The water contained in the wood has three forms, one is water present in the cell cavity and the intercellular space, that is, the water present in the capillary, called free water. The second type is water absorbed by the cell wall, called adsorbed water. The third type is water that constitutes cellular tissue, called chemical water. When the moisture of the moist wood evaporates, the first thing that is lost is free water. When the free water is evaporated and the water is still saturated, the water content is called the fiber saturation point moisture content. The fiber saturation point is the turning point of wood properties. Above the fiber saturation point, the strength of the wood is constant and does not change with the change of water content. At the same time, the wood does not have the volume change of expansion and contraction. When the water content falls below the fiber saturation point, that is, when the adsorbed water in the cell wall begins to evaporate, the strength increases as the water content decreases, and the phenomenon of swelling and shrinkage is also apparent. The water content of different wood fiber saturation points is between 22% and 33%. Humidity and temperature in various parts of nature are relatively stable in different seasons. The wood is in this relative temperature and humidity environment for a long time, and its water content will reach a relatively constant. The water content at this time is called the equilibrium moisture content (for example, the annual equilibrium moisture content in Shanghai is 4.6%). The equilibrium moisture content of wood varies with the temperature and humidity of the environment in which it is exposed. When there is a difference between the equilibrium moisture content and the ambient humidity, the solid wood flooring tends to approach the environment. This creates a phenomenon of swelling and shrinkage of the wood, which is a peculiar physical phenomenon of wood. Wood is an anisotropic body. The moisture content of wood in actual use is below the fiber saturation point, so the gain and loss of water is mainly the adsorption water of the cell wall. Most of the wood cells grow vertically, and their expansion and contraction are perpendicular to the cell wall. As a floor, we can find that there is generally no expansion and contraction in the longitudinal direction, and the expansion and contraction rate in the width direction is generally 3% to 6% (refer to the change in the moisture content of the wood below the moisture content of the fiber saturation point. It is very important to control the moisture content of the floor. It must be paid attention not only in production, but also in the laying, so as not to cause deformation of the floor due to moisture.
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