Effect of Certain Crystalline Substances on Physical Properties of Elastomers I. Stress-Strain behavior

Crystalline organic compounds containing a β-naphthyl group cause pronounced stiffening of rubber vulcanizates under certain conditions. When these materials are removed by extraction, the reinforcing effect vanishes. Reversibility of this effect indicates that the forces involved are not those associated with primary bonds. Reinforcement by at least one of these materials, PBNA, is obtained with vulcanizates made from various elastomers and vulcanizing agents, and is essentially independent of the state of cure. In some instances about five per cent of PBNA, the most effective of these, produces the same stiffness on first extension as 40 phr carbon black. Any PBNA dissolved in the rubber has no effect on stiffness. Thus, less than three per cent crystalline PBNA may produce an isotropic Young's modulus of about 20 kg/cm2 at room temperature. At higher concentrations of PBNA, strain continues to decrease but hardness does not change proportionally. The temperature coefficient of Young's modulus for PBNA reinforced rubber is negative, like that for vulcanizates containing carbon black. However, the increased solubility of PBNA with rising temperature makes quantitative determination of the temperature coefficient difficult. The PBNA-rubber structure is partly destroyed by repeated extensions to twice the original length. However after 5 extensions a substantial enhancement of modulus remains. Addition of PBNA to a rubber vulcanizate does not affect significantly the glass transition temperature. Though the reversible nature of reinforcement with PBNA strongly suggests a physical mechanism, the stiffening cannot be explained by existing theories of physical reinforcement.