Process for preparing flexible flame retarding polyurethane foams

A process for preparing unshrinking flexible and flame retarding polyurethane foams, comprising mixing;

• A. diisocyanate mixture which contains 80 to 45% by weight of diisocyanate and 20 to 55% by weight of urethane-modified diisocyanate obtained by reacting
• A-1. one mole of a dihydroxyl aromatic compound selected from the group consisting of 1,2-dihydroxybenzene, 1,3-dihydroxybenzene, 2,4-dihydroxytoluene, 2,2'-dihydroxydiphenyl, 4,4'-dihydroxydiphenyl, 2,2'-dihydroxybenzophenone and 3,3'-dihydroxydiphenylmethane with
• A-2. 4 to 15 moles of diisocyanate at a temperature from 80.degree. C to 130.degree. C for 3 to 35 hours
• B. a polyhydroxyl compound having at least two active hydrogen atoms and a molecular weight between 1000 and 10000, at least 10% by weight thereof being primary hydroxyl,
• C. a foaming agent, and
• D. a catalyst, and foaming and curing the thus obtained foam in the conventional manner.