Two-step chemolytic delamination and depolymerization of multilayer laminated packaging films into valuable chemicals

Abstract

The mechanical recycling of multilayer laminated packaging films (MLPFs) is considerably challenging due to their compositional heterogeneity and their diversity in adhesives. To address this challenge, this study uses a two-step integrated chemolysis approach to upcycle MLPFs. First, an acid (acetic acid, formic acid, or succinic acid) was used to delaminate MLPFs into separate layers of polyethylene terephthalate (PET) and polyethylene (PE) at 60–90 °C for 10 min. Subsequently, glycolysis using ethylene glycol (EG) was used to depolymerize the delaminated PET layer into bis(2-hydroxyethyl) terephthalate (BHET) and other valuable chemicals. Besides non-catalytical glycolysis, zinc acetate and calcium acetate were employed as catalysts to accelerate the depolymerization of the delaminated PET films, increasing the PET conversion rate from 0% (3 h via non-catalytical route) to nearly 100% (10 min with 30 wt% of zinc acetate). The effects of reaction temperature, reaction time, and catalyst loading on PET conversion and product yields were evaluated. It was found that 1–30 wt% loadings of either catalyst enabled 1 g of 100% PET conversion within 10 min at 190 °C. Under this reaction condition (190 °C and 10 min, which is determined as the optimal condition in this study), zinc acetate with higher loading (30 wt%) led to a higher selectivity towards BHET. However, calcium acetate resulted in a higher yield of mono(2-hydroxyethyl) terephthalate (MHET) as a product over BHET, suggesting different central metal ion influence on the selectivity of glycolysis products. The effect of catalytic hydrates was investigated by considering the anhydrous and mono-hydrated calcium acetates. This study suggests that different metals present in the catalysts can affect the selectivity of glycolysis products. The obtained BHET can be used to synthesize new PET, helping achieve a circular economy. Additionally, a density functional theory (DFT) study provided a mechanistic understanding regarding the role of zinc and calcium acetates on PET glycolysis at the atomistic levels.