Highly fire-retardant optical wood enabled by transparent fireproof coatings

Transparent wood (TW) is a bio-sourced material with superior optical functions. However, the fire safety problem greatly restricts its development and applications. In this paper, a new fire-retardant TW (FRTW) with excellent optical properties and flame retardancy was prepared by impregnating a refractive index matching transparent fireproof coating (phosphate ester-polyethylene glycol (PEAG)) into delignified wood (DLW). TW/PEAG presented record-high transparency with the transmittance of 93% and the haze of 98%. Besides, the TW/PEAG had a 60% drop in peak mass loss rate and a 2.5-fold increment in char residual to those of the natural wood (NW) in ther-mogravimetric analysis indicating enhanced thermal stability. Its peak heat release rate, total heat release, and heat of combustion were decreased by 82.4%, 84.3%, and 80.8% to those of epoxy resin-based TW. Limiting oxygen index and flammability experimental results also indicated the enhanced flame-retardant performance of TW/PEAG. The enhanced flame retardancy of TW/PEAG was due to the PEAG that promoted the thermally insulating char layer formation and reduced the heat release. In addition, the TW/PEAG exhibited excellent mechanical performance as it obtained 153.6 MPa tensile strength and 2.2 GPa elastic modulus. The FRTW presented good optical and flame retardant properties, promising to become a desirable optical material in engineering fields.