ON THE MECHANISM OF SULFUR BEHAVIOR IN RUBBER COMPOUNDS

Mineral sulfur is a widely used ingredient to form cross-links between the rubber chains in the vulcanization process. During compounding, a high elastic state of sulfur at temperatures between 40 degrees C and 70 degrees C promotes elongation of its particles and, next, breaking these thin and weak needles into pieces. This increases the surface a rea of particles and, after melting, creates conditions for intensive diffusion of sulfur into the rubber matrix, which helps to equalize th e distribution of sulfur in cured rubber. Sulfur is relatively uniform ly distributed in the bulk of rubber compound. However, during storage of rubber compounds, a thin layer of sulfur is observed on their surf ace (a bloom). This detrimentally affects processing and has an influe nce on cured rubber properties in use. Up to 40 degrees C, the concent ration of bloomed sulfur increases as the storage time of a rubber com pound is prolonged. For temperatures above 42 degrees C, bloomed sulfu r disappears gradually. The eutectic alloy of sulfur/stearic acid shou ld play an important role in the disappearance of a bloom layer from t he surface of a rubber compound stored at a temperature above 42 degre es C. This is because the melting point for this eutectic, about 46 de grees C, is very close to that at which the change in mechanism of blo oming was noticed. Under this condition, sulfur, as a eutectic, is in a molten state; due to this, it can diffuse into the polymer matrix. F ormation of a eutectic is possible if molten particles of the curing s ystem, containing sulfur, accelerator(s), stearic acid, and zinc oxide , will meet each inside the rubber matrix during the last stage of com pounding.