Method for producing modified wood

Abstract

This method for producing modified wood comprises: a step for impregnating block-shaped wood with an organic acid aqueous solution containing an organic acid; a step for steam-treating the block-shaped wood, which has been impregnated with the organic acid aqueous solution and is in a wet state, at a high temperature and under a high pressure; and a step for drying the steam-treated block-shaped wood and then heating the same in an inert atmosphere, wherein the temperature during heating in the inert atmosphere is 110-180ºC.

Description

Modified wood manufacturing method

[0001] The present invention relates to a method for producing modified wood.

[0002] Since wood repeatedly swells and shrinks as it absorbs and desorbs moisture, changes in moisture content cause dimensional changes. Patent Document 1 discloses a method for suppressing such dimensional changes and imparting dimensional stability.

[0003] Patent Document 1 discloses a method for improving the mold and rot resistance of cellulose products and the dimensional stability of the products. Specifically, Patent Document 1 discloses drying cellulose products to a moisture content level of less than 15% and subjecting them to a heat treatment carried out at high temperature, in which after drying, the product is held in a moist oven supplied with steam at a temperature above 150°C under atmospheric pressure, and the treatment is continued until the product has lost at least 5% of its weight.

[0004] Furthermore, wood may deteriorate, such as by decay, depending on the environment in which it is used. This decay occurs primarily as a result of biological activity and is further accelerated by environmental factors such as moisture and ultraviolet rays. Therefore, wood used outdoors may be impregnated with wood preservatives to inhibit decay. Meanwhile, heat treatment of wood is also being considered to increase its decay resistance.

[0005] Patent Document 2 discloses a method for producing highly durable lumber that does not require special equipment for high-temperature treatment or the use of inert gas, allows easy temperature control, and as a result, can provide highly durable lumber at low cost. Specifically, Patent Document 2 discloses a method for producing highly durable lumber that includes the steps of impregnating wood with an aqueous solution of a salt consisting of a strong acid and a weak base and exhibiting a nearly neutral to weakly acidic pH at room temperature, drying the wood, and heating the wood. It also discloses that the temperature in the heating step is between 120°C and 200°C, and that the salt used is magnesium chloride, calcium chloride, copper sulfate, or ammonium sulfate.

[0006] Japanese Patent No. 3585492 Japanese Patent Application Laid-Open No. 2018-161802

[0007] However, the method of simply drying wood and then treating it with high-temperature steam, as in Patent Document 1, has the problem of promoting the decomposition of cellulose, which is a skeletal component of wood, resulting in a decrease in the strength of the treated wood. If the heat treatment temperature of the wood is lowered to avoid this problem, the hemicellulose in the wood will not be sufficiently decomposed and modified, making it difficult to improve the dimensional stability of the treated wood.

[0008] Furthermore, although the manufacturing method of Patent Document 2 requires a heating temperature of 200° C. or less, it requires the use of a salt such as magnesium chloride, and therefore a manufacturing method with higher safety is desired.

[0009] The present invention has been made in view of the problems of the prior art, and an object of the present invention is to provide a method for producing modified wood that has excellent dimensional stability, mechanical strength, and decay resistance.

[0010] In order to solve the above problems, a method for producing modified wood according to an embodiment of the present invention includes the steps of impregnating a block of wood with an organic acid aqueous solution containing an organic acid, steam-treating the wet block of wood impregnated with the organic acid aqueous solution at high temperature and pressure while it is still wet, and drying the steam-treated block of wood and then heating it in an inert atmosphere, wherein the first temperature during heating in the inert atmosphere is 110°C or higher and 180°C or lower.

[0011] Fig. 1 is an explanatory diagram showing an example of a method for producing modified wood according to the present embodiment. Fig. 2 is a graph showing the relationship between the dimensional change rate and the bending strength for the test pieces of Example 1, Reference Example 1, and Comparative Example 1.

[0012] The modified wood manufacturing method according to the present embodiment will be described in detail below with reference to the drawings. Note that the dimensional proportions in the drawings are exaggerated for the sake of convenience and may differ from the actual proportions.

[0013] [First embodiment] A method for producing modified wood according to this embodiment includes the steps of impregnating a block of wood with an organic acid aqueous solution containing an organic acid, and steam-treating the wet block of wood impregnated with the organic acid aqueous solution at high temperature and pressure while the block is still wet. The method further includes the steps of drying the steam-treated block of wood and then heating it in an inert atmosphere.

[0014] Figure 1 shows the flow of the modified wood manufacturing method according to this embodiment. In this manufacturing method, the first step is to impregnate a block of wood with an organic acid aqueous solution containing an organic acid. The wood may be in the shape of a block, for example, a board. The thickness of the board-shaped wood is not particularly limited, but can be, for example, 10 mm to 40 mm.

[0015] Examples of wood include wood from various tree species used for building materials such as floors, walls, and ceilings, fixtures, furniture, crafts, etc. The wood species is not particularly limited, and for example, at least one selected from the group consisting of cedar, larch, Douglas fir, rubber tree, birch, beech, oak, beech, oak, teak, hard maple, cherry, walnut, white ash, mahogany, and yellow birch can be used. These woods have a luxurious feel and are highly decorative, so by modifying these woods, they can be suitably used for building materials, fixtures, furniture, and crafts.

[0016] Furthermore, fast-growing trees, which grow to large diameters in a short period of time, primarily in Japan and Southeast Asia, can also be used. Specifically, at least one selected from the group consisting of China chinaberry, Chinese lantern tree, Japanese alder, tulip tree, eucalyptus, poplar, Acacia mangium, and falcata can be used. Fast-growing trees grow quickly and are relatively inexpensive, making them suitable for sufficient supply through plantation forestation. However, fast-growing trees have wide ring widths and large ring curvatures, resulting in anisotropic dimensional changes. Therefore, when fast-growing trees are dried, large localized shrinkage stresses occur, making them prone to drying cracks. However, as described below, organic acids and sugars have dimensional stabilizing effects, and impregnating the interior of fast-growing trees with these can suppress shrinkage. Therefore, fast-growing trees can also be suitably used as wood in this embodiment.

[0017] The wood may be in a green state with a high moisture content, or in a dried state with a low moisture content. Even when the moisture content of the wood is high, the moisture in the vessels can be replaced with the organic acid aqueous solution, allowing the wood to be impregnated with the organic acid aqueous solution. Artificially dried wood (KD wood), which has been artificially dried in a drying oven or the like to reduce its moisture content, may also be used. In this case, the moisture content of the KD wood is preferably 7 to 25%. The moisture content of wood can be measured based on Japanese Industrial Standard JIS Z2101 (Testing Methods for Wood).

[0018] Here, raw block-shaped wood (green lumber) may crack or discolor due to drying if stored at room temperature. Logs to be sawn into green lumber may also crack or discolor due to drying, so they are imported between winter and spring when temperatures are low. After sawn into green lumber, they must be stored frozen or refrigerated to prevent cracking and discoloration. In addition, because blocks of lumber are sawn from logs with a roughly circular cross section, yields are low. Furthermore, when logs are imported from overseas and sawn into blocks of lumber, waste is generated during sawing, meaning that the waste is included in the transport of the imported lumber. Therefore, when transporting green lumber and logs over long distances, transportation and storage costs tend to be high.

[0019] In contrast, dried wood (KD lumber) requires a drying process, but because it is in a dry state, there is a low risk of cracking or discoloration. Furthermore, KD lumber can be stored at room temperature. In this way, KD lumber can reduce transportation and storage costs.

[0020] In addition, if the logging site and the location where the manufacturing method of this embodiment is carried out are close to each other, there is no need to transport the logs over long distances. Therefore, in this case, it is preferable to use green lumber as the wood material, rather than KD lumber, which requires a drying process.

[0021] The organic acid aqueous solution to be impregnated into the wood can be prepared by dissolving the organic acid in water. As the organic acid, an organic compound can be used that can improve the dimensional stability and mechanical strength of wood impregnated with the organic acid aqueous solution by heat treating the wood in an inert atmosphere, as described below. Specifically, the organic acid is preferably at least one selected from the group consisting of carboxylic acids, sulfonic acids, and sulfinic acids.

[0022] The organic acid is preferably a carboxylic acid, more preferably a divalent or higher carboxylic acid. When wood impregnated with a carboxylic acid is heat-treated in an inert atmosphere, hemicellulose and lignin, which are constituent components of the wood, tend to deteriorate, thereby further promoting the modification of the wood.

[0023] The carboxylic acid is preferably at least one selected from the group consisting of citric acid, tartaric acid, malic acid, succinic acid, oxalic acid, adipic acid, malonic acid, phthalic acid, sebacic acid, maleic acid, fumaric acid, itaconic acid, glutaric acid (1,5-pentanedioic acid), gluconic acid, glutaconic acid, and pentenedioic acid. Furthermore, the carboxylic acid is more preferably at least one selected from the group consisting of citric acid, malic acid, and succinic acid. By using citric acid, malic acid, and succinic acid, modified wood with good dimensional stability and mechanical strength can be easily obtained. Furthermore, because these carboxylic acids can be obtained from naturally occurring materials, it is possible to reduce the environmental impact.

[0024] In the organic acid aqueous solution, the organic acid content is preferably 3 to 30% by mass, more preferably 3 to 20% by mass, and even more preferably 3 to 10% by mass. When the organic acid content in the organic acid aqueous solution is within this range, the organic acid can easily penetrate into wood, thereby achieving the effects of stabilizing the dimensions of wood and improving its strength.

[0025] It is preferable that the organic acid aqueous solution further contains a sugar. The organic acid aqueous solution contains both an organic acid and a sugar, which can enhance the dimensional stability and mechanical strength of wood. Specifically, by impregnating wood with an organic acid aqueous solution containing both an organic acid and a sugar, and then heat-treating the wood in an inert atmosphere, the dimensional stability and strength can be enhanced by the effect of the organic acid. Furthermore, sugars fill the microvoids in the cell walls of wood instead of water molecules, and can remain in these microvoids without evaporating even during drying. Furthermore, because sugars can maintain the cell walls in a swollen state, the so-called "bulk effect" can suppress shrinkage of the resulting modified wood.

[0026] The saccharide may be at least one selected from the group consisting of monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Examples of monosaccharides include fructose, xylose, ribose, arabinose, rhamnose, xylulose, and deoxyribose. Examples of disaccharides include sucrose, maltose, trehalose, turanose, lactulose, maltulose, palatinose, gentiobiulose, melibiulose, galactosucrose, rutinulose, and planteobiose. Examples of oligosaccharides include fructooligosaccharides, galactooligosaccharides, mannanoligosaccharides, and stachyose. Examples of polysaccharides include starch, agarose, alginic acid, glucomannan, inulin, chitin, chitosan, hyaluronic acid, glycogen, and cellulose.

[0027] Here, the sugar is preferably at least one selected from the group consisting of fructose, maltose, xylose, and sucrose. These sugars are easily available and, together with the organic acid, can further improve the dimensional stability of the modified wood.

[0028] The sugar content in the organic acid aqueous solution is preferably 3 to 30% by mass, more preferably 3 to 20% by mass, and even more preferably 3 to 10% by mass. When the sugar content in the organic acid aqueous solution is within this range, the sugars can easily penetrate into the wood, thereby achieving the dimensional stabilization effect of the modified wood with the sugars.

[0029] Since organic acids and sugars are highly soluble in water, the organic acid aqueous solution does not need to contain an organic solvent. Furthermore, the absence of an organic solvent in the organic acid aqueous solution reduces the environmental impact and increases safety for the human body.

[0030] There are no particular limitations on the method for impregnating a block of wood with an organic acid aqueous solution. For example, wood can be impregnated with the organic acid aqueous solution by immersing the wood in the organic acid aqueous solution and leaving it. In order to speed up the impregnation of the wood with the organic acid aqueous solution, it is preferable to place the wood in a pressure-resistant container filled with the organic acid aqueous solution and apply pressure. In this case, the pressure to be applied is not particularly limited, but is preferably 0.3 to 10.0 MPa, for example.

[0031] When wood is impregnated with the organic acid aqueous solution, the temperature of the organic acid aqueous solution is not particularly limited, but is preferably, for example, 80° C. or less. The temperature of the organic acid aqueous solution can also be room temperature.

[0032] To speed up the impregnation of wood with the organic acid aqueous solution, the wood may be placed in a pressure-resistant container, the pressure reduced, the air inside the wood removed, and then the wood immersed in the organic acid aqueous solution. This allows the organic acid aqueous solution to penetrate more easily into the vessels of the wood, allowing the wood to be quickly impregnated with the organic acid aqueous solution.

[0033] When impregnating a block of wood with an organic acid aqueous solution, it is preferable that the entire wood, i.e., the center of the wood, is impregnated with the organic acid aqueous solution. This allows the organic acid to modify the wood up to the center. However, it is not necessary to impregnate the center of the wood with the organic acid aqueous solution; it is sufficient to impregnate at least the portion of the wood to be modified with the organic acid aqueous solution.

[0034] In the manufacturing method of this embodiment, in the second step, the wood in a wet state impregnated with the organic acid aqueous solution is subjected to a steam treatment at high temperature and pressure while still in the wet state.

[0035] Conventionally, wood has been heat-treated to darken its color, thereby enhancing the wood's natural grain, knots, etc. In this embodiment, wood in a wet state impregnated with an organic acid aqueous solution is subjected to a steam treatment, which facilitates the deterioration of the wood's constituent components, cellulose, hemicellulose, and lignin, thereby enabling the wood to be colored brown.

[0036] Specifically, first, wood impregnated with an organic acid aqueous solution is placed inside, for example, a pressure-resistant container. At this time, it is preferable to place the wood impregnated with the organic acid aqueous solution in a wet state without drying it. If the wood impregnated with the organic acid aqueous solution is dried and then steam-treated, the wood can be colored, but the color may become dull or lose its vividness. Therefore, it is preferable to steam-treat the wood impregnated with the organic acid aqueous solution while it is still wet.

[0037] Next, the wood impregnated with the organic acid aqueous solution is heat-treated in a steam atmosphere by introducing steam while heating the inside of the pressure-resistant container. The heating temperature of the wood is preferably 110°C or higher and 160°C or lower, and the pressure applied to the wood is preferably the saturated steam pressure at that temperature. By heating the wood impregnated with the organic acid aqueous solution in such a high-temperature, high-pressure steam atmosphere, the cellulose, hemicellulose, and lignin, which are constituent components of the wood, are hydrolyzed by the organic acid. This causes the wood to become reddish and darker.

[0038] The heating temperature of the wood is preferably 110°C or higher and 160°C or lower, more preferably 110°C or higher and 150°C or lower, and even more preferably 110°C or higher and 140°C or lower. If wood impregnated with an organic acid and in a wet state is treated with steam at a high temperature exceeding 160°C, deterioration of the wood may progress, resulting in a decrease in strength and durability. Furthermore, if the wood is heated at a low temperature below 110°C, the hydrolysis reaction by the organic acid is unlikely to proceed, and darkening may be insufficient. Therefore, the heating temperature of the wood is preferably within the above temperature range.

[0039] The time for steam treatment of wood impregnated with an organic acid aqueous solution is not particularly limited. For example, increasing the temperature of the steam treatment promotes wood darkening. Furthermore, increasing the concentration of the organic acid contained in the organic acid aqueous solution also promotes wood darkening. The degree of progress of darkening also varies depending on the species of wood or the type of organic acid. Therefore, it is preferable to adjust the time for steam treatment depending on the temperature of the steam treatment, the concentration of the organic acid contained in the organic acid aqueous solution, the species of wood, and the type of organic acid. For example, the time for steam treatment of wood impregnated with an organic acid aqueous solution can be 0.5 hours or more.

[0040] After such steam treatment, the pressure inside the pressure vessel is reduced from the high pressure state to atmospheric pressure, and the wood is removed from the pressure vessel.

[0041] In the manufacturing method of this embodiment, in the third step, the wood impregnated with the organic acid aqueous solution is dried to remove excess moisture from inside the wood. The drying conditions are not particularly limited, but natural drying can be used, for example. The wood may also be dried by heating, for example, at a temperature of 80°C or less, preferably 70°C or less, and more preferably 60°C or less. The drying atmosphere is also not particularly limited, and drying may be performed in the air, for example. The moisture inside the wood may also be removed by gradually decreasing the humidity in the drying atmosphere.

[0042] The drying process will be described in more detail. The drying process may be natural drying as described above, or the wood may be dried using a drying device. An example of such a drying device is a steam drying device that supplies steam to a heating tube in the drying device to gradually increase the temperature inside the drying device while gradually decreasing the humidity (relative humidity) inside the drying device. Alternatively, a dehumidifying drying device equipped with a heat pump dehumidifier or a reduced-pressure drying device that dries by reducing pressure and heating may be used as the drying device. Hot air or a radiant heater may be used during drying.

[0043] The moisture content of the wood dried in the drying step is not particularly limited, but can be, for example, 25% or less. The moisture content of the dried wood may also be 20% or less, or 15% or less. The moisture content of wood can be measured in accordance with JIS Z2101.

[0044] Since the organic acid and sugars remain in the microscopic spaces inside the wood, they can suppress shrinkage of the wood during drying and improve dimensional stability, so that even if wood impregnated with an aqueous organic acid solution is dried as described above, deformation and cracking of the wood can be suppressed.

[0045] In the manufacturing method of this embodiment, the wood that has been subjected to the drying process is subjected to a fourth step of heating in an inert atmosphere. By subjecting the wood that has been impregnated with an organic acid to a heat treatment in an inert atmosphere, the decomposition and modification of hemicellulose can be promoted, thereby improving dimensional stability.

[0046] The heat treatment conditions under an inert atmosphere are preferably heating at 180°C or less, more preferably at 110°C to 180°C, and even more preferably at 140°C to 180°C. The heating time is preferably adjusted depending on the type and size of the wood, but can be, for example, 12 to 72 hours. By keeping the temperature during heating under an inert atmosphere at 180°C or less, modified wood that combines excellent dimensional stability and mechanical strength can be obtained. The inert atmosphere is an atmosphere with a reduced oxygen concentration, and can be, for example, a nitrogen gas atmosphere or a superheated steam atmosphere. Note that superheated steam is a gas obtained by heating saturated steam, and therefore contains almost no oxygen.

[0047] When wood is heat-treated, a decomposition reaction occurs due to the high temperature treatment, which can usually result in volume shrinkage. However, in the modified wood manufacturing method of this embodiment, the wood is impregnated with an organic acid that has a dimensional stabilizing effect, so wood shrinkage can be suppressed even when heat-treated in an inert atmosphere. Furthermore, heat-treated wood generally becomes a material with reduced viscosity. This is because the non-crystalline regions of hemicellulose and other components that cause wood viscosity, i.e., areas that easily adsorb water, are decomposed. However, the modified wood of this embodiment is impregnated with at least an organic acid and may also be impregnated with sugars. It is believed that such organic acids and sugars penetrate between the cellulose fibers and impart flexibility to the wood. Therefore, it is believed that the modified wood of this embodiment can maintain viscosity and increase mechanical strength compared to general heat-treated wood.

[0048] Wood may deteriorate, such as by decaying, depending on the environment in which it is used. This decay occurs primarily as a result of biological activity and is further accelerated by environmental factors such as moisture and ultraviolet light. Therefore, wood used outdoors may be impregnated with wood preservatives to inhibit decay. Meanwhile, chemical-free and non-chemical wood preservation methods are also attracting attention, and heat treatment is a typical example.

[0049] Heat treatments that improve the decay resistance of wood include methods of treating wood at high temperatures in the absence of oxygen or under superheated steam to modify wood components. Known examples of wood with improved decay resistance through such heat treatments include Thermowood, which is heat-treated under superheated steam, Estecwood, which is heat-treated under nitrogen gas, and Platowood, which is steam-dried wood and then heat-treated under nitrogen gas.

[0050] Here, Plato wood is heated at 150°C to 180°C and a pressure of 10 bar (approximately 10 kg / cm ２ ) for 3 to 5 hours, then dried to reduce the moisture content to about 10%, and heated in a heating furnace at 150°C to 190°C for 14 to 16 hours. The heating in the heating furnace is carried out under a nitrogen atmosphere. It has been reported that such Plato wood exhibits high decay resistance in decay resistance tests according to EU standards. On the other hand, a non-patent document reports that evaluation according to JIS K1571 did not confirm any significant improvement in decay resistance. Thus, conventional heat treatments have room for further improvement in decay resistance. Non-patent document: Kensuke Hanada and two others, "Decay and Termite Resistance of Plato Heat-Treated Wood," Wood Preservation, Japan Wood Preservation Association, 2006, Vol. 32, No. 1, pp. 13-19

[0051] The modified wood manufacturing method of this embodiment involves a heat treatment in an inert atmosphere after the organic acid aqueous solution impregnation step, steam treatment step, and drying step. This heating step in an inert atmosphere decomposes and reduces the amorphous regions that are the starting point for wood decay. Therefore, decay resistance can be improved without using wood preservatives to inhibit decay. Furthermore, in the manufacturing method of this embodiment, wood containing an organic acid is subjected to a heat treatment in an inert atmosphere. Organic acids not only promote wood coloring but also promote the decomposition of the amorphous regions of wood, making it possible to proceed with the decomposition of the amorphous regions at low temperatures.

[0052] As described above, modified wood according to this embodiment can be obtained by subjecting wood to the organic acid aqueous solution impregnation step, steam treatment step, drying step, and heat treatment step. The resulting modified wood has high dimensional stability and mechanical strength, as described above. Furthermore, organic acids are widely used as food additives and are highly safe. Furthermore, the modified wood is colored in vibrant colors, giving it a luxurious feel and a highly decorative design, as well as excellent decay resistance. Therefore, the modified wood can be suitably used for a variety of applications, such as building materials, fixtures, furniture, and crafts.

[0053] The method for producing modified wood according to the present embodiment includes the steps of impregnating a block of wood with an organic acid aqueous solution containing an organic acid, steam-treating the wet block of wood impregnated with the organic acid aqueous solution at high temperature and pressure, and drying the steam-treated block of wood and then heating it in an inert atmosphere at a temperature of 110°C or higher and 180°C or lower.

[0054] In the manufacturing method of this embodiment, the organic acid and heat treatment in an inert atmosphere promote the decomposition and modification of hemicellulose in the wood, thereby suppressing moisture adsorption and desorption and improving dimensional stability. Furthermore, the organic acid penetrates between the cellulose fibers in the wood and has the effect of imparting flexibility to the wood, allowing the modified wood of this embodiment to maintain toughness and increase mechanical strength. Furthermore, the organic acid exerts a catalytic effect that promotes the decomposition and modification of hemicellulose in the wood, allowing the heat treatment temperature to be lowered and the heat treatment time to be shortened. Furthermore, in the manufacturing method of this embodiment, wood impregnated with an organic acid is subjected to heat treatment in an inert atmosphere to decompose the hemicellulose that forms amorphous regions, thereby reducing the amorphous regions of the modified wood and improving decay resistance.

[0055] In this way, the modified wood obtained by the manufacturing method of this embodiment may be used as it is for various building materials (floors, walls, ceilings, fixtures, etc.), furniture, wood decks, etc. The modified wood can also be sliced ​​into thicknesses of 1 to 10 mm using a saw or the like, and then bonded to a substrate, to be used as a surface material for various products.

[0056] [Additional Notes] The above description of the embodiments discloses the following techniques.

[0057] (Technology 1) A method for producing modified wood, comprising the steps of: impregnating a block of wood with an organic acid aqueous solution containing an organic acid; steam-treating the wet block of wood impregnated with the organic acid aqueous solution at high temperature and high pressure while the block is still wet; and drying the steam-treated block of wood, and then heating it in an inert atmosphere, wherein the first temperature during heating in the inert atmosphere is 110°C or higher and 180°C or lower.

[0058] This configuration promotes the decomposition and modification of hemicellulose in wood, thereby suppressing moisture absorption and desorption and improving the dimensional stability of wood. Furthermore, because hemicellulose, which forms amorphous regions, is decomposed, the modified wood has fewer amorphous regions and is therefore more resistant to decay. Furthermore, the organic acid penetrates between the cellulose fibers in wood, imparting flexibility to the wood. This allows for the production of modified wood with improved dimensional stability and mechanical strength. Furthermore, because the modified wood is colored in vibrant colors, it has a luxurious feel and can be designed with enhanced aesthetic appeal.

[0059] (Technology 2) A method for producing modified wood according to Technology 1, wherein the second temperature in the steam treatment is 110°C or higher and 160°C or lower, and the pressure is the saturated steam pressure at the second temperature.

[0060] With this configuration, the cellulose, hemicellulose, and lignin in the wood are hydrolyzed by the organic acid, which increases the reddish color of the wood and makes it darker, thereby improving the design properties of the wood.

[0061] (Technology 3) The method for producing modified wood according to Technology 1 or 2, wherein the inert atmosphere is a nitrogen atmosphere or a superheated steam atmosphere.

[0062] With this configuration, wood that has been impregnated with an organic acid and subjected to steam treatment and drying treatment can be easily heat-treated in an inert atmosphere.

[0063] (Technology 4) A method for producing modified wood according to any one of technologies 1 to 3, wherein the organic acid is a divalent or higher carboxylic acid.

[0064] This configuration makes it possible to further promote the modification of wood by heat treating wood impregnated with carboxylic acid in an inert atmosphere, which makes it easier for the wood's constituent components, hemicellulose and lignin, to be altered.

[0065] (Technology 5) The method for producing modified wood according to Technology 4, wherein the carboxylic acid is at least one selected from the group consisting of citric acid, malic acid, and succinic acid.

[0066] This configuration allows the carboxylic acid to easily react with hemicellulose and other components of the wood, making it possible to easily obtain modified wood with good dimensional stability and mechanical strength.

[0067] (Technology 6) The method for producing modified wood according to any one of technologies 1 to 5, wherein the organic acid content in the organic acid aqueous solution is 3 to 30% by mass.

[0068] This configuration allows the organic acid to easily penetrate into the wood, thereby achieving the effects of stabilizing the dimensions of the wood and improving its strength.

[0069] Hereinafter, the present embodiment will be described in more detail with reference to examples, reference examples and comparative examples, but the present embodiment is not limited to these examples.

[0070] [Example 1] (Preparation of test samples) <Example 1 and Reference Example 1> First, four pieces of cedar wood, each 30 mm wide, 30 mm thick, and 200 mm long, were prepared. The pieces were cut with the grain on all four sides. The length direction of the cedar wood was the fiber direction.

[0071] Next, citric acid, which is an organic acid, and water were mixed in the ratio shown in Table 1 to prepare an aqueous solution of Example 1 (Reference Example 1).

[0072]

[0073] Next, the aqueous solution of Example 1 (Reference Example 1) was placed in a pressure-resistant container, and the four cedar lumber pieces were immersed therein. Then, while the cedar lumber was immersed in the aqueous solution, a pressure impregnation treatment was performed in which the atmospheric pressure was maintained at 0.7 MPa for 3 hours. After the pressure impregnation treatment, the cedar lumber was immersed in the aqueous solution and cured for 12 hours under atmospheric pressure. By curing the cedar lumber while immersed in the aqueous solution, the organic acid was reliably impregnated into the cell walls of the cedar lumber.

[0074] Next, the impregnated cedar wood was removed from the aqueous solution and then subjected to steam treatment in a pressure vessel. The steam treatment was performed while the cedar wood was wet. The temperature and pressure of the steam treatment were 120°C and the saturated steam pressure at that temperature, respectively, for one hour.

[0075] The four pieces of cedar wood that had been subjected to the steam treatment were then dried. The drying process was carried out using a steam drying device, with the drying conditions being changed in the order of steps 1 to 7 shown in Table 2.

[0076]

[0077] Then, the four cedar lumber pieces that had been subjected to the drying process were subjected to heat treatments under conditions 1 to 4 shown in Table 3 using a pressure-resistant container. Each heat treatment under conditions 1 to 4 was performed on one cedar lumber piece. The cedar lumber under condition 1 obtained in this manner was used as the test sample for Example 1-1, the cedar lumber under condition 2 was used as the test sample for Example 1-2, the cedar lumber under condition 3 was used as the test sample for Reference Example 1-1, and the cedar lumber under condition 4 was used as the test sample for Reference Example 1-2.

[0078]

[0079] Comparative Example 1 First, four cedar lumber pieces, each 30 mm wide, 30 mm thick, and 200 mm long, were prepared in the same manner as in Example 1. Next, only water was placed in a pressure-resistant container, and the four cedar lumber pieces were immersed in the water. Then, while the cedar lumber pieces were immersed in water, a pressure impregnation treatment was performed in which the atmospheric pressure was set to 0.7 MPa and maintained for three hours. After the pressure impregnation treatment, the cedar lumber pieces were cured for 12 hours under atmospheric pressure while still immersed in water.

[0080] Next, the four pieces of cedar wood after the impregnation treatment were removed from the water and then subjected to steam treatment using a pressure-resistant container, as in Example 1. The steam treatment was performed while the cedar wood was wet. The temperature and pressure of the steam treatment were 120°C and the saturated steam pressure at that temperature, respectively, for one hour.

[0081] Next, the four pieces of cedar wood that had been subjected to the steam treatment were dried using a steam drying device. The drying process was carried out in the same manner as in Example 1, by changing the drying conditions in the order of steps 1 to 7 shown in Table 2.

[0082] The cedar lumber that had been subjected to the drying process was then subjected to heat treatments under conditions 1 to 4 shown in Table 3 using a pressure-resistant container. Each heat treatment under conditions 1 to 4 was performed on one cedar lumber. The cedar lumber obtained under condition 1 was used as the test sample for comparative example 1-1, the cedar lumber under condition 2 was used as the test sample for comparative example 1-2, the cedar lumber under condition 3 was used as the test sample for comparative example 1-3, and the cedar lumber under condition 4 was used as the test sample for comparative example 1-4.

[0083] (Evaluation) <Dimensional change rate measurement> The test samples of Examples 1-1 to 1-2, Reference Examples 1-1 to 1-2, and Comparative Examples 1-1 to 1-4 were cut to a width of 30 mm, a thickness of 30 mm, and a length of 5 mm to prepare four test pieces for each example. The length direction of the test pieces in each example was the fiber direction.

[0084] Next, the test piece of each example was kept under the following moisture absorption conditions to absorb moisture, and the dimensions in the width direction and thickness direction were measured. Next, the test piece of each example was kept under the following drying conditions to dry, and the dimensions in the width direction and thickness direction were measured. Moisture absorption conditions: temperature 40°C, relative humidity 90%, number of days 4 Drying conditions: temperature 70°C, relative humidity 25%, number of days 4

[0085] Next, the dimensional change rate in the width direction and the dimensional change rate in the thickness direction of the test piece of each example were calculated from the following Equations 1 and 2, and the sum of the dimensional change rate in the width direction and the dimensional change rate in the thickness direction was taken as the dimensional change rate of each test piece. The average value of the dimensional change rates of the four test pieces was taken as the dimensional change rate of each example. The dimensional change rates of each example are shown in Tables 4 and 5. [Equation 1] Width direction dimensional change rate (%) = [(Width direction dimension when hygroscopic) - (Width direction dimension when dry)] / [Width direction dimension when dry] x 100 [Equation 2] Thickness direction dimensional change rate (%) = [(Thickness direction dimension when hygroscopic) - (Thickness direction dimension when dry)] / [Thickness direction dimension when dry] x 100

[0086] <Measurement of bending strength> First, the test samples of Examples 1-1 to 1-2, Reference Examples 1-1 to 1-2, and Comparative Examples 1-1 to 1-4 were cut to a width of 11 mm, a thickness of 5 mm, and a length of 90 mm to prepare four test pieces for each example. The length direction of the test pieces in each example was the fiber direction.

[0087] Next, the bending strength of each test piece was measured in accordance with the "bending test" in JIS Z2101:2009 (Testing methods for wood). At this time, the span (distance between supports) was 70 mm, and the load head speed was 10 mm / min. The average value of the bending strengths of the four test pieces for each example was taken as the bending strength of each example. The bending strengths of each example are shown in Tables 4 and 5.

[0088] <Measurement of Dissolution Rate> The test samples of Examples 1-1 to 1-2, Reference Examples 1-1 to 1-2, and Comparative Examples 1-1 to 1-4 were cut to a width of 30 mm, a thickness of 30 mm, and a length of 5 mm to prepare two test pieces for each example. The length direction of the test pieces in each example was the fiber direction.

[0089] Next, the test specimens of each example were left to stand at 100°C for two days to carry out initial drying. Next, the test specimens of each example that had been subjected to initial drying were immersed in water at 20°C for 24 hours. Thereafter, the immersed test specimens were left to stand at 100°C for four days to carry out post-immersion drying.

[0090] Next, the dissolution rate of each test piece was calculated using the following mathematical formula 3. The average value of the dissolution rates for the two test pieces was taken as the dissolution rate for each example. The dissolution rates for each example are shown in Tables 4 and 5. [Math 3] Dissolution rate (%) = [(mass of test piece after initial drying) - (mass of test piece after immersion and drying)] / [mass of test piece after initial drying] x 100

[0091]

[0092]

[0093] 2 shows the relationship between the dimensional change rate and the bending strength for the test specimens of Examples 1-1 to 1-2, Reference Examples 1-1 to 1-2, and Comparative Examples 1-1 to 1-4. As shown in Fig. 2, it can be seen that, for the test specimen of Comparative Example 1, as the heat treatment temperature increases, the dimensional change rate decreases and the dimensional stability increases, but the bending strength also decreases significantly. In contrast, for the test specimens of Examples 1-1 to 1-2 and Reference Example 1-1, as the heat treatment temperature increases, the dimensional stability increases and the bending strength decreases, but it can be seen that the bending strength is less likely to decrease than for the comparative examples.

[0094] In other words, generally, when the heat treatment temperature is increased to improve the dimensional stability of wood, there is a trade-off between the two, in that the strength properties of the wood are significantly reduced. However, by impregnating the wood with an organic acid as in Examples 1-1 and 1-2, it is possible to suppress the reduction in the strength properties of the wood even when the heat treatment temperature is increased to improve the dimensional stability of the wood.

[0095] Furthermore, from Tables 4 and 5, the test specimen of Example 1-1, which was heat-treated at 140°C, was inferior in bending strength compared to the test specimen of Comparative Example 1-1, but its dimensional stability was greatly improved. Moreover, the test specimen of Example 1-2, which was heat-treated at 170°C, was equivalent in bending strength compared to the test specimen of Comparative Example 1-2, but its dimensional stability was greatly improved. Furthermore, the test specimen of Reference Example 1-1, which was heat-treated at 200°C, was improved in both dimensional stability and bending strength compared to the test specimen of Comparative Example 1-3.

[0096] Furthermore, while the dimensional change rate in Example 1-1 was 2.7%, it can be seen from Comparative Examples 1-2 and 1-3 that in order to obtain a similar dimensional change rate without using an organic acid, it is necessary to increase the heat treatment temperature to about 180° C. In this way, by using an organic acid, it is possible to lower the required heat treatment level and improve the efficiency of the heat treatment.

[0097] It can be seen from Table 4 that the elution rates of the test pieces of Example 1 and Reference Example 1 decrease as the heat treatment temperature increases. This is thought to be because the heat treatment causes the organic acid to react with the wood components, immobilizing the organic acid inside the wood. Furthermore, the reaction between the organic acid and the wood components is thought to also suppress the elution of the wood components.

[0098] <Preservative performance test> The test samples of Examples 1-1 to 1-2, Reference Example 1-1, and Comparative Examples 1-1 to 1-3 were evaluated for decay resistance in accordance with "5.2 Preservative performance," "5.2.1 Laboratory tests," and "5.2.1.1 For injection treatment" of JIS K1571:2010 (Wood preservatives - Performance standards and test methods therefor).

[0099] Specifically, each test sample of Examples 1-1 to 1-2, Reference Example 1-1, and Comparative Examples 1-1 to 1-3 was sliced ​​to prepare a plurality of treated specimens each having a width of 30 mm, a thickness of 30 mm, and a length of 4.4 mm. The length direction of the test specimen in each example was the fiber direction. Next, in accordance with "5.2.1.1.3 Test" of JIS K1571:2010, the treated specimens in each example were subjected to weathering, antibacterial, and correction procedures.

[0100] Then, the mass loss rate of each treated specimen was calculated from the mass of the treated specimen before and after the antibacterial treatment or the correction treatment, based on Equation 4 and Equation 5. In other words, the mass loss rate of each treated specimen was calculated by subtracting the average mass loss rate (%) of the correction specimens from the mass loss rate (%) of the treated specimens after the antibacterial treatment was completed. The same evaluation was also performed on cedar sapwood as a control specimen. The evaluation results are summarized in Table 6. [Equation 4] Mass loss rate due to antibacterial treatment (%) = [(mass before antibacterial treatment) - (mass after antibacterial treatment)] / [mass before antibacterial treatment] x 100 [Equation 5] Mass loss rate due to correction treatment (%) = [(mass before antibacterial treatment) - (mass after correction treatment)] / [mass before correction treatment] x 100

[0101]

[0102] As shown in Table 6, when the test fungus was P. palustris, the average mass loss rate of the test samples of Example 1-1 was 27.1%, while the average mass loss rate of the test samples of Comparative Example 1-1 was 40.2%. Furthermore, the average mass loss rate of the test samples of Example 1-2 was 11.4%, while the average mass loss rate of the test samples of Comparative Example 1-2 was 38.9%.

[0103] Similarly, when the test fungus was Coriolus versicolor, the average mass loss rate of the test samples in Example 1-1 was 11.6%, while the average mass loss rate of the test samples in Comparative Example 1-1 was 27.5%.Furthermore, the average mass loss rate of the test samples in Example 1-2 was 2.9%, while the average mass loss rate of the test samples in Comparative Example 1-2 was 22.5%.

[0104] As described above, the test samples of Examples 1-1 and 1-2 have a significantly reduced mass reduction rate compared to the test samples of Comparative Examples 1-1 and 1-2, which indicates that the decay resistance is significantly improved.

[0105] Table 6 also shows that the decay resistance improves as the heat treatment temperature in an inert atmosphere increases. Furthermore, even though the treatment temperature in an inert atmosphere in Example 1-1 is 30°C or more lower than in Comparative Examples 1-2 and 1-3, the decay resistance improved, which suggests that the organic acid accelerated the decomposition of the amorphous regions of the wood.

[0106] Example 2 (Preparation of Test Sample) Example 2 First, dried yellow birch wood (KD wood) measuring 100 mm in width, 300 mm in length, and 28 mm in thickness was prepared as wood.

[0107] Next, aqueous solutions of Example 2 were prepared by mixing citric acid, an organic acid, sucrose, a sugar, and water in the proportions shown in Table 7. These aqueous solutions contained 20% by mass of the organic acid and sugar in total.

[0108] Next, the aqueous solution of Example 2 was placed in a pressure-resistant container, and the above-mentioned yellow birch KD material was immersed in it. Then, while the yellow birch KD material was immersed, a pressure impregnation treatment was carried out by maintaining the atmospheric pressure at 0.3 MPa for 1 hour.

[0109] The impregnated yellow birch wood was then removed from the aqueous solution and subjected to a steam treatment in a pressure vessel. The steam treatment was carried out while the wood was still wet. The temperature and pressure shown in Table 7 were the saturated steam pressure at the temperature, and the treatment time was 75 minutes.

[0110] The steam-treated yellow birch wood was then dried in the following order: (1) at 50°C and 95% humidity for 5 hours; (2) at 45°C and 90% humidity for 118 hours; (3) at 45°C and 79% humidity for 116 hours; and (4) at 50°C and 65% humidity for 240 hours.

[0111] The dried yellow birch wood was then subjected to a heat treatment in a pressure vessel under a nitrogen gas atmosphere at 170°C for 24 hours, thereby obtaining a test sample of Example 2.

[0112] Comparative Example 2 First, a green piece of yellow birch wood measuring 100 mm in width, 300 mm in length, and 28 mm in thickness was prepared. Next, steam treatment and drying were performed using the same process as in Example 2. The dried wood was then heat-treated at 200°C for 24 hours in a nitrogen gas atmosphere using a pressure-resistant container. In this manner, a test sample for Comparative Example 2 was obtained. Note that Comparative Example 2 is a sample that underwent a treatment similar to that for Plato wood. Table 7 summarizes the wood used, the impregnation conditions for organic acids and sugars, the steam treatment conditions, and the heat treatment conditions under a nitrogen gas atmosphere.

[0113]

[0114] (Evaluation) The test samples (test specimens) of Example 2 and Comparative Example 2 were evaluated for decay resistance in accordance with "5.2 Antiseptic performance," "5.2.1 Indoor test," and "5.2.1.1 For injection treatment" of JIS K1571:2010. In this test, no weathering operation was performed, but antibacterial operation and correction operation were performed. Furthermore, the same evaluation was performed on cedar sapwood as a control specimen.

[0115] The mass loss rate was calculated from the masses of the test specimens before and after the antibacterial treatment or the correction treatment based on the above formulas 4 and 5. The corrected mass loss rate of the heat-treated test specimens (test samples of Example 2 and Comparative Example 2) was calculated by subtracting the average mass loss rate of the correction treatment test specimens from the mass loss rate of the antibacterial treatment test specimens. The evaluation results are summarized in Table 8.

[0116]

[0117] As shown in Table 8, when the test fungus was Coriolus palustris, the mass loss rate of the test sample of Example 2 was 9%, while the mass loss rate of the test sample of Comparative Example 2 was 57%, and the mass loss rate of the control specimen was 64%. Also, when the test fungus was Coriolus versicolor, the mass loss rate of the test sample of Example 2 was 1%, while the mass loss rate of the test sample of Comparative Example 2 was 7%, and the mass loss rate of the control specimen was 36%.

[0118] As described above, the test sample of Example 2 has a significantly lower mass loss rate than the test sample of Comparative Example 2, which indicates that the test sample has significantly improved decay resistance. Furthermore, as shown in Table 7, although the treatment temperature in an inert atmosphere in Example 2 is 30°C lower than that of Comparative Example 2, the decay resistance is improved, which suggests that the organic acid promotes decomposition of the amorphous regions of the wood.

[0119] In addition, in JIS K1571, if the mass loss rate for both Coriolus palustris and Coriolus versicolor is 3% or less, the material meets the antiseptic performance standard and is therefore recognized as having antiseptic properties. The test sample of Example 2 meets the standard for Coriolus palustris, but further study is required to meet the standard for Coriolus palustris. However, since the test sample of Example 2 has improved antiseptic performance compared to the test sample of Comparative Example 2, which was treated similarly to Plato wood, it is presumed that the modified wood of this embodiment has improved decay resistance compared to Plato wood.

[0120] [Example 3] In Example 3, the steam-treated wood was not dried and then heated in an inert atmosphere. However, the results show that the organic acid and steam treatments promote the darkening of wood.

[0121] (Preparation of test samples) First, four pieces of dried yellow birch (KD wood) measuring 100 mm in width, 300 mm in length, and 28 mm in thickness were prepared as wood. The wood was obtained by dividing a piece of dried yellow birch (KD wood) measuring 100 mm in width, 1200 mm in length, and 28 mm in thickness into four equal pieces.

[0122] Next, aqueous solutions of Reference Example 3-1, Reference Example 3-2, and Comparative Example 3-2 were prepared by mixing succinic acid, an organic acid, maltose, a sugar, and water in the proportions shown in Table 9. The aqueous solution of Reference Example 3-2 contains 20% by mass of organic acid and sugar in total. In Comparative Example 3-1, no organic acid or sugar was used, and only water was used.

[0123]

[0124] Next, the aqueous solutions of Reference Example 3-1, Reference Example 3-2, and Comparative Example 3-2, and the water of Comparative Example 3-1 were placed in pressure-resistant containers, and the above-mentioned yellow birch KD wood was immersed one by one in the containers. Then, while the yellow birch KD wood was immersed in the aqueous solutions, a pressure impregnation treatment was carried out in which the atmospheric pressure was set to 0.3 MPa and maintained for 1 hour.

[0125] Next, after the impregnation treatment, each yellow birch wood was removed from the aqueous solution and subjected to steam treatment in a pressure vessel. The steam treatment was carried out while the yellow birch wood was still wet. The temperature and pressure of the steam treatment were 140°C and the saturated steam pressure at that temperature, respectively, for 90 minutes.

[0126] Then, each yellow birch material after the steam treatment was naturally dried for 4 days and then further dried at 50°C for 24 hours to obtain test samples of Reference Example 3-1, Reference Example 3-2, Comparative Example 3-1 and Comparative Example 3-2.

[0127] (Evaluation) The dimensional change rate and surface color of each of the test samples of Reference Example 3-1, Reference Example 3-2, Comparative Example 3-1 and Comparative Example 3-2 obtained as described above were measured.

[0128] <Dimensional change rate measurement> For each test sample, the width of the yellow birch KD wood before impregnation with the aqueous solution and the width of the sample after steam treatment and drying were measured. The dimensional change rate of each test sample was then calculated using the following equation 6. The dimensional change rate of each test sample is shown in Table 10. [Equation 6] Dimensional change rate (%) = [(width before impregnation with the aqueous solution) - (width after drying)] / [width before impregnation with the aqueous solution] x 100

[0129]

[0130] <Color measurement> Using a color difference meter, the lightness L of the surface of each test sample was measured. ＊ , and chromaticity a indicating hue and saturation ＊ , b ＊ The lightness L ＊ and chromaticity a ＊ , b ＊The lightness L of each test sample was calculated by measuring three points on the dark part (heartwood) and the light part (sapwood) of each test sample. ＊ and chromaticity a ＊ , b ＊ The measurement results are also shown in Table 10.

[0131] As shown in Table 10, the test sample of Reference Example 3-1 has almost the same dimensional change rate as the test sample of Comparative Example 3-1, but the lightness L ＊ When wood is darkened by heat treatment, the surface of the wood has a lightness L ＊ It is known that the thermal discoloration of wood decreases. Therefore, even under the same steam treatment conditions, Reference Example 3-1, which uses an organic acid, shows accelerated thermal discoloration. Furthermore, although the test sample of Reference Example 3-1 shows accelerated thermal discoloration due to the organic acid, the test samples of Reference Example 3-1 and Comparative Example 3-1 show similar dimensional change rates. Therefore, it is believed that the organic acid also exerts a dimensional stabilization effect on wood.

[0132] As shown in Table 10, the test sample of Reference Example 3-2 had a better dimensional change rate than the test sample of Comparative Example 3-1. This is due to the dimensional stabilization effect of the impregnation with sugars. The test sample of Reference Example 3-2 was darker in color than the test samples of Comparative Examples 3-1 and 3-2. This shows that the use of an aqueous solution containing both an organic acid and a sugar promotes thermal coloration of wood and also improves dimensional stability.

[0133] Although the present embodiment has been described above, the present embodiment is not limited to this, and various modifications are possible within the scope of the gist of the present embodiment.

[0134] The entire contents of Japanese Patent Application No. 2022-114128 (filing date: July 15, 2022) are incorporated herein by reference.

[0135] According to the present disclosure, a method for producing modified wood having excellent dimensional stability, mechanical strength, and decay resistance can be provided.

Claims

1. A step of impregnating a block of wood with an organic acid aqueous solution containing an organic acid; a step of subjecting the block-shaped wood impregnated with the organic acid aqueous solution in a wet state to a steam treatment at high temperature and high pressure while the block-shaped wood is still wet; drying the steam-treated block of wood and then heating it in an inert atmosphere; Including, The method for producing modified wood, wherein the first temperature during heating in the inert atmosphere is 110°C or higher and 180°C or lower.

2. The method for producing modified wood according to claim 1, wherein the second temperature in the steam treatment is 110°C or higher and 160°C or lower, and the pressure is the saturated steam pressure at the second temperature.

3. The method for producing modified wood material according to claim 1 or 2, wherein the inert atmosphere is a nitrogen atmosphere or a superheated steam atmosphere.

4. The method for producing modified wood material according to claim 1 or 2, wherein the organic acid is a divalent or higher carboxylic acid.

5. The method for producing modified wood material according to claim 4, wherein the carboxylic acid is at least one selected from the group consisting of citric acid, malic acid and succinic acid.

6. The method for producing modified wood material according to claim 1 or 2, wherein the organic acid content in the organic acid aqueous solution is 3 to 30 mass %.