Quantification of Micro- and Nanoplastic Formation from Flexible- and Rigid Plastic Products under Photooxidation

When plastic waste is released into the environment, it undergoes weathering processes that lead to the formation of micro- and nanoplastics (MNPs). In recent years, the production mechanisms of secondary MNPs and their surface property changes during weathering have been widely studied. However, information on these fragmented particles remains limited compared to that on parent plastics, particularly with respect to quantitative assessments of particle generation. In this study, we investigated the changes in surface characteristics of flexible- (zipper bags made of low-density polyethylene) and rigid plastic products (single-use plastics made of polypropylene and polyethylene) after photooxidation, and quantified the generated MNPs to calculate their fragmentation rates. Neither plastic exhibited naturally formed surface cracks during exposure. However, both materials became progressively hardened after approximately 100 days of photooxidation and fragmented readily when subjected to external stress. The carbonyl index increased consistently with exposure duration, indicating ongoing photooxidative degradation. These results demonstrate that photooxidation induces polymer embrittlement, which substantially enhances susceptibility to fragmentation under subsequent mechanical abrasion. Generation of MNPs increased markedly when mechanical abrasion followed photooxidation. For flexible plastics, photooxidation alone did not show a clear exposure-dependent trend in particle generation, whereas the application of mechanical abrasion resulted in an estimated annual production of 9,573,818 particles/cm². For rigid plastics, annual particle production increased from 3,251,032 particles/cm² under photooxidation alone to 11,884,373 particles/cm² when combined with mechanical abrasion. This study demonstrates that photooxidation under atmospheric conditions progressively embrittles both flexible and rigid plastics, while subsequent mechanical abrasion accelerates MNP formation. These findings indicate that various weathering factors should be considered when quantifying secondary MNP generation and evaluating polymer-specific fragmentation behavior.

This research was supported by 'Land/Sea-based input and fate of microplastics in the marine environment' of Korea Institute of Marine Science & Technology Promotion (KIMST) funded by the Ministry of Oceans and Fisheries, Republic of Korea (RS-2022-KS221604).