Reactions of Radicals in Filled Rubber Compounds: I. Detection of Free Radicals in Rubbers by ESR

Abstract

ESR measurements were made with vulcanized rubbers of a gum, three carbon black filled and a silica filled compounds. The radicals in the filled rubbers were found to have long lives unlike the gum rubber. Carbon black was found to reduce the sensitivity of ESR due to shielding effect in the magnetic field. The degree of sensitivity reduction appeared to depend on the inter-aggregate distance. Upon exposure to toluene vapor, the ESR intensity of black filled rubbers increased for a short period as swelling widened the inter-aggregate distance and then decayed steadily with further exposure. Such a variation was not found with the gum and silica filled vulcanizates. The decay was faster with higher oxygen content of the environment. In a given environment, however, the decay could be dealt by simple kinetics. Diffusion of oxygen did not seem to play any part. It was inferred that ESR observes radicals only in a thin surface layer of the black filled rubber. The kinetic zero-time radical concentration, obtained by toluene vapor exposure in a nitrogen environment, was considered to be the surface radical concentration in the absence of the shielding effect. Among the rubbers of different black loadings, it showed an exponential relation with the value obtained by direct observation due to the difference in the shielding effect arising from difference in loadings. With the rubber of the same formulation, however, the radical concentration obtained by direct observation showed a linear relation with the kinetic zero-time value, suggesting possibility of quantitative comparison by ESR among the rubbers with the same carbon black content.