How to recycle polymer waste
Abdulrehman Ishfaq | August 19, 2020As polymer manufacturing has increased, so has the need for a sustainable supply of raw material. Mostly, the polymers or plastics are acquired through petroleum resources, which is considered a non-renewable resource. Polymer recycling is key to contemporary and future plastics manufacturing.
The most effective way to reduce the production of polymers is to recycle the polymeric materials, in which the polymers are reheated and reshaped after mixed plastic waste is sorted and cleaned. Nowadays, the recycling of polymers is more advanced and quick. The waste or used polymers are repurposed in the form of recycled polymer products. The recycling of plastics has overcome the waste and energy management issues and also the usage of other energy resources.
The process of recycling involves critical and discrete steps. The steps of recycling can be removed or added according to the needs or conditions. The general steps of polymer recycling are collection, sorting, washing, resizing, identification and compounding.
1. Collection
The first step of polymer recycling is the collection of waste polymer, which is to be processed for recycling. Waste collectors must be sure that the polymer waste is disposed of separately from common waste. Many communities or refuse suppliers offer single-stream recycling, where plastics, metals and papers are collected together, but separate from trash.
2. Sorting
After collection, the plastic waste is conveyed to the processing area where machines are allocated for sorting the material according to the set parameters. The main objective of this step is to ensure the removal of trash and other recyclables from plastic.
Plastics are sorted according to the properties of polymers, including type, color, weight and shape. This step is significant as different types of plastic require different conditions for processing, such as temperature and type of processing machine. Some recycling machines are not suitable or compatible with a specific type of waste plastic. So, if the sorting is not done correctly, ultimately it will produce waste again, which must be reprocessed or recycled again.
3. Washing
Before the step of sizing or shredding, the sorted waste polymer is subjected to washing. As all the material is washed before usage, the waste plastics are also washed through the disinfectors or sanitizers, especially in the case of food and health-related polymer recycled products. The main objective behind performing this step is to clean the plastic from all impurities or microbes from their surface.
Mostly, the plastics or packaging related material has printed or labeled marks, with contamination of glues or adhesives, which must be removed. Sometimes, foodstuff is also removed from the plastic to improve the processing. These impurities are not recyclable and cause processing hurdles, ultimately affecting the quality of recycled products by lowering strength, affecting aesthetics and altering shapes.
4. Resizing
After washing the material, plastic is subjected to shredding through a granulating machine, in which the size of plastic is reduced and optimized according to the inlet of compounding extruder. Another main advantage of reducing the size of the plastic material is to increase the surface area of the plastic. This increase in surface area lowers the amount of energy required to heat, melt, flow through the extruder and ultimately to reshape the plastic in a new product. The reduction in size also overcomes the problem related to the transportation and storage of material.
After the shredding of plastic, the shredded fragments are again sorted based on weight and moved along for further processes where they are reduced again to smaller sizes. Moreover, this process provides the last chance to remove the non-plastic material from the batch to nullify the effect of impurities. The shredded plastic material is passed through the metal detector to eliminate metal content from the plastic batch.
5. Identification
This step of polymer recycling involves the proper testing and characterization of plastic material to estimate the quality, properties, type and class of the polymer material present in the batch. There are various methods through which the polymer is characterized, but the general characterization is done through the density of plastic, air classification, color and melt temperature of the polymer.
The most used quality characterization is density. The polymer particles are separated on the basis of density by immersing them in a large tank full of water. The particles having a density less than water do not sink and stay on the water surface while the particles with a higher density sink down and are separate.
After the density, the most practiced or recommended characterization is air classification. In this classification, the particles are allowed to fly inside a heighted channel. Its main objective is to classify the polymer particles based on their thickness. The more thick material will fly lower and vice versa. Apart from these to characterization, the other two characterizations are melting point and color. These are done by taking a sample from every single batch of polymer particles.
6. Compounding
The main step of recycling of polymer to make a final product is compounding or extrusion. It is considered as the most flexible, tunable and versatile step in polymer compounding. In this step, the pretreated waste polymer particles are entered into the extruder.
The extruder contains three sections. In the first section, the polymer feed particles enter the material. While in the second section, which is also named as transition or compression section, the feed is heated and compressed to make it flow to the next section. In the third or last section, the polymer particles or feed in the molten form are pressurized to flow through a die. The last section of the extruder is also called a metering section. The long cylinder-shaped fibers are extruded out of the die. These fibers are then reduced in pallets to utilize them in further processing to make a new recycled product.
Conclusion
Polymer recycling is a material and energy recovery process in which a polymeric or plastic material is reprocessed to make multiple recycled products. The reprocessing of material, which is obtained from waste, is done in a series of defined sub-processes. The removal of impurities like label inks, metal foils and foodstuffs, is also carried out during these processes as these materials are non-recyclable. Polymer recycling has proven the polymeric or plastic material as a potential renewable source of energy that also reduces the demand for other energy resources, thus provides help in creating a sustainable environment.
Essentially what we were doing 50 years ago in Zambia when every little counted.
What is REALLY needed to properly recycle polymers is to use the ones from bio-organic sources like corn stalks, hemp, lobster shells, seaweed (and etc) and STOP using petro-chemicals. Even when recycled or in the recycling process, petro-chemical polymers produce very environmentally destructive compounds.
Technically correct but economically and thermodynamically negative (need more energy to do than the amount saved)
Comment 2 follows common assumption that plastics in environment are harmful, a popular image that ignores their nontoxicity. The image is fed by fear of miracle-denying science in general and chemistry in particular, and is not going away.
A bio-source is OK if it does the same job (e.g., sugar-based polyethylene), but requires inputs of energy, water and land to grow, harvest and process into plastics, and dispose of ag waste.
Making less of everything (not just plastics) is ecologically useful, but culturally disrupting. Look at the effects of COVID-19 if you need proof.
In reply to #3
More research is clearly warranted. In the meantime, while we are stuck with these materials (plastics are cheap to buy but very costly in the long-run [including all the energy spent mitigating their long-term effects]), we ought to work harder to reduce the use of plastics. While we're doling more research on petro-plastic replacement materials we ought to employ non-petro-plastic options, i.e. returning to more paper and waxpaper use. Albeit these "old-school" methods are less ideal and more expensive on the front end, they are less costly on the backend. We get to through mass market use (yes, including regulation - it IS a public health issue), phase out the plastics, or at least greatly (more than 75%?) reduce their use.
Just sayin
Yes, I KNOW some waxpaper uses petro-parafin, AND I submit feasible (tho again, pricier on the front-end but less costly on the eternal back end) biological/renewable replacements for petro-parafin are availble. Even if it takes a while to phase in a petro-parafin replacement, waxpaper has lot less petrochemicals than the plastic wrap or plastic bad it is replacing.
In reply to #5
Renewable doesn't mean renewed, just as recyclable (which all plastics are) doesn't mean recycled. And recycling isn't necessarily enviro-positive until we know the energy inputs and what the new end-use is. The weight of wax (petrosourced) is more than the .0005" thickness of polyethylene that is on cartons, paper cups, and many other uses, and the PE doesn't crack or pinhole. It takes energy to make paper, too, and even in renewably-managed forests, it takes people and energy to grow and harvest and process the trees successfully. I suspect that PE does its job with lower environmental impact than anything else eligible.
However, that doesn't matter to the critics of plastics. There are technical counter-arguments, and no commercially important polymer is biologically toxic (the long chains resemble fats, and nylons are like proteins, except less reactive). But this is avoided and resented by the part of most all people that needs belief in impossibles (magic, theater, make-believe). Anything natural is usually predictable, even poisons, but plastics are synthesized by people. FDA or not, its us we distrust.
We also follow popular images, and "plastics = bad" is one of them. Belonging to anti-plasticism" makes us feel better, especially if we don't give up their benefits: wire insulation including power transmission, ag irrigation pipe, surgeon's sutures and medical PPE, light computer housing and mice, fishing line, fuel saved because cars and planes are lighter and homes better insulated, all synthetic textiles (nylon is plastic), nonburning Christmas trees, Velcro and all clear adhesive tape, the boxes that keep COVID vaccine super-cold as needed, other boxes that postal workers can handle and stack (1.55 lb HDPE), contact lenses, microwavable trays for takeout meals, toothbrush handles and bristles ....
But, like I said before, it doesn't matter. We learn absolute unexplained authority as babies, and the need for logic is learned later, as little/as much as necessary. We have a built-in fear of judgment as it might contradict what we want to believe. I don't expect to change this, but would like some of us techies at least to see it.
In reply to #6
I don't have the time to go down your defeatist rabbit hole right now. I suspect tho, you are mistaken with your facts in the first paragraph and the next three paragraphs are ad-hominem sophistry.
I'll come back and give you some empirical what-for later.