Stages and Kinetics of Mechanochemical Depolymerization of Poly(ethylene terephthalate) with Sodium Hydroxide

Efficient chemical recycling of consumer plastics (i.e., depolymerization down to monomers) is a crucial step needed to achieve a circular material economy. In this work, depolymerization of poly­(ethylene terephthalate) (PET) via mechanochemical hydrolysis with sodium hydroxide is studied, with complete depolymerization achieved in 20 min. The stages of depolymerization are investigated by monitoring monomer yields and the change in the PET molecular weight over the course of the reaction. The monomer yields initially increase linearly with milling time, up to a yield of roughly 40%. However, the molecular weights of the residual PET decrease concomitantly only slightly, suggesting a reaction scheme analogous to a shrinking core model. As the reaction progresses, a physical transition of the PET/NaOH from a powder to a homogeneous wax and a simultaneous increase in the depolymerization rate are observed. The influence of ball-to-powder mass ratio (BPR) and milling frequency is studied to derive a kinetic rate expression. The linear relationship between BPR and monomer yield and the known relationship between milling frequency are validated for this system.