The Wearing Layer Influences Long-Term Material Loss from Composite Floats for Oyster Aquaculture: An Accelerated-Weathering Comparison of Eight Candidate Materials

Expanded polystyrene floats have been identified as a significant source of marine microplastic in oyster aquaculture, yet no certification criterion assesses the material that the certified alternatives shed. Gravimetric mass loss, normalised to a common exposed area of 7200 mm2, is reported under accredited accelerated weathering to KS F 2274 for eight float materials: expanded polystyrene at three densities, expanded polypropylene, three injection-moulded polymers, and a continuous-fibre basalt-reinforced thermoplastic composite. Specimens were dried to constant mass before weighing, so the quantity measured is a net change: an upper bound on particulate release, not a measure of it. Mass exceeded the initial value in all three stabilised expanded polypropylene specimens. At 3000 h the loss spanned a factor of 54, and every foamed material exceeded every non-foamed one, while tripling the density of expanded polystyrene reduced loss by only 25% (n = 3; p < 0.0001 across the foamed groups). For ten injection-moulded grades the accelerated ranking reproduced that of 2.5 years outdoors (Spearman ρ = 0.89), though the ratio varied from 0.4 to 6.3. A wear model indicates that the pure-polymer barrier providing water tightness contains 905–1180 g L−1 of polymer against 578 g L−1 for the laminate beneath, so the wearing layer, not the bulk material, may govern release in service.