Anomalous thermally stimulated currents in polypropylene films

Thermally stimulated current (TSC) spectra were observed in open-circuit condition for corona-charged polypropylene films from room temperature to about 160°C. In these TSC spectra, two kinds of reversal currents I₁ and I₂ were found: I₁ appeared at room temperature and rapidly decreased with increasing temperature, whereas I₂ increased with temperature to form a TSC band. These anomalous currents practically appeared only for the positively charged samples because of their strong charging-polarity dependence. After I₁ decayed, the same sample was exposed to the humid ambient air for less than 100 s, and then I₁ revived again. Whereas, I₂ was not observed in the sample, which was quickly cooled in, the ambient air just after I₂ was once observed, and reheated again from room temperature. To explain these anomalous behaviors of I₁ and I₂, a simple model was made and three discriminants were also derived from the model for determining hetero-charge´s contribution to I₁ and I₂. From these discriminants, most of I₁ and I₂ were concluded to be due to direct charge flow from the charged surface of the sample to the current-measuring electrode placed above the sample. A tentative explanation for the source of the reversal currents was also made.