Bio-based vanillin-derived nitrogen-phosphorus synergism: A sustainable flame-retardant strategy for high-performance thermoplastic polyurethane

The increasing use of thermoplastic polyurethane (TPU) in wearable electronics, electric vehicle cable sheathing, and flexible building materials, yet raises pressing safety and environmental concerns due to their high flammability and the smoke they emit in a fire. Traditional flame retardants often pose trade-offs in toxicity, sustainability, or mechanical performance, highlighting the need for cleaner, bio-based solutions. In this work, we report a vanillin-derived nitrogen–phosphorus flame retardant (PVD) that addresses these challenges through a green and efficient strategy. Incorporating only 4 wt% PVD into TPU achieves a UL-94 V-0 rating and enhances the limiting oxygen index (LOI) to 27.3 %. At 6 wt%, the TPU/PVD composite reduces peak heat release rate (PHRR) and total smoke production (TSP) by 33.7 % and 43.2 %, respectively, with enhancing mechanical properties. The enhanced flame retardancy of TPU/PVD composites is attributed to a dual-phase mechanism. In the condensed phase, nitrogen and phosphorus elements synergistically promote the formation of a thermally stable, graphitized char layer rich in P O and P O structures, which acts as a physical barrier to heat and mass transfer. In the gas phase, flame inhibition occurs through the quenching of active radicals (e.g., PO and PO 2 ) and dilution of flammable gases by non-combustible gases such as CO 2 and NH 3 . This approach provides a viable pathway toward sustainable, fire-safe, and mechanically robust TPU materials.