Plastic recyclate produced from waste packaging have to meet high sensory requirements in order to be used for new products, and it often has off-odours, some of which have not hitherto been identified.

The Fraunhofer Institute for Process Engineering and Packaging IVV has analyzed 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 avoiding off-odours. The results of this collaborative study with the Chair of Aroma and Smell Research at the Friedrich-Alexander-Universität Erlangen-Nürnberg and the University of Alicante have now been published. 

Separate collection is beneficial for the sensory properties of the plastic waste and hence the quality of the resulting recyclate. 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 the manufacture of high-quality consumer products. Indeed, the off-odours in plastic recyclates prevent a closed cycle for the recycling 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 the Fraunhofer IVV characterizes and optimizes plastics and recyclates. The odorants in HDPE waste and the recyclates produced from packaging from bodycare products and detergents have already been analyzed. Low density polyethylene (LDPE) is one of the most commonly used plastics. It is widely used for packaging materials such as plastic shopping bags. Via various collection systems these bags end up as packaging waste. Part of the current study on post-consumer LDPE shopping bags concerned the effect of the collection strategy on the sensory impairment of the waste.

Identification of odourants - the key step

Identification of the substances causing off-odours is essential in order to be able to take measures for odour optimization. Most of the odourants identified in the study are typical metabolites of microorganisms. Many of these metabolites had a cheese-like or feces-like odour. The odourants included carboxylic acids and sulfur-containing and nitrogen-containing components. The chemical structures of the odourants gave key insight into their origin. Using this information, the pathways into the packaging waste and via the recycling process into the recyclate were identified. Depending on which process step cannot remove an odourant or even results in a new odourant, targeted measures can be taken to reduce odourants or avoid the formation of new odourants.

Odourant reduction via separate collection in the yellow bag

The study showed that the way the packaging waste is collected has a large bearing on the odour quality of plastic shopping bags. There are significant benefits by separate collection in the yellow bag (Germany's standard way of collecting plastic waste). Waste collected in this way had a significantly lower overall odour. In contrast, the waste fraction collected in the general household waste had more intense cheesy-like, sulfur-like, and feces-like odours. The higher organic fraction in the general waste favours the formation of these microbial degradation products. It was also demonstrated that the post-consumer LDPE shopping bags from the separate collection that were washed at 60 degrees had fewer odourants and a reduced overall odour than the unwashed bags.

Methods for odour identification in plastic waste

The Fraunhofer IVV scientists used sensory analytics to identify the odours. Sensory evaluation of the sample materials was first undertaken by a trained sensory panel. The odourants 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 also possible formation pathways and sources of the odorants to be identified. These key findings can now be used to develop customized solutions for optimization of the odour of plastic recyclates, starting at the waste collection stage.

Read more about Fraunhofer IVV's work to adress the packaging issues of today in this interview.