Silica Aerogel Co-Precursor Impregnation Method for Improved Hydrophobicity and Dimensional Stability of Scots Pine and European Beech Wood

This study investigated the effects of silica nanosol co-precursor impregnation using trimethylethoxysilane (TMES) and tetraethoxysilane (TEOS) on the water-related properties of Scots pine (Pinus sylvestris L.) and beech (Fagus sylvatica L.). Five TMES/TEOS molar ratios between 0.50 and 1.50% were applied by vacuum impregnation to promote in situ aerogel formation in the wood tissue. Mass, density, cell-wall bulking, equilibrium moisture content, swelling, and sorption behavior were evaluated. Increasing treatment molar ratio progressively reduced moisture-related dimensional changes. At 1.50% TMES/TEOS, volumetric swelling decreased from 7.30% to 2.82% in Scots pine and from 7.98% to 3.24% in beech, corresponding to anti-swelling efficiencies of approximately 60%. Equilibrium moisture content decreased from 11.5% to 8.98–9.56% in Scots pine and from 13.4% to 8.97–9.29% in beech. Scanning electron microscopy and silicon-specific energy-dispersive X-ray spectroscopy confirmed treatment material deposition within the wood structure, with denser deposits and increased agglomeration at 1.50% TMES/TEOS. TMES/TEOS impregnation improved hydrophobicity and dimensional stability while preserving the hygroscopic nature of wood tissue. This retained moisture responsiveness may support passive indoor humidity regulation, potentially complemented by the moisture-buffering properties of the silica aerogel.