Pressure-sensitive adhesive physical

Microdomain stmcture is a consequence of microphase separation. It is associated with processability and performance of block copolymer as TPE, pressure sensitive adhesive, etc. The size of the domain decreases as temperature increases [184,185]. At processing temperature they are in a disordered state, melt viscosity becomes low with great advantage in processability. At service temperamre, they are in ordered state and the dispersed domain of plastic blocks acts as reinforcing filler for the matrix polymer [186]. This transition is a thermodynamic transition and is controlled by counterbalanced physical factors, e.g., energetics and entropy. 

Block copolymers are widely used industrially. In the solid and rubbery states they are used as thermoplastic elastomers, with applications such as impact modification, compatibilization and pressure-sensitive adhesion. In solution, their surfactant properties are exploited in foams, oil additives, solubilizers, thickeners and dispersion agents to name a few. Particularly useful reviews of applications of block copolymers in the solid state are contained in the two books edited by Goodman (1982,1985) and the review article by Riess etal. (1985). The applications of block copolymers in solution have been summarized by Schmolka (1991) and Nace (1996). This book is concerned with the physics underlying the practical applications of block copolymers. Both structural and dynamical properties are considered for melts, solids, dilute solutions and concentrated solutions. The book is organized such that each of these states is considered in a separate chapter. 

Prior to this discovery, in 1954 Silberberg and Kuhn (62) were first to study the polymer-in-polymer emulsion containing ethylcellulose and polystyrene in a nonaqueous solvent, benzene. The mechanisms of polymer emulsification, demixing, and phase reversal were studied. Wetzel and Hocks discovery would then equate the pressure-sensitive adhesive to a polymer-polymer emulsion instead of a polymer-polymer suspension. Since the interface is liquid-liquid, the adhesion then becomes one type of R-R adhesion (35, 36). According to our previous discussion, diffusion is not operative unless both resin and rubber have an identical solubility parameter. The major interfacial interaction is physical adsorption, which, in turn, determines adhesion. Our previous work on the wettability of elastomers (37, 38) can help predict adhesion results. Detailed studies on the function of tackifiers have been made by Wetzel and Alexander (69), and by Hock (20, 21), and therefore the subject requires no further elaboration. 

Kokubo, T., K. Sugibayashi, and Y. Morimoto. 1991. Diffusion of drug in acrylic-type pressure-sensitive adhesive matrices. I. Influence of physical property of the matrices on the drug diffusion. /. Contr. Rel. 17 69-78. 

The Physical Testing of Pressure-Sensitive Adhesive Systems John Johnston... 

The need for the physical testing of a pressure-sensitive adhesive can vary considerably such reasons include the determination that a given pressure-sensitive adhesive will perform satisfactorily for its intended use, that it meets a specific standard, that uniformity exists within a given population, or between populations, or that it could be to compare one system to other similar systems—all of which demand that any test method must be accurate and reproducible. The thermoplastic nature of pressure-sensitive systems can make this objective very difficult to achieve, without a full understanding of their behavior and without observing a number of precautions. 

Stabilization of Carboxylated Styrene Butadiene (X-SBR) Latices Carboxylated SBR latices are used as adhesives in applications where durability and flexibility are desired. Some of the major uses for X-SBR latex are in tufted carpet backing, paper coatings, wall and vinyl floor tile adhesives, and pressure-sensitive adhesives. Typically, discoloration is the first measure of the degradation of an X-SBR latex. Discoloration of a dried latex film can often be related to a loss of the physical properties and subsequently, to inferior performance in an adhesive formulation. Figure 9 illustrates the effects of adding an effective antioxidant system to an X-SBR latex on the level of discoloration as a result of static oven aging at 150°C (300°F). The addition of AO-4 alone... 

Because of the unique properties of pressure-sensitive adhesives, special tests not applicable to other types have been developed. While standard physical tests such as nonvolatile content, viscosity, and specific gravity are performed to ensure consistency of application, these tests do not predict adhesive performance. For pressure-sensitive adhesives, three critical performance characteristics are usually measured tack, peel, and shear strength. 

One exception of this statement must be noted. These are the so-called pressure-sensitive adhesives which are normally liquids of very high viscosity which do not cure and which are in some cases poorly cross-linked. They adhere on different substrates only by physical bonds and due to the high mobility of their molecules they can repair destroyed bonds in time. This type of adhesion which is called d5mamic adhesion meaning a centipoid effect is unique for this class of adhesives and can be used partly on untreated PP for nonstructural bonds. The adhesion between the pressure-sensitive adhesive and the PP surface in an untreated state is not very high but relatively unproblematic. 

Pressure sensitive adhesives are an exception to what was stated in the very first paragraph of this article, in that they do not harden to a cohesively strong solid. They are in fact viscous liquids, and remain so when incorporated in an adhesive joint. Nevertheless, it is essential that they adhere to substrates, and they will do so by one or more of the mechanisms which have been already described. Physical adsorption will contribute in every case, and in most cases it may be the only mechanism, but chemical bonding via ion-pairs may contribute if the adhesive contains carboxylic acid groups and the substrate is a metal. Static electrification is another possible contributor. 

Pressure-sensitive adhesives form physical bonds with other materials upon brief contact and with light pressure. Examples include self-stick stamps, packaging tape, double-sided tapes, paper labels, and the ubiquitous Post-iE notes. Bond formation results from the polymeric material being able to flow under light pressure, thereby establishing good contact area with the substrate. The debonding step involves deformation of the polymeric material under stress (see Section 12.8.3), followed by separation from the substrate. The adhesives most often involve triblock copolymers such as poly(styrene-h/ock-isoprene-h/ock-styrene) or poly(styrene-Wodc-butadiene-h/ock-styrene), SBR elastomers, natural rubber, or acrylic copolymers (92). 

The acrylic-type pressure-sensitive adhesives are often partly cross-linked, partly linear in composition. Theory and data available suggest that the cross-linked component should be only very lightly cross-linked, so that the Rg calculated from Me of the cross-linked portion is larger than the Rg based on Me of the linear portion see Section 10.2 (93,94), especially Figure 10.11. Then the linear polymer can enter the network structure through reptation, developing physical bonds. 

Normally adhesives are applied in a liquid state to the substrate, wet the surface, and solidify to generate a bond, for example by evaporation of a solvent, cooling of a hot melt, or formation of a cross-linked polymer. Pressure-sensitive adhesives do not change their physical state, but they also must wet the surface of the substrate like a liquid and sustain loads like a solid. There have been many discussions and investigations concerning how they fulfill these conflicting demands. 

Pressure-sensitive adhesion is a complex phenomenon determined by a number of physical processes. A comprehensive overview is given in [211]. 

The particular category of Pressure-Sensitive Adhesives (PSA s) where the polymer exhibit visco-elastic properties able to develop adhesion during the bonding step as well as cohesion to resist to the debonding. No physical or chemical transformation of the adhesive is required to form the bonded joint. 

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