Phosphorylated sodium alginate-filled delignified wood-reinforced transparent wood: Crafting flame-retardant composites

Transparent wood (TW), an emerging sustainable material with high optical transmittance and mechanical strength, is hindered by the inherent flammability of its cellulose-lignin matrix, posing fire risks. To address this, we developed an eco-friendly flame-retardant TW by integrating phosphorylated sodium alginate (PSA)-filled delignified wood with polyurethane acrylic resin (PUA). Delignified balsa wood was freeze-dried into a porous structure, functionalized with PSA, and reinforced with UV-cured PUA. Structural characterization confirmed uniform PSA coating on delignified wood channels, introducing phosphorus groups that enhanced thermal stability and flame retardancy. Combustion tests revealed self-extinguishing behavior in PSA-modified delignified woods, reducing peak heat release rate (pHRR) by 67 % and total heat release (THR) by 80 % versus delignified wood. The final PUA/wood-PSA3 composite demonstrated a 27 % pHRR reduction and 47 % lower THR compared to pure PUA, alongside 61 % less smoke emission. Optical transmittance of composites decreased moderately due to PSA's light absorption, while tensile strengths of PUA/wood composites surged to 30 MPa owing to the aligned cellulose scaffold. The flame-retardant mechanisms were investigated using characterization of the char residue and molecular dynamics simulations. These methods demonstrated the effectiveness of phosphorus-induced char formation and radical quenching in enhancing fire safety while preserving essential material properties.