thermosets, 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. Important thermosets include phenolics, ureas, melamines, epoxies, polyesters, silicones, rubbers, and polyurethanes.