Chain scission distribution function for polypropylene degradation during multiple extrusions

Abstract

The degradation of polypropylene subjected to multiple extrusions was evaluated by following the changes in the molecular weight distribution curves (MWD) obtained by size exclusion chromatography (SEC). The curves shift to the low molecular weight side and we calculate the chain scission distribution function (CSDF) as a function of the initial molecular weight. Assuming that the SEC curves of both, virgin and degraded PP follow Gaussian behaviour the calculated CSDF data would be a straight-line function of the original molecular weight (MW). A random chain scission process gives a constant and positive CSDF value independent of MW. If the process is no longer random the CSDF value increases as a result of the increase in the chain scission probability. Polypropylene subjected to multiple extrusion shows that the chain scission processes during thermo-mechanical degradation display a CSDF curve with a constant positive value in the low molecular weight region which rises in the higher MW region. This indicates that the probability of chain breaking is dependent on the degradation aggressiveness and the molecular weight of the chain. At low MW, it is independent of it, i.e. the chain scission is random, but at higher MW it becomes dependent, increasing with MW. The behaviour has been measured during multiple extrusions and using different screw configurations. The CSDF values are higher the greater the aggressiveness of the degradation, i.e. number of extrusions and screw configuration: kneading elements with 45° (KB45) degrade more than conveying elements (CON), increasing the overall level of chain scission.