Pyrolysis versus hydrolysis behavior during steam decomposition of polyesters using ¹⁸O-labeled steam

Steam decomposition, a method employed for the depolymerization of polyesters, does not require solvents, catalysts, or high pressure. During steam decomposition, the fission of the ester group occurs by hydrolysis, whereas the ester group is cleaved without the action of water during pyrolysis, affording reduced monomer yields. Hence, elucidating the contribution of hydrolysis and pyrolysis to depolymerization in a steam atmosphere, as well as the effect of polyester structure on selectivity, will improve the accuracy of kinetic analyses and maximize monomer yields. In this study, the selectivities for pyrolysis and hydrolysis during the steam decomposition of polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene 2,6-naphthalate (PEN) were quantified using ¹⁸O-labeled steam at different steam concentrations and decomposition temperatures. The decomposition temperature strongly affected the hydrolysis selectivity for PET and PBT, whereas that for PEN was hardly affected. The selectivity for polyester hydrolysis increased with increasing steam concentration for both PET and PEN, with the exception of PBT. These results revealed that the selectivities for both pyrolysis and hydrolysis were significantly affected by the structure of the polyester. In addition, the thermogravimetric kinetic analysis of steam decomposition was consistent with the results of the ¹⁸O-labeling experiments.