Temperature Effect of Electrical Resistivity of Carbon Black Filled Polymers

Abstract

Abstract Very large positive temperature coefficients of resistivity (PTC) found in a narrow temperature range with dispersions of carbon blacks in noncrystallizing polymers are caused by an increase in average particle or aggregate distances of the dispersed black. This effect is the result of their deagglomeration upon temperature increase, induced by the increased Brownian motion, predominantly caused by the large reduction in vehicle viscosity and aided by the increase in segmental motion of the polymeric molecules. Highest PTC effects are found in the range of the steepest resistivity-concentration curves for the carbon black dispersion, differing markedly for each black and each polymer. The range itself is determined by particle size, density, degree of aggregation and agglomeration, and by the presence of surface oxides. In crystalline polymers high PTC effects are predominantly caused by the large volume expansion of the polymer in the melting range, as well as by the formation of a uniform melt from a two phase system of amorphous and crystalline components. Carbon black particles are mostly restricted to the amorphous phase in the solid, but are able to disperse into the entire volume of the melt. The effects of crosslinking the polymer as well as the influence of strain on PTC effects are considered. The decline in PTC effects above the peak temperature is discussed.