Structure and Dielectric Properties of Rubber Mlxtures Containing Carbon Black

One of the fundamental steps in solving the problem of the reinforcement of rubber is that of explaining the nature of the reaction between rubber and fillers. The measurement of heat effects during swelling, in addition to presenting experimental difficulties, does not provide a direct evaluation of the intensity of the rubber-filler bond in rubbers used in industry. In order to measure this bond, the present authors studied the frequency and temperature relations of the dielectric constant ε′ and the tangent of the angle of dielectric losses, tan δ, of unvulcanized natural rubber and sodium-butadiene rubber, rubber-carbon black mixtures, and vulcanized rubber containing different loadings of channel and furnace blacks. The measurements were made in the frequency range of 50 to 2.5×107 cycles per second at 20° C and also in the frequency range of 103 to 4×104 cycles per second at temperatures from −75° to 152° C. As the experiments show (see Figure 1), the coefficient of dielectric loss, ε″=ε′⋅ tan δ, of a raw rubber has no maximum within the frequency range and temperature range studied. At the same time, the rubber-carbon black mixtures are characterized by a maximum ε″ at frequencies of (2−2.5)×103 cycles per second, which depends to a slight extent on the temperature. Vulcanizates containing no carbon black have a maximum ε″ at (2−3.5)×106 cycles per second at 20° C, which is displaced to the temperature range −24° to −27° when the frequency is decreased to 103 cycles per second. Both maxima are observed on the ε″ curves with loaded vulcanizates, the first caused by the heterogeneity of the rubber-carbon black mixture, and the second due to the formation of sulfur dipoles during vulcanization. Consequently, the ε′ curves have two regions of change.