The moisture content of wood is below the fiber saturation point, and the expansion of size and volume due to the increase in water content is called swelling; the shrinkage of size and volume due to the decrease in water content is called shrinkage. This swelling and shrinking property of wood is called expansion and contraction. Wood consists of cells with free water in the cell cavity and sorbed water in the cell wall (hygroscopic water, cell wall water). Only the increase and decrease of absorbing water can cause the wood to expand and contract. When the inhaled water is saturated in the cell wall, the hygroscopicity of the wood is maximized. After that, if the wood continues to be placed in the water, although the water content continues to increase, it can only increase the free water, so it cannot cause the wood to expand and contract. The swellability of the wood is small (smooth texture), the transverse direction (transverse texture) is large, and the chord direction is about twice as large as the radial direction. The amount of swelling of the wood, expressed as the rate of expansion and contraction, that is, the difference between the size of the full dry size of the wood and the moisture content in the air to the fiber saturation point and the percentage of the total dry size. Or expressed by the coefficient of swelling, that is, the average swelling ratio per 1% increase in moisture content. Conversely, when the free water in the wood evaporates and the water continues to be lost, the wood begins to shrink. That is, when the water content is lower than the fiber saturation point, the shrinkage occurs. The size of the wood shrinkage, expressed as the shrinkage ratio, that is, the difference between the difference between the raw material having a water content higher than the fiber saturation point and the size of the dried wood and the wet material size. Or expressed by dry shrinkage, that is, the average shrinkage ratio at every 1% reduction in moisture content. The dry shrinkage of the wood is small and the lateral direction is large (the smaller the angle with the growth wheel, the larger the shrinkage). The size of the shrinkage varies depending on the species. Generally, the normal wood has a longitudinal shrinkage of 0.1% to 0.3%, a diameter of 3% to 6%, a string shrinkage of 6% to 12%, and a volume shrinkage of 9% to 14%. Because wood has the disadvantage of swelling and shrinking, the size and volume of the wood cannot be kept stable, but varies with the humidity and temperature in the air. To maintain the stability of the wood size, the moisture content of the wood should be kept equal to the local equilibrium moisture content. Wood that is dried to an equilibrium moisture content can not only reduce its expansion and contraction. 1. Wood drying can bring wood products below the equilibrium moisture content. The general regulation is 8% to 12%. Due to the large size of China, the humidity varies greatly from place to place. It is best to consider the local equilibrium moisture content. 2. The moisture content of the wood should be determined both at the factory and at the time of purchase. The measurement of wood moisture content is widely applied by electric measurement method, and there are many types of moisture analyzers. There are roughly two types, one is a resistance type moisture meter of a direct current device, and the other is a dielectric moisture meter of an alternating current device. The resistive moisture meter is basically a DC ohmmeter. Using the measured resistance (resistivity), the different resistances of various woods show the moisture content of the wood on the dial. This kind of resistive moisture meter is inserted into the wood with a needle-shaped metal electrode. Because the moisture content of the wood surface is usually low, it is considered that the electrical resistance measured at 1/4 of the thickness of the whole wood can represent wood. The actual resistance, that is, the true moisture content of the wood, when the moisture content of the wood surface is high, the moisture content of the wood surface is indicated by the current short-circuit moisture meter. Although the resistivity moisture meter is inexpensive, the metal needle needs to be inserted into the interior of the wood when the moisture content is measured, and the hard material cannot be inserted, and the surface of the soft wood forms a small hole, which affects the appearance of the wooden floor. The dielectric constant and loss tangent of wood vary with the moisture content of wood. This double-index moisture meter is called the power loss moisture meter, such as using only the single index of wood dielectric constant with the change of wood moisture content. The moisture meter is a capacitive moisture meter. Because the alternating current moisture meter can measure the true moisture content of the wood, the determination of the moisture content of the wood floor is very suitable, although the price is more expensive. Sino-Japanese joint venture Beijing Anli Electronics Co., Ltd. introduced the inductive AD100 type capacitance moisture meter in 1998. It has two types of pointer and liquid crystal display. It also has automatic compensation and zero adjustment function for ambient temperature and humidity. The upper limit of measurement can reach 100%. This product is very popular after the market. In addition, due to the measurement of wood moisture content by the electric moisture meter, it is also related to the structure, density, texture direction and humidity of wood of multiple tree species. Therefore, the instrument should be placed according to regulations. Correction factors for different tree species and their wood thickness. 3. Wood has hygroscopic hysteresis, coupled with the protective layer of paint on the surface of wood products. Therefore, the variation of moisture content caused by changes in climatic conditions of wood products is less than the fluctuation range of equilibrium moisture content, so the final moisture content of wood is generally required. Below the equilibrium moisture content at the time of use. In addition, wood products are subject to expansion and contraction caused by changes in atmospheric humidity, which affects the shape and size of wood products. Experience has shown that the risk of expansion is far less than the hazard of shrinkage, so people technically determine the final moisture content of wood. It is lower than what is required when using it. China has not yet specified drying targets for most wood processing enterprises in various regions. The moisture content of the wooden floor is 8% to 10%, and the maximum is not more than 12%. Measures to prevent warping and deformation of wood: (1) Use a saw blade as much as possible, that is, a plate with an angle of 45° to 90° with the annual ring direction. As can be seen from the lateral warping deformation of the wood, the saw blade is only The size is shrinking, and warping is extremely rare. (2) Select a wood with a small dry shrinkage ratio and a chord dry shrinkage ratio (or differential dry shrinkage, that is, a chordwise dry shrinkage rate divided by a percentage of the radial dry shrinkage ratio). In the same way, starting from the line shrinkage rate of the tree species itself, the wood with a relatively small shrinkage ratio is selected to reduce the shrinkage of the wood; in addition, the warp deformation of the wood with a large diameter of the string diameter is larger, although this theory has not yet Recognized by scholars, but recognized by most scholars. (3) Protective layer such as painting to prevent moisture in the atmosphere from invading the wood. Because the hygroscopicity of the wood is related to the size of the area, it is also painted as far as possible, but the back side does not require wear-resistant materials. It is best to paint on the mouth. (4) After the wood is soaked in water, it is dried at high temperature and high humidity. Although it costs money, it is recommended for some experts. It is said that the dimensional stability of the wood is better after drying. The moisture content of wood flooring products is of the utmost importance, so both manufacturers and consumers who manufacture and distribute must pay great attention. Therefore, manufacturers must use a moisture meter to test before leaving the factory and when purchasing, so as to avoid unnecessary losses due to small loss.
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It maintains dimensional stability and its strength and durability are improved compared to raw materials. Although the average shrinkage rate of the tree species is smaller, the shrinkage of the wood is smaller, but it is impossible to require the wood to expand without shrinking. Only the following necessary measures are taken.
August 31, 2019