The main characteristic of thermosets (literally setting under heat) is that they require curing, when they undergo a molecular cross-linking process which is irreversible and renders them infusible. They therefore offer high thermal stability, good rigidity and hardness, and resistance to creep.

This also means that, once cured, the resin and its laminate cannot be reprocessed except by methods of chemical breakdown, which are currently under development. For practical purposes, therefore, cured thermosetting resins can be recycled most effectively if ground to fine particles, when they can be incorporated into new laminates or other products as fillers.

Thermosetting resins have little use a pure resin, but require addition of other chemicals to render them processable. For reinforced plastics, the compounds usually comprise a resin system (with curing agents, hardeners, inhibitors, plasticisers) and fillers and /or reinforcement. The resin system provides the 'binder,' to a large extent dictating the cost, dimensional stability, heat and chemical resistance, and basic flammability. The reinforcement can influence these (particularly heat and dimensional stability) but the main effect is on tensile strength and toughness. High performance fibres, of course, have a fundamental influence on cost.

Special fillers and additives can influence mechanical properties, especially for improvement in dimensional stability, but they are mainly used to confer specific properties, such as flame retardancy, ultraviolet (UV) stability or electrical conductivity.

Thermosetting resins are normally used in the liquid state and solidify and harden on curing. With some resins it is possible to part-cure and then hold the resin in what is called the B-stage for the cure to be completed at a later time.

The most frequently used thermosetting resins are:

  • polyester;
  • epoxy;
  • phenolic;
  • vinyl ester;
  • polyurethane;
  • silicone; and
  • polyamide and polyamide-imide.