A flame-retardant and optically transparent wood composite

Transparent wood (TW) is an emerging functional wood composite with attractive advantages and promising applications. The versatile functions of the impregnating polymers offer various properties to the TW, but also threaten its fire safety and restrict its applications. Herein, fire retardant TW (FRTW) is prepared by ultraviolet (UV)-assisted lignin modification followed by refractive-index matched transparent fire-retardant coating (TFRC) impregnation. FRTW has a compact structure with a homogeneous interface between the TFRC and cell wall. It presents excellent optical and structural properties with 95% optical transmittance, 88% UV absorption, 96% high haze, and 154.3 MPa tensile strength. Moreover, FRTW shows great flame retardancy against high-temperature heating. It has a 68.2% reduced peak mass loss rate and a 2.8-fold increased char residual than natural wood (NW). Compared to the conventional epoxy-based TW, FRTW has a weaker combustion behavior with an 81.4% reduced peak heat release rate. The enhanced flame retardancy is due to the synergistic flame retardancy (combining cooling, dilution, insulation and radical capture effects) of the TFRC. FRTW provides a new wood composite alternative with excellent optical properties and flame retardancy, which broadens the application potential of conventional TW with excellent fire safety.