|Joan Patterson, doctoral student
in chemical and biomolecular engineering at NC State University, pours PET pellets
into a twin-screw extruder by way of the blue hopper above.
(Photo: Becky Kirkland)
Chemical engineers at North Carolina State University have developed a more efficient way to chemically recycle your soda bottles back into new ones.
Polyethylene terephthalate (PET) is a common plastic used in beverage bottles. Most beverage bottles collected for recycling are reprocessed into non-food products such as fiber and strapping. Only a small percentage of beverage bottles is reprocessed into food-grade PET — plastic packaging, including beverage bottles. Although there is a demand for recycled bottle-grade PET, the high cost of cleaning post-consumer beverage bottles, strict FDA requirements and old technology have favored the use of virgin PET over recycled bottle PET in the manufacturing of beverage bottles.
As part of a National Science Foundation grant, Dr. George W. Roberts, retired professor of chemical and biomolecular engineering at North Carolina State University, Dr. Saad A. Khan, professor of chemical and biomolecular engineering and director of the chemical engineering graduate program at NC State, and Joan Patterson, doctoral student in chemical and biomolecular engineering, have developed a new chemical reprocessing method that can more efficiently convert post-consumer bottle-grade PET into a resin suitable for food-grade applications.
All plastics, including PET, are synthetic polymers. A polymer is a high-molecular-weight chemical compound made up of linked subunits of molecules called monomers. The combining of monomers to form a polymer is called polymerization. Reversing the process is called depolymerization.
Roberts, Khan and Patterson have demonstrated that they can take PET and depolymerize it back to target levels and end up with a material that can be cleaned and repolymerized into bottle-grade PET. “This is one of the few ways of taking food-grade polymers and recycling them back into food-grade applications,” Roberts said.
|The new chemical
reprocessing method can convert post-consumer PET bottles (center) into a material
called a low-molecular-weight oligomer (right), which is suitable for recycling
into bottle-grade PET resin (left).
(Photo: Becky Kirkland)
Instead of the awkward and energy-consuming batch processing tried by some recyclers, the new depolymerization process runs continuously in a machine called a twin-screw extruder. “We think the process is very energy efficient, largely because it operates continuously. There are large energy losses associated with the old technology of throwing [PET] in an autoclave, heating it up, holding it for five hours, and cooling the autoclave down. The main energy here is to drive the extruder screws,” Roberts said.
The extruder is capable of handling a large amount of polymer in a very short time. It melts the PET, which allows the high-molecular-weight polymer to react rapidly with ethylene glycol, reducing the polymer’s molecular weight. Supercritical carbon dioxide lowers the viscosity of the polymer even more as it depolymerizes. The end product is a low-molecular-weight material that is capable of being incorporated into a conventional polyester polymerization process.
According to Roberts, the chemistry of the new process is fairly well known. “We take this high-molecular-weight polymer and break it up into shorter segments by reacting it with ethylene glycol. Once we have the shorter segments, the viscosity of the material is significantly reduced. We can take out any solid, liquid or vapor impurities and wind up with a material that can be sent through a normal polymerization process. What we don’t do is go all the way back to the raw materials from which PET is made. We go back to an intermediate stage—a low-molecular-weight polymer called an oligomer.”
Roberts explained that although they can go back to the monomer stage, there is no need to. “We think it is more economical to make the oligomer because most polyester processes involve two stages, and the second stage starts with an oligomer.”
The technology is nearly ready for commercialization. Final testing with clean flake PET (ground and washed post-consumer bottles) remains to be done. Dr. Ronald A. DiFelice, president of DPoly Systems, is working with NC State University to commercialize the new technology. DiFelice started DPoly Systems in July 2006. The company is one of four startups to receive a 2007 grant from NC IDEA as part of NC IDEA’s commitment to economic development across North Carolina. The funds are designed to help companies move closer to commercialization.
“The process has broad appeal to plastics recyclers,” said DiFelice. “In addition to being applicable to polyesters, the process can be used to depolymerize post-consumer polycarbonates [e.g., plastic used in shatterproof windows] and nylons. The business model is to finish validating the technology and then work with partners to integrate it into high-volume recycling processes.”
According to DiFelice, demand for post-consumer PET is increasing, and there are a number of companies chemically reprocessing PET that would be interested in incorporating this new technology into their line because of the opportunity for increased efficiency and an increase in the value of the end product. This new process results in resin that is purer and therefore more valuable to processors that incorporate post-consumer PET into their products.
— mcblief —
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