Long-term durability and flame retardancy for outdoor wood: Multifunctional enhancement using phenol-formaldehyde resin and monoguanidine phosphate salt

Wood’s flammability, low dimensional stability, and poor fungal resistance limit its application. In this study, Scots pine sapwood ( Pinus sylvestris L .) was treated with a low-molecular-weight phenol–formaldehyde (PF) resin and a monoguanidine phosphate flame retardant (FR) to achieve multifunctional enhancement. Differential scanning calorimetry confirmed no interference of FR with PF curing, while SEM and cell wall bulking analysis demonstrated effective penetration. The treatment improved the modulus of elasticity and rupture, though impact bending strength slightly decreased. Fungal decay resistance reached durability class 1 (EN 350, 2016) based on white- and brown-rot tests (EN 113–2, 2021). Dynamic vapor sorption showed no increase in equilibrium moisture content below 70 % RH. Artificial and natural weathering tests confirmed good FR retention and weathering resistance. Thermogravimetric analysis showed enhanced thermal stability, with higher char residue than FR-only treatments. Cone calorimetry verified sustained flame retardancy even after weathering. This combined treatment offers a durable, multifunctional wood modification approach suitable for structural and outdoor applications. • PF resin and monoguanidine phosphate salt effectively penetrate wood cell walls. • Enhanced mechanical properties, fungal resistance (DC 1), and dimensional stability. • Maintained durability and strong flame retardancy under artificial and natural weathering.