Identifying off-odours in polymers
The material applications of odour-contaminated plastics are limited. In order to use them in the manufacture of new plastic products, their odour levels must be minimised. Analytical methods combined with sensory evaluation can help to identify odour-causing substances and to assess the efficiency and effectiveness of purification processes.
What smells good goes down well, whether it’s a meal, a car or the bloke next door. Stinkers don’t stand much chance of success or career advancement – well, at least not when it comes to waste plastics. This phenomenon is attributable to the property of polymeric materials to adsorb various foreign substances. For instance, they are highly attractive to volatile organic compounds (VOCs), which can accumulate on plastic surfaces.
Due to their volatility, VOCs also include odour-causing substances – that is, compounds that smell pleasant and those that do not, known as off-odours. This means that polymeric materials used to manufacture packaging for cheese, shampoo or fabric softener, to name but a few examples, can take on the odour of the packaged goods either completely or partially.
The resulting odour contamination impairs the further use of the plastic in question. This means that material purity and other physical parameters are not the sole deciding factors for material recycling; the unwanted odour of the resulting recyclate also plays a role. It plays a role that should not be underestimated in the use of post-consumer plastics in the manufacture of new products.
Incidentally: In 2017, around 6.15 million (million) tonnes (t) of plastic waste were generated in Germany, 84.5 per cent of which arose after the plastic had been used, and 15.5 per cent during its manufacture and processing. 2.87 million t (46.7 per cent) of the total plastic waste was recycled into new products or raw materials. 3.24 million t (52.7 per cent), in turn, were used for energy recovery: 2.14 million t of this in waste-to-energy plants, whilst 1.1 million t were used as substitute fuel for fossil fuels, for example in cement works or power stations. The recycling rate for waste from plastics production and plastics processing stood at 67.6 (90.7 per cent). However, only 42.2 per cent of plastic waste from private households was recycled, and only 34.3 per cent of that originating from commercial end-use. The reason for this difference is that plastics in industry are usually very clean and are collected by type. In households and many commercial enterprises, on the other hand, they become contaminated and are also mixed together.
Source: Federal Environment Agency, Plastic Waste, https://www.umweltbundesamt.de/daten/ressourcen-abfall/verwertung-entsorgung-ausgewaehlter-abfallarten/kunststoffabfaelle)
The odour of a plastic is not influenced solely by the packaged contents, but also by other factors, such as the method of disposal. Plastics that are produced as surplus or offcuts during manufacture and processing generally contain little to no foreign odours; depending on the material type and purity, these can be recycled more or less directly on the production line and fed back into the manufacturing and processing cycle. Plastics that have been disposed of as ordinary household waste or originate from commercial end-use, on the other hand, prove to be problematic in terms of odour. There is a high probability that these polymer materials have been exposed to biological or chemical decomposition processes, during which odour-causing substances may be emitted and contaminate the polymers. In order to recycle odour-contaminated plastics as raw materials, cleaning and deodorisation steps are often necessary.
Using aroma analysis techniques
Whether the aim is to assess the odour contamination of used plastic packaging, optimise cleaning processes for waste plastics or recycled materials, or provide recommendations for efficient and effective waste separation: the foundation always lies in the correct analytical strategy and a suitable analytical technique. The Fraunhofer Institute for Process Engineering and Packaging (IVV) in Freising possesses both strategy and technology, as well as extensive expertise in polymer analysis. The results of the IVV’s research are documented in various prestigious scientific publications [1–4].
But what is the correct approach to odour analysis in plastics? Dr Philipp Denk from the IVV explains that guidance can be drawn from the determination of flavour compounds in foodstuffs, a method which can also be applied to other areas of application. A brief overview of the various steps carried out in the laboratory clarifies the process:
Step 1: Sensory classification of the samples. A sensory panel consisting of several people, trained to recognise odours and describe them precisely in words, carries out a sensory evaluation of the samples as part of a ‘classification test’ in accordance with DIN ISO 8587:2010-08. “This makes it possible to initially describe an overall olfactory impression, which is important for the subsequent work,” explains Dr Philipp Denk. This step is followed by gas chromatographic analysis of the samples, combined with a further sensory evaluation using olfactometry (O) via a special port on the GC for sniffing with the nose, in order to characterise the individual odour compounds in detail. To do this, however, the odour compounds must first be extracted from the polymer (see Step 2).
Step 2: Extraction of VOCs from the polymer matrix. Using a suitable solvent, such as dichloromethane, the components – which are often present only in trace amounts in the sample but are highly potent in terms of odour – are extracted from the polymer matrix and separated from the non-volatile constituents using a solvent-assisted distillation technique (Solvent-Assisted Flavour Evaporation, SAFE) under high vacuum. “A gentle approach is essential to prevent ‘unstable odour-active compounds from being degraded or new ones from being formed during processing, which would distort the overall picture of the analysis’,” reports Dr Philipp Denk. The resulting extract is filtered and concentrated, and an odour extract dilution analysis (GEVA) is carried out.
Step 3: Odour extract dilution analysis. It is well known from aroma analysis that, although many different volatile compounds may be present in a sample, often only a very small number of substances determine the characteristic odour. To identify these few relevant components, a sequential series of dilutions of the extracts is separated by gas chromatography and assessed by a sensory expert using olfactometry, that is, via a ‘sniffing’ port (e.g. ODP) at the end of the separation, by a sensory expert using the human nose. ‘Odour-active volatile components can be identified in this way and classified by comparison with reference substances,’ reports Dr Philipp Denk.
Step 4: Identification of odour-causing compounds using 2D-GC-MS/O. The final identification of the compounds particularly relevant to the odour is carried out using gas chromatography–mass spectrometry coupled with olfactometry (GC-MS/O) and, in the event of signal overlap or substances, by means of two-dimensional gas chromatography-mass spectrometry combined with olfactometry (GC-GC-MS/O).
Combining instrumental analysis and sensor technology
After just a little training in olfactory competence, combined with the appropriate technology and a little experience, users can begin to identify compounds solely on the basis of their odour, says chemical engineer Thomas Albinus, an applications chemist and GC-MS/O specialist at a leading supplier of high-performance olfactory detection accessories for GC/MS-O. In its training courses, the company provides both theoretical and practical guidance on how to successfully get started with the analytical technique and on how to use one’s sense of smell to determine and identify the sources of odours. However, it does require in-depth study of the subject to master it, for example when assessing olfactorily contaminated plastics.
The various substances detected, for example in plastic packaging waste, exhibit a wide range of different odours. “These include molecules with mouldy, cheesy or floral odours,” explains Dr Philipp Denk. According to the IVV expert, some of the strongly odourous components have been identified in plastic for the very first time. Furthermore, it has become apparent that, in addition to odour-causing substances from the previous, i.e. packaged, contents such as foodstuffs or cleaning products, other processes can also lead to these unpleasant odours. These include not only microbiological decomposition processes but also ageing processes to which plastics are exposed, as well as the breakdown of minute production residues such as solvent residues.
However, contamination with off-odours is, for example, less pronounced when post-consumer plastics have been sorted by type throughout their life cycle and are generally only slightly contaminated, as is the case, for example, with PET drinks bottles, which remain almost entirely in the cycle for a certain period and can be reused for comparable purposes (keyword: upcycling). Simply collecting packaging waste in the ‘yellow bag’ or ‘yellow bin’ helps with the recovery of polymer materials that are suitable for further use, says Dr Philipp Denk. However, the resulting recyclates are generally not entirely free of unpleasant odours. This means that their reuse requires deodorisation. Using the findings obtained at the IVV, it is possible to evaluate purification steps and to develop targeted strategies for preventing odours in recycled plastics, and to establish these in industrial practice.
What matters when working with the olfactory detector
Effective sensory smelling is a skill that must be learnt. This requires, not least, a suitable technique – which users can, however, acquire through workshops [5] – as well as experience: it is not merely a matter of ‘analytical’ aspects, but also of a certain level of ease of use: With the ‘Olfactory Detection Port 4’ (ODP 4 [6]), for example, the working position can be freely chosen and fixed, which has a positive effect on ergonomics during the sensory evaluation of fragrances using the nose in parallel with MS detection. The new design allows for more comfortable working, not least due to a low surface temperature inherent to its construction, even without a glass funnel, which enables more sensitive perception at lower make-up gas flow rates. The nose positioning mark is adapted to individual requirements: the nose can be positioned optimally, a feature appreciated not only by those who wear spectacles. Furthermore, the ODP 4 offers the advantage of trouble-free transfer of high-boiling compounds. The ‘Sniff&Trap’ option allows the targeted capture of analytes at any point in the chromatogram onto a thermal desorption tube. The scope of delivery includes the Olfactory Data Interpreter (ODI), which enables the rapid and reliable classification and identification of odours, and is also helpful in the evaluation of GEVA (AEDA) data, panel data and cumulative olfactograms.
References
[1] Miriam Strangl, Tanja Fell, Martin Schlummer, Andreas Mäurer and Andrea Büttner, ‘Characterisation of odorous contaminants in post-consumer plastic packaging waste using multidimensional gas chromatographic separation coupled with olfactometric resolution’, Journal of Separation Science 40 (2017) 1500–1507, DOI: 10.1002/jssc.201601077
[2] Miriam Strangl, Martin Schlummer, Andreas Mäurer and Andrea Büttner, Comparison of the odorant composition of post-consumer high-density polyethylene waste with corresponding recycled and virgin pellets by combined instrumental and sensory analysis, Journal of Cleaner Production 181 (2018) 599–607, https://doi.org/10.1016/j.jclepro.2018.01.137
[3] Miriam Strangl, Eva Ordner and Andrea Büttner, ‘Evaluation of the efficiency of odour removal from recycled HDPE using a modified recycling process’, Resources, Conservation & Recycling 146 (2019) 89–97, https://doi.org/10.1016/j.resconrec.2019.03.009
[4] Andreas Cabanes, Miriam Strangl, Eva Ordner, Andreas Fullana and Andrea Büttner, ‘Odorant composition of post-consumer LDPE bags originating from different collection systems’, *Waste Management* 104 (2020) 228–238, https://doi.org/10.1016/j.wasman.2020.01.021
[5] Analysing with all the senses: an introduction to olfactory detection, workshop from 22 to 24 September 2020, details: http://www.gerstel.de/de/workshop-odp.htm
[6] The Olfactory Detection Port (ODP), http://gerstel.de/de/olfactory-detection-port.htm