Developing environmentally relevant test materials for microplastic research through UV-induced photoaging

• UV-driven photoaging produces environmentally relevant test microplastics. • Different photoaging protocols produce true-to-life MPs with distinct properties. • UV-aged fragmented MPs yield reproducible and chemically homogeneous materials. • Environmentally relevant MPs support realistic fate, toxicity, and risk assessment. Microplastics (MPs), defined as plastic fragments smaller than 1 mm, are pervasive pollutants posing considerable ecological and health hazards owing to their durability and potential to cause adverse environmental effects. These particles originate mainly from the breakdown of bigger plastic debris by mechanisms such as UV-induced photodegradation, resulting in fragmentation into micro- and nanoplastics. Appropriate laboratory test materials that simulate naturally degraded plastics are essential for evaluating the environmental impact of MPs, enhancing analytical methods, and assessing remediation pathways. In this study we generated "true-to-life" MPs from commonly utilized plastic products through controlled photodegradation processes designed to accelerate polymer aging. Two aging protocols were developed: (i) UV irradiation of macroplastic fragments for up to eight weeks followed by mechanical milling, and (ii) UV exposure of pre-fragmented MPs over the same period. Five polymers, namely polystyrene (PS), polypropylene (PP), high-density polyethylene (HDPE), polyvinyl chloride (PVC), and polyethylene terephthalate (PET), were chosen for analysis, with PET investigated separately due to the presence of the carbonyl group, which complicates carbonyl index (CI) calculations used as a quantitative index to monitor the photo-oxidation. The surface morphology of aged MPs was examined using Scanning Electron Microscopy (SEM), their chemical composition was investigated by Near-Infrared (NIR) and Fourier-transform Infrared (FTIR) spectroscopy, thermal properties were also evaluated by Thermogravimetric Analysis (TGA). PET degradation was further analyzed using supplementary techniques such as X-Ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC) to assess structural and thermal alterations. These findings demonstrate that the proposed protocols generate MPs with consistent physicochemical properties, providing a model system suitable for studying MP degradation and behavior in laboratory studies, ultimately supporting environmental risk assessment and mitigation strategies.