Charge distribution in Teflon FEP negatively corona-charged to high potentials

The charge distribution in negatively corona-charged Teflon FEP films was investigated with the laser-induced pressure-pulse method. The 12.7 μm and 25.4 μm samples were charged to potentials up to -2500 V and -4500 V, respectively, and thereafter stored at room temperature. Measurements of the charge distribution were taken at time intervals ranging from 1 minute to several days after charging. The results indicated that the corona charge is initially trapped at the surface of the samples. A negative bulk charge, which is distributed over the entire volume and has a density of ~10¹³/cm³, is found for samples kept at the highest voltages for periods of the order of hours. The findings suggest that in highly charged samples intrinsic holes drift to the charged surface, leading to a depletion of holes and thus to the observed negative bulk charge