Time-concentration profiles of tire particle additives and transformation products under natural and artificial aging

Tire and road wear particles (TRWP) are polymer-based microparticles that are emitted into the environment during tire usage. Growing efforts are currently being made to quantify these emissions, characterize the leachates and assess their environmental impact. This study aimed to investigate the effect of aging on TRWP composition. Cryomilled tire tread particles (CMTTP) and TRWP were exposed for different durations to three aging conditions: accelerated thermal and photochemical aging and natural outdoor aging. Particles were then extracted with cyclohexane/ethanol. The time-concentration profiles of 23 additives and transformation products present in these extracts were determined by UHPLC-HRMS. Several chemicals, such as N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6-PPD) or 1,3-diphenylguanidine (DPG), decayed exponentially under all aging conditions, with half-lives of a few days under artificial photoaging versus dozens of days under pure thermal aging at 60 °C. The natural aging profiles lie between those 2 laboratory aging conditions. Other chemicals, such as 6PPD-quinone, presented bell-shaped concentration profiles within CMTTP when particles were exposed to UV light. 6PPD-quinone reached a maximal concentration within a month under natural aging. For TRWP, the initial load of 6PPD-quinone had already reached a maximum prior to the aging experiments and decreased exponentially under natural aging with a half-life below one month. Pure thermal aging induced a significantly slower decay of 6PPD-quinone within TRWP (half-life of half a year), emphasizing a greater stability and persistence in environmental compartments without light. This study highlighted that the more readily accessible CMTTP could be considered a reasonable proxy of TRWP to investigate the fate of chemicals within rubber particles, at least from a qualitative standpoint. Overall, the concentrations of 20 of the evaluated chemicals decreased by >50 % within 50 days under natural aging.