Polyethylene terephthalate (PET) and high-density polyethylene (HDPE) bottles form the largest volume of plastic packaging targeted for recycling worldwide. This mechanical recycling process converts discarded containers back into raw materials without altering the polymer’s fundamental chemical structure. It conserves petroleum resources and lowers the energy demand compared to manufacturing new virgin plastic. The ultimate goal is to create a circular economy where these materials can be reused multiple times, reducing reliance on landfills.
Preparation and Sorting of Collected Bottles
The journey of a plastic bottle begins after collection, when it arrives at a Material Recovery Facility (MRF) often compressed into large bales. These bales are broken apart, and the material stream is subjected to a series of mechanical separation steps. Large screens and air classifiers separate lighter materials like paper and film from the heavier, rigid plastic containers and metals based on weight and size.
Magnets and eddy current separators then remove ferrous and non-ferrous metals, such as aluminum cans, ensuring the plastic stream is free of incompatible items. The remaining plastic containers are funneled toward advanced optical sorting units, which are the technological core of the separation process. These machines use Near-Infrared (NIR) light, which is beamed onto the plastics as they move rapidly along a conveyor belt.
Each type of polymer, such as PET or HDPE, absorbs and reflects the infrared light in a unique pattern, creating a distinct spectral “signature.” A sensor reads this signature, identifying the exact resin type and color of each bottle in milliseconds. Based on this identification, precisely timed jets of compressed air shoot the individual bottles into designated collection bins, effectively separating PET from HDPE and other plastics like polypropylene (PP) or polyvinyl chloride (PVC). The plastic is then compacted into dense bales and shipped to specialized reclamation facilities.
The Mechanical Transformation Process
Upon reaching the reclamation facility, the sorted bales are broken open, and the material undergoes a thorough cleaning process to remove surface contaminants. The whole containers are first shredded into small, uniform pieces, typically ranging from 8 to 12 millimeters, which are referred to as “flakes.” This size reduction increases the surface area, making the subsequent washing steps more effective at removing embedded grime.
These flakes are then subjected to a series of hot washing and friction cleaning steps, often using water heated to temperatures between 60°C and 80°C. The elevated temperature softens and dissolves contaminants like residual liquids, glues, and paper labels that adhere to the plastic. Hot washing is also important for sanitization, removing bacteria and odors, which is a required step for material destined for food-contact applications.
After the initial wash, the flakes are dropped into float-sink tanks to further purify the material based on density. PET flakes are denser than water and sink to the bottom, while lighter plastics, such as residual HDPE caps or PP label fragments, float to the surface and are skimmed off. This density separation ensures a high-purity stream of the desired polymer.
The cleaned flakes must then be dried completely to remove all moisture before melting. Excess moisture can degrade the polymer chains during high-temperature processing, compromising the quality and strength of the recycled plastic. The dry flakes are fed into an extruder, melted into a viscous liquid, and forced through fine-mesh filters to screen out remaining impurities. The molten plastic is extruded into continuous strands, rapidly cooled in a water bath, and then cut into standardized pellets, or resin, known as rPET or rHDPE, ready for manufacturing.
Applications of Recycled Plastic
The resulting recycled resin is incorporated into a wide variety of new products. Recycled PET (rPET) is spun into polyester fiber, which is used in the textile industry to create fabrics for clothing, such as fleece jackets, carpeting, and fiberfill for pillows. In a process known as “bottle-to-bottle” recycling, rPET is also used to mold new beverage containers and food packaging, though this requires stringent decontamination processes to meet food safety standards.
Recycled HDPE (rHDPE) pellets are valued for their durability and chemical resistance, making them ideal for non-food-grade applications. This material is commonly molded into new detergent and household cleaner bottles, as well as milk jugs. Outside of packaging, rHDPE is utilized in the construction sector for creating plastic lumber, decking materials, and pipes, offering a rot-resistant alternative to traditional materials. Both rPET and rHDPE are components in the production of specialized items like automotive parts, strapping materials, and industrial storage containers.