Radiotracer Studies of Carbon Black Surface Interactions with Organic Systems. II. Use of Sulfur-35 in Initial Studies on the Reactivity of “Bound” Surface Sulfur

A broad investigation has been undertaken, using primarily radiochemical techniques, of the reactivity of the insoluble or “bound” sulfur on the surface of carbon blacks. The carbon black samples used in this study were prepared in small pilot plant reactors from sulfur-free commercial feedstocks to which had been added, in separate experiments, sulfur-35 tagged elemental sulfur, dibenzothiophene-S-35 and dibenzyl disulfide-S-35. After careful extraction-purification of the blacks, “chemical probe” studies were undertaken to gain knowledge of the nature of the surface sulfur complexes or functional groups, using reagents such as LiAlH4, trivalent phosphorus compounds, tetramethylthiuram disulfide, and hydrogen. These experiments demonstrated that bound surface sulfur possesses a considerable amount of chemical reactivity. Experiments were next designed to determine whether or not bound surface sulfur becomes involved in vulcanization reactions by, for example, exchange with added curing sulfur or accelerator. In radiotracer studies with SBR, natural rubber, and cis-polybutadiene vulcanizates, the results demonstrated that bound surface sulfur enters into the rubber phase during vulcanization. Finally, results from preliminary rubber physical testing and practical vulcanization studies involving pilot plant blacks (2% wt. sulfur) indicate that surface sulfur has little effect upon the vulcanization or final physical properties of SBR and NR.