Weathering Pathways of PET Consumer Products under Simulated Marine and Photochemical Conditions †

Poly(ethylene terephthalate) (PET) is among the most widely used plastics globally, and its envi- ronmental persistence is strongly influenced by formulation variability and additive content. These factors complicate the identification and characterisation of PET degradation in natural environ- ments, highlighting the need for consistent and robust analytical approaches. The objective of this study was to compare the weathering behaviour of reference PET microplastics and PET mi- croparticles derived from commonly used consumer products following ultraviolet (UV) irradiation and artificial seawater ageing, and to evaluate analytical methods for characterising the resulting changes. Reference PET microplastics along with PET microparticles derived from commonly used consumer products, including beverage bottles, personal care product containers, and takeaway food packaging, were cryogenically milled and characterised before and after ageing. The materials were subjected separately to ultraviolet (UV) irradiation and artificial seawater ageing to simulate en- vironmental weathering in marine conditions. Chemical, structural, and optical changes induced by ageing were investigated using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and diffuse reflectance UV–visible spectroscopy (DRS/UV–Vis). In addition, fragmentation behaviour and degradation pathways under UV and seawater exposure were examined through optical microscopy coupled with image analysis. This work provides comparative insights into the weathering behaviour of PET consumer products and contributes to improved methodologies for assessing PET degradation in marine environments.