Continuous stress relaxation and intermittent stress of diene vulcanizates during oxidative aging

The most important cause of degradation of rubbers is oxidation by atmospheric oxygen. Oxidative stress relaxation has been examined. Oxidation was carried out by continuous stress relaxation and intermittent stress measurements. The effect of sample thickness on the rate of oxidative stress relaxation was evaluated theoretically and experimentally. Extents of chain scission and crosslinking were determined for SBR and NR vulcanizates. The effects of carbon black, antioxidants, and stress on the rate of oxidative stress relaxation were determined.;No antioxidant ;SBR stiffens with aging. Natural rubber with antioxidant stiffens at low aging times, then softens; while natural rubber without antioxidant, softens monotonically with aging. Carbon black (N330) accelerates oxidation for both SBR and NR. The effect of stress on the oxidation of NR gum is minimal up to 200% at 72;Surface cracks were examined after oxidative stress relaxation. Cracks appear sooner; and the depths of surface cracks become shallower for black-filled NR without antioxidant than for a composition with antioxidant. The surprising latter feature is attributed to the higher rate of degradation and stress relaxation for the sample without antioxidant.;Oxidation is diffusion controlled at 72;A unique type of longitudinal cracking was discovered during oxidative stress relaxation: Thin specimens of a black-filled, natural rubber vulcanizate were held in uniaxial tension at 72...