Bio-inspired lightweight pulp foams with improved mechanical property and flame retardancy via borate cross-linking

The ultra-lightweight and porous cellulose foam can be used in a broad range of applications (e.g. structural materials, sound insulation, thermal insulation), but the demands for high strength and fire-retardant properties are still challenges for the end uses. In this work, inspired by the cross-linking function of borate in plants, a lightweight and highly porous pulp foam with improved mechanical properties and flame retardancy was simply prepared via borate cross-linking. It was found that the prepared pulp foams had low density in the range of 13.3–16.4 mg/cm3, high porosity (>98%), and high compressional strength (up to 74.1 kPa, which was 28 times higher than that of pristine foam without borate). More interestingly, the pulp foam could be repeatedly squeezed more than 10 times without failure of its 3D structure. Compared with the readily flammable pristine pulp foam, the resultant pulp foams containing boron with low thermal conductivity (about 0.045 W/(m·K)) showed improved flame-retardant properties and excellent self-extinguishing behavior. When the content of boron was 3.45 wt%, the resultant pulp foam was completely incombustible. Therefore, this study provides a cost-effective and facile route for the fabrication of high-strength and porous pulp foams with excellent flame-resistance and heat insulation properties.