Polyurethane resins are available in both thermoset and thermoplastic formulations. Thermoset polyurethanes are used to pultrude tough new parts, such as marine sheet piling and electrical power poles, and to enhance the rigidity of automotive bumper fascias made by reaction injection molding (RIM; for information about this and other "Resin infusion processes,"see p. 24).
Also available in either form are polyimide resins (the thermoset form of which is described above). In thermoplastic form, polyimides readily release volatiles under heat and pressure, producing parts with fewer voids.
Polyurea polymer formulations are available for reinforced reaction injection molding (RRIM), with the mineral wollastonite as reinforcement. They were the first polymers to withstand the high temperatures in automotive painting processes and also provide a Class A finish.
In this category are two resins that, in thermoplastic form, can be processed, like thermosets, at lower viscosities. A class of cyclic thermoplastic polyesters developed originally at General Electric Co. and marketed by Cyclics Corp. (Schenectady, N.Y.) offers easier processing. Thermoplastic polyester is broken down into a cyclic oligomer form that, when heated to a specified temperature, drops to a water-like viscosity - a significant aid to fiber wetout. When it is catalyzed and then cooled, the oligomer returns to more conventional viscosity and forms a long-chain, high-molecular-weight thermoplastic. The material offers the properties of a thermoplastic but can be processed like a thermoset. Another example is the family of patented thermoplastic polyurethanes (TPUs) developed around 2000 by Dow Chemical Co. (Midland, Mich.) and spun off in 2004 to Midland-based Fulcrum Composites. These TPUs have made possible the commercialization of a thermoplastic pultrusion process. Although pultrusion has been dominated by low-viscosity thermosets, the Dow TPUs have the ability to partially depolymerize at their processing temperature and rapidly repolymerize as they cool. In other words, the monomer molecules in the long polymer chains partially unlink as the resin pellets are heated and melted, then relink again when cooled. This development has made possible the production of pultruded profiles that can be postformed, via thermofoming. or overmolded (via extrusion and/or injection molding) to create products such as threaded rod, without resort to machining processes that damage the pultruded fibers.