Before wood is processed into wood products, certain treatment should be carried out to reduce the moisture content of wood to the national standard, and ensure that the wood will not crack and deform. Wood heat treatment is a method to improve the dimensional stability of wood, which was used as early as in ancient times. It is to treat wood in the low oxygen environment of 160 ℃. At this time, the cell wall and physical properties of wood will change.
Since ancient times, there has been a method to improve the durability and prolong the service life of wood by directly baking the surface of wood with open fire for carbonization. Vikings also began to use this method to treat wood as early as the 10th century, but this method of surface carbonization can not be evenly treated.
From the 19th century to the 20th century, Germany and Canadians used heat treatment to treat wood, and the treatment medium was hot oil. The French also invented the "torrefaction" and "revision" heat treatment processes. Until now, the heat treatment process has been developed more mature, and used in many wood products manufacturing before the treatment steps.
Advantages and disadvantages of wood heat treatment
(1) Improve the appearance of wood. Wood contains carbonyl, carboxyl and other chromogenic groups and auxiliary color groups. After heat treatment, the polysaccharides in wood degrade to form more carbonyl and carboxyl groups, which make the wood darker and form a better appearance. At present, many consumers prefer natural wood color and texture. After heat treatment, the color of wood is deeper and the texture is clearer, which is in line with the aesthetic trend of consumers.
(2) Improve the dimensional stability of wood and reduce the moisture absorption of wood. Dimensional stability of wood is an important quality index in wood processing. The hydroxyl group in wood is the most active and hygroscopic group. After heat treatment, the hydroxyl content of wood can be reduced, the ability of water exchange between wood and the outside world can be reduced, the dimensional stability of wood can be improved, and the deformation can be reduced.
(3) Improve the durability of wood. Wood durability refers to the ability of wood to resist fungi, molds and other microorganisms. Because wood contains fungi, molds and other nutrients, wood is easy to be eroded by microorganisms in the warm and humid environment, resulting in decay and degradation. After heat treatment, the moisture content of wood decreased, and the protein and low molecular sugar content which fungi depend on decreased, which has a certain effect on wood corrosion resistance.
(4) Improve the acoustic performance of wood. After heat treatment, wood can have the effect of "aging" similar to natural wood, improve the acoustic performance of wood, increase the specific dynamic elastic modulus of wood, decrease the loss factor, and improve the mechanical vibration transmission performance.
(5) The surface finishing performance is reduced. The gluing ability of wood after heat treatment will be reduced, so the finishing cost of some furniture made of heat treated wood will be increased. In particular, some water-soluble adhesives are difficult to coat on the surface of wood due to the decrease of moisture absorption of heat-treated wood.
Three heat treatment processes of wood
According to the different heat treatment media, wood heat treatment mainly includes three methods: gas medium heating method, hydrothermal method and heating in oil medium.
(1) Gas medium heating method. The gas medium heating method can be divided into steam heating and inert gas heating. The inert gas is used because oxygen can make the fiber more easily degraded and easily cause wood combustion at high temperature. According to different tree species, the treatment temperature is not the same, generally between 150 ℃ and 250 ℃. Generally speaking, the higher the temperature, the better the durability of wood, the darker the color, but its mechanical strength will be reduced, so the appropriate treatment temperature should be selected.
(2) Hydrothermal method. Hydrothermal method is divided into three stages, the first stage is water pyrolysis, pressurized at 160 ℃ ~ 190 ℃ to split cellulose in wood; 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 the heat treatment stage, the treatment temperature is 170 ℃ ~ 190 ℃.
(3) Oil heating method. The oil can be rapeseed oil, flaxseed oil, sunflower oil and other vegetable oils. In order to ensure uniform heat transfer, one kind of oil should be used instead of mixed use. Generally, the heat treatment temperature is 220 ℃. In order to ensure uniform heat transfer, it is better to isolate wood from air and keep it for 2 ~ 4 hours.
Wood heat treatment is a mature wood modification technology, wood has a perfect technology and theoretical system, and the related research is also very rich.
The biggest disadvantage of wood heat treatment is that it weakens the mechanical properties of wood. Wood enterprises need to solve this problem through process innovation and optimization. In addition to the single use of wood heat treatment process, but also the use of heat treatment combined treatment, to achieve better wood modification effect.