Investigating the surface wettability and surface free energy of sodium silicate-impregnated poplar wood

The surface free energy of wood is an important property that influences later manufacturing process, such as wood gluing and finishing. Sodium silicate modifications of poplar wood inevitably alter wood's surface free energy. The purpose of this study was to determine the effect of various impregnation times on the resulting surface free energy of radial and tangential sections of poplar wood specimens. The contact angles of standard probe liquids (i.e. distilled water, diiodomethane, formamide) on the surface of the wood were measured by contact angle goniometer, and a geometric average method and acid–base interaction method were applied to calculate and analyze the changes in the surface free energy. In addition, Fourier transform infrared spectroscopy was used to analyze the changes in the functional groups on the wood surface; after finishing the wood surface with water-based or oil-based paint, the paint film adhesion properties were also evaluated. The results indicate that sodium silicate modification can effectively improve the surface wettability and surface free energy of poplar wood. The modification treatment gradually creates hydrophilic groups (e.g. hydrogen bonds, free hydroxyl groups) on the wood surface, and their contents increase with increasing impregnation time. Additionally, by introducing Si-O–C moieties, the polarity, dispersion, and acid–base components of the wood surface all increase to varying degrees. Finally, the sodium silicate modification treatment worsens the adhesion of the paint films on the wood surface, although not to a significant extent.