Degradation of polyolefins during melt processing

Abstract

The thermo-oxidative and thermo-mechanical stability of polypropylene and high density polyethylene (Cr-catalyst technology) was studied using multiple extrusions. In the case of PP, chain scission of the macromolecules, caused by β-scission, the breakdown of peroxy radicals and shear, contributed to a decrease in the molecular weight of the polymer. The PE-HD showed a tendency towards crosslinking. The molecular weight distribution and polydispersity of the polymers changed during processing. Functional group concentrations were measured using FT-IR spectroscopy and are related to rheological changes. The measurements obtained using melt flow and low shear melt viscosity techniques were only in good agreement with the results obtained by gel permeation chromatography in the case of PP. The reactions of alkyl radicals, R •, are primarily responsible for molecular weight increase or molecular weight reduction. Upon processing of PP, terminal CH 2C(CH 3)groups are formed by disproportionation reactions of alkyl radicals, R •. The methyl substituent does not allow an alkyl radical addition to this double bond. The increase in molecular weight upon processing of the PE-HD is caused by the addition of an alkyl radical, R •, to CH 2CHgroups. The documented chemistry and reactivity of the radicals formed upon polyolefin processing are in good agreement with the experimental results.