Chemical Recycling, Kinetics, and Thermodynamics of Alkaline Depolymerization of Waste Polyesters

Abstract

Depolymerization of waste poly (ethylene terephthalate) and poly (butylene terephthalate) were undertaken in various alkaline solutions at different reaction parameters at 1 atm in a batch operation. Results were evaluated in terms of weight loss of both the waste polyesters as well as products obtained during depolymerization of polyesters. Poly (ethylene terephthalate) (PET) showed more weight loss as compared to that of poly (butylene terephthalate) (PBT) in various potassium hydroxide (KOH) solutions. Methanolic alkaline solution has showed greater reaction effect as compared to those of aqueous as well as ethanolic solutions for both the waste polyesters. Reaction products composed of ethylene glycol (EG)/1,4-butanediol (BD) and terephthalic acid (TPA) that were analyzed and confirmed. Yields of monomeric products (TPA and EG/BD) were almost equal to those of polyester conversion. To accelerate the rate of reaction cyclohexylamine (CHA), tetrahydrofurane (THF), and dimethyl sulfoxide (DMSO) were introduced during depolymerization of polyesters. A kinetic model is developed and experimental data are fitted with it for validity of the model. The model shows good agreement with the experimental data. Thermodynamics are also undertaken to estimate the rate constants, equilibrium constant, Gibbs free energy, enthalpy, and entropy of reaction that is required to transfer laboratory data through the pilot plant for commercialization. Dependence of rate constant on reaction temperature was correlated by Arrhenius plot that was used to determine activation energy and Arrhenius constant.