invention consist of a mixture of from about 95 to 35 mol preferably 90 to 50 mol of at least one diol of the kind mentioned above and about 5 to 65 mol preferably 10 to 50 mol of at least one monoalkanolamine of the kind mentioned above chain lengthening agent mixtures of ethylene glycol and or butane diol with monoethanolamine and or 1 amino 3 propanol are preferred chain lengthening agent mixtures containing for example about 5 to 35 mol of monoethanol amine and about 95 to 65 mol of ethylene glycol are particularly preferred the molar ratio of higher molecular weight polyesters or polyethers on the one hand to the mixture of low molecular weight chain lengthening agents on the other should be between about 1 1 and 1 6 preferably between about 1 1 5 and 1 5 the polyurethanes are substantially free from reactive end groups they are generally prepared at an nco oh ratio of between about 0 95 and 1 05 preferably between about 0 96 and 1 01 the solvents used for the polyurethanes according to the invention may be either highly polar or less polar solvents known per se or mixtures of such solvents examples include dimethylformamide dimethylacetamide dimethylsulphoxide ethylacetate methylglycol acetate methylethyl ketone acetone cyclohexanone tetrahydrofuran dioxane halogenated hydrocarbons.