Electrical Conductivity of GR-S and Natural Rubber Stocks Loaded with Shawinigan and R-40 Blacks

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Abstract
Measurements have been made of the direct current conductivity of rubbers loaded with carbon black. Shawinigan and Continental R‐40 blacks compounded in natural rubber and GR‐S were studied, and the resistivities were determined as functions of time, temperature, concentration and elongation. Resistance decreased with time, at first very rapidly, then more slowly, approaching an equilibrium value. This behavior seems to be independent of the type of black used. Temperature coefficients of resistance (at 50°C) were positive for Shawinigan stocks, negative for samples containing R‐40, and tended to increase with increasing concentration of black and with increasing extension of the sample. At low concentrations, R‐40 gave higher conductance in GR‐S than did Shawinigan. At high loadings, Shawinigan gave the higher conductance in both GR‐S and natural rubber. Resistance increased with elongation for all stocks containing R‐40. The Shawinigan GR‐S samples showed an inversion; for small elongations the resistance increased on stretching, but for higher extensions the resistance decreased. The results are interpreted on the assumption that the carbon black tends to form chains in the rubber matrix.

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