**Abstract**

Self-adhesive materials are called, in the adhesive industry, pressure-sensitive adhesives (PSAs). PSAs are designed in the shapes of latex, solvent borne resin, rubber solution, or hot melt and are being used for foils and films, tapes, labels, and notepads which can make permanent, removable, or semi-removable adhesive joints for applications of general purpose product assembly by simple contact under light pressure. This special class of adhesives does not undergo any physical transformation or chemical reaction during the bonding process. The end-use properties of PSAs result from the nonlinear viscoelastic behavior of the adhesive material, and the elastomeric polymer basis of PSAs imparts them such a viscoelastic behavior caused by a carefully chosen polymer architecture and monomer composition with the proper addition of small molecules called tackifying resins. They are safe to use and easy to handle and thus are increasingly replacing more conventional types of adhesives. In this chapter, we review adhesion mechanism of PSAs, types of PSAs, adhesion properties and tests, mechanical behavior of joints, and especially different aspects of PSA applications.

**Keywords:** self-adhesives, pressure-sensitive adhesives, viscoelastic behavior, tackifying resins, adhesion mechanism

#### **1. Introduction**

Pressure-sensitive adhesives provide a quick and effective fastening method compared to most mechanical joining techniques. Adhesive bonding has become a preferred fastening technology in applications where dissimilar materials need to be joined. PSAs, with a viscoelastic nature, are composed of several key elements: a fluid, adhesive, tackifier and some kind of structured backing (**Figure 1**) [1].

PSAs have generally an elastomer basis, which can be of the following materials such as natural rubber, vinyl ethers, acrylics, butyl rubber, styrene block copolymers, silicones and nitriles. A tackifier, a low molecular weight resin, is added to the elastomer to increase adhesion and is generally one of the following common resins: terpenes, aromatic resins, hydrogenated hydrocarbon resins and terpene-phenol resins [2].

PSA's in the shapes of latex, solvent borne resin, rubber solution or hot melt are used for foils and films, tapes, labels and notepads which can make permanent, removable or semi-removable adhesive joints for applications of general purpose product assembly, display, tags, labels, and mailers, packaging tapes, masking and paper tapes, tapes for hygiene applications, double-sided tapes, cloth and duct tapes, heavy-duty and reinforced tapes and electrical tape as well as a wide variety of other products (**Figure 2**) [3].

**Figure 1.** *Viscoelastic nature of pressure-sensitive adhesives.*

**Figure 2.** *Various applications of different kinds of pressure-sensitive adhesives.*

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components.

**Figure 4.**

*Schematic of PSA main properties.*

interactions) [2, 4].

procedure (**Figure 5**) [5].

criterion of pressure sensitivity [4, 8, 9].

**2. Adhesion mechanism of PSAs**

*Pressure-Sensitive Adhesive Joints*

*DOI: http://dx.doi.org/10.5772/intechopen.85667*

When considering pressure-sensitive adhesives and their applications, it's useful to keep in mind main adhesion properties such as shear strength, peel adhesion, tackiness and creep (**Figures 3** and **4**) determine the adhesion of PSA system and subsequently, the application it is used for needs a balance of mentioned properties which can be adjusted via polymerization or formulation process and its

In the following chapter, adhesion mechanism of PSAs, types of PSAs, adhesion properties and tests, mechanical and behavior of joints and especially different aspects of PSA applications in the industries will be thoroughly discussed.

To understand how PSAs work, it is necessary to understand not only the properties of the individual components of the adhesive formulation and those of the formulation as a whole, but also the processes that take place upon contact of the PSAs with different types of substrates. Good interfacial interaction with a substrate is essential for PSA performance. Such interaction is caused by forces of different natures (van der Waals, H-bonding, acid–base, and donor–acceptor

At the macro level, this interaction between PSA and substrate results in a wetting– spreading process. Wetting of the substrate by the adhesive is crucial to establish good contact between them. Wetting is also important in the coating

Generally PSAs are used as thin layers; therefore their flow is limited by the physico-mechanical interactions with the solid components of the laminate (liner and face) materials [6, 7]. The term "pressure-sensitive adhesive" itself presumes a definite pressure value necessary to form a strong adhesive joint. Upon converting to dimensionless variables, it would be logical to express pressure as a value relative to the elastic modulus. This is one possible approach to constructing the "dynamic"

The phenomenon of tackiness features a high shear flow of an adhesive material under a compressive force, thus forming the adhesive joint. The flow of the adhesive

**Figure 3.** *Major elements of PSA's adhesion properties: tack, adhesion and cohesion.*

*Pressure-Sensitive Adhesive Joints DOI: http://dx.doi.org/10.5772/intechopen.85667*

*Adhesives and Adhesive Joints in Industry Applications*

*Viscoelastic nature of pressure-sensitive adhesives.*

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**Figure 3.**

**Figure 2.**

**Figure 1.**

*Major elements of PSA's adhesion properties: tack, adhesion and cohesion.*

*Various applications of different kinds of pressure-sensitive adhesives.*

**Figure 4.** *Schematic of PSA main properties.*

When considering pressure-sensitive adhesives and their applications, it's useful to keep in mind main adhesion properties such as shear strength, peel adhesion, tackiness and creep (**Figures 3** and **4**) determine the adhesion of PSA system and subsequently, the application it is used for needs a balance of mentioned properties which can be adjusted via polymerization or formulation process and its components.

In the following chapter, adhesion mechanism of PSAs, types of PSAs, adhesion properties and tests, mechanical and behavior of joints and especially different aspects of PSA applications in the industries will be thoroughly discussed.

### **2. Adhesion mechanism of PSAs**

To understand how PSAs work, it is necessary to understand not only the properties of the individual components of the adhesive formulation and those of the formulation as a whole, but also the processes that take place upon contact of the PSAs with different types of substrates. Good interfacial interaction with a substrate is essential for PSA performance. Such interaction is caused by forces of different natures (van der Waals, H-bonding, acid–base, and donor–acceptor interactions) [2, 4].

At the macro level, this interaction between PSA and substrate results in a wetting– spreading process. Wetting of the substrate by the adhesive is crucial to establish good contact between them. Wetting is also important in the coating procedure (**Figure 5**) [5].

Generally PSAs are used as thin layers; therefore their flow is limited by the physico-mechanical interactions with the solid components of the laminate (liner and face) materials [6, 7]. The term "pressure-sensitive adhesive" itself presumes a definite pressure value necessary to form a strong adhesive joint. Upon converting to dimensionless variables, it would be logical to express pressure as a value relative to the elastic modulus. This is one possible approach to constructing the "dynamic" criterion of pressure sensitivity [4, 8, 9].

The phenomenon of tackiness features a high shear flow of an adhesive material under a compressive force, thus forming the adhesive joint. The flow of the adhesive

**Figure 5.**

*Wetting of the substrates as the crucial item in adhesion by PSAs.*

is needed in the first several seconds to wet the surface of a substrate. Owing to the high viscosity of the PSA, proper wetting cannot be achieved in a reasonable time without compressive force [4].

The final adhesive properties of PSAs require a viscoelastic, non-Newtonian flow behavior which is located at rubbery plateau and related to the supramolecular properties of adhesive. For deeply understanding of viscoelastic behavior, there is the necessity of concluding the most important PSA properties. Totally, adhesives in a linkage have a behavior like a fluid or a solid. PSAs originate from a film-forming, elastomeric material, which combines a high degree of tack with an ability to quickly wet the surface to which it is applied, to provide instant bonding at low-to moderate pressure as a result of its flow characteristics. On the other hand, there is enough cohesion and elasticity in PSAs, in order that in spite of their high tackiness they can be applied with the fingers and easily removed without being left any residue. Furthermore, for achieving bond strength, PSAs must store energy and be elastic. Mainly, there should be a balance between viscous and elastic properties of PSAs. It should also be considered that PSAs have to pass contradictory requirements under different stress rates; at low shear rates, PSAs must flow (bonding) (**Figure 6**) and at high peeling rates there should be an elastic response (debonding) [6, 10–12].

In practical point of view, the relation between adhesion properties and stress rates can be seen by applying peeling test on removable PSAs at different peel rates. The stress rate dependent stiffening is an increase in the elastic contribution to the rheology of the polymer. Major parts of the bond rupture energy are stored when the elastic parts are predominant, causing higher peel and tack properties [13].

Final properties of PSAs are dependent on the nonlinear viscoelastic behavior of the adhesive material being resulted from elastomeric polymer basis of PSAs as viscoelasticity. The same stiffening effect shall be seen by decrease in polymer temperature. This is because the polymer chains are encountered with a restriction in their ability to flow, and the modulus increases. As a result, the adhesive properties of PSAs are also temperature dependent [14].

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*Pressure-Sensitive Adhesive Joints*

behavior of PSAs.

*Schematic of bonding and debonding process.*

**Figure 6.**

*DOI: http://dx.doi.org/10.5772/intechopen.85667*

Pressure-sensitive adhesives must display irreversible work of deformation during bonding and reversible deformation work upon debonding. The ratio of both kinds of deformation work (i.e., of stored and dissipated energy) characterizes the

About the temperature dependence of PSAs' mechanical properties, the storage modulus starts high at low temperatures where all motion within the polymer is

The essential performance characteristics when characterizing the nature of PSAs

To be wetted out high fluidity is implied according to the characterization by an appropriate viscosity of the adhesive. Wetting out, being covered the substrate surface by adhesive fluid, results in bonding formation because of the deformability of viscoelastic PSAs. Vice versa, debonding may be assumed the laminate's deformation, the creation and deformation of two new surfaces. Thus, it may be concluded that for bond-forming a high deformation with a medium elasticity is required, while debonding needs an average deformation with a high elasticity [16].

Viscosity and elastic moduli of PSAs as the properties depended on the experimental parameters used, such as the temperature and time, and the strain rate

are tack, peel adhesion, and resistance to shear. The first property represents the adhesive's ability to adhere quickly (initial grab), and latter one evaluates its ability for not being peeled off, and the last one characterizes the resistance against not being flown when encountering with high shear forces. All in all, the first two characteristics are in direct relation with each other, but inversely related to the last one [15]. Tack is measured in two steps according to Zosel: the contact step and the separation one. At first step, in the surface points contact is made, increasing to a larger area while wetting out, viscous flow, and elastic deformation (**Figure 8**) [6].

At higher temperatures it drops off and exhibits a plateau region which represents the elastomeric response generally encountered at normal end-use temperatures; the storage modulus then decreases further when softening begins. The temperature region through which the polymer changes from a glassy (hard) state into a liquid (rubber-like) state, this second order transition point is called the glass transition temperature (Tg), and is a very important parameter in the characterization of PSAs. In temperatures more than Tg, one can apply the time-temperature superposition principle. The side chain size and mobility of the polymer cause

frozen and the material behaves like a glass (**Figure 7**).

different viscoelasticity around Tg [4].

*Pressure-Sensitive Adhesive Joints DOI: http://dx.doi.org/10.5772/intechopen.85667*

*Adhesives and Adhesive Joints in Industry Applications*

without compressive force [4].

*Wetting of the substrates as the crucial item in adhesion by PSAs.*

**Figure 5.**

is needed in the first several seconds to wet the surface of a substrate. Owing to the high viscosity of the PSA, proper wetting cannot be achieved in a reasonable time

The final adhesive properties of PSAs require a viscoelastic, non-Newtonian flow behavior which is located at rubbery plateau and related to the supramolecular properties of adhesive. For deeply understanding of viscoelastic behavior, there is the necessity of concluding the most important PSA properties. Totally, adhesives in a linkage have a behavior like a fluid or a solid. PSAs originate from a film-forming, elastomeric material, which combines a high degree of tack with an ability to quickly wet the surface to which it is applied, to provide instant bonding at low-to moderate pressure as a result of its flow characteristics. On the other hand, there is enough cohesion and elasticity in PSAs, in order that in spite of their high tackiness they can be applied with the fingers and easily removed without being left any residue. Furthermore, for achieving bond strength, PSAs must store energy and be elastic. Mainly, there should be a balance between viscous and elastic properties of PSAs. It should also be considered that PSAs have to pass contradictory requirements under different stress rates; at low shear rates, PSAs must flow (bonding) (**Figure 6**) and at

high peeling rates there should be an elastic response (debonding) [6, 10–12].

ties of PSAs are also temperature dependent [14].

In practical point of view, the relation between adhesion properties and stress rates can be seen by applying peeling test on removable PSAs at different peel rates. The stress rate dependent stiffening is an increase in the elastic contribution to the rheology of the polymer. Major parts of the bond rupture energy are stored when the elastic parts are predominant, causing higher peel and tack properties [13]. Final properties of PSAs are dependent on the nonlinear viscoelastic behavior of the adhesive material being resulted from elastomeric polymer basis of PSAs as viscoelasticity. The same stiffening effect shall be seen by decrease in polymer temperature. This is because the polymer chains are encountered with a restriction in their ability to flow, and the modulus increases. As a result, the adhesive proper-

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**Figure 6.** *Schematic of bonding and debonding process.*

Pressure-sensitive adhesives must display irreversible work of deformation during bonding and reversible deformation work upon debonding. The ratio of both kinds of deformation work (i.e., of stored and dissipated energy) characterizes the behavior of PSAs.

About the temperature dependence of PSAs' mechanical properties, the storage modulus starts high at low temperatures where all motion within the polymer is frozen and the material behaves like a glass (**Figure 7**).

At higher temperatures it drops off and exhibits a plateau region which represents the elastomeric response generally encountered at normal end-use temperatures; the storage modulus then decreases further when softening begins. The temperature region through which the polymer changes from a glassy (hard) state into a liquid (rubber-like) state, this second order transition point is called the glass transition temperature (Tg), and is a very important parameter in the characterization of PSAs. In temperatures more than Tg, one can apply the time-temperature superposition principle. The side chain size and mobility of the polymer cause different viscoelasticity around Tg [4].

The essential performance characteristics when characterizing the nature of PSAs are tack, peel adhesion, and resistance to shear. The first property represents the adhesive's ability to adhere quickly (initial grab), and latter one evaluates its ability for not being peeled off, and the last one characterizes the resistance against not being flown when encountering with high shear forces. All in all, the first two characteristics are in direct relation with each other, but inversely related to the last one [15].

Tack is measured in two steps according to Zosel: the contact step and the separation one. At first step, in the surface points contact is made, increasing to a larger area while wetting out, viscous flow, and elastic deformation (**Figure 8**) [6].

To be wetted out high fluidity is implied according to the characterization by an appropriate viscosity of the adhesive. Wetting out, being covered the substrate surface by adhesive fluid, results in bonding formation because of the deformability of viscoelastic PSAs. Vice versa, debonding may be assumed the laminate's deformation, the creation and deformation of two new surfaces. Thus, it may be concluded that for bond-forming a high deformation with a medium elasticity is required, while debonding needs an average deformation with a high elasticity [16].

Viscosity and elastic moduli of PSAs as the properties depended on the experimental parameters used, such as the temperature and time, and the strain rate

**Figure 7.** *Temperature dependence of PSAs' mechanical properties.*

**Figure 8.**

*Contact and separation step schematics in tack test: (a) Form loop (b) Clamp tape (c) Push down (d) Pull off.*

cannot be assumed intrinsic material characteristics. So, when speaking of tack, a similar dependence on time, temperature, and the strain rate has to be taken into account which is illustrated by the quite different values of the tack obtained using different experimental techniques namely, quick stick, rolling ball, or loop tack, being evaluated by different time and strain rates, and by the dependence of the adhesive properties to environmental conditions [17].

Peel and peel strength are measured by separating an adhesive applied to a substrate at some angle with respect to the substrate, usually at an angle of 90° or

**95**

**Figure 9.** *Peeling test in PSAs.*

*Pressure-Sensitive Adhesive Joints*

*DOI: http://dx.doi.org/10.5772/intechopen.85667*

before the debonding or peeling step (**Figure 9**) [7, 15].

than during the measurement of the tack [18–21].

(metering roll, drying tunnel) (**Figure 10**) [22].

(**Figure 11**) [4, 14, 17, 24].

180°. Similar to tack, the measurement of the peel adhesion involves a bonding step

The liquid adhesive must be coated onto a release liner or face material. Good coatability implies adequate machinability or processing properties on the coater

Conventional PSAs can be classified as either nonpermanent (2.7–9.0 N/25 mm for 180\_ peel adhesion) or permanent (above 9 N/25 mm for 180\_ peel adhesion)

Repositionable PSPs are a special class of removable pressure-sensitive products (labels and tapes) that stick to various surfaces but remove cleanly and can be reapplied. The final adhesion builds up over a few hours. Non-permanent adhesives are

During production, transport, and coating process, the adhesive fluid is subjected to shear forces resulting in changes of the viscosity. The coated shearthinned adhesive firstly must wet the web, and the wetting out stage depends on the viscosity of PSA. Except for hot-melt PSAs, the coated fluid adhesive film must go through evaporation of the carrier liquid or solvents (drying) which will result in formation of a solid film. Evaporation of the solvents depends on the fact that how it is diffused into the adhesive layer namely, on the adhesive viscosity. As a conclusion, the viscosity of the adhesive, i.e., the time (shear rate)/temperature depen-

dence of the viscosity, influences the coating process of PSAs [23, 24].

Tack as one of the adhesive properties of PSAs is the resistance of the adhesive to being separated after a short time of contact, or by light pressure. Peel is evaluated after an approximately long time of contact after being applied onto the surface of the substrate via a light or medium pressure. The time available for bond forming which means wetting of the surface and then penetration during the first contact step is longer for peeling tests than for tack. After that, flow properties of the adhesive during bonding are less critical for the tack measurement compared to peeling evaluation. Moreover, that how resistant is the PSA in debonding stage is depended on the viscosity/elasticity balance for achieving peelability (removability or repositionability) and on the strain rate influencing the separation resistance substantially

#### *Pressure-Sensitive Adhesive Joints DOI: http://dx.doi.org/10.5772/intechopen.85667*

*Adhesives and Adhesive Joints in Industry Applications*

*Temperature dependence of PSAs' mechanical properties.*

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**Figure 8.**

**Figure 7.**

cannot be assumed intrinsic material characteristics. So, when speaking of tack, a similar dependence on time, temperature, and the strain rate has to be taken into account which is illustrated by the quite different values of the tack obtained using different experimental techniques namely, quick stick, rolling ball, or loop tack, being evaluated by different time and strain rates, and by the dependence of the

*Contact and separation step schematics in tack test: (a) Form loop (b) Clamp tape (c) Push down (d) Pull off.*

Peel and peel strength are measured by separating an adhesive applied to a substrate at some angle with respect to the substrate, usually at an angle of 90° or

adhesive properties to environmental conditions [17].

180°. Similar to tack, the measurement of the peel adhesion involves a bonding step before the debonding or peeling step (**Figure 9**) [7, 15].

Tack as one of the adhesive properties of PSAs is the resistance of the adhesive to being separated after a short time of contact, or by light pressure. Peel is evaluated after an approximately long time of contact after being applied onto the surface of the substrate via a light or medium pressure. The time available for bond forming which means wetting of the surface and then penetration during the first contact step is longer for peeling tests than for tack. After that, flow properties of the adhesive during bonding are less critical for the tack measurement compared to peeling evaluation. Moreover, that how resistant is the PSA in debonding stage is depended on the viscosity/elasticity balance for achieving peelability (removability or repositionability) and on the strain rate influencing the separation resistance substantially than during the measurement of the tack [18–21].

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) (**Figure 10**) [22].

During production, transport, and coating process, the adhesive fluid is subjected to shear forces resulting in changes of the viscosity. The coated shearthinned adhesive firstly must wet the web, and the wetting out stage depends on the viscosity of PSA. Except for hot-melt PSAs, the coated fluid adhesive film must go through evaporation of the carrier liquid or solvents (drying) which will result in formation of a solid film. Evaporation of the solvents depends on the fact that how it is diffused into the adhesive layer namely, on the adhesive viscosity. As a conclusion, the viscosity of the adhesive, i.e., the time (shear rate)/temperature dependence of the viscosity, influences the coating process of PSAs [23, 24].

Conventional PSAs can be classified as either nonpermanent (2.7–9.0 N/25 mm for 180\_ peel adhesion) or permanent (above 9 N/25 mm for 180\_ peel adhesion) (**Figure 11**) [4, 14, 17, 24].

Repositionable PSPs are a special class of removable pressure-sensitive products (labels and tapes) that stick to various surfaces but remove cleanly and can be reapplied. The final adhesion builds up over a few hours. Non-permanent adhesives are

**Figure 9.** *Peeling test in PSAs.*

**Figure 10.** *Coating line of PSAs.*

**Figure 11.** *Categories of adhesives according to the permanency property.*

generally used in the manufacturing process of removable tapes and labels, protective laminates, and other less durable products [25].

For so-called removable adhesives (non-permanent adhesives), the flow properties (viscosity), cohesion of the adhesive and the harbor of the adhesive to the face stock are crucial. Ideally, should the bond to the substrate be nonpermanent, a clean separation from the substrate will be expected and the adhesive remains on the face material. One other requirement for good removability is the low peel value with a permanent character. A clean release from the substrate and no build-up of the peel with time are the minimal requirements for removable PSAs. Based on the adhesive properties, it is possible to formulate the rheological properties required for removable and permanent adhesives [1, 6, 14, 18].

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**Figure 12.**

*Different types of PSAs by their state.*

*Pressure-Sensitive Adhesive Joints*

*DOI: http://dx.doi.org/10.5772/intechopen.85667*

**3. Types of pressure-sensitive adhesives and applications**

coating speed and solvent-like performance simultaneously.

Self-adhesive materials are called, in the adhesives trade, .pressure-sensitive adhesives. PSAs are designed to stick on almost any surface by simple contact under light pressure. Among the different classes of adhesives, pressure-sensitive adhesives (PSAs) are perhaps the most common type found in consumer products. Selfadhesive tapes and labels of all kinds are ubiquitous in everyday life. Despite the fact that PSAs are supposed to adhere to substrates, they are different from other adhesives in many ways. First, PSAs are mainly nonstructural adhesives; they cannot be compared with epoxies for structural applications. Moreover, PSAs mainly adhere to a surface upon contact without any chemical reaction [6, 15, 23, 26].

Pressure-sensitive adhesives can be classified by the type of ingredients used to make them (e.g., rubber-based, acrylic and silicone) and by their state (e.g., organic solvent solution, emulsion, solid (hot melt) and aqueous solution) (**Figure 12**). Emulsions are dispersion of particles containing acrylic polymer in water (latexes). Labels and Packaging Materials (LPM) has manufacturing infrastructures in USA and Europe and wide manufacturing companies to provide latexes around the world. There are also acrylic polymers in petroleum-based solvents—usually of low to moderate solids content. End users prefer the performance of solvent based acrylic polymers although they are more expensive than their water borne counterparts (latexes) [7, 15]. Hot melt PSAs are based on block copolymers. Oils, plasticizers and tackifiers are the necessary components of the formulation needed to obtain appropriate performance. Most hot melts (HM) are dependent on C5/C9 streams from petroleum distilleries. The other PSA acrylic grade UV compositions, which are solventless, produce films with higher thickness, including foam-like adhesives. The two-stage process creating performance advantages in the second stage of curing gives the opportunity of very high molecular weight reactions which can't be cast from solvent. Warm melts (100% solids materials) are either made in solvent and stripped (AC resin) or polymerized to near 100% and coated. They use hot-melt equipment (high coating speeds) and give performance of solvent benefits—high

*Adhesives and Adhesive Joints in Industry Applications*

generally used in the manufacturing process of removable tapes and labels, protec-

For so-called removable adhesives (non-permanent adhesives), the flow properties (viscosity), cohesion of the adhesive and the harbor of the adhesive to the face stock are crucial. Ideally, should the bond to the substrate be nonpermanent, a clean separation from the substrate will be expected and the adhesive remains on the face material. One other requirement for good removability is the low peel value with a permanent character. A clean release from the substrate and no build-up of the peel with time are the minimal requirements for removable PSAs. Based on the adhesive properties, it is possible to formulate the rheological properties required for remov-

tive laminates, and other less durable products [25].

*Categories of adhesives according to the permanency property.*

able and permanent adhesives [1, 6, 14, 18].

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**Figure 11.**

**Figure 10.** *Coating line of PSAs.*
