Influence of the anatomical structure on the moisture sorption and thermodynamic properties of the African tropical woods

The present study summarizes various findings such as equilibrium moisture contents, effect of temperature and relative humidity, better fitted model for wood sorption, heat transfer characteristic and influence of the wood structure. For this purpose, three African tropical medium-heavy woods were used: Sapelli (Entandrophragma cylindricum (Sprague) Sprague), Sipo (Entandrophragma utile (Dawe & Sprague) Sprague), Kosipo (Entandrophragma candollei Harms). Adsorption and desorption isotherms were measured using the static gravimetric method at 30, 40, 50, 60 °C within the range of 5.5 – 83.6% relative humidity. Thermodynamic properties were calculated from the Clausius-Clapeyron and Avramidis equations. Quantitative anatomical parameters of the woods were determined from the SEM images. Results showed that the highest mean values of the measured anatomical parameters were obtained with Sipo and the lowest with Sapelli. The equilibrium moisture content obtained is relatively lower for Kosipo and higher for Sipo at low temperatures (T < 50 °C) and Sapelli at high temperatures (T ≥ 50 °C). The highest values of thermodynamic properties were obtained with Kosipo and the lowest with Sipo. Results of sorption characteristics are correlated with the anatomical structure of the woods studied. Sipo, with lower density (0.64) and higher vessel diameter, vessel proportion and ray size, has higher moisture content and lower thermodynamic properties. However, this trend is not respected between Kosipo and Sapelli. Kosipo, with a higher density and anatomical parameters, has lower moisture content and higher thermodynamic properties than Sapelli. This could be explained by a lower content of amorphous polymers and the presence of silica in some anatomical elements of Kosipo.