# Prediction of Mechanical Performance of Acetylated MDF at Different Humid Conditions
## 3 Results and Discussion  
### Physical Properties  
Moisture content, density and thickness swelling of commercially produced Medite Tricoya(r) MDF samples at three different RH levels are presented in Table 2. As expected, the EMC and density of samples increased with an increase in RH levels. Medite Tricoya(r) samples had 48% lower equilibrium moisture content when compared with commercial indoor MDF samples of similar thickness ([PERSON] et al., unpublished data) at 85% RH. Similar results can be found in a previous work [17]. As the acetylation process is a single site reaction, which means that one acetyl group is attached to one hydroxyl group resulting in a reduction of moisture absorption sites in wood polymers [40], acetylated MDF panels have low EMC even at high humidity level. Only 7.1% thickness swelling (after soaking in water for 24 h) was observed, meaning that acetylated panels do not swell severely when they are exposed to the water. These results showed that acetylation plays a significant role in the thickness swelling of the Medite Tricoya(r) samples. However, the extent of the reduced thickness swelling of acetylated MDF depends on the weight percent gain levels by acetylation process [2].  
Concerning the results of dimensional changes, the acetylation caused the MDF panels to absorb little moisture during the conditioning and are thus dimensionally stable (Table 3). The linear expansion of sample length and thickness swelling values obtained in adsorption conditions (relative humidity change from 65% to 85%) were higher than those values obtained in desorption conditions (relative humidity change from 65% to 35%). The amount of water held by wood fibers at a given temperature and RH depends on the direction from which equilibrium is approached. The moisture adsorbed at high relative humidity exposure is not entirely released when re-drying by lowering the relative humidity levels, and this phenomenon was well observed for wood-based panels in other studies [41; 42]. As a result, acetylation had a major effect on the dimensional stability of Medite Tricoya(r) samples. In addition, the density of panels can also adversely affect the dimensional stability [41]. Average linear expansion and retraction of Medite Tricoya(r) samples were, respectively, 23% and 57% lower than  
\begin{table}
\begin{tabular}{c c c c} \hline
**Properties** & **35\% RH** & **65\% RH** & **85\% RH** \\ \hline EMC (\%) & 4.6 (0.14) & 7.6 (0.14) & 7.9 (0.12) \\ Density (kg/m\({}^{3}\)) & 729.2 (9.59) & 739.6 (1.37) & 742.3 (7.51) \\ \hline Thickness swelling (\%) \({}^{*}\) & & 7.1 (0.36) & \\ \hline \end{tabular}
\end{table}
Table 2: Physical properties of Medite Tricoya® MDF samples at different RH levels. Values in parenthesis are the standard deviations.  
those of standard samples ([PERSON] et al., unpublished data) and for thickness changes, those differences were 67% and 45% (comparison was done from [PERSON] et al., unpublished data). However, no significant differences of relative shrinkage or swelling in thickness and length were observed in the two principle directions (// and \(\perp\) direction of sanding). An exception was seen for the relative swelling in length when RH increased from 65% to 85% where swelling was significantly higher in perpendicular samples. However, the reason for that is not quite clear.