AC and DC zone-induced ageing of HTV silicone rubber

On the way of making efforts to standardize the selection of polymeric materials for high voltage outdoor environments, the resistance to corona and ozone has been recognized as one among twelve parameters to be evaluated within the OGRE Working Group D1.14. Our previous related work reported on the evaluation of the resistance of five commercially available polymeric materials to ac corona-ozone treatment, according to the test procedure recommended by this Working Group. With the same test arrangement in use, this report focuses on comparing the influences of ac and dc corona-ozone exposures on high temperature vulcanized (HTV) silicone rubbers. For this purpose, two types of HTV materials were used, each one characterized by different resistivity levels and tensile strength. For defining the treatment conditions, the ac and dc coronas were characterized by measuring the associated discharge current as well as the power released from the electrode. The intensity of ac corona discharge was found to be higher than that of dc corona, the latter being controlled by electrostatic charging of the treated polymeric surfaces, which also resulted in significant differences of the resulting ozone concentration. Experimental investigations after the treatments concentrated on evaluating dielectric properties (e.g. surface and volume resistivities, dielectric response (DR)), mechanical properties (e.g. tensile strength and elongation at break), as well as chemical and structural analyses (e.g. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR)). Results obtained from the measurements of dielectric properties indicated a strong sensitivity of surface resistivity to the treatments, though the impact of the dc corona remained less pronounced. On the other hand, the dielectric response and volume resistivity did not exhibit a high susceptibility to the exposure. No substantial differences were also found among the ac and dc corona treated samples as regards the mechanical properties. The XPS and FTIR analyses revealed proofs of surface oxidation for the samples treated by ac corona, while the samples treated by dc corona showed much less tendency for oxidation.