Maya Ellison
Plastic is a combination of many materials (polymers) with different chemical compounds and additives, such as pigments or fibres, depending on its use. When recycling plastic, it is critical to separate all polymers. Plastics in the conventional industry must have a polymer content of at least 96% to be recycled. This means that the plastic has to be separated to an almost pure product in terms of chemical composition. Several methods have been developed for the separation of different plastics, including X-ray fluorescence, infrared spectroscopy, colour differences, physical techniques, solvents, and chemical markers. Researchers from Aarhus University have developed breakthrough camera technology that can detect and differentiate between 12 different types of plastics, enabling their separation for recycling.
| Characteristics | Values |
|---|---|
| Technology | Hyperspectral camera, near-infrared technology (NIR), density tests (float/sinks in water) |
| Plastic Types | PE, PP, PET, PS, PVC, PVDF, POM, PEEK, ABS, PMMA, PC, PA12, polyolefin, HDPE, LDPE |
| Plastic Composition | Polymers, chemical compounds, additives (e.g. pigments, fibres) |
| Plastic Purity | Requires at least 96% purity by polymer type for recycling in conventional industry |
| Separation Techniques | Hydrocyclone, triboelectric separation, density difference, X-ray fluorescence, solvent usage, chemical markers |
| Symbols/Codes | Use of symbols and barcodes to identify different plastics |
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Density tests
A similar process can be used to separate a mixture of HDPE, LDPE, PP, PS, and PVC. The mixture is introduced into water, and the plastics that float are separated from those that sink. The floating plastics are then introduced into a mixture of water and alcohol, causing the HDPE to sink while the LDPE and PP remain floating.
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Infrared spectroscopy
The use of spectroscopy is becoming increasingly important in plastics recycling to identify and sort a variety of plastics with a high degree of specificity. This is especially useful as very few types of plastics can be blended into homogeneous mixtures, which makes it necessary to cleanly separate the different polymers in the recycling stream.
Infrared (IR) spectroscopy is a powerful qualitative and quantitative analysis tool. When a plastic absorbs IR radiation, the resulting signal is a spectrum that represents its molecular "fingerprint". Different plastic samples produce different fingerprints that enable plastic identification. Fourier Transform Infrared Spectroscopy (FTIR) can quickly identify and sort plastics for recycling.
Near-infrared (NIR) spectroscopy is suitable for polymer detection and is a rapid, non-destructive analysis method that can be applied to automatic online sorting systems. NIR spectroscopy provides information on the vibration of molecular bonds, such as O-H, C-O, C-H, and N-H, which are the basic structures in many organic compounds, including polymers. NIR spectroscopy is capable of separating plastics from non-plastics in mixtures. However, NIR spectroscopy is not suitable for the identification of black polymers because some colouring additives exhibit very strong absorption in the NIR spectral region.
Mid-infrared (MIR) spectroscopy is used to identify black polymers. The MIR spectral range is applied to black plastics due to the reduced penetration depth of radiation into these materials. The first overtone of the C-H bands between 1600 and 1800 nm can be used to identify plastics from domestic wastes.
Raman spectroscopy can also be used to identify unknown compounds under the right conditions. However, it is not well-suited for the identification of black polymers because carbon black absorbs all the Raman excitation laser light.
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Colour differences
The human eye can differentiate between colours, and this ability can be used to separate plastics. The operators in the shredding plants can separate plastics by simply observing them. This method of differentiation is not sufficient for a good separation of plastics by component.
The colour of plastic is determined by the pigments and dyes added during the manufacturing process. Different types of plastics may have different colours due to the use of different pigments and dyes. For example, plastic bottles are often made of polyethylene terephthalate (PET) and are typically transparent or slightly coloured, while high-density polyethylene (HDPE) is used for milk jugs and is usually opaque or coloured.
In addition to manual colour observation, technology has been developed to identify and separate plastics based on colour differences. Hyperspectral camera technology, for instance, can identify and separate plastics with similar chemical compositions but slight structural differences. This technology uses infrared and machine learning to analyse and categorise plastics, allowing for more accurate separation.
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Solvents
Solvent extraction is an effective method for separating different types of plastics. It is a promising technique for recycling mixed plastics, which have traditionally been difficult to recycle due to technical challenges and insufficient profits. Solvent extraction can address these issues, as it is environmentally friendly and potentially profitable.
The process of solvent extraction involves using solvents to dissolve or swell plastics, allowing for the separation of different plastic types. This method takes advantage of the fact that different plastics have distinct solubility properties. For instance, amorphous (non-crystalline) plastics, such as polystyrene and polycarbonate, are more soluble in common organic solvents, while highly crystalline plastics like nylons and polyolefins are not easily dissolved, even by powerful solvents like tetrahydrofuran (THF).
Sequential extraction processes using mixed solvents can achieve high yields and purities, similar to virgin polycarbonates. This approach can also reduce the energy required for recycling. Additionally, the dissolution/reprecipitation technique and supercritical fluid extraction can produce high-quality recovered plastics comparable to virgin materials.
Solvent-based separation is a well-researched area, with numerous studies published over the last four decades. However, there is a lack of a comprehensive summary of solvent extraction methods for plastics. This gap in knowledge includes the specific conditions, such as temperature and pressure, required for effective solvent extraction.
While solvent extraction is a promising technique for plastic separation and recycling, it is important to note that the process may be complex and influenced by various factors, such as the chemical composition of the plastics and the specific solvents used.
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Chemical markers
The chemical marker method is a process that involves incorporating a specific marker for each type of plastic polymer. As plastic is a combination of many materials with different chemical compounds, additives, pigments, and fibres, the use of chemical markers helps to differentiate between the various types of plastics during the recycling process.
The chemical marker method is particularly relevant for the packaging industry, where the additives incorporated must be completely harmless. This method allows for the identification and separation of different polymers, ensuring that plastics can be recycled effectively.
One example of a chemical marker is the use of cyclohexanoxylene as a solvent. When used, it causes the separation of three distinct phases: one of polystyrene (PS), another of polyvinyl chloride (PVC), and a third of polyolefins. Each phase achieves purities ranging from 96% to 99%. The polymer is then recovered through precipitation using a precipitating agent.
Another proposed method involves the use of specific barcodes for each type of plastic polymer. However, due to the varying sizes and geometries of plastic items, implementing this method has proven challenging. In practice, the identification of the seven most commonly used plastics and their respective symbols has been implemented. This allows for quick and safe separation during the recycling process.
The development of new camera technology, such as hyperspectral cameras that operate in the infrared area, has revolutionized the separation of plastics. This technology, in combination with machine learning, enables the accurate analysis and categorization of different types of plastics, directly on the conveyor belt. This breakthrough has significantly enhanced the accuracy of plastic separation and increased the potential for recycling.
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Frequently asked questions
There are several traditional methods of separating plastics, including hydrocyclones, triboelectric separation, density difference, X-ray fluorescence, infrared spectroscopy, colour differences, and chemical markers.
There are several types of plastics, including PE, PP, PET, PS, PVC, PVDF, POM, PEEK, ABS, PMMA, PC, PA12, polyolefin, and polystyrene.
To separate plastics at home, you can separate them based on their appearance, such as their colour or the number printed on them. You can also separate them based on whether they are recyclable or not. For example, plastic bottles are usually made with recyclable materials, while plastic food trays, egg cartons, and cups do not need to be recycled.
