Compressed wood is a hardened, high-density, high-strength reinforced material made of wood by hot pressing. As early as the beginning of the 20th century, there were wood compression products and patents on the manufacture of compressed wood in Germany and the United States; in the former Soviet Union, in 1932, the hot-press compression method (referred to as the method) and the cooking compression method (referred to as wet method) were developed. In the late 1950s and early 1960s, China also developed compressed wood anchors for coal mines and compressed wood wood shuttles for textiles; in the 1970s, India studied the use of 17 kinds of domestically produced tree species to replace imported hornbeams ( Carpinus) manufactures a weaving machine wooden shuttle. Since the 1990s, with the rapid development of the world timber industry, scholars from various countries have pushed the research on the compression and deformation of wood to a new height on the basis of previous studies, in order to make compressed wood a practical Industrial processing technology has laid a good foundation.
l Wood pretreatment and fixed deformation mechanism
Depending on the requirements of use, different pretreatments of the wood are required prior to compression. Such as hydrothermal treatment, drug treatment, metallization treatment, impregnating resin treatment, microwave heating treatment, etc., corresponding to the production of ordinary compressed wood, drug compressed wood, metalized compressed wood, surface compacted material and compressed plastic wood and other new wood .
It is advisable to compress wood species to select wood with uniform material, straight texture and low water-insoluble extract. Wood with high extract content, such as pine or yellow douglas (Douglas fir), is not suitable for making compressed wood, because resinous substances interfere with the flow and sufficient compression of lignin, and it is difficult to obtain a stable product.
1.1 Softening method
The wood must be softened prior to compression, otherwise the cell wall will be destroyed during compression. Softening methods generally include hydrothermal treatment and chemical treatment. The hydrothermal treatment method includes: boiled, steamed and microwave heat treatment; the drug treatment method includes: liquid ammonia, gaseous ammonia, ammonia water, hydrazine, urea and alkali treatment. Commonly used softening methods are mostly hydrothermal treatment, and also research on compressed wood softened with ammonia water.
1.2 Fixation of compression deformation and its mechanism
Wood, as a natural elastoplastic material, can be compressed under certain conditions without damaging its structure, but its dimensional stability is poor. When immersed in water or placed in damp air, it absorbs moisture and has a tendency to return to its original size. The fixation of compression deformation is the key to achieving soft wood reinforcement and maintaining its physical and mechanical properties. The method mainly includes: impregnating wood with a low molecular weight resin and a non-formaldehyde agent and a non-toxic chemical agent as a catalyst, heat treatment, high temperature and high pressure steaming treatment, and the like.
In recent years, some scholars in Japan and China have studied the mechanism of compression deformation by means of X-ray diffractometer, infrared absorption spectroscopy and chemical analysis of photoelectron spectroscopy. The low molecular weight resins used for the surface compacting materials are PE resin, MF resin, glyoxal resin, etc., and the mechanism of action is chemical crosslinking between resins and between resin and wood radicals during high temperature hot pressing. The effect is to fix the deformation. However, such cross-linking reactions using aldehydes such as formaldehyde as cross-linking agents and inorganic acids as catalysts not only pollute the environment, are harmful to human health, but also reduce the physical and mechanical properties of wood. In recent years, there has also been a formaldehyde reaction catalyzed by gas phase SO2. Although the chemical degradation of wood is reduced, it still pollutes the environment and is difficult to promote in practical applications. Therefore, exploring the non-formaldehyde cross-linking system for wood cross-linking reaction, changing the catalytic process, revealing the reaction mechanism of the new cross-linking agent and catalyst, has become one of the frontier researches of general concern.
The wood is heat-treated under compression, and cellulose and hemicellulose are decomposed and polymerized, β-cellulose is reduced, and a and γ cellulose are increased. The crystallinity of the heat-treated wood is reduced. Compared with the unheated compressed wood, the heat treatment will cause the reduction of MOR and MOE in the direction of the compressed wood fiber. The higher the treatment temperature and the longer the time, the greater the degree of reduction; after heat treatment The density of compression is generally unchanged, the hardness and wear resistance are reduced, the moisture permeability and hygroscopicity are reduced, the hydrophobicity is improved, and the color of the material is darkened. Three kinds of heating methods, such as heating in air, heating in vacuum and heating of molten metal, respectively, the results of research on compression deformation of fixed acacia wood show that the recovery of deformation decreases with the increase of wood weight loss rate. It can be expressed by the hyperbolic equation, and the fixation effect is best when the weight loss rate is 4%.
The high-temperature steaming method means that the wood is treated with steam under a compressed state to fix the compression deformation. The wood is steamed and then compressed, which has different effects from the first compression and then the steaming under compression. The latter is far superior to the former. After treatment with high temperature and high pressure steam, the chemical components in the wood are degraded, and the degradation products are recombined to form a new bond; the amount of alcohol and ether bonds in the cell wall material decreases; the lignin and hemicellulose in the wood degrade. An aldehyde group or a ketone group is produced; although the cellulose structure does not change significantly, part of it is hydrolyzed,
Microfibril cutting, fusion change, crystallinity and crystallization zone width increase. A small amount of graphite carbon exists, so that the arrangement of wood cell wall material is more orderly, the bonding strength is increased, the surface hardness and wear resistance of the wood are increased; the mechanical radical concentration of the pressed compact material after treatment is significantly reduced.
2 Classification and manufacturing methods of compressed wood
2·l universal compression wood
In the first process, the wood block is steamed to make the water content reach 17% to 20%, and the internal temperature rises above 85 °C. The block is then laterally compressed in a hot press, and the platen temperature is generally above 120 °C. When the block is compressed, the method of sectional pressure can be used, and the pressure is increased step by step, usually above 10 MPa (depending on the tree species, softening degree, temperature of the platen); it can also be compressed under a constant pressure to reach the specified thickness. After that, keep warm for 40 to 60 minutes. After that, under the condition of maintaining the pressure, it is naturally cooled until the temperature of the compressed wood center drops to 30 to 40 C. The compressed wood produced by this process has a low heat-pressing temperature and a short time, which is not enough to completely fix the compression deformation, and the compressed wood rebounds in a certain external environment (temperature and humidity). Therefore, the compressed wood produced by this process should be placed in the chamber for a period of time to eliminate its internal stress and then processed; or it can be processed under constant temperature and humidity conditions, and the finished product is immediately immersed or painted. .
Process 2 changes the hot pressing temperature and time in Process 1. The temperature of the hot plate is 160, 180, 200, 220 ° C, and the corresponding holding time is 30, 20, 5, 3 h while maintaining the hot pressing pressure. Then, the temperature is lowered to below 60 C, and the compression deformation is almost completely fixed. After soaking and boiling test, the deformation recovery rate is almost zero. However, prolonged high temperature heat treatment affects the mechanical properties of the compressed wood and leads to changes in color and moisture content.
Process 3: In a closed high-temperature and high-pressure treatment tank with a compression device inside, soften the wood with high-temperature and high-pressure steam, then compress it, and steam it under high pressure and high pressure in a compressed state, and use saturated water vapor of 180C or 200C. Process for 8 min or 1 min. Then forced cooling to below 60 C discharge. After soaking and boiling test, the deformation recovery rate is almost zero, and the compression deformation is almost completely fixed. The mechanical properties, color and moisture content of the treated compressed wood changed only slightly. This process is equally applicable to surface compacted materials and compression shaped wood.
2·2 Surface compacted material
Process flow:
2.3 compression shaping wood
Process flow:
Log drama -> peeling -> microwave heating softening -> four directions compression -> two heat treatment -> cooling -> discharge
Compression and reshaping technology is a new technology developed by Kobayashi Kobayashi in Japan in the early 1990s to utilize medium and small diameter graded fellwood. The method uses a microwave to heat a small-diameter log of 10 to 20 cm, and when the temperature reaches 100 C or more, it is hot-pressed into a square material from four directions at a speed of 5 mm/min in a compressor. The temperature of the compressor hot plate is above 100 °C. Due to the difference in moisture distribution inside the logs, the temperature inside the logs can be as high as 2o °C. Secondary heating is then employed to fix the shaped shape.
3 The nature and use of the compression
After the wood is compacted and compacted, its microstructure and physical and mechanical properties have undergone major changes. As the density of compression increases, its strength, impact toughness and hardness increase linearly, and its wear resistance increases the fastest. However, when the degree of compression exceeds a certain value, the increase in the strength of the wood is slow. In actual production, the density of the compressed wood is determined according to the specific use of the compressed wood. The corrosion resistance of compressed wood did not increase, but the densification slowed the rate of microbial attack. Compressed wood with excellent physical and mechanical properties can be used as a substitute for hard broad-leaved trees for interior decoration, furniture, construction, textile equipment, tool handles and tool models. After the logs are compressed and shaped, the shape and spacing of the annual rings and wood rays vary greatly depending on the degree of compression in each direction and the presence or absence of bark during compression. If used for planing, a variety of thin woods with different textured patterns can be produced. Therefore, compression shaping wood can also be used to make thin wood for wood surface decoration. It also opens up new trails for the high value-added processing and utilization of medium and small-scale plantation wood.
In the 21st century, human beings will pay more attention to their own health and living environment. It can be predicted that the development of short-cycle production processes and equipment for the production of compressed wood with high efficiency and high quality in the future, as well as the production of green products, will be compressed wood. The direction of development.
l Wood pretreatment and fixed deformation mechanism
Depending on the requirements of use, different pretreatments of the wood are required prior to compression. Such as hydrothermal treatment, drug treatment, metallization treatment, impregnating resin treatment, microwave heating treatment, etc., corresponding to the production of ordinary compressed wood, drug compressed wood, metalized compressed wood, surface compacted material and compressed plastic wood and other new wood .
It is advisable to compress wood species to select wood with uniform material, straight texture and low water-insoluble extract. Wood with high extract content, such as pine or yellow douglas (Douglas fir), is not suitable for making compressed wood, because resinous substances interfere with the flow and sufficient compression of lignin, and it is difficult to obtain a stable product.
1.1 Softening method
The wood must be softened prior to compression, otherwise the cell wall will be destroyed during compression. Softening methods generally include hydrothermal treatment and chemical treatment. The hydrothermal treatment method includes: boiled, steamed and microwave heat treatment; the drug treatment method includes: liquid ammonia, gaseous ammonia, ammonia water, hydrazine, urea and alkali treatment. Commonly used softening methods are mostly hydrothermal treatment, and also research on compressed wood softened with ammonia water.
1.2 Fixation of compression deformation and its mechanism
Wood, as a natural elastoplastic material, can be compressed under certain conditions without damaging its structure, but its dimensional stability is poor. When immersed in water or placed in damp air, it absorbs moisture and has a tendency to return to its original size. The fixation of compression deformation is the key to achieving soft wood reinforcement and maintaining its physical and mechanical properties. The method mainly includes: impregnating wood with a low molecular weight resin and a non-formaldehyde agent and a non-toxic chemical agent as a catalyst, heat treatment, high temperature and high pressure steaming treatment, and the like.
In recent years, some scholars in Japan and China have studied the mechanism of compression deformation by means of X-ray diffractometer, infrared absorption spectroscopy and chemical analysis of photoelectron spectroscopy. The low molecular weight resins used for the surface compacting materials are PE resin, MF resin, glyoxal resin, etc., and the mechanism of action is chemical crosslinking between resins and between resin and wood radicals during high temperature hot pressing. The effect is to fix the deformation. However, such cross-linking reactions using aldehydes such as formaldehyde as cross-linking agents and inorganic acids as catalysts not only pollute the environment, are harmful to human health, but also reduce the physical and mechanical properties of wood. In recent years, there has also been a formaldehyde reaction catalyzed by gas phase SO2. Although the chemical degradation of wood is reduced, it still pollutes the environment and is difficult to promote in practical applications. Therefore, exploring the non-formaldehyde cross-linking system for wood cross-linking reaction, changing the catalytic process, revealing the reaction mechanism of the new cross-linking agent and catalyst, has become one of the frontier researches of general concern.
The wood is heat-treated under compression, and cellulose and hemicellulose are decomposed and polymerized, β-cellulose is reduced, and a and γ cellulose are increased. The crystallinity of the heat-treated wood is reduced. Compared with the unheated compressed wood, the heat treatment will cause the reduction of MOR and MOE in the direction of the compressed wood fiber. The higher the treatment temperature and the longer the time, the greater the degree of reduction; after heat treatment The density of compression is generally unchanged, the hardness and wear resistance are reduced, the moisture permeability and hygroscopicity are reduced, the hydrophobicity is improved, and the color of the material is darkened. Three kinds of heating methods, such as heating in air, heating in vacuum and heating of molten metal, respectively, the results of research on compression deformation of fixed acacia wood show that the recovery of deformation decreases with the increase of wood weight loss rate. It can be expressed by the hyperbolic equation, and the fixation effect is best when the weight loss rate is 4%.
The high-temperature steaming method means that the wood is treated with steam under a compressed state to fix the compression deformation. The wood is steamed and then compressed, which has different effects from the first compression and then the steaming under compression. The latter is far superior to the former. After treatment with high temperature and high pressure steam, the chemical components in the wood are degraded, and the degradation products are recombined to form a new bond; the amount of alcohol and ether bonds in the cell wall material decreases; the lignin and hemicellulose in the wood degrade. An aldehyde group or a ketone group is produced; although the cellulose structure does not change significantly, part of it is hydrolyzed,
Microfibril cutting, fusion change, crystallinity and crystallization zone width increase. A small amount of graphite carbon exists, so that the arrangement of wood cell wall material is more orderly, the bonding strength is increased, the surface hardness and wear resistance of the wood are increased; the mechanical radical concentration of the pressed compact material after treatment is significantly reduced.
2 Classification and manufacturing methods of compressed wood
2·l universal compression wood
Process flow:
Sheet softening treatment -> transverse compression -> high temperature treatment -> cooling -> discharge |
->High temperature and high pressure steaming treatment->
Ordinary compressed wood is made by directly compressing wood without special treatment. There are usually three kinds of manufacturing methods: In the first process, the wood block is steamed to make the water content reach 17% to 20%, and the internal temperature rises above 85 °C. The block is then laterally compressed in a hot press, and the platen temperature is generally above 120 °C. When the block is compressed, the method of sectional pressure can be used, and the pressure is increased step by step, usually above 10 MPa (depending on the tree species, softening degree, temperature of the platen); it can also be compressed under a constant pressure to reach the specified thickness. After that, keep warm for 40 to 60 minutes. After that, under the condition of maintaining the pressure, it is naturally cooled until the temperature of the compressed wood center drops to 30 to 40 C. The compressed wood produced by this process has a low heat-pressing temperature and a short time, which is not enough to completely fix the compression deformation, and the compressed wood rebounds in a certain external environment (temperature and humidity). Therefore, the compressed wood produced by this process should be placed in the chamber for a period of time to eliminate its internal stress and then processed; or it can be processed under constant temperature and humidity conditions, and the finished product is immediately immersed or painted. .
Process 2 changes the hot pressing temperature and time in Process 1. The temperature of the hot plate is 160, 180, 200, 220 ° C, and the corresponding holding time is 30, 20, 5, 3 h while maintaining the hot pressing pressure. Then, the temperature is lowered to below 60 C, and the compression deformation is almost completely fixed. After soaking and boiling test, the deformation recovery rate is almost zero. However, prolonged high temperature heat treatment affects the mechanical properties of the compressed wood and leads to changes in color and moisture content.
Process 3: In a closed high-temperature and high-pressure treatment tank with a compression device inside, soften the wood with high-temperature and high-pressure steam, then compress it, and steam it under high pressure and high pressure in a compressed state, and use saturated water vapor of 180C or 200C. Process for 8 min or 1 min. Then forced cooling to below 60 C discharge. After soaking and boiling test, the deformation recovery rate is almost zero, and the compression deformation is almost completely fixed. The mechanical properties, color and moisture content of the treated compressed wood changed only slightly. This process is equally applicable to surface compacted materials and compression shaped wood.
2·2 Surface compacted material
Process flow:
l) Drying of the board -> surface immersion -> microwave heating -> lateral compression -> high temperature treatment -> cooling -> discharge |
-> High temperature and high pressure steaming treatment->
2) Drying of the board -> decompression -> impregnating resin -> drying -> transverse compression -> cooling -> discharging In the 1990s, Japan first conducted research on surface compacted materials. Water or low-molecular-weight water-soluble resin is immersed in the surface of the dried material to a certain depth, and the surface layer is softened by microwave heating, compressed, and shaped to obtain a surface compacted material. The surface compacted material soaked with water has a large increase in wear resistance and hardness as compared with the untreated material, but the compression deformation is almost completely recovered by the hydrothermal recovery test. In order to fix the surface compacting layer, the compression deformation of the surface layer can be completely fixed by the method of the above Process 2 or Process 3. The surface compacted material impregnated with resin has a surface hardness proportional to the amount of compression residual deformation and resin concentration, and the wear resistance is related to the amount of compression residual deformation, regardless of the concentration of the resin. As the resin concentration increases, the weight gain rate (WPG) and the anti-expansion ratio (ASE) increase, and the compression recovery rate decreases. The wood impregnated with the PF resin having a resin concentration of 15% or more is almost completely fixed in the radial compression deformation. The wood is impregnated with MF resin having a resin concentration of 25%. After soaking and boiling test, the compression deformation can also be completely fixed. 2.3 compression shaping wood
Process flow:
Log drama -> peeling -> microwave heating softening -> four directions compression -> two heat treatment -> cooling -> discharge
Compression and reshaping technology is a new technology developed by Kobayashi Kobayashi in Japan in the early 1990s to utilize medium and small diameter graded fellwood. The method uses a microwave to heat a small-diameter log of 10 to 20 cm, and when the temperature reaches 100 C or more, it is hot-pressed into a square material from four directions at a speed of 5 mm/min in a compressor. The temperature of the compressor hot plate is above 100 °C. Due to the difference in moisture distribution inside the logs, the temperature inside the logs can be as high as 2o °C. Secondary heating is then employed to fix the shaped shape.
3 The nature and use of the compression
After the wood is compacted and compacted, its microstructure and physical and mechanical properties have undergone major changes. As the density of compression increases, its strength, impact toughness and hardness increase linearly, and its wear resistance increases the fastest. However, when the degree of compression exceeds a certain value, the increase in the strength of the wood is slow. In actual production, the density of the compressed wood is determined according to the specific use of the compressed wood. The corrosion resistance of compressed wood did not increase, but the densification slowed the rate of microbial attack. Compressed wood with excellent physical and mechanical properties can be used as a substitute for hard broad-leaved trees for interior decoration, furniture, construction, textile equipment, tool handles and tool models. After the logs are compressed and shaped, the shape and spacing of the annual rings and wood rays vary greatly depending on the degree of compression in each direction and the presence or absence of bark during compression. If used for planing, a variety of thin woods with different textured patterns can be produced. Therefore, compression shaping wood can also be used to make thin wood for wood surface decoration. It also opens up new trails for the high value-added processing and utilization of medium and small-scale plantation wood.
In the 21st century, human beings will pay more attention to their own health and living environment. It can be predicted that the development of short-cycle production processes and equipment for the production of compressed wood with high efficiency and high quality in the future, as well as the production of green products, will be compressed wood. The direction of development.
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