Thermoelectric behavior (PTC) of carbon black-containing TPX/UHMWPE and TPX/XL-UHMWPE blends

This article describes the structure and electrical performance of positive-temperature-coefficient/negative-temperature-cofficient (PTC/NTC) effects of the following three-component blends: poly(4-methyl pentene-1)/ultra-high molecular weight polyethylene/carbon black (TPX/UHMWPE/CB), poly(4-methyl pentene-1)/crosslinked-ultra-high molecular weight polyethylene/carbon black (TPX/XL-UHMWPE/CB), and gamma -irradiated, compression-molded plaques of these blends. CB particles are preferentially attracted to the UHMWPE and XL-UHMWPE particles, which constitute the dispersed phase within the TPX matrix, but practically cannot or can only very slightly penetrate them because of their extremely high viscosity. Thus, CB particles initially form conductive networks on the UHMWPE phase; this is followed by distribution in the TPX matrix, electrically connecting the CB-covered UHMWPE particles. This unusual CB distribution results in a reduced percolation threshold of all blends. A double-PTC effect is exhibited by the XL-UHMWPE-containing samples. Irradiation of compression-molded plaques improves their thermoelectric behavior by amplifying the PTC effect and reducing the NTC effect. A schematic model of the double-PTC effect is suggested, describing the morphological changes of 70TPX/30XL-UHMWPE/CB blends at different stages of heating with respect to their thermoelectric behavior. Irradiation of TPX/UHMWPE/CB plaques converts these systems into high-intensity PTC materials free of the NTC effect. (C) 2001 John Wiley & Sons, Inc.