ON THE PRESSURE DEPENDENCY OF THE VISCOSITY OF MOLTEN POLYMERS

A slit viscometer to measure the viscosity of polymer melts under proc essing conditions is described. Along the slit a pressure drop is gene rated by applying a pressure at both the entrance and the exit. In thi s way the pressure in the center can be controlled independently of th e shear rate. The pressure gradient in the slit is measured by means o f three pressure transducers which are mounted in the region of fully developed flow. Results of pressure-dependent viscosity measurements o n polystyrene, polyacrylonitrile-butadiene-styrene, and polypropylene are presented in a shear rate range of five decades. The flow curves o btained at different pressures and temperatures can be shifted onto a master curve. The shear thinning behavior of the three materials is ad equately described with the generalized Cross-Carreau equation, while the zero shear viscosity is modeled with a generalized Arrhenius-W.L.F . relationship, incorporating a pressure dependency. Alternatively, it is possible to describe the zero shear viscosity in terms of the free volume fraction and the temperature.