Full definition
Sulfur is a crucial vulcanizing agent utilized primarily in the rubber industry for diene rubbers that feature carbon-carbon double bonds. These include natural rubber (NR), Styrene-Butadiene Rubber (SBR), Nitrile Butadiene Rubber (NBR), and Butadiene Rubber (BR). The vulcanization process typically occurs at temperatures ranging from 140°C to 180°C, during which sulfur atoms chemically bond with the double bonds in the polymer chains. This reaction results in the formation of sulfur bridges, or cross-links, which enhance the material's elasticity and durability. The nature of these cross-links varies based on the dosage and type of sulfur used, which can range from monosulfidic (with one sulfur atom) to polysulfidic (with multiple sulfur atoms).
The dosage of sulfur is critical in determining the cross-link density and the physical properties of the rubber. Conventional curing typically employs 2 to 3.5 parts per hundred rubber (phr) of sulfur, yielding a high accelerator ratio predominantly creating polysulfidic cross-links. This configuration results in a flexible rubber with good fatigue properties. In contrast, semi-efficient curing utilizes 1 to 1.7 phr of sulfur, while efficient or EV curing employs less than 0.5 phr of sulfur in conjunction with sulfur donors such as Tetramethylthiuram Disulfide (TMTD). This method maximizes the creation of monosulfidic linkages, which are advantageous for heat and compression set resistance.
Sulfur must always be used in conjunction with accelerators and activators, such as zinc oxide (ZnO) and stearic acid, to achieve practical curing rates. The types of sulfur include rhombic sulfur, which consists of S₈ rings and is considered the standard form, and insoluble sulfur, which is a polymeric form of sulfur that does not bloom and is essential for applications in tire components. The use of sulfur in rubber formulations is governed by ASTM D4578, which outlines the testing methods for assessing sulfur levels in rubber compounds.