Identification of contaminants in recycled plastic materials
Within the framework of the European Strategy for Plastics, the European Commission launched the Circular Plastics Alliance (CPA) initiative with the aim of taking measures that involve the entire value chain in order to ensure that 10 million tons of recycled plastic are used in new products before 2025 in the European Union market. In this juncture, the identification of contaminants in recycled plastic materials is key in this drive towards the use of recycled materials.
Decontamination strategies for recycled plastic materials
It is necessary to develop decontamination strategies for plastic materials at the end of their shelf life that allows their quality, their processability in all stages of production and their mechanical properties to be maintained.
In this sense, the aim of the decontamination is to drastically eliminate, or at least reduce, the critical substances in recycled plastic materials because they can limit their field of application. This limitation can be due to various aspects:
- Alteration of the material properties (for example, presence of volatiles that can make the material fragile).
- Presence of substances limited by the legislation of the final material.
- Consumer rejection (for example, because of the presence of unpleasant odours).
Analytical identification and quantification techniques
It is very important to control the presence of these contaminants in recycled materials and thus avoid any limitation or rejection. For this reason, there are different analytical techniques that allow for the identification and quantification of the contaminants present in these materials.
The main analytical techniques, in this case, are chromatographic techniques, not only because they allow all types of organic substances to be analysed, but because they allow very low levels of concentration to be reached that other commonly used identification techniques, for example, spectroscopic techniques (IR, UV) or nuclear magnetic resonance (NMR), are not capable of reaching. The volatile compounds or compounds with intermediate volatility will be analysed through gas chromatography, while the non-volatile compounds will be analysed through liquid chromatography. These techniques coupled with mass spectrometry will allow us to carry out the identification of detected substances.
However, other types of compounds such as metals have to be analysed through other techniques, in this case, inductive coupling plasma (ICP-MS or ICP-OES). This technique allows inorganic elemental and isotopic analysis to be carried out that is capable of determining and quantifying the majority of the elements of the periodic table in a linear dynamic range of 6 orders of magnitude (ng/L – mg/L).
Potential contaminants in recycled plastic materials
The combination of the previous techniques has enabled the identification of the potential contaminants which can be present in recycled material. Some of these contaminants may be oligomers, additives or subproducts of their degradation, chemical substances derived from the use of plastic materials, whether appropriate or not, etc.[1]
- Odour and/or taste, these come from the contact of the plastic in its previous use and, many of them are present due to an intentional use: for example, compounds from mouthwashes, personal hygiene or cleaning products or in the case of PET packaging, linoleum, y-terpinene or p-cymene can be found.
- Circular or linear oligomers, coming from an incomplete polymerisation in the production of packaging or generated by the deterioration of the polymer during the recycling process.
- Additives and their products of degradation, for example, phthalates or UV stabilisers.
- Contaminants derived from non-food plastics and those misuse by the consumer, such as flame retardants, solvents from improper use of food packaging, cross contaminations, etc. These types of contaminants are a very important aspect to control when the final use of the recycled material is to contain food.
- Inorganic elements, mainly heavy metals.
If it is certain that these substances have been detected in the recycled materials, their concentration usually reflects a random distribution, since their presence and identification depend on many factors such as the production and recycling processes, the use and waste management phase, analytical methods applied, etc.
Recycled plastic materials in contact with foodstuffs
The food sector is one of the strictest regarding the incorporation of recycled material in its packages. In this sense, the control of contaminant substances that the recycled material could contain is very important. For this reason, the virgin material is doped with known contaminants of different polarity and volatility (toluene, chloroform, chlorobenzene, phenylcyclohexane, benzophenone, methyl salicylate, methyl stearate, etc.), with the aim of covering the whole spectrum of potential contaminants that the recycled material may have and studying their elimination in the recycling process, which is called a challenge test.
Thanks to chromatography, in this case, gas chromatography, the concentration of these contaminants can be determined before and after the decontamination process and thus studies the efficiency of the recycling process. This makes it possible to evaluate compliance to the EFSA (European Food Safety Authority) and to validate the recycling process.
As previously indicated, one very important aspect to control is odour, as it affects whether the material is accepted or reject, not just in the food sector but in many other sectors. The substances responsible for causing odour in plastic materials present a common characteristic which is their elevated volatility. For that reason, gas chromatography coupled with mass spectrometry is a very efficient analysis tool that enables those substances to be detected.
Furthermore, thanks to the mass spectral library, it provides information about the structure of these molecules. For this purpose, the head-space injection mode will be used. In this technique, the sample is heated in a head-space vial and, after the incubation period, the volatiles are collected with a thermostatic syringe and injected into the chromatographic system for analysis. Furthermore, combined with the solid-phase micro-extraction (SPME), it allows the volatiles to be concentrated before injection into the chromatograph to detect even lower concentrations.
In recent years, other techniques like gas chromatography with olfactometric detection have completed the results of the previous technique, since it has enabled the volatile components to be quantified in addition to identifying those that contribute to the material’s aroma.
All of the previously mentioned techniques for contaminant control in recycled materials can be carried out in the AIMPLAS laboratories. Consult with us if you want to evaluate your plastic materials, both virgin and recycled.