Thermosets are a class of plastic resins that, when cured by thermal and/or
chemical (catalyst or promoter) or other means, become substantially infusible
and insoluble. After cure, a thermoset cannot be returned to the uncured state.
While almost all thermosets in commercial use today are derived from petroleum
feedstocks, R&D is growing in the field of bio-resins. Developed primarily
in an effort to use renewable agricultural feedstocks, bio-resins comprise, in
varying proportions, polyol (from soybeans) and ethanol (from corn). Unsaturated
polyester resins are the most widely used thermosets in commercial,
mass-production applications, thanks to their ease of handling, good balance of
mechanical, electrical and chemical properties, and relatively low cost. Vinyl
ester resins offer a bridge between lower-cost, rapid-curing and easily
processed polyesters and higher-performance epoxy resins (described next). For
advanced composite matrices, the most common thermosets are epoxies, phenolics,
cyanate esters (CEs), bismaleimides (BMIs) and polyimides. Epoxy resins
contribute strength, durability and chemical resistance to a composite. Phenolic
resins are based on a combination of an aromatic alcohol and an aldehyde, such
as phenol, combined with formaldehyde. They find application in flame-resistant
aircraft interior panels and in commercial markets that require low-cost,
flame-resistant and low-smoke products. Cyanate esters are versatile matrices
that provide excellent strength and toughness, allow very low moisture
absorption and possess superior electrical properties compared to other polymer
matrices, although at a higher cost. Among the more exotic of resins,
bismaleimide and polyimide (close relatives, chemically) are used in
high-temperature applications on aircraft and missiles (e.g., for jet engine
nacelle components).