Effect of pressure impregnation with a boron-phenolic composite flame retardant on the combustion performance and mechanical properties of plywood,

Effects of a boron-phenol-based flame retardant were evaluated relative to the combustion performance and mechanical properties of structural plywood manufactured from two domestic softwood species: larch (Larix kaempferi) and Korean pine (Pinus densiflora). The flame retardant was applied using a standardized vacuum-pressure impregnation process, and the retention level, combustion resistance, and structural integrity of the treated specimens were determined. The results showed that the treated specimens met the Korean standard (KS F 3113) requirements for bending strength, modulus of elasticity, and water-resistant tensile-shear strength. Larch plywood exhibited modest changes in combustion and mechanical performance, whereas Korean pine showed significant improvements in flame retardancy, including longer ignition time, lower peak heat release rate, and reduced char length and area owing to the higher flame-retardant retention achieved with Korean pine. Furthermore, flame retardant impregnation did not affect adhesive bonding in either species. This study demonstrates that boron–phenol-based flame retardants can effectively enhance fire resistance in structural plywood without compromising its mechanical performance, thus supporting their applicability in manufacturing flame-retardant wood-based construction materials.