Thermosetting polymer

A Thermosetting resin is a prepolymer in a soft solid or viscous liquid state that changes irreversibly into an infusible, insoluble polymer network by curing. Curing is induced by the action of heat or suitable radiation often under high pressure, or by mixing with a catalyst or crosslinking agent often under atmospheric conditions at ambient temperature.

A cured thermosetting resin is called a thermoset or a thermosetting plastic/ polymer - when used as the bulk material in a polymer composite, they are referred to as the thermoset polymer matrix.

When compounded with fibres they form fibre reinforced polymer composites which are used in the fabrication of factory finished structural composite OEM or replacement parts, and as site-applied, cured and finished composite repair and protection materials. When used as the binder for aggregates and other solid fillers they form particulate reinforced polymer composites which are used for factory-applied protective coating or component manufacture, and for site-applied and cured construction, or maintenance, repair and overhaul (MRO) purposes.

Curing transforms the resin into a plastic or elastomer/rubber by crosslinking through the formation of strong covalent bonds between individual chains of the polymer. Crosslink density varies depending on the monomer/pre-polymer mix used, and how the crosslinking is facilitated:

Thermoset plastic polymers characterised by rigid, three-dimensional structures and high molecular weight, stay out of shape when deformed and undergo permanent or plastic deformation under load, and normally decompose before melting. Thermoset elastomers, which are soft and springy or rubbery and can be deformed and revert to their original shape on loading release, also decompose before melting. Conventional thermoset plastics or elastomers therefore cannot be melted and re-shaped after they are cured which implies that thermosets cannot be recycled for the same purpose, except as filler material. There are developments however involving thermoset epoxy resins which on controlled and contained heating form crosslinked networks that can be repeatedly reshaped like silica glass by reversible covalent bond exchange reactions on reheating above the glass transition temperature. There are also thermoset polyurethanes shown to have transient properties and which can thus be reprocessed or recycled.

...
Wikipedia