JP7712649B2 - Softened wood and its manufacturing method - Google Patents

Description

The present invention relates to softened wood and its manufacturing method.

Wood obtained from forest resources has attracted attention as a sustainable resource with low energy production that can fix _CO2 in the atmosphere, and is widely used for construction and civil engineering, as well as as a raw material for paper, etc. However, with the declining population, demand for these is expected to decrease, and because Japan's forest accumulation is increasing, there is a need to expand demand by developing new functions and uses for wood.

In recent years, various techniques have been reported for softening wood through chemical treatment. For example, Non-Patent Document 1 reports that extremely light wood is treated at high temperature in a sodium hydroxide solution, and then freeze-dried to destroy the wood tissue and turn it into a spongy wood.

However, conventional methods for producing softened wood have been limited in their simplicity and versatility, as they require the use of relatively light wood and the softening process is carried out under harsh conditions that can significantly damage the wood's structure (such as the use of hazardous reagents such as strong acids and strong alkalis at high temperatures).

ACS Nano 2018, 12, 10365-10373

The objective of the present invention is to provide a new method for producing softened wood.

As a result of intensive research conducted by the inventors in consideration of the above problems, the inventors have found that the above problems can be solved by a method for producing softened wood, which includes an impregnation step of impregnating wood with an ionic liquid, and an acquisition step of obtaining softened wood through a heating step of heating the wood impregnated with the ionic liquid. Based on this finding, the inventors have conducted further research and have completed the present invention. That is, the present invention encompasses the following aspects.

Item 1. An impregnation step of impregnating wood with an ionic liquid;
a step of obtaining softened wood by a heating step of heating the wood impregnated with the ionic liquid;
1. A method for producing softened wood, comprising:

Item 2. The manufacturing method described in Item 1, which includes a curing step in which the obtained wood is cured in a humid atmosphere after the heating step to soften the wood.

Item 3. The manufacturing method described in Item 2, in which the humid atmosphere has a humidity of 40% or more.

Item 4. The manufacturing method according to any one of items 1 to 3, which includes a liquid impregnation step of impregnating the wood or softened wood with a liquid after the heating step or the obtaining step.

Item 5. The manufacturing method according to any one of items 1 to 4, wherein the heating temperature in the heating step is 40°C or higher.

Item 6. The manufacturing method according to any one of items 1 to 5, wherein the heating temperature in the heating step is 80°C or higher.

Item 7. The method according to any one of items 1 to 6, wherein the ionic liquid is 1-butyl-3-methylimidazolium acetate.

Item 8. A manufacturing method according to any one of items 1 to 7, in which the wood has a late wood rate of 5% or more.

Item 9. The manufacturing method according to any one of items 1 to 8, in which the cell wall swelling rate of the softened wood produced is 12% or more.

Item 10. Softened wood obtained by the manufacturing method described in any one of items 1 to 9.

Item 11. Softened wood with a cell wall swelling rate of 12% or more.

Item 12. Softened wood according to item 11, having restorability.

Item 13. Softened wood according to item 11 or 12, in which the compression ratio when a compressive force of 0.5 MPa is applied is at least twice the compression ratio when a compressive force of 0.5 MPa is applied to control wood.

Item 14. Softened wood according to any one of items 11 to 13, which has a recovery rate of 80% or more when compressed by 20%.

Item 15. Softened wood according to any one of items 11 to 14, in which the compressive force at a compression rate of 20% is 1.3 times or less the compressive force at a compression rate of 10%.

Item 16. Softened wood according to any one of items 11 to 15, having late wood.

The present invention provides a new method for producing softened wood.

1 is a graph showing the relationship between compressibility and compressive stress measured in Example 1. An example of an electron microscope image of the measurement site in Example 2 is shown. The straight line portion in the image is the measurement length. In this example, the measurement length is for 11 cells. The measurement value is shown below the image. FIG. 13 is a graph showing the relationship between the compressive modulus and compressive stress of test pieces impregnated with [BuMeIm]OAc and heat-treated at different temperatures in Example 3. This shows the relationship between the compressibility and compressive stress of a test piece that was impregnated with [BuMeIm]OAc, heat-treated, and then substituted with pure water in Example 4. "Pure water injection after water washing" shows the results of a test piece that was air-dried and then re-impregnated with pure water.

In this specification, the expressions "contain" and "include" include the concepts of "contain," "include," "consist essentially of," and "consist only of."

1. Manufacturing method of softened wood In one aspect, the present invention relates to a manufacturing method of softened wood (sometimes referred to as the "manufacturing method of the present invention" in this specification) which comprises an impregnation step of impregnating wood with an ionic liquid and a heating step of heating the wood impregnated with the ionic liquid to obtain softened wood. This will be described below.

1-1. Impregnation process The wood to be impregnated in the impregnation process is not particularly limited as long as it is made from felled trees, and a wide variety of wood can be used regardless of the type of woody plant used as the raw material or whether or not it has been sawn (and the extent to which it has been sawn). The wood may be made from one type of woody plant or a combination of two or more types of woody plants.

Woody plants are not particularly limited as long as they can be used as lumber materials, and examples include coniferous trees and broad-leaved trees. More specifically, for example, cedar, Yezo spruce, larch, black pine, Todomatsu, Himekomatsu, yew, Japanese yew, spruce, Japanese spruce, Japanese spruce, Japanese yew, Japanese bean berry, Japanese yew, Japanese asparagus, Japanese cypress, Japanese hemlock, Japanese cypress, Japanese yew, Japanese yew, spruce, yellow cedar, Lawson cypress, Douglas fir, Sitka spruce, Radiata pine, East asparagus, and Japanese yew. Examples of wood materials include coniferous woods such as tan spruce, eastern white pine, western larch, western fir, western hemlock, and tamarack; and hardwoods such as asbestos, American black cherry, yellow poplar, walnut, birch, zelkova, sycamore, silver cherry, ash, teak, Chinese elm, Chinese maple, oak, hard maple, hickory, pecan, white ash, white oak, white birch, red oak, acacia, and eucalyptus. Both sapwood and heartwood can be used as wood materials.

The type of wood is not particularly limited as long as it can be impregnated with the ionic liquid. Examples include logs, timber, planks, solid wood, wood materials, laminated lumber, laminated veneer lumber, plywood, wood boards, particle boards, fiber boards, particulate wood (chips, particles, etc.), fibrous wood, etc. In addition, either green wood or dried wood can be used as the wood.

The wood preferably has latewood. The latewood ratio of the wood is, for example, 1% or more, 2% or more, 5% or more, 7% or more, 10% or more, 12% or more, 14% or more, or 15% or more. The upper limit of the latewood ratio is usually less than 100%, for example, 70%, 60%, 50%, 40%, 30%, or 20%. The wood usually has earlywood (= parts other than the latewood).

The latewood percentage of wood is determined as follows: The latewood portion on the butt end surface of the wood is determined based on Mork's definition, and the latewood percentage (= (latewood width / annual ring width) x 100 (%)) is calculated.

The wood may also be wood that has been treated with a solvent to retain the solvent. The solvent is not particularly limited as long as it is a solvent that can be substituted for the ionic liquid. Specific examples of the solvent include alcohol, water, acetone, acetonitrile, benzene, and toluene. The wood may also retain a drug.

There are no particular limitations on the method for retaining the solvent in the wood, but an example is immersing the wood in the solvent.

Immersion in the solvent may be carried out in any manner as long as the wood can be permeated with the solvent, for example, in which all or part of the wood is immersed in the solvent. Immersion may be carried out under reduced pressure, pressure, or atmospheric pressure. The immersion temperature is not particularly limited as long as the wood can be permeated with the solvent. The immersion temperature can be, for example, 0 to 40°C, preferably 15 to 25°C. The immersion time is not particularly limited as long as the wood can be permeated with the solvent, and is appropriately selected depending on the size of the wood, the pressure and temperature during immersion, etc. For example, the immersion time at atmospheric pressure and room temperature can be about 4 to 24 hours, preferably about 6 to 16 hours.

You can use one type of wood alone or two or more types in combination.

The ionic liquid is not particularly limited as long as it is a salt that is composed of a cation and an anion and is in a liquid state at room temperature. Specific examples of salts that are in a liquid state at room temperature include salts that have a melting point of 40°C or lower, preferably 25°C or lower, more preferably 10°C or lower, and even more preferably -20°C or lower. One type of ionic liquid may be used alone, or two or more types may be used in combination.

The cation is not particularly limited as long as it can be a cation constituting the ionic liquid. Specific examples of the cation include the imidazolium ion represented by general formula (1), the pyridinium ion represented by general formula (2), the ammonium ion represented by general formula (3), the pyrrolidinium ion represented by general formula (4), the phosphonium ion represented by general formula (5), and the sulfonium ion represented by general formula (6), among which the imidazolium ion represented by general formula (1) and the phosphonium ion represented by general formula (5) are preferred, and the imidazolium ion represented by general formula (1) is particularly preferred. The cation may be used alone or in combination of two or more kinds.

In the general formula (1), ^R1 and ^R2 are the same or different and represent an alkyl group which may be substituted (preferably an unsubstituted alkyl group) or a hydrogen atom (however, ^R1 and ^R2 are not hydrogen atoms at the same time). The alkyl group is not particularly limited as long as the cation can form an ionic liquid. The alkyl group may be either branched or straight, but is preferably straight. Examples of the alkyl group include alkyl groups having 1 to 8 carbon atoms, preferably alkyl groups having 1 to 4 carbon atoms, and more specifically, methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, and octyl groups.

Preferred embodiments of general formula (1) include an embodiment in which R ¹ is an alkyl group having 1 to 2 carbon atoms and R ² is an alkyl group having 3 to 5 carbon atoms, and an embodiment in which R ¹ is an alkyl group having 1 to 2 carbon atoms and R ² is an alkyl group having 1 to 2 carbon atoms. Preferable examples of the imidazolium ion represented by general formula (1) include 1-butyl-3-methylimidazolium ion, 1-ethyl-3-methylimidazolium ion, etc., and particularly preferable example is 1-butyl-3-methylimidazolium ion.

In the general formula (2), ^R3 represents an alkyl group which may be substituted (preferably an unsubstituted alkyl group). The alkyl group is not particularly limited as long as the cation can form an ionic liquid. The alkyl group may be either branched or straight, but is preferably straight. Examples of the alkyl group include alkyl groups having 1 to 5 carbon atoms, preferably alkyl groups having 1 to 3 carbon atoms, and more specifically, methyl groups, ethyl groups, propyl groups, butyl groups, and pentyl groups.

In general formula (3), R ⁴ to R ⁷ are the same or different and represent an alkyl group which may be substituted (preferably an unsubstituted alkyl group) or a hydrogen atom (however, R ⁴ to R ⁷ are not simultaneously hydrogen atoms). The alkyl group is not particularly limited as long as the cation can form an ionic liquid. The alkyl group may be either branched or straight, but is preferably straight. Examples of the alkyl group include alkyl groups having 1 to 5 carbon atoms, preferably alkyl groups having 1 to 3 carbon atoms, and more specifically, methyl, ethyl, propyl, butyl, and pentyl groups.

In the general formula (4), ^R8 and ^R9 are the same or different and represent an alkyl group which may be substituted (preferably an unsubstituted alkyl group) or a hydrogen atom (however, ^R8 and ^R9 are not hydrogen atoms at the same time). The alkyl group is not particularly limited as long as the cation can form an ionic liquid. The alkyl group may be either branched or straight, but is preferably straight. Examples of the alkyl group include alkyl groups having 1 to 8 carbon atoms, preferably alkyl groups having 1 to 6 carbon atoms, and more specifically, methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, and octyl groups.

In general formula (5), R ¹⁰ to R ¹³ are the same or different and each represents an alkyl group which may be substituted (preferably an unsubstituted alkyl group), or a hydrogen atom (however, R ¹⁰ to R ¹³ are not simultaneously hydrogen atoms). The alkyl group is not particularly limited as long as the cation can form an ionic liquid. The alkyl group may be either branched or straight, but is preferably straight. Examples of the alkyl group include alkyl groups having 1 to 5 carbon atoms, preferably alkyl groups having 1 to 3 carbon atoms, and more specifically, methyl, ethyl, propyl, butyl, and pentyl groups.

In the general formula (6), R ¹⁴ to R ¹⁶ are the same or different and each represents an alkyl group which may be substituted (preferably an unsubstituted alkyl group), or a hydrogen atom (provided that R ¹⁴ to R ¹⁶ are not simultaneously hydrogen atoms). The alkyl group is not particularly limited as long as the radical polymerizable group-containing cation can form an ionic liquid. The alkyl group may be either branched or straight, but is preferably straight. Examples of the alkyl group include alkyl groups having 1 to 5 carbon atoms, preferably alkyl groups having 1 to 3 carbon atoms, and more specifically, methyl, ethyl, propyl, butyl, and pentyl groups.

In the general formulas (1) to (6), examples of the substituent of the alkyl group include a hydroxyl group, a carbonyl group, a methoxy group, an amino group, a carboxyl group, an aryl group, and a radically polymerizable group. A radically polymerizable group can be used instead of a hydrogen atom. The radically polymerizable group is not particularly limited as long as it can be subjected to addition polymerization by a radical and the radically polymerizable group-containing cation can form an ionic liquid. Specific examples of the radically polymerizable group include a vinyl group, an allyl group, an isopropenyl group, an acryloyl group, a methacryloyl group, and a maleoyl group, and preferably a vinyl group, an allyl group, and an acryloyl group.

The anion is not particularly limited as long as it can be an anion constituting the ionic liquid. Specific examples of the anion include organic carboxylate ions (e.g., organic carboxylate ions having one carboxy group and 1 to 8 carbon atoms (preferably 2 to 4 _{, more preferably 2 to 3)), halide ions, dialkyl phosphate ions, tetrafluoroborate ions (BF4-), BF3CF3-, BF3C2F5-, BF3C3F7-, BF3C4F9- , hexafluorophosphate} ^ions _{( PF6-} ⁾ _{, bis ( trifluoromethanesulfonyl} ) ^{imidate ions ((CF3SO2)2N-} _{) ,} ^perchlorate _ions ^{( ClO4-} ), ^tris ₍ ^{trifluoromethanesulfonyl} ₎ carbonate ions ( _{( CF3SO2} ) _{3C- )} , trifluoromethanesulfonate ions ⁽ _CF3SO3- ), dicyanamide ions (( _{CN ) 2N- )} , and ^{trifluoroacetate} ions ( ^{CF3COO- )} _. ), and ions derived from amino acids. Preferred anions include organic carboxylate ions, halide ions, and the like, and particularly preferred anions include organic carboxylate ions. The anions may be used alone or in combination of two or more.

Ionic liquids can be produced according to known methods (see, for example, Chem. Lett., 2000, p. 922, J. Phys. Chem. B, 103, 1999, p. 4164, etc.). In the present invention, ionic liquids produced according to known methods or commercially available products may be used.

The liquid (impregnation liquid) used in the impregnation step to impregnate the wood with the ionic liquid is not particularly limited as long as it contains an ionic liquid, and may consist of only the ionic liquid, or may contain other solvents and other components in addition to the ionic liquid. From the viewpoint of making it easier to adjust the viscosity, etc., of the impregnation liquid to an appropriate level, it is preferable that the impregnation liquid contains other solvents.

Examples of the other solvent include water and various organic solvents. Examples of the organic solvent include alcohol, acetone, acetonitrile, benzene, and toluene. The other solvent is preferably water. When the other solvent (preferably water) is contained, the total content of the ionic liquid and the other solvent (preferably water) is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and even more preferably 95% by mass or more. The other solvent may be used alone or in combination of two or more kinds.

The manufacturing method of the present invention does not aim to destroy wood tissue to create a sponge-like structure as in the conventional technology, so it is preferable not to use strong acids such as sulfuric acid or strong alkalis such as sodium hydroxide in the impregnation process (or in other processes described below).

Other ingredients are not particularly limited, and examples include various wood treatment agents (e.g., preservatives, anti-termite agents, insect repellents, flame retardants, fire retardants, etc.).

The content of the ionic liquid in the agent of the present invention is not particularly limited. The content is preferably 5 to 60% by mass, more preferably 10 to 50% by mass, even more preferably 20 to 40% by mass, and even more preferably 25 to 35% by mass.

The impregnation of wood with the ionic liquid in the impregnation step of the present invention may be carried out by any method as long as it is possible to impregnate the wood with the ionic liquid. Examples of such methods include a method in which the wood is immersed in an impregnation liquid containing the ionic liquid, or a method in which the impregnation liquid is injected into the raw wood material using an injector.

Impregnation with ionic liquid by immersion may be performed under atmospheric pressure, but it can also be performed by changing the pressure (combining reduced pressure, increased pressure, and atmospheric pressure conditions), and a method of impregnation by alternating reduced pressure and atmospheric pressure conditions is preferred. The temperature of the ionic liquid used for immersion may be any temperature at which the ionic liquid is liquid, for example, 0 to 40°C is preferred, and 15 to 30°C is more preferred. The immersion time of the wood may be any time that allows the wood to be sufficiently impregnated with the ionic liquid, and is appropriately selected depending on the size of the wood, the pressure during immersion, the temperature, and other conditions, for example, 12 to 120 hours is preferred, and 24 to 96 hours is more preferred.

After the wood has been impregnated with the ionic liquid, the wood impregnated with the ionic liquid (i.e., holding the ionic liquid) can be dried. There are no particular limitations on the drying method as long as it does not involve a heating step, and any known method for drying wood can be used. The drying method is usually natural drying.

1-2. Heating step In the heating step, the wood obtained in the impregnation step (wood impregnated with ionic liquid) is heated. The heating temperature in the heating step is preferably 40°C or higher, more preferably 60°C or higher, even more preferably 80°C or higher, even more preferably 90°C or higher, and particularly preferably 100°C or higher, in order to enhance the softening of the wood and further develop and enhance the restorability. The upper limit of the temperature range is not particularly limited, but is preferably 200°C, more preferably 150°C, and even more preferably 120°C. The heating time depends on the heating temperature, but is preferably, for example, 6 hours or more, more preferably 12 hours or more, even more preferably 24 hours or more, and particularly preferably 36 hours or more. The upper limit of the heating time is, for example, 2 months, 1 month, 15 days, 10 days, or 5 days.

1-3. Obtaining step In the obtaining step, softened wood is obtained through a heating step. Immediately after the heating step, the wood is hard and not softened. In the obtaining step, the wood obtained through the heating step is softened. A specific method of softening is to adsorb or impregnate the wood obtained through the heating step with a liquid. Therefore, in one aspect of the present invention, it is preferable that the manufacturing method of the present invention includes a curing step of curing the obtained wood in a humid atmosphere after the heating step to soften the wood, or a liquid impregnation step of impregnating the wood or softened wood with a liquid after the heating step. These are described below.

1-3-1. The humid atmosphere of the curing process is not particularly limited as long as it has a certain humidity or higher. The humidity is, for example, 40% or higher, preferably 50% or higher, and more preferably 55% or higher. The upper limit of the humidity is not particularly limited, and is, for example, 100%, 90%, 80%, or 70%.

The temperature of the moist atmosphere is not particularly limited as long as it is a temperature that does not inhibit the adsorption of moisture into the wood. The temperature is, for example, less than 40°C, preferably 35°C or less, more preferably 30°C or less, and even more preferably 25°C or less. There is no particular lower limit to the temperature, and it is, for example, 0°C, 5°C, or 10°C.

There is no particular restriction on the curing time, so long as it is sufficient for the wood to soften. The time is, for example, 5 hours or more, preferably 10 hours or more, more preferably 15 hours or more, and even more preferably 20 hours or more. There is no particular upper limit on the time, but from the viewpoint of the production efficiency of softened wood, it is preferably 1 week, more preferably 3 days, even more preferably 2 days, and even more preferably 36 hours.

1-3-2. Liquid impregnation process The liquid is not particularly limited as long as it is a solvent other than the ionic liquid that can be substituted for the ionic liquid. Specific examples of the solvent include alcohol, water, acetone, acetonitrile, benzene, toluene, polyethylene glycol, etc. The solvent may be used alone or in combination of two or more. The solvent preferably contains water. The liquid may also contain a drug.

Impregnation can be done by any method that can impregnate the wood with the liquid, such as immersing the wood in the liquid or injecting the liquid into the raw wood using an injector.

Impregnation with the liquid by immersion may be performed under atmospheric pressure, but it can also be performed by changing the pressure (combining reduced pressure, increased pressure, and atmospheric pressure conditions), and a method of impregnation in which reduced pressure and atmospheric pressure conditions are alternately repeated is preferred. The temperature of the ionic liquid used for immersion may be any temperature at which the ionic liquid is liquid, for example, 0 to 40°C is preferred, and 15 to 30°C is more preferred. The immersion time of the wood may be any time that allows the wood to be sufficiently impregnated with the ionic liquid, and is appropriately selected depending on the conditions such as the size of the wood, the pressure during immersion, and the temperature, for example, and is preferably about 12 to 120 hours, and more preferably about 24 to 96 hours.

After impregnation with the liquid, if the wood is treated under conditions that cause the impregnated liquid to evaporate, the degree of softening and restoration of the wood may be reduced. For this reason, it is preferable to use a liquid that does not evaporate easily, such as polyethylene glycol, as the impregnating liquid.

The liquid impregnation process can also be performed on the softened wood after the acquisition process.

2. Softened wood Softened wood is obtained by the manufacturing method of the present invention. The softened wood produced by the manufacturing method of the present invention can be easily compressed. Softened wood (air-dried) from which the ionic liquid has been removed after the heating step can be compressed more easily than untreated wood (air-dried) before impregnation with the ionic liquid. In one aspect, the present invention relates to softened wood obtained (or obtained) by the manufacturing method of the present invention.

The softened wood of the present invention is characterized by swollen cell walls, which is believed to be one of the factors that soften the wood. The cell wall swelling rate in the softened wood is, for example, 12% or more, preferably 14% or more, more preferably 16% or more, and even more preferably 17% or more. There is no particular upper limit to the cell wall swelling rate, and it is, for example, 50%, 40%, 30%, or 25%. The cell wall swelling rate is measured according to the method described in Test Example 2. From this perspective, in one embodiment, the present invention relates to softened wood having a cell wall swelling rate of 12% or more. The softened wood maintains a swollen state of the cell walls even after undergoing a drying treatment (in a dry state).

The manufacturing method of the present invention makes it possible to soften wood without dissolving the cell walls to thin them. In addition, the manufacturing method of the present invention makes it possible to maintain the wood's natural appearance (wood grain, color tone, etc.) without discoloring the wood to white, as occurs with conventional techniques (strong acid or strong alkali treatment).

The softened wood of the present invention has a compressibility when a compressive force (compressive stress) of 0.5 MPa is applied, which is at least twice, preferably at least three times, and more preferably at least five times, the compressibility of untreated wood (control wood) before being impregnated with the ionic liquid when a compressive force of 0.5 MPa is applied. In addition, softened wood impregnated with ionic liquid or other solvents can be compressed more easily than untreated wood impregnated with water.

The softened wood of the present invention has a compression ratio of preferably 20% or more, more preferably 30% or more, and even more preferably 50% or more when a compressive force of, for example, 1.2 MPa, preferably 1.0 MPa, more preferably 0.5 MPa, even more preferably 0.3 MPa, and even more preferably 0.2 MPa is applied.

The softened wood of the present invention has restorability. For example, when the softened wood is compressed by 20%, preferably 50%, and more preferably 75%, the restoration rate is preferably 80% or more, more preferably 85% or more, even more preferably 90% or more, and particularly preferably 95% or more.

The softened wood of the present invention can be compressed more at a constant compression force. For example, the compression force at a compression rate of X% (X=10, 15, 20, 25, or 30) is preferably 1.6 times or less, more preferably 1.5 times or less, even more preferably 1.4 times or less, even more preferably 1.3 times or less, especially preferably 1.2 times or less, and especially more preferably 1.1 times or less of the compression force at a compression rate of (X-Y)% (Y=5, 10, 15, 20, or 25 (where (X-Y) is 5 or more)).

The softened wood of the present invention preferably has a late wood portion. The softened wood of the present invention can exhibit the above-mentioned properties, particularly against forces perpendicular to the lines that make up the late wood portion. On the other hand, the softened wood of the present invention cannot exhibit the above-mentioned properties against forces parallel to the lines that make up the late wood portion.

The present invention will be described in detail below based on examples, but the present invention is not limited to these examples.

Example 1. Production of softened wood 1
The ionic liquid aqueous solutions used are as follows: Four types of ionic liquids were used: 1-ethyl-3-methyl imidazolium chloride ([EtMeIm]Cl), 1-butyl-3-methyl imidazolium chloride ([BuMeIm]Cl), 1-ethyl-3-methyl imidazolium acetate ([EtMeIm]OAc), and 1-butyl-3-methyl imidazolium acetate ([BuMeIm]OAc). The ionic liquids were prepared in pure water to a concentration of 30 mass%.

The test specimens used were as follows: Sapwood of Japanese cedar (Cryptomeria japonica D.don) cut continuously in the direction of the grain to a thickness of 5 mm from an air-dried bar-shaped test specimen with a 15mm x 15mm butt surface with two straight grains was used as the test material (15mm x 15mm x 5mm). These test specimens were dried to a completely dry state. The latewood portion on the butt surface of the test specimen was determined based on Mork's definition, and the latewood rate (= (latewood width / annual ring width) x 100 (%)) was calculated, with an average value of 16.2%.

The test pieces were immersed in a beaker filled with four types of aqueous ionic liquids, and the pressure was repeatedly reduced and normal for about four days at room temperature until the test pieces were fully impregnated (impregnation process). The test pieces that had been fully impregnated with the ionic liquid were allowed to air dry for about a day, and then heat-treated in an environment at 105°C for two days (heating process). Immediately after the heat treatment, the wood was hard and could not be considered softened wood. After the heat treatment, the wood was left to cure for about a day in an environment at a temperature of 20°C and a relative humidity of 60% (curing process).

A compression test was conducted on the test pieces obtained in the curing process using a precision universal testing machine (Shimadzu AG-110kN). Figure 1 shows the relationship between compression ratio and compressive stress. As controls, the results of an untreated test piece in a dry (air-dried) state and an untreated test piece soaked in pure water are also shown.

The dimensions of the test piece immediately after unloading were also measured to determine the return rate (restoration rate). The return rate was calculated using the following formula: return rate (%) = (dimension in the compression direction of the test piece immediately after unloading) / (dimension in the compression direction of the test piece before compression) x 100.

As shown in Figure 1, the air-dried control specimen had a compressive stress of around 3 MPa, while the control specimen soaked in pure water had a compressive stress of around half that, 1.5 MPa, and the compressibility increased (the compressibility remained stable), whereas the specimen soaked in ionic liquid had an increase in compressibility at a weak compressive stress of less than 1 MPa. The lowest compressive stress was observed in the case of [BuMeIm]OAc, which softened to around 0.1 to 0.2 MPa, about one-tenth the strength of the control specimen soaked in pure water, and could be easily compressed with finger pressure.

Table 1 shows the compressive stress at each compression ratio when [BuMeIm]OAc is used.

As shown in Table 1, the increase in compressive stress was gradual even as the compression ratio increased.

When [BuMeIm]OAc, which had the lowest compressive stress, was used, the return rate after unloading was over 97% at a compression rate of 50%, and was only 87% even when the compression rate was increased to 75%.

Example 2. Analysis of the structure of softened wood [BuMeIm]OAc was used as the ionic liquid, and softened wood test pieces were obtained in the same manner as in Example 1. An electron microscope photograph of the surface was taken, and in the photographic image, 4 arbitrary parts where cells (approximately rectangular) were continuously arranged in the late wood part were selected (measurement parts 1 to 4), and the length of 5 to 13 cells (measurement part 1: 12 cells, measurement part 2: 13 cells, measurement part 3: 11 cells, measurement part 4: 5 cells) from the center of one side of a certain cell was measured. For untreated test pieces before immersion in the ionic liquid aqueous solution, measurements were also taken at 4 points so that the number of cells measured was the same as above. The cell wall swelling ratio was calculated according to the following formula: Cell wall swelling ratio = (measurement value of softened wood test piece / measurement value of untreated test piece) x 100 (however, the number of cells measured in both measurements is the same).

An example of an electron microscope image of the measurement site is shown in Figure 2. The measured values and cell wall swelling ratios of each of the four measurement sites, as well as the average cell wall swelling ratio of the four sites, are shown in Table 2.

As shown in Table 2, the cell walls of the softened wood were swollen.

Example 3. Preparation of softened wood 2
[BuMeIm]OAc was used as the ionic liquid, and the test piece was impregnated with the ionic liquid in the same manner as in Example 1. The test piece was naturally dried for about one day, and then heat-treated for two days in environments at 40°C, 60°C, and 105°C. After the heat treatment, the test piece was aged for about one day in an environment at a temperature of 20°C and a relative humidity of 60%. A test piece according to a comparative example was produced in which treatment for two days was performed in an environment at 20°C instead of heat treatment.

The compression test was carried out in the same manner as in Example 1. Figure 3 shows the relationship between the compression ratio and the compressive stress. In the comparative example at a temperature of 20°C, the compressive stress was about the same as that of the test piece impregnated with pure water, but as the treatment temperature (heating temperature) increased, the compressive stress decreased and the specimen softened.

Example 4. Preparation of softened wood 3
[BuMeIm]OAc was used as the ionic liquid, and softened wood test pieces were obtained in the same manner as in Example 1. The test pieces were then immersed in a beaker filled with pure water and stirred for three days using a stirrer to replace the ionic liquid with pure water. The test pieces were dried and weighed to confirm that no ionic liquid remained. Then, an air-dried test piece and a test piece in which the test piece was again impregnated with pure water were produced.

The compression test was carried out in the same manner as in Example 1. Figure 4 shows the relationship between the compression ratio and the compressive stress.

Compared to the air-dried control test piece in Figure 1, the compressive stress of the air-dried test piece from which the ionic liquid was removed in Example 4 was reduced to about half. Furthermore, when pure water was injected into this test piece again, the compressive stress was about the same as that of the test piece from Example 1 from which the ionic liquid had not been removed, and the return rate after unloading also showed a similar trend. In other words, it was revealed that even if the ionic liquid is removed, the softened state can be maintained as long as it is filled with water, and it will return to its original shape even after unloading unless it is significantly compressed.

Claims (6)

An impregnation step of impregnating wood having a late wood rate of 5% or more with an ionic liquid;
a step of obtaining softened wood by a heating step of heating the wood impregnated with the ionic liquid;
a curing step of curing the obtained wood in a humid atmosphere having a humidity of 40% or more for 20 hours or more to soften the wood after the heating step;
Including,
The heating step is a step of heating wood at 80°C or higher for 36 hours or more.
Method for producing softened wood. The manufacturing method according to claim 1, wherein the heating temperature in the heating step is 90°C or higher. The manufacturing method according to claim 1 or 2, wherein the heating temperature in the heating step is 100°C or higher. The method according to any one of claims 1 to 3, wherein the ionic liquid is 1-butyl-3-methylimidazolium acetate. The manufacturing method according to any one of claims 1 to 4, in which the cell wall swelling rate of the softened wood produced is 12% or more. Softened wood obtained by the manufacturing method described in any one of claims 1 to 5.