starch-based materials. ''White glue," used for wood bonding, is a poly(vinyl acetate) emulsion.

Adhesives have several advantages over other joining technologies. In general, adhesives have a lower density than mechanical fasteners, and so weight savings can be realized. Polymer-based adhesives have viscoelastic character and are thus capable of energy absorption. The energy absorption manifests itself in the form of dampening of vibrations and in the increase of fatigue resistance of a joint. Adhesives can be used to join electrochemically dissimilar materials and provide a corrosion-resistant joint. Adhesive joining is limited by the fact that an engineering database is unavailable for most adhesive materials.

The strength and durability of an adhesive bond are subject to the nature of the surfaces to be joined. Part of the reason industrial adhesives have been so successful is that methods have been found to clean and treat surfaces to form good bonds. A better understanding of proper surface preparation for adhesives is needed. The major limitations to the broader use of adhesives in industry are the extreme sensitivity of adhesive bonding to surface conditions and the lack of a nondestructive quality control method.

Adhesive technology can be solidly advanced by the synthesis of new monomers and polymers that extend the range of applicability of adhesive bonding. Thus, new materials should allow adhesives to be more flexible at cryogenic temperatures, more oxidation resistant at high temperatures, stronger at elevated temperatures, and more tolerant of an ill-prepared or low-surface-energy adherent. The engineering aspects of adhesive technology can be solidly advanced by including adhesive technology in university engineering courses and establishing an engineering database. In addition, an easy, nondestructive method of predicting the strength of a joint would be a major advance in the applicability of adhesives. Two drivers of advances in adhesive technology in the near future are economics and the environment. To be environmentally acceptable, new adhesive formulations should contain a minimum of solvent and in some applications should be biodegradable. To be economically attractive, adhesives should be easy to use and should provide a value-added feature to the customer that outweighs the disadvantages cited above.

Blends and Alloys

The time scale for introducing totally new polymers is increasing because the simplest monomers and the processes for converting them into polymers have already been identified and introduced into the marketplace. Furthermore, with increasing regulatory obstacles and the high cost of research, the economic stakes for introducing generically new polymers based on previously unknown chemistry and manufacturing processes have been raised considerably. Because this field was initially dominated by the ready opportunities for chemical innovation, serious development based on the more physical approach of alloying or



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