Beyond size and shape: Physicochemical properties of microplastic test materials generated by cryomilling

Cryomilling is a widely used method to generate microplastic test materials for environmental impact studies. To date, changes in physical, chemical and thermal properties, as well as potential contamination, in cryomilled materials have been inadequately assessed. Here, six polymer samples, including fossil-fuel based polymers (LDPE and PP) and bio-based polymers (PLA, PBS and a PLA/PBAT blend), were cryomilled with their physicochemical properties assessed before and after milling. Particle morphology was influenced by starting material form; pellets produced microplastic fragments whereas films produced thin flakes. Particle size distribution was polymer dependent but smaller size fractions were achieved by using a smaller milling chamber. Cryomilling retained bulk polymer chemical characteristics, including weathering-induced changes, indicating its suitability for generating microplastic test materials without significant alteration during processing. Contamination was negligible from both plastic and metal components of the milling apparatus, validating the use of cryomilled microplastics for leaching and ecotoxicological studies. While overall chemical integrity was preserved, cryomilling caused an increase in the degree of crystallinity in PP, PBS and PLA. These results provide crucial insight into material properties, beyond size and shape, to better understand the influence of physicochemical properties on particle behaviour and draw meaningful associations between generated and environmental microplastics.