Viscoelastic foams having high density

A process for producing viscoelastic polyurethane foams having a density of from 140 g/dm3 to 220 g/dm3 comprising:

• mixing:
• a) polyisocyanate,
• b) polymeric compounds having groups that are reactive toward isocyanates,
• c) optionally chain extenders and/or crosslinkers,
• d) a catalyst,
• e) a blowing agent comprising water, and
• f) optionally additives
• at an isocyanate index of from 50 to 95 to give a reaction mixture, the reaction mixture being placed on a support and reacted to give the viscoelastic polyurethane foam,
• wherein the polyisocyanate (a) has an isocyanate content of from 20 to 31.5% and is obtainable by mixing 4,4′-MDI and oligomers of propylene oxide having from 2 to 8 propylene oxide units, optionally carbodiimide of 4,4′-MDI and optionally higher homologues of MDI having more than two rings, and
• wherein the polymeric compounds (b) comprise, based on the total weight of the polymeric compounds (b):
• b1) from 15 to 40% by weight of at least one polyalkylene oxide having a hydroxyl number of from 20 to 50 mg KOH/g derived from a trifunctional starter molecule and a proportion of ethylene oxide, based on the content of alkylene oxide, of from 4 to 10% by weight and a proportion of primary OH groups of from 50 to 100%,
• b2) from 30 to 70% by weight of at least one polyalkylene oxide having a hydroxyl number of from 20 to 50 mg KOH/g derived from a bifunctional starter molecule and a proportion of ethylene oxide, based on the content of alkylene oxide, of from 4 to 10% by weight and a proportion of primary OH groups of from 50 to 100%,
• b3) from 10 to 30% by weight of at least one polyalkylene oxide having a hydroxyl number of from 400 to 800 mg KOH/g derived from a bifunctional to tetrafunctional starter molecule and a proportion of ethylene oxide, based on the content of alkylene oxide, of from 80 to 100% by weight, and
• b4) from 0 to 30% by weight of a further polyalkylene oxide.