Thermosetting plastics are usually formed from low viscosity liquids that undergo significant cross-linking through the application of an external energy source such as UV light or heat or through the addition of a catalyst.

The most important thermosetting resins for the use in composites are:

  • Phenolics
  • Epoxies
  • Unsaturated polyesters

Unsaturated polyester resins were the first type of matrix material to be used with glass fibres to make polyester GRP (glass reinforced plastics) materials. Polyester resins are still the most widely used matrix materials. They are cheap, easy to work, and cure at room temperature.

Epoxy resins are superior to polyester resins in a number of important ways. They are stronger and stiffer, bond more effectively to fibres, have very little cure shrinkage, and have much higher service temperatures.

Phenolics are not only strong but they also have one of the highest elastic moduli of common plastics and also have good fire resistance.