# Prediction of Mechanical Performance of Acetylated MDF at Different Humid Conditions
## 3 Results and Discussion
### Mechanical Properties
However, acetylation reduces the loss of surface lignin and thus the erosion caused by accelerated weathering [48]. When compared with commercial MDF samples (indoor use), Medite Tricoya(r) samples performed much better in terms of the extent of being weathered by moisture and UV light. Thickness swelling  
Figure 2: Linear relationship between MOEstat and MORstat of the Medite Tricoya® medium density fiberboard (MDF) samples in two different directions conditioned at three different climatic conditions. of the test pieces from indoor MDF was almost twice as much as Medite Tricoya(r) (Figure 4). After the accelerated aging test, fibers in the indoor samples were separated and were possible to shred by finger rub. That implied that no residual strength was left. Medite Tricoya(r) samples showed better resistance against thickness swelling. This could be attributed to the fiber-fiber bonding efficacy of the resin to retain bonding in a very hydrophobic fiber network. After accelerated aging and conditioning (20 \({}^{\circ}\)C, 65% RH), the IB strength of the Medite Tricoya(r) samples was found to be 0.79 \(\pm\) 0.06 MPa. This result showed that Medite Tricoya(r) samples made from acetylated wood fibers were able to retain the initial IB strength of non-weathered samples. Previous results also showed that a higher residual strength was observed in acetylated MDF after a cyclic test [19]. Such retention of strength can also be attributed to other parameters than the acetylation of fibers, such as the adhesive used [49]. Acoustic resonance measurements were used to determine the \(\mathrm{MOE_{dyn}}\) (Figure 5). These values represent the mean stiffness, whilst \(\mathrm{MOE_{stat}}\) represents the local stiffness of the samples at the highly stressed areas of a specific test set-up [50]. \(\mathrm{MOE_{dyn}}\) was found to differ significantly between // and \(\perp\) samples only for the dry condition, i.e., 35% RH. As expected, the increase of RH from dry/standard (35 and 65% RH) to wet (85% RH) conditions resulted in a considerable decrease in \(\mathrm{MOE_{dyn}}\) in both directions.Resonance frequency of MDF samples decreased with the increase in moisture content and thus affected the \(\mathrm{MOE_{dyn}}\). This is because at a dryer state, molecular chains in the amorphous regions of the wood cell wall are distorted with the presence of microvoids between the molecular chains, resulting in lower internal friction, resulting in higher \(\mathrm{MOE_{dyn}}\). On the contrary, when moisture content increases, water molecules are embedded in the microvoids and rearrange the distorted molecular chains in the amorphous region. If the moisture content increases further, water acts as a plasticizer and decreases the cohesive forces between molecules, resulting in a higher internal friction and leading to the decrease in \(\mathrm{MOE_{dyn}}\)[51]. With the increase of RH from 35 to 65%, the average \(\mathrm{MOE_{dyn}}\) reduction of Medite Tricoya(r) samples was negligible, i.e., 1%. When the RH was further raised from 65% to 85%, the reduction of \(\mathrm{MOE_{dyn}}\) was 26%. In addition, \(\mathrm{MOE_{dyn}}\) values in the perpendicular samples were found lower than the corresponding values in the parallel samples. However, those differences were found to be statistically insignificant. A previous study by [PERSON] et al. [26] showed that reduction of stress wave velocity along the perpendicular direction compared to the parallel direction for wood-based panels (plywood, oriented strand board and particleboard) depends on the anisotropic properties  
Figure 4: Medite Tricoya® and commercial standard MDF (for interior use) samples after accelerated aging test. Note the double thickness in the commercial MDF sample. Figure 3: Linear relationship between \(\mathrm{MOR_{stat}}\) and internal bond (IB) strength of the Medite Tricoya® MDF samples conditioned at three different humidity. of the products. Smaller differences between the two directions in MDF samples implies a uniform product.