Abstract:The flame retardant polyurethane prepolymer (FRPUP) was prepared with 4,4′-methylenebis (phenyl isocyanate) and flame retardant polyether polyol, and was employed to toughen phenolic foams. The effects of FRPUP on the mechanical properties, flame retardancy and thermal insulation of phenolic (PF) foams were studied. The results show that the flexural strength and compressive strength of the modified phenolic foam with 2 phr FRPUP increase by 17.3% and 24.5%, respectively compared with the ones of pure PF foam. The limiting oxygen index of the modified PF foam increases to 41.5%, and the thermal conductivity of the modified foam decreases.