Multifunctional Modification of Pinus sylvestris L. Wood Using a Boron-Based Coating Containing CuO Nanoparticles: Flame Retardancy, Char Structure, and Antibacterial Activity

The present study examines the modification of Pinus sylvestris L. wood (PW) using a water-based boron–nitrogen–silica–PVA-CuO NPs flame-retardant (FR) system, and its effects on the flame retardancy, char structure and antibacterial activity. The microwave-prepared FR formulation, containing 0, 1, or 3 wt.% green-synthesized CuO nanoparticles (NPs) dispersed by ultrasonication, was applied to PW by ultrasound-assisted impregnation. The untreated and FR-treated PW were characterized by SEM-EDX, FTIR, XRD and TG/DTG. Fire performance was evaluated by the vertical flame test (VFT), and antibacterial activity was assessed against Staphylococcus aureus. SEM-EDX revealed a continuous FR coating on the wood surface, while FTIR indicated interactions between the coating components and the wood substrate. The FR treatment reduced flammability, decreasing the after-flame time from 37 s for untreated wood to 1–12 s for the treated samples. Increasing the CuO NP content further shortened the after-flame time and enhanced antibacterial activity, although at the highest loading, CuO NPs may catalyze char oxidation during thermo-oxidative decomposition. Raman spectroscopy of the char residues revealed characteristic D and G bands, confirming the formation of a protective carbonaceous char layer. The boron–nitrogen–silica–PVA–CuO FR formulation thus provides a multifunctional approach to wood protection.