Plastic fruit stickers - changes of surface and structure during industrial composting

In the past, large amounts of plastic particles have been found in compost, which often originate from the improper disposal of plastics in organic waste. A so far little-noticed input pathway of plastic in compost are so-called price look-up stickers made of conventional plastic. For example, such fruit stickers remain in the organic material despite sorting processes in the composting plant. However, little is known about alterations of price look-up stickers during industrial composting. Thus, this study aimed to investigate whether an industrial composting process leads to surface and structural changes of fruit stickers. For this purpose, fruit stickers made of polypropylene were placed on banana peels in an industrial composting plant and sampled after pre-rotting (11 days) and main rotting (25 days). Afterwards, composted stickers as well as non-composted stickers (control) were analysed by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), micro- and nano-computed tomography (CT). After industrial composting, all stickers showed signs of surface changes and degradation in the form of cracks, irregularities, and microbial colonisation on both the front and the back. Microbial growth was visible from day 11. Structural changes were observed, with large adhesions penetrating the sticker's surface and an increase in the volume from 16.7 to 26.3% during composting, accompanied by an increase in the carbonyl index. The delamination observed on some stickers after 25 days of composting indicates the formation of smaller microplastic or even submicron plastics.