Effect of freeze-thaw cycles on thermal stability and fire retardancy of P/N/B wood composites

The inherent combustibility of wood represents a critical vulnerability, compromising the structural integrity and fire safety of wooden buildings. This work reports on a highly fire-safe P/N/B wood composite. This material was fabricated based on the freeze-thaw strategy, involving the co-impregnation of amino trimethylene phosphonic acid (ATMP) and boric acid (BA) into the wood substrate. Benefiting from the pore size changes caused by freeze-thaw cycles, the total loading of ATMP and ATMP/BA in wood increased by 37.5% and 21.9%, respectively, as compared the untreated sample. The freeze-thawed wood composite exhibited an ultra-high limiting oxygen index (LOI) of 87.9 % and a V-0 rating was reached in the UL-94 vertical burning test (5 mm). The peak heat release rate (pHRR) and total heat release rate (THR) decreased by 57% and 91%, respectively, as compared to untreated wood. The results showed that the improved fire retardancy performances were attributed to the char layer formed during the combustion, which acts as a thermal barrier, preventing the transfer of heat and mass. SEM, Raman, XPS, FTIR, and XRD results confirmed that phosphates and borophosphates species were formed during burning and acted as reinforcing substances to enhance the strength and stability of the char layer. In summary, this study provides a simple and effective approach for modifying the fire safety of wood, with great potential for construction applications.