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 86-4871 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 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 formula tion, 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 1 B. Classification 3 C. Organization of Book 4 D. Units and Conversion Factors 5 II. Background ..... 5 A. Structural Bonding 6 1. Joining Methods 6 2. Advantages of Structural Adhesives 9 3. Limitations of Structural Adhesives 10 B. Principles of Structural Bonding . . . 11 1. Surface and Interfacial Phenomena 11 2. Phase Change 11 3. Bond Failure . . . . 12 C. Design of Adhesive Joints 12 D. Testing ....... . 13 E. Adhesive Selection 14 III. Markets for Structural Adhesives 14 A. Adhesive Markets ..... 14 B. Markets for Structural Adhesives 17 1. Transportation 17 2. Construction 18 3. Other Rigid Bonding 18 4. Consumer Products 18 IV. Applications 19 References 19 1. Fundamentals of Structural Adhesive Bonding Alphonsus V. Pocius l. Introduction 23 ix x Contents II. Physical Properties of Polymers and Adhesives 24 A. Stress-Strain Properties ..... 24 B. Dynamic Mechanical Properties 30 III. Surface Energy and Aspects of Cohesion 33 A. Thermodynamics ....... . 33 B. Cohesive Strength . . . . . . . . 36 C. Fracture Mechanics for Linear Elastic Solids 40 IV. Theories of Adhesion . . . . . 46 A. Introduction . . . . . 46 B. Wettability/ Adsorption Theory 47 C. Contact Angles and Wettability 47 D. Components of Surface and Interfacial Tension 51 E. van der Waals Forces 55 F. Kinetics of Wetting 56 G. Mechanical Interlocking 58 H. Chemical Bonding at Interfaces 60 I. Acid-Base Interactions . . . . 62 J. The Diffusion Theory of Adhesion 63 V. Criteria for Optimum Bond Strength 65 References ............. . 65 2. Phenolic Resins Janis Robins I. Introduction . . . . . . . . . . . . . . . . . . . 69 II. Mechanism of Phenol-Formaldehyde Reactions 72 A. Reactions under Strongly Acidic Conditions (pH 1-4) 72 B. Reactions under Weakly Acidic Conditions (pH 4-7) 74 1. High ortho-ortho N ovolacs 74 2. Superacidity ..... . 77 3. High ortho-ortho Resoles . 78 4. Dibenzylic Ether Linkages 80 5. Cyclic Polynuclear Species 82 C. Reactions under Basic Conditions (pH 7-11) 82 1. Resoles .......... .... . 82 2. Self-condensation 83 D. Resorcinol and m-Aminophenol Modified Phenolic Resins 85 E. Modified Phenolic Resins ..... . 86 1. Etherified Resoles . . . . . . . . . 86 2. Modification at the Phenolic Hydroxyl 87 III. Curing Reactions . . . 88 A. Curing of Resoles 88 B. Curing of Novolacs 90 C. Thermal Degradation 91 IV. Structural Phenolic Adhesives 92 A. Introduction ..... 92 B. Phenolic Resins for Wood Bonding 92 I. Phenolic Resins . . . . . . . 92 2. Resorcinol Modified Novolacs 93 3. m-Aminophenol Modified Novolacs 94 Contents xi C. Polymer Blends for Metal Bonding 94 I. Phenolic-Epoxy Blends 95 2. Phenolic-Polyvinyl Blends 97 3. Phenolic-Nitrile Blends 99 4. Polyamide Phenolics 101 5. Nylon Phenolics 101 6. Acrylic Phenolics 101 7. Neoprene Phenolics 102 8. Urethane Phenolics 102 D. Cure Conditions and Properties 102 E. Commercially Available Materials 106 References 106 3. Epoxy Structural Adhesives Charles D. Wright and John M. Muggee I. Introduction 113 II. Compounding Epoxy Structural Adhesives 116 A. Overview of Adhesives and Epoxy Raw Materials 116 B. Two-Part Room Temperature Curing Epoxy Adhesives 117 1. Nitrogen-containing Curatives 117 2. Sulfur-containing Curatives 127 3. Carboxyl-containing Curatives 128 4. Lewis Acid Catalysts 128 5. Encapsulation 128 C. One-Part Heat Curing Paste Adhesives 129 1. Catalysts 129 2. Toughening Agents 130 3. Fillers, Filler Treatments, and Surfactants 130 4. Thixotropic Additives 131 5. Adhesion Promoters 131 D. Epoxy Film Adhesives 132 III. Recent Developments in Epoxy Structural Adhesives 134 A. Epoxy Resins 134 B. Room Temperature Amine Curing Agents 137 C. Room Temperature Sulfur Curing Agents 142 D. Room Temperature Acid Catalysts 142 E. Heat Cure Amine Curatives and Catalysts 144 1. Primary Amine Curatives 144 2. Secondary Amine Curatives 146 3. Tertiary Amine Catalysts 149 4. Dicyandiamide and Derivatives 152 5. Imidazole Derivatives 154 F. Heat Cure Carboxylic Acid Curatives 157 G. Heat Cure Anhydride Curatives 159 H. Heat Cure Cationic Catalysts 162 I. FJexibilizing and Toughening Agents 164 1. Flexibilizing Agents 165 2. Toughening Agents 166