automobiles and construction. The resulting glass-reinforced composites are frequently called sheet molding compounds (SMC).
Thermoset materials, although smaller in total volume than the thermoplastics, are used in a number of very high performance applications, such as matrix resins or structural adhesives in composite systems such as those used for aerospace applications. These composites are normally reinforced with glass, aramid, or carbon fibers. Important examples of such matrix materials include the epoxies, bismaleimides, cyanates, acetylenes, and more recently, benzocyclobutene systems. The existing database for matrix resins and structural adhesives is much more established for thermosets than it is for high-performance thermoplastics such as the poly(arylene ether ketones), certain polyaryl imides, and poly(phenylene sulfide). Major research needs in the area of polymer-based composites include better ways to improve the toughness of thermosetting systems and better techniques for processing those formed from high-performance thermoplastics.
Advances in processing and toughening thermosets are occurring on several fronts. Methods for generating the network have been investigated by many organizations. The most conventional methods involve use of a thermal-convection-oven-type curing, often in autoclaves. However, recently there has been considerable effort in electromagnetic (or microwave) processing of high-performance polymeric matrix resins, particularly for structural adhesives and composite structures. An approach for "toughening" that has been investigated over the last 10 years involves the incorporation of either rubbers or reactive engineering thermoplastics into networks, such as epoxies, to develop a complex morphology. Here the added material is dispersed as isolated domains or forms co-continuous morphologies. Most of the original studies focus on rubber toughening, and an extensive body of literature deals with utilization of carboxyl functional nitrile rubbers to toughen epoxy adhesives. More recently, advantages associated with the utilization of engineering thermoplastics have been realized. These include, for example, the ability to retain stiffness and thermo-oxidative stability, as well as in some cases, chemical resistance. These properties are often severely diminished with rubber-toughened thermosetting systems. Fracture toughness can be significantly improved. This is significant in terms of improving the durability of advanced organic materials utilized in structural adhesives and composites. The interfacial adhesion between the separate polymer phases, as well as their proportions, morphology, and molecular characteristics, is of prime significance in improving fracture toughness.
Other forefront areas include the development of new chemistries and, in particular, better characterization of leading candidate materials. The bismaleimides are considered to be somewhat more thermally stable than the epoxy materials and are being seriously considered for various applications, such as the high-speed civil transport airplane, which is planned for commercialization in the next 10 years. Aspects of the flammability of these materials are also crucial.