Polyisocyanurate is a thermoset plastic. It is also called PIR, polyiso, or ISO. It is usually made as a foam. It is used as rigid thermal insulation. It is similar to polyurethane (PUR) except that the proportion of methylene diphenyl diisocyanate (MDI) is higher and a polyester-derived polyol is used in the reaction instead of a polyether polyol. Catalysts and additives used in PIR is also different from those used in PUR.