Quantifying UVC-Induced Aging of Microplastics Using a Multivariate Aging Score

Abstract Microplastics continue to weather as they linger in the environment, yet the roles of polymer type and product formulation in shaping their aging trajectories remain poorly defined. In this work, we examined how commercial polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET) microplastics respond to ultraviolet C (UVC) irradiation across doses from 0 to 40 MJ m -2 . Among the three materials, PP changed the most rapidly: its carbonyl index (CI) rose sharply, its melting temperature (T m ) dropped from 157 °C to 141 °C, and its crystallinity (χ c ) declined from 76% to 52%. In contrast, PE and PET showed only modest alterations in their chemical and thermal signatures. Imaging by scanning electron microscopy further highlights the divergence in aging behavior—PP surfaces developed widespread cracks and generated secondary fragments, whereas the other polymers remained comparatively intact. Given that surface oxidation precedes bulk destabilization, we incorporated an infrared (IR)-based surface-crystallinity index (χ s I) to quantify these early chemical-structural changes. The influence of formulation is investigated using two PP laboratory wastes—transparent centrifuge tubes and blue pipette-tip boxes—both of which show progressive surface cracking, increasing CI, and T m depression as the UVC dose rises, with the colored material aging faster than the transparent PP. Because aging manifests through several properties that do not evolve in parallel, direct comparisons across polymers and products are challenging. An approach based on principal component analysis integrates CI, T m , χ c , and χ s I into a single quantitative aging score. This unified metric provides an approach for harmonized evaluation of aging levels across polymer types, product formulations, and physicochemical properties. The resulting framework facilitates direct comparisons between materials and provides predictive assessment of microplastic transformation under environmental or laboratory exposure conditions.