Vulcanization kinetics of natural rubber and strain softening behaviors of gum vulcanizates tailored by deep eutectic solvents

Rubber processing industries call for ecofriendly, green activators for reducing zinc oxide (ZnO) activator usages and volatile organic compounds emissions. Herein low cost deep eutectic solvents (DESs) are used for regulating vulcanization kinetics of natural rubber (NR) gums and for tailoring mechanical behaviors of the gum vulcanizates. A highly active DES based on choline chloride/thiourea (1/3) pair (CCT) is identified. Its combination with ZnO at lowered amounts allows preparation of high-strength vulcanizates at lowered vulcanization temperature. The 120 °C-cured vulcanizate containing 1 phr (parts per hundred parts of rubber) ZnO and 0.2 phr CCT demonstrates crosslinking density 105 mol/m3 and tensile strength 21 MPa, being higher than those (94 mol/m3 and 16 MPa) of the 140 °C-cured vulcanizate containing 2 phr ZnO activator. Normalization of deformational stress with respect to theoretical elastic modulus of ideal rubbery network reveals that stress (deformation energy) and softening at low strains are closely related to crosslinking density and strain-induced crystallization behavior. Crosslinking density could be independently regulated by introduction of DESs and by variation of vulcanization temperature, paving the way for tailoring elasticity and dissipation of vulcanizates manufactured in an energy-saving efficient vulcanization approach.