Fundamentals of Pressure Sensitivity

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FUNDAMENTALS OF PRESSURE SENSITIVITY

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Handbook of Pressure-Sensitive Adhesives and Products Fundamentals of Pressure Sensitivity Technology of Pressure-Sensitive Adhesives and Products Applications of Pressure-Sensitive Products

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HANDBOOK OF

PRESSURE-SENSITIVE ADHESIVES AND PRODUCTS

FUNDAMENTALS OF PRESSURE SENSITIVITY

EDITED

BY

ISTVÁN BENEDEK MIKHAIL M. FELDSTEIN

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CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2009 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-13: 978-1-4200-5937-3 (Hardcover) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Fundamentals of pressure sensitivity / editors, Istvan Benedek and Mikhail M. Feldstein. p. cm. Includes bibliographical references and index. ISBN 978-1-4200-5937-3 (alk. paper) 1. Pressure-sensitive adhesives. I. Benedek, Istvan, 1941- II. Feldstein, Mikhail M. III. Title. TP971.F86 2009 668’.3--dc22

2008012198

Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

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Contents Preface ................................................................................................ vii Editors ...................................................................................................xi Contributors ...................................................................................... xiii

1

Surface Phenomena on a Solid–Liquid Interface and Rheology of Pressure Sensitivity Oksana A. Soboleva, Alexander V. Semakov, Sergey V. Antonov, and Valery G. Kulichikhin .......................... 1-1

2

Diffusion and Adhesion Costantino Creton and Régis Schach ......................................... 2-1

3

Transition Zones in Adhesive Joints Anatoly E. Chalykh and Anna A. Shcherbina ........................... 3-1

4

Role of Viscoelastic Behavior of Pressure-Sensitive Adhesives in the Course of Bonding and Debonding Processes Christophe Derail and Gérard Marin ........................................ 4-1

5

Viscoelastic Properties and Windows of Pressure-Sensitive Adhesives Eng-Pi Chang ............................................................................ 5-1

6

Probe Tack Costantino Creton and Kenneth R. Shull .................................. 6-1

7

Peel Resistance Hyun-Joong Kim, Dong-Hyuk Lim, and Young-Jun Park ...........7-1

v

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Contents

8

Shear Resistance Sergey V. Antonov and Valery G. Kulichikhin ........................... 8-1

9

Durability of Viscoelastic Adhesive Joints Sergey V. Kotomin ..................................................................... 9-1

10

Molecular Nature of Pressure-Sensitive Adhesion Mikhail M. Feldstein ............................................................... 10-1

11

Significance of Relaxation for Adhesion of PressureSensitive Adhesives Mikhail M. Feldstein, Mikhail B. Novikov, and Costantino Creton ....................................................................11-1

Appendix: Abbreviations and Acronyms ......................................... A-1 Index ................................................................................................... I-1

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Preface In recent years, pressure-sensitive products (PSPs) have reached a maturity that warrants a detailed and critical examination of their science and technology. Based on our experience in both scientific activity and industrial areas, as well as on the special knowledge of outstanding scientists and technologist as contributors, we have addressed all aspects of pressure-sensitive adhesives (PSAs) in the form of a handbook. The huge volume of data accumulated in this field over the past decade presents a delicate problem due to the gap between the fundamentals of pressure-sensitive materials and their practice. The application of PSAs requires a thorough knowledge of basic rheological and viscoelastic phenomena. Adhesive and polymer scientists, however, are not often employed as industrial managers or machine operators. Therefore, a need exists to investigate and summarize the most important features of PSA technology and explain the phenomena scientifically. This book covers the fields of manufacturing, conversion, application and end uses of PSAs using a classic approach to compile a treatise based on the work of various experts, theoreticians, chemists and engineers. The volume and diversification of the data, as well as the boundary between theory and application, imposed the need to impart our treatise in three books. The destination of this handbook is twofold. On one hand, it is addressed to scientists focusing on the fundamental processes underlying the complex phenomenon of pressure-sensitive adhesion; on the other hand, it is intended for industrial researchers who are involved in the practical application of these fundamentals for the development of various products and specialists working in various end-use domains of PSPs. Fundamentals of Pressure Sensitivity contains a detailed characterization of the processes occurring in PSA materials at all stages of the life of an adhesive joint: its formation under compressive force, under service as the bonding force is removed, and under adhesive bond fracture when the major type of deformation is extension. Technology of Pressure-Sensitive Adhesives and Products describes particular features of different classes of PSAs, such as rubber–resin-based adhesives, acrylics, and silicones, and presents a discussion of the synthesis of pressure-sensitive raw materials, their formulation, and the manufacture of PSAs and PSPs. Applications of Pressure-Sensitive Products describes the main classes and representatives of PSPs, their competitors, end use, application domains, application technology, and tests. vii

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Preface

The domain of pressure-sensitive materials includes several fields that would be sufficiently autonomous, complex, and large (e.g., bonding–debonding mechanisms, manufacture, equipment, quality assurance) to be described in separate books. Our goal to create a short vade mecum was made significantly easier because of our previous work in this field, which covered almost all aspects of pressure sensitivity and allowed for their detailed discussion. Separate works have discussed special aspects of this waste domain, such as Pressure-Sensitive Adhesives Technology (I. Benedek, Marcel Dekker, New York, 1997) and Pressure-Sensitive Adhesives and Applications (I. Benedek, Marcel Dekker, New York, 2004), which focused mainly on pressuresensitive labels; Development and Manufacture of Pressure-Sensitive Products (I. Benedek, Marcel Dekker, 1999), which describes the whole domain of self-adhesive products, with or without adhesive; and Pressure-Sensitive Formulation (I. Benedek, VSP, Utrecht, the Netherlands, 2000), which gives a detailed discussion of a special, prac tical segment of pressure-sensitive technology. Advances in PSA materials imposed the need for reediting of these books in cooperation with C. Creton and M.M. Feldstein, allowing a more detailed discussion of the scientific aspects, in Development in Pressure-Sensitive Products (Ed. Benedek, Taylor & Francis, Boca Raton, Florida, 2006), Pressure-Sensitive Design, Theoretical Aspects (Ed. Benedek, VSP, 2006), and Pressure-Sensitive Adhesives and Applications (Ed. Benedek, VSP, 2006). Because these books contained a detailed description of various pressure-sensitive science- and technology-related problems, it was possible to edit our handbook as a lexically constructed work, focused on key problems, which avoids undesired redundancy of aspects described previously in a detailed manner. In Chapter 1 of this book, “Surface Phenomena on a Solid–Liquid Interface and Rheology of Pressure Sensitivity,” rheological characterization of processes occurring in adhesive materials under application of bonding pressure (wetting and spreading flow) is studied as a function of the viscosity of test liquids and viscoelastic properties of a substrate. The rheological response of the PSA material to the application of bonding pressure has been proposed to be characterized in terms of dimensionless parameters, for example, the ratio of the time of adhesive joint formation under bonding pressure to the intrinsic relaxation (or retardation) time. Data are also presented that illustrate the effect of bonding pressure on the change of apparent viscosity and shear rate of a PSA material with time of adhesive bond formation. Chapter 2 of this book, “Diff usion and Adhesion,” is a contemporary, critical reexamination of the diff usion theory of adhesion, in correlation with pressure-sensitive adhesion. When two polymers are not identical and therefore fully miscible, but are partially miscible, they can interpenetrate by a small distance controlled by the thermodynamics of their interaction. In this case, the adhesion energy between the polymer layers depends strongly on the degree of interpenetration at the interface. In Chapter 3 of this book, “Transition Zones in Adhesive Joints,” the structure– morphological classification of transition zones in adhesive joints is presented. The interphase boundary between an adhesive and a substrate is a constituent part of such transition zones. The relationship between the structure and the morphology of the transition zones, with phase diagrams of the adhesive–substrate systems and interdiffusion coefficients, is described. Numerous examples of the structure of

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Preface

ix

transition zones are presented, including amorphous–crystalline and liquid–crystalline equilibriums. Chapter 4 of this book, “Role of Viscoelastic Behavior of Pressure-Sensitive Adhesives in the Course of Bonding and Debonding Processes,” focuses on the effect of viscoelastic properties of PSAs оn their adhesive properties—measured using peeling or probe tack experiments. As demonstrated in Chapter 4, the viscoelastic properties govern, to a large extent, adhesive behavior. Chapter 5 of this book, “Viscoelastic Properties and Windows of Pressure-Sensitive Adhesives,” describes the correlation of the viscoelastic properties of PSAs with industrystandard performance such as peel resistance, tack, and shear resistance. Using a fourquadrant viscoelastic window concept, the possibility of characterizing and aiding in the development of different types of PSAs is further demonstrated. Chapter 6 of the book, “Probe Tack,” is a very illustrative and informative test for adhesion. Structure transformations (cavitation and fibrillation) of adhesive material under a detaching force are discussed. The practical aspects of tack as a common test method for characterization of PSAs and PSPs, as well as its influence on the converting and end-use properties, are also discussed in the other two books. Chapter 7 of this book, “Peel Resistance,” focuses on the measurement of peel resistance as one of the most important characteristics in the evaluation of PSA performance because various factors such as properties of backing materials, surface of the adherend, peeling speed, and test temperature affect peel strength. Much information is provided not only by failure mode detection, but also by drawing up the master curves for peel resistance. The practical aspects of peel resistance as a common test method for the characterization of PSAs and its influence on the converting and end-use properties are discussed in the other two books. Chapter 8 of this book, “Shear Resistance,” describes shear resistance as an important factor affecting the performance of PSAs. The main parameters influencing shear resistance and the correlation among shear resistance, peel resistance, and tack are discussed. The practical aspects of shear resistance as a common test method for the characterization of PSAs and PSPs, as well as its influence on the converting and end-use properties, are also discussed in the other two books. Chapter 9 of this book, “Durability of Viscoelastic Adhesive Joints,” discusses the durability of PSA joints under constant detaching stress for a range of PSAs spanning different classes that have been analyzed using a squeezing–flow technique. The approach to the prediction of adhesive joint durability, one of most important characteristics of PSA performance, is proposed. The role of durability in practice is also discussed in detail in Applications of Pressure-Sensitive Products. Chapter 10 of this book, “Molecular Nature of Pressure-Sensitive Adhesion,” is based on the analysis of peel force in relation to the work of viscoelastic deformation of adhesive fi lm up to break under uniaxial drawing; a simple equation has been derived that represents peel adhesion as a function of the self-diff usion coefficient, relaxation time, and cohesive strength of the adhesive polymer. Chapter 11 of this book, “Significance of Relaxation for Adhesion of Pressure-Sensitive Adhesives,” examines the phenomenon of pressure-sensitive adhesion that is treated as a three-stage process, involving adhesive joint formation under compressive force,

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Preface

followed by relaxation of the adhesive material as the bonding force is removed, and then debonding as a detaching stress is applied. The mechanisms of PSA deformation at each of these stages are different, and the contribution of PSA relaxation to adhesion is also different. The strength of the adhesive joint requires the contribution of slow relaxation processes, which imply the specific importance of both longer relaxation time and large-scale structural rearrangements in the PSA material for proper adhesion. We suggest that our readers use the list of abbreviations and acronyms in the end of this book to facilitate the comprehension of various symbols, whenever they are not sufficiently clear. The role of this book is to provide comprehensive and convenient upto-date information for users in both industry and academia. We were pleased to see the participation of scientists and industrial experts, working in very different areas of the field, on this book. We thank our contributors for their efforts.

The Editors

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Editors István Benedek is an industrial consultant based in Wuppertal, Germany. After exploring his initial interest in macromolecular science, he transferred to the plastics processing and adhesive converting industry as research and development manager, where he has worked for three decades. He is the author, coauthor, or editor of several books on polymers, including Pressure-Sensitive Adhesives Technology (Dekker, New York, 1996), Development and Manufacture of Pressure-Sensitive Products (Dekker, New York, 1999), Pressure-Sensitive Formulation (VSP, Utrecht, the Netherlands, 2000), Pressure-Sensitive Adhesives and Applications (Dekker, New York, 2004), Development in Pressure-Sensitive Products (CRC, Boca Raton, FL, 2006), Pressure-Sensitive Design, Theoretical Aspects (VSP, Leiden, the Netherlands, 2006), and Pressure-Sensitive Design and Formulation, Applications (VSP, Leiden, the Netherlands, 2006), as well as more than 100 scientific research and technical reports, patents, and international conference papers on polymers, plastics, paper/fi lm converting, and web finishing. He is a member of the Editorial Advisory Board of the Journal of Adhesion Science and Technology. Dr. Benedek received his PhD (1972) in polymer chemistry and engineering technology from Polytechnic University of Temeswar. Mikhail M. Feldstein, one of the world’s leading experts in the development of new polymeric composites with tailored performance properties that span pressure-sensitive adhesives and other materials designed for medical and pharmaceutical applications, was born in 1946 in Moscow. In 1969 he graduated with honors from M.V. Lomonosov Moscow State University, Faculty of Chemistry, and in 1972 he earned his PhD in polymer science from the same university for the investigation of polyelectrolyte complexes with ionic surfactants and lipids. His early research interests were associated with the mechanisms of the formation and molecular structure of interpolymer complexes. Since 1972 he has worked in the industry of polymers for medical usage as a developer of hydrophilic pressure-sensitive adhesives for skin application in transdermal therapeutic systems and wound dressings. He received international recognition comparatively late: his earliest contacts with colleagues beyond the borders of former Soviet Union date to 1994 only. In 1999, a famous scientist and vice president of the Russian Academy of Sciences, academician Nicolai A. Platė, invited him to join A.V. Topchiev Institute xi

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Editors

of Petrochemical Synthesis of the Russian Academy of Sciences, one of the most wellknown academic institutes in polymer science. Later that year, Feldstein established long-term and large-scale research cooperation with a leading pharmaceutical company, Corium International, Inc. (CA). In 2005, Feldstein earned his DrSc in polymer science from the A.V. Topchiev Institute of the Russian Academy of Sciences. Since the second half of the 1990s, Feldstein has focused on the molecular origins of pressure-sensitive adhesion and the interrelationship between adhesion and other properties of polymer blends. Based on gained insight into the phenomenon of adhesion at a molecular level, he has developed the first-ever technology for obtaining numerous novel pressure-sensitive adhesives of controlled hydrophilicity and performance properties by the simple mixing of nonadhesive polymer components in certain ratios. Feldstein is the author of nearly 200 research papers, 7 book chapters, and 25 patents. He is a member of Adhesion Society and Controlled Release Society. Feldstein is also an associate editor of the Journal of Adhesion.

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Contributors Sergey V. Antonov

Sergey V. Kotomin

A.V. Topchiev Institute of Petrochemical Synthesis Russian Academy of Sciences Moscow, Russia


Anatoly E. Chalykh

Valery G. Kulichikhin

A.N. Frumkin Institute of Physical Chemistry and Electrochemistry Russian Academy of Sciences Moscow, Russia


Eng-Pi Chang

Dong-Hyuk Lim

Avery Dennison Research Center Pasadena, California

Laboratory of Adhesion & Bio-Composites Seoul National University Seoul, South Korea

Costantino Creton Laboratory of Physical Chemistry and Midfielders Dispersed Unit Joint Paris, France

Christophe Derail IPREM Institute Université de Pau et des Pays de l’Adour Pau, France

Mikhail M. Feldstein A.V. Topchiev Institute of Petrochemical Synthesis Russian Academy of Sciences Moscow, Russia

Hyun-Joong Kim Laboratory of Adhesion & Bio-Composites Seoul National University Seoul, South Korea

Gérard Marin IPREM Institute Université de Pau et des Pays de l’Adour Pau, France

Mikhail B. Novikov A.V. Topchiev Institute of Petrochemical Synthesis Russian Academy of Sciences Moscow, Russia

Young-Jun Park Laboratory of Adhesion & Bio-Composites Seoul National University Seoul, South Korea

Régis Schach Centre Technique de Ladoux Michelin, France

xiii

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Contributors

Alexander V. Semakov

Kenneth R. Shull


Northwestern University Evanston, Illinois

Anna A. Shcherbina A.N. Frumkin Institute of Physical Chemistry and Electrochemistry Russian Academy of Sciences Moscow, Russia

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Oksana A. Soboleva Chemistry Department M.V. Lomonosov Moscow State University Moscow, Russia

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1 Surface Phenomena on a Solid–Liquid Interface and Rheology of Pressure Sensitivity Oksana A. Soboleva M.V. Lomonosov Moscow State University

Alexander V. Semakov Sergey V. Antonov Valery G. Kulichikhin A.V. Topchiev Institute of Petrochemical Synthesis

1.1

1.1 Introduction .............................................................1-1 1.2 Surface Phenomena on a Solid–Liquid Interface ....................................................................1-2 Wetting of Solids by Low-Molecular-Weight Liquids • Wetting of Solids by Polymer Liquids • Wetting of Solids by Multicomponent Liquids • Wetting of Deformable Substrates (Gels)

1.3 Rheology of Pressure Sensitivity .........................1-18 Dimensionless Criteria of Pressure-Sensitive Adhesive Performance • Rheological Properties of Pressure-Sensitive Adhesives under Bonding Pressure

References ........................................................................1-22

Introduction

In accordance with the definition given by the Pressure Sensitive Tape Council, “pressure sensitive” is a “term commonly used to designate a distinct category of adhesive tapes and adhesives which in dry form (solvent/water free) are aggressively and permanently tacky at room temperature and that firmly adhere to a variety of dissimilar surfaces upon mere contact without the need of more than finger or hand pressure” [1]. This definition contains some uncertainties regarding the conditions of formation of the adhesive joint, namely, the exact pressure and duration of its action. It is clear, however, that

1-1

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1-2

Fundamentals of Pressure Sensitivity

because pressure-sensitive adhesives (PSAs) are viscoelastic in nature, these parameters are crucial for their application. The role of rheological characteristics for PSA performance was realized long ago. The main criterion of pressure sensitivity (Dahlquist’s) operates with the rheological parameter (absolute values of elasticity modulus). Attempts have been made [2] to replace this empirical, yet simple and quite reliable criterion with one based on relaxation characteristics. Performance properties of PSAs, especially those related to cohesive strength (e.g., shear resistance), also depend on their rheological behavior. The rheological behavior of adhesive formulations is important for the successful manufacturing of pressuresensitive products. To understand how PSAs work, it is necessary to understand not only the properties of the individual components of the adhesive formulation and those of the formulation as a whole, but also the processes that take place upon contact of the PSAs with different types of substrates. The substrates can be solid or viscoelastic. Good interfacial interaction with a substrate is essential for PSA performance. Such interaction is caused by forces of different natures (van der Waals, H-bonding, acid–base, and donor–acceptor interactions) [3–5]. At the macro level, this interaction between PSA and substrate results in a wetting– spreading process. Wetting of the substrate by the adhesive is crucial to establish good contact between them. Wetting is also important in the coating procedure. In this chapter we will first consider the fundamentals of the wetting–spreading process for simple models (e.g., wetting–spreading of Newtonian liquids on a solid surface). Wetting by polymeric liquids has specific features connected with their high viscosity and elasticity. The viscosity of PSAs upon application is very high. For this reason, spreading (i.e., increase of the wetted surface) proceeds as flow that is governed by the rheological properties. It can be assumed that the rheological characteristics of liquids in thin layers upon wetting should differ from values measured in macroscale by means of conventional rheometers, although the extent of this difference is unknown. To enable the spreading of PSAs onto a substrate’s surface, it is necessary to apply external pressure for a definite time depending on the PSA’s viscosity and elasticity. Spreading of multiphase liquids and influence of surfactants will be also discussed in this chapter. Another point of interest to be considered in this chapter is wetting of the deformable substrates, which is important in some specific applications of PSAs. Finally, we will discuss new rheological approaches highlighting the nature of pressure sensitivity.

1.2 1.2.1

Surface Phenomena on a Solid–Liquid Interface Wetting of Solids by Low-Molecular-Weight Liquids

Let us start with the wetting–spreading processes for low-viscosity Newtonian liquids and absolutely rigid supports, such as metallic surfaces. According to the modern concept [6], spreading of a liquid droplet on a solid surface proceeds through the formation of a primary (or precursor) film. The thickness of this film for different systems is in the range of 0.01–10 μm. The existence of the primary fi lm

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1-3

Surface Phenomena on a Solid–Liquid Interface

has been confirmed by various methods (measurement of electric resistance, ellipsometry, optical and electron microscopy, etc.). Several different mechanisms of precursor film formation have been discussed: surface diffusion, evaporation of a liquid with subsequent condensation on a solid surface, and emission of jets due to the difference in capillary pressure inside the drop and in the vicinity of the three-phase (solid–liquid–air) line. As a rule, in the majority of experiments the secondary spreading (i.e., spreading of a liquid on a surface covered with the precursor fi lm) was studied. Experiments confirmed that the thickness, structure, and composition of the precursor fi lm strongly affected the rate of drop spreading. Kinetic equations of secondary spreading are usually deduced from hydrodynamic models, according to which the spreading rate is limited by the delivery of the liquid to the three-phase line. The initial stage of the spreading process is controlled by the inertial forces in the drop volume [7]. The spreading rate at the end of the inertial stage vin, vin

dr dt

(1.1)

where r is the radius of the wetted surface and t is time, can be estimated from Bernoulli’s law, 1 2 vin Pc 2

(1.2)

where ρ is the density of the wetting liquid and ΔPc is the difference between the values of capillary pressure inside the drop at the beginning and at the end of the inertial stage. If the drop upon spreading has the shape of a spherical segment, ΔPc can be calculated as  1 1  Pc 2   Ri R* 

(1.3)

where σ is the surface tension of the liquid and R i and R* are the curvature radii of the drop surface at the beginning and at the end of the inertial stage of spreading, respectively. It is convenient to express the capillary pressure difference, ΔРс, as a function of the dynamic contact angle Θ. For a spherical segment of a constant volume, Ri  4  2  sin  1 2 cos   2 2  R* 

1

3

(1.4)

Expression of the spreading rate in the inertial stage can be derived from a combination of Equations 1.2 through 1.4. 1 3  4   4  2   v 1  sin 1 2 cos    Ri  2 2     

2 in

(1.5)

The inertial spreading regime is usually observed upon the contact of drops of lowviscosity liquids (hydrocarbons, water) with solid surfaces at ambient temperature.

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1-4

Fundamentals of Pressure Sensitivity 0.8

180 1

150

r (mm)

120 0.4

90 2

Θ (degree)

0.6

60 0.2 30 0 0.000

0.005

0.010

0.015

0.020

0.025

0 0.030

t (s )

FIGURE 1.1 Dependence of the radius of the wetted area (1) and contact angle (2) on time for water drop spreading on a steel surface.

At the initial stage of the spreading process, the surface of the drop is unstable and the drop height changes unpredictably, which is reflected by a “zigzag” dependence of the contact angle on spreading time. An example of such dependence is illustrated in Figure 1.1 for spreading of the water drop on a steel surface. The duration of the inertial stage is very short—about 10 −2 s; therefore, for its registration high-speed shooting [several thousands frames per second (fps)] is needed. At this stage the spreading rate does not depend on the viscosity of the spreading liquid. With the increased radius of the wetted area and reduction in the height of a drop, the role of viscous resistance in a volume of the drop increases. The inertial stage transforms into the viscous stage [7,8]. According to the hydrodynamic model, we can evaluate the kinetic features of viscous spreading, such as the change in radius of the wetted area r with time, by comparing the driving force of spreading, f d 2r (cos 0 cos )

(1.6)

where Θ 0 and Θ are the equilibrium and current values of the contact angle, respectively, with the force of viscous resistance, f r 2 grad v

(1.7)

(v = dr/dt; η is viscosity). If the spreading drop has the shape of a spherical segment of height h, the whole volume is involved in the spreading process and gradv

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1 dr h dt

(1.8)

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1-5


If, otherwise, the drop spreads with a precursor fi lm, the main resistance is located in the layer with thickness δ. As the first approximation, this thickness can be accepted as constant and Equation 1.8 can be rewritten as gradv

1 dr dt

(1.9)

These approaches led to the following expressions for r(t) dependence:  16V  r A1t 0.25   

0.25

t 0.25

(1.10)

for the first case (V is the volume of the drop) and 0. 5

 8  0.5 r A2t 0.5  t  

(1.11)

for the second. The kinetic relationships 1.10 and 1.11 have been verified in Refs 9–11 where spreading of mixtures of two miscible liquids was studied. The use of mixtures of low-viscosity and high-viscosity liquids allowed the viscosity of the spreading drop to vary over a wide range. Complete or partial wetting of the steel substrate was observed in the case of mineral (vaseline) oil solutions in dodecane (viscosity ranged from 1.5 to 150 mPa s, Θ0 = 0°) and glycerol in water (viscosity ranged from 1.0 to 386 mPa s, Θ 0 = 64°). For formulations with viscosity of η ≥ 100 mPa s, the shape of the drop during the process is close to a spherical segment and the propagation of the wetting front is described by a relationship similar to Equation 1.10. On the contrary, for low-viscosity mixtures of nonpolar liquids, the precursor fi lm is formed and the spreading kinetics is described by Equation 1.11. The regularities discussed above are generally applicable to the rather fast spreading inherent to low-molecular-weight liquids. However, PSAs are high-viscosity systems with an essential elastic response. Upon the spreading of liquids with very high viscosity, especially in the case of complete wetting, a very slow rate of wetted radius increase is observed. At this stage, the droplet can be considered as a flat fi lm. The dependence of the radius of the moistened area on time can be described as r ∼t

1

(1.12)

n

where n = 8–12. For polymer droplets n can be even higher [7,12,13]. The following expression of slow spreading was proposed in [11]. 0. 1

  r A3t 0.1   V 0.3t 0.1  

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(1.13)

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1-6


where κ is a correction factor, usually equal to ~10, that describes the increase of viscous friction in a layer with the shape of a spherical segment in comparison with a flat layer. The kinetic laws of the contact angle Θ changing can be deduced from the basic relaxation and rheological models of the spreading process [14]. Various versions of relaxation models are based on the assumption that the rate of wetting d cosΘ/dt is proportional to the deviation of the three-phase substrate/liquid/vapor line from the thermodynamic equilibrium conditions that correspond to the equilibrium contact angle, Θ 0. In accordance with this assumption, cos 0 cos t e cos 0 cos i

(1.14)

where τ is the retardation time of the wetting process and Θi is the initial contact angle. Upon application of this model to experimental results, the dependence, Θ(t), should be linear if plotted in coordinates ln(cosΘ0 − cosΘ) − t. Such linear dependences were experimentally obtained for spreading of dodecane–vaseline oil and water–glycerol mixtures on steel plates. τ values were calculated from the slope of the experimental curves [10]. The Kelvin–Voight model, with parallel connection of viscous and elastic elements [15], can be used to calculate the retardation time of spreading. If such a system is loaded and deformed up to relative deformation εi, the kinetics of deformation after release of the load can be described by the equation 0 t e i 0

(1.15)

where ε0 is the residual equilibrium deformation and τ is the retardation time:

G

(1.16)

where G is the elastic modulus. Upon simulation of drop spreading by the Kelvin–Voight model, it can be assumed that the viscous (Newtonian) element is the liquid with viscosity η inside the droplet, and the elastic (Hooke) element represents the curved surface layer of the droplet with elasticity modulus G. Using some additional assumptions [14], the expression of retardation time can be written as

(1.17)

where λ is a characteristic linear dimension that is close to the radius of droplet Ri for high-viscosity liquids. For low-viscosity liquids, the λ values are rather high (several

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1-7


centimeters), which requires additional modification of the spreading model. So, the rheological model of spontaneous spreading can describe the real process for highviscosity liquids only.

1.2.2

Wetting of Solids by Polymer Liquids

In our experiments we studied the kinetics of spreading r(t) and wetting Θ(t) of drops of oligomeric polyethylene glycol (PEG-400) and low-molecular-weight polyisoprene Isolene 40 (Elementis Specialities Inc., Hightstown, NJ-PI) on polyethylene terephthalate (PET) fi lm (Loparex 7300A, Loparex Inc, Willowbrook, IL). This fi lm has hydrophilic and hydrophobic (silicon coated) sides and was used as an example of conventional release liners. The relationships between the change in radius of the moistened area and contact angle with time are illustrated in Figures 1.2 and 1.3 for wetting of the release liner by the above-mentioned liquids within the first 20–30 s of spreading. The retardation times of contact angle evolution τ are summarized in Table 1.1. Because different sides of the tested release liner (untreated and siliconized) have different polarities (more hydrophilic and more hydrophobic, respectively), wetting of these surfaces depends on the liquid used. Microphotographs showing drops of different liquids on hydrophilic and hydrophobic sides of the release liner are presented in Figures 1.4 through 1.6. Water does not wet both surfaces (Figure 1.4): contact angle on a “hydrophilic” surface is 93.0 ± 2.9°; on a “hydrophobic” surface the contact angle is 105.2 ± 1.7°. 3.0 1

2.5 2

r/r0

2.0

1.5

1.0

0.5

0.0

0

10

20

30

t (s)

FIGURE 1.2 Spreading kinetics of PEG drops on the hydrophilic (1) and hydrophobic (2) surfaces of the release liner.

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1-8

Fundamentals of Pressure Sensitivity 0.0

ln(cosΘ0 − cosΘ)

−0.5 Hydrophilic surface Hydrophobic surface

−1.0 −1.5 −2.0 −2.5 0.0

0.5

(a)

1.0

1.5

2.0

60

80

t (s) 0.0

ln(cosΘ0 − cosΘ)

−0.5 −1.0 −1.5 −2.0 −2.5

0

20

(b)

40 t (s)

FIGURE 1.3 Evolution of contact angles upon spreading of PEG (a) and PI (b) drops on the surface of the PET release liner. TABLE 1.1 Contact Angle Retardation Times and Viscosities of the Spreading Liquids Calculated from Data on Spreading Kinetics Surface PET release liner, hydrophilic side PET release liner, hydrophilic side PET release liner, hydrophobic side

Liquid

τ (s)

η (Pa s)

PEG-400 PI PEG-400

0.83 59.2 0.61

26.6 1600 19.5

Drops of PEG and PI spread on both surfaces (Figures 1.5 and 1.6). Amphiphilic PEG easily wets both surfaces, whereas hydrophobic PI wets the hydrophobic surface much better. The contact angles reach equilibrium values after several hours for the hydrophilic surface and after more than 1 day for the hydrophobic surface (Figure 1.7).

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(a)

1-9

(b)

FIGURE 1.4 Water drops on the hydrophilic (a) and hydrophobic (b) surfaces of the release liner.

(a)

(b)

FIGURE 1.5 PEG-400 drops on hydrophilic (a) and hydrophobic (b) surfaces.

(a)

(b)

FIGURE 1.6 PI drops on hydrophilic (a) and hydrophobic (b) surfaces.

Using Equation 1.17, it is possible to estimate the viscosity, η, of the spreading liquid during the first stage of the spreading process. The calculated values are presented in Table 1.1. The values are quite reasonable, although they are essentially higher than the values measured by conventional rotational viscometry (~0.1 Pa s for PEG and ~30 Pa s for PI). This difference is understandable taking into account the adhesion contact of the PSA with a solid surface. Regarding the real multicomponent, multiphase adhesives, we

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1-10


80

Hydrophilic

60

Θ (degree)

Θ (degree)

80

PEG-400 40 PI

20 0

(a)

0

500

1000 t (min)

60

(b)

PEG-400

40

PI

20 0

1500

Hydrophobic

0

500

1000

1500

t (min)

FIGURE 1.7 Time dependences of PEG and PI contact angles on hydrophilic (a) and hydrophobic (b) surfaces of the release liner.

can expect much slower spreading, which is indeed observed upon PSAs application. Some insight into a possible role of individual PSA components in the wetting–spreading processes can be derived from the analysis of the model situation for multicomponent liquids.

1.2.3

Wetting of Solids by Multicomponent Liquids

Compared with individual liquids, wetting of solids by multicomponent mixtures is a complex process because wetting is accompanied by various processes, such as surface and bulk diff usion, adsorption of the components on different interfaces, mutual dissolution of solids and liquids, and possible evaporation of volatile components (VOCs). The rates of these processes usually differ for various components of a mixture, and frequently these processes control the movement of the three-phase line, predetermining the shape of the drop in the spreading process. Further, we will describe some examples concerning the spreading of mixtures of liquids, the influence of surfactants on spreading, and the role of mutual dissolution and deformability of a substrate in spreading kinetics. Surfactants are traditional components of many multiphase formulations that affect the dispersity, compatibility of components, and the stability of the system in general, as well as within wetting–spreading processes. 1.2.3.1 Spreading of Mixtures of Liquids One of the most important cases of spreading of mixtures of liquids is the case of liquids with different volatility and surface tension. Spreading of viscous hydrocarbons on high-energy surfaces in the presence of various additives was studied in Refs 16 and 17. The following effects were described: 1. Acceleration or deceleration of spreading 2. Reverse motion of liquid from the initially moistened area 3. Catastrophically fast spreading with a rate of two or three orders of magnitude faster compared with drops of individual liquid of similar viscosity The observed effects were explained in terms of the secondary flow caused by the local change in surface tension due to evaporation of one of the components of the mixed solution.

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1-11

This phenomenon is known as the Marangoni effect. Difference in composition inside the drop and in the precursor fi lm, due to more intense evaporation of one of the components from a thin layer, results in the occurrence of additional flow directed either from the drop to the fi lm (if the more volatile component has the lower surface tension value) or from the fi lm to the drop (the reverse situation). The two processes control the acceleration and deceleration of spreading, respectively. The influence of the Marangoni flow on spreading rate was investigated in detail, for example, in Refs 18 and 19. As an example, we can consider the spreading of vaseline oil–dodecane mixtures on a steel plate in the experiments mentioned previously. The instant shape of the drop was registered with a high-speed videocamera. It is possible to detect three stages in kinetic curves of spreading r(t) for drops of different composition, which are described by Equations 1.10, 1.11, and 1.13, respectively. The short-term initial stage proceeds with the maximum rate r = A1t 0.5. This stage was observed for mixtures with low viscosity ( (Dt)1/2, a diff usion zone with phase interface is formed in the PFO layer. At longer observation times, when L 0.9897 2

0 −0.6

−0.4

−0.2

0.0

0.2

0.4

Dissipation (log tan δ at 127°C)

FIGURE 5.1 Correlation of 20 min peel strength with log tan δ at 127°C. (From Dale, W.C., Paster, D.M., and Haynes, J.K., Mechanical Properties of Acrylic PSAs and Their Relationship to Industry Standard Testings, Taylor & Francis, London, 1989. With permission.)

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5-4


3

XLQ

Log shear (h) (kg)/psi at CTH

2

1 r 2 >0.87 0

−1

−2 3.2

3.4

3.6

3.8

4.0

4.2

4.4

Log storage modulus G ′ (Pa) at 127°C

FIGURE 5.2 Correlation of shear hang time with log-log storage modulus at 127°C. (From Dale, W.C., Paster, D.M., and Haynes, J.K., Mechanical Properties of Acrylic PSAs and Their Relationship to Industry Standard Testings, Taylor & Francis, London, 1989. With permission.)

The noteworthy features and contribution of Dale’s work are as follows: • Proposal of a concept of strain–temperature superposition to reconcile the small strain deformation in DMA with the large deformation tensile tests. • Demonstration that a main requirement of good PSA is to possess simultaneously both solid-like strength and liquid-like flow behavior. • Use of log tan δ (positive and negative, respectively) to delineate liquid-like and solid-like properties; cautioned regarding the use of the strain dependence of tan δ. • Emphasis of the importance of the differences in energy integral for adhesive failure (lower boundary stress) and cohesive failure in peel resistance test (i.e., utilizing the full stress–strain curve) adhesives. This identifies the possibility of achieving higher peel strength going from adhesive to cohesive failure (e.g., on substrates of increasingly greater surface energy). This also confirms that the surface adhesion serves primarily to prevent premature separation from the substrate [2,34,35], which results in a low peel resistance. • Confirmation of the major role that bulk viscoelastic properties play in PSA performance and demonstration of the usability of the tensile test and dynamic mechanical methods for correlations with standard peel and shear performance tests.

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Viscoelastic Properties and Windows of Pressure-Sensitive Adhesives

5.2.2

5-5

Work of Tse

By combining the theoretical consideration of Kinloch and coworkers [36–38], the Dahlquist contact criterion, and the proposed bonding and debonding functions, Tse [14,39] presented the PSA performance as T = P0 BD

(5.1)

where T is the adhesion performance and P0 is an intrinsic interfacial failure energy (either the energy required to open up a unit area of PSA–substrate interface in the absence of viscoelastic energy loss or the thermodynamic work of adhesion, which is substrate dependent). B is the bonding function, assumed to be constant when the Dahlquist contact criterion is satisfied (i.e., the plateau modulus is lower than 3.3 × 105 Pa); D is the debonding function, which is the viscoelastic loss component. It is strongly dependent upon the characteristic debonding frequency (i.e., the separation speed of the PSA test). Although viscoelastic measurements involve low strains, whereas PSA adhesion tests involve high strains, Tse’s study confirms many earlier findings of good correlations of viscoelastic properties with PSA adhesion. The noteworthy features and contributions of this work are as follows: • The separation of the bonding and debonding steps in PSA adhesion. • The relationship between viscoelastic behavior at different frequencies and PSA. • Performance and the identification and location of the debonding frequencies for different adhesion tests on the rheological master curves. • The proposal of an end-block polystyrene domain transition temperature (Tdd), which is much lower than the end-block polystyrene domain disappearance (critical) temperature, Tc, proposed by Krause and Hashimoto [40] and the “monophasic” (when the morphology becomes single phase) temperature proposed by Widmaierer and Meyer [41]. The absence of correlation between shear adhesion failure temperature (SAFT) and Tdd suggests that the responsible mechanism for the lower SAFT observed in resin-rich systems is the lowering of the plateau modulus or narrowing of the plateau width. • The evidence for and against the existence of two phases, namely, polyisoprenerich and resin-rich in the mid-block rubber matrix. Such differences can most probably be reconciled by the different sensitivities and thermal histories of the different tests [e.g., differential scanning calorimetry (DSC) versus rheological measurements]. • The inference that resin restricts segmental motion of the rubber mid-block, resulting in a higher monomeric friction coefficient. • The identified criteria for good PSAs, namely, low plateau modulus to facilitate bonding and high energy dissipation at the PSA debonding frequencies and domain integrity. This is consistent with the proposed VWs by Chu [15] and Chang [16,17].

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5-6

5.2.3


Work of Chu

Extending earlier studies on the dynamic mechanical properties of rubber–resin mixtures, Chu [15,52], through his systematic studies, presented a coherent picture of how the interaction of rubber with resins affects the dynamic mechanical properties, which, in turn, affect PSA performance. Compared with previous publications, the noteworthy features and contribution of these papers are as follows: • The identification of the limitations and strengths/capabilities of different rheometrics fi xtures (torsional rectangular and different diameter parallel plates) for dynamic mechanical measurements of PSA materials. For individuals not too familiar with all the testing models, Chu highlighted the caution of instrument compliance at low temperatures or measuring glassy modulus, and he recommended different size parallel plates for different modulus range measurements. • The testing and characteristics of rubber–resin compatibility by the criteria of a pronounced shift in the tan δ peak maximum temperature, together with a decrease in the plateau modulus. • The confirmation that rubbers with a higher plateau modulus and Tg values are much more difficult to tackify than those with lower values. This criterion should compliment the matching of the solubility parameter between rubber and resin commonly used in the PSA industries. • The order of compatibility of different types of resins with different types of rubbers. This is particularly useful for formulation chemists who need guidance on the selection of compatible resins for these rubbers (such a “list” of compatibilities existed before DMA; suppliers published such lists, with compatibility being tested using other than DMA methods; see also Applications of PressureSensitive Products, Chapter 8). • The determination that good PSA systems (based on rubber tackified with an appropriate amount of compatible resins) have a depressed modulus at low frequencies (making bonding favorable) and an elevation in the high-frequency modulus (making debonding more favorable). This also demonstrates the importance of G′ω=100/G″ω=0.1 in achieving good PSA properties (Figure 5.3). • The proposal of room temperature modulus values at different frequencies for PSA systems (e.g., for tapes and labels; see Figure 5.4) and a VW for good PSAs for tapes based on modulus requirements (Figure 5.5). • The correlation of tack versus dynamic mechanical data; specifically G′ω=0.1 and ratio of G′ω=100/G′ω=0.1. This relationship indicates that low G′ω=0.1 and high G′ω=100/G′ω=0.1 ratio are desirable for high tack values. • The proposal of empirical windows for good PSA and good pressure-sensitive labels based on the loci of the room temperature plot of modulus and Tg values determined from the temperature at which tan δ is at the maximum (Figure 5.6).

5.2.4

Work of Chang −2

Using 10 and 102 rad/s as the bonding and debonding frequency, respectively, G′ and G″ values of different PSA samples at these two frequencies were measured, and their

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5-7

G′ (dyn/cm2) at 25°C

107

106

105 0.1 Bonding shear resistance

1.0 10 Frequency sweep data (ω, rad/s) tack

100 Debonding peel

FIGURE 5.3 Room temperature modulus requirement at various frequencies. Shaded area indicates the window with good PSA properties. (From Chu, S.G., Adhesive Bonding, Plenum Publishing, New York, 1991. With permission.)

107

25°C

145 Tape Label PSA

14.5

105

1.45

G′ (dyn/cm2)

Modulus (psi)

106

104 0.1

1.0

ω (rad/s)

10

104 100

FIGURE 5.4 Room temperature modulus values of label and PSA tape at various frequencies. (From Chu, S.G., Adhesive Bonding, Plenum Publishing, New York, 1991. With permission.)

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5-8


1010 ω = 10 rad/s

109

G′ (dyn/cm2)

108 107 106 105 104 103

Melt processing −10

25°C

80

120

150

Temperature (°C) Cold temperature performance

Room temperature performance (application temperature)

Storage (telescoping)

Good shear holding

FIGURE 5.5 Modulus (G′) requirements (windows) for good PSA tapes. (From Chu, S.G., Adhesive Bonding, Plenum Publishing, New York, 1991. With permission.)

VWs were constructed by plotting the four coordinates: (1) G′ at 10−2 rad/s, G″ at 10−2 rad/s; (2) G′ at 102 rad/s, G″ at 10−2 rad/s; (3) G′ at 10−2 rad/s, G″ at 102 rad/s; and (4) G″ at 102 rad/s, G″ at 10 2 rad/s on the log-log cross plot of G′ and G″. Chang [16,17] reported that for most PSAs, the range of G′ and G″ at room temperature within the selected frequencies fell between 103 and 106 Pa. In addition, there was a unique correlation between the adhesion performance of the PSAs and the location of their VWs. A four-quadrant concept was therefore adopted to categorize different types of PSAs. The location of different VWs is illustrated in Figure 5.7 [42], along with their corresponding operative viscoelastic regions. To illustrate the consistency of the VW concept for the different types of PSAs, several key materials in each group are listed below to ascertain their viscoelastic uniqueness. Quadrant 1 (top left-hand quadrant): high G′ and low G″. This quadrant corresponds to high modulus, low dissipation. The bonding and debonding frequencies, in this case, both occur at the plateau region of the rheological master curve. No PSA can be found in this quadrant because of the high bonding modulus (i.e., G′ at 10−2 rad/s) and highly elastic nature (lack of flow) of the material, making the bonding step unfavorable. Some elastomers and release coatings occupy this quadrant. Figure 5.8 illustrates the VWs of a polydimethyl siloxane (PDMS), PDMS + 40% control release agent (CRA, an additive to increase the release force, which is a tri- or tetrafunctional methylsilicate resin; see also Technology of Pressure-Sensitive Adhesives

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5-9


107

145

Vistanex Reichhold (TYLAX) Polysar Union chemical (AMSCO) Dow

SBR Latex

NR

208

SBR1011 L120 8226

G ′, 25°C (dyn/cm2)

3892

6130

30678

222 8244 106

Good PSA

444

14.5

8277 Good label

LM-MS

105 −60

Modulus (psi)

Hartex

1.45 −50

−40

−30

−20

−10

0

10

Tg (°C), tan δ maximum temperature

FIGURE 5.6 Viscoelastic properties of Piccotac HM2162L/Kraton 1107/oil blends and empirical windows required for various labels and PSA tapes. (From Chu, S.G., Adhesive Bonding, Plenum Publishing, New York, 1991. With permission.)

and Products, Chapters 6 and 10), and PDMS + 60% CRA. Because of the high bonding modulus (G′ at 10−2 rad/s) and low dissipation (G″), PDMS is not a PSA, but rather a release coating. However, with the incorporation of an increasing amount of CRA, the modulus is progressively reduced, accompanied by an increase in dissipation or flow. Such a progressive decrease in G′ and increase in G″ results in a progressive increase in the tackiness of the samples. Thus, PDMS, when modified with 60% CRA, is tacky. It can be anticipated that with further increase in CRA concentration, the VW of the resulting sample will move toward the central region, which, as described in a later section, is the location for general purpose PSAs. Quadrant 2 (top right-hand quadrant): high G′ and high G″. This quadrant corresponds to high modulus and high dissipation. The bonding frequency corresponds to

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5-10


106

Quadrant 2

Quadrant 1

Transition−plateu region High modulus High dissipation

Rubbery region High modulus Low dissipation 105 Transition−flow region Medium modulus Medium dissipation General purpose PSA

G ′ (Pa)

Release−Non PSA

104

Plateau−flow region Low modulus Low dissipation

Flow−flow Low modulus High dissipation

Removable PSA 103

High shear PSA

Cold temperature PSA Quadrant 4

Quadrant 3 103

104

105

106

G ′′ (Pa)

FIGURE 5.7 VWs of PSAs as related to different regions on the rheologic master curves. (From Chang, E.P., Viscoelastic Windows of PSAs, Taylor & Francis, London, 1991. With permission.)

106

G ′ (Pa)

105

104

103 103

104

105

106

G ′′ (Pa) Silicone coating

Silicone + 40% CRA

Silicone + 60% CRA

FIGURE 5.8 VWs of release coatings. (From Chang, E.P., Viscoelastic Windows of PSAs, Taylor & Francis, London, 1991. With permission.)

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5-11

106

G ′ (Pa)

105

104

103 103

104

105

106

G ′′ (Pa) HSPSA 1

HSPSA 2

HSPSA 3

FIGURE 5.9 VWs of high-shear PSAs. (From Chang, E.P., Viscoelastic Windows of PSAs, Taylor & Francis, London, 1991. With permission.)

the plateau region, whereas the debonding frequency corresponds to the glass transition region in the rheological master curves for high-shear PSAs. The high bonding modulus, compensated by the high dissipation or flow, makes the bonding marginally efficient. Shear is high because of the high G′ or cohesive strength of the material. Figure 5.9 illustrates the VWs of three high-shear PSAs: HSPSA 1, 2, and 3. All of the VWs occupy the top right-hand corner, which means that these adhesives have comparatively high modulus (G′) and high dissipation (G″) within the application rates (i.e., between 10 −2 and 102 s−1). In general, these adhesives have comparatively high Tg’s and are comparatively highly cross-linked to achieve the high shear performance. Quadrant 3 (bottom left-hand quadrant): low G′ and low G″. This quadrant corresponds to low modulus, low dissipation. The bonding frequency corresponds to the onset of the flow transition, whereas the debonding frequency corresponds to the plateau region in the rheological master curves for removable PSAs. Bonding is facilitated by the low modulus despite the low flow characteristics. Peel values are usually low because of the comparatively low debonding strength and low dissipation. Removable and medical-type PSAs fall within this quadrant. Figure 5.10 illustrates the corresponding VWs for removable adhesives RPSA 1, 2, and 3. The distinct characteristics of this type of adhesives are as follows: • Low bonding modulus so that the adhesive is very contact-efficient • Low dissipation, which implies more elasticity or better removability

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5-12


106

G ′ (Pa)

105

104

103 103

104

105

106

G ′′ (Pa) RPSA 1

RPSA 2

RPSA 3

FIGURE 5.10 VWs of removable PSAs. (From Chang, E.P., Viscoelastic Windows of PSAs, Taylor & Francis, London, 1991. With permission.)

Figure 5.11 illustrates the VWs of some of the removable PSAs used in medical applications (e.g., bandages). Comparing the VWs of these medical removables with those removables in Figure 5.10, one notes that they tend to occupy the lower (better conformability) and farther right (better flow) area of Quadrant 3. Some of the notable differences between the removable and the bandage adhesives are as follows: • The reference temperature for the medical adhesive is the body temperature, 37°C, rather than 23°C in the removable case. This makes the bonding modulus of the medical adhesives even lower (i.e., more conformable) than that of the removables because of the higher reference temperature. This is desirable for contact area considerations because of the rough, frequently varied, and contaminated nature of the skin. • The debonding moduli (top right-hand corner of the window) are usually higher than those of the removables. This, again, is necessary to prevent lift or detachment because of frequent flexing of the skin, especially on curved areas, such as knees and elbows. Quadrant 4 (bottom right-hand quadrant): low G′ and high G″. This quadrant corresponds to low modulus, high dissipation. The bonding frequency corresponds to the flow region, whereas the debonding frequency corresponds to the onset of the flow region in the rheological master curves for very quick or cold-stick PSAs. The low bonding modulus coupled with high flow makes bonding very efficient, thus permitting the material to stick even at low temperatures or very short contact time.

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5-13

106

G ′ (Pa)

105

104

103 103

104

105

106

G ′′ (Pa) MPSA 1

MPSA 2

MPSA 3

FIGURE 5.11 VWs of medical PSAs. (From Chang, E.P., Viscoelastic Windows of PSAs, Taylor & Francis, London, 1991. With permission.)

Three adhesives that have a significant portion of their VWs in the fourth quadrant should have low bonding modulus, G′, as well as good flow or highly dissipative nature, G″. So far, no good example of a PSA has been found with its VW located right in the fourth quadrant. Central area: medium G′ and medium G″. This central area corresponds to medium modulus, medium dissipation. The bonding frequency corresponds to the onset of the flow region, whereas the debonding frequency corresponds to the onset of the glass transition region in the rheological master curves (i.e., usually characterized by the absence of a distinct plateau region) of general purpose PSAs. Figure 5.12 illustrates the corresponding VWs for three general purpose acrylic PSAs, GPPSA l, 2, and 3. They all occupy the central region (overlapping part of the four quadrants), illustrating the general purpose nature of this type of PSAs. 5.2.4.1

Correlation and Prediction of Adhesive Performance with the Viscoelastic Window

Figure 5.13 illustrates the relative position of the VW with respect to the Dahlquist contact criterion line, as well as the diagonal line where G′ = G″ or tan δ = 1. 5.2.4.2 The Dahlquist Contact Criterion Line The Dahlquist line is an important reference line that indicates whether a material would be contact efficient (PSA) or deficient (non PSA). Except for the release coatings, all the different types of the adhesives shown in Figures 5.9 through 5.14 [43] have a bonding

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5-14


106

G ′ (Pa)

105

104

103 103

104

105

106

G ′′ (Pa) Emulsion acrylic

Office tape

Tackified acrylic

FIGURE 5.12 VWs of general purpose PSAs. (From Chang, E.P., Viscoelastic Windows of PSAs, Taylor & Francis, London, 1991. With permission.)

106 tan δ = 1

Dahlquist s criteria line

G′ (Pa)

105

104

103 103

G ′ (100) G ′′ (.01) Removability (elastic)

G ′ (100) G ′′ (100)

Flowability (viscous) G ′ (.01) G ′′ (.01)

G ′ (.01) G ′′ (100)

104

105

106

G ′′ (Pa)

FIGURE 5.13 Relationship of the VW with the Dahlquist contact criteria and tan δ. (From Chang, E.P., Viscoelastic Windows of PSAs, Taylor & Francis, London, 1991. With permission.)

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5-15


Storage modulus (G ′/Pa)

107

106

105 8400

= 21

MW

104

000

203

102 10−5 (a)

00

46

16

103

0

80

3 12

10−4 10−3 10−2 10−1 100 101 Frequency (ω/rad/s)

102

103

104

105

Loop tack (N/cm2)

8

6

4

2

0 (b)

0

2

4

6

8

1/G ′ × 105/Pa

FIGURE 5.14 (a) Storage modulus, G′, as a function of frequency. (b) Loop tack as a frequency of inverse of G′. Stainless-steel substrate (○) and polyethylene substrate (△) for a series of acrylic copolymer samples having the same composition (75 wt % 2-ethylhexyl acrylate, 23 wt % ethyl acrylate, 2 wt % acrylic acid) but different molecular weights. (From Yang, H.W.H. and Chang, E.P., The Role of Viscoelastic Properties in the Design of PSAs. Elsevier/Springer, New York, 1997. With permission.)

modulus (i.e., the base of the application window) much below the Dahlquist line (which means good conformability). In other words, by comparing the position of the base of the window with the Dahlquist line, we immediately know whether the material is a PSA. 5.2.4.3

The G′–G″ Cross-Over Line (tan δ = 1)

The diagonal tan δ = 1 line is another important line of demarcation because it separates regions in which the elastic or storage modulus (G′) is greater or smaller than the

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loss modulus, G″ (i.e., tan δ < 1 and tan δ > 1, respectively). The portion of the window to the left of the line (i.e., tan δ < 1) indicates the more elastic region. In other words, the closer the window is to the top left-hand corner of the G′–G″ plot, the more elastic (or better removeablity) the material characteristics. Conversely, the closer the window is to the lower right-hand corner of the plot, the more viscous (or cohesive failure prone) the material characteristics. Assuming that the adhesive is the only variable in the construction and that the surface effect is negligible, the following adhesion and convertibility performance can be correlated with the shape and location of the VW. 5.2.4.4 Shear Performance The shear performance can be correlated with the following features of the window. The base of the window (i.e., G′ at 0.01 rad/s) usually indicates the value of the plateau modulus (because the plateau modulus typically falls in the bonding frequency). In general, the higher the plateau modulus (provided the Dahlquist contact criteria are still satisfied), the better the shear. The high shear type of adhesive is a good manifestation of this correlation. In addition, if the base of the window (i.e., G′ at 10 −2 rad/s) is the same, or the more extended the plateau (i.e., the difference between G′ values at 0.01 and 100 rad/s is smaller), the better the shear. This is because a more extended or flatter rubbery plateau is indicative of either a higher degree of entanglement due to higher molecular weight or a higher chemical/physical cross-link density. Because the breadth of the plateau is inversely proportional to the height of the window, if the base of the window is the same, the shorter the window, the better the shear performance prediction. 5.2.4.5 Peel Performance Peel performance is dependent upon the efficiency of the bonding step, as well as the separation resistance in the debonding step. The bonding efficiency can be correlated with the plateau modulus at the bonding frequency (∼0.01 rad/s). In other words, the lower the G′ value at 0.01 rad/s (or the base of the window), the more favorable the bonding. The debonding strength comes from two contributing terms, the cohesive strength, which is indicated by the storage modulus, G′, and the energy of dissipation term, which is indicated by the loss modulus, G″. Both of these are measured at the debonding frequency (∼100 rad/s). Thus, the higher the debonding G′ and G″ values (i.e., more toward to the top right-hand corner), the higher the debonding strength. 5.2.4.6 Tack Performance The correlation of tack performance is similar to that of peel, except the bonding frequency for tack is about 1 rad/s, which means that the bonding efficiency relates approximately to the inverse of the half-height of the window. The debonding resistance can be related again to the height of the right-hand corner of the window. The usefulness of Chang’s VW concept is as follows: 1. It can be obtained by just making measurements at two frequencies (i.e., 10−2 and 102 rad/s at the specified temperature). Such simple and rapid measurements will immediately identify the nature and type of the adhesive.

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2. With the VW defined, qualitative information regarding adhesion performance and the mode of failure can be obtained as described. This is particularly useful for comparing, evaluating, and screening different adhesives because the shape and location of their VWs provide comparative qualitative performance information prior to measuring their peel, tack, and shear performances. The limitation of Chang’s VW method is that it only gives the G′ and G″ values at those two frequencies. There is no information for G′ and G″ at the in-between frequencies (e.g., 10−1, 100, and 101 rad/s) or other frequencies as in a master curve, which would be recommended for more quantitative information.

5.2.5 5.2.5.1

Work of Yang and Chang Bonding and G′

Yang and Chang [44] correlated bonding with G′ and debonding with G″ at their corresponding bonding and debonding frequencies. For a PSA material to form a physical bond, two requirements must be met: the bond formation must be thermodynamically favorable, and the contact area must be established upon light pressure within a reasonably short time. The first requirement is a thermodynamic process at the interface and can be expressed as an intrinsic surface energy, I. This energy is the result of thermodynamic interactions, such as dispersion forces and polar interactions. Maximum adhesion occurs when the adhesive and the substrate have similar surface tensions [45]. The second requirement, the bonding term, B, is a kinetic process and depends largely on how easily a PSA material can flow under pressure. If A0 is the total area available for the adhesive material to make a contact and A is the actual contact area established during time, t, then the bonding term, B, is proportional to the ratio A/A0, which is related to the creep compliance, J(t), given by the following equations, A/A0 = 1 − e−J(t) ∼= J(t)

(5.2)

J(t) = {1/G′(ω)} × {1/(1 + tan δ2(ω))} ∼= 1/G′(ω)

(5.3)

when tan δ Gc. The overall picture can be summarized by considering the response of an initial penny-shape interfacial crack (hc/Rd 1), but adhesive failure occurs before extensive deformation of the material into fibrils is possible. This situation corresponds to the behavior of softer rubbers such as PSAs, where E is typically below 0.1 MPa, on silicone release coatings where Gc remains low, although perhaps not as low as the thermodynamic value.40 The peak stress measured in the probe test is indeed controlled by the elastic modulus, or by the external pressure if p0/E > 1, and values of the local contact angle approaching 90° are expected. Because Gc is relatively low, adhesive failure occurs before a fully developed fibrillar structure is able to form, and the overall adhesion energy is relatively low. Nevertheless, a certain degree of fibrillation may still occur, and the overall work of adhesion will be larger than Gc by an amount that depends on the elastic properties of the adhesive. Schematic examples of typical probe test curves obtained for increasing values of Gc/E in this regime are illustrated in Figure 6.8. It is in this regime that the coupling between interface and bulk is stronger.4,31

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6.2.3.3

Large Gc/E

For viscoelastic materials on a high-energy surface, Gc can become significantly higher and Gc/E may become comparable to the adhesive layer thickness. Lateral cavity expansion by motion of the contact line is hindered in this case, so that a fi nely dispersed foam is formed as the walls between cavities are extended vertically into fibrils.19,41,42 The resulting foam of closed cells is observed for all high-performance soft adhesives. If the fibrils are completely elastic, they will stretch according to a nonlinear elastic constitutive equation for rubber elasticity,43 and the stress–strain curves will look like the examples corresponding to the highest values of Gc/E in Figure 6.8. The large strains corresponding to the fibrillation regime (λ > ∼2) complicate the development of a detailed adhesive failure criterion in this regime. The overall behavior is still dominated by a competition between elastic extension and adhesive failure. The complexity arises in the determination of the actual value of G that is operative at the foot of a fibril.44 Qualitatively, one expects that G will be relatively low if the contact angle characterizing the shape of the foot is low (θ < 90°). This correspondence can be attributed to the fact that when θ is low, an incremental decrease in the actual area of contact does not appreciably increase the overall system compliance. As discussed above, this increase in compliance is the overall feature that drives the behavior of both solid and liquid systems.

6.3

Experimental Aspects

Probe tests have been carried out for several decades in the PSA industry and the first well-documented instrument to perform such tests was the Polyken probe tester.45 This instrument, which is still commercially available today, is based on an inverted geometry in which a probe lifts a PSA fi lm deposited on a substrate. The contact pressure is determined by the weight that is attached to the substrate. The probe then retracts at a fi xed velocity and the contact breaks when the PSA substrate is stopped by a fi xed support. This very simple design provides a single value, the maximum retraction force, which is commonly called probe tack. It is, however, poorly reproducible and whereas the probe and the adhesive fi lm self-align during the loading stage, they also will become poorly aligned during the pulloff. It was several decades later that probe tests were significantly improved by the prototype of Zosel at BASF.2 Zosel realized that much information could be obtained from the analysis of the entire force displacement curve, rather than simply from the peak force. His instrument used a very small cylindrical probe (2-mm diameter) and typically very short contact times and low pressures.5,46 Although the reproducibility of the experiment was much better, it still did not integrate a proper way to visualize the contact area and debonding process. This improvement came in the late 1990s with a design illustrated schematically in Figure 6.9, in which a mirror placed at 45° underneath the transparent sample holder allows real-time observation of the bonding and debonding stage. Furthermore, this instrument had a device to control the alignment between the probe and the adhesive fi lm, which greatly improved reproducibility of the force displacement curves. In fact, the most crucial point to obtain reproducible results is to ensure that the only

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Contact) Probe

Mirror

Adhesive layer

Microscope slide

Extensometer

Microscope slide Adhesive layer

Mirror

Steel ball

Lower actuator

Spring

Upper actuator

FIGURE 6.9 Schematic of the experimental design and setup used for the probe tests. (From Creton, C., Roos, A., and Chiche, A., Adhesion: Current Research and Applications, W.G. Possart, Ed., Wiley-VCH, New York, 2005. With permission.)

(

Video

Force cell

Probe Tack 6-13

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deformable part in the measuring chain is the adhesive itself. Therefore, great care must be taken to have a very stiff fi xture and possibly also a very stiff load cell. It must be kept in mind, however, that it is impossible to completely avoid any deformation of the instrument during a test and this deformation must be corrected for the results to be true material properties.40 Although prototypes with very stiff loading fi xtures and load cells may be desirable in the academic lab, commercial instruments such as the texture analyzer are perfectly capable of providing reproducible and reliable data if the sample holder is designed carefully. It is, however, important to ensure that the compliance of the apparatus does not greatly exceed that of the fi lm one wishes to test. This may lead to large differences in the curves that are measured, as illustrated in Figure 6.10.47 It is fi nally useful to discuss briefly the effect of the shape of the probe itself. In 1997, Chuang at Avery advertised a probe test protocol using a spherical probe.48 The main advantage of the spherical probe is the reproducibility because alignment is no longer a problem.7,49,50 However, the detachment of a spherical probe for a fibrillating PSA more closely resembles a peel test than a parallel plates test and imposes a less homogeneous stress field once cavities are fully formed. On the other hand, for weak adhesive forces or interfacial fracture, the spherical geometry is better defi ned and the so-called JKR analysis of the contact problem provides more meaningful data. Several detailed analyses exist for the spherical probe geometry and have been discussed in detail in a recent review. 36

10

Force (N)

5

0

−5

Sample Motor

−10 −50

0 50 Displacement (µm)

100

150

FIGURE 6.10 Measured uncorrected force–displacement curve (dashed line) and corrected adhesive displacement (solid line). In this case, the deformation in the initial stage of the test (compression and tension) is mainly due to the apparatus and sample holder rather than to the adhesive layer itself. Only after the layer fails do the two curves join each other. (From Josse, G., De l’Adhérence à l’Anti-Adhérence à travers le Probe Tack, Paris, Université Paris VI, 2001. With permission.)

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6.4 Analysis of Probe Tack Debonding Curves Obtained for Pressure-Sensitive Adhesives 6.4.1

Cavitation in the Probe Geometry

A distinctive feature of probe test curves is the presence of a relatively sharp maximum in the stress–strain curve. Th is maximum is invariably due to a failure of the adhesive layer by cavitation. Because the factor a/h is usually rather large, the adhesive layer is highly confi ned and the hydrostatic pressure that builds during loading leads to the nucleation and growth of cavities. These cavities will always grow to a size commensurate with the thickness of the layer. 24 Such an example is illustrated in Figure 6.11. Earlier results suggested that the cavitation process was only controlled by elasticity and the peak stress should therefore be proportional to the modulus.6 A later analysis carried out by Chiche et al.9,26 demonstrated that this is only true for adhesives forming large defects at the interface with the probe. An example of the same acrylic-based PSA material debonded from a series of surfaces of increasing mean square roughness is illustrated in Figure 6.12 and demonstrates that the peak stress due to cavitation is lower when the PSA is detached from a rougher surface. The same type of experiments performed on styrene–isoprene–styrene (SIS)-based adhesives demonstrated that the plateau stress after the initial peak is a much more reliable material property than the actual peak stress.15,16,42 Nevertheless, if the probe tests are performed on the same probe, the value of the peak stress is representative of the elasticity of the material. However the peak does not correspond to the appearance of all cavities simultaneously but rather to the point at which the compliance of the

FIGURE 6.11 Images of cavities obtained with SIS-based PSA but three different layer thicknesses. The size of the cavities clearly depends on the thickness of the layer that is being tested. (From Chiche, A., Décollement d’Adhésifs Souples: Cavitation et Fracture, Paris, Université Paris VII, 2003. With permission.)

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0.7

0.15 A C

0.6 B

D

E

0.10

Stress σ (MPa)

0.5 C 0.4

0.05 D

A

B

0.3 E 0.2

0.00

0.6

0.8

1.0

1.2

0.1

0.2

0.4

0.6 Strain ε

1.0

0.8

1.2

Average roughness, RA (nm)

150 8 6

100

4 50 2 0

0 A

B

C

D

Average surface wavelength, λ (µm)

0.0 0.0

E

Polished probe identity

FIGURE 6.12 Representative stress–strain curves of the debonding of a polyethylhexyl acrylate latex at the same debonding velocity of 30 µm/s on rough stainless-steel surfaces. Letters on the probe test curves correspond to the mean square roughnesses reported in the bottom part of the figure. (From Chiche, A., Pareige, P., and Creton, C., Comptes Rendus Acad. Sci. Paris Ser IV, 1: 1197–1204, 2000. With permission.)

layer starts to increase faster due to cavitation than the hardening of the material itself. The process of cavity growth is rather complex, as clearly demonstrated by some recent theoretical arguments and numerical simulations.51–53 Hence, a direct and quantitative interpretation of the peak stress as a cavitation stress should be avoided. Figure 6.13a illustrates the stress–strain curves of three probe tests performed on the same probe surface with three different materials with increasing elastic moduli. The difference in

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Probe Tack

1.4 1.2

2AA 4AA 8AA

Stress (MPa)

1.0 0.8 0.6 0.4 0.2 0.0 0

2

4

6

8

Strain

(a) 107

G (MPa)

106

2 AA 4 AA 8 AA

105

104

103 10−3 (b)

10−1

101 ω*a T

103

105

FIGURE 6.13 (a) Nominal stress as a function of strain for acrylics with variable acrylic acid content. The first digit corresponds to the acrylic acid content (wt %). Probe tests were performed at Vdeb = 100 µm/s and room temperature; (b) master curves of storage modulus G′ as a function of reduced shear rate for three different adhesives with increasing acrylic acid content. Reference temperature: 25°C. (From Lindner, A., Lestriez, B. et al., J. Adhesion 82(3), 267–310, 2006. With permission.)

peak height and plateau stress can be directly attributed to the differences in the moduli, as illustrated in Figure 6.13b. These results are in agreement with the theoretical arguments presented in Section 6.2. Examples of cavities shown in probe tests are given in Figure 6.14. One example (Figure 6.14, left) illustrates the dense population of cavities that is observed for a strong,

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FIGURE 6.14 Images of cavitation of three different PSAs on the same steel surface. An SISbased PSA forming very small cavities that do not grow beyond a size comparable with the thickness of the layer (left). An acrylic all-purpose PSA where substantial lateral propagation occurs (center). A sample of 100 kg/mol linear poly(n-butyl acrylate) as an example of cavitation in a liquid (right).

permanent PSA where the lateral growth of cavities is nearly precluded. When the resistance to interfacial crack propagation is lower, the cavities can grow significantly in the plane of the layer from their original size. An example if this is given next (Figure 6.14, center). Finally, for very soft adhesives that are liquid-like, the cavities tend to nucleate toward the center of the probe and assume irregular shapes that resemble Saff man–Taylor fi ngers observed in fluids (Figure 6.14, right).54,55 As discussed in Section 2.2, it is important to remember that the pressure inside the cavities is nearly zero until air can penetrate from the outside of the layer.11,56 Th is air penetration is accompanied by a drop in nominal stress corresponding roughly to 0.1 MPa. Th is drop is generally not visible for normal PSA (see Figures 6.3 and 6.13a) where detachment of the fibrils occurs before the air pressure can equilibrate, but it nevertheless contributes to the measured stress. For very liquid PSA, on the other hand, such as that shown on Figure 6.14 (right), the probe test curve can indicate a distinctive double plateau, which is characteristic of the breakup of cavity walls without actual detachment of the fibrils.18,57

6.4.2 Weak Adhesion: Interfacial Nucleation and Propagation When Gc/E is very low, debonding occurs by interfacial crack propagation rather than by cavitation and fibril formation. This is generally observed for very weak interfaces, such as those formed between a PSA and a release liner. An example of debonding curves for an acrylic PSA against three different silicone release liner-coated probes is illustrated in Figure 6.15. The probe is removed at the same velocity of 30 µm/s in all three cases. During the loading stage, the probe tester puts the layer under stress and, because the layer is mostly elastic, it stores this elastic energy, which is then released by the fast propagation of interfacial cracks.40 An example of this propagation process is illustrated in Figure 6.16.58

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Probe Tack 0.30 0.25

σ (MPa)

0.20 0.15 Increasing G c 0.10 0.05

0.00 0.0

0.2

0.4

0.6

0.8 ε

1.0

1.2

1.4

FIGURE 6.15 Probe test curves of a removable acrylic adhesive on different surfaces. The arrow indicates increasing Gc values for the surfaces. In all cases the propagation is mostly or completely interfacial, with little deformation of the layer and no clear fibrillar structure. The surfaces are three different release coatings and a steel surface (the most adhesive surface). (Data from Josse, G., Sergot, P., Dorget, M., and Creton, C., J. Adhesion, 80(1–2), 87–118, 2004 and Josse, G., De l’Adhérence à l’Anti-Adhérence à travers le Probe Tack, Paris, Université Paris VI, 2001.)

It is clear from Figure 6.15 that whereas the peak stress is at about the same level for the three surfaces, the decrease in stress after the peak is very different. This difference is due to the very different rates at which the crack propagates.40 In essence, the initiation occurs for the same level of stored elastic energy, but the rate at which this elastic energy can be released back to the system varies and depends on Gc. For very low values of Gc the release rate is very fast and the stress drops almost instantaneously to zero. This rate dependence of Gc is analogous to what is always observed in the field of adhesion of well-cross-linked rubbers, where there is a unique relationship between Gc and crack velocity. One can typically write35,59 n  v  Gc G0  1     v *  

(6.11)

Any variation in Gc can come either from a change in G 0, the component due to interfacial interactions, or from the viscoelastic dissipation component, which is typically more dependent on the dissipative properties of the rubber.60,61 For adhesion of PSA on silicone release liners, the level of adhesion is so weak that changes in the dissipative properties of the PSA do not change Gc much, which is therefore dominated by the value of G0. That is why release liner manufacturers tend to think of adhesion as a phenomenon controlled by the interface.

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FIGURE 6.16 Images of the different stages of propagation of interfacial cracks at the interface between a PSA layer and a silicone-coated probe surface. Time proceeds from top left to bottom right. (From Léger, L. and Creton, C., Adhesion Mechanisms at Interfaces between Soft Polymers, Philosophical Transactions of the Royal Society of London Series a-Mathematical and Physical Sciences, 2008. With permission.)

6.4.3 Strong Adhesion: Fibril Formation and Extension The other well-characterized experimental situation is the case in which Gc/E is larger than the initial thickness of the adhesive layer. The interfacial propagation of the cavities cannot occur easily and the adhesive becomes a foam through the continuous nucleation of additional cavities.19 Once the foam is fi lling the surface of the probe and the walls between the cavities are of the order of half the initial thickness of the layer, the deformation of the layer occurs in the direction of traction and the test starts to be analogous to a tensile test. This analogy between the plateau stress of the probe test and the tensile test is particularly obvious when the adhesive is very elastic.42 This is the case for PSA based on block copolymers and an example of tensile curves and probe test curves

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Probe Tack 1.6 1.4

σ (MPa)

1.2 0% SI

1.0

19% SI

0.8

42% SI

0.6 0.4 54% SI

0.2 0.0 0

5

10

15

ε

(a) 0.8

0% SI

σnominal (MPa)

0.6 19% SI 42% SI 0.4 54% SI 0.2

0.0 0 (b)

5

10

15

20

λ

FIGURE 6.17 (a) Probe tests of the debonding of four PSAs based on SI block copolymers from a steel probe. (b) Tensile stress–strain curves of the same adhesives. The four adhesive contain the same amount of tackifying resin but variable diblock(SI)/triblock(SIS) ratios, as specified. (From Creton, C., Roos, A., and Chiche, A., Adhesion: Current Research and Applications, W.G. Possart, Ed., Wiley-VCH, New York, 2005. With permission.)

on steel for a series of PSAs made from SIS and styrene–isoprene (SI) block copolymers is illustrated in Figure 6.17. The tensile curves demonstrate a distinctive strain hardening that is quantitatively reproduced by the probe test curves. The shape of probe tack curve is typical of strong adhesion and, in this case, the probe test can be used as an approximate characterization of the tensile properties of the PSA. Obviously, in this case the work of adhesion that is measured is rather sensitive to the nonlinear elongational properties of the PSA and not very sensitive to the details of the

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adhesive interactions, as long as they are sufficient to cause the fibril formation and stop interfacial crack growth. Finally, the fibrillar structure will detach from the surface. The quantitative criterion for this process to occur remains relatively poorly understood, but qualitatively the detachment of the fibrils is controlled by the strain hardening of the polymer in the fibrils.44 The more pronounced the strain hardening, the less the fibrils can extend before detaching from the surface of the probe. This can be seen in Figure 6.17 through a comparison of the fibril extension at detachment with the tensile stress–strain curves.

6.4.4

Transition between the Two Regimes

Between these two extreme cases lies the situation in which Gc/E is larger then the typical interfacial defect size (i.e., 500 nm) and smaller than the thickness of the layer h0 (i.e., 100 µm). In this case, the cavities initially formed grow laterally and vertically simultaneously and there is a competition between the two mechanisms. In this regime the resistance to lateral crack propagation is controlled by the dissipative properties of the adhesive, whereas extension is dependent on the nonlinear elastic properties of the adhesive. In this regime the shape of the probe tack curve can depend markedly on the value of Gc/E, with an increasing adhesion energy and maximum fibril extension with increasing values of Gc/E. A typical example of the variation in the shape of the curve with increasing Gc/E is illustrated in Figure 6.18, which demonstrates the same four adhesives in Figure 6.17, but this time detached from an ethylene–propylene surface. A distinctive

0.6

σ (MPa)

0.5 0.4 0.3 0.2 0.1 0% SI

42% SI

19% SI

54% SI

0.0 0

2

4

6

8

10

ε

FIGURE 6.18 Probe tests of the debonding of four PSAs based on SI block copolymers from an ethylene–propylene surface. The four adhesives contain the same amount of tackifying resin but variable diblock(SI)/triblock(SIS) ratios, as specified. (From Creton, C., Roos, A., and Chiche, A., Adhesion: Current Research and Applications, W.G. Possart, Ed., Wiley-VCH, New York, 2005. With permission.)

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Probe Tack

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sign of a mixed failure mode (detachment and extension) is a plateau in stress that decreases with increasing ε. This is typically never observed in tensile tests for elastic materials and demonstrates unambiguously that the fibril-stretching process in this case is not analogous to a tensile test, where the material cannot escape the grips of the tensile tester and typically hardens. In this transition regime between strong and weak adhesion, PSAs are very sensitive to subtle changes in rheological properties. The value of Gc becomes very sensitive to the dissipative properties of the PSA. A recent study indicated that better adhesion can be obtained with a dissipative layer close to the surface, backed by a more elastic layer.31

6.5

Conclusion

If executed and analyzed carefully, the probe test can be a powerful analytical tool to help interpret standard adhesive property tests of PSAs because of its sensitivity to small changes in the molecular structure of the PSA. We have described theoretically the process of cavity nucleation and growth and the transition from interfacial crack propagation to fibril growth. Most information comes from the shape of the stress–strain curve obtained during the debonding stage at different strain rates. A single peak with a sharp drop in force after the peak is indicative of weak adhesion and interfacial crack propagation. As the level of adhesion increases, the stress decrease more progressively after the peak and forms a distinct shoulder, which then turns into a plateau in stress. If adhesion further increases, the plateau in stress will become a second peak at higher elongation, which immediately precedes fibril detachment. The transition from one mechanism to the other can be described by the elastic length Gc/E, which represents the ratio between the critical energy release rate for crack propagation and the elastic modulus. When Gc/E < R0, failure occurs by crack propagation only. At the other extreme, when Gc/E > h0, failure occurs by fibril formation and growth and the adhesion energy is essentially controlled by the nonlinear properties of the adhesive in tension. The transition from one mechanism to the other occurs progressively and in this regime adhesion energy is particularly sensitive to the value of Gc/E.

References 1. PSTC test methods for pressure sensitive adhesive tapes, Pressure Sensitive Tape Council. 2000. 2. Zosel, A., Adhesion and tack of polymers: influence of mechanical properties and surface tensions, Colloid Polym. Sci. 263: 541–553, 1985. 3. Zosel, A., Fracture energy and tack of pressure sensitive adhesives, Adv. Pressure Sensitive Adhesive Technol. 1: 92–127, 1992. 4. Shull, K. R. and C. Creton, Deformation behavior of thin compliant layers under tensile loading conditions, J. Polym. Sci.: Part B: Polym. Phys. 42: 4023–4043, 2004. 5. Zosel, A., Adhesive failure and deformation behaviour of polymers, J. Adhesion 30: 135–149, 1989.

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6. Lakrout, H., P. Sergot and C. Creton, Direct observation of cavitation and fibrillation in a probe tack experiment on model acrylic pressure-sensitive-adhesives, J. Adhesion 69(3/4): 307–359, 1999. 7. Crosby, A. J., K. R. Shull, H. Lakrout and C. Creton, Deformation modes of adhesively bonded elastic layers, J. Appl. Phys. 88(5): 2956–2966, 2000. 8. Webber, R. E., K. R. Shull, A. Roos and C. Creton, Effects of geometric confinement on the adhesive debonding of soft elastic solids, Phys. Rev. E 68: 021805, 2003. 9. Chiche, A., J. Dollhofer and C. Creton, Cavity growth in soft adhesives, Eur. Phys. J. E 17: 389–401, 2005. 10. Gent, A. N., Compression of rubber blocks, Rubber Chem. Technol. 67(3): 549–558, 1994. 11. Lindner, A. and B. Lestriez et al., Adhesive and rheological properties of lightly crosslinked model acrylic networks, J. Adhesion 82(3): 267–310, 2006. 12. Creton, C. and H. Lakrout, Micromechanics of flat probe adhesion tests of soft viscoelastic polymer fi lms, J. Polym. Sci.: Part B: Polym. Phys. 38: 965–979, 2000. 13. Kakavas, P. A. and P. J. Blatz, Effects of voids on the response of a rubber poker chip sample .3, J. Appl. Polym. Sci. 43(6): 1081–1086, 1991. 14. Laun, H. M., M. Rady and O. Hassager, Analytical solutions for squeeze flow with partial wall slip, J. Non-Newtonian Fluid Mech. 81: 1–15, 1999. 15. Roos, A., Sticky Block Copolymers: Structure, Rheological and Adhesive Properties. Paris, Université Paris VI, 2004. 16. Chiche, A., Décollement D’adhésifs Souples: Cavitation et Fracture. Paris, Université Paris VII, 2003. 17. Tirumkudulu, M., W. B. Russell and T. J. Huang, On the measurement of “tack” for adhesives, Phys. Fluids 15(6): 1588–1605, 2003. 18. Poivet, S., F. Nallet, C. Gay, J. Teisseire and P. Fabre, Force response of a viscous liquid in a probe-tack geometry: Fingering versus cavitation, Eur. Phys. J. E 15: 97–116, 2004. 19. Brown, K., J. C. Hooker and C. Creton, Micromechanisms of tack of soft adhesives based on styrenic block copolymers, Macromol. Mater. Eng. 287(3): 163–179, 2002. 20. Gay, C. and L. Leibler, Theory of tackiness, Phys. Rev. Lett. 82(5): 936–939, 1999. 21. Gent, A. N. and P. B. Lindley, Internal rupture of bonded rubber cylinders in tension, Proc. R. Soc. London, Ser. A: Math. Phys. Sci. 249 A: 195–205, 1959. 22. Gent, A. N. and D. A. Tompkins, Surface energy effects for small holes or particles in elastomers, J. Polym. Sci., Part A-2 Polym. Phys. 7: 1483–1488, 1969. 23. Dollhofer, J., A. Chiche, V. Muralidharan, C. Creton and C. Y. Hui, Surface energy effects for cavity growth and nucleation in an incompressible neo-Hookean material—modeling and experiment, Int. J. Solids Struct. 41(22–23): 6111–6127, 2004. 24. Chikina, I. and C. Gay, Cavitation in adhesives, Phys. Rev. Lett. 85: 4546–4549, 2000. 25. Lin, Y. Y., C. Y. Hui and H. D. Conway, A detailed elastic analysis of the flat punch (tack) test for pressure sensitive adhesives, J. Polym. Sci.: Part B: Polym. Phys. 38(21): 2769–2784, 2000.

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26. Chiche, A., P. Pareige and C. Creton, Role of surface roughness in controlling the adhesion of a soft adhesive on a hard surface, Comptes Rendus Acad. Sci. Paris Ser. IV 1: 1197–1204, 2000. 27. Lau, A. W. C., M. Portigliatti, E. Raphaël and L. Léger, Spreading of latex particles on a substrate, Europhys. Lett. (5): 717, 2002. 28. Gent, A. N. and C. Wang, Fracture mechanics and cavitation in rubber-like solids, J. Mater. Sci. 26: 3392–3395, 1991. 29. Fond, C., Cavitation criterion for rubber materials: a review of void-growth models, J. Polym. Sci.: Part B: Polym. Phys. 39: 2081–2096, 2001. 30. Creton, C., J. C. Hooker and K. R. Shull, Bulk and interfacial contributions to the debonding mechanisms of soft adhesives: Extension to large strains, Langmuir 17(16): 4948–4954, 2001. 31. Carelli, C., F. Déplace, L. Boissonnet and C. Creton, Effect of a gradient in viscoelastic properties on the debonding mechanisms of soft adhesives, J. Adhesion 83(5): 491–505, 2007. 32. Williams, M. L. and R. A. Schapery, Spherical flaw instability in hydrostatic tension, Int. J. Fracture Mech. 1: 64–71, 1965. 33. Lawn, B., Fracture of Brittle Solids. Cambridge, Cambridge University Press. 1993. 34. Lin, Y. Y. and C. Y. Hui, Cavity growth from crack-like defects in soft materials, Int. J. Fracture 126(3): 205–221, 2004. 35. Maugis, D. and M. Barquins, Fracture mechanics and the adherence of viscoelastic bodies, J. Phys. D: Appl. Phys. 11: 1989–2023, 1978. 36. Shull, K. R., Contacts mechanics and the adhesion of soft solids, Mater. Sci. Eng. R, Report 36: 1–45, 2002. 37. Zhang Newby, B.-M., M. K. Chaudhury and H. R. Brown, Macroscopic evidence of the effect of interfacial slippage on adhesion, Science 269: 1407–1409, 1995. 38. Chaudhury, M. and B.-M. Zhang Newby, Friction in adhesion, Langmuir 14: 4865–4872, 1998. 39. Amouroux, N., J. Petit and L. Léger, Role of interfacial resistance to shear stress and adhesive peel strength, Langmuir 17: 6510–6517, 2001. 40. Josse, G., P. Sergot, M. Dorget and C. Creton, Measuring interfacial adhesion between a soft viscoelastic layer and a rigid surface using a probe method, J. Adhesion 80(1–2): 87–118, 2004. 41. Creton, C., Pressure-sensitive-adhesives: An introductory course, MRS Bull. 28(6): 434–439, 2003. 42. Creton, C., A. Roos and A. Chiche Effect of the diblock content on the adhesive and deformation properties of PSAs based on styrenic block copolymers. in: W. G. Possart (eds). Adhesion: Current Research and Applications. Wiley-VCH, 2005. 43. Roos, A. and C. Creton, Nonlinear elastic properties of elastomeric block copolymers, Macromolecules 38: 7807–7818, 2005. 44. Glassmaker, N. J., C. Y. Hui, T. Yamaguchi and C. Creton, Detachment of stretched viscoelastic fibrils, Eur. Phys. J. E 25: 253–266, 2008. 45. Hammond, F. H., Polyken probe tack tester, ASTM Spec. Technical Publ. 360: 123–134, 1964.

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46. Zosel, A., The effect of bond formation on the tack of polymers, J. Adhesion Sci. Technol. 11: 1447–1457, 1997. 47. Josse, G., De l’Adhérence à l’Anti-Adhérence à travers le Probe Tack. Paris, Université Paris VI, 2001. 48. Chuang, H. K., C. Chiu and R. Paniagua, Avery adhesive test yields more performance data than traditional probe, Adhes. Age (September): 18–23, 1997. 49. Crosby, A. J. and K. R. Shull, Adhesive failure analysis of pressure-sensitive adhesives, J. Polym. Sci.: Part B: Polym. Phys. 37: 3455–3472, 1999. 50. Crosby, A. J., K. R. Shull, Y. Y. Lin and C. Y. Hui, Rheological properties and adhesive failure of thin viscoelastic layers, J. Rheol. 46(1): 273–294, 2002. 51. Yamaguchi, T. and M. Doi, Debonding dynamics of pressure-sensitive adhesives: 3D block model, Eur. Phys. J. E 21(4): 331–339, 2006. 52. Yamaguchi, T., H. Morita and M. Doi, Modeling on debonding dynamics of pressure-sensitive-adhesives, Eur. Phys. J. E 20(1): 7–17, 2006. 53. Teisseire, J., F. Nallet, P. Fabre and C. Gay, Understanding cracking versus cavitation in pressure-sensitive adhesives: The role of kinetics, J. Adhesion 83(7): 613–677, 2007. 54. Saff man, P. G. and G. Taylor, The penetration of a fluid into a porous medium or Hele-Shaw cell containing a more viscous liquid, Proc. R. Soc. London, Ser. A: Math. Phys. Sci. 245: 312–329, 1958. 55. Derks, D., A. Lindner, C. Creton and D. Bonn, Cohesive failure of thin layers of soft model adhesives under tension, J. Appl. Phys. 93(3): 1557–1566, 2003. 56. Poivet, S., F. Nallet, C. Gay and P. Fabre, Cavitation-induced force transition in confined viscous liquids under traction, Europhys. Lett. 62(2): 244–250, 2003. 57. Lakrout, H., C. Creton, D. Ahn and K. R. Shull, Influence of molecular features on the tackiness of acrylic polymer melts, Macromolecules 34: 7448–7458, 2001. 58. Léger, L. and C. Creton, Adhesion Mechanisms at Interfaces between Soft Polymers, Philosophical Transactions of the Royal Society of London Series a-Mathematical and Physical Sciences 366: 1425–1442, 2008. 59. Shull, K. R., D. Ahn, W. L. Chen, C. L. Mowery and A. J. Crosby, Axisymmetric adhesion tests of soft materials, Macromol. Chem. Phys. 199: 489–511, 1998. 60. Ahn, D. and K. R. Shull, Effects of methylation and neutralization of carboxylated poly(n-butyl acrylate)on the interfacial and bulk contributions to adhesion, Langmuir 14(13): 3637–3645, 1998. 61. Ahn, D. and K. R. Shull, Effects of substrate modification on the interfacial adhesion of acrylic elastomers, Langmuir 14: 3646–3654, 1998.

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7 Peel Resistance 7.1 Introduction .............................................................7-1 7.2 Basic Principles, Theory, and Analysis of Peel Resistance .....................................................7-2 7.3 Peel Resistance Test and Analysis .........................7-7 Test Methods of Peel Resistance • Master Curve and Time–Temperature Superposition • Failure Mode

7.4

Hyun-Joong Kim Dong-Hyuk Lim Young-Jun Park Seoul National University

7.1

Parameters of Peel Resistance ............................7-15 Bulk Properties and Peel Resistance of PressureSensitive Adhesives • Surface Properties of Substrate and Peel Resistance

7.5 Conclusion ..............................................................7-31 References ........................................................................7-32

Introduction

According to a common definition, pressure-sensitive adhesives (PSAs) can be adhered to various surfaces with light pressure within a few seconds. In certain cases, they also can be removed without leaving any residue or contaminating the substrate. Such behavior is due to their ambiguous, solid-like, as well as liquid-like behavior, that is, their viscoelasticity (see also Chapters 4 and 5). The former behavior (called cohesion) gives high strength in the debonding process, whereas the latter behavior (called adhesion) allows wetting of the surface during the bonding process. The adhesive performances of PSAs can be evaluated as tack, peel resistance, and cohesive strength, such as holding power and shear adhesion failure temperature (SAFT) (see also Applications of Pressure-Sensitive Products, Chapter 8). Among these performances, peel resistance is an especially important parameter for PSAs. Peel resistance is the force required to peel off a pressure-sensitive product (label, tape, etc.) from a substrate. According to ASTM D 907 [1], the peel resistance is the average load per unit width of bondline required to progressively separate a flexible member from a rigid member or another flexible member. Control of peel resistance in PSA products is very important for both manufacturers and customers. For instance, in the label industry, the value of peel resistance is one of 7-1

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7-2

Fundamentals of Pressure Sensitivity TABLE 7.1 Classification of Pressure-Sensitive Adhesives versus Peel Resistance and Removability PSA Grade

180° Peel Resistance (N/2.5 cm)

Excellent permanent Permanent Semiremovable Removable and repositionable Excellent removable

>14 10–14 6–8 2–4 T6. Examination of these generic data demonstrates that shifting the curves for temperatures T1 through T5 to the left from the standard temperature, T6, by some amount will cause all of the curves to form a smooth curve. This is the case for most PSAs. The result is known as a master curve and the amount by which a segment is

T1

Rate Temperature

log Va T

FIGURE 7.9 The making of a master curve. (From Pocius, A.V., Adhesion and Adhesives Technology, A.V. Pocius, Ed., Hanser, Munich, 1997. With permission.)

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7-13

Peel Resistance

shifted is known as a shift factor. Thus, a master curve is a plot of the peel strength as a function of a reduced peel rate. The reduced variable is obtained by multiplying the peel rate by the shift factor. Thus, the set of curves illustrated in Figure 7.9a can be reduced to a single curve, illustrated in Figure 7.9 [18]. Although direct testing of peel energy may be limited to a relatively narrow range of test speeds, time–temperature superposition allows prediction of peeling energies. Therefore, we can overcome the limit of the peeling tester and predict the peel resistance in an extended test range. Derail et al. [19] demonstrated that the shift factors derived from peel adhesion master curve construction were equivalent to those derived via rheologic measurement (see Chapter 4). Peel master curves are therefore examples of rheologic master curves. Peeling behavior through the construction of peel master curves can reveal the influence of adhesive molecular features, including molecular weight and composition. Failure mode location in peeling tests depends only on the rheological properties of the PSA at the temperature considered. This is a very important statement for industrial application. The fact that fracture does not depend on the location of crack initiation means that within a controlled range of temperature and deformation rates, the user may trust the adhesive even when some defect initiates a crack in fracture mode, which is different from what is expected.

7.3.3 Failure Mode The failure mode is classified into several types including cohesive failure, adhesive (or interfacial) failure between the adhesive and the substrate, stick–slip failure, and glassy failure between the backing material and adhesive. It is expected that a normal PSA will fail interfacially when tested under standard peel test conditions. Adhesive failure occurs when the adhesive strips are cleanly removed from the adherend, leaving no visually noticeable residue (see Figure 7.10). Some adhesives may fail cohesively, leaving

B-C C

B

A Peel strength (N)

P Time

Viscous

A

Glassy

B B-C

C Log rate (m/s)

FIGURE 7.10 Failure modes as a function of peeling rate. (From Urahama, Y., Tokunaga, Y., and Tanaka, Y., J. Adhesion Soc. Jpn., 23(5), 171–177, 1990. With permission.)

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7-14


3.5

Peel strength (kg/in.)

3.0

2.5

2.0

1.5

1.0 0

1

2

3

4

5

6

7

Length of tape peeled (cm)

FIGURE 7.11 Peel strength–strain curve demonstrating stick–slip peeling. (From Aubrey, D.W., Welding, G.N., and Wong, T., J. Appl. Polym. Sci., 13, 2193–2207, 1969. With permission.)

adhesive residue on the test panel. If the adhesive is not fi rmly anchored to the backing material, it may transfer from the backing material to the test panel, leaving at least part of the backing bare. In the case of transfer tapes, such behavior is designed intentionally; otherwise, it denotes product failure (see Applications of Pressure-Sensitive Products, Chapter 4). The facture shapes of four failure modes in peeling experiments are illustrated in Figure 7.10. Urahama et al. [15] observed that at relatively low peeling speeds below 10 mm/min, the stringiness conformation of adhesives of the porous backing had a honeycomb structure, whereas that of the nonporous backing had a sawtooth-shape structure. In the region of intermediate pulling rate, the mode of failure is stick–slip, which is a regular, jerky peel in which the observed peel force oscillates between well-defi ned limits. An autographic recording obtained in this region is illustrated in Figure 7.11 [20]. In this region, the alternative failure between adhesive and glassy failure was observed. The alternations of force were in step with the alternations of failure mode and, furthermore, the rising and falling parts of the autographic trace corresponded with cohesive and adhesive separations, respectively. The steady rate of pulling is insufficient to sustain peeling with the fast mechanism, so the mechanism reverts to the slow mechanism. This completes the cycle of stick–slip oscillation. In the case of adhesive failure, a general judging standard is the existence of adhesive residue. As previously mentioned, the test method must be selected considering the application of the adhesive (see Applications of Pressure-Sensitive Products, Chapter 4). For example, a special PSA tape is used to fabricate semiconductor chips [21]. Ultraviolet (UV)-curable dicing tape used in the die-bond process for semiconductors must hold tightly upon mounting of a silicon wafer. During the pick-up process, the adhesion strength of the dicing tape must be reduced with no residue so that the diced chip

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7-15

Peel Resistance

can be picked up easily. In this case, human sight is not enough to determine adhesive failure mode with no residue. Additional instrumental analysis like optical microscopy, Fourier transform infrared spectroscopy, or X-ray photoelectron spectroscopy (XPS) may be carried out for an exact determination. Th is is why with the recent rapid development in integrated semiconductor technology, higher reliability is required for the manufacturing process of electronic devices.

7.4 Parameters of Peel Resistance Figure 7.12 presents a simple flow chart of tape processing and peeling tests. We can consider seven factors that can influence on the results of peel resistance by reviewing the tape preparation and test processes. These parameters are summarized in Table 7.3. If we want to acquire correct data influenced by one parameter, we must control the other variables. Table 7.3 will help to control the variables for accurate peeling tests. Detailed effects related to a few select parameters will be described later. PSA adhesion is determined by two factors: the energy of deformation of the viscoelastic adhesive until rupture or separation from the surface occurs and the surface need to be contacted to such a degree that molecular attraction forces come into action.

Preparation of PSA

Coating onto backing material

Cleaning substrate

Preparation of strip

Bonding and sample stabilization

Peeling

Data analysis

FIGURE 7.12

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The flow chart of sample preparation and peel testing.

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7-16 TABLE 7.3

Fundamentals of Pressure Sensitivity Parameters of Processing and Testing that Affect Peel Resistance

Stage of Processing and Testing Preparation of PSA

Coating onto carrier material

Preparation of sample Cleaning substrate

Bonding step and sample stabilization Peeling test

Parameters Affecting Peel Resistance Chemical composition and cross-linking nature and density Viscoelastic properties Miscibility between PSA and other formulation components Modulus of carrier material PSA thickness Thickness of backing material Surface properties of carrier material Sample width Surface properties of substrate (surface energy and roughness) Surface treatment Degree of pollution Bonding pressure Bonding time Peeling angle or geometry and peeling tool Peeling rate Temperature and humidity

7.4.1 Bulk Properties and Peel Resistance of Pressure-Sensitive Adhesives The basic materials of acrylic PSAs are acrylic esters that yield soft and tacky polymers with low Tg. Suitable monomers for PSAs are alkyl acrylate and methacrylate containing 4–17 carbon atoms [22], such as butyl acrylate, 2-ethylhexyl acrylate (2-EHA), and iso-octyl acrylate (an isomer of 2-EHA). Homopolymers of these polymers are soft and tacky materials that cannot be used for good PSAs. They are modified by copolymerization with a small portion of other functional comonomers that affect a wide range of properties and also provide cross-linking sites. The composition of generalized acrylic PSAs is as follows: Main monomer: 50–98% Modifying monomer: 10–40% Monomer with functional groups: 0.5–20% Monomers that yield a homopolymer with low Tg plasticize the adhesive, whereas modifying monomers that yield homopolymers with high Tg increases the adhesive stiff ness. Tg is a useful indicator in choosing a comonomer (see Technology of Pressure-Sensitive Adhesives and Products, Chapter 5, and Applications of Pressure-Sensitive Products, Chapter 7). PSA flexibility and tackiness increase with increasing side-chain length until a certain chain length is exceed and the chains start to form crystalline regions that cause stiffening of the polymer. In the case of long branched side chains, branching decreases the tendency to form crystalline regions. Therefore, branching makes the polymer softer. The comonomer distribution is important and can be affected by the order of monomer addition. Usually, PSAs based on pure acrylics can be regarded as completely random, with the same monomer distribution in every polymer chain.

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7-17

Peel Resistance

100 Peel strength Probe tack SAFT

80

1200 60

40

SAFT (°C)

Peel strength (g/25 mm) and probe tack (g)

1600

800 20

0

400 5

10

15

Acrylic acid contents (wt %)

FIGURE 7.13 Adhesion properties such as probe tack, peel strength, and SAFT with variation in AA content. (From Joo, H.S., Do, H.S., Park, Y.J., and Kim, H.-J., J. Adhesion Sci. Technol., 20(14), 1573–1594, 2007. With permission.)

Comonomers, such as methyl methacrylate and styrene, which increase the Tg of the adhesive, can be used in limited amounts to improve cohesive strength. Much research exists regarding the formulation of acrylates on the properties of PSAs [23–26]. Gower and Shanks [27–29] discussed the formulation for polymerization on adhesive performance and peel master curve. They explained the relationship between comonomers and PSA performance. Figure 7.13 illustrates the effect of chemical composition of PSAs on peel resistance [30]. PSAs with varying amounts of acrylic acid (AA) were investigated, and the expected effects of AA addition were good adhesion to substrates, enhancement of cohesion, and increased viscosity. The Tg of the PSAs increased with increasing AA concentration, because AA has the highest Tg among monomers such as 2-EHA, vinyl–acetate (VAc), and AA. Peel strength slightly increased as AA concentration increased, although PSAs with an AA content of 10 and 15 wt % demonstrated somewhat similar peel strengths. These results can be explained, because the increase in AA concentration affected the viscosity and cohesive strength of synthesized PSA through hydrogen bond formation. Therefore, the peel strength of PSAs increased with AA concentration. The relationship between peel resistance and pulling rate has been examined using a range of poly(butyl acrylate) homopolymer adhesives with different molecular weights [20]. Results are illustrated in Figure 7.14. In the slow peeling region, the peel resistance increases with molecular weight, as would be expected if viscous flow of the adhesive is the controlling factor. The rate at which the transition to stick–slip occurs increases with decreasing molecular weight, although with the two lowest molecular weight adhesives this transition did not occur, even at the highest cross-head speed obtainable. In the fast

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7-18


5

e

d

Force range at stick−slip peeling c

Peel strength (kg/25 mm)

4 Range of force maxima a 3

b

Range of force minima d

b a

2

e e a b

1

f 0 −1.5

−1.0

0.0 0.5 1.0 −0.5 Log rate of jaw separation (cm/min)

1.5

2.0

FIGURE 7.14 Effect of molecular weight of poly(n-butyl acrylate) on peel strength at various rates of jaw separation. (Molecular weight: a > b > c > d > e > f, ○, cohesive failure; ●, adhesive failure between adhesive and glass surface (substrate); □, adhesive failure between adhesive and backing; △, mixture of two types of adhesives.) (From Aubrey, D.W., Welding, G.N., and Wong, T., J. Appl. Polym. Sci., 13, 2193–2207, 1969.)

peeling region, the amount of data available is small but gives no indication of appreciable dependence of peel resistance on molecular weight. To study the relationship between cross-linking density and peel resistance, UVcross-linkable PSAs were synthesized using the solution polymerization method using 2-EHA, VAc, AA, 2-hydroxyethyl methacrylate, and 4-acryloyloxydiethoxy-4-chlorobenzophenone (P-36) as polymerizable photoinititators with double bonds. The effect of photoinitiator content on the PSAs is illustrated in Figure 7.15 [31]. As expected, the higher photoinitiator content in SH3P2 produced a decrease in peel resistance of 69.8% at a UV dose of 210 mJ/cm2, whereas SH3P05 demonstrated only a 9.1% reduction in peel resistance with the same dose. The peel resistance is also influenced by the Tg. If the Tg of the PSA is sufficiently low, the wettability will be high after application to the substrate and, as a result, the peel resistance will also increase. Czech [2] synthesized removable and repositionable water-borne acrylic PSAs. In this study, the effect of copolymerizable emulsifier and plasticizer was investigated. Figure 7.16 illustrates how the influence of vinyl-unsaturated emulsifiers on peel resistance on steel depends on the type of investigated internal emulsifiers and their concentration. The relatively low peel adhesion is observed for the vinyl-unsaturated emulsifier SPMK. The increased amount of SPMK above 5 wt % has very little influence on removability. One possibility of reducing peel resistance in the water-based PSAs lies in the variation of the plasticizer used. The plasticizer, such as di-n-butyl phthalate, diethylhexyl phthalate, and di-n-octyl phthalate, was used in an amount between 10 and 30 wt %. Its influence is illustrated in Figure 7.17.

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7-19

Peel Resistance

4000

Peel resistance (g/25 mm)

PI content (phr) 0.5 1.0

3000

2.0

2000

1000

0 0

500

1000

1500

2000

2500

UV dose (mJ/cm2)

FIGURE 7.15 Change in peel resistance of UV cross-linkable PSAs with varying UV dose (PI, photoinitiator) (*phr:part). (From Do, H.S., Park, Y.J., and Kim, H.-J., J. Adhesion. Sci. Technol., 20(13), 1529–1545, 2006. With permission.)

9

Peel adhesion (N/2.54 cm)

8 7

SEMNa

Semi removable

6 5

AMPSNa

4 3

SPIK

Removable

2 SPMK

1 0 0

1

2

3

4

5

6

7

8

Internal emulsifier concentration (wt %)

FIGURE 7.16 Influence of internal emulsifiers on peel adhesion on steel (SPMK, potassium salt of sulfopropyl methacrylate; SEMNa, sodium salt of sulfoethyl methacrylate; SPIK, bis(3-sulfopropyl)-itaconic acid ester dipotassium salt; AMPSNa, sodium salt of 2-acrylamido2-methylpropyl sulfonic acid). (From Czech, Z., J. Appl. Polym. Sci., 97, 886–892, 2005. With permission.)

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7-20


6

Peel adhesion (N/2.5 cm)

5

Di-n-butyl phthalate

Diethylhexyl phthalate 4

3

Removable

Di-n-octyl phthalate 2

1 10

20

30

Plasticizer concentration (wt %)

FIGURE 7.17 Influence of plasticizer concentration on peel adhesion. (From Czech, Z., J. Appl. Polym. Sci., 97, 886–892, 2005. With permission.)

One effective PSA bulk property control methods is the addition of the tackifier into the elastomer. Adhesion properties such as peel resistance and tack can be easily controlled by the addition of tackifier into a styrenic block copolymer. To investigate the effect of tackifiers on peel resistance, PSAs were blended with various tackifiers such as GA-100 (rosin ester), Hikorez A-1100S (aliphatic hydrocarbon), Regalite R-125 (hydrogenated aromatic hydrocarbon), Quintone U-185 (Modified C5), and Sukorez SU-100 (hydrogenated dicyclopentadiene). Figure 7.18 illustrates the peel resistance of the SIS/ tackifier blends as a function of tackifier contents [32]. The PSAs blended with Sukorez SU100, Hikorez A1100S, and Quintone U-185 exhibit maximum peel resistance at 60 wt % of tackifier content, whereas other PSAs with GA-100 and Regalite R-125 exhibit a peak at 40–50 wt % of tackifier content. The GA-100 and R-125 have a higher softening point and Tg than the other tackifiers. Thus, the maximum peel strength shifts with lower tackifier content, depending on the softening point. Similar behavior is illustrated in Figure 7.19. The hot-melt PSAs (HMPSAs) were prepared by SIS copolymer and tackifier with various Tg: SU-90 (Tg, 39.1°C), SU-110 (Tg, 64.3°C), SU-130 (Tg, 75.4°C). As noted, the peak location of peel resistance shifts to lower tackifier content with increasing tackifier Tg [33]. Considering the fact that the PSAs are a mixture of elastomeric polymers and lowmolecular-weight tackifier resins, it is important to investigate the degree of miscibility between the components, because the phase structures are governed by miscibility; that is, when the components are miscible with each other, the blends must be in a uniform one-phase structure, but when they are immiscible with each other, phase separation occurs. The physical properties and the practical performance of the PSA are dependent on the phase structure of the materials.

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7-21

Peel Resistance

15000

Peel strength (g/25 mm)

12500

10000

7500 5000

2500

0 40 (a)

50

60

70

Wt % of tackifier 15000


12500

10000

7500 5000

2500 0 40 (b)

50

60

70

Wt % of tackifier

FIGURE 7.18 Peel strength of SIS/tackifier blends on SUS substrate (peeling rate, 300 mm/min): (a) Kraton D1107 blends and (b) Vector 4111 blends (○, Hikorez A 1100S; △, Regalite R 125; ▽, Quintone U 185; ◇, Sukorez SU 100; □, GA-100). (From Kim, D.J., Kim, H.-J., and Yoon, G.H., Int. J. Adhesion Adhesives, 25, 288–295, 2005. With permission.)

In the acrylic PSA/tackifier system, Kim and Mizumachi [34] studied systemically the effect of miscibility on adhesion properties. The peel resistance of PSAs is low at a low separation rate, and it gradually increases and decreases again as the separation rate is increased. In the case of a miscible system, incorporation of a tackifier resin results in modification of the bulk properties of the blend; therefore, the peel resistance master curves shift along the rate-axis, whereas the tackifier content changes, as illustrated in Figure 7.20. On the other hand, in the case of a system where the components are

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15000

Peel strength (g)

Vector + SU-90 Vector + SU-110 Vector + SU-130 10000

5000

0 30

40

50

60

70

Tackifier content (wt %)

FIGURE 7.19 Peel strength of vector/tackifier as a function of tackifier content. (From Lim, D.H., Do, H.S., and Kim, H.-J., PSA J. Appl. Polym. Sci., 102, 2839–2846, 2006. With permission.)

3000

Peel strength (gf/cm)

Tackifier content (%)

2000

: : : : :

0 10 20 30 40

1000

0 10−5

10−3

10−1

101

Va T (cm/s)

FIGURE 7.20 Peel master curve dependence on peeling rate in miscible blends. (From Kim, H.J. and Mizumachi, H., Advances in Pressure Sensitive Adhesive Technology-3, D. Satas, Ed., Satas & Associates, Warwick, RI, 1998. With permission.)

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7-23

Peel Resistance

Tackifier content (%) 1500

:

0

: 10 : 20

Peel strength (gf/cm)

: 30 : 40 1000

500

0 10−4

10−2

100

102

VaT (cm/s)

FIGURE 7.21 Peel master curve dependence on peeling rate in immiscible blends. (From Kim, H.-J. and Mizumachi, H., Advances in Pressure Sensitive Adhesive Technology-3, D. Satas, Ed., Satas & Associates, Warwick, RI, 1998. With permission.)

not miscible, physical properties of the two phases are not modified, even if the blend ratio is varied, and the master curve does not shift along the rate-axis, as illustrated in Figure 7.21, because the peel resistance depends mainly upon the properties of the matrix phase. The peak height of the master curve in peel resistance decreases as the amount of the dispersed phase increases. Fujita et al. [35] investigated the effects of miscibility on the peel resistance of natural rubber-based PSAs. In the case of miscible PSAs, the peak position of the pulling rate-peel resistance curve shifted to a lower velocity as the tackifier content increased. Immiscible PSA blends had lower peel resistances than miscible blends and did not exhibit any apparent peak shifts. Viscoelastic properties play a vital role in the study of adhesion behavior, including the application of PSAs, diff usion through interfaces, and internal stress (see Chapter 4). Viscoelastic properties deal with the problem of the action of mechanical forces on the adhesion system, depending on the velocity and loading frequency. Figure 7.22 illustrates the viscoelastic spectrum of typical PSAs [36]. Peeling is a high-rate process and takes place at a speed between 100 and 1000 rad/s. Peel rate can

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8 Tack zone

Shear zone

Log G′, Log G′′ (Pa)

7

6

Peel zone

G′′

5

G′ 4 −3

−2

−1

0

1

2

3

4

Log frequency (rad/s)

FIGURE 7.22 Viscoelastic spectrum of a typical PSA (G′, storage modulus; G″, loss modulus). (From Satas, D., Handbook of Pressure Sensitive Adhesive Technology and Applications, D. Satas, Ed., Satas & Associates, Warwick, RI, 2002. With permission.)

cover both the plateau and the transition zones. If the rate of peel is not too fast, then entanglements will dominate and control the peel strength of PSAs. If peeling occurs at higher rates, then small molecular structures, such as tackifying resins and short chain pendant groups attached to the base elastomer, can contribute to the peel strength of the PSA. The characteristics in a master curve will be described in reference to the characteristics of natural rubber/tackifier blends, as illustrated in Figure 7.23 [37]. In the master curve, three regions of steady peeling and one region of oscillatory (stick–slip) peeling may be distinguished. At high temperatures or very low peel rates, the adhesive experiences predominantly viscous flow, which allows it to be drawn out into long fibrils in the region where the tape curls away from the substrate. These eventually fail in tension, leaving traces of the adhesive on both the tape and the rigid surface, which is known as cohesive failure (see Figure 7.10). At higher peel rate or less elevated temperatures, the adhesive becomes detached from one of the solid surfaces, usually remaining attached to the flexible tape, either before any fibrils have formed or at least before they are well developed. This is characteristic of an interfacial or adhesion failure (see Figure 7.10). Both of these detachment modes are stable and if the peeling rate is kept constant, the peel force will be also. At either high peel rates or low temperatures, the adhesive may undergo transformation into its glassy or brittle form, which now allows deadhesion to proceed by the advance of a crack. Th is may be either stable or unstable and may run either cohesively through the adhesive layer or along the adhesive/substrate interface.

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Peel Resistance

40

T (K) 312 304 296 276 268 258 246 240

Cohesive separation Adhesive separation

P (N)

30

20

B

258 246 240

BC

10 A

C 0 −8

−6

−4

−2

0

2

4

Log Ra ′T (m/s)

FIGURE 7.23 Master curve of peel force against pulling rate at 296 K for the system 6:4 NR: Piccolyte S115, showing superposition of experimental data (A, cohesive failure; B, adhesive failure between adhesive and glass (substrate); C, adhesive failure between adhesive and baking; B-C, stick–slip). (From Satas, D., Handbook of Pressure Sensitive Adhesive Technology and Applications, D. Satas, Ed., Satas & Associates, Warwick, RI, 2002. With permission.)

Figure 7.24 illustrates the rheologic behavior at different temperatures and the corresponding peeling curves [38]. The terminal domain of relaxation corresponds to an interfacial fracture with a crack localized between the adhesive and the rigid substrate (Figure 7.24a). In the intermediate domain (transition region), one can observe stick–slip behavior (Figure 7.24b). Finally, at very low temperatures, the crack is localized between the adhesive and the flexible substrate with a very low peel force (Figure 7.24c). One can conclude that the propagation of the crack, as well as the value of the peeling force, depends largely on the rheological behavior of the adhesive, which is linked to the temperature and the peeling rate. In the case of EVA-based PSAs, Gibert et al. [39] demonstrated that the cohesive-to-interfacial transition observed on a peeling curve appeared at the same time as the liquid-like to solid-like transition induced by crystallization of EVA (Figure 7.25) [39]. Figure 7.25 provides a schematic diagram of the relationship between viscoelastic and peel properties (see Chapter 4). The failure mode does not always follow the four main types: cohesive, adhesive, stick–slip, and glassy failure. The peeling behavior illustrated in Figure 7.26 demonstrates differences. Figure 7.26a exhibits all four fracture domains already described (cohesive, interfacial 1, stick–slip, and interfacial 2), whereas the interfacial 1 fracture domain has disappeared in Figure 7.26b. Cohesive fracture can be shifted directly to a stick–slip behavior as the peeling rate increases. Aubrey and Sherrif [37] derived the same conclusion with another formulation. The addition of resin to the polymer not only diminishes the width of the elastic plateau region but also decreases the plateau

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Log (G′ and G ′′/[Pa])

9

G′ G′′

8 7 6 5 4 3 2

−5 −4 −3 −2 −1 0

1 2 3 ω (rad/s)

(a)

4

5

6

7

8

(a[i]) T = 20°C

5

Log (G′ and G′′/[Pa])

Peel force (N)

8 7 6 5 4 3 2 1 0

2

6

4

8

G ′ T = 20°C G ′′

4

3

2 0

10

−1

0

Time (s)

(b)

1 Log (ω/[rad/s])

2

(b[i])

8

G ′ T = −15°C G′′

6

T = −30°C Log (G′ and G′′/[Pa])

7 Peel force (N)

9

6 5 4 3 2

5

4

1 0

3 0

3 Time (s)

2

1

4

5

0

1 Log (ω/[rad/s])

2

1

2

(c[i])

(c) T = −30°C

8 7 6 5 4 3 2 1 0

Log (G ′ and G′′/[Pa])

Peel force (N)

−1

6

9

8

7

6 0

2

6

4 Time (s)

8

10

G′ T = 20°C G ′′

−1

0

Log (ω/[rad/s])

FIGURE 7.24 G′, G″ versus frequency for SIS + SI blend and peeling curves measured on the tensile machine at various temperatures: (a) terminal domain (rubbery plateau); (a[i]) adhesive fracture; (b) transition region; (b[i]) stick–slip; (c) glassy domain; and (c[i]) glassy fracture. (From Marin, G. and Derail, C., J. Adhesion, 82, 469–485, 2006. With permission.)

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7-27

Peel Resistance

F

F

Crack Crack Peel force

F Crack

Shear modulus

Cohesive fracture

Interfacial fracture

Stickslip

Glassy fracture Peel rate G′ G ′′

Terminal zone

Rubbery state

Glass transition

Glassy state Frequency

FIGURE 7.25 Schematic diagram of the relationship between viscoelastic and peel properties. (From Gibert, F.X., Allal, A., Marin, G., and Derail, C., J. Adhesion Sci. Technol., 13(9), 1029–1044, 1999. With permission.)

modulus value. The gradual disappearance of the type 1 interfacial fracture is probably due to both effects. Changing the resin content not only changes the plateau modulus value and extent, but also changes the Tg, so all these combined effects do not lead to a simple relationship.

7.4.2

Surface Properties of Substrate and Peel Resistance

Good bonding between a substrate and an adhesive is attained when their cohesive energy density or solubility parameters (δ) are matched (see Chapter 10). Figure 7.27 [40] illustrates the effect of thermodynamic compatibility, which is molecular attraction between an elastomer and low molecules on peel strength. Systems with high peel strength and cohesive failure contain adhesives with δ values that are close to that of polyethylene terephthalate (PET) (δ = 10.3 {J/cm3}1/2). Adhesion is low when the δ value of the adhesive is too low or too high, but gradually increases as δ corresponding to that of PET is approached from either side. Hata et al. [41] investigated the influence of the critical surface tension of adherends on the rolling friction coefficient and peel strength of PSAs. In the velocity region where the interfacial failure occurs in the case of acrylic PSAs, the rolling friction coefficient and peel strength have a positive correlation with the critical surface tension of the

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Interfacial 1

160

Interfacial 2

140 Cohesive

Stick-slip

PF TO/ T (N)

120 100 80 60 40 20 0 0

1

2

3

(a)

4 5 6 log (a T·V) (mm/min)

7

8

9

60

PF TO/ T (N)

50 40 30 20 10 0 1 (b)

2

3

4

5

6

7

8

9

log (a T·V) (mm/min)

FIGURE 7.26 Peel force as a function of reduced peel rate: (a) PB:tackifier = 170:30; (b) PB: tackifier = 270:30. (From Aubrey, D.W. and Sherrif, M., J. Polym. Sci. Polym. Chem. Ed., 18, 2597– 2608, 1980. With permission.)

adherends, indicating that both the rate of the bonding process and the failure criterion concerning the interfacial failure are closely related to the critical surface tension of the adherend. Performance of the natural rubber-based PSA was more complicated than that of the acrylic PSA. Kim et al. [32] reported the peel strength of the SIS/tackifier blends at various peeling rates with many substrates having different surface tension values (Figure 7.28). Table 7.4 presents the surface tension of various substrates. High peel strength was

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Peel Resistance

PET = 10.3 = Cohesive failure

44.48

5

22.24

Peel strength (N/25 mm)

Peel strength (lb/in.)

10

0

0 8

9

10

11

12

13

Solubility parameter (δ) of adhesive

FIGURE 7.27 Relation of compatibility to adhesion for Mylar–adhesive–Mylar system. (From Iyengar, Y. and Ericson, D.E., J. Appl. Polym. Sci., 11, 2311–2324, 1967. With permission.)

observed for stainless steel (SUS 304) and glass, medium peel strength for Bakelite, polyvinyl chloride (PVC), and polypropylene (PP), and low peel strength for polyethylene (PE) and Teflon. A similar classification was obtained using surface tension as the distinguishing factor. Although PE exhibits a surface tension similar to that of mediumpeel substrates, it has low peel strength due to the different failure mode. Interfacial failure was observed for PE, whereas cohesive failure was observed for other medium-peel substrates. This may be due to differences in the characteristics of the substrates. In the peel tests performed using PP substrates, cohesive failure occurred at various test temperatures. However, in the peel test performed using the PE substrate, adhesive failure occurred for the Kraton D1107 blends, but stick–slip failure was observed for Vector 4111 blends. After aging at 100°C, cohesive failure occurred in all blends except for the Regalite R-125 blends because PSAs transfer to the substrates during the aging process. A typical stress–strain curve of the Kraton D1107/Sukorez SU-100 blends with the PE substrate is illustrated in Figure 7.29. In the Kraton D1107/Sukorez SU-100 (40/60) blend, a stick–slip type of stress–strain curve was observed (Figure 7.29a), whereas cohesive failure (Figure 7.29b) was observed in the Vector 4111/SU-100 (40/60) blend. Although the curves in Figure 7.29a correspond to stick–slip failure, the adhesive was stripped cleanly from the substrate, leaving no visually residue. Therefore, the surface was clear after the test because of the high cohesion of the SIS-based, HMPSAs with a high SIS content. Not only is the peel force decreased for a higher energy surface, but also the positions of the transitions from cohesive to adhesion and from adhesion to stick–slip failure are changed. That is, the activation of the stiffening behavior of a PSA depends on the substrate to which the adhesive is bonded. Marin and Derail [38] reported similar effects of substrate nature on peel strength and mechanisms of fracture (see Chapter 4).

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7-30


15000


12500 10000 7500 5000 2500 0 0

200

(a)

400

600

Rate (mm/min) 15000


12500 10000 7500 5000 2500 0 0

200

(b)

400

600

Rate (mm/min)

FIGURE 7.28 Peel strength of SIS/Hikorez A 1100s (40/60) blends as a function of peeling rate: (a) Kraton blends and (b) Vector blends (□, SUS; ○, PE; △, PP; ▽, PVC; ◇, Bakelite; ◁, Teflon; and ▷, glass). (From Kim, D.J., Kim, H.-J., and Yoon, G.H., Int. J. Adhesion Adhesives, 25, 288–295, 2005. With permission.) TABLE 7.4

Surface Tension of Various Substrates

Substrates a

Surface tension (γc) (mN/m)

SS

PE

PP

PVC

Bakelite

Teflon

Glass

b

31

33

37

31

18

73

—

Note: SS, stainless steel; PE, polyethylene; PP, polypropylene; PVC, polyvinylchloride. a b

Determined by contact angle measurement. Not available.

Source:

Czech, Z., J. Appl. Polym. Sci., 97, 886–892, 2005. With permission.

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Peel Resistance


6000

4000

2000

0 0

20

(a)

40

60

80

60

80

Distance (mm)


6000

4000

2000

0 0 (b)

20

40 Distance (mm)

FIGURE 7.29 Typical stress–strain curve of Kraton/tackifying resin blends on PE substrate: (a) Kraton D1107/SU-100 = 40/60; (b) Kraton D1107/SU-100 = 60/40. (From Kim, D.J., Kim, H.-J., and Yoon, G.H., Int. J. Adhesion Adhesives, 25, 288–295, 2005. With permission.)

7.5

Conclusion

In the evaluation of PSA performance, the measurement of peel resistance is one of the most important characteristics because various factors, such as properties of backing materials, surface of the adherend, peeling speed, and test temperature, affect the peel strength. Failure mode detection and drawing up the master curves for peel resistance

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provide a great deal of information. Therefore, to gain insight into the peeling mechanism it is helpful to design PSAs and evaluate their performance.

References 1. ASTM D 907. 2. Czech, Z., 2005. Synthesis of removable and repositionable water-borne pressuresensitive adhesive acrylics. J. Appl. Polym. Sci. 97:886–892. 3. Chen W. T. and T. F. Flavin, 1972. Mechanics of fi lm adhesion: Elastic and elastic– plastic behavior, IBM J. Res. Dev. 16(3):203–213. 4. Kaelble, D. H., 1959. Theory and analysis of peel adhesion: Mechanisms and mechanics, Trans. Soc. Rheol. 3:161–180. 5. Kaelble, D. H., 1960. Theory and analysis of peel adhesion: Bond stress and distributions, Trans. Soc. Rheol. 4:45–73. 6. Williams J. A. and J. J. Kauzlarich, 2004. Peeling shear and cleavage failure due to tape restrain, J. Adhesion, 80:433–358. 7. Williams J. A. and J. J. Kauzlarich, 2007. Application of the bulk properties of an acrylic pressure sensitive adhesive to peeling, J. Adhesion Technol. 21(7):515–529. 8. Gent A. N. and E. A. Meinecke, 1970. Compression, bending, and shear of bonded rubber blocks, Polym. Eng. Sci. 10(1):48–53. 9. Satas, D., 2002. Peel. In Handbook of Pressure Sensitive Adhesive Technology and Applications, ed. D. Satas, Satas & Associates, Warwick, RI, pp. 62–86. 10. Horgnies, M., E. Darque-Ceretti, and E. Felder, 2007. Relationship between the fracture energy and the mechanical behavior of pressure-sensitive adhesives, Int. J. Adhesion Adhesives 27:661–668. 11. Kaelble, D. H., 1965. Peel adhesion: Micro-fracture mechanics of interfacial unbonding of polymers, Trans. Soc. Rheol. 9(2):135–163. 12. Kinloch, A. J., C. C. Lau, and G. J. Williams, 1994. The peeling of flexible laminates. Int. J. Fracture 94:79–88. 13. Muny, R. P., 2002. Testing pressure sensitive adhesives. In Handbook of Pressure Sensitive Adhesive Technology and Applications, ed. D. Satas, Satas & Associates, Warwick, RÌ, pp. 139–152. 14. Kawashita, L. F., D. R. Moore, and J. G. Williams, 2004. The development of a mandrel peel test for the measurement of adhesive fracture toughness of epoxymetal laminates, J. Adhesion 80:147–167. 15. Urahama, Y., Y. Tokunaga, and Y. Tanaka, 1990. Morphology at peeling pressure sensitive adhesives (II), J. Adhesion Soc. Jpn. 23(5):171–177. 16. ASTM D3330. 17. PSTC-1. 18. Pocius, A. V., 1997. Basic physico/chemical properties of polymers. In Adhesion and Adhesives Technology, ed. A.V. Pocius, Hanser, Munich. 19. Derail, C., A. Allal, G. Marin, and Ph. Tordjeman, 1997. Relationship between viscoelastic and peeling properties of model adhesive. Part1. Cohesive fracture, J. Adhesion 61:123–157.

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20. Aubrey, D. W., G. N. Welding, and T. Wong, 1969. Failure mechanisms in peeling of pressure-sensitive adhesive tape, J. Appl. Polym. Sci. 13:2193–2207. 21. Do, H. S. and H.-J. Kim, 2006. UV-curable pressure-sensitive adhesives. In Pressure-Sensitive Design and Formulation, Application Volume 2, ed. I. Benedek, VSP, Leiden, Boston, pp. 251–290. 22. Auchter, G., O. Aydin, A. Zettl, and D. Satas, 2002. Acrylic adhesives. In Handbook of Pressure Sensitive Adhesive Technology and Applications, ed. D. Satas, Satas & Associates, Warwick, RI, pp. 445–514. 23. Mayer, A., T. Pith, G. Hu, and M. Lambla, 1995. Effect of the structure of latex particles on adhesion. Part II: Analogy between peel adhesion and rheological properties of acrylic copolymers, J. Polym. Sci. Part B: Polym. Phys. 33(12):1793–1801. 24. Mayer, A., T. Pith, G. Hu, and M. Lambla, 1995. Effect of the structure of latex particles on adhesion. Part I: Synthesis and characterization of structured latex particles of acrylic copolymers and their peel adhesion behavior, J. Polym. Sci. Part B: Polym. Phys. 33(12):1781–1791. 25. Shen, H., J. Zhang, S. Liu, G. Liu, L. Zhang, and X. Qu, 2008. Effect of the chain-transfer-agent content on the emulsion polymerization process and adhesive properties of poly(n-butyl acrylate-co-acrylic acid) latexes, J. Appl. Polym. Sci. 107(3):1793–1802. 26. Rana P. K. and P. K. Sahoo, Synthesis and pressure sensitive adhesive performance of poly (EHA-co-AA)/silicate nanocomposite used in transdermal drug delivery, J. Appl. Polym. Sci. 106(6):3915–3921. 27. Gower M. D. and R. A. Shanks, 2004. The effect of varied monomer composition on adhesive performance and peeling master curves for acrylic pressure-sensitive adhesives, J. Appl. Polym. Sci. 93(6):2909–2917. 28. Gower M. D. and R. A. Shanks, 2004. The effect of chain transfer agent level on adhesive performance and peel master-curves for acrylic pressure sensitive adhesives, Macromol. Chem. Phys. 205(16):2139–2150. 29. Gower M. D. and R. A. Shanks, 2006. Acrylic acid level and adhesive performance and peel master-curves of acrylic pressure-sensitive adhesives, J. Polym. Sci. Part B: Polym. Phys. 44(8):1237–1252. 30. Joo, H. S., H. S. Do, Y. J. Park, and H. –J. Kim, 2007. Adhesion performance of UV-cured semi-IPN structure acrylic pressure sensitive adhesives, J. Adhesion Sci. Technol. 20(14):1573–1594. 31. Do, H. S., Y. J. Park, and H.-J. Kim, 2006. Preparation and adhesion performance of UV-cross-linkable acrylic pressure sensitive adhesives, J. Adhesion Sci. Technol. 20(13):1529–1545. 32. Kim, D. J., H.-J. Kim, and G. H. Yoon, 2005. Effect of substrate and tackifier on peel strength of SIS (styrene-isoprene-styrene)-based HMPSAs, Int. J. Adhesion Adhesives 25:288–295. 33. Lim, D. H., H. S. Do, and H.-J. Kim, 2006. PSA performances and viscoelastic properties of SIS-based PSA blends with H-DCPD tackifier, J. Appl. Polym. Sci. 102:2839–2846. 34. Kim, H.-J. and H. Mizumachi, 1998. Miscibility of acrylic pressure sensitive adhesives. In Advances in Pressure Sensitive Adhesive Technology-3, ed. D. Satas, Satas & Associates, Warwick, RI, pp. 77–128.

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35. Fujita, M., M. Kajiyama, A. Takemura, H. Ono, H. Mizumachi, and S. Hayashi, 1998. Effect of miscibility on peel strength of natural-rubber-based pressuresensitive adhesives, J. Appl. Polym. Sci. 70:777–784. 36. Satas, D., 2002. Dynamic mechanical analysis and adhesive performance. In Handbook of Pressure Sensitive Adhesive Technology and Applications, ed. D. Satas, Satas & Associates, Warwick, RI, pp. 62–86. 37. Aubrey, D. W. and M. Sherrif, 1980. Peel adhesion and viscoelastic of rubber-resin blends, J. Polym. Sci.: Polym. Chem. Ed. 18:2597–2608. 38. Marin, G. and C. Derail, 2006. Rheology and adherence of pressure-sensitive adhesives, J. Adhesion 82:469–485. 39. Gibert, F. X., A. Allal, G. Marin, and C. Derail, 1999. Effect of the rheological properties of industrial hot-melt and pressure-sensitive adhesives on the peel behavior, J. Adhesion Sci. Technol. 13(9):1029–1044. 40. Iyengar, Y. and D. E. Ericson, 1967. Role of adhesive-substrate compatibility in adhesion, J. Appl. Polym. Sci. 11:2311–2324. 41. Hata, T., T. Tsukatani and H. Mizumachi, 1994. The influence of critical surface tension of adherends on the rolling friction coefficient and peel strength of pressure sensitive adhesives, J. Adhesion Soc. Jpn. 30(8):352–357.

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8 Shear Resistance 8.1 Introduction ............................................................ 8-1 8.2 Characterization of Shear Resistance .................. 8-2 8.3 Description of Processes in the Shear Test ...................................................... 8-4 8.4 Behavior of Pressure-Sensitive Adhesives under Compressive Load: The Cold Flow Problem........................................................... 8-9 8.5 Factors Influencing Shear Strength ....................8-11

Sergey V. Antonov Valery G. Kulichikhin A.V. Topchiev Institute of Petrochemical Synthesis

Assembly Time/Preparation Conditions • Load • Temperature • Molecular Weight and Structure • Chemical Composition • Influence of Additives and Miscibility • Method of Polymerization • Cross-Linking Density • Physical Cross-Linking

8.6 Improving the Shear Behavior of Pressure-Sensitive Adhesives ......................... 8-15 References ........................................................................8-16

8.1 Introduction From the rheological viewpoint, pressure-sensitive adhesives (PSAs) can be described in general as viscoelastic materials with a specific complex of rheological properties. Under shear load they demonstrate creep behavior that can lead to failure of the adhesive joint. Shear resistance characterizes a PSA’s ability to resist shearing forces. Shear resistance is also sometimes termed holding power [1] (see also Applications of PressureSensitive Products, Chapter 8). Shear resistance is one of the most significant characteristics of PSAs. The requirements for shear resistance come from the application of PSAs (see also Applications of PressureSensitive Products, Chapter 4). Shear resistance is very important, for example, for packaging tapes and labels, upholstery, films for floor advertising, automobile industry, etc. In some applications PSAs are exposed to shearing forces at elevated temperatures (see also Applications of Pressure-Sensitive Products, Chapter 4) for relatively short times, whereas in other applications the material must withstand shear load for longer periods of time

8-1

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at lower temperatures (see also Applications of PressureSensitive Products, Chapter 4); the actual conditions should A therefore always be kept in mind. Shear adhesion is also essential for the reliability of PSA tapes used for heat shrink attachment in air-cooled electronic assemblies [2]. For this application, estimation of creep behavior in a rather wide temperature range is necessary. Shear deformations may be developed under compressive load as well, which would lead to squeezing the adhesive and its radial flow. This process is often referred to as cold flow of the adhesive (see also Applications of PressureB Sensitive Products, Chapter 8). Understanding the nature of the shear adhesion of PSAs is crucial for controlling their creep behavior. Shear resistance is often regarded as a measure of the PSA’s cohesive strength [1], which seems to be an oversimplified point of view, taking into account their viscoelasticity and the dependence of their mechanical characteristics on time and temperature. Many efforts have been made to relate the shear resistance of PSAs to their structure and properties (chemical composition, visC cosity, storage and loss moduli, miscibility of the components in the formulation, degree of cross-linking, etc.) and develop a model based on these properties that is suitable for predictFIGURE 8.1 Schematic ing the durability of adhesive joints under shear stresses. representation of the static shear test. (A) steel plate, (B) These attempts are examined later in this chapter. Before discussing the impact of different parameters on adhesive tape, and (C) load. shear adhesion, it is reasonable to consider the experimental methods used for estimation of shear resistance (see also Applications of Pressure-Sensitive Products, Chapter 8).

8.2 Characterization of Shear Resistance The most common method of testing shear resistance of PSAs is the static shear test. The standardized methods of the static shear test are similar and are described by ASTM D3654 [3], PSTC-7 [4], FTM 8 [5], and AFERA-4012 [6]. A PSA tape of standard size is applied to the substrate and rolled with a standard weight roller (usually twice). After a specified dwell time necessary for formation of the adhesive joint and stress relaxation, the prepared specimen is fi xed vertically in the shear tester (see Figure 8.1) and the static shear force is loaded to the free end of the tape parallel to the bonding surface. Results are typically reported as holding time, that is, the time to failure of the adhesive joint (in other words, the durability of adhesive joint is measured). The testing conditions used in different standards were summarized in Refs 1 and 7. Typically testing should be accomplished at 23 ± 2°C, the roll weight is about 2 kg, the substrate is steel or aluminum, the sample size is 1 × 1 in., and the load weight is 500–1000 g. The results obtained by

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8-3

Shear Resistance

3

2

5

E

1

6

4

FIGURE 8.2 Schematic design of the apparatus for a modified static shear test. (1) steady steel plate, (2) movable plate or backing, (3) adhesive layer, (4) load, (5) block, and (6) temperature chamber.

different authors using the static shear method can hardly be directly compared due to the differences in testing parameters and thickness of the specimens. Although this method simulates the real behavior of PSA tapes under shear loads, it has a drawback of determining only the “end point” (holding time), thus providing very limited information on the mechanism and kinetics of failure. Several authors [8–11] used a modified version of this test based on a device that is more sophisticated from a rheological viewpoint, known as a shear plastometer. The schematic design of such a device is illustrated in Figure 8.2. The constant shear load is applied via a block to the sample (PSA tape with backing) adhered to the substrate. The backing is sometimes reinforced by a second steel plate [8] to prevent the sample from bending. The sample can be placed in a heated chamber, permitting measurements at different temperatures. This equipment allows the displacement of the one of plates to be registered as a function of time at constant load during the experiment with good resolution (i.e., it is a creep measurement as well). As mentioned previously, devices of this kind have often been used by rheologists for the study of creep behavior and are known as shear (parallel plate, sandwich type, etc.) plastometers [12,13] (see also Chapter 4). In fact, the shear test is a creep test carried to high deformation [14]. Later in this chapter we will discuss a rheological description of the creep test in the shear plastometer. Zosel [8] reports 20% standard deviation of the experimental results obtained for a large number of similar specimens tested on such a device, explained by deviations in sample preparation. Imprecision of the tape’s geometric parameters, especially thickness

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8-4


and incomplete stress relaxation, can be mentioned among the possible reasons for data scattering. The main drawback of the static shear method is that the experiments are quite timeconsuming. Therefore, a “dynamic” test has been proposed (see also Applications of PressureSensitive Products, Chapter 8). A sample similar to that used for the static test is deformed and separated from the substrate, not at constant load but at constant strain rate [8,15]. The experiment can be performed with a tensile tester. The cross-head speed depends on the intended application of the PSA. The maximum force determined during the experiment can be regarded as a measure of shear resistance. The stress–strain curves obtained using this method can provide useful information about the failure of the adhesive joint. In some applications (e.g., packaging, insulation, mounting, or splicing tapes) it is necessary to know the shear resistance at elevated temperatures and the maximum permissible temperature. In the shear adhesion failure temperature (SAFT) method [1,16,17] a sample similar to one described above is loaded with a static weight while the temperature is ramped at rate of 0.4 C/min until the specimen fails. The testing conditions are generally the same as in the static shear test (except for the temperature) and the experiment can be carried out in the same machine if it is equipped with a temperature-controlled chamber. In this method the temperature of the failure of the adhesive joint is considered as a characteristic of shear resistance (see also Applications of Pressure-Sensitive Products, Chapter 8). Reliability of the results obtained in different shear tests depends on many factors. The most common errors in measuring shear resistance include nonuniform thickness of the adhesive layer, the presence of nonrelaxed stresses, the substrate surface is not clean, and the direction of the applied force is not parallel to the interface. Careful inspection of the surface of the adhesive joint after separation to determine the failure mode (cohesive, adhesive, or mixed adhesive–cohesive) can provide valuable information that can be useful for improving performance of the tested PSA.

8.3 Description of Processes in the Shear Test To identify the factors that influence shear resistance and propose ways to improve it, it is necessary to clearly understand the processes that takes place during testing. Satas [18] described three types of adhesion joint failure depending on the proportion between holding power and adhesion. If the holding power is much higher than the adhesion, adhesion failure proceeds. On the other hand, if adhesion is much higher than the holding power, cohesive failure must occur. If adhesion and cohesion forces are comparable, then a “mixed” failure mode (partially cohesive, partially adhesive) is observed. Most of the following discussion regarding holding power and holding time is applicable to the situation of cohesive failure when rheological parameters predetermine the shear resistance of the PSAs. As discussed previously, shear plastometers that are very similar in design to shear testers have long been used by rheologists, although their application was restricted by the region of small deformations for several reasons that will be analyzed later. Let us consider the typical strain versus time curve obtained using this device (Figure 8.3). The first segment (OA) represents a fast and relatively small elastic response. The deformation in the next segment (AB) is a combination of elastic and plastic (flow) modes.

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8-5

Shear Resistance

F = const. (creep) C

F=0 (elastic recovery)

B D Strain

B′

Unrecoverable strain

E A

O Time

FIGURE 8.3 Typical plot of plate displacement versus time in a shear plastometer (small deformations).

In the steady region (segment BC in Figure 8.3), which is characterized by constant slope, the elastic deformation has already passed through its maximum value and further deformation of the sample is connected only with flow (plastic deformation). The rheological parameters in this stage can be calculated as follows. Shear stress is

F A

(8.1)

where F is the force applied to the adhesive tape and A is the overlapping area. Shear rate is

d

(8.2)

where v is the current linear speed of the adhesive tape and d is the thickness of the adhesive layer. The overlapping area decreases with time during the test in accordance with Equation 8.3,  l  A A0  1   l0 

(8.3)

where ∆l is the shift of the adhesive tape from the initial position and l 0 is the initial length of the overlapping area. It is, therefore, clear that shear stress grows with time. Taking into account that

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dl dt

(8.4)

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8-6


it is possible to solve this system and prolong the analysis to the end point when the adhesive tape slides off the substrate. Th is was done by Zosel [8], who tried to predict the holding time from the flow curves of the adhesives. Unfortunately, such a model assumes sliding off of the tape governed by the viscous flow of adhesive as the only mechanism of failure and completely neglects the elasticity of the adhesive. All these equations are valid only when there is no slippage between the tested material and the substrate. It is not surprising, therefore, that this model was able to predict the holding time more or less correctly only for rather low-viscosity non-cross-linked adhesives. By extrapolating the steady segment to the strain axis it is possible to separate elastic and plastic deformations: segment AB′ in Figure 8.3 represents the elastic portion of the total deformation developed in segment AB. If the load is released, then elastic recovery starts with a fast stage (segment CD), followed by slower recovery (DE). The plastic deformation cannot be recovered (unrecoverable strain). Of course, everything noted regarding plastic deformations is applicable only to noncross-linked polymers. All stages of the creep process in polymers can be qualitatively described using Burgers’s model [19,15], which consists of four elements, connected as illustrated in Figure 8.4, where springs elastic constants (moduli) are G 0 and G1 and dashpot viscosities are η 0 and η1. This model combines Maxwell (index 0) and Kelvin–Voigt fluids (index 1). The total stress in this model is equal to the stress of each element: 0 1

(8.5)

η0

G1

η1

G0

FIGURE 8.4

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Burgers’s model.

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8-7

Shear Resistance

and the total deformation is the sum of these parts,

0 1

(8.6)

d d 0 d 1 dt dt dt

(8.7)

and, consequently,

An expression for stress can be derived from Equations 8.5 through 8.7 as follows: G1 1 1

 1 d d

 1  dt  G0 dt 0 

(8.8)

Differential Equation 8.8 can be solved to yield the total deformation:

(t )

t  0 0   1 e 1  0 t G0 G1   0

(8.9)

In this equation λ1 is the retardation time. Let us analyze Equation 8.9. The first term represents an instantaneous elastic deformation. The second term reflects the slow growth of elastic deformation. At t → ∞ τ it becomes equal to ___0 -equilibrium rubbery deformation. The third term, which G1 describes the linear growth of strain over time, demonstrates pure viscous flow. It corresponds to the “steady” region of the creep curve. τ due to the reaction Release of the load leads to instant recovery of deformation by ___ G0 of the spring in the Maxwell element, followed by slower recovery, described by the following expression:

(t )

tt

0 1 t1 0 e 1 0 G1

(8.10)

In Equation 8.10, t > t1, and t1 is the moment of load release. The plastic deformation τ0 developed in the dumper of the Maxwell element during creep stage, __ η 0 t1, is unrecoverable. Although Burgers’s model describes qualitatively the whole creep–recovery cycle, several authors proposed some modifications of this model to enable better compliance with experimental results obtained in shear tests of PSAs. Geiss and Brockmann [15] investigated the creep behavior of SIS-based PSA in cycling creep–recovery tests. The slope of the strain curve increases from cycle to cycle as if the viscosity of the PSA is decreasing. To describe this feature, a time-dependent nonlinear factor was introduced to the reaction of the G1 spring. Kano et al. [10] added a combination of friction and damper elements to extend the application of the model to adhesive failure. The friction element simulates slippage between the adhesive tape and the substrate.

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8-8


Another rheological phenomenon can be very important in the shear resistance of PSAs. Highly structured liquids (e.g., liquids containing fi llers or those with a strong network of H-bonds) may possess the yield stress. It means that at stresses lower than the yield stress value they behave like a solid. Plastic or rubbery deformation of the sample can start only after the yield stress is exceeded. There has been much discussion about the real existence of yield stress [20,21], but, nevertheless, because this phenomenon is observed in the timescale of PSAs application, it can be utilized. We can imagine a PSA that could be adhered to a substrate under some pressure, causing shear stress greater than yield stress, but exploited at lower shear stresses and therefore having excellent shear resistance. An example of a system with yield stress in a creep test was demonstrated in Ref. 22. To estimate typical shear stress in the beginning of the static shear test, let the sample with size 1 × 1 in. (25.4 × 25.4 mm) be loaded with a weight of 1 kg. In accordance with Equation 8.1, τ ≈ 1.5 × 104 Pa, which is comparable with yield stress values observed for many structured systems [21]. As mentioned previously, shear plastometers are used for rheological measurements only at small shear strains. There are several reasons for this. The overlapping area diminishes with time, thus increasing the shear stress (Equations 8.1 and 8.3). Therefore, simple viscoelastic models derived for constant stress conditions become inapplicable for creep description. The shear stress field becomes nonuniform. For example, the material at the edge of the moving plate (point E in Figure 8.2) is exposed not only to shear stresses but also to elongation. Cross-linked materials with limited ability for deformation would lose contact with either of the plates at small strain. The large deformations are, however, inherent for real shear tests. Figure 8.5 illustrates the typical dependence of the displacement of the adhesive tape on time. The diminishing overlapping area noted above has several consequences. First, if the viscosity does not depend on shear stress (Newtonian liquid), this leads to acceleration of the plate movement (see Figure 8.5). For many polymers, however, viscosity decreases with increased shear stress (viscosity anomaly), which means, in this case, even more prominent acceleration. Moreover, as determined by Vinogradov et al. [23], high shear stresses can be easily reached for very viscous systems such as fi lled rubbers, even at small strains, which can induce the forced transition of the polymer melt to a rubbery state, resulting in the loss of adhesion to the substrate and transition to the adhesive mode of joint failure. The elasticity of the adhesive plays a very important role in this process. It can be concluded from information noted previously that the shear resistance of non-cross-linked adhesives, as long as they remain adherent to the substrate, depends mainly on their viscosity and elasticity and, consequently, on the factors influencing the rheological parameters of the adhesive. Elasticity becomes more important at the fi nal stages of the creep test. As for the cross-linked adhesives, their limited ability to deform in the shear field depends mainly on parameters of cross-linking (cross-linking density and their nature).

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8-9

Plate displacement

Shear Resistance

Time

tc

FIGURE 8.5 Typical plot of plate displacement versus time in a shear tester (large deformations). The rectangular area in the left lower corner represents the region of small deformations illustrated in Figure 8.3.

8.4 Behavior of Pressure-Sensitive Adhesives under Compressive Load: The Cold Flow Problem As mentioned previously, shear stresses and shear deformations can develop not only under pure shear conditions, but also under compressive load, forcing the adhesive to squeeze out from a gap between parallel surfaces. Resistance to shear under compressive loading is very important for several applications. For example, wound dressings and patches for transdermal drug delivery should withstand the weight of the patient without adhesive leakage [22] (see also Applications of Pressure-Sensitive Products, Chapter 5). Such leakage is often referred to as cold flow. In this section we will consider the rheology of squeezing between parallel plates. The specimen can be arranged in two ways [24]: The thickness of the specimen is considerably smaller than its diameter; it is larger than the plates and the area under compression is constant. The excessive amount of material can flow freely from the gap. The thickness of the specimen is considerably smaller than its diameter; it is smaller than the plates and the volume of the specimen under compression is constant.

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8-10


F

d

A

h

C

B

FIGURE 8.6

Scheme of a squeeze tester. (A) movable plate, (B) steady plate, and (C) specimen.

We can also consider the situation of a constant applied compressive force and constant squeezing rate. We will, however, limit our discussion in this section to the case of a constant area under compression and constant compressive force because this situation is closer to that observed in practice. Figure 8.6 illustrates the scheme of a squeeze tester. The upper (movable) plate has diameter d. The kinetics of plate movement and the factors influencing it will be the matters of interest. Several assumptions must be made to derive the corresponding equations. • • • •

The tested liquid is incompressible. The plate’s motion is very slow (elasticity is negligible). The material is in immediate contact with the plates. The distance between the plates is small (the velocity component in the perpendicular direction is negligible).

For a Newtonian liquid the kinetics of the distance change can be presented in the form of Stefan’s equation: 1 1 4Ft h2 h02 3R 4

(8.11)

In this equation, h is the current distance between plates, h 0 is the initial distance, F is the applied force, t is time, R is the radius of the smaller plate, and η is the viscosity of the tested liquid. As seen from Equation 8.11, a reduction in the distance between plates slows down in time but never stops completely. In fact, however, even for low-viscosity liquids complete squeezing is impossible due to capillary forces. For a power-law fluid that can be characterized by the expression k n

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(8.12)

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8-11

Shear Resistance

(where k and n are constants), the change in distance between the plates in time is determined by Scott’s equation: 2kRn3  2n 1  (dh/dt )n n3  n  h2n1 n

F

(8.13)

If n = 1 and k = η, Equation 8.13 becomes Equation 8.11. The field of shear stresses in this operating unit is nonuniform. Expressions of shear stress and shear rate at the rim of the smaller plate are as follows: n 3 hF 2 R3

(8.14)

2n 1 ( h )R n h2

(8.15)

R

R

where h˙ = dh∕dt. Unlike in the static shear test, squeezing at constant force proceeds with decreasing shear stress, and for systems with yield behavior even if the initial shear stress is greater than the yield stress, sooner or later (depending on viscosity) the shear stress will approach the yield stress value. Th is approaching is accompanied by further slowing of the plate’s movement and there is a definite minimum distance at every given value of the applied force that corresponds to shear stress equal to the yield stress. As mentioned previously, Equations 8.11 through 8.15 were derived by neglecting the elasticity of the tested liquid. Several attempts to take into account the elastic properties of the material resulted in very sophisticated models [25]. Laun [26], based on the results of squeezing polyethylene (PE) melts, proposed an empirical criterion to eliminate experimental points with significant elasticity: t 40

(8.16)

where t is the time since the beginning of the experiment. Releasing the applied force leads to partial recovery of the sample’s thickness due to its elasticity. Such squeeze–recoil tests were discussed in detail in Ref. 27. The degree of thickness recovery depends mainly on the elasticity of the fluid (molecular weight, physical or chemical cross-linking, H-bond network, etc.), whereas the degree of squeezing is governed by the viscosity of the liquid, its dependence on shear stress, and yield stress.

8.5

Factors Influencing Shear Strength

The main factors influencing the shear strength are discussed in the following sections.

8.5.1

Assembly Time/Preparation Conditions

Typically, shear resistance improves gradually with increasing assembly time [14]. It may be connected with both kinetics of the adhesive joint formation and stress relaxation.

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8-12


8.5.2 Load Kano et al. [10] demonstrated that the holding time decreases significantly with increased dead load in the static shear test for poly(butyl acrylate) and its blends with poly(vinylidene fluoride-co-hexafluoro acetone). Similar results were obtained by Geiss and Brockmann [15] for a styrene–isoprene–styrene (SIS)-based hot-melt adhesive and a commercial high-performance adhesive tape. If a material possesses yield stress and the applied load does not exceed its level, then the creep is very limited [22].

8.5.3

Temperature

Because an increase in temperature usually leads to a drop in viscosity, it is understandable that temperature reduces shear resistance and decreases drastically the time to failure [14,15]. Kim [28] investigated the temperature dependence of the holding power in adhesive joints of an SIS-based hot-melt adhesive with different substrates: stainless steel, glass, Bakelite, polyvinyl chloride, PE, polypropylene, and Teflon. The holding power decreases with temperature for all adhesive joints though the slope of this dependence and absolute values of the holding power were different for each adhesive–substrate pair.

8.5.4

Molecular Weight and Structure

As demonstrated by Tobing and Klein [29], the shear adhesion of gel-free acrylic PSAs increases with molecular weight (MW), presumably due to the increased number of entanglements per molecule and, consequently, the growth of viscosity. Polymers with large side groups, such as non-cross-linked acrylates, have a high molecular weight between entanglements and therefore are characterized by rather high creep and low holding power [30]. The shear resistance of polyisobutylene improves greatly with molecular weight, but is accompanied by a drop in peel strength and detaching force upon probe tack testing. These properties can be compromised by using a polymer with a broad molecular weight distribution [31]. On the other hand, the durability of the joints undergoing adhesive fracture should depend only slightly on MW (if the MW is high enough), because the number of molecular contacts between an adhesive and a substrate, which determines the strength in this case, does not depend on MW [23].

8.5.5

Chemical Composition

Several works were devoted to the relationship between combinations of acrylic monomers in corresponding PSAs and their shear resistance. Kim et al. [14] studied the effect of the ratio of butyl acrylate (BA)/acrylic acid (AA) groups on the shear resistance of BA/ AA copolymers (see also Chapter 7). Increased AA concentration leads to increased holding time. This effect becomes more prominent at AA content greater than 10%. The possible explanation consists of an interaction between AA groups that increases cohesive

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Shear Resistance

8-13

strength. A similar effect was also observed by Demarteau and Loutz [32]. Indeed, it is well known from polymer synthesis that AA is used as a comonomer to increase the cohesion of various acrylic formulations. Park et al. [33] investigated the change in holding power with composition in quaternary copolymers of 2-ethylhexyl acrylate (2-EHA), BA, ethyl acrylate (EA), and vinyl acetate (VA). Increased BA and 2-EHA content leads to a significant decrease in holding time, probably because of the higher flexibility and lower intermolecular interaction introduced by these monomer sequences. Tobing and Klein [29] measured the shear resistance of poly(BA-stat-AA) and poly(2-EHA-stat-AA) copolymers. In all cases, poly(BA-stat-AA) demonstrates higher holding power than (2-EHA-stat-AA), regardless of the method of polymerization. This effect was attributed to a higher number of entanglements in poly(BA-stat-AA) per unit weight or volume. Gower and Shanks [34], however, determined that in BA/2-EHA/methyl methacrylate (MMA)/AA copolymers the holding power increases with 2-EHA concentration. It should be noted that in this case the shear properties were compared not at equal gel content, and the result can be attributed to the increased gel content in formulations with 2-EHA.

8.5.6

Influence of Additives and Miscibility

The influence of different additives on shear strength and correlation of the shear strength with the miscibility of such blends have been studied by several authors [10,11,14,28,35]. Fujita et al. [11] studied the effects of miscible and immiscible tackifiers on the holding power of natural rubber. The holding time of miscible systems tends to decrease with tackifier content. This trend correlates with the changes in plateau modulus. PSA tapes containing 50% or more miscible tackifier started to slip away from the adherent. In the case of immiscible systems, however, the holding time changes unpredictably with cohesive failure. According to Ref. 28, both the holding power and SAFT of the SIS-based PSAs decreased with tackifier level. According to Ref. [14], a completely different behavior is valid for acrylic adhesives modified with miscible and immiscible tackifiers. In this case, the holding time tends to increase with tackifier content, although in the case of miscible tackifiers this trend is much more prominent. Similar results were obtained by Naruse et al. [35] for a BA/AA copolymer modified by different tackifiers. Kano et al. [10] improved the holding power of poly(butyl acrylate) by blending it with poly(vinylidene fluoride-co-hexafluoro acetone). The increased holding time correlates with the increased viscosity of such blends. Unfortunately, data on the miscibility and rheology of studied systems supplied by the authors are insufficient to come to a definite conclusion about the influence of miscibility on shear strength, although formulation practice usually shows a decisive decrease in cohesion-related adhesive properties, especially of the holding time by tackification with compatible resins; see also Technology of Pressure-Sensitive Adhesives and Products, Chapter 8. Th is negative effect increases with tackifier level.

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8-14


8.5.7 Method of Polymerization Tobing and Klein [29] compared the shear resistance of PSAs based on poly(2-EHAstat-AA) obtained via polymerization in solution and in emulsion. In accordance with the industrial practice, solution-borne PSAs demonstrate much higher (one to two orders of magnitude) holding power than emulsion PSAs. It is believed that microgels that formed during emulsion polymerization and their morphology could be retained after fi lm formation. Such discrete morphology could give much lower shear strength than the continuous network morphology that formed from solution (see above). It should be noted that the polymerization technology can include pre- or postpolymerization tackification, which also leads to different adhesion and cohesion properties (see also Technology of Pressure-Sensitive Adhesives and Products, Chapter 1).

8.5.8

Cross-Linking Density

The influence of ultraviolet (UV) dose and cross-linking density on the SAFT of acrylic copolymers of 2-EHA, VA, AA, and 2-hydroxyethyl methacrylate (2-HEMA) was investigated in Ref. [17]. Cross-linking of these copolymers by UV irradiation increased their SAFT from 30 to 150°C. Higher photoinitiator and 2-HEMA contents led to higher SAFT values at lower UV dose (see also Technology of Pressure-Sensitive Adhesives and Products, Chapter 10). Acrylic polymers demonstrated a dramatic decrease in holding power as the gel content decreased [29,34].

8.5.9

Physical Cross-Linking

Hydrogen bonding, ionic bonding, or acid–base exchange (in the general case, donor– acceptor interaction) have an effect similar to the decrease in molecular weight between entanglements, thereby raising the holding power of the PSA. Physical cross-linking has the following advantages: • • • •

No need for curing equipment, curing agents, etc. Thickness of the adhesive does not restrict cross-linking efficiency Cross-linking can be thermally reversible. Tackifiers and other components can be added to adjust properties of the PSA without interfering with the cross-linking mechanism [36].

A complex of high-MW polyvinylpyrrolidone (PVP) and short-chain polyethylene glycol (PEG) can be regarded as an example of physical cross-linking [37]. Th is complex has a “carcass-like” structure, which is characterized by the interaction of functional groups of high-MW polymer (carbonyls of PVP) with complementary groups of low-MW polymer (terminal hydroxyls of PEG) (see also Chapter 10 and Technology of Pressure-Sensitive Adhesives and Products, Chapter 7). Another type of interpolymer complex is formed due to the interaction between complementary groups of two long macromolecules. This type of structure can be called ladder-like. Such a complex is formed as a result of the interaction between a polyacid and polybase. An acid–base interaction was observed for blends of the copolymer of

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Shear Resistance

8-15

dimethylaminoethyl methacrylate (DMAEMA) with MMA and butyl methacrylate (polybase) and the copolymer of methacrylic acid and EA (polyacid) [37]. Everaerts et al. [36] report a significant improvement in the holding power of the isooctyl acrylate (IOA)/AA copolymer by blending it with IOA/DMAEMA. The reason for the improved shear resistance is the intermolecular acid–base interaction between AA and DMAEMA groups and, consequently, an increase in cohesive strength.

8.6

Improving the Shear Behavior of Pressure-Sensitive Adhesives

Improving PSAs’ shear behavior means, first, an increase in their cohesive strength. Therefore, there is a permanent risk of sacrificing tack and peel. Thus, the problem consists of achieving a reasonable compromise between the properties. For cross-linking polymers, the improvement of shear resistance is connected with parameters of cross-linking: as already mentioned, a higher degree of physical and chemical cross-linking increases SAFT and holding time. The choice of monomers is very important for acrylics: monomers with shorter side chains enable higher cohesive strength. The most evident way of enhancing the cohesive strength of melting polymers is to increase their MW. This method increases the viscosity of the PSA and does improve shear resistance, but higher viscosity may make processing difficult. Broad MW distribution can contribute to preserving good peel and tack, while increasing shear resistance [31]. Reinforcement of PSAs with fillers is a valuable method to improve shear resistance, because it can increase both PSA viscosity at application and yield stress. Geiss and Brockmann [15] introduced glass beads to a SIS-based PSA. Samples loaded with the fi ller demonstrated lower creep and higher holding time compared with unfi lled PSAs. This is a typical method in industrial practice to formulate and manufacture carrierless tapes; see also Technology of Pressure-Sensitive Adhesives and Products, Chapter 8. The filler geometry is very important in such applications. Fillers with larger surface (fibers, flakes) are preferable to spherical particles due to higher reinforcing effect at equal loading. Kulichikhin et al. [22] successfully used clay nanoparticles to reinforce the mechanical strength and improve the creep behavior of SIS-based PSAs. An additional advantage of this approach consists of higher water uptake and the transportation of absorbed moisture from the skin (footcare products) to the depths of the adhesive dressing. Introducing fi llers into PSA while improving cohesive strength under application conditions does not necessarily result in increased viscosity at processing temperature and, consequently, complicated processing. Sometimes [38] orientation effects may lead to a drop in viscosity (fi ller geometry is decisive in such applications; therefore, fiber-like fi llers are commonly used in industrial practice). Physical cross-linking by involvement of the PSA components into intermolecular polycomplexes is a new and very attractive method of improving the cohesive strength of noncross-linked PSAs (see Technology of Pressure-Sensitive Adhesives and Products, Chapters 7 and 8). It can combine high shear resistance with hot-melt processibility, often without compromising other properties [36]. To use this method one should select

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8-16


a pair of polymers with complementary functional groups, so groups of one polymer can interact with the groups of the other. For instance, H-bonding can be realized in pairs: hydroxyl–hydroxyl, hydroxyl–carboxyl groups, etc. The interaction between a carboxylic acidic group and a basic amino group is an example of acid–base interaction [37]. Depending on cross-linker functionality [36], it is possible to obtain a thermosetting or thermoreversible complex: primary and secondary amines as cross-linkers yield a thermosetting PSA, whereas tertiary amines produce a thermoreversible network.

References 1. Lim, D.H., H.J. Kim. 2006. General performance of pressure-sensitive adhesives. In Pressure-Sensitive Design, Theoretical Aspects. Volume 1, ed. I. Benedek, Leiden– Boston, VSP, pp. 291–317. 2. Eveloy, V., P. Rodgers, M.G. Pecht. 2004. Reliability of pressure-sensitive adhesive tapes for heat sink attachment in air-cooled electronic assemblies. IEEE Trans. Device Mater. Reliability 4:650–657. 3. ASTM D3654/D3654M-06. Standard Test Methods for Shear Adhesion of PressureSensitive Tapes. 4. PSTC-7. Holding Power of Pressure-Sensitive Tape (revised ed.), 1985. 5. FINAT Test Method 8 (FTM8). 1985. Resistance to shear from a standard surface, 1985. 6. AFERA Test Method 4012. Measurement of shear adhesion, 1979. 7. Brockmann, W., P.L. Geiß. 1997. Creep Performance of Mounting Tapes Based on Hot Melt Pressure Sensitive Adhesives. Proc. 1997 TAPPI Hot Melt Symposium, TAPPI Press, Atlanta, pp. 153–158. 8. Zosel, A. 1994. Shear strength of pressure sensitive adhesives and its correlation to mechanical properties. J. Adhesion 44:1–16. 9. Miyagi, Z., K. Yamamoto. 1987. Viscoelastic analysis of shear adhesion test for pressure-sensitive adhesive tape. J. Adhesion 21:243–250. 10. Kano, Y., S. Akiyama, Z. Miyagi. 1998. Analysis of holding power in the blends of poly(butyl acrylate) with poly(vinylidene fluoride-co-hexfluoro acetone). J. Appl. Polym. Sci. 68:727–738. 11. Fujita, M., A. Takemura, H. Ono, M. Kajiyama, S. Hayashi, H. Mizumachi. 2000. Effects of miscibility and viscoelasticity on shear creep resistance of naturalrubber-based pressure sensitive adhesives. J. Appl. Polym. Sci. 75:1535–1545. 12. Malkin, A.Ya., A.A. Askadsky, V.V. Kovriga, A.E. Chalykh. 1983. Experimental Methods of Polymer Physics. Mir Publishers, Moscow. 13. Dealy, J.M. 1982. Rheometers for Molten Plastics. A Practical Guide to Testing and Property Measurement. Van Nostrand Reinhold Company. 14. Kim, H.-J., H. Mizumachi. 1995. Miscibility and shear creep resistance of acrylic pressure-sensitive adhesives: acrylic copolymer and tackifier resin systems. J. Appl. Polym. Sci. 58:1891–1899. 15. Geiss, P.L., W. Brockmann. 1997. Creep resistance of pressure sensitive mounting tapes. J. Adhesion 63:253–263.

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Shear Resistance

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16. ASTM D4498-00. Standard Test Method for Heat-Fail Temperature in Shear of Hot Melt Adhesives. 17. Do, H.-S., Y.-J. Park, H.-J. Kim. 2006. Preparation and adhesion performance of UV-crosslinkable acrylic pressure sensitive adhesives. J. Adhesion Sci. Technol. 20:1529–1545. 18. Satas, D. 1989. Handbook of Pressure Sensitive Adhesive Technology. Van Nostrand Reinhold, New York. 19. Burgers, J.M. 1935. Mechanical considerations—model systems—phenomenological theories of relaxation and of viscosity. In: First Report on Viscosity and Plasticity, ed. J.M. Burgers, New York, Nordemann Publishing Company. 20. Schramm, G. 1994. A Practical Approach to Rheology and Rheometry. HAAKE GmbH, Karlsruhe, Germany. 21. Barnes, H.A. 1999. The yield stress–a review or ‘παντα ρει’–everything flows? J. Non-Newtonian Fluid Mech. 81:133–178. 22. Kulichikhin, V., S. Antonov, V. Makarova, A. Semakov, A. Tereshin, P. Singh. 2006. Novel hydrocolloid formulations based on nanocomposites concept. In PressureSensitive Design, Theoretical Aspects. Volume 1, ed. I. Benedek, Leiden–Boston, VSP, pp. 351–401. 23. Vinogradov, G.V., A.I. Elkin, S.E. Sosin. 1978. Fracture of uncured linear flexiblechain polymers of narrow molecular mass distribution in triaxial stress (behaviour of elastomers as adhesives). Polymer 19:1458–1464. 24. Oka, S. 1960. The principles of rheometry. In Rheology. Theory and Applications. Volume 3, ed. F.R. Eirich, Academic Press, New York, pp. 17–82. 25. Leider, P.J. 1974. Squeezing flow between parallel discs II. Ind. Eng. Chem. Fundam. 13:342–349. 26. Laun, H.M. 1992. Rheometers towards complex flows: squeeze flow technique. Macromol. Chem., Macromol. Symp. 56:55. 27. Feldstein, M.M., V.G. Kulichikhin, S.V. Kotomin, T.A. Borodulina, M.B. Novikov, A. Roos, C. Creton. 2006. Rheology of poly(N-vinyl pyrrolidone)-poly(ethylene glycol) adhesive blends under shear flow. J. Appl. Polym. Sci. 100:522–537. 28. Kim, D.-J. 1995. Ph. D. Thesis. Performance of SIS-based hot-melt pressure sensitive adhesives. Seoul National University. 29. Tobing, S.D., A. Klein. 2001. Molecular parameters and their relation to the adhesive performance of acrylic pressure-sensitive adhesives. J. Appl. Polym. Sci. 79:2230–2244. 30. Benedek, I. 2006. Principles of pressure-sensitive design and formulation. In Pressure-Sensitive Design, Theoretical Aspects. Volume 1, ed. I. Benedek, Leiden– Boston, VSP, pp. 131–289. 31. Krenceski, M.A., J.F. Johnson. 2004. Shear, tack and peel of polyisobutylene: effect of molecular weight and molecular weight distribution. Polym. Eng. Sci. 29:36–43. 32. Demarteau, W., J.M. Loutz. 1996. Rheology of acrylic dispersions for pressure sensitive adhesives. Progr. Organ. Coatings 27:33–44. 33. Park, W.-H., S.W. Kim, S.C. Shim, I.R. Jeon, K.H. Seo. 2003. Properties of pressure sensitive adhesive made of acrylic quaternary copolymers and their blends with vinyl chloride copolymers. Mat. Res. Innovat. 7:172–177.

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34. Gower, M.D., R.A. Shanks. 2004. The effect of varied monomer composition on adhesive performance and peeling master curves for acrylic pressure-sensitive adhesives. J. Appl. Polym. Sci. 93:2909–2917. 35. Naruse, S., H.-J. Kim, T. Tsukatani, M. Kajiyama, A. Takemura, H. Mizumachi. 1994. Miscibility and PSA performance of acrylic copolymer and tackifier resin systems. J. Adhesion 47:165–177. 36. Everaerts, A., K. Zieminski, L. Nguyen, J. Malmer. 2006. Cross-linking of hotmelt-processible acrylic pressure-sensitive adhesives using acid/base interaction. J. Adhesion 82:375–387. 37. Feldstein, M.M., G.W. Cleary, P. Singh. 2006. Pressure-sensitive adhesives of controlled water-absorbing capacity. In Pressure-Sensitive Design and Formulation, Application. Volume 2, ed. I. Benedek, Leiden–Boston, VSP, pp. 181–230. 38. Kulichikhin, V.G., L.A. Tsamalashvili, E.P. Plotnikova, A.A. Barannikov, M.L. Kerber, H. Fischer. 2003. Rheological properties of liquid precursors of polvnronvlene-clay nanocomnosites. Polym. Sci. (Russia). 45:564–572.

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9 Durability of Viscoelastic Adhesive Joints Sergey V. Kotomin A.V. Topchiev Institute of Petrochemical Synthesis

9.1

9.1 Introduction .............................................................9-1 9.2 Triaxial Stress Tests ................................................ 9-2 9.3 Durability and Adhesion ..................................... 9-13 9.4 Conclusions ............................................................9-17 References ........................................................................9-17

Introduction

Adhesive strength is one of the main characteristics of pressure-sensitive adhesives (PSAs), but how long the adhesive joint can provide contact to the substrate over time and under various external conditions (debonding force, temperature, and relative humidity) is also important. Several methods exist to test the adhesive strength of PSAs, including tack, peel, and shear tests (see Chapters 6 through 8 and Applications of Pressure-Sensitive Products, Chapter 8). These are carried out under standard conditions in accordance with ASTM D1876-01 (Method for Peel Resistance of Adhesives (T-Peel Test) and probe tack test (according to ASTM D2979-01) or using other methods. For long-term durability of PSAs, there is only one standard test—the method for determining the durability of adhesive joints stressed in shear loading (ASTM D2919-01 or PSTC-7). In most publications, discussions of durability properties include the analysis of behavior under various environmental conditions [1,2] (see also Applications of Pressure-Sensitive Products, Chapter 8) or fatigue tests in cyclic mode [3]. The static shear experiment is similar to a creep measurement with a constant shear stress [4–6] (see also Applications of Pressure-Sensitive Products, Chapter 8). In the shear resistance test the contact area is not constant; thus, at a constant load force the shear stress gradually increases in time and there is no control of thickness and deformation of the adhesive layer in the normal direction. These factors can cause large scatter and discrepancies in test results (see Chapter 8). Any slight inclination in the 9-1

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direction of the tear force vector would cause a normal force component, which may influence fracture development for PSAs. The adhesive elastic fi lm under extension in the normal direction is in a triaxial stress state (or under negative hydrostatic pressure) [7]. The phenomenon of internal rupture of tensioned cylindrical rubber samples starting with the formation of microcavities was described 50 years ago by Gent and Lindley [8]. Twenty years previously, Yerzley (1939) first described “yield-point” in the load–extension relation of similar test pieces and attributed it to internal rupture [9]. The recent works of Creton and colleagues [10–12] analyzed the mechanism of cavity formation during fracture of a PSA in a probe tack test. The works of Yamaguchi and co-workers [13,14] are examples of a model approach for the analysis of the cavitation process during the same test using some assumptions.

9.2

Triaxial Stress Tests

Although the durability of viscoelastic adhesive joints in the triaxial stress state definitely has practical importance, to the best of our knowledge only a few works directly focused on this matter [15–17]. First, one must note a fundamental work devoted to elastic adhesive durability performed by Vinogradov et al. 30 years ago [15]. The study of long-term durability of adhesive joints under the action of constant debonding force on polybutadiene (PB) as a PSA model materials yields fundamental information about the structure and properties of adhesives. A description of the experimental device used in this work is presented in Figure 9.1. A support, B (Swedish precision block), is installed in a thermocontrolled unit, A. The adhesive fi lms, C, are placed on the ring edge of a cylinder, D. The ring, D, is fi xed inside the duct, E, of the guiding block, F, and loaded in the axis direction. The material

∆

H

I G F E J D C B A

FIGURE 9.1

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Schematic design of Vinogradov’s adhesiometer.

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Durability of Viscoelastic Adhesive Joints

9-3

of the duct, E, is low-frictional polytetrafluoroethylene (PTFE). The cylindrical ring, D, is hinged, via a rigid tie, G, to the right arm of the balanced lever, H. Weights, I, are suspended from the left arm of the lever, H. A hydraulic damper, J, suppresses the elastic vibrations of the polymer adhesive. Contact between the adhesive and the solid surface was formed at a temperature of 60°C for 30 min under a pressure of 0.5 MPa. The long-term durability t* of the PSA joints in the case of elastic adhesives is related to the stress, σ, by following Bartenev’s equation (Equation 9.1; Ref. 15), t * B()m

(9.1)

or by an extended equation including the influence of temperature and molecular weight, t * B1()m exp

U M kT

(9.2)

where U is the activation energy of the rupture process and B1, m, and α are the constants characterizing the material and type of fracture. The physical meaning of coefficients B, B1, and m is explained in Ref. 18; for epoxy resins at T > Tg constant m = U0/3kTg [19]. Values of the coefficient B and the exponent m according to Ref. 20 are very sensitive to fracture type and decrease by several orders of magnitude as the fracture type changes from cohesive to adhesive. Equation 9.1 noticeably differs from Zhurkov’s classic equation for the durability of solid materials [18], t * 0 exp

U 0 kT

(9.3)

where τ0 is the thermal oscillation time of atoms (for polymers τ0 ∼ 10−12 to 10−13 s); U0 is the activation energy of self-induced rupture of polymer chains at σ = 0. The difference between Equations 9.1 and 9.3 might be explained by the standpoint of fracture mechanisms for brittle solids and elastic materials. In the first case the standpoint is a surface crack propagation; in the second case it is an internal cavity nucleation inside the material. Figure 9.2 illustrates that the t* versus σ* curves for PB in contact with a steel substrate exhibit two regions of durability values with different magnitudes of the indicated constants. These regions represent the different modes of fracture of PB fi lms. In the region of lower values of B and –m (indicated by fi lled symbols in Figure 9.2), cohesive fracture was observed. For cohesive fracture, similar to uniaxial extension, the long time durability reduces with decreasing Mw of the adhesive. Miscellaneous cohesive–adhesive fracture was observed at higher stresses (see symbols ∆ and ◊ in Figure 9.2). In this case, only a few areas of the contact surfaces were covered with the polymer. At stresses above 0.9 MPa the fracture becomes adhesive for two specimens of PB in contact with steel, but tear-off occurs very fast and it is difficult to measure the time

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6 4

4 3 log t ∗ (C)

2 2 1

0

−2 −1.2

−0.8

−0.4

0.0

log σ∗ (MPa)

FIGURE 9.2 Durability t* versus stress σ for PB in contact with PTFE (curves 1 and 2) and steel (curves 3 and 4) at 20°C. For 1 and 2 the unfi lled symbols relate to adhesive failure; for 3 and 4, the black symbols denote cohesive fracture; whereas unfi lled symbols indicate a miscellaneous cohesive–adhesive type of fracture. Mw = 1.5 × 105 g/mol (1, 3) and 6.4 × 105 g/mol (2, 4).

TABLE 9.1 Experimental Parameters B and m in Equation 9.1 at Various Temperatures for PB in Contact with Steel and PTFE (stresses range between 5 × 10−2 and 1 MPa) Coefficient B and m at Different Fracture Modes Cohesive Temperature (°C)

B

Cohesive–Adhesive –m

B

–m

3.2 × 10−1 7.9 × 10−3 5.0 × 10−5 6.3 × 10−4

5.9 15.5 14.7 25.1

4.4 × 10−8

10.0

5 6 20 20* 35 50 60 80

4.0 × 102 5.0 × 101 1.6 × 102 1.0 × 102

4.3 4.0 4.3 4.1

4.0 × 101

4.1

Adhesive B

–m

1.6 × 10−2

3.69

7.2 × 10−11 2.0 × 10−9

11.7 10.2

Mw = 6.4 × 105, except *Mw = 1.5 × 105 g/mol.

to fracture (6.6 × 103 Pa) the plate separation led to compete PB fracture, which is in accordance with the results obtained by Vinogradov et al. [15]. The experimental data proved the validity of Equations 9.1 and 9.2 for the durability of the PB joint for and for the PSA PVP(64%)–PEG, as illustrated in Figure 9.13. The calculated values of the coefficients in Equation 9.1 and activation energy in Equation 9.2 at the indicated temperature and stresses values are presented in Table 9.2. As follows from these data, the activation energy of cohesive fracture for adhesive PVP–PEG hydrogel is much higher than that for PB.

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9-12


FIGURE 9.11 Fibrillization of PVP (64%)–PEG PSA under separation of tester plates.

FIGURE 9.12 Neck formation under plate separation for PVP (50%)–PEG adhesive.

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9-13

Durability of Viscoelastic Adhesive Joints log σ (Pa) 5.8

6.0

6.2

6.4

6.6

4

1/T

log t ∗ (s)

3

log σ 2

1

0 2.8

3.0

3.2

3.4

3.6

1000/T (K−1)

FIGURE 9.13 Durability of PVP–PEG adhesive joints versus debonding stress at fi xed temperature and versus inverse temperature at fi xed tensile stress. (Redrawn from Kotomin, S.V., Borodulina, T.A., Feldstein, M.M., and Kulichikhin, V.G., in Proceedings of the XIIIth International Congress on Rheology, Cambridge, U.K., 4, 44–46, 2000.) TABLE 9.2

Activation Energy for Fracture of PB and PVP–PEG Adhesives

Type of Adhesive PB PB PVP (64%)–PEG

Normal Stress (σ × 103 Pa) 6.6 13.2 33.0

Activation Energy of Fracture, U (kJ/mol K−1) 18.1 9.5 70.1

In the experiments using PVP–PEG hydrogel, a complete fracture of the adhesive joint was not observed, because the elongation of the adhesive during plate separation was limited by the maximum gap value of the instrument cell. The studied adhesives, PVP (67%)–PEG, PVP (50%)–PEG, and PB, may be defined consequently as the strain-hardening, strain-thinning viscoelastic materials and Newtonian liquid, according to the description of the behavior of other PSAs [10].

9.3 Durability and Adhesion In recent work [30] the durability of adhesive joints made from a range of commercially available PSAs was studied. Characterized adhesives are as follow: 1. Gelva 3011—water-based emulsion of cross-linked acrylate copolymer 2. Duro–Tack 87-900A—solution of un-cross-linked acrylic copolymer in EA

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3. PIB—blend of three polymers with different molecular weights (Oppanol B-100, Oppanol B-12, and Indopol H-1900, BASF; see Table 4.2 in Technology of PressureSensitive Adhesives and Products, Chapter 4). Adhesive fi lms were prepared by casting the solutions followed by drying. The durability of adhesive joints was studied using the setup illustrated in Figure 9.14. The set of weights renders the control of the compression or extension of the sample located between flat butt-end surfaces of two steel rods. Varying the weight allows to squeeze the adhesive fi lm between the rods and then to detach them. To compare durability for different PSAs, one must use similar test conditions, such as the load and time of loading. The higher the load, the thinner the adhesive layer and the higher the long-time durability. The dependence of durability on loading time at a pressure of 350 KPa is illustrated in Figure 9.15. Long-time durability for every type of PSA increases gradually and Adhesive film between rods

Weights

FIGURE 9.14 Setup for durability test.

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9-15


500 5

t d (s)

4

250

3 2 1

10

20

30

t l (min)

FIGURE 9.15 Detachment time, t*, vs loading time, t l, for PSAs: (1) acrylic Gelva 3011, (2) PIB, (3, 4) PB, and (5) acrylic DuroTak-87-900A at (1–3, 5) 350 KPa and (4) 490 KPa. (Redrawn from Sadykova, I., B.Sci. thesis: Long-time durability of acrylic PSAs, Moscow, State Academy of Fine Chemical Technology (MITHT), 2007.)

3.5 4 log t (s)

3.0 2 2.5

3

1

2.0 5.62

5.64 5.66 log σ (Pa)

5.68

FIGURE 9.16 Durability versus stress for (1) Gelva, (2) PB, (3) PIB, and (4) DuroTak PSAs.

reaches a certain limit with time. The loading time was 20 min at a normal stress value of 350 KPa and at the same (but negative) extension stress. The dependence of PSA durability on extension stress is illustrated in Figure 9.16 and Table 9.3. The calculated values of coefficients B and m are presented in Table 9.3, along with the data obtained for the PVP–PEG hydrogel, which were taken from Figure 9.11.

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TABLE 9.3 Parameters B and m in Equations 9.1 and 9.2 Calculated from Data Presented in Figure 9.16 along with the Values of Practical Work of Adhesion (W) Derived from the Probe Tack Test Temperature (°С)

Adhesive Gelva DUROTAK 87-900A PIB PB PVP (64%)–PEG

Film Thickness (mm)

−m

B

Maximum Stress (kPa)

Work of Adhesion, W (J/m2)

30

0.13

46.5

7.8

140

1330

26

0.09

68.3

11.5

1020

300

20 29 25

0.13 0.11 0.10

56.8 80.0 20.0

9.6 13.7 0.43

280 300 1200

240 7.5 100

Source: Feldstein, M., Creton, C. 2006. Pressure Sensitive Adhesion as a Material Property and as a Process in Pressure-Sensitive Design, Theoretical Aspects. VSP, Leiden, pp. 22–67 and Sadykova, I. 2007. B.Sci. thesis: Long-time durability of acrylic PSAs, Moscow, State Academy of Fine Chemical Technology (MITHT). 1.2 1.0 0.8 4 0.6

Stress (MPa)

0.8 4

0.4 3

0.2 0.0

1

2

0

0.4

5

10

15

20

3 2 1 0.0 0.01

0.1

1

10

Distance (mm)

FIGURE 9.17 Probe tack curves (time–semilog scale): (1) PB, (2) Gelva, (3) PIB, (4) DuroTak [30]. In the upper right corner of the image the same figure is presented in linear time scale. The bonding force is 10 KN, the contact time is 1 s, and the probe detachment rate is 0.1 mm/s.

Table 9.3 also contains the results of the probe tack test derived from Figure 9.17 [29,30]. In Figure 9.17 the probe tack curves are presented as stress versus probe displacement, both in the linear and in the logarithmic scale, to discern the curves relating to the deformation of various adhesives. The constant rate of debonding was 0.1 mm/s. Table 9.3 indicates that the highest value of the practical work of debonding W is obtained for Gelva adhesive, but for the same PSA, as seen in Figure 9.16, the value of

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9-17

durability t*, as well as the apparent tensile stress, is the lowest. Th is contradiction can be understood if we take into consideration the low value of coefficient m, which provide for Gelva a good durability at higher stresses. The PVP–PEG PSAs have the lowest value of m, which means the best durability at high stresses. These results also mean that the relationship among the durability of various PSA may vary, depending on the value of apparent tensile stress.

9.4 Conclusions The dependence of long-term durability of thin viscoelastic polymer fi lms on temperature and stress is described by equations similar to those derived for cured rubbers, indicating the similarity between the effect of entanglement networks in noncured elastomers and that of a network of chemical bonds in vulcanized rubbers. The elasticity and the viscosity of the polymer play a decisive role in the fracture mechanism of thin adhesive fi lms. The study of adhesion characteristics of noncured polymers demonstrated that in the case of adhesive and miscellaneous cohesive–adhesive fractures, long-term durability is very sensitive to temperature. This feature is associated with the specific temperature dependence value of total deformation and its recoverable and plastic components for noncured polymers in the region of transition from the fluid to the rubber-like state. The behavior of viscoelastic adhesives drastically depends on the rate of deformation. In the case of constant stress, different types of fracture—cohesive, miscellaneous cohesive–adhesive, and adhesive—may be observed, depending on the stress value. The results of durability experiments carried out under triaxial stress proved the applicability of the simple phenomenological equations for the prediction of PSA behavior under various stresses and temperatures. Characteristic coefficients in Equation 9.1 for various PSAs can be determined both for adhesive fracture and for cohesive debonding. Using these coefficients, one can forecast the behavior and long-term durability of PSA at any stress and temperature. At the same time, the comparison of peel and probe tack adhesion with the value of long-term durability requires further study.

References 1. Kinloch A.J. 2002. The durability of adhesive joints. In Adhesion Science and Engineering, vol. 1. eds. D.A. Dillard, A.V. Pocius, Elsevier, Amsterdam, pp. 661–98. 2. Temiz, S., Ozel, A., Aydin, M.D. 2004. A study on durability of joints bonded with pressure-sensitive adhesives. J. Adhesion Sci. Technol. 10:1187–1198. 3. Sohn, S. 2003. A new method based on application of cyclic strain to evaluate the durability of pressure sensitive adhesives. J. Adhesion Sci. Technol. 17:1039–1053. 4. Lim, D.H., Kim, H.J. 2006. General performance of pressure-sensitive adhesives. In Pressure-Sensitive Design, Theoretical Aspects, ed. I. Benedek, Boston, VSP, Leiden, pp. 310–311. 5. PSTC-7. 6. ASTM-D3654/D 3654M-027.

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9-18


7. Lindsey, G.M. 1967. Triaxial fracture studies. J. Appl. Phys. 38:4843–4852. 8. Gent, A.N., Lindley, 1958. Internal rupture of bonded rubber cylinders in tension, Proc. R. Soc. London, Ser. A: Math. Phys. Sci., A 249:195–205. 9. Yerzley, F.L. 1939. Adhesion of neoprene to metal Ind. Eng. Chem. 31:950–956. 10. Creton, C., Lakrout, H. 2000. Micromechanics of flat probe adhesion tests of soft viscoelastic polymer fi lms, J. Polym. Sci.: Part B: Polym. Phys. 38:965–979. 11. Creton, C., Hooker, J.C., Shull, K.R. 2001. Bulk and interfacial contributions to the debonding mechanisms of soft adhesives: extension to large strains. Langmuir 17: 4948–4954. 12. Creton, C. 2005. Mécanismes de déformation, d’endommagement et de rupture de joints collés. Mécanique & Industries 6:37–43. 13. Yamaguchy, T., Doi, M. 2006. Debonding dynamics of pressure-sensitive adhesives: 3D block model. Eur. Phys. J. E 21:331–339. 14. Yamaguchi, T., Morita, H., Doi, M. 2006. Modeling on debonding dynamics of pressure-sensitive adhesives. Eur. Phys. J. E 20:7–17. 15. Vinogradov, V.G., Elkin, A.I., Sosin, S.E. 1978. Fracture of uncured linear flexible-chain polymers of narrow molecular mass distribution in triaxial stress (behavior of elastomers as adhesives). Polymer 19:1458–1464. 16. Kotomin, S.V., Borodulina, T.A., Feldstein, M.M., Kulichikhin, V.G. 2000. Durability and fracture of some viscoelastic adhesives. Proceed. XIIIth Intern. Congress on Rheology, Cambridge, UK, 4:44–46. 17. Kotomin, S.V., Borodulina, T.A., Feldstein, M.M., Kulichikhin, V.G. 2000. Durability and rheology of viscoelastic adhesives. Proceed. 23rd Annu. Meeting Adhesion Soc. pp. 413–415. Myrtle Beach, SC. 18. Vettegren, V.I., Kulik, V.B., Bronnikov, S.V. 2005. Temperature dependence of the tensile strength of polymers and metals at elevated temperatures Tech. Phys. Lett. 31:969–972. 19. Vettegren, V.I., Kulik, V.B., Bashkarev, A.Ya., Lebedev, A.A., Sytov, V.A. 2004. The temperature dependence of the strength of adhesion between steels and epoxyrubber glues and polyamides in a rubberlike state. Tech. Phys. Lett. 30:862–864. 20. Vinogradov, G.V., Malkin, A.Ya., Yanovsky, Yu.G., Borisenkova, E.K., Yarlykov, B.V., Berezhnaya, G.V. 1972. Viscoelastic properties of linear polymers in the fluid state and their transition to the high-elastic state. Polym. Sci., Polym. Phys. Ser. B 10:1061–1076. 21. Good, R.J. 1976. Definition of adhesion, J. Adhesion 8:1–9. 22. Bartenev, G.M. 1984. Strength and Mechanism of Polymers Fracture. Moscow, Khimia (in Russian). 23. Vinogradov, G.V. 1975. Viscoelasticity and fracture phenomenon in uniaxial extension of high-molecular linear polymers. Rheol. Acta 14:942–954. 24. Vinogradov, G.V., Malkin, A.Ya., Volosevitch, V.V., Shatalov, V.P., Yudin, V.P. 1975. Flow, high-elastic recoverable deformations and rupture of uncured high molecular weight linear polymers in uniaxial extension J. Polym. Sci. Polym. Phys. Ed. 13:1721–1735.

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9-19

25. Kotomin, S.V., Borodulina, T.A., Feldstein, M.M., Kulichikhin, V.G. 1999. Squeezerecoil analysis of adhesive hydrohels and elastomer. Polym. Mater. Sci. Eng. 81:425–428. 26. Feldstein, M.M., Lebedeva, T.L., Shandryuk, G.A., Igonin, V.E., Avdeev, N.N., Kulichikhin, V.G. 1999. Polym. Sci., Ser. A 41:867–882. 27. Chalykh, A.E., Chalykh, A.A., Feldstein, M.M. 1999. Effects of composition and hydration on adhesive properties of poly(N-vinyl pyrrolidone)—poly(ethylene glycol) hydrogels. Polym. Mater. Sci. Eng. 81:456–466. 28. Novikov, M.B., Borodulina, T.A., Kotomin, S.V., Kulichikhin, V.G., Feldstein, M.M. 2005. Relaxation properties of pressure-sensitive adhesives upon withdrawal of bonding pressure. J. Adhesion 81:77–107. 29. Feldstein, M., Creton, C. 2006. Pressure-Sensitive Adhesion as Material Property and as a Process, in Pressure-Sensitive Design, Theoretical Aspects. Ed. Benedek I., VSP, Leiden-Boston, pp. 22–67. 30. Sadykova, I. 2007. B.Sci. thesis: Long-time durability of acrylic PSAs, Moscow, State Academy of Fine Chemical Technology (MITHT).

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10 Molecular Nature of Pressure-Sensitive Adhesion 10.1 Toward a Universal Theory of PressureSensitive Adhesion .............................................. 10-2 10.2 Model System to Elicit Supramolecular Structures Responsible for PressureSensitive Adhesion .............................................. 10-4 10.3 Structure–Property Relationships for Model PVP–PEG Hydrophilic Pressure-Sensitive Adhesives ............................ 10-6 Deformation Mechanisms of Model PVP–PEG and Other Adhesives in the Course of Debonding • Tensile Properties of Model PVP–PEG Adhesive Blends • Relationship between Peel Adhesion and the Work of Viscoelastic Deformation up to the Break of the Adhesive Joint • Universal Character of the Viscoelasticity Theory of Pressure-Sensitive Adhesion

10.4 Factors Underlying Pressure-Sensitive Adhesion on a Molecular Scale ....................... 10-14 10.5 Why Do Pressure-Sensitive Adhesives Belong to the Class of Viscoelastic Materials? ............................................................ 10-15 10.6 Fundamental Quantities Outlining the Place of Pressure-Sensitive Adhesives among Other Viscoelastic Polymers .............. 10-16 Correlation of Adhesion with Free Volume • Diff usion Coefficients of Pressure-Sensitive Adhesives • Free Energy for Self-Diffusion and That for Debonding of PVP–PEG Model PressureSensitive Adhesives • Relaxation Times Featured for Pressure-Sensitive Adhesives • Glass Transition Temperatures Responsible for Pressure-Sensitive Adhesion • Correlation of Adhesion with Characteristics of Viscoelasticity • Inconsistency between Small and Large Strain

10-1

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10-2


Mikhail M. Feldstein A.V. Topchiev Institute of Petrochemical Synthesis

10.1

Behaviors: New Consideration of Dahlquist’s Criterion of Tack

10.7 Conclusions ......................................................... 10-37 References ..................................................................... 10-38

Toward a Universal Theory of Pressure-Sensitive Adhesion

For the rational design of new pressure-sensitive adhesives (PSAs), insight into the molecular structures of materials manifesting pressure-sensitive adhesion and the knowledge of quantitative structure–property relationship (QSPR) are keenly needed. The design of adhesives remains mostly empirical because the QSPR are not well known. PSAs exhibit different chemical compositions1,2 and, at first glance, this makes the problem of eliciting the specific features of their molecular structure that provide pressure-sensitive adhesion irresolvable. However, let us recall that PSAs have allied physical properties:3 specified values of glass transition temperatures (Tg),3,4 elasticity modulus (G′),5–7 loss tangent (tan δ),8 solubility parameters,9 and other physical properties. Because a material’s properties influence how its molecular structure functions, similarity in the properties of PSAs signifies closely related structures of PSA materials of various chemical compositions at a supramolecular level. The subject of this chapter is to move toward the emergence of a structure that makes materials tacky and provides the properties of typical PSAs. Adhesion is traditionally defined as the phenomenon in which surfaces of contacting materials are held together by interfacial forces.10 Adhesion may result from the attraction of electrical charges or from molecular forces due to the polarizability of molecules. Tack is the capability of a material to achieve a strong adhesive contact with the surface of a substrate under very light pressure (1–10 Pa) applied over a short time (a few seconds).1 Pressure-sensitive adhesion is a complex and multiform phenomenon that includes tack as a necessary component. However, high tack is a necessary but insufficient condition for pressure-sensitive adhesion.8,11,12 To obtain a high level of adherence, either a liquid-like fluidity or a low elastic modulus of the adhesive material is needed to establish proper contact at the molecular scale with the substrate under applied compressive force. In the stage where the adhesive contact is broken, the elasticity of the adhesive is also of particular importance. This is a characteristic feature of solid materials. A certain degree of elasticity is required to provide a high level of dissipated energy in the course of adhesive bond failure. This liquid–solid duality in the properties of PSAs is difficult to realize in practice when new PSAs are under development. In past years, diverse theories were proposed to explain the driving forces and mechanisms of adhesion. The best known of these theories are the mechanical interlocking, diff usion, adsorption, and electronic theories of adhesion, which were reviewed by Kinloch.13 The mechanical interlocking theory proposes that mechanical keying, or interlocking, of the adhesive into the irregularities at the substrate surface is the main source of intrinsic adhesion. However, the attainment of good adhesion between smooth surfaces exposes the mechanical interlocking theory as not generally widely applicable.

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Molecular Nature of Pressure-Sensitive Adhesion

10-3

The diff usion theory of adhesion, originally advocated by Vojutskii14–17 and further developed by Vasenin18–20 and Chalykh and colleagues, 21–23 states that the intrinsic adhesion of viscoelastic polymers is due to interdiff usion of polymer molecules across the interface. This requires that the macromolecules, or chain segments, possess sufficient mobility and are mutually soluble. The latter requirement may be restated by the condition that they possess similar solubility parameter values. However, when the solubility parameters of the materials are not similar or one polymer is highly cross-linked, crystalline, or below its glass transition temperature, then the interdiff usion is an unlikely mechanism of adhesion. Most recent advances in the diff usion theory of adhesion are reviewed in this book by Creton.24 Deryaguin’s electronic theory of adhesion25 treats the adhesive–substrate joint as a capacitor that is charged due to the contact of two different materials. Separation of the parts of the capacitor leads to a separation of charge and a potential difference, which increases until discharge occurs. Adhesion is presumed to be due to the existence of attractive forces across the electrical double layer. However, it is now established13 that for typical adhesive–substrate interfaces, generated the electrical double layer does not contribute significantly to intrinsic adhesion. Further, any electrical phenomena observed during the joint fracture process arise from the failure event, rather than from intrinsic interfacial forces. Nowadays, the adsorption theory has the widest applicability, whereas each of the other theories may be appropriate only in certain circumstances. The adsorption theory proposes that the materials adhere because of the interfacial interaction forces, which are established between the atoms and molecules in the surfaces of the adhesive and substrate.26 Although for most adhesive–substrate joints the main mechanism of adhesive bond formation is outlined by adsorption theory, this theory cannot be regarded as universal either, because the majority of PSAs adhere to substrates of different chemical nature. All these theories deal with various aspects involved in the attainment of intimate molecular contact at the adhesive–substrate interface. The attainment of such interfacial contact is invariably a necessary first stage in the formation of a strong and stable adhesive bond. The next stage is the generation of intrinsic adhesion forces across the interface, which hold the surfaces of the adhesive and substrate together. Both stages in combination represent the first stage of the complex phenomenon of adhesion, adhesive bond formation, or tack. The second and final stage of the adhesion phenomenon is adhesive joint fracture under applied detaching force that has recently been shown to represent a multistage process in itself.8 Much of the current confusion in the literature concerns the meaning of the term adhesion. According to classic definition noted previously, the phenomenon of adhesion relates rather to instantaneous adherence or tack, a process of adhesive bond formation. Adhesion, therefore, remains an illusive property, and it is suggested that tack is a measure of the capability of the adhesive to form a bond with the substrate.27 Information regarding the magnitude of the intrinsic adhesion forces due to adsorption, diff usion, mechanical interlocking, or electronic mechanisms and the resulting adhesive bond formation may be obtained indirectly, based on geometric factors and surface energy. Direct methods used to measure tack and adhesion (in the classic meaning of the term) rely mainly on measuring the energy required to break the adhesive bond. PSAs are

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10-4


viscoelastic materials that deform under attached debonding force, attaining large strain values. Thus, although the intrinsic adhesion forces affect the strength of the adhesive joint, their contribution is usually significantly obscured by the work of viscoelastic deformation of the adhesive material until the adhesive bond breaks. As demonstrated below, the contribution of viscoelastic deformation of the PSA in the course of debonding prevails against the strength of interfacial interaction forces. The importance of the rheological behavior of PSAs recently received wide recognition. The very name of the phenomenon of pressure-sensitive adhesion implies its rheological nature and the contribution of shear deformation under compressive load to the behavior of PSAs. Thus, although a viscoelasticity theory of pressure-sensitive adhesion seems to be more universal and provides the most productive approach to the comprehension of this phenomenon, it conflicts with the classic definition of adhesion.8

10.2

Model System to Elicit Supramolecular Structures Responsible for Pressure-Sensitive Adhesion

The development of PSAs encounters the problem of obtaining a material that is capable of providing high-strength adhesive joints with various substrates. For this reason, contrary to the classic definition of adhesion, we will always use the term adhesion to mean the actual work necessary to break an adhesive bond, including all dissipative mechanisms. Because adhesion is a macroscopic property that involves numerous processes at a molecular level, eliciting molecular structures underlying the adhesive behavior of materials represents a great challenge. Resolution of this problem is also complicated by the fact that existing PSAs, as a rule, are multicomponent systems, based on elastomers, that are widely varied in their chemical structures. For this reason, elaboration of a model PSA that is suitable for tracing the QSPR becomes a problem of paramount importance. High-molecular-weight, glassy poly(N-vinyl pyrrolidone) (PVP, Mw = 1,000,000; Mn = 360,000 g/mol) has been demonstrated to be easily soluble in low-molecular-weight, liquid poly(ethylene glycol) (PEG, Mw = 400 g/mol),28 yielding single-phase homogeneous blends.29,30 The miscibility of polymers is known to result most frequently from a specific favorable interaction between macromolecules in blends.31 However, as demonstrated by the structures illustrated in Figure 10.1, both PVP and PEG contain only electron-donating groups in monomer units of their backbones, but no complementary proton-donating groups. They are therefore expected to be immiscible. At ambient temperature, PVP is immiscible with high-molecular-weight fractions of PEG (Mw > 600 g/mol)32 and this behavior implies the contribution of proton-donating hydroxyl groups at the ends of PEG short chains to PVP–PEG compatibility.

HO CH2 CH2 O OH

CH2 CH N

n

m

O

FIGURE 10.1 Chemical structure of PVP (left) and PEG (right). m ≈ 10,000; n = 9–10.

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10-5


As demonstrated using Fourier transform infrared (FTIR) spectroscopy, 29,33 PVP’s solubility in liquid short-chain PEG is a result of hydrogen bonding between the terminal OH groups of PEG and the carbonyl C=O groups in PVP repeat units. A schematic view of the proposed structure of the PVP–PEG complex is illustrated in Figure 10.2. Because every PEG chain bears two reactive terminal OH-groups, PEG acts as an H-bonding reversible cross-linker of longer PVP macromolecules. In Figure 10.3, 180° peel adhesion (P) is plotted against the composition of PVP–PEG blends and the content of water absorbed as a vapor from the surrounding atmosphere.11,34

PVP

PEG

FIGURE 10.2 Simplified scheme of the proposed molecular structure of a PVP–PEG H-bonded complex.

Peel force (N/m)

400

300

200

100

2O

10 20 30 PEG c ontent

40 (%)

H

00

(%

)

60 50 40 30 20 10

50

60

0

FIGURE 10.3 Peel adhesion of PVP–PEG blends as a function of PEG concentration and content of absorbed water (percentage of water absorbed per 100% PVP + PEG).

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10-6


Although neither PVP nor PEG-overloaded blends demonstrate any pressure-sensitive adhesion, high adhesion appears in a very narrow range of PEG content (in the vicinity of 36 wt % PEG). Absorbed water affects adhesion in a complicated manner. Dry blends possess no appreciable adhesion. For blends containing less than 36% PEG, the water enhances adhesion, whereas the blends overloaded with PEG (45 wt % and higher) follow an inverse pattern and water sorption inhibits their adhesion. Both PEG and water are good plasticizers and solvents of glassy PVP. 35 They decrease the glass transition temperature of PVP (Tg = 175°C), although the plasticizing effect of PEG is much stronger. The blends containing less than 36% PEG have higher Tg and the plasticizing effect of water promotes adhesion. A PVP blend with 36% PEG is in the viscoelastic state at ambient temperature, and adhesion passes through a maximum at the 12% level of hydration. The blends containing 39% PEG and higher are too fluid-like; the higher the water sorption, the poorer the adhesion. Here, the water acts as a solvent that causes swelling of the PVP–PEG adhesive and dilutes the entanglement structure of the blend, hence obtaining a lower modulus. With the growth of the content of both plasticizers (PEG and water), the mode of adhesive joint failure changes from adhesive to cohesive. The maximum peel strength at 36% PEG corresponds to a transition point.11,34 The peel force behavior illustrated in Figure 10.3 makes the PVP–PEG system a very convenient model from which we are able to elicit the molecular structure responsible for pressure-sensitive adhesion. We have merely to compare the structures and properties of adhesive and nonadhesive PVP–PEG blends. The question, however, is pertinent: taking into account the rather atypical chemical composition of PVP–PEG blends, which has nothing to do with conventional PSAs, which are mainly formulated on the basis of hydrophobic rubbers, is the PVP–PEG system an adequate model? We believe that the common properties determined for the PVP–PEG complex and conventional adhesives might be of particular importance for their adhesive behavior. Consequently, to elucidate general criteria for pressure-sensitive adhesion we must compare the properties of PVP–PEG blends, which provide the best adhesion, with the properties of conventional PSAs and determine any similarities. If common features in the behavior of PVP–PEG and conventional PSAs are of particular importance for the comprehension of necessary conditions for pressure-sensitive adhesion, any distinctions might be due to the contribution of the network of hydrogen bonds, which is only typical for PVP–PEG adhesive hydrogels.

10.3 Structure–Property Relationships for Model PVP– PEG Hydrophilic Pressure-Sensitive Adhesives Both the structure and the properties of the model PVP–PEG adhesive system have received much study in relation to blend composition and the content of absorbed water. The interaction mechanism and the molecular structure have been investigated using FTIR spectroscopy, differential scanning calorimetry (DSC), wide angle x-ray scattering (WAXS), and water sorption techniques. 33,36–41 The stoichiometric

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10-7

composition of PVP–PEG H-bond network complex has been established. 36,39,40 The phase state of PVP–PEG blends has been considered with DSC in a series of research papers. 33,35,42–44 The free volume in PVP–PEG blends is evaluated using positron annihilation.45 Interdiff usion and PVP–PEG miscibility have been characterized with optical microinterference techniques, 30,32,46 whereas self-diff usion of polymer components and absorbed water has been studied using pulsed-field gradient (PFG) nuclear magnetic resonance (NMR).47,48 Rheological and mechanical properties are also described in detail.49–51 All structures and the properties have been related to adhesion and evaluated with peel 34 and probe tack 51,52 tests. The main results were reviewed. 8,11,29

10.3.1

Deformation Mechanisms of Model PVP–PEG and Other Adhesives in the Course of Debonding

In the course of adhesive bond failure under peel or probe tack tests, many PSAs undergo large tensile deformations and form separate fibrils53–58 that elongate up to a few hundred and even thousands of percentages compared with the thickness of the intact adhesive layer. Figure 10.4 illustrates how the debonding process looks in the plane of separation of conventional acrylic (probe tack test, top) and PVP–PEG (36 wt %) adhesive (peel test, bottom).8,11 As a result, the energy expended for the deformation and fibrillation of the adhesive fi lm constitutes by far the largest part of the total energy required for adhesive debonding. The fracture mechanics of adhesive debonding of PVP–PEG PSA in the course of peel testing (Figure 10.4, bottom) involves dramatic stretching and fibrillation of the adhesive layer. The length of the extended fibrils is 10–20 times greater than the thickness of the intact adhesive layer. The fibrils are located throughout the entire width of the adhesive fi lm at nearly equal intervals, implying that the mechanism of fibril nucleation is not random and that the adhesive material is spatially arranged into a three-dimensional network. The entire layer of the adhesive is thus subjected to elongational flow in fibrils, providing resistance to detaching stress and energy dissipation. The failure occurs in the region that is closer to the substrate surface than the backing fi lm. This means that the locus of failure is cohesive.34 The viscoelastic deformation of the adhesive in extension is a major energy-consuming mechanism for all PSAs, as well as adhesive fibrillation and cohesive mode of failure. More comprehensive data on the micromechanics of PSA cavitation and fibrillation can be obtained using the probe tack test data illustrated in Figure 10.5.51 Indeed, as Creton and Fabre demonstrated, 58 the parallel geometry of a flat-end probe is better adapted to examine the details of the debonding mechanisms of soft deformable PSAs than the geometry provided by peel testing. According to Creton and Fabre,58 the microscopic mechanisms involved in the detachment of PSA fi lm from a flat probe can be commonly divided into four parts: 1. Homogeneous deformation before σmax. 2. Cavitation around σmax. These cavities are seen in the microphotographs in Figure 10.5 as light spots.

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10-8


Backing film

Substrate

FIGURE 10.4 Microphotographs of the failure of an adhesive bond in the course of a probe tack (acrylic PSA, top)8 and a peel test (hydrophilic PVP–PEG adhesive, bottom).34 The intact adhesive layer of 0.25-mm thickness is seen in the bottom panel as a light band at the border between the backing film and the PE substrate. (From Feldstein, M.M. and Creton, C., Pressure-Sensitive Design, Theoretical Aspects, Vol. 1, I. Benedek, Ed., VSP, Leiden, 2006; Chalykh, A.A., Chalykh, A.E., Novikov, M.B., and Feldstein, M.M. J. Adhesion, 78(8), 667, 2002.)

3. Rapid lateral growth of the cavities during the steep decrease of nominal σ; then, if there is a plateau in the stress–strain curve, slow growth of these cavities in the direction parallel to the tensile direction; and fi nally 4. Elongation of the walls between cavities (fibrillation). These fibrils eventually either break cohesively or detach from the probe surface, causing complete debonding. As demonstrated by the data in Figure 10.5, the behavior of the PVP–PEG PSA model follows this general description fairly reasonably.51

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10-9


12 10

Force (N)

8 6 4 2 0 0

2

4

6

8

10

12

Strain

FIGURE 10.5 Direct observation of the debonding mechanisms and force-versus-strain curve in the course of probe tack test for a PVP blend with 36 wt % PEG at a debonding rate of 1 µm/s. Each image corresponds to a specific point in the stress–strain curve.. (From Roos, A., Creton, C., Novikov, M.B., and Feldstein, M.M., J. Polym. Sci. Polym. Phys., 40, 2395, 2002. With permission.)

10.3.2

Tensile Properties of Model PVP–PEG Adhesive Blends

Because the major mode of deformation of the adhesive under high-angle peeling is extension (Figure 10.4) and the contribution of shear is negligible,59 it is logical to consider the stress–strain curves under uniaxial extension up to fracture of the model PVP–PEG adhesive blends (Figure 10.6) to compare the behaviors of peel force and the work of deformation of PSA under its uniaxial drawing for PVP–PEG model adhesives. Figure 10.6 illustrates the effect of PEG content upon the stress–strain behavior to break the PVP–PEG blends.49 In general, the type of stress–strain curves obtained for the PVP–PEG adhesive blend in Figure 10.6 is typical of lightly cross-linked highmolecular-weight polymers.60 PEG is a good plasticizer for PVP and the addition of PEG results in increased elongation at break (ε b). With the increase in PEG concentration, the value of ε b increases linearly (Figure 10.7). For PVP blends containing 36% the σ b value declines rapidly with PEG amount (Figure 10.7). The transition from ductile to tight deformation type occurs in a fairly narrow range of PEG content, between 36 and 34% PEG (Figure 10.6). In adhesive behavior, this range of PEG concentration corresponds to the transition from the fibrillar type of adhesive joint failure (36% PEG and higher) to the brittle-like fracture without fibrillation of the adhesive. 34,51 The area under the

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10-10

Nominal stress (MPa)


1.0

31%

0.8

34%

0.6 36%

0.4

39%

0.2 41% 0.0 0

4

8

12

16

20

24

28

Tensile strain

FIGURE 10.6 Tensile stress–strain curves to break the PVP–PEG blends, containing 31, 34, 36, 39, and 41 wt % PEG-400 at 8–9% degree of hydration. The drawing rate is 20 mm/min. (From Novikov, M.B., Roos, A., Creton, C., and Feldstein, M.M., Polymer, 44(12), 3559, 2003. With permission.)

28

100 εb

24

εb, σb (MPa)

wb

16 12

60

σb

40

W b (MJ/m3)

80

20

8 20

4

0

0 30

32

34

36

38

40

42

PEG content (%)

FIGURE 10.7 The total work of viscoelatic deformation to break the PVP–PEG fi lm, Wb, the ultimate tensile strength, σ b, and the break elongation, ε b, as a function of PEG concentration in blends. The extension rate is 20 mm/min. (From Novikov, M.B., Roos, A., Creton, C., and Feldstein, M.M., Polymer, 44(12), 3559, 2003. With permission.)

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10-11


600 100 550

90 80

450

70

400

60

350

50

300

40

W (MJ/m3)

P (N/m)

500

30

250

20 200 10 30

32

34

36

38

40

42

PEG (%)

FIGURE 10.8 Effects of PVP–PEG composition on 180° peel force, P, and the work of viscoelastic deformation of the adhesive fi lm up to break, W, under uniaxial extension. The peel and drawing rates are 20 mm/min. (From Feldstein, M.M., Developments in Pressure-Sensitive Products, 2nd ed., I. Benedek, Ed., CRC-Taylor & Francis, Boca Raton, 2006.)

stress–strain curve that defines the value of the total work of viscoelastic deformation to break the PVP–PEG adhesive blends, Wb, correlates well with both peel34 and probe tack 51 adhesion and reveals a maximum at 36% PEG concentration for the blend demonstrating the best adhesion (Figure 10.8).

10.3.3 Relationship between Peel Adhesion and the Work of Viscoelastic Deformation up to the Break of the Adhesive Joint Taking into account the importance of tensile strain in adhesive bond failure of various PSAs (Figure 10.4), we now consider the relationship between peel adhesion and the work of viscoelastic deformation of model PVP–PEG adhesive blends of various compositions, which provide different levels of adhesion. This relationship is illustrated in Figure 10.8. An evident correlation between peel adhesion and the work of viscoelastic deformation of the adhesive, illustrated in Figure 10.8, signifies the controlling contribution of the process of viscoelastic deformation to adhesive performance. By replotting the values of peel force versus the work of deformation to break the PVP–PEG model PSAs, we obtain an insightful and illustrative relationship presented in Figure 10.9. Analysis of the relationship allows us to gain insight into the factors governing PSA behavior at the most fundamental, molecular level. First, the data in Figure 10.9 establish a demarcation line between the adhesive and nonadhesive PVP–PEG blends. Both PEG-overloaded (41% PEG) and underloaded (31% PEG) blends demonstrate the same, comparatively moderate adhesive capability,

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10-12


800 SIS

Peel force (N/m)

700 600 36%

500 400

39%

300

34% 41% PEG 31%

200 30

40

50

60

70

80

90

100

110

120

Work to break (MJ/m)

FIGURE 10.9 The contribution of the work of viscoelastic deformation of the PVP–PEG model adhesives and SIS-based PSA (Duro-Tak 34-4230, National Starch & Chemical Corp.) into their peel adhesion toward the PET substrate. The contents of PEG in the blends with PVP (in wt %) are indicated. (From Feldstein, M.M., Developments in Pressure-Sensitive Products, 2nd ed., I. Benedek, Ed., CRC-Taylor & Francis, Boca Raton, 2006.)

but only the latter blend belongs to the class of PSAs, whereas the former is, in essence, a tacky liquid. To be a PSA, a tacky material should dissipate an appreciable amount of energy in the course of debonding, and the value of the work of viscoelastic deformation to break the tacky fi lm under its uniaxial extension may be taken as a measure of the dissipated energy (60 MJ/m3 and higher). Second, the linear relationship between the peel force and the work of deformation in Figure 10.9 has, most likely, a general character, spanning not only hydrophilic PVP– PEG, but also hydrophobic rubber-based PSAs. Indeed, the values of peel adhesion and deformation work for traditional PSAs, based on a styrene–isoprene–styrene (SIS) triblock copolymer, are aligned with those for PVP–PEG adhesives. The linear relationship in Figure 10.9 can be described by Equation 10.1, b

P kbl ∫ d

(10.1)

0

where b and l are the width and thickness of the adhesive fi lm, σ and ε are the tensile stress and relative elongation, εb is the maximum elongation of the film at the break, and k is a constant that takes into account the contributions of backing fi lm deformation and interaction between the adhesive and the substrate. If we compare the peel adhesion of various adhesives using the same backing fi lm and a standard high-energy substrate, we can accept k ≈ 1. Assuming further that the deformation of the adhesive fi lm in the

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10-13

course of both debonding and uniaxial drawing follows the linear elastic law, Equation 10.1 can be written as P

b l b2 E

(10.2)

where the σ b is the ultimate tensile strength and E is an approximate tensile modulus of the adhesive material. For a PSA, this is not a bad approximation because they usually soften and then harden at large strains. Equation 10.2 holds for PSAs in the linear elastic region of deformation, whereas for the PVP–PEG blend the deviation of the measured relationship from the law presented by Equation 10.2 was earlier shown to achieve 20%.34 Equation 10.2 is similar to the well-known Kaelble equation,60 P

b l f2 4E

(10.3)

where σf is a critical value of the ultimate stress upon fracture of PSA material under debonding from a substrate with a fixed rate. The implication of the similarity of Equations 10.2 and 10.3 is that the Kaelble equation, Equation 10.3, holds for any type of PSA, including the hydrophilic PVP–PEG. Thus, the rule described by Equations 10.2 and 10.3 is universal.

10.3.4

Universal Character of the Viscoelasticity Theory of Pressure-Sensitive Adhesion

Equation 10.2 can be easily modified to express peel adhesion, P, as an explicit function of the relaxation time, τ, and the self-diff usion coefficient, D, of a PSA polymer. Indeed, let us assume in the first approximation that a PSA represents a viscoelastic material that can be described with a Maxwell model characterized with a single apparent relaxation time, τ, and a microviscosity (or monomer–monomer friction coefficient of polymer chain), η. Taking into account that, according to the Maxwell model, E = 3η/τ, we obtain P b l

2 b 3

(10.4)

The viscosity of the Maxwell model can then be substituted by the classic Einstein expression,61

kT DaN

(10.5)

where N is a number of monomer units of size a in a segment of polymer chain and D is the self-diff usion coefficient of the polymer segment. The substitution of obtained values into the Equation 10.4 yields P b l

aD 2 b 3kT

(10.6)

where a represents the size of the polymer chain segment, k is the Boltzmann constant, and T is temperature.

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10-14


Equation 10.6 is, of course, only qualitatively illustrative, because it makes many crude approximations, including ignoring the existence of the spectrum of relaxation times. It is inappropriate for quantitative calculations of peel force, because it includes immeasurable terms like a (the size of the diff using polymer segment). Nevertheless, it predicts qualitatively the significance of diff usion and relaxation processes (both of which require molecular mobility) for the adhesive behavior of polymers when their debonding is dominated by the formation of fibrils. Equation 10.6 was derived on the basis of the analysis of the deformation contribution to peel adhesion without resorting to the so-called diff usion theory of adhesion.14 Thereby, the rheological approach based on the analysis of viscoelastic deformation of the adhesive material under the debonding process, described here, has more universal character than others mechanisms of adhesion considered previously in this chapter.

10.4

Factors Underlying Pressure-Sensitive Adhesion on a Molecular Scale

According to Equation 10.6, pressure-sensitive adhesion requires a coupling of high molecular mobility, embedded by the high value of the self-diff usion coefficient of the adhesive polymer segment, D, with long-term relaxation processes outlined by large values of relaxation times, τ, and a high cohesive strength of the adhesive polymer, expressed in terms of the ultimate tensile stress upon break of the stretched adhesive under uniaxial drawing, σ b. High molecular mobility is a manifestation of large free volume. A fundamental quantity that underlies a high value of the self-diff usion coefficient at the molecular level is the fraction of free volume, fv ,62  B D A exp    fv 

(10.7)

where A and B are constants. A specific feature of all PSAs is that they should combine a high energy of cohesive interaction with a large free volume. Most commonly, the strong cohesive interaction between macromolecules causes a drastic decrease in free volume, which explains why pressure-sensitive adhesion is a comparatively rare phenomenon. In the model PVP–PEG system these apparently confl icting properties are nevertheless conciliated due to the location of reactive hydroxyl groups at the opposite ends of PEG chains of appreciable length and flexibility (see the scheme in Figure 10.2). In other PSAs of different chemical compositions, these confl icting properties may be conciliated in a variety of alternative ways. High cohesion energy may result from intermacromolecular cross-linking (both covalent and noncovalent and the entanglements of long chains), the addition of tackifiers with high Tg, or to the hydrophobic association of side groups of polymer chains. Large free volume is most frequently provided by the usage of elastomers with a low glass transition temperature, Tg.

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10-15


The glass transition temperature relates to the energy of cohesion and free volume by the equation63 Tg 0.445

z D0 R

(10.8)

where z is the coordination number, a value that is inversely proportional to the free volume, and 〈D 0〉 is the total interaction energy of atoms forming a polymer segment. In acrylic PSAs containing neutralized carboxylic groups, the increase in free volume may result from electrostatic repulsion of the carboxylate anions. In un-cross-linked polyisobutene (PIB) PSA, cohesive strength is a consequence of the presence of a network of long-chain entanglements of high-molecular-weight fraction and is also due to the van der Waals interaction between nonpolar functional groups. In SIS-based triblock copolymers and other thermoplastic elastomers, cohesion is provided by the physical cross-links of high-Tg polystyrene blocks, whereas the free volume is provided by blocks of lower molecular weight polymers. The combination of high cohesion energy and large free volume, featured for all PSAs, is also embedded in such fundamental characteristics of adhesive materials as specific values of solubility parameters, δs, defined as the cohesive energy density or the ratio of the energy of cohesion to the total volume,  H v RT  s    Vm

1

2

(10.9)

where ∆Hv is the molar heat of vaporization, R is the gas constant, T is the temperature (K), and Vm is the molar volume. The heat of vaporization, ∆Hv, is the direct measure of cohesion energy because it is defi ned as the total amount of energy needed to overcome intermolecular attractive forces and transfer a molecule to the vapor phase, in which no intermolecular interaction occurs. The solubility parameter relates to the enthalpic component of the Flory interaction parameter χ between monomer units of two polymer chains i and j (or between a polymer and a solvent) by the equation ij

Vm (i j)2 RT

(10.10)

The smaller the difference between the solubility parameters of an adhesive and a substrate, the greater the peel adhesion.9

10.5

Why Do Pressure-Sensitive Adhesives Belong to the Class of Viscoelastic Materials?

As Equation 10.6 predicts, high adhesion requires a high value of cohesive strength (σ b), high diff usion coefficient (D), and long relaxation time (τ). Although both the diff usion coefficient and the relaxation time are measures of molecular mobility, as illustrated in the scheme in Figure 10.10, they do vary in opposite directions under the transition

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Glasses

Viscoelastic materials

Liquids

τ D

σb

FIGURE 10.10 Directions of varying the values of relaxation time, τ, self-diff usion coefficient, D, and ultimate tensile strength, σ b, under the transition from glassy to liquid state.

from glassy polymer to viscous liquid, for example, with the increase of PEG plasticizer content in blends with PVP. Indeed, the longest relaxation times are featured for glasses (years and centuries), whereas low-molecular-weight liquids relax almost instantaneously. In contrast, the lowest diff usion coefficients are observed for glasses, whereas the highest diff usion coefficients are demonstrated in liquids and gases. According to Equation 10.6, maximum peel strength, P, relates to the maximum magnitude of the τD product. Evidently, this product achieves its maximum magnitude in a certain range of values of relaxation time and diff usion coefficient, which are intermediate between those inherent for liquids and glasses. The materials coupling the properties of the liquids and the solids are in a viscoelastic state, which is why all PSAs are viscoelastic materials.

10.6

Fundamental Quantities Outlining the Place of Pressure-Sensitive Adhesives among Other Viscoelastic Polymers

Equation 10.6 contains the product of the contribution of high cohesion energy and large free volume, whereas such fundamental characteristics of materials as the glass transition temperature (Equation 10.8), the solubility parameter (cohesive energy density, Equation 10.9), or the change in heat capacity at Tg(∆Cp) represent the ratios of cohesion energy and free-volume magnitudes. A question arises regarding what values of these fundamental quantities are typical for PSAs and how we can measure these two contributions separately. What magnitudes of diffusion coefficients (D), relaxation times (τ), and ultimate tensile strength under uniaxial drawing of adhesive films (σ b) are in favor of high adhesion? The following discussion is intended to provide fresh insight into these issues.

10.6.1

Correlation of Adhesion with Free Volume

The study of free volume in polymer systems is of great importance because the size and concentration of unoccupied spaces are thought to affect physical properties, such

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8.0

600

7.5

500

7.0

400

6.5

300

6.0

200

5.5

100

5.0

P (N/m)

f v (%)

as molecular transport, viscoelasticity of polymers, and mechanical, adhesion and other physicochemical properties of the polymers. As the free volume or volume fraction increases or the density of the polymer decreases, the viscoelasticity of the polymer increases. There must be a certain available free volume for a certain size molecule to diff use through. The transition from a glass to a rubber or solvent-swollen polymer can change the rate of diff usion by as much as 10 orders of magnitude, depending on the amount and the size of the penetrants. The free-volume concept was developed to describe the mobility of the polymer and the molecular transport occurrence when a molecule moves into holes with a size greater than some critical volume. The transport process has been described in terms of a redistribution of the local free volume arising from the cooperative motion of neighboring atoms. By the term free volume, we mean the microscopic free volume resulting from the thermal motion of the atoms and molecules that leads to the occurrence of fluctuations in molecular concentration on a nanoscopic scale. Direct experimental measurement of the fluctuational free volume in polymer composites requires the use of costly and not easily accessible techniques. Nevertheless, the free volume in model PVP–PEG blends has been measured using positron annihilation lifetime spectroscopy.45 Figure 10.11 illustrates a comparison of the behaviors of free volume and peel adhesion as functions of the composition of PVP–PEG blends. Fractional free volume ( fv) grows smoothly with PEG concentration in blends from a value of 5.15 vol % for glassy PVP containing 10 wt % absorbed water to 7.02% for a PVP blend with 43 wt % PEG-400. Values of fractional free volume between 6.0 and 8.0% are reported for all polymers in the viscoelastic state.64 The best adhesion is observed in PVP–PEG blends with a free volume fraction between 6.4 and 7.0 vol %. In this range, the average radius of nanoscopic holes has been demonstrated to increase from 2.95 ± 0.01 to 3.08 ± 0.01 Å, whereas their volume rises smoothly from 107.6 ± 0.96 to 122.26 ± 1.09 Å3.

0 0

10

20

30

40

50

PEG (wt %)

FIGURE 10.11 The fraction of free volume ( fv, %), and peel adhesion (P, N/m) of PVP blends with various amounts of PEG-400 at 100% relative humidity of the surrounding atmosphere.

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The PVP–PEG (36 wt %) blend demonstrating maximum peel adhesion is characterized by a free volume fraction of 6.6 vol %, a hole radius of 3.01 ± 0.01 Å, and a volume of 114.77 ± 1.09 Å3. Currently there is a lack of information regarding the free volume in PSAs of other chemical compositions, but taking into account that in the terms of free volume the PVP–PEG model PSA is a typical elastomer, we can assume that other PSAs possess similar free volumes.

10.6.2 Diffusion Coefficients of Pressure-Sensitive Adhesives The diff usion coefficients of various PSA polymers have been reported to be of the order of magnitude of 10−9 to 10−14 cm2/s (or 10−13 to 10−18 m2/s), which Vojutskii14 argues is completely adequate for the formation of an intrinsically strong interface between a PSA and a polymer substrate over a contact time of only a few seconds. As demonstrated in previous chapters of this book, 22,24 for proper characterization of the formation of an adhesive joint the PSA–substrate interdiff usion coefficient is mostly relevant. However, as Equation 10.6 demonstrates, the diff usivity of the adhesive polymer contributes to the amount of mechanical energy necessary to break the adhesive fi lm in the course of stretching that accompanies the process of adhesive bond fracture (see Figure 10.4). Sufficiently high molecular mobility, embedded in the magnitude of the diff usion coefficient, is needed to endow the adhesive material with high compliance and the capability to develop large tensile strain. As far as we know, the role of adhesive polymer diff usivity has not been discussed in adhesion science. The self-diff usion coefficients in model PVP–PEG adhesives have been evaluated with a PFG NMR method as functions of PEG and the absorbed water contents.47,48 In PFG NMR experiments, the incoherent intermediate structure function, Sinc(q, t), of the protons in a system, given by spin echo attenuation, is measured as a function of the generalized scattering vector, q = γδg (where γ is the gyromagnetic ratio of the proton, δ is a fi xed pulse width, and g is the magnitude of the field gradient pulses). The curves of spin echo attenuation are fitted by Equation 10.11, Sinc (q, t ) p1 exp(q 2tD1 ) p2 exp(q 2tD2 )

(10.11)

where p1 are the fractions (populations) of resonating species possessing the self-diff usion coefficient D1. For the PVP–PEG blend that contains 36 wt % PEG-400 and exhibits the best adhesion, only a single population of diffusing species (D = 2.3 10−13 m2/s) was observed 2 days after blend preparation. Within the following 32 days, the diff usion decay evolved into a biexponential behavior with a slower self-diff usion coefficient of 1.5 10 −13 m 2/s and p = 0.87.65 After this time, the diff usion properties of the system were unchanged. Such an aging process is typical for self-assembling supramolecular structures involving the hydrogen bonding and the stoichiometric complex formation.66 The established variation in molecular mobility has only a very slight effect on adhesion. The implication is that the best adhesion in the PVP–PEG system relates to the values of the self-diff usion coefficient ranging from 1.5 10−13 to 2.3 10−13 m2/s.

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To what component of the PVP–PEG blend may the established self-diff usion coefficient be attributed? The binary PVP–PEG blend contains three components: high-molecularweight PVP with an extremely low self-diff usion coefficient, a short-chain PEG with a comparatively high diff usion coefficient, and the product of their interaction—the stoichiometric H-bonded network PVP–PEG complex, whose formation underlies the occurrence of pressure-sensitive adhesion. The PVP–PEG blends may be also treated as concentrated solutions of glassy PVP in liquid PEG-400. Two populations of diff using molecules in the PVP–PEG system were detected. As the PEG concentration in blends decreases, tending to a stoichiometric complex composition ([PEG]:[PVP] = 0.15), the portion of slow-diff using protons, p, approaches unity (Figure 10.12). Such behavior demonstrates that the D value of the slow-diff using component characterizes the segmental mobility in the PVP–PEG complex. As [PEG]:[PVP] → 1, p → 0. Within this composition range, only the fast-diff using component (PEG) is observed. This area of the composition may be defined as a solution of the stoichiometric PVP–PEG complex in PEG. However, at [PEG]:PVP > 1, the fraction of slow-diff using component increases again. This effect may be treated as increasing involvement of PEG chains in the motion of PVP segments, whose self-diff usion coefficient increases in dilute solutions, approaching a value of D ≈ 10−12 m2/s. Figure 10.13 compares the peel adhesion (P) of the PVP–PEG model adhesives with their diff usion behavior expressed in terms of the self-diff usion coefficient of slowly diff using component, measured using PFG NMR techniques,47,48 and the partial diff usion coefficient of PEG-400 in blends with PVP (DPEG ). The latter has been evaluated

[PEG-400]/[PVP] 0.15

1.0

−11.2

0.9

−11.4

0.8 Dfast

log D (m2/s)

−11.6

0.7

−11.8

0.6 Dslow

−12.0

0.5

−12.2

P

−12.4

0.3

pslow

−12.6

0.2

−12.8

0.1

−13.0

0 30

40

50

60

70

80

90

100

PEG-400 (wt %)

FIGURE 10.12 The effect of PEG content (wt %) (bottom axis) in terms of the number of PEG molecules per one PVP repeating unit, [PEG-400]:[PVP], top axis, on the self-diff usion coefficient (D) and population (p) of the fast and slow diff using components at 20°C. The diff usion time is 103 ms. The composition in which the stoichiometric PVP–PEG complex is fully formed and that demonstrates best adhesion is denoted by the dashed line on the left.

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600

1E-11

550 DPEG

500

1E-12 D (m2/s)

Dslow

450 400 350

1E-13

P (N/m)

P

300 250 200

1E-14 20

30

40

50

60

70

80

90

100

PEG (%)

FIGURE 10.13 Partial diff usion coefficient (D PEG) of PEG-400 in blends with high-molecularweight PVP of different compositions, the self-diff usion coefficient of the slow diff using component, and peel adhesion (P) at room temperature.

from the values of the PVP–PEG interdiff usion coefficient (D v) with optical microinterferometry.30,32 DPEG relates to D v by the equation21 DPEG

Dv

PVP

(10.12)

where φPVP is the volume PVP fraction in the binary blend. As established, the values of DPEG exactly meet those of the self-diff usion coefficient of the fast-diff using component by PFG NMR (Figure 10.12). Thus, the fast-diff using component in PVP–PEG blends most likely represents the PEG that is uninvolved in the network complex with PVP. As follows from the data presented in Figure 10.13, the self-diff usion coefficient of PEG-400 within the H-bonded PVP–PEG network at 36% PEG concentration in the blend is DPEG = 1.3 10−13 m2/s. The independent data of the microinterferometry and PFG NMR are in good agreement and outline diff usivity of the model PVP–PEG adhesive that provides the best adhesion on the border between 10−12 and 10−13 m2/s. Such high self-diff usion coefficients are most likely due to low molecular weight and high flexibility of PEG short chains. Needless to say, such high diff usivity is also favorable for fast adhesive bond formation. Thus, the liquid-like diff usion mobility of PSAs is needed not only to provide deep penetration of the PSA polymer into a substrate under slight pressure, but also to develop large tensile strain of the adhesive layer under a detaching force. As Equation 10.6 predicts, high adhesion is associated with high diff usion coefficient values. As direct measurements of diff usivity in the PVP–PEG model PSA demonstrate, the PEG self-diff usion coefficient in this PSA relates to the border between elastomers and viscous liquids, ~10−13 m2/s.

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10.6.3

Free Energy for Self-Diffusion and That for Debonding of PVP–PEG Model Pressure-Sensitive Adhesives

Analysis of the values of self-diff usion coefficients, which are inherent in PSAs, provides only imperfect comprehension of the contribution of diff usion mobility to adhesion strength. Much like pressure-sensitive adhesion, the value of the diff usion coefficient is dictated by the ratio of cohesion energy to the free volume available for diff usion. Whereas the free volume facilitates the diff usion, the forces of intermolecular attraction decelerate it. For closer characterization of diff usive mobility underlying deformation of PSA material under the debonding process, the energetic component of the self-diff usion, the free activation energy for the diff usion, must be taken into consideration. The compositional profi le of PEG self-diff usion activation energy allows defi nition of the borders between the different stages of PVP dissolution in PEG (Figure 10.14). The fi rst stage of the dissolution process (under low PEG content) is PVP plasticization and the formation of a stoichiometric hydrogen-bonded complex, which results in a gradual reduction of the activation energy for PEG self-diff usion. The second stage of the dissolution process (at higher PEG concentrations) is defi ned as a gradual swelling of the H-bonded network in an excess of liquid PEG. Th is stage involves the emergence of mobile PEG in a blend, whose self-diff usion is hampered as indicated by the plateau on the plot of activation energy between 40 and 60% of PEG-400. At higher PEG contents the disentanglement of long PVP chains occurs, and blends containing more than 70% of PEG represent the PVP solution in PEG. Within these blends all the PEG macromolecules demonstrate the activation energy for self-diff usion inherent in the bulky PEG. As follows from Figure 10.14, the maximum adhesion appears as the blend composition (36 wt % PEG) coincides with the composition of the stoichiometric PVP–PEG complex. In this blend, the magnitude of the free energy for PEG self-diff usion is

400 Activation energy Peel force

70

350

60

300

50

250

40

200

30

150

20

100

10

50

0 0.2

0.4

0.6

0.8

Peel force (N/m)

Activation energy for self-diffusion (kJ/mol)

80

0 1.0

PEG weight fraction

FIGURE 10.14 Relationships between PEG content in the blends with PVP, free energy for selfdiff usion, and peel adhesion.

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10-22


55.3 kJ/mol. As the activation energy increases to 69.7 kJ/mol with the decrease in PEG concentration to 30 wt %, the locus of adhesive joint failure of the PVP–PEG PSA with PE substrate changes from miscellaneous (in the maximum of peel adhesion) to adhesive. In contrast, at higher PEG concentrations (40–60% PEG) the activation energy varies between 52.3 and 48.2 kJ/mol. For these blends the locus of failure is always cohesive. Because debonding occurs after a certain time under a fi xed tensile force, the time, t*, required to rupture the adhesive bond characterizes the durability of the joint. The durability of an adhesive joint is a fundamental quantity that characterizes pressure-sensitive adhesion, as described in Chapter 9.67 Temperature dependence of the logarithm of durability follows the Arrhenius relationship fairly reasonably, allowing evaluation of the activation energy for the fracture process of PVP–PEG adhesives under detaching stress. The latter value is plotted versus the composition of PVP–PEG adhesives in Figure 10.15, along with the durability of the adhesive joint and the activation energy for PEG self-diff usion, determined using the PFG NMR method.47,48 As the data in Figure 10.15 demonstrate, the activation energy for adhesive bond failure follows the pattern of the activation energy of the self-diff usion coefficient, climbing sharply with a decrease in PEG concentration below 36%, when maximum adhesion has been achieved. For a blend with maximum adhesion, the activation energy for adhesive debonding is 10–15 kJ/mol higher than the activation energy for self-diff usion. The translational mobility measured in terms of the self-diff usion coefficient takes an appreciable part of the activation energy for adhesive debonding in the blends, which exhibit a miscellaneous (adhesive–cohesive) mechanism of adhesive joint failure.34 For low PEG weight fractions, the activation energy of the debonding process reaches a value of 210 kJ/mol. Within this composition range the lack in 5000 Activation energy (kJ/mol)

200

4000

Adhesive durability 3000

150

2000 100 1000 50

Adhesive durability (s)

ED self-diffusion Ea adhesive debonding

0 0.2

0.4

0.6

0.8

1.0

PEG weight fraction

FIGURE 10.15 Relation of adhesive joint durability and activation energy for adhesive debonding and for PEG self-diff usion to the composition of PVP–PEG adhesive blends. (From Feldstein, M.M., Developments in Pressure-Sensitive Products, 2nd ed., I. Benedek, Ed., CRC-Taylor & Francis, Boca Raton, 2006.)

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10-23

molecular mobility makes a cohesive fracture of the joint impossible and the fracture proceeds mainly by an interfacial mechanism of crack propagation. If one examines the monomer unit level, the activation energy for self-diff usion, ED, should be a function of the product of the cohesive energy density (CED) and the volume of a mole of cylindrical cavities required for diff usion of a polymer chain segment of diameter d over a jump length λ, ED

d 2 CED (1 f v )Ec 4

(10.13)

where Ec is the cohesive energy and fv is the fractional free volume in the polymer. Thus, a phenomenological analysis of the relationship between pressure-sensitive adhesion and the molecular mobility of the PSA, outlined by Equations 10.6 and 10.13, suggests that the two important parameters controlling pressure-sensitive-adhesion and diff usion are the energy of favorable intermolecular interactions (cohesion) and molecular free volume. The difference between adhesion and diff usion is rather quantitative: adhesion occurs only within a very narrow range of the ratio of cohesion energy to free volume, and in this case both the former and the latter magnitudes are to be high. In contrast, diff usion takes a place at any value of cohesion energy and free volume if only the gradient of concentration is available.

10.6.4 Relaxation Times Featured for Pressure-Sensitive Adhesives Diff usion and relaxation represent two sides of the same phenomenon—the molecular mobility of a material. High-diff usion coefficients (in liquids) are always associated with short relaxation times, whereas low-diff usion coefficients (in solids) relate to longer relaxation times (Figure 10.10). Relaxation is a material response to the perturbation of equilibrium structure caused by a temperature jump, magnetic impulse, or (as in the case of pressure-sensitive adhesion) the application of mechanical bonding or detaching stress. In the process of adhesive joint failure, relaxation is a driving force that is directed to the recovery of equilibrium material structure. In the process of relaxation, macromolecules or their segments change their positions, tending to initial, equilibrium structure. Diff usion is a process of spatial drift of molecules due to their kinetic motion. In this connection it is obvious that the relaxation process involves the diff usion as one of the main mechanisms, leading to the recovery of the equilibrium structure. Because pressure-sensitive adhesion is a material response to applied mechanical stress, the role of relaxation in providing both good adhesive contact and adhesive joint strength is quite significant. Paradoxically, the significance of the relaxation processes for pressure-sensitive adhesion remains poorly understood, although this investigation does not require sophisticated methods such as the techniques employed for the study of self-diff usion. To some extent, this can be explained by the abundance of information on the relaxation of elastomers, a family to which all PSAs belong, and recognition of the role of relaxation in well-known effects of bonding time and debonding

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10-24


velocity on the strength of adhesive joints.6 However, Equation 10.6 poses at least two new questions: 1. Are longer relaxation times of special significance for good adhesion? 2. What is the range of relaxation times that provides high adhesion strength of PSAs? Because the values of both the relaxation times and the moduli depend on the type of deformation of adhesive fi lm, which is different under compressive bonding force and at the stage of debonding, more extensive research is necessary before the significance of the relaxation processes for the adhesion can be properly established. For a detailed discussion of this problem, see Chapter 11.68

10.6.5

Glass Transition Temperatures Responsible for Pressure-Sensitive Adhesion

Equation 10.8 indicates63 the glass transition temperature is another fundamental quantity that can be employed to describe the balance of cohesion energy versus free volume in PSAs. As follows from the data listed in Table 10.1, typical PSAs possess glass transition temperatures (evaluated with DSC at a heating rate of 20°/min) ranging between −10 and −113°C.69 The maximum Tg values in this range have been found for acrylic PSAs (between −10.7 and −37.3°C), whereas the minimum Tg values were reported for silicone adhesives (−112.5°C). PIB adhesives occupy intermediate range (around−61°C). All the PSAs (except thermoplastic block copolymer elastomers) demonstrate a single glass transition temperature that certifies their homogeneity. Because the chemical composition of a model PVP–PEG adhesive has nothing to do with typical PSAs, the question now arises of whether the Tg values of PVP–PEG adhesive blends fall within the same range. TABLE 10.1 Glass Transition Temperatures and Relevant Values of Heat Capacity Jumps at the Glass Transition Featured for Typical Pressure-Sensitive Adhesives PSA Acrylic, DuroTak 387-22-87 Acrylic, DuroTak 87-900A Acrylic, DuroTak 180-129a Acrylic, DuroTak 180-129a 0-2 PIB Silicone, Bio-PSA Acrylic, Gelva 3011

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Heat Number

Тg (°С)

∆Ср ⋅ (J/g K)

1 2 1 2 1 2 1 2 1 2 1 2 1 2

−29.9 −30.8 −10.7 −16.4 −34.2 −37.3 −30.4 −31.8 −60.6 −60.6 −110.8 −112.5 −30.9 −32.5

0.27 0.22 0.22 0.35 0.17 0.22 0.36 0.23 0.28 0.31 0.16 0.21 0.35 0.30

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10-25


200

400 Upper Tg Lower Tg Tm Peel force

Tg,Tm (°C)

100

300

200

50 0

100

Peel force (N/m)

150

−50 0

−100 0

20

40

60

80

100

PEG (wt %)

FIGURE 10.16 Peel adhesion and phase state of PVP–PEG systems. Tg is the glass transition temperature and Tm is the melting temperature of PEG in blends with PVP. (From Feldstein, M.M., Developments in Pressure-Sensitive Products, 2nd ed., I. Benedek, Ed., CRC-Taylor & Francis, Boca Raton, 2006.)

Figure 10.16 compares adhesive behavior with the phase state of PVP–PEG blends.11 The PVP–PEG system is plausibly among the first and most illustrative examples of miscible single-phase polymer blends, which reveal two distinct relaxation transitions. These transitions are seen on DSC scans as the heat capacity jumps, resembling glass transitions, and the corresponding glass transition temperatures demonstrate coherent compositional behavior.40 The behavior of the upper Tg in PVP–PEG blends obeys the rule of homogeneous PVP–PEG mixing or glassy PVP dissolution in liquid PEG due to PEG chains H-bonding to PVP through one terminal group only.40 This un-cross-linked and labile PVP–PEG complex requires a small amount of heat for dissociation and is comparatively unstable. In contrast, lower Tg is due to the formation of a hydrogenbonded PVP–PEG network complex (gel), which behaves like a new chemical entity.38–40 In this stoichiometric complex, nearly 20% of PVP repeat units are cross-linked by PEG terminal OH-groups (via H-bonding). The lifetime of the cross-linked PVP–PEG complex is much longer due to multiple hydrogen bonds involved in its formation. Another example of a single-phase system with two Tg is partly denaturated proteins, where a lower Tg relates to glass transition and an upper Tg corresponds to the relaxation of denaturated protein.70 The data presented in Figure 10.16 demonstrate that the Tg values for adhesive PVP–PEG blends range between –55 and –71°C for blends containing 12 wt % of absorbed water.40 The effects of PEG and absorbed water contents on the Tg in adhesive PVP–PEG blends is illustrated in Figure 10.17. If the amount of absorbed water in the PVP–PEG adhesive is from 4 to 8 wt %, the Tg values have been found to vary from –45 to –52°C (Figure 10.17).35 Thus, the Tg values for the model PVP–PEG hydrophilic adhesives represent no exceptions to the rule that was established previously for typical, hydrophobic PSAs.

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PVP dry 150

Tg (°C)

100

PVP hydrated

50 0 −50 0 20

PEG

40 PE

G

60 (%

)

5

80 100

0

15 10 %)

( H 2O

FIGURE 10.17 The glass transition temperature of PVP–PEG adhesive blends as a function of PEG and water content. (From Feldstein, M.M., Kuptsov, S.A., Shandryuk, G.A., Platé, N.A., and Chalykh, A.E., Polymer, 41(14), 5349, 2000.)

10.6.6 Correlation of Adhesion with Characteristics of Viscoelasticity 10.6.6.1 Large Strain Behavior of Model PVP–PEG Adhesive In preceding sections of this chapter we established that pressure-sensitive adhesion results from a specific molecular structure that combines high cohesion energy with large free volume. Direct experimental evaluation of these fundamental quantities for the variety of PSAs is difficult to attain, and it would be highly desirably to find readily measurable quantities that characterize them indirectly. Furthermore, all the above-considered quantities, such as glass transition temperature, solubility parameter, and diffusion coefficient, relate to the ratio of cohesion energy to free volume. The question arises of how can we readily estimate the contributions of cohesion energy and free volume separately. The tensile test data allow us to estimate the cohesion in terms of the ultimate tensile stress at the break, σ b, which is a direct measure of the integral cohesive strength of stretched adhesive material. Tensile strain is large at the moment of adhesive bond fracture for various PSAs, as evident from Figure 10.4 for both probe tack and peel tests, and the advantage of the tensile test is that it provides a feasible tool to measure the viscoelastic properties of adhesives at large strains that approximate the PSA deformation in the course of debonding. Stress–strain curves for PVP–PEG adhesives containing different amounts of plasticizer, PEG-400, are presented in Figures 10.6 and 10.7. The free volume in the PVP–PEG blends has been measured with positron annihilation lifetime spectroscopy.45 Figure 10.18 compares the behaviors of free volume and maximum elongation at the break of adhesive film as functions of the composition of PVP–PEG blends.11 Plasticization of PVP with liquid PEG causes an increase in both

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10-27


30

30

35

40 7.0

25

εb

15 6.0

fv (%)

6.5

20

10 5.5 5 0 0

10

20 30 PEG in blend (%)

40

5.0 50

σb, σb / εb (10−1), σb εb (%)

FIGURE 10.18 Fractional free volume, fv, and maximum elongation of adhesive film at the break of uniaxially stretched material, εb, as functions of PVP–PEG composition. (From Feldstein, M.M., Developments in Pressure-Sensitive Products, 2nd ed., I. Benedek, Ed., CRC-Taylor & Francis, Boca Raton, 2006.)

9

σb

30

6

σb εb

20 εb εb 10

3 σb / εb 0 30

32

34

36

38

40

0 42

PEG (wt %)

FIGURE 10.19 Relationship of ultimate tensile stress (σ b) and maximum elongation at break (εb), their product, and the ratio to the content of PEG-400 in the blends with PVP. (From Feldstein, M.M., Developments in Pressure-Sensitive Products, 2nd ed., I. Benedek, Ed., CRC-Taylor & Francis, Boca Raton, 2006.)

fractional free volume and maximum elongation at break. Consequently, the εb value, which characterizes a process of elongational flow, can be taken as an indirect measurement of the free volume in PVP–PEG blends. With the rise in PEG concentration, the cohesive strength of PVP–PEG adhesives, embedded by the σ b quantity, grows until 36% PEG concentration is achieved and then decreases (Figure 10.19). This implies a twofold role of PEG in the blends. In PVPreached blends, where an H-bonded complex is forming, PEG acts simultaneously both

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as plasticizer, decreasing cohesive strength, and as cross-linker (enhancing cohesive strength). At PEG concentrations higher than 36%, when the PVP–PEG complex is fully formed, the former process dominates and the PEG acts as a plasticizer, decreasing cohesive strength and increasing free volume. As illustrated in Figure 10.19, the product σ b ⋅ εb, which approximates the work of viscoelastic deformation to break, passes through a maximum at 36% PEG content, in full accordance with the data in Figures 10.7 and 10.8 for the work of viscoelastic deformation of the adhesive fi lm up to break (W). The σ b ⋅ εb product is a measure of energy dissipated during the process of deformation of PSA material, and it is therefore not surprising that the maximum of the σ b ⋅ εb product corresponds to the blend with maximum adhesion. The σ b/ε b ratio can be interpreted physically as an average modulus of the adhesive at the moment of fracture. As the data in Figure 10.19 indicate, the maximum energy that must be expended to draw and break a unit volume of the PVP–PEG adhesive (~40–90 MJ/m3) corresponds to the apparent tensile modulus σ b/εb ≈ 2 – 9 × 105 Pa. Let us recall Dahlquist’s criterion of tack, defi ning the elasticity modulus of various PSAs in the order of 105 Pa.5 Although the chemical composition and structure of PVP–PEG blends are absolutely dissimilar compared with those for conventional PSAs, the behavior of PVP–PEG H-bonded network complex near the fracture of adhesive bond obeys Dahlquist’s criterion of tack. This allow us to appreciate the physical meaning of this phenomenological criterion. At the most fundamental, molecular level, Dahlquist’s criterion of tack specifies the ratio between cohesive interaction energy and free volume within PSA polymers. Thus, the analysis of tensile stress–strain curves provides further insight into the factors underlying viscoelastic behavior at a molecular level. 10.6.6.2 Effects of Debonding and Deformation Rates on Adhesion and Viscoelastic Properties It now remains to be elucidated whether tensile test results obtained for the model PVP– PEG adhesive are also typical for other, hydrophobic PSAs. In other words, by comparing the values of ultimate tensile strength (σ b) and maximum elongation at break (εb) for various PSAs, is it possible to make conclusions about the magnitudes of their cohesive strengths and the contents of free volume? Because the tensile stress–strain curves available in literature53,71 often relate to different drawing rates, and taking into account that the relaxation processes dramatically affect both adhesion and viscoelastic behavior, we should first consider the velocity dependence of the adhesion and uniaxial extension of the PVP–PEG model adhesive. The effects of debonding and deformation rates on the mechanism of adhesive joint failure and deformation provide one of the most illustrative examples of these structure–property relationships. As the results of probe tack testing in Figure 10.20 indicated, 51 for the blend containing 36% PEG, a clear transition between debonding without fibrils to debonding with extensive fibril formation occurs without any change in σmax with a rise in probe velocity. Th is result implies that an adhesive could be designed to have a high adhesion energy (defined by the area under the probe tack curve) at low debonding rates but a good release at high debonding rates. Adhesion tests

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2 1 6 4

80

20

σ (MPa)

10 2

5

2

1 µm/s

0.1 6 4 2 0.01 0

2

4

6

8

10

12

14

FIGURE 10.20 Typical stress–strain curves obtained in probe tests for different probe debonding velocities at a constant PEG content (36 wt % PEG). The maximum extension measured (the point where the force drops to zero) increases monotonously with decreasing debonding velocity. The velocities used are (from right to left) 1, 2, 5, 10, 20, and 80 µm/s. (From Roos, A., Creton, C., Novikov, M.B., and Feldstein, M.M., J. Polym. Sci., Polym. Phys., 40, 2395, 2002. With permission.)

performed with the probe method demonstrate an uncharacteristically high sensitivity to the velocity of removal of the probe, with a sharp transition from detachment by fibril formation at low probe velocity to brittle fracture at high probe velocity. This transition occurs in a very narrow velocity range, suggesting the existence of a well-defined relaxation time in the polymer network. This well-defined relaxation time may be related to the breakdown of the hydrogen-bonded network structure formed by the interaction between OH terminal groups of PEG with the carbonyl groups of the pyrrolidone repeat units (see scheme in Figure 10.2). The probe tack data in Figure 10.20 are in good agreement with tensile test results presented in Figure 10.21. As follows from Figure 10.21, a particular feature of the hydrogen bonded PVP–PEG network is the existence of a well-defined time for its structural rearrangement, which in turn depends upon the lifetime of H-bonds under applied mechanical stress. Under slow drawing rates, the PVP–PEG hydrogel behaves as a ductile, un-cross-linked elastomer, whereas at faster extension rates it deforms and breaks as a tight, cured rubber.49,60,72–76 The transition from the ductile to the tight stretching mode occurs in a narrow range of deformation rates (20–50 mm/min). In exactly the same manner, the type of extension changes from ductile to tight, with a very small decrease in the concentration of both plasticizers in blend–PEG (between 36 and 34%; see Figure 10.6) and water (between 6.5 and 3%).49 Taking into account the transient character and fast reformation of the H-bonded network, we believe that under slow extension the intermolecular H-bonds in PVP–PEG blends have time to rupture and reform anew at another place during deformation and do not contribute appreciably to the resistance to strain until the onset of a critical,

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1.6 100 mm/min

Nominal stress (MPa)

1.4 1.2 1.0 50 mm/min

0.8 0.6

20 mm/min 0.4 0.2

10 mm/min

0.0 0

5

10

15

20

25

Tensile strain

FIGURE 10.21 Stress–strain curves to break the PVP–PEG hydrogel, containing 36 wt % PEG400 and 8–9% water, at drawing rates ranging from 10 to 100 mm/min. (From Novikov, M.B., Roos, A., Creton, C., and Feldstein M.M., Polymer, 44(12), 3559, 2003. With permission.)

strain-hardening region, where the final rupture of H-bonded cross-links between the PVP chains occurs (Figure 10.21). In contrast, at higher extension rates the H-bonds have insufficient time for rearrangement at new places and behave like pseudo crosslinks, which must be ruptured to deform the polymer. The narrow transition from ductile to tight stretching with the rise of drawing rate in Figure 10.21 corresponds to a well-defined rate of rearrangement of the H-bonded network under drawing of the PVP–PEG adhesive. Assuming that breakup and reformation of hydrogen bonds forming a PVP–PEG network can occur below the critical deformation rate of 0.05 s–1, we can identify the characteristic time for this process to occur at about 20 s.51 The effects of deformation velocity on adhesion and large-strain viscoelastic behavior of the model PVP–PEG adhesive provide a highly illustrative example of how the relaxation processes control the performance properties of PSAs. 10.6.6.3 Comparison with Typical Hydrophobic Adhesives Figure 10.22 displays the peculiarities of the tensile stress–strain behavior of a PVP– PEG H-bonded network complex in comparison with that of two typical PSA polymers: highly tacky, un-cross-linked low-molecular-weight PIB and elastic SIS triblock copolymer, which is cross-linked physically through glassy polystyrene domains. The latter is fi lled with relevant tackifier to yield a blend coupling high tack at the stage of bonding under compressive force with perfect elasticity and adhesion in the course of debonding. Comparison of the data presented in Figures 10.21 and 10.22 clearly demonstrates the

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900 10 mm/min

Nominal tensile stress (kPa)

800

SIS 700 600 500 400 300 200 PVP−PEG 100 0

PIB 5

0

10

15

20

25

Tensile strain

Nominal tensile stress (kPa)

1400

100 mm/min

1200 1000 800

SIS

PVP−PEG

600 400 200 PIB 0

0

2

4

6

8

10

12

14

16

18

20

22

Tensile strain

FIGURE 10.22 Stress–strain curves to break the PVP–PEG (36%), PIB, and SIS block copolymer (Duro-Tak 34-4230) adhesives at extension rates of 10 and 100 mm/min.

fact that the 10-fold increase in drawing rate affects the ductile–tight behavior of the hydrogen bonded PVP–PEG adhesive to a much greater extent than in conventional hydrophobic adhesives. In other words, the ductile–tight transition in the SIS and PIBbased adhesives is much wider than that in PVP–PEG blends. Thus, the sharp transition between the ductile and tight deformation modes with the change of extension rate is featured only for the H-bonded PVP–PEG system and is atypical of SIS- and PIB-based adhesives. As follows from the comparison of tensile stress–strain curves presented in Figures 10.6 and Figures 10.21 through 10.23 and Table 10.2, according to their tensile behaviors

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20 18

10 mm/min PIB

16

σn (kPa)

14 12 10 8 6 Acrylic

4 2 0 0

10

20

30

40

50

60

70

ε

60

100 mm/min

50

σn (kPa)

40

Acrylic

30 20 10

PIB

0 0

10

20

30 ε

40

50

60

FIGURE 10.23 Stress–strain curves to break the PIB and acrylic (Duro-Tak 87-900A) adhesives at extension rates of 10 and 100 mm/min.

all PSAs can be classified into two main groups. The PSAs of the first group, exemplified by un-cross-linked PIB and acrylic adhesives, represent very soft, compliant materials with low tensile modulus (E, defined as the slope of initial part of the stress–strain curve), which deform like viscous liquids, demonstrating negligible or zero values of ultimate tensile strength (σ b), comparatively low work of viscoelastic deformation up to break (W), and extremely high values of maximum elongation at break (εb). In fact, such adhesives never break in the course of tensile test at the employed values of extension

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Molecular Nature of Pressure-Sensitive Adhesion TABLE 10.2 PSA PVP–PEG SIS PIB Acrylica PVP–PEG SIS PIB Acrylica a

Tensile Properties of Pressure-Sensitive Adhesives Extension Rate (mm/min) 10

100

E (MPa)

σb (MPa)

0.56 1.42 0.12 0.024 3.63 3.51 0.18 0.083

4.2 19.77 0.00006 10.4 28.57 0.063

σy (kPa) 37.9 21.4

84.1 143.4

εb

W (MJ/m3)

16 23 7.5 ∞ 8.6 20 15.8 ∞

17.9 114 0.14 24.6 156 0.88

Duro-Tak 87-900A.

velocities because the distance between tester cross-heads achieves its limit before the breakpoint. Therefore, the characteristic value of ultimate tensile stress, σ b, cannot serve as a measure of cohesive strength for these materials. Instead of σ b, the value of yield stress (the maximum on stress–strain curve), σy, can be used for comparative characterization of the cohesive strength of fluid adhesives (Figure 10.23). The strain-hardened PSAs of the second group, to which physically cross-linked SIS and PVP–PEG adhesives belong, deform like typical rubber-like networks,60,72–74 exhibiting much higher values of tensile modulus (E), work of extension up to break (W), and appreciable resistance to strain, expressed in terms of high tensile strength, σ b (see Table 10.2). Covalent cross-linking of fluid acrylic adhesives with Ti-chelate (namely with Ti-acetylacetonate) has been shown75 to change the type of tensile stress–strain curves to the deformation with pronounced strain hardening that is typical of the adhesives in the second group. In contrast to such quantities as glass transition temperature, the characteristic values of tensile test do not generate any universal range of magnitude for various PSAs. This implies that although the values of ultimate tensile stress and maximum elongation at break are, respectively, indirect characteristics of cohesive strength and free volume, this is true only for an individual PSA. The σ b and εb values cannot be utilized for comparison of cohesive strength and free volume of different PSAs that belong to different groups. The reasoning behind this conclusion is that most likely that the structures of the strain-hardened (cross-linked) and viscous adhesives at large tensile strain are too different to be characterized in terms of tensile test parameters.

10.6.7 Inconsistency between Small and Large Strain Behaviors: New Consideration of Dahlquist’s Criterion of Tack Whereas the preceding section dealt with tensile properties of PSAs at large strain, in this section we consider the viscoelastic behavior of PSAs in the linear elastic region of small deformations as measured with dynamic mechanical analysis (DMA). A considerable amount of research work has been performed on the correlation of dynamic mechanical properties with the accepted parameters of adhesive performance, tack, peel adhesion, and shear performance (reviewed by Satas77). Chu78 and Dale79 established a direct correlation of the dynamic mechanical properties of PSAs with peel adhesion.

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The storage modulus, G′, measures the elasticity of the adhesive. High G′ values are typical for hard polymers with great cohesion energy. The loss modulus, G″, is associated with energy dissipation during deformation. The factor underlying the G″ value at the molecular level is free volume. The greater the G″ value relative to G′, the more dissipative the adhesive. The G″/G′ ratio, defined as the loss tangent, tan δ, is the balance of viscous/ elastic behavior. tan δ = 1 is a limiting value. Above this value the adhesive is generally considered a viscoelastic fluid, whereas below this value, the adhesive can be considered a viscoelastic solid. Once tan δ > 1, the free volume dominates the cohesion energy. Conversely, if tan δ < 1, the contribution of cohesion energy overrides that of free volume. This suggests that all PSAs should possess the specific range of G′, G″, and tan δ values. The DMA technique allows us to estimate indirectly the ratio of cohesion energy to free volume in terms of tan δ values and provides indirect separate characteristics of the cohesion energy and free volume contributions in terms of G′ and G″, respectively. In this section we focus on outlining the range of G′, G″, and tan δ values, which are typical for both conventional hydrophobic and hydrophilic PVP–PEG model adhesives. Based on DMA data, one of the most important critera for a material to display PSA behavior is Dahlquist’s criterion of tack, which states that the elasticity modulus of a PSA should be below 105 Pa.5,6 Typically, at a deformation rate of 1 Hz and ambient temperature, the values of G′ for various PSAs fall in the range 0.01–0.1 MPa (Figure 10.24).80 Typical PSAs are used in a temperature range corresponding to the beginning of the high-temperature rubbery plateau or the end of the transition region (Figure 10.24). Typical values of tan δ for conventional PSAs range from 0.7 to 1.0, implying that either the contributions of free volume and cohesion energy are counterbalanced or the intermolecular cohesion slightly dominates. The temperatures relating to the loss tangent peak vary most frequently between −50 and +5°C.

9.5 tan δ

9.0

1.2

PSA

1.0

8.0 0.8

7.5 7.0

0.6

6.5

0.4

6.0

G ′′ 0.2

5.5 G′

5.0 4.5 −120

tan δ

log G′, log G′′ (Pa)

8.5

−80

−40

0

40

80

0.0 120

160

T (°C)

FIGURE 10.24 Idealized plot of the storage modulus, G′, loss modulus, G″, and tan δ for a typical PSA as a function of temperature.80 ωref = 1 Hz.

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Until recently, Dahlquist’s criterion of tack described fairly reasonably the interrelationship between the adhesive and rheological properties of all PSAs known to date. Recently a new class of PSAs has been obtained, exemplified by a part of triblock thermoplastic elastomers81 (Figure 10.25)82 and hydrophilic PVP–PEG model adhesive (Figure 10.26),8,11,29,49 the behavior of which sometimes disobeys appreciably Dahlquist’s criterion. G′

1E9

G ′′

1E7

1 tan δ

G ′, G ′′ (Pa)

1E8

10

1000000 100000 0.1 10000

tan δ

1000 −50

0

50

100

150

T (°C)

FIGURE 10.25 Temperature sweep curves of the storage modulus, G′, loss modulus, G″, and tan δ for a SIS-based hot-melt PSA (NSC 12602-60H1). (Courtesy of Dr. Y. Hu, National Starch and Chemical.) 9

2.0 G′

8

7

1.0 6

tan δ

log G′, log G ′′ (Pa)

1.5 G ′′

tan δ 5

0.5

4 0.0 −40

0

40

80

120

160

T (°C)

FIGURE 10.26 Temperature dependence of dynamic shear moduli G′, G″, and tan δ for PVP– PEG (36 wt %) PSA at a reference deformation frequency of 1 Hz.

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As illustrated by the curves in Figure 10.25, the values of G′ for SIS-based PSA lie in the range from 1.94 to 0.40 MPa with an increase in temperature from 15 to 40°C. SISbased PSA is used in a temperature range corresponding to the beginning of the hightemperature rubbery plateau and the end of the transition region (Figure 10.25). Only in this connection does its behavior not deviate too much from that of other PSAs. The values of tan δ for the SIS PSA range from 3.1 to 0.4 between 15 and 40°C. The temperature of loss tangent peak is 15°C, tan δ = 3.1. At ambient temperature (20°C), G′ = 0.96 MPa, G″ = 2.16 MPa, and tan δ = 2.2. At increased temperatures, the value of G′ for the PVP–PEG adhesive (Figure 10.26) decreases in the range from 2.95 (15°C) to 0.09 MPa (40°C). At 20°C, G′ = 1.23 MPa, a value clearly incompatible with Dahlquist’s criterion for tackiness, which specifies that an adhesive loses its tack if its elastic modulus at 1 Hz is higher than about 0.1 MPa. The PVP–PEG blend is tacky in a region corresponding to the center of the transition region, where the loss tangent peak occurs (25°C, tan δ = 1.6). The DMA data in Figures 10.25 and 10.26 were measured in the linear elastic regime, that is, at very small strains. Let us refer now back to large strain data for the PVP–PEG adhesive presented in Figure 10.19. The ratio of ultimate tensile stress to maximum elongation at break, σ b/ε b, can be interpreted physically as an average modulus of the adhesive at the moment of fracture under uniaxial extension.49 As the data in Figure 10.8 indicate, the maximum energy that must be expended to draw and break a unit volume of the PVP–PEG adhesive (~40 to 90 MJ/m3) corresponds to the apparent tensile modulus σ b/ε b ≈ 2 – 9 × 105 Pa. The latter values correspond exactly to those specified by Dahlquist’s criterion, taking into account that E = 3G. Thus, Dahlquist’s criterion holds for the PVP–PEG systems at large strains, rather than within the linear elastic region of deformation. In other words, the PVP–PEG blend is very stiff at small strains but undergoes a pronounced softening above 10–20% deformation, which puts it in the range to be a PSA. Similar yet less pronounced inconsistencies between small and large strain behaviors of PSAs are also observed for polymers with more complicated architectures and very pronounced elastic effects.83 For such polymers the elongational properties, involving the orientation of polymer chains, cannot be readily predicted from the linear elastic properties in shear. One such class of polymers is that of diblock and triblock copolymers of styrene and isoprene, which serve as base components of PSA formulations. As recently established by Roos and Creton,73 much like PVP–PEG adhesives, the blends of triblock and diblock copolymers of polystyrene (PS) and polyisoprene (PI) display marked nonlinear viscoelastic behavior and are much more dissipative at low strains than at high strains. Let us turn refer to Figures 10.22 and 10.23 and recall that, in terms of large strain behavior, the PVP–PEG and SIS adhesives form a separate group of PSAs, which elongate as lightly cross-linked rubbers with pronounced strain hardening, whereas other (un-cross-linked) PSAs (PIB, acrylic) deform as viscous fluids. The implication of these observations is that Dahlquist’s criterion of tack holds only for un-cross-linked adhesives with random structure. To become more universal, it must be now extended by taking into consideration the values of storage modulus obtained for cross-linked adhesives and supplemented by the inclusion of loss tangent magnitudes and peak temperatures.

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10-37

10.7 Conclusions In a manner quite similar to that of many other properties of materials, such as glass transition, solubility, or diff usion, pressure-sensitive adhesion results from the counteraction of molecular cohesion and free volume. The length scale where these two factors are important is different: a PSA requires a low density of strong bonds that are relatively far apart and a high density of weak bonds with a high molecular mobility. A large free volume, resulting in liquid-like fluidity, is necessary to facilitate the formation of good adhesive contact as compressive bonding force is applied and to develop large deformations of a PSA under detaching force. High cohesion energy of the PSA material is required to resist the fracture of adhesive bond and dissipate much mechanical energy in the course of debonding. Once the adhesive bond is properly formed, viscoelasticity theory describes the PSA behavior under detaching force fairly reasonably. This viscoelasticity theory claims that the strength of the adhesive joint is controlled by the amount of viscoelastic energy needed to stretch and break the adhesive fi lm cohesively. The viscoelasticity theory seems to be much more universal than other theories, such as mechanical interlocking, diff usion, electronic, and adsorption theories, and it involves the contributions of high cohesive strength, high diff usion coefficient, and long relaxation time. Because the cohesive strength, diff usion coefficient, and relaxation time vary in opposite directions under the transition from glassy materials to liquids, viscoelasticity theory requires that the values of cohesive strength, diff usion coefficient, and relaxation time should be in a specific and rather narrow range of magnitude to make the value of their product and, consequently, adhesion as high as possible. The specific value of the ratio between high cohesion energy and large free volume, featured for the PSAs of various chemical composition and structure, may be expressed in terms of the glass transition temperature, diff usion coefficient, relaxation time, elasticity modulus, and loss tangent. The glass transition temperatures of PSAs fall in the range between −10 and −115°C, the values of diff usion coefficients should be on the border between those typical of elastomers and viscous liquids, ~10−13 m2/s, and the appropriate values of the relaxation times are identified in the next chapter. According to their large strain behavior, all PSAs can be classified into two groups. The majority of PSAs belong to the group of low-modulus, soft polymer materials that flow upon attainment of yield stress in the course of uniaxial drawing. The second group of PSAs consists of comparatively high-modulus, cross-linked covalently (or noncovalently) and highly ordered polymer materials that exhibit a rubber-like type of deformation with pronounced strain hardening. Dahlquist’s criterion of tack, which states that the elasticity modulus of the PSA must be in the range 0.01–0.1 MPa, holds for adhesives in the first group, whereas the PSAs in the second group, such as thermoplastic elastomers and PVP–PEG hydrophilic PSA, based on the network of hydrogen bonds, demonstrate at 20°C the values of storage moduli in the range of 0.6–1.5 MPa, tan δ ≈ 1.2–2.2, and loss tangent peak temperatures around 20°C. The insight gained into the molecular structures responsible for the occurrence of pressure-sensitive adhesion opens the way to the molecular design of new PSAs with optimized performance properties by blending nonadhesive polymers. This approach is illustrated in Technology of Pressure-Sensitive Adhesives and Products, Chapter 7.

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References 1. Satas D. (Ed.) Handbook of Pressure-Sensitive Adhesive Technology, 3rd ed., Satas & Associates, Warwick, RI, 1999. 2. Benedek I., Chemical basis of pressure sensitive products, in: Benedek I. (Ed.) Developments in Pressure-Sensitive Products, 2nd ed., CRC-Taylor & Francis, Boca Raton, 2006, chap. 5. 3. Benedek I., Physical basis of pressure sensitive products, in: Benedek I. (Ed.) Developments in Pressure-Sensitive Products, 2nd ed., CRC-Taylor & Francis, Boca Raton, 2006, chap. 3. 4. Dillard D.A. and Pocius A.V. (Eds.), The Mechanics of Adhesion, Elsevier, New York, 2002. 5. Dahlquist C.A., in: Patrick R.L. (Ed.), Treatise on Adhesion and Adhesives, vol. 2, M. Dekker, New York, 1969, 219 p. 6. Creton C. and Leibler L., How does tack depend on time of contact and contact pressure, J. Polym. Sci.: Part B: Polym. Phys. 34, 545–554, 1996. 7. Zosel A., Structure property relations in pressure sensitive adhesives: a review, in: Proceedings of the 5th European Conference on Adhesion (EURADH’2000), SFV, Lyon, France, 2000, pp. 149–153. 8. Feldstein M.M. and Creton C., Pressure-sensitive adhesion as a material property and as a process, in: Benedek I. (Ed.) Pressure-Sensitive Design, Theoretical Aspects, vol. 1, VSP, Leiden, 2006, chap. 2. 9. Berg J.C., Semi-empirical strategies for predicting adhesion, in: Chaudhury M. and Pocius A.V. (Eds.) Adhesion Science and Engineering: Surfaces, Chemistry and Applications, Elsevier, Amsterdam, 2002, chap. 1. 10. Zisman W.A., Adhesion and bonding, in: Mark H.F., Gaylord N.G., and Bikales N.M. (Eds.) Encyclopedia of Polymer Science and Technology, Interscience Publishers (J. Wiley & Sons), New York, 1964, pp. 445–477. 11. Feldstein, M.M., Molecular fundamentals of pressure-sensitive adhesion, in: Benedek I. (Ed.) Developments in Pressure-Sensitive Products, 2nd ed., CRC-Taylor & Francis, Boca Raton, 2006, chap. 4. 12. Creton, C., Pressure-sensitive-adhesives: an introductory course, MRS Bull. 28(6): 434–439, 2003. 13. Kinloch A.J., Adhesion and Adhesives: Science and Technology, Chapman & Hall, London, 1987, chap. 3. 14. Voyutskii S.S., Autohesion and Adhesion of High Polymers, Wiley Interscience, New York, 1963. 15. Voyutskii S.S., Adhesive Age 5(4), 30, 1962. 16. Voyutskii S.S., Markin Yu.I., Gorchakova V.M., and Gul V.E., How temperature dependence and activation energy affects the adhesion of polymers to metals, Adhesive Age 8(11), 24, 1965. 17. Voyutskii S.S., J. Adhesion 3, 69, 1971. 18. Vasenin R.M., Adhesion, Fundamentals and Practice, McLaren and Son, 1969, p. 29.

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19. Vasenin R.M., Adhesion of high polymers, Adhesive Age 8(5), 18, 1965. 20. Vasenin R.M., Adhesion of high polymers: predicting adhesion, Adhesive Age 8(6), 30, 1965. 21. Chalykh A.E., Diffusion in Polymer Systems, Chemistry, Moscow, 1987 (in Russian). 22. Chalykh A.E. and Shcherbina A.A., Transition zones in adhesive joints, in: Benedek I. and Feldstein M.M. (Eds.) Fundamentals of Pressure-Sensitive Adhesion, Taylor & Francis, Boca Raton, 2009, chap. 3. 23. Arslanov V.V. and Chalykh A.E., Current state and prospects of the development of the theory of adhesion joints, Prot. Met. (USSR). 25(4), 425–431, 1989. 24. Creton C., Diffusion and adhesion, in: Benedek I. and Feldstein M.M. (Eds.) Fundamentals of Pressure-Sensitive Adhesion, Taylor & Francis, Boca Raton, 2009, chap. 2. 25. Deryaguin B.V. and Smilga V.P., Adhesion, Fundamentals and Practice, McLaren and Son, London, 1969, p. 152. 26. Good R.J., Chaudhury M.K., and van Oss C.J., Theory of adhesive forces across interfaces, in: Lee L.H. (Ed.) Fundamentals of Adhesion, Plenum Press, New York, 1991, chaps. 3 and 4. 27. Satas D., Tack, in: Satas D. (Ed.) Handbook of Pressure-sensitive Adhesive Technology, 3rd ed., Satas & Associates, Warwick, RI, 1999, chap. 4. 28. Bϋhler V., Kollidon®: Polyvinylpyrrolidone for the Pharmaceutical Industry, BASF, Ludwigshafen, 1996, p. 20. 29. Feldstein M.M., Adhesive hydrogels: structure, properties and application, Polym. Sci., Ser. A. 46(11), 1265, 2004. 30. Bairamov D.F., Chalykh A.E., Feldstein M.M., Siegel R.A., and Platé N.A., Dissolution and mutual diff usion of poly(N-vinyl pyrrolidone) in short chain poly(ethylene glycol) as observed by optical wedge microinterferometry, J. Appl. Polym. Sci. 85, 1128, 2002. 31. Painter P.C. and Coleman M.M., in: Paul D.R. and Bucknall C.B. (Eds.) Polymer Blends, Vol. 1: Formulation, John Wiley & Sons, New York, 2000, chap. 4. 32. Bairamov D.F., Chalykh A.E., Feldstein M.M., and Siegel R.A., Impact of molecular weight on miscibility and interdiff usion between poly(N-vinyl pyrrolidone) and poly(ethylene glycol), Macromol. Chem. Phys. 203(18), 2674, 2002. 33. Feldstein M.M., Lebedeva T.L., Shandryuk G.A., Kotomin S.V., Kuptsov S.A., Igonin V.E., Grokhovskaya T.E., and Kulichikhin V.G., Complex formation of poly(N-vinyl pyrrolidone) with poly(ethylene glycol), Polym. Sci. 41A(8), 854, 1999. 34. Chalykh A.A., Chalykh A.E., Novikov M.B., and Feldstein M.M., Pressure-sensitive adhesion in the blends of poly(N-vinyl pyrrolidone) and poly(ethylene glycol) of disparate chain lengths, J. Adhesion 78(8), 667, 2002. 35. Feldstein M.M., Kuptsov S.A., Shandryuk G.A., Platé N.A., and Chalykh A.E., Coherence of thermal transitions in poly(N-vinyl pyrrolidone)–poly(ethylene glycol) compatible blends. 3. Impact of sorbed water upon phase behaviour, Polymer 41(14), 5349, 2000. 36. Feldstein M.M., Lebedeva T.L., Shandryuk G.A., Igonin V.E., Avdeev N.N., and Kulichikhin V.G., Stoichiometry of poly(N-vinyl pyrrolidone)-poly(ethylene glycol) complex, Polym. Sci. 41A(8), 867, 1999.

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37. Lebedeva T.L., Kuptsov S.A., Feldstein M.M., and Platé N.A., Molecular arrangement of water associated with poly(N-vinyl pyrrolidone) in the first hydrate shell, in: Iordanskii A.L., Starzev O.V., and Zaikov G.E. (Eds.) Water Transport in Synthetic Polymers, Nova Science Publishers, Inc., New York, 2003, chap. 4. 38. Feldstein M.M., Shandryuk G.A., and Platé N.A., Relation of glass transition temperature to the hydrogen-bonding degree and energy in poly(N-vinyl pyrrolidone) blends with hydroxyl-containing plasticizers. Part 1. Effects of hydroxyl group number in plasticizer molecule, Polymer 42(3), 971, 2001. 39. Feldstein M.M., Kuptsov S.A., Shandryuk G.A., and Platé N.A., Relation of glass transition temperature to the hydrogen-bonding degree and energy in poly(Nvinyl pyrrolidone) blends with hydroxyl-containing plasticizers. Part 2. Effects of poly(ethylene–glycol) chain length, Polymer 42(3), 981, 2001. 40. Feldstein M.M., Roos A., Chevallier C., Creton C., and Dormidontova E.D., Relation of glass transition temperature to the hydrogen bonding degree and energy in poly(N-vinyl pyrrolidone) blends with hydroxyl-containing plasticizers: 3. Analysis of two glass transition temperatures featured for PVP solutions in liquid poly(ethylene glycol), Polymer 44(6), 1819, 2003. 41. Gerasimov V.K., Chalykh A.A., Chalykh A.E., Razgovorova V.M., and Feldstein M.M., Thermodynamic potentials of mixing in poly(N-vinyl pyrrolidone)– Polyethylene glycol system, Polym. Sci. 43A(12), 2141, 2001. 42. Feldstein M.M., Shandryuk G.A., Kuptsov S.A., and Platé N.A., Coherence of thermal transitions in poly(N-vinyl pyrrolidone)–poly(ethylene glycol) compatible blends. 1. Interrelations among the temperatures of melting, maximum cold crystallization rate and glass transition, Polymer 41(14), 5327, 2000. 43. Feldstein M.M., Kuptsov S.A., and Shandryuk G.A., Coherence of thermal transitions in poly(N-vinyl pyrrolidone)–poly(ethylene glycol) compatible blends. 2. The temperature of maximum cold crystallization rate versus glass transition, Polymer 41(14), 5339, 2000. 44. Feldstein M.M., Peculiarities of glass transition temperature relation to the composition of poly(N-vinyl pyrrolidone) blends with short chain poly(ethylene glycol, Polymer 42(18), 7719, 2001. 45. Li Y., Zhang R., Chen H., Zhang J., Suzuki R., Ohdaira T., Feldstein M. M., and Jean Y.C., The depth profi le of free volume in a mixture and copolymers of poly(Nvinyl-pyrrolidone) and poly(ethylene glycol) studied by positron annihilation spectroscopy, Biomacromolecules 4, 1856, 2003. 46. Bairamov D.F., Chalykh A.E., Feldstein M.M., Siegel R.A., and Platé N.A., Mutual diff usion of poly(N-vinyl pyrrolidone) and water, in: Iordanskii A.L., Starzev O.V., and Zaikov G.E. (Eds.) Water Transport in Synthetic Polymers, Nova Science Publishers, Inc., New York, 2003, chap. 3. 47. Vartapetian R.S., Khozina E.V., Karger J., Geschke D., Rittig F., Feldstein M.M., and Chalykh A.E., Self-diffusion in poly(N-vinyl pyrrolidone)–poly(ethylene glycol) system, Colloid Polym. Sci. 279(6), 532, 2001.

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10-41

48. Vartapetian R.Sh., Khozina E.V., Karger J., Geschke D., Rittig F., Feldstein M.M., and Chalykh A.E., Molecular dynamics in poly(N-vinyl pyrrolidone)– poly(ethylene glycol) blends by pulsed field gradient NMR method: effects of aging, hydration and PEG chain length, Macromol. Chem. Phys. 202(12), 2648, 2001. 49. Novikov M.B., Roos A., Creton C., and Feldstein M.M., Dynamic mechanical and tensile properties of poly(N-vinyl pyrrolidone)–poly(ethylene glycol) blends, Polymer 44(12), 3559, 2003. 50. Feldstein M.M., Kulichikhin V.G., Kotomin S.V., Borodulina T.A., Novikov M.B., Roos A., and Creton C., Rheology of poly(N-vinyl pyrrolidone)-poly(ethylene glycol) adhesive blends under shear flow, J. Appl. Polym. Sci. 100, 522, 2006. 51. Roos A., Creton C., Novikov M.B., and Feldstein M.M., Viscoelasticity and tack of poly(N-vinyl pyrrolidone)–poly(ethylene glycol) blends, J. Polym. Sci., Polym. Phys. 40, 2395, 2002. 52. Novikov M.B., Borodulina T.A., Kotomin S.V., Kulichikhin V.G., and Feldstein M.M., Relaxation properties of pressure-sensitive adhesives upon withdrawal of bonding pressure, J. Adhesion 81(1), 77, 2005. 53. Zosel A., Adhesive failure and deformation behaviour of polymers, J. Adhesion 30, 135, 1989. 54. Zosel A., The effect of fibrillation on the tack of pressure-sensitive adhesives, Int. J. Adhes. Adhesives 18, 265, 1998. 55. Christensen S. F., Everland H., Hassager O., and Almdal K., Int. J. Adhesion Adhesives 18, 131, 1998. 56. Chiche A., Zhang W. H., Stafford C. M., and Karim A., Meas. Sci. Technol. 16(1), 183, 2005. 57. Lakrout H., Sergot P., and Creton C., Direct observation of cavitation and fibrillation in a probe tack experiment on model acrylic pressure-sensitive adhesives, J. Adhesion 69, 307, 1999. 58. Creton C. and Fabre P., Tack, in: Dillard D.A. and Pocius A.V. (Eds.) Adhesion Science and Engineering, Vol. I: The Mechanics of Adhesion, Elsevier, Amsterdam, 2002, chap. 14. 59. Kaelble D.H., Theory and analysis of peel adhesion, in: Satas D. (Ed.) Handbook of Pressure-sensitive Adhesive Technology, 3rd ed., Satas & Associates, Warwick, RI, 1999, chap. 6. 60. Ward I.M., Mechanical Properties of Solid Polymers, Wiley Interscience, London, 1983, chap. 2. 61. De Gennes P.G., J. Chem. Phys. 55, 572, 1971. 62. Fujita H., Fortsch. Hochpolym. Forsch., Bd. 3, 1, 1969. 63. Askadskii A.A. and Matveev Yu.I., Chemical Structure and Physical Properties of Polymers, Chemistry, Moscow, 1983, pp. 24–48. 64. Dupasquier A. and Mills A.P., Jr. (Eds.) Positron Spectroscopy of Solids, IOS Press, Amsterdam, 1995.

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65. Feldstein M.M., Borodulina T.A., Vartapetian R.Sh., Kotomin S.V., Kulichikhin V.G., Geschke D., and Chalykh A.E., Correlations between activation energy for debonding and that for self-diff usion in pressure-sensitive adhesive hydrogels, Proceed. 24th Annual Meeting Adhesion Soc. Williamsburg, VA, 137, 2001. 66. Kabanov V.A. and Zezin A.B., Makromol. Chem. (Suppl. 6), 259, 1984. 67. Kotomin S.V., Durability of adhesive joints, in: Benedek I. and Feldstein M.M. (Eds.) Fundamentals of Pressure-Sensitive Adhesion, Taylor & Francis, Boca Raton, 2009, chap. 9. 68. Feldstein M.M., Significance of relaxation for adhesion, in: Benedek I. and Feldstein M.M. (Eds.) Fundamentals of Pressure-Sensitive Adhesion, Taylor & Francis, Boca Raton, 2008, chap. 11. 69. Shandryuk G.A., personal communication. 70. Bessmertnaya L.Ya., Goncharova A.I., Rumsh L.D., Grokhovskaya T.E., and Feldstein M.M., Thermostability and phase behaviour of enzymes in solid state and within polymer matrices, Proceed. Intern. Symp. Control. Release Bioactive Mater. 26, 383, 1999. 71. Ferguson J., Reilly B., and Granville N., Extensional and adhesion characteristics of a pressure-sensitive adhesive, Polymer 38(4), 795, 1997. 72. Meissner B., Tensile stress–strain behavior of rubberlike networks up to break. Theory and experimental comparison, Polymer 41, 7827, 2000. 73. Roos A. and Creton C., Effect of the presence of diblock copolymer on the non linear elastic and viscoelastic properties of elastomeric triblock copolymers, Macromolecules 38, 7807, 2005. 74. Creton C., Roos A., and Chiche A., Effect of the diblock content on the adhesive and deformation properties of PSAs based on styrenic block copolymers, in: Possart W.G. (Ed.) Adhesion: Current Research and Applications, Wiley-VCH: Weinheim, 2005, pp. 337–364. 75. Lindner A., Lestriez B., Mariot S., Creton C., Maevis T., Luhmann B., and Brummer R., Adhesive and rheological properties of lightly cross-linked model acrylic networks, J. Adhesion 82(3), 267, 2006. 76. Creton C., Block copolymers for adhesive applications, in: Matyjaszewski K., Gnanou Y., and Leibler, L. (Eds.) Structure-Property Correlation and Characterization Techniques, vol. 3, 1st ed., Wiley-VCH, Weinheim, 2007, pp. 1731–1752. 77. Satas D., Dynamic mechanical analysis and adhesive performance, in: Satas D. (Ed.) Handbook of Pressure-sensitive Adhesive Technology, 3rd ed., Satas & Associates, Warwick, RI, 1999, chap. 10. 78. Chu S.G., Viscoelastic properties of pressure-sensitive adhesives, in: Satas D. (Ed.) Handbook of Pressure Sensitive Adhesive Technology, 2nd ed., Van Nostrand Reinhold, New York, 1989, pp. 158–203. 79. Dale W.C., Paster M.D., and Haynes J.K., Mechanical property–performance relations of acrylic pressure-sensitive adhesives, in: Satas D. (Ed.) Advances in Pressure Sensitive Adhesive Technology, Satas & Associates, Warwick, RI, 1995, pp. 65–111.

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80. Benedek I., Pressure-Sensitive Adhesives and Applications, 2nd ed., Marcel Dekker, New York, 2004, p. 9. 81. Derail C. and Marin G., Rheology of hot-melt PSAs: Influence of polymer structure, in: Possart W. (Ed.) Adhesion–Current Research and Application, Wiley-VCH, Weinheim, 2005, chap. 16. 82. Courtesy of Dr. Y. Hu, National Starch and Chemical. Unpublished data. 83. Christensen S.F. and McKinley G.H., Int. J. Adhesion Adhesives 18, 333, 1998.

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11 Significance of Relaxation for Adhesion of Pressure-Sensitive Adhesives 11.1

Pressure-Sensitive Adhesion as a Multistage Process .......................................11-2 Introduction and Defi nitions: Time Dependence in Pressure-Sensitive Adhesion • Pressure-Sensitive Adhesion as a Three-Stage Process

11.2

Relaxation under Compressive Load during Adhesive Bond Formation ................... 11-6 Experimental Setup for the Relaxation and Tack Tests • Adhesion in the Absence of PressureSensitive Adhesive Relaxation • Relaxation and Adhesion of Typical Pressure-Sensitive Adhesives • Correlation among Contact Time, Relaxation Time, and Adhesion • Impact of the Relaxation of Entangled and Network Structures on Adhesion • Optimum Range of Longer Relaxation Times Providing Strong Adhesion • Deborah Numbers of PressureSensitive Adhesives • Relaxation and Adhesion of a Model Pressure-Sensitive Adhesive Based on Interpolymer Complex

11.3

Relaxation Properties of PressureSensitive Adhesives upon Withdrawal of Bonding Pressure ..........................................11-26 Approach • Applicability of Burger’s Model of Viscoelastic Body for Elastic Recovery of Pressure-Sensitive Adhesives • Squeeze–Recoil Behaviors of Pressure-Sensitive Adhesives • Retardation Times Featured for Hydrophilic PVP–PEG Adhesives • Retardation Times

11-1

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Fundamentals of Pressure Sensitivity in Hydrophobic Pressure-Sensitive Adhesives • Correlation between Retardation Times and Pressure-Sensitive Adhesion • Relaxation Criteria for Pressure-Sensitive Adhesion • Major Conclusions

11.4

Mikhail M. Feldstein Mikhail B. Novikov A.V. Topchiev Institute of Petrochemical Synthesis

Costantino Creton Unit Joint CNRS-UPMC-ESPCI

Relaxation Properties of PressureSensitive Adhesives in the Stage of Debonding .....................................................11-37 Three-Stage Mechanism of Debonding • Evolution of Pressure-Sensitive Adhesive Structure during the First Stage of the Debonding Process • Mechanisms of Adhesive Relaxation at the Second Stage of the Debonding Process • Comparison of Relaxation Processes in Linear Viscoelasticity and Large-Strain Elongational Geometries • Relaxation Properties of PVP–PEG Model Pressure-Sensitive Adhesives

11.5 General Conclusions .........................................11-57 References ......................................................................11-59

11.1 Pressure-Sensitive Adhesion as a Multistage Process 11.1.1

Introduction and Defi nitions: Time Dependence in Pressure-Sensitive Adhesion

The adhesive and mechanical (rheological) properties of viscoelastic polymer materials are time dependent.1 These properties come into play when the material is deformed in compression2 in the course of adhesive bond formation or when it is deformed during detachment. Both stages require an input of energy. Under an applied force, part of the energy input is irrecoverably dissipated through a mechanism of viscous flow, whereas another part is stored and can be released elastically upon removal of the bonding or detaching force. 3–5 The dissipation never takes place instantaneously.6 As a result, the response of an adhesive material lags behind the application of a deformation force. For this reason, adhesive properties are time dependent and this dependence cannot be ignored when dealing with such materials. Although the mechanical properties of pressure-sensitive adhesives (PSAs) are the subject of extensive study and have been reviewed in relevant books,7,8 as well as in earlier chapters of this book, the significance of relaxation properties for pressure-sensitive adhesion are still not adequately understood.9 Although, in principle, all mechanical and adhesion properties are time dependent, they can often be treated as if they had a purely elastic or a purely viscous nature. Whether they can or cannot depends on a characteristic ratio of times, called the Deborah number, nD

CRC_59378_C011.indd 2

mat exp

(11.1)

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Significance of Relaxation for Adhesion of PSA

11-3

which is the ratio of the time scale of the material rearrangements, τmat, to the time scale of experimental observation, τexp (see Ref. 10, p. 35). Even some apparent solids “flow” if they are observed long enough. The origin of the name is the line “The mountains flowed before the Lord,” in a song by prophetess Deborah recorded in the Bible (Judges 5:5). When the Deborah number approaches zero, the material may appear purely viscous to the observer and purely elastic when it approaches infi nity. Real materials fall in between and are viscoelastic. In particular, polymeric materials have Deborah numbers around unity and are the viscoelastic materials par excellence. In fact, methods for dealing with time dependence in mechanical properties have developed largely with the development of polymeric materials. Time-dependent mechanical properties are traditionally characterized in terms of so-called response times and almost always by a distribution of such times.1 If the material is strained to a fi xed value, which is then held constant, the corresponding stress relaxes and the response times are called relaxation times. On the other hand, if a stress is applied and kept fi xed, the strain is retarded and the response times are called retardation times. Because the context nearly always makes it clear which are under discussion, both relaxation and retardation times are customarily designated by the same symbol, τ. However, the relaxation and the retardation times are not identical. When ranking both sets in ascending or descending order, they alternate (i.e., the retardation times are intercalated between the relaxation times).10 Each response time is associated with a corresponding spectral strength that may be a modulus, Gi, or a compliance, Ji. The time dependence of a material is thus revealed in a finite, discrete set of response times and their associated spectral strengths. When a shear strain is fi xed and a stress relaxation occurs, this set is {Gi, τi},3,11 in

Gt Geq ∑ Gi exp(t/i )

(11.2)

i1

where Geq is the equilibrium relaxation modulus, τi is the-relaxation time, and Gi is the relaxation modulus associated with τ. When a stress is removed and a strain recovery occurs, this set is {Ji, τi},3,11 in

J J 0 ∑ J i (1 et / i )

(11.3)

i1

where Ji is the compliance (Pa−1) in the i element of a structure and τi is the retardation time (s). As t → ∞, J0 → 0. The corresponding value of relaxation modulus can be evaluated with Equation 10.3 as the reciprocal of the compliance, Gi = 1/Ji. Most well-known examples of time dependence in pressure-sensitive adhesion are the effects of contact bonding time12–15 and debonding velocity16,17 on the strength of adhesive joints. The former effect is often explained by the molecular diff usion of the PSA into the substrate.18–20 However, as Equation 10.6 (Chapter 10) predicts, both diffusion and relaxation processes contribute to the work of viscoelastic deformation of the adhesive fi lm and failure of the adhesive bond that control peel adhesion.21 In the following discussion we focus on this contribution.

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11-4


The well-known Dahlquist’s criterion of adhesion specifies that the values of elasticity moduli of various PSAs should be in the range between 0.01 and 0.1 MPa.22 As demonstrated previously, 22,23 this criterion is incapable of predicting adhesive behavior from the linear elasticity data at small strains for the PSAs, which are either crosslinked (covalently or physically) or based on ordered supramolecular structures. One such PSA is a hydrogen-bonded stoichiometric network complex of high-molecularweight poly(N-vinyl pyrrolidone) (PVP) with a short-chain polyethylene glycol (PEG), which serves in our study as a model PSA.24,25 Another type of PSA departing from Dahlquist’s criterion (although to a less pronounced extent) are the adhesives based on diblock and triblock copolymers of styrene and isoprene, which are physically crosslinked by the glassy domains composed of polystyrene chains with a higher glass transition temperature (Tg; see Technology of Pressure-Sensitive Adhesives and Products, Chapter 3). For both such PSAs the large strain elongational properties, involving the orientation of polymer chains, cannot be readily predicted from the linear elastic properties in shear at small strains. As demonstrated in Chapter 10, 22 for these PSAs, it is the large strain modulus (the derivative of the stress–strain curve at strains above 100%) that is in agreement with Dahlquist’s criterion, rather than the linear elastic properties at small strains. The implication of this fact is that the supramolecular structure of PVP-PEG and styrene–isoprene–styrene (SIS) adhesives under an applied detaching stress undergoes transformation in such a way that the behavior typical of a PSA is a result of the large deformation behavior. This means that the interpretation of the phenomenon of pressure-sensitive adhesion solely as an equilibrium material property cannot always be adequate and, to gain further insight into this phenomenon, adhesion should be also treated as a process. Adhesion is traditionally defined as the phenomenon in which surfaces of contacting materials are held together by interfacial forces.26 In this chapter, the term phenomenon is treated as any event, series of experimental facts, or experience that is detectable by our senses and measuring instruments and that can be scientifically described. In turn, the term property means a characteristic quality regarded as being possessed by a material or a group of materials and that is common to all the members of this group. Process is defined as a continuing development involving many changes or transformations of the structure and properties of the material. Within this framework, the phenomenon is a conjunction of the property and the process. In other words, the term property can be defi ned as an equilibrium feature of the molecular structure independent of time. To reduce the concept of pressure-sensitive adhesion to such a defi nition would be a gross simplification, because adhesion is a phenomenon that is due to a nonequilibrium process. Th is phenomenon consists of a series of transformations of the structure of the adhesive material under an applied bonding and detaching stress, that is, it involves the process of evolution of material structure, geometry (e.g., cavitation and formation of fibrils), and properties. The order and arrangement in time of these transformations has great importance for the perception of the phenomenon as a whole. In the following section we present a rheological and structural description of all stages of the process of pressure-sensitive adhesion.

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11.1.2

Pressure-Sensitive Adhesion as a Three-Stage Process

The process of making and breaking a PSA bond can be divided into three stages: (I) adhesive bonding under a compressive force, (II) relaxation upon the removal of the bonding pressure, and (III) rupture of the adhesive bond under a tensile force. The squeeze–recoil test 27 provides a simple but adequate characterization of all three stages (Figure 11.1; see also Chapter 9 in this volume).28 During the first stage (I), the adhesive film is compressed between a fi xed bottom plate and an upper cylindrical rod of the Squeeze–recoil Tester under a fi xed squeezing force applied to the upper rod. The rod displacement (the gap between the plates, h(t)), is measured as a function of time. Under a compressive load, the material is squeezed from a gap between the upper and the lower plates (squeezing flow) and the deformation of the tested material is registered in terms of ∆h. Adhesive bonding under a compressive force (I) is followed by the removal of bonding pressure and the relaxation of the adhesive material (II), after which the application of the detaching stress brings the process to an end when the fracture of the adhesive joint occurs (III) at high elongations via adhesive or cohesive mechanisms. Because the three consecutive stages of adhesive bonding under applied pressure, material relaxation, and bond fracture under a tensile force form a unique continuous process, it is not surprising that the detailed conditions of every preceding stage can affect appreciably the mechanism of the debonding process and the value of the practical work of adhesion evaluated with a probe test. The strength of a PSA adhesive joint can be at least a function of both contact time and contact pressure.9,12–15 It is, therefore, important to discuss the mechanism by which the process of adhesive bond formation can contribute to the process of adhesive debonding. As Figure 11.1 clearly illustrates, pressure-sensitive adhesion is a permanent process including three indivisible consecutive stages. This implies that the second (relaxation)

I Compression (bonding)

II

F

h (mm)

1

III

F

F=0

0.1

Relaxation 0

250

500

750 1000 Time (s)

Debonding 1250

1500

1750

FIGURE 11.1 Typical protocol of squeeze–recoil testing of PVP–PEG (36%) adhesive fi lm. Compressive and debonding forces are 0.2 N. (From Feldstein, M.M., in Benedek, I. Ed., Developments In Pressure Sensitive Products, 2nd ed., Chap. 4, CRC-Taylor & Francis, Boca Raton, 2006.)

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11-6


and the third (debonding) stages cannot be studied separately if the adhesive joint in the course of the first stage is never formed. The PSA behavior at each subsequent stage is affected by the conditions of earlier stages, providing a “memory effect.” In other words, at each stage of the process the PSA “remembers” the way as an earlier stage has been performed. In fact, only the second stage, relaxation, does not always occur if compressive bonding load is immediately followed by application of a detaching force, avoiding the stage of PSA relaxation (II; Figure 11.1). As illustrated below, the third stage, adhesive debonding, is a multistage process itself, as evidenced by structural and geometric transformations of adhesive material in the course of a probe tack test. In the following discussion we describe the relaxation properties of PSAs at each subsequent stage of the process and compare them with adhesive behavior to identify the values of relaxation times and corresponding moduli that produce strong pressuresensitive adhesion.

11.2 Relaxation under Compressive Load during Adhesive Bond Formation 11.2.1 Experimental Setup for the Relaxation and Tack Tests To evaluate the relaxation properties of adhesives under bonding pressure and adhesion, the probe tack test is most appropriate. The probe tack test is intended to mimic, in a more reproducible and quantifiable way, the process of touching the adhesive fi lm surface with a finger and sensing the force and energy required to remove it 29 (see also Applications of Pressure-Sensitive Products, Chapter 8). The probe tack experiment can be performed in two different ways. First, when the compression force remains constant during contact time (Figure 11.2a), and second, when the deformation of the adhesive layer is kept constant during the contact time and the material is able to relax in the course of bond formation (Figure 11.2b). If the compressive stress remains constant during the contact time (Figure 11.2a), the stage of relaxation is missed. The sample comes into contact with the tester probe until the predetermined contact force (10 N, in our case) is attained. Then the motor maintains a constant contact force during the specified bond formation time, which in turn decreases the thickness of the adhesive. At the end of the contact time, the probe is then separated from the adhesive with a constant velocity and a tensile force is recorded as a function of time (displacement).13 When, on the other hand, the deformation of the adhesive layer is kept constant, the probe test, as well as the squeeze–recoil test presented in Figure 11.1, can be divided into three successive stages (Figure 11.2b). The first stage is compression, when the flat cylindrical probe approaches the adhesive layer with a constant velocity, penetrates 0.1 mm into its depth, and then stops. The averaged thickness of all tested samples was ∼0.5 mm; thus, the deformation of the adhesive layer never exceeded 20%. The second is the stage of relaxation, when the adhesive material under the probe relaxes during the predetermined contact time (we varied contact times from 1 to 1000 s). The third stage is debonding, when the probe is removed with a constant debonding rate of 0.1 mm/s. Nominal stress (σn) and strain (ε) curves are obtained using the values of the initial film thickness (h0) and the initial contact area (A): σ = F(t)/A and ε = (h(t) − h0)/h0.

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11-7


σ (MPa)

t (s)

Compression

Debonding

(a)

Debonding

Relaxation

Compression

σ (MPa)

t (s)

(b)

FIGURE 11.2 Nominal stress versus time (displacement) curves of a typical probe test. (a) The compressive stress remains constant during the contact time and (b) the strain of the adhesive layer is kept constant during the contact time and the adhesive is allowed to relax in the course of bond formation.

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11-8


Of course, the more a PSA is viscoelastic and the thicker the layer is, the more the details of the bonding stage will affect the debonding stage.

11.2.2

Adhesion in the Absence of Pressure-Sensitive Adhesive Relaxation

As illustrated in Figure 11.3, if the adhesive is not allowed to relax during or after the process of bond formation, the value of contact time has no effect on the mechanism of adhesive deformation in the course of the debonding process. The sole parameter that demonstrates smooth growth with contact time is the value of maximum stress, whereas the practical work of adhesion (the area under the stress–strain curve) tends to increase insignificantly with contact time. The most plausible explanation for the observed rise of peak stress is the improvement of probe contact with the adhesive layer. At the microscopic level this means that the average size of the contact defects (microbubbles) decreases with contact time.23 Another possible reason behind the increase of maximum debonding stress is that the initial thickness of adhesive fi lm decreases slightly with contact time.

11.2.3

Relaxation and Adhesion of Typical Pressure-Sensitive Adhesives

An alternative procedure for the probe tack test includes the relaxation of compressive stress during contact formation, while the probe position (and material deformation) remains fi xed with contact time (Figure 11.2b). This procedure has the advantage of keeping the layer thickness independent of contact time and therefore avoiding any

0.8

500 s

σ (MPa)

0.6

300 s 0.4 50 s 1s

0.2

0.0 0

2

ε

4

6

FIGURE 11.3 The effect of contact time on the curves of probe separation for the acrylic adhesive Gelva 3011. Bonding pressure is constant (0.8 MPa) and debonding velocity is 0.1 mm/s.

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dependence of the debonding process on the layer thickness itself.30 The relaxation curves for PIB (Oppanol B15), silicone (BIO-PSA® 7-4302), and SIS-based Duro-Tak® 34-4230 PSAs are illustrated in Figure 11.4.31 By fitting the exponential relaxation curves in Figure 11.4 with the sum of exponents (Equation 11.2), it has been established that three populations of relaxation times and corresponding moduli can approximate the curves of stress relaxation in Figure 11.4. Values are presented in Table 11.1. For all adhesives examined, the following ranges of relaxation times and moduli have been established: Geq = 0–0.6 MPa, G1 = 0.08–0.8 MPa, τ1 = 1–6 s, G 2 = 0.06–0.5 MPa, τ2 = 5–50 s, G3 = 0.02–0.2 MPa, τ3 = 30–800 s. Figures 11.5 through 11.8 illustrate the effects of contact bonding time on the tack behavior for a variety of PSAs.31 All employed adhesives can be classified into two 0.35 0.30 0.25

σ (MPa)

0.20 SIS

0.15 0.10 0.05

Silicone PIB

0.00 0

20

40

60

80

100 t (s)

120

140

160

180

200

FIGURE 11.4 Compressive stress relaxation curves for PIB (Oppanol B15), silicone (BIO-PSA 7-4302), and SIS-based (Duro-Tak 34-4230) adhesives. TABLE 11.1 Relaxation Times and Corresponding Moduli for Acrylic (Gelva 3011), PIB (Oppanol), Silicone (BIO-PSA 7-4302), and SIS-Based (Duro-Tak 34-4230) Adhesives Obtained by Fitting the Compressive Stress Relaxation Curves with Equation 11.2 Adhesive Type

Geq (MPa)

G1 (MPa)

τ1 (s)

G2 (MPa)

τ2 (s)

GB (MPa)

τ3 (s)

BIO-PSA 0.025 ± 0.0007 0.8 ± 0.02 3.3 ± 0.1 0.5 ± 0.02 21.4 ± 0.8 0.2 ± 0.005 150 ± 5.4 7-4302 Oppanol 0 0.57 ± 0.01 1.2 ± 0.03 0.4 ± 0.01 7.2 ± 0.2 0.07 ± 0.003 48 ± 1.8 B15 Oppanol 0 0.2 ± 0.01 1.8 ± 0.04 0.17 ± 0.01 5 ± 0.4 0.02 ± 0.005 35 ± 3.4 B12 Duro-Tak 0.6 ± 0.005 0.2 ± 0.006 5.3 ± 0.3 0.1 ± 0.004 44.5 ± 2.9 0.2 ± 0.004 770.7 ± 56.1 34-4230 Gelva 3011 0.035 ± 0.0006 0.08 ± 0.003 5.1 ± 0.3 0.06 ± 0.002 43.4 ± 3.04 0.05 ± 0.001 355.7 ± 20.3

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0.8 0.6

σ (MPa)

0.4 0.2 0.0 −0.2 −0.4 0

200

400 Time (s)

600

800

FIGURE 11.5 Effect of contact time on probe tack curves of the silicone adhesive BIO-PSA 7-4302. The initial bonding pressure is 0.8 MPa and the velocity of probe separation is 0.1 mm/s. 0.15 0.10

σ (MPa)

0.05 0.00 −0.05 −0.10 −0.15 0

50

100

150

200 250 Time (s)

300

350

400

200 250 Time (s)

300

350

400

0.3

σ (MPa)

0.2 0.1 0.0

−0.1 −0.2 −0.3

0

50

100

150

FIGURE 11.6 Kinetics of nominal compressive stress relaxation during adhesive bond formation, followed by the debonding process for PIB Oppanol B12 (top) and Oppanol B15 (bottom).

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0.3 0.2

σ (MPa)

0.1 0.0 −0.1 −0.2 −0.3 0

FIGURE 11.7

200

400

600 Time (s)

800

1000

1200

Probe tack curves of Duro-Tak 34-4230 obtained under different contact times. 0.12 0.08

Stress (MPa)

0.04 0.00 −0.04 −0.08 −0.12 0

FIGURE 11.8

200

400

600 Time (s)

800

1000

1200

Effect of contact time on probe tack of acrylic adhesive Gelva 3011.

groups: (1) fully relaxing PIB adhesives (Figure 11.6), such as Oppanol B15 and Oppanol B12 PSAs, described in Technology of Pressure-Sensitive Adhesives and Products, Chapter 4, and (2) adhesives that are able to store energy in the course of deformation and exhibit a residual (unrelaxed) stress during the contact time. SIS-based (Duro-Tak34-4230, Figure 11.7) and, to a somewhat lesser extent, the acrylic Gelva® 3011 PSA (Figure 11.8), belong to the second group. Silicone adhesive BIO-PSA 7-4302 (Figure 11.5) takes an intermediate position.

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As a rule, the appearance of the second maximum on probe tack curve results from a network structure of adhesive material. For instance, in the SIS-based Duro-Tak-344230 adhesive, the network structure is provided by the glassy domains of polystyrene blocks. Polymer networks frequently reveal an apparent yield stress that is defi ned as the minimum pressure at which the material flows extremely slowly under an applied stress.28 Such behaviors are typical for SIS-based Duro-Tak 34-4230 (Figure 11.7) and, to a less pronounced extent, for acrylic Gelva 3011 (Figure 11.8) adhesives. As follows from the data in Table 11.1, 31 the relaxation times of SIS DURO-Tak 34-4230 and acrylic Gelva 3011 PSAs are appreciably higher than those for PIB Oppanol B12, Oppanol B15, and silicone BIO-PSA 7-4302. Thus, the longer relaxation processes are mostly associated with the appearance of a pronounced plateau on the debonding curves (BIO-PSA 7-4302, Figure 11.5), or even a second maximum (SIS Duro-Tak 34-4230, Figure 11.7, and acrylic Gelva 3011 PSA, Figure 11.8). Fully relaxing, soft viscous adhesives such as Oppanol B12 or B15 have low relaxation times and a liquid-like debonding mechanism typical of fluid adhesives. The values of characteristic modulus associated with shortest relaxation times, G1, are generally higher than G 2 and G3. The equilibrium relaxation modulus, Geq, is a direct measure of the stored elastic energy in a polymeric material. Fluid polyisobutene (PIB) adhesives (Oppanol B12 and Oppanol B15) do not reveal any apparent yield stress and, consequently, no equilibrium modulus. The complex geometry of the probe tack test does not allow us to present a straightforward physical meaning of the two shorter relaxation time values, because in addition to the viscoelastic response of the adhesive material and small-scale recovery of the material structure, these values account for the formation of adhesive contact and instrument compliance. However, let us take into consideration that the longer relaxation time has been earlier demonstrated to contribute to high adhesive strength (see Equation 10.6 in Chapter 10).

11.2.4

Correlation among Contact Time, Relaxation Time, and Adhesion

As the data presented in Figures 11.5 through 11.8 illustrate, 31 the contact time does not affect appreciably the mechanism of the debonding process. The peak stress increases with increasing contact time during the fi rst 20 s and achieves its limiting value after 50 s for PIB and after 100 s for all other examined PSAs. The value of the practical work of adhesion, W, is demonstrated in Figures 11.9 through 11.11, along with the maximum debonding stress as a function of contact time. The work of debonding is an increasing function of contact (relaxation) time. The maximum practical work of adhesion is achieved if the adhesive material is allowed to relax for 200 s or longer. For fluid adhesives (PIB Oppanol B12 and B15) the time to achieve a steady-state regime of W values is shorter: 25 and 50 s, respectively. Th is indicates that for maximum adhesive strength the contact time of adhesives should be comparable with the longer relaxation time. Of course, many PSAs have very long relaxation times because they are very solidlike and the contact time dependence more reflects the annealing of surface defects

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1.0 700

W

W (J/m3)

σmax

600

0.6

550

σmax (MPa)

0.8

650

0.4 500 0.2

450 0

100

200 300 Time (s)

400

500

FIGURE 11.9 Debonding energy (W) and maximum stress values (σmax) versus the contact time for silicone adhesive BIO-PSA 7-4302.

120 Oppanol B12

W (J/m2)

100

Oppanol B15

80

60

40

20 0

50

100

150 Time (s)

200

250

300

FIGURE 11.10 Practical work of adhesion (W) versus the contact time for PIB Oppanol B12 and Oppanol B15 adhesives.

than a true relaxation of the material per se, although the relaxation process contributes to the driving force for such annealing. At best, this example is illustrative of the fact that, in general, it is not wise to ignore the effect of the bonding stage when analyzing the debonding stage. This is particularly true when the PSA is very fluid-like with an important viscous component.

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0.40 550 0.36

W

500 450

0.28

400

0.24

350 Peak 2

0.20 Peak 1

0.16

W (J/m2)

Stress (MPa)

0.32

300 250 200

0

200

400 600 Time (s)

800

1000

FIGURE 11.11 Effect of contact time on the work of debonding and the values of two stress peaks for SIS-based Duro-Tak 34-4230 PSA.

11.2.5

Impact of the Relaxation of Entangled and Network Structures on Adhesion

Although it is difficult to attribute unequivocally the shorter and intermediate values of the relaxation times to the relaxation of polymer segments and macromolecules due to the complex geometry of the probe tack test, the longer relaxation times of the order of 50 s and higher relate most likely to the effect of a complex polymer architecture, network structure, and the effect of polymer chain entanglements.32 Let us compare first the relaxation and debonding curves for two PIB adhesives of different molecular weights illustrated in Figure 11.10.31 Oppanol B12 is reported to have Mw = 51,000 g/mol and Oppanol B15 has Mw = 88,000 g/mol. The entanglement molecular weight of PIB is reported to be 8700 g/mol.33 As evident from Figure 11.10, for lower Mw PIB the practical work of adhesion, W, achieves its steady-state values of 100–110 J/m2 almost instantaneously, whereas for the higher Mw fraction this process takes appreciable time that is comparable with the value of longer relaxation time for this polymer (50 s). Fluid adhesives relax faster than the elastic adhesives; however, the observed difference in behavior of these two PIB adhesives is not only due to the difference in their MW. The aggressive tack of highly soft, low-MW PIB hastens the formation of good adhesive contact and less time is required to achieve the maximum value of the practical work of adhesion. The correlation between relaxation and adhesion is presented in Figures 11.12 through 11.14 for acrylic Gelva 3011, silicone BIO-PSA 7-4302 and for SIS-based physically cross-linked Duro-Tak 34-4230 adhesives. The former (Figure 11.13) represents the behavior of fluid, fully relaxing adhesives with very low values of equilibrium relaxation modulus, Geq, whereas the latter stores mechanical energy under bonding pressure, demonstrating a high value of equilibrium relaxation modulus (see Table 11.1) and appreciable unrelaxed residual stress (Figure 11.14). As demonstrated by the stress relaxation curves, the initial period of fast relaxation (20–35 s) is followed by an intermediate period and

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11-15


1000

0.15 W

0.14

900 800

0.12 700

Peak 2

0.11

600

Peak 1

0.10

W (J/m2)

Stress (MPa)

0.13

500

0.09

400

0.08 200

0

400 600 Time (s)

800

1000

FIGURE 11.12 The contact time dependence of the work of debonding and the values of two stress peaks on probe tack curve for acrylic Gelva 3011 adhesive.

0.25 W

0.05

600

Intermediate

0.10 Fast

σ (MPa)

0.15 Slow

W (J/m2)

700

0.20

500

0.00 0

200

400 600 Time (s)

800

1000

FIGURE 11.13 Effect of contact time on the practical work of adhesion compared with the bonding stress relaxation curve for silicone BIO-PSA 7-4302 adhesive.

the process of slow relaxation (since 125–150 s). Shorter relaxation times dominate within the period of fast relaxation, whereas the longer relaxation times govern the steady-state (equilibrium) adhesion of both types of PSAs. In both cases, achieving maximum adhesion falls the end of the intermediate relaxation period and the onset of slow relaxation. Thus, in full agreement with Equation 10.6 presented in the Chapter 10, high adhesion is associated with longer relaxation time. The relaxation mechanism provides the links between all the stages of the process of pressure-sensitive adhesion.

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600 W

400

Intermediate

0.15

0.10 0

W (J/m2)

500

Fast

σ (MPa)

0.20

300 Slow 200 200

400 600 Time (s)

800

1000

FIGURE 11.14 Comparison of the bonding stress relaxation curve with the change in the practical work of adhesion as a function of contact time for SIS-based Duro-Tak 34-4230 PSA.

11.2.6

Optimum Range of Longer Relaxation Times Providing Strong Adhesion

As demonstrated in Chapter 10,22 for high adhesion a compromise must be reached among the values of the cohesion energy, diff usion coefficient, and relaxation times of PSAs. Figure 11.15 establishes the correlation between the practical work of adhesion and the values of longer relaxation times measured for the examined adhesives. Adhesion appears only when the longer relaxation times exceed 50 s and increases, passing through a maximum (acrylic Gelva 3011 PSA) at τ3 = 330–380 s. A further increase in longer relaxation times results in a gradual decline in adhesion. Good adhesion is assured as long as the longer relaxation time varies in the range from 150 to 800 s.

11.2.7

Deborah Numbers of Pressure-Sensitive Adhesives

Standard viscoelastic polymers have well-defined relaxation times such as a reptation time, rouse time, or entanglement time. Such relaxation times do not depend on the observation time and the Deborah number (for a given material) can only be changed by changing the observation time or the temperature. On the other hand, PSAs have very complex relaxation spectra and the approximate results obtained from the fits of Equations 11.2 and 11.3 to experimental data depend markedly on the experimental window used. Figures 11.16 and 11.17 illustrate the effect of the observation window, tobs, on the values of the fitted value of the longer relaxation time, τ3, for SIS-based Duro-Tak 34-4230, acrylic Gelva 3011, silicone BIO-PSA 7-4302, and PIB Oppanol B12 and B15 adhesives. As illustrated by Figures 11.16 and 11.17, for all PSAs examined except the most fluid low-molecularweight PIB (Oppanol B12), the fitted longer relaxation time is an increasing linear function of the observation window, although the points for the smaller observation window

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900

900 τ

800

700

700

600

600 W

500

500

400

400

300

300

200

200

100

100

0

W (J/m2)

τ3 (s)

800

O B1 ppa 2 no

l

l O B1 ppa 5 no

B 7− IO43 PS 02 A ®

G 30 elv 11 a ®

D 34 uro − 4 -Ta 23 k ® 0

0

FIGURE 11.15 Longer relaxation times and practical work of adhesion for SIS-based Duro-Tak 34-4230, acrylic Gelva 3011, silicone BIO-PSA 7-4302, and two grades of PIB adhesives (Oppanol B12 and B15). Observation time is 1000 s.

800 700

SIS

600

τ3 (s)

500 Acrylic

400 300 200

Silicone

100 0 200

400

600 tobc (s)

800

1000

FIGURE 11.16 Effect of the observation window on the values of longer relaxation time, τ3, for SIS-based Duro-Tak 34-4230, acrylic Gelva 3011, and silicone BIO-PSA 7-4302 adhesives.

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60 55 50

3 (s)

45

Oppanol B15 nD = 0,06

40 35 30

Oppanol B12 25 20 15 100

150

200

250

300

t obs (s)

FIGURE 11.17 Effect of the observation window on the values of longer relaxation time, τ3, for PIB Oppanol B12 and B15 adhesives.

TABLE 11.2 Values of Deborah Numbers, nD, Determined for SIS-Based (Duro-Tak 34-4230), Acrylic (Gelva 3011), Silicone (BIO-PSA 7-4302), and PIB (Oppanol B15) Adhesives by Treating the Relationships between the Longer Relaxation Times and Observation Times PSA SIS (Duro-Tak 34-4230) Acrylic (Gelva 3011) Silicone (BIO-PSA 7-4302) PIB (Oppanol B15)

Deborah Number (nD)

R2

Notes

0.857 ± 0.090 0.989 ± 0.089 0.388 ± 0.061 0.155 ± 0.008 0.058 ± 0.027

0.9788 0.9921 0.9765 0.9952 0.8378

Complete data set Taking four last points only Taking four last points only Complete data set Complete data set

tend to deviate from the linear relationship for the most elastic, cross-linked SIS Duro-Tak 34-4230 and acrylic Gelva 3011 adhesives. Interestingly, however, the slopes of the linear parts of the curves presented in Figures 11.16 and 11.17 are constant and can be related to the Deborah number, nD, of the PSAs. The values of the Deborah numbers are listed in Table 11.2, along with parameters of linear regression, R2. All PSAs are viscoelastic materials that couple the properties of solids and liquids. Values of the Deborah number are informative regarding the liquid-like and solid-like contributions. As nD = 1, a material is 50% solid and 50% liquid. Th is is the case of physically cross-linked SIS-based adhesive characterized by the magnitude of the Deborah number that approaches unity (Table 11.2). For all other adhesives considered, the liquid-like viscous contribution dominates the solid-like elastic contribution. Whereas the elastic contribution is appreciable for the chemically cross-linked acrylic Gelva 3011

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11-19

adhesive (nD = 0.388), silicone BIO-PSA 7-4302 and PIB adhesives are typical viscous liquids. For most liquid-like, lower-MW PIB fractions (Oppanol B12), the fitting of stress relaxation curves with Equation 11.2 at observation times less than 200 s does not yield plausible values of relaxation times (Figure 11.17). Cross-linked adhesives (SISbased and acrylic), as well as silicone PSA, which exhibit the best adhesion, demonstrate Deborah numbers between 0.15 and 1. The values of the Deborah number described in this section are only approximate and are calculated with longer relaxation times. They are only used to give an idea of the type of behavior (liquid or solid) of the PSA examined. In reality, the spectrum of the relaxation times in the linear viscoelastic region is a well-defined material property that does not depend on observation time. However, such viscoelastic properties as moduli G′ and G″ depend on both the relaxation time and the observation time.

11.2.8

Relaxation and Adhesion of a Model Pressure-Sensitive Adhesive Based on Interpolymer Complex

11.2.8.1 Model Pressure-Sensitive Adhesive Employed To gain improved insight into the relaxation properties that are responsible for the high adhesion of PSAs, the quantitative structure–property relationship (QSPR) is useful. In turn, the QSPR investigation requires the development of a model adhesive. Variation of the model adhesive composition offers a convenient tool to manipulate simultaneously the structure, adhesion, and relaxation properties. To evaluate the molecular nature of relaxation mechanisms that govern the adhesive behavior of PSA materials, in the present work we studied the relaxation and adhesive performance of a polybase–polyacid intermolecular complex as a function of its composition. The polybase was a copolymer of dimethylaminoethyl methacrylate (DMAEMA) with alkyl methacrylate (AMA), whereas the polyacid was a copolymer of methacrylic acid (MAA) with ethylacrylate (EA). Triethylcitrate (TEC) served as a plasticizer for the poly(DMAEMA-co-AMA)– poly(MAA-co-EA) blends. We studied the blends with different plasticizer content (25–45 wt %). In this particular region of the plasticizer concentration, the complex demonstrates pressure-sensitive character of adhesion.34 The molecular structure, adhesion, and viscoelastic properties of the complex are described in Technology of PressureSensitive Adhesives and Products, Chapter 7.35 11.2.8.2 Adhesive Properties The adhesive properties of the interpolymer complex versus its plasticizer content were studied using the probe tack method. Figure 11.18 illustrates the effect of TEC content on the values of maximum stress and the practical work of adhesion (area under the probe tack curve) for the interpolymer complex.36 Whereas σmax is a decreasing function of the plasticizer content, the work of adhesion passes through a maximum at 35 wt % TEC. The adhesive behavior of the blend with 35 wt % TEC corroborates the fact that the pressure-sensitive type of adhesive behavior requires a specific balance between solidlike and liquid-like properties.

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44

0.45 0.40

40

W

σmax

32 0.30 28 0.25

W (J/m2)

σmax (MPa)

36 0.35

24

0.20

20 24

28

32 36 TEC (wt %)

40

44

FIGURE 11.18 Values of maximum stress and the practical work of adhesion versus plasticizer (TEC) concentration for a model PSA adhesive based on a polybase–polyacid complex. Contact time, 1 s. Debonding rate, 0.1 mm/s. (From Novikov M.B., Kiseleva T.I., Anosova J.V., Singh P., Cleary G.W., and Feldstein M.M., Proceed. 30th Annual Meeting Adhesion Soc. Tampa, FL, 2007.) 1.6 25% TEC 1.4 30% TEC

σ (MPa)

1.2 1.0 0.8 35% TEC 0.6

40% TEC

0.4

45% TEC

0.2 0

50

100 Time (s)

150

200

FIGURE 11.19 Relaxation curves obtained in the course of adhesive joint formation for the blends of an interpolymer complex with different amounts of plasticizer (TEC). Contact time, 200 s. (From Novikov M.B., Kiseleva T.I., Anosova J.V., Singh P., Cleary G.W., and Feldstein M.M., Proceed. 30th Annual Meeting Adhesion Soc. Tampa, FL, 2007.)

11.2.8.3 Relaxation Properties Relaxation curves obtained in the course of adhesive joint formation between an adhesive layer and a probe are illustrated in Figure 11.19. The results of fitting the curves in Figure 11.19 with Equation 11.2 are listed in Table 11.3. We realize that the time chosen for this adhesive contact time of 200 s is insufficient to precisely identify very long relaxation

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11-21

Significance of Relaxation for Adhesion of PSA TABLE 11.3 Relaxation Properties of a Model PSA Made Up of a Polybase–Polyacid Interpolymer Complex Containing Different Amounts of Plasticizer (TEC) % TEC 25 30 35 40 45

Geq (MPa)

G1 (MPa)

1.342 ± 0.004 1.238 ± 0.004 0.515 ± 0.003 0.354 ± 0.002 0.180 ± 0.004

0.051 ± 0.003 0.110 ± 0.004 0.135 ± 0.002 0.058 ± 0.003 0.252 ± 0.011

τ1 (s) 1.29 ± 0.13 2.84 ± 0.13 0.66 ± 0.02 0.67 ± 0.08 2.55 ± 0.22

G2 (MPa) 0.091 ± 0.002 0.186 ± 0.003 0.314 ± 0.006 0.376 ± 0.002 0.421 ± 0.009

τ2 (s) 12.54 ± 0.56 16.29 ± 0.57 20.83 ± 0.38 11.3 ± 0.15 16.18 ± 0.77

G3 (MPa) 0.164 ± 0.002 0.106 ± 0.002 0.415 ± 0.003 0.526 ± 0.001 0.377 ± 0.009

τ3 (s) 161.12 ± 8.35 145.8 ± 15.18 111.99 ± 3.14 105.27 ± 1.31 91.33 ± 4.64

Note: Observation time is 200 s.

160 140

Time (s)

120 100 80 60 40 20 0 25

FIGURE 11.20 mer complex.

30

35 TEC (wt %)

40

45

Relaxation times versus TEC concentration in a polyacid–polybase interpoly-

processes within the polymer system. However, using the exponential Equation 11.2 and the Kelvin–Voigt model, we are able to define the short and large-scale relaxation processes within this time region. As illustrated in Figure 11.19, there is a sharp transition in the relaxation behavior of the blends that contain 30 and 35 wt % TEC. Although the interpolymer complexes containing 25 and 30 wt % TEC demonstrate a highly pronounced residual stress on the relaxation curves, which is a characteristic feature of crosslinked and ordered structures, the relaxation curves of the blends with 35 wt % TEC and more demonstrate a gradual decrease in stress that is rather typical of viscous liquids. Adequate fitting of the relaxation curves in Figure 11.19 with Equation 11.2 is possible using a sum of three exponents. The effect of TEC concentration on relaxation times is illustrated in Figure 11.20. The longer relaxation time is a decreasing function of plasticizer content. Accordingly, the values of the equilibrium relaxation modulus reduce with the increase in TEC concentration (Table 11.3). Faster relaxation processes, τ1 and τ2, are unaffected by the concentration of the plasticizer (Figure 11.20) that controls the

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adhesive properties (compare with the data in Figure 11.18). Based on this observation, a logical deduction can be drawn that the large-scale relaxation processes, characterized by the value of the longer relaxation time, contributes more to PSA performance. 11.2.8.4

Effect of Contact Time on Adhesion

Figure 11.21 illustrates the effect of contact time on the typical curve of nominal compressive stress relaxation during adhesive bond formation, followed by the debonding process, for the model adhesive based on an interpolymer complex and plasticized with 35 wt % TEC. The variation in contact time does not change the mechanism of the debonding process. The curve presented in Figure 11.21 is typical and relates to the blend that exhibits the best adhesion (compare with Figure 11.18). Figures 11.22 and 11.23 demonstrate the effect of contact time on the value of the practical work of adhesion, W, and maximum stress. Both σmax and W achieve their limiting values at contact times of ∼50 s. This time corresponds to the beginning of the domination of slow relaxation processes (compare with Figure 11.19) and, as a consequence, to the onset of large-scale rearrangements within the structure of the adhesive polymer. However, this tendency is less pronounced for the blend that contains 45 wt % TEC (Figures 11.22 and 11.23). Indeed, this blend exhibits a liquid-like behavior that is characterized by a faster relaxation. The σmax values increase with increasing contact time, whereas the increase in plasticizer concentration in the interpolymer complex results in a decrease of maximum stress values (Figure 11.22). However, the values of σmax for the blend containing 35 wt % TEC for a contact time longer than ∼50 s are higher than for other blends. It is important

0.8

Stress (MPa)

0.4 0.0 −0.4 −0.8 −1.2 −1.6 0

50

100 Time (s)

150

200

FIGURE 11.21 Effect of contact time on the curves of bonding stress relaxation, followed by probe separation from adhesive fi lm surface under detaching force for the model PSA based on an interpolymer polybase–polyacid complex containing 35 wt % TEC.

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35%

25%

0.6

30% 0.5 σmax (MPa)

40% 0.4

45%

0.3

0.2 0

50

100 150 Contact time (s)

200

FIGURE 11.22 Effect of contact time on the maximum values of probe detaching stress, σmax, for a plasticized interpolymer polyacid–polybase complex. (From Novikov M.B., Kiseleva T.J., Anosova J.V., Singh P., Cleary G.W., and Feldstein M.M., Proceed. 30th Annual Meeting Adhesion Soc. Tampa, FL, 2007.)

100

35% 30%

W (J/m2)

80

25%

60

40%

45% 40

20 0

50

100 150 Contact time (s)

200

FIGURE 11.23 Effect of contact time on the practical work of adhesion (W) for the plasticized interpolymer complex. (From Novikov M.B., Kiseleva T.J., Anosova J.V., Singh P., Cleary G.W., and Feldstein M.M., Proceed. 30th Annual Meeting Adhesion Soc. Tampa, FL, 2007.)

to note that the dependence of adhesion parameters on contact time for the blend that contains 45 wt % TEC is less pronounced than that for blends with a lower plasticizer content. Indeed, the relaxation of the most fluid blend containing 45 wt % TEC occurs much faster than for other blends. The values of practical work of adhesion for the blend containing 45 wt % TEC are much lower than for the blends containing 30 or 35 wt % TEC (Figure 11.23). On the other hand, the interpolymer complexes with still lower

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160

100

W (J/m2)

80

120

60 80 Contact time 1 s

Relaxation time (s)

Contact time 200 s

40 40 20 25

30

35 TEC (wt %)

40

45

FIGURE 11.24 Effect of TEC content on the values of longer relaxation time, τ3, and practical work of adhesion, W, for the model PSA based on an interpolymer complex.

plasticizer contents demonstrate solid-like behavior, with a highly pronounced maximum on the debonding curve and a low value of maximum elongation. 11.2.8.5 Comparison of the Composition Dependence of Adhesion and Relaxation Time Figure 11.24 illustrates the general conclusion that can be derived from the analysis in this work. Large-scale relaxation processes within the polymer system, characterized by the values of the longer relaxation time, τ3, predominantly govern pressuresensitive adhesion performance. This result confirms the prediction of Equation 10.6 in Chapter 10, which states that longer relaxation times are of particular importance for high adhesion. The best adhesion is observed for interpolymer complex blends with longer relaxation times between 100 and 145 s. These values are appreciably longer than those obtained earlier for a range of commercial adhesives (Figure 11.25) that possess a somewhat higher adhesion at a comparable observation time of 200 s. 11.2.8.6 Main Conclusions Relaxation properties and adhesion of PSAs have been studied with the probe tack method under conditions corresponding to adhesive bond formation. Typical examples of various PSA classes were examined: adhesives based on the SIS block copolymer, PIB of two molecular weights, and acrylic and silicone PSAs. In addition, an interpolymer complex between a polybase and a polyacid has been employed as a model PSA to elicit the structure–property relationship. Under the conditions corresponding to the process of adhesive bond formation under compressive force, for which the mode of deformation is typically in shear, PSAs reveal three retardation times, which, in their magnitudes, are about 1 decade apart. Only the

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900

3

80

800

600 500

40

W

400

W (J/m2)

Relaxation times (s)

700 60

300 20

200 2

100

1

PI B

D Acr 87 uro ylic -9 -Ta 00 k A

A G cryl e 30 lva ic 11

SI

Si lic on e

0 S

0

FIGURE 11.25 Comparison of relaxation times at observation time of 200 s and practical work of adhesion for a series of commercial adhesives (SIS, silicone, cross-linked acrylic Gelva 3011, un-cross-linked acrylic Duro-Tak® 87-900A, and PIB).

longer retardation times (150–800 s) are significant for high adhesion of various PSAs and they relate mainly to the energy-dissipating processes and chain entanglements, which in turn are associated with translational movement (self-diff usion) of polymer segments and entire macromolecules in the course of large-scale structural rearrangements. The minimum values of longer relaxation times are typical of fluid adhesives. Whether chemically or physically cross-linked, network adhesives reveal much greater values of longer relaxation times. Compressive stress relaxation in the course of adhesive bonding defines the mechanism of adhesive joint failure during debonding under a tensile detaching force. The adhesives exhibiting complete stress relaxation debond mainly as fluids, with or without a pronounced plateau on the probe tack stress–strain curves. In contrast, network adhesives, such as SIS and covalently cross-linked acrylic PSAs, are capable of storing mechanical energy during the bonding stage. They demonstrated the occurrence of residual, unrelaxed stress and typically have two peaks on the debonding stress–strain curves. For all PSAs, the practical work of adhesion achieves its maximum value as the contact time becomes comparable with the longer relaxation time or, more precisely, as the mechanism of slow relaxation is rendered dominating. If the stress during adhesive bonding is not allowed to relax, the mechanism and the energy of debonding are independent of the contact time. However, if the bonding stress can relax, the contribution of the contact time to the work of adhesive debonding is appreciable. Correlation between the adhesion and relaxation time for all PSAs examined can be described fairly reasonably with Equation 10.6 of Chapter 10, which relates peel adhesion to the relaxation time and translational mobility of adhesive polymer.

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11.3


Relaxation Properties of Pressure-Sensitive Adhesives upon Withdrawal of Bonding Pressure

11.3.1 Approach In this section we consider the relaxation properties of PSAs in the second stage of the process of pressure-sensitive adhesion, defined in Figure 11.1 as relaxation (elastic recovery) upon withdrawal of compressive bonding stress. The squeeze–recoil technique has been used for this purpose and has been described in detail in a range of publications.27,28,37 As model PSAs, we employed a hydrophilic adhesive based on a hydrogenbonded complex of high-molecular-weight PVP with short-chain PEG, 22,24,25,35 as well as the hydrophobic PSAs prepared by blending SIS triblock copolymer, butyl rubber (BR), and PIB with relevant tackifiers and plasticizers. The compositions of hydrophobic adhesives are listed in Table 11.4.37 The retardation times and characteristic moduli of the tested adhesives were estimated by nonlinear fitting the experimental squeeze–recoil curves with Equation 11.3. For slow relaxation processes, the total measuring time of the sample thickness, h, was limited to 6000–7000 s. This total observation time corresponds to an average time of elastic recovery of 2000 s upon the withdrawal of the compressive force.

11.3.2

Applicability of Burger’s Model of Viscoelastic Body for Elastic Recovery of Pressure-Sensitive Adhesives

Burger’s model of viscoelastic liquids can be applied to describe the squeeze–recoil curves of some PSAs. Burger’s model represents the combination of springs and dashpots outlined by the Kelvin–Voigt model of a viscoelastic solid and the Maxwell model of a viscoelastic liquid, which are linked to each other in series.11 Figure 11.26 illustrates the squeeze–recoil profi le of the idealized viscoelastic liquid according to Burger’s model. When the material is subjected to a compressive stress, three different strain responses can be observed.11 1a. Instantaneous step of elastic response due to the Maxwell spring. 2a. Gradual strain development related to the Kelvin–Voigt element, which reaches its equilibrium value with time tending to infi nity. 3a. Purely viscous response of Burger’s model related to the Maxwell dashpot that occurs as the Kelvin–Voigt element has attained its equilibrium. The slope of the strain–time curve is then constant and is equal to the shear rate. TABLE 11.4 Sample SIS + I SIS + R SIS + R + I B + PIB

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Compositions of Hydrophobic Polymers Examined Composition SIS Vector 4111 (57% wt)/Isolene 400 (43% wt) SIS Vector 4111 (50% wt)/Regalite R9110 (50% wt) SIS Vector 4111 (36.4% wt)/Regalite R9110 (27.2% wt)/Isolene 400 (36.4% wt) BR 065 (60% wt)/PIB Vistanex LM-MH (40% wt)

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Viscous flow 1a Strain / deformation

3b 2b

2a

Elastic recovery

3a

1b

Creep phase

Recovery phase

t1

Time

FIGURE 11.26 Typical view of the squeeze–recoil profi le according to Burger’s model. (From Novikov, M.B. et al. J. Adhesion, 81, 77–107, 2005.)

SIS

0.00

−0.04 h − h0 /h0

PVP-PEG 36% −0.08

−0.12

−0.16

0

1000

2000

3000 4000 Time (s)

5000

6000

7000

FIGURE 11.27 Squeeze–recoil profi les for SIS and PVP blends with 36% PEG under stepwise increasing compressive force of 0.5, 1, 2, and 5 N, respectively. (From Novikov, M.B. et al. J. Adhesion, 81, 77–107, 2005.)

When the compressive stress is removed, Burger’s model recovers in a two-step manner: 1b. Strain reduces instantaneously by the elastic response. 2b–3b. Strain reaches a value that is equal to the permanent, nonrecoverable strain and represents the viscous flow of the Maxwell dashpot. Agreement between the behaviors of the idealized Burger model (Figure 11.26) and real adhesives is illustrated in Figure 11.27 and provides qualitative evidence that the examined adhesives behave like linear viscoelastic systems, at least for intermediate

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times, implying the applicability of the squeeze–recoil test for characterization of the relaxation properties of adhesives upon the removal of compressive force.

11.3.3 Squeeze–Recoil Behaviors of Pressure-Sensitive Adhesives Typical squeeze-flow displacement–time curves for the SIS triblock copolymer and for the PVP–PEG adhesive blend are illustrated in Figure 11.27. A remarkable qualitative agreement is observed between the squeeze–recoil behaviors of real materials and the ideal Burger model of a viscoelastic body illustrated in Figure 11.26. As a fixed compressive force is applied to the sample, the gap (h) between the upper and lower plates of the tester, equal to the sample thickness, decreases gradually (Figure 11.27). The higher the squeezing stress, the more deformed the SIS rubber and the PVP–PEG hydrogel. Under a comparable compressive force, the PVP–PEG adhesive is compressed to a greater extent than the SIS rubber, indicating that the PVP–PEG blend is softer. The deformation of the samples under squeeze flow is partly recoverable. As the compressive force is removed, the sample tends to return to its initial shape. The profi le of the squeeze–recoil is indicative of the elastic contribution and relaxation properties of material. As evident from the curves in Figure 11.27, the SIS rubber recovers its initial thickness better compared to the PVP–PEG adhesive model PSA. For the latter, the viscous dissipation of mechanical energy is much more pronounced. Figure 11.28 illustrates the effect of PEG concentration on the squeeze–recoil profi les of PVP–PEG blends under a stepwise increasing compressive force. The higher the

0.0 31% PEG 36% PEG

h − h0 /h0

−0.1

−0.2

39% PEG

−0.3

41% PEG

−0.4 0

1000

2000

3000

4000 5000 Time (s)

6000

7000

8000

FIGURE 11.28 Effect of PEG content on squeeze–recoil profi les of PVP–PEG blends under stepwise increasing compressive force of 0.5, 1, 2, and 5 N. (From Novikov, M.B. et al. J. Adhesion, 81, 77–107, 2005.)

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PEG content, the greater the contribution of plastic deformation. Th is then requires a longer time to recover the equilibrium thickness of the hydrogel upon removal of compressive stress, indicating that the retardation time increases with the rise in PEG concentration.37

11.3.4 Retardation Times Featured for Hydrophilic PVP–PEG Adhesives For evaluation of the relaxation properties of adhesives from the data in Figures 11.27 and 11.28, the values of relative displacement at a recovery step (h − h 0)/h 0, have been taken with a positive sign and plotted against time in Figures 11.29 and 11.30. The (h − h 0)/h 0 value divided by the removed stress yields the compliance, J. The points represent the measured values, whereas the lines are the results of the data fitted with Equation 11.3. As follows from Figures 11.29 and 11.30, Equation 11.3 provides a fairly reasonable fit with a regression coefficient that is always no less than 0.98. Adequate fitting is obtained taking into account two terms in Equation 11.3, whereas adding a third and fourth terms does not improve the fit and yields corresponding retardation times that fall within the range of deviations from the values found with Equation 11.3 in the two-term form. Using Equation 11.3 with a single retardation time does not, however, provide adequate fitting (relevant regression coefficients lie normally around 0.96).

0.08 0.07

5N

0.06 2N h − h0 /h0

0.05 0.04 1N

0.03 0.02 0.01 0.00 0

100

200

300

400

500

600

700

800

900

1000

Time (s)

FIGURE 11.29 Impact of compressive force on the kinetics of strain recovery upon the removal of compressive force for PVP–PEG (36 wt %) adhesive. (From Novikov, M.B. et al. J. Adhesion, 81, 77–107, 2005.)

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0.35 0.30 41% PEG 0.25 h − h0 /h0

39% PEG 0.20 0.15 0.10

36% PEG 34% PEG

0.05

31% PEG

0.00 0

500

1000

1500 2000 Time (s)

2500

3000

FIGURE 11.30 Effect of PEG concentration on the kinetics of strain recovery for PVP–PEG blends. (From Novikov, M.B. et al. J. Adhesion, 81, 77–107, 2005.) 100

6 5

80

60 τ1 (s)

G1 (MPa)

4 3 40 2 20

1

0

0 30

32

34 36 38 PEG content (%)

40

42

FIGURE 11.31 Shorter retardation time and corresponding modulus as a function of the concentration of plasticizer (PEG) in PVP–PEG adhesive blends. The data are averaged for two values of compressive force (1 and 2 N). (From Novikov, M.B. et al. J. Adhesion, 81, 77–107, 2005.)

The coefficients of regression with Equation 11.3 (G1 and G2 moduli) for the squeeze– recoil profiles, illustrated in Figures 11.29 and 11.30, are illustrated in Figures 11.31 and 11.32 as the functions of PEG content in adhesive blends with PVP. Two reliably different retardation times are determined for the PVP–PEG adhesives, which differ

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11-31

5

600

4

500

3

400

2

300

1

200

τ2 (s)

G2 (MPa)


0

100 30

32

34 36 38 PEG content (%)

40

42

FIGURE 11.32 Longer retardation time and corresponding modulus plotted versus PEG concentration in PVP–PEG adhesive blends. The data are averaged for two values of compressive force (1 and 2 N). (From Novikov, M.B. et al. J. Adhesion, 81, 77–107, 2005.)

in their magnitudes by about 1 decade: the shorter time, τ1, is in the range of 10–110 s and the longer time, τ2, is ∼120–950 s. Within the framework of Burger’s model of viscoelasticity, the shorter retardation or relaxation time is mainly associated with an elastic contribution of the spring element into strain recovery, whereas the longer time characterizes the behavior of a coupled dashpot and spring elements of the model and the rate of strain recovery during the dissipation of the initially applied energy. The mechanism of the latter process involves large-scale rearrangement of the structure of the polymeric material via long-range motion (diff usion) of polymer segments and entire macromolecules. This process takes normally a much longer time than the elastic recovery of polymer chain conformations. The longer process may also be associated with the entanglements between polymer chains. The shorter retardation time relates mainly to the restoration of the original conformation of polymer segments between entanglements. As demonstrated in Figures 11.31 and 11.32, with increasing PEG concentration in blends, the shorter retardation time is nearly constant, whereas the longer time increased gradually and the relevant values of corresponding moduli decreased gradually with PEG content. The increase in the longer retardation time as a function of PEG content is also easily observable in Figure 11.28 and reflects the slowing down of strain recovery upon the removal of compressive force. Under relatively moderate compressive forces the retardation times and corresponding moduli are practically independent of the applied compressive stress.37 However, at comparatively high shear stresses (compressive force of 5 N and higher) both the retardation times and the corresponding moduli tend to increase. The higher the stress, the larger the molecular rearrangements occurring in the strained material and the longer the time required for relaxation. The values of retardation times and moduli at relatively moderate compressive forces of 1 and 2 N can be considered material characteristics.

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5

G1, τ1

31

G2, τ1

31 34

4

31 34

G (MPa)

31

36

3

2

36

31

34 36 34

39

41

41

36 1

39

39

39 41 0 0

100

200

300

400

500

τ (s)

FIGURE 11.33 The relationship between retardation times and the corresponding moduli for PVP–PEG blends. The PEG contents (wt %) are indicated. (From Novikov, M.B. et al. J. Adhesion, 81, 77–107, 2005.)

As a rule, to demonstrate the effects of composition on the spectra of retardation times in the examined PSAs, we use the values of retardation times and associated modulus averaged for compressive forces of 1 and 2 N. The retardation times and corresponding moduli are not fully independent material characteristics, but are correlated to each other. As follows from the data in Figure 11.33, in the course of PVP plasticization with PEG, the higher values of the longer retardation time are usually associated with lower values of the corresponding modulus. On the other hand, the shorter retardation time is independent of G1 (Figure 11.31). The inverse proportionality between the retardation time and the corresponding modulus, which is the case for all polymer blends studied in this work, suggests the applicability of the Maxwell, Kelvin–Voigt, and Burger models of viscoelasticity (G = η/τ) to describe the behavior of the hydrophilic PVP–PEG and hydrophobic adhesives. Although the comparison of the idealized behavior of Burger’s model illustrated in Figure 11.26 with the squeeze–recovery profiles of real adhesives (Figure 11.27) characterizes qualitatively these adhesives as linear viscoelastic systems, the data in Figure 11.33 provide quantitative support in favor of this observation and imply that the values of retardation times and corresponding moduli evaluated in this work can be treated as true material constants.

11.3.5 Retardation Times in Hydrophobic Pressure-Sensitive Adhesives Many hydrophobic elastomers have been used to produce PSAs, but generally the elastomers must be blended with tackifiers and plasticizers to obtain optimized adhesion.

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In a PSA formulation of that type, the rubbery polymer provides the elastic component, whereas a low-molecular-weight tackifying resin and a plasticizer constitute the viscous components. Most parent elastomers per se do not have the proper rheology to be PSAs. Typically, the addition of a tackifier raises the glass transition temperature, Tg, lowers the plateau modulus by diluting the chain entanglements of the elastomer, and increases the ratio of viscous to elastic response of the elastomer/tackifier blend, improving both the bond-making and the bond-breaking processes. Plasticizers demonstrate similar effects on rheology, but cause reduction in Tg. It is, therefore, of particular interest to trace how the formulation process affects the relaxation properties of a composite PSA. In this work we use a SIS block copolymer and BR as base elastomers. A hydrocarbon resin (Regalite R9100, R) has been employed as a tackifier. R is a partially hydrogenated resin with a specific balance of aliphatic and aromatic groups. As a plasticizer for the SIS, a low-molecular-weight polyisoprene rubber, Isolene (I), has been used. The pressure-sensitive adhesion in BR is provided by mixing the BR with a low-molecularweight PIB (Vistanex). The compositions of the samples examined in this work are presented in Table 11.4. As evident from the data in Figures 11.34 and 11.35, mixing elastomers (SIS and BR) with plasticizers (I for SIS and low-molecular-weight PIB for BR) and tackifying resin (R) results in an appreciable increase in retardation times (Figure 11.34) and a corresponding decrease in the corresponding moduli (Figure 11.35). However, their effects on the values of the shorter retardation time are less marked compared with the dramatic changes in the longer retardation time. The joint effect of the plasticizer and tackifying resin on the retardation time and corresponding modulus of SIS are much more pronounced than the

P

Retardation times (s); P (N/m)

700 600 500 400 300 200 100

τ1

τ2

0 SIS

SIS+I

SIS+R SIS+I+R

DT

BR

BR+PIB

FIGURE 11.34 Effects of plasticizers [isolene (I) and low-molecular-weight PIB] and tackifier resin [regalite (R)] on retardation times and peel adhesion (P) of SIS and BR compared with DuroTak 34-4230 (DT), used in this work as a typical hydrophobic PSA. (From Novikov, M.B. et al. J. Adhesion, 81, 77–107, 2005.)

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11-34


G1 G2 (102 MPa); P (N/m)

700

P

600 500 400 300 200

G2 G1

100 0 SIS

SIS+I

SIS+R SIS+I+R

DT

BR

BR+PIB

FIGURE 11.35 The impact of plasticizers (I and low-molecular-weight PIB) and tackifier resin (R) upon the retardation moduli and peel adhesion (P) of SIS block copolymer and BR compared to Duro-Tak 34-4230 (DT), used in this work as a typical hydrophobic PSA. (From Novikov, M.B. et al. J. Adhesion, 81, 77–107, 2005.)

separate effects of the plasticizer–tackifier mixture (compare the SIS + I + R system with the SIS + I and the SIS + R blends in Figures 11.34 and 11.35).

11.3.6 Correlation between Retardation Times and Pressure-Sensitive Adhesion The phenomena of tack, peel, and shear have been reported to depend upon the relative participation of the two primary molecular mechanisms of deformation: viscous flow that proceeds by diff usion via free volume and the elastic distortion that stores free energy.38 These two mechanisms are characterized by different time scales. Whereas the process of viscous flow requires appreciable time, elastic flow dominates at shorter time scales. To appreciate the significance of the relaxation properties for the adhesive behavior of polymers we must compare the effects of composition on relaxation and pressure-sensitive adhesion. Figures 11.34 and 11.35 illustrate the correlation between adhesive and relaxation properties for SIS blends with a plasticizer (I) and a tackifier (R), as well as BR plasticized with low-molecular-weight PIB. Unblended SIS and BR reveal no or low adhesion. Mixing the SIS with a plasticizer provides initial tack but comparatively low adhesion, whereas a plasticizer (low-molecular-weight PIB) significantly improves both the tack and the adhesion of BR. In SIS blends with a tackifier the adhesion is much improved. High adhesion is also reported for the ternary SIS blends containing both tackifier R and plasticizer I. As a reference, for hydrophobic PSAs in this work we employ a SIS-based Duro-Tak 34-4230, which demonstrates 180o peel adhesion of 775 N/m. The relaxation properties of this reference sample, tested under comparable conditions, are characterized by τ1 = 17 s, G1 = 0.74 MPa, and τ2 = 356–400 s, G2 = 2.48 MPa (Figures 11.34 and 11.35).

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11-35


600 P

P (N/m) τ (s)

500 400 300

τ2

200 100

τ1

0 30

32

34

36 38 PEG content (%)

40

42

FIGURE 11.36 180o peel adhesion, P, and retardation times of PVP–PEG adhesives as a function of PEG concentration at 50% relative humidity of the surrounding atmosphere. (From Novikov, M.B. et al. J. Adhesion, 81, 77–107, 2005.)

Let us compare now the values reported for the hydrophobic adhesives with values featured for hydrophilic PVP–PEG PSAs. The PVP–PEG system provides an appropriate model to illustrate the effect of the molecular structures underlying pressure-sensitive adhesion, because its adhesive behavior can be related to the changes in structure, interaction mechanism, phase state, and other physical properties as the PEG content is varied. The effects of PEG concentration on adhesive and relaxation properties of PVP– PEG blends expressed in terms of 180° peel force, 39 retardation times, and corresponding moduli are presented in Figures 11.36 and 11.37. Whereas the shorter retardation time is nearly constant, the longer time is a monotonously increasing function of PEG content, and the corresponding moduli decrease with the increase in PEG concentration between 31 and 41% PEG, peel adhesion comes through a maximum at 36% PEG concentration in the blends. The maximum adhesion relates to the PEG concentration at which a stoichiometric PVP–PEG H-bonded complex is completely formed within the PVP–PEG blends.22,24,25,40 This complex demonstrates properties that are not typical of both parent polymers. As evident from the data in Figures 11.36 and 11.37, the maximum adhesion in PVP blends with 34–39% PEG is observed when the shorter and longer retardation times range within 10–65 and 120–450 s, respectively, whereas the values of the corresponding moduli vary between 0.5–2.6 and 0.8–3.8 MPa, respectively. The relaxation properties of the blend containing 36% PEG and providing the best adhesion are characterized by τ1 = 26–46 s, G1 = 1.3–2.17 MPa, and τ2 = 325–430 s, G 2 = 2.94–3.3 MPa. Within the PEG concentration region (31–34%), where debonding occurs through a predominantly adhesive type of bond failure, 38 the gain in adhesion is always associated with an appreciable rise in the value of the longer retardation time (Figure 11.36).

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11-36


8

600 G1 G2

500

P

400

4

300

2

Peel force (N/m)

G (MPa)

6

200 0 30

32

34

36 38 PEG content (%)

40

42

FIGURE 11.37 Effect of PEG concentration on 180o peel adhesion, P, and moduli corresponding to shorter (G1) and longer (G 2) retardation times for PVP–PEG adhesives at relative humidity of 50%. (From Novikov, M.B. et al. J. Adhesion, 81, 77–107, 2005.)

However, as the PEG concentration reaches 36% and higher, the type of debonding becomes miscellaneous (adhesive–cohesive) and this rule no longer holds. For these blends, the longer retardation time continues to increase more smoothly, whereas peel adhesion begins to decrease. In the same manner as that established above for hydrophobic blends based on SIS and BR (Figures 11.34 and 11.35), for hydrophilic adhesives greater adhesion is associated with the values of longer retardation time ranging from 325 to 445 s (Figures 11.36 and 11.37).

11.3.7

Relaxation Criteria for Pressure-Sensitive Adhesion

Summing up the data in Figures 11.34 through 11.37, we come to the relaxation criteria for pressure-sensitive adhesion, which can be stated in a preliminary form as follows:37 1. To be a PSA, polymer compositions preferably possess two retardation times of 10–70 and 300–660 s, respectively. 2. For proper adhesion, the relaxation modulus, G 2, relating to the longer retardation time, is preferably higher than the modulus, G1, corresponding to the shorter retardation times. Because the G 2 and G1 values are the measures of energy dissipated, respectively, for predominantly large-scale and small-scale viscoelastic mechanisms of squeeze–recoil, and because the amount of energy dissipated in the course of the debonding process is the measure of adhesion, this requirement illustrates the prevailing importance of the larger-scale mechanism (that requires appreciable molecular mobility) for pressure-sensitive adhesion. 3. Optimum adhesion is achieved as the absolute values of the G 2 and G1 moduli range between 2.5–3.3 and 0.70–2.20 MPa, respectively.

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11-37

It is evident that further work is needed to demonstrate whether the established values of retardation times and relevant moduli are also typical of the entire variety of PSAs currently available. Furthermore, more data should be obtained to trace quantitative correlations between the adhesion and relaxation characteristics within the window outlined by the above criteria. As follows from the data in Figures 11.34 and 11.35, the retardation times and G1 modulus for SIS + I adhesive fall within the relaxation criteria for PSAs, yet the SIS + I adhesive exhibits a relatively low peel force. Th is is most likely due to the fact that the modulus G 2 = 1.61 MPa for the SIS + I blend is below the lower limit outlined by the relaxation criterion (G 2 = 2.5 MPa). It implies also that there exist different combinations of retardation times and corresponding moduli that are either favorable or unfavorable for high adhesion.

11.3.8 Major Conclusions Under the conditions imitating the removal of compressive force upon adhesive bond formation, for which the mode of deformation is typically in shear, PSAs reveal two retardation times, which are about 1 decade apart in their magnitudes. The shorter retardation times (10–70 s) define the rate of release of stored energy due to the recovery of conformation of polymer chains. The longer retardation times (300–660 s) relate mainly to the energy-dissipating processes and chain entanglements, which are associated with translational movement (self-diff usion) of polymer segments and entire macromolecules in the course of larger-scale structural rearrangements. Both plasticizers and tackifying resins increase the values of retardation times; however, their effects on the longer retardation time are much more pronounced compared with the shorter time. Correlation between adhesion and the retardation time for both hydrophilic and conventional (hydrophobic) PSAs can be described fairly reasonably with Equation 10.6 in Chapter 10, 22 which relates peel adhesion to relaxation time and the translational mobility of the adhesive polymer.

11.4 Relaxation Properties of Pressure-Sensitive Adhesives in the Stage of Debonding 11.4.1 Three-Stage Mechanism of Debonding Debonding is the third and final stage of the process of adhesion (see Section 11.1.2) and, in turn, can be treated as a three-stage process. The peel test geometry does not provide any detailed insight into the mechanisms of debonding of the adhesive layer from the hard surface and, in particular, it is not able to separate the small strain deformation (in the linear regime) from the large strain deformation (in the nonlinear regime) taking place in the adhesive layer during the debonding process. The parallel geometry of flat-end probe tack tests is better adapted for studying the details of the debonding mechanisms of thin layers of soft deformable adhesives under tensile stress.14,17,41,42 In these tests, the adhesive layer is submitted to a uniform distribution of stress and deformation in a confined geometry, making it easier to break up the debonding process into elementary steps. According to a universally accepted description,17 the first stage

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Stress σ (MPa)

0.8

σmax

0.6 0.4

250 µm

σbf

0.2

Wadhesion ~ 100−400 J/m2

0.0 0

1

2 Strain ε

3

4

1 mm

FIGURE 11.38 Direct observation of the debonding mechanisms and stress versus strain curve in the course of probe tack test for SIS-based PSA and corresponding images illustrating transformations of the PSA structure in the plane normal to the adhesive layer at different stages of the debonding process. Debonding velocity 0.1 µm/s. T = 22°C, contact time 1 s. (From Creton, C. et al. in Adhesion: Current Research and Applications, Possart, W.G., Ed., Weinheim, Wiley-VCH, 2005. With permission.)

of the debonding process is a homogeneous small tensile deformation of the adhesive layer until the initiation of failure mechanism occurs through the formation of cavities or cracks at the interface or in the bulk of the adhesive. This stage corresponds to the increase in stress and the appearance of a peak on the probe tack curve (Figure 11.38).43 The second stage of debonding represents the formation of a foamed structure of cavities in the direction normal to the plane of the adhesive fi lm. As a result, the tensile stress decreases, leading to a gradual decline or the formation of a plateau on the probe tack curve. The occurrence of the plateau relates to the elongation of fibrils, which initially represent the walls between neighboring cavities. The third and final stage of debonding involves separation of the adhesive and probe surfaces, either by failure of the fibrils (cohesive failure) or by detachment of the foot of the fibrils from the surface of the substrate (probe). The general features of a stress–strain curve obtained in a probe test of a PSA are characterized typically by four parameters (Figure 11.38): (1) maximum stress, σmax; (2) stress corresponding to the beginning of the fibrillation process, σ bf; (3) maximum extension, εmax; and (4) work of separation, W, defined as the integral under the stress– strain curve multiplied by the initial thickness of the layer h0. Adhesive polymer relaxation is involved in all three stages of the debonding process. In the course of debonding, the elongation of the fibrils often achieves many hundreds and thousands of percents. For the characterization of polymer relaxation as a material property, only the linear elastic region of very small deformations is usually taken into consideration. However, let us recall that adhesion is a process rather than a material property (see Section 11.1.1) and that debonding occurs at very high tensile strains.

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11-39

Consequently, in the following discussion we must consider the relaxation of adhesive polymers at large tensile deformations in various stages of the debonding process. We first consider the transformation of the structure of adhesive material during the debonding process.

11.4.2

Evolution of Pressure-Sensitive Adhesive Structure during the First Stage of the Debonding Process

Because over the course of a probe tack test a shear deformation in the plane of the adhesive layer is coupled with tensile strain in the direction normal to the adhesive fi lm, it is difficult to interpret the corresponding relaxation processes in terms of relaxation moduli and times. Nevertheless, the probe tack test allows an illustrative visualization of the mechanisms of transformations of the PSA structure and geometry during a process of elastic recovery at different stages of the probe tack test. To interpret correctly the probe tack stress–strain curves as characteristics of the process of debonding, it is essential to identify the transformation of the microstructure occurring in the adhesive material over time. With this purpose, we give a qualitative description of the structural changes observed in the adhesive fi lm if probe detachment is stopped for a certain time during different stages of the debonding process. 1. In the course of the first stage of homogeneous deformation, stress increases linearly with deformation under a fi xed debonding rate (Vdeb) before the stress reaches its peak.44 2. In the course of the second stage, intensive cavitation is observed, resulting in decreased stress.16,17,43 Figure 11.39 illustrates the displacement of motors and a typical force curve as a function of time during a relaxation experiment carried out in a probe tack setup. The displacement of the motors driving the probe was stopped at a given moment during different stages of the debonding process. The system, under tensile stress, was then left to relax for a given time, tstop. During this time, relaxation of the force occurs. At the end of the stop the displacement of the probe was resumed until complete debonding of the adhesive occurred.44 The model adhesives used in this study consisted of a series of acrylic copolymers based on 2-ethylhexyl acrylate as a base monomer. As comonomers, they contained increasing amounts (2, 4, and 8 wt %) of acrylic acid (AA). Adhesives are referred to as 2AA, 4AA, and 8AA, respectively. The advantage of using polyacrylates is that the pure polymers exhibit PSA properties without any need for additional formulation ingredients such as tackifying resins. Figure 11.40 illustrates the nominal stress as a function of time for a nonstop test and for three relaxation tests on a steel probe. The relaxation tests were always performed during the increase in force at the beginning of the debonding process before catastrophic failure was observed. The stops from stop 1 to stop 3 were performed at increasing values of stress at the beginning of the relaxation. The level of tensile stress at which the test is stopped and the adhesive allowed to relax is referred to as σ 0. Because this early loading stage of the debonding process is essentially elastic at the

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11-40


Displacement (motors) Vdeb Vdeb

tstop Time

Force

Stop Time

FIGURE 11.39 Force and displacement of the motors for a relaxation test as a function of time. The motors are stopped at a given moment, and the adhesive is allowed to relax for a given time, tstop. (From Lindner, A., Maevis, T., Brummer, R., Lűhmann, B., and Creton, C., Langmuir 20, 9156, 2004. With permission.) 0.30 Without stop

Stress (MPa)

0.25 0.20 0.15

Stop 3

0.10

Stop 2 Stop 1

0.05 0.00 0

50

150

100

200

250

t (s)

FIGURE 11.40 Stress, σ, as a function of time for 2AA acrylic PSA. A test without stop and three tests with stops at different initial values of σ 0: Stop 1, σ0 = 0.08 MPa; Stop 2, σ 0 = 0.16 MPa; Stop 3, σ0 = 0.25 MPa. For all stops, tstop is 180 s. (From Lindner, A., Maevis, T., Brummer, R., Lűhmann, B., and Creton, C., Langmuir 20, 9156, 2004. With permission.)

deformation rates used here, increasing the initial values of σ 0 corresponds to an increase in the amount of elastic energy stored in the adhesive layer at the beginning of the stop. Figure 11.40 illustrates that significant stress relaxation is taking place during the stop.44

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Figure 11.41 (top) illustrates the stress relaxation of the 2AA adhesive on a high-energy steel probe for the three different stops. Figure 11.41 (bottom) demonstrates the same relaxation process for 2AA but on an apolar low-energy poly(ethylene-co-propylene) (EP) substrate. As evident from these data, relaxation of the tensile stress is strongly affected by the nature of the substrate. This implies that stress relaxation is not only a property of the adhesive material, but also the property of the adhesive–adherent pair. In other words, the boundary conditions at the probe–adhesive interface contribute greatly to the relaxation of stress in the course of the debonding process. As follows from Figure 11.41 (top), the decrease in stress slows down toward the end of the stop and finally reaches a nearly constant value, as occurs for viscoelastic materials

0.25

Stress (MPa)

0.20 0.15 Stop 3 0.10

Stop 2 Stop 1

0.05 0.00 0

50

100 t (s)

150

200

150

200

0.25

Stress (MPa)

0.20

0.15 Stop 3 0.10 Stop 1

Stop 2 0.05

0.00 0

50

100 t (s)

FIGURE 11.41 Stress relaxation during probe stops for 2AA acrylic adhesive on high-energy steel (top) and low-energy EP substrates (bottom). (From Lindner, A., Maevis, T., Brummer, R., Lűhmann, B., and Creton, C., Langmuir 20, 9156, 2004. With permission.)

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possessing a yield stress. When visualizing the debonding process, it is obvious that most of the cavity growth and cavitation takes place during the first 60 s. The further comprehension of structural evolution process is possible if the characteristic times of the structural evolution are taken into consideration. To describe precisely the deformation mechanism during the debonding process observed in the probe tack test, we must define the terms cavitation and propagation. Confined fi lms of PSA under a tensile stress develop cavities. These cavities appear where surface defects were initially present,44 and we can optically detect them when their size becomes of the order of a few micrometers. The cavities then grow rapidly to a size of the order of the thickness of the fi lm. At this stage of the debonding process, two different mechanisms can be observed on video images made in the course of deformation and relaxation: either the cavity stops growing and new cavities progressively nucleate, eventually fi lling the space previously occupied by the adhesive fi lm, or the cavity continues growing laterally in a disk-like shape until it comes in contact with an adjacent disk-like cavity.45,46 The process of nucleation of new cavities under constant stress is called cavitation, whereas the growth of existing cavities in the plane perpendicular to the tensile direction is referred to as propagation. Video captures taken during the force relaxation process (Figures 11.42 and 11.43) reveal whether cavitation or propagation takes place. These observations reveal whether the cavities eventually coalesce or whether individual cavities persist and form a foamlike structure when the adhesive layer is stretched further. This last question is of particular importance to the long-term durability of the adhesive bond, because a coalescence of individual cavities leads to rapid complete detachment of the adhesive. 1a

1b

2a

2b

FIGURE 11.42 Snapshots taken during the relaxation process for 2AA (left) and 8AA (right) on steel.44 The pictures at the top are always taken at the beginning of the stop (t = 0) and the pictures at the bottom are taken at the end of the stop (t = 180 s). (1a) A stop at a low initial stress level (σ0 = 0.08 MPa, σ0/σmax = 28%). (1b) A stop at higher initial stress (σ0 = 0.25 MPa, σ0/σmax = 85%) for 2AA. One observes little cavitation, but some growth of the existing cavities. (2a) A stop at a low initial stress level (σ0 = 0.14 MPa, σ 0/σmax = 25%). (2b) A stop at a higher initial stress for 8AA (σ0 = 0.45 MPa, σ0/σmax = 81%). In this case one observes the nucleation of new cavities. (From Lindner, A., Maevis, T., Brummer, R., Lűhmann, B., and Creton, C., Langmuir 20, 9156, 2004. With permission.)

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a

b

c

FIGURE 11.43 Snapshots taken during the relaxation process of 2AA acrylic adhesive on a lowenergy EP substrate. The images at the top are always taken at the beginning of the stop, whereas the images at the bottom are made at the end of the stop (t = 180 s for a and b, t = 15 s for c). (a) Stop at a low initial stress level (σ0 = 0.13 MPa, σ 0/σmax = 51%). (b) Stop at intermediate initial stress (σ0 = 0.15 MPa, σ 0/σmax = 59%). (c) Stop at high initial stress level (σ0 = 0.22 MPa, σ 0/σmax = 86%). On the low adherence surface, no cavitation but substantial growth is observed. (From Lindner, A., Maevis, T., Brummer, R., Lűhmann, B., and Creton, C., Langmuir 20, 9156, 2004. With permission.)

When the adhesives were bonded to a steel surface, both mechanisms of cavity nucleation and growth were observed, whereas on low-energy EP surfaces, only crack propagation occurred. This result demonstrates that unless the sensitivity of these specific mechanisms to the molecular structure of the adhesive is understood, there will be no hope for predicting the lifetime of the bond. Because cavitation and crack growth are processes that entail locally large strains of the adhesive, it is highly unlikely that linear viscoelastic properties alone will be able to predict the nucleation or growth of these cavities. The sharp difference in behavior between the two substrates highlights the importance of adhesive interactions. On steel, resistance to crack propagation is high, so the most important property, that the adhesive must have, is good resistance to the formation of cavities. On the contrary, on EP surfaces, resistance to cavitation is not very important because failure occurs by crack propagation. It is the balance between these two properties that must be optimized.

11.4.3 Mechanisms of Adhesive Relaxation at the Second Stage of the Debonding Process The relaxation data described in the previous section relate to the probe tack test, which was stopped at the stage of homogeneous deformation of the adhesive before the peak

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stress, when a fibrillar foam-like structure is not yet formed. Here, we consider the results of a similar relaxation test performed by stopping the driving motor of the probe tack apparatus and monitoring the force relaxation under constant displacement conditions while the fibrillar foam is well formed, the peak stress is covered, and a strainhardening effect is observed, indicating the formation of strong fibrils (Figure 11.44).47 The adhesive used in this case is based on SIS triblock copolymer. As indicated by the data in Figures 11.44 and 11.45, stress does not relax to zero, as one would expect for a viscous liquid capable of flow, but to ∼70% of its initial value before remaining constant. A similar relaxation curve with appreciable residual unrelaxed stress was obtained for an SIS-based adhesive at the conditions imitating adhesive bond formation under bonding pressure (compare with Figures 11.4 and 11.7). This relaxation behavior is typical of viscoelastic adhesives possessing an apparent yield stress and clearly demonstrates that the fibrils are able to store elastic energy during their formation and stretching; only during detachment or fracture is this elastic energy released.47 The video images of the debonding process, illustrated in Figure 11.46, clearly indicate that the plateau stress for the SIS adhesive corresponds to the formation and elongation of the fibrillar foam.48 The images demonstrated that the average cell size and the amount of cavities are fairly independent of the relaxation time so that observed differences in measured stress cannot be due to a different microscopic structure of the foam. Comparatively negligible changes in the structure of SIS adhesive occur during the relaxation process, emphasizing the very elastic nature of the polymer in the fibrils. Such an elastic fibrillating behavior has also been reported in an even more pronounced way for soft physical gels, which are composed of block copolymers swollen in a preferential solvent for the midblock.44

1.0

σ (MPa)

0.8

0.6

0.4

0.2

0.0 0

50

100

150 Time (s)

200

250

300

FIGURE 11.44 Stress-versus-time curves for probe tack tests with intermediate stops at different levels of initial stress for SIS adhesive. The test is stopped for 120 s and then resumed. Note that the stress–strain curves are almost identical for all tests. (From Roos, A. and Creton, C., Macromol. Symp. 214, 147, 2004. With permission.)

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0.5

σN (MPa)

0.4

0.3

0.2

0.1

0.0 0

100

200

300

400

500

600

t − t stop (s)

FIGURE 11.45 Stress-versus-t − tstop curves obtained by stopping the driving motors in the fibrillation regime at three different stress levels for the SIS adhesive. The test was stopped here for 600 s. (From Roos, A. and Creton, C., Macromol. Symp. 214, 147, 2004. With permission.)

t = tstop

t = tstop + 120

FIGURE 11.46 Snapshots taken during the relaxation process for SIS adhesive. The left image was taken at the beginning of the stop, whereas the right image relates to the end of the stop. The horizontal full scale is 1.5 mm. (From Roos, A., Ph.D. Thesis, Universite Paris VI, Paris, 2004. With permission.)

11.4.4 Comparison of Relaxation Processes in Linear Viscoelasticity and Large-Strain Elongational Geometries Due to a complex mechanism of PSA deformation in the course of the probe tack test, it is pertinent to consider it under simpler conditions. Recently, relaxation tests for SIS triblock copolymer plasticized with diblock PS–PI copolymer were performed.49 Shear geometry was used to test the relaxation of physically cross-linked SIS-based systems in the linear regime and elongational geometry to test them in the nonlinear range. In shear, the deformation from 1 to 5% was applied to the sample and the relaxation

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modulus was measured as a function of time G(t) for 103–104 s. In elongation, the relaxation tests were performed at a deformation of 500%. It took 9 s for the cross-head to go to this elongation (Vt = 500 mm/min). Tests were performed at room temperature, and the relaxation of stress was measured as a function of time for 1000 s. The relaxation moduli normalized by the moduli at the beginning of the relaxation (G 0) are presented in Figure 11.47 for the four model resin blends.49 G 0 is independent of the PS–PI content, but always lower than G′. The material relaxes more when there is more diblock in the blend, namely, from 30 to 85% of the stress after an imposed deformation of 2%. In all cases, the relaxation seems to be over after 1000 s. Figure 11.48 illustrates the relaxation of stress normalized by the level of stress at the beginning of relaxation for both pure SIS blends and resin blends.49 Unlike in the linear regime, the levels of stress at the beginning of relaxation decrease with increasing diblock and resin contents. This is not surprising because in the tensile geometry the stretch to 500% deformation takes several seconds, and some relaxation can take place already in this loading stage. Similar to what is observed in the linear regime, the more diblock in the blend, the more rapid and pronounced the relaxation. However, the similarity ends here. After 1000 s, the material only relaxes 18–27% of the initial applied stress for pure polymer blends and 18–34% of the stress for adhesive formulations. In addition, in all cases the relaxation process is not over after 1000 s. These two experiments demonstrate that there is a remarkable difference in the kinetics of relaxation at small and large strains. When the sample is stretched to large strains, the relaxation of polymeric chains occurs in a strongly oriented network at a rate that probably depends mainly on the relaxation of the triblock chains bridging between PS domains. On the other hand, at small strains the network of polymer chains is not oriented to such an extent and most of the stress, and then the relaxation, occurs between entanglements in the polyisoprene domains, which relax faster than the central PI blocks of the triblocks.49 The SIS blends with a PS–PI diblock copolymer display marked 1 9 8 7 6

4

54→0

G(t)/G0

5

3

2

0.1 10−1

100

101

102

103

104

t (s)

FIGURE 11.47 Relaxation modulus in shear for SIS blends with 60 wt % resin after a deformation of 2%.49 The content of PS–PI diblock copolymer varies between 54 and 0 wt %. (From Roos, A. and Creton, C., Macromolecules, 38, 7807, 2005. With permission.)

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11-47


1.0

54→0

σN(t )/σ0

0.9

0.8

0.7

0.6 (a)

2

4

6 8

1

2

4

6

10

8

2

4

6

100

8

1000

t (s) 1.0

54→0

σN(t )/σ0

0.9

0.8

0.7

0.6 (b)

2

1

4

6 8

2

4

10

6

8

100

2

4

6

8

1000

t (s)

FIGURE 11.48 Relaxation of the stress in elongation after a deformation of 500% for (a) SIS blends without resin and (b) blends with 60% resin. The content of the PS–PI diblock copolymer varies between 54 and 0 wt %. (From Roos A. and Creton C., Macromolecules 38, 7807, 2005. With permission.)

nonlinear viscoelastic behavior and are much more dissipative at low and intermediate strains than at very high strains. This suggests that free diblock chains dangling from PS domains may have very long relaxation times, which dominate the viscoelastic behavior at low and intermediate strains, whereas at high strains the behavior is dominated by the bridging chains provided by the triblocks. The significance of the relaxation data presented in this section for pressure-sensitive adhesion emerges from the fact that the adhesion of SIS blends increases with increasing contents of resin and PS–PI diblock copolymer. As evident from Figures 11.47 and 11.48, both tackifying resin and PS–PI plasticizer accelerates the stress relaxation, both in the linear elastic deformation region and at large strains. For a more insightful analysis, the values of relaxation times and corresponding moduli are to be taken into consideration. With this purpose in mind, the relaxation properties of another physically cross-linked model PSA based PVP–PEG H-bonded complex have been studied.

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11.4.5 Relaxation Properties of PVP–PEG Model Pressure-Sensitive Adhesives 11.4.5.1

Relaxation Spectrum of PVP–PEG Adhesive in Linear Elastic Shear

The dynamic mechanical properties of the PVP–PEG adhesives in the linear viscoelastic regime were measured on a parallel plate rheometer.28 The amplitude of deformation was chosen to be in the linear region over the whole range of temperatures. For our PVP–PEG blends this zone corresponds to a deformation varying from 0.1 to 1%, depending on the temperature. The relaxation spectrum in Figure 11.49 illustrates three different groups of relaxation times that are typical for the rheological behavior of the PVP–PEG adhesive under shear stress: about 10−5, 1–50, and 1000–3000 s. These values are in fairly reasonable agreement with the results of the direct evaluation of the retardation times of PVP– PEG blends recently measured with a squeeze–recoil test under conditions imitating the removal of a compressive force in the course of adhesive bond formation, which are presented in Section 11.3. Two values of retardation times were established in this work: the shorter retardation time of 10–70 s and the longer time of 300–660 s. Whereas the relaxation time of ∼10−5 s most likely refers to the transition from a glassy solid to a viscoelastic state, both longer times are supposed to be associated with the rearrangement of the network of H-bonds. In full agreement with the prediction of Equation 10.6 in Chapter 10, the longer retardation time has the most significance for pressure-sensitive adhesion.22

8

7

log H (Pa)

6

5

4

3 −12

−10

−8

−6

−4

−2 0 log τ (s)

2

4

6

8

FIGURE 11.49 Relaxation spectrum featured for the PVP adhesive blend with 36% PEG at 20°C. (From Feldstein M.M. et al., J. Appl. Polym. Sci., 100, 522–537, 2006. With permission.)

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11.4.5.2

11-49

Tensile Stress Relaxation at High Elongations: The Statement of the Problem

As demonstrated in Section 10.3.3 of Chapter 10, peel adhesion of the PVP–PEG adhesive blends is controlled by the work of viscoelastic deformation and fracture of adhesive fi lms under large uniaxial extension.22 This renders the study of relaxation properties of this model PSA in the course of stretching reasonable. Large-strain relaxation in entangled polymers has been examined in some detail.50,51 For a large step in strain, the relaxation modulus has been established to depend on the magnitude of the initial step in strain. However, the maximum relaxation time has been proved to be independent of strain, even outside the range of linear elasticity. Tensile stress relaxation of PSAs at large strains raises some specific questions. Indeed, in the course of a typical relaxation test, a material should be instantaneously stretched to a predetermined deformation before the relaxation of the stress is recorded. Quantities evaluated using this methodology are generally regarded as characteristics of the material. In our approach we consider pressure-sensitive adhesion as a process rather than as a material property. We are mostly interested in the characterization of relaxation under the conditions that approximate closely the scenario of an adhesion test. In the course of such an adhesion test, the PSA is usually loaded initially with a finite strain rate, sometimes very small, and a partial relaxation occurs during this loading stage. Measured in such a manner values cannot be regarded as the properties of the material solely, but also as the characteristics of the process of testing. To avoid any confusion with true values of the relaxation times and moduli, in further discussion we refer to the evaluated properties of the relaxation process as the characteristic times of elastic recovery, τ, and the characteristic moduli, E. In the present study we attempt to answer some questions that usually do not arise during a more classic examination of relaxation: How is relaxation affected by the drawing rate in the course of loading? How does relaxation depend on the maximum elongation reached? Another distinctive feature of our approach is that relaxation is always seen within the context of adhesion. This is necessary to identify the values of relaxation times and corresponding moduli associated with high adhesion. The PVP–PEG adhesive represents a convenient model PSA because its adhesion may be easily manipulated by a change in composition. Short-chain PEG serves simultaneously in binary blends with PVP as a noncovalent cross-linker of longer PVP macromolecules and as a plasticizer, affecting the balance between the energy of intermolecular cohesion and the free volume.25,52 The blend containing 36 wt % PEG exhibits maximum adhesion, whereas in the blends underloaded with PEG the increase in cohesion strength dominates the increase in free volume. On the contrary, in PEG-overloaded blends the increase in free volume dominates the increase in cohesion, resulting in the fluidity of the adhesive blends. Such behavior is obvious from Figures 10.6 and 10.21 in Chapter 10, which illustrate the effects of PVP–PEG blend composition and drawing rate on tensile stress–strain curves of adhesive films up to break.22,25,52 As follows from Figures 10.6 and 10.21, a specific property of the PVP–PEG model PSA is the occurrence of a surprisingly sharp transition from the ductile type of deformation that is typical of uncross-linked rubbers to a more elastic type of extension with pronounced strain hardening featured for cross-linked elastomers. This transition is observed in a very narrow range of the decrease in PEG content (Figure 10.6, between 36 and 34 wt % PEG)

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and increase in extension velocity (Figure 10.21, between 20 and 50 mm/min). The narrow transition from ductile to tight stretching with the increased drawing rate in Figure 10.21 corresponds to a well-defined rate of rearrangement of the H-bonded network during the deformation of the PVP–PEG adhesive. Assuming that the cooperative breakup and reformation of hydrogen bonds forming this PVP–PEG network can only occur below the critical deformation rate of 0.05 s−1, we can identify the characteristic time for this process to occur at about 20 s.53 What is of particular importance for the following discussion is the equivalence between the effects of the increase in plasticizer (PEG) content and the decrease in stretching rate on the tensile and, consequently, relaxation properties. Figure 11.50 represents the effect of drawing rate on tensile stress relaxation curves for the most tacky PVP blend with 36 wt % PEG containing 8% of absorbed water at a fi xed elongation, ε = 3. The relaxation follows the exponential law described by Equation 11.2 and written as in

Et Eeq ∑ Ei exp(t /i )

(11.4)

i1

where Ei are the tensile relaxation moduli. Similar to what is observed for SIS adhesives in the shear regime (Figures 11.4 and 11.47), the PVP–PEG relaxation in Figure 11.50 illustrates the occurrence of a residual unrelaxed stress, Eeq. The lower the drawing rate in the loading stage, the more rapid and pronounced the relaxation and the lower the equilibrium value of the corresponding modulus, Eeq. The curves relating to a comparatively fast relaxation process can be satisfactorily described by Equation 11.4 with two characteristic times of elastic recovery. This is the case for blends containing 41 wt % PEG at low (10 mm/min) loading rates. In contrast, the blends containing 36 wt % PEG and less, as well as the samples deformed with higher extension rates, have relaxation curves that can be

340 300

E (kPa)

260 220 100

180

50

140

20

100 60 20

10

0

200

400

600 800 Time (s)

1000

1200

FIGURE 11.50 Relaxation curves recorded for the PVP blend with 36 wt % of PEG-400 that is stretched to a deformation of 300% (ε = 3) with a rate ranging from 10 to 100 mm/min. The content of absorbed water in the blend is 8 wt %.

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adequately fitted with Equation 11.4, including three populations of characteristic recovery times, τ. For this reason, in the following discussion we consider the effects of relative elongation on relaxation, which have been measured for the elastic PVP blend with 36% PEG, in terms of two characteristic recovery times, whereas the effects of stretching velocity and PEG concentration are treated with three different populations of recovery times. 11.4.5.3

Effects of Relative Elongation and Extension Rate on Tensile Stress Relaxation

In this section we discuss the relaxation properties of PVP–PEG model PSA under experimental conditions corresponding to adhesion testing. Figures 11.51 through 11.53 illustrate the effects of tensile strain and strain rate on the relaxation of the PVP blend with 36 wt % PEG that exhibits maximum adhesion. 80 100 mm/min

70 60

τ1 (s)

50 40 30 20 10 mm/min 10 1.0

1.5

2.0

2.5

3.0

ε

3.5

4.0

550 500 100 mm/min 450

τ2 (s)

400 350 300 250 10 mm/min

200 150 1.0

1.5

2.0

2.5

ε

3.0

3.5

4.0

FIGURE 11.51 Effects of relative elongation and extension velocity on characteristic times of elastic recovery for the PVP–PEG (36 wt %) model PSA.

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11-52


160 140

100 mm/min

Eeq (kPa)

120 10 mm/min

100 80 60 40 20 0 1.0

1.5

2.0

2.5 ε

3.0

3.5

4.0

FIGURE 11.52 Impacts of relative elongation, ε, and drawing rate on the values of equilibrium characteristic recovery modulus, E eq, for the PVP–PEG (36%) model PSA.

By fitting the relaxation curves with Equation 11.4, one can extract two characteristic times of recovery, which increase linearly with increasing elongation (Figure 11.51). The increase in strain rate during loading makes this relationship more pronounced. At a comparatively low velocity of stretching (10 mm/min), the shorter characteristic time varies between 18 and 75 s, whereas the longer time ranges from 200 to 520 s. The 10-fold increase in strain rate results in an appreciable decrease and narrowing of the characteristic time ranges: τ1 = 7–15 and τ2 = 145–230 s. The values of characteristic equilibrium moduli are presented in Figure 11.52. At the low strain rate of 10 mm/min the Eeq is an increasing function of elongation. The most growth is observed in the range of elongations between ε = 1–2, wherein the Eeq increases from zero to an almost constant value. At high strain rate (100 mm/min) the equilibrium characteristic modulus tends to increase linearly with tensile strain. Within a strain range ε = 2–4, the values of the equilibrium characteristic moduli are almost identical and independent of strain rate (Figure 11.52). The E eq value characterizes the amount of mechanical energy stored by the adhesive material in the process of loading. At low tensile rates the material has enough time to relax and the amount of stored energy is negligible. This behavior is typical of viscous liquids and un-cross-linked linear polymers. In contrast, at strain rates of 100 mm/min the material behaves as a cross-linked elastomer, storing elastic energy in the course of stretching. The transition from the liquid-like to rubber-like behaviors occurs in very narrow range of tensile rates that is due to the well-defined relaxation time of the network of hydrogen bonds in the PVP–PEG complex.

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350 300

E1 (kPa)

250 200 100 mm/min 150 100 10 mm/min 50 0 (a)

1.0

1.5

2.0

2.5

3.0

3.5

4.0

3.0

3.5

4.0

ε

280

240

E2 (kPa)

200

160 100 mm/min 120

80 10 mm/min 40

0 (b)

1.0

1.5

2.0

2.5 ε

FIGURE 11.53 Relation of the characteristic moduli of elastic recovery to the relative elongation and drawing velocity of an adhesive PVP blend with 36 wt % PEG.

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11-54


As follows from Figure 11.53, the characteristic moduli corresponding to both shorter and longer relaxation times tend to increase with the increase of relative elongation at high drawing velocity, whereas the moduli, corresponding to the slow extension, decrease. Figures 11.54 through 11.56 illustrate the effect of strain rate on the relaxation of a PVP–PEG PSA at a fixed relative elongation of 300% (ε = 3). The treatment of relaxation

500 τ3 400

300

200

100

τ1

τ2

0 0

20

40

60

80

100

Tensile rate (mm/min)

FIGURE 11.54 Effect of loading tensile rate on the values of characteristic recovery times of PVP–PEG PSA; ε = 3. 45

Eeq (kPa)

40 35 30 25 20 0

20

40 60 80 Tensile rate (mm /min)

100

FIGURE 11.55 Equilibrium characteristic recovery modulus as a function of the tensile rate in the course of loading of a PVP–PEG (36%) model PSA; ε = 3.

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11-55


140 E3

120

E (kPa)

100 E2

80 60

E1

40 20 0 0

20

40 60 80 Tensile rate (mm/min)

100

FIGURE 11.56 The effect of extension velocity on the characteristic moduli of elastic recovery of a PVP–PEG PSA; ε = 3.

curves was performed using Equation 11.4 with three populations of characteristic recovery times. In agreement with the data in Figures 11.51 through 11.53, the 10-fold increase in strain rate from 10 to 100 mm/min does not affect the values of shorter and intermediate characteristic times (Figure 11.54), whereas the longer recovery time tends to decrease with the increase in strain rate. The equilibrium characteristic recovery modulus increases at lower deformation rates, achieving its limiting value since the velocity of 50 mm/min (Figure 11.55). Characteristic recovery moduli corresponding to shorter and intermediate recovery times, E1 and E2 , are increasing functions of the tensile rate, whereas the value of E3 decreases with the increase in loading tensile velocity (Figure 11.56). The results presented above suggest that the conditions of adhesion testing, such as strain rate and strain amplitude, affect appreciably the relaxation properties of adhesive materials. The effect of maximum strain on relaxation is in synergy with the impact of strain rate. The relaxation behavior is controlled by two major factors: the longer characteristic time of elastic recovery and the value of the equilibrium characteristic recovery modulus. The former value increases with strain and decreases with increasing strain rate; the latter increases in both cases. 11.4.5.4 Impact of Plasticizer Concentration and Relation to Adhesion As Figure 11.57 illustrates, the longer characteristic recovery time decreases with increasing PEG content in the blends, whereas both shorter and intermediate recovery times are unaffected by PEG content. The equilibrium characteristic recovery modulus decreases with PEG concentration (Figure 11.58), as well as the moduli associated with

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11-56


300 τ3 250

τ (s)

200

150

100

τ2

50 τ1 0 32

36 PEG (wt %)

40

FIGURE 11.57 Characteristic recovery times plotted versus the composition of PVP–PEG PSA. The composition displaying maximum adhesion is marked by a dotted line. The extension rate is 100 mm/min; ε = 3.

80

Eeq (kPa)

60

40

20

0 30

32

34

36 PEG (wt %)

38

40

42

FIGURE 11.58 The effect of PEG content in blends with PVP on the value of the equilibrium characteristic recovery modulus. The dotted line designates the composition exhibiting maximum adhesion. The extension rate is 100 mm/min; ε = 3.

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11-57


50

E2

40

E (kPa)

30 E1 20

E3

10

0 30

32

34

36 PEG (wt %)

38

40

42

FIGURE 11.59 Values of the characteristic recovery modulus for PVP–PEG adhesives of various composition. The extension rate is 100 mm/min; ε = 3.

the corresponding characteristic times (Figure 11.59). This behavior implies that PEG is a good plasticizer of PVP. It is instructive to compare the effects of PEG concentration in PVP–PEG blends on relaxation properties, illustrated in Figures 11.57 through 11.59, and on the retardation time and relaxation modulus, presented in Figures 11.36 and 11.37. With the increase in PEG content, the relaxation and recovery moduli decline, but the longer retardation time increases in contrast to the behavior of the characteristic recovery time that is, in essence, the relaxation time. This distinction accounts for a fundamental difference between the relaxation and the retardation processes, defined by the Equations 11.2 and 11.4 on the one hand and Equation 11.3 on the other. All PVP–PEG blends considered in this section demonstrate some level of pressuresensitive adhesion. However, the PVP blend with 36% PEG exhibits the best adhesion. Its relaxation properties indicate which values of the characteristic recovery times and corresponding moduli are associated with a high level of pressure-sensitive adhesion.

11.5 General Conclusions Pressure-sensitive adhesion can be seen as a process of transformation of the structure and properties of the adhesive material under an applied mechanical strain history, including three indivisible consecutive stages: (1) adhesive bond formation under a bonding pressure, when the main type of deformation is in shear, (2) relaxation upon the withdrawal of compressive stress, and (3) debonding under a detaching tensile force.

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In turn, the third stage, adhesive debonding, is a multistage process itself, including the steps of homogeneous deformation under a tensile stress, followed by cavitation and fibrillation of the adhesive material. Relaxation mechanisms accompany all stages of the adhesion process and provide the links between the stages. In terms of relaxation properties, all PSAs can be classified into two groups: (1) fully relaxing (fluid) adhesives and (2) elastic adhesives that are able to store energy in the course of bond formation and exhibit residual (unrelaxed) stress during the contact time. In terms of molecular structure, the group of fluid adhesives includes un-crosslinked, entangled polymers with low glass transition temperatures, whereas the elastic PSAs are most often networks of polymers that are cross-linked covalently or physically. The elastic PSAs of the second group demonstrate a much slower relaxation than the fluid PSAs. The longer relaxation processes are mostly associated with the appearance of a pronounced plateau or even of a second maximum on the probe tack curves. The equilibrium relaxation modulus is a direct measurement of the stored elastic energy in a polymer material and relates to its yield stress. Fluid adhesives do not have a yield stress or, consequently, an equilibrium modulus. As the comparison of the relaxation and adhesive properties of various PSAs indicates, high adhesion is associated with longer relaxation times. This conclusion is in full agreement with Equation 10.6 in Chapter 10, which predicts improved adhesion with the increase in longer relaxation time of the adhesive material. In turn, the longer values of relaxation time are typical of large-scale entangled and network supramolecular structures. This implies that large-scale rearrangements of molecular structures in the course of relaxation govern a high level of pressure-sensitive adhesion. The relationship between peel adhesion and the molecular structure of the adhesive material, predicted by Equation 10.6 in Chapter 10, also suggests that the greater the size of the relaxing molecular structures in adhesive material, a, the higher the adhesion. As predicted using Equation 10.6 in Chapter 10, for high adhesion a compromise must be reached among the values of cohesion energy, diff usion coefficient, and relaxation time of PSAs. It is, therefore, not surprising that a direct correlation between the practical work of adhesion and the values of longer relaxation times has been established for the examined PSAs. Adhesion appears with the rise of longer relaxation times values above 50 s and increases, passing through a maximum at τ3 = 330–380 s. A further increase in the longer relaxation times results in a gradual decline in adhesion. Good adhesion occurs when the longer relaxation time varies in the range of 150 to 800 s. Relaxation properties of the PSAs depend on the observation window. If the observation time is long enough, a PSA seems to be more fluid than at shorter observation time. PSAs are viscoelastic materials that combine the properties of solids and liquids. Relative contributions of the viscous and elastic behaviors can be estimated in terms of the Deborah numbers that relate the time scale of structural rearrangement in the material to the time of experimental observation. Because the viscous and elastic contributions are perfectly counterbalanced, the Deborah number is unity. Very fluid, quickly relaxing materials possess values of Deborah number tending to zero. Useful PSAs have demonstrated Deborah numbers between 0.15 and 1 if the longer relaxation time is chosen as the characteristic relaxation time of the adhesive.

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11-59

The relaxation behavior during the stage of strain relaxation upon withdrawal of the bonding pressure has been characterized in terms of retardation times. Retardation times are not identical to relaxation times. Whereas the relaxation times increase with plasticization, the retardation times decline. Two populations of retardation times have been determined for PSAs: shorter retardation times of the order 10–70 s and longer retardation times in the range of 300–660 s. The longer retardation times relate mainly to the energy-dissipating processes, which, in turn, are associated with large-scale rearrangements of supramolecular structures involving polymer chain entanglements and translational movements of polymer segments and entire macromolecules through a self-diff usion mechanism. The observed correlation between high adhesion and the values of longer retardation times supports the validity of Equation 10.6 in Chapter 10. The large-scale rearrangements of the structure of adhesive materials during relaxation can be easily visible and characterized if the probe tack tester is stopped at different stages of the debonding process, when the main mode of PSA deformation is in tension. When a soft adhesive is kept under a tensile load during the debonding process, two important failure mechanisms are observed: nucleation of new cavities and the growth of existing cavities as cracks at the interface between the adhesive and the adherent. The plateau stress in probe tack curves corresponds to the formation and elongation of a fibrillar foam. The stop of the probe at this stage for elastic adhesives demonstrated that the size and number of cavities remain the same in the course of relaxation. To describe quantitatively PSA relaxation in the course of debonding, when large tensile strain dominates, the method of stretching adhesive fi lms is useful. The work of viscoelastic deformation up to fi lm fracture under uniaxial extension has been established to control peel adhesion. If the relaxation properties of PSAs under a large tensile strain are expressed in terms of characteristic times of elastic recovery and corresponding characteristic moduli, the effect of debonding conditions, such as loading rate and tensile strain amplitude, on relaxation becomes evident. The effect of maximum tensile strain on relaxation is in synergy with the effect of strain rate. The relaxation behavior at large strains is controlled by two major factors: the longer characteristic time of elastic recovery and the value of equilibrium characteristic recovery modulus. The former value increases with strain and decreases with strain rate; the latter increases in both cases. The data presented in this chapter demonstrate the significance for pressure-sensitive adhesion of relaxation properties typical of PSAs, as well as the development and transformations of these properties in the course of adhesion testing.

References 1. Tshoegl N.W., Time dependence in material properties: An Overview, Mechan. Time-Depend Mater., 1, 3, 1997. 2. Moonan W.K. and Tschoegl N.W., The effect of pressure on the mechanical properties of polymers. 4. Measurements in torsion, J. Polym. Sci., Polym. Phys. Ed., 23, 623, 1985. 3. Ferry J.D., Viscoelastic Properties of Polymers, 3rd ed., Wiley & Sons, New York, 1980.

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4. Aklonis J.J. and MacKnight W.J., Introduction to Polymer Viscoelasticity, 2nd ed., Wiley & Sons. New York, 1980. 5. Christensen R.M., Theory of Viscoelasticity, 2nd ed., Academic Press, New York, 1982. 6. Matsuoka S., Relaxation Phenomena in Polymers, Oxford University Press, New York and Carl Hauser Verlag, Munich, 1992. 7. Satas D. (ed.), Handbook of Pressure-Sensitive Adhesive Technology, 3rd ed., Satas & Associates, Warwick, RI, 1999. 8. Dillard D.A. and Pocius A.V. (eds.), The Mechanics of Adhesion, Elsevier Science, New York, 2002. 9. Zosel A., The effect of bond formation on tack of polymers, J. Adhesion Sci. Technol. 11, 1447, 1997. 10. Tschoegl N.W., The Phenomenological Theory of Linear Elastic Behavior, SpringerVerlag, Heidelberg, 1989, p. 126. 11. Schramm G., A Practical Approach to Rheology and Rheometry, Haake Rheometers, Karlsruhe, 1994, p. 104. 12. Zosel A., The effect of fibrillation on the tack of pressure-sensitive adhesives, Int. J. Adhesion Adhes. 18, 265, 1998. 13. Zosel A., Fracture energy and tack of pressure-sensitive adhesives, in: Satas D (ed.) Advances in Pressure Sensitive Adhesive Technology, vol. 1, Satas & Associates, Warwick, RI, 1992, chap. 4. 14. Zosel A., Adhesion and tack of polymers: Influence of mechanical properties and surface tension, Colloid Polym. Sci., 263, 541, 1985. 15. Pickering J.P., Van Der Meer D.W., and Vancso G.J., Effects of contact time, humidity, and suface roughness on the adhesion hysteresis of polydimethylsiloxane, J. Adhes. Sci. Technol., 15(12), 1429, 2004. 16. Lakrout H., Sergot P., and Creton C., Direct observation of cavitation and fibrillation in a probe tack experiment on model acrylic pressure-sensitive adhesives, J. Adhesion 69, 307, 1999. 17. Creton C. and Fabre P., Tack, in: Dillard D.A. and Pocius A.V. (eds.) Adhesion Science and Engineering, Vol I: The Mechanics of Adhesion, Elsevier, Amsterdam, 2002, chap. 14. 18. Creton C. and Leibler L., How does tack depend on time of contact and contact pressure, J. Polym. Sci.: Part B: Polym. Phys. 34, 545–554, 1996. 19. Creton C. and Schach, R., Diff usion and adhesion, in: Benedek I. and Feldstein M.M. (eds.) Fundamentals of Pressure Sensitivity, Taylor & Francis, Boca Raton, 2008, chap. 2. 20. Voyutskii S.S., Autohesion and Adhesion of High Polymers, Wiley Interscience, New York, 1963. 21. Dahlquist C.A., in: Patrick R.L. (ed.) Treatise on Adhesion and Adhesives, vol. 2, M. Dekker, New York, 1969, p. 219. 22. Feldstein M.M., Molecular nature of pressure-sensitive adhesion, in: Benedek I. and Feldstein M.M. (eds.) Fundamentals of Pressure-Sensitive Adhesion, Taylor & Francis, Boca Raton, 2008, chap. 10.

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23. Feldstein M.M. and Creton C., Pressure-sensitive adhesion as a material property and as a process, in: Benedek I. (ed.) Pressure-Sensitive Design, Theoretical Aspects, vol. 1, VSP, Leiden, Boston, 2006, chap. 2. 24. Feldstein M.M., Molecular fundamentals of pressure-sensitive adhesion, in: Benedek I. (ed.) Developments in Pressure-Sensitive Products, 2nd Edition, CRC-Taylor & Francis, Boca Raton, 2006, chap. 4. 25. Feldstein M.M., Adhesive hydrogels: structure, properties and application, Polym. Sci., Ser. A., 46(11), 1265, 2004. 26. Zisman W.A., Adhesion and bonding, in: Mark H.F., Gaylord N.G., and Bikales N.M. (eds.) Encyclopedia of Polymer Science and Technology, Interscience Publishers (J. Wiley & Sons), New York, 1964, pp. 445–477. 27. Kotomin S.V., Durability of adhesive joints, in: Benedek I. and Feldstein M.M. (eds.) Fundamentals of Pressure Sensitivity, Taylor & Francis, Boca Raton, 2009, chap. 9. 28. Feldstein M.M., Kulichikhin V.G., Kotomin S.V., Borodulina T.A., Novikov M.B., Roos A., and Creton C., Rheology of poly(N-vinyl pyrrolidone)-poly(ethylene glycol) adhesive blends under shear flow, J. Appl. Polym. Sci., 100, 522, 2006. 29. Satas D., Tack, in: Satas D. (ed.) Handbook of Pressure-Sensitive Adhesive Technology, 3rd ed., Satas & Associates, Warwick, RI, 1999, chap. 4. 30. Chiche, A., Dollhofer J., and Creton C., Cavity growth in soft adhesives, Eur. Phys. J. E, 17, 389–401, 2005. 31. Novikov M.B., Gdalin B.E., Anosova J.V., and Feldstein M.M., Stress relaxation during bond formation and adhesion of pressure sensitive adhesives, J. Adhesion 84, 164, 2008. 32. Bartenev G.M. and Barteneva A.G., Relaxation Properties of Polymers, Chemistry Publishers, Moscow, 1992, p. 384. 33. O’Connor A.E. and Willenbacher N., The effect of molecular weight and temperature on tack properties of model polyisobutylenes, Int. J. Adhes. Adhes., 24, 335, 2004. 34. Feldstein M.M., Cleary G.W., and Singh P., Pressure-sensitive adhesives of controlled water-absorbing capacity, in: Benedek I. (ed.) Pressure-Sensitive Design and Formulation, Application, vol. 2, VSP, Leiden, Boston, 2006, chap. 3. 35. Feldstein M.M., Cleary G.W., and Singh P., Hydrophilic adhesives, in: Benedek I. and Feldstein M.M. (eds.) Technology of Pressure-Sensitive Adhesives and Products, Taylor & Francis, Boca Raton, 2008, chap. 7. 36. Novikov M.B., Kiseleva T.I., Anosova J.V., Singh P., Cleary G.W., and Feldstein M.M., Contribution of relaxation processes into pressure-sensitive adhesion of interpolymer complexes, Proceed. 30th Annual Meeting Adhesion Soc., Tampa, Florida, USA, 54, 2007. 37. Novikov M.B., Borodulina T.A., Kotomin S.V., Kulichikhin V.G., and Feldstein M.M., Relaxation properties of pressure-sensitive adhesives upon withdrawal of bonding pressure, J. Adhesion, 81(1), 77, 2005. 38. Rohn Ch., Rheology of pressure-sensitive adhesives, in: Satas D. (ed.) Handbook of Pressure-Sensitive Adhesive Technology, 3rd ed., Satas & Associates, Warwick, RI, 1999, chap. 9.

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39. Chalykh A.A., Chalykh A.E., Novikov M.B., and Feldstein M.M., Pressuresensitive adhesion in the blends of poly(N-vinyl pyrrolidone) and poly(ethylene glycol) of disparate chain lengths, J. Adhesion, 78(8), 667, 2002. 40. Feldstein M.M., Roos A., Chevallier C., Creton C., and Dormidontova E.D., Relation of glass transition temperature to the hydrogen bonding degree and energy in poly(N-vinyl pyrrolidone) blends with hydroxil-containing plasticizers: 3. Analysis of two glass transition temperatures featured for PVP solutions in liquid poly(ethylene glycol), Polymer, 44(6), 1819, 2003. 41. Zosel A., Adhesive failure and deformation behaviour of polymers, J. Adhesion, 30, 135, 1989. 42. Shull K.R. and Creton C., Deformation behavior of thin, compliant layers under tensile loading conditions, J. Polym. Sci., Polym. Phys., 42, 4023, 2004. 43. Creton, C., Roos A., and Chiche A., Effect of the diblock content on the adhesive and deformation properties of PSAs based on styrenic block copolymers, in: Possart W.G. (ed.) Adhesion: Current Research and Applications, Weinheim, Wiley-VCH, 2005, 337p. 44. Lindner A., Maevis T., Brummer R., Lűhmann B., and Creton C., Sub-critical failure of soft acrylic adhesives under tensile stress, Langmuir 20, 9156, 2004. 45. Shull K.R., Flanigan C.M., and Crosby A., Fingering instabilities of confined elastic layers in tension, J. Phys. Rev. Lett., 84, 3057, 2000. 46. Creton C., Hooker J.C., and Shull K.R., Bulk and interfacial contributions to the debonding mechanisms of soft adhesives, Langmuir, 17, 4968, 2001. 47. Roos A. and Creton C., Linear viscoelasticity and nonlinear elasticity of block copolymer blends used as soft adhesives, Macromol. Symp., 214, 147, 2004. 48. Roos A., Ph.D. Thesis, Universitė Paris VI, Paris, 2004. 49. Roos, A. and Creton, C., Effect of the presence of diblock copolymer on the non linear elastic and viscoelastic properties of elastomeric triblock copolymers, Macromolecules, 38, 7807, 2005. 50. Osako K., in: Nagasawa M. (ed.) Molecular Conformation and Dynamics of Macromolecules in Condensed Systems, Elsevier, Amsterdam, 1988, p. 175. 51. Isono Y., and Nishitake T., Stress relaxation and change in entanglement structure of polyisobutylene in large shearing deformations, Polymer, 36(8), 1635, 1995. 52. Novikov M.B., Roos A., Creton C., and Feldstein M.M., Dynamic mechanical and tensile properties of poly(N-vinyl pyrrolidone)–poly(ethylene glycol) blends, Polymer, 44(12), 3559, 2003. 53. Roos A., Creton C., Novikov M.B., and Feldstein M.M., Viscoelasticity and tack of poly(N-vinyl pyrrolidone)–poly(ethylene glycol) blends, J. Polym. Sci., Polym. Phys., 40, 2395, 2002.

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Appendix: Abbreviations and Acronyms Generally Accepted Abbreviations and Symbols aT AA BA DMA DMAEMA DSC EA FTIR G′ G″ MAA MMA Mn MW, Mw NMR PB PDMS PE PEG PET PI PIB PMMA PP PSA

shift factor acrylic acid butyl acrylate dynamic mechanical analysis dimethylaminoethyl methacrylate differential scanning calorimetry ethyl acrylate fourier transform infrared (spectroscopy) dynamic shear storage modulus dynamic shear loss modulus methacrylic acid methyl methacrylate number average molecular weight molecular weight nuclear magnetic resonance polybutadiene poly(dimethyl siloxane) polyethylene poly(ethylene glycol) polyethylene terephthalate polyisoprene polyisobutylene poly(methyl methacrylate) polypropylene pressure-sensitive adhesive

A-1

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A-2

PSP PTFE PVC PVP SAFT SIS t T tan δ Tg VA

Chapter 1

δ ε ε0 εi ρ σ τ Θ Θ0 Θa Θi Θr E fd fη G h h0 P ∆Pc PC PH Pst r R*

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Appendix: Abbreviations and Acronyms

pressure-sensitive product poly(tetrafluoroethylene) polyvinyl chloride poly(N-vinyl pyrrolidone) shear adhesion failure temperature styrene–isoprene–styrene triblock copolymer time temperature loss factor G″/G′, loss angle tangent glass transition temperature vinyl acetate

Surface Phenomena on a Solid– Liquid Interface and Rheology of Pressure Sensitivity thickness of the layer of the liquid current deformation residual equilibrium deformation initial deformation density surface tension retardation time current contact angle equilibrium contact angle advancing contact angle initial contact angle receding contact angle Young modulus driving force of spreading force of viscous resistance elastic modulus current gap between plates at squeezing initial gap between plates at squeezing pressure capillary pressure difference at the beginning and at the end of inertial stage of spreading capillary pressure hydrostatic pressure polystyrene radius of the wetted surface curvature radius of drop surface at the end of the inertial stage of spreading

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Ri v V vin VOC

A-3

curvature radius of drop surface at the beginning of the inertial stage of spreading spreading rate volume of drop spreading rate at the inertial stage of spreading volatile components

Chapter 2 Diffusion and Adhesion χ ε η η0 γ1, γ2 γ12 φ φ(aTv) ρ σ τd a c De E EPDM Gc G N0 h0 Ip k N, NA, NB SBR SEC Sty tc Tref V vd w Wadh Wrev

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Flory interaction parameter strain viscosity zero-shear rate viscosity surface tension interfacial tension volume fraction dissipative factor in adhesion density stress terminal relaxation time of the polymer monomer segment length constant with value 6 or 9 Deborah number Young’s modulus ethylene–propylene diene rubber critical energy release rate plateau modulus initial thickness of the adhesive fi lm polydispersity index Boltzmann constant degree of polymerization of the polymer poly-styrene-r-butadiene of 80 kg/mole molecular weight size exclusion chromatography styrene contact time reference temperature probe debonding velocity probe debonding velocity interfacial width adhesion energy reversible work of adhesion

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A-4


Chapter 3 Transition Zones in Adhesive Joints α η µm τ A Аcoh d d∝ D ED Ei erf(z) L L cr ni P PEU PFO PVDF R S tD Тiso Tm UCST wi w′i wi″ x z

Chapter 4

ε η η0 ω Φ σ τ

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numerical constant viscosity microns numerical constant peel strength of adhesive joint cohesive strength average pore diameter pore diameter at t∝ diff usion coefficient activation energy of diff usion energy of an adhesive bond Gaussian integral thickness of adhesive layer impregnation depth number of ith type links per unit area of interphase contact pressure polyester urethane phenol formaldehyde oligomer polyvinylidene fluoride pore radius contact area time of diff usional relaxation isotropization point melting point upper critical solution temperature concentration expressed in weight or volume fraction concentration of ith component in the first phase concentration of ith component in the second phase diff usion coordinate tabulated value of Gaussian integral

Role of Viscoelastic Behavior of PressureSensitive Adhesives in the Course of Bonding and Debonding Processes extension dynamic viscosity newtonian viscosity circular frequency amplifying factor tensile stress relaxation time

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τrep E F Fmin and Fmax G G* Gc Grep(t), GrA(t), GrB(t), GHF(t) G(t) JG JR J(t) Me Sr top Tref U V W W0

Chapter 5 ω ρ A A0 ATPSA B CRA CTPSA D DE Gn GPPSA HSPSA L Me P0 rad/s RPSA Tc

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A-5

reptation time elastic modulus peeling force minimum and maximum forces in the stick-slip domain fracture energy complex shear modulus critical energy release rate the various relaxation domains of the relaxation function relaxation function instantaneous compliance retardational creep compliance creep (compliance) function molecular weight between entanglements strain rate open time reference temperature of the master curves opening of the crack peeling rate thickness of the adhesive thermodynamic work of adhesion

Viscoelastic Properties and Windows of Pressure-Sensitive Adhesives frequency in rad/s density of the blend actual area of contact total area available for contact all temperature pressure-sensitive adhesives bonding function control release additives cold temperature pressure-sensitive adhesives debonding function dissipation energy per cycle of deformation plateau modulus general purpose pressure-sensitive adhesives high shear pressure-sensitive adhesives applied strain amplitude molecular weight between entanglements intrinsic interfacial energy radians per second removable pressure-sensitive adhesives domain disappearance (critical) temperature

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A-6


Tdd Vp VW

domain transition temperature volume fraction of polymer in the polymer–resin blend viscoelastic window

Chapter 6 ε εmax γ λ λ1, λ 2, λ 3 λr µ σ σmax σN σzz θ a a0 E F fel(λr) g go gc h h0 JKR p p0 PEHA pel r R0 Rc Rd Rp SI Uel v v* V

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Probe Tack

strain maximum extension of the fibrils surface tension extension ratio extension ratio along the three principal directions extension ratio of the cavity shear modulus stress maximum stress during a tensile debonding test nominal tensile stress normal stress in the tensile (z) direction contact angle of the cavity with the surface diameter of the contact area diameter of the probe Young’s modulus load elastic strain energy density function for the growth of a cavity energy release rate limiting value of Gc at a vanishing crack velocity critical energy release rate thickness of the layer initial thickness of the layer Johnson-Kendall-Roberts pressure external pressure poly(2-ethylhexyl acrylate) elastic inflation pressure distance along the radial direction initial radius of the cavity radius of the cavity initial projected radius of the cavity on the probe surface projected radius of the cavity on the probe surface poly(styrene-b-isoprene) elastic strain energy density crack velocity critical value of crack velocity above which dissipative effects are important volume of the cavity

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V0 Vdeb Wadh

A-7

initial volume of the cavity debonding velocity of the probe adhesion energy

Chapter 7 Peel Resistance δ ψ θ AA C1 C2 2-EHA G G0 Gc 2-HEMA P P-36 S SBC Ts VAc W WD WT

solubility parameters viscoelastically dissipated energy peel angle acrylic acid constants constants 2-ethylhexyl acrylate fracture energy energy required to propagate a crack adhesive fracture energy 2-hydroxyethyl methacrylate adhesion force 4-acryloyloxydiethoxy-4-chlorobenzophenone shape factor styrenic block copolymer standard temperature vinyl acetate peel work work of deformation non-recoverable work of translation

Chapter 8 Shear Resistance η γ γ˙ γ˙R λ τ τR A 2-EHA F G h h0 IOA k

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viscosity deformation shear rate shear rate at the rim of the plate at squeezing retardation time shear stress shear stress at the rim of the plate at squeezing overlapping area in the shear test 2-ethylhexyl acrylate shearing force elastic modulus current gap between plates at squeezing initial gap between plates at squeezing isooctyl acrylate consistency coefficient in the power law

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A-8

Δl n R UV v


displacement of the adhesive tape in the shear test power law index plate radius at squeezing ultraviolet speed of the adhesive tape in the shear test

Chapter 9 Durability of Viscoelastic Adhesive Joints α σ τ0 B B1 k m t* U U0

power constant at molecular weight in extended Bartenev’s equation tensile stress thermal oscillation time of atoms for polymers constant in Bartenev’s equation constant in extended Bartenev’s equation Boltzmann constant power constant long-term durability activation energy of the rupture process activation energy of self-induced rupture of polymer chains at σ = 0

Chapter 10 χ δ δs ε εb η γ λ ω φ σ σb σf σmax σn σy τ a b CED ∆Cp

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Molecular Nature of PressureSensitive Adhesion

Flory interaction parameter fi xed pulse width in PFG NMR solubility parameter relative elongation maximum elongation of the fi lm at the break microviscosity (monomer-monomer friction coefficient of polymer chain) gyromagnetic ratio of proton jump length of a polymer segment deformation frequency volume fraction tensile stress ultimate tensile strength ultimate stress at fracture of PSA under debonding maximum debonding stress in probe tack test nominal stress yield stress (maximum on tensile stress-strain curve) relaxation time size of monomer units in polymer chain segment width of adhesive fi lm cohesive energy density change in heat capacity

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A-9

d diameter of cavities required for diff usion of a polymer chain segment D self-diff usion coefficient interaction energy of atoms forming a polymer segment Dv interdiff usion coefficient E elasticity tensile modulus Ec cohesive energy ED activation energy for self-diff usion F force fv free volume fraction g magnitude of field gradient pulses ∆Hv heat of vaporization k Boltzmann constant l thickness of adhesive fi lm N number of monomer units in a segment of polymer chain P 180° peel force, resistance, adhesion PALS positron annihilation lifetime spectroscopy PFG pulsed-field gradient pi populations of resonating species possessing specific diff usivity q generalized scattering vector QSPR quantitative structure–property relationship R gas constant Sinc(q, t) incoherent intermediate structure function t* durability of the joint Tm melting temperature Vm molar volume W practical work of adhesion (the area under probe tack curve) Wb work of viscoelastic deformation to break adhesive fi lm in tensile test WAXS wide angle x-ray scattering z coordination number

Chapter 11 ε η σ σ0 σmax σn τ τmat τexp A AMA

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Significance of Relaxation for Adhesion of Pressure-Sensitive Adhesives

relative elongation viscosity stress level of tensile stress at which the probe tack test is stopped maximum debonding stress in probe tack test nominal stress relaxation (retardation) time time scale of the material rearrangements time scale of experimental observation contact area alkyl methacrylate

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A-10

BR D E Eeq EP F G0 Geq h H I J nD P PFG PS QSPR R R2 t* TEC Tm Vdeb W

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butyl rubber self-diff usion coefficient elasticity tensile modulus equilibrium elasticity tensile modulus poly(ethylene–co–propylene) substrate force relaxation moduli normalized by the moduli at the beginning of the relaxation equilibrium relaxation shear modulus rod displacement (a gap between the upper and bottom plates of squeeze-recoil tester), adhesive fi lm thickness relaxation spectrum isolene, polyisoprene rubber compliance Deborah number 180° peel force, resistance, adhesion pulsed-field gradient polystyrene quantitative structure–property relationship regalite R9100 hydrocarbon resin regression parameter durability of adhesive joint triethylcitrate melting temperature debonding rate practical work of adhesion (the area under probe tack curve)

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Index A Acrylics acid content of, 6-17, 7-17 based polymers, 5-20 composition of, 7-16 emulsions of, 4-15 fibril elongation of, 7-6 to 7-7 removable, 6-19 water solubility of, 4-4 Additives, 8-13 Adherence, process of, 4-12 to 4-15 Adherents, crystalline, 3-13 to 3-14 deformable, wetting out of, 1-13 to 1-18 surface properties of, 7-27 to 7-31 Adhesion (See also Peel resistance; Viscoelastic materials) composition dependence of, 11-24 contact time for, 11-12 to 11-15, 11-22 to 11-24 defined, 4-3, 10-2, 11-4 durability of, 9-13 to 9-17 energy of, 2-4 to 2-11, 2-14 to 2-16, A-3, A-7 experiments, 4-9 to 4-10 free volume in, 10-16 to 10-18 open time for, 4-14 as peel resistance free volume in, 10-17 master curve for, 4-6 performance in, 5-16 phase state influence in, 10-25 plasticizer concentration, role of, in, 7-20 of PVP blends, 9-11

of PVP-PEG blends, 10-5 rate and failure modes in, 7-13 to 7-14 stick-slip, 7-14 viscoelastic deformation in, 10-11 to 10-13 performance , 4-3 to 4-15 process, 4-12 to 4-15, 10-5, 11-2 to 11-6, 11-26, 11-37 to 11-47, 11-49, 11-57 to 11-59 properties, 7-17, 11-19 to 11-20 relaxation time in, 11-12 to 11-14 scientific research for, 4-2 strength of, 3-9 to 3-10, 6-18 to 6-22 theories of, 3-7, 10-3 of typical pressure-sensitive adhesives, 11-8 to 11-12 weak, 6-18 to 6-20 Adhesive (See also Pressure-sensitive adhesives) bandage of, 5-12 classification of, 3-3 to 3-29 combined transition zones in, 3-25 to 3-29 concentration gradient transition zones, 3-12 to 3-24 amorphous phase separation systems in, 3-12 to 3-13 complex amorphous-crystalline equilibrium systems, 3-14 to 3-20 crystalline substrate for, 3-13 to 3-14 diff usion and adhesive behaviour in, 3-21 to 3-24 ever-tacky, 3-3 joints with crystalline surface, 3-13 to 3-14 overview of, 3-3 to 3-5

I-1

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I-2 Adhesive (Contd.) structure-gradient transition zones in, 3-8 to 3-11 structure-mechanical transition zones in, 3-5 to 3-8 debonding, cavitation in, 4-15 to 4-17, 6-3 to 6-8, 6-15 to 6-23, 10-7 to 10-9, 11-38 to 11-39, 11-42 to 11-43 contact time for, 11-14 curves of, 6-15 to 6-23 definition of, 5-1 deformation mechanisms of model PVP- PEG, 10-7 to 10-9 energy of, 11-13 free, for self-diff usion of, 10-21 to 10-23 mechanisms, 10-9, 11-38 model PVP-PEG in, 9-10, 10-28 to 10-30 probe tests for, 6-15 to 6-23 stages for, 11-37 to 11-57 stress-strain curves for, 6-16 to 6-17 velocity in, 11-3 drug delivery, 7-33, 8-9 failure definition of, 9-6 during probe tack, 10-8 mechanism of, 2-10 to 2-11 modes, 7-13 to 7-15, 7-25 properties interpolymer complex for, 11-19 to 11-20 performance and viscoelastic window, 5-13 viscoelastic polymer materials as, 11-2 Adsorption theory, 10-3 Aging process, 10-18 Aluminum foil tape, 7-3 to 7-4 oxide layer of, 3-5 Amorphous phase separation systems, 3-12 to 3-13 Analysis of peel resistance, 7-2 to 7-7 Annealing, 3-10, 3-18 to 3-21, 11-12 to 11-13 Aqueous dispersions, 1-12 to 1-13, 1-24 Assembly time, 8-11 B Bandage adhesives, 5-12 Bartenev’s equation, 9-3

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Index Bending zone, 7-5 to 7-6 Bernoulli’s law, 1-3 Block copolymers, 4-9 Bond, formation of, 5-1 strength of, 7-7 Bonding, and G′, 5-17 to 5-19 pressure withdrawal in, 11-26 to 11-37 Burger’s model, 8-7 to 8-8, 11-26 to 11-28 C Carrier materials, 7-2, 7-16 Carrierless tapes, 8-15 Cavitation, 6-5 to 6-18 in debonding, 4-15 to 4-17 definition of, 11-42 formation process of, 6-7 images of, 6-18 interfacial, 6-9 to 6-10 probe geometry for, 6-15 to 6-18 in SIS-based PSA, 6-15 stages of, 6-5 to 6-8 Chang`s approach, for, 5-6 to 5-20 bonding and G′, 5-17 to 5-19 Dahlquist Contact Criterion Line, 5-13 to 5-15 fundamental polymer parameters, 5-19 to 5-20 G′-G″ Cross-Over Line, 5-15 to 5-16 peel performance, 5-16 PSA performance, 5-19 to 5-20 shear performance, 5-16 tack performance, 5-16 to 5-17 Chemical structures, 8-12 to 8-13, 10-4 Chu`s approach, 5-6 Cohesion energy, contribution of, to adhesion, 10-11 to 10-14 Cohesion energy contribution of to adhesion, 10-11 to 10-14 in diff usion and adhesive behavior, 3-21 to 3-24 overview of, 3-12 Cohesive-adhesive fracture, 9-6 Cohesive failure, 3-22, 7-25 Cold flow problem, 8-9 to 8-11 Combined transition zones, 3-25 to 3-29 Complex amorphous-crystalline equilibrium systems, 3-14 to 3-20

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Index Complex shear modulus, in linear viscoelasticity , 4-17 to 4-20 Compliance in linear viscoelasticity, 4-21 Composition dependence of adhesion, 11-24 Compressive load relaxation, 11-6 to 11-25 Concentration gradient transition zones, 3-12 to 3-24 in amorphous phase separation systems, 3-12 to 3-13 in complex amorphous-crystalline equilibrium systems, 3-14 to 3-20 for crystalline substrate, 3-13 to 3-14 Confi nement efect, 6-4 to 6-5 Contact angle retardation times, 1-8 to 1-10, 1-17 Contact time, for adhesion, 11-12 to 11-15, 11-22 to 11-24 for debonding, 11-14 in probe tack curves, 11-10 Coupling elastomers, 3-25 Creep-recovery tests, 1-19, 8-7 Cross-linking density, 7-18, 8-14 physical, 8-14 to 8-15 polymers, 8-15 Crystalline substrate, 3-13 to 3-14 Crystallization, 3-9 to 3-11 Curl, ability to, 7-24 Curl, properties of, 7-24 D Dahlquist’s Contact Criterion Line, 5-13 to 5-15, 10-28, 10-33 to 10-36, 11-4 Dale`s approach, 5-3 to 5-4 Dale`s approach in, 5-2, 5-4 to 5-6 Dead load, 8-12 Debonding cavitation in, 4-15, 6-15 to 6-18 contact time in, 11-14 curves of, 6-15 to 6-23 with cavitation in probe geometry, 6-15 to 6-18 fibril formation and extension in, 6-20 to 6-22 interfacial nucleation and propagation in, 6-18 to 6-20 transitions in, 6-22 to 6-23

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I-3 definition, 5-1 deformation mechanisms of model PVPPEG, 10-7 to 10-9 energy, 11-13 free energy for self-diff usion, 10-21 to 10-23 mechanisms of, 10-9, 11-38 model PVP-PEG for, 9-10, 10-28 to 10-30 probe tests for, 6-15 to 6-23 stages of, 11-37 to 11-47 first, 11-39 to 11-43 linear viscoelasticity and large-strain elongational geometries in, 11-45 to 11-47 relaxation properties in, 11-37 to 11-57 second, 11-43 to 11-45 three-stage mechanism of debonding, 11-37 to 11-39 stress-strain curves of, 6-16 to 6-17 velocity, 11-3 Deborah number, 2-9 definition of, 2-9 origin of, 11-2 to 11-3 of pressure-sensitive adhesives, 11-16 to 11-19 Deformation curves of, 1-20 of model PVP-PEG, 10-7 to 10-9, 10-28 to 10-30 stages of, 6-2 to 6-12 in cavitation, 6-5 to 6-8 by fibril formation and growth, 6-8 to 6-12 homogeneous, 6-4 to 6-5 Dendritic spreading, 1-12 Detachment time, 9-15 Dicing tape, 7-14 Diferential scanning calorimetry (DSC) in test of molecular nature of pressuresensitive adhesion, 10-6 to 10-7 in test of phase state, 10-25 Diff usion, 2-16 to 2-19 activation energy values in, 3-17 and adhesion, 2-2, 3-21 to 3-24, 10-13 to 10-14 in adhesion energy from probe-tack of immiscible polymers, 2-11 to 2-16

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I-4

Index

Diffusion (Contd.) for adhesion energy from probe tests, 2-14 to 2-16 for adhesive behavior of transition zones, 3-21 to 3-24 coefficients of, 10-18 to 10-20 in cohesive failure, 3-22 definition of, 10-23 experimental details of, 2-2 to 2-11 interfacial width of from neutron in materials, 2-12 overview of, 2-11 to 2-12 partial, 10-20 PVC-BNR system profiles in, 3-18 PVC-EVA 30 system profiles in, 3-19 reflectivity, 2-12 to 2-14 as self-diffusion, 2-2 to 2-11 for tack properties tests of, 2-4 to 2-11 theory of, 10-3 through polymer interfaces, 2-11 to 2-12 DMA (See Dynamic mechanical analysis) Drug delivery adhesives, 7-33, 8-9 Drum peel, 7-9 to 7-11 DSC (See Diferential scanning calorimetry) Durability, of adhesion, 9-13, 10-22, 11-42 definition of, 1-20 of joint, 9-1 to 9-17 molecular weight for, 8-12 normalized, 1-21 overview of, 9-1 to 9-2 in PVP-PEG systems, 9-13 stress and, 9-4 to 9-5, 9-15 test of, 9-14 triaxial stress tests for, 9-2 to 9-13 Dynamic mechanical analysis, in formulation and evaluation of complex PSA systems, 5-20 Dynamic peel test, 7-10 Dynamic shear test, 8-4

Elongation effects, 11-51 to 11-55 relative, 11-51 to 11-55 tensile stress in, 10-27, 11-49 to 11-51 viscosity in, 4-22 to 4-23 Emulsifiers, 7-19 Energy of adhesion, 2-4 to 2-11, 2-14 to 2-16 of fracture, 3-6 release rate of, 6-10, A-6 Entangled structures, 11-14 to 11-16 Ethylene-vinyl acetate, 3-10, 4-20 Ever-tacky adhesives, 3-3

E

G

Elasticity, 6-5 to 6-6 Elastomers coupling of, 3-25 fibril elongation in, 7-6 to 7-7 thermoplastic, 10-15, 10-35 Electronic theory of adhesion, 10-3

G′-G″ Cross-Over Line, 5-15 to 5-16 Gaussian integral, 3-18, A-4 Gel, deformable, wetting of, 1-12 to 1-18 General purpose PSAs, 5-14, A-5 Glass transition temperature, 10-24 to 10-26

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F Failure, during probe tack, 6-2 to 6-23, 10-8 mechanism of, 2-10 to 2-11 modes of, 7-13 to 7-15, 7-25 Fibril, defi nition of, 4-12 elongation of, 7-6 to 7-7 formation of, definition of, 9-10 extension of, 6-20 to 6-23 growth of, 6-8 to 6-12 length distribution of, 7-6 to 7-7 in PVP fibrillation, 9-12 Flory`s interaction parameter, 2-11 to 2-14, 2-18, 2-20, 10-15 Flow, cold, 8-9 to 8-11 properties, 7-2 Foaming test of PSAs, 6-12, 6-20, 11-38 to 11-44 Fractional free volume, 10-27 Fracture energy in adherence performance, 4-3 Free energy for self-diff usion, 10-21 to 10-23 Free volume, correlation of with adhesion, 10-16 to 10-18 fractional, 10-27

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I-5

Index adjusting of, 4-14 Chang’s research on, 5-11, 5-13 Chu’s research on, 5-2 of tackifers, 11-33 H Haynes theory, 5-3 to 5-4 Heteroepitaxy, 3-10 HMPSAs (See Hot-melt pressure-sensitive adhesives) Holding time, 8-2 to 8-4, 8-13 Homogeneous deformation, 6-4 to 6-5 Hot-melt pressure-sensitive adhesives, 4-9, 4-13 to 4-14 Humidity, 7-16 Hydrophilic surfaces, PI drops as, 1-8 to 1-10 PVP-PEG adhesives as, 1-7 to 1-8, 11-29 to 11-32 spreading on, 1-13 to 1-18 time dependence of, 1-10 Hydrophobic surfaces, compared with model PVP-PEG, 10-30 to 10-33 PEG drops as, 1-7 to 1-8 PI drops as, 1-8 to 1-10 polymers as, 11-26 retardation times for, 11-32 to 11-34 time dependence in, 1-10 I Immiscible blends, 7-23 Interfacial cavity, energy release rate of, 6-10 propagation of, 6-20 schematic of, 6-9 Interfacial nucleation and propagation, 6-18 to 6-20, 6-22 to 6-23 Interfacial slippage, 2-17 Interfacial width, In neutron reflectivity, 2-12 to 2-14 Interpolymer complex model, 11-19 to 11-25 adhesive properties of, 11-19 to 11-20 composition dependence of, 11-24 effect of contact time in, 11-22 to 11-24 as model pressure-sensitive adhesive, 11-19

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J Joint, and adhesion, 9-13 to 9-17 durability of, 9-1 to 9-17, 10-22, 11-42 overview of, 9-1 to 9-2 triaxial stress tests for, 9-2 to 9-13 K Kelvin-Voight model, 1-6 to 1-7, 11-21 L Labels, 7-1 to 7-2 Ladder-like structure, 8-14 to 8-15 Large-strain relaxation, 11-49 Linear elastic shear, 11-48 Linear viscoelasticity, 4-17 to 4-21 complex shear modulus in, 4-17 to 4-20 compliance in, 4-21 domain of, 4-5 to 4-9 large-strain elongational geometries in, 11-45 to 11-47 zero-shear viscosity for, 4-20 to 4-21 Liquids, spreading mixtures, 1-10 to 1-12 wetting out of, on solids, of low-molecular-weight, 1-2 to 1-7 of multicomponent, Marangoni effect of, 1-10 to 1-11 by polymer, 1-7 to 1-10 spreading for mixtures of liquids, 1-10 to 1-12 spreading of surfactant solutions, 1-12 to 1-13 for surfactants, 1-10 Load, compressive, relaxation by, 11-6 to 11-25 definition of, 8-12 behaviour of PSA by, 8-9 to 8-11 Loop tack, 4-9, 5-18 M Mandrel peel test, 7-10 Marangoni effect, 1-10 to 1-11 Master curve, 7-11 to 7-13 Maxwell model, 10-13 Medical PSAs, 5-13 Migration, 1-11

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I-6 Mirror test, 6-12 to 6-13 Miscible blends, 7-22, 8-13 Molecular structure, in correlation of adhesion with free volume, 10-16 to 10-18 in Dahlquist’s Criterion of Tack, 10-33 to 10-36 in pressure-sensitive adhesion, 10-1 to 10-37 design of, 4-20 mobility of, 10-14 to 10-15 for other viscoelastic polymers, 10-16 to 10-36 influence of on tensile properties, 10-9 to 10-11 for PVP-PEG H-bonded complex, 10-5 for PVP-PEG hydrophilic PSAs, 10-6 to 10-14 role of in deformation mechanisms of, 10-7 to 10-9 in relationship between peel adhesion and viscoelastic deformation, 10-11 to 10-13 and glass transition temperatures, 10-24 to 10-26 relaxation times of, 10-23 to 10-24, 11-1 to 11-59 role of in diff usion coefficients for, 10-18 to 10-20 in self-diff usion, 10-21 to 10-23 as supramolecular structures, 10-4 to 10-6 underlying factors, 10-14 to 10-15 in universal theory of pressure-sensitive adhesion, 10-2 to 10-4 Multicomponent adhesives, 1-9 to 1-11, 3-25 to 3-29 N Neo-Hookean material, 6-6 Network structures, 11-14 to 11-16 Neutron reflectivity, 2-12 to 2-14 In 90° peel, 7-5 to 7-11 NMR (See Nuclear magnetic resonance) Nonlinear domain, 4-9 to 4-10 Nuclear magnetic resonance (NMR), 2-13, 10-7, A-1 O Open time, 4-14, A-5

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Index P Packaging products, 5-1, 8-1, 8-4 Paper, 7-2 Partial diff usion, 10-20 Paster, 5-3 to 5-4 Patch, skin, (See Transdermal drug delivery system) Peel adhesion, master curve of, 4-6 performance of, 5-16 phase state in, 10-25 plasticizer concentration, role of in, 7-20 of PVP blends, 9-11 of PVP-PEG blends, 10-5 rate and failure modes in, 7-13 to 7-14 role of free volume in, 10-17 stick-slip in, 7-14 viscoelastic deformation in, 10-11 to 10-13 Peel force of aluminum foil tape, 7-3 to 7-4 pulling rate in, 7-25 relation of to cohesion strength, diff usion and relaxation, 10-13 to 10-14 Peel properties, 7-27 Peel resistance, 7-1 to 7-32, 7-15 to 7-31 (See also Adhesion) basic principles of, definition of, 2-2 dependence of, on bulk properties, 16 to 7-27 cross-linking density, 7-18 descriptive assumptions on, 7-3 emulsifiers, 7-19 failure mode, 7-13 to 7-15 pulling rate, 7-17 to 7-18 retardation times, 11-35 to 11-36 surface properties of substrate, 7-27 to 7-31 flowchart for, 7-15 influence of, on removability, 7-2 of labels, 7-1 to 7-2 overview of, 7-1 to 7-2 parameters of, 7-15 to 7-31 of PVP-PEG composition, 10-11 relation of to, cohesion strength, 10-13 diff usion, 10-13 relaxation, 10-13 test,

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Index master curve and time-temperature superposition, 7-11 to 7-13 methods for, 7-9 to 7-11 90° peel, 7-5 to 7-11 180° peel, 7-2 to 7-11 theory of, 7-2 to 7-7 tools for, 7-10 of UV cross-linkable PSAs, 7-19 Peel strength, 3-21, 5-18 for blends of, SIS/tackifer, 7-21 Vector/tackifer, 7-22 PEG (See polyethylene glycol) Performance evaluation, in adhesion, 4-3 to 4-15 as peel, 5-16 of pressure-sensitive adhesives, criteria of, 1-18 to 1-21 factors affecting it, 7-9 fundamental polymer parameters for, 5-19 to 5-20 of shear, 5-16 of tack, 5-16 to 5-17 with viscoelastic window, 5-13 PIB (See Polyisobutylene) Plasticizer concentration effects of, 11-33 to 11-34 impact and relation of to adhesion, 11-55 to 11-57 in interpolymer complex, 11-20 to 11-21 on peel adhesion, 7-20 Plate, displacement of, 8-5, 8-9 separation of, 9-10, 9-12 Polydisperity, 2-2 to 2-3 Polyethylene glycol, chemical structure of, 10-4 (See also Model PVP-PEG) Polyisobutylene, cohesive strength of, 10-15 durability study for, 9-14 Polyken probe tester, 6-12 for adhesion energy, 2-4 to 2-11, 2-14 to 2-16 contact time for, 11-10 curves for, 6-19, 9-16, 11-7, 11-11 debonding for, 6-21 to 6-22 fibril formation for, 4-12 parameters of, 4-16 schematic of, 6-2 to 6-3, 6-13

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I-7 setup of, 11-6 to 11-8 stress-strain curve of, 11-38 stress-versus-time curves for, 11-44 to 11-45 theoretical background of, 6-2 to 6-12 use of, 2-2 Polymer, interfaces of, 2-11 to 2-12 Polymerization method, 8-14 Polypropylene, epitaxial structures of, 3-10 peel strength of, 7-29 to 7-30 Polysaccharides, 1-13 Polyvinyl chloride, 3-15, 7-29, A- 2 Polyvinyl pyrrolidone, A-2 (See also PVPPEG model) blends, chemical structure of, 10-4 durability of, 9-13 fibrillation in, 9-12 peel adherence in, 9-11 PP (See Polypropylene) Preparation conditions, 8-11 Pressure-sensitive adhesion, contribution of cohesion, diff usion and relaxation to, 10-8 to 10-15 criteria of performance for, 1-18 to 1-21 bonding pressure in, 1-21 to 1-22 definition of, 1-1 as multistage process, 11-2 to 11-6 with three-stages, 11-5 to 11-6 time dependence of, 11-2 to 11-4 relaxation criteria in, 11-36 to 11-37 retardation times in, 11-34 to 11-36 rheology of, 1-18 to 1-22 Pressure-sensitive adhesive, bandage of, 5-12 bonding pressure for, 1-21 to 1-22 bulk properties of, influence on peel resistance, 7-16 to 7-27 cohesive failure for, 2-22 compressive load behavior, 8-9 to 8-11 Deborah number of, 11-16 to 11-19 ever-tacky, 3-3 failure of, 7-13 to 7-14 for general purpose, 5-14, A-5 improving shear behaviour of, 8-15 to 8-16 joints with crystalline substrate, 3-13 to 3-14 layer deformation of, 6-2 to 6-4 medical, 5-13 model of, 11-19 molecular nature of, 10-1 to 10-37

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I-8 Pressure-sensitive adhesive (Contd.) overview of, 1-1 to 1-2 peel test schematic for, 7-5 performance criteria for, 1-18 to 1-21 factors affecting, 7-9 fundamental polymer parameters for, 5-19 to 5-20 relaxation for, 11-38 removable, 5-11 to 5-12, 6-19 rheological properties of, universal theories for, 10-2 to 10-4 with contributions of cohesion, diff usion and relaxation, 10-8 to 10-15 universal theory of, 10-2 to 10-4 viscoelastic window for, 5-13 Probe tack, 6-1 to 6-23 adhesive bond failure in, 10-8 confinement effect in, 6-4 to 6-5 debonding curves for, 6-15 to 6-23 cavitation in probe geometry for, 6-15 to 6-18 fibril formation and extension in, 6-20 to 6-22 interfacial nucleation and propagation for, 6-18 to 6-20 transitions in, 6-22 to 6-23 experiments for, 6-12 to 6-14 fibril in, definition of, 4-12 elongation of, 7-6 to 7-7 extension, 6-20 to 6-23 formation of, 9-10 growth of, 6-8 to 6-12 length distribution of, 7-6 to 7-7 Propagation, definition of, 11-42 interfacial cracks in, 6-20 PSA, performance of, and fundamental parameters of, 5-19 to 5-20 tack for immiscible, 2-11 to 2-16 PSAs (See Pressure-sensitive adhesives) PSA/tackifer system, 7-21 to 7-23 Pulling rate, for peel force, 7-25 for peel resistance, 7-17 to 7-18 PVC (See Polyvinyl chloride) PVP, fibrillation in, 9-12 overview of, 6-1 to 6-2

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Index PVP (See Polyvinyl pyrrolidone) PVP-PEG model, comparison of, with typical hydrophobic adhesives, 10-30 to 10-33 Dahlquist’s Criterion of Tack for, 10-33 to 10-36 debonding effects and deformation rates for, 10-28 to 10-30 deformation of, 10-7 to 10-9, 10-28 to 10-30 for hydrophilic pressure-sensitive adhesives, 10-6 to 10-14 free volume of, 10-21 to 10-23 deformation mechanisms of, 10-7 to 10-9 peel adhesion and viscoelastic deformation of, 10-11 to 10-13 tensile properties of, 10-9 to 10-11 viscoelasticity theory, use of, for, 10-13 to 10-14 large strain behaviour for, 10-26 to 10-28 elongation/extension rate on tensile stress relaxation of, 11-51 to 11-55 relaxation properties of, 11-48 to 11-57 linear elastic shear for, 11-48 plasticizer concentration and relation to adhesion of, 11-55 to 11-57 tensile stress relaxation of, 11-49 to 11-51 Q QSPR (See Quantitative structure-property Relationship) Quantitative relationships, 4-10 to 4-12 Quantitative structure-property relationship, 10-2, A-9 to A-10 R Relaxation, 11-1 to 11-59 in adhesion, 11-12 to 11-14 in adhesive bond formation, 11-6 to 11-25 approach to, 11-26 and bonding pressure withdrawal, 11-26 to 11-37 composition dependence of, 11-24 for compressive load, 11-6 to 11-25 contact time effect for, 11-16 to 11-19

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I-9

Index criteria for pressure-sensitive adhesion, 11-36 to 11-37 curves of, 11-20 Deborah number in, 11-16 to 11-19 of entangled and network structures, 11-14 to 11-16 influence of, on, adhesive properties, 11-19 to 11-20 for interpolymer complexes, 11-19 to 11-25 large-strain, 11-49 for model pressure-sensitive adhesive, 11-19 in multistage process of pressure-sensitive adhesion, 11-2 to 11-6 as three-stage process, 11-5 to 11-6 optimum range of times of, 11-16 in polymers, 11-38 of pressure-sensitive adhesives, 11-8 debonding stage of, first, 11-39 to 11-43 linear viscoelasticity and large-strain elongational geometries in, 11-45 to 11-47 second stage of debonding process, 11-43 to 11-45 three-stage mechanism of debonding, 11-37 to 11-39 process of, 11-42 to 11-43 in PVP-PEG model, 11-48 to 11-57 in tensile stress, 11-49 to 11-51 influence of, extension rate on, 11-51 to 11-55 linear elastic shear, 11-48 plasticizer concentration, 11-55 to 11-57 retardation times in, for hydrophilic PVP-PEG adhesives, 11-29 to 11-32 for hydrophobic PSAs, 11-32 to 11-34 and pressure-sensitive adhesion, 11-34 to 11-36 and squeeze-recoil behavior, 11-28 to 11-29 in SIS adhesive, 11-45 in tack tests, 11-6 to 11-8 times of, for acrylics, 11-9 Release force, 5-8 to 5-9 Release liners, 2-10, 6-18 to 6-19 Removability, 7-2 Removable acrylics, 6-19 Removable adhesives, 5-11 to 5-12, 6-19, A-5

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Repositionability, 7-2 Reptation concept, 4-19, A-5 Research on adhesion science, 4-2 Retardation times, for contact angle, 1-8 to 1-10, 1-17 correlation of with pressure-sensitive adhesion, 11-34 to 11-36 definition of, 11-3 for hydrophilic PVP-PEG adhesives, 11-29 to 11-32 for hydrophobic pressure-sensitive adhesives, 11-32 to 11-34 Rheoadsorption theory of adhesion, 3-7 Rheological properties, 11-2 and bonding pressure, 1-21 to 1-22 master curve for, 4-7 performance criteria for, 1-18 to 1-21 in pressure-sensitive adhesion, 1-18 to 1-22, 4-18 structure-mechanical transition zones as, 3-7 of viscoelastic polymers, 11-2 Rubber, 3-25 to 3-29 S SAFT (See Shear adhesion failure temperature) SBC (See Styrene block copolymer) Schematic tack curves, 6-11 Scott’s equation, 8-11 Secondary spreading, 1-3 Self-diff usion, free energy in, 10-21 to 10-23 influence of, on, tack, 2-2 to 2-11 of viscoelastic materials, 10-16 Shear, deformation in, 8-2 hang time in, 5-3 to 5-4 as holding time, 8-2 to 8-4 linear elastic, 11-48 performance, 5-16 plastometer for, 8-3, 8-5 rate of, 1-22, 8-5, A-7 resistance of, 8-1 to 8-16 characterization of, 8-2 to 8-4 cold flow problem in, 8-9 to 8-11 dynamic test for, 8-4 static test for, 8-2 to 8-4 strength, factors influencing it, 8-11 to 8-15

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I-10 Shear (Contd.) additives, 8-13 assembly time/preparation conditions, 8-11 chemical composition, 8-12 to 8-13 cross-linking density, 8-14 load, 8-12 method of polymerization, 8-14 miscibility, 8-13 molecular weight and structure, 8-12 physical cross-linking, 8-14 to 8-15 temperature, 8-12 improving of, for pressure-sensitive adhesives, 8-15 to 8-16 overview of, 8-1 to 8-2 stress, 1-22, 8-5, 8-8, A-7 test methods for, 8-4 to 8-9 viscosity, 1-22 Shear adhesion failure temperature, 5-5, 8-4, 8-13 to 8-15, A-2 SIS-based PSA, 6-15, 11-45, A-2 Solid-liquid interface surface phenomena, 1-2 to 1-18 Solids, wetting out, of, by low-molecular-weight liquids, 1-2 to 1-7 Marangoni effect in, 1-10 to 1-11 by multicomponent liquids, 1-10 by polymer liquids, 1-7 to 1-10 spreading mixtures of liquids in, 1-10 to 1-12 spreading surfactant solutions in, 1-12 to 1-13 with surfactants, 1-10 Solubility, parameter of, 10-15, 10-1 to 10-37 Solvent-based PSPs, 4-4 Spreading, dendritic, 1-12 on gels, 1-13 to 1-18 kinetics of PEG drops, 1-7 to 1-8 of mixtures of liquids, 1-10 to 1-12 secondary, 1-3 of surfactant solutions, 1-12 to 1-13 Squeeze-flow technique, 1-21 to 1-22 Squeeze-recoil, behaviors of, 11-28 to 11-29 profi les of, 11-27 to 11-28 test of, 9-9, 11-5 to 11-8 Squeeze tester schematic, 8-10 Standard types of test, 6-1 Static peel test, 7-10

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Index Static shear test, 8-2 to 8-4, 9-1 to 9-2 Steel, 3-25 to 3-29, 6-18 Stefan’s equation, 8-10 Sticking, 3-1 to 3-2 Stick-slip failure, 7-14, 7-25 Storage modulus and bonding, 5-17 to 5-19 Strength of adhesion, 10-1 to 10-37 Stress, normal, 7-8 to 7-9 role of, in, durability, 9-4 to 9-5, 9-15 Stress-strain curves, in adhesion energy determination, 2-15 in debonding, 6-16 to 6-17 for deformation, 4-16 diblock content, influence of, on, 4-10 to 4-11 parameters of, 11-38 for resin blends, 7-31 schematic of, 6-3 in viscoelasticity, 10-29 to 10-32 Stress-versus-time curves, 11-44 to 11-45 Structure-gradient transition zones, 3-8 to 3-11 Structure-mechanical transition zones, for adhesive classification, 3-5 to 3-8 in “rheoadsorption” theory of adhesion, 3-7 in rheological theory of adhesion, 3-7 Structure-property relationships, for model PVP-PEG hydrophilic PSAs, 10-6 to 10-14 deformation mechanisms of, 10-7 to 10-9 for peel adhesion and viscoelastic deformation, 10-11 to 10-13 for tensile properties, 10-9 to 10-11 in viscoelasticity theory of pressuresensitive adhesion, 10-13 to 10-14 Styrene block copolymers, 2-2, A-7 Substrates, crystalline, 3-13 to 3-14 deformable, wetting out of, 1-13 to 1-18 surface properties of, 7-27 to 7-31 Supramolecular structures, 10-4 to 10-6 Surface properties of substrate, influence of on, peel resistance, 7-27 to 7-31 Surface tension, 7-30, A-2, A-6 Surfactants definition, 1-10 spreading solutions of, 1-12 to 1-13

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Index T Tack, curves of, 2-4 to 2-7 between immiscible polymers, 2-11 to 2-16 performance of, 5-16 to 5-17 self-diff usion in, 2-2 to 2-11 test of, 11-6 to 11-8 Tackifers, 8-13 Tapes, 7-14, 8-15 TDS (See Transdermal drug delivery system) Temperature, dependence on, 10-35 of glass transition, adjusting of, 4-14 for tackifiers, 11-33 work of Chang, 5-1, 5-13 work of Chu, 5-2 for modulus requirements, 5-6 to 5-8 as shear strength factor, 8-12 sweep curves, 10-35 Tensile, properties, of model PVP-PEG adhesive blends, 10-9 to 10-11 of PSAs, 10-33 strength, 10-16 stress, in elongation, 10-27 in relaxation, 11-49 to 11-51 strain curves, 6-21, 10-10 Test, of, creep-recovery, 1-19, 8-7 durability, 9-14 dynamic peel, 7-10 dynamic shear, 8-4 failure mode, 7-13 to 7-15 foam, 6-12, 6-20, 11-38 to 11-44 mandrel peel, 7-10 90° peel, 7-5 to 7-11 180° peel, 7-9 to 7-11 peel resistance, 7-7 to 7-15 probe tack, 6-2, 11-6 to 11-8 squeeze-recoil, 9-9, 11-5 to 11-8 static peel, 7-10 static shear, 8-2 to 8-4, 9-1 to 9-2 T-peel, 7-10 Triaxial stress, 9-2 to 9-13 Termodynamic work of adhesion, 4-3, A-5 Termoplastic elastomers, 10-15, 10-35 Time parameter,

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I-11 as contact time, 11-12 to 11-15, 11-22 to 11-24 dependence on, 11-2 to 11-4 of cavity growth, 6-7 of complex shear modulus, 4-17 to 4-20 of compliance, 4-21 of Deborah number, 11-2 to 11-3 in linear viscoelasticity, 4-17 to 4-21 in reptation concept, 4-19 for zero-shear viscosity, 4-20 to 4-21 master curve of, 7-11 to 7-13 as open time, 4-14 as relaxation time, 11-12 to 11-14 for acrylics, 11-9 for adhesion strength, 11-16 to 11-18 correlation of with contact time and adhesion, 11-12 to 11-14 definition of, 11-3 for pressure-sensitive adhesives, 10-23 to 10-24 for viscoelastic materials, 10-16 as retardation time, for contact angle, 1-8 to 1-10, 1-17 correlation of with pressure-sensitive adhesion, 11-34 to 11-36 definition of, 11-3 for hydrophilic PVP-PEG adhesives, 11-29 to 11-32 for hydrophobic pressure-sensitive adhesives, 11-32 to 11-34 time-temperature superposition, 7-11 to 7-13 T-peel test, 7-9 to 7-11 Transdermal drug delivery system, 8-9 Transfer tapes, 7-14 Transient creep function, 4-21 Transition zones (See also Concentration gradient transition zones) adhesion theories for, 3-2 adhesives classification for, 3-3 to 3-29 combined, 3-25 to 3-29 structure-gradient related, 3-8 to 3-11 structure-mechanical, 3-5 to 3-8 definition of, 3-2 role of in diff usion and adhesive behavior, 3-21 to 3-24 overview, 3-1 to 3-3 Triaxial stress tests, 9-2 to 9-13 Trouton behavior, 4-22 Tse`s approach, 5-5

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I-12 U Ultraviolet light, cross-linkable PSAs, 7-19 induced curing, 7-14 Universal theory of pressure-sensitive adhesion, 10-2 to 10-4 V Vinogradov’s adhesiometer, 9-2 Viscoelastic materials (See also Linear viscoelasticity; Peel adhesion; Peel resistance) adherence performance of, 4-3 to 4-1 linear viscoelastic domain of, examples, 4-5 to 4-9 nonlinear domain of, adherence experiments in, 4-9 to 4-10 overview of, 4-3 to 4-4 process and material properties of, 4-12 to 4-15 quantitative relationships for, 4-10 to 4-12 relationships between, 4-4 to 4-12 behaviour of, 4-1 to 4-24 in cavitation, 4-15 to 4-17 overview of, 4-1 to 4-3 as soft polymers, 4-17 large deformations of and nonlinear aspects, 4-22 to 4-23 reptation concept of, 4-19 time dependent effects for and linear viscoelasticity of, 4-17 to 4-21 contact time for, 11-12 to 11-15, 11-22 to 11-24 Dahlquist contact criteria for, 5-14 to 5-15 debonding rates of, 10-28 to 10-30 definition of, 4-3, 10-2, 11-4 deformation of, 10-10 to 10-13, 10-28 to 10-30 durability of, 9-13 to 9-17 durability and adhesion for, 9-13 to 9-17 overview of, 9-1 to 9-2 triaxial stress tests for, 9-2 to 9-13 energy of, 2-4 to 2-11, 2-14 to 2-16 experiments with, 4-9 to 4-10 free volume for, 10-16 to 10-18 future research for, 5-20

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Index as hydrophobic adhesives, 10-30 to 10-33 joint durability for, 9-1 to 9-17 linear domain for, 4-5 to 4-9 measurement for, 4-18 model PVP-PEG adhesive based on, 10-26 to 10-28 nonlinear domain and adherence of, 4-9 to 4-10 quantitative relationships for, 4-10 to 4-12 typical hydrophobic adhesives of, 10-30 to 10-33 overview of, 4-3 to 4-4, 5-1 to 5-2 peel properties of, 7-27 performance of, 4-3 to 4-15, 5-13 polymers, as, 10-15 to 10-16 adhesive and rheologic properties of, 11-2 adhesion and free volume for, 10-16 to 10-18 Dahlquist’s Criterion of Tack for, 10-33 to 10-36 diff usion coefficients for, 10-18 to 10-20 examples of, 11-8 to 11-12 general purpose, 5-14, A-5 high shear, 5-11 medical, 5-13 glass transition temperatures for, 10-24 to 10-26 pressure-sensitive adhesives as, 10-16 to 10-36 relaxation times for, 10-23 to 10-24 self-diff usion for, 10-21 to 10-23 theoretical background of, 10-13 to 10-14 processing of, 4-12 to 4-15 quantitative relationships for, 4-10 to 4-12 relaxation time of, 11-12 to 11-14 release coatings for, 5-10 as removable adhesives, 5-11 to 5-12 research background for, 4-2, 5-3 to 5-20 Chang, 5-6, 5-17, 5-20 Chu, 5-6 Dale, 5-3 to 5-4 Hayne, 5-5 Paster, 5-5 Tse, 5-5 Yang, 5-17 to 5-20 soft polymers as, 4-17 to 4-23

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I-13

Index large deformations and nonlinear aspects of, 4-22 to 4-23 time-dependent effects and linear viscoelasticity of, 4-17 to 4-21 Trouton behaviour of, 4-22 spectrum of, 7-24 strength of, 3-9 to 3-10, 6-18 to 6-22 structural properties of, 7-17, 11-19 to 11-20 theoretical overview of, 3-7, 10-3, 10-13 to 10-14 weakness of, 6-18 to 6-20 Viscosity, by elongation, 4-22 to 4-23, A-3 to A-4, A-7, A-9 of, PEG-400, 1-17 of, spreading liquids, 1-8 to 1-10 Volume, free, correlation of, with adhesion, 10-16 to 10-18 fractional, 10-27 W Water-based/soluble PSPs, 4-4, 7-18 Weak adhesion, 6-18 to 6-20 Wettability, test of, 1-23 to 1-24, 7-18 Wetting,

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of, deformable substrates (gels), 1-13 to 1-18 of, solids, by low-molecular-weight liquids, 1-2 to 1-7 by multicomponent liquids spreading mixtures of liquids, role in, 1-10 to 1-12 by spreading surfactant solutions, 1-12 to 1-13 by polymer liquids, 1-7 to 1-10 William-Landel-Ferry equation, 7-11 to 7-12 Y Yang`s approach, for bonding and G′, 5-17 to 5-19 polymer parameters, 5-19 to 5-20 for PSA performance and fundamental Yield-point, 9-2 Yield stress, 10-33, 10-37, 11-12, 11-33, 11-42, 11-58 Z Zero-shear viscosity, 4-20 to 4-21 Zhurkov’s equation, 9-3

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