Rigid substantially closed cell foams are made by reacting an organic isocyanate with a mixture of high molecular weight polyols in the presence of water, a catalyst and other optional materials such as chain extenders, crosslinking agents and flameproofing agents. The reactants are used in quantities such that the isocyanate to hydroxyl group index is from about 90 to about 150. The mixture of high molecular weight polyols is composed of (1) a polyester having at least two hydroxyl groups and an OH number greater than 150 and/or a polyether having at least two hydroxyl groups and an OH number greater than 200 and (2) a polyether having at least two hydroxyl groups and an OH number below 100. The foams produced in accordance with this process are useful as insulating materials and as structural reinforcing materials.
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