How to recycle polymer wasteAbdulrehman Ishfaq | August 19, 2020
As 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.
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.
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.
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.
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.
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.
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.
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.