Chloe Zhang
Plastic is a defining material of the modern world, used in everything from food packaging to children's toys. However, plastic waste has become a significant environmental concern, with its detrimental effects on ecosystems and human health. Recycling plastic waste is crucial to mitigating these issues and conserving resources. The process of recycling plastic involves several steps, from collection to sorting, shredding, melting, and finally, reformation into new products. Mechanical recycling is the most prevalent form of plastic recycling, but other methods, such as chemical recycling and homemade plastic-forming, are also gaining traction.
| Characteristics | Values |
|---|---|
| Plastic recycling methods | Mechanical recycling, feedstock recycling, chemical recycling, burning, landfill |
| Plastic recycling process | Sorting, washing, shredding/grinding, melting, extrusion, reforming |
| Plastic recycling challenges | Sorting by colour and polymer type, contamination, cost, incompatibility of additives, degradation |
| Plastic waste impact | Environmental pollution, microplastic release, health risks, climate change |
| Plastic waste reduction strategies | Reuse, recycling, reduction, legislation, lobbying, taxation |
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What You'll Learn
Sorting and cleaning
The sorting process begins with the collection of plastic waste by local authorities or independent waste collectors, who transport it to transfer stations or sorting installations. At these locations, the plastic waste is torn open from their bags and then moved through an array of sensors and optical scanners that identify and categorize the plastic items based on factors like density, colour, and behaviour when exposed to specific wavelengths of light. Pneumatic systems then use strategically placed air jets to divert the plastic items into different collection bins. While automated systems have significantly improved the accuracy of recycled plastic sorting, manual intervention is sometimes necessary for irregularly shaped or unusually coloured items that pose a challenge for the sensors. Skilled workers play an important role in ensuring that the correct sorting decisions are made and that non-recyclable plastics, such as PVC, are identified and separated.
After sorting, the plastic waste undergoes an extensive washing process to ensure that impurities and the last contaminants are removed. Plastic waste is generally cleaned with hot or cold water, with the assistance of caustic agents or detergents. Washing is important because any contamination could ultimately affect the quality of the final recycled product. For example, plastic heavily contaminated with food waste may not be picked up by the near-infrared sensors (NIR), leading to its disposal even though it could have been recycled. Rinsing plastic waste before disposal is beneficial and can also help prevent odours from recycling containers.
The washing process is often integrated with the next steps in the recycling process, such as shredding or granulating the plastic into smaller pieces. The washing step may be followed by a sink-float sorting step, and it is then usually followed by further sorting and processing steps. Sorting and cleaning are just the first steps in the plastic recycling process, which also includes steps such as extrusion (melting the plastic and forcing it through an extruder to form pellets or bars that can be sold to manufacturers) and moulding the plastic into new products.
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Shredding and melting
Shredding is the first step in recycling rigid plastics, which are large, voluminous plastic residues that are not recyclable at the curbside. These include plastic toys, sandboxes, kiddie pools, canisters, bowls, and laundry tubs. The plastic is fed into a shredder, which can be a single-shaft shredder in the case of the WEIMA shredder, and is uniformly shredded to a homogeneous particle size. The size of the material is defined by the screen size used, which is usually 40-80mm for hard plastics. The material is then transported to the next step, which can be washing, drying, or extrusion.
Extrusion is the process of melting down the shredded plastic and forcing it through an extruder. The plastic is cut as it comes out of the extruder to form pellets, which are sold to manufacturers. The melting and reforming of plastic into other items can cause polymer degradation at the molecular level, and the waste must be sorted by colour and polymer type before processing, which can be complicated and expensive.
The pellets produced from the extrusion process are then purchased by manufacturers and incorporated into new products. The recycled plastic can be used for compression moulding, highly reinforced composites, powder coating, 3D printing, and other demanding applications. The recycled plastic can also be used to create pellets that are then sold as raw materials for manufacturing.
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Forming pellets
The process of forming recycled plastic pellets involves several steps. First, the raw material, which comes from discarded plastic products, needs to be cleaned to remove any residual pollutants. This is followed by crushing or shredding the plastic into smaller pieces to meet the processing standards of the pellet-making machine. The plastic is then fed into the pellet-making machine, where it is mixed and re-plasticized under high temperature and pressure. This process, known as extrusion, involves melting the plastic and forcing it through an extruder. As the plastic exits the extruder, it is cut into small pieces called pellets. These pellets are then cooled through a cooling system to harden them.
The recycled plastic pellets are small granulates that serve as the primary material in various production processes. They are used in machines such as Blown Film Machines, Injection Machines, and Plastic Molding Machines. These machines create products like plastic films, bottles, containers, bags, and more. The pellets are also used for injection blow moulding and the production of garbage bags, completing the recycling circle.
It is important to note that the recycling process may vary slightly depending on the specific technology and equipment used. Additionally, the quality of the final product is influenced by the pellets, highlighting their significance in the plastic production process.
Overall, the formation of recycled plastic pellets involves cleaning, crushing, melting, extrusion, cooling, and pelletizing steps. These pellets are then utilised in various manufacturing processes, contributing to the recycling and reuse of plastic materials.
To ensure the effectiveness of the recycling system, it is crucial to have collaboration between different organisations, including the public, manufacturers, designers, brands, and retailers. By working together, we can promote sustainability and reduce the environmental impact of plastic waste.
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Moulding into new products
Once the plastic has been shredded into flakes or pellets, it can be moulded into new products. This is done through a process called injection moulding, which involves injecting molten plastic into a mould cavity under high pressure, where it cools and solidifies, taking the shape of the mould. The moulds consist of two halves, the core and the cavity, which are precisely machined to create the desired product shape. Injection moulding is widely used due to its versatility, cost-effectiveness, and ability to produce intricate designs.
Blow moulding is another manufacturing process used to create hollow objects such as bottles, containers, and automotive components. It involves melting plastic resin and then inflating it into a mould cavity using compressed air. The compressed air expands the plastic against the mould walls, allowing it to cool and solidify into the final product shape. Blow moulding offers advantages such as cost-effectiveness, high production rates, and the ability to create complex geometries.
Compression moulding is a technique used for moulding thermosetting plastics. It involves placing a precise amount of plastic material into the mould cavity and applying pressure to evenly distribute the material. Heat is then applied to cure the plastic into a solid state. Compression moulds are ideal for producing strong, intricate plastic components with excellent dimensional accuracy and surface finish.
Rotational moulding, or rotor-moulding, is used for creating sizable, hollow plastic parts. The manufacturing process starts with a heated mould that rotates on two axes. Liquid plastic material is poured into the mould, and the rotation spreads the plastic evenly across the interior surfaces.
Localised plastic recycling is a movement to decentralise the recycling of plastic waste, with small businesses collecting and sorting plastic waste on a community scale before turning it into new products. This movement is led by organisations like Precious Plastic, which uses small-scale injection moulding machines to produce new products from shredded plastic waste. However, creating moulds for this process can be challenging and expensive. To address this, 3D printing has been explored as a more accessible and affordable option for mould-making, allowing for greater experimentation and innovation in the types of products that can be created from locally recycled plastic.
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Environmental impact
The environmental impact of recycled plastic has both positive and negative aspects. Recycling plastic waste reduces fossil fuel utilisation, power consumption, and landfilling. This leads to a decline in the emission of greenhouse gases, lowering carbon footprints. Reprocessing 1 ton of plastic can save up to approximately 130 million kilojoules of energy. Recycling also contributes to the global economy and creates jobs.
However, the recycling process can also be toxic and harmful to human, animal, and environmental health. Sorting plastic is more complicated than sorting other materials because it comes in a greater range of forms. Errors in sorting can lead to material with inconsistent properties, rendering it unappealing to industry. Even the most efficient filtration systems cannot prevent the release of microplastics into wastewater. Additionally, almost all recycling involves melting and reforming plastic into other items, which can cause polymer degradation at the molecular level.
Furthermore, the global recycling rate for plastic is only 9%, which has been ineffective in offsetting the impact of rising global plastic production of more than 400 million metric tonnes annually. The use of plastic is anticipated to triple by 2060 compared to 2019, which will create a significant unintended environmental leakage.
Some companies have started to reuse waste to make new products from old plastic, textiles, and other materials that would otherwise fill landfills and oceans. Examples of products made from recycled plastic include portable chargers, weighted blankets, and chairs.
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Frequently asked questions
The first step in recycling plastic is to collect discarded plastic materials. This can be done through various channels, such as curbside collection, drop-off centres, or specialised recycling programs.
After collection, the plastic waste undergoes sorting, shredding, and cleaning to remove contaminants such as dirt, labels, or residual content.
The shredded and cleaned plastic is then melted down and formed into small pellets through a process called extrusion. These pellets become the raw material for manufacturing new products.
Recycled plastic can be used to create a wide range of products, including plastic bottles, packaging materials, containers, furniture, textiles, and more. Additionally, plastic bags can be reused and melted down at home to create new items.
