Doctoral thesis viva voce : Myriam Pire

Epoxidized Natural Rubber and Crosslinking by Dicarboxylic Acids: Chemistry, Kinetics and Mechanical Properties

This work focuses on the crosslinking of epoxidized natural rubber (ENR) by dicarboxylic acids via epoxy sites. The curing reaction at 180 °C of the reactive functional rubbers containing 10 to 50 mol % epoxide groups was followed by rheology, and cured materials were characterized by stress-strain experiments, dynamic mechanical analysis and DSC measurements.

Study of binary blends of ENR and dodecanedioic acid showed that a specific ratio of crosslinking agent gave rise to optimum mechanical properties, which depends on the epoxidation level of the rubber. The slow cure process had detrimental effects to mechanical properties and was found to be tremendously accelerated (from 3 hours to 20 minutes) in presence of 1,2-dimethylimidazole. An equimolar amount of accelerator and carboxylic functions was required to reach the maximum tensile properties. This was associated to the formation of a reactive imidazolium dicarboxylate, soluble in the elastomer matrix.

Mechanism of the accelerated reaction was further investigated by coupling two methods: solid-state NMR measurements on crosslinked samples and synthesis of liquid epoxidized rubber that can be analyzed by solution NMR. These experiments proved the ester nature of the crosslinks, highlighted the role of soluble imidazolium intermediate, and gave some insights into the regioselectivity of the reaction. Industrial development of the formulation was performed by adding fillers and antioxidants.

Results of thermal stability and fatigue life experiments on these charged materials are comparable to the ones obtained with standard sulphur and peroxide crosslinked samples.