hardwoods have more hemicellulose than softwoods (Holtzapple 2003 García Esteban et al. Since softwoods and hardwoods show a different hemicellulose content, i.e. So, hydroxyl groups of hemicellulose catch water molecules by forming hydrogen bonds and are responsible for moisture entering the wood mass. However, because cellulose is mostly in crystalline form that is not accessible to water, hemicellulose is considered to have a greater affinity to water (Schirarend 1986 Berry and Roderick 2005 Kulasinski et al. Cellulose and hemicellulose have a higher content of hydroxyl groups than lignin. 2013).Ī central role in the hygroscopic behavior of wood can be ascribable to the hydroxyls of its constituents. A sorption isotherm is a discrete representation of equilibrium moisture states of wood with its surrounding environment and each state is attained after either adsorption or desorption of water molecules (Engelund et al. At constant temperature (T) and relative humidity (RH), wood reaches an equilibrium state characterized by a certain equilibrium moisture content (EMC) (Glass Samuel and Zelinka 2010). Moisture content (MC) of wood also affects its resistance to decay and insects as well as its preservative treatment (Tsoumis 1991). While moisture in the cell lumens only produces an increment of weight (Tsoumis 1991), moisture contained in cell walls influences the mechanical and thermal properties of wood (Bartolucci et al. fibers.įurthermore, due to its hygroscopicity, wood can absorb moisture from the surrounding environment in the form of vapor or liquid water. vessels, and strength-giving elements, i.e. Indeed, its microstructure is made up of conducting elements, i.e. Different from softwood, hardwood has more complex anatomical features and greater structural variation (Stagno et al. Softwood has a quite homogeneous microstructure mostly dominated by one kind of structure, called tracheid (Capuani et al. Wood is classified as softwood and hardwood. In particular, each wooden species has pores, such as pits, perforations, and the lumen of vessels, tracheids, and fibers, with peculiar dimensions and spatial organization (Capuani et al. Wood can be also described as a porous system with strongly variable and multi-scale porosity. Among them, cellulose can be considered the most abundant component of wood (Alesiani et al. It is characterized by fibers with a cell wall composed of natural polymers, such as cellulose, hemicellulose, and lignin. Wood is a biological heterogeneous and anisotropic porous material (Capuani et al. The main result of this study was that at RH = 94% the relaxation time T 2,2, associated with the cell wall bound water, can be used as a marker to discriminate among softwoods and hardwoods. Importantly, the study was performed using a portable low-cost NMR instrument with which it is possible to investigate wood samples of any size. This work aimed to test the hypothesis of discriminating among softwoods and hardwoods of different botanical species and identifying further sub-clusters of woods based on the NMR proton spin–spin ( T 2) and spin–lattice ( T 1) relaxation times of their cell wall water in the hygroscopic moisture range. Proton Nuclear Magnetic Resonance (NMR) is a relative method used for the evaluation of moisture content distribution in wood and NMR relaxation is an excellent tool to study the hygroscopic behavior of different woods below the fiber saturation point. Below the fiber saturation point, wood moisture is only stored in the cell walls. Since hemicellulose shows the greatest water affinity, it can be considered the main responsible for the ingress of water into the wood mass. The hygroscopic behavior of wood can be attributed to the hydroxyl groups of its constituents. Moisture affects all properties of wood, as well as its preservative treatment. Wood is a hygroscopic material that can reach an equilibrium moisture content when ambient temperature and relative humidity are constant.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |