Functional composite wood integrating fire warning and intelligent protection

Wood holds irreplaceable value in various applications, but its flammability restricts its broader utilization potential. Inspired by biological self-protection, an intelligent composite wood with integrated fire warning and active protection was developed based on a reversible color-changing mechanism of methyl blue (MB). Under the action ofthe cationic surfactant cetyltrimethylammonium bromide (CTAB), the conjugated quinoid benzene rings of MB undergoes reversible electronic changes, causing a color transition from deep blue to pale blue. The addition of Ca 2+ accelerates this decolorization, forming a colorless MB/CTAB/CaCl 2 (MCC) solution. Incorporating these functional units of a polyacrylamide-based hydrogel (HAH) into wood endows the composite wood (BW/HAH/MCC) with high flame retardancy. Upon fire exposure, HAH/MCC rapidly provides a visual fire alarm. If the fire persists, the composite triggers further protection. The synergistic effect of CaCl 2 and combustion residues forms a dense physical barrier layer on the surface of the material, thereby significantly improving its flame retardancy through modification. The peak heat release rate (PHRR) and total heat release rate (THRR) of composite wood decreases by 89.9% and 71% respectively, with a ignition delay of 585 s. The limiting oxygen index (LOI) exceeds 60% for the composite wood. The flexural and compressive strength increase from 56.6 to 9.56 MPa to 64.3 and 11.19 MPa, respectively. In water immersion tests, the composite wood shows lower leaching of Ca 2+ (4.6%) and MB (36.8%), confirming its good weather resistance. This strategy integrates fire warning, passive hygroscopic reversal and passive flame-retardant functions, offering a promising design concept.