Conduction and charge-injection in polypropylene films aged by corona discharge with streamers

The nature of the surface plays a critical role in the charging ability and the insulating character of insulating materials. By submitting the polymer (for instance polypropylene) to corona discharges with streamers, the corona-activated species change the chemical and morphological properties of the surface. The physico-chemical properties had previously been investigated by electron-micrography and infrared absorption spectroscopy, and both techniques have clearly shown that nodules and crystallites of oxidized constituents of the polymer appear on the surface. By using both surface potential decay and return voltage analysis, the present work will be shown to reveal a significant correlation between the physico-chemical properties of the treated polymer and the transparency of the surface to charges deposited on it by a direct current corona. Whilst the voltage of charged untreated samples decays rapidly and substantially, it decays less, and less quickly, for treated samples. Conversely, whilst the return voltage for untreated samples is hardly measurable, it is quite appreciable for treated samples. These observations cannot be explained in terms of one-dimensional charge transfer models but their interpretation may be based on the defects mentioned above, which modify the local field on the sample during charge deposition.