A single-step, extrusion-based process to depolymerize poly(ethylene terephthalate) (PET) is being investigated. The depolymerization is effected by reacting ethylene glycol with the polymer (glycolysis). The ethylene glycol attacks the ester linkages along the polymer backbone, causing chain scission. 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 34mm counter-rotating twin-screw extruder have shown substantially faster depolymerization rates than typical batch glycolysis reactions, shortening reaction times from hours to minutes. Oligomers with number-average molecular weights of 2,500 g/mol (DP_n =13) can be produced continuously from the starting bottle-grade PET resin (M_n = 19,600 and DP_n = 102) 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. The influence of temperature and pressure on the reaction also has been studied.

Preliminary studies on a ¾” single-screw extruder have shown that addition of scCO₂ decreases the reaction time. The results of current studies with scCO₂ in the twin-screw extrusion system will be discussed.