pressure-sensitive adhesives and applications

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

Istvan Benedek Wuppertal, Germany

M A R C E L

MARCEL DEKKER, INC.

D E K K E R

NEW YORK - BASEL

The rst edition of this book was published as Pressure-Sensitive Adhesives

Technology, Istvan Benedek and Luc J. Heymans (Marcel Dekker, Inc., 1996).

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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, Pressure-

Sensitive Adhesives Technology (Marcel Dekker, 1996), the rst of these

books, constitutes the main step on the way to understanding adhesive-

based 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 pressure-

sensitive products. Other scientic 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

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

Istvan Benedek

iv Preface to the Second Edition

Preface to the First Edition

Since their introduction half a century ago, pressure-sensitive adhesives havebeen successfully applied in many elds. They are used in self-adhesivelabels, and tapes and protective lms, as well as in dermal dosage systemsfor pharmaceutical applications, the assembly of automotive parts, toys,and electronic circuits and keyboards. They have experienced an astonishinggrowth rate, and the installed manufacturing and converting capacity hasalso sharply increased. A specic engineering technology for pressure-sensitive 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 thoroughknowledge of basic rheological and viscoelastic phenomena. Adhesive andpolymer scientists, however, are not very often employed as industrialmanagers or machine operators. Therefore the need arises to investigate andsummarize the most important features of pressure-sensitive adhesivetechnology and to explain the phenomena scientically. This book coversall the elds of manufacturing, conversion, and application and end-uses ofpressure-sensitive adhesives.

The classical approach would be to compile a treatise based on thework of various experts, theoreticians, chemists, and engineers, therebycoming up with a book consisting of a series of papers with a common titleonly. We have, however, chosen a dierent approach. Based on ourexperience as engineers (in both scientic activity and industrial areas) andusing the available technical literature, we have addressed all aspects ofpressure-sensitive adhesives. We have included the scientic basis ofsuitability for specic applications (i.e., chemical and physical, rheology),the raw materials, the manufacture (formulation) of the adhesive and of thelabelstock (converting the adhesive). We have selected self-adhesive labelsas the most complex self-adhesive laminate; we mainly discuss labels, but,

v

whenever possible, a comparison with and extension to other applications isincluded. In order to illustrate the dierent topics and issues discussed, wehave referred to a number of commercially available products. It should bekept in mind that these products are only mentioned in order to clarify thediscussion and in no way does it constitute any judgment about inherentperformance characteristics or their suitability for specic applications orend-uses.

It is not the aim of this book to establish or complete the science ofpressure-sensitive adhesives, nor does it constitute a series of recipes. Ratherit serves as a practical aid to converters and those involved in the design anduse of pressure-sensitive adhesives.

Istvan BenedekLuc J. Heymans

vi Preface to the First Edition

Contents

Preface to the Second Edition iiiPreface to the First Edition v

1. Introduction 1References 3

2. Rheology of Pressure-Sensitive Adhesives 51 Rheology of Uncoated PSAs 5

1.1 Properties of PSAs 61.2 Inuence of Viscoelastic Properties on the

Adhesive Properties of PSAs 111.3 Inuence of Viscoelastic Properties on the

Converting Properties of PSAs and PSA Laminates 251.4 Inuence of Viscoelastic Properties on

End-Use Properties of PSAs 261.5 Factors Inuencing Viscoelastic Properties of PSAs 31

2 Rheology of PSA Solutions and Dispersions 402.1 Rheology of PSA Solutions 412.2 Rheology of PSA Dispersions 44

3 Rheology of the Pressure-Sensitive Laminate 523.1 Inuence of the Liquid Components of

the Laminate 543.2 Inuence of the Solid Components of

the Laminate 583.3 Inuence of the Composite Structure 81

References 82

vii

3. Physical Basis for the Viscoelastic Behavior ofPressure-Sensitive Adhesives 891 The Role of the Tg in Characterizing PSAs 90

1.1 Values of Tg for Adhesives 911.2 Factors Inuencing Tg 921.3 Adjustment of Tg 1041.4 Correlation Between the Main Adhesive, End-Use,

and Converting Properties of PSAs and Tg 1072 Role of the Modulus in Characterizing PSAs 110

2.1 Factors Inuencing the Modulus 1132.2 Adjustment of the Modulus 1242.3 Modulus Values 126

3. Contact Physics 1293.1 Contact Mechanics for Elastic Materials 1293.2 Contact Mechanics for Viscoelastic Materials 1303.3 Micromechanical Considerations 132

4 The Role of Other Physical Parameters inCharacterizing PSAs 137

References 138

4. Comparison of PSAs 1471 Comparison of PSAs with Thermoplastics and Rubber 147

1.1 Cold Flow 1481.2 Relaxation Phenomena 1551.3 Mechanical Resistance 156

2 Comparison Between PSAs and Other Adhesives 158References 159

5. Chemical Composition of PSAs 1611 Raw Materials 162

1.1 Elastomers 1631.2 Viscoelastomers 1761.3 Viscous Components 1871.4 Components for In-line Synthesis 1891.5 Components for Special Pressure-Sensitive

Formulations 1971.6 Other Components 1981.7 Release Coatings 199

2 Factors Inuencing the Chemical Composition 2052.1 Synthesis 2052.2 Formulation 210

viii Contents

2.3 Physical State of PSAs 2112.4 End-Use 2162.5 Coating Method 2202.6 Solid State Components of the Laminate 2212.7 Product Build-Up 2222.8 Environment Related Considerations 223

References 224

6. Adhesive Performance Characteristics 2351 Adhesion-Cohesion Balance 235

1.1 Tack 2351.2 Peel Adhesion 2611.3 Shear Resistance (Cohesion) 307

2 Inuence of Adhesive Properties on OtherCharacteristics of PSAs 320

2.1 Inuence of Adhesive Propertieson the Converting Properties 320

2.2 Inuence of Adhesive Properties on End-UseProperties 320

2.3 Inuence of Peel Adhesion 3202.4 Inuence of Shear 321

3 Comparison of PSAs on Dierent Chemical Bases 3213.1 Rubber-Based Versus Acrylic-Based PSAs 3213.2 Acrylics and Other Synthetic Polymer-Based

Elastomers 322References 341

7. Converting Properties of PSAs 3531 Convertability of the Adhesive 353

1.1 Convertability of Adhesive as a Function ofthe Physical State 353

1.2 Convertability of Adhesive as a Function ofAdhesive Properties 361

1.3 Convertability of Adhesive as a Function of theSolid State Components of the Laminate 361

1.4 Convertability of Adhesive as a Function ofCoating Technology 361

1.5 Convertability of Adhesive as a Function ofEnd-Use Properties 362

2 Converting Properties of the Laminate 3622.1 Denition and Construction of the Pressure-Sensitive

Laminate 363

Contents ix

2.2 Printability of the Laminate 391References 419

8. Manufacture of Pressure-Sensitive Adhesives 4251 Manufacture of PSA Raw Materials 425

1.1 Natural Raw Materials 4261.2 Synthetic Raw Materials 427

2 Formulating PSAs 4292.1 Adhesive Properties 4302.2 Formulating Opportunities 4322.3 Tackication 4332.4 Rosin-Based Tackiers 4612.5 Hydrocarbon-Based Tackiers 4682.6 Special Tackier Resins 4762.7 Tackication with Plasticizers 4772.8 Cohesion Regulation 4802.9 Coating Properties 4812.10 Converting Properties 4842.11 End-Use Properties 4892.12 Inuence of Adhesive Technology 5052.13 Technological Considerations 5412.14 Comparison Between Solvent-Based,

Water-Based, and Hot-Melt PSAs 543References 549

9. Manufacture of Pressure-Sensitive Labels 5591 Coating Technology 5602 Coating Machines 563

2.1 Adhesive Coating Machines 5642.2 Coating Devices/Coating Systems 5652.3 Choice of Coating Geometry 5852.4 Other Coating Devices 591

3 Coating of Hot-Melt PSAs 5943.1 Roll Coaters for Hot-Melt PSAs 5943.2 Slot-Die Coating for Hot-Melt PSAs 595

4 Drying of the Coating 5984.1 Adhesive Drying Tunnel 598

5 Environmental Considerations 6026 Simultaneous Manufacture of PSAs and PSA Laminates 612

6.1 Radiation Curing of PSAs 6126.2 Siliconizing Through Radiation 613

x Contents

7 Manufacture of the Release Liner 6167.1 Nature of the Release Liner 6167.2 Coating Machines for Silicones 6197.3 Technology for Solvent-Based Systems 6197.4 Technology for Solventless Siliconizing 619

8 Rehumidication/Conditioning 621References 621

10. Test Methods 6291 Evaluation of the Liquid Adhesive 629

1.1 Hot-Melt PSAs 6301.2 Solvent-Based PSAs 6301.3 Water-Based PSAs 630

2 Evaluation of the Solid Adhesive 6392.1 Test of Coating Weight 6392.2 Other Properties 640

3 Laminate Properties 6433.1 General Laminate Properties 6443.2 Special Laminate Properties 687

References 700

Abbreviations and Acronyms 7091 Compounds 7092 Terms 711

Index 715

Contents xi

1Introduction

Adhesives are nonmetallic materials [1] used to bond other materials, mainlyon their surfaces through adhesion and cohesion. Adhesion and cohesionare phenomena which may be described thermodynamically, but actuallythey cannot be measured precisely. It was shown [2] that the most importantbonding processes are bonding by adhesion and bonding with pressure-sensitive adhesives (PSAs). For adhesives working through adhesionphenomena the adhesive uid is transformed after bonding (i.e., the buildup of the joint) into a solid. In the case of PSAs, the adhesive conservesits uid state after the bond building too. Thus its resistance to debondingis moderate and the joint may be delaminated without destroying thelaminate components in most cases.

Pressure-sensitive adhesives have been in wide use since the late 19thcentury, starting with medical tapes and dressings. The rst U.S. patentdescribing the use of a PSAfor a soft, adhering bandagewas issuedin 1846 [3]. Ninety years later Stanton Avery developed and introduced theself-adhesive label [4]. Two major industries resulted from these innovations:pressure-sensitive tapes and labels. Industrial tapes were introduced inthe 1920s and 1930s followed by self-adhesive labels in 1935. About tenyears after that, pressure-sensitive protective lms were manufactured. Thehistory of PSAs was described by Villa [5]. First, solvent-based PSAs usingnatural rubber were developed (19th century). In the 1940s hot-meltadhesives were introduced. Pressure-sensitive adhesives are adhesives thatform lms exhibiting permanent tack, and display an adhesion whichdoes not strongly depend on the substrate [6]. The term PSA has a veryprecise technical denition and has been dealt with extensively in thechemical literature [7,8]. However, as discussed in [9], the technical termof PSA in dierent languages (e.g. pressure-sensitive adhesive, auto-collants, Haftkleber, etc.) is not completely clear. The recent development

1

of pressure-sensitive products without a coated pressure-sensitive adhesivelayer, makes the denition of this product group more dicult [9,10].The build up and classication of pressure-sensitive products have beendiscussed in detail in [9].

The function of PSAs is to ensure instantaneous adhesion uponapplication of a light pressure. Most applications further require thatthey can be easily removed from the surface to which they wereapplied through a light pulling force. Thus PSAs are characterized by abuilt-in capacity to achieve this instantaneous adhesion to a surface withoutactivation, such as a treatment with solvents or heat, and also by havingsucient internal strength so that the adhesive material will not breakup before the bond between the adhesive material and the surfaceruptures. The bonding and the debonding of PSAs are energy-drivenphenomena. Pressure-sensitive adhesives must possess viscous properties inorder to ow and to be able to dissipate energy during the adhesive bondingprocess. However, the adhesive must also be elastic (i.e., it must resistthe tendency to ow) and, in addition, store bond rupture energy in orderto provide good peel and tack performance. Pressure-sensitive adhesivesshould possess typical viscoelastic properties that allow them to respondproperly to both a bonding and a debonding step. For satisfactoryperformance in each of these steps the material must respond to a deformingforce in a prescribed manner.

Polymers employed as PSAs have to fulll partially contradictoryrequirements; they need to adhere to substrates, to display high shearstrength and peel adhesion, and not leave any residue on the substrateupon debonding. In order to meet all these requirements, a compromiseis needed. When using PSAs there appears another dierence from wetadhesives, namely the adhesive does not change its physical state becauselm forming is inherent to PSAs. Thus, PSAs used in self-adhesive laminatesare adhesives which, through their viscoelastic uid state, can build up thejoint without the need to change this ow state during or after application.On the other hand, their uid state allows controlled debondinggiving a temporary character to the bond. Because of the uid characterof the bonded adhesive, the amount of adhesive (i.e., the dimensions ofthe adhesive layer) is limited; the joint works as a thin-layer laminate orcomposite. Because of this special, thin-layer structure of the composite,the solid state components of the laminate exert a strong inuence onthe properties of the adhesive in the composite. Therefore, there exists adierence between the measured properties of the pristine adhesive and ofthe adhesive enclosed within the laminate.

Adhesives, in general, and PSAs, in particular, have to build upa continuous, soft (uid), and tacky (rubbery) layer. The latter will adhere

2 Chapter 1

to the substrate. On the other hand, the liquid adhesive layer of the PSAsworking in the bond has to oer a controlled bond resistance. Thisspecial behavior requires materials exhibiting a viscoelastic character. Theproperties which are essential in characterizing the nature of PSAs comprise:tack, peel adhesion, and shear. The rst measures the adhesives ability toadhere quickly, the second its ability to resist removal through peeling, andthe third its ability to hold in position when shear forces are applied [11].

These properties will be discussed in more detail in Chap. 6, whichdescribes the adhesive properties of PSAs. In order to understand theimportance of these properties, it is absolutely necessary to answer thefollowing questions:

What does the viscoelastic character of a PSA comprise?What is the material basis (main criteria) for the viscoelastic behaviorof a PSA?

REFERENCES

1. DIN 16921.

2. R. Kohler, Adhasion, (3) 90 (1970).

3. J.A. Fries, New Developments in PSA, in TECH 72, Advances in Pressure-

Sensitive Tape Technology, Technical Seminar Proceedings, Itasca, IL, May,

1989.

4. Der Siebdruck, (3) 69 (1986).

5. G.J. Villa, Adhasion, (10) 284 (1977).

6. Vinnapas, Eigenschaften und Anwendung, 7.1. Teil, Anwendung, Wacker

GmbH, Munchen, 1976.

7. R. Houwink and G. Salomon, Adhesion and Adhesives, Vol. 2, Chapter 17,

Elsevier Co., New York, 1982.

8. D. Satas, Handbook of Pressure Sensitive Technology, Van Nostrand

Rheinhold Co., New York, 1982.

9. I. Benedek, Development and Manufacture of Pressure-Sensitive Products,

Marcel Dekker, New York, 1999.

10. I. Benedek, Pressure-Sensitive Formulation, VSP, Utrecht, 2000.

11. J.P. Keally and R.E. Zenk. (Minnesota Mining and Manuf. Co., USA), Canad.

Pat. 1224.678/10.07.92 (US Pat. 399350).

Introduction 3

2Rheology of Pressure-SensitiveAdhesives

Pressure-sensitive adhesives are viscoelastic materials with ow propertiesplaying a key role in the bond forming; their elasticity plays a key role inthe storage of energy (i.e., the debonding process). The balance of theseproperties governs their time-dependent repositionability and bondingstrength (i.e., their removability). Their ow properties are useful in coatingtechnology and at the same time detrimental to the converting technology oflabels.

Generally PSAs are used as thin layers, therefore their flow is limitedby the physico-mechanical interactions with the solid components of thelaminate (liner and face) materials. On the other hand the solid componentsof 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) maydiffer from those of the pure (unbonded) adhesive. Therefore in this chapterthe rheology of pure and coated PSAs will be dealt with separately.

1 RHEOLOGY OF UNCOATED PSAs

It remains dicult to examine the properties of pure (i.e., uncoated orunbonded) PSAs, and to obtain generally valid information. Pressure-sensitive adhesives are seldom used as thick layers between motionless rigidsurfaces (i.e., as uids). On the other hand, as known from industrialexperience, the nature of the face stock material or of the substrate used,and their characteristics and dimensions may signicantly inuence theproperties of the PSA laminate. Practically, this disadvantage is eliminatedby the use of normalized or standard solid state components. However, atheoretical approach may be used for the investigation of pristine PSAs.

5

1.1 Properties of PSAs

The adhesive and end-use properties of PSAs require a viscoelastic, non-Newtonian ow behavior which is based on the macromolecular nature ofthe adhesive. In order to understand the needs and means of viscoelasticbehavior one needs to summarize the most important material propertiesspecically related to PSAs. Generally, adhesives in a bond behave like auid or a solid. Fluids are characterized by their viscosity which inuencestheir mobility, whereas solids are characterized by their modulus whichdetermines their deformability. In an ideal case, for Newtonian uids (or forsolids obeying Hookes law) the applied force (load) will be balanced bythe materials own mechanical characteristics, that is, the viscosity or theYoungs modulus E:

= 0 2:1E = 2:2

where and are the applied stresses, and and 0 are the strain and shearrate, respectively.

As indicated earlier, PSAs originate from a lm-forming, elastomericmaterial, which combines a high degree of tack with an ability to quickly wetthe surface to which it is applied, to provide instant bonding at low-to-moderate pressure as a result of its ow characteristics. On the other hand,PSAs possess sucient cohesion and elasticity, so that despite theiraggressive tackiness they can be handled with the ngers and removedfrom smooth surfaces without leaving any residue. Moreover, in order toachieve bond strength they have to store energy (i.e., they must be elastic).Fundamentally PSAs require a delicate balance between their viscous andelastic properties. One should note that PSAs have to satisfy thesecontradictory requirements under dierent stress rate conditions, that is,at low shear rates they must ow (bonding) and at high peeling rates theyhave to respond elastically (debonding).

Consequently, according to their adhesive and end-use properties,PSAs cannot be Newtonian systems: they do not obey Newtons 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 orshear rate:

= 0 2:3

That is:

a n 2:4

6 Chapter 2

where a is the apparent viscosity, and n denotes the ow index [1]. ForNewtonian systems the exponent n is one, which implies that the viscositydoes not depend on the shear rate. As pointed out, the viscosity of PSAsdoes depend on the shear rate. This is possibly due to their macromolecularcharacter. Pressure-sensitive adhesives are polymers containing long-chainentangled molecules with intra- and intermolecular mobility. At low strainrates, the viscous components of the polymer dissipate energy, and as aresult resistance to debonding forces is low. At higher strain rates, themolecules have less time to disentangle, and to slide past one another; inthis case viscous ow is reduced, but the elastic modulus or stiness of thepolymer increases [2]. This behavior results in additional stored energy, andthe debonding resistance intensies accordingly.

Practically, the dependence of the adhesive performance character-istics on the stress rate may be observed by peeling o removable PSAs atdierent peel rates: at higher rates paper tear may occur. The stress rate-dependent stiening is an increase in the elastic contribution to the rheologyof the polymer. When the elastic components are predominant more of thebond rupture energy is stored, resulting in higher peel and tack properties.

The end-use properties of PSAs result from the nonlinear viscoelasticbehavior of the adhesive material, and the elastomeric polymer basis ofPSAs imparts them such a viscoelastic behavior. It is evident that the samestiening eect is apparent when the polymer temperature decreases. In thiscase the polymer molecules are again restricted in their ability to ow, andthe modulus increases. Consequently the adhesive properties of PSAs arealso temperature dependent. Thus one always has to take into accountthat the viscoelastic properties of PSAs are strain-rate and temperaturedependent. Zosel [3] demonstrated that the separation or debondingenergy of the adhesive joint is a function of the thermodynamical work ofadhesion and of a temperature and rate-dependent function (dependingon the viscoelastic properties). Accordingly, PSAs would absorb less ormore energy depending on the rate (frequency !) of the applied stress.Practically, end-use situations with dierent stress rates may be simulatedexperimentally by applying a strain to a thin sample of the material andmeasuring the output stress. If the material is an ideal solid, its response iscompletely in phase with the applied strain. A viscoelastic uid, such as aPSA, displays a mixture of solid-like and liquid-like responses. Therefore theoutput stress curve is deconvoluted into an in-phase part (related to energystorage) and an out-of-phase part (related to energy loss). The coecients ofthe in-phase and out-of-phase parts are called the energy storage modulusand the energy loss modulus [4].

According to the theory of Lodge [5] the rheological state of a viscousliquid subject to a sinusoidal deformation will be described by the following

Rheology of Pressure-Sensitive Adhesives 7

equation:

12 ! 0 ! cos! t 2:5

The response of the elastic solid may be described as follows:

12 G! 0 sin! t 2:6

where G is the shear modulus. For the reversible and irreversible work ofdeformation one can write:

12 G! 0 sin! t rev 0 rev ! cos! t 2:712 irr 0 irr ! cos ! t 2:8

where denotes the stress, ! the angular speed, the viscosity, and 0 theamplitude of the deformation.

The storage modulus G0 increases with the frequency:

G0 rev ! G cos 2:9

whereas the viscosity decreases with the frequency; is the loss angle.Pressure-sensitive adhesives must display irreversible work of defor-

mation during bonding and reversible deformation work upon debonding.The ratio of both kinds of deformation work (i.e., of stored and dissipatedenergy) characterizes the behavior of PSAs. In general the energy state of theviscoelastic polymer may be described as follows:

t G0! cos ! t G00! sin ! t 2:10

where G0 is the storage modulus and G00 the loss modulus, and

loss tan loss modulus=storage modulus G00=G0 2:11

Tan is a damping term and is a measure of the ratio of energydissipated as heat, to the maximum of energy stored in the material. One cansuppose that the term loss tan , as an index of the amount of stored orlost energy (i.e., of the contribution of the elastic and viscous part of thepolymer) may also characterize the adhesive properties. It was shown thatloop, peel, and quick stick show a good correlation with loss tan [4]. It wasdemonstrated that PSAs intended for similar application also exhibit similar

8 Chapter 2

rheological properties. The correlation between adhesive properties and thedynamic shear storage modulus appears quite good [6]: hence the conceptof window of performance as a function of the storage modulus of theadhesive 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 motionwithin the polymer is frozen and the material behaves like a glass. At highertemperatures it drops o and exhibits a plateau region which represents theelastomeric response generally encountered at normal end-use temperatures;the storage modulus then decreases further when softening begins. The tem-perature region through which the polymer changes from a glassy (hard)state into a liquid (rubber-like) state, this second order transition point (witha continuous dierential of the free enthalpy, but discontinuous, secondorder dierential of the Gibbs free energy) is called the glass transitiontemperature (Tg), and has a special signicance in the characterization ofPSAs. Above the Tg the time-temperature superposition principle can beapplied. Dierences in viscoelastic parameters around the glass transitioncan 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 .


At Tg there occurs a change in the thermodynamic state which can be relatedto 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 betweenthe glassy and rubbery regions. Ferry [8] pointed out that in this region oftransition there is no change in the thermodynamic state. The loss tan peakis the midpoint of this transition zone where the ratio of loss modulus andstorage modulus reaches a maximum. The energy loss maximum at thispoint has considerable inuence on the tack of the system. The storagemodulus denition can be simplied as a hardness parameter [9]. OptimumPSA performance can be quantied using the storage modulus; ideally thevalue of the storage modulus should vary between 20 and 80 kPa.

Chu [7] correlated PSA performance and dynamic mechanicalperformance (DMA) properties, whereby PSA performance, especially fortapes, was related to the storage modulus G0 at room temperature and theloss tan peak temperature of the system. Chu also showed that the PSAapplication 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 10C. Optimum G0 values forpermanent PSA labels were determined to be around 20 kPa at roomtemperature.

Pressure-sensitive adhesives were dened using viscoelastic applicationwindows relating the storage modulus G at room temperature to the loss tan peak of the adhesive. In water-based adhesives the viscoelastic relationshipis not as simple, that is, it was determined that for the most commonly usedpolymers in water-based dispersions these relationships may not apply. Inhot-melt and solvent-based PSAs a close and predictable relationship existsbetween the loss tan peak, dened as the dynamic Tg, and the Tg asmeasured by dierential scanning calorimetry (DSC). For water-basedadhesives the relationship varies depending upon the polymer type used.The loss tan peak temperature of an acrylic can dier from the Tg (DSC)by as much as 30C. The phenomenon is much less pronounced for styrene-butadiene rubber (SBR)-type polymer dispersions. This variation is alsovalid for an adhesive dispersion containing a tackier. The consequence ofthis is that the loss tan peak temperature cannot be used to predict anddene PSAs performance in a viscoelastic application window. According toBamborough [9] and applying the rule proposed by Chu, it may appear thatan SBR would require considerably more compatible resin than an acrylicpolymer; a soft acrylic (AC) PSA (like Acronal V 205) would require ahigher amount of compatible resin than a hard one (like Acronal 80 D).Adhesive formulators know this not to be the case. Experienced adhesiveformulators know that for Acronal V 205 an optimum concentration oftackier would be 30 parts per 100 parts polymer whereas for Acronal 80 D

10 Chapter 2

one needs 80 parts (see Chap. 6). The loss tan peak temperature for water-based adhesive systems is not a reliable predictor of PSA performancecharacteristics.

It is proposed that the loss modulus peak temperature of the adhesiveshould be 50C below the operating temperature of the adhesive.Bamborough [9] proposed using the loss modulus peak temperature in theglass transition region instead of the loss tan peak temperature as a meansof predicting PSA performance for water-based adhesives. Despite theabove-illustrated discrepancies concerning DMA for adhesive characteriza-tion, the use of dynamic mechanical spectroscopy to measure moduluschanges, and dierential scanning calorimetry to measure shifts in the Tg ofthe adhesive are now common methods for rheological studies of adhesives[10]. In order to understand the practical benets of such investigations therheology of PSAs needs to be studied.

As discussed above PSAs are special products allowing instantaneousbonding due to their liquid-like ow and solid-like debonding resistancedue to their elasticity. Such viscoelastic behavior can be characterizedrheologically by the main parameters of a liquid (viscosity) and a solid(modulus) taking into account their dependence on the time andtemperature (see storage and loss modulus ratio). It is evident that thetemperature domain allowing such viscoelastomer-like behavior has to betaken into account also. Therefore as a supplemental rheological parameterthe value of the Tg should be used also. Extending the use of the William-Landel-Ferry time-temperature superposition principle allowed an easierrheological characterization of PSAs. It is known that the viscoelasticbehavior of amorphous polymers is a function of the time-temperaturedependence, the dependence of the ratio between recoverable energy duringa given deformation and energy losses on the experimental conditions,characterized by the stress rate and temperature, that is of the validity ofthe time-temperature superposition principle. This principle states that theviscoelastic properties at dierent temperatures can be superposed by a shiftof the isotherm data along the logarithmic time-frequency scale. Asdiscussed in [11], by replacing the time with the temperature it waspossible to develop full plastic PSPs, i.e., polymer lms having built-inpressure sensitivity.

1.2 Influence of Viscoelastic Properties on the AdhesiveProperties of PSAs

The essential performance characteristics when characterizing the natureof PSAs are tack, peel adhesion, and resistance to shear. The rst propertyrepresents the adhesives ability to adhere quickly (initial grab), the second


measures its ability to resist removal by peeling, and the third characterizesthe adhesives ability to resist ow when shearing forces are exerted.Generally speaking the rst two characteristics are directly related to eachother, but inversely related to the third one [12].

Tack and peel tests imply a high loading rate, whereas shear will bemostly measured in a static manner. The rst phase in the use of PSAs(bonding) generally occurs slowly, whereas the second step (debondingduring converting or end-use) imposes higher stress rates. The balance ofthe adhesive properties, and of the adhesive/converting/end-use propertiesreects at the same time the need for the balance of the viscoelasticcharacteristics. In this chapter the inuence of the viscoelastic properties onthe adhesive properties will be briey examined.

Influence of Viscoelastic Properties on the Tack of PSAs

According to Rivlin [13] the separation energy after a short contact time andlow pressure is a measure of the tack. A short contact time and low pressureduring application of PSAs imply a high wetting ability. For bonding tooccur there is an a priori need for wetting of the substrate. As conrmed bySheri et al. [14] and by Counsell and Whitehouse [15] tack is a function ofwetting. Good wetting supposes sucient uidity of the adhesive, anduidity is characterized by viscosity.

Tack Dependence on the Viscosity. According to Zosel [16] tack ismeasured in two steps, namely the contact step and the separation step.During the rst step, contact is made in the geometrical surface points,which increase to a larger area through wetting out, viscous ow, and elasticdeformation. Wetting out implies high uidity, as characterized by anadequate viscosity of the adhesive. Wetting out (i.e., covering the surfaceby the uid adhesive) is followed by bonding due to the viscoelasticdeformation of PSAs. On the other hand, debonding assumes the defor-mation of the laminate, the creation of two new surfaces, and deformationof the new surfaces. Thus, it may be concluded that for bond-forming a highdeformation with a medium elasticity is required, whereas for debonding amedium 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 amountof deformation energy, which dissipates on the break of the bond [19]:a high ability to store energy implies elasticity, a high energy at break meanshigh cohesion. Thus, tack depends on elasticity and cohesion. Therefore,for high tack, a low bonding viscosity, a high debonding viscosity, and highelasticity are required. Factors inuencing the viscosity and the elasticity ofthe polymer will also inuence the tack. The polymers own characteristics

12 Chapter 2

and the environmental conditions (experimental parameters) inuence 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 correlatewith 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

Dahlquists 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 sucient 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 to1 105 Pa, and the Tg lies between 10 to 10C [25].

Hamed and Hsieh [26] showed that for a given total bonded area, test

specimens containing noncontact regions of suciently small scale can

exhibit a higher peeling force than those in which the noncontact ones are

large. For an elastomer there is a critical aw size below which adhesive

strength will remain unchanged. Rubbery materials with higher elasticity

have a smaller critical aw size. Thus the modulus inuences the tack.

Rubbery materials with a more elastic response exhibit tack properties that

are more sensitive to interfacial aws, compared to those that respond more

by viscous ow. When failure occurs by viscous ow, the tack is relatively

insensitive to interfacial aws, but when the strain rate is suciently high, so

that the elastomer responds elastically, stresses may be concentrated at the

edges of interfacial aws causing a reduction of the strength.

Tack Dependence on Experimental Parameters. Viscosity and elasticmoduli 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 dierent values of the tack obtained using dierent

experimental techniques (rolling ball, quick stick, or loop tack) which are

characterized by dierent time and strain rates, and by the sensitivity of the

adhesive properties to environmental conditions (see Chap. 10). Dierent


tack methods are adequate for dierent PSPs [27]. With the probe tack testthe load is regulated, as in the lamination of protective lms. Polyken tack isless strongly aected by the resin softening point (it is applied under load!)during tackifying. Loop tack simulates the actual application conditions oflabels, which are blown onto a surface by using air pressure. Rolling balltack is more complex, it is friction related.

Tack Dependence on Temperature. Most general-purpose adhesivesare formulated to have tack at room temperature. If the adherent tempera-ture is lower than room temperature, a higher degree of adhesive cold ow isrequired to provide proper wet-out. In reality it is very dicult to apply PSAlabels at low temperature conditions. Deep freeze label (or tape) adhesivesmust be specially formulated, with a low viscosity at low temperatures [28].This behavior is due to the limited ow of the adhesive at lower tem-peratures, a phenomenon governed by the strong temperature dependenceof the viscosity and of the elastic modulus. Deep freeze labels shoulddisplay the same viscoelastic properties at 40C as at 20C [29]. It isrecommended that such adhesives have a lower storage modulus value thancommon labels (104 Pa). As shown by Hamed and Hsieh [26] tack is afunction of test temperature and strain rate, and the experimental data maybe shifted from a master curve. However, the tack behavior as a function ofthe temperature and strain rate is more complex than that of the cohesion.

Tack Dependence on Strain Rate. It was shown earlier that tackdepends on the bonding and debonding process, on debonding (separation)work, and thus tack also depends on the strain rate. This phenomenon maybe observed by testing the tack using methods measuring the debondingresistance (force), like loop tack or Polyken tack. Tack varies as theseparation speed of the Polyken test is changed [4]. The dependence of thetack value on the increasing speed was conrmed by Hamed and Hsieh [26]too. They observed a nonlinear, discontinuous increase of the tack with themeasurement speed. Tack values rise to a rst maximum and then, after aperiod of slight decrease, rise continuously with the measurement speed.Data obtained from Polyken tack measurements show a correlation betweentack and loss tan peak values [4]. Tack dependence on the debonding ratewas also conrmed by McElrath [21]. He demonstrated that loss modulusvalues depend on the applied frequency, and dierent tack test methods(loop, quick stick, and probe tack) exhibit maxima at dierent frequencies.

Influence of Viscoelastic Properties on the Peel of PSAs

Peel and peel strength are measured by separating an adhesive applied to asubstrate at some angle with respect to the substrate, usually at an angle

14 Chapter 2

of 90 or 180 [30]. Similar to tack, the measurement of the peel adhesioninvolves a bonding step before the debonding or peeling step.

Tack measures the resistance to separation of the adhesive after ashort contact time, or by light pressure. Peel is measured after a relativelylong or very long contact time (at least 102 longer contact time than in thetack test) after application of a light or medium pressure. The time availablefor bond forming (wetting and penetration of the surface) during the rstcontact step is longer for peel measurements than for tack. It follows thatow properties of the adhesive during the bonding step are less critical thanfor the tack measurement. On the other hand, the debonding resistancewill depend on the viscous nature/elasticity balance requiring its preciseadjustment in order to achieve peelability (removability or repositionability)and on the strain rate, which inuences the separation resistance in a morepronounced manner than during the measurement of the tack.

Peel Dependence on Viscosity. Like tack, peel implies a bonding anddebonding step, with the time for the latter lasting longer. Supposedly theinuence of the viscosity on the bonding step during a peel measurementis less important than for the tack. On the other hand, the debondingresistance of the joint is increasingly proportional to the viscous ow of theadhesive (i.e., a high peel needs a solid-like adhesive). In formulationpractice the regulation of the peel by (bonding) viscosity is used for special,rubber-based, low peel products (e.g., tapes and protective lms), wheremechano-chemical destruction of the base elastomer (mastication) is carriedout [31, 32] in order to manufacture a soft adhesive. The value of the peel isalso a criterion for the distinction between removable and permanent labels.The peel dependence on the viscosity (modulus) and its theoretical basis willbe discussed in more detail in Section 1.4.

Peel Dependence on the Elastic Modulus. Special PSAs intended formedical 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 onthe substrate. Creep compliance values greater than 2.3 10 Pa1 are notpreferred.

The peel force is related to the storage modulus G0 of the adhesive.High tack, removable adhesives should have a low G0, and the storagemodulus should not vary much with the peel rate. On the other hand, thedebonding takes place in a much higher frequency range than the bondingprocess. In order to maximize the peel force, the highest possible G0 value inthe high frequency range is needed. Satas [6] showed, that solution polymerswith a higher G0 slope at higher frequencies than emulsion polymers, exhibithigher peel adhesion, as is generally the case with acrylic solution polymers.


Changing the chemical composition (e.g., sequence distribution) may lead toa change (increase) in modulus and eventually to a decrease of the peel force

[34]. Regulating the diblock/triblock ratio in segregated copolymers mayreduce/increase the plateau modulus, i.e., soften or harden the polymer, andinuence the adhesive and converting properties [35].

Peel Dependence on Dwell Time and Strain Rate. As known fromplastic lm processing and use, adhesion and self-adhesion depend on the

contact time. Self-adhesive (i.e., adhesiveless) protective lms may build upa 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 phenomenondue to the macromolecular nature and viscoelasticity of such polymer lms.In a similar manner contact forming or bonding of the adhesive assumes itsviscoelastic deformation. Viscous, slow ow is time dependent and thusbond forming will also be time dependent. According to [36] peel build-up asa 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, thespacing of the asperities, and the surface properties. For certain applicationconditions the time for self-healing (i.e., no external loading) is given by:

theal A,mLCo2oCo=C11=m=W 2:12

where A(,m) is a dimensionless constant depending on m, the creepexponent, and a dimensionless parameter:

A 2

2=p

L=hp

CoWp

2:13

where W/Coo2 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 ofthis materials that have identical creep behavior and work of adhesion canhave very dierent healing times. The healing also depends on the height ofthe asperities through the parameter . This parameter has the followinginterpretation: if the viscoelastic polymer is replaced with an elastomer with

modulus 1/Co then the two surfaces will jump in contact with each otherwith no externally applied load and the contact area will continue toincrease until the surfaces are in full contact if was larger than somecritical value. This critical value is typically of order 1. The formulation ofthis work implicitly assumed that is below the critical value. For typicalvalues of h, L, andW, the elastic modulus which would cause the surfaces to

16 Chapter 2

jump in contact and a self-heal is of order 0.1MPa. Therefore this work ismore relevant for viscoelastic adhesives or uncrosslinked rubbers of highermodulus.

The time dependence of the bonding step is illustrated by the inuenceof the dwell time on the peel values. As discussed in Chap. 10, applicationpressure and temperature inuence the peel and its build-up too. Therefore,as proposed in [37] for special PSPs static and dynamic dwell time shouldbe tested.

Like the pressure used during application, the contact time (i.e., theinterval between bonding and debonding) takes into account the relativelylow mobility (ow rate) of PSAs. A higher pressure or a longer dwell timeshould help adhesive ow and increase the peel resistance. It is known fromPSA characterization, that for the FTM-4 high speed release test thesamples are placed between two metal plates under a pressure of 6.87 kPa toensure good contact. In a similar manner for PSPs with very low tack andinstantaneous peel (e.g., protective lms) bonding depends on the appliedpressure and temperature also. A special class of such products (warmlaminating lms) is used at increased laminating temperature and pressure[38]. Foam-like transfer tapes are also used under pressure to increase initialbond strength [39]. As discussed in [40] time-dependent peel control is one ofthe main requirements for PSA. Generally, until equilibrium is reached, thepeel 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 dwelltime of the adhesive, that is, the viscous and elastic deformation of theadhesive need a period of time (depending on the viscosity and experimentalconditions). As will be discussed later, the time dependence of the bondingimposes the use of normalized dwell times for peel measurement purposes.Generally the peel/dwell time dependence is not linear [42], but partiallycrosslinked silicone rubber exhibits peel values which increase linearly with

Table 2.1 Peel Adhesion as a Function of Increasing Dwell Times

Peel Adhesion (N/25mm)

Dwell Time (week) Sample 1 Sample 2 Sample 3

0 0.32 0.12 0.09

1 0.44 0.24 0.21

2 0.47 0.30 0.30

3 0.55 0.35 0.35

4 0.60 0.40 0.47


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

Figure 2.2 Dependence of the peel values on the dwell time. Peel frompolyethylene as a function of the coating weight for dierent dwell time values.

Dwell time of 1) 20min; 2) 0min.

18 Chapter 2

and a log peel rate between 1.0 and 2.6 cm/min [46]. Another inuence of thedwell time may be observed, when examining the inuence of the storageand aging on the peel adhesion value. In general there is a build-up of thepeel resistance with time.

It should be emphasized that stress transfer during debonding occursby means of the solid components of the laminate. Therefore strain ratedepends upon the carrier and substrate. The denaturation of the peel valuesby carrier deformation (i.e., strain rate) is discussed in detail in [47].

Influence of the Peeling Rate. At low peel rates the viscous ow, andthe deformation of the adhesive layer are dominant for the peel resistance.Therefore the peel resistance increases with increasing peeling rate. At highpeeling rates the elastic character of the adhesive dominates, thus in thisregion the peel resistance no longer depends on the peeling rate. Of practicalimportance is the very pronounced dependence of the peel force on thepeeling rate (Fig. 2.3); the peel force increases with the peel rate [48]. Thisbehavior requires the use of normalized peeling rates for the measurement ofthe peel adhesion force.

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

measured.


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 givenfrequency value. Removable adhesives having a plateau region in thestorage 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; theyalso 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 dierentwater-based PSAs displays large dierences after storage at 100C. Asshown in [25] adhesive break (in the adhesive layer) appears above a certainpeeling rate (2.5mm/min), or below a certain temperature (25C) only.

It may be possible to remove a label from a paper substrate if it ispeeled o very slowly, but the same label will certainly tear if it is peeled

o quickly. Since the peel force is related to the storage modulus of theadhesive, high-tack removable adhesives should possess a low storage

modulus which does not vary much with frequency (rate of peel). At anygiven temperature peel adhesion is observed to increase as the peel rate isincreased [2]. At low strain rates the peel forces are much lower. Under these

conditions, the viscous components of the polymer dissipate signicantamounts 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 pastone another. This behavior results in more stored energy, and peel forcesintensify 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 peelstrength implies a certain modulus value and viscosity. Both modulus and

viscosity depend on the temperature. With the increase of the temperaturethe viscosity of PSAs decreases. Therefore the increase of the temperature

improves the tack and instantaneous peel, and exerts a negative inuence onthe cohesion. Such an inuence 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 untackied PSA at 0C isabout 210% lower than the peel value at 23C. For tackied formulationspeel reduction at 0C may attain 300% [50]. The importance of laminatingtemperature of acrylic PSA coated protective lms on plastic plates wasdemonstrated in [51]. The dependence of the peel value and of the break

nature on the peel rate is a common phenomenon observed in thedelamination of PSPs during their application also. Such dependence

20 Chapter 2

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

Rw f Tw, vw 2:14

Generally the unwinding force depends on a number of parameterssuch as the adhesion force, the moduli of elasticity of the adhesive andcarrier lm, the thicknesses of the adhesive and lm, and the width of thetape. The modulus of the elasticity of the adhesive depends on the time andtemperature, 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 theresistance of adhesive joints to shear stress, and is measured as a force perunit area at failure. The shear force is applied in a plane parallel to theadhesive joint.

Dierent authors have formulated a denition of the cohesivestrength. If deformation by shearing is considered like a ow, the owlimit (FL) is given by the following correlation [53]:

FL E We b2=4h3 2:15

where We denotes the maximum elastic deformation of the sample, b thewidth of the adhesive surface in the stress direction, and h the thickness ofthe adhesive layer. It can be seen from the above relation that the ow 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 thesefactors may be formulated as follows [54]:

F =h2 t 2:16

The inuence of the viscoelastic properties on the adhesive character-istics of PSAs depends on the nature of the stresses applied during tack, peel,or cohesion measurements. Kohler [55] demonstrated that tensile or shearstresses produce dierent levels of deformation in PSAs. When applying a


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

P 3 R4

4 t h22:17

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

0 2:18

where 0 1 sec1, only a 2.5 kg/cm2 force is needed. Thus it may beconcluded that shear-stressed pressure-sensitive joints need lower forcelevels in order to undergo a deformation (i.e., pressure-sensitive joints areweaker than classical ones).

As illustrated by the relations (2.15)(2.18) the shear resistancedepends on the adhesives viscosity and elasticity. If the adhesive isconsidered a Newtonian uid, the shear resistance SH is given as a functionof the viscosity , the adhesive thickness h, and the tensile rate :

SH =h 2:19

If the solid obeys Hookes law the shear resistance depends on the shearmodulus G and the tensile rate :

SH G=h 2:20

Shear Dependence on Viscosity. Toyama and Ito [56] used the data ofcreep testing (shear resistance) to calculate viscosity. Increasing themolecular weight of CSBRs increases the Mooney viscosity [20]. At thesame time an increase in the molecular weight will improve the cohesivestrength (shear) of carboxylated SBR. For styrene block copolymers the roleof viscosity in shear measurement is observed at higher temperatures wherethe molecular association does not work. At room temperature shearresistance is due to the elasticity of the polymer, determined by segregationand its parameters (e.g., sequence length and distribution), at highertemperatures it is given by viscous ow inuenced by copolymercomposition and global molecular weight.

22 Chapter 2

Shear Dependence on the Modulus. According to Woo [57] the shiftfactors used to plot shear measurements at dierent temperatures on a

master curve were almost identical to the shift factors used for moduluscurves. Thus the time to fail in shear can be predicted from the viscoelastic

function. Correlations between the viscoelastic and PSAs performanceproperties can be made by looking at those temperature regions whichcorrelate (via the time-temperature superposition principle) to the time

scales of the adhesive performance parameters. The magnitude of G0 in theupper temperature range is indicative of internal strength or cohesion.

Initial peel, which is largely dependent upon the wet-out characteristics ofthe 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 roomtemperature viscoelastic properties [58]. The decrease of the modulus at high

temperature reduces the shear resistance. Milled rubber possesses a lowerelastic modulus and shorter modulus plateau than rubber from dried latex

[25]. For instance the G0 value at 20C for milled smoked sheet is about105 Pa. The thermo-mechanical degradation of the rubber through millingdoes not change the tan peak temperature (Tg). It does reduce the modulusat high temperatures. This modulus reduction relates to the lower shearperformance of solvent-based systems.

The possibility to characterize application eld-related properties of

PSAs and to correlate them with the rheology of the adhesive is illustratedby formulating adhesives for medical tapes [59]. The adhesive used formedical tapes is characterized by a dynamic shear modulus of about

12 104 Pa, a dynamic loss modulus of 0.60.9 104 Pa, and a modulusratio or tan of about 0.40.6 as determined at an oscillation frequencysweep of 1.0 rad/sec at 25% strain and 36C. Adhesives with moduli higherthan the acceptable range display poor adhesive strength, while adhesives

with moduli below the acceptable range exhibit poor cohesive strength andtransfer large amounts of adhesive to the skin upon removal [59].

Shear Dependence on Time and Strain Rate. The shear resistance ofthe adhesive depends on its internal cohesion. The cohesion is a function ofthe inherent viscosity or modulus and thus it depends on the parameters

inuencing the viscosity or modulus. Both viscosity and modulus are notintrinsic 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 labelingpractice this behavior is illustrated by the low temperature resistance of the

adhesive joints and by the dierences between statical and dynamical shear.As is known, the viscosity depends on the loss modulus (viscous ow is the


phenomenon that loses the energy). It also depends on the shear rate(frequency):

0 G00=! 2:21

At low frequencies 0 and the steady-state viscosity are the same. A staticshear 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 tovariable shear rates, the plot of the shear stress/shear rates no longer showsa linear relationship [60]. In a diagram with double logarithmic scaling thiscurve becomes a straight line. The mathematical equivalent of these twocurves is given by the following equations:

K D00 2:22ln lnK n lnD 2:23

where D denotes the shear rate, K is a viscosity related coecient, and n isan exponent of the power law equation dening the shear rate dependenceon the viscosity. This exponent is known to vary for polymers between0.31.0. Practically, it was demonstrated that an increase of the test rateproduces an increase of the cohesion and of the peel up to a critical value

[26]. As shown for cohesive strength measurements as a function oftemperature and shear rate [26], the principle of strain rate-temperatureequivalence can be applied.

The strain rate inuences the shear resistance by bonding too. Bondingis a diusion and time dependent process. This is illustrated by the pressuredependence of the lap shear adhesion for adhesives; the higher the pressurethe greater the bond strength [61].

Shear Dependence on Temperature. As discussed earlier, the cohesivestrength increases with increasing test rate or with decreasing temperature.

Shear data obtained as a function of the test temperature and shearing ratecan be shifted horizontally to form a single master curve, illustrating theprinciple of time-temperature equivalence. Hamed and Hsieh [26] founda good agreement between experimental and calculated values, using theuniversal Williams-Landel-Ferry relationship for an amorphous rubber.The nding that data can be shifted to form a master curve is evidenceof the importance of chain segmental mobility in controlling the shearstrength. If the chain segmental mobility (i.e., the ability to relax) is high

24 Chapter 2

(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 ConvertingProperties 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 nite products. Before application

they have to be nished. In this case nishing 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

uidity required for processing purposes) is dierent 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 uid 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

diusion 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, inuences 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

dierent from that of the converted material. The coating-related rheology

will be covered in Section 3.


Convertability of Laminate

Convertability of the PSA laminate means its ability to be processed intonished products (labels, decals, etc.) by operations which inuence itsdimensions (e.g., slitting, cutting or die cutting, embossing, folding,perforating etc.) or its surface quality (e.g., printing, laquering, etc.). Theow properties of the adhesive inuence the migration or penetration,oozing, and cold ow, 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 PSAlaminate. It should be mentioned that the converting properties of thelaminate 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-UseProperties of PSAs

The most important end-use properties of PSAs are the propensity tolabeling (dispensing) and bonding behavior. For some special PSPs thedebonding characteristics have to be taken into account too. Theapplication technology of other PSPs (e.g., tapes and protective lms) isless 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 inuenced by the adhesive properties (peel and tack) and by thedispensing properties of the label. Removability or peeling o is inuencedby the adhesive properties. Thus the parameters inuencing the adhesiveproperties will also characterize the end-use properties.

Label Application Technology

Label application technology refers to labeling. Labels are either applied byhand or with mechanical processes. Generally the label dimensions aredecisive for the choice of the application technology. Large labels will only beapplied by hand. On the other hand, according to the application technology,reel and sheet laminates are manufactured. For PSA reels and sheets, a quitedierent adhesive/cohesive balance is required (i.e., quite dierent tack/peel/shear values or ow properties). High speed labeling guns need high tackPSAs (so-called touch blow labels use no mechanical contact in labelapplication), whereas for high speed cutting, high modulus and high cohesionPSAs are required. Dispensing and labeling were discussed in detail in [63].

In the label industry a basic dierence exists between roll andsheet supplied laminates (face material/adhesive/release liner) [40].Pressure-sensitive adhesives for sheet applications must resist ying knife

26 Chapter 2

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 ow within the polymer is

signicant during the converting operation, poor die cutting or poor

guillotining (knife fouling) can result.

Removability of PSAs

Conventional PSAs can be classied as either nonpermanent (2.79.0N/

25mm for 180 peel adhesion) or permanent (above 9N/25mm for 180 peeladhesion). Repositionable PSPs are a special class of removable pressure-

sensitive products (labels and tapes) that stick to various surfaces but

remove cleanly and can be reapplied. The nal 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 ow 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 requiredthat 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


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 ow and

elastic motion of the macromolecules. The major danger is that if the viscousow is too pronounced, bonds will fail in the adhesive mass. (It should be

mentioned that for special removable protective lms the regulationabsorbtion 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 ow and low build-up of the peelduring storage [66]. On the other hand, a certain degree of cold ow is needed

in order to obtain removability. Adding 220% plasticizer during the

polymerization of acrylics leads to a removable adhesive but the plasticizermay migrate out after storage [67]. Furthermore cohesive strength is

necessary to avoid stringing. It must be stated that the adhesion dependson both the rheological features and chemical anity. 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 Teon surface. The adhesionof these adhesives must be attributed largely to their rheological features

rather than to selective wettability.

How to Achieve Removability? A special adhesion/cohesion and/orplasticity/elasticity balance is required in order to achieve removability.

Flow properties allow rearrangement, relaxation, and transformation of thedebonding energy into viscous ow, therefore softening of the adhesive is

a prerequisite for removability. It is well known from the eld of caseine-based 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 12%).

Flow properties are correlated to creep which, in turn, is a function of

molecular weight and plasticizer loading. Higher molecular weights and lowplasticizer levels reduce the tendency to creep. For removable PSAs the

molecular weight should be limited. Permanent adhesives need no uidity

during debonding, as they must display debonding resistance when subjectto 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 ow. 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 usingstress-resistant polymers, llers, and primers; stress-resistant polymers are

those which develop a controlled crosslink density or are soft internally.

Primers are somewhat exible and generally promote good anchorage to theface stock. Thus, they can absorb expansion or impact stresses without

28 Chapter 2

adhesive failure. Generally, as discussed in [44], removability can be achieved

by regulating the chemical composition of the adhesive, and the structure andgeometry of the adhesive, and the carrier properties and by means of the

manufacturing technology. The main methods used to change the chemicalcomposition (and macromolecular characteristics) of the adhesive compo-nents, e.g., tackifying, softening, and hardening (crosslinking, lling, etc.)

inuence the removability too. Formulation windows with boxes shifted tothe left and up (i.e., for lower frequencies) are harder [70].

Influence of the Viscosity/Modulus on Removability. As shownearlier, 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 removableadhesives, namely PSAs which wet the surface poorly and PSAs which

adhere easily but are easily removed. The rst type of PSAs is highly elasticand has low tack. Its removability depends on the adhesive not establishinga good contact with the surface. Sometimes the poor contact is built-in

mechanically. These PSAs might have inclusions that prevent the adhesivefrom achieving full contact with the surface, as the adhesive contains

physical particles that do not deform completely and therefore limit thecontact area between adhesive and substrate. These PSAs possess a high

modulus. Thus, hardening of the polymer (modulus increase) is a possibilityto obtain removable adhesives.

As discussed in [71] the build-up of ordered multiphase structurescan modify the chain mobility. Such structures include llers, crosslinks,

crystallites, and molecular associations. Fillers modify the Tg also. Reallysuch hardening of the adhesive using a composite structure works like other

methods for the control of the contact surface (e.g., pattern-like contactpoints, pattern-like nonadhesive points, decrease of the coating weight etc.).It should be mentioned that in lled hard adhesive formulations the

absorbtion of the debonding energy is improved too [72]. As illustrated in[73] the degree of lling (e.g., lling, lling and foaming) strongly inuences

the adhesive characteristics and adhesion build-up. The rheology of suchreinforced systems and its inuence on the adhesive and end-use properties

were discussed in detail in [74]. The most important llers used in PSAs andtheir functions are listed in [40]. According to [75] current removableadhesive formulations include high molecular weight polymers that reduce

ow 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


formulations use plasticizers or other low molecular weight viscous com-

ponents (paranic oil, polyisobutylene etc.). The presence of 8% plasticizer

may improve the elongation by 3000% [66]. The softening eect isillustrated by the lowering of the minimum lm-forming temperature

(MFT) (e.g., 6% plasticizer decreases the MFT from 23C to 0C), or thedecrease of the tensile yield strength (e.g., 6% plasticizer decreases thetensile yield strength from 13.0 to 1.0N/mm2). The elongation of PVAc

PSAs (normally 610% at break) was signicantly increased by adding

plasticizers to up to 1001000% [76]. Dierences in plasticity may be due tothe molecular weight of the plasticizer or dierent interactions between

chain segments and plasticizers. Long-chain plasticizers exert a strong

plasticizing eect. Generally plasticizers decrease the modulus value andinuence the frequency dependence of the loss tan [4]. For nonplasticizedadhesives loss tan is low at low frequencies. For plasticized adhesives losstan shows a maximum at lower frequencies, and peel and quick stick alsoexhibit maxima at lower frequencies.

Plasticizers may be incorporated in peelable PSAs to soften the

adhesive and thus improve peelability. However, some plasticizers can exerta tackifying eect 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 withthe main adhesive polymer and the tackier 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 1020% by weight are added [6]. Other plasticizers which can be

employed, include the well-known extruder oils (aromatic, paranic, or

naphthenic) as well as a wide variety of liquid polymers. Satas [6] showedthat 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 lms. Such crosslinked natural rubber formulationsare 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 appliedones (i.e., the peel resistance increases with the dwell time). This build-up of

30 Chapter 2

the peel resistance is due to the ow 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-dened (normalized) dwell time.

1.5 Factors Influencing Viscoelastic Properties of PSAs

The material characteristics, chemical composition/structure, and environ-mental and experimental conditions inuence the viscoelastic propertiesof PSAs. The inuence of the material characteristics on the viscoelasticproperties of the PSAs will be discussed rst. Numerous empiricalcorrelations between the molecular structure, molecular weight, molecularweight distribution (MWD), branching, and rheology are generally valid forchemically quite dierent polymers [79].

Influence of Material Characteristics on ViscoelasticProperties of PSAs

The molecular character of the base polymer, its chemical composition, andstructure inuence its viscoelastic properties. The inuence of the molecularweight on the viscoelastic properties of the PSAs will be examined rst.Viscosity and the elastic modulus are the most important parameterscharacterizing the viscoelastic behavior of PSAs. Both parameters dependon the molecular weight of the base polymer.

Dependence of the Viscosity of PSAs on the Molecular Weight. Theviscosity of macromolecular compounds is a function of the molecularweight. Their zero viscosity 0 depends on the weight average molecularweight according to a correlation of the form [80,81]:

0 f MW 2:24

where 3.4.Zosel [16] demonstrated the strong inuence of the molecular weight

of acrylic PSAs on their viscosity and on their tack. The increase of themolecular weight in the range of 103107 produces an increase of theviscosity from 102 to 1010 Pas. Zosel also showed the existence of amaximum in the tack/molecular weight graph for polyisobutylene. Thepronounced increase of the viscosity with the molecular weight limits theusable molecular weight range for hot-melt PSAs, limiting (unfortunately)their cohesion as well. (According to [82] for such adhesives 80 Pas is thepreferred maximum viscosity value for high speed coating.) It is well knownto skilled formulators that block copolymers have lower viscosities thannatural rubbers. (Their viscosity depends on sequence distribution and


branching too). The solution viscosity of the high polymers also increaseswith their molecular weight. Thus a viscosity-imposed balance betweenprocessing and end-use properties, that is, a solid content/molecular weightbalance for solvent-based PSAs is always a limiting factor, keeping in mindthat the solid content of common solvent-based PSAs does not exceed3040%. On the other hand the additives in an adhesive formulation, theirnature, and their concentration also inuence the viscosity of the blend.Practical examples are given by plasticizers and tackiers. Micromoleculartackiers (plasticizers) may be considered as diluting agents. Thus theviscosity of the polymer solution obeys an exponential law, depending onthe polymer concentration C, namely

Z

C 2:25

At rst rubber resin formulations were used for PSAs. To achieveviscoelastic behavior, the elastic component (rubber) had to be mixed(formulated) with a viscous component. Either macromolecular ormicromolecular compounds can be used as viscous components. The bestknown are resins and plasticizers; both are suggested as tackiers.Generally, low molecular weight resins and relatively high molecularweight solvents are used as tackiers. Common HMPSA formulationsincorporate a high level of plasticizer, usually naphthenic oil or liquid resin.The level of the tackier resin inuences the resulting viscosity. As anexample, Zosel [16] demonstrated that the zero viscosity of tackied PSAsdecreases up to 80% resin loading level, resulting in better ow properties.The diluting eect of the tackier resin depends on its own viscosity too.Generally, soft (liqu

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