A material that will undergo or has undergone a chemical reaction through the application of heat and pressure, catalysts, ultraviolet light, etc., leading to a relatively infusible state. Typical of the plastics in the thermosetting family are the aminos (melamine and urea), most polyesters, alkyds, epoxies, and phenolics. (Plastics Engineering Handbook of The Society of the Plastics Industry, Inc., edited by Michael L. Berins, 1991).