2001 TSE: NSF/EPA Partnership for Environmental Research: Source-Based Pollution Prevention through Depolymerization of Poly(ethylene terephthalate) with Carbon Dioxide

North Carolina State University, Raleigh NC

Investigators

Abstract

Source-Based Pollution Prevention through Depolymerization of Poly (ethylene terephthalate) with Supercritical Carbon Dioxide Technical Description: This research represents a new approach to pollution prevention through elimination of waste polymer that is generated during the manufacture of poly (ethylene terephthalate) (PET). A one-step process will be investigated to depolymerize PET and recover purified monomeric and oligimeric units for repolymerization. The key features of the process are: 1) use of a twin-screw extruder to continuously create the fresh surface that is required for a depolymerizing agent (methanol or ethylene glycol) to penetrate into polymer; 2) use of supercritical carbon dioxide (scCO2) as a processing aid. Molten polymer is contacted with a mixture of scCO2 and either methanol or ethylene glycol in a twin-screw extruder. The methanol or ethylene glycol causes the PET to depolymerize as it passes through the extruder. The scCO2 plasticizes the PET, facilitating penetration of the methanol/ethylene glycol into the polymer. The scCO2 also reduces the viscosity of the melt and enhances chain mobility, improving the overall reaction rate. Finally, the scCO2 extracts organic contaminants from the polymer. The scCO2 containing dissolved contaminants,unreacted ethylene glycol or methanol, and some of the monomers produced is removed in a vent on the extruder. The potential advantages of this process are: (1) elimination of the solvents and processing steps that otherwise are required for separation of monomers and oligomers from contaminants; (2) much higher depolymerization rates; (3) integration of multiple processes, i.e., depolymerization and extraction, into a single step. The process parameters will be studied include the depolymerizing gent, the depolymerizing agent/polymer ratio, pressure, temperature, CO2/polymer ratio, and average residence time in the extruder. Broader Significance: Approximately 100 million pounds of waste PET is generated annually in U.S. manufacturing facilities. While some of this waste is used in low-value products such as fillers, a large portion remains unusable. Successful development of this new technology could help to solve a serious waste disposal problem and increase the competitiveness of U.S. PET manufacturers. A fundamental understanding of the depolymerization of PET could provide a basis for several technology extensions. First is the application to post-consumer waste PET, the annual volume of which is about 30 times the volume of in-plant waste. Second, this new process should be applicable to other step-growth polymers such as poly (bisphenol A carbonate) and various nylons.