paints, ether/maleic acid copolymer is soluble in water and strippable cobweb coatings (see sec. 2.2.7) as well as in organic solvents ; the vinyl methyl for protecting instruments and- machinery ether homopolymer is soluble in cold but not in against moisture and corrosion during ship- hot water. ment and storage. A water- and fuel-resistant The chief application for the polyvinyl alkyl lacquer [150] based on a vinylidene chloride- ethers has been in pressure-sensitive tapes, acryonitrile copolymer is covered by Mil. Spec. polyvinyl ethyl ether being particularly useful MIL-L-18389. in this respect in combination with phenolic varnish resins. In the coatings field, the poly- 3.6.7. Polyvinyl Ethers vinyl ethers are finding some use as plasticizing agents in chlorinated rubber paints and cel- Among the polyvinyl ethers, only the alkyl lulose nitrate lacquers, and as binding agents types are of sufficient importance to warrant in paints and impregnating solutions. Poly- brief mention here. These include the methyl, vinyl methyl ether can be used as a nonionic ethyl, isobutyl, and n-butyl polyvinyl ethers, heat-sensitizing or softening agent in latex and a copolymer of vinyl methyl ether with paints to improve film coalescence and adhe- maleic anhydride. The polyvinyl alkyl ethers sion.

3.7. Acrylic Resins

3.7.1. Properties on their composition and molecular weight, as noted above, they are, in general, soluble in The acrylic resins of chief importance in aromatic and chlorinated hydrocarbons, esters, coatings are the polymers and copolymers of and ketones. A few special types are available acrylic and methacrylic acid esters. They are which are soluble in alcohol and in mineral thermoplastic, addition-type polymers that spirits, but in most instances the alcohols and range in physical properties from soft, sticky aliphatic hydrocarbons are non-solvents. The semiliquids to hard, machinable solids, depend- acrylic resins have only limited compatibility ing on their composition and molecular weight. with each other and with other classes of

The acrylates are softer, more extensible, and resins ; however, their monomers are readily co- more soluble than the corresponding methacry- polymerized with other addition-type mono- lates. Softness, extensibility, and solubility are mers. Some types of acrylic resin are compat- also greater with increasing number of carbon ible with nitrocellulose, ethyl cellulose, and atoms in the alcohol side-chain. With the wide some of the vinyl resins. They are not com- range of properties obtainable by varying the patible with drying oils, oleoresinous varnishes, alcohol side-chain, and with the additional op- or oil-modified alkyds. Where plasticization is portunities for variation afforded by copoly- desired for special purposes, the acrylic ester merization of various acrylates and methacry- resins may be combined and are compatible lates, it is possible to produce resins having al- with some of the common chemical plasticizers most any desired degree of flexibility without such as dibutyl phthalate, tricresyl phosphate, the use of plasticizers. The latter is an impor- and dibutyl sebacate. tant advantage, since the unmodified acrylic The acrylic resins are available in emulsion ester polymers possess a clarity and a resist- form as well as solution-type resins. In emul- ance to discoloration by aging, heat, and ultra- sion form they have found wide acceptance in violet light that is unsurpassed by any other latex paints and primer-sealers for both inte- resin. Other desirable properties of acrylic rior and exterior applications. The character- resins which have led to their wide use in coat- istic heat and light stability and chemical re- ings and plastics since their introduction sistance of the solution-type acrylic resins are around 1931 are: wide solubility in common also found in the latex form. A discussion of organic solvents, permitting ease and economy acrylic latex paints is given in section 2.3.5.6. of formulation; excellent resistance to acids (except concentrated oxidizing types), alkalis, 3.7.2. Uses water, alcohols, aliphatic hydrocarbons, oils, and greases ; low acid number and low reactiv- Because of their versatility and excellent ity with pigments ; and good adhesion to a vari- combination of properties, the acrylic resins ety of surfaces. Although acrylic coatings air- find many uses in the coatings field. In addi- dry satisfactorily, optimum film properties are tion to their large-scale use in latex paints, they developed by baking. are used in protective and decorative lacquers While the degree of solubility (and the so- for paper, fabrics, leather, plastics, wood, and lution viscosity) of the acrylic resins depends metal. Clear acrylic coatings are used to pro-

52 tect paintings and other art objects, to pre- on washing machines, stoves, and dishwashers. serve important documents, to prevent the tar- A water-thinnable vehicle for industrial baking nishing of silverware, to protect bright metal finishes utilizing a new thermosetting acrylic against outdoor corrosion, and as electrically emulsion polymer [123] is reported to have a insulating coatings. White baking enamels hardness, impact resistance, adhesion, and dur- having excellent resistance to chemicals and ability superior to that of premium melamine- chemical fumes can be made v^^ith acrylic resins. alkyd baking finishes while being equal to mela- Acrylic vehicles are also used in luminescent mine-alkyds in gloss and stain resistance. paints. Recently a high-gloss, hard, weather- Since the acrylic ester monomers are readily resistant finish based on acrylic resins has been copolymerized with other vinyl monomers, an developed and adopted for use on automobiles. important use for the acrylics has been to pro- Thermosetting types of acrylic resins have been vide internal plasticization for polymers of developed [124] which cross-link upon heating vinyl chloride, vinylidene chloride, vinyl ace- to form hard, insoluble and infusible coatings tate, acrylonitrile, styrene, and others. An that are suitable for appliance finishes that acrylic-nitrocellulose gloss lacquer [138] is must withstand severe service, such as those covered by Mil. Spec. MIL^L-19537.

3.8. Polystyrene and Its Copolymers

The styrene monomer is readily polymerized butadiene and with drying oils, however, find by bulk, solution, suspension, or emulsion extensive use in coatings. methods (see sec. 3.1.3) in the presence of a peroxide catalyst and mild heat. Very high 3.8.2. Styrene-Butadiene Copolymers molecular weights (greater than 1,000,000) can from be obtained; however, most commercial poly- Styrene-butadiene copolymers range brittle solids to soft, rubbery materials, styrenes are considerably lower in molecular hard, butadiene. weight (about 125,000 for molding grades and depending on the ratio of styrene to butadiene types constitute the widely used about 35,000 for grades used in surface coat- High GR-S (now termed SBR) synthetic rubbers. ings). Styrene is also readily copolymerized containing about 60-80 with a variety of other materials, including The high styrene types, percent styrene, are used in coatings. Being drying oils, butadiene, phenolics, vinyls, acryl- non-polar hydrocarbon resins, they are insolu- ics, maleic esters, and various unsaturated di- ble in water, alcohols, other strongly polar carboxylic acids. Substituted styrenes, such as and solvents, but they are soluble in aromatic and a-methyl styrene, vinyl toluene, etc., also polym- chlorinated hydrocarbons in mixtures of erize readily. The polystyrene family of resins and aromatic hydrocarbons with aliphatic hydro- is therefore a large one, and a variety of prop- carbons and ketones. They are resistant to erties is obtainable depending on the composi- acids, alkalis, mineral oils, animal tion and the degree of polymerization. and and vegetable fats and oils. Although thermoplastic and possessing a low softening temperature 3.8.1. Polystyrene Homopolymer (about 125 °F [52 °C] for the solution types), they are heat stable at temperatures as high Polystyrene itself is a hard, glasslike, color- as 300 °F (149 °C). Both flexibility and ad- less, transparent, thermoplastic resin. It is hesion are greater than that of the polystyrene soluble in aromatic and chlorinated hydrocar- homopolymer. bons and in some esters and ketones. It is The chief application for styrene-butadiene highly resistant to acids (except strong oxi- copolymers in the coatings field has been in the dizing types), alkalis, alcohols, and vegetable water-base latex form. The first latex paints fats and oils. Although somewhat permeable to be developed were of the styrene-butadiene to water vapor, its water sorption is extremely type, and these are still among the leading ma- low, and it is highly resistant to and dimen- terials in the huge gallonage of interior latex sionally stable in water. It has excellent elec- paints now marketed (see sec. 2.3.5). To a trical properties. The chief use for unmodified lesser, but still considerable, extent the styrene- polystyrene is in the plastics molding field. butadiene copolymers also find use in exterior Polystyrene is not widely used alone as a coat- latex paints. They are also used as grease- ing because of its brittleness, although it is proof, chemical-resistant, heat-sealable coat- readily applied from solution for coatings ap- ings for paper and fabrics. plications in which flexibility is not a require- Solution-type styrene-butadiene resins also ment (e.g., as a grease-, stain-, and perspira- find wide use in coatings. Their combination tion-resistant protective coating on hard book of excellent water and chemical resistance, covers, stiff leather and fabrics, labels on rigid toughness and abrasion resistance, good adhe- substrates, and as an electrically insulating sion, and fast drying properties have led to coating) . The copolymers of polystyrene with their use in traffic paints, masonry paints, con-

53 Crete floor enamels, swimming pool paints, rene-isoprene copolymers with properties some- metal primers, industrial maintenance paints, what similar to styrene-butadiene copolymers and general-purpose exterior coatings. have been produced. Styrene-maleic anhydride copolymers find use as emulsifying and stabilizing agents in latex formulations. 3.8.3. Styrenated Oils and Alkyds The polymers of styrene and acrylonitrile are resistant to aromatic hydrocarbons but soluble in ketones they Although polystyrene cannot be physically ; not been used mixed with drying oils because of incompata- have much as coatings. bility, the styrene monomer is readily copolym- 3.8.4.2. u-Methyl styrene is useful as a er ized with drying oils having conjugated un- moderating comonomer in the mass copolymer- saturation (e.g., tung oil and dehydrated cas- ization of styrene with the more reactive dry- tor oil). Non-conjugated drying oils, such as ing oils by slowing the polymerization rate, it linseed or soybean oil, may be successfully ; permits better control of the process. styrenated if they are first mixed with oils of the conjugated type. The styrenated oils yield 3.8.4.3. Vinyl toluene polymers and co- fast-drying, hard, moisture-resistant films that polymers are a promising and increasingly use- have fair durability. ful class of resins for coatings. The name Improved durability is obtained by copolym- "vinyl toluene" refers to the ortho-, meta-, and erizing styrene with alkyds. The alkyd may be para-substituted methyl styrenes, as distin- styrenated directly or a styrenated oil may be guished from a-methyl styrene. The vinyl used (see sec. 3.4.2.1). Styrenated alkyds, toluene polymers are more flexible and more while exhibiting some of the sensitiveness of widely soluble and compatible than their poly- polystyrene to aromatic solvents, have better styrene counterparts. Like styrene, vinyl tol- acid and alkali resistance than ordinary oil- uene can be copolymerized with drying oils, oil- alkyds. Although not as durable on exterior modified alkyds, and various addition-type mon- exposure as the pure oil-modified alkyds, the omers, yielding a very versatile and useful class styrenated alkyds find considerable use in low of resins having a wide range of desirable cost household and industrial finishes where properties. The reaction of vinyl toluene with fast drying is required along with good pro- drying oils in the presence of divinyl benzene tective properties. Long-oil styrenated alkyds as a cross-linking agent produces alkyd-like also find use as plasticizers for vinyl, amino, resins that are more stable and alkali-resistant and cellulose nitrate lacquers. than ordinary alkyds because they have a carbon-to-carbon instead of an ester-type cross- 3.8.4. Other Styrene Derivatives linkage. Among the more recent developments in vinyl toluene resins are copolymers with bu-

3.8.4.1. Copolymers of styrene with phenolic tadiene and other dienes ; these are soluble in resins are used in coatings for porous surfaces organic solvents and constitute a promising and in household maintenance finishes. Sty- class of coatings resins.

3.9. Silicone Resins

3.9.1. Pure Silicone Resins The low molecular weight linear polymers are Silicone polymers are organosilicon com- heat- and oxidation-resistant fluids. The high pounds of high molecular weight, consisting of molecular weight linear polymers are elastom- an inorganic backbone chain of silica (alternate ers. The crosslinked types, in a range of mo- Si and 0 atoms) with organic radicals attached lecular weights, are the silicone resins used as to this main chain through C-Si linkages. The coatings. Crosslinking is generally accom- silicon basic silicone (siloxane) structure is: plished by introducing trifunctional H H H atoms (having a third oxygen atom attached in place of one of the organic substituent I I I -Si-O-Si-O-Si-0- groups) at intervals along the siloxane chains to form oxygen bridges between trifunctional I I I H H H atoms on different chains. Ways have been de- Replacement of the hydrogen atoms with vari- veloped also for accomplishing the crosslinking ous alkyl and aryl groups yields a very large through polyfunctional organic side-groups. and very useful class of polymers (organopoly- Although the solubility, compatibility, and siloxanes) that are characterized by outstand- chemical resistance of the silicone resins vary ing heat resistance, excellent water resistance with the nature of the side groups and the mo- and water repellency, excellent electrical prop- lecular weight, they are generally soluble in erties, and very good weatherability. The sili- chlorinated and aromatic hydrocarbons, in mix- cones have been available since about 1944. tures of aromatic and aliphatic hydrocarbons, Both linear and crosslinked types are produced. and in many ketones, esters, and glycol ethers. They may be cold-blended or copolymerized oxies, acrylics, amino resins, and cellulose de- with a variety of other resins as described rivatives. Both baking enamels and air-drying later. They are resistant to acids, alkalis, wa- coatings are available. Suitable driers may be ter, corrosive atmospheres, greases, and oils. used in the baking types to permit lower bak- Silicone resins require baking at elevated ing schedules if necessary. In air-drying types, temperatures (300-500 °F [149-260 °C]) to the organic constituent is usually the major develop their optimum film properties, and they component. As little as 25 percent of silicone are capable of thousands of hours of service resin can considerably increase the heat resist- at such temperatures. The decomposition tem- ance and weatherability of organic resins that perature for most silicone resins is around would quickly deteriorate if used alone. At the 650 °F (343 °C). However, when pigmented same time, some physical properties (e.g., ad- with heat-resistant pigments such as aluminum hesion, toughness, color, solvent resistance) or zinc, they afford protection against weath- may be enhanced by the organic resin. ering and corrosion at operating temperatures up to 1000 °F (538 °C) . The silicone resin will 3.9.2.1. SiLicONE-ALKYDS.—Silicone-modified gradually volatilize on prolonged heating at this alkyd resins have been particularly useful in temperature, but a silica matrix remains which the coatings field. Baking enamels of this type holds the pigment in place. Incorporation of combining a silicone with a non-drying oil ceramic frits into the formulation permits even alkyd retain their color and gloss for long pe- higher operating temperatures—to 1400 °F riods of continuous service at temperatures up (760 °C). to 400 °F (204 °C). Air-drying aluminum paints based on silicone-modified medium-oil Uses. Because of their exceptional heat re- alkyds have excellent corrosion resistance and sistance, the silicone resins find their main use weatherability, and will withstand tempera- in heat-stable maintenance coatings and indus- tures up to 800 °F (427 °C) for limited periods. trial finishes for severe high-temperature ap- Short-oil alkyds containing about 30 percent plications such as hot stacks, oven walls, com- silicone have been used to make durable, gloss- bustion engine mufflers, incinerator interiors, retentive automotive finishes. Exterior trim arc light reflectors, and wall and space heaters. paints having excellent chalk resistance can be Several Government specifications [58, 59, 60, made with silicone-modified long-oil alkyds. 61] for high temperature-resistant paints are Heat- and corrosion-resistant primers for use based on, or can be met by the use of, silicone on metals under silicone-modified topcoats are resin vehicles. Although used primarily for usually based on a silicone-alkyd vehicle with their high temperature resistance, silicone res- suitable heat- and corrosion-resistant pigments, ins also perform well at low temperatures. Ap- such as zinc chromate, aluminum paste, or zinc plied from dilute solution to masonry surfaces, dust (also see sec. 3.4.2.2). the silicones provide excellent water repellency. Some silicone fluids are used as additives in 3.9.2.2. Other silicone-modified resins protective coatings to promote flow-out and that are the basis for durable coatings com- prevent bubbling, and to produce hammertone bining many of the desirable properties of each finishes (sec. 4.1.4.5) ; these fluids should not component include combinations of silicones be confused with the film-forming silicone with phenolic resins to produce weather- and resins. water-resistant maintenance paints ; with acrylic resins or melamine-formaldehyde resins for

3.9.2. Modified Silicone Resins durable white enamels ; with chlorinated rubber to make tough, heat- and corrosion-resist- Where the maximum heat resistance obtain- ant industrial maintenance paints; with spoxy able with the pure silicone resins is not re- resins for strong, heat-resistant electrical in- quired, more economical yet heat-resistant coat- sulating coatings; and with cellulose deriva- ings can be made with modified silicone resins tives, such as ethyl cellulose and cellulose ni- that have been blended, or copolymerized, with trate, to make heat- and weather-resistant organic resins such as alkyds, phenolics, ep- lacquers.

3.10. Epoxy Resins

Epoxy resins, since their introduction in linkages and is therefore quite stable, with a 1949, have found important and steadily in- considerably greater degree of chemical resist- creasing use in the coatings field because of ance than is obtained with ester linkages. their unsurpassed combination of excellent ad- Epoxy resins have two types of reactive groups, hesion, toughness, and chemical resistance. the terminal epoxide groups and the hydroxy! They are the condensation products of epichlo- groups along the polymer chains. They are rohydrin with &is-phenol. The chain structure therefore readily crosslinked with heat-reac- consists only of carbon-to-carbon and ether tive resins or polyfunctional amines to form

55 . highly resistant, thermoset polymers. The hy- rosive atmospheres. Thick epoxy coatings can droxyl and epoxide groups may also be reacted be applied to irregularly shaped objects and with organic acids; esterification with long- grill-work by the fluidized resin bed technique chain fatty acids such as linseed, soy, or cocoa- (see sec. 5.4.6) nut fatty acid yields alkyd-like resins with the inherently more stable ether linkage of the 3.10.2. Unmodified Epoxy Air-Dry Resins epoxy chain as the backbone instead of the ester linkage of the alkyd. The amine-cured epoxy resins combine most Because of the wide spacing between the re- of the advantageous properties of a baked film active epoxide and hydroxyl groups, even the with the convenience of an air-dry coating. crosslinked epoxy polymer retains an unusual Reactive amines, such as ethylene diamine or degree of flexibility along with its strength and diethylene triamine, reduced with a suitable chemical stability. Etherification of the chro- amount of thinner, are stirred into the vehicle mophoric phenolic hydroxyl group during the shortly before use at a concentration of about condensation with epichlorohydrin greatly re- 6 to 8 parts per hundred parts (by weight) of duces yellowing tendencies in the epoxy pol- resin. Once the amine hardener has been ymer. The polar nature of the epoxy molecule added, the pot life of the mixture is from 8 to accounts for its strong adhesion to a wide vari- 60 hours depending on the nature of the resin ety of surfaces. It also accounts for its solu- and the amine used. Where possible, an induc- bility in oxygenated solvents such as ketones, tion period of a half-hour to an hour should be esters, and ether-alcohols and in chlorinated allowed after mixing before applying the coat- hydrocarbons. Aromatic hydrocarbons and al- ing [62]. Because of the caustic, volatile, and cohols are latent rather than active solvents toxic nature of reactive organic amines, care for epoxy resins. must be exercised to avoid contact with the The various grades of epoxy resins offered skin and to provide adequate ventilation dur- commercially differ primarily in molecular ing their use. The lower molecular weight weight, the solubility and compatibility usually epoxy resins are well suited for cold-curing ap- decreasing with increasing molecular weight. plications. Their better solubility permits a In general, the epoxies are compatible with high solids vehicle which can be applied in a pure phenolic resins, urea-formaldehyde resins, heavy coat in one operation. Since curing pro- chlorinated biphenyls, and polyvinyl formal. ceeds by direct crosslinking rather than by They are incompatible with rosin-modified phe- oxidation, thick coats are evenly hardened nolics, cellulose derivatives, and most alkyds. throughout without difficulty, provided the sub- The lower molecular weight epoxies are com- strate is porous enough to permit escape of patible with polyvinyl acetate and have limited solvent. On nonporous surfaces, thickness compatability with a few natural resins and should be limited to about 2 mils and sufficient ester gums. time (1 to 2 days) should be allowed before From the standpoint of composition and use, over-coating, to avoid entrapment of solvent the epoxy resins fall into three categories: (1) that might migrate to the interface and impair unmodified (unesterified) baking types, (2) un- adhesion. The cured coating is almost color- modified (unesterified) air-drying types, and less and is satisfactory for white and pastel (3) ester ified types for either air-dry or bak- finishes as well as for deep colors and clear ing finishes. These are discussed below. coatings. The amine-catalyzed cold-cured epoxy resins 3.10.1. Unmodified Epoxy Baking Resins possess very good chemical resistance, toughness, and durability and are finding increas- The maximum adhesion, durability and re- ingly greater use in industrial maintenance sistance properties of the epoxies are realized paints, architectural paints and enamels, spar with the higher molecular weight, unmodified varnishes for marine applications, and clear (unesterified) epoxy baking resins. These are coatings for floors, furniture, concrete, and dissolved in suitable solvents and blended with metal. They may be applied by brush, spray, certain urea-formaldehyde or phenol-formalde- or flow-coating methods. For spray applica- hyde resins. Upon baking, they cure to tough, tion a special type of spray gun [63] is avail- hard, thermoset coatings that exhibit outstand- able which mixes the resin and hardener in ing adhesion, fiexibility, and chemical resist- the head of the gun at the instant of use (see sec. 5.4.3.6). ance (especially to alkalis) . These coatings find extensive use in applications such as cor- Epoxy-polyamide coatings are discussed un- rosion- and abrasion-resistant tank and drum der Polyamide Resins (sec. 3.11.3). linings, can coatings, appliance primers, refrigerator and washing machine finishes, hos- 3.10.3. Esterfied Epoxy Resins pital furniture, collapsible tube linings, wire enamels, clear metal finishes, and industrial fin- Epoxy resins can be esterified by reaction ishes for equipment that must withstand cor- with fatty or rosin acids in heating kettles.

56 . in a manner somewhat similar to that by which and durability for coatings applications. Long- alkyd resins are made. By the use of different oil epoxy esters find use in air-dry furniture types and amounts of organic acid, a variety and floor finishes, trim and trellis paints, and of air-dry or bake-type vehicles can be pre- general-purpose interior and exterior enamels. pared. The baking types are often blended Short-oil baking types combined with urea- or with amino resins to improve their chemical melamine-formaldehyde resins are used for ap- resistance. The epoxy esters are less expen- pliance finishes, industrial baking enamels and sive and more widely soluble and adaptable primers, metal cabinet enamels, and decorative than the unmodified epoxy resins, but they are enamels. not as chemical- or weather-resistant. They Styrenated epoxy ester resins can be pre- soon chalk on outdoor exposure, but their ulti- pared by adding styrene monomer to the par- mate film life is greater than that of medium- tially esterified epoxy ester and refluxing the length alkyds, and they have better resistance mixture. The resulting resins, while somewhat to mildew and dirt collection. similar to styrenated alkyds, are superior to Where the maximum chemical resistance ob- the latter in outdoor durability, chemical re- tainable with the unmodified epoxies is not resistance, and mar resistance. They find use in quired, the epoxy esters offer a very useful fast-baking enamels, force-dry wood finishes, combination of adhesion, flexibility, toughness. and quick-drying varnishes.

3.11. Polyamide Resins

Polyamide resins are the polymeric conden- phenol, m-cresol, and formic acid. Nylon 6 is sation products of dibasic organic acids and soluble also in aqueous isopropanol and higher polyamines, with recurring amide groups as an alcohols. Nylon resins are strong, tough, and integral part of the main chain. Although the abrasion-resistant. Their low coefficient of different polyamides constitute a large family, friction gives them excellent wear resistance those of chief importance are of three types: even in the absence of lubrication. The excel- (1) the nylons, (2) thermoplastic fatty acid- lent combination of properties possessed by the diamine polymers, and (3) thermosetting fatty nylon resins has led to their wide and impor- acid-polyamine polymers. These types are distant use as a textile fiber and for molded plas- cussed below. tic articles and mechanical parts. However, they have found only limited use in the coat- 3.11.1. Nylon Resins ings field because of their poor solubility and The nylon resins are linear polyamides made high softening temperature. Nylon 6, because by the condensation of di-carboxylic acids with of its wider solubility and lower softening tem- diamines. They are capable of being formed perature, is finding use in both solution and into strong, oriented filaments on stretching. dispersion form as a coating for textiles, The oldest and most widely used type of nylon leather, wire, and heat-sealing foils. is that based on adipic acid and hexamethylene diamine (called nylon 6/6 because both the 3.11.2. Thermoplastic Polyamides acid and amine components have six carbon atoms) . Another major type is nylon 6/10, in Linear, thermoplastic polyamide resins can which sebacic acid is used instead of adipic be made hy the condensation of dimerized fatty acid to yield a softer, more flexible polymer acids with diamines (e.g., ethylene diamine) with lower water absorption and greater stiff- Unlike the nylons, these are fairly soluble and ness at high humidities. A newer type, nylon compatible with a variety of solvents, plasti- 6, is made by heating caprolactam (a six-car- cizers, and resins. Combinations of alcohols bon ring-type amide) under conditions in which and aromatic hydrocarbons are the best sol- the ring opens to form a linear polymer; this vents. They are soluble also in isopropanol type has a lower softening temperature and a and higher alcohols, in hot aromatic hydro- wider solubility than the other types of nylon. carbons, in chloroform, and in acetic acid. Although the nylons are thermoplastic resins, They are not soluble in glycols, aliphatic hy- their heat-distortion temperature (about 270- drocarbons, ketones, or esters. They are com- 360 °F [132-182 °C]) is considerably higher patible with phenolic, maleic, and rosin resins, than that of most other thermoplastic resins but are incompatible with most vinyl resins, and many thermosetting ones. In addition to urea- and melamine-formaldehyde resins, pe- their heat resistance, they have fairly good re- troleum resins, coumarone-indene, and cellu- sistance to sunlight (although they yellow lose derivatives. Although they may be blended slightly) , and they are resistant to weak and and are to a limited extent reactive with epoxy strong alkalis, dilute acids, and most organic resins, the more reactive, thermosetting types solvents. Among the very few good solvents of polyamide are more often used for this pur- for ordinary nylons (nylon 6/6 and 6/10) are pose, as discussed later.

57 The properties of the various polyamide resins, the polyamide serving as both curing resins can be varied over a wide range of flexi- agent and modifier for the epoxy resin. The bility and hardness by blending with each other resulting resin blend yields coatings having ex- or with various liquid plasticizers or compat- cellent gloss, hardness, flexibility, impact re- ible resins. Polyamide coatings offer a good sistance, and abrasion resistance, plus very combination of properties, including tough- good resistance to solvents, chemicals, and out- ness, water and chemical resistance, grease re- door weathering. They are superior to straight sistance, good electrical properties, and excel- epoxies in resistance to continuous water im- lent adhesion to a variety of surfaces, includ- mersion. Although resembling amine-cured ing difficult ones such as cellophane, glassine, epoxies in their ability to cure at room tem- and polyethylene. They are useful as heat- perature without requiring oxygen, the epoxy- sealing coatings for paper, fabrics, metal foils, polyamides have a considerably longer pot life. and many plastics. They also find use in high In many cases they may be applied as soon as gloss, overprint varnishes and lacquers, and in mixed without the induction period recom- lacquer-type printing inks. A recent and in- mended for epoxy resins. Epoxy-polyamide creasingly important use for the thermoplastic coatings exhibit outstanding adhesion to almost polyamide resins is in the formulation of thixo- any type of surface—metals, plastics, glass, tropic paints; for this purpose, they appear to paper, fabrics, leather, wood, ceramics, rub- be superior to the metallic stearates and soap ber, masonry, etc. Effective corrosion-inhibit- solutions used in the past. The polyamides ing primers using zinc chromate pigment can also make good general-purpose adhesives. be made with epoxy-polyamide vehicles provided Polvamide coatings may be applied as a hot the pigment is dispersed in the epoxy resin be- melt, from solution, or as an aqueous disper- fore adding the polyamide. The epoxy-polya- sion. mides also make very good vehicles for zinc-rich paints (see sec. 2.3.1.5). Epoxy-polyamides 3.11.3. Thermosetting Polyamides are used as chemical-resistant can coatings, drum and pipe linings, weather- and water-resistant marine finishes, abrasion- and corrosion- The thermosetting polyamide resins are the resistant coatings for industrial equipment, floor polyfunctional condensation products of dimer- coatings, and in many other applications where ized fatty acids with polyamines (e.g., diethyl- their combination of excellent adhesion, tough- enetriamine) . They range from soft, tacky ness, and chemical resistance are utihzed to resins to pourable fluids. They may be re- advantage. Several Military specifications for acted with epoxy, phenolic, or other resins to epoxy-polyamide coatings are cited in the bibli- produce thermoset resins with very useful ography of chapter 8 [129, 130, 131, 132]. properties. In addition to their usefulness in the coatings field, epoxy-polyamide resins also find use 3.11.3.1. Epoxy-polyamide resins are of par- as general adhesives, as plastic solders and seal- ticular importance. They are obtained by ants, and in casting, potting, and laminating blending reactive polyamide resins with epoxy applications.

3.12. Polyurethanes

The polyurethanes are among the most re- The fundamental urethane reaction, discov- cent and promising resins to achieve promi- ered by Wurtz in 1848, is that of an isocyanate nence in the coatings field. First developed in with an alcohol: (Germany during World War II, the knowledge H was transferred to the United States in 1945, I and manufacture of the necessary diisocyanate R-N = C = 0 + R'-O-H ^R-N-C = 0 raw materials started in this country in 1950. Since 1954, with the establishment of several 0-W large companies for the manufacture and development of polyurethane resins, the use of isocyanate alcohol urethane this versatile class of polymers has grown steadily, with applications in the fields of flex- The active hydrogen atom of the hydroxyl ible and rigid foams, elastomers, synthetic group is transferred to the nitrogen atom of fibers, adhesives, and coatings. the isocyanate group, while the alcohol residue

58 (alkoxy group) becomes attached to the car- type, and a so-called Polyisocyanate type. Both bon atom of the isocyanate group. In the are formulated for room temperature cure, but general urethane reaction, the active hydrogen curing may be hastened and hardness and atom can be furnished by any hydroxyl-bear- toughness of the coating increased by baking. ing compound, including castor oil polyols, carboxylic acids, water, and selected polyesters 3.12.1.1. The PREPOLYMER TYPE of vehicle con- and polyethers—or by analogous compounds in sists of isocyanate-terminated polymers to- which the active hydrogen is attached to a gether with a substantial proportion (about 2- nitrogen or sulfur atom instead of an oxygen 15%) of free toluene diisocyanate (TDI), the atom. Through the reaction of polyfunctional usual isocyanate raw material. Since TDI is isocyanates (usually toluene diisocyanate) very volatile and highly toxic, such prepolymer with polyols, either linear or crosslinked poly- vehicles retain some toxicity until cured. Cur- mers can be formed, depending on the function- ing is accomplished by mixing the prepolymer ality and ratio of the reactants. The proper- with polyfunctional, reactive amines and poly- ties of the polyurethane are dependent also on ols shortly before use. The pot life of the the structure of the polyol employed; for ex- mixed ingredients and the time required for ample, a simple triol such as glycerol will yield the coating to cure depend on the nature and a hard, brittle polymer, while a castor polyol concentration of the curing agents and on the or a polyester, with greater spacing between temperature. Normally, a pot life of 8 hours hydroxyl groups, will yield a flexible polymer. or more can be expected. The usual polyure- The ultimate film properties are affected also thane prepolymer coatings dry in an hour or by the solvent system employed and by the two and may be recoated in four to six hours. curing temperature and humidity. Since iso- Maximum hardness and resistance properties cyanate groups are reactive with water and are developed in about a week. hydroxyl-bearing materials, care must be exercised to use solvents which are water- and 3.12.1.2. The so-called polyisocyanate type alcohol-free. Pigments, fillers, and dyes also is a prepolymer that has been specially proc- must be chosen with care if good package sta- essed to produce a largely tri-functional poly- bility is to be maintained. mer with little if any residual, unreacted TDI. The combination of different isocyanates with Since there is virtually no free TDI in such the large number of available polyols, in vari- vehicles, their toxicity is no longer a problem. ous NCO/OH ratios, makes possible the for- They are cured by mixing with suitable reac- mation of a wide variety of polymers with tive polyols, including the castor polyols, and properties "tailored" to specific applications. various hydroxyl-bearing polyethers and poly- Polyurethane coatings formulated for optimum esters [146]. To a lesser extent, polyamines performance are characterized by an outstand- are used to effect the curing. Depending on ing combination of desirable properties, includ- the nature of the polyol and the drying condi- ing hardness with flexibility, high gloss, and tions, about 2 to 15 hours may be required for excellent resistance to abrasion and chemicals. drying. As with all polyurethanes, the prop- They may be applied by dipping, knife, or erties of the final coating are determined large- spray, ly by the nature of the modifying polyol ; that Polyurethane coatings are available either is, by the number and spacing of hydroxyl as two-component systems that are mixed short- groups in the polyol. As has already been ly before use and cure by direct crosslinking, pointed out, either linear, branched, or cross- or as single-package materials that cure when linked polymers can be obtained, depending exposed as a film to moisture, oxygen, or heat. on the functionality and ratio of the reactants. The reaction between isocyanate groups and As a matter of convenience in mixing and use, active hydrogen atoms proceeds readily at the proportioning of the polyisocyanate pre- room temperature without catalysts; however, polymer and polyol curing agent can readily the reaction is greatly accelerated by the ad- be worked out in advance by the manufacturer dition of small amounts of tertiary amines, so- to permit the mixing of equal volumes of poly- dium alkoxides, or Friedel-Crafts catalysts, or isocyanate and polyol on the job for optimum by heating. The two-component polyurethane results. systems at present provide the maximum versatility and best properties obtainable with this 3.12.2. Single-Package Urethane Systems class of resins, although continuing research on one-component and on solventless systems Single package polyurethane coating sys- may change this situation. tems are formulated to cure by application of heat, by oxidation, or by reaction with atmos- 3.12.1. Two-Component Urethane Systems pheric moisture.

Two-component polyurethane coating sys- 3.12.2.1. Heat-cured one-package polyure- tems are of two types: a so-called Prepolymer thane vehicles can be prepared by temporarily

59 . blocking the reactivity of the -NCO groups in in about an hour by solvent evaporation and a polyisocyanate prepolymer by loosely asso- then cures by reaction with atmospheric mois- ciating them with a reactant (e.g., phenol) ture to a crosslinked coating. Recoating is us- that is driven off at elevated temperatures. ually possible in four to six hours, and optimum While the isocyanate groups are blocked off film properties are developed in about a week, at room temperature, suitable polyol or amine as with the two-package systems. However, curing agents may be added vi'hich will not be under low humidity conditions (below 30% effective until the composition is heated to R.H.) longer curing times may be required. reactivate the isocyanate groups. Such heat- cured systems have been particularly useful as 3.12.3. Properties and Uses of the wire coatings that permit soldering without the Polyurethanes need for stripping the insulation. It has been stated that a wide variety of polyurethane coatings for particular applica- 3.12.2.2. Air-cured one-package polyurethane tions is possible, depending on the nature of vehicles may appropriately be termed urethane- the reactants employed. For example, by vary- alkyds, since they are made—like alkyd resins ing the molecular weight of the polyol (and —by cooking together a polyol and a drying hence the distance between cross-links), ex- oil, with toluene diisocyanate as the other major tremely hard or soft flexible polymers can be reactant instead of phthalic anhydride. Such obtained. Tough polyurethane coatings com- vehicles are sometimes called urethane oils. bining hardness with flexibility can be pre- They dry by oxidation of the carbon double- pared which are well-suited as floor varnishes bonds in the drying oil and hence require the for severe service, such as on gymnasium floors, use of ordinary amounts of conventional paint and as porch and deck enamels on wood or driers in order to cure within a reasonable concrete. Their excellent mar and abrasion length of time. The performance of such coat- resistance is also utilized in furniture finishes. ings is quite similar to that of conventional A polyurethane coating [147] for use on poly- alkyds. ester-fiberglass gunstocks over a polyamide- epoxy primer [148] is covered by Mil. Spec. 3.12.2.3. Moisture-cured one-package poly- MIL-P-46057. Their water and corrosion re- urethane vehicles utilize the ready reactivity sistance makes them useful for marine paints of isocyanate groups with water to effect cure and for metal primers and enamels. Their of the coating. The functionality and molar weather resistance is utilized in clear varnishes ratios of the reactive ingredients are impor- for boats and exterior wood siding (the poly- tant factors in the successful formulation of urethanes do not adhere satisfactorily to oil such vehicles. The reaction between water and based wood fillers and stains but adhere well isocyanate first produces an amine and carbon to alcohol- or water-based stains and fillers) dioxide gas. The amine reacts with more iso- The excellent alkali resistance of the polyure- cyanate and gradually builds up a polymeric thanes finds use in masonry coatings and swim- structure, while the carbon dioxide gas diffuses ming pool paints. Although polyurethane coat- out of the film. The cured film has properties ings tend to darken somewhat on exterior approaching those of the two-component sys- exposure, they have excellent gloss and gloss tems, but the rate of cure depends on the pre- retention in applications where prolonged dec- vailing weather conditions. In a typical one- orative as well as protective qualities are de- package system, the film is laid down from a sired. The polyurethanes are still relatively water-free solution of an isocyanate prepolymer expensive materials but are becoming cheaper in organic solvents. The film dries tack-free as their volume of use steadily grows.

3.13. Polyethylene

The outstanding properties of polyethylene systems to effect coatings application. Though are its chemical inertness, solvent resistance, generally inert, polyethylene is attacked by hal- flexibility, toughness, excellent dielectric and ogens and concentrated nitric acid and is solu- related properties, and low cost. Although its ble in hot aromatic hydrocarbons. It is resist- widest use is in the form of film, sheet, and ant to other mineral acids including hydro- molded articles—its coatings applications be- fluoric, and is unaffected by strong caustic. ing limited by its complete insolubility in organic solvents at room temperature—the excel- 3.13.1. Low- and High-Density Polyethylene lent properties of the resin have been utilized in the coatings field by employing hot melt or Polyethylene is prepared by the addition poly- flame-spraying methods (sec. 5.4.3.4), a fluid- merization of the ethylene monomer. The ized resin bed technique (see sec. 5.4.6 on properties of polyethylene, like all polymers, Methods of Application), or water-dispersed vary with molecular weight and structure and

60 with molecular weight distribution. In the cept at elevated temperatures) to strong acids case of polyethylene, an additional parameter, (except concentrated nitric) and alkalis, sol- density, is an important factor in determining vents, greases, and oils. Polyethylene-coated the properties of the material. The density of aluminum foil is heat-sealable and completely polyethylene may range from 0.92 (g/cc) in moisture-proof. Polyethylene-coated cellophane the older, conventional, low-density type to and polyester film are finding increasing 0.96 in the newer, linear, high-density type. acceptance. Cardboard cartons for milk and Conventional polyethylene is made by a high- other liquids are being coated with poly- pressure process, usually by mass polymeriza- ethylene plasticizers and other modifiers are ; tion, but sometimes by a "solvent" or slurry unnecessary in polyethylene, and it is odorless, process; the resulting polymer has a branched tasteless, and nontoxic. Polyethylene is ex- structure which reduces the tendency toward truded onto fabrics to make them waterproof. crystallization, the degree of crystallinity be- Nonwoven mats of cellulose fibers coated with ing about 60-65 percent. The new, linear type polyethylene are used to make disposable baby of polyethylene is virtually free from branch- diapers and hospital sheets. Irregularly shaped ing, so that greater alignment and close-pack- metal parts can be coated with polyethylene by ing of polymer chains is possible, and this type a fluidized-bed technique (see sec. 5.4.6). A has a degree of crystallinity of 80-85 percent; heavy coating of polyethylene can be applied it is manufactured by a low-pressure proc- to metal surfaces by flame-spraying (see sec. employing ess of solution polymerization the 5.4.3.4) ; thus, tough chemical- and abrasion- new stereospecific catalysts (see sec. 3.1.4.5 on resistant coatings can be applied to chemical

Stereoregular Polymers) . The greater crystal- storage tanks, metal bench tops, drums, and linity of linear polyethylene gives it a higher pipe. The excellent electrical properties of density, a higher melting point, greater im- polyethylene are utilized in coatings for wire permeability to moisture and gases, greater and cable. stiffness and tensile strength, and greater sol- For elevated temperature or outdoor appli- vent and chemical resistance than is possessed cations, polyethylene must be stabilized against by the conventional, low-density type. Molec- oxidation. Effective stabilizers are available. ular weights as high as 2,000,000 can be Carbon black is a particularly effective light achieved in high-density polyethylene, whereas stabilizer for polyethylene subjected to exterior a molecular weight of about 50,000 is the high- exposure. Heat stabilization is accomplished est practically obtainable in the conventional by incorporation of an antioxidant. type. A medium-density polyethylene with proper- 3.13.2. Irradiated Polyethylene ties intermediate between those of the low- and the high-density types is also being manufac- Polyethylene can be crosslinked by a few tured, by both the high- and the low-pressure seconds exposure to sufficiently intense high- processes with recently-developed modifica- energy radiation. By regulating the radiation tions. All three types of polyethylene (high-, dose, the degree of crosslinking can be con- medium-, and low-density) are readily melted, trolled so that the melting point of the resin molded, extruded, or heat-sealed. The heat- is raised as much as 50 °F (28 °C) above that sealing temperature for the linear, high-density of normal polyethylene without greatly alter- type is about 50 °F (28 °C) higher than that ing its basic mechanical properties. Very high of the conventional, low-density type. dose levels will render the resin infusible at Since the cost of high-density polyethylene is temperatures up to the decomposition point. It greater than that of the low-density type, the is interesting to note that the increase in soft- choice of type is dictated by the requirements ening temperature induced in conventional of the application. For example, where a high polyethylene by irradiation is about the same degree of impermeability is desired, a medium- as that achieved in going from low-density to density material is greatly superior to the low- high-density polyethylene. The high-density density type, but the degree of impermeability material also can be improved by irradiation. is not much increased by further increasing the density. the other hand, where maximum On 3.13.3. Chlorosulfonated Polyethylene hardness and heat resistance are required, a high-density polyethylene will give the best re- Chlorosulfonated polyethylene is an elasto- sults. meric material produced by the simultaneous Uses. The most important application for chlorination and sulfonation of already-poly- polyethylene in the coatings field is in packag- merized relatively high molecular weight lowing. Extruded in a thin film onto a moving density polyethylene, by treatment with gaseous sheet of paper, aluminum foil, or other pack- chlorine and sulfur dioxide in the presence of a aging material, it provides a clear, strong, flex- free-radical initiator (e.g., a peroxide) as cata- ible, abrasion-resistant, heat-sealable coating of lyst. The chlorine atoms reduce the crystallin- low permeability that is highly resistant (ex- ity (and hence the stiffness) of the polyethylene

61 without greatly changing its desirable prop- Solution coatings of chlorosulfonated poly- erties. The sulfonyl chloride groups provide ethylene can be formulated in a variety of reactive sites through which subsequent cross- colors, including white. They can be applied linking (vulcanization) can take place. By to virtually all other elastomeric compositions varying molecular weight, chain branching, to provide protection against weathering and and degree of crystallinity and cross-linking ozone. The coatings may be used without cur- through control of chlorine and sulfur content, ing if a light stabilizer and detackifying resin a variety of chlorosulfonated polyethylene de- are incorporated into the formulation. How- rivatives can be prepared. The product offered ever, a heat cure is required for maximum re- commercially is usually derived from a poly- sistance properties. Coatings that will cure at ethylene having a molecular weight of about room temperature can be prepared by using 20,000, and contains about 27 percent chlorine special accelerators. Unpigmented lacquers and 1.5 percent sulfur. Curing is effected based on chlorosulfonated polyethylene can be through the sulfonyl chloride groups by em- applied to rubber sporting goods and footwear ploying curing systems based on a polybasic to provide a durable, lustrous, protective finish. metal oxide or metal salt of a weak acid (e.g., Cotton and synthetic fabrics coated with this litharge or magnesia), an organic acid, and an resin exhibit excellent weathering resistance. organic accelerator. The cured polymer has Chlorosulfonated polyethylene coatings are excellent resistance to abrasion, heat, weather- proving useful in rugged service applications ing, and cracking, and is completely resistant to such as upholstery fabric, awnings, tarpaulins, ozone; it has good resistance to mineral oils heavy duty gloves, and as industrial mainte- and to corrosive chemicals such as oxidizing nance coatings in corrosive [141] or abrasive acids, alkalis, chrome plating baths, and hydro- environments. Coatings of chlorosulfonated gen peroxide. The litharge-cured coating sys- polyethylene have excellent adhesion to chlori- tems have good resistance to prolonged water nated rubber-primed [140] sandblasted steel. immersion, but the magnesia-cured systems are They may also be applied to wood and masonry inferior in this respect. surfaces. (See also sec. 3.15.2).

3.14. Fluorocarbon Resins

The fluorocarbon resins are substituted ethyl- perature (650 °F [343 °C]) to which the resin ene plymers in which hydrogen atoms have particles as manufactured must be heated to been replaced with fluorine atoms or both fuse them into a continuous mass. The resin is fluorine and chlorine atoms. The resulting available in the form of granules for compres- polymers are characterized by their chemical sion molding, as a powder for extrusion mold- inertness, low coefficient of friction, heat re- ing, and as an aqueous dispersion for coatings sistance, and strength and flexibility over a applications; in each case, fusion of the resin wide temperature range. Of chief interest are particles is achieved by sintering as indicated polytetrafluoroethylene and polychlorotrifluoro- above. ethylene. Polytetrafluoroethylene has a translucent, white, or grayish color, and a waxy feel. Noth-

3.14.1. Polytetrafluoroethylene ing will stick to the untreated surface ; however, the surface can be dulled by treatment with a Polytetrafluoroethylene is a completely fluori- molten sodium-ammonia mixture and thus ren- nated, linear polymer having a dense, symmetri- dered capable of being bonded with various cal, highly crystalline structure that is com- commercial adhesives. Polytetrafluoroethylene pletely resistant to chemical attack by all known has a very low coefficient of friction—the low- substances except molten alkali metals, and est of all known solids—and the static coeffi- fluorine at high pressures. It is the most heat- cient is lower than the dynamic coefficient ; thus, resistant organic polymer known, being capable bushings made of or coated with the resin ex- of continuous service at temperatures up to 500 hibit smooth, non-binding motion and long wear °F (260 °C). Although it is thermoplastic and without the need for lubrication. Polytetra- softens at about 620 °F (327 °C), it does not fluoroethylene retains its good strength and melt, and at about 750 °F (399 °C) it decom- flexibility over a very wide temperature range, poses slowly to yield the monomer and lesser from -150 to 500°F (-101 to 260°C), and it amounts of various gaseous fluorine deriva- has very high impact strength. It is nonflam- tives. The decomposition products are danger- mable. Its water absorption is practically zero. ously irritating and toxic and should not be in- It has a specific gravity of about 2.2. Although haled. Small amounts of gaseous decomposition possessing excellent stability, it exhibits some products are given off also at the sintering tem- cold flow (non-elastic, permanent deformation

6^ under stress) at moderately elevated tempera- to produce polymers ranging from low molec- tures, the heat distortion temperaure for the ular weight oils, greases, and waxes to high bulk material beging 250 ° F (121 ° C) at 66 psi. molecular weight (300,000 to 400,000) molding The electrical properties of polytetrafluoro- and dispersion resins. ethylene are outstanding. It has a very low Although not as extremely inert and ther- dielectric constant and very low dissipation fac- mally stable as polytetrafluoroethylene, poly- tor which remain virtually unchanged in the chlorotrifluoroethylene finds many similar uses. frequency range from 60 cycles to 10,000 mega- It is resistant to all corrosive chemicals (in- cycles. Dielectric strength and arc resistance cluding fuming oxidizing acids) except molten are excellent. alkali metals, fluorine, and highly halogenated To attain in a coating the maximum perform- compounds. It is insoluble in all organic liquids ance properties of which polytetrafluoroethylene at room temperature, but is swelled slightly by is capable, it is necessary that the coating be halogenated solvents, toluene, diethyl ether, and free of pinholes. Since the resin does not melt ethyl acetate. At elevated temperatures, it is or flow during the fusion process, pinholes are swelled by still other liquids and can be dis- likely to be present unless multiple coats (at solved by a few, such as highly halogenated least three) are applied. compounds. The softening temperature of the molding and dispersion resins vary Despite its high cost, the unique combination may from about to of desirable properties possessed by polytetra- 350 about 420 °F (177 to 216 °C), depending on their molecular weight and degree fluoroethylene makes its use both practical and of crystallinity. Polychlorotrifluoroethylene is economical for severe applications in which capable of service maximum chemical resistance and heat stability continuous at temperatures up to 400 °F °C), while retaining an ap- are required. Aside from its uses in molded, (204 preciable machined, and extruded forms, polytetrafluoro- degree of flexibility at temperatures around —200 It is ethylene in the form of dispersion coatings may °F (— 129 °C) or lower. resistant to be applied to metals, glass, ceramics, wire, fab- weathering and aging, and is nonflammable, strong, tough, and abrasion-resist- rics, and mats. Sintering at about 650 °F (343 ant. It °C) fuses the particles into a continuous, tough, has greater resistance to cold-flow than the tetrafluoro extremely resistant film that can be used in polymer. Its electrical prop- such applications as a lining for corrosive-reac- erties are excellent; it combines high dielectric strength and arc resistance with a low dissipation vessels ; as a chemical-resistant coating for tion factor (the latter is not as low, however, as drums and tanks ; as a protective coating on ma- that of polytetrafluoroethylene, polyethylene, chinery and equipment subjected to heat, fumes, and spillage; as an anti-adhesion coating on and polystyrene). processing equipment and moving parts subject For coating purposes, dispersions of finely to clogging and misfunction as a result of un- divided polychlorotrifluoroethylene in volatile desired buildup of materials (e.g., buildup of organic solvents are generally employed. These resins may be applied by spraying, spreading, glue on packaging machines) ; as a low-friction, wear-resistant coating for machine parts that or dip-coating to metals, ceramics, glass, wire, cannot be lubricated ; and as an insulating coat- and certain fabrics and other materials. Spe- ing for wire and electrical parts for severe or cial primers are available to increase adhesion critical electronic applications. where necessary. After evaporation of the organic solvent the coating is fused by heating to 3.14.2. Polychlorotrifluoroethylene (Kel-F) the melting point to form a tough, permanent, continuous film that has all the properties of Polychlorotrifluoroethylene is a completely the basic resin. Since the resin actually melts halogenated, linear polymer containing, on the and flows during the baking process, a pinhole- average, one chlorine atom for every three free coating can be obtained. Despite the high fluorine atoms. The fluorine imparts chemical cost of polychlorotrifluoroethylene resins, they inertness, thermal stability, and almost zero are widely used as corrosion- and chemical- water absorption to the polymer, while the resistant coatings for metals in severe industrial chlorine in part by reducing molecular regu- — applications where their exceptional resistance larity and crystallinity—contributes a lower properties provide long-term savings in main- softening temperature, meltability and mold- tenance and replacement costs. Typical indus- ability, reduced cold-flow, and greater trans- trial applications parency. The degree of crystallinity depends include coatings for troughs, on the temperature treatment received during tanks, vats, hoppers, mixers, paddles, pumps, the manufacturing quick quenching of and conveyors. Fabrics coated with the resin process ; the heated polymer yields a relatively amor- are rendered waterproof and resistant to heat, phous material with much smaller crystallites, chemicals, fungi, weathering, and abrasion, while slow cooling produces a more crystalline while retaining flexibility at very low tempera- polymer. Molecular weight also can be varied tures.

63 3.15. Synthetic Rubbers

Synthetic rubbers, in addition to their useful- erators (basic metal oxides, such as magne- ness as substitutes for natural rubber, also find sium oxide or zinc oxide) are generally em- important uses as adhesives, and in coatings ployed to shorten the baking time or to lower where resilience, abrasion resistance, distensi- the baking temperature. Satisfactory room- bility, and elasticity are of prime importance. temperature curing neoprene coating formula- Among the synthetic elastomers available are tions also have been developed and are commer- a number of materials which combine excellent cially available. heat, chemical, and weathering resistance along Neoprene coatings are usually supplied as so- with their desirable elastomeric properties. lutions of the uncured polymer in aromatic Those of particular interest to the coatings field solvents. Catalysts or accelerators are added are discussed in the following sections. immediately prior to use. Heat-curing types are usually syrupy liquids that are best applied 3.15.1 Polychloroprene (Neoprene) by brush or dip-coating, or they may be soft pastes that can be applied by troweling. Air- Neoprene is a tough, resilient material pos- drying types may usually be applied by brush, sessing outstanding resistance to oils, alkalis, dip, spray, roller, or flow. Depending on the heat, weathering, and abrasion. It will with- nature of the formulation, thicknesses of 3 to 6 stand continuous exposure to sulfuric and phos- mils per coat are readily obtained by the afore- phoric acids and many other inorganic and or- mentioned methods. Troweling yields coating ganic acids, but is decomposed by very strong thicknesses several times greater per coat. oxidizing compounds such as nitric and chromic Where maximum protection, durability, and re- acids, sulfur trioxide, and hydrogen peroxide. silience are required (especially on sharply con- Neoprene is swelled and softened by aromatic toured surfaces) coatings up to 14 in thick- , inch and halogenated hydrocarbons, aromatic alco- ness can be obtained with multiple trowel coats. hols, esters, ketones (except acetone), turpen- An adequate period (several hours) of air dry- tine, and carbon disulfide. It is very resistant ing should precede baking to prevent blister to oils, aliphatic hydrocarbons, aliphatic alco- formation from too rapid an escape of solvent. hols, acetone, water, and most salts. Ethers In general, neoprene coatings are utilized at and aldehydes, in general, have only a mild thicknesses of 10 to 15 mils. A primer is re- swelling effect on the polymer. Neoprene required for maximum adhesion. Chlorinated tains most of its desirable properties at tem- rubber-base primers [140] have been very ef- peratures as high as 150 °F (66 °C) and is fective for this purpose. Neoprene coatings serviceable at temperatures up to 250 °F (121 are generally available only in gray or dark °C) for some applications. It remains flexible colors, chiefly black and green. down to —20 °F (—29 °C), becoming brittle at lower temperatures. Long exposure to sunlight Uses. The excellent combination of resist- and weathering has little effect on neoprene ; it ance and durability properties possessed by tends to become hard and dry after long ex- neoprene has been proved in many severe in- posure, rather than soft and sticky as does dustrial applications since 'its introduction in natural rubber. Neoprene is virtually non- 1932. Neoprene coatings are widely used to flammable. provide lasting protection against corrosion Neoprene is prepared by the emulsion poly- and abrasion of industrial equipment such as merization (see sec. 3.1.3.3) of chloroprene in ductwork, pipelines, storage tanks, scrubber an alakaline medium. The polymerization is towers, tumbling drums, coal chutes, pump and halted when about 30 percent complete, while blower blades, ship propeller shafts, tool han- the polymer is still thermoplastic. The polymer dles, and machinery. Neoprene coatings may is recovered by acidifying the emulsion to a be applied to wood and concrete flooring, ramps, point just short of coagulation and then freeze- and steps to provide protection against wear coagulating the polymer onto a brine-cooled and chemical splash or spillage ; anti-skid prop- drum, from which it is stripped, washed, and erties can be imparted by incorporating grit or dried. Several types of neoprene, differing in sand in the applied coating. Neoprene finds use uncured properties and manner of cure, are as a chemical- and abrasion-resistant coating manufactured. Also, several types are mar- for textiles, rubber, and leather for such items keted in latex form. The partially polymerized as industrial gloves, heater and brake hose, belts, polymer can be milled and compounded like raw and conveyors. As a coating for ignition wire, rubber, and then vulcanized (crosslinked) by neoprene affords excellent resistance to oil and heat and/or catalysts to complete the polymer- heat. Neoprene coatings on military aircraft ization. Although heat alone suffices to cure have exhibited excellent resistance to abrasion neoprene quite rapidly (in an hour or two) at and erosion, although the v/eight of the thick temperatures around 270 °F (132 °C), accel- coatings employed (about 10 mils) constitutes

64 somewhat of a drawback in aerodynamic appli- by aromatic and chlorinated hydrocarbons and cations. Neoprene coatings are widely specified are swelled by oils and greases. Styrene-buta- maintenance finishes in oil refineries, chemical diene rubber is also used in general-purpose ad- plants, garages, filling stations, and aircraft hesives. installations.

3.15.4. Butadiene-Acrylonitrile Rubber 3.15.2. Ghlorosulfonated Polyethylene (Nitrile Rubber)

Chlorosulfonated polyethylene, was discussed The butadiene-acrylonitrile copolymers (ni- previously in section 3.12.3 under Polyethylene. trile rubbers) are similar to the butadiene-sty- It may be useful here, however, to point out the rene rubbers in general appearance and physi- considerable similarity between the properties cal properties, but nitrile rubbers have better and applications of chlorosulfonated polyethyl- resistance to most solvents (although they are ene and those of neoprene, while indicating a soluble in ketones), greater resistance to the basis for choosing between them. Both are deteriorative effects of heat and sunlight, and highly suitable for applications in which resist- excellent resistance to oils, greases, and waxes. ance to heat, chemicals, weathering, and abra- The oil resistance increases with increasing sion are required. Neoprene is less expensive nitrile content. Vulcanization is accomplished than chlorosulfonated polyethylene and is supe- with sulfur or sulfur-bearing materials, as rior in oil resistance. Chlorosulfonated poly- with butadiene-styrene rubbers. Nitrile rub- ethylene, however, possesses superior resistance bers have found use in the coatings field as to strong oxidizing agents such as nitric [141] nonvolatile, non-extractable plasticizers for and chromic acids, hypochlorite and chlorine phenolic resins and for polyvinyl chloride res- dioxide solutions, hydrogen peroxide, and ozone ins. In latex form, nitrile rubbers have been —agents which soon deteriorate neoprene. It used as coatings and impregnants for paper, is available in a wide range of durable colors, fabrics, and leather products. both light and dark, whereas neoprene is limited to gray or dark colors. Also, chlorosul- 3.15.5. Isobutylene-Isoprene Copolymer fonated polyethylene coatings are capable of (Butyl Rubber) service at higher temperatures than neoprene, the limit upper for continuous service being Butyl rubber consists of about 98 percent about 220 °F (104 °C) as compared with about isobutylene copolymerized with about 2 percent 150 °F (66 °C) for neoprene. Neoprene, there- isoprene. The unsaturation imparted by the fore, is the general-purpose material, while isoprene renders the copolymer vulcanizable, a chlorosulfonated is its polyethylene used where property not possessed by 100 percent polyiso- specific superior properties justify the higher butylene. This degree of unsaturation is very cost. low, however, compared to that found in natural rubber, and accounts for the chemical inertness 3.15.3, Styrene-Butadiene Rubber (SBR) of butyl rubber. Vulcanization, though more difficult than with diene polymers, can be ac- Styrene-butadiene copolymers of high buta- complished with combinations of sulfur and diene content are soft, elastomeric materials, as thiuram accelerators or with special methylol- distinguished from the harder, high styrene containing resins. types usually used in the coatings field, pre- Butyl rubber is highly impermeable to gases dominantly in latex paints. The high styrene/ (a property that has led to its wide use for low butadiene resins were discussed in section automobile inner tubes). It has a "dead" feel, 3.8.2 under Styrene-Butadiene Copolymers. its resilience being strikingly lower than that of The styrene-butadiene copolymer elastomers natural rubber. usually contain about 75 percent butadiene; Butyl rubber is highly resistant to concen- however, copolymers ranging all the way from trated acids and alkalis, corrosive salts, and 50 percent butadiene to 100 percent polybuta- fresh and salt water, and it has found use as diene are manufactured, and are available both linings for chemical tanks, pumps, and hoses. as solid polymers and as latexes. They are Its good electrical insulating properties cou- vulcanized with sulfur or sulfur-bearing mate- pled with heat resistance to about 212 °F (100 rials. Although the chief use for styrene- °C) and excellent resistance to aging and ozone butadiene copolymers is for SBR synthetic rub- make it useful as a wire coating. Because of f ber (formerly called GR-S), they also find some its paraffinic hydrocarbon nature, it is resistant use in coatings where their elastomeric prop- to esters, ketones, alcohols, animal fats, and erties are advantageous, as in coatings for vegetable oils, but it is attacked by petroleum j

I rug backing, paper, fabrics, and leather. Such hydrocarbons, aromatic hydrocarbons, chlorin- coatings are tough, flexible, abrasion-resistant, ated solvents, and carbon disulfide. It remains

1 ' and acid- and alkali-resistant, but are attacked flexible at temperatures as low as —400 °F

65 (—240 °C), even after prolonged aging, and is alkalis, and steam. Their electrical properties proving useful in cryogenic applications. Its and their resistance to weathering are excellent. flexibility has made it useful as a coating for Silicone rubber coatings are available as solu- fabrics. Recently, efforts have been made to tions, pastes, and water dispersions, for appli- utilize the resistance of butyl rubber to aging, cation by spraying, brushing, dip-coating, or weathering, sunlight, and ozone in solution-type spreading. They are applied to fabrics (in- industrial maintenance coatings being offered cluding glass fabric), ceramics, asbestos paper, for metal and masonry surfaces. metals, and wire to provide resilient, flexible, durable coatings that retain their desirable 3.15.6. Polysufide Rubber properties over a very wide temperature range.

Organic polysulfide polymers result from the 3.15.8. Polyurethane Rubber condensation of organic halides with alkali polysulfides. They range from viscous liquids The general chemistry and properties of the to rubber-like solids. The polysulfide rubbers polyurethanes have been discussed in section are processed and handled like natural rubber. 3.12. There it was noted that a wide variety of Curing is accomplished with metallic oxides products can be made from the wide choice of (usually zinc oxide). Polysulfide rubbers have polyols available. For the production of poly- excellent resistance to organic solvents, includ- urethane elastomers, hydroxyl-bearing polyes- ing most aliphatic and aromatic hydrocarbons, ters or polyethers are employed for reaction alcohols, ketones, and esters. Coatings based with the diisocyanate (usually toluene diiso- on polysulfide rubber have low permeability to cyanate) . Because of the wide spacing between gases, good electrical properties, excellent re- reactive groups in such polyols, a flexible struc- sistance to aging, sunlight, and ozone, and af- ture is produced. Polyurethane rubber pos- ford good protection against fresh and salt sesses high tensile and tear strength and ex- water. Their resistance to acids and alkalis is cellent abrasion resistance superior to that of only fair. Although the tensile strength and the best cold rubber. Resistance to oxygen, abrasion resistance of polysulfide rubbers is ozone, and ultraviolet is excellent. Resilience generally lower than that of some other syn- and wear properties are outstanding. Optimum thetic rubbers, specially formulated coatings elastomeric properties, however, are not re- based on polysulfide rubber exhibit excellent tained over a wide temperature range; poly- abrasion resistance. Blends of polysulfide rub- urethane rubber hardens at moderately low ber with vinyl resins have produced durable and temperatures (below —20 °F [—29 °C]) and useful coatings. Polysulfide rubber coatings weakens when subjected to high temperatures, have found use as solvent-resistant linings for steam, or hot water. Good resistance to sol- chemical tanks and equipment, and as solvent- vents, alkalis, and acids can be built into the resistant coatings for paper, fabrics, leather, polymer. In addition to its uses as a premium wood, and concrete. specialty rubber and as a resilient, durable, foam or sponge rubber, polyurethane rubber is 3.15.7. Silicone Rubber used as a coating to provide wear-resistant ve- neers on tires, soles, heels, belts, rubber and Silicone elastomers are high molecular plastic goods, and flooring. Fabrics coated with weight, linear organosilicon polymers com- polyurethane rubber are flexible, wear-resistant pounded with an inorganic filler and a vulcan- and weather-resistant. izing agent (usually a peroxide). Recently, vinyl curing systems, involving the introduction 3.15.9. Chlorinated Rubber of reactive pendant groups into the polymer and use of a specifically-reactive peroxide, have been Chlorinated rubber (or Parlon®, as it is developed. The general chemistry and prop- widely known in the trade) is prepared by the erties of silicone resins have been discussed in addition and substitution of chlorine atoms into section 3.9.1. Silicone elastomers are outstand- the polyisoprene molecule to yield a material ing in their retention of flexibility over a wide containing 63 to 67 percent chlorine. The chlo- range of temperatures, from —100 to 500 °F rinated product is a thermoplastic resin rather (—73 to 260 °C). Whereas most organic elas- than an elastomer and is used principally in tomers soon soften and deteriorate at tempera- coatings, inks, and adhesives. Chlorinated rub- tures above 200 °F (93 °C), the silicone elas- ber is outstanding in its resistance to water and tomers are virtually unaffected by continuous common corrosive chemicals. It possesses a service at a temperature of 300 °F (149 °C high degree of impermeability to water vapor and will withstand a temperature of 600 °F and a very low water absorption. It is highly (316 °C) for a brief period. Although very re- resistant to strong alkalis and acids, strong sistant to ozone and to hot oils, the silicone elas- bleaches, corrosive vapors and fumes [140], tomers are poor in resistance to hydrocarbon soaps and detergents, mineral oils, mold and solvents, and they are attacked by acids, strong mildew. It is deteriorated by heat and ultra-

66 violet light, but can be stabilized to improve its resistant to many oils, even when hot, they are resistance to these influences. It is odorless, not resistant to aromatic solvents nor to glycols tasteless, nontoxic, and non-flammable. or steams. They are not well-suited for low Chlorinated rubber is readily soluble in chlo- temperature applications. In both solution and rinated aromatic hydrocarbons and has found latex form, the polyacrylic ester rubbers are important use in corrosion-resistant paints and finding use in the coatings field. primers, industrial maintenance coatings, traffic paints, swimming pool paints [35], and con- 3.15.11. Gyclized Rubber crete and masonry paints [34] . Other uses are Cyclized rubber is made by treating natural in flame-retardant coatings, textile coatings, rubber with strongly acidic reagents (e.g., sul- printing inks, and adhesives. Properly formu- furic or sulfonic acids) which reduce the orig- lated chlorinated rubber coatings have proven inal unsaturation through the formation of very effective as adherent, corrosion-resistant cyclic structures. The resulting product is primers for metals which are to receive neo- hard, resinous, and thermoplastic instead of prene topcoats. Paints, enamels, and lacquers rubbery. While remaining soluble, like natu- based on chlorinated rubber are very quick- ral rubber, in hydrocarbon solvents, it is ren- drying compared with most other film-forming dered much more resistant to water, acids, materials. and alkalis. Cyclized rubber is supplied in the form of granules or powders that can be applied 3.15.10. Polyacrylic Ester Rubber as hot melts or from solution in organic solvents to give adherent, flexible coatings having low Polyacrylic ester rubbers are specialty poly- water penetration and good chemical resist- mers that can be cured with special vulcanizing ance. It finds use in coatings for paper and agents, such as polyfunctional amines, to give metals, in acid- and alkali-resistant paints, in products highly resistant to weathering, dis- printing inks, and as a strong metal-to-rubber coloration, ozone, and gas diffusion. Although adhesive.

3.16. Water-Soluble Resins

3.16.1. Thermoplastic Types high film integrity of solution-type coatings. Such coatings are based on a water-soluble Several classes of water-soluble resins of the resin that can be converted by heat, oxidation, thermoplastic type have been discussed in ap- and/or chemical agents into a crosslinked, in- propriate sections earlier in this chapter. For soluble, thermoset material. A general discus- example, methyl cellulose and other soluble sion of water-solution type coatings, indicating cellulose derivatives were described in section background, properties, formulation, and use, 3.2.3. and polyvinyl alcohol was described un- was presented in section 2.3.6. A more specific der Vinyl Resins in section 3.6.4. Methyl cellu- discussion of the resins themselves is given lose and other soluble cellulose derivatives are here. typically employed as thickening, stabilizing, The thermosetting water-soluble resins gen- and emulsifying agents in water-base paints erally contain hydroxyl and/or carboxyl groups, more often than as coatings in themselves. the latter sometimes reacted with a volatile or- Polyvinyl alcohol, while used for the above pur- ganic base such as morpholine, to render them poses, also finds use as a film in its own right water-soluble. These same polar groups im- because of its excellent resistance to organic part a chemical reactivity to the polymer mole- solvents and its high degree of impermeability cule which permits it to be crosslinked by reac- to vapors and gases. Moreover, although poly- tion with a suitable resin or drying oil, under vinyl alcohol itself is a linear thermoplastic the influence of heat, oxygen, and/or catalysts. polymer, it can be crosslinked with suitable Amino resins have been particularly effective chemical agents to insolubilize it, as noted in for this purpose. section 3.6.4. A water-soluble melamine-acrylic resin for industrial finishes has been described [123] 3.16.2. Thermosetting Types which is available as a clear solution in water, requiring only the addition of pigment to pro- The success of water-thinned latex paints in duce a ready-to-spray enamel. When baked, the consumer field has led to intensive effort on the acrylic and melamine components become the part of resin producers and paint manufac- crosslinked to form a hard, durable, high gloss turers to develop practical and durable water- finish said to be comparable to the best mela- base industrial coatings that will combine the mine-alkyds. safety and convenience of latex paints with the Water-soluble baking resins composed of

67 trimellitic anhydride, a glycol, and a dicar- peratures as low as 250 °F (121 °C) to yield

boxylic acid have been described [64] ; these are coatings having high gloss and good mar resist-

self-curing vehicles vv^hich, on baking at 350 to ance ; such resin blends are somewhat less flexi- 500 °F (177 to 260 °C), are reported to yield ble and less resistant to detergents than the coatings having high gloss, good flexibility and self-cured vehicles. impact resistance, high hardness, good mar re- Technological advances from development sistance, excellent salt-spray resistance, and work now in progress can be expected to yield good resistance to hot detergents. Vehicles in an increasing number of water-soluble resins which the trimellitic resin is combined with a for industrial applications in the not-distant water-soluble amino resin can be baked at tem- future.

3.17. Miscellaneous Resins

3.17.1. Resin Combinations ing (though still important) use in the coatings field because of their replacement by the more The scope of this work has permitted consid- uniform and versatile synthetic resins, natural eration of only the main classes of resins and resins have been adequately discussed else- resin combinations. Obviously, many other where [65, 66]. combinations are possible through chemical reaction or physical blending, and have yielded a 3.17.4. Furan Resins wide variety of materials for specific coatings applications. The essential properties of this These are black, thermosetting resins made at-first-glance bewildering array of materials by the polymerization of furfuryl alcohol in can often be assessed, for practical purposes, the presence of acidic catalysts. Strong acids, from a knowledge of the properties of the indi- both mineral and organic, cause the polymeri- vidual constituents, many of which have been zation to occur with explosive violence. Furan discussed in this chaper. resins are usually supplied as a partially polymerized, neutralized liquid that remains stable 3.17.2. Resins Discussed Elsewhere on storage, until cured to an insoluble and infusible resin by addition of dilute acid at the In some cases, information concerning the time of use. Because of their dark color, the properties and uses of particular materials has uses of furan resins are primarily functional been given elsewhere in the text, where this ap- rather than decorative. The fully cured resins peared logical and appropriate; for example, are resistant to acids (except strong oxidizing terpene resiyis and coumarone-indene resins, be- types), alkalis, corrosive fumes, and solvents, ing primarily varnish resins, were discussed in and find use as industrial maintenance coatings, section 2.2.2 on Varnish and have been omitted tank and digester linings, anl laboratory bench from the present chapter. In other cases, in- tops. For the latter and similar applications, formation relating to a particular class of the resin may be polymerized in situ by apply- resins has been distributed among several ing alternate coats of furfuryl alcohol and places in the text to make it available where it dilute mineral acid, thereby building up a jet- was most appropriate or needed thus, those black, chemically resistant finish. ; aspects of polyvinyl acetate and styrene-butadiene resins which relate directly to latex 3.17.5. Polypropylene paints are discussed in section 2.3.5, while the general properties of these resins are discussed The polypropylene of current importance is in sections 3.6.1 and 3.7.2 of the present chap- a recent member of the new family of stereo- ter. Similarly, water-soluble resins have been regular polymers (see sec. 3.1.4.5) having an discussed at several places in the text (see sec- isotactic (highly ordered), highly crystalline tions 2.3.6 and 3.16). In such instances, the structure. (The atactic form is not made treatment has been selective rather than repeti- commercially.) Its exceptional properties re- tious, with appropriate cross-references within semble, but surpass, those of high-density the text. The comprehensive Index will provide polyethylene (see sec. 3.13.1). Although ther- a ready guide to all the information relating moplastic, it possesses unusual heat resistance. to a particular resin or subject within the scope Its melting range is from 325 to 340 °F (163- of this publication. 171 °C), and it is capable of continuous service at 300 °F (149 °C). It is stronger and harder 3.17.3. Natural Resins than high-density polyethylene, and has greater resistance to organic solvents. Resistance to These have received only brief treatment acids, alkalis, and salt solutions is excellent, herein (sec. 2.2.2). Aside from their diminish- even at elevated temperatures. Water absorp-

68 tion is practically nil, and electrical properties 3.17.6. Polycarbonate Resins (Lexan) are excellent. While likely to find its widest This is another of the new and unusual use in molded articles and film, polypropylene thermoplastic resins that is now commercially can also be applied as a coating in much the available. It is tough, heat-resistant, dimen- same manner as polyethylene. It can be applied sionally stable with a low water absorption, to metals by dipping the heated object into the and has good electrical properties. It is un- powdered resin (see fluidized resin bed, sec. stable in the presence of alkali. Its softening

5.4.6) and then completing the fusion and flow- range is from 420 to 440 °F (216 to 227 °C) , and out process by heating in an oven at about it exhibits excellent thermal stability ; it is vir-

482 °F (250 °C) . The coating obtained is heat- tually unaffecetd by an hour of heating at sealable, clear, tough, chemically resistant, and 570°F (299 °C), as might be required during highly impermeable to many vapors and gases. processing. It is self-extinguishing when Wire coatings, applied by extrusion, exhibit burned (ASTM D 635). Its transparency im- excellent electrical and mechanical properties, parts excellent colorability. Its chief applica- articles with high resistance to heat, cracking, and tions, at present, are for molded such as housings for business machines, electrical abrasion. Polypropylene can also be extruded apparatus, and portable tools parts for appli- onto paper and fabrics to provide a flexible, ; ances and heating apparatus, heat-resistant durable coating, in a variety of colors. Stabili- lenses and instrument windows, and molded zation against sunlight is accomplished, as with replacements for die-cast metal parts. The polyethylene, by incorporation of carbon black resin can also be extruded into heat-sealable or other suitable opacifying agents. Although sheet or film and as a wire coating. Although still too to have achieved use, poly- new wide polycarbonate resin is still too new to have propylene is among the most promising of the found much application in the coating field, its new plastic materials, and is rapidly becoming combination of desirable properties coupled more available and lower in cost as commercial with its thermoplastic nature may lead to coat- production is stepped up. ings applications in the future.

Figure 3.1. Roberts jet abrades- for measuring abrasion resistance as described in Fed. Test Method Std. No. lAla, Test Method 6193. (Photo courtesy of Kameras Instruments.)

69 Figure 3.2. Microknife parallel-groove adhesion tester as used in ASTM Method D 2197-67T. (Photo courtesy of Gardner Labora- tory.)

Figure 3.3. Infrared spectrophotometer is used to identify hinders, solvents, and pigments in organic coatings.

70 4. Selection of Coating Systems

Selection of the optimum coating system for ing material is wastefully expensive if it fails a given purpose involves the careful considera- to perform its service function, while an ini- tion of a number of concurrent and sometimes tially expensive material that does the neces- conflicting requirements. The final choice often sary job may prove quite economical in the long involves compromises in which moderate short- run. The old adage about avoiding "penny- comings in secondary properties must be ac- wise but pound-foolish" practices definitely cepted in order to obtain maximum performance applies in the coatings field. Moreover, if the vi^ith respect to properties of primary impor- only coating system that will meet the require- tance to the intended application. Sometimes ments of an essential service application is one there may be several equally good choices from that is difficult or hazardous to apply, then the a performance standpoint; in such situations difficulties will have to be met and the risks other factors, such as cost or availability, w^ill taken (with all possible precautions) in order govern the selection. Seldom does one find the to achieve the performance properties without ideal coating system ; that is, one possessing which all else is naught. every desired performance property along with for The properties of all important classes of low cost, ease of application, and freedom from synthetic in fire and toxicity hazards. resins for coatings were discussed Despite the basic importance of the cost fac- detail in Chapter 3. It now remains for us to tor and the universal desirability of safe and consider how this information can best be uti- easy application properties, all of these are lized for the selection of suitable materials for secondary to the actual performance properties particular applications involving specific types of the coating system. Even the cheapest coat- of substrate and service environment.

4.1. Basis for Selection

When one is confronted with the complex official to know or to specify exactly how a problem of selecting the best coating system given resin is to be formulated, unless a gov- for a particular application, the basic task is ernment or other specification is available to decide which of the many available resins which spells out such detailed requirements. will best meet the performance requirements. Very few specifications are so restrictive many ; It is at this perplexing and critical point in the merely stipulate performance requirements, planning of an adequate coating system that making the burden of proper resin selection the responsible engineer, architect, maintenance and formulation the responsibility of the manu- superintendent, or procurement officer most facturer ; others specify the basic type of resin needs complete, concise, and reliable informa- (and perhaps certain key modifying ingredi- tion to guide his decision, regardless of whether ents) to be employed, and leave to the manu- he himself must specify the particular material facturer the task of formulating these basic to be used or whether he must merely choose ingredients into a finished coating material that wisely from among various candidate materials will meet performance requirements. A con- offered by different manufacturers or suppliers. venient guide to organic coating materials cov- Although any resin chosen as the film-form- ered by Federal Specifications is provided by ing base can be formulated in a great many the guide list and the tabular summary given ways with all sorts of modifications to enhance in chapter however, most of the newer syn- 8 ; or diminish certain properties, the fundamental thetic resin coatings are not yet covered by character of the resin continues to be the basic Federal Specifications. factor in determining the properties achieved. Application of the information about syn- Once the decision as to the basic resin to be thetic resins given in chapter 3 to the problem employed has been made, the manufacturer can of coating selection will be greatly facilitated generally be relied upon to supply the best, or by a logical approach involving: at least an adequate, formulation of that type (1) Consideration of such factors as service for the particular application. Implicit here environment, nature of substrate, basic func- is the necessity for the procurement official to tion to be provided, appearance desired, appli- inform the coatings manufacturer to the fullest cation limitations, and cost—leading to extent possible of the requirements of the appli- (2) Designation of various factors or prop- cation; for only the manufacturer (or his well- erties as being of either primary or secondary informed suppliers) knows just what goes into importance in the intended application. his own formulation and what its detailed per- (3) Scanning of the information on specific formance capabilities are. It is usually im- resins given in chapter 3, to select those resins practical and unnecessary for the procurement which possess all (or as many as possible) of

71 the properties of primary importance. Such With a general approach to the problem of scanning may be minimized by limiting it to coatings selection now in mind, let us consider those classes of resins which are indicated in in greater detail those factors which constitute subsequent sections of this chapter to be suit- a basis for the checklist of primary and second- able for use on the specific substrate and in the ary properties to be evaluated in conjunction particular environment of the intended appli- with the information on synthetic resins given cation; it should be recognized, however, that in chapter 3. many less commonly used but potentially applicable materials may be found in the greater 4.1.1. Service Environment detail of chapter 3. (4) Final selection of the most suitable resin A detailed consideration of the type of serv- (or resin blend, if necessary) on the basis of ice to be encountered will yield many of the the number of desirable secondary properties items needed for our checklist of properties. it possesses in addition to the primary prop- Is the service environment to be an interior or erties that led to its preliminary selection. an exterior one? What are the durability or The above points will be developed in subse- longevity requirements—is maximum longevity quent discussion. desired, or is a temporary coating or a strip- Where the problem is to determine whether pable coating needed ? What specific endurance or not a specific type of coating material being properties are needed—resistance to heat, cold, ofi'ered for a particular application is basically sunlight, general weathering? Is resistance to suitable for that application, the above approach chemicals required—if so, to which types—sol- provides a ready answer, since it is necessary vents, acids, alkalies, detergents, continual to refer only to that section of chapter 3 in water immersion? Will the coating be sub- which the resin in question is discussed. jected to alternate soaking and drying, in which Where the task is to specify or select the case a material of low water absorption and most suitable coating for a particular applica- good dimensional stability will be necessary. tion, the problem is obviously more diffuse and What mechanical properties are needed—should difficult, since, theoretically, one has the whole the coating be hard, flexible, tough, rubbery? field of synthetic resins to choose from. Even Must it be resistant to abrasion, impact, flex- here, however, the approach suggested above ing? What are the adhesion requirements? can provide a solution. Often, to one who has The answers to questions such as the foregoing some familiarity with organic coatings, several will help to bring the problem into focus. candidate materials will immediately suggest themselves (see section 4.6 on Functional Coat- ings). It is then necessary only to check the 4.1.2. Nature of Substrate appropriate sections of chapter 8 against the check-list of desired properties in order to select While seeking a coating having properties the most suitable of these materials. If the that will enable it to withstand the rigors of "most suitable" material thus chosen does not the service environment to which it will be possess all of the required properties, or if it exposed, one must also consider the nature of is desired to make certain that a better material the substrate to be coated. For example, a has not been overlooked, it will be necessary coating capable of giving excellent service in to study the information in chapter 3 more a particular environment when applied on a fully. In such cases, and in situations where metal surface may fail badly in that same en- a group of potentially suitable materials is not vironment if applied on wood, partly because known, the broad field of organic coatings can of the poorer dimensional stability and greater be narrowed by considering the nature of the coating elasticity requirements of wood sur- substrate, as discussed in subsequent sections faces. On the other hand, a satisfactory coat- 4.2, 4.3, 4.4, and 4.5 of this chapter. These ing for wood may be lacking in adhesion to a sections indicate classes of materials that are nonporous surface such as metal, or it may fail suitable for use, respectively, on wood, metal, to provide adequate corrosion resistance. Coat- masonry, plaster, and wallboard surfaces. ings for masonry or plaster must be resistant At most, the task of selection will require to the alkalinity in such surfaces, and must a careful reading of all of chapter 3 as well as give good holdout (uniformity of coverage and the present chapter. The reader will thus gain appearance) despite the porous and non- a knowledge of the properties of synthetic uniform nature of such surfaces. resins sufficient to enable him to immediately It is evident, therefore, that both the choice eliminate some materials as unsuitable, while of the film-forming resin and its formulation pointing to others as potentially suitable mate- into a suitable coating must take into account rials from among which a final selection can the chemical and physical nature and the sur- be made on the basis of a detailed study of their face topography of the substrate. Smoothness, properties in relation to the needs of the appli- porosity, dimensional stability, corrodibility, cation. dampness, alkalinity, hydrophilic or hydro-

72 phobic character—all of these attributes of the heat-resistant and weather-resistant. A coat- surface to be coated affect the choice of a ing for a home space heater must be both heat proper coating system. In many instances, the stable and color stable at elevated tempera- diverse requirements of substrate and environ- tures. Floor coatings must be tough (hard yet mental factors necessitate a dual or multiple flexible) to withstand the scuffing and abrasion coating system in which a primer of specific received in service. composition is used to satisfy substrate ad- Color and surface texture (gloss) are prime hesion and corrosion-resistance requirements factors in determining the heat and light re- while a coating of a different composition is flectivity or absorptivity of a surface ; it is well- used as the topcoat to withstand environmental known that light colors and glossy surfaces re- conditions. Sometimes an intermediate or tie- flect heat and light, while dark surfaces absorb coat is used between primer and topcoat to such radiation. For example, an outdoor stor- improve the bond between them or to provide age tank exposed to the heat of the sun will be special properties such as filling, hiding, and maintained at a considerably lower tempera- bodying. ture if a light-colored rather than a dark- In general, it may be stated that a coating colored paint is applied to its surface. Heat material for metal surfaces should at least have build-up in such a tank can also be lessened good wetting properties and good specific ad- (though not to the extent achieved with a white hesion, as well as good film integrity to aid paint) by employing a metal-pigmented coating corrosion resistance. Coatings for wood must (e.g., an aluminum paint), which possesses have, among other properties, good flexibility a high specular reflectivity. Aluminum-pig- and elasticity and, in many applications, a mod- mented paints are useful further for their very erate degree of permeability to water vapor in good weathering properties. order to perform satisfactorily. Coatings for Other examples of functional coatings are masonry must be water- and alkali-resistant, low-modulus or soft materials for acoustical have good sealing properties, and be permeable applications, rubbery coatings to provide re- to water vapor. Coatings for plaster, in addi- silence, coatings of high dielectric strength for tion to being moisture- and alkali-resistant, electrical insulation, high visibility coatings in- must have a controlled degree of penetration to corporating fluorescent pigments, and coatings provide both good adhesion and adequate top- with a high degree of imperviousness to vapors coat holdout. and gases for packaging applications. Specific information on coatings for various Some types of functional coatings depend types of substrates is given in sections 4.1 strongly upon certain additives in addition to through 4.5 of this chapter. the film-forming resin to achieve their purpose. For example, despite the importance of the 4.1.3. Basic Function resin in a high-visibility paint, it is the fluorescent material (see sec. 2.3.1.8) that contributes In selecting a suitable coating for a partic- the basic functional property. Similarly, fire- ular application, it is helpful to keep in mind retardant coatings (sec. 2.3.8) depend heavily the basic function (protective, decorative, or on special types of pigments, fillers, plasticizers, functional) that the coating is to fulfill, for this and flame-retardant additives to achieve their can aid considerably in the compilation of an purpose. Anti-static (explosion-proof) floor appropriate checklist of required properties. If coatings for hospital operating rooms, in addi- the basic purpose of the coating is to protect a tion to being based on an abrasion-resistant surface from harmful environmental agents resin, are formulated with sufficient carbon to (for example, weathering, fumes, or chemi- render them adequately conductive. Fungus- cals), it is obvious that specific resistance to resistant coatings often contain a fungistat or these environmental factors is of primary im- a fungicide as well as a resistant resin. So- portance while other factors, such as appear- called sanitary or hygienic coatings incorporate ance, are secondary. If the coating is to pro- a bacteriostatic substance to keep the surface vide mainly a decorative function, the emphasis "germ-free". Functional coatings such as the shifts to factors such as color and gloss, and foregoing emphasize the importance of the en- the retention of these properties in service; tire formulation as well as the resin itself in here, the character of the pigment (see sec. producing a suitable coating material for a 2.3.1) and the pigment volume concentration given application. (PVC) of the formulation play an important role along with the resin itself. 4.1.4. Appearance In cases where the coating is to fulfill one or more definite functional purposes (see sec. 4.6), Coating appearance is a consideration in certain specific properties can be listed imme- most coatings applications and becomes a factor diately as of primary importance. Thus, a of primary importance in cases where the coat- coating for an industrial smokestack must be ing serves a decorative function. Aside from

73 the color itself, the appearance of a coating is 4.1.4.2. Some resins normally yield TRANS- determined by its degree of transparency, LUCENT COATINGS. Among these are the fluoro- translucency, or opacity; by its degree of gloss carbon resins, polyamides, and conventional or flatness; and by its surface topography polyethylene (high-density polyethylene is rela- which may be, for example, textured to simu- tively clear). Ordinarily clear coatings can late fabric or leather, wrinkled or hammer- be made translucent by incorporating small toned to provide special effects, or multicolored quantities of a finely divided extending pigment to give interesting and pleasing color effects or filler (see sec. 2.3.1.3), or a somewhat in- while hiding substrate imperfections. compatible resin or plasticizer, to produce a colloidal suspension that will scatter the trans- 4.1.4.1. Clarity and whiteness.—With the mitted light. exception of clear coatings, white coatings, and a few specialized finishes (e.g., wrinkle and 4.1.4.3. Opacity is achieved by incorporating multicolor finishes), most of the appearance hiding pigments into the formulation. The properties of a coating are determined more by opacity or hiding power increases with increas- the manner of formulation and application than ing pigment volume concentration and increas- by the nature of the resin itself. Clear, coloring index of refraction of the pigment (see less coatings are required where a natural finish sec. 2.3.1.2). is desired to protect an already attractive surface such as beautiful-grained wood or bright 4.1.4.4. A GLOSSY SURFACE is the rule with metal. Outstanding clarity and transparency, clear films laid down from solution in a good together with maximum retention of these solvent. Glossy surfaces are the result of good properties on aging, can be obtained with coat- coating flow-out and leveling and good film in- ings based on cellulose acetate butyrate and on tegrity, resulting in an optically smooth sur- acrylic resins. However, many other resins face. Some resins are inherently capable of which yield clear, colorless films are described yielding films with exceptionally high gloss; in Chapter 3, including cellulose derivatives, notable among these are polyurethanes, alkyds, various vinyl resins, and polystyrene, and cir- and epoxy-polyamides. Poor gloss (surface cumstances often dictate the choice of one of dulling) of unpigmented coatings may result these. For example, in an application in which from improper formulation or from poor appli- a clear coating is desired which must also with- cation techniques or conditions. For example, stand a considerable amount of abrasion, a if the solvent portion of a sprayed coating is too clear vinyl coating might be more suitable, be- lean in active solvent, or excessively fast in cause of its inherently greater toughness, than evaporation rate, the deposited film may be too an acrylic coating of better clarity but lesser viscous for good flow-out or too dry for good abrasion resistance. coalescence, so that an optically or visually White coatings should be based on colorless rough surface is produced. "Blushing" of the or nearly colorless resins that do not tend to film under high humidity conditions, or "bloom- yellow with age. Excellent retentive white ing" due to migration of an incompatible con- finishes can be made with resins such as mela- stituent to the surface or resulting from an un- mine- or urea-formaldehydes, vinyls, acrylics, desirable chemical reaction at the surface, also selected alkyds, and various cellulose deriva- may destroy gloss. More information on the tives. Good white finishes can also be made relationships between formulation, application with resins that have only a pale yellow color, conditions, and film properties is presented in such as epoxies and drying-oil alkyds. Prop- section 5.3.2, and in section 7.2. erly formulated polyurethane coatings give The degree of gloss exhibited by a pigmented good results as clear finishes for boats and ex- coating depends primarily on the pigment vol- terior wood siding, but they tend to darken ume concentration (PVC) and, in general, de- somewhat during exterior exposure and hence creases with increasing pigmentation. Thus, enam.els are formulated within a range of do not make the best white finishes. Phenolic PVC 15 to 30 percent, with the highest gloss enamels resins also are transparent enough to yield satin the low PVC portion of this range. Gloss isfactory spar varnishes and floor coatings; paints are usually in the PVC range of 30 to 45 however, their yellow color, which darkens percent. Semi-gloss paints are in the 40 to 55 considerably with age, limits their usefulness percent PVC range. Flat paints are formu- as clear unsuitable coatings and makes them lated with PVC's generally in the 50 to 65 per- for white or pastel finishes. In general, even cent range. Above this range, the critical PVC resins with only a slight yellow or amber cast (beyond which the paint properties undergo a are unsuitable for applications where maximum pronounced and usually undesirable change) is clarity and transparency are required, as in liable to be exceeded, although the actual criti- coatings for artistic paintings, important docu- cal PVC depends on the nature of the materials ments, silver objects, and optical devices. (vehicle, pigments, extenders, etc.) involved.

74 :

Although pigment volume concentration is the 4.1.4.5.3. Pebble finishes are obtained by chief factor determining the gloss of pigmented spraying a high viscosity coating of high solids coatings, other factors such as fineness of content and fast setting time onto a smooth base grind, compatability of vehicle components, coat in such manner as to produce blobs of coat- vehicle dispersive power, and other formula- ing material that set before flow and leveling tion variables also affect the degree of gloss ob- can take place. Through control of viscosity, tained. As in the case of clear films, the gloss air and fluid pressure, and speed of travel of of pigmented coatings also depends on the man- the spray-gun, the pattern of blobs can be ner of application and on the prevailing appli- varied to give a variety of textured surfaces, cation conditions, inasmuch as both of these from a uniformly pebbled finish to one resem- affect the structure of the film laid down. bling leather grain. Pebble finishes can be formulated with high quality resins such as vinyls, epoxies, and alkyds that do not require 4.1.4.5. Textured coatings having a wrin- further protective overcoating. Simulated kled, crackled, pebbled, hammertoned, or other leather finishes having excellent chemical re- intentionally "faulty" film structure or finish sistance, abrasion resistance and general dur- find use both for their pleasing appearance and ability can be made with vinyl plastisols (see their serviceability. Such coatings can provide sec. 3.6.2) ; these must be given a flash bake to an attractive and durable finish on physically fuse the applied coating into a continuous film. imperfect surfaces that might require costly surface preparation if coated with conventional 4.1.4.5.4. Hammertone finishes are generally smooth finishes. They are based on special one-coat, sprayed coatings formulated with a resins and formulating ingredients and spe- small amount (1 or 2 ounces per gallon of ve- cially balanced solvent mixtures. The effect hicle) of a fine grade of non-leafing aluminum achieved also depends importantly on the man- pigment plus minimal amounts of such other ner of application. Various kinds of textured colors as may be desired. The solvent mixture coatings are described below is a combination of fast and slow evaporating solvents that permits the coating to set fast enough to prevent running and sagging while 4.1.4.5.1. Wrinkle finishes are formulated for maintaining good fluidity long enough for a fast drying that will form a surface "skin" and vortex action to be set up in the drying and cause partial setting of the film before much cooling coating. The swirl of aluminum parti- flow and leveling occur. Continued rapid cur- cles in the convection currents leads to the ing and escape of solvent sets up shrinkage typical hammertone configuration in the set stresses and strains that cause the film to wrin- film. After a short period of air-drying, ham- kle. It is apparent that the film must be flexible mertone finishes are baked for final cure. Since if it is to wrinkle rather than crack under the hammertone finishes are largely the result of high drying stresses developed. Wrinkle fin- the solvent balance and manner of application ishes are usually based on an alkyd or varnish of a coating, they may be based on a variety of vehicle in which the modifying oil is of a very different resins selected on the basis of the fast drying type, usually tung oil or oiticica oil. protective and durability requii-ements to be Phenolic resin-tung oil combinations have been met. notably successful in this type of finish. To essen- achieve the necessary fast dry and cure, high 4.1.4.5.5. Multicolor finishes consist tially of discrete particles of or dif- proportions (10 to 20 times greater than usual) two more ferent colored, organic solvent-base materials of efficient driers such as manganese and cobalt suspended in solution of a water- are used in the formulation, and a high propor- an aqueous soluble polymeric stabilizer. discussion of tion of low-boiling fast-evaporating solvents is A employed. To further develop the wrinkle such finishes is given in section 2.3.7. They are being used with particular success as interior effect, the coating is generally applied in a architectural finishes for wood, masonry, plas- single thick coat and is baked to complete the ter, including difficult surfaces cure. Fine rather than coarse wrinkle finishes and wallboard— can be obtained by applying the above prin- such as basement walls and ceilings of hospitals well as walls furniture in ciples in less drastic fashion. and garages as and hotels, industrial plants, public buildings, and homes. Durable types for use on exterior ma- 4.1.4.5.2. Crackles finishes are obtained by sonry or metal also are available. In addition over-coating an ordinary protective base lac- to their generally good durability and pleasing quer with an over-pigmented, brittle coating appearance, multicolor finishes offer easy ap- that will crack under the stresses developed plication with a minimum of surface prepara- during drying. The base coat is usually of a tion, good hiding of surface imperfections, uni- contrasting color to that of the crackle coat. form masking of areas of different porosity or A clear protective topcoat may be applied over texture and easy touch-up of damaged areas the crackle coat if desired. when necessary.

75 — 4.1.5. Application Limitations is the inevitable item of cost. Despite the basic importance of the cost factor, it must always In selecting a coating material for a given in any honest appraisal of ultimate economy of purpose, it is necessary to take into account coating operations—be weighed against the per- such characteristics as toxicity, flammability, formance properties and requirements of the odor, drying speed, moisture sensitivity, brush- coating system. A low-cost coating that fails ability or sprayability, and other inherent prop- to perform its function is a completely erties that affect the manner of application. wasted expense. For example, toxic or flammable coating mate- Considering the relatively high cost of rials cannot be used in areas where, for op- labor involved in applying protective coatings, erational or economic reasons, it is not feasible a short service life of a cheap but inferior to shut down the operation. Nor should toxic material may make ultimate coating and main- or flammable materials be used without ade- tenance costs far higher than if a more expen- quate safety equipment and precautions, par- sive but more durable coating had been em- ticularly in confined areas. A desirable coating ployed in the first place. This is not to say system that requires several days for applica- that cheap coatings are necessarily inferior, or tion and cure would not pose a problem in a that an expensive coating automatically will new building not yet occupied however, in an ; perform well. Obviously, it depends on the re- occupied hospital or in a busy restaurant, an quirements of the application. Where, for ex- odorless fast-drying coating such as a latex ample, a relatively inexpensive rosin-modified paint might be far more feasible. The ability phenolic will perform satisfactorily in an in- to dry very quickly is a necessary requisite for terior application, it would be a useless ex- traffic paints one which excludes many other- — travagance to use the more expensive pure wise satisfactory materials. phenolic type. The latter, on the other hand, Other examples of application limitations af- would be more economical, in the long run, for fecting the broad choice of film-forming resin or type of formulation may be given: When an exterior application. Similarly, an expen- painting must be done in cold weather (never sive premium material such as a fluorocarbon desirable but sometimes necessary), a latex resin would be quite inferior as a coating for paint might be unsatisfactory because of poor a masonry wall compared to a much less ex- coalescence (or even freezing) at low tempera- pensive latex paint. tures. Plastisols require a flash fusing to form Aside from the ultimate cost of a coating a continuous film. Heat-convertible coatings material in relation to its suitability and dur- cannot be used unless adequate oven or infrared ability in a given application, and apart from baking facilities are available. Oil paints do the intrinsic cost of the film-forming resins and not perform well if applied over damp or alka- other ingredients that make up the formula- line surfaces. Latex paints require oil-base tion, a number of specific application factors primers before application to wood surfaces. Polyurethanes adhere better to water-base must be taken into account in arriving at an stains than to oil-base stains. Phenolic paints over-all cost estimate. Thus, in addition to the (sec. 2.3.2.2) require thorough surface prep- gallon cost of the material as purchased, one aration and limited inter-coat drying periods to must also consider the spreading rate (i.e., the give good adhesion. Multicolor finishes must area adequately covered by a unit volume of be applied by spraying—the high shear in- coating material, usually expressed as square volved in brushing would destroy the emulsion feet per gallon), the probable application time and discrete character of the particles of col- and its equivalent in labor costs, the investment ored lacquer. Two-component coatings, such in regular or special equipment required for as air-drying epoxies, which must be activated application of the material, and finally the ex- just before use, require (unless brushing is to service of the coating. be employed) a special type of spray gun that pected useful life will meter and mix the components at the gun In the absence of actual experience with a at the instant of spraying. And so on (also see particular material, it is apparent that much of sec. 7.2.4.3). the information needed for a cost estimate must In short, in choosing a coating material, due come from the coatings manufacturer. Infor- consideration should be given to its application mation on surface preparation, spreading rates, properties in relation to available facilities and application methods, drying time, type of serv- the environmental conditions under which it ice for which recommended, and an indication will be applied. of the order of durability to be expected in the intended application is usually furnished by 4.1.6. Cost the manufacturer, and is generally reliable if Last but far from least among factors to be the manufacturer is a reputable one (also see considered in the selection of coating systems sec. 7.2.2).

76 4.2. Coating Systems for Wood

Wood is a porous, hydrophilic material of 4.2.1. Exterior Coatings for Wood relatively poor dimensional stability. It swells and shrinks, expands and contracts, with Linseed oil house paints, properly formulated changes in moisture content and temperature. and pigmented, remain one of the best and A coating that is to perform successfully on a most durable coatings for exterior wood struc- wood surface must therefore be distensible tures in ordinary environments. The linseed and elastic enough to follow the dimensional oil film owes its effectiveness to a unique com- changes in the wood without cracking. Good bination of desirable properties (see sec. 2.3.2.1 wetting and controlled penetration of the por- on Oil Paints), including good adhesion, elas- ous wood surface are also requirements for ticity, permeability, and weather resistance. obtaining good adhesion and adequate holdout Despite limitations such as relatively poor of topcoats. On exterior wood surfaces that water-, chemical-, and abrasion resistance, lin- are subject to absorption and migration of seed oil paints are excellent in general dur- moisture from within, as exterior wood siding ability and performance when applied to ex- on a house, the coating m.ust have the additional terior wood siding on homes and other build- property of permeability to water vapor; ings. otherwise, the entrapped moisture is likely to On exterior wood surfaces that are free from cause blistering and peeling of the paint film. the problem of moisture entrapment and the Even so, excessive amounts of moisture origi- related blistering of the paint film, oleoresinous nating inside a structure must be controlled to coatings (see sec. 2.3.2.2) such as oil-modified ensure satisfactory paint performance. alkyds or oil-modified phenolic types may be A satisfactory coating system for wood gen- used. Long oil-alkyds make excellent archi- erally requires the use of a primer or under- tectural paints for exterior trim and trellis coater to lightly penetrate and seal the wood, applications. Alkyd paints dry faster and followed by one or more topcoats of a coating harder than straight oil paints, and have bet- that will provide the necessary protective prop- ter gloss and color retention along with excel- erties and desired appearance. For exterior lent general durability. Paints based on sili- use, the primer is usually a specially formu- cone-modified alkyd and phenolic resin vehicles lated oil or oleoresinous type, as described in also are finding use on exterior wood. section 2.4.1 and typified by Federal Specifica- Where a clear finish is desired on exterior tion TT-P-25 however, self-priming (thinning wood surfaces, phenolic-tung oil spar varnishes ; with linseed oil) is sometimes resorted to as a (and to a lesser extent, alkyd varnishes) are matter of convenience, or when a lead-free used. Clear coatings are considerably less dur- coating system resistant to sulfide fumes is re- able than pigmented ones (see sec. 2.3.1.1). quired. On interior wood, the first coat is fre- Where a good oil or oleoresinous paint may quently a long oil-alkyd enamel undercoat, such give four or five years of satisfactory service, a as that described by Federal Specifications TT- clear finish based on the same vehicle may fail E-543 or TT-E-545; alternatively, when con- in a year or two, or even sooner. venience dictates, an appropriate topcoat mate- Woods exposed to marine or immersion con- rial may be made into a self-primer by thinning ditions, as on boats and over-water structures, with one pint per gallon of a suitable thinner have traditionally been protected with tung oil- (see sec. 5.3.2). Other federal specifications phenolic spar varnishes, particularly those for wood primers are TT-P-636 and TT-P-659 based on 100 percent phenolic resins. Not yet (see tabular summary in chapter 8). fully evaluated, but very promising in these The above remarks relating to wood primers applications, are newer coatings based on apply primarily to conventional coating sys- epoxy resins, epoxy-polyamide resins, poly- tems for wood in which the topcoat is an oil- esters and polyurethanes. base or oleoresinous material. The newer syn- On new wood surfaces, including exterior thetic resin coating systems, based on resins wood siding, latex paints based on acrylic or on such as cellulose acetate butyrate, polyesters, polyvinyl acetate resin emulsions are proving and many others may require different types of satisfactory, provided the wood is first treated primers, most frequently a thinned (self- with a zinc-free oil-base primer. With some- primed) version of the topcoat material. The what greater difficulty, latex paints may also directions furnished by the manufacturer of be applied over previously painted wood sur- such coatings are the best guide to the type of faces, if loose chalk is first removed and an primer or sealer required. A discussion of the oil-base primer applied in accordance with the types of materials suitable as the main body or manufacturer's instructions. finish coatings for wood is given in the follow- Wood surfaces, such as porches and decks, ing sections. that are subjected to considerable abrasion and

77 wear as well as weathering require coatings lated with less durable and less expensive based on tough, durable vehicles. The most resins, pigments, and additives than are re- widely used vehicles for this purpose have been quired for exterior finishes. medium or long oil alkyd or phenolic varnishes, For interior woodwork, cabinets, and furni- particularly tung oil-phenolic types. A typical ture, the most thoroughly proven and widely oleoresinous floor and deck enamel is described used paints are those based on alkyd resins, by Federal Specification TT-E-487. More re- particularly medium oil soya-modified types. cently several of the newer synthetic resins, Several Federal specifications for interior especially epoxies, polyurethanes, and polyes- alkyd-base paints are included in the summary ters, are finding increasing use in floor and given in chapter 8. deck finishes properly formulated and applied ; (sec. 5.3.3.) they are capable of excellent serv- For the clear finishes generally desired on ice in such applications, although more expen- furniture, oleoresinous varnishes (see sec. 2.2.2) sive than oleoresinous types. and cellulosic lacquers are widely used. Most of the latter are based on cellulose nitrate modified Wood furniture that is to be subjected to out- with suitable resins and plasticizers. door exposure can be protected with coatings Cellulose acetate butyrate lacquers also are useful in this based on alkyd or phenolic varnish vehicles, application. Polyester coatings, rather cellulose acetate butyrate lacquers, polyester though difficult to apply, are finding increasing use as resins, polyurethane finishes, and epoxy resins. premium furniture finishes of great beauty and Federal specifications for organic coatings durability. Epoxy resins and polyurethanes for exterior wood surfaces are limited chiefly may be used as furniture coatings ; their tough- to a variety of oil, oleoresinous, and alkyd ness and durability, however, have led to their types. The newer synthetic resins to which ref- wider use as clear finishes for wood surfaces erence has been made, while of great poten- (e.g., gymnasium floors) that must withstand tial usefulness in this area, have not yet been severe abrasion and wear. described by formal Federal specifications. Ex- Clear coatings for interior wood paneling isting Federal specifications for exterior wood may be based on the same resins indicated above for coatings may be readily located by means of furniture finishes. Bleached shellac varnish the classified list and summary charts given in [151] is often used as an attractive, non-dark- chapter 8. ening finish for wood paneling, although it is For maximum usefulness, the information more brittle and less durable and chemical- given in this and other sections of this chapter resistant than many other materials. The on the suitability of various types of coatings shellac may be used as a sealer for the wood, for use on a specific substrate in a particular and then over-coated with a more durable and environment should be supplemented by re- resistant finish such as an oleoresinous varnish. ferring to the detailed information about these Shellac is used also as a floor sealer, with a resins which is given in chapter 3. For ex- protective overcoat of hard wax. durable ample, such reference to the information on A floor sealer based on a tung oil-phenolic varnish phenolic resins will indicate that while mod- resin is described by Federal Specification ified phenolic resins are used to some extent in TT-S-176. Floor sealers are designed to pene- exterior finishes (in cheaper finishes, or in trate and seal the wood, leaving only a thin film small proportions to improve the adhesion of a on the surface which does not show wear as pure phenolic resin), it is the 100 percent readily as thicker varnish coatings. Durable phenolic types which have the best exterior polymeric floor finishes are available based on durability. a variety of resins or blends of resins such as maleic-modified alkyds, resin-modified phenolics, 4.2.2. Interior Coatings for Wood cellulose derivatives, and natural resins. A con- Interior coatings for wood, in addition to the ventional floor and trim varnish is covered by general requirements for good adhesion and Federal Specification TT-V-71. For maximum adequate coverage, are usually expected to be durability with minimum maintenance in severe relatively quick-drying, reasonably free from service applications such as bowling alleys, obnoxious odors and toxic ingredients, attrac- gymnasium floors, and areas of heavy foot traf- tive in color and appearance, stain-resistant, fic, polymeric coatings based on polyurethane or washable, and scuff-resistant. Good flexibility epoxy-polyamide resins are proving highly use- is a necessary property, but elasticity require- ful. Where resilience and oil and chemical ments are less severe than for exterior coatings. resistance are paramount factors, neoprene Since resistance to sunlight and weather is not coatings may be used on floors. needed, interior coatings may often be formu- A coating for laboratory bench tops, with

78 excellent resistance to acids, alkalis, and other Most of the resins mentioned above in con- corrosive chemicals, can be built up with furan nection with clear coatings may serve also as resins (see sec. 3.17). the basis for pigmented paint vehicles.

4.3. Coating Systems for Metals

Although metals are impermeable to moisture 7.2.2.3). Compatibility implies that each part and gases and are relatively stable dimension- of the system will bond properly to other parts ally, the common structural metals—particu- and to the substrate in such manner as to larly iron and steel—are very susceptible to achieve a composite system that will remain corrosion in a moist atmospheric environment intact and perform its protective function for (above 60 per cent relative humidity at ordi- the maximum possible time. nary temperatures), particularly under acidic The information to be presented represents conditions such as occur in industrial atmos- established good practice in the selection of pheres. Corrosion is an electrochemical process coating systems for metals, particularly steel. in which chemical changes are produced in the Adherence to the suggested practice will ensure metal by the flow of current caused by differ- a compatible coating system capable of excellent ences in electrical potential arising between dis- service in the intended application. In cases similar metals or between dissimilar areas in where it is not possible to follow the optimum or around the same metal. The flow of current procedures indicated, deviations are permissible dissolves away the anodic (corrodible) material provided precautions are taken to avoid possible while affording protection to the cathodic mate- difficulties. For example, although it is ordi- rial. Moisture on the metal surface provides narily not good practice to apply a phenolic top- a conductive path through which corrosion cur- coat over an oil-base primer because of the rents can flow. The presence of soluble salts or danger of lifting the undercoat, this may be other electrolytes on the metal surface greatly done successfully if the oil-base primer is accelerates the corrosion process and may lead allowed to dry thoroughly before application to severe localized corrosion in the form of pit- of the topcoat. More information of this kind ting. Oxygen or oxygenated areas are in effect is presented in section 7.2.2.3. cathodic toward iron and steel, so that rusting (oxidation) takes place rapidly in the presence A satisfactory coating system for metal of oxygen and moisture. The rust formed is begins with the application of a suitable cor- permeable to moisture and is strongly cathodic rosion-inhibiting primer to the clean (and often to the underlying iron or steel, so the corrosion chemically pretreated) surface. The primer, proceeds until the metal is destroyed. Mill scale ideally, should have good wetting properties also is cathodic to iron and steel, and is usually (see sec. 7.2.4.1) to provide adequate adhesion cracked or loosely bonded; hence the need to to the nonporous metal surface and to ensure remove it, if possible, before painting, especially penetration into crevices and into areas of rust if the steel will often be wet or immersed during or scale difficult to completely remove (as on service (see sees. 6.3.1 and 7.2.3). structural steel) . It should also, of course, provide a strong bond with the topcoat. So long The corrosion of metal which is in continuous as the primer remains intact, excellent protec- contact with a conducting medium such as moist tion against corrosion is afforded the substrate. soil or water can be suppressed by imposing an An important function of the topcoat (body or electric current sufl^icient to render the metal finish coats) therefore, is to protect the primer completely cathodic. Such cathodic protection , against wear and deterioration so that it can is particularly applicable to the exterior of continue to perform its corrosion-inhibiting buried pipelines, the interior of water tanks, function. Additionally, the topcoat should have and the exterior of ship hulls. In general, how- a high degree of imperviousness to moisture, ever, the most practical and by far the most give the desired appearance, and possess ade- widely used method of protecting metal against quate over-all durability in the environment in corrosion is by the proper application of a suit- which it is to serve. able organic protective coating system that will prevent moisture and oxygen from reaching the The primary purpose of this section, as with surface of the metal and will act as a barrier others in this chapter, is to provide information against galvanic current (see sec. 7.2.3). All on the selection of coating systems, by indicat- elements of such a system—surface preparation ing classes of resins or coatings which are par- and pretreatment, primer, intermediate and fin- ticularly well-suited for use on a given surface ish coats—must be selected with the type of under various specified conditions. Important service in mind, and with care to ensure that adjunct information is given at appropriate all parts of the system are compatible (see sec. places elsewhere in the text, as follows : The

79 formulation and selection of primers for ferrous 4.3.1.1. Oil-base and oleoresinous systems. metals, galvanized surfaces, aluminum, and —A common problem in the painting of struc- magnesium are discussed in section 2.4.2. Cor- tural steel surfaces is the presence of harmful rosion-inhibiting primers derive their special rust and mill scale impractical to remove com- protective qualities from the corrosion-inhibit- pletely. Such steel requires a linseed oil-base ing pigments which they contain ; these pig- primer to provide adequate wetting and pene- ments and the manner in which they function tration of pores and crevices. The primer must are described in section 2.3.1.5. Information also contain corrosion-inhibiting pigments to on the surface preparation and pretreatment of passivate the steel surface. A typical excellent metals is covered in section 6.3. In general, primer for this purpose is the straight red-lead- once the metal has been adequately prepared hsLse linseed oil paint covered by Federal Speci- and the proper corrosion-inhibiting primer ap- fication TT-P-86, Type I. Where faster drying plied, the topcoat materials that may be used is necessary and some sacrifice in wetting abil- are similar for most metals. The selection of ity can be tolerated, the type II material of the topcoat materials possessing the good film above specification may be used instead of type I. integrity and other attributes necessary for The type II primer is based on a red lead-iron good performance on metals is discussed below. oxide-extender mixed pigment in an alkyd var- Oil-base, alkyd, phenolic, and vinyl types are nish-raw linseed oil vehicle that will" air dry in widely used. Also useful for particular appli- about 16 hours as compared to about 36 hours cations on metal surfaces are coatings based on for the straight oil base (type I) material. epoxy resins, polyamides, epoxy-polyamides, On smooth, thoroughly clean steel, quick- chlorinated rubber, neoprene, polysulfides, poly- drying corrosion-inhibiting primers based on polyurethanes, cellulose esters, styrene- esters, poorer-wetting but more durable alkyd or phe- butadiene copolymers, and other synthetic nolic varnish vehicles may be used. A typical resins; these often require special primers red lead-alkyd varnish (linseed oil-modified) which should be applied in accordance with the primer that dries in about 6 hours is described instructions of the manufacturer. by TT-P-86, Type III. The straight alkyd varnish vehicle possesses better water and 4.3.1. Coatings for Structural Steel chemical resistance than the straight oil base (type I) and oil-fortified alkyd varnish (type The everpresent need for protecting from II) vehicles, but it should not be used as a corrosion the vast quantities of structural steel primer on incompletely cleaned steel because of in use has led to the development of effective its limited wetting properties. protective coating systems for these materials. Very good durability under conditions of Chief such systems from the standpoint among fresh-water immersion, high humidity and con- of wide use are the time-tested oil-base and oleo- densation, or moderately severe chemical atmos- resinous (alkyd and phenolic) types. Of proven pheres is obtained with corrosion-inhibiting usefulness among newer types of coating mate- primers based on a straight phenolic-tung oil rials are systems based on vinyl resins; these varnish vehicle such as that typified by TT-P-86, have been particularly efi'ective under severe Type IV. The latter contains a red lead-plus- corrosive conditions, such as immersion in sea extender mixed pigment. While having excel- water or exposure to industrial and chemical lent durability under adverse conditions, its environments. Also proving useful under con- limited wetting ability restricts its use to thor- ditions of continual water immersion and severe oughly clean steel, preferably steel that has been marine service are the new zinc rich paints (see blast cleaned or pickled. sec. 2.3.1.5) based on high percentages of zinc dust in epoxy, epoxy-polyamide, polyurethane, An alternative to the red lead type primers and other alkali-resistant vehicles [128]. discussed above are primers utilizing a zinc yellow-iron oxide mixed pigment as the corro- Where high resistance to strong acids and sion-inhibiting base in an oleoresinous vehicle. alkalis is required, excellent protection is af- The three types described by Federal Specifica- forded by chlorinated rubber coatings. Chlori- tion TT-P-57 are typical ; the pigment portion nated rubber is also an excellent primer for contains some zinc oxide and a substantial pro- metal that is to receive neoprene coatings in portion of siliceous extenders in addition to the applications requiring exceptional resistance to main pigments. The vehicle for type I is a abrasion, such as structures exposed to wind- 50/50 blend of a long oil-alkyd resin with raw driven sand. The choice of an optimum coating linseed oil; its slow drying and good wetting system that will provide adequate protection at properties permits its use on steel limited to minimum cost requires consideration especially wire-brush cleaning. Type II is based on a faster of the condition of the surface to be painted and medium oil length alkyd vehicle ; although the nature of the exposure environment. drying and more durable than type I, its poorer

80 wetting properties permit it to be used only on 7.2.2.3. A variety of oil-base and oleoresinous smooth, thoroughly cleaned steel ; it is intended intermediate and finish coat paints and enamels, for use as a factory or industrial primer rather in many different colors, are covered by Federal than for structural steel. Type III is based on specifications. The classified list given in chap- a straight phenolic-tung oil vehicle that pro- ter 8 will provide a ready guide to the letter- vides good durability under conditions of severe number designations of these specifications; humidity or fresh water immersion; however, composition and performance information for its limited wetting properties restrict its use most of them can be found in the tabular sum- to thoroughly cleaned steel (sandblasted or mary of that same chapter. pickled), preferably steel that has received a Among finish coat materials suitable for use, phosphate pretreatment. over the aforementioned primers, on structural A corrosion-inhibiting primer based on the steel and other exterior metal surfaces, the fol- unique basic lead silico chromate pigment de- lowing are typical : A linseed oil paint incorpo- scribed in section 2.3.1.5 is available under Fed- rating a titanium-lead-zinc type pigmentation, eral Specification TT-P-615, in four types that is covered by Federal Specification TT-P-102; are similar in application to the four types of it is available in white and light tints and has red lead-base paint described in TT-P-86 and good resistance to weathering in ordinary en- referred to above. The type I vehicle is a 4:1 vironments. A lead-free, fume-resistant linseed blend of raw linseed oil with a long-oil alkyd oil paint (white only) is described in TT-P-103. Black linseed oil resin ; it provides good wetting of metal despite paints with very good dura- the presence of small amounts of corrosion bility are covered by TT-P-27 and TT-P-61. products impractical to remove. The type II A durable red iron oxide paint (so-called "roof vehicle is a 1:1 blend of raw linseed oil with and barn paint") based on a varnish-fortified linseed oil vehicle is specified in TT-P-31. A a medium-oil alkyd resin ; although its wetting properties are not as good as that of type I, it chrome-green long oil-alkyd paint (known as may be used where faster drying is necessary. a "trim enamel paint") is covered by TT-P-71. The type III vehicle is an alkyd varnish that Where the greater resistance of an alkyd resin may be used only on thoroughly clean steel be- is needed in a coating for relatively smooth cause of its fast drying and low wetting prop- steel that has received an oleoresinous primer, erties. Type IV is based on a phenolic-tung oil an alkyd gloss enamel such as that described by vehicle that provides good durability under TT-E-489 offers excellent durability in ordi- severe humid conditions or fresh water immer- nary atmosphere; a semigloss alkyd enamel is covered by TT-E-529, and a lustreless alkyd sion ; however, its fast drying and limited wetting ability restricts its use to steel that has enamel by TT-E-527. For severe service under been thoroughly cleaned, preferably by sand- conditions of continuous condensation or fresh blast. The basic lead silico chromate pigment water immersion, in conjunction with phenolic may also be incorporated, in combination with varnish-base primers, a highly durable (weath- conventional pigments and extenders, into a er- and water-resistant) aluminum phenolic variety of vehicles to produce intermediate coat paint can be prepared by adding two pounds of and finish coat paints with corrosion-inhibiting aluminum pigment paste (TT-P-320, Type II, properties. Class B) to one gallon of phenolic resin spar varnish conforming to Federal Specification For the protection of potable water tanks, TT-V-119; however, aluminum-pigmented ve- a zinc dust pigmented primer and paint based hicles are not suitable for use in strongly alka- on a phenolic varnish vehicle is very suitable. line or acidic environments. There, inert pig- Such a paint [152] is covered by Military Speci- ments are required. fication MIL-P-15145 (Formula No. 102).

4.3.1.2. Vinyl systems.—For the protection 4.3.1.1.1. Intermediate and finish coats.— of structural (and other) steel exposed to Intermediate coats for application over the severely corrosive conditions in industrial or primers that have been discussed may be the same as the primer coat or the same as the finish marine atmospheres, chemical environments, coat material to be used. Finish coats should condensation, and alternate or continuous im- be selected with system compatibility kept in mersion in fresh or salt water, coating systems mind. Thus, as a general rule, oil-base primers based on vinyl resins give excellent service. should be over-coated with oil-base topcoats, For optimum results the steel should be cleaned alkyd primers with alkyd topcoats, and phenolic by blast or pickling to remove rust, mill scale, primers with phenolic topcoats. Variations and all traces of oil or grease. The clean sur- from the rule are permissible if special preface must be pretreated with a vinyl washcoat cautions are taken to ensure compatibility and or wash primer such as that specified in Mili- intercoat adhesion, as discussed in section tary Specification MIL-C-15328 (Navy Stand-

81 ard Formula No. 117) before application of the able in coal tar enamel systems). In noncorro- regular vinyl primer. The wash primer pro- sive soil, an asphalt varnish such as that cov- vides a mild phosphating action in addition to ered by Federal Specification TT-V-51 may depositing a thin vinyl film that provides an suffice. All bituminous coatings are resistant anchor for the corrosion-inhibiting primer to water immersion, but not all are suitable for which follows. The most widely used vinyl potable water. (For the latter application, coal priming paint for steel is a red lead type [143, tar enamel has been widely used.) Although 145] such as that conforming to MIL-P-15929 bituminous coatings are resistant to many (Formula No. 119). A zinc chromate vinyl chemicals, they are attacked by many organic priming paint [144] such as that conforming to solvents and by some chemicals and must be MIL-F-15930 (Formula No. 120) may also be used with discretion. For example, the coal used with good results. tars have good resistance to aliphatic liquids The intermediate coat may be the same as but poor resistance to acids, while the reverse the priming coat or the same as the finish coat. is true for the asphalts. The most durable and chemical-resistant finish coats are based on straight vinyl resins, typically a vinyl chloride-acetate copolymer resin, 4.3.1.4. Galvanized steel may often be together with inert pigments. Aluminum-pig- painted without undue difficulty if the galva- mented vinyl paint will give excellent service nized surface is well-weathered but free of dirt in marine atmospheres or fresh water immer- and rust. However, new galvanized steel must sion but should not be used in strongly alkaline be solvent-cleaned to remove all oil and grease or acidic environments. Straight vinyl resin and for best results (if not factory chromated) paints incorporating suitable inert pigments are resistant to direct liquid contact with severely should be pretreated with a basic zinc chromate corrosive chemicals such as inorganic acids, washcoat or cold phosphate pretreatment (see alkalis, and salts they are resistant also to ali- ; sec. 6.3.3.1). A zinc dust primer such as that phatic liquids, oils, greases, and alcohols ; how- covered by Federal Specification TT-P-641 ad- ever, they are attacked by strong oxidizing acids heres well and provides good protection for gal- and by many aromatic organic solvents (see sec. 3.6.3). vanized surfaces ; it will also perform well as the finish coat except in acidic environments. Where the maximum durability and chemical The TT-P-641 zinc dust paint is available in resistance of straight vinyl paints is not re-

three types : Type I, based on a raw linseed oil quired, excellent service under severe exposure vehicle, provides the best wetting but is slow to conditions (as on the boot-topping areas or on dry. Type II, the type most often used, is based the topsides of marine vessels) may be obtained on a long oil, linseed-modified, alkyd resin ve- with vinyl-alkyd paints such as those described hicle that has moderately good wetting prop- in a series of Military specifications comprising erties and is faster drying than the straight oil the Standard Formula 122 series [68]. These type. Type III is based on a pure phenolic resin are a series of paints ranging in shade from spar varnish vehicle that is suitable for use black through gray to white, in either gloss or under severe moisture conditions or underwater dull finish. Their numerical designations in- exposure; however, its limited wetting proper- clude the following: MIL-P-15932 thru 15936, ties require that the steel be thoroughly cleaned -16188, -16501, -16502, -16738 [55]. and given a phosphate pretreatment before In the case of the 4.3.1.3. Bituminous coatings (also see sec. application of the primer. I while 2.5) may be used on iron and steel subjected type and type II materials, pretreatment, to severe corrosive atmospheres or underground beneficial, is optional. burial. For best results the steel should be blast Badly rusted galvanized steel should be cleaned or pickled. For very severe exposure cleaned like ordinary rusted steel before paint- a wash primer pretreatment is helpful before ing is attempted. Where it is not possible to application of cold-applied asphalt or coal tar remove all corrosion products, a wetting oil mastics. Coal tar mastic must be over-coated (thinned linseed oil) pretreatment may be used with coal tar emulsion to prevent checking and to ensure a bond between the primer and the cracking on exposure to sunlight. For under- steel. The wetting oil treatment permits use of ground use in very corrosive soil, best results the faster drying primers (types II and III) are obtained with a coal tar primer topcoated which cannot ordinarily be used on incompletely with coal tar enamel (wash primer is not suit- cleaned surfaces.

82 .,

4.3.2. Industrial Coatings for Steel on smooth clean metal—for refinishing or as the original finish—on vehicles, construction equip- Factory-coated steel products such as rail- ment, metal signs, railing, etc. road cars, vehicles, machinery, tools, garden Other industrial coatings for steel include equipment, appliances, metal furniture, etc., those based on polyamide and epoxy-polyamide may be protected or decorated with a wide resins, polyurethanes, polyesters, and neoprene. variety of organic coatings. The opportunity Coating systems suitable for steel fuel tanks afforded in the factory for thorough surface and salt water tanks are described [142] in preparation and pretreatment, coupled with Mil. Spec. MIL-P-23236. selection of a primer and finish coat system All the coating systems which have been dis- suited to the service environment, leads to excel- cussed require proper cleaning and in some lent service performance. Most factory-applied instances pretreatment of the metal surface, as coating systems utilize quick-drying, baking- well as suitable primers to provide corrosion- type primers and finish coats. Baking makes inhibition and topcoat adhesion. The instruc- possible a rapid production line coating operations furnished by the manufacturer usually tion and results in the development of maximum provide the best guide to their application. adhesion and optimum film properties. For general use in ordinary environments, 4.3.3. Coatings for Other Metals both interior and exterior, medium- to short-oil alkyd resin vehicles of the baking type are most Coatings for aluminum, magnesium, brass, widely used. exceptional whiteness and Where copper, and other metals are, in general, similar color good retention together with very mar once the metal has been properly cleaned, pre- resistance are required, as on refrigerators, treated, and primed. Pretreatments and prim- washing machines, hospital equipment, etc., ers for aluminum and magnesium and their non-drying oil alkyds modified with urea- or alloys were discussed in sections 2.4.2.3 and melamine-formaldehyde resins give excellent 2.4.2.4. Additional detailed information on service. detailed discussion of the applica- A various metals is presented in section 6.3 of tions the various of alkyd resins for which types chapter 6 on Surface Preparation and Pretreat- are well-suited is given in sections 3.4.1 and ment. Copper (section 6.3.6.3), after being 3.4.2. cleaned and perhaps roughened slightly or pre- Baked phenolic resin coatings, particularly treated, may be primed with an aluminum var- those based on pure phenolic resins of the alco- nish such as TT-V-81 or with a zinc dust prim- hol-soluble, thermosetting type (see sec. 3.3.2.3) ing paint such as TT-P-641. Thinned, carbon- are highly resistant to corrosive atmospheres, pigmented oil paints such as TT-P-61 also may organic solvents, and organic and mineral acids be used as a primer on copper. Terneplate (sec. (except strong oxidizing types) they have ; 6.3.6.6), or "roofing tin," may be painted with proved durable also when submerged in water ordinary iron oxide-pigmented paint such as or buried in soil. TT-P-31 ; new terne plate must be wiped free Excellent chemical and abrasion resistance is of all oil or grease, using solvents such as tur- achieved with baked epoxy resins (see sec. 3.10) pentine or gasoline; the paint should be well Many of the properties of the baked resin are brushed out, since thick coats are prone to crack retained in the amine-catalyzed types which later. cure at room temperature. Zinc-rich paints When coatings based on synthetic resins (see sec. 2.3.1.5) based on epoxy-type vehicles (epoxies, polyurethanes, polyesters, neoprenes, are giving good service as corrosion-resistant etc.) are selected, the manufacturer's instruc- coatings for the undersides of automobiles. tions with respect to choice of primer and man- Vinyl coatings, especially vinyl chloride-ace- ner of application should be closely followed to tate copolymer types, are widely used for the achieve optimum results. protection and decoration of metal, in both The list of finish coat materials that may be smooth and textured finishes (see sec. 3.6.3, and used over suitably primed metal is a long one the discussion of textured finishes in sec. that includes many of the synthetic resins dis- 4.1.4.5). Plastisols based on polyvinyl chloride cussed in detail in chapter 3. Mention has resins (sec. 3.6.2) yield tough, chemical- and already been made, in connection with the dis- abrasion-resistant coatings for severe service cussion of coatings for steel, of oil-base and applications such as plating baths, dish-washer oleoresinous coatings, vinyls, epoxies, poly- racks, and chemical tanks. amides, polyurethanes, polyesters, neoprene,

1 Lacquers based on cellulose nitrate, cellulose chlorinated rubber, cellulose esters, and other

! acetate butyrate, or acrylic resins are often used film-forming resins. The reader is referred to for the protection or decoration of metal. Gen- the information given in section 4.1 of this chap- I eral purpose nitrocellulose lacquers are deter for guidance in a systematic basis for utiliz- scribed in Federal Specifications TT-L-26 and ing the comprehensive information on synthetic TT-L-31. The latter is suitable for exterior use resins presented in chapter 3.

83 4.4. Coatings for Concrete and Masonry

The chief characteristics of concrete and able to moisture but unaffected by dampness or masonry surfaces that must be considered in alkalinity. They may therefore be used above the formulation and selection of satisfactory grade outside, and either above or below grade coating systems are their porous, often rough inside, on aged or unaged concrete and masonry texture and the alkaline nature of the portland surfaces. Cement-water paints have good hid- cement contained in concrete, stucco, asbestos- ing and decorative qualities but become some- cement shingle, and in the mortar used in brick* what translucent and darker in color when and cinder block construction. Superimposed wetted. Upon drying, the original opacity and on the inherent porosity of these materials is color are regained, but there may be a period a surface topography which may vary from the of mottled shading while the drying is taking relative smoothness of cast concrete to the gross place. Cement-water paints will not adhere irregularity of stucco. An additional charac- well to previously painted or chalky surfaces teristic that must be considered is the hydro- and will be weak, brittle, and prone to cracking philic nature of concrete and masonry surfaces, if not kept damp during cure, especially during as well as the fact that wherever portland the first 48 hours after application. cement is present, some of the water originally used to prepare it may be present in both hydrated and adsorbed form. 4.4.2. Oleoresinous Paints

A coating that is to perform satisfactorily on Oleoresinous paints, while attractive in ap- concrete and masonry surfaces therefore must pearance, are capable of satisfactory perform- be sufficiently free-ftowing to follow and coat ance only when applied on dry, well-aged (8 to the macroscopic surface roughness of the and 12 months) concrete or masonry. Their limited yet not penentrate deeply into the pores ; other- resistance and permeability to moisture does not wise, the pores would soak up so paint much permit their use below grade or in other appli- that good hold-out of topcoats be difficult would cations where moisture behind the film can and costly to obtain. Additionally, the coating cause it to blister and flake. A suitable oleo- should be insensitive to the moisture and alka- resinous paint for application to thoroughly linity likely to be present in such substrates, aged, dry, exterior concrete surfaces is described particularly those which are unaged. by Federal Specification TT-P-24; an alumi- Before applying paints to concrete and ma- num-pigmented paint based on TT-V-81 mix- sonry, the surface must be properly cleaned and, ing varnish also may be used. Oleoresinous preferably, in the case of open-textured surfaces paints suitable as both primer and finish coat such as cinder block, filled with a suitable grout on aged, dry, interior concrete and masonry sur- coat. Surface preparation is discussed in sec- faces are covered by Federal Specifications tion 6.4. A primer or primer-sealer is applied TT-P-30 and TT-P-47 as well to TT-P-24. to the prepared surface before application of Alternatively, for interior masonry, if the sur- finish coats, except when cement-water paints face is sealed with an oil-base primer-sealer, or whitewash are used. The primer is often such as that described by Federal Specification simply a thinned version of the finish paint. A TT-P-56, and given a suitable enamel under- detailed discussion of primers for concrete and coat (such as TT-E-543), then almost any con- masonry is given in section 2.4.3. ventional oleoresinous interior paint may be (Additional infor- Coating systems for concrete and masonry applied as the finish coat. mation on oleoresinous paints is given in sec. may be a cement-water type, an oleoresinous type, a latex-base (water-thinned) type, or a 2.3.2.2.) synthetic resin type. These are discu.-^sed below (for whitewash, see sec. 2.3.9.2). 4.4.3. Latex Paints

their durability 4.4.1. Cement-Water Paints Latex paints, because of good and relative insensitivity to dampness and alkalinity along with ease of application and Cement-water paints, the oldest type, are clean-up, have become the most widely used composed chiefly of portland cement with some coatings for both exterior and interior concrete lime (see sec. 2.3.9.1), and are low in cost. and masonry surfaces. (A comprehensive dis- When properly applied and damp-cured, they cussion of latex paints is given in sec. 2.3.5.) acrylic, can provide a very durable finish that is perme- All three main types of latex paint— polyvinyl acetate, and styrene-butadiene—may * Painted brick surfaces can be durable only if the brick itself en- be used on either exterior or interior concrete dures. In new construction, the proper grade of brick for the environment should be specified independently of subsequent painting or masonry; however, best results on exterior considerations. For exterior use, grade SW or MW as defined in ASTM Standard C 62-62 T should be used [160]. exposure are obtained with the acrylic and the

84 polyvinyl acetate types, while the unmodified are used for this purpose. A synthetic rubber- styrene-butadiene type is generally restricted to base paint for interior concrete floors exposed interior use. The finish coat material, when to dampness is described by TT-P-91. Also thinned, may serve also as the primer-sealer. useful for chemical- and abrasion-resistant As with other masonry paints, it is important service on concrete and masonry are coatings that the surface be adequately prepared (clean, based on synthetic rubbers such as neoprene, chalk-free, and filled if necessary) before paint chlorosulfonated polyethylene, and polysulfides. is applied. Since the paint itself is water-based, Traffic paints for concrete (and bituminous) dampness of the surface is not a problem. The surfaces on streets, highways, and airfields are good resistance of latex paints to alkalinity per- formulated with a variety of quick-drying resins mits them to be used on unaged (but not on and solvents, including short-oil alkyd and phe- fresh, uncured) concrete and masonry. Federal nolic varnishes, styrenated-alkyds, chlorinated Specifications for exterior latex paints include rubber, and solution-type styrene-butadiene TT-P-19 (acrylic), TT-P-55 (polyvinyl ace- resins, together with tate), and TT-P-99 (styrene-butadiene). An suitable pigments and plasticizers to interior latex paint of unspecified resin type is produce a tough, adherent, covered by TT-P-29. weather-resistant coating. Federal Specifica- tions for traflic paints include TT-P-85, TT-P- 87, and TT-P-115. 4.4.4. Synthetic Resin Coatings Multicolor coatings (see sec. 2.3.7) can pro- Synthetic resin coatings, other than latex vide an attractive and durable finish for con- paints, which are being used successfully for crete and masonry surfaces. They may be severe service on concrete and masonry include formulated for either interior or exterior use. epoxy resins, polyesters, polyurethanes, and A multicolor lacquer based on an alkyd-modified various synthetic rubbers. Filled epoxy resins cellulose nitrate vehicle is covered by Federal and filled or glass-fibre reinforced polyester Specification TT-L-45; type I is for interior resins can be applied in thick mastic-like coats use and type II for exterior. to yield hard, glossy, tile-like coatings having The suggested uses for the various synthetic great mechanical strength, high chemical resist- resins indicated above are not mutually exclu- ance, and great beauty. Polyurethane resins sive ; many of the resins may be used with good produce tough, durable coatings for concrete results for applications for which others have floors; they are also used in swimming pool been mentioned as outstanding. Reference paints because of their good water and alkali should be made to the detailed information given resistance. Of proven durability and widest use for each resin in chapter 3 and to the guide- among swimming pool paints are those based lines given in section 4.1 of this chapter to pro- on a chlorinated rubber vehicle; such a paint vide a sound basis for the selection of an is covered by Federal Specification TT-P-95. optimum coating material for the intended Solution-type styrene-butadiene coatings also application.

4.5. Coatings for Plaster and Wallboard

The key to successful painting of plaster and quately. Whenever possible, new plaster should wallboard is in the selection and proper appli- be allowed to dry for at least two weeks before cation of a suitable primer-sealer (see sec. applying even a primer-sealer. Latex-base

2.4.4) . Primer-sealers are specially formulated primer-sealers, such as that covered by Federal materials designed to properly seal the porous Specification TT-P-650, are particularly well- surface, bond well to the topcoat, and provide suited for application on gypsum wallboard good topcoat hold-out. They are available either ("sheet rock") because of their nonsensitivity as oleoresinous or latex-base types. The oleo- to the alkalinity in the plastered tape joints em- resinous types offer the good brushability, level- ployed and because they do not raise the nap of ing, and adhesion typical of oil-base paints, and the outer paper layer as do oleoresinous types. provide better wetting and adhesion on chalky A phenolic emulsion-type dry wall primer also surfaces than is obtained with the latex types. is available for use on gypsum wallboard. They should be applied only on dry, well-aged Composition-board (fiberboard) and other (60 to 90 days) plaster. The latex-base primer- non-alkaline wallboards (see sec. 6.5.2) may be sealers are characterized ease application, by of sealed with either the latex or oleoresinous type fast drying, and good water and alkali resist- of primer-sealer with good results. (Primer- ance. The latter permits their use, if necessary, sealers are also discussed in sec. 2.4.4.) even on damp or only briefly aged (but not freshly applied) plaster, provided topcoating is Either the latex-base or oleoresinous type of deferred until the primed plaster has dried ade- primer-sealer may be over-coated with virtually

85 ; any conventional type of finish coat—latex, oil- E-543 or TT-E-545 (odorless). A one-coat base, or alkyd. A latex paint suitable for in- oleoresinous finish is provided by TT-P-47; terior use both as primer-sealer and finish coat however, such a coating should not be expected is described by Federal Specification TT-P-29 to provide the appearance and durability obtain- "self-priming" is accomplished by thinning with able with multiple-coat systems. water. An oleoresinous primer-sealer is pro- Multicolor finishes (see sec. 2.3.7) may be vided by TT-P-56, and an alkyd flat paint suit- applied on plaster and wallboard with attractive able for use both as primer-sealer and finish results. They are particularly effective in coat is covered by TT-P-30. A wide variety of masking surface imperfections and obliterating oil-base and alkyd-base coatings suitable for differences between dissimilar substrates. A application on properly primed plaster and wall- multicolor lacquer based on an alkyd-modified board are covered by Federal specifications and cellulose nitrate vehicle is covered by Federal may be readily identified through the quick Specification TT-L-45. guide list and the tabular summary presented In short, in coating plaster and wallt)oard, in chapter 8. apply the right primer-sealer to the properly In a three-coat oleoresinous system, the mid- aged and prepared substrate, and almost any dle coat may be an enamel undercoater such as conventional type of finish coat may be applied that conforming to Federal Specification TT- over it without difficulty.

4.6. Functional Coatings

The preceding sections of this chapter have considerations are minimized and excessive dealt with the problem of selection of coating overlapping of candidate materials is avoided. systems chiefly the standpoint of the type from In making use of the information given, one of substrate involved and whether the service should guard against the temptation to "over- environment was an exterior or interior one. select"; that is, against choosing a premium In general, this is effective a practical and material where an ordinary coating would suf- approach to the problem of coatings selection. fice. Over-selection unnecessarily restricts the There are times, however, the importance when number of potentially suitable materials, thereby of one or functional properties over- more lessening the opportunity for obtaining a coat- shadows ordinary environmental and substrate ing that meets the minor as well as the major considerations when the problem of selection — requirements of the application. The fact that can best be narrowed first examining those by a particular resin appears repeatedly in the materials which are capable of providing one functional listings characterizes it as a material or more all-important functional properties. of exceptionally good all-round properties but This is particularly true when the functional does not necessarily make it the best material requirement is a very rigorous one. For exam- for every job or even for most jobs. Often, ple, if complete inertness to all known chemicals those materials which are outstanding in a num- at temperatures up to 500 °F °C) is re- (260 ber of important functional properties may have quired, then the quest for a suitable material can disadvantages in other respects, such as high at once be narrowed to polytetrafluoroethylene cost, difficult application properties, limited pot provided that one knows that this material — life, the need for baking or fusing, etc. An possesses such resistance. objective appraisal of the properties needed for As part of the philosophy and systematic a given application, along the lines suggested approach to the problem of coatings selection in section 4.1, is essential to ensure a satisfac- offered in section 4.1 of this chapter, it was sug- tory coating system at minimum cost. gested that when the requirements of the appli- The ability to associate certain properties cation are considered by one who has some with certain materials can be very helpful in familiarity with organic coatings, several can- narrowing and solving the problem of coatings didate materials will often immediately suggest selection. The following discussion seeks to themselves, thus providing a practical starting provide this facility with respect' to a selected point for a detailed check of properties among group of functional properties. a limited group of materials. For those who may not yet have this familiarity, the information which follows should provide a useful step 4.6.1. Chemical Resistance toward acquiring it. Our treatment of this coating materials aspect of coatings selection is not exhaustive While some organic may appropriately be called chemical-resistant be- and is desirably limited to indicating only those cause of their relative inertness to a wide spec- materials which are generally recognized as out- trum of corrosive agents, the term "chemical- standing with respect to the particular property resistant" is more meaningful and useful when under consideration. In this way, subjective the types of chemicals to which the coating is

86 ,

resistant are specified. For example, baked to oxidation under conditions of exposure to (thermoset) phenolic resin coatings are gen- sunlight and weathering is obtained with sili- erally considered to have excellent chemical cone resins, polysulfied rubber, cellulose acetate resistance, and they do—to most acids, fats, butyrate, acrylic resins, polyurethane rubber, oils, organic solvents, and water—but they may polyacrylic ester rubber, polyvinyl acetate, be quickly destroyed by strong oxidizing acids selected alkyds (e.g., melamine- or silicone-al- alkalis. Generally good chemical kyds) vinyl chloride-acetate resins (stabilized) or strong , resistance may be the rule for a particular epoxy resins, cyclized rubber, furan resins, pure material; yet the exception can be costly and phenolic resins, polyesters, and polyurethanes. dangerous. 4.6.1.4. Resistance to organic solvents.— The only organic material that is completely Complete resistance to all known organic sol- inert to attack by all known chemicals (except vents is provided by polytetrafluoroethylene. molten alkali metals, and fluorine at high pres- Excellent resistance to a wide spectrum of sures) is polytetrafluoroethylene. All others, organic solvents is obtained with water-soluble however resistant, have an "Achilles heel" of resins such as polyvinyl alcohol and methyl which the potential user should be aware—by cellulose. Polyethylene is highly resistant to conducting his own tests if necessary. Limita- all organic solvents except hot aromatic and tions of knowledge and space preclude a pin- chlorinated hydrocarbons. Polypropylene is pointing of all such weaknesses here. However, even more resistant than polyethylene. Excel- the considerable information offered will sim- lent resistance to most organic solvents is pro- plify and expedite the task of coatings selection. vided by baked pure phenolic resins, baked epoxies, and furan resins. Polychlorotrifluoro- 4.6.1.1. Acid resistance. Excellent resist- — ethylene is insoluble in all organic solvents at ance to dilute and concentrated acids (except room temperature but is swelled by halogenated oxidizing types) is provided by coatings strong solvents, toluene, diethyl ether, and ethyl ace- of neoprene, polyvinyl chloride, polyvinylidene tate; at elevated temperatures it is swelled by chloride-vinyl chloride copolymer, polypropyl- still other materials and will dissolve in a few ene, polyethylene, and butadiene-acrylonitrile of them. Very good resistance to a wide va- (nitrile rubber) . Excellent resist- copolymer riety of organic solvents can be obtained with ance even to strong oxidizing acids is provided polysulfide resins. Good resistance to many by polytetrafluoroethylene, polychlorotrifluoro- organic solvents is also obtainable with prop- ethylene, chlorosulfonated polyethylene [141], erly formulated polyurethanes, polyesters, vi- isobutylene-isoprene copolymer (butyl rubber), nylidene chloride copolymers with vinyl chlo- chlorinated rubber Good resistance and [140]. ride or acrylonitrile, and epoxy-polyamide poly- to non-oxidizing acids is also afforded by resins. styrene, furan resins, solution-type styrene- Excellent resistance to aliphatic hydrocar- butadiene copolymers, silicones, cyclized rub- bons, fats, oils, greases, and waxes is obtainable ber, polyesters, and acrylic resins; also, by with a wide variety of coating resins, notably baked thermoset coatings of pure phenolic the fluorocarbons, polyethylene and polypropyl- resins, unmodified epoxy resins, amino and ene, neoprene and chlorosulfonated polyethyl- resins. ene, polyvinyl formal and polyvinyl alcohol, cellulose esters, acrylic resins, and most thermo- 4.6.1.2. Alkali resistance.—Excellent re- set resins such as epoxies, melamines, poly- sistance to dilute and concentrated alkalis is urethanes, polyesters, etc. provided by coatings of polytetrafluoroethylene, The wide variations in solvent resistance that polychlorotrifluoroethylene, polyethylene, poly- may result from differences in resin composi- propylene, neoprene, baked epoxies (unmodi- tion and formulation, together with the wide fied), polyamides, butyl rubber (isobutylene- variety of organic solvents that are available, isoprene copoylmer), polyvinyl formal, poly- make it necessary to interpret and use data on vinyl chloride, vinyl chloride-vinylidene chloride solvent resistance with discretion backed up copolymer, and chlorinated rubber. Good re- by laboratory testing as may be required. It sistance to alkalis is also afforded by polysty- is evident, nevertheless, that the guidelines pro- rene, furan resins, amino resins, ethyl cellulose, vided above can minimize the work required chlorosulfonated polyethylene, styrene-butadi- by permitting a judicious preliminary selection ene resins, butadiene-acrylonitrile copolymer of potentially suitable materials. (nitrile rubber), polyurethanes, acrylic resins, 4.6.1.5. Water resistance. cyclized rubber, and coumarone resins. —Outstanding resistance to water and very low water absorption with excellent dimensional stability are pro- 4.6.1.3. Oxidation resistance. In addition — vided by the fluorocarbon resins, polyethylene to the materials listed as resistant to strong and polypropylene, polyvinylidene chloride co- oxidizing acids (sec. 4.6.1.1), good resistance polymers (saran), polystyrene, chlorinated rub-

87 ber, and certain silicone resins. Good resist- 4.6.5. Impermeability ance to water is also provided by cellulose nitrate, cellulose acetate-butyrate, ethyl cellu- Outstanding impermeability to gases and lose, phenolic resins, vinyl chloride and vinyl vapors is exhibited by vinyl chloride-vinyli- chloride-acetate resins, acrylic resins, epoxies dene chloride copolymers, polytetrafluoroethyl- and polyamides, polyurethanes, and polyesters. ene, isobutylene-isoprene copolymer (butyl rubber), polyvinyl alcohol, polypropylene, polyethylene, polysulfide 4.6.2. Heat Resistance chlorinated rubber, rubber, and polyacrylic ester rubber. The need for heat resistance immediately brings to mind polytetrafluoroethylene, silicone 4.6.6. Transparency resins, and polycarbonate resin as the outstanding heat-stable resins. Very good heat resist- Outstanding clarity, transparency, and non- ance compared to most resins is provided by yellowing properties are exhibited by acrylic coatings of neoprene, polychlorotrifluoroethyl- resins, polystyrene, cellulose acetate butyrate, ene , baked phenolic resins, chlorosulfonated polyvinyl butyral, polyesters, and polycarbon- polyethylene, polyvinyl formal, cellulose ace- ate resin. tate, ethyl cellulose, isobutylene-isoprene copolymer (butyl rubber), polypropylene, amino 4.6.7. Fire Resistance resins, furan resins, polyurethanes, epoxies, and styrene-butadiene resins. Polymers w^ith Among nonflammable resins with self-extin- low softening temperatures but good resistance guishing properties are the fluorocarbons, to discoloration or degradation by heat within chlorinated rubber, polycarbonate resin, poly- their working range include acrylic resins, cel- vinyl chloride, polyvinyl chloride-acetate, and lulose acetate butyrate, and polystyrene. certain silicone resins.

4.6.3. Abrasion and Mar Resistance 4.6.8. Radiation Resistance

To be resistant to abrasion a coating mate- A definitive statement on the high energy rial must either be tough and resilient enough radiation resistance of organic coating mate- to "ride with the punch," or else hard enough rials is difficult because of the limited, often to be immune to the destructive stresses to which empirical, and sometimes contradictory nature it is subjected. The resilient type of abrasion of the available experimental and performance resistance is provided by many of the synthetic data. Moreover, most of the information con- rubbers (see sec. 3.15), notably neoprene, cerning organic coatings must be extrapolated chlorosulfonated polyethylene, and polyure- or inferred from data obtained on much thicker thane rubber. Outstanding toughness and sheet or bulk plastics. To the extent that oxy- abrasion resistance also is exhibited by poly- gen (and perhaps other environmental agents) vinyl formal and by polytetrafluoroethylene. may affect the behavior of the material under Hard coatings with excellent mar resistance irradiation, such extrapolation may be mislead- include baked epoxy resins, amino resins (es- ing because the thickness of the material, to- pecially melamines), polyurethanes, baked phe- gether with its permeability, determines the nolic resins, and polycarbonate resin. Tough quantity of oxygen that can diffuse into it and abrasion-resistant coatings are also obtained be available for reaction where radiation ef- with polyvinyl chloride plastisols, polypropyl- fects are being induced below the surface. ene, polyethylene, polychlorotrifluoroethylene, Thus, a thin film (i.e., a coating) of an oxygen- epoxy-polyamides, ethyl cellulose, and various sensitive polymer—even a relatively imperme- vinyl and styrene-butadiene resins. able one—may be quickly destroyed by irradiation because of diffusion of oxygen throughout of 4.6.4. Electrical Properties the material whereas a thick sheet or block the same material may suffer only surface Excellent electrical properties, including high degradation. Nevertheless, a few basic facts dielectric strength and arc resistance and about the effects of high energy radiation on low dissipation factor are exhibited by poly- polymeric materials can be stated here, and tetrafluoroethylene, polyethylene, polypropyl- certain polymers may be broadly classified on radiation. ene, polystyrene, polyvinyl formal, and poly- the basis of their resistance to such detailed information, the reader is re- vinyl chloride. Also very useful for electrical For applications are neoprene, polychlorotrifluoro- ferred to the literature [155, 156]. ethylene, silicone resins, epoxy resins, polya- It is known that high-energy ionizing radia- mides, ethyl cellulose, isobutylene-isoprene rub- tion (x-ray, y-ray, /3-particles, etc.) induces ber (butyl rubber), polysulfide rubber, and either crosslinking, chain scission, chain strip- polycarbonate resin. ping (removal of chain substituents), or a

88 combination of these processes in high poly- ticularly phenyl substituted types). Polymers meric materials into which the radiation is that may be considered as moderately resis- absorbed. These effects occur regardless of the tant to radiation include styrene-acrylonitrile source of the high energy radiation—whether and styrene-butadiene-acrylonitrile copoly- generated by high voltage electrical machines mers, polyacrylic acid and esters, polyacryla- such as a Van de Graaff generator or a linear mide, polyurethanes, diallyl phthalates, accelerator, or emanating from radioactive iso- polychloroprene (neoprene) , chlorosulfonated topes such as cobalt-60 obtained by exposing polyethylene, and SBR rubber (formerly called the metal to pile radiation in a nuclear reactor, GR-S) . Natural rubber, too, can be vulcanized or from the atomic pile itself. Nor are the (crosslinked) with high energy radiation. In effects significantly different whether the high the case of all the polymers named above, it energy radiation is electromagnetic (y-rays) in should be remembered that in the presence of nature or in the form of high speed electrons unlimited amounts of oxygen the crosslinking in reaction may be offset or even outweighed by (/3-particles) . Crosslinking, as explained section 3.1.1, is a basically favorable process increased chain scission. the physical and which may improve many of Finally, it may be helpful to point out those chemical properties of the polymer, such as classes of polymers that are known to undergo imparting greater hardness, better solvent re- chain scission and/or chain stripping rather sistance, and a higher softening point. Even- than crosslinking when exposed to high energy tually, however, under prolonged irradiation radiation and that consequently soon deterio- even the most resistant polymers finally em- rate under irradiation. Such "negative" in- brittle and deteriorate. Chain scission or strip- formation may be particularly useful when ping, since it degrades the polymer, is detri- applied to polymers that are noted for their mental from the start. Whether a given outstanding chemical inertness and might polymer is improved or degraded by moderate therefore have been expected to be radiation- doses of high energy radiation depends on resistant also. The fluorocarbon resins, in- whether the cross-linking or the scission (and/ cluding polytetrafluoroethylene, polychlorotri- or stripping) reaction predominates, as de- fluoroethylene, and polyvinyl fluoride, all are termined by the chemical structure of the degraded by high energy radiation, especially polymer; that is, by the nature and bond in the presence of oxygen. Polymethyl metha- strengths of the main chain and side groups, crylate and cellulose acetate butyrate, though steric factors, and the shielding effect of sub- noted for their excellent resistance to weather- stituent groups. For example, it has been ing and ultraviolet radiation, suffer severe found that pendant benzene rings, perhaps be- degradation when exposed to high energy cause of their efficient dissipation of energy, radiation. Similarly, polyisobutylene, despite exert a shielding effect that protects the main its chemical inertness, undergoes rapid chain chain from scission and promotes crosslinking scission under high energy irradiation, even rather than degradation. Such is the case in in the absence of oxygen. Other common poly- polystyrene which exhibits the greatest resist- mers that have shown [157] poor resistance ance to high energy radiation of any of the to high energy radiation include polyvinyl common plastics. On the other hand, poly- chloride homopolymer and copolymers, polyvinyl chloride is quickly degraded even by low vinylidene chloride, styrene-butadiene copoly- doses of high energy radiation because of its mer, polyethylene terephthalate, polycarbonate high susceptibility to chain stripping. resin, cellulose esters and ethers, and amino The radiation resistance of high polymers is resins and phenolic resins unless mineral filled. of interest both from the point of view of Mineral filled epoxy resins also have with- obtaining improved (crosslinked) materials stood high energy radiation fairly well. It is through manmade irradiation and from the apparent that reinforcement with inorganic standpoint of the use of polymeric materials fillers may permit the use of some polymers in environments where they must withstand under radiation conditions that would quickly exposure to moderate amounts of radiation, as destroy the unmodified polymer. It is possible in a space capsule or satellite. From both also that even the less radiation-resistant ma- points of view, crosslinking rather than chain terials may be suitable in certain applications scission is the desirable effect. Polymers that where the degradation suffered is not critical tend primarily to crosslink under high energy (for example, when irradiation results in the radiation and may therefore be classed as rela- loss of elongation or tensile strength in an tively radiation-resistant include polystyrene, application where neither property is impor- polyethylene, polypropylene, linear polyamides tant because the material needs only to remain (nylon resins), polyesters, and silicones (par- in place without physical strain).

89 4.7. Summary

Briefly, this chapter has indicated both a tion of a suitable material for a given applica- philosophy and a systematic approach to the tion in the great majority of cases. As an problem of coatings selection, together with added feature, to provide the potential user specific recommendations regarding coating with a facility for associating certain resins systems for various substrates and environ- with certain functional properties, section 4.6 ments. The guidelines provided, in combination with the detailed information on the prop- has presented a listing of typical coating properties of coating materials presented in chapter erties and the resins which are outstanding in 3, should furnish a sound basis for the selec- providing these properties.

Figure 4.1. Xenon-arc accelerated weathering machine. Operating conditions are described in ASTM Method E 240-64T.

Figure 4.2. Interior of carbon-arc accelerated weathing machine. Principles and operating conditions are given in ASTM Method E 188-63T.

90 5. Storage, Safety, and Application Methods

The proper storage and handling of organic be observed in dealing with potentially flam- coating materials and the usual methods for mable, toxic, and sometimes "perishable" ma- applying them are adequately discussed in a terials such as paints and other organic number of Departmental and Bureau paint coatings. It is also appropriate to present manuals (see references [40] and [41], and sufficient information about the various meth- General Bibliography in chapter 9). It is not ods of applying coatings—particularly the less a purpose of this publication to duplicate these commonly used or newer methods such as detailed treatments of the subject. It is de- fluidized bed and airless spray—to enable the sirable, however, within the comprehensive yet reader to understand the principle of opera- concise scope sought in this volume, to point tion and the chief advantages and disadvan- out the basic precautions that should always tages of each method.

5.1. Storage of Coating Materials

While different coating materials vary greatly troublesome in this respect. Such difficulties in their storage stability—some remaining can be minimized by inverting containers at useable for years while others deteriorate in intervals of a month or two. a few months—all organic coating materials Maintenance of proper identification of paints should be stored in tightly closed containers in in storage will save time and avoid costly a covered, well-ventilated area where they will mistakes. Containers should be relabeled not be exposed to excessive heat, fumes, sparks, when necessary to ensure legibility. Pre- flame, or direct sunlight. Heat accelerates the viously opened containers, in particular, should thickening or hardening of reactive vehicles be checked for tight closure and legibility of during storage and increases the fire hazard identification. normally present with coating materials con- Paint solvents and thinners should be stored taining volatile, flammable solvents. Freezing with the same or greater care than the paints destroy the use- may break the emulsion and themselves. fulness of latex paints, although some may Application should withstand a few freeze-thaw cycles; water- equipment be cleaned promptly base coatings should be protected against freez- and thoroughly with a suitable solvent after each use, ing. Low temperatures down to 0 °F (—18 °C) and stored in a clean place. thicken but do not ordinarily harm solvent-base Brushes that receive daily use may be kept in coatings; however, the material must be good condition by suspending them in a suitable warmed before use to restore a normal vis- solvent between periods of use; however, this cosity for proper application. practice may create a fire hazard when volatile Pigmented materials tend to settle in the solvents are employed unless care is taken to container and may form a cake that is difficult keep the containers well-covered with foil or to redisperse. Red lead paints are notably film or stored in a special cabinet.

5.2. Safety Precautions

The combustible and toxic nature of most safety, the subject of toxicity is discussed here organic solvent-base coating materials should in somewhat greater detail than is found in be kept in mind whenever such materials are most paint manuals; however, the treatment stored, handled, or applied. Containers should is necessarily limited. Detailed authoritative be opened carefully in a direction away from information on the toxicity of specific solvents, oneself or others, since volatiles or unexpected pigments, resins, and other constituents of or- gaseous reaction products may sometimes build ganic coatings is available from the U. S. Public up pressure inside the container. Open flames Health Service (Department of Health, Edu- should be kept away from the area where the cation, and Welfare, Washington, D.C.), in the paint is being used, and sparks should be Chemical Safety Data Sheets issued by the avoided. Adequate ventilation should be pro- Manufacturing Chemists Association (Wash- vided indoors to prevent the build-up of toxic ington, D.C.), and in a number of compre- or explosive concentrations of solvent vapors. hensive reference works [69, 70], Government Special precautions are necessary when spray- and industrial pamphlets [71, 72], and manu- ing, whether indoors or outside, to avoid in- facturers' technical bulletins. halation of toxic fumes and dusts. Even common solvents that are ordinarily considered innocuous may be harmful if inhaled 5.2.1. Toxicity in high concentrations or for prolonged periods. For example, turpentine can cause narcotic Because of its importance to the health of poisoning and kidney damage when excessive the worker and to those responsible for his amounts are inhaled. Systemic damage may

91 : . also result from an acute dose (sufficient to Sandblasting, in particular, requires a suitable produce unconsciousness), or from repeated mask, goggles, and protective clothing. Some excessive inhalation of such common solvents paint additives, such as the mercurial com- as the acetate esters (methyl-, ethyl-, butyl-, pounds used to impart fungicidal properties, and amyl acetate), amyl alcohol, methyl iso- may be toxic if ingested. butyl ketone, and possibly methyl ethyl ketone. The toxicity hazards associated with organic Solvents such as toluene, xylene, ethyl, ether, coating materials have been pointed out not cyclohexanone, butyl alcohol, and the nitro- to discourage their use or arouse fear but to paraffins have a relatively low chronic toxicity, emphasize the importance of adequate safety but their narcotic effect in high concentrations measures to prevent accident and injury. With can lead to unconsciousness followed by respira- adequate precautions based on knov/ledge of tory failure and death unless the victim is the properties of these materials, they may promptly restored to fresh air. Even the least be handled confidently and safely. The basic toxic of the common paint and lacquer sol- precautions to be observed are simple and vents—including mineral spirits, petroleum practical naphtha, gasoline, acetone, isopropyl alcohol, (1) Avoid unnecessary or prolonged in- and ethyl alcohol—although well-tolerated in halation of fumes and dusts; always provide relatively large concentrations and having little adequate ventilation, or use a suitable mask or no chronic ill effects, can be dangerous in or respirator (remember that gas mask can- very high concentrations because of their nar- isters will absorb only limited amounts of cotic effect, excepting the alcohols which are noxious vapor and must be renewed at frequent irritants (to the eyes and upper respiratory intervals) tract) rather than toxicants. (2) Avoid unnecessary or prolonged con- Some common solvents—usually not used tact with the skin; the use of protective clothing directly in paints but often encountered in and skin creams will eliminate the need for paint removers and cleaning liquids—are dan- scrubbing the skin with toxic solvents. gerously toxic even at relatively low concen- (3) Avoid ingestion of toxic materials; trations; for example, methyl alcohol when in- wash hands carefully before eating or smoking; haled or ingested repeatedly or to the point change clothing that has soaked up toxic of unconsciousness may cause death or perma- materials. nent blindness. Benzene is a dangerous, cumu- (4) Know the material with which you lative poison that causes severe damage to the are working; read and heed the precautions on liver and blood-forming tissues of the body. the label—it will tell you when an unusual Many chlorinated solvents, particularly chlo- hazard exists and how to avoid it. rinated aliphatic hydrocarbons, are toxic; (5) Make safety "second nature"; don't chloroform, for example, has poisonous after- sneer at safety regulations or at the fellow effects, while carbon tetrachloride and now- who observes them; be alert to possible haz- seldom-used tetrachloroethane are dangerously ards, avoid them, and stay healthy. toxic with severe chronic, cumulative effects Water-base paints are free from the toxicity on the kidneys, liver, and lungs. Since solvent- hazards associated with organic solvents and type paint removers frequently contain highly are generally based on non-toxic resins. They toxic solvents, they should be used only with may contain pigments or additives, however, adequate ventilation or a respirator. which may be harmful, so that ingestion should The toxic systemic effects of organic solvents be avoided. The manufacturer's label is a can be produced by absorption through the useful guide in such matters. skin as well as by inhalation. Acute or chronic dermatitis or systemic poisoning may result 5.2.2. Fire from repeated or prolonged contact of toxic solvents with the skin. Sometimes, the vehicle Most organic coating materials are com- resin itself may be irritating or toxic to the bustible (water-base coatings excepted), pri- skin while the resin is in the uncured state. marily because of the flammable solvents and The catalysts or hardening agents (e.g., organic thinners which they contain. They are safe amines) used in two-component coating sys- while stored in tightly closed containers away tems are frequently volatile and toxic materials from heat and flame. When containers are until they have been incorporated into the opened and while the coating is being applied, resin. open flames and heated objects (lighted ciga- Some of the pigments used in organic coat- rettes, for example) should be kept out of the ing materials are toxic. Lead compounds and area. chromates are particularly dangerous, whether When applying organic-solvent base coat- ingested or inhaled. Lead fumes generated ings indoors, adequate ventilation should be while burning off old paint are very poisonous. provided to prevent the build-up of explosive Inhalation of the dust and debris from sanding concentrations of vapor. Nevertheless, flame and brushing operations is a health hazard. and spark should be avoided. Smoking should

92 be prohibited. Blow-torches should not be followed by explosion as containers become used even in other parts of the building into over-heated, and that many burning paints and which solvent vapors may diffuse. Care should solvents give off toxic fumes. Chlorinated be taken to extinguish all pilot flames on heat- hydrocarbons, for example, while usually non- ers and stoves. Sparking motors should not flammable or less flammable than most other be in operation. Even an electric light switch solvents, give off deadly fumes when heated or the spark from a metal tool can be a source to decomposition. While it may be worthwhile of danger if explosive concentrations of vapor to fight a small incipient fire with a hand ex- are present—hence the need for adequate tinguisher while awaiting the fire department, ventilation. it should be realized that a small fire can de- Rags used to wipe up linseed oil or other velop into a very large fire in a few seconds, drying oils or oxidizable liquids are a frequent with consequent risk to life. source of spontaneous combustion when left lying around. They should be disposed of by 5.2.3. Falls and Spillage controlled burning, or by drenching them with water and putting them in a covered metal container. A large proportion of the injuries suffered by painters is the result of falls from ladders iVhen burning old paint off wood, the torch or scaffolds, frequently involving paint spillage must be handled with care to avoid setting fire as an accompanying hazard. Such accidents to the substrate. A water hose should be kept are usually the result of carelessness in the ready for use if needed. rigging, maintenance, or inspection of scaffolds Paint and solvent fires usually cannot be and accessory equipment, or in the placement of fought with water. Water is immiscible with most paint materials; where large quantities ladders. Even a short fall can be painful and dangerous. of flammable liquids are involved the water One should be certain that ladders, scaffolds, ropes, in proper will only further spread the flame because the and moorings are condition, and that mooring are firmly lighter liquids float on the water surface. hooks attached at a location strong to Such fires are most effectively fought with amply enough carbon dioxide, with foam, or with dry-chemical support the weight. Although these precau- extinguishers, which smother them. This tions appear self-evident, accidents continue to occur because they are ignored or overlooked. should be remembered if there is ever occasion to use a hand fire extinguisher on burning In the event of a major spillage of volatile solvents while waiting for the fire department flammable liquid, don't wait for a fire—call to arrive. It should be remembered also that your disaster control unit or the fire depart- fire in a paint storage area may be quickly ment immediately.

5.3. Mixing, Thinning, and Activation

Most of the coatings in use today are ready- top lids from cans with the creation of a fire mixed materials—ready for use as supplied by hazard. Zinc dust paints also are supplied the manufacturer except for the need for: with pigment and vehicle in separate packages (1) stirring or mixing pigmented materials to because of the tendency toward gassing when uniformly redisperse the solids in the vehicle, mixed with the vehicle and stored. (2) appropriate thinning when necessary for Before mixing or thinning paint for use, it priming purposes or spray or other application, should be brought, if necessary, to the proper and (3) proper activation in the case of two- application temperature. Cold paint is most component reactive systems. conveniently and safely warmed by letting it In a few instances, the pigment and vehicle stand in a warm room. Where more rapid of a pigmented coating system are furnished heating is required, a hot water bath or electric in separate containers for mixing just prior to blanket may be employed, but the warming use in order to avoid undesirable settling, jacket should not be above 90 °F (32 °C). caking, flocculation, or deteriorative changes Organic coating materials should never be during storage. For example, aluminum paints heated over an open flame because of the great [73] are sometimes prepared by mixing the danger of localized overheating, decomposition, aluminum paste or powder into the appropri- explosion, and fire. ate vehicle just before the paint is applied. This procedure is designed to overcome loss 5.3.1. Mixing of Paints of leafing properties of the aluminum pigment and gassing in the container during storage; Ready-mixed paints are usually furnished in both of these undesirable characteristics have cans of five gallons or less, to permit conveni- been encountered with improperly formulated ent mixing and to avoid the deterioration that aluminum paints. Gassing (hydrogen forma- may occur in large containers or drums which tion) may result in the blowing off of friction are used up more slowly and acquire a large

93 — air space as the contents dwindle. Thorough stirred gently to restore a uniform mixture. mixing of pigmented coating materials just Vigorous stirring, particularly shaking, should before use is necessary to redisperse settled or be avoided, since it introduces air bubbles that caked pigment and ensure a smooth, uniform will impair the appearance and serviceability mixture that incorporates all the original pig- of the deposited film. If air bubbles are in- ment into the vehicle. Hand-mixing methods, advertently introduced, the coating material power stirring, or automatic shaking or vibra- should be allowed to stand before use until the tion may be employed. bubbles have escaped. Automatic shaking, where available, is convenient and rapid; usually, from three to fif- 5.3.2. Use of Thinners teen minutes is required. The violent agitation, however, produces many fine bubbles in the paint which may be objectionable, particu- The most common reason for thinning paints larly in enamels. It may be necessary to let and other organic coating materials is to re- the paint stand for some time to let the bubbles duce the viscosity—either for the purpose of escape and then give it a final mixing by rendering the material suitable for use as a hand before using it. Final hand-mixing is priming coat, or to render it suitable for appli- desirable in any event to ascertain that no un- cation by a means different from that for dispersed material remains on the bottom or which it was initially formulated. For ex- sides of the container. ample, oil-base paints are usually suitable for Ready-mixed paint in the usual size con- brush application as furnished; however, they tainers (up to five gallons) may be hand-mixed may be thinned further (self-primed) for use conveniently and effectively in the following as the first coat on bare wood—or they may be manner: Pour off the thin supernatant mate- thinned to permit their application by spraying. rial into a clean container; if only light set- Lacquers, on the other hand, are generally fur- tling has occurred, pour off about one-third to nished in a viscosity ready for spray applica- one-half of the paint. Stir the remaining portion, since this is the preferred method of appli- tion with a paddle or power stirrer until a uni- cation. Brushing lacquers also are available form consistency is obtained, making sure that but they must be applied with awareness of all material settled on the bottom or sides of their quick-setting characteristics which limit the container is re-incorporated. If necessary, their brushability to only three or four strokes* stir the poured-off portion also. Slowly, with immediately after application. continual stirring, return the poured-off por- Quick-drying coating materials such as lac- tion to the original container. Finally, "box" quers, which undergo considerable cooling due the paint by pouring it back and forth between to the rapid evaporation of solvent from the the two containers several times. While the applied film, may require the addition of some paint is being used, it should be stirred at fre- blush-retardant thinner when applied under quent intervals to maintain a uniform mixture adverse conditions of high humidity. "Blush" and to prevent skinning. is a whitish discoloration of the film caused by When a previously opened container is used the condensing of moisture from the air into again, the same mixing procedure indicated the drying, cooling film. The visual appearance above is used, after first carefully removing is the result of: (1) the presence of fine drop- any skin that may have formed. If pieces of lets of water incompatible with the organic the skinned material fall into the paint, or if solvents in the coating, and (2) the actual pre- a few small clumps of hard-to-disperse material cipitation, in very fine form, of some of the are present in the paint, these can be removed film-forming material by the nonsolvent water. by straining through clean cheesecloth or fine Although the deposited water will eventually wire screen. Paint containing a large number evaporate, the precipitated film material will of such clumps, or a considerable quantity of remain as a discoloration and discontinuity settled, caked material that cannot be redis- which impairs the usefulness of the coating. persed, is not suitable for use and should be Blush can be avoided if sufficient high-boiling discarded. solvent is present in the final stages of the dry- When tinting is to be done, the base paint ing process to redissolve the precipitated mate- should be thoroughly mixed before adding the rial. Blush-retardant thinners contain a high tinting liquid. The latter should be added proportion of high-boiling active solvent which slowly with continual stirring, and the mix- not only lessens the blushing tendency by reducing should be continued until a completely uni- ing the drying rate and the resultant cooling of form distribution of the tinting color is ob- the applied film but also provides additional ac- tained. tive solvent for redissolving any precipitated Clear coatings, such as varnishes or clear film material during the final stages of drying. lacquers, do not ordinarily require stirring. Blush retarders must be used sparingly and However, if the material near the bottom of with discretion because too much can upset the the can is thicker than the rest, it should be original solvent balance of the formulation and

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create application difficulties (e.g., running, former and produce a solution of high solids sagging, prolonged tackiness) while also af- content at the proper application viscosity. fecting, sometimes adversely, the structure and The diluents, although nonsolvents, are well- properties of the film that is laid down. tolerated and serve to extend the solvents for Yet another reason for thinning an organic economy while contributing to the adjustment coating material is to replenish solvent lost of viscosity, drying rate, and gel properties. through evaporation from the container while The amount of volatile liquid contained at the the material is in use or between usage. Nor- gel stage is an important factor in determining mally, such adjustment is best made with a the structure of the film that is laid down as thinner the composition of which is essen- the residual liquid evaporates. In general, the tially the same as that employed in the original low-boiling constituents of the volatile vehicle solvent formulation. It should be recognized, contribute solvent power and fast setting, the however, that in the case of a composite thinner medium-boiling components control the drying covering a range of boiling points, the bulk rate to allow good leveling, the diluents (usually of the solvent loss will be in the lower-boiling medium-boiling liquids) regulate the on-set of constituents. Therefore, if the solvent loss has the gel stage, and the presence of some medium- been excessive—as indicated by a pronounced to high-boiling active solvent imparts blush increase in the viscosity of the paint in the resistance and ensures solubility of the film- absence of chemical reactivity—it is advisable former in the final stages of the drying. To that the restoring thinner be richer than nor- meet the concurrent and often" conflicting re- mal in low boiling constituents. The problem quirements of (1) high solids content for good of proper thinning is most difficult with criti- film build per coat at a workable viscosity, (2) cally balanced complex solvent formulations good flow and leveling without runs or sags, such as those used in lacquers and dopes; it is and (3) fast drying without the development least critical with paints that utilize single of film faults such as blush or cratering, it is solvents or simple blends of similar solvents, essential to employ a suitably formulated mix- such as the turpentine and/or mineral spirits ture of active solvents and diluents carefully which constitute the volatile portion of oil- balanced for proper solvent power, evaporation base or oleoresinous paints. rate, and gel stage composition. To ensure that the performance character- 5.3.2.1, Principles of proper thinning.— istics so painstakingly built into the original Whether the purpose of thinning be to reduce formulation will not be nullified by improper viscosity, retard blush, replenish lost solvent, or thinning, the following principles and precau- some other function, certain basic principles tions should be observed should be kept in mind to ensure satisfactory re- (1) Use thinners only when specified or nec- sults. Every organic coating material is a spe- essary, and in the minimum amount that will cially formulated material designed for a par- do the job. Even an ideal thinner may ad- ticular type or area of service. The volatile versely affect the application properties, portion of the vehicle has a major role in spreading rate, and ultimate durability of the determining the solids content and application coating if used where it is not needed. viscosity of the material, its drying rate, the (2) Whenever possible, use only that thin- structure and adhesion of the deposited film, ner or type of thinner which is specifically and the resultant effects on appearance, me- recommended by the manufacturer of the coat- chanical properties, and durability. When the ing material. original material must be thinned, for whatever (3) When the manufacturer's thinner, or a reason, the essential solvent balance must be known substitute, is not available, make sure maintained in order to obtain the desired film that the thinner to be used is completely com- properties for which it was designed. patible by adding it to a test quantity of the The principles and importance of proper sol- original coating material and noting if any vent balance in determining the properties and flocculation, separation, or other abnormality performance of the coating may be seen from occurs in the thinned paint and in a film pre- consideration of a typical lacquer formulation pared from it. (the general characteristics of a lacquer are (4) In choosing or preparing one's own discussed in section 2.2.4). The nonvolatile thinner, use a composition approximating that portion of a lacquer typically may consist of of the volatile vehicle of the original coating the film-forming base, a modifying resin, a material. Compositional information, if not plasticizer, and possibly a stabilizer or other available from the label, may usually be ob- additive—plus suitable pigments where color tained from the covering specification. Fed- or opacity are desired. The volatile vehicle is eral specification thinners for various types of a mixture of active solvents (including latent coatings are listed in section 8.1.9. solvents) and diluents, in a range of boiling (5) If the composition of the volatile vehicle points and associated evaporation rates. The is not known and its nature cannot be inferred active solvents are needed to dissolve the film- from its odor, prepare a thinner on the basis

95 : of known solvents and nonsolvents for the (1) Many of the catalysts or curing agents film-forming resin, keeping in mind the for- employed are toxic prior to incorporation into mulating principles previously discussed. (In- the resin; in such cases, inhalation, skin con- formation on the solubility of resins is avail- tact, or ingestion should be avoided. able in chapter 3), If the identity of the resin (2) It is important that the two components base also is not known, then trial and error be uniformly mixed in the proportions recom- testing of small batches with various classes mended by the manufacturer to achieve opti- of solvents may reveal a compatible solvent for mum film properties. emergency use. (3) The activated mixture has a limited pot (6) Before thinning, thoroughly mix the life—usually from one to eight hours, de- original coating material to a uniform consist- pending on the amount of catalyst used, the ency as described earlier, add the necessary temperature, and the nature of the materials amount of thinner, and again stir to a uniform involved. Follow the manufacturer's instruc- consistency. tions. (7) When using thinners, observe all due (4) The curing reaction frequently is ex- precautions against fire and toxicity hazards as othermic; hence the coating may seem quite described in section 5.2. thin or be too thin for some applications shortly after being activated, because of the viscosity 5.3.3. Activation of Two-Package Systems reduction resulting both from the addition of liquid reagent and from the temperature rise. Two-package coating systems are those in In the case of epoxy resins, for example, an which the film-forming resin and the curing or induction period of about 45 minutes after ad- hardening agent are furnished in separate condition of the catalyst is recommended before tainers for mixing just prior to use. Typical using the coating material. two-package systems include amine-cured (5) In some instances, the thickness of the epoxy resins, styrene-plus-peroxide-cured poly- deposited film has a marked effect on the rate esters, amine- or polyol-cured polyurethanes, and extent of the cure achieved. Polyester coat- and metallic oxide-cured neoprenes and chloro- ings, for example, may not cure satisfactorily sulfonated polyethylenes. Of these, the epoxies, in thicknesses of less than six mils (0.006 polyesters, and polyurethanes cure at room inch) because of excessive loss of volatile cata- temperature whereas the last two are usually lyst, air inhibition, and reduced reaction tem- heated after the catalyst or accelerator has been perature because of loss of the heat generated added. by the exothermic reaction. Activation of two-package systems poses no (6) As a rule, follow the manufacturer's special problems if the following precautions mixing and application instructions and take are taken the recommended precautions.

5.4. Application Methods and Equipment

The choice of an appropriate application suppliers of such equipment. Therefore, in the method from among the many available for interests of adequate yet concise coverage, the applying organic coatings depends on a variety objective in the sections which follow is to pre- of factors, including the nature of the coating sent the essential details of each method and material, the type and texture of the surface on to provide a sound basis for the selection of an which it is to be applied, the shape and size of appropriate application method. Toward this the object or area to be coated, environmental end, there is given for each method a descrip- considerations, the available eauipment and fa- tion of the principle involved, the basic tech- cilities, the skills available, fire and toxicity niques and tools or equipment required, the hazards, the time factor, and—last but not main advantages afforded, and the chief limi- least—labor costs. tations. Obviously, the choice of method also depends In general, application in the field is limited upon an awareness of what methods are avail- to brushing, rolling, and conventional cold or able and upon a knowledge of the technique hot spraying methods, while factory coating involved and the advantages afforded by each. operations are generally accomplished with Detailed information about the commonly used more elaborate types of spraying (electro- methods of brushing, spraying, dipping, and static, airless, etc.) or by special techniques spreading is readily available in many pub- such as fluidized bed. lished books, manuals, pamphlets, and specifications [81, 96, 122, 1541. Similarly, detailed 5.4.1. Brush Application instructions for use of the newer or less well- known methods, such as airless spray and fluid- Applying paint by brush is an ancient tech- ized bed, are provided by the manufacturers or nique that still is used for a substantial portion

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of the painting done in the field. While con- should be brushed out thoroughly. Quick-dry- siderably slower than spraying, it gives excel- ing paints should be brushed only enough to lent results when done with skill using the spread them evenly. Very quick-setting coat- proper brush and technique. Brushing works ings such as lacquers must be applied quickly the paint into pores and crevices in a way that in only two or three strokes to avoid disturbing spraying does not accomplish. At the same the gelled film—for this reason, lacquers are time, the brushing works surface dust and usually applied by spraying or dipping rather foreign matter into the paint, in effect clean- than by brushing. Near the end of each stroke, ing away surface contaminants and thereby the brush should be gradually lifted to prevent promoting good adhesion. For this reason, roughness and brush marks. The paint should brushing is generally the preferred method of be stirred frequently during use to avoid set- applying primers, especially on rough or im- tling. Since brushing "works" the paint, it is perfectly cleaned surfaces. essential—to avoid lifting—that an applied A high-quality brush of the proper size and coat be thoroughly dry before brushing on shape is essential for good work. In the long another coat. run, the best brush—though initially expensive—is the most economical. The best brushes 5.4.2. Roller Application of today are made of specially selected Chinese hog bristles, sorted and bundled by skilled craftsmen to produce a quality tool. The bristles The roller applicator is a fast and convenient of such brushes have split or flag ends which (and therefore economical) means for apply- are indispensable for holding an adequate quan- ing various types of pigmented coatings to tity of paint and distributing it smoothly over large, flat surfaces that are reasonably free of the surface; the bristles continue to split as interference from architectural protrusions they wear down, thus maintaining their effi- and mounted fixtures. It gives excellent results ciency. Good nylon brushes with bristles hav- on porous or textured surfaces such as brick, ing artificially flagged ends are also available; cinder block, and plaster. And it is one of the while they do not have the lasting paint-holding best methods available for painting wire fence and brushing quality of the best bristle brushes, and grillwork. While it does not lend itself to they will outwear bristle about four-to-one and the fine workmanship that can be achieved with are particularly useful for painting rough sur- skilled brush application, the roller is an excel- faces such as brick and cinder block. Nylon lent and proven tool which is capable of good brushes are useful also with water-thinned results in the hands of either the skilled painter paints because they are unaffected by water or the do-it-yourself-er. The roller applicator which tends to make bristle brushes soggy and may be used with oil paints, alkyd enamels, less latex paints, and workable ; however, they are not suitable for various other synthetic resin- applying lacquers because the synthetic bristles base coatings that do not set up too quickly. may be attacked by the strong solvents used Quick-setting materials such as lacquers cannot in these formulations. be applied by roller. A high-quality paint brush is an expensive The usual roller consists of a metal cylinder tool that deserves proper care to maintain its or cylindical frame that rotates on an axle usefulness. A new brush should be broken in supported at the ends by a frame to which a before use by shaking and combing it free of handle of any convenient length is attached. loose hairs and dust, soaking it in linseed oil The cylinder is covered with a replaceable for a day or two, and finally rinsing it in paint "sweater" of a suitable napped material thinner. The brush should be suspended so that usually wool, mohair, nylon, or acrylic fiber. the bristles do not rest on the bottom of the Covers for virtually any type of job are avail- container during soaking. When painting, the able. For use with water-base paints, a type brush should not be used sideways or forced of roller cover is available in which the napped into corners or tight places; such abuse will material is bonded to a stiff hollow core of a ruin its shape and destroy its efficiency. suitable plastic or a resin-impregnated card- Brushes are available in a wide variety of sizes board which can be easily slipped on and off and shapes and textures for almost any paint the roller frame. The length and texture of the job. After each use, brushes should be cleaned nap or pile are of prime importance in de- thoroughly with a suitable thinner and stored termining the application properties of the in a clean place ; if stored in oil or thinner be- tool. Rough surfaces require long-nap roller tween usage, the brush should be suspended as covers. The longer the nap, the more the mentioned above (also see section 5.1). amount of paint that can be held, and' the Brush application of paint is a skill that must greater the stippling or "orange peel" effect. be acquired by practice and experience ; never- Wool covers hold more paint than other kinds theless, certain basic principles should be fol- and produce an attractive stipple that makes lowed. The paint should be brushed on in one them preferred for most interior painting of direction, then cross-brushed. Oil-base paints walls and ceilings with oil- or alkyd-base

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paints. Lamb's wool covers with naps of brushing. When skillfully done, it produces various lengths are used for both smooth and a coating of uniform thickness and appearance rough surfaces—a %-inch nap gives good re- unobtainable by brushing. Since the paint is sults on light stucco or rough plaster. Mohair merely laid on without being worked into the covers are used where a particularly smooth substrate, it is essential that the surface be finish is desired, as in the application of gloss clean to obtain good adhesion. or satin enamel. Covers of nylon and acrylic Successful spray painting is dependent on pile are useful with latex paints because they skill developed through knowledge and experi- do not mat or wilt when water-soaked; they ence. Although a variety of different spray also wear well when used on rough-textured methods are available, all require clean paint, surfaces such as masonry. Care should be clean equipment, and a basically similar move- taken not to use synthetic resin covers in strong ment of the gun. The gun should be held per- solvents that may attack them. Polyester pendicular to the surface at all times, at a resins can be efficiently applied with roller distance of six to ten inches, and should be covers of wool frieze (having a shaggy pile). moved in a straight line at uniform speed, with Types of rollers are available which carry proper "triggering" at the beginning and end a supply of paint inside the cylinder, the paint of each stroke. Proper triggering consists of feeding through the napped cover as it is used starting the stroke before the trigger is pulled however, it has been difficult to achieve a uni- and releasing the trigger just before the end form flow of paint by this method, and it has of the stroke to prevent the build-up of ex- not found wide favor. Pressure feed also has cessive amounts of paint at the beginning and been used—with the paint fed from source to end of each pass. Care should be exercised flexible roller on a continuous basis through not to tilt the gun or move it through an hose; this method also has found only limited arc ; both these faults will result in a distorted application, mainly for coating very large spray pattern and a non-uniform coating. areas. Arcing of the gun can be avoided by proper In the conventional or dipping method of wrist action and body motion. Correct lapping roller application, the coating material is placed is very important in achieving a uniform thick- in a tray having a sloping bottom, and the ness and appearance. The lapping should be roller is dipped into the tray and rolled back done in such manner that the center of the and forth until it is loaded with the paint. spray strikes the bottom of the previous pass. Excess paint is rolled out on the grid of the Corners should be sprayed by directing the dip tray, and the paint is then applied to a gun toward the corner at a 45-degree angle to strip of surface by rolling back and forth each side so that both sides of the corner are slotvly until complete coverage is obtained. equally coated at the same time as the gun is Rolling should not be continued too long, lest moved along the corner (on vertical corners, be ruined. During roller the paint set up and the gun must be turned on its side—the idea application, adjacent strips should be lapped as is to utilize the long dimension of the elliptical in other methods of painting. Since the roller spray pattern for accomplishing the double- will not fit into corners, the edges of walls and edged coating operation) ceilings must be painted ("cut in") by brush The chief disadvantages of conventional before beginning the roller application. The spraying methods are those associated with roller should follow the brushing promptly to the overspray caused by air and paint rebound avoid lap marks. from the surface being painted. The overspray To keep rollers and covers in good condition, wastes paint, necessitates the careful masking it is essential that they be thoroughly cleaned of adjacent areas, may contaminate surround- after each use by squeezing out excess material ing objects (at a considerable distance when and washing in a suitable solvent. Latex wind-borne), and may build up dangerous con- paints can be washed out with soap and water, centrations of toxic or fiammable vapors and with thorough rinsing. Roller covers should mists (see section 5.2 for Safety Precautions). be dried on their rollers to avoid shrinkage. To minimize these drawbacks, new methods Wool covers may be combed while damp to such as electrostatic and airless spraying have minimize matting. been developed and are coming into widespread use, particularly for factory and industrial 5.4.3. Spray Application painting operations. Both conventional and newer methods of More paint is applied by spraying than by application are described in the sub- any other method because of its general appli- spray follow. cability to most painting situations, and be- sections which cause of its speed and resultant savings in time and labor costs. Spraying is particularly 5.4.3.1. Conventional cold spray is the convenient for overhead work, for large areas, application method most widely used in the and for reaching into areas inaccessible to field. While possessing the disadvantages

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associated with overspray (see above), the 5.4.3.2. Airless spray is one of the recent method is fast and economical compared to developments in methods of spray application. brushing, especially on large jobs or rough Hydraulic pressure rather than air is used to surfaces. The equipment required is relatively accomplish the atomization of the coating ma- simple and inexpensive compared to that of terial, in much the same way that a water more elaborate methods. A typical system con- spray is produced at the nozzle of a garden sists of an air compressor capable of providing hose. High pressures—around 2500 lb/in- up to about 90 lb/ in- at the gun, an air trans- are required to force liquids of paint viscosity former to regulate the pressure to the gun through the very small, precision orifice em- while filtering out dust and dirt and trapping ployed. Nozzles, valves, hoses, and other parts out oil and water, flexible hosing to carry air must be designed to withstand high pressure and paint to the spray gun and its attached and erosion; hoses are usually lined with reservoir, and the spray gun itself. The latter Teflon and wrapped with wire; nozzles are is provided with an on-off trigger and with often made of tungsten carbide. A spray pat- means for .adjusting the flow of air and fluid tern similar to that obtained in air atomization to the spray head to obtain a desirable flow is produced by a precisely milled slot on the rate, proper atomization, and the desired spray face of the orifice. The high pressure required pattern. The spray head is a removable as- is usually generated by an air-powered fluid sembly that includes a fluid-controlling needle, pump having a high fluid-to-air pressure a hollow fluid tip, and an air cap. The fluid ratio that permits hydraulic pressures of 2500 tip and air cap are chosen to suit the vis- lb/in- to be obtained with only about 100 cosity and solvent balance of the coating to be lb/ in- of air pressure at the input. The com- sprayed; it is essential always to use the type pressed air serves only as a safe and convenient of fluid tip and air cap specifically recom- source of power for driving the fluid pump mended for the material to be sprayed. motor; air does not contact or become incor- Before spraying, the coating material should porated into the coating material. be thinned in accordance with the manufac- The coating material is carried to the gun turer's instructions, and the air and fluid by a single hose; the flow is controlled by an pressures at the gun should be set and bal- ordinary high pressure valve. The gun itself anced to give an adequate flow of material with does not have a trigger and has no means for proper atomization and a normal spray pat- varying the spray pattern; however, a variety tern. If the ratio of air pressure to fluid of spray caps for use with materials of dif- pressure is too low, the spray pattern may be ferent viscosities are available. Once the air too heavy at the center or splotchy due to in- pressure to the fluid pump has been adjusted, sufl^cient atomization. Too high an air-to-fluid the flow of coating material to the gun is con- pressure ratio results in over-atomization with trolled only by the valve, which must be turned the production of excessive fog and mist during on and off at the beginning and end of what is the spraying operation. ' Misting can also be essentially a continuous spraying process. Al- caused by holding the gun too far from the though the problem of balancing air and fluid surface being sprayed, with the result that the pressures is not present, the lack of fine con- paint particles become too dry enroute and trol over the spraying process reportedly re- are blown off the surface as a powder instead sults in a rougher film than that obtainable of adhering to it and coalescing. The thinner by conventional spray. Airless spraying is the liquid, the lower are the air and fluid therefore better suited for applications where pressures required for spraying; however, a protection rather than appearance is the para- it ; material should never be thinned more than is mount concern, and is most feasible where recommended merely to make spraying easier relatively large surfaces and objects or a con- I excessive thinning can produce its own quota tinuous production line is involved. of serious film faults, such as runs, pinholing, Both portable and fixed types of airless spray cratering, and blushing. Overspray can be re- equipment are available. Also, airless spray duced by using the lowest atomization pressure may be combined with hot spray techniques that will do the job satisfactorily. As a gen- (see section 5.4.3.3) to permit spraying at eral guide, the common paints and enamels lower pressures through a triggered gun. With are usually satisfactorily air hot airless spray, the paint must either be re- I sprayed at pressures of 60 to 70 lbs/ in- and fluid pressures of circulated between gun and reservoir or the

I all j 15 to 20 lb/in-, while lacquers and other low- feed hose must be hot-water jacketed the

I solids liquids may be sprayed at air pressures way to the gun in order to maintain the re-

I of 40 to 50 lb/in- and fluid pressures of 10 to quired temperature and viscosity. (At least 15 lb/in2. one manufacturer of hot airless spray equip- I The techniques to be used in spray applica- ment identifies his process by the term "airless tion have been discussed in the introductory spray" alone.) portion of section 5.4.3. The chief advantage of airless spray over

99 conventional spray lies in the great reduction instantly liquefied as the resin particles, sus- in overspray achieved by elimination of the pended and propelled by compressed air, atomizing air. Since most of the sprayed paint emerge from the flame-ringed nozzle of a spe- stays on the surface to which it is directed, cially constructed gun. Upon striking the sur- better coverage is obtained, paint vâste is mini- face, the liquefied resin particles immediately mized, fog and mist are virtually eliminated, "freeze" or congeal into a continuous coating. and the hazards of toxicity and fire are con- Although the flame-spraying technique has siderably reduced. With less solvent lost be- been used chiefly for applying polyethylene twêen the spray gun and the surface, the coat- coatings to metal surfaces, the technique is ap- ing reaches the surface in a more fluid state, plicable to other thermoplastic resins (e.g., flu- and most coating materials can be sprayed at orocarbons and polysulfides) which are chemi- package viscosity vîthout the time and expense cally stable at their melting point for the brief of thinning. Highly viscous materials may be second during which the resin is in the liquid difficult to atomize by airless spray. Also, state. Metal surfaces permit the option of coatings containing particles too large to pass heating the surface also, when practicable, to through the orifice of the gun (e.g., fiber- achieve better coalescence and a smoother film. containing paints and most multicolor lacquers) Coatings applied by flame-spraying to clean, cannot be sprayed by the airless method. How- properly prepared surfaces adhere quite well ever, with most materials, a high rate of ap- and provide a means for utilizing as coatings plication and high film build per coat are a number of highly chemical-resistant resins possible with airless spray. which cannot be conveniently applied by solu- Because of the operational convenience, speed, tion or dispersion techniques (the latter re- and economy of airless spray, it is finding wide quires fusing, which may not be feasible in use in industrial finishing and maintenance some applications). painting. 5.4.3.5. Electrostatic spray utilizes the at- 5.4.3.3. Hot spray, in which the coating traction between electrically charged materials material to be sprayed is first heated to a of opposite polarity as a basis for the rapid temperature in the vicinity of 150-175 °F, application of uniform coatings, even on com- (66-79 °C), is applicable to both airless and plex shapes, with a minimum of overspray. conventional spraying techniques. The paint The coating particles are charged to a high is safely and rapidly heated by passage through negative potential (up to 100,000 volts) as they a hot water heat exchanger supplied from a contact or pass through a suitably shaped elec- the directly heated boiler and is carried to trode. The object or surface to be coated is gun by a heated hose. Heating reduces the grounded, so that it is effectively at a high viscosity of the material, lessens the amount of positive potential with respect to the negatively thinner required, and permits spraying at a charged coating particles. The atomized par- higher solids content. The result is heavier ticles are therefore drawn swiftly to the sur- film build per coat, with attendant economy face and captured, with very little overshoot of time and labor. Hot spray is accomplished even on objects with an open structure. The at lower pressures than conventional spray, method requires that the coating material be with a considerable reduction in the amount essentially nonconductive so that its charge will of overspray and its accompanying evils. The not be dissipated by current flow between higher spraying temperature and lesser amount particles, and that the surface to be coated be of thinner to be evaporated virtually eliminate electrically conductive enough to remain at the tendency toward blushing, even under ad- ground potential so that its attraction for the verse humidity conditions, so that year-round negatively charged particles can be maintained. spraying is possible. Several different methods of electrostatic Coatings for hot spray application must be spraying are available. While all utilize the specially formulated for proper solvent bal- basic principle of electrostatic attraction by ance and viscosity; otherwise, the coating may giving the coating particles a high negative suffer from poor coalescence, dusting, pinhol- charge while maintaining the surface at ground ing, and other defects. With proper formula- potential, they differ in the manner in which tion sol- (e.g., elimination of fast-evaporating the coating material is atomized and directed. vents), hot spray coatings are less prone than Also, the original use of electrostatic spray conventionally sprayed coatings to such defects for applying liquid coating materials has re- as running, sagging, orange peel surface, etc. cently been supplemented by an electrostatic method for applying dry powder coatings. 5.4.3.4. Flame spraying is a technique that permits thermoplastic materials that are insoluble in organic solvents to be applied to 5.4.3.5.1. Liquid electrostatic spray may be metal and other surfaces. The coating ma- of the air-atomized type or the airless type. A terial in the form of a powder is almost conventional spray gun is used in the air-

100 atomized type but is operated at the lowest charged particles leaving the nozzle are at- pressure (about 12 Ib/in^) that will atomize tracted to any electrically grounded object in the paint and start it from the gun. The atom- their general path and will adhere to the surized coating material is sprayed into a nega- face long enough to permit a heat cure that tively charged field where the particles too fuses the particles into a continuous film. Uni- become negatively charged and are drawn to form coatings up to 20-mils thick can be ap- the grounded (effectively positive) object to plied in a single pass of the spray gun, even be coated. The conventional spray gun permits on complex shapes. Objects with open struc- triggered control of the spraying process. tures, such as wire mesh and fan guards, can be coated on both sides at the same time by 5.4.3.5.2. Airless electrostatic spray uses ei- using an ungrounded surface on the far side ther centrifugal methods or hydraulic pressure as an electrostatic reflector to repel particles to atomize the coating material, and the ma- back to the object. An application rate equiva- terial is applied in a continuous rather than a lent to laying down a three-mil-thick coating triggered operation. In a typical centrifugal at the rate of one square foot per second is method, the paint is pumped from a small hole possible. in the center of a negatively charged spinning Since the coating is all solids, the fire and disc that imparts a similar charge to the paint. toxicity hazards and the expense of volatile sol- The paint becomes atomized as it is thrown vents are completely eliminated. Spray booths from the edges of the fast spinning disc. The are not needed, and the method is well suited charged particles are then drawn to the for either field or factory application. Despite grounded objects which are positioned in a the high voltage employed, the method is com- circle around the spinning disc. The objects pletely safe because of the very low current may be slowly rotated while the disc is moved involved (only 0.2 milliampere) . Any type of up and down to provide complete coverage on powder that is essentially nonconducting can be all sides. In another centrifugal method, the sprayed onto any surface that is at least mildly paint is fed into a negatively charged spinning electrically conductive. Thermoplastic (heat- bell that gives the paint a negative charge and fusible) materials such as polyvinyl chloride, directs the atomized particles driven from its cellulose derivatives, fluorocarbons, polyethyl- rim toward the grounded object to be coated. ene, and polypropylene are readily applied by

The centrifugal methods are useful chiefly for this method ; however, the method is applicable industrial finishing, where the necessary spray to thermosetting resins such as epoxies only if equipment and automatic positioning and con- a curing agent inactive at room temperature veyor facilities can be set up on a permanent but effective when heated can be incorporated basis. into the material. Inorganic fusible powders The most widely applicable method of air- such as talc, vitreous frit, and metallic oxides less electrostatic spray combines the normal also can be applied by dry electrostatic spray. airless spray technique discussed in section Surfaces that have been successfully coated in- 5.4.3.2 with the electrostatic principle described clude metals, glass, heated plastics and ceram- in this section. Here, the negatively charged ics, and moist paper and cloth. coating material is atomized by hydraulic pres- It may be noted that the dry electrostatic sure as it leaves the orifice of the airless spray spray process is most useful with many of the gun and the charged particles are drawn to same resins employed in the fluidized bed tech- the grounded (positive) surface to be coated nique (see section 5.4.6). However, large ob- by electrostatic attraction as before. This jects (e.g., automobile bodies) are more easily method is adaptable for field application as coated by electrostatic spraying, and thin coat- well as for industrial finishing. ings can be applied with a greater degree of All wet electrostatic spray methods afford uniformity. similar advantages of minimum overspray with a great reduction in the hazards, waste, and 5.4.3.6. Dual-gun spraying provides a prac- inconvenience normally associated with over- tical means for spraying two-component coat- spray, together with substantial savings in ing systems that must be activated at the time time, material, and labor costs compared to of use and have only a limited pot life after conventional methods of application. activation. Epoxies, polyesters, and polyurethanes are typical of the coating materials that 5.4.3.5.3. Dry electrostatic spray is a com- are being successfully applied by the dual-gun paratively recent development [741 in which technique. Such materials cannot be applied the coating material is sprayed in the form of by conventional spray gun because their con- a dry powder that goes on as a one-hundred- tinually changing viscosity makes them difficult percent-solids coating. The powder is charged to control, and the gun would be capable of to a high negative potential (about 90,000 only limited operation before it would have to volts) and then fed at very low air pressure be thoroughly cleaned to avoid permanent clog- (about 1 lb/in-) through a spray nozzle. The ging with hardened resin. Specially designed

101 dual spray guns are now available [63] that certain point), and the less uniform the coat- are capable of continuously metering and mixing tends to be. In general, best results are ing the two components (resin and catalyst) in obtained by withdrawing the object at a slow, the correct proportion at the instant of spray- uniform speed that permits most of the drain- ing. Resin and catalyst are fed to the spray age to take place as the withdrawal proceeds. gun through separate hoses, one or both of However, when a second coat is to be applied which may be heated if necessary to reduce the on a material that is quickly attacked by the viscosity of the material. Built-in metering solvents in the formulation (as happens with devices of almost any needed precision deliver many lacquers) , it may be ncessary to perform the two materials to a chamber in the spray the dipping quickly to avoid redissolving some head, where they are intimately mixed in the or all of the previous coat. exact proportions required for proper cure and The dip tank should be no larger than is then sprayed from the orifice under sufficient necessary to accommodate the object and should hydraulic pressure to produce atomization (cf., have a minimum cross-sectional area to mini- airless spray, section 5.4.3.2). mize solvent evaporation and coating deterioration. An agitator should be provided for 5.4.4. Dipping and Flow Coating the continual stirring of pigmented materials. Clear coatings should be stirred occasionally 5.4.4.1. Dipping is a fast and economical and whenever the material in the tank is re- method of industrial finishing—one that lends plenished. The tank should be kept covered itself to a continuous conveyorized operation when not in use to reduce the fire hazard. from initial cleaning of the surface to final curing of the coating. Despite the seeming 5.4.4.2. Flow coating is a process in which simplicity and ease of dip coating, it is actually the coating material is flowed onto the object a difficult process because of the many variables from a hose, after which the excess material that must be closely controlled to obtain a satis- is allowed to drain off into a shallow tank or factory finish. Because of these difficulties, drip pan and recovered for reuse. Flow coat- combined with the need for a relatively large ing is very similar to dipping in its require- volume of material in the dip tank, dipping is ments for close control of the coating composi- not usually a convenient or economical way of tion, viscosity, drying rate, and drainage coating objects in the field, especially on an characteristics—but it has the advantage of intermittent or sporadic basis. not requiring a large dip tank. The coating ma- Of basic importance in determining the thick- terial is supplied from a central reservoir ness and uniformity of the coating are the which can feed several hoses at a time if decomposition and viscosity of the coating ma- sired, while permitting convenient sampling terial, the rate of withdrawal of the object, and control of the coating composition at its and the effectiveness of the drainage in pre- source. venting drips and fat edges in visible areas. In both the dipping and the flow coating Control of viscosity requires control of both processes, the manner in which the object is temperature and solvent evaporation in the dip hung (particularly if it is of complex shape) tank. Loss of solvent does not merely raise is of the utmost importance in achieving a the viscosity ; through the differential evapora- uniform coating without unsightly drip beads tion of lower-boiling constituents, the solvent or fat edges. balance also is changed—and along with it the drying and drainage behavior of the coating. 5.4.5. Knife and Roller Coating Adding more of the original thinner can restore the proper viscosity but does not restore the original solvent balance. In industrial Knife coating and roller coating are high- finishing, a practical remedy for the evapora- speed industrial coating methods for applying tion problem is to maintain a nearly solvent- a wide variety of coating materials to flat sur- saturated atmosphere in the dipping room; faces on a continuous production line basis. however, this greatly increases the risk of fire Both are essentially spreading methods. and requires every possible precaution against sparks and flame. Water-thinned coatings are 5.4.5.1. Knife coating is similar in prin- free from the problems of differential evapora- ciple to the doctor-blade technique used in the tion and fire hazard but may introduce cover- laboratory for the application of uniform films. age and drainage problems of their own. It can be used for coating rigid, flat sheet ma- The shape and surface texture (e.g., porosity) terial or for materials that can be held flat on of the object and the rate of its withdrawal a rigid substrate while being coated. Only one from the dip tank have a direct effect on drain- side can be coated at a time. The coating ma- age and on the thickness and uniformity of terial is flowed on and spread evenly over the the coating. The faster the object is with- surface as the latter is drawn beneath the drawn, the greater is the thickness (up to a knife or doctor-blade. The coating thickness

102 .

will vary with nonuniformity in the thickness occur with irregularly shaped particles. Both of the sheet stock. powder and gas must be clean and dry to ensure free mobility of the particles. Their 5.4.5.2. Roller coating utilizes hard rubber mobility is enhanced by a small electrostatic or steel rolls to apply the coating to flexible, charge which keeps the particles apart. The flat sheet stock of metal, cloth, paper, and thin fluidized powder occupies three to four times composition board. The coating material is the volume of the original powder. A prop- applied and spread evenly over the surface of erly fluidized bed looks and behaves like a liquid the material by a roller rotating in a direction and offers negligible resistance to the movement material being opposite to that of the strip of of an object immersed in it. coated, while the strip is held against the coat- The article to be coated is preheated to a ing roller by tension furnished by the driving temperature above the melting point of the rolls. The coating roller receives its paint from resin and then dipped into the fluidized bed, a feed roll that dips into a paint reservoir as where it acquires a uniform coating of the it offsets onto the rotates and then the paint material as the particles melt onto it. (Ir- coating roller. Coating speeds of fifty feet per regularly shaped or undercut objects should be minute are readily achieved. If desired, both moved through the bed to obtain complete sides of the strip can be coated at the same coverage.) Upon withdrawal, the coating con- time. The method is not applicable to thick geals into a more or less continuous film which stock that will not bend over the rolls. may be further annealed in an oven to ensure good leveling and a pinhole-free coating and 5.4.6. Fluidized Bed Coating to complete the curing of a thermosetting resin. The coating thickness obtained depends on the Fluidized bed coating is one of the recent preheat temperature, the shape of the object, innovations in the coatings field. It is a rapid, convenient, and economical method for apply- and the duration of the dip. Thin-walled sections present diflflculties because of ing solventless (100 percent solids) coatings on may some quick cooling in transit objects of metal, glass, ceramic, plastics, and while from preheat to other materials capable of withstanding the dip. Since the object must be suspended while being heating and fluxing temperatures involved in dipped into the fluidized bed, an uncoated area is left at the point of suspension which the process. It also makes possible the utilization of insoluble resins that cannot be readily can be touched up by filling it with powder and applied as a coating by ordinary methods. The then fusing it with a blast from a hot-air gun. While the fluidized bed method is most widely technique is particularly suitable for coating applicable are small objects and for applying thick, uniform to thermoplastic resins which coatings even on complex or undercut shapes. readily fused into a continuous film, it can also be used with thermosetting resins that can be The method is applicable to both clear and pigmented coatings. liquefied by heat prior to being finally cured after-heating; resins are notable In the fluidized bed process, the coating ma- by epoxy a example. Thermoplastic resins which have terial in the form of a fine (50-150 mesh) dry been successfully applied by the fluidized bed powder is "fluidized" by suspending the coat- technique include virtually insoluble resins such ing particles in a controlled upward stream of as polyethylene, polypropylene, nylon, the gas (usually compressed air or nitrogen) and fluorocarbons, as well as soluble resins such This is accomplished in a vessel that is divided into an upper and lower chamber by a porous as polystyrene, vinyls, acrylics, and cellulosics. membrane that retains the powder while per- When using the fluidized bed method, it mitting passage of the gas. The coating par- should be borne in mind that the powdered ticles must be spherical in shape to present a material presents health and safety hazards uniform surface resistance in the gas stream unless adequate facilities are available for and to avoid the agglomeration that tends to hooding, exhaust, and dust collection.

103 Figure 5.1. Roller-coated sheet metal stock ("coil coating") passes through final inspection station to recoiling equipment. (Photo courtesy of Industrial Finishing.)

Figure 5.2. Electrostatic spray gun paints chain link fence from one side with veTij little overspray. (Photo courtesy of Ransburg Elec- tro-Coating Corporation.)

104 :

6. Surface Preparation and Pretreatment

The common denominator in all surface prep- is to receive a wash primer pretreatment; and aration is thorough cleaning of the surface. new wood siding that is to receive an oil paint To the clean surface, pretreatments may be may need no cleaning at all (although knot- applied to enhance adhesion and (in the case sealing and puttying of nail-holes may be of metals) to provide a barrier against the necessary) . The important point is that what- spread of corrosion. Whereas the purpose of ever method of surface preparation is selected the basic surface preparation is to achieve a must be suited to the job and thoroughly per- physically clean surface, the pretreatment is formed skimping in either a ; the selection or the chemical process, the functions of v^hich are execution of a method of surface preparation (1) to change the original surface to one that is an invitation to trouble. possesses a greater surface area and/ or a A variety of chemical pretreatments, many greater specific affinity for the primer to be of them of a proprietary nature, are available applied, and (2) in the case of metals, to passi- for use on specified types of surfaces. Pre- vate the surface and retard corrosion. treatments for metal find the widest use; they The choice of a practical method of surface are based on various chemical agents, including preparation depends on the nature and condi- phosphating, chromating, and oxidizing agents, tion of the surface, its size and shape, the formulated for maximum effectiveness on the equipment available, the type of coating to be particular metal to be treated. Pretreat- ments in acidic applied, and the service environment. Methods —usually the form of agents or resinous surface conditioners are also useful range from light brushing or a simple solvent — on concrete and masonry surfaces. Plastics, wash to heavy sandblasting. Dirty or corroded particularly those with low energy (non-adher- surfaces, coatings of limited v\^etting ability, ent) surfaces such as polyethylene and the and severe service environments require vigor- fluorocarbons, may be treated with strong acid, ous, thorough cleaning methods; for example, caustic,' or oxidizing agents to etch the surface rusty steel that is to receive a phenolic coating for better coating adhesion. system to withstand marine exposure must be In the sections that follow, the basic impor- thoroughly cleaned by sandblasting before the tance of proper surface preparation is stressed, coating is applied if good service performance and specific information on the preparation and is to be achieved. On the other hand, a solvent pretreatment of various kinds of surfaces is wash may suffice for an aluminum surface that presented.

6.1. Importance of Surface Preparation and Pretreatment

The importance of proper surface prepara- the workmanship of the professional painter tion to the durability of any coating system or responsible coatings official. cannot be overemphasized. Without proper A coating can provide its function only so surface preparation, the finest paint—applied long as it remains intact and firmly bonded with the greatest skill—will fall short of its to the surface it is to protect or decorate. Good maximum performance capabilities or may even adhesion requires that the coating material be fail miserably. Considering the proportion- brought into intimate contact with the surface ately high cost of the labor involved in applying itself (or a firmly bonded chemical pretreat- protective coatings as compared to the cost of ment layer) , without intervening corrosion materials, it would appear to be axiomatic that products or contaminants, to permit the inter- only the most durable materials applied in a action of molcular forces of attraction directly manner that achieves maximum service should between the coating and the substrate material. ever be used. Yet there are some who, while Thus, a properly prepared surface is essentially recognizing the value of high-quality materials a clean surface, but not necessarily the ultimate and workmanship, fail to grasp the funda- in cleanness. It is frequently impractical, for mental importance of surface preparation as economic reasons or because of equipment lim- the foundation upon which all else depends. itations, to achieve a perfectly clean surface; There seems to exist in some cases a sub- nor is this usually necessary. However, ade- conscious willingness to take a chance when it quate surface preparation can and must be comes to the hard work of surface preparation. achieved in all cases if the coating is to adhere But while an attitude of "the old surface isn't firmly and durably. An adequately prepared going to show, so why should I bother with it" surface not only provides a good anchor for may be understandable in the unversed do-it- the coating but also ensures a surface free of yourselfer, such misconception has no place in corrosion products or contaminants which

105 — might shorten the life of the film by spreading film that is formed on the metal surface. Not along the coating-substrate interface and de- only does such a film provide an excellent base stroying adhesion, or by actually breaking for coatings adhesion, but it also resists the through the coating. spread of corrosion beneath it, so that damage The value of chemical pretreatment, par- to the substrate from a break in the film is ticularly for metals, lies in the thin, tightly not as likely to progress beyond the area of adherent, ideally roughened, paint-receptive immediate damage.

6.2. Preparation of Wood Surfaces

6.2.1. Bare Wood good knot sealer should be brushed on, such as the WP-578 knot sealer developed by the 6.2.1.1. General. — Wood surfaces to be Western Pine Association and covered by Mili- painted should be clean, free of cracks and tary Specification MIL-S-12935 [149]. This splinters, and properly seasoned (moisture con- knot sealer consists of a ten-to-one blend of tent below 15 percent for exterior wood, and a phenolic resin and polyvinyl butyral in de- less than 10 percent for interior wood). Ex- natured alcohol solution. It has proven more cessive moisture in the wood is accompanied by effective than shellac or aluminum paint for large dimensional changes as it dries, which this purpose. may cause cracking of the paint film; also, the The puttying of nailholes and cracks is an entrapped moisture may cause blistering of the important part of the preparation of exterior film, with early flaking or peeling. Green wood wood surfaces for painting; however, it should should be allowed to cure for at least a week be done after the priming coat has been ap- in clear, warm weather before painting is at- plied and dried, as indicated in section 6.2.1.1, tempted. Seasoned wood that has become wet The putty should be allowed to dry thoroughly should be allowed to dry until it is dry to the before the finish coats are applied, touch before painting. Cracks and nailholes should be filled with 6.2.1.3, Interior wood should be well sea- putty or plastic wood. Puttying must be pre- soned, clean, and dry when painted. It is ceded by application of a primer to prevent essential that the surface be sanded to a smooth loss of oil from the putty by absorption into texture before painting to obtain a good fin- the wood. The putty should be allowed to dry ished appearance. The type and amount of for at least 48 hours in the case of shallow sanding required depends on the initial rough- holes or cracks, and for a period of from one to ness of the surface. To avoid unnecessary three weeks for deep fillings. Plastic wood scratching or gouging of the wood with a re- although more expensive and more difficult to sultant increase in the amount of labor ulti- handle than putty—may be used directly on the mately required to achieve a smooth surface, bare wood and hardens in a few hours. When sandpaper of the minimum coarseness that will dry, the filled areas should be lightly sanded do the job should be used. Where an initially before painting. rough surface is involved, sanding should start Loose dust, dirt, and sandings should be with a more or less coarse grade of sandpaper brushed away. Scraping or wire brushing may and proceed through intermediate grades to a be used to dislodge mortar scale, pitch streaks, fine grade for finishing. The sanding should and other tightly adherent foreign substances. be done in the direction of the wood grain. Grease and oil can be removed by washing with Sanding of woodwork and cabinet work may mineral spirits or white gasoline (caution). be accomplished by starting with a medium Shop markings may be removed by sanding or grade sandpaper (No. 1) and finishing with a solvent wash. Smoothing may be done fine sandpaper (No. 0 or finer). On furniture, with sandpaper, steel wool, or power sanding or where the finest finish is desired, the final machines. sanding should be done with a very fine grade of sandpaper (No. 4/0). 6.2.1.2. Exterior wood should be properly Fine steel wool may be used instead of sand- seasoned, clean, and dry before painting. Al- paper and is particularly useful on rounded or though a smooth surface is not necessary, the irregularly shaped surfaces. wood should be brushed, scraped, washed, or Floors are best sanded by machine, working sanded as may be required to produce a surface in the direction of the wood grain. For the free of dust, dirt, oil, grime, sap streaks, first rough sanding, coarse sandpaper is used splinters, projections, and very rough areas. (No. 21/2 for hardwood and No. 3 for softwood Knots require special treatment to minimize floors), followed by No. li/o sandpaper, and the possibility of discoloration, cracking, and finishing with a fine grade (No. 0) for hard- premature failure of the paint applied over wood and a medium grade (No. 1) for soft- them. As much sap as possible should be re- wood (fine sandpaper becomes gummed by soft moved from the knot by scraping and by wash- woods under power sanding conditions). ing with turpentine. When the area is dry a Wherever nails are used, they should be

106 countersunk and the nailholes filled with plastic excessive or if the old paint is soiled, washing wood or putty (the surface must be primed or with detergent followed by a thorough hosing sealed before puttying, as indicated in sec. down with water may suffice. In some in- difficult stances, a final light sanding of the old paint 6.2.1.1) . Plastic wood—although more to handle than putty— is often preferred for may be desirable. Small deteriorated areas its more natural appearance and quick-drying may be scraped, wire-brushed or sanded down properties. As previously noted, putty may re- to bare wood and spot-primed before over-all quire days or weeks of drying before it can be repainting. In such cases, the boundary of painted satisfactorily. When thoroughly dry, intact paint should be sanded to a feather edge. the fillings should be sanded smooth before If deterioration of the paint has progressed painting. to the point where flaking and peeling, or wide- Where stain is to be used, the stain is ap- spread cracking and edge curling, have taken plied directly to the bare wood surface. The place, then the old paint will have to be entirely stain, when dry, is followed by a wash coat removed. On exterior surfaces, this is prob- (frequently one pint of shellac varnish thinned ably best accomplished by burning off the paint, with four to seven parts of denatured alcohol) with due precautions against the hazards of and given a further fine sanding. At this point, fire and toxic fumes (see sec. 5.2). The burn- open-grain woods, such as oak and walnut, are ing off should be done by an experienced given a coat of paste wood filler thinned to worker. The paint is heated sufficiently to brushing consistency. The filler is allowed to soften it and cause it to blister so that it can set for about twenty minutes, and the excess be removed with a scraper. Care must be is then removed by wiping across the grain. taken not to char the wood surface. Finally, After the wood filler has dried, any remaining the wood will require sanding to complete its excess is sanded away, leaving the stained, preparation for painting. filled wood surface ready for the clear finish Where burning off is not practicable, scrap- coats. ing and wire brushing may be used to remove the old paint, although the process may be 6.2.2. Previously Painted Wood long and tedious. Liquid paint removers (such as those covered by Federal Specification TT- The repainting of wood surfaces is most R-251 or Military Specification MIL-R-46073) easily accomplished when it is done at the may be used effectively; however, they are proper stage in the life of the coating—that relatively expensive and difficult to handle, and is, when the coating is well-worn but still repeated applications and scraping may be re- basically sound. At this stage, little if any quired. Liquid paint removers are more ef- surface preparation is required other than the fectively utilized indoors, where burning off brushing or washing away of loose chalk, dust, is not feasible, and where a smooth final sur- dirt, or grime. On the other hand, if the old face is desired. They give best results when paint is allowed to deteriorate to the point applied on horizontal surfaces; vertical sur- where large areas of the paint are flaking and faces require a viscous remover of the semi- peeling, or the paint is cracked and the edges paste type. The wax film that forms on the curled, then it will have to be removed com- surface of the paint remover should not be pletely to prepare the surface properly for re- disturbed, since its function is to seal in the painting. Complete removal of the old paint volatile active solvents until they have done is a laborious, expensive task that must be their job of softening the paint. The directions accomplished with torches or paint removers, of the manufacturer should be carefully fol- accompanied and followed by thorough scrap- lowed when using liquid paint removers. Since ing and sanding of the surface. Obviously, it they nearly always contain toxic volatile sol- j I pays to repaint at the proper time. This is vents, adequate ventilation should always be best determined through periodic close inspec- provided during their use. When removal of tion of the surface. the paint has been effected, the residual wax

; The preceding remarks are particularly ap- must be removed from the surface by washing

1 plicable to exterior painted surfaces. Most with mineral spirits, followed by a detergent white outside paints, and tinted paints to a wash and thorough rinsing with water. (Wax- lesser extent, are formulated to give controlled free solvent-type paint removers are available chalking of the coating which assists in keeping but are generally not as effective as the wax- it clean. The ideal time to repaint such sur- containing types.) faces is when the paint is worn thin to the After the basic surface preparation has been f point where bare wood begins to show, or when accomplished, nailholes, cracks, and other de- (after in 1 minute checking and crumbling begin. Such fects should be puttied priming) as

; surfaces take paint well and need little surface the case of initially bare surfaces. preparation. If chalking has kept the paint Interior paint coatings, unless worn thin by

I clean, brushing with a painter's duster may abrasion or repeated washing, are usually re- be all the preparation required. If chalking is painted because they have become dull, dis- il

107 — colored, or dirty—or because a change of color ough rinsing with water. If the old paint is is desired for decorative rather than functional glossy, it should be lightly roughened with reasons. Such surfaces are relatively easy to sandpaper or steel wool to improve paint prepare for repainting. The most important adhesion. requirement is that the surface be free of loose In all repainting, care dirt and grease. Indoor surfaces often become should be taken not to build too great a thickness coated with a film of grease or oil from cooking up by applying vapors and from contact with people or objects. too many coats or by repainting too frequently. The new paint will not adhere long to a greasy A thick paint coat is prone to cracking and surface. All such contaminants should be re- chipping off because it becomes too rigid to moved by thorough washing, first with mineral follow the normal dimensional changes in the spirits, then with a detergent, followed by thor- wood.

6.3. Preparation and Pretreatment of Metal Surfaces

The basic requirement in the preparation of provides the optimum surface for painting metal surfaces for painting, as with other sur- structural steel, the labor and high cost of this faces, is cleanness. However, the degree of type of cleaning is ordinarily justified only cleanness which is practicable or necessary when maximum performance under severe particularly in the case of steel surfaces service conditions is required. Maximum per- depends on the condition of the surface, the formance in difficult environments usually in- type of coating system that is to be applied, volves synthetic resin-base coatings of high the nature of the service environment, and the durability but limited wetting power; such cost factor in relation to the improved dura- coatings require thoroughly clean (and often bility to be expected from more thorough sur- pretreated) surfaces to develop adequate and face preparation. The choice of a suitable lasting adhesion. Examples of such coatings are method of surface preparation also depends on the phenolic and vinyl paints recommended in the location, size, and shape of the surface to section 4.3.1 for severe marine service and be painted, and on the cleaning equipment and immersion conditions. Oil paints, on the other facilities that are available. Structural steel hand, although suitable only for mild exposure often must be painted in the field, under con- in ordinary atmospheric environments, possess ditions involving certain limitations of surface inherently good wetting properties that permit preparation and coating application. Indus- their use even on incompletely cleaned steel trial steel products coated at the factory can from which it has not been practical to remove be prepared for painting by thorough clean- all surface rust and corrosion products. ing and pretreatment processes that yield opti- While the wetting ability of the primer to be mum results. used is the immediate factor determining the Before considering the detailed information degree of surface preparation required, the given in the following sections on the prepara- choice of primer in turn depends to a large tion of specific types of surfaces, the reader extent on the requirements for compatibility should consult the general information in the and good adhesion of the topcoat. Therefore, introductory sections of this chapter on the the surface preparation actually depends on basic importance and purpose of adequate sui'- consideration of the coating system as a whole. face preparation and pretreatment. As part of the information presented in chap- Because of the enormous tonnage of steel ter 4 on Selection of Coating Systems, the gen- used in structural applications and manufac- eral type of surface preparation required or tured articles, and because of the continual considered adequate for particular types of need for protecting this strong but corrosion- coating systems and service environments was susceptible metal from damage under a wide indicated. In the present section, the details variety of environmental conditions, the meth- about the various methods of surface prepara- ods of surface preparation employed for steel tion and pretreatment are given. encompass virtually the entire spectrum of available methods. We shall therefore describe 6.3.1.1. Solvent cleaning with organic sol- the various basic methods of surface prepara- vents is a very effective method for removing tion in the discussion of steel surfaces which oils, greases, waxes, shop markings, drawing follows, and shall merely make reference to and cutting compounds, and other solvent- these methods as needed in subsequent sections soluble foreign matter from the surface to be dealing with other metals. painted. The removal of such contaminants is usually prerequisite to the effective use of me- 6.3.1. Gleaning of Steel (and Iron) Surfaces chanical methods for removing rust, mill scale, and heavy dirt encrustations that cannot be Although sandblasting to "white" metal removed by solvent cleaning. Mechanical (often followed by chemical pretreatment) cleaning methods, short of sandblasting to

108 —

"white" metal, do not completely remove such contaminants from the surface. These may oily contaminants and may spread them over be washed away with water, steam, deter- the surface in a film that can seriously impair gents, emulsions, or alkaline cleaners. the adhesion and durability of the applied coating. 6.3.1.2. Steam cleaning in combination with detergents, while comparable in cost to 6.3.1.1.1. Wiping, dipping, or spraying.— solvent cleaning, is an effective means for re- Since organic solvents remove oils, greases, and moving heavy soil as well as greases and oils w^axes by dissolving them, the cleaning liquid from metal surfaces ; it does not remove rust and soon becomes contaminated with the removed mill scale. If a strongly alkaline cleaner is products and must be purified or renewed to used, the action of the caustic together with avoid redepositing an oily residue on the work the high temperature and high velocity of the when the solvent evaporates. This is particu- steam will remove old paint as well as surface larly true when wiping, dipping, or spraying soil and grime. Steam and/or hot water at a methods are employed; in these cases, it is temperature of about 300 °F (149 °C) and a essential that the final rinsing be done with pressure in the vicinity of 200 lb/ in-, and con-

clean solvent. ' The rather high cost of organic taining the desired detergent, is directed solvents, the large quantities required for sol- against the work from a distance of about six vent cleaning, the expense of solvent purifica- inches, with the nozzle tilted slightly in the tion, and the need for precautions against fire direction of travel. Speed of travel, number and toxicity hazards combine to make organic of passes, and time between initial wetting solvent cleaning a relatively expensive though and subsequent cleaning should be adjusted to effective method of removing oily and waxy the amount and nature of the soil to be re- contaminants. moved. The time between initial passes and later return allows the detergent to act upon 6.3.1.1.2. Vapor degreasing is the most ef- and loosen foreign matter to permit its com- fective and economical type of solvent clean- plete removal during the final passes. Any ing; it is also the least hazardous. In the alkaline residue remaining on the surface after vapor degreasing process, the article to be the cleaning operation must be removed by cleaned is suspended in the solvent vapor above thorough rinsing with fresh water. Steam a boiling cleaning liquid (usually stabilized cleaning is particularly well-suited for cleaning trichloroethylene or perchloroethylene, because in the field, where the location, size, and shape of their virtual nonflammability and their rela- of objects and structures frequently do not tively low toxicity as compared to other chlori- lend themselves to efficient cleaning by other nated hydrocarbons). The hot, distilled sol- methods. Workers engaged in steam cleaning vent vapors condense upon the relatively cool operations should be protected against burns object, dissolving oils and greases and carrying and chemical injury to the eyes and skin. them away as the condensed droplets fall back into the parent liquid. Cold water pipes or 6.3.1.3. Emulsion cleaning with emulsi- coils are often placed in the upper part of the fiable solvents combines some of the cleaning degreasing tank or chamber to effect further action of both water and solvents. The emulsi- condensation of vapor and to provide a vapor fiable solvents used are composed essentially ceiling (above which vapors will not rise). of an organic grease solvent, an emulsifying The unique advantage of vapor degreasing over agent, blending agents, and a petroleum thin- other organic solvent cleaning methods is that ner such as kerosene or mineral spirits. The the article being cleaned is continuously washed work is immersed in the emulsifiable solvent with clean, distilled solvent, while the con- (or the solvent may be brushed or sprayed onto taminants accumulate in the liquid below. the work) and allowed to remain for several Eventually, of course, the parent solvent must minutes to dissolve oil and grease and to loosen be purified by distillation, preferably in an dirt. The dissolved and loosened oil and soil auxiliary still, after first removing any un- are then emulsified and fiushed away by a pres- dissolved water by a separatory process. To surized hot-water rinse. A small residue of maintain an efficient degreasing operation, emulsion usually remains on the surface and purification of the solvent should take place will leave a thin film of oil if not removed. hefore the concentration of contaminants Removal of this residue is readily accomplished reaches about 25 percent. An incidental but by washing with steam, hot water detergents, important benefit of vapor degreasing is that or solvents. Cleaning with emulsifiable sol- the degreased work emerges warm and dry vents permits removal of heavy oil and grease and ready for any additional cleaning, pre- contamination at lower cost than with organic treating, or finishing to solvents alone also, corrosive salts and other operations that are ; follow. water-soluble contaminants are washed away Organic solvents do not remove corrosive during the pressurized rinse. Since most of salts, soaps, and other harmful water-soluble the surface soil is emulsified and washed away

109 during the rinsing process, the emulsifiable Workers should wear splash goggles, protective solvent itself becomes contaminated only slowly rubber gloves and garments, and respirators and requires only occasional renewal. It when preparing or using caustic solutions. should be kept in mind that the hydrocarbon In case of accidental contact of caustic soda solvents contained in these cleaners present solution with the eyes or skin, the affected the usual fire hazard associated with the use parts should be flushed immediately with large of organic solvents. amounts of water, and a physician should be consulted promptly. 6.3.1.4, Alkali cleaning provides an economical and effective means for removing oil, 6.3.1.5. Acid cleaning or pickling is a grease, wax, corrosive salts, and other soil very effective method for completely removing (except rust and mill scale) from metal sur- all rust and mill scale from steel surfaces by faces. The strength of the cleaner can be chemical and/or electrolytic action. The acids varied from mild to caustic by varying the commonly employed are phosphoric, sulfuric, type and concentration of alkali employed. and hydrochloric—either singly or in combina- Among commonly used alkaline cleaners are tion. Occasionally, in special cases involving trisodium phosphate, sodium tetraborate (bo- stainless or other alloy steels, hydrofluoric or rax), sodium meta- and ortho-silicate, sodium nitric acid also may be used for brightening carbonate (soda ash), and—where maximum the surface or for scale removal. saponifying strength is needed—sodium hydroxide (caustic soda). Most proprietary al- 6.3.1.5.1. Phosphoric acid cleaning is an ex- kaline cleaners employ blends of these ma- cellent time-tested method for removing oil, terials to obtain a more versatile and more grease, soil, corrosive salts, and light rust from effective cleaning action ; surface active (wet- metal surfaces. The phosphoric acid solution ting) agents and emulsified hydrocarbon sol- is combined with detergents and wetting agents vents are sometimes added to increase the to assist in soil removal and contains organic grease-removing action of the cleaner. solvents such as alcohols and ketones to aid in Alkaline cleaners must be used hot (about the removal of oils and greases. Where heavy 175 °F [79 °C]) for maximum effectiveness. grease or oil contamination is present, it is de- They accomplish their cleaning action by sa- sirable first to remove most of this by cleaning ponifying oils and greases to form soluble with organic solvents in order to avoid rapid soaps, and by loosening other types of soil by fouling of the phosphoric acid cleaner. Al- a detergent action that displaces the soil from though it is possible to remove even heavy rust the surface. The saponified oils and the loos- and mill scale from steel surfaces by using hot, ened and suspended soil are readily flushed strong phosphoric acid pickling solutions (15- away by thorough rinsing with hot water. 50% phosphoric acid) for a long period of Strong alkaline cleaners based largely on so- time, it is preferable to employ a sulfuric acid dium hydroxide are effective paint strippers pickling method (as described in the following for removing old paint from steel surfaces. section) for the eflJicient chemical removal of Alkaline cleaners may be utilized by immers- heavy corrosion products. ing the article to be cleaned in a soak tank of Cleaning with phosphoric acid solutions can hot cleaner, by pressure-spraying the cleaner be accomplished by dipping the work into a over the object, or by an electrolytic process tank of the hot or cold cleaner, or by flowing which generates gas bubbles that aid greatly or brushing the cleaner onto the work. After in dislodging soil. Following the use of akla- removal of soil and rust, the surface should be line cleaners, care must be taken to rinse all washed off with clean water (or wiped off alkaline material from the surface; even small with clean, damp rags if rinsing is not prac- traces of alkali, soap, or detergent left on the tical because of the size or location of the surface are detrimental to coating adhesion structure. Military Specification MIL-M- and durability. To ensure neutralization of 10578 describes two types of phosphoric acid any alkaline residue, a passivating dichromate metal cleaner, both of which must be diluted or dilute chromic acid final rinse may be used, with three times their volume of water before unless a phosphating or other suitable pre- use [78]. The type I cleaner, before dilution, treatment is to follow. contains about 47 percent by volume of phos- While alkali cleaning is free from the fire phoric acid (85%), and is used where the sur- and toxicity hazards of organic solvent clean- face can be washed off with water after the ing (unless emulsified solvents have been in- cleaning process. The type II cleaner, before corporated), the corrosive effects of alkaline dilution with water, contains about 14 percent materials on the skin and on ordinary clothing by volume of phosphoric acid (85%), and may must be guarded against. Caustic soda, in be used where washing with water is not prac- particular, can cause serious burns to the skin ticable and the surface is merely to be wiped and eyes and is extremely irritating to the off with rags. nasal and bronchial membranes if inhaled. Unlike alkali cleaning or strong acid pickling,

110 .

phosphoric acid cleaning does not leave a is limited to objects that will fit into the avail- harmful residue on the washed or wiped sur- able pickling baths. It is therefore largely a face. Instead, the phosphoric acid gently factory technique that finds its widest use on etches the surface and reacts chemically with structural steel leaving the mill. Workers en- it to produce a minute layer of iron phosphate gaged in pickling activities must be carefully which provides both a mechanically and chem- protected against acid splash or inhalation by ically receptive surface for subsequent coating. wearing goggles, respirators, and suitable rub- (The thin phosphate layer formed on the sur- ber clothing—including boots, gloves, and face during phosphoric acid cleaning or pick- aprons. ling is not a substitute for—and should not be Electrolytic pickling is more rapid than or- confused with—the heavy phosphating pre- dinary chemical pickling for removing mill treatments which are described in sec. 6.3.2.) scale because of the greater evolution of hydrogen and the resultant greater agitation and 6.3.1.5.2. Pickling with strong acids—usually "blasting" action. There is little difference in sulfuric, but sometimes hydrochloric (muri- acid consumption between the electrolytic and atic) or other acids as already noted—is chemical processes, provided the latter is prop- accomplished in dilute solution (usually at a erly inhibited to avoid unnecessary solution of concentration of 5 to 10 percent by weight) clean metal. and at a temperature of 140 °F (60 °C) or above. Rust is readily dissolved by this treat- 6.3.1.6. Hand cleaning methods are suit- ment, forming ferrous salts which remain in able for removing loose rust, loose mill scale, solution. Mill scale is attacked more slowly; dirt incrustations, and loose paint from metal removal of mill scale is dependent largely on surfaces that are to receive paints of good wet- penetration of cracks and crevices by the acid ting ability (e.g., oil paints) that are destined which thus works its way beneath the scale for mild, noncorrosive atmospheric exposure. where hydrogen is evolved and "blasts" the Hand cleaning usually suffices also for interior scale from the metal. The rate of rust and surfaces and for the spot cleaning and priming scale removal increases with increasing tem- involved in most maintenance painting where perature, and with increasing acid concentra- the surface has not been allowed to deteriorate tion up to a certain point (about 50 percent badly. Hand cleaning methods do not remove by weight for sulfuric acid, and about 30 per- heavy or tightly bound rust and mill scale and cent for hydrochloric acid) . Adequate agita- therefore are not satisfactory where coatings tion must be provided to maintain a normal of limited wetting ability (e.g., phenolic var- pickling rate. Inhibitors must be present in nish paints) are to be used or where severe the acid bath to prevent excessive attack upon atmospheric exposure or water immersion con- the metal itself in cleaned areas while rust and ditions are encountered. Oil and grease, if scale removal is still taking place in other present, must be removed by solvent cleaning areas; inhibitors also minimize surface bub- (sec. 6.3.1.1) before commencing hand clean- bling and "hydrogen embrittlement" of the ing operations, since the latter may further steel by controlling the amount of hydrogen spread oily contaminants over the surface. evolved. The buildup of ferrous salts in the Hand cleaning is accomplished with a variety pickling solution slows down the rate of rust of hand tools, including wire and stiff bristle and scale removal; when this can no longer brushes, scrapers (often with special shapes be compensated by increasing the temperature for reaching into difficult areas), impact and and acid concentration, the pickling solution chipping tools such as hammer and chisel, and must be discarded and a fresh bath prepared. abrasive materials (emery cloth, sandpaper, Before pickling is begun, oily contaminants and steel wool). Normally, chipping and (grease, wax, etc.) should be removed from scraping precede wire brushing. Finally, all the surface by solvent cleaning, alkaline clean- loosened matter should be brushed or swept off ing, or other suitable degreasing method (see and the surface blown clean with compressed preceding subsections) . After the pickling is air (vacuum cleaning also is suitable). Work- completed, the work should be promptly and ers should be protected with goggles and dust thoroughly rinsed with clean cold water; any respirators. If cleaning must be done in areas acid residue will cause rapid rusting of the steel where fiammable vapors may be present, non- surface. The cold water rinse should be fol- sparking tools must be used to avoid the danger lowed by a dilute dichromate, chromic acid, of explosion. or phosphoric acid rinse to passivate the sur- Since hand cleaning is slow and laborious face (chromium compounds should not be used (hence expensive) and of only limited efficacy, if phosphate pretreatments are to follow ; how- it is used primarily for small or noncritical ever, prompt treatment with dilute phosphoric jobs, for jobs that are inaccessible to power acid is highly desirable in any event) tools, where power tools are uneconomic or While pickling is an effective method for unavailable, where the surface is not very dirty removing all rust and mill scale from steel, it or corroded (as in maintenance painting done Ul ;

at the right time) , and where the coating sys- blasting and shot blasting are very costly meth- tem to be applied can tolerate or overcome the ods, the use of which is largely restricted to residues remaining after hand cleaning. shop work or vacuum blasting where the expensive abrasive materials can be recovered 6.3.1.7. Power tool cleaning removes little for reuse; moreover, these methods usually more dirt, rust, and mill scale than hand tool yield a greater surface roughness (higher an- cleaning; however, it is less laborious and chor pattern) than is desirable for most coat- much more rapid. Like hand cleaning, it re- ing applications. The present section will be moves only loose mill scale and rust. (Blast limited to a discussion of sandblasting as a cleaning—as described in the following sub- most effective method of preparing steel and section—is required to remove heavy rust and other surfaces for painting. tight mill scale.) Since not all corrosion The height of the anchor pattern, or the products are removed from the surface by maximum height of profile of a blasted sur- power cleaning, the method is limited to coat- face, is the distance from the bottoms of the ing systems that have good wetting ability for lowest pits to the tops of the highest peaks. incompletely cleaned surfaces, or to coating The larger the size of the abrasive employed, systems that will not be subjected to severe en- the higher is the anchor pattern or the greater vironments or immersion conditions. Power is the surface roughness produced. For good cleaning is adequate also in most maintenance durability, the overall thickness of the dry coat- painting situations where the surface is in rea- ing must be great enough to cover even the sonably good condition and only spot cleaning peaks with an adequately thick protective film. to the bare substrate is necessary. Thus, surfaces that are to receive a thin coat- The tools required may be electric or pneu- ing require a shallower anchor pattern and matic powered. They include large rotary hence must be cleaned with a finer abrasive power wire brushes, power grinding wheels than surfaces that are to receive a thick coat- and sanding machines, and power-driven ro- ing. Thicker coatings will not only tolerate a tary or piston type hammers and scalers for deeper anchor pattern but may benefit in ad- chipping and descaling. Of these, power wire hesion and durability from the greater sur- brushes in a variety of sizes, textures, and face contact and better "tooth" provided by shapes are the most widely used; cup shapes, the rougher surface. There is, of course, a radial types and disc shapes are available. practical limit to the surface roughness that Oil and grease should be removed from the can be tolerated by films of practical thickness surface by solvent cleaning (sec. 6.3.1.1) be- peaks that are too thinly coated become focal fore beginning power tool cleaning, to avoid points for wear and corrosion and premature spreading such contaminants over the sur- coating failure. The effectiveness of sand- face. Chipping and scaling operations, if blasting as a means of preparing steel surfaces needed, should precede wire brushing. Great for painting lies in the ready availability and care is required, when using power impact tools, relatively low cost of sand having a particle to avoid gouging sound metal. Excessive wire size conducive to production of an excellent brushing of unremovable mill scale will bur- anchor pattern on the cleaned surface. nish the surface and greatly reduce the adhesion of the applied coating. Areas not accessi- 6.3.1.8.1. Sandblasting to "white" metal, while ble to power tools should be cleaned by hand not always feasible or economical, is the most methods. After completion of the mechanical thoroughly effective method available for pre- cleaning, all lessened material should be swept paring steel surfaces for painting. It removes from the surface, and the surface should be all rust, mill scale, corrosive salts, dirt, grease, blown clean with compressed air or cleaned oil, wax, and other foreign matter from the by vacuum. Goggles and dust respirators metal—no matter how heavy the corrosion or should be worn for protection. If power clean- contamination—leaving a completely clean uni- ing must be done in areas where flammable form surface with an ideally roughened texture vapors may be present, nonsparking motors or profile (anchor pattern) for maximum coat- and tools should be used to avoid the danger ing adhesion and durability under the most of explosion. severe service conditions. Although sandblasting is a relatively expensive method of surface 6.3.1.8. Sandblasting is a form of blast preparation, the greater coating durability cleaning in which sand is employed as the abra- obtainable under adverse service conditions will sive which is propelled by compressed air often make it the most economical method in through a nozzle directed against the sur- the long run. This is especially true in severe face to be cleaned. Other forms of blast clean- exposure environments, such as chemical atmos- ing utilize crushed iron grit (grit blasting) or pheres or water immersion. Thorough sand- iron shot (shot blasting) as the abrasive; blasting is, in fact, the only method of surface these may use either compressed air or centri- preparation that will ensure maximum coating fugal wheels to propel the abrasive. Grit performance under such rigorous conditions.

112 There may be occasional situations, however, in tem. Wet sandblasting is not widely used now which the pattern of corrosion would not justify as a method of preparing steel surfaces for complete sandblasting to white metal even for painting. severe service exposure. For example, to remove the last traces of rust from a deeply pitted 6.3.1.8.6. Dry sandblasting equipment and surface may be impracticably expensive because techniques.—An efficient sandblasting operation of the extra labor involved and the excessive requires the right combination of nozzle type, removal of sound metal required to reduce the hose size, abrasive size, air pressure and flow, entire surface to the level of the pits. In gen- and sand handling capacity—also, a competent eral, sandblasting can advantageously be em- nozzleman. Nozzles of widely different durabil- ployed wherever the greater coating durability ity and cost may be made of ceramics, cast iron, obtainable by this method justifies the extra steel alloys, tungsten-carbide, and other suitable time and cost. materials. Nozzle size is limited by the requirement that the hose be 3 or 4 times larger in 6.3.1.8.2. In less severe service environments, diameter than the nozzle and yet be reasonably where maximum coating durability is not re- flexible and manageable. For general sand- quired, lesser degrees of sandblasting than blasting work, a %-inch nozzle, requiring a cleaning to white metal are usually satisfactory. 114, -inch hose, is the largest size that is practi- So-called commercial blast cleaning envisions cal. However, nozzles up to %-inch diameter a good but not perfect cleaning job in which can be used on large flat surfaces where their nearly all rust, mill scale, and contaminants are rigidity and unwieldiness do not seriously af- removed from the surface, but not necessarily fect the overall production rate. A %-inch to the point where a surface of uniform color, nozzle operated at an air pressure of 90 lb/in ^ texture, and cleanness is obtained. This degree requires about 210 ft ^/min of air and will pass of cleaning is significantly lower in cost than about 1500 pounds of sand per hour. Both sand blast cleaning to white metal, and is generally and air must be dry and oil-free. An optimum adequate for all but the most rigorous types anchor pattern for painting is obtained with an of service exposure. abrasive size of 20 to 50-mesh. During blasting the nozzle is usually held at a slight angle 6.3.1.8.3. To meet certain situations in cor- from the perpendicular at a distance of 12 to rosive environments where commercial blast 18 inches from the surface; however, an angle cleaning is inadequate but white metal blast of about 45 degrees may be more effective for cleaning is prohibitively expensive, an inter- dislodging loose corrosion products or loose mediate grade called "near-white" blast clean- paint. ing, permitting a small specified amount of streaking and shadowing, has recently been 6.3.1.8.7. Vacuum blast cleaning systems are established by the Steel Structures Painting available for specialized use where dust is not Council [153] after successful use in industrial, permissible or when the abrasive is to be re- state, and federal trials. covered. The blast nozzle is enclosed in a hol- 6.3.1.8.4. So-called "brush-off" blast cleaning is low cone having a brush at one end to trap the a rapid type of superficial sandblasting intended abrasive and cleaning debris while the vacuum to remove all loose rust, loose mill scale, and exhaust is applied through the side of the cone. loose paint from the surface, leaving only tightly adhering, intact mill scale and rust. This de- 6.3.1.8.8. Cost and Production Rate.—The overall cost unit area of sandblast cleaning gree of surface preparation is comparable or per superior to the best that can be accomplished is determined primarily by the production rate, the abrasive. by hand or power tool cleaning methods, and the cost of labor, and the cost of The production rate (surface area cleaned per is adequate for coating systems with good wet- will greatly with the nature ting ability which are to withstand only mild unit time) vary exposure. and amount of corrosion products to be removed, with the degree of surface cleanness to 6.3.1.8.5. Wet sandblasting employs a slurry of be achieved, with the type of equipment and sand and water instead of dry sand as the abra- blasting conditions employed, and with the skill sive. Heavy rust and tight mill scale are effec- of the nozzleman. The available figures for tively removed by this method, and the dust production rates obtainable by the various de- problems and hazards of dry sandblasting are grees of sandblasting (viz, white metal, com- largely eliminated. However, the clean, wet mercial, and brushoff) are not in good agree- surface begins to rust again almost immediately ment; however, as a rough guide-line, under unless corrosion inhibitors are incorporated in ordinary conditions, sandblasting to white met- the blast water. Even when inhibitors are al may proceed at the rate of 50 to 150 square used, some doubt exists as to the efficacy of the feet per hour, while the production rate for passivated surface in promoting maximum ad- commercial blast cleaning may be about 3 times hesion and durability of the applied coating sys- greater, and the production rate for brushoflf

113 . — blast cleaning may be 6 to 8 times greater. metal, aided by the "explosive" action of water Obviously, cleaning to white metal is consider- vapor generated beneath the scale, breaks it ably more expensive than commercial blast loose from the surface so that it can be removed cleaning and is many times more costly than by scraping and wire brushing. Rust is con- brushofF blast cleaning. The degree of clean- verted to a black powder that may then be ing to be done should be adequate for the par- brushed from the surface. Heavily rusted and ticular coating system and service environment heavily scaled steel may require a slow traverse involved. Cleaning beyond this point is un- or several passes of the flame to accomplish necessary and uneconomic, while less than the the removal. Tightly bonded mill scale is not required degree of cleaning ultimately results removed by flame cleaning. in higher costs by shortening the service life Immediately following the flame treatment, of the coating. the surface should be power wire brushed or scraped to remove all loosened rust, scale, and 6.3.1.8.9. Safety.—Sandblasting is a hazard- foreign matter, and then swept or brushed free ous operation unless adequate precautions are of any residual dust and debris. Promptly taken to protect personnel and property against after the flame treatment and cleaning—while harm from dust and flying chips and particles. the metal is still warm and well above the tem- The nozzleman should be equipped with a U.S. perature of the surrounding atmosphere—the Bureau of Mines approved helmet connected to a prime coat should be applied. Unless the prime source of clean, compressed air. Other workers coat is applied promptly to the warm metal, in the area should wear goggles and filter- before any moisture becomes absorbed on the type air respirators. Sandblasting is a con- surface, most of the benefits of the flame clean- stant menace to unprotected eyes, and inhala- ing will be lost and the results will be little if tion of the dusts may cause silicosis. Blasting any better than power tool cleaning alone. The should not be done in areas where explosive requirement that priming follow so closely upon concentrations of volatile vapors may be pre- the heels of the flame cleaning and wire brush- sent. Blast hoses should be grounded to dis- ing creates both scheduling problems and a pos- sipate static charges. Nearby property should sible fire hazard, since the primers employed be protected by covering, masking, or wrapping. usually contain volatile, flammable solvents. Ropes used for scaffolding should be heavily The priming must be conducted in a manner wrapped to prevent damage. Wet sandblasting, that will not result in the buildup of explosive while free of dust hazards, presents a hazard concentrations of solvent vapor in the area of from the toxic, soluble chromium compounds the flame. (corrosion inhibiting agents) contained in the If oil or grease are present on the surface, mist. For detailed information on safe prac- they should be removed by solvent cleaning tice in sandblasting, reference may be made to and the solvent allowed to evaporate—before the codes or manuals of various national associ- using the flame cleaning method. Grease and ations concerned with this problem [75,76,77] oil may char under the rapidly moving flame, leaving an objectionable carbon deposit on the 6.3.1.8.10. After sandblasting is completed, surface. traces of residual dust, debris, or abrasive any When properly done, flame cleaning is con- removed by brushing with should be completely sidered to give results better than that obtain- clean brushes, blowing with clean compressed able by power tool cleaning alone, but it is not fresh, sandblasted, air, or vacuuming. The a substitute for sandblasting to a "commercial" is very susceptible to corro- bare metal surface surface or to "white" metal. sion and will rust quickly if wet or exposed Flame cleaning is very useful in maintenance to high humidity. The clean surface must be painting where it is necessary to remove all treated or primed promptly, before rusting or the old paint from the metal. For this purpose, occur. Even in ordinary contamination can slow traverse of the flame head and multiple it is highly desirable that mild atmospheres, passes are usually required. While the usual the surface be primed or chemically treated oxy-acetylene flame cleaning equipment is suit- within eight hours after sandblasting. able, other types of torches utilizing propane, gasoline, or other fuels capable of hot flame 6.3.1.9. Flame cleaning, depending on the may be used. Coatings that soften with heat manner in which it is employed, can be used can be removed with a scraper immediately either to remove all loose mill scale and rust flame. For coatings that are not from unpainted steel or to remove old paint following the from metal surfaces. For the removal of mill softened appreciably by heat, the flame must scale and rust from unpainted steel, an intensely be held at one place until the coating is burned hot, high-velocity, oxy-acetylene flame is passed away, after which the ash and any carbona- over the metal surface at the rate of about 15 ceous residue must be removed by scraping or to 35 feet per minute. The rapid heating and wire brushing. The considerable labor involved differential expansion of the mill scale and base in the removal of such residues can be reduced

114 by the use of special flame cleaning equipment with the metal to form the desired gray-white in which the oxy-acetylene flame is supple- phosphate coating—before washing or wiping mented by a jet of oxygen which assists the off the excess acid and any loose residue. burning and at the same time blows the residue Suitable cold phosphating solutions contain- from the surface as it forms. ing phosphoric acid, an oil solvent, a wetting agent, and water are described in Military 6.3.2 Chemical Pretreatment of Steel Specification MII^M-10578 [78]. This specifi- (and Iron) Surfaces cation covers two types of metal conditioning compounds, both of which must be diluted with previously, metal surfaces are As indicated three times their volume of water before use. to a corro- given chemical pretreatments form The two types differ primarily in phosphoric sion-inhibiting, paint-receptive coating on the acid concentration. Type I, after dilution for chemical reaction with the surface through a use, contains about 10 percent by weight of conversion coatings metal. In practice, such phosphoric acid type II contains about 3 per- insoluble ; for steel and iron are nearly always cent by weight of phosphoric acid after dilution. phosphates, although chromate and oxide metal The stronger, type I solution is used where application. Phosphate coatings find limited some light rust is present and where the sur- capable coatings, though insoluble in water, are face can be washed off to remove all reacted of absorljing small amounts of corrosion inhibi- phosphoric acid. Free phosphoric acid on the dilute solu- tors (notably, chromic acid) from surface is detrimental to paint life. After tion to enhance corrosion resistance and further the treated surface has been thoroughly paint. It is for increase the life of the applied washed, it should be giver, a final rinse with a this reason that most phosphatizing processes dilute chromic acid or dilute chromic-phos- specify a final rinse in dilute chromic acid or phoric acid mixture to passivate the surface. a dilute chromic-phosphoric acid mixture. A chromic acid concentration of about 0.02 to In addition to the direct phosphating types of 0.05 percent by weight is satisfactory. inorganic pretreatment, wide and effective use The weaker, type II conditioner is used is made of the wash primer or washcoat type where it is not practicable to rinse off the of organic pretreatment coating. The latter treated surface. The conditioner, after proper provides a mild phosphatizing action on the dilution (with three times its volume of water, surface while laying a thin, very adherent down or more if necessary) , should be applied spar- organic film. Organic pretreatment coatings ingly to the clean metal surface and allowed should not be used over surfaces that have al- to react with the surface and dry upon it to ready received a phosphate pretreatment. form a grayish-white deposit of insoluble phos- The chief types of inorganic phosphate pre- phate. If the proper strength and amount of treatment and the organic coating (wash pri- solution are used, all the phosphoric acid will mer) type of pretreatment are described in the be reacted and it will not be necessary to wipe following subsections. off or further treat the surface. However, if any unreacted phosphoric acid remains on the 6.3.2.1. Cold phosphate pretreatment surface (as evidenced by a dark, sticky resi-

ACID TYPE) : This type of pre- (PHOSPHORIC due) , it must be wiped off with damp rags or treatment utilizes relatively dilute phosphoric with rags wet with an alcohol-water mixture acid solutions to etch the surface lightly and to remove all traces of stickiness, and the sur- convert a very thin layer of the metal to an face again allowed to dry. The dried surface insoluble phosphate coating weighing about 8 should have a white appearance; however, any milligrams per square foot of surface. The excessive or loose powdery deposit should be solutions employed are similar to those used in brushed off with dry brushes or dry rags, or the phosphoric acid cleaning of metals as de- removed with clean compressed air or by scribed in section 6.3.1.5.1; however, when vacuuming. these materials are used as a pretreatment, the The treated surface should be given a prim- emphasis is upon chemical conversion of the ing coat of paint as soon as practicable, before metal rather than on cleaning. The surface any rusting or contamination of the surface should previously have been cleaned free of all occurs. oil, grease, dirt, mill scale, and rust—except for Cold phosphate pretreaments are not in- very light rust which may be converted during tended for use in maintenance painting where the treatment. If used over mill scale, the the acids employed may contact and damage scale may later loosen and lift the paint. Un- existing paint. The solutions should not be like acid cleaning processes in which the solu- stored or mixed in uncoated steel or galvanized tions may be used hot and profusely and for steel containers; glass or earthenware vessels relatively long periods of time, the technique are satisfactory. in pretreating is to apply only the necessary Precautions are necessary to protect work- amount of conditioner and to leave it on only men from accidental splashes of the acid solu- for a brief period—until the acid has reacted tions. The solutions are particularly danger-

115 ; ous to the eyes; splashproof eye shields or Hot phosphate pretreatments require a com- goggles should be worn to prevent painful, pletely clean bare surface, preferably one that serious injury. Acid-resistant rubber gloves, has been sandblasted if heavy rust or mill aprons, and boots should be provided where scale was present. In any event, the surface needed. should be free of oil, grease, dirt, paint, rust, and mill scale. Federal Specification TT-C- 6.3.2.2. Cold phosphate pretreatment 490 [79] specifies that the clean metal should (ZINC type) : Proprietary zinc phosphate be exposed to the hot phosphating solution for phosphatizing solutions containing zinc phos- at least three minutes if a dip method is used phate, phosphoric acid, and an activating agent or for at least 1 minute if spraying is employed. applied unheated are available which can be The equipment used should be constructed of to steel or galvanized surfaces, using ordi- materials resistant to the action of the phos- nary application methods such as brush, spray, phating solution and should not contain cop- dip, or flow coating. The surface to be treated per alloy fittings or brazing where they may should first be cleaned by an appropriate come in contact with the solution. Fog sprays sec. remove any oil, method (see 6.3.1) to should be provided on both dip tanks and spray grease, dirt, rust, and mill scale. The dilute, equipment to prevent the solution from drying unheated phosphatizing solution is then ap- on the work surface before the necessary sub- plied in accordance with the manufacturer's sequent washing with water. Hot phosphate instructions and allowed to remain on the residues are detrimental to paint life and must surface until the zinc phosphate coating is be removed by thorough washing with hot or formed (usually for two to five minutes). cold clean water, followed by a final rinse with The surface is then thoroughly washed with a dilute chromic acid or chromic-phosphoric clean water. final rinse in dilute chromic A acid mixture to enhance corrosion resistance acid or a dilute chromic-phosphoric acid mix- and paint life. The work should remain in the ture is usually specified in order to achieve max- chromic rinse for a minimum of 1 minute when imum corrosion resistance and paint life. The dipped or for at least 30 seconds in spray proc- priming paint should be applied to the phos- esses. TT-C-490 specifies that the chromic phated surface as soon as practicable after it acid rinse solution be maintained at a tem- is dry. perature of 140 to 210 °F (60 to 99 °C) and a 6.3.2.3. Hot phosphate pretreatment pH of 2 to 4. Industry recommendations do not necessarily with given in (zinc phosphate type) : These are proprietary coincide those processes based on specially formulated, bal- TT-C-490, and the instructions of the manu- anced solutions of phosphoric acid saturated facturer or supplier should be consulted in the or supersaturated with acid zinc phosphate case of specific proprietary materials. Indus- salts (plus accelerating agents) that deposit a try recommendations frequently call for lower heavy, visibly crystalline coating of zinc phos- temperatures (room temperature to about phate (weighing about 150 to 400 milligrams 160 °F [71 °C]) and a higher, more strin- per square foot) on the surface of the metal. gently controlled pH (4 to 5) than those given By providing substantial protection against the in TT-C-490. spread of corrosion beneath the paint film as The zinc phosphate coating should be contin- well as a physically receptive surface for ex- uous, uniform in texture, evenly deposited, and cellent paint adhesion, the heavy phosphate gray to black in color. It should not be mottled coating significantly increases the durability in appearance nor show any smut, powder, cor- of the applied paint, particularly under severe rosion products, or white stains from dried service conditions involving corrosive atmos- phosphating solutions. Light brown or orange pheres or water immersion. (For mild expo- stains from the chromic acid rinse do no harm sure conditions, where it is neither economic however, deep brown stains may be evidence of nor necessary to thoroughly clean the metal heavy concentrations of chromic acid which surface by sandblasting, the benefits of hot might cause blistering of some paint films. The phosphatizing may not be sufficient to justify minimum weight of the phosphate coating the additional cost of such treatment.) Hot should be 150 milligrams per square foot for zinc phosphate pretreatments are largely lim- spray processes and 300 milligrams per square ited to objects of a size and shape that can be foot for dip processes. treated in dip tanks or by spraying in appro- Care should be taken to prevent burns from priate enclosures. They are therefore particu- the hot solutions or toxic effects from the larly suitable for the pretreatment of fabri- chromic acid rinse. Splash-proof eye shields cated steel on the production line. There have should be worn to avoid painful damage to the been some developments in special thioxotropic eyes. or gelled phosphate solutions for field applica- The phosphated surface should receive a tion, but these have not been widely used. Hot priming coat of paint as soon as practicable, phosphate treatments are not used in mainte- before any contamination or deterioration of nance painting. the surface occurs.

116 6.3.2.4. Hot phosphate pretreatment any contamination or deterioration of the sur-

(iron phosphate type) : These are proprie- face occurs. tary processes based on balanced aqueous solutions of phosphoric acid and acid phosphate 6.3.2.5. Organic pretreatment coating salts, with or without accelerating agents, (WASH primer) : The wash primer or wash which build up a coating of insoluble amorphous coat type of pretreatment provides a mild phos- iron phosphate on the surface of the metal. phatizing action on the metal surface while The iron phosphate coatings thus obtained are laying down a thin, highly adherent organic film not as heavy as those yielded by the hot zinc which provides an excellent base for most or- phosphate processes. A minimum coating ganic coating systems. The wash primer type weight of 40 milligrams per square foot is usu- of pretreatment is particularly useful where ally specified, and the coating weight that can the size and shape of the work preclude the use be built up usually does not exceed 100 milli- of hot phosphate solutions or where mixed grams per square foot. The thinner, amorphous metal assemblies are to be treated. The wash coating of iron phosphate is more flexible than primer pretreatment is of maximum effective- the crystalline zinc phosphate type and hence ness and outstanding benefit when used on is j)referred in applications where the metal is thoroughly cleaned steel (sandblasted or to oe formed after painting. Like zinc phos- pickled) which will be exposed to severely cor- phate coatings, the iron phosphate coatings rosive atmospheric conditions or to water im- provide substantial protection against the mersion. It also improves the durability of spread of corrosion beneath the paint film as conventional paints on hand cleaned steel ex- well as a physically receptive surface for exposed to corrosive atmospheres or water im- cellent paint adhesion, thus contributing sig- mersion. For hand-cleaned steel to be ex- nifically to increased paint life, especially posed in mild, ordinary atmospheres, the wash under severe service conditions such as cor- primer pretreatment—while beneficial—may rosive atmospheres or water immersion. not be economically justifiable. Wash primer Hot iron phosphate pretreatments are lim- should not be used on phosphated steel or over ited to objects of a size and shape that can be other pretreatments. treated in dip tanks or in specially designed spray enclosures. They are particularly suit- 6.3.2.5.1. Two-component wash primer.— able for the factory pretreatment of produc- The most widely used and proven type of wash tion line items. As with other hot phosphate primer organic pretreatment coating is the pretreatments, the surface to be treated must two-component type comprising (1) a base be completely clean—free of oil, grease, dirt, material containing an alcohol solution of poly- paint, rust, and mill scale. The treatment should vinyl butyral resin pigmented with insoluble be continued until a phosphate coating is pro- basic zinc chromate, and (2) an activating duced which is insoluble in water and has a diluent in the form of an alcohol solution of color ranging from a golden yellow to purple. phosphoric acid. This is the type of pretreat- Federal Specification TT-C-490 [79] specifies ment commonly known as WP-1, or as Bureau that the items be exposed to the phosphating of Ships Formula No. 117 as covered by Mili- solution for at least three minutes in dip proc- tary Specification MII^C-15328 [30] . A simi- esses or for at least 1 minute in spray processes. lar composition providing a smoother finish for The article should then be rinsed in clean aircraft use is covered by MIL-C-8514 (Aer) water, followed by a final rinse with a dilute [80] . Application methods for this type of pre- chromic acid (or dilute chromic-phosphoric treatment are given in MIL-C-8507 [81]. acid) solution similar to that used in zinc phos- The resin base and the acid diluent must be phate processes (see sec. 6.3.2.3). The hot mixed just before use and should be applied phosphate residues are detrimental to paint life within 4 hours after mixing. The fresher the and should not be allowed to dry upon the sur- mixture, the better are the results obtained. face prior to removal by washing. Since the phosphoric acid reacts with the vinyl The insoluble, golden yellow to purple iron resin and the pigment, as well as with the phosphate coating should be continuous, uni- metal surface, the activated mixture may gel, form in texture, and evenly deposited. There or, in any event, become useless for its chief should be no smut, power, corrosion products, function of promoting good adhesion if it stands or white stains due to dried phosphating solu- for more than about 8 hours. It is essential tions on the surface. that the base material be thoroughly mixed by Precautions should be taken to prevent burns vigorous mechanical shaking or stirring to re- from the hot solutions or toxic effects from the incorporate all settled pigment or caked ma- chromic acid rinse. Splashproof eyeshields terial before adding the activating diluent (the should be worn to prevent painful, serious dam- latter also should be shaken before use). age to the eyes. For use, one part by volume of the phosphoric The phosphated surface should receive a coat acid activator-diluent should be added sloivly, of priming paint as soon as practicable, before ivith continuous stirring to prevent localized

117 gelling, to four parts of the resin base. The is enjoyed by the two-component wash primers, resin base should never be added to the diluent. and the one-package wash primers have not The activated mixture should be thinned to found wide use. spraying consistency with alcohol ; generally, a volume of thinner equal to that of the added 6.3.3. Galvanized Surfaces diluent will suffice, but up to double this volume is permissible if necessary to obtain a thin, wet, Serious difficulty has often been experienced uniform film. The added alcohol should nor- in obtaining good adhesion of conventional mally be either ethyl alcohol or isopropyl alco- paints to galvanized steel, especially to new gal- hol; however, under high humidity conditions, vanized surfaces. All too often, the paint peels some butyl alcohol may be substituted for a or flakes off after a short period of exposure. portion of the lower boiling alcohols to an ex- A chief cause of such early adhesional failure, tent sufficient to prevent blushing. The dry apart from improper cleaning of the surface, film thickness should be 0.2 to 0.3 mil for opti- is thought to be the reactivity of new zinc with mum results and should not exceed 0.5 mil in many paint vehicles. While the nature or me- any case. At the correct thickness, the film chanism of this reaction is not well understood, will have a greenish-yellow translucent ap- practical means for dealing with the problem pearance through which uneven coloring in the are available. Good adhesion and durability of substrate will show. Wash primer should paints applied on galvanized surfaces can be never be applied so thick as to hide the sub- ensured by: (1) thorough cleaning of the sur- strate. It should be emphasized that the face, and adequate weathering and /or washcoat must be laid down wet to obtain a (2) chemical pretreatment of the surface to form a continuous film; otherwise, the coating will be paint-receptive barrier layer that insulates the powdery and its adhesion seriously impaired. paint film from the reactive zinc while shielding Brush application, while generally less satis- the zinc from further attack atmospheric factory than spray, may be used on rough sur- by corrosive agents. faces or for touching up small areas. When spot treating previously painted surfaces, the wash primer should ordinarily be applied only 6.3.3.1. New galvanized surfaces fre- to bare areas, although overlap of the surround- quently are coated with factory processing coming paint is not harmful provided the washcoat pounds, temporary protective oils, and other adheres well to the old paint and does not lift it. contaminants harmful to paint adhesion unless The wash primer pretreatment should not be removed prior to pretreatment or painting. In thought of as a coat of paint. While it may addition, most of the galvanized sheet and strip afford temporary protection for several weeks made today is given a chromate treatment of under mild, dry conditions (more than is af- one kind or another at the mill to protect against forded by an inorganic phosphate coating), it white stain, particularly if the metal is destined is not intended for use as a shop coat or to pro- for warehouse storage. Surfaces thus treated tect against exposure. For optimum adhesion may not be receptive to some types of paints and best results, the wash primer should re- and cannot be further pretreated by the meth- ceive a coat of priming paint as soon as it is ods usually prescribed for galvanized steel. completely dry—usually in from one to four Where it is desired to use a particular pretreat- hours after the pretreatment. Intercoat adhe- ment (e.g., hot phosphating) on galvanized sion will be impaired either by premature appli- steel, the problem is best avoided by specify- cation of the primer or by unnecessarily long ing "no treatment" on the mill order. When aging of the washcoat. Virtually any type of the metal is obtained from a warehouse and primer and many types of coatings can be used the nature of the factory treatment is unknown, over the wash primer pretreatment with good probably the best practice is to solvent-clean results; certain lacquers and vinyl paints are the surface (using a spectrum of solvents if among the exceptions that require special necessary) and then apply a zinc dust primer primers or intermediate coats. such as that covered by Fed. Spec. TT-P-641 [29], followed by the desired finish. The prob- 6.3.2.5.2. Single-package tvash primers based lem has been considered by the American Iron on lead chromate pigment or on chromic-phos- and Steel Institute [83] and by the American phate pigments have recently become available. Society for Testing and Materials [84]. A wash primer of this type for use on steel, Zinc dust primers generally give good results aluminum, and magnesium is covered by Fed- on clean galvanized surfaces with or without eral Specification MIL-P-14504 [82]. These pretreatment. However, pretreatment is bene- are proprietary materials, and the directions ficial and is recommended where the maximum of the manufacturer should be followed in their possible service life is sought, provided that the use. While the convenience of a single-package galvanized surface has not previously been wash primer is obvious, these materials do not given an interfering factory chromate treat- yet have the long history of successful use that ment.

118 . :

For optimum results, new galvanized sur- processing compounds, and protective treat- faces should be thoroughly solvent-cleaned by ments (including factory chromate treatments) one of the methods described in section 6.3.1.1 —have been largely removed from the surface to produce a surface completely free of oil, may frequently be painted satisfactorily with grease, processing compounds, temporary pro- conventional paint systems provided the sur- tective substances and other foreign matter. face is first cleaned with solvent to remove The clean surface, if not factory chromated as any residual oil or soil. For reliable results, noted above, should then be given one of the however, chemical pretreatment (if no residual following pretreatments chromate coating is present) and/or the use (1) Hot zinc phosphate pretreatment similar of zinc dust primer is highly recommended. to that described in section 6.3.2.3 but specially If the surface is too soiled for adequate clean- formulated for galvanized surfaces, provided ing with solvents alone, it may be brushed that the item is of a size and shape that can lightly with steel wool to assist in removal of be treated in hot dip tanks or in special spray the soil and then brushed, blown, vacuumed, enclosures. A pretreatment of this type is or again solvent washed. Rusted areas on covered by specification MIL-T-12879, Type I, galvanized steel should be hand cleaned or wire Class 1 [85]. brushed in the same manner as rusty steel. (2) Cold zinc phosphate pretreatment of the- If large areas of the galvanized surface are brush-on or wash type described in section badly rusted, it will be necessary to clean the 6.3.2.2. entire surface by the same vigorous methods (3) Wash primer type of organic pretreat- (e.g., acid cleaning or sandblasting) that are ment coating as described in section 6.3.2.5. used for rusted steel (see sec. 6.3.1). (4) Cold phosphate pretreatment of the dilute phosphoric acid type as described in sec- 6.3.3.3. Cadmium surfaces may be cleaned, tion 6.3.2.1. pretreated, and successfully painted by the (5) Chromate pretreatment of the general same methods that are employed for galvanized nature described in section 6.3.6.1 and as cov- (zinc-coated) surfaces (also see sec. 6.3.6.2). ered by specification MIL-T-12879, Type I, Class 2 [85]. 6.3.4. Aluminum and its Alloys The best choice among the above recommended pretreatments is largely a matter of conven- Aluminum and its alloys should be solvent- ience as determined by the size and shape of the cleaned by one of the methods discussed in object, its location, and the application equip- section 6.3.1.1 to remove all oil, grease, soil, ment and facilities available. The most gen- and other foreign matter. The use of mechani- erally applicable and most widely used pre- cal cleaning methods is damaging to the sur- treatment is the wash primer type; it gives face, particularly where a thin cladding of excellent results in virtually all situations. The aluminum is present on a substrate of a harder proprietary zinc phosphate pretreatments, alloy. Where the surface is too soiled or de- either hot or cold, also give excellent results; teriorated to be adequately cleaned with sol- where the hot zinc phosphate pretreatment is vents alone, the minimum amount of brushing with stiff bristle feasible—as in factory coating operations— it or aluminum wool that will is probably the best pretreatment available. dislodge the soil should be used.* All debris The dilute phosphoric acid type of cold phos- from the mechanical cleaning- operations should phate pretreatment, while less effective than be swept from the surface or removed with the others, may be the expedient choice in sit- clean, compressed air or by vacuuming, and uations where the treated surface cannot be any residual oily contaminants should be re- rinsed off or where other application limita- moved by another solvent wash. tions exist. The clean surface should be pretreated im- Chromate treatments are more often used for mediately by one of the following methods: the protection of zinc surfaces against white (1) Wash primer type of organic coating stain rather than as a paint base; to ensure pretreatment described in section 6.3.2.5 and the paintability of a chromate-treated surface, covered by MIL-C-15328 [30]. it is essential that a prepaint type of treatment (2) Chemical coating pretreatment covered be specified [85] by Military Specification MIL-C-5541 [86]. "Home cure" treatments such as washing the (3) Anodic coating pretreatment by the galvanized surface with vinegar, cider, acetic chromic acid process or the sulfuric acid proc- acid, muriatic (hydrochloric) acid, or copper ess, both of which are covered by MIL-A-8625 sulfate solution have been shown to be useless as Type I and T^pe II, respectively or even harmful and should be avoided. [87].

steel wool or wire brushing should be avoided, since it may 6.3.3.2. Old galvanized surfaces that have result in the embedment of fine steel particles in the soft aluminum surface that are difficult to dislodge and may cause serious galvanic weathered for six months or more—so that oils. rusting and pittine.

119 Of the above methods of pretreatment, the 6.3.5. Magnesium and its Alloys wash primer type is best suited for use in the field. It gives excellent results and is widely The corrosion resistance and the paintability used in both field and industrial applications. of magnesium and magnesium-base alloys can For best results, the wash primer should re- be greatly increased by application of a suitable ceive a coat of priming paint within about 1 chemical or electrochemical pretreatment prior to 4 hours—after it is adequately dry but before to application of a suitable primer. (Primers it ages unnecessarily. A zinc yellow primer for use on magnesium surfaces are usually of such as that covered by Fed. Spec. TT-P-666 the zinc chromate type. They must be carefully [88] is highly recommended for use over the selected to provide adequate adhesion (see sec. wash primer on aluminum surfaces. Other 2.4.2.4)). Most of the paint-base treatments primers in which zinc yellow constitutes a ma- are of the chromate type. Before chemical pre- jor portion of the pigment, such as that covered treatment, the magnesium surface should be by TT-P-645 [89], may also be used. thoroughly cleaned to remove all oil, grease, The chemical "films" covered by MIL-C-5541 welding flux, soil, oxides, or other foreign are intended to be chemical pretreatment coat- matter. ings designed to improve paint adhesion and to inhibit underfilm corrosion. Products meet- 6.3.5.1. Cleaning.—Since magnesium, un- ing these requirements^ usually produce a like aluminum, is not attacked by caustic solu- chromate-type coating on the surface of the tions, highly alkaline cleaners similar to those aluminum, either by chemical reaction with used for the heavy duty cleaning of steel (see the surface or by deposition of the coating sec. 6.3.1.4) may be employed, and this is the from solution or combination of the two by a preferred method for cleaning ordinary grease processes. Despite the wide and effective use and soil from magnesium-base surfaces. The of these chromate-type coatings, their exact alkaline cleaner should be maintained at a chemical composition is not reliably established. above 11 for general cleaning and at a pH They have been variously described as chro- above 13 for the removal of graphite lubri- mium chromates, chromate-chromic oxides, and cants ; the latter is accomplished by soaking the chrome-aluminum oxide complexes however, ; part for 10-20 minutes in the hot caustic (about all of them are derived from acidic hexavalent 200 °F [93 °C]), followed by thorough rinsing chromium solutions of the general nature de- in cold water and immersion in a chromic- scribed in section 6.3.6.1, and they all produce nitrate pickle to complete removal of burned-on thin amorphous corrosion-inhibiting films hav- lubricant. Old paint coatings may be removed ing excellent paint-base qualities on aluminum with alkali paint removers such as that covered surfaces. In an older, less widely used in- by Federal Specification TT-R-230 (see sum- dustrial pretreatment process, coatings consist- mary chart in sec. 8.2). ing essentially of zinc and aluminum phosphates are formed on the aluminum surface by 6.3.5.1.1. Electrochemical cleaning consider- treating it with zinc phosphate solutions con- ably shortens the cleaning time. Using an taining fluoboric acid; these are crystalline alkaline cleaning bath at a temperature of phosphate coatings similar to those formed on about 200 °F (93 °C) and a pU above 11, the steel or zinc by other proprietary processes (see parts are cathodes in the bath employing sees. 6.3.2.2 and 6.3.2.3). While many of the made chemical pretreatments which have been men- a current density of 10 to 40 amperes per square foot at 6 volts for 3 to 10 minutes. tioned are designed for industrial use, others may be used in the field; the instructions of the manufacturer should be followed in their 6.3.5.1.2. Alkaline cleaning of any type must selection and use. always he folloived by thorough rinsing in cold The anodic coating type of pretreatment ivater. Alkaline residues are very detrimental (covered by MIL-A-8625), using chromic acid to paint life. Even when acid pickling is to be or sulfuric acid as the electrolyte in an anodiz- used subsequent to alkaline cleaning, it is essential rinse all alkaline or residues, ing bath, is limited to industrial applications. to away soapy The oxide coatings produced by anodic treat- since soaps carried over into the acid bath will liquid ments are denser and less porous than most form an oily layer on the surface of the which will contaminate the work as it is im- chemical conversion coatings, but they may still be improved by sealing with a hot water treat- mersed or withdrawn. oily greasy con- ment containing dilute amounts of chromate or Where large amounts of or sur- dichromate salts. taminants are present on the magnesium face, they should be removed by solvent clean- 1 See Qualified Products List QPL-5541 established for Military ing (see sec. 6.3.1.1) prior to alkaline cleaning Specification MIL-C-5541 by the Bureau of Naval Weapons, Depart- ment of the Navy, copies of which can be obtained by writing the to avoid premature fouling of the alkaline Commanding Officer, Naval Supply Depot, 5801 Tabor Avenue, Philadelphia, Pennsylvania 19120. cleaner.

120 :

6.3.5.1.3. Acid pickling is used to remove designations in the discussion which follows. oxide layers, old chemical finishes, burned-on Before application of chemical treatments, the drawing and forming lubricants, and other surface should be thoroughly cleaned by the water-insoluble or non-emulsifiable substances methods indicated in the preceding section. from magnesium-base surfaces. Chromic acid Five types of chemical pretreatment are de- pickling is preferred for general cleaning and scribed in MIL-M-3171 ; these are summarized for the removal of surface oxidation and cor- below rosion products; this type of pickle does not result in dimensional loss and may be used on 6.3.5.2.1. Type I Chrome-Pickle Treatment parts with tolerance limits. A chromic-nitrate (Dow No. 1) is of general applicability and pickle is used primarily for the removal of is the most widely used chemical treatment for burned-on graphite lubricants. Sulfuric acid magnesium surfaces. In addition to providing pickling is used on magnesium sand castings a suitable base for paint, it is used for the to remove the effects of blasting operations; temporary protection of parts during storage alternatively, a nitric-sulfuric acid pickle may and shipment, and for touching up previously be used for this purpose. A chromic-nitric- treated work. The treatment is applicable to hydrofluoric acid pickle may be used for the all magnesium alloys (including those contain- pickling of castings, particularly die castings. ing steel or brass inserts) when close toler- A strong phosphoric acid pickle also may be ances are not required; the treatment removes used for cleaning all types of die castings. For about 0.0006 inch from the surface of the metal. the removal of mill scale, particularly from The treatment utilizes a sodium dichromate- sheet material, and to ensure maximum cor- nitric acid solution for wrought magnesium, rosion protection, an acetic-nitrate pickle is with the addition of a small amount of sodium, employed; this pickle normally removes from potassium, or ammonium acid fluoride to the 0.0005 to 0.001 inch of metal from the surface. bath for treating molded or cast magnesium. Acid cleaning must always be folloived by thor- Dipping is the preferred method of applica- ough rinsing in cold running tvater. tion, but brushing may be employed on work Further details about the aforementioned too large for immersion. Close control of acid pickling and alkaline cleaning materials immersion time (normally 30 seconds to 2 are given in Military Specification MIL-C-3171 minutes) in relation to bath composition is

[90] ; this specification also describes methods essential. After immersion, the part should be for the chemical pretreatment of magnesium allowed to drain for a few seconds before being surfaces. Useful information on the cleaning washed thoroughly in cold running water fol- and pretreatment of magnesium surfaces may lowed by a hot water dip to facilitate drying. also be found in the publication "Magnesium In brush application, fresh solution should be Finishing" [90a] and in the Metal Finishing generously applied and remain on the surface Guidebook [91]. for at least one minute before being thoroughly washed off with cold running water. Excessive 6.3.5.1.4. Mechanical cleaning methods (see immersion, or failure to keep the surface wet sees. 6.3.1.6, 6.3.1.7, and 6.3.1.8), including with solution during brush application, will sanding, grinding, wire brushing, and grit-, produce a powdery coating that is unsatisfac- shot-, or sand-blasting, may be used on rough tory as a paint base. When properly applied, unmachined castings that are discolored or cor- the Type I treatment produces a matte gray roded. Blasting methods may result in surface to yellow-red, iridescent coating that exhibits a contamination which will greatly increase the pebbled etch finish under magnification. A initial corrosion rate of magnesium. Hence, bright, brassy, relatively smooth surface (the blast cleaning should always be followed by result of excessive nitric acid or nitrate salt pickling in a sulfuric acid or sulfuric-nitric build-up in the bath) is not suitable as a acid pickle until 0.002 inch of surface has been base for paint, although it is satisfactory etched away. for the protection of parts during shipping and storage.

6.3.52. Chemical pretreatments for in- 6.3.5.2.2. Type II Sealed Chrome-Pickle creasing the corrosion resistance of magnesium Treatment (Dow No. 10), while less often alloys and for producing a paint-receptive sur- specified than Type III, may be used for the face are covered in detail in Military Specifica- general long term protection of all magnesium- tion elsewhere MIL-M-3171 [90] and [90a, base alloys when close dimensional tolerances 91]. Since virtually all the processes now in are not required. It provides a good paint base use for the pretreatment of magnesium sur- in addition to its protective qualities. While faces (except the HAE process) were devel- most commonly used for wrought products, it oped by a single manufacturer [90a], these may also be used on castings. The treatment processes have been identified for the sake of is a combination of chrome pickle and dichro- clarity by their commonly known proprietary mate boil. The chrome pickle treatment is

121 applied as described under Type I, except that 6.3.5.2.4. Type IV Galvanic Anodizing Treat- the parts need not be dried after rinsing. The ment (Dow No. 9) may be applied to all freshly chrome pickled parts are then immersed magnesium-base alloys for long term protection for 30 minutes in a bath of boiling sodium and to provide a good base for painting. The dichromate solution saturated with calcium or treatment causes no dimensional change and magnesium fluoride and maintained at a pH may be applied after machining operations. of 4.0 to 5.5. After the immersion, the parts The parts are first cleaned as indicated in sec- should be thoroughly rinsed in cold running tion 6.3.5.1 and then given the hydrofluoric acid water, followed by a hot water dip to facilitate or acid fluoride dip specified in the type III drying. (The dichromate treatment must not treatment. The work is then galvanically ano-

be applied to old chrome pickled surfaces ; the dized for 10 to 30 minutes in a solution contain- old chrome pickle film should have been re- ing 4 oz. of ammonium sulfate, 4 oz. of sodium moved by chromic acid pickling and/or heavy dichromate, and 1/3 fl. oz. of ammonium hy- duty alkaline cleaners as described under Clean- droxide per gallon of solution. The pH of the ing in sec. 6.3.5.1). The coating produced has bath is maintained between 5.3 and 6.0 by addi- the brown color of the dichromate superposed tions of a chromic-sulfuric acid solution con- on the varying matte gray to yellow-red iri- taining 5 percent by weight of each. The descence of the chrome pickle. anodizing is carried out at a bath temperature of 120-140 °F (49-60 °C) and a current density 6.3.5.2.3. Type III Dichromate Treatment of 2-10 amperes per square foot. The tank, if (Dow No. 7) is suitable for the long term steel, is made the cathode; if the tank is non- protection of all magnesium-base alloys except metallic, steel cathode plates must be provided. those containing manganese or cerium, includ- The work is electrically connected to the cathode ing work for which close dimensional tolerances through an external circuit that includes an are required. It provides a very good combina- ammeter and rheostat; the work must not be tion of paint base and protective qualities. The allowed to contact the cathode directly. Ordi- treatment does not appreciably affect dimen- narily, 70 to 150 ampere-minutes per square sions and normally is applied after machining. foot of work is sufficient to produce a uniform Parts containing bearings and inserts of brass, coating. A properly applied type IV coating bronze, cadmium plate, and steel are not affected is usually uniformly black or dark brown in by the treatment, but aluminum is rapidly at- color. Gray and nonuniform coatings indicate tacked during the hydrofluoric acid dip which that articles were not properly cleaned before is an important step in this treatment. Where treatment or that the anodizing solution was aluminum inserts or rivets are used, an acid depleted. After immersion, the parts should fluoride dip is employed instead of the hydro- be thoroughly rinsed in cold running water, fol- fluoric acid dip. The hydrofluoric acid or acid lowed by a hot water dip or exposure to heated fluoride dip both cleans and activates the mag- air to facilitate drying. The specified paint nesium surface (however, the surface should coating (usually a zinc chromate primer) have previously been cleaned by the methods should be applied as soon as practicable after indicated in sec. 6.3.5.1). The hydrofluoric the treated parts are thoroughly dry. acid bath must be maintained between 10 and 20 percent HF; if allowed to become weaker 6.3.5.2.5. Type V Caustic Anodizing Treat- than 10 percent HF, the magnesium will be ment, while of some benefit for the protection severely attacked. It is recommended that of magnesium-base alloys, is no longer recom- wrought alloy containing 3 percent aluminum mended because of the availability of newer and 1 percent zince be immersed for 30 seconds and more satisfactory anodic treatments, such and that all other wrought alloys and castings as those described in the following section. be immersed for 5 minutes at a temperature of 70-90 °F (21-32 °C). After immersion, the 6.3.5.3. Anodic or electrochemical treat- parts should be thoroughly rinsed in cold run- ments are generally considered to provide the ning water to minimize fluoride carry-over into maximum in corrosion resistance and paint the dichromate bath which would render it in- adhesion. Hard coatings with good abrasion operative. After the fluoride treatment, the resistance also can be obtained. Anodic proc- parts are immersed for 30 minutes in a boiling esses produce relatively thick, dense, adherent sodium dichromate solution saturated with cal- protective coatings on magnesium alloys. It cium or magnesium fluoride and maintained is essential that the work be clean and that at a pH of 4.0 to 5.5 (as described for the good electrical contacts be established to ensure

type II treatment) . Properly applied coatings a uniform coating. vary from light to dark brown depending on the -alloy. After the dichromate treatment, the 6.3.5.3.1. The HAE process [92, 93], devel- parts should be given a thorough rinsing in cold oped at Frankford Arsenal, is an electrochemi- running water, followed by a hot water dip to cal process for producing inorganic coatings facilitate drying. having excellent corrosion resistance and paint

122 base qualities on all magnesium alloys. The post-treatment may be omitted and the anodic processing bath contains potassium hydroxide, coating instead rinsed thoroughly in cold then potassium fluoride, trisodium phosphate, alumi- hot water followed immediately by dyeing with num hydroxide, and potassium manganate in special solutions. Type I, Class B is the so- aqueous solution. The bath can be maintained called "low voltage coating" produced by treat- indefinitely by replenishing chemicals. The ment for 15 to 20 minutes in a 140 to 150 °F solution is strongly oxidizing and must not be (60 to 66 °C) bath at 9 volts and a current allowed to come in contact with organic ma- density of about 40 amperes per square foot. terials. The processing tank is made of black The treatment is applicable only to aluminum- iron or unlined steel, with provision for bearing magnesium alloys. The low voltage circulating and cooling the electrolyte. For coating is smooth, olive-drab in color, and the bifluoride-dichromate post-treatment often builds up to about 0.0004 inch but is somewhat specified, a tank lined with polyethylene, a softer and more porous than the Class A coat- vinyl, or other inert material is required. ing. It is given a hot sodium dichromate The magnesium alloy should be suitably (0.3% solution) post-treatment and dried with- cleaned (see subsec. 6.3.5.1) before chemical out rinsing. pretreatment. Heavy oxide films or corrosion Type II, Class A is the heavy, hard, brown products should be removed by a chromic acid coating obtained by a full HAE treatment in pickle (other acid pickles should be avoided). which the work is immersed for 60 to 90 min- Otherwise, a hot alkaline cleaning and cold utes at room temperature at a current density water rinse is usually adequate. Light chro- of 15 to 20 amperes per square foot maintained mate films left on the magnesium surface do over a voltage range from 0 to 85 volts. The not interfere with formation of the heavy coating builds up to about one mil (0.001 (Type II) HAE coating; however, such films inch) in thickness and provides the maximum should be removed from surfaces that are to in corrosion resistance and abrasion resistance receive the light (Type I) HAE coating. The as well as an excellent paint base. Corrosion process utilizes closely controlled alternating resistance is enhanced by post-treatment in a current. Electrical connections to the work and hot sodium chromate solution (1%) or, to an to bus bars must be firmly made with mag- even greater degree, by post-treatment in an nesium connectors and leads ; the portion of the ammonium bifluoride-sodium dichromate bath leads at the liquid-air interface should be pro- (10% bifluoride, 2% dichromate, room tem- tected with vinyl-type electroplater's tape. perature) ; the post-treatment is followed by Parts to be processed are divided into two drying without rinsing. An additional degree groups of approximately equal area, each group of corrosion resistance to salt spray is imparted constituting one of the electrodes. Coatings of if the post-treatment includes artificial aging different hardness and thickness, and of char- for several hours in a humidified enclosure acteristic color, can be produced from the same heated to about 185 °F (85 °C). chemical bath by varying the operating condi- Among the HAE coatings described above, tions (current, time, and/or temperature). the heavy coating with bifluoride-dichromate The current density must be closely controlled post-treatment (Type II, Class A, Grade 3 of throughout the process by the use of suitable MIL-M-45202) is the one most frequently control circuits which increase the voltage as specified and the treatment considered to be required to maintain a constant current as the the most economical and effective as a base for electrical resistance increases with the build-up organic finishes. of the anodic coating. Three classes and various grades of HAE 6.3.5.3.2. Acid Chromate Anodic Process coatings, comprising light and heavy types, (Dow No. 17).—This is a proprietary process are covered by specification MIL-M-45202 [94] which is applicable to all magnesium al- [93]. Type I, Class A is a light, smooth, tan loys. Several classes of these coatings are coating having a thickness of about 0.0002 inch, covered by specification MIL-M-45202 [93]. produced by treatment for 6 to 8 minutes in a The process produces coatings that have ex- bath at room temperature employing a current cellent paint base and corrosion protection density of 18 to 20 amperes per square foot qualities, along with good hardness and abra- maintained over a voltage range from 0 to 80 sion resistance. The process utilizes a hot volts. The anodic coating is then given a hot aqueous electrolytic bath containing ammonium sodium chromate (1% solution) post-treatment bifluoride, sodium dichromate, and phosphoric and dried without rinsing. This type of coat- acid. Either alternating or direct current may ing is particularly useful for providing a good be employed, at voltages up to 110 volts. Treat- paint base and good corrosion protection where ment time depends on the current density (5 tolerance requirements, time or cost limitations, to 50 amp /ft-) and may vary from a minute or lack of substrate rigidity preclude use of to an hour. Three types of coating may be the full, hard brown coating. Alternatively, if produced, depending on the voltage employed; a clear coating is to be applied, the chromate these are: (1) W-volt clear coating used as a

123 . base for transparent lacquers or varnishes to acid, dichromates, chromates, or a combination impart a final metallic appearance similar to of these), a mineral acid (usually sulfuric), clear anodizing on aluminum; (2) 60 to 75-volt and sometimes an organic acid also. The low voltage thin coating (light green, about chromate coatings are formed non-electrolyti- 0.0003 inch in thickness) for a good combina- cally by simple immersion of the parts in the tion of paint base and corrosion protective chromating bath at room temperature or qualities; and (3) 75 to 90-volt regular full slightly higher, for periods ranging from a coating (dark green and about 2 mils in thick- few seconds to a few minutes depending on the ness) for the best combination of abrasion re- type of treatment. Spraying or brushing tech- sistance, corrosion protection, and paint base niques may also be used in some instances qualities. Before treatment, parts should be where immersion is not feasible. The chromate cleaned in an alkaline cleaner (see subsection coatings produced by these treatments are very 6.3.5.3.1) and thoroughly rinsed with cold thin (0.00002 inch or less) amorphous films water. After anodizing, the parts are rinsed with excellent corrosion-inhibiting properties in cold water, followed by a hot water dip or that enhance paint life while protecting the rinse to facilitate drying. (If the anodized metal. parts are to he left unpainted, they should be A number of systems for the preparation sealed by immersion for 15 minutes in an and painting of various metal surfaces are aqueous sodium silicate bath maintained at specified in MIL-STD-171 (ORD) [96]. 200-212 °F (93-100 °C) followed by rinsing in cold water then hot water and drying) 6.3.6.2. Zinc and cadmium.—These surfaces should be thoroughly solvent-cleaned to produce 6.3.6. Other Metals a surface completely free of oil, grease, dirt, processing compounds, and temporary protec- The surface preparation and pretreatment of tive compounds. Vapor degreasing is pre- some of the less frequently painted metals are ferred. The clean surface, if not previously briefly considered in the following subsections. chromated at the factory, should then be given one of the five pretreatments indicated in the 6.3.6.1. General.—Metals that are difficult discussion of galvanized surfaces in section to paint or pretreat chemically should be thor- 6.3.3. (Sec. 6.3.3 also deals with the problems oughly cleaned and, in many cases, roughened involved in the painting of factory-chromated before painting. galvanized surfaces.)

6.3.6.1.1. The cleaning process should remove 6.3.6.3. Copper, brass, and bronze.—The all oil, grease, soil, dirt, and other foreign surface should be cleaned free of all oil, grease, matter from the surface. Solvent cleaning dirt, and other foreign matter and then rough- (see sec. 6.3.1.1) is of general applicability, ened by sanding, light sandblasting, or phos- although other cleaning methods (section 6.3.1) phoric acid etching (sec. 6.3.1.5). Alterna- may be used where appropriate or necessary. tively, the clean surface may be given a wash Sandblasting accomplishes both cleaning and primer pretreatment as described in section roughening; where sandblasting is unfeasible 6.3.2.5, or a chromate treatment such as that or too severe, the use of emery paper can described in section 6.3.6.1. (Also see sec, often provide the degree of roughening needed 4.3.3). for adequate paint adhesion. 6.3.6.4. Silver may be protected against 6.3.6.1.2. The wash 'primer type of pretreat- tarnishing by application of a suitable chro- ment described in section 6.3.2.5, in addition to mate treatment or a protective lacquer or both. its wide and effective use on steel, zinc, and The silver should first be cleaned free of oil, aluminum, is also applicable to other metals, grease, fingerprints, tarnish, polishing com- including cadmium, copper and its alloys, stain- pounds, and other foreign matter. A pro- less steel, nickel, chromium, titanium, lead, and prietary treatment which produces a clear terneplate—but not necessarily under all types chromate coating on silver is available as indi- of topcoats. cated in section 6.3.6.1. The chromate coating affords a good base for a clear lacquer if 6.3.6.1.3. Proprietary chrornate treatments desired. are available [91a, 95] that provide excellent corrosion protection and impart good paint re- 6.3.6.5. Tin should be solvent-cleaned to re-

j ceptivity to surfaces of zinc, cadmium, copper, move all oil, grease, dirt, and other foreign | brass, bronze, silver, aluminum, and magnesium matter from the surface. For hot dip tin (chromate treatments for the latter were cov- no further pretreatment is necessary. Electro- ered in sec. 6.3.5.2). These chromate treat- deposited tin (deposited from an alkaline stan- ments employ acidic solutions containing hex- nate bath) should be treated with a hot chromic avalent chromium ions (derived from chromic acid-phosphoric acid aqueous solution (contain-

124 , ing equal weights of each acid and maintained appearance. The surface should be cleaned and at a pR of 2.0 to 3.0 at 160 to 180 °F [71 to roughened before receiving paint. A wash 82 °C]) before application of a protective coat- primer pretreatment (sec. 6.3.2.5) may be used ing. Baked lacquers make effective coatings. to obtain increased adhesion.

6.3.6.6. Terneplate should be cleaned (usu- 6.3.6.8. Titanium and titanium alloys that ally by solvent cleaning) free of oil, grease, are to receive organic coatings should be sol- dirt, and other foreign matter. If feasible, an vent cleaned to remove oil, grease, and other oxalate or chromate base pretreatment coating foreign matter and then sandblasted. If sand- should be applied. Otherwise, a wash primer blasting is not feasible, the surface should be pretreatment coating (see sec. 6.3.2.5) may be given a wash primer pretreatment (as de- applied to the clean surface. Alternatively, a scribed in sec. 6.3.2.5). clear varnish size coat is sometimes applied to the clean surface before application of a 6.3.6.9. Stainless steel, nickel, and chro- finish coat. Adhesion is improved if the sur- mium, because of their noncorrosive nature and face is roughened before application of organic intrinsically pleasing appearance, are seldom coatings. A conventional iron oxide "roof and painted. When an organic coating is desired, barn paint," such as that described by Federal the surface should be thoroughly cleaned Specification TT-P-31, provides an economical (preferably by solvent-cleaning) to remove all and reasonably durable coating for "roofing oil, grease, dirt, and other foreign matter and tin" (as terneplate roofing is commonly called) perhaps roughened slightly with emery paper to promote adhesion. The wash primer type 6.3.6.7. Lead, while needing little protection, of pretreatment (sec. 6.3.2.5) may also be used is frequently painted to obtain a more pleasing on these metals to obtain improved adhesion.

6.4. Preparation of Concrete and Masonry Surfaces

As with other surfaces, a basic requirement preparing concrete and other masonry surfaces in the preparation of concrete and masonry for for painting. In some cases, as with glazed painting is that the surface be clean: free of asbestos-cement shingle, weathering for sev- dust, dirt, oil, grease, efflorescence, chalk, and eral years is virtually the only practical way to loose material. Additional problems in the obtain a surface suitable for painting. Aging painting of concrete and masonry surfaces, par- permits the concrete to dry out, neutralizes the ticularly when new or insuflliciently aged, arise surface alkalinity, and reduces the inner, free from: (1) unbound moisture within the con- alkalinity through continuing cure and by crete (and in the mortar joints of other moisture-borne migration of alkaline substances masonry) remaining from the original mixing to the surface where they are gradually neu- with water, (2) the presence within the ma- tralized by reaction with the carbon dioxide terial of soluble alkaline substances that are in the atmosphere. brought to the surface by the outward move- With the escape of moisture and the reduc- ment of moisture and deposited as efflorescence, tion in the amount of soluble alkaline sub- (3) possible contamination with form oil or stances within the material, efflorescence de- concrete curing compounds, (4) glazed areas creases and eventually ceases for practical resulting from casting against a smooth, non- purposes. However, efflorescence may recur absorbent form. even after extensive aging if the concrete Depending on the condition of the surface should again become wet internally. and the nature of the coating system to be During weathering, oily contaminants are applied, the proper preparation of concrete and gradually washed and worn away, and the masonry surfaces for painting may involve one surface acquires a weathered roughness that or more of the following steps: (1) aging or provides a good mechanical bond for paint. weathering, (2) cleaning, (3) patching, grout- While all this is well and good, many months ing, and filling, (4) roughening, (5) pretreat- or even years may be required for the practical ment, (6) surface conditioning. These factors completion of this aging process, depending on are discussed in the following subsections, and the thickness and porosity of the material and the extent to which they must be considered in the nature of the service environment. It is applying various types of coating systems is frequently impractical to delay painting for indicated. such long periods of time. Fortunately, although long aging is necessary for the safe 6.4.1. Aging and Weathering application of oil-base paints, there is now available a variety of water- and alkali-resistant Where time permits, aging is probably the types of coatings which require relatively little most convenient and effective single method of aging of the substrate.

125 6.4.1.1. Latex (water emulsion) paints paint. A moderate degree of chalking is not (see sec. 2.3.5), because of their relative in- a problem with oil-base paints, since these sensitivity to water and alkalinity, may be effectively penetrate and bind the chalk. applied on damp, only briefly aged (but not fresh, uncured) masonry surfaces. As a gen- 6.4.2.1. Dust, dirt, and loose material fre- eral rule, at least 4 weeks aging is desirable quently can be removed from the surface merely whenever possible. Although the surface may by bristle-brushing or by hosing down with be damp, it should not be wet when the latex clean water. When necessary, scraping, wire- paint is applied. brushing, or sandblasting may be employed to

dislodge foreign matter ; such mechanical meth- 6.4.1.2. Cement-water paints (see sec. ods should be followed by hosing down with 2.3.9.1) are completely resistant to the moisture water. and alkalinity in concrete and may be applied virtually as soon as the concrete has set. Little 6.4.2.2. Form oil, grease, and other oily con- or no aging is required. taminants are eventually removed by weathering, although many months may be required, 6.4.1.3. Alkali-resistant synthetic resin depending on the degree of contamination and COATINGS (e.g., epoxies, polyesters, polyure- the severity of the weather. In the absence of thanes, and various synthetic rubbers) may be adequate weathering, oily contaminants should applied without long aging of the surface in be removed either by (1) scrubbing with a solu- most instances however, it is usually essential ; tion of trisodium phosphate (4 oz. per gallon of that the surface be dry. The instructions of water) followed by thorough rinsing of the the manufacturer usually provide the best surface with clean water, or (2) etching the guide to the of materials. use such surface with a 5-10 percent solution of muriatic (hydrochloric) acid in the manner described 6.4.1.4. Oil-base and oleoresinous paints below for the removal of efflorescence. When may be safely applied only on dry, thoroughly the latter (acid) treatment is used, it must aged concrete or paints masonry. Where such be followed either by thorough rinsing with must be applied to concrete that has aged less clean water or by neutralization with trisodium than a year, it has been suggested that a [97] phosphate solution or ammonia water (10%) pretreatment solution consisting of an aqueous followed by thorough rinsing with plain water. containing 2 percent zinc chloride 3 percent and The choice as to whether an acid or an phosphoric acid be applied to the surface and alkaline cleaning method should be used, and allowed to dry upon it without rinsing. After whether—in the case of acid cleaning—to use drying, loose salts should light be removed by only a clean water rinse or to neutralize before dusting, but the in- adherent encrustation of rinsing, depends on the nature of the coating soluble salts should be left on the surface. The system to be applied. Since many latex emul- application of dilute zinc sulfate solution to sions are alkaline in nature and therefore concrete to reduce the surface alkalinity, al- acid-sensitive (such is the case with styrene- though in past, is a popular practice the no butadiene latexes and with acrylic latexes), longer considered to be an effective treatment alkaline cleaning or neutralization of acid with because it may leave soluble salts on the sur- trisodium phosphate solution is usually recom- face that may increase the tendency toward mended as a precaution against breaking of blistering of the paint film. the emulsion when it is applied to the surface. On the other hand, this may not be a critical 6.4.2. Gleaning factor with polyvinyl acetate latexes which are normally slightly acidic. With oil-base or oleo- The cleaning methods used should remove resinous coatings, which are alkali-sensitive, dirt, dust, oil, grease, efflorescence, loose chalk, alkaline washes are best avoided. Where spe- and other loose material or foreign matter cialized synthetic resin coatings are to be used, from the surface (see applicable parts of sec. the instructions of the manufacturer should 6.3.1). Latex paints have good resistance to be followed. moisture and alkalinity, but because of their Oily contaminants can be removed from the nonpenetrating nature and poorer wetting surface by mechanical methods if desired. properties they require more thorough surface Traces of oil can be removed by rubbing the preparation than solution-type coatings, par- surface with steel brushes or coarse abrasive ticularly with respect to removal of all loose stones. However, if the surface is generally chalk. Even a moderately chalky condition is contaminated, it is much more effective to not a satisfactory base for latex paint; a suit- lightly sandblast the area to be painted, or able surface conditioner (see sec. 6.4.6) that else to postpone painting until the oil has been will wet and penetrate the chalk must be used removed by the action of the weather. Me- to obtain satisfactory adhesion of the latex chanical cleaning methods should be followed

126 . by hosing down the surface with water to wash powdering. When a latex paint is to be used, away all loose debris. it is preferable to employ a special fill coat made by combining white portland cement, washed 6.4.2.3. Efflorescence, when present, must silica sand, and water with a mixing liquid be completely removed before painting is containing the same latex polymer that is used attempted; otherwise, it will spoil both in the latex paint to be applied. In some cases, the appearance of the paint job and its per- some of the latex paint itself is added to the formance. It has already been indicated that fill coat mixture. Formulas for fill coats are efflorescence is caused by the moisture-borne given in the applicable Federal Specifications migration of soluble alkaline salts from the for latex paints [5, 6, 7]. Such fill coats do not interior of the material to the surface where require damp curing; however, they should be the salts are deposited as a whitish bloom or allowed to dry for 24 hours before painting. encrustation. The removal of efflorescence is Since both fill coats and grout coats such as best accomplished by sandblasting, if possible; those described above are alkaline in nature, otherwise, abrasive stones or wire brushing it is unsafe to use them under oil-base or oleo- may be employed, followed by treatment with resinous paints unless adequate time for aging dilute muriatic (hydrochloric) acid. An acid (at least 90 days) can be allowed (see pre- concentration of 5 to 10 percent is usually em- ceding sec. 6.4.1) ployed, but concentrations as high as 20 percent are sometimes used. The surface is first wet 6.4.4. Roughening with water, then the acid is applied liberally and allowed to remain on the surface for about Some concrete surfaces are so dense, or five minutes. The surface is then scrubbed glazed from having been cast against a smooth with a stiff brush and thoroughly rinsed with or nonabsorbent form, that good paint adhesion clean water to remove all traces of acid. If is difficult to obtain. Such surfaces should be necessary, the treatment is repeated, until all roughened before painting. This is best accom- efflorescence has been removed. A neutralizing plished by light sandblasting where feasible. rinse with trisodium phosphate solution or Alternatively, the surface may be etched with ammonia water is often recommended when a 5 to 10 percent solution of muriatic (hydro- certain types of latex paint are to be applied, chloric) acid. The surface should be wet down as indicated in the preceding subsection. The with water before applying the acid. The acid neutralizing solution, in turn, should be thor- etching process should be continued until the oughly rinsed away with clean water. surface acquires the texture of a fine sandpaper. The surface should then be rinsed thor- 6.4.2.4. Mildew or mold if present (usually oughly with clean water. Where specified for only on previously painted or organically con- the particular type of paint to be used, the sur- taminated surfaces) should be removed by face may be given a neutralizing alkaline rinse scrubbing with trisodium phosphate solution with dilute trisodium phosphate solution or (4 oz. per gallon of water) or with a 5 percent ammonia water, as discussed in the preceding solution of sodium hypochlorite such as the subsection 6.4.2.2 on cleaning, before rinsing common household chlorine bleach, followed in thoroughly with clean water. either case by thorough rinsing with clean When neither light sandblasting nor acid water. etching are feasible, the glazed or dense surface may be roughened by rubbing it with coarse 6.4.3. Patching, Grouting, and Filling abrasive stones.

Large cracks, holes, and other damaged areas 6.4.5. Pretreatment should be repaired by patching with a suitable material such as the commercially available Chemical pretreatment of concrete and ma- cement-sand mixtures, latex-concrete mixtures, sonry surfaces, other than treatment with stucco patch, or other cementitious materials hydrochloric acid as already mentioned for that are formulated for this purpose. Ade- etching the surface or for the removal of quate time for drying or aging of repairs should efflorescence, is largely limited to situations in be allowed before painting. Open-textured which oil-base or oleoresinous paints are to be masonry surfaces such as cinder block should applied to surfaces which have not been ade- be given a grout coat or a fill coat, depending quately aged. In such cases, the pretreatment on the type of coating that is to be applied. A with zinc chloride-phosphoric acid solution de- grout coat consisting of a 50:50 cement-sand scribed above under Aging (sec. 6.4.1) is mixture provides an excellent base for cement thought to be beneficial on the basis of avail- water paints. The grout coat, like cement able evidence [97]. This pretreatment, how- water paint, must be kept damp while curing ever, has not had wide use because only a for the first 48 hours to ensure proper hydration limited amount of oil-base or oleoresinous paint of the cement and freedom from cracking or is used on masonry surfaces today, now that

127 the water- and alkali-resistant latex paints are or multiple layers of weak cement-type paints; available. At best, pretreatment must be con- in such cases, sandblasting is the only effective sidered as less effective than thorough aging method of preparing the surface (also see sec. in producing a paintable surface. 6.6.4.1).

6.4.6. Conditioning of Chalky Surfaces 6.4.7. Summary

The nonpenetrating character of latex paints The nature and degree of surface preparation which gives them their excellent sealing prop- required for concrete and masonry before paint- erties on porous surfaces of concrete and ing depends to a large extent on the nature of masonry is associated with low wetting prop- the coating system that is to be applied. In erties that prevent the development of good ad- all cases the surface should be sound, free of hesion to chalky surfaces. Previously painted dirt, oil, efflorescence, and loose material, and concrete or masonry surfaces that exhibit rough enough to provide for good mechanical chalking not practicable to remove by sand- bonding of the coating. Where practicable, blasting should be brushed as free as possible light sandblasting provides the best surface of loose chalk and then treated with an alkali- preparation; it is not, of course, a substitute resistant, penetrating type of surface condi- for aging where the latter is required. Open- tioner before application of latex paint. A textured masonry surfaces should receive a typical surface conditioner suitable for this grout or fill coat before painting. Cement- purpose may be a relatively low viscosity coat- water paints may be applied on damp, unaged ing based on a tung-oil phenolic resin or a concrete and masonry and on surfaces pre- soya- or linseed-oil alkyd of long oil-length in an viously painted with cement-water paint, even organic solvent thinner, possibly together with if chalking or dusting is present, so long as the a small proportion of pigments and extenders old coating is firmly adherent. Latex paints (also see sec. 2.4.3). A number of proprietary may be applied on damp, relatively briefly aged products for this purpose are available and surfaces that are clean and free of chalk ; when are usually included as part of the manu- all loose chalk cannot be removed, a penetrating facturer's line of latex coating systems. The liquid surface conditioner should precede appli- surface conditioner penetrates and binds the cation of the latex paint. Oil-base and oleo- chalk and provides an adherent base for the resinous paints may be safely applied only on Litex paint. Even the use of a surface con- dry, well-aged concrete and masonry surfaces ditioner is not adequate when the masonry sur- that are not likely to become internally wet face is in poor condition with heavy chalking again during service after painting.

6.5. Preparation of Plaster and Wallboard Surfaces

Plaster and wallboard surfaces require rela- where large cracks or holes are involved, it is tively little preparation for painting; however, desirable to undercut the edges of the area to as in any painting, it is essential that the be filled and to wet the edges with water to pre- surface be sound and clean. Since plaster and vent excessive loss of water from the patching wallboard are porous materials, the clean sur- material into the adjacent plaster. (Very face must be properly sealed to obtain good large holes or areas of loose plaster should, of hold-out and uniform appearance of topcoats. course, be replastered before painting.) After Latex primer-sealers are particularly suitable the repaired areas have dried for at least three for this purpose because of their nonpenetrat- days, they should be sanded smooth with fine ing nature, but their lower wetting ability ne- sandpaper. cessitates a cleaner surface than is required for solution-type coatings. Both latex-type and 6.5.1.1. New plaster, because of its high oleoresinous primer-sealers are discussed in moisture content and alkalinity, should be sections 2.4.4 and 4.5. Application of the allowed to age for 60 to 90 days Isefore appli- proper primer-sealer allows the use of virtually cation of oleoresinous paints. Where latex any type of topcoat. paints are to be applied, at least two weeks of aging is highly desirable, and a month or more 6.5.1. Plaster should be allowed whenever practicable. Apart from aging, new plaster frequently needs only To achieve a good finished appearance, it is to be dusted to prepare it for painting. necessary that cracks, holes, indentations, and other defects be repaired before painting. This 6.5.1.2. Old plaster or plaster that has be- can be accomplished by filling the damaged come soiled should be washed with clean water. areas with spackling compound or patching If a latex primer-sealer is to be applied, it is plaster. To obtain a good anchor for the patch permissible to use a mild detergent such as

128 :

trisodium phosphate solution as a cleaning aid, or plasterboard), and (2) non-alkaline, organic followed by a clean water rinse. Excessive composition boards composed of compressed wetting or soaking of the plaster should be and/or resin-bound wood or cellulose fibers in avoided. a variety of physical forms, and known by such Oily contaminants, if present, must be re- designations as beaverboard, fiberboard, hard- moved to obtain good paint adhesion. Although board, Masonite, pressed wood, etc. this can be accomplished by washing the surface with a detergent solution, it is usually pre- 6.5.2.1. Gypsum wallboard consists of a ferable, especially where an oleoresinous coat- core of gypsum plaster sandwiched between ex- ing system is to be applied, to "dry clean" the terior skins of heavy, felted paper. In dry surface by wiping or washing it with a suitable wall construction, the sheets of wallboard are solvent such as mineral spirits. Wet surfaces butted together and nailed to the studs. The must be allowed to dry thoroughly before appli- nails should be countersunk and covered with cation of oleoresinous paints. Latex paints joint cement. All joints should be filled with may be applied while the surface is still damp, joint cement, taped, and plastered. When dry, but not wet. Painting with latex coatings may, the assembly should be sanded smooth leaving therefore, advantageously begin soon after feathered edges. Care in sanding is required washing of the surface is completed, thus mini- to avoid roughing up the paper surface. After mizing the possibility of recontamination. sanding, the surface should be wiped free of

dust, preferably with a damp sponge ; it is then ready to receive an emulsion-type primer- 6.5.2. Wallboard sealer. Solution-type oleoresinous primer-sealers are not satisfactory for use on gypsum of different kinds of wallboard A number wallboard because they tend to raise the nap wallboard) available (including prefinished are of the outer paper layer. under a somewhat confusing variety of generic and proprietary names. For practical painting 6.5.2.2. Fiberboard, hardboard, and other purposes, these be considered within two may nonalkaline wallboards based on wood or cellu- categories : ( ) which are essen- broad 1 Those lose fibers should have all nails countersunk, tially paper-covered inorganic plasters, fre- the holes filled with spackling compound, and quently alkaline in nature, or which are joined sanded smooth. The surface^hould be brushed or finished with alkaline plasters, as typified or vacuumed to remove all dust and is then by the common gypsum wall board or "dry ready to receive either a latex primer-sealer or wall" construction (also known as "sheet rock" an oleoresinous solution-type primer-sealer.

6.6. Preparation of Previously Painted Surfaces

The amount of surface preparation required (1) Periodic careful inspection of the painted for a previously painted surface depends on the surface, with touch-up painting where needed condition of the old paint, the nature and con- for damaged or exceptionally worn areas; (2) dition of the substrate, and the kind of paint repainting at the right time, when the old paint to be applied in repainting. Surface prepara- is well-worn but not so badly deteriorated as tion is an integral part of the general problem to require extensive or complete removal, and of maintenance painting discussed below. before the substrate itself is exposed to deteriorative influences; (3) adequate surface 6.6.1. Maintenance Painting prepartion which will remove all loose paint and foreign matter, leaving a clean, sound sur- All organic coatings eventually deteriorate face for the new paint; and (4) selection of a or wear away and therefore require periodic suitable recoating system that will bond prop- renewal. Since the service life of the coating erly to the old paint, or to the substrate where will vary widely with the type of coating and it is exposed. Provided that the old paint sys- the nature of the service environment, no fixed tem gave satisfactory service the choice of a interval between repainting can be stated. similar system will assure compatibility. However, when accurate records of paint jobs Repainting at the right time minimizes the are kept, the history can provide a good esti- labor and expense of surface preparation. mate of when repainting may be needed, for Paint Avorn thin but still firmly bonded to the purposes of advance planning. The actual substrate need only be dusted or washed to time to repaint should be determined by peri- ensure a clean surface before repainting. On odic examination of the surface, since the cost the other hand, in situations where the old paint of repainting is closely tied to the condition of has been permitted to deteriorate to the point the coating and substrate. of extensive cracking, curling, or peeling—or The essential elements of an effective and where the substrate has been laid bare and economical maintenance painting program are damaged or corroded—it may be necessary to

129 — expend a great deal of effort and time in com- by washing with solvents as described in section pletely removing the old paint and in cleaning 6.3.1.1. The clean and spot-primed surface is dirt or corrosion products from the substrate. then ready to receive a full coat of paint, pref- In the latter case, the cost of surface prepara- erably of the same type as before to assure tion may be considerably greater than the cost compatibility with the old paint. of paint and painting. When the coating failure is general, as evi- Painting too often or too soon also should be denced by numerous and widely distributed avoided. Not only is this uneconomical but it rust spots, or by widespread peeling, flaking, ultimately results in the build-up of an ex- blistering, or exposure of bare substrate, it is cessively thick coating that may be brittle and necessary to remove the old paint completely prone to cracking and chipping, particularly on and to clean the metal surface properly before dimensionally unstable substrates such as wood repainting can be done successfully. Removal or thin metal. of the old paint and cleaning of the surface can Mildew or mold may form on coatings in be accomplished simultaneously by the methods warm, damp climates or locations. Before of sandblasting, flame cleaning, or alkali clean- repainting is done, the fungus should be re- ing described in section 6.3.1 (subsecs. 6.3.1.8, moved by scrubbing the surface with a solu- 6.3.1.9, and 6.3.1.4, respectively). Alkali clean- tion of trisodium phosphate (4 oz. per gallon ing does not remove rust. Of the several of water) or with a 5 percent sodium hypo- methods of paint removal and surface prepara- chlorite solution such as the common household tion mentioned, sandblasting is by far the most chlorine bleach, followed in either case by effective where feasible. thorough rinsing with clean water. It is desirable to remove mildew or mold whenever it is noticed during the periodic maintenance in- 6.6.4. Previously Painted Concrete and spections, since if allowed to remain it will Masonry Surfaces cause progressive destruction of the coating. The criteria which determine the right time 6.6.4,1. Latex (emulsion) paint.—If the for repainting, and the problems encountered old paint is well-worn but in basically good and procedures to be followed in maintenance condition—that is, free of excessive chalking, painting, are somewhat different for different flaking, blistering, or peeling—the necessary types of substrates in various environments. surface preparation before repainting with Surface preparation for the maintenance paint- latex paint may involve only a thorough washing of the various types of substrate is dis- ing with clean water, or with a detergent such cussed in the following sections. as trisodium phosphate solution followed by a thorough clean water rinse, to remove dust, 6.6.2. Previously Painted Wood Surfaces dirt, oily contaminants, and loose material from the surface. The preparation of previously painted wood Loose or heavily chalked surfaces, particu- surfaces for repainting has been fully discussed larly of cement-type paints, cannot be satisfac- in section 6.2.2. torily repainted. Not only is paint adhesion to such surfaces poor but failure will occur 6.6.3. Previously Painted Metal Surfaces within the chalk layer itself through crumbling and distintegration. Lightly chalked surfaces Preventive maintenance in the form of peri- may sometimes be satisfactorily prepared by odic touch-up painting of localized areas that removing as much chalk as possible by brushing show excessive wear, damage, or rusting is an and washing followed by treatment of the sur- important factor in obtaining maximum service face with a penetrating type surface conditioner life from coatings on metal surfaces. The lo- as described in section 6.4.6. However, the calized bad spots can be cleaned by wire brush- only completely effective way to prepare a ing, then spot-primed and repainted with very heavily chalked surface or a surface with mul- little difficulty or expense. tiple layers of weak cement paint for satisfac- At the stage in the life of the coating when tory repainting is to remove the chalk or it is well-worn but still performing its protec- cement paint thoroughly by sandblasting. tive function in all but a few localized areas, Where sandblasting is not feasible, power wire the protection can be prolonged and the labor brushing methods may be employed, but with and expense of extensive surface preparation greater labor and less satisfactory results. can be avoided by application of a single over- Complete removal of the old paint in the area all coat of paint. In such cases, localized bad of failure is also indicated when the paint is spots should be wire-brushed and spot-primed, blistering, flaking, or peeling. Such failure, and the entire surface should be cleaned free particularly if accompanied by efflore'^cence or of dust, dirt, oil, grease, and foreign matter by discoloration, is usually caused by excessive washing with a mild detergent solution or moisture coming from within the concrete or where considerable oil or grease is present masonry surface. Where the source of the

130 moisture is from outside the masonry (e.g., hydrochloric acid before repainting. Cement- from the ground on the exterior of a wall be- water paints will not ordinarily adhere to or- low grade) then the source will have to be ganic coatings; the latter must be completely eliminated through better drainage or by ex- removed, preferably by sandblasting, before terior waterproofing before successful repaint- repainting. ing can be accomplished. 6.6.5. Previously Painted Plaster and 6.6.4.2. Solvent-thinned paints of the oil- Wallboard Surfaces base or oleoresinous type, while less moisture- and alkali-resistant than latex paints, have The surface to be repainted should be clean better wetting and penetrating properties and and sound. Damaged areas should be repaired therefore require less stringent surface prep- by filling with spackling compound or patching aration. Moderately chalked areas of firmly plaster and sanding smooth. All loose or blis- attached paint (including old but sound ce- tered paint should be removed by scraping and ment-water paint) ordinarily require only the surrounding area sanded to a feather edge. brushing to remove loose chalk and soil from Dust, dirt, and moderate amounts of oily con- the surface. If oily contaminants are present, taminants may be removed by washing with a they can be removed by washing with mineral detergent such as trisodium phosphate solution spirits or other suitable solvent. The surface (4 oz. per gallon of water) . Where consider- must be thoroughly dry when repainting. able oily contamination is present, it is pref- Blistering and peeling or other evidence of erable to solvent-clean the surface (see sec. moisture failure preclude the application of 6.3.1.1) with a grease solvent such as mineral oil-base or oleoresinous paints unless the source spirits. "Dry cleaning" with solvent is espe- of the moisture can be completely eliminated. cially desirable if oleoresinous paint is to be Loose paint in localized areas should be re- applied, since it eliminates the need to allow moved by brushing or scraping. Where the time for thorough drying. If all chalk cannot paint is in generally poor condition, with wide- be removed, an oleoresinous paint may provide spread flaking or peeling, complete removal by better adhesion than the latex type. The lat- sandblasting or power wire-brushing is inter, however, may be applied while the surface dicated. is still damp (but not wet) from washing. When repainting is to be done with solvent- If blistering and efflorescence are in evidence, thinned or 100-percent-solids coatings based the presence of excessive moisture in the wall on specialized synthetic resins, the instructions is indicated, and the source of the moisture of the manufacturer concerning surface prep- should be located and eliminated, if possible, aration should be closely followed. before repainting. If the paint is in poor condition as evidenced 6.6.4.3. Cement-water paints (see sec. by general scaling, cracking, blistering, peeling, 2.3.9.1) may be applied directly over firmly etc., then the old paint should be entirely re- adherent cement-type paints, even if chalking moved with a solvent-type paint remover such or dusting is present; brushing with a stiff as that covered by Fed. Spec. TT-R-251 or brush is usually the only surface preparation Mil. Spec. MIL-R-46073. Such paint removers required. However, if the old cement paint is contain toxic volatile solvents which may also crumbling, flaking, or scaling, it should be be flammable, and they must be used with cau- completely removed by sandblasting, or by tion and adequate ventilation. If the paint re- treatment with hydrochloric acid in the manner mover contains wax, as is usually the case, the described for the removal of efflorescence (sec. stripped surface must be washed off with min- 6.4.2). Old coatings of whitewash (see sec. eral spirits before proceeding with sealing and 2.3.9.2) also should be completely removed with painting.

131 6.7. Summary

The foregoing chapter has emphasized the painted surfaces, the preparation of previously fundamental importance of proper surface painted surfaces has also been considered, in preparation in obtaining maximum service life relation to the general problem of maintenance from the applied coating and has presented painting. When the old paint is completely detailed instructions for the cleaning and pre- removed, the principles of cleaning and pretreatment (if necessary) of virtually all types treating the bare surface are similar to those of surfaces likely to be painted. Cleaning for previously unpainted surfaces. methods have been described in detail primarily As a general rule to be adhered to v^henever in the discussion of steel surfaces (sec. 6.3.1) possible, surface preparation should be fol- where they find their v^^idest spectrum of appli- lowed as soon as practicable by application of cability; thereafter, to conserve space, these the priming paint, before the clean surface can cleaning methods have been referred to as again become soiled or corroded. needed to indicate their applicability to other types of surfaces. The pretreatment of metal The detailed information that has here been surfaces has been comprehensively covered and presented, in conjunction with the general in- includes not only steel but also surfaces of formation given in chapter 4 on the basic types aluminum, magnesium, zinc, copper, and other of surface preparation required or considered metals. adequate for particular types of substrates and While the bulk of the presentation has dealt environments, should provide a practical basis with the preparation of new or previously un- for preparing virtually all surfaces for painting.

Figure 6.1. Glossmeter measures specular reflectance of surfaces and coatings as described in ASTM Method D 523-66T.

132 Figure 6.2. Portable impact- type slipperiness tester for measurement of relative slipperiness of different walkway surfaces for various types of footwear, as described in ASTM Method E 303-66T. A mechanical heel of leather or rubber forms the lower part of the pendulum and sweeps over the surface when the pendulum is released. Height of swing beyond surface indicates "antislip coeflScient."

7. Consolidation of Coatings Information

7.1. A Glance Backward and Forward

Previous chapters of this publication have mary is preceded by a quick guide list that pro- dealt with major areas of the organic coatings vides a key to the information in the tables; field, including: A description of the types of together, they constitute a comprehensive coatings that are available (chapter 2) ; a de- source of information relating to Federal Speci- tailed discussion of the properties of synthetic fications for organic coating materials. resins on v^^hich most modern coatings are Considerable information relating to the na- based (chapter 3) ; presentation of a basis for ture and arrangement of the subject matter the selection of coating systems for specific presented in this publication, and guidance in purposes (chapter 4) ; instructions for the its effective utilization, is given in the Preface storage, safe handling, and efficient applica- and in Chapter 1. A careful reading of these tion of organic coating materials (chapter 5) ; sections will prove very worthwhile. In sec- and detailed information on the preparation tion 1.3 is given a description, with examples, and pretreatment of surfaces that are to receive of the manner in which the Index has been or- organic coatings (chapter 6). ganized to provide maximum coverage and Subsequent chapters present a tabular sum- utility with a minimum number of entries. mary of Federal Specifications for organic The present chapter rounds out the material coating materials (chapter 8), and a selected presented by providing important supplemen- bibliography of v^orks in the coatings field tary information and by furnishing examples (chapter 9). Finally, a comprehensive Index of use of the monograph in solving typical coat- is provided. The aforementioned tabular sum- ing problems.

7.2. Supplementary Information

The following sections present general infor- below the dew point and result in the deposition mation, important details, and helpful hints of dew or frost upon the uncured paint film. not covered elsewhere in the text. While latex paints are not sensitive to moisture in the same sense that organic solvent- 7.2.1. Painting Conditions type paints are, they will not dry well under humid conditions and will not withstand rain- The ideal time for painting is during warm, fall occurring during or shortly after their ap- dry weather. In any case, the painting should plication. Although they may be applied with- be done under favorable atmospheric conditions out difficulty on damp or lightly wet surfaces, that will promote proper drying and avoid con- they should not be applied to materials that are densation of moisture on the uncured film. As water-soaked or to surfaces covered with heavy a rule, exterior painting should be done only dew or with frost. (Cement-water paints pref- in clear, dry weather at moderate temperatures erably are applied to surfaces that have been (45 to 95 °F [7 to 85 °C] are the nominal lim- thoroughly dampened with water, but they can- its usually specified) . When the surface and the not be used in freezing weather.) Latex paints paint are cold, there is a greater possibility of may suffer from poor coalesence at tempera- condensation, drying is slower, the tendency tures below 50 °F (10°C), and their use at to run or sag is greater, and the extra thinner near-freezing temperatures is completely pre- required to reduce the paint viscosity for proper cluded. application may unbalance the formulation The upper temperature limit for satisfactory with adverse ejffects on spreading rate and film painting, while nominally given as 95 °F properties (see sec. 5.3.2 for a discussion of the (35°C), is actually determined by the nature principles of Iproper thinning) . Under high of the coating material. Neither the atmos- humidity conditions, even in warm weather, pheric temperature nor the temperature of the again drying is slowed, condensation is very surface to be painted should be so high as to likely to occur, and the addition of blush-re- cause blistering or pinholing of the paint film tardant thinner may adversely effect the film through too-rapid evaporation of solvent, or properties. Outside painting should never be poor leveling and faulty surface appearance be- done in rain or snow, or when dew or frost is cause of abnormally fast setting of the film. In present on the surface. On days when a sharp practice, this seldom happens except with fast- drop in temperature of 20 degrees Fahrenheit drying lacquers or when paints are applied to or more is expected, it is inadvisable to do any surfaces exposed to the hot sun ; such surfaces, painting later than mid-afternoon. Such a particularly if metal, may be considerably hot- temperature drop will frequently chill the air ter than the ambient atmosphere. It is for this

135 : . reason that painting in the hot summer sun is The dry film thickness can be calculated by not considered to be good practice. multiplying the wet film thickness by the vol- Interior painting can often be done when ume fraction of nonvolatile vehicle plus pig- weather conditions preclude outdoor painting; ment in the whole paint. however, it is generally desirable to do interior In general, the thicker the coating is, the painting during warm, clear weather when greater are the durability and protection pro- windows and doors can be left open to provide vided, up to the point where the coating be- the ventilation needed to prevent dangerous comes so thick and inflexible that it is prone build-up of toxic or flammable vapors. In the to crack and chip away from the substrate. latter respect, latex paints with their freedom Also, several thin coats provide better insur- from fire and toxicity hazards are more suit- ance against pinholes and holidays than a sin- able for year-round painting. gle thick coat because of the improbability that In general, painting should begin as soon as such flaws would occur at exactly the same possible after surface preparation (including point in successive layers of paint. Coatings adequate time for drying) has been completed that serve chiefly a decorative function in mild —before the clean surface can again become service environments need not be as thick as soiled or corroded. In multiple-coat systems, coatings that must withstand rigorous service it is desirable that subsequent coats be applied in corrosive or abrasive environments. For without prolonged delays which might contami- optimum service performance, it is important nate the surface or adversely affect intercoat that the coating be applied at the spreading adhesion. rate recommended by the manufacturer or in the applicable specification, so that the proper 7.2.2. General Coating Requirements final film thickness will be obtained. Recom- mended spreading rates normally take into account the nature of the surface and differences 7.2.2.1. Thickness.—The number of coats in coverage requirements with succeeding coats, of paint required for a particular application as well as the composition and function of the is determined by the film thickness needed to coating material the service accomplish the desired protective or decorative and the nature of environment. While spreading rates therefore function and by the film build per coat. The may vary widely with different materials and latter in turn depends on the spreading rate and applications, the following discussion of typi- solids content of the applied coating. cal surfaces and coating materials may serve as a guide while emphasizing the importance 7.2.2.1.1. Spreading rate is the area covered of adhering to recommended spreading rates. by a unit volume of coating material, usually expressed in terms of square feet per gallon. For adequate film build on rough, porous Spreading rate is directly related to wet film surfaces such as concrete masonry, a con- thickness by the equation and siderably lower (heavier) spreading rate is re- 231.0 "~ quired for the first or primer coat than for 144.0 xS subsequent coats, to allow for absorption and where Tw is the wet film thickness in inches, for the filling of pores and crevices. For exam- and S is the spreading rate in square feet per ple, the priming paint (perhaps a properly gallon. Tables and graphs giving the com- thinned latex paint) might be applied at a puted relationship are readily available in spreading rate of only 20 square feet per gallon, various paint manuals and handbooks [40, 98] while the unthinned finish paint might be For convenience, a few representative values spread at the rate of about 600 square feet per are given in the table below gallon. Wood, also, is a porous material which re- Calculated Calculated quires a lower spreading rate for the priming Wet film spreading Spreading wet film thickness rate rate thickness paint than for subsequent coats. For com- (mils) (ftygal) (mils) (ftygal) monly used paints on exterior wood surfaces, 1.0 1604 1600 1.00 the optimum total dry film thickness is 4 to 5 1.5 1069 1300 1.23 mils (0.004 to 0.005 inch). This is readily 802 1000 1.60 2.0 comprising 2.5 642 700 2.29 achieved with a three-coat system 3.0 535 650 2.47 one primer coat and two finish coats. The 3.5 458 600 2.67 primer is normally applied at a spreading rate 4.0 401 550 2.92 of about 450 square feet per gallon, and each 4.5 356 500 3.21 coat of finish paint at about 650 square feet 5.0 321 450 3.56 6.0 267 400 4.01 per gallon. Two-coat systems (one coat of 7.0 229 350 4.58 primer and one finish coat) capable of build- 8.0 201 300 5.35 ing up a 4-mil dry coating thickness also are 9.0 178 250 6.42 available. Such systems utilize specially for- 10.0 160 200 8.02 150 10.69 mulated paints having a relatively high solids

136 content and applied at a low (heavy) spread- in terms of coating system durability and ap- ing rate. In practice, heavy spreading rates pearance if the paint is not evenly spread. In without the development of runs, sags, or other critical coating applications or where doubt faults are difficult to achieve except by skilled exists, it is advisable to measure the wet film painters working under favorable tempera- thickness at intervals during the painting proc- ture and humidity conditions. ess; also, if feasible, the dry film thickness The spreading rates of paints commonly ap- should be measured in a representative number plied on plaster and wallboard surfaces are, in of locations to provide a final check on the general, similar to the requirements for wood quality of the job. surfaces. On smooth nonporous surfaces such as metal, 7.2.2.1.2. Thickness measurement.—Some of optimum spreading rates may be very different the instruments that are useful for the meas- for different primers and finish coats, depend- urement of coating thickness are described ing on their composition and function and on briefly in the following pages. Detailed discus- the over-all thickness requirements of the ap- sions of such instruments may be found in the plication. Thus, a zinc chromate primer on specific references cited and in references [98] aluminum is most effective when applied in a and [161]. very thin coat (0.4 to 0.6 mil dry film thickness) Wet film thickness can be measured readily at a spreading rate of about 1600 square feet and nondestructively on any reasonably smooth, per gallon, while a zinc yellow-iron oxide flat, rigid surface by standard methods [99] primer on steel may be spread at the rate of employing readily available test instruments, about 650 square feet per gallon, and the primer such as the Interchemical or Pfund gages. The coat in a vinyl resin coating system may be Interchemical Wet Film Thickness Gage is es- applied at only 250 square feet per gallon. Fin- sentially a triple wheel, having an eccentric ish coats in an oleoresinous paint system typi- recessed inner wheel concentric with and af- cally may be applied at a spreading rate of fixed to circular outer rolling wheels ; when about 550 square feet per gallon, while body the gage is rolled over the wet surface, the wet and finish coats in a heavy duty vinyl coating film thickness is indicated by the point at which system may be applied at only 150 to 200 square the paint first contacts the eccentric wheel. The feet per gallon. Dry film thickness require- Pfund Gage utilizes a convex lens of calibrated ments may range from 3 mils in a two-coat curvature, which is pushed through the wet film paint system for mild exposure, to 5 mils or to the substrate; from the known radius of more in a typical four-coat oleoresinous paint curvature and the diameter of the paint spot system for severe atmospheric exposure, and produced on the convex lens, the wet film thick- to about 12 mils in the six-coat vinyl resin coat- ness can be calculated. ing system for steel subjected to severe expo- Dry film thickness on non-metallic substrates sure under continuous immersion in salt water. cannot be measured nondestructively by pres- With so many variables affecting spreading ently available methods. On sheet or panel rate, the value of having and of following a spe- material amenable to measurement by dial gage cification or the manufacturer's recommenda- or micrometer [100, 101], the nominal coating tions is quite apparent. thickness can be computed as the difference be- When repainting, the thickness of new paint tween the thickness of the coated and the un- applied should, ideally, merely replace the coated (or stripped) material. Other devices, paint that has been worn away, to restore the such as the Gardner Scratch (Saw-tooth) original coating thickness. As already noted, Thickness Gage, employ calibrated scratching the build-up of excessive film thickness can re- or penetrating wheels or tools as a means of sult in stresses within the coating and at the measurement. A simple, pencil-like "go-no go" coating-substrate interface that may cause pre- gage [103] equipped with three fine teeth mature coating failure by cracking and chip- scaled to scratch the paint surface to specified ping. Excessive thickness is most likely to depths has been proposed for use in the field cause failure on dimensionally unstable sur- by the Federal Housing Administration. A re- faces, such as wood. When doing interior recently-introduced Paint Inspection Gage [104] painting, it should be remembered that indoor combines a precision V-groove cutting tool with coatings do not wear away like coatings ex- a 50-power gaging microscope that permits ob- posed to the weather, and repeated repainting servation of film thickness and other visual may eventually build up a very thick unsatis- characteristics of the applied film. While these factory film. It is a good rule, when repainting instruments (dial gage and micrometer ex- little-worn interior surfaces, to limit the thick- cepted) for measuring dry film thickness on ness of the applied paint to the minimum needed nonconductive substrates have not yet been to hide the old surface properly. adopted as standard methods, they represent In all painting, care should be taken to ap- useful approaches to a basically difficult prob- ply the coating at as uniform a thickness as lem. possible. Spreading rates have little meaning On metallic or conductive substrates the meas-

137 urement situation is much better. Measure- rent principle are now available which greatly ments of dry film thickness on such surfaces can simplify the above type of measurement while be made non-destructively, with ease and speed, extending the range, speed, and precision of by standard methods employing readily avail- the measurements. Typical of these are the able instruments that operate on magnetic or Boonton, Dermitron, Elcotector, and Perma- electronic principles, in conjunction with refer- scope instruments. They indicate the magni- ence standards of known coating thickness. On tude of the inductive effect related to the coat- magnetic (ferrous) substrates, an instrument ing thickness in terms of an immediate meter such as the Aminco-Brenner Magne-Gage, oper- reading which is readily converted to film ating on the magnetic principle, can be used thickness by the use of known thickness stand- [102, 105]. In essence, this instrument when ards and a calibration chart; or, the meter calibrated measures the force required to pull scale may be calibrated in terms of known a permanent bar magnet from the coated sur- thickness standards to give direct readings of face. Since the magnetic attractive force is a coating thickness. While these modern instru- function of the distance between the bar mag- ments have not yet been incorporated into the net and the magnetic substrate, it is similarly body of standard test methods, evaluative work related to the thickness of the intervening non- now in progress points to their early adoption magnetic coating material, as given by a cali- in this respect. bration curve obtained from measurements on It should be noted that although the eddy standard panels of known coating thickness. current-type instruments were designed espe- While the Magne-Gage is capable of a high cially to meet the need for a nondestructive order of precision, even on very thin coatings, method for measuring the thickness of organic it is basically a laboratory instrument which coatings on nonmagnetic metal substrates, this requires a level, vibratinn-free location that is does not preclude their use on magnetic sub- free of stray magnetic fields. strates or any other type of electrically con- For easy use in the field on magnetic sub- ductive substrate. strates, a variety of less precise but convenient hand-held magnetic-type instruments are avail- 7.2.2.2. Drying time.—The time required^ able. Typical of these is the Elcometer, which for a freshly applied coating to dry sufficiently incorporates a strong permanent magnet and to permit satisfactory recoating and placement utilizes the magnetic flux across an air gap in into service varies widely with the type of coat- the instrument as a measure of the proximity ing and with the prevailing atmospheric condi- of its probe feet to a ferrous surface. A pointer tions. It depends also on the thickness and num- attached to an armature in the air gap responds ber of coats in the coating system and on the to the changes in the magnetic flux with chang- severity of the intended service environment. ing distance, thus indicating the thickness of For example, under favorable atmospheric the interposed coating on a direct-reading cali- conditions, the drying time between coats may brated scale. vary from one hour for a latex paint to 48 hours On nonmagnetic metal substrates, electronic for an oil-base paint; however, considerably instruments operating on the "eddy current" longer drying times may be required for these principle can be used to measure dry coating same materials under conditions of high hu- thickness. Such instruments depend on the midity, low temperature, or poor air circula- effect which the conductive base metal exerts tion. A five-coat paint system would normally on the inductance of a probe coil connected into require a longer drying time between coats than a suitable electronic sensing circuit. The mag- a three-coat system. Coatings destined for nitude of the inductive effect depends on the severe exposure in corrosive or abrasive en- distance between the coil and the metal and is vironments should be more thoroughly cured therefore also related to the thickness of the before being placed into service than coatings intervening coating, as given by a calibration that are to serve under mild conditions. In curve generated from measurements on stand- cases—contrary to usual practice—where quick- ard panels of similar substrate material and drying coatings containing strong solvents are known coating thickness. An early instrument to be applied over slow-curing coatings (e.g., of this type, the Filmeter [106], senses the when a phenolic topcoat is to be applied over inductive effect on the probe coil as a change an oil-base primer), it is essential that the in the frequency of an associated oscillator cir- undercoat be allowed enough additional dry- cuit, as measured by restoring the original fre- ing time to assure a thorough cure in order to quency through "zero-beating" against a fixed avoid wrinkling or lifting by the topcoat. reference oscillator. A dial reading corresponding to the frequency change is converted While it is apparent frcm the foregoing that to film thickness by use of a calibration chart absolute requirements for drying time cannot furnished with the instrument together with be laid down, general guidance is usually pro- reference panels of known thickness. vided in the applicable paint specifications, on Im.proved instruments based on the eddy cur- manufacturers' labels, and by the criteria for

138 stages of drying (dry-hard, dry-through, etc.) base, oleoresinous, and various other types of defined in Method 4061 of Federal Test Method coatings. Many coating systems based on syn- Standard No. 141 and elsewhere. The nominal thetic resins, such as vinyls, neoprenes, epoxies, minimum times for recoating specified in the polyesters, and others, depend upon special Federal Specifications for various organic coat- primers for the development of good adhesion ing materials may be seen from the tabular to the substrate and require close adherence to summary presented in chapter 8. the manufacturer's recommendations regarding While an adequate drying period between materials and procedures to obtain a compati- successive coats of paint is essential, exces- ble coating system capable of maximum per- sively long periods of drying between coats of formance. a multicoat system are undesirable because the Under some conditions it may not be prac- exposed surface may become contaminated or ticable or desirable to adhere to the general the coating may cure to a very hard film with rule of using basically similar materials adverse effects upon intercoat adhesion. throughout the coating system. Such might be the case, for example, when incompletely 7.2.2.3, Coating system compatibility.— cleaned steel requiring an oil-base primer must A compatible coating system is one in which serve in a severe environment involving high each applied coat contributes to the over-all humidity and water immersion conditions for performance capability of the system by bond- which an oil-base topcoat would not be suitable. ing properly to the substrate or coating beneath In such cases, the broad principle of compati- it without the development of adverse effects bility outlined above can still be applied with such as lifting, wrinkling, mottling, bleeding, successful results by selecting a coating sys- etc. Compatibility, in this sense, begins with tem in which there is a progressive transition consideration of the nature and condition of in properties from inner to outer coats. In the the substrate and the selection of a suitable above example of conflicting requirements be- primer coating that will have good adhesion to tween primer and finish coats, satisfactory both the substrate and to the topcoat while pro- results could be achieved by following the viding the particular functional properties oil-base primer with an alkyd varnish base needed on the given substrate (e.g., sealing of intermediate coat and finishing with a phe- wood or masonry surfaces, wetting and cor- nolic varnish-base topcoat, provided sufficient rosion-inhibition of metal surfaces, and so on additional drying time for thorough curing —as described in detail in those sections of this were allowed between each applied coat. volume dealing with coating systems for each An exception to the general rule of choosing important type of substrate (chapter 4) . In- chemically similar materials for multicoat sys- termediate and finish coats should be of a type tems occurs in cases where a dark undercoat that will wet (and perhaps slightly attack) the may attack or be attacked by a chemically sim- preceding coats to ensure good adhesion with- ilar light-colored topcoat, resulting in bleeding out the development of the aforementioned film of dark material into the light coating. In such defects. cases, the remedy is to use a topcoat sufficiently As a general rule, compatibility of a multi- different in chemical nature that it will not coat system can be assured by using basically interact with the undercoat. For example, an the same chemical type of coating material asphaltic coating will bleed into an oleoresinous throughout when practicable. For example, topcoat (both being hydrocarbon in nature) if the nature and condition of the surface have when the asphalt is attacked by the hydrocar- dictated the choice of an oil-base primer (as bon solvents in the oleoresinous coating at the might be the case on hand-cleaned steel), then time it is applied; later, the asphalt may con- it would be good practice to use oil-base inter- tinue to attack and slowly migrate (bleed) into mediate and finish coats, provided the latter the oleoresinous topcoat. On the other hand, a would meet the exposure requirements. On shellac varnish based on an alcohol solvent may clean, sandblasted steel permitting the use of be applied over asphalt without danger of lift- a fast-drying oleoresinous primer, compatibil- ing or bleeding eith,er at the time of applica- ity could be assured by using oleoresinous-type tion or later. When employing different types intermediate and finish coats. An all-latex of materials in the same system, care must be paint system would be a compatible coating sys- taken to ensure adequate intercoat adhesion. tem for masonry. The principle involved in all Materials too widely different in chemical na- these cases is to achieve approximately the ture will not bond properly to each other, and same kind of drying behavior, solvent action, the coating system may delaminate when solvent resistance, and chemical nature in each placed into service. part of the coating system to minimize the A mixed coating system is desirable also in risk of incompatibility. Other examples may cases where the substrate contains extractable be given : Lacquers normally should be applied matter which may bleed into the applied paint. only over other lacquers, since they contain For example, it is not feasible to apply some strong solvents that would severely attack oil- latex paints directly on redwood because the

139 water in the paint may extract soluble colored avoidance of the waste and unsightliness of matter from the wood and cause staining of drips and spillage, and protection of adjacent light topcoats. If an oil-base primer is first or nearby surfaces and objects by the use of applied to the redwood, however, to provide a drop cloths or masking tape as needed. barrier against water-extractable stains, then Finally, the essence of good workmanship latex-type finish paints may be used satisfac- in painting may be epitomized in the instruc- torily. tion to mix thoroughly, thin properly, and ap- In short, compatibility is generally favored ply uniformly at the recommended spreading by employing chemically similar vehicles in rate. each part of the coating system ; however, certain types of different coatings may be inter- 7.2.3. Application of Gorrosion-Inhibitive mixed when required, if precautions are taken Primers to ensure compatibility. Information concerning corrosion-inhibiting 7.2.2.4. Workmanship and general ap- pigments and primers has been given at sev- pearance.—A coating system that has been eral places in the text. The pigments them- carefully selected to meet the needs of a particu- selves and the manner in which they function lar application deserves the careful workman- were described in section 2.3.1.5. Their formu- ship necessary to ensure its full performance lation into primers for various metals was capability. Shoddy workmanship can spoil discussed in section 2.4.2, and their role in both the appearance and the service perform- the composite coating system was indicated in ance of even the finest coating materials. the discussion of Coating Systems for Metals Adequate time, skill, and care must be given given in section 4.3 and its various subsections. to every aspect of coatings application, from The theory of corrosion was considered in the preparation of the surface to application and introductory portion of section 4.3, wherein the cure of the final finish coat. Workmen should galvanic nature of the corrosion process was not only be skilled and conscientious but should pointed out. It remains now to point out cer- also be experienced in the use of the particular tain important principles which must be borne coating materials which they are applying. in mind in the application of corrosion-inhibi- Even basically skilled personnel need instruc- tive primers. tion and training in the use of new or unfamil- When applying corrosion-inhibitive primers iar materials requiring unusual techniques or to metal surfaces, it is essential that the entire precautions before they can be expected to do surface area be coated. Otherwise, the cor- a good job in the field. rosion at small, localized uncoated areas (pin- Good workmanship begins with the realiza- holes and holidays) may be intensified far be- tion that high quality in materials is not a sub- yond that which would have occurred without stitute for high quality in workmanship, and any coating at all. The reason for this lies in vice versa. Nor does skill in using the tools of the galvanic or electrochemical nature of the his trade give the workman the knowledge or corrosion process. Differences in electrical right to alter a carefully designed paint formu- potential arising between dissimilar metals or lation by thinning or mixing on the job in any dissimilar areas in or around the same metal manner except that recommended in the appli- cause a flow of electric current that dissolves cable specification or by the coatings manufac- away the anodic (corrodible) material while af- turer. Indiscriminate thinning or mixing of fording protection to the cathodic material. If materials on the job may later result in a poor the anodic areas are very small compared to appearance or impaired performance even the cathodic area, all the current which flows when no outright incompatibility is noticed at will be concentrated at these small anodic areas the time of the thinning or mixing. and they will rapidly become deeply pitted or A properly applied coating should cover the perforated. substrate uniformly and present a neat, uni- As an example, consider the case of a brass form appearance, free from runs, sags, ridges, valve connected between lengths of steel pipe. lap marks, unnecessary brush marks, or other The steel pipe, being anodic to the brass, will evidences of faulty workmanship. There corrode while protecting the brass. If one should be no pinholes, holidays, mottling, should be tempted to apply a protective coating blush, cratering, dusting, orange peel, or other only to the corroding steel pipe, a situation signs of unfavorable application conditions, could arise in which a pinhole in the pipe coat- improper thinning, or incorrect formulation. ing would become the anode for all the galvanic The finished paint job should show no gouges, current furnished by the much larger brass scratches, thumb or finger prints, embedded cathode, and perforation of the pipe would soon dirt, or other marks of carelessness. result. The best way to avoid such a potentially Good workmanship is fostered by neat work disastrous corrosion situation is to paint both habits and a concientious attitude which as- the anodic and the cathodic surfaces. sure the proper care of tools and equipment, While the above principles have been pre-

140 sented in terms of a coating system employing by forwarding a purchase order to the Speci- a corrosion-inhibiting primer because of the fications Activity, Printed Materials Supply general use of such primers on metal exposed Division, Building 197, Naval Weapons Plant, to corrosive environments, the principles are Washington, D.C. 20407. Non-government ac- equally applicable to any barrier coating ap- tivities may purchase copies of the Index from plied to a metal substrate. the Superintendent of Documents, U. S. Gov- ernment Printing Office, Washington, D.C. 12A. Helpful Hints 20402. The specifications and other documents listed in the Index must be obtained, however, from the Specifications Activity identified The following is a collection of miscellaneous above. Single copies of product helpful hints and useful facts not given or not specifications required for bidding purposes be obtained emphasized elsewhere in this publication. may without charge from GSA Business Service Centers serving regional areas in various parts 7.2.4.1. Wetting properties of common of the country, as identified in the Index of COATINGS.—Among the oil-base, oleoresinous, Federal Specifications. quick guide and vinyl coatings that have proved widely A to and summary of Federal Specifications for organic useful for the protection of steel (see sec. 4.3.1) coating materials are given in chapter 8. there is a progressive increase in resistance to corrosive environments (and, happily, in dry- 7.2.4.2.2, Military Specifications are listed in ing speed) as one progresses from straight oil- the Department of Defense Index of Specifica- base vehicles to alkyd-base vehicles to phenolic- tions and Standards which is for sale base vehicles to vinyls. Unforunately, this [68] by the Superintendent of Documents, Govern- progressive, advantageous increase in corrosion U.S. ment Printing Office, Washington, D.C. 20402. resistance is offset by a progressive decrease in The specifications and other documents listed the surface-wetting ability of these vehicles. in the Index are available to qualified activities Thus, the oil-base vehicles which are the least on request from the Commanding Officer, resistant to corrosive environments are the Naval Supply Depot, 5801 Tabor Avenue, Philadel- slowest drying and have the best wetting prop- phia, Pennsylvania, Attn: Code CDS. Specific erties, whereas the vinyl vehicles which are references to a selected group of Military the most corrosion-resistant and most rapid Speci- fications for organic coating materials are drying have the poorest wetting ability. The given in the bibliography of Chapter 9. lower the wetting ability of the paint, the more thorough must be the surface preparation to 7.2.4.2.3, Federal Test Methods for organic ensure adequate adhesion and durability. coating materials are compiled in Federal Test While one should always select a coating sys- Method Standard No. Paint, Varnish, tem which will have adequate durability in the HI for Lacquer, and Related Materials; Methods of environment to which it is to be exposed, it Inspection, Sampling, and Testing which may is obviously uneconomical to apply the more be purchased from General Services Adminis- corrosion-resistant coating systems in mild en- tration, Business Service Center, vironments where the expense of extra surface Washington, D.C. 20405, for approximately $3.00 each. preparation necessitated by their lower wetting ability cannot be justified on the basis of 7.2.4.2.4, ASTM Test Methods may be pur- increased service life. chased by Government activities through the Federal Supply Schedule (FSC Group 76, Part 7.2.4.2, Sources of specifications.—Some I, Item No. 36-1). Nongovernment activities uncertainty and confusion exist regarding pro- may purchase them directly from the curement sources of Government and other American Society for Testing and Materials, 1916 Race specifications. The following is presented as a Street, Philadelphia, Pennsylvania general guide. 19103.

7.2.4.2.5, ASA Standards* may be purchased 7.2.4.2.1. Federal Specifications are prepared by both Government and nongovernment activi- under the direction of the Administrator of ties directly from the American Standards General Services by appropriate technical Association, Inc., 10 East 40th Street, agencies of the Government and are promul- New York, New York 10016. gated by the Administrator. All Federal Speci- fications, including interim specifications, are 7.2.4.3. Application hints (also see listed in the Index of Federal Specifications, sec, 4.1.5). Standards, and Handbooks [67] which is prepared by the General Services Administration 7.2.4.3.1. When applying multicoat paint sys- of the Federal Supply Service. Government tems, the use of a contrasting shade or color for activities should obtain copies of this Index and the periodic cumulative supplements through * Now called USASI Standards, since the name of the American Standards Association was recently changed to United States of departmental channels where established, or America Standards Institute.

141 . — successive coats will provide visual help in these subjects reference should be made to assuring the uniform and adequate application standard works such as the Gardner Paint Test of each coat. Manual [98], the Book of ASTM Standards, Federal Test Method Standard No. 141, and 7.2.4.3.2. When painting metal, care must be the works of Mattiello. Information is given exercised to avoid leaving any exposed metal or also in the books by Payne, by von Fischer, by holidays which could become focal points for Burns and Bradley, and by others listed in the the start and spread of corrosion. Inadvertent General Bibliography of chapter 9. This man- exposure of bare metal or inadequate paint ual does, however, present information on thickness is most likely to occur in difficult methods for measuring thickness, as an inte- paint-holding areas such as sharp edges, rough gral part of the discussion of coating thickness or unclean welds, sharp prominences, and pits requirements given in section 7.2.2.1. or crevices in the surface. In such cases, careful brush application particularly of the first — 7.2.6. Paint Film Defects and Failures or primer coat—generally gives better results than other application methods because it pro- Paint film defects and failures are beyond vides an opportunity to work the paint into the scope of this volume. This subject has or onto the irregular surface (see sec. 5.4.1) been covered in depth by Hess [107] and in the work of the Research Committee of the 7.2.4.3.3. When applying latex paints, it Federation should be remembered that while these may be of Paint, Varnish, and Production Clubs [108]. The subject also has been con- applied without difficulty on damp or lightly sidered by Chatfield others, in wet surfaces, they should not be applied to [109] and and a number of articles in the literature. Defini- water-soaked surfaces or to substrates contain- tions and pictorial examples specific types ing large amounts of internal moisture, such of of failure (e.g., blistering) as fresh concrete or fresh plaster. Normally, may be found among the Standards of the American Society for at least 4 weeks of drying is desirable before Testing and Materials (see Bibliogra- painting the latter surfaces, to allow for a re- General phy) . pictorial duction of the free moisture and excessive Good examples of paint failures on wood surfaces are given in the paint alkalinity. Under exceptional circumstances, manual of the Corps of Engineers this drying time may be reduced to only two [41] and in the older National Bureau of Standards weeks before applying a first coat of latex paint Paint or primer-sealer, provided that application of Manual [110] which is superseded by the present publication. the topcoat is deferred until the primed plaster or concrete has dried sufficiently by diffusion of moisture through the still-permeable first coat. 7.2.7. Basic Information The application of latex paints has been discussed at several places in the text, including As part of the consolidation of information sections 2.3.5.3, 2.4.3, 2.4.4, 4.4.3, 4.5, 6.4, 6.5.1, which is a prime objective of the present chap- and 7.2.1. ter, it may be useful and appropriate to list- apart from the Index or the Table of Contents 7.2.4.3.4. Vinyl coating systems for the pro- specific sections in the text where important tection of steel under severely corrosive condi- fundamental or basic information is given: tions in industrial or marine environments depend upon special primers for the development Subject Section NATURE of: of maximum adhesion and corrosion resistance. Organic coatings 2.1 The most widely used primer for this purpose Drying oils 2.2.1 is a vinyl-red lead type such as that described Primers 2.4 by Military Specification MIL-P-15929, A zinc FORMULATION of: chromate-vinyl type primer such as that de- Strippable coatings 2.2.7 scribed by Military Specification MIL^P-15930 Latex paints 2.3.5.4 Multicolor finishes 2.3.7 also used. It be emphasized that may be must Fire-retardant coatings 2.3.8.1, .2, .3 both of these primers require the use of a vinyl Textured coatings 4.1.4.5 wash primer or washcoat pretreatment such as FUNCTION of: that described by Military Specification MIL- Pigments in paint 2.3.1.1, .3, .5 C-15328 (see sees. 4.3.1.2 and 6.3.2.5). Functional coatings. 4.6 POLYMERS: Polymeric nature of synthetic resins 3.1 7.2.5 Test Equipment and Test Methods Relation between polymer structure and properties 3.1.1 The subjects of test equipment and test The functionality concept in polymerization 3.1.2 METHODS cover a broad field which (with the Methods of polymerization 3.1.3 exception noted below) is beyond the scope Types of polymers according to structure. 3.1.4 of the present volume. For information on Classification of synthetic resins 3.1.5

142 THEORY: Effects of high energy radiation 4.6.8 Formation of a latex film 2.3.5.5 Factors determining gloss of a coating 4.1.4.4 Theory of corrosion 4.3, 7.2.3 Importance of surface preparation and Principles of proper thinning 5.3.2 pretreatment 6.1 MISCELLANEOUS: Structure and use of index 1.3 Basic objectives and arrangements 1.2 Toxicity of coating materials 5.2.1 Coating system compatibility 7.2.2.3 Quick guide to Federal Specifications for Cost of painting 4.1.6 organic coatings 8.1, 8.2

73. Use of the Text to Solve Specific Coating Problems

The following sections are intended to illus- tions (sec. 8.1.3) reveals a half dozen oil-base trate the usefulness of the monograph in solv- paints. Further reference to the summary ing coating problems by indicating suitable chart of Federal Specifications (sec. 8.2) and coating systems for a variety of typical appli- a glance down the "for use on wood" column cations, together with specific cross-references leads to three exterior oil paints for wood, to the places in the text where this informa- among which TT-P-102 is seen to be appropri- tion can be found. The examples given serve ate for general use in ordinary environments. the dual purpose of furnishing specific solutions The wood surface should be clean and dry to typical coating problems while providing an when the paint is applied; prepare the surface insight into effective utilization of the informa- in the manner described in section 6.2.1, Ap- tion contained in the text. plication by brush (5.4.1) is preferred (see It should be emphasized that the recommen- "application method" column in summary dations made, while assuring a suitable coating charts of section 8.2). The painting should system for the intended application, do not be done in favorable weather (7.2.1). The preclude appropriate modifications or the use undercoat should be applied at a spreading rate of other types of coating systems which may of about 450 square feet per gallon and the be equally suitable. The examples given are topcoats at about 650 square feet per gallon typical and illustrative rather than unique solu- to yield an over-all dry film thickness of 4 to 5 tions to the problem at hand. mils (7.2.2.1). A drying time of 48 hours Before considering specific solutions, it will should be allowed between each coat, as men- be helpful to review the general approach to tioned in section 7.2.2.2 and as specifically indi- the problem of coatings selection which is given cated in the summary chart (8.2) for Fed. Spec. in the introduction and section 4.1 of chapter 4. TT-P-102. The paint should be applied in a The examples to be given represent the practi- workmanlike manner that produces a good final cal application of those basic principles. appearance (7.2.2.4). In using the text, the information on selec- Note: To avoid unnecessary repetition, the copious tion of coating systems will be found in chapter cross-references provided in this first example are not repeated in such detail in subsequent examples. 4, application methods are described in chapter 5, and surface preparation is covered in chapter 6, In all painting operations, the safety pre- 7.3.2. Example No. 2: cautions emphasized in section 5.2 should be Selection and Application of a Coating System kept in mind and adhered to. A quick guide for Structural Steel to be Exposed in a Moderate to and summary of Federal Specifications re- Industrial Environment ferred to is given in chapter 8. In the examples which follow, all the infor- A satisfactory coating system for steel begins mation given may be found in the text, as indi- with the application of a suitable corrosion- cated by appropriate cross-references to spe- inhibiting primer to the clean metal (4.3). cific sections in the text. Exposure in a moderate industrial environment suggests the use of an alkyd resin-base system, 7.3.1. Example No, 1: which is intermediate in durability between a Selection and Application of a Suitable Coating straight oil-base system (for mild exposure) System for New Exterior Wood Siding on a and a phenolic resin system (for severe ex- Home in an Ordinary Urban Environment posure under conditions of condensation or fresh water immersion). To ensure a com- Wood surfaces require the use of a primer patible system (7.2.2.3), it is normally desir- or undercoater for best results (4.2) . For able that the alkyd-base topcoat be applied over exterior use, a specially formulated oil or oleo- an alkyd varnish-base primer. The latter re- resinous type such as that described in section quires smooth, thoroughly clean steel for the 2.4.1 and typified by Federal Specification development of adequate adhesion, as would be TT-P-25 is highly desirable. The undercoat obtained by sandblasting to "white metal" should be followed by one or preferably two (6.3.1.8.1). Assuming that only "brush-off" topcoats of a linseed oil paint (4.2.1). Refer- blast cleaning (6.3.1.8.4) or power tool clean- ence to the Quick Guide to x^'ederal Specifica- ing (6.3.1.7) is feasible in this instance, the

143 . choice of a suitable primer is narrowed to suitable (non-toxic) phenolic varnish vehicle. either an oil-base type or a raw linseed oil- Such a paint is covered by Military Specifica- fortified, long oil-alkyd type, with the latter tion MIL-P-15145 (Formula No. 102). preferred because of its better resistance and its better compatibility with the alkyd topcoat 7.3.5. Example No. 5: to be employed. All the foregoing information Selection and Application of a Coating for a is contained in section 4.3.1.1, which also indi- Cinder-Block Basement Wall cates that suitable primers having this general Select a cement-water paint of the desired composition are covered by Federal Specifica- color (sees. 2.3.9.1 and 4.4), The surface tions TT-P-86, Type II or TT-P-57, Type I. should not previously have been painted with Reference to the summary chart of section 8.2 an organic coating and should be free of chalky also indicates the composition of these primers, or crumbling, old cement paint (6.6.4.3) and further reveals that the TT-P-86, Type II ; otherwise, it must be prepared by power wire- primer is somewhat faster-drying than the brushing to remove all the old coating. TT-P-57, Type I primer. Assuming that the Dampen the surface before applying the TT-P-86 primer is selected for its faster dry- cement-water paint, and keep the surface ing and because of the proven performance of damp while the coating cures, especially for the first red lead-type primers, it is desirable that the 48 hours (4.4.1). cement water-paint, prop- 16-hour nominal drying time given in the chart A erly-applied and cured, provides a very durable be increased to at least 24 hours to ensure com- finish that is permeable to moisture but un- patibility with the alkyd-base topcoat to be ap- affected by dampness or alkalinity. Federal plied. A high quality alkyd gloss enamel Specification TT-P-21 covers a cement-water suitable for the topcoat is described by Federal paint (located reference to "quick guide" Specification TT-E-489, as indicated under by in section with further details in sum- "Intermediate and Finish Coats" in section 8.1.3), mary chart of 8.2) 4.3.1.1.1.

7.3.6. Example No. 6: 7.3.3. Example No. 3: Selection and Application of a Coating System Selection and Application of a Coating System for Exterior Concrete for Galvanized Steel to be Exposed in an Ordinary Environment A latex paint system is suitable for application to a properly prepared exterior concrete Galvanized steel that is well-weathered and surface (4.4). Either an acrylic type such as free of rust, oil, or dirt can be coated with that covered by Federal Specification TT-P-19 conventional paints without undue difficulty, (see summary chart, sec. 8.2) or a polyvinyl but the use of a zinc dust primer such as that acetate type such as that conforming to Federal covered by Federal Specification TT-P-641 is Specification TT-P-55 may be used. The con- recommended (4.3.1.4). In this instance the crete should be allowed to age as long as prac- Type II material, based on a long-oil, linseed- ticable, but not less than 30 days, before paint- modified, alkyd resin vehicle provides a good ing (6.4.1). The surface should be cleaned combination of wetting properties, drying time free of dirt, curing compounds, loose chalk, and (nominally 18 hours as indicated in the sum- other foreign matter by the methods described mary chart for TT-P-641 in section 8.2), and in section 6.4.2. Efflorescence, if present, durability. New galvanized steel must be thor- must be completely removed—by sandblasting oughly washed with solvent (6.3.1.1) to remove (6.3.1.8) when feasible; otherwise, by treat- processing contaminants from the surface. If ment with dilute muriatic (hydrochloric) acid. it is known that the galvanized steel has not re- A primer coat (usually simply a thinned ver- ceived a factory chromate treatment (6.3.3.1), sion of the topcoat) should be applied to the best results will be obtained by use of a cold clean surface, followed by one or two coats zinc phosphate pretreatment of the brush-on of the unthinned topcoat material (2.4.3 and or wash type (6.3.2.2) or by applying a wash 4.4.3). It is usur.lly advantageous to dampen primer type pretreatment coating (6.3.2.5) , be- the concrete surface before applying the primer fore applying the zinc dust primer. The type or primer-sealer (2.4.3, 7.2.1, and 7.2.4.3.3). TT-P-641 material may also be used as the Drying time between coats need be only an topcoat in this system. hour or two in clear, dry weather (summary chart, 8.2), 7.3.4. Example No. 4: Selection of a Coating System for the Inside of a 7.3.7. Example No. 7: Potable-Water Tank Coating System for Gypsum Wallboard (Dry Wall) For the protection of potable water tanks, section 4.3.1.1 recommends the use of a zinc Section 4.5 indicates that a latex system is dust-pigmented primer and paint based on a suitable for use on gypsum wallboard and

144 recommends use of a latex primer-sealer such as gymnasium floor, bowling alley, or areas of that covered by Federal Specification TT-P-650 heavy foot traffic (4.2.2). Epoxy-polyamide to avoid raising the nap of the outer paper coatings also provide good durability in such layer. The primer-sealer may be follovêd vvîth applications. A conventional floor varnish of a topcoat of latex paint conforming to Fed. moderate durability is covered by Fed. Spec. Spec. TT-P-29. Alternatively, oil-base or TT-V-71 (see summary chart, sec. 8.2). alkyd-base paints may be applied over the latex primer-sealer. 7.3.10. Example No. 10: Acid-Resistant Coating 7.3.8. Example No. 8: Selection and Application of a Coating System Consult section 4.6 which indicates outstand- for Aluminum Exposed to a Moderately ing materials for specific functional purposes. Corrosive Atmosphere Acid-resistant coatings are named in section 4.6.1.1. Possible choices are (1) polyvinyl Solvent-clean the surface to remove all oil, chloride coating for resistance to all except grease, and soil (6.3.1.1). Pretreat the sur- strong oxidizing acids, and (2) chlorinated face viîth viâsh primer such as that conforming rubber coating for resistance even to strong to Military Specification MIL-C-15328 in the oxidizing acids. If required, even greater acid manner prescribed in section 6.3.2.5. Follov^ resistance could be obtained with coatings such with a zinc yellow primer (6.3.4) such as that as polytetrafluoroethylene or cholorotrifluoro- covered by Fed. Spec. TT-P-666. The zinc ethylene. chromate primer may be over-coated with any one of a wide variety of coatings, including 7.3.11. Example No. 11: oil-base or oleoresinous types, vinyls, epoxies, Fire-Retardant Coating System cellulose esters, etc., as indicated in section 4.3.3 and as described in chapter 3. The function, performance capability, and classification of fire-retardant coatings are dis- 7.3.9. Example No. 9: cussed in section 2.3.8. A basis for their selec- Coating for Wooden Gymnasium Floor tion is given in section 2.3.8.5, wherein reference is made to a number of Federal and A polymeric finish such as a polyurethane or Miliary Specifications for fire-retardant coat- an epoxy coating will provide excellent dura- ings (see Bibliography, references 19 through bility in a severe service application such as a 24).

7.4. Summary and Conclusion

Preceding chapters of this publication have The two remaining chapters present a quick described its objectives and arrangement, dis- guide and a summary of Federal Specifications cussed the early history of paint and varnish, for organic coating materials, and a selected described the types of coatings that are avail- bibliography of works in the coatings field. able, given the properties and uses of all the While the monograph has endeavored to be important synthetic resins available today, pro- a comprehensive guide rather than a specific directive for the of coatings, vided a basis for their selection, and given use organic much of the information presented information on application methods, surface may be the basis for such instructions, particularly where preparation, and pretreatment. the newer materials are to be utilized. The present chapter consolidates the infor- It is hoped that the publication will serve as mation given and presents important supple- a comprehensive, unifying treatise in the field mentary information. of organic coatings.

145 Figure 7.1. Eddy-current thickness gage for nondestructive measurement of thickness of nonconductive coatings on metal substrates, as described in ASTM Method D H00-67T. (See text, p. 138.)

Figure 7.2. Magne-Gage measures thickness of norv- magnetic coatings on magnetic (ferrous) substrates (see text, p. 138) as described in ASTM Method D 1186-53. (Photo courtesy of American Instrument Co.)

146 Figure 7.3. Interchemical wet film thickness gage, with holder; when gage is rolled over wet surface, thickness is indicated by point of first contact of paint with recessed, eccentric, calibrated inner wheel, as described in ASTM Method D 1212-5U (also, see text, p. 137). (Photo courtesy of Gardner Laboratory.)

Figure 7.4. Balanced-beam scrape-adhesion tester determines load required to remove coating with V-shaped stylus, as described in ASTM Method D 2197-63T.

147 )

Figure 7.6. Sward hardness rocker; the harder the surface, the greater the number of swings obtained before the rocking amplitude decays to a specified level.

( Photo courtesy of Gardner Laboratory. 8. Summary of Federal Specifications for Organic Coatings

This chapter presents a concise summary of merical listing, the classified list provides an important Federal specifications for organic entree to the more comprehensive tabular coating materials in the form of two complemen- summary which follows it. tary tables. The first is a list of letter-number The second table offers a compendium of designations of Federal specifications classified the essential technical according to generic type and use for quick, information contained in a selected list of important Federal convenient reference. While this table is com- specifications plete, at this writing, with respect to organic for organic coatings. The concise tabular ar- coatings, the listing of accessory materials is rangement provides a rapid and convenient limited to a few of the more important thin- means for obtaining at a glance the key in- ners, paint removers, and other auxiliary items. formation in a given specification, or for finding In addition to being more convenient and in- a Federal specification for a coating material formative than a straight alphabetic or nu- to meet the needs of a particular application.

8.1. Quick Guide to Federal Specifications for Organic Coating Materials

The following list gives only letter-number Cold water: P-22, P-23 designations^ of Federal specifications, but it Fire-retardant: P-26, P-34 groups them under generic headings that facilitate the finding of a particular type of ma- Heat-resisting: P-28 terial. The full titles of these specifications may be found in the Index of Federal Specifi- Iron oxide type: P-31 cations [67] . More comprehensive information LATEX—Acrylic: P-19 about many of these materials can be found Polyvinyl acetate: P-55 in the tabular summary of important Federal specifications which follows the classified list. Styrene-butadiene: P-0099 Type unspecified: P-29 8.1.1. Enamels Oil-base (no resin): P-20, P-27, P-61, ALKYD—Gloss: E-489, E-491, E-505, E-560 P-102, P-103, P-104 Semi-gloss: E-485, E-509, E-529 Oil-alkyd: P-53, P-59, P-71, P-81 Lustreless: E-527 Rust-inhibiting: E-485 Oil-base or Oleoresinous: P-14, P-47, P-51 Heat-resistant: E-496 Undercoat: E-543, E-545 Oleoresinous emulsion: P-18, P-88 Floor and deck: E-487 Phosphorescent: P-54 Red-lead-base: P-86 8.1.2. Lacquers Rubber-base: P-91, P-95 Acid-resistant: L-54 Stencil: P-98 Cellulose nitrate: L-26, 1^31, 1^40 Traffic: P-85, P-0087, P-115 Cellulose nitrate-Acrylic: L-50 (aerosol type) Multicolor: L-45 8.1.4. Primers Rubbing: L-57 For Wood: P-25, P-636, P-659 Spraying (general purpose) : L-58 For Ferrous Metals: P-57, P-86, P-615, 8.1.3. Paints P-636, P-645, P-659, P-662, P-664

Alkyd, flat: P-30 For Aluminum and Magnesium and their Alloys: P-645, P-666 Alkyd resin emulsion: P-18 Cement-water: P-21 For Galvanized Metal: P-641 — For Masonry, Plaster, and Wallboard: P-29, 1 For the sake of brevity, the prefix "TT ", which applies to all specifications listed, has been omitted. P-30, P-56, P-650

149 8.1.5. Sealers

Varnish type: S-176 Lacquer type: S-171, S-190

Sealer and lubricant (for wood) : S-180

8.1.6. Stains

Opaque, exterior: S-706 Semi-transparent, exterior: S-708

Interior (oil type): S-711

8.1.7. Varnishes Asphalt: V-51 Damar: V-61

Mixing (for aluminum paint) : V-81 Oleoresinous: V-71 Rubbing: V-86 Shellac: S-300 Spar:V-109, V-119, V-121

8.1.8. Miscellaneous Coatings Calcimine: C-96 Rust-inhibitive compound: C-530

Strippable: C-517 Surfacer: S-810 Vehicle underbody: C-520

8.1.9. Accessory Materials

Aluminum-pigment powder and paste: P-320

Caulking compound (plastic) : C-598 Driers: D-643, D-651

Fillers: F-320, F-336, F-340 Paint Removers: R-230, R-243, R-251 Putty: P-781, P-791 Rubbing Compound: R-771 THINNERS—Dope and lacquer: T-266 Mineral spirits (petroleum): T-291 Mineral spirits (odorless): T-295 Synthetic enamel: T-306 Turpentine: T-801

Notes: (1) For full titles of specifications, or for a straight alphabetic or numerical listing, consult the published Index of Federal Specifications [67].

(2) For more comprehensive information on a selected group of important organic coating Figure 8.1. Improved dip-coater prevents test panel from turning and scraping against sides of narrow materials from the above list, see the tabular sum dip tank. (Roberts, A. G., & R. S. Pizer, Improved mary which follows in Section 8.2 of this chapter. dip-coater, Anal. Chem. 26, 790, Apr. 195i).

150 :

8.2. Tabular Summary of Important Federal Specifications for Organic Coatings

8.2.1. Description and Use (2) To find a specification or a type of coating suitable for interior or for exterior use (or The tabular summary lists the specifications both) on a particular type of substrate. For in sequence by letter and number, so that simi- example, if one needs a coating for interior lar materials are generally grouped together. wallboard, a glance down the "wallboard" It will be noted here and in the Index of Federal column will indicate immediately, by code letter "1", Specifications [67] that Federal specifications a number of potentially suitable materials for organic coating materials (e.g., TT-L-31) from among which a further selection may be bear the general group designation "TT", fol- made on the basis of the additional information lowed by a subgroup letter denoting the general given as to type of coating, nature of vehicle, type of coating (for example, E for enamel, properties, and use. If, for instance, TT-E-543 L for lacquer, P for paint or primer, etc.) and is considered, the table indicates further that finally an identifying number. For each speci- this is an enamel-type undercoat and that the fication, there is tabulated a brief description wallboard must oe sealed with a suitable of the material and its basic purpose, the nature primer-sealer before application of the under- of the coating vehicle, information on which coat. surfaces and under what conditions the mate- (3) To find a specification for a coating in- rial is used, the method of application and tended for a certain type of use, such as an cure, the drying time, outstanding properties oil-base paint (house paint), a floor and deck of the material, and notes on its intended use. enamel, a spar varnish, a latex paint, a primer Basically, as stated in the introduction to for aluminum, a traffic paint, a swimming pool this chapter, the tables may be used either to paint, etc. Such coatings can be found readily obtain at a glance the key information con- by scanning the material description blocks in tained in a particular specification, or to find the first column. a Federal specification for a coating material (4) To locate a coating based on a particular to meet the requirements of a particular appli- type of vehicle, such as a drying oil, an oleo- cation. More specifically, among the ways in resin, an alkyd resin, cellulose nirate, polyvinyl which the tables may be used are the following acetate, rubber, etc. As in (3), this is readily the material described by a (1) To identify accomplished by scanning the material de- specification which only the letter-number for scription blocks in the first column. These designation is known, and to give the essential blocks usually list the basic type of vehicle properties and applications of the material. even when this is not given in the formal speci- For example, the table quickly indicates that fication title or in conventional title indexes. TT-L-31 describes a cellulose nitrate gloss lacquer, alkyd-resin modified, for use as a Alternatively, one may locate specifications weather-resistant protective coating on smooth, relating to the purposes of (3) and (4) above by referring to classified list of specifica- clean, exterior metal; also, that the lacquer is the tions given in 8.1 of this chapter. applied by spraying and air-dries suffici- Section ently for recoating in six hours, plus other The tabular summary charts are on the fol- information. lowing pages.

Figure 8.2. Modem outdoor exposure racks can be set for any angle in a single longitudinal plane. Hinged cross bars are adjustable to retain specimens of different lengths.

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