Wood moisture and material properties
I. Source and state of moisture in wood When a living tree grows, its roots continuously absorb water from the soil, and the xylem of the trunk sends water to various organs of the tree, while at the same time the nutrients produced by photosynthesis of the leaves It is transported from the phloem of the trunk to various parts of the tree. Moisture is not only an essential material for tree growth, but also a carrier for trees to transport various materials. After the live tree is felled and sawn into various specifications, the majority of the moisture remains in the wood. This is the wood. The main source of moisture in the wood, while the wood also absorbs some moisture into the interior during storage, transportation or use.
For different tree species, there is a difference in the moisture content of the xylem in the trunk. Even in the same growing season, the water content of the xylem varies. The water content of various parts of the xylem, such as heartwood, sapwood, roots, trunks and treetops, is also different, so the water distribution in the wood is very uneven. When the atmospheric conditions around the wood change, the moisture content will change accordingly. The water can be divided into three types: free water, sorbed water and combined water due to its different position in the wood: (1) free water exists in the In the large capillary system in which the perforations on the wall of the wood cell or the perforation at the end of the catheter and the cell cavity and the interstitial space communicate with each other, the free water is physically combined with the wood, and the binding is not tight, and this part of the water easily escapes from the wood. It is easy to inhale. The wet wood is placed in the dry air, and the first thing to evaporate is the free water. For the newly cut wood, the free water content of the wood of different tree species varies greatly, generally 60~70% to 200~250. Between %.
(2) sorption of water is present in the microcapillary system formed between the microfibrils and large filaments in the cell wall of the wood or on the free control groups of the cellulose molecules in the surface of the microcrystals and in the amorphous region. The content varies little among the tree species, and the maximum content of wood sorption water is generally between 23% and 31%, with an average of about 3%. The sorption water is tightly combined with the wood material, and this part of the water does not easily escape from the wood, only when The free water in the wood evaporates and the partial pressure of water vapor in the wood is greater than the partial pressure of water vapor in the surrounding wall, which can be evaporated from the wood.
(3) The combined water and cell wall constituents are in a strong chemical state. The amount of water in this part of the wood is small, negligible, and cannot be removed under normal dry conditions.
Second, the moisture of the wood and the indoor environment People live in the indoor environment, do not want excessive humidity fluctuations, should be stable within a certain range, which is very beneficial for the health of people and the preservation of objects. Some research data show that the best relative humidity range for preventing wet mold is 0-80%; the pest prevention is 0-70% or 80-100%; the preservation book is 40-60%; the bacterial infection is prevented from 55-60%; The mortality rate is at least 60-70%, and the relative humidity of the human living environment should be around 60. There are many factors that cause changes in indoor humidity. For example, changes in external temperature or changes in indoor temperature can cause changes in humidity; water vapor flowing in or out from a louver or a vent, water vapor permeable from a wall, water vapor coming in from a kitchen, etc. It also causes changes in humidity.
The humidity control property of wood is one of the unique properties of wood. It is also the advantage of its interior decoration materials and furniture materials. The decoration of wood or wood materials has a much longer change in humidity than that of concrete houses or wallpaper decoration houses. small. The so-called humidity control property of wood is based on the moisture absorption and desorption of the wood itself, which directly mitigates the humidity change in the indoor environment. When the relative humidity of the indoor environment is lowered, the water vapor partial pressure balance between the indoor environment and the wood is broken, and the partial pressure of water vapor inside the wood is higher than the partial pressure of water vapor in the indoor environment, and the moisture inside the wood will escape. Increase the relative humidity of the indoor environment, which is the desorption process. Conversely, when the relative humidity of the indoor environment increases, the wood absorbs moisture from the indoor air, which is a moisture absorption process. The wood progresses very strongly at the beginning of the desorption or moisture absorption process and then gradually slows down until a new dynamic equilibrium is reached. Here, wood plays a role in the storage of water, just like the natural regulator of indoor air humidity. People have long been used to living in the environment of wooden furniture and wooden interior materials.
Third, the moisture of the wood and the shape and size of the wood The amount of wood moisture affects the strength, rigidity, hardness, corrosion resistance and mechanical processing properties, combustion value, thermal conductivity, electrical conductivity, etc. of the wood within a certain range.
When the wet wood is placed in a dry environment, since the partial pressure of water vapor in the wood is greater than the partial pressure of atmospheric water vapor, moisture will be emitted from the interior of the wood to the atmosphere, and the first is free water. When the free water in the wood evaporates and the water is still saturated, the moisture content of the wood is called the fiber saturation point, and the fiber saturation point is the turning point of the change in the properties of the wood. When the moisture content of wood changes above the fiber saturation point, the change of water only has the increase and decrease of the amount of free water, and the increase or decrease of the amount of free water has little effect on the properties of wood, only affecting the weight, combustion value, thermal conductivity and Electrical 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 of the amount of water absorbed has a great influence on the properties of wood, such as the mechanical properties of wood, shrinkage, swelling, thermal conductivity and electrical conductivity. The changes are accompanied by the increase and decrease of the absorbing water. It can be said that the amount of water absorbed in the wood is the main factor affecting the nature of the wood.
In addition, when there is a suitable environment such as air, nutrients, temperature, humidity, etc., the spores of wood rot fungi infest the wood to cause harm to the wood. The optimum temperature for growth of most wood rot fungi is 25~30°C, and the moisture is 35~50%. If the moisture content of wood is reduced to 2%, the activity of wood rot fungus is inhibited, so drying also prevents wood discoloration. Effective measures of decay.
In our daily life, the deepest experience is the shrinkage and swelling characteristics of wood, which is a distinctive feature of wood. After the newly cut trees are sawn into square plates, they will begin to emit moisture in a relatively dry environment. First, the 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, the sorption water in the wood cell wall is discharged from the wood, and the size of the wood decreases. This is because the gap between the microfibrils and the large filaments in the cell wall is reduced by the discharge of the absorbing water, so that the cell wall is thinned, causing the wood to shrink. Conversely, in the process of gradually wetting the wood from the fully dry state to the fiber saturation point, the phenomenon of wood swelling is observed.
The shrinkage and swellability of the wood makes the size of the wood product unstable. Due to the anisotropy of wood structure, wood shrinkage varies greatly depending on the direction, tree species, density, late wood rate, growth wheel width, stress wood, and chemical composition of the cell wall. Therefore, the shrinkage and swelling of the wood also exhibit anisotropy, which is mainly manifested in the fact that the longitudinal shrinkage rate along the trunk direction is small, and the radial shrinkage rate along the radius of the trunk is about 0.10~0.20. 6%; the chordwise shrinkage rate of wood along the annual ring direction is about twice the radial shrinkage rate, about 6~12%, because the shape of the parts changes due to uneven shrinkage. When the raw material is dry, due to the joint effect of the radial and chord-to-dry shrinkage differences, various deformations such as warping, bending, twisting, and lateral bending of the end surface of the square material of the sawing of the log are caused. Due to the unevenness of drying and the shrinkage of the various directions, the wood causes cracks, end cracks, surface cracks, heart cracks, honeycomb cracks, wheel cracks and other defects.
The equilibrium moisture content of wood is of great significance in the processing and utilization of wood. Its role is to make people realize that wood must be dried to the equilibrium moisture content of the wood in accordance with the temperature and humidity of the air in the area before it is made into wood. In this way, changes in the moisture content of the wood due to the influence of temperature and humidity in the area of ​​use can be avoided, and the change in the size or shape of the wood is not caused, thereby ensuring the quality of the wood product. Our country has a vast territory, and the water content of timber in each region varies greatly depending on the climate of each region. Even in the same region, the equilibrium moisture content of wood varies in different seasons. For example, the annual average moisture content of wood in Beijing is about 11.1%, the highest in August is 15.6%, and the lowest in April is about 8.5%. The annual average moisture content of wood in Guangzhou is about 15.1%, the highest in China from April to May is 17.6%, and the lowest in November is about 12.4%. If a factory in Hunan manufactures a batch of wood products for each of Beijing and Guangzhou, if the woods are not dried to the appropriate extent according to the climatic conditions of Beijing and Guangzhou, the wood products will crack due to shrinkage when used in Beijing. When used in Guangzhou, it will not open and close due to expansion. In order to ensure the shape and size of the wood products, in addition to the wood, the use of wood with less shrinkage and shrinkage rate, and more importantly, the wood is dried to an equilibrium moisture content suitable for the environment, so that the wood It will not cause cracking and deformation due to rapid desorption or moisture absorption, which will affect the shape and dimensional stability of wood products. This is because the moisture content in the wood must be considered when processing or purchasing wood products.
These premium quality tablewares bundle with cascading metallic dot design are created to effortlessly match the rest of your decorations. The disposable plates are a perfect pairing with the paper cups and napkins supply and are suitable for any season.
Perfect for graduation-themed parties
Graduation Party Supplies--paper platesnapkin cocktail napkin are designedwith graduate elements, such as grad capdiplomabook and so on are clearly printed.
As elegantgraduation decorationsthey will dress your table special and suprise your guests to make the party more fun and attractive.
Tableware,Toilet Mat,Stain Resistant Toilet Rug,Classic Woodgraining Toilet Rug,U Shaped Toilet Rug
HISMOK(SHENZHEN)TECHNOLOGY CO.,LTD , https://www.willtrademat.com