Polypropylene compositions

Polypropylene which contains residues of Ziegler catalysts, and especially crystalline polypropylene which contains significant amounts of low molecular weight or atactic polymer, is improved by addition of a very low concentration of finely divided zinc oxide. Such zinc oxide not only acts as acid acceptor, making it possible to omit the conventional calcium stearate acid acceptor, but also interferes with migration of atactic and low molecular weight polymer to the surface, thus preventing plate-out of such material on processing equipment. In the thermal processing of polyproplene compositions containing metal stearate as acid acceptor or organic stearates as antistatic additives, the presence of such zinc oxide in the composition prevents plate-out on processing equipment and improves corrosivity of the compositions; it does not eliminate the antistatic effect.