Surface potential on LDPE and PP films after removal of the upper disk electrode

The potential distribution was measured on the free surface of low-density polyethylene (LDPE) and polypropylene (PP) films which had been inserted between plane electrodes after the removal of the upper brass disk electrode. The polarity of the surface potential was always the same as that of the applied step voltage which had been subjected to the upper electrode for a while and then shorted. The absolute value of the surface potential tended to increase to a constant value of ~650 V with increasing applied voltage and application time for both polarities. There was no appreciable difference in the surface potential between PP and LDPE when the applied voltage and its application time changed. However, the increase in the shortcircuit time tended to increase the surface potential for LDPE only, while it did not change that for PP. The effect of a longer shortcircuit on the surface potential increase for the LDPE films for both polarities could be explained by bipolar injection