Thermosetting polymers

A thermoset, also called a thermosetting plastic, is a plastic that is irreversibly cured from a soft solid or viscous liquid, prepolymer or resin. The process of curing changes the resin into an infusible, insoluble polymer network, and is induced by the action of heat or suitable radiation often under high pressure, or by mixing with a catalyst.

Thermoset resins are usually malleable or liquid prior to curing, and are often designed to be molded into their final shape, or used as adhesives. Others are solids like that of the molding compound used in semiconductors and integrated circuits. Once hardened a thermoset resin cannot be reheated and melted to be shaped differently.

Thermosetting polymers may be contrasted with thermoplastic polymers, which are commonly produced in pellets, and shaped into their final product form by melting and pressing or injection molding.

Curing a thermosetting resin transforms it into a plastic, or elastomer (rubber) by crosslinking or chain extension through the formation of covalent bonds between individual chains of the polymer. Crosslink density varies depending on the monomer or prepolymer mix, and the mechanism of crosslinking:

Acrylic resins, polyesters and vinyl esters with unsaturated sites at the ends or on the backbone are generally linked by copolymerisation with unsaturated monomer diluents, with cure initiated by free radicals generated from ionizing radiation or by the photolytic or thermal decomposition of a radical initiator – the intensity of crosslinking is influenced by the degree of backbone unsaturation in the prepolymer;

Epoxy functional resins can be homo-polymerized with anionic or cationic catalysts and heat, or copolymerised through nucleophilic addition reactions with multifunctional crosslinking agents which are also known as curing agents or hardeners. As reaction proceeds, larger and larger molecules are formed and highly branched crosslinked structures develop, the rate of cure being influenced by the physical form and functionality of epoxy resins and curing agents – elevated temperature postcuring induces secondary crosslinking of backbone hydroxyl functionality which condense to form ether bonds;

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