polyesters mentioned above such processes are described for example in german auslegeschriften 1 168 075 and 1 260 142 and in german offenlegungsschriften 2 324 134 2 423 984 2 512 385 2 513 815 2 550 796 2 550 797 2 550 833 2 550 862 2 633 293 and 2 639 254 however it is also possible in accordance with u s pat no 3 869 413 or german offenlegungsschrift 2 550 860 to mix an already formed aqueous polymer dispersion with a polyhydroxyl compound and subsequently to remove the water from the mixture the precondition for the effectiveness of the carbodiimides containing isocyanate groups used in accordance with the invention is the possibility that carboxylic acids may be formed during thermal or hydrolytic degradation of the polyurethanes other plastics suitable for stabilization in accordance with the invention are polyterephthalates which can be synthesized for example from terephthalic acid dimethyl ester or other such terephthalate esters and butanediol and or ethanediol and may also incorporate other components such as diols and or dicarboxylic acids and which have average molecular weights m n of at least 10 000 particularly suitable polyterephthalates include polybutylene terephthalate and polyethylene terephthalate materials such as these are used as high quality synthetic materials or fibers in elasticized form they may even contain relatively large proportions of a relatively long chain diol such as a c 4 polyether products such as these are commercially available.