of about 80 and about 20 respectively and also about 65 and about 35 respectively mixtures of tdi and polymeric mdi preferably in the proportion by weight of about 80 tdi and about 20 of crude polymeric mdi to about 50 tdi and about 50 crude polymeric mdi and all polyisocyanates of the mdi type for rigid foams the preferred isocyanates are e g polyisocyanates of the mdi type and preferably crude polymeric mdi the amount of polyisocyanate included in the foam formulations used relative to the amount of other materials in the formulations is described in terms of u201c isocyanate index u201d u201c isocyanate index u201d means the actual amount of polyisocyanate used divided by the theoretically required stoichiometric amount of polyisocyanate required to react with all the active hydrogen in the reaction mixture multiplied by one hundred 100 see oertel polyurethane handbook hanser publishers new york n y 1985 the isocyanate indices in the reaction mixtures used in the process of this invention generally are between 60 and 140 more usually the isocyanate index is for flexible tdi foams typically between 85 and 120 for molded tdi foams normally between 90 and 105 for molded mdi foams most often between 70.