Screening p-Phenylenediamine Antioxidants, Their Transformation Products, and Industrial Chemical Additives in Crumb Rubber and Elastomeric Consumer Products

Recently, roadway releases of N,N '-substituted p-phenyl-enediamine (PPD) antioxidants and their transformation products (TPs) received significant attention due to the highly toxic 6PPD-quinone. However, the occurrence of PPDs and TPs in recycled tire rubber products remains uncharacterized. Here, we analyzed tire wear particles (TWPs), recycled rubber doormats, and turf-field crumb rubbers for seven PPD antioxidants, five PPD-quinones (PPDQs), and five other 6PPD TPs using liquid chromatography-tandem mass spectrometry. PPD antioxidants, PPDQs, and other TPs were present in all samples with chemical profiles dominated by 6PPD, DTPD, DPPD, and their corresponding PPDQs. Interestingly, the individual [PPDQ]/[PPD] and [TP]/[PPD] ratios significantly increased as total concentrations of the PPD-derived chemical decreased, indicating that TPs (including PPDQs) dominated the PPD-derived compounds with increased environmental weathering. Furthermore, we quantified 15 other industrial rubber additives (including bonding agents, vulcanization accelerators, benzotriazole and benzothiazole derivatives, and diphenylamine antioxidants), observing that PPD-derived chemical concentrations were 0.5-6 times higher than these often-studied additives. We also screened various other elastomeric consumer products, consistently detecting PPD-derived compounds in lab stoppers, sneaker soles, and rubber garden hose samples. These data emphasize that PPD antioxidants, PPDQs, and related TPs are important, previously overlooked contaminant classes in tire rubbers and elastomeric consumer products.

Automated summary

Recently, the presence of N,N'-substituted p-phenylenediamine (PPD) antioxidants and their transformation products (TPs) in recycled tire rubber products has gained attention. These compounds, including the toxic 6PPD-quinone, were found in tire wear particles, recycled rubber doormats, and turf-field crumb rubbers. The study also revealed higher concentrations of PPD-derived chemicals compared to commonly studied industrial rubber additives in elastomeric consumer products like lab stoppers, sneaker soles, and rubber garden hoses. Overall, this highlights the importance of PPD antioxidants and related TPs as previously overlooked contaminants in tire rubbers and elastomeric consumer products.