Artificial weathering of plastics used in oyster farming

Abstract

With the omnipresence of plastic litter from oyster farming in marine coastal areas, the objective of this work was to better understand the weathering of plastics used in this field, focusing on oyster spat collectors. During their use, around fifteen years, collectors made of polypropylene (PP) undergo numerous degradations, alternatively submerged, emerged in seawater, and stored outdoor until the next cycle. They weaken, crack, break, end up fragmenting and disseminated in the environment as microplastics associated to persistent organic pollutants. In this work, a comparison of 55 months of in situ weathering with five months of artificial weathering in air or in artificial seawater in a homemade UV chamber was conducted to better understand the mechanisms involved. Chemical, thermal and surface characterizations of virgin and weathered samples were conducted using Fourier Transform Infrared Spectroscopy (FTIR), Differential Scanning Calorimetry (DSC) and Environmental Scanning Electron Microscopy (ESEM). After 55 months of in situ weathering, collectors were notably damaged with large fissures and loss of microplastics (MPs) associated with an increase of carbonyl index values and a decrease of melting temperatures and crystallinity rates. Considering only UV irradiation, five months of artificial weathering at 30 °C under continuous irradiation of 6.9 W/m2 under UV lamps (295–400 nm) reproduced approximately 4.4 months of natural sunlight. Artificial weathering confirmed that photooxidation by combined effects of UV rays and oxygen was the main weathering mechanism and was reduced in seawater. These results help to understand the mechanisms involved in the weathering of these collectors in the marine environment and provide valuable information for industrials and professionals. Our study suggests a better storage away from UV rays and a reduction of the duration of use compared to current practices.