Structure and Properties of Loaded Rubber Mixtures. IX. Modification of Carbon Structures by Repeated Deformations

1. The structure of the carbon black phase of a rubber-carbon black vulcanizate is characterized first of all by a specific number of rubber-carbon black and carbon black-carbon black bonds (or contacts) and, in turn, by a specific proportion of these two types of bond. 2. The total number of bonds in the carbon structure, or the degree of their development, is indicated by the specific electric resistivity ρ of the vulcanizate, which decreases with the development of this structure. 3. Measurement of the proportion of carbon-carbon bonds in the structure establishes the factor n of the equation I=CVn, relating the energy of the current which flows through the test-specimen to the constant by the difference of the potentials. If n=1, all the bonds in the carbon structure which take part in the transmission of the current are of the carbon-carbon type, and the rubber mixture possesses a purely ohmic conductivity; in all other cases n>1. 4. During the deformation of loaded vulcanizates, changes of the specific resistivity ρ, and also of the factor n, take place. In the first cycle of stretching, ρ at first increases and then decreases slightly. During recovery after stressing, the electric resistivity sharply increases, reaching after the stress is removed a value several times greater than the maximum on the p curve of the first cycle of deformation. In succeeding deformation cycles, the change of resistivity proceeds with relatively slight hysteresis effects. 5. In the deformation of loaded rubbers, the weaker bonds are largely destroyed, and consequently the proportion of bonds of the stronger type increases. In cases where the carbon-carbon bonds are stronger than the rubber-carbon bonds, the value of n after deformation is smaller than that of n0 before deformation, or n0/n>1; in the opposite case, n0/n<1. 6. Both parameters (n and ρ) depend on the type of rubber; their greatest values, corresponding to a less developed structure with a small proportion of carbon-carbon bonds, are observed in the case of butadiene-styrene rubber; in sodium-butadiene rubbers, the degree of structural development and proportion of carbon-carbon bonds are much higher; the most highly developed carbon structure and the greatest proportion of carbon-carbon bonds are found in Butyl and natural rubbers. Hence, the value of n0/n is hardly related to the type of rubber. 7. Both parameters also depend on the type of carbon black. The most highly developed structures, with a large proportion of carbon-carbon bonds, are observed with channel carbon black, where these bonds are stronger than the rubber-carbon bonds, i.e., n0/n>1. The least developed structure, with a small proportion of carbon-carbon bonds, is observed with nozzle black and lampblack, in which cases these bonds are weaker than the rubber-carbon bonds, i.e., n0/n<1. Furnace blacks occupy an intermediate position. Thus, the carbon blacks studied are classified according to the value of n and the relation n0/n in the same sequence as when classified according to their reinforcing effects. The possible causes of this distribution are discussed. 8. The great strength of the bond between the particles of more active (channel) carbon blacks is one of the reasons for the greater heat formation in rubbers containing these carbons. Heat formation in rubbers containing less active carbon blacks (nozzle black, lamp black) which possess a weaker bond between their particles when all other conditions are equal, is much less.