Preparation of porous thermoresponsive coating (PU@AC-Al) and its synergistic mechanism of early fire warning and flame retardancy

Early fire warning is vital to prevent fire escalation, yet conventional smoke-based alarms suffer from high activation temperatures and delayed responses. To overcome these issues, a thermoresponsive polyurethane (PU)-based smart coating with early warning capability was developed by incorporating ammonium carbamate (AC) and nano-aluminum oxide (n-Al₂O₃) via a two-step synthesis. The resulting thermoresponsive coating (PU@AC-Al) exhibits dual-mode functionality, providing both active gas-releasing fire warning and passive flame-retardant protection. Experimental results demonstrate that n-Al₂O₃ in the coating induces the formation of a porous structure in the polyurethane matrix via an interfacial pore-forming effect, which enhances the decomposition of the thermosensitive component and promotes gas release. The AC component exhibits dual functionality during thermal disturbance through a solid–gas phase transition reaction: it absorbs phase change enthalpy to delay temperature rise in the coating and simultaneously releases characteristic NH₃ gas as an early warning signal. When the n-Al₂O₃ content reaches 5 wt%, the coating's thermal response sensitivity is triggered at 72 ℃, and the ignition alarm time under open flame conditions is reduced to 12 s. The initial gas release temperature precedes the typical smoke generation temperatures of common construction materials by 66.5–200 ℃, demonstrating excellent early warning performance.