Degradation rates and ageing effects of UV on tyre and road wear particles

Tyre and road wear particles (TRWPs) are estimated to be the largest source of microplastics in the environment and due to the intrinsic use of tyres in our society this will continue to grow. Understanding their degradation mechanisms and subsequent accumulation over time is important to gain insights into the fate and impact of these particles in the environment. Accelerated UV-ageing was performed on cryomilled tyre tread particles and TRWPs from a road simulator to investigate the abiotic degradation of rubber. Degradation was followed with thermogravimetric analysis (TGA) that led to an average abiotic degradation rate of 0.025 day^-1 when corrected for the acceleration factor. Static light scattering (SLS) showed that during degradation, the average particle size reduced by 0.03 μm day^-1 and smaller particles <10 μm were formed. Further characterisation with scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX) confirmed these findings and showed that the sulphur content is reduced through UV-ageing suggesting that crosslinking breakage may be a mechanism of degradation. Analysis with gas chromatography and mass spectrometry (GC-MS) showed a substantial decrease in chemical additives by UV-induced oxidation and breakdown. Finally, with measurements in the field TRWP particle sizes and accumulation times were studied, confirming the experimentally determined degradation mechanisms.