Electrically Conductive Rubber

Carbon particles in rubber are at first distributed as individual particles by the shearing action of milling, but the individual particles are in a state of Brownian movement due to the kinetic energy of the system. This causes the particles to drift about at a rate determined by the size of the particles, or particle aggregates, and the effective viscosity of the segments of the rubber molecules which is less than one hundredth of that of bulk rubber. The particles thus soon come into contact with each other, and since they are of a disordered crystalline structure and possess relatively high free surface forces, they cohere. The mobility of the aggregates of the particles is much less than that of the individual particles, due to their large size and size relative to that of the rubber molecules, so finally they are relatively immobilized into a scaffoldlike structure of carbon particles. This structure can be broken by external forces, and the broken structural units reform in vulcanized rubber to a structural state at a rate, and to an extent determined by the kinetic energy of the system. The idea of increasing structure formation with rise in temperature is apparently contrary to the kinetic theory, but the persistance of a stable conductivity value in passage from a high to a lower temperature rules out particle motion as the prime cause of conductivity. Furthermore the attainment of a discrete value for the conductivity (Figure 19) at any temperature rather than an alteration of the rate of change towards some maximum value, suggests that temperature activated energy barriers exist—possibly between the carbon and the rubber—which have to be broken before the carbon particles are free to move. Such a system explains the conductive properties of carbon-rubber mixes containing carbon particles of 250 to 300 A.U. diameter, but the properties of systems containing carbons of greatly different particle size may be considerably different. There is some evidence that the cohesion of carbon particles into structural units is sufficiently high to withstand the shearing forces involved during milling and processing treatments of unvulcanized rubber, and so carbon, structure formation from individual particles is of an irreversible nature. As the structural units grow in size under the influence of the kinetic energy of the system they become increasingly less mobile. This gives rise to permanent reduction in conductivity if the structure built up is violently disturbed; for example, if unvulcanized rubber is remilled after appreciable structural formation has taken place.