A study of reactive flow of polypropylene in single- and twin-screw extruders

Abstract

This work brings together extruder flow simulations, peroxide-initiated reactive degradation of polypropylene (PP), and mixing efficiency analysis of flows. A steady-state, reactive extrusion process model incorporating the controlled degradation reaction is presented, in both a differential model formulation and a reduced dimension macroscopic formulation for self-wiping co-rotating twin-screw (CSCO) extruder systems. The differential formulation is solved using the finite element method for a single-screw extruder system, and both formulations are solved for CSCO extruder systems. Simulation results for both reactive and non-reactive PP flows are obtained, to study the effects of the reaction and other processing parameters on the model predictions of the extruders' behaviour, as well as on the mixing ability of the flows within their channels, as characterized by the area and flow mixing efficiencies. For the single-screw extruder system, it is demonstrated that the mixing abilities of the channel flows are very similar to those of two-dimensional flows, on the basis of average flow efficiencies along the channels. Both pressure-to-drag flow ratio and the channel aspect ratio are found to have a significant influence on these values. For the twin-screw extruder system, it is demonstrated that the mixing abilities of the flows in fully-filled forward conveying screw element channels are again similar to those of two-dimensional flows. The average flow efficiencies in the intermeshing region are less than those in the translation region, due to extensional flows caused by the channel shift. The pressure-to-drag flow ratio is found to have a significant influence on the flow efficiencies in the translation region of the channels, and the magnitude of the channel shift in the intermeshing region is found to influence the values there. Also for the twin-screw system, the macroscopic composite process model for CSCO extrusion is solved, yielding profiles of average values of the process degrees of freedom along the extruder axis. Predicted PP extrudate molecular weights for reactive extrusion simulation runs at 6 levels of peroxide addition at one screw speed and mass throughput level are found to compare closely with the measured molecular weights for the same conditions. A response surface of average residence times against screw speed and mass throughput is found using the results of a residence time distribution experiment on a laboratory scale CSCO extruder. The regression model is found to show significant lack of fit, particularly at high average residence times. By performing simulations for the same conditions as the experiments, it is demonstrated that the macroscopic twin-screw extrusion process model can accurately predict polymer average residence times in a co-rotating twin-screw extruder. Finally, several recommendations for future work are made.