Kinetics of depolymerization of poly(ethylene terephthalate) in a potassium hydroxide solution

The depolymerization of poly(ethylene terephthalate) (PET) flakes in a potassium hydroxide solution was carried out in a stirred batch reactor at 120, 140, and 160°C, below its melting point and under pressures of about 1.7, 2.9, and 4.6 atm, respectively. After the reactions, the residual solids almost remained in flake shape and their molecular weights were close to that of PET before the reaction. The products composed of ethylene glycol and terephthalic potassium salt were in the liquid phase. They were separated by acidification (to obtain solid terephthalic acid) and filtration processes and subsequently were analyzed quantitatively by potentiometric titration, elementary analysis, and gas chromatography. The results of kinetic analysis showed that the depolymerization reaction rate was first order to potassium hydroxide and first order to the PET concentration. This indicates that the ester linkages on the surface of the solid PET flakes sequentially reacted with potassium hydroxide in the solution to produce ethylene glycol and terephthalic potassium salt. A mechanism for the major reaction occurring on the polymer chain end section on the solid PET surface was proposed in this research. The dependence of the rate constant on the reaction temperature was correlated by the Arrhenius plot, which shows an activation energy of 69 kJ/mol and an Arrhenius constant of 419 L/min/cm².