Full definition
The most widely used vulcanizing (cross-linking) agent for diene rubbers containing carbon-carbon double bonds — natural rubber (NR), SBR, NBR, and BR. During vulcanization at 140-180°C, sulfur atoms react with the double bonds in adjacent polymer chains, forming sulfur bridges (cross-links) of varying length: monosulfidic (C-S-C, 1 sulfur atom — short, thermally stable), disulfidic (C-S₂-C), and polysulfidic (C-Sₓ-C, x=3-6 — long, flexible, better fatigue resistance). Dosage determines cross-link density and type: conventional cure (2-3.5 phr sulfur, high accelerator ratio, mostly polysulfidic — flexible, good fatigue), semi-efficient (1-1.7 phr), and efficient/EV (<0.5 phr sulfur + sulfur donor like TMTD, mostly monosulfidic — better heat and compression set resistance). Sulfur must be used with accelerators and activators (ZnO + stearic acid) for practical cure rates. Types: rhombic sulfur (S₈ rings, standard), insoluble sulfur (polymeric, non-blooming — essential for tire components). Per ASTM D4578 for sulfur in rubber.