Imidazole-promoted acceleration of crosslinking in epoxidized natural rubber/dicarboxylic acid blends

The crosslinking of epoxidized natural rubber (ENR) with dodecanedioic acid was tremendously accelerated in presence of 1,2-dimethylimidazole (DMI). The curing reaction of ENR containing 10 or 25 mol % epoxide groups was followed by rheology at 180 °C or 160 °C and the mechanical properties of the cured materials were examined by stress–strain experiments. An equimolar amount of accelerator and carboxylic functions was required to reach optimum tensile properties. Among a variety of usual accelerators, DMI was found the only one efficient, which was associated to the formation of reactive imidazolium carboxylates. The presence of these intermediates was supported by DSC experiments. Instead of a catalytic mechanism, the activation of the crosslinking agent is enabled by the synergetic association of DMI and diacid forming a soluble species in the rubber matrix. This simple system thus enables the efficient crosslinking of functionalized natural rubber without the use of sulphur or peroxides.