For most people, PET bottle recycling means placing plastic bottles such as water bottles, soda bottles, oil containers, and similar products into the blue recycling bin. For others, it means bringing these bottles to a local recycling center. While this simple act of “PET bottle recycling” represents the first step, numerous additional processes are required before PET bottles can be truly recycled and transformed into new products such as polyester fiber, PET sheets/boards, or even back into new PET bottles.
Collecting PET Bottles
The initial step in PET recycling is the collection of bottles. As mentioned above, a significant portion of collection occurs through “curbside” recycling programs, where trucks collect the blue recycling bins, as well as through recycling center drop-offs. In curbside recycling, often referred to as “single stream” recycling, all types of recyclables are placed into a single bin and transported to a MRF (Materials Recovery Facility) for sorting. These recyclable materials typically include glass, aluminum cans, plastic bottles, and paper/cardboard. This mixed stream of recyclables undergoes both manual sorting and processing through specialized recycling machinery such as trommels, eddy current separators, and other equipment.
Once collected and sorted at the MRF, the plastic bottles are compressed into large compacted bales (pictured above) and shipped to a PET bottle washing plant, either domestically or internationally, for processing into “hot-washed” PET flakes. Meanwhile, the separated glass, metals, and paper/cardboard are packaged and sent to their respective recycling facilities for further processing.
PET Bottle Washing/Recycling Plant
In this section, we’ll focus on the process of cleaning PET bottles and processing them into recycled PET plastic, or RPET for short.
When plastic bottles arrive at a PET recycling/washing plant, they typically come in large bales that have already been separated by color. The primary objective of a PET bottle washing plant is to recover these dirty plastic bottles and thoroughly clean them for reuse. During this process, various contaminants must be separated, including plastic labels wrapped around each bottle and bottle caps, which are not made from PET plastic.
To clean PET bottles effectively, a series of recycling machinery must be used. This series of equipment, either fully or partially automated, is commonly referred to as a PET bottle washing line or recycling line. The bottles go through three distinct phases: sorting, washing, and drying.
In a standard, fully automated PET washing line, the first piece of equipment is a “debaling machine” that breaks apart large, compacted bales of PET bottles, allowing the bottles to flow freely onto a belt conveyor. This conveyor leads to a large trommel separation machine.
The trommel, a large rotating tunnel with holes smaller than the standard PET bottle, serves as robust machinery for removing small contaminants from the material stream. Since most plastic bottles come from MRFs (Material Recovery Facilities), the bales often contain various small contaminants such as broken glass, metals, rocks, paper/cardboard, etc., leftover from the sorting of “single stream recycling” materials. These small contaminants fall through the holes on the trommel while PET bottles move forward in the washing line in a steady flow.
Manual sorting provides an accurate method for removing contaminants from the material stream. For example, a 1-gallon milk bottle (made from HDPE) might pass through the trommel process but is considered a contaminant since it’s not made of PET plastic. The material stream now enters long rows of belt conveyors where attendants standing on both sides manually remove contaminants before the bottles are cut into small pieces.
A granulator cuts PET bottles into small pieces of plastic shred often referred to as “PET flakes” that are 12-18mm in size. Wet granulators are commonly used in this process, where a stream of water is sprayed onto the bottles as they are being cut. This makes the wet granulator the first stage in the washing process.
At this stage, the PET flakes still require significant processing before they can be recycled into new products. To continue the sorting process, the flakes pass through an air classifier where lighter materials are separated from heavier ones. This is accomplished by blowing the material stream with a column of pressurized air. In PET recycling, the air classifier effectively blows away the plastic film labels from the heavier PET plastic.
With the bottle labels removed, the next step is removing the bottle caps, which are made from PP/PE plastic (polypropylene and polyethylene). This is accomplished through a relatively simple process where the material stream enters a large tank of water. In a PET washing line, this tank is often called a float/sink separation tank. PET flakes have a density greater than water and sink to the bottom, while PP/PE plastic floats. The separation tank has screw conveyors at the bottom that transport the PET flakes to the next piece of machinery.
At this point, the relatively pure stream of PET flakes enters the first official washing machinery: the hot boiler/steamer washer. This equipment uses hot water with the addition of a caustic or similar solution to remove glues (used for attaching labels), grease, oils, and residual liquids/foods stuck to the bottles. As these contaminants are broken down in the hot washer, a secondary cold friction washer further scrubs the PET flakes. Through these two pieces of washing equipment, the PET flakes become perfectly clean and ready for drying.
To reuse the PET flakes effectively, they must be thoroughly dried, as melting PET flakes with water inside would create air bubbles in the final product—an undesirable outcome. Therefore, an elaborate drying system must be implemented to efficiently remove all water. The first step involves a de-watering machine that uses centrifugal force to spin away a large portion of the water. Further drying is accomplished through a dehydration process where thermal heaters reduce the moisture content of the PET flakes to below 1 percent.
At the end of a PET bottle recycling line, the result is clean, contaminant-free PET flakes ready for use in manufacturing various recycled products.
RPET Products
With clean PET flakes now available, what applications can they serve? The first and probably largest use of PET flakes is in the production of polyester staple fibers, which can be woven into clothing and various applications such as carpets, handbags, etc.
RPET can also be used in manufacturing PET sheets/boards, PET strapping, thermoforming applications such as laundry scoops, engineering applications such as reinforced components for automobiles, and even back into PET bottles for packaging applications. In these scenarios, the PET flakes must first undergo an extrusion process often called “pelletizing,” where PET flakes are melted and shaped into pellets. Once PET pellets are produced, they can be manufactured into a variety of products through injection molding or other processes.
