This section of the document will cover all the stages of the recycling process in detail, and move onto what exactly can be created with the finished materials after the recycling process has been completed.
Stages of the Plastic Recycling Process
The plastic recycling process is highly diverse, detailed, and a multistage system. This process is an integral part of this research and will be covered extensively. Plastic recycling has become largely in demand, as the heavy strain of plastic manufacturing has placed an large burden on our environment, and resources. Recycling has long gone overlooked, and in some instances, poorly implemented. This research, and vision of this written work, aims to change this pitfall, and pave the way for the future of the plastic recycling industry.
Stage 1: Collection
The first stage in the plastic recycling process is collection. This phase involves plastic being collected at various points, to be recycled in the future. This stage has a large weakness, and it is completely reliant on individuals to place their plastics in the correct disposal areas to be collected. Unfortunately, this is not the most reliable, and a testament to this is that plastic waste is rather prominent in our environment, and in garbage landfills where it is mixed with other garbage. There are various programs in place, government related, that do entail having collections appearing at residencies, or places of business to help in the collection process, but this can only go so far and is still not entirely reliable.
There are many different types of collection programs that are implemented today:
- Curbside Recycling – This method is regarded highly as a low risk method, which has high potential to reduce waste volume through consumer convenience. Plastic material is collected into large bins outside of residencies, which will await pickup from contracted companies. This program often yields high amounts of recycled plastics in compared to other methods and requires the resident to manually separate the plastics from their garbage.
- Drop-off Recycling – This method required large community recycling centers for residents to travel to in order to deposit their materials. These collection facilities are usually located in common areas like parking lots, or malls. The collection containers located at these centers have different locations for plastic material type.
- Buy-back centers – These centers are generally operated by 3rd parties and offer funds to consumers who drop off plastic. These centers usually have specific instructions for the consumer to sort plastic by type before depositing and receiving funds. These centers usually have an unpredictable yield, even with an incentive, and this is due to a convenience factor. A pro to this method is that the plastic is usually well sorted as a result of the financial incentive and can allow the 3rd party to immediately begin the recycling process with minimal sorting needed.
- Deposit or Refund Programs – These can usually be found in local grocery centers. These programs allow the user to place their bottles into machines which will give them partial returns, or in some cases, full returns for their product as denoted on the plastic bottle.
Stage 2: Sorting
After the plastic collection process has been completed, the plastic is then transported after pickup, to a local recycling facility where it will be sorted through by resin code. Machines at the facilities will also sort plastics based upon their properties, the sorting method, and preference which varies per facility depending on what exactly is to be produced after the plastic is broken down. Plastic can be sorted in a multitude of ways, not limited to: type of plastic (resin code, material originally manufactured with), color, or how it was manufactured. Sorting can be either done manually, by machine, or even both if needed.
The plastic sorting process is crucial for the remaining steps, and for good reason. All plastics, as mentioned previously, must be processed with varying methods depending on the resin, and how it was manufactured. This fact remains true within the recycling process, as when the recycled plastic is moved to manufacturing, improperly mixed plastics can generate weaknesses with the final product and defects. Further increasing importance, some facilities are only capable of handling specific classes of plastic making it crucial to have the correct processes in place, in order to avoid having to restart the resorting process or risking a defective product. Sorting through the plastic also helps to remove any metals, glass, paper, and various other materials that could harm other stages down the process flow.
A company located in France named Paprec, has created their own automated sorting system which removed plastic bottle caps from bottles. First, Paprec’s system takes the bales of collected plastic from selected sorting, which are placed on a conveyor and split by machine. Once the bottles have been jarred from the bales they are rotated onto a toothed shaft. This shaft then separates the bottles facilitating their paths to their respective conveyor belts. Within this system, Paprec has also incorporated two large magnets (Overbands) which removed further pollution from the plastic. There are many methods to be considered for the sorting process as well:
- Manual Sorting – plastics, and unwanted particles are moved, or removed by human operators. This has plastics from various polymer family trees placed onto a conveyor belt and handled by the operators.
- Post Grinding Waste Sorting – Usually applied to PVC plastics which are ground before being sorted, rather than after. The PVC is ground using a multitude of blades inside a cylinder, with a grille at the bottom reducing the granules to a desired size.
- Optical Waste Sorting – separation and pollution removal are refined, utilizing capillary action for higher quality. This method is useful to also help differentiating between clear, light blue, dark blue, green, and various other colored PET plastics.
- Floatation Waste Sorting – Plastics are fed into underwater grinders, which reduce the plastics to flakes, and separated by buoyancy.
- Densitmetric Waste Sorting – This sorting method uses a densimetric table to separate the heavy fraction of material, from the light.
There are also automatic sorting systems that allow the plastics to be easily distinguished from one another. A system called Fourier-transform near-infrared, or FT-NIR, spectroscopy for polymer type analysis, while utilizing optical color recognition cameras to sort belts of plastic into clear, and colored groups. There are various other detectors that can be utilized in automating the sorting process. X-Ray detection is yet another method used for sorting out PET containers by detecting the 59% chlorine within them, skewing their weight as well. Ballistic separators, hydrocyclones (can be in multiple stages), and air-classifiers allow flexible plastics, packaging, and labels to be collected (How Is Plastic Recycled: Step by Step.).
Sorting machines involved within the sorting stage are highly complex and must come equipped with a multitude of features to assure the sorting process is completed efficiently. To operate at such a high level sorting machines are equipped with: sensors as discussed before, ejectors which use precision air injection units to group resins by type, computing systems that supply the algorithms to identify and sort different polymers while offering a control interface, and user interfaces which allow operators to interact with the machinery (How Is Plastic Recycled: Step by Step).
Stage 3: Washing
Washing is the 3rd plastic recycling process stage. Washing plastic is essential to remove any impurities on the resin, and anything resting on the plastic, which is not made from plastic itself and will interfere with the remaining stages of breakdown, and re-manufacturing. This will include any sort of labeling, food particles, adhesives, or any other foreign substance that could potentially cause errors to future products and aren’t meant to be handled by the machinery. Any sort of substance that can be removed will, undoubtedly, cause the final manufactured product to have instabilities in its design.
Different washing processes must be utilized depending on the state of the plastic, and class. For heavily contaminated, or customized materials with adhesives for example, plastics are placed in a hot boiler or steamer using hot water mixed with caustic (or a like solution) to remove glues, grease, oil, food or other residues from the surfaces. Once these materials are broken down in the hot wash, the plastics are then passed into a cold friction washer to further scrub the flakes. At this stage, the resin is now ready for drying; it is imperative that all washed flakes are dried before the resizing or melting stage commences, as this will leave abnormalities in the final product (Nearsay.com. (2018)).
Machines that are used in the washing stage are: water baths, friction washers, or washing lines. Washing lines spray a continuous hot spray over the plastic material which is enough to remove any debris, labeling, or materials that are not needed for the remaining stages. Disinfecting agents and detergents are applied to either machine to decrease the chances of contaminated material appearing further down the line (PET Bottle Recycling).
Stage 4: Resizing (Shredding)
This stage within the recycling process is an integral part of remanufacturing the plastic into reusable materials. Here is where, after the collected material has been properly sorted and cleaned, the plastic will be shredded or granulated into small particles. This process can usually be accomplished by various plastic shredders or grinders. This is usually done by transporting the plastic into large funnels, or hoppers lined with shredding knives, or metal teeth, that grind the particles into small pellets. During this stage, the surface area of the plastic will increase, which allows it to become more malleable, processable, and transportable. This stage can be utilized as another opportunity to identify and eliminate any plastic waste that has made its way into the 4th stage, and can be done with special equipment like metal detectors or even magnets (Nearsay.com. (2018).
Machines that are essential for this stage are single or multi-shaft shredders, and granulators. Multi-shaft shredders resize the plastic using scissor cutting techniques with a series of rotating blades; a con to multi-shaft shredders is that they tend to be inconsistent with the sizes that the plastic is being shredded into. Single shaft performs in a similar fashion but in a tearing motion as opposed to a scissor like cut at a slower pace. Both machines can handle dirtied material before processing. Granulators, as previously referenced, are fashioned with a rotor attached to blades which rotate in a chamber containing a grid floor. Granulators have a higher consistency of precise shredding sizes, but need to be maintained at a high rate, as their blades will wear faster than the shredders (Nearsay.com. (2018)).
Stage 5: Identification and Separation of Plastics
Nearing the end of the recycling process, identification and separation allows for the plastic particles to be tested to determine their quality and class. There are 3 main quality tests that will aid in identifying these important traits.
The first of these tests is the density test. Within this test, the plastic particles are placed into a large tank of water. The particles that are less dense than the water will remain buoyant at the top of the tank, while those that are denser will sink to the bottom. The second test is that of air classification. Air classification allows for the determination of a plastic particles thickness, or thinness. This test is commenced in a wind tunnel, where the smaller particles are lifted to the top, and the larger ones are pushed down to a lower altitude. During this stage, some plastics are also melted. Once melted into a liquid state, they can also be chopped into pellets. Stages can vary by facility, but this pelleting process will be covered in both Step 5 and 6 for clarity (How Is Plastic Recycled: Step by Step).
The third test is that of quality relative to the melting point, and color, of the plastic. This is conducted by collecting and analyzing small samples from the large batches of plastic.
Machines which are utilized in this stage vary between two methods which are wet and dry. Wet methods are accomplished by a float tank, predominantly, which separate material based upon density. Hydrocyclones utilize centrifugal force to separate the material out by weight. Dry methods are completed by air classification machines which separates materials based off of mass; and mechanical separators that divide the plastics based off of size and shape. Mechanical separators can be designed with three differing variations: flat, circular, or inclined. There are also various other machines that employ a more futuristic approach to identifying and separating plastic types. Laser spectral analyzers are one of these futuristic machines that utilize spectroscopic detection to convey precise levels of contamination that may be within the plastics. Inclusively there are also melt separation machines that move plastic strips along a conveyor, or hot roller, with a high level of heat applied that gets the plastics to their melting point, which is unique for each resin type (How Is Plastic Recycled: Step by Step).
Step 6: Compounding and Extrusion Tactics
Compounding is the final step within the plastic recycling process. This is where the plastic particles are manufactured into useful material for future production, more specifically plastic pellets, which are integral for the creation of new plastic products. The small plastic particles will be smashed, then melted together to form these pellets. In some cases, equipment can also be used to melt down the plastic flakes, usually created during the shredding process, into granules (or nurdles) or mold them into new shapes.
More specifically, pellets, flakes, particles, or powders are usually brought into a container which converts the material into a molten plastic base substance. Through the blending process, we get a base plastic material. This plastic material is then extruded from the container and chopped into pellets. These pellets are the final product of the plastic recycling process and are used in manufacturing plastic products. Additionally, there are various amounts of fillers and additives that can be introduced into the compounding process to change the base structure of the plastic which is being remanufactured into pellets, giving it new traits; this is not always needed (Tynan, K. C., & Allen, J).
Machinery that is incorporated in the pelletizing stage are usually more common than varied. Single or double screw extruders are the most common machines that are used in active compounding. Single screw extruders utilize a pumping action and a shear that shapes the plastic into pellets; double screw extruders pelletize plastics by a mixing function with a lower shearing force allowing it to create compound materials. It is important to note that the length and design of the extruders can vary based upon the resin that is being processed at the facility. Ventilation and vacuum pumps/shafts may also be required to regulate the gasses that may arise from the compounding machines (Tynan, K. C., & Allen, J).
These mentioned stages are general, and vary by company. There are many different types of processes, equipment, and methods within those stages in recycling plastic. There are two widely used processes for recycling plastic products which are heat compression and monomer. Heat compression is especially popular in the United States and allows for all types of plastic to be recycled at once. During heat compression, all forms of plastic are collected without being sorted by type, cleaned, and then mixed in large tumblers that churn the whole collection. Monomer recycling reverses the polymerization reaction in order to recycle the same type of condensed polymer, purifying while cleaning the plastic waste simultaneously.
Citations:
How Is Plastic Recycled: Step by Step. (2018, May 24). Retrieved from https://greentumble.com/how-is-plastic-recycled-step-by-step/
Nearsay.com. (2018). 4 Steps of the Plastic Recycling Process - Green Earth LLC - Linville. [online] Available at: https://nearsay.com/c/397370/234930/4-steps-of-the-plastic-recycling-process
PET Bottle Recycling. (2013, June 05). Retrieved from http://www.petbottlewashingline.com/pet-bottle-recycling/
Tynan, K. C., & Allen, J. (2018, October 23). What is Plastic Compounding? Retrieved from http://www.wisegeek.com/what-is-plastic-compounding.htm