PLASTICS DESIGN LIBRARY (PDL)
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`Series Editor: Sina Ebnesajjad, PhD
`President, FluoroConsultants Group, LLC
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`The PDL Handbook Series is aimed at a wide range of engineers and other professionals working in the plastics
`industry, and related sectors using plastics and adhesives.
`PDL is a series of data books, reference works and practical guides covering plastics engineering, applications,
`processing, and manufacturing, and applied aspects of polymer science, elastomers and adhesives.
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`Recent titles in the series
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`

`

`HANDBOOK OF ADHESIVES AND
`SURFACE PREPARATION
`Technology, Applications and Manufacturing
`
`Edited by
`
`Sina Ebnesajjad
`
`President, FluoroConsultants Group, LLC
`
`Amsterdam (cid:129) Boston (cid:129) Heidelberg (cid:129) London (cid:129) New York (cid:129) Oxford
`Paris (cid:129) San Diego (cid:129) San Francisco (cid:129) Singapore (cid:129) Sydney (cid:129) Tokyo
`
`William Andrew is an imprint of Elsevier
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`William Andrew is an imprint of Elsevier
`The Boulevard, Langford Lane, Kidlington, Oxford, OX5 1GB, UK
`30 Corporate Drive, Suite 400, Burlington, MA 01803, USA
`
`First published 2011
`
`New material included in this work
`Copyright Ó 2011. Published by Elsevier Inc. All rights reserved.
`Previously published material appearing in this work
`Copyright Ó 2006, 2008, 2011. Published by Elsevier Inc. All rights reserved.
`Copyright Ó 2005, 2006. Published by Elsevier Ltd. All rights reserved.
`Material in this work originally appeared in Adhesives Technology Handbook (Second Edition), edited by Sina
`Ebnesajjad (Elsevier Inc., 2008, ISBN 978-0-8155-1533-3), Surface Treatment of Materials for Adhesion Bonding, edited
`by Sina Ebnesajjad and Cyrus F. Ebnesajjad (Elsevier Inc., 2006, ISBN 978-0-8155-1523-4), Handbook of Adhesives and
`Sealants, Volume 1, edited by Philippe Cognard (Elsevier Ltd, 2005, ISBN 978-0-0804-4554-0), Handbook of Adhesives
`and Sealants, Volume 2, edited by Philippe Cognard (Elsevier Ltd, 2006, ISBN 978-0-0804-4708-7) and Applied Plastics
`Engineering Handbook, edited by Myer Kutz (Elsevier Inc., 2011, ISBN 978-1-4377-3514-7).
`
`No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical,
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`This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as
`may be noted herein).
`
`Notices
`Knowledge and best practice in this field are constantly changing. As new research and experience broaden our
`understanding, changes in research methods, professional practices, or medical treatment may become necessary.
`
`Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any
`information, methods, compounds, or experiments described herein. In using such information or methods they should
`be mindful of their own safety and the safety of others, including parties for whom they have a professional
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`To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any
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`ClearCorrect Exhibit 1042, Page 3 of 418
`
`

`

`In this work I have selected a range of material from
`recent books published by Elsevier, including titles in
`the William Andrew imprint. Thus, I would like to
`offer my recognition to the authors of the selected
`material, and editors of the books from which I have
`sourced this material: John Bishopp, Christian Decker,
`Cyrus F. Ebnesajjad, Guy Rabilloud, Peter G. Pape,
`Philippe Cognard, editor of the Handbook of Adhesives
`and Sealants and Myer Kutz, editor of the Applied
`Plastics Engineering Handbook.
`I would like to offer my deepest appreciations to
`Dr. Kayvon Modjarrad (MD, PhD), Infectious Disease
`Specialist and Dr. Peter E. Dross, neuroradiologist for
`the review and revision of Chapter 14. Their patience
`in explaining medical
`topics to me was beyond
`generous. My thanks go to Dr. Norman Rocheleau,
`DDM for the review of the sections on dental appli-
`cations of adhesives.
`My sincere appreciation goes to my colleague,
`manager and friend Mr. Matthew Deans for his tireless
`
`Acknowledgments
`
`support, knowledge, good nature, and patience. His
`support of this project has been essential to the mate-
`rialization of the book.
`I would like to thank my editorial assistant Miss
`Kristin N. Dross for her work on the contents of this
`book. She reviewed, edited and corrected the writings
`of the content, raised questions, and helped me find
`answers to them.
`the
`for
`Lisa Jones was the project manager
`production of this book and has my heartfelt appreci-
`ation for her diligence and unlimited patience.
`I wish to thank all the companies that have gener-
`ously contributed to this book by allowing the author
`use of their data, information, illustrations, photographs
`and product and process. I have done my best to ensure
`the name of each company is cited for each use.
`
`Sina Ebnesajjad
`Chadds Ford, Pennsylvania
`
`xvii
`
`ClearCorrect Exhibit 1042, Page 4 of 418
`
`

`

`Dedicated to the memories of my dear friend
`William Andrew Woishnis
`Who left the world too early, too young.
`
`ClearCorrect Exhibit 1042, Page 5 of 418
`
`

`

`Preface
`
`A basic industrial process is the bonding of similar
`and dissimilar materials to each other. Bonding using
`an adhesive has proven to be an effective means of
`attaching materials together.
`There are numerous books about adhesives. Several
`excellent books are available that deal with the subject
`of adhesives from various viewpoints. Some have
`looked at adhesives from the perspective of synthesis,
`chemistry, or bonding techniques. Others have treated
`the subject from a practical standpoint. Of these, most
`are attempts to describe adhesion in relation to
`a variety of materials, including plastics, metals, wood,
`and so on. A few books regarding applications of
`adhesives are highly specialized in particular indus-
`tries, such as metals and construction.
`What is different about this book? The present book
`is focused on practitioners of adhesion technology
`from an end-user’s perspective, thus covering most
`substrates, such as plastics, metals, elastomers, and
`ceramics. The information is aimed at allowing readers
`to select the right adhesive and successfully bond
`materials together. Other
`than the choice of
`the
`appropriate adhesive, surfaces must be pretreated
`according to specific methods and prior to the appli-
`cation of adhesives. By including generous selections
`from the recent back catalog of both Elsevier and
`William Andrew publishing (now part of Elsevier), I
`aim to provide to an audience of engineers and other
`professionals working with adhesives, a wide-ranging
`and practical handbook.
`This book describes treatment methods that must be
`applied to a material surface before successful adhe-
`sive bonding is possible. There are numerous exam-
`ples, ranging from wallpaper in a house and paint on
`surfaces to parts used in the construction of aircraft.
`The aim of the contents is to explain, in an accessible
`yet complete manner, all that is required to achieve
`successful adhesion bonding of different materials.
`Fundamental material considerations have been
`given priority to facilitate the use of the contents of this
`book in different industries. The book is both a refer-
`ence and a source for learning the basics. Additionally,
`
`it is useful for all involved in the product value chains,
`and it offers information helpful to engineers, chem-
`ists, students, and all others involved in material
`adhesion and processing.
`Every attempt has been made to enhance the
`accessibility of the information to create a reader-
`friendly text. In the balance of practical and theoretical
`subjects, practical has been given definite precedence.
`This is a trade-off that the author readily acknowl-
`edges. There are numerous good books and sources for
`the study of the theory and science of adhesion and
`adhesives.
`The references listed at the end of each chapter
`serve as both bibliography and additional reading
`sources. Most of the basic practical technology of
`adhesives was developed decades ago. Older refer-
`ences have been retained wherever they represent the
`preferred source of information for a specific topic.
`Readers can find a wealth of information and reports
`that have been declassified by the Defense Technical
`Information Center (www.dtic.mil), most of which
`date to the 1960s.
`The book consists of four parts. Part I contains two
`introductory chapters that describe the fundamental
`concepts of surface treatment and adhesion. Part II is
`comprised of chapters that describe surface tension
`(energy) concepts, surface characterization techniques,
`and surface preparation methods for metals and ther-
`moplastics, thermosets, and elastomers.
`Part III describes the characteristics of adhesives
`from the standpoints of chemical
`structure and
`application. Heat-resistant and UV-cure adhesives are
`discussed in separate chapters because of
`their
`importance to the adhesive industry.
`Part IV of the book describes the applications of
`adhesives with respect to special adherends. Applica-
`tions of adhesives in aerospace, electronic, and
`medical/dental have been described in separate chap-
`ters because of the importance and breadth of adhesion
`use in these industries. There are few, if any, adhesives
`that are not used by these industries, thus rendering the
`information in the chapters useful to nearly any other
`
`xix
`
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`

`

`xx
`
`PREFACE
`
`industry. After all, the aerospace industry has been
`a pioneer in the use of structural adhesive bonding in
`critical applications.
`Appendix A discusses Safety, Environmental,
`Economic Aspects, and Future Trends. Appendix B
`provides an exact copy of the FDA Guidance for Tissue
`Adhesive for the Topical Approximation of Skin
`issued in May 2008. A glossary section rounds off the
`book.
`I hope this book is useful to those who practice the
`art of adhesion as a profession. None of the views or
`
`information presented in this book reflect the opinions
`of any of the companies or individuals that have
`contributed to the book. If there are errors, they are
`an inadvertent oversight on the part of the author.
`A note indicating suggestions or specific errors to the
`publisher, for the purpose of correcting future editions,
`would be much appreciated.
`
`Sina Ebnesajjad
`Chadds Ford, Pennsylvania
`
`ClearCorrect Exhibit 1042, Page 7 of 418
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`

`

`1 Introduction and Adhesion Theories
`Sina Ebnesajjad
`
`1.1 Definition of Adhesives
`and Adhesive Bonding
`
`An adhesive is a material that is applied to the
`surfaces of articles to join them permanently by an
`adhesive bonding process. An adhesive is a substance
`capable of forming bonds to each of the two parts
`when the final object consists of two sections that are
`bonded together.1 A feature of adhesives is the rela-
`tively small quantities that are required compared to
`the weight of the final objects.
`Adhesion is difficult to define, and an entirely
`satisfactory definition has not been found. The
`following definition has been proposed by Wu.2
`“Adhesion refers to the state in which two dissimilar
`bodies are held together by intimate interfacial
`contact such that mechanical force or work can be
`transferred across the interface. The interfacial forces
`holding the two phases together may arise from van
`der Waals forces, chemical bonding, or electrostatic
`attraction. Mechanical strength of the system is
`determined not only by the interfacial forces, but also
`by the mechanical properties of the interfacial zone
`and the two bulk phases.”
`types of adhesive
`There are two principal
`bonding: structural and nonstructural. Structural
`adhesive bonding is bonding for applications in
`which the adherends (the objects being bonded) may
`experience large stresses up to their yield point.
`Structural adhesive bonds must be capable of
`transmitting stress without losing of integrity within
`design limits.3 Bonds must
`also be durable
`throughout the useful service life of a part, which
`may be years. In addition to possessing significant
`resistance to aging, a structural bond is defined as
`having a shear
`strength greater
`than 7 MPa.
`Nonstructural adhesives are not required to support
`substantial loads but merely hold lightweight mate-
`rials in place. This type of adhesive is sometimes
`called a “holding adhesive.” Pressure-sensitive tapes
`and packaging adhesives are examples of nonstruc-
`tural adhesives.
`
`The distinction between structural and nonstruc-
`tural bonds is not always clear. For example, is a hot
`melt adhesive used in retaining a fabric’s plies
`structural or nonstructural? One may argue that such
`an adhesive can be placed in either classification.
`However, the superglues (cyanoacrylates) are clas-
`sified as structural adhesives even though they have
`poor resistance to moisture and heat.
`
`1.2 Functions of Adhesives
`
`The primary function of adhesives is to join parts
`together. Adhesives accomplish this goal by trans-
`mitting stresses from one member to another in
`a manner that distributes the stresses much more
`uniformly than can be achieved with mechanical
`fasteners. Adhesive bonding often provides struc-
`tures that are mechanically equivalent to or stronger
`than conventional assemblies at
`lower cost and
`weight. In mechanical fastening, the strength of the
`structure is limited to that of the areas of the members
`in contact with the fasteners.4 Obtaining adhesive
`bonds that are stronger than those of the strength of
`adherends is not unusual.
`Smooth surfaces are an inherent advantage of
`adhesively joined structures and products. Exposed
`surfaces are not defaced and contours are not
`disturbed, as happens with mechanical fastening
`systems. This feature is important in function and
`appearance. Aerospace structures,
`including heli-
`copter rotor blades, require smooth exteriors to
`minimize drag and to keep temperatures as low as
`possible. Lighter weight materials than are used with
`conventional
`fastening can often be used with
`adhesive bonding because the uniform stress distri-
`bution in the joint permits full utilization of the
`strength and rigidity of the adherends.4 Adhesive
`bonding provides much larger areas for stress transfer
`throughout
`the part,
`thereby decreasing stress
`concentration in small areas.
`are
`Dissimilar materials,
`including plastics,
`readily joined by many adhesives, provided that
`
`Handbook of Adhesives and Surface Preparation, ed. Sina Ebnesajjad. DOI: 10.1016/B978-1-4377-4461-3.10001-X
`Ó 2011 Elsevier Inc. All rights reserved.
`This chapter has been adapted from ‘Introduction and Adhesion Theories’ in Adhesives Technology Handbook (2nd ed.),
`Ebnesajjad Ó 2008 Elsevier Inc.
`
`3
`
`ClearCorrect Exhibit 1042, Page 8 of 418
`
`

`

`4
`
`HANDBOOK OF ADHESIVES AND SURFACE PREPARATION
`
`proper surface treatments are used. Adhesives can be
`used to join metals, plastics, ceramics, cork, rubber,
`and combinations of materials. Adhesives can also be
`formulated to be conductive. The focus of this book
`is on adhesives for bonding plastics, thermosets,
`elastomers, and metals.
`Where temperature variations are encountered in
`the service of an item containing dissimilar materials,
`adhesives perform another useful function. Flexible
`adhesives are able to accommodate differences in the
`thermal expansion coefficients of the adherends, and
`therefore prevent damage that might occur if stiff
`fastening systems were used.
`Sealing is another important function of adhesive
`joining. The continuous bond seals out liquids or gases
`that do not attack the adhesive (or sealant). Adhesives/
`sealants are often used in place of solid or cellular
`gaskets. Mechanical damping can be imparted to
`a structure through the use of adhesives formulated for
`that purpose. A related characteristic, fatigue resis-
`tance, can be improved by the ability of such adhesives
`to withstand cyclic strains and shock loads without
`cracking. In a properly designed joint, the adherends
`generally fail in fatigue before the adhesive fails. Thin
`or fragile parts can also be adhesively bonded. Adhe-
`sive joints do not usually impose heavy loads on the
`adherends, as in riveting, or localized heating, as in
`welding. The adherends are also relatively free from
`heat-induced distortion.4
`
`1.3 Classification of Adhesives
`
`Adhesives as materials can be classified in a number
`of ways, such as chemical structure or functionality.
`Adhesives are categorized into two classes: natural and
`synthetic. The natural group includes animal glue,
`casein- and protein-based adhesives, and natural
`rubber adhesives. The synthetic group has been further
`divided into two subcategories industrial and special
`compounds. Industrial compounds include acrylics,
`epoxies, silicones, etc. An example of the specialty
`group is pressure-sensitive adhesives.
`
`1.4 Advantages and
`Disadvantages of Joining
`Using Adhesives
`
`The previous discussion highlighted a number
`of advantages of adhesive bonding. This section
`
`will cover both advantages and disadvantages of
`adhesive bonding, and some points are reiterated.
`
`1.4.1 Advantages
` Uniform distribution of stress and larger stress-
`bearing area5,6
` Joins thin or thick materials of any shape
` Joins similar or dissimilar materials
` Minimizes or prevent electrochemical (galvanic)
`corrosion between dissimilar materials
` Resists fatigue and cyclic loads
` Provides joints with smooth contours
` Seals joints against a variety of environments
` Insulates against heat
`transfer and electrical
`conductance (in some cases adhesives are
`designed to provide such conductance)
` The heat required to set the joint is usually too
`low to reduce the strength of the metal parts
` Dampens vibration and absorb shock
` Provides an attractive strength/weight ratio
` Quicker and/or cheaper to form than mechanical
`fastening
`
`1.4.2 Disadvantages
` The bond does not permit visual examination
`of the bond area (unless the adherends are
`transparent)5e7
` Careful
`required to
`surface preparation is
`obtain durable bonds, often with corrosive
`chemicals
` Long cure times may be needed, particularly
`where high cure temperatures are not used
` Holding fixtures, presses, ovens, and autoclaves,
`not usually required for other fastening methods,
`are necessities for adhesive bonding
` Upper
`service temperatures are limited to
`
`approximately 177
`C in most cases, but special
`adhesives, usually more expensive, are available
`
`for limited use up to 371
`C
` Rigid process control, including emphasis on
`cleanliness, is required for most adhesives
` The useful life of the adhesive joint depends on
`the environment to which it is exposed
`
`ClearCorrect Exhibit 1042, Page 9 of 418
`
`

`

`1: INTRODUCTION AND ADHESION THEORIES
`
`5
`
` Natural or vegetable-origin adhesives are subject
`to attack by bacteria, mold, rodents, or vermin
` Exposure to solvents used in cleaning or solvent
`cementing may present health problems
`
`1.5 Requirements of a Good Bond
`
`The basic requirements for a good adhesive bond
`are:6
` Proper choice of adhesive
` Good joint design
` Cleanliness of surfaces
` Wetting of surfaces that are to be bonded
`together
` Proper adhesive bonding process (solidification
`and cure)
`
`1.5.1 Proper Choice of Adhesive
`There are numerous adhesives available for
`bonding materials. Selection of the adhesive type and
`form depends on the nature of adherends, perfor-
`mance requirements of the end use, and the adhesive
`bonding process.
`
`1.5.2 Good Joint Design
`Imparting strength to a joint by design is possible.8
`A carefully designed joint yields a stronger bond than
`one not carefully designed when advantages of the
`mechanical design are combined with adhesive bond
`strength to meet the end use requirements of the
`bonded part.
`
`1.5.3 Cleanliness
`To obtain a good adhesive bond, starting with
`a clean adherend surface is essential. Foreign mate-
`rials such as dirt, oil, moisture, and weak oxide layers
`must be removed, lest the adhesive be bonded to
`weak boundary layers rather than to the substrate.
`Various surface treatments exist
`that remove or
`strengthen the weak boundary layers. Such treat-
`ments typically involve physical or chemical pro-
`cesses, or a combination.9
`
`1.5.4 Wetting
`Wetting is the displacement of air (or other gases)
`present on the surface of adherends by a liquid phase.
`The result of good wetting is greater contact area
`
`between the adherends and the adhesive over which
`the forces of adhesion may act.10
`
`1.5.5 Adhesive Bonding Process
`Successful bonding of parts requires an appro-
`priate process. The adhesive must not only be
`applied to the surfaces of the adherends; the bond
`should also be subjected to the proper temperature,
`pressure, and hold time. The liquid or film adhesive,
`once applied, must be capable of being converted
`into a solid in one of three ways. The method by
`which solidification occurs depends on the choice of
`adhesive. The ways in which liquid adhesives are
`converted to solids are:6
` Chemical reaction by any combination of heat,
`pressure, and curing agents
` Cooling from a molten liquid
` Drying as a result of solvent evaporation
`
`Requirements to form a good adhesive bond, as
`well as processes for bonding, analytic techniques,
`and quality control procedures, are discussed in this
`book.
`
`1.6 Introduction to Theories
`of Adhesion
`
`interlocking, electro-
`Historically, mechanical
`static, diffusion, and adsorption/surface reaction
`theories have been postulated to describe mecha-
`nisms of adhesion. Theories have recently been
`postulated for adhesive bonding mechanisms (Table
`1.1). It is often difficult to fully ascribe adhesive
`bonding to an individual mechanism is often diffi-
`cult. A combination of different mechanisms is
`most probably responsible for bonding. The extent
`of
`the role of each mechanism may vary for
`different adhesive bonding systems. An under-
`standing of these theories is helpful to those who
`work with adhesives.
`An important facet of adhesion bonds is the locus
`of the proposed action or the scale to which the
`adhesive and adherend interact. Table 1.1 shows
`a scale of action for each mechanism, which is
`intended to aid in the understanding of these mech-
`anisms. Of course, adhesiveeadherend interactions
`always take place at the molecular level, discussed
`later in the chapter.
`
`ClearCorrect Exhibit 1042, Page 10 of 418
`
`

`

`6
`
`HANDBOOK OF ADHESIVES AND SURFACE PREPARATION
`
`Table 1.1 Theories of Adhesion
`
`Traditional
`
`Recent
`
`Scale of Action
`
`Mechanical interlocking
`
`Mechanical interlocking
`
`Microscopic
`
`Electrostatic
`
`Diffusion
`
`Electrostatic
`
`Diffusion
`
`Adsorption/surface reaction
`
`Wettability
`
`Chemical bonding
`
`Macroscopic
`
`Molecular
`
`Molecular
`
`Atomic
`
`Weak boundary layer
`
`Molecular
`
`The microscopic
`in
`interest
`of
`parameter
`mechanical interlocking is the contact surface of the
`adhesive and the adherend. The specific surface area
`(i.e., surface area per unit weight) of the adherend is
`an example of one such measure. Surface roughness
`is the means by which interlocking is thought to
`work. It can be detected by optical or electron
`microscopy. In the electrostatic mechanism,
`the
`surface charge is the macroscopic factor of interest.
`The charge in question is similar to that produced in
`a glass rod after rubbing it with a wool cloth. Diffu-
`sion and wettability involve molecular and atomic
`scale interactions, respectively.
`Readers who wish to gain an in-depth understanding
`of the interaction forces, adhesion mechanism, and
`thermodynamics of adhesion are recommended to
`consult Fundamentals of Adhesion, edited by Lieng-
`Huang Lee.11 This reference provides a qualitative and
`quantitative treatment of adhesion, complete with
`derivation of force interaction equations.
`
`1.6.1 Mechanical Theory
`According to this theory, adhesion occurs by the
`penetration of adhesives into pores, cavities, and other
`surface irregularities on the surface of the substrate.
`The adhesive displaces the trapped air at the interface.
`Thus that an adhesive penetrating into the surface
`roughness of
`two adherends can bond them is
`concluded. A positive contribution to the adhesive
`bond strength results from the “mechanical inter-
`locking” of the adhesive and the adherends. Adhesives
`frequently form stronger bonds to porous abraded
`surfaces than they do to smooth surfaces. However,
`this theory is not universally applicable, for good
`adhesion also occurs between smooth surfaces.12
`Enhanced adhesion after abrading the surface of
`an adherend may be due to (1) mechanical
`
`interlocking, (2) formation of a clean surface, (3)
`formation of a highly reactive surface, and (4) an
`increase in contact surface area. It is believed that
`changes in physical and chemical properties of the
`adherend surface produce an increase in adhesive
`strength.13 It can be debated whether mechanical
`interlocking is responsible for strong bonds or an
`increase in the adhesive contact surface enhances
`other mechanisms. Thorough wetting and extensive
`chemical bonding are expected consequences of
`increased contact surface area.
`There are supportive data in the literature that
`relates joint strength and bond durability to increased
`surface roughness. There are also observations indi-
`cating, contrarily, that increased roughness can lower
`joint strength.14
`
`1.6.2 Electrostatic (Electronic)
`Theory
`This theory proposes that adhesion takes place due
`to electrostatic effects between the adhesive and the
`adherend.15e18 An electron transfer is supposed to take
`place between the adhesive and the adherend as a result
`of unlike electronic band structures. Electrostatic
`forces in the form of an electrical double layer are thus
`formed at the adhesiveeadherend interface. These
`forces account for the resistance to separation. This
`theory gains support by the fact
`that electrical
`discharges have been noticed when an adhesive is
`peeled from a substrate.13
`The electrostatic mechanism is a plausible expla-
`nation for polymeremetal adhesion bonds. The
`contribution of the electronic mechanism in nonme-
`tallic systems to adhesion has been calculated and
`found to be small when compared to that of chemical
`bonding.19,20
`
`ClearCorrect Exhibit 1042, Page 11 of 418
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`

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`1: INTRODUCTION AND ADHESION THEORIES
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`7
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`1
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`2
`
`3
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`15
`
`10
`
`5
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`Peel strength, lb/in
`
`50
`
`150
`100
`Bonding temperature, ºC
`
`200
`
`Figure 1.1 Peel strength of polypropylene and butyl
`rubber vs. bonding temperature: (1) adhesive failure;
`(2) adhesive/cohesive failure; (3) cohesive failure.2
`
`the adherend is called wetting. For an adhesive to wet
`a solid surface, the adhesive requires a lower surface
`tension than the critical surface tension of the solid.
`This is precisely the reason for surface treatment
`of plastics,
`increasing their surface energy and
`polarity.
`Van der Waals forces are extremely sensitive to the
`distance (r) between molecules, decreasing by the
`inverse of the seventh power (1/r7) of the distance
`between two molecules and the cubic power of the
`distance between two adherends. These forces are
`normally too small to account for the adhesive bond
`strength in most cases.
`Figure 1.2 illustrates complete and incomplete
`wetting of an adhesive spreading over a surface.
`Good wetting results when the adhesive flows into
`the valleys and crevices on the substrate surface. Poor
`wetting results when the adhesive bridges over the
`valley and results in a reduction of the actual contact
`area between the adhesive and the adherend, result-
`ing in a lower overall joint strength.13 Incomplete
`wetting
`generates
`interfacial
`defects,
`thereby
`reducing the adhesive bond strength. Complete
`wetting achieves the highest bond strength.
`Most organic adhesives readily wet metal adher-
`ends. On the other hand, many solid organic
`substrates have surface tensions lower than those of
`common adhesives. Criteria for good wetting require
`adhesives to have a lower surface tension than that of
`the substrate, which explains, in part, why organic
`adhesives such as epoxies have excellent adhesion
`to metals but offer weak adhesion on untreated
`
`1.6.3 Diffusion Theory
`This theory suggests that adhesion is developed
`through the interdiffusion of molecules in between
`the adhesive and the adherend. The diffusion theory
`is primarily applicable when both the adhesive and
`the adherend are polymers with relatively long-chain
`molecules capable of movement. The nature of
`materials and bonding conditions will
`influence
`whether and to what extent diffusion occurs. The
`diffuse interfacial (interphase) layer typically has
`a thickness in the range of 10e1,000 A˚ (1e100 nm).
`Solvent cementing or heat welding of thermoplastics
`occurs due to diffusion of molecules.13
`No stress concentration is present at the interface
`because no discontinuity exists in the physical
`properties. Cohesive
`energy
`density
`(CED,
`Eqn (1.1)) can be used to interpret diffusion bonding,
`as defined by Eqn (1.2). Bond strength is maximized
`when solubility parameters are matched between the
`adhesive and the adherend.
`
`CED ¼ Ecoh
`r
`ffiffiffiffiffiffiffiffiffi
`V
`
`d ¼
`
`Ecoh
`V
`
`(1.1)
`
`(1.2)
`
`Ecoh is the amount of energy required to separate
`the molecules to an infinite distance, V is the molar
`volume, and d is the solubility parameter.
`A relevant example is the adhesion of polyethylene
`and polypropylene to a butyl rubber. The adhesive
`bond is weak when two polymers are bonded at
`temperatures below the melting point of the polyolefin.
`Bond strength increases sharply when the adhesion
`process takes place above the melting temperature of
`
`
`polyethylene (135
`C) and polypropylene (175
`C).
`Figure 1.1 illustrates the bond strength (peel strength)
`as a function of bonding temperature. Inferentially,
`interdiffusion of polyolefins and butyl rubber increases
`at elevated temperatures, generating higher bond
`strength.
`
`1.6.4 Wetting Theory
`This theory proposes that adhesion results from
`molecular contact between two materials and the
`surface forces that develop. The first step in bond
`formation is to develop interfacial forces between the
`adhesive and the substrates. The process of estab-
`lishing continuous contact between the adhesive and
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`ClearCorrect Exhibit 1042, Page 12 of 418
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`8
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`HANDBOOK OF ADHESIVES AND SURFACE PREPARATION
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`Figure 1.2 Examples of (a) good and (b)
`poor wetting by an adhesive spreading
`across a surface.13
`
`polymeric substrates such as polyethylene, poly-
`propylene, and fluoroplastics.13 The surface energy
`of plastic substrates can be increased by various
`treatment techniques to allow wetting.
`
`1.6.5 Chemical Bonding
`This mechanism attributes the formation of an
`adhesion bond to surface chemical forces. Hydrogen,
`covalent, and ionic bonds formed between the
`adhesive and the adherends are stronger than the
`dispersion attractive forces. Table 1.2 lists examples
`of these forces and their magnitudes. In general, there
`are four types of interactions that transpire during
`chemical bonding: covalent bonds, hydrogen bonds,
`Lifshitzevan der Waals
`forces, and acidebase
`interactions. The exact nature of the interactions for
`a given adhesive bond depends on the chemical
`composition of the interface.
`Covalent and ionic bonds (Table 1.2) are examples
`of chemical bond