Partial glycolytic depolymerisation of poly(ethylene terephthalate) (PET) in the solid state: Modelling the contribution of time and temperature

Abstract

[EN] Glycolysis is one of the most explored approaches to the chemical recycling of poly(ethylene terephthalate) (PET). Apart from complete depolymerisation, which is the traditional strategy in this field, there is also the possibility of partial depolymerisation of PET in the solid state so that instead of reaching the monomer, oligomeric and polymer chains with a tuneable polymerisation degree are formed. This study monitored and modelled the partial depolymerisation of PET using an excess of ethylene glycol (EG) in the absence of an additionally added catalyst (only remnant production catalysts) at different times (from 10 to 120 min) and temperatures (from 160 to 210 degrees C) as a feasible approach for tailoring physico-chemical performance of PET fractions. Under these conditions, glycolysis involved the heterogeneous process concerning the diffusion of EG into the bulk PET particles and attack of amorphous regions, coexisting with the beginning of the homogeneous stage with the further generation of oligomers and a minor number of BHET molecules. Monitoring glycolysis highlighted variations in the solid fraction of PET: a significant molar mass reduction, an increase in polar hydroxyl groups, and promotion of low-lamellar thickness crystalline population. These changes were empirically modelled using a Boltzmann equation and offering the possibility of tailoring the physico-chemical performance of PET as a function of immersion time and temperature for this particular system. The glycolysed fractions of PET waste can be considered appropriate for closed-loop, semi-closed, and open-loop chemical recycling, with different applications through a simple, green, and versatile process.