Pressure-Sensitive Adhesives: An Introductory Course
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Pressure-Sensitive
Adhesives: An Introductory Course Costantino Creton
Abstract Self-adhesive materials are called, in the adhesives trade, “pressure-sensitive adhesives” (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 everyday life. However, until recently, the understanding of the materials science and engineering of PSAs and, in particular, the specific role played by the different components in them was very limited outside of the companies involved in their manufacture, and the interested reader had to refer to general technological texts.1,2 Although 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 typically stick to a surface upon contact without 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 adhesion is then rather insensitive to the compressive pressure applied upon bonding. This property makes PSAs particu-
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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, establish 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 temperature, 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
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