Relationship between different mixing conditions and DC breakdown strength of high density polyethylene compound

In this work, we present results of the short-time dielectric breakdown test in high-density polyethylene (HDPE) containing 2.0 wt% of carbon black and 0.5 wt% of titanium dioxide mixed in different processing equipment, in a single screw extruder and in a co-rotational twin screw extruder. These additives are commonly used as pigments in polyolefin materials used as insulation in medium voltage cables. Thin film specimens were obtained by hot-compression, from materials compounded in these extruders. For the dielectric breakdown test, a DC voltage ramp rate of 500 V/sec. was applied to the samples which were immersed in a silicone oil bath at room temperature. The results were analyzed by the Weibull distribution. The Weibull shape and scale parameters were determined by Maximum Likelihood and Graphical Methods. The homogeneity of the pigments in the compounds was analyzed by optical microscopy. These results show that the breakdown strength depends on the mixing conditions. The best results were obtained in materials mixed in a twin screw extruder.