SmellStop: Odour reduction as a key technology for the use of recycled post-consumer polyethylene

Use of recyclates in new packaging

The packaging sector processes the largest amount of plastic of any industrial sector. Compared to durable products in the construction and automotive sectors, packaging such as films, cups and bottles have a short lifespan. Currently, most plastic packaging is made from fossil-based virgin materials. This linear use of plastic products leads to high greenhouse gas emissions and significant environmental pollution. New laws and regulations in Germany and the EU require increased use of recyclates in new packaging. Until now, the majority of recyclates have been used for lower value products or thermally utilised. During the conception phase of the project, the “SmellStop” research team identified the strong odour of recyclates as a major obstacle to the widespread use of high levels of recyclates in packaging products, especially during processing. “SmellStop" aims to minimize the odour of PCR materials to increase product acceptance and achieve a sustainable, resource-efficient circular economy in the field of lightweight packaging.

Reducing odour via process engineering

The aim of the project is to reduce the odour activity of the recyclates via process engineering as well as preventing it in the long term by specifically adapting the printing inks. The odour reduction of the post-consumer polyethylene films is planned in several steps: thermal pre-treatment in the infrared rotary drum; targeted degassing in the twin-screw extruder; decontamination under the influence of temperature and water vapour, and optimised process parameters during film extrusion.

The odour is analysed at all stages of the process using gas chromatography-mass spectrometry (GC-MS) to identify the odour-causing substances and to avoid them in the production of the primary packaging. Targeted contamination of virgin material will also be used to draw conclusions about odour-active compounds in recycled materials. The focus is primarily on printing inks, adhesives, and biotic contaminants. The GC-MS analysis is further used to calibrate mobile sensors that can detect odours during production operations and in the product.

Potential for a truly circular economy

The results have a major impact on the circular economy of lightweight packaging. By establishing the developed processes, recyclates can be used in a variety of products.

The Institute of Plastics Processing (IKV) at RWTH Aachen University will initially carry out laboratory-scale processing tests and GC-MS analyses of the odour-causing substances. In scale-up trials, the companies Coperion GmbH (regranulation), Kreyenborg GmbH & Co. KG (thermal pre-treatment in an infrared rotary kiln), Reifenhäuser Cast Sheet Coating GmbH & Co. KG (flat film production) and RKW SE (blown film production) validate the results on an industrial scale. Siegwerk Druckfarben AG & Co. KGaA supply contaminants in the form of printing inks and adhesives and continuously optimizes the formulations to achieve low odour levels in the recycling process. 3S GmbH optimizes and calibrate the sensors for the inline detection of odours in recyclate processing, enabling the quick and easy quantification of odours during processing and in the product.

In total, the entire odour reduction process will be performed twice, with the odour measurement and odour reduction strategies being further optimised in the second run.

Removal of odour-active components from recyclates in the recycling process (Status April 2025)

The SmellStop project is investigating the process-related odour reduction of post-consumer recyclates (PCR) from flexible polyethylene-based packaging (PE). The main focus is on processes after washing and shredding, such as drying, preparation and further processing into new film products. For economic reasons, some decontaminating processes in recycling (such as melt degassing in reprocessing or silo storage) are sometimes omitted nowadays.

The first step after sorting, shredding and cold washing the PCR packaging flakes is to process the material on a twin-screw extruder. This is characterised by its high mixing and kneading effect as well as its good degassing performance. During degassing, plastic is first melted and then subjected to a vacuum. Gaseous components, some of which are odour-forming, are thus released from the melt more quickly and can be removed in a targeted manner. Odour-forming substances may be degradation products, for example, or may have migrated into the product during the life of the packaging.

Tests were carried out on a twin-screw extruder of the type ZSK26Mc from Coperion GmbH to determine the reduction of gaseous components in the recyclate. The quantity of volatile organic compounds (VOCs) contained in the recyclate was analysed by varying the temperature and material shear applied, as well as the number of degassing positions and an entrainer feed in the form of distilled water. The analysis was carried out using gas chromatography with mass spectrometry (GC-MS) of the type GC-2030 with GCMS QP2020NX from Shimadzu. To simulate a realistic outgassing of VOCs in the recyclate and thus a realistic emission situation, the GC-MS samples were outgassed at a temperature of 80 °C for 10 minutes and the released components were measured.

The figure shows two GC-MS spectra of processing trials, which were processed at a temperature of 200 °C, a speed of 300 min-1 and a throughput of 3 kg/h.T he results show that the amount of gas can be significantly reduced. The quantity of odour-active molecules such as butyric acid, styrene, limonene and menthol can also be drastically reduced. Melt degassing is therefore an essential component in the processing of low-odour recyclates.