Reactive extrusion of polypropylene II: Degradation kinetic modeling

Abstract

AbstractA kinetic model for the free‐radical‐initiated molecular weight degradation of polypropylene was further developed. The model has a single variable parameter, the initiator efficiency, f. Assumptions were detailed, new comparisons with experimental data presented, and model sensitivity to the value of f evaluated. The model was found to provide a good description of both molecular weight distribution and molecular weight average data from degradations carried out in a single‐screw extruder at 200 and 220°C. Data at 0.04 wt% initiator feed concentration were fit and the resulting f value used to predict results at 0.01 wt% and 0.02 wt%. In accordance with observation, the model predicted that temperature would have no effect on the molecular weight of the extrudate because the comparatively long (≥ 2.8 min) residence time in the extruder permitted degradation reactions to go to completion. The model predictions were found to depend upon the change in molecular weight distribution rather than the absolute value of the distribution data. Predictions were therefore unaffected by concentration correction in size exclusion chromatography interpretation. The model was determined to have a low sensitivity to the value of f. Hence, f was estimated to only ± 25% and it is anticipated that attempts to use the model for comparing different initiators would be limited by this characteristic. Also, because the minimum residence time in the extruder was 2.8 min, the model has yet to be tested against data at times less than this value.