polyester prepared in advance are also usable further compounds of special type whose constituent unit is an organosiloxane and which contain at least two hydroxyl groups at the ends or in the side chains of their molecules can also be used in the present invention since the process of the present invention is to be applied to textile belts attention should be given not to cause yellowing of the textile belts from this point of view preferred examples of the above mentioned organic polyisocyanate are aromatic aliphatic isocyanates such as xylene diisocyanate alicyclic isocyanates such as isophorone diisocyanate and aliphatic isocyanates such as hexamethylene diisocyanate in case where yellowing of the textile belts does not matter in practical use aromatic isocyanates such as tolylene diisocyanate and diphenylmethane diisocyanate may also be used examples of the chain extender are for instance glycols polyhydric alcohols amino alcohols diglycols amino or mercapto group containing compounds and water the urethane prepolymers can be produced by known methods also the blocking of the free isocyanate group of the urethane prepolymer can be conducted in known manners the blocked urethane prepolymer used in the present invention is in the form of an aqueous composition namely an aqueous solution or aqueous emulsion in water alone or a mixed solvent of water and a water miscible organic solvent such as a lower alcohol or.