INNOVATION | ODOUR REDUCTION


tometry (2D-HR-GC-MS-O). In total, 32 odourous substances were identified. These odourants can stem from fragrances used in cosmetics and washing/cleaning agents that are not thoroughly eliminated during standard recycling steps and are carried over in the recycled polymer. “Comparative odour extract dilution analysis


Above: The GC-O process at Fraunhofer IVV


ble requires both a selective and sensitive analytical approach, she says, as potent molecules are often present at very low concentration levels. “This is especially challenging in plastic matrices where the target analytes are often co-eluting with other polymer-specific non-odourous volatiles. Conse- quently, standard methods such as general screen- ing of VOCs by gas chromatography-mass spec- trometry (GC-MS) are not comprehensive enough for full clarification of such odours,” says Strangl. “We take a further step by using a comprehen-


Right: Mixed recyclates


sive approach that combines sensory evaluation from a trained human panel with instrumental olfactometric analyses, GC-MS, coupled with an olfactometric detection, GC-MS-O. Because the human nose is a particularly sensitive detector, this smell-guided chemo-analytical technique allows the detection of powerful odourants having a low perception threshold for the human nose, even though their concentration might fall beneath the detection limit of GC-MS instruments.” High-density polyethylene (HDPE) now plays an essential role in many packaging applications, meaning that a recycling solution for this material is particularly sought. “Odourous contaminants are a common problem when looking to achieve high-quality HDPE recycled goods,” says Strangl. “By knowing the differences in the odourant composition of waste, recycled pellets and virgin pellets, we can gain an insight into the odour sources and formation pathways, which supports the development of optimised avoidance strate- gies. For this reason, the odour of post-consumer HDPE waste, resulting recycled HDPE pellets and virgin HDPE granules was analysed by comprehen- sive sensory analysis, as well as instrument analysis using high resolution gas chromatography-olfac- tometry followed by two-dimensional high-resolu- tion gas chromatography-mass spectrometry-olfac-


16 PLASTICS RECYCLING WORLD | July/August 2018


(cOEDA) revealed that the conventional recycling process applied for this study resulted only in a slight reduction of odour contamination. However, it was also demonstrated that no new odourants were generated during the recycling process to any relevant extent. In conclusion, further opti- mised strategies for odourant removal from post-consumer recycled HDPE material are still required.” (See: Strangl M, Schlummer M, Maeurer A & Buettner A. J Clean Prod. 181, 599-607 (2018)). According to Fraunhofer IVV, the majority of the


more than 30 odourants identified belonged to the group of terpenes or phenylpropanoids, which usually cause citrus-like, soapy, fruity or floral odours. Since these substances could only be detected in the HDPE waste and recycled pellets, migration processes of fragrances used in pack- aged cosmetics and washing/cleaning agents are assumed to be mainly responsible for the differ- ences in the total odour between the HDPE waste/ recycled pellets and the virgin HDPE granules. In contrast, the moieties of cheesy-smelling carboxylic acids and fatty-smelling unsaturated aldehydes were detected in all samples. Therefore, in addition to migration, degradation processes of the polymers or of the goods in the packaging might add compounds that affect the specific smells of the specimen materials (Strangl M, Fell T, Schlum- mer M, Maeurer A & Buettner, A. J Sep Sci. 40, 1500-1507, (2017)). The Fraunhofer IVV approach to odour assess- ment starts with the smell of a sample material being evaluated by the trained sensory panel. This


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PHOTO: FRAUNHOFER IVV


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