VISCOSITY CHANGES DURING URETHANE POLYMERIZATION WITH PHASE SEPARATION.

The rheology of an MDI based three component polyurethane system, similar to those used in reaction injection moulding (RIM), was investigated. The viscosity rise for uncatalyzed samples was obtained with a Rheometrics mechanical spectrometer. For highly catalyzed systems an on-line viscometer was used. The viscosity was found to rise during the reaction much more rapidly than expected from molecular weight considerations. This is thought to be due to the fact that the hard segments (short diol and diisocyanate) from domains that phase separate and act as physical crosslinks. A viscosity model useful for process modelling within the accuracy of experimental methods is presented. The effect of the catalyst on viscosity rise seems to be just to change the time scale by increasing the reaction rate but not to affect its shape.