9 июля, 2015 
Malyar David N.-S. Hon
Clemson University, Clemson, South Carolina, U. S.A.
Adhesives now form an inextricable part of modern life. In fact it is difficult to imagine our everyday existence without these bonding substances. Consider packaging, building, automobiles, dentistry, medicine, consumer goods, and food industries—all these activities rely heavily on adhesive materials. Use of adhesives in today’s technology continues to grow at a rapid pace throughout the world and research in polymer science continues to mushroom, producing a plethora of new adhesives.
For a successful application of an adhesive to a substrate (adherent), to reach a maximum adhesion, it must be able to flow, penetrate, wet, and set. The adhesive in its final state must be a solid, high-molecular-weight polymer that is able to carry and transfer mechanical forces. In order to meet these four criteria, there are three ways in which adhesives are prepared: (1) A polymeric adhesive can be dissolved or dispersed in a solvent; after application, the solvent is allowed to evaporate, leaving behind the high- molecular-weight polymer. (2) If the adhesive is a thermoplastic polymer, it can be heated to melt and apply in a molten state; after application, it is allowed to cool and solidify in the glueline. (3) An adhesive can be prepared by chemical reactions via a polymerization process to convert monomer (low-molecular-weight starting unit) into polymer (high — molecular-weight material). Many natural adhesives are prepared based on the first method, and most of the synthetic adhesives are prepared by the third method.
In essence, an adhesive is any substance, inorganic or organic, natural or synthetic, that is capable of bonding substances together by surface attachment. The bonding power of an adhesive depends heavily on its molecular weight or size of the molecules. Under a proper bonding process, the adhesive with higher molecular weight provides a stronger bond. Hence the adhesive is a high-molecular-weight substance, commonly known as a polymer.
A polymer is composed of repeating units (i. e., monomers) that are linked together into long chains that can be linear, branched, or cross-linked. If a polymer contains two different types of monomers, it is a copolymer. A linear polymer is a thermoplastic. At elevated temperatures it melts and flows as a liquid. In a cross-linked polymer, the repeat units are actually linked into a three-dimensional network of macroscopic size. It is a thermoset. Once the polymerization is completed, the cross-linked polymer cannot be softened or melted. It is hard, infusible, and insoluble. Hence a thermoset adhesive is the most durable but is also difficult to characterize as compared to a thermoplastic one.
Adhesives are a very diverse and complex group of materials. They can manifest themselves in many shapes and forms—they can be viscous liquids, powders, or cured products. Analysis or characterization is an essential step in working with adhesives. As a rule, such efforts are directed toward a specific purpose that may focus on structural determination, curing reaction, size of the molecule, material design at a molecular level, process control, or failure analysis. In this chapter we provide a general review of several physical methods frequently used for analysis of adhesives. In view of the prolific literature on the subject as well as the space constraints, it is not intended to give a comprehensive treatment of the theory and experimental aspects. The examples chosen for this review are illustrative and not exhaustive.
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