Polyisocyanurate (PIR) is formed through a chemical reaction between PMDI (polymethylene diphenyl isocyanate) or MDI and polyol. What distinguishes polyisocyanurate from polyurethane are the mixed quantities of MDI and polyol components. The amount of MDI used in PIR production is higher, and they can react with themselves apart from the polyol. As a result, isocyanurate chains with greater chemical stability form at high temperatures. These chains are more resistant than normal polyurethane chains. Due to the structural difference, its flammability and smoke formation are significantly lower than those of PUR. Thanks to its closed-cell structure, it can provide superior thermal insulation.