The polyetherols c) are preferably polyalkylene glycols, e.g. polyethylene glycols, polypropylene glycols, polytetrahydrofurans etc., block copolymers of ethylene oxide and propylene oxide or block copolymers of ethylene oxide, propylene oxide and butylene oxide which comprise, in copolymerized form, the alkylene oxide units in random distribution or in the form of blocks.