Effect of water on the accelerated sulfur vulcanization of natural rubber

Water present in natural rubber was found to play a role in forming carbon-carbon crosslinking junctions before forming carbon-sulfur crosslinking junctions when its effect on the accelerated sulfur vulcanization was investigated by analyzing low–molecular weight products through various instrumental analytical techniques. Samples were prepared by mixing deproteinized natural rubber (DPNR) with various moisture contents with stearic acid, ZnO, sulfur, and N-tert-butylbenzothiazole-2-sulfenamide, and they were vulcanized at 150 °C for various cure times. Low–molecular weight products such as 2,2′-dibenzothiazole disulfide (MBTS), 2-mercaptobenzothiazole (MBT), ZnMBT, Zn2+ and sulfur linked to the rubber were extracted from the vulcanized natural rubbers using benzene and dioxane, and subsequently measured by high-performance liquid chromatography, UV–visible spectroscopy, inductively coupled plasma atomic emission spectroscopy, and combustion analysis. The water present in natural rubber was found to strongly affect the production and formation rate of the low–molecular weight products in the accelerated sulfur vulcanization of natural rubber. As the result it suggests that the water promotes the formation of the C-C linkages, and then it sequentially promotes the formation of the C-S linkages.