What is the relationship between the moisture content of wood and the shape and size of its products
The amount of water content in wood affects its strength, rigidity, hardness, corrosion resistance, mechanical processing performance, combustion value, thermal conductivity, and conductivity within a certain range.
When wet wood is placed in a dry environment, due to the fact that the partial pressure of water vapor inside the wood is greater than the partial pressure of atmospheric water vapor, water will be emitted from the inside of the wood into the atmosphere, and the first thing to evaporate is free water. When the free water inside the wood evaporates completely and the absorbed water is still in a saturated state, the moisture content state of the wood is called the fiber saturation point, which is the turning point of changes in wood properties. When the moisture content of wood changes above the fiber saturation point, the change in moisture only affects the increase or decrease in the amount of free water, and the increase or decrease in the amount of free water has little effect on the properties of wood, only affecting its weight, combustion value, thermal conductivity, and conductivity. On the contrary, when the moisture content of wood changes below the fiber saturation point, there is no free water inside the wood, and the change in the amount of absorbed water has a significant impact on the properties of the wood. For example, the mechanical properties, dry shrinkage, moisture expansion, thermal conductivity, and conductivity of the wood change with the increase or decrease of absorbed water. It can be said that the amount of absorbed water in the wood is the main factor affecting the properties of the wood.
In addition, when there are suitable environments such as air, nutrients, temperature, and humidity, the spores of wood rot fungi can infect wood and cause harm to it. The optimal temperature for the growth of most wood rot fungi is 25-30 ℃, with a moisture content of 35-50%. When the wood moisture content is reduced to 2%, the activity of wood rot fungi is inhibited. Therefore, drying is also an effective measure to prevent wood discoloration and decay.
In our daily lives, what we deeply experience is the dry shrinkage and wet expansion characteristics of wood, which are a significant feature of wood. After newly cut trees are sawn into square timber, they will begin to emit water when placed in a relatively dry environment. Firstly, free water in the wood begins to escape, and the size of the wood does not change. When the free water in the wood evaporates completely, the absorbed water inside the wood cell wall is discharged from the wood, and the size of the wood decreases. This is due to the reduction of the gap between microfibrils and large filaments in the cell wall due to water absorption and excretion, resulting in thinning of the cell wall and drying shrinkage of the wood. On the contrary, during the process of wood gradually wetting from fully dry state to fiber saturation point, the phenomenon of wood swelling can be observed.
