Process for preparing flexible foams

A process for preparing a flexible foam by reacting

• 1) at least two organic polyisocyanates with
• 2) an isocyanate-reactive compound containing at least two isocyanate-reactive hydrogen atoms and having a number average molecular weight of 1000 to 12000;
• and optionally with
• 3) an isocyanate-reactive compound containing at least two isocyanate-reactive hydrogen atoms and having a number average molecular weight of 60 to 999;
• and with
• 4) water;
• optionally in the presence of
• 5) a catalyst and
• 6) other auxiliaries and additives known per se, wherein
• a) one organic polyisocyanate (a1) is an isocyanate-terminated prepolymer having an NCO value of 5-18% by weight, prepared by reacting an excessive amount of a diphenylmethane diisocyanate with an isocyanate-reactive compound containing at least two isocyanate-reactive hydrogen atoms and having a number average molecular weight of 1000 to 12000;
• and wherein another organic polyisocyanate (a2) is a polyisocyanate having an NCO value of at least 25% by weight;
• b) 20-95 parts by weight of isocyanate-reactive compound 2) is used per 100 parts by weight of organic polyisocyanates 1);
• and
• c) the reaction is conducted at an index of 40-130 with the proviso that the weight ratio of (a1):(a2)=5:1 to 1:1, the weight ratio of organic polyisocyanate 1):2)+3)+4)=4.7:1 to 1:1 and the weight ratio of 2)+3):4)=20:1 to 2:1.