Optimization of Hydrothermal and Oleothermal Treatments for the Resistance of Dabema (Piptadeniastrum africanum (Hook.f.) Brenan) Wood

This study evaluates the effects of hydrothermal and oleothermal treatments on the physical, colorimetric, and mechanical properties of Dabema wood. Samples were heated at 100, 160, and 220 °C for 2, 3.5, and 5 h. Equilibrium moisture content decreased from 13.16% in untreated wood to approximately 43% lower after hydrothermal treatment at 160 °C for 5 h and to 64% lower after oleothermal treatment at 220 °C for 5 h. Water absorption decreased from 78% in untreated samples to 25%–64% following hydrothermal treatment and to 17%–44% after oleothermal treatment. Hydrothermal treatment caused significant darkening, whereas oleothermal treatment maintained a lighter, more stable color. Mechanical properties improved substantially: in compression, MOE increased by 113% after oleothermal treatment at 220 °C for 5 h. In bending, MOR and MOE rose by 25%–35% under optimal oil-heat conditions. In tensile, MOE increased by 30%, and maximum tensile stress improved by up to 130%. Oleothermal treatments yielded the most stable enhancements, whereas severe hydrothermal treatments sometimes reduced mechanical performance despite improving moisture resistance. Multivariate analysis (PCA) and response surface methodology (RSM) indicate that oleothermal treatment at 160 °C for 3.5–5 h provides the best compromise between stiffness and color stability. Thermogravimetric analyses (TG/DTG) show hydrothermal treatment promotes hemicelluloses degradation, whereas oleothermal treatment stabilizes the cellulose–lignin network. Overall, hydrothermal treatment enhances dimensional stability, while oleothermal treatment achieves an optimal balance of stiffness, mechanical performance, and color retention. Deep color changes from furanic resin formation under hydrothermal conditions are strongly suppressed by oil during oleothermal processing, yielding lighter and more durable wood. For commercial applications such as furniture and structural components, oleothermal treatment is recommended, whereas hydrothermal treatment is more suitable when dimensional stability is prioritized over mechanical performance.