Characterising the ageing effects of microwave exposure on microplastics using multivariate data analyses

Microplastics (MPs) are ubiquitous environmental contaminants, yet their responses to common household treatments such as microwave (MW) heating remain poorly understood. Clarifying MW-induced ageing mechanisms in MPs is essential for accurately assessing their potential risks to ecosystems and human health. However, current knowledge of how MW exposure impacts different MP polymers at the molecular and morphological levels is limited. In this study, a household microwave oven operated at 1000 W (2.45 GHz) for 3 – 15 min was used to represent a domestic appliance; exposures were performed on dried pristine MPs to investigate direct microwave-driven ageing without matrix (food, solvent, etc.). MW-induced ageing of polypropylene (PP), nylon, polymethyl methacrylate (PMMA), and polyethylene terephthalate (PET) MPs was systematically evaluated using optical photothermal infrared (O-PTIR) spectroscopy, image-based morphological analysis, and advanced chemometric techniques. Our results indicated significant morphological and chemical alteration in PP and nylon MPs after MW exposure, with PP exhibiting the most severe degradation. Parallel factor analysis (PARAFAC) enabled the identification of polymer-specific and time-dependent degradation patterns, revealing subtle spectral variations that were not clearly resolved by principal component analysis (PCA). PMMA MPs demonstrated moderate morphological changes but maintained notable chemical stability, while PET MPs exhibited minimal molecular changes detectable primarily by the PARAFAC model. This study provides the first application of PARAFAC for analysing IR spectral data of MPs, enabling a deeper understanding of subtle polymer ageing phenomena. The findings demonstrated that MW exposure could cause polymer-specific ageing effects in MPs.