218584288 pressure sensitive adhesives and applications

1.1 Properties of PSAsThe adhesive and end-use properties of PSAs require a viscoelastic, Newtonian flow behavior which is based on the macromolecular nature ofthe adhesive.. Consequently

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 publi-cation, shall be liable for any loss, damage, or liability directly or indirectly caused oralleged to be caused by this book The material contained herein is not intended toprovide specific advice or recommendations for any specific situation.

Trademark notice: Product or corporate names may be trademarks or registeredtrademarks and are used only for identification and explanation without intent toinfringe

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The growing interest in the advances described in this book have called forthis second edition of this book In the past few years pressure-sensitiveproducts have reached a maturity that warrants a detailed and criticalexamination of their science and technology This is a vast domain, and Ihave tried to cover some of its special aspects in separate works The volumeDevelopment and Manufacture of Pressure-Sensitive Products (MarcelDekker, 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, Sensitive Adhesives Technology (Marcel Dekker, 1996), the first of thesebooks, constitutes the main step on the way to understanding adhesive-based pressure-sensitive products

Pressure-In the past decade advances in contact physics and mechanics haveallowed us to correlate the macroscopic aspects of adhesive bondingand debonding with the macromolecular basis of the viscoelastomers Themost important elements of this progress are described in a separate section

of the revised book Developments in the practical examination and qualityassurance of pressure-sensitive products required an enlarged discussionand reformulation of Chapter 10, ‘‘Test Methods.’’ Environmentalconsiderations made necessary the discussion of recycling methods, andbiodegradability of raw materials, product components, and pressure-sensitive products Other scientific and industrial advances (i.e., new rawmaterials and improved coating technology) are also included in thesecond 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 first single-author book on general aspects of pressure-sensitiveadhesive technology, and it has been my pleasure to assist in its success.

Istva´n Benedek

Since their introduction half a century ago, pressure-sensitive adhesives havebeen successfully applied in many fields They are used in self-adhesivelabels, and tapes and protective films, 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 specific 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 scientifically This book coversall the fields 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 different approach Based on ourexperience as engineers (in both scientific activity and industrial areas) andusing the available technical literature, we have addressed all aspects ofpressure-sensitive adhesives We have included the scientific basis ofsuitability for specific 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 labels

as 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 different 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 specific 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 Rather

it serves as a practical aid to converters and those involved in the design anduse of pressure-sensitive adhesives

Istva´n BenedekLuc J Heymans

Preface to the Second Edition iii

1.2 Influence of Viscoelastic Properties on the

1.3 Influence of Viscoelastic Properties on the

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

1.5 Factors Influencing Viscoelastic Properties of PSAs 31

3.1 Influence of the Liquid Components of

3 Physical Basis for the Viscoelastic Behavior of

1.4 Correlation Between the Main Adhesive, End-Use,

1.5 Components for Special Pressure-Sensitive

2.3 Physical State of PSAs 211

2 Influence of Adhesive Properties on Other

2.1 Influence of Adhesive Properties

2.2 Influence of Adhesive Properties on End-Use

3.2 Acrylics and Other Synthetic Polymer-Based

1.1 Convertability of Adhesive as a Function of

1.2 Convertability of Adhesive as a Function of

1.3 Convertability of Adhesive as a Function of the

1.4 Convertability of Adhesive as a Function of

1.5 Convertability of Adhesive as a Function of

2.1 Definition and Construction of the Pressure-Sensitive

2.2 Printability of the Laminate 391

2.14 Comparison Between Solvent-Based,

6 Simultaneous Manufacture of PSAs and PSA Laminates 612

7 Manufacture of the Release Liner 616

Introduction

Adhesives are nonmetallic materials [1] used to bond other materials, mainly

on 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 fluid is transformed after bonding (i.e., the build

up of the joint) into a solid In the case of PSAs, the adhesive conservesits fluid state after the bond building too Thus its resistance to debonding

is 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 first U.S patentdescribing the use of a PSA—for a soft, adhering bandage—was issued

in 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 films 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 films exhibiting permanent tack, and display an adhesion whichdoes not strongly depend on the substrate [6] The term PSA has a veryprecise technical definition and has been dealt with extensively in thechemical literature [7,8] However, as discussed in [9], the technical term

of PSA in different languages (e.g pressure-sensitive adhesive, collants, Haftkleber, etc.) is not completely clear The recent development

auto-1

of pressure-sensitive products without a coated pressure-sensitive adhesivelayer, makes the definition of this product group more difficult [9,10].The build up and classification 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 havingsufficient internal strength so that the adhesive material will not break

up 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 flow 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 flow) and, in addition, store bond rupture energy in order

to 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 fulfill 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 compromise

is needed When using PSAs there appears another difference from wetadhesives, namely the adhesive does not change its physical state becausefilm forming is inherent to PSAs Thus, PSAs used in self-adhesive laminatesare adhesives which, through their viscoelastic fluid state, can build up thejoint without the need to change this flow state during or after application

On the other hand, their fluid state allows controlled debondinggiving a temporary character to the bond Because of the fluid character

of 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 influence onthe properties of the adhesive in the composite Therefore, there exists adifference 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 up

a continuous, soft (fluid), and tacky (rubbery) layer The latter will adhere

to the substrate On the other hand, the liquid adhesive layer of the PSAsworking in the bond has to offer 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 first measures the adhesive’s 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 behavior

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)

Generally PSAs are used as thin layers, therefore their flow is limited

by the physico-mechanical interactions with the solid components of thelaminate (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) maydiffer from those of the pure (unbonded) adhesive Therefore in this chapterthe rheology of pure and coated PSAs will be dealt with separately

It remains difficult 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 fluids) 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 significantly influence theproperties of the PSA laminate Practically, this disadvantage is eliminated

by 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, Newtonian flow 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 propertiesspecifically related to PSAs Generally, adhesives in a bond behave like afluid or a solid Fluids are characterized by their viscosity which influencestheir mobility, whereas solids are characterized by their modulus whichdetermines their deformability In an ideal case, for Newtonian fluids (or forsolids obeying Hooke’s law) the applied force (load) will be balanced bythe material’s own mechanical characteristics, that is, the viscosity
or theYoung’s modulus E:

at low shear rates they must flow (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 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 orshear rate:

That is:

where
ais the apparent viscosity, and n denotes the flow 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 flow is reduced, but the elastic modulus or stiffness of thepolymer increases [2] This behavior results in additional stored energy, andthe debonding resistance intensifies accordingly.

Practically, the dependence of the adhesive performance istics on the stress rate may be observed by peeling off removable PSAs atdifferent peel rates: at higher rates paper tear may occur The stress rate-dependent stiffening is an increase in the elastic contribution to the rheology

character-of the polymer When the elastic components are predominant more character-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 samestiffening effect is apparent when the polymer temperature decreases In thiscase the polymer molecules are again restricted in their ability to flow, 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 (depending

on the viscoelastic properties) Accordingly, PSAs would absorb less ormore energy depending on the rate (frequency !) of the applied stress.Practically, end-use situations with different 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 fluid, 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 coefficients 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

12 ¼ Gð!Þ
0
sin ! t þ
rev0 rev
! cos ! t ð2:7Þ

12 ¼
irr
0 irr
! cos ! t ð2:8Þ

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

The storage modulus G0increases with the frequency:

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:

where G0is 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 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

rheological properties The correlation between adhesive properties and thedynamic shear storage modulus appears quite good [6]: hence the concept

of ‘‘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 off 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 (with

a continuous differential of the free enthalpy, but discontinuous, secondorder differential of the Gibb’s free energy) is called the glass transitiontemperature (Tg), and has a special significance in the characterization ofPSAs Above the Tg the time-temperature superposition principle can beapplied Differences 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 Tgthere occurs a change in the thermodynamic state which can be related

to a mechanical energy loss function such as the loss modulus The loss tanpeak 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 peak

is 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 influence on the tack of the system The storagemodulus definition can be simplified as a hardness parameter [9] OptimumPSA performance can be quantified 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 G0values at room temperature between 50 kPa and 200 kPa with loss tan peak temperature limits between 10 and þ10C Optimum G0 values forpermanent PSA labels were determined to be around 20 kPa at roomtemperature

Pressure-sensitive adhesives were defined 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 relationship

is 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, defined as the dynamic Tg, and the Tg asmeasured by differential scanning calorimetry (DSC) For water-basedadhesives 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 30C The phenomenon is much less pronounced for butadiene rubber (SBR)-type polymer dispersions This variation is alsovalid for an adhesive dispersion containing a tackifier The consequence ofthis is that the loss tan peak temperature cannot be used to predict anddefine PSAs performance in a viscoelastic application window According toBamborough [9] and applying the rule proposed by Chu, it may appear that

styrene-an SBR would require considerably more compatible resin thstyrene-an styrene-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 oftackifier would be 30 parts per 100 parts polymer whereas for Acronal 80 D

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

water-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 means

of 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 differential scanning calorimetry to measure shifts in the Tgofthe adhesive are now common methods for rheological studies of adhesives[10] In order to understand the practical benefits of such investigations therheology of PSAs needs to be studied

As discussed above PSAs are special products allowing instantaneousbonding due to their liquid-like flow 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 Tgshould 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 during

a 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 different temperatures can be superposed by a shift

of 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 films having built-inpressure 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 propertyrepresents the adhesive’s ability to adhere quickly (initial grab), the second

measures its ability to resist removal by peeling, and the third characterizesthe adhesive’s ability to resist flow when shearing forces are exerted.Generally speaking the first 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 first 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 propertiesreflects at the same time the need for the balance of the viscoelasticcharacteristics In this chapter the influence of the viscoelastic properties onthe 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 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 confirmed bySheriff et al [14] and by Counsell and Whitehouse [15] tack is a function ofwetting Good wetting supposes sufficient fluidity of the adhesive, andfluidity 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 first step, contact is made in the geometrical surface points,which increase to a larger area through wetting out, viscous flow, and elasticdeformation Wetting out implies high fluidity, as characterized by anadequate viscosity of the adhesive Wetting out (i.e., covering the surface

by the fluid 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 deformation

of 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 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 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 influencing the viscosity and the elasticity ofthe polymer will also influence the tack The polymer’s own characteristics

and the environmental conditions (experimental parameters) influence itsviscous and elastic behavior Studying the performance of carboxylatedstyrene-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 goodagreement 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 thetransition zone is important for PSA performance In accordance withDahlquist’s criterion for a minimum value of compressive creep compliance

to achieve tack, Class and Chu said their data indicated a maximummodulus value Dahlquist [23, 24] related tack to modulus, showing that thecompression modulus should not be much higher than 105 Pa Very highmodulus adhesives do not possess sufficient conformability to exhibitpressure-sensitive tack The optimum tack properties of PSAs are obtainedwhen the room temperature modulus falls within the range of  5  105to

1  105Pa, and the Tglies between 10 to þ10C [25]

Hamed and Hsieh [26] showed that for a given total bonded area, testspecimens containing noncontact regions of sufficiently small scale canexhibit a higher peeling force than those in which the noncontact ones arelarge For an elastomer there is a critical flaw size below which adhesivestrength will remain unchanged Rubbery materials with higher elasticityhave a smaller critical flaw size Thus the modulus influences the tack.Rubbery materials with a more elastic response exhibit tack properties thatare more sensitive to interfacial flaws, compared to those that respond more

by viscous flow When failure occurs by viscous flow, the tack is relativelyinsensitive to interfacial flaws, but when the strain rate is sufficiently high, sothat the elastomer responds elastically, stresses may be concentrated at theedges of interfacial flaws 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, andthe strain rate) Thus, a similar dependence of the tack on time, temperature,and the strain rate has to be taken into account This dependence isillustrated by the quite different values of the tack obtained using differentexperimental techniques (rolling ball, quick stick, or loop tack) which arecharacterized by different time and strain rates, and by the sensitivity of theadhesive properties to environmental conditions (see Chap 10) Different

tack methods are adequate for different PSPs [27] With the probe tack testthe load is regulated, as in the lamination of protective films Polyken tack isless strongly affected 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 flow isrequired to provide proper wet-out In reality it is very difficult 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 flow of the adhesive at lower tem-peratures, a phenomenon governed by the strong temperature dependence

of the viscosity and of the elastic modulus Deep freeze labels shoulddisplay the same viscoelastic properties at 40C as at þ20C [29] It isrecommended that such adhesives have a lower storage modulus value thancommon labels (104Pa) As shown by Hamed and Hsieh [26] tack is afunction of test temperature and strain rate, and the experimental data may

be 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 may

be 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 confirmed by Hamed and Hsieh [26]too They observed a nonlinear, discontinuous increase of the tack with themeasurement speed Tack values rise to a first 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 confirmed by McElrath [21] He demonstrated that loss modulusvalues 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 asubstrate at some angle with respect to the substrate, usually at an angle

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 102longer 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 firstcontact step is longer for peel measurements than for tack It follows thatflow 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 influences 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 theinfluence of the viscosity on the bonding step during a peel measurement

is less important than for the tack On the other hand, the debondingresistance of the joint is increasingly proportional to the viscous flow 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 films), 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 will

be 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 G0value inthe high frequency range is needed Satas [6] showed, that solution polymerswith a higher G0slope 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 to

a 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, andinfluence the adhesive and converting properties [35]

Peel Dependence on Dwell Time and Strain Rate As known fromplastic film processing and use, adhesion and self-adhesion depend on thecontact 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 phenomenondue to the macromolecular nature and viscoelasticity of such polymer films

In a similar manner contact forming or bonding of the adhesive assumes itsviscoelastic deformation Viscous, slow flow is time dependent and thusbond 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 contactbuild-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:

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

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, applicationpressure 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., theinterval between bonding and debonding) takes into account the relativelylow mobility (flow rate) of PSAs A higher pressure or a longer dwell timeshould help adhesive flow 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 films) bonding depends on the appliedpressure and temperature also A special class of such products (warmlaminating films) 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

the dwell time [43] The adhesion build-up over time for the main types

of 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 force

in 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 effect’’ 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 time

is 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 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

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

and a log peel rate between 1.0 and 2.6 cm/min [46] Another influence of thedwell time may be observed, when examining the influence 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 occurs

by means of the solid components of the laminate Therefore strain ratedepends 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, 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 aredifferent tackified water-based acrylic formulations; the peel adhesion from glass wasmeasured

The dependence of the peel on the peeling rate may also be observedfor very low peel forces (peeling from release liner) [49] McElrath [21]showed that (as theoretically supposed) the loss modulus depends on thefrequency, 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 peelrate [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 testrates or lower ambient temperatures produce maxima at lower resin levels,lower resin softening points, or lower elastomer Tg The peel of differentwater-based PSAs displays large differences after storage at 100C Asshown in [25] adhesive break (in the adhesive layer) appears above a certainpeeling rate (2.5 mm/min), or below a certain temperature (25C) only.

It may be possible to remove a label from a paper substrate if it ispeeled 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 theadhesive, high-tack removable adhesives should possess a low storagemodulus 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 theseconditions, the viscous components of the polymer dissipate significantamounts of energy and, as a result, resistance to peel forces is low At higherstrain 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, eachadhesive 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 andviscosity depend on the temperature With the increase of the temperaturethe viscosity of PSAs decreases Therefore the increase of the temperatureimproves the tack and instantaneous peel, and exerts a negative influence onthe cohesion Such an influence is illustrated in the end-use of several PSPs

It was shown for ethylene acrylic acid copolymers that peel adhesiondepends on the temperature [30] The peel of an untackified PSA at 0C isabout 210% lower than the peel value at 23C For tackified formulationspeel reduction at 0C may attain 300% [50] The importance of laminatingtemperature of acrylic PSA coated protective films on plastic plates wasdemonstrated in [51] The dependence of the peel value and of the breaknature on the peel rate is a common phenomenon observed in thedelamination of PSPs during their application also Such dependence

causes the so-called inversion of the adhesive break when tapes are unwound

at 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)

Generally the unwinding force depends on a number of parameterssuch as the adhesion force, the moduli of elasticity of the adhesive andcarrier film, the thicknesses of the adhesive and film, 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

Different authors have formulated a definition of the cohesivestrength If deformation by shearing is considered like a flow, the flowlimit (FL) is given by the following correlation [53]:

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 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 thesefactors may be formulated as follows [54]:

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

As illustrated by the relations (2.15)–(2.18) the shear resistancedepends on the adhesive’s viscosity and elasticity If the adhesive isconsidered a Newtonian fluid, the shear resistanceSHis given as a function

of the viscosity
, the adhesive thickness h, and the tensile rate :

of 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 flow influenced by copolymercomposition and global molecular weight

Shear Dependence on the Modulus According to Woo [57] the shiftfactors used to plot shear measurements at different temperatures on amaster curve were almost identical to the shift factors used for moduluscurves Thus the time to fail in shear can be predicted from the viscoelasticfunction 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 timescales of the adhesive performance parameters The magnitude of G0in 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, butare associated with faster relaxation times and thus correlate with roomtemperature viscoelastic properties [58] The decrease of the modulus at hightemperature 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 about

105Pa The thermo-mechanical degradation of the rubber through millingdoes not change the tan peak temperature (Tg) It does reduce the modulus

at high temperatures This modulus reduction relates to the lower shearperformance of solvent-based systems

The possibility to characterize application field-related properties ofPSAs and to correlate them with the rheology of the adhesive is illustrated

by formulating adhesives for medical tapes [59] The adhesive used formedical tapes is characterized by a dynamic shear modulus of about1–2  104Pa, a dynamic loss modulus of 0.6–0.9  104Pa, and a modulusratio or tan  of about 0.4–0.6 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 adhesiveswith 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 parametersinfluencing 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, theshear resistance will depend on the temperature and the time: in labelingpractice this behavior is illustrated by the low temperature resistance of theadhesive joints and by the differences between statical and dynamical shear

As is known, the viscosity depends on the loss modulus (viscous flow is the

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

At low frequencies
0 and the steady-state viscosity are the same A staticshear test is based on low frequency deformation; therefore creep may beregulated by viscosity

The cohesive strength (shear) increases with increasing test rate or withdecreasing temperature [26] When non-Newtonian liquids are subjected tovariable shear rates, the plot of the shear stress/shear rates no longer shows

a 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:

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 between0.3–1.0 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 influences the shear resistance by bonding too Bonding

is a diffusion 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] found

a good agreement between experimental and calculated values, using theuniversal 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 shearstrength If the chain segmental mobility (i.e., the ability to relax) is high

(high temperature or low test rate) then the fracture stress is small On theother 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 toaccommodate the applied stress This statement appears valid for peeladhesion too One should recall that the removability of a PSA labelstrongly 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 aremanufactured generally as web-like products Most of them are laminates.They are applied as web-like laminates or finite products Before applicationthey have to be finished In this case finishing means the transformation

of the continuous web-like product that has the optimal geometry formanufacture into a product that has the optimal characteristics for use.Convertability is the sum of the convertability of the adhesive and that ofthe laminate The converting properties of the adhesive and of the laminatedepend on the rheology of the adhesive It has to be pointed out that, exceptfor hot-melt PSAs, the rheology of the uncoated adhesive (with an inherentfluidity required for processing purposes) is different from that of theconverted 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 Thecoated 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 layerhas to allow the elimination of the carrier liquid (drying) in order to form

a solid adhesive layer Evaporation of the carrier liquid depends on itsdiffusion 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 (shearrate)/temperature dependence of the viscosity, influences the coatability andconvertability of PSAs It was mentioned earlier that, except for hot-meltPSAs, the other PSAs are dispersed or diluted systems, their rheology beingdifferent from that of the converted material The coating-related rheologywill be covered in Section 3

Convertability of Laminate

Convertability of the PSA laminate means its ability to be processed intofinished products (labels, decals, etc.) by operations which influence itsdimensions (e.g., slitting, cutting or die cutting, embossing, folding,perforating etc.) or its surface quality (e.g., printing, laquering, etc.) Theflow 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 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-Use

Properties 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 films) 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 influenced by the adhesive properties (peel and tack) and by thedispensing properties of the label Removability or peeling off is influenced

by the adhesive properties Thus the parameters influencing 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 quitedifferent adhesive/cohesive balance is required (i.e., quite different tack/peel/shear values or flow 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 difference exists between roll andsheet supplied laminates (face material/adhesive/release liner) [40].Pressure-sensitive adhesives for sheet applications must resist flying knife

and guillotine cutting A poor selection of the adhesive results in part inoozing of the adhesive (gum deposits) on the cut surfaces and smearing ofthe 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 rollstock, and guillotining of sheet stock Formulating approaches that improvethe high frequency modulus of the adhesive will enhance converting [2] sincedie cutting and guillotining are such high frequency processes The lessviscous 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 issignificant during the converting operation, poor die cutting or poorguillotining (knife fouling) can result

Removability of PSAs

Conventional PSAs can be classified as either nonpermanent (2.7–9.0 N/

25 mm for 180peel adhesion) or permanent (above 9 N/25 mm for 180peeladhesion) Repositionable PSPs are a special class of removable pressure-sensitive products (labels and tapes) that stick to various surfaces butremove cleanly and can be reapplied The final adhesion builds up over a fewhours Adhesives included in the nonpermanent category are used in themanufacture of removable tapes and labels, protective laminates, and otherless durable products [64]

For nonpermanent, so-called removable adhesives, the flow propertiesand cohesion of the adhesive as well as the anchorage of the adhesive to theface stock are critical In an ideal case, if the bond to the substrate isnonpermanent, then a clean release from that substrate is encountered andthe adhesive remains on the face material Another requirement for goodremovable adhesives is the low peel level with a permanent character (i.e., nobuild-up of the peel in time is acceptable) A clean release from the substrateand no build-up of the peel with time are the minimal requirements forremovable PSAs On the basis of the adhesive characteristics it is possible toformulate the rheological criteria necessary for removable and permanentadhesives

Criteria for Removability For certain applications PSPs are requiredthat display a low peel force and give a clean, deposit-free separation fromthe substrate The criteria for removability were discussed in detail in [44].Generally a special balance between tack, peel adhesion, cohesion, andanchorage is required in order to ensure removability Removability requires

a breakable bond at the adhesive/substrate interface Bond breaking is anenergetic phenomenon For removability the whole debonding energy should