# Effect of Variable Conditions of Exposure on the Physical and Mechanical Properties of Blockboards
## 3 Results and Discussion
### Physical Properties
Nevertheless, after 3 months, the increases in MC (19% for V blockboards and 52% for HDF blockboards) were comparable with those obtained in the kitchen. These could be attributed to the dynamics of swelling and shrinkage of panels in the environmental conditions with high variability of RH. MC increased slower in HDF structures compared to veneered ones, but, in time, the fibres absorbed more humidity from the atmosphere and, thus, reached a higher MC after 3 months. In the climatic chamber, the dynamics of the parameter variations were different than in the other two exposure climates (Table 5), being determined by the programmed cyclic changes in RH. After three months of exposure, the thickness and weight increases were 1.8 and 1.1 times higher for V blockboards and 4.4 and 0.4 times higher for HDF blockboards compared to values recorded in the kitchen. The highest thickness swelling (TS) values were achieved by blockboards with HDF faces in the climatic chamber (4.29%) and the lowest ones by blockboards with V face sheets in the kitchen (0.77%) after three months of exposure. TS values recorded for the tested boards in the kitchen and bathroom increased after one month by 0.11% and 0.49%, respectively, for V blockboards and by 0.39% and 0.73% for HDF blockboards. After two months, slight decreases were registered, indicating lower RH changes in the environment. Cyclic changes in RH had an effect on the TS increase in the climatic chamber after one month with 1.82% for V blockboards and 3.6% for HDF blockboards. These increases continued until the end of the exposure period. TS values were much higher for blockboards with HDF face sheets compared to those with V face sheets at the end of the exposure period. It can be attributed to the low moisture diffusion in the veneer faces with transverse grain orientation, which inhibits deeper moisture penetration: a fact also observed in plywood by other researchers [26]. \begin{table}
\begin{tabular}{l r r r r r r} \hline \hline  & & \multicolumn{2}{c}{**Ba-V**} & \multicolumn{3}{c}{**Ba-HDF**} \\  & **t, (mm)** & **w, (g)** & **MC, (\%)** & **t, (mm)** & **w, (g)** & **MC, (\%)** \\ \hline \multirow{3}{*}{Before testing} & 18.12 & 938 & 8.35 & 17.68 & 1305 & 6.98 \\  & (0.05) & (1.6) & (0.2) & (0.09) & (2.5) & (0.02) \\
1 month & 18.21 & 959 & 8.46 & 17.81 & 1331 & 7.37 \\  & (0.08) & (2.2) & (0.5) & (0.11) & (3.9) & (0.4) \\
2 month & 18.18 & 955 & 8.83 & 17.80 & 1328 & 7.9 \\  & (0.07) & (2.5) & (0.3) & (0.09) & (5.3) & (0.3) \\
3 month & 18.35 & 963 & 9.95 & 17.95 & 1321 & 10.6 \\  & (0.09) & (5.7) & (0.4) & (0.11) & (6.0) & (0.9) \\ \hline \hline \end{tabular} Values in the parenthesis are standard deviations.  
\end{table} 
Table 4: Variation of the blockboards’ physical parameters during their exposure in the bathroom conditions (average values recorded for each month). The accelerated weathering in climatic chamber more rapidly influenced the moisture uptake by all blockboards (Figure 4). The higher the RH, the higher the observed increase in moisture content. The panels exposed in the climatic chamber (Table 5) exhibited the highest increase of MC: 41% for V blockboards and 82% for HDF blockboards after three months of exposure, followed by those exposed in the bathroom and kitchen. There were small differences in moisture content uptake by the boards with the same structure exposed in the kitchen and bathroom. In the climatic chamber, MC increased rapidly after one month of exposure, then the increase was diminished as a result of reaching equilibrium inside the structure. The increase in MC was higher for the structures with veneered faces in all situations compared to those with HDF faces after two months of exposure. It seems that the higher density of HDF face sheets, at least at the beginning of exposure, limited and slowed down the penetration of moisture due to the high compression of the fibres.