| Abstract: | Following a survey of polyurethane ionomers, their technical importance, methods of preparation and characteristics, the methods of preparation of aqueous polyurethane dispersions are presented. Dispersion by strong shearing forces and emulsifiers is compared to the spontaneous dispersion of polyurethane ionomers from organic solution or as liquid isocyanate prepolymers. As a new method for the preparation of polyurethane dispersions, the spontaneous dispersion of ionomer melts with following poly-condensation in two-phase systems is described. The principle is that molten ionic modified polyester or polyether prepolymers containing terminal -NCO groups are reacted with urea to give bisbiuret, methylol groups generated by aqueous formaldehyde in a homogeneous phase, and the plasticised melt of methylolised ionic oligourethane bisbiurets so obtained dispersed in water at 50-130 °C. Thereafter, a formaldehyde poly-condensation forming high-molecular polyurethane polybiuret is achieved by lowering the pH of the dispersion. The reaction takes place in the dispersed phase. The procedure yields from the monomers, simply and without complicated apparatus, emulsifier free aqueous dispersions of ionic polyurethanes. The reactions occurring in the process are discussed. |
