For different tree species, there are differences in xylem moisture content. Even if the same tree grows in different seasons, its xylem water content is also different.
The moisture content of different parts of xylem, such as heartwood, sapwood, root, trunk and treetop, is also different, so the water distribution in wood is very uneven. When the atmospheric conditions around wood change, its moisture content will also change. Because of its different position in wood, moisture can be divided into free water, adsorbed water and combined water
( 1) free water exists in the macrocapillary system composed of pores on the cell wall or perforation at the end of the vessel, cell cavity and intercellular space, which communicate with each other. Free water is physically combined with wood, but not closely. This part of water is easy to escape from wood and easy to inhale. When wet wood is placed in dry air, the first thing that evaporates is free water. For the newly felled raw wood, the free water content of different tree species varies greatly, generally between 60 ~ 70% and 200 ~ 250%.
( 2) adsorbed water exists in the capillary system composed of microfibrils and macrofilaments in the cell wall of wood, or adsorbed on the free control group of cellulose molecules on the surface of microcrystalline and amorphous region. The difference of absorbed water content in wood is small among tree species, and the larger water absorption content of wood is generally between 23% and 31%, with an average of about 3%. The adsorbed water is closely combined with the wood material, and this part of water is not easy to escape from the wood. Only when the free water in the wood is evaporated and the partial pressure of water vapor in the wood is greater than that in the surrounding wall gas, the water can be evaporated from the wood.
( 3) the combined water and the components of cell wall are in a strong chemical state. The content of this part of water in wood is very small, which can be ignored, and can not be removed under general drying conditions.
The moisture control property of wood is one of the unique properties of wood, and it is also the advantage of wood as interior decoration material and furniture material. The humidity change of houses decorated with wood or wood materials is much smaller than that of houses decorated with concrete or wallpaper. The so-called humidity control characteristics of wood is to rely on the moisture absorption and desorption of wood itself to directly ease the humidity change of indoor environment. When the relative humidity of the indoor environment decreases, the water vapor partial pressure balance between the indoor environment and the wood is broken. When the water vapor partial pressure in the wood is higher than that in the indoor environment, the moisture inside the wood will escape, which will increase the relative humidity of the indoor environment. This is the desorption process.
On the contrary, when the relative humidity of indoor environment increases, the wood will absorb moisture from the indoor air, which is a hygroscopic process. At the beginning of desorption or moisture absorption process, wood is very strong, and then gradually slow down until a new dynamic equilibrium is reached. Here wood plays a role in the storage of moisture, just like the natural regulator of indoor air humidity. For a long time, people have been used to living in the environment of wooden furniture and wooden interior decoration materials.