Exposure conditions control plastic degradation in marine environments: evidence from UV, floating, and intertidal experiments

Plastic debris entering marine environments undergoes progressive weathering that promotes fragmentation into microplastics, yet the relative influence of polymer type and environmental exposure pathways on degradation dynamics remains insufficiently quantified. This study experimentally evaluated the degradation of six common consumer plastics (PET, HDPE, PVC, LDPE, PP, and PS) under three environmentally relevant scenarios: controlled UV-A irradiation in seawater, buoyant exposure simulating floating ocean debris, and intertidal coastal deployment subject to tidal cycles. Over periods of three to six months, physical and optical deterioration was assessed using a standardized six-criterion index encompassing discoloration, gloss loss, surface erosion, microcracking, particle detachment, and porosity, supported by colorimetry, image analysis, and scanning electron microscopy. Bayesian cumulative ordinal regression was applied to jointly model degradation responses across treatments. Results revealed marked polymer-specific and environment-dependent degradation patterns, with intertidal conditions producing the most severe surface damage owing to the combined action of photochemical, mechanical, thermal, oxidative, and biological stressors. Extensive microcracking, porosity development, and particle release indicate active fragmentation pathways likely to generate secondary microplastics. Floating conditions produced moderate degradation, while laboratory UV exposure alone induced measurable but comparatively limited deterioration. These findings identify intertidal coastal zones as critical hotspots for secondary microplastic generation and emphasize that laboratory UV experiments alone cannot fully capture degradation dynamics under realistic marine exposure conditions.