Patent ID: 8916620

Claim:
A method which comprises preparing a polyurethane-modified polyisocyanurate (PU-PIR) foam by reacting polymeric MDI, an aromatic polyester polyol, and water in the presence of a blowing agent, a urethane catalyst, a trimerization catalyst, a surfactant, and a flame retardant composition, wherein at least 33 wt. % of the flame retardant composition is crystal phase II ammonium polyphosphate, wherein the PU-PIR foam is formulated at an NCO/OH index within the range of 1.8:1 to 4:1, and wherein the PU-PIR foam has a thermal stability as measured at 800° C. under anaerobic conditions that is at least 10% higher than the thermal stability of a PU-PIR foam prepared under identical conditions at the same flame retardant level in the absence of the ammonium polyphosphate.