EP4596199A1

EP4596199A1 - Production method of compressed wood

Info

Publication number: EP4596199A1
Application number: EP23871692.2A
Authority: EP
Inventors: Kenji Mori; Michinobu MASUNO
Original assignee: Panasonic Intellectual Property Management Co Ltd
Current assignee: Panasonic Housing Solutions Co Ltd
Priority date: 2022-09-29
Filing date: 2023-08-30
Publication date: 2025-08-06
Also published as: JP2024050277A; EP4596199A4; WO2024070452A1

Abstract

Provided is a method for producing compressed wood, including: an impregnation step of pressure-impregnating a block of wood having a thickness of 3 mm or more with an organic acid aqueous solution containing an organic acid; a drying step of drying the wood, in a wet state, which has been impregnated with the organic acid aqueous solution; a compression step of compressing the wood, which has been dried, while heating to 140 °C or more; and a heating step of heating the wood, which has been compressed, without compression.

Description

TECHNICAL FIELD

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

BACKGROUND ART

Conifers, such as Japanese cedar and cypress, are used as furniture materials and building materials because they grow quickly and are readily available. However, conifers are softer and have lower strength than broad-leaved trees. Thus, a Technique has been studied to increase the specific gravity of soft wood through compressive deformation so as to mainly improve surface hardness. This Technique has attracted attention as it extends the range of use of wood having low specific gravity and low surface hardness, particularly Japanese cedar wood.
In contrast, it is known that the compression of compressed wood obtained through compressive deformation may decompress due to moisture absorption or water absorption, and returns to the state before compression. A Technique has been studied to reduce such a decompression phenomenon called springback.
Non-Patent Literature 1 discloses a Technique to permanently fix the compressive deformation by performing heat treatment on Japanese cedar wood. Specifically, Non-Patent Literature 1 discloses the following. First, water was injected into Japanese cedar sapwood under reduced pressure, and it was heated with hot water at 95 °C. Then, it was compressed using a press with a hot plate temperature set at 105 °C, and dried on a hot plate for 3 hours. Then, the obtained compression test piece was kept dry, and subjected to heat treatment using a hot air dryer. By performing the heat treatment at 180 °C for 20 hours, 200 °C for 5 hours, or 220 °C for 3 hours, the springback of the compressed wood decreased, accordingly, and the compression shape almost completely fixed.

CITATION LIST

PATENT LITERATURE

Non-Patent Literature 1: Masafumi Inoue and Misato Norimoto, "Permanent fixation of compressive deformation in wood by heat treatment", Wood Research and Materials, Kyoto University Wood Research Institute, 1991, No. 27, p. 31-40

SUMMARY OF INVENTION

However, in the method of Non-Patent Literature 1, it is necessary to perform heat treatment at a high temperature for a long period of time after the wood is dried while being compressed. Thus, in order to reduce the production cost of compressed wood, a method to control the springback of compressed wood through low temperature and/or short time treatment has been desired.
The present disclosure has been made in consideration of the above issues, which are inherent in the related art. An object of the present disclosure is to provide a compressed wood production method capable of controlling springback of compressed wood using a simple method.
In response to the above issues, a method for producing compressed wood according to an embodiment of the present disclosure includes: an impregnation step of pressure-impregnating a block of wood having a thickness of 3 mm or more with an organic acid aqueous solution containing an organic acid; a drying step of drying the wood, in a wet state, which has been impregnated with the organic acid aqueous solution; a compression step of compressing the wood, which has been dried, while heating to 140 °C or more; and a heating step of heating the wood, which has been compressed, without compression.

BRIEF DESCRIPTION OF DRAWINGS

[Fig. 1] Fig. 1 is an explanatory diagram illustrating an example of a method for producing compressed wood according to the present embodiment.
[Fig. 2] Fig. 2 is a graph illustrating the relationship between compression temperature in a compression step, and a percentage change in thickness of compressed wood before and after a heat treatment in a steam atmosphere, regarding each compressed wood of examples and a comparative example.

DESCRIPTION OF EMBODIMENTS

Referring to the drawings, a description is given below of a method for producing compressed wood according to the present embodiment. Note that dimensional ratios in the drawings are exaggerated for convenience of the description and are sometimes different from actual ratios.

[First embodiment]

A 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 production method further includes a compression step of compressing the dried wood while heating, and a heating step of heating the compressed wood without compression.
Fig. 1(a) illustrates a flow of the method for producing the compressed wood according to the present embodiment. In the method for producing the compressed wood according to the present embodiment, a first step S1 is to impregnate a block of wood with an organic acid aqueous solution containing an organic acid (impregnation step). It is sufficient that the wood have a block shape, and an example is wood processed into a plate shape.
The thickness of wood in a plate shape is preferably 3 mm or more. By treating wood having such a thickness, as described below, compressed wood having a thickness of a few millimeters can be obtained. Compressed wood having a thickness of a few millimeters can be preferably used, for example, as a surface material attached to the surface of plywood. Note that the thickness of wood in a plate shape is preferably 3 mm or more, and may be 10 mm or more, or 12 mm or more. The upper limit of the thickness of wood in a plate shape is not particularly limited, and can be 40 mm, for example.
Examples of wood include wood of various tree species used for building materials, such as floors, walls, and ceilings, fixture members, furniture, crafts, and the like. The tree species of wood is not particularly limited, and not only conifers but also broad-leaved trees can be used. Specifically, as the wood, at least one selected from the group consisting of Japanese cedar, Japanese larch, Douglas fir, rubber tree, birch, beech, oak, beech, oak, teak, hard maple, cherry, walnut, white ash, mahogany, and yellow birch can be used. The wood described above has a high-grade appearance and high design characteristics, and thus by modifying them, they can be suitably used for building materials, fixture members, furniture, and crafts.
In addition, fast-growing trees that grow to large-diameter trees in a short period of time mainly in Japan and Southeast Asia can also be used as wood. Specifically, as the wood, at least one selected from the group consisting of chinaberry, Nepali hog-plum, Japanese alder, tulip tree, eucalyptus, poplar, acacia mangium, and falcata can be used.
Note that the wood may be in a raw state having a high moisture content, or in a dry state having a low moisture content. Even when the moisture content of wood is high, water in vessels can be replaced with an organic acid aqueous solution, and thus the inside of the wood can be impregnated with an organic acid aqueous solution. Note that artificially dried wood (KD wood) that is artificially dried in a drying kiln or the like to reduce the moisture content may be used as wood. In this case, the moisture content of the KD wood is preferably 7% to 25%. The moisture content of wood can be measured in accordance with the Japanese Industrial Standard JIS Z2101 (Methods of test for woods).
An organic acid aqueous solution impregnated into the above wood can be prepared by dissolving an organic acid in water. As the organic acid, as will be described below, an organic compound can be used that is capable of controlling springback by compressing the wood impregnated with the organic acid aqueous solution while heating. Specifically, the organic acid is preferably at least one selected from the group consisting of a carboxylic acid, a sulfonic acid, and a sulfinic acid.
Note that the organic acid is preferably a carboxylic acid, and more preferably a divalent or more carboxylic acid. When wood impregnated with a carboxylic acid is heated, components of wood tend to change in quality, and this makes it possible to further promote the modification of wood.
The carboxylic acid is preferably at least one selected from the group consisting of a citric acid, a tartaric acid, a malic acid, a succinic acid, an oxalic acid, an adipic acid, a malonic acid, a phthalic acid, a sebacic acid, a maleic acid, a fumaric acid, an itaconic acid, a glutaric acid (1,5-pentanedioic acid), a gluconic acid, a glutaconic acid, and a pentenedioic acid. The carboxylic acid is more preferably at least one selected from the group consisting of a citric acid, a malic acid, and a succinic acid. By using a citric acid, a malic acid, or a succinic acid, compressed wood can be obtained through low temperature and/or short time treatment. Moreover, these carboxylic acids can be obtained from naturally derived materials, and it becomes possible to reduce environmental load.
In an organic acid aqueous solution, the content of the organic acid is preferably 3% to 30% by mass, more preferably 3% to 20% by mass, and even more preferably 3% to 10% by mass. Since the content of the organic acid in the organic acid aqueous solution is within these ranges, the organic acid can easily penetrate into the wood, and the effect of fixing the compressed wood due to the organic acid can be obtained.
As will be described in a second embodiment, the wood may be impregnated with both an organic acid and a saccharide by adding the saccharide to the organic acid aqueous solution. However, by impregnating the wood with only the organic acid, compressing it while heating, and further heating, the compressed wood can be fixed and springback can be sufficiently controlled. Thus, the organic acid aqueous solution may contain at least an organic acid, and need not contain a saccharide.
Since organic acids are highly soluble in water, organic acid aqueous solutions need not contain an organic solvent. Moreover, when an organic acid aqueous solution does not contain an organic solvent, the environmental load can be reduced and the safety to the human body can be enhanced.
The method for impregnating a block of wood with an organic acid aqueous solution is not particularly limited. For example, wood can be impregnated with an organic acid aqueous solution by immersing it in the organic acid aqueous solution and leaving it to stand. Note that to speed up the impregnation of wood with an organic acid aqueous solution, it is preferable to put wood into a pressure-resistant container filled with an organic acid aqueous solution and to pressurize it. In this case, the pressure for pressurizing is not particularly limited and is preferably set to 0.3 to 10.0 MPa, for example.
When wood is impregnated with an organic acid aqueous solution, the temperature of the organic acid aqueous solution is not particularly limited and is preferably set to 80 °C or less, for example. The temperature of the organic acid aqueous solution can also be room temperature.
In order to speed up the impregnation of an organic acid aqueous solution into wood, the wood may be immersed in the organic acid aqueous solution after the air inside the wood is removed by decompressing the wood in a pressure-resistant container. This makes it easier for the organic acid aqueous solution to penetrate into the inside of vessels in the wood, and thus the wood can be impregnated quickly with the organic acid aqueous solution.
When impregnating a block of wood with an organic acid aqueous solution, it is preferable that the organic acid aqueous solution impregnate the whole wood, that is, to the center of the wood. In this way, it is possible for the wood to be modified to the center of the wood by the action of the organic acid. However, it is not always necessary to impregnate the center of the wood with an organic acid aqueous solution, and it is sufficient that at least the part of the wood to be modified be impregnated with the organic acid aqueous solution.
In the production method according to the present embodiment, as a second step S2, wood in a wet state, impregnated with the organic acid aqueous solution, is dried to remove excess water inside the wood (drying step). The drying condition is not particularly limited, but it can be natural drying, for example. The wood may 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. Moreover, the drying atmosphere is not particularly limited and, for example, drying may be performed in the air. In addition, moisture inside the wood may be removed by gradually reducing the humidity in the drying atmosphere.
The drying step will be described in more detail below. The drying step may be natural drying as described above, or the wood may be subj ected to drying treatment using a drying apparatus. An example of the drying apparatus is a steam type drying apparatus controlled in such a manner that the humidity (relative humidity) in the drying apparatus is gradually decreased while the temperature in the drying apparatus is gradually increased by supplying steam to a heating tube in the drying apparatus. The drying apparatus may be a dehumidifying drying apparatus equipped with a heat pump type dehumidifier, or a decompression drying apparatus that performs drying through decompression and heating. Hot air or a radiation type heater may be used for drying.
The moisture content of the wood dried in the drying step 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%. Note that the moisture content of wood can be measured in accordance with JIS Z2101.
Here, since organic acids stay in minute spaces inside the wood, shrinkage of the wood during drying can be controlled and dimensional stability can be enhanced. Thus, even when the wood impregnated with the organic acid aqueous solution is dried as described above, deformation and cracking of the wood can be controlled.
In the production method according to the present embodiment, as a third step S3, the dried wood is compressed (compression step). The compression method is not particularly limited, and the wood may be compressed after holding the wood with a jig, for example. Specifically, as illustrated in Figs. 1(b) and 1(c), wood 1 can be compressed by holding the wood 1 with a pair of metal plates 10, for example, and then by pressing the metal plates 10 from above and below. Note that a compression apparatus for compressing wood is not limited to a press device using flat plates, but a roll press device for continuously compressing wood by passing the wood between a pair of rolls can also be used. The compression of wood can be performed in the air.
When the dried wood is compressed, the compression percentage is not particularly limited, but can be 30% or more, or 50% or more. The upper limit of the compression percentage is not particularly limited, but can be 70%, for example. Note that in this specification, the compression percentage is a value calculated using equation 1. Compression percentage (%) = [(thickness T1 of wood before compression) - (thickness T2 of wood after compression)]/[thickness T1 of wood before compression]
Here, in the present embodiment, when wood is compressed, it is necessary to compress the wood while heating. Specifically, the wood 1 is compressed while heating the metal plates 10, which is a jig. The heating temperature of the wood during compression while heating is preferably 140 °C or more, more preferably 160 °C or more, even more preferably 180 °C or more, particularly preferably 200 °C or more, and most preferably 220 °C or more. Note that the heating temperature of the wood during compression while heating can be 250 °C or less. Thus, by compressing the wood containing the organic acid while heating, restoration of the compressed wood can be controlled when performing the next heating step. In addition, since the wood can be softened by compressing the wood while heating, the compression step can be performed easily.
Note that the compression time during compression while heating the wood can be adjusted depending on the type and size of the wood, the amount of the organic acid impregnated into the wood, and the heating temperature, and it can be set to 1 to 120 minutes, for example.
In the production method according to the present embodiment, as a fourth step S4, the wood compressed in the third step S3 is heated without compression (heating step). Specifically, as illustrated 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 held and heated in the compressed state using a fixing jig. Instead, the wood is heated as is without using a fixing jig for compression. As described above, in the compression step, the wood containing the organic acid is compressed while being heated, and thus the wood in the compressed state is fixed to some extent so that the springback is controlled. Therefore, heat treatment can be performed on the wood in the compressed state without using a fixing jig.
The heating atmosphere for the heat treatment on the wood in the compressed state can be under air, a steam atmosphere, or an inert atmosphere. The inert atmosphere is an atmosphere with reduced oxygen concentration, for example, a nitrogen gas atmosphere, or a superheated steam atmosphere. Since the superheated steam is a gas obtained by heating a saturated steam, it contains little oxygen. However, the heating atmosphere for the heat treatment on the wood in the compressed state is preferably a steam atmosphere, and more preferably a saturated steam atmosphere. By performing the heat treatment in a steam atmosphere, the reaction between the wood and the organic acid is promoted, and the fixation to control the springback can be performed in a short time.
The temperature for performing the heat treatment on the wood in the compressed state is preferably a temperature where the wood is fixed. Specifically, the heating temperature is preferably 130 °C or more and 250 °C or less, and more preferably 150 °C or more and 220 °C or less.
The heating time for performing the heat treatment on the wood in the compressed state is not particularly limited because it can be adjusted depending on the type and size of the wood, the amount of the organic acid impregnated into the wood, and the heating temperature and the heating atmosphere, but it can be set to, for example, 1 minute to 24 hours. Note that when the heating atmosphere is under air, the heating time is preferably from 2 to 24 hours. When the heating atmosphere is a steam atmosphere, the heating time is preferably from 1 minute to 3 hours.
Then, by cooling the wood after the heat treatment, the compressed wood of the present embodiment can be obtained. The compressed wood thus obtained can maintain a state of high density and improved surface hardness for a long period of time because the springback is controlled even when moisture and water are absorbed.
The reason why the springback of compressed wood is controlled, and the compressed wood is fixed, by using the production method according to the present embodiment, is not necessarily clear, but the following mechanism can be considered. Note that the technical scope of the present embodiment is not limited to the embodiment in which the effect is manifested using the mechanism.
First, the springback of compressed wood due to moisture absorption and/or water absorption will be described. Conventional compressed wood is obtained by drying wood in a deformed state by applying an external force in the direction of compression. At this time, hydrogen bonds are formed in and between cell walls, and thus it is difficult to return to the original thickness when the external force is unloaded. In contrast, since hydrogen bonds are held in and between cell walls due to hydrogen bonds in this state, hydrogen bonds are broken due to moisture absorption and/or water absorption, and springback occurs in which wood returns to the original thickness. In other words, when wood is compressed and deformed, it is thought that deformed cells have a restoring force (residual stress) to return to their original state, but this is controlled by hydrogen bonds.
In contrast, as described in Non-Patent Literature 2, it has been confirmed that performing heat treatment (high-temperature treatment in the dry state) or steam treatment has an effect to control the decompression of compressed wood due to moisture absorption and/or water absorption (fixation). The mechanism of this phenomenon is unclear, but it is assumed as follows. It is known that a decomposition reaction of non-crystalline regions of hemicellulose and cellulose occurs in wood through high-temperature treatment. It is assumed that by performing the above heat treatment in the compressed state, components of wood are decomposed, and the restoring force of the above-described wood cells is reduced, and the restoration during moisture absorption and/or water absorption is controlled. As described in Non-Patent Literature 3, in wood fixation, steam treatment is known to be more effective in a shorter time than high-temperature heat treatment in a dry state. This is thought because the decomposition reaction of wood is proceed more readily in the presence of water, and in addition, the steam treatment facilitates formation of crosslinking bonds.
In the present embodiment, it is assumed that the organic acid promotes the decomposition through the above heat treatment, and furthermore, the organic acid crosslinks in the wood, so that the fixation of compressed wood is shortened.

[Non-Patent Literature 2] Inoue M, Norimoto M, Tanahashi M, Rowell R.M., Steam or heat fixation of compressed wood, Wood and Fiber Science, 25(3), (1993) pp. 224-235
[Non-Patent Literature 3] Takashi Higashihara, "Permanent Fixation of Transversely Compressed Wood by Steaming and its Mechanism" Research Report of Forest Tree Breeding Center, Forestry Agency, Forest Tree Breeding Center, No. 23, 2007, p. 255-308

As described above, the method for producing the compressed wood according to the present embodiment includes an impregnation step of pressure-impregnating a block of wood having a thickness of 3 mm or more, with an organic acid aqueous solution containing an organic acid. The method further includes a drying step of drying the wood, in a wet state, which has been impregnated with the organic acid aqueous solution, a compression step of compressing the wood, which has been dried, while heating to 140 °C or more, and a heating step of heating the wood, which has been compressed, without compression. In the production method according to the present embodiment, since the wood containing the organic acid is compressed and then heated, it is possible to obtain compressed wood where control of springback is possible through low temperature and/or short time treatment. In addition, in the production method, since the wood containing the organic acid is compressed while being heated in the compression step, restoration of the compressed wood can be controlled in the heating step. Consequently, the heating step can be greatly simplified because it is not necessary to use a fixing jig for maintaining the compressed state of the wood in the heating step.
Furthermore, the compressed wood obtained in the present embodiment is densified and further has an improved surface hardness. In addition, the organic acid remaining in the compressed wood is often used as food additives and has high safety. Thus, the compressed wood can be suitably used for various applications, such as various building materials (floors, walls, ceilings, decorative members, etc.), furniture, and crafts. The compressed wood can also be used as a surface material for various products by bonding the compressed wood to a substrate.

[Second embodiment]

Next, a method for producing compressed wood according to a second embodiment will be described in detail. Note that descriptions that overlap with those in the first embodiment will be omitted.
The production method according to the present embodiment includes an impregnation step of impregnating wood with an organic acid aqueous solution containing an organic acid and a saccharide, and a drying step of drying the wood impregnated with the organic acid aqueous solution. The production method further includes a compression step of compressing the wood, which has been dried, while heating, and a heating step of heating the wood, which has been compressed, without compression.
In the production method according to the first embodiment, the wood is impregnated with only the organic acid, then compressed while being heated, and then heated without compression to obtain compressed wood with controlled springback. However, in the present embodiment, since the wood is impregnated with an organic acid and a saccharide, and then heated, it is possible to obtain compressed wood with increased hardness and controlled springback.
In the production method according to the present embodiment, in the first step S1, an organic acid aqueous solution containing an organic acid and a saccharide is impregnated into a block of wood. The shape, thickness, and tree species of the wood can be the same as those described in the first embodiment. The type of the 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 the wood can be prepared by dissolving the organic acid and the saccharide in water. As the saccharide, at least one selected from the group consisting of a monosaccharide, a disaccharide, an oligosaccharide, and a polysaccharide can be used. Examples of the monosaccharide include fructose, xylose, ribose, arabinose, rhamnose, xylulose, and deoxyribose. Examples of the disaccharide include sucrose, maltose, trehalose, turanose, lactulose, maltulose, palatinose, gentiobiulose, melibiulose, galactosucrose, rutinulose, and planteobiose. Examples of the oligosaccharide include a fructooligosaccharide, a galactooligosaccharide, a mannan oligosaccharide, and stachyose. Examples of the polysaccharide include starch, agarose, an alginic acid, glucomannan, inulin, chitin, chitosan, a hyaluronic acid, glycogen, and cellulose.
Here, the saccharide is preferably at least one selected from the group consisting of fructose, maltose, xylose, and sucrose. These saccharides are readily available and in addition, make it possible to enhance the strength of the compressed wood.
In the organic acid aqueous solution, the content of the saccharide 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 content of the saccharide in the organic acid aqueous solution is within these ranges, the saccharide easily permeates into the wood, and thus the strength improvement effect of the compressed wood can be obtained.
In the production method according to the present 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 inside the wood. Furthermore, in the third step S3, the wood subjected to the drying step is compressed while being heated. In the fourth step S4, the compressed wood is heated without compression. These second to fourth steps can be performed in the same manner as in the first embodiment.
The compressed wood obtained using the production method according to the present embodiment can maintain a state of high density and improved surface hardness for a long period of time because springback is controlled even in the case of moisture and water absorption, due to the effect of the organic acid. In addition, the saccharide can fill in micropores in the cell walls of the wood instead of water molecules, and remains in the micropores without evaporating even during drying. Since a saccharide can keep the cell walls in a swollen state, what is known as "bulk effect" can control shrinkage of the wood in the drying step. In addition, the organic acid and a saccharide penetrate between fibers of cellulose and also have the effect of imparting flexibility to the wood. Therefore, it is though that the compressed wood of the present embodiment can maintain resilience and increase mechanical strength, compared with general compressed wood.
As described above, the method for producing the compressed wood according to the present embodiment includes an impregnation step of pressure-impregnating a block of wood having a thickness of 3 mm or more with an organic acid aqueous solution containing an organic acid and a saccharide. The production method further includes a drying step of drying the wood, in a wet state, which has been impregnated with the organic acid aqueous solution, a compression step of compressing the wood, which has been dried, while heating to 140 °C or more, and a heating step of heating the wood, which has been compressed, without compression. In the production method according to the present embodiment, as in the first embodiment, the wood containing the organic acid is compressed while being heated, and then heated without compression, and thus compressed wood where control of springback is possible can be obtained using a simple method. In addition, the obtained compressed wood has high density and improved surface hardness, and further, the mechanical strength is also improved by the effect of the saccharide. Therefore, the compressed wood can be suitably used for various applications, such as building materials, decorative members, furniture, crafts, and surface materials.

(Note)

With the above description of the embodiments, the following Techniques are disclosed.

(Technique 1) A method for producing compressed wood, comprising:
- an impregnation step of pressure-impregnating a block of wood having a thickness of 3 mm or more with an organic acid aqueous solution containing an organic acid;
- a drying step of drying the wood, in a wet state, which has been impregnated with the organic acid aqueous solution;
- a compression step of compressing the wood, which has been dried, while heating to 140 °C or more; and
- a heating step of heating the wood, which has been compressed, without compression.
With this structure, compressed wood with controlled springback can be obtained through low temperature and/or short time treatment. The compressed wood thus obtained can maintain a state of high density and improved surface hardness over a long period of time because springback is controlled even when moisture and water are absorbed. Therefore, the compressed wood can be suitably used for various applications, such as building materials, decorative members, furniture, crafts, and surface materials.
(Technique 2) The method for producing the compressed wood according to Technique 1, wherein the heating step is a step of heating the wood under a steam atmosphere.
Since the reaction between the wood and the organic acid is promoted by performing the heat treatment under a steam atmosphere, fixation for controlling the springback can be performed in a short time.
(Technique 3) The method for producing the compressed wood according to Technique 1 or 2, wherein the drying step is a step of setting a moisture content of the wood within a range of 5% to 30%.
With this configuration, excess moisture in the wood is removed, and thus the reactivity of the organic acid to the wood can be enhanced.
(Technique 4) The method for producing the compressed wood according to any one of Techniques 1 to 3, wherein the organic acid is at least one selected from the group consisting of a citric acid, a malic acid, and a succinic acid.
With this configuration, the organic acid tends to act on hemicellulose of the wood, and thus the compressed wood can be obtained where the springback can be controlled through lower temperature and/or shorter time treatment.
(Technique 5) The method for producing the compressed wood according to any one of Techniques 1 to 4, wherein a content of the organic acid in the organic acid aqueous solution is within a range of 3% to 30% by mass.

With this configuration, the organic acid easily penetrates into the wood, and thus the effect of controlling the spring back of the wood can be obtained.

EXAMPLES

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

[Preparation of test samples]

(Example 1)

First, multiple pieces of Japanese cedar wood having a width of 30 mm, a thickness of 20 mm, and a length of 25 mm were prepared. The wood grain was set to be across in the width direction, with the fibers running along the length direction of the wood. Note that the following tests were conducted using continuous test pieces.

Next, as illustrated in Table 1, a citric acid, which is an organic acid, and water were mixed to prepare an impregnation solution having a citric acid concentration of 5% by mass.

[Table 1]

	Example 1	Example 2	Comparative example
Impregnation	Yes	Yes	Yes
Type of impregnation solution	Citric acid aqueous solution	Citric acid aqueous solution	Water
Concentration of organic acid	5% by mass	10% by mass	-

Next, the impregnation solution was placed in a pressure-resistant container, and the multiple pieces of wood were immersed. Then, a pressure-impregnation treatment was performed with the wood immersed in the impregnation solution, where the atmospheric pressure was held at -0.09 MPa for 20 minutes, and then at 0.8 MPa for 3 hours. After the pressure-impregnation treatment, the wood was held under atmospheric pressure for more than 12 hours while immersed in the impregnation solution. In this way, by holding the wood while immersed in the impregnation solution, it is possible to ensure the impregnation of the wood cell wall with the organic acid.
Next, the wood after the impregnation treatment was removed from the impregnation solution, and the wood was dried. Note that the drying step was performed by changing drying conditions in the order of steps 1 to 7 listed in Table 2 using a steam type drying apparatus. As a result of measuring the moisture content of the wood after the drying step, according to JIS Z2101, it was approximately 8%. [Table 2]

Step Temperature (°C) Relative humidity (%) Time (Hr)

1 50 95 3

2 45 90 12

3 45 80 12

4 50 70 12

5 55 45 6

6 60 30 6

7 70 15 6
Then, the wood subjected to the drying step was subjected to a compression treatment using a hand press equipped with hot plates where the temperature can be controlled at the top and bottom. Specifically, the wood after the drying step was placed in contact with hot plates heated to 180 °C, 200 °C, 220 °C, or 240 °C, and preheating was performed on the wood for 3 minutes. Then, spacers having a thickness of about 10 mm were placed on both sides of the wood between the hot plates, and the wood was compressed. Note that the compression to the wood was performed until a sufficient load was applied to the spacers on both sides, and was held for 3 minutes. Then, the compressed wood was taken out from between the hot plates.
Next, the compressed wood was subjected to a heat treatment in a saturated steam atmosphere at 150 °C for 5 minutes. Here, the wood was heated without being compressed using a fixing jig. Then, after the heat treatment was completed, the wood was cooled to the level of human skin, thereby obtaining pieces of compressed wood of example 1 whose compression temperatures in the compression step were 180 °C, 200 °C, 220 °C, and 240 °C.

(Example 2)

In the same process as in example 1, except that the concentration of the citric acid contained in the impregnation solution was 10% by mass, pieces of compressed wood of example 2 were obtained whose compression temperatures in the compression step were 140 °C, 160 °C, 180 °C, 200 °C, 220 °C, and 240 °C.

(Comparative example)

In the same process as in example 1, except that water was used as the impregnation solution, pieces of compressed wood of comparative example were obtained whose compression temperatures in the compression step were 140 °C, 160 °C, 180 °C, 200 °C, 220 °C, and 240 °C.

[Evaluation]

The percentage change in thickness of the compressed wood of each of examples 1, 2, and comparative example was measured. Specifically, the dimension of the compressed wood of each of examples 1, 2 and comparative example was measured in the thickness direction before the heat treatment in a saturated steam atmosphere. Furthermore, the dimension of the compressed wood of each example was measured in the thickness direction after the heat treatment in a saturated steam atmosphere.

Then, the percentage change in thickness of the compressed wood of each example in the thickness direction was calculated using equation 2 below. Table 3 illustrates the dimension in the thickness direction before the heat treatment, the dimension in the thickness direction after the heat treatment, and the percentage change in thickness of each example of compressed wood. Note that for reference, table 3 also lists the dimension of wood in the thickness direction before immersion in the impregnation solution, for the compressed wood of each example.

Percentage change in thickness direction (%) = [(dimension in thickness direction after heat treatment) - (dimension in thickness direction before heat treatment)] /[dimension in thickness direction before heat treatment]

[Table 3]

	Impregnation solution	Compression temperature (°C)	Wood thickness before immersion (mm)	Steam heat treatment
	Impregnation solution	Compression temperature (°C)	Wood thickness before immersion (mm)	Thickness before treatment (mm)	Thickness after treatment (mm)	Percentage change in thickness (%)
Comparatie example	Water	140	20.15	10.4	19.3	85.6
Example 2	Citric acid 10%	140	21.24	11.09	19.91	79.5
Comparatie example	Water	160	20.61	10.23	18.82	84.0
Example 2	Citric acid 10%	160	20.89	10.26	17.92	74.7
Comparatie example	Water	180	20.58	10.19	17.71	73.8
Example 1	Citric acid 5%		20.75	9.81	16.31	66.3
Example 2	Citric acid 10%		21.19	9.84	15.67	59.2
Comparatie example	Water	200	20.64	10.2	17.53	71.9
Example 1	Citric acid 5%		20.85	9.81	15.68	59.8
Example 2	Citric acid 10%		21.05	9.73	13.58	39.6
Comparatie example	Water	220	20.72	10.27	16.87	64.3
Example 1	Citric acid 5%		21	9.8	12.99	32.6
Example 2	Citric acid 10%		20.96	9.71	12.17	25.3
Comparatie example	Water	240	20.63	10.41	15.2	46.0
Example 1	Citric acid 5%		20.93	9.78	11.19	14.4
Example 2	Citric acid 10%		21.09	9.8	10.74	9.6

Fig. 2 illustrates the relationship between the compression temperature during the compression step and the percentage change in thickness of test pieces, regarding each compressed wood of examples 1, 2, and comparative example. From Fig. 2, it is evident that the percentage change in thickness of the compressed wood of example 1 impregnated with the organic acid significantly decreases compared with that of the compressed wood of comparative example impregnated only with water. Especially, when the compression temperature is 220 °C or more, it is evident that the percentage change in thickness of each compressed wood of examples 1 and 2 significantly decreases compared with that of comparative example.
In contrast, the percentage change in thickness of the compressed wood of the comparative example exceeded 80% at the compression temperature of 160 °C or less, and the percentage change in thickness exceeded 60% even at the compression temperature of 220 °C. Since the compressed wood in the comparative example does not contain the organic acid, when the heat treatment in the steam atmosphere is performed after the compression step, the compressive deformation is decompressed at the initial stage of heating, due to the moisture contained in the steam. Therefore, it is thought that the percentage change in thickness increased.
As described above, it is evident that by impregnating the wood with the organic acid, restoration of the compression state can be controlled in the heating step. In other words, it is evident that there is no need to keep the wood compressed using a fixing jig in the heating step, and heating can be performed without a jig. Therefore, compared with the conventional production method, the wood can be heated without using a fixing jig to maintain the compression state, and the production process can be greatly simplified.
Note that in the examples, the temperature of the steam heat treatment was set at 150 °C. However, it can be easily assumed that the reaction between the wood and the organic acid is promoted by raising the temperature of the heat treatment, and the fixation treatment can be performed in a shorter time.
In the examples, the compression treatment was performed at 140 °C, 160 °C, 180 °C, 200 °C, 220 °C, and 240 °C, and the holding time after preheating and compression was 3 minutes each. However, if the heating time after preheating and compression is shorter, the drying of wood and various bonds remain insufficient, and the wood may decompress in the thickness direction during releasing the pressure. Therefore, it is necessary to adjust pressing temperature, time, and pressure-releasing temperature according to the size of the wood and the moisture content adjusted. In addition, the impregnation with the organic acid is considered to promote the fixation during the compression treatment. Therefore, the fixation level can be designed according to the heat treatment after pressure-releasing, and the combination of temperature and time of the compression treatment.
Although the embodiments have been described above, the embodiments are not limited to these descriptions, and various modifications are possible within the scope of the gist of the embodiments.
The entire contents of Japanese Patent Application No. 2022-157043 (filed September 29, 2022 ) are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The present disclosure is capable of providing a method for producing compressed wood where the springback of the compressed wood can be controlled using a simple method.

REFERENCE SIGNS LIST

1: Wood

Claims

A method for producing compressed wood, comprising:
an impregnation step of pressure-impregnating a block of wood having a thickness of 3 mm or more with an organic acid aqueous solution containing an organic acid;

a drying step of drying the wood, in a wet state, which has been impregnated with the organic acid aqueous solution;

a compression step of compressing the wood, which has been dried, while heating to 140 °C or more; and

a heating step of heating the wood, which has been compressed, without compression.
The method for producing the compressed wood according to claim 1, wherein the heating step is a step of heating the wood under a steam atmosphere.
The method for producing the compressed wood according to claim 1 or 2, wherein the drying step is a step of setting a moisture content of the wood within a range of 5% to 30%.
The method for producing the 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 a citric acid, a malic acid, and a succinic acid.
The method for producing the compressed wood according to any one of claims 1 to 4, wherein a content of the organic acid in the organic acid aqueous solution is within a range of 3% to 30% by mass.