Effect of screw element type in degradation of polypropylene upon multiple extrusions

The screw profile of a twin-screw extruder can be designed to contain kneading and conveying elements inducing different levels of degradation in the polymer melt. In this work, the level of degradation in polypropylene has been measured after multiple extrusions-for various screw profiles-using size exclusion chromatography and IR spectroscopy. The average molecular weight and the polydispersity have been reduced and the carbonyl and unsaturation indexes increase as the number of extrusions and the aggressivity of the screw profile increase. The kneading element with 90degrees caused the greater level of degradation. On the other hand, the addition of left-hand conveying elements reduces the level of degradation because of the extra volume of molten polymer held in the screw, reducing the viability of oxygen inside the barrel. (C) 2002 Wiley Periodicals, Inc.