Plastic recyclates produced from waste packaging have to meet high sensory requirements for them to be used for new products. Plastic recyclates often have off-odours, some of which have yet to be identified.
The Fraunhofer Institute for Process Engineering and Packaging IVV has analysed the sensory properties of post-consumer shopping bags made of low-density polyethylene (LDPE) and originating from different collection systems. More than 60 odorous substances were identified using combined chemo-analytical methods. The information gained provides a targeted strategy for preventing off-odours.
The results of this collaborative study with the Chair of Aroma and Smell Research at FAU and the University of Alicante have now been published.
In order to meet the targets of the new EU Packaging and Packaging Waste Directive concerning the recycling of packaging waste, new markets for recyclates produced from waste plastic packaging must be found. Recyclates produced from waste plastic packaging must have no off-odours if they are to be used as secondary raw materials for manufacturing high-quality consumer products. Indeed, the off-odours in plastic recyclates prevent a closed cycle in the recycling process of plastic packaging materials. Currently, there is a particularly high reusage rate for recyclates produced from polyethylene terephthalate (PET) bottles.
LDPE packaging waste in focus
The Sensory Analytics department at Fraunhofer IVV characterises and optimises plastics and recyclates. The odorants in HDPE waste and the recyclates produced from packaging from personal-care products and detergents have already been analysed. Low-density polyethylene (LDPE) is one of the most commonly used plastics. It is widely used for packaging materials such as plastic shopping bags. These bags end up as packaging waste via various collection systems. Part of the current study on post-consumer LDPE shopping bags focused on the effect of the collection strategy on the sensory impairment of the waste.
Identification of odorants – the key step to prevention
The identification of the substances that cause off-odours is essential to enable the relevant measures for odour optimization to be taken. Most of the odorants identified in the study are typical products of metabolism of microorganisms, many of which had a cheesy or faecal odour. The odorants included carboxylic acids and components containing sulphur and nitrogen. The chemical structures of the odorants gave key insights into their origin. The pathways into the packaging waste and via the recycling process into the recyclate were identified using this information. Targeted measures can be used to reduce odorants or prevent the formation of new odorants, depending on which process step is unable to remove an odorant or even results in a new odorant.
Odorant reduction via separate collection in the yellow bag (‘Gelber Sack’)
The study showed that the way the packaging waste is collected has a significant effect on the odour quality of plastic shopping bags. Separate collection using yellow bags (the ‘Gelber Sack’ system used in Germany) showed significant benefits. Waste collected in this way had a significantly lower level of overall odour. In contrast, the proportion of waste collected with general household waste had more intense cheesy, sulphurous, and faecal odours. The higher proportion of organic matter in general waste promotes the formation of these microbial decomposition products. The study also demonstrated that the post-consumer LDPE shopping bags from the separate collection that were washed at 60 degrees had fewer odorants and a lower level of overall odour than the unwashed bags.
Methods for odour identification in plastic waste
The scientists at Fraunhofer IVV used sensory analytics to identify the odours. Firstly, a trained sensory panel carried out sensory evaluation of the sample materials. The odorants were then identified using chemo-analytical methods such as gas chromatography-olfactometry and with two-dimensional coupling with mass spectrometry. This enabled the chemical structures to be determined and identified the possible formation pathways and sources of the odorants. These key findings can now be used to develop customised solutions for optimising the odour of plastic recyclates, starting at the waste collection stage.