The properties of shrinkage and swelling of wood are among the main characteristics that must be considered in the use of wood for different reasons. The market for other building materials, including wood, is growing stronger by the day. They aren’t afraid in marketing their products as superior to wood, not by looking at the potential benefits of their products, but rather by looking at the potential disadvantages of wood.
In particular, shrinkage/dimensional movements and different kinds of deformations in wood are used as an argument. This is why it is crucial to create building materials made of wood that have these issues diminished to a minimum in order in order to keep or increase the market share of wood-based products.
If the moisture content of wood fluctuates in the hygroscopic range, shrinkage and swelling can be observed in the process of desorption and adsorption respectively. It is crucial to consider the dimensional movements of wood in relation to its moisture content while making various wood products.
Definitions
The moisture content in products should be in a way that minimizes the dimensional change as the wood comes into contact with the varying conditions in the surrounding.The wood shrinks when water molecules depart the molecular structure of cells’ walls.
Wood is an anisotropic material that causes different swelling and shrinkage potentials in the three principal directions of the wood. Different wood characteristics can be a contributing factor to the various variations in the wood’s structure when it’s dried at a lower temperature than the saturation point for the fiber. In this section, the most crucial aspects of swelling and shrinkage of wood are discussed. As an introduction, definitions of the characteristics are explained along with instructions about how to measure them.
In addition, the author provides explanations regarding the different levels of shrinkage and swelling in the three principal directions of wood and how the swelling and shrinkage differ for different wood characteristics. The possible deformations that can occur during the drying of wood are explained, as brief information on the stabilization of dimensional wood is presented.
Why would wood shrink and swell?
The effects of swelling and shrinkage on the wood are crucial characteristics of the wood. They can cause a decrease and a growth in the dimensions of wood and size, respectively when the moisture content changes below the point of fiber saturation. The changes in moisture content that are beyond the fiber saturation point result in no change in the dimensions of the wood.
But, if dry wood has been dried to a green state there are a variety of stresses that within the wood could be so strong that cell walls are unable to stand up to these forces (e.g. the capillary tension force). This will result in the cell’s collapse wall structure and changing dimensionality of the wood even when there is a moisture level higher than the saturation point of fibers.
The swelling and shrinkage in wood exhibit an anisotropic quality, which means that the amount is different across the three major directions of the wood. The results are greater when they are in the direction of tangential, but lower in the radial direction, whereas they are much less in the longitudinal direction.
How do you measure shrinkage/swelling, and warp?
In ISO 4858 (1982a): Wood measurement of shrinkage in volume ISO 4469 (1981): Wood determination of radial and tangential shrinkage ISO 4860 (1982b): Wood assessment of swelling in the body as well as ISO 4859 (1982c): Wood determination of radial as well as expansion of tangentially.
To measure shrinkage and swelling, either a micrometer or slide caliper that is precise is a good way to gauge the changes in dimensionality in various directions of drying the wood. To gauge the change in volume, the measurements made in the three directions can be mixed.
Another option is to employ the method of immersion to determine the number of wood pieces. This technique is helpful in the event that the wood is of a particular shape, which makes it difficult to measure dimensional dimensions. However, it is essential to recognize that this method could be a little inaccurate because the liquid may get absorbed into wood throughout the time of immersion. This is a minor issue in greenwood, but it could be an issue when dry wood is used.
The rules for measuring warp are laid out by European guidelines (EN 1310). Bow, twist, and spring are measured by the largest deviation across a two-meter length of the plank. Cup is defined as the highest variation within the width of the plank. The requirements for the twist and cup depend on the width of the plank. A few examples of the requirements to allow warp are listed in the appendix.
Shrinkage and swelling
In conditions of green, the cell walls of the wood are typically saturated with water. When the wood is dried below the point of saturation for fibers the wood begins to shrink, and it will decrease in volume until the moisture of the piece is at zero.
Swelling and shrinkage are both completely reversible for small pieces of wood that are stress-free apart from the initial desorption process. In large pieces of wood that are solid, the process of shrinkage or swelling may not be fully reversible due to the presence of internal drying stress. Cellulose is among the major components of the cell walls of wood (43-52 percent of the dry substance).
The cell wall structure is extremely organized and is typically found as fibrils of elementary origin that behave similarly to threads. The fibrils of the beginning are the cells of the cellulose molecule are arranged in alternating crystalline and amorphous areas. The areas of crystalline are hydrophobic, meaning that molecular interactions between cellulose molecules are extremely robust. This is why water adsorption occurs in the amorphous areas, not in the crystallized areas.
Wood-water relations
A variety of fundamental fibrils (20-60) form microfibrils that measure 10-30m in diameter. Many microfibrils (20-50) create microfibrils. Microfibrils create the lamellae which form walls of cells. In the interfibrillar space between the micro, elementary, and macro fibrils is a meshy network that is partly made up of amorphous material like hemicellulose, lignin, and pectins. The heartwood is home to a variety of extractives. are also present.
In these amorphous regions as well as on the surfaces of the crystalline regions water molecules exist in the shape of bound water within the cell wall. When there is adsorption or removal of water from these areas the size of the wood material changes as it will expand or shrink.
The anisotropic nature that causes shrinkage and increase in the size of wood
Shrinkage and swelling are different across the three main branches of wood. The lower longitudinal shrinkage when compared to cross-sectional shrinkage is been attributed to the cell wall’s composition.
The second cell wall layer is comprised of three layers: S1, S2, and S3. This S1 as well as the S3 layers are comparatively thin when compared with the S2 layer, and consequently have less influence on shrinkage and swelling when compared to the S2 layer. S2 layer.
Shrinkage, swelling, and warping caused by changes in moisture 93 microfibrils are oriented more evenly within the layer of S2 in comparison to the other two layers. In the S2 layer, the longitudinal direction of fibrils is nearly in a parallel direction to that of a cell (a deviation of around 10-15deg for maturing wood).
The swelling or shrinkage is the result of the decrease or rise in the number of water molecules in and between fibrils. This is primarily due to dimension shifts in the cross-sectional direction in the cells’ walls. Due to the above-mentioned divergence between the fibril direction inside the S2 layer as well as the longitudinal orientation of cells, some swelling or shrinkage occurs in the cell wall layer as well.
Shrinkage/swelling in various species of wood
Certain fundamental distinctions exist in the characteristics of shrinkage among species. In general, the case of tangential shrinkage is approximately double the amount of radial shrinkage. In addition, the volumetric shrinkage tends to increase with density, though there are many variations in the general pattern.
The coefficients of shrinkage for a variety of species. The intervals shown in the table need to be considered to be simply a measure of the magnitude of mean values. This implies that the simplest values for coefficients may be in excess of the levels listed. This means that the variance in the values of shrinkage and expansion for the species can be significant.
The effect of the various wood properties
The wood’s behavior when drying is directly related to the shape of cell walls as well as the fiber matrix. The most important factors for mechanical properties are the S2 layer on the cell wall as well as the density of the wood. Its thickness S2 layer is between 30 fibrils in the earlywood to 150 fibers in latewood. Even if the two exhibit different shrinkage characteristics in pure samples the wood behaves as a homogenous, uniform cell matrix that interacts with one another (and with the radiations) in large-scale samples.
Density, grain angles, along with the fiber angle vary between mature and juvenile wood, and consequently also, characteristics of distortion and shrinkage. The greater angle of fibril in wood that is juvenile results in an increase in that longitudinal shrinkage ratio as well as the dominant left-handed grain angle causes a unidirectional twist. Compression wood from softwoods differs substantially from regular wood by having greater density, smaller cell walls, as well as a greater fibril angle.
Compression wood exhibits more volumetric shrinkage, and significantly more longitudinal shrinkage as opposed to regular wood. Planks containing a portion of compression wood may be susceptible to significant distortions when drying.
The deformations and shrinkage occur when drying full-size lumber
It is widely recognized that deformations can occur in sawn timber after drying to a moisture content lower than the point of saturation for fibers. To comprehend the mechanism that causes the various types of deformation that occur during and after the drying of wood, a variety of factors need to be considered.
The first is that the potential of shrinkage differs based on the three principal directions within the wood. In addition, the potential for shrinkage differs for different kinds of wood, e.g. high-density and low-density as well as compression wood as well as juvenile wood.
Second, changes in the moisture content within the hygroscopic spectrum cause internal stresses within the wood. Wood can undergo deformations due to creep when it is stressed particularly if it is split or cleaved after this procedure, the constant creep of the wood causes different types of deformations.
Dimensional stabilization through modification
The ability to bind water molecules is an intrinsic characteristic of every wood. The ability to shrink and expand is directly connected to this characteristic.
In certain situations, it is essential to limit the volumetric changes. This can be accomplished by reducing hygroscopicity. Additionally, a lower moisture content could increase biodegradability resistance. A variety of strategies have been suggested as well as a few will be briefly discussed.
Conclusion
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