Exit effects in the wall slip of polymeric concentrated suspensions

In this work we have studied the rheological properties of a 50 wt.% wood flour filled polypropylene. Experiments have been performed with an instrumented slit die attached to a single screw extruder and kept at 195°C in order to reduce possibility of natural fiber degradation. The slit die could be mounted with several gap heights in order to use the Mooney procedure to measure wall slip. Non-Newtonian effects have also been taken into account using the Rabinowitsch correction procedure. As a result, if one uses these slip law and flow rule and solves them to check on the experimental pressure readings, it can be found that the theoretical pressures overestimate the experimental measurements and moreover that the experimental pressure drop is non-linear close to the exit. This result can be explained by assuming that slip velocity increases in the last portion of the slit die, i.e. close to the exit, as a consequence of pressure dependent wall slip. A pressure dependent slip model has been then proposed, its specific equations numerically solved and the results fitted to the experimental pressure data, yielding a better fit. As pressure dependence of slip velocity induces a non-fully developed flow, it is clear that one of the hypotheses which are at the basis of the Mooney procedure is not satisfied by this fluid, thus strictly speaking the Mooney procedure is inapplicable.