Laboratory-Simulated Photoirradiation Reveals Strong Resistance of Primary Macroplastics to Weathering

The photodegradation of macroplastics in the marine environment remains poorly understood. Here we investigated the weathering of commercially available plastics (tabs 1.3 x 4.4 x 0.16 cm), including high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), and polycarbonate (PC), in seawater under laboratory simulated weathering, UVa and mechanical agitation, for 3-9 months, which is equivalent to up to 100-300 years of natural sunlight coastal exposure. After the exposure, the physical integrity and thermal stability of the tabs remained relatively intact, suggesting that the bulk polymer chains were not severely altered despite the strong irradiation, likely due to their low specific surface area. In contrast, the surface layer (~1 μm) of the tabs was highly oxidized and eroded after 9 months of weathering. Several antioxidant additives, such as butylated hydroxytoluene, were identified in the plastics through low temperature pyrolysis coupled with gas chromatography mass spectrometry (Pyr-GC/MS), suggesting that these antioxidants may have played a role in slowing down the photodegradation. The Pyr-GC/MS results also revealed many new oxygen-containing compounds formed during the photodegradation, and these compounds indicated the dominance of chain scission reactions during weathering. These findings highlight the strong resistance of industrial macroplastics to weathering, emphasizing the need for a broader range of plastics with varying properties and sizes to accurately estimate plastic degradation in the marine environment.