Removing hazardous additives from elastomer manufacturing

Elastomers known as rubber are ubiquitous in industrial applications, but they often contain chronic additives, such as zinc oxide, tetramethylthiuram monosulfide (denoted TMTM), and copper dimethyldithiocarbamate (CDD) that has a higher melting point than the common vulcanization temperature. These additives are released into the environment either through the wear and tear of tires and the landfilling of waste rubber. It is imperative to identify and adopt safe, cost-effective alternatives to replace zinc oxide and TMTM, both of which have moderate chronicity rating. Styrene-butadiene rubber (SBR) in this study is cured by using sulphur, zinc stearate, and dipentamethylenethiuram hexasulfide (TRA). The curing characteristics and the morphology and mechanical properties of the cured SBR are investigated. Zinc stearate and TRA exhibit a commendable rating of zero in terms of both chronicity and toxicity, making them promising candidates for substituting chronic additives. Adding 0.25 phr of zinc stearate into SBR can significantly enhance the crosslinking density while exhibiting anti-reversion performance, in comparison with a recipe that includes 5 phr of zinc oxide and 8 phr of TMTM. Transmission electron microscopy reveals that the zinc oxide (nano) particles are not soluble in SBR, and thus only the particle surface contributes to vulcanization. TRA is “dissolvable” in SBR, making it an ideal replacement for CDD which is insoluble due to its high melting point. Therefore, we strongly advise against the utilization of curing additives with melting points that exceed the vulcanization temperature. This work contributes to the green manufacture of elastomers.