Charge storage in corona-charged polypropylene films analyzed by LIPP and TSC methods

Negatively corona-charged 50-μm-thick polypropylene (PP) film is measured using laser-induced pressure pulse (LIPP) and thermally stimulated current (TSC) in order to study the charge storage mechanism in the PP film. The LIPP can reveal the space-charge distribution in the depth direction of the PP films and the TSC can be used to measure the energetic depth of the charge trap. The LIPP shows that negative charge is deposited on the charged surface of the sample. Almost all surface charges are removed by soaking the sample in ethanol. However, about 5% of the surface charge is injected into the sample up to a depth of about 7 μm from the surface. The injected charge is not removed by the dip-in-ethanol method because the ethanol does not penetrate into the sample. The injection of the surface charge increases with corona-charging temperature. Besides the negative charge injection, the injection of positive charge from the opposite surface is also observed when the sample is charged at higher than 60°C. In addition, negative bulk charge is formed when the sample is charged at higher than 80°C. The LIPP profile is compared with the TSC spectrum. It is shown that the space charge observed using LIPP disappears when the temperature of the sample exceeds 80°C. However, TSC is observed at even higher than 80°C. This indicates that the TSC is observed even after the disappearance of the space charge measured using LIPP.