If recycled polymers recovered from packaging waste are used for the manufacture of new products, they must meet stringent organoleptic requirements. However, recycled products often have unpleasant odors, many of which cannot be identified. The Fraunhofer Institute for Packaging Technology (IVV) analyzed the organoleptic properties of used LDPE bags obtained from various collection systems. Using combined methods of analytical chemistry, the researchers were able to identify more than 60 substances with a pronounced odor. These results will help implement technologies aimed at eliminating unpleasant odors.
In order to achieve the goals set by the new EU directive on the recycling of packaging waste, it is necessary to develop new markets where recycled materials can become a popular raw material. To be reused for high quality products, the secondary must be free from unpleasant odors, which can hinder the creation of a closed cycle in the packaging sector. At present, recycled granules obtained from PET bottles have high rates of reuse.

Focus on LDPE waste

The Organoleptic Analysis Division of the Fraunhofer Institute for Packaging Technology and Packaging is engaged in the study of the properties of plastics and recycled materials and the optimization of their use. The institute has already analyzed odoriferous substances in HDPE waste and recycled products from packaging of detergents and body care products. Recently IVV, in cooperation with the Department of Fragrance Studies at the Friedrich-Alexander University of Erlangen and Nuremberg and with the University of Alicante, carried out the same work for LDPE.
One of the most commonly used polymeric materials is also high density polyethylene (LDPE). A wide variety of packaging is made from it, for example, shopping bags. After entering the waste collection system, they are sent for recycling. Using the example of used LDPE shopping bags in the IVV study, the influence of the separate collection strategy on the indicators of the organoleptic properties of materials was also shown.

Identification of odorous substances

The identification of substances causing an unpleasant odor is the main prerequisite for taking measures to improve the odor of raw materials and products. Most of the odor substances identified in the IVV study are common metabolic products of microorganisms. This is why waste often has a cheesy and fecal aroma. Also, carboxylic acids, sulfur-containing and nitrogen-containing components were identified. Information about the origin of the detected odorous substances is provided primarily by their chemical structure. Based on this data, it is possible to trace the path of the packaging from getting to waste through the recycling process into secondary material. Depending on at which stage of the process the removal of the odor substance is not possible or there is a risk of a new odor developing, targeted measures can be taken to combat these phenomena.

Separate collection in “yellow bags”

Research has shown that the way in which packaging waste is collected decisively affects the odor quality of shopping bags. The concept of separate collection in “yellow bags” provides significant advantages. Waste collected in this way has a much less pronounced odor. Conversely, the fractions of waste from general household waste containers have a more intense smell of cheese, sweat and feces. The higher organic matter content of the residual waste favors the formation of microbial decomposition products. In addition, the study showed that separately collected LDPE shopping bags that were washed at 60 ° C had fewer odors and a lower overall odor than unwashed bags.

Odor identification methods

To identify odors, the team of scientists used methods of organoleptic analysis. Odor-causing substances have been determined using analytical chemistry methods such as gas chromatography-olfactometry combined with mass spectrometry. Thus, the chemical structure and possible sources of odorous substances were identified. This data can now be used to develop customized solutions to optimize the odor of recycled plastics even before waste is collected.