From passive protection to active sensing: Wood coating integrating flame retardancy and fire warning

The application of flame-retardant coatings represents a straightforward and effective strategy for mitigating the fire risk of wood. However, conventional halogen-based coatings only provide passive protection, with their high environmental toxicity further limiting safe application. To integrate flame retardancy and early fire warning, this study proposed a halogen-free “polyphenol-mediated” strategy. First, a nitrogen‑phosphorus synergistic structural matrix was prepared from biomimetic shell-like tannic acid; nickel and polypyrrole were then introduced to obtain the flame retardant TPPNy with conductive capability. TPPNy was blended with a self-made waterborne polyacrylate (WPA) to form a compatible organic-inorganic hybrid coating (TPSA), which is designed to enhance the adhesion between the flame retardant and wood substrates as well as improve the coating toughness. During combustion, TPPNy releases flame-retardant gases and forms a dense, low-intumescence char, which not only provides a barrier effect but also forms a conductive carbon framework. Among them, polypyrrole can release inert gas while jointly optimizing the conductive network structure together with nickel. Since Ni 2+ is easily oxidized to NiO at high temperatures, it can act as an inorganic conductive filler in the conductive network structure, reduce conductive breakpoints, and synergistically achieve flame retardancy and fire warning performance. The optimal coating TPSA 4 (containing 4 wt% TPPNy) exhibits a LOI of 32%, reaches the UL-94 V-0 rating, reduces the peak heat release rate by more than 75%, triggers early warning within 10 s under 6 V, and maintains stable performance after being placed under constant temperature and humidity conditions for 72 h. This eco-friendly coating provides an integrated flame-retardant and early-warning solution for wood fire safety.