The Role of Artificial Weathering Protocols on Abiotic and Bacterial Degradation of Polyethylene

Plastic pollution poses significant environmental challenges due to its persistence and contribution to the microplastic formation, with polyethylene being among the materials more abundantly found. Understanding how different artificial weathering protocols influence the degradation of plastics is crucial for assessing their environmental impact. This study investigates the effects of three distinct artificial weathering protocols-continuous UV-A irradiation (M_L), cyclic UV-dark exposure (M_C[L→D]), and sequential UV-dark phase (M_L→D)-on the physicochemical properties of plastics, using oxo-low-density polyethylene as the model material. Surface oxidation, measured by quantification of the carbonyl index, was most pronounced under the M_C[L→D] protocol despite the shortest time of overall UV exposure, indicating that oxidative reactions continue during the dark phases. Vinyl group formation, however, required continuous or cyclic UV exposure, highlighting the critical role of light in this chemical process. Alterations in the surface hydrophilicity, measured by contact angle, and changes in molecular weight were quantified and found to closely link to the weathering conditions, with increased oxidations enhancing the surface hydrophilicity and the chain scission balanced by crosslinking with extended UV durations. These findings emphasize the importance of weathering protocols when trying to simulate conditions in the lab that are closer to the ones in the environment to understand plastic degradation mechanisms. Biodegradation experiments with Rhodococcus rhodochrous demonstrated that weathered oxo-LDPE samples with higher surface oxidation levels (ΔCI > 1) supported an increased CO₂ production by Rhodococcus rhodochrous, with the M_C[L→D]-360 h protocol yielding the highest biodegradation rates-31-43% higher than the control.