Method of preparing polyurethane-modified polyisocyanurate foam

Rigid foams having excellent compression strength, dimensional stability, flame retardance and adhesion are produced by reacting an active hydrogen compound having at least two functionalities with a polyisocyanate compound in the presence of a catalyst and a blowing agent comprising either water alone or a mixture of water and a low boiling compound, wherein (1) the polyisocyanate compound is a prepolymer obtained by reacting a polymeric MDI with 5 to 30% by weight, based on the polymeric MDI, of a polyether polyol and/or polyester polyol having a hydroxyl value of at most 100 mg KOH/g, and (2) the number of isocyanate groups in the polyisocyanate compound is at least 1.5 times by mole as large as the number of active hydrogen atoms in the active hydrogen compound and water.