What Is a Thermosetting Plastic? Definition and Examples

Thermosetting Plastics Definition
Heat changes a thermosetting plastic from a liquid or soft solid into a hard solid. The process is irreversible. (image credit: Cjp24)

A thermosetting plastic is a polymer that is irreversibly hardened by heat. Thermosetting plastics are also known as thermosets, thermosetting polymers, or thermosetting resins. The starting material for a thermoset is a liquid or soft solid. Heat provides energy for covalent bond formation, cross-linking the polymer subunits and curing/hardening the plastic. Sometimes the heat is applied externally, but it may come from the chemical reaction of mixing ingredients. Adding pressure, a catalyst, or a hardener can increase the curing rate. Once cured, a thermosetting plastic cannot be re-melted, so it is formed into its final shape by injection molding, extrusion molding, compression molding, or spin casting.

Thermosetting Plastic Examples

Many plastics encountered in daily life are thermosetting plastics. Examples include:

  • Bakelite (phenolic)
  • Cyanate esters
  • Duroplast
  • Epoxy resin
  • Fiberglass (a fiber-reinforced thermoset)
  • Melamine
  • Polyester resin
  • Polyurethane
  • Silicone resin
  • Vinyl esters
  • Vulcanized rubber

Difference Between Thermosetting Plastic and Thermoplastic

Heat makes a thermosetting plastic irreversibly rigid, but it makes a thermoplastic moldable or pliable. A thermoplastic then hardens again upon cooling. Thermosetting plastics tend to be stronger than thermoplastics because of internal cross-linking via covalent bonds. For the same reason, thermosetting plastics tend to have higher corrosion resistance and hardness. On the other hand, thermosets are more likely to permanently deform under a load and are more brittle than thermoplastics. Thermosets cannot be reshaped, but they are perfect for high-temperature applications, including electronics and appliances. Thermoplastics can be remolded and recycled. Their strength, flexibility, and shrink-resistance make them suitable for high-stress parts and plastics bags and containers.


  • Ellis, B. (ed.) (1993). Chemistry and Technology of Epoxy Resins. Springer Netherlands. ISBN 978-94-010-5302-0.
  • Goodman, S. H.; Dodiuk-Kenig, H. (eds.) (2013). Handbook of Thermoset Plastics (3rd ed.). USA: William Andrew. ISBN 978-1-4557-3107-7.
  • IUPAC (1997). “Thermosetting Polymer”. Compendium of Chemical Terminology (2nd ed.) (the “Gold Book”). doi:10.1351/goldbook.TT07168