Omar Khalid
Sorting plastic is a critical step in the recycling process, as different types of plastics have distinct properties and require different treatments. The separation and classification of plastics can be complex, but advancements in technology have improved the accuracy of sorting. Sorting methods include manual, automated, and near-infrared (NIR) systems, each with its advantages and limitations. Understanding the different types of plastics, such as PET, PVC, HDPE, and PP, and their unique characteristics, is essential for effective sorting and recycling. Proper sorting ensures the success of the recycling process and helps reduce plastic pollution.
| Characteristics | Values |
|---|---|
| Sorting Technology | Near infrared (750–2500 nm) systems, optical detection systems, fluorescent markers, pneumatic systems, manual sorting |
| Plastic Type | Polyethylene (PE), Polypropylene (PP), Polyethylene Terephthalate (PET), Polyvinyl Chloride (PVC), High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE) |
| Plastic Properties | Density, flexibility, colour, smell, food contamination, exposure to sunlight, melting point, toughness, adaptability, transparency |
| Plastic Applications | Food packaging, detergent bottles, drink containers, milk cartons, consumer packaging, auto-parts |
| Plastic Contamination | Glue, ink, paint, strong cleaning agents, food contamination |
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What You'll Learn
Sorting plastic by density
Density-based sorting is the most common technique for separating plastic waste due to its high throughput, separation efficiency, and cost-effectiveness. This method takes advantage of the differences in the densities of polymers to separate them into several fractions.
The density of a substance is independent of the size of the sample. Thus, density is a property by which one substance can be distinguished from another. For example, the density of PVC is 1.40 g/cm3 at 20°C, while the density of PP is 0.91 g/cm3.
In commercial firms, the sorting of plastics is often done by flotation, where heavy plastics sink in a brine (salt) solution, while lighter ones float and can be separated. Different brine solutions can be used to separate all the major plastics.
In addition to density-based sorting, sensor-based sorting techniques such as X-Ray Transmission (XRT) or X-Ray Fluorescence (XRF) can also detect differences in density between materials. These methods can be used to remove certain polymers from a mix of plastic waste prior to Near Infrared (NIR) separation. NIR systems are widely used to sort mixed plastics by polymer type and benefit from having density-based sorting steps upstream in the sorting procedure, as they limit the number of different polymer types in the input.
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Sorting plastic by charge
Sorting plastics by charge involves the triboelectric charge, which is a dry technique that does not require the use of water or hazardous chemicals. This method is based on the principle of charging plastics through friction and then utilizing the differences in charge to separate them.
The triboelectric series (TES) is a list of materials arranged according to their ability to acquire a positive or negative charge when they come into contact with other materials. When two materials with different triboelectric properties come into contact and are then separated, an electrostatic charge is created. This charge can be used to separate different types of plastics.
In the case of sorting plastic waste, a triboelectrostatic separator is used to collect positively charged plastic fractions and produce negatively charged fractions for further treatment. For example, in a mixture of acrylonitrile-butadiene-styrene (ABS), polypropylene (PP), and polyvinyl chloride (PVC), the triboelectrostatic separator collects the ABS flakes as the positively charged fraction and the PP and PVC flakes as the negatively charged fraction.
Another example is the separation of low-density polyethylene (LDPE) and high-density polyethylene (HDPE) using a roll-type electrostatic separator. The plastics are charged through particle-to-particle and particle-to-wall collisions inside a rotating tube, and the electrostatic separator recovers the LDPE and HDPE in separate collectors based on their electrostatic charge.
It is important to note that electrostatic sorting is just one method used in the complex process of sorting different types of plastics. Other techniques include manual sorting, near-infrared (NIR) spectroscopy, air sorting, float-sink separation, and selective dissolution, each with its own advantages and limitations.
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Sorting plastic by colour
There are two main methods for sorting plastic by colour: manual and automated. Manual sorting involves human labour and is often used for larger pieces of plastic or when the type of plastic is unknown. However, it is labour-intensive and less accurate than automated methods. Automated sorting techniques, such as optical, magnetic, and near-infrared (NIR) spectroscopy, are typically faster and more efficient, using light and sensors to identify and separate plastics by colour.
Optical sorting has been used effectively in the glass industry for some time and is now becoming the norm in plastic recycling. Automated systems use optical scanners and sensors to identify and sort plastics by colour, density, and other attributes. NIR spectroscopy involves shining near-infrared light onto plastic items and analysing how the light is reflected to determine the resin type. This method is highly accurate and can differentiate between plastics with unique molecular structures, such as PET and HDPE.
In addition to optical and NIR methods, other technologies are being developed to sort plastics by colour. For example, visible reflectance spectroscopy (VIS) has been proposed for sorting polypropylene (PP) plastics by colour. This method uses a "Three-Filter" identification algorithm to recognise colours and sort plastics accordingly.
Overall, sorting plastic by colour is a critical step in the recycling process, and the development of advanced technologies has improved the accuracy and efficiency of this process.
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Sorting hard and soft plastics
Sorting plastics is an important step in the recycling process. Different plastics have distinct properties, and they must be sorted thoroughly before recycling. Sorting to a high degree of purity means polymers can be reused and retain their value.
Hard and soft plastics must be sorted into the same container for plastic waste at home. However, there are exceptions where soft plastics must be handed in at the recycling station, especially if they contain soft PVC, which is extremely harmful to the environment when burned. It is crucial to ensure that the plastic is clean and dry before sorting it into the waste bin. Examples of soft plastics include bubble wrap, wrapping plastic from toilet paper, fruit, and vegetables. Chip bags and coffee packaging should not be sorted as soft plastics due to the presence of metal in their packaging.
Hard plastics, such as milk and juice cartons, can be included in the plastic waste container at home. However, it is important to scrape and rinse these containers to remove any food or drink residue before placing them in the recycling bin.
At recycling facilities, light plastics, such as polyethylene (PE) and polypropylene (PP) films, are separated from heavier and rigid plastics using near-infrared systems that distinguish between the reflectivity and individual wavelength signatures of different polymers. Optical detection systems further separate plastics by colour.
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Sorting plastic by polymer type
Manual sorting is often used for larger pieces of plastic or when the type of plastic is unknown. Automated sorting techniques include optical, magnetic, and infrared sorting. Infrared sorting, for example, detects the spectral distribution of light energy emitted by polymer molecules when exposed to IR light. This distribution is unique to each material type, allowing plastics to be distinguished from one another.
Another automated sorting technique involves the use of flotation tanks. This method separates plastics according to their specific gravity, with lighter materials floating to the top and heavier materials sinking to the bottom. This is particularly useful for separating polyolefin fractions (PP, LDPE, HDPE) from other polymer fractions, as these plastics have densities below 1 g/cm3.
The use of digitalisation and artificial intelligence is also being explored to improve the accuracy and efficiency of plastic sorting. For example, Tomra has employed deep learning, a type of AI that learns by analyzing thousands of images of objects to be sorted. This technology was first used to separate polyethylene (PE)-silicon cartridges and two-component adhesives, ensuring the purity of the desired PE stream.
Overall, sorting plastic by polymer type is a complex process that requires vigilance and the utilization of various techniques to ensure the safe and effective recycling of plastics.
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Frequently asked questions
The seven standard classifications for plastics are:
- PET (used for water bottles)
- Polyvinyl chloride (PVC) (used for water bottles)
- Polypropylene (PP)
- High-density polyethylene (HDPE) （used for milk containers)
- Low-density polyethylene (LDPE) (used for plastic bags)
- Polystyrene (PS)
- Polyethylene (PE)
Sorting plastic is a complex process that involves a combination of cutting-edge technology and human involvement. Automated systems use sensors to identify specific types of plastic, such as PVC, based on their distinct properties. Pneumatic systems then use air jets to divert the plastic into different collection bins. In some cases, manual intervention is required for irregularly shaped or unusually coloured items.
Hard and soft plastic waste can be sorted into the same container at home, as long as the packaging has not been in contact with chemicals such as paint or strong cleaning agents. Plastic that has contained food or drink should be scraped clean before being placed in the plastic container. Milk and juice cartons should also be sorted with plastic containers.
Different types of plastic have different properties, and some cannot be recycled. Sorting plastic ensures that non-recyclable plastics, such as PVC, are separated from recyclable plastics. Additionally, some types of plastic can cause dangerous chemical reactions when mixed, so it is essential to keep them separate.
