Investigation of Hydrothermal Aging on Polyvinyl Chloride (PVC) Used in Medium Voltage Cables

Abstract

This investigation deals with the effect of hydrothermal aging on the electrical insulating properties of polyvinyl chloride used in medium voltage cables. The evolution of dielectric properties (dielectric loss factor, dielectric constant, dielectric strength and volume resistivity) as a function of aging time and temperature has been studied. After that, the mechanical characteristics (elongation at break and tensile strength) were also determined. Physico-chemical analysis was performed to highlight the structural changes induced by hydrothermal aging. Infrared spectroscopy using the Fourier transform (ATR-FTIR) and thermogravimetric analysis (TGA-DTA) have been performed. The results obtained show that the loss factor and dielectric constant increase gradually with the increase of temperature, while the volume resistivity decreases. At 100℃, the general shape of the curves giving the evolution of dielectric properties as a function of aging time shows that both loss factor and dielectric constant increase while those of volume resistivity and dielectric strength decrease. In the case of 80℃, the loss factor decreases slightly and the dielectric constant remains almost constant with aging time, whereas the volume resistivity and dielectric strength increase. The activation energy varies with aging time. The impact of hydrothermal aging on the properties of the material has been established in this study. At the beginning of aging, the degradation is mainly due to the elimination of molecular HCl. At more advanced stages of aging, the degradation is attributed to the elimination of HCl and double bonds formation followed by a change in color. Other consequences of the degradation like crosslinking and swelling of the samples are also noticed.