Thermochemical and physical properties of two fast-growing eucalypt woods subjected to two-step freeze–heat treatments

• Two-step combined treatments showed different results for each step separated.
• Use of freezing reduced the degradation of the woods’ chemical compounds.
• FTIR analysis proved the decrease of OH adsorption sites.
• The apparent contact angle confirmed the hydrophobicity degree of wood surface.

In this study, the effect of two-step freeze–heat treatments on the thermochemical and physical properties of two Eucalyptus wood were investigated. Five treatments were performed and compared to untreated control: freezing (WF), freezing followed by heat treatment at 180 and 200 °C (WFT180 and WFT200) and heat treatment at 180 and 200 °C (WT180 and WT200). The freeze step was performed at −22 °C for 72 h and the heat step was performed at 180–200 °C for 3.5 h. Thermochemical (wet chemical analysis, FTIR and TGA), physical (water absorption and ASE) and surface properties (wettability) were measured. The two-step freeze–heat treatments changed the holocellulose and extractives contents, and stabilized the 1% NaOH solubility. Dimensional stability increased and hygroscopicity of thermally treated eucalypt wood decreased. The two-step treatments resulted in lower chemical changes than the application of a heat treatment only. The apparent contact angle of thermally treated wood increased, confirming the improvement of wood hydrophobicity.