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Dielectric Breakdown in Polyethylene at Elevated Temperatures

A two-terminal dielectric test cell for the study of the dielectric properties of viscous polymers above their softening temperature was used to perform 60 Hz breakdown measurements in polyethylene between 23 and 150°C. Molded specimens of polyethylene with embedded electrodes of parallel-plane configurations were employed. The interelectrode spacing was kept constant over the entire temperature range by suitable compensation of differential thermal expansion effects in the test cell. Low density (0.919 g/cm3) polyethylene was used for this study. The results show a marked decrease of the breakdown strength as a function of temperature between room temperature and the crystalline melting temperature, Tm. Above Tm the breakdown strength appears to be only weakly dependent on temperature. The results are analyzed with the help of Weibull statistics. The temperature dependence of the breakdown strength is discussed in the light of Artbauer's theory of free volume breakdown.