Electrical conductivity of carbon black-polyethylene composites - Experimental evidence of the change of cluster connectivity in the PTC effect

The electrical resistivity of carbon black filled high density polyethylene increases significantly when the composite is heated to the melting temperature of the matrix. The paper aims to provide new insights on this feature, known as the Positive Temperature Coefficient (PTC) effect for resistivity. The influence of the volume fraction cb of carbon black and of the temperature on the DC conductivity is analyzed and compared to their influence on matrix expansion. Measurements of the dependence on frequency of the AC conductivity of carbon black-polyethylene composites, at different loadings and at different temperatures, are performed. They yield experimental evidence of a change of connectivity of the conducting network during heating near the melting temperature of polyethylene. It is shown that the initial volume fraction of carbon black strongly affects the PTC mechanism. (C) 2001 Elsevier Science Ltd. All rights reserved.