From macro- to microplastic: Accelerated weathering of polystyrene by simulated solar radiation and mechanical stress

, Meides Nora, Menzel Teresa, Pötzschner Björn, Strohriegl Peter, Altstädt Volker, Senker Jürgen.

To date, the presence of microplastics (MP) is verified in almost every environmental compartment. The fragmentation of macro- into microplastic is rated as one of the most important pathways for secondary MP into the environment. As natural degradation is challenging to monitor over long time scales, artificial weathering represents a promising, but still hardly recognized, alternative. Here, we present a long-term accelerated weathering study on the Polystyrene (PS) grade PS 158 N. Tensile bars were exposed to simulated solar radiation and MP particles (125-200 µm) additionally to mechanical abrasion under laboratory-controlled conditions in a commercial weathering chamber. We studied the mechanical and chemical degradation of PS materials using a variety of analytical techniques such as GPC, SEM, EDX and tensile testing as a function of exposure time. Additionally, solid state 13C MAS NMR spectroscopy was used to investigate chain defects induced by photooxidation. Furthermore, Monte Carlo simulations of the chain scission and crosslinking processes were performed. A quantification of the reaction products as a function of exposure time could be completed. Our results helped us to reconstruct the process of PS degradation, coupling both geometries investigated and methods utilized. We identified two stages, ultimately leading to the formation and degradation of MP. Stage I is dominated by photochemical oxidation in a near-surface layer. In stage II, the degradation is accelerated by the formation of microcracks and particle rupturing. The ratio and intensity of both environmental stress factors influence the life times of both morphologies dramatically. Small MP particles with high proportions of carboxyl, peroxide and keto groups are constantly released into the environment. Their increasing polarity facilitate biofilm formation and the uptake by and interaction with organisms. We expect this two-stage model to be general, allowing a transfer of the degradation mechanisms to other commodity plastics.

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