 Plastics are generally divided into two categories , which are " thermosets " and " thermoplastics . " A thermoset is a plastic in which a chemical reaction sets the molecular bonds after first forming the plastic , and then the bonds cannot be broken again without degrading the plastic . <unk> cannot be melted , therefore , once a thermoset has set it is impossible to weld it . Examples of thermosets include epoxies , silicone , vulcanized rubber , polyester , and polyurethane . 
