Flame-retardant, heat-insulating and char formation properties of densified Pinus sylvestris treated with different compression pressures

Densified Pinus sylvestris wood was prepared via densification treatment at different compression pressures, and the flame retardancy and heat-insulating property of the resulting densified Pinus sylvestris wood are investigated by various analytical methods. The results verify that the densification treatment has a negligible effect on the composition and chemical structures of the wood, while the flame retardancy, smoke suppression and heat-insulating properties of wood are significantly increased. However, an excessive compression pressure will destroy the porous structure of wood, which results in the decrease in flame-retardant efficiency of the densified wood. When the compression pressure reaches 10 MPa, the resulting specimen presents the lowest fire hazard, which has 9.8% reduction in total heat release (THR), 37.1% reduction in total smoke release (TSR) and 25.9% reduction in equilibrium backside temperature at 2400 s compared to natural wood. Char formation analysis shows that the enhanced flame retardancy of densified wood can be attributed to a lower contact area between the wood and high-temperature environment accompanied with the generation of a strong and thermally stable char layer rich in aromatic and cross-linking structures. These attractive features of densified wood effectively reduce the transfer of heat and combustion rates when exposed to flame, resulting in better fire-retardant and heat-insulating properties during combustion.