PTC PROPERTY IN HEAT-TREATMENT PROCESS OF ISOTACTIC POLYPROPYLENE FILLED WITH CARBON BLACK

It has been well-known that the electrical resistivity of conductive particles filled insulating crystalline polymer composites shows a large positive temperature coefficient (PTC) below the melting temperature of the matrix polymers and a rapid decrease above it. This PTC property is decided by the degree of dispersion of fillers in matrix and the thermal expansion of polymers. Here the change of PTC property, microstructure of matrix polymers and the dispersion state of fillers in heat-treatment process were investigated, and the relationship among them was discoursed. The PTC intensity (logarithmic ratio of electrical conductivity at room and at melting temperature) increases with annealing temperature up to 120°C. In the range of annealing temperature 120-155°C, the electrical conductivity at room temperature and the PTC peak temperature increases, but the PTC intensity decreases. The former is due to the increase in the matrix polymer crystallinity accompanied with a transformation of the smectic phase into the monoclinic phase, the latter is due to the agglomeration of fillers in the partially melting process at annealing temperature more than 120°C. This agglomeration of fillers was confirmed through the measurement of the surface areas of fillers in polymers, and of time dependence of quenched samples at various temperature, and scanning electron microscopy observation.