Natural Weathering Reduces Spectroscopic Identifiability of Common Plastics

Assessing the full extent of microplastic pollution requires understanding of when weathering transforms plastics beyond reliable identification; however, systematic multi-polymer natural weathering studies are lacking. Acrylonitrile butadiene styrene (ABS), polycarbonate (PC), high-density polyethylene, polyethylene terephthalate (PET), polylactic acid, polypropylene (PP), high-impact polystyrene (HIPS), polytetrafluoroethylene, and polyvinyl chloride were exposed to 12 months of outdoor weathering. Prior to analysis, surfaces were cleaned to remove debris that could compromise polymer identification. Optical microscopy showed that ultrasonic cleaning induced surface cracking; the simple alternative of removing debris by wiping with ethanol minimised surface alteration. Confidence in polymer identification based on the fit of spectra to the Open Specy reference library decreased with increasing weathering for ABS, PC, PET, PP and HIPS; Raman spectra became less readily identifiable after less weathering than FTIR spectra. Reductions in polymer identification, changes in morphology detected by scanning electron microscopy, and peak height changes were most marked for ABS, PP and HIPS. The most pronounced changes in peak heights typically occurred for vinylidene, C-O and C=O groups, consistent with oxidative degradation. Evidence also suggested potential crystal structure changes in Sb₂O₃ additives and that black plastics are more resistant to oxidation than white plastics. Fragmentation of macroplastics was found to directly generate microplastics, with particle size varying by polymer type. These findings indicate that natural weathering progressively reduces reliable polymer identification, suggesting systematic underestimation of plastic pollution. Accurate environmental monitoring must therefore account for polymer-specific degradation, analytical technique, and pigmentation when estimating microplastic abundance.