A process for the production of polyurethane and polyisocyanurate foams

The invention b) characterized in the presence of blowing agents c), characterized a process for the preparation of polyurethane or polyisocyanurate foams by reacting polyisocyanates a) with compounds having at least two reactive with isocyanate groups, hydrogen atoms that, as polyisocyanates a) prepolymers containing isocyanate groups, having an NCO content in the range of 25 to 31 wt .-%, based on the weight of the prepolymer, prepared by reacting ai) mixtures of monomeric polymeric diphenylmethane diisocyanate with at least one compound with more than one reactive toward isocyanate hydrogen atom aii) used the compounds having at least two reactive isocyanate groups with hydrogen atoms, b), and contain at least one polyester alcohol bi) which contains at least an output component that is hydrophobic.