Preparation of chlorofluorocarbon-free flexible polyurethane foams using diphenylmethane diisocyanate-based polyisocyanate mixtures containing urethane groups, and modified polyisocyanate mixtures of the type

A process for the preparation of chlorofluorocarbon free flexible polyurethane foams and flexible, elastic polyurethane molded foams by reacting

• A) liquid diphenylmethane diisocyanate-based polyisocyanate mixtures containing bonded urethane groups with
• B) relatively high-molecular-weight polyhydroxyl compounds and
• C) in the presence or absence of low-molecular-weight chain extenders and/or crosslinking agents,
• in the presence of
• D) blowing agents, and
• E) catalysts
• wherein the liquid polyisocyanate mixtures A) containing bonded urethane groups contain from 20 to 30 percent by weight of NCO groups, based on the total weight, and are obtained
• I) by reacting
• a) a mixture (I) of diphenylmethane diisocyanates and polyphenylene-polymethylene polyisocyanates containing from 35 to 94 percent by weight, based on the total weight, of diphenylmethane diisocyanate isomers, with
• b) a polyether-polyol mixture (b) which contains
• b.sub.1) at least one polyoxypropylene glycol having a hydroxyl number of from 120 to 25, and
• b.sub.2) at least one trifunctional polyoxypropylene-polyoxyethylene-polyol having a hydroxyl number of from 120 to 25 and containing 30.1 to 90 percent by weight, based on the weight of polymerized alkylene oxide units, of polymerized ethylene oxide units,
• or
• II) by diluting
• c) a quasi-prepolymer containing urethane groups and containing from 10 to 19% by weight of NCO, obtained by reacting 4,4'-diphenylmethane diisocyanate or a mixture of diphenylmethane diisocyanate isomers (II) with the polyetherpolyol mixture (b), which contains
• b.sub.1) at least one polyoxypropylene glycol having a hydroxyl number of from 120 to 25, and
• b.sub.2) at least one trifunctional polyoxypropylene-polyoxyethylene-polyol having a hydroxyl number of from 120 to 25 and containing from 30.1 to 90 percent by weight, based on the weight of polymerized alkylene oxide units, of polymerized ethylene oxide units, using a mixture (I) of diphenylmethane diisocyanates and polyphenyl-polymethylene polyisocyanates.