2 5 2 8 2 5 :: II/F·8 11111 . :: 11111 . 11111 . 3 2 1.0 I~ 11111 . I~ ~1I13.2 .2 I:.i I" I.:: ~III~ W ~1If36 I I~ L:li I::... ~~ 2.0 ...I:: I~ .. ~ ~ L:. 1.1 it:.:L:.U 1.1 La.:.~

4 111111.25 ""'1. 111111.6 111111.25 lilll1.4 111111.6

MICROCOPY RESOLUTION TEST CHART MICROCOPY RESOLUTION TEST CHART NATIONAL BUREAU Of STANDARDS·1963-A NATIONAL BUREAU OF STANDARDS.1963-A foJo l{ '::-3'" f

ADHESIVE BONDING OF

>­ 0:: ~ -1 0') m. ..- Technical Bulletin No. 1~H2 ::::J 0­ en t­ UJ -1 r­ Lr.J C,.) C!:l z: 0 <: c,-) 0 -J

August 1975

U. S. Department of Agriculture Forest Service BONDING OF WOOD

By M. L Selbo, retired, formerly Chemical Engineer, Forest Products Laboratory-Forest Service U. S. Department of Agriculture

Technical Bulletin No. 1512

Washington, D. C. August 1975 Selbo, M. L. 1975. Adhesive bonding of wood. u.s. Dep. Agr., Tech. Bull. No. 1512, p. 124.

Summarizes current information on bonding wood into dependable, long­ lasting products. Characteristics of wood that affect gluing are detailed, as well as types of and processes to be used for various conc\itions.

KEY WORDS: Bonding wood; .tdhesives; glues; glue types; glued products; gluing techniques; glulam.

Oxford No. 824.8

For sale by the Superintendent ofDocuments, u.S. Government Printing Office, Washington, D.C., 20402. Price $1.55. Stock No. 001-000-03382

ii FOREWORD

More than four decades ago Thomas R. Truax wrote USDA Bulletin No. 1500, "Gluing of Wood." In this bulletin, Tru~.x laid down sound principles that have stood the critical tests of time. But adhesive technology has expanded enormously and there are many building blocks to be added to the solid foundation Truax laid down in the 1920's. When Truax' bulletin was published, synthetic adhesives had not been introduced and practically all wood gluing was done with glues formulated or compounded from naturally occurring materials. Some of these glues (based on casein, blood, starch, and animal extracts) are still being used, but in quantities far overshadowed by synthetics such as phenol-, resorcinol-, urea-, and melamine-furmaldehyde resins, as well as vinyl resins of various types. Furniture was the major glued wood product when Truax wrote his technical bulletin; , suitable only for interior use, was in its infancy. Currently, gluing is involved in practically all branches of the wood-using industry. In housing, gluing is employed extensively, particularly in prefabrication, but also on the building site; plywood is mass produced in more than half of the States of the Union. Structural laminated timbers are produced for spans well over 300 feet and for structures as di­ vergent as churches and minesweepers. The technology of adhesives and gluing has come a long way. With some synthetic resins, joinrs can be produced that withstand the ravages of the elements fully as well as wood itself.

ACKNOWLEDGMENT

The author appreciates the cooperation ofgluing firms, associations, and equipment suppliers in providing photographs and granting permission to publish them. Photographs were provided by Rilco Laminated Products; Industrial Machine Company; American Plywood Association; Ashdee Division, George Koch Sons, Inc.; Carter Products Company, Inc.; Chas. Smith Enterprises, Inc.; Newman Machine Company; James L. Taylor Manufacturing Company; Evans Division, Royal Industries; Globe Machine Manufacturing Company, Inc.; Black Brothers; and Toreboda Limcni" (Sweden).

iii Use oftrade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval of any product or service by the U.S. Department of Agriculture to the exclusion of other~ that may be suitable.

Mention of a chemical in this publication does not constitute a recommendation; only those chemicals registered by the U.S. Environmental Protection Agency may be recommended, and then only for uses as prescribed in the registration-and in the manner and at the concentration prescribed. The list of registered chemicals varies from time to time; prospective users, therefore, should get current information on registration status from Pesticides Regulation Division, Environmental Protec­ tion Agency, Washington, D.C. 20460.

Requests fOr copies of illustrations contained in this publication should be directed to the Forest Products Laboratory, USDA Forest Service, P.O. Box 5130, Madison, Wis. 53705.

iv CONTENTS

Page Introduction ...... 1 Wood Properties Important in Adhesive Bonding ...... 4 Density ...... 4 Shrinking and Swelling ...... 7 Adhesives for Wood ...... 10 Synthetic Adhesives ...... 10 Adhesives of Natural Origin ...... 27 Improving Performance of Wood Through Gluing ...... 35 Crossbanded Construction ...... 36 Laminated Construction ...... •...... 45 End and Corner Joint Construction ...... 49 Preparing Wood for Gluing ...... 56 Moisture Contenr ...... 56 Drying and Condiri0ning ...... 58 Storage Before Gluing ...... 59 Surfacing 'Wood For Gluing ...... 60 Machining Special Types ofJoints ...... 61 Cutting and Preparing Veneer ...... 62 Adhesives and Bonding Processes for Various Producrs ...... 65 Plywood ...... 65 Laminated Timbers ...... 69 Furniture ...... 70 Ship and Boat Construction ...... 76 Doors ...... 77 Sparring Goods ...... 81 Particleboard ...... 81 Housing and Housing Components ...... 82 New Products ...... 84 Gluing Operadon ...... 87 Mixing Adhesive ...... 87 Spreading Adhesive ...... 88 Assembling Parts ...... 90 Assembly Time ...... 90 Pressing or Clamping ...... 91 Curing Adhesive ...... 92 Conditioning Glued Products ...... 96 Adjustments in Adhesives and Gluing Procedures ...... 96 Gluing Treated Wood ...... 98 Wood Treated With Oil-Soluble Preservatives ...... 101 Wood Treated With \'V'aterborne Preservatives ...... 10 I Wood Treated With Fire-Retardant Chemicals ...... 101 Quality Control ...... 102 Glossary ...... 109 Index ...... 120

v ADI-IESIVE BONDING OF WOOD

MUS 676 Figure 1.-These glued-laminated beams stretching skyward are but one striking example of today's profitable partnership between wood and adhesives. Soon these beams will hold up the roof of a sports arena.

INTRODUCTION

Bonding of wood with glue is known to common (fig. l)--but the gluing process date back to the Pharoahs and in alllikeli­ has never become static. hood the first use of glue with wood was This publication brings together cur·· much further back in antiquity. Since rent information on use of adhesives for then, glued wood producrs have become bonding wood, so it can serve as a guide

1 in production of more dependable glued bonds between them when cooled to freez­ products. The more important types are ing temperatures. So, an adhesive appar­ emphasized because new synthetics are ently must "wet" wood surfaces and sub­ appearing almost daily and to discuss all sequently solidify to make a strong'bonded synthetic and "natural" adhesives would joint. be an impractical and almost impossible Putting a drop of water on wood and task. observing the rate at which it is absorbed The information presented here is based has been proposed as a test for gluability. on research carried out at the Forest Prod­ This theory holds in most cases; however, UCtS Laboratory and elsewhere, as well as there are exceptions such as southern on the author's experience both in research treated with creosote to an 8 pounds per and production gluing. A list of selected cubic food retention. Actually the pine references follows each major section. was glued adequately to serve more than Factors that affect the adequacy of the 25 years in bridge stringers, yet the oily glue bonds are emphasized, rather than creosote certainly would have made the theories of adhesive bonding which, un­ water absorption test misleading. fortunately, still remain in a somewhat One of the more successful attempts to nebuous state. Even the w 'dd-famous explain adhesive bonding of wood was l scientist Debye steps lightly when ap­ made in 1929 when Truax, 1 Browne, and proaching the subject of adhesion: "The Brouse discussed the theories ofmechanical forces between two molecul('s are supposed and specific adhesion. Further theoretical to consist of a universal attraction, which clarification undoubtedly will evolve. But increases with diminishing distance until in the meantime, some practical engineer­ the two molecules touch." ing principles must be applied to assure Blomquistl states that"... the actual dependability in glue joints. adhesion is more probably due to chemical It is well known that numerous factors or physical forces ..." and"Adhesion is (such as pressure, temperature, and assem­ assumed first to require actual wetting of bly time) play an important part at some the adherend by the adhesive ...." time during the formation of a glue bond. There seems to be general agreement If these factors are controlled within a that a prime prerequisite for good bonding reasonable range about the optimum for is that the adhesive must wet the surfaces each, high-quality glue bonds will result. to be joined. A related example is that But if borderline conditions are used for water generally wets clean, freshly ma­ one or more of these factors~in other chined wood surfaces and also forms strong words, ifno substantial factors ofsafety are

OUT OF CONTROL .,PLANT PRODUCTION employed~then the end results can be , I catastrophic. Also, since the interactions t-.~ . i IN CONTROL , I, f I­ "--1 between the various factors are often ill­ I f defined, aiming toward optimum condi­ I I f I tions is the safest practice. I I I-n-J I In figure 2, good results are indicated I LABORATORY i by the flat (horizontal) portion ofthe curve CONTROL i and decreasing joint quality by the down­ I ward sloping part at left. Under laboratory I I' I I conditions good results can consistently be .--.------'-~__.._._L._...... _ INCREASE IN SAFETY FACTORS-'~- obtained even when operating near the M IJ6 ~40 breaking point of the curve. In plant pro­ Figure 2.-Gluing variables require more duction, the control of the factors is usu­ control in plant production than might ally less exact and variable results may be indicated from laboratory experi­ ments. I See rererence on Page 3.

2 occur (indicated as out of control on the Adhesives available today cover a wide figure), unless greater margins ofsafety are area in properties and performance charac­ allowed. teristics, and the producer of glued wood In bonding wood with adhesives one products must be keenly aware of these must be aware that wood is not a uniform facts when switching from one wood substance, but a complex material that species to another, from one adhesive to varies significantly in many properties­ another, and from one pro :!UCt to another. density, for instance, which may range In general, the serviceability ofa glued from lower than 0.30 to higher than 0.80­ wood assembly depends upon (1) the kind and it would be mere chance if the same ofwood and its preparation for use, (2) .he bonding material and procedure would be type and quality of the adhesive, (3) com­ suitable for the entire range of wood patibility of the gluing process with the species. wood and adhesive used, (4) type of joint Use of adhesives for bonding wood has or assembly, and (5) moisture-excluding increased enormously over the past decades effectiveness of the finish or protective and glued products vary in size from tiny treatment applied to the glued product. wood jewelry to giant laminated timbers In addition, conditions in use naturally spanning hundreds of feet. No single ad­ affect the performance of a glue bond. For hesive has ever been formulated, and prob­ adequate performance, a glue joint should ably none ever will be, that will meet the remain as strong as the wood under the various requirements of all the innumer­ service conditions to which the glued prod­ able applications ofadhesive bonding. It is uct is exposed. If it does not, it becomes therefore important that the user has the the weakest link in the assembly and the proper background information to choose point at which failure first will occur. and evaluate the adhesive best suited for a particular application. " During the more than 30 years the author has been involved in wood gluing­ SELECTED REFERENCES in plywood production, industry adhesive research, and Government research on adhesives and glued prod ucts-great Blomquist, Richard F. changes and progress have occurred in this 1963. Adhesives-past, present, and future. field. The plywood industry, by far the Edgar Marburg Leer., Am. Soc. Test. largest user of wood adhesives, has grown Mater., Philadelphia, Pa. 34 p. to become an extremely important factor Browne, Fred L. and Brouse, Don in the construction field. The structural 1929. Nature of adhesion between glue and laminating industry has also shown a wood. Ind. and Eng. Chern. 21:80-84. Jan. healthy growth as improved glues and de­ Debye, P.]. W. sign information have become available. 1962. Interatomic and intermolecular forces Adhesives for furniture have shifted more in adhesion and cohesion. III Adhesion and and more from those based On the natural­ Cohesion, edited by Philip Weiss, p. 1­ occurring materials to synthetics. The 17. Elsevier Publ. Co., New York bonding of wood with adhesives-gen­ Truax, Thomas R. 1929. The gluing of wood. USDA Bull. erall y far more efficient than the use of No. 1500. 78 p. mechanical fasteners-has made possible a U.S. Forest Products laboratory, Forest Service wide range ofproducts and uses for which 1974. Wood handbook; Wood as an engi­ wood was considered unsuitable a few short neering material. U.S. Dep. Agric., decades ago. Agric. Handb. 72, rev., 432 p.

3 WOOD PROPERTIES IMPORTANT IN ADHESIVE BONDING

Various properties of wood affect its Tahle 1.-Range in specific gravity values I gluing characteristics. Perhaps the most of some common species of wood (continued) important is wood's density, but the amount of shrinking and swelling with Species changes in moisture content is also an im­ Sp. g. portant factor, particularly where long­ term serviceability of glue joints is re­ (A cer sp.); quired. In certain cases, pitch content, Sugar (A. saccharllm) ...... 56 oiliness, and the presence of other exuda­ Black (A, nigrlll1l) ...... 52 Red (A. rllbrlll1l) ...... 49 tion products and extractives also have Silver (A. saccharinllm) ...... 44 some influence on gluability. , yellow (Be/llfa alfeghaniensis) ...... 55 DENSITY Ash, white (Fra:cinm americana) ...... 55 Pine (Pinlls sp.); Longleaf (P. paluSlris) ...... 54 Two blocks of wood of equal volume Loblolly (P. /aeda) ...... 47 may vary a great deal in weight, even ifthe Shortleaf (P. echina/a) ...... 47 blocks are of the same species. Weight of Ponderosa (P. ponderosa) ...... 38 wood is generally expressed either in Eastern white (P. s/robm) ...... 34 Sugar (P. lamber/iana) ...... 34 pounds per cubic foot or as a comparison with the weight of an equal volume of , American (Ulmus americana) ...... 46 water (specific gravity, or sp. g.). , western ...... ,48 Table 1 shows the great range in specific Tupelo, black (Nyssa sylva/ica) ...... 46 gravity among a number of the more im­ Sweetgum (Liqllidambar s/yraciflua) ...... 46 portant commercial species of the United Douglas·, Coast (Pseudo/Jllga menzies;; States. In general, strength properties of var. menziesii) ...... 45 wood increase with specific gravity. In a Hemlock, western (TslIga he/erophyl/o) ...... 42 Yellow-poplar (Liriodendron ll1lipifera) ...... 40 l T

4 r-;SPECIF/C GRAVITY 90 a12_ 0.14 al6 0.18_ a20 0.22 80 a24 0.26 a2e a.30

a.34 70 ---,­

h: ~ ~ ~ 0.50 Q. '­ ~ ~ 50 ~ '-'.\ ("J ~ 40 Vi ~ <:{

30

20r------~~--~b_---

OL--L__ L--L __ L-~~_L--l __~~__~~__~~~~~ o 10 20 30 MOISTURE CONTENT (PERCENT) M 137 285 Figure 3.-Relation between air "pace in woed and specific gravity at various mois­ ture contents.

5

, ; .,;. '<> .' I "-. . '. . • ( • / 0 " - • '" ..." • ~.. ('J' • n • similar manner, the glue.bond quality re­ quired for a dense species is greatet than for a lighter one. Hence, the chartindicates the relative glue-bond quality required for the species listed and for other species falling within the density range given. The solid wood substance of all species has about the same specific gravity (1.45), but in high-density species less of the vol­ ume in the capillary structure of dry wood is occupied by air. As moisture is added to th~ wood, the air space decreases (fig. 3). When wood of different spt:cies is ex­ amined with the naked eye, the ratio of wood substance to air space is not readily seen. Under the microscope, the difference in such characteristics as cell wall thick­ ness, cell diameters, and pore space is easily noticed. Figures 4 to 8 are photo­ micrographs of species covering a wide range in these characteristics. The same magnification is used for each photo. The enlarged cross section of western M 139600 redccdar (fig. 4) shows that slightly more Figure S.-Aspen (quaking), 33 X (wood substance about 27 pet. and air space 73 pet.).

M 139603 M 139602 Figure 4.-Cross sedion of weste", red­ Figure 6.-Douglas-flr, 33 X (wood sub~ cedar, 33>< (wood substance about 22 stance about 32 pet. and air space pd. and air space 78 pet.). 68 pet.).

6 than one-fifth ofthe area is wood substance and the remainder is air space. A transverse section of aspen (fig. 5) indicates this species contains slightly more than one­ quarter wood substance and about three­ quarters air space. Throughout the cell structure of this wood, numerous vessels are about evenly dispersed (diffuse-porous). A transverse section of Douglas-fir (fig. 6) shows this species has about one-third wood substance and two-thirds air space. In Douglas-fir latewood the cell walls are thick and the cell diameters relatively small. In the earlywood the cell openings increase and the cell wall thickness de­ creases. Another diffuse-porous wood, sugar maple (fig. 7), has about 42 percent wood substance and 58 percent air space. One of the most dense native species, hickory, is shown in cross section in figure

M 139604 8. Hickory surpasses practically all other Figure 7.-Sugar maple, 33 X (wood commercial native species in shock resis­ substance about 42 pet. and air space tance and in some other strength proper­ 58 pet.). ties. Hickory averages about 50-50 air space and wood substance. Since hickory is about 50 percent wood substance, com­ pared to 20 percent for western redcedar, it is reasonable to assume that "splicing" of hickory requires different bonding agents and procedures than "splicing" of western redcedar.

SHRINKING AND SWELLING

In ordinary use, wood shrinks as it gives off moisture and swells as it absorbs mois­ ture. These dimensional changes generally put stresses on joints in glued products, the higher the stresses, the stronger the glue joints must be to avoid bond failure. Figure 9 shows the approximate change in volumetric shrinkage of wood of various specific gravities with changes in moisture M 139601 content ftom bone-dry to the fiber satura­ Figure S.-Hickory (true) 33 X (wood tion point (the point at which further in­ substcmce about 48 pet. and air space crease in moisture content causes no swell­ 52 pet.). ing or change in volume). These data also

7 18~~~---4----4----t----+----+----"~--~----~--~

4 I---=-...±-"

3 6 9 ~ ~ m ~ ~ 27 30 MOISTURE CONTENT (PERCENT) M 137286 Figure 9.-Relation between volumetric shrinkage and specific gravity as the mois­ ture content changes, indicate the need for higher quality glue density and greatest shrinkage (white oak) and stronger glue joints as the density and developed the largest amount of failure in shrinkage potential of the wood increase. the glue joints; the joints in the lightest Figure 10 illustrates how joints made species (Sitka spruce) developed the least with three types ofglue performed on three glue failure. Obviously, the glue and species of various densities and shrinking gluing condition that have given excellent and swelling characteristics during three bonds on a light species such as Sitka soak-dry cycles. With each adhesive type, spruce may not be adequate for a dense the joints in the species of the highest wood such as white oak.

8 SELECTED REFERENCES Stamm, A. J. 1946. Passage of liquids, vapors, and dis­ Northcott, P. l. solved materirds rhrough softwood. U.S. 1964. Specific gravity influences wood bond Dept. Agric., Tech. Bull. No. 929. 80 p. durability. Adhes. Age 7(10):34-36. Truax:, T. R., Browne, F. L., and Brouse, D. Perty, D. A., and Choong, E. T. 1929. Significance of mechanical wood 1968. Effect of surface aging and extraction joints for the selection of woodworking rreatmenr on gluability of southern pine glues. Ind. and Eng. Chern. 21:74-79. veneer. La. State Univ. Wood Uril. Note U.S. forest Products Laboratoty, forest Service No. 13, 3 p. La. Sch. Forest. 1974. Wood handbook: Wood as an engi. Rice, J. T. neering material. U.S. Dept. Agric., 1965. Effect of urea·formaldehyde resin Agric. Handb. 72, rev. 432 p. viscosity on plywood wood bond dura­ bility. For. Prod. J. 15(3):107-112.

100 LEGENO: ~ W =WHITE OAI! fZZ3M = ...... _5 =SITKA SPRUCE ~ ... 80 ~ ~ ~ ~ ~ ~ '" :8 ~ 60 i:: ~ ~ ~ 'q ~ ~ 1%~ C:i 40 .... ~ ~~ ~ ~ ~ t%~ ~ 20 i:2l t% W ~ ~~ %~ 0 ~MS ~ II RESORCINOL UREA CASEIN TYPE OF GLUE Figure 10.-Effect of species on glue-joint delamination in gusset-type assembly joints made of white oak, mahogany, and Sitka spruce members and Douglas-fir plywood gussets. The specimens were exposed to three soak-clry cycles (ASTM D 1101-59).

9 ADHESIVES FOR WOOD

Until nearly the middle of the 20th cen­ small proportions to improve working tury, glues based on naturally occurring properties such as viscosity ofthe adhesive. materials were the principal adhesive Walnut shell flour is the most commonly bonding agents for wood. The basic in­ used filler. gredients for these ge.lerally were byprod­ Extenders are low-cost materials (wheat uCts of meat processing (for animal and flour, for example) added to resins to re­ blood glues), or ..:asein, soybean, and duce the cost of the adhesive. Highly ex­ starch. tended urea resins are ofter used for ply­ In the early 1930's, synthetic resin ad­ wood where low moisture resistance or hesives began to appear on the woodwork­ durability is adequate. When phenol ing scene; because of their versatility and resins are used for bonding interior-type othe.r advantages, they found widespread plywood, they are commonly extended use in the woodworking industry. Some with materials such as lignin, dried blood, synthetic resin adhesives, when properly and specially treated Douglas-fir bark. used, will produce joints that remain as The chief advantage of some synthetic strong as the wood even in unprotected resin adhesives is their excellent durability, exposure to the weather. More of them, making glued wood products serviceable and most of the "naeural" glues, will pro­ under more severe exposures than was duce adequate joints for normally dry in­ possible with the nonresin glues. When terior use. properly used, most synthetic adhesives are capable ofproducing side grain-to-side SYNTHETIC ADHESIVES grain joints as strong in shear as the wood itself with most species native to conti­ The more important adhesives for wood nental United States. Some are capable of are cl!rrently produ.ced by chemical syn­ maintaining their strength under prac­ thesis. The general synthesis of resin glues tically any condition ofservice where wood is discussed in numerous textbooks and is a suitable material. Others have only other publications and will not be repeated moderate resistance to heat or moisture or here. Production details may vary among both and are not suitable for critical or manufacturers and are usually not dis­ severe use conditions. Between these two closed except in the patent literature. extremes in durability, a wide range of A hardener or setting agent is usually synthetic resin. adhesives is available. required to convert synthetic adhesives Development of synthetic resin ad­ from liquid to solid. These agents may be hesives has facilitated manufacture of furnished separately for addition to the many important glued wood products. resin before use, or they may be present Among these are laminated bridge tim­ (particularly with spray-dried powdered bers, ship keels and frames, and other resins) in the resin as supplied. Hardeners laminated members for use under severe sometimes fall in the class of catalysts service; plywood for boats, signs, railroad which ip.crease the rate of curing but are cars, and other exterior uses; and com­ not consumed in the reaction. ponents for houses and similar structures. Use offillers with synthetic adhesives is Most of the synthetic woodworking rather common. Fillers are generally inert adhesives currently in use set or cure by material' "hat are added to the resins in chemical reaction. The rate ofcuring, like

10 Figure 11.-Test for evaluation of glue-bond durability in plywood. The Forest Service maintains four such test areas to determine durability of adhesives under different climatic conditions. One test area is near Madison, Wis.; one south of Olympia, Wash.; one at San Joaquin Experimental Range, Calif.; and the fourth at the Harrison National Forest, La. Various types of glued wood products are exposed at each site. that of all chemical reactions, depends on The following generalizations are based rem perature, in this case rhe rem perature on numerous exposures ofglued specimens, of the glue. Raising the glue line tempera­ both laboratory-controlled and exposed to ture speeds the rate of curing as well as the weather (fig. 11), and on service rec­ the rate of srrength development of the ords from various parts of the country joint. This property is used ro advantage in (fig. 12). high-frequency heating, steam-heated While the synthetic re3:I'IS can be cc;')­ platens, and other means of heating in sidered in a variety of groupings, they are high-speed production processes. discussed here under general headings. One of the most important differences These include phenolics, ureas, melamines, among the various types of resin adhesives polyvinyl resin emulsions, hot melts, is their durability, or resistance to deterior­ epoxies, conwcrs, mastics, and various ation under various service conditions. For combinations of specific adhesives. some types-the phenols, resorci nols, ureas, melamines, and polyvinyls­ Phenolic Resins considerable data and service records indi­ cate their durability. With other types, Phenolic resins are formed by reacting laboratory data and experience are much formaldehyde with phenol in what is called more limited and hardly justify long-term a condensation reaction. These may be con­ performance forecasts. sidered in four categories: High-tempera­

11 M1Nl2Z Figur~ 12.-Partial view of 11 creosoted laminated southern pine bridge stringers in!talled on the Texas & Pacific Railroad near Woodlawn, Tex., 1944. No joint s()paration or other sign of deterioration is apparent. The light gray material shown on the stringers is sandy silt that has seeped down from the ballast above.

tun:~setting phenolics, iucermediare­ of chI.' phenolics is generally similar, this temperature-setting phenol ics, resorcinols, phase is summarized after the individual ilntI phenol-resorcinuls. Because durability types are discussed.

12 HIGH -TEMPERATURE-SETTING spread use in structural plywoud and mher PHENOLICS applications. The alkaline phenol resin adhesive~ normally cure at 2650 to 310° F. As a Phenol-formaldehyde adhesives were result, their use is restricted almost first introduced in film form (about 1920). entirely to gluing the more durable types In production of this type of adhesive, the of plywood and related thin products that resin is deposited on a tissuelike paper and can be heated co these temperatures in a the solvent removed by drying. The film practical time period. Phenol resins have form of phenolic was particularly con­ been formulated to cure at temperatures venient for making plywood from thin as low as 2400 F. for hot pressing. bur veneers since no increase in moisture such formulations are nOt in common use content was involved. in the United States. As softwood plywood approached maSS The major use of liquid phenol re~in production status, phenolic resins became "u­ hesives is for bonc.ling exterior softwood available in liquid form, making extetiot plywood, incluc.ling boat hull ply\\Jod and plywood a reality. This development per­ products for other marine uses. Their m itted application of adhesive by roll u~t: in plywooc.l manufacture is mnn: spreaders and variation in spreads as limited. Liquid phenol resim formlllattu veneer quality required. specially for sofrwood plywoou produui,m Phenolic resins are also available as are generally quitt.: reactive:; .IS a result tl:q spray-dried powders to be mixed in water have relatively shllrt storage lives. \X'b, re or water-alcohol solutions before use. longer stotage is n:quired, the powdered These phenolic resins (both liquid and film phenol resins arc: otten us<:J. They arc prt­ form) require heat for curing to complete pared for use by dissolving in W.lter \If i\1 polymerization; thus they prompted de­ water and alcohol and in Slime c.l~es lll.•} velopment of hot presses. require adJition of separate hardenu;.. Neat phenolic resins as used in the earli­ est production of exterior plywood had a Phenol resins are: commonly used \\ ith tendency to "bleed-through" or penetrate some walnut shell flour or other filler. I q( the wood excessively, particularly in loose­ withour extenJers where highest joim cut veneer. Incorporation of fillers such as durability is required. In recent years, LOn· walnut shell flour or powdered oat hull siderable amounts of ioce;rior-t),pe $I:-ft­ residue produced a more workable adhesive wood plywood also have been made witil by reducing penetration. Small amounts phenol re"in to which wete added t:lirlr of wheat flour or heat-treated dry blood high proportiuns uf exrenuers and fjJkr~ also have been used with phenolic resins, such as ground bark, wood or walnut ~hdl reportedly decreasing the cure time. Re­ flour, dry soluble blood, or certain other cently, it has been reported that neat agricultural residues. These phenul ad­ resins, with higher solid contents applied hesives replaced conventional protein ~lu<,s with thinner spread than the filled resins, used [or interior plywood for scv<:r,ll are performing very satisfactorily and also decades. permit higher moisture concent in veneers. Phenol r~sins can be formui.tted, within The phenolic resins used for making limits. to suit rhe manuf".lcturing cp(;fa~ plywood are generally alkaline and require dons of the gluet! products. The <.:urinil high temperatures for proper curing. Acid cycle or length ofth~ prcssure [1~rwd in the phenolic glues were also developed to set at hot press can be concmlleJ ar le.lst Far­ moderate-to-room temperatures, but acid dally by che proper amounc .lnd type of types have not found volume ust!. The ex­ catalyst and by the way in which the H'sitl cellent durability ch.lracteristiCS of the is made. Assembly periods depend some­ alkaline phenolics prompred their wide­ what on reactivity ofthe adhesive, but thei

13 can generally be controlled within practical content with this type of glue is about 8 limits. With some formulations for high­ percent. speed, flat plywood production, assembly periods as long as 15 minutes at 700 to 800 F. are permissible. For other formu­ INTERMEDIATE-TEMPERATURE­ lations, an aU-open assembly period of SETTING PHENOLICS several hours or days can be allowed, as may be necessary in bag opera­ The intermediate-temperature-setting tions where a long layup period is un­ types of phenol resin adhesives were de­ avoidable. Adhesives for bag molding (see veloped as durable glues that could be Pressing and Clamping) must permit as­ cured at 2100 F. or less, such as in heated sembly of nearly tack-free, glue-coated chambers or electrically heated jigs. vene:ers and yet later flow adequately when Special formulations of phenol resins were heat and pressure are applied in the final offered for thi~ purpose, being more reac­ curing operation. tive at the lower curing temperatures be­ The dry film form of phenol resin ad­ cause of rather highly acid catalysts. Thus, hesive is well adapted to gluing thin, and this type ofadhesive has often been referred particularly crotch, veneers because there to as acid-catalyzed phenol resin. Some is no problem involved in controlling formulations are suitable for gluing ply­ 0 spread. Moreover, the danger of bleed­ wood at temperatures as low as 80 F. if through is almost nil. Since the film the pressure periods are overnight or longer weighs about 12-1/2 pounds per 1,000 and ifseveral days of additional condition­ feet and approximately one-third ing are allowed before subjecting the ply­ of this weight is paper, a relatively light wood to severe service. spread of resin is obtained when a single The acid-catalyzed phenol resin ad­ sheet is used per glueline. If film glues hesives have been used to a limited extent are to be used successfully, the veneer fot gluing sandwich panels, prefabricated must be well cur, smooth, and uniform in house panels, and truck panels. They are thickness. Since the film glue conta.ins normally supplied as liquid resins with the little or no water, all moisture needed for acid catalyst furnished separately for addi­ softening the resin and providing the tion at the time of use. Acid-catalyzed necessary flow during pressing must come phenol resins do not glue as well on wood from the veneets. For this reason, the con­ at 6 percent moisture content as at 10 to trol of moisture content in the veneers is 12 percent. even mOte critical with a phenol resin film Since the introduction of the resorcinol adhesive than with conventional glues and phenol-resorcinol resin adhesives, the applied in liquid form. acid-catalyzed phenol resin adhesives have not been extensively used, although they Film adhesives normally do not give are generally somewhat cheaper and are good results on veneer at moistute contents lighter colored than the phenol-resorcinol of less than 6 percent. The most satisfac­ and resorcinol resins. The acid-catalyzed tory moisture content of veneer for gluing phenol resins are not considered as durable with phenol resin films varies somewhat as the resorcinol resin types for long-time with the species and veneer thickness; in severe service and elevated-temperature general, good results are obtained in the exposures. range of 8 to 12 percent. Too high a moi:·ture content may cause blisters, ex­ cessive bleed-through, and starved joints; RESORC/NOLS one that is toO low usually results in dried joints of low strength. For furniture and Adhesives based on resorcinol-formalde­ similar interior uses, optimum moisture hyde resins were firSt introduced in 1943.

14 Almost immediately they found wide glueline temperature has been necessary. application in gluing laminated members If facilities for raising the temperature of such as keels, stems, and frames for naval the gluelines co such levels are not avail­ vessels, and for assembly gluing and able, adequate bonds can be obtained by laminating in wood aircraft where the extending curing periods. combination of high durability and These glues, as well as phenol-resorcinol moderate-temperature curing was' ex­ modifications, have earned an outstanding tremely important. reputation for performance under severe The resorcinol resins bear many resem­ service conditions. Bridge timbers lami­ blances to phenol resins. A principal nated with them are still in excellent con­ difference is the greater reactivity of the dition after more than a quarter century of resorcinol resins, which permits curing at service. Laminated oak timbers for mine­ lower temperatures. Resorcinols are sup­ sweepers have gained a similar reputation. plied in two components as a dark reddish Resorcinol resin adhesives are not affected liquid resin with a powdered, or at times by the commonly used preservative treat­ with a liquid, hardener. These glues cure ments, which permits treating the glued at 70° F. or higher, but usually are not timbers for long-term service. They also recommended for use below 70° F. with bond treated wood, making it feasible co and generally require somewhat treat the and then glue the assem­ higher cure temperatures with dense hard­ blies co the desired size and shape. This is . Straight resorcinol resin adhesives particularly advantageous where large, have storage lives of at least a year at 70° curved members that cannot be treated in F. Their working lives are usually from 2 cylinders are involved. Special formula­ to 4 hours at 70° F. tions have also been developed for gluing Assembly periods are not coo critical fire-retardant-treated wood. on softwoods as long as the glue is still fluid when gluing pressure is applied. On dense , such as oak, the assem­ PHENOL-RESORCINOLS bly period must be adjusted (usually ex­ tended) ro give a rather viscous glue at the Phenol-resorcinol resins are modifica­ time the assembly is pressed. One- co 2­ tions of straight recorcinol resin adhesives hour assembly periods have been used wirh produced by polymerizing the twO resins good results when gluing oak at 70° co (phenol- formaldehyde and resorcinol­ 80° F., but the actual assembly time for formaldehyde). The principal advantage a particular formulation depends on the of the copolymer resins over straight resor­ age and viscosity of the glue at the rime cinol resin is their significantly lower cost, of spreading as well as the temperature, because the price of phenol is much lower absorpciveness of the wood, and other than that ofresorcinol. This COSt advantage factors. Resorcinol adhesives are ideal for apparently is achieved without any signifi­ laminating large timbers that require con­ cant losses in joint performance. For wood siderable time co assemble and bring under gluing, the volume of phenol-resorcinol pressure. Very short assembly periods with used now far exceeds that ofstraight resor­ dense woods can result in "starved" joints cinol. Proportions of the two resin com­ and should be avoided. ponents in the copolymer are not generally Resorcinol resin glues will cure ade­ revealed by the manufacturers. Like their quately on thin plywood and other light components, the copolymer resins are dark constructions of medium- co low-density reddish liquids and are prepared for use by species at about 70° F. For such high­ adding powdered hardeners. The hard­ density hardwoods as white oak, used for eners generally consist ofparaformaldehyde laminating ship timbers and similar items, and walnut shell flour, mixed in equal curing for several hours at about 150° F. parrs by weight.

15 Phenol-resorcinol resins generally have or more at 1500 F. as when it was heated shorrer storage lives than straight resorci­ for about 2,500 hours at 800 F. nol resins, usually somewhat under 1 year Originally, resorcinol and phenol­ at 700 F. Many manufacturers formulate resorcinol adhesives were rather costly, phenol-resorcinols to fit prevailing tem­ which limited their use almost exclusively perature conditions-fast-setting resins to the most severe service conditions. for cool weather, somewhat slower setting Within recent years, however, the price ones for warmer weather, and still slower has come to about a third of what was ones for hot weather. This is particularly common several decades ago. helpful for the laminating industry, where the curing time could become prohibit­ DURABILITY OF PHENOLIC RESINS ively long with a slow-setting resin during cooler weather and the permissible assem­ The durability of moderately alkaline bly time could be difficult to meet with phenol resin, resorcinol resin, and phenol­ fast-setting resins during hot weather. resorcinol resin adhesives is essentially Research has shown that, as the curing similar. When these glues are properly temperature is increased, the required used, they are capable of producing joints curing period decreases logarithmically. that are about as durable as the wood itself Figure 13 shows the relation [)2tween under various severe service conditions curillg temperature and curing time for studied. Properly made joints will with­ fi'!~ different resorcinol and phenol­ stand, without significant delamination or resorcinol adhesives. With glues A, C, and loss in strength, prolonged exposures to E, about the same joint quality could be cold and hot water, to alternate soaking obtained in laminated white oak when the and drying, to temperatures up to those glueline was heated for about 6 minutes that seriously damage the wood, to high

10 CURINIJ TIME {HOURS}

M 11224; Figure 13.-Minimum curing times required at different temperatures for five resor­ cinol and phenol-resorcinol glues on white oak. A and Bare phenol-resorcinols; C, 0, and E are resorcinols.

16 relative humidities where many untreated Urea Resins species decay, and to outdoor weathering without protection from the elements. U rea-formaldehyde resin adhesives The joints between lumber laminations came on the market in the middle to late or plies of plywood made with these ad­ 1930's. By using different types and hesives will not separate when exposed to amounts of catalyst, they can be formu­ fire. The glues are not weaken'" -l by fungi, lated either for hot-pressing or for room­ bacteria, or other micro-organisms and are temperature curing. They are compatible avoided by termites. These adhesives, with various low-cost extenders or fillers, however, do not offer any significant pro­ thus permitting variation in both quality tection to the adjacent wood. Conse­ and cost. Even the hot-press formulations quently, wood products glued with these set at appreciably lower temperatures than adhesives should be considered no more the alkaline phenolic adhesives. Being decay- or insect-resistant than solid woods light in co lot or slighdy tan, urea adhesives of the s':rne species. form a rather inconspicuous glueline. But Glued wood is subject to shrinkage exposure to moist conditions, and particu­ stresses and, even if the glue joints are larly to warm, humid surroundings, leads durable, splitting and checking might to deterioration and eventual failure of occur adjacent to or away from the glue urea resin adhesive bonds. Durability of joints. For severe service, therefore, it is urea-resin adhesives is summarized at the important to employ treatments that pro­ end of this section. tect against wood-degrading organisms Major uses for urea resins are in hard­ and also impart water repellency, thus re­ wood plywood, particleboard, and furni­ ducing shrinking and swelling stresses In ture manufacture. They arc also available the glued member. in the retail trade for home workshop use. Completely cured phenolic-type glue Urea resins are generally marketed in joints (made with neutral or moderately liquid form (as water suspensions) where alkaline resins) are highly resistant to the large-scale use is involved and shipping action of solve~ts, oils, acids, alkalies, distances are not excessive. They are avail­ wood preservatives, and fire-retardant able with solids contents from about 40 to chemicals. Thus, in general, well-made 70 percent. They are also marketed as dry joints bonded with phenol, phenol-resorci­ powders, with or withour catalyst in­ nol, and resorcinol resin glues are difficult corporated. The powdered ureas are to destroy without destroying the wood prepared for use by mixing with water or itself. However, as with other types of with water and catalyst if the catalyst is glues, joints poorly made with these supplied separately. In general, powdered durable adhesives may fuil in service. urea resin adhesives with separate catalysts have longer storage lives than the liquid Acid-catalyzed phenol resins have urea resins or the powdered types with shown good durability in such applica­ catalysts incorporated. tions as bonding honeycomb paper core to plywood faces of sandwich panels. With Urea resins are generally more versatile dense species, such as white oak, much than some other resin adhesives; the same lower shear strength was obtained with resin, as received from the manufacturer, acid-catalyzed phenol glue than with can be used for either hot-pressing or room­ phenol-resorcinols. Under exposure to temperature cure by addition of the proper elevated temperatures such as 158° F., catalyst. Some manufacturers, however, the joint quality was reduced more than for supply different resins for hot-pressing and the conventional alkaline phenol resin for room-temperature curing. Sp,c ial adhesives. formulations have been developed for SHe h

17 uses as high-frequency curing and for tape­ Urea resins are normally not recom­ less splicing of veneers. mended for use on wood below 6 percent Urea resin adhesives can be extended in moisture content. This limitation with cereal flour to reduce cost where the appears to be related to the porosity of the Jomt strength and durability attainable species and to the rate at which moisture with unextended glue are not required, is absorbed from the adhesive by the wood. such as in mild exposures with low­ shrinkage woods. Wheat and rye flours are most commonly used for extenders, and ex­ HOT-PRESS UREA RESINS tensions up to 100 parts by -;"'~;ght of flour to 100 parts resin solids (100 pet. Hot-press urea resin adhesives are 0 0 extension) are used with room-temperature­ normally cured at 240 to 260 F. curing formulations for bonding hard­ Assembly periods vary considerably for the different formulations. Many typical ad­ wood plywood. Extensions up to 150 parts flour per 100 parts resin solids (150 pct hesives are formulated for assembly periods excension) are sometimes used in hot­ of 10 to 30 minutes, but special formula­ pressing hardwood plywood. tions may permit assembly periods of 24 hours or more. Because of their relatively Various grades of wheat flour affect the high reactivity, some urea resin adhesives working properties of the adhe~ive differ­ precure on hot cauls or platens before full ently, particularly consistency and ren.­ gluing pressure is applied. This can be dency to foam, and may also influence the avoided by use of cooled cauls and proper effect of the catalyst used. Flour exr.ension sequence in the spreading and press­ genera.lly makes the adhesive more viscous; loading operations. the degree of change depends on the Urea r(;·,,(ns cure much faster than amount and type of flour and also on the phenol resins at the same temperature. protein content of the flour (formaldehyde When this advantage of ureas is added to reacts readily with protein). their lower COSts and lack of color, they are atrracrive for gluing furniture and A small amount ofsodium bisulfite (1 to architectural plywood for interior use 2 pet. by weight) is sometimes added to where the greater durability of the phenol the flour during mixing with the resin to resin glues usually is not required. Typical help overcome differences in flours and to recommendations for curing hot-press urea reduce the amount of additional water resins are 2 to 5 minutes for panels with that might otherwise be needed to make a a total thickness of 3/16 inch or less and spreadable mixture. The addition of 8 to 10 minutes for I-inch panels with sodium bisulfite may affect the catalyst about 1/2-inch cores when rhe platen system of some adhesives, and the user temperature is 2600 F. and one panel is should obtain the recommendations of the glued per press opening. glue supplier for the type and amount of For certain products and service condi­ extension suitable for a particular product. tions, the durability of hot-press urea Extended glues often require somewhat resins can be improved by adding more heavier spreads and shorter assembly durable resins or special resin-forming periods than the corresponsing unextended ingredients. These additives are generally glues. referred ro as fortifiers and rhe resultant Urea resin adhesives for edge gluing, glues as fortified urea resin glues. The most assembly, veneer splicing, and laminating widely used fortifiers are the melamine of furniture parts are not normally ex­ resins, but crystal resorcinol has also been tended with cereal flours, but they do employed. The amount of fortifier varies con.tain some walnut shell flour as filler considerably. Under more severe condi­ to improve working properties. rions, including outdoor wearhering of

18 plywood, durability has generally im­ temperacure and somewhat on the mois­ proved as the amount of fortifier is in­ cure content of the wood, amount of ex­ creased. Because of the special interest in tension, and the amount of glue spread. melamine-urea resin adhesives, these ace The minimum pressure period depends described sepacately. No entirely adequate on the type ofGlued product and 1.lpon the room-temperacure-setting fortified urea temperacure of the wood and the room, for resin glues have yet been introduced for these temperacures control the speed of the industrial use. curing reaction. Because ofslow heat trans­ fer through the wOOci, room-tern perature­ setting urea resin glues generally cure in­ adequately if the glue is spread on cold ROOM-TEMP~RATURE-SETTING wood and then clamped for only a short UREA RESINS time at 70° to 80° f. At 70° F. a pressing period ofat least 4 hours is gener­ Urea resins classified as room-tempera­ ally required on thin, flat members, such ture-setting are formulated ro cure at as plywood, and at least 6 to 8 hours is temperacures of 70° F. or higher. They required on heavy or curved members. were the first synthetic adhesives devel­ Longer pressing periods ace generally re­ oped for practical use at n:>rmal mom quired for heavy species than for lighter temperacures. They were extensively used ones. In no case should pressure be released in assembly gluing of aircraft pacts, truck until the squeezeout is hacd. body pacts, and similar items before the Room-temperacure-setting urea resin introduction of the more durable room­ formulations ace often used in special heat­ temperacure-setting resorcinol resin glues. curing operations with heated jigs and In addition to their use in cold-pressing high-ftequency curing to get faster setting plywood (with hydraulic presses, I-beams, than is possible with conventionai hot­ and retaining clamps to maintain the press formulations. But ifassembly periods pressure after removal from the press), ace excessive, the adhesive may precure be­ room-temperacure-setting ureas ace now fore gluing pressure is applied. A room­ used for edge gluing on clamp cacriers, temperacure-setting glue must be fluid at in various assembly operations, and for the time pressure is applied to adequately laminating furnicure parts. Their availabil­ transfer glue to the un spread surface. ity in small retail packages as dry powders These glues will hacden at temperacures that require only the addition of water below 70° F. but at a very slow rate, makes them very convenient for small job and joints with erratic strength and dura­ and home workshop uses. bility may result. Curing below 70° F. The working life of a glue of this type is is therefore generally not recommended. usually from 3 to 5 hours at 70° F. and In special application's where rapid less at higher temperacures. Special slow­ strength development at room tempera­ acting catalysts increase the working life ture is of primary importance, the normal of room-temperature-setting urea resins room-temperacure-setting urea resin for­ during hot weather and make them more mulation without catalyst may be applied practical for use during summer months .in to one wood surface and a strong acidic plywood and other commercial applica­ catalyst applied to the mating surface. tions. Sometimes the liquid catalyst is applied in Assembly periods with these adhesives advance and air dried. Tho, joint is then ace fairly short, usually with maximum assembled and. pressure quickly applied. closed assembly of 30 minutes at 70° F. The sepacately applied catalyst is assumed fot critical applications. The maximum to penetrate into the glue and to cause permissible assembly period depends on rapid setting. Such strong catalysts cannot

19 be incorporated in the glue before spread­ order of decreasing durability, by the ing because they shorren pot life. hot-press urea resins, room-temperature­ This technique is referred to as the setting urea resins, and highly extended "separately applied catalyst process." urea resins. Tests made with hot-press urea vPhen properly conducted, it results in resin glue extended with rye flour showed rapid developll'ent of joint strength, thus that the wet joint strength falls off slowly permitting a shorter pressing period than as more flour is added. No important de­ with adhesives having catalyst mixed with crease in wet joint strengths was apparent the resin. The process has not been widely until 50 to 100 percent of extender, based accepted, however, because it is difficult upon the weight of dry resin, had been to obtain uniform mixture of catalyst and added. Under dry conditions, the dry joint resin. Uneven penetration results in erratic strength decreased still more slowly, and joint quality; moreover, the highly acid joints containing twice as much rye flour glueline does not seem to be as durable as as resin exhibited high strength. gluelines made with adhesive of the same Under conditions conducive to develop­ type without separately applied catalyst. ment ofmold and other micro-organisms, joints made with extended urea resin glues also lost strength more rapidly than unex­ DURABILITY OF UREA RESINS tended glues. The loss was noticeable with as little as 10 percent flour and particularly In general, well-made urea resin glue rapid for glues having gteater extensions. joinn develop high original dry strength Tests with preservatives showed that the and wood milures with almost all U.S. mold resistance of flour-extended urea commercial species, good resistance to con­ resins could be increased by adding chlor­ tinuous soaking in cold water, and mir inated phenols to the glue in amounts resistance to continuously high relative equal to 5 percent of the weight of the humidity and alternatively high and low flour. (Concentrations lower than 5 pet. relative humidity. Nevertheless, a com­ appeared to offer less protection, but there bination of high relative humidity and seemed to be little advantage in increasing high temperature deteriorates urea resin the concentration above 5 per.) These glue bonds in a relatively short time. preservatives seem to delay the effect ofthe Resistance to cyclic soaking and drying micro-organism damage, but they are exposures is reasonably good if the test unable to prevent it over long periods of pieces are plywood or thin members, but exposure to high moisture conditions. only moderate to low resistance is obtained Thus, except for highly fortified types, ifthe pieces are. heavy laminations ofdense the urea resins as a group are low in dura­ wood. This applies to shorr-term expos­ bility under conditions involving high ures. Over the long term, urea resin glue temperatures and humidities. At high joints deteriorate under the exposures men­ temperatures and extremely low relative tioned, and the rate of deterioration usually humidity the joints are more durable, but increases with the density of the species. this is mainly of academic interesr because Urea resin glue bonds are generally des­ such conditions rarely exist where glued troyed by boil tests (generally 4 hr. boil­ wood products are used. Gradual weaken­ ing, 20 hr. drying 0 at 145 ± 50 F., ing of room-temperature-setting and hot­ 4 hr. boiling, cooling, and testing wet, press urea resin glue joints occurs lmder Prod. Std..PS 1-66) 0 as used for glue-joint dry conditions at 160 F. A much less evaluation of exterior-type plywood. significant weakening of room-tempera­ The fOrtified urea resins are more dur­ ture-setting urea resin glue joints has able under practically all of the exposure been observed in birch plywood under conditions named. 0 They are followed, in continuous exposure at 80 F. and 65

20 percent relative humidity. The rate of they have not been widely used. Some of strength loss is increased by high humidiry the high-remperature-setting melamines at 80" F. will cure adequately at tem":,,ratures from Delamination usually occurs within a 1400 to 1800 F. if the curing period few hours in boiling water. Urea resin is extended to 10 hours or more. Lamin­ bonds tend to break down at tempera­ ated Douglas-fir beams bonded with these tures that char wood; therefore, when cer­ glues and cured overnight at 1400 F. tain urea resin-bonded are ex­ have shown excellent performance in posed to fire, even for short periods, the outside exposure for up to 20 years. plies may delaminate. Plywood panels Most of the melamine resin glues are made with unfortified room-temperature­ marketed as powders that are prepared for setting and hot-press urea resin adhesives use by mixing with water and sometimes have shown considerable delamination with a separate hardener. Those using after 2 to 3 years of outdoor exposure at hardeners or catalysts will set much more Madison, Wis. Panels made with fortified rapidly or at lower temperatures than urea resins have shown much less delami­ those cured withour hardeners. There have nation in the same length of time. Under been indications, however, that the exterior exposure and where high tempera­ catalyzed melamines do not have the same tures with or without high relative resistance to weather that the uncatalyzed humidities are involved, urea resin glues ones have. are markedly less durable than phenol, Pure melamine resin adhesives are al­ resorcinol, and melamine resin glues. (This most white, but the addition of filler does not imply that melamine glues have usually gives them a light tan color similar the same durability characteristics as to the urea resins. The filler is usually phenul and resorcinol resins.) walnut shell flour, but occasionally wood Admittedly, urea resin glue joints (in flour is used. unfinished spec imens-no lacquer, Melamine resins have been used to a , or ) have shown much larger limited extent for gluing hardwood ply­ decreases in strength than phenol, resorci­ wood where the darkness of phenol resins nol, phenol-resorcinol, and melamine is objectionable and durability approach­ resin glue joints after several years' expos­ ing that ofphenol resins is required. Mela­ ure to less severe laboratory-controlled mine resins are consider:tbly more ex­ conditions. Nevertheless, high-quality pensive than phenol or urea resins. urea resin glue joints do appear to be Uncatalyzed melamine resin glues also sufficiencly durable for nonstructural have been investigated for gluing heavy interior applications within the !',uman laminated ship timbers at curing tempera­ comfort range of temperature and humid­ tures of 1400 to 1900 F. On Douglas­ ity conditions. On the other hand, particu­ fir they showed promising results, bur on larly with high shrinkage, dense species, oak the glue bond deteriorated when the the more durable resin adhesives would specimens were soaked in salt water assure longer trouble-free service life. (simulating sea water) for 15 years. Current commercial applications of mela­ Melamine Resins mine resin glues in structural wood lamin­ ating include bonding interior finger Melamine resin adhesives are normally joints, laminated decking, and laminated of the hor-press type, curing at 2400 to beams with 60:40 melamine-urea combin­ 2600 F., similar to the hot-press urea­ ations and high-frequency curing. The resin glues. Special formulations have melamine resins have been used success­ sometimes been offered for curing at fully in high-ftequency edge gluing where temperatures as low as 1400 F., but a durable, colorless glueline is required.

21 As a group, melamine resin adhesives gen­ durable than urea resins, cheaper than erally have a pot life of at least 8 hours straight melamine or resorcinol resins, and at 700 F. and they tolerate rather long capable of curing at lower hot-press tem­ open and closed assembly periods. peratures than conventional phenol resin Slow-curing, uncatalyzed melamine glues. resin glues have shown good durability characteristics on laminated Douglas-fir Polyvinyl Resin Emulsions beams exposed for several decades to the weather. Similar glues used for laminating Polyvinyl resin emulsions are thermo­ white oak failed almost completely after plastic, softening when the temperature is 15 years of soaking in salt water. Rapid­ raised to a particular level and hardening setting, catalyzed melamine resin glues, again when cooled. They are prepared by in limited tests, have not shown the same emulsion polymerization of vinyl ace rate durability as indicated with uncatalyzed and other monomers in water under con­ melamines on softwood species. trolled conditions. Since individual types of monomers are not identified by the Melamine-Urea Resins manufacturer, this group is simply referred to as polyvinyl resins or PYA's. In the Melarnine-urea resins are a spec ial emulsified form, the polyvinyl resins are group of hot-press adhesives produced by dispersed in water and have a consistency either dry blending urea and melamine and nonvolatile content generally com­ resins or by blending the twO separate parable to the thermosetting resin glues. resins in liquid solution and then spray­ They are marketed as milky-white fluids drying the mixture. In either case, the to be used at room temperature in the resins are supplied by the manufacturer as form supplied by the manufacturer, powders, to be prepared by adding water normally without addition of separate and catalyst. Reportedly, the adhesive pro­ hardeners. duced by spray-drying a mixture of the two The adhesive sets when the water of the resins produces somewhat more durable emulsion partially diffuses into the wood bonds than the one produced by blending and the emulsified resin coagulates. There the twO powdered resins. At present, the is no apparent chemical curing reaction, as most common combinations are said to with the thermosetting resins. contain 40 to 50 percent by weight of Setting is comparatively rapid at room melamine resin and 50 to 60 percent of temperature, and for some constructions urea resin on a solid basis. it may be possible to release the clamping In finger- lumber for structural pressure in half an hour or less. Limited laminated timbers, a 60:40 melamine-to­ tests indicate that some of these glues urea ratio is used. Such joints, when prop­ set in most wood joints at 75 0 F. at a erly produced, are considered adequate for rate comparable to that of hot . normally dry interior service but are not The polyvinyl resins have indefinitely recommended where long-term exterior long storage (in tight containers) and use is involved. The melamine-urea com­ working lives (at normal room tempera­ binations are used in much the same way tures) and cail be used as long as the resin as the hot-press ureas and melamine glues, remains dispersed. Coagulation in storage curing at 2400 to 2600 F. in manufac­ by evaporation or freezing must be avoided, ture of plywood. In finger-jointing opera­ although special emulsions have been tions, they are generally cured by high­ offered that are said to withstand repeated frequency heating. The melamine-urea freezing and thawing. The set resins are resin glues offer advantages for hardwood light in color, often transparent, and result plywood in that they are colorless, more in gluelines that are barely visible.

22 A considerable amount of variation glues appear to be suitable for edge-gluing has been observed in the performance of applications. However, not all glues show the different glues of this type. Some of such improvement and no quick ali,-j the poorer ones gave considerably lower simple screening test is yet available for joint strengths and developed litc1e or no checking this property. Therefore, the. user wood failure compared to other types of must exercise caution in selecting such a resin and nonresin glues. On the other glue for edge-gluing, particularly of dense hand, joints produced with some of the species. Screening tests, by cycling panels newer polyvinyl resins gave unusually high made with different glues between high shear strengths although generally not and low humidity conditions, might be ad­ high wood failures, particularly with visable before using PYA's in full-scale denser species. Such results might be ex­ production. pected of rather elastic-type adhesives, be­ At the British Forest Products Labota­ cause the load ptobably will be distributed tory, joints made with 39 brands of PYA more uniformly over the entire joint area were teseed in an atmosphere of 25° C. under test than with brittle glues. The (77° F.) and 60 percent relative humidity polyvinyl resin adhesives have litc1e dulling under approximately one-third ultimate effect on the sharp edges of cucting , load and normal loading rate. Joints with but a tendency to foul has been 37 brands failed within 6 months; joints reported. with the other two brands survived and Some PYA's soften and lose a portion of were still intact after 24 months. their strength as the temperature in­ Studies have indicated that some poly­ creases above normal room temperature, vinyl resin emulsion glues are promising and the strength of many of them is in assembly joints such as , mortise appreciably reduced at about 160° F. and tenon, and lock-corner. Their fast They are also generally weakened more by setting is of benefit and their elasticity higher relative humidity conditions than may be an additional advantage where the are the thermosetting resin glues. dimensional changes in the joint are nomi­ Probably the most serious limitation in nal. the use of these adhesives in woodworking In terms of durability, polyvinyl resin is the lack of resistance to continuously emulsion adhesives are considerably less applied loads. Such "cold flow" is the resistant to warm, moist, or humid condi­ tendency for a glue to yield to, rather than tions than the thermosetting resins. The resist, the stresses exerted on the joint at PVA's lack resistance to water and high normal room temperature. This limitation relative humidities, and a number tend to has been most serious when polyvinyl soften at temperatures as low as 110° F. resins have been used for edge-gluing Even at normal room temperatures, creep lumber for solid stock, particularly high­ or yield of the bond (cold flow) might density hardwoods, which will not be sub­ become a problem if heavy stress is con­ sequently veneered. Wfhen such a st(;d: is tinued on the joint. The low resistance of exposed to low humidities, moisture con­ polyvinyl resin emulsions to water and tent changes most rapidly through the end moisture limit PYA use primarily to non­ grain, with resultant shrinkage stresses structural interior applications, as in cer­ across the ends of the panels. When these tain types of furniture joints. stresses are of appreciable magnitude and duration, the glue often fails, resulting in Thermosetting Polyvinyl open joints. Emulsions This cold-flow limitation has encour­ Thermosetting polyvinyl emulsions, aged considerable reformulation of poly­ also identified as catalyzed PYA emulsions vinyl resins 50 several currently available and cross-linked PYA's, have been avail­

23 able for a decade. They are modified PYA recommended for structural applications emulsions and ,generally have heat and because of creep (fig. 14). moisture resistance superior to ordinary On the Qther hand, in tests On hot­ PYA's, particularly when cured at ele­ pressed plywood made with a cross-linked vated temperatures. PYA, the joints performed almost as well Room-temperature cure of these ad­ as those made with phenolic glues. hesives has been insufficient to prevent Further research has shown that these creep when glued specimens were stressed adhesives cannot be classed with resorcinols during exposure at 150° F. and high as being room-temperature-curing and re.1ative humidity. Therefore, they are not suitable for general structural applications.

Fi':!ure 14.-Cross section of laminated oak glued with thermosetting or cross­ linked PYA and subjected to vt:l(uum-pressure soaking, steaming, and drying. The bridging in the joint at the center might explain why PYA glues have performed well in cyclic tests on and dowel joints. Had a brittle glue been used, fractures would probably have occurred either in the bond or adiacent to the glueline. The elastic PYA yielded enough to retain the bond between the join! surfaces.

24 They are, however, markedly superior to plasticizers and other ingredi "'ItS ro ordinary PYA's in resistance co moist improve working pwperries. conditions, and there is reason ro believe Hot melts have melting points .:overing they would perform well in most non­ a rather wide range. Tnlnsicion P011ltS Irom structural interior uses. In common with solid to a sott mass or liquid ha\'..:. been some other adhesives, they would not be reported from .tS low as 150" F. lU as expected to perform as well On dense, high as 390" F .• although wI)rkil';,' telll­ high-shrinkage species as on lighter ones. penltureS in the range of )75" to 410° F. are supposed to be more <.:OmlUon. Some hot melts are reported co be Hot Melts water resistant and provide somewh.tr (-{,IS­ ric gluelines; however, their resisrantt w Hot-melt adheSives for wood are furn~ heat is generally poor. For beSt rl:sults, ished in solid form, usually as pellets, good concrol is required of wood illld t:1ue chunks, granules, or in cord form on reels. temperatures as well as the rate ofapl'hra­ They involve a wide variety of thermo­ cion. plastic mixtures that are converred by heat ro spreadable consistency and applied Epoxy Resins while hot and fluid; they set almost instan­ taneously as the heat dissipates from the Epoxy resin adhesives became a\',li[able thin glue film to the greater mass of the in the 1940's and found a major uSc for substrate. Pressure is applied on the joint metal bonding in the aircraft industry. during formation of the bond. The bond However, epoxies do adhere to a variety of forms very rapidly. depending upon the substrates and in relent years h.1\'1.' been temperature difference between the glue employed as bonding age rs in numerous and the parts being joined. Setting times special applications. They are probably the as brief as a fraction ofa second have been most versatile adhesives t"urrenrly available reported. in th,lt they adhere ro more dm~n !It sub­ One of the primary uses of hot melts in strates than other synthetic ,lr Ihuurally wood gluing has been for of occurring bonding agents. Th... y have nOt, panel products. Machines are increa.<;ingly however, found extensive USI.' fu,: [wilJing common in the furniture industry ro apply wood. edge banding co panels with hot melts at Epoxy resin refers in it bro,ld SU1W tn a about 60 to 100 linear feet per minute. wide variet}' of poLymersrhanlcterized In The process is reported to lend itself to their simplest form by an oxyg(;n atom application of veneer and thicker edge linked to each of two adjacent (arbon bands to lumber and particleboard cores. acorns on a chain. as in ethylene m.idr. The Hot melts are also being used to some earlkr epoxy resins used for mem! bnn4illlg extent for bonding decorative overlays or were condensation prodults ofbispht'n,)1 A films to particleboard for counter and furn­ and epichlorohydrio, Curing ap.cnrs li.[ iture topS and shelves,and for coaring these resins were vacious amines ;Iod ;It ld panel products. Methods ofapplication in­ anhydrides. Improvements in thl.' working clude roll coating, blade coating, and cur­ charaCteristics of epoxies have bel.'n m.l~L tain coating. over the years and ,I wide ,arlcty of ti'f(;Jul­ The composition of hot-melt adhesives arions ,Ife now .lvail,lbk. The}' Lure' by varies a great deal and may include poly­ additional polymeriz.uioo ,...·[th Y<:ry litde mers, such as ethylene vinyl acetate copol}'­ volume change or shrinbge while rhey mers, polyamides, polyolefins, and poly­ harden. esters, as well lIS other resins or copoly­ An imporranr advanr.tge of epoxy .1<1­ mers. These are generally modified with hesives is that they can be formul,l[('d to

25 meet a variety of use conditions. They are Rubber adhesives are unique in that available as elevated- and room-tempera­ they develop considerable strength imme­ ture-setting; their POt life can be varied diately upon contact of the surfaces to be from a few minutes to an hour or more; bonded. Full joint strength, however, de­ they can be used with numerous types of velops rather slowly, and the ultimate fillers; and can be modified with poly­ strength is generaliy much lower than for sulfide and natUral or synthetic rubber co ordinary woodworking glues. change their elasticity. As practically no Emulsion-type rubber-base adhesives solvent or other product is given off during are also available and their performance is the setting of epoxy adh,esives, they have similar to the solvent type in many res­ very little shrinkage; thus, they can coler­ pects. However, the emulsion types have ace much chicker gluelines and are more less resistance to moisture. gap-filling than ordinary lldhesives. For wood gluing, use ofepoxy resins has Mastic Adhesives been limited mostly co such special appli­ cations as repair work, sometimes in com­ One ofthe definitions for mastic is "any bination with gJass fiber for reinforcement. of various quick-drying pasey cements used Clean, sanded surfaces have provided for cementing riles to a wall." To the bener bonds for such applications (in the author's knowledge, the term "mastic ad­ author's experience) than smoothly planed hesive" was first used in connection with surfaces, In gluing white oak, Douglas-fir, wood bonding to describe thick, pasty soy­ and Alaska-cedar with a number of com­ bean glue for hot-press plywood. Various mercial epoxy adhesives, better results adhesives of "mastic" consistency have werc invariably obtained on sawn sur(.1.ces been marketed over the years. Their basic than on smoorhly planed surfaces. In short­ ingredient was often rubber, bur lately term soaking tests (vacuum-pressure im­ compositions based on materials such as pregnation), epoxy adhesive bonds failed , polyesters, silicones, and on white oak, but several formulations epoxies have come into use. Mastics are showed prom ising results on rhe cwo soft­ sometimes matketed as "construction woods. adhesives," which could be misleading Since epoxy adhesives are available in so because they generally provide less rigid many varieties for many different applica­ bonds than commonly used in laminated tions, and in consisrencies from free flow­ timbers and other structural applications. ing ro rhixot

26 than that of conventional thermosetting finely divided forms absorb water more wood adhesives, but. this would not neces­ rapidly and can be dissolved more easily sarily limit the usefulness of mastics in than the cake and flake (')fms. The higher applications where high joint strength is grade glues in the flake form are usually not a prerequisite for good performance. light in color and nearly transparent. Gap-filling properties and ability to retain Lower gr'1de glues tend to be dark and resilency over the long rerm can be highly opaque. important. Color and transparency, however, are not dependable indications of quality be­ ADHESIVES OF NATURAl. cause low-grade glues are sometimes ORIG:N bleached. Also, foreign substances such as zinc white, chalk, and similar materials are Adhesives of natural origin-such as frequently added to transparent glues to animal, casein, soybean, starch, and blood produce what are technically known as glues--are Still being used to bond wood opaque glues. The added materials, while in some plants and shops, but are being they apparently do no harm, do not in­ replaced more and more by synthetics. crease the adhesive qualities. Aside from Animal glue is probably the "natural" the fact that they give '- ;nconspicuous adhesive most widely used, although glueline in light-colo! ~ woods, the casein glue is being used a great deal for "opaque" or whitened glues have no ap­ structural laminating. parent advantage over other glues of the same grade. Marked improvements have been made Animal Glue over the years in the standardization of methods for grading animal glues for Animal glue is a gelatin adhesive ob­ woodworking. The definitely established rained from waste or byproducts of the tests and specifications give the user of meat processing and tanning industries. animal glues means to insure uniformity The most common raw materials are hides and to secure a product suited ro his or trimmings of hides, sinews, and bones operating needs. of cattle and othet animals. Trimmings from the leather industry (from tanned PREPARING ANIMAl. GLUE FOR hides) are also utilized. USE Glue made from hides is generally of higher grade than glue derived from bones In preparing animal glues for use, a and tendons. However, there is consider­ number ofprecautions must be observed if able variation in the quality or grades of satisfactory results are to be obtained. The glue from hides as well as from the other proportion of glue and water should be sources. Glues for woodworking, as well varied to meet manufacturing conditions. as most other uses, are commonly blends When the right proportions have been of two or more batches from the same worked out, they should be used consis­ stock or from different classes of stock. tently. The glue and the water should be Source is important only insofar as it weighed out whenever a barch is prepared. affects grade. Clean, cold water should be used and the Each class of glue is sold in cake, flake, mixture thoroughly stirred at once to allow ground, pearl, shredded, and other forms; a uniform absorption ofwater by the dry but the form of the glue is no particular glue and prevent the formation of lumps. indication of quality. The chief difference The batch should then stand in a cnol between the various forms is in the time place until the glue is thoroughly water required to put the glue into solution. The soaked and softened. The soaking may take

27 only an hour or twO or longer, the time for pressing exists when tbe glue is thick depending upon the size of the particles. enough to form short, thick strings when The glue should then be melted over a couched with a finger, but not thick water bath at a temperature not higher than enough to resist an imprint or a depression 1500 F. High temperature and long, con­ readily. The thickening time or assembly tinued heating reduce the strength of ani­ period is ususally fixed by the operating mal glue solutions and are co be avoided. conditions thac dictate how much time The glue pot should be kept covered as shall elapse between spreading and press­ much as possible co prevent the formarion ing. The grade of glue and the proportion of a skin or scum over the glue surface. of water added in mixing become, there­ Strict cleanliness should be maintained fore, the variables by which the manufac­ for glue pOts and spreading equipment as turer lan fit the glue mixture [0 his well as tables and floors in the glue room. operating conditions. When once estab­ Old glue soon becomes foul and provides lished, the glue grade and proportion of a breeding place for bacteria that cause water should be adhered to except when decomposition, exposing the fresh batches temperature changes in the glue room or to the constant danger of becoming con­ wood require a change in the mixture. taminated, Glue POtS should be washed When the assembly period is fixed by every day and o::lly enough glue for a day's the operation, and the temperature in the run should be prepared at a time. If .these glue room rises, an adjustmene must be simple sanitary precautions are not ob­ made to accelerate the speed of thickening. served, poor joints are likely co result. This adjustmenr can usually be made most easily by mixing less water with the glue. Strong joines may be made with a num­ STRENGTH AND DURAS/lITY OF ber of grades of animal glue, but different ANIMAL GLUE JOINTS gluing conditions must be used according to the grade of the glue. If wood joint Making uniformly strong joints depends tests are made with glues of different primarily upon having the proper correla­ grades under a uniform set ofgluing condi­ tion of gluing pressure and glue viscosity tions, the grade ofglue that gives the best at the moment pressure is applied. With results is the one best adapted to the par­ animal glue solutions, the consistency de­ ticular gluing conditions employed. The pends on cooling and drying effects. For joint tcst results are not necessarily an the first few minutes after the animal glue accurate measure of the inherent strengths has been spread on the wood, the cooling of the other grades tested. effect is much more important than the With respect to maintaining strength drying; this tempetature-viscosity rela­ over the long term, animal glue in three­ tionship varies with the grade and with the ply birch plywood joints showed po signi­ concentration of the glue solution. High­ ficant loss in strength after 5 years' ex­ grade animal glues thicken to the proper posure at 800 F. and 65 percent relative pressing consistency quicker and at higher humidity. Cycling of similar specimens temperatures than do low-grade glues of between 6S percent relative humidity and equal concentration. Assuming glues of 30 percent relative humidity produced equal grade, one mixed with less water very little strength loss in the joints. This will thicken more rapidly than one mixed is the apprOldmate change in moisture con­ with a greater quantity of water. tene that can be expected in interior wood­ Warm animal glues, as they normally work in normal use in heated buildings in exist in the spreader, are roo thin for press­ the northern part of the United States. ing and some thickening must occur before In this type of service, properly designed pressure is applied. The best consistency and well-made joints of animal glue should

28 give long-term satisfactory performanct!, precipitated by mineral acids, such as hy­ particularly if the glued products have a drochloric or sulfuric, and by renner. In reasonably good moisture-excluding finish. preparing the glue, caseins precipitated by Such a finish retards moisture changes and different methods require different thus reduces the rate of stresses induced by amounts of water to produce solutions of shrinking and swelling. similar viscosity. Satisfactory glues can be Furniture and other products glued with produced from caseins precipitated by any animal glues often serve satisfactorily in of these methods, provided the casein is of spite of occasional exposures to relative good quality. humidities up to 80 percent or more. The starting point in the manufacture of Protection afforded by the finish usually casein is skim milk-that is, whole milk prevents the moisture content of the wood from which the fat has been removed in from reaching equilibrium values, particu­ the form of cream. The usual steps in the larly if the exposure to dampness is not manufacturing process are: (I) Precipita­ prolonged. Degradation of the joints is tion of the casein; (2) washing the curd to more apt to occur with dense, high-shrink­ remove the acid and other impurities; age species than with lighter species that (3) pressing the damp curd, wrapped in exert lower stresses on the glue joints. cloth, to remove most of the water; (4) dry­ ing the curd; and (5) grinding it to a Casein Glue powder. The care with which these steps are carried OUt determines the quality of Casein glue has been used in Europe for the finished product. at least a century and in the United States for more than two-thirds ofa century. The FORMULATION OF basic constituent of casein glue is dried CASEIN GLUES casein which, when combined with al­ kaline chemicals (usually lime and one or The principal ingredients of a casein more sodium salts), is water soluble. glue are casein, water, hydrated lime, and For some uses, the principal require­ sodium hydroxide. A properly propor­ ments of casein glue are water and mold tioned mixture of casein, water, and hy­ resistance combined with adequate dry drated lime will yield a glue of high water strengths. For other applications, it is resistance, but its working life will be very desirable to formulate a less expensive shore. A glue can also be prepared of casein glue that possesses low staining casein, water, and sodium hydroxide. tendencies, long working life, high dry When properly prepared, such a glue will strength, or good spreading characteristics, have excellent dry strength and a long even at some sacrifice of water resistance. working life, but it will not have rhe The glue supplier can produce, therefure, water resisr.ance ordinarily associated with a variety of casein glues of different prop­ casein glu. .!s. By adjusting the proportions erties from which the user may choose of sodium hydroxide and lime, glues of according ro his needs. high water resistance and convenient work­ ing life may be obtained. PREPARATION Of CASEIN Casein glues containing sodium hy­ droxide and hydrated lime cannot be mixed When milk becomes sour, it separates in dry (solid) form and shipped. The hy­ into curd, the chief protein constituent, groscopic properties of sodium hydroxide and whey. The curd, after being washed prevent storing a casein glue containing it and dried, is the casein of commerce. without danger of decomposition. The When formed in this way, it is known as alkali can be introduced in an indirect naturally soured casein. Casein is also manner, however, so thar rhe casein can

29 be mixed with all the necessary ingredi­ that will have good dry strength and ents, except water, in the form of a dry water resistance. powder thilt can be handled and stored Sodium silicate may be used in place of conveniently. One way is to replace the sodium hydroxide or in place of dry sodium hydroxide with chemically equiv­ sodium salts, and a glue so prepared will alent amounts of calcium hydroxide and a differ from one prepared by the formula substance that, when dissolved in water, above. Particularly, a much longer work­ reacts with the calcium hydroxide to form ing life is obtained with a glue using sodium hydroxide. Any convenient sodium silicate and having alkalinity sodium salt of an acid whose calcium salt equal to that obtained by the use ofsodium is relatively insoluble may be used, hydroxide or other sodium salts. There is a provided it is not hygroscopic and does considerable range ofpermissible lime con­ not react with the lime or the casein tent (above that necessary to react with the when the mixture is dry. sodium silicate); however, the working life Prepared casein gilles. -Most manufac­ decreases as the proportion of calcium turers of wood glues furnish casein glues hydroxide increases. containing the required ingredients in A small amount of cupric chloride in powder form ready to mix with water. casein glue has been found effective in They are prepared for use by merely sifting increasing the water resistance. This im­ them into the proper amount of water and provement is most striking in glues that stirring the mixture. They usually contain do not contain as much lime as required the essential ingredients of casein, hy­ for optimum water resistance. It is not drated lime, and sodium salt, and are always advisable to use the maximum occasionally formulated to reduce staining, amount of lime because high-lime glues hardness, or to impart other properties. almost invariably have a short working life. Many of the formulas were protected by In such cases, it may be expedient to ob­ patents, mosr of which are now ourdated. rain high water resistance by adding Directions for mixing these glues with copper chloride rather than the maximum water are usually furnished by the manu­ amount of lime. fucturer. lYle/-mixed casein gilles. -Some glue users may prefer to mix the ingredients USE CHARACTERISTICS OF directly from the basic materials-casein, CASEIN GLUE sodium hydroxide, and lime. Appwxi­ mately the following proportions of Casein glue sets as a result of chemical ingredients should be mixed in this order. reaction and loss of moisture to wood and air. Hence, its rate ofsetting is affected by the temperature ofthe wood and surround­ Ingredients Parts by weight ing atmosphere, the moisture content of Casin 100 the wood, and other factors. Longer setting Water 150 time is required in a cold shop than in a Sodium hydroxide 11 warm one, and wood high in moisture Water 50 Calcium hydroxide content will retard the setting rate. (hydrated limel 20 Casein glue wiII set at a temperature Water 50 almost as low as the freezing p0i nt ofwater, but the setting period required to develop strong joints at such temperatures varies This glue remains usable for some 6 to 7 from several days to several weeks. The hours at temperatures between 70° and time depends also on the species glued. 75° F. It is capable of producing joints The wet strength developed at low tem­

30 peratures may never be as good as that preservatives are sometimes added to casein developed at normal room temperatures. glues. Federal Specification MMM-A-125 A pressing period of 4 hours at 70° F. gives minimum requirements for water­ is considered a minimum for straight and mold-resistant casein glues. Prolonged members; for curved members, a some­ exposure to conditions favorable to mold what longer period is desirable. growth or other micro-organisms, how­ Casein glue will produce adequate bonds ever, will eventually result in failure even with wood at a wide range in moisture in joints made of casein glue containing content-from about 2 to 18 percent. To preservative. avoid serious shrinking or swelling stresses Outdoors or where high humidities, on the joints, however, the moisture con­ either continuous or intermirrent, are in­ tent of the wood at the time of gluing volved, casein glue joints are not durable. should be slightly lower than the average Ca~ein glues containing preservatives have expected in service. shown greater resistance to high humidi­ ties than have unpreserved caseins, but the DURABILITY OF CASEIN GLUE preservative did not prevent eventual destruction of the glue bonds under damp Well-made casein glue joints will de­ conditions. Consequently, casein glue is velop the fi..ll strength of most low- and not considered suitablt for glued products medium-density woods in shear parallel to intended for exterior use, or for interior the grain and will retain a large part of use where the moisture content ofthe wood their strength even when submerged in may exceed 18 percent for repeated or water for a few days. With dense woods, prolonged periods. Voluntary Product however, casein glue develops only me­ Standard PS 56 for structural glued dium to low wood failure percentages laminated timbers limits casein-glued when the joints are tested in shear (fig. 15) material to service where the equilibrium To improve resistance to deterioration moisture content of the wood does not caused by molds or other micro-organisms, exceed 16 percent.

CASEIN UREA PHENOL - RESORCINOL SPECIES SPRUCE. StTKA __ - MAHOGAN'f____ _

DOUGLAS - FIR__ _ - ASH. WHITE__ _ - BIReM. YELLOW__ - OAK, RED_____ OAK, WHITE __ _ - MAPLE, SUGAR__ HICKORY____ _ I o 40 BO o 40 BO o 40 BO • WOOD FAILURE (PERCENT) M 132 4}4 Figure i 5.-Percentage failure in wood Clf various species glued with ca!oein, urea resin, and phenol-resorcinol resin when joints were tested in block shear (ASTM D 905). With both resins, the joints were about as strong as the wood; with casein, a large percentage of th2 failures in the denser species were in the glue, indicating that the glue bond was the weakest link.

31 .\I 118 199-4 Figure 16.-Building erected in 1935 with casein glued-laminated arches. It is cur­ rently used for packaging research at the Forest Products Laboratory. Arches Clre in excellent condition.

Casein glue joints have demonstrated or more in the United States (fig. 16). good resistance to dry hear. Results of test In Europe, similar structures that are much exposures to temperatures as high as 1580 older are not uncommon. This should be F. for periods up co 4 years have indi­ adequate basis for confidence in casein glue cated that the glue bonds in birch ply­ as a structural bonding agent for softwood wood are about as resistant as the wood co laminates used under normally dry interior this type of exposure. Temperatures that conditions. char and burn wood cause decomposition In joints where the grain of the pieces of casein glue. Charred wood exposed co bonded is not parallel, casein glue has not fire, however, conducts heat co its interior performed nearly as well, particularly with very slowly so that softening of casein dense wood having high shrinkage. glue joints takes place only next co the zone of char. Soybean Glue Laminated softwood structural mem­ bers bonded with casein glue have given Soybean glue was introduced co the excellent service when protected from ex­ plywood industry in the Pacific Northwest terior and damp conditions for 35 years during the early 1920's, and for many

32 years was the major glue used for making casein and soybean glues are used for mak­ softwood plywood (interior type-no prac­ ing plywood of the same species, soybean tical exterior glue had yet been developed). glue generally shows the poorer water The protein constituents of soybean glue resistance. However, it is appreciably that supply the adhesive properties are superior to starch glues in resistance to somewhat similar to those of casein. moisture and high humidity. The basic adhesive material in this glue As the standards for performance of is the protein from soybeans. The oil is plywood have become stricter over the first removed from the bean by expeller or years, it increasingly has become common solvent processes. The coarse meal is chen practice ro fortify soybean glue with a usually passed through a roller mill certain amount of dried blood or occa­ (smooth rolls) to crush the shell loose from sionally casein--casein if the plywood is the kernel. The kernel is ground to the cold pressed and blood ifit is hot pressed. desired particle size, usually in a hammer­ Since softwood plywood is being produced mill. The flour is mixed with small more and more by hot pressing, the blood­ amounts of chemicals and is then ready for fortified soybean glues are predominant. shipment (usually in lOO-lb. bags) to the plywood plant. Soybean glue almost always is used as a wet-mix glue. Usually, the glue powder is Blood Glue first mixed with sufficient water to make a smooth dough free of dry lumps. Then Glues made of soluble dried blood or additional water is added slowly with the blood albumin have been used to some mixer running. If the required additional extent in the United States, bue they are water is added all at once, the dough more common in some European and might break up into lumps, making it Asiatic COuntrih. nearly impossible to obtain a final smooth Blood albunlln, a slaughterhouse by­ mixture. Slaked lime, caustic soda solu­ product, coagulates and sets firmly when rion, and sodium silicatt> are usually added heated to a temperature of about 1600 F. to the mix in that order with short periods It then shows a significant resistance to the of mixing between rhe addirion of each softening effect of water. This characteris­ ingredient. To prevent or reduce foaming, tic makes it a desirable material for glue a small amount of pine oil or other de­ to use in products such as plywood. foaming agent is usually added to the glue A number of patents have been granted mix. on glue formulations based on blood. As Directions for mixing and the amount with other protein glues, alkalies such as of each ingredient to be added are furn­ caustic soda, hydrated lime, sodium sili­ ished by the glue manufacturer. cate, or combinations of these are em­ Softwood plywood well glued with soy­ ployed in formulating blood glues. Ther­ bean glue is roughly comparable in wat!'r mosetting resins (usually phenolic) are also resistance to birch or similar density ply­ sometimes incorporated to increase the wood bonded with casein glue, and is resistance of the glue bonds to degrading generally considered satisfactory for nor­ influences. mally dry interior service. Soybean glue Hot-press blood glues are probably the has not proved entirely satisfactory for most resistant of the protein-type glues to gluing hardwoods, particularly the denser weathering and similar severe service but ones. are not recommended for long-term Soybean glue is generally not recom­ exterior use as are the phenols and resor­ mended for hardwood plywood; if both dnols and some other synthetics.

33 SELECTED REfERENCES Kreibich R. E., and Freeman, H. G. 1965. 'Testing adhesives for creep can provide data on adhesive systems which wjJ.l help improve structural bondants. Adhes. Age 8(8):29-34. American Society fur Testing and Materials Lee, Henry, and Neville, Kris St~.ndard method of test f9r strength prop­ 1967. Handbook of epoxy resins. McGraw­ erties ofadhesive bonds in shear by com­ Hill Book Co., New York. pression loading. Des. D 905. (See current McGrath, J. J. edition) Philadelphia, Pa. 1967. Hot melts featured at adhesive meet­ Blomquist, R. F. ing. For. Prod. J. 17(4):22. 1964. Durability offortified urea-resin glues Northcott, P. L., and Hancock, W. V. exposed to exterior weathering. For. Prod. 1966. Accelerated tests fur deterioration of J. 14(10):461-466. adhesive bonds in plywood. Durability of Blomquist, R. F., and Olson, W. Z. Adhesive Joints, SPec. Tech. Pub!. No. 1957. Durability of urea-resin glues at 401. 62-79. Am. Soc. Test. Mater., elevated temperatures. For. Prod. J. Philadelphia. 7(3):266-272. Olson, W. Z. Carlson, Herbert E. 1955. Polyvinyl-resin emulsion woodwork­ 1962. Bonding with hot melts. Adhes. Age ing glues. For. Prod. J. 5(4):219-226. 5(11):32-33. Olson, W. Z., and Blomquist, R. F. Carroll, M. N., and Bergin, E. G. 1962. Epoxy-resin adhesives for gluing 1967. Catalyzed PV A emulsions as wood wood. For. Prod. J. 12(2):74-80. adhesives. For. Prod. 17(11):45-50. J. Picotte, Gordon L Carruthers, J. S. 1965. "Toughness" of structural adhesives. 1958. The comparative durability of assem­ Adhes. Age 8(2):21-23. bly glues in England and Nigeria. Dep. of Rundle, V. A. Sci. and Ind. Res., For. Prod. Res. Lab., 1969. Hot-melt coatings for wood products. Princes Risborough, England For. Prod. J. 19(9):73-80. Cheo, Y. C Selbo, M. L. 1969. Hot melt coatings for wood products. 1969. Performance ofsouthern pine plywood For. Prod. J. 19(9):73-79. during 5 years' exposure to weather. For. Cone, Charles N. Prod. J. 19(8):56-60. 1959. Resin-blood glue and process of Selbo, M. L. making the same. U.S. Pat. No. 1965. Performance of melamine resin adhe­ 2,895,928. 8 p. July 21. sives In various exposures. For. Prod. J. Forsyth, Roberr S. 15(12):475-483. 1962. New developments in synthetic resin Selbo, M. L. hot melts. Adhes. Age 5(8):20-23. 1958. Curing rates of resorcinol and phenol­ Freeman, H. G., and Kreibich, R. E. resorcinol glues in laminated oak mem­ 1968. Estimating durability of wood adhe­ bers. For. Prod. J. 8(5):145-149. sive in vitro. For. Prod. J. 18(7):39-43. Gillespie, R. H., Olson, W. Z., and Blom­ Selbo, M. L. 1949. Durability of woodworking glues fur quist, R. F. .. dwellings. Proc. For. Prod. Res. Soc. 1964. Durability of urea-reSIn glues modi­ fied with polyvinyl acetate and blood. For. 3:361-380. Prod. J. 14(8):343-349. Selbo, M. L. Hartman, Seymour , 1949. Glue joints durable in beams lami­ 1970. High temperature adhesive. For. nated ofcommon lumber. South Lumber­ Prod. J. 20(12):21-23. man 179(2244):60-62. Kopyscinski, Walter, Norris, F. H., and Selbo, M. L., Knauss, A. C, and Worth, H. E. Hen••an, Stedman 1965. Glulam timbers show good perform­ 1960. Synthetic hot melr adhesives. Adhes. ance after two decades of service. For. Age 3(5):31-36. hod. J. 15(11):466-472. Kreibich, R. E., and Freeman, H. G. Selbo M. L., and Knauss, A. C 1970. Effect of specimen stressing upon 1958. Glued laminated wood construction durability of eight wood adhesives. For. in Europe. Proc. Am. Soc. Civil Eng. Prod. J. 20(4):44-49. 84(ST 7). 19 p. Nov.

34 Selbo, M. 1., Knauss, A. c., and Worth, H. E. Twiss, Sumner B. 1966. Twenty years of service durability of 1965. Structural adhesive bonding. Part 11: pressure-treated glulam bridge timbers. Adhesive classification. Adhes. Age Wood Res. News 44(3):5-16, Part I; 8(1):30-34. 44(4): 10-14, Part H. U.S. Department of Commerce Selba, M. 1., and Olson, W. z. 1973. Structural glued laminated lumber. 1953. Durability of woodworking glues in Voluntary Prod. Stand. PS 56-73. Natl. different types of assembly joints. For. Bur. Srand., Washington, D.C. Prod. J. 3(5):50-57. U.S. Forest Products Laboratory, Forest Service Simpson, W. T., and Soper, V. R. 1967. Casein glues: Their manufacture, 1970. Tensile stress-strain behavior of flexi­ preparation, and application. U.S. For. bilized epoxy adhesive film. USDA For. Servo Res. Note FPL-0158.15 p. U.3. Servo Res. Pap. FPL 126. 13 p. U.S. Dept. Agric., For. Serv. For Prod. Lab., Madison, Wis. Dept. Agric. For. Servo For. Prod. Lab., Madison, Wis. U. S. Forest Products Laboratory, Forest Service 1963. Durability of water-resistant wood­ Skeist, Irving working glues. Rep. 1530. 41 p. U.S. 1964. Modern structural adhesives for use Dept. Agric. For. Servo For. Prod. Lab., in the building industry. Adhes. Age Madison, Wis. 7(4):21-26. U.S. General Services Administration Troughton, G. E. 1969. Adhesive, casein-type water and mold 1967. Kinetic evidence for covalent bonding resistant. Fed. Specif. MMM-A-125. berween wood and formaldehyde glues. Fed. Supply Serv., Washington, D.C. Inf. Rep. No. VP-X-26, 22 p. For. Williamson, D. V. S. Prod. Lab., Vancouver, B.C. 1965. Hot melt adhesives in Europe. Adhes. Truax, T. R., and Selbo, M. L. Age 8(8):24-27. 1948. Results of accelerated tests and long­ Williamson, F. 1., and Nearn, W. T. term exposures on glue joints in laminated 1958. Wood-to-wood bonds with epoxide beams. Trans. Am. Soc. Mech. Eng. resins-species effect. For. Prod. J. 8(6): 70:393-400. May. 182-189.

IMPROVING PERFORMANCE OF WOOD THROUGH GLUING

To produce high-quality, adhesive­ Both resistance to splitting and uni­ bonded wood products, it is not only im­ formity in strength properties can be im­ portant to know and understand the prop­ proved by gluing toget.her sheets or layers erties and use characteristics of the of wood with the grain in adjacent layers adhesive, it is equally important to know how at approximately 90° (plywood). to select, prepare, and use the wood so that it By gluing together layers all having the will serve to tbe best advantage. grain approximately parallel (laminating), the strength in bending and intension in Wood has good stability and excellent the direction of the grain can be improved. strength properties in the grain direction. Two-by-fours laminated from low-grade Across the grain it is stable at constant veneers can be produced with much more moisture content but shrinks with de­ uniform strength properties than solid creases in moisture and swells with in­ structural 2 by 4's. This is accomplished creases in moisture. Normal straight­ by dispersion ofstrength-reducing defects. grained wood of some species may also By end-joint gluing, material of any split or check along the grain when sub­ desired length can be obtained; and by jected to rapid reductions in moisture edge gluing, any desired width is obtain­ content. able.

35 CROSS8ANDED (housing, concrete forms, packaging). CONSTRUCTION Hardwood plywood goes to furniture, paneling, and other uses. Several types of Crossbanded construction includes a plywood are shown in figures 18, 19, and large variety of panel products consisting 20. Veneer plywood is most commonly usually ofan odd number oflayers ofwood made and used as flat panels (figs. 18 and glued together with the grain in adjacent 19). It may also be made as flat panels layers at an angle of about 90°. Ply­ and later bent or formed within limits as wood, the most common form of cross­ may be required (fig. 20, A). Plywood banded construction, is defined as a with sharp or compound curves may be crossbanded assembly made of layers of formed by pressing the veneers (coated veneer or veneer in combination with with glue) in molds of the desired shape lumber or other core materials and joined and curing the glue with radio-frequency with an adhesive. The plywood construc­ energy or other- types of heat. As a rule, tions probably most widely used are veneer curve.:J plywood formed to the desir-ed plywood and lumber core plywood. The shape during manufacture is more stable term "veneered pane!s" is often used for than plywood bent to form later. lumber core plywood. A veneered panel Most plywood is three or five ply. In could also have a particleboard core (fig. thr-ee-ply construction, the twu outside 17), with or without crossbands. Flush plies are called faces, or face and back, doors are generally of cross banded con­ and are usually laid at right angles to struction. the grain of the center ply or core (fig. The bulk of softwood plywood produc­ 18, A). In five-ply panels the outside plies tion goes to structural applications are also called faces, or face and back, and the center- ply is the core (fig. 18, B). The second an-:i fourth plies are termed the crossbands and are usually at right angles to the grain of the face, back, and core. Plywood construction other than three­ or five-ply may be used, but odd numbers of plies ar-e generally symmetrically arranged on each side of the core. The core may be veneer, lumber, or various combinations of veneer or- lumber lamin­ ated so that they act as a single ply (fig. 19, A, B, C, D). In recent years it has been found advantageous to make panels offoqr veneers. The two center ones are glued together with the grain parallel and serve as the core in a three-ply panel. In a similar manner three or more laminated veneers could serve as core. Panels may range in total thickness from less than one-eighth inch to more than 3 inches. They may vary in number and thickness ofplies, kinds and combinations ofwoods, and durability of the glues required for various service conditions. Figure 17.-Three- and flve-ply veneered The chiefadvantages ofplywood as com­ panels with particleboard cores. pared with solid wood are: (1) Greater

36 M 138743 Figure l8.-Three constructions of all-veneer plywood: A, three-ply; B, five-ply; and C, seven-ply.

F"C~ C S Ross lll"'IN 9"NDs

"'~D CaRt: M 138746 Figure 19.-Three different constructions of lumber (ore plywood and one with lam­ inated veneer core: A, three-ply; B, five-ply, with veneer edge banding; C, seven­ ply, with laminated veneer COfe (used where showthrough must be avoided and good stability is required); and 0, extra thick lumber core plywood. Crossband at middle of core reduces dimensional changes of COrl'.

37 M 138744 Figure 20.-Three types of curved plywood: A, conventillnal five-ply, cll-veneer plywood bent to form; B, laminated core with veneer crossbands and faces; and C, five-ply, spirally wrapped plywood tubing.

resistance to splitting and checking, right angles to the plies. Along planes (2) more nearly equal strengrn properties parallel to the plies, the splitting charac­ along the length and width of the panel, teristics resemble those of solid wood. (3) dimensional changes with changes in The glued, crossbanded product, there­ moisture content rhat are more nearly fore, is more nearly constant in width and equal in length and width and distinctly length under varying moisture conditions. less than the changes of solid lumber in It is not necessary that the cross plies be width, and (4) the plywood production thick or that they occupy a very large parr processes utilize wood more efficiently. of the total thickness of the crossbanded These advantages are present because product. In a five-ply veneered panel with the direction of the grain of each ply is a core of nominal l-inch lumber and fuce generally at right angles to that ofadjacent and back of 1/28-inch veneer, for example, plies. The strength ofa piece of lum ber or the crossbands are frequently of 1/20-inch veneer along the grain is much greater veneer. The choice of thickness depends on than the strength across the grain. When the tensile strength of .the species. The pieces are glued together with the direc­ crossbands must be sufficiently strong in tion oftheir grain at right angles, the high tension parallel to grain to withstand, strength and dimensional stability of each without breaking, the stresses developed piece along the grain resist the stresses and by the core when it tends to expand or movement of adjacent pieces across the COntr~ct as the moisture coment changes. grain, and the strength and stability of the To realize fully the advantages of cross­ panel in the two directions are, in effect, banded construction, the panels must be equalized. The result is a more nearly properly designed and glued. The ten­ homogeneous product than solid wood. dency of panels to cup and twist may be Because of the cross plies, plywood panels greater in improperly constructed plywood are very resistant to splitting in planes at than in the average panel of solid wood of

38 the same thickness. Cross banded panels same species, of similar density, and cut may be considered to be relatively free in the same manner. from stress at the time the glue sets. Since the outer plies of a crossbanded When the moisture content changes construction are restrained on only one thereafter, however, adjoining plies try to side, changes in moisture content induce shrink or swell in directions at right angles relatively larger stresses in the outer glue to each other but each ply restrains the ply joints. The magnitude of stresses depends or plies next to it. Since the moisture in the upon such factOrs as thickness of plies, panel during service is rarely distributed density, shrinkage of the woods involved, as it was when the glue set, plywood and the amount of the moisture coment panels may be considered as continually changes. In general, one-eighth inch is the under stresses that tend to rupture the glue maximum thickness of face plies that can joints or to distort the panel. The further be held securely in place when moderately the moisture content departs from that ex­ dense woods are used and large moisture isting when the glue set, the greater will changes occur. For panels where face be the stresses developed. The develop­ checking would be objectionable, such as ment of these stresses cannot easily be in doors and furnicure, thin mce veneers prevented but their magnitude and effect (1/28 in. or 1/32 in.) are preferable to can be largely controlled by choice of thicker ones. species, proper design, well-glued joints, and control of moisture content at the time Quality of Plies of gluing. In thin plywood, the quality of all the rn crossbanded products that are prop­ plies affects the shape and permanence of erly designed, the forces exerted by the form of the panel. For greatest stability all plies on one side of the core balance in plies should be straight grained, smoothly magnitude and in direction the forces ex­ cut, and of sound wood that is of uniform erted by the plies on the other side of the growth and texture. core. This balance is pardy accomplished In thick, five-ply (lumber core) panels by the use of an odd number of plies so the cross bands in particular affect the arranged that for any ply on one side ofthe stability and quality of the panel. Imper­ core there is a corresponding parallel ply fections in the crossbands, such as marked on the other side at the same distance differences in the texture of the wood, from the core. irregularities in the surface, or even pro­ In addition to being correctly spaced nounced lathe checks, may show through from the core, the wood in corresponding thin face veneers as imperfections in the plies should have the same shrinkage and surface of the panel. density properties to obtain a balanced Figured veneer cut from burls, ctotches, eftecr. The shrinkage of wood varies with stumps, and similar irregular material is the species and the method of cutting, not straight grained but is used because of and the stresses developed vary with the its attractive appearance. It shrinks both density. In some cases, the difference in with the width and length of the sheet, shrinkage between edge-grained wood and whereas plain veneer shrinks chiefly in flat-grained wood of the same stOck is width. This difference in shrinkage be­ greater than between similarly cut wood of tween the two types ofveneer causes warp­ different species. Consequently, flat­ ing when they are used as opposing plies grained and quartered material ofthe same in thin panels. With combinations of wood may not balance so closely as woods plain and figured veneer, it is not practical ofdifferent species. For best results, there­ to have a strictly balanced construction and fore, corresponding plies should be of the the effect of the unsymmetrical arrange­

39 ment must be compensated for in some For cerrain types of panels that are held other way. Ordinarily, by laying figured securely in place by mechanical means, veneer over a thick and properly cross­ tendencies toward warping might be un­ banded core, the construction is made sriff important. For others, such as lids and enough to prevent the unbalanced stresses doors thar generally are mechanically exerted by rhe rhin faces from excessively fasrened at one edge, even a small amount warping or disrorting rhe panel. Thick, of warp might be objectionable. In panel five-ply veneered panels (fig. 19, B and C) products, twisting and cupping are the make it possible co use a figured veneer on most common types of warping. rhe panel face and a straight-grained veneer on the back. TWISTING

Causes and Prevention of Corresponding plies on opposite sides of Warping rhe core should not only swell or shrink in the same direction, but rhe stresses In a panel that is symmetrical and should be of the same magnitude. If rhe balanced about its central , opposing stresses are not balanced with respect to plies must have abour rhe same moisture direcrion, the distorrion that resulrs content when glued. Variations in mois­ frequently is twist, a form of warping rure content of corresponding plies ar the in which the four corners of the panel will rime of gluing bring about shrinkage not rest simultaneously on a flat surface. differences rhat may result in warping. Generally, twisting in plywood is re­ Large changes in the moisture contenr of lated to grain direction. While factors rhe wood after gluing shouid be avoided orher than grain direction can cause twist­ because they induce inrernal stresses of ing, they are not so frequenrly encountered large magnitude that could cause warping, in practice. If plywood or veneered cops, checking, and weakening of joints. unattached to supporring members, are Warping may be expected if the panel twisted, it is most likely that grain direc­ contains veneer that is partially decayed, or tion of the panel plies is at fault. veneer rhar has ll,bnormal shrinkage In five-ply veneered panels with com­ characteristics, such as exhibited by com­ paratively thick cores and thinner cross­ pression wood or tension wood. bands and faces, the cross bands are the Cross grain or shorr grain that runs most essential element in maintaining sharply through the crossband veneer from a panel free from twist. In this construc­ one surface to the other often causes the rion, the grain ofthe crossband on one side panels to cup. Cross grain that runs should be parallel to the grain of the diagonally across the crossband veneer is crossband on the other side of the core. likely ro cause the panel to twist unless The amount of variation from this condi­ the two opposing crossbands are laid with tion that may occur without twisting the grain parallel to each other. Failure to depends upon such factors as thickness of observe this simple precaution is the cause core, density of core, moisture content of of much warping in crossbanded construc­ the core at the time of gluing, and tion. While it is impractical to eliminate change in moisture content after gluing. all crossgrained veneer, that showing an With thin, experimental panels, a varia­ excessive amount. of cross grain should be tion of 5° in grain direction of opposing rejected for most plywood manufacture. crossbands has caused distinct twisting. It can be used for purposes where its Examination of commercial, five-ply effect is not harmful (cabinet backs, for veneered panels has often disclosed pro­ exnmple). nounced twisting with a variation of 15°.

40 While the crossbands of five-ply con­ when plywood panels were fastened struction are usually the critical elements, rigidly (particularly ifglued) to supporting the faces are critical in three-ply construc­ members or frames whose shrinkage tion. If the crossbands of five-ply, thick characteristics differed from those of the core construction are properly laid, varia­ plywood panel. tions in the direction of grain of the faces seldom cause objectionable distortion. In five-ply, chin core construction, however, CUPPING parallel grain is important in the faces as well as in the crossbands. Ordinarily, the exact cause of cupping Ordinarily, the causes of twisting are is much more difficult to establish than easily detected. To avoid or reduce twist­ the cause of twisting. However, cupping ing that occurs when grain direction of difficulties are often more easily elimi­ plies is not parallel, changes in manufac­ nated in com.mercial operatio.ls than are turing procedure are usually required. One the causes of twisting. of the simplest and least cosrly methods Cupping generally results from. forces of reducing twisting is to select for cross­ that restrain the core unequally on the two bands such species as basswood, aspen, and sides. If, for example, a cross band was yellow-poplar that generally produce glued to only one side of a core, the core reasonably straight-grained stock. If the would be greatly restrained on one side value of the product justifies the added in its movements with moisture changes cost, the veneer should be clipped and but not at all on the other side, and cup­ trimmed parallel and perpendicular to ping would surely result. The direction of the grain rather than parallel and per­ the cupping would depend on whether the pendicular to the axis of the veneer bolt. core increased above or dried below the Since adjacent pieces of sliced veneer are moisture content it had when the glue set. very similar in grain formation, twisting If the crossband on one side differs dis­ may be reduced or avoided by using two rinccIy in shrinkage characteristics or adjacent pieces ofsliced veneer for the two strength properties (along the grain) from crossbands ofone panel. They must be laid the corresponding crossband on the other with the grain parallel to each other in side, cupping may be expected. A few the panel. The same principle could be of the more common causes are: applied to rotary-cut veneer for cross band­ 1. Crossband on one side thicker than ing by properly marking and arranging the on the other. When the core attempts to veneer as it comes from the lathe to insure change dimensions under changes in mois­ that matching sheets would be used for ture content, the movement of the core the two crossbands of a panel. Such pre­ will be restricted more strongly on the cautions, however, mayor may not be side with the thicker crossb~nd (assuming practical in a commercial operation, de­ both crossbands are of the same or similar pending on the cost of the final product. species) and cupping will result. Unequal If a panel changes in moisture content sanding of the faces of a three-ply panel to a marked degree at the edge while the produces an effect similar to that caused center changes very little, the stresses de­ by the use of faces of unequal thickness. veloped may cause twisting. This condi­ Minor variations in the thickness of thin tion can be detected by determinacion of faces on five-ply, lumber-core panels will the moisture content at the edges and at not ordinarily cause objectionable dis­ the center of the panel. Much of the twist­ tortion. ing will probably disappear when the panel 2. Short-grained crossband on one side is reconditioned to a uniform moisture and a straight-grained crossband on the content, Twisting has also been observed other. When the grain of a sheet of

41 veneer dips abruptly through the sheet or scoring the plywood or because of the from one surface to the other, the sheet way in which the plywood is built into the will have greater shrinkage in length than finished item. a sheet in which the grain is parallel to the One rather common cause of warping plane of the sheet. If such a short-grained that is not related co grain direction or sheet is laid as one crossband and a quality of plies is permitting plywood to straight-grained sheet as the opposing dry (or CO regain moisture) more rapidly ccossband. the shore-grained sheet will not from one side than from the other. It has offer the same resistance co the movement been observed frequently that the cop panel of the core as the straight-grained one and ofa pile ofpanels will be warped because it cupping will result. dried more rapidly from the upper surface 3. Partially decayed crossband on one than from the bottOm. When panels are side and a sound ccossband on the other. piled solidly, the tOp of the pile should When the core shrinks or swells under be kept covered co prevent rapid loss or moisture changes, the decayed ccossband regain of moisture by the tOp surface. If offers less resistance co the dimensional considerable change in moisture content is changes of the core than the sound cross­ expected, it is ofren desirable to protect band and cupping results. the ends and edges ofthe panels from rapid 4. Reaction wood in one ccossband and changes in moisture content. normal wood in the other. One of the When a finish that is highly resistant to characteristics of reaction wood is a high the passage of moisture is applied to one degree of longitudinal shrinkage as com­ side ofthe panel and either no finish or one pared with normal wood. If a sheet of low in resistance co moisture movement is veneer containing reaction wood is laid as applied t(' the other, moisture will move in one crossband and a sheet of veneer of or out ofone surface more rapidly than the normal wood as the other, the sheet I;on­ other. In this case, cupping may result taining reaction wood will tend co shrink JUSt .liS when panels are allowed co dry or swell longitudinally while the corres­ more rapidly from one surface than from ponding ccossband of normal wood will the other. tend co remain more nearly fixed in the While plywood shrinks and swells lengthwise direction. Consequently, the much less than normal wood does in either core will be restrained unequally on the the tangential or radial direction, it two sides and cupping will result. If a shrinks and swells more than normal wood crossband contained both reaction wood does in a longitudinal direction. A ply­ and normal wood, distortion in the form wood panel that is fastened firmly co a of combined twisting and cupping might longitudinal supporting member, there­ result. fore, may warp or pull loose from the In exterior flush doors it is conceivable fastenings under severe changes in mois­ that longitudinal shrinkage of the inner tute content. If the design requires a face and longitudinal swelling of the outer fastening between plywood and framing face, during the cold season, could also members, provision should be made contribute co cupping. wherever possible to permit a slight move­ ment of the plywood relative to the supporting member JUSt as a solid table cop HANDLING AND is ordinarily fastened to the frame co permit FABRICATION a slight swelling or shrinking of the top. Softwood plywood glued co studs in walls It is quite possible that well balanced of houses or mobile homes usually doe:: and properly constructed plywood will not change enough in moisture content to warp because ofmethods used in handling cause any warping problems.

42 REQUIREMENTS fOR used for crossbanding although they are CROSSSANDS not so inherently straight in r;rain as might be desired. Birch and maple have been used If the flatness of plywood is an im­ although they are comparatively high in portant consideration, as often is the case specific gravity and somewhat irregular in with furniture plywood, one of the grain direction. Even though the less essential requirements of good crossband­ desirable species are used for crossbanding, ing veneer is straightness of grain. Of the satisfactory items can be produced if the straight-grained species that have been characteristics of the species are recognized available in quantity and sizes suitable for and the operating procedures adjusted to veneer cutting operations, yellow-poplar compensate for some of the deficiencies. has been a favorite. For instance, a thinner veneer of high If the crossbanding is ro be laid under tensile strength can be substituted for a thin face veneers, it should be uniform in thicker one lower in strength. texture and free from defects. Species that Table 2 shows the average shrinkage and show marked contrast between the early­ density values of some woods commonly wood and latewood, such as Douglas-fir used for plywood and veneered panels. and southern pine, are less desirable than Shrinkage data for quartered (radial) and those in which the contrast between early­ rotary-cut (tangential) stock are shown wood and latewood is slight, such as bass­ since some species are manufactured and wood, aspen, and yellow-poplar. The used extensively in both forms. The table defects that can be permitred in the permits selection ofspecies that have about crossbanding depend upon the thickness the same density and percentage of of the face veneer and upon the quality of shrinkage. Differences in density between finish demanded. A high-gloss finish will twO woods can be compensated for by accentuate minor surface irregularities varying the thickness of the plies in in­ much more than a matte finish. If the verse proportion ro their specific gravities. thickness of the face veneer is about one This method of compensation results in twenty-eighth inch, as often used, and if using a proportionately thicker ply of the a finish that shows no irregularities under lighter species, which might be advan­ reflected ligh' is desired, the crossbanding tageous in some cases. The practice, must be essentially free from defects and however, requires thorough knowledge of the edge joints must be tightly glued. the properties of the wood and is not When the plywood panels are thin (one­ common. fourth inch or less, they can be more easily disrorted by the shrinking or swelling of the crossbands, and low shrinkage charac­ REQUIREMENTS fOR CORES teristics and low specific gravity of cross­ bands become desirable properties. For A high percentage of core roral ply­ lumber core panels, the specific gravity wood thickness helps maintain a flat, un­ and the shrinkage characteristics of the warped surface. In general, the core should cross bands are probably less important so comprise five- to seven-tenths of the total long as one crossband balances the other thickness of a five-ply panel where flatness and stresses do not cause rupture of ad­ is important. jacent glue bonds. When crossbands and face veneers are The limited supply of species that pos­ relatively thin, the cores for high-grade sess all or nearly all of the desirable charac­ panels must be practically free from knots, teristics for crossbanding necessitates the knotholes, limb markings (local areas of use of less desirable species. Sweergum cross grain occurring in the region of and tupelo, for example, are frequently knots), and decayed wood. Unless re­

43 TabJe 2 -Average Jhril/kage arid del/rily valuer of wood commonly glued'

Common species name Shrinkage' Densiry"

Radial Tangential

Har:dwoods Percent PerCeIlI

t~~dert ted 'O ...... 'O ~ ...... ~ '0 ..... ' ...... 'O."'" 4.4 7.3 Ash, 0.41 white 0 ...... •• 'O .. 'O ..... ~'O'O ...... ~ •• 'O ...... 4.9 7.8 .60 Aspen, quaking • 'O ••• 'O •• 'O'O • '" ...... 'O.~ o. 3.5 6.7 Basswood .38 ..... ~'O.'O ...... 6.6 9.3 .37 .. 'O 'O ...... ~ ...... 'O'O ...... 'O ... 'O ...... 5.5 11.9 Birch, yellow .64 -"'00 •••• _" ...." ...... 7.3 9.5 CottOnwood, .62 eastern ...... ~ ...... " 3.9 9.2 Elm, American .40 ...... 4.2 7.2 .50 Mahogany ISwielenia sp.) ...... 3.7 5.1 Maple, red ...... ,...... 4.0 8.2 Maple, sugar .54 ...... ~ • 'O ••••• 4.8 9.9 Oak, northern red .63 ...... 'O ...... ~ 4.0 8.6 Sweergum .63 .. ···,.·····,,········ ...... 4 •••••• 5,3 10.2 S)'camore .52 ...... " .... ~ .. 0­ ...... 5.0 8.4 Tupelo, black .49 ..... -...... -'~""'" 5,-1 8.7 Tupelo, .50 water ...... 4.2 7.6 Walnut. .50 black ...... ~ .. ~ .... ~ ...... , 5.5 7.8 Yellow-poplar .55 ...... 4.6 8.2 .42 Softwoods Douglas-fir, coast .. ~. -.. _...... 4.8 7.6 Fir white .48 t ...... 3.3 7.0 Hemlock, ,39 western ...... 4.2 7.8 Larch, western .45 ...... *'0 ...... ~ 4.5 9.1 Pine, ponderosa .52 • ~ ... ~ ...... ~,,~~ ~ .. ~ k~ ~ 4, ~ ~ ~ •• ~ 3.9 6.2 Pine, AO shordeaf ...... 4.6 7.7 Pine, sugar .51 ~~."' ••• ~. ~~ •• ""~'o...... 2.9 5.6 .36

Redwood, ~ old-g(owrh •• •• o.,,'''.'' _...... _ 2.6 4.4 Spruce, .40 Sitka ...... ~~ ...... o...... ~. o..o. 4.3 7.5 Ao

I Data from \'\food Handbook. "Shrinkage from green to overdry condition expressed in. percent of dimensions when green. 3Density expressed 35 specific gravity based on ovcndry weight and volume at 12 pet. moisture content.

moved, such defeccs may be visible on the viewed in reflected light that causes even a faces of panels afeer they have received a minor irregularity in the surface to show finish. The size of defects that may occur clearly; on the sides and ends of the same in cores without showing upon the finished piece, minor irregularities in the surface faces depends largely upon the thickness may be much less conspicuous. Decayed of the crossband and mce veneers. Prevail­ wood has a different shrinkage rate than ing commercial practice varies as to the sound wood and under moisture changes maximum size of knots and blemishes this Shrinkage djfference may cause permitted, depending in part upm the noticeable irregularities on highly finished quality of finish demanded from the item surfaces. and in part on how readily minor Edge-grained cores are better than flat­ irregularities in the surface may be de­ grained cores because oftheir lower shrink­ tected during the com mon use of the item. age in width. With most species a core A veneered tablerop, for example, is often made of aU quartersawed or of all-flat­

44 sawed material remains more uniform in able for cores. As a general rule, however, thickness with moisture content changes the less desirable the species, the more than one made by combining these twO care wiII be required to fabricate satisfac­ types of material. Use of narrow core Strips tory panels. makes surface irregularities caused by mixed grain (flat sawn and vertical) less obvious. For high-grade cores of softwood LAMINATED CONSTRUCTION it is desirable to use quartersawed stock. If both edge- and flat-grained material or even all flat-grained lumber is used in Glued-laminated or paraUel-grain softwood cores, the panels are more likely construction, as distinguished from to show wavy and irregular surfuces than if plywood or other cross banded construc­ the stock is all edge grained. Edge-grained tion, refers to twO or more layers of wood material is more desirable than flat-grained joined with an adhesive so that the grain for softwood core stock for the additional of all layers or laminations is approximately reason that the hard bands of latewood are parallel. The size, shape, number, and less likely to show through thin fuce ven­ thickness of the laminations may vary eers. Mixing spe(;es in the same core greatly. Glued-laminated construction invites irregularities in the surface because may be used as a base or core for veneer, of the differences in the shrinkage charac­ as in doors or tabletops and other furniture teristics of the different species. It is im­ panels, or it may be used withOllt veneer portant, of course, that all pieces in any covering for chair seats, tabletops, bleacher one core be at the same moisture content seats, and bench topS, structural beams when the core stock is surfaced. Otherwise, and arches (fig. 21), boat timbers, lamin­ when the moisture content of the stock ated decking, airplane propellers, spars, equalizes in service, some pieces will swell spar flanges, or for sporting goods items or shrink more than others and irregulari­ such as baseball bats and bowling pins ties in the surface will result. (fig. 22). The best. core woods for high-grade While the properties ofglued-laminated panels have low density, low shrinkage products are generally similar to those of characteristics, slight contrast between the earlywood and latewood, and are easily glued. Yellow-poplar and basswood as well as several foreign species are desirable core woods. Edge-grained redwood is very satisfactory. Sweetgum and tupelo are used extensively for cores. Ponderosa pine is used extensively for cores in doors. Such cores are commonly built from small pieces ofwood which are byproducts of the manufacture of sash and ocher millwork and are faced with thick veneers. Pon­ derosa pine, because of its lower density and shrinkage characteristics, is less apt to cause showthrough than denser species. Many other species are used successfully for core stock even though they may lack ~f 118468 some of the most desirable properties. Figure 21.-Model of laminated arches Satisfactory panels can be fabricated even used in churches, gymnasiums, super­ if the most desirable species are not avail­ markets, and similar structures.

45 longer available in solid timbers (fig. 23). The manufacture of glued-laminated beams, arches, and trusses has become a very important segment of the wood in­ dustry. Glued structural members ehat may be used in full exposure to weather as well as those intended for dry service are being produced. Laminated utility poles for power transmission lines are further examples of expanding application of the laminating technique. It seems probable that increasing scarcity of large timbers will lead to further expansion in the use oflaminated products even through production of laminated items involves skills and equipment not necessary in pro­ ducing items from solid wood.

M Figure 22.-Laminated bowling pin (un­ finished). solid wood of similar quality, manufac­ ture by gluing permits production oflong, Figure 23.-Laminatad white wide, and thick items out of smaller and frame. less expensive material and often with less waste of wood than if solid wood alone was used. Curved members may be fabri­ Selection of Species and Grades cated by simultaneously bending and gluing thin laminations to shapes that For many glued-laminated items, the would be very difficult or impossible to selection of species is partially limited by produce from solid wood. The essentially the properties desired in ;:he finished parallel direction of grain of the wood article. White oak is desired in ship tim­ to the longitudinal axis of these laminated bers, for example, because it combines products gives them strength that is often excellent properties co hold fastenings with far superior to solid wood cut to the high strength and durability under wet same size and shape. Boat timbers, for ex­ exposures; Sitka spruce is often specified ample, are laminated in sizes that are no for booms, spars, propellers for wind

46 tunnels, and masts because of its high Sapwood is as durable as heartwood strength-weight ratio; and yellow birch is under continuously dry conditions, but als.) sometimes favored for small aircraft under moist service conditions the sap­ propellers because of roughness. wood of even the durable species is sus­ Softwood species, principally Douglas­ ceptible to attack by wood-degrading fir, southern pine, western larch, and hem­ fungi and by insects. When the laminated lock, are used largely in laminated arches product must be durable under moist and beams because of favorable COSt, avail­ exposures, the wood should be treated with a.bility, and adequate strength properties. a suitable preservative. These softwood species are the mainstay ofthe strucrurallaminating industry while others find special applications where their properties are desired. Occasionally, Stresses in laminated Members gluing characteristics and resistance to ad­ verse use conditions may affect the choice Differences in shrinking or swelling are of species, although tests have shown that the fundamental causes ofinternal stresses. glue joints of long-term durability and Within a single member, adjacent lamina­ high strength can be produced with prac­ tions should shrink and swell in about the tically all domestic species. To attain this same amounts and in the same direction. high joint strength and permanence, Laminations, therefore, should have some­ however, the gluing procedure must be ad­ what similar shrinkage properties (table 2) justed more carefully and the adhesive and be at about the same moisture content must be of bener quality for some species when glued. than for others. Maximum lengthwise shrinkage in a Product quality or grade requirements straight-grained piece of normal wood is are often established by design criteria or only about one-third of 1 percent, but the by use requirements. DefeCts permitted in shrinkage across the grain may be 10 to 30 spars or propellers, for example, are times more. Cross grain as well as knots; sharply limited. Severely curved parrs of burls, and other growth characteristics high-strength laminated members gener­ affect the strength of laminations. For this ally require clear and straight-grained reason, slope of grain and knots are wood, free of significant defects, so that restricted in laminated timbers. In bend­ the laminations may be bent to the desired ing members, laminations with smaller curvature without breaking. Defects such knots and straighter slope of grain are as large holes, knots, and decay reduce usually placed in the outer laminations effective glue-joint area. Surfaces contain­ with the grade decreasing toward the ing pitch, cross grain, and knots do not center of the member. glue so well as clear wood. Medium- to If twO or more pieces having different large-sized knots and knotholes aggravate shrinkage values are glued together, even glue-joint delamination when the exposure though they are straight grained, a mois­ involves alternate wetting and drying. ture content change will cause them to Lower grades of lumber, consequently, are shrink or swell in different amounts and less adapted to laminating timbers for ex­ thus set up stresses. Flat-grained or plain­ terior use than for interior use' in which sawed lumber shrinks or swells more in they are kept dry and undergo less severe width with moisture changes than vertical­ changes in moisture content. If the mem­ grained or quartersawed lumber. If bers are well treated (after gluing) with an internal stresses are to be avoided, there­ oil-borne preservative, lower grades might fore, flat-grained wood should nor be be used for exterior service if strength re­ glu!'d to edge-grained wood. In softwood quirements are met. species for structural use, matching of

47 grain in adjacent laminations is much less woods can be bent more severely than soft­ critical than with dense high-shrinkage woods of the same thickness. species. The top and bottom laminations on a laminated member are less apt to face Relief of Stresses check ifthe pith side is turned out because its shrinkage is less than the sap side The stresses that develop in glued­ (fig. 24). Where the laminations come laminated members due ro differences in from small trees, this might be a worth­ while practice to follow. If adjacent laminations differ in mois­ ture content at the time ofgluing, stresses in the glue joint and irregularities in the surface will develop when the laminations later come to a common moisture content. The pieces should therefore be conditioned to about the same moisture content before being glued. For structural members, a range in moisture content no greater than 5 percent between laminations in a single assembly is suggested. If exact trueness of surface is imporrant, as it may be in furni­ ture, even this range may prove excessive. Stresses will also be created if the interior portion of anyone board differs greatly in moisture content from the ourer portion or shell, and it is suggested that such differences not exceed 5 percent. Laminations up to about 2 inches thick are most commonly used in gluing straight timber~, provided that suitably dry srock is available. Withi.1 this 2-inch limit, the thickness of the lamination does not affect the performance or durability of welI­ glued joints, so that different thicknesses may safely be glued in the same laminated assembly. It may sometimes be desirable to use more than one thickness of stock in flat assemblies to attain maximum utility from the lumber supply or to fubri­ cate a laminated member to close dimen­ sions. For curved members, the maximum thickness of laminations is usually gov­ M 138756 erned by the curvature to which the lami­ Figure 24.-End section of laminated nations are bent. The minimum radius to beam hailing the pith side out in top which dry, clear, straight-grained lumber and bottom laminations. Particularly in boards from small trees, the pith can be bent without breaking is about 100 face has less checking tendencies than to 125 times its thickness and varies a great the sap face. (Fourth lamination from deal with the species of wood as well as bottom shows section through vertical within the same species. In general, hard­ finger ioint.)

48 the properties of the various laminations will gradually disappear ifthe glued article is kept for a long time at a constant moisture content. This is because of stress relaxation and creep characteristics ofwood with time. Stresses due to moisture difference between laminations, or within laminations, at the time ofgluing will not reappear after having once been relieved. Stresses due to cross grain or to differences in the shrinkage properties of the ad­ jacent members, however, will reappear if the moisture content is changed after the stresses have once been relieved.

END AND CORNER JOINT CONSTRUCTION

When the end grain surfaces of twO pieces of wood are glued together, a is formed. Mitered joints are usually cut at a 45° angle with the grain and must essentially be treated as butt joints for glu­ ing purposes. If twO pieces of plywood are glued edge to edge or if the edge of one piece of plywood is glued to the face or surface of another, only partially effective glue bonds are obtainable since edge grain of certain plies only is bonded to edge M 13M 532 grain of plies of adjoining pieces. Several Figure 2S.-Various types of corner types of corner joints are shown in figure joints: A, slip or lock corner; S, 25. The importance of moisture control tongue and ; C, blind dovetail; where miter and other corner joints are 0, dovetail; E, dowel; f, mortise and involved is illustrated in figure 26. A tenon; G, shouldered corner; and H, somewhat different corner joint where butt end to side grain. moisture c('ntrol and choice of low-shrink­ age material is important is shown in Figure 27. To obtain acceptable strength in pieces No gluing technique has yet been de­ spliced together endwise, it is necessary vised to make square-end butt joints (fig. to make a scarf, finger, or other sloped 25, H) sufficiently strong and permanent joint (fig. 29). The plain scarf with a low to meet the requirements of ordinary slope generally develops the highest service, and no adhesive has been offered strength, but is also the most wasteful of for commercial bonding of such joints. material and requires considerable care Figure 28 compares bending strength both in machining and gluing to obtain (by twO methods) ofsevera! types of glued consistently high-quality joints. If the corner joints made of particleboard. A grain of a board to be spliced makes an with a plywood spline appears angle with the face of the board, the the most promising. scarf should be cut with the slope of the

49 EFFECT OF MOISTURE CONTENT CHANGES ON MITER ,JOINT

-l_~'__.._G_R_A_IN_D_I_R_£_C_r._1O_N_O__-t'/l

WOOD AT14% "f.C. WOOD AT8% M.C. WHEN GLUED

WOOD AT 2% M.C.

M 136 S39 Figure 26.-Miter joints can open when high-shrinkage material is used and wide variations in moisture content Occur. A glued spline or other reinforcement can reduce or prevent joint separation. Vertical-grained meterial, having lower shrink­ age in width, will reduce chances for separation of this type of joint.

grain rather than against it to more nearly FLAT GRAIN approach side grain on the scarfed surface.

During the gluing operation, end slippage B . ~o 0-C.. C _0,.:>:c.- A should be prevented to keep the paces in proper alignment, and a slight overlap is ~.. ~ desirable to insure adequate pressure on the joint (fig. 30). If the members slip end­ wise during the pressing operation, the B~~~~WOOD HAS'~:RUNK ~c~~~, joint will not receive sufficient and uniform WOOD HAS SWOLLEN pressure and erratic strengths may be ex­ pected. Even plain scarf joints with a low EDGE GRAIN slope are not as strong as clear wood (ofthe same quality) 10 tension paraIIel to the grain. Tests on specimens containing scarf joints stressed In tension indicated the average strength ratios given in this tabula­ tion: WOOD HAS SHRUNK Strength ratio WOOD HAS SWOLLEN Slope ofScarf (jointed/nonjoirlled) ( Pet.) Ii 138 461 1 in 12 and less steep 90 Figure 27.-Corner joint showing effect I in IO 85 1 in 8 of serious moisture changes after fab­ 80 rication (somewhat exaggerated to 1 in 5 65 emphasize need for control of mois­ ture content). The advantage of edge Adequate data are lacking on the dura­ grain over flat grain in this type of bility of glued scarf joints with very steep construction is also evident.

50 -

TYPE 01" JOINT 7::1-< JOINT STRENGTH BASEO ON BENOING STRENGTH OF PARTICLEBOARO (PCT.)

MITER 28 TO GLIIEO b.

MITER WITH 3 38 T3 OOll'ELS ~

MlrER WITH 22 15 PLASTIC ANGLE 6

BIITT WITH 3 36 ';7 OOWELS 6

MITER WITH b "2 73 PLYIltOOO SPLINE

111137679 ...... 28.--Comparison of relati.,,~ bending strength of different types of comer joints in particleboard. In ad­ dition to the mechanical reinforcement in four of the joints, they were also glued.

M 138 ~27 slopes. Ifstrength is important, therefOre, Figul"Cl 29.-End joints for splicing lum­ it is probably advisable to avoid scarfs ber (occasionally also used for panel steeper than 1 in 10 for exterior use or prodUcts): A, (when used with a low slope, can develop almost other severe exposure. the full strength of wood); B, horizon­ Approximate tensile strengths (ex­ tal (cui' pa~allel to wide pressed as a percentage of clear wood) face of board); C:, vertical finger joint of various types of end joint5 are shown in (cut perpendicular to wide face of figure 31. board). Type C is the most commonly Finger joints (fig. 29, R and C) lend used finger joint for structural pur­ themselves mo~e readily to rapid produc­ pose$. tion and uniform quality rhan do plain scarf joints and are more practical where their strength is sufficient for the design result. With short fingers, this effect is not involved. Figure 32 illustrates a suggested as pronounced as with long fingers. method for applying pressure when gluing Figure 33 shows a number of finger finger joints. When pressure is applied joints where the slope (angle between axis only in the longitudinal direction, unre­ of member and sloping joint) and tip Liable bonds in the outer fingers may thickness (tip of finger) are held constant,

51 but the pitch (distance center-to-center of ~--~~-. -----' ~ finger tips) increases ftom three-sixteenth ~ .- J CORRECT inch to one-half inch and consequently the length also increases. The tensile strengths of such joints. made with various slopes, are shown in figure 34. The sloping joint area (per TOO MUCH OVERLAP square inch of section) is included for comparison. Reasonably good conelation between strength and joint area is indi­ cated. This is logical because wood is roughly 10 rimes as strong in tension as in shear, and the number of srrengrh­ INSUFFICIENT OVERLAP reducing finger tips decreases with in­ creased joint area (fig. 33). Figure 30.-Scarf joints should be prop­ The joint geomerry is as important as erly alined for gluing. good glue and gluing practices to produce high-sttength finger joints. or planks. CUtS the fingers, and applies Finger-joior machining lends itself glue to the fingers in rapid succession. quite well to mechanized continuous Whete strength is important, joints operation. Figure 35 shows equipment such as the plain end to side grain (fig. 25, that u-ims (or squares) the ends of boards H) have proved entirely inadequate because

/0% 20% 30% 70% 85%

M 138748 Figure 31.-Approximate percentage of the tensile strength of clear wood obtain­ able with different types of end joints. (Nearly the full tensile strength of a low­ density species has been obtained with butt joints in laboratory experiments but no practical glue or gluing procedure have been developed to do this commer­ ciQlly.)

52 internal stresses from moisture changes. In the manufacture of such parrs, it is therefore necessary to reinforce the end­ to-side-grain joints wirh devices such as and tenons (fig. 25, E and F). Even then, the stresses that recur with seasonal changes put a severe strain on the joints, which makes it very desirable to prorect such joints against changes in moisture concent. The strength and permanence of all types ofend-to-side-grai n joints depend Figure 32.-Suggested method for ap­ upon the type of glue and gluing tech­ plying pressure (P) to finger joints nique, the accuracy of the machining, and while glue is curing the design and fit of the parts. In manufacturing wood assemblies, such as furniture, it is often necessary to they are commonly subjected to unusually fasten together twO pares that shrink and severe stresses in service. Under changing swell differently with moisture changes. moisture conditions, the end-grain surface Tables and desks, for example, are usually of the joint tends to swell or shrink in all designed so that the topS can be expected dimensions of the joint while the side­ to swell or shrink differently than the grain surface of the joinr swells or shrinks frames. Even if feasible, it would be un­ only in one direction. Joints that are not desirable to glue the tOps rigidly to the subjected to much external stress may frames because the differences in shrinkage serve satisfactorily, for example joincs characteristics would tend to distort the made by gluing fucing strips of veneer on cables. Chests are often designed so thar the end edges of a crossbanded tabletop the topS and bottoms clln be expected to (fig. 19, 8). In furniture parts, however, shrink or swell appreciably in width, while stresses might easily exceed the strength of on the ends the runs length­ end-to-side-grain joints in service. Ulti­ wise across the width and no significanc mate failure of the joints usually results change in this dimension can be expected. when external stresses are combined with If the tOpS and bottoms of such items are •

M 120 740 Figure 33.-Finger joints with constant slope (1 :14, angle between axis of member and sloping joint) and tip thickness, ;"ut with increasing (from left to right) pitch and length of fingers.

53 txXJC:t.As-nR 12~-__"_~~ _ ~r• • _~ ______~ TIP aa4'~ 12 , SL~ I.G SLOPE I 8 Sl.OP£ ,,0 1Ot--- -.~-,~ .. ! " '~'-i1O

8~~_.~__ ~ "8 ,. -6[

; ~ ,,~t f .2 ~ j , ...... il! 1:C-iktht~~Q; 0-....- 1. ".1.. -.. .1 .- ~ ~ ~ 1 '-;J .1.. - 5 .. , I , I , , , ',0 ~ § J64/61116Z 16416 i. F 16 • .. l~:r II .. 16 r ~H~ '14~ SLOPE ,-,2 ~ SLOPE" J 14 SLOPE 116 il! ~ :e12>--"'~" ~.. - r-"'2~ ~JOi ..... ·- , , , .... '" ;:; ~ -~JO~ 1!J ~ 8t.··...... I-- " I" · ..... S ~

6 ..---" "!:l 1---6 ~ 4:~- 1'- 1---1. 2r---- ~. I·· '..-,2 , , ! 0- .. .. ! ~~. L .1 , , '" 0 I 4 .1 T I , $ T , 16 8 ~ f ,1 .. • 7 Ii r ,i .. i Ii 7 PITCH" liNCH) " M 123420 Figure 34.-Tensile strength of Douglas-fir finger joints with constant tip thickness and six different slopes. For each slope the pitch varied from three-sixteenth to one-half inch (see fig. 33). glued or fastened rigidly to the ends, vices, such as slotted screw holes, that subsequent swelling or shrinking with permit normal shrinking and swelling moisture changes can be expected ro cause when twO elements differing in shrinkage splits and disrortion, to break the joinr, properties musr be joinred together or to or ro rupture the wood. In designing wood insure that all joining elements have items, it is important to provide for de- similar shrinkage properties.

SELECTED REFERENCES

Bohannan, Billy, and Selbo, M. L. Freas, A. D., and Selbo, M. L. 1965. Evaluation of commercially made 1954. Fabrication and design of glued lami­ joints in lumber by three test methods. nated wood strucrural members. U.S. U.S, For. Servo Res. Pap. FPL 41, 41 p. Depr. Agric., Tech. Bult. No. 1069. U.S. Depr. Agric. For. Serv. For. Prod. 220 p. Lab., Madison, Wis. Haskell, H. H., Bair, W. M., and Donaldson, Bryant, Ben S., and Stensrud, R. K. William. 1954. Some factors affecting .the glue bond 1966. Progress and problems in rhe southern quality of hard-grained Douglas-fir ply­ pine plywood industry. For. Prod. J. wood. For. Prod. J. 4(4):158-161. 16(4): 19-24. Cass, S. B., Jr. Heebink, B. G. 1961. A comparison of hor-pressed interior 1963. Importance of balanced construcrion plywood adhesives. For. Prod. J. J 1(7): in plastic-meed wood panels. U.S. For. 285-287. Serv. Res. Note FPL-021. 5 p. U.S.

54 .. 138686 Figure 35.-Finger-iainting equipment: A, trim ; B, (utter (ar shaper) head; and C, glue applic!;ltar.

Dept. Agdc. For. Serv., For. Prod. Lab., Selbo, M. L. Madison, Wis. 1963. The effect of joint geometry on tensile Heyer, O. C, and Blomquist, R. F. strength of finger joints. For. Prod. J. 1964. Stressed-skin panel performance. U.S. 13(9):390-400. For. Serv. Res. Pap. FPL lB. 12 p. U.S. Stensrud, R. K., and Nelson, J. W. Dept. Agric. For Serv. For. Prod. Lab., 1965. Importance of overlays to the forest Madison, Wis. products indusrry. For. Prod. J. 15(5): Jarvi, R. A. 203-205·. 196B. Laboratory test for prepre;sing of ply­ Strickler, M. D. wood. For. Prod. J. 18(3):53-54. 1970. End gluing of lumber. For. Prod. J. 20(9):47-5 1. Lamburh, A. L Strickler, M. D. , Pellerin, R •. F., and 1961. Prepressing plywood assemblies. For. Talbott, W. Prod. 11(9):416-4 19. J. J. 1970. Experiments in proof loading Struc­ Raknes, E. tural and end-jointed lumber. For. Prod. 1967. Finger jointing with resorcinol glue at J. 20(2):29-35. high wood moisture coneent. Norwegian U.S. Forest Products Laborarory, Forest Service Inst. of Woodworking and Wood Tech. 1966. Some causes of warping in plywood Rep. No. 32. and veneered products. U.S. For. Serv. Raknes, E. Res. Note FPL-0136. B p. U.S. Dept. 1969 • Finger jointing strucrural timbers Agric. For. Serv, For. Prod. Lab., Madi­ with a high moisture contene. Norsk son, Wis. Skogind 23(G}:lB4-190. U.S. Forest Products Laboratory, Forest Service Roth, A. 1974. Wood handbook: Wood as an engi­ 1970. A new type of finger joint. Pap. ja neering material. U.S. Dept. Agric., Puu 52(1):25-2B. Agric. Handb. 72 rev. 432 p.

55 PREPARING WOOD FOR GLUING

Glues vary in properties and use charac­ ofglued members, A change in moisture teristics as well as in quality, but in most content generally develops stresses on the instances failute of glue joints can be glue joints; the magnitude ofthese stresses traced to impwper preparation of the is roughly proportional to the magnitude wood. Among the various fuctors causing of the change with a particular species. inadequate glue bonds, the most prevalent A certain percentage change in a dense is lack of proper moisture control, both wood develops greater stresses than the before and afrer gluing. same change in a light species. Glue joints will remain most nearly free MOISTURE CONTENT from stresses' ifthe moisrure content of the glued parts (when the glue sets) equals the The moisture content of wood at the average moisture content the product will time of gluing is important because it attain in service. The moisture content of affects the quality of the bond and the wood for gluing, when increased by rhe performance of the glued product in water from the glue, should be as near service. as possible to the average moisture content Satisfactory adhesion to wood is ob­ that the glued member will have in tained with most adhesives when the wood service. is at moisture contents of about 6 to 17 The moisture content of dry wood in percent, and with some glues well beyond service above grade in dwellings in the this range (up to 25 pet. has been re­ United Srates commonly varies from about ported for resorcinol adhesives). The pre­ 4 percent to about 13 percent. The wood cise upper and lower limits vary with ad­ in a chair in a dwelling in northern hesive type and formulation. Satisfactory Minnesota, for example, may have a joints have been made experimentally with moisture content as low as 4 percent in the wood that was near the fiber saturation winter and as high as 10 percent in the point (with resorcinol-type glues) and at same room in the summer. 'Wood in a simi­ very low moiscure contents (with casein lar item in a dwelling along the GulfCoast glue). may maintain a moisture content varying The moisture content of the wood little from 13 percent throughout: the affects the rate of change of viscrsity of year. Except for the coastal areas and many adhesives during the ?"ssembly certain arid inland areas, the moisture period. It also affects the rate of setting content of wood in heated buildings will and, in hot pressing, it affects markedly average about 7 or 8 percent for the 'if:~" the tendency to form blisters (unbonded Dry wood in protected but ur.he:(~,!d areas caused by formation of steam in the shelters has an average ofabout 12 percent joinc when moisrure content is roo high). moisture throughout a large part of the Consequently, the moisture content of the United States, but the average is less in the wood may necessitate adjustments com­ arid areas and more along the coasts. patible with the gluing procedure. The amount ofwater added to the wood Most importantly, moisture content of in gluing varies fru;n less than 1 percent the wood at the time of gluing has much in lumber 1 inch thick to over 60 percent to do with the final strength of the joines, in thin plywood where the amount ofwood the development of checks in the wood, is small in proportion to the amount of and the stability (freedom from warping) glue. Calculated percentages of moisture

56 added to wood in gluing are given in table where P = percentage of water added 3 for a number ofspecies in constructions. w = pounds of water in 100 Thickness of the wood, number of plies, pounds of mixed g~ue density of the wood, glue mixture, and G = pounds of mixed glue used quantity of glue spread all affect the in­ per thousand square feet of crease in the moisture content of the wood glue-joint area when the glue is spread. If pertinent tables L = number of laminations are not readily available, the percentage L-1 = number ofgluelines in glued of water added with the glue may be cal­ assembly culated from the following formula: 2 T = average laminarion thickness (in inChes) 0.000192WG L ­ 1 S = specific gravity ofwood (dry) p TS L Thin veneer, even if dried almost free of moisture, will take up so much water 'Both this formula and the percentages listed in from wet glue that its moisture coment table 3 are based on the assumption that all water will become higher than the probable added by the glue is absorbed by the wood. The average for the plywood in service. Very 2Ssumption is somewhat .:rroneous, but the method yields resules sufficiendy accurate to guide in select­ dry veneer is easily split or cracked and is ing suitable moisture contents. difficult to handle before and during the

Table 3.-Calculated percentages of moistllre' (Jdded 10 UI()()d ill glui/lg (fil't-ply cOllstrlldioll)

Adhesive Glue Lami- Wood species spread' nacion Or ply Hard White Mahogany Douglas- Somhern thick- maple oak fir pine ness

Inch Pet. Pet. Pc/. Pel. Pet. Casdn 65 3/4 1.4 1.3 1.8 1.9 1.8 Do ...... 95 3/4 2.1 2.0 2.7 2.7 2.6 Do ...... 65 3/8 2.8 2.7 ,.6 3.7 3.5 Do ...... 95 3/8 4.2 3.9 5.3 5.5 5.1 Urea resin 45 3/4 .6 .6 .8 .8 .7 Do ...... 65 3/4 .9 .8 1.1 1.1 1.0 Do ...... 45 3/8 1.2 1.1 1.5 1.5 1.4 Do ...... 65 3/8 1.7 1.6 2.2 2.2 2.1 Resorcinol or intermediate­ temperature­ setting phenol 45 3/4 .4 .4 .6 .6 .5 Do ...... 65 3/4 .6 .6 .8 .8 .8 Do ...... 45 3/8 .9 .8 1.1 1.2 I.l Do ...... 65 3/8 1.3 1.2 1.6 1.7 1.6

I Moisture calculated on the basis of average specific gravity values as follows: Hard maple, 0.63; white oak, 0.67; mahogany, 0.49; Douglas-fir, 0.48; southern pine, 0.51. Glue mixtures used were: Casein, 1 p2tt solids co 2 parts water; urea resin, 1 part solids to 0.65 part water; resorcinol and inter­ mediate-remperature-setting phenol, 70 pet. solids. 21n pounds per square foot ofjoint area.

57 gluing operation. It also very quickly Lumber reabsorbs moisture during handling. Therefore, it is not practical to dry it In drying lumber, although the desired below 2 or 3 percent moisture content. average moisture content may be reached, Furthermore, experience has shown that a individual differences may be considerable; moisture content of 5 percent or less in variations can occur in the moisture con­ the veneer at the time of gluing is satis­ tellt ofindividual boards and even between factory for furniture and similar uses. different parts of the same board. It is Lumber with a moisture content of 6 to desirable to reduce these differences as 8 percent is satisfactory lor gluing into much as possible before gluing. In laminat­ furniture and similar items that will be ing nominal I-inch boards, for example, used in most of the areas of the continental moisture content should vary no more than United States. Lumber for use in unheated 5 percent between members in a single buildings or shelters, and in partially pro­ assembly or between different pares of the tected installations, should generally con­ same board. tain about 11 percent moisture before To obtain such uniformity, a condition­ gluing. The moisture added in gluing will ing period after kiln or air drying is then bring the toral moisture to about 12 usually desirable .. Conditioning is best ac­ percent. In moist, wet, or unprotected ex­ complished in a stOrage room in which the terior exposures, glutd members may temperature and humidity are controlled qevelop moisture contents well above 12 to maintain the desired moisture contenr. percent. A moisture content range of 12 to The time required for conditioning de­ 15 percent, howevet, is generally satisfac­ pends upon the species, dryness, method of tOry for such gluing. piling, and circulation of air, as well as The manufacturer shipping glued arti­ upon the temperature and relative humid­ cles to various parts of the country and ity of the stOrage room. A conditioning making products for various uses cannot period of 1 week in open piling is benefi­ provide for all the variations to be met cial. Dense species generally require longer in service. He must, therefore, aim at conditioning than lighter ones and the approximate averages. Wherever feasible, moisture content equalizes more rapidly a finish that effectively excludes moisture with increased temperatures. will guard against checking and joint failure because it slows down the rate of Veneer moisture changes. Veneer is generally dried in mechanical dryers of various types, but may occasion­ DRYING AND ally be kiln or air dried. The internal CONDITIONING stresses chat develop in drying veneer are generally noc as great a problem as with Wood free fnm casehardening and heavier StOck. Occasionally, however, other internal stresses will be best for glu­ internal stresses do cause wrinkling, check­ ing. Internal stresses, which generally ing, or honeycombing of the veneer. These result from drying, may prevent a good defects are easily recognizable and usually fit of the mating surfaces and mean warp­ can be avoided by improving the drying ing and checking after the wood is glued. conditions. Dried wood should be tested for the pres­ In plywood plants where veneer is cut, ence of such stresses before it is removed it is custOmary to glue it soon after drying. from the kiln. If wood is casehardened or For general purposes, veneer is in condition otherwise stressed, it should be treated to for gluing if it is reasonably flat, tightly relieve the stresses. and smoothly CUt, free from defects not

58 permitted by the grade, and at a moisture veneer is about 8 percent, so thar chance content suitable for the glue and gluing of subsequent drying and checking is ptocess. For most cold-press gluing, the reduced or eliminated. moisture COntent of veneers should be " Wet veneer is likewise glued to paper percent or less. For hoc pressing (with facings to form a paper-veneer combina­ liquid glues), thin veneers should be 5 tion for t;olltai ners. percent or less in moisture content; mois­ cure content of thicker v('nee'r m:w be STORAGE BEFORE GLUING slightly higher. depending upon the species, the amount ofglue spread. md the temperature of the press platens. If the It is nor sufficient only to drr and con­ moiscure content is too high. steam blisters dition wood ro the ptoper moisture con­ will form when the pressure i~ released. tent for gluing. Because a stOrage period Veneer may take on moiscure in lengthy is generally required between drying and shipment or storage, and it i~ gond prac­ final machining and gluing, provisions tice to redry it JUSt before gluing. Fine must be made to prevent appreciable mois­ hardwoods are often redried in har place ture changes during this time. dryers in which the veneer remaw,> bt·­ tween ~he plates until it is sufficienrly drr Lumber and flat. In the best ptactice, the veneer is then piled so that it will scay flat and The ideal condition is to provide storage will cool before gluing, but so ic will not space for dried lumber with humidity and reabsorb much moisture from the air. temperature control to maintain the mois­ Pancy veneer, such as burl, crotch, or ture content in the stock at a level suitable other shore-grained pieces, is more likely for gluing--such as 7 percent for most in­ to wrinkle and check than straight-grained door use and about 12 percent for outdoor veneer. Film glue is well suited for fancy service. If the temperature is controlled at veneers and they may be hot pressed with about 75° F. and the relative humidity at the veneers at a moisture content of 8 to 10 35 percent (wet-bulb depression about 16° percent. Fragile figured veneers are some­ to 17° F.), the moisture content of the times toughened by dipping chem in a wood will be suitable for interior wood­ liquid sizing solution and chen redrying. work. Several different sizing solutions have If controlled temperature and humidity been used, but one satisfactory solution is: storage is not possible, maintaining the temperature in the storage room about Ing1"edimt Parts by weight 20° F. above the outdoor temperature will Water 63 provide a moisture content. in the lumbef Animal glue 16 of6 to 8 percent in most parts ofthe United Alcohol 16 States. Similarly, if the lumber is stored Glycerine 5 under roof in an unheated area, the mois­ ture content will be fidrly close to 12 per­ It is often desirable to reinforce highly cent when the lumber is. stored for 2. suffi­ figured veneer to reduce dimensional cient rime to come to equilibrium. It is changes and facilitate handling. This is very important, however, that duriIlg cold sometimes done by bonding the figuted weather the lumber be brought inro a face veneers to a thin but strong veneer warm room at least 24 hours before ma­ backing (as 1/40-in. birch or maple) using chining and gluing; cold lumber will slow a film glue. Film glue adds no moisture to down the rate of cure and generally result the veneer and permits forming the bond in inferior glue b(lnds. This is important when the moisture comenc of the face: when using resin. adhesives, but even a

59 protein-base glue such as casein has given in the layer should not exceed 0.010 inch. more dependable results on lumber Experience has indicated that cup in inches warmed co room temperature before glu­ in boards after final surfacing preferably ing. should not exceed one ninety-sixth of the Warming lumber of the denser species ratio ofwidth to thickness. Thus, for lami­ before gluing is especially important. nations 6 inches wide, a maximum cup is suggested as one-sixteenrh inch in a board Veneer 1 inch thick, one-eighth inch in a 112­ inch board, and one-fourth inch in a 1/4­ Veneer is usually glued shortly after dry­ inch board. ing co minimize the chance for appreciable Preferably, machining should be done moisture changes. Gluing hot veneer, just before gluing so that the surfaces are however, muse be avoided co prevent the kept clean and are not distorted by mois­ glue from becoming too dry before pres­ ture changes. Where the four sides of a sure is applied. piece are to be glued, it is beSt to glue in When 2 to 3 days lapse between drying twO operarions and machine just before and gluing, provision must be made to each operation. prevent moisture changes. Even though Surfaces made by are usually the veneer is solid stacked, the ends and the rougher than those made by planers, joinr­ top ofthe loads may gain moisture rapidly. ers, and other machines equipped with If it is impossible to Store veneer at the cutter heads. Modern saws freshly sharp­ proper equilibrium moisture condition ened, well ali ned, and skillfully operated (less than 5 pet. when hot pressing with are capable of producing surfaces adequate liquid phenolics), covering the loads com­ for gluing many products and provide a pletely with moistureproof film (poly­ saving in time and labor. Except where the ethy lene) provides considerable prorecdon. saws are usually well maintained, how­ ever, glue joinrs between sawed surfaces SURFACING WOOD FOR are weaker and more conspicuous than GLUING those between well-planed or jointed sur­ faces. Consequently, if inconspicuous Careful machining is essential in prepar­ glue joints of maximum strength are re­ ing wood for gluing. For strongest joinrs, quired, planed or jointed surfaces are gen­ wood surfaces should be machined smooth erally more reliable. and true with sharp tools, and be essen­ Machine marks, caused by feeding the daIly free from machine marks, chipped or stock through a planer toO fast for the loosened grain, and other surface irregu­ speed of the knife, prevent complete con­ larities. To provide uniform distribution of tact ofthe joint faces when glued. Machine gluing pressure, each lamination or ply marks in cores of thinly veneered panels should be of uniform thickness. A small are likely to show through the finished variation in thickness among laminations surface. Unequal thickness or width, or plies may cause considerable vanation which cause unequal distribution ofgluing in the thickness of the assembly. In the pressure and usually result in weak joints, production of glued-laminated members, may be due to the grinding, setting or for example, the differences in thickness wearing of machine knives. Knives that are throughout a lamination consisting of a dull or improperly set or ground may single board should not exceed 0.016 inch. produce a burnished surface that interferes When the lamination consists of tWO or with gluing or formation of rhe strongest more boards, laid side by side or end to glue bonds. end, differences in thickness at the edge Wood surfaces are sometimes inten­ and end joints between any two boards tionally roughened by rooth ,

60 scratching, or sanding with coarse sand­ grained surfaces can be made substantially paper in the belief that r.::;ugh surfaces are as strong as the wood itselfin shear parallel better for gluing. However, comparative to the grain, tension across the grain, and strength tests at the Forest Products Labo­ cleavage. The tongue-and- and ratory failed to show better results with other shaped joints have the theoretical roughened than with smooth surfaces. advantage of larger gluing surfaces than Also, studies ofthe penetration ofglue into the plain joint, but the extra surface is not wood have shown the theoretical benefit of needed because adequate plain joint~ can the roughened surface to be improbable. be produced easily by normal commercial Light sanding has proved an advantage in gluing procedures. Furthermore, the preparing for gluing such surfaces as resin­ impregnated wood, laminated paper plastic, plywood that has been pressed at high temperatures and pressures, or wood that has been glazed from dull tOols or by being pressed excessively against smooth, hard surfaces. Within recent years, significant devel­ opments in sanding equipment have been reported. Advantages of so-called planing in preparing wood for gluing are reported to be deeper cutS in a single pass, close tolerances, and improved surface quality for gluing.

MACHINING SPECIAL TYPES Of JOINTS

Plain side-grain-tO-side-grain joints are generally prepared with planers and joint­ ers equipped with rotary curter heads and knives. With such equipment, clean-cut smooth surfaces for gluing are relatively easy to obtain when the machines are well maintained. With special or irregularly shaped joints, ideal surfaces for gluing are often more difficult to obtain.

Side-Grain Surfaces

Plain, tongue-and-groove, circular tOngue-and-groove, and dovetail are four of the more common types of edge joints used in gluing boards into wider pieces (fig. 36). As knowledge of adhesives and gluing techniques has increased, the plain M 118 HI edge joint has become far more common Figure 36.-Various types of edge joints. than any of the others. With most species The plain (top) is the most commonly ofwood, straight plain joints between side­ used.

61 theoretical advantage is often lost, wholly well-machined and well-glued, is less than or in part, because the shaped joints are the average strength of solid wood in more difficult to machine to a perfect fit tension. Presumably strength is lower than are plain joints. because ofthe difficulty in accurately align­ Experience has shown that the lack of a ing the structural wood elements of one perfect fit in tongue-and-groove or dove­ piece with those of the joining piece and tail construction often results in joints that because of stress risers at the tip of the are weaker than plain joints. The principal scarfs. advantage of the tongue-and-groove joint is that the pieces of wood to be glued are End-to-Side Surfaces more easily aligned and held in place during assembling. This makes possible End-to-side-grain joints are difficult to faster clamping and less slipping of the machine properly and to glue adequately parts under pressure, which are advantages for ordinary requirements. Such joints are so important that some form of tongue and subjected in service to unusually severe groove is often used in edge gluing where stresses as a result of unequal dimensional pressure is applied by clamps. A shallow changes in the twO members of the joint (one-eighth inch or less) as the moisture content changes. It is is as useful in this respect as a deeper cut therefore necessary to use dowels, tenons, and is less wasteful of lumber. or other devices to reinforce the joint by bringing side grain into contact with side End-Grain Surfaces grain (fig. 25, E and F). In dowel, mortise­ and-tenon, dado, tongue-and-groove, It has proved practically impossible to rabbet, slip, and dovetail construction, an make end-grain butt joints sufficiently imperfect fit of the parts often results in strong or permanent for ordinary service. only partial adhesion in the joints. In most With certain synthetic resins (epoxies and of these joints, complete contact over urethanes) it has been possible to approach poorly fitted portions cannot be obtained the tensile strength of weaker species, but by ordinary gluing. Furthermore, pressure such end gluing is of little practical value is often applied but momentarily in gluing because ofIow bending strength and cum­ and the glue does not set during the pres­ bersome gluing procedures. To approach sure period. Careful machining of irregu­ the tensile strength of various species by larly shaped joints is therefore highly end jointing, use a plain scarf, finger joint, important to obtain maximum strength or other form of joint that exposes a certain and durability in service. amount of side-grain surface (fig. 29). A serrated scarf appears advantageous in CUTTING AND PREPARING providing greater gluing area than a plain VENEER scarf, but has never been extensively used because of greater difficulties in machin­ Veneer is commonly rotary Cut, sliced, ing it. Even the plain scarf has essentially or sawn. The total quantity of rotary-cut been replaced by finger joints, except in veneer, however is much larger than the cases where maximum strength is needed. combined amount of sawed and sliced Careful machining to insure an accurate veneer produced. Most rotary-cut veneer fit of the surfaces is essential to develop is produced in large sheets by revolving the the maximum potential strength of the log against a knife. The flat-grained veneer joint. With available glues the plain scarf produced in this manner is peeled off in a with a low slope generally will produce continuous sheet very much like unrolling the highest strength. Even with very low paper. The half-round and back-cutting slopes the average strength of scarf joints, processes produce highly figured veneer

62 from stumps, burls, and other irregular stock, thin cores (five-sixteenth inch and parts of logs. These processes consist of less), for cross-banding, and for curved placing a part of a log or bolt off center laminated members. Most veneer that is in a lathe, usually with an auxiliary device glued ranges in thickness from one-fourth called a stay log, and rotary cutting the bolt to one thirty-second inch. Veneer thinner into small sheets of veneer. As the veneer than about one thirty-second inch is dif­ is bent away from the log during cutting, ficult to bond with liquid glues because the the open (knife) side often develops checks. sheets are fragile and curl readily when the This open side is likely to show defects glue is spread. Thin veneers can be glued in finishing and should be the glue side without prohibitive difficulty, however, whenever possible. with film glues. Rotary-cut veneer is produced in thick­ Veneer surfaces, particularly the loose nesses ranging from about five-sixteenths sides, are often somewhat rough and ir­ to one one-hundredth of an inch. regular, but by heating the logs and care­ Sliced veneer is cut to obtain a definite ful cutting, veneer can be produced that figure and is produced in long, narrow is comparatively smooth and firm on both sheets by moving a flitch or block against sides. Since veneer is seldom resurfaced a heavy knife. The veneer is forced abruptly before it is glued, the care with which away from the flitch by the knife, thus it is cur is important to good gluing. If causing fine checks or breaks on the knife it is equally well cut, veneer produced by side. The checked or open side should be any of the three processes can be glued the glue side whenever possible. equally well. In book-matching face stock where the In gluing operations where full-sized open side of every other sheet must be the sheets of veneer are available, the sheets finish side, the veneer must be well cur. may be glued immediately after drying. Mahogany, walnut, and other prized hard­ Cutting to size is preferably done bef0re woods are commonly sliced for the furni­ the final drying_ Cutting after dryi ng ture trade, and slicing softwood for vertical allows more opportunity for the veneer to grain stock is becoming common practice. reabsorb moisture from the air. Further, Most sliced veneer is Cut in thicknesses very dry veneer is easily damaged and ranging from about one-sixteenth to one should be handled as little as possible. one-hundred twenty-fifth or an inch, but Whenever a face for a high-go.de for special orders veneer one-eighth inch or veneered panel is made of two or more thicker can be cur. sheets of veneer, careful edge jointing is Veneer is also sawed from flitches, usu­ necessary to make the joints inconspicuous. ally to get a desirable figure or grain. It This type of joint is made by placing the is produced in long, narrow strips which dried veneer in piles of several sheets and are essentially of the same quality and then running the piles over a special veneer appearance on both sides. Being equally which makes the individual veneer firm and strong on both sides, alternate edges smooth and true. These sheets are pieces may be turned over to match them then laid in the desired position and either for figure to serve as the faces of veneered glued together in a special machine called panels. Sawed veneer usually ranges in a tapeless veneer splicer or taped tightly thickness from one-fourth to one-thirtieth together by taping machines. In recent ofan inch. Because sawing wastes material, years, the taping process has been in­ many mills have discontinued production creasingly replaced b}' a machine that glues of sawed veneer. a thread (usually glass fiber) in a zig-zag Sliced and sawed veneers are used princi­ fashion across the veneer joint (fig. 37)­ pally for faces in plywood and veneered The glu~ is a hot melt, permitting very panels. Rotary-cur veneer is used for £1.ce rapid edge bonding of veneers.

63 M 138 26~·1l Figure 37.-Machine for edge-bonding veneers by gluing hot-melt saturated threads across the faces of adjacent s!rips. A zig-zag thread pattern is used when tighter edge bonding is required.

SELECTED REfERENCES For cores, crossbands, and sometimes \ even for faces, perfectly tight joints be­ Davis, E. M. tween the edges ofthe veneer are not neces­ 1962. Machining and related characteristics of United States hardwoods. U.S. Oep. sary. Such items may be jointed satisfac­ Agric., Tech. Bull. No. 1267. 68 p. torily on a veneer clipper. For general McMillen, J. M. utility plywood, where thick veneers are 1963. Stresses in wood during drying. U.S. used, the veneer sheets are merely laid in For. Prod. lab. Rep. 1652. 33 p. U.S. position without fastening of any kind. Oep. Agric. For. Serv. For. Prod. Lab., The spaces between the sheets or even Madison, Wis. lapped edges, which often result, are of Peck, E.C. 1932. Moisture content of wood in dwell­ minor importance in this grade of panels. ings. U.S. Oep. Agric. Circ. 239. 24 p. If a very high-quality finish or a very Rasmussen, E. F. high degree of resistance to severe service 1961. Dry kiln operator's manual. U.S. Dep. is desired, edge gluing of all veneer joints Agric., Agric. Handb. No. 188. 197 p. is justified. If the tapeless splicer is used Mar. for this purpose, the amount ofglue spread Rietz, R. c., and Page, R. H. 1971. Air drying of lumber: A guide to must be carefully controlled lest excess industry practices. U.S. Oep. Agric., glue squeeze out on the surface of veneer Agric. Handb. 402. 110 p. and interfere with subsequent gluing. Ex­ Simpson, W. T., and Soper, V. C. cessively high pressures or temperatures 1970. Tensile stress-strain behavior of flexi­ of the shoe of the tapeless splicer may bilized epoxy adhesive films. USDA For. burnish the edges of the veneer sheets Servo Res. Pap. FPL 126. 13 p. U.S. Oep. Agric. For, Serv. For. Prod. lab., Madi­ enough to cause a weakly bonded streak in son, Wis. the plywood. If weather resistance is im­ Sisterhenm, G. H. portant, the adhesive used to splice the 1961. Evaluation of an oil-fired veneer sheets should be as durable as the adhesive dryer-its effect on glue bond quality. used in bonding the plywood. For. Prod. J. 11(5):207-210.

64 ADHESIVES AND SONDING PRO.CESSES fOR VARIOUS PRODUCTS

As indicated earlier, adhesives that are ant (lower than exterior but gr.eater than excellent for bonding certain wood species interior), and (3) exterior adhesive, with may not be equally well suited for others. veneers chat may be of lower grade than In a similar manner, and particularly be­ required for exterior plywood. cause production procedures vary, an ad­ Minimum requirements for each type, hesive well suited for one product may be developed through correlation with results entirely impractical for another. As an ex­ of long-term exposures, are detailed in ample, alkaline phenolics have for years Product Standard PS l. been the mainstay in production of ex­ High-temperature-setting (alkaline) terior-type sofrwood plywood, but their phenol resin adhesives are used almost ex­ high-temperature-curing requirements clusively for exterior softwood plywood. keep them from being practical for lami­ Formulations vary and what is suitable for nated timbers. Such timbers would ex­ one species may not necessarily be the ideal plode from steam formation in the interior adhesive for other species. Adhesives that if heated to the temperatures required to had been used successfully for years in cure phenolic-type adhesives used for production ofDouglas-fir plywood did not plywood. give the same trouble free performance on No detailed discussion will be attempted southern pine, and reformulation of phe­ on the manufacture and composition of nolic adhesives for southern pine became different types of adhesives. The user is necessary. Higher glue spreads were more concerned with how well the adhesive generally required for southern pine ply­ adapts to his process and the dependability wood than for Douglas-fir plywood. of his products. Hence, good communica­ For moisture-resistant glue bonds in tion between supplier and user is more interior plywood, soybean glues, georally important than knowledge of the type of fortified wIth some blood, or highly ex­ hardener, filler, extender, solvent, and tended phenol resin glues are usually em­ fortifier that mayor may not be employed ployed. For intermediate moisture­ in formulating the adhesive. resistant interior plywood, extended phe­ A brief discussion of adhesives and pro­ nol resin adhesives or blood glues fortified duction procedures for major segments of with phenol resin are commonly used. the wood gluing industry follows. Both exterior and interior plywood glue bonds are cured in multiopening hot presses; steam is employed to raise the press PLYWOOD platens to the required temperature. Interior plywood is sometimes also made Softwood plywood is generally produced by cold pressing. The glue is then usually in twO types, interior and exterior. Exterior mixtures of soy flour and spray-dried plywood is bonded with completely water­ blood, often of low soluhility, or even proof adhesive that must be able to with­ straight blood. There is still some straight stand temperatures that char the wood soy flour glue used, but the quantity is without separating joints. Interior ply­ quite small. Figures 38 to 43 illustrate wood is produced with three levels ofglue­ the major steps custom:.>.!. il y employed in bond quality: (1) Interior or moisture softwood plywood production. Within resistant, (2) intermediate moisture resist­ recent years more auromated layup systems

65 M 118 190 figure 41.-Spreading glue on veneer with a conventional double-roll glue spreader and assembling the veneers for plywood.

MIlS 193 Figure 38.-Rotary cutting of Douglas­ fir vei1eer.

M 138352 Figure 42.-Equipment for applying ad­ hesive to veneer by curtain coating. A, Veneer sheet entering coater; B, veneer sheet coated with adhesive Figure 39.-Clipping softwood veneer going to layup table. to width and cutting out objectionable defects.

M 138 191 figure 43.-Glued and assembled ve­ M 138 197 neers being transferred to automatic Figure 40.-Drying softwood veneer in press loader. Arrow points to 20­ roller-conveyor dryer. opening steam-heated hot press.

66 have been explored where the core pieces cular to the face (fig. 19); this will reduce are joined edge to edge with hot-melt­ or eliminate cupping tendencies that are coated threads (fig. 37) and then cut to more apt to occur with wide, flat-grained panel size. Glue is applied by rubber­ core boards. covered roll spreader (fig. 41), spray With particleboard cores (fig. 17), the systems, or by curtain coating (fig. 42). crossbands are sometimes eliminated, Curtain coating ;~ reported to result in depending upon various fuctors such as more uniform spread as well as less waste shape of particles, size of the panel, and of adhesive. shrinking and swelling characteristics both While softwood plywood finds its major of the core and the veneer. For large panels uses in structural applications such as hous­ such as tablerops, crossbands are usually ing, most hardwood plywood is produced employed; for smaller panels, and partic­ for furniture, wall paneling, door skins, ularly if both the particleboard and the and similar uses. Hardwood plywood is face veneers are from woods of low or also classified according to glue-bond moderate shrinkage, crossbands are often quality: Type I and technical, fully water­ eliminated. Fine particles on the core faces proof; type II, water resistant' and type III, are important when only face veneer is em­ moist;.l[e resistant. ployed. Large particles are more likely to Adhesives used for type I and technical cause showthrough. On the other hand, are generally phenol resin, melamine, or boards made of thin flakes generally shrink melamine-urea combinations. This does and swell less in width and length than not imply that these two adhesive types boards made of particles such as slivers, are considered equally resistant or durable shavings, or . in long-term exterior service. Type II is Materials for lumber-core panels are bonded with urea resin (sometimes mod­ generally selected to obtain stable, smooth erately extended), as well as other bonding material that will not contribute to the agents of moderate moisture resistance. warping of the panel nor contain defects Type m is generally bonded with highly that might show through the faces. Woods extended urea, but occasionally with casein with a. relatively low density, low shrink­ glue. age characteristics, uniform texture, and a Hardwood plywood is produced either reputation for staying flat in service are by hot pressing or cold pressing, depend­ preferred. ing on the equipment available. Type I Correct and uniform moisture content and technical, because ofthe adhesives em­ in the core at the rime of gluing is also ployed, are hot pressed. Types II and III importane. About 7 percene is suitabie for may be either hot or cold pressed depend­ most of the United States. ing on the adhesive used (ureas are formu­ A simple method of determining lated both for hot and cold pressing; casein changes in moisture coneent that might is generally cold pressed). have occurred in the core after the panel Much hardwood plywood, particularly was glued is illustrated in figure 44. A that going ineo furniture, is made with a strip is cut from the panel in the direction thick core of lumber or particleboard, and of the grain of the crossbands, and with occasionally other material. A common a thin the cross bands are released construction is made up of a nominal l­ from the core for a diStance of about 10 inch core, crossbands of veneer frequendy inches. In figure 44, A, the core was in one-twentieth inch thick, and veneer f.'lces tension across the grain before releasing the one twenty-fourth or one twenty-eighth crossbands, and had been at higher mois­ inch thick. Lumber care is often made up ture coneene when the panel was glued. In ofnarrow Strips edge glued to the required figure 44, B, the core had been in compres­ width and with the annual rings perpendi­ sion before being released from the cross­

67 A

I \ - o. S M 137693 Figure 44.-lf the panel had 7 fo 8 percent moisture content when veneers (cr(\S5­ bands and faces) were separated from the core, it is safe to assume that: A, moisture content of core was too high at time of gluing; a, moisture content in core was foo low when panel was glued. band and had picked up moisture since the Careful inspection ofpanel surfaces, par­ panel was glued. By using the average ticularly fot furniture panels, is also im­ shrinkage value for the species, the ap­ portant in various stages of production. proximate moisture content of the core at The sooner surface defects can be detected, the time the panel was glued can be calcu­ the less labor is expended if the panel must lated. be rejeCted. Incident lighting fixtures such

M l}S 3)0 Figure 45.-lncident lighting for detecting defects such as showthrough, check­ ing, or other surface blemishes in panels.

68 r?1-LlGHT BOX I r ! I 1 LIGHT RAYS;--'--1 '\ :rIRROR

Figure 46.-Lighting arrangement used in some plywood plants to facilitate inspec­ tion of panel surfaces as panel emerges from . as illustrated in figure 45 are very useful exceed 16 pct. for an appreciable length for such inspections. Figure 46 is a sche­ of time) and phenol-resorcinol is used m~.tic sketch of a lighting device used at where the moisture content in use gen­ inspection stations in some plywood erally exceeds 16 percent. plants. For end jointing of laminations, mela­ mine-urea (in a 60:40 ratio) is used for LAMINATED TIMBERS most interior laminates, and resorcinol or phenol-resorcinol is the preferred glue Adhesives that set at room temperatures when the product is intended for exterior or at moderately elevated temperatures are use. Co-sprayed melamine-urea has shown most practical for laminated timbers of better durability than the twO resins appreciable cross section. Casein and mechanically mixed. phenol-resorcinol are used for normally Some of the major operations in the dry interior service (where moisture production of laminated timbers are illus­ content of the wood is not expected to trated in figures 47 to 54.

M 116 8-1 7 .2 Figure 47.-Laying out template for an arch on mold-loft floor. Figure 48.-Settins to fit template.

69 ~ lE-~ <~~

!tijj!CPI ! . aiki:&bL:Sti::iC!

~!fJli!ti • -iii¥;-..... ~ '-5 - ±:

I!ii!SJFT ti7 -- 'T . j -,iii

\1 J 1- HII Figure Sl.-Phenol-resurcinol adhesive M 116847-2 applied by riJ,b(>f! sp!~ader. In the Figure 49.-Ribbon glue spreader. A, laminating i;;.:iu5iry, ~!;.. laminations Plank receiving adhesive; B, ribbons are often Illaced on edge (wide face of glue extruded from equally spaced vertical) during layup and pressure holes on spreader pipe. application; hence, the adhesive must be thixotropic to avoid running to bot­ tom edge.

M 118 \9U Figure SO.-Ribbon spreading of glue.

FURNITURE M 118112 A greater variety of species and JOint Figure S2.-Tightening clamps on glued designs are used in making furniture than assemblies with nut runner operated in any other segment of the wood-using by compressed air. industries. Some of the denser species are hickory, oak, pecan, sugar maple, beech, density and amount ofshrinkage and swell­ birch, ash, walnut, elm, hackberry, and ing the joint is exposed to. In the author's cherry. In the medium and lower density opinion, more furniture joints fhil because ranges are gum, poplar, ponderosa pine, ofintetnal stresses than because ofexternal alder, basswood, and mahogany. load, or they weaken to a degree where The external load a furniture joint must external load brings on f.'lilure. It is .im­ withstand is usually difficult to determine. portant, therefore, that the furniture The internal stresses, induced by moisture designer be intimately acquainted with the changes, generally increase with wood properties of the different woods. and

70 M 136844·6 Figure S3.-Surfacing sides of slightly E cambered hlminated beams. MllS'll Figure SS.-Various types of furniture joints used in a study on effect of joint design and exposure on the per­ !'ormance of different types of glues: A, Dowel; B, mortise and tenon; C, blocked; D, slip (or lock); E, end grain tl) side grain; F, side grain to side grain.

least affected by cyclical high-low humid­ ity exposures, and the block corner joint M 138311 Figure S4.-Marking curved laminated showed the greatest deterioration (fig. 56). member for trimming to outline of Of the adhesives evaluated, resorcinol and template. phenol-resorcinol generally performed best. Animal glue and casein glue were in the upper range on side-grain-co-side­ grain joints but were generally the least know enough about adhesives co make the durable in other types of joints. Acid­ proper combination ofwood, joint design, catalyzed phenol and urea resin glues were and bonding agent. Finish also plays an generally intermediate in most joines, important part in the performance ofglued although there was considerable variation wood products. in the performance of the three ureas in­ In a study on performance of certain cluded in the study. Polyvinyl emulsion types of furniture joints (fig. 55) bonded adhesive performed well in dowel and slip with different adhesives, the side-grain-co­ or lock joines bm poorly in side-grain-co­ side-grain joint, as would be expected, was side-grain joines.

71 go o ~O 40 60 80 Zet 40 60 too

Urea 1 [A;;'1""l 2 1 Urea l c:id-phenol Urea l c..eta Urea J 1 Urea J Ruorc!nol 1 1 Phenol-re.orc1nol I Resorcin"l Acid-phenol benol-re,JoreJnul PolyvinyJ StDE-GRAIN-To-SIDE-CRAIN JOHnS SLIP OR LOCK JOl:tTS

r==c:rln 1 1-r---cl__...J.:;'urea 3 A"L21Ani,.u.ll 1 II Anf~.1 21 I .ase!n 1An-I...l 1 U.e.l I U-ru 2 1 Urea 1 Polyvinyl o;fA«:!d-phenol Urea 1 1 ~I-C-"-'-2 1 --'-1 I [ Resordnal 1 1 Rt-sardn",l l I Atld-pbenol 1 Ph .... ntll ... rl'sorc.'i nol Phenol-resorC"lnol i i OO'.:rL JOINTS

Ant.!l 2 I I I Anl~i::: t: 1 -t I Polvvtnyl Casein Crea ;! I I Ca ••in I Add-phenol Ad-d-phenol 1 Uliea l t Anl..11 ! I Phenol-resorcInol IJrc.1 1 f Animal 2 I '"al I I '! t Polyvinyl 11 rhl"ni'l"res,'r~ inol I" ~ResorC"ino! Rt's(tr.lnd I f R I M 138928 Figure 56.-Comparison of percentages of control strength values I'etained by dif­ ferent types of assembly joints after 36 months of exposure to a repeating cycle consisting of 4 weeks in air at 90 percent relative humidity followed by 4 weeks in air at 30 percent relative humidity, both at 80 0 F. (Two different commercial animal glues were tested, and urea adhesives ;'rom three different manufacturers were tested.)

Figures 57 and 58 illustrate the per­ sugar maple withom any finish or surface formance of two types of joints In two coating and no load was applied during levels of exposure. Under the humidity cycling. exposure of 65-30 percent-approxi­ Urea resin is probably the adhesive most mately that for normal interior furniture widely used for furniture. If good-quality use--side-grain-tO-side-grain joints held urea adhesive is employed, satisfactOry per­ up well with all glues. Where the grain formance can be expected in reasonably was crossed, however (mortise and tenon, well-maintained furnirure. fig. 58), the casein glue (and others) dete­ Fat moist and tropical conditions, boil­ riorated appreciably in the milder exposure proof adhesives such as resorcinol and phe­ and seriously in the more drastic humidity nol-resorcinol would provide the best changes. The evaluations were made on long-rl'rm performance.

72 JOINT SntE:'ctH (PERCENT)

0 20 0 60 80 100 120 0 20 4O 60 lon I1n aeaorc:1nol Resorcinol 1 ! Add-phenol Acid-phenol ! f I Urea 011 Urea 012 Phenol-re.arclnol 1 Phenol-resarelDOl Urea D2 ContreI. Una 02 Polyvinyl Polyvinyl I AnllaAl 2 Animal 2 fUfea DlJ Ul'U OJ. 'l I Cudo Calleln I IAn1a.. 1 Animal 1 I , I I 1 I I I I I I I I I I I 1 ! I Urea D2 pnlYVlnvl ~ ACid-phenol Urn ))12 Resorcinol ' Resorcinol i PhenoI-ulIOreinol Animal 2 I Ure.a 012 lZ Cudn Uru D13 cont Ueea 02 i Ania.al 2 Urea 013 JI Ani...l 1 Phenol-resuccinol 11 Polyvinyl AnilllAt 1 C••etn Add-ohenal I I I I I I I I I I I I ,I I ! polYVinyl I 1 Urea D12 . t I I Add-phenol Ueea 012 I I I ! Reaorcinol Add-phenol l; I Anlr:ul 2 Urea 02 24 I Palyvlnyl Rellocr-fnnl I I C10nth, ! I I Urea D2 Animal 2 i t!rea nIl Url'ol D13 11 ! Phenol-resorcinol easdn U CaseIn Phenol-cesore inol i Ani:al 1 Anima.l 1

I I ! I ! I I I 1 i I ! 1 ! I PolyVinyl i ACid-phenol I ! I I Pher,ol~ resate!no1 i Ucell Dl2 I j I L'cca D12 Urea DI I J I , Urea 1)[ -~\ I Anloal ;. I 1 PQlyvlny: J" Mid-phenol I men )i j Resorcinol Ca5clt1 I trea Dl; 1 ~Anical 2 ","11:1;\1 1 1 Casein Retwr.'1nol i ;Animal 1 ~ Ph~rh.' l-re8('1n~ i nol 11 i I

M 13B 929 Figure 57.-Performance of various glues in side-grain-to-side-grain joints exposed to repeating cycles between 65-30 and 90-30 percen' relative humidity, all at BO~ F.

Polyvinyl resin emulsions, because of ordinary PYA emulsions. However, they their flexibility, have performed well in are reported to be subject to creep and certain types of joints (dowel, slip or lock probably would not be desirable for joints joints). But where joints are continually under appreciable continued enernalload. stressed, polyvinyl resin emulsions should Animal glues are still used to some be avoided because of tendencies to creep. extent for assembling furniture; if the Also, their moisture resistance is low, proper care is taken both in gluing and which makes them unsuitable where high finish upkeep, good performance can be hunlidity prevails. expected in normally dry interior use. Thermosetting polyvinyls, particularly Although their moisture resistance is low when cured at elevated temperature, are and they deteriorate under high humidity far superior in moisture resistance to the exposure, in side-grain-ro-side grain joints

73 JOINT STRDIGTII (PERCENT)

o 20 40 60 80 100 20 40 60 80 100 I Cllse{n Phen(ll-resorcinol 1 Phenol-resorc lnol t.'rea 1)2 I Animal 1 Anical 1 I Urea 02 Co:1trols I Add-phenol I Acid--phenol I Polyvinyl-res.1n Polyvinyl-resin 1 Anit:k1.1 2 Animal 2 1 Resorcinol Resorcinol 1 Crt!H btl Urea nu 1 t:rea D12 .Y.r.~11 Dlt 1 I I I 1 ] I l Cas\":in I Cascin I t:rea D2 Animal 1 II Pheno,l-resor.;inul Urea 02 I Urea DtJ 1 An~t:lat 1 Acid-phenol 1 12 1 P lyvinyl-rcsfn I I:lollchs­ Animal 2 aesuuinul Resorcinol :;re~ Dll L'rea v12 trca au 1 Phenol-resor.c:{nol Polvvlnyl-resln I 1 ! I I I 1 .I ) 1 I L,.J.ijCl." , i I Ii Anlmal 2 ]J II I ".~ . Coil I 1 1 1f I'H'l D13 1 I .:::.. [rt'".t n2 1 ::llll!1.ttlt> I ~ ,t"!Uv·tt,s~ .. tl'Sl,n 'l'n',l m.:­ 1 !',·j'Vll". t-n..·.tn 1 l'llo'\·,IT"'r.',,,,1" ~n(11 1 ,\. l<.i-pht"!l.)l I I I I I 1 ~lcaSf.'"in Urea 013 An!r:t.ll 2 t\OllIlai 1 I I I l' l'''ea D12 I 1 m,Jothoj P"lVVlnvl .. rt>sln I ~r(>a r,2 1 R('sC'f, '"'''' 1 I A. 111- 'Ill 1\•• 1 1 1 ~'lItmol-n'sun ~'lnl

";-10 90-30 M 1\8930 Figure S8.-Performance of various glues in mortise-and-tenon joints exposed to repeating cycles between 6S-30 and 90-30 percent relative humidity, 1;/11 at 80 0 F, animal glues hold up reasonably well for, In operations where high-frequency shorr periods even at the higher humid­ heating or othet means for elevated tem­ ities. perature curing are available, melamines Use of hot melts is rapidly increasing and melamine-ureas would certainly because of the automated and increased deserve consideration in furniture manu­ production feasible with them. Many types facture. Of the twO, melamine-urea cures and formulations are available, leaving faster and is lower in cost. little basis for any general statement on One of the older methods of gluing lon~-term durability at this time. furniture panels, bur still acommonly used

74 Figure 59.-Edge gluing oak backs of church pews. Systems of this tYPE: (glue wheel) have been in use for decades.

,\[ tv.. !lO!-1 Figure 61.-Edge-glued middle and end supports for church pews. t. ~ _:r, ~ #- ",'~;;~~·;;~~':~~·;~~p:+'l~~:,~v~: .. " ~ -" ...... ~-.. ~..... ~.~-.'

Figure 60.-Sfack of laminated curved backs for church pews made of two 3J4-inch layers of particleboard and faced with oak veneer. Jig used for ~j'IIS 261.(, gluing is in background. Figure 62.-Lay-up table for single­ opening batch-process, edge-gluing press. Adhesive is heat cured. one, is illustrated in figure 59. It is applic­ able to room-temperarure-serting glues, but moderate hear can also be applied while the "glue wheel" completes a cycle. Figures 62 and 63 iIIustrare edge-gluing Figure 60 shows slightly curved backs operations for furniture panels produced b}r for church pews made by laminating two hot pressing. The panels are bonded wirh layers of particleboard t:'lced with thin urea resin adhesive. Figure 64 shows J. veneers, and figure 61 shows middle and bacrery of cold presses for mass production end suppOrtS (up-rights) for church pews of furniture panels and figure 6'5 shows a produced by edge gluing. con rin uous-feed, s ream- hea red edge­

75 . - .. ~-:;-: ~ ... ~:::. "'~--,': • ,.-l • Figure 6S.-Continuous-feed, steam­ • ~. 4

.... \ • '<'" ...... ~. ~ heated press for edge gluing. Edges M HS 264-S of lumber spread with glue travel by Figure 63.-Single-opening hot press conveyor to operator's p::lsition. for edge-gluing panels. The cured panels are ejected as the new batch goes in.

Figure 66.-Machine for applying edge banding to furniture panels with hot­ melt glue.

gluing press. Figure 66 shows a machine for continuous edge banding of panels, employing hor-melt adhesive.

SHIP AND BOAT CONSTRUCTION

M 13907 2 Gluing operations for ship and boar Figure 64.-Cold-pressing panels for building faIl essentiaIly in three caregories: cabinet doors glued with modified (1) Laminating structural members (keels, PVA. frames, and deck beams-figs. 47 co 54

76 M 9$198 F Figure 68.-Construction of V-bottom Figure 67.-Jig for gluing U-snaped boat with frames joined at keel, chine, ship frame. The smaller members and deck with glued and bolted ply­ shown inside the frame were glued wood gussets. and clamped elsewhere and brought to the curing area by overhead crane. The enclosed heating unit and fan for gluing pressure in assembly gluing of circulating air are .shown in back­ boats. Predrilled holes slighdy reamed oue ground. A metal cover is placed on on the contacting surfaces permit drawing top of jig forming a complete en­ the glued surfaces into closer contact. closure during curing. For laminated ship and boat members, white oak is often used because of its high impact resistance and durability (fig. 69). and fig. 67), (2) assembly gluing (plywood White oak is one of the higher density gussets for joining frames at chines and for native species and requires high-quality joining deck beams to frames-fig. 68), adhesive, generally ofthe phenol-resorcinol and (3) production of marine plywood for type. Surficient assembly perio..:! should be bulkheads, decks, outer skin or planking, used to allow the viscosity of the glue to and superstructure (figs. 38 to 43). build up to the proper level. Elevated For marine plywood, hot-press phenolic curing temperatures, about 1500 F. for adhesives similar to those used for exterior 6 hours, are generally required with plywood are employed. The only substan­ current adhesives, bur lower curing tizl difference between the two types of temperatures may be used with extended plywood is that cereain defects permitted curing periods (fig. 11). in veneers for exterior plywood are not allowed in the marine grade. In ship and boat component production, DOORS such as laminating and assembly gluing, phenol-resorcinol adhesives are used al­ Wood doors vary in size from small most exclusively. Since elevated tempera­ cabinet doors to large garage -,r Wa! .:-house tures for curing often are not feasible in units, and in construction from :ush assembly gluing, it is advisable to check panels with hollow or solid cores (figs. 70 with the glue manufacturer .to determine to 72) to panels with frames, usually called if the adhesive is room temperature setting stiles and rails. Details will not be dis­ on the species involved. Rustproof screws cussed here, but a few precautions will be or bolts are generally used for applying pointed OUt to aid in avoiding pitfalls that

77 M 94561 F Figure 69.-Laminated white oak frames used in construction of Navy minesweeper.

STILE

A B c M 138750 Figure 70.-Three types of flush doors. A, Five-ply, solid-core; B, seven-ply, solid­ I;ore. The three-ply faces (door skins) are usually preglued; C, seven-ply, hollow­ core door. Core material in this case is wood shavings produced by special process.

78 STILE

A B c \; '.'8749 Figure 71.-Flush doors with three-ply plywood faces. A, Expanded cell-type core; S, mineral composition corf'; C, particleboard core.

have resulted in unsatisfactory door per­ blocks. This can result from placing formance. vertical-grain blocks adjacent to flat­ Flush doors are often made with "door grain blocks in the core or from using skins" for faces. These door skins are gen­ blocks of unequal moisture content at the eral!.y rhree-ply plywood about l/S inch time the door is glued. Low-shrinkage thick. The inner ply or core of this ply­ species such as ponderosa pine are less wood is generally of a lower grade than the likely to cause showthrough of core blocks faces. When such plywood is used for door than higher shrinkage species such as sI

79 D M 96381 Figure 72.-Typical core types used in hollow-core doors. A, Lattice; B, ladder; C, tube; D, honeycomb. warping caused by shrinkage and swelling casein). Exterior doors, unless completely of the panels. protected by wide overhangs, should have Adhesives in doors for interior use are waterproof glue bonds of the quality used generally ofthe water-resistant types (urea, for exterior plywood (phenolics).

80 SPORTING GOODS The risk involved in using a moisture­ sensitive adhes:ve would be if such lami­ nated equipment, intended for normally Glued products used for sportS include dry use, would be stored in a damp ware­ house for an extended period. bowling pins, t{!nnis rackets, snow skis, water skis, hockey sticks, and various types Where facilities for curing at elevated temperatures are available, a melamine- or of gym equipment. I.aminated baseball bats (fig. 73) have also been produced. resorcinol-fortified urea would provide a greater margin of safety than straight urea These items generally are subject to resin as far as durability of glue bonds is rough usage and must be made of tough, concerned. strong woods. Such woods usually exert W'here steel or fiberglass are combined high stresses on the glue joints under loss with wood, as in some snow skis, an epoxy or increase in moisture content. For long­ formulated for this purpose may be the best lasting, safe products, adhesives of good choice. quality are needed. For items such as water skis, a water­ PARTICLEBOARD proof bond is definitely required to obtain reasonable service life for the product. Urea resin is used almost exclusively as Bowling pins are subject to severe im­ binder for interior particleboard. Since the pactS and a tough adhesive would be ex­ wood is broken down into small particles, pected to give the best performance. Bur the stresses on the minute bonds are prob­ because bowling pins never get wet, and ably lower with changes in moisture generally have a heavy plastic coating, the content than in a solid wood-to-wood ultimate in water resistance is not needed. joint. On the other hand, it is well known One manufacturer of laminated bowling that urea-bonded particleboard deteriorates pins successfully used a separate applica­ in a few years when exposed directly to the tion of urea resin (catalyst applied to one weather. This indicates that urea resin is face and the resin to the other) for many not a suitabie binder for particleboard years. where damp ot humid use conditions are involved. For exterior boards, phenolic binder is employed but no substantial use has been made of particleboard for exterior service in this country. Particleboard has also been made with melamine resin binder, and at least on an experimental basis, with extracts from bark. Binder generally is applied by air spraying or airless spraying with agitation of the particles. When veneering partirleboard or bond­ ing particleboard to itself or to wood an Id 84744 F adhesive fully as durable as the binder ~sed Figur~ 73.-'-Laminated baseball bats in the boards should be used. For normally with center lamination of hickory for dry interior applications, urea resin should improved impact strength ane! the re­ be adequate; for uses such as light cabinet maining sections of ash with edge doors, high-quality polyvinyl glue has grain e~posed on the surface of the bat. Edge-grained surfaces make the been reported to give good service. A good bat more r:?~istant to shelling. moisture-excluding finish reduces stresses

81 on the glue bonds and is a good safety factOr, particularly when panels are used in. Kitchens and bathrooms where inter­ ·1 mittent high humidity often occurs.

HOUSING AND HOUSING COMPONENTS

Glues for floor and wall panel applica­ tions (figs. 74 and 75) are generally of the elastOmeric or mastic type and arc based on rubber, polyurethanes, and other mate­ rials. They are usually furnished ready for use in small carrridges (cylinders) that fit calking guns; they may be applied as a to studs and joists, or to smooth walls when wood paneling is applied to existing walls (fig. 76). Pneumatic glue guns are also available for more efficient application. The glues are smoothed by the pressure (or by hand rollers where no nails are used); but because of their gap-filling properties, a chin, uniform glue film as obtained with well-fitted joints formed under pressure is not necessarily required. M 138 198 On an experimental basis, paneling has Figure 75.-Application of mastic ad­ hesive to studs for bonding wall panels.

also been applied merely by pressing the panel (by roller) against the wall to smooth out the glue without the benefit of nails. Wall panels are sometimes nailed only at tOp and bottom (where nail holes will be covered by molding or ) and the remainder of the panel is brought into close contact with the studs with hand rollers to establish the glue bond. Advantages of using glue in applying wall panels include providing racking resistance to the walls and, where decora­ tive panels are involved, avoiding un­ sightly marring of beautiful panels by nailing and nail popping.

M 138 196 Nail-glued plywood floors reportedly Figure 74.-Application of mastic ad­ permit wider joist spacing or smaller joists hesive for bonding plywood to joists. than floors only nailed. Another important The plywood is also nailed, which advantage claimed for this system is elimi­ furnishes gluing pressure. nation or reduction of floor squeaks.

82 M 13B 597 Figure 76.-Application of press-dried lumber paneling to plastered wall with mastic-type adhesive. Left: Fitting panel to the adjacent one. Right: Applying adhesive to wall with calking gun.

Housing components such as trusses and produce adequate glue bonds in certain wall and floor sections have been factory­ members by nail-gluing and allowing the made for many years. Because of adverse resin adhesive to SPt at room temperatures. exposure that can often occur during ship­ The nail-glued truss shown in figure 77 ment and erection, a waterproof adhesive is a typical building unit produced by this (resorcinol or phenol-resorcinol) is recom­ method. mended for gluing such components. Two major advantages ofpreglued com­ Since com binations oflumber and plywood ponents are reduced labor cost at the site are often involved, appreciable stresses on and higher quality building units (im­ the joints with seasonal moisture changes proved strength by gluing). In the factory, are almost unavoidable. This is another jigs can be employed to provide both uni­ reason for advocating highly durable adhe­ formity of dimensiuns and rapid assembly sives for housing component manufacture. of parts. The conditions for obtaining Various means for pressing and curing high-quality glue joints are also much the glue joints in components have been more favorable in the plant where tem­ devised. Low-voltage heating is one of the perature of both materials and surround­ common methods. It is also possible to ings can be controlled.

83 ZM 96227 F Figure 77.-Light truss with plywood gusset plates glued to framing members. The gussets were V2"inch, five-ply, eXferior-grade Douglas-fir plywood; framing mem­ bers were 1 % by 3% inches in cross section; and ninepenny nails (indicated by + on sketch) were used to apply gluing pressure.

NEW PRODUCTS Experimentally, bonding various types of overlays to wood has improved appear­ With use of adhesives steadily on the ance, paintability, ~lnd other properties, increase, new bonded wood products-or and heavier decorative overlays for kitchen combinations of wood and other mate­ tables and cabinet tops have been produced rials-are continuously coming on the for several decades. Quite a range of adhe­ marker. sives from ureas to resorcinols, depending Wood "jewelry" in many varieties is on the moisture resistance required, bond generally made by bonding the shaped these overlays to panel products. Contact wood parts to metal clips or similar fasteners WIth epoxy adhesive. When a clear epoxy is used, a complete coating of the wood pan can also provide a durable finish. Laminated flooring of various construc­ tions has been made for many years, but new adhesive bonding teChniques are developed from time to rime. A method of obtaining twO three-ply flooring boards by gluing and pressing one five-ply plank is illustrated in figure 78. This type of flooring, made of softwoods with oak top face, is produced in Scandinavia. Details of construction of a four-ply, laminated flooring produced for many years in Europe are shown in figure 79. Oak-faced flooring for use in permanent M 138530 construction should be bonded with an Figure 78.-Single gluing operation and adhesive at least as durable as fortified urea. resawing yield two three-ply flooring Where appreciable fluctuations in mois­ boards (top and bottom of sketch) ture content or where high humidity and from one five-ply unit (center). Oak temperature conditions. prevail, a phenol­ lamination is sawn at dashed line and resorcinol would provide greater assurance the surface of each half becomes the of long-term satisfactory perfor'11ance. top flooring surface.

84 /SOFTWOOD VENEER // SOFTWOOD LUMBER / CORE /SOFTWOOD VENEER .'

Figure 79.-Sketch of glued flooring made with softwood lumber core faced with and a top layer made up of narrow strips of oak laid in various parquet designs. The finished boards are furnished in about 6- by 120-inch strips, tongued-and-grooved, and varnished on the top surface.

adhesives applied to both the overlay and As with products of established per­ the panel products also are widely used for formance, species, finish, use conditions, this purpose; they are particularly con­ and expected service life must be con­ venient for do-it-yourself and on-the-job sidered when choosing an adhesive for a applications where equipment for applying new product. pressure over large areas is usually not available. Use of vinyl overlays for such items as moldings and furniture parts has been in­ creasing the past few years. These overlays (vinyl films) are produced with wood grain patterns; thus woods not usually suitable for molding can be given the appearance of walnut or other high-quality wood. These films are furnished with or without adhesive applied to the film and the adhe­ sive composition is usually no" disclosed. M 138 lH Figure 80 illustrates one type of equip­ Figure SO.-Machine for applying flexi­ ment for applying flexible vinyl overlay. ble overlay (vinyl film) to molding and With adjustable soft tolls, the film can be similar stock. Arrow pr.,,,lts to over­ applied to molding and other items of a laid stock coming through t!1~ ma­ variety of profiles. chine.

85 SELECTED REFERENCES Lehmann, \VI. F. 1965. Improved particleboard through bener resin efficiency. For. Prod. J. Adltesives Age 15(4):155-161. 1964. Fibrous overlay bonded to wood ar Lehmann, \VI. F. high speeds. Adhes. Age 7(5):25. 1968. Resin distribution in flakeboards Adhesives Age shown by ultraviolet light photOgraphy. 1965. Glued walls for a new building. For. Prod. J. 18([0):32-34. Adhes. Age 8(2):36-37. Adhesives Age Lehmann, \VI. F. 1965. Spray able adhesive reduces drywall 1970. Resin efficiency in particleboard as lamination coSts. Adhes. Age 8(6):27. influenced by density, ammization, and Anderson, A. B., Breuer, R. J., and Nicholls, resin coment. For. Prod. J. 20(1 1):48-54. G. A. Miller, D. G. 1961. Bonding particleboards with bark 1953. Curved pl)'wood, its production and extracts. For. Prod.}. 11(5):226-228. application in the furniture industr),. For. Bergin, E. G. Prod. J. 3(2):22-26. 1969. The strength

86 GLUING OPERA rlON

The gluing operation generally consists which the powder is added gradually with of these steps: (1) Mixing the ingredients the mixer running to prevent formation of that make up the glue, when ready for use; lumps. After a smooth, homogeneous mix­ (2) spreading the glue on one or both joint ture is obtained, the remaining water is surfaces to be bonded; (3) assembling the added slowly with the mixer running. If individual parts in the order planned for all the remaining water is added at once, the bonded product; (4) allowing the the doughlike mix might break into large spread gh.~ to thicken and penetrate the lumps; such lumps, particularly with low­ wood surfaces for a certain period (usually viscosity adhesives, may be extremely referred to as the open and closed assembly difficult to break up even with vigorous periods and as a rule specified by the mixing. supplier); (5) applying preSsure to bring This procedure is also sometimes neces­ the spread surfaces into close contaCt; (6) sary when a powdered hardener (often is a retaining pressure until the bond gains mixture of the actual hardener and an inert sufficient strength to perm it safe h'lntlling powder such as walnut shell nour or woo,] of the glued product; and (7) condition­ flour) is mixed with a liquid resin. It is ing the glued stock to complete adhesive often easiel' to obtain a homogeneous mix cure and allow any solvent to diffuse if the hardener is first mixed with part of throughout the glued assembly, the resin until a smooth mixture is ob­ Each step will be discussed in more tained, and then the remainder of the resin detail, but inasmuch as the gluing proce­ is added gradually with stirring. dures vary considerably for different prod­ The glue supplier generally furnishes ucts, the discussion must necessarily be instructions for mixing and weighing the somewhat general. It is suggested there­ ingredients to be mixed. Mechanical fore that the adhesive user follow the manufacturer's instruction very closely, mixers of various types (figs. 81 and 82) are invariably used in industrial operations; and that the manufacturer's technical in the home workshop, small amounts can serviceman familiarize himself with the customer's process and product so he will be mixed satisfactorily by hand, using a clean metal or glass container and a paddle be able to give sound advice to the cus~ for stirring. Since many adhesives are tomer. either mildly acid or alkaline, containers MIXING ADHESIVE not affected by acid or alkali should be used. Strict cleanliness of gluing equip­ ment is important for extraneous materials Some adhesives, such as the film types can easily lower the bonding quality of the and the straight polyvinyls, are furnished ready for use and hence require no mixing. adhesive. A mixer suitable both for labora­ tory and small shop use is shown in figure Others, as the ready-to-use case.ins and some powdered meas, need only to be 83. mixed with water, as prescribed by the Some resins must be kept cool during glue supplier. storage and also at the time of mixing. In this operation, usually part of the Instructions to this effect are generally water is first run into the mixer, after furnished by the supplier.

87 I Figure 82.-Blender-typa mixer for resin adhesives. The mixer is available with or without water jacket for cooling or warming the mix.

M tHllS,) Figure 81.-Counter-rotating paddle­ type mixer for protein and resin ad­ hesives. Mixers are available in vari­ ous sizes.

Sometimes, parricularly during cool weather, resin adhesives should be allowed M 48436 F to mature for a shorr period between mix­ Figure 83.-Three-speed mixer with two sizes of paddles and mixing bowls, ing and use. During hoc weather the re­ suitable for laboratory and shop use. action period usually can be omitted. To avoid exceeding the working life of the mixed glue, mix smaller batches during hot weather than in the cooler seasons. SPREADING ADHESIVE This will prevent shutdowns for cleaning spreaders and other equipment because glue has exceeded its working life. Jacketed Various methods are used to apply adhe­ mixers permit the temperature of the mix sive to joint surfaces when bonding wood, to be controlled by running water of the depending largely on the type and amount required temperature through the jacket of glued product and also to some extent (fig. 82). on the adhesive. Automatic mixers are also available for In the small workshop, application by certain applications, as shown in figure 84. brush is often practical. 'When larger

88 and 50) is said to save adhesive and result in more uniform spread and greater rate of production. Ribbon spreading permits the laminations to travel under the extruder at a greater rate of speed than through a toll spreader. Since the spread surfaces are often placed in a vertical position during the assembly period, ribbon spread­ ing requires a thixotropic adhesive that . will not sag or run to the bottom edge of the laminations bffore gluing pressure is applied (fig. 51). The adhesive also must remain sufficiently fluid to smooth out in a uniform film when gluing pressure is Figure 84.-Glue maxlng and spreading applied. flquipment. A, Unit that automatically The amount ofspread (usually expressed mixes liquid urea and powdered cata­ in pounds of wet glue per 1,000 square lyst in the right propertions; B, glue spreader. feet of glue-joint area) varies considerably with the type of adhesive used, product surfaces are involved, a mohair paint roller being bonded, species, moisrure content of works well with many adhesives and is rhe wood, and the temperature and humid­ more efficient than a brush. ity of the gluing area. In general, appre­ Mastic adhesives used for bonding ciably higher spread is required with panels to studs and joists are applied in casein glue than with most synthetics. beads by hand-op;:rated calking guns (figs. Small items that can be assembled rapidly 74, 75, 76) or by compressed air-operated can often be bonded satisfactorily with guns for more efficient operation. less spread than large members requiring In furniture manufacture where poly­ a long assembly period. Often adjustments vinyl glues are sometimes used extensively, in the adhesive (such as setting rate) must the liquid glue is distributed by pumps or be made for different size products. Dense gravity feed through pipes, often applied woods generally require heavier spreads from nozzles conveniently located within rhan lighter ones (assembly time and other the workmen's reach. factors may also need adjustment). Wood In larger gluing operations, such as in at low moisture content absorbs the solvent plywood and laminated timber produc­ from the adhesive faster than wood at tion, application by double-roll spreaders higher moisture content; this makes in­ (fig. 41) equipped with doctor tolls for creased spread necessary, unless assembly close control of the spread has been time is short. common for decades. High temperature and low humidity in In recent years, curtain coating, a the gluing area also suggest increases in method similar to the process used for pre­ the glue spread. This again depends on finishing plywood, has come into use in whether the assembly period can be plywood manufacture (fig. 42). Curtain shortened to compensate for the faster dry­ coating is claimed to result in more uni­ ing and "skinning" over of the glue film. form spread and less waste of adhesive. As a rule, only one of the mating sur­ Reportedly, adhesives are also applied by faces of a joint is spread with adhesive. extruders in some softwood plywood With certain products, however, such as plants. large laminated members that require con­ In the laminating industry, ribbon siderable time to assemble, spreading Doth spreading or extrusion spreading (figs. 49 surfaces ofeach lamination can be advanta­

89 Figure 85.-Spreading resin glue on both faces of board with a rubber­

covered double-roll spreader. Spread­ M 1368·16·8 er has adjustable speed to accommo­ Figure 86.-Glue spreading mechanism date different types of resins. for finger joints. Glue is dispensed at arrow. geous. This is called "double spreading" nuously through both machines to the lay­ (fig. 85). up station. Special joines, such as finger joines, re­ quire spreading mechanisms of the same profile as the joine for unifvrm application ASSEMBLY TIME of adhesive. Such a spreader is shown in figure 86. The interval between spreading the adhesive and the application of full gluing ASSEMBLING PARTS pressure is called assembly time. If wood surfaces coated with glue are exposed freely Because of the large variety of glued to the air, solvene evaporation and changes wood products, only a few will be briefly in adhesive consistency occur much more mentioned to give a general idea of the rapidly than if the joine surfaces are in assembly operations involved. contact. Free exposure of the coated sur­ The key to success in most industries t1Ces is called "open assembly;" surfaces today is auromation, and significant break­ in coneact, "closed assembly." thtoughs have been made in recene years Proper adjustment of the assembly time in fields such as plywood manufacture is very importane and often has significant where layup efficiency has improved im­ effect on the quality of the glue joints. mensely. In a similar manner, layup of A too-short assembly period often results large assemblies in the more progressive in "starved" glue joints, particularly with lumber-laminating plants is being done low-viscosity adhesives and dense species with hardly a piece of lumber being that absorb moisture from the glue slowly. touched by hand. Too long an assembly period (particularly In some plants the planer and glue open assembly) can easily result in "skin­ spreader are arranged in tandem with ning" over or drying out of the glue film. synchronized rates of speed. The lamina­ The result is inadequate transfer ofadhesive tions, end-jointed to length, run conti- from the spread to the unspread surfaces.

90 PRESSING OR CLAMPING

Glue-joint surfaces must be brought into close cOntact to enable the adhesive to form a bond between them. Hence the application ofadequate and uniformly dis­ tributed 'pressure to the joint at the proper time is essential in production of consist­ ently high-quality bonded joints. Pressure N 87206 P must smooth the adhesive to a continuous, Figure 88.-C-type rockerhead clamp fairly thin layer between the wood surfaces, with adjustable span used for lami­ and hold the parts in close contact while nating and other gluing pressure ap­ the adhesive is setting or curing. plications. The optimum thickness ofglue films in joints varies with the type of adhesive and wood species. Cured films as thin as 0.002 between the clamp and the glued assembly inch have resulted in good bonds with urea to distribute pressure to the areas between adhesives, and those as thick as 0.0 10 inch clamps. These cauls must be thick enough resulted in good quality joints with resor­ to distribute the pressure uniformly, as cinol adhesives used with dense species. well as being flat and smooth. The damps. For best results, pressure should be applied must be sufficiently close together to evenly over the entire joint area. Fluid produce adequate pressure between as well pressure, such as used in bag molding with as under the points of contact. Gluing thin veneers, comes dosest to being com­ pressure in the range of 100 to 200 pounds pletely uniform. per square inch is usually adequate for most In hot-pressing plywood, multi­ operations, with viscous adhesives and opening hydraulic hot presses (fig. 43) dense species generally requiring the top apply pressure of about 175 pounds per of the range. square inch to the veneers while the glue 'JV'hen clamps are used to apply gluing is curing. In laminating, retaining clamps pressure, tOrque wrenches and similar of various types (figs. 52, 87, and 88) devices may be used to determine the are cornmonly used. Caul boards are laid amount of pressure applied. Figure 89 shows a panel press where pressure is ap­ plied by compressed air hoses. Satisfactory gluing for certain construc­ tions can also be accomplished with nail pressure, provided the nails are spaced and driven properly and the proper precautions are taken with regard to assembly time. The pressure obtained with nails is rela­ tively low; hence the adhesive must be a I fairly fluid when the nails are driven. -. No general rules have been developed M H292 for relating nail size and spacing to insure Figure 87. - Double-bolt rockerhead adequate pressure. The nailing pattern and clamps used to apply gluing pressure spacing shown for the light plywood­ to laminated assemblies. The rocker­ lumber truss in figure 77, however, has head equalizes the pressure across the given adequate glue-bond quality. The assembly. adhesive was a phenol-resorcinol.

91

shaped articles such as wood carvings has ,come into use. Both vacuum molding and fluid pressure molding (fig. 90) are suit­ able for application of plastic films to up­ grade wood surfaces.

CURING ADHESIVE

Curing requirements of adhesives com­ monly used ror wood range from normal room temperatures to about 3000 F. Some adhesives, such as the ureas, are formulated both for room-temperature and elevated­ temperature curing. The hot-setting ureas generally cure in the range of 2400 to 2600 F. The melamines cure in about the same range but will also cure at lower temperatures with extended curing Figure 89.-Compressed air periods. inating press for cold pressing panels. For assembly operations, adhesives such as polyvinyls, ureas, resorcinols, and Bag-molded plywood for aircraft and animal glues are generally used at normal light boats was produced during World room temperatures. The thermosetting War II and later. Recently, application of polyvinyls can be cured both at normal wood-grained vinyl film to irregularly room temperatures and at elevated tem-

BAG

HEAT AND r;::::3--- PRESSURE FLUID

HEAT AND CYLINDER PRESSURE FLUID

~~=I!;>- BLEEDER

DRAIN M 142395 Figure 90.-Three methods of forming bag~molded plywood.

92 .1\ 9684~ F .1\ 9684,1 F Figure 91.-Curing glue in scarf joints Figure 92.-Curing phenol-resorcinol in with low-voltage electric heating. The scarf joints with continuous rubber heating elements are embedded in pad heated with low-voltage electric silicone rubber pads and aluminum current. Thin sheets of aluminum sep­ foil separates the pads from the arate pad from boards to prevent boards to prevent contamination 'of contamination of pad by squeezed­ the pads by glue squeezeout. out glue. peracures, but provide more durable bonds nated member by low-voltage hearing is when heat cured. Resorcinols provide illustrated in figure 93. This method re­ durable bonds on many species when cured quires a transformer and somewhat heavy at room temperatures, but denser species leads. such as oak require elevated temperatures Preheating wood before spreading and to provide bonds as durable as the wood then using the stored heat to cure the adhe- within a reasonable time period (fig. 11). The rate of cure of resin adhesives depends both on the type of catalyst used and the curing temperature. The higher this temperature for a given glue, the more rapid is the curing reaction and the shorter the rime required to complete the cure. Alkaline phenolic resins, the type used almost exclusively for exterior plywood, cure in the range of about 2650 to 3100 F, Acid-catalyzed phenols are formulated to set at temperatures as low as room tem­ perature. Curing equipment ranges from large, steam-heated hot presses for plywood (fig. 43) to small, low-voltage heating pads where resistance wire is embedded in silicone rubber (figs. 91 and 92) to generate heat. Thin wires or other condu::tive mate­ rial in theglueline also have been used as Figure 93.-Curing glue in slightly curved heating elements with low-voltage electric laminated member with low-voltage current. Curing ofa slightly curved lami­ electric current.

93 HEAT INSUL

B

A

~LATOR ELECTROOE

~CONCE~m. GLlJE LINE

o

c M 89990 F

Figure 94.-Various electrode .arrangements for applying high-frequency electricc:1I energy to glued assemblies. A, Assembly between electrodes, with electric fiellcl perpendicular to plane of glue joints; S, sandwich method, with high-voltage electrode between the two assemblies being glued; C, electrodes arranged for parallel .or selectiv.e heating of glue joints; D, stray-field heating arrangements of electrodes. sive is a technique sometimes used for Because wood is a good heat insulator, special applications such as finger-jointing this process is somewhat time consuming. lumber and for laminating timber decking High-frequency (H-F) heating (fig. 94) in a continuous process. is probably the method most widely used For large, lamiuated members, en­ for elevated-temperature curing the glue in closures formed with canvas or other mate­ members that do nor lend themselves to rials over the clamped assemblies (fig. 67) hot pressing, particularly for smaller items are supplied with heat from steam pipes such as furniture parts. H-F heating has or by other means for curing the glue. been used extensively for such operations

94 Figure 95.~Finger-iointed lumber, A, spread with glue traveling on a con­ veyor toward an H-F unit, B, for curing. M 136846-6 Figure 96.-Finger joints stopped by electronic memory system, A, between electrodes of high-frequency genera­ 'tor. B, indicates location of electrodes as edge gluing of lumber and curing glue and the curing area. in finger joints (figs. 95 and 96). It is also being used in Europe for laminating beams by continuous operation (fig. 97). Curing In high-frequency curing, the resin the glue in steps (each step equals the adhesives for wood are generally rated from length ofpress) in laminated beams by H-F easiest to use to most difficult in this order: heating has been practiced by at least two Ureas, melamine-ureas, thermosetting laminators in the United Srates for a num­ polyvinyls, melamines, resorcinols, ber of years. phenol-resorcinols, and phenols. The H-F curing cycle depends on such mctors as generator capac ity, type of glue and glue joint area, and the arrangement of the electrodes in .relation to the glue joims. Parallel heating (fig. 94, C) is gen­ erally the most efficient method since the larger part of the energy is converted to heat in rhe gluelines. The level of moisture content in the wood is an important factor. The higher the moisture content the more conductive the wood becomes; thus, the more energy is dissipated throughot.t the wood instead of being concentrated at the glue joints. Close control of the variables involved in the gluing operation is required for suc­ M 138 n; cessful H-F curing. Accurate machining, Figure 97.-High-frequency curing .of uniform moisture content in the wood, and adhesive in laminated beams by con­ tinuous process. Gluing pressure is ap­ uniform glue spread are some of the more plied in the electrode area by blocks important factors. Technical knowledge in fastened to belt moving along endless the generation and use of high-frequency track. Parallel heating is employed currents is also of vital importance with and the uppar electrode can be seen this type of curing. on top of the beam.

95 CONDITIONINGGLU.ED that sunken joints will be minimized or PRODUCTS eliminated in edge-glued panels: 7 days at 80° F. and 30 percent rela­ tive humidity

It is usually not economical to maintain 0 gluing pressure or continue curing under 4 days at 120 F. and 35 p.ercent rela­ tive humidity pressure until the adhesive joints have reached their ultimate strength. A condi­ 24 hours at 1600 F. and 44 percent tioning period after gluing pressure has relative humidity been released is beneficial in many ways. 16 hours at 2000 F. and 55 percent It allows moisture, if introduced by the relative humidity glue, to diffuse away from the glue joints and equalize throughout the member. It These recommendations are based on permits the glue to continue to set and the appearance of edge~glued panels with approach its ultimate bond strength. a high-gloss finish. With panels given a Stresses set up in the glued article during matte finish or panels covered with veneer, the gluing and curing operation will tend shorter conditioning periods generally to be relieved and die out. would be sufficient. If there is an appre­ Because hot pressing generally lowers ciable layover period between the sutfacing the moisture content of a panel and cold and the sanding and finishing operations, pressing increases the moisture content, the conditioning period can probably be conditioning panels and other products somewhat shortened, since some surface under controlled humidity and tempera­ irregularities are removed by rhe sanding. ture is generally desirable and also most For edge-glued furniture panels that are efficient. subsequently covered both with crossbands A typical example of inadequate condi­ (usually one-sixteenth or one-twentieth tioning is represe!1ted by the "sunken inch) and face veneers (usually one twenty­ joints" sometimes found in edge-glued eighth inch), it is expected that the condi­ lumber panels. They are often caused by tioning times shown above could be appre­ surfacing the stock too soon after gluing. ciably shortened at the different tempera­ The wood adjacent to the joint absorbs tures, perhaps to as much as one-half the water from the glue and swells. Ifthe panel time indicated. is surfaced before this excess moisture is distributed, more wood is removed along the joints than at intermediate points. ADJUSTMENTS IN ADHESIVES Then during equalization of the moisture, AND GLUING PROCEDURES greater shrinkage occurs at the joints tha,] elsewhere, and permanent depressions are The strength and quality of a glue joint formed. This condition is illustrated in depend not only on the type of wood or figure 98 where panels were surfaced im­ the quafity of the glue used but also on mediately after gluing pressure was re­ the gluing procedure in making the joint. leased. When improperly conditioned Often the same glue is .entirely adequate panels are veneered, showthrough of for a wide range of species provided the sunken joints and similar defects mars. gluing conditions are adjusted to the re­ surface appearance. quirements of the particular species in­ Based on re~earch at the Forest Products volved. Laboratory, th.e following conditions A specific example from production maintained in a room with good circula­ illustrates the type of adjustments that tion should provide reasonable assurance are required at times. White oak ship

96 Figure 98.-Yellow-poplar panels edge-glued witll! urea resin cured byl:ligh­ frequeno;:y dielectric heating and with animal glue set at room temperature. Upper panels (left, urea; right, animal gJue) were surfaced immediately after gluing pre!>sure was released. Lower panels (left, urea; right, animal glue) were condi­ tioned 7 days at room temperature before they were surfaced. Note sunken joints on panels surfaced without conditioning. frames were laminated in a plant where the where the same adhesive was used to lami­ temperature generally exceeded 900 F. nate similar frames, the temperature during summer days. The glue was freshly ranged from 600 to 700 F. To obtain accept­ mixed shortly before spreading. The lami­ able glue bonds under these conditions, nations were assembled and clamping pres­ the mixed glue had to be aged at least sure applied in rapid succession. The glue half an hour before spreading and full bonds were excellent. In another plant, gluing pressure was not applied for at .least

97 2 hours after spreading. The longer closed SELECTED REFERENCES assembly time was required (at the lower temperature) to allow the glue to penetrate Adhesives Age 1964. Edge glue spreader with vertical axis the dense wood surface and.reach the proper application roll. Adhes. Age 7(9):29. viscosity before .applying fulI gluing Bellosillo, S. B. pressure. 1970. Nail-gluing of lumber-plywood When bonding a dense wood, it appears assemblies-a literature ·review. Inf. Rep. that the glue must be viscous at the time OP-X-29. 70 p. Can. For. Serv., Dep. pressure is applied on the joint. With a of Fisheries and Forest., Ottawa, Canada. Chow, S. Z., and Hancock, W. V. iight, porous species much more latitude 1969. Method for determining degree of in glue viscosity is permissible. A light cure ofphenolic resin. For. Prod. J. 19(4): wood is generally more absorbent; hence, 21-29. a .st9sved joint condition (glue too thin Currjer, Raymond A. at time of pressure application) is less apt 1963. Compressibility and bond quality of to occur. Also, a .lower gluing pressure western softwood veneers. For. Prod. J. usually can be employed than with dense 13(2):73-79. Freeman, Harlan G. species. 1970. Influence of production variables on A good correlation has been noted be­ quality of southern pine plywood. For. tween .the viscosity of urea resin and bond Prod. J. 20(12):28-31. durability in plywood made from sliced Rayner, C. A. A. hard maple, with the higher viscosities 1965. Cascade gluing in plywood manumc­ giving the higher durability. The wood­ turing.. Adhes. Age 8(2):33-35. Rice, J. T. worker of years past touched the spread 1965. Effect of urea-formaldehyde resin vis­ animal glue film with his fingertips; when cosity on plywood wood bond durability. the glue was sufficiently tacky to stick to For. Prod. J. 15(3):107-112. the fingers and puil off the strings, he Selbo, M. 1. knew it was the proper time to bring the 1952. Effectiveness ofdifferent conditioning mating pieces together and apply gluing schedules in reducing sunken joints in edge glued lumber panels. For. Prod. pressure. J. 2(1):110-112. Webb, David A. 1970. Wood laminating adhesive system for ribbon spreading. For. Prod. J. 20(4): 19-23.

GLUING TREATED WOOD

Productio.n of laminated glued wood durable bond at moderate temperatures products suitable for unprotected exterior made possible the production of large use dates back to the development ofresor­ laminated timbers suitable for exterior use cinol and phenol-resorcinol adhesives­ from the standpoint of the bonded joint. about 1943. Glues available before that To impart durability to the wood under time either lacked necessary water resist­ exterior service, however, preservative ance or required very high curing tempera­ treatment is sometimes required. In some tures such as those used for exterior-type instances, the laminations are treated plywood. before gluing, and this necessitated devel­ The ability of resorcinol or phenol­ opmentofprocedures for bonding preserv­ resorcinol adhesives to provide a highly ative-treated wood.

98 Figure 99.-Bridge on logging railroad near Longview, Wash., built with laminoted stringers pressure treated with creosote-oil mixture befor.e erection.

Preservative-treated structures can also more, when only the outer layer ofa mem­ be produced by treating the glued mem­ ber is treated, checks that sometimes bers, and numerous structures of this type develop later in service may allow decay have been built (fig. 99). Treatment of to Statt. On the other hand, bridge timbers glued members permits application of produced by this method (pressure-treated preservative after all cutting, boring, and with creosote or creosote and oil mixtures other framing has been done, to assure a after gluing) have been found to be in ex­ protective coating on all exposed surfaces. cellent condition after more than 25 yeats Material handling at the treating plant is of service. often simplified when the .finished mem,­ Wood pressure-treated with preserv­ bers, rather than the lumber, ate treated. atives can be used to produce members of Probably the most serious .disadvantage of practically any size and shape that ate this method is the limited size of treating thoroughly impregnated. By prvper selec­ cylinders, which precludes treatment of tion of materials, thin laminations can be larger timbers and particularly of large given complete penetration with preserv­ curved ones. Preservative penetration is ative chemicals; this as a rule is notpossible blocked by gluelines to some extent and with larger timbers. Laminated mem­ this, of course, is a disadvantage ..Further­ bers produced from such treated srock can

99 be safely shaped and bored without ex­ followed. This type ofmaterial was glued posing untreated material. commercially as early as 1945 (fig. 100). When a plant stocks treated lumber at Data show that certain combinations of the proper moisture contene, .it can usually glue and preservative tteatments are com­ fill an order for glued treated wood much patible under prescribed conditions of moreprompdy than when the members gluing, whereas others require further must be laminated and then shipped to study-both on laboratory and commer­ a treating plant. cial scale-before definite production Of the twO methods, treatment after procedures can be formulated. The advice gluing has been most used. However, of the glue manufacturer should be sought when laminated members do not lend before gluing wood created with a partic­ themselves to treatment because of their ular preservative. size and shape, gluing treated material is All combinations of preservatives and the only known method co produce ade­ glues do not perform equalJy well and the quately treated members. conditions that lead to good, durable Studies on gluing of wood created with bonds on untreated wood do not always wood preservatives and fire-reta" :lant apply to treated wood. chemicals were undertaken during the Certain basic principles that apply to latter part of World War II and yeats that gluing untreated wood do hold true to a

... , .. -"" .;:..; .. - ~~~}...- ",";''''

M 12452, Figure lOO.-Laminated southern pine stringers in 60-foot, open-deck trestle on Atlantic CO.astline Railroad .s-:.uth of Palmetto, Fla. lurnber used in stringers was treQted with fluor-chrome-arsenate-phenol (Wolman salt) before gluing. The stringers on the opposite side of the trestle were glued from creosote~tredted southern pine.

100 certain extent for gluing treated wood. For is required for satisfactory bonding. The exampie, in gluing untreated wood there type of solvent also affects gluability. is usually considerable difference in the Volatile solvents such as naphtha and gluing properties ofthe different .species­ mineral spirits caus!: less interfe.rence with the denser woods in general requiring bonding than heavier solvents such as fuel scronger adhesion to the wood and greater oil. Wood treated with pentachloro­ cohesive strength in the glue than the phenol in liquefied fuel gas is reported to lighter ones. This also applies to treated cause practical1y no gluing problem. wood, although bonding treated wood further depends on the concentration of preservative on the surface ar the rime of WOOD TREATEDW'TH gluing and the chemical effect of the pre­ WATERBORNE PRESERVATIVES servative on the glue. In general, somewhat more curing (higher temperature or longer time) is re­ When wood is treated with waterborne guired when gluing treated than untreated chemicals, the moisture COntent of the wood. wood is appreciably increased and redrying A reasonably clean joint surface is re­ is necessary. Upon being redried, the guired in the bonding of untreated wood, lumber generally is somewhat distOrted, and this appears LO apply also to treated covered with deposits ofchemicals to some wood. There also seems to be fairly good extent, and too variable in thickness to be evidence that, wher.e gluing of treated suitable for good gluing. Resurfacing be­ lumber is involved, surfacing alier treating comes necessary. \\, hen the stock is re­ (preferably shortly before gluing) is re­ surfdced immediate1y before gluing, there quired. Where the glued members are appears to be, generally, less problem in intended to withstand exterior exposure, gluing wood treated with waterborne pre­ the joints should pass the tests prescribed servatives than in gluing wood treated in Voluntary Product Standard PS 56-73 with oil-borne preservatives. Laminated for structural glued-laminated timber. bridge timbers glued from lumber treated Where the treatment .is intended mainly with waterborne preservatives have given for resistance to termites and similar satisfactOry service for about a quarter hazards, and the glued members are century. protected from the weather, the tests pre­ Treating large laminated timbers with scribed for interior service of glue joints waterborne preservatives after gluing .is in laminated timbers might be sufficient. generally not recommended because of checking and dimensional changes .that occur during drying. WOOD TREATED WITH OIL-SOLUBLE PRESERVATIVES WOOD TREATED WITH Because wood treated with oil-soluble FIRE-,RETARDANTCHEMICAlS preservatives usually goes into exterior or similar types ofservice, only adhesives suit­ Because fire-retardant-treated wood is able for severe exposure conditions, such as often used in relatively dry .exposure for resorcinol and phenol-resorcinols, should such purposes as veneered doors, wall be used. Asa general rule, woods that take panels, and partitions, adhesives only treatment well, such as southern pine, can moderately resistant to moisture might also be glued satisfactorily. Those difficult occasionally be suitable. For maximum fire to treat are more problematic, and a "clean resistance (as far as the glue is concerned), treatment" (by steaming or other means) it probably is necessary to use phenol,

1()1 resorcinol, and melamine resins that do SELECTED REFERENCES not permit the wood to delaminate or separate when it is charred. Many fire­

QUALITY CONTROL

The author's first experience with evalu­ Up to recent years, shear block tests ation of glue-joint quality was gained by (ASTM D 905, Standard Method of Test splitting apart the edge and end trims of for Strength Properties of Adhesive Bonds plywood panels as the panels came through in Shear by Compression Loading) on hard trim saws. If the failures were mostly in maple were a common requirement in glue the wood, it was a good indication that the specifications. This test has mer

102 within a species, as an added safety feature, The person in charge of quality control 1:heglue might also be evaluated on a must be very knowledgeable, both in wood denser species than the one to be used in technology and in the physical and chemi­ production. cal characteristics of adhesives..There are The well-worn phrase that dense wood is product standards, industry standards, "mote difficult to glue" does not neces­ commercial standards, ASTM standards, sarily mean that the same adhesive devel­ Federal specifications, and military specifi­ ops high wood failure in a light wood and cations 1:hat generally specifY minimum low wood failure in a dense wood. The glue performance requirements under different may not have been used under the opti­ tests, and the procedures for carrying our mum conditions required for the denser the tests are fairly routine. But the inter­ wood; or .it may not be strong enough to pretation of the test results, including the cause failure in the denser material. visual examination of the test specimens, As in any manufacturing operation, often requires a great deal of knowledge control ofquality ofglue joints is extremely and experience to determine their meaning important, particularly since the raw and consider improvements or changes. materials-wood and glue-are character­ Needless to say, quality concrol does not istically somewhat variable. involve only tests and evaluations of the

100 LEGEND: ~ I:: ACCELERAT£DVACUUM-PRESSURE ~ SOAKING AND DRYING CYCLES fZ2LIlI:: SALT WATER SOAKING AND DRYING CYCLES I es 80 I-­ ~:: HIGH-TEMPERATURE LOW-HUMIDITY AND ~ ROOM-TEMPERATURE HIGH-HUMIDITY CYCLES ~ ~ ~ 60 ~ .~ ~ .'" / ~ f: .... 40 ~ ~11l ~ ~ I t% ~ 20 1/ ~ R:~ 1 ~ /.~ 0 ~.I1l ~ RESORCINOL UREA CASEIN TYPE OF GLUE ZM 73979F Figure 101.-Effect of three types of accelerated laboratory tests on glue bonds in plywood-to-Iumber .gusset joints made with three types of adhesives. The vacuum-pressure, soak-dry cycles resulted in the largest amount of .joint separa­ tion with each type ,of glue. The data indicate that two of the glues are unsuitable for severe exposures.

103 2"/ A I:f-/

2M 69813 F Figure 102.-Standard block,.A, and .stair-step type, B, shear specimens for evaluat­ ing glue-joint strength and quality. The stair-step is convenient for testing .succes­ sive joints in .a laminated timber.

104 final products, but must begin with each type ass.embly joints are illustrated in ingredient that goes into the glued prod­ figure lOI. uct. Tests such as the compression block This is particularly important where a shear (figs. 102 and 103) and the plywood number of woodspedes are used and a tension shear (figs. 104 and 105) have been variety ofproducts are made. The adhesive employed to evaluate glue joints for used for one spedes may not necessarily decades. They give a good indication of be adequate for another, or at least might initial quality and workmanship in pro­ require modification in the gluing proced­ ducing the joints when tested dry. For ure. Also, modifications might be re­ determinadon of long-term durability, quired when changing from one product harsher treatments are required, and twO 4-hour boil cycles interspersed with 20 to another. The standards and specifications fot .the hours of drying ofthe spedmens (generally different products generally specify one, referred to as the boil test) has been the and more often several, test requirements most widely used test for exterior plywood. that a product must meet. Detailed procedures for this and other tests Although test methods must be used are given in many standards and specifica­ that will result in accelerated degradation tions for plywood and adhesives. of glue joints that would eventually fail Vacuum-pressure, soak-dry tests similar under normal service conditions, the teStS to ASTM 2559 have been used for about must be reasonable for the type of bond 3 decades to evaluate glue bonds in lami­ involved. Even the very best casein-glued nated construction (fig. 106) and have joint, for instance, will fail after a few cycles of vacuum-pressure, soaking, and drying. Casein .glue is not capable of form­ ing exterior-type bonds; hence, test methods designed for such bonds are not applicable to casein-glued joints. Effects of three different accelerated test methods on three types of glue bonds in gusset-

I I I I I I ... .. I I /" 11 I: 1 r-'''~ ~ "1 b'''j ~1!i!!\\\\l!lil\ll!l!l!U\\!I!\II!II\I!\\lililnll\lI!11!l!li!I!"liIia \. 3[ .\ M 76703 Figure 103.--.Block-shear testing of glue Figure 104.--.Tension-shear specimen joints in universal testing machine. from three-ply plywood.

105 within recent years also been adopted for plywood and particleboard. They are more indicative of weatherability and soak-dry resistance of glue joints and are also less time consuming than soaking and drying at atmospheric pressure.

Tension tests are generally considered the most reliable for glued end joints. A rectangular specimen shown in figure 107 is easily prepared and rapidly tested, Im­ poccant features in quality control.

Figure 108 illustrates an electronic universal testing machine capable of Figure 105.-Hydraulically operated, plotting stress-strain curves and suitable quick acting, tension-shear test ma­ for use in compression and tension testing chine for plywood. of a wide range ofspecimen types.

M 59284 P Figure 106.---.Laminated oak beam section "after completion of vacuum-pressure, soak-dry cycles (ASTM D 2559). "Glue joints are still intact "although w.ood is .badly checked from severe drying str"esses inflicted by the test.

106 .SELECTEDREFERENCES

American Institute of Timber Construction Standard specifications for structural glued laminated timber ofDouglas-fir, western larch, and CalifOrnia redwood. AlRC 203 (see current edition). Englewood, Colo. American Society for Testing and Mater.ials Standar:l specification for adhesives for struc­ tural laminated wood products for use und·,r exterior (wet use) exposure condi­ tions. ASTM D 2559 (see current edition). Philadelphia. AmC':rican Society for Testing anJ Materials Srandard method of test for strengrh flroP­ erties of adhesive bonds in shear by com­ pression loading. ASTM D 905 (see current edition). Philadelphia. Callahan, Richard C. 1953. Quality control in furniture manufac­ ture. For. Prod. J. 3(5):19-24. Kreibich, R. E., and Freeman, H. G. 1968. Development and design of an accel­ erated boil machine. For. Prod. J. 18(12): 24--26. National Woodwork Manufacturers Association 1969. Industry Standard 1.S. 1-69. Wood flush doors; hardwood veneered including hardboard and plastic faced flush doors. July. Ripley, Robert M. 1953. Effective quality control on end prod­ UCt. For. Prod. J. 3(5):25-28. Dec. Selbo, M. L. 1962. A new method for testing glue joints oflaminated timbers in service. For. Prod. J. 12(2):65-67. Selbo, M. L. 1964. Rapid evaluation of glue joints in laminated timber. For..Prod. J. 14(8): 361-365. Selbo, M. L. 1964. Tests fOr quality of glue bonds in end-jointed lumber. ASTM Special Tech. Figure 107.-Finger-jointedstrip-tension Pub!. No. 353: 1962 Symposium on specimen in test grips for testing. Timber, pp. 78-86. Am. Soc. Tesr. Mater. Philadelphia. U.G. Department of Commerce Proposed Product Standard for hardwood and decorative plywood. PS 51 (see mosr U.S. Department of Commerce recent issue). Structural glued laminated timber. Comma. U.S. Department of Commerce Stand.CS 253 (see latest edition). Softwood plywood construction and indus­ L S. Department of Commerce trial. U.S. Prod. Stand ..PS 1 (see latest Wood double-hung window units. Commer. edition). Srand. CS 190 (see latest edition),

107 M 138766-11 Figure 108.-..,Electronic universal testing machine capable of plotting stress-strain curves.

U.S, Deparrment of Defense U.S. General Services Adminisrrarion Adhesive; phenol and resorcinol resin base Adhesive; vinyl acerare resin emulsion. Fed. (for marine use only). Mil. Spec if. MIL­ Specif. MMM-A-193c.Fed. SupplyServ. A-22397. Der. Supply Agency (see laresr (see larest edition). edirion). U,S. Deparrment of Defense U.S. General Services Administration Adhesive; modified epoRy resin with poly­ Adhesive; natural or synrhetic-natural amine curing agent. Mil. Spec if. MIL-A­ rubber. Fed. SpeciE. MMM...,A-139. Fed. 81253(1). Def. Supp),y Ageflcy (see latesr Supply Servo (see latest edition). edirion). U.S. Deparrment of Defense U.S. General Services Adminisrration Adhesive; epoxy resin with polyamide curing Adhesive; synrhetic, epoxy resin base, pasre agent. Mil. Specif. MlL-A-81235(2), form. general purpose. Fed. SpeciE. Def. Supply Agency (see laresr edition). MMM-A-187a. Fed. Supply Servo (see U.S. General Services Administration latesr edidon). Adhesive; urea resin type (liquid and West Coast Adhesive Manufacturers Associa­ powder). Fed. Specif. MMM-A-188b. tion's Technical Committee Fed. Supply Servo (see laresr edition). 1966. A proposed new test for accelerated U.$. General Services Administration aging of phenolic resin bonded particle­ Adhesive; COntact. Fed. Spec if. MMM-A­ board. For.. Prod. J. 16(6): 19-23. 130a. Fed. Supply Servo (see larest edi­ tion). West Coast Adhesive Manufacturers Associa­ U.S. General Services Adminisrration tion Adhesive; animal glue. Fed. Spec if. MMM­ 1970. Accelerared aging of phenolic resin A-lOOc. Fed. Supply Servo (see laresr edi­ bonded particleboard. For. .Prod. J. tion). 20(0):26-27.

108 GLOSSARY

Absorptiveness.-The ability of a s:Jlid to air, steam, water, or vacuum, to a flex­ absorb a liquid or vapor, or the rate at ible cover which, sometimes in conjunc­ which the liquid or vapor is absorbed. tion with a rigid die, completely en­ Aged (Matured).-The condition ct closes the material to be bonded. which the reaction between the active Baseboard.-A board placed against the ingredients of an adhesive has reached wall around a room next to the floor the proper stage for spreading. to finish properly between floor and Airseasoning (Air drying). -Theprocess plaster or gypsum board. of drying green lumber or other wood Blade-coaling-Application ofa film of a products by exposure to prevailing liquid material (liquid resin) on a panel atmospheric conditions outdoors or in surface by scraping the straight edge of an unheated shed. a steel blade, or other material, over the Annual ring.-The growth layer put on panel. a tree in a single growth year, including Blistering.-Fonnation of vapor pocket earlywood and latewood. in a plywood panel because of toO wet Architectural plywood. -Plywood having veneer, tOO much solvent in adhesive, esthetic appeal, attractive grain pattern. too high adhesive spread, or tOo high Assembly joints.-Joints for bonding cure temperature for the adhesive used. variously shaped parts such as in wood Blood albrlmin.-Complex protinaceous furniture (as opposed to joints in ply­ material obtained from blood. wood and laminates that are all qUIte BoilprGof adhesive-Adhesive that will similar). not fail after many hours of boiling. Assembly lime.-Interval between Bond failure. -Rupture of adhesive spreading the adhesive on the surfaces bond. to be joined and the application ofpres­ Book matching.-Matching veneer by sure to the Joint Or joints. turning over alternate sheers. Note-For assemblies involving Boom.-A spar extending ftom a mast to multiple layers or parts,the assembly hold bottom of sail outstretched; also time begins with the spreading of the used for loading and unloading pur­ adhesive on the first adherend. poses. (1) Open assembly time is the .time Bowing.-Distortion whereby the faces of interval between the spreading of the a wood product become concave or adhesive on the adherend and the com­ convex along the grain. pletion of assembly of the parts for Burnished.-A glazed surface with which bonding. it may be difficult to obtain a satisfac­ (2) Closed assembly time is the time .tory bond. interval between completion of assem­ Burl.-Burls come from a warry growth bly of the parts for bonding and the generally caused by some injury to.the application of pressure to the assembly. growing layer JUSt under the bark, This Bacteria.-One-celled micro-organisms injury, perhaps. due to insects or which have no chlorophyll and multiply bacteria, causes the growing cells to by simple division. divide abnormally, creating excess Bag molding.-A method of molding or wood, that finds room for itselfin many bonding involving the application of little humps. Succeeding growth fol­ fluid or pressure, usually by means of lows these Contours. Cutting across

109 these humps by the half-round method Convex. --Curved like a section ofthe out­ brings them out as little swirl knots side of a sphere. or eyes. Copolymer.-Substance obtained when Butt joint.-An end joint formed by two or more monomers polymerize. gluing together the squared ends oftwo Clamping pressure.-Pressure developed pieces. Because of the inadequacy and by clamps of various desigtJs to bring variability in strength of butt joints joint surfaces into close COntact for glue when glued, such joints are generally bond formation. not depended on for strength. Cleavage.-Splitting or dividing along Calking gun.-Device for dispensing a the grain. bead of calking material, mastic glue, Closed side.-Side of veneer not touching etc. knife as it is peeled from log (also called Capillary structure.-An inclusive term tight side of veneer). for wood fibers, vessels, and other ele­ Coagulation.-The process by which a ments of diverse structure making up liquid becomes a soft, semisolid mass. the material wood. Cobesion.-The state in which the par­ Casehardening.-A stressed condition in ticles of a single s.ubstanceare held a board or timber chara.~terized by com­ together by primary or secondary va­ pression in the oueer layers accompanied lence forces. As used in the adhesive by tension in the center or core, the field, the state in which the particles result of too severe drying conditions. of the adhesive (or the adherend) are Catalyst.-A substance that markedly held together. speeds up a chemical reaction such as Cold flow.-Tendency to yield or "flow" the cure of an adhesive when added in under stress at normal room tempera­ minor quantity as compared to the ture (see also Creep). amounts of the primary reactants. Cold pressing.-Pressing panels or lami­ Cell wall.-Enclosing membrane for the nates Without application of heat for minute units of wood structure. curing the glue. Cereal flour.-Flour from grain used as Compressometer. -Device used for meas­ food. Char.-To scorch or reduce to charcoal uring pressure. Consists essentially of by burning. a cylinder, piston, and a pressure gage. Chetking.-A lengrhwise separation of Oil in the cylinder transmits the pres­ the wood that usually extends across sure applied to the piston to the gage. the rings of annual growth and com­ Compression wood.-Abnormal wood monly results from stresses set up in formed on the lower side of branches wood during seasoning. and inclined trunks of softwood trees. Chemical synthesis.-The formation of a Compression wood is identified by its complex chemical compound by com­ relatively wide annual rings, usually bining two or more simpler compounds, eccentric, relatively large amount of radicals, or elements. earlywood, sometimes more than 50 Condensation reaclion.-A chemical percent of the width of the annual rings reaction in which two or more mole­ in which it occurs, and its lack of de­ cules combine with the separation of marcation between earlywood and late­ water or some other simple substance. wood in the same annual rings. Com­ If a polymer is furmed, the process is pression wood shrinks excessively called polycondensation. lengthwise, as compared with normal Continuous feed press.-Press in which wood. panels are moving ahead (under pres­ Concave.--Curved like a section of the sure) while glue is setting. inside of a sphere.

llO Co-spray dried.-Dried by spraying two time weight and volume are deter­ resins simultaneously into the same mined. drying chamber from atomizing nozzles. Design criteria.-Standard rules for (See also Spray dried.) design. Creep.-The dimensional change with Diagonal-grain wood.-A form of cross time of a material under load, follow­ grain where the longitudinai elements ing the initial instantaneous elastic or run obliquely but parallel to the surface; rapid deformation. Creep at room tem­ i. e., the growth layers are not parallel perature is sometimes called cold flow. to the edge of the piece as viewed on a Creosote.-Oily liquid used, among other quartersawed surface. things, as preservative for wood. Doctor roll.-Smooth roll whose position Critical expoaure.-Exposure ro harsh in relation to spreader roll is adjustable conditions (see also Severe exposure.). for regulating amount of ,glue spread. ,Cross grain.-A general term for any Door skins.-Thin plywood, usually grain deviating considerably from the three-ply, used for faces of flush doors. longitudmal axis of a piece of timber Double spreaditzg.-Applying adhesive and emerging at an angle from a face to both mating surfaces of a joint. or edge. Dovetail.-Joint shaped like a dove's taiL Cross-/ink.-An atom or group connect­ Dowel.-Wood peg fitted into corre­ ing parallel chains in a complex mole­ sponding holes in twO pieces to fasten cule. them rogether. Crotch veneer.-Veneer cut from fork of Earlywood.-The portion of the annual tree to provide pleasing grain, figure, growth ring formed during the early and contrast. growth period. EarJywood is less dense Cup.-Distorrion whereby a board be­ and mechanically weaker than latewood. comes concave or convex across the grain. Edge gluing.-Bonding veneers or boards Curing (Cure).-To change the physical edge to edge with glue. properties of an adhesive by chemical Elasticity.-The capacity of bodies to reaction, which may be condensation, return to their original shape, dimen­ polymerization, or vulcanization; usu­ sions, or positions on the removal of a ally accomplished by the action of heat deforming force. and catalyst, alone or in a combination, Elastomer. -A material that at room tem­ with or without pressure. perature can be stretched repeatedly to Curtain coating.-Applying adhesive to at least twice its original length and, wood by passing the wood under a thin upon immediate release of the stress, falling curtain of liquid. will return with force to its approximate Dado.-A rectangular groove across the original length. width of a board or plank. Electrodes.-In radiofrequency heating, Delamination.-The separation of layers metal plates or other devices for apply­ in laminated wood or plywood because ing the electric field to the material of failure of the adhesive, either in the being heated. adhesive itself or at the interface between the adhesive and the adherend. Elevated temperature setting. -An adhe­ sive that requires a temperature at or Density.-Weight per unit volume, 0 generally expressed in pounds per cubic above 310 C. (87 F.) to set (see also foot. For wood, since changes in mois­ Room temperature setting). ture content affect its weight alld Emulsion.-A mixture in which very volume, it is necessary to specify the small droplets of one liquid are sus­ moisture condition of the wood at the pended in another liquid.

111 End grain.--The grain of a cross sec­ Film adhesive.-Describes a class ofadhe­ tion of a tree, or the surface of such a sives furnished in dry film form with or section. without reinforcing tissuelike paper or Endjoint.-A joint made by gluing two fabric. pieces of wood end to end, commonly Finger joint.-An end joint made up of by a scarf or finger joint. several meshing fingers ofwood b Dnded Equilibrium moistllre content. -The together with an adhesive. moiscure content at which wood neither Fire 1·etardant.-A chemical or prepara­ gains nor loses moisture when sur­ tion ofchemicals used to reduce flamma­ rounded by air at a given relative bility or to retard spread of fire. humidity and temperature. Flat-grained ll1mber.-Lumber that has Expeller.-Device that removes oil from been sawed in a plane approximately bean by crushing (See also RolJer mil/). perpendicular to a radius of the log. Extetlder. -Asubscance, generally having Lumber is considered flat grained when some adhesive action, added to an adhe­ the annual growth rings make an angle sive to reduce the amount ofthe primary of less than 45° with the surface of the binder required per unit area. piece. Exterior service.-Service or use in the open (exposed w weather), F/ow.-In gluing, the state of a substance Externalload.-Load applied externally. sufficiently liquid to penetrate pores and External stresses.-Scresses imposed by minute crevasses when pressure is external load. appIied. Extract/ves.-Any substance in wood, FirJid preSS1Jre.--:Pressure applied by an not an integral part of the celiular struC­ inflated bag or similar means. cure, that can be removed by solution Flush pane/s.-Flat panels as on a flush in hot or cold water, ether, benzene, door (no contorted or shaped parts). or other solvents that do not react chem­ Fortifier.-Material improving certain ically with wood components. qualities in adhesives, such as water Extrusion spreading. -Adhesive forced resistance and durability. through small openings in spreadel; head Flmgi.-Simple forms of nongreen plants (see also Ribbon spreadillg). consisting mostly of microscopic Exudation products.-Tars and similar threads (hyphae) some of which may products that migrate to the wood sur­ attack wood, dissolving and absorbing face. substrate mate.rials (cell walls, celJ Fiber satflrationpoint.-The stage in the contents, resins, glues, etc.) which the drying or wetting of wood at which the fungi use as food. cell walls are sacurated with water and the cell cavities are free of water. Also Gap-filling adhesive.-Adhesive suit­ described as the moisture level above able for use where the surfaces to be which no dimensional changes take joined may not be in dose or continuous place in wood. It is usually taken as COntact owing either to the impos­ about 30 percent moisture content, sibility of applying adequate pressure or based on the weight when ovendry. to slight inaccuracies in matching Figured veneer.-General term for deco­ mating surfaces. rativ,~ veneer such as from crotches, Glazed.-Worn shiny by rubbing. buds, and stumps. Glossy finish.-Shiny finish, reflects Filler.-A relatively nonadhesive sub­ light. stance added to an adhesive to improve Gluabi/ity.-Term indicating ease or its working properties, permanence, difficulty in bonding a material with strength, or other qualities. adhesive.

112 Glue laminating.-Producdon of struc­ High-freque1lcy wring. -Setting or tural or nonstructural wood members by curing adhesive with high-frequency bonding two or more layers of wood electric currents. together with adhesive. Hollow-core construction.-A panel con­ Glueline.-The layer of adhesive affect­ struction with facings ofplywood, hard­ ing union (bond) between any twO ad­ board, or similar material bonded to a joining wood pieces or layers in an framed core assembly of wood lattice., assembly. paperboard rings, or rhe like, which GIlle wheel.-Continuous, caterpillar­ support the facing at spaced intervals. type device or machine used for edge Honeycomb core.-A construction of thin gluing panels or laminating small items sheer material, such as .resin impreg­ such as table legs. nated paper or fabric, which has been Gluing pressure.-Pressure to bring the cortugated and bonded, each sheet in surfaces spread with glue intO close opposite phase ro the phases ofadjacent contact for bonding. sheets, to form a core material whose Grain direction.-Fiber direction (essen­ cross section is a series ofmutually conti­ tially parallel to pith of tree). nuous r.ells similar to natural honey­ Gravity feed.-Moves ahead by virtue of comb. its own weight. Honeycombing.-Fissures in the interior GlIsset.-A flat piece of wood, plywood, of a piece of wood generally caused by or similar type member used to provide drying stresses resulting from case­ a connection at the int~rsection of wood hardening. members. Most commonly used at Hot press.-A press in which the platens joints ofwood trusses. They are fastened are heated to a prescribed temperature by nails, screws, bolts, or adhesives or by steam, electricity, or hot water. w.ith adhesive in combination with Humidity cycling.-Exposure to high nails, screws, or bolts. humidity followed by lvw humidity (or vice versa) for various periods. Hammermill. -Consists of horizontal or Hygroscopic.-Term used to describe a vertical shaft rotating at high speed on substance, such as wood, that absorbs which crushing elements, hammers, and loses moisture readily. bars, or rings, are mounted. Incident lighting.-Light rays falling Hardener.-A substance or mixture of on a surface at a low angle or almost substances added to an adhesive to pro­ parallel to the surface. mote or control the curing reaction by Interior service. -Used in the interior (of taking part in it. The term is also used a building) protected from outdoor to designate a substance added to control weather. the degree of hardness of the cured film. Internal stress.-Stresses set up from in­ Hardwood.-A conventional term for the ternal conditions, such as differential timber of broad-leaved trees, and the shrinkage, aside from external loads trees themselves, belonging to the applied to a member. botanical group Angiospermae. Inverse proportion.-A relation between Heartwood.-The wood extending from variables in which one increases as the the pith to the sapwood, the cells of other decreases. which no longer participate in the life Jacketedmixer.-Double-wall mixer per­ processes of the tree. Heartwood may mitting cooling or heating liquid to cir­ be infiltrated with gums, resins, and culate between the walls. other materials that usually make it Jig.-A device for holding an assembly i.n darker and more decay resistant than place during gluing or machining sapwood. operations.

113 .Jointer.-Machine equipped with rotary Mastic adhesive.. -A substance with cutter and flat bed permitting surfac­ adhesive properties, generally used in ing one side of a member at a time. relatively thick layers that can.be readily Joint geometry.-Shape or design of joint formed with a trowel or spatula. (for example, a finger joint). Matte finish.-Dull finish. Joist.-Gne of a series of parallel beams, Mature.-{see Aged). usually nominally 2 ,inches thick, used Mechanical adhesion. -Adhesion ef­ to support floor and ceiling loads, and fected by the interlocking action of an supported in turn by larger beams, adhesive that solidifies within the girders, or bearing walls. cavities of the adherend. Keel.-The chief timber or steel member Mechanical fasteners. -Nails, screws, extending along the encire length of the boIts, and similar items. bottom of a boat or ship to which the Mesh sieve.-The size of openings in a frames are attached. sieve as designated by the number of Kiln .drying.-The process of drying meshes (openings) per linear inch. wood products in a dosed chamber in Mitered joint.-Joint cut at.3. 45° angle which the temperature and relative with fiber direction. humidity of the circulated air can be Mixed grain. -Mixture of flat-sawn and controlled. quartersawn pieces. Lacquer.-A clear finishing material Mold.-A fungus growth on wood prod­ cl);:-.sisting of shellac or gum resins dis­ UCts at or near the surface and, there­ solved in alcohol and other quick-drying fore, not typically resulting in deep solvents, with or without nitrocellulose. discolorations. Mold discolorations are Laminated member.-A wood member usually ash .green to de.ep green, glued up from smaller pieces of wood, although black is common.. either in straight or curved form, with Molding.-Shaping or forming to desired the grain ofall pieces essentially parallel pattern or form. to the length of the member. Monomer.-A relatively simple com­ Laminated timber.-Synonymous to pound which can react to form a laminated member, but usually implies polymer. structural member. Mortise.- A slot cut in a board, plank, Latewood.-The denser, smaller celled or timber, usually edgewise, to receive parr of the growth layer formed late in tenon ofanother board, plank, or timber the growing season. to form a joint. Layup.-Assembled parts placed in posi­ Mttltiopening press.-Press having a don they occupy in final product. number ofplatens between which panels Lignin.-The noncarbohydrate, struc­ can be pressed. tural constituent of wood and some other plant tissues, which encrusts the Nailed glued.-A .laminate for which cell walls and cements the cells together; gluing pressure .is obtained by .nailing now believed to consist of a group of together the pieces spread with glue. closely related polyme.rs of cer.tain Nail popping.-Protrusion of nailheads phenylpropane derivatives. because of shrinking and swelling of Lo.w-voltage heating._Heating by pass­ wood. ing low-voltage electric current through Natural adhesive.-Adhesive produced resistance elements. from naturally occurring products such Marineplywood.-Plywood made of as blood and casein. veneers of grades specified for marine Neoprene.-Synthetic rubber. use and bonded with waterproof adhe­ Nominal lumber.-The rough-sawed sive (usualJy phenolic type). commercial size by which lumber is

114 known .and ·sold in the market; for Pitch. -In finger joints, the distance example, a 2 by 4. between midpoint of one fingertip and Nonvolatile.-Portion of a solution that the midpoint of the adjacent finger.tip. is not easily vaporized. Pith side.-Side nearest to pith (and usu­ Oil-borne preurvative.- Preservative ally center of tree). dispersed or dissolved in oil carrier or Planer.-Machine equipped with cutter vehicle. rolls and feed rolls for surfacing or Oil-soluble preservative. -Preservative planing wood. chemical dissolved in oil carrier. Plasticizer.-A liquid or solid chemical Ope". assembly.-The time interval be­ added to a compound to impart soft­ ·tween the spreading of the adhesive On ness or flexibility, or both, to it. the adherend and the complet:on of Platens.-Steel plates constituting the assembiy of the parts for bonding. (See pressure elements in a single- or multi­ also Assembly time.) opening hot press. Open piling.-..sra:::king wood products Plywood.-A composite product made up layer by layer, separated by strips of of crossb~nded layers of veneer only or wood inserted between layers to permit veneer in combination with a core of air circulation. lumber or ofparticleboard bonded with Open side. -Side of veneer next to knife an adhesive. Generally the grain of as it comes off the log (al,50 called loose adjacent plies is roughly at right angles side). and an odd number of plies is usually Organic solvents.-Solvents based on used. carbon compounds. Pneumatic,-FiJled with compressed air. Original dry stnmgth. -Shear strength Polymerization.-A chemical reaction in developed by glue joints when tested which the molecules of a monomer are dry and before aging or exposure to linked rogether to form large molecules deteriorating conditions. whose molecular weight is a multiple of Overhang. -Part of roof extending be­ that of the original substance. When yond the outer wall. two or more monomers are involved, the Overlay.-Plastic film or one or more process .is called copolymerization or sheets ofpaper impregnated with resin heteropolymerization. and used as face material, mainly over PoIYllrethane.-A versatile chemical used plywood but also on lumber or other for adhesives, sealing compounds, products. Overlays can be classified as finishes, and other purposes. masking, decorative, ors.tr.uctural, Porosity.-The ratio of the volume of a depending on their purpose. material's pores to that of its solid Parallel heating. -Electric or high­ content. frequency field parallel to adhesive Pot Ii/e.-Usable life of adhesive after joints. mixing (see also Working Life). Partick.board.-A generic term for a panel manufactured from lignocellulosic Precipitated.-Separated oue (addition of materials-.--commonly wood-in the acid to milk causes curds to separate out form ofparticles (as distinctfrom fibers) from whey). which are bonded .together with a syn­ Precuring.-Condition .of too much cure thetic binder (or other) under heat and or set of the glue before pressure is pressure by a process wherein the .inter­ applied, resulting in inadequate flow particle bonds are created wholly by the and glue bond. .added binder. Prefabricated. -Factory-built, standard­ Pearl glue.-Animal glue dried in the ized sections or components for ship­ form of round pearls. ment and quick assembly, as for a house.

115 'Pr2servative.-Any substance that, for a when subjected to stress, usually has :reasonable length of time, is effective a sofrening or meltin.g range, and usu­ in preventing the development and ally fractures conchoidally. action of wood-destroying fungi, borers Resurfacing. -Planing agPin to obtain a of various kinds, and harmful insects freshly clean surface for gluing. that deteriorate wood when the wood Ribbon spreading.-Spreading a glue in has been properly coated or impregnated parallel ribbons instead of a uniform with it:. film. Qtlartersawed.-Sawn so the ann11,:l1 Roll coating.-Application of a film of a rings are essentially perpendicular to th!: liquid material (liquid resin) on a sur­ wide face of the board. Lumber is con­ face with rolls. sidered quartersawed when the annual Roller mill.-Device for crushing beans growth rings form an angle of 45° to by passing them between smooth rolls, 90° with the wide surface of the piece. thereby separating oil. Rabbet.··-A type of joint for fitting one Room-temperature-setting adhesive.­ wood member to another (for example, An adhesive that sets at temperatures planking to keel and stem of a boat). between 20° and 30° C. (68° to 86° Racking. -Application of pressure to the F.)-the limits for standard room tem­ end of a wall anchored at the base but perature specified in ASTM D 618. free to move at tOp. Rotary cut.-Veneer cur on a lathe which Radiofrequencyenergy.-Electrical _Otates a Jog or bolt, chucked in the energy produced by electric fields alter­ center, against a fixed knife. nating at radiofreguencies. Sandwich panel.-A layered construction Rail.-BottOm or top horizontal member comprising a I;vmbinacion of relatively of a door. high-strength, thin, facing materials Reactioi; ·wood. -Common term for intimately bonded to and acting inre­ tension wood in hardwoods and com­ grally with a low-density core materiaL pression wood in softwoods. Sapwood.-The living wood of pale colur Rcactive.-Adhesives that cure orser near the outside of the log. Under mOSt rather fast (opposite to sluggish or slow conditions the sapwood is more suscep­ curing). tible to decay than heartwood. Reconditio11cd.-Brought back to a Sash.-A frame for holding the glass pane previous condition (for example, pre­ or panes for a window. vious moisture level), Scarfjoints.-Sloping joint between ends Relative hllmidity.-Ratio ofthe amount of two wood members. of water vapor present in the air to that Setting.--Hardening (see also Curing). which the air would hold at saturation Severe exposure. -Exposure to harsh at the same temperature. It is usu,tlly weather conditions or to harsh tests such considered on the basis of the weight as boiling and drying at low humidities. of the vapor but, for accuracy, should Shear. -The relative displacement of be considered on the basis of vapor woody tissues following fracture as a pressures. result of shearing stress. Re1znet. -A preparation or extract used to Sht:. r block test (also called glue block curdle milk (as in cheesemaking). sbeartest}.-A means of testing a glue Resiliency.-The quality of being resil­ joint in shear (ASTM D 905). ient or elastic. Shearparallelto grain.-Stresses applied Resin.-A solid, semisolid, or pseudo­ in a manner to cause shear failure along solid organic material that has an in­ the grain. definite and often high molecular Shear strength.-The capacity of a body weight, exhibits a tendency to flow to resist shearing stress.

116 Shoe (Tapeless splicer). -Device for fie gravity of wood is usually based on bonding veneers edge to edge with glue the green volume. (no .tape). Sp/ine.-Thin piece of wood or plywood Short grain.-Term used for cross grain often used to reinforce a joint between as when end grain is exposed on face two pieces of wood. of veneer. Spray dried..-Dried under vacuum of Showthrough.-Term used when effects atomized particles of a liqu.id resin. of defects within a panel can be seen on Squeezeout.-Bead of glue squeezed out the face. of a joinr when gluing pressure is Sizing.-The process of applying diluted applied. animal glue or similar material to the Starved joint.-A joint that is poorly face or faces of a panel to reinforce fuzzy bonded because insufficient acihe~ive has fibers and facilitate sanding. remained in it as a result of excessive Skinning.-Formation of a skin on the pressure on the joint or toO low vis­ adhesive surface due to evaporation of cosity, or both; the adhesive is forced !'olvent. out from between the surfaces to be Sliced veneer.-Veneer that is sliced off joined. .a log, bolt, or flitch with a knife. Stem.--Condnuation of the keel to form Slip joint.-Type of corner joint with the prow of a boat or ship. interlocking ··fingers." Stiles.-Vertical pieces in a panel or Slope of grain.-Angle between grain frame, as of a door or window. direction and axis of piece. Storage life.-The period of time during Soak-dry cycles.-Type of test where which a packaged adhesive can be stored specimens are alternately soaked and under specified temperature conditions dried. and .remain suitable for use. Sometimes Softwood.-A conventional term for both called shelf life. timber and the trees belonging to the Straight-grained wood.-Wood in which botanical group Gymnospermae. the fibers run parallel to the axis of the Solids content.-Thepercentage by piece. weight of the nonvolatile matter in an Stress.-The force (per unit area) devel­ adhesive. oped in resistance to loading or, under s.olid .core.--Core with no open spaces as certain conditions, self-generated in the occur in hollow cores. piece by internal variations of moisture Solvent.-The medium within which a content, temperature, or both. substance is dissolved, most commonly Stress risers.-Points of concentrated applied to liquids. Used to bring partic­ stress. ular solids into solution. Strtlctural plywood. -Plywood for struc­ Spar. -Round wood member used on tural use, such as flooring, -siding, and ships for loading and unloading, also roof sheathing. for keeping sails outstretched. Stud.-One of a series of slender, vertical Spar f/ange.-Upper or lower member of structural members placed .as support­ a spar made in the form of an I-beam. ing elements in walls and partitions. Specific adhesion.-Adhesion effected by Sunken joint.-Depression in wood valence forces (of the same type as those surface .at glue joint caused by surfacing that effect cohesion) acting between the edge-glued mater.ialtoo soon after adhesive and the adherend. gluing. (Inadequate time allowed for Specific gravity.-In wood technology, moisture added with glue to diffuse the ratio of the ovendry weight of a away ftom the joinr.) piece of wood to the weight of an equal Synthetic adheJives.-Adhesives pro­ volume ofwater at 4°C. (39° F.). Speci­ duced by chemical synthesis.

117 Tack.-The property of an adhesive that Torque wrench.-Wrenchequipped with' enables it to form a bond ofmeasurable indicating device for measuring torque. strength immediately after .adhesive and Transverse section. -Wood cut in a direc­ .adherend arehrought into contact under tion perpendicular to the grain, pro­ low pressure. ducing an end-grain surface. Tape/ess splicer.-Machine for joining T redting cylinder. -Cylindrical-shaped veneers edge to ecge with glue only and vessel equipped with vacuum and no .tape. pressure pumps used in preservative Tenon.-Aprojecting part cur on the end pressure treatment of wood. df a piece of wood for insertion into a Truss. -A frame or jointed structure corresponding hole in another piece to designed to act as a .beam of long span, make a joint. while each member is usually subjected Tensile strength.~The capacity ofa body to longitudinal stress only, either to sustain tensile loading (resistance to tension or compression. lengthwise stress). In wood,tensile Twist.-A distortion caused by the turn­ strength is high .along the grain and low ing or winding of the edges of a board across the grain. so that the four corners of any' face are Tension parallel to grain.-Stress on a no longer in the same plane. material (wood) in the long direction of Uncata/yzed.-Nocatalyst employed or its fibers. added. Tension wood.-An abnormal form of Underlayment.-A material placed under wood [,,-,und in leaning trees of some finish coverings, such as flooring or hardwood species and characterized by shingles, to provide a smooth, even the presence of gelatinous fibers and surface for applying the finish. excessive longitudinal shrinkage. Vacuum mo/ding.-Process of molding Tension wood fibers hold together a thin plywood or laminate to desired tenaciously, so that sawed surfaces usu­ shape by use of rubber bag, etc., from ally have projecting fibers and planed which air can be evacuated. surfaces often are torn or have raised Vacuum pressure. -Term describing grain. Tension wood may cause warping. pr.ocess ofapplying vacuum and pressure Texture.-The arrangement of the parti­ alternately. cles or constituent parts of material, Varnish.-A thickened preparation of such as wood, metal, etc. (Uniformly drying oil or drying oil and resin suit­ textured wood-not a great difference able for spreading on surfaces to form between earlywood and latewood.) continuous, transparent coatings or for Thermal softening.~Softens with heat. mixing with pigments to make enamels. Thermopl.stic.-Softens or becomes Veneer.-Thin sheets ·of wood made by plastic with sufficient heat. rotary cutting or slicing of a log. Thixotropy.-A property of adhesive Veneer clipper. -Machine for cutting systems to thin upon isothermal agita­ veneers into desired sizes. tion and to thicken upon subsequent Viscosity.-That property of a fluid ·rest. material by virtue of which, when flow Tongue-and-groove.-A kind of joint occurs inside it, forces arise in such a in which a tongue or rib on one board direction as to oppose flow. fits into a groove .on another. Waterborne chemical.-In wood preserv­ Tooth planing.-Planing resulting in a ing, a chemical dissolved in water ,to ridged .ortoothed surface which was facilitate penetration into wood. thought to give abetter anchorage for Water so/ub/e.-Substance that can be glue Illan a smooth surface. dissolved in water.

118 Weathering.-The mechanical or chemi­ (curds) .after coagulation, as in cheese­ cal disintegration of the surface ofwood making. that is caused by exposure to light, the Wood failure.-The rupturing ( - wood action ofdust and sand carried by winds, fibers in strength tests on bonded ~peci­ and the alternate shrinking and swelling mens, usually expressed as the percent­ ofthe surface fibers with continual varia­ age of the total area involved which tion in moisture content brought by shows such failure. changes in the weather. Weathering Wood flour.-Very finely divided wood, does not include decay. as produced by grinding in a ball mill. Wet~bulb temperature.--The tempera­ It is graded according to the mesh it ture indicated by any temperature­ must pass. measuting device, the sensitive element Working life.. -Theperiod.chime during of which is covered by a smooth, clean, which an adhesive, after mixing with soft, water-saturated cloth (wet-bulb catalyst, solvent, or other compounding wick). ingredients, remains suitable for use. Wet joint strength.-Shear stress resisted by joints after exposure to water soaking Zinc white.-Zinc oxide used as a pig­ or in wet condition. ment. Wbey.--.:The thin, watery part of milk Zone of char. -Zone burned to a char which separates from the thicker parts (see also Char).

119 INDEX

Acid-catalyzed phenol resin adhe­ Curing temperatures for adhe­ sives ....••...... _..•...... 14, 93 sives ...... _...... 77 Adjustments in adhesives and Acid phenolic glues ...." ...;.. 13 gluing procedures ...... 96 Alkaline phenol resins ...... 13 Alkaline phenol resins ...... 13, 65, 93 Blood glues ...... 33 Animal glue ...... 27 Intermediate-temperatu re­ ,Durability ...... 28 setting phenol resins ...... 14 Mixing ...... 27 Melamine resin adhesives ...... 21 Used ar room remperature ..... 92 Resorcinol resins ...... 15 Used in furniture manufacture . 71 Thermosetting polyvinyl emul­ Assembly time ...... 90, 109 sions ...... _...... 24 Closed assembly ...... 87, 90, 109 Urea resin adhesives ...... 18 For animal glues ...... 28 Cutting and preparing veneer .... 62 Fot phenol resin adhesives ..... 13 Density ...... 4, 111 For resorcinol resins ...... 15 Double spreading ...... 89, III For room-ternperature-setting Dovetail edge joint ...... 62 urea resins ...... 19 Drying lumber ...... 58 Open assembly ...... 87, 90, 109, 115 Drying veneer ...... 58 Bag molding ...... 14, 109 Durability of Blood glue ...... 33, 65 Alkaline phenol resins ...... 16 Bonding dense woods ...... 98 Animal glue joints ...... 28 Bonding laminated timbers ...... 69, 97 Casein joints ...... 31, 32 Bonding light, porous species .... 98 Melamine resins ...... 22 Bonding plywood ...... 65 Phenol resorcinol resins ...... 16 Butt joint ...... 49, 110 Polyvinyl resin emulsions ...... 24 Calking gun ...... 82, 89 Resorcinol resins ...... 16 Casein glues ...... 29 Urea resins ...... 17, 19,20 Durability ...... 31, 32 Durability of syntherk adhe­ Formulation ...... 29 sives ...... 10, 24 Preparation ...... 29 Edge gluing ...... 35, 63, 111 Prepared ...... 30 Elevated temperature curing of Use characteristics ..." ...... 30 glue ...... 92 Used for furniture bonding .... 71 End-joint gluing ...... 35, 112 Used for laminated timbers ... 69 Circular tongue-and-groove .... 61 Wet-mixed casein glue ...... 30 Dado ...... 62 Causes and ptevention ofwarping . 40 Dovetail ...... 62 Causes of plywood cupping ...... 4 I Mortise-and-tenon ...... 62 Circular tongue-and-groove edge Plain ...... 62 joint ...... 61 Rabbet ...... 62 Clamping glue joints ...... 91 Slip ...... 62 Closed assembly ...... 87, 90, 109 Tongue-and-groove ...... 62 Conditioning glued producrs ..... 96 End and corner joint .construc­ Contacr adhesives ...... 26, 85 tion...... 49, 50 Corner joints ...... 49 Epoxy tesin adhesives ...... 25, 81 Crossbanded construction ...... 36 Extenders used in synthetic adhe­ Cupping of plywood ...... 41 sives ...... 10, 112 Curing adhesives ...... 92 Exrwders used in applying adhe­ Elevated temperature curing .. 92, 93 sive: ...... 89 Equipment ...... 93, 94 Factors affecting gluing ...... 2 Hot-setting ureas ...... 92 Assembly time ...... 2 Low voltage heating ...... 83 Pressure ...... 2 Melamines ...... 92 Temperature ...... 2

120 Figured veneer ••..•.•.•...... •. 39, 112 Machine marks on lumber ...... 60 Fillers used in synchetic adhe­ Machining special types ofjoints .. 62 sives ...... 10, 13, 112 End-grain surfaces ...... 62 Finger joints ...... 18, 51, 94, 112 End-to-side-grain surfaces ..... 62 Flush doors ...... 79 Side-grain surfaces ...... 61 Fortifiers ...... , 18, 112 Marine plywood ...... 77, 114 Furnirure ...... 70 Mastic adhesives ...... 26, 82, 114 Glues used ...... 29, 72, 73 Melamine resin adhesives ...... 21, 22 Species used ...... 70 Melamine urea resins ...... 22, 74, 81, 92, Gluability cest ...... 2 95, 102 Glue block shear tesc ...... 116 Mixing adhesives ...... 87. 88 Gluing doors ...... 79 Moisrure content of wood ...... 56 Gluing hardwood plywood ...... 67 At time ofgluing • ...... 56 Gluing housing and housing com­ Gluing furniture .,...... •.•. 56 ponents ...... 82, 83 Gluing hardwood plywood ..... 67 Gluing jewelcy ...... 84 Gluing lumber ...... _...... 58 Gluing laminated flooring ...... 84 Gluing veneer ...... 58 Gluing operations ...... 87 In service ...... 56 Applymg gluing pressure ...... 91 Using casein glue ...... 30 Assembling parts ...... 9') Nail gluing ...... 82, 83 Assembly time ...... 90 Natural origin adhesives ...... ,. 27 Conditioning glued stock ...... 96 Animal.,...... 27,73 Mixing ...... 87 Blood ...... 33 Spreading ...... 88 Casein ...... 29 Gluing particleboard ...... 81 Soybean ...... 32 Gluing preservacive-treated wood • 99 Open assembly...... 87, 115 Treated with fire-retardant Overlay...... 115 chemicals ...... lD 1 Overlays bonded to wood ...... 85 Treated with oil-soluble preser­ Pearl glue ...... 115 vatives ...... 10 1 Phenol-formaldehyde adhesives ... 13 Treated with resorcinol-resin Phenol resins ...... 11. 65 adhesives ...... 17 Phenol-resorcinol resins ...... 15, 16, 72, 77. Treated with waterborne pre­ 80, 84, 92, servatives ...... 10 1 95, 101 Gluing pressure ...... 91 Plain edge joint ...... 61 Gluing sporting goods 81 Plywood advantages vs. solid wood Gluing ueaced wood ...... 98 construction ...... 36 Gluing unueated wood ...... 101 Plywood conscruction ...... 36, 38 Gluing woods treated with oil­ Requirements ...... 43 soluble preservatives ...... 10 1 Species preferred ...... 46 Gluing woods treated with water­ Plywood cores ...... 43 borne preservatives ...... 10 1 Requirements ...... 43 Hardener fur synthetic adhesives .. 10 Polyvinyl resin emulsions ...... 22, 23, 73, 89, Hardwood plywood ...... 67 92 Construction ...... 67 POt life ...... 115 Moisrure content when gluing . 67 For epoxy adhesives ...... 26 High-frequency heating ...... 18, 74. 94, 113 For melam;ne resin adhesives ... 22 Hot-melt adhesives ...... 21, 25, 63, 74 heparing wood for gluing ...... 56 Hoc-press urea resin adhesives ... 18, 92 Pressing and curing glue joints .• 83 Intetmediate-temperarure-secting Pres.ing or clamping ...... 91 phenol resins ...... 14 Properties important in bonding 4 Jig ...... 83, 113 Quality conuol ...... 102 Laminated construction ...... 45 Tension testS ...... 106 Bonding ...... 69 Vacuum-pressure, soak-dry Properties ...... 45 tescs ...... 106 Selection ofspecies and grades .. 46 Rate of curing or setting ...... 10, 11 Stresses in ...... 47 Requirements (or crossbands ....: 43 Uses ...... 45 Resorcinol-resin adhesives ...... 14, 16, 72, 78, Laminated flooring . , ...... 84 83, 84, 93, Laminated ship and boat mem­ 95, 101, 102 bers ...... 76 Ribbon spreading ...... 116

121 Room-temperature-setting urea Sunken joints ...... 96 resins ••••..•• .... .••...... ••• 17, 92, 116 Surfacing wood for gluing •.••.•.• 60 Rotary-cut veneer .•.•.....••...... 62, 63 Synthetic adhesives ...... 10 Scarf joints ...... 49, 116 Advantages .•..•.••.•••.••••...•. 10 Serrated scarf joint ...... 62 Durability ...... 10 Shear block test •...... ••• 116 Melamines ...... 21, 22 Ship and boat construction ..,.... 76 Phenols •.•...•....•••.••.•••••••.• 13 Adhesives used ...... '" ...... •• 77 Polyvinyls •....•••••••.••••.••.•.. 22,23 Gluing operations • •••• •••••...•. 77 Resorcinols ...... ~ ..._...... ~~.'" ... .. 14, 15, 24 Species desired ...... 46 Ureas ...... 17, 18, 19, 72 Use of melamine resin glues •. 21 Used in manufucturing ."...... 10 Use of resorcinol resins ...... 15 Tenon ...... 118 Shrinking and swelling " .•••••.•• 7 Thermosetting polyvinyl emul­

Sliced and sawed veneers ...... 63 sions ...... i ...... ~ ...... " ...... ~ 23,73 Sottwood plywood ...... 65 Tongue·and-groove ...... 61, U8 Exterior' ...... 65 Twisting of plywood .•••....•....• 40 Interior • .••• .••• ...... ••..... 65 Urea-formaldehyde resin adhe­ Uses ...... ' ••.• 67 sives ..... "- ...... "-"- ...... ~ •. ~ ..., ...•.... 17, 18, 19, 20, Solid cores fur flush doors •.•..••. 79 72, 81, 84, Soybean glue ...... 27, 32, 65 92 Spreading adhesives . ••.•••..•...... 88 Vacuum molding . '" ...... 118 Starved glue joint ...... 90, 117 Vinyl overlays ...... 85 Storage of lumber ...... ,.... 59 Warping. causes and prevention • 40 Storage of veneer ...... 60 Wood "jewelry" ...... 84 Stress relief in laminated construc­ tion ...... 49

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