PROCESSO PARA A PREPARACAO DE SOLUCOES AQUOSAS OU DISPERSOES DE POLIURETANAS,SOLUCOES OU DISPERSOES OBTENIVEIS DE ACORDO COM ESTE PROCESSO,BEM COMO SUA APLICACAO PARA A PRODUCAO DE REVESTIMENTOS,LACAS OU PELICULAS

The present invention relates to a process for the preparation of aqueous solutions or dispersions of polyurethanes, in which process prepolymers which have at least two free isocyanate groups and contain chemically-fixed hydrophilic groups and/or external emulsifiers which are not chemically fixed are reacted with chain lengthening agents in the aqueous phase, characterized in that the isocyanate prepolymer which is hydrophilically modified and/or contains an external emulsifier is mixed with chain lengthening agents selected from the group consisting of azines and hydrazones in the absence of water and the mixture obtained is then mixed with water. The present invention also relates to the dispersions or solutions which can be obtained by this process. Lastly, this invention relates to the use of the dispersions or solutions which can be obtained by this process for coating flexible substrates.