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.

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The physical characteristics of silica pigments covering a wide range of properties were determined and related by regression analysis to the static and dynamic mechanical properties of SBR vulcanizates reinforced with these silicas. Three morphological parameters were found to determine the vulcanizates' characteristics: the external surface area by CTAB adsorption, the structural index, and the DBP absorption number. The adherence between silica and elastomer was studied by means of swelling in solvents, by determination of the volume increase on deformation, and by scanning and transmission electron micrography. It was found that dewetting, the formation of vacuoles on deformation, is a most important factor, limiting reinforcement by silica at loadings above 40 phr. Equally, at such loadings the pH of the silica has an important influence : the higher the acidity, the less the adherence between pigment particle and elastomer. Silane coupling agents not only increase adherence, but also improve the degree of dispersion of the silica, which is frequently characterized by the presence of large, partly unwetted particle clumps under conventional conditions of incorporation, thus severely limiting the strength properties of the vulcanizates. The curing system employed and the accelerators used are also of paramount significance for adherence.

The low electrical resistance of vulcanizates loaded with conductive carbon blacks in a given particle size range is cause by the low density of the particles, high structure, or both. In addition, the smaller the particle size of the black, the lower the resistivity. The same factors also cause high modulus and, in liquid dispersions, high viscosity. It is shown that acetylene black owes its lower resistivity in vulcanizates to its high structure. Vulcan XC 72 owes its even lower resistivity to both high structure and low particle density, while EC Black owes its lowest resisitivity to an extremely low particle density caused by the presence of a large number of hollow-shell particles, as indicated by electron micrography. It was shown that the resistivity of vulcanizates loaded with EC Black is practically unaffected by deformation up to 150%, in contrast to other conductive blacks at comparable resistivities. It has been observed that increasing the degree of loading leads to a lower limit of resistivity, about equal for all of the blacks. The various phenomena could be explained by the concept that the resistivity of carbon black loaded vulcanizates resides in the gaps between particles or aggregates and is determined by a process of electron tunneling.