Continuous Depolymerization of Poly(ethylene terephthalate) via Reactive Extrusion.

Abstract

A single-step, extrusion-based process to depolymerize poly(ethylene terephthalate) (PET) has been investigated. The depolymerization 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. A key feature of this process is that it employs the use of a twin-screw extruder as a reactor. The twin-screw extruder conveys the polymer and continuously creates a fresh surface area that facilitates penetration of the depolymerizing agent into the polymer. This research could play a role in reducing the considerable amount of bottle-grade PET resin that is manufactured from virgin materials, resulting in a significant environmental and economic impact.

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 DPn of the final product can be controlled at essentially atmospheric pressure.