What is "polyisocyanurate"

n. (context organic chemistry English) A form of polyurethane polymer, based on cyanuric acid, formed from a polyhydroxy alcohol and a diisocyanate; it forms foams that are used for thermal insulation

Polyisocyanurate, also referred to as PIR, polyiso, or ISO, is a thermoset plastic typically produced as a foam and used as rigid thermal insulation. Its chemistry 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 formulations also differ from those used in PUR.