Thermal Aging Behaviors of Rubber Vulcanizates Cured with Single and Binary Cure Systems

Rubber compounds are crosslinked by a sulfur, peroxide, or resole cure system. The sulfur vulcanization is the most popular method. In general, an accelerated sulfur cure system consists of elemental sulfur (S8), one or two cure accelerators, and cure activators. Crosslink density of a rubber vulcanizate determines the physical properties. By increasing the crosslink density, the modulus, hardness, resilience, and abrasion resistance increase, whereas the elongation at break, heat build-up, and stress relaxation decrease. Sulfur linkages are composed of monosulfide, disulfide, and polysulfides. Sulfur linkages, especially polysulfides, are dissociated by heating and this brings about decrease of the crosslink density. Curatives remained in a rubber vulcanizate make new crosslinks and this results in increase of the crosslink density. Crosslink density of a rubber vulcanizate is changed by thermal aging. In general, crosslink density of a sulfur-cured rubber vulcanizate increases with increase of the aging temperature. However, for a rubber vulcanziate with an elemental sulfur-free cure system, the crosslink density after thermal aging at high temperatures decreased. In the present work, we studied the thermal aging behaviors of the rubber vulcanizates with single and binary cure systems which have one and two cure accelerators, respectively. The single cure system had one cure accelerator of N-tert-butyl-2-benzothiazole sulfenamide (TBBS) and binary cure system had two cure accelerators of TBBS and 1,6-bis(N,N'-dibenzylthiocarbamoyldithio)hexane (DBTH). Scheme 1 shows the chemical structures of TBBS and DBTH. The binary cure system has faster cure rate and better reversion resistance than the single cure system. The NR vulcanizates with different crosslink densities were prepared to investigate the influence of the initial crosslink density on the thermal aging behaviors. The aging temperatures (50-90 C) and aging times (4 and 8 days) were also varied to investigate the influence of the aging temperature and time on the thermal aging behaviors. Figures 1 and 2 show variations of the crosslink density changes by the thermal aging as a function of the aging temperature. The crosslink density change was calculated by dividing the difference in the crosslink densities of the vulcanizates after and before the thermal aging by the initial crosslink density; ΔXc (%) = 100 × (Xc aged – Xc )/Xc , where the Xc aged and Xc ini indicate the crosslink densities of the vulcanizates after and before the thermal aging, respectively. The crosslink densities after the thermal aging at 50-90 C on the whole increased. The increased crosslink density after