Interfaces in Composites. Reinforcement of Elastomers by Carbon Black

Basic features of carbon black-aggregation of particles into structure, particle size and morphology, and surface activity-are reviewed. Carbon black reinforcement of vulcanizates is first examined in the example of tearing, and the influence of hysteresis is considered. The dynamic properties of vulcanizates containing two major types of reinforcing carbon black are compared. While particle size gives the best correlation with tensile strength of vulcanizates, surface activity is shown to be the key to reinforcement. The role of these properties of carbon black in dissipating rupture energy is discussed. The relation between work to tensile break and hysteresis to break in gum rubbers can be applied to black-reinforced vulcanizates by use of a strain amplification factor. The complication introduced by stress-softening is explained in terms of localized stress relaxation. Abrasion reinforcement can also be explained in terms of hysteresis. The Flory-Rehner relationship of modulus of elasticity of swollen vulcanizates to physically-effective cross-linking applies to unswollen vulcanizates only after prestressing. Black-reinforced vulcanizates involve application of the strain amplification factor. The concept of mobile linkages to rubber chains at the surface of black particles is related to the influence of strain magnification and strain rate magnification in the reinforcing mechanism. These linkages result in formation of “shell” rubber adjacent to carbon black particles. The slippage of rubber chains relative to carbon black aggregates allows stress-sharing by highly-stressed chains. Bound rubber results from reaction of elastomer free radicals generated during mastication with carbon black. There is a relation between bound rubber and reinforcement which is fully developed only after vulcanization. Formation of bound rubber results from the surface activity of carbon black rather than its structure. Its contribution to reinforcement of the vulcanizate may be as important as cross-linking.