If plastic recyclates from packaging waste are to be used to manufacture new products, they must meet high sensor requirements. However, plastic recyclates often have odors, some of which have so far not 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) from different collection systems. More than 60 odor-active substances were identified using combined chemo-analytical methods. They provide information for a targeted strategy to prevent odors.
The study has now been published in cooperation with the Chair for Aroma and Smell Research at FAU and the University of Alicante.
In order to meet the targets for recycling packaging waste required by the new EU packaging directive, new markets for recyclates made of plastic packaging have to be opened up. So that recyclates made from plastic packaging waste can be reused as secondary raw materials in high-quality products, they must not have any odors. In this way, a bad smell in recycled plastic prevents a closed cycle of packaging recycling. There is currently a high rate of reuse of packaging waste, especially in the recycling of bottles made of polyethylene terephthalate (PET).
Focus on LDPE packaging waste
The department of analytical sensors at Fraunhofer IVV deals with the characterization and optimization of plastics and recyclates. The odorous substances in HDPE waste and recyclates from packaging of personal care products and detergents have already been analyzed. Low-density polyethylene (LDPE) is one of the commonly used plastics. Many packaging such as B. Shopping bags consist of it. They are recycled as packaging waste via the collection systems. Based on the investigated post-consumer LDPE shopping bags, the study at hand also focused on the influence of the collection strategy on the sensory impairment of the waste.
Identification of odorants - the basic step to avoid
The identification of the substances responsible for the malodour is the basic prerequisite for taking measures to optimize odors. The majority of the odorants found in the study are typical metabolites of microorganisms. A large number of the odorants therefore had a cheese-like and fecal smell. Among the odorants, carboxylic acids and sulfur-containing and nitrogen-containing components were determined.
Above all, the chemical structure of the odorants found provides information about their origin. Based on this information, the entry paths into the packaging waste and the recycling process into the recyclate can be traced. Depending on the process step at which an odorant cannot be removed or even arises, targeted measures for reducing or avoiding new formation can be derived.
Less odorants when collected separately in the yellow sack
The study has shown that the way the packaging waste is collected already has a decisive influence on the quality of the smell of the shopping bags. The separate collection in the yellow sack has clear advantages. The waste collected in this way had a significantly lower total odor load.
In contrast, the waste fraction, which was collected in the general household waste, had more intense cheese-like, sweaty and fecal smell notes. The higher organic content in the residual waste favors the creation of these microbial breakdown products.
Furthermore, it could be shown that the examined post-consumer LDPE shopping bags from the separate collection, which were washed at 60 degrees, had fewer odorants and a lower overall odor than the unwashed ones.
Methods of odor identification in plastic waste
The team of scientists used methods of analytical sensor technology to identify the smells. As part of the study, the sensory evaluation of the sample materials was carried out by a trained sensor panel. The causative odorants were elucidated using chemo-analytical methods such as gas chromatography olfactometry and two-dimensionally coupled with mass spectrometry. In this way, the chemical structure was determined as well as possible pathways and sources of odorants were derived.
These fundamental findings can now be used to develop tailor-made solutions for the odor optimization of recycled plastics right from the collection.
Odorant composition of post-consumer LDPE bags originating from different collection systems. Andrea Cabanes et al. Waste Management, Volume 104, 1 March 2020, Pages 228-238, https://doi.org/10.1016/j.wasman.2020.01.021.
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