Study on the chemical recycling of PET to monomers by depolymerization

Abstract

The current proliferation of polyethylene terephthalate (PET) as fibres, soft drink bottles and films poses serious environmental and economic concerns to the world. For the treatment of waste PET, its recycling is a promising option for the conservation of resources and environment. Various polymer recycling methods using chemical, mechanical, and thermal processes are available, but only chemical recycling conforms to the principle of sustainable development because it enables the formation of monomers. Among chemical recycling processes employing hydrolysis, methanolysis, glycolysis, and others, glycolysis is the simplest, oldest, and least capital intensive and was selected for further investigation in this set of studies. PET was depolymerized to its monomer, bis(2-hydroxyethyl) terephthalate (BHET), using excess ethylene glycol (EG) in its sub- and supercritical state. High monomer yield ($\gt 90 mol%$) was obtained at short reaction times with supercritical glycolysis because of liquid-like high diffusivity, and gas-like high solubility of EG in supercritical state. To investigate the effect of catalyst on the depolymerization, PET was depolymerized to BHET using EG in the presence of a catalyst (pure metal oxide, mixed oxide) at low temperature and pressure. The same high level ($\gt$ 90 mol %) of monomer yield was obtained as in the case of sub- and supercritical glycolysis with less severe reaction conditions. Metal oxides as glycolysis catalysts could provide a better alternate to conventional catalysts (acetates of zinc, lead, and manganese) in view of high monomer yield, high mechanical strength, high melting points, flexibility of usage in fixed and fluidized beds, possibility of regeneration, long shelf life, and extremely cost effective. The catalysts were characterized by SEM, TEM, XRD, and BET surface area while the monomer BHET was characterized by HPLC, $^1H$ NMR, $^{13}C$ NMR, DSC, and TGA. To further decrease the reaction temper...