Corrugating Medium Reference

Home , Heated roll laminator, Paper machine, Shive (papermaking)

Its Influence on Box Plant Operations and Combined Board Properties and Package Performance

Project 3808

Report To The Technical Division of the Containerboard and Kraft Paper Group of The American Forest and Paper Association

May 2, 1994

Institute of Paper Science and Technology Atlanta, Georgia

By Joseph J. Batelka 1993 Acknowledgment

The author wishes to express his sincere appreciation to the Containerboard and Kraft Paper Group of the American Forest & Paper Association for providing the funding for this undertaking, and to the Institute of Paper Science and Technology for providing the logistical support needed to complete this project.

iii Table of Contents

Corrugating Medium

ii IPST Mission Statement ii IPST Accreditation Statement ii IPST EEO Statement ii IPST Notice and Disclaimer Statement iii Acknowledgement v Table of Contents vii List of Figures and Tables

Introduction

Chapter I Introduction ...... I

Fluting

Chapter 2 Flute Forming Theory ...... 3

Chapter 3 High/Low Flutes ...... 9

Chapter 4 Fractured Flutes ...... 23

Chapter 5 Medium Strength Loss By Fluting ...... 35

Bonding

Chapter 6 Bonding Theory ...... 53

Chapter 7 Single-Face Bonding ...... 67

Chapter 8 Double-Face Bonding ...... 81

Combined Board

Chapter 9 Combined Board Issues ...... 93

v Chapter 10 Flexural Stiffness ...... 95

Chapter 11 Edge Crush Test ...... 109

Chapter 12 Flat Crush Test ...... 135

Package Performance

Chapter 13 Package Performance Issues ...... 157

Chapter 14 Package Compressive Strength ...... 161

Chapter 15 Package Rough Handling ...... 179

Reference

Chronological Bibliography ...... 193

Author Index ...... 203

Subject Index ...... 205

vi List of Tables and Figures

Chapter 2 Flute Forming Theory Fig. 2.1. Schematic Diagram of Forces Acting During Flute Forming in the Single-Facer Operation ...... 4 Table 2.1. Relative Tensile Forces Generated in the Medium Web by the Various Mechanical Elements in the Single-Facer Operation ...... 5 Fig. 2.2. Compression and Permanent Caliper Loss of Medium During the Flute Forming Operation ...... 6 Fig. 2.3. Effect of the Medium Web Tension on the Medium Draw Factor ...... 7

Chapter 3 High/Low Flutes Fig. 3.1. Regression Equation Relating the High/Low Flute Defect to Combined Board Edge Crush Test ...... 10 Table 3.1. Vibration Amplitudes in Single-Facer ...... 11 Fig. 3.2. Effect of Single-Facer Process Variables on the Springback of Fluted Medium ...... 12 Fig. 3.3. Effect of Single-Facer Process Variables on the High/Low Flute Defect ...... 13 Fig. 3.4. Effect of Single-Facer Process Variables on the High/Low Flute Defect ...... 14 Fig. 3.5. Effect of Single-Facer Process Variables on the High/Low Flute Defect ...... 15 Table 3.2. Major Variables Affecting the High/Low Flute Defect ...... 16 Fig. 3.6. Effect of Corrugator Speed and Medium Basis Weight on the High/Low Flute Defect ...... 17 Fig. 3.7. Regression Equation Relating Medium Friction and Formation to the High/Low Flute Defect ...... 18 Fig. 3.8. Effect of Corrugator Speed and Medium Basis Weight on the High/Low Flute Defect ...... 19 Fig. 3.9. Effect of Medium Moisture and Corrugating Roll Temperature on Springback of Fluted Medium ... . 20 Table 3.3. Effect of Lubricant on Friction and the High/Low Flute Defect ...... 21 Fig. 3.10. Effect of Refining and Wet Pressing on Corrugating Medium Properties ...... 22

Chapter 4 Fractured Flutes Fig. 4.1. Effect of Flute Fracture on Flat Crush and Edge Crush ...... 24 Fig. 4.2. Effect of Medium Web Tension on the Flute Fracture Defect ...... 25 Fig. 4.3. Effect of Single-Facer Process Variables on the Flute Fracture Defect ...... 27 Fig. 4.4. Effect of Single-Facer Process Variables on the Flute Fracture Defect ...... 28 Fig. 4.5. Regression Equation Relating Medium Properties to the Flute Fracture Defect ...... 29 Fig. 4.6. Theoretical Model Relating Single-Facer Process Variables and Medium Properties to the Flute Fracture Defect ...... 30 Fig. 4.7. Verification of Theoretical Model in Fig. 4.6...... 31 Table 4.1. Summary of Major Process and Material Factors Affecting the Flute Fracture Defect ...... 26 Table 4.2. Effect of Lubricant on Friction and the Flute Fracture Defect ...... 32 Fig. 4.8. Effect of Medium In-Feed Rolls on the Flute Fracture Defect ...... 33

Chapter 5 Medium Strength Loss By Fluting Fig. 5.1. Compression and Permanent Caliper Loss of Medium During the Flute Forming Operation ...... 36

vii Table 5.1. Effect of Heat Treatment on the Physical Properties of Corrugating Medium ...... 37 Fig. 5.2. Effect of Temperature and Moisture Content on the Tensile Strength and Tensile Stretch of Corrugating M edium ...... 38 Fig. 5.3. Effect of Single-Facer Process Variables on the Springback of Fluted Medium ...... 39 Fig. 5.4. Effect of Medium Moisture and Corrugating Roll Temperature on Medium Springback of Fluted Medium ...... 40 Fig. 5.5. Effect of Medium Web Moisture Content During Fluting on the Fluted Medium Tensile Strength .... 41 Fig. 5.6. Effect of Corrugating Roll Temperature on Medium Concora Strength ...... 42 Fig. 5.7. Regression Equation Relating Corrugating Conditions and Medium Properties to Flat Crush ...... 43 Fig. 5.8. Effect of Medium Properties on Medium Crush Strength Retention After Fluting ...... 45 Fig. 5.9. Regression Equation Relating Medium Elastic Properties to the CD Crush Strength Retention After Fluting ...... 46 Fig. 5.10. Relationship Between MD STFI Compression of Fluted Medium and Flat Crush ...... 47 Fig. 5.11. Relationship Between Fluted Medium Springback and Medium Concora Strength ...... 48 Fig. 5.12. Effect of Refining and Wet Pressing on Flat Crush Strength Retention After Fluting ...... 49 Fig. 5.13. Effect of Refining and Wet Pressing on Edge Crush Strength Retention After Fluting ...... 50 Table 5.2. Major Corrugating Conditions and Medium Properties Affecting Medium Strength Retention A fter Fluting ...... 51

Chapter 6 Bonding Theory Table 6.1. Seven Major Steps in the Corrugator Bonding Process ...... 53 Fig. 6.1. Energy Requirements of the Various Bond Setting Stages of the Corrugator Starch Adhesive ...... 55 Fig. 6.2. Corrugator Starch Adhesive Viscosity Changes During the Bond Setting Process ...... 56 Fig. 6.3. Failure Modes of Corrugator Green-Bonds and Cured Bonds ...... 58 Fig. 6.4. Effect of Paper Moisture Content on the Elapsed Time to Achieve a Fiber Tear Bond Failure Mode ... 59 Fig. 6.5. Relationship Between the Single-Facer Cured Bond Failure Mode and the Pin Adhesion Strength for Commercially Produced Board ...... 60 Fig. 6.6. Effect of Medium Properties on the Transfer of Adhesive to the Flute Tip from the Glue Applicator Roll ...... 62 Fig. 6.7. Effect of Medium Properties on the Penetration of Starch Adhesive into the Corrugating Medium .... 63 Fig. 6.8. Permanent Transverse Compression of Medium Due to the Corrugating Roll Pressure ...... 64 Fig. 6.9. Effect of Permanent Transverse Compression of the Medium on Adhesive Transfer to the Flute Tip and on the Penetration of Starch Adhesive into the Medium ...... 65 Table 6.2. Starch Adhesive Penetration into the Medium of Commercially Produced Corrugated Board ...... 66

Chapter 7 Single-Face Bonding Fig. 7.1. Single-Facer Green-Bond Development Curve ...... 69 Fig. 7.2. Effect of the Medium Preconditioning Level on the Single-Facer Green-Bond Development Rate .... 70 Fig. 7.3. Effect of Corrugating Variables on the Single-Facer Green-Bond Development Rate ...... 71 Fig. 7.4. Relative Effectiveness of Corrugating Variables on Changing the Single-Facer Green-Bond Developm ent Rate ...... 72 Fig. 7.5. Effect of Corrugator Process Variables and Medium Properties on the Single-Facer Cured Bond Pin Adhesion Strength ...... 73 Fig. 7.6. Effect of Single-Facer Speed on the Cured Bond Pin Adhesion Strength ...... 74 Fig. 7.7. Effect of Single-Facer Speed and Medium Basis Weight on the Cured Bond Pin Adhesion Strength . . . 75

viii Fig. 7.8. Effect of Adhesive Application Rate on the Cured Bond Pin Adhesion Strength ...... 77 Fig. 7.9. Effect of the Single-Facer Speed on the Adhesive Application Rate and the Cured Bond Pin Adhesion Strength ...... 78 Fig. 7.10. Effect of the Medium Water Drop Test on the Cured Bond Pin Adhesion Test ...... 79 Fig. 7.11. Effect of the Single-Facer Medium Preconditioning Level on the Green-Bond Strength and the Cured Bond Strength ...... 80

Chapter 8 Double-Face Bonding Fig. 8.1. Double-Face Green-Bond Development Curve Obtained on the Double-Backer Simulator Developed at the Institute of Paper Science and Technology ...... 82 Fig. 8.2. Effect of Bonding Time on the Strength of the Double-Face Green-Bond ...... 84 Fig. 8.3. Interrelationship Between the Hot Plate Temperature and the Bonding Time on Achieving the Fiber Tear Stage of the Double-Face Bond...... 85 Fig. 8.4. Effect of the Double-Face Green-Bond Induction Time on the Maximum Corrugator Speed Attainable While Maintaining Good Bonding ...... 86 Fig. 8.5. Effect of the Hot Plate and Adhesive Temperature on the Double-Face Green-Bond Induction Time... 87 Fig. 8.6. Effect of the Hot Plate and Adhesive Temperature on the Double-Face Green-Bond Bonding Rate .... 88 Fig. 8.7. Effect of the Adhesive Gel Point on the Elapsed Bonding Time Required to Achieve a Constant Double-Face Bond Strength ...... 89 Fig. 8.8. Effect of Preheating at the Double-Backer on the Induction Time and Bonding Rate of the Double-Face Green Bond ...... 90 Fig. 8.9. Regression Equation Relating Medium Properties to the Double-Face Green-Bond Induction Time . . . 92

Chapter 10 Flexural Stiffness Fig. 10.1. Schematic Description of Corrugated Board Flexural Stiffness Test Method ...... 96 Fig. 10.2. Effect of Medium Basis Weight on Combined Board 4-Point Beam Flexural Stiffness with 26 lb/msf Linerboard Facings ...... 98 Fig. 10.3. Effect of Medium Basis Weight on Combined Board 4-Point Beam Flexural Stiffness with 42 lb/msf Linerboard Facings ...... 99 Fig. 10.4. Effect of Medium Basis Weight on Combined Board 4-Point Beam Flexural Stiffness with 90 lb/msf Linerboard Facings ...... 100 Fig. 10.5. Effect of Medium Basis Weight on Combined Board 4-Point Beam Flexural Stiffness Influenced by the Linerboard Facings Basis Weight ...... 101 Fig. 10.6. Relative Effect of Medium and Linerboard Basis Weight Changes to the Combined Board 4-Point Beam Flexural Stiffness Strength ...... 102 Fig. 10.7. Effect of Linerboard Basis Weight on the Combined Board 4-Point Beam Flexural Stiffness Strength . 103 Fig. 10.8. Effect of Medium Basis Weight on the Combined Board 4-Point Beam Flexural Stiffness Strength. . . 104 Fig. 10.9. Effect of the Flat Crushing of the Flutes of Combined Board on the 4-Point Beam Flexural Stiffness Strength ...... 105 Fig. 10.10. Effect of Medium Basis Weight on the Combined Board 3-Point Beam Flexural Stiffness Strength ... 107

Chapter 11 Edge Crush Test Fig. 11.1. The Effect of Medium Basis Weight on the Combined Board Edge Crush Test ...... 110 Fig. 11.2. A Relationship Between the CD Ring Crush Test and the CD STFI Crush Test, and the

ix Relationship Between the Medium Basis Weight and the Combined Board ECT ...... 112 Fig. 11.3. A Relationship Between the Medium CD Ring Crush Test and the Combined Board CD ECT, and a Relationship Between the Medium CD STFI Crush Test and the Combined Board CD ECT . .. 113 Fig. 11.4. A Relationship Between the Medium CD Ring Crush Test and the Combined Board CD ECT for C-Flute and B-Flute Board ...... 114 Fig. 11.5. The Effect of Flute Fracture on the Edge Crush Strength of the Corrugating Medium ...... 115 Fig. 11.6. The Effect of Corrugator Adhesive Gaps and Blisters on the ECT of the Combined Board ...... 116 Fig. 11.7. A Regression Equation Relating Box Plant Process Defects to the Combined Board ECT Strength ... 117 Fig. 11.8. The Effect of Combined Board Crushing on ECT ...... 119 Fig. 11.9. The Relationship Between the Degree of Actual Combined Board Crushing and the Measured Crushing Determined by Combined Board Caliper Measurements ...... 120 Fig. 11.10. The Effect of Medium Basis Weight on the Caliper Recovery of Combined Board After Crushing of the Flutes ...... 121 Fig. 11.11. Effect of Nip Hardness on the Caliper Recovery of Combined Board After Crushing of the Flutes . .. 122 Fig. 11.12. Effect of Multiple Flute Crushing on the Caliper Recovery of Combined Board ...... 123 Fig. 11.13. The Rate of Caliper Recovery of Combined Board After Crushing of the Flutes ...... 124 Fig. 11.14. Effect of Medium Properties and Corrugating Process Conditions on the ECT of the Combined Board 125 Fig. 11.15. Effect of the Medium Elastic Properties on the Retention of the Medium Edge Crush Strength After Fluting ...... 126 Fig. 11.16. Effect of Pulp Refining and Paper Machine Wet Pressing on the Retention of the Medium Edge Crush Strength After Fluting ...... 127 Table 11.1. The Cyclic Humidity Creep Rate and the Hygroexpansivity of NSSC, Green Liquor, and Recycled M ediums ...... 128 Fig. 11.17. Relationship Between Relative Humidity and Moisture Content for NSSC Corrugating Medium .... 129 Fig. 11.18. The Effect of the Medium Moisture Content on the MD and CD Short-Span (STFI) Crush Strength of the Medium ...... 130 Fig. 11.19. The Effect of Medium Moisture Content on the CD Compressive Strength of NSSC and Recycled M ediums ...... 131 Fig. 11.20. The Relative Effect of Medium Moisture Content on the Compressive Strength and Tensile Strength of Corrugating Medium ...... 132

Chapter 12 Flat Crush Test Fig. 12.1. Typical Stress/Strain Curve for the Flat Crush Strength of Corrugated Board ...... 136 Fig. 12.2. Effect of Medium Concora Strength on the Flat Crush of Corrugated Board ...... 138 Fig. 12.3. Effect of Medium MD Ring Crush Strength on the Flat Crush Strength of Corrugated Board ...... 139 Fig. 12.4. Relationship Between the MD STFI Crush Strength of the Sidewalls of Fluted Medium and the Flat Crush Strength of the Corrugated Board ...... 140 Fig. 12.5. Regression Equation Relating Corrugated Board Flat Crush Strength to Medium Properties and to Corrugating Process Variables ...... 141 Fig. 12.6. Effect of the Medium Web Moisture During Fluting on the Resultant Corrugated Board Flat Crush Strength ...... 142 Fig. 12.7. Effect of Corrugating Roll Temperature on the Concora Strength of the Fluted Medium ...... 143 Fig. 12.8. Effect of Corrugating Medium Material Properties on the Flat Crush Strength Retention After Fluting ...... 144 Fig. 12.9. Effect of Pulp Refining and Paper Machine Wet Pressing on the Flat Crush Strength Retention of the Medium After Fluting ...... 145

x Fig. 12.10. Effect of Fractured Flutes on the Flat Crush Strength of Corrugated Board ...... 147 Fig. 12.11. Effect of Combined Board Crushing on the Flat Crush Strength of Corrugated Board ...... 148 Fig. 12.12. Effect of the Corrugating Medium Concora Strength on the Measured Caliper Loss of Flat Crushed Combined Board ...... 149 Fig. 12.13. Effect of Flute Size on the Measured Caliper Loss After Flat Crushing of Combined Board ...... 150 Fig. 12.14. Effect of Combined Board Crushing on the Flat Crush Strength and Measured Caliper of Corrugated Board ...... 151 Fig. 12.15. Flat Crush Strength Stress/Strain Curves for PreCrushed Corrugated Board ...... 152 Fig. 12.16. Actual Crushing Versus Measured Crushing for Flat Crush Stressed Corrugated Board. The Effect of Crushing Roll Nip Hardness on the Measured Caliper Loss in Flat Crush Stressed Corrugated Board ...... 153 Fig. 12.17. Effect of Corrugating Medium Basis Weight and Multiple Crushing on the Measured Caliper Loss of Flat Crush Stressed Combined Board ...... 154

Chapter 13 Package Performance Issues Fig. 13.1. The Seven Major Corrugated Packaging Performance Criteria ...... 158

Chapter 14 Package Compressive Strength Fig. 14.1. Corrugated Box Top-To-Bottom Compressive Strength Prediction Model ...... 162 Fig. 14.2. Corrugated Box End-To-End Compressive Strength Prediction Model ...... 163 Fig. 14.3. Effect of Corrugating Medium Basis Weight on the Top-To-Bottom Compressive Strength of Corrugated Board Tubes ...... 164 Fig. 14.4. Effect of Corrugating Medium Basis Weight on the Top-To-Bottom and End-To-End Compressive Strength of Commercially Made Corrugated Paperboard Boxes ...... 165 Fig. 14.5. Effect of Corrugating Medium Basis Weight and Flute Size on the Relative Top-To-Bottom and End-To-End Box Compressive Strengths ...... 167 Fig. 14.6. Effect of Corrugating Medium Basis Weight on the Top-To-Bottom Compressive Strength of Corrugated Boxes ...... 168 Fig. 14.7. Effect of Unbalanced Linerboard Facings on the Compressive Strength of Corrugated Board Panels. . 169 Fig. 14.8. The Balance Between Linerboard and Medium in Achieving Box Compressive Strength at the Lowest Total Fiber Usage ...... 170 Fig. 14.9. Effect of Corrugator Bond Gaps on the Compressive Strength of Corrugated Board Boxes ...... 172 Fig. 14.10. Effect of Combined Board Flat Crush Stressing on the Top-To-Bottom and End-To-End Package Compressive Strength ...... 173 Fig. 14.11. Relative Effect of Combined Board Flat Crush Stressing on the Top-To-Bottom and End- To-End Box Compressive Strength ...... 174 Fig. 14.12. Effect of Water Resistant Corrugator Adhesive on the Top-To-Bottom Compressive Strength of Boxes Under Various Relative Humidity Conditions ...... 175 Fig. 14.13. Effect of Water Resistant Corrugator Adhesive on the Stacking Creep Failure Rate at Various Box Moisture Content Levels ...... 176

Chapter 15 Package Rough Handling Fig. 15.1. Effect of Corrugating Medium Basis Weight on the Comer Drop Test Performance of RSC-Style Boxes ...... 180

xi Fig. 15.2. Effect of Corrugating Medium Elmendorf Tear Strength on the Comer Drop Test Performance of RSC-Style Boxes ...... 181 Fig. 15.3. Effect of Flute Size on the Comer Drop Test Performance of RSC-Style Boxes ...... 182 Fig. 15.4. Effect of Combined Board Flute Crushing on the Flat Crush Strength Stress/Strain Curve a nd the Flat Crush Energy Absorption ...... 184 Fig. 15.5. Effect of Multiple Flat Crush Impulses on the Permanent Caliper Loss in Corrugated Board ...... 185 Fig. 15.6. Effect of the Number of Flat Crush Impulses on the Maximum Deceleration Force of Corrugated Board Cushioning Material ...... 186 Fig. 15.7. Shock Force Absorbing Characteristic of Corrugated Board Cushioning Material with Varying Flat Crush Strength and Varying Number of Corrugated Layers ...... 187 Fig. 15.8. Effect of Medium and Linerboard Basis Weight, and the Number of Plies on the Cushioning Characteristics of Corrugated Board Pads ...... 188 Fig. 15.9. Effect of Medium and Linerboard Basis Weight, and the Number of Plies on the Cushioning Characteristics of Corrugated Board Pads ...... 189 Fig. 15.10. Effect of Corrugated Pad Design on the Cushioning Characteristics of the Structure ...... 191 Fig. 15.11. Effect of Impact Loads on the Cushioning Characteristics of Various Style Corrugated Cushioning Pads ...... 192

xii Chapter 1

Introduction

The purpose of this publication is to provide a the facings being attached. The idea for fluting paper comprehensive review of the technical information was an offshoot of the method used to maintain the available on the corrugating process, combined board ruffled collars worn by the 18th century gentlemen, strength, and package performance as impacted by the (106). corrugating medium. The technical information re- The rapid growth of corrugated packaging technol- viewed covers both the effect of medium material ogy then followed. A patent for single-faced corru- properties and the effect of the corrugating process gated, used primarily for protecting glass lamp chim- variables as they interact with the medium. While this neys, was issued to Oliver Long in 1874, and was fol- publication emphasizes the work done at the Institute of lowed by a patent for singlewall corrugated granted to Paper Science and Technology, it also includes exten- H. B. Meech in 1879. The first use of corrugated ship- sive information from other published sources. A bibli- ping containers was in 1903, for transporting dry cereal ography of 178 articles spanning the period from 1939 by rail. The Western railroads established a Rule 41 for to 1993 and which includes subject matter related to corrugated shipping containers in 1906. This grudg- corrugating medium was compiled. ingly recognized the corrugated case as an alternative to This publication is not a comprehensive encyclo- wooden crates. The only requirement was that the pedia on corrugating medium. It does, however, discuss package weigh no more than 100 pounds. The Southern the major observations and conclusions from this body railroads joined the Western railroads in accepting of research with enough detail to qualitatively and products in corrugated boxes in 1910. Rule 41 was quantitatively demonstrate cause and effect relation- modified to establish a corrugated board grade structure ships. The reader should keep in mind that the quanti- based on box size, package weight, mullen burst tative relationships are specific to the experimental strength, and the caliper of the facings, (106). conditions used in each reference, and while the trends It took a decision by the Interstate Commerce shown are probably valid for a wide range of opera- Commission in 1912, to require all railroads in the tions, each specific operation may find the magnitude United States to accept, without prejudice, corrugated of the effects to be greater or smaller. The reader can paperboard shipping containers as an alternative to obtain the detailed experimental conditions, the detailed wooden crates. This was known as the Prindham Case, mathematical equations, and the detailed experimental and it established a uniform, nationwide grade structure data from the references cited. for corrugated, a United States Rule 41. The legal abil- The reference bibliography is presented in ity of the railroads to set these packaging specifications chronological order starting with the most recent publi- was based on the anti-trust exemption granted to the cation. A bibliographical subject index and a biblio- Uniform Classification Committee by the U.S. Con- graphical author index are included to assist you in re- gress. When truck carriers became prevalent, a similar searching specific subjects. The references cited in the arrangement was established for this mode of transpor- bibliography may also be a source of additional refer- tation, and its packaging specification is known as Item ences not included in this bibliography. 222. Paper, as we know it, made from a slurry of plant Over the years, additional grades were added to fibers filtered through a mesh, pressed, and then dried, Item 222/Rule 41, such as doublewall and triplewall, was first produced in China in the second century B.C. and other minor changes were made. However, during It then took 21 centuries for corrugated paperboard to the 71 years between 1912 and 1993, only two major be invented. Albert L. Jones obtained a patent in 1871 changes to the corrugated board grade structure specifi- which covered the production of fluted paper without cation format were made. Linerboard basis weight was 2 / Chapter 1 Introduction substituted for the linerboard caliper in 1944. This condition in the steam showers but remain nonabsor- change was in response to the increasing use of the bent enough to form and maintain strong corrugator fourdrinier paper machines as replacements for the bonds. It must be slippery enough to minimize the cylinder paper machines in the 1940s. The linerboard tensile forces developed in the single-facer operation, produced on the fourdrinier was able to meet the mul- but have a high enough coefficient of friction to pre- len burst specification at a lower basis weight and, vent the rolls from telescoping while being handled. therefore, lower caliper than the cylinder machine lin- The key to a "good" corrugating medium is the balance erboard. This reduction in basis weight allowed the of properties. linerboard productivity and capacity improvements that The format used in this publication is to follow the were needed to provide sufficient corrugated boxes to life story of corrugating medium starting with the flute support the material logistics of the World War II ef- formation in the box plant and ending with the me- fort. The second major change occurred in 1991, when dium's contribution to the field performance of the cor- Edge Crush Test was adopted as an alternative to the rugated box. Each chapter is devoted to a specific step combined board mullen burst test and the linerboard in this total medium process. basis weight, (6, 106). This change reflected the impor- I have attempted to use my experience and judge- tance of the top-to-bottom compressive strength of the ment in interpreting the significance of the information box for meeting today's performance requirements. given in the references. However, in the few instances You have probably noted that, except for a mini- when the literature indicated conflicting conclusions mum basis weight of 26 lb/msf and a minimum caliper that could not be logically reconciled, both conclusions of 9 mils for the medium, the Item 222/Rule 41 corru- are presented for the reader's consideration. Again, the gated board grade specifications involved physical reader should keep in mind that the quantitative rela- properties related only to the linerboard component of tionships shown are specific to the experimental condi- corrugated board until 1991, when the Edge Crush Test tions and materials used in each reference. While the option was added. The medium was considered an trends shown are probably valid for a wide range of "ugly stepchild," at best, by the non-technical commu- operations, each specific operation may find the magni- nity of our industry. It is a component not readily visi- tude of the effects to be greater or smaller than shown ble in the finished product. You might say "out of sight, by the specific data. out of mind." Recycled or Straw medium was the product of the day for 75 years. The first commercial semichemical medium, Neutral Sulfite Semichemical (NSSC), was not readily available until 1946, (6). Corrugating medium is still not considered a very glamorous product and is still not a very attractive looking paper. However, glamorous or not, attractive or not, the corrugating medium is what makes the product a corrugated board. Without the medium, the product would be either a solid fiber box or a paper bag. Knowledgeable people have opined that corrugated board is a sandwich structure in which the medium is the meat of that structure, (7). It has also been stated that "The quality of corrugated board is determined on the single-facer because it is here that the operation should develop the maximum potentials of the medium," (129). The medium must maintain the separation of the linerboard facings to form a sandwich structure. It must do this after being burned on preheater drums; scalded with steam showers; pulled, bent, and squashed in the corrugating rolls; doused with a watery starch mixture; and flat crush compressed in the hot plate section of the double-backer and in the finishing department. The life of the corrugating medium is not an easy one. It must be weak enough to flute, but strong enough to withstand the tensile, the flat crush, and the edge compression forces. It must be absorbent enough to

______. Chapter 2

Flute Forming Theory

It has been suggested that the quality of the corru- The Bonding criterion also includes several sub- gated board is determined at the single-facer. The ob- categories such as blisters, fluff out, and loose edges. jective of the single-facer operation should be to main- The bonding criterion will be discussed in later chap- tain the maximum strength and quality potential of the ters. medium, (129). A simplified schematic representation of the sin- In the common parlance of the corrugated industry, gle-facer corrugating process is shown in Figure 2.1. the performance of the medium in the single-facer op- The total process with regard to runnability includes eration is known as "runnability." The concept of run- factors existing at the roll-stand, the splicer, the pre- nability encompasses only the needs and requirements heater, the steam shower, the single-facer, and the vari- of the corrugating crew and corrugating operation. A ous web spans and idler rolls, (154). good runnability medium does not necessarily indicate The process starts with the roll of medium in the a good performance package since the material may be roll-stand. The process variables at this point include weak in strength properties. A poor runnability medium the medium material properties, the medium roll quality does indicate a potential for a poor quality package. In attributes, and the roll-stand braking system. The me- some instances, the corrugator crew can overcome the dium properties that have been related to medium run- potential package quality problems associated with the nability include MD tensile, MD stretch, caliper, coef- poor runnability by making compensating adjustments ficient of friction, MD modulus of elasticity, ZD to other factors in the corrugating process, such as modulus of elasticity, MD/ZD shear modulus, and slowing the corrugator speed. In this event, the package compressibility, (24, 78, 84, 134, 154). The medium quality does not suffer, but the medium still has poor roll quality attributes that are of concern include out-of- runnability in the eyes of the operator and in the eyes of round rolls, uneven hardness across the roll, and roll the box plant cost accountant. edge damage. An out-of-round roll will lope as it un- If a specific medium is said to have "good run- winds and will cause tensile force spikes in the medium nability," there is no problem in understanding that the web. The uneven roll hardness will cause variability in medium performed well and without problem during the tensile force across the web width. Edge damage to corrugation. Unfortunately, the term "poor runnability" the roll can initiate web breaks. does not convey a specific definition as to what prob- The roll-stand braking system affects the overall, lems were encountered. There are many symptoms each average web tension by the adjustment of the brake of which, by themselves or in combination with other setting. "Catching" of the braking system due to a me- symptoms, represents poor runnability. chanical defect will cause tensile force spikes in the The term "runnability" encompasses two major medium web. The diameter of the medium roll interacts performance criteria: Flute Formation and Bonding. with the roll-stand braking system, depending on the The Flute Formation criterion includes two subcatego- specific system design, to increase the medium web ries: Fractured Flutes and High/Low Flutes. Both of tensile force as the roll diameter decreases. The me- these defect categories are influenced by the medium dium roll is turned on the roll-stand by the pull force of physical properties and attributes, by the corrugating the medium web as it feeds through the corrugating process settings, and by the mechanical condition of the rolls. If the roll-stand brake resistance force is fixed and corrugating equipment. The fluting process is consid- constant, the leverage arm of the medium web pulling ered the key element for box plant productivity and the force (roll radius) decreases as the paper is consumed structural performance of the package, (24, 25, 26, 27, and the roll diameter decreases. The medium web ten- 31, 38, 78, 84, 129). sion must then increase proportionally to the decreasing r

FIGURE 2.1

Schematic Diagram of Single-Facer Forces 10 and Material Factors

Friction Wetting Friction Cooling Medium / Web Drying Heating Friction Friction

Flutter

Brake Setting Drying Medium Materia# Roll Diameter Heating Caliper Out-Of-Round Roll Labyrinth Friction MD Tensile Uneven Roll Hardness Labyrinth Flexing MD Stretch Labyrinth Compression Elastic Moduli Top Corrugator Roll teeth Coef. Of Friction I- Corrugating Medium / 5 leverage arm in order to keep the pulling force needed the flute is formed. Fourth, the medium is then com- to overcome the brake force constant and to keep the pressed as the teeth of the upper and lower corrugating roll unwinding. rolls come together at the nip. The splicers are generally designed so that the hy- The compression of the medium due to the corru- draulic system controlling the festoon adjusts the pres- gating roll pressure is shown in Figure 2.2. The me- sure to keep the idler rolls properly spaced. These ad- dium exhibits a permanent caliper loss of between 30% justments can add tensile force spikes to the medium and 40% in the flute tip area and between 5% and 15% web. The splicer idler rolls, as well as all of the other in the flute sidewall areas. The data indicate that the idler rolls in the single-facer, increase the web tension permanent caliper loss due to the medium compression due to friction in the bearings. The tensile loading that is equal in magnitude for both the leading and trailing is of concern is the tensile force per inch of medium sidewalls of the flute. The average permanent caliper web width. The roll-stand braking resistance for a given loss and the relative loss between the tip and the side- setting and the frictional resistance at the bearings are wall do vary with different flute size corrugating rolls. generally independent of the medium web width. The direction of the observed differences, however, is Therefore, the tensile force per inch of the medium not consistent with the difference in flute height. It ap- increases as the medium web width decreases. A one- pears that the differences are associated with the spe- setting-fits-all approach to medium web tension control cific design of the corrugating roll flute contour, clear- at the single-facer is not a recommended operating ance, and radius of curvature, rather than flute size, A, strategy to follow as the medium web width varies. The B, or C, (145, 149). free spans of the medium web between idler rolls also The compaction of the fluted medium also appears add to the tensile force due to the force of gravity on to be influenced by the corrugator speed. In general, the the mass of the medium. Typical, relative tensile forces permanent caliper loss appears to decrease as the corru- for a single-facer are shown in Table 2.1. gator speed increases. This speed effect observation may be explained by the effect of speed on the translational movement or bounce of the upper corrugating Table 2.1. Relative Tensile Forces Generated In The roll as the flutes of the two corrugating rolls mesh. This Single-Facer, (154) translational movement has been shown to occur twice per flute, once at the flute apex and once at the flute root. The amplitude of this translational movement de- Relative creases as the corrugator speed increases, and the flutes Process Tensile are less completely formed, (149, 152). Stage Contribution Figure 2.3 shows the effect of the medium web tensile force on the measured draw factor. The data Roll-Stand & Roll 300 units indicate that the draw factor increases as the web ten- Preheater 15 units sion increases, with the measured draw factor increas- Idler Rolls (Total) 23 units ing by 0.96% when the web tension was increased from 0 lb/inch to 1.5 lb/inch. Intuition would indicate that the opposite shuld occur. Logic would indicate that the The preheater drum will also add to the web ten- medium stretches more as the tensile force is increased sion due to the medium having to turn the drum. Some and, therefore, should provide more linear footage to plants lock the preheater drum so that it does not turn. make flutes. The experimental data appear to be an This greatly increases the web tension since the me- artifact of the measurement technique used. The linear dium must now be pulled over the heated surface. The footage of the medium was determined by the number locking of the preheater drum is usually done to com- of revolutions of a medium web idler roll, and the pensate for a mechanical problem with the preheater. combined board linear footage was determined by the The steam shower also increases the medium web ten- number of revolutions of the corrugating rolls, (134). sion since the medium must slide across the surface of The increased medium length due to more stretch at the the device. higher tensile loading causes an increase in the idler The single-facer variables are more complex. First, roll rotation. A similar increase in the corrugating roll the medium web must be dragged across the flute tips rotation did not occur. This indicates that the stretch in of the upper corrugating roll so as to provide the mate- the medium web up to the point of flute formation is rial required to make the flute shape (draw factor ef- overshadowed by the stretching required to form the fect). Second, the medium needs to be pulled over the actual flute. This indicates that the medium web tension tip and sidewalls of both corrugating rolls as the flute is used in the single-facer process should be set solely on actually being formed. Third, the medium is flexed as the basis of controlling the medium web and on the 6 / Chapter 2 Flute Forming Theory

r FIGURE 2.2 Permanent Thickness Compression of Medium During Fluting

Permanent Transverse (ZD) Compression of Medium During Fluting, (145)

16-

a 14 B-Flute W C-Flute M A-Flute

i 12 ...... P e -12% -11% r 1 0 ......

m 8 -3% 1

4 Original Leading Tip Trailing Sidewall Sidewall LI Flute Profile Position

Permanent Transverse (ZD) Compression of ii Medium During Fluting, (149)

C 770 a I I 6 i * A-Flute + B-Flute P . e I50 ...... --. r 440 + ...... L o v 30-Flute Tip - Flute Sidewalls i s s 2 o0-...... /...... ' 10 S- < ^ / %

i_ u - n-______400 500 600 700 800 900 1Single-Facer Speed, fpm * Percerit Of Original Caliper r-

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... 8 / Chapter 2 Flute Forming Theory basis of controlling the medium runnability. Using excessive medium web tensile force will not reduce material cost by reducing medium consumption. The qualitative nature of the stresses on the medium during the flute forming process has been discussed in the literature. The medium web enters the single-facer by contacting a flute tip on the top portion of the upper corrugating roll. More linear footage of the medium is required to form the flute contour than is represented by the outer circumference of the upper corrugating roll. The medium must, then, be pulled over the tips of the upper corrugating roll teeth as it enters the labyrinth formed by the two corrugating rolls. As the medium enters the labyrinth, it contacts the teeth of the lower corrugating roll, and the medium web starts to be bent to form the flute shape. Based on the flute profile, the medium would have to be stretched by 8% to form the flute if no other strain re- lief mechanisms were present. Medium fails in tensile at approximately 1% stretch. Fortunately, there are other mechanisms. The bending and flute forming strains are accommodated by the medium pulling into the labyrinth, by the medium stretching, and by the MD/ZD shear of the medium, or, in the worst case, by fracture of the medium, (24, 129, 154). There are also changes in the moisture content and temperature of the medium web due to the influences of the preheater, the steam shower, and the heated corrugating rolls. These changes can affect the runnability characteristics of the medium, (38, 78, 84). The specific effect of moisture content and temperature changes on the medium runnability is discussed in the following chapters. Many of these forces applied to the medium during fluting alter the physical properties of the medium. These changes in the medium properties may help to explain why it is sometimes difficult to correlate the properties of the original medium to the medium runnability and the combined board properties with the degree of precision one would like. The mechanical condition of the corrugator can also influence the observed runnability quality of the medium. Defective bearings can cause an increased tensile force on the medium web because of increased friction and can cause tensile spikes in the web if the bearings "catch." Run out in idler rolls and other rolls can also cause tensile spikes in the web. Chapter 3

High/Low Flutes

The term "High/Low Flutes" refers to the variation glewall board in water to separate the double-face lin- in the height of the fluted medium component of the erboard from the medium. The double-face linerboard single-faced web. The quantitative level of high/low is then sprayed with an iodine/iodide solution in order flutes is generally expressed as the difference in height to stain the starch glue lines and to make them more between adjacent flutes. For a given sample of single- visible. The presence of high/low flutes is indicated by face board, it can be expressed as the average differ- the variability in the observed glue line widths. This ence in flute height, or it can be expressed as the per- procedure is valid only if the hold-down roll pressure cent of flute height differences that exceeded a certain on the double-backer glue machine has not been ex- critical value. The most common critical value level cessively increased so as to crush the tall flutes and, used for comparisons is 4 mils. thereby, obtain a uniform adhesive application. The high/low flute defect is important because of Statistical analysis indicates that approximately its adverse effect on combined board strength proper- 101 consecutive flute height measurements are required ties and package performance. The variation in the flute in a given sample in order to obtain a reasonable height of the single-faced web results in either a vari- quantitative measurement of the high/low flute defect. able strength double-backer bond or in excessive The difference in height between adjacent flutes is then crushing of the flutes, (115). The variable double- obtained by subtraction to obtain a set of 100 data backer bond strength is caused by the low height flutes points. The population of flute height differences is the receiving less adhesive applied to them than the taller high/low flute data. flutes in the double-backer glue machine. Excessive One experimental technique is to make the flute crushing results when the corrugator crew increases the height measurements using a mechanical caliper gage. double-backer hold-down roll pressure so as to achieve The measurement is made on the single-faced web and a more uniform adhesive application. The increased includes the thickness of the single-face linerboard. pressure crushes the taller flutes to the height of the This experimental technique is very time- consuming shorter flutes. and introduces extraneous data variability by incorpo- The effect of the high/low flute defect on the rating the linerboard caliper variability into the meas- combined board Edge Crush Test (ECT) is shown in urements. Figure 3.1 for a 42-26-42 lb/msf, C-Flute corrugated An alternative flute height measurement technique board construction. The data show that the ECT de- was developed at the Institute of Paper Science and creases as the percentage of high/low flutes increases. Technology. The method uses a infrared laser dis- Sensitivity analysis using the regression equation shows placement gage that measures the distance from the top that an increase in percentage of high/low flutes having of the unbonded flute tip to the bottom of the adjacent a height difference of 4 mils or more from 0% to 100% flute roots. The speed of the laser device allows numer- results in an 11% loss in ECT, (5). Based on the McKee ous intermediate height measurements on the flute box compression model, an 11% decrease in ECT will sidewalls to be collected at the same time. The laser result in an 8% decrease in the top-to-bottom compres- measurement data are fed into a computer program sive strength of the package made from the lower ECT which calculates the sample high/low flute property in strength corrugated board. any form desired (average, % of flutes above 4 mils, % Many corrugator crews are familiar with a tech- of flutes above 3 mils, a histogram of the flute height nique used to qualitatively evaluate the corrugated difference distribution, etc). The computer program board for high/low flutes during production on the cor- also allows the flute profiles to be viewed and exam- rugator. The technique involves soaking a piece of sin- ined for flute shape abnormalities. This laser measure- 10 / Chapter 3 High/Low Flutes

j: 11I II II FIGURE 3.1 11 11 I Effect of High/Low Flutes on il I I Edge Crush Test. (5) I I I i I I Effect of Box Plant Process Variables I on Combined Board Edge Crush Test, (5) I I II ECT = 33.7 - a(A) + b(B) - c(C) - d(D) - e(E) I 2 r = 0.750 n = 216 I I iI II I I ECT - Edge Crush Test, lb/inch | I a - 0.0397 A - Leaning Flute Angle, Deg. I I i b - 0.150 B - Single-Face Pin Adhesion, lb

c - 0.0534 C - High/Low Flutes @ 4 mils Or Greater, %

d - 0.134 D * Actual Crushing, mils

e - 0.130 E " Pressure Roll Cutting, % Mullen Loss I

I I I II I Effect of High/Low Flutes on Combined Board Edge Crush Test, (5) Sensitivity Curve I II 49 :. I I 484

I 46 b 4 5 ...... 1...... I n I II c 43 42- I 0 10 20 30 40 50 60 70 80 90 100 I High/Low Flute, Percent of Flutes With a I Difference of 4 mils or More I II I -1 Corrugating Medium / 11

ment technique is quick, precise, and accurate, and is adjustment of these fingers is important to controlling capable of making measurements at webs speeds in high/low flutes. The development of fingerless single- excess of 1000 linear feet per minute. The device is facers has reduced the high/low flute problem, but suitable for use in obtaining continuous high/low meas- based on field experience, it has not completely elimi- urements on commercial corrugators, (24, 31, 72). nated the problem. Vibration of the upper corrugating A high/low flute survey of 16 commercial corruga- roll, stresses at the pressure roll nip, and stresses asso- tors was conducted in 1962. The results of the survey ciated with the angle of the web leaving the pressure showed that all of the corrugators exhibited the roll nip also influence the tendency to have high/low high/low flute phenomenon and that there was a distinct flutes, (138, 149, 152). pattern or periodicity to the high/low flutes on all 16 Research has shown that the high/low flute defect corrugators. The periodicity of the high/low patterns is a result of single-facer mechanical conditions, sin- differed from corrugator to corrugator. These observa- gle-facer operation settings, and the physical properties tions suggest that the presence of high/low flutes is of the corrugating medium. A number of papers have more strongly related to the mechanics of the single- been published which describe the influence of the facer operation than to the nature of the corrugating various effects. medium material. The study also shows that A-Flute, Figure 3.2 summarizes the results of a high/low B-Flute, and C-Flute samples exhibit approximately the flute study which utilized a modified laboratory con- same severity of high/low flutes and that the severity of cora tester. A strain gage was placed inside the concora high/low flutes can vary across a single-faced web at tester and was used to measure the spring-back force of the same machine direction position, that is, a cross the fluted medium. The measurement was made at the direction effect, (145). exit side of the corrugating gears and simulated the Studies of the frequency of the corrugating me- response of the medium occurring in the gap between dium web tension variability show that the dominant the end of the lower corrugating roll fingers and the vibration component is at the primary flute forming pressure roll nip in the single-facer process. The re- frequency (corrugating roll teeth frequency) and its searchers investigated the effect of medium moisture second, third, and fourth harmonics. The maximum content, corrugator roll temperature, corrugator roll amplitude of vibration occurs at different harmonics for pressure, and medium web tension. All four variables different corrugator speeds. This suggests that the had an effect on the spring-back force of the fluted maximum amplitude is produced by the combined medium. The order of impact of the variables, greatest resonant frequencies of a number of corrugator compo- to least, is the order as listed above. The high/low flute nents. The relative amplitude of vibration of selected defect was reduced by a higher medium moisture con- corrugator components is shown in Table 3.1. The tent, a higher corrugating roll temperature, a higher largest effect is associated with top corrugating roll corrugating roll pressure, and a lower medium web pressure loading, (98). This effect is most likely related tensile force, (81). to the interaction of the corrugator roll pressure and the Figure 3.3 summarizes the results of a high/low translational bounce of the upper corrugating roll asso- flute study conducted on a pilot size single-facer. Eight ciated with the rotational meshing of teeth with the different commercial mediums were used in the study. lower corrugating roll. The effect of the process variables of corrugating roll pressure, medium web tensile force, medium steam shower application, corrugating speed, and web take- Table 3.1. Relative Vibrational Amplitudes for off angle was evaluated. Take-off angle refers to the Various Corrugating Components, (98) angle of the singlewall web leaving the pressure roll nip. The angle is expressed as deviation from the nip tangent line. All five variables had an effect on Component +/- Half high/low flutes, and the order of impact of the vari- Amplitude ables, greatest to least, is the order as listed above. The Top Roll Acceleration 0.96 high/low flute defect was reduced by a higher corrugat- Top Roll Pressure 3.34 ing roll pressure, a lower medium web tensile force, a Web Tension 0.25 greater amount of preconditioning steam, a lower corrugator speed, and a lower take-off angle, (115). High-speed camera studies have shown that the Figure 3.4 and Figure 3.5 summarize the results of high/low flute defect is associated with the process area a high/low flute study conducted on a pilot-size single- located between the end of the lower corrugating roll facer using several commercial mediums. The corrugat- fingers and the pressure roll nip where the fluted me- ing medium materials evaluated in the experiment in- dium is suspended in air, (138). The proper positional cluded semichemical, kraft, and recycled fiber fur-

I - FIGURE 3.2 . Effect of Corrugating Variables on _|

Spring-back of Fluted Medium I '

Effect of Corrugating Roll Pressure Effect of Web Tension on the Spring-back Force of the on the Spring-back Force of the Fluted Medium, (81) Fluted Medium, (81)

400- 400 -:- - 350- -...... S 350 . - S F 3 - P F 300. 0 0 r o0 .. .. 20 - , I I r 250-. - r 250 - ...... 200 b 150--." ; . I " '." a m 100 ...... c ...... 50 a N - k 50-_ k 0------o0------, __, 4 6 8 10 12 14 16 18 20 0 50 100 150 200 250 300 350 400 Corrugating Roll Pressure, kN/m Medium Web Tension, N/m 26 Ib/maf Medium 26 lb/mef Medium

Effect of Corrugating Roll Temperature Effect of Medium Moisture Content on the Spring-back Force of the on the Spring-back Force of the Fluted Medium, (81) Fluted Medium, (81)

- 400- 400 S 350 .- S 350 - F 300 ...... - p F 300 .- . o ro 3 . . . r . I r 250 \ I r 250 n e 200 . b e2 000...... - . .... b 150 - a m 100.-..... **!** . .,,....' ...... a m 100- k 501.- ...... k 50- 0- :; ------14 80 90 100 110 120 130 140 150 160 170 180 190 200 2 4 6 8 10 12 Corrugating Roll Temperature, Deg. C Medium Moisture Content, % 26 lb/mef Medium 26 Ib/msf Medium

------Corrugating Medium / 13

il I FIGURE 3.3 Effect of Corrugator Process Conditions on High/Low Flute Defect

Effect of Corrugating Process on High/Low Flute Formation, (115)

A 3.0- e 2.8- + 16% + 25% + 37% r 2.6 ...... 187......

, 1.6 Lrcs CodCnto.... mi 1.21.4 s 1.0 Speed, fpm Steam Shower, psi Corr. Roll Pres. Process Condition /Inch

Effect of Corrugating Process on High/Low Flute Formation, (115)

A 2.2 V e + 28% + 4% r 2 .0 - ...... a 1.75 g 1.8 ...... e H 1.6 / 0.50 0 20

m 1.2

I 1.0 Web Tension, lb/inch Take-Off Angle, Deg. Process Condition

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(9qz.) 'peedS BuIle~niJoo (9aL) 'juawuuSHv lIou~ BuI)Slh0on -seiuePue.1 mOl/q61H 94nlj1 uo - 9010usPuJ.1 mO-1/14BH elni~j uo suojllPuO0 BsOOJdI OuI)eBnfjjoo JO 409JJ0 8uOllPuO03 98eo0M 5UaUnjjoCBj JO 400JJ3 loelea] 91nI-J MOj/Ljb!.H uo salqeiueA DU1bulleOnjoo lo e0 To 3ufloH 16 / Chapter 3 High/Low Flutes nishes. Medium basis weight levels of 26 and 33 lb/msf being corrugated using the concora tester. A greater were included. The influence on high/low flutes of length increase was taken to represent a material re- medium web tension, medium preconditioning steam, laxation and dimensional instability that is related to the medium preheater wrap, corrugating roll pressure, out- high/low flute defect. The data show that the high/low of-parallel corrugating rolls, speed, take-off angle, and flute defect is reduced by a higher corrugating medium medium web orientation was evaluated. All eight proc- moisture content and by a higher corrugating roll tem- ess variables had some effect on high/low flutes. The perature, (95). listing of the variables in the order of their impact, The major single-facer process variables and cor- greatest to least, and the direction of change in the vari- rugating medium material properties that affect the able needed to reduce the high/low flute defect were: high/low flute defect are summarized in Table 3.2. It is decreased medium web tensile force, decreased speed, obvious that some of the listed variables are more rea- increased corrugating roll pressure, increased use of sonable candidates for selection as process control tools medium preconditioning steam, a negative take-off for the high/low flute defect than others. For instance, angle, and increased medium preheater wrap. Slightly the medium basis weight needs to be based on the issue less high/low flutes occurred when the medium was of cost-effective packaging and not the high/low flute corrugated with the felt side toward the single-facer issue. Corrugator speed is another example. It would be bond. This is most likely due to a minor difference in a illogical to assume that the corrugated industry will surface property of the medium between the two sides, move in the direction of reducing corrugator speed in such as the coefficient of friction. The greater out-of- order to minimize the probability of high/low flute parallel corrugating roll condition actually reduced formation. However, it is also illogical to assume that high/low flutes. This seems contrary to logic and may the corrugator crew should be allowed to push the ma- only indicate that the "normal" corrugating roll bearing chine beyond its designed mechanical speed. To do so position setting for this particular pilot-size single-facer would aggravate the mechanical vibrations that produce was really out-of-parallel, (126). medium web tension spikes, and would aggravate the Figure 3.6 summarizes the results of a high/low bounce of the upper corrugating roll, both of which flute study on a pilot-size single-facer using 26 and 33 increase the probability of producing high/low flutes. lb/msf commercial mediums. The experimental results Increase bounce adversely affects the flute profile show that the high/low flute defect is reduced by a definition. slower corrugator speed and a lower basis weight corrugating medium, (17). Figure 3.7 summarizes the experimental results of Table 3.2. Major Process & Material Factors Affect- a high/low flute study run on a pilot-size single-facer ing the High/Low Flute Defect using 21 samples of 26 lb/msf commercial corrugating mediums and A-Flute corrugating rolls. The multiple Direction of Change Needed regression analysis of the data indicates that the to Reduce H/L high/low flute defect is reduced by a corrugating me- Variable Flutes References dium having a lower coefficient of friction (measured Medium Web Tension Decrease 68,81, 115, against a heated steel surface), a more uniform forma- 126 tion, and a higher alcohol/benzene extractive content, Corrugator Speed Decrease 17,78,84, 126 (113). The favorable effect of a more uniform corrugat- Corrugating Roll Pres- Increase 68, 78, 81, 84, ing medium formation on reducing the high/low flute sure 115,126 defect is confirmed by other research publications, (17, Corrugating Roll Tem- Increase 81,95 perature 66, 69, 115). Preheater Wrap Increase 126 Figure 3.8 summarizes the test results obtained for 115,126 a high/low flute study run on a pilot-size single-facer Medium Steam Shower Increase Medium Moisture Con- Increase 81,95 using commercial corrugating mediums ranging in ba- tent sis weight from 26 to 40 lb/msf. The data show that the Medium Basis Weight Decrease 17,25, 26,27, high/low flute defect is reduced by a slower corrugating 78,84 speed and by a lower corrugating medium basis weight, Medium Caliper Decrease 17 (25, 26, 27). Medium Coef. of Fric- Decrease 17,113 Figure 3.9 summarizes the experimental results of tion Against Heated a high/low flute defect study which utilized a laboratory Steel Medium Formation More Uniform 17, 66, 69, 113, concora tester. The tendency of high/low flutes was 115 characterized by the measurement of the increase in the Medium MD Stretch Increase 17,25,26,27 length of the fluted medium test specimen strip after

I _ .- ...... FIGURE 3.6 Effect of Single-Facer Speed and Medium Basis Weight on High/Low Flutes, (17)

40 -

35- X 26 Ib/ .. . -. . ...- .. .7. . - H msf i fmsf ...... . ..I. ! g 30- i 33 lb/ h i / 25 - ...... L

....i ...... w 20 ......

F 15 - I i u t 10 - e S 5

0 0- - I II I I 1 (*) 100 200 300 400 500 600 700 800 900 1000 1100 5. Single-Facer Speed, fpm UQ

* % of Flutes Over 4 mils. "- __ ___ - __ _'_

-. 0-0 FIGURE 3.7 Medium Properties Affecting High/Low Flute Defect, (113) (jD

H/L = 1.39 + a(A) - b(B) - c(C)

r 2 = 0.578

a = 6.25 A = Coef. of Kinetic Friction b = 0.38 B = Alcohol-Benzene Extraction, % c = 0.083 C = Thwing Formation, units

I- FIGURE 3.8 Effect of Medium Basis Weight on High/Low Flute Defect, (25, 26, 27)

50 45 H 40 9 h 35 / L 30 0 w 25 F I I u 15 - t e 10 s 5 (*) 0 0 - 0 Om 100 200 300 400 500 600 700 800 900 1000 1100 Single-Facer Speed, fpm

9- * % of Flutes Over 4 mils. 00g.et

1. ------\o 20 / Chapter 3 High/Low Flutes

II II1f ii 11 FIGURE 3.9 fl| Effect of Corrugator Process Variables it on Fluted Medium Spring-back

Effect of Medium Moisture Content on Fluted Medium Spring-back, (95)

F u % 85- t e i 0 d f 80- . 0 L r 75 .. e i n g g . 70- t h 65 0 2 4 6 8 10 12 14 16 18 20 Medium Moisture Content, % II Actual Data Not Shown Iii I!

Effect of Corrugating Roll Temperature on Fluted Medium Spring-back, (95)

F 90- I '~~~~~~~~~.^^~ -;~ : : .- u % 85- t eO0 ...... de °f 808 0.. ...

L r 75- ...... - e i ng g 70 .70...... t 7 0 . :: . : h 65 10 20 30 40 50 60 70 80 90 100 110 120 Corrugating Roll Temperature, Deg.C Actual Data Not Shown Corrugating Medium / 21

The experimental data are consistent with regard to the corrugator. Refining did not affect the ZD exten- the beneficial effect of a higher medium moisture con- sional stiffness, but it was increased by wet pressing, tent and a higher medium temperature at the point of (15). flute formation. High/low flute defects are reduced by a Wet pressing and refining are methods commonly higher medium rollstock moisture content, by the used at mills to improve the compressive strength of greater use of medium steam showers, by the greater medium. These compressive strength improvement use of medium preheating, and by higher corrugating techniques, however, may adversely affect the medium roll temperature. It is hypothesized that the higher me- properties important to high/low flutes. This point is dium temperature and moisture content decrease the raised to remind the reader of the observation made in stiffness of the medium and increase its stretch, (25, 26, Chapter 1. There are many extraordinary demands 27). It is also hypothesized that the higher moisture placed on corrugating medium, and there is an impor- content reduces the plastic temperature of the medium tant need to understand these demands, to compromise fibers,(78, 87), and allows the flute forming strains to these demands (but only when absolutely necessary), be dissipated by the MD/ZD shear of the medium, (38). and to control the papermaking and corrugating proc- The higher medium moisture content also results in a esses so as to achieve the best total corrugated board lower caliper for the fluted medium, (69). This com- product. paction of the fiber as the flute formed may also assist In summary and with consideration of only the in maintaining the fluted shape. issue of high/low flutes, the total body of technical in- Medium web tension and medium formation both formation available indicates that the medium mills and affect the high/low flute defect. These two effects may the box plants should consider the feasibility of the help to explain the cross corrugator direction high/low following actions to reduce the probability of encoun- flute pattern that has been observed, (145). Formation tering the high/low flute defect. and basis weight streaks and a nonuniform cross machine web tension could produce this CD effect. The medium mills should consider: The coefficient of friction can be reduced on the corrugator by the use of a lubricant applied to the me- * Increase MD Stretch. dium web, Table 3.3. A low molecular weight, low density, nonemulsifiable polyethylene lubricant is the * Improve Formation. most effective type. It is an inexpensive, solid material which can be applied by having the medium web rub * Increase Moisture Content. against a bar of the lubricant as it feeds into the single- facer. A lubricant application rate of 0.0084 lb lubricant * Reduce Hot Coefficient of Friction. per msf medium is sufficient to produce the results shown in Table 3.3, (94, 96). * Reduce Caliper.

The box plants should consider: Table 3.3. Effect of Medium Lubrication on the Co- efficient of Friction and High/Low Flutes, (94, 96) * Reduce Medium Web Tension.

* Increase Medium Without With Lu- Steam Preconditioning. Property Lubricant bricant * Increase Medium Preheating. Coefficient of Friction 0.23 0.08 Against Hot Steel * Increase Corrugating Roll Pressure. High/Low Flutes (% of 100% 73% value without lubricant) * Increase Corrugating Roll Temperature.

Figure 3.10 shows the effect of pulp refining and * Unlock Preheater Drums. paper machine wet pressing on the medium properties of MD tensile, MD tensile stretch, ZD extensional stiff- * Properly Maintain Mechanical Equipment to ness, and porosity. Both refining and wet pressing serve Minimize Vibration and Friction Forces. to increase the density and the fiber bonding of the medium, which increases the tensile strength and the * Properly Maintain Steam System to Raise Medium tensile stretch. However, it also produces a less porous Temperature and Moisture Content. sheet which may be less receptive to preconditioning on r

"11 FIGURE 3.10 Q Effect of Paper Mill Refining and F Wet Pressing on Medium Properties C Effect of Refining and Wet Pressing on Effect of Refining and Wet Pressing on Medium MD Tensile Strength Medium MD Tensile Stretch for 40 Ib/msf Medium, (15) for 40 Ib/msf Medium, (15)

110 2.1 - M 10 - - ...... M 2.0 - . . . D 1000 t 1.9 n 1988...... e 1.8 -

an 80 ...... I c

e 70

400/Low 400/Medium 325/Low 326/Medlum 400/Low 400/Medium 325/Low 325/Medium Process Condition, Freeness/Wet Pressing Process Condition, Freeness/Wet Pressing Process Condition, Freenesa/ Wet Pressing

Effect of Refining and Wet Pressing on Effect of Refining and Wet Pressing on Medium ZD Extentlonal Stiffness Medium Gurley Porosity I for 40 Ib/msf Medium, (15) for 40 Ib/msf Medium, (15)

E 50- P275-- 0 250 - r 225 - 400/Low 400/ 325/Low 325/ um 4 0 ...... a 175 - t) 400/Low ...... 400/ 325/Low 325/ t 150 y 125 , 100 f 30on, Freeness/Wet Pressing 5 75. e 25, e 50' c 25 s 20 0 f (Ez t) 400/Low 400/Medium 325/Low 325/Medium 400/Low 400/Medium 325/Low 325/Medium Process Condition, Freeness/Wet Pressing Process Condition, Freeness/Wet Pressing Chapter 4

Fractured Flutes

The term "Fractured Flutes" refers to the physical longer than 1/4 inch. Fracture failure generally occurs separation of the corrugating medium fiber network in the flute sidewall areas, although the fractures can during the flute forming process in the single-facer. occur at the flute tips in heavy basis weight medium, Material engineering theory describes two possible (46). A method for testing for fracture was not found in types of separation that might be expected. One separa- the literature. A simple method,. used by the author, is tion type is that due to forces acting in the in-plane ma- the "Thumb Nail Test." It consists of rapidly rubbing chine direction of the medium, and the other separation the thumb nail over the flutes of a single-faced sample, type is due to a shear force acting in the machine direc- one time. The rubbing should be in the machine direction/thickness (caliper) direction of the medium. The tion and should be done using very little pressure. The in-plane type of failure in paper is usually associated thumb nail force will cause the fractured area to sepa- with a tensile or tear type of force. The shear force type rate and make the fracture more visible. Looking at the of failure in paper is usually associated with a bending sample over a light box accentuates the failure lines. force that causes delamination. Both tensile and bend- For those with delicate hands, the rounded end of a ing forces exist in the flute forming process, (24, 126, utility knife handle can be used in place of the thumb. 129, 154). It may seem that the bending of the medium It was pointed out in Chapter I that the fluted me- occurs only to the medium material located at the tips dium is the heart of the corrugated board structure and of the final flute. This is not correct. The bending force that the quality of the corrugated board is determined at is applied to the total surface area of the medium web the single-facer, (7, 129). Figure 4.1 shows the effect as the medium is drawn into the corrugating roll laby- of fractured flutes on the combined board flat crush rinth, (46). strength and on the fluted medium edge crush strength. Examination of medium fractured during flute The degree of fracture represented by the data in Fig- forming in commercial corrugating operations indicates ure 4.1 is the least possible. It represents the point that the failure is an in-plane separation, specifically a where fracture has just begun to occur in the medium. MD tensile failure. Shear deformation forces are at Flat crush is reduced by 9.7%, and the medium edge work during the bending of the medium to form the crush strength is reduced by 14.7%, (129). On average, flute shape. The main contribution of shear is to help the medium contributes about one-third of the total dissipate some of the tensile strain, (24, 126, 129, 154). combined board Edge Crush Test. Based on this ratio The only time that the author has observed shear frac- and on the McKee box compression model, the 14.7% ture failure of the medium after corrugating was when a reduction in the medium fluted edge crush would result prelaminated two-ply medium was used. The medium in an estimated 3.6% loss in box compression. The separated at the lamination glue line and produced two author believes that the actual reduction in box com- single-faced corrugated sheets rather than one single- pression would be considerably greater than 3.6% since wall sheet. the combined board flexural stiffness would also be The severity of the flute fracture defect can vary adversely affected by the flute fracture. It is the strong widely. In the worst case, the medium is literally shred- opinion of the author that no amount of flute fracture is ded. Pieces of medium fall out of the web, and it is tolerable. Any degree of fracture makes the board generally impossible to feed the single-faced web commercially unacceptable. through the double backer. Flute fracture failure is of- Figure 4.2 shows the effect of medium web ten- ten difficult to detect by eye at the point where it first sion and flute size on flute fracture. The experiments starts to occur. The fracture failure lines propagate in were conducted on a pilot-size single-facer using a the cross machine direction and are generally not commercial medium. The data show that a higher me- 24 / Chapter 4 Fractured Flutes r- I-I !j FIGURE 4.1 11 i Effect of Fractured Flute Defect on !i Corrugated Board Properties

Effect of Flute Fracture on Combined Board Flat Crush, (129)

1| F o40 iI t38" C 36 ' -.. ^ ^ ^ - ...... 1 I r34- I u 32 . I h 30- I ,28- I 26- s 2 4 - i22 . 20 / No Fracture Fractured Flutes Flute Fracture Condition I Effect of Flute Fracture on Fluted Medium Edge Crush, (129) I l I I 64 ./...... I

i b 56. 6/ .14.. 71 Ch ...... I

h An G7-- No Fracture Fractured Flutes Flute Fracture Condition I j FIGURE 4.2 Effect of Medium Tension on Flute Fracture Speed, (24) w 5 e b T 4 e n s 3 i 0 n 2 I b / 1 n c h 0 0 q on 0 100 200 300 400 500 600 700 800 900 1000 0A Speed at Fracture, fpm 5'J B

LA 26 / Chapter 4 Fractured Flutes dium web tension produces flute fracture at slower cor- formation would be beneficial to reducing the flute rugator speeds. At a given medium web tension, A- fracturing tendency, (92, 93). Flute fractured at a slower speed than B-flute, and B- Figure 4.6 is an equation for predicting the frac- Flute fractured at a slower speed than C-Flute, (24). ture speed of a medium based on the corrugating proc- The fact that fracture speed does not correlate with flute ess variables of roll-stand brake tension, radii of curva- height suggests that the design of the flute profiles ture of the flute tips on the corrugating roll and the ef- (radii, angles, and clearances) and/or the surface char- fective wrap angle of the medium web in the corrugat- acteristics of the corrugating rolls are more controlling ing roll labyrinth, and on the medium properties of MD than flute height. tensile strength, MD stretch, soft platen caliper, and Figure 4.3 and Figure 4.4 summarize the results of coefficient of friction against a heated steel surface. a flute fracture study conducted on a pilot-size single- The model indicates that the flute fracturing tendency is facer using several commercial mediums. The corrugat- decreased by reducing the roll-stand brake force (lower ing medium materials evaluated in the experiment in- web tension) and by a medium having a higher MD cluded semichemical, kraft, and recycled fiber fur- tensile strength, a higher MD stretch, a lower caliper, nishes. Medium basis weight levels of 26 and 33 lb/msf and a lower hot coefficient of friction. The predicted were included. The effect on flute fracture of the vari- fracture speed based on the model is compared to ex- ables of medium web tension, medium preconditioning perimental flute fracture data obtained for three differ- steam, medium preheater wrap, out-of-parallel corru- ent commercial mediums using a pilot-size single-facer. gating rolls, speed, take-off angle, and medium web The comparison is shown in Figure 4.7 and it indicates orientation was evaluated. The listing of the variables that the model is reasonably accurate and is a good that affect fracture in the order of their impact, greatest predictive tool, (26, 26, 27). to least, and the direction of change in the variable The major single-facer process variables and the needed to reduce the probability of flute fracture were: major medium material properties that affect flute decreased web tension, increased medium precondition- fracture are summarized in Table 4.1. As was discussed ing steam, and increased medium preheater wrap. The in Chapter 3, slowing the corrugator speed is not sug- corrugating roll pressure, out-of-parallel corrugating gested as a long-term strategy for the control of flute rolls, and take-off angle had little or no effect on flute fracture. On the other hand, it is also not suggested that fracture. The effect of the medium moisture content the corrugator speed be increased beyond the mechani- indicates an optimum operating plateau region of 6% to cal design limit of the specific corrugator. Doing so will '1 12%. Medium moisture content levels below 6% and increase the mechanical vibrations and result in me- above 12% resulted in increased fracture tendencies. dium web tension spikes that can cause fracture. The The web orientation effect (wire side or felt side toward excessive speed will also cause bounce in the upper the single-face bond) is most likely due to differences corrugating roll and poor flute forming, (98, 149). in surface properties, (126). Figure 4.5 summarizes the results of a study to Table 4.1. Major Process & Material Factors Af- determine the effect of medium physical properties on fecting The Flute Fracture Defect flute fracture. The experiments were done on a pilot- Dii -ection of Change Needed size single-facer using medium produced on a pilot-size Variable to Reduce Flute Fracture References paper machine. A full factorial experimental design was Medium Web Tension Decrease 24, 25, 26, 27, used. The fracture tendency in this study is defined as 126 the highest medium web tension attained at a corrugator Corrugator Speed Decrease 24, 126 speed of 600 fpm without producing fractured flutes. Roll-Stand Brake Decrease 25, 26, 27 Medium Preheater Wrap Increase 126 The multiple regression equation developed from the Medium Temperature Increase 38, 78, 84 experimental data includes interactive terms, such as Medium Steam Shower Increase 126 the interaction of the CD elastic modulus and the CD Medium Moisture Content Increase 38, 78, 84, 126 tensile stretch. (Note: It seems more logical to the Medium MD Tensile Increase 25, 26, 27, 46 author that the MD elastic modulus and MD stretch Medium MD Stretch Increase 25, 26, 27, 46 would be related to fracture. There is a possibility that Medium Caliper Decrease 25, 26, 27 the CD effects described in the reference are typo- Medium Coef. of Friction Decrease 25, 26, 27, 46, graphical errors.) The interactive terms make it very Against Heated Steel 78, 129, 134 difficult to quantify the relative importance of the in- Medium Formation More Uniform 92, 93 Shives in Medium Decrease 110, 114 dividual variables. However, it can be inferred that a Medium Porosity Decrease (Less 92, 93 medium that is less porous, that has a lower CD elastic Porous) modulus and CD stretch, and which has a more uniform FIGURE 4.3 Effect of Corrugator Process Variables on Flute Fracture Defect

Effect of Corrugating Process Conditions Effect of Corrugating Process Conditions on Flute Fracture Tendencies - on Flute Fracture Tendencies - Web Tension, (126) Steam Shower, (126)

F 900- 900- r 800 - ...... -...... r 800 a 4 ' 70 t7 700- * r 8 0 0 - ...... ; ...... r e 500 e 500-

S 400- S 400 e 300 ...... e 300 e e 2 0 0 ...... d 200 ------(fpm) 0.0 0.5 1.0 1.5 2.0 2.5 (fpm) 0 5 10 15 20 25 30 Web Tension, Ib/inch Medium Steam Shower, psi

Effect of Corrugating Process Conditions Effect of Corrugating Process Conditions on Flute Fracture Tendencies - on Flute Fracture Tendencies - Pre-Heater Wrap, (126) Corrugating Roll Pressure, (126)

F 900 --- F 900- r 800 ...... - .

e soo ; u eoo ...... u eoo-00 ......

S 4oo n 400' e 00-- e 300 - 200 - d 200 - 5' (fpm) 0 10 20 30 40 60 60 70 80 90 100 (fpm) 100 200 300 400 500 600 Medium Pre-Heater Wrap, % Of Maximum Corrugating Roll Pressure, lb/inch b. 5'I

__4 -1 tw I 00 FIGURE 4.4 SD U,, Effect of Corrugator Process Variables on r 't Flute Fracture Defect Fa I Effect of Corrugating Process Conditions Effect of Corrugating Process Conditions on Flute Fracture Tendencies - on Flute Fracture Tendencies - Corrugating Roll Alignment, (126) Medium Moisture Content, (126)

900 - F 9 0 0 - i r 800 - . a a .-. c 700 ...... C 700 u 600 - u 600 - - . r r e 5 0 0 - ...... e 500 / S 400 :

e 3 o o ...... e 300 - e d 200. d 200 -, (fpm) 0.00 0.05 0.10 0.15 0.20 (fpm) 0 5 10 15 20 25 Corr. Rolls Out-Of-Parallel, Inch Medium Moisture Content, %

Effect of Corrugating Process Conditions Effect of Corrugating Process Conditions on Flute Fracture Tendencies - on Flute Fracture Tendencies - Take Off Angle, (126) Web Orientation, (126)

F 900 - 900- F r 800- s .. .. a c 700- B 700 ...... t u 600 _ d 4 0 : ...... r e 500 .... P IcI0 i S 400. f 300 - ...... e e 300 -.. ire m 200 -, ------' d 300 Wire Felt -20 -16 -10 -6 0 6 10 16 20 200 P Take Off Angle *.Deg. (fpm) Bonding Side Of Medium * Web Angle Exiting the Pressure Roll Nip ...... 11 FIGURE 4.5 Effect of Medium Properties on Flute Fracture, (92, 93)

Fract. = a(A) - b(B) - c(C) -d(D) + e(E) - f(F) + g(G) + 12.79

r -2 0.76 Fract. - Lowest Web Tension Causing Fracture @600 fpm

a = 0.0440 A - Air Resistance, sec.

b = 0.0275 B = CD Mod. of Elast., Psi

c = 2.458 C = CD Stretch, %

d = 0.0853 D = Formation, units e = 0.00628 E = B times C f - 0.1657 F = A times C

g = 0.000023 G = A times B times C

=_ -~0.n w FIGURE 4.6 & n Medium Fracture Speed Model, (25, 26, 27) ID P

CP to 297 4.895 Tt /10 FS -( I Tf , e e e ) (R+t /2) C

FS = Fracture Speed, fpm Tf = MD Tensile Strength, lb/inch t = Soft Platen Caliper, inch A Coef. Hot Friction, Steel surface 0 = Effective Wrap Angle In Labyrinth, radians T o = Brake Tension, lb/inch E = MD Stretch, % R - Radius of Curvature of Flute Tip FIGURE 4.7 Validation of the Flute Fracture Model. (25. 26. 27)

1200 F C r a a 1000 I C C t 800 U u I r 600 a e t e S 400 d P e 200 e (~ d 0 0 o (fpm) 0 200 400 600 800 1000 1200 5- OT

Observed Fracture Speed, fpm ' See FIGURE 4.6 for the Model. -

Ei» 32 / Chapter 4 Fractured Flutes

Fracture occurs when the tensile stress and the flute fracture occurs on a pilot-size single-facer, Figure tensile strain in the medium web due to flute formation 4.8. (146). bending forces, frictional forces, and roll-stand brake forces exceed the tensile strength and tensile stretch of the medium in the corrugating roll labyrinth, (24, 25, Table 4.2. Effect of Medium Lubrication on the Co- 26, 27, 38, 78, 84, 129, 154). The beneficial effect of a efficient of Friction and Fractured Flutes, (94, 96) higher medium MD tensile strength and stretch is due to the ability of the material to withstand greater corru- Without With Lu- gating stresses and strains before failing. Reducing the Property Lubricant bricant roll-stand braking force and using a corrugating me- Coefficient of Friction 0.23 0.08 dium with a lower hot coefficient of friction both re- Against Heated Steel duce the fracture tendency by reducing the medium Maximum Corrugator 550 +1000 web tension. A more uniform formation in the medium Speed Without Fractured and a reduction in the occurrence of shives in the me- Flutes dium both reduce the probability of localized tension spikes. A shive is a bundle of fibers or a small piece of underpulped wood. A lower caliper medium is benefi- In summary and with consideration of only the cial to reducing fracture because it increases the relative issue of flute fracture, the total body of technical in- clearances of the medium in the sidewall area of the formation available indicates that the medium mills and corrugating roll teeth. the box plants should consider the feasibility of the The probability of flute fracture is reduced when following actions to reduce the probability of encoun- the medium web entering the corrugating roll flute tering the flute fracture defect. forming labyrinth has a higher moisture content and a higher temperature. The higher moisture content and The Medium Mills should consider: temperature "soften" the web by reducing its stiffness, * Increasing MD Tensile Strength. increasing its MD stretch at tensile failure, and reducing the out-of-plane shear modulus. These are favorable * Increasing MD Stretch. changes. The higher web moisture content also in- * Improve Formation. creases the medium's hot coefficient of friction and reduces its tensile strength. These are unfavorable * Reduce Shive Count. changes. The combined data from the literature indicate * Increase Moisture Content. I that, on balance, the favorable effects out weigh the unfavorable effects, (38, 46). * Reduce Hot Coefficient of Friction. Two box plant process methods have been in- * Reduce Caliper. vented to reduce the probability of flute fracture, particularly at high corrugator speeds. The first method The Box Plants should consider: consists of using a lubricant applied to the medium web as it feeds into the single-facer. The lubricant serves to * Increase Medium Preheating. reduce the friction during the actual flute forming proc- Increase Medium Steam Preconditioning. ess, (78, 84, 94, 96, 147). It is important that the lubricant selected does not impair the porosity or absorb- * Increase Corrugating Roll Temperature. ency of the medium since bonding may be affected, * Reduce Medium Web Tension. (78, 84). A low molecular weight, low density, nonemulsifiable polyethylene lubricant is the most effec- * Unlock Preheater Drums. tive type. It is an inexpensive, solid material that can be * Properly Maintain Mechanical Equipment to applied by having the medium web rub against a bar of Minimize Vibration and Friction Forces. the lubricant. A lubricant application rate of 0.0084 lb lubricant per msf medium is sufficient to produce the * Properly Maintain Steam System to Raise Medium results shown in Table 4.2, (94, 96). Temperature and Moisture Content. The second method for reducing and controlling the medium tension consists of medium web in-feed rolls located close to the corrugating rolls. The in-feed rolls act as a web tension buffer against the upstream sources of tension. The device was shown to be very effective in increasing the corrugator speed at which - - ____Aw_ -,

.- - = FIGURE 4.8 Effect of Medium In-Feed Rolls on Flute Fracture Defect. (146)

k. F OVER r W i a t c h f ...... t U F IIf r e e e 900 d e S r 600 P e e f P 300 d m

0 0 0 100 200 300 400 500 600 700 800 900 1000 K (1) Fracture Speed Without In-Feeder, fpm UQ5-4

w -____ -______---- ...... __ ..J wI Chapter 5

Medium Strength Loss By Fluting

It is the ultimate desire of package designers and technical terms, the medium is exposed to in-plane packaging engineers to be able to accurately predict the tensile strain, bending shear strain, and out-of-plane field performance of corrugated packages based solely transverse compression strain, (25, 26, 27, 30, 38, 129, on the measured physical properties of the linerboard 154). As stated in Chapter 1, "the life of the corrugating and medium rollstock used to make the box, and from medium is not an easy one." the size and design of the package. It is the opinion of The medium plays several key roles in the struc- the author that those designers and engineers who be- tural performance of the corrugated board and the cor- lieve that they can now do so with a great degree of rugated package. Based on the information available in accuracy are very much mistaken. the references, the discussions in this chapter will be As this chapter will show, it is currently very diffi- mainly limited to the properties associated with the cult to predict just the combined board strength prop- combined board flat crush and edge crush strengths. erties, which are dependent on the fluted medium, The flat crush strength of the corrugated board is im- solely from the strength of the starting medium and the portant to the panel bulge resistance, the compressive flute size, except as an overall average effect. There are strength, and the cushioning characteristics of the cor- too many corrugating process factors that affect the rugated package. The effect of flat crush on panel bulge relationship. and the box compression is through the flexural stiff- The corrugated medium in the combined board is ness strength. The medium must be able to maintain the exposed to many different stresses and strains between structural integrity of the corrugated board by keeping the single-facer medium roll-stand and the exit side of the linerboard facing as far apart as possible. The edge the single-facer pressure roll nip. The medium web's crush strength of the corrugated board is important to temperature and moisture content are altered by the the compressive strength of the corrugated package. preheater drum and the preconditioning steam shower. Table 5.1 demonstrates the effect of heat treatment The medium web is exposed to MD tensile forces from on the physical properties of corrugating medium. The the roll-stand braking system and the various sources of medium was heated for 2 seconds at a temperature of mechanical friction. The greatest effect, however, is in 300 deg.F. The samples were then conditioned at 73 the actual flute forming part of the process. deg.F. and 50% RH before testing. The 2-second treat- The medium web is heated by the corrugating rolls, ment time corresponds to a linear contact distance of and then stretched and bent to form the flute shape in 8.3 feet for a medium web travelling at 1000 fpm. The the corrugating roll labyrinth. Once the flute is formed, treatment conditions are, therefore, representative of the it is than squeezed and compressed in the tip and side- medium preheating that occurs in a commercial singlewall areas as it reaches the tangent point between the facer operation. upper and lower corrugating rolls. The data show that the heat treatment produces a Figure 5.1 shows that the medium is permanently 14% increase in the medium material CMT (flat crush compressed, on average, by 37% in the flute tip area effect), a 10% increase in CD ring crush (edge crush and by 11% in the flute sidewall area, (145, 149). This effect), a 14% increase in MD tensile strength, a 6% compression may actually help to reestablish the fiber- increase in stretch, and an 18% increase in tensile en- to-fiber bonding that was disturbed by the flute forming ergy adsorption. The MD fold resistance is the only stresses, (30, 95). Approximately 15% water, by tested property that exhibited a reduction, (75). This weight, is then added to the medium by the application shows that the properties of the medium web entering of the starch adhesive slurry, and the medium flute tip the corrugating rolls are significantly different from the is again compressed in the pressure roll nip. In more properties of the medium web at the roll-stand. 36 / Chapter 5 Medium Strength Loss By Fluting

-i aI FIGURE 5.1 Ill Permanent Thickness Compression of SI Medium During Fluting

I Permanent Transverse (ZD) Compression of Medium During Fluting, (145)

16-

I! C 14 II a 14 | B-Flute '- C-Flute

P e - 12% 11% r 10-

6-6 ..... 3..%...

Original Leading Tip Trailing Sidewall Sidewall Flute Profile Position I

Permanent Transverse (ZD) Compression of Medium During Fluting, (149)

C 70 a ...... _____.. _____

P 50 ...... e 5060i * A-Flute * B-Flute 4 0 - ......

2 0 - ......

%F ... * ......

400 500 600 700 800 900 Single-Facer Speed, fpm * Percent Of Original Caliper Corrugating Medium / 37

tendency of a fluted paper to attempt to elongate Table 5.1. The Effect of Heat Treatment on Medium (straighten out) is representative of stresses remaining Properties, (75) (2-second Treatment at 300 deg.F) in the paper. The experiment was conducted using a modified concora test instrument. A strain gage was Property Untreated Treated % used to measure the spring-back force of the fluted web Value Value Change as it was leaving the nip between the corrugating rolls. Concora Medium 240 275 + 14% Commercial mediums were used for the experimental Test, N testing. CD Ring Crush, 0.88 0.97 + 10% kN/m The data show that higher corrugating roll pres- MD Tensile, kN/m 8.7 9.9 + 14% sure, higher corrugating roll temperature, and higher MD Tensile Failure 1.7 1.7 + 6% medium web moisture content all produced a more Stretch, % permanently formed flute shape, as indicated by the Tensile Energy Ad- 89 105 + 18% lower measured spring-back force. Higher medium web sorption, J/sq.m tension increased the spring-back force. The data indi- MD Fold Endurance, 100 43 - 57% cate an interactive effect between the corrugating roll no. folds pressure and the medium web tension, and between the corrugating roll temperature and the medium web moisture content. The spring-back was independent of Figure 5.2 shows the effect of the medium web the medium web tension at corrugating roll pressures temperature and moisture content on the MD tensile above 13 kN/m. The medium web moisture variable strength and MD stretch at tensile failure. The medium had a greater magnitude effect on spring-back at higher moisture content is represented by the 50%, 70%, and corrugating roll temperatures, (81). 90% Relative Humidity levels used to condition the The data shown in Figure 5.4 were also generated samples prior to tensile testing. The three relative hu- using a concora test instrument and handsheets made midity levels correspond to average moisture content with corrugating medium pulp. In this experiment, the levels of 7.0%, 9.6%, and 13.9%, respectively. The spring-back force of the fluted medium was quantified tensile test instrument was equipped with a heating de- by measuring the final equilibrium length of the unsup- vice that allowed the test specimen to reach the desired ported, fluted test strip. The data are expressed as the test temperature in 1 second. The medium test speci- equilibrium length of the fluted strip as a percent of the mens were encased in aluminum foil to prevent mois- original, unfluted test specimen length. A higher per- ture loss in the specimen during heating and testing. cent value indicates a greater spring-back tendency. This experiment was designed to simulate the MD The data show that both a higher corrugating roll tem- tensile property changes that occur in the medium web perature and a higher medium web moisture content due to the preheating and the steam preconditioning on reduced the spring-back, (95). the corrugator. Figure 5.5 shows the effect of the medium web The data shown in Figure 5.2 are the average ef- moisture content during fluting on the MD tensile fect for three different commercial mediums. The data strength of the fluted medium. The tensile strength of demonstrate that the tensile strength of the medium the fluted medium decreases as the moisture content of decreases with increasing temperature and with increas- the medium web being fluted increases. This indicates ing moisture content. The MD tensile failure stretch that a higher medium web moisture content at the in- increases with increasing moisture content. The stretch feed side of the corrugating rolls makes the medium increases with increasing temperature up to about 220 web more prone to MD tensile failure during fluting, deg.F. and then decreases, (16). The author is not cer- (95). tain whether this is a real effect or an artifact of the Figure 5.6 shows the effect of the corrugating roll experimental technique. It is possible that the samples temperature on the concora strength of the fluted me- lost moisture at the higher temperature levels and, dium. The experimentation was done using a concora therefore, had a reduced measured stretch. test instrument and handsheets made from corrugating The next several references address the issue of the medium pulp. The concora strength of the fluted me- effect of the single-facer process variables on the char- dium increases with increasing corrugating roll tem- acteristics of the fluted medium. perature, (95). Figure 5.3 shows the effect of corrugating roll Figure 5.7 is a multiple regression equation which pressure, medium web tension, corrugating roll tem- correlates selected corrugating process variables to the perature, and medium moisture content on the spring- flat crush strength of the fluted medium. Since the ini- back of the flutes. Spring-back is indicative of the abil- tial unfluted medium concora strength appears in the ity of the paper to retain the molded flute shape. The regression equation and is a constant for a given me- 38 / Chapter 5 Medium Strength Loss By Fluting

II FIGURE 5.2 Effect of Temperature and Humidity on iI Medium MD Tensile and Stretch ill 11 Effect of Temperature and I Relative Humidity on Medium I i Machine Direction Tensile Strength (16) I I

h e -20 -.....

-80 -80-- - - - ,- ; -; 60 90 120 150 180 210 240 270 300 330 360 Temperature, Deg. F Iii Average of Three Different Mediums Ii

Effect of Temperature and Relative Humidity on Medium Machine Direction Tensile Stretch (16)

140 -

h t 100------a r n t 80- . *--\i,--I n - . _ g c ..... , ...-...., .... , e h 60 -\. , 4 0 - * /t...... 5...... i ......

n 0 90 120 150 180 210 240 270 300 330 360 60 90 120 150 180 210 240 270 300 330 360 Temperature, Deg. F Average of Three I Different Mediums I ------:.:., FIGURE 5.3 Effect of Corrugating Variables on Spring-back of Fluted Medium

Effect of Corrugating Roll Pressure Effect of Web Tension on the Spring-back Force of the on the Spring-back Force of the Fluted Medium, (81) Fluted Medium, (81)

400 ! 400- - - - 3 5 0 -1 - * * .- .. . - ...... 1- ...... S 350 ...... ;...... F ...... S F 3003 0 0 --- ...... b ' 310 60 - ...... r .o ...... r r 250 ...... - ...... I r 250 ..-- i . e 200 b ' 150 ' ...... 1e50 am 1 0 0 - ...... k 50- k 0-1------i 4 6 8 10 12 14 16 18 20 0 50 100 150 200 250 300 350 400 Corrugating Roll Pressure, kN/m Medium Web Tension, N/m 26 Ib/maf Medium 26 lb/mat Medium

Effect of Corrugating Roll Temperature Effect of Medium Moisture Content on the Spring-back Force of the on the Spring-back Force of the Fluted Medium, (81) Fluted Medium, (81)

-_ 400 - 400 - .- -_____~~ _- _ S 350 : . S 350 '

p F 3 0 0 p F 300 - r o . r o I r 250 I r 250 n c nc e 200-...... ne 200.... ' .- r .- **j'^^s. .. -... - j...... 0 b> ' 1501650 - . .*| ' i b 150- a m10oo0 c N ...... k 5 0 ...... k 50- . 'Q

80 90 100 110 120 130 140 150 160 170 180 190 200 2 4 6 8 10 12 14 Corrugating Roll Temperature, Deg. C Medium Moisture Content, % 26 lb/maf Medium 26 lb/mal Medium

.0 40 / Chapter 5 Medium Strength Loss By Fluting

!I i! FIGURE 5.4 Effect of Corrugator Process Variables on Fluted Medium Springback

Effect of Medium Moisture Content on Fluted Medium Spring-back, (95)

F 90 - .

u % 85 - ......

te 0: 5 : ii . d f 8 0 · ......

Lo 065

ng . g . 7 0 ......

h : ! 0 2 4 6 8 10 12 14 16 18 20 Medium Moisture Content, % Actual Data Not Shown 11

Effect of Corrugating Roll Temperature iI on Fluted Medium Spring-back, (95)

F Ii 90-

u % 85- t eO de f 80

L r 75 . . .. . - ...... ei n g g 70 . : t h 65- 10 20 30 40 50 60 70 80 90 100 110 120 Corrugating Roll Temperature, Deg.C Actual Data Not Shown FIGURE 5.5 Effect of Medium Web Moisture Content on MD Tensile of Fluted Medium, (95)

7.2 - F T 70-7.0 I e 6.8- n i 5 6.6 - e 1 6.4- d e 6.2

M S 6.0 t r 5.8- d i n 5.6- u g m t 5.4- h 5.2 ---

0 (lb per 5.0- 30

1/2 O~q inch) 2 4 6 8 10 12 14 16 18 20 O0

Medium Web Moisture Content (LQ(D During Fluting, % 0- Data Points Not Shown I

4t --- , --- ...... FIGURE 5.6 cF, Effect of Corrugating Roll Temperature N., on Medium Concora Strength. (95) t-

46-

q 44 -

C 42 - 0 n 40- c o r 38 - 7 a ' 36 I b 34-

32 -

30- --I 10 30 50 70 90 110 130 150 170 190 Corrugating Roll Temperature, Deg.C Actual Data Not Shown L =1 FIGURE 5.7 Effect of Corrugating Conditions and Medium Properties on Flat Crush. (16)

FC = a(A) + b(B) - C(c) 2 e(E) + f(F)- g(G) + h(H) - 37.5

a = 0.00168 r 2= 0.82 A = Speed, fpm b = 1.58 B = Nip Moisture Content, % c = 0.0443 C = B d = 0.292 D = Nip Temperature, Deg.F e = 0.00088 E = D f = 1.35 F = Roll Moisture Content, % g = 0.0854 G =F o0 OQ h - 0.555 H = Concora, lb 0Q

4a (D FC = Flat Crush, psi pi I

- 44 / Chapter 5 Medium Strength Loss By Fluting dium material, a higher calculated flat crush strength The test data show that the MD STFI compressive for the fluted medium represents an improvement of the strength (Flat Crush Test related) is reduced by an av- retention of this property after fluting. The data were erage 42% (35% to 50% range) by fluting the medium. generated on a pilot-size single-facer using commercial The CD STFI compressive strength (Edge Crush Test corrugating mediums. The regression equation indicates related) is reduced by an average 18% ( 15% to 20% that a higher medium moisture content and a higher range) by fluting the medium. The MD short-span medium temperature at the point of fluting are benefi- tensile strength was reduced by an average of 29% due cial for retaining flat crush strength, (16). to the fluting process. This is considerably less of a Figure 5.8 shows the relationship between the re- strength loss than the 42% loss in MD short-span com- tention of crush strength after fluting and properties of pressive strength. The thickness (caliper) direction the original medium. The crush retention is expressed tensile strength (ZDT) also shows a reduction in as the ratio of the fluted to unfluted medium compres- strength due to the fluting process, (30). These two ob- sive strength. While the paper is not clear, it appears the servations indicate that the loss in medium strength due crush strength referred to is the MD flat crush. The ex- to fluting is due mainly to the breakage of the fiber-to- perimental data were generated using 26 lb/msf com- fiber bonds. This debonding is apparently not fully re- mercial corrugating mediums, and 26 and 40 lb/msf covered by the transverse compression of the medium Formette handsheet mediums made from commercial at the tangential contact point between the two corru- medium furnishes. The corrugating was done on a pi- gating rolls, (see Figure 5.1). lot-size single-facer. A total of 19 different medium Figure 5.10 is a plot of the MD STFI compressive materials were tested. The results show that the com- strength of the fluted medium against the measured flat pressive strength retention of the medium, after fluting, crush of the same fluted medium. The good correlation is correlated to the MD elastic modulus, the ZD elastic between the two properties shows that the change in the modulus, the basis weight, and the density of the un- short-span STFI compression test values between the fluted corrugating medium. The crush strength reten- unfluted and fluted medium is a good predictor of flat tion is improved by a higher ZD modulus and density, crush strength change expected due to fluting, (30). and by a lower MD modulus, basis weight, and caliper, Figure 5.11 shows that the medium spring-back (29). measurement is a good predictor of the fluted medium Figure 5.9 is an experimentally generated regres- concora strength. This indicates that the effects of the sion equation which correlates unfluted medium prop- corrugating process variables on fluted medium spring- erties to the CD crush strength retention of the medium back shown in Figure 5.3 and Figure 5.4 should di- after fluting. The CD crush strength retention is ex- rectly translate into fluted concora strength effects, pressed as the ratio of the medium fluted edge crush to (95). the medium unfluted ring crush. A higher ratio is fa- Figure 5.12 shows the effect of the paper mill vorable. The equation is based on data generated by the process variables of pulp refining and paper machine corrugation of 26 lb/msf commercial mediums and wet pressing on the retention of MD flat crush strength formette handsheets, made from commercial medium after fluting on the corrugator. Figure 5.13 is a similar furnish, on a pilot-size single-facer. The medium fur- plot for the retention of CD edge crush strength. nishes evaluated were NSSC, Green Liquor, Caustic The data show that increased refining improved the Carbonate, and Recycled. The regression equation indi- retention of the MD flat crush potential of the original cates that the CD medium crush retention is improved medium by 3 percentage points, and increased the re- by a higher MD elastic modulus, a higher CD elastic tention of the CD edge crush potential of the original modulus, a lower density, and a higher in-plane poisson medium by 24 percentage points. Similarly, the data ratio, (1). show that increased wet pressing decreased the reten- The STFI short-span compression test instrument tion of the MD flat crush potential of the medium by 6 was used to measure the change in the compressive percentage points, and increased the retention of the strength of the medium resulting from the fluting op- CD edge crush potential of the medium by 5 percentage eration. The STFI test uses a specimen test span of 0.7 points. The data show that the combined densification mm. This small span allowed compression tests to be of the medium by refining and wet pressing improves measured on the sidewall areas of the flutes. The com- CD strength retention after fluting by 29 percentage pression values for the fluted material were compared points, and decreases the MD strength retention by 3 to those for the same corrugating medium before flut- percentage points, (15). ing. The fluting was done on a pilot-size single-facer The author is tempted to speculate on the implica- using four different 26 lb.msf commercial mediums, tions of these results with respect to the relative impor- (30). tance of fiber-to-fiber bonding versus fiber strength, but FIGURE 5.8 Effect of Medium Properties on Medium Crush Strength Retention, (29)

1.0

C Ex =MD Elastic Stiffness...... '...... r 0.9 E= Z ...... D.....i EzC'"S ZD . Elastic Stiffness.tif f n e S s.. u s BWt = Medium Basis Weight. h 0.8 R e 0.7 t e n 0.6 t ...... i '''''''''r 0.86: ''' ''' ''' '' o 0.5 n (units) 0.4 I 0 0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 ((Ex/Ez)(BWt)/(Density) frq

01K Actual Data Points UQv2 0.CD Not Shown 5' B 4h, A" I FIGURE 5.9 Effect of Medium Elastic Properties on Retention il of CD Crush Strength After Fluting, (1) 1

w0 CR = 1.35 - a(1/A) - (b + c(C))(1/D) - e(1/E) + f(F)(G)

r 2 = 0.636 CR = (Fluted Edge Crush)/(CD Ring Crush), Both In lb/6 inch

a = 0.385 A = MD Modulus, GPa b = 0.326 c = 0.365 C = Density, g/cubic cm D = CD Modulus, GPa

e = 0.0060 E = ZD Modulus, GPa f = 0.00037 F = XY Poisson Ratio

G = Corrugating Speed, fpm

==z ...... - FIGURE 5.10 Relationship of MD STFI Compression of Fluted Medium to Flat Crush, (30)

F 37 I a t 35 C r u 33 s h 31 p s i 29

0 27 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 MD STFI Compression of Fluted Medium, !:' (Ig 26 Ib/msf Medium kN/m j -i- ...... --.------y -P, CD ob FIGURE 5.11 Relationship Between Medium Spring-back o.SU )-- r,3 3 and Medium Concora Strenath. (95) to 0 w w(A 1< Itn CT 44 M OQ e d 42 i u m 4 0 C o 38 n c ° 36 r a ' 34

b 32 68 70 72 74 76 78 80 82 84 86 88 Medium Fluted Springback, % of Original Fluted Length Both Varied By Changing Corrugating Roll Temperature. FIGURE 5.12 Effect of Refining and Wet Pressing on Flat Crush Retention After Fluting. (15)

M 0.50 D C 0.45 r u s 0.40 h R 0.35 a i 0.30

* 0.25

0.20 400/Low 400/Medium 325/Low 325/Medium

Process Condition, Freeness/Wet Pressing (' No0 * Flat Crush (psi) Divided By Concora Crush (lb) Jo O FIGURE 5.13 iD Effect of Refining and Wet Pressing on Edge Crush Retention After Fluting e-t 0 w

5' 1.4 " 0Q C ---- D 1.2 - 1. At 50% RH * At 90% RH C r 1- 40 Ib/msf Medium u s h 0.8-

R 0.6- a t i 0.4- 0 * 0.2-

400/Low 400/Medium 325/Low 325/Medium Process Condition, Freeness/Wet Pressing * ECT (lb/inch) Divided By CD Ring Crush (lb/6 inch) ------Corrugating Medium / 51

he will leave this analysis to the reader. It has been suggested that the medium needs to have adequate in- Table 5.2. Major Corrugating Process Variables terfiber bonding and fiber flexibility, good plasticity and Medium Properties Affecting Medium Strength with heat and moisture, and good formation, (95). Retention After Fluting The paper mills have been moving in the direction of increased wet pressing to improve the medium CD Direction of and MD crush strength and to improve the paper ma- Change in chine productivity. Wet pressing improves both MD Variable to and CD average crush strength of the medium rollstock. Improve the The data shown in Figure 5.12 and Figure 5.13 sug- Strength Re- gest, however, that the relative strength improvement Variable tention References actually achieved in the corrugated board, due to wet pressing effects, will be less than supposed for flat Corrugating Increase 68, 81 crush and greater than supposed for CD edge crush. Roll Pressure This may explain some of the anecdotal experiences Medium Web Increase 16, 38, 68, 81, heard from the box plant personnel about some of the Temperature 95 new "high crush" medium grades that are being offered. Medium Web Increase 16, 81, 95 Moisture Some statements have been offered that the ECT is fine Content but that the Flat Crush is low. Medium Web Decrease 68, 81 The major corrugating process and corrugating Tension medium properties affecting the strength retention in Medium Increase 16, 29, 31 the corrugating medium after fluting are summarized in Tensile Table 5.2. Strength The summary of the data discussed in this chapter MD Tensile Increase 16 indicates that more of the inherent strength of the cor- Stretch at rugating medium will be retained in the corrugated Failure board by employing the proper control of selected sin- Medium Increase 29, 38 gle-facer process conditions. These are the major proc- Density ess factors which enhance the flute molding character- Medium Cali- Decrease 15, 29, 38 istics of the corrugating medium. The information in per the literature identifies these major process factors as heat, moisture, forming pressure and web tension. Im- provement in strength retention after fluting can also be affected by those medium properties which minimize the flute forming stresses. The information in the literature identifies these major material properties as reduced caliper and increased stretch at tensile failure. There was significant debate in the industry, during the consideration of Alternate Item 222/Rule 41, concerning the "correct" quantitative relationship between the linerboard and medium component CD crush- strength and the combined board Edge Crush Test. It seemed that each person had his/her own equation. It is interesting to speculate on how much of the disagreement can be attributed to the effect of the fluting process variables discussed in this chapter. The same type of conjecture can be made as to disagreement over predicting the flat crush strength of combined board from the medium concora strength. Chapter 6

Bonding Theory

Bonding or adhesion is simply the process of hav- The bonding theory that will be discussed applies ing one material stick to another material, (119). For equally to both the single-face bond and to the double- the corrugator bond, this involves having the tips of the face bond. The one exception is the type and duration fluted medium stick to the linerboard facings at two of the pressure applied to the bonded area as the bond locations, the single-facer and the double-backer. These sets. The single-face bond is under pressure only at the bonds make the combined material a corrugated struc- nip between the lower corrugating roll and the pressure ture and the package a corrugated box. Without the roll in the single-facer for a fraction of a millisecond. bonds, the package would be a three-ply multiwall bag. The double-face bond is under lower pressure for a The objective of the corrugator bonding process is to minimum of 6 seconds as the board passes through the obtain a consistent, strong, tough (not brittle) bond that hot plate and cooling sections of the double-backer. will lower waste costs, increase corrugator productivity, and provide a more consistent performance package product, (12, 14, 47, 71, 79, 83). The corrugator pro- Table 6.1. Seven Major Steps in the Corrugator ductivity is affected by the bonding process since the Bonding Process corrugator speed is generally governed by the rate of adhesive tack development and the rate of bond strength development, (12, 67, 76). 1. Select Adhesive. Some people have suggested a fourth objective, specifically, to minimize starch consumption and cost. 2. Select and Treat Substrate (Heat, Moisture). I They point out that the application of too much adhesive can cause poor print quality if the combined board 3. Apply Adhesive to Substrate. exhibits washboarding, and can promote excessive crushing if the medium is still damp during slotting and 4. Adhesive "Wets" the Substrate. scoring, (14, 71, 79, 83). A thicker adhesive application also increases the bond setting time, (148). The author 5. Adhesive Penetrates the Substrate. agrees that an application of an overabundance of adhesive should be avoided for the reasons stated above 6. Adhesive Starts to Set (Green-Bond). and also because of the adverse effect on warp. How- ever, the starch adhesive is one of the least expensive 7. Cured Bond Forms. materials used in a box plant. The author believes that attempts to absolutely minimize the adhesive application and thereby save pennies will produce dollars of A silicate adhesive was used for bonding on the added cost due to excessive box plant waste and poor corrugator from 1879 until the mid-1940s, at which package performance time a starch-based adhesive replaced the silicate adhe- Studies have shown that the formation of the cor- sive. The starch-based adhesive is still in use today. On rugator bond is a complex process that is affected by average, the starch adhesive slurry contains 78% water, the properties of the paper and adhesive, and by the 21% starch, 0.5% borax, and 0.5% caustic. The starch corrugator process variables of heat, moisture, pres- portion consists of 3 percentage points of cooked, pre- sure, and speed (time), (20, 21, 22, 47). The corrugator gelled carrier starch and 18 percentage points of un- bonding process can be described by seven major steps, cooked, raw pearl starch. One function of the carrier as shown in Table 6.1, (21, 24, 35, 47, 67, 119, 148). starch is to hold the pearl starch granules in suspension, 54 / Chapter 6 Bonding Theory

(45, 67, 76). The quality of the adhesive slurry is gen- 591 and TAPPI T-419. The other method utilizes a co- erally controlled by measuring its solids content, its balt tracer in the adhesive, (71, 79, 83). Stein-Hall cup viscosity, and its gel point. It is interest- A liquid adhesive needs to wet the surface of the ing to note that an adhesive application rate of 2.2 substrate materials and to increase the area of contact lb/msf starch solids on a 42-26-42 lb/msf C-flute con- by spreading over the surface and flowing into the sub- struction adds 6.7% water to the combined board. strate pores in order to form a strong bond. (18, 24, The substrates consist of the single-face linerboard, 119, 148). The relative ability of a liquid to wet the the fluted corrugating medium web, and the double- substrate is determined by measuring the contact angle face linerboard. The temperature of the three webs is of a bead of the liquid against the substrate when con- controlled by the use of linerboard, medium and single- tact is first made. The contact angle is the angle be- faced web preheaters, by the heated corrugating rolls tween the flat substrate and the bottom of the adhesive and pressure roll, and by the hot plates of the double- bead surface touching the substrate. A contact angle of backer. The moisture is affected by the various heating 0 deg. indicates a spontaneous wetting and spreading of sections previously mentioned and by the steam show- the liquid over the substrate surface. A contact angle of ers and the water associated with the adhesive slurry. 90 deg. or greater indicates that the liquid will not wet In general, the adhesive slurry is applied to the tips the substrate surface and, therefore, will not bond to the of the fluted medium by an applicator roll. The film substrate. Contact angles less than 90 deg. indicate thickness on the applicator roll is controlled by the use varying wetting potentials, (24). The contact angle is of a metering roll. The adhesive film thickness on the reduced by a lower viscosity and/or a lower internal applicator roll is affected by gap setting between the free energy of the adhesive, (67, 76). two rolls, by the ratio of the surface speeds of the me- Figure 6.1 shows a stylized representation of the tering roll to applicator roll, by the corrugator speed, stages that a corrugator starch adhesive goes through and by the interaction of the ratio and the corrugator from the time that it is applied to the flute tip until the speed, (47, 57, 60, 61, 71, 79, 83). There is a critical time that the final cured adhesive is formed. The plot is roll speed ratio at which the adhesive film thickness is shown in terms of the energy required per pound of independent of the corrugator speed. This critical speed adhesive slurry and the temperature of the adhesive. ratio is related to the high shear viscosity of the adhe- The first 60 BTU, or 8.3% of the total process energy is sive. The Stein-Hall cup viscosity, commonly used in required to raise the adhesive temperature from ambient box plants for adhesive quality control purposes, does to the gel point. This energy is for an adhesive with a not measure the high shear viscosity of the adhesive, gel point of 159 deg.F. A lower gel point adhesive (47, 60, 61). An instrument is available to measure the would require less energy input in this stage, and a high shear viscosity of the starch adhesive slurry in the higher gel point adhesive would require more energy box plant environment, (61). input. The temperature then remains constant as the raw Adhesive film instability patterns can also occur on pearl starch granules in the adhesive slurry gel. The the applicator roll surface. The two most common types gelling requires 20 BTU or 2.8% of the total process are "ringing," which consists of bands of varying adhe- energy. The temperature of the adhesive then increases sive film thicknesses running around the circumference to the 212 deg.F. vaporization temperature of water by of the applicator roll and a "mottle" pattern, (70). Run- absorbing 62 BTU, or 8.6% of the total process energy. l out in the applicator roll or in the metering roll, due to The water then evaporates from the bond area until the defective bearings or bent shafts, will cause the adhe- final dry, cured bond forms. This evaporation requires sive film thickness to vary around the circumference of 582 BTU, or 80.4% of the total process energy, (24). the applicator roll. This defect is often visible as a re- This type of energy plot does not directly address petitive "strobe-like" barring on the surface of the the development of the actual strength of the bond. The < turning applicator roll. The ringing and mottle defects bond strength development stages are shown in Figure also are generally visible on the turning applicator roll. 6.2, where bonding time is plotted against the adhesive Adhesive consumption determinations are made in viscosity. The adhesive viscosity is indicative of the the box plant by measuring the volume of adhesive tackiness of the adhesive. A higher viscosity has more slurry consumed for a known area of several thousand tack or stickiness. The representation is not shown to square feet of corrugated board. This method yields an scale. average application rate but does not provide informa- The adhesive viscosity actually decreases for a tion on application variability due to the instability short period of time as the temperature of the slurry patterns described above. Two laboratory starch adhe- increases, (45). The viscosity of free water decreases sive measurement techniques are available for quantify- with increasing temperature. Once the gel temperature ing the smaller pattern variability in adhesive applica- has been reached, the uncooked raw starch granules tion. An iodide/iodine method is described in ASTM D- begin to absorb water and gelatinize; the viscosity and -W

FIGURE 6.1 Bond Setting of Corrugator Starch Adhesive, (24)

275 T e 250 m 225 p e 200 r a 175 t u 150 r e 125 100 D 75 e g 50 25 F n 0 0 0 100 200 300 400 500 600 700 800

Heat Energy, BTU/lb Adhesive ($I

LA LA1 A -

FIGURE 6.2 O 0O 0'(' Green-Bond Development Theory. (45) qz a,"

Wetting, Green-Bond Drying Diffusion, Formation Cured - Bond A V Forming d i h S e c Adhesive X Observed s o Fiber i s Tear Raw v i Starch e t Gelling y

Bonding Time

- Corrugating Medium / 57 internal adhesion strength begins to increase rapidly; two components, the paper and the adhesive, as a func- and the adhesive begins to become tacky. This is the tion of time. The internal self-bonding strength of the bond formation stage commonly referred to as the adhesive gradually increases with time as the adhesive "green-bond," (21, 24, 35, 45, 67, 76, 119, 148). The passes through the five stages described above. The green-bond can be defined as the point where the paper, on the other hand, loses some of its internal self- "thermoplastic" adhesive film can withstand the shear bonding strength as the water from the adhesive wets and tensile forces due to the mechanical stresses in- the paper and disrupts the fiber-to-fiber bonding. At duced by the corrugating equipment, (45). The start of some stage, the internal strength of the adhesive ex- the green-bond stage on one corrugator may or may not ceeds the strength of the paper and fiber tear occurs. correspond to that of another corrugator, depending on The top graph in Figure 6.3 represents a "good" bond their relative mechanical condition. situation. The adhesive strength remains above that of The viscosity increase that occurs at the gel point is the paper in the cured bond stage of the process. The associated with several different mechanisms. First, the bottom graph depicts a "false good" bond situation. The conversion of the raw starch into cooked or gelatinized adhesive strength exceeds the paper strength for some starch increases the viscosity of the total adhesive. Sec- period in the green-bond region but does not exceed the ond, the cooked carrier portion of the original adhesive paper strength in the cured bond region. The cured slurry can form a real adhesive film. Third, the raw bond in this case would exhibit an adhesive bond fail- starch acts as a "water sink" as it absorbs several times ure mode or, as commonly stated in the industry a its weight in water as it swells. This reduces the free "brittle bond," (18, 24, 67, 76, 119). It has been sug- water content of the total adhesive and increases its gested that this type of "false good" bond condition is total viscosity, (45, 67, 76). Fourth, when the raw increasing in the field with the advent of the highly starch absorbs water, the cooked carrier starch releases densified, high crush strength paper grades and with the some of its originally bound water, due to equilibrium increased use of recycled fiber. It has been reported that forces. The carrier starch concentration increases, and it the "brittle bond" is not seen in the sheets exiting the releases film forming lower molecular weight starches corrugator but appears after the sheets have remained in to the free adhesive liquid. Fifth, the gelatinization of the stacks for a period of time and have dried out, (11). the raw pearl starch releases lower molecular weight Figure 6.4 shows the effect of paper moisture starch molecules into solution. Under the correct heat content on the bonding time required to achieve fiber and moisture conditions, these low molecular weight tear. The data are based on a dextrin adhesive that sets fractions can form an adhesive film, (67, 76). Micro- solely through the loss of moisture from the adhesive. scopic examination of the cured glue lines has shown The data show that the time to observed fiber tear in- that various areas of the bond contain different propor- creases as the moisture content of the paper increases. tions of gelled and ungelled starch granules, varying This is caused by the reduction in the rate of moisture starch macromolecule structures, and differing starch migration away from the glueline, (148). It is not un- molecular weight fractions, (21, 35, 48, 102, 117). Both common for a corrugator crew to examine the double- the carrier and raw starch portions of the original starch back bond at the edges of the corrugator and to see adhesive slurry are important to the bond formation separation of the bond in the adhesive. The adhesive process, (67, 76). tends to be thick and tacky, and the crew assumes that The final stage of bond development involves the the bond will "setup" in the stacks. This assumption removal of water from the bond site either by evapora- may or may not be correct. It may require considerable tion or by migration of the water into the paper, (21, 35, time before the true quality of the cured bond is known. 67, 76, 119, 148). Once this has occurred, the final Figure 6.5 is a representation of the corrugator cured bond is formed and cannot get any stronger. Most bond failure patterns observed in a large number of people in the industry qualitatively judge the bond samples of commercial corrugated board. The nature of quality and potential strength by tearing apart a sample the failure modes are related to the measured pin adhe- of combined board and looking for fiber tear. A deep sion strength of the cured bonds. Bond failure can oc- linerboard fiber tear is considered especially desirable. cur in five different zones, within the glue film (brittle Medium fiber tear or decapping of the flute tips is gen- bond), at the glue/medium interface (little fiber pull), erally considered less desirable bond failure modes. It within the medium material (decapping), at the is felt that the bond can be no stronger than the ZD glue/linerboard interface (little fiber pull), and within tensile strength of the linerboard. In fact, this may or the linerboard material (generally considered a good, may not be true depending on the moisture content of "tough" bond). The data indicate that a minimum pin the sample at the time that the inspection is made. adhesion strength, using TAPPI sample conditioning The qualitative bond strength development curves and testing methods, of 6.5 psi is needed to generate in Figure 6.3 show the internal adhesion strength of the linerboard fiber pull failure, (14). 58 / Chapter 6 Bonding Theory

I I

I i FIGURE 6.3 I I. !II 11 I Bond Development Theory 11 I Failure Modes of Cured Bonds I iI Bonding Development Theory, (24) I, 'Paper Failure' Mode of Cured Bond

Green-Bond Area Adhesive B i 0 n d Paper

S i t r e 1 / Cured Bond Area n //

t h

Time

Ii Bonding Development Theory, (24) 'Adhesive Failure' Mode of Cured Bond

Green-Bond Area B 0 n Paper d

S .--- / Adhesive t r e Cured Bond Area_, n g t

Time

ii I iI _j FIGURE 6.4 Effect of Paper Moisture Content on Time to Fiber Tear, (148)

80 F 70 T b e 60 r m e T 50 e a T 40 0 r 30 s e c 20

10 0

3 4 5 6 7 8 9 10 11 12 13 ('39Oq Paper Moisture Content, % (IQ0 2 Bleached Linerboard CD & Dextrin Adhesive So

LtA I'D .1--- . O or

FIGURE 6.5 5"E5, 0 Single-Facer Bond Failure Modes. (14) D "! q a,

P 10.0 i n 9.0 A

d ...... / .... h 8.0 e S 7.0 i 0 n 6.0 e - I 5.0 I Failure Mo0e Key b / 4.0 .Glue Film. sS 2 = Glue/Medium Interface. q 3 - Medium. 3.0 4 a Glue/Linerbo ard interface i n 5 = Linerboard. 2.0 I 1 2 3 4 5 Bond Failure Mode --- 1 Corrugating Medium / 61

The effect of medium properties on the volume of the flute tip area is reduced, on average, by 35%, as is adhesive transferred to the medium from the applicator shown in Figure 6.8. This compression may affect the roll is shown in Figure 6.6. The experiment was con- receptivity of the medium to the adhesive and the ducted using a bench top laboratory device. Three 26 penetration of the adhesive into the medium. lb/msf commercial corrugating mediums exhibiting The same three medium materials that were used to different smoothness and liquid receptivity characteris- generate the data shown in Figures 6.6 and 6.7 were tics were selected for testing. The Roughness Index, calendered using heated steel rolls. A caliper reduction Receptivity Index, and the Void Volume values for the of 32% was achieved. This calendering did not appre- three mediums are given in Figure 6.6. The Roughness ciably change the Receptivity Index of the three mate- Index varied by a ratio of 3:1; the Receptivity Index rials, but it did increase the smoothness. After calender- varied by a ratio of 12:1; and the Void Volume varied ing, all three materials had approximately the same by 4.8%. smoothness. The effect of the calendering on the adhe- The data show that the adhesive volume transferred sive transfer to the medium and on the penetration of to the non-receptive medium did not increase with the starch into the medium is shown in Figure 6.9. The contact time after the first I second. This can be attrib- nonreceptive medium still picked up less adhesive than uted to the lack of sufficient wetting of the medium the receptive mediums. The percentage difference was surface by the adhesive. The two receptive mediums the same as was observed for the uncalendered medium showed increases in adhesive transfer with increasing samples. contact time, and the rate of increase was approxi- All three calendered mediums showed much higher mately equal for the two (equal correlation line slopes). percentages of the starch located closer to the surface of The smoother sample, however, picked up 4.5 units less the medium than was observed for the respective uncal- adhesive volume at all contact times, (104, 120). These endered samples, (117). The results indicate that the results suggest that the adhesive application to a recep- caliper reduction which occurs at the flute tips does not tive medium, at constant glue station settings, will in- affect the volume of adhesive transferred from the glue crease as the corrugator speed decreases (increased roll to the flute tip. Receptivity tests on various medium contact time with adhesive film on the glue roll). At rollstock materials will be indicative of the relative per- constant corrugator conditions, a rougher surface me- formance, with respect to adhesive application, that is dium will have more adhesive transferred to it from the expected for the materials in the actual corrugating glue roll than will a smoother surface medium, pro- process. The compaction at the flute tip during fluting vided both are equal in receptivity, (18, 47, 119, 120). does reduce the adhesive penetration into the medium The penetration of the adhesive into the medium is and negates the effect any surface smoothness differ- I shown in Figure 6.7 for the same experimental materi- ences measured for different uncorrugated medium als and conditions shown in Figure 6.6. The data materials. shown are based on the average values for the com- Other researchers have investigated the starch bined 0.06 and 0.73 contact times. The bars represent penetration into the medium component of commer- the cumulative percent of total adhesive between the cially produced corrugated board. The results of these described depth into the medium and the application studies indicate that most of the starch remains near the surface of the medium. The starch content data were surface. Only the cooked starch components penetrate generated by surface grinding of the medium samples the medium to any great extent, (11, 14, 117). The re- and by the TAPPI T-419 starch analysis method. All sults of starch penetration analysis on commercial cor- three samples showed no starch presence beyond a rugated board are summarized in Table 6.2, (14). These depth of 6 mils. The only major difference in starch studies confirm the laboratory results discussed above, penetration between the different medium samples oc- (117, 120). curs at the 1.2 mils depth. The smoother surface sample These experimental results serve to support the had more surface starch, (120). It must be kept in mind prior conclusion that the depth of starch penetration that a deeper penetration of starch does not necessarily into the corrugating medium at the bond site is not a mean an improved bond strength. The starch deeper strong indicator of a good, strong, tough bond. Micro- inside the medium may be low molecular weight com- scopic examination of the single-facer bond in components which do not contribute to bonding. mercially produced corrugated board shows three zones The data shown in Figures 6.6 and 6.7 are based containing adhesive. The first zone is at the flute tip; on the medium material before it has gone through the the second zone is on the exterior of the flute sidewall; fluting process. As was discussed in Chapter 2, the and the third zone is in the fillet region between the medium at the flute tip, where the adhesive is eventu- medium flute and the linerboard facing. The adhesive at ally applied, is permanently compressed by the action the flute tip contains both cooked and uncooked starch. of the corrugating rolls. The caliper of the medium in The uncooked starch is present because of the lack of -7

w~bs/,D 'wfl!pa8LAJ ' dn PGei3!d OA!SGIPV 'Oes 'awilj JOJ0 91 t O1v OTTZ O'Z 91 0L TO0 0,0 i I I- I I I

a w ...... IIleH8I . * B/ooT 'ownIQA P!OA n egg, VLO. 9 m.x~pul Al!A~ideboet L 0 - xapuI ssauqJ~nou '-9L A

...... a A

GA!jda9O9-uON/q6flot - .- 91. I a GA!1d~o3eU/LqfloU ...... p GAlldao3U/LUoowS - V

(OZI-) wnipE)V 04 jolsuej1 GAISeLIPV UO SO!1HJaOid wfipaey lo 408fl3 (N 0 909 3uflou - -- ...... 41 FIGURE 6.7 Effect of Medium Properties on Adhesive Penetration Into Medium. (120)

A d 120- h e s 100 - i V e 80--

60- A b 0 40-- V e D 20 -- e I P 0 t 0- O h 1.2 2.4 3.6 4.8 6.0 00 Depth Into Medium, mils 0\ GQ'D pi

El FIGURE 6.8 O a

Permanent Transverse (ZD) Compression |^o-t of Medium During Fluting. (145) S, o

C 11 a I i 10 P ep 9 r 8

m 7 i $ 6

4 Original Leading Tip Trailing Sidewall Sidewall Average of A, B & Flute Profile Position C-Flute Results. Corrugating Medium / 65

Ii i

FIGURE 6.9 I

I; Effect of Medium Calendering on Adhesive I

Ii I Transfer and Penetration I I I Effect of Medium Flute Tip Compression on Adhesive Transfer, (117)

26 - A 2 4 -

dAd 242 ...... | 1...... Origina. lE Calendered

T 16 ...

Calendering Resulted In A 32% Caliper Reduction Caliper Reduction

Effect of Medium Flute Tip Compression on Adhesive Penetration, (117)

I 140 -Calendering Reduced Caliper By 32%

p 120 - .. 1 Original 1 Calendered A e ... -.-.-.--...---...--...... Withind 2.4100 mils - Of The Surface.

e t 8 0 ...... I S r 6 t

n 20 ...... 0 Smooth/Receptive Rough/Receptive Rough/Non-Receptive Original Medium Characteristics - Percent Of Total Applied Adhesive Within 2.4 mils Of The Surface

11H -1 66 / Chapter 6 Bonding Theory

sufficient water needed to gel the starch. The water is lacking in this area due to the mechanical dewatering caused by the pressure roll nip. This flute tip bond region has very little cured bond strength. It is thought, however, that this region provides some green-bond strength. The adhesive on the sidewalls of the flute do not contribute to the bonding since it is not in contact with the linerboard. The adhesive in the fillet zone is almost 100% gelled and provides the major portion of the total, cured bond strength, (18, 21, 35, 67, 76, 102).

Table 6.2. Adhesive Starch Penetration Into Me- dium in Commercially Corrugated Board, (14)

Distance Into Medium Description of Starch (mils) Observed

0 Thick Surface Film 1 Abundant Starch 2 Few Starch Granules Visible 4 Trace of Solubilized Starch 6 No Starch

In summary, the formation of the corrugator bonds is a complex process. Obtaining uniformly strong and tough bonds requires the consideration of the following parameters. The influences of the linerboard facing material and the corrugator speed are not included.

1. The properties of the adhesive, particularly the solids content, gel point, and high-shear viscosity (not Stein-Hall viscosity).

2. The mechanical condition of the adhesive application system, particularly roll run-out and the ta- pered speed controls.

3. The properties of the medium, especially its receptivity to the adhesive slurry. This receptivity can be characterized by surface energy measurements.

4. The moisture content of the medium at the point in the process where the adhesive is applied.

5. The heat and pressure applied to the bond area.

6. The mechanical condition of the corrugator with respect to the shear and tensile forces transmitted to the green-bond areas. Chapter 7

Single-Face Bonding

The quality of the single-face bond is important The single-faced web is exposed to many shear and because of its potential effect on the box plant waste tensile forces during its trip to the double-backer. The costs, on the box plant productivity, and on the per- web is exposed to flutter as it moves from the pressure formance of the corrugated package, (12, 14, 24, 47, roll nip, up the elevator, to the bridge. The web is then 67, 71, 76, 79, 83). The development of the single-face fan folded on itself as it enters the bridge and is pulled, bond is a complex process that involves the properties with fits and starts, until it is unfolded at the web guide of the medium, the corrugating process conditions, and and enters the double-backer glue machine. All in all, it the mechanical condition of the corrugator, (20, 21, 22, is a very bumpy ride. The single-face bond cannot be 24, 35, 47, 67, 119, 148). reformed if it separates due to these mechanical forces. Some people have suggested that minimizing the Any such separation produces defects commonly called consumption and cost of the starch adhesive should also "blisters," "fluff-out," and "loose edges," (3, 8, 10). be a top priority objective of the bonding process, (14, The development of the single-face bond strength 71, 79, 83). The author takes exception to this philoso- is a continuous process starting when the adhesive is phy. The starch adhesive is one of the least costly ma- applied and ending when the finished combined board terials used in a box plant. The objective should be to leaves the corrugator and reaches temperature and control the bond strength at a high and uniform level, moisture equilibrium with the ambient environment. (47). The corrugator bond strength directly affects the The initial stages of the bond development, before sig- compressive strength of the corrugated board. A 10% nificant fiber tear is detected, is commonly referred to reduction in pin adhesion strength produces a 3.3% loss as the "green-bond" stage. Once general fiber tear is in the Edge Crush Test of the combined board and a observed, it is the practice to refer to it as the "cured" 2.5% loss in top-to-bottom box compression, (5). It is bond. However, as discussed in Chapter 6, the differ- the opinion of the author that an overabundance of ad- entiation between the green-bond stage and the cured hesive should not be used because of its adverse effect bond stage is not that simple. Fiber tear may be ob- on warp and other properties of the corrugated sheet, served in the green-bond stage due to the wetting and however, a miserly application rate should also not be disruption of the fiber-to-fiber bonds of the paperboard used. Too little adhesive will increase the probability of by the water present in the adhesive slurry. This is a waste and poor package performance. It is the author's "false" cured bond. opinion that trying to operate at single-facer dry adhe- The objective of the single-face bonding process sive application levels much below 1.0 lb/msf is penny- should be to obtain a balance between the two stages, wise and dollar foolish. (14). Neither a good green-bond strength that results in The single-face corrugator bond begins its devel- a corrugated product with a weak, brittle cured bond, opment the instant that the starch adhesive slurry is nor a corrugated product with a strong, tough cured applied to the tip of the medium flute. The adhesive- bond, but blistered areas due to a weak green-bond covered flute tip is then brought into contact with the separation is acceptable. single-face linerboard web in the nip between the lower The mechanisms involved in the development of corrugating roll and the pressure roll. The bond exiting the single-face bond are discussed in detail in Chapter this nip must be of sufficient strength to hold the two 6. This chapter deals with the available technical in- paper webs together until the final, cured bond has formation that relates the effect of the medium physical formed. The bond site is not exposed to any additional properties to the strength of the green-bond and cured heat or pressure until it enters the double-backer, (3, 8, bond stages. The effect of medium preconditioning is 10). also included. 68 / Chapter 7 Single-Face Bonding

A typical green-bond strength development curve, the different medium materials. observed for the initial stages of the bond forming These results show that the use of proper medium process, is shown in Figure 7.1. The bonding time preconditioning levels can overcome most of the effect shown covers the range from 0 milliseconds to 200 of differences in the medium material properties on the milliseconds after the single-faced web has left the green-bond strength development. This emphasizes the pressure roll nip. The green-bond strength was quanti- importance of the proper maintenance and proper utili- fied by applying a mild, constant, mechanical stress to zation of the medium preheaters and steam showers in the edges of the web after it exited the pressure roll nip. the single-facer process. This is especially true during The stress causes varying degrees of debonding to oc- speed changes on the corrugator. The preconditioning cur, originating at the edges of the web and extending should be reduced simultaneously with reducing the toward the centerline of the web, depending on the corrugator speed and increased simultaneously with strength of the green-bond. A strong green-bond results increasing speed, (8, 10). in no debonding. A very weak green-bond results in The green-bond strength measurement technique complete separation of the linerboard from the medium. described above was used to determine the relative ef- Intermediate strength green-bonds result in varying fect of the medium and linerboard moisture contents, degrees of separation. The experiments were conducted the medium and linerboard temperatures, and the on a pilot-size corrugator, (3, 8, 10). bonding time on the green-bond strength development. Figure 7.1 shows the bonded area plotted against The medium web's temperature and moisture content the bonding time. The bonding curve can be character- were measured with sensors located as close as possible ized by four parameters. The induction time is the to the in-feed side of the corrugating rolls. The bonding elapsed bonding time before any measurable bond time was varied by maintaining a constant corrugator strength is detected by the methods used in this study. speed and moving the debonding stress device to dif- This does not imply that there is no bond strength at all ferent distances from the pressure roll nip, (3). during this time interval, only that any bond strength Figure 7.3 shows the multiple regression equation that exists is too weak to resist the mild mechanical relating the green-bond strength, as indicated by the force that was applied to the web. Once a measurable percent bonded area, to the variables of moisture con- bond develops, the strength of the bond increases line- tent, temperature, and time. The effect of all five vari- arly with time. The slope of this linear section of the ables was statistically significant at greater than the bonding curve is called the bonding rate. The induction 99% probability level. The equation constants indicate time and the bonding rate can be combined to produce that the medium moisture content has more of an effect a calculated minimum time to a 100% bond. The than the medium temperature. However, this is an arti- bonding time in these experiments was varied by fact of the difference in the numerical values of the two changing the corrugator speed. The decrease in the ob- variables. The absolute numerical value for the me- served bond strength in some samples, after the maxi- dium temperature is about 25 times the numerical value mum was reached, is attributed to the overheating and of the medium moisture content, 188 deg.F. versus crystallization of the starch adhesive at the slower cor- 5.5% MC. Figure 7.4 shows the relative impact of the rugator speed, (3, 8, 10). The green-bond strength de- five variables based on a constant 10% change in the velopment is improved by a shorter induction time and variable. On this basis, the medium temperature has a higher bonding rate. approximately three times more of an effect than the Figure 7.2 shows the measured induction time, medium moisture content,. and the bonding time has bonding rate, and calculated time to 100% bond for the approximately twice the effect of the medium moisture 15 commercial mediums evaluated. The mediums in- content, (3) cluded NSSC, Caustic Carbonate, Green Liquor, and Figure 7.5 shows a regression equation which re- Recycled materials. The green-bond development char- lates the cured bond pin adhesion to a number of me- acteristics were measured for each medium at two dif- dium properties and single-facer process conditions. ferent medium preconditioning levels, normal and re- The equation contains 11 terms, all of which were duced. The data show a relatively wide variability in found to be statistically significant at the 95% or green-bond strength development for the 15 mediums greater probability level. The equation contains inter- under reduced preconditioning levels. Increasing the active terms which make a direct comparison of the preconditioning level to normal reduced the induction relative effect of each variable difficult to quantify. In time by an average of 16%, increased the bonding rate general, however, the data indicate that the cured bond by an average of 47%, and decreased the calculated pin adhesion strength increases as corrugator speed time to 100% bond by an average of 26%. The added decreases, as the medium nip temperature decreases, preconditioning also reduced the magnitude of the dif- and as the nip moisture content increases, (16). ferences in green-bond development observed between Figures 7.6 and 7.7 show that the cured bond pin FIGURE 7.1 SF Green-Bond Development Curve, (8, 10)

100

90 B 80 0 n 70 d e 60 d 50 A r e 40 a 30 % 20 10

0 0 0 20 40 60 80 100 120 140 160 180 200 Bonding Time, milliseconds

I' (Varied By Changing Corrugator Speed) TO 70 / Chapter 7 Single-Face Bonding

Ii il il iI FIGURE 7.2 i il 11 Ii Effect of Medium Preconditioning qI I 11 on Green-Bond Development I! I

11 I !I I Effect of Preconditioning on the ii,;I Green-Bond Development - Subnormal I i Medium Preconditioning, (8, 10) 1 - Caustic Carbonate Medium 2 * NSSC Medium I *3Green Liquor Medium 4 - Recycled Medium 11 150 i 135- ....

T 1 1203- M Induct. Time ~ Bonding Rate I i 0 105- .-. . iI e 7 5 ...... i B 60 - I d 30 I (m s) 5 1 i 10e 2 3 1 4 I1 1 4 1 2 3 4 2 3 Medium Material i

I Effect of Preconditioning on the i I Green-Bond Development - Normal II Medium Preconditioning, (8, 10)

1 * Caustic Carbonate Medium 2 - NSSC Medium I 3 * Green Liquor Medium 4 * Recycled Medium I 150 135- T 1 120- Induct. Time E Bonding Rate I i 0 105-

iI (sTo) 15° '' '' '''' '

1 2 3 1 1 4 1 1 4 1 2 3 4 2 3 I Medium Material

I I H 11:i I k i FIGURE 7.3 Effect of Corrugating Variables on Single-Facer Green-Bond Development

Bond = a(A) + b (B) + c(C) + d(D) + e(E) - 257.7 r2 = 0.806 Bond = Percent Bonded Area

a = 0.461 A = Linerboard Temperature, Deg.F b = 0.544 B = Medium temperature, deg.F c = 3.94 C = Linerboard Moisture Content, % d = 6.06 D = Medium Moisture Content, %

0 e = 0.474 E = Bonding Time, millisec. 5 Q

0_ -

-g 1

FIGURE 7.4 CD Single-Facer Green-Bond Development Sensitivity to Process Conditions, (3) |"

e 0 Lv1 188 7 .32% --- 5.73% .---100-s

B 66.0. 0 ...... 0

40-e 48i:

Base Case LB Temp. Med. Temp. LB MC Med. MC Time Process Condition Increased By 10% FIGURE 7.5 Effect of Corrugating Conditions and Medium Properties on Cured Pin Adhesion, (16)

PA - 110.6 - a(A) - b(B)3- c(C)2 + d(D) - e(E) + f(F) - g(G) - h(H) + i(I) 2 + j(J) - k(K) + I(L) r 2= 0.89 PA - Cured Pin Adhesion, psi

a = 20.1 A = Speed/100, fpm b = 0.0439 B = A c = 10.0 C = Medium Nip Temperature/100, Deg.F d = 11.7 D = A times C e = 36.3 E = Water Drop/100, sec. f = 9.34 F = E times Soft Platen Density (Ib/msf per mil) g = 0.888 G = Gurley Porosity, sec. h = 251 H = Soft Platen Density minus Hard Platen Density i = 185 I = H ('3 O0~o j - 4.14 J = Medium Nip Moisture Content, % fq

k a 0.155 K = J 2 D I = 3.87 L = Medium Basis Weight, Ib/msf

L, FIGURE 7.6 |'- Effect of Single-Facer Speed on Single-Face Cured Pin Adhesion. (119) B '.

P 767 8 -- ...... A

b 66 - ......

64 -i----- 100 200 300 400 500 600 700 Single-Facer Speed, fpm

.1 ------. FIGURE 7.7 Effect of Single-Facer Speed and Medium Basis Weight on Cured Pin Adhesion, (17)

130

P i 120 n A 110 d h e 100 S i 0 n 90

80 b

OqCm 70 P0 100 200 300 400 500 600 700 800 900 1000 1100 5' (IQq Single-Facer Speed, fpm CL (1 0-

Lhl 76 / Chapter 7 Single-Face Bonding

adhesion strength decreases with increasing corrugator entering the corrugating rolls. A higher medium speed. The studies were conducted on a pilot-size sin- temperature and a higher medium moisture content gle-facer, (17, 119). These results confirm the speed favor quicker green-bond strength development. effect predicted by the regression equation shown in Figure 7.5. Figure 7.8 shows that the cured pin adhe- 2. Medium material properties appear to have only a sion strength decreases with decreasing adhesive appli- second-order effect on the green-bond strength de- cation, (14, 58, 120). It may be hypothesized that the velopment when adequate medium preconditioning observed decrease in cured bond pin adhesion with is used. A more porous medium and a less wettable increasing speed is due to a lower adhesive application medium favor a quicker green-bond strength de- rate at the higher speed. The effect of speed on the ad- velopment. hesive film thickness on the applicator roll is discussed in Chapter 6. Figure 7.9 shows the results of a study 3. The cured bond strength is related mainly to the which measured both the cured pin adhesion strength amount of adhesive applied, and to the receptivity and the adhesive application rate as a function of the of the medium to wetting by the starch adhesive. corrugator speed. The experiment was conducted on a The medium should be neither extremely absorbent pilot-size corrugator. The data show a direct correlation nor extremely nonreceptive. between the pin adhesion strength and the amount of applied adhesive in the corrugator speed range of 200 4. The temperature and moisture content of the me- to 400 fpm. Above the 400 fpm speed, the pin adhesion dium entering the corrugating rolls are secondary strength continued to decrease while the adhesive ap- effects for the cured bond strength. The cured bond plication rate increased, (85). This behavior would sug- strength is improved by a lower medium tempera- gest that the adverse effect of the faster corrugating ture and a higher medium moisture content. speed on the cured bond strength is, in part, related to a reduction in the amount of adhesive applied, but also 5. Faster corrugator speeds are detrimental to both the may be due to the increased mechanical stresses on the green-bond and the cured bond strength. The speed green-bond at the faster speeds. effect appears to be associated with a reduced ad- Figure 7.10 shows the effect of the medium water hesive transfer to the medium flute tip and with the drop test property on the cured bond pin adhesion increased mechanical stress on the green-bond. strength. The data indicate that there is an optimum range for the water drop property. It would appear that 6. The single-facer medium preconditioning should a medium that is neither extremely absorbent nor ex- be controlled primarily to enhance the green-bond tremely nonreceptive will achieve the strongest cured formation. Preconditioning should be increased bond, (104). There is some controversy about the mer- simultaneously with corrugator speed increases and its of the water drop test as an indicator of medium should be reduced when the corrugator speed is quality, (85). slowed. The effect of medium preconditioning at the single-facer on both the green-bond strength and the cured bond strength is shown in Figure 7.11. Using less than normal preconditioning for the medium web reduced the green-bond strength development by 53%, but had only a 3% negative effect on the cured bond strength, (8, 10). This indicates that the medium preconditioning should be set so as to maximize the green-bond development. The medium preconditioning should not be adjusted, as so often happens, based on observation of the cured or "false" cured bond at the dry end of the corrugator. In summary, the experimental data presented in this chapter support the following observations.

1. The green-bond strength development, within 200 milliseconds after the single-faced web leaves the pressure roll nip, is affected primarily by the temperature and moisture content of the medium web

J FIGURE 7.8 Effect of Adhesive Application on Cured Pin Adhesion, (71, 79, 83)

74 p i 72 - i ' ... ' ' I I n f iI . -'. . - -1 I. .. - A 70- d . h e 68 - s ...... i 66- 0 *1 n ,....M ...... 1...... , ...... |' . . ,..,.. , 64- b 62-

0 60 - F - 2

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 El' (q Single-Facer Adhesive, Ib/msf 2 CD a

--I = FIGURE 7.9 °|

Effect of Speed on Cured Pin Adhesion CD and Dry Adhesive Application Level, (85) l'

76 0.73 D C 74-\ ..- * 0.72 r u y e 72 -... 0.71 A d d lb Ib 70 00.70 eh pelb per per sq. 68 0.69i msf in. A e d 66 - 0.68 eh A s 64- 0.67 p l'~ ~31<~~~~l Cured Pin Adhesion I

60 i----- 0.65 100 200 300 400 500 600 700 800 Single-Facer Speed, fpm A-Flute

__ . . ------FIGURE 7.10 Effect of Medium Water Drop Test on Cured Pin Adhesion, (104)

C 60 u r e d 55 P i 50 n

A 45 d h e S 40 i 0 n 35

O b 30 0 200 400 600 ' ' ' ++600 qQ At 600 fpm Water Drop Test, sec. Single-Facer Speed

-o-i I' (En 00 FIGURE 7.11 5.o Effect of Preconditioning on SF OQ Green-Bond and Cured Bond, (10) § -4 zn on

110- _*on 110 % 100- [-Normal Precond.-- 100 P G 90- Subnormal Precond. A 90 i n e 80- 80 A ne 70- 70 d h I 60- 60 e B 50- 50 S o i n 40- 40 0 d n 30-- 30 I A r 20- 20 I e b a 10- 10 0- 0 % Green Bond Cured Pin Adhesion Chapter 8

Double-FaceBonding

As was stated in Chapter 7 for the single-face contains both gelled and ungelled (raw granules) starch. bond, the double-face bond is important because of its This zone does not contribute to the bond strength since potential impact on the box plant waste costs, on the the adhesive does not contact the linerboard facing. The box plant productivity issues, and on the functional fillet area is where the adhesive has been squeezed out performance of the corrugated paperboard package, and fills the angle between the fluted medium and the (12, 13, 14, 24, 47, 67, 71, 76, 79, 83). The double-face linerboard. This zone contains almost 100% gelled green-bond development rate is the factor that generally starch. The contribution of the fillet zone to bond governs the speed of the corrugator. The double-face strength can vary. It often contains void spaces that green-bond must be strong enough to pass through the reduce its bonding effectiveness. The high bonding corrugator slitter/scorer unit and the cut-off knife with- zone is where the adhesive is in intimate contact with out causing permanent bond separation in those areas both the medium and the linerboard and where the of the board where the forces are applied. It is generally starch is fully cooked and gelled. This zone provides felt that the final, cured double-face bond will form in the preponderance of the bond strength, (13, 18, 67, the board stacks under the pressure of the sheet weight, 76). (12, 18). The cured bond strength directly affects the The details on the mechanisms involved in the de- Edge Crush Test strength of the corrugated board and velopment of the corrugator starch slurry into an adhe- the compressive strength of the corrugated box. A 10% sive were discussed in Chapter 6. They will not be re- reduction in the cured pin adhesion strength of the peated in this chapter. double-face bond will cause a 3.3% reduction in the A schematic representation of strength develop- Edge Crush Test and a 2.5% loss in top-to-bottom box ment in a typical double-face bond is shown in Figure compression. 8.1. The data were generated using a double-backer The double-face bond formation process is some- simulator developed at the Institute of Paper Science what simpler than that for the single-face bond forma- and Technology. The simulator can reproduce the tem- tion process. Experiments and experience have shown peratures, pressures, adhesives, adhesive application that the physical properties, varying over a wide range, rates, and speeds attained in full-size commercial cor- of the corrugating medium do not play a major role in rugator operations. It is also equipped with a stress the development of the double-face green-bond. The gauge that can measure the bond strength of each dou- green-bond formation is controlled mainly by the rate ble-face glue line leaving the double-backer hot plate of heat conduction to the double-face bond site. Higher section, (13, 18, 24). hot plate temperatures, lower gel point adhesives, and The double-face bond strength development curve, thinner double-face linerboard materials all improve the Figure 8.1 is similar in nature to the single-face green- rate of double-face green-bond formation, (12, 24, 71, bond development curve shown in Figure 7.1 of 79, 83). Other process variables which influence the Chapter 7. They both have induction times, during double-face green-bond rate of formation include ad- which no measurable bond strength occurs; a linear hesive viscosity, adhesive solids, adhesive application bond strength increase with time (bonding rate), once rate, adhesive slurry temperature, preheating of the the bond has started to form; a maximum bond double-face linerboard and the single-faced web, and strength; and a drop off in bond strength after the the pressure applied in the double-backer hot plate sec- maximum strength has been achieved. The most sig- tion, (24). nificant difference between the two bonding curves is The double-face bonded area exhibits three major the magnitude of the times involved. The typical single- zones. The adhesive on the exterior sides of the flute face induction time was about 20 milliseconds. The 1. FIGURE 8.1 ?s Double-Face Bond Development Curve, (24) IX

nk~~ ~ ~ ~i ni~~~~~ ~~dU'c'' t"-' ~ ! , , Bonding m B Rate n Induction i | -- | I d | TimeT i m X S . t r e n g t n t S

0 5 10 15 20 25 30 Bonding Time, seconds u Corrugating Medium / 83 typical double-face induction time is 5 seconds, a 250- Figure 8.4 shows the calculated effect of the dou- fold increase. The typical single-face bonding rate is ble-face green-bond induction time on the maximum 7.1 percent bond per millisecond. The typical double- achievable corrugator speed. The calculation indicates face bonding rate is 5.7 percent bond per second, a that an induction time of 5 seconds or less is required to 1250 fold decrease. Some of these difference may be achieve the fastest design speeds of the current new attributed to the effect of the pressure roll nip and corrugators, (24). heated corrugating rolls in the single-facer process. The effect of the hot plate temperature and the ad- However, the interpretations of the physical events, hesive slurry temperature on the double-face green- occurring at the various stages, also differ. bond induction time is shown in Figure 8.5. The data The indicated measurable bond strength in the in- were developed using the previously described double- duction time stage is not a true experimental bond backer simulator. The experimental data show that the strength but an artifact of the experimental technique. induction time is decreased by higher hot plate tempera- The bonding rate stage represents the viscosity increase tures and by higher adhesive slurry temperatures. The and setting of the starch base adhesive. The saw-tooth average magnitude of the hot plate temperature effect shape of the bond strength curve is caused by the alter- on the induction time is 0.162 seconds/deg.F. The aver- nation of flute tips (plot peaks) and between flute tip age magnitude of the adhesive slurry temperature effect areas (plot valleys) passing the stress gauge. The on the induction time is 0.081 seconds/deg.F., (18). maximum bond strength represents the point at which The effect of the hot plate temperature and the ad- linerboard fiber tear occurs. This probably is not the hesive slurry temperature on the double-face green- cured bond strength since the linerboard would have a bond bonding rate is shown in Figure 8.6. The data high moisture content at the bond site due to the water were developed using the previously described double- associated with the adhesive slurry. The high linerboard backer simulator. The experimental data show that the moisture content disrupts the fiber-to-fiber bonding and bonding rate is increased by higher hot plate tempera- lowers the force required to produce fiber tear. The tures and by higher adhesive slurry temperatures. The decrease in bond strength after the maximum strength average magnitude of the hot plate temperature effect has been reached is not a true physical occurrence. It is on the bonding rate is 0.0073 lb/sec. per deg.F. The an artifact of the termination of the experimental test, average magnitude of the adhesive slurry temperature (13, 18, 24). effect on the bonding rate is 0.0027 lb/sec. per deg.F., Improvement in the double-face green-bond devel- (18). opment process is achieved by reducing the induction Figure.8.7 shows the effect of the adhesive slurry time stage and by increasing the bonding rate. The gel point on the bonding time required to achieve a double-face cured bond strength, assuming no brittle bond strength of 150 mJ and 300 mJ. The experimenta- "zipper" bond, will be governed by the ZD (thickness tion was done on a modified Strohlein apparatus which direction) fiber-to-fiber bond strength of the linerboard, was originally developed for testing gummed tape. The (13, 18). data show that the bond strength development time is Figure 8.2 shows the double-face green bond reduced by decreasing the adhesive slurry gel point. strength as a function of bonding time. The data repre- The data show that the effect reaches a minimum pla- sent the results of laboratory experiments. As shown in teau at a gel point of 51 deg.C., below which no further the previous figure, the bond strength increases linearly bond development improvement occurred, (12). The with time. The bond rate development shown by the gel point must also be controlled so as to maintain the data is 29.7 mJ per second, (12). stability of the adhesive slurry in the glue pan. Too low Figure 8.3 shows the effect of the hot plate tem- a gel point could cause premature gelling of the adhe- perature on the bonding time required to achieve liner- sive in the adhesive application equipment. board fiber tear at the bond sites. The data show that the Figure 8.8 shows the effect of contact time with bonding time required decreases as the hot plate tem- the preheater on the double-face green-bond induction perature increases and as the double-face linerboard time and bonding rate. Increased preheater contact time basis weight decreases. The bonding time to fiber pull translates into a higher single-faced web temperature at failure is 0.135 seconds per lb/msf linerboard basis the time the adhesive is applied to the flute tips, and a weight at a hot plate temperature of 250 deg.F., and higher double-face linerboard web temperature at the 0.066 seconds per lb/msf linerboard basis weight at a time that contact is made between the double-face lin- hot plate temperature of 350 deg.F., (71, 79, 83). These erboard and the single-faced web flute tips. Longer data support the previous observations which indicated preheater contact time (higher web temperature) de- that the double-face bond development is primarily creases the induction time and increases the bonding controlled by the speed with which heat is conducted to rate, (13). the bond site. As was stated at the beginning of this chapter, the FIGURE 8.2 Effect of Bonding Time on the C- o Double-Face Green-Bond Strength, (12) CTlS-1 D :00 a,&i, oo G r 600 e e n 500 B n° 400 d S 300 t r e 200 n g h 100 m 0 J 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Time, Se Ii 7. -- ...I- 00

d 9eo 'ainj~eadwajL aeld 1 009 09V OOV 098 O0e O9z 00Z -0*0

U~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~...... 0

0Q9 p

;sw/ql 9Z 0 4q~~!eM p~~oqjeur 0,9

-06 (69 &6L 'LL) 'luawdOIGA90 puoEi 83ojeiqqqnoQ3 lo 915PI 11nd ja~ pieoqjaull le ainjeieawal. Gleld IOH PuI3 GW!1 DUI.PUOG 1O U01.1ula 6~'9 3 d no .7 FIGURE 8.4 0s o 0

Effect of Double-Face Green-Bond

(Q C 1000 o r 900 r u 800 g a 700 t o 600 r 500 S p 400 e e 300 d 200 f 100 m 0 0 2 4 6 8 10 12 14 16 18 20 22 24 Green-Bond Induction Time, seconds Calculated Speed Values. Corrugating Medium / 87

II qfl FIGURE 8.5 ;! Effect of Hot Plate and Adhesive Temperature on Double-Face Green-Bond Induction Time

Effect of Hot Plate Temperature on Double-Face Green-Bond Induction Time, (18)

9- I n 8- ...... ud7-

T 3- m 2- e (sec.) 1 220 230 240 250 260 270 280 Hot Plate Temperature, Deg. F

Effect of Adhesive Temperature on Double-Face Green-Bond Induction Time, (18)

9.0 - ., I 8.8 n 8 .8 ...... d 8.6 u c 8.4 8.2' i - o 8.0 n 7.8 T i 7 .6 ...... m 7 .4 - - ...... e (sec.) 7.2- 90 92 94 96 98 100 102 104 Adhesive Temperature, Deg. F 88 / Chapter 8 Double-Face Bonding

[i.I FIGURE 8.6 Effect of Hot Plate and Adhesive Temperature on Double-Face Green-Bond Bonding Rate

Effect of Hot Plate Temperature on Double-Face Green-Bond Bonding Rate, (18)

1.4 - BI o b 1.3 --

d p 1.2- i r n 1.1 ...... g s c 1.0

t d 0.9 : : .

e 0.8-, 220 230 240 250 260 270 280 Hot Plate Temperature, Deg. F

1II Effect of Adhesive Temperature on Double-Face Green-Bond Bonding Rate, (18)

0.70- B I 0 b 0 .6 8 - -...... i b 0.686- . ... n r:

g s 0.64- ...... e Rc t 0.62 -

0.60 - 90 92 94 96 98 100 102 104 Adhesive Temperature, Deg. F

-1

I -J FIGURE 8.7 Effect of Adhesive Gel Point on Time to Achieve a Given Level of Double-Face Green-Bond Strength, (12) 30-

25 -

T 20- m e *I , 15- ...... - ...... I...... - ...... -- ..I... S e c 10- -\ - -- Bond Strength X 300 mJ . I...... - _ -..... -- ... I -- I..I... .. © 150 mJ 5- | : ! 0 O-_ O

(mQK 46 48 50 52 54 56 58 60 5o Adhesive Gel Point, Deg. C 0.

kO 90 / Chapter 8 Double-Face Bonding

11 It FIGURE 8.8 ji Effect of Double-Back Preheating :1 II on Double-Face Green-Bond Development i Effect of Double-Back Preheating on Double-Face Green-Bond Induction Time, (13)

9.0- n d 8.5- u c t 8.0- ...... 4 i 0 n 7.5- T i 7.0 ...... m e 6.5 (sec.) 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 Preheater Contact Time, Sec.

Effect of Double-Back Preheating on Double-Face Green-Bond Bonding Rate, (13)

R 3.0- a 2.9 ...... t ...... e 2.8 - - e 2 7...... 02.6

n 2.3 nd 2. -2 / .. ... ------

g 2.0 (lb/sec.) 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 i Preheater Contact Time, Sec.

I Corrugating Medium / 91 corrugating medium properties do not have a great ef- sure in the double-backer. fect on the double-face bond strength development, (12. 24, 71, 79, 83). However, the medium properties d. Lower double-face linerboard basis weight. do have some influence as shown in Figure 8.9. The data used to develop the multiple regression e. Lower double-face linerboard caliper, (in- equation were obtained with the double-backer simula- creased density). tor. The equation indicates that the double-face green- bond induction time is reduced by a medium with a f. Lower adhesive slurry gel point, (less heat lower flute tip water drop test, by a medium with a needed to gel adhesive). higher T-819 water penetration test, and by a less porous medium, (18). Both of the water receptivity tests g. Higher adhesive slurry solids content and vis- are based on the time required for the medium material cosity, (less water). to absorb liquid. The equation indicates opposite effects for the two receptivity tests, and is, therefore, not clear h. Lower adhesive application rate, (less water). as to the actual effect of this material property. The technical information presented in this chapter i. Higher adhesive slurry temperature. on the development of the double-face bond strength supports the following observations. 6. Slower corrugator speeds and longer double- backer hot plate and cooling sections do not affect 1. The speed at which the double-face bond strength the double-face bonding rate. They do, however, develops can affect the box plant waste and pro- increase the bonding time available before the ductivity costs. The strength of the green-bond de- bonds are stressed at the corrugator slitter/scorer termines the maximum permissible corrugator unit and cut-off knife. speed. The bond must not separate as it passes through the corrugator slitter/scorer unit and the cut-off knife.

2. The final, double-face bond strength is important to the compression strength and field performance of the corrugated box.

3. The double-face bond development curve exhibits an induction time, during which no measurable bond strength can be determined, and a rate of bond strength increase that is linear with bonding time, once the bond has started to form.

4. The medium material properties have little effect on the double-face bond strength development. There is one indication that a less porous medium may be beneficial.

5. Most of the double-face bond development characteristics are controlled by the process conditions which influence getting heat to the bond sites, and by the properties of the adhesive. An increased rate of double-face bond strength development can be achieved by:

a. Increased preheating of the double-face linerboard and the single-faced webs.

b. Higher hot plate temperature.

c. Higher hot plate ballast roll or plenum pres- FIGURE 8.9 Effect of Medium Properties on 0 D Double-Face Green-Bond Induction Time, (18) o - 5w 0 '"

(Q IT = 9.94 + a(A) - b(B) - c(C)

r 2= 0.792

IT = Green-Bond Induction Time, sec. a = 0.0534 A = Flute Tip Water Drop Test, sec. b = 0.0533 B = T-819 Water Penetration, sec. c = 0.107 C = Gurley Porosity, sec. Chapter 9

Combined Board Issues

There are many tests that can be performed on cor- The corrugating medium physical properties and rugated board samples. Some of the test results are in- attributes can affect the results obtained in 15 of the 30 fluenced by the physical properties of the corrugated tests listed. Those tests involved with basis weight and medium used to manufacture the combined board and caliper do not require much technical discussion. Those some are not. The following is a partial listing of vari- tests involved with bonding are discussed in Chapters ous combined board tests. The tests that are preceded 6. 7. and 8. by an asterisk are those that are influenced by the The discussion of the remaining nine test parame- physical properties and attributes of the corrugating ters is limited by the amount of technical information medium. available and found in the literature. The following three chapters cover the contribution of the corrugating Combined Board Caliper medium properties and attributes to the characteristics Flute Height of the combined board strength properties of Flexural Linerboard Caliper Stiffness Test, Edge Crush Test, and Flat Crush Test. * Medium Caliper It is important to keep in mind that the relation- * Combined Board Basis Weight ships between medium properties and combined board Linerboard Basis Weight properties described in Chapters 10. 11. and 12 are, * Medium Basis Weight unless indicated otherwise, based on having good box Coefficient of Friction plant fabrication quality. The results shown are not in- MD Edge Crush Test fluenced by obvious fabrication defects such as * CD Edge Crush Test high/low flutes, leaning flutes, fractured flutes, or poor MD Flexural Stiffness corrugator bond strength. The effects of these process * CD Flexural Stiffness defects were discussed in previous chapters. * SF Pin Adhesion * DF Pin Adhesion Slide Angle * Flat Crush * MD Torsion Tear * CD Torsion Tear Scoreline Fold * Water Resistant Bond Water Absorption Rate Mullen * Klemm Test Hydrostatic Mullen * Puncture Scuffing Test MD Scoreline Tensile * CD Scoreline Tensile Scoreline Fold Cracking * Tarnishing Chapter 10

Flexural Stiffness

The Flexural Stiffness Test is designed to measure from the center line of bending is so important to stiff- the bending resistance of the corrugated board, Tappi ness, flute height has a large effect. For a given grade T-820. The bending resistance of the corrugated board of corrugated board, A-flute is much stiffer than C- is directly related to the degree of side panel bulge that flute, and C-flute is much stiffer than B-flute. will occur in a box during stacking in a warehouse or A schematic representation of the 4-point beam, due to a flowable product packed inside the box, such Flexural Stiffness Test is shown in Figure 10.1. The 3- as resin pellets or the liquid held in a bag-in-box type of point beam Flexural Stiffness Test uses just one load package. The combined effect of both the MD and CD point located midway between the two support points. flexural stiffness is important to box panel bulge resis- The 3-point beam test is not used as a measurement tool tance, (36, 99). The flexural stiffness strength is also for predicting box compression because of the fact that one of the two corrugated board physical properties that the shear properties of the medium are included as a determines the top-to-bottom compressive strength of a factor in the measured stiffness value for the combined corrugated box, assuming adequate corrugator bond board. quality. The other corrugated board property is Edge Shear refers to the ability of the various plies in the Crush Test. A 10% change in the flexural stiffness sample to move with respect to each other. In the case strength of the combined board will result in a 2.5% of singlewall corrugated board, the plies in question are change in box compression, based on the model devel- the two linerboard facings and the fluted medium. The oped and published by Mr. Robert McKee in 1963, (19, easiest way to envision shear is to consider a stack of 24, 36, 73, 82, 99). copier paper. The stack is rectangular in shape with the Corrugated board behaves the same as any other ends being square with the top and bottom surfaces of multi-ply structural material with respect to bending the stack. As the stack of paper is flexed, the individual strength. The stiffness contribution of a given ply to the paper plies slide against each other, as shown by the total stiffness of the structure is equal to the product of ends of the stack forming an angle to the bowed top and the elastic modulus of the material in the ply times the bottom surfaces. This is shear, and allowing shear to moment of inertia of the ply. The stiffness of the total occur makes the stack of paper easier to bend. If all of structure is equal to the sum of the stiffness contribu- the sheets in the stack were glued together, shear could tions of its parts. This assumes that there is no shear not occur, and it would be much harder to bend the deformation between the plies in the structure. The stack of paper. moment of inertia of a given ply is related to approxi- The side panels of a box are bordered by the flap mately the cube of its distance from the bending center scores and the body scores. When the box is setup and of the total, multi-ply structure. Since the linerboard the flaps are sealed, the four scorelines, defining a side plies are further away from the center of the corrugated panel of the box, in effect, clamp the edges of the two board structure than the medium, they contribute more linerboard facing together and prevent shear from oc- than the medium to the flexural stiffness of the board. curring. Shear does not occur in the 4-point beam test In term of flexural stiffness, the medium serves primar- on corrugated board. This is why the 4-point beam test ily to keep the linerboard facings separated, (36, 99). In is the correct method to use for corrugated box applica- fact, the linerboard plies contribute between 90% and tions. 95% of the total flexural stiffness of a typical singlewall The objective of the Flexural Stiffness Test is to corrugated board grade. This assumes, of course, that measure the elastic region bending strength of the the medium does not have a major quality problem, sample. The term elastic region simply means that the such as fractured flutes. Since the distance of a ply measured deflection remains linearly proportional to =I FIGURE 10.1 |' Flexural Stiffness Test

w) 0 I' Load Side View of Test Load

Support Support Deflection Gauge

Top View of Corrugated Board Test Samples -....-...... Flute Direction

CD Test Direction MD Test Direction

z= Corrugating Medium / 97 the load, that is, one unit more load produces one unit 90 lb/msf linerboard facings. The data are summarized more deflection. It is necessary to achieve a sufficiently in Figure 10.5. All of the corrugated board was C-flute, high deflection in the sample during testing to minimize combined on a commercial corrugator, and made with the experimental error due to random variation. If, for the same rolls of varying basis weight mediums. All of example, the deflection gauge reading error was plus or the data, except the MD, 90 lb/msf linerboard case, minus 2 mils, a maximum deflection end point for a test show little effect of increasing medium basis weight on specimen of 4 mils would have a 50% error probability. the combined board flexural stiffness, (82). The MD On the other hand, a deflection end point that is too flexural stiffness of the board made with 90 lb/msf lin- high can produce values that exceed the elastic region erboard was the highest level achieved in this study. for the sample and cause erroneously low flexural stiff- The deviant data for this experimental condition may ness values. be explained by the span/load/crushing test instrument Higher deflections also require higher test loading effect discussed above. forces. Higher loading forces can result in crushing of The legitimate assumption in 1976, when this the sample at the support points, and the crushing will study was conducted, was that a higher basis weight lower the entire test specimen with respect to the de- equalled a proportionately higher strength medium. flection gauge. This will cause erroneously low deflec- Since 1991, Alternate Item 222/Rule 41 no longer re- tion measurements and erroneously high measured quires a minimum medium or linerboard basis weight. flexural stiffness values. The experimental results discussed in this chapter all The key to achieving valid flexural stiffness meas- use basis weight as the experimental design variable. urements is the use of the proper span distances be- "Basis weight" can be redefined as "medium strength," tween the two support points, between the two load for the purpose of interpreting the significance of the points, and between the support and load points. Larger data to today's corrugated board strength performance span distances produce larger sample deflections at criteria. lower applied load levels. The equation for calculating The relative contribution of the medium and liner- the flexural stiffness includes the span distances, so board to the combined board flexural stiffness is shown changing the span dimension does not affect the stiff- in Figure 10.6. The data show that increasing the liner- ness measurement. The span distances used should be board basis weight (strength) has much more of an ef- based on the stiffness strength of the material. Stiffer fect on improving the flexural stiffness of the corru- materials should have greater span distances. The spans gated board than an equal increase in the medium basis should be selected so that the load ranges used are weight (strength). The slopes of the medium trend lines about equal for all samples. are much lower than the slope of the linerboard trend The cross direction (CD), flexural stiffness speci- line, (82). This confirms the observations, discussed mens have the flutes running parallel to the length of above, that the linerboard contributes more than 90% to the sample. This CD orientation provides direct support the flexural stiffness strength of corrugated board. from the fluted medium against crushing at the support Figure 10.7 shows the effect of linerboard basis and load points of the test instrument. The CD meas- weight on the flexural stiffness of C-flute corrugated urement direction also requires the fluted medium to board. The information shown is based on the results of bend as part of the total corrugated structure being a mathematical model, (19). The curvilinear relation- flexed. The medium, then, does contribute the CD flex- ship shown may reflect a reduction in the elastic ural stiffness. The machine direction (MD), flexural modulus of linerboard with increasing basis weight or a stiffness specimens have the flutes running perpendicu- span/load/crushing test procedure anomaly discussed lar to the length of the sample. This MD orientation above. provides less support from the medium against crushing Figure 10.8 compares the flexural stiffness of two or deflection of the linerboard between flute tips at the C-flute corrugated board materials made on a commer- support and load points of the test instrument. Also, the cial corrugator using identical linerboard facings and medium can deform in an accordion manner during the two different basis weight grades of medium, 23 lb/msf MD flexural stiffness testing, and, therefore, does not and 37 Ib/msf. The data show that a 61% increase in contribute directly to the measured MD stiffness medium material (strength) increased the flexural stiff- strength. The medium only serves to maintain the ness by only 9%, (36). spacing distance between the linerboard facings, (99). Figure 10.9 shows the effect of flat crushing of the Figure 10.2 shows the effect of medium basis combined board flutes on the retention of flexural stiff- weight on the flexural stiffness of C-flute board made ness. The data show that the MD flexural stiffness is with 26 lb/msf linerboard facings. Figure 10.3 contains affected to a greater degree by crushing than is the CD identical plots for the board made with 42 lb/msf liner- flexural stiffness. An actual 50 mil crushing of the cor- board facings, and Figure 10.4 for the board made with rugated board produces a 39% loss in MD flexural 98 / Chapter 10 Flexural Stiffness

11 H II FIGURE 10.2 i I Effect of Medium Basis Weight on Combined Board Flexural Stiffness - 26 Ib/msf Linerboard

Effect of Medium Basis Weight on Combined Board Flexural Stiffness - With 26 Ib/msf Linerboard, (82)

100-. :

9 0 ...... I t 70- ......

r e ......

19 20 21 22 23 24 25 26 27 Medium Basis Weight, Ib/msf Commercially Produced C-Flute Board I

Effect of Medium Basis Weight on Combined Board Flexural Stiffness and Caliper - 26 Ib/msf Linerboard, (82)

Ivv 180 90- ...... ,,..:...... 170 B 0 FS 80- 160 a I t 70- .. . :. -160 r e f 60- ...... _tf.. ..A . ,. ver...... :... -140 d 50- -130 C U n 40- . : ...... -120 a r e 30- -110 a s ...... I...... 20- -100 p I s Geom. Aver. Stiff. -+ Caliper, mils 10- 90 e (Ib-in) A . 80 r 19 20 21 22 23 24 25 26 I27 (mils) Medium Basis Weight, Ib/msf Commercially Produced C-Flute Board

I Corrugating Medium / 99

.1r ilI ij!I ji 1; ji FIGURE 10.3 I 11 i ii, Effect of Medium Basis Weight on Combined i I I I Board Flexural Stiffness - 42 Ib/msf Linerboard I I I I I I Effect of Medium Basis Weight on I Combined Board Flexural Stiffness - i With 42 Ib/msf Linerboard, (82) I I I I 240- 220- . I I i e 140 - ...... X f 120-120...... u n 10-...... r e 80 ...... a n l 4860 00 e- - ...... |...... I 240 -

(Ib-in) 20-0: ...... ; ...... 19 20 21 22 23 24 25 26 27 Medium Basis Weight, Ib/msf Commercially Produced C-Flute Board

Effect of Medium Basis Weight on Combined Board Flexural Stiffness and Caliper - 42 Ib/msf Linerboard, (82) I

140 _ - 1.U 130- -170 B 0 FS 120- ...... -...... : -160 a I t 110- ...... ! ...... -150 r I 100- ...... - 140 d e f I x f 90- ...... -130 C ...;...... !..... U n 80- * i - 120 a r e 70 -tO a s 60 - 100 p I s | Geom. Aver. Stiff. Caliper, mils I 560...... 90 e (Ib-in) An - V, 80 I I 19 20 21 22 23 24 .-.. 25 26 2*27 (mils) Medium Basis Weight, Ib/msf I Commercially Produced C-Flute Board I

1,'i i Li 100 / Chapter 10 Flexural Stiffness

I7 I 111 i I FIGURE 10.4 iii. I Effect of Medium Basis Weight on Combined I I Board Flexural Stiffness - 90 Ib/msf Linerboard

Effect of Medium Basis Weight on Combined Board Flexural Stiffness - I With 90 Ib/msf Linerboard, (82) I

340

...... 323 2 0 .-...... S-...... I F S 2803000 - ...... F S 240 - ...... i...... ef 240- 1 I ~(l~~b-in)- ..! ;~~20; ... . ___! tiffness! I 19 20 21 22 23 24 25 26 27 Medium Basis Weight, Ib/msf Commercially Produced C-Flute Board I

Effect of Medium Basis Weight on Combined Board Flexural Stiffness and Caliper - 90 Ib/msf Linerboard, (82)

250 - 200 24 0 - ...... 1 9 0 B 0 F S 230 - -- - **;-- *- *-- **- ^ **- -*:.--^J_^-- - 180 a r e 210-- ...... - 160 d I X 200 ...... , ...... - 10 C 140 a s 1 80 -...... -..-...... 1 4 0 I ar e ISO- .. ii , t7 |0o- :._- Geom. Aver. Stiff. - Caliper, mils i P I e :,n) 0 r : ! 1 10nnv I ,wv~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 19 20 21 22 23 24 25 26 27 (mils) Medium Basis Weight, Ib/msf I Commercially Produced C-Flute Board

I Corrugating Medium / 101

FF !I! :Iq FIGURE 10.5 I: I;i II Effect of Medium Basis Weight on il Combined Board Flexural Stiffness 1i .1 Effect of Medium Basis Weight on I MD Flexural Stiffness, (82)

4001 M 3501 -- 26 Ib/mst LB _ 42 Ib/msf LB - 90 Ib/msf LB | I D I S 300- ...... F t I I i 250- ...... e f x f 200-

...... _ ...... r s a s 100- ___...... _ ...... _.___ I ...... -...... (Ib-inJ 50- ,E i 0] I 19 20 21 22 23 24 25 26 27 Commercially Produced Medium Basis Weight, Ib/msf -11 I C-Flute Corrugated Board

Effect of Medium Basis Weight on I CD Flexural Stiffness, (82) I 200 , .

C 180 - - 26 Ib/mef LB -+- 42 Ib/maf LB -*- 90 Ib/msf LB D 1 6 0 : ...... S F t 140- -- - = - - --- I i 12 0 - ...... - - . ..I .- .-.....-..... I ...... ! I x f 10 0 -. . n u e 8 0 ...... I r s 0 ...... i ...... ^ a s I 40- - - ______- ;__.· - ._ (Ib-in. 20 -- - -...... I 0ol -- i i------i -- I IIIf 19 20 21 22 23 24 25 26 27 Commercially Produced Medium Basis Weight, Ib/msf i C-Flute Corrugated Board

I i 3 _

FIGURE 10.6 X ,n

1i. Effect of Medium and Linerboard Basis En01ti 1 O-_-* Weight on the Geometric Mean CE - Flexural Stiffness. (82) 220 - Average 200 M-- 26 Ib/msf LB in'erib'ard '.' G F 180 ...... E ~ ffe~t...... -... e I ...... -±-...... 442 _...~2 ...... Ib/msfb / mj...... LB ...... xe 160 * 90 lb/msf LB m -...... , t 140 r S 120 i t 100 c i f ...... a pois within each lierb...... ard ...... grade f 80 M n 60 e e a s 40 n s 20 - -representfour-different medium basis weight grades: 20.0, 21.7, 23.3 and 26.2 Ib/msf. (lb-in.) 0 60 80 100 120 140 160 180 200 220 240 Total Basis Weight, Ib/msf Commercially Produced C-Flute Corrugated Board I I------FIGURE 10.7 Effect of Linerboard Basis Weight on 4-Pt. Beam Flexural Stiffness. (19)

40 F I 35 e x 30 u r a 25 I 20 S

t (-I i 15 f f 10 n e S 5 s 0 (Nm) 0 5 0 10 20 30 40 50 60 70 80 90 100 CQ Linerboard Basis Weight, Ib/msf Data Points Not Shown. - 0 U) - -- ..'...... =-.: 104 / Chapter 10 Flexural Stiffness

i Li I I

!I li 1} FIGURE 10.8 I I Effect of Medium Basis Weight on I

Combined Board Flexural Stiffness I

i Effect of Medium Basis Weight on Geometric Average Flexural Stiffness, (36)

20 - I 18 .....- - .- -.1... --.. . I...... I- - 7 16 I ...... I ...... - - .. ...... - . F S 9% ...... I t 14 e f 12 I...... I. ...- ...... I ..... -.1 .... - x f 10 ...... I. u n 8 ...... I...... - ...... r e ...... a s 6 I s 4- I...... -...... I . I I...... (Nm) 2- ...... 0- 23 37 Medium Basis Weight, Ib/msf C-Flute, 41 Ib/msf Linerboard

Effect of Medium Basis Weight on Combined Board Caliper, (36)

5.5 -

C 5.0-1 a ... p 4.5 --- - - 4

e r 3.5

23 372.5

2.0 23 37 Medium Basis Weight, Ib/msf C-Flute, 41 Ib/msf Linerboard

-i -I

I FIGURE 10.9 Effect of Combined Board Crushing on Combined Board Flexural Stiffness, (124)

110

0 100 f 90 U 80 n c 70 r u 60 s h e 50 d 40 V a 30 I 20 0 e 10 co 0 10 20 30 40 50 60 70 80 90 100 110 120 5' Actual Crushing, mils C-Flute Board

i1- 106 / Chapter 10 Flexural Stiffness stiffness and an 18% loss in CD flexural stiffness. The 7. Flexural stiffness is adversely affected by flat percentage loss in flexural stiffness in both directions is crushing of the combined board flutes. The MD much greater than the measured caliper loss, (124). flexural stiffness is more sensitive to crushing than These data suggest that the empirical short form of the the CD flexural stiffness. A measured 10% reduc- McKee box compression equation, which substitutes tion in caliper caused by crushing results in an ap- combined board caliper for combined board flexural proximate 20% loss in CD flexural stiffness and a stiffness, may be a much less sensitive model than the 48% loss in MD flexural stiffness. original McKee equation for recognizing the effect of box plant flute crushing quality defects on box com- 8. This effect of crushing on the flexural stiffness pressive strength. strength demonstrates that the short form of the Figure 10.10 shows the effect of medium basis McKee box compression equation, which substi- weight on the 3-point beam flexural stiffness. Based on tutes combined board caliper for combined board this stiffness measurement method, a 61% increase in flexural stiffness, does not adequately reflect the medium basis weight produces a 17% increase in the loss in box compression due to the crushing of the flexural stiffness, (99). This is approximately twice the medium flutes. effect shown in Figure 10.9 for the 4-point beam test. The difference can be attributed to the added effect of medium basis weight (strength) on reducing the shear strain occurring during the 3-point beam test. The data on flexural stiffness presented in this chapter support the following observations.

1. The 4-point beam test method should be used for corrugated board measurements of flexural stiffness since it eliminates the effect of shear in the medium on the measured strength. The 3-point beam test should not be used.

2. The span distances between the support points, between the load points, and between the support and load points should be adjusted based on the stiffness of the sample being tested. Greater distances should be used for stiffer samples so as to minimize the probability of crushing the corrugated board at the load and support points.

3. The flexural stiffness measurement should reflect the bending resistance of the sample in its elastic region.

4. The flexural stiffness strength of combined board is determined primarily by the linerboard facings and the height. Stronger linerboard and higher flutes improve the flexural stiffness of the corrugated board. This assumes no major corrugating defects in the medium, such as flute fracture.

5. The main contribution of the corrugating medium to the combined board stiffness is to keep the linerboard facings separated and fixed.

6. In singlewall board, a 61% increase in medium material only produces an approximate 9% increase in flexural stiffness.

- J Corrugating Medium / 107

IF il |! i| 11 ii FIGURE 10.10 i! n1 Effect of Medium Basis Weight on Combined Board Flexural Stiffness

Effect of Medium Basis Weight on 3-Point-Beam Flexural Stiffness, (99)

8.0 ,

7 .5...... X 7.0 - a

10 12 14 16 18 20 22 24 26 28 30 32 34 36 et s 4.0.4.5 10 12 14 16 Medium18 20 Basis22 Weight.24 26 lb/msf 28 30 32 34 36 - lb per 3 Inch width. Averaged over 3 linerboard basis weights. !i i

Flute Height and Apparent Density of Corrugating Mediums, (99)

190 ...... F 188 A 1 186 -4.0 P u P t 184 a r 182 -3.5 e n e 180- t g 178 -3.0 D h 176 e.: .p: . er...... : ::: :..:: ::::< 1::.- ::::::::::::...... n ,174- -2.5 s 18...... :...... ie m 172- t 1170 - ,~n Y -|- Flute Height App. Density 168- 166 , Ii , I ;i I I I I , - 1.57 10 12 14 16 18 20 22 24 26 28 30 32 34 36 Medium Basis Weight, lb/msf * Ib/msf per mil medium caliper. Chapter 11

Edge Crush Test

The Edge Crush Test, ECT, of corrugated board is that a 10% change in ECT strength causes a 7.5% change designed to measure the pure compressive strength of the in box compressive strength, (36, 40, 116). material. The TAPPI Official Test Method T-811 speci- The CD ECT of corrugated board has been related to fies a test specimen height of 1.25 inch for B-flute, 1.50 the sum of the compressive strengths of the linerboard inch for C-flute, and 2.00 inch for A-flute. The heights and medium materials used in its construction. The con- were selected to ensure that the test specimens will be tribution of the corrugating medium, of course, includes stiff enough to resist bending or buckling during testing, the effect of the flute draw factor, (16, 23, 39, 50, 54, 56, and fail in pure compression. The only reason that the 82, 99,). test procedure does not specify a 1.25 inch height for all Figure 11.1 shows the effect of the medium basis of the flute sizes is a safety concern. The original proce- weight on the combined board ECT. The effect shown is dure required the use of a circular saw to cut the test linear for medium basis weights above 19 lb/msf, and specimens. The objective of the test method was to allow decreases more rapidly for medium with basis weights as tall a test specimen as possible so as to keep fingers as below 19 lb/msf, (99). At the time that this study was far away from the saw blade as possible. Since C-flute is made, corrugating medium was considered a commodity stiffer than B-flute, the C-flute specimen can be taller and item in which changes in basis weight were assumed to still not bend or buckle during testing. The same is true be directly proportional to changes in compressive for A-flute versus C-flute. The top and bottom edges of strength, (54, 56, 64). This may or may not be true in the test specimens are reinforced with wax to prevent test today's technological environment. specimen edge failure, which is not representative of the It has been pointed out that the measured ECT can true compressive strength of the material. be less than the ECT calculated by the summation of the The ECT can be measured in both the cross direc- crush strength of the components. If the components tion, CD, and the machine direction, MD. The corrugat- reach their maximum strength at significantly different ing medium contributes directly to the compressive deflections, they will not reach their maximum strength strength of the corrugated board in the CD test since the at the same time in the ECT test. In this event, the meas- flutes are oriented vertically in the test specimen. The CD ured ECT will be lower than predicted, (50). This con- test is the most commonly run ECT test since most cor- cept of the effect on ECT of differences in the load de- rugated boxes have the flutes oriented vertically. The flection characteristics of linerboard and medium is medium does not contribute directly to the ECT of the similar to that of a box with an inner partition. A knowl- MD test. The compression load must be supported solely edgeable box designer specifies the height of the divider, by the linerboard facings since the fluted medium can which reaches a maximum compressive strength at about flex like a bellows. The medium does help to support the 0.1 inch deflection, so that it relates to the relative height linerboard facings at the bonded area and does tie the of the taller box, which reaches its maximum compres- linerboard facings together so they act as a unified struc- sive strength at about 0.5 inch deflection. If the two com- ture. The bond sites, in effect, divide the linerboard into a ponents of the package were made to the same height, the series of short segments stacked on each other. This ef- partition would fail in compression before the box fect is controlled by the spacing of the flutes rather than reached its maximum load bearing capacity, and the by the strength of the medium itself. package would not be as strong as it could have been if The ECT strength is important because of its effect the proper relative heights were used. on the top-to-bottom compression strength of a corru- Unfortunately, the height of the medium with re- gated box. The box compression strength calculation spect to the height of the linerboard cannot be adjusted in model, published by Mr. Robert McKee in 1963, shows corrugated board. They must be the same height. How- MQ O FIGURE 11.1 -ID Effect of Medium Basis Weight on CD CDs-i C Combined Board Edae Crush Test. (99)

140

130 E C T 120 I 110 b / 100 i n 90 C h 80

70 10 12 14 16 18 20 22 24 26 28 30 32 34 36 Medium Basis Weight, Ib/msf Averaged over three linerboard basis weights.

-I Corrugating Medium / 111 ever, the characteristics of the compression stress/strain flute grades responded equally, that is, the slopes of the curves of materials can be used as a design tool to prop- two regression lines are the same, (82). The difference in erly match materials. off-set between the lines is greater than can be explained The two most common ways currently used to by just the difference in draw factor between the flute measure the compressive strength of linerboard and cor- sizes. The greater than expected difference may be ex- rugating medium is the Ring Crush Test, TAPPI T-822, plained by differences in the corrugating quality (bond and the Short-span (STFI) Test, TAPPI T-826. One pub- strength, crushing, flute lean, fracture, etc.) that might be lished relationship between the two test methods for cor- expected in a commercial process. rugating medium is shown in Figure 11.2. The data show Figure 11.5 demonstrates the effect of the medium a reasonable linear relationship, (39). flute fracture defect on the ECT of the combined board. The author wishes to state that, in his opinion, an The experiments were run on a pilot size corrugator us- absolute and universal correlation between the two tests ing commercial medium. The degree of fracture repre- will never be possible. A good empirical correlation can sented by the data was not severe. It was just at the start be obtained for a given set of samples, but the correlation of fracture, close to the breakpoint between good and can change if the fibers are changed or if the bonding defective corrugated board. This low degree of fracture between the fibers is changed by a paper machine proc- reduced the ECT by almost 15%, (129). ess change. The two tests are measuring two different The effect of corrugator adhesive gaps, like the old aspects of the compression strength of paper. Virtually all fingerline gap, is shown in Figure 11.6. The experiments physical properties of paper can be related to the strength were conducted on a pilot-size corrugator using com- of the individual fibers in the paper, and to the amount of mercial medium. The various size gaps were induced by bonding between the fibers and the strength of those using a scrapper blade on the glue applicator roll in the bonds. Typical hardwood fibers are about 0.9 mm long, single-facer. The relationship shows a slightly curved and typical pine fibers are about 3 mm long. The STFI shape. A 0.1 inch gap reduces the combined board ECT Test uses a span of 0.7 mm. The Ring Crush Test uses a by 4%, a 0.4 inch gap by 17%, and a 1.0 inch gap by specimen height of 12.7 mm. The STFI Test emphasizes 32%, (116). The 0.5 and 1.0 inch glue gaps very seldom the individual fiber strength over bonding strength, rela- occur in real life. However, blisters do occur in this size tive to the Ring Crush Test, and vice versa. Which is range, and a blister is equivalent to a glue gap since the better? The author will not state his opinion! bond between the medium and the linerboard is not The relationship between medium basis weight and formed even though the adhesive is present. The effect the combined board ECT is shown in Figure 11.2. The shown in Figure 11.6, therefore, applies to blisters, fluff- linerboard was held constant, and the combined board out, and loose edge defects as well. was manufactured on a pilot-size corrugator. Forty-nine Figure 11.7 presents a regression equation which different samples of commercial corrugating medium, relates the corrugated board quality defects of leaning ranging in basis weight from 26 lb/msf to 52 lb/msf, were flutes, single-face pin adhesion bond strength, high/low used in the study. The relationship is shown to be linear, flutes, actual crushing, and pressure roll cutting to the with a correlation coefficient of 0.974. Figure 11.3 combined board ECT. The experiments were done on a shows the relationship between the medium CD ring pilot-size corrugator using commercial medium and lin- crush strength and the combined board CD ECT, and erboard. A full factorial experimental design was used so between the medium CD STFI crush strength and the that any interactive effects could be determined. No such combined board CD ECT. Both show linear relation- interactions were shown by the data. The variables were ships. The correlation coefficient, for this set of data, was found to be additive. For example, the loss in ECT due to slightly higher for the Ring Crush Test than for the STFI crushing will be directly added to the loss in ECT due to Crush Test, being 0.995 and 0.977, respectively. Other leaning flutes, even though both defects reduce the cali- studies by the same researchers have shown that the STFI per of the combined board, (5). An interactive effect Test was a better indicator of ECT than the Ring Crush means that two variables act together in a way that makes Test for extremely densified medium products. This later their combined effect greater or less than the sum of their study used Formette handsheets and a pilot-size corruga- individual effects. tor for the experiments, (29, 39). All five defects had a statistically significant effect Figure 11.4 shows the relationship between the cor- on ECT. All had an adverse effect, except for the pres- rugating medium CD ring crush strength and the com- sure roll cutting which had a positive effect. It is hy- bined board CD ECT for both C-flute and B-flute board. pothesized that the favorable effect of the pressure roll The experiments were conducted using commercial me- cutting is due to a slight decoupling of the linerboard and dium materials and a commercial corrugator. The data medium at the bond sites, which allows the linerboard show a directly proportional increase in ECT with in- and the medium to reach their maximum strength at creasing medium ring crush strength. The C-flute and B- closer to the same ECT deflection, as was discussed in 112 / Chapter 11 Edge Crush Test

FIGURE 11.2 Relationship Between CD Ring Crush and CD STFI & Between Medium Basis Weight and Edge Crush Test

Relationship Between Cross Direction Ring Crush and STFI Crush for Corrugating Medium, (39)

C 30- D 2 8 ...... s 26 -...... ,; T 24 - 2 2 - ...... 20- 1 186- - . ... b 16 n 114 2 .4...... : ...... : . -

. 10 35 45 55 65 75 85 95 105 115 CD Ring Crush, lb/6 inch

Relationship Between Medium Basis Weight and Combined Board Cross Direction Edge Crush Test, (39)

60- E C56058 - . ;--. .-. ...-.. ;.--..-.. i-..-..|...... -|.-...... C 5455 6 6 - ...... - ...... :...... - .

52 - c 50 .. 4...... i - !

i 48 n c 42 h 4 2 ...... 40-.. 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 Medium Basis Weight, Ib/msf

______I Corrugating Medium / 113

I FIGURE 11.3 Relationship Between CD Ring Crush and CD Edge Crush Test & Between CD STFi and CD Edge Crush Test

I Relationship Between Medium CD Ring Crush and Combined Board Cross Direction Edge Crush Test, (39)

60 E 58--1- - - -- C 5 6 ...... T 54 52 50- b / 4 8 -...... :......

4 ...... 2......

40 ----- , 35 45 55 65 75 85 95 105 115 Medium CD Ring Crush, lb/6 inch

Relationship Between Medium CD STFI Crush and Combined Board Cross Direction Edge Crush Test, (39)

60- .....

C 5C5 6 4 6...... :?. ** ...... , ......

52 ......

f .,2 - - ...... ECT;...... *0.782(CD...... STFI .. .. Crush) ...... +134.2 40 ......

10 12 14 16 18 20 22 24 26 28 30 Medium CD STFI Crush, lb/inch

:1H J1

I oo - FIGURE 11.4 D -

Relationship Between Medium Ring Crush » 0 En ::r and Combined Board Edge Crush Test. (82) 'n 0

E 57 C T I 55 b 53 / i 51 n c 49 h 47

45 6 7 8 9 10 11 12 CD Ring Crush, lb/inch 11 FIGURE 11.5 Effect of Flute Fracture Defect on Fluted Medium Edge Crush Strength. (129)

66 - M F 640 - 6Fractured e Flutes

u d 56 eFlute Fracture 58Condition CD ur 5052 (lb/in.)h 444648. 42 ?

No Fracture Fractured Flutes I. Flute Fracture Condition afini-iv q43UI 'GAISeLqpV ul deE 0'l 6'0 S9O L'Q TO TO VWO ' 0,0

I LI

09 jq

(9LL) -Ise±L snio aDp~j ao3I0fpeog-~upqwo3 uo saeE eAISeqpV 1o 109113 - 0

I2-w 9L L 3UflDid FOGURE 11.7 Effect of Box Plant Process Variables on Combined Board CD Edge Crush Test, (5)

ECT = 33.7 - a(A) + b(B) - c(C) - d(D) - e(E)

rr2 = 0.750 n = 216

ECT = Edge Crush Test, lb/inch a = 0.0397 A = Leaning Flute Angle, Deg. b = 0.150 B = Single-Face Pin Adhesion, lb c = 0.0534 C = High/Low Flutes @4 mils Or Greater, % d = 0.134 D = Actual Crushing, mils 0 0 e = 0.130 E = Pressure Roll Cutting, % Mullen Loss IQ

- 118 / Chapter 11 Edge Crush Test the sixth paragraph of this chapter. The researcher does pressibility effect is the principle of the so-called "no- not recommend that a box plant use pressure roll cutting crush" feedrolls that are being sold commercially. Figure to increase ECT because of other package quality issues, 11.12 shows the effect of repeated crushing of the same such as scoreline splitting, mullen strength, and puncture sample of corrugated board. Above an actual crushing resistance, (5). level of 20 mils, repeated crushing results in larger, per- The regression equation shown in Figure 11. 7 indi- manent, measured caliper loses, (130). cates a 1.5 lb/inch reduction in ECT for every 10 lb de- The speed of the caliper recovery in corrugated crease in single-face pin adhesion. While the study did board after crushing is shown in Figure 11.13 for both a not include the double-face bond strength, there is no 15% and a 28% actual crushing level. The material used reason to believe that the effect would be any different in the study was C-flute board made with 26 lb/msf me- than that shown for the single-face bond. Having a good dium. The data show that more than 90% of the caliper quality and a strong corrugator bond is an inexpensive recovery occurs within 1 minute after crushing. This re- way to add to compressive strength. Conversely, an at- covery rate is much quicker than the time it takes a ma- tempt to save on adhesive costs by excessively decreas- chine operator in a box plant to cut and measure a sample ing the adhesive application rate and thereby reducing the of board for crushing, (5). strength of the corrugator bond is a very costly way to The data indicate that actual crushing of 25 mils or experience box failure, (5, 7). less in a commercial box plant manufacturing operation Another researcher reported an initial increase in is not statistically detectable at the 95% probability level, ECT with crushing before it leveled off and then de- even under the best of circumstances, by the use of creased, Figure 11.8. No hypothesis was offered by the combined board caliper measurements. An actual crush researcher to explain this experimental observation, (7, of 25 mils will produce a 3.4 lb/inch reduction in ECT, or 124). The author has no explanation to offer and is of the 7.8% of the most common singlewall grade. This esti- opinion that some experimental error or data transposi- mate is based on the regression equation shown in Figure tion error may have occurred. 11.7. An alternative approach for a box plant operation to Box plant personnel frequently use combined board consider for controlling the crushing defect may be to caliper measurements to judge the degree of crushing measure and control the actual gap clearances at the vari- taking place in the box plant process. Figure 11.9 shows ous nip points in the corrugated board and boxmaking the relationship between the actual crushing and the processes. measured crushing of 42-26-42 lb/msf C-flute corrugated A regression equation relating corrugating process board. The crushing was done in a laboratory using a variables and corrugating medium properties to ECT is rubber-to-steel roll nip, and with the flutes of the com- presented in Figure 11.14. The data were generated us- bined board being oriented perpendicular to the nip axis ing a pilot-size corrugator and commercially produced during the crushing. The actual crush is represented by corrugating mediums. The equation shows that the ECT the measured gap clearance at the crushing nip. The data decreases with decreasing medium basis weight, decreas- show that the measured crush is considerably less than ing medium STFI crush strength, increasing corrugator the actual crush. On average, the difference is a ratio of speed, decreasing corrugator bond strength, and increas- approximately 1:7. It also shows that caliper measure- ing severity of high/low flutes, (16). The quantitative ments on corrugated board are very variable, even under effect of the high/low flute defect on ECT in this study is controlled laboratory conditions and using the average approximately equal to the magnitude of the effect de- value of 10 measurements on each small size sample. scribed in the regression equation shown in Figure 11. 7. Figure 11.10 shows the actual crushing versus The two equations show a greater difference in the pre- measured crushing for A-flute corrugated board. It also dicted effect of the corrugator bond strength, (5, 16). shows the influence of medium basis weight on the dif- Figure 11.15 shows the effect of corrugating me- ference between the actual and the measured crushing. dium properties on the retention of CD crush strength in The difference between actual and measured crush for A- the medium after fluting. The experimental data were flute is approximately the same magnitude as was shown generated on a pilot-size corrugator using both commer- for C-flute board in Figure 11.9. The heavier basis cial mediums and Formette handsheet mediums. The weight medium (stiffer medium) shows a greater caliper fiber furnishes included NSSC, Green Liquor, Caustic recovery than the lower basis weight medium, (130). Carbonate, and Recycled. The equation predicts that a Figure 11.11 shows the difference between a steel-to- medium with higher MD, CD, and ZD (thickness) elastic steel roll nip and a rubber-to-steel roll nip. The steel-to- modulus properties will retain more of its CD compres- steel nip shows an average 6 mil greater measured crush sive strength after fluting, (1, 39). than the rubber-to-steel roll nip, at equal measured nip Figure 11.16 shows the impact of paper mill pulp clearances. The difference can be attributed to the "give" refining and paper machine wet pressing on the retention or compressibility of the rubber roll, (130). This com- of the corrugating medium CD crush strength after flut- = - .... -.. -'. --.- FIGURE 11.8 Effect of Combined Board Crushing on CD Edge Crush Test, (124)

C 130- -130 a 125 ...... a . . 1 1 5

% 110o . ------*--- X--*---- - 110 r

0 100 -510

U 9oX0 9

r 80% 80 n

h 70 - - ECT ±'Caliper . - 70 u e ...... - 65...... 65 s^ 0 10 20 30 40 50 60 70 80 90 100 110 d

Actual Crushing, mils5 C-Flute Board

i-* ...... 11

FIGURE 11.9 C^ Relationship Between Actual Combined I Board Crush and Measured Crushing. (5)

14---- * e M 1.1 2 -* a 10

!4-C ......

eu -] i I! | i i [ :: d u -2 ...... ;i ...... ,.~.> ...... i--..~...i....~'.~¢'~::. *I . . ..

C(mils)4 i

0 5 10 15 20 25 30 35 40 45 50 55 60 Actual Crushing, mils 42-26-42, C-Flute Construction. Rubber/Steel Roll Crushing Nip. II FIGURE 11.10 Effect of Medium Basis Weight on Measured CaliDer After Crushina. (130)

50 M e 45 a s 40 u r e 35 d 30 C 25 r u 20 s h 15

M 10 i 5 I s 0 5' 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 cmOQ

Actual Crushing, Mils :L A-Flute Corrugated Board. -. I-J m2 :>9 00- FIGURE 11.11 m) Effect of Roll Nip Hardness (b» ni EA :: on Measured Crushing, (130)

M S 25- e e I,' ~ 6 mil / .- " a e 20 - Difference -' s I u / r t 15 - ^ >' /i \;,.,.--"" . " . .'.'. . - """.'f e 0 d S ...... , . - t 10- ...... , , ' Each Data Point Represents the C e .. " , pairing of equal nip clearances r e 5- ...... for the Two Nip Hardness Conditions. u I X ,.--*' s h 0 - (mils) 0 5 10 15 20 25 30 Measured Crush, mils Rubber/to/Steel FIGURE 11.12 Effect of Multiple Crushing on the Measured Crushing, (130)

M 60 ae 55 ..-- ...... i...... s 50 .. .. 2 Crush Pass ... ru 45 - 1--- + 2 Crush Passes e 40 d 35 C 30 r u 25 h 20 . 15 m 10 i 5

s 0 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 3 I.p Actual Crush, mils I o Average for A-Flute, CD 26 & 30 Ib/msf Medium. ow(i 124 / Chapter 11 Edge Crush Test

i fI I!I FIGURE 11.13 i Rate of Caliper Recovery of Corrugated Board After Crushing By Rubber-To-Steel Roll Nip I I II !i Caliper Recovery of Combined Board ii Crushed 23.3 mils or 15% i;1 Rubber-to-Steel Crushing Nip, (5) i

180 170 Original Uncrushed Caliper M 1 0 -- -- ...... dt - - e.. n...... a. 1 6 0...... C 160 .-......

d 140 - * ...... e n...... 15 0 - ...... -...... ;...... I

I - . : . ;i i 1 0 ...... - ...... 120 ...... : . !

100-1100 - I I I i * i i I i - , ! I I I - i - I 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Recovery Time, min.

IL i i

i Caliper Recovery of Combined Board Crushed 43.7 mils or 28% Rubber-to-Steel Crushing Roll Nip, (5)

180 .-. - : M 170 Original Uncrushed Caliper .ii 1 7 0 - ...... 160..... ^.. ra 15 0 ...... - ...... , -...... -...... j ...

U ...0 ; t3 .-

Cr i 01 -.., , ..- ......

e110 ...... - 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Recovery Time, min. 42-26-42, C-Flute Corrugated Board.

-I r FIGURE 11.14 Effect of Corrugating Conditions and Medium Properties on Combined Board CD Edge Crush Test. (16)

ECT = a(A) + b(B) - c(C) - d(D) + e(E)

ECT = Edge Crush Test, lb/inch r =0.78

a = 0.406 A = CD STFI, lb/inch b = 0.0201 B = Pin Adhesion, lb c = 0.0497 C = High/Low Flutes Over 4 Mils, % d = 0.00088 D = Speed, fpm n e = 0.561 E = Medium Basis Weight, Ib/msf 0 aQ0

(m5,

OL = :1 ------1

FIGURE 11.15 Ct Effect of Medium Elastic Properties on the e Retention of CD Crush Strength After Fluting, (1) g

CR = 1.35 - a(1/A) - (b + c(C))(1/D) - e(1/E) + f(F)(G) r 2 = 0.636 CR = (Fluted Edge Crush)/(CD Ring Crush), Both In lb/6 inch

a = 0.385 A = MD Modulus, GPa b = 0.326 c = 0.365 C = Density, g/cubic cm D = CD Modulus, GPa e = 0.0060 E = ZD Modulus, GPa

f = 0.00037 F = XY Poisson Ratio G = Corrugating Speed, fpm =-- --... FIGURE 11.16 Effect of Refining and Wet Pressing on Edge Crush Retention After Fluting for 40 Ib/msf Medium, (15) 1.1 - _ C 1.0- D 0.9- ...... At .50%. RHRH.. m At 90% RRH...... H ...... C 0.8- r u 0.7- h 0.6- R 0.5- a 0.4- t 0.3- 0 0.2- * 0.1-

0.0- 0 400/Low 400/Medium 325/Low 325/Medium 5 Process Condition, Freeness/Wet Pressing OQ

0~Ma-4 * ECT (Ib/in ch) Divided By N. CD Ring Cru sh (lb/6 inch) I 128 / Chapter 11 Edge Crush Test ing. The term "retention" refers to the combined board properties of the paper. The data show that the compres- ECT strength as compared to the CD ring crush or STFI sion strength decreases at a faster rate than tensile crush of the unfluted medium rollstock material. A strength with increasing moisture content up to the 10% higher ratio or retention means more bang for the buck. moisture content level. Above 10% moisture content, the The experiments were conducted using Formette hand- crush strength decreased at a lower rate than the tensile sheets corrugated on a pilot- size corrugator. The data strength, (37). show that a more highly refined (lower freeness) and a Work done at the USDA, Forest Products Labora- more highly wet pressed (more densified) medium re- tory in Madison, Wisconsin, has shown that the stacking tains more of its CD crush strength after fluting. The life of paper products is more adversely affected by cy- relative improvement of refining and wet pressing is seen cling moisture content than by a constant moisture con- at 90% relative humidity as well as at 50% RH, (15). tent at the highest end of the cycle. The term "cyclic These results are in agreement with the predicted effects humidity creep failure" has been used to define this ob- of the regression equation shown in Figure 11.15. served physical behavior. Creep failure refers to the Luckily, more refining of the pulp furnish and more gradual deflection of a material with time, when under a wet pressing on the paper machine also increase the compression load, until compression failure occurs. starting CD crush strength of the medium. That is, they Creep failure is the reason that a corrugated box with a increase the pounds of crush strength per Ib/msf of me- 1000 lb compressive strength cannot remain stacked in a dium basis weight, (23). For a given medium basis warehouse for an infinite time when the bottom box of weight, increased pulp refining and wet pressing increase the stack has only 500 lb of weight on top of it. Over the CD crush strength of the medium rollstock and in- time, the box will continue to settle down; the side panels crease the degree to which that strength is passed on to of the box will continue to bulge; and one day the box the ECT of the combined board. will fail in compression. The next several figures deal with the effect of rela- Three types of commercial corrugating medium tive humidity and moisture content on the crush strength were measured for their cyclic humidity creep character- of medium. Figure 11.17 shows the relationship between istics, NSSC, Green Liquor, and Recycled, at the Forest relative humidity and moisture content for a commercial Products Laboratory. Only one random sample of each NSSC corrugating medium paper. Both the adsorption medium type was tested. All were 26 Ib/msf basis weight and the desorption curves are shown. The medium dis- grades. The results of the study are shown in Table 11.1 plays the hysteresis (separation of the adsorption and in terms of the maximum creep rate (higher is worse), in desorption curves) that is typical for paper, (40). terms of the hygroexpansivity (sample size changes with The effect of moisture content on the CD and MD RH changes), and the ratio of the two measurements. The STFI crush strength of commercial NSSC corrugating ratio will be a constant value if hygroexpansivity is an medium is shown in Figure 11.18. On average, the me- absolute indicator of the cyclic humidity creep rate ex- dium loses proportionately less CD crush strength than pected for different corrugating mediums. The data show MD crush strength as the paper moisture content in- that the Recycled medium had the lowest creep rate, fol- creases, (40). The author hypothesizes that this difference lowed by the NSSC medium, and then the Green Liquor is related to the relative effect of moisture on the individ- medium, (4). ual fiber strength versus its effect on the fiber-to-fiber bond strength. Figure 11.19 compares the effect of moisture con- Table 11.1. Cyclic Humidity Creep Characteristic of tent on the crush strength of NSSC medium handsheets Various Corrugating Medium Types to that of Recycled medium handsheets. The compressive strength of the medium was measured using the STFI Ratio crush strength method. The data show that there is no Maximum (Creep difference in the crush strength response of NSSC and Creep Rate Hygro- ex- Rate to Recycled medium to moisture content over the moisture Corrugating (m/m per pansivity Hygro. Medium Type hour) Strain (m/m) Strain) content range of 5% to 15%. Below 5% moisture content, NSSC -0.002644 0.003139 -0.832 the recycled medium crush strength increases at a faster Green Liquor -0.003682 0.004233 -0.863 rate, and above 15% moisture content, the recycled me- Recycled -0.002185 0.003350 -0.632 dium loses strength at a slower rate. Both of the medium types attained the same moisture content levels when exposed to the identical humidity and temperature condi- It must be emphasized that the results in Table 11.1 tions used to condition the samples, (37). cannot be used as a generalization about the relative Figure 11.20 compares the effect of corrugating creep performance of the various medium types. Each medium moisture content on the compression and tensile medium was represented by only one grab sample. The FIGURE 11.17 Relationship of Relative Humidity and Moisture Content for Corrugating Medium. (40)

M 20 - . . - . | M o 18 ...... i ...... s 16...... u 14-...../ r e 12- . . / - / . 10-Desorption

n ....Adsorption ; ...... t 4 - ......

Relative Humidity, % @ 23 Deg. C C t. ,~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ts e 6 ...... ; .... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^ 0 10 20 30 40 50 60 70 80 90 100

,'J OQ 0 FIGURE 11.18 (D Effect of Medium Moisture Content Is -H D-i on STFI Crush Strength, (40) CD

100 % The CD has a 1.8 percentage point 90 higher average STFI retention than f 80 the MD. V 70 S a 60 T I 60 F u 50 I e S~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~...... '~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.''.''.' .... ./...... 40 ;I MD: ...... ip~ ~ ...... 1 r1...... 30 . . ii~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~, 0 % 20 ......

M 10 '. . I C ...... 0 .,,; i . 1 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Moisture Content, % Actual Data Points Not Shown

L -1 I FIGURE 11.19 Effect of Medium Moisture Content on the CD Compressive Strength, (37)

200 180 C 0 160 m R 140 e r I e 120 s a s t i 100 i 0 80 v n e I 60 40 * 20

0q 0 0 2 4 6 8 10 12 14 16 18 20 0~ Medium Moisture Content, % ao ('w * Percent Of Value At 7.5% MC _W FIGURE 11.20 CDtl-a" Comparative Effect of Moisture Content .oi2 1-n on Medium Tensile and Compressive wCD CD Strengths. (37) 180 Each data point is at 0% I' *...... C 160 ...... matching moisture; content level.l -i "~ 2.5% R mo 140 e p I r a e 120 ...... t s s 100 5.0%°...... I v o e n 80 ...... i Moisture =| ^ - Content % 60 '------Level

40 _720.0.5%% || t7.5% 20 i I I 30 40 50 610 70 80 c)0 100 110 120 Relative Tensile, % * * Percent Of Value At 7.5% MC

I Corrugating Medium / 133

data do indicate, however, that mediums can and do have board caliper measurements. The combined board different cyclic creep characteristics, most likely related regains its caliper too fast to make it a sensitive to the effect of a wide range of furnish and process vari- process control tool. ables. The data presented in this chapter support the fol- 10. A 25 mill actual crushing level results in a 7.8% lowing observations concerning the interaction and inter- reduction in ECT for the largest volume corrugated relationship between the corrugating medium and the board grade produced by the industry, 200 lb Test combined board CD Edge Crush Test. SW or 32 lb/inch ECT SW.

1. The Edge Crush Test, ECT, is designed to measure 11. A possible alternative approach to controlling flute the pure edgewise compressive strength of corru- crushing in the box plant is to measure and control gated paperboard. the gap settings at the various pinch points in the process. 2. The ECT strength of corrugated board is important because of its effect on the top-to-bottom compressive strength of corrugated boxes.

3. A 10% change in ECT produces a 7.5% change in box compression.

4. The CD edge crush strength of the corrugating medium contributes directly to the CD ECT of the combined board. A stronger compressive strength medium, due to a higher basis weight or to a paper mill strength improvement process change, will increase the ECT of the combined board made from that medium, all other things being equal.

5. Increasing the elastic moduli of the corrugating medium improves the starting strength of the material and the ability of the medium to retain its strength during the flute forming process.

6. Increased medium pulp refining and increased wet pressing of the medium web on the paper machine both increase the elastic moduli of the finished medium paper rollstock.

7. Box plant process variables have a significant effect on ECT. The ECT is adversely affected by flute fracture, high/low flutes, leaning flutes, lower corrugator bond strengths, and crushing of the combined board flutes.

8. The effect of these medium material properties and these box plant process variables on the ECT may partially explain why the industry has not been able to agree on a universal, accurate and precise correlation equation between the medium CD crush strength and the combined board CD ECT.

9. The composite data show that it is not statistically possible, at the 95% probability level, to detect actual combined board crushing of less than 26 mils in a box plant environment by the use of combined Chapter 12

Flat Crush Test

The purpose of testing packaging materials is to production of a cost-effective flat crush strength. The provide a basis for predicting the physical properties and typical corrugating rolls are designed to accommodate the field performance of the final package products, (59). the "standard" 26 Ib/msf, 9-point medium, (32, 34, 130). The ultimate goal is to produce a cost-effective package The combined board flat crush strength is also influenced that provides the required protection of the product dur- by the properties of the corrugating medium and the cor- ing the packing, transportation, storage, and consumption rugating process variable, particularly the moisture con- phases of its life cycle, (123). tent and temperature of the medium at the time that the The most distinguishing feature of corrugated pa- flutes are being formed, (16, 34). perboard packaging material is the flute structure formed The Flat Crush Test has long been regarded as an by the corrugating medium, (129). The corrugated board indicator of the strength of the fluted structure in the flute structure can be damaged by the crushing forces package and as an indicator of the degree of crushing when it passes between the rollers and belts during the damage suffered by the corrugated board, (124, 130). corrugated container making process, (124). These proc- Crushing implies damage, and damage implies a quality ess pinch points include the single-faced web preheater problem, (130). The corrugated board flute structure can drum at the double-backer (if the flutes are against the be crushed in processing, lose caliper, feel relatively soft drum); the double-backer adhesive application nip; the to the hand, and still show no loss in the measured flat belted and pressure loaded hot plate and cooling sections crush strength. The Flat Crush Test measures the maxi- of the double-backer; and the various feedrolls, printing mum failure load supported by the medium flutes before units, die cutting units, and folding belts associated with the complete collapse of the flute structure occurs, (124). the boxmaking operation, (59, 124, 130). The flutes are The process crushing damage, prior to actual flat crush also exposed to crushing forces in the package filling line failure, affects the quality of the combined board by (start and stop forces of the product against the box causing permanent change to the elastic and nonelastic walls), during package handling (dropping) and during regions of the flat crush stress/strain curve characteristics, transportation of the filled boxes (acceleration and decel- (130). eration of the loads in the truck or rail car). A typical Flat Crush Test stress/strain (load/deflec- The Flat Crush Test measures the maximum force tion) curve is shown in Figure 12.1 for a 26 lb/msf A- that can be supported by the corrugating medium flute flute corrugated board grade. The curve exhibits six dis- structure when the force is applied perpendicular to the tinct zones. The first zone is the elastic region where the combined board surfaces. The flat crush strength is the deflection per unit load is linear and where full caliper, one corrugated board property discussed in this reference flute shape, and flute strength recovery will occur once publication that is controlled entirely by the fluted me- the loading force is removed. The second zone is where dium portion of the combined board structure. The liner- the initial flute deformation starts. The flute tip starts to board facings do not contribute to the flat crush strength flatten, and the sidewalls start to assume a more vertical in any fashion, except by the effect of the bond sites in orientation under the influence of the increasing flat keeping the flute spacing. The flat crush strength of cor- crush load. The third zone is the nonelastic deformation rugated board is influenced by the corrugating roll flute area, where the stress/strain relationship assumes a design characteristics of flute size (sidewall height), curved rather than linear shape. This zone represents the shape (flute tip radius of curvature and sidewall angle), continuation of permanent damage to the flute structure. and the clearances provided between the meshing teeth of The fourth zone is the flat crush failure point. It is the the upper and lower corrugating rolls (fluting damage to maximum load that the fluted structure can support. The the medium). A good flute design should provide for the fifth zone is where the failed flutes keep buckling and E FIGURE 12.1 X - Flat Crush Stress/Strain Curve r'- for A-Flute. 26 Ib/msf Medium, (124) COC

450

400

350 S t 300 r e 250

s 200

150 *b 100

50 (Loading) 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 Strain (Crushing), % Of Original Caliper (Deflection)

j Corrugating Medium / 137

folding, and this zone is followed by the sixth zone, dium, (30, 95). where the medium is in full contact with the linerboard, Figure 12.5 shows a regression equation relating the and the force is now attempting to compress the fibers in flat crush strength of the combined board to the strength the paper, (124). of the medium and to corrugating process variables. The Figure 12.2 shows the relationship between the data show that the combined board flat crush strength is medium Concora Test and the combined board Flat related to the medium concora crush strength, but, that Crush Test for both B-flute and C-flute corrugated board. the amount of concora strength retained after fluting is The medium basis weights used in the study ranged from controlled by the corrugating process variables of me- 20 Ib/msf to 26 lb/msf. The data demonstrate that B-flute dium moisture content and medium temperature at the yields a higher flat crush strength at a given medium point in time when the flutes are actually being formed. concora strength than does C-flute. This is due to the fact A higher medium rollstock moisture content, a higher that B-flute has more flute walls per unit area of com- medium web temperature, and more presteaming mois- bined board than C-flute, and the fact that the B-flute ture addition to the medium all improve the retention of sidewall height is shorter than that for C-flute and less flat crush strength during fluting, (16, 95). The specific prone to buckling under load. The data also show that the influence of the medium rollstock moisture content and flat crush strength does not increase linearly with increas- moisture added to the medium web by the steam showers ing medium concora strength. A similar curvilinear rela- is shown in Figure 12.6. The experimentation was done tionship between the corrugating medium MD Ring using commercially produced medium and a pilot-size Crush Test and Flat Crush Test is shown in Figure 12.3 corrugator. Between the range of 5% to 13% moisture for commercially produced mediums varying in basis content, a 1 percentage point increase in the medium web weight from 26 lb/msf to 54 lb/msf, and fluted on a pilot- moisture increases the flat crush strength by an average size corrugator, (39). These two experiments indicate that of 1 psi or 2.8%, (16). heavier basis weight mediums lose more flat crush The effect of the medium temperature on flat crush strength during fluting. This can be attributed to the in- retention during fluting is shown in Figure 12.7. The creased flute sidewall damage caused in the thicker me- experiment was done using handsheets and a modified dium by the reduced relative clearance between the concora test instrument. The data show that, on average, meshed teeth in the upper and lower corrugating rolls, a 20 deg.F. increase in temperature increases the flat (15, 34, 39, 73, 82, 130). crush strength retention during fluting by 1.25 lb or The medium Concora Test is used to measure the 3.3%, (95). flat crush potential of medium rollstock before actual With these effects, it is no wonder that the data in corrugating. It was developed as a tool to predict the flat Figures 12.2 and 12.3 show that increased medium basis crush strength expected in the final combined board. The weight does not always 'translate into higher combined Concora Test involves the simulation of the commercial board flat crush strength. Neither experiment was de- corrugating operation by using a small, bench top corru- signed to minimize the strength loss due to fluting of the gating unit. The half-inch wide sample of medium is heavier weight medium materials. Most corrugating rolls fluted, faced with an adhesive covered tape, and crush and corrugator process conditions are geared to 26 lb/msf tested. In actuality, there are several significant differ- or lower basis weight medium, (15, 32, 34, 39, 73, 82, ences between the Concora Test and commercial corru- 130). gating. The Concora Test uses A-flute-shaped heated The effect of medium properties on the retention of corrugating gears; the medium is not preheated or flat crush strength during fluting is shown in Figure 12.8. presteamed before being fluted; and the corrugating is The experiment was conducted using Formette hand- done at an extremely slow speed, (15). sheets, ranging in basis weight from 26 lb/msf to 40 Figure 12.4 shows the relationship between the MD lb/msf, fluted on a pilot-size corrugator. The data indicate STFI crush strength of the sidewall areas of the fluted that the flat crush retention is improved by a corrugating medium and the flat crush strength of the combined medium material with a higher density, a lower basis board. The study used commercial 26 lb/msf semichemi- weight, a lower MD elastic modulus, and a higher ZD cal and recycled mediums combined on a pilot-size cor- (thickness direction) elastic modulus, (29, 33). rugator. The data show a linear relationship between the One objective at the paper mill is to improve the flat two test values over the range of strength covered by the crush cost-effective quality of the corrugating medium. study. This indicates that both tests reflect the fluting However, that quality improvement must carry through damage that occurred during corrugation. Comparison of to the combined board after high-speed corrugating if it the fluted and unfluted MD STFI crush strengths shows a is to have true commercial value, (30). Figure 12.9 30% to 40% MD strength loss due to fluting. It is hy- shows the influence of paper mill pulp refining and paper pothesized that the effect is due to shear forces that dis- machine wet pressing on the retention of flat crush rupt the fiber-to-fiber bonding in the corrugating me- strength during fluting. The experiment used Formette F --

FIGURE 12.2 C0 -- = IC)Q Relationship Between Medium Concora °~ sO co F VI! and Combined Board Flat Crush, (73, 82) fSj

50- F I a 45- t 40- I---- :7: * C r u 35-

S ; 41 h 30- .. ..- . : ....- . i

Pp 25- * C-Flute S i 20- © B-Flute t 15- 35 40 45 50 55 60 65 70 75 Medium Concora Strength, lb. FIGURE 12.3 Relationship of Medium MD Ring Crush to Combined Board Flat Crush. (39)

50 48 ...... ' ' :...... ' ...... C F 54.2 0 I 46 -...... ' .' ...... 33.l8-...... 8 ...... m a _~~~~~~~~~~~~~~~~~~~:i...... ,L,...... b t 44 - i C 42 40.8, , n r e u 40 d s h 38 Numbers are measured basis weights B 36 ,/ ...... iof corrugating medium used in the 0 i26 I4; I experiment. a s 34 ..2 6 .4 ...... r i d 32 0 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 (rc~o

Unfluted Medium MD Ring Crush, lb/in. C'D

CLkid EltI-

I w0 .... - ...... -- I" 0 -- FIGURE 12.4 3 g Relationship of MD STFI Compression W , of Fluted Medium and Flat Crush, (30) t'1

39 38 F I 37 a 36 t 35 C r 34 U sS 33 h 32 I 31 Pp s 30 i 29 28 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 MD STFI Compression of Fluted Medium, kN/m 26 Ib/msf Medium

U .I FHGURE 12.5 Effect of Corrugating Variables and Medium ProDerties on Flat Crush. (16)

FC = a(A) + b(B) - c(C)2 e(E) 2 + f(F) - g(G) + h(H) - 37.5 r 2 = 0.82 a = 0.00168 A = Speed, fpm b = 1.58 B = Nip Moisture Content, % c = 0.0443 C = B d = 0.292 D = Nip Temperature, Deg.F e = 0.00088 E = D f = 1.35 F = Roll Moisture Content, %

g = 0.0854 G = F ('3 h = 0.555 H = Concora, lb 'Q

C F f. 0 I 38- m a b t Roll C 36 - Moisture .. n r Content ...... e U d s h 34- Roll Moisture B Content Plus 3% 0 Moisture Added to p ..el-1-Z a s 32 - *dium By Steam Shower r d

30 I 4 5 6 7 8 9 10 11 12 13 14 Medium Web Moisture Content During Fluting, % 1, 1

FIGURE 12.7 Effect of Fluting Gear Temperature on Medium Concora Strength, (95)

46 -

44 - ...... -

C 42-- ..- .. .. -.- II. - I... 0

n 40 ..... "' " ' c 0 r

b 34- .f .-

32 - ...... I.! ...... .. - ...... q i 0

30- i 11I I I I 9 K 10 30 5S0 70 90 110 130 150 170 190 (rQ5, 4 Corrugating Roll Temperature, Deg.C P.0 Actual Data Not Shown 9 4 I W i1i FIGURE 12.8 - Effect of Medium Properties on the i Flat Crush Strength Retention After Fluting. (29) F 1.0 -

ta 0.9 C. .. . IEx ...... MD Elastic... Stiffness. r 0.8 ...... Ez ZD Elastic Stiffness. u . BWt = Medium Basis Weight. h 0.7 R tR 2 e r = 0.86 n ni0 n 04.4 (units) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 ((Ex/Ez)(BWt)/(Densi ty) Actual Data Points Not Shown Ii . I~~~~~~~~~~~~~

..... FIGURE 12.9 Effect of Pulp Refining and Wet Pressing on Flat Crush Retention After Fluting 40 Ib/msf Corrugating Medium, (15)

M 0.50 D C 0.45 r u s 0.40 h R 0.35 a t 0.30 0

* 0.25

0.20 400/Low 400/Medium 325/Low 325/Medium 5 Process Condition, Freeness/Wet Pressing Oq CD~ * Flat Crush (psi) Divided By a1 Concora Crush (lb) 146 / Chapter 12 Flat Crush Test handsheets and a pilot-size corrugator. The data indicate Measured caliper and measured Flat Crush Test are that lower wet pressing and increased pulp refining are poor indicators of the actual degree of flat crush strength beneficial, (15). Others have concluded that increased damage caused by crushing of the combined board. They wet pressing is beneficial because of the reduced caliper do not accurately reflect the permanent changes that have of the medium, (23, 24, 26, 39). Both increased refining occurred to the characteristics of the flat crush and increased wet pressing benefited the CD edge crush stress/strain curve, Figure 12.15. Combined board can be strength retention during fluting, Figurell.16 in Chapter crushed up to 40% without any major change to the 11. This is, then, an area that may require controlling the measured, maximum flat crush strength or to the deflec- papermaking process to obtain a reasonable balance be- tion of the sample at Flat Crush Test failure. Yet, the tween two combined board properties important to pack- elastic flat crush strength zone of the material has been age performance, Edge Crush Test and Flat Crush Test. destroyed, and the non-elastic zone severely deformed. The effect of fractured flutes on flat crush strength is These changes can cause the board to feel soft and to be shown in Figure 12.10. The degree of fracture repre- less effective in its cushioning properties, (59, 124). sented by the data is very slight, being just at the break- Figure 12.16, top graph, compares actual combined point between fractured and unfractured medium flutes. board flute crushing to measured crushing. The data This level of defect reduces the flat crush strength by show that caliper measurement in a box plant is a poor almost 10%, (129). indicator of actual crushing and crushing damage. The The effect of crushing of the combined board flutes rapid rebound in caliper after crushing, more than 90% on flat crush strength is shown in Figure 12.11 for A- recovery within the first minute after the crushing force is flute board made with 26 Ib/msf medium. The data show removed, and the relatively high variability in combined that a measured crush of 5 mils reduces the flat crush board caliper measurements indicate that actual crushing strength by 5%, (130). However, it should be noted that a below 25 mils will not be statistically detectable at the measured crush of 5 mils represents an actual crush of 95% probability level, in the best of corrugated box approximately 33 mils, (5). plants, (5). The degree of measured crush caused by a given nip Figure 12.16, bottom graph, also shows that the force is affected by the concora strength of the medium, relationship between actual and measured crush depends as shown in Figure 12.12. The experiment used four on the nature of the nip point. The use of rubber-covered commercially produced corrugating mediums ranging in rolls that can compress and absorb some of the crushing basis weight from 20 to 26 lb/msf. The combined board deflection reduces the actual corrugated board crushing, was produced on a commercial corrugator and crushed in (130). With steel-to-steel rolls, where the gap is mechani- the laboratory using a rubber-to-steel roll nip. The data cally set, all corrugated board is equally crushed, regard- show that lower concora strength mediums are more sus- less of its flat crush stiffness, (24). ceptible to crushing under conditions where the rubber- Figure 12.17, top graph, shows that a higher basis covered nip roll can compress and reduce the crushing weight medium has a higher caliper recovery than a done to the corrugated board, (73, 82). With steel-to-steel lower basis weight medium. This effect can be attributed rolls, where the gap is mechanically set, all corrugated to the greater flute rigidity provided by the heavier basis board is equally crushed, regardless of its flat crush stiff- weight medium. Figure 12.17, bottom graph, also shows ness, (24). The use of compressible rubber rolls is the that multiple crushing of the same area of the combined basic concept of the so-called "no-crush" systems being board results in a higher degree of nonrecoverable caliper sold commercially to box plants. The data also show that loss, (130). higher crushing forces cause larger permanent caliper In summary, the technical information presented in losses (flute deformation) in the combined board. The this chapter supports the following observations concern- difference between B-flute and C-flute effects is shown ing the flat crush strength of corrugated board. in Figure 12.13. B-flute combined board has a higher flat crush strength than C-flute board because of the closer 1. Corrugated board is exposed to numerous flat crush spacing of the flutes and because of the lower height of stresses during the manufacture of the package and the sidewalls of B-flute. This gives B-flute a greater resis- during the field use of the package. tance to the crushing forces. Similarly, A-flute is less resistant to crushing than C-flute, (73, 82). 2. The Flat Crush Test measures the maximum load Figure 12.14 also shows data on the effect of flute that can be supported by the corrugating medium crushing on flat crush strength. These data do not agree flutes before total destruction of the flute structure with the previous data, (124). No explanation was given occurs. The characteristics of the entire flat crush in the publication, and the author has no rationale to offer strength stress/strain curve are more important to except that there may be some technical errors in these package performance than just the maximum failure data. strength.

I FIGURE 12.10 Effect of Fractured Flutes on Combined Board Flat Crush Strength. (129)

F 40 I a 38 t36 C 34 r u 32 s h 30 ' 28 P 26 i 24 22 0 o0 20 No Fracturee Fractured Flutes S' Flute Fracture Condition (ACL El9-

3 s o FIGURE 12.11 o-~ Effect of Flute Crushing on Combined

Board Flat Crush Strength, (130) t3

100 F U I n 95 Ca a r u 90 S C h 85 r e u d 80

h r 75 , i 70 % 9I n 65 o a f I 60 0 2 4 6 8 10 12 14 16 18 20 Measured Crushing, % of Original Caliper A-Flute, 26 Ib/msf Medium Actual Data Points Not Shown I, FIGURE 12.12 Effect of Medium Concora Strength on Nonrecoverable Crushing of Flutes. (73)

5 C 0 h a -5 n g e -10 I -15 n -20 C a -25 I i -30 p e r -35 (mils) 5 -40 0;' 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 Oo et Medium Concora Crush, psi r- El Average of A & B-Flute.

L »10 ...... ------I..- ...i1 FIGURE 12.13 a- Effect of Flute Size on .| Nonrecoverable Caliper Loss, (73) t

L 45 - s 40 -...... s 40- : C-Flute I 35 B- Flute

C a 25 Average C-Flute Flat Crush = 33.1 psi ep Average B-Flute Flat Crush = 38.4 psi r I, 10----- m 5- I 0- s 0 10 20 Crushing Force, psi Average Of Four Different Concora Crush Strength Mediums. j] FIGURE 12.14 Effect of Combined Board Crushing on Flat Crush Strength. (124)

110 110 M 0 105 105 e f a F 100 100 s 0 I r 0 95 95 I a f e i t 90 ...... 90 d g U i C n 85 ...... 8 5 C r n r r a 80 80 U u u s I s s 75 75 h h h I C e 70 n a d 70 rush -k- Caliper g I 65 - --.- ..- - ...... -I. ______...... 6 5 i 0 60 -- l l 60 % p e 0 10 20 30 40 50 60 70 80 90 100 110 S' r (OQ Actual Crushing, mils (LD C-Flute Board

Z== ,paeog pglbnjIoQ 9;nIli-o ww 'Uleil 09's 00T~ OT.Z 0o'?g 091- 00L 0O*0 00*0

..- 09L

.. 001 d S

- 0 9 ~

N Z , (69) 'DuiqsnflJdei PJBOG pauiqwoo lo I En 0 2 F- PaO3r :Sv9Ajflo uieilS/ssGj4S LIsnflelA~I u In -- UE N - 9VZ;L 3Uflno 'n 4! 4. Corrugating Medium / 153

iI FIGURE 12.16 Effect of Combined Board Crushing on Post-Crushing Combined Board Caliper

Relationship Between Actual Combined Board Crush and Measured Crushing. (5)

M e 14 a 12 s * u 10 ...... e 8 d 6 .. C 4 ...... * , ...... r 2 .'g....'l*'** ; ~.. t..''~.'..'.'.'.~'. ... ' -.. u s 0 h -2 _,,,,,,,,._,,,,_,,.,_,_,_...... _ ... ^ ..-..- -, -...... -..... - -...-.. .. : ..._ ...... _...... n -4 9 0 5 10 15 20 25 30 35 40 45 50 55 60 (mils) Actual Crushing, mils 42-26-42,

Effect of Roll Nip Hardness on Measured Crushing, (130) R 0 30-

S 25 ...... -. tC e r - ...... eu 2 0 ;

s 1 ......

e 5 -- ; . -.... o Cu ..i ..-...... , ...... i ...... e m : 5- 0 s 0 5 10 15 20 25 30 Rubber-To-Steel Roll Crushing, mils

i. II 154 / Chapter 12 Flat Crush Test

ii r- FIGURE 12.17 Effect of Combined Board Crushing on il Postcrushing Combined Board Caliper

Effect of Medium Rasis Weight on Measured Caliper After Crushing, (130) I

50- . .

M 445 o - : 2 6 : ...... i...... ;...... :...... i...... e 35- 26 lb/msf ...... a 33 0 5 - -...... -...---- ...... -...... --... r 300 . .. . . i . e 2 5 . -

C 15 - r u 10 h 5 I (mils) 0 ti 0 10 20 30 40 50 60 70 80 90100110120130140150 Actual Crushing, mils Ij A-Flute Corrugated Board. I

Effect of Multiple Crushing on Measured Crushing, (130)

60- ' ·. - Me 55. . . -..- ...... a 50 - . . 1 Crush Pass s 4 5 - ...... u 40 ...... 2 Crush Passes ...... r 35 ..... e 30 - --- T----y------...... : ...... -.-......

C 20 ......

(is 0 0 10 20 30 40 50 60 70 80 90100110120130140150 Actual Crush, mils Average for A-Flute, 26 & 30 Ib/msf Medium.

|| Corrugating Medium / 155

3. The flat crush strength of the combined board is 10. Flute fracture adversely affects the flat crush influenced by the flute design of the corrugating strength. rolls, by the physical properties of the corrugating medium, and by corrugating process variables. 11. Combined board caliper measurements in the box plant are a poor and insensitive method for judging 4. Approximately 30% to 40% of the inherent flat the flat crush strength damage done by crushing of crush strength of the medium is destroyed during the the corrugated board. fluting process. Stronger or heavier basis weight mediums may or may not result in increased flat crush strength in the combined board. Most corrugating rolls are designed for 9-point, 26 lb/msf corrugating medium.

5. The major flute design variables affecting the resultant combined board flat crush strength include clearance allowances in the flute flank areas of the meshed corrugating rolls, the flute flank design angle, and the radius of curvature of the flute tips. Larger clearances and larger radii of curvature are favorable for flat crush strength retention. The flute flank angle is a complex issue.

6. For a given corrugating medium and a given fluting efficiency, B-flute has a higher flat crush strength than C-flute, and C-flute is stronger that A-flute. The differences are attributable to the effect of shorter flute sidewall height (less buckling) and the greater number of flutes per foot (more support) for B-flute over C-flute, and C-flute over A-flute.

7. A higher medium web temperature and moisture content reduce the flat crush strength loss during fluting. A I percentage point change in moisture content affects the flat crush strength by 2.8%, and a 20 deg.F. change in temperature affects the flat crush strength by 3.3%.

8. The flat crush strength retention during fluting is favorably affected by a higher medium density, a lower medium MD elastic modulus, and a higher ZD (thickness) elastic modulus. Added refining and wet pressing in the paper mill improves both the inherent, unfluted medium flat crush strength and the percent of flat crush strength retained in the combined board after fluting. The wet pressing effect is not as clear as the refining effect.

9. The Concora Test, while useful, is not consistent in accurately predicting the flat crush strength of the commercially fluted combined board. The Concora Test, while superficially similar to actual corrugating, does not include the factors of flute design, medium preconditioning, and fluting speed found in commercial operations. These factors have a large effect on damage caused by fluting.

I I Chapter 13

Package Performance Issues

The corrugated packaging industry is thought, by quires an analysis of the functional requirements of cor- some, to be a commodity-type business. Indeed, the lin- rugated packaging and the role, if any, that the medium erboard and medium segment of the industry has en- plays in meeting each of the requirements. gaged, for years, in material swap programs between Figure 13.1 lists the seven major corrugated box companies in order to reduce freight costs. A swap pro- performance criteria considered critical by box custom- gram consists of the Company-A mill shipping rollstock ers. The seven criteria are: to contain the product, to pro- to a nearby Company-B box plant rather than to a far tect the product, to stack the product, to advertise the away Company-A box plant, which is located near a product, to operate reliably on high-speed packing lines, Company-B mill. The Company-B mill then reciprocates to meet regulatory requirements, and to meet any indi- by shipping an equal amount of rollstock to the Com- vidual customer specifications agreed upon. These seven pany-A box plant. Swapping was done under the as- major criteria may vary in importance depending on the sumption that all linerboard and all medium of a speci- individual packaging application; however, they are fied grade produced in the United States were equivalent, equally important in terms of the overall corrugated as long as they produced combined board that met the packaging business. Item 222/Rule 41 specifications. Under the pre-1991 There are four mechanisms by which the box can regulations, the specification parameters for linerboard fail to contain the product. First, the manufacturer's joint. included only mullen and basis weight, and for medium can open. The failure can occur in the linerboard-to- included only basis weight and caliper. "A 26 lb/msf linerboard glue joint made when the box was folded and medium was a 26 lb/msf medium, was a 26 lb/msf me- glued. This failure mode does not directly involve the dium." This has changed. The industry is now looking corrugating medium. The failure can occur in the corru- very closely at the other quality attributes of the liner- gator bond between the linerboard and the medium. The board and medium, quality attributes which influence influence of the corrugating medium on this type of fail- those functional performance properties of the corrugated ure mode is discussed in Chapters 6. 7. and 8. package which are important to the package user. Second, the flaps of the sealed and filled box can The box plant segment of the industry has always open. This type of failure involves the linerboard and been a job shop type of business. The boxes made for a adhesive application. It does not involve the medium. given customer are unique to that customer. Indeed, the Third, the box scorelines can separate. Research boxes are generally unique for each product line mar- work done at the Institute of Paper Science and Technol- keted by a given customer. The uniqueness can involve ogy has shown that scoreline separation failure is initi- differences in the combined board construction, such as ated when the stress on the scoreline exceeds the tensile singlewall, doublewall, or triplewall; such as A-flute, C- energy absorption (TEA) of the combined board at the flute, B-flute, E-flute, or F-flute; and/or such as the liner- scoreline. Once the failure has been initiated, further board and medium grades. The differences can also in- propagation of the scoreline separation is governed by volve other box attributes, such as box size and style; the tear strength of the corrugated board at the scoreline. such as the use of nonskid, nonabrasive, or moisture bar- The corrugating medium will contribute to the TEA of rier coatings; such as the print logo; and/or such as the the combined board in the flap score areas but not in the unitizing pattern. body score areas. It does not contribute to the body score This publication deals with the subject of corrugat- TEA since the flute structure will expand like an accor- ing medium and its influence on box plant operations, dion, and the tensile force will be applied only to the combined board properties, and package performance. linerboard components. The medium will contribute to The effect of the medium on package performance re- the tear resistance of both the flap and body scorelines 'T3 - FIGURE 13.1 W _ CD 0 Major Corrugated Box Performance Criteria

K - o (i CD V) -kContain the Product. C XA GI L* Protect the Product.

* Stack the Product.|

* Advertise the Product.

*| Perform on High-speed Packing Lines.

| * Meet Regulatory Requirements.

* Meet Customer Specifications. Corrugating Medium / 159 and, therefore, influence the severity of the scoreline singlewall board. The effect of the corrugating medium separation. on the combined board flexural stiffness is discussed in Fourth, the corrugated board can be punctured. The more detail in Chapter 10. corrugating medium adds directly to the puncture resis- The medium does not influence the ability of the box tance of the combined board. The puncture of the corru- to effectively advertise the product. The printing is done gated board will increase directly with the increasing tear on the linerboard surface. The flute size does have some strength of the medium and with the increasing height of effect on the print quality. The linerboard, particularly a the flute. For a constant linerboard and medium material, low basis weight linerboard facing, can deflect in the the puncture energy will be the highest for A-flute and areas located between the flute tips (washboarding) and followed in decreasing energy by C-flute, B-flute, E- produce an uneven surface for printing. The tendency to flute, and F-flute. washboard is reduced as the spacing between the flute The protection of the packaged product is influenced tips decreases. F-flute would be expected to have the by the cushioning characteristics of the combined board. most even surface, and A-flute would be expected to The ability of corrugated board to absorb and dissipate have the most uneven surface. shock impact forces is governed by the flat crush charac- Assuming proper box dimensions and proper vac- teristics of the fluted medium. The influence of the cor- uum systems on the box setup machine, the most impor- rugating medium on the combined board flat crush is tant factors affecting the performance of boxes on high- discussed in Chapter 12. speed packing are the flatness (lack of warp), stiffness The stacking strength of a corrugated box is deter- (rigidity) of the corrugated board, and well- defined mined by many factors, including those related to mate- scorelines. The medium has very little effect on warp and rial strength, package design, and field use conditions. should provide sufficient rigidity to the board, provided The scope of this publication is limited to the material that there are no blatant quality problems, such as frac- strength factors influenced by the corrugating medium. tured flutes or unbonded areas between the linerboard The box compressive strength model developed by Rob- and medium (blisters or loose liner defects). ert McKee and published in 1963 shows that the top-to- The medium can influence the ability of the box to bottom compressive strength of RSC-style boxes is de- comply with various regulatory requirements. Specifica- termined by the CD Edge Crush Test (ECT) and the MD tions dealing with compressive strength and drop testing and CD Flexural Stiffness of the corrugated board used are particularly influenced by the corrugating medium. to manufacture the box. The different regulations (Item 222, Rule 41, DOT, FDA, The corrugated medium contributes directly to the USDA, Military, United Nations, etc.) have specification combined board CD ECT. The CD ECT is proportional requirements that are too complex and varied to discuss to the sum of the CD edge crush strength (Ring Crush or in detail in this publication. This is also true of the indi- STFI Crush) of the linerboard and medium materials vidual customer specifications. The reader will have to used to manufacture the corrugated board. The effect of seek this information elsewhere. the medium on CD ECT is discussed in Chapter 11. In summary, the major box performance criteria The flexural stiffness of the combined board is equal most strongly influenced by the corrugating medium are to the sum of the stiffness contribution (Elastic Modulus "Stack the Product," "Contain the Product," and "Protect times Moment of Inertia) of the various linerboard and the Product." These three areas are discussed in more medium components. This assumes that the corrugated detail in Chapters 14 and 15. board acts as a unified, sandwich- type structure. The It is important to keep in mind that the relationships corrugating medium serves to bridge and bond the liner- between medium properties and combined board proper- board plies together to form the unified structure. The ties described in Chapters 14 and 15 are based on having effect of the medium on the strength of its bonding to the good box plant fabrication and converting quality. The linerboard is discussed in Chapters 7 and 8. results shown are not influenced by the obvious fabrica- The direct stiffness contribution of the medium to tion and converting defects, such as high/low flutes, the total combined board stiffness is relatively small as fractured flutes, leaning flutes, poor corrugator bond compared to the contribution of the linerboard. In fact, strength, poor scoring, excessive crushing, or out-of- the medium does not contribute to the MD Flexural square boxes. The effects of these process defects were Stiffness Test because of the ability of the flute structure discussed in prior chapters. to flex in this direction. The medium does contribute to the CD Flexural Stiffness Test. However, the medium contribution is relatively small because of its small Mo- ment of Inertia, being located in the center portion of the sandwich structure. The medium contributes only between 5% and 10% to the total Flexural Stiffness Test in Chapter 14

Package Compressive Strength

The performance standard for corrugated paperboard lies between the pure compression and the elastic buck- packaging is generally based on compressive strength ling physical performance regions of the corrugated and the ability of the package to retain its strength and board, (36, 37,44, 55, 123). rigidity under humid conditions. The stacking capability Figure 14.2 shows a similar mathematical model for is the major characteristic that differentiates a corrugated predicting the end-to-end compressive strength of a cor- box from a paper bag, a plastic bag, a paper wrap, a plas- rugated box. This style of box is typically used when unit tic wrap, or any other type of flexible package. Box loads of filled boxes are handled during transportation compression strength is more relevant today because of and warehousing with trucks equipped with side-to-side the changes that have occurred in the handling and ware- clamping devices. In an end-to-end compression design housing of packaged goods. Rule 41 and Item 222 were box, the outer flaps are the main contributors to the total changed in 1991 to recognize an alternate, compressive box compressive strength. Compression failure generally strength-based specification for defining acceptable cor- initiates in the outer flaps, in the area located between the rugated packaging. Box compression, in conjunction inner flaps. The box length, width, and depth appear in with flat crush strength and puncture resistance, is also a this equation. The top-to-bottom compression model good indicator of the rough handling endurance of the includes only the length and width. The corrugated board package, (2, 123). strength factors included in the model are the combined The paper industry is continuing to move in the di- MD and CD flexural stiffness and the MD Edge Crush rection of using heavily loaded or long nip wet presses on Test. The MD edge crush strength is determined only by paper machines to improve the strength levels obtained the edge crush strength of the linerboard facings. The from each pound of paper fiber. The industry is reducing medium does not directly contribute to MD ECT as it the weight of packaging and eliminating overpackaging. does to CD ECT. However, the medium does, in MD Corrugated packaging is now more consistent in com- ECT, act to reinforce the linerboard at the bond sites and pressive strength performance. The corrugating medium to act as a bridge between the linerboard facings so that plays several key rolls in determining the compressive they act as a unified structure. The reported average abso- strength performance of boxes. It adds directly to the lute error of the estimate for the end-to-end box com- edge compressive strength of the corrugated board. It pression model is 8.2%, as compared to the reported top- contributes marginally to the bending stiffness of the to-bottom compression model average absolute error of combined board. It is the one and only component that 6.1%, (141). can keep the linerboard facings separated for stiffness The effect of the corrugating medium basis weight and yet bound together to form a unified structure, all at on the top-to-bottom compressive strength of corrugated the same time, (9, 24). tubes is shown in Figure 14.3. The tubes were produced Figure 14.1 shows a mathematical model for pre- on a pilot-size corrugator using commercially produced dicting the top-to-bottom compressive strength of RSC- linerboard and medium. The effect of medium basis style corrugated boxes. It was published by Mr. Robert weight and flute size on the top-to-bottom and end-to- McKee in 1963. The model shows that the box compres- end compressive strength of boxes is shown in Figure sive strength is determined by the size of the box (as in- 14.4. The boxes were produced in a commercial box dicated by the box perimeter), by the pure compressive plant operation using commercially produced linerboard strength of the combined board (as indicated by the CD and medium, (73, 82, 99). Edge Crush Test), and by the elastic buckling resistance All three plots show that the effect of the medium of the corrugated board (as indicated by the combined basis weight on package compression strength is not lin- MD and CD Flexural Stiffness). Box compression failure ear. The data indicate that increasing the medium basis o~ tQj

FIGURE 14.1 (D 0 Top-To-Bottom Box Compression Equation en :i 0 . Tg w(A^

w CT,^' CD

0.746 0.127 0.492 P = 2.028 (Pm) [(Dx)(Dy)] (Z)

P =Box Compression Strength, lb. P = Corrugated Board CD Edge Crush Test, lb/inch. Dx = Corrugated Board MD Flexural Stiffness, lb-inch. Dy = Corrugated Board CD Flexural Stiffness, lb-inch. Z = Box Perimeter, inch.

R. McKee, Paperboard Packaging, August 1963 ...... FIGURE 14.2 End-To-End Box Compression Equation. (141)

I"1R 0 .787[(Dx)(Dy) mBV, 0.512 . 0.574 P = 0.15 (Pmx)(2d) + 3.10(Pmx) [(Dx)(Dy)] (1+ L ) (W) P = Box Compressive Strength, lb. Loading Pmx = MD Edge Crush Test, lb/inch. Force d = Box Depth, inch. Dx = MD Flexural Stiffness, lb-inch. Dy = CD Flexural Stiffness, lb-inch. W = Box Width, inch. L = Box Length, inch.

0O K L m~ Inner I sH(t & Flaps CD

0Jw FIGURE 14.3 {a

Effect of Medium Basis Weight on the CD Top-To-Bottom Tube Compressive Strength. (99) .

T 2000------u .. ... I...... ,...... , . j ,, ^ ^. . .. bLI 19001 9 0 - ...... - j -.j -

C~ 1800~ ~ ~0 ~ ~~ ....~... .. ~~~ ~~~~ *- **...... -..-... . 1.... . I...... ^ ...... i -...... i...... C 1700m160 0 - - -- 1------**- -j--- ...... *--* )* - -t..^^ .****- j- * -- -- i- -..-.. m0 n -(1600 -lo o - -...... - .....j.. .,/ ...., j...... i...... ,.. Pr 1500 e 1400 ...... s s 1300 o 1200 - n 1100 1000 b 900 , , 1 , , 10 12 14 16 18 20 22 24 26 28 30 32 34 36 Medium Basis Weight, Ib/msf Averaged over three linerboard basis weights. L J Corrugating Medium / 165

ii11 I! H FIGURE 14.4 !II

Effect of Medium Basis Weight on Top-To-Bottom II and End-To-End RSC Box Compressive Strength Iii: ,I,1 Effect of Medium Basis Weight on Top-To-Bottom RSC Box Compression, (82)

I B 800 - o 775o o ......

C 725-- Cm 7 70 02 °5- ...... i- +-......

~ear~~~~,//~ 657~~05 ~ ...... s 625- ...... 600 - ...... n 575 ;- , , I (lb) 19 20 21 22 23 24 25 26 27 Medium Basis Weight, Ib/msf 42 Ib/msf Linerboard Facings. Average of Two Box Sizes.

Effect of Medium Basis Weight on End-To-End RSC Box Compression, (82) I

B 850:- 0 ......

e 600 - ,.". : s 550 -...... -.. ..-- - ...... -...... I + B-Flute C-Flute 50 I ' o 550: n 450-21 2 2 , 19 20 21 22 23 24 25 26 27 Medium Basis Weight, Ib/msf 42 Ib/msf Linerboard Facings. Average of Two Box Sizes. .i :i il 166 / Chapter 14 Package Compressive Strength weight has a diminishing benefit on the observed pack- to-bottom box compression increase of 29%. The total age compressive strength. This observation can be at- fiber usage, linerboard and medium, increased by only tributed to the fact that the medium contributes little to 17%. The data are based on commercially produced the combined board flexural stiffness strength term in the boxes, (36). models. It may also be attributed, in part, to the fact that A corrugated box is a complex structure to which the the higher basis weight medium may be damaged more traditional engineering concepts of stress, strain, equilib- by the fluting process. Most corrugating rolls are de- rium, and compatibility can be applied. The engineering signed to operate most effectively with 26 lb/msf, 9-point objective is to have all of the components act as a unified medium material, (32, 34, 130). structure and to have each component reach its maximum It should be noted that C-flute board yields a higher load bearing capability at the same time so that the total top-to-bottom compressive strength box than B-flute structure failure occurs at the highest possible loading, board, all other material and quality factors being equal. (50, 52, 53, 58). The top-to-bottom compression force on However, the opposite is true for the end-to-end com- a RSC-style box must be supported by the four vertical pression where B-flute boxes are stronger than C-flute panels of the box. The load causes the panels to buckle boxes. In both cases, the C-flute board has a higher flex- and bend in the center while the comers of the box, ural stiffness than the B-flute board, and a higher flexural formed by the junction of two panels, remain vertical. As stiffness will increase the box compressive strength, as the compression load increases, the vertical edges of the shown by the equations in Figures 14.1 and 14.2. How- panels must support a higher percentage. of the total. load ever, the flute size has little effect on the CD edge crush on the box. strength of the combined board, but the flute size has a Assuming that the box side panels bulge outward, large effect on the MD edge crush strength. With MD the inner linerboard experiences a compression force, and ECT, the closer flute spacing in the B-flute board serves the outer linerboard facing experiences a tensile force. to better reinforce the compression resistance of the lin- The maximum compression stress (load) occurs in the erboard facings. inner linerboard facing at the side panel comers formed The MD ECT test specimen can be envisioned as a by the two panels and the box flaps. The maximum ten- series of tiny linerboard plate segments piled one on top sile stress occurs in the outer linerboard facing at the side of another. The top and bottom of each "plate" are repre- panel comers formed by the two panels and the box sented by the corrugator bond gluelines. The shorter flaps. The linerboard compression strain (deflection) at height "plates" that result from the closer flute spacing failure is less than the linerboard tensile strain at failure. with B-flute increase the reinforcement of the linerboard Actual box compression failure, therefore, initiates in the facings against buckling between the flute tips. This inner linerboard facing at the comer of the side panel, I added reinforcement produces a higher maximum edge assuming no major quality problems, such as a blisters, crush strength. The edge crush strength has a much exist. The typical bowed failure lines seen on the outside greater relative effect on box compression than does the of the box are actually post-compression failure symp- flexural stiffness strength, (73, 82, 99), and this causes toms. This primary box compression failure mode is also the B-flute boxes to be stronger in end-to-end compres- true for internal forces exerted on the panels of a box by sion than C-flute boxes of the same size, material, and a flowable product, such as resin pellets or a liquid, that quality. Similarly, C-flute boxes are stronger than A-flute is packed inside the box, (52, 53, 58). I boxes in end-to-end compression. This failure mode is demonstrated in Figure 14.7 Because of these effects, the medium basis weight where the top-to-bottom compression failure load for two (can be interpreted as strength) affects the end-to-end box unbalanced linerboard constructions is compared. The compression slightly more than the top-to-bottom box 42-26-69 lb/msf C-flute combined board had, on aver- compression. This experimental observation is demon- age, a 31% higher compressive strength than the 69-26- strated in Figure 14.5 where the change in end-to-end 42 Ib/msf C-flute construction, (52, 53, 58). box compression due to changes in the medium basis The application of engineering approaches to corru- weight is plotted against that for the respective top-to- gated paperboard packaging raises the question about the bottom compression. The slope of the regression lines for balance in properties and/or basis weight between the both the B-flute and C-flute constructions documents a linerboard facings and the medium. Figure 14.8 shows greater rate of compression change with medium basis one approach to handling this linerboard/medium balance weight for the end-to-end compression-style package, issue for singlewall corrugated board boxes. It is pre- (82). sented only as an example of the concept of material bal- Figure 14.6 shows the results of another experiment ance. This author does not take a position as to the valid- to determine the effect of medium basis weight on box ity of the conclusions based on this particular computer compressive strength. The data indicate that a 61% in- model. crease in medium basis weight (strength) produces a top- The researcher used the box compression model

I -- =::- -:- - -1 FIGURE 14.5 Relative Effect of Medium Basis Weight on T/B and E/E Box Compressive Strength, (82)

900 850 B 0 800 x 750 E n C 700 d 0 m 650 P T r 600 0 e s 550 E s 500 n i 0 450 d n 400 I (lb) 0r) 350 0 3 350 400 450 500 550 600 650 700 750 800 850 900 UO 4 Top-To-Bottom RSC Box Compression, lb 0

a% --j LI 21 ---i

FIGURE 14.6 CD I Effect of Medium Basis Weight on Box Compressive Strength, (36) a

o Medium Weight = + 61% +29%

Total Fiber Weight = + 17% C 5.0 - 0 m 4 5 . . p4.5- r e 4.0- S s i 3.5 - 0 n 3.0

N 2.5-

2.0 23 37 Medium Basis Weight, Ib/msf C-Flute, 41 Ib/msf Linerboard -I-...- - FIGURE 14.7 Effect of Inner Linerboard Basis Weight on Top-To-Bottom Compressive Strength of a Corrugated Box Panel. (52, 53, 58) 1.6 F 1.5 .;...... \ 442-26-69 Ib/msf a 1.4 i i \ ! i C -Flute i i i ' I 1.3 u 1.2 r e 1.1 L 1.0 ...... ' , ' o 0.9 ...... " ...... a ."- ...... C -F . u t e ...... ". d 0.8 0.7 69-26-42 Ib/msf P 0.6 I Flute I

C} i 0.5 0

0.4 -T1 r t~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

9 10 11 12 13 14 15 16 17 18 19 20 21 0q- Square Panel Size, inch

9- -,,4 go(B - FIGURE 14.8

Balance of Linerboard and Medium iCD0 0C UlrUl3 ElIC to Achieve Minimum Fiber Box (/)" Compressive Strength, (55) CD W e 3.5 i g h 3.0 t R 2.5 a t i 2.0

L 1.5 L B 1.0 / M e 0.5 d 100 300 500 700 900 1100 1300 1500 1700 1900 2100 2300 Box Compression, lb A-Flute Boxes. Computer Model Results. Weight Ratio Based On Two Liners And 1.55 Medium Draw Factor. Corrugating Medium / 171 shown in Figure 14.1, and assumed a continuous basis house. There are no easy answers to this knotty transla- weight grade structure for both the linerboard and me- tion problem that this author can offer, except for experi- dium. A higher ratio value in Figure 14.8 denotes rela- ence and caution. The laboratory is not the "real world;" tively more linerboard usage and relatively less medium it is just a powerful tool for trying to deal with the com- usage. The traditional 42-26-42 lb/msf C-flute corrugated plexities of real life. board grade has a ratio of 2.24; the 69-26-69 lb/msf C- The effect of the crushing of the flutes in the side flute grade has a 3.68 ratio; and the 33-26-33 lb/msf C- panels of the box on box compression strength is shown flute grade has a ratio of 1.76. The calculations are based in Figure 14.10. The ratio of the corrugated board flat on achieving the defined box compressive strength at the crush strength divided by the crushing force is shown lowest fiber cost. The calculations indicate that a larger plotted against the box compressive strength expressed as box should have a higher ratio value (more linerboard, a percent of that of the uncrushed box. A low ratio value less medium). This result can be attributed to the signifi- represents worse crushing. The boxes were produced in a cant effect of the linerboard on the flexural stiffness term commercial box plant. The data show that the end-to-end of the box compression equation model. The calculations box compressive strength is less sensitive to low levels of for both size boxes show a maximum ratio in the mid- crushing force than is the top-to-bottom box compres- compressive strength range, and a decreasing ratio par- sion. The top-to-bottom strength is affected almost im- ticularly on the higher compressive strength side of the mediately, even at a flat crush force ratio of 4.0 to 4.5. range. This indicates that higher compressive strength The end-to-end box compression strength was not af- requirements for a given size box should consider in- fected until the ratio decreased to 3.0. However, once the creasing the medium basis weight proportionately more end-to-end compressive strength started to be affected, it than the linerboard basis weight, (36, 50, 55, 64, 99). dropped at a faster rate than that of the top-to-bottom. The literature contains many other approaches This difference in rate of compression loss is shown in (computer models) that address this balancing act. They Figure 14.11, (73). are not included in this reference publication since it The effect of the water resistant corrugator adhesive would be impossible to judge their relative merits. This on box compressive strength at various relative humidity author will just repeat the familiar warning given for all levels is shown in Figure 14.12. The test boxes were computer model applications, "Garbage In = Garbage 20.5 x 13.5 x 12.5 inch in size and were manufactured in Out!" a commercial box plant. The data show no detectable Figure 14.9 shows the effect of corrugator bond effect of the water resistant adhesive under the standard glue skips on the top-to-bottom compressive strength of laboratory box compressive testing methods. Figure boxes. The corrugated board was produced on a pilot- 14.13 shows the stack life creep failure tests for the same size corrugator using commercially produced linerboard boxes. This creep failure test more closely represents the and medium. The adhesive gaps were produced by using field stacking conditions existing during warehouse stor- a wiper blade on the applicator roll. The gap, therefore, age periods. That is, the boxes are evaluated for com- ran around the complete perimeter of the box. The data pression performance while under load for an extended show no effect on the box compression until the gap was time period. The creep failure data show that the use of approximately 1/3 inch wide. A 1/2 inch gap reduced the the water resistant corrugating adhesive greatly reduced box compression by 11%, and a 1 inch gap reduced the the rate at which the boxes failed under stack loading box compression by 27%, (116). While the experiment conditions. The magnitude of the box performance im- was conducted by removing a strip of adhesive, similar provement increases with increasing box moisture con- results would be expected for MD- oriented corrugator tent. The water resistant adhesive improved the stacking blisters. This author wishes to point out that these results life by 6% at a box moisture content of 7.5% are based on the standard laboratory box compression (approximate TAPPI standard conditioning), and by 12% test procedure. This author expects that the effect of bond at a box moisture content of 12.5%. Similar experiments gaps on the actual stacking performance of the box in the were conducted to evaluate the effect of wet strength field will be much worse than that indicated by these grades of linerboard and medium. The use of wet laboratory test data. strength components did not significantly affect either the This issue of the translation of laboratory test results laboratory compression test strength or the compression into the expected field performance of corrugated boxes creep failure rate, (7, 87). should always be of top concern to packaging engineers In summary, the technical information presented in and package producers. An example, often used by this this chapter supports the following observations concern- author to make the point, is the placement of a 2-by-4 ing box compressive strength. wooden stud in the center of a corrugated box. The laboratory compression test will show unbelievably high 1. The stacking capability is the major characteristic strength levels. The box will be a disaster in the ware- that differentiates a corrugated box from a paper ainji-v 40UI 'OAISe8lpv u 1 de E 01- 60 TO L0O 9*O 9-0 VWO CIO ZO V0 O00 OOL q 09L 009 U 099 0

006 S S 096 9

w 0901- 0 3 001-1I x 091-1I 0 El 00Z L

0 (9LL) UO6issaidwoD Rog wofl09oj3-pa-oj En rU uo sauD puoS .IoleDfl~Joo lo 4031J .=U 6't'L 3Ufluid N Cu

LI- Corrugating Medium / 173

I I: i i FIGURE 14.10 I Ii i

Effect of Box Panel Crushing on I Box Compressive Strength I i

Effect of Flat Crush Strength on Top-To-Bottom Compression Retention After Crushing, (73) B o % 110-

0 _ ......

1005 -...... + ^ -......

e9C 75- ..... / ...... C-Flute i r 70s- .../* .. '...... + B-Flute i 5 ...... oh 60- n 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Flat Crush Strength/Crushing Force Ratio Both in psi Units li Effect of Flat Crush Strength on End-To-End Compression Retention After Crushing, (73) B o % 110 - . . x I 05 - ...... - ...... *...... x 190 ...... -...... O1^ 8500 --** -...... : ...... 9 50 - ...... s e 85865 .-...... ' ' Pe zr 80C-70 - Flu7...... r o 75 ......

0 - " .....r ' ...... :' r 6 - ---;- -. --- .. -- -.....-..----B-Flute...-- .---...... i s 55- ...... o h 50-, , ; - n 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Flat Crush Strength/Crushing Force Ratio Both in psi Units i

j Pz co FIGURE 14.11 10D- 0 Relative Effect of Panel Crushing on Box M - Compression - T/B Versus E/E. (73) CD - CD

E en0CD S n 110 d 0 f T 100 0 U n ...... E c 90 n r d u 80 s C h e 100%/ Agreement / 0 70 d \ . ,. - Regression m p S 60 r t *-' .1'/8 |i *- C-Flute r e e 50 '"'"'"'"",,,...... - B-Flute s n sS g t 40 i h 0 40 50 60 70 80 90 100 110 n Top-To-Bottom Compression, % of Uncrushed Strength FIGURE 14.12 Effect of Water Resistant Corrugator Adhesive on Box Compression. (87)

B 1800- ...... [. Reg ula r Ad hesive .I ...... x 1600- Regular Adhesive C 1400- ...... Water Resistant Adh. 0 '.'.''.X'.'.''...''.'.''...... R A...... m 1200- r 1000 - e s 800- s i 600- ''.....''...... n 400- 200- b 0 0 o 73/50 73/85 90/90

(1) Conditioning, Deg.F Temp. / % RH or5 90-26-90 Ib/msf Construction, CD C-Flute Boxes

LAl =7:7-- .I FIGURE 14.13 o Effect of Water Resistant Adhesive on Bi Box Stacking Life Creep Failure. (87) 1

0.65 -+6 0.65-- Regular Adhesive 0.60 -

, -I l I I + 12% - Water Resistant Adh.

a o.50 t 27%

0.40 - 0.35- 0.30 - - . ; . 0.25 -.. - -- .. L. -.-- 0.20 i I i I 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Box Moisture Content, % * Ratio is applied stack load divided by the box compressive strength at the moisture content shown. ii Corrugating Medium / 177

bag, a plastic bag, a paper wrap, a plastic wrap, or measured with a standard laboratory compression any other type of flexible package. tester, even under high humidity conditions. The water resistant adhesive does have a very large 2. Mathematical models are available for predicting the beneficial effect on the compression creep failure top-to-bottom and end-to-end compressive strength rate, which is more closely related to warehouse of boxes. Both models incorporate various forms of stacking performance. the box size, the pure compressive strength of the corrugated board, and the elastic bending resistance 9. Wet strength linerboard and medium components of the combined board. did not affect either the laboratory box compression test strength or the compression creep rate. 3. The effect of the medium basis weight on package compression strength is not linear. The data indicate 10. This issue of the translation of laboratory test results that increasing the medium basis weight has a dimin- into the expected field performance of corrugated ishing benefit on the observed package compressive boxes should always be of top concern to packaging strength. This observation can be attributed to the engineers and package producers. There are no easy fact that the medium contributes little to the com- answers to this knotty translation problem that this bined board flexural stiffness strength. author can offer, except for experience and caution. The laboratory is not the "real world;" it is just a 4. Assuming all material and other quality issues re- very powerful tool for trying to deal with the com- main constant, A-flute board produces a higher top- plexities of real life. to-bottom box compressive strength than C-flute board, and C-flute board boxes are stronger in compression strength than B-flute boxes. The flute size effect can be attributed to the influence of the corrugated board thickness on the flexural stiffness strength.

5. The opposite is true for end-to-end box compressive strength. B-flute boxes are stronger than C-flute boxes, and C-flute boxes are stronger than A-flute boxes. This flute size effect can be attributed to the large effect that the closer flute spacing has on the MD Edge Crush Test strength of the combined board. The ECT effect overshadows the flexural stiffness effect.

6. Assuming that the box panels bulge outward under compression loading, box compression failure is initiated by compression failure of the inner linerboard facing near a comer of the panel. The typical bowed failure lines seen on the outside of the box that has failed in compression follow the maximum stress lines but are actually postcompression failure symptoms. Box compression strength can be improved by placing the strongest linerboard facing on the inside of the box, all other things being equal.

7. The crushing of the flutes in the side panels of the box has a large detrimental effect on box compression. The effect is proportionately greater than that indicated by box plant caliper measurements on the combined board.

8. The use of the water resistant corrugator adhesive has little effect on the box compressive strength as Chapter 15

Package Rough Handling

It is estimated that approximately 90% of the pack- basis weight. The data indicate that box drop strength aged goods shipped to consumers in the United States are performance improves as the corrugating medium basis packaged in corrugated containers. The product must weight increases. However, there is considerable scatter arrive at the consumers location undamaged and in one in the data points, (73, 82). I piece, if the package is to be judged as having performed Figure 15.2 shows the same box drop test data plot- its function. The package must block and cushion the ted against the average Elmendorf tear strength of the product and protect it from drop or impact forces, (41). It medium. The data show that a stronger tear strength me- must also contain the product. It is not nice to have cans dium improves the drop test performance of the box. of soup bouncing all over a parking lot. There is much less scatter in this strength data than was As mentioned in the prior chapters, the handling of seen for the basis weight data in Figure 15.1, (73, 82). packaged products has changed dramatically over the This reinforces the point, that was made several times past 30 years. Packaged goods are now usually unitized before in this reference publication, that material per- on a pallet or slip sheet and moved by the use of ware- formance should be based on material strength levels and house trucks. In the distant past of the 1960s, many boxes not basis weight. were still handled one at a time. Package testing labora- The effect of flute size on the box drop test perform- tories were equipped with large revolving drums, fitted ance is shown in Figure 15.3. The same linerboard and on the inside with baffles. Boxes were routinely tumbled medium materials were used in both flute size boxes, and in these drums to see if the package could withstand the the boxes were manufactured at the same time in a com- rigors that existed in the field distribution cycle prevalent mercial box plant. The C-flute boxes performed better in in those years. Today, most packaging engineers have the comer drop test than the B-flute boxes, (73, 82). never seen the drum test being run. The ability of corrugated board to act as a cushion- However, scoreline tearing still occasionally occurs. ing material is a more complex issue. The cushioning Research at the Institute of Paper Science and Technol- qualities of corrugated board involve the flat crush char- ogy has shown that the scoreline separation failure in a acteristic of the fluted medium and the design of the corrugated box starts by the tensile failure of the com- package. The cushioning function is provided by both the bined board at the scoreline. Once the failure has started, primary shipping case and any special inserts that may be it continues to progress by tear failure along the score- used inside the box to protect the product, (28, 41, 103, line. The corrugating medium, because of its fluted 123). structure, contributes to the CD tensile strength of the A material acts as a cushion by absorbing kinetic combined board, but not to the MD tensile strength. In a energy in a controlled manner with time. Kinetic energy typical RSC-style box, the medium would then contribute is the energy that any moving object has due to the com- to the tensile strength of the flap scores of the box, but bination of its mass and its velocity, (28). A simplistic not to the body scores of the box. The medium contrib- example of the interaction of these properties and forces utes to the combined board tear strength in both direc- may be useful. An electric light bulb can be dropped on a tions. concrete floor from a height of 1/2 inch, and it will Figure 15.1 shows the effect of corrugating medium probably not break. If the same light bulb is dropped on basis weight on the ability of the filled box to withstand the same concrete floor from a height of 4 feet, it will being dropped without breaking open. The boxes were probably shatter into hundreds of pieces of glass. If the dropped from a height of 12 inches onto a comer of the same concrete floor is covered with an inch thick, plush box. The plot shows the number of drops that were made carpet, and the same light bulb is dropped on it from a before box failure occurred as a function of the medium height of 4 feet, the bulb probably will not break. 0 00 FIGURE 15.1 oM O

7 C 0 r 6 n e r 5 D r 0 P 4 S

N 3 0

2 19 20 21 22 23 24 25 26 27 Medium Basis Weight, Ib/msf 12 Inch Drop Height. 2 1/2 Can Case Size Box.

------FIGURE 15.2 Effect of Medium Elmendorf Tear Strength on Box Corner Drop Performance. (82)

7 C 0 r 6 n e r 5 D r 0 P 4 s

N 3 0 n

2 0 50 55 60 6 5 70 75 80 85 90 95 100 105 110 115 120 o I (3 12 Inch Drop Height. Medium Elmendorf Tear Strength, -MD x CD , g 2 1/2 Can Case Size Bo) (. 9o ......

P)-0 00- as FIGURE 15.3 (D O C

Effect of Flute Size on x 0 Box Corner Drop Test, (82) r:L 5C

C 0 r 7 n e, 6 r 5 D r o 4 P S 3

N 2 o 1

0 C-Flute B-Flute Flute Size 12 inch Drop Height. 2 1/2 Can Case Size Box. Corrugating Medium / 183

The kinetic energy of the light bulb at the time that it leveling off indicates that the entire flute structure has hits the floor depends on the drop height. The kinetic been destroyed. The falling object has "hit bottom." energy is imparted to the light bulb by the force of grav- The effect of the corrugated board flat crush strength ity. At a low drop height, the bulb has a low kinetic en- and the number of corrugated board plies on the maxi- ergy when it hits the floor because the force of gravity mum static stress that can be sustained at a deceleration has had only a short time to act on the bulb. The higher level equal to 200 times the force of gravity (200 G) is the drop height, the higher the kinetic energy, and the shown in Figure 15.7. Static stress is a fancy engineering higher the kinetic energy, the greater the probability that term for the load per unit area. The data show that a cor- the light bulb will break. However, the light bulb has the rugated board with a higher flat crush strength has better same kinetic energy when dropped from 4 feet whether it cushioning ability. More layers of corrugated board also lands on concrete or on a carpet. The light bulb does not improve the cushioning ability. More plies of corrugated break when it hits the carpeted floor because the carpet board provide a thicker structure that can be crushed by can absorb some of the kinetic energy by having the nap the falling object. This provides more time to stop the of the carpet deform. As the carpet deforms, the light object and allows a lower maximum stopping force (for bulb has more time to slow its speed and to stop falling. the true "Techy" types, Force x Distance x Time = A longer deceleration time requires a lower maximum Work). The data show that a higher flat crush strength deceleration force to stop the fall of the bulb. With a can be traded off against the number of corrugated board lower deceleration force, the light bulb has a lower prob- plies needed to achieve a specified level of cushioning. ability of breaking. The concrete floor has no give. The Based on the data shown in Figure 15.7, two layers of light bulb must stop almost instantaneously. The instan- corrugated board with a flat crush strength of 3.0 taneous stopping requires that a very high deceleration kp/sq.cm. can be substituted for three layers of corru- force be applied to the light bulb by the concrete floor. gated board with a flat crush strength of 1.0 kp/sq.cm., The high deceleration force breaks the light bulb. Life (123). should always be this simple. Figures 15.8 and 15.9 show the effect of medium Figure 15.4 shows typical flat crush stress/strain basis weight, linerboard basis weight, and the number of curves for C-flute corrugated board where the flutes have corrugated board layers on the maximum measured de- been precrushed to varying degrees. The combined board celeration force with a 1 psi static load being applied. flat crush property that is important to cushioning is not The I psi static load is well within the elastic region of only the maximum flat crush strength, but also the flat the flat crush stress/strain curve for the corrugated board crush energy. The flat crush energy is equal to the area grade used in this study. The data show that increasing under the curve, that is, the stress times the strain. The the medium basis weight from 26 lb/msf to 33 lb/msf original, zero precrushed combined board has a flat crush resulted in a 94% increase in the maximum measured energy of 17.5 units. A 20% precrush reduces this flat deceleration force. The effect of the linerboard basis crush energy to 14.0 units, a loss of 20%. Similarly, a weight and the effect of the number of corrugated pad 40% precrush reduces the energy by 38%, and the 50% plies were much less, (103). Based on the prior data, it precrush reduces the energy by 83%, (59). If the kinetic would appear that a lower deceleration force would be energy of the falling object exceeds the flat crush energy advantageous, and fewer pads, lower basis weight liner- of the combined board in the area contacted by the falling board facings, and lower medium basis weight would be object, the flute structure will be completely collapsed. better. The falling object will "hit bottom" and may very well be That is true if a product is being dropped on a piece damaged. The major conclusion from the data shown in of corrugated board. However, the product in question is Figure 15.4 is that the crushing of the flute structure of usually packed inside of the corrugated box, and the box corrugated board has a very adverse effect on the cush- acts as a buffer between the forces of the outside world ioning capability of the combined board. and the valuable product. In this scenario a corrugated Figure 15.5 shows that the crushing damage to the board that can absorb more energy will better protect the flute structure is made worse by repeated application of a product from an outside impact force. As a rule of constant crushing force to the same area of the corru- thumb, a soft, giving corrugated pad is best for inner gated board, (130). Figure 15.6 shows the effect of re- packing since it acts as a cushion for the product against peated impacts on the maximum stress (deceleration transportation shocks, such as acceleration and decelera- force) imparted to the falling object by the corrugated tion. However, it must have enough guts (energy absorp- board acting as a cushion material. A higher deceleration tion power) so that it does not crush down completely maximum stress level increases the probability of dam- and lose all of its cushioning properties. The corrugated age to the falling product. The maximum stress increases box, on the other hand, acts as a buffer to the impact rapidly with repeated impacts and then starts to level off forces of the cruel outside world, such as a person kick- in this experimental example after 10 impacts, (128). The ing the side of the box. This buffer role requires the high % 00 FIGURE 15.4 goq 4 Effect of Combined Board Crushing on 00ca (D-

V. the Flat Crush Stress/Strain Curve, (59) 3 ( qr- 300

250

S t 200 r e s s 150

k 100 a (Load) 50

0 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 Strain (Defection), mm C-Flute Corrugated Board.

Lt- FIGURE 15.5 Effect of Multiple Crushing of Combined Board on Measured Caliper Loss, (130)

M 60- e ....-- ...... a 55- s 50- -. - 1 Cr ush Pass i~~~~~~~~~~~~~~~~~~~~~~~ ...... r 45- -- 2C Irush Passes e 40- d ...... , ...... [...... :...... [...... ~ ...... 35- i T C 30- 1._...... '-' ;. ....- [ .I...... -.. ' ...... '' ' ''...'."'...... i ....'.' r . - I.- I ...I .I.. ...:.1 I...... :..... -I. I u 25- ...... ,...... Li ../}./.-.

i - _- -- .-....I...... - -.-- .. ------; -....-- ...... h 20- i ...... ' .._...... I , 15- ...... / i , i M 10- ...... i 5- ......

0 s 04 P' I 'y' I i -I-- I I o 0 40 50 60 70 80 90 100 110 120 130 140 150 10 20 30 5' Actual Crush, Mils Average for A-Flute, 26 & 30 Ib/msf Medium. (Oh ...... --

I 00 g o*3 CM Cb() FIGURE 15.6 CD n

Effect of the Number of Impacts 0 i on the Maximum Measured Stress. (128) 33_ S: 210- (IQ

i i M i ...... I --; ...... I...... a 190- i x i m 170 -1 u

m 150 - . . S t 130 - ...... r e S 110- s

90- G

I I

70- I I I I I II II I I 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Number of Impacts 42-26-42 Corrugated Board Construction. Corrugating Medium / 187

I i1 FIGURE 15.7 Shock Force Absorbing Characteristics of i ;i Corrugated Board - Effect of Combined .i Board Flat Crush & Number of Layers. (123)

I 0.20- 10 Pad S 0 Layers t 0.18- .. .. - -. a i0. 1 6

ct . ; r 0.12- . e s 0 .1 2 0...... ' ...... s 0.10- - - - ...-...-- i^IL ' ^ 5 Pad Jl m 0.02- ...... ql; /0.06

. 0 .0 2 ...... 2.

Flat Crush, kp/sq. cm.

Box Drop Tests From 61 cm. and 200 G Peak Deceleration. I

nI FIGURE 15.8 Ua Effect of Medium & Linerboard Basis 0I Weight and Number of Pads on Cushioning. (103)

Corrugated Board Construction, Ib/msf. A-Flute 70 --- M 65 LI-I 26-26-26 BE 26-33-26 E 42-26-42 ME 42-33-42 .. a x 60- .. m u 5 0 ......

G 4

F ...... 5 Pads 10 Pads...... 0 r 25 C 20

5 Pads 10 Pads Number of Pads * At 1 psi Static Stress. - . FIGURE 15.9 Effect of Medium & Linerboard Basis Weight and Number of Pads on Cushioning, (103) 65- Linerboard Medium Number 60- Basis' Weight M Basis Weight . f Pads a 55- ...... ((- I bb/m / m sf-)s f ) ...... -...... +94% i 50- m 45-- u +16% +9%.... m 40- :" h i: ..... d 0 : o G 3 5 - F 30- o 25- r 20- e 15 10- 5- 0) 0- 26 42 26 33 5 10

(Ib/msf) 00 * At 1 psi Static Stress. '9 A-Flute Variable Level 190 / Chapter 15 Package Rough Handling energy absorption capacity provided by the higher flat age size. The "Folded Pad" design becomes more ef- crush strength medium. It was mentioned before that fective as the package size decreases. cushioning is a very complex subject. Figures 15.10 and 15.11 show the effect of corrugated board pad designs on cushioning capability. There are literally hundreds of different designs in use today. The two shown, "Spring Pad" and "Folded Pad," are two of the more common designs currently being used commercially. The experimental data show that the cushioning effectiveness of the spring pad design is independent of the package size. The folded pad design becomes more effective as the package size decreases. The technical information presented in this chapter supports the following observations with regard to package rough handling characteristics.

1. A package must block and cushion the product and protect it from drop or impact forces. It must also contain the product.

2. A stronger tear strength medium improves the drop test performance of the box. C-flute boxes performed better in the comer drop test than the B-flute boxes.

3. The ability of corrugated board to act as a cushioning material is a very complex issue. The cushioning qualities of corrugated board involve the flat crush characteristic of the fluted medium and the design of the package. A material acts as a cushion by absorbing kinetic energy in a controlled manner with time.

4. The combined board flat crush property that is important to cushioning is not only the maximum flat crush strength, but also the flat crush energy. The flat crush energy is equal to the area under the flat crush stress/strain curve.

5. Crushing of the flute structure of corrugated board has a very adverse effect on the cushioning capability of the combined board. The crushing greatly reduces the available flat crush energy.

6. As a rule of thumb, a soft, giving corrugated pad is best for inner packing since it acts as a cushion for the product against transportation shocks, such as acceleration and deceleration. The corrugated box, on the other hand, acts as a buffer to the outside impact forces, such as a person kicking the side of the box. This buffer role requires the high energy absorption capacity provided by the higher flat crush strength medium.

7. The cushioning effectiveness of the "Spring Pad" design inner packaging is independent of the pack- Corrugating Medium / 191

ii FIGURE 15.10 ;i Effect of Pad Design on Cushioning

Side View of Cushioning Pad Designs

I Spring Pad - 2 Ply

Folded Pad

Effect of Cushioning Pad Design on Peak Acceleration at Varying Static Stresses, (41) .i

P 230 e a 210 ...... - -...... 4...... k 190 ...... ,,...... , , , ,.., ...... c 170 c 150 _...,...... !...... J...... -...... -...... | i : Spring Pad ' i i I ...... ee 130 r 110 a _I- ...... ^ ...... t 90 i o 70 -_ ...... n 50 ''' '''...-''' \. -''Folded-. ""-'-- ...... -.,, = g 30 ¢).01 0.05 0).1 0.5 1 2 Static Stress, psi 42-26-42 Ib/msf, C-Flute Corrugated Board Construction 192 / Chapter 15 Package Rough Handling

,II it 11 i

I FIGURE 15.11 I I

Impact Load Effect on Peak Acceleration I for Various Cushioning Pad Designs. (41) .i I i 140 130 ...... ; ...... *X Spring Pad, 5 Ply p 120 '*--+ ©Folded Pad e 1 1 0 A - ...... k 100 A 90 C c 80 e ...... I 70 e

...... i r 60 ...... a ...... ------...... - t 50 ...... ' ...... 0 40 n * 30 ~~~~~~~~~~~~...... 7...... 9 20 ...... 10 . . . !...

0 _i I I Ii I I i 0 5 10 15 20 25 30 35 40 45 50 55 Load, lb.

i 42-26-42, C-Flute I I Corrugated Board

I CHRONOLOGICAL BIBLIOGRAPHY

1. Jones, G. "The Effect of Medium Properties on the Retention of Crush Strength After Corrugating." Project 3747, Final Report to the Containerboard and Kraft Paper Group of the American Forest and Paper Association from the Institute of Paper Science and Technology, July 1993.

2. Jopson, R. N. "Saturation Technology for Corrugated Containers." Tappi J. 76(4): 207-214, April 1993.

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13. Whitsitt, W. J.; Marcille-Lorenz, M. "Double-Backer Bonding Technology." Tappi J. 73(5): 137-142, May 1990.

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17. Whitsitt, W. J.; Smith, C. N. "Single-Face Runnability and Bonding." Project 2696-22, Report One to the Fourdrinier Kraft Board Group of the American Paper Institute from The Institute of Paper Chemistry, February 1989.

18. Whitsitt, W. J.; Marcille-Lorenz, M. M. "Double-Backer Bonding Technology." Project 2696-24, Report One to the Fourdrinier Kraft Board Group of the American Paper Institute from The Institute of Paper Chemistry, January 1989.

19. Nordkvist, B. "Optimizing Fluting and Liner Proportions." Paperboard Packaging 73(10): 91-96, October 1988.

20. Daub, E.; Gottsching, L. "Gluing abd Gluability of Corrugating Medium and Linerboard - Part 2B." Papier 42(19): 551- 563, October 1988.

21. Daub, E.; Gottsching, L. "Gluing abd Gluability of Corrugating Medium and Linerboard - Part 2A." Papier 42(7): 346- 352, July 1988.

22. Daub, E.; Gottsching, L. "Gluing and Gluability of Corrugating Medium and Linerboard - Part 1." Papier 42(6): 274- 285, June 1988.

23. Whitsitt, W. J. "Papermaking Properties Affecting Box Properties" IPC Technical Paper Series No. 288: The Institute of Paper Chemistry, Appleton, WI, May 1988.

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25. Whitsitt, W. J. "Relationship Between Runnability and (Corrugating) Medium Properties." TAPPI Corrugated Contain- ers Conference Proceedings: 151-156, Chicago, IL, October 1987.

26. Whitsitt, W. J. "Runnability and Corrugating Medium Properties." TappiJ. 70(10): 99-103, October 1987.

27. Whitsitt, W. J. "Relationship Between Runnability and Medium Properties." IPC Technical Paper Series No. 238: The Institute of Paper Chemistry, Appleton, WI, May 1987.

28. Liu, J. Y.; Laundrie, J. F. "Cushioning Properties of Corrugated Pads in Edge and Comer Drop Tests." Boxboard Con- tainers 94(10): 28-33, May 1987.

29. Whitsitt, W. J.; Baum, G. A. "Compressive Strength Retention During Fluting." TappiJ.70(4): 107-112, April 1987.

30. Sprague, C. H.; Whitsitt, W. J. "Compressive Strength Retention During Fluting of Medium - Strength Losses in Flut- ing." Tappi J. 70(2): 91-96, February 1987.

31. Whitsitt, W. J.; Schrampfer, K. E.; Baum, G. A. "High-Low Monitor." Project 2692-5, Report One to the Fourdrinier Kraft Board Group of the American Paper Institute from The Institute of Paper Chemistry, January 1987.

32. Chow, D. K. "Design of Cost-Effective Flute Features in Corrugating Medium." Tappi J. 69(10): 126-128, October 1986.

33. Whitsitt, W. J.; Baum, G. A. "Compressive Strength Retention During Fluting (of Medium)." TAPPI Corrugated Con- tainers Conference: 11-15, New Orleans, LA, October 1986.

34. "Fluting of Heavy Weight Mediums." Project 2696-23, Status Report to the Fourdrinier Kraft Board Group of the American Paper Institute from The Institute of Paper Chemistry, October 1986. Corrugating Medium / 195

35. Hoke, U.; Daub, E. "Absorptive Behavior of Medium and Liner." Papier 40(10A): V76-V87, October 1986.

36. Augustin, H. E. "Optimizing of Fluting and Liner Proportion in Corrugated Board." TAPPI Corrugated Containers Con- ference: 35-38, Kansas City, MO, October 1985.

37. Gunilla, M.; Richardson, K.; Back, E. "Note on the Compression Strength of Fluting Over a Temperature and Moisture Range." Papier 39(7): 302-305, July 1985.

38. Whitsitt, W. J.; Smith, C. N. "Improved Utilization of Corrugator Preconditioning." Project 2696-21, Report One to the Fourdrinier Kraft Board Group of the American Paper Institute from The Institute of Paper Chemistry, February 1985.

39. Whitsitt, W. J. "Relationship Between Elastic Properties and End-Use Performance." Project 2695-23, Report One to the Fourdrinier Kraft Board Group of the American Paper Institute from The Institute of Paper Chemistry, January 1985.

40. Fellers, C.; Brange, A. "The Impact of Water Sorption on the Compression Strength of Paper." Papermaking Raw Mate- rials (Punton, ed.), Vol. 2: 529-539, Mechanical Engineering Publications, Ltd., 1985.

41. Liu, J. Y.; Laundrie, J. F. "Measure Cushioning Values of Corrugated Pads." Packaging 31(1): 58-63, 1985.

42. Worster, H. E. "Factors Affecting Corrugating Quality: A Status Review." Southern Pulp & Paper 47(4): 12-14, April 1984.

43. Batelka, J. J. "Effect of Flat Crushing on Edge Crush Test Strength." Proceedings of the Box Makers Seminars, the American Paper Institute, New York, NY, 1984.

44. Hoke, U.; Gottsching, L. "Crush Tests of Corrugating Medium and Liners and Their Relation to Properties of Corru- gated Board and Boxes." Papier 37(10A): V67-V76, November 1983.

45. Pickens, T. "Changing Paper Makeup Effects Boxmaker's Craft." Southern Pulp & Paper 46(3): 28-31, March 1983.

46. Sprague, C. H.; Whitsitt, W. J. "Medium Fracture and Strength Losses in Fluting." Tappi J 65(10): 133-134, October 1982.

47. Sprague, C. H.; Whitsitt, W. J. "Improving Bonding Consistency." Tappi J. 65(6): 133-135, June 1982.

48. Wilkins, R. "Laboratory Studies on the Gluing Process in the Corrugating Unit of Corrugators." Allgemeine Papier- Rundschau: 455-165, April 16, 1982.

49. Sprague, C. H. "Achieving Adequate Bonds on a Consistent Basis, Precision in Starch Application." Tappi J. 65(4): April 1982.

50. Urbanik, T. J. "Principle of Load-Sharing in Corrugated Fiberboard." Paperboard Packaging 66(11): 122-128, Novem- ber 1981.

51. Back, E. L.; Salmen, L.; Wilken, J. E. "The Corrugatability of NSSC and Waste Based on Corrugator Medium." Medd. Svenska Traforskningsinst B 572: 1981.

52. Peterson, W. S.; Fox, T. S. "Workable Theory Proves How Boxes Fail in Compression." Paperboard Packaging 65(10): 136-144, October 1980, and 65(11): 68-76, November 1980.

53. Peterson, W. S. "Unified Container Performance and Failure Theory." Tappi J. 63(10): 75-79, October 1980, and 63(11): 115-120, November 1980.

54. Johnson, M. W.; Urbanik, T. J.; Denniston, W. E. "Maximizing the Top-To-Bottom Compression Strength." Paperboard Packaging 65(4): 98-108, April 1980. 196 / Chronological Bibliography

55. Peterson, W. S. "Minimum-Cost Design for Corrugated Containers Under Top-To-Bottom Compression." Tappi J. 63(2): 143-146, February 1980.

56. Johnson, M. W.; Urbanik, T. J.; Denniston, W. E. "Optimum Fiber Distribution in Singlewall Corrugated Fiberboard." Research Paper FPL 348, United States Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI, 1979.

57. Fox, T. S.; Jurewicz, J. T. "A Study of Bonding Mechanism of Corrugating Medium." Project 2696-17, Report Two to the Fourdrinier Kraft Board Group of the American Paper Institute from The Institute of Paper Chemistry, November 1978.

58. Peterson, W. S. "Component Properties and End Use Performance." Kraft Mill Process Production Engineering, Pro- ceeding of the Southeast Conference: 47-92, October 1978.

59. Nordman, L.; Kolhonen, E.; Toroi, M. "Investigation of the Compression of Corrugated Board." Paperboard Packaging: 48-54, October 1978.

60. Jurewicz, J. T. "The Dynamics of a Corrugating Glue Machine." Tappi J 61(6): 39-41, June 1978.

61. Ford, T. L. "The Viscous Behavior of Corrugating Adhesive." Final Report on Student Research Project A-291, The Institute of Paper Chemistry, Appleton, WI, May 1978.

62. Jaeger, T. "A Study of Pressure in the Nip of a Reverse-Roll Adhesive Applicator." Master's Degree Thesis, The Insti- tute of Paper Chemistry, Appleton, WI, May 1978.

63. Biorseth, E. J. "Controlling Adhesive Application Variables for Better Uniformity." Boxboard Containers: 43, January 1978.

64. "Some Aspects of Corrugated Case Performance and Heavyweight Fluting." Converter 14(8): 16-19, August 1977.

65. Fox, T. S.; Jurewicz, J. T. "A Study of Bonding Mechanisms of Corrugated Medium." Project 2696-17, Report One to the Fourdrinier Kraft Board Group of the American Paper Institute from The Institute of Paper Chemistry, July 1977.

66. Gottsching, L.; Otto, W. "Runnability Characteristics of Corrugating Medium. Part V: Relationship Between Paper Properties and Runnability." Papier 31(5): 169-179, May 1977.

67. Williams, R. H.; Leake, C. H.; Silano, M. A. "Influence of Carrier Starch on Green Bond Strength in Corrugating Me- dium." Tappi J. 60(4): 86-89, April 1977.

68. Gottsching, L.; Otto, W. "Runnability Characteristics of Corrugating Medium. Part IVb Dimensional Stability as a Function of Web Tension, Temperature, and Nip Pressure." Papier 31(4): 129-136, April 1977.

69. Gottsching, L.; Otto, W. "Runnability Characteristics of Corrugating Medium: Part IVa." Papier 31(3): 85-94, March 1977.

70. "Study of the Film Stability of an Adhesive on the Applicator Roll." Project 2696-17, Research Plan to the Fourdrinier Kraft Board Group of the American Paper Institute from The Institute of Paper Chemistry, March 1977.

71. Gartaganis, P. A. "The Cobalt Tracer Method for the Direct Measurement of Starch Consumption on the Corrugator." Paperboard Packaging 62(2): 47-48, February 1977.

72. Gottsching, L.; Otto, W. "Running Characteristics of Corrugating Medium. Part III: The Importance of Structural Sta- bility and Its Characterization." Papier 31(2): 45-53, February 1977. Corrugating Medium / 197

73. Fox, T. S.; Kloth, G. R. "Comparative Evaluation of B- and C-Flute Boxes and Combined Board Fabricated with Corru- gating Medium of Various Concora Values." Project 2695-18, Report Two to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, February 1977.

74. Salmen, L.; Back, E. L. "Simple Stress-Strain Measurements on Dry Papers From -20 deg.C to 250 deg.C." Svensk Pap- perstidning 80(6): 178, 1977.

75. Back, E. L.; Stenberg, L. E. "Wet Stiffness by Heat Treatment of the Running Web - Properties of Treated Liner and Corrugating Medium." Pulp and Paper Canada 77(12): T264-T270, December 1976.

76. Williams, R. H.; Leake, C. H.; Silano, M. A. "Influence of Carrier Starch on Green Bond Strength in Corrugating Me- dium." TAPPI Corrugated Container Conference Papers: 53-61, Cincinnati, OH, November 1976.

77. Gottsching, L.; Otto, W. "Runnability Characteristics of Corrugating Medium. Part IIa: Deformability Under the Influ- ence of Corrugator Speed and Medium Moisture." Papier 30(10): 417-425, October 1976.

78. Toroi, M.; Kolhonan, E. "Runnability of Fluting." Pulp & Paper Canada 77(10): 65-68, October 1976.

79. Gartaganis, P. A. "The Cobalt Tracer Method for the Direct Measurement of Starch Consumption on the Corrugator." Pulp & Paper Canada 77(7): 69-72, July 1976.

80. Gottsching, L.; Otto, W. "Runnability Characteristics of Corrugating Medium. Part I: Introduction." Papier 30(6): 221- 228, June 1976.

81. Jonsson, P.; Hulteberg, A. "A New Technique for Determination of Fluting Runnability Characteristics." Paperboard Packaging 61(3): 52-65, March 1976.

82. Fox, T. S.; Whitsitt, W. J. "Comparative Evaluation of B- and C-Flute Boxes and Combined Board Fabricated with Various Corrugating Medium Weights." Project 2695-18, Report One to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, February 1976.

83. Gartaganis, P. A. "The Cobalt Tracer Method for the Direct Measurement of Starch Consumption on the Corrugator." CPPA Annual Meeting 628: 57-62, Montreal, Quebec, Canada, January 1976.

84. Toroi, M.; Kolhonan, E. "Runnability of Fluting." CPPA Annual Meeting 62A: 195-199, Montreal, Quebec, Canada, January 1976.

85. McKee, R. C.; Whitsitt, W. J.; Smith, C. N. "Study of Bonding of Corrugating Medium." Project 2696-17, Report One to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, January 1976.

86. Gottsching, L.; Otto, W. "Running Characteristics of Corrugating Medium. Part Iib: Deformability Under the Influence of Web Tension, Temperature and Linear Pressure of the Corrugating Rolls." Das Papier 30(11): 457-465, 1976.

87. McKee, R. C.; Whitsitt, W. J. "Effect of Relative Humidity on the Stacking Performance of Boxes Made with Water- Resistant Adhesive and with Regular and Wet-Strength Components." Project 2695-16, Report One to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, November 1975.

88. Koning, J. "Metering Characteristics of Starch Corrugating Adhesive." USDA Forest Service Internal Report, Forest Products Laboratory, Madison, WI, 1975.

89. Gartaganis, P. A. "Strength Properties of Corrugated Containers." Tappi J. 58(11), 1975.

90. Anderson, R. G.; Back, E. L. "A Method of Increasing Wet Stiffness of Corrugated Board by Means of Batchwise Hot Air Treatment - Board Properties." Tappi J. 58(6: 88, 1975. 198 / Chronological Bibliography

91. Anderson, R. G.: Back, E. L. "A Method of Increasing Wet Stiffness of Corrugated Board by Means of Hot Air Treat- ment-Design and Cost of Process." TappiJ.58(8): 156, 1975.

92. Koning, J. W.; Fahey, D. J. "Papermaking Factors that Influence Runnability of Corrugating Medium." TAPPI Spring Corrugated Containers Conference: 9-16, New Orleans, LA, June 1974.

93. Koning, J. W.; Fahey, D. J. "Papermaking Factors that Influence Runnability of Corrugating Medium." Tappi J 57(6): 65-68, June 1974.

94. McKee, R. C.; Whitsitt, W. J.; Corbett, H. M. "Investigation of Solid Lubricants as an Aid in Corrugating." Paperboard Packaging 58(12): 44-47, December 1973.

95. McKenzie, A. W.; Yuritta, J. P. "The Runnability of Corrugating Media: The Effect of Roll Temperature and Medium Moisture Content." Appita 27(2): 106-111, September 1973.

96. McKee, R. C.; Whitsitt, W. J.; Corbett, H. M. "Investigation of Solid Lubricants as an Aid in Corrugating." Project 2696-10, Report Two to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, January 1973.

97. Gartaganis, P. A.; Davy, R. G. "Machine-Direction (Linear) Corrugating Medium by the Dryformed Route." Tappi J. 56(12): 96-101, 1973.

98. McKee, R. C.; Whitsitt, W. J.; Laughlin, M. J. "Investigation of the Nature and Magnitudes of the Cyclic Fluctuations in the Web Tension and Top Corrugating Roll Molding Force Components and Their Relationship to High-Lows." Project 2696-8, Report Two to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, October 1972.

99. Kellicutt, K. Q. "How Liner/Medium Weight Relationships Affect the Strength of Corrugated." Boxboard Containers 79(8): 51-56, March 1972.

100. Brecht, W.; Colpe, D. "Improvement of Structural Stability of Corrugating Medium." Das Papier 26(10): 497-505, 1972.

101. Olsen, H. C. "A Test Method for the Analytical Determination of Starch Consumption." Tappi J. 55(7): 1091-1093, 1972.

102. Thayer, W. S.; Thomas, C. E. "Analysis of the Glue Liner in Corrugated Board." TappiJ. 54(11): 1853-1858, Novem- ber 1971

103. Stem, R. K. "How Variation in Corrugated Pad Composition Affect Cushioning." Package Engineering 16(7): 50-53, July 1971.

104. McKee, R. C.; Swanson, J. W.; Becher, J. J.; Hoffman, G. R. "Fundamental Study of Adhesion of Corrugated Board." Project 2696-4, Report Four to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, July 1971.

105. Jordan, C. A. "Testing of Cushioned Loads." Modem Packaging 44(7): 59-66, July 1971.

106. Daly, G. J. "The Corrugated Container Industry." Harry J. Bettendorf, Publisher, Oak Park, IL, 1971.

107. McKee, R. C.; Whitsitt, W. J.; Smith, C. N. "Evaluation of Corrugator Operating Conditions with Respect to High-Low Flute Formation." Paperboard Packaging 56(6): 36-45, 1971.

108. McKee, R. C.; Swanson, J. W.; Becher, J. J.; Hoffman, G. R. "Fundamental Study of Adhesion of Corrugated Board." Project 2696-4, Report Three to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, November 1970. Corrugating Medium / 199

109. "Spectral Analysis of Corrugating Variables as Related to High-Lows." Project 2696-8, A Preliminary Report to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, September 1970.

110. Simmonds, F. "Corrugating Furnishes and Mediums." Paperboard Packaging 55(1): 52-61, January 1970.

111. "Effect of Atmospheric Moisture Content Upon Shock Cushioning Properties of Corrugated Fiberboard Pads." USDA Forest Service research Paper FPL-129, Forest Products Laboratory, Madison, WI, 1970.

112. Toroi, M. "The Influence of Corrugated Board Components on Board Properties and Some Manufacturing Problems." Papierverabeiter 4(12): 36-43, December 1969.

113. McKee, R. C.; Whitsitt, W. J. "Relationship Between High-Low Flute Formation and the Properties of the Corrugating Medium." Project 2696-6, Report One to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, De- cember 1969.

114. Simmonds, F. "Corrugating Furnishes and Mediums - Their Microscopy, Sheet Properties and Runnability." Southern Pulp & Paper Manufacturers 32(12): 66-76, December 1969.

115. McKee, R. C. "Evaluation of Corrugator Operating Conditions and Medium Properties with Respect to High-Low Flute Formation." Project 2696-7, Report One to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemis- try, December 1969.

116. Koning, J. W.; Moody, R. C. "Effect of Glue Skips on Compressive Strength of Corrugated Fiberboard Containers." TappiJ. 52(10): 1910-1915, October 1969.

117. McKee, R. C.; Swanson, J. W.; Becher, J. J.; Hoffman, G. R. "Fundamental Study of Adhesion of Corrugated Board." Project 2696-4, Report Two to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, September 1969.

118. Hung, J. Y.; Nelson, R. W.; Van Eperen, R. H. "Remarks on the Surface Receptivity of Paper as These Relate to Liquid Film Application." TappiJ. 52((0: 1732-1734, September 1969.

119. McKee, R. C.; Gander, J. W.; Whitsitt, W. J. "Study of the Fundamental Properties of Corrugating Medium Which Govern Bonding and Method of Measurement." Project 2696-3, Report One to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, February 1969.

120. McKee, R. C.; Swanson, J. W.; Becher, J. J.; Hoffman, G. R. "Fundamental Study of Adhesion of Corrugated Board." Project 2696-4, Report One to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, January 1969.

121. Jordan, C. A. "Testing Corrugated Corner Pads." Modem Packaging 42(9): 121-126, 1969.

122. Back, E. L.; Didriksson, I. "Four Secondary and the Glass Transition Temperatures of Cellulose, Evaluated by the Sonic Pulse Technique." Svensk Paperstidning 72: 687, 1969.

123. Langaard, 0. "Optimation of Corrugated Board Construction with Regard to Compression Resistance of Boxes." Inter- national Paper Board Industry: 18-23, November 1968.

124. Crisp, C. J.; Stoot, R. A.; Tomlinson, J. C. "Resistance of Corrugated Board to Flat Crushing Loads." Tappi J. 51(5): 80A-83A, May 1968.

125. Stem, R. K. "Tests Show Corrugated Pads' Performance as Cushioning." Package Engineering 13(2): 71-75, February 1968. 200 / Chronological Bibliography

126. McKee, R. C.; Whitsitt, W. J.; Hubert, W. N. "Effect of Operational Variables on Runnability and High-Low Corruga- tions." Project 2696-1, Report One to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, January 1968.

127. "Flat-Crush Cushioning Capability of Corrugated Fiberboard Pads Under Repeated Loading." U.S. Forest Service Re- search Note FPL-0183, Forest Products Laboratory, Madison, WI, 1968.

128. Mustin, G. S. "Theory and Practice of Cushion Design." Department of Defense, Washington, D.C., 1968.

129. McKee, R. C.; Gander, J. W. "Properties of Corrugating Medium Which Influence Runnability." Tappi J. 50(7): 35A- 40A, July 1967.

130. Staigle, V. H. "Corrugated Board - Is It Crushed During Fabrication." TappiJ.50(1): 45A-47A, January 1967.

131. Back, E. L. "Thermal Auto-Crosslinking in Cellulose Material." Pulp & Paper Magazine of Canada 68: T-165, 1967.

132. Jordan, C. A.; Stem, R. K. "New Tests Probe Cushioning Properties of Corrugated Boards." Package Engineering 10(12): 76-94, December 1965.

133. Hintermaier, J. C.; White, R. E. "The Splitting of a Water Film Between Rotating Rolls." Tappi J. 48(11): 617-625, November 1965

134. McKee, R. C. "Study of the Relationship of Draw Factor and Runnability." Project 1108-22, Report Eight to the Four- drinier Kraft Board Institute from The Institute of Paper Chemistry, June 1965.

135. Schultek, R. "Critical Factors in Corrugated Steam Systems." Paperboard Packaging 50(3): 57-58, March 1965.

136. Moody, R. C. "Edgewise Compression Strength of Corrugated Fiberboard as Determined by Local Instability." USDA Forest Service, FPL Research Report-46, Forest Products Laboratory, Madison, WI, 1965.

137. Zahn, J. J. "Local Buckling of Orthotropic Truss-Core Sandwich." USDA Forest Service, FPL Research Paper-220, Forest Products Laboratory, Madison, WI, 1965.

138. McKee, R. C.; Smith, C. N. "Behavior of Fibrous and Nonfibrous Components in the Corrugating Operation. Part V: State of Stress at Pressure Roll Nip." Project 1108-22, Report Eight to the Fourdrinier Kraft Board Institute from The Insti- tute of Paper Chemistry, December 1964.

139. Velarde, J. J. "Influence of Moisture Content of Corrugating Medium on the Properties of Corrugated Board." ATCP 4(6): 33-434, November/December 1964.

140. McKee, R. C. "Behavior of Fibrous and Nonfibrous components in the Corrugating Operation. Part IV-B: Effect of Finger Design and Clearance on Flute Profile of Single-Faced Board." Project 1108-22, Report Seven to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, June 1963.

141. "End-Load Box Compression." Project 1108-4, A Preliminary Report to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, June 1963.

142. Back, E. L. "Reactions in Dimensional Stabilization of Paper and Fibre Building Board by Heat Treatment." Svensk Paperstidning 66: 745, 1963.

143. Goring, D. A. "Thermal Softening of Lignin, Hemicelulose and Cellulose." Pulp & Paper Magazine of Canada 64: T- 157, 1963.

144. Schneider, E. S. "Automatic Web Control on the Corrugator." Paperboard Packaging 47(5): 54-56, May 1962. Corrugating Medium / 201

145. McKee, R. C.; Smith, C. N. "Behavior of Fibrous and Nonfibrous Components in the Corrugating Operation. Part IV: Analysis of Commercial Boards for High-Low Corrugations." Project 1108-22, Report Four to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, March 1962.

146. McKee, R. C. "Investigation of Medium Feeders as Means of Improving Runnability of Corrugating Medium." Project 1108-22, Report Five to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, March 1962.

147. McKee, R. C. "Behavior of Fibrous and Nonfibrous Components in the Corrugating Operation. Part IV-A: Analysis of Flute Profile of Aluminum Foil." Project 1108-22, Report Six to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, March 1962.

148. Bristow, J. A. "Factors Influencing the Gluing of Paper and Board." Svensk Papperstidning 64: 775-796, November 1961.

149. McKee, R. C.; Gander, J. W. "Behavior of Fibrous and Nonfibrous Components in the Corrugating Operation. Part III: A Study of the Dynamics of the Upper Corrugating Roll." Project 1108-22, Report Three to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, March 1961.

150. Peters, W. "Measuring Forces that Cause Production Problems." Paperboard Packaging 46(2): 60-62, February 1961.

151. Kellicutt, K. 0. "How Paperboard Properties Affect Corrugated Container Performance." Tappi . 44(3): 201A-204A, 1961.

152. McKee, R. C. "Behavior of Fibrous and Nonfibrous Components in the Corrugating Operation. Part II: Behavior of Medium in Single-Facer." Project 1108-22, Progress Report Two to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, May 1960.

153. McKee, R. C. "Corrugating Variables and the Effect On Combined Board Characteristics." Tappi J. 43(3): 218A-228A, March 1960

154. McKee, R. C.; Gander, J. W. "Behavior of Fibrous and Nonfibrous Components in the Corrugating Operation. Part I: Analysis of Stress and Strain in Medium During Formation of the Flutes." Project 1108-22, Progress Report One to the Fourdrinier Kraft Board Institute from The Institute of Paper Chemistry, February, 1960.

155. Reynolds, J. G.; Limerick, J. M. "The Bathurst Method for the Determination of Starch Consumption." Tappi J. 43(12): 204A-206A, 1960.

156. McKee, R. C. "Corrugating Variables and the Effect on Combined Board Characteristics." Tappi J. 43(3): 218A-228A, 1960.

157. Spaulding, R. O.; Wallis, S. W. "Owens-Illinois Develops New C-Flute Contour." Fiber Containers 44(6): 44-46, June 1959.

158. Wilson, H. W. "W-S Flute Design Brings New Concept of Corrugating." Fibre Containers 44(4): 69-72, April 1959.

159. Koenig, J. J. "Single Facing Defects, Causes and Effects on Strength of Corrugating Containers." Tappi J. 39(7): 148A- 151A, 1959.

160. Kellicutt, R. Q. "Relationship of Moment of Inertia to Stiffness of Corrugated Board." Package Engineering 44(10): 80-105, 1959.

161. Nitchie, C. D. "Further Studies on Flute Contour." Tappi J. 40(11): 181 A-184A, November 1957.

162. Nitchie, C. D. "Flute Contour Studies." Fibre Containers 42(4): 50-52, April 1957.

J 202 / Chronological Bibliography

163. Goetsch, W. M. "Adhesive Application on the Single-Facer and Double-Backer." Tappi J. 39(7): 143A-145A, July 1956.

164. Wilson, H. W. "An Operators Thought on Flute Contour." Tappi J. 39(7): 146A-148A, July 1956.

165. Koenig, J. J. "Single-Facing Defects, Causes, and Effects on Strength of Corrugated Containers." Tappi J. 39(7): 148A- 151A, July 1956.

166. Nissan, A. W. "The Rheological Properties of Cellulose Sheets: Retrospect and Synthesis." TappiJ. 39(2): 93-97, Feb- ruary 1956.

167. Wilson, H. W. "How and Why Flute Contour Affects Corrugated Quality." Fibre Containers 40(11): 69-75, November 1955.

168. Harrison, P. "The Role of the Steam System in Corrugating." Tappi J. 37(6): 184A-188A, June 1954.

169. Magnuson, A. L. "Corrugating Problems and Their Solution." Fibre Containers 39(6): 68-79, June 1954.

170. "Oil Mist Lubrication of Single Facer Improves Corrugated Board Production." Paper Trade Journal 138(20): 134, May 1954.

171. Cox, H. L. "Computation of Initial Buckling Stress for Sheet-Stiffener Combinations." Journal of the Royal Aeronauti- cal Society 58: 634-638, 1954.

172. Werner, A. W. "Contemporary Flute Design." TappiJ. 36(5): 167A-170A, May 1953.

173. Skiver, F. E. "High-Low Corrugations." Tappi J. 36(1): 54A-56A, January 1953.

174. Andersson, O.; Berkyto, E. "Some Factors Affecting the Stress-Strain Characteristics of Paper." Svensk Papperstidning 154(13): 437-444, July 1951.

175. Norris, C. B. "Strength of Orthotropic Materials Subject to Combined Stresses." USDA Forest Service Report No. 1816, Forest Products Laboratory, Madison, WI, July 1950.

176. Steenberg, B. "Behaviour of Paper Under Stress and Strain." Pulp & Paper Magazine of Canada 50(3): 207-220, 1949.

177. McCready, D. W. "Effect of the Adhesive on Strength Characteristics of Corrugated Fibreboard." Fibre Containers: 20- 27, February 1939.

178. Nikkel, W. A.; Limerick, J. M. "The Determination of Corrugator Starch Consumption." Pulp & Paper Canada (C) 60.

J eph J. Batelka Group Leader Container Research Author Index

(Numbers Refer to Bibliography Listing.)

Anderson, R. G., 90, 91. Harrison, P., 168. Andersson, O., 174. Augustin, H. E., 36. Hintermaier, J. C., 133. Hoffinan, G. R., 104, 108, 117, 120. Back, E. L., 37, 51, 74, 75, 90, 91,122, 131, 142. Hoke, U., 35, 44. Batelka, J. J., 3, 5, 8, 10, 43. Hubert, W.N., 126. Baum, G. A., 29; 31, 33. Hulteberg, A., 81. Becher, J. J., 104, 108, 117, 120. Hung, J. Y., 118. Berkyto, E., 174. Biorseth, E. J., 63. Jaeger, T., 62. Brange, A., 40. Johnson, M. W., 54, 56. Brecht, W., 100. Jones, G., 1. Bristow, J.A., 148. Jonsson, P., 81. Jopson, R. N., 2. Chow, D. K., 32. Jordan, C. A., 105, 121, 132. Colpe, D., 100. Jurewicz, J. T., 57, 60, 65. Considine, J. M., 4. Corbett, H. M., 94, 96. Kellicutt, K. Q., 99, 151, 160. Cox,H.L., 171. Kloth, G. R., 73. Crisp, C. J., 124. Koenig, J. J., 159,165. Kolhonan, E., 59, 78, 84. Daly, G. J., 106. Koncel, J. A., 9. Daub, E., 20, 21,22, 35. Koning, J. W., 88, 92, 93, 116. Davy, R. G., 97. Kroeschell, W. O., 7, 14. Denniston, W. E., 54, 56. Didriksson, I., 122. Langaard, O., 123. Laundrie, J. F., 28, 41. Ellis, R. L, 10. Laufenberg, T. L., 4. Evans, J. W., 4. Laughlin, M. J., 98. Leake, C. H., 67, 76. Fahey, D. J., 92, 93. Limerick, J. M., 155, 178. Fellers, C., 40. Liu, J. Y., 28, 41. Fitt, L.E., 11. Ford, T. L., 61. Magnuson, A. L., 169. Fox, T. S., 52, 57, 65, 73, 82. Marcille-Lorenz, M., 13, 18. Martinez, A., 12. Gander, J. W., 119, 129, 149, 154. McCready, D. W., 177. Gartaganis, P. A., 71, 79, 83, 89, 97. McKee, R. C., 85, 87, 94, 96, 98, 104, 107, 108, 113, Goetsch, W. M., 163. 115, 117, 119, 120, 126, 129, 134, 138, 140, Goring, D.A., 143. 145, 146, 147, 149, 152, 153, 154, 156. Gottsching, L., 20, 21, 22, 44, 66, 68, 69, 72, 77, 80, 86. McKenzie, A. W., 95. Gunilla, M., 37. Moody, R. C., 116, 136. Mustin, G. S., 128. Hall, M. S., 16. 204 / Author Index

Nelson, R. W., 118. Wilken, J.E., 51. Nikkel, W. A., 178. Wilkins, R., 48. Nissan, A. W., 166. Williams, R. H., 67, 76. Nitchie, C. D., 161, 162. Wilson, H. W., 158, 164, 167. Nordkvist, B., 19. Worster, H. E., 42. Nordman, L., 59. Norris, C. B., 175. Young, S., 11. Yuritta, J. P., 95. Olsen, H. C., 101. Otto, W., 66, 68, 69, 72, 77, 80, 86. Zahn, J. J., 137.

Peters, W., 150. Peterson, W. S., 52, 53, 55, 58. Pickens, T., 45.

Reynolds, J. G., 155. Richardson, K., 37.

Salmen, L., 51, 74. Schneider, E. S., 144. Schrampfer, K. E., 31. Schultek, R., 135. Silano, M. A., 67, 76. Simmonds, F., 110, 114. Skiver, F. E., 173. Smith, C. N., 3, 5, 16, 17, 38, 85, 107, 138, 145. Spaulding, R. O., 157. Sprague, C. H., 30, 46, 47,49. Staigle, V. H., 130. Steenberg, B., 176. Stenberg, L. E., 75. Stem, R. K., 103, 125, 132. Stoker, D. L., 4. Stoot, R. A., 124. Swanson, J. W., 104, 108, 117, 120.

Thayer, W. S., 102. Thomas, C. E., 102. Tomlinson, J. C., 124. Toroi, M., 59, 78, 84, 112.

Urbanik, T. J., 50, 54, 56.

Van Eperen, R. W., 118. Velarde, J. J., 139.

Wallace, J. R., 11. Wallis, S.W., 157. Wennerblom, A., 12. Werner, A. W., 172. Westerlind, B., 12. White, R. E., 133. Whitsitt, W. J., 13, 16, 17, 18, 23, 25, 26, 27, 29, 30, 31, 33, 38, 39, 46, 47, 82, 85, 87, 94, 96, 98, 107, 113, 119, 126. Subject Index

(Numbers Refer to Bibliography Listing)

A Combined Board: 15, 44, 54, 56, 82, 99, 153, 156. Compression Creep: 4, 87. Additives: 2, 87, 94, 96, 104, 108, 117, 119, 147, 154, Concora Effects: 16, 73. 177. Conditioning: See "Preconditioning." Adhesion Theory: 20, 21, 22,24,45, 67, 76, 119, 148. Contact Angle: 119. Adhesive Adsorption: 13, 18,20,21,22, 35, 117,118, Corrugating: 42, 46,47, 85, 144, 146, 150, 153, 156, 119, 120. 165, 168. Adhesive Application - Single-Facer: 14, 47, 49, 60, 61, Corrugating Defects: 95, 115, 116, 159, 165. 62, 63, 65, 70, 71, 79, 83, 85, 88, 102, 118, 119, Corrugating Roll Pressure: 5, 68, 78, 81, 84, 86, 89, 109, 120,133,152,163. 115, 126. Adhesive Application - Double-Backer: 57, 60, 61, 65, Corrugating Strength Loss: See "Fluting Damage." 70, 163. Corrugator Roll Stack: 98, 109, 126, 149, 152, 154. Adhesive Consumption: 47, 71, 79, 83, 85, 101, 117, Creep Failure: See "Compression Creep." 119, 120, 155, 178. Crushing: 5, 7,43, 59, 73, 121, 124, 130. Adhesive Penetration: 14, 25, 48, 117, 120. Crush Recovery: See "Measured Crush." Adhesive Wetting: 15, 16, 18, 20, 21,22, 24, 47, 104, Cushioning: 24, 28,41, 103, 105, 111, 121, 123, 124, 108,117,119,120,148. 125, 127, 132.

B D

Basis Weight Effects: 15, 16, 19, 24,25, 26,27, 34, 36, Density Effects: 1, 7, 11, 14, 16, 23, 29, 33, 110, 114, 39, 55, 73, 78, 82, 84, 99, 103. 117. Bending Strain: 55, 154. Dimensional Stability: 1, 4, 66, 68, 69, 80, 142. Bonding - Double-Backer: 12, 13, 18, 24,35, 45, 67, 76, Draw Factor: 126, 134. 163, 177. Drop Test: 82. Bonding - Medium Properties: 8, 10, 20, 21, 22, 47, 117, 120. E Bonding - Single-Facer: 3, 8, 10, 11, 14, 17, 24, 35, 45, 47,48,67,76, 85, 102, 104, 108, 116, 117, 119, Edge Crush Test: 1,2, 5, 7, 9, 16, 19, 23, 24, 37,38, 39, 120, 152, 163, 177. 43, 50, 54, 55, 54, 59, 82, 90, 91, 99, 116, 123, Bonding Time: 3, 8, 10, 13. 124,129, 136, 137, 141, 146. Bond Gaps: 116. Elastic Properties: 1,24, 29, 33, 38, 39, 54, 56, 134. Bond Setting: 3, 8, 10, 13, 18, 24, 45, 67, 76, 117, 148. Ene-To-End Compression: See "Box Compression." Box Compression: 19, 24, 36, 44, 52, 53, 54, 55, 56, 58, Extractives: 113. 59, 64, 73, 82, 87, 89, 99, 116,123,124,136, 137, 141,153,159,165. F Box Crushing: See "Crushing." Box Drop Test: See "Drop Test." Fingers: 138, 140. Buckling: 44, 54, 56, 116, 136, 137, 171. Flat Crush: 16,23, 29, 31, 32, 37, 38, 39, 59, 82, 95, 123, 124, 129, 130, 146. C Flat Crush Stress/Strain: 39, 59, 103. Flexural Stiffness: 19,24, 54, 55, 56, 59, 82, 124, 141, Caliper Effects: 25, 26, 27, 34, 39, 54, 56, 69, 73, 82, 85, 160. 99, 146. Flute Forming: 1, 15, 17, 24,25, 26, 27, 29, 30, 33, 34, 206 / Subject Index

37,38,39,51,74,77,78,80,84,86,95, 100, Preconditioning: 1, 3, 8, 10, 24, 30, 37, 38, 46, 47, 68, 134, 145, 146, 149, 152, 154. 69,74, 86, 107, 115, 126, 135, 154. Flute High/Low: See "High/Low." Pressure Roll: 115, 138, 152. Flute Height: 31, 55, 126, 140, 145, 146. Flute Leaning: See "Leaning Flute." R Flute Shape: 31, 32, 34, 50, 152, 157, 158, 161, 162, 164, 167, 172. Relative Humidity Effects: 40. Fluting Damage: 1, 5, 15, 16, 17, 29, 30, 33, 34, 77, 95, Relative Humidity v. Moisture: 40. 100, 149. Roll Alignment: 126. Formation: 17, 69, 92, 93, 95, 110, 113, 114, 115. Roll Stack: See "Corrugator Roll Stack." Fracture: 17, 24, 25, 26, 27, 46, 78, 84, 92, 93, 95, 96, Recycled Fiber: 1, 2, 4, 11, 37, 51, 59, 69, 77, 80, 81, 110,114,126,129,134,146,154. 110, 114, 126,149. Friction: 46, 96, 113, 134, 147, 154. Roughness: 13, 18, 104, 108, 117, 119, 120. Runnability: 13, 16, 17, 18, 24, 25, 26, 27, 31, 34, 38, 66, G 77,78,81,84,92,93,95, 96, 110, 114, 115, 126, 134, 146, 150, 154, 169. Gaylord Shower: See "Preconditioning." Rough Handling: See "Drop Test." Green-Bond: 8, 10, 13, 18, 24,45, 67, 76. S H Shear: 154. Heat Response: 13, 16, 18, 75, 95, 117, 120, 122, 131, Shives: 92, 93, 110, 114. 142, 143, 166. Speed Effects: 16, 17, 47, 78, 84, 85, 110, 114, 115, 117, High/Low: 5, 16, 17, 24, 25, 26, 27,31, 66, 69, 72, 78, 120, 126, 146, 149. 80, 81, 84, 95, 96, 98, 107,109,113,115, 126, Static Stress: 54, 56, 41, 123. 138, 140, 145, 147, 152, 173. Steam Box: See "Preconditioning." History: 6, 106. STFI: 16. Hygroexpansivity: See "Dimensional Stability." Strength Improvement: 1, 2, 5, 15, 46, 75, 90, 91, 97, 131,142, 146. L Strength Retention: See "Fluting Damage." Stress/Strain: 1, 24, 39, 50, 52, 53, 54, 55, 56, 58, 59, 74, Leaning Flute: 5, 149, 152. 92,93, 103, 113, 116, 124, 136, 137, 154, 171, Lubricants: 1, 78, 84, 94, 96, 170. 174, 175, 176. Stretch: 46, 75, 92, 93, 110, 114. M Surface Features: 110, 114.

Measured Crush: 5, 24, 59, 130. T Medium/Liner Balance: 19, 36, 50, 55, 64, 73, 82, 99, 123. Tear Strength: 73, 82. Modulus of Elasticity: See "Elastic Properties." Temperature of Web: 3, 16, 24, 30, 37, 38, 46, 74, 78, Moisture Effects: 3, 40, 75, 87, 111, 139, 154. 81, 84, 86, 95, 107, 109, 154. Moisture of Web: 1, 3, 16, 24, 30, 37, 38, 46, 74, 78, 81, Tension of Web: 24, 25, 26, 27, 46, 66, 78, 80, 81, 84, 84, 86, 95, 107, 126. 86,98, 107, 110, 114,115,126,146,154. Moisture v. Compression: 40. Top-To-Bottom Compression: See "Box Compression."

P V Pads: 28, 41, 103, 121, 124, 125, 127, 132. Physical Properties: 4, 15, 16, 17, 25, 26, 27, 40,42, 44, Vibration: 98, 109. 66,80,110,113,114,115,117, 119,126, 139, Void Volume: 117, 120. 153,174, 175, 176. Pin Adhesion Effects: 5, 8, 10, 14, 16, 17, 49, 63, 71, 79, W 82, 85, 104, 108, 117, 120. Poisson Ratio: 1. Water Drop: 15, 16, 18, 24,29, 85, 104, 117, 119, 120. Porosity: 8, 10, 15, 16, 92. Water Resistant Adhesive: 87. Corrugating Medium / 207

Web Orientation: 115, 126. Wetting: See "Adhesive Wetting." Wet Strength Medium: 87.

Z-Direction Tensile: 85.