# Prediction of Mechanical Performance of Acetylated MDF at Different Humid Conditions
## 1 Introduction  
Medium density fiberboard (MDF) is manufactured with wood fibers bonded with water resistant adhesives such as phenol formaldehyde, urea formaldehyde, isocyanate resin, etc. [1; 2]. MDF is primarily used in furniture, as a building material and for laminate flooring, since it has good strength and stiffness and it is easy to process. Compared to plywood, MDF panels generally swell more and may not be recovered after drying due to the inherent hygroscopicity of the wood fibers, the residual stresses formed in the fiber mat during hot pressing and some loss of the glue bonds [3]. As a result, when the MDF panel is exposed to any form of water, its constituent wood fibers swell and some of that residual stress is released, resulting in an increase of thickness of the panel. Thickness swellingmarkedly weakens the product [4], and the mechanical properties that are most directly affected are shear strength and moduli of elasticity and rupture [5]. Exposure to outdoor environment with varying climate conditions can result in dimensional changes and strength loss, and thus MDF panels are generally not recommended for exterior applications. In terms of computational modelling, these phenomena are considered to be the rheological behavior of the material [6]. Several studies have been carried out using various adhesive systems [7; 8; 9], post treatments [10], heat treated fibers [11], alternative fibers [12] and recycled adhesives [13] to improve strength and water resistance of MDF panels. In addition, chemical modification was also used to improve the material properties, such as moisture-related properties, durability and weathering resistance [14; 15; 16]. The most described chemical modification method for improved dimensional stability of wood particles and fibers to produce panel products is acetylation with acetic anhydride [17; 18]. In acetylation process, acetic anhydride substitutes the hydroxyl (-OH) groups in the wood cells with acetyl groups, resulting in decreased hygroscopicity and increased dimensional stability; while reductions in some mechanical properties also occur depending on the extent of temperature and time of the modification reaction [18; 19; 20]. Several explanations of the strength loss were given, as for example the type of the adhesive [21], bondability [22], press pressure [23], etc. Higher bondability is required if the panels are used under severe conditions for a long time. Thus, the effect of weathering on the dimensional changes and mechanical properties of acetylated MDF would be beneficial for predicting its long-term service behavior. Mechanical strength and stiffness along and perpendicular to the sanding direction should be also available to achieve better and proper assembly of acetylated MDF panels in outdoor applications. Fiberboards can be exposed to a range of environmental conditions during service life. Moisture content of MDF, similar to other lignocellulosic materials, changes with the change of surrounding humidity and temperature. Therefore, it is important to know the relationship between moisture content and strength properties of MDF if used as a structural member subjected to these environmental conditions. Use of non-destructive acoustic testing could provide a rapid and reliable measurement of strength properties of MDF panels. Usually, acoustic testing is carried out by using time-of-flight (TOF) and resonance methods [24]. TOF methods use propagation time of a pulse of ultrasound or a stress wave across the material. On the other hand, resonance methods use the free vibration frequency of the material under forced harmonic vibration. Resonance methods provide more information on the elastic properties of materials and are thus considered more reliable than the TOF methods [25]. Previous studies showed a very good to strong relation between dynamic bending properties measured by acoustic tools with the static bending properties of wood panels [26; 27].