Process for the preparation of low density flexible polyurethane foams and flexible elastic moulded foams devoid of fluorochlorohydrocarbons and polyisocyanate mixtures on difenylmethane diisocyanate basis modified by urethane groups for use therewith

摘要

The invention relates to a process for the production of CFC-free flexible (moulded) polyurethane foams of low density by reacting A) liquid polyisocyanate mixtures containing bonded urethane groups and containing from 22 to 30% by weight of NCO groups, these mixtures themselves being prepared by reacting a mixture of diphenylmethane diisocyantes and polyphenylpolymethylene polyisocyanates (crude MDI) with at least one polyoxypropylene-polyoxyethylene-polyol having a functionality of from 2.5 to 3.5, a hydroxyl number of from 50 to 90 and a content of polymerised ethylene oxide groups of from greater than 30 to less than 50% by weight, based on the weight of polymerised ethylene oxide and 1,2-propylene oxide groups, or by reacting 4,4'-diphenylmethane diisocyanate (MDI) or an MDI isomer mixture with said polyoxypropylene-polyoxyethylene-polyol to give an NCO-containing quasi prepolymer and diluting this quasi prepolymer with crude MDI, with B) relatively high-molecular-weight polyhydroxyl compounds and optionally C) low-molecular-weight chain extenders and/or crosslinking agents in the presence of D) blowing agents, E) catalysts and optionally F) auxiliaries and/or additives, and the urethane group-modified polyisocyanate mixtures (A) which can be used according to the invention.