Numerical modelling of mass transfer and the plasticization effect of supercritical CO2 on polypropylene during extrusion

Abstract

Supercritical CO₂ (sCO₂) is a known plasticizer and has been used to replace conventional blowing agents in the processing of polypropylene. Few numerical models are available that consider advanced transport phenomena in polymer extrusion. This work presents a computational fluid dynamics (CFD) model that considers multiphase flow by employing a mixture model, accounting for heat and mass transfer, and the effects of these physics on the viscosity of polypropylene throughout the extruder domain by modifying the polymer's apparent viscosity with the corresponding rheological shift factors. The results of this model show that the mixture model produces a spatial distribution of sCO₂ which is comparable to an earlier study which employs an interface tracking method, indicating that a mixture model can provide comparable levels of resolution. Secondly, the viscosity profile and mass transfer are strongly associated with throughput – more so than varying the rotational speed of the extruder screws, except for the case where the average droplet size is of the order of 10^-3 to 10^-2 [m], in which case hydrodynamic effects increase the mass transfer coefficient as the rotational speed of the screws is increased. It is concluded that this model presents a step forward in comprehensively modelling multiphase flow in (twin screw) extruders.