A kinetic study of the depolymerisation of poly(ethylene terephthalate) by phase transfer catalysed alkaline hydrolysis

BACKGROUND: Chemical or tertiary recycling of waste polymers including PET, poly(ethylene terephthalate), leads to the formation of raw starting monomers by different depolymerisation routes. This work was focused on the identification of the catalytic behaviour, if any, of a series of quaternary phosphonium and ammonium salts as phase transfer catalysts for the alkaline hydrolysis of PET, and on the determination of the kinetics of the phase transfer catalysed process. RESULTS: Among the salts examined tributylhexadecylphosphonium bromide was found to be the most effective catalyst. The proposed kinetic model accounted for the uncatalysed and catalysed reactions and predicted a linear correlation for the reaction rate with the concentration of the quaternary salt. The notable increase in the phase transfer catalysed reaction rate was related mainly to the greater value of the pre-exponential factor while the value of the activation energy was hardly modified by the presence of the quaternary phosphonium salt, thereby suggesting a similar mechanism for the alkaline hydrolysis with or without phase transfer catalyst. CONCLUSIONS: The use of selected phosphonium quaternary salts exhibited a remarkably positive effect on the experimental conditions under which the depolymerisation of poly(ethylene terephthalate) by alkaline hydrolysis can be carried out, especially in terms of low operating temperature. Kinetic correlations provided a reliable mathematical reaction model for this recycling process, which is in agreement with the principles of sustainable development. (C) 2008 Society of Chemical Industry