Hotspot-Catalytic Site Synergy Enables Photothermal Closed-Loop Upcycling of Polycarbonate into High-Performance Flame-Retardant Composites

Engineering plastics are essential to modern industry, yet their recycling remains challenging because conventional processes are energy-intensive and often degrade material value. Polycarbonate (PC), a representative engineering plastic, exemplifies this problem due to its widespread use and growing waste accumulation. Here, we propose a hotspot-catalytic site synergy strategy for the photothermal closed-loop upcycling of waste PC into high-performance flame-retardant composites. In the designed CNT-PEI hybrid, carbon nanotubes generate localized photothermal hotspots under light irradiation, while polyethyleneimine provides amine-rich catalytic sites for carbonate-bond cleavage. This synergy markedly accelerates depolymerization under mild conditions, affording bisphenol A (BPA) in >90% yield within 40 min under a light intensity of 400 mW/cm², with depolymerization efficiency improved by 138.0% and 54.8% relative to thermal depolymerization and CNT-based photothermal depolymerization, respectively. Furthermore, the recovered BPA was repolymerized into PC, followed by blending with recovered CNT-PEI to fabricate regenerated PC/CNT-PEI composites. As expected, PC/CNT-PEI exhibit a flame-retardant rating of UL-94 V-0, 18.9% higher tensile strength, and 71.0% higher thermal conductivity than pristine PC. Moreover, the PC/CNT-PEI composites can be depolymerized again to BPA in 97.5% yield, demonstrating a closed-loop upcycling process. This work provides an energy-efficient strategy for upcycling waste PC into high-performance flame-retardant materials.