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Causes of wood cracking and improvement methods

The reason and improvement of antiseptic wood cracking

The existence of moisture in wood can be divided into five states,

Green wood state: the newly cut wood is in the green wood state. Both the cell cavity and cell wall contain water. The amount of free water varies with the seasons, and the bound water is saturated.

Fiber saturation point: if the raw wood is placed in the environment of 100% relative humidity, the free water in the cell cavity slowly evaporates. When there is no free water in the cell cavity, and the binding water in the cell wall is saturated, this state is called fiber saturation point. Generally, the moisture content of fiber saturation point is 30%.

Air drying state: when the raw wood is naturally dried in the atmospheric environment, the final water balance state is called air drying state. Generally speaking, the air dry water content in South China is higher, about 15% ~ 16%, while that in North China is about 12% ~ 13%.

Absolutely dry state: when the free water and bound water in the wood are completely removed, the state of the wood is called the absolutely dry state.

The fiber saturation point is a critical state. Generally, the amount of free water has little effect on the physical properties of wood, but the amount of combined water has a great influence on the physical and mechanical properties of wood.

The phenomenon of wood shrinkage and swelling refers to the phenomenon of wood cell wall shrinkage or swelling caused by the desorption or absorption of water from the absolute dry state to the initial saturation point. When the moisture content of wood is higher than the fiber saturation point, the change of moisture content will not cause wood shrinkage and swelling. Macro performance is the shrinkage and swelling of wood size.

The reasons for cracking of preservative wood are as follows

When the wood is naturally dried in the atmosphere, the moisture content on the surface will evaporate quickly, the moisture content will be low first, and the surface will begin to dry and shrink. However, the moisture content of the inner layer is still high, and there is no drying shrinkage. In this way, if the outer layer wants to shrink and the inner layer does not shrink, the internal stress of external tension and internal compression will occur. When the tensile stress exceeds the tensile strength limit of wood, the wood structure will be damaged and cracked.

In addition, wood shrinkage and swelling anisotropy, axial shrinkage of 0.1% ~ 0.3%, radial shrinkage of 3% ~ 6%, tangential shrinkage of 6% ~ 12%. Because of the difference between tangential shrinkage and radial shrinkage, internal stress will occur and cracking will occur.

The change of moisture content is the main reason of wood cracking. When the wood is used indoors, because it has been dried to the equilibrium moisture content, the moisture content in the room is relatively stable, so there are few cracks. When the wood is used outdoors, it is greatly affected by the weather, especially in the rain rich south. If the sun is exposed immediately after a long-term rainy weather, the moisture content inside and outside of the wood varies greatly and changes rapidly. If this cycle is carried out for a long time, the wood cracking will be accelerated. In addition, large-size wood is more likely to crack because the interior of large-size wood is more difficult to dry, and the anisotropy of drying shrinkage and swelling is more obvious in large-size wood.

How to improve the cracking of preservative wood?

According to the causes of wood cracking, keeping the relative stability of wood moisture content and reducing the internal stress of wood are the ways to reduce wood cracking.

Due to the great changes of the environment of wood used outdoors, the moisture content of the environment can not be controlled, so the actual contact between wood and water can only be reduced. For example, paint on the surface of wood to reduce the contact between wood and water; In addition, waterproof agent can be used to treat wood to reduce its water absorption. In addition, in a stable environment, drying wood, in the process of drying slowly release some of the wood's internal stress, to a certain extent, can also reduce cracking.

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