Functionalized ionic liquid supported on expandable graphite for achieving efficient fire retardancy and high thermal conductivity in thermoplastic polyurethane

Ionic liquids (ILs), as a new class of environmentally friendly and highly efficient flame retardants, have been rarely applied to thermoplastic polymers due to their intrinsic liquid nature. In this study, a facile and low-cost strategy was developed to introduce a phosphorus-containing ionic liquid (PIL) into the structure of expandable graphite (EG), forming a hybrid flame retardant (PIL@EG) for improving the fire safety and thermal conductivity of thermoplastic polyurethane (TPU). The results demonstrated that by combining physical ( i.e., EG) and chemical ( i.e., PIL) intumescent strategies, the TPU composite containing 15 wt% PIL@EG achieved a UL-94 V-0 rating and exhibited an increased limiting oxygen index (LOI) of 30.8%. Cone calorimeter test (CCT) results revealed that, compared with neat TPU, the peak heat release rate (PHRR) of the TPU/15 EG and TPU/15PIL@EG composites decreased by 82.6% and 82.8%, respectively, while the corresponding total heat release were reduced by 29.4% and 47.3%, respectively. The incorporation of PIL@EG effectively retarded the thermal degradation rate of TPU, enhanced the thermal stability, and suppressed the emission of volatile degradation products during combustion. Meanwhile, the thermal conductivity of TPU/15PIL@EG was significantly enhanced, reaching 0.82 W/m·K, which corresponds to increases of 204% and 22.4% compared with neat TPU and TPU/15 EG, respectively. Overall, this study not only demonstrates the great potential of ionic liquids in thermoplastic systems but also provides new insights into their functionalization and multi-field applications.