Polyisocyanurate, also referred to as PIR, polyiso, or ISO, is essentially an improvement on polyurethane (PUR). The proportion of methylene diphenyl diisocyanate (MDI) is higher than for PUR and instead of a polyether polyol, a polyester derived polyol is used in the reaction. Catalysts and additives used in PIR formulations also differ from those used in PUR. This isocyanurate polymer has a relatively strong molecular structure, because of the combination of strong chemical bonds, the ring structure of isocyanurate and high cross link density, each contributing to the greater stiffness than found in comparable polyurethanes. The greater bond strength also means these are more difficult to break, and as a result a PIR foam is chemically and thermally more stable: breakdown of isocyanurate bonds is reported to start above 200°C, compared with urethane at 100 to 110°C. Depending on the product application greater stiffness, chemical and/or thermal stability may be desirable in an attempt to achieve optimal end use performance.

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