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`
`Pressure-Sensitive
`Adhesives: An
`Introductory Course
`
`Costantino Creton
`
`Abstract
`Self-adhesive materials are called, in the adhesives trade, (cid:147)pressure-sensitive
`adhesives(cid:148) (PSAs). PSAs are designed to stick on almost any surface by simple contact
`under light pressure. This special class of adhesives does not undergo any physical
`transformation or chemical reaction during the bonding process. Because of this, the
`rheological properties of the adhesive must be finely tuned for the application, combining
`a carefully chosen polymer architecture and monomer composition with the proper
`addition of small molecules called tackifying resins. PSAs are soft, deformable solids
`and, depending on the formulation, easily form bridging fibrils between two surfaces
`upon debonding. They are safe to use and easy to handle and thus are increasingly
`replacing more conventional types of adhesives. In this article, we review both the
`primary material characteristics of PSAs and the main physical principles that make
`them work effectively.
`
`Keywords: acrylics, block copolymers, peel, polymers, pressure-sensitive adhesives,
`tackifying resins.
`
`Introduction
`Among the different classes of adhesives,
`pressure-sensitive adhesives (PSAs) are
`perhaps the most common type found in
`consumer products. Self-adhesive tapes and
`labels of all kinds are ubiquitous in every-
`day life. However, until recently, the under-
`standing of the materials science and
`engineering of PSAs and, in particular, the
`specific role played by the different com-
`ponents in them was very limited outside
`of the companies involved in their manu-
`facture, and the interested reader had to
`refer to general technological texts.1,2 Al-
`though PSAs are designed to join two
`surfaces together, they differ from other
`adhesives in several ways. First, PSAs are
`typically used as nonstructural adhesives;
`they do not compete with epoxies for
`structural applications. Second, PSAs typi-
`cally stick to a surface upon contact with-
`out any chemical reaction.3 It is interesting
`to note that the term pressure-sensitive
`really should be pressure-insensitive, since
`PSAs do not need the application of much
`pressure to stick, and the measured ad-
`hesion is then rather insensitive to the
`compressive pressure applied upon bond-
`ing. This property makes PSAs particu-
`
`larly easy and safe to use, since no solvent
`evaporation or chemical reaction takes
`place and bonding can be done at room
`temperature.
`Similar to all classes of adhesives, PSAs
`must be able to form a bond, that is, estab-
`lish molecular contact (even on a rough
`surface) and then sustain a minimum level
`of stress upon debonding. All other classes
`of adhesives, however, form the bond in
`the liquid state and then are tested in the
`solid state, with the transition occurring
`by chemical reaction, change in tempera-
`ture, UV irradiation, or another change in
`the structure of the adhesive. By contrast,
`modern PSAs are soft, viscoelastic solids
`that obtain their unique properties simply
`from the hysteresis of the thermodynamic
`work of adhesion. That is, there is a differ-
`ence (adhesion hysteresis) between the
`energy gained in forming the interactions
`and the energy dissipated during the frac-
`ture of these same bonds. At least for short
`contact times, the only interface forces ac-
`tive in PSA adhesion are van der Waals
`forces.4–6
`In order to possess these unique charac-
`teristics, however, the mechanical proper-
`
`ties of these adhesives must be much more
`finely tuned than those of conventional
`adhesives. It is the goal of this article to
`give a general picture of the main material
`requirements for obtaining PSA properties.
`
`Classes of Pressure-Sensitive
`Adhesives
`In practice, as will be developed in more
`detail in the section on material require-
`ments, all commercial PSAs are based on
`polymers, mainly coming from three fami-
`lies: acrylics, styrenic block copolymers,
`and natural rubber. There are also niche
`markets for silicone PSAs, where low-
`temperature use or high-temperature sta-
`bility is required and cost is not an issue.
`Historically, the first PSAs were rubber-
`based. They remain the cheapest to pro-
`duce and also the simplest to formulate,
`since they are typically compounds of
`natural rubber and a low-molecular-weight
`tackifying resin, miscible with the rubber
`in approximately equal proportions. Earlier
`versions were not cross-linked, but today
`a cross-linking step is generally performed
`to avoid flow.
`Acrylic PSAs provide more latitude for
`optimization and formulation. They are
`typically random copolymers of a long
`side-chain acrylic (n-butyl acrylate or
`2-ethylhexyl acrylate) with a low glass-
`transition temperature Tg, a short side-chain
`acrylic such as methyl acrylate to adjust
`the Tg, and acrylic acid to improve adhesion
`and optimize elongational properties (i.e.,
`their mechanical response to deformation
`in uniaxial extension). Small-molecule ad-
`ditives such as tackifiers can be included,
`essentially to adjust the Tg and optimize
`dissipative properties, but they are not
`necessary for a useful acrylic PSA. As for
`natural rubber PSAs, a cross-linking step
`(electron-beam or UV irradiation), once the
`adhesive has been coated, is generally used
`to prevent creep.
`PSAs based on styrenic block copolymers
`are the latest type to have come on the
`market. They are generally blends of styrene-
`isoprene-styrene (SIS) triblocks and styrene-
`isoprene diblocks compounded with a
`low-molecular-weight but high-Tg resin
`based on C5 rings that is miscible with the
`isoprene phase but immiscible with the
`styrene phase. It is important to note that
`a tackifying resin is a necessary compo-
`nent for this class of PSA.7–10
`In order to obtain usable PSA properties,
`the proportion of styrene in the com-
`pounded PSA must be on the order of
`4–12%, the molecular weight of the styrene
`block must be above 10–11 kg/mol to stay
`immiscible with the isoprene phase, and
`the weight fraction of polymer in the
`blend must typically vary between 25%
`
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`
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`
`EX. 1020
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`

`Pressure-Sensitive Adhesives: An Introductory Course
`
`and 45%. Because of the use of mutually
`immiscible blocks, these PSAs are really
`nanophase-separated, with styrene do-
`mains dispersed in an isoprene matrix.
`These styrene domains provide physical
`cross-links, which give these PSAs a supe-
`rior resistance to creep.
`
`How Do They Work?
`For many years, the mechanisms by
`which PSAs adhere to almost any sub-
`strate remained rather mysterious. Com-
`panies proceeded mostly empirically from
`formulation to application, using a variety
`of trade tests designed to closely match
`the conditions the PSA would be sub-
`jected to in a particular application but the
`results of which could not be easily trans-
`ported to another. The usual recipe one
`could use was that the PSAs should have
`an elastic modulus below a certain thresh-
`old called the Dahlquist criterion11 and a
`large value of the viscous component of
`the elastic modulus to dissipate energy
`upon debonding.12 Furthermore, due to the
`complicated nature of the materials (some-
`thing between a solid and a liquid), few
`academic studies went beyond making
`correlations between a specific molecular
`structure and a macroscopic property. It is
`chiefly thanks to a BASF scientist, Albrecht
`Zosel, that a mechanistic approach was
`developed toward the understanding of
`PSA adhesion.5,13,14
`Like all adhesives, PSAs work because
`the separation of two surfaces requires a
`certain amount of energy dissipation. This
`energy dissipation can be readily measured
`with a peel test, but early experiments
`quickly showed that the peel force de-
`pended on peel velocity, peel angle, and
`adhesive thickness, implying a strong
`coupling between geometry and mechani-
`cal properties. Zosel developed a simple
`test in which a cylindrical flat-ended
`probe is indented into a thin layer of
`adhesive and subsequently removed, as
`schematically described in Figure 1. The
`force and position of the probe are continu-
`ously recorded during the test, and the
`entire curve can be analyzed. The distinc-
`tive plateau observed after the maximum
`force peak shown in Figure 1 was clearly
`identified as being caused by the forma-
`tion of fibrils during detachment.13 Only
`those PSAs displaying high peel forces
`showed the formation of these fibrils.
`However, the molecular parameters con-
`trolling the first peak and the mechanism
`responsible for the drop in the force were
`only elucidated much later, when a probe
`tester fitted with a video camera was
`developed. The real-time observation tool
`showed that the force peak was due to the
`formation of cavities growing from the
`
`stantial cavity growth along the interface,
`followed by a coalescence process only
`occurred on low-adhesion surfaces; and
`finally, the formation of fibrils was only
`possible if coalescence of the cavities did
`not occur. The final elongation of the fibrils
`at detachment was directly related to the
`elongational properties of the adhesive, as
`discussed by others in the interpretation
`of peel test results.21–24 Given this general
`picture of how PSAs work, let us now con-
`sider what the key material requirements
`are to control the debonding mechanisms.
`
`Material Requirements
`From an applications point of view, PSAs
`need to possess three important proper-
`ties. First of all, some degree of stickiness:
`in order to form a good bond on almost
`any surface, a PSA must be sticky upon
`simple contact, a property generally called
`tack. Second, all PSAs are peeled from a
`surface, either before use or during use, if
`they are removable: a controlled peel force
`as a function of peel velocity and precise
`control of the residue left on the surface
`are also requirements. Finally, permanent
`PSAs are subjected to stresses for long
`periods of time, and creep must be mini-
`mized. Each one of these application re-
`quirements can only be met with specific
`material properties, and often commercial
`formulations are the result of a compro-
`mise. The main requirement in terms of
`molecular structure is that all PSAs must
`
`Figure 1. Schematic illustration of a
`tack test performed with a cylindrical,
`flat-ended probe.
`
`interface with the probe into the bulk of
`the layer, as shown in Figure 2.15 The
`widespread presence of cavities caused
`the force to drop, and if the cavities did
`not coalesce, the walls between these cavi-
`ties became the fibrils observed by Zosel.
`It therefore became obvious that the de-
`bonding mechanism was rather compli-
`cated and could be divided into stages:
`initiation by cavitation, cavity growth,
`and finally fibril formation and stability.15
`Several years of further studies6,9,16–20 pro-
`vided, little by little, a more complete pic-
`ture of the main controlling factors: the
`first peak of stress was due to a process of
`cavity growth in a rubbery elastic medium
`and therefore was chiefly controlled by
`the elastic modulus of the adhesive; sub-
`
`Figure 2. (a) Schematic illustration of a probe test setup, showing possible observation
`angles for the (b) cavitation and (c) fibrillation processes occurring during debonding of
`pressure-sensitive adhesives (PSAs).
`
`MRS BULLETIN/JUNE 2003
`
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`Pressure-Sensitive Adhesives: An Introductory Course
`
`be based on polymers well above their Tg.
`Typically, in order to be tacky at a given
`usage temperature, the Tg of the adhesive
`must be 25–45⬚C below that temperature
`and be as broad as possible to maximize
`viscoelastic dissipation at the low-modulus
`end of the glass transition. Differences in
`properties can only then be obtained with
`variations in Tg, molecular weight, or mo-
`lecular architecture and supramolecular
`structure. In addition, formulation plays
`a significant part in the optimization of
`viscoelastic properties, and small molecules
`are invariably added to the base polymers.
`The stickiness to the touch of a PSA is
`related to its ability to form bridging fibrils
`when the PSA is pulled away, as demon-
`strated by a probe test (Figure 2). The re-
`quired property for obtaining this fibril
`formation on a standard test surface such
`as steel is a resistance to fracture by crack
`propagation. In other words, if failure is
`initiated by a small crack (at the edge or in
`the center of the contact area), it should
`not be able to propagate but rather should
`blunt.25,26 If adhesive forces are provided
`by van der Waals forces alone, this crack
`blunting, which causes the formation of
`fibrils, is observed for materials with a
`Young’s modulus E below 0.1 MPa. Such
`a low modulus puts PSAs in the category
`of elastomers or gels. As discussed previ-
`ously, the same requirement of low modu-
`lus is dictated by the necessity to form a
`good bond with a rough surface under a
`light pressure.27,28
`If one recalls that the elastic modulus of
`an elastomer is related to its average mo-
`lecular weight between entanglements
`(Me),29,30 it becomes clear that suitable elas-
`tomers for a PSA application must have a
`large Me value.14 This can be achieved by
`polymers such as poly(n-butyl acrylate)
`or poly(2-ethylhexyl acrylate) or by
`adding miscible small-molecule tackifiers
`to elastomers such as natural rubber or SIS
`block copolymers. These tackifiers essen-
`tially dilute the entanglement network
`and lower the modulus of the PSA in the
`plateau region, as shown in Figure 3. The
`test frequency was 1 Hz in this figure.31,32
`However, a low elastic modulus, while
`being necessary for tackiness, is not suffi-
`cient for control of the peel force, and
`therefore two more properties guiding
`molecular design must be included:
`1. For the material to be a useful PSA, the
`viscous component G⬙ of the shear modu-
`lus at the test frequency must be relatively
`high (on the order of the elastic compo-
`nent G⬘ for permanent PSAs and ⬃10–30%
`of the elastic component for the removable
`ones). This requirement presupposes a ma-
`terial that will dissipate energy through
`deformation, causing a moderate to high
`
`Figure 3. Elastic component of the
`shear modulus G⬘ for a pure styrene-
`isoprene-styrene (SIS) triblock copolymer
`containing 30 wt% triblock (open
`circles), and a 40⬊60 blend of SIS
`triblock and tackifying resin (open
`triangles).
`
`peel force. Often, this is achieved by a sig-
`nificant broadening of the G⬙ peak associ-
`ated with the glass transition.9
`2. The PSA must be able to strain-harden
`at high levels of strain (viscous fluids such
`as honey or tar are sticky but are not use-
`ful PSAs). This property fulfills the typical
`requirement of an adhesive to fail without
`leaving a sticky residue on the surface.
`Although all PSAs exhibit some degree
`of strain hardening in elongation, the level
`of elongational strain where strain harden-
`ing sets in and how progressive this strain
`hardening is are key material parameters
`in PSA design. For homopolymers and
`random copolymers, the degree of strain
`hardening is controlled through a suitable
`choice of molecular-weight distribution
`and degree of cross-linking.33 Experimen-
`tally, the optimum degree of cross-linking
`corresponds approximately to slightly
`above the gel point: a typical PSA contains
`about 50–70% of insoluble fraction. The
`gel point is defined as the degree of cross-
`linking at which the elastic and viscous
`moduli are equal over a range of test fre-
`quencies. By increasing or decreasing the
`gel fraction, it is possible to modify the
`onset of strain hardening and control fibril
`extension and, therefore, the peel force.
`Finally, another property that some
`PSAs must have is good resistance to a
`continuous shear force, since many appli-
`cations, while not being strictly speaking
`structural, must sustain a moderate level
`of stress for a very long time. Examples
`include PSAs used to fasten components
`inside a digital camera, and the PSA that
`holds the rear-view mirror of your car in
`place. This requirement assumes PSAs to
`be solids and therefore to have no meas-
`urable Newtonian viscosity at the usage
`temperature. Again, this can be achieved
`
`through two main molecular design tools:
`controlled chemical cross-links or physical
`cross-links. However, the requirements to
`maximize resistance to creep are not the
`same that maximize peel force. In a shear
`experiment, no fibrils are formed, and the
`optimum degree of cross-linking is dis-
`placed toward more cross-linking than for
`the peel force. The upper limit in degree of
`cross-linking is given there by the resistance
`to crack propagation: if the PSA is too
`highly cross-linked, it becomes too elastic
`(as determined by the ratio tan ␦苷 G⬙/G⬘
`in a rheology experiment), and the adhe-
`sion hysteresis is not large enough to cause
`crack blunting. Therefore, the inevitable
`presence of a small defect leads to a quick
`adhesive failure.
`
`Nanostructured PSAs
`It is worthwhile to spend a little time
`discussing the structure and specific prop-
`erties of PSAs based on block copolymers.
`These PSAs are widespread in the adhesive-
`tape industry, essentially because of their
`ease of production in thin-film form by the
`hot-melt process (as discussed later in the
`article). The first studies on their proper-
`ties date back to the late 1970s.8,34–36 They
`are, however, also used because of their
`unique properties as adhesives.
`As we now understand, SIS block co-
`polymers undergo a microphase separa-
`tion if the respective blocks are sufficiently
`long.37 Compounding the block copoly-
`mer with a tackifying resin miscible with
`the isoprene phase yields a structure of
`physical cross-links provided by the
`glassy styrene domains in an elastomeric
`isoprene⫹resin matrix. This type of struc-
`ture gives, at room temperature, a PSA
`that is very solidlike at low frequencies
`(absolutely no Newtonian viscosity) while
`at the same time very viscoelastic at high
`frequencies. These “nanostructured” mate-
`rials possess a superior resistance to creep
`while maintaining a very high peel force
`and acceptable tack. In terms of molecular
`design, an important parameter is the
`ratio between isoprene chains bridging
`two styrene domains (typically triblocks)
`and nonbridging isoprene chains (triblocks
`of SIS in a loop or “hairpin” configuration
`or diblocks). Large-strain tensile deforma-
`tion is greatly influenced by this connec-
`tivity between domains, and the strain
`hardening is controlled by the extent and
`length of the bridges between these
`styrene blocks,10,38 as shown in Figure 4.
`It is also worthwhile to note that there are
`current efforts to design heterogeneous
`nanostructured acrylic PSAs using emul-
`sion polymerization to make core-shell
`particles that retain their original structure
`once the PSA film is formed.39,40
`
`436
`
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`
`EX. 1020
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`

`Pressure-Sensitive Adhesives: An Introductory Course
`
`Easy Release of PSAs
`from Surfaces
`An interesting and complementary
`problem arising from the preparation of
`PSAs is the issue of protecting the PSA
`before use. By definition, PSAs are sticky
`and, therefore, dust particles in the air can
`easily stick to the surface, making manipu-
`lation difficult once the sticky surface is ex-
`posed. Adhesive-tape manufacturers solve
`the problem by adhering the sticky side of
`the tape to the back surface of the tape in
`rolls, while adhesive labels normally are
`sold in contact with a protective film.
`How does the back surface of adhesive
`tape or the protective film of a label work?
`In this case, it is absolutely necessary not
`only for the adhesive to peel cleanly from
`the protective surface but also to retain its
`properties once it is applied to the target
`surface. In practice, adhesion must be sig-
`nificantly higher on the back of a tape than
`on the protective film of a label (other-
`wise, the tape would unroll under its own
`weight), and accordingly, different mate-
`rials are used for both cases. Poly(vinyl
`carbamates) provide good surfaces for the
`back side of tapes, while highly cross-
`linked silicones are used for the protective
`films for labels.
`How does a release coating work? Re-
`calling the section on the mechanisms of
`debonding of PSAs, the high adhesion
`comes essentially from the formation of
`
`fibrils, so a low-adhesion surface must be
`able to suppress this fibrillation process.
`Recent experiments20 have shown that this
`lack of fibrillation implies that the multiple
`cracks nucleating at the interface (Fig-
`ure 2) no longer blunt but coalesce. Such a
`coalescence process is shown in Figure 5
`for tensile probe test curves of tack. This
`lack of crack blunting leads to a complete
`or partial suppression of the fibrillation
`stage and is therefore a very good way to
`control a macroscopic adhesion parameter
`such as the peel force. At a more mecha-
`nistic level, separation of the adhesive
`from the substrate is generally caused by a
`tensile force applied in the direction per-
`pendicular to the interface. For cavities to
`coalesce, the lateral crack-propagation ve-
`locity must be high relative to the vertical
`crack-propagation velocity, and this means
`a low level of instantaneous energy dissi-
`pation by viscous flow at the edge of the
`crack: this is where adhesion is important.
`Interestingly, surfaces causing high cavity-
`growth rates and therefore low adhesion
`have in common a low resistance to inter-
`facial friction.41,42 This low resistance to
`interfacial friction requires a high level of
`molecular mobility of the surface,43 so that
`the PSA effectively is adhered to a molecu-
`larly fluid but macroscopically solid layer.
`Removable adhesives work under the
`same principle, that is, suppression or at
`least minimization of the fibrillation stage.
`
`However, in this case, the goal must be
`achieved by varying the composition and
`molecular architecture of the adhesive
`alone. This typically implies a reduction in
`the value of G⬙ relative to G⬘ so that crack
`propagation becomes less dissipative and
`fibrillation is minimized. Basically, one
`seeks to obtain the minimization of fibril
`formation on any given surface.
`
`Manufacturing Adhesives
`Although we have focused here on the
`properties of the adhesives, many require-
`ments in terms of molecular structure or
`formulation actually stem from the neces-
`sity of producing PSA products at high
`speed and reasonable cost.
`PSA films are produced essentially by
`three methods:
`䊏 From solution by coating and solvent
`evaporation. This is the more traditional
`method, but it is now only used where
`specific performance of solution systems
`cannot be obtained by other means.
`䊏 From latexes, that is, emulsions of small
`particles by coating and water evapora-
`tion. This is the typical method used for
`labels based on acrylic polymers.
`䊏 From hot melts. This method implies
`that the manufacturing is performed with-
`out solvent. In order to work, the viscosity
`must be reasonably low during the coating
`process and achieve its end-use properties
`later on through a decrease in temperature
`
`Figure 4. Tensile probe test curves
`of nominal stress (␴苷 F/A0) versus
`nominal strain [␧ 苷 (h ⫺ h0)/h0, where
`h and h0 are the deformed and
`undeformed thicknesses of the adhesive
`film, respectively] for three polymer
`blends. All tests were performed at a
`probe retraction velocity of 100 ␮m/s on
`a 100-␮m-thick layer. The percentage of
`diblock styrene-isoprene in the polymer
`part of the blend was varied from 0% to
`19% to 42%, while the weight ratio of
`total polymer (diblock ⫹ triblock) to
`resin was kept at 30⬊70 for all three
`samples. Note the important effect of
`the variable amount of diblock on the
`plateau part of the curve representing
`the fibrillation process.
`
`Figure 5. Tensile probe test curves of nominal stress versus nominal strain (“tack”) for the
`same type of PSA as shown in Figure 4, based on a triblock copolymer and a tackifying
`resin, on two surfaces. The bottom micrograph is polished steel, and the top is a release
`surface; both have a diameter of 10 mm. Note the very different debonding patterns for
`both surfaces and the complete absence of a fibrillation plateau on the ␴–␧ curve of
`the adhesive on the release surface.
`
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`
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`Pressure-Sensitive Adhesives: An Introductory Course
`
`or a cross-linking and polymerization re-
`action. It is the preferred method for all
`PSAs based on block copolymers, in which
`case the phase-separated structure under-
`goes a “melting” transition when the tem-
`perature is raised above the Tg of the
`styrene phase.
`For the reasons explained earlier, the
`first two coating methods must be fol-
`lowed by a cross-linking step that is nor-
`mally accomplished either by temperature
`or radiation. Radiation curing can be per-
`formed either by UV exposure, in which
`case the polymerization and cross-linking
`proceed simultaneously and suitable photo-
`initiators must be used, or by electron-beam
`irradiation, where typically the polymer is
`already formed. Radiation curing has the
`distinct advantage over thermal curing of
`allowing very short reaction times.44
`
`Applications
`For many people, the quintessential PSA
`application is adhesive tape or the self-
`adhesive label. While these applications
`represent the largest volume of sales, they
`generally do not require a fine-tuning of
`properties, and the emphasis is on mini-
`mizing production costs. However, a variety
`of other applications where more finely
`tuned properties are needed are now
`widely available. The applications can
`be roughly divided in three categories
`based on performance requirements: semi-
`structural permanent, permanent with no
`real structural requirement, and removable.
`Among the applications for removable
`adhesives, a large industrial application
`is the temporary protection of surfaces
`(automobile bodies, windshields, and pre-
`fabricated elements used in construction),
`as illustrated in Figure 6. In this case, the
`key technical problem is to maintain the
`easy removability of the protective film
`and the lack of adhesive residue on the
`car’s surface, even after three months of
`outdoor aging in harsh conditions. An-
`other important application of removable
`PSAs is masking tape for painting appli-
`cations. Finally, one should mention the
`most consumer-friendly application of PSAs
`in our opinion, the Post-It note. In this
`case, a thin layer of adhesive must stick to
`paper and be easily removable. This requires
`a particularly low level of adhesion, since
`the paper fibers are easily ripped apart.
`Among permanent nonstructural PSAs,
`the main applications are labels in all their
`forms and packaging tapes. In both cases,
`the requirement is good adhesion on paper
`or cardboard, which really means that de-
`tachment cannot occur without permanent
`damage to the surface of the cardboard or
`to the label. PSAs based on block copoly-
`mers have a large segment of the market
`
`Figure 6. Temporary protection of automobile paint and other components is routinely
`provided by the use of polymer sheets (white areas in photograph) coated with a PSA, thus
`offering safe adhesion and easy removability without affecting the optical surface quality of
`the car even after prolonged contact time under outdoor conditions. (Courtesy of tesa AG.)
`
`for tapes, and acrylic water-based poly-
`mers dominate the label market. How-
`ever, high-end labeling applications and,
`in particular, outdoor applications are
`dominated by solution acrylics, with their
`superior resistance to aging.
`Finally, double-sided tapes and foams
`are increasingly used as an alternative to
`fastening with conventional chemically
`reactive soft glues. In particular, the micro-
`electronics industry uses a large number
`of PSA layers for the construction of
`components.
`In this case, the adhesive is often pre-
`foamed to introduce defects (small bubbles),
`which, by breaking the confinement with-
`out having to nucleate a cavity, have supe-
`rior strength. Defects introduced throughout
`the adhesive layer reduce the stress on
`interfacial defects, which typically fail cata-
`strophically. These are high-end applica-
`tions where maximizing the resistance to
`creep of the adhesive under stress is very
`important. The phase-separated structure
`of block-copolymer-based PSAs gives them
`a competitive advantage for this type of
`application.
`
`Future Trends
`PSAs are now increasingly replacing
`more traditional adhesives because of their
`ease of use and safe manipulation. Despite
`the developments of energy-efficient
`solvent-recovery methods, environmental
`legislation drives research toward alterna-
`tive production methods to coating from
`solution. Water-based acrylic emulsions are
`widespread for general-use PSAs. How-
`
`ever, the presence of surfactants, which
`are not easy to eliminate,45,46 and the lesser
`control that one obtains over the molecular
`structure, precludes for the moment their
`access to the most demanding applica-
`tions. Ongoing research is aimed at im-
`proving properties to the same level as
`solution-cast acrylics.
`Along the same lines, an increasingly
`widespread production method (at least
`for tapes) is hot-melt, or solventless, tech-
`nologies. In this type of manufacturing
`process, the viscosity must be lowered
`enough for an easy and homogeneous
`coating without the use of any volatile
`compound while maintaining properties
`of the final product. Superior recyclability
`of this type of product is likely to become
`important in the future, due to environ-
`mental pressures.
`From the point of view of end-use
`properties alone, the future trend for PSAs
`will be in the direction of heterogeneous
`polymer structures and also on the incor-
`poration of additional functionalities such
`as thermal or electrical conductivity or
`controlled drug release, as in nicotine
`patches, into a PSA matrix without alter-
`ing its self-adhesive properties.
`
`Acknowledgments
`We thank Dr. Bernd Luhmann from
`tesa AG for his careful reading of the
`manuscript and helpful suggestions. We
`acknowledge the financial support of the
`European Commission, under contract
`G5RD-CT-2000-00202-DEFSAM.
`
`438
`
`MRS BULLETIN/JUNE 2003
`
`EX. 1020
`APPLE INC. / Page 5 of 6
`
`

`

`Pressure-Sensitive Adhesives: An Introductory Course
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