Abstract: A process for the preparation of polyurethane foams, preferably semirigid polyurethane foams, having an improved foam structure involves reactinga) organic and/or modified organic polyisocyanates withb) polyhydroxyl compounds containing at least 2 reactive hydrogen atoms andc) crosslinking agents which comprise at least one polyoxyalkylene-polyol having a mean functionality of from 3 to 8 and a hydroxyl number of from 200 to 1300 ppm and a content of alkali metal ions of from 150 to 1200 ppm,in the presence ofd) blowing agents,e) if desired catalysts,f) inorganic and/or organic acids as additives andg) if desired auxiliaries.

Abstract: The present invention provides an isocyanate prepolymer prepared by reacting an organic di-isocyanate with an unsaturated alcohol dimer having 16 to 24 carbon atoms and/or a hydrogen addition product of the unsaturated alcohol dimer and a process for preparing the same.