Sulfur is best known industrially as the feedstock for sulfuric acid, and its far smaller but chemically essential role in rubber vulcanization is governed by a precise ratio: research measuring rubber networks directly shows that the amount of sulfur relative to accelerator changes not just how much crosslinking occurs, but what kind. This page covers that research and where sulfur actually comes from.
What is it?
Sulfur is a byproduct-recovered element used mainly to produce sulfuric acid, with a separate, chemically essential role as the crosslinking agent in rubber vulcanization. Per the U.S. Geological Survey’s Mineral Commodity Summaries 2025, U.S. sulfur production in 2024 was estimated at 8.2 million tons (7.5 million tons recovered as elemental sulfur), recovered “in descending order of tonnage, at petroleum refineries, natural-gas-processing plants, and coking plants,” with Louisiana and Texas accounting for about 52% of domestic production. Roughly 90% of sulfur consumed was in the form of sulfuric acid.
Why should I use it?
- It’s the fundamental crosslinking agent that makes rubber vulcanization possible in the first place — see “How it’s used.”
- The ratio of sulfur to accelerator gives direct, measurable control over the chemistry of the crosslinks formed, not just how many form — see Limitations.
How it’s used
In rubber compounding, sulfur acts as the crosslinking agent, typically combined with an accelerator to control the vulcanization reaction.
Limitations — when it isn’t the right choice
- The sulfur-to-accelerator ratio doesn’t just control how much crosslinking happens — it determines the type of crosslink formed, and getting this wrong is a real formulation risk. A 2022 peer-reviewed NMR study tested natural rubber and polyisoprene networks with sulfur levels of 1, 2, and 3 parts per hundred rubber (phr) against a fixed 3 phr accelerator (TBBS), giving accelerator-to-sulfur ratios of 3:1, 1.5:1, and 1:1, with measured crosslink densities ranging from 2 to 8 × 10⁻⁵ mol/g. The study found that “by increasing the A/S ratio, the efficiency of the vulcanization reaction is improved and a higher number of sulfur bridges of a shorter length are produced” — meaning the sulfur dosage itself shifts the balance toward shorter (mono- and disulfidic) or longer (polysulfidic) sulfur bridges, not just the total crosslink density. This means choosing a sulfur level isn’t just about reaching a target crosslink density — it’s a decision about crosslink chemistry itself, and these findings are specific to the natural rubber/polyisoprene system and TBBS accelerator tested in this study; they shouldn’t be assumed to hold identically for other accelerator systems.