BACKGROUND: Chemical engineers at North Caroline State University have developed a more efficient way to chemically recycle your soda bottles back into new ones. Many companies are interested in the new process, both in the US and internationally, because of its potential for increased efficiency and increased value of the end product. NCSU is working with a startup company called DPoly Systems to commercialize the technology.
WHAT IS PET? All plastics are synthetic polymers, a high-molecular weight chemical compound made up of linked molecules called monomers. The combining of monomers to form a polymer chain is called 'polymerization'. Polyethylene perepthalate (PET) is a common plastic used in beverage bottles. Like most plastics, the bottles are non-biodegradable and will just sit in landfills if we don't recycle them. In addition, the PET bottle market continues to grow rapidly; in Europe, even beer is packaged in plastic bottles. PET is made out of petroleum, so more efficient recycling of old PET bottles would help reduce out dependence on oil.
THE PROBLEM: Recycling is an excellent concept, but we often waste more energy in reprocessing our recyclables than we are gaining. Furthermore, to date no one has found a cost-effective means of recycling food containers into new food containers. More efficient processes will bring us closer to the goal of not wasting our resources. Although there is a demand for recycled bottle-grade PET, the high cost of cleaning post-consumer beverage bottles, strict FDA requirements, and outmoded technology have favored the use of virgin PET over recycled bottle PET in the manufacturing of beverage bottles. Instead, most beverage bottles collected for recycling are reprocessed into non-food products such as fiber and strapping.
HOW IT WORKS: The NCSU researchers have developed a new chemical reprocessing method that uses a twin-screw extruder as a chemical reactor. The solid plastic pieces are melted down and then react chemically with ethylene glycol, reducing the molecular weight. Supercritical carbon dioxide lowers the viscosity even more, so that the plastic emerges as something more similar to water, rather than a strand of plastic. This is called 'depolymerization'. They then remove impurities and 're-polymerize' the material, resulting in resin that is purer and therefore more valuable to processors that incorporate post-consumer PET into their products. The technique takes 10 minutes, compared to two to four hours for conventional batch processing, in which the PET is placed in an autoclave, heated for five hours, them cooled down. The extruder runs continuously, reducing energy losses, and is capable of handling large amounts of polymer in a very short time frame.