Adhesives are materials that bond together the surfaces of two other materials. The basic features of the structure, properties, and modification of polymeric adhesives are discussed as they relate to adhesive behavior. Adhesive properties and bond performance, including durability, are derived from chemical composition (functional groups), molecular organization (branching, molecular weight distribution, cross-linking) and physical state (elastomer, thermoplastic, thermoset, crystalline). The various types of adhesives include those derived from naturally occurring polymers—carbohydrates, proteins, and natural rubber—and those derived from the multitude of synthetic polymers—phenolics, epoxies, acrylics, elastomers, urethanes, etc. The transformation of liquid adhesive to a solid bond calls for curing reactions of reactive components or film formation from emulsion, solution, or melt. The properties of adherend surfaces play a critical role in the wetting and bonding of substrates as do modifications of interfacial interactions by chemical surface treatments and coupling agents. Resistance to environmental effects of humidity, temperature, microbial attack, etc. determines the durability of bonds.
