A thermosetting plastic, also known as a thermoset, is polymer material that irreversibly cures. The cure may be done through heat (generally above 200 °C (392 °F)), through a chemical reaction (two-part epoxy, for example), or irradiation such as electron beam processing.
Thermoset materials are usually liquid or malleable prior to curing and designed to be molded into their final form, or used as adhesives. Others are solids like that of the molding compound used in semiconductors and integrated circuits (IC). Once hardened a thermoset resin cannot be reheated and melted back to a liquid form.
According to IUPAC recommendation: A thermosetting polymer is a prepolymer in a soft solid or viscous state that changes irreversibly into an infusible, insoluble polymer network by curing. Curing can be induced by the action of heat or suitable radiation, or both. A cured thermosetting polymer is called a thermoset.
The curing process transforms the resin into a plastic or rubber by a cross-linking process. Energy and/or catalysts are added that cause the molecular chains to react at chemically active sites (unsaturated or epoxy sites, for example), linking into a rigid, 3-D structure. The cross-linking process forms a molecule with a larger molecular weight, resulting in a material with a higher melting point. During the reaction, the molecular weight has increased to a point so that the melting point is higher than the surrounding ambient temperature, the material forms into a solid material.
Uncontrolled reheating of the material results in reaching the decomposition temperature before the melting point is obtained. Therefore, a thermoset material cannot be melted and re-shaped after it is cured. This implies that thermosets cannot be recycled, except as filler material.
Thermoset materials are generally stronger than thermoplastic materials due to this three dimensional network of bonds (cross-linking), and are also better suited to high-temperature applications up to the decomposition temperature. However, they are more brittle. Since they are "set" (non-reformable), they tend not to be recyclable.
Some examples of thermosets are:
- Polyester fibreglass systems: sheet molding compounds and bulk molding compounds)
- Polyurethanes: insulating foams, mattresses, coatings, adhesives, car parts, print rollers, shoe soles, flooring, synthetic fibers, etc. Polyurethane polymers are formed by combining two bi- or higher functional monomers/oligomers.
- Vulcanized rubber
- Bakelite, a phenol-formaldehyde resin used in electrical insulators and plasticware
- Duroplast, light but strong material, similar to bakelite used for making car parts
- Urea-formaldehyde foam used in plywood, particleboard and medium-density fiberboard
- Melamine resin used on worktop surfaces
- Epoxy resin used as the matrix component in many fiber reinforced plastics such as glass-reinforced plastic and graphite-reinforced plastic)
- Polyimides used in printed circuit boards and in body parts of modern aircraft
- Cyanate esters or polycyanurates for electronics applications with need for dielectric properties and high glass temperature requirements in composites
- Mold or mold runners (the black plastic part in integrated circuits or semiconductors)
Some methods of molding thermosets are:
- Reactive injection molding (used for objects such as milk bottle crates)
- Extrusion molding (used for making pipes, threads of fabric and insulation for electrical cables)
- Compression molding (used to shape most thermosetting plastics)
- Spin casting (used for producing fishing lures and jigs, gaming miniatures, figurines, emblems as well as production and replacement parts)
See also 
- The Open University (UK), 2000. T838 Design and Manufacture with Polymers: Introduction to Polymers, page 9. Milton Keynes: The Open University
- Roberto C. Dante, Diego A. Santamaría and Jesús Martín Gil (2009). "Crosslinking and thermal stability of thermosets based on novolak and melamine". Journal of Applied Polymer Science 114 (6): 4059–4065. doi:10.1002/app.31114.