Why does wood swell and shrink and how to prevent

2022-07-22
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From the absolutely dry state, wood can absorb moisture or water, and the wood will expand until it reaches fiber saturation. This phenomenon is called wet swelling

when the water in the cell cavity of green or wet wood evaporates completely, and the absorbed water (also known as cell wall water) of the cell wall does not dissipate; Or when the cell wall of dry wood is full of water, that is, when the water in the cell wall reaches a saturated state, but there is no water in the cell cavity, the water state of wood at this time is called fiber saturation (also known as fiber saturation point). During the air drying process of green or wet wood, from the beginning to the fiber saturation, the evaporated water is the free water in the cell cavity, which has no effect on the shape, size and properties of wood. But from the fiber saturation, continue to dry, the absorbed water in the wood cell wall begins to evaporate, and the wood begins to shrink until the wood reaches absolute dryness. This phenomenon is called dry shrinkage. Similarly, from the absolutely dry state, wood can absorb moisture or water, and the wood will expand until it reaches fiber saturation. This phenomenon is called wet swelling. In the actual drying process, because the wood has a certain thickness, the internal and external moisture content is uneven, and the moisture content of the whole board is above the fiber saturation, the board surface has begun to shrink

the moisture content of fiber saturation varies with different tree species, usually about 30% on average, generally between 20-35%

the dry shrinkage and wet expansion of wood often change with different wood sections, tree species, drying methods, etc. In terms of the section direction of wood, the chord shrinkage is about 6-10%, and the radial shrinkage is about 3-5%, that is, the chord shrinkage is twice the diameter shrinkage; The longitudinal contraction is very small, about 0.1%, which can be ignored. The volume shrinkage is the largest, about 13.8%. Due to the inconsistency between tangential shrinkage and radial shrinkage, wood drying shrinkage is one of the reasons for deformation and cracking

how to reduce the expansion and contraction of wood? The dry shrinkage and wet expansion of wood below fiber saturation are the characteristics of wood. However, processing can reduce the expansion and contraction of wood to the minimum, that is, improve its dimensional stability, which is a great progress in the wood industry

1. Try to use the radial saw board

the radial saw board in the wood industry refers to the board surface that forms an angle of 45-90 degrees with the direction of the growth ring. The chord saw plate is 0-45 degrees

2. High temperature drying

after high temperature and low humidity drying, the drying shrinkage of wood is smaller than that of the usual manual drying method. If it is dried at low temperature, the wood shrinkage can reach the maximum. Due to the uneven section shrinkage during rapid drying, large internal stress is generated, which causes residual plastic deformation and hinders the normal drying shrinkage of wood

3. Paint

the wood is made into finished products after drying, and then painted to prevent the wood from contacting with the air, which can reduce the moisture absorption or dehumidification of the wood

4. Soaked

wood such as lotus and Liquidambar is easy to deform. In rural areas, people often put raw wood into ponds or rivers to soak for a period of time, and then dry it. The effect is better. Another example is Casuarina equisetifolia, which is particularly easy to crack and deform after processing. Coastal people have an experience that sawing as soon as possible while raw wood can effectively reduce cracking; There are also logs or plates immersed in seawater, which is also good for reducing deformation

5. Other treatment methods

wood is made into plywood, glued wood, wood laminated plastic for use, or wood is chemically treated. The so-called wood plasticization is an effective method to reduce swelling and shrinkage. In particular, after impregnation with monomer and unsaturated polyester or irradiation with radiation, the wood is plasticized, which completely changes the original properties of the wood, greatly improves the dimensional stability, and changes the low-grade wood into high-grade wood for special purposes. It is a new advanced technology both at home and abroad

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