MODELING OF POLYPROPYLENE DEGRADATION DURING REACTIVE EXTRUSION WITH IMPLICATIONS FOR PROCESS-CONTROL

This paper presents the development of a model for free radical initia ted polypropylene degradation during reactive extrusion that combines a kinetic model of the polypropylene degradation reaction with a simpl ified model of the melting mechanism in the extruder. The free radical initiated degradation of polypropylene is characterized by a narrowin g of the molecular weight distribution (MWD) and a decrease in the mol ecular weight averages. A high temperature SEC is used to determine MW D's for three different commercially available polypropylenes degraded at various initiator concentrations in a 1.5 inch single screw extrud er (L/D = 24:1). The predictions of the kinetic model alone and the co mbined kinetic-melting model are compared with the experimentally dete rmined MWD's and molecular weight averages for the degraded polypropyl enes. The predictions of a modified kinetic model that includes the po ssibility 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 i n comparison to the original kinetic model. A viscosity-molecular weig ht relationship is also developed, which is then used to determine the gain of the degradation process as a function of the initiator concen tration from the molecular weight averages predicted by the kinetic-me lting model. Earlier work has shown such prior knowledge of the proces s gain can be used to significantly improve the performance of process control schemes for the degradation process.