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Polymer Science and Engineering: The Shifting Research Frontiers
1991). This trend is expected to continue but will require greater sophistication in terms of process technology, characterization, and structure-property relationships (especially modeling) than has been required in the past.
Thermoset materials are broadly defined as three-dimensional, chemically resistant networks, which in various technologies are referred to as gels, vulcanizates, or "cured" materials. Applications as diverse as coatings, contact lenses, and epoxy adhesives can be cited. Thermosets are defined here as rigid network materials, that is, as materials below their glass transition temperature. Thermosets are formed when polyfunctional reactants generate three-dimensional network structures via the progression of linear growth, branching, gelation, and postgelation reactions. The starting monomers must include at least some reactive functionality greater than two, which will ensure that as the reaction proceeds, the number of chain ends will increase. They will eventually interconnect to produce a gelled network material. This process may be followed by observing the viscosity increase as a function of time or from the percent reaction completed. In many cases, this can be predicted mathematically. As the gel begins to form, the soluble fraction decreases and eventually is eliminated altogether.
An important consideration with respect to rigid thermosetting networks is the extensively studied interrelationship between reactivity, gelation, and vitrification. As the reaction proceeds, the glass transition temperature rises to meet the reaction temperature, and the system vitrifies; that is, the motion of the main chain stops. At this point, the reaction essentially stops for all practical purposes. This has been conveniently described in terms of a time-temperature-transformation cure diagram. Thermosetting systems can be formed either by chain or step polymerization reactions. The chemistry of thermoset materials is even now only partially understood, because they become difficult to characterize once they reach the three-dimensional insoluble network stage. Thermal and dynamic mechanical methods have been widely used to characterize these materials, and solid-state nuclear magnetic resonance (NMR) has begun to have some impact on this problem.
Thermoset materials make up approximately 15 percent of the plastics produced in the United States. Figure 3.4 shows recent data on the production of the various types of thermosets and their uses. Phenolics make up the largest class of thermoset materials. Some polyurethanes are classified as thermosets, although many urethane and urea materials can be produced in linear thermoplastic or soluble forms, such as the well-known elastomeric spandex fibers. Urea-formaldehyde-based materials continue to be significant and, in fact, were the systems used in the first "carbonless" paper. Unsaturated polyesters are derived from maleic anhydride and propylene glycol, which are then dissolved in styrene and cross-linked into a network. They have gained significant importance in