Structural Adhesives Chemistry and Technology

TOPICS IN APPLIED CHEMISTRY Series Editors: Alan R. Katritzky, FRS

Kenan Professor of Chemistry University of Florida, Gainesville, Florida

Gebran J. Sabongi Technical Manager, Graphic Research Laboratory 3M, St. Paul, Minnesota

STRUCTURAL ADHESIVES Edited by S. R. Hartshorn

A Continuation Order Plan is available for this series. A continuation order will bring delivery of each new volume immediately upon publication. Volumes are billed only upon actual shipment. For further information please contact the publisher.

Structural Adhesives Chemistry and Technology

Edited by

S. R. Hartshorn 3M St. Paul, Minnesota

Plenum Press • New York and London

Library of Congress Cataloging in Publication Data

Structural adhesives.

(Topics in applied chemistry) Includes bibliographies and index. 1. Adhesives. l. Hartshorn, S. R. II. Series.

TP968.S83 1986 624.1'899 ISBN 978-1-4684-7783-2 ISBN 978-1-4684-7781-8 (eBook) DOl 10.1 007/978-1-4684-7781-8

© 1986 Plenum Press, New York Softcover reprint of the hardcover 18t edition 1986 A Division of Plenum Publishing Corporation 233 Spring Street, New York, N. Y. 10013

All rights reserved

86-4871

No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission from the Publisher

Contributors

Charles W. Boeder, Adhesives, Coatings and Sealers Division, 3M, St. Paul, Minnesota 55144

B. H. Edwards, Adhesives, Coatings and Sealers Division, 3M, St. Paul, Minnesota 55144

S. R. Hartshorn, Adhesives, Coatings and Sealers Division, 3M, St. Paul, Minnesota 55144

G. H. Millet, Industrial and Consumer Sector Research Laboratory, 3M, St. Paul, Minnesota 55144

John M. Muggee, Adhesives, Coatings and Sealers Division, 3M, St. Paul, Minnesota 55144

Alphonsus V. Pocius, Adhesives, Coatings and Sealers Division, 3M, St. Paul, Minnesota 55144

Gene B. Portelli, Industrial and Consumer Sector Research Laboratory, 3M, St. Paul, Minnesota 55144

Janis Robins, Adhesives, Coatings and Sealers Division, 3M, St. Paul, Minnesota 55144

K. P. Subrahmanian, Adhesives, Coatings and Sealers Division, 3M, St. Paul, Minnesota 55144

Philip H. Thompson, Formerly with 3M. 2445 Londin Lane #206, St. Paul, Minnesota 55119

Charles D. Wright, Adhesives, Coatings and Sealers Division, 3M, St. Paul, Minnesota 55144

v

Preface

Adhesives in general and structural adhesives in particular are the subjects of much academic interest as well as commercial importance. Structural bonding, as a method of joining, offers a number of advantages over mechanical fastening. However, in order to achieve satisfactory results, the proper adhesive must be selected and the appropriate bonding procedures followed.

The purpose of Structural Adhesives: Chemistry and Technology is to review the major classes of structural adhesives and the principles of adhesion and bonding as these relate to structural joints. Each chapter provides an overview of the topic under discussion with a list of references to the relevant literature. In addition to describing the chemistry involved, other aspects of structural adhesive technology are covered, such as formula­tion, testing, and end uses.

Some structural adhesives, especially epoxies and phenolics, have a long history of successful use and are now widely employed. Others, such as the structural acrylics and cyanoacrylates, are beginning to gain industrial acceptance. Urethanes and anaerobics have limited but important uses, while high-temperature adhesives are still largely in the research and development stage.

A number of chapters in this book are devoted to topics that are of more general interest. Thus one chapter covers the fundamentals of adhesion and discusses the principles that provide a necessary background for an understanding of adhesive bonding. Another deals with the important subject of environmental resistance and the factors affecting the durability of structural adhesive joints. The design, analysis, and testing of adhesive joints also play essential roles in the successful use of structural adhesives. Finally, some consideration is given to methods of application, to indicate how structural adhesives are handled in typical industrial situations and to mention some of the factors that need to be considered when choosing a method of application.

vii

viii Preface

It is not the intention to provide a thorough coverage of the subject; that would not be possible in a single volume of this size. Rather, it is hoped that this book will serve as an introduction to structural adhesives and structural adhesive bonding and that it will be a useful source reference to the subject.

S. R. Hartshorn St. Paul, Minnesota

Contents

Introduction S. R. Hartshorn

I. Definitions . . . . . . A. Structural Adhesives B. Classification C. Organization of Book D. Units and Conversion Factors

II. Background ..... A. Structural Bonding

1. Joining Methods 2. Advantages of Structural Adhesives 3. Limitations of Structural Adhesives

B. Principles of Structural Bonding . . . 1. Surface and Interfacial Phenomena 2. Phase Change 3. Bond Failure . . . .

C. Design of Adhesive Joints D. Testing ....... . E. Adhesive Selection

III. Markets for Structural Adhesives A. Adhesive Markets ..... B. Markets for Structural Adhesives

1. Transportation 2. Construction 3. Other Rigid Bonding 4. Consumer Products

IV. Applications References

1. Fundamentals of Structural Adhesive Bonding Alphonsus V. Pocius

l. Introduction

ix

1 3 4 5 5 6 6 9

10 11 11 11 12 12 13 14 14 14 17 17 18 18 18 19 19

23

x

II.

III.

IV.

Physical Properties of Polymers and Adhesives A. Stress-Strain Properties ..... B. Dynamic Mechanical Properties Surface Energy and Aspects of Cohesion A. Thermodynamics ....... . B. Cohesive Strength . . . . . . . . C. Fracture Mechanics for Linear Elastic Solids Theories of Adhesion . . . . . A. Introduction . . . . . B. Wettability/ Adsorption Theory C. Contact Angles and Wettability D. Components of Surface and Interfacial Tension E. van der Waals Forces F. Kinetics of Wetting G. Mechanical Interlocking H. Chemical Bonding at Interfaces I. Acid- Base Interactions . . . . J. The Diffusion Theory of Adhesion

V. Criteria for Optimum Bond Strength References ............. .

2. Phenolic Resins Janis Robins

I. II.

III.

IV.

Introduction . . . . . . . . . . . . . . . . . . . Mechanism of Phenol-Formaldehyde Reactions A. Reactions under Strongly Acidic Conditions (pH 1-4) B. Reactions under Weakly Acidic Conditions (pH 4-7)

1. High ortho-ortho N ovolacs 2. Superacidity ..... . 3. High ortho-ortho Resoles . 4. Dibenzylic Ether Linkages 5. Cyclic Polynuclear Species

C. Reactions under Basic Conditions (pH 7-11) 1. Resoles .......... .... . 2. Self-condensation

D. Resorcinol and m-Aminophenol Modified Phenolic Resins E. Modified Phenolic Resins ..... .

1. Etherified Resoles . . . . . . . . . 2. Modification at the Phenolic Hydroxyl

Curing Reactions . . . A. Curing of Resoles B. Curing of Novolacs C. Thermal Degradation Structural Phenolic Adhesives A. Introduction ..... B. Phenolic Resins for Wood Bonding

I. Phenolic Resins . . . . . . . 2. Resorcinol Modified Novolacs 3. m-Aminophenol Modified Novolacs

Contents

24 24 30 33 33 36 40 46 46 47 47 51 55 56 58 60 62 63 65 65

69 72 72 74 74 77 78 80 82 82 82 83 85 86 86 87 88 88 90 91 92 92 92 92

93 94

Contents

C. Polymer Blends for Metal Bonding I. Phenolic-Epoxy Blends 2. Phenolic-Polyvinyl Blends 3. Phenolic-Nitrile Blends 4. Polyamide Phenolics 5. Nylon Phenolics 6. Acrylic Phenolics 7. Neoprene Phenolics 8. Urethane Phenolics

D. Cure Conditions and Properties E. Commercially Available Materials

References

3. Epoxy Structural Adhesives Charles D. Wright and John M. Muggee

I. Introduction II. Compounding Epoxy Structural Adhesives

A. Overview of Adhesives and Epoxy Raw Materials B. Two-Part Room Temperature Curing Epoxy Adhesives

1. Nitrogen-containing Curatives 2. Sulfur-containing Curatives 3. Carboxyl-containing Curatives 4. Lewis Acid Catalysts 5. Encapsulation

C. One-Part Heat Curing Paste Adhesives 1. Catalysts 2. Toughening Agents 3. Fillers, Filler Treatments, and Surfactants 4. Thixotropic Additives 5. Adhesion Promoters

D. Epoxy Film Adhesives III. Recent Developments in Epoxy Structural Adhesives

A. Epoxy Resins B. Room Temperature Amine Curing Agents C. Room Temperature Sulfur Curing Agents D. Room Temperature Acid Catalysts E. Heat Cure Amine Curatives and Catalysts

1. Primary Amine Curatives 2. Secondary Amine Curatives 3. Tertiary Amine Catalysts 4. Dicyandiamide and Derivatives 5. Imidazole Derivatives

F. Heat Cure Carboxylic Acid Curatives G. Heat Cure Anhydride Curatives H. Heat Cure Cationic Catalysts I. FJexibilizing and Toughening Agents

1. Flexibilizing Agents 2. Toughening Agents

xi

94 95 97 99

101 101 101 102 102 102 106 106

113 116 116 117 117 127 128 128 128 129 129 130 130 131 131 132 134 134 137 142 142 144 144 146 149 152 154 157 159 162 164 165 166

xii

1. Fillers ..... . I. Reinforcing Fillers 2. Glass Fillers 3. Corrosion-inhibiting Fillers 4. Adhesion-promoting Fillers 5. Cure-promoting Fillers 6. Electrical Conductivity-promoting Fillers 7. Silica Fillers 8. Flow Control Fillers

K. Additives . . . . . . 1. Adhesion Promoters 2. Adhesive-strengthening Additives 3. Heat and/ or Moisture Resistance Additives 4. Film-forming Additives 5. Dispersing Additives

References

4. Polyurethane Structural Adhesives B. H. Edwards

I. II.

III.

IV.

Introduction . . . . . . . . . . . . . . . . Polyurethane Chemistry . . . . . . . . . . . Structure-Property Relationships in Polyurethanes A. General Concepts B. Effect of Soft Segment C. Effect of Hard Segment D. Chemical Crosslinking E. Stability of Polyurethanes

I. Thermal Stability 2. Oxidative Stability 3. Solvolytic Stability 4. Stabilization

Polyurethane Structural Adhesive Applications A. Introduction B. Two-Part Adhesives C. One-Part Adhesives

I. Moisture Cure 2. Heat Cure

D. Comparison of One-Part and Two-Part Systems E. Plastic and Metal Bonding

I. Trends . . . . ... 2. Substrate Preparation 3. Bond Testing

F. Wood Bonding References

5. Anaerobic and Structural Acrylic Adhesives Charles W Boeder

I. II.

Introduction Anaerobic Adhesives

Contents

168 168 169 169 169 169 169 170 170 171 171 172 173 173 173 174

181 182 185 185 187 188 190 190 190 191 192 192 194 194 195 196 196 197 200 200 200 202 203 205 209

217 218

Contents

III.

A. Background . . . . . . B. Formulation Components

1. Monomers 2. Initiators . . . . . 3. Accelerators 4. Stabilizers/ Inhibitors 5. Modifiers

C. Chemistry of Anaerobic Adhesives D. End Use Applications ..... E. Anaerobic Products and Performance Structural Acrylic Adhesives A. Background . . . . . . B. Formulation Components

1. Monomers 2. Catalyst Systems 3. Tougheners

C. Chemistry D. Applications E. Commercial Product Performance

References

6. Cyanoacrylate Adhesives G. H. Millet

I. II.

III.

Introduction . . . . . Cyanoacrylate Chemistry A. General Description B. Monomer Synthesis

1. Alkyl Cyanoacrylates 2. Biscyanoacrylates 3. 2-Substituted-l-Cyanoacrylates

C. Properties of Cyanoacrylate Monomers D. Polymerization of Cyanoacrylate Monomers

1. Introduction 2. Initiation 3. Propagation 4. Chain Transfer and Termination 5. Bulk Polymerization .

E. Polymerization Modifiers 1. Initiators . . 2. Accelerators 3. Inhibitors

F. Properties of Polycyanoacrylates Cyanoacrylate Adhesive Properties A. Bond Strength .... .

1. Introduction .... . 2. Bond Strength Modifiers

B. Durability 1. Introduction 2. Heat Durability

xiii

218 218 219 224 226 229 230 231 233 233 235 235 236 236 237 238 240 241 243 243

249 250 250 252 252 254 254 255 255 255 257 258 258 259 261 261 262 263 266 269 269 269 272

278 278 279

xiv

IV.

V.

3. Heat Resistance Promoters . . . . . 4. Moisture Durability . . . . . . . .

C. Other Adhesive Properties and Modifiers 1. Viscosity . . . . . . . . . . . . . 2. Colorants and Dyes . . . . . . . . 3. Miscellaneous Properties and Modifiers

Cyanoacrylate Application and Uses A. Bondable Substrates B. Surface Preparation

I. Metals . . . . . 2. Polymers . . . . 3. Glass and Ceramics 4. Wood ..... .

C. Toxicology and Safety 1. Handling Precautions 2. Toxicology . . . . .

D. Containers and Storage . E. Application Methods and Equipment

1. Adhesive Application 2. Application Equipment

F. Typical Industrial Applications Summary .......... . A. Advantages of Cyanoacrylate Adhesives B. Disadvantages of Cyanoacrylate Adhesives C. A Comparison of Cyanoacrylates with Other Adhesives D. A Summary of Recent Improvements to Cyanoacrylate Adhesives

References

7. High-Temperature Polymers and Adhesives K. P. Subrahmanian

I. Introduction II. High-Temperature Polymers

A. Theory of Thermal Stability B. Measurement of Thermal Stability C. Heat-Resistant Polymers

I. Aromatic Conjugated Polymers 2. Ladder Polymers 3. Stepladder Polymers 4. Aromatic Linked by Aliphatic or Functional Group

III. High-Temperature Adhesives A. Condensation-Type Adhesives

I. Polyimides 2. Polyphenylquinoxaline

B. Addition-Type Adhesives I. Norbornene End Groups 2. Acetylene End Groups 3. Maleimide End Groups

C. Thermoplastic Adhesives D. Miscellaneous Other Polymers for Adhesive Applications

Contents

280 285 288 288 290 292 293 293 293 293 294 294 294 294 294 295 296 296 296 297 297 298 298 299 300 303 303

309 309 309 311 311 312 312 313 316 317 318 318 320 321 323 325 328 329 331

Contents

IV.

V.

VI.

Bonding Technology of High-Temperature Adhesives A. Substrates and Surface Preparation ..... .

1. Surface Preparation of Titanium ..... . 2. Surface Preparation of Aluminum and Other Adherends

B. Comparative Properties of High-Temperature Adhesives Applications of High-Temperature Adhesives A. General Considerations B. Applications

1. Aerospace 2. Electronics 3. Electrical 4. Automotive 5. Miscellaneous

Trends and Conclusions References

8. The Durability of Structural Adhesive Joints S. R. Hartshorn

I. Introduction II. Factors Affecting Durability

A. Nature of the Adhesive 1. General Considerations 2. Resistance to Hydrolysis 3. Mechanical Properties 4. Effect of Adhesive Formulation 5. Adhesives for Aluminum 6. Adhesives for Wood 7. Other Structural Adhesives

B. Nature of the Adherends 1. Metal Adherends 2. Wood Adherends 3. Plastic Adherends 4. Thermal Stability of the Adhesive 5. Hydrolytic Stability of the Adhesive 6. Cure of the Adhesive

C. Nature of the Surface 1. Effect of Surface Preparation 2. Characterization of Oxide Layer Structure 3. Hydration of Oxide Layer 4. Stabilization of the Oxide Interface

D. Environmental Conditions E. Processing Conditions

III. The Effect of Water A. Uptake of Water

1. Diffusion Coefficients 2. Equilibrium Water Uptake 3. Uptake of Water by an Adhesive Joint

B. Effect of Water on Joint Strength

xv

331 331 332 335 335 340 340 340 340 341 341 342 342 342 343

347 348 348 348 349 350 352 354 359 361 363 363 364 364 365 365 365 367 367 374 375 377 379 379 380 380 380 382 383 384

xvi

IV.

C. Effect of Water on Joint Durability 1. Thermodynamic Considerations 2. Irreversible Effects ..... 3. A Mechanism for Environmental Failure of Adhesive Bonds

Assessment of Durability A. Sustained Load Methods

I. Individual Specimens 2. Multiple Specimens

B. Endurance Limit C. Cyclic Stress Testing . D. Hot Water Soak . . . E. Fracture Mechanics Tests

I. Fracture Energy 2. Failure under Sustained Loads 3. Cyclic Loading

F. Prediction of Service Life 1. Reaction Rate Method 2. Statistical Approaches

V. Summary References

9. Testing, Analysis, and Design of Structural Adhesive Joints Gene B. Portelli

I. II.

III.

IV.

Introduction . . . . . . . . . . . Mechanical Testing of Adhesive Joints A. Introduction B. Tensile Tests C. Shear Tests

1. Lap Shear Tests 2. Compressive Shear Tests 3. Torsional Shear Test

D. Peel Tests ..... E. Other Tests Stress Analysis of Adhesive Joints A. Introduction B. Tensile Loads C. Shear Loads D. Peel Loads Fracture Mechanics of Adhesive Joints A. Introduction ........ . B. Fracture Mechanics Theories C. Fracture Mechanics Applied to Adhesive Joints

V. Adhesive Joint Design A. Introduction B. Butt Joints C. Lap Joints D. Scarf and Modified Joints E. Design Criteria

VI. Summary References

Contents

386 386 387 388 389 390 391 391 392 394 395 396 397 398 399 400 400 401 402 403

407 409 409 410 413 414 418 418 419 422 425 425 426 426 434 436 436 437 439 441 441 441 442 443 444 445 446

Contents

10. Industrial Application Methods Philip H. Thompson

I. II.

III.

IV.

Introduction . . . . . . . . . . . . . . . . . . . Factors Affecting the Choice of a Method of Application A. Adhesive Properties

I. Physical Form 2. Curing Mechanism

B. Substrate Properties I. Material . 2. Condition 3. Shape

C. Conditions of Manufacture Application Methods and Equipment A. Dispensing

I. Liquids and Pastes 2. Films .... . 3. Powders ... .

B. Handling Multiple-Component Adhesives 1. Pumps .... . 2. Hoses .... . 3. Mixers and Valves 4. Cleaning . . .

C. Heat Curing 1. Hot Air Ovens 2. Localized Hot Air 3. Infrared Ovens 4. Localized Infrared 5. Electrical Induction 6. RF and Microwave Energy 7. Hot Oil Baths 8. Condensing Vapors 9. Autoclaves . . .

10. Hot Platen Press Conclusion

xvii

451 452 452 452 454 455 455 455 456 456 457 457 457 462 462 463 463 464 465 466 467 467 468 468 469 469 470 471 471 472 472 472

Appendix I: Standard Definitions of Terms Relating to Adhesives 475

Appendix II: SI Units and Conversion Factors 485

Appendix III: ASTM Standards Relating to Adhesives 491

Index ................................... 495