Polypropylene chain scissions and molecular weight changes in multiple extrusion

Polypropylene degradation, during multiple extrusion cycles at different temperatures, is studied by following chemical and molecular weight changes. The former was measured using FTIR and the latter was determined by size exclusion chromatography. The molecular weight and polydispersity reduction, and the lack of carbonyl groups and gel formation, show that chain scission and not oxidation is the dominant degradation reaction under the experimental conditions. Molecular weight distribution curves suggest that chain scissions occur close to the middle part of the macromolecule, in agreement with the Bueche theory for mechanical degradation. Three zones with different degradation-temperature behavior were observed: no appreciable degradation occurred between melt temperature and 230 degrees C; from 230 to 250 degrees C there was a rapid increase of chain scissions with rise of temperature; and finally from 250 to 270 degrees C, the number of chain scissions did not change with temperature increase. (C) 1998 Published by Elsevier Science Limited. All rights reserved.