Development of nitrogen-phosphorous and nano-SiO2-based fire retardant system for improved thermal, mechanical, and flame retardant properties in wood

Among several fire retardant (FR) additives, the nitrogen-phosphorous-based systems are most preferred in timber applications. These compounds together exhibit excellent synergy in reducing flammability. However, their application also produces certain undesired side effects in wood, such as loss in strength. As a possible solution to this problem, the nano-SiO2 sol (1.0, 2.5, and 5.0 %) was impregnated in wood along with nitrogen-phosphorous compounds, i.e., di-ammonium hydrogen orthophosphate (15, 20, and 25 %) + dicyandiamide (3 %), and their synergistic effect on flammability, thermal, and mechanical properties was studied in this research. The dispersion of nano-SiO2 in wood was examined by scanning electron microscopy (SEM). The interaction between SiO2 nanoparticles, FR chemicals (DAP/DCD), and wood was studied by Fourier transform infrared spectroscopy (FTIR). Limiting oxygen index (LOI) and cone calorimetry were used to evaluate the fire performance of treated and untreated wood. The optimum FR concentration was recorded as 20 % di-ammonium hydrogen orthophosphate (DAP), 3 % Dicyandiamide (DCD) and 2.5 % nano-SiO2 (NS). The result showed that at optimum concentration (NS2.5/DAP20/DCD3), the flexural strength was increased by 39 % and 54 % and flexural modulus by 59 % and 37 % in Hevea brasiliensis and Pinus roxburghii wood, respectively, as compared to pure FR (DAP20/DCD3)-treated samples. Thermogravimetric analysis (air atmosphere) results indicated initiation of early charring and reduced mass loss rate in NS/DAP/DCD treated samples, as compared to pure FR-treated and control specimens. Based on the mechanical properties, thermal stability, and flammability results, this study confirmed an excellent synergy between nitrogen-phosphorous compounds and nano-SiO2 as FR.