Depolymerization of polyesters in sub- and supercritical water

Depolymerization behavior polyesters (poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalene dicarboxylate) (PEN)) in suband supercritical water was investigated for the purpose of monomer recycling. In case of the depolymerization of PET, the yield of terephthalic acid (TPA) increased to 90 % with increasing reaction temperature up to 693 K while the maximum yield of ethylene glycol (EG) achieved was only 60 % at 573 K. For PEN depolymerization, the yield of 2,6-naphthalene dicarboxylic acid (2,6-NPA) also increased to 97 % with increasing reaction temperature up to 693 K and the maximum yield of EG obtained was also 60 % at 573 K. The higher yield of 2,6-NPA than that of TPA was ascribed to the higher stability of 2,6-NPA than that of TPA. The lower yields of EG than the corresponding monomers (TPA and 2,6,-NPA) in the decomposition of the two kinds of polyesters were caused by the catalytic dehydroxylation of EG by protons derived from dicarboxylic acids.