A Self-Catalytic Bio-Platform for Upcycling of PET Plastic into Oligoesters for Polyurethane Synthesis

This study presents a green approach for polyethylene terephthalate (PET) upcycling using a biphasic system of subcritical water and castor oil. This system enables efficient conversion without an external catalyst and facilitates product separation. Hydrolysis of castor oil generates fatty acids in situ, which catalyze PET conversion to selectively produce low-molecular-weight oligoesters (Mn ≈ 1500 g/mol). These oligoesters are inherently immiscible with the bio-medium at room temperature, allowing straightforward separation by centrifugation. Orthogonal experiments show that temperature is the dominant factor affecting PET conversion, with the optimal conditions being 200 °C, a water-to-oil mass ratio of 1:5, and a reaction time of 10 h for complete conversion. Under the practical optimum condition, the castor oil phase remained highly effective over five consecutive depolymerization cycles. The functionalized oligoesters were used in polyurethane synthesis. At 6% loading, they gave adhesives with a T-peel strength of 8.09 N/15 mm and tensile strength of 25.14 MPa, and excellent damp-heat stability (only 0.62% loss in T-peel and 0.47% in 180° peel after aging). Thermogravimetric analysis confirmed enhanced thermal stability, with increases of 19.52 °C in T5% and 16.50 °C in T50% compared with the unmodified system. These results demonstrate the practical potential of the obtained oligoesters in high value adhesive applications.