Single-catalyst reactions from depolymerization to repolymerization: Transformation of polyethylene terephthalate to polyisocyanurate foam with deep eutectic solvents

Abstract

Polyisocyanurate (PIR) has been used as a flame retardant insulation material because of its improved thermal insulation and flame-retarding effects due to its cyclic structure with an isocyanate trimer group. When a deep eutectic solvent (DES) is used as a catalyst, PET glycolysis produces bis(2- hydroxyethyl terephthalate) (BHET) and its derivatives. These recycled polyols react with isocyanate to form PIR foam (PIRF) in the presence of a physical blowing agent. DES-based glycolysis is a low-energy process with a relatively low temperature and short reaction time. In this study, a potassium-based DES system was applied for the sequential depolymerization and repolymerization of waste PET into PIRFs. Potassium-based DESs were used as hydrogen bond acceptors (HBAs), and ethylene glycol (EG), glycerol, and urea were used as hydrogen bond donors (HBDs). The PIRF was obtained through one-pot depolymerization and repolymerization with a single-catalyst DES system. All DES catalyst systems effectively decomposed PET; the PET conversion efficiency reached 95% after 2 h reaction. The recycled PIR was more thermally stable (approximately 26.9% limiting oxygen index (LOI) and approximately 68.8 W/g heat release rate (HRR)). This paper presents an environmentally friendly and robust upcycling process for the transformation of waste PET into PIRF.