Investigation of PP and PTFE film electrets stored at low pressure

Polypropylene (PP) and polytetrafluorethylene (PTFE) electrets 20 μm thick obtained in positive and negative corona discharges were studied. Each sample was placed on a metal pad. After charging the electrets to different initial surface potentials (350, 500, 650, 800 and 950) V they were put into a vacuum chamber under various pressures (0.1, 1, 10, 20, 66, 132 and 1000) mbar for 1 h and the surface potential was measured again. The dependence of electret surface potential on electret storage pressures after charging was investigated for four groups of experiments. As pressure was reduced from 1000 mbar to 0.1 mbar a sharp decay of surface potential was observed. The results obtained show that the main parameter determining the start of surface potential sharp decay is the pressure to initial surface potential ratio. Thus for each group of experiments only one generalised curve could be plotted. A comparison between PP and PTFE electrets was done.