Electrical strength and chemical surface analysis of polypropylene after exposure to external partial discharges

Films of isotactic polypropylene have been subjected to external partial discharges (PDs) at 2.25 kV and 1 kHz, using both needle and plane circular electrodes. The time to breakdown has been recorded for samples of different thicknesses (80 to 190 μm), at different temperatures (25 to 100°C), and in different atmospheres (air and He). Data by FMIR-IR spectroscopy, contact angle measurements, ESCA (electron spectroscopy of chemical analysis), surface profile analysis, and SEM (scanning electron microscopy) on samples subjected to PD are reported. It is show that the PD breakdown occurs more readily in He than in air. The surface roughness increases more rapidly on PF exposure in He in air. Aldehydes, ketones, hydroxyl, and vinyl groups are formed in samples exposed to PD in air, whereas samples subjected to PD in He exhibit a more significant production of α,β-unsaturated ketones. It is also shown that the oxidation level of the surface as assessed by carbonyl index (FMIR-IR), O/C ratio (ESCA), or contact angle is a maximum after 0.5 to 1 h of PD exposure in air, whereas in He the oxidation level increases monotonously with exposure time until breakdown occurs