# Effect of Variable Conditions of Exposure on the Physical and Mechanical Properties of Blockboards
## 3 Results and Discussion
### Mechanical Properties  
The results of the mechanical properties of the tested panels are presented in Table 6. Initial values (before testing the boards) of MOR and MOE were close to those mentioned in the literature for blockboard exposed under stable indoor conditions [30,31]. For the majority of tested boards, higher MOR initial values were registered for blockboards with HDF face sheets (Table 6). After one month of exposure, the flexural properties started to decrease by 1% until 7.5%, with no noticeable differences between the panels exposed in the kitchen and bathroom. A slight effect of moisture content on MOR and MOE after one month of exposure was observed for both blockboard types. In contrast, the exposure in accelerated climate (CC) determined a decrease of strength after one month, within the range of 4% and 13%, more evident for blockboards with HDF faces.  
\begin{table}
\begin{tabular}{l c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Kitchen**} & \multicolumn{2}{c}{**Bathroom**} & \multicolumn{2}{c}{**Climatic Chamber**} \\
**Time of** & **MOE,** & **MOR,** & **MOE,** & **MOR,** & **MOE,** & **MOR,** \\
**Exposure** & **[N/mm\({}^{2}\)]** & **[N/mm\({}^{2}\)]** & **[N/mm\({}^{2}\)]** & **[N/mm\({}^{2}\)]** & **[N/mm\({}^{2}\)]** & **[N/mm\({}^{2}\)]** \\ \hline \hline \multicolumn{8}{c}{Structure 1, with veneer face sheets} \\ \hline Before exposure & 6820 & 44.1 & 7120 & 43.6 & 7133 & 48.2 \\  & (620) & (5.5) & (450) & (2.1) & (550) & (5.4) \\ \(1\) month & 6750 & 42.2 & 6820 & 41.8 & 6830 & 45.6 \\  & (570) & (4.2) & (660) & (2.7) & (733) & (4.5) \\ \(2\) months & 6440 & 37.4 & 6680 & 38.6 & 6223 & 41.6 \\  & (540) & (3.9) & (450) & (3.2) & (426) & (5.7) \\ \(3\) months & 6230 & 36.5 & 6470 & 32.1 & 5665 & 32.8 \\  & (230) & (2.2) & (340) & (4.2) & (399) & (1.9) \\ \hline \hline \multicolumn{8}{c}{Structure 2, with HDF face sheets} \\ \hline Before exposure & 6380 & 48.3 & 6860 & 49.2 & 6630 & 44.3 \\  & (510) & (5.3) & (678) & (3.6) & (613) & (3.7) \\ \(1\) month & 5950 & 46.8 & 6530 & 45.5 & 5783 & 40.2 \\  & (420) & (4.5) & (546) & (1.4) & (733) & (4.3) \\ \(2\) months & 5746 & 43.5 & 6080 & 38.4 & 5365 & 36.4 \\  & (254) & (5.6) & (453) & (2.7) & (486) & (4.8) \\ \(3\) months & 5480 & 38.2 & 5746 & 34.2 & 4700 & 28.7 \\  & (240) & (3.6) & (380) & (2.5) & (385) & (2.1) \\ \hline \hline \end{tabular} Values in the parenthesis are standard deviations.  
\end{table}
Table 6: Mechanical properties of the blockboards exposed in different climate conditions (average values).  
Figure 10: Deflection values and warpage of blockboard with HDF faces tested in the climatic chamber: (**a**) at the beginning of the experiment; (**b**) after three months of exposure in the climatic chamber.  
The boards exposed in the climatic chamber had an accentuated decrease of the mechanical properties compared to the panels exposed in the kitchen or bathroom. The increase of moisture content in the structures caused the reduction of bending strength and modulus of elasticity by approximately 32% and 21% for V blockboards and by approximately 35% and 20% for HDF blockboards.  
The higher decrease of mechanical properties was observed on blockboards with HDF face sheets compared to those with veneers due to the highest MC uptake. The noticeable decreases of the MOR and MOE values over 65% relative air humidity are in line with the results obtained by other researchers [17], who observed that these reductions are more intensive for fibrous panels of higher density when the relative air humidity is raised from 65% to 95%. A similar tendency for strength reduction for different wood-based composites when MC increases was also found by other researchers [32; 33; 34; 35].