Volatile organic contaminants in HDPE milk bottles along the mechanical recycling value chain, revealing origins and contamination pathways

To progress towards closed-loop recycling of plastic packages it is vital to understand the origin of contaminants and to develop effective mitigation strategies. High density polyethylene (HDPE) milk bottles were sampled at nine different locations along the recycling value chain and analysed with two gas chromatography – mass spectroscopy methods to study the presence of volatile organic compounds. The total approximated concentration of volatiles reduced over time from the production of the bottles up to the cross-docking facility for the separately collected lightweight packaging waste (from about 250 to 100 ppm), as alkanes and alkenes evaporate from the bottle. A clear maximum was observed in the number of identified of volatiles and in the total approximated concentration in milk bottles at the sorted HDPE product. Here contaminants peaked that originate from the milk, from other packaging components (labels, caps, inlays), from other packages and from the surrounding atmosphere. When only the milk bottles were manually separated out from this sorted product and these were mechanically recycled, flakes were obtained that contain the least amount of volatile compounds, much less than the freshly produced bottles (about 25-50 ppm). The type of volatiles present was, however, markedly different. In the freshly produced bottles alkanes, alkenes and intentionally added anti-oxidant were found, whereas in the recycled flakes mostly contaminants were found that originated from: the milk, the other packaging components, other packages and the surrounding atmosphere. Seven different contamination pathways were discerned. The gathered knowledge facilitates the future development of food-grade recycled HDPE.