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The wood moisture content refers to the ratio of the moisture content of the wood to the dry weight of the wood, also known as the water content. The water content directly affects the volume and strength of the wood.
When the wood is cut down, the cell gap is filled with water. When the site is stacked, the water in the cell cavity evaporates first. At this time, the total weight of the wood is reduced, but the volume and strength are not changed. At some point, the water in the cell cavity has evaporated, and the cell wall is still full of water. This condition is called "fiber saturation." At this time, the water content is about 30%. For the sake of convenience, the industry has specified a water content of 30% as "the saturation point of the fiber". The water contained in the cell wall continues to evaporate, causing changes in the cell wall. At this time, the wood not only loses weight, but also begins to shrink in volume, and the intensity begins to increase.
Wood strength varies with water content because the colloids between the cell walls are "hydrophilic". After the water evaporates, the colloidal plasticity decreases, the cementing force increases, and the fiber can resist the external force. The change of water content has little effect on the tensile strength of the grain, and has a great influence on the compressive strength and bending strength of the grain. For example, the compressive strength of pine at the fiber saturation point is about 3KN/CM2.
The phenomenon of volume shrinkage caused by the decrease of water content in wood is called dry shrinkage. Dry shrinkage is also called “anisotropicâ€. When the wood texture is not straight and uneven, the evaporation speed of surface and internal water is inconsistent, and the degree of shrinkage of each part is different. Cracks, twists and other irregular deformations occur, and cracks may occur when the shrinkage is uneven.
The change of wood strength and shrinkage have brought many inconveniences to the use of wood. It is impossible for us to eliminate this objective and unfavorable change, but we can understand the law of change and control this change. Corresponding to a certain air humidity and relative humidity, there is a wood moisture content value, which is called "balanced water content". For example: local indoor average humidity of 32. The relative humidity is 55%. From the figure, it is found that the equilibrium moisture content is 10%, the furniture high-grade materials, the general moisture content is 15%, the general wood products (including wooden packaging), the relevant departments set about 18%~25% for the standard products, because of the wood Under the moisture content of 18% or less, wood rot fungi cannot survive and multiply.
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