Thermosetting Plastic Definition

Epoxy resin is an example of a thermosetting plastic.
Epoxy resin is an example of a thermosetting plastic.

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A thermosetting plastic is a polymer that irreversibly becomes rigid when heated. Such a material is also known as a thermoset or thermosetting polymer. Initially, the polymer is a liquid or soft solid. Heat provides energy for chemical reactions that increase the cross-linking between polymer chains, curing the plastic. The rate of curing may be increasing in many cases by increasing pressure or by adding a catalyst.


Many common plastics are thermosets. They include:

  • Vulcanized rubber
  • Fiberglass (a fiber-reinforced polymer composite)
  • Polyester resin
  • Polyurethane
  • Melamine
  • Bakelite
  • Silicone resin
  • Epoxy resin


  • Ellis, B. (ed.) (1993). Chemistry and Technology of Epoxy Resins. Springer Netherlands. ISBN 978-94-010-5302-0
  • IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). "Thermosetting Polymer". doi:10.1351/goldbook.TT07168