Enhancing wood stability and fire retardancy through citric acid and phosphorylated sucrose stearate cross-linking modification

Citric acid (CA) is commonly used as a green and cost-effective crosslinker to enhance the physical and mechanical properties of fast-growing wood species. However, the flammability of CA-treated wood remains high and hinders its use in areas where fire resistance is critical. Therefore, it is essential to find a sustainable and efficient fire retardant that can be combined with CA to prepare wood with exceptional overall performances. In this study, we synthesized phosphorylated sucrose stearate (SUP) and used it to modify poplar wood together with CA, creating a cross-linking network. The resulting wood's physical, mechanical properties, and fire resistance were evaluated while analyzing CA-SUP network's fire-retardancy mechanism. Results have demonstrated that successful cross-linking modification was achieved through esterification, resulting in improved dimensional stability and water resistance. Furthermore, SUP has been found to homogenously disperse in the wood structure, reducing the degree of cell wall damage caused by the CA treatment, while further enhancing the water resistance and MOE of wood. Whilst the CA treatment improved the thermal stability of wood, it resulted in a dramatic release of smoke when the CA-treated wood was flamed. However, the addition of a small quantity of SUP, specifically in the CA-SUP-5% sample, was able to increase the limited oxygen index and promote quick extinguishing when flamed. Notably, compared to the CA-treated wood, the heat release rate and smoke production rate of the CA-SUP-5% sample reduced remarkably by 38.5% and 71%, respectively. The exceptional fire retardancy and smoke suppression were achieved through the synergy of CA and SUP, as they dehydrated, decarboxylated, and catalytically carbonized, promoting the release of water and CO2, thus diminishing the release of CO and generating dense char residue. Consequently, this study presents a sustainable, cost-effective, and efficient method to prepare high-quality wood.