Bulk Modification of Wood Using a Combination of Hydrophobic Silica Sol–Gel and Nanoclay to Enhance Dimensional Stability and Durability

Wood modification aimed at reducing moisture sensitivity and biological degradation should improve dimensional stability and durability without unacceptable deterioration of mechanical performance. This study investigated bulk impregnation of European beech (Fagus sylvatica L.) and Scots pine (Pinus sylvestris L.) using vinyltriethoxysilane (VTES), Cloisite 30B nanoclay (C30B), and NaOH, and the combination of VTES and C30B with emphasis on the different NaOH concentrations (VTES + CN-1, VTES + CN-2, and VTES + CN-3). Treatment uptake and retention, volumetric bulking coefficient, dimensional stability, mechanical properties, decay resistance, color, and FTIR spectral changes were evaluated. The combined CN treatments provided the greatest dimensional stabilization. VTES + CN-2 produced the highest volumetric anti-swelling efficiency, reaching 60.51% in beech and 60.05% in Scots pine. Longitudinal compressive strength was not significantly affected, although hardness decreased after some treatments. The CN formulations also reduced fungal mass loss in both species by up to 40.9% compared to the control. Overall, the results show that combined VTES-C30B-NaOH modification can improve dimensional stability and biological resistance, but its effectiveness depends strongly on formulation and wood species.