Any of the customary urethane catalysts, either of the amine type or the metallic type, i.e., materials such as triethylenediamine, lead octoate, tin octoate and dibutyl tin dilaureate and other compounds of the same class, are suitable; the choice depending on the reactivity of the components entering into the reaction and the mass to be reacted, i.e., the exotherrn to be expected.