Double positive temperature coefficient effects of carbon black-filled polymer blends containing two semicrystalline polymers

Carbon black (CB)-filled polypropylene (PP)/ultra-high molecular weight polyethylene (UHMWPE) composites were prepared by the conventional melt-mixing method. The effects of the PP/UHMWPE weight ratio, CB content and CB particle size on the positive temperature coefficient (PTC) and negative temperature coefficient (NTC) effects, as well as the room temperature resistivity of the composites were elucidated in detail. When the PP/UHMWPE weight ratio is larger than 3/7, the PTC and NTC behaviors of 10 wt% CB-filled PP/UHMWPE composites are similar to those of a CB-filled neat PP composite. However, when the weight ratio equals or is smaller than 3/7, the composites exhibit a PTC effect that is similar to that of a CB-filled neat UHMWPE composite. The elimination of the NTC effect in CB-filled polymer blend composites can be achieved by using a very high viscosity polymer as one of its components. The results of an optical microscopic study indicate that the CB particles are selectively localized at the interface between the PP matrix and the UHMWPE particles and in the PP matrix of the composites. This selective localization of the CB particles is attributed to the fact that the viscosity of the UHMWPE particles is so high that the CB particles cannot go inside the UHMWPE particles. In addition, a novel physical phenomenon-double-PTC effect-is observed and quantitatively characterized. The balance between the PTC intensity and the resistivity at room temperature can be achieved by using a mixture of large and small CB particles. (C) 2000 Elsevier Science Ltd. All rights reserved.