Related: Organic Chemistry

materials that can not be softened on heating. In thermosetting polymers, the polymer chains are joined (or cross-linked) by intermolecular bonding. Thermosets are usually supplied as partially polymerized or as monomer-polymer mixtures. Cross-linking is achieved during fabrication using chemicals, heat, or radiation; this process is called curing or vulcanization.

Thermosets are also used in some forms of plastics processing, including fibre-reinforced composites, and are the most common resins in formulated products like paints, adhesives, and inks. Examples include phenol formaldehyde, urea formaldehyde, epoxy, polyurethane, unsaturated polyester, alkyd, and silicone. These resins are cured via a chemical reaction which is generally not reversible. Whereas thermoplastics soften and can be reprocessed using heat, thermosets generally undergo decomposition when heated. For this reason, recycling of thermosets is difficult.