The aim of this study was to enhance the water repellency and dimensional stability of thermally modified wood by combining the preimpregnation of paraffin wax emulsion. To achieve this, Southern pine (SP; Pinus spp.) samples were first impregnated with paraffin wax emulsion (with 2.0% solid content) and then subjected to thermal modification (TM) in an oven at 180°C and 220°C for 4 and 8 h. The contact angle (CA), surface free energy, water absorption rate (WAR), anti-swelling efficiency (ASE), and bending properties of the control and modified samples were investigated. Moreover, the chemical and morphological alterations were analyzed by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Both the wax impregnation and TM decreased the surface wettability, water absorption, and tangential swelling, while the combination of wax and thermal treatment exhibited the best water repellency and dimensional stability, indicating the synergism between the two procedures. However, the wax preimpregnation did not affect the mechanical properties of thermally modified wood. The FTIR, SEM, and XPS analyses confirmed that the synergistic effect is mainly due to the redistribution of the paraffin wax during TM rather than its impact on the chemical changes caused by thermal degradation.

Natural wood is susceptible to moisture during its practical use, limiting not only service life but also the range of uses. In this study, plant extracts, specifically linseed oil and carnauba wax (both environmentally friendly and water-repellent substances), were examined as a means to mitigate limitations of natural wood. Stable and homogenous epoxidized linseed oil/carnauba wax emulsions with an average particle size less than 230 nm were used as a modifier to enhance the performance of wood. Weight percentage gain, bulking coefficient, micromorphology, chemical structure, moisture adsorption, contact angle, water repellency, and dimensional stability of treated wood were measured. Using a scanning electron microscope (SEM), it was observed that carnauba wax and epoxidized linseed oil acted as a mechanical barrier and could partially cover or block vessels, pits, and wood rays. Furthermore, the hydrophobicity of wood increased significantly after emulsion impregnation. However, there was no obvious chemical reaction between epoxidized linseed oil and the main components of wood in the Fourier transform infrared spectroscopy (FTIR) results. The combined effect of epoxidized linseed oil and carnauba wax was apparent in both decreased water absorption and dimensional deformation. Poplar wood impregnated with an epoxidized linseed oil/carnauba wax complex emulsion shows potential for improving water-related properties. Thus, the complex emulsion modification can improve the inherent shortcoming of poplar wood effectively.