WO2024070451A1

WO2024070451A1 - Method for producing compressed wood

Info

Publication number: WO2024070451A1
Application number: PCT/JP2023/031507
Authority: WO
Inventors: 健次森; 充伸枡野
Original assignee: パナソニックＩｐマネジメント株式会社
Priority date: 2022-09-29
Filing date: 2023-08-30
Publication date: 2024-04-04
Also published as: JP2024050254A

Abstract

A method for producing compressed wood, comprising an infiltration step in which an aqueous organic-acid solution containing an organic acid is pressurized and infiltrated into block-shaped wood having a thickness of 3 mm or larger, a drying step in which the wet wood impregnated with the aqueous organic-acid solution is dried, a compression step in which the dried wood is compressed, and a heating step in which the compressed wood is heated while being kept in the compressed state.

Description

Manufacturing method of compressed wood

This disclosure 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 investigated that increases 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 a 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, returning to its pre-compression state. Methods for reducing this recovery phenomenon, known as springback, have been investigated.

Non-Patent Document 1 discloses a technology 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.

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.

This disclosure has been made in light of the problems inherent in the prior art. The purpose of this disclosure is to provide a method for manufacturing compressed wood that is capable of suppressing springback in compressed wood using a simple method.

In order to solve the above problems, the method for producing compressed wood according to the disclosed 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, 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, and a heating step in which the compressed wood is heated while maintaining the compressed state.

FIG. 1 is an explanatory diagram showing an example of a method for producing compressed wood according to the present embodiment. FIG. 2 is a graph showing the relationship between the time of steam heating treatment and the rate of change in thickness of the compressed wood pieces of the Examples, Reference 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 proportions in the drawings have been exaggerated for the sake of explanation and may differ from the actual proportions.

[First embodiment]
The method for producing compressed wood according to the present embodiment includes an impregnation step of impregnating a block of 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, and a heating step of heating the wood while maintaining it in a compressed state.

(a) in Figure 1 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.

Wood can be any of a variety of wood species used for building materials such as floors, walls, and ceilings, fixtures, furniture, and crafts. There are no particular limitations on the wood species, and not only conifers but also broadleaf trees 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 wood species have a luxurious feel and are highly decorative, so by modifying these wood species, they can be suitably used for building materials, fixtures, furniture, and crafts.

Furthermore, as for the wood, fast-growing trees that grow to large diameters in a short period of time, mainly found in Japan and Southeast Asia, etc., can 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 dried 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. 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 KD wood is preferably 7-25%. 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 heat treating the wood impregnated with the organic acid aqueous solution while it is in a compressed state, 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 heat-treated while in a compressed state, the compositional components of the wood are more likely to change, which makes it 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 and then heating it in a compressed state, 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 are no particular limitations on 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 to stand. 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.

In order to speed up the impregnation of 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 inside the wood, allowing the organic acid solution to be quickly impregnated into the wood.

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 now 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. Other drying devices that may be used include a dehumidifying drying device equipped with a heat pump dehumidifier, and 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 dried wood may also be 20% or less, or 15% or less. The lower limit of the moisture content of 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, which helps prevent the wood from shrinking when it dries and improves its 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 prevented.

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 in a jig. Specifically, as shown in (b) and (c) of FIG. 1, for example, the wood 1 can be compressed by clamping the wood 1 between a pair of metal plates 10 and then applying pressure to the metal plates 10 from above and below. 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.

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]

The wood can be compressed in the atmosphere. The temperature at which the wood is compressed is not particularly limited. The wood may be compressed at room temperature, or the wood may be compressed while being heated. Specifically, the wood 1 may be compressed while the metal plate 10, which serves as the jig, is heated. The heating temperature at which the wood is compressed while being heated is also not particularly limited, and can be, for example, 100°C or higher and 250°C or lower. Compressing the wood while heating it can soften the wood, making the compression process easier.

In the manufacturing method of this embodiment, in the fourth step S4, the wood compressed in the third step S3 is heated while maintaining the compressed state (heating step). Specifically, the compressed wood 1 is sandwiched between a pair of flat plates (metal plates 10) that serve as a fixing jig, and then the pair of flat plates are connected to each other using a connecting device, thereby maintaining the wood 1 in a compressed state. At this time, the compression rate of the wood can be 30% or more and 70% or less, as described above. Note that the connecting device is not particularly limited, and for example, a bolt and nut can be used. Then, as shown in FIG. 1(d), the wood 1 is heated together with the fixing jig using a heating device 20.

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

The temperature at which the compressed wood is heated is preferably set to a temperature at which the wood is fixed. Specifically, the heating temperature is preferably set to 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.

Then, the wood is heated while still in a compressed state, and then removed from the fixture to obtain the compressed wood of this embodiment. The compressed wood obtained in this manner is able to maintain its high density and improved surface hardness for a long period of time, since springback is suppressed even when the wood absorbs moisture and water.

As described above, the heating atmosphere when heat-treating the compressed wood can be air, steam, or an inert atmosphere. When heat-treating in a steam atmosphere, the wood 1 is heated while clamped by a fixture to prevent the compressed wood 1 from absorbing moisture during heat-treating and springing back. However, when heat-treating in a dry state such as air or a nitrogen gas atmosphere, the wood 1 absorbs less moisture during heat-treating and is less likely to spring back. Therefore, when heat-treating in a dry state, the wood 1 may be heated while clamped by a fixture, or may be heated without clamping the wood 1 by a fixture. When heat-treating in a superheated steam atmosphere, condensed water may adhere to the wood 1 at the beginning of the heat-treating, so it is preferable to heat the wood 1 while clamping it by a fixture. However, if the heating device is preheated to a state where condensed water is unlikely to be generated, the wood 1 may be heat-treated without being clamped by a fixture.

The reason why the manufacturing method of this embodiment suppresses and stabilizes the springback of compressed wood is not entirely 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, we will explain springback caused by moisture and/or water absorption in compressed wood. 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 their 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). The mechanism of this phenomenon is not clear, but 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, reducing the restoring force of the above-mentioned wood cells to return to their original state, 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, and a heating step in which the compressed wood is heated while maintained in a compressed state. In the method for producing compressed wood of this embodiment, the wood containing an organic acid is heated while still in a compressed state, so that compressed wood capable of suppressing springback can be obtained by low-temperature and/or short-time treatment. Furthermore, the obtained compressed wood has high density and also has improved surface hardness. In addition, the organic acid remaining inside the compressed wood is often used as a food additive, and is 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, the compressed wood can be used as a surface material for various products by adhering it to a substrate.

[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 an organic acid aqueous solution containing an organic acid and a sugar is impregnated into a block of wood, 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, and a heating step in which the wood is heated while being kept in a compressed state.

In the manufacturing method of the first embodiment, compressed wood with reduced springback is obtained by impregnating the wood with only organic acid and then heating it in a compressed state. However, in this embodiment, compressed wood with reduced springback and increased hardness is obtained because the wood is impregnated with organic acid and sugars and then heated in a compressed state.

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 wet wood impregnated with the organic acid aqueous solution is dried to remove excess moisture from inside the wood. Furthermore, in the third step S3, the dried wood is compressed. Then, in the fourth step S4, the compressed wood is heated while kept in the compressed state. These second to fourth steps can be carried out in the same way as in the first embodiment.

The compressed wood obtained by the manufacturing method of this embodiment is able to maintain a high density and improved surface hardness for a long period of time because the organic acid prevents 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 keep the cell walls in a swollen state, the so-called "bulk effect" can prevent the wood from shrinking during the drying process. It is also believed that the organic acid and sugars penetrate between the cellulose fibers and give the wood flexibility. Therefore, it is believed 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 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, and a heating step in which the compressed wood is heated while maintained in a compressed state. As in the first embodiment, the method for producing compressed wood of this embodiment also heats wood containing an organic acid while it is in a compressed state, so that compressed wood capable of suppressing springback can be obtained by low-temperature and/or short-time treatment. 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 Notes]
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 compressing step of compressing the dried wood;
A heating step of heating the compressed wood while maintaining the compressed state;
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, 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. This compressed wood can therefore be used for a variety of purposes, including 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 compressed state 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 for fixation to be achieved in a short period of time in order 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 easily react with the hemicellulose in the wood, making it possible to obtain compressed wood that can suppress springback by treating it at a lower temperature and/or for a shorter period of time.

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

This composition allows the organic acid to easily penetrate the wood, which helps to suppress springback in the wood.

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

[Preparation of test samples]
(Example 1 and Reference 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.

Then, 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 at -0.09 MPa and held for 20 minutes, and then at 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 for at least 12 hours under atmospheric pressure 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 liquid 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 in accordance with JIS Z2101 and was found to be approximately 8%.

Then, the wood that had been dried was 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 first placed between the top and bottom hot plates that were heated to 200°C so that they were in contact with each other, and the wood was then 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 then held for three minutes.

Next, a pair of stainless steel metal plates with holes drilled in the four corners were prepared as fixing jigs. The compressed wood was then sandwiched between the metal plates, and the four corners of the pair of metal plates were then fixed using stainless steel screws and nuts. In this way, the wood was held in a compressed state by being sandwiched between the pair of metal plates.

Then, the wood sandwiched between the metal plates was subjected to a heat treatment in a saturated water vapor atmosphere at 150°C. The heat treatment times were 0, 5, 15, 30, 60, and 120 minutes. After the heat treatment, the wood was cooled to body temperature and the fixture was removed to obtain compressed wood of Example 1, which had been heated for 5, 15, 30, 60, and 120 minutes, and compressed wood of Reference Example 1, which had been heated for 0 minutes.

(Example 2 and Reference Example 2)
The same process as in Example 1 and Reference Example 1 was used except that the concentration of citric acid contained in the impregnation liquid was 10 mass %. Compressed wood of Example 2 was obtained with heat treatment times of 5 minutes, 15 minutes, 30 minutes, 60 minutes, and 120 minutes, respectively, and compressed wood of Reference Example 2 with a heat treatment time of 0 minutes.

Comparative Example
Comparative examples of compressed wood were obtained by the same process as in Example 1 and Reference Example 1, except that water was used as the impregnation liquid, and the heat treatment times were 0, 5, 15, 30, 60, and 120 minutes, respectively.

[evaluation]
A hot water immersion test was carried out on the compressed wood of Examples 1-2, Reference Examples 1-2, and Comparative Example. Specifically, first, the dimensions in the thickness direction of the compressed wood of Examples 1-2, Reference Examples 1-2, and Comparative Example were measured. Next, the compressed wood of each example was immersed in boiling water for one hour. Then, the dimensions in the thickness direction of the compressed wood of each example after immersion were measured.

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

Figure 2 shows the relationship between the time of steam heating treatment and the thickness change rate of the test pieces for the compressed wood of the Examples, Reference Examples, and Comparative Examples. From Figure 2, it can be seen that the compressed wood of the Examples, which was impregnated with organic acid, had a significantly lower thickness change rate compared to the compressed wood of the Comparative Example, which was impregnated with water only. In particular, it can be seen that the thickness change rate is significantly reduced by impregnating with organic acid, even if the heating treatment time is as short as 30 minutes or less. This shows that by impregnating wood with organic acid, it is possible to carry out a fixation treatment to suppress springback in a shorter time than with conventional manufacturing methods.

In this example, the heat treatment temperature was 150°C, but it is easy to imagine that by increasing the heat treatment temperature, the reaction between the wood and the organic acid will be accelerated, allowing the fixation process to be completed in a shorter time.

In this example, the compression process was carried out at 200°C, and the preheating and holding times after compression were each 3 minutes. However, if the heating times after preheating and pressing are short, the wood will not dry sufficiently and various bonds will not be sufficiently formed, which may result in recovery in the thickness direction when released. For this reason, 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 even during compression due to the impregnation with organic acid. For this reason, it is possible to design the fixation level by combining the heating process after release and the temperature and time of the compression process.

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

The entire contents of Patent Application No. 2022-157002 (filing date: September 29, 2022) are hereby incorporated by reference.

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

1 Wood

Claims

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 compressing step of compressing the dried wood;
and a heating step of heating the compressed wood while maintaining the compressed state.
The method for producing compressed wood according to claim 1, wherein the heating step is a step of heating the wood in a compressed state in a water vapor atmosphere.
The method for manufacturing 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 any one of claims 1 to 3, 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 any one of claims 1 to 4, wherein the organic acid content in the organic acid aqueous solution is 3 to 30 mass %.