Pressure-Sensitive Adhesives and Applications

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Pressure-Sensitive Adhesives and Applications Second Edition, Revised and Expanded

Istvan Benedek Wuppertal, Germany

MARCEL

MARCELDEKKER, INC. D E K K E R

-

NEWYORK BASEL

The ﬁrst edition of this book was published as Pressure-Sensitive Adhesives Technology, Istva´n Benedek and Luc J. Heymans (Marcel Dekker, Inc., 1996). Although great care has been taken to provide accurate and current information, neither the author(s) nor the publisher, nor anyone else associated with this publication, shall be liable for any loss, damage, or liability directly or indirectly caused or alleged to be caused by this book. The material contained herein is not intended to provide speciﬁc advice or recommendations for any speciﬁc situation. 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 A catalog record for this book is available from the Library of Congress. ISBN: 0-8247-5059-4 This book is printed on acid-free paper. Headquarters Marcel Dekker, Inc., 270 Madison Avenue, New York, NY 10016, U.S.A. tel: 212-696-9000; fax: 212-685-4540 Distribution and Customer Service Marcel Dekker, Inc., Cimarron Road, Monticello, New York 12701, U.S.A. tel: 800-228-1160; fax: 845-796-1772 Eastern Hemisphere Distribution Marcel Dekker AG, Hutgasse 4, Postfach 812, CH-4001 Basel, Switzerland tel: 41-61-260-6300; fax: 41-61-260-6333 World Wide Web http://www.dekker.com The publisher oﬀers discounts on this book when ordered in bulk quantities. For more information, write to Special Sales/Professional Marketing at the headquarters address above. Copyright ß 2004 by Marcel Dekker, Inc. All Rights Reserved. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microﬁlming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher. Current printing (last digit): 10

9 8 7 6 5

4 3 2 1

PRINTED IN THE UNITED STATES OF AMERICA

Preface to the Second Edition

The growing interest in the advances described in this book have called for this second edition of this book. In the past few years pressure-sensitive products have reached a maturity that warrants a detailed and critical examination of their science and technology. This is a vast domain, and I have tried to cover some of its special aspects in separate works. The volume Development and Manufacture of Pressure-Sensitive Products (Marcel Dekker, 1998) describes the whole domain of self-adhesive products; Pressure-Sensitive Formulation (VSP, Utrecht) gives a detailed discussion of a special, practical segment of pressure sensitivity. However, PressureSensitive Adhesives Technology (Marcel Dekker, 1996), the ﬁrst of these books, constitutes the main step on the way to understanding adhesivebased pressure-sensitive products. In the past decade advances in contact physics and mechanics have allowed us to correlate the macroscopic aspects of adhesive bonding and debonding with the macromolecular basis of the viscoelastomers. The most important elements of this progress are described in a separate section of the revised book. Developments in the practical examination and quality assurance of pressure-sensitive products required an enlarged discussion and reformulation of Chapter 10, ‘‘Test Methods.’’ Environmental considerations made necessary the discussion of recycling methods, and biodegradability of raw materials, product components, and pressuresensitive products. Other scientiﬁc and industrial advances (i.e., new raw materials and improved coating technology) are also included in the second edition. Thus, after undergoing a major revision, the second edition of this book remains a comprehensive and convenient up-to-date source of

iii

iv

Preface to the Second Edition

information for users in industry and academia. So far as I am aware, this is the ﬁrst single-author book on general aspects of pressure-sensitive adhesive technology, and it has been my pleasure to assist in its success. Istva´n Benedek

Preface to the First Edition

Since their introduction half a century ago, pressure-sensitive adhesives have been successfully applied in many ﬁelds. They are used in self-adhesive labels, and tapes and protective ﬁlms, as well as in dermal dosage systems for pharmaceutical applications, the assembly of automotive parts, toys, and electronic circuits and keyboards. They have experienced an astonishing growth rate, and the installed manufacturing and converting capacity has also sharply increased. A speciﬁc engineering technology for pressuresensitive adhesives, surprisingly a special science, appears to be lacking. Very few books deal with intrinsic features of pressure-sensitive adhesives. The application of pressure-sensitive adhesives requires a thorough knowledge of basic rheological and viscoelastic phenomena. Adhesive and polymer scientists, however, are not very often employed as industrial managers or machine operators. Therefore the need arises to investigate and summarize the most important features of pressure-sensitive adhesive technology and to explain the phenomena scientiﬁcally. This book covers all the ﬁelds of manufacturing, conversion, and application and end-uses of pressure-sensitive adhesives. The classical approach would be to compile a treatise based on the work of various experts, theoreticians, chemists, and engineers, thereby coming up with a book consisting of a series of papers with a common title only. We have, however, chosen a diﬀerent approach. Based on our experience as engineers (in both scientiﬁc activity and industrial areas) and using the available technical literature, we have addressed all aspects of pressure-sensitive adhesives. We have included the scientiﬁc basis of suitability for speciﬁc applications (i.e., chemical and physical, rheology), the raw materials, the manufacture (formulation) of the adhesive and of the labelstock (converting the adhesive). We have selected self-adhesive labels as the most complex self-adhesive laminate; we mainly discuss labels, but, v

vi

Preface to the First Edition

whenever possible, a comparison with and extension to other applications is included. In order to illustrate the diﬀerent topics and issues discussed, we have referred to a number of commercially available products. It should be kept in mind that these products are only mentioned in order to clarify the discussion and in no way does it constitute any judgment about inherent performance characteristics or their suitability for speciﬁc applications or end-uses. It is not the aim of this book to establish or complete the science of pressure-sensitive adhesives, nor does it constitute a series of recipes. Rather it serves as a practical aid to converters and those involved in the design and use of pressure-sensitive adhesives. Istva´n Benedek Luc J. Heymans

Contents

Preface to the Second Edition Preface to the First Edition

1.

Introduction References

2.

Rheology of Pressure-Sensitive Adhesives 1

Rheology of Uncoated PSAs 1.1 Properties of PSAs 1.2 Inﬂuence of Viscoelastic Properties on the Adhesive Properties of PSAs 1.3 Inﬂuence of Viscoelastic Properties on the Converting Properties of PSAs and PSA Laminates 1.4 Inﬂuence of Viscoelastic Properties on End-Use Properties of PSAs 1.5 Factors Inﬂuencing Viscoelastic Properties of PSAs 2 Rheology of PSA Solutions and Dispersions 2.1 Rheology of PSA Solutions 2.2 Rheology of PSA Dispersions 3 Rheology of the Pressure-Sensitive Laminate 3.1 Inﬂuence of the Liquid Components of the Laminate 3.2 Inﬂuence of the Solid Components of the Laminate 3.3 Inﬂuence of the Composite Structure References

iii v

1 3 5 5 6 11 25 26 31 40 41 44 52 54 58 81 82 vii

viii

3.

Contents

Physical Basis for the Viscoelastic Behavior of Pressure-Sensitive Adhesives Role of the Tg in Characterizing PSAs Values of Tg for Adhesives Factors Inﬂuencing Tg Adjustment of Tg Correlation Between the Main Adhesive, End-Use, and Converting Properties of PSAs and Tg 2 Role of the Modulus in Characterizing PSAs 2.1 Factors Inﬂuencing the Modulus 2.2 Adjustment of the Modulus 2.3 Modulus Values 3. Contact Physics 3.1 Contact Mechanics for Elastic Materials 3.2 Contact Mechanics for Viscoelastic Materials 3.3 Micromechanical Considerations 4 The Role of Other Physical Parameters in Characterizing PSAs References

1

4.

The 1.1 1.2 1.3 1.4

Comparison of PSAs 1

Comparison of PSAs with Thermoplastics and Rubber 1.1 Cold Flow 1.2 Relaxation Phenomena 1.3 Mechanical Resistance 2 Comparison Between PSAs and Other Adhesives References

5.

Chemical Composition of PSAs 1

2

Raw Materials 1.1 Elastomers 1.2 Viscoelastomers 1.3 Viscous Components 1.4 Components for In-line Synthesis 1.5 Components for Special Pressure-Sensitive Formulations 1.6 Other Components 1.7 Release Coatings Factors Inﬂuencing the Chemical Composition 2.1 Synthesis 2.2 Formulation

89 90 91 92 104 107 110 113 124 126 129 129 130 132 137 138 147 147 148 155 156 158 159 161 162 163 176 187 189 197 198 199 205 205 210

Contents

6.

2.3 Physical State of PSAs 2.4 End-Use 2.5 Coating Method 2.6 Solid State Components of the Laminate 2.7 Product Build-Up 2.8 Environment Related Considerations References

211 216 220 221 222 223 224

Adhesive Performance Characteristics

235 235 235 261 307

1

Adhesion-Cohesion Balance 1.1 Tack 1.2 Peel Adhesion 1.3 Shear Resistance (Cohesion) 2 Inﬂuence of Adhesive Properties on Other Characteristics of PSAs 2.1 Inﬂuence of Adhesive Properties on the Converting Properties 2.2 Inﬂuence of Adhesive Properties on End-Use Properties 2.3 Inﬂuence of Peel Adhesion 2.4 Inﬂuence of Shear 3 Comparison of PSAs on Diﬀerent Chemical Bases 3.1 Rubber-Based Versus Acrylic-Based PSAs 3.2 Acrylics and Other Synthetic Polymer-Based Elastomers References

7.

ix

Converting Properties of PSAs 1

Convertability of the Adhesive 1.1 Convertability of Adhesive as a Function of the Physical State 1.2 Convertability of Adhesive as a Function of Adhesive Properties 1.3 Convertability of Adhesive as a Function of the Solid State Components of the Laminate 1.4 Convertability of Adhesive as a Function of Coating Technology 1.5 Convertability of Adhesive as a Function of End-Use Properties 2 Converting Properties of the Laminate 2.1 Deﬁnition and Construction of the Pressure-Sensitive Laminate

320 320 320 320 321 321 321 322 341 353 353 353 361 361 361 362 362 363

x

8.

Contents

2.2 Printability of the Laminate References

391 419

Manufacture of Pressure-Sensitive Adhesives

425 425 426 427 429 430 432 433 461 468 476 477 480 481 484 489 505 541

1

Manufacture of PSA Raw Materials 1.1 Natural Raw Materials 1.2 Synthetic Raw Materials 2 Formulating PSAs 2.1 Adhesive Properties 2.2 Formulating Opportunities 2.3 Tackiﬁcation 2.4 Rosin-Based Tackiﬁers 2.5 Hydrocarbon-Based Tackiﬁers 2.6 Special Tackiﬁer Resins 2.7 Tackiﬁcation with Plasticizers 2.8 Cohesion Regulation 2.9 Coating Properties 2.10 Converting Properties 2.11 End-Use Properties 2.12 Inﬂuence of Adhesive Technology 2.13 Technological Considerations 2.14 Comparison Between Solvent-Based, Water-Based, and Hot-Melt PSAs References

9.

Manufacture of Pressure-Sensitive Labels 1 2

3

4 5 6

Coating Technology Coating Machines 2.1 Adhesive Coating Machines 2.2 Coating Devices/Coating Systems 2.3 Choice of Coating Geometry 2.4 Other Coating Devices Coating of Hot-Melt PSAs 3.1 Roll Coaters for Hot-Melt PSAs 3.2 Slot-Die Coating for Hot-Melt PSAs Drying of the Coating 4.1 Adhesive Drying Tunnel Environmental Considerations Simultaneous Manufacture of PSAs and PSA Laminates 6.1 Radiation Curing of PSAs 6.2 Siliconizing Through Radiation

543 549 559 560 563 564 565 585 591 594 594 595 598 598 602 612 612 613

Contents

7

Manufacture of the Release Liner 7.1 Nature of the Release Liner 7.2 Coating Machines for Silicones 7.3 Technology for Solvent-Based Systems 7.4 Technology for Solventless Siliconizing 8 Rehumidiﬁcation/Conditioning References

10.

Test Methods 1

Evaluation of the Liquid Adhesive 1.1 Hot-Melt PSAs 1.2 Solvent-Based PSAs 1.3 Water-Based PSAs 2 Evaluation of the Solid Adhesive 2.1 Test of Coating Weight 2.2 Other Properties 3 Laminate Properties 3.1 General Laminate Properties 3.2 Special Laminate Properties References Abbreviations and Acronyms 1 Compounds 2 Terms Index

xi

616 616 619 619 619 621 621 629 629 630 630 630 639 639 640 643 644 687 700 709 709 711 715

1 Introduction

Adhesives are nonmetallic materials [1] used to bond other materials, mainly on their surfaces through adhesion and cohesion. Adhesion and cohesion are phenomena which may be described thermodynamically, but actually they cannot be measured precisely. It was shown [2] that the most important bonding processes are bonding by adhesion and bonding with pressuresensitive adhesives (PSAs). For adhesives working through adhesion phenomena the adhesive ﬂuid is transformed after bonding (i.e., the build up of the joint) into a solid. In the case of PSAs, the adhesive conserves its ﬂuid state after the bond building too. Thus its resistance to debonding is moderate and the joint may be delaminated without destroying the laminate components in most cases. Pressure-sensitive adhesives have been in wide use since the late 19th century, starting with medical tapes and dressings. The ﬁrst U.S. patent describing the use of a PSA—for a soft, adhering bandage—was issued in 1846 [3]. Ninety years later Stanton Avery developed and introduced the self-adhesive label [4]. Two major industries resulted from these innovations: pressure-sensitive tapes and labels. Industrial tapes were introduced in the 1920s and 1930s followed by self-adhesive labels in 1935. About ten years after that, pressure-sensitive protective ﬁlms were manufactured. The history of PSAs was described by Villa [5]. First, solvent-based PSAs using natural rubber were developed (19th century). In the 1940s hot-melt adhesives were introduced. Pressure-sensitive adhesives are adhesives that form ﬁlms exhibiting permanent tack, and display an adhesion which does not strongly depend on the substrate [6]. The term PSA has a very precise technical deﬁnition and has been dealt with extensively in the chemical literature [7,8]. However, as discussed in [9], the technical term of PSA in diﬀerent languages (e.g. pressure-sensitive adhesive, autocollants, Haftkleber, etc.) is not completely clear. The recent development 1

2

Chapter 1

of pressure-sensitive products without a coated pressure-sensitive adhesive layer, makes the deﬁnition of this product group more diﬃcult [9,10]. The build up and classiﬁcation of pressure-sensitive products have been discussed in detail in [9]. The function of PSAs is to ensure instantaneous adhesion upon application of a light pressure. Most applications further require that they can be easily removed from the surface to which they were applied through a light pulling force. Thus PSAs are characterized by a built-in capacity to achieve this instantaneous adhesion to a surface without activation, such as a treatment with solvents or heat, and also by having suﬃcient internal strength so that the adhesive material will not break up before the bond between the adhesive material and the surface ruptures. The bonding and the debonding of PSAs are energy-driven phenomena. Pressure-sensitive adhesives must possess viscous properties in order to ﬂow and to be able to dissipate energy during the adhesive bonding process. However, the adhesive must also be elastic (i.e., it must resist the tendency to ﬂow) and, in addition, store bond rupture energy in order to provide good peel and tack performance. Pressure-sensitive adhesives should possess typical viscoelastic properties that allow them to respond properly to both a bonding and a debonding step. For satisfactory performance in each of these steps the material must respond to a deforming force in a prescribed manner. Polymers employed as PSAs have to fulﬁll partially contradictory requirements; they need to adhere to substrates, to display high shear strength and peel adhesion, and not leave any residue on the substrate upon debonding. In order to meet all these requirements, a compromise is needed. When using PSAs there appears another diﬀerence from wet adhesives, namely the adhesive does not change its physical state because ﬁlm forming is inherent to PSAs. Thus, PSAs used in self-adhesive laminates are adhesives which, through their viscoelastic ﬂuid state, can build up the joint without the need to change this ﬂow state during or after application. On the other hand, their ﬂuid state allows controlled debonding giving a temporary character to the bond. Because of the ﬂuid character of the bonded adhesive, the amount of adhesive (i.e., the dimensions of the adhesive layer) is limited; the joint works as a thin-layer laminate or composite. Because of this special, thin-layer structure of the composite, the solid state components of the laminate exert a strong inﬂuence on the properties of the adhesive in the composite. Therefore, there exists a diﬀerence between the measured properties of the pristine adhesive and of the adhesive enclosed within the laminate. Adhesives, in general, and PSAs, in particular, have to build up a continuous, soft (ﬂuid), and tacky (rubbery) layer. The latter will adhere

Introduction

3

to the substrate. On the other hand, the liquid adhesive layer of the PSAs working in the bond has to oﬀer a controlled bond resistance. This special behavior requires materials exhibiting a viscoelastic character. The properties which are essential in characterizing the nature of PSAs comprise: tack, peel adhesion, and shear. The ﬁrst measures the adhesive’s ability to adhere quickly, the second its ability to resist removal through peeling, and the third its ability to hold in position when shear forces are applied [11]. These properties will be discussed in more detail in Chap. 6, which describes the adhesive properties of PSAs. In order to understand the importance of these properties, it is absolutely necessary to answer the following questions: What does the viscoelastic character of a PSA comprise? What is the material basis (main criteria) for the viscoelastic behavior of a PSA? REFERENCES 1. 2. 3.

4. 5. 6. 7. 8. 9. 10. 11.

DIN 16921. R. Ko¨hler, Adha¨sion, (3) 90 (1970). J.A. Fries, ‘‘New Developments in PSA,’’ in TECH 72, Advances in PressureSensitive Tape Technology, Technical Seminar Proceedings, Itasca, IL, May, 1989. Der Siebdruck, (3) 69 (1986). G.J. Villa, Adha¨sion, (10) 284 (1977). Vinnapas, Eigenschaften und Anwendung, 7.1. Teil, Anwendung, Wacker GmbH, Mu¨nchen, 1976. R. Houwink and G. Salomon, Adhesion and Adhesives, Vol. 2, Chapter 17, Elsevier Co., New York, 1982. D. Satas, Handbook of Pressure Sensitive Technology, Van Nostrand Rheinhold Co., New York, 1982. I. Benedek, Development and Manufacture of Pressure-Sensitive Products, Marcel Dekker, New York, 1999. I. Benedek, Pressure-Sensitive Formulation, VSP, Utrecht, 2000. J.P. Keally and R.E. Zenk. (Minnesota Mining and Manuf. Co., USA), Canad. Pat. 1224.678/10.07.92 (US Pat. 399350).

2 Rheology of Pressure-Sensitive Adhesives

Pressure-sensitive adhesives are viscoelastic materials with ﬂow properties playing a key role in the bond forming; their elasticity plays a key role in the storage of energy (i.e., the debonding process). The balance of these properties governs their time-dependent repositionability and bonding strength (i.e., their removability). Their ﬂow properties are useful in coating technology and at the same time detrimental to the converting technology of labels. Generally PSAs are used as thin layers, therefore their flow is limited by the physico-mechanical interactions with the solid components of the laminate (liner and face) materials. On the other hand the solid components of the laminate are generally thin, soft, viscous, and/or elastic layers, allowing a relatively broad and uniform distribution of the applied stresses. Thus the properties of the bonded adhesive (i.e., its flow characteristics) may differ from those of the pure (unbonded) adhesive. Therefore in this chapter the rheology of pure and coated PSAs will be dealt with separately.

1

RHEOLOGY OF UNCOATED PSAs

It remains diﬃcult to examine the properties of pure (i.e., uncoated or unbonded) PSAs, and to obtain generally valid information. Pressuresensitive adhesives are seldom used as thick layers between motionless rigid surfaces (i.e., as ﬂuids). On the other hand, as known from industrial experience, the nature of the face stock material or of the substrate used, and their characteristics and dimensions may signiﬁcantly inﬂuence the properties of the PSA laminate. Practically, this disadvantage is eliminated by the use of normalized or standard solid state components. However, a theoretical approach may be used for the investigation of pristine PSAs. 5

6

1.1

Chapter 2

Properties of PSAs

The adhesive and end-use properties of PSAs require a viscoelastic, nonNewtonian ﬂow behavior which is based on the macromolecular nature of the adhesive. In order to understand the needs and means of viscoelastic behavior one needs to summarize the most important material properties speciﬁcally related to PSAs. Generally, adhesives in a bond behave like a ﬂuid or a solid. Fluids are characterized by their viscosity which inﬂuences their mobility, whereas solids are characterized by their modulus which determines their deformability. In an ideal case, for Newtonian ﬂuids (or for solids obeying Hooke’s law) the applied force (load) will be balanced by the material’s own mechanical characteristics, that is, the viscosity or the Young’s modulus E: ¼ = 0

ð2:1Þ

E ¼ =

ð2:2Þ

where and are the applied stresses, and and 0 are the strain and shear rate, respectively. As indicated earlier, PSAs originate from a ﬁlm-forming, elastomeric material, which combines a high degree of tack with an ability to quickly wet the surface to which it is applied, to provide instant bonding at low-tomoderate pressure as a result of its ﬂow characteristics. On the other hand, PSAs possess suﬃcient cohesion and elasticity, so that despite their aggressive tackiness they can be handled with the ﬁngers and removed from smooth surfaces without leaving any residue. Moreover, in order to achieve bond strength they have to store energy (i.e., they must be elastic). Fundamentally PSAs require a delicate balance between their viscous and elastic properties. One should note that PSAs have to satisfy these contradictory requirements under diﬀerent stress rate conditions, that is, at low shear rates they must ﬂow (bonding) and at high peeling rates they have to respond elastically (debonding). Consequently, according to their adhesive and end-use properties, PSAs cannot be Newtonian systems: they do not obey Newton’s law (i.e., there is no linear dependence between their viscosity and the shear rate). Their viscosity is not a material constant, but depends on the stress value or shear rate: ðÞ ¼ = 0

ð2:3Þ

That is: ¼ a n

ð2:4Þ

Rheology of Pressure-Sensitive Adhesives

7

where a is the apparent viscosity, and n denotes the ﬂow index [1]. For Newtonian systems the exponent n is one, which implies that the viscosity does not depend on the shear rate. As pointed out, the viscosity of PSAs does depend on the shear rate. This is possibly due to their macromolecular character. Pressure-sensitive adhesives are polymers containing long-chain entangled molecules with intra- and intermolecular mobility. At low strain rates, the viscous components of the polymer dissipate energy, and as a result resistance to debonding forces is low. At higher strain rates, the molecules have less time to disentangle, and to slide past one another; in this case viscous ﬂow is reduced, but the elastic modulus or stiﬀness of the polymer increases [2]. This behavior results in additional stored energy, and the debonding resistance intensiﬁes accordingly. Practically, the dependence of the adhesive performance characteristics on the stress rate may be observed by peeling oﬀ removable PSAs at diﬀerent peel rates: at higher rates paper tear may occur. The stress ratedependent stiﬀening is an increase in the elastic contribution to the rheology of the polymer. When the elastic components are predominant more of the bond rupture energy is stored, resulting in higher peel and tack properties. The end-use properties of PSAs result from the nonlinear viscoelastic behavior of the adhesive material, and the elastomeric polymer basis of PSAs imparts them such a viscoelastic behavior. It is evident that the same stiﬀening eﬀect is apparent when the polymer temperature decreases. In this case the polymer molecules are again restricted in their ability to ﬂow, and the modulus increases. Consequently the adhesive properties of PSAs are also temperature dependent. Thus one always has to take into account that the viscoelastic properties of PSAs are strain-rate and temperature dependent. Zosel [3] demonstrated that the separation or debonding energy of the adhesive joint is a function of the thermodynamical work of adhesion and of a temperature and rate-dependent function (depending on the viscoelastic properties). Accordingly, PSAs would absorb less or more energy depending on the rate (frequency !) of the applied stress. Practically, end-use situations with diﬀerent stress rates may be simulated experimentally by applying a strain to a thin sample of the material and measuring the output stress. If the material is an ideal solid, its response is completely in phase with the applied strain. A viscoelastic ﬂuid, such as a PSA, displays a mixture of solid-like and liquid-like responses. Therefore the output stress curve is deconvoluted into an in-phase part (related to energy storage) and an out-of-phase part (related to energy loss). The coeﬃcients of the in-phase and out-of-phase parts are called the energy storage modulus and the energy loss modulus [4]. According to the theory of Lodge [5] the rheological state of a viscous liquid subject to a sinusoidal deformation will be described by the following

8

Chapter 2

equation: 12 ¼ ð!Þ 0 ! cos ! t

ð2:5Þ

The response of the elastic solid may be described as follows: 12 ¼ Gð!Þ 0 sin ! t

ð2:6Þ

where G is the shear modulus. For the reversible and irreversible work of deformation one can write: 12 ¼ Gð!Þ 0 sin ! t þ rev 0 rev ! cos ! t

ð2:7Þ

12 ¼ irr 0 irr ! cos ! t

ð2:8Þ

where denotes the stress, ! the angular speed, the viscosity, and 0 the amplitude of the deformation. The storage modulus G0 increases with the frequency: G0 ¼ ðrev ! þ GÞ cos

ð2:9Þ

whereas the viscosity decreases with the frequency; is the loss angle. Pressure-sensitive adhesives must display irreversible work of deformation during bonding and reversible deformation work upon debonding. The ratio of both kinds of deformation work (i.e., of stored and dissipated energy) characterizes the behavior of PSAs. In general the energy state of the viscoelastic polymer may be described as follows: ðtÞ ½G0 ð!Þ cos ! t þ G00 ð!Þ sin ! t

ð2:10Þ

where G0 is the storage modulus and G00 the loss modulus, and loss tan ¼ loss modulus=storage modulus ¼ G00 =G0

ð2:11Þ

Tan is a damping term and is a measure of the ratio of energy dissipated as heat, to the maximum of energy stored in the material. One can suppose that the term ‘‘loss tan ,’’ as an index of the amount of stored or lost energy (i.e., of the contribution of the elastic and viscous part of the polymer) may also characterize the adhesive properties. It was shown that loop, peel, and quick stick show a good correlation with loss tan [4]. It was demonstrated that PSAs intended for similar application also exhibit similar

Rheology of Pressure-Sensitive Adhesives

9

rheological properties. The correlation between adhesive properties and the dynamic shear storage modulus appears quite good [6]: hence the concept of ‘‘window of performance’’ as a function of the storage modulus of the adhesive was developed [7]. These moduli, the storage and the loss moduli, can be displayed as a function of the temperature (Fig. 2.1). The storage modulus starts high at low temperatures where all motion within the polymer is frozen and the material behaves like a glass. At higher temperatures it drops oﬀ and exhibits a plateau region which represents the elastomeric response generally encountered at normal end-use temperatures; the storage modulus then decreases further when softening begins. The temperature region through which the polymer changes from a glassy (hard) state into a liquid (rubber-like) state, this second order transition point (with a continuous diﬀerential of the free enthalpy, but discontinuous, second order diﬀerential of the Gibb’s free energy) is called the glass transition temperature (Tg), and has a special signiﬁcance in the characterization of PSAs. Above the Tg the time-temperature superposition principle can be applied. Diﬀerences in viscoelastic parameters around the glass transition can be directly related to the side chain size and mobility of the polymer.

Figure 2.1 Dependence of the modulus on the temperature. 1) Storage modulus; 2) loss modulus; 3) tan .

10

Chapter 2

At Tg there occurs a change in the thermodynamic state which can be related to a mechanical energy loss function such as the loss modulus. The loss tan peak does not occur at the glass transition but in the transition zone between the glassy and rubbery regions. Ferry [8] pointed out that in this region of transition there is no change in the thermodynamic state. The loss tan peak is the midpoint of this transition zone where the ratio of loss modulus and storage modulus reaches a maximum. The energy loss maximum at this point has considerable inﬂuence on the tack of the system. The storage modulus deﬁnition can be simpliﬁed as a hardness parameter [9]. Optimum PSA performance can be quantiﬁed using the storage modulus; ideally the value of the storage modulus should vary between 20 and 80 kPa. Chu [7] correlated PSA performance and dynamic mechanical performance (DMA) properties, whereby PSA performance, especially for tapes, was related to the storage modulus G0 at room temperature and the loss tan peak temperature of the system. Chu also showed that the PSA application window for a high cohesive strength tape adhesive requires G0 values at room temperature between 50 kPa and 200 kPa with loss tan peak temperature limits between 10 and þ10 C. Optimum G0 values for permanent PSA labels were determined to be around 20 kPa at room temperature. Pressure-sensitive adhesives were deﬁned using viscoelastic application windows relating the storage modulus G at room temperature to the loss tan peak of the adhesive. In water-based adhesives the viscoelastic relationship is not as simple, that is, it was determined that for the most commonly used polymers in water-based dispersions these relationships may not apply. In hot-melt and solvent-based PSAs a close and predictable relationship exists between the loss tan peak, deﬁned as the dynamic Tg, and the Tg as measured by diﬀerential scanning calorimetry (DSC). For water-based adhesives the relationship varies depending upon the polymer type used. The loss tan peak temperature of an acrylic can diﬀer from the Tg (DSC) by as much as 30 C. The phenomenon is much less pronounced for styrenebutadiene rubber (SBR)-type polymer dispersions. This variation is also valid for an adhesive dispersion containing a tackiﬁer. The consequence of this is that the loss tan peak temperature cannot be used to predict and deﬁne PSAs performance in a viscoelastic application window. According to Bamborough [9] and applying the rule proposed by Chu, it may appear that an SBR would require considerably more compatible resin than an acrylic polymer; a soft acrylic (AC) PSA (like Acronal V 205) would require a higher amount of compatible resin than a hard one (like Acronal 80 D). Adhesive formulators know this not to be the case. Experienced adhesive formulators know that for Acronal V 205 an optimum concentration of tackiﬁer would be 30 parts per 100 parts polymer whereas for Acronal 80 D

Rheology of Pressure-Sensitive Adhesives

11

one needs 80 parts (see Chap. 6). The loss tan peak temperature for waterbased adhesive systems is not a reliable predictor of PSA performance characteristics. It is proposed that the loss modulus peak temperature of the adhesive should be 50 C below the operating temperature of the adhesive. Bamborough [9] proposed using the loss modulus peak temperature in the glass transition region instead of the loss tan peak temperature as a means of predicting PSA performance for water-based adhesives. Despite the above-illustrated discrepancies concerning DMA for adhesive characterization, the use of dynamic mechanical spectroscopy to measure modulus changes, and diﬀerential scanning calorimetry to measure shifts in the Tg of the adhesive are now common methods for rheological studies of adhesives [10]. In order to understand the practical beneﬁts of such investigations the rheology of PSAs needs to be studied. As discussed above PSAs are special products allowing instantaneous bonding due to their liquid-like ﬂow and solid-like debonding resistance due to their elasticity. Such viscoelastic behavior can be characterized rheologically by the main parameters of a liquid (viscosity) and a solid (modulus) taking into account their dependence on the time and temperature (see storage and loss modulus ratio). It is evident that the temperature domain allowing such viscoelastomer-like behavior has to be taken into account also. Therefore as a supplemental rheological parameter the value of the Tg should be used also. Extending the use of the WilliamLandel-Ferry time-temperature superposition principle allowed an easier rheological characterization of PSAs. It is known that the viscoelastic behavior of amorphous polymers is a function of the time-temperature dependence, the dependence of the ratio between recoverable energy during a given deformation and energy losses on the experimental conditions, characterized by the stress rate and temperature, that is of the validity of the time-temperature superposition principle. This principle states that the viscoelastic properties at diﬀerent temperatures can be superposed by a shift of the isotherm data along the logarithmic time-frequency scale. As discussed in [11], by ‘‘replacing’’ the time with the temperature it was possible to develop ‘‘full plastic’’ PSPs, i.e., polymer ﬁlms having built-in pressure sensitivity. 1.2

Influence of Viscoelastic Properties on the Adhesive Properties of PSAs

The essential performance characteristics when characterizing the nature of PSAs are tack, peel adhesion, and resistance to shear. The ﬁrst property represents the adhesive’s ability to adhere quickly (initial grab), the second

12

Chapter 2

measures its ability to resist removal by peeling, and the third characterizes the adhesive’s ability to resist ﬂow when shearing forces are exerted. Generally speaking the ﬁrst two characteristics are directly related to each other, but inversely related to the third one [12]. Tack and peel tests imply a high loading rate, whereas shear will be mostly measured in a static manner. The ﬁrst phase in the use of PSAs (bonding) generally occurs slowly, whereas the second step (debonding during converting or end-use) imposes higher stress rates. The balance of the adhesive properties, and of the adhesive/converting/end-use properties reﬂects at the same time the need for the balance of the viscoelastic characteristics. In this chapter the inﬂuence of the viscoelastic properties on the adhesive properties will be brieﬂy examined. Influence of Viscoelastic Properties on the Tack of PSAs According to Rivlin [13] the separation energy after a short contact time and low pressure is a measure of the tack. A short contact time and low pressure during application of PSAs imply a high wetting ability. For bonding to occur there is an a priori need for wetting of the substrate. As conﬁrmed by Sheriﬀ et al. [14] and by Counsell and Whitehouse [15] tack is a function of wetting. Good wetting supposes suﬃcient ﬂuidity of the adhesive, and ﬂuidity is characterized by viscosity. Tack Dependence on the Viscosity. According to Zosel [16] tack is measured in two steps, namely the contact step and the separation step. During the ﬁrst step, contact is made in the geometrical surface points, which increase to a larger area through wetting out, viscous ﬂow, and elastic deformation. Wetting out implies high ﬂuidity, as characterized by an adequate viscosity of the adhesive. Wetting out (i.e., covering the surface by the ﬂuid adhesive) is followed by bonding due to the viscoelastic deformation of PSAs. On the other hand, debonding assumes the deformation of the laminate, the creation of two new surfaces, and deformation of the new surfaces. Thus, it may be concluded that for bond-forming a high deformation with a medium elasticity is required, whereas for debonding a medium deformation with a high elasticity are required. The tack may also be characterized as separation energy [17,18]. During debonding high tack means that the adhesive absorbs a high amount of deformation energy, which dissipates on the break of the bond [19]: a high ability to store energy implies elasticity, a high energy at break means high cohesion. Thus, tack depends on elasticity and cohesion. Therefore, for high tack, a low bonding viscosity, a high debonding viscosity, and high elasticity are required. Factors inﬂuencing the viscosity and the elasticity of the polymer will also inﬂuence the tack. The polymer’s own characteristics

Rheology of Pressure-Sensitive Adhesives

13

and the environmental conditions (experimental parameters) inﬂuence its viscous and elastic behavior. Studying the performance of carboxylated styrene-butadiene rubber (CSBR) latexes, Midgley shows that tack depends on the Mooney viscosity (i.e., on the molecular weight, MW) [20]. Tack Dependence on the Modulus of Elasticity. Loss moduli correlate with PSA debonding tests; McElrath [21] studied the debonding frequency of PSA tests and their location on the loss modulus master curve. A good agreement between adhesive properties and loss modulus was demonstrated. Although absolute correlations have not been established, Class and Chu [22] suggested that the location and the shape of the modulus curve in the transition zone is important for PSA performance. In accordance with Dahlquist’s criterion for a minimum value of compressive creep compliance to achieve tack, Class and Chu said their data indicated a maximum modulus value. Dahlquist [23, 24] related tack to modulus, showing that the compression modulus should not be much higher than 105 Pa. Very high modulus adhesives do not possess suﬃcient conformability to exhibit pressure-sensitive tack. The optimum tack properties of PSAs are obtained when the room temperature modulus falls within the range of 5 105 to 1 105 Pa, and the Tg lies between 10 to þ10 C [25]. Hamed and Hsieh [26] showed that for a given total bonded area, test specimens containing noncontact regions of suﬃciently small scale can exhibit a higher peeling force than those in which the noncontact ones are large. For an elastomer there is a critical ﬂaw size below which adhesive strength will remain unchanged. Rubbery materials with higher elasticity have a smaller critical ﬂaw size. Thus the modulus inﬂuences the tack. Rubbery materials with a more elastic response exhibit tack properties that are more sensitive to interfacial ﬂaws, compared to those that respond more by viscous ﬂow. When failure occurs by viscous ﬂow, the tack is relatively insensitive to interfacial ﬂaws, but when the strain rate is suﬃciently high, so that the elastomer responds elastically, stresses may be concentrated at the edges of interfacial ﬂaws causing a reduction of the strength. Tack Dependence on Experimental Parameters. Viscosity and elastic moduli of PSAs are not intrinsic material characteristics (i.e., they depend on the experimental parameters used, such as the temperature and time, and the strain rate). Thus, a similar dependence of the tack on time, temperature, and the strain rate has to be taken into account. This dependence is illustrated by the quite diﬀerent values of the tack obtained using diﬀerent experimental techniques (rolling ball, quick stick, or loop tack) which are characterized by diﬀerent time and strain rates, and by the sensitivity of the adhesive properties to environmental conditions (see Chap. 10). Diﬀerent

14

Chapter 2

tack methods are adequate for diﬀerent PSPs [27]. With the probe tack test the load is regulated, as in the lamination of protective ﬁlms. Polyken tack is less strongly aﬀected by the resin softening point (it is applied under load!) during tackifying. Loop tack simulates the actual application conditions of labels, which are blown onto a surface by using air pressure. Rolling ball tack is more complex, it is friction related. Tack Dependence on Temperature. Most general-purpose adhesives are formulated to have tack at room temperature. If the adherent temperature is lower than room temperature, a higher degree of adhesive cold ﬂow is required to provide proper wet-out. In reality it is very diﬃcult to apply PSA labels at low temperature conditions. Deep freeze label (or tape) adhesives must be specially formulated, with a low viscosity at low temperatures [28]. This behavior is due to the limited ﬂow of the adhesive at lower temperatures, a phenomenon governed by the strong temperature dependence of the viscosity and of the elastic modulus. Deep freeze labels should display the same viscoelastic properties at 40 C as at þ20 C [29]. It is recommended that such adhesives have a lower storage modulus value than common labels (104 Pa). As shown by Hamed and Hsieh [26] tack is a function of test temperature and strain rate, and the experimental data may be shifted from a master curve. However, the tack behavior as a function of the temperature and strain rate is more complex than that of the cohesion. Tack Dependence on Strain Rate. It was shown earlier that tack depends on the bonding and debonding process, on debonding (separation) work, and thus tack also depends on the strain rate. This phenomenon may be observed by testing the tack using methods measuring the debonding resistance (force), like loop tack or Polyken tack. Tack varies as the separation speed of the Polyken test is changed [4]. The dependence of the tack value on the increasing speed was conﬁrmed by Hamed and Hsieh [26] too. They observed a nonlinear, discontinuous increase of the tack with the measurement speed. Tack values rise to a ﬁrst maximum and then, after a period of slight decrease, rise continuously with the measurement speed. Data obtained from Polyken tack measurements show a correlation between tack and loss tan peak values [4]. Tack dependence on the debonding rate was also conﬁrmed by McElrath [21]. He demonstrated that loss modulus values depend on the applied frequency, and diﬀerent tack test methods (loop, quick stick, and probe tack) exhibit maxima at diﬀerent frequencies. Influence of Viscoelastic Properties on the Peel of PSAs Peel and peel strength are measured by separating an adhesive applied to a substrate at some angle with respect to the substrate, usually at an angle

Rheology of Pressure-Sensitive Adhesives

15

of 90 or 180 [30]. Similar to tack, the measurement of the peel adhesion involves a bonding step before the debonding or peeling step. Tack measures the resistance to separation of the adhesive after a short contact time, or by light pressure. Peel is measured after a relatively long or very long contact time (at least 102 longer contact time than in the tack test) after application of a light or medium pressure. The time available for bond forming (wetting and penetration of the surface) during the ﬁrst contact step is longer for peel measurements than for tack. It follows that ﬂow properties of the adhesive during the bonding step are less critical than for the tack measurement. On the other hand, the debonding resistance will depend on the viscous nature/elasticity balance requiring its precise adjustment in order to achieve peelability (removability or repositionability) and on the strain rate, which inﬂuences the separation resistance in a more pronounced manner than during the measurement of the tack. Peel Dependence on Viscosity. Like tack, peel implies a bonding and debonding step, with the time for the latter lasting longer. Supposedly the inﬂuence of the viscosity on the bonding step during a peel measurement is less important than for the tack. On the other hand, the debonding resistance of the joint is increasingly proportional to the viscous ﬂow of the adhesive (i.e., a high peel needs a solid-like adhesive). In formulation practice the regulation of the peel by (bonding) viscosity is used for special, rubber-based, low peel products (e.g., tapes and protective ﬁlms), where mechano-chemical destruction of the base elastomer (mastication) is carried out [31, 32] in order to manufacture a soft adhesive. The value of the peel is also a criterion for the distinction between removable and permanent labels. The peel dependence on the viscosity (modulus) and its theoretical basis will be discussed in more detail in Section 1.4. Peel Dependence on the Elastic Modulus. Special PSAs intended for medical and surgical applications should display a low value of the modulus (and a high value of the creep compliance) in order to allow removability [33]: the higher the creep compliance, the greater the adhesive residue left on the substrate. Creep compliance values greater than 2.3 10 Pa1 are not preferred. The peel force is related to the storage modulus G0 of the adhesive. High tack, removable adhesives should have a low G0 , and the storage modulus should not vary much with the peel rate. On the other hand, the debonding takes place in a much higher frequency range than the bonding process. In order to maximize the peel force, the highest possible G0 value in the high frequency range is needed. Satas [6] showed, that solution polymers with a higher G0 slope at higher frequencies than emulsion polymers, exhibit higher peel adhesion, as is generally the case with acrylic solution polymers.

16

Chapter 2

Changing the chemical composition (e.g., sequence distribution) may lead to a change (increase) in modulus and eventually to a decrease of the peel force [34]. Regulating the diblock/triblock ratio in segregated copolymers may reduce/increase the plateau modulus, i.e., soften or harden the polymer, and inﬂuence the adhesive and converting properties [35]. Peel Dependence on Dwell Time and Strain Rate. As known from plastic ﬁlm processing and use, adhesion and self-adhesion depend on the contact time. Self-adhesive (i.e., adhesiveless) protective ﬁlms may build up a 100% increase of their adhesion as a function of the end-use conditions [36]. The build-up of the adhesion with the time is a general phenomenon due to the macromolecular nature and viscoelasticity of such polymer ﬁlms. In a similar manner contact forming or bonding of the adhesive assumes its viscoelastic deformation. Viscous, slow ﬂow is time dependent and thus bond forming will also be time dependent. According to [36] peel build-up as a function of the time is a result of contact build-up in time. Full contact build-up is referred as self-healing and in general the self-healing time (theal) depends on the viscoelastic properties of the material, the aspect ratio, the spacing of the asperities, and the surface properties. For certain application conditions the time for self-healing (i.e., no external loading) is given by: theal ¼ Að, mÞLCo o2 ½Co =C1 1=m =W

ð2:12Þ

where A(, m) is a dimensionless constant depending on m, the creep exponent, and a dimensionless parameter: pﬃﬃﬃﬃﬃﬃﬃﬃ pﬃﬃﬃﬃﬃﬃﬃﬃ pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ A ¼ ð2 2=Þ ð L=hÞ ð Co W Þ

ð2:13Þ

where W/Co o2 is a characteristic length of the local bonding process and [C1/Co]1/m is the characteristic relaxation time of the material. Eq. (2.12) implies that the healing is very sensitive to the cohesive stress. Because of this materials that have identical creep behavior and work of adhesion can have very diﬀerent healing times. The healing also depends on the height of the asperities through the parameter . This parameter has the following interpretation: if the viscoelastic polymer is replaced with an elastomer with modulus 1/Co then the two surfaces will jump in contact with each other with no externally applied load and the contact area will continue to increase until the surfaces are in full contact if was larger than some critical value. This critical value is typically of order 1. The formulation of this work implicitly assumed that is below the critical value. For typical values of h, L, and W, the elastic modulus which would cause the surfaces to

Rheology of Pressure-Sensitive Adhesives

17

jump in contact and a self-heal is of order 0.1 MPa. Therefore this work is more relevant for viscoelastic adhesives or uncrosslinked rubbers of higher modulus. The time dependence of the bonding step is illustrated by the inﬂuence of the dwell time on the peel values. As discussed in Chap. 10, application pressure and temperature inﬂuence the peel and its build-up too. Therefore, as proposed in [37] for special PSPs static and dynamic dwell time should be tested. Like the pressure used during application, the contact time (i.e., the interval between bonding and debonding) takes into account the relatively low mobility (ﬂow rate) of PSAs. A higher pressure or a longer dwell time should help adhesive ﬂow and increase the peel resistance. It is known from PSA characterization, that for the FTM-4 high speed release test the samples are placed between two metal plates under a pressure of 6.87 kPa to ensure good contact. In a similar manner for PSPs with very low tack and instantaneous peel (e.g., protective ﬁlms) bonding depends on the applied pressure and temperature also. A special class of such products (warm laminating ﬁlms) is used at increased laminating temperature and pressure [38]. Foam-like transfer tapes are also used under pressure to increase initial bond strength [39]. As discussed in [40] time-dependent peel control is one of the main requirements for PSA. Generally, until equilibrium is reached, the peel resistance increases with increasing contact pressure and time [26,41] (Table 2.1; Fig. 2.2). As can be seen from Fig. 2.2, the peel force increases with the dwell time of the adhesive, that is, the viscous and elastic deformation of the adhesive need a period of time (depending on the viscosity and experimental conditions). As will be discussed later, the time dependence of the bonding imposes the use of normalized dwell times for peel measurement purposes. Generally the peel/dwell time dependence is not linear [42], but partially crosslinked silicone rubber exhibits peel values which increase linearly with

Table 2.1

Peel Adhesion as a Function of Increasing Dwell Times Peel Adhesion (N/25 mm)

Dwell Time (week) 0 1 2 3 4

Sample 1

Sample 2

Sample 3

0.32 0.44 0.47 0.55 0.60

0.12 0.24 0.30 0.35 0.40

0.09 0.21 0.30 0.35 0.47

18

Chapter 2

Figure 2.2 Dependence of the peel values on the dwell time. Peel from polyethylene as a function of the coating weight for diﬀerent dwell time values. Dwell time of 1) 20 min; 2) 0 min.

the dwell time [43]. The adhesion build-up over time for the main types of PSPs (e.g., permanent, repositionable, and removable adhesives) was discussed in detail in [44]. It has been demonstrated that peel build-up depends on the adherend surface quality [37]. The build-up of the peel force in time has a special importance in the design of removable adhesives. The dependence of the separation energy on the contact time was also demonstrated by Zosel [3]. The ‘‘memory eﬀect’’ in the adhesion of rubber to rigid substrates is well known [45]. After readhering the adhesive, the peel force is lower. After initial peeling of the adhesive, it regains its original peeling resistance only after a period of time (or much faster with the help of some solvent). The memory as a function of the dwell time is associated with some rearrangements of the molecular structure of the rubber at the interface. In a special case (e.g., diaper closure tapes) the removable adhesive allows reliable closure and refastening. In this case the time dependence of the ﬂow/deformation of the adhesive and its inﬂuence on the peel may be observed during the debonding process. Such tapes have to exhibit a maximum peel force at a peel rate between 10 and 400 cm/min

Rheology of Pressure-Sensitive Adhesives

19

and a log peel rate between 1.0 and 2.6 cm/min [46]. Another inﬂuence of the dwell time may be observed, when examining the inﬂuence of the storage and aging on the peel adhesion value. In general there is a build-up of the peel resistance with time. It should be emphasized that stress transfer during debonding occurs by means of the solid components of the laminate. Therefore strain rate depends upon the carrier and substrate. The denaturation of the peel values by carrier deformation (i.e., strain rate) is discussed in detail in [47]. Influence of the Peeling Rate. At low peel rates the viscous ﬂow, and the deformation of the adhesive layer are dominant for the peel resistance. Therefore the peel resistance increases with increasing peeling rate. At high peeling rates the elastic character of the adhesive dominates, thus in this region the peel resistance no longer depends on the peeling rate. Of practical importance is the very pronounced dependence of the peel force on the peeling rate (Fig. 2.3); the peel force increases with the peel rate [48]. This behavior requires the use of normalized peeling rates for the measurement of the peel adhesion force.

Figure 2.3 Dependence of the peel force on the peeling rate. 1 through 8 are diﬀerent tackiﬁed water-based acrylic formulations; the peel adhesion from glass was measured.

20

Chapter 2

The dependence of the peel on the peeling rate may also be observed for very low peel forces (peeling from release liner) [49]. McElrath [21] showed that (as theoretically supposed) the loss modulus depends on the frequency, as does peel adhesion, and displays a maximum for a given frequency value. Removable adhesives having a plateau region in the storage modulus/frequency plot, exhibit a peel force independent of the peel rate [6]. Kendall et al. [25] studied the dependence of the peel on the Tg; they also stated that the peel maxima change with the testing rate. Higher test rates or lower ambient temperatures produce maxima at lower resin levels, lower resin softening points, or lower elastomer Tg. The peel of diﬀerent water-based PSAs displays large diﬀerences after storage at 100 C. As shown in [25] adhesive break (in the adhesive layer) appears above a certain peeling rate (2.5 mm/min), or below a certain temperature (25 C) only. It may be possible to remove a label from a paper substrate if it is peeled oﬀ very slowly, but the same label will certainly tear if it is peeled oﬀ quickly. Since the peel force is related to the storage modulus of the adhesive, high-tack removable adhesives should possess a low storage modulus which does not vary much with frequency (rate of peel). At any given temperature peel adhesion is observed to increase as the peel rate is increased [2]. At low strain rates the peel forces are much lower. Under these conditions, the viscous components of the polymer dissipate signiﬁcant amounts of energy and, as a result, resistance to peel forces is low. At higher strain rates the molecules have less time to disentangle and to slide past one another. This behavior results in more stored energy, and peel forces intensify accordingly. One can conclude that at least theoretically, each adhesive may be considered as a low peel adhesion adhesive (see Chap. 6) provided a very low peel rate is applied. Temperature Dependence of the Peel. As is known, a maximum peel strength implies a certain modulus value and viscosity. Both modulus and viscosity depend on the temperature. With the increase of the temperature the viscosity of PSAs decreases. Therefore the increase of the temperature improves the tack and instantaneous peel, and exerts a negative inﬂuence on the cohesion. Such an inﬂuence is illustrated in the end-use of several PSPs. It was shown for ethylene acrylic acid copolymers that peel adhesion depends on the temperature [30]. The peel of an untackiﬁed PSA at 0 C is about 210% lower than the peel value at 23 C. For tackiﬁed formulations peel reduction at 0 C may attain 300% [50]. The importance of laminating temperature of acrylic PSA coated protective ﬁlms on plastic plates was demonstrated in [51]. The dependence of the peel value and of the break nature on the peel rate is a common phenomenon observed in the delamination of PSPs during their application also. Such dependence

Rheology of Pressure-Sensitive Adhesives

21

causes the so-called inversion of the adhesive break when tapes are unwound at too low a temperature (lower than 10 C) or with too high running speed. It is well known in packaging applications that the winding resistance for tapes depends on the temperature and the winding rate (i.e., the peel is temperature dependent). As shown in [52] the unwinding resistance (Rw) is a function of temperature (Tw) and unwinding speed (vw). Rw ¼ f ðTw , vw Þ

ð2:14Þ

Generally the unwinding force depends on a number of parameters such as the adhesion force, the moduli of elasticity of the adhesive and carrier ﬁlm, the thicknesses of the adhesive and ﬁlm, and the width of the tape. The modulus of the elasticity of the adhesive depends on the time and temperature, i.e., on the unwinding speed and temperature. Influence of Viscoelastic Properties on the Shear of PSAs Cohesive strength is measured as shear or shear strength, which is the resistance of adhesive joints to shear stress, and is measured as a force per unit area at failure. The shear force is applied in a plane parallel to the adhesive joint. Diﬀerent authors have formulated a deﬁnition of the cohesive strength. If deformation by shearing is considered like a ﬂow, the ﬂow limit (FL) is given by the following correlation [53]: FL ¼ ðE We b2 Þ=4h3

ð2:15Þ

where We denotes the maximum elastic deformation of the sample, b the width of the adhesive surface in the stress direction, and h the thickness of the adhesive layer. It can be seen from the above relation that the ﬂow limit (i.e., the cohesion) is a function of the elastic modulus of the adhesive. Considering that the strength of a PSA joint depends on the viscosity , the adhesive layer thickness h, and time t, the interdependence of these factors may be formulated as follows [54]: F ¼ =ðh2 tÞ

ð2:16Þ

The inﬂuence of the viscoelastic properties on the adhesive characteristics of PSAs depends on the nature of the stresses applied during tack, peel, or cohesion measurements. Ko¨hler [55] demonstrated that tensile or shear stresses produce diﬀerent levels of deformation in PSAs. When applying a

22

Chapter 2

tensile force on a circular, laminated PSA layer, with a viscosity of 103 Pa, according to the relation: P¼

3 R4 4 t h2

ð2:17Þ

where P is the applied force, R is the sample radius, t is the time, and h is the thickness of the adhesive layer, one needs a force of 100 kg/cm2 during 1 sec in order to achieve a certain deformation; but applying a shear stress, according to the Newtonian relation: ¼ 0

ð2:18Þ

where 0 ¼ 1 sec1, only a 2.5 kg/cm2 force is needed. Thus it may be concluded that shear-stressed pressure-sensitive joints need lower force levels in order to undergo a deformation (i.e., pressure-sensitive joints are weaker than classical ones). As illustrated by the relations (2.15)–(2.18) the shear resistance depends on the adhesive’s viscosity and elasticity. If the adhesive is considered a Newtonian ﬂuid, the shear resistance SH is given as a function of the viscosity , the adhesive thickness h, and the tensile rate : SH ¼ ð Þ=h

ð2:19Þ

If the solid obeys Hooke’s law the shear resistance depends on the shear modulus G and the tensile rate : SH ¼ ð GÞ=h

ð2:20Þ

Shear Dependence on Viscosity. Toyama and Ito [56] used the data of creep testing (shear resistance) to calculate viscosity. Increasing the molecular weight of CSBRs increases the Mooney viscosity [20]. At the same time an increase in the molecular weight will improve the cohesive strength (shear) of carboxylated SBR. For styrene block copolymers the role of viscosity in shear measurement is observed at higher temperatures where the molecular association does not work. At room temperature shear resistance is due to the elasticity of the polymer, determined by segregation and its parameters (e.g., sequence length and distribution), at higher temperatures it is given by viscous ﬂow inﬂuenced by copolymer composition and global molecular weight.

Rheology of Pressure-Sensitive Adhesives

23

Shear Dependence on the Modulus. According to Woo [57] the shift factors used to plot shear measurements at diﬀerent temperatures on a master curve were almost identical to the shift factors used for modulus curves. Thus the time to fail in shear can be predicted from the viscoelastic function. Correlations between the viscoelastic and PSAs performance properties can be made by looking at those temperature regions which correlate (via the time-temperature superposition principle) to the time scales of the adhesive performance parameters. The magnitude of G0 in the upper temperature range is indicative of internal strength or cohesion. Initial peel, which is largely dependent upon the wet-out characteristics of the polymer, is governed by G0 and tan in this same temperature range. Similarly, loop tack and quick stick properties are wet-out dependent, but are associated with faster relaxation times and thus correlate with room temperature viscoelastic properties [58]. The decrease of the modulus at high temperature reduces the shear resistance. Milled rubber possesses a lower elastic modulus and shorter modulus plateau than rubber from dried latex [25]. For instance the G0 value at 20 C for milled smoked sheet is about 105 Pa. The thermo-mechanical degradation of the rubber through milling does not change the tan peak temperature (Tg). It does reduce the modulus at high temperatures. This modulus reduction relates to the lower shear performance of solvent-based systems. The possibility to characterize application ﬁeld-related properties of PSAs and to correlate them with the rheology of the adhesive is illustrated by formulating adhesives for medical tapes [59]. The adhesive used for medical tapes is characterized by a dynamic shear modulus of about 1–2 104 Pa, a dynamic loss modulus of 0.6–0.9 104 Pa, and a modulus ratio or tan of about 0.4–0.6 as determined at an oscillation frequency sweep of 1.0 rad/sec at 25% strain and 36 C. Adhesives with moduli higher than the acceptable range display poor adhesive strength, while adhesives with moduli below the acceptable range exhibit poor cohesive strength and transfer large amounts of adhesive to the skin upon removal [59]. Shear Dependence on Time and Strain Rate. The shear resistance of the adhesive depends on its internal cohesion. The cohesion is a function of the inherent viscosity or modulus and thus it depends on the parameters inﬂuencing the viscosity or modulus. Both viscosity and modulus are not intrinsic material characteristics, they depend on the temperature and time (i.e., on the nature and time history of the applied forces). Therefore, the shear resistance will depend on the temperature and the time: in labeling practice this behavior is illustrated by the low temperature resistance of the adhesive joints and by the diﬀerences between statical and dynamical shear. As is known, the viscosity depends on the loss modulus (viscous ﬂow is the

24

Chapter 2

phenomenon that ‘‘loses’’ the energy). It also depends on the shear rate (frequency): 0 ¼ G00 =!

ð2:21Þ

At low frequencies 0 and the steady-state viscosity are the same. A static shear test is based on low frequency deformation; therefore creep may be regulated by viscosity. The cohesive strength (shear) increases with increasing test rate or with decreasing temperature [26]. When non-Newtonian liquids are subjected to variable shear rates, the plot of the shear stress/shear rates no longer shows a linear relationship [60]. In a diagram with double logarithmic scaling this curve becomes a straight line. The mathematical equivalent of these two curves is given by the following equations: ¼ K D00 ln ¼ ln K þ n ln D

ð2:22Þ ð2:23Þ

where D denotes the shear rate, K is a viscosity related coeﬃcient, and n is an exponent of the ‘‘power law equation’’ deﬁning the shear rate dependence on the viscosity. This exponent is known to vary for polymers between 0.3–1.0. Practically, it was demonstrated that an increase of the test rate produces an increase of the cohesion and of the peel up to a critical value [26]. As shown for cohesive strength measurements as a function of temperature and shear rate [26], the principle of strain rate-temperature equivalence can be applied. The strain rate inﬂuences the shear resistance by bonding too. Bonding is a diﬀusion and time dependent process. This is illustrated by the pressure dependence of the lap shear adhesion for adhesives; the higher the pressure the greater the bond strength [61]. Shear Dependence on Temperature. As discussed earlier, the cohesive strength increases with increasing test rate or with decreasing temperature. Shear data obtained as a function of the test temperature and shearing rate can be shifted horizontally to form a single master curve, illustrating the principle of time-temperature equivalence. Hamed and Hsieh [26] found a good agreement between experimental and calculated values, using the universal Williams-Landel-Ferry relationship for an amorphous rubber. The ﬁnding that data can be shifted to form a master curve is evidence of the importance of chain segmental mobility in controlling the shear strength. If the chain segmental mobility (i.e., the ability to relax) is high

Rheology of Pressure-Sensitive Adhesives

25

(high temperature or low test rate) then the fracture stress is small. On the other hand, a large stress is required to rapidly tear apart a PSA sample, since the chains have little time to rearrange their microstructure in order to accommodate the applied stress. This statement appears valid for peel adhesion too. One should recall that the removability of a PSA label strongly depends on the peeling rate.

1.3

Influence of Viscoelastic Properties on the Converting Properties of PSAs and PSA Laminates

The converting properties of PSPs were discussed in detail in [62]. PSPs are manufactured generally as web-like products. Most of them are laminates. They are applied as web-like laminates or ﬁnite products. Before application they have to be ﬁnished. In this case ﬁnishing means the transformation of the continuous web-like product that has the optimal geometry for manufacture into a product that has the optimal characteristics for use. Convertability is the sum of the convertability of the adhesive and that of the laminate. The converting properties of the adhesive and of the laminate depend on the rheology of the adhesive. It has to be pointed out that, except for hot-melt PSAs, the rheology of the uncoated adhesive (with an inherent ﬂuidity required for processing purposes) is diﬀerent from that of the converted material. Converting Properties of the Adhesive The liquid adhesive must be coated onto a release liner or face material. Good coatability implies adequate machinability or processing properties on the coater (metering roll, drying tunnel). During manufacturing, transport, and coating, the adhesive ﬂuid is subjected to shear forces, going through a more or less pronounced change of the viscosity. The coated shear-thinned adhesive has to wet the web, and the wet-out depends on its viscosity. Except for hot-melt PSAs, the coated liquid adhesive layer has to allow the elimination of the carrier liquid (drying) in order to form a solid adhesive layer. Evaporation of the carrier liquid depends on its diﬀusion through the adhesive layer (i.e., on the viscosity of the adhesive). It may be concluded that the viscosity of the adhesive, i.e., the time (shear rate)/temperature dependence of the viscosity, inﬂuences the coatability and convertability of PSAs. It was mentioned earlier that, except for hot-melt PSAs, the other PSAs are dispersed or diluted systems, their rheology being diﬀerent from that of the converted material. The coating-related rheology will be covered in Section 3.

26

Chapter 2

Convertability of Laminate Convertability of the PSA laminate means its ability to be processed into ﬁnished products (labels, decals, etc.) by operations which inﬂuence its dimensions (e.g., slitting, cutting or die cutting, embossing, folding, perforating etc.) or its surface quality (e.g., printing, laquering, etc.). The ﬂow properties of the adhesive inﬂuence the migration or penetration, oozing, and cold ﬂow, thereby limiting the convertability of the laminate. Thus the viscosity of the adhesive and its time/temperature dependence (i.e., a nonlinear character) determine the converting properties of the PSA laminate. It should be mentioned that the converting properties of the laminate really depend on the interaction of the PSA-laminate components. These properties will be discussed in more detail in Section 3.2. 1.4

Influence of Viscoelastic Properties on End-Use Properties of PSAs

The most important end-use properties of PSAs are the propensity to labeling (dispensing) and bonding behavior. For some special PSPs the debonding characteristics have to be taken into account too. The application technology of other PSPs (e.g., tapes and protective ﬁlms) is less sophisticated therefore this chapter describes the label application only. The end-use properties of various PSPs have been discussed in detail in [62]. Labeling is inﬂuenced by the adhesive properties (peel and tack) and by the dispensing properties of the label. Removability or peeling oﬀ is inﬂuenced by the adhesive properties. Thus the parameters inﬂuencing the adhesive properties will also characterize the end-use properties. Label Application Technology Label application technology refers to labeling. Labels are either applied by hand or with mechanical processes. Generally the label dimensions are decisive for the choice of the application technology. Large labels will only be applied by hand. On the other hand, according to the application technology, reel and sheet laminates are manufactured. For PSA reels and sheets, a quite diﬀerent adhesive/cohesive balance is required (i.e., quite diﬀerent tack/peel/ shear values or ﬂow properties). High speed labeling guns need high tack PSAs (so-called touch blow labels use no mechanical contact in label application), whereas for high speed cutting, high modulus and high cohesion PSAs are required. Dispensing and labeling were discussed in detail in [63]. In the label industry a basic diﬀerence exists between roll and sheet supplied laminates (face material/adhesive/release liner) [40]. Pressure-sensitive adhesives for sheet applications must resist ﬂying knife

Rheology of Pressure-Sensitive Adhesives

27

and guillotine cutting. A poor selection of the adhesive results in part in oozing of the adhesive (gum deposits) on the cut surfaces and smearing of the edges of the label stock with subsequent poor feed to the printing press. The requirements are generally less critical for roll applications. Converting in the paper label industry involves processes such as die cutting of roll stock, and guillotining of sheet stock. Formulating approaches that improve the high frequency modulus of the adhesive will enhance converting [2] since die cutting and guillotining are such high frequency processes. The less viscous and the more rigid the response of the polymer during the processes, the cleaner the process tends to be. If viscous ﬂow within the polymer is signiﬁcant during the converting operation, poor die cutting or poor guillotining (knife fouling) can result. Removability of PSAs Conventional PSAs can be classiﬁed as either nonpermanent (2.7–9.0 N/ 25 mm for 180 peel adhesion) or permanent (above 9 N/25 mm for 180 peel adhesion). Repositionable PSPs are a special class of removable pressuresensitive products (labels and tapes) that stick to various surfaces but remove cleanly and can be reapplied. The ﬁnal adhesion builds up over a few hours. Adhesives included in the nonpermanent category are used in the manufacture of removable tapes and labels, protective laminates, and other less durable products [64]. For nonpermanent, so-called removable adhesives, the ﬂow properties and cohesion of the adhesive as well as the anchorage of the adhesive to the face stock are critical. In an ideal case, if the bond to the substrate is nonpermanent, then a clean release from that substrate is encountered and the adhesive remains on the face material. Another requirement for good removable adhesives is the low peel level with a permanent character (i.e., no build-up of the peel in time is acceptable). A clean release from the substrate and no build-up of the peel with time are the minimal requirements for removable PSAs. On the basis of the adhesive characteristics it is possible to formulate the rheological criteria necessary for removable and permanent adhesives. Criteria for Removability. For certain applications PSPs are required that display a low peel force and give a clean, deposit-free separation from the substrate. The criteria for removability were discussed in detail in [44]. Generally a special balance between tack, peel adhesion, cohesion, and anchorage is required in order to ensure removability. Removability requires a breakable bond at the adhesive/substrate interface. Bond breaking is an energetic phenomenon. For removability the whole debonding energy should

28

Chapter 2

be absorbed by the adhesive itself in such a manner that no failure occurs in the adhesive mass. In this case the energy is used only for the viscous ﬂow and elastic motion of the macromolecules. The major danger is that if the viscous ﬂow is too pronounced, bonds will fail in the adhesive mass. (It should be mentioned that for special removable protective ﬁlms the regulation absorbtion of the debonding energy is achieved using a deformable carrier material [65].) Some low tack, crosslinkable dispersions are good removable PSAs. Their good elasticity yields low cold ﬂow and low build-up of the peel during storage [66]. On the other hand, a certain degree of cold ﬂow is needed in order to obtain removability. Adding 2–20% plasticizer during the polymerization of acrylics leads to a removable adhesive but the plasticizer may migrate out after storage [67]. Furthermore cohesive strength is necessary to avoid stringing. It must be stated that the adhesion depends on both the rheological features and chemical aﬃnity. It was shown [68] for certain PSAs that the apparent separation energy was approximately the same when pulled away from either a glass or a Teﬂon surface. The adhesion of these adhesives must be attributed largely to their rheological features rather than to selective wettability. How to Achieve Removability? A special adhesion/cohesion and/or plasticity/elasticity balance is required in order to achieve removability. Flow properties allow rearrangement, relaxation, and transformation of the debonding energy into viscous ﬂow, therefore softening of the adhesive is a prerequisite for removability. It is well known from the ﬁeld of caseinebased adhesives [69] that polyethylene glycols added to caseine and dextrine glues control the humidity absorption, and thus the tear of gummed papers at low environmental humidity, and avoid blocking at high humidity, thereby acting as plasticizers (at a loading rate of 1–2%). Flow properties are correlated to creep which, in turn, is a function of molecular weight and plasticizer loading. Higher molecular weights and low plasticizer levels reduce the tendency to creep. For removable PSAs the molecular weight should be limited. Permanent adhesives need no ﬂuidity during debonding, as they must display debonding resistance when subject to high stress rates and large forces. They need to display a modulus and viscosity increase as a function of the stress rate. On the other hand, removable adhesives need easy debonding and viscous ﬂow. The applied stress must be able to cause movement within the bulk of the material. Theoretically the applied stress for debonding can be minimized by using stress-resistant polymers, ﬁllers, and primers; stress-resistant polymers are those which develop a controlled crosslink density or are soft internally. Primers are somewhat ﬂexible and generally promote good anchorage to the face stock. Thus, they can absorb expansion or impact stresses without

Rheology of Pressure-Sensitive Adhesives

29

adhesive failure. Generally, as discussed in [44], removability can be achieved by regulating the chemical composition of the adhesive, and the structure and geometry of the adhesive, and the carrier properties and by means of the manufacturing technology. The main methods used to change the chemical composition (and macromolecular characteristics) of the adhesive components, e.g., tackifying, softening, and hardening (crosslinking, ﬁlling, etc.) inﬂuence the removability too. Formulation windows with boxes shifted to the left and up (i.e., for lower frequencies) are harder [70]. Influence of the Viscosity/Modulus on Removability. As shown earlier, hardening of the adhesive increases its modulus (i.e., decreases its ability to creep and its ability to wet the surface), and thus decreases the tack (i.e., peel adhesion after very short dwell times) as well as the peel resistance. It can therefore be assumed that hard adhesives display low peel adhesion (i.e., are removable). According to [6] there are two basic types of removable adhesives, namely PSAs which wet the surface poorly and PSAs which adhere easily but are easily removed. The ﬁrst type of PSAs is highly elastic and has low tack. Its removability depends on the adhesive not establishing a good contact with the surface. Sometimes the poor contact is built-in mechanically. These PSAs might have inclusions that prevent the adhesive from achieving full contact with the surface, as the adhesive contains physical particles that do not deform completely and therefore limit the contact area between adhesive and substrate. These PSAs possess a high modulus. Thus, hardening of the polymer (modulus increase) is a possibility to obtain removable adhesives. As discussed in [71] the build-up of ordered multiphase structures can modify the chain mobility. Such structures include ﬁllers, crosslinks, crystallites, and molecular associations. Fillers modify the Tg also. Really such hardening of the adhesive using a composite structure works like other methods for the control of the contact surface (e.g., pattern-like contact points, pattern-like nonadhesive points, decrease of the coating weight etc.). It should be mentioned that in ﬁlled hard adhesive formulations the absorbtion of the debonding energy is improved too [72]. As illustrated in [73] the degree of ﬁlling (e.g., ﬁlling, ﬁlling and foaming) strongly inﬂuences the adhesive characteristics and adhesion build-up. The rheology of such reinforced systems and its inﬂuence on the adhesive and end-use properties were discussed in detail in [74]. The most important ﬁllers used in PSAs and their functions are listed in [40]. According to [75] current removable adhesive formulations include high molecular weight polymers that reduce ﬂow on the surface and prevent adhesion build-up. The second type of removable PSAs displays high tack and is characterized by a low modulus. In order to soften the adhesive such

30

Chapter 2

formulations use plasticizers or other low molecular weight viscous components (paraﬃnic oil, polyisobutylene etc.). The presence of 8% plasticizer may improve the elongation by 3000% [66]. The softening eﬀect is illustrated by the lowering of the minimum ﬁlm-forming temperature (MFT) (e.g., 6% plasticizer decreases the MFT from 23 C to 0 C), or the decrease of the tensile yield strength (e.g., 6% plasticizer decreases the tensile yield strength from 13.0 to 1.0 N/mm2). The elongation of PVAc PSAs (normally 6–10% at break) was signiﬁcantly increased by adding plasticizers to up to 100–1000% [76]. Diﬀerences in plasticity may be due to the molecular weight of the plasticizer or diﬀerent interactions between chain segments and plasticizers. Long-chain plasticizers exert a strong plasticizing eﬀect. Generally plasticizers decrease the modulus value and inﬂuence the frequency dependence of the loss tan [4]. For nonplasticized adhesives loss tan is low at low frequencies. For plasticized adhesives loss tan shows a maximum at lower frequencies, and peel and quick stick also exhibit maxima at lower frequencies. Plasticizers may be incorporated in peelable PSAs to soften the adhesive and thus improve peelability. However, some plasticizers can exert a tackifying eﬀect on adhesive polymers and this may limit the amount used. Typical plasticizers include phthalate esters and polyalkylene ether derivatives or phenols, the principal requirement being compatibility with the main adhesive polymer and the tackiﬁer so as to avoid or minimize migration of the plasticizer. It is possible to add quite large quantities of plasticizer, even up to 50% by weight on the solid adhesive; however, usually 10–20% by weight are added [6]. Other plasticizers which can be employed, include the well-known extruder oils (aromatic, paraﬃnic, or naphthenic) as well as a wide variety of liquid polymers. Satas [6] showed that the addition of a plasticizing oil which is compatible with the mid-block of a styrene-isoprene-styrene (SIS) block copolymer results in an adhesive with lower peel force. Such plasticizers are used for HMPSAs. In some cases softening and hardening of the adhesive are used together. Mechano-chemical destruction of the macromolecules of natural rubber (mastication) followed by postcrosslinking leads to PSAs used for removable protective ﬁlms. Such crosslinked natural rubber formulations are softer than certain common noncrosslinked acrylic recipes [77]. Internal crosslinking by special comonomers and the simultaneous use of plasticizers have also been suggested [78]. Influence of Viscoelasticity on Applied Labels After some time labels show a higher peel resistance than freshly applied ones (i.e., the peel resistance increases with the dwell time). This build-up of

Rheology of Pressure-Sensitive Adhesives

31

the peel resistance is due to the ﬂow and contact build-up of the adhesive. Therefore, one should pay attention when evaluating the peel force, that is, peel should always be measured after a well-deﬁned (normalized) dwell time. 1.5

Factors Influencing Viscoelastic Properties of PSAs

The material characteristics, chemical composition/structure, and environmental and experimental conditions inﬂuence the viscoelastic properties of PSAs. The inﬂuence of the material characteristics on the viscoelastic properties of the PSAs will be discussed ﬁrst. Numerous empirical correlations between the molecular structure, molecular weight, molecular weight distribution (MWD), branching, and rheology are generally valid for chemically quite diﬀerent polymers [79]. Influence of Material Characteristics on Viscoelastic Properties of PSAs The molecular character of the base polymer, its chemical composition, and structure inﬂuence its viscoelastic properties. The inﬂuence of the molecular weight on the viscoelastic properties of the PSAs will be examined ﬁrst. Viscosity and the elastic modulus are the most important parameters characterizing the viscoelastic behavior of PSAs. Both parameters depend on the molecular weight of the base polymer. Dependence of the Viscosity of PSAs on the Molecular Weight. The viscosity of macromolecular compounds is a function of the molecular weight. Their zero viscosity 0 depends on the weight average molecular weight according to a correlation of the form [80,81]: 0 ¼ f ðMW Þ

ð2:24Þ

where ¼ 3.4. Zosel [16] demonstrated the strong inﬂuence of the molecular weight of acrylic PSAs on their viscosity and on their tack. The increase of the molecular weight in the range of 103–107 produces an increase of the viscosity from 102 to 1010 Pas. Zosel also showed the existence of a maximum in the tack/molecular weight graph for polyisobutylene. The pronounced increase of the viscosity with the molecular weight limits the usable molecular weight range for hot-melt PSAs, limiting (unfortunately) their cohesion as well. (According to [82] for such adhesives 80 Pas is the preferred maximum viscosity value for high speed coating.) It is well known to skilled formulators that block copolymers have lower viscosities than natural rubbers. (Their viscosity depends on sequence distribution and

32

Chapter 2

branching too). The solution viscosity of the high polymers also increases with their molecular weight. Thus a viscosity-imposed balance between processing and end-use properties, that is, a solid content/molecular weight balance for solvent-based PSAs is always a limiting factor, keeping in mind that the solid content of common solvent-based PSAs does not exceed 30–40%. On the other hand the additives in an adhesive formulation, their nature, and their concentration also inﬂuence the viscosity of the blend. Practical examples are given by plasticizers and tackiﬁers. Micromolecular tackiﬁers (plasticizers) may be considered as diluting agents. Thus the viscosity of the polymer ‘‘solution’’ obeys an exponential law, depending on the polymer concentration C, namely Z ¼

ðCÞ

ð2:25Þ

At ﬁrst rubber resin formulations were used for PSAs. To achieve viscoelastic behavior, the elastic component (rubber) had to be mixed (formulated) with a viscous component. Either macromolecular or micromolecular compounds can be used as viscous components. The best known are resins and plasticizers; both are suggested as tackiﬁers. Generally, low molecular weight resins and relatively high molecular weight solvents are used as tackiﬁers. Common HMPSA formulations incorporate a high level of plasticizer, usually naphthenic oil or liquid resin. The level of the tackiﬁer resin inﬂuences the resulting viscosity. As an example, Zosel [16] demonstrated that the ‘‘zero viscosity’’ of tackiﬁed PSAs decreases up to 80% resin loading level, resulting in better ﬂow properties. The diluting eﬀect of the tackiﬁer resin depends on its own viscosity too. Generally, soft (liquid) resins impart aggressive grab and quick stick, while hard resins help retain good cohesive strength and creep resistance. Thus resins with a softening point below about 50 C impart tack, but poor cohesive strength, while those above 70 C lead to poor tack properties [83]. It was shown earlier that the peel adhesion depends on the viscosity. Peel is a function of molecular contacts which depend on the diﬀusion. The rate of the adhesive diﬀusion into the face stock is inversely proportional to (MW)2. Thus the number of contact points decreases with (MW)2, and thus tack and peel decrease with the molecular weight [26]. On the other hand the chain elasticity increases with the molecular weight as well as the so-called ‘‘critical ﬂaw size,’’ resulting in a lower peel at a higher molecular weight. Dependence of the Elastic Modulus on the Molecular Weight. Raw materials for PSAs are characterized by two special features: the Tg and the modulus of elasticity. Both of these properties and their dependence on the

Rheology of Pressure-Sensitive Adhesives

33

base polymer will be examined in Chap. 3. The dependence of the elastic modulus on the molecular weight will be studied in this chapter. It should be mentioned that toughness properties of polymers generally increase with increasing molecular weight up to a point, then level oﬀ. An extension towards higher temperatures of the plateau region of the dynamic modulus master curve can be achieved by the increase of the molecular weight of CSBR [20]. Influence of Chemical Composition/Structure on Viscoelastic Properties of PSAs As discussed earlier and detailed in the next chapter, the rheological properties of PSAs are characterized by viscosity and modulus, and the Tg of the base polymers. All these parameters depend on the chemical composition/structure of the PSAs. The dependence of the viscosity on the chemical composition/structure is discussed next. Viscosity of the Base Elastomer. As known from polymer chemistry the inherent viscosity of polymers depends on the intermolecular mobility of the polymer molecules. This mobility is a function of the chain length (molecular weight) and structure (e.g., branching) and reciprocal interaction of the polymer molecules (polar forces). It must be noted that both the viscosity and its dependence on the temperature are a function of the chemical composition/structure of the polymer. The ﬂow activation energy (e.g., Arrhenius) depends on branching [80]. The chemical composition of the base elastomer also inﬂuences the viscosity of the PSAs. Acrylic hot-melt PSAs with non-Newtonian behavior diﬀer from compounded hot-melt PSAs and from conventional solvent-based rubber/resin PSAs, and allow for the development of high shear adhesives with a low processing viscosity [84]. Styrenic block copolymers have a relatively low solution or melt viscosity, since in solution or at temperatures above the Tg of polystyrene, the systematic polystyrene domain within the block copolymer no longer exists. Generally the viscosity of the adhesive is a function of the raw material class, but viscosity changes are possible in the same raw material class also. For instance the saturated styrene block copolymers (e.g., Kraton G) possess higher solution viscosity than the common unsaturated ones [85]. Influence of Tackifier Resin. Pressure-sensitive formulations are based on an elastomer or viscoelastomer and a viscous component. The most important viscous components are tackiﬁer resins. Exhibiting a surface free energy close to that of the elastomer, the resin (ﬁller) is perfectly wetted by the organic matrix, and as a consequence, good dispersions (i.e., lower viscosities) are obtained [86]. Addition of resins to rubber-based adhesives

34

Chapter 2

decreases the viscosity [87]. The addition of low melting point resins to a hot-melt formulation with block copolymers (Kraton GX 1657) decreases the melt viscosity [88]. As discussed in Chap. 5, the mutual compatibility of the formulation components strongly inﬂuences their rheological properties.

Influence of Environmental and Experimental Conditions on Viscoelastic Properties of PSAs The coating of PSAs, the converting of the laminate, and the end-use of the pressure-sensitive labels occur under very diﬀerent temperature and stress rate conditions. Because of their macromolecular chemical basis, the properties of the PSAs are time/temperature dependent. Thus, for adequate manufacturing and use their time/temperature sensitivity must be studied. For a better understanding of this dependence the temperature and time dependence of the viscoelastic properties will be examined separately. First, the dependence of the viscosity and of the modulus as a function of the temperature will be reviewed. The physical and mechanical characteristics of a polymer depend on its chemical composition/structure. Chemical and macromolecular characteristics determine the internal mobility of the polymer chains. On the other hand, the ambient temperature inﬂuences the rate of chemical and physical interactions (i.e., the chain mobility) and thus the viscoelasticity of the polymer. Liquid characteristics (viscosity) or solid behavior (modulus) of the macromolecular compound are both functions of the temperature. Adhesion properties require a special balance of the viscous liquid/elastic solid state. Therefore the temperature also inﬂuences the adhesive properties. Dependence of Viscosity on Temperature. In general the viscosity is a function of temperature. For low molecular weight, nonassociated liquids (Newtonian systems) the viscosity depends on the temperature according to the equation of Arrhenius: ¼ A exp ðEo =RT Þ

ð2:26Þ

where Eo represents the activation energy, R is the universal gas constant, and T the absolute temperature. For highly viscous liquids [89]: ln ¼ A exp ½B=ðT þ CÞ þ D

ð2:27Þ

Rheology of Pressure-Sensitive Adhesives

35

where A, B, and D are experimental parameters. For non-Newtonian systems the Williams-Landel-Ferry equation is valid, that is: ¼ A exp ½B=ðT To Þ þ

Eo RT

ð2:28Þ

where A and B are experimental parameters, and To is the temperature at which the free volume is zero. This equation remains valid for T < Tg þ 100 K. Generally the temperature-dependent shift factor aT for the viscosity is given by [90]: aT ¼ o ðTÞ=o ðTo Þ

ð2:29Þ

where o is the viscosity at zero shear rate. For amorphous polymers (e.g., PSAs): ln aT ¼ ½C1 ðT To Þ=½C2 þ ðT To Þ

ð2:30Þ

where C1 and C2 are material-dependent parameters. For partially crystalline polymers (e.g., resins): ln aT ¼ Eo =Rð1=T 1=To Þ

ð2:31Þ

For the temperature dependence of the dynamic viscosity, two models generally are considered. The model of Hirai and Eyring [91] indicates the existence of an energy barrier between holes in a liquid and this barrier is the rate determining step of viscous ﬂow; the model applies transition state theory to explain the temperature dependence of viscosity, namely: ¼ 0 expðE=RTÞ

ð2:32Þ

The constant 0 includes both the molecular volume of the liquid and the activation entropy of ﬂow and is generally assumed constant. Another model [92] supposes the formation of holes (i.e., the free volume as the rate determining step), introducing the expression: ¼ A0 exp ðB Vo =Vf Þ

ð2:33Þ

A0 and B are two constants, and Vo and Vf are the occupied and the free speciﬁc volume of the liquid, respectively. The temperature dependence of the viscosity is explained by that of the volumes; by rearranging the

36

Chapter 2

well-known Williams-Landel-Ferry equation [93]: log

C1 ðT Tg Þ log T ¼ o C2 þ ðT Tg Þ

ð2:34Þ

where the constants C1 and C2 depend on the fractional free volume. Influence of Time on the Viscoelastic Properties of PSAs. The intermolecular mobility of the macromolecules is a time-dependent, relatively slow process. This means that molecular rearrangements require a certain time. Practically, this time may be longer, equal to, or (rarely) shorter than the duration of the external stress. Therefore, internal (molecular) motion does not always occur synchronously to the external one. Macroscopic ﬂow behavior and elastic recovery are characterized by viscosity and modulus. Both of these properties are the result of internal microscopic interactions (i.e., of the time needed for these interactions). Therefore, the viscoelasticity of PSAs depends on the time and on the rate of the applied external stress. Williams, Landel, and Ferry [93] deﬁned a shift factor allowing the correspondence between time and temperature inﬂuences. The shift factor depends on the Tg and on the reference temperature [94]. At relatively low strain rates, such as those acting during initial bonding or wet-out of the surface, the polymer molecules have time to slide past one another and undergo viscous ﬂow [2]. This eﬀect, as would be expected, is more pronounced at higher temperatures, where molecular mobility is greater. Storage and end-use of PSAs are both low strain processes. Alternatively, strain rates are signiﬁcantly higher during peel measurements. Here the molecules are not able to ﬂow past one another suﬃciently fast, and the polymer stiﬀens. This stiﬀening constitutes an increase in the elastic contribution to the rheology of the polymer. When the elastic components predominate, more of the bond rupture energy is stored resulting in a higher peel and tack. High speed converting and end-use operations (e.g., die cutting, removal of labels) are high strain processes [2]. The time and temperature dependence of the viscoelastic behavior of polymers may be interpreted by the corresponding dependence of the relaxation spectra and by conformational changes within the macromolecular chains as shown by Schneider and Cantow [95]. The corresponding contributions to the viscoelastic functions are expressed by the frequency shift factor aT which allows the superposition of viscoelastic isotherms into composite curves at To, the reference temperature [95]: T ¼ ½2 o T To =½2 o To T

ð2:35Þ

Rheology of Pressure-Sensitive Adhesives

37

From Eq. (2.33) it follows that the temperature dependence is connected mainly to the translational friction coeﬃcient per monomeric unit o although the mean square end-to-end distance per monomeric unit a2 may vary signiﬁcantly with temperature for most polymers. The shift factor is related to the dynamic viscosity 0 according to: T ¼ ð0 To o Þ=ð0o TÞ

ð2:36Þ

where is the density of the polymer. Intermolecular polymeric motion inﬂuences the polymer ﬂow, and this motion is a function of time and temperature. From the mechanics of ﬂuids or solids, it is known that mechanical properties decrease as temperature increases (i.e., the molecular motion is enhanced). In a similar manner there is an opportunity for molecular motion if the rate of the applied force is lowered. Thus high temperature and low forces act similarly on the molecular motion and on phenomena related to it (e.g., viscosity). Dependence of the Viscosity on the Shear Rate. Pressure-sensitive adhesives are viscoelastic materials, that is, their mechanical properties are temperature and time dependent. The material characterization requires the measurement of a material function such as viscosity or modulus as a function of temperature and time. As can be seen from Fig. 2.4, the viscosity is a function of the shear rate. At low shear rates, there is a short interval where the viscosity does not depend on the shear rate (this is the so-called zero-viscosity). At higher shear rates, there is a linear or nonlinear decrease of the viscosity as a function of the shear rate (Newtonian or non-Newtonian systems). The decrease of the viscosity with the increase of the shear rate is given by the elastic component of the adhesive [96]. The existence of a shear rate-dependent viscosity domain implies that static methods for testing the cohesive strength of PSAs cannot yield real information about their behavior at high stress rates (peel, cutting). This statement can be conﬁrmed through the deﬁnition of the adhesion failure energy (separation energy W), upon debonding through peel or loop tack [97]: Z W¼

F v dt

ð2:37Þ

where v is the debonding (separation) rate and F the applied force. The exponent value for a Newtonian liquid that has a shear rate-independent viscosity is n ¼ 1 [see Eq. (2.4)]. Changes in the polymer structure may be associated with changes in the viscosity-shear rate dependence. Detailed

38

Chapter 2

Figure 2.4 Dependence of the viscosity on the shear rate. Decrease of the Brookﬁeld viscosity for diﬀerent (1 through 3) water-based acrylic PSAs as a function of the stirring rate.

studies of elastomers showed mastication increased n from 0.25 to 0.5 [98]. Mastication in a mixer yields less change of the polymer elasticity than mastication on a roll mill. Consequently rubber-based PSA formulations manufactured without a priori calendering or roll mixing exhibit a higher viscosity and a very broad range of viscosity values (see Fig. 2.5) as a function of the solid content. The viscosity of PSAs inﬂuences the contact build-up of the liquid adhesive. For easy bonding low viscosity values are required. In order to obtain a high debonding resistance (high peel and shear) high viscosity values are needed. Thus one can wonder what low and high viscosity values mean? What should be chosen, the low or the high viscosity version of the adhesive? Generally, with common formulations the elastic component of the polymer predominates. That is, high peel and high shear as exhibited by high viscosity polymers are used, and bonding (ﬂow) of the adhesive is obtained by the use of longer dwell times. For those highly viscous polymers the value of the viscosity is given indirectly as cohesion (shear). On the other hand, removable PSAs are more plastic and possess a lower viscosity,

Rheology of Pressure-Sensitive Adhesives

39

Figure 2.5 Range of solution viscosity values. Tackiﬁed, unmilled rubber/resin PSA: solution Brookﬁeld viscosity as a function of the solid content.

causing stringing in many cases. Viscosities higher than 105 Pas are required in order to avoid this phenomenon. Generally, viscosities of PSAs vary between 104 and 105 [55]. Dependence of the Modulus on the Stress Rate. Like the viscosity the modulus of the PSAs also depends on the stress rate. As an example, Zosel [16] measured the modulus of tackiﬁed PSAs as a function of the resin concentration, at a rate above 102 sec, and demonstrated the existence of a maximum at a resin concentration of 50%. On the other hand Dahlquist, working at faster rates (t < 102 sec) showed the existence of a maximum at 60% resin concentration [23]. Influence of the Elasticity/Plasticity Balance on the Properties of PSAs. If the elastic components are predominant in the rheology of the polymer, more of the bond rupture energy is stored, resulting in higher peel and tack properties [2]. Some of the energy supplied by the application of stress to a material is stored energy, in contrast to that which is dissipated in overcoming viscous drag. The two recognized means of storing mechanical energy are inertia (kinetic) and elasticity (potential) of which the latter commands the most attention as the determining factor which allows us to distinguish permanent adhesives from removable ones. If the time scale of the measurement is short enough, any liquid will respond elastically to prevent relaxation of the applied stress (e.g., high speed peel).

40

2

Chapter 2

RHEOLOGY OF PSA SOLUTIONS AND DISPERSIONS

In order to be coated onto the face stock material the adhesive must be in the liquid state. This state may be achieved by melting (hot-melt PSAs), dissolving (solvent-based adhesives), or dispersing (adhesive dispersions) the PSAs. Dissolving and dispersing the adhesive produce diluted adhesive systems, where ﬂow properties are due to the carrier liquid. The rheology of diluted PSAs also depends on the carrier liquid. Generally, there are aqueous and solvent-based adhesive solutions, and aqueous or solvent-based adhesive dispersions. The most important are the organic solvent solutions of adhesives (solvent-based adhesives) and the water-based dispersions of adhesives (water-based adhesives). In fact most solvent-based adhesives like rubber resin adhesives, contain some dispersed material, and most of the dispersion-based aqueous adhesives contain some dissolved substances (e.g., thickeners, protective colloids, tackiﬁers, etc.). The rheology of PSAs in the liquid state (molten or diluted) is very important for the coating process. As stated in [99] the main processing parameter by coating is the viscosity of the adhesive. In this case one needs to be concerned with coating rheology. Wetting out, coating rheology, coating speed, and versatility are the most important ﬂow parameters inﬂuencing the convertability (coatability) of the adhesive. All of these parameters depend on the physical state of the adhesive, either as a diluted liquid system, yielding an adhesive layer by evaporation of the carrier liquid (water or solvent), or as a 100% solid system, giving a solid adhesive layer by a physical (hot-melt) or chemical [ultraviolet (UV)- or electron beam (EB)-cured] process. Therefore the coatability and the coating rheology of diluted and undiluted systems will be discussed separately. The molten state of hot-melt PSAs diﬀers from the coated, solid state by the value of their viscosity only. Therefore, it may be assumed that the rheological properties of hot-melt PSAs during converting do not diﬀer from those of solid PSAs. Hot-melt ﬂow is not an isothermal process; it is more sensitive to thermal transfer and temperature than the room temperature cold ﬂow of the solid, coated adhesive layer. However, it is designed to possess an adequate ﬂuidity at a given temperature (i.e., an optimized viscosity). Because of the relatively high viscosity of the ﬂuid hotmelt PSA and the short period of the liquid state, phenomena encountered by diluted PSA systems (e.g., wetting out, migration, mechanical stability, and volatility of low molecular weight components) appear less important. Diﬀusion problems for hot-melt PSAs are due not to the carrier liquid (and do not occur during the coating) but mainly to the low molecular weight polymers. Hence there is no need to discuss the coating rheology of hot-melt

Rheology of Pressure-Sensitive Adhesives

41

PSAs separately. Unfortunately, the nature and concentration of the carrier liquid in a diluted PSA play a decisive role in their rheology. Thus it appears necessary to discuss the rheological features of those systems. Because of the basic diﬀerences between solution adhesives and polymer dispersions, considerable diﬀerences in rheology exist and it is necessary to discuss the coating rheology of both these diluted PSA systems separately. 2.1

Rheology of PSA Solutions

The rheological behavior of polymer solutions is a result of solute/solvent concentration; it depends on the properties of the dissolved PSAs and of the solvent system. The inﬂuence of the dissolved polymers and their chemical composition and macromolecular characteristics will determine their interaction with the solvent. On the other hand, like the rheology of the solid polymer, the ﬂow characteristics of the polymer solution also depend on time and temperature. Coating rheology includes the ﬂow properties of the wet and dried adhesive, leading to a wettable, stable, running timeindependent liquid layer, with a smooth surface after drying under the applied shear forces. Coating rheology of solvent-based PSAs is the rheology of a true solution, thus mainly viscosity driven and more simple than that of water-based systems. Such viscosity depends on the nature and concentration of the formulation components [100]. Typical viscosity values of acrylic solvent-based PSAs are situated in the range of 1700– 25,000 mPas. The ﬁrst step of the coating process is the wet-out of the solid surface by the ﬂowing adhesive. Generally, wetting out is a function of surface tension, viscosity, and density of the adhesive layer. For coating of solvent-based formulations a surface tension higher than 38 mN/m is required. For coating of aqueous formulations the surface tension must be higher than 46 mN/m [101]. It should be kept in mind that solvent-based adhesives are usually highly viscous systems (more than 10,000– 15,000 mPas) with low surface tension solvents as liquid carrier, and being (at least partially) directly coated on high surface energy webs. Thus, the wet-out of solvent-based PSAs is less diﬃcult and less critical than that of water-based ones. Surface tension depends on polymer and solvent nature, viscosity, and density. Synthetic raw materials for solvent-based adhesives are homogeneous, whereas natural raw materials are not. In general rubber/resin adhesives display a broader variation of the solution properties than acrylic-based ones. On the other hand solution polymerization allows better chemical control, thus some macromolecular properties of solvent-based polymers are superior to those of water-based ones (e.g., MWD, sequence distribution, stereoregularity, gel content, etc.). The use of special solvents or solvent mixtures constitutes an additional possibility for

42

Chapter 2

surface tension control of solvent-based adhesives. Generally solvents used for solvent-based PSAs have a low surface tension (25–30 mN/m), much lower than that of water (72 mN/m) or even of aqueous dispersions (usually 35–42 mN/m). The surface tension of solvent-based PSAs causes less concern than that of aqueous dispersions. Viscosity control of solventbased PSAs can also be realized by the use of ﬁllers. Special Features of the Rheology of PSA Solutions Polymer solutions are multicomponent systems, where the rheological properties of the components and their mutual interactions may play an important role. The rheology of the basic polymer and of the solvent-system interface is a function of the nature and concentration of the components. In some cases the procedure for dissolving the basic polymers, respectively for eliminating the carrier solvent, also inﬂuences the ﬁnal adhesive properties. These aspects will be examined in more detail in Chap. 9. In this section the most important features of the rheology of polymer solutions are summarized. The ﬂow properties of PSA solutions are important for their coating behavior. The coating rheology of the polymer solutions may be diﬀerent from that of the polymer without the solvent carrier, due to the multicomponent (system) character, and to the quite diﬀerent manufacturing and end-use conditions (stress rate and temperature) for PSA solutions. Polymer-Dependent Characteristics. Polymer solutions are viscoelastic, with a non-Newtonian character, their behavior being determined by the nature and concentration of the components. Theoretically the most important rheological characteristics of PSA solutions will be described by a correlation depending on the macromolecular and chemical characteristics of the polymer (solute) and the chemical nature and concentration of the solvents used. The time/temperature dependence of the viscosity is also pronounced. Solvent-based adhesives are mostly solutions of rubber/resin blends or acrylic copolymers. Their viscosity depends on the chemical composition and molecular weight of the base polymers. Mechano-chemical degradation of the base polymer, e.g., mastication of natural rubber, reduces the molecular weight and its dispersion. As discussed in [102] the ratio of the viscosities of adhesive compositions based on unmilled or milled rubber is about 5–10. For rubber/resin-based PSAs viscosity control is limited to the change of the MW/gel content ratio. Because of the high dispersion of these values, the use of an additional 10–15% of solvent in these formulations is common practice. Resins tend to reduce viscosity and increase solids content [103]. The molecular weight of solvent-based adhesives is lower than that of water-based dispersions. However, these

Rheology of Pressure-Sensitive Adhesives

43

relatively low molecular weights are high enough to yield high viscosities. As shown earlier viscosity directly inﬂuences the wetting out. The wet-out is a function of surface tension, viscosity, and density. Surface tension depends on the solvents used, and on the density of both polymer and solvent. Solvent-Dependent Characteristics. Generally polymer solubility depends on solute/solvent interactions, thus the viscosity of PSA solutions depends on the nature and concentration of the solvents used. The viscosity of solvent-based PSAs depends on the molecular weight, chemical nature, and solids content of the formulation. With suitable molecular weight values and a solids content of 20–25%, solvent-based PSAs display viscosities of more than 10,000 mPas. High solids solvent-based acrylic PSAs have a solids content higher than 60% and a Brookﬁeld viscosity of 15,000 mPas (at 25 C) at 65% solids [104]. Gravure cylinders can be used for high solids (50–80%) highly viscous solutions having a viscosity of more than 50 Pas and for low viscosities of 12–15 mPas too [105]. It should be kept in mind that solvent-based PSAs diﬀer from the water-based PSAs not only by their higher viscosity but also through the easier viscosity control. Problems may appear for crosslinked systems (for instance for crosslinkable, solvent-based acrylates) where diluting may aﬀect the stability of the adhesive [106]. As discussed in detail in [99] for such systems viscosity is pot life related. As is known from practice, it is possible to formulate solvent-based PSAs according to their viscosity (for doctoring ease) or to their solids content (for high drying speeds). Rubber/resin PSAs mainly use hydrocarbon-based solvents, thus the choice of the solvent as an aid cannot play an important role in the control of their viscosity. Currently binary or ternary solvent mixtures allow better viscosity control. On the other hand, the rheology of the solvent-based PSAs is sensitive towards drying speed (i.e., the rate of the diﬀusion of the solvents). Technology-Dependent Characteristics. The rheology of the polymer solutions may depend on the manufacturing and coating technology. Solvent-based PSAs are made by dissolving the basic polymer. Because of the sensitivity of the macromolecular compounds towards mechanochemical destruction, the dissolving technology will inﬂuence the spread of the viscosity values. On the other hand the same PSA solution may give coatings of diﬀerent quality, according to the geometry (shear) of the metering device [99]. As stated in [99] the main parameters of the wet ﬁlm quality for solvent-based adhesives are the surface tension, solids content, viscosity, solvent characteristics, and temperature. The quality of the dry coating depends on the evaporation rate of the solvent, which is a function of the coating thickness, solids content, solvent characteristics, and drying

44

Chapter 2

conditions. Another feature is given by the sensitivity of the coated adhesive layer towards drying speed (rate of diﬀusion of the solvents used). Mainly for rubber/resin adhesives based on hydrocarbon solvents, superﬁcially stripped, textured surfaces may be obtained if the solvent ‘‘boils’’ during drying and the ﬂuidity of the dry layer is lost. Adherend-Dependent Characteristics. Liquid components of solventbased PSAs may interact (physically or chemically) with the contact surface of the adherent, resulting in better adhesion. Time/Temperature Dependence of the Viscosity of PSA Solutions A shear-induced viscosity decrease of polymer solutions is governed by molecular orientation and disentangling. For Newtonian liquids at low shear rates it is possible to restore the polymer network in the solution through relaxation. High shear implies the decrease of the network points. Therefore, the shear modulus G is given, as a function of the network points vc, G ¼ vc RT

ð2:38Þ

The dependence of the viscosity on the shear is a general phenomenon also valid for polymer (adhesive) dispersions. It should be mentioned, however, that in a manner diﬀerent from adhesive dispersions, the mechanical stability of the system for polymer solutions is viscosity (or solids content) independent. 2.2

Rheology of PSA Dispersions

The coatability of liquid adhesives is a function of wetting out, coating versatility, coating rheology, and coating speed. On the other hand the coating rheology inﬂuences the wet-out and the coating versatility, and depends on the coating speed. The wetting out of water-based systems constitutes a diﬃcult problem because of the high surface tension of the carrier liquid (water). Solvent/solute interactions in ‘‘real’’ solutions give rise to the increase in viscosity. Water-based dispersions generally possess a lower viscosity while ready-to-use PSA dispersions possess water-like low viscosities. Their low density and the transfer coating technique used also contribute to the coating diﬃculties. In a diﬀerent manner from solvent-based PSAs, improvement of the wet-out by the use of surface active agents causes irreversible changes in the adhesive properties. Thus an adequate coating rheology appears important. Water-based dispersions have a limited shelf life and thus the coating rheology also depends on the age of the dispersion. This is a function of the

Rheology of Pressure-Sensitive Adhesives

45

dispersed polymer particles (macromolecular and dispersion characteristics) and of the carrier liquid as well. Only small changes in the composition of the carrier liquid are allowed. It may be assumed that the two main parameters inﬂuencing the coating rheology of the polymer solutions (polymer and solvent nature) have a reduced importance for water-based PSA dispersions. More important seem to be the dispersion nature and the inhomogeneous character of the liquid, dispersed adhesive. The ﬂow of a dispersion depends on the concentration and characteristics of the dispersed solid particles (particle size and particle size distribution). The correlation between solids content and viscosity includes the particle size and particle size distribution. On the other hand, the inhomogeneous, anisotropic character of the adhesive layer persists after the coating (wetting out and drying) of the liquid layer. The ﬁnal water content of the dried, coated adhesive (residual humidity) depends on the humidity of the environment (i.e., it reaches an equilibrium value). The dried dispersion-based PSAs will never have the rheology of the bulk adhesive (the adhesive from the polymer particles) but remain in an intermediate state between the rheology of the dispersed system and that of the base polymers. Furthermore the wetting additives left in the adhesive layer also contribute to the composite character of the PSA layer. Therefore it can be assumed that the rheology of the water-based PSA layer is that of a composite material (i.e., polymer particles surrounded by a layer of additives). From experimental practice it is known that the rheology of water-based PSAs is shear sensitive (i.e., dynamic wetting needs a certain level of coating speed and a certain level of shear on the metering/coating device). The most important rheological parameter of a water-based PSA dispersion is its viscosity. The viscosity inﬂuences the coatability, the ﬁlm-forming characteristics, and the properties of the adhesive layer thus formed. Viscosity of PSA Dispersions In order to be transfer coated onto the face stock (or release liner) the adhesive dispersion must wet-out the solid surface. Furthermore, the coated adhesive has to permit the withdrawal of the carrier liquid (water), and the coalescence of the solid, dispersed particles (ﬁlm forming). The wet-out of the adhesive and the ﬁlm building process are strongly inﬂuenced by the viscosity of water-based PSAs. On the other hand, the coating technology (coating machines and coating speed) is viscosity dependent too. Coating devices are very sensitive towards viscosity, thus the coating versatility (i.e., the ability of an adhesive to be used on diﬀerent coating machines) depends on the viscosity. The coating speed contributes to the wet-out process. Excessive speed may produce foam or adhesive build-up on the machine.

46

Chapter 2

Both phenomena are a function of the viscosity of the adhesive. The adhesive’s ﬂow on the metering device (i.e., the viscosity) also inﬂuences the coating weight of the PSAs. Because of the lower level of interactions between the solute and solvent, generally the viscosity of the dispersions is lower than that of true solutions. Wet-Out of Water-Based Adhesives As stated in [99], of the technological properties of dispersed adhesive systems, wetting ability is the most critical performance characteristic for their coating. As will be shown later, the wet-out is a function of the quality of the face stock and/or release liner. Water-based PSAs are coated mainly by transfer, that is, coated onto the release liner and subsequently transferred onto the face stock. The surface quality of the release liner is a function of the siliconizing technology (condensation/addition; solventless/solventbased/water-based silicones). The fast curing rate at low temperatures (ca. 5 sec at 110 C) of the addition-based systems allows mild coating conditions for the paper (better paper/moisture equilibrium) [107]. A decade ago, solventless systems were developed, permitting better release properties and economical advantages. Today, they represent about 40% of the world usage in self-adhesive applications [55]. These systems are based on very low molecular weight polymers, which can only be applied to very high grade papers in terms of pore sealing and smoothness of the surface, in addition to which special (better) wetting properties of the water-based adhesives are required. Generally, suitable water-based PSAs have to display good wetting on the worst liners (i.e., on siliconized liners) and especially on solventless silicone-coated release liners. Theoretical Basis of Wetting Behavior. When in its liquid state, the adhesive must be able to uniformly wet the web. The amount of work expended under reversible or equilibrium conditions to disrupt the interface, i.e., to coat it, is a function of the surface energy of the latex. Equilibrium spreading occurs when the surface energy of the substrate ySV is higher than the surface tension of the latex yLV and the internal surface tension between latex and substrate surface ySL, as summarized by the following formula and illustrated in Fig. 2.6: SV ¼ LV cos þ SL

ð2:39Þ

where is the contact angle. Thus for a given substrate, wetting is facilitated if the surface tension of the latex or/and the internal surface tension is minimized. Therefore the surface tension of the adhesive has to be decreased. As typical surface tension values for wetting out, 42 105 mN/m, to

Rheology of Pressure-Sensitive Adhesives

Figure 2.6 surface.

47

The contact angle of a high surface tension liquid on a nonpolar

46 105 mN/m are required [108]. Wetting out on silicone coated release is more diﬃcult. Silicones have a surface tension of about 20 mN/m. The surface tension of water-based dispersions depends on their formulation. For instance the surface tension of SBR lattices is lower than that of acrylates. Industrial coating requires not only wetting but also the stability of the coated liquid layer. Unfortunately, this liquid layer formed by wetting the surface is not stable, it suﬀers dewetting. Two types of dewetting phenomena are possible, namely retraction of the latex coating from the edge and surface cracking. Static/Dynamic Wet-Out. In industrial practice the wetting failure of greatest concern is the dewetting of the adhesive from the primary web, between the gravure coating station and the drying tunnel. In the process of latex coating at high speed the dynamic aspect of the wetting/dewetting is also critical. The real wetting/dewetting behavior of the latex cannot be approximated in the laboratory because of shearing of the dispersion on the coating (metering) roll, which improves wetting, and the competition between dewetting and drying imposed coalescence. Therefore dewetting on the

48

Chapter 2

coating machine is diminished by shearing of the dispersion layer and coalescence of the adhesive. Industrial wetting is always better than on the laboratory scale. The surface tension of the latex and its contact angle inﬂuence the wetting/dewetting process [109,110]. Dewetting problems can be overcome by reducing both the surface tension and the contact angle of the latex. In order to evaluate the coatability both parameters (surface tension and coating angle) have to be measured. Generally the contact angle of the latex is plotted against the surface tension of the latex for diﬀerent surfactant systems. The best wet-out is obtained for the lowest contact angle and surface tension. Other factors aﬀecting wetting/dewetting are the viscosity and the density of the liquid adhesive. Surface Tension (Static/Dynamic Values). The inﬂuence of the lower surface tension values is especially noticeable, when trying to coat onto low surface energy substrates. If the surface energy of the web is too high, and the formula is not thickened, defects show up in the ﬁlm (e.g., cratering or pinholes). Reducing the surface tension of the latex helps to get good wetting. On the other hand, the coated liquid layer tends to retract. In order to avoid ﬁlm retraction surfactants have to migrate rapidly to the interface between the adhesive and the release liner so as to avoid dewetting. Thus for PSA applications where the coating process contantly creates new interfaces, a low static surface tension and a low dynamic surface tension are needed. Generally the surface tension is a function of the following parameters, namely the age of the latex [111], the polymerization yield [112], the solids content of the latex [113], the temperature [113], the viscosity [113], the particle size and particle size distribution [114]. The inﬂuence of the viscosity on the surface tension can be determined by means of surface amplitude decay through the Orchard [115] equation for leveling: Z lnðAo =A1 Þ ¼ ð164 =3 4 Þ 0 h31 dt= ð2:40Þ where Ax denotes the surface amplitude at any given time, denotes eﬀective surface tension, h is the mean ﬁlm thickness, l is the wave length, and is the dynamic viscosity. From the above relationship it may be concluded that the surface tension s is correlated to the viscosity: s ¼ f ðÞ

ð2:41Þ

Contact Angle. A low contact angle enhances the wetting out. For the evaluation of the coatability the contact angle needs to be measured. Unfortunately measuring of the contact angle faces some diﬃculties as the

Rheology of Pressure-Sensitive Adhesives

49

contact angle depends on the surface tension and on the smoothness of the surface; furthermore the contact angle is time dependent (i.e., there is a wetting and a dewetting angle) and the contact angle also depends on the dimension of the adhesive drop. Influence of Viscosity on Wetting Dewetting may be reduced by increasing the standing ability of the liquid adhesive layer. The mobility of the liquid layer is inversely proportional to its viscosity and density. This is conﬁrmed by thickening and incorporating ﬁllers into the water-based formulations. Only slight density diﬀerences may be obtained, hence wetting out will depend mainly on the viscosity. Highly viscous aqueous dispersions are characterized by good wetting properties, and generally the increase of the viscosity is a method to improve the wet-out. Thus, one can write: Wetting out ¼ f ðviscosityÞ

ð2:42Þ

For aqueous solutions of surface active agents Kurzendo¨rfer [116] formulated a special dependence for the amount of rest liquid G (i.e., liquid which does not dewet) and dynamic viscosity : G ¼ ð8=3Þr h3=2 ð0:5 0:5 =g0:5 Þ ðtw td Þ0:5

ð2:43Þ

where r is the average cylinder radius (for the test equipment used for vertical immersion-wetting out tests), h is the pull-out length of the cylinder, is the speciﬁc weight of the liquid, tw is the pull-out time of the cylinder, td is the dewetting time of the liquid, and g is the gravity constant. Thus, G ¼ C1 0:5 d 0:5 ðtw td Þ0:5

ð2:44Þ

This correlation conﬁrms the dependence of the wet-out on the viscosity and density of the latex. As will be shown in Section 1.4 of Chap. 3, aqueous dispersions are characterized by a minimum ﬁlm-forming temperature. This temperature depends on the viscosity of the bulk polymer in the interior of the emulsion particles. Influence of Viscosity on Coating Versatility. As discussed in [100] the wetting out and viscosity are the main parameters for the coatability of water-based PSAs. Wetting out is strongly inﬂuenced by surface tension, viscosity depends on the diluting ability of the system. Coating versatility refers to the ability of water-based PSAs to be coated by diﬀerent coating techniques. Generally, coating technology is a function of the chemical

50

Chapter 2

composition, physical state of the adhesive, product class, and product build-up. As discussed in [117] coating technology and a particular coating device can require a certain viscosity and solids content and inﬂuence the formulation. As listed [118] the viscosity range of water-based PSAs covers 150–9000 mPas. The application equipment for water-based PSAs diﬀers from that used for solvent-based formulations. Generally it works with low viscosity. In Europe two methods dominate the coating technology, namely the metering bar system and the reverse gravure. The metering bar system (Meyer bar) is used mainly for tapes and protective ﬁlms. Reverse gravure is recommended for labels. Slot die coating was developed for coating of water-based dispersions also because of the very broad range (0.1–10 Pas) of viscosities which can be used [119]. For coating by reverse gravure the high solids content (64%) has to be reduced. For speciﬁc viscosity ranges diﬀerent coating devices are used. Reverse roll coating by rotogravure needs low viscosities of 150–400 mPas (17–30 sec Ford Cup 4; 17–23 sec with DIN Cup 4 mm [120]); thickening makes this interval broader. Reverse roll coaters are suggested up to a viscosity of 40 Pas [121]. As stated in [99] water-based adhesive systems may require a diﬀerent gravure set up of the coaters. The manufacturing versatility of selected coating methods is discussed in detail in [38, 100]. The base viscosity of the dispersion is a function of the polymerization recipe. For instance, water-based acrylics are synthesized with viscosities of around 100 to 500 mPas. Thus they are adequate for gravure and reverse gravure coating [122]. Adjusting the viscosity of an aqueous dispersion requires knowledge of the thickening/ diluting response of the dispersion. Ideally the adhesive should allow large viscosity changes without important variation in solids content and mechanical stability. Another problem concerns the shear sensitivity of the viscosity. Because of the shear sensitivity of the adhesive on the machine the apparent viscosity in the machine diﬀers from the viscosity measured under laboratory conditions. For water-based dispersions a high thixotropy ratio is required. On the other hand, dispersions with diﬀerent viscosities may be coated directly or by transfer. The coating technology (transfer or direct) inﬂuences the anchorage and migration of the adhesive and therefore the end-use properties. Therefore an indirect inﬂuence of the viscosity on these properties must be taken into account. Influence of Viscosity on Coating Weight. As discussed in [38] diﬀerent coating methods and coating weights are used for the main PSP (e.g., label, tape, protective ﬁlms etc.). For labels coating weight values of 10– 30 g/m2, for tapes 5–40 g/m2, and for protective ﬁlms 0.5–15 g/m2 are applied. It is evident that the diﬃculty in achieving a standard tolerance value of the coating weight depends on its absolute value. To keep the coating weight

Rheology of Pressure-Sensitive Adhesives

51

tolerance in the usual range within a maximum of 5% the viscosity of the emulsion must be kept constant. For a PSA dispersion on a reverse gravure coater it is ideal to have a high solid content with a low viscosity. A low viscosity implies a high coating weight. The minimum viscosity depends on the surface tension of the adhesive too. In practice the coating weight depends on several parameters such as the viscosity, running speed, and solids content. Viscosity is a function of the solids content; on the other hand it inﬂuences the coating speed. The interdependence of these parameters, and their inﬂuence on the coating weight is illustrated in Fig. 2.7. As can be seen from Fig. 2.7 the interdependence of the coating parameters (running speed, solids content, and viscosity) is not linear. The coating weight increases with the solids content, with the running speed, and with decrease of the viscosity up to a maximum level, then decreases. During the coating process disturbing phenomena may occur such as adhesive build-up on the machine, coagulum build-up on the machine, and in the coated layer, turbulences in the coated adhesive layer and changes in the viscosity and foam formation. These phenomena depend on the rheology of the diluted system. Because of their practical importance, they will be discussed in Chap. 9.

Figure 2.7 The interdependence of the coating weight, the solid content, the viscosity, and the coating speed of PSA.

52

Chapter 2

Factors Influencing the Viscosity of Aqueous Dispersions. The following factors inﬂuence the viscosity of aqueous dispersions [116], namely: temperature; solids content; pH; dispersing agents, particle form, size, and size distribution; viscosity of the dispersing media; and shear forces. In general, the viscosity decreases with increasing temperature. The increase of the solids content implies an increase of the viscosity. The dependence of the viscosity on the pH is more complex, but generally it can be assumed that viscosity increases with increasing pH. The dependence of the viscosity on the dispersing agent is a function of the nature and concentration of the dispersing agent. A broad particle size distribution imparts low viscosity due to the mobility of the particles. Generally waterbased dispersions contain a lot of diﬀerent additives, which inﬂuence the viscosity of the dispersing media. Shear forces produce in most cases a pronounced decrease of the viscosity (shear thinning). Too high a shear force may destroy the system producing coagulum. Too low an emulsion viscosity can cause wetting problems, foam problems, or adhesive bleedthrough when the emulsion is directly coated onto the face stock. The emulsion viscosity can also be controlled by either controlling the emulsion particle size, or by subsequent thickening/diluting.

3

RHEOLOGY OF THE PRESSURE-SENSITIVE LAMINATE

Pressure-sensitive adhesives are mostly used in a coated, bonded state enclosed within a laminate. Generally the adhesive acts as an intermediate layer between face stock and release liner, and ﬁnally, between face stock and substrate. Because of the relatively small thickness of the adhesive layer (Fig. 2.8) its ﬂow properties are strongly inﬂuenced by the properties of the delimiting solid surfaces. On the other hand, at least one component of the laminate (the face stock), but mostly both (face stock and release liner or substrate) are ﬂexible, soft materials allowing no uniform distribution of the applied forces, and undergoing a pronounced deformation during converting or end-use. Thus one can suppose that the rheology of the bonded PSAs will be strongly inﬂuenced by the components, structure, and manufacture of the laminate. Pressure-sensitive adhesives act like permanently liquid elastomeric layers enclosed between two thin, ﬂexible, soft but solid surfaces. The ﬂow of the adhesive ﬁlm is strongly inﬂuenced by the adhesive/surface interactions, and by the stress/temperature loading transmitted by and through these delimiting surfaces. The adhesive/face stock, adhesive/release liner, and adhesive/substrate interactions depend on the adhesive nature and

Rheology of Pressure-Sensitive Adhesives

Figure 2.8

53

Geometry of the PSA paper laminate.

on the characteristics of the solid components. One can conclude that the rheology of the adhesive in the laminate depends on its thickness, and the dimensions on the continuous/discontinuous nature of the laminate components, and on the laminate build-up. It is relatively easy to determine the rheological properties of the adhesive or of the materials used as face stock or release liner, but there is no means for direct measurement of the rheology of the pressure-sensitive laminate. Practically, one can investigate the adhesive, converting, and end-use properties of the coated (bonded) PSAs, and thus indirectly evaluate the rheology of the bonded PSAs. Both the uncoated adhesive and the solid components inﬂuence the rheology of the coated adhesive. In Section 2 the most important features of the rheology of the adhesive are summarized. The inﬂuence of the solid laminate components on the rheology of the bonded adhesive will be described indirectly through the most important adhesive properties (i.e., tack, peel adhesion, and shear). Rheology depends both on the nature of a ﬂuid and on its ﬂow conditions; these are a function of the relative volumes of the ﬂuid and, in the special case of coated adhesive, on its coating weight. Therefore before examining the inﬂuence of the solid state components on the adhesive properties, the inﬂuence of the coating weight needs to be considered ﬁrst.

54

3.1

Chapter 2

Influence of the Liquid Components of the Laminate

Earlier the rheology of uncoated PSAs was examined. The conclusions of this chapter may be taken as valid for the liquid component (adhesive) of the laminate. However, these considerations must be completed by the examination of the inﬂuence of the adhesive coating weight on the rheology of pressure-sensitive laminates. The importance of the ﬂow of the adhesive in achieving pressuresensitive properties has been outlined. Flow properties depend on the ﬂow conditions, that is, ﬂuid volume, rate, and interfaces. Pressure-sensitive adhesives are thin-layer coatings, where surface roughness and ﬂuid volume may have a pronounced inﬂuence on the motion of the adhesive. Surface inﬂuences may be at least partially avoided by increasing the distance of the ﬂuid in motion from the solid surface (i.e., by increasing the coating weight of the adhesive). Changes of the coating weight will inﬂuence the adhesive and converting properties of the PSAs. Influence of Coating Weight on Adhesive Properties The main adhesive properties of PSAs may be described as tack, peel, and shear, and each one of these properties depends in a diﬀerent manner on the coating weight. As is known from labeling practice, a certain coating weight is necessary in order to obtain a measurable tack. Diﬀerent tack measurement methods show a diﬀerent sensitivity towards the coating weight. On the other hand the nature of the solid components of the label also inﬂuences the relation tack/coating weight. A detailed investigation of RB tack dependence on the adhesive and carrier nature and thickness was carried out in [27]. As demonstrated RB tack values increase with the increase of the coating weight and the decrease of the ﬁlm carrier thickness. Such dependence is a function of the adhesive nature too. For elastic materials the energy ﬂow to the crack tip per unit area of crack extension is given by the following correlation [123]: G ¼ K12 =2E

ð2:45Þ

where the parameter K1 is called the mode one stress intensity factor. For viscoelastic materials, in contrast to elastic materials, K1 depends on the history of contact and on the details of the crack opening and closing process [124]. According to Schapery [125] in this case the rate of energy ﬂow to the crack tip is given by: 1 GðtÞ ¼ CðtÞK12 ðtÞ 2

ð2:46Þ

Rheology of Pressure-Sensitive Adhesives

55

This equation is similar to the elastic criterion [Eq. (2.45)] except that the time dependent C(t) replaces the constant E, thereby making G a dynamic quantity. The history of contact and the details of crack opening have to be taken into account by measuring the peel too. Dwell time (or theal) depends on the application conditions. The coating weight also inﬂuences peel adhesion. Generally the peel increases with increasing coating weight. This is a nonlinear dependence, and its relation depends on the substrate to be adhered to [126,127]. It was recognized in the ﬁrst years of the development of PSAs that controlling the coating weight allows modiﬁcation of the permanent/removable character of the adhesive. A decrease of the coating weight from 20–25 g/m2 to 15–20 g/m2 can yield a removable label [58]. The use of less than 7 g/m2 of dry adhesive for labels removable from paper was developed [66]. Coating weight reduction is the main method to ensure removability of the slightly crosslinked adhesives used for protective ﬁlms [44]. For acrylic PSAs used for common protective ﬁlms, coating weight values of 1.7–12.0 g/m2 are used. As discussed in [44] for an ideal adhesive the adhesion should not depend on the coating weight. On the other hand the peel value attains a measurable value only above over a certain coating weight. Contact build-up needs adhesive ﬂow. For given application conditions, the surface roughness, carrier deformability, and adhesive cold ﬂow are the main parameters of contact build-up. Therefore the dependence of the peel force on the coating weight can be described by a simpliﬁed plot, with the linear part of the dependence shifted over the critical coating weight value: P ¼ Cw

ð2:47Þ

where ¼ f (adhesive, carrier, substrate, time, temperature) and is a conformation factor, which depends on the parameters inﬂuencing the contact build-up between adhesive and substrate surface and the so-called free ﬂow region of the PSA. That means that for too low coating weight or too rough surface no adhesion builds up. It is evident that rough, nonpolar surfaces and harder adhesives on rigid nonconformable carrier materials lead to higher values of ; in such cases the critical coating weight is higher. Like the tack dependence on the coating weight, the dependence of the peel is strongly inﬂuenced by the nature and geometry (surface and bulk characteristics of the carrier material, see Section 3.2 also). As stated in [44] the resulting peel is a complex function of carrier deformation ("c ): P ¼ f ð"c ="c Þ

ð2:48Þ

56

Chapter 2

where and are exponents that account for the inﬂuence of the deformation work of the peel force. For thick mechanically resistant or elastical materials the value of is low. As seen from the relation (2.48) both and depend on the adhesive, carrier, substrate, time, and temperature. For a given application and the same PSP (same adhesive, coating weight, and carrier) on a given substrate and can be considered as parameters that depend only on the time and temperature. Therefore the peel value depends on the application conditions only, i.e., laminating pressure, time, and temperature. Thus for the application of classic PSPs (tapes and labels) the intrinsic adhesive properties of the product (regulated by the adhesive nature and coating weight) are more important for peel build-up. For protective ﬁlms the application conditions are determinant. It should be noted that contact build-up with the adherent depends on the adhesive geometry too. Thus, as demonstrated in [45] special crosslinked adhesives coated onto protective ﬁlms with diﬀerent coating devices (e.g., gravure cylinder line 30, line 40, and Meyer bar) and with diﬀerent coating weight, give the same peel value. Shear resistance tends to be inversely proportional to the coating weight [56]. It is to be mentioned that because of the roughness of the solid components a minimum coating weight may be required in order to achieve a good shear strength. For special applications (e.g., packaging, splicing, or mounting tapes) the high speed application and conformability of the product require a thick adhesive layer (50–90 mm) [127]. In order to achieve high shear values such compositions have to be ﬁlled or crosslinked. It should be mentioned that the coating weight inﬂuences the shear test also. A suitable test for the measurement of shear properties of an adhesive is the thick adherend shear test (TAST) since it is conducted using a tensile testing machine [128]. However, ﬁnite element analysis shows that the stress distribution in the thick adherend shear test is not pure shear. The values of maximum shear strain gained from the thick adherend shear test are not comparable to those obtained from tests in pure shear. The role of the coating weight on the energy dissipation will be discussed in Chap. 6. Influence of Coating Weight on Converting Properties The coating weight strongly inﬂuences the convertability of the laminate. As discussed in detail in [63] printing related product characteristics (shrinkage, lay ﬂat, curling, stiﬀness, and wrinkle build-up), confectioning properties (cutting, die cutting, perforating, embossing, folding, and winding), and dispensing/labeling properties are coating weight dependent. This dependence will be examined in Chap. 7. Here, the inﬂuence of the coating weight on the cold ﬂow of the adhesive will be discussed. The cold ﬂow of the adhesive in laminates is hindered by the marginal interaction of the

Rheology of Pressure-Sensitive Adhesives

57

adhesive with the ‘‘walls’’ of the laminate. It depends on the wetting and anchorage of the adhesive. Because of the local character of this interaction the middle layer displays free ﬂow. For an ideal adhesive, cold ﬂow should be maximum during bonding and minimum during debonding. That means that high loss modulus (peak) at the practical frequency of bonding helps obtain adequate bonding. On the other hand at the highest debonding frequency the elastic behavior of the adhesive (storage modulus) is required. Cutting, slitting, and die cutting are high frequency operations. The surface properties (roughness, porosity, and surface tension) inﬂuence the wetting out, but there exists an indirect inﬂuence of the adhesive on converting properties through the coating weight as well. It is evident that the thickness of the free ﬂowing layer depends on the coating weight and on the thickness of the anchored layer. The anchored layer depends on the coating technology and on the nature of the solid laminate components; the free ﬂowing layer depends on the coating weight (Cw) and coating technology: Cold flow ¼ f ðfree flowÞ ¼ f ðCw , Coating technologyÞ

ð2:49Þ

Therefore, in a ﬁrst approximation, the cuttability depends on the coating weight, as follows: Cuttability ¼ f ðCoating technology=Cw Þ

ð2:50Þ

Figure 2.9 illustrates the dependence of the cuttability on the coating weight (i.e., the cuttability decreases with increasing coating weight). The most important factors inﬂuencing the coating weight are the face stock material, the substrate to be adhered to, the permanent/removable character of the adhesive, and the coating conditions. As discussed in [27], the parameter in Eq. (2.47) describing the dependence of the peel on the coating weight is a function of the adhesive, carrier, substrate, and time/ temperature. Generally it may be stated that the optimum coating weight value depends on the chemical composition and end-use of the PSA. The adhesive performance also inﬂuences the coating weight to be used. The coating weight of selected PSPs is listed in [129]. Solvent-based acrylics are coated for diﬀerent product classes with the following coating weight values: 5–10 g/m2 for protective ﬁlms; 25–40 g/m2 for ﬁlm-based tapes; 50–100 g/m2 for foam tapes; and 20–30 g/m2 for labels and tags. For labels peel is regulated by formulation, not by coating weight. Labels can be considered a special case of PSPs where coating weights are relatively high (above the critical domain). On the other hand for the same product class various

58

Chapter 2

Figure 2.9 Dependence of the cuttability on the coating weight. Cuttability of soft PVC ﬁlm labels: 1) PSA with low PE-peel, medium tack, high cohesion; 2) PSA with high PE-peel, low tack, low cohesion; 3) PSA with low peel, very low tack, high cohesion; 4) PSA with high peel, high tack, low cohesion.

adhesive formulations (with diﬀerent base elastomer and physical status) use diﬀerent coating weights. Protective ﬁlms coated with solvent-based acrylics or rubber resin formulations display a lower coating weight than dispersion coated ones [63]. All these parameters and their inﬂuence on the coating weight will be examined in Chap. 6. 3.2

Influence of the Solid Components of the Laminate

Some PSAs are self-sustaining (e.g., sealant tapes, mounting tapes, selfadhesive protective ﬁlms etc.). Such formulations do not need a solid state carrier material. Their adhesive properties depend on the rheology of the nonreinforced PSA. Most PSPs possess a carrier material (e.g., face stock for labels). Both, the bulk and surface properties of the solid state carrier material (for labels: face stock) inﬂuence the adhesive, converting, and end-use properties of the pressure-sensitive products. The solid laminate components aﬀect the distribution of the applied stresses on the adhesive (Fig. 2.10) and the ﬂow of the liquid adhesive in the laminate.

Rheology of Pressure-Sensitive Adhesives

59

Figure 2.10 The inﬂuence of the solid state laminating components (face stock and release liner) on the active and passive forces in the adhesive’s ﬂow: FA; active forces, compression, and impact stress on the laminate: Fp1; passive forces, resistance against PSA ﬂow, given by the adhesive/face stock interaction: Fp2; resistance against PSA ﬂow, given by adhesive/release liner interaction.

The interaction between the PSAs and the delimiting surfaces acts like a brake or a resistance to ﬂow. The stress distribution depends on the elasticity, hardness, and ﬂexibility of the solid component (i.e., on its bulk properties) and dimensions. Control of the ﬂow is a function of the quality of the delimiting surfaces, that is of the surface properties of the solid components. As a solid component inﬂuencing the adhesive properties the adherent (i.e., the substrate) should be considered as well. In order to elucidate the inﬂuence of each one of the solid components of the PSA label (applied or not) a separate discussion of these components has to be carried out. First the role of the face stock in the design of the label will be examined in order to clarify its inﬂuence on the rheology of the adhesive. The inﬂuence of the face stock material on the rheological characteristics of the adhesive (modulus and viscosity) will be discussed while characterizing the face stock material by its own rheological properties (modulus). The best approach to describe these phenomena is their indirect examination through the adhesive and converting characteristics. The properties of self-adhesive labelstock are primarily concerned with the printability of the face stock, the conversion characteristics of the

60

Chapter 2

Figure 2.11 Face stock inﬂuence on the adhesive and converting properties. The inﬂuence of the stiﬀness, elasticity, roughness, porosity, and surface tension on the face stock directly and through coating weight on the adhesive and converting properties of the label material.

laminate, and end-use adhesive performance. All these characteristics depend on the face stock material used. As shown in Fig. 2.11, the face stock material inﬂuences the properties of the PSA label through its bulk and surface properties. The bulk properties of the face stock material inﬂuence the adhesive and converting properties directly and indirectly. The indirect inﬂuence is exerted by the coating weight. The role of the porosity in the migration is well known. If the adhesive wets the adherent (face stock) surface, its performance is largely inﬂuenced by the bulk properties of the face stock material. The enormous importance of the surface quality of the solid components of the PSA laminate on the properties of the PSA is illustrated by the recent rheological studies concerning high and low surface energy substrates. In the case of low surface energy substrates the rheological approach of the properties is often not valid. The plasticity/elasticity balance of the face stock material, its stiﬀness, and its capability to absorb energy, inﬂuence the adhesive and converting properties of the pressure-sensitive laminate. Influence of the Bulk Characteristics of Carrier Material on the Adhesive Properties of the Pressure-Sensitive Laminate The dependence of the most important adhesive characteristics (peel, tack, and shear) on the bulk properties of the face stock material will be

Rheology of Pressure-Sensitive Adhesives

61

investigated. It should be pointed out that some of the tests of the adhesive properties of PSAs involve deformation of the face stock material (e.g., tack, peel); others (e.g., shear) are characterized by the deformation of the pure adhesive layer only. Thus the bulk properties of the solid components of the pressure-sensitive laminate inﬂuence more the tack and the peel of the adhesive. Peel adhesion depends on the face stock used as well as on testing conditions [44]. This strong dependence is due to the simultaneous deformation of the face stock material and adhesive during peeling. Also, some tack measurements induce face stock straining. In fact most of the tack measurements may be considered as peel, shear, or tensile strength measurements, and therefore they will be discussed in Chap. 6. Face Stock Material and Peel Adhesion. From an energy point of view debonding is associated with energy dissipation. Energy dissipation during debonding is dependent on such parameters as interfacial adhesion, polymer rheology, rate of peel, temperature, angle of peel, web stiﬀness, and coating weight. Interfacial adhesion and web stiﬀness are characteristics of the face stock material. They also inﬂuence the angle of the peel. On the other hand, the peel rate, especially in the starting phase or onset of peeling, or during manual peel, strongly depends on the plasticity and elasticity of the face stock material (Fig. 2.12). Figure 2.12 presents the dependence of the peel on the face stock material for tackiﬁed PSAs dispersions with diﬀerent tackiﬁer levels. As can be seen from Fig. 2.12, the value of the peel is quite diﬀerent for a soft, plastic face stock material. Adhesive formulations display quite diﬀerent peel adhesion values when coated on diﬀerent face stock materials. Thus face stock ﬂexibility and plasticity are the main bulk characteristics inﬂuencing the rheology and the peel. From release force measurements, it is well known that the peel force from a silicone release liner depends on the peel angle p and the peeling rate : Peel force ¼ f ðp , Þ

ð2:51Þ

The peel angle is really a function of the ﬂexibility fFS of the face stock: p ¼ f ðflexibilityÞ

ð2:52Þ

Thus: Peel force ¼ f ðp , fFS Þ

ð2:53Þ

The peel angle is normalized (90 or 180 ) but the real peel angle depends on the ﬂexibility of the material (Fig. 2.13), and its deviation from the theoretical angle is diﬀerent for 90 and 180 peel tests.

62

Chapter 2

Figure 2.12 Dependence of 180 peel adhesion force on the nature of the face stock material. 1) PVC; 2) aluminum.

Figure 2.13

Theoretical and real peel angle during 180 peel measurement.

Rheology of Pressure-Sensitive Adhesives

63

For peel testing at 80 the delaminated length A of the peeled bond is given as a function of the modulus of the face stock Ef, the modulus of the adhesive Ea, the thickness of the adhesive coating ha, and the thickness of the face stock hf [130]: A

qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ 4 ðEf =Ea Þ h2a hf

ð2:54Þ

A more detailed description of the inﬂuence of the face stock elasticity (stiﬀness) on the peel is given in Chap. 6. Stress transfer to the adhesive and from the adhesive is inﬂuenced by its plasticity. The experimental data from Fig. 2.14 illustrate this behavior. Both phenomena—face stock stiﬀness and elasticity (resistance to elongation)—were studied by Gent and Kaang [68]. When adhesive labels (or tapes) are pulled away from a rigid substrate, the force required depends upon both the strength of adhesion and the resistance of the label to stretching [68]. The pull-oﬀ force F may be correlated with the work of detachment Ga and the eﬀective tensile modulus E of the tape and its

Figure 2.14 The inﬂuence of the elasticity/plasticity of the face stock on the peel. A paper label material (A) is being peeled through a 180 angle from a substrate, while the peel stress is being transmitted via diﬀerent materials (B). B1: paper (peel force 7.4 N/25 mm); B2: PET (7.7); B3: soft PVC (6.9); B4: LDPE (6.9).

64

Chapter 2

thickness t. The pull force is found to be neither proportional to Ga as might at ﬁrst be expected, nor is it proportional to (EGa)1/2 as found in many linearly elastic systems. Instead it is found to be proportional to (EG3a )1/2, a result which emerges directly from the analysis as a consequence of the particular relation between the force F and the corresponding elastic displacement of the label where no further debonding occurs. It is to be noted that the values of detachment energy Ga obtained from pull-oﬀ experiments are generally lower than those obtained from peeling experiments. It was stated that: F ð8Ga =3Þ3=4 ðE tÞ1=4

ð2:55Þ

That is, F ¼ f ðt1=4 Þ

ð2:56Þ

A further prediction of the theory is that the product F (where is the detachment angle) will be independent of the stiﬀness of the tape (label). Diﬀerences in Ga from pull-oﬀ and from 90 peel measurements are attributed to additional energy losses (by peeling) due to the severe bending deformations imposed on the label. A possible cause is nonlinear elastic behavior of the label in tension. The eﬀective tensile modulus E at small strains is greater than at large ones. The simplifying assumption of linearly elastic behavior is quite inadequate for ﬁlm labels or for tapes which undergo large deformation as well as plastic yield. The work to bend the label away from the substrate may be important too. In some circumstances this contribution can be both large and strongly dependent upon the magnitude of the peel angle [131]. Later the shaft loaded blister test SLBT of Wan and Mai [132] was used to determine the apparent strain energy release rate (G), for a PSA tape in order to examine the inﬂuence of the solid state carrier material [133]. Samples consisting of stacked ﬁlms (either 1, 2, or 4 plies thick) were tested to determine if the backing’s tensile rigidity had any eﬀect on the measured G. The G values measured from the SLBT were compared to values obtained using the pull-oﬀ test [68]. For high energy aluminum substrates (i.e., for high adhesion values) results from a 90 peel test were an order of magnitude greater than the SLBT, probably due to the plastic deformation of the peel arm. A measure of the fracture toughness of an adhesive bond is the applied or apparent strain energy release G. The expressions used to calculate G are typically derived using linear elastic fracture mechanics (LEFM), that is by assuming that the adherent and adhesive are loaded within the elastic range, except locally at the crack tip,

Rheology of Pressure-Sensitive Adhesives

65

where plastic deformation occurs. In practice adhesives behave viscoelastically or elastic-plastically, and under high stress yielding may occur. For thin ﬁlms the eﬀects of plastic deformation and yielding may be signiﬁcant given their small load bearing capacity, associated with the small thickness, and the relatively strong adhesion [134]. Energy transfer from the face stock to the adhesive depends on the anchorage of the adhesive layer and on the existence of an energy absorbent layer on the face stock materials. Both are surface dependent characteristics, thus the surface of the face stock material will inﬂuence the peel of PSAs (especially for removable and nonpolar adherent applications). Indirectly the anchorage of the PSAs and the existence of diﬀerent adhesive layers on the surface or in the face stock material depend on the migration of the adhesive or adhesive components in the face stock material. This phenomenon depends on the surface and bulk properties of the face stock material. Influence of the Substrate. The nature of the substrate inﬂuences the peel adhesion, and thus the removability of the PSA label. Certain PSAs may be classiﬁed (depending upon the substrate) as removable or permanent adhesives. The most important parameters of the substrate that inﬂuence the removability of the PSA label are its ﬂexibility, its elasticity/plasticity balance, and the anchorage of the adhesive onto the face stock. The peel does not depend on the deformability of the adhesive, but on the ﬂexibility of the face stock and substrate [130]. The deformability of the adherents inﬂuences the stress distribution in the bond [56]. If the adherents are not deformable, the adhesive would be stressed uniformly. If the solid components of the joint are deformable the stress distribution varies along the joint. Mathematically this phenomenon may be described by a factor n, where n is the ratio between maximal and mean stress ( max, ): n ¼ max = ¼

pﬃﬃﬃﬃﬃﬃﬃﬃﬃ pﬃﬃﬃﬃﬃﬃﬃﬃﬃ =2 coth =2

ð2:57Þ

where is given by the following correlation: ¼ ðG l 2 Þ=ðEabÞ

ð2:58Þ

where a is the thickness of the adherent, b is the thickness of the adhesive, G is the modulus of the adhesive, E is the modulus of the adherent, and l is the overlap length. The inﬂuence of the deformability of the adherents on the stress distribution by peeling has also been demonstrated [135]. The adhesive is under tension at the interface and under compression just behind it. The tensile strength is a function of the stiﬀness of the combined adherents and

66

Chapter 2

adhesive, as well as of the thickness of the system. The compressive load appears greatest at 180 peel between ﬂexible and rigid adherents, less at 90 peel under the same circumstances, and the peel force is the lowest when measured between two ﬂexible adherents. Face Stock Material and Shear Strength. There are no experimental data available about the inﬂuence of the bulk properties of the face stock material on the shear resistance. Diﬀerences in shear strength of PSAs coated on diﬀerent face stock materials are due to the surface of these materials. Influence of the Face Stock on the Converting Properties of the Pressure-Sensitive Laminate The most important converting properties of pressure-sensitive labels are the printability and the cuttability. The rheology of the adhesive strongly inﬂuences the cuttability of the label. Next the dependence of the cuttability on the bulk properties of the face stock material will be analyzed, and in particular the inﬂuence of the rheological characteristics of the face stock material. During the die-cutting operation, the mechanical strength of the face stock and release liner materials inﬂuences the distribution of the stresses from the cutting edge to the adhesive layer. Generally, this mechanical resistance is a function of the mechanical properties of the bulk material, and of the dimensions of the face stock and release liner: Rc ¼ f ðK1 K2 Þ

ð2:59Þ

where Rc is the mechanical resistance of the face stock material, K1 denotes material characteristics and K2 material dimensions. In some cases (sheet material) a certain number of superposed laminate sheets are cut during the same operation; thus the distribution of the force may be inﬂuenced by laminate slip (Fig. 2.15). Therefore the laminate strength will depend on the motion of the laminate layers (face stock and release liner respectively), with respect to one another (i.e., on the surface characteristics of the laminate: face stock, release liner) K3: Rc ¼ f ðK1 K2 K3 Þ

ð2:60Þ

where K3 is a surface-dependent factor inﬂuencing the laminate slip. As can be seen from Fig. 2.16, the motion and cold ﬂow of the adhesive will be a function of stress magnitude and distribution, but smearing of the cutting knife and of the cut (side, front) surface also depends on the porosity of this surface.

Rheology of Pressure-Sensitive Adhesives

Figure 2.15

67

The motion of the laminate layers during cutting.

Figure 2.16 Flow and smearing of the adhesive during cutting. (A) through (E) are the diﬀerent steps during the cutting operation.

68

Chapter 2

Thus the cutting behavior and the cuttability C of the laminate will depend on the mechanical properties of the bulk material (face stock, release liner) K1, on the surface properties of these materials K3, and on the dimensions (inﬂuencing the real value of the forces and the cut area) of the solid components of the laminate K2: C ¼ f ðK1 K2 K3 Þ

ð2:61Þ

In order to identify the most important mechanical properties of the bulk material which may inﬂuence the cuttability, the nature of the forces and deformation during cutting have to be examined. Diﬀerent kinds of stresses are present in a material, as demonstrated by creasing and folding paper board (Fig. 2.17) [136,137]. One can conclude that the same stresses (i.e., tensile, compression, and shear) are working when cutting a pressure-sensitive laminate. In fact creasing the face stock constitutes the ﬁrst operation during cutting. Material characteristics related to tensile, compression, or shear resistance of the face stock material will inﬂuence the cuttability. The distribution of the stresses, the uniformity of the force acting on the laminate, and the compression stress character of the ﬁrst cutting step are determined by the rigidity of the face material (Fig. 2.18).

Figure 2.17 Stress distribution during creasing (cutting) of the face stock (label) material. 1) Tensile, 2) compression, 3) shear stresses.

Rheology of Pressure-Sensitive Adhesives

69

Figure 2.18 Stress distribution during cutting as a function of the nature (rigidity) of the solid components of the laminate.

The rigidity (stiﬀness) of the material is a function of its modulus. An incompressible liquid PSA may support a compressive stress well. In the ﬁrst step of the cutting operation the force is acting perpendicularly on the whole laminate. The situation changes only through the concentration of the local stresses, and because of the weakness of the solid material, giving rise to the ﬂexure of the face and shearing of the adhesive. Hence the stiﬀness of the solid laminate components inﬂuences the stress distribution and the creep resistance of the adhesive. The value of the modulus of elasticity E of the face stock material (or/and release liner) inﬂuences the cuttability of the material. Thus: C ¼ f ðEÞ

ð2:62Þ

On the other hand, assuming that the cutting edge (knife) is acting on a partially supported material, ﬂexure of the material occurs ﬁrst (i.e., the ﬂexural modulus of the material inﬂuences the cuttability). In a ﬁrst approximation the cuttability C depends on the tensile, compression, ﬂexural, and shear moduli (El, Ec, Ef, Esh) of the material and the value of these moduli may or may not diﬀer from one another, as a function of the

70

Chapter 2

materials used (e.g., paper, plastic, or metal) and of the practical conditions: C ¼ f ðEi Þ

ð2:63Þ

Cuttability also depends on the material dimensions. It should be mentioned that the stress distribution dependence on the thickness of the face stock material may vary according to the nature of the applied stress. It may be stated that the cuttability improves with material thickness, that is: C ¼ f ðEi , hÞ

ð2:64Þ

where h is the material thickness and is the exponent taking into account the improvement of the cuttability by the increased cut surface for a voluminous (thick) laminate component. For hygroscopic materials like paper, bulk properties may depend on the environmental humidity. Because of the ﬁberlike structure of the material, the surface texture may also change as a function of the humidity. Surface texture inﬂuences the smoothness (friction) of the surface and the smoothness inﬂuences the cuttability. The surface friction of papers increases with increasing air humidity [138]. On the other hand, because of the hygroscopic nature of the face paper, there is a continuous transfer of water between the adhesive and face material, inﬂuencing the composite character of the adhesive and its rheology (water-based adhesives). A more detailed analysis of the parameters inﬂuencing the cuttability will be presented in Chap. 7. Here the inﬂuence of the bulk properties of the face stock material on the distribution of the stresses acting on the laminate, and of the rheology of PSAs on the laminate will be discussed. Influence of Surface Properties of the Carrier Material Coatability is characterized by wetting out and anchorage of the coated adhesive on the carrier material. Both are inﬂuenced by surface polarity and porosity. Figure 2.11 showed that the surface properties of the face stock material inﬂuence the adhesive and converting properties of the pressure-sensitive laminate, either directly or indirectly. The direct inﬂuence is based on the anchorage phenomenon at the surface of the face stock. Indirect inﬂuences are reﬂected by the porosity related adhesive penetration in the face stock and the changes in the coating weight caused by penetration. Special problems are encountered with nonpolar plastics where no chemical aﬃnity exists between the polar PSA and the nonpolar, nonporous carrier surface. For textured ﬁber-like porous surfaces adhesive migration and breakthroughs may improve anchorage but denaturate the top surface of the face stock and at the same time change the coating weight. Fillers or surface-active agents built into the carrier material of protective ﬁlms or tapes can act as release enhancing agents, but they disturb the

Rheology of Pressure-Sensitive Adhesives

71

adhesive contact. Fillers can modify the relaxation spectra of the polymers [139]. On the other hand for special PSPs (e.g., self-adhesive ﬁlms) migration of an adhesive component from the plastic carrier (e.g., polybutylene) due to its incompatibility ensures the adhesive properties of the product [140]. Influence of Face Stock on Migration. It is diﬃcult to store labelstock for any length of time, due to possible bleeding and/or oozing problems. A lot of problems exist with high gloss label material bleedthrough. Adhesive bleedthrough often interferes with the printing. Migration (penetration or bleeding) is due to the diﬀusion of the adhesive into and through the face stock material. This phenomenon depends on the properties of the adhesive and of the face stock material, on the coating technology, and on the processing (storage, converting) conditions: Migration ¼ f ðadhesive, face stock, coating technology, processingÞ ð2:65Þ Migration of the adhesive in the face stock material or of the additives from the face stock material in the adhesive produces changes in the composition of both materials and in their mechanical/rheological properties. In many health care and other applications it is desirable to have a PSA ﬁlm coated onto a porous web. Hot-melt PSAs oﬀer a relatively easy route to coating PSAs onto fabric and nonwoven webs with a minimum of adhesive penetration into the web. The penetration of the adhesive in the porous web can be avoided with a release liner used as an intermediate web surface (transfer coating) for some processes, but during storage of the product diﬀusion in the porous web may occur. Evidently, migration (penetration) of the adhesive into the face stock depends on the adhesive’s nature, but also on the face stock porosity. The dimensions of the pores strongly inﬂuence the penetration. It was demonstrated that pore diameters of 0.01–0.05 mm allow good penetration [141]. Theoretically, penetration under pressure and capillary penetration in paper are diﬀerent phenomena [142]. Penetration through pressure Pp would depend on the pressure p, the radius of the pores r, and the viscosity : Pp ¼ f ð p, r, Þ

ð2:66Þ

Capillary penetration Pc (mainly water penetration) depends on the humidity of the paper H and its pH value: Pc ¼ f ðH, pHÞ

ð2:67Þ

72

Chapter 2

Porosity inﬂuences positively the anchorage of the adhesive through migration. Where pores, crevices, and capillaries are accessible in the surface of the adherent the adhesive will penetrate to some extent, and so increase adhesion. The capillary rise h is a function of the porosity [143]: h ¼ k LV cos =R

ð2:68Þ

where R is the equivalent radius of the capillary, k is a constant, and is the density of the liquid; h reaches a maximum when: 1 LV ¼ ðc þ 1=bÞ 2

ð2:69Þ

where c is the surface tension of the liquid. One can conclude that the dimensions (radius) of the pores, the wettability of the material (surface tension), the humidity of the paper, and the pH of the paper are major factors inﬂuencing the migration. This is also conﬁrmed by the following correlation dL=dt ¼ ð cos rÞ=ð4 LÞ

ð2:70Þ

where dL/dt denotes the penetration rate, is the viscosity of the liquid, r is the capillary radius, is the surface tension of the liquid, is the wetting angle of the capillary wall, and L is the penetration length. Generally, the penetration rate of the adhesive in the face stock depends on the capillary radius (i.e., on the paper density) [144]. Influence of Face Stock Porosity on Coating Weight. It is well known in the ﬁeld of cold-seal adhesives that blocking is a function of the adhesive coating weight and of the face material. For a porous web (paper) a coating weight of about 6 g/m2 is recommended; nonporous face stock (ﬁlm or aluminum) needs no more than 3 g/m2. Similarly the dry coating weight (the weight of the adhesive applied per unit surface area) can vary substantially depending upon the porosity and irregularity of the face and of the substrate surface to which the face material is to be adhered. For instance higher PSA loadings are preferred for adhering porous, irregular ceramic tiles to porous surfaces, while lower adhesive loadings are required to manufacture tapes, ﬁlms, and other articles from relatively nonporous, smooth surface materials such as synthetic polymer ﬁlms and sheets. When the adhesive is applied to nonporous polymeric or metallic face materials, intended for adhesion to nonporous polymeric or metallic surfaces, adhesive coating weights of about 7–70 g of dry adhesive per m2 of treated surface are generally adequate. A good adhesion for tapes manufactured from continuous polymeric materials can usually be achieved with dry adhesive coating weights of about 15–30 g

Rheology of Pressure-Sensitive Adhesives

73

of adhesive per m2, while coating weights of about 30–70 g/m2 of adhesive are usually employed for the manufacture of paper-backed tapes, such as masking tapes. In order to avoid a decrease of the coating weight due to the porosity of the face stock material or to prevent plasticizer migration, it is also possible to apply a barrier coating to the paper [145]. Influence of the Face Stock on the Adhesive Choice. Face stock sensitivity towards PSAs requires an adequate choice of the adhesive system. Shrinkage of soft PVC face stock requires the use of shrinkage resistant adhesive systems [146]. To this end crosslinked acrylics are generally employed. On the other hand, plasticizers in the vinyl ﬁlm can have a pronounced eﬀect on the adhesive properties during the aging process [147]; plasticizer resistant adhesives have to be selected. These examples show that the face stock nature and its sensitivity towards certain adhesives may inﬂuence the choice of the adhesive and thus the rheology of the adhesive layer. Influence of the Face Stock on the Coating Technology. In the ﬁrst instance the mechanical and chemical characteristics of the face stock materials determine the choice of the coating technology, the coating process (direct coating/transfer coating), use of cold/hot adhesives (solventbased/hot-melt) or chemically aggressive or inert adhesive systems (solventbased/water-based), and coating machines (metering device, drying system, web transport system, etc.). The inﬂuence of the face stock material on the choice of the coating technology is illustrated in Fig. 2.19. As can be seen in Fig. 2.19 the web is usually supported when making paper-based laminates via transfer coating, using a gravure roll as a metering device, and solvent-based, water-based, or hot-melt PSAs. For ﬁlm coating a Meyer bar and/or gravure roll is suggested, usually using water-based PSAs. Bonding (or anchorage) supposes the coating of a substrate (or face stock) material with the liquid adhesive, that is the spreading and wet-out of the adhesive on the surface of the solid material. After wetting the surface the adhesive penetrates into the cavities of the solid material and physical and chemical contact points are formed. Both phenomena, bonding and debonding, depend on the quality of the surface. Wetting Out as a Function of the Face Stock Surface. Earlier the impor-tance of the wetting out, and the parameters inﬂuencing it were discussed, considering in particular the liquid component (adhesive). Next the characteristics of the solid components inﬂuencing the wet-out are examined. It was shown earlier that the bulk properties of the face stock materials on the one side, and the surface properties on the other side are determinant for the properties of the pressure-sensitive laminate. The surface quality inﬂuences the wetting out of the adhesive. Wetting the solid surface by the adhesive is an

74

Chapter 2

Figure 2.19 The inﬂuence of the face stock material on the coating technology. Dependence of the coating technique (direct/transfer), of the nature of the PSA (water-based, solvent-based, hot-melt), of the coating device, and of the drying technology used, on the nature of the face stock materials (paper/ﬁlm).

initial necessary condition for the anchorage of this adhesive to the solid surface (e.g., face stock material) [54], and is a function of the surface tension of the solid and liquid. Liquids, and for that matter solids, at the boundary between two phases (liquid/paper or liquid/solid) possess properties which are diﬀerent from those within the mass of the liquid itself. This diﬀerence results from an imbalance between the attractive faces of the surface layer of molecules and the molecules contained within the liquid (or solid) [148]. The molecules on the surface layer compensate for charge imbalance by pulling themselves close together, thus creating the phenomenon of surface tension. In the most extreme case when the surface tension of the adhesive diﬀers greatly from the initial surface tension of the ﬁlm or paper (face stock or liner) ﬁsh eyes and cratering will result (when the face stock surface tension is less than the adhesive surface tension). To obtain optimal wet-out the surface tension of the adhesive must be equal to or less than the critical surface tension of the face stock or liner (a contact angle of 0 ) (Fig. 2.6). As can be seen in Fig. 2.6, a high surface tension liquid beads up on nonpolar, low energy surfaces [109]. Table 2.2

Rheology of Pressure-Sensitive Adhesives Table 2.2

75

Surface Energy Values of Common Face Stock/Substrate Materials

Material

Surface Energy, mN/m

References

Aluminum Cellulose acetate Cellulose triacetate Glass Iron Polyamide

45.0 39.0 29.0 47.0 46.0 36.0 40.0–46.0 46.0 33.0–46 42.0–48.0 43.0 32.0 26.0 46.0 14.1 29.0–31.0 33.0 31.0 37.0–43.0 33.0 16.0 27.0 31.0 39.0 29.8 33.0–44.0 38.0 28.0–30.0 29.0 30.0 31.0 33.0–43.0

109,148 151 142–154 109,148 148 155 156 151 148 53,109 151–154 151 154 154 100,152,153, 53,152–156 52,151–156 151 109, 157 151 53,149 151 151 151 152,154 109,141 151 53 151,155 127,151,151 148 53 53,101, 152–154 156 151 141 35,151–153 151 151 152–154

Polyamide

(PA-6) (PA-11) Polybutadiene Poly(butyl methacrylate) Polycarbonate Polydimethylsiloxane Poly(ethylene) (HDPE) (LDPE) Poly(ethyleneterephthalate) P(EVAc) Poly(hexaﬂuoropropylene) Poly(isobutylene) Polyisoprene Poly(methyl acrylate) Poly(methyl methacrylate)

Poly(propylene)

Poly(styrene)

Poly(tetraﬂuoroethylene) Poly(triﬂuoroethylene) Poly(vinyl acetate)

33.0 30.0–35.0 33.0–35.0 18.5 22.0 25.0 24.9

(continues)

76 Table 2.2

Chapter 2 Continued

Material

Surface Energy, mN/m

References

Poly(vinyl alcohol) Poly(vinyl chloride)

37.0 35.0–41.0 36.0 39.5 33.0–38.0 39.0–40.0 41.9 35.0–37.0 29.0–31 40.0 20.0 18.0–22.0 45.0 60.0

151 53, 157 147 148 148 53,109 151 151 151 53 152 153 109 109

Hard Soft Poly(vinylidene chloride) Silicones Steel, stainless Steel, chrome plated

lists the surface energies of materials commonly used as face stock and substrate. Situations can also arise where the adhesive is absorbed on the surface of the face stock (e.g., ﬁlm) by chemisorption, with a zero contact angle (good wet-out). However, the mass of the liquid does not bond to this chemisorbed layer [148]. Except for hot-melt PSAs (and some warm coated masticated rubberbased formulations used for medical tapes [149]), PSAs are coated onto the face stock (or release liner) as liquids. The wetting out of liquid PSAs depends on the quality of the face stock surface, on the surface tension, and on the roughness of the surface. The dependence of the wetting out on the roughness is a general phenomenon. The wet-out time on cardboard (for an aqueous dispersion) may vary between 0.1–0.4 sec, depending on the roughness of the surface [142]. Moreover, Bikermann [150] stated that adhesion is due to the inherent roughness of all surfaces. Molecular forces of attraction cause an adhesive to wet and spread on the surface. But once achieved the mechanical coupling between the adhesive and the inherently rough surface is more than enough to account for the bond strength. Influence of Carrier on the Anchorage of PSAs. Interfacial adhesion is a prerequisite for bonding and debonding. Stable bonds are formed when there is a strong interaction between the adhesive and the face stock (or substrate). The diﬀerent types of possible interactions include van der

Rheology of Pressure-Sensitive Adhesives

77

Waals’ forces, polar interactions, electrostatic interactions, and chemical bonding. Eﬀective bonding between the face material and the adhesive is critical to the performance of PSAs; given an adhesive with internal integrity, the adhesive properties exhibited by that system will be proportional to the aﬃnity of the adhesive for the face stock material [64]. A useful approach for evaluating the compatibility of the adhesive and face stock is to measure the free energy of these surfaces. Theoreticians have formulated adhesion models based on the thermodynamic or reversible work of adhesion WA. This is the change of free energy when materials are brought into contact, which represents the amount of energy released under reversible or equilibrium conditions to disrupt the interface. The expression is given by the following equation: WA ¼ SV LV SL

ð2:71Þ

In practical terms the face stock material and the adhesive should have comparable surface energy properties. Solvent-based PSAs consist of a polymer dissolved in a solvent. The solvent-based PSA adhesion or anchorage on plastic ﬁlms essentially concerns the study of how polymers and solvents interact. In order to explain the phenomena which occur during the coating of solvent-based adhesives the nature of this interaction between polymers and solvents is to be examined. Solvent molecules are attracted to other solvent molecules. Similarly, polymer chains possess attractive forces within a chain, and from one chain to another. A polymer dissolves in a solvent when the net eﬀects of the solvent/solvent and polymer/polymer attractive forces are reduced. Hildebrandt [158] deﬁned a solubility parameter as a measure of the internal attractive forces of the solvent, deﬁned as heat of evaporation EV per unit molecular volume VM; is a measure of the energy density of the solvents: 2 ¼ EV =VM

ð2:72Þ

Hansen et al. [159,160] proposed an alternative to the single valued solubility parameter, namely: 2 ¼ 2d þ 2p þ 2h

ð2:73Þ

where d is the attractive molecular force from temporary dipole formations, p is the attractive force from dipole/dipole interactions, and h is the attractive force from hydrogen bonding. This concept allowed for the weighted eﬀects of the highly polar and hydrogen bonding solvents.

78

Chapter 2

Polymers, unlike solvents, have a range of acceptable solubility parameters due to the eﬀects of molecular weight distribution, areas of crystallinity, diﬀerent monomer units on the same chains, etc. Hansen [159,160] developed a method for rating the solubility parameter ranges for polymers using the three-component parameter system. One can then deﬁne the solubility parameter range of a polymer in terms of a three-dimensional volume. Then any solvent or solvent blend with a solubility parameter that falls within the described volume will dissolve the polymer. For our purposes, an adhesive/solvent blend that falls within the solubility parameter area of a plastic ﬁlm will develop good adhesion or anchorage. For simplicity, one can make a good approximation of the adhesive/substrate or face material interaction using a two-dimensional graph. Carrier Surface and Removability. The nature of the face stock inﬂuences the peel and thus the removability of a PSA label. Due to the strong inﬂuence of the carrier material some PSAs may be classiﬁed (depending upon the face stock) as removable or permanent adhesive. The main criteria for remova-bility are tack, peel, cohesion, and anchorage. The most important parameters of the face stock inﬂuencing the removability of the PSA label are its ﬂexibility and the anchorage (adhesion) of the adhesive to the face stock. The inﬂuence of the carrier surface on the anchorage was discussed above. The peel does not depend on the deformability of the adhesive only, but also on the ﬂexibility of the face stock [127,44]. The inﬂuence of the bulk properties of the face stock on the peel, and the removability will be examined in Chap. 6. The inﬂuence of the surface properties of the face stock on the removability of PSAs will be discussed next. An adequate anchorage of the PSAs on the carrier imparts good removability from the adherent. Some special adhesives display built-in release from untreated polypropylene. This means that the anchorage on treated polypropylene is higher than on the untreated one, and thus no need for release coating exists when coated on the corona-treated side of polypropylene tape [157]. For tapes and protective ﬁlms the back side of the product can work with supplemental release coating or, with an appropriate carrier material with low level of adhesivity (e.g., polyoleﬁns) as an uncoated liner [161]. Biaxially oriented multilayer PP ﬁlms for adhesive coatings have been manufactured to improve their adhesion to the adhesive coating by mixing the PP with particular resins. The resins add up to 25% by weight. The special importance of the anchorage as the main parameter for removability is illustrated for protective ﬁlms. As discussed in detail in [27] the use of a primer improves the anchorage of the adhesive in such a manner that for primed coatings a higher coating weight may be used, allowing the same removability.

Rheology of Pressure-Sensitive Adhesives

79

Energy dissipation during debonding depends on the interfacial adhesion [109]. Interfacial adhesion is a function of the surface quality. In the case of removable adhesives high rate peel forces must be dampened by high energy dissipation (i.e., good anchorage of the adhesive on the face material is needed). Adhesion to paper and stainless steel is greater than the anchorage to polyester ﬁlm; therefore the bond fails due to a breakdown within the adhesive layer when paper is used; when polyester ﬁlm is used it peels away leaving the adhesive layer virtually intact on the steel plate. On the other hand, peel characteristics change on plasticized PVC [162]. A thermodynamic approach considers adhesion between two materials as the result of two diﬀerent interfacial interactions, namely a dispersive (Id) one and a polar (Ip) one [163]. The reversible adhesive work Wh is described as the sum of these two types of physical interactions: Wh ¼ Id þ Ip

ð2:74Þ

In addition to these physical interactions there exist some chemical interactions as well. Both physical and chemical interactions depend on the quality, polarity, and chemical aﬃnity of the face stock (or adherent) material. In general a peel energy measurement yields a greater value than the corresponding reversible work. On the other hand [163] peel energy depends both on macroscopic factors of energy dissipation f(R) and on the molecular factors of energy dissipation g (Mc): PF ¼ Wh gðMcÞ f ðRÞ

ð2:75Þ

where PF is the peel energy and Wh is the reversible work of adhesion. Regarding the role of the macroscopic factors, the dependence of the dissipation energy (peel) on the polymer thickness has been shown. Polymer thickness implies coating weight, and as shown earlier, surface porosity inﬂuences the coating weight. These results refer to hot-melt PSAs, but one can admit their general validity for the PSA domain. The inﬂuence of the anchorage on the peel is illustrated by the use of polyester (PET) as standard face stock material for PSA tests. Paper diﬀers according to the manufacturing conditions and storage conditions. Therefore a dimensionally-stable material (e.g., PET) is used as standard face stock material in laboratory tests. Unfortunately, it is diﬃcult to wet [164] and anchorage onto PET remains poor. Thus it was observed in many cases [165] that if the adhesive to be tested was applied on a polyester ﬁlm, the ﬁlm peels away, leaving the adhesive layer initially intact on the steel plate; however, if paper is used the bond fails due to breakdown within the

80

Chapter 2

Table 2.3

Dependence of the Peel on the Face Stock Material 180 Peel Adhesion on PMMA at Room Temperature, N/25 mm

Coating Weight, g/m2 2.0 2.5 3.0 3.5 4.0 4.5 5.0

White

Transparent

0.89 1.04 1.19 1.34 1.50 1.65 1.81

0.62 0.87 1.02 1.17 1.32 1.47 1.62

Face stock: 70-mm, blown, low density polyethylene film, white and transparent. Adhesive: Crosslinked water-based acrylic PSA.

adhesive layer (adhesive transfer). Generally, peel tests using PET face stock give lower values than the same tests using paper. The data from Table 2.3 illustrate the inﬂuence of the face stock material on the peel (removability). For removable PSAs the adhesive must adhere better to the face stock material than to the substrate. This adherence or anchorage may generally be improved using primers [166–168,27]. The inﬂuence of the substrate surface quality on the peel was studied indirectly as the separation energy dependence on the surface tension of the substrate [16]. Converting Characteristics of the Pressure-Sensitive Laminate. It is evident that the surface properties of the face stock material inﬂuence its converting characteristics, as well as the converting characteristics of the pressure-sensitive laminate. Convertability of the face stock material means in a ﬁrst approximation its coatability (printability). Convertability of the pressure-sensitive laminate includes its printing ability, confectioning characteristics (cuttability, die-cuttability, perforating, embossing, and folding and winding) and its (end-use related) dispensing and labeling ability. The distribution of forces during the cutting operation depends on the slip of the laminate layers. The surface quality and smoothness of the face stock surface inﬂuence the cuttability [see Eq. (2.60), parameter K3]. On the other hand surface smoothness inﬂuences the anchorage of the adhesive (i.e., its ﬂow between the ‘‘walls’’ of the laminate). Cuttability increases with the anchorage of the adhesive and decreases with the smoothness of the face stock. a C ¼ f ð1=Sm Þ

ð2:76Þ

Rheology of Pressure-Sensitive Adhesives

81

where Sm denotes smoothness and a is an exponent taking into account the dependence of the cuttability on external slip surfaces (between various laminates) and internal ﬂow surfaces (enclosed within a laminate). Face Stock Geometry (Label Thickness) and Pressure-Sensitive Properties From labeling practice it is apparent that the label geometry inﬂuences the adhesive properties of the label. Generally it is diﬃcult to ﬁnd suitable PSAs for labels to be applied around sharp corners [169]. The label thickness inﬂuences the peel properties, whereas peel adhesion depends on the face thickness [170]. The inﬂuence of the thickness of the face stock material is given by the change in ﬂexibility. The ﬂexural resistance depends on the width b and thickness h of the sample (moment of inertia). Flexural resistance inﬂuences the peel angle. In practice, however, the peel angle is diﬀerent from the angle theoretically supposed (90 or 180 ). The face stock (label) form (width) also inﬂuences the peel force [171], and peel adhesion increases linearly with increasing label width. It must be mentioned that the global geometry of the pressure-sensitive laminate must be taken into account for convertability tests (i.e., the whole thickness of the adhesive/ solid component sandwich). Similar to the formula given by Whitsitt [172] for corrugated board construction, where the stiﬀness S is a function of the elastic modulus of the liners, t is the liner thickness, and H is the combined board caliper, one can write: S ¼ ðE tH 2 Þ=2

ð2:77Þ

and thus the global laminate thickness inﬂuences the stiﬀness related properties (peel, tack, cuttability) of pressure-sensitive laminates. Thickness changes of 4–5% for paper may give 15% changes of the ﬂexural resistance [173]. There exists an exponential dependence between thickness and the ﬂexural resistance (exponent ¼ 2.5). The ﬂexural resistance is not a material characteristic, but a result of the construction of the laminate. The eﬀect of the face stock thickness on the peel was studied [174]; peel adhesion increased up to a limit with the thickness of the face stock (ﬁlm). Especially at higher peeling rates the peel value increases with the thickness of the face stock ﬁlm used. 3.3

Influence of the Composite Structure

The structure of the PSA label inﬂuences the rheology of the bonded adhesive. Generally the continuous/discontinuous character of the PSAs, the symmetry of the build-up, and the multilayer structure of the PSA sandwich

82

Chapter 2

determine the adhesive and rheological properties of the bonded PSA. As shown earlier the pull-oﬀ force of a label depends on the elastic modulus E, width w, and thickness t of the label [Eq. (2.55)]. Experimental values of F were plotted against N1/4 where N is the number of layers of tape (label) applied on top of another and pulled away together. The eﬀective label thickness was proportional to N. In practice if the tape (or ﬁlm label) face material stretches so that the detachment angle becomes increasingly large, then two or more ﬁlm layers should be applied [68]. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

W. Hoffmann, Kautschuk, Gummi, Kunststoffe, (9) 777 (1985). S. W. Medina and F. W. Distefano, Adhesives Age, (2) 18 (1989). A. Zosel, Colloid Polymer Sci., 263, 541 (1985). Adhesives Age, (9) 37 (1988). A. S. Lodge, Elastic Liquids, Academic Press, London and New York, 1964, p. 72. D. Satas, Adhesives Age, (8) 28 (1988). S. G. Chu, Viscoelastic Properties of PSA, in Handbook of Pressure Sensitive Technology (D. Satas, Ed.), Van Nostrand-Rheinhold Co., New York, 1988. J. Ferry, Viscoelastic Properties of Polymers, 2nd Edition, J. Wiley and Sons, New York, 1970. A. W. Bamborough, 16th Munich Adhesive and Finishing Seminar, 1991, p. 96. Adhesives Age, (11) 40 (1988). I. Benedek, Development and Manufacture of Pressure-Sensitive Products, Marcel Dekker, New York, 1999, Chapter 1. J. P. Keally and R. E. Zenk, (Minnesota Mining and Manuf. Co., USA), Canad. Pat., 1224.678/10.07.82 (US Pat. 399350). A. S. Rivlin, Paint Technology, (9) 215 (1944). M. Sheriff, R. W. Knibbs and P. G. Langley, J. Appl. Polymer Sci, 17, 3423 (1973). P. J. Counsell and R. S. Whitehouse, in Development in Adhesives (W. C. Wake, Ed.), Vol. 1, Applied Science Publishers, London, 1977, p. 99. A. Zosel, Adha¨sion, (3) 17 (1966). J. Johnston, Adhesives Age, (12) 24 (1983). R. Bates, J. Appl. Polymer Sci., 20, 2941 (1976). W. Retting, Kolloid Zeitschrift, (210) 54 (1966). A. Midgley, Adhesives Age, (9) 17 (1986). K. McElrath, Coating, (7) 236 (1989). J. Class and S. G. Chu, Org. Coat. Appl Polymer Sci., Proceed., 48, 126 (1989). C. A. Dahlquist, Tack, in Adhesion Fundamentals and Practice, McLaurin and Sons Ltd., London, 1966. C. A. Dahlquist, in Handbook of Pressure Sensitive Adhesive Technology (D. Satas, Ed.), Van Nostrand-Rheinhold Co., New York, 1988, p. 82.

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37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52.

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84

Chapter 2

53. 54. 55. 56.

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57.

58.

59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77.

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82.

83. 84. 85. 86. 87.

88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. 101. 102. 103. 104. 105. 106.

85

Jpn. Patent A 8231792; in P. Gleichenhagen, E. Behrend and P. Jauchen (Beiersdorf A. G., Hamburg, Germany), EP 0149135B1/24.07.87. Kautschuk, Gummi, Kunststoffe, (6) 556 (1977). G. Menges and V. Lackner, Kunststoffe, 82 (2) 106 (1992). H. Schuch, ‘‘Anwendung einer Mischungsregel fu¨r die dynamische Viskosita¨t von Polymerschmelzen,’’ German Rheological Society, Annual Meeting, Erlangen,Germany, 27 April 1987; in Kautschuk, Gummi, Kunststoffe, 40 (9) 860 (1987). C. Donker, R. Ruth and K. van Rijn, ‘‘Hercules MBG 208 Hydrocarbon Resins: A New Resin for Hot Melt Pressure Sensitive (HMPSA) Tapes,’’ 19th Munich Adhesives and Finishing Seminar, 1994, p. 64. Adhesives Age, (10) 24 (1977). Adha¨sion, (9) 352 (1966). E. H. Otto, 4th PTS Adhesive Seminar, Munich, Germany, PTS Reports, Nr. 03, 1984, p. 39. J. B. Bonnet, M. J. Wang, E. Papirer and A. Vidal, Kautschuk, Gummi, Kunststoffe, (6) 510 (1986). P. A. Mancinelli, J. A. Schlademann, S. C. Feinberg and S. O. Norris, ‘‘Advancement in Acrylic HMPSAs via Macromer-Monomer Technology,’’ PSTC XVII Technical Seminar, May 4, 1986, Woodsfield Schaumburg, IL; in Coating, (1)12 (1986). Coating, (6) 198 (1984). D. Hadjistamov, Farbe und Lack, (1) 15 (1980). Kautschuk, Gummi, Kunststoffe, (6) 545 (1987). N. Hirai and H. Eyring, J. Polymer Sci., 37, 51 (1959). A. K. Doolittle, J. Appl. Physics, 23, 236 (1958). J. D. Ferry and E. R. Fitzgerald, Proc. 2nd International Conf. Rheology, Butterworth, London, 1953, p. 140. T. Timm, Kautschuk, Gummi, Kunststoffe, (1) 15 (1986). H. A. Schneider and H. J. Cantow, Polymer Bulletin, (9) 361 (1983). Kautschuk, Gummi, Kunststoffe, (9) 31 (1985). A. N. Gent and A. J. Kinloch, J. Polymer Sci., A-2 (9) 659 (1971). Kautschuk, Gummi, Kunststoffe, (8) 34 (1987). I. Benedek, Pressure-Sensitive Formulation, VSP, Utrecht, 2000, p. 24. I. Benedek, Pressure-Sensitive Formulation, VSP, Utrecht, 2000, Chapter 2. K. W. Gerstenberg, Corona Treatment for Wetting and Adhesion on printed Materials, European Tape and Label Conference, Brussels, p. 173, 1994. I. Benedek, Development and Manufacture of Pressure-Sensitive Products, Marcel Dekker, New York, 1999, p. 487. Adhesives Age, (3) 36 (1974). P. Tkaczuk, Adhesives Age, (8) 23 (1988). F. Fikentscher, Coating, (4) 86 (1972). ‘‘Wiederablo¨sbare druckempfindliche Kleber und Schutzfilme,’’ Duro-Tak, Specialty Adhesives, Product Data, 8/89, p. 9, National Starch and Chemical B. V., Adhesives Division, Zutphen, The Netherlands.

86 107.

108. 109. 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. 122. 123.

124. 125. 126. 127. 128.

129. 130. 131. 132. 133.

134.

Chapter 2 A. Fau and A. Soldat, Silicone Addition Cure Emulsions for Paper Release Coating, in TECH 12, Advances in Pressure Sensitive Tape Technology; Technical Seminar Proc., Itasca, IL, 3–5 May, 1989, p. 7. Coating, (9) 340 (1995). A. C. Makati, Tappi J., (6) 147 (1988). D. W. Mahoney and J. W. Hagan, PSTC Proceedings, Pressure Sensitive Tape Council, Glenview, IL, 1982, p. 82. Coating, (2) 39 (1970). Adha¨sion, (10) 273 (1975). J. Hausmann, Adha¨sion, (4) 21 (1985). H. Hadert, Coating, (2) 35 (1985). S. E. Orchard, J. Oil, Lab. Chem. Assoc., 44, 618 (1961). C. P. Kurzendo¨rfer, T. Altenscho¨pfer and H. J. Volker, Henkel Referate, (20) 33 (1984). I. Benedek, Development and Manufacture of Pressure-Sensitive Products, Marcel Dekker, New York, 1999, p. 361. I. Benedek, Pressure-Sensitive Formulation, VSP, Utrecht, 2000, p. 37. H. Hardegger, Coating, (6) 193 (1992). J. Voges, Klebstoffrohstoffe, Beschichtungstechnikum, BASF, Ludwigshafen, Germany, 08.01.1993. H. D. Patermann, Coating, (6) 224 (1988). B. W. McMinn, W. S. Snow and D. T. Bowmann, Adhesives Age, (12) 34 (1995). M. Giri, D. Bousfield and W. N. Unertl, ‘‘Adhesion Hysteresis in Viscoelastic Contacts,’’ Proc.24th Annual Meeting of Adh. Soc., Feb. 25–28, 2001, Williamsburg, VA., p. 15. C. K. Hui, J. M. Baney and E. J. Kramer, Langmuir, (14) 6570 (1988). R. A. Schapery, Intl. J. Fracture, (11) 141 (1975). I. Benedek, Development and Manufacture of Pressure-Sensitive Products, Marcel Dekker, New York, 1999, pp. 275, 294–296. I. Benedek, Development and Manufacture of Pressure-Sensitive Products, Marcel Dekker, New York, 1999, p. 481. R. D. Adams, R. Thomas, F. J. Guild and L. F. Vaughan, ‘‘Thick Adherend Shear Tests,’’ Proc. 24th Annual Meeting of Adh. Soc., Feb. 25–28, 2001, Williamsburg, VA., p. 59. I. Benedek, Development and Manufacture of Pressure-Sensitive Products, Marcel Dekker, New York, 1999, p. 267. J. Wilken, Ally. Papier Rundschou, (42) 1490 (1986). A. N. Gent and G. R. Hamed, J. Appl. Polymer Sci., 21, 2817 (1977). Kai Tak Wan and Yiu-Wing Mai, International Journal of Fracture, 74, 181, 1995. E. O. Brien, T. C. Ward, S. Guo and D. Dillard, ‘‘Characterizing the Adhesion of Pressure-Sensitive Tapes Using the Shaft Loaded Blister Test,’’ Proc. 24th Annual Meeting of Adh. Soc., Feb. 25–28, 2001, Williamsburg, VA., p. 113. Yeh-Hung Lai and D. A. Dillard, J. Adhesion, 56, 1996.

Rheology of Pressure-Sensitive Adhesives 135. 136. 137. 138. 139. 140. 141. 142. 143. 144. 145. 146. 147. 148. 149. 150. 151. 152. 153. 154. 155. 156. 157. 158. 159. 160. 161. 162. 163. 164. 165. 166. 167. 168. 169.

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Adhesives Age, (6) 32 (1986). J. D. Hine, Verpackungsrundschau, Technisch wissenschaftliche Beilage, 9–15 (1964). H. Grosmann, Verpackungsrundschau, (4) 1799 (1986). Ally. Papier Rundschou, (42) 1152 (1988). V. I. Pavlov, T. P. Muravskaya, R. A. Veselovskiy and M. T. Stadnikov, Kompoz. Polym. Mater., 37,14 (1988). I. Benedek, Development and Manufacture of Pressure-Sensitive Products, Marcel Dekker, New York, 1999, p. 346. Adha¨sion, (2) 43 (1977). E. Brada, Papier und Kunststoffierarbeitung, (5) 170 (1986). J. Skeist, Handbook of Adhesives, 2nd Edition, Van Nostrand-Rheinhold Co., New York, 1977. J. Wilken, Papiertechnische Rundschau, (4) 4043 (1988). E. Park, Paper Technology, (8) 14 (1988). R. Lowman, Finat News, (3) 5 (1987). R. A. Lombardi, Paper, Film and Foil Converter, (3) 76 (1988). J. Pennace, Screen Printing, (7) 65 (1988). I. Benedek, Development and Manufacture of Pressure-Sensitive Products, Marcel Dekker, New York, 1999, p. 324. J. J. Bikermann, J. Colloid Sci., (2) 174 (1947). Softal Electronic, Report, (107) 1 (1996). B. Martens, Coating, (6) 187 (1992). H. P. Seng, ‘‘Printing Properties of PP-Tapes,’’ European Tape and Label 146 Conference, 28.04.1993, Brussels, Belgium. Softal Electronic, Report, (102) 1 (1996). Deutsche Papierwirtschaft, (1) IV (1988). U. Zorll, Chem. Ing. Tech., 60 (3) 162 (1988). Solvay & Cie, Beschichtung von Kunststoffolien mit IXAN, WA, Prospect Br.1002d-B-0, 3-0979. J. Hildebrandt, J. M. Prausovitz and R. L. Scotty, Regular and Related Solutions, Van Nostrand-Rheinhold Co., New York (1970). C. M. Hansen, J. Paint TechnoL, (33) 104 (1967). C. M. Hansen and K. Skaarup, J. Paint. Technol., (39) 511 (1967). Vorwerk & Sohn GmbH & Co KG, Wuppertal, DE 4040917/02.07.1992. Adhesives Age, (3) 36 (1986). H. K. Mu¨ller and W. G. Knauss, Trans. Soc. Rheol, 15, 217(1971). J. Lin, W. Wen and B. Sun, Adha¨sion, (12) 21 (1985). British Petrol, Hyvis, Prospect (1985). E. Djagarowa, W. Rainow and W. L. Dimitrow, Plaste u. Kaut., (2) 100 (1970); in Adha¨sion, (12) 363 (1970). Adha¨sion, (1/2) 27 (1987). Coating, (4) 123 (1988). Cham Tenero, 2nd International Cham-Tenero Meeting for the PressureSensitive Materials Industry, 1990, Locarno.

88 170. 171. 172. 173. 174.

Chapter 2 A. W. Aubrey, G. N. Welding and T.Wong, J. Appl. Chem., (10) 2193 (1969). J. R. Wilken, apr, (5) 122 (1986). W. J. Whitsitt, Tappi J., (12) 163 (1988). G. Renz, apr, (24–25) 960 (1986). J. Sehgal, Fundamental and Practical Aspects of Adhesive Testing, Adhesives 85; Conference Papers, Sept. 10–12, 1985, Atlanta.

3 Physical Basis for the Viscoelastic Behavior of Pressure-Sensitive Adhesives It was shown earlier that PSAs are required to bond rapidly to a variety of substrates under conditions of low contact pressure and short contact time. This characteristic feature of a PSA is called tack. During bond formation contact on a molecular dimension between the adhesive and the adherent is established, in isolated points of the geometrical contact area, the number and size of which increase with the contact time through deformation and ﬂow, as well as by wetting. Tack is a characteristic of amorphous high polymers above the glass transition temperature (Tg) only. In terms of physical properties the Tg represents the temperature range through which the polymer changes from a hard, glassy state into a liquid, rubber-like state. In general a polymer with low viscosity (low Tg) will be able to wet a substrate surface and establish intimate contact with the adherent. An increase in Tg will lead to a stiffer polymer, decreased wettability, and most likely a decline in adhesive properties. The adhesive polymer must however possess sufficient cohesive strength to resist further flow and subsequent deformation. The proper flow of the adhesive ensures quick coverage of the substrate through wetting, but a solid-like response to applied stresses. Efficient wetting implies a high deformability (i.e., a low modulus), whereas a solid-like response to stress means low deformability (i.e., a high modulus). Thus one can postulate: Wetting out ¼ f ðE 1 Þ

ð3:1Þ

End-use ¼ f ðEÞ

ð3:2Þ

89

90

Chapter 3

It can be concluded that the viscoelastic flow properties of PSAs materials are characterized by the value of the elastic modulus E and the Tg. The interdependence between the macromolecular structure and adhesive properties has been studied by means of rheology since the mid-1960s by Hoechst [1]. The interdependence between Tg and auto-adhesion has been investigated. Investigations of the viscoelastic properties of many commercial labels and tapes showed that Tg and E at the application temperature are the most important features for a good PSA performance. The Tg may be measured from the peak of the tan curve (see Chap. 2). Several methods are used to determine the glass transition of a polymer material; the most commonly used ones are mechanical/rheological, calorimetric, dilatometric, and dielectric methods. The calorimetric method has gained wide acceptance; the most common instrument used is the differential scanning calorimeter (DSC).

1

THE ROLE OF THE Tg IN CHARACTERIZING PSAs

Adhesive bonding is only possible if the ﬂuidity of the materials involved can ensure enough contact areas, in which chain segments would diﬀuse into each other. In general, the Tg is one of the most important parameters in order to determine the minimum usage temperature for polymeric materials; it is in fact the temperature below which large scale molecular motion does not occur. Below this temperature polymers become glassy and brittle, and develop a high modulus; above this temperature the polymer behaves like a more or less rubber-like elastic ﬂuid. Its tensile strength and modulus decrease above the Tg (i.e., the solid begins to liquefy). Polymers for use in adhesives must act like a continuous, tacky, and elastic layer. A continuous adhesive layer is formed by coalescence; the real area of contact of an elastic ﬁlm on a rough surface depends on two parameters: the elastic modulus at the bonding frequency (G0 ) and the relaxation properties of the polymer which govern the change of the real area of contact; tack is the result of a bonding (viscous ﬂow, wetting out, elastic deformation) and debonding (deformation and failure) process, where elasticity and viscoelasticity refer to energy storage and dissipation. One of the contributing mechanisms to polymer adhesion is said to be diﬀusion of chain elements across the polymer/substrate interface. The concept makes sense when polymers in close contact are above their respective Tg. During the stage of adhesive bonding the molecular mobility of the PSA polymer contributes to the process of polymer relaxation, which in turn accounts for the increase of the strength of adhesive joints

Physical Basis for the Viscoelastic Behavior of PSAs

91

with PSA-substrate contact times. All these requirements imply ﬂuidity of the adhesive at operating temperatures above the Tg. On the other hand the internal strength of the adhesive (i.e., the cohesion) must be high. The break energy of the polymers depends on their molecular motion mechanism. This molecular motion is a function of the Tg and of the molecular weight. Thus, the Tg and molecular weight values are critical for the use of a polymer as an adhesive. This statement was conﬁrmed by Midgley [2] and Foley et al. [3]; they demonstrated that the Tg, molecular weight, and molecular weight distribution are the basic properties to characterize elastomer latexes used in adhesives. It will be shown later that the Tg is a function of molecular weight as well as of other chemical and macromolecular characteristics. The importance of the Tg as a characteristic of the molecular motion will be described in a detailed manner in Section 1.2. Also, it should be stressed that it is not the aim of the book to provide a detailed discussion of the chemical or macromolecular basis of PSAs, but rather an understanding of the adhesive process; there are some fundamental correlations between the physical basis and characteristics of PSAs which need to be discussed. First the value of the Tg, the factors inﬂuencing the Tg, and the adjustment of the Tg will be reviewed. 1.1

Values of Tg for Adhesives

Generally the desired value of the Tg for coatings (ﬁlm-like polymers) depends on their application and end-use temperature, quality criteria, and working mechanism. Paints and varnishes possess a Tg range of þ3 to þ16 C — this is a relatively high Tg range — that yields hard polymer surfaces [3]. Similarly dispersion-based oﬀset printing inks show a minimum ﬁlm-forming temperature (MFT) of þ5 to þ10 C (i.e., also a relatively high Tg range). It should be mentioned that the required Tg of coatings also depends on the face surface. Coatings on soft face materials need a lower Tg. As an example, acrylic binders for absorbent nonwovens have a Tg below 0 C [4]. Adhesive coatings have to be softer than lacquer coatings or printing inks (their Tg is lower also), but their Tg depends on the substrate in a similar manner. Contact adhesives display a Tg range of 10 to 0 C. These adhesives need ﬂuidity only for ﬁlm and bond forming. They must not display ﬂuidity in the bonded state. Pressure-sensitive adhesives have to be liquid at room temperature and within the bond, thus they should have a Tg of about 15 to 5 C or less. In fact the Tg of some PSA base polymers is much lower (Tables 3.1 and 3.2). The Tg of PSAs is generally the result of a formulation of a low-Tg base elastomer and high-Tg additives (see Chap. 8). As an example, the

92

Chapter 3

Table 3.1

Glass Transition Temperature for Different Adhesive Types

Adhesive Type Press (contact) adhesives Low energy curing adhesives Hot-melt adhesives Wet adhesives Acrylic PSA Acrylic PSA Rubber/resin PSA Vinyl acetate-acrylic PSA Electron beam-cured PSA Solvent-based PSA for protective ﬁlms PSA generally

Tg ( C)

References

5 to þ15 40 to þ30 þ5 to þ100 40 to þ15 20 40 55 35 to þ5 25 20 15 to þ5 30–70 C below the use temperature

5 5 5 5 5 6 2 7 8, 9 10 11 12

current Tg range of rubber/resin PSAs (15 to 5 C) is a combination of the low Tg of the rubber—less than 20 C (Table 3.2)—and the high Tg of the tackiﬁer resin—more than þ10 C (Table 3.3)—as shown in Table 3.4. In this case the loading level of both components is almost the same, or of the same order of magnitude. Another example shows that the relatively low amounts of high-Tg additives used in the synthesis of low-Tg base elastomers may have a decisive inﬂuence on the Tg of the product. Plasticizer-free vinyl acetate homopolymer (VAc) dispersions display a Tg of þ30 C, and build a hard, brittle ﬁlm at room temperature. This is due (at least partially) to the protective colloid used. As is known, in this case a polyvinyl alcohol (PVA) with a Tg of þ70 C is used as a protective colloid. The high value of the Tg of PVA has a negative inﬂuence in other cases of formulation of adhesive polymers too; PVA used as a thickener lowers the tack of PSA formulations. The values of the Tg for common PSAs have changed as a function of the raw material development. Some years ago, a typical PSA had a Tg of about 40 C, now common acrylic PSAs possess a Tg range of 40 to 60 C (see Table 3.1); the best performing styrene-butadiene-rubber latexes used for PSAs are those with a Tg of 35 to 60 C [11]. 1.2

Factors Influencing Tg

The Tg is a function of chain mobility and ﬂexibility. Chain mobility depends on internal interactions, the internal forces between polymer chains

Physical Basis for the Viscoelastic Behavior of PSAs Table 3.2

93

Glass Transition Temperature Values for Base Materials for PSA Labels

Polymer

Tg ( C)

References Applications

85 13 70 11, 13, 14 62 15 Polyisobutene 60 16 Polyisoprene, low 72 17 molecular weight 65 Natural rubber 64 18 56 Poly(styrene-butadiene) 59 19, 20 carboxylated 54 Poly(styrene-butadiene) 55 21, 22 50 Poly(ethylene-butylene) 53 23 Poly(ethylhexylacrylate- 49 24 acrylic acid) SBR latex 17 (tan ) 46% ST 25 18 44% ST 25 46 25% ST 3 Poly-n-triethylacrylate 54 13 46 15 Poly(ethylene-vinyl 40 26 acetate-acrylate) Polyvinylchloride, 40 18 plasticized 0 Poly(ethylene-vinyl 35 25 acetate-dioctyl 25 maleinate) Poly (SIS) 28 25 Polybutadiene Polyethylhexylacrylate

Poly(propene-butene)

16

27

Poly(propene-octene) Polyethylacrylate Polypropylene amorphous

25 22 14 11 14

28, 29 13 15 30 27

Poly(ethylenevinylacetate)

10 þ 15

18

Rubber/resin PSA Acrylic PSA PSA, sealants PSA PSA — — UV light curable PSA PSA WB PSA

Acrylic PSA — PSA Face stock PSA

EB curable HM on Kraton D-1320X basis Semi-pressuresensitive adhesive PSA Acrylic PSA Face stock Semi-pressuresensitive PSA, face stock (continues)

94 Table 3.2

Chapter 3 Continued

Polymer

Tg ( C)

References

Applications

Polymethylacrylate

10 þ16 þ29 þ55

16 15 13 31

Acrylic PSA

þ75–105 þ85 þ 100 þ 100, þ110 þ 105 þ 106 þ 185

16 32 16 13 16, 33 13 13

Polyvinylacetate Poly(hydroxyethyl methacrylate) Polyvinylchloride Polyacrylnitryl Polystyrene Polymethylmethacrylate Polyacrylic acid Polymethacrylic acid

— Radiation curable PSA Face stock Acrylic PSA Hot-melt, face stock Acrylic PSA Acrylic PSA Acrylic PSA, thickener

Table 3.3 Glass Transition Temperature and Ring and Ball Softening Temperature for some Tackifier Resins Resin type

Tg ( C)

Regalrez 1065 Regalrez 1078 Regalrez 1094 Regalrez 1033 Escorez 9241

17 23 33 7 5

R&B softening temperature ( C) 65 76 94 33 —

Table 3.4 Changes in the Tg Due to Tackification— Aqueous Dispersions Components PSA Tackiﬁer resin dispersion Concentration, parts (w/w)

Tg ( C)

100 85 70 55 60 60 50

54.7 53.7 38.7 26.4 17.1 22.8 27.8

0 15 30 45 40 40 50

Physical Basis for the Viscoelastic Behavior of PSAs

95

and segments. Intermolecular forces are a function of the nature and dimension of the polymer chains (i.e., of chemical composition, structure, and molecular weight). Equations can be derived that express the dependence of the second order transition temperature on molecular weight, plasticization, degree of crosslinking and copolymerization. Such equations may be reduced in form to equations derivable from the free volume theory. From the ﬁnding that the maximum in debonding force corresponds to the minimum value of the Cp Tg product for PSA polymers, the conclusion was derived that this product (deﬁned as a measure of heat that has to be expended to provide the polymer transition from the glassy to the viscoelastic state, and to impart translational mobility to the polymeric segments, constituted from contributions of free volume and the energy of cohesion) can be used as a predictive criterion of adhesive behavior for various polymers [34–36]. This argues in favor of the interplay of free volume against cohesive energy in polymers as a general determinant of pressure-sensitive adhesion. The Tg depends mainly on the chemical composition and structure of the adhesive and the molecular weight of the base polymer. The chemical composition and structure of the polymers may be inﬂuenced by synthesis and formulation. As a general principle one can say that reduction of the intermolecular forces improves chain mobility and therefore lowers the Tg. Dependence of Tg on Molecular Weight Concerning the composition dependence of the Tg of compatible polymer blends, the parameters of an extended Gordon-Taylor equation are not only polymer speciﬁc, but also molecular weight dependent [37]. Chain mobility decreases with increasing molecular weight. The Fox-Flory equation relates Tg to the number average molecular weight Mn [38]: Tg ¼ Tg C Mn1

ð3:3Þ

where C denotes a constant. This dependence is not a linear function; the Tg dependence on molecular weight is more pronounced in the low molecular range. On the other hand PSA formulations are mainly mixtures of high molecular base elastomers and lower molecular additives. In some cases the base elastomer itself behaves like a mixture of two macromolecular compounds with diﬀerent MW, due to a special sequence distribution. Therefore, it is more appropriate to discuss the dependence of the Tg on the molecular weight for the base elastomer and the lower molecular formulating additives separately.

96

Chapter 3

Molecular Weight of the Base Elastomers. Because of the signiﬁcant inﬂuence of the molecular weight on the Tg a relatively low degree of polymerization (DP) is recommended for polymers used as adhesives (e.g., for polyvinyl acetate, the degree of polymerization amounts to several thousands); for PSAs the molecular weight should be lower. The polymer molecular weight has a signiﬁcant eﬀect on the balance of pressuresensitive adhesive properties [3]. As discussed in [39] shear is roughly proportional to molecular weight up to relatively high molecular weights at which the shear resistance drops oﬀ dramatically in some polymers. Tack is typically high at very low molecular weights and decreases gradually as the molecular weight is increased. Peel adhesion typically exhibits a discontinuous behavior, increasing with molecular weight up to moderate molecular weight levels and then gradually decreasing as the molecular weight further increases. For polyisobutene a maximum of the peel resistance has been found in a narrow range of molecular weights [40]; the molecular weight for PSA base polymers is situated in the range of 1000–2000. As shown earlier [41] typically the molecular weight of solventbased PSA polymers is limited to about 100,000, with a solids content ranging between 12 and 25%. At this molecular weight level the cohesive strength remains marginal and crosslinking appears necessary for better performance. High molecular weight polymers oﬀer high shear and good thermal stability. An acrylic acid copolymer with a molecular weight of 70,000–80,000 is proposed for better wet adhesion properties [42]. Polyisoprene used for PSAs has a molecular weight of 4 103–5 106. The Tg of low molecular weight polyisoprenes diﬀers from that of natural rubber (Table 3.2). Whereas the molecular weight of solvent-based acrylics is about 100,000, water-based ones may have molecular weight values as high as 1,000,000 [5]. It is relatively simple to obtain low or standard molecular weight values for linear, uncrosslinked polymers. It is more diﬃcult to limit the value of molecular weight for partially crosslinked natural or synthetic rubbers used as PSAs. In this case the molecular weight is described by the assembly of linear and crosslinked polymer chains. As an example, for PSAs based on carboxylated styrene-butadiene (CSBR) latexes, the weight percentage of the polymer having a molecular weight of about 300,000 or more is employed as a relative measure of the eﬀective molecular weight [5]. In such styrene-butadiene copolymers higher gel content is an indication for higher molecular weight. Above 30% gel, tack and peel resistance decrease rapidly. The degree of crosslinking, i.e., the length of the mobile chain sequences may inﬂuence macroscopic viscoelasticity. The mechanical properties of crosslinked polymer networks are determined by crosslinking density, the molecular weight between junctions (Mc), and the Tg. The

Physical Basis for the Viscoelastic Behavior of PSAs

97

elastic properties of the polymeric network depend on the length of the polymeric ‘‘bridges.’’ At suﬃciently low frequency of deformation, linear chains begin to disentangle by the so-called ‘‘reptation process’’ and the elastomer ﬂows. Thus, entanglement molecular weight Me aﬀects the mechanical and rheological properties of the polymer. The time constant for reptation for linear molecules is proportional to MW to the third power at constant temperature. The acrylics are inherently (elastic and) tacky due to their high entanglement molecular weight (Me), without the need of adding tackiﬁer [43,44]. A physical model correlating polymer viscoelastic behavior to PSA performance has been developed [45] to explain the relationship between Tg, Me, and PSA performance In a similar manner the minimum length of the soft sequences in thermoplastic elastomers (and polymer blends) inﬂuences the properties of the elastomer [46]. In such cases, both the global molecular weight and the molecular weight of the elastomer (or plastomer) chain sequences inﬂuence the glass transition temperature. As will be discussed later, the dimensions of the polymer sequences aﬀect compatibility also, and thus delimit the regulating possibilities of the Tg. In conclusion, the molecular weight, the molecular weight between entanglements (Me), the molecular weight of the crosslinking bridges (Mc), and the molecular weight of the polymer sequences (Ms) aﬀect the Tg (and the modulus) of the polymers. Molecular Weight of Tackifier Resins. The tackiﬁer must have a lower molecular weight than the base polymer, but a higher Tg than the base polymer, and it must be compatible. Tackiﬁer addition can cause an increase in entanglement molecular weight. For instance for an acrylic polymer Me increased by tackifying from 30 to 93 kg/mol. For this polymer the weight average molecular weight (Mw) of the linear polymer was 68 k, and the average molecular weight between crosslinks (Mc) was 52 k [43]. Tackifying resins used in common adhesive formulations possess a molecular weight ranging from 300 to 3000 where the Tg strongly depends on the molecular weight [47–49] (see Chap. 8, Section 2.3 too). The resins’ molecular weight controls the location and sharpness of the loss/storage peak (tan ) in tackiﬁed styrene block copolymers (SBCs). It is admitted that by tackiﬁcation (for a given softening point and concentration of the resin) the higher the increase of tan the better the compatibility. According to [50] acid resins exhibit a narrow molecular weight distribution, ester resins have a broader MWD and somewhat lower Tg (4–6 C).

98

Chapter 3

Dependence of Tg on Chemical Composition and Structure The bonding/debonding characteristics of PSAs depend on their ﬂuidity, that is, on the internal mobility of the polymer chains. Internal mobility is a function of the internal forces acting on a sterical (volume and structure) or polarity basis. Thus, in a ﬁrst instance, polymer mobility may be regulated by changing the polymer (chain) composition through the choice of sterical/ polarity characteristics of the monomers. These changes are reﬂected by the value of the Tg. First the dependence of the Tg on the chemical composition will be reviewed. Flexible Main Chain. The proper choice of the base monomers and/ or those acting via polar forces permits adjustment of the Tg. A detailed discussion of the interdependence of the base monomers will be given in Chap. 5. The inﬂuence of the base monomers on the Tg will be discussed ﬁrst, going from general problems (polymers) to special ones (PSA). Theoretically, the unpolar, low-volume chain of polyethylene (PE) should display a high mobility, and thus a low Tg. In fact PE has a medium to low Tg(30 C) but a very high tendency to crystallize. Therefore, it does not fulﬁll the criteria for tack, namely to be amorphous as referred to earlier. In order to avoid crystallinity and ordering of polymer chain segments on an ethylene basis, sterical hinder appears necessary, namely, that voluminous (branched) monomer units have to be inserted between ethylene units or inversely ethylene units should be inserted as ‘‘plasticizer’’ units between other vinyl, acryl, or other monomer units. This is illustrated (for common plastics) by the plasticizing eﬀect of ethylene in copolymers with propylene [30]. In this case the Tg decreases from 262 K to 214–222 K. A similar, plasticizing eﬀect of ethylene may be observed for common, non-pressure-sensitive adhesives [51]. The same Tg decreasing eﬀect for polyvinylacetate is produced by the addition of 32% butyl acrylate; adding 18% ethylene decreases the Tg to 0 C [52]. The Tg of polyvinylacetate (PVAc) dispersions usually amounts to þ30 C. Polydibutylmaleinate dispersions have a Tg of 5 to þ10 C, and poly(vinylacetateethylene) dispersions have a Tg range of 10 to þ5 C [53]. In the synthesis of PSAs cases exist where an adequate lowering of the Tg may be obtained by the use of ethylene only. Vinyl acetate-dioctyl maleate copolymers with a dioctyl maleate content as high as 50% do not possess enough tack and peel [54]. The use of a monomer ratio of 15–30 wt% of ethylene with 35–50% 2-ethyl hexyl maleate, dioctyl maleate, or fumarate, introduces suﬃcient tack and peel. It should be mentioned that other low-Tg materials than ethylene-based ones show this crystallizing tendency as well. A striking example is given by the elastomeric sequences of rubbery block copolymers (hot-melt PSAs), where PE-like sequences are interrupted by butylene units.

Physical Basis for the Viscoelastic Behavior of PSAs

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Side Groups. Flexible main polymer chains with a low volume and unpolar side groups, yield a low Tg. Generally, in order to avoid intermolecular, segmental contact of the chains and their build-up into structures, sterical hindrance has to be incorporated. According to the length, bulk volume, and polarity of the side groups an increase or decrease of the Tg may be obtained. This eﬀect is used in the synthesis of special plastomers too. Due to the presence of long chain branching the so-called metallocene resins (very low density polyethylenes) display a combination of elastomeric and thermoplastic properties. Such polymers give a tacky ﬁlm [55]. The eﬀect of the side groups on the Tg can be illustrated in the ﬁeld of the acrylics. Deﬁnite trends in physical properties of the polymers can be observed, when higher molecular weight acrylic esters are used; only a few acrylic monomers are adequate for PSAs. The most important of these are ethyl hexyl acrylate and butyl acrylate. As the ratio of low molecular weight (short-branched) monomers is increased, the following changes in properties generally occur:

Tackiness increases as Tg is lowered; Hardness decreases; Tensile strength decreases; Elongation at break increases.

According to the above scheme (at least theoretically), it is possible to classify the base monomers into softer (plasticizer) or stiﬀer (hardener) ones. The harder ones are short side chain (bulky or polar) acrylates. As an example illustrating the eﬀect of the side chain length on the Tg the ethyl acrylate-methyl methacrylate copolymers should be mentioned: 75% ethyl acrylate forms polymers with a Tg of 5 C, but the decrease of the soft monomer concentration to 25% leads to a Tg of þ 80 C. The principal monomers used in emulsion polymerization can be divided into two main groups: those producing hard polymers such as vinyl acetate, styrene, and vinyl chloride, and those capable of softening hard polymers or producing soft polymers such as ethylene or butyl acrylate [54]. Generally soft polymer compositions must be hardened by polar monomers (see Chap. 5) which may interact physically or chemically, such as acrylic acid, glycidyl methacrylate, N-vinyl-pyrrolidone, methacrylamide, acrylonitrile, and methacrylic acid [56]. Because of the very low Tg of ethyl hexyl acrylate, which is a principal monomer for PSAs, it is recommended to copolymerize with acrylonitrile (e.g., Acronal 80D, 81D), styrene, vinyl acetate, or methyl methacrylate in order to increase the cohesion (i.e., the Tg). The level of the stiﬀening (hard) monomers should be kept as low as possible. As an example, the usual styrene content of CSBR latexes varies between about 40 and 70%; with the Tg the values range from about 30 C

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to room temperature [11]. Generally, the concentration of the most commonly used carboxylic acids lies between 3 and 6%. It should be mentioned that the inclusion of voluminous side groups may be carried out by homopolymerization also (e.g., in the polymerization of butadiene the concentration of 1,2-butadiene units inﬂuences the Tg). On increasing the 1,2-butadiene content, the Tg increases also because of the hindrance of the internal rotation and lowering of the chain ﬂexibility. The inﬂuence of the length of the side groups of the monomer on the Tg was studied in a detailed manner by Zosel [57] for common acrylic esters used as raw materials for PSAs. A strong dependence between side chain length and the Tg of the PSAs was demonstrated, namely, the Tg decreases with increasing side chain length. Oleﬁn-based monomers with neo structure (e.g., vinyl neodecanoate and vinyl neododecanoate) H2C¼CH-O-CO-C(R1)(R3)-R2 in which R1þR2þR3 average 8–10 carbons, possess a particularly branched structure and (compared to acrylic polymers) the homopolymers of these monomers exhibit unusually high entanglement molecular weight (Me) at comparable Tg [58]. Polar Monomers. Polar monomers are copolymerized with the base elastomer in order to improve the mechanical properties (cohesion) of the adhesives. Their inclusion increases the Tg. This may be explained by increasing dipole interaction, or enhanced hydrogen bonding. Next the inﬂuence of the small scale parameters of the polymer structure, mainly the microstructure, the morphology, and the linear/crosslinked character of the polymer will be reviewed. Influence of the Microstructure. Many variations can be made in the build-up of a thermoplastic rubber e.g., molecular weight, styrene content, the number of blocks in the chain (i.e., two, three, or multiblocks), and furthermore in their conﬁguration (e.g., a linear polymer, or one that is branched or radial). Varying the structural parameters has a direct inﬂuence on their physical properties, hardness, and viscosity. As is known the electron beam-crosslinkable SIS rubber has a branched molecular structure. Ethylene copolymers are directly synthesized in high pressure equipment, and the comonomer is randomly distributed along the polymer chain [59]. For copolymers containing large amounts of softening comonomer, the PE segments are short and the melting point and amount of crystallinity are depressed, reducing the heat resistance. Graft or block copolymers are ideal as adhesives because their twophase structure provides a broad operating range. They can be designed as structures containing microphases and separated into a tough, rubbery, low-Tg phase and a hard reinforcing phase (either a glass or a crystalline material). In some cases (e.g., release coatings) a reduced segmental mobility

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is required. It has been shown that the methylsilicate functioning as a high release agent (HRA) to modify the viscoelastic properties of polydimethylsiloxanes (PDMS) eﬀectively reduces the segmental mobility of the ﬂexible PDMS backbone [60]. Sequence Distribution and Length. Within the block copolymers used for hot-melt PSAs there are amorphous polymer segments with Tg higher than room temperature (RT), and other, amorphous polymer segments with Tg RT [61]. The Tg of the thermoplastic component (polystyrene) inﬂuences the stiﬀness of the whole polymer. Its value may be changed, thereby modifying the length of the sequence. The Tg of polystyrene is a function of its molecular weight up to a value of about 20,000, after which the Tg remains constant at 100 C [13]. As an example, the molecular weight of the polystyrene block in SIS used for hot-melt PSAs is about 104, and its Tg is about 85 C [62]. Generally, the sequence length of the elastomer varies between 500–700 monomer units (with Tg at 70 C), where the styrene blocks have a length of 200–500 units [63]. According to these macromolecular characteristics, in SIS copolymers the polyisoprene (PI) phase shows a peak for the loss modulus in the temperature range of 40 C [33]. Another peak around 100 C is linked to the softening point of the pressuresensitive domains. In a similar manner a separate Tg was observed for the hard phase (105 C) and for the soft phase (45 C) in acrylic hot-melt PSAs [64]. In each case the high-Tg endblock will cause a high cohesion/stiﬀness at room temperature. The thermal stability improves by tackifying these elastomers with resins that have a melting point higher than the Tg of the polystyrene domain. As stated by [65] investigating the eﬀect of the di/tri ratio and of the temperature, for diﬀerent materials (81% SIS þ 19% SI) and (5.7% SIS þ 54.3% SI), a broadened glass transition is observed and a second relaxation process appears at low frequencies (high temperatures) while the SI content is increased. Sequence distribution is important because the additivity of the Tg is valid for random copolymers only. Kendall, Foley, and Chu [3] showed that the Tg of a random styrene-butadiene copolymer is approximately related to the styrene content, through the following equation: 1 1 Tg1 ¼ w1 Tg1 þ w2 Tg2

ð3:4Þ

where w1 and w2 are the weight fractions of each copolymer. The ‘‘inverse’’ additivity of the Tg is very important for adjusting the Tg (see Section 1.3). A general applicability of the proposed PSA model (based on free volume considerations [34–36]) is conﬁrmed by the fact that the Tg of such systems obeys fairly well the Kovacs [66] equation concerning the additivity of glass transition temperatures, and relating the Tg of the system to the Tg of the

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components, the fractional free volume at the glass transition temperature, and the thermal expansion coeﬃcient at Tg (See Chap. 5). Influence of Morphology on Tg. The nature and build-up of the polymer chain inﬂuences the polymer morphology. Intramolecular forces may promote crystallization. Crystallization reduces chain mobility, and thus increases the Tg. By crystallization or crosslinking through the main valencies there appears a hindrance of the chain mobility [67]. Crystalline or crosslinked high polymers are not tacky, even above their Tg. This is an important characteristic not only for adhesives, but also for face stock materials, or substrates to be bonded. Diﬀerent adhesion levels can be achieved on high-density polyethylene (HDPE), which has high crystallinity, and low-density polyethylene (LDPE), which has low crystallinity. Because of its crystallinity, HDPE only displays one third of the contact area of LDPE. As described in [68], very low-density polyethylene ﬁlms manufactured by casting (i.e., with a greater amorphous content) and having a low ﬁlm thickness (conformability) display self-adhesivity at room temperature. Thus, diﬀerent adhesion on polyethylene types with diﬀerent crystallinity inﬂuences the anchorage of PSAs applied on face stock materials or substrates made from these polymers. On the other hand, crystallization phenomena may also inﬂuence the adhesive ﬂexibility. As an example one can refer to the design of raw materials for hot-melt PSAs. Here the elastomer segment composition in a rubbery block copolymer must be designed to adequately suppress crystallinity by interrupting the polyethylene-like chains (with a butylene monomer unit) without at the same time enriching the polymer in butylene, in such a way that the Tg is substantially raised. The purpose of this restriction is to obtain a saturated oleﬁn rubber block, with the lowest possible Tg and the best rubbery characteristics. Another possibility is to insert isoprene units. Each isoprene monomer addition to the growing chain incorporates two methylene and one propylene sequence (or one ethylene and one propylene for ethylene propylene rubber, EPR, or ethylene propylene diene rubber, EPDM, respectively). Although isoprene can also polymerize through the 3,4 carbons, the resulting hydrogenated structure which has an isopropenyl side group, does not provide any crystallinity or substantial increase of the Tg. In a similar manner, the increase of the acrylic acid content (decrease of the crystallization tendency) improves the peel values, through the lowering of the Tg [63]. Common amorphous polypropylene shows a crystallization tendency, and may be used for semi-pressure-sensitive adhesives only. They lose their performances because of crystallization. The copolymerization of propylene with hexene or octene allows the synthesis of pressure-sensitive raw materials (see Chap. 5). Leibler et al. [69] studied side chain ﬂuorinated

Physical Basis for the Viscoelastic Behavior of PSAs

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acrylic copolymers. This family of polymers organizes at low temperature into very structured lamellar phases, with in addition, the side chain groups crystallized on a hexagonal array inside the layers. They organize into isotropic phases at around 35 C, depending on their composition. When the temperature is raised and the transition between the crystallized lamellar and the isotropic phase occurs, the tack energy goes from zero to a typical value of 50 J/m2, on a temperature range of 2–5 C. The tack energy G then decreases again as temperature is further increased, like in a conventional adhesive. The inﬂuence of the sequence build-up (and morphology) on the rheological characteristics is illustrated by linear and radial block copolymers. Because of their more ordered structure, the radial copolymers (SBCs also) generally show a longer linear portion of the rubbery plateau and lower values of tan . The two tan peaks characteristics of block copolymers are more extreme (87 and þ85 C) for a radial styrene block copolymer than for the linear one (82 and þ75 C). The butadiene used in these types of block copolymers normally has a loss tangent peak at 90 C, the styrene one at þ100 C. Influence of Crosslinking. Steric hindrance by crystallization or crosslinking reduces chain mobility and thus increases the Tg. Relaxation in the glass transition region involves cooperative motion of molecular segments. An approach pioneered by Adam and Gibbs involves the concept of cooperativity where the conﬁgurational entropy at a given temperature is lower than that expected for independent relaxation of the primitive molecular segments. Cooperativity is a theoretical concept that describes the scale over which intramolecular forces are being propagated in a relaxation event and the strength of coordinated motions among polymer chains needed for the relaxation to occur. Fracture toughness characterizes the glassy state. The role of the network density on glass transition for vinyl ester polymers was studied by [70]. For vinyl ester polymers the narrow range in molecular weights between crosslinks, did not make apparent any correlation between the fracture toughness and molecular weight between crosslinks (Mc). No correlation was found between fracture toughness and the domain size at Tg. In the case of crosslinking the eﬀect on Tg depends on the crosslink density. The crosslink density is a function of the crosslinking method too. For instance the modulus of polybutadiene networks crosslinked chemically or by radiation diﬀers [71]. Common electron beam-curing polymers are not recommended for PSAs because of the high crosslinking density, which imparts hardness and a tack-free character. For PSAs it is recommended to have a Tg lower than 20 C and a low crosslink density. Crosslinking

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agents in an adhesive inﬂuence the Tg and thus the minimum ﬁlm-forming temperature. Woodworking glues having trivalent chrome nitrate as the crosslinking agent (for better water resistance) display a MFT increase of 3–4 C. Generally, the high temperature performance of hot-melt adhesives is limited by the Tg of the endblocks of the base polymers. Better temperature resistance may be achieved by crosslinking the polymers [72]. 1.3

Adjustment of Tg

For practical use adhesives must be ﬂuids (i.e., they must be applied at a temperature above the Tg). An end-use temperature higher than the Tg can be achieved by melting the adhesive (e.g., hot-melt PSAs) or by changing (at least temporarily) its Tg. Fluidity may be achieved by other ways, such as using a volatile plasticizer like a solvent (solvent-based adhesives), or a dispersing agent (water-based dispersions). These methods permit the application of the adhesive, but a continuous adhesive layer is formed above the Tg only. Thus adjusting the Tg appears to be required. The composition of the adhesive is determined by polymer synthesis or most frequently through formulation. Polymer synthesis regulates intermolecular forces by the nature and polarity of the chain building monomer units. Polymer formulation regulates intermolecular interactions by changing the internal structure of the adhesive polymer. Polymer Synthesis Earlier a short description of the parameters inﬂuencing the Tg was given. The macromolecular and chemical characteristics of polymers (molecular weight, chemical composition, structure) can be adjusted through the polymer synthesis, by the choice of the raw materials and of the polymerization technology. In the case of solvent-based adhesives and hot-melt PSAs, the ﬁnal composition of the adhesive layer is that of the bulk material (i.e., the polymer does not contain important amounts of other nonpolymeric additives). In the case of water-based dispersions, the ﬁnal adhesive composition may diﬀer from that of the bulk adhesive, by the additives used, and/or the water included in the adhesive composition. Thus there are diﬀerences between the Tg of the synthesized, uncoated (bulk) polymer and the Tg of the applied adhesive. Water-based adhesives are almost always formulated oﬀering the advantage of Tg adjustment by both procedures (changing the composition of the bulk or of the dispersed adhesive). As a function of the level of formulating agents, the Tg diﬀers more or less from that of the base polymer. On the other hand, it was shown that for natural rubber latices,

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the Tg of natural rubber latex is eﬀectively indistinguishable from that of the dry rubber separated from the same latex [73]. Adjusting the Tg by Formulating In polymer synthesis diﬀerent copolymerization techniques of various monomers giving diﬀerent chain ﬂexibility and intermolecular forces (i.e., a diﬀerent macromolecular order) lead to adequate, low-Tg PSAs. Another possibility to change the macromolecular order is the change of the intermolecular forces by the change of the ‘‘packing density’’ through the use of macromolecular or micromolecular diluting agents such as tackiﬁers or plasticizers. Use of Plasticizers. Plasticizers change the degree of order of the molecules, also changing their entropy and enthalpy. The use of plasticizers for current thermoplastic materials (e.g., polyvinyl chloride, or PVC) is well known. Using high amounts of plasticizer, it is possible to drastically reduce the Tg as an example the addition of 40% plasticizer to PVC decreases its Tg to 40 C. Such a plasticized PVC used as the face material for pressuresensitive labels, possesses a Tg in the same temperature range as the PSAs applied on it, which contributes to very good anchorage of the PSAs on the face stock material. Low stiﬀness and thus poor cuttability are the drawbacks associated with low Tg. Skin softeners used in transdermal applications can also plasticize the medical adhesive thus lowering the Tg and reducing the peel strength. The addition of DMSO reduced the Tg by 19 C [74]. The decrease of the Tg by a micromolecular plasticizer yields increased plasticity and thus better tack of the adhesive, thus plasticizers act like tackiﬁers. Use of Macromolecular Tackifiers. In a manner diﬀerent from plasticizers, the use of macromolecular tackiﬁers results in a higher Tg. In this case the better ﬂow properties and improvement of the tack are due to the decrease of the modulus. Earlier a special case to forecast the Tg for random styrene-butadiene copolymers was mentioned. Additivity of the Tg as described for this polymer is a general phenomenon. Generally the Tg is additive, that is, the Tg of polymer compounds is a function of the nature of the components and their ratio. Practically, this is only true for compatible polymers, and thus polymer compatibility also inﬂuences the adjustment of the Tg. The dynamic mechanical properties of polymers are basically determined by their mutual solubility. If both are compatible and soluble in one another, the properties of the mixture are approximately those resulting from the random copolymer of the same composition. Many

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polymer composites, however, form two phases due to their natural insolubility. In that case the tan temperature graph will show two peaks, which identify the Tg of each of the components. As an example, a good compatibility between natural rubber and glycerine ester of a hydrogenated colophonium was demonstrated by Zosel [57]; the Tg of the mixture increases for a higher resin content, but no second Tg is observed in the composition range of 80–200% rubber. In a similar manner the increase of the Tg and the absence of distinct melting points was observed for nitryl rubber-phenolic resin and EPDM-hydrocarbon resin [75]. For compatible adhesive components (most rubber-resin systems) an acceptable estimate of the tan peak temperatures can be made using the Fox equation [37,76,77]. As Schneider and Leikauf [78] have shown the Fox equation [79] results in a ﬁrst approximation from the Gordon-Taylor expression, which was deduced in supposition of additivity of the speciﬁc volumes. As Kovacs [80] demonstrated, the supposition of the speciﬁc volume additivity requires that the respective free volumes be additive. For miscible polymer blends the Gordon-Taylor [81] equation may also be used [82]: Tg ¼ ðw1 Tg1 þ w2 Tg2 Þðw1 þ kw2 Þ1

ð3:5Þ

where Tg, Tg1, and Tg2 refer to the peak temperature of tan for the blend and the pure components, w1 and w2 are the weight fractions of the components, and k is a polymer constant. For compatible polymers deviations from the additivity rule is less than 5 C [83]. It should be mentioned that diﬀerent addition rules may give diﬀerent results. The Tg for hot-melt PSAs calculated according to the Fox equation or by the mixing rule may yield diﬀerent results [83]. On the other hand it was indicated that many miscible blend systems exhibit phase separation upon heating, caused by the existence of a lower critical solution temperature [84]. This incompatibility caused by heating may inﬂuence the tack and peel adhesion of stored PSAs or the ﬂuidity of PSAs during cutting. Compatibility is a function of the molecular weight and this statement is very important for the tackiﬁcation process of PSAs. Generally, higher softening point resins are less miscible with the base polymer, and any resin added after a certain saturation level is reached, does not contribute to raising the Tg of the system. Instead a second phase consisting of pure resin or a resin-rich blend is formed. The existence of this incompatible phase becomes noticeable by the appearance of a second glass transition point. The structure of the low molecular weight resin is very important to its compatibility with elastomers, and consequently to its eﬀect on the viscoelastic properties and performance as PSAs. A completely aromatic

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resin such as polystyrene exhibits poor compatibility with natural rubber, but is compatible with styrene-butadiene rubber (SBR). A cyclo-aliphatic resin such as poly(vinylcyclohexane) is compatible with natural rubber and is incompatible with styrene-butadiene rubber. An alkyl-aromatic resin, such as poly-t-butyl styrene is incompatible with both elastomers. Use of Blends of Viscoelastomers. The most used method for suppliers to regulate the Tg of adhesive raw materials is the mixing of viscoelastomers having diﬀerent glass transition temperatures. As discussed in [85] the most common solvent-based acrylates for protective ﬁlms have a Tg of 20 C. Such formulations generally contain at least two components. The base polymer is harder (with a Tg of about þ7 C). As suggested in [86] a nontacky but ﬂexible acrylic copolymer, with a Tg value of 14 C can be used as a ‘‘cohesion improving agent’’ for PSA formulations. Polyisobutene is manufactured from oily to hard products with a molecular weight of 1000–200,000. Mixtures of polyisobutene with high and low molecular weight should be used to regulate the adhesion-cohesion. As discussed in detail in [87] the so-called formulation by mixing of high polymers includes also the use of plastomers. 1.4

Correlation Between the Main Adhesive, End-Use, and Converting Properties of PSAs and Tg

As discussed earlier, the Tg characterizes the rheological behavior of the polymers and of the adhesive formulation, thus it inﬂuences the end-use and converting properties also. Adhesive Properties and Tg Room and elevated temperature cohesive strength, adhesion to various substrates, ﬂexibility, etc., are all related to the ﬁlm-forming ability of the adhesive (i.e., to its Tg) [88]. The viscoelastic properties of the polymer ﬁlm determine its adhesion-cohesion balance (i.e., its adhesive and end-use properties). The value of the glass transition temperature determines the main adhesive properties. An adequate choice of the Tg allows optimization of the adhesive and end-use properties for a speciﬁc application area. Generally, low Tg values ensure very high tack, whereas medium ones give an optimum peel combined with an acceptable cohesion. The holding power has been shown to be controlled by zero shear viscosity which is related to Mw, Me, and Tg by the following equation [58]: log o ¼ 3:4 log Mw 2:4 log Me þ A=ðT Tg þ 70Þ þ B

ð3:6Þ

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where A and B are constants. This equation shows that while o is reduced by Me, it is also increased by the Tg of the polymer (since it is more rigid). For polymers exhibiting higher Me and glass transition temperature, we can expect that PSA peel, tack, and holding power can all be improved. The Tg appears particularly useful when comparing the ﬂexibility of several latexes, that is, the latex grade with the lower Tg can be expected to impart a softer bond at a given temperature and to remain ﬂexible at lower temperatures (see Chap. 8. on the choice of latexes for general/deep freeze labeling). But care should be exercised when comparing latexes belonging to diﬀerent polymer groups, as the nature of the polymer can also inﬂuence the ﬂexibility. On the other hand a more complex situation arises by the synthesis of the new core shell latexes where particle structure is important. Here it is possible to identify the composite structure of the latex by determining the Tg and the MFT. The optical examination of the crosssection may show a random distribution of the soft and hard polymer phases, but unusual Tg values indicate the special structure. It must be stressed that Tg values of adhesive latexes are necessary but not suﬃcient to characterize these polymers. Particle characteristics (morphology) are also important. Furthermore the Tg is not an absolute measure of an adhesive’s suitability for use, but it is a good predictor. Once a material reaches temperatures below its Tg, the viscous component of its viscoelastic character is eliminated [89]. Therefore it can be explained why siliconebased PSAs suﬀer no delamination at 75 C, like acrylics do at 25 C. The Tg range of silicone-based PSAs is situated at 100 to 125 C, while organic PSAs possess a Tg range of 40 to 25 C. The eﬀect of the Tg on the peel adhesion at constant molecular weight was studied by Kendall et al. [3]. They stated that the peel adhesion increases with increasing Tg (at constant tackiﬁer resin level) and passes through a maximum. This maximum occurs at a Tg of about 55 C. In some instances maximum peel strengths are apparent at constant Tg and increasing resin level. Schrijvers [40] found that peel and tack increase with increasing Tg. Converting Properties and Tg Converting and laminating properties of the adhesive refer to its ability to be coated in order to form a continuous ﬁlm on the face material. The ﬂuidity required to coat the adhesive may be achieved using diﬀerent procedures. Through the use of higher temperatures (hot-melt PSAs) or dispersing agents the viscosity of the adhesive can be reduced to a convenient level. Practically, the dispersed (or dissolved) adhesive will form a continuous ﬁlm

Physical Basis for the Viscoelastic Behavior of PSAs

109

after drying, if its particles are soft enough to be deformed, to exhibit coalescence only. Coalescence is only possible above the Tg. Near this temperature the coalescence is theoretically possible, but the viscosity of the polymer remains too high to achieve full coalescence. The coalescence depends on the viscosity of the bulk polymer in the polymer particles [90]: ¼ 3 tð dÞ1

ð3:7Þ

where is the contact angle of the particles (Fig. 3.1), g is the surface tension of the dispersion, is the viscosity of the bulk polymer in the particles, d is the particle diameter, and t denotes time. Pressure-sensitive adhesives are very low-Tg polymers in a continuous ﬂuid state with low viscosity. Thus PSA coalescence above the Tg is given at all temperatures. Therefore for these materials the deﬁnition of a minimum ﬁlm-forming temperature has no sense nor importance. For adhesives other than PSAs the negative inﬂuence of the surface active agents (decrease of the surface tension, see above formula) and the much higher viscosity in the vicinity of the Tg leads to a higher MFT value than the Tg. Thus between the glass transition temperature of these adhesives and their MFT, discrepancies exist [91], that is: Tg 6¼ MFT

Figure 3.1

Coalescence of polymer particles.

ð3:8Þ

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These discrepancies are due not only to the dispersion nature of the adhesive, but also to the test methods used for measuring the MFT; the MFT depends on particle size also. Excellent ﬁlm-forming properties are obtained with small particle size dispersions. On the other hand, the particle size dependence of the MFT, makes it sensitive towards the face stock porosity. The MFT measured on porous surfaces (real conditions) is higher than the standard value measured on aluminum [92]. End-Use Properties and Tg It is well known that diﬀerent adherents require diﬀerent adhesives (i.e., harder or softer formulations). Adhesives for vinyl substrates have a Tg value of 10 C in order to obtain an optimum bonding [5]. For adhesion to wooden plates a Tg of 3 C is required. Steel can be bonded optimally with an adhesive exhibiting a Tg of þ7 C. The bonding technology will also inﬂuence the Tg range. Wet laminating needs Tg values between 40 and þ15 C, bonding by pressure requires a Tg range of 5 to þ15 C. Bond forming at high temperatures is possible for adhesives with a Tg range of þ5 to þ 100 C, and low energy curing (UV, electron beam) requires a Tg range of 40 to þ30 C. As discussed earlier, PSAs generally display a Tg range lower than 20 C. 2

ROLE OF THE MODULUS IN CHARACTERIZING PSAs

The Tg is an important characteristic of PSAs, allowing the selection of raw materials for PSA applications. Its value deﬁnes the tack of PSAs; a low Tg is a prerequisite for tacky materials. On the other hand the Tg alone does not permit us to obtain a real image of the adhesive performance. As an example, Zosel [57] showed that poly(ethyl hexyl acrylate) (PEHA) and polyisobutylene (PIB) have about the same Tg values (64 and 60 C, respectively), but PIB is less tacky because its modulus is 20–300 times higher than that of the PEHA. On the other hand hydroxyethylmethacrylate (with a Tg of þ55 C) and ethyl hexyl acrylate have the same inﬂuence on the adhesive properties of radiation cured formulations. Thus it can be concluded that knowledge of both Tg and modulus is necessary for characterizing PSAs. The interdependence between the creep compliance and tack was demonstrated by Dahlquist [93,94]. As discussed earlier (see Chap. 2) PSAs may be described as viscoelastic ﬂuids, with the plastic, viscous part of the system characterized by viscosity (viscous ﬂow occurring above the Tg) and the elastic deformation governed by the modulus. The Tg and modulus both characterize the adhesive ﬂow and its mechanical response. During

Physical Basis for the Viscoelastic Behavior of PSAs

111

application of PSAs bonding occurs ﬁrst; bonding implies adhesive deformability and wetting of the substrate. Deformability by wetting includes the building of contact areas, anchorage, and penetration. Thus: Bonding ¼ f ðfluidityÞ ¼ f ð1=EÞ

ð3:9Þ

Anchorage ¼ f ðfluidity, 1=fluidity Þ ¼ f ½ð1=EÞ , E

ð3:10Þ

Penetration ¼ f ðfluidityÞ ¼ f ð1=EÞ

ð3:11Þ

Good wettability to the adherend relates to a low elastic modulus (G1). Fukuzawa and Uekita [95] examined the modulus of various pressuresensitive products and their components, and the adhesive performances related to modulus. For instance they found G1 values of 1.42–2.04E þ 5 for labels, 1.3–1.68E þ 5 for Kraft paper, 1.42–2.09E þ 7 for OPP, and 6.60E þ 4 for cellophane. Ideally the most important property of an adhesive is the bond strength, or the debonding resistance. The bond strength increases with the increase of the modulus: Bond strength ¼ f ðEÞ

ð3:12Þ

This general statement was made for adhesives, using more or less complex mathematical correlations. In Chap. 2, Eq. (2.15) showed that the ﬂow limit is correlated to the value of the modulus [96]. A formal physical signiﬁcance of the b/"b ratio (ultimate strength-break elongation), is an apparent ultimate tensile modulus at polymer fracture. PSA polymers obey also Dahlquist’s criterion of tack, which establishes that PSA elasticity requires a modulus value of the order of 105 Pa. The fact that the b/"b ratio is 2–9 105 Pa for several PSA polymers allows us to appreciate the physical meaning of Dahlquist’s phenomenological criterion [97]. At the most fundamental, molecular level, Dahlquist’s criterion of tack speciﬁes the ratio between cohesive interaction energy and free volume (see above, the correlation between Tg and free volume) within pressure-sensitive polymers, since the cohesive toughness and free volume are the fundamental characteristics of any polymer materials. Earlier studies [45,98–100] have shown that the modulus G0 value at low frequency can be related to the wetting and creep behavior (bonding) of the adhesive and the modulus at high frequency (100 rad/sec) can be related to the peel or quick stick (debonding). Using the relationship developed by Chu [99], general correlations were developed between tape properties

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(release and peel adhesion). The peel strength is proportional to the ratio of G00 and G0 at the respective debonding and bonding frequencies [58]: P / 1 G00 ð!1 Þ=G0 ð!2 Þ

ð3:13Þ

Since PSA bonding typically occurs at the plateau region of the viscoelastic curve, the bonding is facilitated by lowering the G0 at this region. G0 can be eﬀectively lowered by increasing the Me of the polymer. PSA debonding, however, happens at a much higher frequency, typically between 100–12,000 rad/sec. The loss modulus at this frequency is in the transition region of the viscoelastic curve and its value is highly dependent on the Tg of the polymer. Increasing the Tg results in higher G00 in this region. As discussed in the next chapter concerning contact physics and mechanics, debonding of pressure-sensitive materials is characterized on a macroscopical (visible) scale by cavitation build-up and ﬁbrillation. The driving force for the cavitation process in a predominantly elastic medium is the release of elastic energy [101,102]. If this idea is extended to a viscoelastic material, one expects the critical stress for unstable growth of cavities to be proportional to the real G0 of the dynamic shear modulus of the adhesive. The maximum stress is the parameter most directly related to the cavitation process. Pressure-sensitive adhesives intended for similar applications also exhibit similar rheological properties [103]. The correlation between adhesive properties and dynamic shear storage modulus is quite good. The above given short examination of the dependence of bonding and debonding on the modulus clariﬁes the role of the modulus as the main parameter inﬂuencing the adhesive properties. Detailed theoretical considerations about the inﬂuence of the modulus on the rheology of the adhesion were provided in Chap. 2. Chapter 6 describes the dependence of adhesive characteristics on the modulus. Dynamic mechanical analysis (DMA) currently is used in many application areas, especially in the adhesive industry. The relationship between rheological properties, as measured by DMA, and end-product performance was deﬁned for many types of adhesives, particularly PSAs (see 0 Chap. 1, Section 1). The storage modulus G is a measurement of hardness [Eq. (2.9)]. At G’ ¼ E þ 0.8 Pa and above, the hot-melt PSA is approaching the glassy region. A typical value of G0 for a PSA at 23 C is between 1 E þ 0.5 and 2 E þ 0.4 Pa [104]. The loss modulus G00 is associated with energy dissipation; the greater the value related to G0 , the more ﬂexible the PSAs. Tan is the balance of the viscous/elastic behavior; in practice it can be correlated to the cohesive strength of the adhesive; for hot-melt tan ¼ 1 is a limiting

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value. Above this value the hot-melt PSA can ﬂow without induced deformation; below this value the hot-melt PSA has inherent cohesion. 2.1

Factors Influencing the Modulus

Generally the modulus is a material characteristic, but for materials in a composite structure it may depend on the structure and component characteristics of the composite. An example is that the elastic modulus of epoxy coatings is higher than that of the uncoated material [92]. Material Characteristics For common plastics the modulus is not a constant as it depends on the molecular weight distribution (MWD) and on the manufacturing process [48]. This is generally valid for adhesives and PSAs in particular. Here the modulus is a function of the molecular weight and its distribution, the chemical composition, and the structure. It should be noted that in certain cases, when evaluating adhesive characteristics, the creep compliance (1/E), not the modulus, and its dependence on the polymer need to be considered (see Chap. 2, Section 2). As an example, special PSAs (e.g., skin adhesives) use grafted polymers; the number and composition of the attached polymeric moieties on the polymer, and the inherent viscosity (molecular weight) of the polymer are manipulated to provide an adhesive composition with a creep compliance value greater than 1.2 105 Pa1 [6]. Modulus and Molecular Weight. Molecular weight is a parameter inﬂuencing the most important practical properties such as tensile strength, elongation, modulus, Tg, viscosity, green strength, wettability, pot life, chemical resistance, and processability of the adhesives. For instance, it has been shown for PIB, that tack and adhesive fracture energy decrease monotonically for MW > 45 kg/mol [105,106]. Schlademan and Bryson [107] examined the eﬀect of MW and aromatic content of the resin, for tackiﬁed formulations. As stated, the number average molecular weight showed a strong correlation with shear adhesion values. On the other hand, the molecular weight eﬀect on the molten viscosity was small. It was diﬃcult to determine whether the eﬀect is primarily due to a direct viscosity (modulus) inﬂuence, an indirect compatibility (entropy) inﬂuence, or both. There is a plateau of the modulus corresponding to rubbery/elastic behavior of the material as a function of the temperature. The existence of this plateau and its position (Tg) depend on the molecular weight. If the molecular weight and the molecular weight between entanglements of the polymer do not attain a critical value, no rubbery, elastic plateau appears.

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That means that for low molecular weight polymers a ‘‘pressure-sensitive’’ bonding may appear, but pressure-sensitive debonding properties do not exist. Generally PSAs are polymer blends, thus the inﬂuence of the molecular weight on the modulus must be taken into account from the point of view of the mutual solubility — compatibility — (see Section 1.3 and Chap. 8, Section 2.3). Considering the tackiﬁed elastomer as a polymer solution in the resin, and the polymer solution as ideal (zero excluded volume eﬀect), the plateau modulus ðG0n Þ decreases as a function of the volume fraction of the polymer (Vp), the density of the blend (), and the molecular weight between the entanglements (Me) according to the following equation: G0n ¼ ðRT =Me Þ Vp2

ð3:14Þ

where R is the universal gas constant. The gas constant appears in the correlation due to the hypothesis that the ideal rubber behaves like an ideal gaseous conformation network. The role of the Me for the Tg was discussed earlier. Me can be calculated from the plateau modulus. For instance, as found by [108], in the rubbery plateau region, the storage moduli G0 (!, T) of all poly-n-butylacrylate polymers (having diﬀerent molecular weight) were constant at about G0n 0.15 MPa, thus the average molecular weight between entanglements was 22 kg/mol. In principle the failure of the ﬁbril may occur by: disentanglement of the polymer chain, fracture of the polymer chain, or debonding of the ﬁbril from the adherend. In the ﬁrst process the maximum strain of the ﬁbril "max is controlled by the product dd"/dt where d is the terminal relaxation time of the polymer ( / M3 and decreases with increasing temperature), and d"/dt is the local strain rate (d"/dt / Vdeb) [108]. This means that both the ﬁbril stress ﬁbril and "max increase continuously as VdebM3 increases because disentanglement becomes more diﬃcult. If the failure mechanism is not disentanglement but a debonding process, the critical stress sustainable by the ﬁbril-adherend interface is the rupture criterion. One expects r to be constant in this regime; "max decreases with VdebM3 since "max / r/VdebM3. An entanglement model can be used to estimate the critical molecular weight using the characteristic ratio of the polymer, the number of bonds per monomer, and the monomer molecular weight. A classical example of the modulus dependence on molecular weight is given by rubber/resin formulations. Here nonmilled rubber from dried natural latex shows a larger plateau modulus and a higher elastic modulus (G0 ) than milled natural rubber. Milled rubber possesses a lower molecular

Physical Basis for the Viscoelastic Behavior of PSAs

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weight because of the mechano-thermal degradation of the elastomer. For a CSBR latex, the increase of the molecular weight is accompanied by an extension of the plateau region of the dynamic modulus in the master curve towards higher temperatures [62]. Although the modulus increases with increasing molecular weight, its adjustment by the molecular weight remains limited. It was shown [2] that the maximum of the modulus is situated at about 500–1000 atoms in the main polymer chain. Longer molecules do not give higher strength because their various segments act independently of each other. On the other hand, an excessive increase of the MW is not recommended because of the poor wetting out (low ﬂow) and decrease of the ability of the adhesive to desorb energy (adhesive break). It was shown earlier that Tg related parameters (chain mobility) limit the molecular weight as well. Another parameter is the molecular weight distribution, which may be characterized by dynamical rheology (i.e., the rheology is very sensitive towards MWD) [109]. This sensitivity is also valid for PSAs; in this case the MWD inﬂuences the loss tan peak. McElrath [110] demonstrated that the molecular weight (and its distribution) of a tackiﬁer inﬂuences the peak of the loss modulus and its location as a function of the frequency. For tackiﬁed formulations the loss tan peak is sharper for those resins with a narrow MWD. It is very broad and diﬀuse for the broad MWD obtained by blending two resin fractions [33]. In a diagram representing tan peaks for a resin-rich phase (on a log scale) the broad peak (large MWD, resin mixtures) does not equal zero. At zero, the loss modulus equals the storage modulus. Above zero there is more energy loss than storage. On the other hand, for tack it is important to dissipate energy during the debonding step in order to achieve high tack. Energy dissipation is higher for a broad MWD (i.e., for resin mixtures); this consideration is very important for the choice of the tackiﬁer resins. Commercial acrylic adhesives like PEHA have a very broad spectrum of relaxation times. In such products low MW fractions provide the fast relaxation times necessary to achieve a large real contact area within a short contact time (typically 1 s). As shown in [108] in the case of monodisperse poly-n-butylacrylates these low MW fractions are absent, and the contact area depends mostly on the elastic properties of the polymer at bonding frequencies. One can conclude that cavitation by debonding of PSAs is indeed governed by the viscoelastic properties of the adhesive and the low molecular fractions in PSAs are important for the formation of the contact. Experimental results have shown that a large spectrum of relaxation times is a necessary condition for the performance of PSAs. If short relaxation times are absent (i.e., no low MW fraction) the contact with the substrate is not achieved within short contact times, while if long relaxation times are absent, a ﬁbrillar and adhesive fracture cannot be

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achieved. It is to be noted that the role of the relaxation times is taken into account in the modiﬁed form of the Kaelble equation (for the peel) which can be presented in the form [111]: P ¼ b l ½ða N DÞ=12RT f2

ð3:15Þ

where f is the critical tensile stress of polymer at ﬁbril failure, D is the selfdiﬀusion coeﬃcient of the polymer segment, b is the width and l the thickness of the adhesive layer, a is the size of the diﬀusing polymer segment, N is the Avogadro number, and is a segmental relaxation time. The inﬂuence of the molecular weight and MWD on the modulus should be taken into account for rubber blends too. For instance, it was shown [112] that mastication (used for natural rubber) will have a relatively small eﬀect on long chain molecules and on high molecular weight elements. If they survive mastication to some extent, then their relative contribution to the elasticity will increase. Therefore in mixtures with fractions having diﬀerent molecular weights, the inﬂuence of the high molecular weight factors is more pronounced. For tackiﬁed blends the molecular weight of the resin is also signiﬁcant for the compatibility. In this case an optimum of the viscoelastic properties is linked to molecular weight below 1000 with a narrow MWD. The dependence of the modulus on the mutual solubility was conﬁrmed by Kendall, Foley, and Chu [3]. However, in certain cases the use of low modulus polymers is dependent on coating technology. Low viscosity requirements imply low molecular elastomers as raw materials for hot-melt PSAs; therefore the toughness of hot-melt PSAs is also low [113]. Opportunities to adjust the molecular weight of the base polymers for adhesives allow the adjustment of the modulus as well. Theoretically the molecular weight may be modiﬁed by increasing the polymerization degree for the linear polymer, or by crosslinking. It was shown earlier that increasing the polymerization degree is limited because of the decrease of the ﬂow properties (for processing and end-use purposes). Thus an adequate adjustment of the molecular weight should be achieved through crosslinking. Chemical Composition. The modulus is a function of the chemical composition; polymers based on diﬀerent monomers possess diﬀerent modulus values. For instance, of block copolymers with diﬀerent rubbery blocks (but similar block molecular weights and about 30% styrene), the SEBS had the highest modulus [114]. On the other hand minor compositional aspects such as sequence distribution also inﬂuence the modulus (see later).

Physical Basis for the Viscoelastic Behavior of PSAs

117

Zosel [57] tested the dependence of the modulus of acrylates on the side chain length. Polymethyl-, polyethyl-, n-butyl-, i-butyl-, and ethyl hexyl acrylate were studied in order to estimate the inﬂuence of the chain length (branching) on the Tg and E. Shorter side chains in acrylates impart higher modulus values, the E ¼ f (temperature) plot being moved to the right (higher Tg values) with a higher plateau. In vinylacetate-butylacrylate copolymers the maximum of the curves (peel force-peel rate) corresponds to the change in mode of failure from cohesive (at lower peel rates and a VAc content <50%) to adhesive (at higher debonding rates and a higher VAc concentration) as a result of stiﬀening by vinylacetate [115]. According to the theory of rubber elasticity the shear modulus GA depends on the number of crosslinks Vc. GA ¼ Vc R T

ð3:16Þ

where R and T are the universal gas constant and the temperature, respectively. For tackiﬁed PSAs the addition of resin, the dilution of crosslinks, and the destruction of the elastomeric network decrease the modulus, but the modulus increases again at higher resin loadings. The inﬂuence of the crosslinking through molecular weight increase and natural aging was discussed earlier. Here another special eﬀect of the crosslinking on the modulus, due to the lack of interaction between gel and tackiﬁer, will be examined. Rubber shows a highly reversible deformability combined with a low modulus (the modulus of unﬁlled rubber is 103 lower than that of plastics). Additionally rubber exhibits a technologically important property, namely the ability to undergo selfreinforcement [116] through formation of strain-induced crystalline structures; this imparts excellent tensile properties to rubber. Selfreinforcement by crystallization is a well-known possibility to improve the properties (modulus) of rubber-like synthetic polymers used as raw materials for hot-melt PSAs. In this case soft polyethylene butylene (PEB) segments (matrix) are reinforced by polystyrene segments at the end of the chain; this gives physical crosslinking [117]. Chemical crosslinking is characteristic for CSBR latexes, as they consist of a mixture of polymer species with linear chains, branched chains, and a crosslinked network. Many natural rubber solvent-based adhesive formulations also contain a crosslinking agent. The eﬀect of crosslinking may be illustrated by the increase of G0 (elastic modulus). The adhesive with more crosslinking possesses a higher modulus and higher shear holding power. Classical rubber elasticity theory predicts that the plateau modulus G0n for a diluted polymer is proportional to the square of the volume fraction of

118

Chapter 3

the polymer. Scaling law interpretations by De Gennes [118] suggests that G0n should be proportional to the 2.25 power of the polymer volume fraction [see correlations (3.16) and (3.18) also]. Plots of SBR blends indicate higher values of 2.45–2.65 for the exponent. A possible explanation is that the SBR contains gels (see Chap. 5) which do not participate in the dilution by the resin. Therefore the resin concentration in the amorphous phase is higher than expected, resulting in an apparent higher power for the polymer volume fraction, and as a practical consequence for SBR tackiﬁcation in a higher tack. The inﬂuence of the tackiﬁer concentration on the modulus was studied in [39]. It was shown that the minimum G0 value (at 25 C) occurs at about 40–60% resin loading for rubber/resin systems. For a detailed examination of the modulus dependence on tackiﬁers, the reader is referred to Chap. 8. Dependence of Modulus on Sequence Distribution. The sequence distribution exerts a strong inﬂuence on the modulus of block copolymers used as raw materials for hot-melt adhesives. By changing the sequence length, volume, and polarity, it is possible to modify the modulus, as well as the adhesion/cohesion balance of the polymers; here polar styrene blocks reinforce these polymers [117]. It should be mentioned that a similar hardening eﬀect of certain monomers was observed in some statistical copolymers as well [119]. For partially hydrolyzed ethylene vinyl acetate (EVAc) copolymers the dependence of the modulus on the distribution of the hydroxy groups was shown [120]. Theoretically SBCs have a lower modulus value (106 Pa) in comparison to unvulcanized rubber (107 Pa). Therefore, in principle, their tackiﬁcation seems to be easier. Really, because of the segmented construction of such elastomers, their tackiﬁcation (as the increase of the tack) concerns the diene sequences only, i.e., only the ‘‘half ’’ of the molecule can be tackiﬁed. However, the most important practical use of sequence regulation occurs in the synthesis of thermoplastic rubbers used as raw materials for hot-melt PSAs. These materials are comparatively low molecular weight polybutadiene and polyisoprene rubbers, intertwined with short polystyrene chains. When they are cooled from the melt the (incompatible) polystyrene end blocks separate out and gather to form small hard domains of polystyrene. Since the polystyrene blocks are covalently bonded to the rubber, phase separation is restricted; this forces the copolymer into a special microphase morphology [121] (see Chap. 5). Special sequence distribution allows increasing of the molecular weight without the loss of the tack. Thus segmented EVAc copolymers with very high molecular weight are adequate raw materials for hot-melt PSAs [122].

Physical Basis for the Viscoelastic Behavior of PSAs

119

Softer, more tacky adhesives are obtained with polymers with a low polystyrene content such as Cariﬂex TR 1107 [123]. Some applications, however, require a high shear resistance (i.e., a higher polystyrene content). For lower viscosity hot-melt PSAs, which are easier to process at lower temperatures, SIS block copolymers with high melt ﬂow rates containing a relatively high proportion of diblocks were developed. These low modulus polymers also promote better die-cuttability of PSAs used in label stock applications. It can be expected that in the future SIS polymers will be designed with even more variations in the diblock/triblock ratio, which will enable the manufacture of still softer hot-melt PSAs with a more aggressive tack. The higher modulus polystyrene-polybutadiene-polystyrene (SBS) block copolymers are mainly used in nontacky adhesives. For tacky hotmelt PSAs with better shear, low MW, low viscosity SBS grades with a high polystyrene content are used [2]. The size and morphology of the polystyrene domain in styrene block copolymers inﬂuence the cohesive strength of the system. However, if one increases the polystyrene domain by increasing the polystyrene content of the copolymer the storage plateau modulus increases, thus making the polymer considerably more diﬃcult to tackify [124]. On the other hand it is well known that in tape applications the diblocks act as an elastically ineﬀective diluent in the formulated adhesive. A high diblock content limits the cohesive strength of the base polymer of the adhesive system. The tensile strength and the modulus decrease as the diblock content increases. The diblock has a molecular weight which is half that of the triblock. Increasing the SI content is equivalent to replacing one triblock with two diblocks. This loss of connectivity causes a loss of elastic strands. The measured max in probe tests decreases as the amount of diblock increases. In [65] the shear properties of styrenic block copolymers were measured in the linear viscoelastic regime (small strain) on a parallel plate rheometer. As demonstrated, diﬀerences due to the change of the SI content occur at 5 103 to 5 101 rad/sec, where an increase in the SI content leads to a decrease in the elastic modulus G0 , and to an increase in the loss modulus G00 . In this range the blends are more dissipative if there is more SI. Tensile tests show that decreasing the SI content or increasing the crosshead velocity leads to a higher stress for all extensions and to a more pronounced strain hardening. For label stock adhesives the presence of diblocks is currently considered desirable for obtaining the die-cutting and skeleton waste stripping performance. The optimum level of required diblock depends upon the type and speed of the converting machine, the label stock face material used, together with the other components in the adhesive formulation.

120

Chapter 3

Modulus and Tg The tensile strength and elastic modulus decrease above Tg [125]. This general observation has a special implication for polymer blends. For certain polymer blends a phase separation temperature exists, the value of which depends on the Tg. A diﬀerent dynamic mechanical response is observed below and above this phase separation temperature (Tc). For temperatures below Tc a thermorheologically simple behavior is observed and the time/temperature (frequency/temperature) superposition principle can be applied [126]. For these temperatures master curves are obtained according to: Gred ðTo , !Þ ¼ GðT, !Þ To o Tg1

ð3:17Þ

where Gred is the modulus (G0ðxÞ or G00ðxÞ ) reduced to the reference temperature To. For polymer blends where no strong interactions occur between the polymers (i.e., one of the components acts as a low molecular weight solvent) the plateau modulus Gon2 is given by the following equation: Gon2 ¼ Gon2 ð¼1Þ 2:25 A

ð3:18Þ

where A is the polymer volume fraction. For phase-separated mixtures (and chemically crosslinked systems) an !1/2 power law was observed (usually G0 !2 and G00 !). Filler Effect on the Modulus. The use of ﬁllers with elastomers yields increased stiﬀness, modulus, and rupture energy [127]. In the presence of reinforcing ﬁllers the elasticity modulus of elastomers at small and moderate deformation increases in the ﬁrst approximation according to the GuthSmallwood equation: E ¼ Eo ð1 þ 2:5 þ 2 Þ

ð3:19Þ

where Eo is the modulus of elasticity of the unﬁlled elastomers and is the volume fraction of the ﬁller. This eﬀect of the ﬁller is due to a hydrodynamic viscosity increase, an increased stiﬀness, and a lower Tg of part of the rubber phase at the interface with the ﬁller, given by surface interaction. A covalent bonding of network segment to ﬁller surface, and a tightening of the short chains between aggregates must also be considered. For ﬁlled elastomers the mechanical work of deformation (the change of entropy and internal energy) should include the parameter . The ﬁller particles, which may be as small as 1.5 105 mm (about 30 polymer chain

Physical Basis for the Viscoelastic Behavior of PSAs

121

diameters), may also interface with the motion of the polymer molecules [128]. Generally the modulus increases by increasing the concentration and activity of the ﬁllers [129]. Fillers increase the modulus proportional to their concentration [130]. The relative modulus increase was calculated as a function of the concentration. These correlations are calculated for ﬁllers acting as chemically inert substances. Proactive ﬁllers (such as metallic oxides or salts) may interact with the base polymer, actually crosslinking it. This addition of some oxide to carboxylated compounds dramatically reduces the pressure-sensitive nature of the adhesive [131]. On the other hand, tackiﬁer resins act partially as ﬁllers. Exhibiting a surface free energy close to that of the elastomer, the ﬁller is perfectly wetted by the organic matrix and, as a consequence, good dispersions (i.e., lower viscosities) are obtained [127]. In the case of PSAs ﬁllers can have a negative eﬀect on the tack, but they are sometimes added to provide additional functionality. As demonstrated in [132], a ﬁller volume fraction of 0.5 substantially stiﬀens the adhesive, while also reducing the overall deformation. An approximate characteristic value of the energy release can be obtained assuming that the nonlinear eﬀects arise from the presence of a plastic displacement in addition to the elastic displacement (by tack) corresponding to linear elasticity. The overall adhesion energy is much larger than Gest (energy release rate), indicating that most of the energy is dissipated by bulk deformation of the adhesive, with a relatively small fraction available for crack propagation. Filler particles reduce the energy release rate required for crack propagation, but have a more complicated eﬀect on the deformability of the adhesive, and on the resultant work of adhesion. The overall extension of the adhesive initially increased with increasing ﬁller concentration, but was reduced substantially at the highest concentration. Crystalline polymers are two-phase systems also consisting of both crystalline and amorphous regions. They can be considered as networks in which crystallites are the solid ﬁller and act as multifunctional crosslinks. Such structures exist in paper, they may arise by strain-induced crystallization in special elastomers, or in plastomers used as carrier materials, as discussed in detail in Chap. 2, describing the rheology of the carrier material [133]. As is known, multiphase rubbery polymers can be considered as elastic networks or ﬁlled systems. The stress-strain behavior of such polymers can be adequately modeled by the Mooney-Rivlin equation as an elastic network (considering the trapped entanglements as ﬁnite crosslink junctions) or using the Guth-Gold equation relating the stress level (modulus) to the hydrodynamic eﬀects of the styrene blocks, considered

122

Chapter 3

as a ﬁller. In this case the plateau modulus is given by the equation [see correlation (3.18) also]: Gon ¼ ð=Me Þ RTð1 þ 2:5 þ 14:12 Þ

ð3:20Þ

According to [134] after drying an emulsion-based PSA the adhesive behaves as a ﬁlled polymer melt. Even though emulsion particles are internally crosslinked to get the appropriate viscoelastic behavior, the particles only interact via entangled dangling ends at the interface between particles. Crosslinking Effect on the Modulus. To increase the peel energy it is common to add a slight degree of crosslinking. By manipulating the gel/sol ratio a higher performance adhesive can be designed. On the other hand, crosslinking is generally inﬂuenced by the initial molecular weight of the polymer. Depending on the network mesh size, additive molecules (e.g., tackiﬁer) are either trapped (low Mc) or not trapped (high Mc) in the network. Such segregation is important for the modulation of the adhesive properties. Most polymers have a built-in crosslinking, in other cases postsynthesis crosslinking is carried out. For instance, research has shown that emulsion polymerization of acrylates carried out to complete conversion, produces signiﬁcant amounts of microgels inside the particles due to chain transfer to the polymer via hydrogen abstraction [135]. As stated by Van Holde and Williams [136] high gel content could compensate for low molecular weight. This was not true of the peel behavior. As observed, adhesion failure was favored by high molecular weight, high gel content, and high peel rate. Cohesive failure leaving tacky adhesive on the substrate was favored by low molecular weight, low gel content, and low peel speed. When the molecular weight was low, but the gel content was high, a thin residue on the adherend was observed. The creep compliance both at short times and at long times is governed by a combination of the initial molecular weight and the gel content. The compliance decreases and the viscosity increases as the molecular weight increases or the gel content increases. The eﬀect of gel content becomes less important at the high molecular weights. The molecular weight of the soluble portion of partially crosslinked adhesives is always dominated by the initial MW, even at high gel content. Creep resistance can be achieved either through molecular weight increases or gel content increases and to a certain extent one can compensate for low initial molecular weight with greater crosslinking as far as creep resistance is concerned.

Physical Basis for the Viscoelastic Behavior of PSAs

123

Dependence of the Modulus on the Tackifier Resins. It was shown earlier that the use of tackiﬁer resins lowers the plateau modulus of adhesive blends. This eﬀect depends on the nature and concentration of the tackiﬁer. The dependence of the modulus on the tackiﬁer concentration was shown by Sherrif et al. [63] and Kamagata and Kosaka [137]. Zosel [57] demonstrated that the modulus reaches a minimum for 50% resin in the mixture (natural rubber/glycerine ester of colophonium). Ullmann and Sweet [138] investigated silicone PSAs. As stated, a minimum concentration of resin is required to tackify the polymer so that it exhibits PSA properties, however too much resin will result in poor tack, and loss of adhesion. As the level of the resin was increased, the storage modulus G0 increased in the entire frequency range. Good adhesion for tapes is achieved when G0 is low at low frequency rates, i.e., 0.1 rad/sec, and a relatively high slope exists for G0 as the frequency is increased [139]. Increasing the modulus G0 results in adhesives with higher adhesion and lower tack. Adhesives with lower creep compliance (higher G0 ) will have better cohesive strength (creep resistance). However, if the modulus is too high, the adhesive may not rapidly wet the substrate. As discussed earlier the molecular weight and MWD of the tackiﬁer resin strongly inﬂuence the modulus of the mixture. According to the ﬁller theory, the modulus of a diluted polymer (e.g., thermoplastic rubber in resin and oil) is given by: Gon ¼ 22 ð=Me Þ RTð1 þ 2:5 þ 14:12 Þ

ð3:21Þ

where is the fraction of the polymer in the polydiene phase and 2 is the fraction of the polystyrene block in the entire composition [see Eq. (3.20) also]. The modulus depends on the plasticizer too. Low molecular weight fractions can play the role of the plasticizer also. As stated in [140] the fraction of the polymer that has the molecular weight less than twice the entanglement molecular weight (Me) will act as a plasticizer. Dependence of the Modulus on Other Factors The modulus of adhesive joints depends on the adhesive’s thickness [141]. In fact increasing the thickness of the adhesive layer decreases its modulus. This behavior may be explained by the special multilayer structure of the adhesive and the inﬂuence of the surface of the solid state components. The modulus of the laminate varies close to the interface with the adhesive. The width of this zone depends on the adhesive; for certain adhesives it varies between 0.04–0.2 mm [142].

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It must be noted that the investigations on the inﬂuence of the modulus on the joint resistance, the dependence of the modulus, and adhesive joint resistance on the adhesive layer thickness were mostly carried out for thick-wall sandwiches and adhesives other than PSAs. However, it should be noted that the thickness (coating weight) of the adhesive would also exert a special inﬂuence on the adhesive resistance for PSA laminates. Increasing the coating weight increases the thickness of the mobile middle adhesive layer only (i.e., the cold ﬂow of the adhesive). According to the paradox of Griﬃth [143] a ﬁber-like material possesses a much higher strength than the same material in another form. The paradox of tensioning or stressed length inﬂuences the eﬀect of the thickness of the adhesive and of the primer also. Another parameter inﬂuencing the modulus of plastics is their degree of orientation [104]. Actually there are no data available about a similar modulus dependence for PSAs. One should note the dependence of the modulus on the composite structure of the laminate or laminate components. Special attention must be paid to the inﬂuence of humidity on the modulus of the paper. The modulus of the paper depends on its chemical composition, temperature, physical structure, and moisture content. When analyzing the elastic properties of paper, it is necessary to take into account the fact that paper is composed of both crystalline and amorphous cellulose [141], the latter undergoing softening as the moisture content increases. The eﬀect of moisture on the modulus of cellulosic materials is also aﬀected by temperature. A moisture/temperature equivalence exists, that is, for lower moisture contents the softening eﬀect occurs at higher temperatures. The decrease in relative modulus with increasing moisture content is diﬀerent for papers of diﬀerent crystallinity. Theoretically the modulus is a function of the number of hydrogen bonds (n) absorbing the applied strain: E ¼ k n1=3

ð3:22Þ

where k is the average value of the force constant and E is the elastic modulus of the paper. The decrease of the eﬀective number of the hydrogen bonds results from moisture absorption, resulting in the decrease of E. 2.2

Adjustment of the Modulus

On the basis of the interdependence between modulus and adhesioncohesion balance of PSAs there is a need to adjust the modulus value. As discussed earlier, the modulus is a polymer characteristic and depends on its chemical and macromolecular properties; thus the modulus should be

Physical Basis for the Viscoelastic Behavior of PSAs

125

modiﬁed by changing these properties. In a similar manner to the Tg it is possible to change the modulus of the adhesives during the polymer synthesis or through formulation. Synthesis The choice of the monomers and the control of the sequence distribution and polymer structure allow an adequate regulation of the modulus. As an example, the Tg and modulus of the acrylic latexes can be adjusted by polymerizing monomeric acrylates at diﬀerent rates [3]. On the other hand the choice of the adequate rubbery sequence for block copolymers for hotmelt PSAs allows the modiﬁcation of the polymer stiﬀness. Jacob [144] pointed out that although more economical, there are only a few cases where the use of SBS block copolymers for hot-melt PSAs applications appears appropriate. This may be explained by the high modulus of these compounds compared to SIS, and by the higher stiﬀness and limited tackifying ability (compatibility). Formulation On the basis of a few adhesive groups/polymers, a large number of PSAs are compounded according to the very diﬀerent requirements concerning the adhesion/cohesion balance. This is possible through the use of formulation. Formulation refers to the design of the adhesive properties through compounding. Designing of the adhesive properties involves mainly tackiﬁcation (i.e., the increase of tack and peel) or, in a few cases, the improvement of the shear and cohesion (see Chaps. 6 and 8). Tackiﬁcation imparts better ﬂow properties and molecular mobility. This is achieved by the use of diluting agents, either micromolecular (plasticizer) or macromolecular (tackifying resins). Thermoplastic rubbers are unique in the way they bring together both rubber and plastic [145]. They comprise two inherently incompatible, but chemically connected segments which attempt to separate at service temperatures; this results in their having a morphological structure in which thermoplastic end blocks such as polystyrene form domains holding together a rubbery network (e.g., polybutadiene; see earlier). Materials with ﬁne basic morphologies can thus be produced. In addition to these opportunities provided by changing the basic polymer structure, further modiﬁcation of the material properties is possible during compounding. For example, by using low molecular weight compounding ingredients which demonstrate preferential compatibility with, or solvency for one or the other of the two phases in the base polymer, it is possible to change the eﬀective ratio of the thermoplastic to

126

Chapter 3

rubber phase. More speciﬁcally it is often possible to move from a nonresilient morphology to a soft material by increasing the volume of the rubbery phase through the addition of a suitable oil, or by decreasing the amount of the polystyrene compatible aromatic resin in an SBS copolymer/ oil/resin hot-melt adhesive formulation. The plasticizing agents typically decrease both the dynamic viscosity and the elastic modulus of the adhesive matrix. Crosslinking Crosslinking yields a higher modulus and higher shear resistance. A noncrosslinked soft acrylic polymer has a storage modulus of about 103 Pa, and this value may be increased to 104 Pa by irradiation [146]. In the range of the crosslinked adhesives those with the highest modulus are the most crosslinked and have the highest shear and holding power [147]. Theoretically, on the basis of crosslinking data it should be possible to calculate the shear modulus of the polymer. Practically, in a few cases it is possible with the aid of crosslinking data to predict the gel point and the shear modulus for polyurethanes [148]. The theoretical basis for the dependence of the modulus on the crosslinking is given by the dependence of the modulus on the polymer chain network, and on the number of crosslinked sites [119]. For small deformations (according to Fox and Flory [38], deformation means transition from a real network to a phantom network) there is a correlation between the modulus and the number of crosslinked points (entanglement): G ¼ 2C1 þ 2C2 ¼ ðvCH vC Te ÞkTðhr2 i=hr2o iÞ

ð3:23Þ

where G is the modulus, 2C1 is a function of crosslinking density, ve is the number of network points in the uncrosslinked polymer (polymer-speciﬁc content), and Te is the trapping factor (i.e., the number of rigid network points); it is a function of molecular weight distribution and chemical crosslinking density. hr2 i is the mean square end-to-end distance of a network in the undistorted state and hr2o i is the mean square end-to-end distance of the corresponding free chains. Ideally, the chain length of the crosslinking agents also inﬂuences the modulus [149]; longer-chain crosslinking agents allow more elongation. 2.3

Modulus Values

The value of the modulus at a temperature above Tg characterizes the viscoelastic behavior of PSAs raw materials. The location of the rubbery

Physical Basis for the Viscoelastic Behavior of PSAs

127

plateau is very important. In order to obtain suitable adhesive properties at temperatures above Tg (room temperature or less), a low-E plateau is needed. The dependence of the modulus on the temperature and stress rate is well known. Thus it becomes very diﬃcult to compare diﬀerent polymers on the basis of single E data only. However, these values provide a ﬁrst indication of the possible application ﬁeld of the polymers. Pressuresensitive adhesives are soft and mobile under certain conditions. Even the most crosslinked PSAs do not match any property of a thermoset plastic as the value of their modulus is low, and hence their ﬁlms are not meant to be used without a face material. However, in the last decade carrierless pressure-sensitive products were developed also. Such products are described in detail in [150]. First PVC and butyl rubber were used for carrierless products. Their plasticizing allows a substantial decrease of the modulus and its regulation. Later special acrylic adhesives were developed. They are used mostly as mounting and sealing tapes. Their modulus is increased by ﬁlling (with continuous or discrete ﬁllers) and crosslinking. Therefore, for such products, an adequate peel value can be obtained with high coating weight only. Table 3.5 summarizes some modulus values for polymers used for PSAs or related materials. A maximum tack implies a modulus of 1 105 to 2 104 Pa [151]. For a good wet-out a modulus of about 3 105 Pa is required [152]. For diﬀerent kinds of stresses diﬀerent modulus values must be taken into account. Generally cold ﬂow is characterized by the creep modulus Ec: Ec ðtÞ ¼ =ðtÞ

ð3:24Þ

where is the stress (constant in time) and e(t) is the time dependent strain: E ¼ f ðT, stressÞ

ð3:25Þ

Generally for small deformations (" < 0.8%) the stress dependence of the deformation may be eliminated. Pressure-sensitive adhesives are subject to higher deformation levels, thus the stress dependence of the strain cannot be ignored and the value of the creep modulus is not a constant. On the other hand, for high deformation rates (e.g., peel or cutting) the dynamic modulus must be considered: E 0 ¼ 2G0 ð1 þ vÞ

ð3:26Þ

128 Table 3.5

Chapter 3 Modulus Values for Materials Used for PSA Laminates Modulus Value (N/mm2)

Material Nature Butadiene-styrene copolymer copolymer, block, radial copolymer, block, linear Cellulose acetate Ethylene-vinylacetate copolymer

Characteristics

MD

Mean

TD

Ref. 153

Bulk

—

7–11

—

Film Bulk, grafted Bulk, 50–70% VA

— — 24–45 — —

5 25 — 280 200–1600 130–700 2–8

— — — — —

0.9 2920–3700 2100–2400 200–500 700–1400 28–35 50–130 — — — — — 3200–3250 4000

— — — — — — — 2200 4000 2500 2500 2100

— 1–5

61

Isoprene-styrene copolymer, block, linear Natural rubber Bulk Paper — Polycarbonate Film Polyethylene LD, ﬁlm HD, ﬁlm MD, ﬁlm Polyethyleneterephthalate Film, oriented Polypropylene BOPP, ﬁlm BOPP, ﬁlm BOPP, ﬁlm BOPP, ﬁlm BOPP, ﬁlm Polystyrene Film Polyvinylchloride Hard, unplasticized, ﬁlm Plasticized, ﬁlm Rubber Unﬁlled

— — — — — —

1400 2400 1500 2000 1200

67

154 122 155

— 153 156 116 116 116 155 157 157 157 157 158 116 159 — 116

where v is Poisson’s ratio. As discussed earlier (Chap. 2, Section 1.2), Dahlquist [93] has taken the 1/E value (creep compliance) as an index for PSAs, showing that for general PSAs this value should exceed 1 l06 Pa1 but a maximum tack requires a lower modulus (i.e., a higher creep compliance) [93,94]. The creep compliance for PSAs for medical and surgical applications should be about 1.2 10 5 Pa 1 , preferably 1.3 105 Pa1 [13]; the higher the creep compliance, the bigger the adhesive residue left on the substrate. The fundamentals of creep compliance as they relate to polymeric materials and in particular to viscoelastic polymers are covered in [110].

Physical Basis for the Viscoelastic Behavior of PSAs

3

129

CONTACT PHYSICS

As stated in Chap. 1, Section 1 the adhesive properties of PSAs require a viscoelastic non-Newtonian ﬂow behavior. The rheology of such materials is characterized by the time and temperature dependent modulus, and its components. In application practice such behavior is manifested by the time, stress, and temperature dependent bonding (e.g., dwell time) and debonding (e.g., adhesive or cohesive break of the joint as a function of the stress rate) of the adhesive. To understand the nature of this special bonding/debonding process, contact physics and mechanics were applied for its study. The scope of such investigations is to correlate the end-use adhesive characteristics (e.g., peel strength, tack, and shear resistance) with the data obtained by rheological and mechanical measurements and calculations (see Chap. 6 also). Classical mechanics oﬀers well-known test and calculation methods which can be extrapolated. Thus, simplifying the problems of pressure-sensitive adhesive bonding and debonding to those of fracture mechanics on the one hand, and correlating the properties of the stressed ﬁnite polymer elements with the their macromolecular build-up on the other hand, it is possible (at least theoretically) to explain the practical behavior of PSAs. The viscoelastic contact problem can be divided in two phases: advancing contact or the bonding phase, and receding contact or the debonding phase. In the bonding phase the contact radius increases with increasing time, whereas during receding contact, the contact radius decreases with increasing time. Contact problems were studied ﬁrst for the most simple case of elastic materials; later the results of such investigations were extrapolated for viscoelastic materials. 3.1

Contact Mechanics for Elastic Materials

The study of the bonding phase for elastic materials started some time ago. For such materials the lack of creep allows the examination of the bonding/ debonding as a reversible process where stress and strain occur simultaneously. Derjaguin was the ﬁrst to consider the eﬀect of elastic contact deformation on adhesion by assuming that the particle acts as a Hertzian indenter. The driving force for elastic contact is the work of adhesion. Maugis and Pollock [160] generalized the adhesion model to include plastic deformation and predicted that the contact area was determined by the hardness H of the deformed material. F þ Fa ¼ a2 H

ð3:27Þ

130

Chapter 3

where F is the external force and Fa is the adhesion force between particle and surface. The adhesion energy for the contact mechanics data is obtained using the following equation: G ¼ fða3 KÞ=ðR PÞg2 =6a3 K

ð3:28Þ

given by Maugis [161]. The adhesion to elastomeric substrates generally can be described quite well by the Johnson-Kendall-Roberts (JKR) theory [162,163]. According to the JKR theory [161,164] the contact radius a between contacting spherically symmetric bodies under an applied load is given by: a3 ¼ ðR=KÞfP þ 3WR þ ½6WRP þ ð3WRÞ2 1=2 g

ð3:29Þ

where W is the work of adhesion, P is the applied (contact) load, and Ri, Ei, and i are the radius of curvature, modulus, and Poisson’s ratio of the ith cylinder (lenses or sphere) respectively. 3 1=K ¼ ð1 21 Þ=E1 þ ðð1 22 Þ=E2 Þ 4

ð3:30Þ

In the above correlations the modulus of elasticity is the material characteristic which is determinant for the deformation of the items in contact. G ¼ ða3 K=R pÞ=6a3 K

ð3:31Þ

The eﬀective adhesion energy G can be calculated using Eq. (3.31), when the crack propagates at the interface and gross displacements are purely elastic [165]. This parameter is similar to the fracture mechanics concept of strain energy release rate. A desirable feature of a contact mechanical experiment is the very low crack propagation speed. Another feature is its capability to reveal both thermodynamic and kinetic information through loading and unloading cycles. 3.2

Contact Mechanics for Viscoelastic Materials

For viscoelastic materials, from the energetical point of view, contact buildup and debonding are not fully reversible. The deformation energy is (partially) dissipated by viscous ﬂow. Such ﬂow depends on the time, therefore there is a delay between the stress and the strain. In practice, contacting needs a (dwell) time. Creep appears during bonding/debonding.

Physical Basis for the Viscoelastic Behavior of PSAs

131

Ideally the time dependence of the viscous ﬂow may be a linear function. Generally it is not. On the other hand (in practice) bonding and debonding occur at diﬀerent stress rates, i.e., the time-temperature dependent material characteristics diﬀer also. Debonding is strongly aﬀected by the hydrodynamics of liquids and fracture mechanics, manifested through microscopical deformation of the material under ﬂow (cavitation and ﬁbrillation, see later), which depend on the experimental conditions too. Bonding seems to be more easily described by rheological parameters. The contact mechanics problem for a viscoelastic material including the eﬀects of adhesion has not been solved. The basis of viscoelastic adhesion to solid surfaces has been lain by Creton and Leibler [166]. The viscoelastic nature is accounted for by using a time dependent modulus E(t) of the adhesive. Hui et al. [167] extended the model to a rough surface in which the surface topology is modeled by a Gaussian distribution of spherical asperities. In both cases for wetting, elastic rather than viscous ﬂow dominated. Elmendorp et al. [134] postulate, that for PSAs the actual wetting mechanism for slow processes is viscous wetting rather than viscoelastic deformation. The most comprehensive theory for the bonding phase is due to Schapery [168] and Hui et al. [169]. In this theory the rate of contact growth is related to a reference stress intensity factor, which depends on the loading history and material viscoelasticity. This theory was conﬁrmed by numerical simulations [170]. Unfortunately this theory cannot be applied to the debonding phase, so a comprehensive theory for adhesion hysteresis is still lacking. Viscoelasticity leads to lower adhesion energy from loading and higher adhesion energy from unloading than the equilibrum value [171]. Johnson [172] proposed a theory for adhesion hysteresis. This theory assumes that the rate of loading and unloading are suﬃciently slow, so that the bulk material is in its relaxed state. Barquins and Maugis [173] have approached this problem from the viewpoint of fracture mechanics for the special case that viscoelastic losses are localized at the crack tip. Viscoelastic response is assumed to be proportional to the thermodynamic work of adhesion, and creep was completely ignored. The theory of Ting [174] and experimental results suggest creep eﬀects can be signiﬁcant. Using the Maxwell and Voigt/Kelvin model, the most striking feature of both models is that the time of maximum contact radius tpeak does not coincide with the time of maximum load as it does for elastic materials. Instead maximum radius always occurs after the maximum load. This delay occurs because the material continues to respond by creep even during the unloading cycle. The best sensitivity to creep is obtained if the loading cycle is as short as possible and the unloading time comparable to the relaxation time. Using FEM, Lin and Hui [170] showed that Johnson’s solution works well for slow

132

Chapter 3

loading/unloading rates, but fails to capture adhesion hysteresis at moderate or high rates of loading. In addition Johnson’s theory cannot be used if the material exhibits long time ﬂuid behavior. Such material behavior is often used to ﬁt experimental data [175,176]. Lin and Hui [177] investigated materials with creep compliance of the form: CðtÞ ¼ Co þ C1 tm

0 < m41

ð3:32Þ

where t is time, C1 is a material constant, and Co is the instantaneous compliance. The special case of m ¼ 1 corresponds to a linear viscous material. Using a cohesive zone theory, it was possible to simulate adhesion hysteresis for two linear viscous spheres in contact. Johnson [172] has shown that the relevant time scales for the two processes are usually very diﬀerent. It has been found [178], that the creep eﬀect should dominate the viscoelastic response of the most viscoelastic materials with eﬀective moduli smaller than about 10–100 MPa and W (work of adhesion) < 100 mJ/m2. As discussed in Chap. 1, Section 2, according to Dahlquist, for PSAs the compression modulus should not be much higher than 105 Pa (i.e., 0.1 MPa). That means, that the creep eﬀect dominates the viscoelastic bonding/ debonding behavior of common PSAs. Although in some cases there is a direct connection between contact mechanics and peel strength data (see Chap. 6), and the extrapolation of the contact mechanics data (at low rate) matches the peel data (at high rate), the above described theoretical approaches (in their actual status) cannot describe the behavior of PSAs. Supplementary problems arise from the time dependent character of the peel tests (e.g., adhesive or cohesive debonding), and in many cases other approaches (than the very frequently used indenterbased bonding-debonding) give better results, showing that the fundamental hypothesis concerning the mechanics of the joint is not valid.

3.3

Micromechanical Considerations

As mentioned earlier, in the past few years the parameters of pressure sensitivity were completed by criteria observed during the microscopic examination of the adhesive joint. Such characteristics refer to the visible (physical) modulus of the debonding of strained polymer joints, to the hydrodynamics of material ﬂow, and to the fracture mechanics. Rheologically they are related to the modulus of the material. Adhesion between materials can be considered to be the result of two contributing factors: the chemical work of adhesion (Wa) which is related to

Physical Basis for the Viscoelastic Behavior of PSAs

133

the bonding across the interface, and the energy dissipation due to the deformation of the two materials (), generally expressed by: Gc ¼ Wa ð1 þ Þ

ð3:33Þ

where Gc is the strain energy release, a quantitative measure of the adhesion [179]. The work of adhesion W is composed of contributions from a thermodynamic component Wo and a viscoelastic component Wo [(R,T)] [180,181]: W ¼ Wo þ Wo ½ðR, TÞ

ð3:34Þ

The work of adhesion is the energy associated with either formation or disruption of an interface under equilibrium conditions, which by deﬁnition, allows no dissipation of energy. The work of interfacial fracture is the energy of disruption of the interface between two solids. Since separation of two solids cannot be done at equilibrium, some dissipation must occur. The dissipation is of two kinds: a surface dissipation in the vicinity of the interface due to the separation, and a second type of dissipation due to the bulk deformation of the viscoelastic adhesive. The thermodynamic work of adhesion represents the bond strength (or the intimate wetting of an adhesive on a substrate), and results from chemical bonds, physical interactions, acid-base interactions, or other adhesive mechanisms. The work of adhesion also shows temperature and time dependence, thus reﬂecting the viscoelastic properties of the adhesive. For energy dissipation a special role is played by the deformation, microscopical ﬂow (strain) behavior of the adhesive polymers. Cracks in soft materials or at the interface between a soft and a stiﬀ material sometimes blunt rather than propagate. Delamination of PSAs is accompanied by cavitation in the PSA, and the formation of an extensive cohesive zone behind the debond tip. The presence of such large scale bridging may provide additional energy dissipation and increased resistance to delamination. The transition from propagation to blunting leads to a substantial improvement in the performance of pressure-sensitive adhesives. According to [182] blunting in a soft elastic solid has essentially to do with large deformations. Blunting is predicted when maximum cohesive stress and Young’s modulus are of similar magnitude. Taking the example of an indenter which is removed from the adhesive (e.g., during probe tack, or loop tack), generally the microscopical phenomena of ﬂow related to debonding include cavitation build-up, build-up of air ﬁngers, and ﬁbrillation (Fig. 3.2). As the probe

134

Figure 3.2

Chapter 3

Debonding by cavitation and ﬁbrillation.

withdraws, the ﬁlm must sustain a negative hydrostatic pressure and there is a strong driving force to reduce the degree of conﬁnement of the ﬁlm. This reduction of the conﬁnement occurs through the formation of Saﬀmann-Taylor air ﬁngers coming from the outside of the probe. These air ﬁngers cover rapidly the entire contact surface, coalesce, and no adhesive remains attached to the probe [183]. In practice, cavitation and ﬁbrillation strongly depend on the geometry of the experimental devices. When a ﬂat punch is pulled away from the adhesive layer, the contact area remains constant until cavities form at the punch/adhesive interface. These voids grow rapidly to an equilibrium size at which point the loading energy is used to extend an array of ﬁbrils. For ﬂat punches the hydrostatic stress within the adhesive is large enough to lead to nucleation and growth of voids. In most commercial PSAs this ﬁbril drawing process is responsible for the high adhesion energies typically measured. For spherical punches the most energetically favorable debonding mode is crack propagation from the contact perimeter. As the punch is pulled away from the adhesive, mechanical energy is stored in the adhesive materials as it is deformed. The energy release rate quantiﬁes the amount of energy which is recovered when an incremental area of the adhesive detaches from the punch. Part of this energy is stored as surface energy, in the new surface which is created, but most of it is

Physical Basis for the Viscoelastic Behavior of PSAs

135

dissipated irreversibly within the adhesive in a region near the crack tip. For a linearly elastic material the values of G are given by: G ¼ ½ðP0 PÞ2 =4a dC=da

ð3:35Þ

where a is the contact radius, P is the applied load, P0 is the load corresponding to a given contact radius in the absence of adhesive interaction, and C is the compliance of the adhesive layer. As the thickness of the adhesive layer (h) decreases, its compliance decreases as well, as does its derivative and the resultant driving force for crack propagation. For very large values of a/h the compliance of the adhesive layer is inversely proportional to the bulk modulus of the material. Experimentally cavitation is observed for values of the average tensile stress that are comparable to Young’s modulus of the elastic layer ( avg/E ¼ 1). A critical value of G referred to here as Go must be exceeded in order for a ﬁnite crack velocity to be observed. The minimum possible value of Go is the thermodynamic work of adhesion, which is typically of the order of 0.05 J/m2 for polymers. It is to be noted, that such phenomena related to ﬂow instability possess a general character. They can be observed in the ﬂow of other highly viscous or viscoelastic ﬂuids also. For instance, the build-up of cavitations is well known to specialists in the processing of molten plastic ﬁlms (blown ﬁlms, bubble instability); ﬁbrillation is also observed by stretching and splitting of polymer ﬁlms (ﬁlm/ﬁlament transition) and from the spinning of high polymers. In these technologies such phenomena appear if the strain rate is higher than a critical value (see Chap. 4 also). Over the last few years appreciable progress has been achieved in the quantitative description of the micromechanics of PSA debonding [184,185]. These works consider the nucleation of cavities within the PSA polymer and the extension of ﬁbrils as the major factors leading to the dissipation of applied energy [186]. For strong adhesives the peel edge propagates as a process of cavitations and ﬁbrillation [187]. Such failure has been observed in tack tests [185,188,189] and also in a series of 180 peel tests where the cover sheet is polyethylene terephthalate and the adhesive is a SIS. Since the peel edge can be regarded as the tip of an advancing crack, the adhesive ahead of the advancing peel front is subjected to very large hydrostatic tension due to the lateral constraint imposed by the substrate and the cover sheet. The resistance in PSA layers appears to be dominated by the cavitation behavior of the adhesive. This observation is apparent from stress-separation tests, where the strongly rate-sensitive peak stress (cavitation stress, c) dictates the area under the stress-separation curve. The cavitation behavior of PSAs can be understood in terms of the

136

Chapter 3

well-established mechanics model [190]. For strong adhesives, the interface does not debond so that these cavities can grow vertically perpendicular to the interface. New cavities will form as long as the average spacings between cavities are large enough to provide the necessary lateral constraint as pointed out [188]. As more cavities are formed, the spacing between them is reduced to a level below which the hydrostatic stresses are too low to further nucleate. There is a lateral hydrostatic stress on the ﬁbrils causing their spinning, like a tensioned sample under tensile strength, but there is another vertical hydrostatic pressure caused by compression of the carrier also. Fibrillation then proceeds by the concurrent vertical growth of these cavities. The hydrostatic pressure is responsible for the formation of cavities which grow in the ﬁlm and progressively become elongated in the tensile direction to form a ﬁbrillar structure. For an acrylate PSA the cavities initially appear at the interface between the probe and the ﬁlm [191]. The ﬁbrils are modeled as elastic strings. It is assumed that the ﬁbrils break down at a critical length. According to [65] the strain hardening observed in the ﬁbrillation part of the probe test curves for styrene block copolymers can be correlated to tensile tests. For a material obeying the simple kinetic theory of rubber elasticity, the critical pressure at which a cavity will grow without limit (tear to form an internal crack, or ﬁbrillate) has been shown to be proportional to the initial elastic modulus of the material. For instance (for an SIS-SI copolymer), if the cavitation stress obtained from stress-separation tests is plotted as a function of strain rate, the stress exhibits the same trend as the frequency dependent shear storage modulus (G0 ). These data indicate that the cavitation stress in PSAs is indeed proportional to the elastic modulus. Thus, the cavitation criterion of Gent and Lindley [192], according to which the cavitation in elastic rubbers should occur when 3G0 , is quantitatively satisﬁed. However, although energy dissipation through cavitation and ﬁbrillation is mainly modulus dependent, experimental data demonstrate, that it depends also on the adherend surface [183,193]. Therefore the chemical work of adhesion will inﬂuence the Gc too, i.e., a pure, mechanoenergetic description of debonding is not possible. As discussed in [194] PIB, a base polymer for PSAs does not fulﬁll the Dahlquist criterion and does not show the ﬁbrillation phenomenon typical for PSAs. According to [69], from the energetic point of view, cavitation and ﬁbrillation are not equivalent. On the other hand the debonding force computed from ﬁbrillation depends on the geometry of ﬁbrillation too [193]: Z G ¼ ½aðL eÞ= emax

"max

ð", "0 , TÞd"

ð3:36Þ

Physical Basis for the Viscoelastic Behavior of PSAs

137

where "0 is the elongation rate, "max is the maximum elongation of the ﬁlament, l is the periodicity of the ﬁlaments, a is the ﬁlament width, L is the ﬁlament height, e is the adhesive thickness, and is the stress given by Lodge’s model. Thus, one can conclude, that in many PSA systems the relation between the microscopic structure of a material, its physical response to large deformations in tack experiment, and its adhesive properties is not completely elucidated [195,196]. In particular the link between bond formation and the tack energy, or between the tack energy and the microscopic processes occurring during separation, such as cavitation and ﬁbrillation have been the subject of many studies and still raise a lot of questions.

4

THE ROLE OF OTHER PHYSICAL PARAMETERS IN CHARACTERIZING PSAs

As discussed earlier the glass transition temperature and the modulus are the main parameters in characterizing the versatility of PSA raw materials to be used as viscoelastic compounds having pressure-sensitivity. Other parameters may contribute to the investigation of such materials also. Knowledge from applied physics (mechanics) or special domains of polymer physics (e.g., compatibility) may help in characterizing PSAs. The mechanical properties of pressure-sensitive products depend on the chemical basis and rheological behavior of the macromolecular compounds. Glass transition temperature, viscoelastic response, and yield behavior of crosslinked systems are explained by extending the statistical mechanical theory of physical aging, taking into account the transition of the WLF dependence to an Arrhenius dependence of the relaxation time in the vicinity of Tg. As discussed in detail in [197] for classical PSPs the mechanical properties of the nonadhesive carrier material and of the adhesive are of diﬀerent orders of magnitude. The carrier is designed to resist stresses without deformation; the adhesive is designed to allow a high degree of deformation without being destroyed. For the main applications, the plastic carrier material was considered in a ﬁrst approximation as nondeformable. In the past decade new pressure-sensitive products were developed using thin carrier materials (e.g., protective, separation, or security ﬁlms) or carrier materials having self-adhesive properties. For such products the mechanical properties of the carrier strongly inﬂuence pressure-sensitivity. Testing of the mechanical properties is probably the most perfected area of materials science. Therefore such testing was extended for the domain of adhesives and adhesive raw materials although it allows a comparison of order of magnitude only. Although the correlation

138

Chapter 3

between mechanical and adhesive properties is not universal, cohesion measurements show a good agreement with tensile strength (see Chap. 10 too). For instance, a styrene-isoprene (SI) diblock copolymer possesses a tensile strength of 20 psi whereas a fully coupled SIS triblock exhibits 300 psi tensile strength. A shear holding time of 387 days was obtained for a pure triblock polymer in comparison with 86 days for a commercial one, illustrating that unentangled and uncrosslinked molecules do not contribute to the mechanical properties of the polymer [198]. The mechanical properties were discussed in detail in our book concerning pressure-sensitive products [197] (i.e., the adhesive-carrier assembly), and some special aspects are described in Chap. 6. The role of the compatibility of polymers will be discussed in Chap. 9. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

9.

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4 Comparison of PSAs

There is no special science dedicated to the study of PSAs and there is no special education geared to this particular ﬁeld. Therefore specialists from other related ﬁelds work on the complex technical problems involved in the manufacture and use of PSA laminates. Pressure-sensitive adhesives are macromolecular compounds and thus one can suppose that a better understanding of this ﬁeld requires the input of a polymer scientist or engineer. In fact certain mathematical correlations or approximations describing the dependence of the polymer behavior on the experimental nature and structure are also valid for PSAs. But the problem appears more complex because of the viscoelastic behavior of PSAs which depends on the experimental conditions; PSAs may obey correlations valid for plastics (viscous materials) or rubber (elastic materials). Doing mostly empirical work, the PSA expert tries to act on a theoretical basis. This theory is based on the knowledge from processing plastics and elastomers, or from the more thoroughly investigated ﬁeld (relative to PSAs) of general purpose adhesives. Therefore, a better understanding of PSA practice requires a quintessence of the similarities/ diﬀerences in the properties/rheology of plastomers/elastomers, adhesives, etc. Generally, making a comparison between the rheological properties of plastomers/elastomers, PSAs, and of classical adhesives, one can observe certain similarities and diﬀerences. Similarities are given by the (limited) ﬂow of plastomers/elastomers; the diﬀerences arise from the (partially) ordered structure of these materials. 1

COMPARISON OF PSAs WITH THERMOPLASTICS AND RUBBER

Similar to plastics and rubber, PSAs are macromolecular compounds; their properties depend on their chemical composition and macromolecular 147

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characteristics. But in a diﬀerent manner from elastomers, plastomers, or common adhesives, the physical state and not the chemical composition plays a determinant role for PSAs. For plastics and rubber the ﬂuid state is only required for processing purposes. For PSAs ﬂow is a permanent need. Properties related to ﬂow are the lack of crystallinity, the lack of orientation, the need for cold ﬂow, the high rate of relaxation, and the ductility. It should be pointed out that in comparison to the ﬂow of plastics or rubber during processing, the ﬂow of PSAs occurs isothermally. On the other hand the PSA ﬂuid is always a composite, containing solids, liquids, or volatile components coming from the converting process (ﬁllers, solvents, wetting agents, etc.). The level of additives is never zero, but remains relatively low (less than 20–30% by weight). Generally the rheological similarity of PSAs, thermoplastics, and rubber is observed by examining the practical behavior (e.g., cuttability, re-adhering behavior, mechanical destruction, etc.) of the material. These properties, starting from similar practical processing or test conditions for polymers and from the parameters inﬂuencing the polymer behavior are discussed next. 1.1

Cold Flow

Pressure-sensitive adhesives must exhibit a good resistance to stringing, edge ﬂow, and bleeding [1]. Even though PSAs are highly viscous, they nevertheless display cold ﬂow [2]. The rheology of the adhesive determines its converting and end-use properties. In the industrial practice of laminateconverting, the storage (as a converting-related phase) and the cuttability of the laminate are aﬀected by the ﬂow of the adhesive (see Chap. 2, Section 3.1). As far as the end-use properties are concerned the plasticity/elasticity balance is important. Both storage and cutting may be accompanied by edge ﬂow (oozing) and bleeding of the adhesive. This phenomenon is due to the viscous ﬂow of PSAs and occurs also at low (room) temperature. The cold ﬂow is a general characteristic of plastics. In order to understand its importance and the parameters inﬂuencing it, the ﬂow of plastics and the similarities and diﬀerences between the cold ﬂow of plastics and that of PSAs need to be examined. The cold ﬂow of plastics is generally taken into account in the design of plastic elements (pieces) and where safety factors are required; when long-time, dynamical, or thermal stresses are expected, lower values of the relevant material properties are used. The nature of the ﬂow behavior does not change for amorphous plastics at higher temperatures, therefore their molten state and ﬂow in the molten state may be taken as a simpliﬁed model for the ﬂow of PSAs. The investigation of the parameters inﬂuencing the ﬂow of plastics may yield

Comparison of PSAs

149

information about the ﬂow of PSAs during processing. The inﬂuence of the coating weight, shear rate, and the viscosity on the ﬂow of PSAs will be reviewed based on the processing of plastics.

Coating Weight and Cold Flow There are tests in polymer processing to determine the calendering ability of a polymer; these are based on the shear resistance of the material [3]. Here the maximal shear rate _ max depends on the linear speed of the cylinder , and the thickness of the layer ho: _max ¼ 2v h1 o

ð4:1Þ

that is v ¼ _max ho =2

ð4:2Þ

Assuming a similarity between the material volume squeezed out during cutting of pressure-sensitive laminates and calendering of plastics (push-out of the plastic web) one can estimate that the rate of edge bleeding (i.e., the volume of the adhesive oozing out from the laminate) depends on the adhesive’s thickness (i.e., on its coating weight) [4], or: Bleeding ¼ f ðcoating weightÞ

ð4:3Þ

For polymers (plastics) the minimum pressure for ﬂow can be estimated from the knowledge of the melt ﬂow index (MFI); the MFI depends on the temperature and this dependence is a function of the temperature range used. With knowledge of the geometric parameters of the mold, temperature, and thermal properties of the melt, the minimum pressure requirement for ﬂow may be calculated. On the other hand the thermal properties are polymer dependent (see Chap. 2, Section 1.5) [5]. For rubber blends there is a ﬂow (creep) limit, tested with a Ho¨ppler consistometer [6], depending on sample volume V, modulus G, and sample thickness hL, according to the following equation: 1=2 To ¼ 9h5L G2 =4V 3

ð4:4Þ

150

Chapter 4

Viscosity and Cold Flow During the extrusion of plastics the volume V of the extruded elastomer depends on its viscosity s [7]: V ¼ ½i ðBH=2Þ fs v2 ½i ðBH 3 =12s Þ fs ðdp=dzÞ

ð4:5Þ

where i, BH, v, fs, and p are polymer- and machine-dependent parameters. Assuming a similarity between the material squeezed out by extrusion and by the cutting of PSAs laminates, one can conclude that: Oozing ¼ f ðViscosityÞ

ð4:6Þ

Thus, on the basis of Eqs. (4.1)–(4.5) describing the ﬂow of plastomers during processing, one can suppose a similar dependence of the ﬂow of PSAs on the viscosity, shear rate, and layer thickness; cuttability also depends on these parameters (see Chap. 2). Other complex features of the polymer ﬂow also show similarities with PSAs. An example is given by the melt ﬂow viscosity of polymer blends. Here an ‘‘interfacial slip viscosity’’ is deﬁned because of the observed minima in the viscosity. A multilayer structure is assumed for blends where the steady state shear viscosity of the blend at a constant shear stress may be written as follows [8]: 1 i ¼ 1 =1 þ 2 2 þ i i

ð4:7Þ

where i are the viscosities of the adjacent layers and i are the volume fractions. Measurements of i were lower by a factor of 10 with respect to the lowest viscosity of the mixed homopolymers. The minimum in viscositycomposition plots of the blends disappeared on the addition of selected block copolymers to the blend. Thus it may be concluded that interfaces with low frictional resistance may be formed and must be considered when modeling the behavior of polymer blends. In tackiﬁed PSAs similar viscosity-lowering phenomena occur, which may be partially avoided using polymeric additives. Shear Rate as a Parameter for Cold Flow Some rubber blends that exhibit good processability during pressing do not display good properties during injection molding because of the diﬀerent shear rates applied in both processes. Since the viscosity of these rubber

Comparison of PSAs

151

blends is a function of the shear rate, the tests of processability have to be carried out at diﬀerent shear rates according to the real shear conditions. Molten thermoplastics undergo high shear straining during processing; extrusion of polymer melts is carried out at shear rate values of 102 to 105 s1 [9]. At such a high rate, high shear processing may increase the polymer temperature and may decrease the polymer viscosity. Experimental work has developed a mathematical basis for this phenomenon. The amount of mechanical energy transformed into frictional heat (also called dissipation D) depends on the viscosity and shear rate [10]. D ¼ f ð _ 2 Þ

ð4:8Þ

where and _ denote viscosity and shear rate, respectively. The increase of the temperature of a sheared polymer sample depends on the shear stress , the shear rate _ , the density , the heat capacity Cp, and the shear time t. In a similar manner it may be assumed that the cutting of pressure-sensitive laminates and the shear during cutting depend on the same parameters, that is, that the rate of the cutting and cutting time inﬂuence the bleeding and smearing of the adhesive during cutting, because (at least partially) of the increase of the temperature. Thus the equation given for the temperature increase of plastics will be valid for PSAs: ¼ ð _ tÞ=ð Cp Þ

ð4:9Þ

where , _ , Cp, and t are the above-mentioned parameters. In the processing of plastics the decrease of the viscosity at higher shear rates and the selfheating of the material are positive phenomena; for PSA-converting purposes, however, these phenomena constitute a disadvantage [11]. Flow Test of Plastics and PSAs The shear rate must be examined by the test of the ﬂow properties of the plastomers. The amount of the material yield V in a plastometer is given by Eq. (4.10), where the shear rate _ plays an important role [12]: V¼

ð R3 _ TÞ 4

ð4:10Þ

where R and T are the universal gas constant and the temperature, respectively. Unfortunately in laboratory scale tests the melt ﬂow of thermoplastic polymers is characterized by the melt ﬂow rate (MFR), which depends on the molecular weight and molecular weight distribution

152

Chapter 4

measured under low shear conditions [13]. Thus no (or only limited) information about the real molding behavior may be obtained from the melt ﬂow rate. Extrapolated to the ﬁeld of PSAs, the MFR corresponds to a static, long cold ﬂow time. It is interesting to note that like the MFR the shear of PSAs depends on the molecular weight. This statement is valid and important for tackiﬁed PSAs, where shear resistance may decrease exponentially with the level of low molecular weight tackiﬁer resin. As with MFR values, static or low speed dynamical shear tests provide only limited information due to the high speed shear resistance (i.e., the cuttability of PSAs). In fact, the melt ﬂow of polymers is both pressure (shear) and temperature dependent [14]. Therefore the same temperature dependence for holding power tests, or other low stress rate tests (e.g., Williams-plasticity) may be assumed for PSAs. A similar temperature dependence of the ﬂow characteristics may be observed in the evaluation of elastomers (e.g., hot-set test: tensile strength at high temperature and less than 75% elongation) [15]. Practically, there is a relation between the time/temperature dependence of the tested polymer characteristics and the processing behavior of the material. As an example Shenoy [15] demonstrated the dependence of the minimum pressure P necessary to ﬁll a cavity during processing, on the polymer characteristics [i.e., f (n), K, C, and n] and the melt ﬂow index (MFI): Pmin n ¼ f ðÞ K C 3n R1þn MFIn

ð4:11Þ

where and K are rheological parameters; for the dependence of MFI on the temperature, the Arrhenius or Shenoy equation [Eq. (4.11)] should be used. In a similar manner the dependence of the PSA bleeding (or migration, especially for hot-melt PSAs) on the polymer rheological characteristics and temperature should be evaluated. Generally there are many test methods available to investigate the temperature dependence of certain properties of plastics and elastomers (e.g., dependence of the tensile modulus on temperature [16], or of the critical tear energy [17]), which should be considered for similar measurements on PSAs. There are also similarities in the low temperature range and the low shear rate tests of plastics/PSAs. In the case of polymer deformation by elongation (tensile) the viscosity depends on the tensile rate (draw ratio) [18]: c ðtÞ ¼

ðtÞ ¼ tensile strain=elongation rate _

ð4:12Þ

Comparison of PSAs Table 4.1

153

Test of the Adhesive Flow via Tensile Strength and Static Shear Values for Diﬀerent Formulations

Performance Characteristics Shear resistance (min) Tensile strength (N/mm2) Elongation (%)

1

2

3

4

5

900 8.0 400

200 3.3 1020

50 2.9 1150

170 3.5 700

30 1.4 1550

The strain rate is low (below 1 s!) and it enables the slow mechanical response of the material to be examined [19]. Therefore it may be assumed that tensile tests occur at similar low shear rates to static shear measurements (holding power). Such tests would not give any relevant data about high shear behavior (cutting) of PSAs. Table 4.1 illustrates the ﬂow (cohesion) behavior of a PSA tested by tensile strength and statical shear (holding power). As can be seen from Table 4.1, the tensile strength of the adhesive is proportional to its shear strength but only for certain formulations. Rheology of Plastics/PSAs There is a special chemical basis of PSAs that provides the whole assembly of rheological properties (see Chap. 2). The main polymer characteristics (e.g., modulus and viscosity) may be controlled through chemistry. The basic knowledge of the interdependence of polymer composition/structure and rheological parameters is known for plastomers/elastomers. Adhesive chemists have to apply these general methods to the special ﬁeld of PSAs. The most important molecular factors inﬂuencing the rheological behavior of polymer melts are the chemical composition of the monomers, the average molecular weight, the MWD, and the branching of the chains [20]. These same parameters also inﬂuence the rheology and the adhesioncohesion balance of PSAs. Other more complex similarities also exist; the Tg is an example. The chemical composition of the monomer unit provides the ﬂexibility and ﬂuidity of the adhesive. Because of the volume dependence of the Tg, substituted high-volume resins provide less tack. The chemical composition of the monomer makes crosslinking possible. The crosslinking density inﬂuences tensile strength and tension set [21]; in a similar manner it inﬂuences the peel and cohesion of PSAs. In certain tackiﬁed rubbers, gel formed by crosslinking gives higher tack, but generally crosslinking leads to lower tack (nontackiﬁed mixtures). An increase in molecular weight of plastics may lead to incompatibility in plastic blends. In a similar manner

154

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tackiﬁer resins with a molecular weight higher than 1000 cause incompatibility problems. On the other hand this behavior may explain the higher tackifying eﬀect of low molecular weight liquid resins. As discussed earlier (Chap. 3, Section 3), trials were made to correlate the pressure-sensitive debonding with the visible morphology of the debonded surfaces, i.e., with the build-up of surface and bulky defects in the adhesive and the development of ﬁbrillar structures. The fracture of such structures was compared with the break of macroscopic, plastic (elastic or viscoelastic) items, using the well-known test and calculation methods of classical mechanics. The mechanical performances can be related to the modulus of the material, and to its macromolecular characteristics. Such ﬁbrillar structures are known from diﬀerent technologies working with ﬂuidlike materials (polymer solutions, molten plastics, plastics under creep, etc.). The ‘‘legging’’ of PSAs was described also. According to [22,23] the spinning of polymeric ﬁbers, the processing of numerous foodstuﬀs, and the peel and tack characteristics of adhesives are all associated with the formation, stability, and ultimately the longevity of the ﬂuid ‘‘strands.’’ This tendency to form strands is usually described in terms of tackiness of the ﬂuid or by concepts such as stringiness. The dynamics of such processes are complicated due to spatially and temporally nonhomogeneous growth of extensional stresses, the action of capillary forces, and the evaporation of volatile solvents. Fibrillation as an engineering phenomenon is the result of material and processing characteristics. Theoretically it can be described by dimensional analysis. Although such analysis was yet not developed, trials were made to describe ﬁbrillation as a general phenomenon (including PSAs also). Tripathi et al. [22] discuss the application of a simple instrument referred to as a capillary break-up extensional rheometer [24] that can be used to readily diﬀerentiate between the dynamical response of diﬀerent PSAs. The device relies on the quantitative observation of the rate of extensional thinning or ‘‘necking’’ of a thin viscous or viscoelastic ﬂuid ﬁlament. The high resolution measurement of the radial proﬁle provides a direct indication of the ultimate time to break-up, of the ﬂuid ﬁlament. This critical time is a sensitive function of the rheological properties of the ﬂuid and the mass transfer characteristics of the solvent, and can be conveniently reported in terms of a dimensionless processability parameter (P). Experiments with a new variant of the ﬁlament rheometer using a viscoelastic polymer solution with a nonvolatile solvent showed a good agreement with numerical simulations of the elasto-capillary thinning process. The computations indicate that PSA samples with a processability number of P 3 104 show rapid necking and break-up corresponding to good processability, whereas PSA samples with P 3 103 are prone to strand formation and have poor processing characteristics.

Comparison of PSAs

155

As discussed in detail in [25,26] there are special adhesiveless selfadhesive products (e.g., protective ﬁlms, separation ﬁlms, etc.) which exhibit auto-adhesion due to their chemical composition and to the application conditions. Such self-adhesive carrier materials are plastic ﬁlms having plastic or plasto-elastic character. As described in [27] their self-adhesive performances are given by improved molecular freedom to freely rotate. Generally their Tg is higher than the glass transition temperature of common PSAs but lower than their application temperature. Such materials are used mainly as removable products, where removability is achieved by the low level of adhesion and by the energy dissipation during deformation of the thin polymer ﬁlm. Developments in ﬁlm-forming polymers allowed the increase of the conformability of such ﬁlms (i.e., better bonding characteristics), thus self-adhesive carrier materials compete with adhesive coated ones in application domains requiring higher peel adhesion values. Actually there is no theoretical approach comprising self-adhesivity achieved by means of diﬀerent technologies (e.g., adhesive coated and adhesiveless products). 1.2

Relaxation Phenomena

For all elastomers, reinforced or not, the stress to be applied to obtain a given elongation is less important on a second elongation cycle [28]. This so-called stress-softening or Mullins eﬀect [29] results from several mechanisms (i.e., not only the structure of the elastomeric network, but also the interactions of the polymer with the solid components of the laminate). The stress-softening eﬀect should be taken into account for re-adhering adhesives or for the repeated (periodical) compression-yieldshear stresses when cutting pressure-sensitive laminates. The dependence of the peel force and rate for rubbery materials on the frequency of subsequent bonding and debonding (e.g., reuse, renewed application of the adhesive label), has also been described as a ‘‘memory eﬀect.’’ An applied label loses this memory eﬀect after some time or when in a solvent. The memory as a function of the dwell time eﬀect is associated with some rearrangements of the molecular structure of the rubber at the interface. This memory eﬀect is important when re-adhering repositionable labels. The role of the relaxation phenomena and their inﬂuence on the mechanical properties of plastics is well known. The dependence of the mechanical properties of plastic blends on the relaxation ability was demonstrated. Applying cyclical stresses to polymer/polymer mixtures leads to a dissipation of the energy in the elastic parts of the mixture; the tensile strength depends on the viscoelasticity and relaxational behavior. Similar phenomena exist for PSAs where a balance of elastic/viscous parts

156

Chapter 4

(given by a certain formulation) may allow a soft or hard debonding (see Chap. 2, Section 1.4) as a result of the relaxation. As discussed earlier (Chap. 3, Section 3) ﬁbrillation supposes the existence of a minimum molecular weight (higher than Me). The cohesive strength of the ﬁbrils/ lamellae increases with increasing chain relaxation time. The area of contact of an elastic ﬁlm on a rough surface depends on the relaxation properties of the polymer also. 1.3

Mechanical Resistance

The mechanical resistance of plastics, rubber, and adhesives is quite diﬀerent. As can be seen from the modulus values (see Chap. 2), the modulus of the face stock materials (paper or plastics) diﬀers by a factor of 103 from the modulus of rubber, or by 104 from the modulus of the adhesive. Rubber is an ideal raw material for PSAs. Rubber displays auto-adhesion and shows strain-induced crystallization [30]. Amorphous rubber is very mobile during application (bonding) and strain-crystallized materials yield high shear. But PSAs are mostly mixtures of resin and rubber in a 1:1 ratio, thus it is as yet not clear if the eﬀects of the strain-induced crystallization can be extrapolated to PSAs. A more thorough examination of the main rheological and mechanical characteristics of plastics/elastomers and PSAs reveals a lot of diﬀerences in their properties. Generally the values of the modulus E, the tensile strength, and the elasticity are much lower for PSAs than those of the elastomers (or plastomers) [25]. High modulus values as encountered with elastomeric block copolymers (e.g., 250–350 kg/cm2), high elastic recovery values (of about 80%), or high elongation at break (800–1300%) cannot be obtained for PSAs. Their ﬂow characteristics are very pronounced. There is a model rubber (DIN 53516) that is used to characterize rubber and it is possible to characterize the creep under compression of the elastomers in a ‘‘creep unit.’’ If the deformation < 0.8% (see Chap. 3, Section 2.3), then for plastics one can write: E 6¼ f ðÞ

ð4:13Þ

If the modulus does not depend on the stress value, the yield, compression, and ﬂexural modulus are identical, Et ¼ Ec ¼ Ef No model PSA exists as deformation values for PSAs are higher than 0.8% and the modulus values for diﬀerent kinds of stresses vary. On the

Comparison of PSAs

157

other hand, not only the deformation value but also the mechanism of the deformation and destruction of plastics/elastomers diﬀer from those of PSAs. Plastics are viscoelastic materials and do not obey linear elastic fracture mechanics (LEFM) unless as an approximation where the plastic zone remains small [20]. For ductile fracture the F integral was suggested as a fracture criterion for large-scale plasticity. In a similar manner PSAs undergo ductile fracture; however, in the design of removable PSAs the formula for the critical fracture toughness Kc given by Griﬃth [31] should also be considered: Kc ¼ c a1=2

ð4:14Þ

where depends on the geometry of the specimen, a denotes the crack length, and c is the critical value of the remotely applied stress at which the crack begins to grow. It may be assumed that a similar correlation exists between a specimen geometry a, ‘‘leg’’ length qa, a critical value of the applied stress ca, and critical tear toughness Kca for a removable PSA [32]: Kca ¼ f ðca qa

aÞ

ð4:15Þ

The test and calculation methods used for the mechanical performances of industrial plastics were extrapolated for PSAs also. In some cases the common mechanical characteristics, e.g., tensile strength, tear strength etc., are related to the adhesive performances. For instance, by the analysis of PSA fracture mechanics in the course of peeling, Feldstein et al. [33] established that the peel force P relates to tensile strain and stress by the following equation: l p¼b 2

Z ðÞ d

ð4:16Þ

0

where b is the width and l is the thickness of the adhesive ﬁlm. At low tensile rates the adhesive fracture occurs at comparatively small tensile stress values, but at signiﬁcantly greater values of elongation, . With the increase in extension rate, the critical magnitude of tensile stress at fracture ( f) varies rapidly, approaching a limiting value which corresponds to a maximum cohesive strength of the PSA ﬁlm c. A similar behavior was found in [34]. As stated, the peel strength is strongly dependent on the

158

Chapter 4

eﬀective rate of peeling. At low rates the elastomer layer could not be detached cleanly. Instead it split apart leaving rubber behind (cohesive failure). Under these circumstances the peel strength is the same as the tear strength measured by tearing apart a thin sheet of the elastomer. A detailed discussion of the test methods will be given in Chap. 10. 2

COMPARISON BETWEEN PSAs AND OTHER ADHESIVES

Common adhesives have to be ﬂuid only during the coating process. Similar to PSAs, the ﬂow of common adhesives during application is obtained through the aid of a dispersing agent (solvent or dispersing media) or by melting the bulk adhesive. After coating the adhesive bond is achieved by the increased viscosity of the adhesive (in comparison with the application/ viscosity) and, in some cases, by the chemical interaction of the adhesive with the adherend. In special cases after bond forming, the adhesive itself undergoes a chemical transformation (e.g., crosslinking) that results in a higher adhesion/cohesion balance with respect to the uncoated adhesive. This is possible by changing the viscous/elastic balance and increasing the value of the modulus. In this case the adhesive layer has similar rheological and mechanical properties to those of the adherend, and therefore debonding (without material destruction) or rebonding is no longer possible. The properties of such adhesive joints are similar to plastics or elastomers. Similarities with the rheology of PSAs are observed for the uncoated, or coated but unbonded adhesive only. On the other hand, the chemical basis of the common adhesives is almost the same as that of PSAs. The chemical composition of PSAs and common adhesives may be the same, although the role of the built-in fractional groups may be diﬀerent. The presence of itaconic and/or acrylic acid in vinyl acetate (VAc) terpolymers imparts excellent freeze-thaw stability to these emulsions. The same monomers in PSAs are used to crosslink and to impart special adhesion. The same statement is valid for the formulation. Plasticizers or tackiﬁers used in PSAs in order to achieve tack and peel give polyvinyl acetate (PVAc) adhesives better bonding ‘‘speed.’’ Classical adhesives and PSAs diﬀer in the reason why one uses plasticizers and tackiﬁers. For classical adhesives plasticizers and tackiﬁers modify the application viscosity and green tack, whereas for PSAs they modify tack and peel. The functional groups in classical adhesives serve mainly as crosslinking sites, while in PSAs they are adhesion promoters. Thus in hot-seal adhesives the resin improves the adhesion to diﬃcult substrates, decreases the blocking resistance, and lowers the seal

Comparison of PSAs

159

temperature [35]. Diﬀerences in the use of additives for water-based formulations may be noted also. Thus polymeric colloids for use in PSAs have a relatively low Tg (i.e., in the range of about 40 to 20 C), while the colloids for use in laminating adhesives have a Tg value within a range of 25 to 10 C. Classical (non-pressure-sensitive) adhesive joints are designed and tested according to the theory and calculation methods of common mechanics. Such methods were used for PSAs also. For instance, Brockmann and Geiß [36] extrapolate the calculation methods used for common adhesive joints to PSAs, admitting that joint failure occurs after a critical value of the creep. For the evaluation of this critical load limit, creep measurements have been carried out using single lapped shear joints. Three stadiums of creep were observed. It starts with an elastic strain and decelerated creep. When the acceleration becomes zero, i.e., at the beginning of constant creep velocity, the deformation turns from viscoelastic-plastic to viscoelastic. The third creep stadium is deﬁned by a starting acceleration and ends with total failure of the specimen. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Adhes. Age, (3) 8 (1987). H.G. Koch, Adha¨sion, (11) 312 (1976). J. Guillet and V. Verney, Polymer J., (8) 773 (1984). R.R. Lowman, FINAT News, (3) 24 (1987). D.R. Saini and A.V. Shenoy, Plastics and Rubber Proc. Appl., (3) 178 (1983). B. Poltersdorf and D. Schwambach, Kautschuk, Gummi, Kunststoffe, (5) 454 (1988). H.J. Laake, Kautschuk, Gummi, Kunststoffe, (6) 506 (1989). J.L. Jorgensen, L.D. Thomson, K. Rasmussen and K. Soendergard, Internat. Polymer Process, (3–4) 122 (1988). F. Kurihara and S. Kimura, Polymer J., 17, 863 (1985); in Kautschuk, Gummi, Kunststoffe, (3) 256 (1985). H.Y. Tang and J.E. Mark, Macromolecules, (17) 2616 (1984). H. Kehler and W.M. Kulickje, Chem. Ing. Techn., (10) 802 (1986). Kautschuk, Gummi, Kunststoffe, (9) 31 (1985). S. Cartasegna, Kautschuk, Gummi, Kunststoffe, (12) 1188 (1986). F. Grajewski, A. Limper and G. Schwarter, Kautschuk, Gummi, Kunststoffe, (12) 1188 (1986). G. Bolder and H. Meier, Kautschuk, Gummi, Kunststoffe, (8) 715 (1986). Y. K. Kawasaki and N. Watanabe, Kautschuk, Gummi, Kunststoffe, (2) 119 (1987). T. Gehman, Kautschuk, Gummi, Kunststoffe, (6) 493 (1989). T.S. Ng, Kautschuk, Gummi, Kunststoffe, (9) 830 (1986).

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19.

J.N. Hay, Coaters Scientific Conference Communications, Surface Coating Evaluation and Performance, 1988. H.M. Laun, Kautschuk, Gummi, Kunststoffe, (6) 554 (1987). G. Bolder and H. Meier, Kautschuk, Gummi, Kunststoffe, (3) 196 (1984). A. Tripathi, G.H. McKinley, and P. Whittingstall, ‘‘Using Filament Stretching Rheometry to Predict Strand Formation and Processability of PressureSensitive Adhesives and Other Complex Fluids,’’ Proceedings of the 23rd Annual Meeting of the Adhesion Society, Myrtle Beach, SC, Febr., 20–23 2000, p.119. Cr.I. Simionescu, N. Asandei and I. Benedek, Rev. Roum. Chim., (7) 1081 (1971). M.I. Kolte and P. Szabo, J. Rheol, 43 (3) 609 (1999). I. Benedek, Development and Manufacture of Pressure-Sensitive Products, Marcel Dekker, New York, 1999, Chapter 3. I. Benedek, E. Frank and G. Nicolaus (Poli-Film Verwaltungs GmbH, Wipperfu¨rth, Germany), DE 4433626 A1/21.09.1994. I. Benedek, Development and Manufacture of Pressure-Sensitive Products, Marcel Dekker, New York, 1999, Chapter 4. J.B. Bonnet, M.J. Wang, E. Papirer and A. Vidal, Kautschuk, Gummi, Kunststoffe, (6) 510 (1986). L. Mullins, J. Rubber Res., (16) 275 (1947). G. Hammed, Rubber Chem. Technol., (5) 576 (1981). A. Griffith, in Coaters Scientific Conference Communications, Surface Coating Evaluation and Performance (J.N. Hay, Ed.), 1988. R. Dorpelkus, Allgemeine Papier-Rundschau, (16) 456 (1986). A.A. Chalykh, A.E. Chalykh,V. Yu Stepanenko and M.M. Feldstein, ‘‘Viscoelastic Deformations and the Strength of PSA Joints Under Peeling,’’ Proceedings of the 23rd Annual Meeting of the Adhesion Society, Myrtle Beach, SC, Febr., 20–23, 2000, p. 252. A.N. Gent, G.R. Hamed and W.J. Hung, ‘‘Adhesion of Elastomers: Dwell Time Effects and Adhesion to Fillers,’’ Proceedings of the 23rd Annual Meeting of the Adhesion Society, Myrtle Beach, SC, Febr., 20–23, 2000, p. 6. E.A. Theiling, Kunstharznachrichten, (1) 14 (1972). W. Brockmann and P.L. Geiß, J. Adhesion, 63, 253 (1997).

20. 21. 22.

23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.

34.

35. 36.

5 Chemical Composition of PSAs

A typical PSA is derived from a ﬁlm-forming elastomeric material such as natural rubber, styrene-butadiene, butyl, silicone, nitrile, and acrylic rubber. The elastomer provides ﬂexibility, ease of tackiﬁcation, and the desired bond strength when compounded with compatible tackiﬁers, pigments, plasticizers, waxes, and oils. Special polymers were developed as raw material for PSAs which work with or without tackiﬁers. Natural rubber represents 45% of the elastomeric raw materials; block copolymers and water-based acrylics amount to 40%, and the remaining part is mainly covered by solvent-based acrylics. Although new elastomers have been identiﬁed as base materials for PSA applications, natural rubber is still the most preferred polymer. From a chemical point of view classical PSAs are elastomers that exhibit viscoelastic properties due to their low Tg (40 to 60 C) and modulus. Their macromolecular basis is built on long-chain polymers, with a certain degree of branching and with or without crosslinking. The balance between plastic and elastomeric properties is governed by the polymer nature and structure, and molecular weight. For classical rubber-based PSAs, structure and molecular weight adjustment occurs via crosslinking or mechano/chemical destruction of the network (e.g., mastication, tackiﬁcation, plasticizing). Synthetic raw materials for PSAs can be tailored in order to achieve the chemical characteristics required to obtain the desired adhesive properties. A detailed study of the interdependence between the chemical basis and physical structure of PSAs was carried out by Krecenski, Johnson, and Temin [1]. As discussed earlier (Chap. 2) the rheology of the pressure-sensitive product depends on its solid components and the interaction between the liquid and solid components too. The chemical composition of the whole pressure-sensitive construction—polymer carrier and macromolecular adhesive—was discussed in [2]. A detailed analysis of the formulation of PSAs is given in [3]. 161

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Pressure-sensitive adhesives are amorphous, viscoelastic materials, and their ﬂow behavior is characterized by the viscosity and the modulus (see Chap. 4). On the other hand a special structure and temperature dependence of the internal phase transitions serves as the basis for the required rheology. Since the viscosity, modulus, and glass transition temperature are dependent on the chemical composition, changes in the composition and/or in the macromolecular characteristics of the basic elastomers may inﬂuence the rheology of PSAs. From a theoretical point of view it is more accurate to discuss the inﬂuence of the compositional and macromolecular factors on the properties of PSAs separately. However, it is not within the scope of this book to present a detailed, purely theoretical approach to PSAs; therefore the chemical basis (composition and macromolecular characteristics) will be discussed together and, if relevant, related to adhesive practice.

1

RAW MATERIALS

Early PSAs were based on organic solvent solutions of natural rubber that were tackiﬁed by ‘‘some type of resinous material’’ [1]. These so-called rubber-resin (RR) adhesives are still widely used. With regard to their chemical basis and application technology, PSAs may be classiﬁed into dispersed systems and 100% solids. Dispersed systems may include solutions and emulsions; 100% solids cover hot-melt adhesives and radiation-cured materials (oligomers and prepolymers). The most important solutions are solvent-based systems. They may include rubber/resin adhesives, acrylics (thermoplastic and crosslinked), and silicones. Hot-melt PSAs are based on (mainly styrene-diene) block copolymers and acrylics. The radiation (electron beam and UV)-cured materials can be either acrylic- or rubberbased. Finally emulsions (water-borne adhesives) comprise acrylics, natural and synthetic rubber latexes, and ethylene vinyl acetate dispersions. The original compositions were heterogeneous physical mixtures of one or more elastomers (rubbers) with tackiﬁers and/or plasticizers. The second approach, essentially homogeneous polymers based mainly on acrylate esters, was introduced in the mid-1950s. Both techniques were reﬁned and their usefulness extended as a result of extensive research and development on new raw materials and crosslinking methods, and both are currently available in water-borne and 100% solids form as well as in the original solvent solution. Deﬁciencies persist, however, especially as the use of PSAs expanded into new, more demanding markets. Particular needs exist for PSAs with good aging characteristics, with broader speciﬁc adhesion, higher cohesion and heat resistance, adherend independent removability, and reduced plasticizer-induced performance losses when used on ﬂexible

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polyvinyl chloride (PVC) face stock materials or special substrates. Some alternative compositional approaches have shown considerable improvement, especially for silicones, but these are too expensive for general use, as are urethane-based PSAs. 1.1

Elastomers

A main characteristic of PSAs is bond forming (tack). There are three fundamental criteria that must be met in order for an elastomer to exhibit tack [4]. The polymeric chain must come into molecular contact with the surface, thus the chains must interdiﬀuse across the interface and become entangled with one another. After bond formation the elastomer must display high cohesive strength to resist easy separation upon application of a force. This property distinguishes a tacky elastomer from a low molecular weight liquid. This combination of partially contradictory characteristics will be embodied by a viscous/elastic, rubber-like material. Therefore the most important component of a PSA formulation is the elastomer. Elastomers are high polymer materials. They are crosslinked and possess a steel-like elastic behavior. Rubber-like elasticity is characterized by values of the compression modulus which do not depend on the temperature and by pronounced reversible deformability. Elastomers do not exhibit viscous ﬂow. From the theoretical point of view it is possible to transform an elastomer in a viscoelastomer by blending it with viscous components. The main part of PSA formulation is based on this principle. PSAs based on natural or synthetic elastomers are tackiﬁed formulations, i.e., multicomponent systems. A partially crosslinked elastomer (i.e., natural rubber) was used as a starting material for PSA formulations. Rubber-like synthetic materials, with a rubber-like (hydrocarbon-based) chemical composition were developed later and used with or without natural rubber. The development of soft and elastic polymer coatings (usually acrylic-based) opened up new directions in the synthesis of elastomers. Later, multicomponent copolymerization of nonpolar monomers was developed, which yielded a new class of soft and elastic, thermoplastic elastomers. Such compounds with a segregated structure work like a crosslinked elastomer. Heterocyclic compounds exhibiting a special bond stability function as the elastomeric backbone of the formulation (e.g., silicones). Thus there is a broad range of elastomers available for PSA formulations. Natural Rubber Natural rubber was used almost exclusively as the base elastomer for PSAs in the early stages of PSA manufacturing. Diene-based natural and synthetic

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rubbers always contain more or less reactive side groups, or branching, and thus they always possess some crosslinked structure. However, their solidlike and elastic behavior is not given (it is inﬂuenced only) by such structure, and they are (at least partially) soluble. Mechano-chemical destruction of the base elastomer (by mastication or dissolution) allows the modiﬁcation of the molecular weight or molecular weight distribution. The good compatibility of natural rubber with diﬀerent tackiﬁers or plasticizers allows an easy adjustment of the adhesion/cohesion balance. In some cases more cohesion is required; therefore blends of natural rubber with styrenic block copolymers are preferred. Natural rubber is a well-known component of traditional rubber/resin systems, but its use (in latex form) in emulsion-based PSAs for permanent label stock applications is less widely appreciated [5]. Natural rubber latex can be used in SBR/tackiﬁer resin systems to modify the balance of adhesive properties. High molecular weight natural latexes contribute to improved shear resistance, while the lower molecular weight ones enhance the tack (especially when natural latex is used at less than 10% of the polymer content in the formulation). Furthermore it was claimed that when SBR and natural rubber latexes are blended, a product with better aging characteristics results. Here the SBR latex crosslinks and hardens under oxidative aging, whereas the natural rubber latex undergoes chain scission and softens, whence a balance is achieved [6]. Natural rubber latex-based PSAs oﬀer an advantage over solventbased systems (e.g., milled smoked sheet) with regard to the molecular weight diﬀerence between the two. The high molecular weight portion of natural rubber is insoluble in solvents, therefore it cannot be used in solventbased adhesives. Natural rubber must be milled to a Mooney viscosity of 53 or below to obtain complete solubility (at 30% solids) [7]. The natural rubber from dried latex shows a wider plateau of the modulus and a higher elastic modulus value at 100 C than the milled natural rubber. The degradation does not change the tan peak temperature, but it does reduce the modulus at high temperatures. This modulus reduction relates to the low shear performance of solvent-based systems. An adequate rubber latex has a gel content low enough to allow good quick stick, but high enough to give good holding power when properly formulated [8]. A detailed discussion about the use of natural rubber per se (solid state) or in latex form will be presented in Chap. 8. Synthetic Elastomers Batch-to-batch inconsistency and staining are two problem areas and/or disadvantages of rubber/resin PSAs. Molecular weight and molecular

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weight dispersion both inﬂuence rubber compatibility with tackiﬁers and the adhesion/cohesion balance. Unfortunately the molecular weight and its distribution depend on the natural rubber quality and on the processing parameters. Therefore the scattering of the adhesive performance values remains high for natural rubber-based adhesives. For a better reproducibility synthetic rubber-like products were tested. First pure hydrocarbonbased polymers (polydienes or diene-styrene copolymers) were synthesized; later acrylic and vinyl monomers were combined. Natural rubber-like stereoregulated polydienes, diene-styrene random and block copolymers, and thermoplastic elastomers on styrene-diene, styrene-oleﬁn, ethylenepropylene-diene, acrylic, etc. basis have been synthesized and tested. As discussed in detail in [9], unlike natural rubber, certain of these products can also be used for noncoated pressure-sensitive products. Various raw materials (monomers) and polymerization methods were used to prepare rubber-like products. Synthetic, hydrocarbon-based elastomers, homo- or copolymers, and random or block copolymers (twocomponent or multipolymers), stereoregulated or not, are synthesized and applied as raw materials for PSAs. Low speed crystallization of polyoleﬁns permits their use as semi-PSAs [10]. Macromolecular compounds based on dienes were tested too. Stereoregulated 1,4-cis-polyisoprene is recommended for special formulations [11]. On the other hand mono-oleﬁns like atactic polypropylene are also used. The best results were obtained by copolymerizing styrene with mono- or dioleﬁns. The ﬁrst products were designed for hot-melt PSAs; later carboxylated styrene-butadiene rubber (CSBR) dispersions were synthesized (see Chap. 8). In order to improve the adhesion of polybutadiene-based PSAs, these were modiﬁed with isopropyl dicarboxylate [12,13]. Other synthetic rubbers (butylene- and isobutylene rubber) were tested as tackiﬁers. The use of methyl rubber and polybutylene in adhesives was described in the literature [14]. Development of special products for speciﬁc end-uses, clariﬁcation of the structure/ property relationships, and introduction of new products such as styreneethylene-butylene-styrene thermoplastic elastomers (TPE) and crosslinkable derivatives were documented [15]. As discussed in detail in [9], concerning their chemical build-up and processing technology, synthetic elastomers used as raw materials for PSAs can be divided into polymers with either nonsegregated or segregated structure (i.e., common and thermoplastic elastomers). Nonsegregated Elastomers for PSAs. Synthetic rubber-based PSA formulations were developed as solvent-based, water-based, and 100% solids recipes. The most widely known synthetic elastomers are the styrenediene copolymers, but butyl rubber, polyisobutene, acrylnitrile-diene

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copolymers, halogenated diene-diene copolymers, silicones, and polyurethanes are employed too. The ﬁrst trials to synthesize natural rubber-like elastomers used dienes (alone or as the main comonomer) for the build-up of a mostly linear, noncrosslinked polymer. Later diﬀerent comonomers were added and ethylene-, propylene-, and diene copolymers were manufactured having an elastomer- or viscoelastomer-like behavior. Such synthetic elastomers possess a random sequence distribution. Styrene-butadiene copolymers were developed for the rubber industry to achieve hard and abrasion resistant highly ﬁlled vulcanizable composites for technical products. Synthetic styrene-butadiene latexes were ﬁrst produced and sold into adhesive end-uses in 1946 [16]. Generally, SBR latexes have a solids content of 41–54% with a butadiene content of 70–75%. For PSAs the most eﬀective SBR latexes are those with a styrene level of about 25–35% and a Tg between 60 to 35 C [17]. Low temperature synthetic butadiene rubbers (GR-S) with 30% styrene are soft and tacky; high temperature ones (with 45–80% styrene) are harder and less tacky [16]. The most important properties of SBR latexes used for PSAs include their molecular weight, molecular weight distribution, gel content, Tg, and styrene content [18]. Styrene-butadiene rubbers with 23% styrene are proposed for solvent-based PSAs. In such adhesives the styrene-butadiene rubber exhibits less cohesion and more tack than natural rubber. Styrene-butadiene rubber can be made with or without carboxylic groups. Polar styrene-butadiene latices are manufactured by emulsion polymerization of styrene, butadiene, and a small amount of a functional monomer such as vinyl-carboxylic acid (e.g., acrylic, methacrylic, itaconic, or fumaric acid) either alone or in combination [18]. The functional monomers stabilize the product. They are usually present at a level of about 1.0–3.0%. When COOH groups are not present in the polymer chain, SBRs are usually poststabilized with about 1–4% of surfactants; antioxidants and pH-adjusting agents are also added [16]. The CSBR dispersions possess small (typically 0.15 mm) particles with narrow particle size dispersion. Butadiene can polymerize via two diﬀerent modes of addition, namely the 1,2- or 1,4- conﬁguration. Both conﬁgurations contain residual double bonds capable of further polymer growth, either branched or crosslinked. The resulting three-dimensional network structure improves the mechanical properties apparently without aﬀecting the Tg. The gel content may be adjusted through the ratio of styrene to butadiene and the molecular weight. Tack increases with increasing butadiene content (50–70%), but changes little at higher levels. When measuring peel adhesion the failure mode is adhesive at lower butadiene levels and cohesive at high butadiene levels.

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Shear strength is higher as the styrene content increases. Above 30% gel content (70% butadiene content) the tack decreases rapidly. The usual range of hot carboxylated SBR latexes runs from about 40% styrene to 70% styrene; the normal range for PSAs latexes generally lies below 45% styrene. For many systems the Tg is ﬁxed at 55 C for use in combination with an 80 C softening point resin [19]. The molecular weight and the Tg of carboxylated butadiene polymers constitute major variables that determine the pressure-sensitive properties. Carboxylated SBR latexes consist of a mixture of polymer species having linear chains, branched chains, and crosslinked materials. The relative ratio of the species can be controlled by changing the concentration of molecular weight regulators, and by varying factors such as conversion, reaction temperature, or polymer particle number [19]. In some cases, for the characterization of CSBR, the acid equivalent (carboxyl equivalent) for 100-g latex solids is given [8]. Carboxylated neoprene latexes have the following advantages [8,20,21]: mechanical stability, resistance to electrolytes, crosslinking at room temperature, thermal stability, good polyethylene (PE) adhesion, high shear, low peel, removability, temperature resistance, and conformance with FDA regulations (e.g., 175.105 for adhesives). Aqueous acrylic and modiﬁed acrylic PSAs possess certain limitations, including poor adhesion to polyethylene and polypropylene surfaces. Attempts to improve their performance by the incorporation of tackiﬁers have met with only limited success [18]. Tackiﬁed carboxylated styrenebutadiene rubber latex systems, on the other hand, seem to be able to overcome the apparent inherent limitations of water-based systems and yet match the superior performance characteristics of the solvent-based rubber/ resin systems. Until the 1980s, relatively few SBR latexes were available with the right characteristics necessary for PSAs. The most important factors in the design of CSBRs for PSAs and their interaction with the other components of the system were discussed in detail [18]. When using CSBR latexes it is better to produce a low viscosity latex and control the ﬁnal rheology of the adhesive through the addition of thickeners. A comparative evaluation of the formulation and end-use properties of CSBR and acrylic PSAs was summarized by Benedek [22] and will be discussed in Chap. 6. Some years ago contact adhesives on neoprene basis were developed [21]. Later carboxylated neoprene latexes with better tack for PSAs applications were synthesized [20]. There is a diﬀerence between dienestyrene and diene-polar comonomer copolymers. Polychloroprene exhibits strain-induced crystallization (like natural rubber, but uncontrolled). Styrene-butadiene copolymers do not display such comportment. As a raw material for PSAs, functionalized chloroprene latices are more important.

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The formulation and end-use of carboxylated latexes will be reviewed in Section 2.9 of Chap. 8. Polymers and copolymers of C3 and C4 alkenes include a broad range of plastomers, viscoelastomers, and viscous compounds which can be used as various components in PSPs. The most important products are poly(butenes/isobutenes) and amorphous polyalphaoleﬁns. Ideally diene polymerization supposes 1,4-addition of the diene units with the formation of a natural rubber-like polydiene. In industrial practice 1,2-polymerization may occur, i.e., vinyl units may be inserted in the main polymer backbone. In an extreme situation the polymer backbone is built up from vinyl units and a low level of diene units. Butyl rubber is such a copolymer of isobutylene and 1–2% isoprene. Butyl rubber may be used as a base elastomer in PSAs. Its molecular weight and molecular weight distribution inﬂuence the adhesive performance. Generally blends of butyl rubber having diﬀerent molecular weight are used. Polyisobutylene is an elastomer or plasticizer too. Polyisobutylene (MW 3000) is liquid whereas high molecular weight polyisobutylene (MW 200,000) is a clear solid. The isotactic polymers are to be distinguished from polymers of isobutene (normally termed polybutene) which are used as oily additives, and from amorphous, atactic poly(1-butene) polymers which range from viscous oils to rubbery polymers. The polybutenes used as plasticizers are isobutene-butene copolymers synthesized from macromolecular mono-oleﬁns with low isoparaﬃn content. Polybutenes are saturated therefore they exhibit aging resistance. Amorphous poly(alphaoleﬁns) encompass a group of usually low molecular weight polyoleﬁns which are obtained by coordination polymerization [23]. Atactic polypropylene (APP) is a byproduct in the production of isotactic polypropylene, using ﬁrst-generation Ziegler-Natta catalysts. The atactic polypropylene obtained was ﬁrst used as a raw material for HMPSAs. Later ethylene-propylene copolymers were manufactured. Such polymers cannot be used as adequate raw materials for HMPSA. Their tackifying leads to softening, but such products do not have the required viscoelasticity. The raw materials used for PSAs are the distinctive product types of APAO made by direct reactor synthesis, namely homopolymers of propylene, copolymers of propylene and ethylene, and copolymers of propylene and 1-butadiene [23]. Proper selection of comonomers and synthesis conditions has led to the development of oleﬁn copolymers which are pressure-sensitive [24,25]. The copolymerization of propene with long-chain oleﬁns leads to amorphous and tacky products. The best results have been obtained with hexene and octene as comonomers. Because these materials are neat oleﬁn polymers, they oﬀer several advantages over typical styrene block copolymer-based,

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tackiﬁed PSAs. These include the absence of oils which can bleed into the substrate, fewer skin sensitivity problems, a good thermal stability (<10% viscosity decrease after 100 hr at 172 C in a forced-draft oven), low color and odor, the ability to be blended with most oleﬁn compatible adhesive ingredients for common applications, and ﬁnally a lower density than conventional rubber-based hot-melt PSAs leading to a decreasing usage of raw materials. These raw materials are mainly amorphous terpolymers of alpha and omega oleﬁns. A range of properties can be obtained using this new amorphous polyoleﬁn (APO) technology. Examples include a Tg between 10 to 40 C, a probe tack of 0–800 g, and a static shear ranging from 1 to 30 h. The 180 peel strength (PSTC-1), 90 quick stick (PSTC-5), and shear resistance (hr) amount to 2.5, 0.8, and 8, respectively. New, mainly amorphous ethylene-propylene-butene-(l)-terpolymers can be made tacky quite easily and they exhibit a low speed of crystallization. There are propene-rich and butane-rich copolymers. Propylene/ hexene copolymers were also tested for hot-melt PSA applications [15]. There are pressure-sensitive and semi-pressure-sensitive products. The semipressure-sensitive products display limited pressure-sensitivity in time. They lose their pressure-sensitivity due to crystallization. With hot-melt adhesive formulations with a long ‘‘open time’’ one takes advantage of the slow crystallization. During cooling these products exhibit properties like those of a hot-melt PSA for a long period; afterwards, however, they are no longer tacky. They appear quite suitable for application by spraying. Hot-melt PSAs with an excellent price/performance ratio are based on a highly viscous, amorphous poly(alphaoleﬁn) (APAO), rubber, resin, and a combination of plasticizers. The latter consist of a mineral oil and polyisobutylene (see Section 1.2). High molecular weight elastomers based on acrylates have been produced by emulsion polymerization. Such compounds do not contain carbon-carbon double bonds as polydienes do. Their backbone includes a saturated alkyl and alkoxyacrylate (95–99%) and 1–5% curing monomers, having a chlorine or hydroxyl functionality in the side chain. The main polymer chain is based on ethyl hexyl or butyl acrylate. These polymers can be used for HMPSAs (reactive hot-melts), solvent-based PSAs, pressuresensitive sealant tapes, and sealant caulks. They are used in tackiﬁed or plasticized formulation [26] (see Section 1.2 also). Silicones are used as raw materials for PSAs as well as coated on release liners. In some cases silicone-grafted acrylic copolymers were used in order to achieve good removability. Silicone-based PSAs were developed in 1953 [27]. Silicone PSAs are based on the combination of a ‘‘gum’’ component (highly ﬂexible, linear siloxane rubber) and a (extensively branched) resin [28]. The gum component is a polysiloxane gum, such as methyl phenyl

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polysiloxane; the resin is also an organopolysiloxane. Speciﬁc resins are reacted with linear, high viscosity organopolysiloxane ﬂuids [29]. For instance, for an HM silicone PSA, the resin/silicone ratio was adjusted from 54/45 to 65/53 [30]. Industrially, polydimethylsiloxane and polydimethyldiphenylsiloxane are the main polymers for the production of silicone PSAs. For elevated thermal stability phenylsilicones are suggested, and for better adhesion characteristics and improved ﬁller loading capability methylsilicones are suggested. Such polymers can be crosslinked with benzoyl peroxide. First low solids (60%) silicone adhesives were manufactured. New silicone PSAs are based on a silylvinyl-silylhydride reaction (hydrosilylation). Such addition-based crosslinking is catalyzed by platinum. Such formulations possess 80–90% solids. Silicone polymers have inherently ﬂexible siloxane backbones and a low Tg. Polydimethylsiloxane exhibits a Tg of about 120 C. In comparison acrylics show a glass transition temperature of about 65 C to 40 C. Both contribute to excellent performance at low temperatures; they also oﬀer good weatherability [29]. The performance properties of silicone PSAs can be designed by changing the resin/gum ratio [30]. A comparison was made between a typical dimethyl-based silicone and an acrylic-based organic PSA [31]; the silicone PSAs display good tack, peel, and shear at temperatures as low as 20 C. Problems with silicone PSAs may arise concerning their shelf life and the availability of adequate release liners. While silicone PSAs have existed for over 15 years, there was no adequate release technology available for them; their use was limited to industrial tapes. Several products on the market utilize dimethyl silicone in combination with diphenyl silicone PSAs. While diphenyl silicone PSAs have some advantages in speciﬁc applications, dimethyl silicone PSAs represent the bulk of silicone PSAs because of product cost. New release liner technology based on ﬂuorosilicones was developed, providing a release surface for several types of silicone PSAs. A low energy surface does not suﬃce to oﬀer release; this condition must be satisﬁed, but other conditions that favor a trend towards a low release energy are a contact angle greater than 30 and a negative spreading coeﬃcient. The surface tension of the adhesive must be higher than that of the release coating. Fluorosilicones have critical wetting surface tensions of 15–18 mN/m or lower, but in general do not perform as release surfaces for PSAs because they do not display suﬃcient ﬂexibility and ability to reorient their dimethyl groups like polydimethyl silicones. Crosslinked silicones may also be used as a release layer for silicone PSAs [32]. The organopolysilicones may be produced directly by the hydrolysis of organic substituted halosilicones and depolymerization, or by hydrogenation of

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171

higher molecular weight organopolysilicones and heat treatment. Special silicone groups can be built into common polymers too. Such formulations are discussed in detail in [33]. Segregated Elastomers for PSAs. As discussed in detail in [9] and [34] the properties of polymers can be signiﬁcantly improved by build-up of a multiphase, composite structure. Elasticity requires a reversibly deformable network. The existence of a network does not allow viscous ﬂow. This dilemma can be avoided by synthesizing and formulating viscoelastic compounds that have partially segregated structures and behave like elastic, crosslinked materials or viscous polymers depending on the temperature and their formulation. In such heterogeneous systems one polymer exists above the Tg, while the other exists below its Tg. The one component is glassy, the other is rubbery. The main segregated polymers used as raw materials for PSAs include: styrene block copolymers, and acrylic, polyoleﬁn, silicone, polyurethane, and polyester block copolymers. Thermoplastic elastomers have been known since the 1960s [35]. In 1960 Shell developed Kraton in the USA, and in 1974 Cariﬂex; later, ethylene butylene copolymers (SEBS) were developed. The ﬁrst styrenic thermoplastic elastomers were introduced to the market in 1964 [36]. The styrene block copolymers used today usually have a molecular weight in the range of 60,000–200,000 and a styrene content of 15–45% [37]. Because of their hard segments such polymers are always harder than natural rubber. Since their introduction styrenic block copolymers have found their way into numerous and varied application ﬁelds in the adhesives industry. Their molecular structure allows these thermoplastic rubbers to be formulated and coated in solvent-free hot-melt systems which are widely accepted for a variety of ﬂexible assembly adhesives and as well as PSAs for labels and tapes. Many variations can be made in the structure of these block copolymers which has led to a wide range of grades that diﬀer in physical properties and are well suited for more speciﬁc applications. The most commonly used block copolymers for hot-melt PSAs still are the linear SIS grades with low polystyrene content, although interest is growing in the use of low viscosity SBS grades in combination with newly developed tackifying resins. Styrene-isoprene block copolymers are more expensive than natural rubber. The chemistry of styrene block copolymers is discussed in detail in [26]. Generally block copolymers used for PSAs contain three blocks A-B-A [37] where A is an amorphous polymer with the Tg above room temperature (thermoplastic), and B is an amorphous polymer with Tg much lower than room temperature (in rubber, for example, A is polystyrene and B polydiene). Because of the incompatibility of polystyrene endblocks

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and elastomeric midblocks, there are two microdomains in the polymer; a discontinuous phase and an agglomeration of polystyrene endblocks is formed through van der Waals forces. The ﬁrst thermoplastic elastomers (TPE) were of the linear triblock form, SBS or SIS [38]. The key to their function is what was called a ‘‘spaghetti and meatballs’’ morphology. At an appropriate balance of endblock polystyrene molecular weight vs. those of the elastomer midblock, a two-domain structure is formed which consists of polystyrene islands in a rubbery ocean. Polystyrene is not miscible in polydienes and the polystyrene blocks at the end of each polymer molecule associate with each other, forming an ‘‘associative crosslink’’ at room temperature which behaves like a covalent vulcanized bond. If the polystyrene content is lower than 50%, polystyrene domains are formed through the incompatibility of the polystyrene with the polybutadiene sequence. In general, thermoplastic elastomers are block copolymers or polyblends with separate phases [36]. In these copolymers with an A-B-A structure, the A sequences contain 200–500 units and the B sequence 500–700 units [39]. The basic polymer parameters of SBCs include the nature of the diene and vinylaromatic monomer (e.g., butadiene, isoprene, styrene, -methyl-styrene, etc.), molecular structure (e.g., linear or radial), molecular weight, sequence distribution (e.g., diblock, triblock, etc.), and styrene content. The main midblock sequences are built up from polybutadiene (in SBS), polyisoprene (in SIS), ethene-buthene (in SEBS), ethene-propene (SEPS), and functionalized (maleinized or silanized) dioleﬁn derivatives, with polar groups exhibiting better adhesion to polar substrates [40]. The most commonly used rubber is of the SIS type, such as Kraton D 1107 [38]. The SIS types are widely used in hot-melt PSAs, but are too soft for non-pressure-sensitive hot-melt PSAs [41]. Because of the higher stiﬀness of the polybutadiene blocks such polymers are recommended mostly for non-pressure-sensitive applications. The molecular structure of SIS polymers allows them to be dissolved, formulated, and applied in solvent, or to be formulated and coated in a hot-melt system at temperatures higher than the Tg of the styrene endblocks. Styrene block copolymers can be used as high solids (60–70%) solutions, or as hot-melt. Following evaporation of the solvent or cooling down of the hot-melt, the physical crosslink structure rebuilds to provide the desired strength and performance characteristics. As thermoplastic elastomers have a less stable elastic domain and a second Tg (identical with the melting point) they act like thermoplastics above this temperature. Unfortunately, the physical bonding is less stable [42]. Moreover, the nature of an SIS polymer prevents its use in PSAs that must withstand solvent or high-temperature exposure at or above Tg of the polystyrene endblock.

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Recently new SIS polymers were introduced which can be crosslinked through electronbeam (EB) curing. An example of these new radiationsensitive polymers (Kraton D 1320 X rubber) shows good crosslinking with an electron beam dose of 5–7 Mrad [37,38]; it is a multiarmed SIS block copolymer with only 10% styrene content. Further development of the styrene block copolymers concerns branching, the sequence distribution, and the soft monomer unit. As discussed in [26] side chain segregation depends on the ﬂexibility and length of the branches. The compatibility of the polystyrene endblocks and of the elastomeric midblocks depends on their molecular weight and their volume fractions in the mixture. Phase separation in the radial block copolymer is higher. Therefore physical crosslinking is higher. ABA and AB block copolymers of the same type of composition in another geometry (star-shaped or radial copolymers) have also been developed [43]. According to the structure and placement of the side chains, branched, radial teleblock, and multiarm star block copolymers are known [44]. Such polymer structures are described by the terms ‘‘branched,’’ ‘‘radial,’’ and ‘‘star.’’ Radial and teleblock copolymers with isoprene branches were prepared, but butadiene-based armed polymers have been proposed also [45]. The number and molecular weight of the arms inﬂuences the solid state morphology of such polymers. Structures with more than eight arms are bicontinuous and ordered. According to [44] a formulation with a tackiﬁed star block copolymer gives a higher shear resistance. The mechanical properties of a radial block copolymer are better. A desirable melt viscosity and shear resistance can be obtained by selecting only star block copolymers with six or more arms. Star-shaped styrene block copolymers have an (apparently) higher molecular weight. Therefore they are more reactive to radiation curing. Because of their star structure, partial crosslinking does not modify (essentially) the ﬂexibility of the macromolecules. Polystyrene-polybutadiene-polystyrene polymers are completely amorphous and therefore show limited compatibility with waxes; the polybutadiene midblock is unsaturated and therefore exhibits limited stability during hot-melt processing. The saturated midblock thermoplastic elastomers are commercial ABA-type block copolymers in which the polyisoprene or polybutadiene is hydrogenated. The strength and stiﬀness of the block copolymer depend on the chemical nature of the segments and their mutual interaction. Block copolymers having the same styrene content, about the same block molecular weight, but diﬀerent types of midblock display diﬀerent mechanical properties. The SEBS polymer has the highest strength and modulus. The commercial SEBS polymers have a much better stability during hot-melt processing because their midblock is a saturated polyoleﬁn rubber. The EB rubber midblock of the commercial SEBS

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polymers is essentially a random copolymer of ethylene and 1-butene. In the commercial SEBS polymers the 1-butene content is controlled at a level high enough to make the EB copolymer midblock almost totally amorphous, making the copolymer thoroughly rubbery and soluble in hydrocarbon solvents. In SEBS the saturated polyoleﬁn confers excellent resistance to light, ozone, and heat to the polymer. Styrene block copolymers are usually synthesized via a three-step process. A new process produces styrene block copolymers via a two-step process using a difunctional initiator instead of a monofunctional one [36]. In this new technology the midblock (e.g., the polyisoprene or polybutadiene) is built ﬁrst and the polymer chain grows in two directions at the same time, with two reactive sites, one at each end. In the second step two polystyrene blocks of the SIS (or SBS) are built at the same time. As a result a pure 100% triblock polymer is formed. Uncrosslinked and unentangled polymer chains do not contribute to the strength of the elastomer. Therefore pure triblock SIS copolymers have also been manufactured. Such pure triblock copolymers have a high-Tg endblock [46]. Diblock units work as low cohesion diluting agents and decrease the cohesion. SAFT decreases with the diblock content. Increasing the diblock content improves peel and tack. A range of copolymers with up to 42% diblocks have been synthesized. In [47] blends with up to 54% SI were studied. Diﬀerences due to the change of the SI content occur in the frequency range of 5 103 – 5 101 rad/sec, where an increase in the SI content leads to a decrease of the elastic modulus G0 and to an increase in the loss modulus G00 . In this range the blends are more dissipative if there is more SI. Tensile tests show that decreasing the SI content or increasing the crosshead velocity leads to a higher stress for all extensions and to a more pronounced strain hardening. Other substituted styrene derivatives (e.g., p-t butylstyrene, n-propylstyrene sulfonate, sulfonated SEBS block copolymer, etc.) were investigated also [48–50]. Another regulating possibility of the copolymer performance is given by the nature of the hard monomer. Generally the hard segment size inﬂuences the melt transition temperature of thermoplastic elastomers. For instance -methylstyrene has a higher Tg and therefore provides better thermal properties than styrene [51]. In the early 1980s HMPSAs based on acrylic block copolymers were introduced. Phase-separated acrylic block copolymers display a thermally reversible crosslinking. Unfortunately such experimental copolymers possess a high viscosity, low tack, and shear. The hard blocks of acrylic copolymers can contain styrene derivatives also. Polyurethane-acrylate hotmelt adhesives have been prepared too. As discussed in detail in [26] many attempts were carried out to prepare segregated acrylic block copolymers; however, their industrial applicability is limited. A better approach is given

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by the use of acrylic oligomers and prepolymers able to be macromerized by polymerization or crosslinking (see later). Special, segmented EVAc copolymers have been developed as raw materials for HMPSAs. They have a higher vinyl acetate content, and can be considered elastomers. Such polymers possess alternative segments with low and high vinyl acetate content. As mentioned earlier, synthetic hetero block copolymers were developed also. Such nonhydrocarbon-based TPEs contain heteroatoms (e.g., silicones, polyurethanes, etc.). The mixing of siloxane resins with a silicone elastomer in 1953 led to the discovery of silicone PSAs [27]. As described in [52] the high solids silicone adhesives are multicomponent systems which contain at least four components: a base elastomer, a tackiﬁer resin, and at least two crosslinkers. Such silicones have a block structure in comparison with the random build-up of peroxide-cured silicones. Polyurethane adhesives are used for special applications. Twocomponent and single-component polyurethanes were developed. The adhesive polymers are composed of end-functionalized soft segments (polyether or polyester macroglycols) connected via a urethane linkage and a spacer to an isocyanate group. The spacer is typically an aromatic unit arising from 4,40 -methylene bis(phenylisocyanate) (MDI) or toluene diisocyanate (TDI) used during synthesis. Two-component-based reactive polyurethanes result from the combination of polyol-polyisocyanate or -isocyanate terminated prepolymers cured with polyamines or polyols. The polyether or polyester is ﬂexible. Single part polyurethane adhesives are polymers of low molecular weight, whose chains are terminated with isocyanate groups. These NCO groups are free to react with moisture, either present already on the web or added as water fog at the nip station. Polyurethane adhesives with 100% solids, solvent-based ones, and dispersions were developed. Emulsiﬁer free and (cationic, anionic, and nonionic) emulsiﬁer containing polyurethane dispersions can be manufactured. Thermoplastic urethanes are synthesized from diisocyanates, short chain diols, and polymeric diols. Generally the isocyanate can be regarded as a stiﬀ component of the copolymer, while the polyol may be regarded as a relatively ﬂexible component. The stiﬀ segments tend to form laterally ordered groups through intermolecular bonding. Flexible groups are disordered and randomly oriented. They are necessary to provide toughness and ﬂexibility. If the ﬂexible backbone is suﬃciently long, a certain amount of coiling occurs, giving rise to rubbery domains. Polyesterdiols and polyetherdiols can be used as soft segments for thermoplastic urethanes. Polyetherdiols have various chemical bases, structure, and molecular weight. They include mainly C3 and C4 ethers [53].

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The Tg and morphology of such compounds are inﬂuenced by the length and concentration of the polyethers. A foamed adhesive for tapes was manufactured from a mixture containing an isocyanate terminated monomer or oligomer, and a polymer comprising a backbone of polymers such as polybutadiene, polyesters, and polyether which must contain at least two active hydrogens capable of reacting with the isocyanate terminated monomer and expandable ﬁller. Such foam can be manufactured in situ [54]. Special polyurethanes contain vinyl unsaturation as the reactive site. Thermoplastic elastomers may contain oxygen as a heteroatom in block copolymers based on esters. Such a copolymer can have hard segments of polybutylene terephthalate and ﬂexible segments of polyalkylene oxide terephthalate. It displays insuﬃcient thermal resistance and inadequate pressure sensitivity. Better results were obtained with PSA formulations based on radiation curable macromers (see later). Special TPEs were discussed in detail in [55]. As discussed above elastomers are used in two-component PSA formulations. As described in [56,57] in some special cases certain base elastomers are used per se, without tackiﬁcation. Natural rubber latex, CSBR, or butene derivatives can be employed for special products (mostly with low adhesivity) without tackiﬁcation. 1.2

Viscoelastomers

The special structure of the diene monomers (e.g., butadiene, isoprene, etc.) leads to macromolecular compounds displaying rubber-like properties. In order to achieve viscoelastic properties the natural, synthetic diene-based elastomers must be combined with tackiﬁers. In order to achieve ﬂuidity at the processing temperature and a good adhesion/cohesion balance at room temperature, synthetic block copolymers (with a crosslinked structure) were synthesized. Both tackifying or internal crosslinking are used in order to achieve balanced PSA properties. The mostly nonpolar backbone of the hydrocarbon-based elastomers requires formulating in order to achieve a suitable adhesion/cohesion balance. The use of polar monomers allows the synthesis of single-component, unformulated elastomers with an adequate viscoelastic balance. The most common monomers used for this purpose are acrylic and vinyl monomers. It is evident that acrylics have the most sophisticated chemical basis and therefore may satisfy the extreme requirements concerning the diﬀerent physical state (solvent-based, dispersed, or hot-melt PSAs) and end-use properties (permanent/removable, paper/ﬁlm). On the other hand, the new ethylene multipolymers will win new ﬁelds of application due to their dual (technical/economical) advantages.

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Acrylic Viscoelastomers It is generally agreed that PSAs based on acrylics are the most important adhesives for the production of label stock. In the past several years new classes of raw materials for PSAs (e.g., CSBR, ethylene vinyl acetate copolymers) were developed. There was a considerable interest recently in a comparative evaluation of acrylics and other raw materials in order to forecast development trends. The development of acrylic polymers allowed the synthesis of pure elastomers that are not crosslinked and may present a viscoelastic behavior. Later, other comonomers, usually vinyl acetate and its derivatives, were copolymerized with acrylics mainly for economical reasons. The search for a low-cost, low-Tg monomer forced the development of ethylene vinyl acetate elastomers and, related to these compounds, a new renaissance of the maleinate copolymers. Actually there is a broad range of uncrosslinked elastomers and viscoelastomers that are suitable as base materials for PSAs. Pure Acrylics. Acrylics have been used in PSA applications since 1928 [58]. In a manner diﬀerent from natural and synthetic rubbers, hydrocarbonbased acrylates (AC) are synthesized from esters of polar, organic acids, mainly acrylic and methacrylic acid [59]. The comonomers are the acids or other nonacrylic monomers. The choice of the main monomer determines the Tg of the polymer and its characteristics, especially the tack. Introduction of comonomers allows the (re)adjustment of the adhesion/cohesion balance. Comonomers with reactive functional groups allow crosslinking and thus the improvement of the shear resistance. An advantage of the acrylics is their synthesis-based pressure-sensitive character (i.e., they possess pressuresensitive properties without additional tackiﬁers and they display stable storage and aging properties). The acrylics are inherently tacky due to their high entanglement molecular weight (Me), without the need to add tackiﬁer [60]. Moreover, acrylics are not sensitive towards oxidation. Their adhesion, cohesion, and tack do not change after long aging periods at high temperatures. Another advantage of acrylics is their lack of sensitivity towards UV light; acrylics are colorless and they do not change their performance characteristics during aging; they also display high transparency. Acrylics are resistant towards plasticizer migration; they do not contain oligomers and therefore they do not give rise to migration. Acrylics are polar, so they display a good adhesion towards polar substrates. Thanks to an adequate adhesion/cohesion balance, acrylics usually display good wet-out properties and die-cuttability. Because of their wide chemical basis and well-known chemistry it is possible to polymerize acrylics in solution, dispersion, or in the bulk state;

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there are solvent-based, water-borne, hot-melt, and radiation-curable acrylic PSAs. The relatively low molecular weight and linear, gel-free structure of solvent-based acrylics allows the use of a broad range of solvents, while the chemically inert nature of these solvents makes the use of diﬀerent crosslinking agents possible. On the other hand the polarity, water insolubility, and high reactivity of the main acrylic monomers allow the synthesis of mechanically stable, protective, colloid-free, ‘‘grit-free’’, aqueous dispersions with a high solids content. At the present time hot-melt acrylic PSAs are at the developmental stage; this is also the case for radiation-cured, solventless adhesives. Polyacrylates are used mostly as solutions or dispersions. Solid state polyacrylate rubbers are also known [61]. They are produced by emulsion polymerization technology to form a latex, which is subsequently coagulated, washed, and dried in sheet form. Polyacrylic rubbers possess excellent physical characteristics. They are resistant to a wide range of chemicals and oils, have good low and high temperature resistance, and a high degree of resistance to weather, atmospheric oxidation, and UV due to the saturated backbone. Polyacrylates do not contain double bonds (as in diene rubber). They are basically saturated copolymers containing 95–99% backbone monomers consisting of alkyl, or alkyl and alkoxy acrylates such as ethyl, butyl, methoxy ethyl, and ethoxy ethyl, and 1–5% cure site monomers with chlorine or hydroxy functionality for vulcanization purposes. The main applications of polyacrylate rubbers are solvent-based and hot-melt PSAs (100% solids) pressure-sensitives. New solid and liquid polyacrylates with many functional groups (e.g., hydroxy, carboxy, epoxy, and isocyanate groups) for possible crosslinking were also developed. Physical characteristics of polyacrylic elastomers include resistance to temperature (40 to 200 C), petroleum, synthetic oils, aliphatic hydrocarbons, oxidation, ozone, and UV light. Even though it is not the aim of this book to discuss the synthesis and monomer basis of PSAs, a short overview of the acrylic components follows. Long-chain polyacrylates do not exhibit suﬃcient cohesive strength; their cohesion may be improved by copolymerizing with n-butyl acrylate, acrylic acid, and other polar comonomers such as glycidylmethacrylate, N-vinylpyrrolidone, methacrylamide, acrylonitrile, etc. [62]. The patent literature generally identiﬁes alkyl acrylates and methacrylates of 4–17 carbon atoms as suitable monomers for PSAs [63]. The most commonly used monomers are 2-ethyl-hexyl acrylate, butyl acrylate, ethyl acrylate, and acrylic acid. Monomeric acrylates or methacrylate esters of a nontertiary alcohol (having 1–14 carbon atoms) with the average number of carbon atoms between 4 and 12 may be used [64]. Additional monomers (e.g., itaconic acid, methacrylic acid, acrylamide, vinyl acetate, etc.) can be included in minor

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amounts (e.g., a 74:20:6 iso-octyl acrylate:n-butyl acrylate:acrylic acid monomer ratio or a model composition having 97.5/2.5 wt% of EHA or BuAc and AA [65]). Similarly other acrylates derived from alcohols with 4–14 carbon atoms may be included. As discussed earlier in Chap. 2, deﬁnite trends in physical properties of the polymers can be observed, when higher molecular weight acrylic esters are used. As their ratio to low molecular weight monomers is increased, the following changes in properties occur, namely, the tackiness increases, the hardness decreases, the tensile strength decreases, the elongation increases, and the water absorbtion decreases. The term hard monomer refers to a monomer which when homopolymerized yields a polymer with a Tg above 25 C, preferably above 0 C. Among such monomers are methyl acrylate, alkyl methacrylates (e.g., methyl methacrylate, ethyl methacrylate, and butyl methacrylate), styrenic monomers (e.g., styrene and methyl styrene), and unsaturated carboxylic acids (e.g., acrylic acid, methacrylic acid, itaconic acid, and fumaric acid). The term soft monomer refers to a monomer which when homopolymerized yields a homopolymer with a low Tg (i.e., less than 25 C). Examples are the alkyl acrylates (e.g., butyl acrylate, propyl acrylate, 2-ethyl hexyl acrylate, iso-octyl acrylate, and isodecyl acrylate). Presently EHA and/or butyl acrylate are preferred [66]. Soft monomers, like the diester of fumaric acid (dibutyl fumarate) with 2–8 atoms in the ester group or alkyl acrylate with 2–10 carbon atoms (e.g., EHA), should be incorporated at 50–85% in PSAs [67]. Hard monomers like alkyl methacrylate (e.g., MMA) with 2–6 carbon atoms and unsaturated carboxylic acid (e.g., acrylic acid) with 2–8 carbon atoms should be incorporated at 0–10% in PSAs. As an ideal formulation for solvent-based and water-based PSAs, the following composition was given [68]: 70–90% soft monomers, 10–30% hard monomers, and 3–6% functional monomers. Kuegler [69] discussed copolymerization of acrylic esters with a minimum amount of fumarate diesters. Copolymers containing octyl acrylate, ethyl acrylate, vinyl acetate, and maleic anhydride were patented by the National Starch and Chemical Corporation some years ago. These PSAs have a Tg range of 45 to 65 C [70]. Emulsion acrylic PSAs from acrylicbased fumaric ester interpolymers may be synthesized when peel and tack values can be modiﬁed without aﬀecting the cohesive strength [67]. It should be noted, that some acrylic comonomers play a special role in PSA synthesis. Such monomers assure enhanced chemical reactivity or a physical reinforcing eﬀect. Such monomers are: acrylic acid, acrylnitryl, and certain polyfunctional acrylates. As discussed earlier, acrylic acid is incorporated in PSA compositions in order to improve the polarity and/or to allow crosslinking reactions. The role of the carboxyl (COOH) sticker

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groups in the peel adhesion strength of elastomers is well known. Adhesion went through a maximum at some optimal concentration of such groups (ca. 3 mol%) [71]. For polymers with acrylic acid a much higher adhesion hysteresis is obtained as compared to copolymers without AA [72]. This might be partly due to the hydrogen bonding formation and surface rearrangements during contact when AA is present. Acrylic acid content dramatically increases the adhesion and adhesion hysteresis of acrylic copolymers. The addition of AA also decreases the critical crack propagation speed above which a much stronger rate dependence of the adhesion energy is observed. Pocius et al. [73] have studied the release behavior of acrylic tapes from release coating based upon long alkyl side-chain polycarbamates. Pressure-sensitive adhesive-like networks with 10% AA show diﬀerent release behavior from those without acrylic acid. The work of adhesion and the interfacial energy diﬀer for PSAs with or without acrylic acid. As described in [73] the use of PSAs containing AA on vinylcarbamate release coating may lead to blocking. Acrylnitryl is incorporated in PSA copolymers in order to improve their cohesion (through the special physical interactions exerted by the nitryl group). As discussed in detail in [74–85] such an eﬀect can be observed in various acrylic or vinylaromatic, heterocyclic or diene copolymers of AN and acrylates. In some cases a special sequence distribution can be achieved also [80,81]. Polyfunctional acrylates are used generally as crosslinking monomers. For instance 1,6-hexane diol diacrylate (HDDA) was proposed for acrylic model compounds [73]. Such monomers are used in the oﬀ-line or in-line synthesis of PSA raw materials (see Section 1.3). The polymerization technology inﬂuences the properties of the acrylic copolymers. Generally acrylates can be polymerized in solution or emulsion. Solution or water-based emulsion polymerized products diﬀer concerning their macromolecular characteristics (molecular weight, molecular weight distribution, and gel content). Research has shown that emulsion polymerization of acrylates carried out to complete conversion produces signiﬁcant amounts of microgels inside the particles due to chain transfer to the polymer via hydrogen abstraction [86]. According to [65], for gel-free, low-Tg acrylics (based on EHA or BuA) there was no signiﬁcant diﬀerence between emulsion vs. solvent-borne PSA ﬁlm adhesive performances. On the other hand emulsion and suspension polymerization give diﬀerent PSA raw materials as a function of the particle size [65,87]. Solution polymerization carried out using batch or continuous feed, with constant or variable comonomer ratio led to diﬀerent products [88]. For instance, a critical fraction of EHA was determined for which there is a transition between adhesive and cohesive failure. The critical fraction depends on the

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181

polymerization process. Depending on the polymerization yield the comonomer content varies also. Another parameter of inﬂuence is if tackiﬁcation was carried out during polymerization or during formulation [89]. The phase structure of PSAs mixed in the monomer state and solution polymerized was diﬀerent than that expected from the phase diagrams of solution blended systems. The lower molecular weight of acrylates and some reactions of acrylates with rosin caused the ﬁrst system to be more compatible. Natural rubber was used almost exclusively as the base elastomer for PSAs, in the early stages of PSA manufacturing [90]. It remains diﬃcult to synthesize high molecular rubber-like acrylics with a low softening point range, although polyacrylate elastomers have been known for about 20 years. There are several patents including such recipes, but their practical use is limited. For instance acrylic hot-melt PSAs with an improved creep resistance at ambient temperatures and desirable melt viscosity at elevated application temperatures are prepared by copolymerizing acrylic and methacrylic acid and alkyl esters with 10–40 wt% of an acrylate- or methacrylate-terminated vinyl aromatic monomer-based macromolecular compound [91]. In recent years acrylic-based adhesives which possess a balance of high tack, high peel, and high shear properties were prepared by coating a blend of monomers (such as iso-octyl acrylate and acrylic acid) on a web, maintaining an inert atmosphere, and polymerizing the blend in situ. The commercial preparation of such products, however, requires a substantial investment in unconventional manufacturing equipment [92]. Currently lowvoltage electron beam accelerators with high output are used [93]. Acrylics play a determinant role in this technology too. As stated in [94] most EB curable PSAs are acrylic functionalized, they are based on (1) postcrosslinking of SIS-HMPSA; (2) acrylate functionalized polyester prepolymers; (3) acrylate functionalized liquid rubber; or (4) acrylate prepolymers. Ultraviolet light-induced on-line curing of special prepolymers oﬀers a less expensive technology for acrylic hot-melts. Such acrylic hot-melts are prepared by solution polymerization of the monomers along with a small amount of UV-reactive comonomer [95,96]. Under the action of the UV light the photoreactive groups crosslink the acrylic polymer backbone molecules by a chemical grafting (see Section 1.3 also). Acryl-Vinyl Copolymers. The main competitors to acrylic monomers are the acrylic copolymers; they are macromolecular compounds with a relatively high (more than 10% wt/wt) content of a second, main comonomer such as vinyl acetate (AC/VAc) or styrene (AC/S).

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Vinyl-Based Viscoelastomers Among the nonacrylic-based copolymers, one needs to mention the vinyl acetate and ethylene copolymers and the vinyl ethers (VE). Ethylene copolymers are elastomers with a second, main comonomer such as vinyl acetate (EVAc), vinyl acetate and acrylics (EVAc AC), or maleic acid derivatives. In order to produce vinyl acetate copolymers maleic acid derivatives are usually copolymerized with vinyl acetate, in order to lower the Tg of the poly(vinyl acetate/maleinate). Water-borne adhesives based on polyvinyl acetate were discussed in detail [97]. Vinyl Acetate Copolymers. Polyvinyl acetate homopolymers have been used as adhesive dispersions since 1940 [98]. Later they were plasticized using alkyl maleinates and acrylates as comonomers. Vinyl acetate copolymers display excellent versatility for carrier materials and adhesive or nonadhesive coatings. Unfortunately, such copolymers are not suitable for high performance pressure-sensitive use. The ﬁrst copolymers with ethylene were synthesized in 1960. From the range of vinyl acetate copolymers usable for pressure-sensitive adhesives and pressure-sensitive products, ethylene-vinyl acetate (binary or multicomponent) copolymers are the most important products. These are used as bulk materials (for hotmelts) or as dispersions. Functionalized ethylene-vinyl acetate copolymers have been synthesized also. Special products include maleic acid as comonomer. Ethylene copolymers are described in the next subsection. Vinyl acetate-vinyl pyrrolidone copolymers are used mainly in water dispersible or soluble formulations as base elastomers [57]. Polyvinyl Ethers. Polyvinyl-, ethyl-, and isobutyl ethers were used some years ago as a main component of PSAs [99]. They are soluble in a broad range of solvents (e.g., ethanol, acetone, butyl acetate, toluene, and ethylacetate); methyl ether also is soluble in water. This polymer has a good adhesion on polyethylene, is not sensitive towards atmospheric humidity, and therefore has been used on a large scale for roll stock materials. Because of their resistance to plasticizers, polyvinyl ethers (PVEs) were proposed as a tackiﬁer for acrylic-based formulations for PVC [100]. The water-soluble copolymer of methyl vinyl ethers and maleic anhydride is used as a thickener [101]. The most common polyvinyl ethers are based on vinyl methyl ethers, vinyl ethyl ethers, and vinyl isobutyl ethers [102]. The polymers are supplied either solvent free, as solutions, or as dispersions; the vinyl ether polymers may also be coated in the molten state [102]. Water vapor transmission of PVEs has the same value as for human skin [100]; therefore PVEs are recommended for medical tapes. Because of

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their water solubility PVEs are used for water-removable labels and as well as on humid surfaces [99]. Because of their compatibility with plasticizers PVEs are migration resistant and are recommended for PSAs coated on soft PVC. Polyvinyl ethers may also be used as tackiﬁers for acrylic dispersions, and polyurethane- and electron beam-cured PSAs; thus, polyvinyl ethers are used as tackiﬁers in crosslinked PUR-based PSAs and for electron beam-cured acrylic PSAs [103–105]. Polyvinyl ethers are mainly used in water-based PSAs in order to impart a hydrophilic character or as a tackiﬁer [102]. Unfortunately they are storage sensitive and have to be stabilized using protective agents. Polyvinyl ethers also are used as adhesives applied on PE [99,105]. Ethylene-Based Viscoelastomers In order to use ethylene polymers as raw materials for PSAs, their plastomeric character must be changed into an elastomeric one, and they must display a good compatibility with viscous components to achieve the unique viscoelastic behavior of PSAs. Through copolymerization branched structures are introduced into the ethylene linear backbone, thus it is made less compact. A level of 20% vinyl acetate makes it is PE practically crystalline, a level of 40% produces a fully amorphous material. Above 30% vinyl acetate, EVAc copolymers behave like elastomers [106]. Recognizing the potential advantages of EVAc for use in PSAs, a lot of research and development work was carried out in order to come up with practical, commercial systems. Ethylene-vinyl acetate (EVAc) copolymers have been available for more than 25 years; their wide use in non-pressure-sensitive bonding applications is based to a large extent on improvements over other adhesives in those areas where traditional PSAs were deﬁcient. In addition EVAc feed stocks were relatively low in cost and readily available [107]. According to [108] the ethylene content of an ordinary EVAc latex is about 15%; such a dispersion is manufactured by emulsion polymerization at medium pressure (4 MPa). The most important methods used for producing ethylene-vinyl acetate latex with high ethylene content and long sequence ethylene-ethylene structure include: emulsion polymerization at high pressure (20 MPa); dissolution of an ordinary EVAc polymer in an organic solvent which is added to water containing emulsiﬁer at high stirring rate, and then evaporation of the solvent; direct emulsiﬁcation of an ethylene-vinyl acetate copolymer at high speed and high pressure; and dissolution of an EVAc copolymer in a monomer mixture of VAc and acrylate. In ethylene-vinyl acetate copolymers ethylene is the softening component; its softening eﬀect is stronger than that of dibutyl maleinate.

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For an equivalent softness of EVAc copolymer ﬁlms a level of 43% dibutyl maleinate, 45% butyl acrylate, 73% ethyl acrylate, or only 25% ethylene is necessary. As little as 18% ethylene content decreases the Tg of polyvinyl acetate down to 0 C. For the same eﬀect 20% vinyl maleate, 30–32% dibutyl maleinate, or 33–34% butyl acrylate are required [109]. The development of soft, tacky PSAs, starting from the hard vinyl acetate homopolymer is illustrated in Fig. 5.1. Dupont introduced the ﬁrst commercial products under the Elvax trademark in the early 1960s for use in hot-melt PSAs. These proved to exhibit excellent properties compared to the polyethylene-based hot-melt PSAs and the aqueous adhesives such as dextrine and polyvinyl acetates. Water-borne EVAc for laminating adhesives were patented and introduced by Air Products in 1973. They quickly became the standard for many applications, particularly as replacements for externally plasticized polyvinyl acetate emulsions. Polymerization of EVAc-containing polymers with intrinsic pressuresensitive properties is a more basic approach. Ethylene-vinyl acetate chemistry requires specialized, high pressure polymerization equipment and most of the work was carried out at companies having prior expertise.

Figure 5.1

Transition from hard, non-tacky to very soft, tacky PSA.

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185

Wacker Chemie GmbH was awarded the ﬁrst U.S. patent [110]. Wacker’s claims covered a copolymer blend of the following composition: Monomer

% by weight

Ethylene Acrylic ester Vinyl acetate Methacrylamide Others

10.0–30.0 29.0–69.0 20.0–55.0 0.2–8.0 0–12.0

The Tg range is speciﬁed at 20 to 60 C. The National Starch and Chemical Corporation is the other current holder of U.S. patents for EVAc-based interpolymer PSAs. A diﬀerent approach was taken at National Starch; drawing on extensive EVAc experience in the packaging ﬁeld, it was decided to maximize the ethylene content in the copolymer. A high EVAc content provides optimum speciﬁc adhesion and plasticizer tolerance when the adhesives were used on ﬂexible PVC. While the original National Starch EVAc materials retained their peel value when aged on plasticized PVC face stock material, the cohesive strength was low. Development work resulted in a new U.S. patent [56], covering the following composition: Monomer Vinyl ester of alkenoic acid Ethylene Di-2-ethyl hexyl maleate or di-n-octyl maleate or the corresponding fumarates Monocarboxylic acid

% by weight 30–70 10–30

20–40 1–10

Wacker Chemie GmbH continued developmental work and high solids vinyl acetate ethylene dispersions (with 60% solids) were synthesized [111]. The tackifying inﬂuence of maleic acid and maleic acid derivatives on vinyl acetate adhesives was recognized since the early development of vinyl acetate copolymers [112]. The ﬁrst trials of incorporating polar monomers into ethylene (or ethylene vinyl acetate) copolymers were carried out with acrylates. Unfortunately, faster reacting acrylates (relative to ethylene or vinyl acetate) have a tendency towards block copolymerization with vinyl acetate.

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The disparity in the reactivity ratios between ethylene and acrylates with respect to vinyl acetate also limits ethylene incorporation into a terpolymer system. Maleates (or fumarates), on the other hand, do not readily homopolymerize, leading to alternating copolymers with vinyl acetate; this, coupled with a much more favorable reactivity towards vinyl acetate and ethylene, enables the production of terpolymers with an increased ethylene content. The two preferred comonomers among EVAc terpolymers for optimal pressure-sensitive performance are dioctyl maleate and 2-ethyl hexyl acrylate. The higher molecular weight of dioctyl maleate allows a lower mole fraction, permitting a higher ethylene content. Thus polymers commercialized by National Starch contain 20% or more ethylene by weight. Acrylic and ethylene copolymers may be incorporated into aqueous ethyl hexyl acrylate-based PSA systems [113]. Water-borne EVAc multipolymers, resulting from high pressure polymerization of ethylene and vinyl acetate with additional monomers, constitute a new class of PSAs. Their properties combine the most desired features of acrylic copolymers and tackiﬁed rubbers: high quick stick, resistance to oxidation and discoloration, clarity, adhesion to low energy surfaces, and compatibility with plasticized PVC material [114]. A comprehensive treatment by Benedek [22] covered attempts to utilize EVAc dispersions as basis for formulating PSAs. Two decades ago Benedek compared tackiﬁed EVAc- and SBR-based PSAs with acrylates and concluded that, with the exception of shear strength, the adhesive properties of EVAc and SBR PSA dispersions are inferior to those of acrylate dispersions. This statement is still valid. Acrylics exhibit lower peel adhesion and tack than ethylene-vinyl acetate dioctyl maleate copolymers. Tackiﬁed SBR, although exhibiting the tack of these polymers, generally displays lower initial peel strength [115]. Generally the monomers are polymerized in an aqueous medium under pressure not exceeding 100 MPa. The quantity of ethylene entering into the polymer is inﬂuenced by the pressure, agitation, and viscosity of the polymerization medium. Thus in order to increase the ethylene content of the copolymer, higher pressures are to be employed. A pressure of at least 10 MPa is common [115]. As mentioned earlier, ethylene-vinyl acetate copolymers were used since the beginning of hot-melt adhesive technology as raw materials for hot-melt adhesives. The positive inﬂuence of the vinyl acetate comonomer units on the sealability of the ethylene-based plastics is well known for those familiar with the bonding technology of polymers. Ethylene-vinyl acetate as a semicrystalline polymer imparts internal strength, ﬁlm-forming characteristics, and ﬂexibility to hot-melt systems [116]. Unfortunately EVAc copolymers do not possess the agressive tack required for hot-melt PSAs.

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In the past decade some development was achieved through the synthesis of segmental EVAc copolymers, containing sequences with high vinyl acetate content and sequences with low vinyl acetate content [117]. These hot-melt PSAs possess the well-known UV and oxidation resistance of the ethylene-vinyl acetate copolymers. As discussed in detail in [118] ethylene-vinyl acetate copolymers have been suggested as detackiﬁers, and antiblocking and release agents also. 1.3

Viscous Components

Rubbers are elastic materials, exhibiting a pronounced self-recovering character. As PSAs are viscoelastic formulations, a change of the elastic character is possible by decreasing the plateau modulus. A decrease of the plateau modulus is produced by the use of low modulus and/or low-Tg formulating additives. The latter are viscous materials, providing a pronounced viscoelastic character to the rubber/additive blend. Their use as a reducing agent of the modulus is well known from PVC chemistry where plasticizers are used to transform hard and brittle materials into soft, elastic ones. Later on they were used to soften the polyvinyl acetate-based adhesives. Plasticizers The addition of a plasticizer has an eﬀect similar to that of a tackifying resin. Plasticizers improve the ability of PSAs to ﬂow under bond formation conditions, but reduce the higher frequency modulus, thereby improving tack and peel. A detailed discussion of the plasticizers used in PSAs formulations is given in Chap. 8. Tackifiers Pressure-sensitive adhesives are usually blends of rubbers with low molecular weight resins. The resin is described as a tackiﬁer if added to the rubber, as the tough dry rubber is converted into a product with PSA properties. As stated in [119] tackiﬁer resins are compounds having a low or medium molecular weight, used to change and control the rheology of base elastomers and viscoelastomers in pressure-sensitive formulations. According to the best known theory of tackiﬁcation the resin is a solvent of the polymer; in tackiﬁed blends multiphase structure exists. However, there are reactive resins also which can chemically modify the base elastomers or viscoelastomers [120]. The molecular weight and MWD, chemical nature, polarity, and color of the resins play an important role in their use. Diﬀerent natural or synthetic resins, e.g., rosin derivatives,

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hydrocarbon resins, coumarone indene, polyterpene, terpene-phenol, and phenolic and ketone resins are used as tackiﬁers. Tackiﬁer resins were described in a detailed manner by Rich [121]. Reactive resins and hybrid resins are employed too. Reactive resins are functionalized common resins or special synthetic products with reactive groups, hybrid resins are mixtures of chemically diﬀerent tackiﬁers. Colophonium derivatives, acid resins, resinates, and resin esters, and their chemistry, properties, applications, and suppliers were discussed by Jordan [122,123]. The properties of hydrocarbon resins are covered by Jordan [124,125] and the use of resins in the rubber industry was reviewed by Fries [126] who describes the structure, properties, and use of diﬀerent resins (hydrocarbon resins, rosins, and phenolformaldehyde resins) in a comparative manner. A short review of tackiﬁers is given in [127]. Among natural resins the rosin derivatives are the most widely used ones. Rosin is a thermoplastic acid resin obtained from pine trees. The rosin group consists of rosin, modiﬁed rosin, and derivatives. The most widely used rosin acids include abietic acid, neoabietic acid, primaric acid (neoprimaric acid), dehydroabietic acid, dihydroabietic acid, and tetrahydroabietic acid [126,128]. The storage stability and aging stability of the resins may be improved by hydrogenation, disproportionation, dimerization, and esteriﬁcation [128]. There are diﬀerent methods to extract the rosin. Gum rosin is obtained as oleoresin from the living tree. Wood rosin originates from aged pine, whereas tall oil rosin is obtained from tall oil which is a byproduct of the paper industry. Diﬀerent kinds of resins (e.g., disproportionated tall oil rosins, polymerized tall oil rosins, cured rosins, hydrogenated wood rosins, and polymerized wood rosins) were suggested [129]. The dimerized rosin acid content (35–60%) may yield harder or softer colophonium resins [130]. The range of resins used in formulating is illustrated in Table 5.1. Pentaerythritol esters of hydrogenated rosins with a ring-and-ball softening point of 104 C and an acid number of 12 are also used. Hydrocarbon resins are by far the preferred tackiﬁers in PSA formulations, representing about 80% of the market. Hydrocarbon-based resins are low molecular weight polymers based on petroleum, pure monomers, natural terpenes, or coal derivatives [128]. The most important raw materials are aromatic C9–Cn fractions, C4–C5 dioleﬁns, styrene, methyl styrene, vinyl toluene, dicyclopentadiene, alpha and beta pinene, dlimonene, isoprene, and piperylene. Generally, hydrocarbon resins are classiﬁed as follows: aliphatic, aromatic, alkyl aromatic, and hydrogenated hydrocarbon resins. Low color, inert, low molecular weight, thermoplastic hydrocarbon resins produced from the terpene monomer alpha pinene with a ring-and-ball softening point of 112–119 C were suggested also.

Chemical Composition of PSAs Table 5.1

189

The Range of Resins used for Formulation Purposes Softening point ( C)

Resin type

Trade name

Gum rosin Wood rosin Tall oil rosin Polymerized rosin Hydrogenated rosin Pentaerythritol wood rosin Glycerine-hydrogenated rosin

— — — Poly-Pale Staybelite Pentalyn A Staybelite Ester 10, Dermulsene DEG Pentaerythritol-hydrogenated rosin Pentalyn H Glycerine-highly stabilized rosin Foral 85 Pentaerythritol-highly stabilized rosin Foral 105 Hydroabietyl phthalate Cellolyn 21 Oleﬁn Hydrolin Cycloaliphatic hydrogenated oleﬁn Permalyn Aliphatic petroleum hydrocarbon Piccopale/Piccotac Modiﬁed aromatic hydrocarbon Hercotac AD Dicyclopentadiene Piccodiene Mixed oleﬁn Statac, Super Statac, Wingtack Alpha pinene Piccolite Beta pinene Croturez, Piccolites Terpene Zonarez, Nirez 100 Alpha-methyl styrene-vinyl toluene Piccotex Alpha-methyl styrene Kristalex Styrene Piccolastic Terpene phenolic Piccofyn Dermulsene DT 75 Coumarone-indene Cumar

Drop

R&B

— 81 — 102 75 111 — 83 104 82 104 63 — — — — — — — — — — — — — — — —

78 73 80 95 68 — — — — — — — 100–135 85–135 70–122 85–115 73–140 75–105 — 10–135 10–135 10–145 75–120 25–120 5–190 100–135 — 100

In some cases special chemically reactive synthetic resins such as phenolformaldehyde or melamine condensation products can be used. A detailed discussion of resins will be given in Chap. 8. 1.4

Components for In-line Synthesis

The manufacture of the adhesive can be carried out oﬀ-line or in-line. Oﬀline manufacturing includes mixing of the adhesive components followed by

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coating of the ready to use PSA. In-line manufacture of the PSA consists of simultaneous coating and curing or postpolymerization, i.e., it is an in situ manufacture of the adhesive. In this case a special ‘‘ready to coat ’’ mixture of polymerizable or crosslinkable monomers, oligomers, or polymers (e.g., radiation cured hot-melts) is ﬁrst applied onto a carrier. Such a reaction mixture is transformed after coating into a ready to use adhesive. The postcoating synthesis of the PSA must be carried out by the converter. In some cases the in-line transformation of the coated layer concerns only the modiﬁcation of an adhesive, in order to improve its performance (e.g., crosslinking of solvent-based acrylates used for protective ﬁlms, in order to achieve removability). Therefore the raw materials used for in-line synthesis include the whole range of components used for the manufacture of pressuresensitive adhesives (e.g., monomers, oligomers) and for their formulation (e.g., elastomers, viscoelastomers, viscous components, and additives) and other special components (e.g., initiators, crosslinking agents, etc.). Such a formulation recipe depends on the chemical/physical technology used for the adhesive manufacture. As discussed in [131] the full or partial postmanufacture of the PSA is the result of the chemical development induced by the trend toward solvent-free fabrication. Such formulations with 100% solids include hot-melts and radiation-cured reaction mixtures. Technological reasons and insuﬃcient progress in acrylic-based hot-melts forced the development of oligomer- and macromer-based curable formulations. The main procedures for in-line synthesis include crosslinking and polymerization. They can be combined also. The polymeric raw materials investigated for postpolymerization are PSAs, PSAs with unbalanced adhesive properties, or nonadhesive oligomers. Monomers (as main components or as additives) can be used also. It should be noted that in-line synthesis (modiﬁcation) of the adhesive supposes functionality. Curable Raw Materials Monomers, oligomers, and prepolymers can be polymerized, and constitute together with polymers the range of curable raw materials. As discussed earlier tack, peel, and shear resistance of PSAs depend on the cohesive strength. However, enhancing the cohesive strength, greatly decreases the chain mobility needed to form a bond and hence the measured tack decreases. On the other hand, natural rubber can be processed to a relatively low molecular weight but, upon straining, it crystallizes, and hence resists separation. Induced strengthening mechanisms are favorable if a high tack is desired. Thus for high tack, the elastomers used as PSA components should have a low cohesion at low strain rates to facilitate bond formation, but a high cohesive strength at high strains to resist bond breakage.

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The classical way to obtain a higher cohesive strength is to use crosslinkable base materials. Thus a viscoelastomer can be transformed into an elastomer. For instance viscoelastic polyisobutylene was transformed into an elastomer by crosslinking. This approach was initiated by the formulation of the old rubber/resin-based PSAs, where natural rubber with an inherent gel content was used as the base elastomer. Later on chemically crosslinked or crosslinkable synthetic rubbers containing reactive functional groups (e.g., CSBR) were prepared. One other possibility was given by the synthesis of sequenced, physically crosslinked block copolymers (i.e., SIS and SBS). Acrylics are actually the most suitable PSAs raw materials on the market, but also the most expensive ones. In this context ease of tackiﬁcation of the HC rubber-based materials should be emphasized. First, chemical crosslinking based on built-in reactivity was practiced. Later radiation curing was developed. Next a short description of the most important representatives of the chemically crosslinked viscoelastomers and elastomers is presented. The advantages of crosslinking comprise increases of the cohesive strength, the temperature, the chemical resistance, and the anchorage. Chemical Crosslinking. The crosslinking of acrylic-based copolymers (used for adhesives) has been discussed in many industrial papers [132]. Other crosslinked compounds (e.g., polyurethanes) also may be used as base material for PSAs [133]. The ability of acrylic monomers to polymerize with each other leads to many applications. These polymers can crosslink with epoxy resins, amines, reactive resins, as well as alkyl urea derivatives [134]. Although the suitable polymers can contain functional, crosslinking monomers, such as N-methylol acrylamide, such monomers may release formaldehyde upon curing or cause the loss of tack and adhesion. Thus the preferred polymers contain less than about 1% of N-methylol acrylamide monomer units. According to [65] in an acrylic emulsion 0.5–1.0 wt% IBMA (isobutoxy methylacrylamide) was copolymerized to provide interlinking of the microgels in the ﬁlm upon heating. In some polymer systems the unsaturated carboxylic acid is the eﬀective ingredient for modifying polymer properties (e.g., cohesive strength); these modiﬁcations can be controlled through crosslinking with a suitable agent such as chromium acetate. Multifunctional comonomers containing ethylene units, such as dialkyl maleate, trialkyl cyanurate, tetraethylene glycol dimethacrylate, etc., can be crosslinked by eliminating the unsaturation. The crosslinking of polyvinyl acetate dispersions using metal salts was discussed by Homanner [135]. Pressure-sensitive adhesivelike networks were prepared using the following starting compositions: 90% EHA and 10% 1,6-hexanediol diacrylate (HDDA); 80% EHA, 10% AA, and 10% HDDA [73]. Such polymers possess a crosslink density similar to

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commercial acrylic PSA. The mechanism of crosslinking for solvent-based acrylics was studied by Milker and Czech [136]. They used formulations with hydroxyethyl acrylate and acrylic acid as reactive monomers, and titanates and acetyl acetonates (Al, Co, Ti, Zn, Zr) as crosslinking agents. Solvent-based acrylates containing 5% wt of hydroxyethylmethacrylate (HEMA) were crosslinked with PUR [137]. As an example of crosslinking for a solvent-based acrylic PSA the following procedure is described: 1. 2. 3. 4.

An organic solution of a PSA, a hindered phenol antioxidant, and a tackifying rosin ester are blended. An organic solution of N,N0 -bis-l,2-propenylisoftalimide crosslinker is added to the blend. A thin layer of the adhesive solution is coated onto a sheet backing. The coated sheet is heated to remove the solvent and to cure the polymer and crosslinker [138].

In another example an acrylic copolymer with N-methylol acrylamide as the reactive comonomer was cured using ethyl acetoacetate diisopropoxyaluminum [139]. Crosslinking may be carried out using metal complexes, isocyanates, amino resins, polyamide, polyamine, epichlorhydrine resins, and other compounds [136]. Metal acetyl acetonates and orthotitanic acid esters are used together with alcohols as stabilizing agents. Crosslinking can only be obtained provided that the polymer chain is equipped with acid groups. The hydroxyl group alone does not contribute to crosslinking; however, in combination with COOH groups, an unexpected synergism reﬂected in a higher shearing strength follows. The application of isocyanates remains limited in practice, since the pot life of PSAs being crosslinked with polyisocyanates is rather short. The special feature of amino resins used as a crosslinker is the low crosslinking speed at room temperature. A new class of polyaziridine derivatives giving a low crosslinking density has been developed. According to their chemical composition and to the nature of built-in reactive sites, quite diﬀerent crosslinking agents may be used. For general use polyaziridines were proposed as crosslinking agents [140]. An organic solvent solution of N,N0 -bis-l,2-propenylisoftalimide may be used as a crosslinker. A chrome stearic acid complex (0.1 wt% of the dry adhesive) also was suggested [141]. Crosslinking via aluminum acetate/acrylic acid reactions was suggested for ethylene/ maleinate copolymers [115]. Curing of silicone-based PSAs is done using benzoyl peroxide. The additive trialkyl cyanurate is recommended for the electron beam crosslinking of polymers. This component allows a controlled adjustment of

Chemical Composition of PSAs

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crosslinking density and improves mechanical characteristics. Macromolecular compounds like polystyryl ethyl methacrylate also may be used. The chemical crosslinking of rubber derivatives was developed also. Water-soluble polyamide-epichlorohydrine-type materials are eﬀective as crosslinking agents for CSBR latexes [142]. Although many SBR latexes are self-crosslinking, special crosslinking agents can be used. When modifying by crosslinking it is common to add a solution of some suitable zinc compounds. A careful balance must, however, be considered as the addition results in decreased tack. It should be noted that addition of zinc oxide to carboxylated neoprene latex dramatically reduces the pressure-sensitive character of the adhesive [143]. A crosslinkable PSA composition was formulated on the basis of a butadiene acrylated rubber; it is crosslinked with aluminum propoxide or sec-butoxide [144]. Radiation Curing. Generally polymerization by radiation is similar to the conventional process of thermal (free radical) polymerization initiated by peroxides or azo compounds. Diﬀerent kinds of high energy beams (IR, UV, or electron beams) may be used as a source of energy. The crosslinking of UV light-curable PSA formulations can be initiated by laser beam, too [145]. Similarities between UV- and EB-curing exist also, but each has its special features. In the UV curable formulation free radicals are generated from the photoinitiator, in the EB curable formulation the free radicals are generated on the reactive functional sites upon exposure to an EB source. The electron beam is a much higher energy source than UV light and promotes a higher level of crosslinking, therefore UV light-curable formulations cannot be used unchanged for EB curing. The EB-induced polymerization supposes the reaction of a vinyl unsaturation with electrons of a suitable energy level. Because of the lower energy of UV radiation generally a photoinitiator is needed, but initiatorless UV light-curable systems were developed also [146]. Such technologies cover various domains of the converting. The use of electron beam-curing in the paint industry was discussed in [147,148]; the development of silicone release manufacturing via electron beam-curing was described by Pilar [149]. Electron beam-curing may also be used for lacquers and laminating adhesives [150–152]. Ultraviolet light-induced curing of laquers and printing inks was tested industrially since 1973 [153,154]. Water-based electron beam-cured coating systems were discussed by Loutz [155]. As discussed in detail in [156] pressure-sensitive adhesives crosslinked by ultraviolet light or electron beams have been available for 15 years or more, but their application has not been a total success. Radiation-cured tapes were patented in 1954 [157]. Such tapes were manufactured ﬁrst with a solvent-based pressure-sensitive adhesive composition, dried, and cured with an electron beam [158].

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Radiation curing of PVC carrier ﬁlms for tapes was introduced in the 1980s [159]. Proposals and guidelines for the formulation of beam-curable PSAs were given in [160]. The ﬁrst rubber-based adhesives were beam-cured. In Europe a plant at Tesa-Werk Oﬀenburg (Beiersdorf) for the curing of tapes was started [161] where classical adhesives are being used. The methods and possibilities of electron beam-curing for continuous webs were also discussed by Holl [147]. A comparative study of the costs for electron beam and water-based coating is given by Pagendarm [162]. Actually silicone release curing by radiation and curing of printed coatings is a common technology. As prognosticated by [163], in 2000 about 70–80% of label printing machines used UV light-induced curing. Hybrid systems for combined UV light/electron beam or hot air/electron beam were introduced for oﬀset and ﬂexo printing [164]. Progress in UV curable acrylic hot-melt precursors promises a larger use of this technology for PSAs. Generally, formulated adhesives are cured after their coating, i.e., the curability of the main elastomer or viscoelastomers is strongly inﬂuenced by the performances of the other formulating components. Ewing and Erickson [36] studied the response of various resins and stabilizers to electron beam radiation. Their results show that proper selection of the formulating ingredients can minimize the required radiation dosage for the adhesive, while maintaining the desired balance of tack, shear, and solvent resistance. A detailed discussion of the formulation used in radiation curable PSAs is given in Chap. 8. Next, a short description of the main components is given. Polymerizable Precursors As discussed earlier, crosslinking allows the (build-up or) postmodiﬁcation of the elastic network necessary to regulate the adhesive-cohesive balance of the PSA. It supposes the existence of crosslinkable polymeric structures. Thus segregated constructions are made. Another possibility to create such composites is given by polymerization. This procedure uses monomers to synthesize polymers (viscoelastomers) or polymeric and polymerizable (or curable) compounds (macromers) which are built into an elastical network. For this process the converter can choose to use either a classic polymerization technology (i.e., thermal, free radical initiated polymerization, polyaddition, or polycondensation) or a radiation-induced reaction. It is evident, that in some cases this choice is limited by the chemical nature of the (pre-) polymer. It is to be emphasized that in certain cases (i.e., acrylic hot-melt development), postpolymerization of an oligomer is a necessity because of the lack of suitable macromolecular compounds. According to [165] the main possibilities to formulate radiation curable formulations include the syrup approach (oligomer/monomer mixture), the reactive

Chemical Composition of PSAs

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oligomer approach, and crosslinking of pressure-sensitive adhesives. For monomer- and oligomer-based radiation curable formulations polymerization and postcrosslinking may occur simultaneously. Monomers for In-line Synthesis. Generally, special monomers are used together with polymeric compounds for in-line synthesis of the adhesive. In principle the liquid monomers are applied to the carrier, which is an endless belt, and the polymerization is completed thermally or in an oven by UV light. Electron beam-cured formulations are composed of oligomers, prepolymers (50–100%), multifunctional monomers, and additives. The most important oligomers used are polyester acrylics, epoxy acrylates, urethane acrylates, and polyether acrylates. Hexanediol diacrylates, trimethyl allylpropane triacrylates, tripropylene glycoltriacrylates, and pentaerythritol tetra-acrylate are used as monomers (called reactive diluting agents). The term reactive diluent generally refers to an unsaturated ethylene monomer which is miscible with the principal oligomers, which reduces the viscosity of the composition, and which reacts with the oligomer to form a copolymer. In some cases the addition of the reactive diluent not only reduces the viscosity of the uncured diluent composition but also increase the elongation of the cured coating [166]. Ethyl acrylate, butyl acrylate, glycidylmethyl acrylate, and acrylic acid may be used as monomers for prepolymers [167]. In some cases vinyl lactone is built-in as a sensitive comonomer [168]. The reactive diluents used to reduce the viscosity usually have hydroxyl groups as active sites. An example is hydroxylated caprolactam acrylate [169]. Copolymerizable photoinitiators have been studied for the curing of functionalized acrylates also. Such monomers may contain vinyl unsaturation, diol, or epoxide groups [170,171]. Benzocyclobutenone acrylamide monomer has been incorporated into polymers also. Under UV radiation benzocyclobutenone (BCPO) readily reacts with itself and also with an alcohol to produce an ester. Thus UV light-induced crosslinking occurs without an initiator. Solvent-based adhesives with polymers containing 2-hydroxyethylacrylate and BCPO were prepared by solution polymerization and they were mixed 1/1, giving upon curing enhanced adhesion. Postinduced covalent crosslinking is superior to ionic reactions and proceeds without the need of any initiator [172]. Oligomers for In-Line Synthesis. Oligomers were developed for in situ manufacture of pressure-sensitive adhesives. Such oligomers (50–10% of the recipe) can possess various chemical compositions (hydrocarbonsor heteroatom-based) and diﬀerent functionalities. Both operations—the preparation of the prepolymer and its postpolymerization—can be eﬀected by classical (polymerization, polyaddition, or polycondensation) or radiation-induced polymerization. Possible formulations for UV-cured

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PSAs include acrylic oligomers; unfortunately their shear resistance is too low [173]. Liquid, saturated copolyesters with one terminal acrylic double bond per 3000–6000 molecular weight units are used in formulations for UV- and electron beam-curable PSAs [174]. Acrylated polyester oligomers (with a molecular weight of 3000–8000) were tested as hot-melt PSAs [155]. According to Harder [175] a HMPSA on acrylic basis has been prepared by solution polymerization. Low molecular weight, low viscosity polymers have been synthesized which can be coated by 90–140 C and postcrosslinked, using EB or UV curing. For instance, Acronal DS 3429 is a UV-curable acrylate copolymer for HMPSAs [176]. It is a highly viscous clear liquid, which can be processed at 100–140 C. It possesses a viscosity of about 15,000 mPa.s at 120 C. The photoinitiator is built into the polymer; no C ¼ C double bonds exist in the raw material. Therefore this product is not suitable for EB curing. As discussed earlier (Section 1.1), hydrogenated styrene block copolymers have excellent light and heat stability but display high viscosities and cannot be crosslinked. Therefore new, low viscosity, curable raw materials were synthesized. Ultraviolet light-curable reactive oligomers were manufactured. They are low molecular weight products having special functional groups which participate in UV-light crosslinking (specially by cationic UVcuring). A poly(ethylene-butylene) rubber bearing a primary hydroxyl functionality on one end and epoxidized polyisoprene functionalities on the other end, with a Tg of 53 C was developed (Kraton EKP-200) by Shell. The grade L-1203 is a linear poly(ethylene-butylene) rubber bearing a terminal aliphatic primary hydroxyl group on the same backbone and possesses a Tg of 63 C. Another macromer possesses a terminal hydroxyl functionality on both ends. Generally a blend of such products having diﬀerent functionalities is used. Other special oligomers are described in detail in [33]. Polymers for In-Line Synthesis. The most important requirements for the polymers used as the electron beam-curing material for PSAs are a low crosslinking density and a Tg below 20 C [116]. Ultraviolet light-cured hybrid systems were discussed by Barisonek and Froelic [178]. Storagestable latently curable acrylic formulations based on triethylene glycol dimethacrylate and polymeric hydroperoxides were patented [178]. Generally the initiator system for UV-cured PSAs may be free radical or cationic [179]. Crosslinking via polymer bonded benzophenone groups has been studied also. There is a correlation between the activity and structure of polymeric photoinitiators containing side-chain benzophenone or chromophores. Such built-in photosensitizers have been tried for styrene-butadiene copolymers also.

Chemical Composition of PSAs

197

Photoinitiators. By photoinitiated polymerization the light energy produces reactive species (radicals, cations, or anions) which can initiate thermal chain reactions or short life intermediates (e.g., radicals, carbenes, or nitrenes) which can produce crosslinking reactions. Because of the lower energy level of UV radiation a photoinitiator is needed. Benzophenone, thioxanthone, benzoin ether, benzylketals, -acyloxime esters, and dialkoxyacetophenone are the most used photoinitiators. Diﬀerent photoinitiators act through diﬀerent mechanisms. For instance, benzoin derivatives, benzyland acetophenone-ketals, aliphatic azoderivatives, peroxides, aromatic disulﬁdes, perhalogenides, -benzoinoxime ester and benzoinphosphine oxides act via intermolecular bond association. Aromatic ketones, especially benzophenone, thioxanthones, quinones, and heterocyclical compounds act through internal hydrogen abstraction. They need accelerators (like amines or alcohols) which easily release hydrogen atoms. Ketones with amines, allylthiocarbamides, etc., act through photoinitiated electron transfer.

1.5

Components for Special Pressure-Sensitive Formulations

As discussed earlier (Chap. 3, Section 1.2) in the last decade research in the ﬁeld of bioadhesives allowed the development of a special class of products which display pressure-sensitivity although their components (alone) are not elastomers or viscoelastomers [180,181]. Such systems can be prepared by formulation only. They display pressure-sensitivity due to the mutual eﬀect of plasticizing and crosslinking of the formulation components in the presence of water. This new class of pressure-sensitive products includes hydrogels. Hydrogel (containing 4–12% water) based miscible blends of high molecular weight PVP with a short-chain PEG reveal a pressuresensitive character of adhesion within a narrow range of PVP-PEG compositions [182]. In this case adhesion is a result of hydrogen bonding of both polyethyleneglycol (PEG) terminal hydroxy groups to the carbonyls in polyvinylprrolidone repeating units. The PVP-PEG hydrogels behave like typical rubbers and couple the properties of PSAs and bioadhesives. According to [182] bioadhesion can be deﬁned as a pressure-sensitive character of adhesion toward highly hydrated, soft substrates, comprising mucin (the glycoprotein predominant in the mucous layer). In contrast to bioadhesives conventional PSAs are mainly hydrophobic elastomers providing much stronger adhesion to dry substrates. However, they lose their adhesion as the substrate or adhesive is exposed to bulk water. The distinctive feature of bioadhesives is a plasticizing eﬀect of water. As stated in [180] the Vinogradov equation [183] (derived originally for elastomers),

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can be applied to describe the debonding mechanics of PVP-PEG adhesive hydrogels. In the PVP-PEG mixtures pressure-sensitivity arises within a narrow range of compositions and is also aﬀected by blend hydration. Later it was demonstrated, that there is a general method to tailor PSAs by mixing nontacky hydrophilic polymers with short-chain plasticizers, which bear complementary functional groups at their ends. Although chemical composition and structure of PVP-PEG hydrogel have nothing to do with those for conventional PSAs, nevertheless the PVP-PEG hydrogen-bonded network obeys also Dahlquist’s criterion of tack. There is a general applicability of the PVP-PEG model to other hydrophilic polymers and plasticizers. Adhesive hydrogels can be obtained by PEG blending with such long-chain complementary hydrophilic polymers as other poly(N-vinyl amides), particularly poly(N-vinyl caprolactam), their copolymers, polyacrylic acid etc. The Kovacs equation (Chap. 3, Section 1.2) predicts that a high adhesion has to appear in the polyethyleneglycol blends with polyvinylcaprolactam and other complementary long-chain partners with the same composition range as it occurs for PVP-PEG blends (35 wt% PVP and 5 wt% PEG). This prediction is also conﬁrmed experimentally [184,185]. PEG is not a unique short-chain telechelic plasticizer and cohesive strength enhancer. Tacky blends can be obtained by mixing PVP with other hydroxyl or carboxyl terminated short-chain plasticizers, ethylene glycol and its polymers, with molecular weight of 200–600 g/mol, low MW propylene glycol, and alkane diols (propane diol up to hexane diol). The highest peel strength has been found for PVP with glycerol which possesses the maximum density of hydroxyl groups per molecule. Strictly speaking carbonic diacids are not plasticizers, but noncovalent crosslinking agents acting as cohesive interaction enhancers. Those can be applied in combination with plasticizers which reduces Tg. Water is an appropriate plasticizer to impart adhesion to PVP blends with carbonic acids. The key result is that the free volume and cohesive strength of the polymer need to be of appropriate magnitude, and in a strictly speciﬁed ratio to each other for adhesion to appear. A similar conclusion was previously derived from the ﬁnding that the maximum in the debonding force corresponds to the minimum value of the CpTg product. This is demonstrated for PVP-PEG blends over the range of compositions and for other (common) PSA base polymers (e.g., PIB, natural rubber, etc.) [186].

1.6

Other Components

The most important single components in a PSA are the polymers (elastomers and tackiﬁers). Other micromolecular components such as

Chemical Composition of PSAs

199

plasticizers, antioxidants, ﬁllers, crosslinking agents, initiators, detackiﬁers, solubilizers, primers etc., are also included. Depending on the application technology carriers (solvents or water) may be required. Depending on the coating technology wetting agents are used; converting and end-use properties of the label may require special additives. Such compounds can be divided into chemical and technological additives. A detailed discussion of these components is given in Chap. 8.

1.7

Release Coatings

The release coating does not constitute a component of the pressuresensitive adhesive but it is a part of the pressure-sensitive product, and thus it inﬂuences the performance of the pressure-sensitive adhesive. Therefore a short description of the release coatings will be given. The energy dissipation during debonding is of two kinds: a surface dissipation in the vicinity of the interface due to the separation and a second type of dissipation due to the bulk deformation of the viscoelastic adhesive and of the release coating. It is expected that the surface dissipation is mainly controlled by the nature of the interface, i.e., the composition of the release coating, while the bulk dissipation would be mainly controlled by the viscoelastic response of the adhesive. The release coating is very thin relative to the adhesive layer and, as it is more elastic than the adhesive, the energy dissipation during the debonding mainly takes place in the adhesive. As stated by Gordon et al. [187–189] until recent studies the release liner was simply viewed as a low surface energy platform from which to release organic PSAs. While empirical correlation appeared to exist between silicone bulk rheological properties and absolute release magnitudes, it was not yet clear as to whether this was truly a bulk or interfacial rheological eﬀect. It is evident, that the viscoelastic properties of the release inﬂuence the magnitude of the release force also. The release mechanism involves a low polarity surface or low surface tension and incompatibility of the release surface and adhesive surface. Generally the adhesive raw materials are high polymers bearing nonpolar or/and voluminous side chains or functional groups. The adhesive eﬀect of low polarity is well demonstrated by silicones and fatty acid derivatives. The release surface will normally be provided by a layer of a silicone polymer or by a material providing similar release properties coated on the liner substrate (see Chap. 3). Non-silicone release coatings were developed too. The various release coating technologies available on the market and their strengths and weaknesses were discussed by Craig [15].

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Silicone-Based Release Coatings Silicones are the best known dehesive macromolecular compounds. They can be imbedded into plastomers and elastomers too. As stated in [190] the most used release coatings with general applicability are based on silicone polymers. Such products were developed many years ago for greaseproof and release papers and antiblocking coatings. The use of silicone as a release coating is based on its low surface energy (22–24 mN/m), where the surface energy of most adhesives averages 30–50 mN/m. This diﬀerence prevents the adhesive from wetting the silicone surface. The adhesion energy of the acrylic adhesive on silicone release coating is 10 times smaller than that on steel [191]. The basic release liner is comprised of paper with a very thin silicone coating layer, adhering suﬃciently to the adhesive to hold the laminate together, but enabling peeling oﬀ of the release paper from the face material [192]. Silicone release resins are linear polymers or prepolymers in liquid form, with or without solvents. They are coated onto paper, ﬁlm, or other backing materials and then cured typically by heat and/or surface catalytic action to form solid, nontacky, crosslinked polymers in situ. The release resins are formed from halosilicones and consist predominantly of repeated units of the structure: R j O Si j R where R is a hydrogen or a hydrocarbon radical, usually a lower alkyl or aryl (typically methyl) group, O is oxygen, and Si is silicon. The degree of polymerization is such as to produce a liquid linear prepolymer material with no signiﬁcant crosslinking. It is believed that the currently known silicone linear release prepolymers consist of at least 95% repeating units of this structure with reactive end groups, but that small quantities of other modifying units may be present, if so desired [192]. For silicone-based PSAs, crosslinked silicone release liners are suggested [193]. Siloxane grafted vinyl copolymers may also be used as release layers [194]. Silicone coatings can be divided into two main groups, namely, thermally cured and radiationcured ones. Thermal curing occurs through condensation or addition reaction. Thermally cured silicone release systems consist of a reactive polymer, a crosslinker, and a catalyst. The radiation curing process is based on the chemical eﬀect of radiation. Moisture curing silicone release coating technology is only at the developmental stage. More than 20 years of research were devoted to the concept of silicone addition curing systems

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(using a platinum-based catalyst) for paper release applications [195]. However, it was not until the 1980s that this technique really began to develop after better control over the problem of inhibition had been achieved. Although more expensive than the condensation system, additive curing oﬀers several advantages [196]. It allows greater control over release characteristics. There is no postcure; once the coating is properly cured, there is little tendency to block. Diﬀerential release can be achieved using diﬀerent polymers and additives, and, as there is no postreaction, the release force remains stable with time. Actually 1–2 m thick silicone release is coated with a speed of >5 m/sec and displays acceptable cure in <4 sec [187]. Catalyst System. With a tin catalyst system the curing is performed via a condensation reaction. The coating materials are supplied as two components which are crosslinked in the coating process; component A is a silicone polymer and component B contains the tin catalyst. The curing reaction acts as a postcure; on double-sided coatings, migration may occur which increases blocking [197]. With the platinum catalyst system the curing occurs via an addition reaction. The coating materials are supplied as two components which are crosslinked in the coating process; component A is a vinyl functional siloxane and acts as the catalyst, and component B contains a silane hydride functional polymer. For addition cure emulsions a platinum or rhodium component dispersed in a methyl vinyl siloxane polymer acts as a catalyst. The oxidation state of the metal, the nature of the binder, and the content of platinum will determine the level of reactivity of the catalyst and its shelf life stability. The base polymer consists of one or several dimethylpolysiloxanes which contain reactive vinyl groups. The portions of these groups, their number, environment, and accessibility will help to determine the reactivity of the system and the type of material obtained after cure, which constitutes the skeleton of the system [15]. For these systems a crosslinker composed of one or several methyl hydrogenopolysiloxanes may be used. In order to inhibit the reaction at room temperature stabilizers (e.g., acetylenic alcohol, acetylenic ketone dialkyl maleates, and azodicarboxylates) may be used [15]. Controlled Release. The main goal of release formulation is to increase release so as to be able to eﬃciently ‘‘dial a release.’’ In principle the silicone release coating is a polydimethylsiloxane (PDMS)-based silicone network. Its adhesive or nonadhesive performances can be modulated either by PMDS chemical modiﬁcation or by adding additive molecules. As discussed in [187] adhesion theory might lead one to expect that interfacial absorbtion, molecular interdiﬀusion, and entanglements are mechanisms

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envisioned as a mean of controlling the release force. The industrial solution to release control is the inclusion of a methylsilicate in a manner analogous to tackiﬁer resins in the organic adhesive they are in contact with. A tri- or tetrafunctional resin is used as a control release agent (CRA). The properties of the silicone release coating are tuned by introducing a siloxane-based resin to create a diﬀerential, which is the ratio of the base polymer release to a higher release due to high release additives (HRAs) in PDMS. The additive molecules are called MQ molecules. Adhesion increases with increasing amount of resin present in the elastomer. How does the resin act? The role of the methylsilicates functioning as HRAs to modify the viscoelastic properties of PDMS was highlighted by Gordon et al. in [198]. It has been shown that the methylsilicate eﬀectively reduces the segmental mobility of the ﬂexible PDMS backbone. According to [199] the network density increases (Mc decreases) as the MQ amount increases in the network. The network is more and more dense. The inﬂuence of the molecular weight is more complex. The network density is higher for the medium molecular weight, whatever the additive amount, and the Mc is higher for the higher polymer length. For higher molecular weight of the base polymer the additive content does not seem to have a great inﬂuence, whereas for a molecular weight equal to 11,000 g/mol the adhesive and attractive forces vary dramatically when the additive content is equal to 20%. For additive molecule amounts higher than 30% in the polymer blend, the presence at the interface of MQ molecules induces polarity increase and stress transfer at the interface. The consequence is an increasing of both the speed exponent (n) of the viscoelastic function , and of the intrinsic adhesive strength Go in the Gent and Schultz equations [200] relating the viscoelastic response to the interfacial response: Gp ¼ Go ðT,vÞ

ð5:1Þ

Gp ¼ Go Rnp

ð5:2Þ

where Gp is the adhesive strength and Rp is the peel speed. Increasing the molecular weight between crosslinks of the linear PDMS chain in the network increases the release force. Likewise, increasing the concentration of the methylsilicate HRA increases the release force. However, the largest impact of the HRA on the release force is typically at low to intermediate peel velocities (0.005 m/s to 0.17 m/s) while increasing the molecular weight between crosslinks mainly inﬂuences release at high peel rates [201,202]. Chaudhury and Whitesides [203] proposed a release mechanism for cured PDMS-based release coatings in which interfacial slippage minimizes the

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bulk shear deformation experienced by organic adhesives. Commercially cured PDMS release coatings exhibit interfacial slippage. Traditional HRAs used in release coatings ‘‘freeze out’’ interfacial slippage resulting in increased adhesive deformation. The HRA reduces the segmental mobility of the PDMS chain within the cured network leading to increased shear stress and an increase in the practical peel force. At lower MQ content the PDMS viscous surface layer contributes to the very low adhesion behavior due to the existence of a surface weak boundary layer. Concerning the molecular weight eﬀect, it appears that, depending on the network mesh size, MQ additive molecules are either trapped (low Mc) or not (high Mc) in the network. Such segregation is important for the modulation. MQs participate in the crosslinking reactions too. Crosslinking reactivity between the curing agent and PDMS or MQ molecules depends on the initial PDMS chain length. As a consequence, for intermediate molecular mass (11 k) a surface weak boundary layer of free PDMS chain can exist. According to [191] the proportion of resin in the silicone release coating controls the propagation velocity of the cracks and therefore the maximum deformation of the adhesive before debonding. Radiation-Cured Systems. Ultraviolet- and electron beam-cured coatings are one-component solventless systems. The process is based on the chemical eﬀects of radiation; in the UV system it is necessary to use some heating energy [204]. Ultraviolet-cured silicones have been proposed for narrow webs; for broad webs EB-curing has been recommended [205]. Although beam-curing (UV and electron beam) silicones are available on the market this technology has not succeeded yet (apart from a few exceptions). The main advantages are the low space requirements and minimum substrate strain compared with thermal systems. The advantages/ disadvantages of beam-curing will be discussed in Section 1.4 of Chap. 5. Silicone coating technologies may be divided into three major categories, according to their application form [206]: solvent-based (addition and condensation products) silicones, emulsions, and solventless (thermal or electron beam-cured) silicones [204]. In 1986 about 15% of silicone coatings were carried out with solvent-free silicones [205], while solventless silicones covered 40% of the whole consumption. In the United States two electron beam-cure silicone release coating machines were installed in 1983 [147]. In Europe the ﬁrst release coating machine using electron beam-curing was installed in 1984 [207]. Radiationcurable release coatings may be prepared with a low adhesive material of limited compatibility in the liquid, so that a thin layer migrates to the surface of the ﬁlm [208]. A controlled release eﬀect may be achieved using a diﬀerent polydimethylsiloxane level [209]. Acrylic or epoxy derivatives of

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silicone may be used as active components. Generally polyfunctional acrylic or mercapto groups are used as reactive sites on the dimethylpolysiloxane [210]. Non-Silicone-Based Release Coatings As discussed in [190] for some products there is no need for the high level of adhesive performance aﬀorded by silicones. For adhesive coatings with very low tack and peel the use of silicones would give laminates having insuﬃcient bond strength. For these applications non-silicone release layers are suggested. Various compounds such as CMC, alginates, and shellac have been tested [211]. Natural rubber latex [212], fatty acid derivatives, melamine, Werner complexes, complexes of basic chromium chloride with fatty acids, metal complexes of ﬂuorinated derivatives, starch and watersoluble ﬂuorinated compounds, vinyl esters having long pendant groups containing tetraﬂuoroethylene and hexaﬂuoropropylene oligomers, etc., were proposed also [213–215]. The usual non-silicone release coatings for PSAs are based on polyvinyl carbamates, acrylic ester copolymers, polyamide resins, and octadecyl vinyl ether copolymers. As an example, acrylonitrile-stearyl acrylate copolymers, chlorinated polyoleﬁns, and ethylene-vinyl acetate copolymers may be used as solutions [216]. The aqueous release coatings (which are not silicone-based) presently marketed are based on either acrylic ester copolymers or vinyl acetate copolymers and are available in crosslinkable and self-crosslinking types. The selfcrosslinking polymers have found application where heat resistance is required [217]. From the solvent-based, non-silicone release coatings carbamates have a broad application spectrum. As discussed in [218] polyvinylcarbamate is the main release agent used for protective ﬁlms. It can be applied alone or in mixtures with chlorinated polyoleﬁns. Kinning [219] has studied the release behavior of acrylic tapes from release coating based upon long alkyl sidechain polycarbamates [polyvinyl-N-octadecyl carbamate (PVNODC) and polyvinyl-N-decylcarbamate (PVNDC)]. The surface energy of carbamate is the same or lower than that of a silicone [203]. The lower surface energy of such release coatings is probably due to self-assembly of the side chains into a liquid crystalline order, thus presenting only methyl groups to the air. As described by Kinning [219] the use of PSAs containing acrylic acid on polyvinylcarbamate release coating may lead to blocking. For these products the interfacial energy is almost the same, but the work of adhesion (with acrylic adhesives without acrylic acid) diﬀers (it is 40 mJ/m2 for PVNDC and 30.7 mJ/m2 for PVNODC). Polyvinyl-N-decylcarbamate is much more likely to provide blocking compared to polyvinyl-N-octadecyl

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carbamate. This was ascribed to a melting and loss of structure in the release coating as the temperature was increase [73].

2

FACTORS INFLUENCING THE CHEMICAL COMPOSITION

The most important means to adjust the chemical composition of a PSA are the synthesis of the base materials and the blending or formulation of the components. However, other factors may inﬂuence the chemical composition of the PSA, such as the physical state of the synthesized elastomer, the coating technology, and the solid state components of the laminate. As pointed out earlier a diﬀerent end-use requires a diﬀerent rheology; thus the end-use properties also determine the chemical composition of the adhesive.

2.1

Synthesis

The synthesis of the raw materials for PSAs provides a straightforward way to modify the properties of PSAs through the chemical composition. Such synthesis includes polymerization and polymer analogous reactions. Polymerization The polymerization process which leads to the raw materials for pressuresensitive adhesives or to the pressure-sensitive adhesive itself is a function of the polymerization system employed and of the polymerization conditions. The polymerization system is determined principally by the monomers and initiators used, which impose the mechanisms of the polymerization, but other additives may aﬀect the characteristics of the macromolecular compounds synthesized. Generally the choice of the monomers and of the polymerization technology ensures an adequate change of the adhesive properties. For special products and (in-line) manufacture technology postpolymerization based on polymer analogous reactions was developed. Therefore, as discussed in detail in [156,220] formulation has to cover oﬀ-line synthesis and in situ synthesis too. In principle, polymer synthesis can lead to raw materials for PSA (e.g., elastomers and viscoelastomers) or to PSAs. The synthesis of polymeric pressure-sensitive raw materials is generally based on pure monomers and is carried out as oﬀ-line synthesis. The synthesis of pressure-sensitive adhe-sives based on monomers, oligomers, and polymers may occur in-line

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or oﬀ-line. The oﬀ-line manufacture covers synthesis of reactive base materials and it may include the partial use of such reactivity. The Choice of the Monomers. Previously the importance of the Tg and modulus for PSAs was discussed (see Chap. 3). The ﬁrst step in the design of an adhesive system is the selection of the monomers to build-up the polymer backbone. The comonomers, molecular weight, and degree of crosslinking or plasticizing of the polymer strongly inﬂuence its Tg. Ethylene is the least expensive monomer usable to decrease the Tg. According to their functionality the comonomers used for ethylene copolymers can be classiﬁed as follows: softening, hardening, polar, and reactive comonomers (see Section 1.2 too). The requirement for highly branched soft monomers (e.g., ethyl hexyl acrylate or butyl acrylate) as the main polymer backbone was pointed out earlier. On the other hand it may be desirable for various reasons to incorporate any of several modifying comonomers as part of the terminally unsaturated vinyl monomeric portions of the adhesive system. For example, acrylonitrile imparts hardness and solvent resistance, t-butyl styrene improves tack, methyl methacrylate makes the adhesive harder, octyl vinyl ether softens the adhesive, etc. The synthesis of the main polymer may inﬂuence the chemical composition of the adhesive through a built-in reactivity which can be activated at a later time. Functional groups are usually incorporated into the polymer for chemical reactivity reasons (e.g., crosslinking). This type of chemical reaction is desired to minimize the adhesive ﬁlm’s thermoplastic response and maximize its tensile properties. As an example, as the polymer contains carboxyl groups, the bond strength and other properties can be modiﬁed and improved by adding multivalent metal ions in a suitable form to promote ionic crosslinking. As discussed in [220] the following performance characteristics of PSAs are improved by carboxy groups: adhesion, cohesion, crosslinking, and dispersion stability. Carboxy functionality is preferred for acrylates and for styrene-butadiene latices. Unfortunately the carboxy groups can cause side reactions too. For instance, the carboxy groups from the PSA can react with Si-H groups [221] or with the carbamate release (as discussed earlier). The acrylic acid content increases adhesion and adhesion hysteresis of acrylic copolymers. The addition of acrylic acid also decreases a critical crack propagation speed above which a much stronger rate dependence of the adhesion energy is observed (see Chap. 3). Hydroxy functionality may be used for curing also. For instance, Takemura et al. [137] used a solvent-based acrylate with 5 wt% HEMA and PUR crosslinker. The main built-in crosslinkable monomers with such functionality are listed in [220] as follows: acrylic acid, itaconic and methacrylic acid (1–4 wt%), methylol acrylamide, and hydroxypropyl

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acrylate. Stabilizing hydrophilic comonomers e.g., ammonium acrylate, sodium acrylate, styrene sulfonate, sodium sulfonate-hydroxypropyl acrylate, and sodium-2-sulfoethyl-methacrylate were suggested also. Hydrophilic comonomers e.g., partially neutralized methacrylic acid, hydroxyethyl acrylate, and hydroxypropyl acrylate were employed for water-soluble compositions. As described in [220] special monomers may work as antioxidants, surfactants, biocides etc. and may improve removability, ﬂame resistance etc. For example, latexes were made using allyl ammonium linear-alkylbenzene sulfonate as a surface active comonomer, to improve the wettability and water resistance of the PSA. The contact angle of the latex ﬁlm produced with the non-migrating surfactant not only had a higher original contact angle (125 vs. 80 ) but the contact angle did not drop even after several minutes [222]. The Choice of the Polymerization Procedure. The polymerization procedure inﬂuences the composition, molecular weight, macrostructure, and microstructure of the polymers. The main polymerization methods include free radical, ionic, and ionic-coordinative initiation, and chain growth mechanisms. Their versatility for diﬀerent monomers is quite diﬀerent. Ionic and ionic-coordinative polymerization lead principally to polymers with higher structural order. Technological synthesis conditions (e.g., gas, liquid, or solid state, solvated or dispersed systems) can inﬂuence the polymerization reaction also. Ionic or ionic-coordinative polymerization allows the regulation of the macro- and microstructure of the polymers. Monomer insertion, sequence distribution, chain branching, and sterical structure of the polymers can be ﬁnely regulated. The control of the monomer insertion allows the build-up of side-chain unsaturated crosslinkable polymers; sequence distribution and chain branching provide segregated polymer structures. Free radical polymerization is the main procedure for the synthesis of raw materials for PSAs. It is carried out in solution, or in water-based dispersions. Solution polymerization is carried out to produce linear polymers which may be subsequently crosslinked; emulsion or suspension polymerization are carried out to synthesize linear or crosslinked polymers. Ionic and ionic-coordinative polymerizations are carried out in solution. Living (solution) polymerization with anionic catalysts provides a greater control of molecular structures and composition of synthetic rubber than radical emulsion or solution. With adequate catalysts the control of the MW and MWD, the sequence distribution of the monomers, the isomeric structure of the diene units, and either a linear or branched (e.g., star) molecular structure was achieved. It should be noted that the nature of the monomers may have a determinant role for the same polymerization procedure (e.g., solution) and catalyst system. For instance,

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soluble metal-organic catalysts may lead to diﬀerent tacticity (e.g., atactic, syndiotactic) and sequence distribution (e.g., random or block) as a function of the chemical nature of the monomers used (acrylics, vinylaromatics, heterocyclics, and dienes) [74–85]. The choice of the comonomers in combination with special polymerization conditions inﬂuences the ﬁnal chemical composition. This general statement is illustrated by the synthesis of special ethylene-maleinate multipolymers [130] or special styrenic block copolymers (Dexco triblock copolymers). As discussed in [223] in the MMA-EHA copolymer, the EHA content increases vs. conversion diﬀerently as a function of the polymerization method. Molecular weight adjustment during synthesis oﬀers another way to inﬂuence the chemical basis and adhesive properties. Here solvent-based or emulsion-based polymerization of the same monomers provides another possibility to inﬂuence the molecular weight. Emulsion kinetics diﬀer from those of mass, solution, and suspension polymerization which show an inverse relationship between the rate of polymerization and the number average degree of polymerization. For instance, as discussed in [89] solution polymerized acrylates have a MW of 200,000 to 300,000 compared to emulsion polymerized acrylates with a molecular weight of 150,000–450,000; however, mixtures of acrylate and tackiﬁer polymerized together as the same constitution have a molecular weight of 60,000–100,000 due to transfer through the tackiﬁer. The PSA mixed in a monomer state and solution polymerized was no longer a simple blend of polymer and tackiﬁer. The polymeric conversion of the mixture was also lower. According to [65] the main reason for the much lower shear resistance for emulsion-based PSAs is due to the discrete microgel morphology of such polymers in contrast to solvent-borne acrylics formed as a continuous network morphology. Interlinking of the microgels by covalent bonds in the ﬁlm is needed. This can be achieved by the aid of a functional monomer, e.g., IBMA. Aqueous dispersions and solutions of the useful polymers can be manufactured by procedures known in the art to be suitable for the preparation of unsaturated oleﬁn carboxylic acid ester polymers, such as acrylic ester polymers. For instance, aqueous polymer dispersions can be obtained by gradually adding each monomer simultaneously to an aqueous reaction medium at rates proportional to the respective percentage of each monomer in the ﬁnished polymer, then initiating and continuing the polymerization with a suitable polymerization catalyst. Promoters are free radical initiators and redox systems such as hydrogen peroxide, potassium or ammonium peroxydisulfate, dibenzoyl peroxide, lauroyl peroxide, ditertiary butyl peroxide, 2,20 -azobisisobutyro-nitrile, etc., either alone or

Chemical Composition of PSAs

209

together with one or more reducing components such as sodium bisulﬁte, sodium metabisulﬁte, glucose, ascorbic acid, etc. The acrylate monomer may include an alkyl group with 1 to about 10 carbon atoms. Suitable alkyl acrylate monomers are methyl acrylate; ethyl acrylate; N-propyl; isopropyl acrylate; butyl acrylate; 2-ethyl hexyl acrylate; and heptyl, octyl, nonyl, and decyl acrylate. The preferred alkyl acrylate comonomers are butyl acrylate and 2-ethyl hexyl acrylate. Acrylic acid used as a comonomer improves the cohesion [224]. Acrylic esters were copolymerized with small portions of monomers such as acrylic acid, methacrylic acid, itaconic acid, acrylamide, methacrylamide, acrylic esters, vinyl esters, -alkoxy-alkyl unsaturated carboxylic acid amides, half esters, half amides, amide esters, amides and imides of maleic anhydride, and the alkylaminoalkylene monoesters of maleic, itaconic, or citraconic acids. If the average number of carbon atoms in the longest alkyl chain exceeds 12, the adhesive tends to become waxy and lacks suﬃcient adhesion. The useful PSA polymers contain a suﬃcient amount of one or more of the described functional monomers to increase the cohesive strength of the adhesive, relative to an otherwise identical PSA in the absence of such functional monomers. Detectable enhancement of cohesive strength is found in many polymers at functional monomer concentrations as low as 0.05 wt%. Working with a butyl acrylate/vinyl acetate/methacrylic acid model terpolymer system, Brooks [225] used computer aided regression analysis to identify key factors inﬂuencing the properties and PSA performance of acrylic emulsion polymers. Experiments were designed to study the inﬂuence of the acid comonomer level, the chain transfer agent, surfactants, and buﬀer concentration on peel, shear, and tack values. Molecular weight, polydispersity, and gel content were also measured. The strongest inﬂuences were found with acid comonomer level and buﬀer concentration. The choice of the monomers is discussed in detail in [220]. The Choice of the Additives. Additives inﬂuence the chemistry and the physical interaction of the formulation components. They aﬀect the base polymer (e.g., initiators, crosslinker, ﬁller) or the dispersed polymeric system (e.g., surfactant, ﬁller). The nature of the initiators inﬂuences the polymer structure in free radical polymerization too. For instance, peroxides cause more hydrogen abstraction resulting in more branching. Azo compounds yield more linear polymers. Emulsion polymerization leads to a polymer with a chemical composition which is strongly inﬂuenced by the components of the dispersing medium. Its stability as a disperse system is aﬀected by such components too. Generally the stability of the emulsion depends on the following parameters: electrical charges, elastic recovery, and gel structure. Such parameters are a function of the monomers used and depend

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on the technological additives employed during manufacture of the dispersed systems. The role of the additives will be described in detail in Chap. 8. Polymer Analogous Reactions The polymer analogous reactions can be carried out on macromolecular raw materials or on the ﬁnished product components in-line or oﬀ-line. Such reactions lead to new polymers having special characteristics. They are aﬀected by built-in functionality and by the modiﬁcation method used. Such reactions include the functionalization of polymers and the use of the existing functional groups for chemical reactions. The main synthesis procedures based on polymer analogous reactions are grafting and crosslinking. Functionalization. Functionalization allows the build-up of reactive groups on polymer backbones. Industrially functionalization is carried out during the synthesis of macromolecular products (mainly elastomers or plastomers) or as a postreactor modiﬁcation of plastomers used as carrier webs for various pressure-sensitive products. It will be described in Chap. 8. Grafting. Grafting is a general method for functionalization and production of segregated structures. It was developed as a synthesis and postreactor modiﬁcation method. Grafting of reactive polar groups onto elastomers or plastomers can be carried out oﬀ-line as a synthesis method for polymers and/or it can be employed during the manufacture of pressuresensitive products, mainly as a surface treating method [226]. Although grafting achieved a special importance for pressure-sensitive products due to its use for the functionalization of the plastic carrier materials (graft polymerization, plasma and chemical corona treatment [227]), most developments in its use for PSAs have an experimental character [120]. For instance, PSAs having a vinyl polymer backbone grafted with polysiloxane moieties exhibit repositionability [228]. Crosslinking. The importance of crosslinking was discussed previously in Section 1.4. A detailed description will be given in Chap. 8. 2.2

Formulation

Some adhesives typically are used without or with quite low levels of tackifying resins, but tackifying resins can contribute signiﬁcantly to tack and peel. In fact, formulation does not only mean tackiﬁcation. Formulation is the general notion for a recipe containing all the components necessary to impart to an adhesive the combination of the ﬁnal (desired)

Chemical Composition of PSAs

211

adhesive, coating, converting, and end-use properties. As described in [229] ‘‘in the industrial practice formulation is the blending of diﬀerent components of a recipe in order to achieve adequate technological and end use performance characteristics.’’ In principle formulation is carried out to achieve the end-use properties or to fulﬁll the requirements of a given manufacturing technology. The end-use properties include the adhesive and converting performances; the manufacture technology covers the manufacture of the adhesive and of the PSP too. Details of formulation are described in Chap. 8. An exhaustive discussion of formulation is given in [230].

2.3

Physical State of PSAs

As discussed in [229] formulation inﬂuences the adhesive coating technology and vice versa. Generally the chemical compositions of solvent-based, water-based, and hot-melt PSAs are quite diﬀerent. As discussed earlier for solvent-based and water-based PSAs, their formulating freedom is a given, that is, their composition does not have any limiting factors other than the Tg and modulus to be achieved. On the other hand hot-melt PSAs have to be ﬂuid at the coating temperature, that is, their melting point has to be lower than their thermal resistance and, at the same time, they must display a low viscosity at the (high) coating temperature, but a high viscosity at the end-use temperature. This means that within the design of the chemical composition of hot-melt PSAs the processability of the adhesive should be considered. For this purpose conventional elastomers are not adequate. In a similar manner the high coating temperature requires aging and temperature-resistant tackiﬁers. In the case of waterbased PSAs, the adhesive should possess water solubility or dispersability (i.e., the base elastomer should be designed so as to incorporate hydrophilic monomers).

Raw Materials for Hot-Melt PSAs Twenty years ago only solvent rubber-based PSAs existed. Natural, high molecular weight rubber was used as the base elastomer. In order to achieve better solubility and/or special adhesive properties, masticated or unmasticated rubber is preferred. Even a highly masticated natural rubber (with a low gel content) remains too viscous in the molten state in order to be coated per se, and has too low a temperature resistance/cohesion at the enduse temperature. [It should be mentioned that for special medical products

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‘‘dryblending’’ (warm or in the molten state) and coating of the blend by roll pressing on a porous web were carried out [231].] For hot-melt PSAs low melt viscosity rubbers are needed (actually viscosities of 7.5 Pa.s are common); for these polymers an acceptable cohesion level may be achieved only by physical crosslinking. Therefore thermoplastic elastomers are used [232]. In general hot-melt PSAs are based on a thermoplastic elastomer, an end compatible resin, a midblock compatible resin, oil, and antioxidant [233]. The most commonly used block copolymers for hot-melt PSAs are linear SIS block copolymers with a polystyrene content usually ranging between 10 and 25%. They possess a relatively low modulus and are easier to tackify than SBS types. SIS block copolymers display better tack, compatibility, and aging resistance; they have a lower viscosity in hot-melt PSAs and remain soft after aging. Current developments in the ﬁeld of thermoplastic elastomers have broadened the available raw material base for hotmelt PSAs (Section 1.1). The main raw materials used in hot-melt adhesives (e.g., polyoleﬁns and EVAc copolymers, polyesters, and polyamides) are described in [234]. Dupont’s ethylene-vinyl acetate copolymers were developed for use as a base resin in hot-melt PSAs. They are easily blended and have an excellent thermal oxidation stability. Unfortunately they do not provide much tack and peel. For hot-melt PSAs an unstatistical EVAc copolymer with a higher VAc content, higher molecular weight, and a sequential structure was evaluated as a raw material [234]. Although solvent and aqueous emulsion acrylic PSAs were in use for some time, it is only recently that hot-melt acrylics were developed. The wide variety of acrylic monomers available and their reactivity provide endless opportunities to tailor polymers to suit speciﬁc applications. Reactive hot-melt PSAs can be developed using solid and liquid functionalized polyacrylic elastomers, in combination with special hardeners or catalysts containing isocyanate for heat curing or UV curing with appropriate agents such as -chlorobenzophenone or others. Such acrylates can be processed at 120 C and have a viscosity of 8 Pa.s [235]. Polyesters are also used as raw materials for hot-melt PSAs. Many epoxy and carboxyl components can be used, hence the raw material base of polyester-based hot-melt PSAs is broader than that of block copolymers. The saturated copolyesters with 1–2 radiation curable double bonds have an application temperature of about 100 C and a viscosity of 2–10 Pa.s. Through synthesis or by postfunctionalization other reactive groups (e.g., carboxyl-, alkyl-, isocyanate-, acryl-, or alkoxy-silyl groups) may be built into the polymer. Copolyesters are not dangerous to human health (some have FDA approval) and are insensitive to plasticizer migration [236]. The longer the chain length, the better the cohesion and adhesion, but the higher the melt viscosity.

Chemical Composition of PSAs

213

Raw Materials for Solvent-Based PSAs Solvent-based acrylic adhesives represent about 40% of the total solventbased adhesive label stock production. Such systems are appreciated for their high performance level and quality, but remain the most expensive. They are increasingly being replaced by water-based adhesives but will likely remain preferred for demanding applications (e.g., outdoor usage, high humidity conditions). In the future, they may also be replaced by new hotmelt or reactive solventless systems. Rubber-based solutions still account for 55% of the solvent adhesives, but there is little research or development going on in this area. Through the choice of the chemical base for solvent-based PSAs, the designer may use a broad range of chemically quite diﬀerent raw materials. Theoretically all the base components suitable for molten or water-dispersed PSAs are soluble in organic solvents (i.e., they may be used for solventbased PSAs as well). End-use and commercial considerations may play a determining role. Solution polymers generally have a narrower molecular weight distribution (MWD) than emulsion polymers; therefore the versatility of solution polymers allows a great deal of latitude [42]. Batch polymerization can theoretically yield a MWD approaching 1.0; in practice levels of 1.4–1.8 are more common. Continuous polymerization results in a MWD of 1.8–2.0 in most cases. The MWD along with the molecular weight exerts a very strong inﬂuence on processability. Raw Materials for Water-Based PSAs Among the various coating technologies water-based adhesives have grown from a small usage in the 1970s to the status of preferred technology, representing 63% of the whole label stock production in 1988. Actually they cover more than 80%. Solvent- and hot-melt-based adhesives are estimated at respectively 10% and 10% of the total production. Taking into account the growing importance of the water-based PSAs technology a broad range of raw materials was developed. It should be mentioned that unlike solventbased adhesives—where a rubber-based technology was consolidated and where only a late development in polymer synthesis (acrylics) made the manufacture possible of nonrubber-based PSAs—it was not possible to develop an aqueous technology based on natural or synthetic rubber latex in the ﬁeld of water-based PSAs. The emulsions typically contain about 40–70% polymer when manufactured, whereas preferred latexes typically have a content of about 40–60 wt% polymer solids. The correlation between solids content and viscosity includes the particle size and particle size distribution (PSD). A bimodal or multimodal PSD increases the solids content. The dispersed polymer particles can be of any size suitable for the

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intended use, although a particle size of at least about 120 nm is presently preferred. Most often the latexes have particles with a diameter in the range of about 120–1000 nm. Decreasing the particle size (at very low particle size) the viscosity increases exponentially. Introducing polydispersity in an acrylic dispersion might result in a lower viscosity at the same volume fraction than monodisperse spheres. Dispersions with spherical particles display lower viscosities than other particle forms. Acrylics as raw materials for water-based PSAs did not have any competitors for a long period of time. Only lately has the development of vinyl acetate copolymers led to a new, price attractive class of water-based dispersions as raw materials for PSAs. Some years ago water-based and hydrocarbon-based (rubbery) dispersions were synthesized on the basis of (carboxylated) styrene-butadiene copolymers. Natural rubber, CSBR, and acrylics make up the main base polymers for water-based PSAs. Adhesives based on acrylic dispersions represent over 90% of the European production of water-based label stock adhesives. The success of water-based adhesives, particularly acrylics, is due to several factors. First water-based adhesives eliminate the use of solvents and their associated environmental concerns. Furthermore water-based adhesives display good coating characteristics with diﬀerent types of coating equipment and oﬀer good convertability to the converters. Blending with tackiﬁer emulsions allows the development of products with speciﬁc adhesive performance characteristics, although adhesion on polyoleﬁn substrates oﬀers potential for improvement. Raw Materials for Radiation-Cured PSAs As discussed earlier (Section 1.4) the main possibilities to formulate radiation curable formulations include the syrup approach (monomer/ oligomer mixture), the reactive oligomer approach, and crosslinking of pressure-sensitive adhesives. Electron beam-cured adhesives are usually acrylate-based systems, formulated from oligomers with reactive diluents in addition to the additives required to achieve the functional properties [237]. Generally prepolymers and reactive diluting agents are used [238], but the danger of phase separation (especially at concentrations lower than 10% of the diluting agent) remains [237]. The purpose of the reactive diluent monomer in the UV-curable adhesive is to provide a solubility medium for the resin system [239]. Such monomers used include 2-ethyl hexyl acrylate, hydroxyethyl methacrylate, cyclopentadiene acrylate, and tetrahydrofurfuryl acrylate. Diluent monomers may also eﬀect the PSA characteristics. A major obstacle in the application of radiation-curing technology for adhesives was the development of ﬁlm-forming properties without adversely

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aﬀecting the tack. Therefore multifunctional monomers were used in limited quantities in the adhesives, because they minimize the segmental motion in the polymer and substantially reduce adhesive properties. On the other hand too high a radiation dose may lead to damage of the face stock and/or loss of tack, whereas too low a dose produces uncrosslinked monomers. There are, however, multifunctional monomers with physical and functional characteristics that provide positive contributions to the adhesive properties. Such a monomer is tetraethylene glycol diacrylate which displays enhanced polarity and molecular ﬂexibility. When incorporated into the UV-curable PSA formulations as an adhesion modiﬁer, a permanent type adhesive is obtained exhibiting 2.5–3.0 lb of peel adhesion with more than 2 h of shear strength [239]. Substitution of tetraethylene glycol diacrylate with other multifunctional acrylates (e.g., pentaerythritol triacrylate, hexanediol diacrylate, trimethylol propane) imparts a nonpermanent characteristic. As discussed earlier (Section 1.4) a range of 100%-solid hot-melt PSAs on acrylic basis was developed using macromers [155,240]. Polyester oligomers were suggested too. Low molecular weight polybutadienes (MW ¼ 3000–5000) are able to make rigid resins more ﬂexible and impart rubber-type properties. In addition to reactive side group vinyl unsaturation they possess a terminal functionality which can provide additional reactivity and/or property enhancement; they can contain amine, vinyl, carboxyl, and hydroxyl terminal functionalities, which can be incorporated in UV-curable adhesives at 15 wt%. A variety of photoinitiator systems were tested for radiation-curing PSAs, including: benzophenone, isobutyl benzoin ether, diethoxyacetophenone, and benzoin ethyl ether [239]. Using the same formulation but a diﬀerent radiation method (UV or electron beam) the adhesive characteristics are quite diﬀerent [239]. The adhesive values for electron beam-curing are overall quite low compared to data acquired for the same formulations cured via the UV process. This is probably due to the fact that the electron beam is a much higher energy source than UV, thus promoting a higher level of crosslinking within the system. The ﬁrst radiation curable styrene block copolymer (Kraton D-1320X) was processed like a common thermoplastic elastomer. This is a star-shaped SIS block copolymer. Classical hot-melt formulations do not respond similarly when subjected to irradiation. To obtain optimum crosslinking with a minimum of radiation, the selection of suitable resins and plasticizers is necessary; for example, PSAs based on the newly developed SIS copolymers crosslink much faster with saturated resins than with unsaturated resins and plasticizers. Polybutadiene alone needs extremely high doses; blends with acrylic esters do not display acceptable PSA properties. Only

216

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built-in hydroxy and epoxy groups and double bonds (e.g., ethyl hexyl acrylates) give good results [238]. The best adhesive properties are achieved with a dose of 30–40 kJ/kg; unsaturated polyesters and monomers with alkylamino groups give good results for a radiation dose of 80 kJ/kg [238]. 2.4

End-Use

High speed automatic label application requires moderate to high initial tack in order to prevent label delamination in the production cycle. Moderate to high shear properties are necessary on squeezable containers to allow the label to remain in contact with the bottle during deformation [241]. Good wet-out characteristics are extremely important for proper application of clear label stock materials. See-through labels (no-label look) also require water-like clarity of the adhesive system; therefore solventbased acrylics usually are used. Some applications using opaque label materials require rubber-based systems. These few examples illustrate how very diﬀerent end-use requirements may inﬂuence the design of the chemical composition. Next the features of composition design for the most important PSA categories are discussed. Raw Materials for Reel and Sheet Label Stock Roll labels (usually on paper) are estimated at about 77% of the total label volume; sheet labels and other products represent 20% and 3%, respectively, of the overall production [242]. Quite diﬀerent application (labeling) technologies are used for small labels and decals, for machine (gun) labeling, and hand applied labeling. In most cases the adhesive tack and release force (peel force from the release liner) need to be closely monitored. Reel labels require an aggressive tack, while sheet labels need more cohesion and less tack. Therefore the base elastomer for reel/sheet labels will be selected in a quite diﬀerent manner. Cohesive, low tack, high molecular weight polymers of short (side) chain acrylics, and ethylene vinyl acetate copolymers are adequate for such sheet applications. Soft, very tacky polymers of ethyl hexyl acrylate or butyl acrylate (i.e., long side-chain acrylics) are preferred for reel labels. These may be used as solvent-based or water-based adhesives. Competitive formulations for reel labels may also be designed on a hot-melt PSA basis. Water-based formulations are high tack, tackiﬁed acrylics, or CSBR-based, tackiﬁed PSAs. The use of special ethylene vinyl acetate-maleinate multipolymers also was suggested [130]. The ﬁrst-generation ethylene vinyl acetatedioctyl maleate copolymers oﬀered good tack and peel strength, broader speciﬁc adhesion than acrylates, and excellent PVC plasticizer resistance.

Chemical Composition of PSAs

217

Cohesive strength remains limited however, and the product is used mainly in roll label applications at moderate to high service temperatures. Raw Materials for Film Labels Clarity considerations together with a high UV resistance limit the choice of raw materials for ﬁlm labels. Face stock/adhesive interactions also act as limiting factors. Therefore only a few materials appear suitable as PSAs on ﬁlm face stock materials; the choice is restricted to the range of acrylics (usually solvent-based ones). Raw Materials for Permanent and Removable Labels As discussed in Chap. 2, removability implies a low peel level and no adhesion build-up. These criteria require low tack, high cohesion adhesives, and good plasticizer compatibility. Because of the relatively low cohesion of hot-melt PSAs and of the high migration tendency of water-based PSAs (and their limited crosslinking) solvent-based PSAs were used as removable PSAs. The latest developments in water-based acrylics allow the formulation of removable PSAs on a dispersion basis. However, care should be taken concerning the choice of the base elastomer. As an example it was observed that ethylene vinyl acetate copolymers used for ﬁlm coatings interact with the soft PVC face stock material. Therefore aged peel values for removable formulations are higher than desired. A detailed description of the chemical basis of PSAs used for removable labels is given in Chap. 8. As far as the pressure-sensitive adhesive component is concerned, the properties which contribute to peelability are principally limited tack, a low build-up factor, and a relatively soft adhesive. These properties can be achieved in a conventional way by omitting or only using a low level of tackiﬁer, including tack deadeners such as waxes, ﬁllers, or special agents, and by including plasticizers. The chemistry of the polymer also has an eﬀect but, apart from choosing among polymers which are commercially available, the chemistry is controlled primarily by the manufacturer of the polymer rather than the adhesive formulator. Small amounts of a highly polar monomer, such as acrylic acid, cause adhesion build-up. On the other hand, small portions of acrylonitrile and methacrylonitrile improve removability. An amount of about 30% natural rubber latex added to a vinyl acetate-maleinate dispersion yields a removable PSA, without leaving any adhesive residues upon debonding [243]. Raw Materials for Other Requirements As discussed in detail in [230] there are numerous pressure-sensitive products tailor made for special applications. In the past decade, to the main product

218

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classes of labels and tapes, were added protective ﬁlms, forms, and special products. In each such product range there are special requirements for the environmental stability and regulation of the environmental stability [244]. These performance characteristics include temperature resistance and water resistance as the main criteria. Both will be discussed in detail in Chap. 8. Next a short description of the problem is given. Temperature Resistance. The standard temperature resistance of PSA formulation is given by the rheological characteristics of the components. The stability in time of such performance characteristics is aﬀected by the chemical resistance of the raw materials. For instance butyl systems should perform well at low temperatures, while acrylics are superior in a room temperature environment [245]. Low temperature peel adhesion for silicones increases at 75 C; for organic PSAs the peel value is zero below 25 C [31]. Hence silicone PSAs are recommended for low temperature conditions. Water Resistance. Whereas wash-oﬀ labels were required on the market some years ago [246], conventional PSAs are hydrophobic in nature [141]. As such, they adhere poorly to wet or damp surfaces and generally cannot be eﬀectively removed with water, even using automated cleaning procedures which employ detergents, warm water, or caustics. Benedek [247] discussed the problem of the water resistance of PSAs in detail. The water resistance of water-based adhesives depends on the chemical composition of the base polymer and on the composition of the emulsion. Generally the water resistance Rw is the sum of the synthesis given water resistance Rws and of a formulation given water resistance Rwf [248]: Rw ¼ Rws þ Rwf

ð5:3Þ

Thus the postregulation of the water resistance can be achieved by modiﬁcation of the base polymer or (for water-based adhesives) of the dispersed system. The inﬂuence of the monomers and of the surface active agents used during the polymerization was described. As pointed out, formulating depends on the adhesion-water resistance balance. Generally, for common pressure-sensitive products water resistance is required. A special performance is the water resistance of PSAs based on water-based dispersions. Such products possess a heterogeneous, composite structure, including migrating and water-soluble components (e.g., surface active agents, monomers and polymers). Water-soluble surfactants remain in the dried ﬁlm and give water-sensitive ﬁlms. By migrating to the ﬁlm-air interface surfactants cause the polymer ﬁlm to be moisture sensitive or hydrophilic; by migrating to the ﬁlm-substrate interface they can cause

Chemical Composition of PSAs

219

a reduction in adhesion. Prevention of this migration can be achieved using copolymerizable or volatile surfactants. By neutralizing a traditional anionic surfactant with a reactive moiety, for example allylamine, one can render the surfactant a reactive surfactant [249]. Allyl-acrylic latexes were produced [250]. Ammonium salts of styrene-maleic anhydride lead to copolymers with improved water resistance [105]. In some cases enhanced ﬁlm hydrophobicity has also been found to enhance water whitening resistance [251,252]. For example Feng and Winnick [252] noted that polybutylmethacrylate latex exhibited a high degree of water whitening whereas polybutylmethacrylate-co-methacrylic acid latex copolymer did not. Miller and Barnes [253] have found a strong eﬀect of water-soluble low MW species on both water absorbtion and water whitening. Water resistance can be improved by the choice of the monomers too. As stated by Yang [254] a beneﬁt from vinyl neoester comonomer is the improved water resistance for copolymers containing vinyl acetate. Water-soluble (wash-oﬀ) formulations are limited by a narrow raw material basis and low adhesion levels. The practical requirement for washoﬀ labels were identiﬁed as the need to have a temperature-dependent water solubility and wet tack (i.e., adhesion on condensed water coated surfaces). A relatively simple formulation exhibiting water solubility can be designed by changing the ratio of water soluble/insoluble components in a common formulation (pseudo-solubility). Time- and storage-independent wash-oﬀ formulations generally need more than 20 wt% special water-soluble components. Adhesives based on polyvinyl methyl ether were among the earliest available water-soluble PSAs. While they showed fair adhesive properties and are used for limited applications, several deﬁciencies restrict their suitability, mainly that they did not absorb water rapidly enough to adhere to wet surfaces. Adhesives based on polyvinyl methyl ether, however, were selected for wash-oﬀ labels [141]. Partial esters such as polyvinyl methyl ether/maleic anhydride with nonionic surfactants of the nonyl phenol ethylene oxide adduct type have also been used. Polyvinyl ethers do not adhere to wet surfaces and maleic anhydride copolymers are very humidity-sensitive. Other water-soluble polymers such as polyvinyl pyrrolidone (PVP) with water-soluble plasticizers, copolymers of acrylic acid and alkyl acrylates, and others also were used [255]. A need exists for an adhesive which has good adhesion to both wet and dry surfaces, whether cold or warm, whether polar or nonpolar, with a suﬃcient permanent character to be retained as tamper-proof when combined with label papers, but yet easily removed when desired, with warm or cold water, or with detergents and alcohols used in commercial or domestic cleaning operations. Such a formulation may be given by acrylic acid-based polymers with a broad MWD [141].

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Water-soluble hot-melt PSAs are made from polyethylene oxide (MW ¼ 50,000) and polyalkylene oxide (MW ¼ 190–15,000) [256]. Other formulations contain a water-soluble ester of acrylic acid and polyethylene or polypropylene glycol [257]. Water-soluble PSA is made up of 2-ethyl hexyl acrylate or other tackifying monomers (optionally methyl methacrylate or other diluent monomers) and 7–30 wt% hydrophilic monomers selected from methacrylic acid (and salts thereof), hydroxy alkyl acrylates, and hydroxy alkyl methacrylates [258]. Other water-activatable or soluble hot-melt PSAs are based on vinyl pyrrolidine/vinyl acetate copolymers [259]. New generation acrylic water-based PSAs exhibit enough tack and peel after immersion in hot water [260]. Adhesion on Porous Substrates. In the area of health care tapes and disposable products, acrylate polymers are innocuous, in general, when exposed to human skin. Many health and other tape (label) applications exist where it is desirable to have a pressure-sensitive ﬁlm coated onto a porous web. Acrylic hot-melt PSAs oﬀer a relatively easy route to coating PSAs onto ﬁlm or nonwoven webs, with a minimum of adhesive penetration into the web [261]. Acrylic hot-melt PSAs have better initial tack after aging than classical, rubber-based ones [262]. They have a viscosity of 26,000–30,000 mPa.s at 180 C and display good peel adhesion values on diﬀerent substrates [263]. A detailed comparison of the PSA properties based on diﬀerent raw materials will be given in Section 2 of Chap. 6. 2.5

Coating Method

The use of direct or transfer coating in the manufacture of pressure-sensitive products depends on technical and economical considerations. The nature of the product, its build-up, the nature of its components, and the technical equipment aﬀect the choice of a coating technology. The coating method (direct or transfer coating) can inﬂuence the chemical composition of the adhesive. One can assume that direct coating on porous surfaces allows a rapid penetration of the adhesive in the face stock material. Evidently, low molecular compounds migrate preferentially (see Section 2.7 of Chap. 8). The viscosity of the coated adhesive and its components inﬂuences both the wet-out and the bleedthrough. For instance a special water-based dispersion for paper masking tapes, designed to be direct coated on crepe paper with a coating weight of 38 g/m2, has 60% solids content and a high viscosity (5000 mPa.s) [264]. For easy-to-wet, directly coated face stock materials low viscosity dispersions may be used if bleedthrough is not an important issue. For diﬃcult surfaces (release liners) or/and porous materials a higher application viscosity appears to be

Chemical Composition of PSAs

221

required. The viscosity of the PSAs to be coated may be controlled when formulating (e.g., by choice of the solvents used, solids content, thickness, pH, etc.). The viscosity of the bulk material (rheology) is composition and molecular weight dependent. Chemical Basis for Directly Coated PSAs. Each class of raw materials (e.g., natural or synthetic rubbers, acrylics, ethylene vinyl acetate copolymers, etc.) with or without tackiﬁer may be used for direct coating of diﬀerent face stock materials. In each physical state (molten, dissolved, or dispersed) materials may be used for direct coating onto paper face stock materials. Restrictions must be taken into account concerning the porosity/ viscosity balance (usually for water-based PSAs) and the thermal sensitivity of special face papers. As with ﬁlm coatings, this acts as a limiting factor when using hot-melt PSAs. Almost all rheological behavior (i.e., a high ﬂuidity or a shear-induced viscosity decrease) may be tolerated for direct coating of paper or ﬁlms. In order to avoid penetration (i.e., bleedthrough of hot-melt PSAs) the application viscosity should amount to 17,000–20,000 mPa.s at 180 C [262]. Hot-melt PSAs are also suggested for coating onto nonwoven substrates [265]. Raw materials also diﬀer as a function of the coating machine. As an example aqueous acrylic tape adhesives are designed as a function of the coater geometry [266]. The required viscosities are: for rotogravure, 75–100 mPa.s; for Meyer Rod, 300–800 mPa.s; and for knife-over-roll, 4000–8000 mPa.s. On the other hand foam generation is higher for Meyer Rod than for rotogravure, and even higher for knife-over-roll. A detailed discussion of the formulation diﬀerences as a function of the coating method is given in Section 2.3 of Chap. 9. Chemical Basis for Indirectly (Transfer) Coated PSAs. Indirect (transfer) coating implies good wetting at low surface tension, or high viscosity. Both are intrinsic properties of hot-melt PSAs or some solventbased PSAs. Diﬃculties appear mostly with the use of water-based PSAs, where special wetting agents and thickeners must be used. 2.6

Solid State Components of the Laminate

Quite diﬀerent plasticizer levels are required for the formulation of adhesives used for ﬁlms (50%) or other materials (2–6%) because of the diﬀerent ﬂexibility and porosity of these substrates [267]. As illustrated by this example the solid state components of the laminate inﬂuence the choice of the adhesive components by their diﬀerent aﬃnity towards these components and by changing the adhesive composition through chemical interaction with the PSAs.

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Pressure-sensitive adhesives for ﬁlm coating must meet some special, face stock-related requirements. Pressure-sensitive adhesives for PVC coating should meet the general requirements for ﬁlm coating in addition to some special PVC-related requirements, as the requirements for soft PVC are quite diﬀerent from those for hard PVC; similarly, ﬁlled (opaque) PVC possesses diﬀerent properties from clear PVC. Sensitive ﬁlm face stock materials need a nonaggressive adhesive formulation; for example, soft PVC usually requires tackiﬁer-free adhesive formulations. Stabilizers present in the PVC (e.g., Pb, Sn) produce oxidative degradation of the tackiﬁer resin which is associated with the loss of the tack. Polyoleﬁn carriers impose improved wettability and anchorage, and aging properties. For such materials dimensional stability is aﬀected thermally. Porous, primerless papers as face stock material need nonmigrating adhesive formulations. Generally, migration of the low molecular weight components from the adhesive (e.g., plasticizer, tackiﬁer, surface active agent, water, etc.) into the paper, or from the ﬁlm (plasticizer) or paper (water, special chemicals like components of thermal papers) into the PSAs, may alter the chemical composition of the coated adhesive. These selected examples illustrate how the chemical composition may inﬂuence the solid state components of the laminate and vice versa. Not only the adhesive properties, but also the coating characteristics of the PSAs may be inﬂuenced by the solid state laminate components. As an example, carboxylated neoprene latex is preferred to other latexes because it provides better speciﬁc adhesion to aluminum foil. Tables 5.2 and 5.3 summarize the quite diﬀerent requirements imposed on PSAs as a function of the face stock material.

2.7

Product Build-Up

Pressure-sensitivity is the result of special chemical and macromolecular build-up, but macroscopical product build-up may contribute to pressuresensitivity too. By transfer coating, the surface quality of the silicone layers aﬀects the smoothness of the PSA layer. The high gloss surface ﬁnish of the release coating (with OPP) liner transfers to the adhesive increasing its tack level by about 30% [268]. On the other hand transfer coating on such release needs special formulation. Numerous pressure-sensitive products require multiple (adhesive and nonadhesive) coated layers. In in-line coating the coated web may become a complex structure, for instance: releasecorona-primer-PSA-corona-primer-PSA. As discussed in [269] the coating technology for in-line manufacture is needed for special labels, tapes, or forms. In such cases it is preferred to have the same vehicle for the diﬀerent coating layers (or to use 100% solids) and the adhesives should exhibit

Chemical Composition of PSAs

223

Table 5.2 PSA for Labels: Main Requirements as a Function of the Face Stock Material Face stock Required Performance Characteristics Adhesive properties Converting properties End-use properties Water resistance Shrinkage resistance Aging resistance Migration resistance Clarity

Paper

Film

þþ þþ

þþ þþ

–– –– þ– –– þþ

þþ þþ –– þþ ––

þ þ very important, þ – important, – – not necessary

Table 5.3 Performance Characteristics of Different PSAs as a Function of the Face Stock Material Characteristics as a Function of the Face Stock Material Film

Paper

Clarity Aging resistance Water whitening resistance Shrinkage resistance Migration resistance

CSBR

EVAc

AC

þþ –– þ– þþ ––

þþ þþ –– þ– þ–

þþ þþ þ– þþ þ–

þ þ very important, þ – important, – – not necessary

coating versatility for direct and transfer coating too. A special formulation is required for double-side coated and carrier less tapes [270]. 2.8

Environment Related Considerations

Environmental considerations inﬂuence the chemical composition (formulation) of the PSA also [269]. The most important problems concern recycling during manufacture of PSAs and recyclability of pressure-sensitive adhesive coated products, but advances in so-called ‘‘bio-based’’ pressuresensitive adhesives have been made too (see Chaps. 9 and 10).

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6 Adhesive Performance Characteristics

1

ADHESION-COHESION BALANCE

Pressure-sensitive adhesives possess adhesion, required for bonding and debonding, and cohesion necessary against debonding. Adhesion is characterized by tack and peel, whereas cohesion is described by shear resistance (and partially by peel). The special balance of these properties, the adhesion/cohesion balance, embodies the pressure-sensitive character of the adhesive. For any PSA, tackiﬁed or not, both tack and peel adhesion can be considered as the end result of two distinct processes, namely, a bonding process and a bond breaking process. The eﬃciency of the bonding process is related to the adhesive’s ability to exhibit viscous ﬂow. In order to achieve peel adhesion the bonding stage involves some dwell time. During this time the adhesive must ﬂow in the absence of any externally applied forces. The more liquid-like the behavior of the polymer under these conditions, the more pronounced the degree of bond formation. The debonding process involves a more rapid deformation of the adhesive mass. The polymer’s resistance to deformation at higher strain rates becomes very important; the higher this resistance, the higher the force which must be applied to separate the adhesive from the adherend (i.e., the peel resistance). Therefore, high tack, high peel strength adhesives should exhibit good ﬂow at low strain rates, but good resistance to ﬂow at higher strain rates. 1.1

Tack

In Chapter 2 a short deﬁnition of the tack was given and the interdependence of tack/adhesive rheology was discussed; this chapter 235

236

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describes the external aspects of the tack. Tack of PSAs is not an exactly deﬁned, physical characteristic; it may be deﬁned as a separation energy. Tack is a function of T (i.e., Tack > 0 if T > Tg). The tack is inversely proportional to the elasticity modulus [1]. On the other hand, the modulus depends on the branching, as does the Tg. Tack is the resistance oﬀered by an adhesive ﬁlm to detachment from a substrate; it is the measure of the stretchiness of an adhesive, or the ability to form an instant bond when brought into low pressure contact with a substrate to which the adhesive is to adhere [2]. Tack, and the methods used to measure it were discussed by Johnston [3]. The concept of tack remains diﬃcult to deﬁne. It was called tack, wet tack, quick stick, initial adhesion, ﬁnger tack, thumb tack, quick grab, quick adhesion, and wettability. The Pressure-Sensitive Tape Council prefers quick stick [4] and deﬁnes it as ‘‘that which allows a PSA to adhere to a surface under a very slight pressure.’’ The American Society for Testing and Materials [5] deﬁnes tack as the force required to separate an adherent and an adhesive at the interface shortly after they were brought rapidly into contact under a light pressure of short duration. Measurement of the Tack Tack is measured either by touch or quantitatively by means of a loop tack tester. Loop tack, deﬁned as the ‘‘quick stick’’ tack value, is the force required to separate at a speciﬁc speed, a loop of material (adhesive surface facing outwards) which was brought into contact with a speciﬁc area of a standard surface (in the absence of signiﬁcant pressure). Tack also may be measured by the rolling ball or rolling cylinder method (for a detailed description of the methods see Chap. 10). In the last decade the development of contact mechanics favored the use of probe tack-like tack test methods. Asymmetric adhesion tests using either ﬂat or spherical punches played an important role in understanding the fundamental mechanism of adhesion. The geometry of a spherical punch pressing against a ﬂat substrate has been used in fundamental studies of the tackiness of crosslinked elastomers [6–9]. Flat punch tests have often been the choice of industrial research labs and quality control and are the basis of commonly used ASTM standards. Tack Measured as Coefficient of Friction. It is believed that the resistance to the rolling motion of a ball on a PSA reﬂects the tackiness of the adhesive, because the motion must be closely related to bonding and debonding processes, which occur simultaneously at the contact surface. This way of expressing tack is useful in some practical cases, but the physical meaning of the measured values is not necessarily clear. It is more scientiﬁc

Adhesive Performance Characteristics

237

to express the tack in terms of a rolling coeﬃcient of friction which depends on the physical properties of the materials [10]. The rolling coeﬃcient of friction can be determined experimentally from the ‘‘pulling cylinder’’ method more easily than from the ‘‘rolling ball’’ method, and one can theoretically calculate the rolling coeﬃcient of friction by making some assumptions concerning the deformation and failure mode of PSAs. The velocity of the rolling ball changes at every moment and, at the same time, the rolling coeﬃcient of friction varies as a function of velocity. Assuming that F is the static frictional force, N is the normal force, and I are the angle of rotation and moment of inertia of a cylinder, respectively, f is the rolling coeﬃcient of friction of the PSA, P and Mg are the force components in parallel with and perpendicular to the rolling cylinder, respectively, and if the cylinder is pulled at a constant velocity, P is given by the following equation: P ¼ ðf =RÞMg

ð6:1Þ

The rolling coeﬃcient of friction of a viscoelastic material can be written as the sum of two terms: f ¼ fc þ fa

ð6:2Þ

where fc represents the rolling friction caused by the compressive deformation of the substrate material and fa that caused by adhesion or extensional deformation of the substrate, which are shown in Fig. 6.1. In the case of PSAs fa must be much larger than fc. If compressive deformation of the adhesive is neglected, the strain of the adhesive " at is expressed as [10]: " ¼ ðR=hÞð1 cos Þ

ð6:3Þ

and the rate of strain "0 is: "0 ¼ ðR =hÞ sin ¼ ðv=hÞ sin

ð6:4Þ

where h and v are the original thickness of the adhesive layer and velocity of the cylinder, respectively. For a mechanical model, the stress generated by the elongation of the adhesive can be expressed as a function of , where f is the angle at the moment of failure. Assuming that the moment caused by the extended part of the adhesive is equal to fa Mg (according to the deﬁnition of the rolling coeﬃcient of friction) fa may be expressed as a

238

Chapter 6

Figure 6.1 Rolling friction caused by compressive deformation of the substrate (adhesive) and by its extension deformation during rolling of the ball.

function of the cylinder characteristics (i.e., width b, radius R, moment, and stress): Z

fa ¼ ðR2 b=MgÞ

ð Þ cos sin d

ð6:5Þ

0

Graphs of fa versus log v are diﬀerent for diﬀerent viscoelastic models and failure criteria [10]. Mizumachi calculated the fa/v functions for a single Voigt element, a single Maxwell element, two Maxwell elements, multiple Maxwell elements, and multiple Maxwell elements in parallel (Fig. 6.2). In some cases a dependence of fa on the modulus E, viscosity , velocity v, adhesive thickness, and cylinder radius was found. It would be reasonable to expect that a curve of f versus log v for a PSA generally exhibits a peak value or values that become small when the velocity becomes extremely high or extremely low [10]. For adhesive practice it is very important to summarize the dependence of the tack on the adhesive rheology (modulus/ viscosity) and on the coating weight Cw: Tack ¼ f ðE,,Cw Þ

ð6:6Þ

The inﬂuence of the rolling ball or cylinder geometry on the measured tack value may be derived from the general conditions of the adhesion,

Adhesive Performance Characteristics

239

Figure 6.2 Viscoelastic model elements for PSA. 1) A single Maxwell element; 2) two Maxwell elements in parallel connection; 3) a single Voigt element.

friction, and wear of elastomers [11]. Equilibrium conditions and the kinetics of adherence between a single hard spherical asperity and a smooth surface of a soft elastomer were studied using fracture mechanics concepts (see Chap. 3, Section ‘Contact Mechanics’). According to Greenwood [12], when an elastic cylinder rolls over a plane, the movement of the area of contact can be regarded as a propagating crack at the trailing edge (fracture) and a healing crack at the leading edge. If the cylinder is truly elastic and the process of forming or destroying free surface reversible, then there will be no contribution to the rolling friction from the surface energy. In practice, with an imperfectly elastic roller, the friction depends both on the viscoelastic properties of the roller and on the surface energy; surprisingly the two seem to be not additive but multiplicative. It has long been believed that only the fracture process at the trailing edge is signiﬁcant, and that the energy recovered from the destruction of free surface at the leading edge is negligible. Conditions for the appearance of reattachment folds and detachment waves were established as a function of several parameters; namely, the normal applied load, the sliding or rolling speed, and the radius of the curvature. The radius aH of the contact area is given by: aH ¼ ðPR=KÞ=3

ð6:7Þ

240

Chapter 6

where P is the normal applied load, R is the radius of the ball, and K ¼ ð4E=3Þð1 2 Þ

ð6:8Þ

where E and are the modulus and Poisson’s coeﬃcient of the adhesive. The contact area is given by two diﬀerent ﬁelds, one of compression in front of the ball and another of tension (smaller than that of compression) behind the ball [13]. The friction force of the ball depends on the temperature and on the velocity of the ball, and this dependence is similar to that of the loss modulus on the frequency and temperature (i.e., it may be described by the Williams–Landel–Ferry equation) [14]. The friction is given by the propagation of the small debonding ‘‘waves’’ (Fig. 6.3), which are related to the peeling oﬀ of the ball, VR ¼ nF

ð6:9Þ

where is the velocity of the ball, R is the local resistance against the motion of the ball, V is the relative speed of the ball, and F is the peel force. As can be seen from Eq. (6.9), rolling ball measurements can be formulated as peel measurements. (Since the peel edge can be regarded as the tip of an advancing crack, the adhesive ahead of the advancing peel front is subjected to very large hydrostatic tension due to the lateral constraint imposed by the substrate and the cover sheet [15]. The ﬁbrils during peeling are modeled as elastic strings and the ﬁbril break down at a critical length like by the rolling of the ball. It is evident that peel adhesion will depend on the relaxation of the adhesive material. The friction is the result of the competition between the entrainment of the adhesive by the ball and the interfacial relaxation due to the molecular motion. This interfacial relaxation depends on the rheology of the PSA, but at the same time it depends on the thickness of the adhesive

Figure 6.3

Friction given by the propagation of small debonding ‘‘waves.’’

Adhesive Performance Characteristics

241

and its anchorage onto the face stock material. If this holds, the nature of the face stock surface has a strong inﬂuence on the tack measured by the rolling ball. This conclusion is very important because of the quite diﬀerent dependence of the tack measured as loop tack on the face stock material. In this case the ﬂexibility of the surface plays a determining role on the measured tack value. It should be mentioned that, on the basis of our knowledge about peel adjustment with the aid of the face stock surface (primer), it is quite normal to have the same dependence for tack measurements by rolling ball, whereas shown earlier, peeling phenomena occur. The examination of the friction force allows a correlation between tack and shear resistance. The friction force Ff is the sum of two components [16]: Ff ¼ Fadh þ Fdef

ð6:10Þ

where Fadh is the adhesion component and Fdef the deformation component. The latter is a function of the internal cohesion of the material, therefore tack will be a function of the cohesion as well. Tack Measured as Cohesion. Hamed and Hsieh [17] measured tack as cohesion. In the debonding step tack is a function of the cohesion; the upper limit of the tack of an elastomer is its cohesive strength. If during bond formation, complete contact and interdiﬀusion occur, then the interface will be modiﬁed, and the measured tack must be identical to the cohesive strength of the elastomer [17]. Hamed and Hsieh proposed ‘‘relative tack’’ (tack divided by cohesive strength) as a measure of the completion of the tack bond. If the relative tack equals zero, then no tack bond was formed. Relative tack is rate dependent; Hamed and Hsieh measured tack as the average peel force per unit width. At low test speeds the ratio of tack to cohesive strength is less than the same ratio at higher test rates [17]. At suﬃciently low stress rates the short inter-diﬀused chains have time to relax, relieving the applied stress. On the other hand, the cohesive strength is controlled by the longer chains, which cannot slip past one another very easily. At higher strain rates even the short molecules may have little time for the molecular rearrangements needed to relieve stress; hence bond strength will increase rapidly as there is insuﬃcient time for failure by chain slippage and chains may rupture. Tensile tests of SIS block copolymers show that decreasing the SI content or increasing the crosshead velocity leads to a higher stress for all extensions and to a more pronounced strain hardening [18]. The strain hardening observed in the ﬁbrillation part of the probe test curves can be correlated to tensile tests; in both cases the lower the SI content, the more pronounced the strain hardening. Experimental results show, that the beginning of the probe test (where the cavitation process takes place)

242

Chapter 6

quantitatively corresponds to the small strain shear properties of the adhesives (i.e., tack depends on the cohesion) while the end of the test (ﬁbrillation, strain hardening) can be qualitatively—so far—related to the large strain elongational processes. Tack and Adhesive Fracture Energy. Adhesion and tack of polymers are not fundamental material properties like the modulus or the viscosity; they strongly depend on the test methods used and the measurement conditions. Adhesion performance is characterized by the adhesive fracture energy. Two stages characterize the tack test that is based on the measurement of the strength vs. time: the contact phase and the separation phase. The tack energy (G) increases with the aptitude of the adhesive to deform by cavitation and stringing during the debonding process. For ‘‘nonstringing adhesives’’ (low viscous dissipation in the bulk of the adhesive) G is sensitive mainly to the surface energy and to the surface roughness of the probe. According to [19] a constant contact load is applied during a given contact time then the probe is pulled up at constant debonding rate; a constant contact area is applied for very short contact time (less than 1 sec) and the probe is pulled up. The force and the contact area are monitored vs. time. The product of the area subtended by the debonding force vs. time by the pulling rate gives the tack energy. Actual contact area increases with the thickness and then applatizes. Tack energy is a square function of the wet area. First Zosel [1] developed an instrument which measures the fracture energy and allows the adjustment of the most important parameters during the bonding and debonding processes, such as contact time, contact pressure, rate of separation, and temperature. The (bonding/debonding) force vs. time plot of the instrument can easily be transformed into a stressstrain curve, giving the tensile stress as a function of the tensile strain during bond formation and separation. An appropriate measure of adhesive bond strength and tack is the energy of separation per unit area of surface (i.e., the adhesive failure energy) [20–22]. This apparatus resembles the probe tack tester and a very high correlation between both methods was found. According to [23] the use of a Mechano Optical Tack tester allows the determination of the tack strength (F ), tack energy (G), and the actual contact area (A), for tests at controlled contact force (Fc), contact time (tc), and release rate (r) [24–29]; G (J/m2) is given by the relation: Z G ¼ ðr=AÞ

tf

FðtÞ dt

ð6:11Þ

Tack curves are load displacement curves generated during asymmetric adhesion test. Several past investigations have shown that for elastomeric

Adhesive Performance Characteristics

243

adhesives the relation between the crack tip velocity and G is a unique function for each combination of substrate and adhesive [30,31]. Empirically one suggested form of this unique function is: G ¼ Go ½1 þ ðaT , . . .Þ

ð6:12Þ

where ¼ (/*)n, Go describes the interfacial contributions to the adhesion, and includes the quantities pertaining to bulk dissipative processes near the crack tip. Theoretical models were proposed by Gay and Leibler [32] and Creton and Leibler [33] to account for the eﬀect of the surface energy, surface roughness, and the adhesive Young’s modulus on the cavitation and ﬁbrillation process of stringing adhesives. Peel Test Used for Estimating the Tack. A modiﬁed 90 peel test (now known as PSTC Test Method 15 for Quick Stick) is used for tack measurements [34,35]. Tack Measured as Plasticity. The measurement of the tack is not as reproducible as the measurement of the Williams plasticity [36]. Practically tack is measured as quick stick, loop tack, or rolling ball tack. None of these methods is suﬃcient to characterize tack alone (see Chap. 10). Therefore the tack index was deﬁned as [37]: Tack Index ¼

A þ B þ 50ð17 CÞ 3

ð6:13Þ

where A is the value measured in a 90 quick stick test, B is the value of a loop tack test (g/cm), and C is the value of the rolling ball tack in cm (up to a maximum of 15 cm). This index represents an attempt to take into account the three measures simultaneously. Tack Level. The main adhesive properties (tack, peel, and cohesion) depend on the state of the polymer (pure or formulated), the coating technology (direct/transfer), the coating weight, and the face stock. Of course the most important parameter remains the chemical composition of the polymers. Thus estimating the tack level supposes a comparison of PSAs on the basis of diﬀerent polymers. Acrylics are polymers of long-chain alkylacrylates. There are only a few commercially available low-Tg acrylates (e.g., ethyl hexyl- and butyl acrylate). High tack implies a high level of these ﬂuidizing comonomers. The obtained polymers are tacky, but they lack the necessary cohesive strength. Hence one can conclude that common (nonformulated) acrylic PSAs are not tacky enough and that special acrylic PSAs are not cohesive enough. On the other hand, the tack of pure acrylic

244

Chapter 6

PSAs is lower than that of classical (tackiﬁed) rubber-based PSAs, but higher than that of other untackiﬁed adhesives (except polyvinyl ethers): Tack : AC ðunformulatedÞ > SBR ðunformulatedÞ

ð6:14Þ

> EVAc ðunformulatedÞ

The superior tack level of unformulated acrylic PSAs is illustrated in the Table 6.1, comparatively to water-borne EVAc or CSBR. It can be seen that acrylic PSAs possess a better tack at any given coating weight. Factors Influencing the Tack Tack is inﬂuenced by the nature and amount of the adhesive (coating weight) and face stock material; as shown earlier test methods and conditions also inﬂuence the measured tack value. Influence of the Nature of the Adhesive. Tack may diﬀer according to the chemical composition or state of the adhesive. Pressure-sensitive adhesives that have diﬀerent chemical bases (e.g, natural/synthetic rubber, acrylics, vinyl acetate copolymers, maleinates) exhibit diﬀerent tack levels. On the other hand, PSAs within the same class of monomers (e.g., acrylic PSAs) may display diﬀerent tack, depending on the speciﬁc monomer characteristics, chain structures, and molecular weight. However, most PSAs are formulated and the formulating additives (tackiﬁers, plasticizer, etc.) change the tack. Water-based PSAs need special formulating additives. Thus the nature of the uncoated adhesive (solvent-based, water-based, or hot-melt) will also inﬂuence its tack. Generally, additives such as tackiﬁers (resins or plasticizers) improve the tack; their inﬂuence depends on their compatibility. As an example, polybutenes added as a solution in toluene generally improve the rolling ball tack, whereas the use of Acronal 81 D causes a marginal decrease in tack [38].

Table 6.1 Tack of Unformulated PSAs on Different Chemical Bases. Rolling Ball Tack of Water-Based Acrylic, CSBR, and EVAc PSAs Rolling Ball Tack (cm) Coating Weight (g/m2)

AC

CSBR

EVAc

22 40

1.5 1.5

5.8 5.8

17.0 8.5

AC, V 205 (BASF); EVAc, 1360 (Hoechst); CSBR, 3703 (Polysar).

Adhesive Performance Characteristics

245

Influence of Molecular Weight. Tack is a function of the molecular weight. Fibrillation supposes a molecular weight higher than the entanglement molecular weight. Previous studies [39,40] on PIB showed that tack and adhesive fracture energy decrease monotonically for MW > 45 kg/mol. Zosel [1] demonstrated that the tack of polyisobutylene increases with increasing molecular weight, up to a maximum for MW ¼ 5 104. Midgley [41] noted a slight tendency for tack to depend on increasing molecular weight. According to Midgley, pressure-sensitive adhesive-like blends containing longer chains do not show a strong dependence of the work of adhesion (Wa) on MW, temperature, and mixing ratios, especially as Mv exceeds 1000 kg/mol. This seems to be in contrast to the pronounced tack maximum at Mw/Me around 4 observed by Tobing et al. [42,43]. The useful range of molecular weight in which good PSAs can be identiﬁed is limited. The upper limit of molecular weight corresponds to adhesive failure caused by a lowered ability of the adhesive to absorb energy before the interfacial bond is broken or by poor wetting because of the lowered ﬂow of the adhesive. For solvent-based acrylic PSAs, the viscosity is dependent on molecular weight, and thus limited by it. For water-borne acrylic PSAs there is no technological molecular weight limit, but there is an optimum molecular weight for superior tackiﬁcation. At a suﬃciently short contact time the bond strength (tack) is due primarily to the interdiﬀusion [17]. The self-diﬀusion rate is inversely proportional to the second power of the molecular weight; thus tack decreases with increasing molecular weight. On the other hand, adhesive diﬀusion in the face stock material inﬂuences the coating weight and tack depends on the coating weight. Therefore the molecular weight will have a contradictory eﬀect on the tack. Relatively low molecular weight base elastomers and relatively high molecular weight tackiﬁer resins should be used for a good tack. The dependence of the tack on the molecular weight is demonstrated by the aging of SIS rubbers [44]. In this case chain scission occurs and tack increases. In general, resins with a softening point below about 50 C impart tack, but give poor cohesive strength, while resins with a softening point above 70 C give good cohesive strength but poor tack [45]. Tack properties at a given tackiﬁer level are superior with the lower melting point tackiﬁer [46]. For a natural rubber/rosin ester mixture a resin with a lower melting point (e.g., Staybelite E 10, melting point of 75 C) imparts the maximum tack at a higher level (52%) than a higher melting point resin (e.g., Pentalyn H, melting point of 95 C) at a 33% loading level. At the same time, for maximum peel adhesion, an even higher tackiﬁer level (70%) is needed.

246

Chapter 6

For CSBR tackiﬁcation, increasing the melting point of the resin improves the quick stick of the adhesive [47]. New, high temperature tackiﬁers for formulating PSAs were proposed [48]. Narrower MWD resins contribute to a higher tack than broad MWD resins [49]. A detailed description of the resin characteristics and the correlation resin melting point/tack is presented in Chap. 8. Effect of Crosslinking and Sequence Distribution. Crosslinking yields high shear and lower tack properties [50]. As demonstrated for formulations containing an electron beam crosslinkable rubber, the tack is good and remains fairly constant for the Polyken probe and loop tack, regardless of the applied electron beam dose. However, the rolling ball tack values tend to become slightly poorer as the radiation dosage is increased. Generally, crosslinking reduces the chain mobility with increasing Tg, thus decreasing the tack. As an example, crosslinkable water-based acrylic PSAs (e.g., Acronal 29 D, 30 D) suﬀer a loss of tack after thermal crosslinking. The tack of silicone PSAs depends on their coating weight and curing [51]: Tack ¼ f ðCoating weightÞ,ðCuringÞ1

ð6:15Þ

Lower peroxide levels and a greater adhesive thickness increase the tack. High peroxide levels increase crosslink density and cohesive strength [generally 0.5–3.0% benzoyl peroxide (BPO) is used]. The modiﬁcation of the tack by crosslinking is a function of the materials used, and of practical experience. Despite the reactivity of Cymel resins, the tack of the modiﬁed neoprene latex remains quite excellent [52]. Aqueous PUR dispersions were used to improve the adhesion of acrylic adhesives, for better ﬁlm formation and mechanical properties. The use of nonionic PUR yielded higher shear strength. The tack of formulation was also higher [53]. Tordjeman et al. [23] examined tack as a function of the manufacture of the adhesive (batch, continuous feed with constant and with variable comonomer ratio). Tack strength as a function of the time gives a peak for batch and a bimodal curve (shoulder) for mixture. Tack is higher for batch. This behavior can be correlated with the heterogeneous composition and rather high cohesion of the batch type PSA. A very long plateau is observed on the rheological master curve for batch type, which behaves as a crosslinked elastomer. Styrene block copolymers are physically crosslinked systems. In such compounds the sequence distribution inﬂuences the tack. As stated by Roos and Creton [18] the measured max in probe tests decreases as the amount of diblock increases.

Adhesive Performance Characteristics

247

Tack of Uncoated PSAs. Surface-active agents in water-based formulations reduce the tack [41]. This inﬂuence appears to be less pronounced for the other adhesive characteristics. With the exception of a slight change in rolling ball tack, little diﬀerence was found between the same adhesives cast from water- and solvent-based formulations. In fact, the diﬀerent tack level of solvent- or water-based PSAs may be observed mainly at low coating weight (undercoating). At higher coating weight the tack level of waterbased acrylic PSAs approaches that of the solvent-based ones (Table 6.2). The obtained tack level also depends on the nature and amount of the surface-active agents [54]; ﬁllers (e.g., silica) also decrease the tack [55]. Influence of the Coating Weight. The thickness of the adhesive layer inﬂuences the ﬂow conditions. Therefore the coating weight will inﬂuence the tack; in general the tack is directly proportional to the coating weight. The exact nature of this dependence also is inﬂuenced by the physical state of the adhesive. Figure 6.4 illustrates the dependence of the tack on the coating weight for a solvent-based and water-based adhesive, respectively, namely, that tack increases with increasing coating weight of the PSA. Solvent-based adhesives display a higher tack at lower coating weights. The dependence of the tack on the coating weight is not linear; tack depends on the nature of the adhesive, face stock, and substrate, Tack ¼ f ðCw Þ

ð6:16Þ

where 6¼ 1.

Table 6.2 Tack of Solvent-Based Acrylic PSA Vs. WaterBased Acrylic PSA as a Function of the Coating Weight Rolling Ball Tack (cm) Coating Weight (g/cm2) 8.0 10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5

Solvent-based PSA

Water-based PSA

4.30 3.20 2.80 2.40 2.20 2.10 2.00 1.95 1.30

6.9 6.0 5.2 4.0 3.5 2.8 2.3 2.0 1.9

248

Chapter 6

Figure 6.4 Dependence of the tack on the coating weight. Rolling ball tack as a function of the coating weight for 1) water-based and 2) solvent-based PSAs.

Influence of Time/Temperature on Tack. Greenwood and Johnson [56] have predicted that by increasing the velocity of separation, viscoelastic eﬀects extend the length over which the surface forces act, thereby increasing the total force of adhesion. Probe tack values increase with the applied load; even after a 100 sec dwell time the tack value is still increasing, indicating that optimum contact has not yet been achieved [57]. Bates [58] proposed that tack could be described by an equation of the Arrhenius type such that the tack energy T: T ¼ Aemx

ð6:17Þ

where A and m are constants which involve the eﬀect of load and dwell time, and x is the separation rate of the probe. Tack is known to go through a maximum around Tg þ 60 C [43]. Since the glass transition temperature of PIB is approximately 60 C, tack measurements were performed at 10–25 C. Zosel [1] studied the tack dependence of polyacrylates on the temperature and found that the tack increased with increasing temperature going through a maximum at a given temperature as a function of the acrylate structure. Branching produces a shift of the modulus towards lower temperatures, that is, branched

Adhesive Performance Characteristics

249

acrylates display the same modulus at lower temperatures. According to [18] increasing the temperature from 22 C to 60 C leads to lower max and "max values in probe tack tests regardless of the percent diblock content of the base polymer. At high temperature the interfacial crack propagation becomes easier and debonding proceeds quickly by crack propagation. The most useful signals in PSA analysis are G0 (the shear modulus) and 00 G (the loss modulus). G0 is a direct measurement of the tack strength, and G00 is a direct measurement of the peel strength [59]. It is assumed that the application of the adhesive (tack test) would occur at an angular frequency near 0.1 rad/sec. It is further assumed that the peel test would approximate the higher frequency of 10 rad/sec. The G0 range of 50,000 to 200,000 Pa is accepted as an ideal PSA performance. Frequency dependent F-D measurements (force-distance curves) with friction force microscopy (FFM), were made as they represent a nanoscale analogous to the macroscopic probe tack test; lateral modulation measurements are being developed, too. Influence of Test Methods and Conditions. Diﬀerent test methods give rise to diﬀerent tack values. On the other hand, labeling practice points towards a strong inﬂuence of the labeling conditions (mainly of the temperature) on the tack. As discussed in detail in [60] an ideal PSA gives instantaneous tack and peel on a rigid substrate. Certain pressure-sensitive products must adhere to soft substrates and provide enough adhesion after a given time under special application conditions (temperature, pressure, dwell time, etc.) only. For such products (e.g., protective ﬁlms) ‘‘application tack’’ characterizes better the adhesivity of the product. Chap. 2 gives a detailed description of the tack dependence on the experimental conditions (time/temperature). Influence of Face Stock Material on Tack. As observed when measuring the tack with diﬀerent methods, the nature and dimensions of the face stock material inﬂuence the value of the tack. Loop tack measurements are sensitive to the face stock ﬂexibility, while rolling ball tack is inﬂuenced by the anchorage of the adhesive on the face stock (i.e., by the face stock surface). Wet-out on the face stock material inﬂuences the tack of the pressure-sensitive laminate; however, experimental results show some discrepancies; Toyama et al. [61] found maximum tack for a surface tension of the face stock equal to the surface tension of the adhesive. On the other hand, Sherrif et al. [62] and Counsell and Whitehouse [63] found that tack increases with increasing surface tension of the face material. The surface inﬂuences the tack values measured as loop tack (Table 6.3) [64]. Tack characterizes the short bonding time and debonding behavior of the adhesive. During bonding the pressure-sensitive laminate must conform

250 Table 6.3

Chapter 6 The Influence of the Face Stock Material on the Tack Loop Tack (N)

Coating Weight (g/m2) 22 24 43 46 56

Soft PVC

Paper

7.3 5.8 3.3 2.5 3.3

5.7 5.5 3.2 3.8 4.2

A water-based EVAc PSA was used.

to the substrate in order to establish a maximum contact area. A high debonding resistance is favored by a large contact area. Thus the ﬂexibility (i.e., the bulk properties) of the face material will strongly inﬂuence the tack of the PSAs. This behavior is illustrated by the sensitivity of the tack test methods to the ﬂexibility of the face stock material. In rolling ball tack tests the adhesive does not merely contact the ball, but climbs up the back of the ball just prior to release, very much like a low angle peel. The more ﬂexible the face, the worse the situation becomes, especially if the self-adhesive tape specimen is not ﬁrmly secured prior to the test. Kaelble [65] showed that there is an area of compression just behind the peel front. This results in automatic pressure application of the tape to the test panel, no matter how minimal the initial application pressure. A polyethylene (PE) ﬁlm, unlike other ﬁlm tapes, does not have a tack plateau but continues to climb in tack value [3]. At maximum load the PE ﬁlm contributes to the value of tack obtained due to its extremely high extensibility. In industrial practice the low adhesivity of special protective ﬁlms is the resultant of the simultaneous reduction of the coating weight and of the thickness of the carrier ﬁlm. Thus the coating weight can be decreased without the loss of the peel and tack [60]. Tack Values. In adhesive practice there is a need for reference values of the tack since the formulator and end-user have to compare the obtained tack values with their target performance. This remains a diﬃcult question because tack values depend on the measurement methods and conditions, the face stock material used, and the coating weight. Tack values obtained for adhesives with a diﬀerent chemical basis or physical state diﬀer. However, for those skilled in the formulation and use of PSAs, there are some target values of the tack which need to be achieved. Table 6.4 summarizes some tack values for PSAs with diﬀerent chemical bases and measured with diﬀerent methods.

Adhesive Performance Characteristics

251

Table 6.4 Tack Values for PSAs on Different Chemical Bases, Measured with Different Methods Chemical Basis

Loop Tack (N/25 mm)

Base polymer Supplier

Rolling Ball Chemical nature Tack (cm)

Glass

PE

1360 FC88/PC80 FC88 V205 V208 3703 3703/V205 3703/V205

EVAc AC AC AC AC CSBR AC/CSBR AC/CSBR

6.9 13.1 13.3 6.0 7.9 5.3 6.2 8.4

3.4 7.6 8.0 4.2 7.9 2.7 1.8 7.6

Hoechst UCB UCB BASF BASF Polysar Polysar/BASF Polysar/BASF

17.0 7.0 7.5 2.0 > 30.0 25.0 20.0 6.5

Tackified emulsions were used.

Improvement of the Tack On the basis of theoretical considerations and practical experience one can assume that hot-melt and solvent-based PSAs possess a higher tack than water-based ones (cf. the lower molecular weight for the base polymers of hot-melt PSAs and no poor tack additives for solvent-based PSAs); soft base polymers (ﬂexible, branched chain) yield better tack. Low molecular weight polymers and uncrosslinked polymers (or with a low degree of crosslinking) display higher tack levels. Using nonmigrating surfactants (NMS) improvement in peel, tack, and shear were realized [66]. Latexes prepared with NMS exhibited a loop tack increase from 1.67 to 4.0 (psi). Since PSA bonding typically occurs at the plateau region of the viscoelastic curve, bonding is facilitated by lowering the G0 at this region. G0 can be eﬀectively lowered by increasing the entanglement molecular weight of the polymer; PSA debonding, however, happens at much higher frequency, typically between 100–12,000 rad/sec. The loss modulus at this frequency is in the transition region of the viscoelastic curve and its value is highly dependent on the Tg of the polymer. Increasing the Tg results in higher G00 in this region. The tack properties will be improved. Increasing of the glass transition temperature can be achieved by special branched comonomers, e.g., vinyl neodecanoate and vinyl neododecanoate [67]. Tackiﬁed polymers possess better tack as do plasticized polymers. Finally, higher coating weights result in higher tack levels (see Fig. 6.4). The coating weight also inﬂuences the peel adhesion and shear (see Sections 1.2 and 1.3), as well as other characteristics like wettability (see

252

Chapter 6

Chap. 2). Therefore, before discussing other adhesive characteristics, a short examination of the coating weight as a quality parameter is presented. Role of Coating Weight and Parameters Influencing It Benedek [68] summarized the inﬂuence of the coating weight on the PSA label quality and its importance for the screening and testing of PSAs. The coating weight inﬂuences the geometry and the quality of the PSA layer. Its inﬂuence on the quality of the PSA layer is indirect, via drying [69]. The coating weight inﬂuences the drying of the adhesive layer. The drying rate R is given as a function of the mass transfer coeﬃcient cm, the heat transfer coeﬃcient ch, and the latent heat of evaporation L [70] (i.e., as a function of mass and heat transfer): R ¼ f ðch s tÞ=L ¼ f ðcm s pÞ

ð6:18Þ

where t ¼ t ts p ¼ ps p ts ¼ surface temperature ps ¼ vapor pressure at ts p ¼ partial pressure s ¼ surface area For the drying of aqueous adhesive layers, one can assume that the drying rate R also depends on the layer thickness h, concentration c, c, and speciﬁc gravity [69]: R ¼ ðh t cÞ=ð L hÞ

ð6:19Þ

For solvent-based adhesives, the drying rate depends on the diﬀusivity D, concentration c, and layer thickness h [69]: R ¼ D c=4h2

ð6:20Þ

Generally, the drying rate is inversely proportional to the layer thickness (i.e., to the coating weight Cw): R ¼ f ð1=Cw Þ

ð6:21Þ

As shown in Fig. 6.5 for low coating weight values, the exponent is smaller than 1, whereas for high coating weights, the exponent should be larger than 1 (i.e., for drying PSAs with a high coating weight, the layer

Adhesive Performance Characteristics

253

Figure 6.5 Dependence of the drying speed on the coating weight. Weight loss (%) as a function of the time, for diﬀerent coating weights, and (wet adhesive thickness, m: 1) 60; 2) 100; 3) 200) during drying of a water-based PSA.

thickness is the most important rate determining parameter). The dependence between the wet coating weight, solids content, and drying time was studied by Hansmann [70]. The strong inﬂuence of the coating weight on the drying has an indirect eﬀect on the adhesive characteristics. The equilibrium water content of the paper and water-based PSAs layer depends on the drying degree (i.e., on the coating weight). On the other hand, residual humidity aﬀects tack, peel, and shear (see Section 1.2). The direct inﬂuence of the coating weight on the peel and shear will be discussed in Chap. 10. As a general statement, the coating weight has a positive eﬀect on tack and peel, and a negative one on the shear and drying. In a ﬁrst approach tack T and peel P are proportional to the coating weight, shear S and the drying speed R are inversely proportional. Thus: T ¼ f ðCw Þ

ð6:22Þ

P ¼ f ðCw Þ

ð6:23Þ

254

Chapter 6

S ¼ f ðCw1 Þ"

ð6:24Þ

R ¼ f ðCw1 Þ

ð6:25Þ

In summary the dependence of the adhesive properties Padh on the coating weight (Cw) can be represented as follows: Padh ¼ f ½ðCwþ Þ ðCw"þ Þ1

ð6:26Þ

namely, the coating weight inﬂuences the versatility of the adhesive in a complex manner. Evidently, the development of the PSA chemistry leads to better adhesives (i.e., with a higher tack and peel adhesion) at a lower coating weight. The most important factors inﬂuencing the coating weight are the face stock material, the substrate to be adhered to (adherent), the permanent/ removable character of the adhesive, and the coating conditions. In general it can be stated that the optimum coating weight value depends on the chemical composition and on the end-use of the PSAs. On the other hand, the coating weight must be correlated to the uniformity of the coated layer and to the coating (ﬁlm-forming) quality, depending on the combination of the following parameters:

Machine characteristics: speed and sense of rotation of the metering cylinders, concentricity of the rolls, gap widths, properties of rolls (e.g., surface finish, stability of material, deflection), configuration, number and combination of rolls, hydraulic pressure, and temperature control Adhesive characteristics: cohesive strength, flow, and rheological properties of the adhesive.

Chemical Composition and Coating Weight. According to their chemical nature, synthesis, or formulation, diﬀerent PSAs possess a diﬀerent adhesion-cohesion balance. Tack and peel may be improved by increasing the coating weight. Thus, the chemical nature of the PSAs will inﬂuence the required coating weight. On the other hand, as discussed in detail in [71] regulation of the crosslinking degree allows changes in the peel and tack for the same coating weight and adhesive. Influence of Coating Conditions on the Coating Weight. The most important parameter is the coating technology (i.e., the direct/indirect nature of the coating operation). Direct coating supposes depositing the

Adhesive Performance Characteristics

255

adhesive on the face stock and thus adhesive bleeding through the porous surface remains possible. Hence the measured coating weight could be lower than the one being deposited. A detailed analysis of the inﬂuence of the coating conditions (coating technology) on the coating weight will be given in Section 2.2 of Chap. 9. A lot of parameters inﬂuence the coating weight [68]. For laboratory tests it is important to describe the dependence of the coating weight on the knife opening (clearance) and viscosity. For a coating device with a blade, the pressure diﬀerence p depends on the viscosity , width of the blade b, web velocity Vw, and blade angle as follows [72]: p ¼ ð4 Vw Þ=ðb Þ

ð6:27Þ

For the simplest case of a parallel blade, the blade angle may be replaced by the blade opening (i.e., the distance between the blade and the cylinder). In a ﬁrst approximation a linear correlation is found between coating weight and blade opening (Fig. 6.6). On the other hand, the dependence of the coating weight on the viscosity appears more complex. The coating weight or layer thickness Hw

Figure 6.6 The dependence of the coating weight on the clearance (blade opening). Unﬁlled, water-based PSA formulation.

256

Chapter 6

for a Newtonian ﬂuid depends on the surface tension ST, blade opening d, viscosity , and speciﬁc gravity [73]: Hw ¼ f ðST ,d,,Þ

ð6:28Þ

There is a complex dependence of the coating weight on the viscosity (Fig. 6.7). Unthickened dispersions behave quite diﬀerently from thickened ones; thickened dispersions give a higher coating weight at higher viscosities than unthickened ones (Fig. 6.8). The viscosity also depends on ﬁllers and pH; adjusting the coating weight with ﬁllers (viscosity) or through the pH also is possible. Table 6.5 demonstrates that ﬁlled dispersions do not display a linear dependence of the coating weight on the blade opening. Surfactants may change the viscosity of the aqueous dispersions and hence the coating weight. Storage time and temperature also inﬂuence the viscosity and thus the coating weight. In fact practical coating conditions diﬀer quite a bit from laboratory ones. Shear thinning and thickening and ﬂow under diﬀerent centrifugal forces may inﬂuence the coating weight. Laboratory coating conditions may be regarded as static whereas industrial conditions are dynamic. Here the web velocity and shear conditions are the

Figure 6.7 The dependence of the coating weight on the viscosity. Water-based, unthickened PSA formulation.

Adhesive Performance Characteristics

257

Figure 6.8 Coating weight dependence on the viscosity. Water-based, thickened PSA formulation.

Table 6.5 Dependence of the Coating Weight on the Clearance (Blade Opening) for Water-Based Acrylic PSAs Containing Filler Coating Weight (g/m2) Filler content (parts/100 parts, wet weight) Clearance 60 100

0

5

10

20

30

25.5 43.0

20.0 36.0

33.0 40.0

44.0 38.0

29.0 45.0

most important parameters of the coating weight for a given adhesive and coating device geometry. Practically, during coating, in order to keep the coating weight tolerance within maximum 5% the viscosity of the emulsion must be kept at a constant level [74]. For an emulsion PSA on a reverse gravure coater, it is ideal to have a high solids content with a low viscosity. Low viscosity

258

Chapter 6

means a high coating weight; conversely, high viscosity means a low coating weight. The minimum level of the viscosity also depends on the surface tension of the adhesive; common viscosities for reverse gravure range from 17–24 DIN sec. Pressure-sensitive adhesive solutions or dispersions that have diﬀerent chemical bases may have a diﬀerent solids content and viscosity. Thus, for the same viscosity and wet coating weight, a quite diﬀerent dry coating weight and tack level may be obtained. This is illustrated by Fig. 6.9. For aqueous dispersions the coating weight also depends on the porosity of the web (paper). Therefore direct or transfer coating will result in diﬀerent coating weights (i.e., for the same coating weight the thickness of the adhesive layer on the coated side will diﬀer) (Table 6.6). The obtained coating weight depends on the type and characteristics of the metering device used. A gravure cylinder with a line screen of 40, 70 m depth, yields a coating weight of 5–7 g/m2 [75]. When coating a 50%-solids adhesive with adequate viscosities, the rheology of the system is such that to achieve a 3–4 g/m2 solid coating weight, a 35-m gravure cell is suggested, while 2–2.2 g/m2 solids requires a 22-m deep gravure cell.

Figure 6.9 Tack dependence on the coating weight. Rolling ball tack as a function of the coating weight for water-based PSAs on diﬀerent chemical bases: 1) CSBR tackiﬁed; 2) CSBR; 3) acrylic.

Adhesive Performance Characteristics Table 6.6

259

Dependence of the Coating Weight and Peel on the Coating Method Coating Method

PSA 1 2 3 4

Coating Weight (g/m2)

Direct

18.0 20.0 19.5 21.0 15.0 21.0 10.0 15.0

Transfer

Peel on glass (N/25 mm) 17.0 17.0 PT 21.0 21.0 PT 12.0 19.0 PT 7.0 9.0

Water-based acrylic PSA coated on paper face stock material. PT ¼ paper tear.

For a metering cylinder, the coating weight will depend on the blade opening between blade and cylinder, the machine speed, and the ﬁlm transfer from the cylinder to the substrate [76]. Machine speed inﬂuences the coating weight; higher velocities yield higher coating weights [77]. As discussed in [60] the coating device aﬀects the nominal coating weight value also. For instance, using a Meyer Bar a lower eﬀective coating weight of 1.8 g/m2 gives the same adhesivity obtained with the gravure cylinder and 2.4 g/m2 adhesive. Influence of the End-Use of the Label on Coating Weight. As discussed in detail in [71] the product class (e.g., label, tape, protective ﬁlm, etc.) inﬂuences the coating weight. Protective ﬁlms are manufactured with 1–15 g/ m2 coating weight, labels use 15–25 g/m2 PSA, tapes have generally coating weight values above 20 g/m2. For the same product class the end-use aﬀects the coating weight value also. The end-use of the label requires PSAs with a permanent or removable character. On the other hand, the end-use of the label determines its application and labeling technology. This also depends on the adhesion/cohesion balance. Labeling guns require very tacky adhesives; for decals a high tack is less important. Peel is a function of the coating weight. In Chap. 2 it was shown that removability depends on peel adhesion and on the coating weight. According to the permanent/removable character of the designed label, one can use a higher or lower coating weight. Mounting, insulating, and splicing tapes possess coating weight values higher than 30 g/m2. Removable tapes like protective ﬁlms are coated with less than 10 g/m2 adhesive. The coating weight values used for diﬀerent pressure-sensitive products are listed in [71].

260

Chapter 6

Influence of the Face Material on the Coating Weight. The dry coating weight (i.e., the weight of the dry adhesive applied per unit surface area) can vary substantially depending upon the porosity and irregularity of the face material and of the substrate surface to which the face stock is to be adhered. For instance, higher adhesive loadings are preferred for adhering porous, irregular ceramic tiles to porous surfaces, while lower adhesive loadings are required to manufacture tapes, ﬁlms, and other articles from relatively nonporous smooth-surfaced materials such as synthetic polymer ﬁlms and sheets. When the adhesive is applied on nonporous polymeric or metallic face materials intended for adhesion to nonporous polymeric or metallic surfaces, adhesive loadings of about 8–85 g of dry adhesive per m2 are generally adequate. High adhesion in tapes manufactured from continuous sheet polymeric substrates can usually be achieved with dry adhesive coating weights of about 16–32 g/m2, while coating weights of about 14–28 g/m2 of adhesive are usually employed for paper-backed tapes, such as masking tapes. Textile carrier materials for labels or tapes need coating weight values above 40 g/m2. The anchorage of the adhesive on the carrier material inﬂuences the coating weight also. As discussed in [71] primed removable products work with less coating weight. The conformability and deformability of the face stock material aﬀects the coating weight also. For instance, soft and thin LDPE carrier for protective ﬁlm requires lower coating weight values than HDPE or PET. Influence of the Substrate on Coating Weight. The adhesive ﬂow is inﬂuenced by the nature of the contact surface (liner as adherent). Chemical, physical, and mechanical interactions with the solid state component may hinder the adhesive ﬂow. Therefore the chemical nature (e.g., aﬃnity and polarity) of the surface and the physical nature (e.g., porosity and roughness) will inﬂuence the interaction of the adhesive with the substrate. The adhesive ﬂow inﬂuences the bonding and debonding characteristics. According to Tordjeman [19] in probe tack tests the actual contact area increases with the thickness and then applatizes. The contact area Ac is given as a function of the contact force Fc and the apparent modulus Ea as follows: Ac Nhai2 þ Fc =Ea

ð6:29Þ

where the apparent modulus Ea takes into account the competition between the thickness eﬀects and the roughness hai: Ea ¼ Eð1 þ hai2 =2 h2 Þ

ð6:30Þ

Adhesive Performance Characteristics

261

The coating weight determines the real thickness of PSAs and the thickness of the adhesive layer which can ﬂow freely. Thus the coating weight will depend on the nature of the substrate. As a practical example, adhesion onto HDPE is generally lower than adhesion onto LDPE [78]; therefore the coating weight should be increased for adhesion to HDPE. The inﬂuence of the carrier material and of the substrate on the peel will be discussed in detail in the Section 1.2. Dependence of Coating Weight on Adhesive Quality. The coating weight may also depend on the quality of the adhesive. Under certain conditions the emulsion polymerization of water-soluble monomers may yield suspensions (i.e., the monomer is homopolymerizing) with bulky, nondispersed polymer particles remaining (coagulum, grit). This phenomenon is especially encountered with acrylnitryl copolymers; Acronal 81 D may contain so much coagulum that a coating weight of at least 100 g/m2 is recommended (i.e., more than the diameter of the largest solid particle in the ﬂuid adhesive layer) [79]. Coating Weight Values. There are a lot of factors inﬂuencing the value of the coating weight. On the other hand, the development of the PSA chemistry has led to adhesives with improved tack and peel adhesion (i.e., with lower coating weight). Some years ago coating weight values of 30–100 g/m2 were recommended [80]; actually 15–20 g/m2 are more common. Table 6.7 gives some typical values of the coating weight used for labels on diﬀerent face stock materials and for diﬀerent end-uses. Techniques for measuring the coating weight and tolerances are covered in Chaps. 8, 9, and 10. 1.2

Peel Adhesion

Peel adhesion is the force required to remove a PSA-coated ﬂexible material from a speciﬁed test surface under standard conditions (e.g., speciﬁc angle and rate). It gives a measure of adhesive or cohesive strength, depending on the mode of failure [88]. Johnston [57] stated as an analogy that the adhesive consists of many little springs, as depicted in the Maxwell and Voigt models to simulate viscoelastic behavior, and that these springs range from strong to weak. The former contribute to the adhesion (peel), the latter contribute to tack. In fact, the measured peel energy includes a signiﬁcant contribution from energy dissipation within the bulk of the polymer. Thus for a given extensible polymer, the higher the work of adhesion (due to the interaction at the surface), the higher the dissipation energy (due to polymer deformation) and the higher the peel energy [89]. As discussed earlier, in

262 Table 6.7

Chapter 6 Coating Weight Values of Industrial Labelstock Adhesive characteristics Nature

Chemical Physical Acrylic Acrylic Acrylic — — Acrylic Acrylic Acrylic CSBR Acrylic Acrylic Acrylic

Hot-melt Water-based Solvent-based — — Water-based Water-based Water-based Water-based Water-based Hot-melt Solvent-based/water-based

Adhesive layer Coating Face Coating weight (g/m2) thickness (m) stock Refs. — 25 20 30 26–29 27–29 25 25 25–28 — — —

25 — — — — — — — — 20 25–150 25

Paper Paper Paper — — Film Paper Paper Film Paper Paper —

46 81 81 82 83 2 84 85 38 86 87 45

Chap. 3, Section 3, based on contact physics and mechanics, debonding is related to cavitation and ﬁbrillation of the PSA, and its modulus. Measurement of the Peel Force Peel is measured as the force required to remove a PSA-coated material which was applied to a standard test plate, at a speciﬁc rate and angle. In some cases not only the value of the peel force, but also the failure mode (deformation or destruction of the components of the pressure-sensitive laminate or of the substrate) has to be examined in order to obtain a correct appraisal of the removability of PSAs. The results of a peel test are very useful in comparing diﬀerent adhesives, but due to a complicated stress distribution, they lack the ability to quantitatively investigate the molecular origins of adhesion. For this reason asymmetric adhesion tests are favored in studying the interfacial and bulk contributions to adhesion. Due to the disadvantages of the ﬂat punch (alignment diﬃculty and ﬁbrillation upon pull-oﬀ ) Crosby and Shull [30] use the spherical geometry. This test provides adequate data to generate tack curves. Pocius et al. [90] studied the release behavior of acrylic tapes from release coating based upon long alkyl side-chain polycarbamates by the

Adhesive Performance Characteristics

263

methodology of contact mechanics and peel strength data (90 peel of PSA tapes measured on glass plate coated with release). The extrapolation of the contact mechanics data (at low rate) matches the peel data (at high rate). They studied the adhesion energy as a plot of crack speed (nm/s). On the basis of ﬁbrillation, the debonding force is computed by Verdier and Piau [91] using the following correlation: G ¼ F=l

ð6:31Þ

where F is the force and l is the bond width. Z G ¼ ½a ðL eÞ= "max

"max

ð","0 ,TÞ d"

ð6:32Þ

where "max is the maximum elongation of the ﬁlament, l is the periodicity of the ﬁlaments, a is the ﬁlament width, L is the ﬁlament height, and e is the adhesive thickness. The elongation rate ("0 ) is taken to be V/e; is the stress given by Lodge’s model. According to Lin and Hui [15] the ﬁbrillated zone length is determined by the fracture criterion h ¼ hc and s ¼ so. The dependence of the normalized peel velocity V/E*A on the normalized peel force (F/E*I1/3) is shown for diﬀerent values of normalized hc; normalized peel velocity is proportional to normalized peel force; A is the rate constant associated with ﬁbril deformation. The normalized peel height (L/2I1/3) is plotted against normalized peel force for diﬀerent hc/(2I )1/3 values. The peel height decreases with increasing peel force. In addition the dependence of the normalized peel height on the normalized peel force is independent of the critical ﬁbril length. For a given peel force the ﬁbrillated zone size increases with critical ﬁbril failure length. According to [92] the peel force P relates to tensile strain " and stress by the following equation: Z

"

P ¼ ðbl=2Þ

ð"Þ d"

ð6:33Þ

0

where b is the width and l is the thickness of the adhesive ﬁlm. The speciﬁc work of PSA ﬁlm tensile straining (A) is identical to the work of adhesion: Z A ¼ ð"Þ d" ð6:34Þ As follows from Eqs. (6.33) and (6.34), the mode of adhesive joint failure is determined by the ratio between the two limiting tensile stress

264

Chapter 6

values a (which characterizes the strength of the adhesive bond) and c) which relates to the cohesive strength of the PSA polymer). For a > c the failure is cohesive, whereas an adhesive type of debonding occurs at c > a. The slip-stick point position corresponds to the condition of a c. (Slip-stick is the phenomenon when the adhesive separation front can move faster than the rate of testing.) In the case of interfacial failure the peel curve is noisy but approximately constant [93]. According to [94] when failure is cohesive, the peel force is rather steady; for uncrosslinked PSA the peel force drops when the mode of failure changes from cohesive to adhesive [95]. The 180 peel test is a combination of tensile strength and shear, while a 90 peel is of tensile strength only [96]. Approximating the deformation of adhesive in the peel front by uniaxial elongation, the peel force is given by integration of the tension across the peel front [97]: Z

xb

o ðxÞ dx

P¼

ð6:35Þ

0

where P is the peel force per peel strip width, o is the tension at the adhesive/substrate interface, x is the horizontal position in the peel front, and xb is the position at which the adhesive detaches or breaks cohesively. Christensen [97] compares the experimental and theoretical peel forces and demonstrates the validity of Eq. (6.35) and of the assumption of uniaxial deformation. Kendall [98] compares the adhesion force for diﬀerent kinds of adhesive joints and debonding tests, e.g., peel test (with ﬂexible carrier materials at large angle; with rigid carrier-disk sample; with semiﬂexible carrier Obreimoﬀ [99] geometry for cleavage) lap joint failure, and small angle peel. As stated by Johnson [100] for contact of elastic spheres, the elastic modulus (E ) does not aﬀect the pull force (F ) (see Chap. 3, Section 3). According to Kendall [98] the sphere adhesion result is rather similar to that of the peel test. In the peel test the shape, and therefore the strain energy, remains constant for an elastic material, the adhesion force turns out to be independent of the elastic modulus just as in the JKR (Johnson–Kendall–Roberts) solution. For a disk sample (rigid cleavage) the force increases in proportion to (EW)1/2, the interface toughness. This supports the idea that stiﬀening adhesive joints usually leads to more strength. That means that by cleavage peel the force depends on the E: F ¼ f ðEWÞ1=2

ð6:36Þ

Adhesive Performance Characteristics

265

Using Obreimoﬀ geometry, to pull a cantilever strip from the surface the force is given by the following correlation: F ¼ bðWEdÞ1=2 ðd 2 =6c2 Þ1=2

ð6:37Þ

where b is the width and c is the length of the joint. For the lap joint the strength increases with sheet thickness (d ) and E. F ¼ bð4WEdÞ1=2

ð6:38Þ

For low angle peeling: F ¼ bð2WEdÞ1=2

ð6:39Þ

which is the equation for lap failure of a ﬂexible ﬁlm in contact with a rigid surface. That means that E does not inﬂuence F for 180 peel but it inﬂuences it for other kinds of debonding. The strain energy release rate G, during debonding of a cantilever beam sample, containing at its midline a thin PSA layer, was utilized to quantify the adhesion of PSA by Taub and Dauskardt [101]. One might expect the value of the adhesive fracture energy (Gc) from the peel tests to be signiﬁcantly lower than that determined from CDB (Double Cantilever Beam) specimens, as was observed in the case of values of Gc deduced using the FEA-CZM (cohesive zone model) analysis of the peel tests [102]. Factors Influencing Peel Adhesion Bonding plays an important role in the peel resistance. Parameters inﬂuencing the adhesive ﬂow during the bonding step will inﬂuence the peel adhesion. In a manner similar to tack, peel is inﬂuenced by the PSA nature and geometry, and by the nature of the face stock material. The inﬂuence of the adhesive thickness, viscosity, backing rigidity, and peeling velocity in the cohesive regime was stated [103]. Test methods and conditions modify the peel value as well. More pronounced for the peel force are the inﬂuence of the dwell time and of the adherent. Influence of the Adhesive’s Nature on Peel. Similar to tack, peel resistance is a function of the chemical nature and macromolecular characteristics of the base polymers. On the other hand, and in contrast to tack, peel adhesion increases with the cohesion (i.e., with the molecular weight) up to a limit. A quite diﬀerent adhesion/cohesion balance is required for good peel adhesion onto polar or nonpolar substrates. Regarding the inﬂuence of the adhesive’s nature, one should diﬀerentiate between the

266

Chapter 6

chemical composition of the base elastomers, the chemical composition of the formulated adhesive, and the technology dependent composition of the coated PSAs; one should also note the inﬂuence of aging, the chemical composition of the PSA, and solid state components of the laminate, because of the environmental and reciprocal interactions between the adhesive and the neighboring components (e.g., face stock and release liner). Influence of the Chemical Composition of the Base Elastomer on the Peel. The anchorage of the adhesive on the substrate is governed by a physical and a chemical component. Chemical interactions between the adhesive and the substrate result in adhesion build-up; the chemical composition of the base elastomers strongly inﬂuences this adhesion build-up. Small portions of highly polar monomers (e.g., acrylic acid) produce adhesion build-up. On the other hand, PSAs with a vinyl polymeric backbone and with grafted polysiloxane moieties produce initially repositionable labels [104]. The inclusion of polar carboxylic groups in the adhesive may inﬂuence its peel value and change its aﬃnity towards polar surfaces [105]. In a similar manner such groups inﬂuence the peel of the adhesive through water sensitivity. Built-in carboxyl groups may produce a change in the peel value from special substrates through pH changes [106]. Introducing polar, functional groups into ethylene vinyl acetate copolymers improves their adhesion [107]. The role of the carboxyl sticker groups on the peel adhesion strength (G) of linear polybutadiene to aluminum surfaces coated with amine and other receptor groups was investigated in [108]; G went though a maximum at some optimal concentration of the sticker groups (ca. 3 mol %). According to [109] the acrylic acid content dramatically increases adhesion and adhesion hysteresis of acrylic copolymers. The addition of acrylic acid also decreases a critical crack propagation speed above which a much stronger rate dependence of the adhesion energy is observed. The contribution to adhesion strength by these groups is evaluated by surface energy measurements and by peel strength measurements (energy dissipation due to polymer deformation). Earlier studies [110–112] have shown that the modulus G0 value at low frequencies can be related to the wetting and creep behavior (bonding) of the adhesive and the modulus at high frequencies can be related to the peel or quick stick (debonding). A general correlation was developed between tape properties (release and peel adhesion). Good adhesion for tapes is given when G0 is low at low frequency rates, i.e., 0.1 rad/sec and a relatively high slope exists for G0 as the frequency is increased [113]. The peel strength is proportional to the ratio of G00 and G0 at the respective debonding and bonding frequencies. Gordon et al. [114–116] presented an empirical correlation between the bulk viscoelastic properties of commercial adhesive and

Adhesive Performance Characteristics

267

the release proﬁle observed from peel force vs. peel velocity measurements. It was shown that the bulk viscoelastic properties of the adhesive play a dominant role in controlling release proﬁles in a 180 peel test. The general shape of the release proﬁle was consistent with the shape of the adhesive’s viscoelastic function—speciﬁcally the loss tangent (tan delta) as a function of the dynamic frequency—in the peel window previously deﬁned: ! ¼ 2=h

ð6:40Þ

where ! is the angular frequency in the viscoelastic test, is the delamination velocity in the peel test, and h is the thickness of the deformed region of the viscoelastic material. It was evident, that the viscoelastic properties of the release inﬂuence the magnitude of the release force at any given peel velocity. A power law relationship was observed in coating formulations in which the linear PDMS chain segments were greater than the entanglement limit [117]. Adhesives with lower creep compliance (higher G0 ) will have better cohesive strength (creep resistance). However, if the modulus is too high, the adhesive may not rapidly wet the substrate. The real area of contact of an elastic ﬁlm on a rough surface depends on two parameters: the elastic modulus at the bonding frequency (G0 ) and the relaxation properties of the polymer which govern the change of the real area of contact with time for a given imposed displacement. Increasing the modulus G0 results in adhesives with higher adhesion and lower tack. In the correlation proposed by Bikermann [118] for the energy release rate, the Young moduli of the backing and adhesive respectively are included. According to Fukuzawa and Uekita [119] the increase of the elastic modulus of the PSA caused a decrease of the peel adhesion. As discussed in Chaps. 2 and 3 the rheological properties depend on the chemical composition and molecular weight of the base polymers. Influence of the Chemical Composition of the Formulated Adhesive on Peel The eﬀect of the formulating additives on the peel will be reviewed in Section 2.9 of Chap. 8. Some of these inﬂuences are caused by the chemical interaction of the viscous and elastic components of the formulation (e.g., elastomer, tackiﬁer, plasticizer) and exist in a freshly coated adhesive layer, but others are due to the interaction of adhesive/environment or adhesive/laminate components. Changes in the peel value given by adhesive/ environment interactions may be irreversible (aging) or reversible, such as the inﬂuence of the atmospheric humidity on the peel of tackiﬁed (with aqueous resin dispersions) or untackiﬁed water-based PSAs.

268

Chapter 6

Tackiﬁcation remains the most pronounced formulating eﬀect on the peel. Peel adjustment through tackiﬁcation will be discussed in a detailed manner in Chap. 8. Here it should be mentioned that for removable (low peel adhesives) no tackiﬁer or a low level of tackiﬁer should be used. More than 50% tackiﬁer will make the adhesive nonpeelable; ﬁllers also inﬂuence the peel adhesion. Generally, they improve the modulus, but decrease the contact area and diﬀusion rate. Therefore over 10–15% ﬁller concentration (on wet/wet basis) decreases the peel [106]. In hot-melt PSA formulations peel values sharply decrease with the use of fully saturated mineral oils [120]. Effect of Crosslinking on Peel Adhesion. The 180 peel strength of PSA formulations based on crosslinkable rubber (e.g., SIS, electron beamcurable) remain fairly constant over the entire range of electron beam doses [50]. Yarusso et al. [95] studied as a model PSA, anionic polyisoprene. The eﬀects of the initial molecular weight and of the degree of crosslinking were studied. As stated, high gel content could compensate for low MW. This was not true for peel behavior. Three diﬀerent failure modes were observed in the adhesion testing. Adhesion failure with no apparent change in the appearance of the stainless steel plate was favored by high molecular weight, high gel content, and high peel rate. Cohesive failure leaving tacky adhesive both on the tape and on the panel was favored by low molecular weight, low gel content, and low peel speed. When the molecular weight was low, but the gel content was high, and the peel speed was low, we observed a very thin residue on the stainless steel plate, which was often of a bluish or brownish color and was nontacky. Generally, crosslinking exerts a more complex inﬂuence on the peel. A low degree of crosslinking improves the cohesion and thus the peel, whereas a high crosslinking degree lowers the tack and the peel. As an example one can consider the increase of the peel for Cymel crosslinked neoprene latex and the dramatic reduction of the peel upon adding zinc oxide to the formulation [52]. For the crosslinked adhesive more complex three-dimensional ﬂow patterns are obtained by peeling. At very high velocities stick release is observed [91]. Peel data for moderately crosslinked, epoxidized natural rubber (ENR) are shown to be time-temperature superposable, while those for a lightly crosslinked ENR are not. Strain crystallization, because of the resultant hysteresis, could provide the additional dissipative mechanism and hence the anomalously high peel strength at elevated temperature for the lightly crosslinked ENR [121]. Tobing [122] has shown that for gel free, low-Tg acrylics (based on 2-EHA) there was no signiﬁcant diﬀerence between emulsion vs. solventborne PSA ﬁlm adhesive performances. However, commercial products have a high level of gel and low shear. Interlinking of the microgels

Adhesive Performance Characteristics

269

by covalent bonds in the ﬁlm (using a functional monomer, e.g., IBMA) is needed. In special acrylic emulsions 0.5–1.0 wt% IBMA (isobutoxymethylacrylamide) was copolymerized to provide interlinking of the microgels in the ﬁlm upon heating. When the MW of the linear polymer was greater than 2Me and the Mc of the microgels was greater than Me, a synergistic eﬀect was obtained. Influence of the Molecular Weight of the Adhesive on the Peel. The inﬂuence of the molecular weight of the polymer components of the PSAs on the rheology of the PSAs was examined in Chap. 3. The rheology of the adhesive inﬂuences its adhesive characteristics; therefore, the peel also is a function of the molecular weight [123]. The increase of the modulus with the molecular weight reinforces the peel; on the other hand, it reduces the chain mobility and diﬀusion rate, and decreases the peel [124]. Physical adhesion can develop from adequate wetting at a well-deﬁned interface or by diﬀusional interpenetration of segments across the interface, when this is thermodynamically possible. The fracture energy G of a joint follows a relation of the form [125]: G ¼ f ðD tÞa

ð6:41Þ

where D is the diﬀusion coeﬃcient, which is inversely proportional to the molecular weight, and t is the time. For adhesive formulations tackiﬁed with butyl rubber, peel adhesion increases with the molecular weight of the tackiﬁer. On the other hand, when tackifying CSBR, higher melting point resins decrease the adhesion on polyethylene. The tendency to transfer adhesive or residue to the substrate is especially sensitive to the initial molecular weight even at high gel content. The dispersion of the molecular weight plays an important role in debonding also. Commercial acrylic adhesives, like rubber very broad spectrum of relaxation times. And in particular low MW fractions provide the fast relaxation times necessary to achieve a large real contact area within a short contact time (typically 1 sec). In the case of monodisperse polymers these low MW fractions are absent and the contact area depends mostly on the elastic properties of the polymer at bonding frequency. The distinctive feature of these linear monodisperse samples is that the transition from cohesive to adhesive failure with increasing strain rate always corresponded to a maximum on the value of the "max vs. strain rate [126]. However, the strain rate at which the cohesive-adhesive transition and the maximum in "max occurred increased when the molecular weight decreased. If failure proceeds through disentanglement of the polymer chain, both the ﬁbril stress ﬁbril and "max increase continuously as VdebM3 increases because

270

Chapter 6

disentanglement becomes more diﬃcult. If the failure mechanism is not disentanglement but a debonding process the critical stress sustainable by the ﬁbril-punch interface is the rupture criterion. On expects r to be constant so in this regime "max decreases with VdebM3 since: "max / r/ VdebM3. The two limiting cases are interfacial dewetting (Vdeb <<1) and brittle interfacial fracture (Vdeb >> 1). Experimental results have shown that a large spectrum of relaxation times is a necessary condition for the performance of PSAs. If short relaxation times are absent (i.e., no low MW fraction) the contact with the substrate is not achieved within short contact times, while if long relaxation times are absent, a ﬁbrillar and adhesive fracture cannot be achieved. Influence of the Adhesive Geometry on the Peel Adhesion. Adhesive geometry includes the dimensions and form of the adhesive layer; the former depend on the coating weight. The coating weight strongly inﬂuences the value of the debonding force. In general peel adhesion is a function of the thickness of the adhesive [127] and, like tack, peel is very sensitive towards the coating weight. Generally, at low coating weights, the peel increases with the coating weight, implying a debonding through adhesive failure. Concerning the inﬂuence of the coating weight on the peel resistance, the peel force P depends on the peel stress s, sample width b, layer thickness h, and creep modulus Ec as follows [105]: P ¼ ðs2 b hÞ=Ec

ð6:42Þ

As can be seen in Fig. 6.10 there exists a complex dependence of the peel on the coating weight; generally there is an inﬂection point in the plot. Up to this point peel increases rapidly with the coating weight; after this point peel depends less on the coating weight. The shape of the plot depends on the adhesive and face material. In order to avoid quality consistency deviations caused by a variation of the coating weight, the coater should use a coating weight above this critical value. It should also be mentioned that in the upper domain (over the critical value), the peel dependence on the coating weight is less dependent on the face stock. Generally, the dependence of the peel on the coating weight may be formulated as follows: P ¼ f ðCwb Þ

ð6:43Þ

where b 6¼ 1 and possesses quite diﬀerent values in the over and under critical domains. As discussed in Chap. 2 for given application conditions, surface roughness, and carrier deformability, the dependence of the peel force on

Adhesive Performance Characteristics

271

Figure 6.10 Dependence of the peel on the coating weight. 1) Peel on polyethylene; 2) peel on glass.

the coating weight can be described by a simpliﬁed plot [see Eq. (2.43)], which is a linear dependence (like the Kaelble equation), valid over the critical coating weight. Taub and Dauskardt [128] shifted the so-called R curves—energy release rate G (J/m2) ¼ f (PSA thickness)—towards lower values of G with increasing layer thickness (in the range of 30–250 m) until a critical layer thickness is obtained. For thicknesses greater than this critical thickness, the R curves become relatively insensitive to changes in the layer thickness. This insensitivity to adhesive layer thickness is consistent with data reported in the literature for cohesive failure in PSA peel tests [129]. By the analysis of PSA fracture mechanics in the course of peeling, Chalykh et al. [92] established that the peel force P relates to tensile strain " and stress by an equation which includes b (the width) and l (the thickness) of the adhesive ﬁlm. The peel force P, to rupture the adhesive bond obeys the modiﬁed Kaelble equation (3.15) [130,131]. Bikermann [118] also proposed a derivation of the energy release rate as follows: F ¼ 0:3799beð1=4Þ ð2hÞð3=4Þ ðE=Þð1=4Þ

ð6:44Þ

272

Chapter 6

where F is the force (F ¼ bG), b is the adhesive width, is the rupture stress, e is the adhesive thickness, 2h is the backing thickness, and E and Y are the Young moduli of the backing and adhesive respectively. The adhesion energy calculated from indenter tests is given by the follo-wing correlation [18]: Z Wadh ¼ ho

ð"Þd"

ð6:45Þ

where ho is the initial ﬁlm thickness, the nominal stress, and " the strain. As discussed earlier, tack also is measured as peel adhesion. Tack increases with the coating weight. Figure 6.11 illustrates the simultaneous increase of tack and peel with the coating weight; however, if the debonding occurs by break in the adhesive layer (cohesive failure) the peel does not depend on the coating weight [132]. The peel strength depends on the thickness of the adhesive layer and of the face stock material. Peel increases with increasing thickness of the adhesive. This increase of

Figure 6.11 The inﬂuence of the coating weight on the tack and peel. The dependence 1) of the rolling ball tack and 2) of the peel on polyethylene on the coating weight for a water-based acrylic PSA formulation.

Adhesive Performance Characteristics

273

Table 6.8 Dependence of the Peel on the Thickness of the Adhesive Layer Wet Adhesive Coating Weight 2 Layer Thickness (m) (g/m ) 60 70 100 130 160

22 24 24 25 43 46 46 47 56 67

Peel (N/25 mm) PVC

Paper

7.3 — 5.8 — 3.3 — 2.5 — 3.3 —

— 5.6 — 5.5 — 3.2 — 3.8 — 4.2

A 200-mPa.s EVAc water-based PSA was coated on soft PVC via an 80 g/m2 paper (transfer coating).

the peel with the adhesive thickness is illustrated in Table 6.8. The inﬂuence of the thickness of the adhesive layer on the peel was also studied by Schlimmer [133], who found that peel adhesion does not depend on the layer thickness. The use of large amounts of adhesive leads to a relatively strong bond between the adhesive and the target substrate, thus making the product less peelable. Generally, one ﬁnds that lower coating weights make peelability of the product in use easier to achieve, but make a uniform coating more diﬃcult to obtain. For paper coating with acrylic PSAs the coating weight amounts to at least 12 g/m2, and usually 14–20 g/m2 depending on the nature of the adhesive. The inﬂuence of the coating weight on the peel was studied by several authors [3,123,134]. The 90 peel adhesion test (FTM 2) results in a higher (usually more than double) peel adhesion from PE than the 180 peel test [47] (see Section 3.1 of Chap. 10). For the 90 peel a nonlinear dependence of the peel on the coating weight was established; for 180 peel adhesion a linear dependence of the peel force on the coating weight is given. Peel dependence on the coating weight also is a function of the substrate, face stock, and adhesive. For silicone-based adhesives, in most cases a coating weight-independent adhesion value on steel is obtained at coating weights over 4–6 g/m2. Table 6.9 shows that peel adhesion increases with the coating weight; after a certain value of the coating weight (which depends on the adhesive and substrate) peel adhesion increases at a slower rate and eventually reaches a plateau.

274

Chapter 6

The existence of a critical coating weight value plays an important role in the end-use of pressure-sensitive products with low nominal coating weight, e.g., protective ﬁlms, where the coating weight may attain the critical value. Figure 6.12 illustrates the dependence of the peel resistance on the coating weight for two diﬀerent adhesives. At low nominal coating weight values (0.5–1.5 g/m2) changes of 25–35% of the coating weight lead to pronounced (50–60%) reduction of the peel value. As seen, the reduction Table 6.9 Dependence of the Peel on the Coating Weight (Peel on Glass and Polyethylene as a Function of the Coating Weight) Coating Weight 2 (g/m )

Peel (N/25 mm) Glass

PE

9.4 18.8 33.5 39.6 61.8

17.0 PT 17.0 PT 22.0 PT 25.0 25.0

15.0 16.0 14.0 17.0 19.0

A tackified water-based PT ¼ Paper tear.

acrylic PSA

was

used.

Figure 6.12 Dependence of the peel resistance on the coating weight and adhesive nature: (1) and (2) are diﬀerent adhesive grades.

Adhesive Performance Characteristics

275

of the coating weight decreases the peel resistance and below a critical coating weight value (which is adhesive and substrate dependent) joint failure may occur. Dependence of the Peel on the Shape of the Adhesive Layer. In general, defects in an applied continuous PSA layer should be avoided for common adhesive coatings. However, there are special coatings with a discontinuous character of the adhesive layer, mostly in order to avoid the build-up of high peel resistance. On the other hand, the structure of the continuous adhesive layer also inﬂuences the peel resistance [132]. As discussed in [60] the coating device aﬀects the shape of the coated adhesive layer. Crosslinked adhesives coated with a gravure cylinder give peel values which diﬀer from those obtained with a Meyer Bar. Regulating the shape of the PSA allows the control of the adhesion for removable adhesives too [135]. See Chap. 2 for a more detailed discussion of the inﬂuence of the adhesive form on the peel. Influence of Coating Technology on Peel. Direct coating achieves better anchorage of the PSAs on the face stock, and therefore better removability. For optimum anchorage, sheet-stock PSAs are often directly coated on to the face material, rather than transfer coated via the release liner. The state of the adhesive during the coating operation also inﬂuences the peel. Water-based PSAs contain diﬀerent water-sensitive and hygroscopic components; hence they may contain less or more humidity than the laminate. The residual water content acts like a plasticizer and reduces the peel [78]. Hyde and Yarusso [136,137] discovered that under certain hotmelt coating conditions PSA blends of NR and miscible aliphatic tackifying resins can also undergo strain-induced crystallization. This phenomenon would not be observed in coatings prepared from solvent- or water-based adhesives. In this case the molecular orientation of the rubber phase imparted by the strain-induced crystallization process leads to a reduction of tack and an anisotropy in the peel adhesion performance. The peel adhesion is substantially lower in the coating machine direction compared to the transverse direction. This property allows for repositioning of the tape. Influence of Face Stock Material on the Peel. The rheology of the PSA depends on the solid state components of the laminate. Peel is a function of the rheology of the adhesive and it also depends on the face stock material. Both the bulk properties of the face stock material and its surface quality aﬀect the peel resistance. Peel is determined by the deformability not only of the adhesive, but also of the face stock [138]; the stiﬀness (ﬂexural modulus) of the face stock inﬂuences the peel angle and thus the peel value [132]. The eﬀect of the face material becomes part of the result; as an example one can

276

Chapter 6

consider a 180 peel adhesion test, where work is being done not only to separate the adhesive from the applied surface but also to bend the face stock through 180 [3]. The same adhesive at the same coating weight can display a variety of peel values when coated onto diﬀerent face stock materials. Depending on the substrate, adhesives may be classiﬁed as removable or permanent. The same substrate material may have diﬀerent surface properties; for example, yellow plastic items may be labeled less easily than white ones because of the ﬁllers used [139]. The bulk characteristics of the face stock material, its stiﬀness, and plasticity/elasticity aﬀect the transfer of the debonding forces (see Chap. 2). On the other hand, the chemical composition of the face stock material inﬂuences the interaction between the adhesive and the face stock material. This inﬂuence should be taken into account especially for PVC foils [132]. The geometry and dimensions of the face stock can aﬀect the peel resistance as well. Mechanical eﬀects in the peel test were studied by Kim et al. [140]. The peel strength measured by the peel test method is a practical adhesion (an engineering strength per unit width) and does not represent the true interfacial adhesion. The measured value is a combination of the true interfacial adhesion strength and forces required for plastic and elastic deformation of the adhesive ﬁlm and substrate. The major controlling factors in the peel strength are thickness, Young’s modulus, yield strength, the strain hardening coeﬃcient of the adhesive ﬁlm, compliance of the face material, and interfacial adhesion strength. The mechanical characteristics of the face stock material, its ﬂexibility, and elasticity (plasticity) inﬂuence the transfer of the debonding energy to the adhesive layer (see Chap. 2). The decrease or damping of the peel force is caused by the deformation (elongation) of the face stock material under tension, changing the direction or distribution of the force (change of the debonding angle), and by the stiﬀness of the material; these phenomena decrease the peel force [3]. Thus, one can write: Peel ¼ f ½ðStiffnessÞ1 , ðPlasticityÞ1

ð6:46Þ

The stiﬀness depends on the ﬂexural modulus Ef whereas elongation depends on the elongation modulus Ee of the face stock. Thus: Peel ¼ f ½1=ðEf ,Ee Þ

ð6:47Þ

This equation is conﬁrmed by practical data from the labeling industry. Stiﬀ aluminum foils used as face stock material exhibit a lower peel value. Similarly, opaque soft PVC gives a lower peel value than clear (softer) PVC.

Adhesive Performance Characteristics

277

For some adhesives, in spite of the weak adhesion, there is a possibility of achieving high peel strength values if the adhesive ﬁlm exhibits a low modulus [141]; this means that: Peel ¼ f ðEf ,Ee1 Þ

ð6:48Þ

According to [142] a disadvantage of the peel testing is the signiﬁcant plastic deformation that the peel arm undergoes. The extent of plastic deformation is a function of peel angle and peel rate and therefore the measured Gc (fracture energy) values are said to be geometry dependent. Bikermann [118] proposed a derivation of the energy release rate see [Eq. (6.44)] where h (the backing thickness) and Y (the Young moduli of the backing) take into account the inﬂuence of the carrier material. In the formula discussed by Kaelble [143] the factor Eh3 takes into account the backing rigidity. Piau et al. [103] stated that for steel as carrier, lower peel and lower increase of the peel with the velocity. In the peeling master curve they represent the quantity 3/7G as a function of V, where

¼ Ve2/(Eh3) is a dimensionless number representing the eﬀect of the adhesive viscous forces compared to the rigidity eﬀects of the backing. Fukuzawa and Uekita [119] suggested a theoretical formula of peel adhesion (P): P ¼ b½Wa þAX 2 ðeB=X þ B=X 1Þ þ 2Es Is ð! sin !Þ=ð1 cos !Þ=ð1 cos !Þ

ð6:49Þ

which includes in its parameters A and B the work of adhesion Wa, between the PSA and the adherend, the work of deformation of the PSA (Wd), and the work of bending of the backing Wb. The modulus of the PSA (Ea), the modulus of the backing Es, and the moment of inertia of the backing Is (Is ¼ bt3s /12; where b is the width of the PSA tape, ts is the thickness of the backing) are taken into account also. An increase in the modulus of the backing causes an increase in the peel adhesion. If the modulus of the backing attains 39 E 9 dyn/cm, peel increases. Fracture energy values measured from the SLBT were compared to values obtained using the pull-oﬀ test (and were of the same order of magnitude for diﬀerent substrates). However, for high energy substrates results from a 90 peel test were an order of magnitude greater than the SLBT, probably due to the plastic deformation of the peel arm [142]. For thin ﬁlms the eﬀects of plastic deformation and yielding may be signiﬁcant given the small load bearing capacity, associated with the small thickness, and the relatively strong adhesion [144].

278

Chapter 6

According to Williams et al. [145] the current challenge is to model accurately any extensive plastic deformation which may occur in the ﬂexible peeling arm, since if this is not accurately modeled then the value of Gc deduced may suﬀer a high degree of error. Since the cover sheet width is much greater than its thickness, the deformation during peeling is independent of the out-of-plane coordinate. The cover sheet is assumed to be linearly elastic with Young modulus E and Poisson’s ratio respectively. Plastic yielding of the cover sheet will not occur as long as it is suﬃciently thick [146], so that: pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ 6E FR=H <

ð6:50Þ

where F is the peel force per unit width, is the yield stress of the cover sheet in shear, and E* ¼ E /(1–2). The cover sheet is modeled as elastic, so that: M ¼ E*Ik, where I ¼ H3/12, k is the radius of curvature of the cover sheet at P (x, y), and M is the moment at P [15]. As discussed above, reduction of the coating weight to values under the critical weight causes joint failure. In principle such reduction may be the result of carrier (and adhesive) deformation also [147]. Comparative industrial experiments with protective ﬁlms having carrier ﬁlms with standard and reduced thicknesses for the same adhesive and adhesive coating weight, conﬁrm the decrease of the peel resistance with the reduction of the thickness of the carrier ﬁlm too (Table 6.10). Films with strongly reduced thickness show very low adhesion. The mean peel

Table 6.10 The Influence of the Carrier Thickness on the Peel Resistance of Adhesive Coated Films Adhesive Code 1 2 3 4 5

Carrier Thickness (m)

Peel Resistance on Stainless Steel 180 (N/10 mm)

50 25 80 40 80 45 45 35 40 30

0.15 0.05 2.50 0.90 2.50 0.80 1.10 0.80 0.40 0.01

Adhesive Performance Characteristics

279

resistance value of about 0.6 N/10 mm is reduced to 0.1–0.06 N/10 mm. That means that such ﬁlms, although they possess a standard adhesive coating weight, do not work like standard products. The adhesion of such protective ﬁlms with very thin carrier material is extremely low and does not allow their standard use. Tensile stresses in the carrier ﬁlm may produce bond failure. The deformability of the carrier (reduction of its thickness by stretching) depends on its thickness. The deformability of the carrier produces two eﬀects. In general, reduction of the carrier thickness causes the decrease of the peel force. In the extreme case, an excessive reduction of the carrier thickness leads to bond failure. Astonishingly this phenomenon occurs at a carrier thickness which is suﬃcient for self-supporting or processing of the laminate. That means, that the pressure-sensitive laminate cannot be used although its components (the adhesive and the carrier) work. The adhesive ﬁlm can be laminated on the product to be protected and the adhesion between the ﬁlm and substrate is suﬃcient if no in-plane tensioning of the carrier appears, but stretching of the ﬁlm causes debonding. Such pressure-sensitive products with very low (dynamically tested) adhesion behave under statical use adequately. However, their end-use quality cannot be evaluated by means of standard tests. As stated, bond failure appears at a critical carrier deformability (Fig. 6.13). It is supposed that in this case the simultaneous, carrier-induced deformation of the adhesive leads to

Figure 6.13 Dependence of the peel resistance on the maximum tensile force (Fm) in the carrier ﬁlm.

280

Chapter 6

an apparent reduction of the coating weight which causes bond failure (Fig. 6.14). For thin plastic ﬁlms the maximum tensile strength (Fm) is preferred as an index of the carrier deformability. Thus, the peel resistance can be described as a function of the coating weight (Cw) and the parameter (Fm) of the carrier deformability which aﬀects the coating weight according to the following correlation: P ¼ Cw Fm

ð6:51Þ

The dependence of the peel resistance on the carrier thickness is given in a general form by the following correlation: P ¼ f ðFm k2 Þ

ð6:52Þ

where the coeﬃcient k2 takes into account the minimum ﬁlm thickness required for dimensional stability. The correlation between adhesive coating weight, carrier strength, and peel resistance can be written in the following form: P ¼ f ½ðCw k1 Þ,ðFm k2 Þ

ð6:53Þ

Figure 6.14 Bond failure and deformation of the removable, adhesive coated, thin plastic carrier.

Adhesive Performance Characteristics

281

There is a critical deformability which leads to a critical coating weight: Cwcr ¼ f ðFmcr Þ

ð6:54Þ

Our experience suggests that the critical value of Fm (situated between 8–9 N) is attained by mono-extrudates at a ﬁlm thickness of about 35 m. Co-extrudates display such values at a lower thickness, of about 20 m. A theoretical minimum peel is observed for a 180 peel angle. In reality, minimum peel values were obtained for lower peel angles. These discrepancies may be explained by the low ﬂexibility of the face stock material; however, gradually increasing face stiﬀness and thickness decreases the peel angle and so the peel adhesion [3]. On the other hand, the extra work needed to bend the face material will raise the observed adhesion value. In order to overcome this, a ‘‘reverse adhesion test’’ can be performed by mounting the tape to be evaluated, adhesive surface up, onto a standard test panel, to eliminate the backing eﬀect. The 180 peel adhesion values (under-layered) show a linear dependence on the thickness of the face material [3]. Table 6.11 contains typical 180 and 90 peel adhesion values for labels with diﬀerent face stock materials and peel angle. As can be seen from Table 6.11, the stiﬀness of the face stock exerts a strong inﬂuence on the peel values. The dependence of the peel value on the peel angle and dwell time is shown in Table 6.12. In fact, not only the stiﬀness but [as discussed above, see correlation (6.53)] also the resistance to elongation inﬂuence the peel value (Table 6.13). In these measurements, the transfer of the peel force was achieved with the aid of a diﬀerent material (i.e., the face stock had a combined paper/ material construction) where the paper layer was contacting the adherent and the material layer (paper, ﬁlm, etc.) was subjected to tension. Thus by using diﬀerent force transferring materials the peel force was more or less

Table 6.11 Peel Adhesion Values as a Function of the Face Stock Material and Peel Angle (Peel Force, N/25 mm) Face Material PSA Tackiﬁed, water-based acrylic Water-based acrylic Tackiﬁed, water-based CSBR Rubber-resin solvent-based Rubber-resin solvent-based removable

Peel Angle ( )

Paper

Film

90

180

x — x x x

— x — — —

8.3 8.0 8.4 12.5 4.6

25.0 PT 12.0 25.0 PT 23.0 13.0

282

Chapter 6

Table 6.12 Dependence of the Peel on the Peel Angle and the Dwell Time (Peel Value, N/25 mm) PSA

Peel Angle ( )

Face Material

Chemical nature

Physical state

Paper

Film

90

180

Dwell Time

Tackiﬁed acrylic Acrylic CSBR acrylic Rubber-resin Rubber-resin Acrylic Acrylic Acrylic

Water-based Water-based Water-based Solvent-based Solvent-based Water-based Water-based Water-based

x — x x x — — —

— x — — — x x x

8.3 8.0 8.4 12.5 4.6 5.7 6.0 10.0

25.0 PT 12.0 25.0 PT 23.0 13.0 5.2 12.0 14.0

0 0 0 0 0 0 15 sec 24 hr

Table 6.13 Dependence of the Peel on the Bulk Properties of the Face Stock Material. The Influence of the Mechanical Properties of the Face Stock Material (Maximal Tensile Strength of a PE Film) on the Peel Force Ultimate Tensile Strength (N/cm) 8.0 9.0 10.0 11.0 12.0 13.0 14.0 16.0 18.0

Peel (N/cm) 0.13 0.23 0.30 0.40 0.54 0.60 0.70 0.87 1.07

A nontackified acrylic PSA was used. 180 peel adhesion on stainless steel was measured.

dampened according to the plastic/elastic character of the material. Soft, plastic materials (e.g., ﬁlm) exhibit less peel strength than resistant ones (e.g., paper) (Table 6.12). As an example for the peel dependence on the face stock stiﬀness or rigidity, aluminum-laminated paper is removable without primer (using an adequate PSA), and a solvent-based silicone release liner should be used in order to get an appropriate release force. Peel strength increases with the thickness of the face stock material, according to a power law dependence [148]. With gradually increasing face

Adhesive Performance Characteristics

283

material thickness one needs to consider two additional features that aﬀect the result, namely, the rapidly decreasing peel angle, which will drop the observed peel value, and the extra work needed to bend the face material, which will raise the observed peel adhesion value [3]. As an example PET face ﬁlm shows a drop in peel adhesion from around 2 mil onwards, aluminum shows a continual rise, and polyethylene ﬁlm shows little change or a slight decrease. To overcome this, a ‘‘reverse adhesion test’’ can be performed [3]; a standard PET ﬁlm strip is applied and rolled down to standardize the face material eﬀect. A plot of adhesion vs. face thickness can then be drawn which can be extrapolated (for diﬀerent PET thicknesses) indicating the value of adhesion without the added inﬂuence of the face stock. The inﬂuence of the carrier rigidity is evidenced by special debonding tests which are aﬀected by the stiﬀness (ﬂexibility) of the carrier material. Such a characteristic test is the shaft loaded blister test (SLBT). The shaft loaded blister test may have both the ease and convenience of peel testing and the plastic deformation characteristic for a pressurized blister test [142]. The theoretical framework for measuring the fracture energy using the shaft loaded blister test was developed by Wan and Mai [144]. For small angle of deﬂection the following relation exists between P (the shaft load), wo (the central shaft displacement), a (the crack length), h (ﬁlm thickness), and material properties (E): Ehw3o =8 Pa2 0:5 W

ð6:55Þ

where the interfacial adhesion (W) can be calculated as a function of P. Unfortunately, the experimental results are contradictory. Samples consisting of stacked ﬁlms (either 1, 2, or 4 plies thick) were investigated utilizing the shaft loaded blister test to determine if the backing’s tensile rigidity had any eﬀect on the measured G (apparent strain energy release rate) [149]. The results suggest that the number of plies had no eﬀect on the measured G when an experimental compliance calibration is incorporated into the calculations. G values measured from the SLBT were compared to values obtained using the pull-oﬀ test (and were of the same order of magnitude for diﬀerent substrates). In a similar manner the (calculated) value of W is not a function of the number of stacked ﬁlms, n. On the other side the sensitivity of W to the ﬁlm thickness h reﬂects that the actual system is a viscoelastic bimaterial made up of the polymer backing and PSA, whereas the theoretical system is a single elastic material. In the peeling master curve Piau et al. [103] represent the quantity

3/7G as a function of V, where ¼ Ve2/(Eh3) is a dimensionless number

284

Chapter 6

representing the eﬀect of the adhesive viscous forces compared to the rigidity eﬀects of the backing. Miyagi et al. [150] relate the stiﬀness of the laminate to the shear modulus of the adhesive. The stiﬀness is deﬁned as P/ where P is the load applied at the center of the support and is the resulting displacement of the support. Their analysis provides the following expression for stiﬀness: P= ¼ ð4E= Þ fb½2ðt þ hÞ3 =L3 g

ð6:56Þ

where is a function of the adhesive thickness (t), carrier thickness (h), length (L), and sample width (b); E and G are the tensile and shear moduli of the carrier, and Ga is the shear modulus of the adhesive. The stiﬀness increases with the adhesive shear modulus, as a function of t/h. The surface properties of the carrier material aﬀect the adhesive performances of the pressure-sensitive products too. When a laminate is being debonded, there are at least four possible modes of failure, including: adhesive failure with debonding of the adhesive from the web, adhesive failure with debonding of the adhesive from the substrate, cohesive failure or splitting of the adhesive itself, and paper/ﬁlm tear or failure of the web itself. The failure mode is inﬂuenced by the surface quality of the face stock [151,152]. Clay-coated papers exhibit paper tear if the surface strength of the layer is not suﬃcient [153]; water-based dispersions of polymethylacrylate can be used for removable paper labels, but they are not removable from PVC and lacquered surfaces [84]. The substrate nature inﬂuences the peel and removability; for example, a two-year warranty was given for Fascal 1800, except from PMMA, nitrolacquer, and polystyrene surfaces. In an ideal case, the correlation between the anchorage of PSAs to the face stock material AF, cohesion of the adhesive C, and adhesion of the PSAs on the substrate As should be valid [138]: AF > C > AS

ð6:57Þ

In the case of permanent adhesion As should be higher than the mechanical resistance of the face stock material or of the substrate. For removable adhesives, As should be less than the minimal forces producing a deformation or deterioration of the face stock material or of the substrate. Energy considerations imply an adequate anchorage of the PSA onto the face stock material, but this will depend on the surface quality of the face stock material. In fact the surface characteristics of the face stock material inﬂuence the anchorage of the PSAs on the face stock material through the

Adhesive Performance Characteristics

285

wetting of the surface and the anchorage of the coated, wetted adhesive on this surface. The ﬁrst step in the bonding process is the wet-out of the surface by the adhesive; surface roughness and energy aﬀect this wettability. Wettability is an initial prerequisite to high adhesion. Adhesion is at a maximum at zero interfacial energy. The inﬂuence of the surface properties of the carrier material and of the adherent on the adhesive performances will be discussed in the next sections together with the eﬀect of the experimental conditions. Influence of the Release Liner on the Peel. The surface and bulk properties of the release liner inﬂuence the peel resistance of the PSA. The structure, nature, and thickness of the adhesive layer aﬀect the peel value [138]. Silicone transfer to PSAs, contamination of the adhesive layer through crosslinking catalysts, or mechanical damage of the PSA layer (as a replica of the release layer) may negatively inﬂuence the peel resistance. At a given peel rate release force decreases as coating thickness is increased (1.4–4.2 m) [117]. This is consistent with earlier understandings of varying adhesive thickness. The increasing thickness h of the deformable layer within the PSA laminate results in shifting the viscoelastic window to lower frequencies. Traditional high release agents (HRAs) used in release coatings ‘‘freeze out’’ interfacial slippage resulting in increased adhesive deformation. The HRA reduces the segmental mobility of the PDMS chain within the cured network leading to increased shear stress and an increase in the practical peel force. In a manner similar to the face stock the bulk properties of the liner inﬂuence the peel of a label from the release liner. When increasing the thickness of the release liner, the high speed release values are approximatively doubled when the liner is removed from the label. This conﬁrms that the nature of the release liner (probably its stiﬀness) has a considerable inﬂuence on the high speed release force. Influence of Label Laminate Construction on Peel Adhesion. The energy absorption during peeling decreases the value of the peel force. Therefore, multilayer labels, with more deformable liquid adhesive layers and with the ability to absorb the debonding energy by viscous ﬂow, show lower peel values than single-ply labels (Table 6.14). The peel resistance depends on the rheology of the adhesive layer R and on the peel force F (see Chap. 2). The rheology of the adhesive layer depends on its thickness h. In the case of multilayer labels, the adhesive thickness is the sum of the diﬀerent layers or: h ¼ hi

ð6:58Þ

286

Chapter 6

Table 6.14 Peel Dependence on the Laminate Structure. Stiffness and Peel Values for Mono- and Bilayer Laminates, with the Same Face Stock and Different Adhesives Laminate Structure Face stock First layer Second layer

PSA Soft PVC Paper

Untackiﬁed acrylic Tackiﬁed water-based acrylic Tackiﬁed water-based acrylic/CSBR Tackiﬁed water-based acrylic Tackiﬁed solvent-based rubber-resin

Characteristics Stiﬀness (mN/cm)

Peel on glass (N/25 mm)

— 698 480

— 17.4 17.8

515 483

16.0 10.5

The untackified acrylic adheres to the test substrate.

Therefore the increased layer thickness improves the ﬂow properties of the adhesive (i.e., its energy absorption capacity). Thus it is assumed that the peel is inversely proportional to the number N of the adhesive layers, Peel ¼ f ðN 1 Þ

ð6:59Þ

On the other hand, increasing the layer number increases the stiﬀness of the laminate (i.e., it changes the debonding angle) and thus decreases the peel too (see the inﬂuence of the face stock on the peel). Therefore, it is more accurate to assume a pronounced eﬀect of the number of layers through viscous ﬂow (higher adhesive thickness) and peel angle (higher laminate stiﬀness), or: Peel ¼ f ð1=N Þ

ð6:60Þ

where > 1. As shown in Table 6.15, this hypothesis is conﬁrmed by experimental data. Increasing the number of layers increases the stiﬀness of the laminate and at the same time decreases the peel value. Increased stiﬀness requires more energy for the ﬂexure of the face stock material (i.e., the peel force increases). Thus, the peel will depend on the number of layers according to a more complex correlation. Peel ¼ f ðN =N Þ

ð6:61Þ

Adhesive Performance Characteristics

287

Table 6.15 Dependence of the Peel on the Number of Layers Laminate Characteristics Number of layers 2 4 6 8

Stiﬀness (mN/cm) 18 60 100 152

Peel (N/25 mm) On glass On PE 4.3 3.9 3.6 3.7

2.9 2.6 2.6 1.5

Water-based acrylic formulation. Film face stock material.

where >> . A multilayer ﬁlm-based label may increase the energy absorption by the viscous ﬂow of the solid state components (face stock included) (i.e., the peel decreases again). Thus, one can assume that: Peel ¼ f ½ðN Þ=ðN N Þ

ð6:62Þ

where takes into account the adhesive slip and the change of the debonding angle, denotes the increase of the mechanical resistance with the number of layers, and accounts for the deformation of special ﬁlmbased face stock materials. For multilayer ﬁlmic labels the dependence of the peel on the number of layers may be summarized by the following equation: Peel ¼ f ðN =N Þ

ð6:63Þ

where N ¼ N Ng (i.e., N > N). It may be assumed that the peel decrease with increasing number of layers is more pronounced for ﬁlm labels than for paper labels. This hypothesis is conﬁrmed by the data in Table 6.16. Paper laminates show a less pronounced peel decrease with increasing number of layers (Table 6.17). In a similar way paper layers in hybrid (paper/ﬁlm) labels impart increased peel adhesion. In some experiments the inﬂuence of the stiﬀness deformation resistance of multilayer laminates was studied. In these tests a ﬁlm label was reinforced with paper labels coated with diﬀerent PSAs. The distribution of the peel forces depends on the deformability of the ﬁlm face stock, or on the slip between the labels (i.e., the adhesive nature). As seen from Table 6.16, reinforcing the ﬁlm label with an elongation-resistant

288

Chapter 6

Table 6.16 The Influence of the Number of Layers on the Peel Adhesion of Film Laminates Number of Layers 2 4 6 8

Stiﬀness (mN/cm)

Peel on Glass (N/25 mm)

39 236 408 644

23.5 19.5 12.0 8.0

Water-based PSA coated on soft PVC.

Table 6.17 Peel Dependence on the Number of Layers for a Multilayer Paper/Film Laminate Laminate Construction PSA PSA PSA PSA

Film – – – – Film PSA Film – – Film PSA Paper – – Film PSA Film PSA Paper

Laminate Stiﬀness Peel on Glass (mN/cm) (N/25 mm) 46 130 245 483

8.4 6.4 8.0 7.8

material (paper) improves the peel value. On the other hand, the peel value of multilayer laminates increases with the internal rigidity (no motion) of the sandwich (i.e., with the adhesion between the laminate face components). The slight anchorage of a rubber-resin adhesive on soft PVC dramatically decreases the peel value of the sandwich. Control of the viscous ﬂow can be achieved by superimposing adhesive layers. Influence of Experimental Conditions on Peel. The surface tension, surface structure (roughness), and thickness of the substrate inﬂuence the peel resistance [138]. Test conditions such as dwell (contact) time, contact pressure, debonding rate, debonding angle, and temperature also aﬀect the peel resistance. At lower speed, viscous ﬂow is the predominant inﬂuence on adhesion (peel value), while at higher speed, the elasticity determines the adhesion; at lower speed, peel adhesion increases. Some of these parameters were discussed earlier (see Chap. 2). Concerning the stress rate and its inﬂuence on the peel, an increase of the temperature allows increased molecular mobility resulting in increased tack and reduction in shear resistance. A high rate of stress is equivalent to a

Adhesive Performance Characteristics

289

drop in temperature. Adhesive transfer may occur at higher unwinding force and rate; faster peeling rates may transfer the adhesive or break the label. Physical and chemical interactions between the adhesive and the substrate result in a build-up of the adhesion (i.e., the peel increases with the dwell time) [138]. This is a general behavior observed for PSAs with diﬀerent chemical compositions (e.g., acrylics or neoprene latices) [58]. The build-up of the peel depends on the chemical composition and time [105,106,154]. Because of the slow ﬂow of the adhesive, adhesion tests require at least 0.5 sec of dwell time [155]. The surface of a PSA can be aﬃxed to a substrate to which it will be adhered, but once aﬃxed, adhesion builds to form a strong bond. Removable adhesives can be formulated so as not to display adhesive build-up, that is, their adhesion to the substrate does not increase to the point where the label (tape) cannot be removed cleanly, even after exposure to heat in a drying oven. Adhesion build-up requires a long time [3]. Diﬀerent peel values are obtained when testing immediately after application (PSTC 1), after a 20 min dwell time (ASTM D-1000), and after a 24 h dwell time. The adhesion can still be climbing after 30 days. Peel adhesion is a function of the dwell time [156]; after 10, 30, 60, 180 min, and 24 h dwell times the following peel values were obtained: 4.0, 4.6, 5.3, 5.5, and 8.0 N/2 cm. Diﬀerent methods specify diﬀerent dwell times: AFERA 4001 requires 10 min, PSTC 1 and FINAT specify 20 min and 24 hr, respectively [139]. In order to test adherent ﬁber tear, tape is applied to cardboard with a 2 lb roller, immediately pulled oﬀ at a 90 angle, and rated for ﬁber tear. Roughness of the carrier surface and of the adherent inﬂuences bonding and debonding also. As discussed earlier dwell time depends on the surface quality too. Time dependent peel adhesion is commonly assumed to be due to the limited rate at which the rough adhesive layer can conform to the ﬂat substrate; the applied label retains a hazy look for a substantial time when applied to clear transparent substrates. Generally the substrate is rough too. According to Elmendorp et al. [157] the paper backing although highly calendered, is never perfectly ﬂat for two reasons: the distribution of wood ﬁbers in the paper (the formation) is never perfect, and the humidity cycles the paper undergoes during adhesive coating and drying can lead to microcockling, a slight waviness. Surface roughness with an amplitude of several microns with a wavelength of 2–5 mm is typically seen. Nardin and Ward [158] discovered a simple relationship between the surface roughness (i.e., the depths of the pits or valleys on the ﬁber surface) and the degree of interfacial adhesion due to the mechanical interlocking. Hui et al. [159] showed that when the dimensionless parameter pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ ¼ ð2= Þ 2W=E > 1

ð6:64Þ

290

Chapter 6

where is the aspect ratio of a typical surface proﬁle (or asperity) and is the number of asperities per unit length, the surfaces will jump into contact with each other with no applied load, and the contact area will continue to expand until the two surfaces are in full contact. Self-adhesion can be avoided by increasing the aspect ratio of the asperities, by decreasing the work of adhesion, by decreasing the density of the asperities, or by increasing the modulus of the polymer. In the coating and testing practice, adhesive contact is needed, i.e., the PSA has to conform onto substrate or carrier. According to Tordjeman et al. [19] the actual contact area is a function of the contact force (Fc) and the apparent modulus (Ea) which takes into account the competition between the thickness eﬀects and the roughness [see correlation (6.29)]. Other phenomena can compete with adhesive conformation during the dwell time also. The study of the adhesion of butyl rubber to rigid counter surfaces showed [160] that after 1000 hr of contact at 80 C the strength of adhesion become about ten times larger than the value measured after short contact times, and still showed no signs of reaching an equilibrium level. This increase is attributed to slow molecular rearrangements at the interface rather than to development of speciﬁc bonds, because it took place to a similar degree for butyl rubber and chlorobutyl rubber, against diﬀerent substrates. A similar but somewhat smaller increase in adhesion also occurred for vulcanized layers. For tackiﬁed adhesives the overall work of adhesion was measured as a function of the storage time [161]. Generally it decreases with increasing storage time; the resin migrates to the surface. The increase of the domain’s size with increasing time suggests that at long times there will be a surface layer of tackiﬁer-enriched material, making a more rigid surface that will inhibit wetting and degrade the overall adhesive performance. The strength of an adhesive bond formed with PSA is generally recognized to be deﬁned by two major contributions: speciﬁc interaction forces across the adhesive-substrate interface and the work of viscoelastic deformation of the PSA polymer to rupture the adhesive bond. Within the frameworks of the diﬀusion theory of adhesion, the well-known eﬀect of adhesive strength enhancement with the time of adhesive-substrate contact is treated as the time required to provide the penetration of PSA macromolecules into the substrate [162,163]. One of the contributing mechanisms to polymer adhesion is said to be diﬀusion of chain elements across the polymer/polymer interface. The concept makes sense when polymers in close contact are above their respective Tg. The common features of various PSA polymers are high diﬀusivity and translational molecular mobility as well as ﬁbrillation of the polymer at cohesive failure

Adhesive Performance Characteristics

291

under applied debonding stress. Diﬀusion across the interface contributes to bond strengths in joints involving ﬂexible polymer chains, where dispersion forces dominate at the interface, and where favorable acid-base interactions abet the dispersion forces [164]. Chalykh et al. [165] studied the kinetics of interfacial zone formation in the closure of adhesive bonding to a polyethylene substrate and showed the lack of PSA interpenetrating into the substrate and vice versa: no interdiﬀusion across the interface occurs. During the stage of adhesive bonding the molecular mobility of PSA polymers contribute to the process of polymer relaxation, which in turn accounts for the increase of the strength of adhesive joints with PSAsubstrate contact times. There is no need to advocate the diﬀusion theory of adhesion to explain the relevance of enhanced molecular mobility for the adhesive behavior of PSA polymers [163]. As discussed in Chap. 2, the time-temperature strongly aﬀects the peel. When increasing the temperature the viscosity of the adhesive decreases and its anchorage to the substrate will be improved [105]; on the other hand, adhesion increases and thus peel reaches a maximum for a given temperature. Temperature inﬂuences the eﬀect of roughness also. As demonstrated in [166,167] the debonding mechanism of SIS-based PSA is strongly inﬂuenced by roughness; at normal temperatures surface roughness is detrimental for good contact. High numbers of defects are trapped at the interface, relaxation is incomplete. At higher temperatures roughness prevents the cavities from rapidly expanding in the plane of the ﬁlm and increases the adhesion energy. At low temperatures adhesion decreases and the peel adhesion decreases as well [106]. One should take into account the poor peel adhesion properties of most current PSA labels at low temperatures (deep-freeze labels). The rate of separation and the temperature are related to the adhesive strength in a complicated manner, and these factors can dramatically inﬂuence whether or not adhesive or cohesive failure will occur. Peel decreases with decreasing temperature [3]. Cohesion and auto-adhesion of styrene-butadiene elastomers were studied by Hamed and Hsieh [17], through peel tests at diﬀerent peeling rates and temperatures. Peel resistance depends on the diﬀerence between the application temperature and the Tg of the adhesive. For instance, at 0 C the peel resistance value of an untackiﬁed acrylic adhesive is about 210% lower than the peel resistance value of the same adhesive at 23 C [168]. By unwinding of tapes so-called adhesive inversion may occur also [169]. In this case the separation (debonding) by unwinding of the tape does not occur at the interface of adhesive/printed carrier but between adhesive and primed carrier. Such behavior can be observed especially at temperatures lower than 10 C and at high unwinding speeds. The eﬀect of temperature on the 180 peel force in the temperature range between 20 C and þ40 C could be

292

Chapter 6

predicted from the proﬁle of the velocity spectrum of the rolling adhesive moment [170]. The dependence of the peel force on the peel rate is complex. It has been shown that several regimes can be obtained as a function of the peel rate [167]. The ﬁrst regime is the cohesive regime and it can be divided into three diﬀerent ones (as a function of the peeling rate). A two-dimensional one (at very low speeds), a three-dimensional one showing regularly spaced ribs, and ﬁnally a three-dimensional one, showing the breaking of ribs at the edge of the free surface [103]. Assuming that one can add the cohesion energy wc, Piau et al. [103] propose the following relation for the ﬁrst regime for low velocities: G ¼ wc f1 þ 0:1ðEh3 =Ve2 Þ3=7 ðV=s Þg

ð6:65Þ

For the dependence of the peel force on diﬀerent parameters Kaelble [143] proposed a law using Bikermann’s approach, for a Newtonian adhesive: qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ F=b ¼ 2 2 ðEh3 =eÞ1=21=2 ðK=tm Þ1=2

ð6:66Þ

where is the viscosity of the adhesive, K/tm is a peeling velocity factor which is not so well deﬁned, but tm possibly will vary like e/V. Fukuzawa and Uekita [119] proposed a theoretical formula of peel adhesion see [see Eq. (6.49)] as a function of the peel angle and peel rate. At low peel rates the peel adhesion depends mainly on the work of adhesion, but at high peel rates it is inﬂuenced by the work of deformation. According to [92] the maximum of the curves (peel force-peel rate) corresponds to the change in mode of failure from cohesive (at lower peel rates, < 300 mm/min) to adhesive at higher debonding rates. At low tensile rates the adhesive fracture occurs at comparatively small tensile stress values, but at signiﬁcantly greater values of elongation, ". With the increase in extension rate, the critical magnitude of tensile stress at fracture ( f) grows rapidly, approaching a limiting value which corresponds to a maximum cohesive strength of the PSA ﬁlm ( c). At the same time the work of PSA tensile straining varies, passing through a maximum. As conﬁrmed in [160] also the peel strength is strongly dependent on the eﬀective rate of peeling. At low rates the elastomer layer could not be detached cleanly. Instead it split apart leaving rubber behind (cohesive failure). Under these circumstances the peel strength is the same as the tear strength measured by tearing apart a thin sheet of the elastomer. As the peel

Adhesive Performance Characteristics

293

rate was increased, however, an abrupt transition occurred to interfacial failure at much lower peel forces, only about one twentieth as high. This transition is attributed to failure of entanglements to ﬂow apart at a critical peel rate. After the transition the peel strength again increased with peel rate paralleling the cohesive strength of the polymer itself but now only about one tenth as high. This increase is attributed to increasing viscous losses in peeling away the elastomer as it approaches the glassy state. According to [94] the diﬀerence between stick and slip peel force diminishes as the rate increases. There are two interfacial debonding zones as a function of the rate. The ﬂatness of the peel force vs. peel rate response is optimal at intermediate molecular weight with very high molecular weight leading to poor adhesion at low rates [95]. In the correlation used by Gent and Schultz [21] and later by Carre´ and Schultz [171]: G ¼ wo gðMc Þ ðVÞ

ð6:67Þ

where wo is the cohesive energy, g(Mc) is a molecular dependent term, and is a dissipative function of the velocity, the function (V ) has been determined in a few cases to be a power law of the velocity. Using a statistical software package, peel strengths were measured for PSA tapes, according to the following correlation [172]: Load ¼ 0:55 þ 0:88 rate þ 1:17 103 time

ð6:68Þ

It has been demonstrated, that single pull peel testing does not always predict the joint failure that occurs as a result of cyclic loading at subcritical loading levels [173–175]; adhesive joints when critically loaded at levels below the load at ﬁrst yield in the single pull experiment, can open up at appreciable rates. The application conditions of pressure-sensitive products and their inﬂuence on the end-use properties are discussed in detail in [176]. Peel Force from the Release Liner. A special case of peeling is the separation of the label from the release liner (i.e., labeling). As discussed above the peel resistance depends on the peeling rate and angle. As stated by [177] the peel decreases with the angle: 90 peel is about twice of that of 180 peel. In a similar manner the peel force to separate the PSA from the release liner depends on the peeling rate and angle, and adhesive/release nature [177]: Peelforce ¼ f ðV, Þ

ð6:69Þ

where V and denote peeling rate and angle. Separation rates of 200 m/min are common during labeling. At these rates condensation silicone release

294

Chapter 6

layers display an increase of the peel. In the FTM 10 test method a much lower test rate is used, while the FINAT method is designed for a rate of 10– 300 m/min with the ﬁrst test apparatus built in 1984. For testing purposes a sample of 2.5 70 cm or 5 70 cm was proposed; samples have to be stored under pressure (70 g/m2, 20 hr, 23 C) before testing. Performance characteristics after aging should be tested upon storage at 40 and 70 C, respectively [177], because the release nature and age also inﬂuence the peel. There are some new materials that are supposedly insensitive to release nature and age (e.g., Nacor 80). Acrylic PSAs require a separate study for each formulation with regard to the level of the release force and its stability. For current water-based acrylic PSAs on solventless release liners, release forces average 0.05–0.06 N/25 mm (FTM 3). Interaction between waterbased acrylic PSAs can increase this value to more than 0.1–0.15 N/25 mm (e.g., Vinacryl 4512). Increasing the concentration of the methylsilicate HRA, increases release force. However, the largest impact of the HRA on release force is typically at low to intermediate peel velocities (0.005 to 0.17 m/s) while increasing the molecular weight between crosslinks mainly inﬂuences release at high peel rates [117]. Textiles, nonwovens, and surfaces coated with alkyd-, PUR-, acryl-, epoxy-, nitro-, and powder-lacquers, and plastisols be used as contact surfaces for testing purposes. Crystal glass is proposed as substrate by FINAT, steel by AFERA, and glossy stainless steel by ASTM or BWB-TL [178]. These materials are used in order to eliminate the inﬂuence of the adherent on the peel. If the only factor governing the peel force were the total extent of contact (i.e., the number and type of interactions across the interface) then the scale of the contact zone would not inﬂuence the measured peel strength [17]. In general this is not true, particularly for soft vulcanized elastomers, whose strengths are a measure of dissipative energy losses that occur while they are being peeled away. Adhesive strength to paper and stainless steel is greater than to polyester ﬁlm [83]. Therefore the bond fails due to breakdown within the adhesive layer when paper is used. When polyester ﬁlm is used as face material it peels away, leaving the adhesive layer virtually intact on the steel plate. The inﬂuence of the adherent is illustrated by PSA labels applied on PVC (where the peel value changes in plasticized PVC) or by removability from fragile substrates (e.g., lightweight papers) [179]. Peel and tack should be tested on diﬀerent substrates (e.g., cellulose, polyamide, glass, cardboard, metal) [180]. Temperature resistance, the lowest application temperature, and the suggested operating end-use temperature should be determined. The inﬂuence of the adherent surface on the peel is shown in Table 6.18. The mechanical strength of the substrate inﬂuences the peel resistance too. Paper possesses an anisotropic structure. Cellulose ﬁbers which have the

Adhesive Performance Characteristics

295

Table 6.18 The Influence of the Adherent Surface (Substrate) on the Peel. Peel Values of PSAs with Different Chemical Bases, as a Function of the Substrate Used Chemical Composition

Peel (N/25 mm)

Base components

Supplier

Glass

Cardboard

PE

1360 V205 80D CF52 FC88 PC80 CF52 FC88 CF52 V205 80D CF52 V205 V208 80D CF52

Hoechst BASF BASF A&W UCB UCB A&W UCB A&W BASF BASF A&W BASF BASF BASF A&W

12.0 PT

2.0

11.0

15.0 PT

11.0 PT

13.0

18.0 PT

11.0 PT

12.0

18.0 PT

—

15.0

14.0 PT

10.0

13.0

typical dimensions of 4 20 2000 m, are layered with most of the ﬁbers lying like ribbons in the plane of the paper sheet. Therefore paper is very resistant to in-plane stresses and very sensitive to out-of-plane or z directional stresses. It is susceptible to delamination. Such delamination occurs at higher peel rate [181]. The peel force initially rises until the peel force applied to the paper surface is high enough to remove ﬁbers after which the force drops to a low steady-state value, corresponding to delamination (paper failure). Therefore paper delamination is characterized by a high initiation force and a relatively low propagation force. Microscopic observation reveals that paper failure starts with the lifting of an individual ﬁber. Bikermann and Whitney [182] reported that paper is much more likely to delaminate in peel if the tape overlaps the edge of the paper. In peeling the actual lifting forces on the paper surface are less than the measured forces because of viscous dissipation in the PSA layer. The actual forces were calculated on the paper surface in peel from the observed stretching of paper in 90 peel. Comparing peel force and the actual force at the paper surface, as a function of the peel rate the two types of forces were equal at low peel rates, whereas the forces experienced by the paper were about 2/3 of the

296

Chapter 6

measured force at high peel rate. Yamauchi and coworkers reported peel forces and failure modes as a function of peel rates and paper types [183]. Pelton et al. [184] studied the peel force as a function of log (peel rate), and found quite diﬀerent interdependences for glassine, copy paper, and ﬁlter paper. Glassine (tracing) paper, a smooth, nonporous substrate gave interfacial failure only and peel force increased with peel rate. Filter paper showed paper failure and low peel forces. Copy paper showed increasing peel forces with peel rate until the failure mode changed to paper failure giving low peel forces. Filter paper, which is strong by conventional measurements, gave only paper failure. Thus roughness, porosity, and the extent of adhesive penetration dictate whether paper failure occurs with peel. According to [185] the deformability of the substrate must be taken into account. For small values of the adherend thickness, the assumption of bending-dominated crack propagation becomes invalid when the opening forces become signiﬁcant. At the other extreme the peak stress in the interface region aﬀects the plastic deformation of the adherends. Thus, peel force depends on the adherend thickness. For a given macroscopic energy release rate the plastically deforming adherends do not allow high levels of opening stresses to be developed within the adhesive. Thus the appropriate stress levels for development of a microcracking zone are reached before the adhesive fails cohesively at the crack tip. In contrast the elastic adherends induce much higher levels of opening stress over an extended region in the adhesive layer. Thus before cohesive failure at the crack tip occurs, large enough stress are developed ahead of the crack tip to allow a microcracking zone to be developed. This change in mechanism means that toughness of joints may not be the same for substrates that remain elastic as opposed to cases where they deform plastically. The deformability of the substrate has to be taken into account by the choice of the peel test methods also. For instance T-peel is used for ﬂexible adherends [186,187] (see Chap. 10 too). Chalykh et al. [92] investigated the strength of the PSA joints on diﬀerent substrates (PE, PP, PVC, PET, PA, and hydrate cellulose). The strength of the adhesive bond for the substrate was found to be ordered like the corresponding magnitudes of the thermodynamic work of adhesion. Wa : HC > PA > PVC > PP > PE

ð6:70Þ

Piau et al. [103] measured for steel as carrier, lower peel and lower increase of the peel with the velocity. The adhesion energy of the acrylic adhesive on silicone release coating is 10 times smaller than that on steel. Moreover the stress-strain curves have diﬀerent shapes on steel [188].

Adhesive Performance Characteristics

297

In peeling tests described in [91] diﬀerent substrates (PyrexTM, PlexiglasTM and PMMA) were used. Four mechanisms of cohesive failure have been found as the reduced velocity increased. The peeling mechanism on Plexiglas also led to the creation of another mechanism not observed on Pyrex. Peel values on various substrates (stainless steel, cardboard, and polyethylene) for diﬀerent adhesives are listed in [189]. Other Factors. Peel adhesion does not depend on the number of passes with a standard application roller and it does not depend on the solvent used to clean the test plates. On the other hand, peel increases with the application pressure and temperature [132]. Rheology is frequency dependent and strain frequency inﬂuences the peel; repeated, cyclical peeling oﬀ reduces the peeling force (see Chap. 3). Readherence onto a paper substrate decreases as the number of peeling trials grows. In a lot of experiments the peel of an experimental PSA from newspaper as substrate was tested. The following data for the peel force were collected as a function of the number of peel trials:

Peel from paper, first trial: 110 g/12 mm. Peel from paper after 50 peel trials: 85 g/12 mm. Peel from paper after 100 peel trials: 75 g/12 mm.

The readherability of a pressure-sensitive product was tested after repeated lamination and delamination [190]. The dependence of the peel on the cohesion is very complex. For some adhesive formulations based on natural rubber and SBR a direct proportionality between peel and cohesion was established [191]. Humid environments can adversely aﬀect bond strength. Soaking conditions, even high humidity, often cause irreversible loss of adhesive strength or failure. According to Czech [192] for splicing tapes the peel adhesion strongly depends on the relative humidity. Release transfer to the adhesive surface can reduce the peel resistance also. Druschke [193] found for a noncovered PSA layer a peel value of 12.3 N/20 mm which changed to 9.7 N/20 mm for the same adhesive after release paper contact. The subsequent adhesion, i.e., the peel resistance value after contact with release paper depends on the initial peel resistance of the adhesive and on the substrate [194]. Another problem is peel retention after aging. The requirements concerning peel retention after aging diﬀer as a function of the product build-up and end-use. Improvement of Peel Adhesion Improving the peel means optimizing it in such a way that it meets the enduse requirements. Regulating means increasing or decreasing the peel value

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and changing the character of the bond rupture related to the requirements of a permanent (paper tear) or removable (adhesion break) bond. Permanent/Removable PSAs. Pressure-sensitive adhesive-coated materials are functionally divided into two broad classes: permanent and removable [79]. The ﬁrst of these is represented by the so-called permanent materials in which the properties of PSAs are selected so as to form an adhesive bond with the target substrate which is strong and, apart from degradation, does not weaken signiﬁcantly with time. The second broad class consists of ‘‘removable’’ or ‘‘peelable’’ PSA-coated materials in which the PSAs form an adhesive bond, of functionally adequate strength with the target, but which after an extended period of adhesion (typically days to months) can be peeled away from the substrate, without damaging it, without leaving any residue of PSA on the target substrate, and without the adhesive-coated material tearing itself apart. To be peelable PSA-coated materials require a combination of properties. The PSAs must form a bond of adequate strength with the target substrate, but which does not subsequently either dramatically weaken, in which case the adhesive-coated material might fall oﬀ the substrate, or signiﬁcantly increase in strength, as this would tend to limit peelability. From these considerations it follows that the adhesive should possess an adequate cohesive strength in order to minimize any tendency of the adhesive ﬁlm itself to rupture, as well as a high bonding strength to the face material (anchorage) onto which the adhesive is coated. It is clear that the formation of a strong bond between a PSA and its face material and a relatively much weaker bond between adhesive and target substrate are in conﬂict. Consequently, many of the peelable PSAs currently on the market do not fully meet these contradictory requirements; eﬀectively, they are only less permanent than typical permanent PSA-coated materials. The reduction of the peel is a relatively simple procedure which (at least theoretically) can be achieved by increasing the viscous component of the PSA, that is, the component converting the high speed bond-breaking force in permanent adhesive deformation (viscous ﬂow). Unfortunately this viscous ﬂow may exhibit adhesive break (i.e., after bond breaking adhesive residue is left on the adherent). Such a peel reduction is inadequate to obtain a convenient removable PSA. Summarizing, a limited peel value and a high shear strength are required for removable adhesives [86]. A typical value for removable pressure-sensitive tapes is 10 N/25 mm [195]; lower values (2.5–5 N/25 mm) are required for labels. Shear values of at least 100 min at room temperature were suggested [196]. In fact these values should be considered as guidelines only since paper tear occurs at 20–25 N/25 mm. Removable PSAs should exhibit a peel adhesion value of 1.5 N/25 mm [195]; 1–6 N/20 mm are

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299

suitable values for removable labels [197]. In general, the same removable water-based acrylic PSAs for labels may be used for tapes. Removable PSA labels are a special type of permanent label, where low peel force levels, as a function of dwell time, and a pure adhesive break from the substrate are assumed. Removable behavior depends on energy dissipation which inﬂuences the debonding (Fig. 6.15). The adhesive (its bulk properties, formulation, nature, structure, and molecular weight), laminate components, laminate geometry, and debonding conditions inﬂuence the removability. During peeling tests at least four possible modes of failure may occur [198]: 1. 2. 3. 4.

Adhesive failure (primary): debonding of the adhesive from the face material (i.e., not removable). Adhesive failure (secondary): debonding of the adhesive from the substrate (i.e., removable). Cohesive failure: splitting of the adhesive itself (i.e., not removable). Web or film tear: failure of the face material itself, hence not removable.

Energy dissipation during debonding depends on such parameters as interfacial adhesion, polymer rheology, rate of peel, temperature, angle of peel, face stiﬀness, and coating weight. Several requirements have to be fulﬁlled in order to achieve good removability [199]. These include

Figure 6.15 Schematical presentation of stress dissipation as a function of the time for permanent and removable PSAs.

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self-adhesion on the substrate (tack), peel adhesion (in order to assure bonding), cohesion in order to avoid legginess or stringing, and superior adhesion to the face stock rather than to the substrate (anchorage). For suitable peelable labels adhesive failure from the substrate is desired. As discussed earlier removability implies lower stresses, and lower stresses are possible using stress-restricting polymers, ﬁllers, or a primer coating [200]. Peel depends on the nature and geometry of the bonding components (i.e., the self-adhesive label and substrate). For a given adhesive, face stock, and substrate, the interaction of the adhesive with both solid state components of the joint will depend on the interface adhesive/solid state component. The type of face material is governed by the end-use of the label; therefore the label-maker should optimize the interface between the face stock and the PSA. The most important parameters of the face stock inﬂuencing the removability of the PSA label are its ﬂexibility and the anchorage (adhesion) of the adhesive to the face stock. Eﬀective bonding between the face and the adhesive is critical to the performance of the PSAs; for example, given an adhesive with internal integrity, the adhesive properties exhibited by that system will be proportional to the aﬃnity of the adhesive for the face material. Face stock interactions bring the deﬁnition of ‘‘permanent’’ and ‘‘nonpermanent’’ PSAs much more into focus by emphasizing the fact that if a permanent bond is formed between the label and the substrate, the failure mode will be face oriented and the result is 100% transfer of the adhesive to the substrate. Conversely, if the bond to the substrate is nonpermanent, then a clean release from that substrate is encountered and the adhesive remains on the face material. Repositionability and readherability are performance characteristics that depend on the peel build-up. Both allow the debonding of the PSP after its application and its rebonding. Repositionability is an aging-related characteristic; it needs a slow build-up of the adhesion on the substrate. In this case peel build-up is delayed in time but is not limited as an absolute value. Repositionable labels may also be permanent. According to a simpliﬁed theory bonding and debonding can be controlled through the value of G0 and through the dependence of this value on the frequency. For removable products G0 possesses the same (small) value at diﬀerent frequencies. Thus a main possibility for regulating the peel resistance is the control of the G0 of the PSA. Use of Primers. Primers are necessary when using removable PSAs [201,202]; they modify the face stock-PSA interaction. Polyoleﬁn face stock materials are generally top coated to improve printability; in a similar manner primer coatings are applied on polyoleﬁn face stock materials to increase the anchorage of the adhesive [203]; PVC-based tapes also need

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a primer coating [204,205]. Soft PVC should be primed when used with rubber-resin-based PSAs [206]. There are a few studies describing the use of primers in order to obtain removable adhesives [207–209]. The use of the primer generally decreases the peel value [210]. Primers are generally used to promote a stronger bond between PSAs and face materials, to absorb stress, and to strengthen the face stock, especially when paper labels are concerned. There are very diﬀerent recipes for primers. As in the case of adhesive base elastomers, primer coatings use more crosslinking agents, harder resins, resin solutions, or self-crosslinked elastomers. A primer coating for polyethylene face stock material typically is composed of a chloroprene rubber (20–60 pts, 60% chlorine), an EVAc copolymer (40–80 pts, with 25% VAc), and chloropolypropylene (1–15 pts with 25–50% chlorine) [209]. While stearic acid is eﬀective in preventing adhesive build-up, it leaves a greasy deposit referred to as a ‘‘ghost’’ which could stain the substrate; the metallic stearates do not do this [210]. Contact cements (polymers with high cohesive strength and a strong tendency for two surfaces of the polymer to adhere to one another when placed in contact) are preferred as primers; isoprene polymers and polychloroprene are particularly good as contact cement. The use of a crosslinked contact cement as a primer improves peelability and increases bonding on the carrier from 0.5 to 2.4 kg [211,212]. More about primer formulations can be found in Chaps. 7 and 8. A detailed discussion of primer formulations for diﬀerent pressure-sensitive products is given in [213]. One approach is to use a key coat including particulate matter, so as to make the surface of the coating rougher, and provide a better key for PSAs; primers provide a bond between adhesive and face stock [214,215]. Primed face stock ensures more uniform peel values and thus better removability (Table 6.19). In order to achieve a maximum anchorage of the adhesive to the face material, a silicone release coating may be used as a primer for silicone adhesives [51]. In making PSA tapes, polyacrylamide may be used as the binder between the hydrophilic cellulose and the hydrophilic butadienestyrene latex. The primer also improves the anchorage for natural rubberbased PSAs [216]; the primer modiﬁes the separation energy of the surface. Primers are somewhat ﬂexible and generally have high anchorage to the face material. Thus, they can absorb impact stresses without adhesion failure [200]. There are removable PSA formulations with a primer coating that contain a contact cement with the same contact cement in the PSA as well. The ﬂexible sheet label has on one side a primer coating ﬁrmly bonded to the face stock, which includes a contact cement, and above the primer coating is a peelable PSA (including the same contact cement). The contact cement serves to ﬁrmly bond the primer coating and the adhesive coating

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Table 6.19 The Influence of the Primer on the Removability of PSA Labels Removability Substrate

With primer

Without primer

Steel Copper Aluminum Glass PE HPVC PMMA PA PET Average

2 2 1 2 2 2 2 2 1 1.8

3 2 3 4 5 3 2 5 5 3.4

Removability was evaluated subjectively. The following removability indices were used: 1— very low peel, no residues; 2 — low peel, no residues; 3 — low peel, legging; 4 — medium peel, no residues; 5 — high peel, no residues.

together. Contact cements are a class of adhesives which function by exploiting the properties of certain polymers, which exhibit a high cohesive strength and a natural tendency for two surfaces of the polymer to adhere when placed into contact. Examples include polymers such as isoprene, natural rubber, polychloroprene, and one class of acrylic polymers. A low Tg is a property common to polymers which are used in contact cements. The stress absorption capacity is a function of the primer’s own softness. As shown in Table 6.20, adhesives with a diﬀerent ‘‘hardness’’ (peel) act as a primer coating in diﬀerent manners. The soft (removable) ones do not cause any peel decrease; the peel adhesion decrease is proportional to primer stiﬀness. In these experiments diﬀerent adhesives were used as the primer (ﬁrst) coating. The second layer (i.e., the PSA contacting the substrate) has always been the same (a permanent, tacky PSA). The necessity of stress absorption (i.e., energy absorption) is illustrated by the data of Table 6.21. It can be seen from Table 6.21 that the peel value of the removable adhesive layer increases with the coating weight of the primer coating. A primer coating weight equivalent to 5–10% of the permanent adhesive weight is able to change the peel of removable PSAs. The peel force will be inﬂuenced by the material (PSA) in the noncontact area as well [17]. When the elastomer is more viscous, the strength will be less aﬀected by ﬂaws (noncontacted zones) compared to an elastomer

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303

Table 6.20 The Influence of the Softness of the Primer on the Peel Value/Removability of PSA Primer

Peel (N/25 mm)

Soft hot-melt PSA Removable rubber-resin solvent-based PSA Hard hot-melt PSA Permanent rubber-resin solvent-based PSA None

20.0 23.0 24.0 24.0 PT 24.0 PT

A tackified water-based acrylic PSA was used.

Table 6.21 Influence of the Primer Structure/ Geometry on the Peel Value/Removability Removability Relative thickness of the layer (%) Substrate

100

75

50

0

Steel Copper Aluminum Glass PE PMMA PET Average

2 2C 1 2 2 2 2 1.85

1 1 2 1 2 1 1 1.3

1 1 2 1 2 1 1 1.3

3 2 2 3 5 2 5 3.1

Removability was evaluated subjectively. The following removability indices were used: 1 — very low peel, no residues; 2 — low peel, no residues; 3 — low peel, legging; 4 — medium peel, no residues; 5 — high peel, no residues; C — cohesive failure.

that responds more elastically. As discussed above, primers may improve stress distribution. Another stress-reducing possibility is the choice of the adequate polymer (PSA) formulation or the use of adequate ﬁllers [200]. The primer coat typically is present on the coated face stock in an amount ranging from 2–20 and more often 5–10 g/m2. The lower limit is determined by the requirement to ensure complete coverage of the coated face material, with a good coating pattern, and the upper limit is determined

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primarily by costs as there appears to be no signiﬁcant beneﬁt in increasing the coating weight of the primer beyond the limit indicated; generally 2–5 g/m2 of primer is used [86]. For tapes with an adhesive layer of 16–20 m, the thickness of the primer coating should be 1–1.5 m [217]. The thickness of the primer (manufactured via an electron beam) from a monomer blend should be less than 5 m, preferably less than 1 m, and ideally less than 0.5 m. Indeed it is believed that thicknesses approaching a single molecular layer would function eﬃciently [218]. The use of polyaziridine as a primer shows the eﬀect of the crosslinking on the removability of PSAs. Peel adhesion of the crosslinked PSA (after 24 hr) is 0.51 kg/cm versus 0.67 kg/cm for the uncrosslinked one. The crosslinking of PSAs shows similarities with the use of lightly crosslinked rubbers in thermoplastics for controlled soft-phase dispersions. Readhering adhesives permit removal of the coated face material and allow readherence to another surface. Paper substrates which were precoated with removable, readhering adhesives are commercially available. One commonly known product of this type is marketed under the trade name ‘‘Post It’’ as note pads [219]. In order to achieve a good removability, many diﬀerent procedures are used, including the use of hard (Tg > 10 C) suspension particles, crosslinking the matrix, and low coating weights (7 g/m2) [199]. Stress-Resistant Polymers. Stress-resistant polymers are those which develop a controlled crosslink density or which are internally soft. This means that crosslinking agents or plasticizers may improve the removability. Like craze deformation for plastic-elastomeric blends, PSAs are subject to energy dissipation via deformation work; thus shear deformation is necessary [200]. Flexibilizers. Polymers exhibit two kinds of fracture mechanisms: ductile and brittle. Ductile fracture involves overall yielding of the specimen and since the volume of yielded material is large, this failure mechanism absorbs a great deal of energy; however, it is slower than brittle fracture. Removable PSAs undergo ductile fracture in that they absorb all the break energy. A number of superimposed adhesive layers having diﬀerent gradients of shear/creep compliance can meet the requirement of releasable adhesion to plastic surfaces such as polyethylene. Flexibilizers are plasticizer compounds which react with the polymer; because of their volume they are able to keep the chain segments apart. These plasticizers increase tack, but lower cohesion [200]. Adding a 15% butyl dispersion to a water-based PSA formulation may improve its removability properties [220]. Flexibilizers, like

Adhesive Performance Characteristics

305

plasticizers introduce creep, which is a function of molecular weight and plasticizer loading, or Creep ¼ f ½ðPlasticizerÞ, ðMolecular weightÞ1

ð6:71Þ

A high molecular weight and a low plasticizer level reduce creep. Plasticizers may be incorporated in the peelable PSAs to soften the adhesive and thus improve peelability. Care should be exercised as some plasticizers can have a tackifying eﬀect on adhesive polymers and this may limit the amount to be used; however, it is possible to use quite large quantities of plasticizers, even up to 50 wt% on the solid adhesive, but more often they are used at 10–20 wt%. Generally, plasticizers impart a fast tack increase, a slower peel increase, and very fast decrease of the cohesive strength. Fillers. Stress reduction by ﬁllers (i.e., reducing peel adhesion) is well known in the tape industry (e.g., 0.5–10% ﬁller, particle size 150 m, in rubber-based PSAs) [221]. Peel adhesion decreases in formulations containing ﬁllers [106]. Rubber dispersions also may be used as ﬁllers for water-based PSAs [220]. Strengthening of the Face Stock. Paper face materials can lead to paper tear, when peeling oﬀ the label. Reinforcing the ﬁber structure of the paper face stock with a primer can strengthen the surface layer in order to avoid ﬁber tear. High strength face stock materials (e.g., rubber lateximpregnated papers) are often used. In some cases the primer strengthens the face stock material [216]. Reduction of the Contact Surface. Peel adhesion depends on the surface area between the label and the adherent. Reducing the contact surface between the label and the adherent improves peelability or removability. Peel resistance and removability can be regulated by modifying the ratio between the adhesion surface and the application surface [222]. There are diﬀerent methods to reduce the contact area between the adhesive and the substrate, including: reducing the coating weight of the adhesive, increasing the stiﬀness of the face stock (or substrate), applying a discontinuous coating on the face stock material, and removing contact points or areas between adhesive and substrate. Discontinuous adhesive coatings (i.e., ungummed areas) reduce the contact area since in some cases, it suﬃces to coat only a minor portion of the face stock surface with adhesive [223–226]; a number of adhesive and nonadhesive zones may be used [219,227]. There are several patents concerning the use of ﬁlled PSAs that decrease the adhesive/adherent

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contact area. Some use glass beads as ﬁller [228]; microspheres also may be used [229]. The reduction of the contact surface may be achieved by overlapping the contact surface of the PSAs with powdery materials [230]. As an example, adhesive tapes applied for sealing windows and bonding side moldings onto cars use UV-cured acrylic adhesives containing 2–15 parts per hundred (phr) hydrophilic silica-glass microspheres [231]. Adding an expandable ﬁller to the formulation reduces the peel resistance. Regulation of the contact surface can be achieved also by crosslinking the whole adhesive surface and making it suﬃciently fragile to allow crack migration of the uncrosslinked adhesive. Coating Weight Adjustment. As far as the PSA component is concerned the properties which contribute to peelability are principally a limited tack, a low build-up factor (lack of age hardening), and a relatively soft adhesive. These properties can be achieved conventionally, by omitting or only using a low level of tackiﬁer, or by including tack deadeners, such as waxes, and by including plasticizers. The most important parameter is the adjustment of the coating weight. A way to reduce the peel (i.e., to get a removable PSA) is the reduction of the coating weight. Reducing the coating weight from 20–25 to 15–10 g/m2 allows removability for PSA formulations that are based on acrylic, vinyl ether, and plasticizer. For PSAs coated on paper the coating weight for repositionable labels may be as low as 7 g/m2. In this case an initial peel value of 110 g/12 mm (180 peel at 300 mm/min) is enough to resist a vertical application (5 hr debonding method) [199]. Shear Strength for Removable Adhesives. Permanent adhesives generally possess a higher cohesive and shear strength than removable ones. On the other hand, several high-shear adhesives used for tapes also are proposed for removable labels (e.g., Ashland, Aroset 2530-W-50). For removable adhesives a shear value of 50–1000 min at 70 C or 50–1000 min at 175 F is suggested [232]. The minimum level of shear for a removable PSA also depends on the coating weight. From a good removable PSA no adhesive transfer can be accepted. Problems Concerning Removability. The most frequently encountered problems for removable PSAs are too high peel forces, the change of the peel force in time (build-up), adhesive residue left on the substrate, low tack, and pronounced bleeding, smearing, and migration. It is diﬃcult to ﬁnd removable labels which can be removed without leaving any adhesive residues behind. Generally, the migration or bleedthrough of removable PSAs is more pronounced than for permanent PSAs. This will be discussed in more detail in Chap. 8.

Adhesive Performance Characteristics

307

Peel Adhesion Values Improved peel adhesion requires a higher tackiﬁer loading than tack. Higher melting point resins are suggested. Permanent and removable labels/tapes, labels for hand-applied labeling or gun labeling, labels for room temperature or high temperature use, labels for use on plastics or metals, etc., have quite diﬀerent peel adhesion values. Table 6.22 summarizes some of the performance characteristics speciﬁed by manufacturers of PSA laminates. Detailed lists with peel values for various pressure-sensitive products are given in [71,176,233]. 1.3

Shear Resistance (Cohesion)

Pressure-sensitive adhesives possess typical viscoelastic properties which allow them to respond to both a bonding and a debonding step. For permanent adhesives the most important step is the debonding one; the adhesive should not break under debonding (mainly shear and peel) forces (i.e., permanent adhesives must provide a higher level of cohesive or shear strength than removable adhesives). This is an inverse requirement with respect to Dahlquist’s criterion (see Section 2.3 of Chap. 3) for a minimum value of the compressive creep compliance to achieve tack. Creep (ﬂuidity under low forces) results in edge bleeding, migration, and poor diecuttability; high resistance to ﬂuidity allows low instantaneous ﬂow (i.e., low tack). The cohesion (shear) is important as an index for label-processing characteristics (bleeding, migration, die-cutting) and also end-use properties. Shear resistance is measured as a force required to pull the pressuresensitive material parallel to the surface to which it was aﬃxed with a Table 6.22

Typical Peel Values 180 Peel on Steel

Adhesive Nature Chemical Physical

End-use of the laminate

Acrylic Acrylic — SIS SBS — — Acrylic Acrylic

Label 90–105 oz/in Transfer tape >7 lb/in Highly aggressive label/tape 60–100 N/100 mm Packaging tape 10–18 N/25 mm Double-sided carpet tape 8.5 N/cm Permanent label 69–77 oz/in Removable label 11–14 oz/in Application tape/ﬁlm 1.3–5.0 N/25 mm Protective ﬁlm 100–200 cN/cm

Hot-melt PSA Solvent-based — Hot-melt PSA Hot-melt PSA Hot-melt PSA Hot-melt PSA Water-based Water-based

Value

Units

Ref. 46 234 235 236 237 238 239 240 241

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deﬁnite pressure. Practically one measures the holding time under standard conditions. Unfortunately there are no standard values of the time until shear failure occurs. Measurement of the Shear Resistance Test conditions inﬂuence the shear resistance also. Holding power is basically a viscosity eﬀect. It was found that the shift factors used to plot shear measurements at several temperatures on a master curve were almost identical to the shift factors used for G0 and G00 master curves. The shear resistance of PSAs may be measured statically or dynamically. Currently, static shear test methods use a constant load at longer test times; they generally show poor reproducibility and need very long measurement times. Better results are obtained with the hot shear test, where the cohesion of the sample is measured at an elevated temperature. Dynamic shear tests measure the cohesion of the sample in a tensile tester under increasing load (force). It has previously been indicated, that molecular disentanglement can take place at a testing rate of 0.0100 /0.25 mm per minute [96]. A dynamic shear test can be set up on an adhesion tester at this slow speed. While the width of the sample can be of any standard width, the height of the sample needs to be limited, so that it does not become a tensile test of the backing. If a hot shear test is carried out in such a way that the test temperature is gradually raised, and if the temperature at which the bond fails is taken as a characteristic value, specialists speak about a dead load hot strength test or shear adhesion failure temperature (SAFT). There also are shear test methods that measure neither the time until debonding nor the temperature of the debonding but rather the deformation of the sheared sample (i.e., the slip of the sample after a given time is measured) [234]. A combined peel and shear strength of the adhesive can be tested (e.g., 20 hold strength of the adhesive to corrugated board). The important shear properties of adhesives are the shear strength or the shear stress at failure, the shear modulus or the shear stress/shear strain ratio, the creep modulus or the shear stress/shear strain ratio at time t, the shear stress or the load/ bond area ratio, and the shear strain or the shear slip/adhesive thickness ratio [241]. A detailed description of the shear measuring systems is included in Chap. 10. Here the importance of hot shear measurements only will be highlighted. They exhibit a lower discrepancy of the test values and allow extrapolation of the data for long end-use periods. Room temperature shear values display variations ranging from 100–1000%. Hot shear values normally do not vary more than 100–200%. On the other hand, better die-cuttability requires hot shear values to be higher by an order of magnitude (1000%); hence, a real improvement of

Adhesive Performance Characteristics

309

Figure 6.16 Interdependence: room temperature shear/hot shear. 1) Test using heated panel; 2) test using climatized atmosphere.

the cohesion can only be evaluated via hot shear measurements. Figure 6.16 illustrates the measuring errors for the most commonly used hot shear test methods (relative to room temperature shear tests); the preheated plate method is better. Note that the value of shear or hot shear alone is not important, but rather it is the interdependence between them. At times the hot shear values may be converted into room temperature data. Using the Williams–Landel–Ferry time/temperature superposition shift factors developed for SIS triblocks the 80 C data may be extrapolated to yield a shear holding time at 25 C of 387 days for a pure triblock versus 86 days for a commercial product. The cohesion is a real measure of the internal structural resistance of the polymer. Generally, the mechanical properties of a polymer depend on its cohesion. Diﬀerent methods exist to determine the mechanical resistance of a polymer, such as the measurement of the tensile strength, elongation at break, etc. Generally, adhesives are used as thin layers, where the adherents undergo shear deformation during debonding and delamination. Therefore, the measurement of the shear was accepted as a criterion for the cohesion. Cohesion also can be measured by the Williams plasticity [242]. A measure of the Williams plasticity is given by the thickness of an adhesive

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pellet (mm) after 14 min compression at a ﬁxed temperature and under a ﬁxed load. Peel adhesion measurements also provide information about the cohesion. Shear may be also measured as tension. For polymers obeying Hooke’s and Newton’s laws, shear is directly proportional to tensile strength [243]. The evaluation of the viscosity is an index for holding power, telescoping, oozing, and die-cutting properties [244]. Interdependence of Adhesive/Cohesive Properties. Generally, the practical use of PSA labels requires an adequate adhesion/cohesion balance, that is, a certain aggressiveness of the adhesive, characterized by instant tack, a time-dependent ﬁnal adhesion (bonding strength) characterized by peel and shear, according to the method of debonding (i.e., the end-use stresses). The fast, instantaneous ﬂow required for high tack does not allow less ﬂow and lower deformation when debonding (i.e., shear is inversely proportional to tack and peel is partially so). On the other hand, the adhesion/cohesion equation is more sophisticated. A good peel adhesion supposes a certain ﬂow and a low to medium cohesion, but a high peel needs high cohesion when debonding. There is an internal relationship between peel adhesion and shear strength (Fig. 6.17). For PSA speciﬁcation and current production control purposes both values should be measured. For

Figure 6.17

Interdependence: hot shear/peel. Tackiﬁed water-based acrylic PSA.

Adhesive Performance Characteristics

311

Acronal V 205 minimum and maximum hot shear values (70 C, coated on PET) are 25 min and 100 min, respectively, which correlates with a peel from steel of 6–8 N/25 mm. The adhesion-cohesion balance and its regulation for various pressure-sensitive products is discussed in detail in [232]. Factors Influencing Shear Resistance Like tack and peel adhesion, shear is inﬂuenced by the characteristics of the adhesive and of the laminate, as well as by the debonding conditions. Nature of the Adhesive. Special built-in functional groups, crosslinking, and high molecular weight can ensure high shear and low tack properties. The use of harder monomers also improves the cohesion. For example, a higher acrylic acid level in solvent-based acrylic PSAs yields higher shear values [245]. Cohesive strength is independently controlled by the hard monomer content and increases as the concentration of this hard monomer increases [2]. Cohesive properties of a polymer can be aﬀected by introducing polar groups that interact by forming secondary bonds; such interactions are classiﬁed as hydrogen, dipole-dipole, and dipole-induced bonding. Monomer-reinforced copolymers are suitable as PSAs, since they possess high shear strength [246]. Minor amounts of unsaturated oleﬁn carboxylic acids and/or sulfoalkyl esters of such carboxylic acids improve the cohesive strength of PSAs; it is preferred that the polymer contains at least about 0.1 wt%, usually about 0.1–10 wt% of these components. The higher the percentage of iso-octyl acrylate, the tackier the adhesive; conversely, the higher the percentage of acrylic acid, the higher the shear properties. Permanent adhesives generally provide a higher level of cohesion shear strength than removable adhesives. Rubber-resin adhesives ensure a compromise between high cohesive strength for conformance to curved containers and good quick stick required in automatic labeling [247]. Acrylic adhesives provide a high level of adhesion to surfaces that are diﬃcult to adhere to. However, most acrylic-based adhesives do not have a cohesive strength equal to normal rubber-resin systems and may display cold ﬂow. Showing up as a halo of adhesive around the die-cut label, cold ﬂow can inhibit eﬃcient labeling. All adhesives used in automatic labeling systems require a level of quick stick that will provide for eﬃcient adhesion with a minimal application pressure. It is the general ﬁnding of the industry that for the same tack and peel values acrylic emulsion PSAs show much lower shear holding power compared to that of their solvent-based counterparts [248]. The diﬀerence in shear holding power between emulsion vs. solvent-based PSAs is about 1–2 orders of magnitude, depending on the monomeric composition used.

312

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Tobing and Klein [248] have shown that for gel free low-Tg acrylics (based on 2 EHA or BuAc) there was no signiﬁcant diﬀerence between emulsion vs. solvent-borne PSA ﬁlm adhesive performances. However, commercial products have a high level of gel and low shear. The four-fold decrease of shear by tackifying was attributed to the lack of entanglement between the linear polymer chain and the microgels. In a novel acrylic emulsion 0.5–1.0 wt% IBMA (isobutoxymethylacrylamide) was copolymerized to provide interlinking of the microgels in the ﬁlm upon heating. Shear Dependence on Coating Weight. Shear depends on the coating weight. A suitable test for the measurement of shear properties of an adhesive is the thick (6 mm) adherend shear test (TAST) since it is conducted using a tensile testing machine. However, ﬁnite element analysis shows that the stress distribution in the thick adherend shear test is not pure shear. The values of maximum shear strain gained from the thick adherend shear test are not comparable to those obtained from tests in pure shear [249]. Shear Dependence on Molecular Weight. Cohesion increases with the molecular weight. The ultimate tensile strength of a polymer reaches a maximum at about 500–1000 main chain atoms. Longer molecules do not impart higher strength because their various segments act independently of each other. The inﬂuence of the molecular weight on the shear resistance is illustrated by the changes of the shear resistance due to adhesive aging (i.e., destruction and depolymerization). SIS block copolymers undergo oxidative degradation at elevated temperatures by a mechanism which leads predominantly to scission of the polymer chains. This leads to a fall in molecular weight and a resulting decrease in viscosity and holding power [250]. The composition of the adhesive (i.e., its nature and molecular weight) inﬂuences the shear; shear measured as SAFT is directly dependent on the softening point (i.e., the molecular weight of the resin) [251]. The dependence of the shear on the molecular weight is illustrated by the manufacture of hotmelt PSAs. When high shear mixers are used the degradation depends on the mixing environment. In contact with open air, degradation occurs rapidly and polymer breakdown is observed. A dramatic reduction in melt viscosity and holding power results from even immediate degradation [252]. Polymer viscosity and shear are interdependent [253]. The latter depends on the molecular weight and its distribution. A broad MWD polymer may have a lower cohesive strength than one with a narrow MWD and lower molecular weight. The dependence (increase) of the cohesive strength as a function of the molecular weight was described by Schrijver [254]. For HMPSA, the eﬀect of MW and aromatic content of the resin on the shear resistance was observed. The number average molecular weight showed a strong correlation

Adhesive Performance Characteristics

313

with shear adhesion values [255]. The holding power has been shown [256] to be controlled by zero shear viscosity which is related to Mw, Me, and Tg by the following equation: Log o ¼3:4 log Mw 2:4 log Me þ A=ðT Tg þ 70Þ þ B

ð6:72Þ

where A and B are constants. This equation shows that while o is reduced by Me it is also increased by Tg of the polymer. Copolymers containing vinyl neoester units exhibit higher Me and Tg, thus one can expect that peel, tack, and holding power can all be improved. As illustrated in [67] shear for EHAbased copolymer was 3.9–1.6 hrs (100 0.500 1 kg); the neo-based polymer gave shear values of 16–21 hr. The shear resistance is improved by the increase of the melting point of the resin [47]. Gordon et al. [257], using diﬀerent Escorez (manufactured by Exxon Chemical) resins with diﬀerent softening points, list the SAFT data of 60/40 adhesive formulas. The SAFT improves with increasing softening point of the resin. In a similar manner shear increases with the increase of the ring-and-ball softening point of Bevitack (Bergvik-Arizona) resins used as tackiﬁer for hot-melt PSAs [258]. For tackiﬁcation of Neoprene Latex 102, the 32 C holding power increases if the 49 C softening point (SP) resin is replaced by a 72 C SP resin. Table 6.23 illustrates the increase of the shear resistance with the increase of the molecular weight. Influence of Crosslinking on the Shear. Crosslinking aﬀects polymer properties in a manner similar to molecular weight increases but in a more pronounced way [50]. Crosslinking imparts high shear and lower tack [83]. The phenomenon is well known from the ﬁeld of plastics where shear is proportional with the degree of crosslinking for polyethylene (i.e., the peroxide concentration) [259]. Similarly, increasing the amount of the resin in a silicone adhesive increases its crosslink density and results in greater

Table 6.23

Shear Dependence on the Molecular Weight Shear [min at temperature ( C)]

Molecular Weight of the Base Elastomer 50 60 70 1 M1 1.5 M1 3 M1

50

60

70

20 420 > 1200

2 120 > 1200

1 60 780

A CSBR was used as base elastomer.

314

Chapter 6

cohesive strength and higher peel adhesion values [260]. Physical crosslinking has the same inﬂuence as chemical crosslinking. The beneﬁcial inﬂuence of a 100% pure triblock in hot-melt PSA formulations is demonstrated in creep testing. Here the PSA is subjected to a dead load shear, and if every molecule of the rubber is able to fully participate in the associative crosslinking shear, bond holding times are excellent. Reactive resins with methylol groups in the ﬂuid state are eﬃcient tackiﬁers but also crosslink with the neoprene rubber used as elastomer, and lead to high strength bonds [45]. Aqueous PUR dispersions were used to improve the adhesion of acrylic adhesives for better ﬁlm formation and mechanical properties [53]. The molecular interaction between acrylic and polyurethane not only inﬂuences the product stability but also governs the adhesion performance. The use of nonionic PUR yielded higher shear strength (from 28 kN/m2 to 65 kN/m2), but the use of anionic PUR resulted in very high shear strength (700 kN/m2). Anionic PUR behaved like a crosslinker. Shear resistance in acrylic formulation improved as crosslinker concentration increased. Electron beam-cured crosslinkable rubbers are used for PSAs also; the SAFT increases for these polymers with increasing gel content, but the relationship between gel content and SAFT is a gradual slope [50]. A polymer gel content greater than 0.8% is required to increase the high temperature holding power above 300 min. Crosslinking increases the modulus; therefore crosslinked adhesives have the highest shear holding power (Table 6.24). Unfortunately crosslinking is associated with the loss of tack (and peel) and thus it may not be suitable for PSA modiﬁcation, except under strictly controlled conditions (e.g., solvent-based adhesives). Ulrich [261] addressed the need to improve both the adhesion and the heat resistance at 70 C of an acrylate/acrylic acid adhesive when applied to low energy surfaces; this can be achieved by employing a tackifying resin for better

Table 6.24

Influence of the Crosslinking on the Shear

Adhesive Nature Chemical Acrylic Acrylic Acrylic Acrylic

Crosslinking Agent

Shear

Physical

Nature

Level (%)

Value

Units

Temperature ( C)

Ref.

Solvent-based Solvent-based Solvent-based Solvent-based

AZ AZ — —

0 1 — 1

0.1 1.0 1.0 5000

hr hr min min

RT RT 70 70

— — 9 9

AZ, aziridine; RT, room temperature.

Adhesive Performance Characteristics

315

adhesion and by crosslinking the tackiﬁed PSA for better heat resistance at 70 C. The new class of acrylic hot-melt PSAs that contain reactive methacrylic end groups that can be crosslinked, imparts good temperature stability [253]. Conventional natural rubber adhesives do not have the required temperature resistance for masking tape; to impart the required degree of heat resistance to the adhesive, it must be crosslinked. Shear Dependence on Composite Status. Generally, the composite structure of the PSA layer contains technological (polymerization or formulating) additives necessary for converting the liquid adhesive, and additives for improving the convertability of the laminate. Additives related to the water-based nature of the adhesive (e.g., surfactants, defoamer, and water) generally decrease the cohesion level of the adhesive. On the other hand converting additives (ﬁllers) improve the cohesion level. Thus: Cohesion ¼ f ½ðFillersÞ,ðWater, SurfactantsÞ1

ð6:73Þ

The resistance of separating liquid-bonded plates depends on the layer thickness and viscosity [see Eq. (2.14)]. Concerning the inﬂuence of the coating weight on the shear, the ﬂow (creep) limit (FL) may be formulated as a function of the adhesive modulus, the elastic work, and the dimensions of the sample. Shear (deﬁned as the limit of the slip) also depends on the layer thickness. Experimental data reveal a complex dependence of the shear on the coating weight. Above a minimum coating weight level the shear values decrease with increasing coating weight. Therefore the following dependence between coating weight and shear S can be written: S ¼ f ðCwd Þ1

ð6:74Þ

where d 6¼ 1. Since the coating weight inﬂuences the drying rate, and vice versa, therefore the shear strength depends on the drying as well (Fig. 6.18). Generally, the thickness of the adhesive layer inﬂuences the strength of the adhesive joint [133]: ¼ tan ¼ v=D

ð6:75Þ

where g is the shear resistance, is the displacement of the component of the joint, and D is the thickness of the adhesive layer. The strain rate is given by the following equation: 0 ¼ d=dtðk=DÞ

ð6:76Þ

316

Figure 6.18

Chapter 6

The dependence of the shear on the drying of the PSA coating.

On the other hand, the mechanical resistance of adhesive joints depends on the length of the overlap [262]. For adhesives obeying Newton’s and Hooke’s laws, the shear is inversely proportional to the thickness of the adhesive layer (i.e., the coating weight) [243]. Shear really depends on the adhesive’s intrinsic cohesion and on the adhesive/label geometry (i.e., shear resistance measurements should be carried out with well-deﬁned samples). Influence of Substrate on Shear. Shear depends on the substrate surface also. Therefore for packaging tapes cardboard shear (in hours) and cardboard adhesion (%FT) are tested too. The inﬂuence of the substrate on shear is illustrated in Fig. 6.19 [217]. Shear Resistance and Face Stock Material. The evaluation of the shear strength should be carried out relative to the face stock used. Unmodiﬁed PSAs are used mostly for ﬁlm coating, while tackiﬁed PSAs are used mainly for paper labels. Consequently, ﬁlm and paper laminates display diﬀerent shear levels. Shear values for the same adhesive diﬀer when coated on diﬀerent face stock materials. Common, untackiﬁed acrylic PSAs display less cohesive strength when coated on paper than on polar ﬁlms. The shear resistance of the joints depends on the surface tension of the substrate [263]. A surface treatment (i.e., an increase of the surface tension)

Adhesive Performance Characteristics

317

Figure 6.19 The inﬂuence of the substrate nature on the shear. Shear values for samples on glass, polyethylene, stainless steel, and PVC as adherend.

Table 6.25 Dependence of the Shear on the Surface Nature of the Face Stock Material SAFT Values ( C) PSA Formulation A B C D

PET

Paper

90 60 70 75

60 45 55 60

may improve shear characteristics. Table 6.25 illustrates the dependence of the shear resistance (measured as SAFT) on the face stock surface. Because of the static nature of the shear tests in general, the deformability, the elasticity of the face stock material is less important, but may nevertheless inﬂuence the test results. Therefore shear tests are carried out with nondeformable, dimensionally stable face stock materials (like paper or PET) or by reinforcing the plastic label on the back side (see Section 3.1 of Chap. 10). The applied coating technology inﬂuences the

318

Chapter 6

coating weight and smoothness of the adhesive layer, and thus the shear resistance. Improving Shear Resistance The values of the shear resistance depend on the cohesion of the adhesive and on the laminate geometry. Cohesion may be improved with hard monomers and by increasing the molecular weight (crosslinking). The laminate geometry inﬂuences the adhesive ﬂow. Upon improving the anchorage of the adhesive to the face stock the adhesive bond fails via cohesive break (i.e., within the adhesive layer). Therefore a low coating weight (an optimum as a function of the roughness of the solid state components of the label) will improve the shear resistance. Cold ﬂow may be limited by use of ﬁllers; in some cases the use of reactive ﬁllers improves the shear resistance also. The homogeneity of the adhesive strongly inﬂuences the resulting shear strength (see Chap. 8). Shear Values There are many data available about shear strength. Unfortunately most of them are static shear values at room temperature. For these the method used (i.e., the sample, dimensions, weight, adherent) inﬂuences the measured absolute values. On the other hand, and in a quite diﬀerent manner from tack and peel values, shear test results for PSAs with diﬀerent chemical bases do not allow a real comparison of their cohesion-related properties (e.g., cuttability); they are used more as an internal check of the adhesion/ cohesion balance for small changes in a given formulation. Test methods and test conditions strongly inﬂuence the measurements. For instance the SIS block copolymers have such high room temperature shear values, that only highly plasticized formulations can be evaluated in a convenient period of time [255]. For SIS copolymers the shear properties were measured in the linear viscoelastic regime (small strain) on a parallel plate rheometer. Diﬀerences due to the change of the SI content occur in the range 5 103–5 101 rad/sec, where an increase in the SI content leads to a decrease in the elastic modulus G0 and to an increase in the loss modulus G00 [18]. Shear values obtained for hot-melt PSAs are generally lower than those for solvent-based PSAs (Table 6.26). Dispersion-based PSAs may exhibit lower or higher shear values as a function of the formulation. High shear, high molecular weight solvent-based acrylic PSAs are superior in shear to rubber-resin-based PSAs; CSBR exhibits higher shear than common acrylic PSAs. Crosslinking improves the cohesion even more.

Chemical Basis

Typical Shear Values

— — — — — — — — — — — — — — x x

—

x x x — —

x x x x — x x x x —

— — — — x — — — — —

— — — x x

x

Styrene Acrylic Rubber-resin rubber-resin

PSA

Table 6.26

— — x x — x x — — —

— — x — —

—

x x — — — — — — — —

x x — — —

—

— — — — x — — — — —

— — — x x

x

Waterbased Hot-melt

Physical State

Kraton GX/ Regalrez 100/150 — — Crosslinked Crosslinked — — — — Removable —

Crosslinked Self-crosslinked

Electron beamcured SIS

Note

5–10 0.5–2 > 200 20–100 100 > 16 0.7–2 83 16–83 4–5

300 > 20 260 4.6 > 5000

300

Value

min min min min hr hr hr hr hr hr

min hr hr d hr

min

PSTC 7 PSTC 7 — — l00 l00 1000 g

PSTC 7

RT — 70 — RT — 70 — 88 — RT 0.500 0.500 , 1000 g RT 0.500 0.500 , 500 g 70 1.27 2.54 cm, 1000 g 70 — RT —

RT RT RT RT RT

95

Method

Temperature Unit ( C)

o

Shear Resistance

267 267 267 267 268 46 2 269 — —

50 — 264 265 266

50

Ref.

Adhesive Performance Characteristics 319

320

2 2.1

Chapter 6

INFLUENCE OF ADHESIVE PROPERTIES ON OTHER CHARACTERISTICS OF PSAs Influence of Adhesive Properties on the Converting Properties

Wettability depends on the adhesion-cohesion balance; high shear, low tack adhesives exhibit better wettability than soft, tacky ones. Printability of the laminate depends on its stiﬀness, porosity, and surface characteristics; all these properties depend on the stiﬀness, anchorage, and migration of the adhesive (see Section 2.2 of Chap. 7 and Chap. 8).

2.2

Influence of Adhesive Properties on End-Use Properties

The dependence of the removability on the adhesive properties is well known. High peel adhesion and peel build-up do not allow removability. On the other hand, low shear may cause legging or adhesive residue on the substrate after peeling. Labeling properties depend on the label adhesion to the release liner (i.e., on the release or peel force). For gun labeling they also depend on the tack and peel. Repositionable adhesives need low initial peel adhesion, whereas tamper-proof ones require a high initial peel adhesion level. A detailed discussion of the inﬂuence of the adhesive properties on the end-use properties for the main pressure–sensitive products (e.g., labels, tapes, protective ﬁlms etc.) is given in [176].

2.3

Influence of Peel Adhesion

The inﬂuence of the peel on the end-use properties is evident. For permanent/removable or tamper-proof labels the peel value remains the main property. In a similar manner peel inﬂuences the labeling process, where the release force (peel from the release liner) has to be diﬀerent for labels not used in dispensers (where a high release force would not be of great consequence), and those used in dispensing guns (where a high peel force of the label from the release would indicate poor conversion properties, or poor labeling ability). The build-up of the peel resistance as a function of the time is very important for the design of removable (no build-up), repositionable (slow build-up), and tamper-proof (fast build-up) labels. Diﬀerent peel levels are required for labels with a multilayer construction (i.e., similar to application and masking tapes). The peel level has to be quite diﬀerent for smooth, polar surfaces (e.g., glass or steel) or rough ones (e.g., cardboard).

Adhesive Performance Characteristics

2.4

321

Influence of Shear

Low cohesion limits the applicability of the label/decal PSA grade with respect to PVC ﬁlm shrinkage both on the release liner during storage and when applied to a substrate, with regard to wing-up (ﬂagging) on curved surfaces (mandrel hold), stringing, and edge ooze during converting. Shear strength for ﬁlm laminates is important for removable adhesives in order to avoid the build-up of adhesive residues; for permanent adhesives cohesion ensures good die-cutting properties and avoids oozing and bleed through the face material.

3

COMPARISON OF PSAs ON DIFFERENT CHEMICAL BASES

It is evident that on the basis of their chemical composition and rheological behavior PSAs with diﬀerent chemical bases can display quite diﬀerent properties. Next a comparative examination of PSAs on a diﬀerent chemical basis is presented. First the classical rubber-resin solvent-based PSAs will be compared to acrylic PSAs. In a separate discussion the advantages of water-based acrylic PSAs over other water-based PSAs is examined. Although the whole range of properties is covered, it remains clear that the most important properties of a PSA label are those correlated with the adhesive characteristics. 3.1

Rubber-Based Versus Acrylic-Based PSAs

Table 6.27 summarizes the most important diﬀerences (i.e., the general adhesion properties, the special adhesive properties, and the stability) between rubber-resin or acrylic-based PSAs. As can be seen from Table 6.27, rubber-resin adhesives possess very good general adhesive properties, but remain defensive in shear. Acrylic PSAs display better speciﬁc adhesion properties. Generally, the advantages of acrylic systems include chemical and water resistance, UV and oxidative stability, heat resistance, tack at varying temperatures, and the balance of adhesive and cohesive properties. Acrylicbased PSAs oﬀer a unique combination of performance advantages relative to hydrocarbon-based rubber-resin adhesives, and are used extensively in end-use markets that demand excellent color and clarity, weatherability, durability, and plasticizer migration resistance (i.e., overall versatility of the adhesive). Acrylic PSAs do not exhibit yellowing and display good chemical resistance [270]. Rubber-resin adhesives provide a compromise of high

322

Chapter 6

Table 6.27

Comparison of Rubber-Resin PSA Versus Acrylic PSA Chemical Basis of the PSA Acrylic

Performance Characteristics General Tack Peel Shear Speciﬁc Cuttability Resistance to migration Aging Thermal stability UV stability

Rubber/resin

Solvent-based Water-based Solvent-based

Hot-melt

Fair Fair Good

Fair Fair Good

Good Good Good

Good Good Fair

Good Good

Fair Fair

Fair Good

Insuﬃcient Insuﬃcient

Good Good

Good Good

Fair Fair

Insuﬃcient Insuﬃcient

cohesive strength for conformance to curved containers and good quick stick required in automatic labeling, but they exhibit a varying batch-to-batch consistency and can cause staining problems. Staining and softening usually result when using rubber-based PSAs on PVC substrates. Low energy face stock surfaces and soft rubber-based PSAs bond fairly well (anchorage), but they are diﬃcult to convert, whereas acrylic PSAs convert well, but exhibit low bond strength (anchorage). Properties that are correlated with the chemical instability of the rubber-resin basis (e.g., temperature and light resistance) and the pronounced liquid character of the rubber-resin mixture (e.g., cold ﬂow and shear) remain inferior for rubber-resin PSAs as compared to acrylic PSAs. 3.2

Acrylics and Other Synthetic Polymer-Based Elastomers

In order to make a competitive evaluation of the properties of acrylics used as raw materials for PSAs, the advantages and disadvantages of acrylics as well as their main technical features and adhesive properties should be examined. This study deals with the evaluation and examination of the adhesive and end-use properties of acrylic-based PSAs as compared to other PSAs. First tack, peel adhesion, cohesion, and aging resistance are used as examination criteria. The main adhesive properties (tack, peel, and cohesion) depend on the state of the polymer (pure or formulated), the coating weight, and on the face stock used.

Adhesive Performance Characteristics

323

Tack Common acrylic PSAs are not tacky enough and special acrylic PSAs are not cohesive enough. Therefore acrylic PSAs need compounding or tackifying. The tack of unformulated acrylic PSAs is lower than that of tackiﬁed rubber-based PSAs (their main competitor) but higher than that of the other untackiﬁed adhesives, except polyvinyl ethers [see Eq. (6.14)]. The better tack level of unformulated acrylic PSAs, compared to other unformulated PSAs (e.g., EVAc and CSBR) is illustrated in Table 6.28. These data show that acrylic PSAs possess a better tack at any coating weight level. In order to evaluate the tack of formulated acrylic PSAs, the following examination criteria will be considered: ease of tackiﬁcation, the tackiﬁer level, and the tack level. Ease of Tackification/Compatibility. Acrylic PSAs display good overall compatibility with common tackiﬁers (plasticizers or tackifying resins). Both solvent-based and water-based tackifying resins can be fed into water-based acrylic PSAs. It is not possible to add molten tackiﬁers to water-based acrylic PSAs. Tackifier Level. A level of 30–40% (by wet weight) tackiﬁer resin is commonly used in formulated acrylic-based PSAs and a tackiﬁer level of 10–20% suﬃces for tack improvement of acrylic-based PSAs (Table 6.29). This is very important because adhesive formulations for soft ﬁlms should contain a minimum resin level in order to avoid resin/plasticizer interaction. As discussed in detail in [271] acrylics need a lower tackiﬁer concentration for the maximum tack than CSBR or vinylacrylics and the tack maximum (as a function of the tackiﬁer concentration) is broader. Tack Level. In general, the ease of tackifying and the obtained tack level depend on the chemical composition, structure, and molecular weight of the acrylic PSAs. With respect to the chemical composition and structure of the polymer, there exists a broad range of possibilities. For solvent-based acrylic PSAs viscosity is dependent on molecular weight; thus the molecular weight constitutes a limiting factor. For water-borne acrylic PSAs there is no technological molecular weight limit, but the molecular weight should be limited because of compatibility reasons. There is an optimum molecular weight in order to achieve good tackiﬁcation. Table 6.30 illustrates the low tackifying response of high molecular weight acrylic dispersions. Acrylic-based PSAs need lower tackiﬁer levels than CSBR and EVAc copolymers. The tack level of tackiﬁed acrylic PSAs is superior

— — 22.0 27.0 — — 21.0 25.0 35.0 PT 35.0 PT

10 15 20 25 30 35 40 50 55 60

— — 20.0 20.0 — — 14.0 15.0 17.0 22.0

PE — — 1.5 1.5 — — 1.5 1.5 1.5 1.5

RB tack (cm) — 10.0 PT 10.0 PT — 14.0 PT 13.0 PT 15.0 PT — — —

Glass — 1.0 1.0 — 1.0 1.0 1.0 — — —

PE

Peel (N/25 mm)

EVAc

AC: Acronal V205 (BASF); EVA: 1360 (Hoechst); CSBR: 3703 (Polysar); RB: rolling ball.

Glass

Coating Weight (g/m2)

Peel (N/25 mm)

AC

Chemical Composition

— 18.5 17.0 — 17.0 15.0 7.0 — — —

RB tack (cm)

10.0 PT 10.0 PT 12.0 PT 16.0 PT — — 21.0 — — —

Glass

3.0 5.0 12.0 14.0 — — 14.0 — — —

PE

Peel (N/25mm)

CSBR

17.0 8.0 5.5 4.9 — — 6.0 — — —

RB tack (cm)

Table 6.28 Comparison of Acrylic PSAs Versus EVAc and CSBR PSAs. Tack and Peel of Unformulated WB PSA on Different Chemical Basis, as a Function of the Coating Weight

324 Chapter 6

Adhesive Performance Characteristics

325

Table 6.29 Tack as a Function of the Tackifier Level Tackiﬁer Level (parts per 100 PSA wet weight)

Tack (quick stick; g/cm)

20 40 60 80 100

270 570 710 800 880

Quick stick measured for tackified waterbased PSA (25 g/m2 on PET).

Table 6.30 Tack Dependence on the Molecular Weight. Influence of a High Molecular Weight Acrylic PSA on the Tack of a CSBR-Based, Tackified Water-Based PSA Formulation. Chemical Composition of the PSA (parts, wet weight) Compound code Supplier Nature

3703 Polysar CSBR

20094 Syntho Acrylic

CF 52 A&W Tackiﬁer

V205 BASF Acrylic

RB Tack (cm)

32 32 32 32

33 23 13 —

35 35 35 35

— — — 33

25.0 20.0 6.5 2.5

RB, rolling ball.

to that of tackiﬁed EVAc, but lower than the tack of tackiﬁed CSBR, namely: Tack : CSBRðformulatedÞ > ACðformulatedÞ > EVAc ðformulatedÞ ð6:77Þ as far as the tack properties are concerned. As discussed in Chap. 2, adhesive properties also depend on the face stock material. A short examination of the performance of acrylic-based PSAs in comparison with other adhesives coated on paper and ﬁlm, follows next. Permanent/Removable Paper Label Applications. Generally, removable adhesives exhibit low tack. Common acrylic-based removable PSAs are

326

Chapter 6

untackiﬁed formulations. Rubber-based, solvent-based, or water-based removable formulations, generally are tackiﬁed and thus they are tackier than acrylic-based ones (Table 6.31). On the other hand, the removability assumes a primer coating and/or a low coating weight. Film Application. SBR has low light and aging stability and EVAc has a limited water resistance. These disadvantages make them inadequate for ﬁlm coating applications. On the other hand, both need tack improvement (i.e., a tackiﬁer). The use of soft PVC as the main face stock material implies no or a low tackiﬁer level. Thus, only acrylic-based PSAs meet the whole range of quality requirements for foil coating; they show the best unformulated tack, another important feature for ﬁlm-coating adhesives (Table 6.32). Adhesion to Polar/Nonpolar Surfaces. Acrylic-based PSAs are the best unformulated materials in the whole range of required peel forces for polar substrates. Unfortunately, the peel of untackiﬁed acrylic PSAs on polar surfaces is not suﬃcient (Table 6.33). The peel of other PSAs on nonpolar substrates is low too, except for the new class of ethylene-vinyl acetate-maleinate and ethylene-vinyl acetate-acrylic copolymers [272]. Peel of Tackified Acrylic PSAs Tackiﬁcation improves adhesion to polar and nonpolar surfaces. Polymers like SBR and EVAc need more tackiﬁer than acrylic PSAs (Table 6.34). A detailed discussion about tack and required tackiﬁer level is included in Chap. 8. Formulations with less than 30 wt% tackiﬁer do not provide enough peel on polyethylene for common acrylic formulations (Table 6.35). On the other hand, there are some new acrylic PSAs which display adequate peel adhesion on polyethylene at low (20 wt%) tackiﬁer loading levels (Table 6.36).

Table 6.31 Rolling Ball Tack (cm) of Rubber-Resin-Based Formulation Versus Acrylic PSA. Dependence of the Tack on the Coating Weight Chemical Composition of PSA Coating Weight (g/m2) 8.6 18.4 28.0

Solvent-based rubber/resin

Water-based tackiﬁed acrylic

4.0 2.1 1.9

6.4 2.4 1.9

Adhesive Performance Characteristics Table 6.32

327

PSAs used for Film Coating

Chemical Composition

Adhesive Characteristics

Basis

Code

Supplier

Parts (wet)

Acrylic

V205 V205 V208 PC80 FC88 ECR555 FC88 PC80 FC88 880D V205 20084 FC88 2313 1360 1360 1370 913 1360 FC88 V205 1360 EAF60 VU895 Nacor 90

BASF BASF BASF UCB UCB Exxon UCB UCB UCB BASF BASF Syntho UCB Nobel Hoechst Hoechst Hoechst UCB Hoechst UCB BASF Hoechst Wacker Wacker National

100 70 30 100 100 100 80 20 70 30 80 20 70 30 100 70 30 50 50 30 30 30 100 100 100

EVAc

Acrylic-EVAc blend

EVAc-acrylic EVAc-maleinate

Table 6.33

Peel (N/25 mm)

RB tack (cm)

0 15.0

5.0 2.5

6.0 8.0 5.0 —

3.5 3.5 1.0 3.5

10.0

6.5

11.0

4.5

11.0

4.5

4.0 —

6.0 3.0

—

7.0

10.0

3.5

— 11.0 7.0

10.0 2.5 8.5

Typical Peel Values of Untackified PSA on Polyethylene

Code

Supplier

Nature

Peel (N/25 mm)

V205 80D 85D 1360 EAF60 3703 20094

BASF BASF BASF Hoechst Wacker Polysar Syntho

Acrylic Acrylic Acrylic EVAc EVAc CSBR Acrylic

20.0 3.8 10.3 5.0 7.0 7.0 0.2

Instantaneous peel was measured; coating weight 20 g/m2.

328

Chapter 6

Table 6.34 Required Tackifier Level for AC PSA Versus CSBR PSA. The Dependence of the Peel and Tack on the Tackifier Level AC Formulation Tackiﬁer Level (parts/100 wet weight) 20 33 43 50

Peel (N/25 mm) Glass 20.0 22.0 22.0 22.0

PT PT PT PT

CSBR Formulation

PE

RB tack (cm)

15.0 17.0 20.0 PT 19.0

2.5 2.5 2.0 1.5

Peel (N/25 mm) PE

RB tack (cm)

10.0 11.0 16.0 PT 14.0 PT

3.0 2.5 1.5 1.5

Glass 14.0 15.0 17.0 19.0

PT PT PT PT

AC: an 80/20 wet weight parts blend of Acronal V205/80D; CSBR: 3703 (Polysar); tackifier: CF52(A&W); PT: paper tear; RB: rolling ball.

Table 6.35 Required Tackifier Level for AC PSA Formulations. Peel Dependence on the Tackifier Level (Peel Adhesion on PE Film and Plate) Peel (N/25 mm) PE ﬁlm (coating weight, g/m2)

Tackiﬁer Level (% wet weight) 16 20 24 29 33

Table 6.36

PE plate (coating weight, g/m2)

10

20

10

20

6.0 15.0 20.0 17.0 15.0

2.0 5.0 5.0 8.0 8.0

23.0 20.0 15.0 20.0 PT 21.0 PT

6.0 7.0 5.0 5.0 20.0 PT

Peel on Polyethylene for Low Level Tackified Special WB PSA Chemical Composition, Parts (wet weight)

Code Supplier

5650 Hoechst

ECR-576 Exxon

Exoryl 2001 Exxon

SE-351 A&W

Peel on PE

Nature

AC 100 100 — — — —

AC — — 100 100 — —

AC — — — — 100 100

Tackiﬁer — 20 — 20 — 20

(N/25mm) 16.5 PT 19.5 PT 17.5 PT 18.0 PT 5.2 15.4

Adhesive Performance Characteristics

329

Proposals from tackiﬁer suppliers include the use of 25–50% tackiﬁer resin for aggressive, soft, acrylic dispersions on an ethyl hexyl acrylate basis (Fig. 6.20). Other base elastomers require higher tackiﬁer levels. Figure 6.21 and Table 6.37 illustrate the low tackifying response of CSBR-based PSAs as compared to acrylic-based PSAs. For common waterbased PSA formulations with standard adhesive performance characteristics, EVAc and SBR require higher tackiﬁer loadings than acrylic PSAs (Fig. 6.21). In a similar manner tackiﬁed acrylic PSAs show a more pronounced response to tackiﬁcation than ethylene vinyl acetate copolymers (Table 6.38). Generally, higher peel adhesion is obtained with less tackiﬁer for acrylic PSAs; the absolute value (level) of the achieved peel also is higher (Table 6.39). One can conclude that the peel of tackiﬁed acrylic PSAs on polar or nonpolar surfaces is superior: Peel : AC ðTackifiedÞ > CSBR ðTackifiedÞ >> EVAc ðTackifiedÞ ð6:78Þ

Figure 6.20 Suggested tackiﬁer level for PSA formulation on diﬀerent chemical bases. 1) Acrylic; 2) vinylacrylic; 3) SIS; 4) SIS; 5) SIS; 6) acrylic; 7) SBR; 8) vinylacrylic; 9) EVAc.

330

Chapter 6

Figure 6.21 Ternary composition diagram. Common PSA formulation for 1) CSBR, 2) EVAc, and acrylic formulation. Water-based tackiﬁed PSA.

Table 6.37

Tackifying Response of Water-Based CSBR PSA Formulations

Chemical Composition, Parts (wet weight)

Adhesive Characteristics Peel (N/25 mm)

PSA 3703 Polysar 50 57 67 80

Tackiﬁer CF 52 (A&W)

On glass

50 43 33 20

19.0 PT 17.0 PT 15.0 PT 14.0

On PE 15.0 PT 16.0 PT 11.0 10.0

RB tack (cm) 1.5 1.5 2.5 3.0

Cohesion of Acrylic-Based PSAs Except for some special adhesives with improved low temperature (and room temperature) ﬂuidity (e.g., deep-freeze adhesives), common unformulated acrylic PSAs possess enough intrinsic shear strength to be converted and processed without problems. Other polymers (e.g., EVAc, CSBR) have better cohesion (measured as shear) than acrylic PSAs, but their

Adhesive Performance Characteristics Table 6.38

331

Tackifying Response of Water-Borne EVA-Based PSA Formulations

Chemical Composition, Parts (wet weight) Code Supplier

1360 Hoechst 50 50 70 50 50 50

1370 Hoechst 50 30 50 50 30 30

Adhesive Properties CF52 A&W

Peel (N/25mm)

RB tack (cm)

100 100 100 70 70 50

7.0 3.0 15.0 PT 6.0 3.0 8.0

>30 >30 20 >30 >30 >30

Table 6.39 Peel/Tackifier Level Correlation for Water-Based PSAs on Different Chemical Bases PSA Code

Supplier

Nature

Level

3703 3703 3703 Nacor 90 Nacor 90 Nacor 90 FC88 FC88 FC88 V205 80D V205 80D V205 80D 3703 80D 3703 80D 3703 80D 1360 1360 1360

Polysar Polysar Polysar National National National UCB UCB UCB BASF BASF BASF BASF BASF BASF Polysar BASF Polysar BASF Polysar BASF HOE HOE HOE

CSBR CSBR CSBR EVAcM EVAcM EVAcM AC AC AC AC AC AC AC AC AC CSBR AC CSBR AC CSBR AC EVAc EVAc EVAc

50 66 77 50 66 77 50 66 77 40 10 49.5 16.5 57.75 19.25 25 25 35 35 38.5 38.5 50 66 77

Tackiﬁer Level

Peel (N/25 mm)

50 33 23 50 33 23 50 33 23 50

19.0 15.0 14.0 20.0 20.0 18.0 16.0 8.5 7.0 19.0

33

10.5

23

9.0

PT PT PT PT PT PT

50

27.0 PT

33

25.0 PT

23

23.0 PT

50 33 23

15.0 12.0 6.5

The concentration (level) of the components is given in parts wet weight. Tackifier: CF52 from A&W.

332

Chapter 6

shear-correlated properties like cuttability are not superior. As an example for the upper limit, crosslinked, solvent-based acrylic PSAs exhibit shear levels in excess of 260 hr [150]. On the other hand, soft, high tack acrylic PSAs show 15 min shear times. Table 6.40 summarizes characteristic shear values for PSAs on diﬀerent chemical bases; it shows that acrylic PSAs display the best shear values in the range of unformulated polymers with similar tack. This statement is very important for ﬁlm label applications, where new ethylene-maleinate multipolymers display very good nonpolar peel adhesion, but low cohesion. Shear Strength of Tackified Acrylic PSAs Common tackiﬁer levels lead to a decrease in cohesion. This is a function of the nature of the acrylic, the nature of the tackiﬁer, and the mixing ratio. Shear loss is less pronounced for CSBR and EVA. Table 6.41 illustrates the variation of the shear through tackiﬁcation for diﬀerent classes of raw materials. Unfortunately higher shear does not lead to better converting properties for CSBR/EVAc. Aging Resistance For common PSAs generally, time and temperature stability of the adhesive/laminate at room temperature is required. There are special enduses where high temperature stability or low temperature performance is necessary. Table 6.42 summarizes data concerning UV and temperature stability of diﬀerent PSAs. Because of their bonding ability (accentuated ﬂow) common acrylic PSAs show only a limited temperature stability in PSA labels. High molecular weight or partially crosslinked acrylic PSAs display an improved thermal stability; partial crosslinking may be obtained with special built-in monomers (e.g., acrylonitrile). Therefore some common ethyl hexyl-based acrylic PSAs with acrylonitrile as comonomer will display higher than normal shear values. Because of the temperature resistance of these nitrilebased crosslinks, especially high hot shear values are obtained. Laminate Properties Some aspects of the laminate properties are dealt with in Chaps. 2 and 8. The adhesive/solid state component interactions inﬂuence the stability of the laminate properties. First, these interactions may be restricted to mutual interactions between adhesive and paper-based face stock material. The most important mutual interaction takes place between the porous, rough

x x x x x

RR

x x

EVAc

x x x x

Yes x x x x x

No

Crosslinked

x x x x x

SB

x x

x x x x

WB

x x

x

HM 0.7–2 5–10 >200 40–90 1 5000 >10,000 260 >200 500–4500 3 4.6 50–400 300 100 1 1

Value hr min min min min min min hr min min hr d min min hr hr hr

Unit RT RT RT RT 70 70 RT RT RT RT RT RT RT 95 88 RT RT

Temperature (oC)

Shear Resistance

500 g

500 g

500 g

2 267 267 — 269 269 46 264 267 46 273 265 159 20 274 275 276

0.500 0.500 , — — 0.500 0.500 , — — 0.500 0.500 , — — — 0.500 0.500 , — — — — — —

500 g

Ref.

Method

AC: acrylic; RR: rubber-resin; EVAc: ethylene-vinyl acetate; SB: solvent-based; WB: water-based; HM: hot-melt; RT: room temperature.

x x x x x x x x x x

AC

Chemical basis

Physical State

Shear Values as a Function of the Physical State and Chemical Nature of the PSA

Chemical Characteristics

Table 6.40

Adhesive Performance Characteristics 333

334

Chapter 6

Table 6.41 Influence of the Tackification on the Shear of Water-Based PSA on Different Chemical Bases Tackiﬁer Level PSA V205 80D Hot shear RT shear 3703 V205 80D Hot shear RT shear 3703 80D Hot shear RT shear 3703 Hot shear RT shear

Supplier

Nature

20

25

30

35

40

45

50

BASF BASF

AC AC

Polysar BASF BASF

CSBR AC AC

Polysar BASF

CSBR AC

41 39 9 100 40 20 20 — 250 40 40 > 1500

38 37 7 80 — — — — — 37.5 37.5 > 1200

36 34 — 55 — — — 19 — 35 35 240

33 32 7 39 — — — 22 — — — —

38 22 4 27 30 22.5 7.5 13 25 30 30 2

— — — — — — — — — 22.5 22.5 2

25 25 — 55 25 19 8 — 14 25 25 2

Polysar

CSBR

—

—

70

—

—

55

50

1300

500

368

Shear data given in minutes. Hot shear was tested at 50 C/min. An acid rosin was used as tackifier. RT: room temperature; AC: acrylic.

Table 6.42

Aging Stability of PSA on Different Chemical Bases Adhesive Properties After aging

Adhesive

Initial

Code

Supplier Nature

2382 2343 1338 10066 1247 EAF60 V205

Syntho Syntho Syntho Syntho Syntho Wacker BASF

AC VAc-AC CSBR S-AC CSBR EVAc AC

Thermal treatment

Peel Tack (N/25 mm) (cm) 15.0 14.6 20.4 22.0 — 25.0 PT 25.0 PT

2.5 2.5 5.5 7.5 — 4.0 2.5

Peel (N/25 mm)

Tack (cm)

Light exposure

14.0 24.0 PT 15.0 20.0 — 25.0 PT 25.0 PT

4.5 2.5 8.0 10.0 — 6.0 4.0

1 — 5 5 5 1 1

1 ¼ good, 5 ¼ poor; AC, acrylic; VAc, vinyl acetate; CSBR, carboxylated styrene-butadiene rubber; S-AC, styrene acrylic; EVAc, ethylene vinyl acetate.

Adhesive Performance Characteristics

335

face stock material and the ﬂuid adhesive (i.e., a physical interaction between paper and the PSA); this is known as migration. Migration Resistance. The migration resistance is a complex characteristic. It depends on the adhesive, on the carrier, and on the manufacturing method. As discussed in detail in [277] the parameters of the migration include the nature, molecular weight, and gel content of the base elastomer, the nature and softening point of the tackiﬁer used, and the nature and boiling point of the plasticizer. For dispersed systems the solids content, the surfactant, and the thickener aﬀect the migration too. The migration resistance of untackiﬁed acrylic PSAs remains superior to other formulations, with solvent-based ones better than water-based dispersions. Tackiﬁed acrylic PSAs show better migration resistance than competitive raw materials: Migration : AC ðTackifiedÞ EVAc ðTackifiedÞ CSBR ðTackifiedÞ ð6:79Þ There are some acrylic PSAs with lower migration resistance (e.g., removable and deep-freeze PSAs). A high emulsiﬁer loading decreases the migration resistance of water-based PSAs. Anchorage. Anchorage is the phenomenon of adhesion between face stock material and adhesive. For ﬁber forming, textured supports (e.g., paper) this is a physical phenomenon also. Physical interaction and ﬂow of the adhesive are the main components for the anchorage of PSAs onto rough, textured substrates. Common, tacky, unformulated acrylic PSAs display good anchorage on paper, even if transfer coated. Hard, low tack acrylic PSAs do not exhibit enough anchorage on paper. Removable, unformulated acrylic PSAs give less anchorage on unprimed surfaces than tackiﬁed formulations. Anchorage also is a function of the emulsiﬁer level; therefore solvent-based acrylic PSAs display better anchorage than water-based ones. Untackiﬁed EVAc or CSBR exhibit poorer anchorage than acrylic PSAs; untackiﬁed acrylic PSAs give better anchorage than competitive materials. Tackiﬁcation improves the anchorage. Tackiﬁed acrylic PSAs and tackiﬁed rubber display better anchorage than tackiﬁed EVAc. With respect to the chemical interaction between acrylic PSAs and paper, untackiﬁed acrylic PSAs do not interact chemically with normal paper. A similar behavior is observed for competitive base polymers. Tackiﬁed formulations may interact with special (e.g., thermal) papers, due to the tackifying resin. Most plastic ﬁlms are compounded materials (i.e., they contain additives). Therefore there are possibilities for an interaction between micromolecular components of the ﬁlm (e.g., PVC or polyoleﬁns) and the

336

Chapter 6

adhesive. Plasticizer and emulsiﬁer from the ﬁlm, slip agents, or antifogs could migrate into the adhesive. On the other hand, monomers, oligomers, and surface agents from the adhesive can migrate into the face material, causing stiﬀening of the face stock (or its shrinkage) as well as the loss of the adhesive properties. Because of their low UV stability rubber-resin adhesives cannot be used for ﬁlms. EVAc copolymers may interact with some ﬁlm materials and cause shrinkage. Thus only acrylic PSAs are recommended for ﬁlm coating. In a similar manner the anchorage of unformulated acrylic PSAs on nonpolar, untreated ﬁlm is better than the anchorage of its competitors (except for ethylene-maleinate multipolymers and Nacor 90). Acrylic-based PSAs exhibit varying shrinkage on soft PVC. There are some leading acrylic PSA dispersions that show shrinkage values of 0.3–0.9% (Table 6.43). A detailed discussion of these characteristics is presented in Chap. 10. It is evident that adhesives interact with the release liner as well. Acrylic PSAs with diﬀerent formulations exhibit diﬀerent release levels and release level stability. Ethylene vinyl acetate-maleinate copolymers display less sensitivity towards the nature and age of the release liner. Unformulated acrylic PSAs display the best die-cuttability. In the range of formulated products, crosslinked, tackiﬁed, solvent-based adhesives show the best diecuttability (see Chapters 8 and 9). Generally, acrylic formulations require less surface active agents for stabilizing and wet-out, thus the humidity content of the laminate is lower. Therefore acrylic-based laminates are superior compared to CSBR/EVAc-based laminates with respect to lay-ﬂat, die-cuttability, and water resistance. Special Features of Water-Based Acrylic PSAs as Compared to Other Water-Based PSAs Water-based PSAs show diﬀerent characteristics according to their chemical nature and manufacturing technology. The properties of the aqueous dispersions as well as those of the coated layer and of the laminate vary widely. In general, water-based acrylic PSAs possess a higher solids content and a higher solids content/viscosity ratio than common CSBR/EVAc dispersions; they also respond better to diluting. Water-based acrylic PSAs exhibit a lower surface tension than common EVAc or CSBR dispersions. Current surface tension values range from 34–40 mN/m, whereas EVAc dispersions show values above 45 mN/m. Rubber-based dispersions have a surface tension between 45–60 mN/m. Furthermore water-based acrylic PSAs show higher coagulum (grit) than CSBR dispersions, although they display a better mechanical stability than CSBR. Water-based acrylic PSAs generate less

Nature

Test Conditions

Shrinkage Values for PSA-coated PVC (Different PSAs) Shrinkage (%, Face Stock)

WB WB WB WB WB WB SB SB WB WB WB WB WB

AC AC AC AC AC AC AC AC EVAc EVAc AC EVAc AC

70 70 70 70 67 67 67 67 70 70 67 70 70

7 7 7 7 7 1 7 1 7 7 7 7 7

0.20–0.30 0.50 — — — — — — — — — — —

0.15 0.40 0.40 — — — — — — — — — —

WB: water-based; SB: solvent-based; AC: acrylic; EVAc: ethylene vinyl acetate.

01 02 03 04 05 05 06 06 07 07 08 09 10

— 0.85 0.85 1.70 6.00 17.0 8.0 11.0 0.50 2.60 0.40 2.0–3.0 0.5–1.0

— 0.60 0.50 1.20 — — — — 0.40 1.40 0.40 — —

x — — — — x — x x — — — —

— x x x x — x — — x x x x

— — — — 278 278 278 278 279 279 280 275 275

White PVC Transparent PVC Test method Temperature Time Code Status Chemical basis ( C) (days) Longitudinal Transverse Longitudinal Transverse Mounted Unmounted Ref.

PSA

Table 6.43

Adhesive Performance Characteristics 337

338

Chapter 6

foam formation than CSBR and EVAc. Finally, water-based acrylic PSAs show better wet-out properties than EVAc- and CSBR-based PSAs, and can be converted at higher speeds than CSBR and EVAc dispersions. Special Features of Solvent-Based Acrylic PSAs as Compared to Solvent-Based Rubber PSAs Solvent-based acrylic PSAs have some special characteristics compared to classical rubber-resin PSAs. First, a higher solids content (more than 20–30%) is possible and the solids content/viscosity ratio can be adjusted exactly. Usually special solvents are used and both solutions or dispersions can be produced. In general fewer or no plasticizers and antioxidants are used; ﬁnally crosslinking is possible. Modification of Adhesive Properties The main factors inﬂuencing the adhesion are: the properties of the adhesive, the properties of the carrier (face stock, liner), the construction and geometry of the laminate, the coating weight, the coating technology, and the age of the laminate. The adhesive properties of the PSAs can be inﬂuenced through polymer synthesis, formulation, and converting. Polymer Synthesis. There exists a broad range of diﬀerent functional acrylic/methacrylic monomers which can be used for PSA synthesis. Acrylic PSAs have the broadest monomer basis. Some of the monomers impart adhesive properties, others are used as emulsiﬁer or stabilizer monomers. A butyl acrylate (BuAc) base provides a higher cohesion level than ethyl hexyl acrylate (EHA); other nonacrylic comonomers also can be used. For example acrylonitrile imparts hardness and solvent resistance, styrene and alpha-methyl styrene impart ﬁrmness and improve peel adhesion, t-butyl styrene improves tack, methylmethacrylate makes the adhesive ﬁrmer, vinyl acetate improves adhesion to certain plastic surfaces, etc. Compared to acrylic-based PSAs, rubber- and EVAc-based copolymers possess a narrower raw material basis. EVAc-based PSAs must incorporate more than 18% ethylene, while SBR-based PSAs contain less than 25–35% styrene. Thus, their formulating freedom remains limited. Most vinyl acetate copolymers need protective colloids as their particle size and particle size distribution are limited; they have a predetermined viscosity and solids content/viscosity ratio, and a built-in water resistivity. EVAc-based PSAs do not have crosslinked structures, whereas acrylic-based PSAs may have them. Styrene-butadiene rubber-based PSAs must always have a gel/sol ratio as their properties (peel, zshear, migration) depend on this.

Adhesive Performance Characteristics

339

A pressureless well-known polymerization technology is available for acrylic PSAs. Styrene-butadiene rubber PSAs need medium pressure and EVAc PSAs require high pressure reactors. For solvent-based acrylic PSAs a crosslinking technology was developed; for water-based acrylic PSAs branching and grafting procedures were established. Polymer Formulations. A general compatibility exists between diﬀerent acrylic PSAs, as well as between acrylic PSAs and other polymers. However, no similar compounding freedom exists for rubber- or EVAcbased polymers. Plasticizers and tackiﬁers can be used for acrylic PSAs and other polymers. Crosslinking by built-in comonomers, metal complexes, or PUR is possible for acrylic PSAs; EVAc PSAs have no similar crosslinking possibilities. Attempts were made to crosslink CSBR using polyisocyanates. Converting. Theoretically there are three converting technologies for acrylic PSAs: hot-melt, solvent-based, and water-based. Practically, only a few experimental hot-melt acrylic PSAs exist (like experimental hot-melt EVAc-based PSAs). Rubber-based PSAs also can be converted using all of the above technologies. The coating technology is a function of the adhesive, face stock, release nature, and of converting, processing, and end-use requirements. Acrylic-, rubber-, and EVAc-based PSAs allow direct or transfer coating. Acrylic solutions are coated directly on paper and special ﬁlms, and indirectly on soft PVC. Solvent-based rubber PSAs are directly or transfer coated on paper. Water-based acrylic PSAs are directly or transfer coated on ﬁlms. Through direct/transfer coating, anchorage, die-cutting, water resistance, and release can be adjusted. High viscosity (and high solids) water-based and solvent-based acrylic PSAs are available, as well as low viscosity (ready-to-use) ones. Primers provide better anchorage and less migration, and increase the removability. Rubber-based PSAs need primers; acrylic PSAs can be converted with/ without primer. Common acrylic dispersions need more wetting agents for transfer coating than for direct coating. They need more wetting agents on solventless silicones than on solvent-based silicones. Wet-out depends on the siliconized paper. The age of the siliconized liner inﬂuences the wetting also. Anchorage of the coated adhesive also depends on the age of the laminate. Special Features of Acrylic PSAs. Next the main attributes of acrylic PSAs are highlighted. From the point of view of the end-user acrylic PSAs provide the best balance between adhesive and cohesive properties, the best independence from the face stock and substrate, the best aging properties (e.g., oxidative stability, heat resistance, UV stability), and the best tack

340

Chapter 6

at varying temperatures. In addition, they exhibit the best machinery properties (adhesion/release balance), as well as FDA and BGA approval. The advantages of acrylic PSAs for the converter include a broad range of raw materials, a broad compatibility with other polymers, and diﬀerent tackifying possibilities (e.g., high polymers, resins, plasticizers). They accept a broad range of tackifying resins, and display at low tackiﬁer level a more pronounced tackiﬁer response. They are generally suitable for diﬀerent face stocks and for permanent and removable applications in rolls and sheets. Acrylic PSAs possess excellent converting properties (e.g., wetting out, drying speed, low foam, high mechanical stability) and allow many diﬀerent ways for adjusting peel adhesion and shear. The disadvantages of acrylic PSAs for the end-user are relatively lowadhesion on untreated, nonpolar surfaces, relatively low die-cuttability if formulated for high tack and coated on lightweight laminate components, and relatively low water resistance for water-based PSAs. Disadvantages of acrylic PSAs for the converter include the lack of available acrylic hot-melt PSAs and the lack of or limited compatibility with rubber-based adhesives. Solvent-based acrylic PSAs are superior to water-based PSAs. Although acrylic PSAs are the most expensive PSAs, they are used with increasing dynamism. There are only a few manufacturers formulating monomers for acrylic PSAs (mainly for EHA and BuAc). The most important component for acrylic PSAs are the tackiﬁer resins. In this ﬁeld there is an abundant supply of new and classical products. Formulating Ease and Costs. Acrylic PSAs are multipolymers, so their properties can be varied using diﬀerent monomer feeds and polymerization technologies. Solvent-based technology is more expensive but has the advantage of crosslinking (higher quality at a higher price level). Water-based technology presents the advantage of high solids, seed and branching, and particle size distribution control. Depending on the end-use requirements, it is possible to synthesize or buy acrylic PSAs for compounding, resulting in more or less expensive products. Acrylic PSAs can be tackiﬁed using a medium level of tackiﬁer loading only; they have a reserve of adhesive properties and theoretically they can be modiﬁed with ﬁllers. The price of the laminate depends on the following components: the face stock, the release liner, the adhesive, and the choice of the converting (laminating) technology. Paper. Tackiﬁed acrylic PSAs show good anchorage and low migration (solvent-based or water-based), thus they also can be used with inexpensive non-surface-treated paper. On the other hand, water-based acrylic PSAs need special (primered) face stock in order to obtain a good

Adhesive Performance Characteristics

341

die-cuttability. Heavy, voluminous papers increase the die-cuttability also. Removable water-based acrylic PSAs also need primered papers. Film. Water-based and solvent-based acrylic PSAs can be used without restrictions for diﬀerent ﬁlm materials. The same acrylic can be used for paper as well. Direct coating on nonpolar ﬁlms requires a surface treatment. Acrylic PSAs can be coated on paper-based release liners as well as on ﬁlm-based release materials. Solvent-based or solventless siliconizing can be used. Direct/transfer coating provides another possibility for release level modiﬁcation. Transfer coating of water-based acrylic PSAs on solventlesssilicones remains the cheapest way. Solvent-based acrylic PSAs are the most expensive adhesives. Hence:

The use of filled solvent-based acrylic PSAs decreases the adhesive costs. Compounding solvent-based acrylic PSAs also decreases the adhesive cost. Tackifying solvent-based/water-based adhesives decreases the costs. Common water-based acrylic PSAs are less expensive than solventbased ones (price ratio 2/1). Tackified water-based acrylic PSAs are the most cost-effective materials within the range of acrylic PSA adhesives.

Converting Costs. Converting acrylic PSAs is possible in solution or as an aqueous dispersion. Converting solvent-based acrylic PSAs is more expensive than water-based acrylic PSAs, whereas converting solventbased rubber-resin PSAs is less expensive than acrylic PSAs. Converting low viscosity water-based acrylic PSAs costs less than coating highly viscous ones. As pointed out before, acrylic PSAs can be converted directly or by transfer, and transfer coating remains less expensive than direct coating. REFERENCES 1. 2. 3. 4. 5. 6.

A. Zosel, Colloid Polymer Sci., 263, 541 (1985). C.M. Chum, M.C. Ling and R.R. Vargas (Avery Int. Co, USA), EP 12257927/ 18.08.1987. J. Johnston, Adhes. Age, (11) 30 (1983). Glossary of Terms Used in Pressure Sensitive Tapes Industry, PSTC, Glenview, IL, 1974. ASTM, D-1878–6 IT; Pressure-Sensitive Tack of Adhesives, American Society for Testing and Materials, Philadelphia. M. Barquins and D. Maugis, J. Adhesion, 13, 53 (1981).

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9. 10. 11. 12. 13. 14. 15.

16. 17. 18.

19.

20. 21. 22. 23.

24. 25. 26. 27. 28. 29. 30.

31. 32.

Adhesive Performance Characteristics 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43.

44. 45. 46.

47. 48. 49. 50. 51. 52. 53.

54. 55. 56. 57. 58. 59.

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262. 263. 264. 265.

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Novamelt Research GmbH, Technical Bulletin, 1999. Poli-Film Kunststofftechnik GmbH, Wipperfu¨rth, Germany, Technical Bulletin, 1994. Novacel GmbH, FRG, Technical Bulletin, 1994. Adhes. Age, (12) 43 (1981). Tappi J., (9) 104 (1983). K.N. Nakao, J. Adh. Soc. Japan, (6) 6 (1970). K. Fukuzawa and T. Uekita, ‘‘New Methods of Evaluation for PressureSensitive Adhesives,’’ Proc. of the 22nd Annual Meeting of the Adhesion Society, Panama City Beach, Florida, Febr. 21, 1999, p.78. J. Andres, Allg. Papier Rundschau, (16) 444 (1986). US Pat., 4,563,389; in S. Krampe and C.L. Moore (Minnesota Mining and Manuf. Co., St.Paul, MN, USA), EP 0,202,831 A2/26.11.1986. A.W. Norman, Adhes. Age, (4) 36 (1974). S.D. Tobing and A. Klein, ‘‘Synthesis and Structure Property Studies in Acrylic Pressure-Sensitive Adhesives,’’ Proc. 24th Annual Meeting of Adh. Soc., Febr.25, 2001, Williamsburg, VA, p.131. R.D. Adams, R. Thomas, F.J. Guild and L.F. Vaughan, ‘‘Thick Adherend Shear Tests,’’ Proc. 24th Annual Meeting of Adh. Soc., Febr., 25, 2001, Williamsburg, VA, p.59. R.A. Fletscher, ‘‘The Temperature Factor in Compounding Thermoplastic Rubber-based HMA,’’ 11th Munich Adhesive and Finishing Seminar, Munich, Germany, 1986, p.36. A. Sustici and B. Below, Adhes. Age, (11) 17 (1991). A. Bell, Shell Bulletin, TB, RBX/73/8/6. J.A. Schlademan, Coating, (1) 12 (1986). L.M. Schrijver, Coating, (3) 70 (1981). J.A. Schlademan and J.G. Bryson, Proc.of the 21st Annual Meeting of the Adhesion Society, Savannah, GA, Febr., 1998. A. Pocius and C. Dahlquist, Adhesion and Adhesives, ACS Audio Courses (1986); in H.W. Yang, ‘‘Effect of Polymer Structural Parameters on PSAs,’’ Proc. of the 22nd Annual Meeting of the Adhesion Society, Panama City Beach, FL, Febr. 21, 1999, p.63. J.M. Gordon, G.B. Rouse, J.H. Gibbs and W.M. Risen Jr., J. Chem. Phys., (66) 4971 (1977). G. Meinel, Papier u. Kunststoffverarb., (10) 26 (1985). H.J. Boob and H.M. Ulrich, Kautschuk, Gummi, Kunststoffe, (3) 209 (1984). L.A. Sobieski and T.J. Tangney, Adhes. Age, (12) 23 (1988). M. Ulrich, US Pat., 24. 906; in Handbook of Pressure Sensitive Technology (D. Satas, Ed.), Van Nostrand-Rheinhold Co., New York, 1982, p.203. Coating, (2) 43 (1987). W.J. Whitsitt, Tappi J., (12) 163 (1988). Tappi J., (5) 182 (1988). L. Krutzel, Adhes. Age, (9) 21 (1987).

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266. 267. 268. 269.

Coating, (7) 186 (1984). Adhes. Age, (11) 27 (1983). C.K. Otto, Allg. Papier Rundschau, (16) 438 (1986). J.P. Keally and R.E. Zenk (Minnesota Mining and Manuf. Co., St. Paul, MN, USA), Canad. Pat., 1, 224, 678/19.07.1982 (USP. 399, 350). Adha¨sion, (2) 43 (1977). I. Benedek, Pressure Sensitive Formulation, VSP, Utrecht, 2000, Chapter 3. R.P. Mudge (National Starch Chem. Co., Bridgewater, USA), US Pat., 4, 753, 846/28.06.1988. Tappi J., (9) 105 (1984). Allg.Papier Rundschau, (5) 228 (1987). R.P. Mudge (National Starch Chem. Co., Bridgewater, USA), EP 0, 225, 541/ 11.12.1985. J.A. Fries, ‘‘New Developments in PSA,’’ Tappi HM Seminar, 2 June, 1990, Toronto, Canada. I. Benedek, Pressure Sensitive Formulation, VSP, Utrecht, 2000, p.55. Paper, Film and Foil Converter, (3) 76 (1988). Vinamul GmbH, Nacor 90, Data sheet, 1988. D.G. Pierson and J.J. Wilcynski, Adhes. Age, (8) 53 (1990).

270. 271. 272. 273. 274. 275. 276. 277. 278. 279. 280.

7 Converting Properties of PSAs

Convertability is the ability of the adhesive to be used for coating and laminating, and the property of the laminate to be used for label (or tape) manufacturing. The most important factors which contribute towards good convertability of a self-adhesive laminate include the nature of the face paper, the type of the adhesive and silicone, the release force, the properties of the siliconized release liner, and the die-cutting properties of the adhesive and of the face paper. There are two diﬀerent aspects of the convertability of an adhesive, namely the convertability of the ﬂuid adhesive (solvent-based, water-based, or hot-melt), and the convertability of the coated adhesive (processability); good convertability implies the suitability of the adhesive ﬂuid to be coated without any problem, and the capability of the laminate to be processed (cutting, die-cutting, printing, labeling, etc.) without any problem. As discussed in detail in [1] and in Chap. 3, Section 1.4, convertability in practice is the sum of the convertability of the adhesive and that of the laminate. 1

CONVERTABILITY OF THE ADHESIVE

Because of the diﬀerent materials used as face stock materials and the different technologies and adhesives used, the coatability of the liquid adhesive is a function of many diﬀerent parameters (e.g., the physical state of the adhesive, the nature of the face stock, the coating technology, the laminate structure, the end-use properties, the cost, and the nature of the release). 1.1

Convertability of Adhesive as a Function of the Physical State

The convertability (coatability) of the adhesive is characterized by the parameters such as the wet-out, the coating rheology, coating speed, and 353

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versatility. All of these parameters depend on the physical state of the adhesive, whether it concerns a diluted liquid system, yielding an adhesive layer through the evaporation of the carrier liquid (water or solvent) or a 100%-solid system, giving a solid adhesive layer by a physical (hot-melt) or chemical (UV or electron beam) process. Hence the coatability of diluted and undiluted systems needs to be discussed separately. Coatability of Diluted Systems Diluted adhesive systems are deﬁned as solutions or dispersions of adhesives. There are aqueous or solvent-based adhesive solutions, and aqueous or solvent-based dispersions. The most important ones are organic solvent solutions of adhesives (solvent-based adhesives) and water-based dispersions of adhesives (water-based adhesives). In fact most of the solventbased adhesives also contain (at least partially) dispersions (like rubber-resin adhesives), and most of the dispersion-based aqueous adhesives contain some dissolved materials (e.g., thickeners, tackifying polymers, etc.). In Chap. 2, the special features of the wetting out of solvent-based adhesives as well as their coating rheology were discussed. For coating practice more information is given by the versatility of the PSAs. Coatability of Solvent-Based Adhesives Factors inﬂuencing the coatability of solvent-based systems include the inﬂuence of the solvent content on the coating thickeners, coating appearance, and the mechanical-hydraulic forces in the coating nip [2]. Solventbased adhesives are mostly solutions of rubber-resin mixtures or acrylic copolymers. Their physical properties depend on both components, namely on polymer and solvent, and on the composition of the adhesive solution. Solvent-based adhesives are generally highly viscous systems with low surface tension solvents as the carrier; they are (at least partially) directly coated onto high surface energy webs. The wet-out of solvent-based PSAs is less difficult than with water-based PSAs. The coating rheology of solventbased PSAs concerns the rheology of a true solution, thus is mainly viscosity driven and more simple than that of water-based systems. Coating versatility is the capability of an adhesive to be coated on different machines, using different coating technologies without changes in its composition or performance characteristics. Because of the high viscosity of the solutions, there are only a limited number of coating devices able to use solvent-based PSAs. Coating speed is the most important coatability-related property of solvent-based PSAs. The key issues include:

different coating devices for different viscosities; base viscosity as a function of the polymerization recipe;

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thickening/diluting response of the adhesive; different technologies (transfer/direct) for different adhesives; different technologies for different end-uses. In general the most important parameters of the liquid solvent-based adhesive (solids content and viscosity) can be easily and precisely adjusted because the (mechanical) stability of the system is independent of viscosity (or solids content). On the other hand, there is no need for an exact adjustment of these properties because of the higher viscosity. Thus solventbased PSAs are suitable to be used on different coating devices. The (possible) different nature of the used solvents only acts as a limiting factor. Therefore, Versatility : Solvent-based PSAs > Water-based PSAs

ð7:1Þ

The coating speed remains the most important converting property of solvent-based adhesives. In contrast to water-based PSAs, there is no limit (minimum speed) for the coating speed because of the unlimited wet-out of these adhesives. The limits of the running speed for a given machine are dictated only by the explosion limits of the solvents. Theoretically, a high solids content and low boiling point solvent allow high drying speeds; thus an increase in solids content and the appropriate choice of adequate solvents ensure better running speeds. The newest experimental products generally contain a higher solids content. Theoretically, a 100%-solids content (at very low molecular weight) is possible; however, the most concentrated aqueous dispersions have 67–69% solids content (theoretically 75% is possible). Thus ways to achieve higher coating speeds appear to be quite different for solvent-based or water-based PSAs. Coatability of Water-Based Systems The coatability of water-based adhesives also is a function of the wet-out, coating versatility, coating rheology, and coating speed. As stated by [3] the driving force for dynamic wetting is in fact the surface tension of the adhesive rather than the work of adhesion. Thus, the wet-out of water-based systems constitutes a diﬃcult problem because of the high surface tension of the carrier (water), the low viscosity of the most important ready-to-use formulations, the low density, and the transfer technology used for coating. The use of co-solvents is limited, while higher viscosities need diﬀerent coating devices. In contrast to solvent-based PSAs, an improvement of the wet-out characteristics (with surface active agents) induces irreversible changes to the adhesive properties. Thus the coatability of water-based formulations is more sensitive than that of solvent-based PSAs.

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For water-based PSAs, the coating rheology concerns a dispersion, not a solution, and is thus time dependent. No changes in carrier material are allowed and the dispersed polymer is of high molecular weight; coalescence yields a composite, anisotropic layer. Coating devices are very sensitive towards viscosity and thus their coating versatility is reduced; the coating speed depends on solids content as well as on dispersion characteristics. A minimum level of surface tension is required for dynamic wetting, the maximum speed for a machine is dictated by rest water content (humidity) of the dried layer. Rest humidity content is an equilibrium value which depends on the wet-out additives; thus wetting out is the most important convertability parameter. The coating speed is more dependent on the coating weight for water-based PSAs than for solvent-based PSAs. In addition, the formulating freedom with coating speed remains very limited, unlike with solvent-based PSAs. The special features of the wetting out for aqueous dispersions were discussed in Chap. 2 concerning the rheology of water-based dispersions. Summarizing, the most important key issues concerning the wet-out are:

wet-out as the combination of wetting/dewetting; dynamic versus static wetting; factors acting against dewetting (shear, coalescence); parameters influencing wetting/dewetting (surface tension, contact angle, viscosity, density); surface tension and parameters influencing it; contact angle, and the problems of measuring this parameter; the influence of the viscosity (versatility, wet-out, coating weight).

The theoretical aspects of the coating rheology of water-based adhesives were discussed in Chap. 2. There is a qualitative difference between the coating rheology of the liquid and that of the semisolid (dried) adhesive layer. The former is like a solvent-based (solute) rheology, whereas the latter is more that of a reinforced matrix. For the first step of the coating, temporary and reversible shear sensitivity of the water-based PSAs determines their behavior on and after the metering roll. In the second stage, the diffusivity of the coated layer and its flow determine the drying speed and the aspect of the dried, coalesced adhesive film. Coating rheology of solvent-based adhesives is a function of polymer and solvent quality, whereas coating rheology of waterbased adhesives is a function of the dispersion structure. On the other hand, the influence of the metering device, and of the drying ‘‘regime’’ is more pronounced than for solvent-based PSAs. Slight shear rate changes may avoid ‘‘stripe’’ structures, air and heating adjustment may avoid adhesive blow-away, viscosity control may help adjust the smoothness of the final

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coating. Unlike solvent-based adhesives, formulating only has a limited influence on the coating rheology of the adhesive. Key parameters include: adhesive build-up on the machine, mechanical stability, due to insufficient flow; coagulum build-up on the machine and during formulation (mechanical stability); coating uniformity (striped, textured structures, insufficient flow); stability of the viscosity (thixotropy and decrease of the viscosity, either reversible or irreversible); foam formation. Coating versatility refers to the suitability of the water-based PSAs to be coated by diﬀerent coating technologies. Today in Europe two coating methods for emulsion PSAs dominate their respective ﬁelds [4], namely: metering bar systems for tapes and protective ﬁlms, and reverse gravure for labels. The coating technology will be discussed in more detail in Chap. 9. The mechanical stability of the water-based adhesive on the coating machine is characterized by the following parameters: coagulum build-up on the machine, grit build-up in the dispersion, stability of the viscosity, and foaming. Several latex adhesives have a tendency for coagulum build-up on the metering roll of the reverse roll coater. Such coagulum may arise either from poor mechanical stability under shear loading or from imperfect doctoring leading to a thin coating on the roll, which dries and cannot be redispersed so that these particles eventually transfer to the applicator roll [5]. Dispersions with a high solids content and a high surface tension tend to build up dry, tacky residues. Frequently changing the rheology (diluting and thickening) leads to deposit-free water-based adhesives. Often the adhesive layer build-up on the machine is associated with insufficient fluidity of the ‘‘almost dried’’ adhesive, giving a coating appearance that is textured with stripes. Diluting/thickening also helps to avoid this phenomenon. Unlike adhesive build-up, grit (coagulum, discrete deposits) build-up is associated with insufficient mechanical stability of the dispersion (i.e., an insufficient stabilizing agent level in the formulation). According to [6] tackified acrylic emulsions are more susceptible to high shear induced coagulation. A higher acrylic acid level in the copolymer reduces the coagulum. Generally, tackified formulations are more sensitive (the IR spectrum of the coagulum showed that it is 3 times higher in tackifier than is the formulation); rosin-ester tackified PSAs are more sensitive than acid-resin tackified formulations (e.g., an acid-rosin with 60 ppm standard coagulum content does not influence negatively current paper coating formulations; rosin-ester dispersions must possess less than 20 ppm coagulum).

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Theoretically, high shear stirring tests show the grit build-up and the lack of mechanical stability; generally, most PSA formulations withstand this test. Real data can be obtained with a two-cylinder laboratory device, or practically, a 3-day running test on the production line. Most unthickened PSA dispersions exhibit only slight thixotropy (i.e., changes in the viscosity after formulation that are generally irreversible). It is to be noted that 0.5–1.5 s (Ford Cup sec) are normal absolute values for observed changes in the viscosity during 8–24-hr production runs. Foaming does not really constitute a problem as most machines do not attain critical speeds. Most dispersions possess viscosities below 500 or above 5000 mPa s. Also there are a variety of very efficient defoamers. Water-based PSAs require longer drying times (and hence longer dryers) than solvent-based adhesives, and 15 to 150 m dryers are currently in use. The coating speed of the adhesive not only has economical importance, but ease of adjustment also influences the tolerances of the coating weight. For gravure roll coating a short contact time between web and pressure roll means a low coating weight, whereas a long contact time (low speed) leads to a high coating weight. With a given gravure size it is possible to change the coating weight within a range of approximately 10% [8]. Aqueous systems require more drying heat. The heat of evaporation of water amounts to 540 cal/g, while that of typical coating solvents, such as methyl ethyl ketone (MEK) and toluene, is 100–200 cal/g. On the other hand water-based systems are dried more efficiently with large volumes of air rather than with air of higher temperature [7]. This difference translates into longer ovens, lower line speeds, and higher utility costs [8] for water-based systems. Therefore the running speed has its economical importance, possibly leading to lower costs and higher productivity (Fig. 7.1).

Figure 7.1

Parameters of the PSA laminate inﬂuenced by the coating speed.

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Originally several technical parameters were evaluated in order to improve the drying speed of the diluted systems, including an increase of the solids content and the transfer coating technology. With transfer coating a high coating speed of the non-water-sensitive web was possible; an increase of the solids content allows (at least theoretically) a reduction of the volatiles and a higher drying speed. The speed limiting steps are the foaming, the appearance of the coated layer, and the residual water content of the dried adhesive. It is evident that for a water-based PSA exhibiting an adequate rheology the upper limit of the running speed is: Running speed ¼ Maximum drying speed for normal dispersions ð7:2Þ For high solid dispersions: Running speed ¼ Maximum speed leading to a smooth coated layer (7:3Þ For medium viscosity dispersions: Running speed ¼ Maximum speed which limits foam formation ð7:4Þ The physical state of the PSA influences its drying properties [9]. Solvent-based and water-based PSAs display a quite different evaporation behavior (i.e., they require a quite different temperature gradient during drying) (Fig. 7.2). The coating and drying speed will depend on the proper adjustment of the drying (oven) temperature. Key aspects include the economical/technical importance of the running speed, the built-in characteristics of the water-based PSAs influencing the running speed, and the technical limits of the running speed. Coatability of Hot-Melt PSAs According to their physical state the coatability of hot-melt PSAs is limited. Their viscosity depends on the temperature and on their chemical composition/aging. Because of the restricted range of raw materials available for their formulation, there is a narrow temperature interval suitable for them to be coated. On the one hand, this interval is situated at relatively high viscosities. Thus special metering devices only (see Section 2.2 of Chap. 9) may be used; on the other hand, the high viscosity in the

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Figure 7.2 Temperature gradient required for drying of: 1) solvent-based PSA coating; 2) water-based PSA coating.

molten state allows a good wet-out. Contactless coating methods for HMPSAs have been developed to allow the use of hot spray technology in the nonwovens market. Coating Versatility. The coating versatility of hot-melt PSAs remains limited. Generally, special agents (e.g., waxes, oils) are used in the formulation (see Section 2.9 of Chap. 8) in order to extend the coatability range of the adhesive. Coating Rheology. Hot-melt PSAs based on Kraton G are coated at a viscosity of 25,000–80,000 mPa s (at 177 C) [10]. Acrylic hot-melt PSAs possess a viscosity range of 8000–30,000 mPa s [11]. As illustrated by these examples the viscosity of current hot-melt PSAs is of an order of magnitude higher than that of solvent-based PSAs, and even higher than water-based PSAs. It is not possible to change this with dilution agents or shear forces (machine). Coating Speed. Because of the lack of carrier or solvent, hot-melt PSAs are ideal for the application of thick ﬁlms onto webs at speeds of hundreds of ft/min [7] versus acrylic PSAs coated at 170 m/min. Other data

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show that in comparison with water-based PSAs (200–250 m/min) hot-melt PSAs can be coated at higher speeds (300 m/min) [12] (see Chap. 9). Warm-Melts. The development of new, low cohesion acrylic warmmelts requires the use of a postcrosslinking agent [13–15]. The large scale application of UV postcuring is enhanced by this raw material development. Before crosslinking, the adhesive to be coated is a highly viscous ﬂuid (at room temperature) and can be processed at 120–140 C (its viscosity is about 10–20 Pa s). A special feature of this radiation crosslinking is its anisotropy. The radiation is partially absorbed, partially transmitted, and partially reﬂected. The maximum radiation is given at the top of the adhesive layer. Therefore direct and transfer coating give diﬀerent adhesive characteristics. Such UV-curable polymers are suitable for diﬀerent end-uses (e.g., deep freeze or removable adhesive) as a function of the UV dose [16]. The crosslinking level is high enough to cut and stamp without edge bleeding. 1.2

Convertability of Adhesive as a Function of Adhesive Properties

The adhesive properties inﬂuence the coatability of PSAs. A certain level of initial ﬂow (tack and peel adhesion) is needed to achieve the anchorage of the adhesive onto the face stock or release liner. In some cases a primer coating is required. A pronounced inﬂuence of the tack and shear may be observed during the converting of the laminate. 1.3

Convertability of Adhesive as a Function of the Solid State Components of the Laminate

The coatability of the adhesive depends on the face stock and release liner. Wetting out, drying parameters, and running speed depend on the surface characteristics of the face stock and release liner. Machine parameters also are a function of the bulk properties (mechanical characteristics) of the web and inﬂuence the coatability of the PSAs. Other bulk properties such as porosity, chemical aﬃnity, aging resistance, etc., also inﬂuence the coatability. 1.4

Convertability of Adhesive as a Function of Coating Technology

The convertability of the adhesive depends on the coating technology. For all types of PSAs and independent of their physical state, it is theoretically possible to use direct or transfer coating. Practically, the selected coating technology (direct or transfer) supposes quite diﬀerent rheological behavior. As an example, without thickening, a low viscosity conventional latex will

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not completely wet a silicone-coated release liner. In this case the application, convertability of the adhesive depends on the coating technology [5]. 1.5

Convertability of Adhesive as a Function of End-Use Properties

Transfer coating is preferred for heat-sensitive or porous face stocks; transfer-coated PSAs have to be formulated for better wet-out. Hence the convertability of the adhesive is a function of its end-use properties. On the other hand, a better anchorage (see Section 2.2 of Chap. 9) is obtained through direct coating. Certain adherend require a special adhesive smoothness or texture, which is a function of the metering device. As described in detail in [17] there are some special pressure-sensitive products containing an adhesive imbedded in the carrier material. Carrierless tapes were developed also. In such cases special manufacturing technologies and converting characteristics are required. 2

CONVERTING PROPERTIES OF THE LAMINATE

In the present context laminating means the production of a layered material from two or more webs using a bonding medium. In the case of PSA laminates, the face stock and release liner are connected together temporarily by means of a PSA. The PSA-coated material will be processed (converted) and used in the form of the laminate. Generally, the face stock will be printed either in-line or oﬀ-line, but after coating with a PSA; the liner also may be printed. The coating properties of the solid state components of the laminate (with adhesive, release, or inks) are discussed in detail in our book concerning the manufacture of pressure-sensitive products [1]. The product class (e.g., label, tape, protective ﬁlm etc.) related special features are described too. A coated, laminated material usually starts as a continuous roll which is then cut in discrete pieces. Cut material (printed before or after cutting) will be used for labeling purposes. Labeling (separation of the liner from the adhesive and aﬃxing the adhesive-coated label on the ﬁnal substrate) can be done manually or mechanically, this process being inﬂuenced by the face material-adhesive-release properties. Hence the convertability of the laminate will be inﬂuenced by the printability, confectionability (ability to be slit, cut, die-cut, perforated, embossed etc.), and labeling properties. As discussed in detail in [1] the winding performances aﬀect such characteristics also. All of these parameters depend on the solid state components of the pressure-sensitive laminate and on the adhesive. Therefore a detailed examination of these

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parameters requires a short description of the pressure-sensitive laminate, its components, and its construction. 2.1

Definition and Construction of the Pressure-Sensitive Laminate

Generally, pressure-sensitive laminates are sandwich structures where two (diﬀerent or identical) solid state components (generally ﬂexible sheets) are temporarily bonded together with the aid of a PSA. The pressure-sensitive composite is a temporary structure where one of the solid state components is acting as a temporary shield only, in order to avoid the deterioration of the PSA layer. This part, known as the release liner, is in some cases (mainly for tapes) identical with the face stock material. Such a construction is possible and necessary for pressure-sensitive tapes used in continuous reels. Pressuresensitive labels are discontinuous materials where the release layer has to be built in as a separate solid state component. Therefore, it may be assumed that pressure-sensitive labels contain the following main components: a face stock material, a pressure-sensitive adhesive, and a release liner. All these components are relatively thin; paper and soft or hard plastic ﬁlms are used as solid components. Soft plastic ﬁlms can be bonded only by using PSAs where the use of rigid adhesives is not possible because of stress concentration. Main Laminates The components of the PSA laminate and its build-up depend on the end-use, on the application technology of the labels, and on the PSA-coated laminates. Sheet Material. Pressure-sensitive labels are temporarily built-up composites. The end-use (application) of the label assumes peeling oﬀ the label from the release liner and the subsequent bonding of the free adhesive surface onto a substrate. This end-use called labeling is carried out by hand or automatically, with the aid of labeling guns. The main criterion concerning the feasibility of hand applied or automated labeling is the dimension of the label. It is evident that large sized labels have to be considered as sheetlike materials. On the other hand, economical considerations also inﬂuence the labeling. Unicates or small series of labels have to be manufactured by technologies using sheet-like materials. Big series of relatively small and low cost labels have to be applied automatically with labeling guns. Printing, cutting, and debonding conditions for sheet and reel materials are diﬀerent and therefore the quality requirements for both these label categories also diﬀer. Sheet materials were used with priority some years ago; ﬁlms as face

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stock material were tested for sheet-like labels ﬁrst. Because of the relatively big surface and the low delaminating and labeling speed of these labels, the face stock quality and the cohesion of the adhesive play an important role for sheet-like pressure-sensitive labels. Roll Material. Labels applied from a reel have to be tacky in order to ensure a suitable adhesion onto the adherend after a very short dwell time; at the same time they have to be very slightly bonded to the release liner in order to display a low debonding (delaminating) resistance. Laminate Build-Up. Theoretically, a PSA laminate consists of at least two solid state components (face stock and release liner) and a liquid phase (PSA) present as a continuous layer. The face stock or release liner itself may have a composite structure, the components of the laminate may show a discontinuous nature, and the laminate may possess a multilayer structure. Therefore the construction of the laminate exerts a strong inﬂuence on its properties. A detailed description of the pressure-sensitive product classes and their build-up is given in [18]. Laminate Components The pressure-sensitive layer (a liquid adhesive layer) is enclosed between two solid state sheet (forming) materials. Generally, there is only one liquid state component in the laminate, namely the pressure-sensitive adhesive layer between the face stock material and release liner. In some cases an additional primer coating is applied between the face stock material and the PSAs. Because of its reduced and limited ﬂow properties, the primer coating may be considered (like the release coating) as a solid state component of the laminate. The main solid state components of a pressure-sensitive label are the face stock material and the release liner. There are also sophisticated sandwich structures which possess a lot of face stock and release layers, where in some cases one or more face stock materials should also act as the release liner. Multiweb constructions can include solid-state components and adhesives that differ in their chemical nature and build-up. There are multilayer self-adhesive labels which comprise a carrier layer containing a silicone layer, an adhesive layer, a printed message, a carrier layer (e.g., polyester), a release layer (e.g., silicone), a carrier layer (e.g., polyethylene), and a printed message. These labels are useful when a printed message is to be attached to two different objects [19]. Forms are labels that have a multiweb, multilaminate structure, where continuous and discontinuous carrier materials and PSAs with different adhesive properties are laminated together, to allow stress and time dependent controlled delamination [18,20].

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Face Stock Materials. The face stock material is the solid state component of the pressure-sensitive label with a permanent character (i.e., the adhesion of the PSAs on the face stock material—the anchorage— should be higher than its bonding to the release liner). Generally, the adhesion of the PSAs to the face stock material (i.e., anchorage) should be superior than its bonding to the substrate. The most important face stock materials are paper, cloth, plastic ﬁlm, metallized ﬁlm, metal foils, elastomeric foams, and nonwoven materials. Face materials include specially modiﬁed papers, as well as plastic ﬁlms such as PVC, cellulose acetate, polyoleﬁns, polyester, and polystyrene, and aluminum foil as well as metallized papers and ﬁlms. Market requirements impose certain characteristics of the label material [21]. Labels for oil containers need to display durability, visual impact, and ﬂexibility. Toiletries and cosmetics require a ‘‘no label’’ look, a high consumer image, and stain resistance; the electrical market needs include visibility, durability (e.g., vinyls, PET), and heat resistance. Toys need nontoxic labels with long life and visual impact. The chemical industry requires resistance to chemicals, visibility, and durability. Thus quality requirements for the label indicate the need for adequate face stock materials. Laminate manufacturers must make an adequate choice of the face stock material to be coated. In most cases this choice is limited, either imposed by the customers or, in some cases, determined by the manufacturers’ own capabilities (i.e., coating machine or adhesive system). The most important decision is the choice between a paper or filmbased face stock material and in each of these domains between permanent or removable labels. Different mechanical performance characteristics are needed for tear-debonded (peeled) permanent and easily removable products. Tamper evident labels require carrier materials having low mechanical stability. A different material stiffness is required for squeezable or ‘‘rigid’’ labels or for conformable face materials. For special uses (e.g., medical products) preferred face stocks are those which permit transpiration of moisture; for other applications (e.g., pharmaceutical packaging) migration-resistant barrier layers are needed. On the other hand, storage or weather conditions may point towards quite different materials. Processing of the laminate (as sheet or roll material) and its printing determine the machining ability and surface properties. Various printing methods or writeability require quite different film carrier materials. Economical considerations may interfere with technical requirements. The following parameters influence the choice of face stock materials: conformability, grain direction, appearance, printing method, mechanical properties, chemical and environmental resistance, and primer or barrier coating [22,23].

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Edge lifting of labels can occur if a label stock is too stiff to conform to a curved surface. For foil laminates and whenever possible, the layout should be in the long-grain direction to provide maximum conformability. Many plastic containers are labeled with 60-lb high-gloss paper or foil-laminated papers selected for their attractive appearance. Special inks and printing techniques may be used because of the label end-use. Cosmetic bottles, for example, are commonly labeled with a PVC-coated foil laminate which resists alcohol [22]. The required tear and tensile strength of the face material of the label depend upon the label converting method and the method of applying the finished labels. Additional factors to be considered include UV light resistance and stability, cold temperature application and resistance, water immersion or high humidity resistance, and resistance to various chemicals. A primer coating which improves the anchorage of the adhesive to the label stock may also provide a barrier to prevent components in the adhesive from penetrating the label stock. Figure 7.3 summarizes the main criteria for the choice of the face stock material for PSA laminates. Through the choice of the face stock materials new trends may be observed [24]. Some years ago, price and productivity factors were predominant. Today environmental, esthetical, chemical resistance, flexibility, and recycling considerations appear more important. The most important face stock materials used are fibrous materials and films. Fibrous materials are mostly paper-like products (paper and synthetic paper), textiles, and nonwoven materials. Films may be made of plastic, metal, or a combination thereof (metallized plastic) [25,26]. The most

Figure 7.3

The main criteria for the choice of the face stock material.

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commonly employed film face stock materials in PSAs are polyester, polypropylene, polyethylene, polyvinyl chloride and, of course, reinforced paper [27]. Paper was the traditional face material for PSA labels for most of the almost fifty years of the existence of the industry. This is changing, the estimate for nonpaper materials running as high as 40% by 1990 [28], and actually covering more than 50%. There are several reasons for this trend; pulp prices have risen dramatically while petroleum prices have declined. Changing packaging concepts such as the squeezable plastic bottles dictate the use of flexible face materials. Eye-catching labels compete more effectively. Durability of the label and retention of its appearance also are factors, with the ‘‘last look’’ prior to discarding the package forming a mental image which can influence future buying decisions [29]. Previously, the only opportunity for film-based primary labels was in situations where the package might be exposed to harsh environments or where superior graphics were necessary [30]. Actually the packaging industry was looking for striking graphic effects and chose relatively expensive plastic face stocks because of their smoother, more uniform printing surface [31]. Paper Used as Face Stock. Paper is the main face stock and release liner material actually used. Its good wettability, dimensional stability during coating, printability, and cuttability as well as its well-known technical history as an information carrier made it possible to use it from the beginning of the label industry; an ideal face stock paper possesses a high gloss, a high surface strength, a good wet-out, and good anchorage for printing inks [26]. A high surface strength (tear) may be achieved with a high binder level. Gloss and wetting out are inﬂuenced by the pigments used. The most important mechanical properties of paper type as a function of its weight are listed in Table 7.1. Noncoated papers, clay-coated papers, colored papers, paper-film laminates, and special papers also are used as face stock material [28]. The most important factors for base papers used for pressure-sensitive face materials are the required quality level, quality consistency, and efficiency [31]. Special paper grades can be selected which exhibit barrier properties (e.g., to prevent the paper surface being marred by bleedthrough of adhesive components). The pressure-sensitive label papers must be suitable for reelto-reel conversion, reel-to-reel printing, electronic data processing (EDP) label printing, and for sheet printing. Demands on label paper from the converter’s point of view are printability, die-cutting, lay-flat, and matrix stripping properties. Suitable pressure-sensitive label papers must be printable in all commercial printing systems: letterpress, flexo, rotogravure, offset, silk screen printing. Nonimpact printing methods were developed also and are used specially for narrow web printing. Important growth in

368 Table 7.1

Chapter 7 Quality Criteria for Face Stock Paper Properties Tensile strength (N/cm)

Weight Thickness

MD

CD

MD

CD

Bekk smoothness Bursting (sec) pressure 2 Top Bottom (N/cm )

50 60 70 80 80 110 125 150 330

40 40 40 60 45 100 100 80 250

14 25 18 25 20 50 50 60 100

4 4 4 4 4 20 20 7 4

6 6 6 6 6 20 20 18 6

2000 1500 300 1500 200 — — — 50

0.050 0.050 0.075 0.080 0.085 0.150 0.180 0.150 0.430

Elongation at break (%)

200 400 30 500 200 — — — 10

15 15 18 25 18 — — — —

the label industry has come from blank labels or nearly blank labels, for which more printing is done at the point of use. Therefore combined printing methods have been among the main processes used. Years ago, either simple, uncoated grades or cast-coated papers were used for pressure-sensitive labels. The properties of different paper qualities have been studied [32]. The sensitivity of paper towards humidity should also be taken into account (see Chap. 3) since paper is made up of both crystalline and amorphous cellulose, with the latter undergoing softening as the moisture content increases. The elastic modulus of dry paper is usually considered to be temperature dependent. The decrease in relative modulus with increasing moisture content is different for papers of different crystallinity [33] (see Chap. 3, Section 2.1 too). There are very different paper qualities used as face stock, and there are many quality criteria recommended for selecting adequate face stock materials. In a first step, the paper weight is used as a simple quality index. Almost without exception, standard labels are printed in color on 85–100-g paper. A high stiffness can be obtained by using a high weight or a more voluminous paper grade. The strong influence of the calendering should also be noted. Papers used as face stock material have a weight ranging from 30 to 250 g/m2 and weigh generally about 80 g/m2 [25,28]. Paper for data systems is primarily uncoated, engineered for strength, and has a high absorbency to capture ink from a computer terminal [34]. Coated papers constitute the larger market segment, ranging from cast-coated sheets to enamel and matte finish grades. These include thermal

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sensitive and other specially treated coated products. Specialty structures are another major market that includes fluorescent-coated papers, latex impregnated papers, polyethylene and paper laminations, and metallized laminates [35]. Uncoated and coated papers are used in impact printing: flexography, gravure, dot matrix, ultraviolet light-cured letterpress, and offset. Both types of papers also are used in nonimpact printing, laser, inkjet, ion deposition, magnetography, and direct thermal and thermal transfer printing. In thermal transfer two contradictory properties are required: a high degree of smoothness and a surface that at the same time will absorb ink. For special uses there are different kinds of paper; humidity resistance, fat resistance, and a flameproof character are characteristics of these papers. Laminated papers are manufactured in order to achieve chemical resistance. Metallized papers are used to get a better lay-flat; aluminum/ paper laminates (7 m aluminum, 50-g paper) also are used. For dispersion coating, paper heavier than 80 g/m2 should be used in order to ensure enough dimensional stability [35]. On the other hand, the porosity of paper is more critical when direct coating. For labels for high speed printing applications, the absorbency and porosity of the paper should be improved [36]. The technical requirements for thermal papers were described by Park [37]. Paper types used include calendered Kraft for paper roll labels with the following advantages: good surface hold-out, smoothness, dimensional stability, and an average weight of 65 g/m2 [38]. The effect of super-calendering on paper properties is better gloss, smoothness, and ink hold-out; super-calendering increases the stiffness of the paper [39]. Clay-coated paper is used for sheet labels with the following advantages: smoothness, dimensional stability, lay-flat during printing and converting, with an average weight of 85 g/m2 [28]. The stiffness of machine-finished, machine-glossed papers is higher than that of colored clay-coated and calendered paper [40]. Polymer-coated papers for transfer coating of aqueous adhesives [38] are suitable for very low coating weights, possess a waterproof layer under the silicone, and exhibit good solvent resistance, as well as good dimensional stability with a double-sided coating. Clay-coated papers make up more than 40% of the market for paper face stock materials [41]. The overlayer of coated papers may contain chemically very different components such as kaolin, casein, acrylics, polystyrene-butadiene latex, clay, starch, etc. [42]. The general requirements for paper as carrier material and the special requirements for paper used as carrier material for different pressure-sensitive products are discussed in detail in [43]. Films Used as Face Stock. In addition to the traditional base materials within the range of high quality papers, ﬁlms made of thermoplastic materials are becoming more and more important. The

370

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volume of PSA labels made of paper face stocks remains signiﬁcantly larger than the volume of labels made with ﬁlm face stocks. This is primarily due to the relative cost diﬀerences between the face stocks. Hence the beneﬁts of ﬁlm substrates for PSA labels are centered on appearance and performance, and the dominance of paper in the PSA label market is being challenged; this trend is more pronounced in Japan and Europe [44]. The advantages of face stock materials other than paper include weather resistance, durability, resistance against packaged ﬂuids, squeezability, esthetics, and transparency [45]. The following materials are used as film face stock: polyamide, polyamide/aluminum, polyvinyl chloride, oriented polypropylene, polyethylene, polyethylene terephthalate, and metallized plastics [45,46]. In Europe, in the 1980s, 12% of the labels were film labels, on the bases of PVC, PET, polystyrene, polypropylene, and extended polyethylene [21]. Other films used as face stock materials include cellulose acetate, cellulose triacetate, cellulose acetobutyrate, and cellulose hydrate [47]. According to Thorne, at the end of the 1980s, nonpaper products would possess a market share of 12% by volume and 25% by value in Western Europe [48]. Actually more than 20% of the face stock materials (in volume) are film based. It was estimated that the cost of using synthetics is 4–5 times that of paper. Therefore the first synthetics were only used when the properties of strength, fine print acceptance, and moisture resistance were essential. In the last decade the premises of the use of films were substantially changed. Less expensive films were developed, and the paper price increased. Coatability and printability of films were strongly improved. Films achieved a well-defined standard quality together with other plastics-based web-like materials, e.g., foams, nonwovens, etc. Thus for PSA labels with synthetic face materials, the nature, caliper, gloss, opacity, and corona treatment are detailed in the specification. Generally, films must display good coatability and convertability in a laminate, using the same manufacturing conditions as paper-based laminates. The wetting out of the film materials is more difficult because of the smoothness, the lack of porosity, the low surface tension, and additive migration from the film. Coatability is influenced by the high deformability (elongation) and low temperature resistance of most films; convertability of the laminate is influenced by the plasticity/elasticity balance of the material. The films most commonly used as face stock materials are PVC, polypropylene, and PET [49]. Special polyethylene face stock materials were developed also. Such carrier materials are top-coated or pretreated to allow smooth transport through the printer without jamming and damaging the fusing drum in the case of laser printers. Packaging tapes are dominated by oriented polypropylene (OPP) and PVC [50]; 150 m PVC is also used

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for labels [51]. Calendered PVC and biaxially oriented polyester face stocks have dominated film applications in primary labels over the last ten years. Table 7.2 lists the plastic materials used (alone or as laminate) as face stock materials for PSAs. Proprietary synthetic papers such as DuPont’s Tyvek, Arjo Wiggins’ Synteape, and Polyart from Arjobex also find wide usage. Generally, both soft PVC (20–50% plasticizer) and hard PVC are used as face stock materials. For tapes 25–40 m PVC, polypropylene, and PET were used [49, 52]. PVC (polyvinyl chloride), or ‘‘vinyl’’ was the first major plastic to be used as face stock material. It was first used in the 1950s and remains a significant factor in the market place [44]. Its primary disadvantage results from the need to add plasticizers to achieve flexibility and stabilizers to prevent degradation. Plasticizers and stabilizers can migrate and affect the adhesive performance, so careful selection and testing are required. The problem of changes in peel value by plasticizer migration is a common phenomenon for removable PSA-coated soft PVC. The properties of hard PVC depend on the manufacturing procedure. Suspension-made

Table 7.2

Plastics used as Face Stock Materials Primed

Commercial Products

Material

Yes No Trade name

Cellulose acetate BOPP OPP OPP PE PE PET PET Polyoleﬁn Polyoleﬁn PE/PS Polyoleﬁn Polyoleﬁn Polyoleﬁn Polyoleﬁn PS PVC PVC

—

— X

X x X X X X x X X x X X x x x x

— Rayoface Kimdura Oppalyte Compucal Fle — Polylaser Prymalin Opticlear Polyart Maclitho IMAGin Megarex Teslin Opticite — FlexmarkW-400-FW

Supplier converter — UCB Kimberley-Clark Mobil Flexcon Flexcon Decora Raﬂatac 3M Mactac BXL Mactac Mactac x-ﬁlm Impact Dow — Flexcon

Alternative to Ref. PVC PVC PVC — — — — Paper — — — Paper PVC PVC Paper — —

[44] — [44] [44] [44] [44] [44] [53] [44] [44] — [44] [54] [55] [53] [44] [44] [56]

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PVC has superior electrical properties to emulsion-based PVC. Hard PVC possesses a good resistance against chemical oils and greases with good mechanical properties and inflammability resistance [43]. The drying temperature of soft PVC should not exceed a maximum of 45–50 C, for hard PVC 75 C. Printability depends on the nature of plasticizers used in the PVC recipe [57]. Generally, monomeric plasticized PVC is used as face stock for labels [58]. Lay-flat for paper requires the control of the humidity with a precision of 0.3–0.6%. Lay-flat of printed PVC film (coated with PSAs) depends on the thickness of the film, the thickness of the ink layer, the bonding forces between label and release, and the elasticity of the film and ink [59]. Film face stock materials may be coated directly or by transfer. Although new, high productivity machines use the transfer process, there remain some older special machines using the direct process. The surface tension of the web and the surface tension of the water-based PSAs influence the wet-out. In the most extreme case, when the surface tension of the dispersion differs greatly from the critical surface tension of the film, fish eyes and cratering will result (when the film surface tension is less than the dispersion’s surface tension). Fortunately PVC displays an increased surface tension (40 mN/m) [60] as compared to the surface tension of the common water-based acrylic PSAs (28–38 mN/m). Soft and hard (plasticized) PVC exhibit different levels of critical surface tension. The critical surface tension of rigid PVC is 39 mN/m whereas flexible soft PVC has a critical surface tension with values of 33–38 mN/m [61]. The most important disadvantages of soft PVC films are the shrinkage and the flagging of the edges. Both phenomena are most apparent for extruded PVC film. Shrinkage concerns the change of the original dimensions during storage at room or higher temperatures due to chemical or physical (environmental) influences. Shrinkage is caused by the residual stresses introduced in the material (i.e., by relaxation). In the manufacture of a PSA laminate and during PSA coating or laminating, the PVC film is stretched and strained; the strained film will be laminated with the dimensionally more stable paper-based release liners. During storage relaxation occurs and the strained material returns to its original length (i.e., it shrinks) (Fig. 7.4). The shrinkage of soft PVC does not depend on the tension of the film. It is caused by the laminate manufacture. Another part of the shrinkage is induced during the manufacture of the film. This part is more pronounced for extruded films. Usually during the manufacturing of the base film, heat is applied to convert dry powder blends or granules into a molten state for calendering or extrusion into a plastic film [62]. Whilst still warm the unsupported film is often subject to tension which can cause stretching in the longitudinal direction. This manifests itself as shrinkage (recovery of

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373

Figure 7.4 Schematical representation of the shrinkage of a PSA laminate: A) nonprinted material; B) printed material; 1) release paper; 2) release layer; 3) adhesive layer; 4) face stock material; 5) printing ink.

strain) when the product is reheated to the temperature at which the tension was applied. The laminator must act to eliminate this hazard with film made under controlled conditions. Additionally, product shrinkage can be induced by residual solvents after printing, or through plasticizer migration out of the film into the adhesive or substrate. Flagging or wing-up of the labels on curved surfaces due to insufficient adhesion of the PSAs to the substrate is different from flagging after coating (i.e., the loss of the lay-flat). This phenomenon is due to the chemical influence of the adhesive or printing inks [59]. Wing-up depends on the following factors: the adhesive/printing ink composition, the thickness of the film, the adhesive properties of the PSAs, the thickness of the adhesive/ink film, the differences in the modulus of coating/coated material, and the coating width (full or limited). Figure 7.5 illustrates flagging after printing of a PSA film. Another disadvantage of PVC used as face stock material is the requirement for special coating machines to avoid tensile forces during coating and laminating [63]. Plasticizer migration from the PVC may influence not only the face stock shrinkage and thickness, but also the adhesive properties. Peel adhesion values change when removable PSAs are used on plasticized face stock [64]. An additional disadvantage of the use of PVC as face stock is its sensitivity towards adhesive rheology. This is a general phenomenon also characteristic for other film face stock materials.

374

Figure 7.5 applied.

Chapter 7

Flagging after printing of a PSA ﬁlm: 1) nonprinted; 2) printed; 3)

On the other hand, this phenomenon is induced by and related to the adhesive. Practically, the poor coating rheology typical of conventional latex adhesives manifests itself in several ways: first, there is a tendency in roll applications to form ridges parallel to the direction of coating. Ridges adversely affect adhesive performance by reducing the effective area of contact with the secondary substrate, but more importantly they substantially detract from the appearance of a lamination when a clear film is used (see Section 2.9 of Chap. 8). PVC has a temperature resistance of about 70 C, which is low compared to cellulose acetate (120 C). Processing of PVC needs special machines because of its high elongation. Peel values on plasticized PVC may vary [65]. Stabilizers of PVC (e.g., Pb, Sn) cause oxidation of the tackifier resin and thus loss of the tack. Thickness variations of vinyl labels often cause problems in the applications [66]. Generally, thickness variations of 10% are observed during the manufacture of PVC film. Other data show that 8% profile tolerances are common [67]. The environment and product resistant properties of film labels are significant advantages [68]. These advantages are obvious in high humidity environments and they also provide a significant marketing advantage. The polar surface of PVC provides good printability (i.e., PVC displays good processability and printability). PVC films used as face stock materials may be cured via electron beam [69]. They may be coated with silicones using electron beam-curing. The back side of the film may be coated via screen printing with a PSA; thus a ‘‘mono-web’’ material may be manufactured.

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In the range of plastic films PVC displays the best balance between coating, converting, and end-use properties (e.g., good wettability, good anchorage, high drying temperature resistance, good printability, good weatherability, and nonflammability). Its replacement with polyolefins is due mainly to environmental considerations. There are some differences between the properties of clear and opaque PVC: Soft, clear PVC contains more plasticizer; consequently, shrinkage values are higher for clear PVC than for opaque film. Soft, clear PVC is more flexible than opaque film, thus bonding wet adhesion (BWA) values are better. Soft, clear PVC is more ‘‘plastic’’ than opaque film, hence cutting properties are poorer than those for opaque film. Soft, clear PVC has lower surface tension values, therefore wet-out is better on opaque film. The range of polyoleﬁns used as face stock materials is shown in Table 7.3, with each kind of plastic ﬁlm displaying its special advantages; in parallel to PVC other PSA ﬁlm materials were developed [70]. The most dynamic segment of other face stock materials is that of the polyolefins, mainly polyethylene and polypropylene. The properties of face stock materials used for tapes were summarized by Meinel [71]. A detailed description of the polyolefin films used as packaging and face stock material is given by Placzek [72–74] and their printability is discussed by Verseau [47]. Polyolefin carrier materials for different PSPs and their manufacture are presented in [17]. Polyethylene face stocks offer excellent chemical, solvent, moisture, and shrinkage resistance. Because of its flexibility, polyethylene was used successfully as squeezable label material on plastic packages [30]; there are different kinds of polyethylene, the most important materials being

Table 7.3 Performance Characteristics of Different Plastic Films Used as Face Stock Materials Performance Characteristics of Plastic Films Criteria Stiﬀness Stiﬀness Die-cuttability Thermal resistance Ductility Tear resistance Coatability

LDPE

HDPE

OPP

Hard PVC

PET

Low Very low Very low High High Low

Medium Medium Low Medium Medium Low

Good Good Good Low Very low Low

Very good Very good Good Low Low Very good

Very good Very good Very good Low Very low Medium

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LDPE and HDPE. LDPE is clear and HDPE has a slight white color (opacity) [75]. HDPE is generally used for labels dispensed with labeling guns; it is used for plastic bottle labeling (oils, chemicals, cosmetics). LDPE possesses a good reversible deformability and is adequate for squeeze bottles. Polyethylene is sensitive towards solutions of surface active agents and environmental stress cracking [57]. It is temperature sensitive, therefore the drying temperature should be less than 50 C during printing. Polyethylene is used as face stock material alone or as a composite (co-extrudate or co-laminate). Polyethylene-coated paper (cardboard) or nonwoven coated paper are used for special labels. The use of polyethylene as release liner will be discussed later (see Section 1.7 of Chap. 9). It should be mentioned that top-coated polyethylene may be used also. Polypropylene films offer high clarity and good resistance properties. Polypropylene labels offer potential as lower cost alternatives to polyester labels in numerous applications [30]. Polypropylene is stiffer, cleaner, and has better temperature resistance than polyethylene. Special polypropylenes may be subjected to radiation sterilization. The drying temperature when printing on polypropylene may attain 60 C [57] oriented polypropylene may withstand 110 C. Oriented polypropylene face stock materials are manufactured with a resulting thickness range of 19–60 m [76]. Generally, OPP face stock materials for labels display the following benefits [63]: strength and durability, versatility, choice of finishes, printability, cost effectiveness, compatibility of polypropylene label and container, environmental acceptability, a noncrosslinked character, and excellent die-cutting properties. There are matte and glossy, transparent, white, and pearlized, coronatreated (on both sides) films, with a gauge of 55–80 m [77]. A range of oriented polypropylene (OPP) labeling films for in-mold and self-adhesive applications has been introduced. These films offer a choice of finishes. Printable with all usual methods, they are expected to be suitable for end-uses ranging from toiletries to food, car care, and gardening products [78]. Mobil is manufacturing clear and opaque OPP films as face stock material (Labe-LyteÕ ), with a gauge of 19–60 m, used in pressuresensitive label applications. They are said to be adequate for label converters using different printing, and cutting techniques, to ensure sensitive roll stock laminates of superior resistance to tearing and moisture, and breakage reduction during stripping [79]. There are different polypropylene film qualities used as face stock material, each with its advantages. A comparison of the main properties of polyethylene and cast and oriented polypropylene used as face stock material is given in Table 7.4. Polypropylene films manufactured via the chill roll procedure show fewer thickness variations than blown film (Table 7.4). A surface treatment on blown polypropylene films lasts longer than on cast polypropylene

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Table 7.4 CPP/OPP/PE, Used as Face Stock Materials: Comparison of the Main Properties Face Stock Material Properties

CPP

OPP

PE

Thickness Proﬁle Transparency Shrinkage Storage dimensional stability Strain resistance Price

þþ —þ þ— þ— þ— þ— þþ

—— þþ þþ þþ þþ þþ ——

þþ —— —— —— —— —— þþ

Table 7.5 The Choice of the Film Face Stock Material as a Function of the EndUse of the Laminate Performance Characteristics, Flm Material End-Use Requirements Chemical resistance Moisture resistance Shrinkage resistance Flexibility Clarity

PE

PS

PP

Soft PVC

Good Very good Good Good Low

Medium Very good Very good Low Very good

Good Very good Medium-low Medium-low Very good

Low Medium Low Very good Low

films [47]. Taking into account its mechanical and thermal characteristics (stiffness and temperature resistance) as well as the optical ones, polypropylene seems more appropriate as a replacement for PVC than polyethylene. A detailed discussion of PVC versus polyolefins as face stock material was given by Hammerschmidt [80]. For large surface labels cast PVC should be selected; a lower price alternative is calendered PVC. Polyester displays better temperature resistance, cellulose acetate better clarity. In any case, the choice of an adequate material also is influenced by its stiffness (E modulus) (Table 7.5). In the same range of materials, the required thickness may influence the formulation. As an example, for a thickness of up to 60 m hard PVC is used; above 80–120 m soft PVC is suggested. Low Surface Energy Face Stock Materials. Materials with low surface energy such as polyethylene and polypropylene are diﬃcult to be

378

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coated. Wetting out on these materials is problematic and they exhibit a low bond strength (anchorage) unless the surface is pretreated to improve wettability and adhesion (see Chap. 2, Section 2 also). The surface tension of water is 72 mN/m, while a typical solvent has a surface tension of 25– 30 mN/m. With many substrates having surface free energies in the range of 30–40 mN/m wetting proves to be problematic. In the case of water-based PSA technology, the surface energy of the web must be raised 7–10 units above the dyne level of the coating [81]. Polyethylene for tapes and labels has to be treated [82]. Numerous pretreatment methods in order to improve the wettability and adhesion on the surface of nonpolar face stock materials are known [83]. Generally, they improve the polarity of the material through physical or chemical eﬀects [84]. The most common classical treatment methods are chemical treatment [85], flame (thermal) treatment [86,87], and corona treatment [88,89]. Chemicals or flames produce a dehydrogenation/oxidation of the surface. No universal theory exists for corona treatment, but the number of CO groups increases as a function of the electrical energy (power) used [90]; for example, about 2–3 104 carboxyl groups per cm2 are formed for an energy level of 500 J/m2. Corona treatment works by setting up an electrical field around the film to be treated. This field ionizes the air molecules directly in its vicinity. These ionized air molecules, which are usually in the form of ozone and/or nitrous oxides, actually oxidize the surface of the film. The untreated surface is composed entirely of carbon/hydrogen repeating units. After the corona treatment, the carbon/hydrogen units are oxidized to include hydroxyls, carboxylic acid, and occasionally amine, or nitrous groups. The chemical change of the surface accounts for the increase of the critical surface tension. The effect of this oxidation only penetrates 5–10 A˚ beneath the surface of the material; this may not yield a permanent change of the film surface tension as plastic films are in a constant state of change and untreated polymers can in time migrate to the surface and cover the treated polymers. Generally, different frequencies between 10–80 kHz are used; higher frequencies yield a better treatment. The final surface tension depends on the material treated. Polypropylene is more difficult to be treated than polyethylene. A disadvantage of the corona treatment is its short duration. The surface tension of a corona-treated blown film decreases from 56 to 36 mN/m; this effect is due to the migration of slip additives [91]. Untreated polyethylene and other polyolefins are difficult face materials for acrylic adhesives, and a large amount of effort has gone into the search for a technique which will easily and reliably increase anchorage without a significant change in the bulk properties. The most extensively studied techniques for enhancing adhesive bonding are treatment with helium

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gas plasma, oxygen gas plasma, or chromic acid. These and other surface modification procedures suffer from the same common shortcoming as corona treatment: the short life time of the treatment [92]. Corona treatment is possible for different face stock materials, including polymers [93–98], aluminum [99], or other similar materials [100,101]. During corona treatment (in a 12–20 kV, 20 kHz electrical field) chain scission and oxidation occur. Energy-rich electrons and ions break carbon-carbon (C-C) and carbon-hydrogen (C-H) levels (3.7 and 4.3 eV, respectively). The distance of the electrodes from the web influences the efficiency of the processes; the best results are obtained with a gap of about 1 mm [102]. Although corona treatment is a complicated subject, one can conclude from current data that corona treatment generates polar sites and then increases the polar surface component of the free energy [103]. The possible functional groups that may be formed are shown in Table 7.6. The surface tension of polypropylene amounts to 28 mN/m; corona treatment raises the surface tension up to 50 mN/m [102]. For printing and adhesion the surface tension should be 40 mN/m [107]. HDPE is more difficult to treat than LDPE. After a day the concentration of oxygen

Table 7.6 The Influence of the Corona Treatment on the Polyolefinic Surface. Possible Polar Functional Groups Formed [104–106] Oxygen

Nitrogen

Halogen

R–OH

RNH

RC