Pressure dependent wall slip of wood flour filled polymer melts

The rheological properties of a 50 wt.% wood flour filled polypropylene have been measured at 195°C using an instrumented slit die attached to a single screw extruder. Wall slip and non-Newtonian effects have been corrected through the Mooney and Rabinowitsch procedures, thus equations for viscosity and wall slip have been obtained. Interestingly, the pressure drop profiles calculated using these equations are not perfectly consistent with the experimental pressure measurements, in that the theoretical curves overestimate the experimental data. An explanation has been sought by assuming that slip velocity increases in the last portion of the slit die, i.e. close to the exit, where pressure drops down considerably. A pressure dependent slip model has been then proposed, its specific equations numerically solved and the results fitted to the experimental pressure data. The fitting is better than the one coming from the model that comprises slip depending on shear stress only, thus, for the fluid that has been studied, the assumption of pressure dependent slip appears to be justified.