Process for the preparation of fluorochlorohydrocarbon-free low density polyurethane soft foams and soft elastic polyurethane molding foams and polyisocyanate mixtures modified with urethane groups and diphenylmethane-diisocyanate base

The method of producing mixtures of liquid polyisocyanate-containing-urethane groups in a bound form on a diphenylmethane-diisocyanate base, having an NCO group content of 22 to 30 weight percent, which comprises the steps of:

• reacting
• a mixture of diphenylmethane-diisocyanate and polyphenyl-polymethylene-polyisocyanates having a diphenylmethane-diisocyanate isomers content of 35 to 94 weight percent with respect to the total weight, and
• at least one polyoxypropylene-polyoxyethylene-polyol having a functionality of 2.5 to 3.5, a hydroxyl number of 50 to 90 and a polymerized ethylene oxide groups content of more than 30 to less than 50 weight percent with respect to the weight of polymerized ethylene oxide and 1,2-propylene oxide groups, or
• reacting
• a 4,4'diphenylmethane diisocyanate or
• a mixture of diphenylmethane diisocyanate isomers with the aforementioned polyoxypropylene-polyoxyethylene-polyol to yield a urethane-group containing quasi prepolymer having an NCO content ranging from 10 to 19 weight percent, and
• diluting same with the aforesaid mixture consisting of diphenylmethane-diisocyanates and polyphenyl-polymethylene-polyisocyanates having a diphenylmethane-diisocyanate-isomers content of 35 to 94 weight percent, with respect to the total weight.