Thermosetting plastics (thermosets) are polymer materials that cure, through the addition of energy, to a stronger form. The energy may be in the form of heat (generally above 200 degrees Celsius), through a chemical reaction (two-part epoxy, for example), or irradiation.
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, when 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. Subsequent uncontrolled reheating of the material results in reaching the decomposition temperature before the melting point is obtained. A thermoset material cannot be melted and re-shaped after it is cured.
Thermoset materials are generally stronger than thermoplastic materials due to this 3-D network of bonds, and are also better suited to high-temperature applications up to the decomposition temperature of the material. They do not lend themselves to recycling like thermoplastics, which can be melted and re-molded.
Some examples of Thermosets are:
- Vulcanized rubber
- Bakelite, a Phenol Formaldehyde Resin (used in electrical insulators and plastic wear)
- Urea-formaldehyde foam (used in plywood, particleboard and medium-density fibreboard)
- Melamine resin (used on worktop surfaces)
- Polyester Resin (used in glass-reinforced plastics/fibreglass (GRP))
- Epoxy Resin (used as an adhesive and in fibre reinforced plastics such as glass reinforced plastic and graphite-reinforced plastic)
- Polyimides used in printed circuit boards and in body parts of modern airplanes
Methods used to mold thermosets
- Reactive Injection molding (used for objects like milk bottle crates)
- Extrusion molding (used for making pipes, threads of fabric and insulation for electrical cables)
- Calendering (used for making large sheets of plastic)
- Compression molding (used to shape most thermosetting plastics)
- Blow molding (used for bottles)
- Spin casting (used for producing fishing lures and jigs, gaming miniatures, figurines, emblems as well as production and replacement parts)