Electrochemically exfoliated MXene reinforced WPU/PNO composite coating with enhanced flame retardancy and fire warning performance

Waterborne polyurethane (WPU) is widely used but its flammability limits high-end applications. Herein, we report a synergistic strategy to fabricate a multifunctional WPU-based composite coating with enhanced flame retardancy and ultra-fire warning capability. Reductive MXene nanosheets (prepared via green electrochemical exfoliation with lysine functionalization to inhibit oxidation) were integrated with a phosphorus‑nitrogen hybrid (PNO) within the WPU matrix. The PNO/MXene synergy established a dual condensed/gas‑phase flame‑retardant system. In the gas phase, decomposition releases CO₂ as the predominant product, which dilutes combustible volatiles and acts as an inert barrier. However, it is worth noting that although the production of carbon monoxide and carbon dioxide has both decreased, the experiment still observed a slight increase in smoke production. This phenomenon can be attributed to carbon particles produced by incomplete combustion, which also indirectly indicates that a carbon layer has formed on the surface of the coating. In the condensed phase, TiO₂ catalyzes the formation of a compact char layer, providing a solid barrier against heat and oxygen. At elevated temperatures, this char develops a highly conductive pathway (e.g., via graphitization), enabling an ultra‑rapid fire warning through a sudden resistance drop. Combustion tests showed significant reductions in peak heat release rate (40.2%) and total heat release (77.4%), alongside remarkable enhancements in mechanical strength (79.5%) and toughness (120.8%). Critically, the MXene‑derived conductive network triggers an alarm within 3 s upon flame contact. This work provides a promising strategy for developing multifunctional WPU composites integrating mechanical reinforcement, high-efficiency flame retardancy, and intelligent fire with potential applications in smart homes, construction, and public safety.