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

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. Chemical recycling scheme of “polymer to polymer”: polyethylene terephthalate to polyisocyanurate foam