Melt Glycolysis of Poly(ethylene Terephthalate) Using Co2-assisted Extrusion

A single-step, extrusion-based process to depolymerize poly(ethylene terephthalate) (PET) is being investigated. The depolymeriza tion is effected by reacting ethylene glycol (EG) with the high molecular weight polymer (glycolysis). The ethylene glycol causes chain scission by attacking the ester linkages along the polymer backbone. The objective is to recover purified low molecular weight oligomers of bis(hydroxyethyl) terephthalate (BHET) for repolymerization. The process has two novel features: 1) the use of a twin-screw extruder to convey the polymer and to continuously create the fresh surface that is required for the depolymerizing agent to penetrate into the polymer, and 2) the use of supercritical carbon dioxide (scCO2) as a processing aid. Results of glycolysis experiments on a Leistritz 34 mm counter-rotating twin-screw extruder have been characterized by substantially faster depolymerization rates than typical batch glycolysis reactions, shortening reaction times from hours to minutes. Oligomers with number-average degrees of polymerization (DPn) as low as 13 (Mn = 2,500) can be produced continuously from the starting bottle -grade PET resin (DPn = 102 and Mn = 20,000) in residence times of just 10 minutes. By changing operating conditions, e.g. feed rate and EG:PET ratio, the DP of the final product can be controlled at essentially atmospheric pressure.