Electrical Charging Characteristics of Spherulitic Polypropylene

Surface potential and thermally stimulated currents (TSC) were examined for negatively corona-charged spherulitic polypropylene (PP) which was isothermally crystallized at 140°C for 2 or 4 hours from the 190°C melt of PP. It was found that the surface potential of the 4 hours-crystallized PP containing many large spherulites was limited to below about -900 volts. However, untreated PP and the 2 hours-crystallized spherulitic PP showed no surface potential saturation up to -1800 volts, which is the limit of the charging apparatus used in this study. From the results on the TSC of these samples, their surface potential saturation can be explained in terms of decrease in the number of charge trapping sites which are located in the interspherulitic amorphous regions of these samples. It was also demonstrated that the internal structure of spherulites did not contribute significantly to the charging saturation of spherulitic PP.