The combined effects of carbon black and carbon fiber on the electrical properties of composites based on polyethylene or polyethylene/polypropylene blend

Composites were prepared of high-density polyethylene (HDPE) with carbon black (CB) and carbon fiber (CF) (HDPE/CB/CF), and of polymer blends of HDPE and polypropylene (PP) with CB and CF fillers (HDPE/PP/CB/CF). The combined effects of CB and CF on the conductivity and resistivity-temperature behavior of the two composites were investigated. The volume resistivities of HDPE/CB/CF and HDPE/PP/CB/CF (2 wt% CF) composites were, respectively, lower than those of HDPE/CB and HDPE/PP/CB composites at the same total filler content. In both composites, electrons are transported over long distances by CFs with little loss of energy, whereas CB particles improve the interfiber contact by forming CB particle bridges. Scanning electron microscopy revealed two network structures that can both increase the density of conducting paths, which reduces the resistivity of the composites. The intensity of the positive temperature coefficient (PTC) and the temperature coefficient of resistivity of both the HDPE/CB/CF and HDPE/PP/CB/CF composites increase appreciably with increasing CF loading. This finding indicates that adding a mixture of CB and CF is a promising way to improve the performance of PTC composites.