Since ancient times, there has been a method to improve the durability and prolong the service life of wood by baking the surface of wood directly with open flame for carbonization. Vikings began to treat wood in this way as early as the tenth century, but this surface carbonization method could not be evenly treated.
From19th century to 20th century, Germans and Canadians used heat treatment to treat wood, and the treatment medium was hot oil. The French also invented "baking" and "remelting" heat treatment processes. Up to now, the heat treatment process has developed quite maturely, and it has been used in the treatment steps before the manufacture of many wood products.
Advantages and disadvantages of wood heat treatment
1, improve the appearance of wood. Wood contains chromophoric groups and chromophoric groups such as carbonyl group and carboxyl group. After heat treatment, the polysaccharides in wood are degraded to produce more carbonyl and carboxyl groups, which makes the color of wood darker and forms a better appearance. At present, many consumers prefer the color and texture of natural wood. After heat treatment, the color of wood is deepened and the texture is clearer, which conforms to the aesthetic trend of consumers.
2. Improve the dimensional stability of wood and reduce the hygroscopicity of wood. The dimensional stability of wood is an important quality measurement index in wood processing. The hydroxyl group in wood is the most active and hygroscopic group in chemical reaction. After heat treatment, the hydroxyl content of wood can be reduced, thus reducing the ability of wood to exchange water with the outside world, improving the dimensional stability of wood and reducing deformation.
3. Improve the durability of wood. Wood durability refers to the ability of wood to resist microorganisms such as fungi and molds. Because wood contains nutrients such as fungi and molds, it is easy to be eroded, decayed and degraded by microorganisms in a warm and humid environment. After heat treatment, the moisture content in wood decreased, and the contents of protein and low molecular weight sugars on which fungi depend decreased, which had a certain impact on the corrosion resistance of wood.
4. Improve the acoustic performance of wood. After heat treatment, wood can play an "aging" effect similar to natural wood, improve the acoustic properties of wood, increase the specific dynamic elastic modulus of wood, reduce the loss factor and improve the mechanical vibration transmission performance.
5, the surface smoothness is reduced. After heat treatment, the bonding ability of wood will decrease, so the finishing cost of furniture made of partially heat-treated wood will increase. Especially, some water-soluble adhesives are difficult to be coated on the surface of wood due to the decrease of hygroscopicity of heat-treated wood.
Three kinds of wood heat treatment processes
According to different heat treatment media, wood heat treatment mainly includes three methods: gas phase medium heating method, hydrothermal method and oil medium heating method.
1, gas phase medium heating method. The gas phase medium heating method can be divided into steam heating method and inert gas heating method. Inert gas is used because oxygen easily degrades fibers and easily causes wood to burn at high temperature. According to different tree species, the treatment temperature is different, generally between 65438 050℃ and 250℃. Generally speaking, the higher the temperature, the better the durability and the darker the color of wood, but its mechanical strength will decrease, so it is necessary to choose a suitable treatment temperature.
2. Hydrothermal method. Hydrothermal method is divided into three stages. In the first stage, the cellulose in wood is pyrolyzed by water under the pressure of 160℃~ 190℃. The second stage is to reduce the moisture content of wood to about 10% to prepare for the follow-up work; The third stage is heat treatment at 170℃~ 190℃.
3. Oil heating method. Wood is treated with oil medium, and the oil can be vegetable oil such as rapeseed oil, linseed oil and sunflower oil. In order to ensure uniform heat transfer, one oil should be used instead of mixed use. The general heat treatment temperature is 220℃. In order to ensure uniform heat transfer, it is best to isolate wood from air for 2~4 hours.
Wood heat treatment is a mature wood modification process, wood has a perfect technical and theoretical system, and related research is also very rich.
The biggest disadvantage of wood heat treatment is that it weakens the mechanical properties of wood to some extent. Wood enterprises need to innovate and optimize technology to solve this problem. In addition to a single wood heat treatment process, combined heat treatment can be used to achieve better wood modification effect. (Source: Shuming Design Exchange Conference)
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