Synthesis of Hydrogenated Natural Rubber Having Epoxide Groups Using Diimide

Epoxidized natural rubber (ENR) with 50% mol of epoxide groups was synthesized using performic acid generated from the reaction of formic acid/hydrogen peroxide in latex form followed by hydrogenation using diimide generated from hydrazine (N₂H₄) and hydrogen peroxide (H₂O₂) with boric acid (H₃BO₃) as a catalyst. The resulting products (hydrogenated epoxidized natural rubber, HENR) were characterized by proton nuclear magnetic resonance spectroscopy (¹H-NMR), gel testing, transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The effects of reaction parameters such as N₂H₄ amount, H₂O₂ amount, H₃BO₃ amount, gelatin amount, reaction time, and reaction temperature on the percentage of hydrogenation degree and gel content were investigated. The transmission electron micrographs of the particles confirmed a core/shell structure consisting of a highly unsaturated concentration region as the core encapsulated by a low carbon-carbon double bond concentration region as the shell, which indicated that the rubber particle seemed to be modified from the outer layer to the center of the rubber particle. Overall, the data showed that an increase in the amount of the individual chemicals, reaction time, and temperature increased the hydrogenation degree. However, a higher level of gelatin retarded an increase in the percentage of hydrogenation degree. As the hydrogenation degree increased, the gel content increased due to the ether linkage and the crosslinking reaction triggered through hydroxyl radicals. From DSC measurements, the glass transition temperatures of hydrogenated products increased above those of original rubbers. The thermal stability of hydrogenated products was improved, demonstrated by a decomposition temperature shift to a higher temperature than ENR, as shown by the results from the thermogravimetric analysis. Therefore, the hydrogenated ENR (HENR) exhibited good thermal stability, which could extend the applications of ENR in the automotive and oil industries.