(PIR) Rigid Insulation

In polyisocyanurate (PIR) foam synthesis, an increased proportion of methylene diphenyl diisocyanate (MDI) results in unreacted isocyanate (–N=C=O) functional groups. These groups undergo trimerization, a catalytic reaction in which three isocyanate monomers form a cyclic isocyanurate ring. This trimeric ring structure introduces thermoset rigidity and enhanced thermochemical stability due to the inherent strength of the isocyanurate ring, which resists depolymerization under high temperatures.

In contrast, polyurethane (PUR) foam is synthesized primarily with polyether-based polyols that react with MDI or other diisocyanates to form linear urethane linkages (–NH–CO–O–), resulting in a more flexible, lower thermal stability polymer network. The use of polyester-based polyols in PIR formation instead introduces additional ester linkages (–CO–O–) within the polymer backbone, contributing to an increased cross-link density and improved mechanical performance at elevated temperatures.

The resultant three-dimensional, cross-linked polymer network in PIR, enriched with isocyanurate rings, displays a higher glass transition temperature (Tg) and reduced flammability, making PIR particularly advantageous for thermal insulation applications where dimensional stability and fire resistance are critical.