Production of foam webs from high-temperature-resistant polysulfones or polyether sulfones

A process for producing a foam sheet having a thickness of more than 0.5 cm, said process comprising mixing thermoplastic polymers selected from polysulfone or polyether sulfone or mixtures thereof and from 1 to 10% by weight, based on the thermoplastic polymers, of a volatile blowing agent, at above 300° C. under pressure, and extruding the mixture into the open atmosphere using a slot die, wherein the blowing agent is a mixture of water with up to 200 parts by weight, based on 100 parts by weight of water, of an alkanol or of an aliphatic ketone.