Sustainable curing system of nitrile butadiene rubber vulcanizates and their rheological behaviors tailored by ionic liquid

A green vulcanization of nitrile butadiene rubber (NBR) gums was developed using the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]) as a potentially more sustainable co-curing agent in combination with sulfur, without using the conventional toxic activators zinc oxide and stearic acid. Structural and elemental characterizations witnessed homogeneous sulfur distribution and robust ionic interactions between IL and the NBR backbone. Further, density functional theory (DFT) frontier-orbital analysis indicated enhanced reactivity of the IL (HOMO–LUMO gap 2.12 eV) relative to sulfur and NBR, supporting a catalytic/ionic interaction role for [EMIM][OAc]. FTIR band shifts (notably the C≡N stretch) and XPS nitrogen signals confirmed strong dipolar/ionic interactions between IL and the nitrile groups. The best 5 phr IL formulation (NBR-IL5) exhibited a 23% reduction in optimum curing time (44.1 to 34.3 min), 57% improvement in maximum torque (420 to 663 kPa), and 38% improvement in tensile strength compared to sulfur-alone vulcanizates. Comprehensive rheological characterization and dynamic softening indeed confirmed enhanced network elasticity, reduced damping as evidenced by a low tan δ, improved strain tolerance, and greater thermal stability up to 150°C. Nonlinear strain sweeps and KDR analysis showed suppressed Payne-type softening and higher strain tolerance. For NBR-IL5, the G″ₘₐₓ/G″₀ value reached 8.78, while the experimentally measured tan δ went down to approximately 0.0594. All this results in a highly elastic and low damping ionically mediated network. The IL-assisted sulfur cure system is thus a green and scalable alternative to the traditional route of vulcanization and confers added performance improvements without increasing its environmental footprint while furthering sustainable rubber processing technologies.