Conversion of polyethylene terephthalate to high-quality terephthalic acid by hydrothermal hydrolysis: the study of process parameters

A new circular economy concept is presented for the textile sector to convert unwearable polyester textile waste into valuable chemical feedstock. The idea behind it is to develop a new circular economy concept for the most used material in the textile industry, that is, polyester. Hydrothermal hydrolysis, an environmentally friendly process, has been studied for recovering polyester monomeric units. Under high-temperature and high-pressure conditions complete chemical depolymerization of pure poly(ethylene terephthalate) (PET) to terephthalic acid (TPA) was achieved at high yield. The produced TPA was characterized by potentiometric titrations, Fourier transform infrared spectroscopy, proton nuclear magnetic resonance spectroscopy and elemental analysis. A series of experiments were performed on the PET material with different intrinsic viscosities to define the appropriate depolymerization conditions related to the temperature ( T), time ( t) and PET:H ₂ O ratio, which enables total conversion of the polymer. Maximal conversion (92%) to TPA was defined at 250℃, pressure of 39–40 bar, PET:water ratio of 1:10 and hydrolysis time of 30 min after reaching steady-state conditions in the reactor. The applied depolymerization route resulted in moderate purity of the originated TPA, which was applied successfully in a laboratory-scale two-step re-polymerization to produce PET resins.