Critical Condition of Nanostructured Magnesium Oxide for Accelerated Sulfur Vulcanization of Natural Rubber

An attempt to use nanostructured magnesium oxide (MgO) particles was made for the accelerated sulfur vulcanization of natural rubber by varying their amount and Brunauer–Emmett–Teller (BET)-specific surface area. The nanostructured MgO particles in a ratio of 0–20 parts per hundred rubber (phr) were mixed with natural rubber together with sulfur, vulcanization accelerator, and stearic acid. The resulting rubber composite was vulcanized at 150 °C. The effect of the nanostructured MgO particles was investigated by cure characteristic measurement at 150 °C. The resulting natural rubber vulcanizates were characterized by swelling measurement, tensile test, and rubber-state nuclear magnetic resonance (NMR) spectroscopy. The vulcanization was promoted by increasing the amount and BET-specific surface area of the nanostructured MgO particles. Cross-link density and tensile properties depended on the amount and BET-specific surface area. Rubber-state NMR spectroscopy revealed that the nanostructured MgO particles positively promoted the ionic mechanism of the vulcanization and effectively mitigated radical mechanisms and side reactions. This dual effect, enhancing vulcanization while suppressing undesirable side reactions, underscored MgO's potential as a pivotal agent for elevating the overall quality and performance of natural rubber vulcanizates. A critical threshold value of the surface area of MgO was 4.5 m2/g-rubber. Above this threshold value, a discernible enhancement in vulcanization was observed. This critical threshold value emerged as a key determinant, signifying the crucial role of the surface in the vulcanization kinetics of natural rubber in the presence of MgO.