Modeling of polypropylene degradation during reactive extrusion with implications for process control

This paper presents the development of a model for free radical initiated polypropylene degradation during reactive extrusion that combines a kinetic model of the polypropylene degradation reaction with a simplified model of the melting mechanism in the extruder. The free radical initiated degradation of polypropylene is characterized by a narrowing of the molecular weight distribution (MWD) and a decrease in the molecular weight averages. A high temperature SEC is used to determine MWD's for three different commercially available polypropylenes degraded at various initiator concentrations in a 1.5 inch single screw extruder (L/D = 24:1). The predictions of the kinetic model alone and the combined kinetic‐melting model are compared with the experimentally determined MWD's and molecular weight averages for the degraded polypropylenes. The predictions of a modified kinetic model that includes the possibility of termination by combination are also examined. The kinetic‐melting model is found to provide significantly improved predictions of the experimentally determined MWD's and molecular weight averages in comparison to the original kinetic model. A viscosity‐molecular weight relationship is also developed, which is then used to determine the gain of the degradation process as a function of the initiator concentration from the molecular weight averages predicted by the kinetic‐melting model. Earlier work has shown such prior knowledge of the process gain can be used to significantly improve the performance of process control schemes for the degradation process.