JP2024050277A - Manufacturing method of compressed wood - Google Patents

Abstract

[Problem] To provide a method for manufacturing compressed wood that can suppress springback of compressed wood by a simple method. [Solution] The method for manufacturing compressed wood includes an impregnation step in which an organic acid aqueous solution containing an organic acid is pressurized and impregnated into a block-shaped piece of wood having a thickness of 3 mm or more, a drying step in which the wood in a wet state impregnated with the organic acid aqueous solution is dried, a compression step in which the dried wood is compressed while being heated to 140°C or higher, and a heating step in which the compressed wood is heated without being compressed. [Selected Figure] Figure 1

Description

The present invention relates to a method for producing compressed wood.

Coniferous trees such as cedar and cypress grow quickly and are easy to obtain, so they are used as furniture and building materials. However, coniferous trees are softer and less strong than hardwoods. For this reason, technology is being considered to increase the specific gravity of soft wood by compressing and deforming it, primarily to improve the surface hardness. Such technology is attracting attention as a way to expand the range of uses for wood with low specific gravity and low surface hardness, such as cedar.

On the other hand, it is known that compressed wood, which is obtained by compressing and deforming wood, recovers from compression by absorbing moisture or water, and returns to its pre-compression state. Methods for reducing this recovery phenomenon, known as springback, have been investigated.

Non-Patent Document 1 discloses a technique for permanently fixing the compression deformation by heat treating cedar wood. Specifically, Non-Patent Document 1 discloses the following. First, water is injected under reduced pressure into the cedar sapwood, and then it is heated with hot water at 95°C. It is then compressed using a press with a hot plate temperature set to 105°C and dried in the hot plate for 3 hours. The obtained compressed test piece is then kept in a dry state and heat treated using a hot air dryer. As a result, the springback of the compressed wood was reduced and the compressed shape was almost completely fixed by performing the heat treatment at 180°C for 20 hours, 200°C for 5 hours, or 220°C for 3 hours.

Masafumi Inoue and Kyo Norimoto, "Permanent Fixation of Compressive Deformation by Heat Treatment", Wood Research and Materials, Kyoto University Wood Research Institute, 1991, No. 27, pp. 31-40

However, the method described in Non-Patent Document 1 requires that the wood be compressed and dried, and then heat-treated at high temperature for a long period of time. Therefore, in order to reduce the manufacturing costs of compressed wood, there has been a demand for a method to suppress springback in compressed wood by heat-treating it at a low temperature and/or for a short period of time.

The present invention was made in consideration of the problems inherent in the conventional technology. The object of the present invention is to provide a method for manufacturing compressed wood that can suppress springback in compressed wood using a simple method.

In order to solve the above problems, the method for producing compressed wood according to the present invention includes an impregnation step in which an organic acid aqueous solution containing an organic acid is pressurized and impregnated into a block-shaped piece of wood having a thickness of 3 mm or more, a drying step in which the wood in a wet state impregnated with the organic acid aqueous solution is dried, a compression step in which the dried wood is compressed while being heated to 140°C or higher, and a heating step in which the compressed wood is heated without being compressed.

This disclosure provides a simple method for producing compressed wood that can suppress springback in compressed wood.

FIG. 2 is an explanatory diagram showing an example of a method for producing compressed wood according to the present embodiment. 1 is a graph showing the relationship between the compression temperature during the compression process and the thickness change rate of the compressed wood before and after heat treatment in a water vapor atmosphere for the compressed wood of the examples and comparative examples.

The manufacturing method for compressed wood according to this embodiment will be described in detail below with reference to the drawings. Note that the dimensional ratios in the drawings have been exaggerated for the sake of explanation and may differ from the actual ratios.

[First embodiment]
The method for producing compressed wood according to the present embodiment includes an impregnation step of impregnating wood with an organic acid aqueous solution containing an organic acid, and a drying step of drying the wood impregnated with the organic acid aqueous solution. The method further includes a compression step of compressing the dried wood while heating it, and a heating step of heating the compressed wood without compressing it.

Figure 1 (a) shows the flow of the method for producing compressed wood according to this embodiment. In the method for producing compressed wood according to this embodiment, the first step S1 is to impregnate a block of wood with an organic acid aqueous solution containing an organic acid (impregnation step). The shape of the wood may be block-like, and an example of the shape of the wood may be one processed into a board.

The thickness of the plank-shaped wood is preferably 3 mm or more. By processing wood having such a thickness as described below, compressed wood having a thickness of several millimeters can be obtained. Compressed wood having a thickness of several millimeters can be preferably used as a surface material to be attached to the surface of plywood, for example. The thickness of the plank-shaped wood is preferably 3 mm or more, and may be 10 mm or more or 12 mm or more. There is no particular limit to the upper limit of the thickness of the plank-shaped wood, but it can be, for example, 40 mm.

Examples of wood include wood of various tree species used for building materials such as floors, walls, and ceilings, fixtures, furniture, and crafts. There are no particular limitations on the type of wood, and hardwoods as well as conifers can be used. Specifically, the wood can be 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. 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.

As for the wood, fast-growing trees that grow to large diameters in a short period of time, mainly in Japan and Southeast Asia, can also be used. Specifically, the wood can be at least one selected from the group consisting of China chinaberry, Chinese lantern tree, alder, tulip tree, eucalyptus, poplar, Acacia mangium, and Falcata.

Wood may be in a raw state with a high moisture content, or in a dry state with a low moisture content. Even when wood has a high moisture content, the moisture in the vessels can be replaced with an organic acid solution, so the wood can be impregnated with the organic acid solution. In addition, artificially dried wood (KD wood) that has been artificially dried in a drying oven or the like to reduce its moisture content may be used as the wood. 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).

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 that can suppress springback by compressing the wood impregnated with the organic acid aqueous solution while heating, as described below, can be used. Specifically, the organic acid is preferably at least one selected from the group consisting of carboxylic acids, sulfonic acids, and sulfinic acids.

The organic acid is preferably a carboxylic acid, and more preferably a divalent or higher carboxylic acid. When wood impregnated with a carboxylic acid is heated, the compositional components of the wood are more likely to change, so that it is possible to further promote the modification of the wood.

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. 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, compressed wood can be obtained by low-temperature and/or short-time heat treatment. Furthermore, these carboxylic acids can be obtained from naturally occurring materials, which makes it possible to reduce the environmental impact.

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 the wood, and the organic acid can have a fixing effect on the compressed wood.

As described in the second embodiment, the wood may be impregnated with both the organic acid and the sugar by adding sugar to the organic acid aqueous solution. However, by impregnating the wood with only the organic acid, compressing it while heating, and then further heating it, the compressed wood is fixed and springback can be sufficiently suppressed. Therefore, the organic acid aqueous solution needs to contain at least an organic acid, and does not need to contain sugar.

Since organic acids are highly soluble in water, the organic acid aqueous solution does not need to contain an organic solvent. By not including an organic solvent in the organic acid aqueous solution, the environmental burden can be reduced and safety for the human body can be increased.

There is no particular limit to the method for impregnating a block of wood with the 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 pressurize it. In this case, the pressure to be applied is not particularly limited, but it is preferable to set it to, for example, 0.3 to 10.0 MPa.

When impregnating wood with an organic acid aqueous solution, the temperature of the organic acid aqueous solution is not particularly limited, but it is preferable to set it to, for example, 80°C or lower. The temperature of the organic acid aqueous solution can also be room temperature.

To speed up the impregnation of the wood with the organic acid solution, the wood can 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 solution. This allows the organic acid solution to easily penetrate into the vessels of the wood, allowing the wood to be quickly impregnated with the organic acid solution.

Here, when impregnating a block of wood with the organic acid aqueous solution, it is preferable that the entire wood, that is, the center of the wood, is impregnated with the organic acid aqueous solution. This allows the action of 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, and it is sufficient to impregnate at least the part of the wood that is to be modified with the organic acid aqueous solution.

In the manufacturing method of this embodiment, in the second step S2, the wood in a wet state impregnated with the organic acid aqueous solution is dried to remove excess moisture from inside the wood (drying step). The drying conditions are not particularly limited, but for example, natural drying can be used. 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. Furthermore, the drying atmosphere is not particularly limited, and for example, drying may be performed in the air. The moisture inside the wood may also be removed by gradually decreasing the humidity in the drying atmosphere.

The drying process will be described in more detail. The drying process may be natural drying as described above, but the wood may also 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. In addition, the drying device may be a dehumidifying drying device equipped with a heat pump dehumidifier, or a reduced pressure drying device that dries by reducing pressure and heating. Hot air or a radiant heater may be used when drying.

The moisture content of wood dried by the drying process is not particularly limited, but can be, for example, 30% or less. The moisture content of the dried wood may be 20% or less, or 15% or less. The lower limit of the moisture content of the dried wood is not particularly limited, but may be 1% or 5%. The moisture content of wood can be measured based on JIS Z2101.

Here, the organic acid remains in the tiny spaces inside the wood, suppressing the shrinkage of the wood during drying and improving dimensional stability. Therefore, even if wood impregnated with an aqueous organic acid solution is dried as described above, deformation and cracking of the wood can be suppressed.

In the manufacturing method of this embodiment, the dried wood is compressed in the third step S3 (compression step). The compression method is not particularly limited, and for example, the wood may be compressed after being clamped with a jig. Specifically, as shown in (b) and (c) of FIG. 1, for example, the wood 1 is clamped between a pair of metal plates 10, and the metal plates 10 are pressed from above and below to compress the wood 1. Note that the compression device for compressing the wood is not limited to a press device using flat plates, and a roll press device that continuously compresses the wood by passing it between a pair of rolls can also be used. Furthermore, the compression of the wood can be performed under atmospheric pressure.

When compressing the dried wood, the compression ratio is not particularly limited, but may be 30% or more, or may be 50% or more. The upper limit of the compression ratio is not particularly limited, but may be, for example, 70%. In this specification, the compression ratio is a value calculated by Equation 1.
[Equation 1]
Compression rate (%) = [(thickness of wood before compression T1) - (thickness of wood after compression T2)] / [thickness of wood before compression T1]

Here, in this embodiment, when compressing the wood, it is necessary to compress the wood while heating it. Specifically, the metal plate 10, which is a jig, is heated while compressing the wood 1. The heating temperature of the wood when compressing while heating is preferably 140°C or higher, more preferably 160°C or higher, even more preferably 180°C or higher, particularly preferably 200°C or higher, and most preferably 220°C or higher. The heating temperature of the wood when compressing while heating can be 250°C or lower. In this way, by compressing the wood containing an organic acid while heating it, it is possible to suppress the restoration of the compressed wood during the next heating process. In addition, by compressing the wood while heating it, it is possible to soften the wood, making it easier to carry out the compression process.

The compression time when compressing wood while heating can be adjusted depending on the type and size of the wood, the amount of organic acid impregnated into the wood, and the heating temperature, but can be, for example, 1 to 120 minutes.

In the manufacturing method of this embodiment, in the fourth step S4, the wood compressed in the third step S3 is heated without being compressed (heating step). Specifically, as shown in FIG. 1(d), the wood 1 is heated using a heating device 20. Specifically, the wood compressed in the third step S3 is not heated while being held in a compressed state using a fixing jig, but is heated as is without using a fixing jig for compression. As described above, in the compression step, the wood containing an organic acid is compressed while being heated, so that the compressed wood is fixed to a certain extent and springback is suppressed. Therefore, the compressed wood can be heat-treated without using a fixing jig.

The heating atmosphere when heat treating compressed wood can be air, steam or an inert atmosphere. 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. Superheated steam is a gas obtained by heating saturated steam, and therefore contains almost no oxygen. However, the heating atmosphere when heat treating compressed wood is preferably a steam atmosphere, and more preferably a saturated steam atmosphere. By carrying out the heat treatment in a steam atmosphere, the reaction between the wood and the organic acid is promoted, and fixation for suppressing springback can be achieved in a short period of time.

It is preferable that the temperature during the heat treatment of the compressed wood be a temperature at which the wood is fixed. Specifically, the heating temperature is preferably 130°C or higher and 250°C or lower, and more preferably 150°C or higher and 220°C or lower.

The heating time when heat treating compressed wood can be adjusted depending on the type and size of the wood, the amount of organic acid impregnated into the wood, and the heating temperature and heating atmosphere, so it is not particularly limited, but can be, for example, 1 minute to 24 hours. If the heating atmosphere is air, it is preferably 2 hours to 24 hours. If the heating atmosphere is a water vapor atmosphere, it is preferably 1 minute to 3 hours.

The compressed wood of this embodiment can be obtained by subjecting the wood to a heat treatment and then cooling it. The compressed wood obtained in this manner is able to maintain its high density and improved surface hardness for a long period of time because springback is suppressed even when the wood absorbs moisture and water.

The reason why the manufacturing method of this embodiment suppresses and stabilizes the springback of compressed wood is not necessarily clear, but the following mechanism is thought to be the cause. Note that the technical scope of this embodiment is not limited to embodiments in which the effect is achieved by such a mechanism.

First, springback caused by moisture and/or water absorption in compressed wood will be explained. Conventional compressed wood is obtained by applying an external force in the compressive direction to wood and drying it in a deformed state. During this process, hydrogen bonds are formed within and between the cell walls, making it difficult for the wood to return to its original thickness when the external force is removed. However, in this state, hydrogen bonds are held within and between the cell walls, so moisture and/or water absorption breaks the hydrogen bonds, causing springback, which causes the wood to return to its original thickness. In other words, when wood is compressed and deformed, the deformed cells still have a restoring force (residual stress) that tries to return to the original state, but this is suppressed by hydrogen bonds.

In response to this, as described in Non-Patent Document 2, it has been confirmed that heat treatment (high-temperature treatment in a dry state) or steam treatment has the effect of suppressing the recovery of compressed wood due to moisture absorption and/or water absorption (fixation). Although the mechanism of this phenomenon is not clear, it is speculated as follows. It is known that high-temperature treatment of wood causes a decomposition reaction of the amorphous regions of hemicellulose and cellulose. It is speculated that the above-mentioned heat treatment in a compressed state decomposes the components of wood, and the restoring force of the above-mentioned wood cells to return to their original state is reduced, thereby suppressing the recovery during moisture absorption and/or water absorption. In addition, as described in Non-Patent Document 3, it is known that steam treatment is effective in fixing wood in a shorter time than high-temperature heat treatment in a dry state. This is thought to be because the decomposition reaction of wood is more likely to proceed in the presence of water, and steam treatment makes it easier to form cross-links.

In this embodiment, it is presumed that the organic acid promotes decomposition by the heat treatment as described above, and further cross-links the wood, thereby shortening the time required for fixation of the compressed wood.
[Non-Patent Document 2] Inoue M, Norimoto M, Tanahashi M, Rowell RM, Steam or heat fixation of compressed wood, Wood and Fiber Science, 25(3), (1993) pp.224-235
[Non-Patent Document 3] Takashi Higashihara, "Deformation fixation of compressed wood by steam treatment and its mechanism," Forest Tree Breeding Center Research Report, Forestry Agency Forest Tree Breeding Center, 2007, No. 23, pp. 255-308

In this way, the method for producing compressed wood of this embodiment includes an impregnation step in which an organic acid aqueous solution containing an organic acid is pressurized and impregnated into a block-shaped piece of wood having a thickness of 3 mm or more. The method further includes a drying step in which the wood in a wet state impregnated with the organic acid aqueous solution is dried, a compression step in which the dried wood is compressed while being heated to 140°C or higher, and a heating step in which the compressed wood is heated without being compressed. In the method for producing compressed wood of this embodiment, the wood containing an organic acid is compressed and then heated, so that compressed wood capable of suppressing springback can be obtained by low-temperature and/or short-time treatment. In addition, in the method for producing compressed wood, the wood containing an organic acid is compressed while being heated in the compression step, so that the restoration of the compressed wood can be suppressed in the heating step. As a result, since there is no need to use a fixture to maintain the compressed state of the wood in the heating step, the heating step can be greatly simplified.

Furthermore, the compressed wood obtained in this embodiment has a high density and also has improved surface hardness. In addition, the organic acids remaining inside the compressed wood are often used as food additives and are highly safe. Therefore, the compressed wood can be suitably used for various applications such as various building materials (floors, walls, ceilings, fittings, etc.), furniture, and crafts. In addition, by adhering the compressed wood to a substrate, it can also be used as a surface material for various products.

[Second embodiment]
Next, a method for producing compressed wood material according to a second embodiment will be described in detail. Note that the description overlapping with the first embodiment will be omitted.

The manufacturing method of this embodiment includes an impregnation step in which wood is impregnated with an organic acid aqueous solution containing an organic acid and a sugar, and a drying step in which the wood impregnated with the organic acid aqueous solution is dried. The manufacturing method further includes a compression step in which the dried wood is compressed while being heated, and a heating step in which the compressed wood is heated without being compressed.

In the manufacturing method of the first embodiment, wood is impregnated with only organic acid, compressed while heated, and then heated without compression to obtain compressed wood with reduced springback. However, in this embodiment, wood is impregnated with organic acid and sugars and then heated, so compressed wood with increased hardness can be obtained while suppressing springback.

In the manufacturing method of this embodiment, in the first step S1, an organic acid aqueous solution containing an organic acid and a sugar is impregnated into a block of wood. The shape, thickness, and species of the wood can be the same as those described in the first embodiment. In addition, the type of organic acid and the content of the organic acid in the organic acid aqueous solution can also be the same as those described in the first embodiment.

The organic acid aqueous solution to be impregnated into wood can be prepared by dissolving an organic acid and a sugar in water. The sugar can 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, galtooligosaccharides, mannanoligosaccharides, and stachyose. Examples of polysaccharides include starch, agarose, alginic acid, glucomannan, inulin, chitin, chitosan, hyaluronic acid, glycogen, and cellulose.

Here, the sugar is preferably at least one selected from the group consisting of fructose, maltose, xylose, and sucrose. These sugars are easy to obtain and can further increase the strength of the compressed wood.

In the organic acid aqueous solution, the sugar content is preferably 3 to 30% by mass, more preferably 3 to 20% by mass, and even more preferably 3 to 10% by mass. By having the sugar content in the organic acid aqueous solution within this range, the sugars can easily penetrate the wood, thereby improving the strength of the compressed wood.

In the manufacturing method of this embodiment, in the second step S2, the wood in a wet state impregnated with the organic acid aqueous solution is dried to remove excess moisture from inside the wood. Furthermore, in the third step S3, the wood that has been subjected to the drying step is compressed while being heated. Then, in the fourth step S4, the compressed wood is heated without being compressed. These second to fourth steps can be carried out in the same manner as in the first embodiment.

The compressed wood obtained by the manufacturing method of this embodiment can maintain a high density and improved surface hardness for a long period of time because the organic acid suppresses springback even when the wood absorbs moisture and water. Furthermore, sugars fill the microvoids in the cell walls of the wood instead of water molecules, and can remain in the microvoids without evaporating even when drying. Since the sugars can maintain the cell walls in a swollen state, the so-called "bulk effect" can suppress the shrinkage of the wood during the drying process. In addition, the organic acid and sugars are thought to have the effect of penetrating between the cellulose fibers and imparting flexibility to the wood. Therefore, it is thought that the compressed wood of this embodiment can maintain its toughness and increase its mechanical strength compared to general compressed wood.

In this way, the method for producing compressed wood of this embodiment includes an impregnation step in which an organic acid aqueous solution containing an organic acid and sugars is pressurized and impregnated into a block-shaped wood having a thickness of 3 mm or more. The method further includes a drying step in which the wet wood impregnated with the organic acid aqueous solution is dried, a compression step in which the dried wood is compressed while being heated to 140°C or higher, and a heating step in which the compressed wood is heated without being compressed. As in the first embodiment, the method for producing compressed wood of this embodiment also involves compressing wood containing an organic acid while being heated, and then heating it without being compressed, so that compressed wood capable of suppressing springback can be obtained by a simple method. Furthermore, the obtained compressed wood has a high density and improved surface hardness, and furthermore, the mechanical strength is improved due to the effect of the sugars. Therefore, the compressed wood can be suitably used for various purposes such as building materials, fittings, furniture, crafts, and surface materials.

(Additional Note)
The above description of the embodiments discloses the following techniques.

(Technology 1) An impregnation step in which an organic acid aqueous solution containing an organic acid is pressurized and impregnated into a block-shaped wood having a thickness of 3 mm or more;
a drying step of drying the wood in a wet state impregnated with the organic acid aqueous solution;
A compression step of compressing the dried wood while heating it to 140°C or higher;
a heating step of heating the compressed wood without compressing it;
A method for producing compressed wood, comprising:

This configuration allows compressed wood with reduced springback to be obtained by low-temperature and/or short-time processing. Furthermore, the compressed wood obtained in this manner is able to maintain a high density and improved surface hardness for a long period of time, since springback is reduced even when the wood absorbs moisture and water. Therefore, the compressed wood can be used for a variety of purposes, such as building materials, fixtures, furniture, crafts, and surface materials.

(Technology 2) The method for producing compressed wood described in Technology 1, in which the heating step is a step of heating the wood in a water vapor atmosphere.

By carrying out the heat treatment in a water vapor atmosphere, the reaction between the wood and the organic acid is accelerated, allowing immobilization to be achieved in a short period of time to prevent springback.

(Technique 3) The method for producing compressed wood described in Technique 1 or 2, in which the drying process is a process for reducing the moisture content of the wood to 5-30%.

This configuration removes excess moisture from the wood, increasing the reactivity of the organic acid with the wood.

(Technology 4) The method for producing compressed wood described in any one of Technology 1 to 3, wherein the organic acid is at least one selected from the group consisting of citric acid, malic acid, and succinic acid.

This composition allows the organic acid to more easily react with the hemicellulose in the wood, making it possible to obtain compressed wood that suppresses springback by treating at a lower temperature and/or for a shorter period of time.

(Technology 5) The method for producing compressed wood described in any one of Technology 1 to 4, wherein the organic acid content in the organic acid aqueous solution is 3 to 30 mass %.

This configuration allows the organic acid to easily penetrate the wood, thus suppressing springback in the wood.

The present embodiment will be described in more detail below with reference to examples and comparative examples, but the present embodiment is not limited to these examples.

[Preparation of test samples]
Example 1
First, a plurality of pieces of cedar wood with a width of 30 mm, a thickness of 20 mm, and a length of 25 mm were prepared as wood. The grain of the wood was in the width direction, and the length direction of the wood was in the fiber direction. The following tests were performed using continuous test pieces.

Next, as shown in Table 1, an impregnation solution with a citric acid concentration of 5% by mass was prepared by mixing citric acid, which is an organic acid, with water.

Next, the impregnation liquid was placed in a pressure-resistant container, and multiple pieces of wood were immersed in it. Then, while the wood was immersed in the impregnation liquid, the atmospheric pressure was set to -0.09 MPa and held for 20 minutes, and then 0.8 MPa and held for 3 hours, to carry out a pressurized impregnation process. Furthermore, after carrying out the pressurized impregnation process, the wood was aged in an atmospheric pressure atmosphere for 12 hours or more while immersed in the impregnation liquid. In this way, by aging the wood while immersed in the impregnation liquid, it is possible to reliably impregnate the cell walls of the wood with organic acids.

Next, the impregnated wood was removed from the impregnation solution and then dried. The drying process was carried out using a steam drying device, with the drying conditions changed in the order of steps 1 to 7 shown in Table 2. The moisture content of the wood after the drying process was measured according to JIS Z2101 and was found to be approximately 8%.

The wood that had been dried was then compressed using a hand press equipped with hot plates that could control the temperature above and below. Specifically, the wood after the drying process was placed between the top and bottom hot plates that were heated to 180°C, 200°C, 220°C, or 240°C so that it was in contact with the wood, and the wood was preheated for three minutes. Next, spacers with a thickness of approximately 10 mm were placed on both sides of the wood between the hot plates, and the wood was then compressed. The wood was compressed until a sufficient load was applied to the spacers on both sides, and this was maintained for three minutes. The compressed wood was then removed from between the hot plates.

The compressed wood was then subjected to a heat treatment for 5 minutes in a saturated steam atmosphere at 150°C. During this process, the wood was heated without being compressed in a fixed jig. After the heat treatment was completed, the wood was cooled to body temperature, yielding compressed wood of Example 1, in which the compression temperatures in the compression process were 180°C, 200°C, 220°C, and 240°C, respectively.

Example 2
The compressed wood of Example 2 was obtained by the same process as in Example 1, except that the concentration of citric acid contained in the impregnation liquid was 10 mass %, and the compression temperatures in the compression process were 140°C, 160°C, 180°C, 200°C, 220°C, and 240°C, respectively.

Comparative Example
Comparative examples of compressed wood were obtained by the same process as in Example 1 except that water was used as the impregnating liquid, and the compression temperatures in the compression step were 140°C, 160°C, 180°C, 200°C, 220°C, and 240°C, respectively.

[evaluation]
The thickness change rate of the compressed wood of Examples 1 and 2 and the Comparative Example was measured. Specifically, the dimensions in the thickness direction of the compressed wood of Examples 1 and 2 and the Comparative Example were measured before heat treatment in a saturated steam atmosphere. Furthermore, the dimensions in the thickness direction of the compressed wood of each Example were measured after heat treatment in a saturated steam atmosphere.

The thickness change rate of each example of compressed wood in the thickness direction was calculated from the following formula 2. Table 3 shows the dimensions of each example of compressed wood in the thickness direction before heat treatment, the dimensions of each example of compressed wood in the thickness direction after heat treatment, and the thickness change rate. For reference, Table 3 also shows the dimensions of each example of compressed wood in the thickness direction before immersion in the impregnation solution.
[Equation 2]
Thickness change rate in thickness direction (%) = [(thickness dimension after heat treatment) - (thickness dimension before heat treatment)] / [thickness dimension before heat treatment]

Figure 2 shows the relationship between the compression temperature during the compression process and the thickness change rate of the test pieces in the compressed wood of Examples 1-2 and the Comparative Example. From Figure 2, it can be seen that the compressed wood of the Examples, which was impregnated with an organic acid, had a significantly lower thickness change rate than the compressed wood of the Comparative Example, which was impregnated with water only. In particular, when the compression temperature was 220°C or higher, it can be seen that the compressed wood of both Examples 1 and 2 had a significantly lower thickness change rate than the Comparative Example.

In contrast, the compressed wood of the comparative example has a thickness change rate of over 80% when the compression temperature is 160°C or lower, and even at a compression temperature of 220°C, the thickness change rate is over 60%. Because the compressed wood of the comparative example does not contain organic acids, when it is subjected to heat treatment in a water vapor atmosphere after the compression process, the moisture contained in the water vapor causes the compression deformation to recover in the initial heating stage. This is thought to be why the thickness change rate became so large.

In this way, it can be seen that impregnating wood with an organic acid can suppress the return to the compressed state during the heating process. In other words, it is not necessary to use a fixing jig to keep the wood in a compressed state during the heating process, and it can be heated without a jig. Therefore, compared to conventional manufacturing methods, it is possible to significantly simplify the manufacturing process by heating wood without using a fixing jig to maintain the compressed state.

In this example, the temperature of the steam heat treatment was 150°C, but it can be easily assumed that increasing the heat treatment temperature will promote the reaction between the wood and the organic acid, allowing the fixation process to be completed in a shorter time.

In this example, the compression process was performed at 140°C, 160°C, 180°C, 200°C, 220°C, and 240°C, with preheating and holding times after compression of 3 minutes each. However, if the heating time after preheating and pressing is short, the wood may not dry sufficiently and the various bonds may not be fully formed, causing recovery in the thickness direction when released. Therefore, it is necessary to optimize the pressing temperature and time, and the release temperature according to the dimensions of the wood and the moisture content to be adjusted. Also, it is believed that fixation is progressing during compression due to the impregnation with organic acid. Therefore, it is possible to design the fixation level by combining the heating process after release and the temperature and time of the compression process.

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

1 Wood

Claims (5)

an impregnation step of pressurizing and impregnating an organic acid aqueous solution containing an organic acid into a block-shaped wood having a thickness of 3 mm or more;
a drying step of drying the wood in a wet state impregnated with the organic acid aqueous solution;
A compression step of compressing the dried wood while heating it to 140°C or higher;
a heating step of heating the compressed wood without compressing it;
A method for producing compressed wood, comprising: The method for producing compressed wood according to claim 1, wherein the heating step is a step of heating the wood in a water vapor atmosphere. The method for producing compressed wood according to claim 1 or 2, wherein the drying process is a process for reducing the moisture content of the wood to 5 to 30%. The method for producing compressed wood according to claim 1 or 2, wherein the organic acid is at least one selected from the group consisting of citric acid, malic acid, and succinic acid. The method for producing compressed wood according to claim 1 or 2, wherein the organic acid content in the organic acid aqueous solution is 3 to 30 mass %.

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