Study on time-dependent resistance of carbon black-loaded high-density polyethylene composites during the isothermal course

Carbon black (CB)-loaded high-density polyethylene composites were prepared using conventional blending. The resistance and temperature (R-T) relations under constant heating rates and the resistance and time (R-t) relations at different isothermal temperatures have been studied. The results of the R-T and differential scanning calorimetry (DSC) curve demonstrated a correlation between the positive temperature coefficient/negative temperature coefficient transition and the melting course. At isothermal temperatures below T_PTC/NTC, the resistance displayed a sharp increase and thereafter a mild decrease with time. The time to reach the highest resistance became shorter with rise in the isothermal temperature. The ratio between highest resistance and initial resistance was maximum at T_peak of the DSC curve. When the isothermal temperature was higher than T_PTC/NTC, the resistance attenuated with time. The attenuation fits to a first order exponential decay function. The calculated time constant τ decreased with rise in isothermal temperature. The attenuation discrepancy under different isothermal temperatures reduced as the heating rate before the isothermal courses was higher. A model based on polymer chain diffusion and CB movements at high temperature is proposed. The model can explain the results obtained in R-T and R-t measurements.