Method for manufacturing modified wood materials

Abstract

This invention provides a method for manufacturing modified wood materials that improves dimensional stability during water absorption and long-term performance stability. [Solution] The method for producing modified wood material includes an impregnation step of impregnating the wood material with a modified aqueous solution, a drying step of drying the wood material after the impregnation step, and a heat treatment step of heat-treating and hardening the wood material after the drying step, wherein the modified aqueous solution contains citric acid and an alkanediol having 3 to 8 carbon atoms and having only two primary hydroxyl groups.

Description

Technical Field

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

Background Art

[0008] Furthermore, the alkanediols used in this invention are limited to those having only two primary hydroxyl groups, and do not include those having secondary or tertiary hydroxyl groups.

[0009] Furthermore, the alkanediol may be 1,3-propanediol or 1,4-butanediol.

[0010] Since 1,3-propanediol and 1,4-butanediol can be produced from biomass resources, the above composition can suppress the generation of greenhouse gases and contribute to preventing global warming.

[0011] Furthermore, the alkanediol may be 1,5-pentanediol, 3-methyl-1,5-pentanediol, or 1,6-hexanediol.

[0012] Since 1,5-pentanediol, 3-methyl-1,5-pentanediol, and 1,6-hexanediol can be produced from petroleum raw materials, the above composition can reduce the production cost of modified wood materials.

[0013] Furthermore, the concentration of the modified aqueous solution should be between 10% and 40%.

[0014] Tests have confirmed that the above configuration ensures dimensional stability during water absorption and long-term performance stability.

[0015] Also, when the number of carboxy groups of the citric acid in the modified aqueous solution is set to 1, the number of hydroxyl groups of the alkanediol is preferably 0.6 or more and 2.5 or less.

[0016] According to the above configuration, it has been confirmed by tests that dimensional stability during water absorption and long-term performance stability can be ensured.

Brief Description of the Drawings

[0017] [Figure 1] It is a table showing the results of Test 1 of the modified wood material. [Figure 2] It is a table showing the results of Test 2 of the modified wood material. [Figure 3] It is a graph showing the results of Test 2 of the modified wood material. [Figure 4] It is a table showing the results of Test 3 of the modified wood material. [Figure 5] It is a table showing the results of Test 4 of the modified wood material. [Figure 6] It is a table showing the results of Test 5 of the modified wood material. [Figure 7] It is a table showing the results of Test 6 of the modified wood material. [Figure 8] It is a table showing the results of Test 7 of the modified wood material. [Figure 9] It is a photograph of the sample of Test 7 of the modified wood material.

Embodiments for Carrying Out the Invention

[0018] Hereinafter, embodiments of the present invention will be described. The description of the following embodiments is merely illustrative in nature and is not intended to limit the present invention, its applications, or its uses.

[0019] The manufacturing method of this embodiment includes an impregnation step of impregnating a wood material with a modified aqueous solution, a drying step of drying the wood material that has undergone the impregnation step, and a heat treatment step of heat-treating and curing the wood material that has undergone the drying step. Before these steps, a step of preparing the wood material is performed. Additionally, steps such as washing, drying, and cooling may be additionally performed.

[0020] The wood material to be modified is not particularly limited and may be for building materials or used for other purposes. It may be a coniferous tree, a broad-leaved tree, a solid wood material, a glued laminated timber, a plywood, etc.

[0021] In the impregnation step, the wood material is impregnated with the modified aqueous solution. The impregnation is performed, for example, by immersing the wood material in the modified aqueous solution and by normal pressure impregnation, reduced pressure impregnation, pressure impregnation, or reduced pressure-pressure impregnation. The impregnation step may be performed as a combination of 30 minutes of reduced pressure impregnation at -0.08 Mpa followed by 1 hour of pressure impregnation at 1 Mpa. It is preferable that the wood material is dried before the impregnation step.

[0022] In the drying step, the wood material that has undergone the impregnation step is dried. The drying step may be performed at room temperature or at a high temperature. The drying step may be performed by drying at 40°C for 1 day and drying at 60°C for 1 day.

[0023] In the heat treatment step, curing by heat treatment of the wood material that has undergone the drying step is performed. The heat treatment is performed in an environment of 100°C to 200°C. The heat treatment step may be a step of heat-treating at 120°C for 1 day.

[0024] The modified aqueous solution contains citric acid and an alkanediol with 3 to 8 carbon atoms and two primary hydroxyl groups. It is believed that impregnating wood material with this modified aqueous solution and performing a heat treatment process causes dehydration condensation (esterification) between the carboxyl group of citric acid and the hydroxyl groups of the alkanediol or wood, thereby imparting dimensional stability. Since citric acid is a trivalent carboxylic acid, it is thought that a cross-linked structure is formed in the modified wood material, and the presence of this cross-linked structure is also thought to contribute to dimensional stability and performance stability.

[0025] In this embodiment, the alkanediols contained in the modified aqueous solution are limited to those having only two primary hydroxyl groups, and those having secondary and tertiary hydroxyl groups are not included. According to the results of Test 1 described later, when polyhydric alcohols having secondary and tertiary hydroxyl groups are included, problems arise with the dimensional stability during water absorption and the long-term performance stability of the modified wood material.

[0026] Furthermore, the alkanediols contained in the modified aqueous solution are limited to those with 3 to 8 carbon atoms, and those with 2 carbon atoms (e.g., ethylene glycol) or 9 or more carbon atoms are not included. According to the results of Test 1 described later, when the number of carbon atoms is 2, problems arise with the dimensional stability during water absorption and the long-term performance stability of the modified wood material. Also, when the number of carbon atoms is 9 or more, it may become poorly soluble in water, which is undesirable.

[0027] The alkanediols contained in the modified aqueous solution are preferably 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, or 1,6-hexanediol. Test 1, described later, has confirmed that using these polyhydric alcohols results in superior dimensional stability during water absorption and long-term performance stability of the resulting modified wood material.

[0028] The concentration of the modified aqueous solution is preferably between 10% and 40%. Here, the concentration of the modified aqueous solution is defined as the value obtained by dividing the total mass of citric acid and alkanediol contained by the mass of the modified aqueous solution. Test 2, described later, has confirmed that if the concentration of the modified aqueous solution is within this range, the resulting modified wood material exhibits excellent dimensional stability during water absorption and long-term performance stability. The concentration of the modified aqueous solution may exceed 40%. However, if the concentration of the modified aqueous solution exceeds 50%, cracks are more likely to occur on the surface due to hardening shrinkage during the heat treatment process. Therefore, the concentration of the modified aqueous solution is preferably 50% or less.

[0029] Regarding the mixing ratio of citric acid and alkanediol in the modified aqueous solution, it is preferable that the number of hydroxyl groups in the alkanediol is between 0.6 and 2.5, with the number of carboxyl groups in citric acid being considered as 1. In other words, it is preferable that the mixing ratio of citric acid to alkanediol is carboxyl groups of citric acid to hydroxyl groups of alkanediol = 1:2.5 to 1.5:1. It has been confirmed by tests 3 and 4 described later that if the mixing ratio is within this range, the resulting modified wood material exhibits excellent dimensional stability during water absorption and long-term performance stability.

[0030] Furthermore, some of the citric acid may be replaced with other carboxylic acids. For example, it has been confirmed in Test 4, described later, that performance stability (water resistance) can be ensured even when some of the citric acid is replaced with succinic acid.

[0031] Furthermore, other additives such as preservatives, termite repellents, flame retardants, and dyes may be added to the modified aqueous solution, provided that they do not inhibit the effects of the present invention.

[0032] [Verification of effectiveness through testing] [Test 1] As a wood material, sapwood of Japanese cedar measuring 30 mm in length, 15 mm in thickness, and 100 mm in width was prepared, and the following measurements and treatments were performed. The details and results are shown in the table in Figure 1. Samples 01-18 and 56 are samples in which the types of carboxylic acid and polyol contained in the modified aqueous solution differ. Note that the polyol "PEG200" in sample 13 is polyethylene glycol with an average molecular weight of 200, and the same applies to samples 14 and 15. Sample 00 is a sample treated without containing carboxylic acid or polyol in the modified aqueous solution.

[0033] (Step 1) The mass was measured in a dry state (this measurement result will hereafter be referred to as the "initial total dry mass").

[0034] (Step 2) An impregnation process with a modified aqueous solution was performed. The types of carboxylic acids and polyols contained in the modified aqueous solution are shown in Figure 1. The concentration of the modified aqueous solution was 20%. The impregnation process was carried out under the conditions of 30 minutes of reduced pressure impregnation at -0.08 MPa followed by 1 hour of pressurized impregnation at 1 MPa.

[0035] (Step 3) The drying process was carried out. The drying process was performed under the conditions of drying at 40°C for 1 day and drying at 60°C for 1 day.

[0036] (Step 4) A heat treatment process was carried out. The heat treatment process was performed at 120°C for 1 day.

[0037] (Step 5) Mass and width measurements were performed (these measurement results will hereafter be referred to as "total dry mass" and "total dry width").

[0038] (Step 6) To evaluate dimensional stability during water absorption, the material was impregnated with water. The water impregnation was performed under the conditions of 30 minutes of reduced pressure impregnation at -0.08 MPa followed by 1 hour of pressurized impregnation at 1 MPa.

[0039] (Step 7) The width was measured (this measurement result will hereafter be referred to as "water absorption width").

[0040] (Step 8) To evaluate the long-term performance stability, boiling was performed for 3 hours.

[0041] (Step 9) Drying was carried out at 100°C until a constant weight was reached (i.e., until no further change in mass occurred).

[0042] (Step 10) Mass measurement was performed (this measurement result will hereafter be referred to as "total dry mass after boiling"). ).

[0043] To evaluate dimensional stability during water absorption and long-term performance stability, the dimensional change rate and the residual drug rate were calculated using the following formula. The results are shown in the table in Figure 1. Dimensional change rate = (Absorbed width - Total dry width) / Total dry width Drug residual rate = (Total dry mass after boiling - Initial total dry mass) / (Total dry mass - Initial total dry mass)

[0044] A large dimensional change rate indicates that the dimensions of the wood material have changed significantly due to water absorption. The dimensional stability during water absorption was evaluated using the dimensional change rate according to the following criteria. The results are shown in the table in Figure 1. Less than 2.00%: Pass (Excellent, indicated by "◎" in the table) 2.00% or more and less than 3.00%: Pass (Good, indicated by "○" in the table) 3.00% or more but less than 4.00%: Fail (Poor quality, indicated by "△" in the table) 4.00% or higher: Fail (Poor quality, indicated by "×" in the table)

[0045] If the residual chemical value is low, it means that the chemical leached out of the wood material during boiling in step 8. In this case, concerns arise regarding the long-term stability of quality. The long-term performance stability was evaluated using the residual chemical value according to the following criteria. The results are shown in the table in Figure 1. Greater than 90.0%: Pass (Excellent, indicated by "◎" in the table) Greater than 82.5% but 90.0% or less: Pass (Good, indicated by "○" in the table) Greater than 75.0% but 82.5% or less: Fail (Poor quality, indicated by "△" in the table) Below 75.0%: Fail (Poor quality, indicated by "×" in the table)

[0046] As shown in the table in Figure 1, the results of Test 1 indicate that samples 01-04 and 56 passed both the dimensional stability upon water absorption and the long-term performance stability test, while samples 05-18 failed both the dimensional stability upon water absorption and the long-term performance stability test, or either one of them. Therefore, Test 1 demonstrated that modified aqueous solutions containing citric acid as the carboxylic acid and 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, or 1,6-hexanediol as the polyol can improve both the dimensional stability upon water absorption and the long-term performance stability test.

[0047] The polyols in the modified aqueous solutions used in samples 01-04 and 56 have two primary hydroxyl groups and no secondary or tertiary hydroxyl groups. On the other hand, the polyols in the modified aqueous solutions used in samples 05-11, 16, and 18 have secondary hydroxyl groups. From this, it is thought that the presence of secondary hydroxyl groups in the polyols contained in the modified aqueous solution adversely affects dimensional stability during water absorption or long-term performance stability.

[0048] The polyols in the modified aqueous solutions used in samples 01-04 and 56 are alkanediols. On the other hand, the polyols in the modified aqueous solutions used in samples 13-16 have ether bonds and are not alkanediols. From this, it can be concluded that if the polyol contained in the modified aqueous solution is an alkanediol, dimensional stability during water absorption and long-term performance stability can be ensured.

[0049] The polyols in the modified aqueous solutions used in samples 01-04 and 56 are alkanediols with 3, 4, 5, and 6 carbon atoms, respectively. On the other hand, the polyol (ethylene glycol) in the modified aqueous solution used in sample 12 has 2 carbon atoms. From this, it can be concluded that if the polyol contained in the modified aqueous solution has 3 or more carbon atoms, dimensional stability during water absorption and long-term performance stability can be ensured.

[0050] The carboxylic acid in the modified aqueous solutions used in samples 01-04 and 56 was citric acid. On the other hand, the carboxylic acid in the modified aqueous solutions used in samples 17-18 was malic acid. From this, it can be concluded that when the carboxylic acid contained in the modified aqueous solution is citric acid, dimensional stability during water absorption and long-term performance stability can be ensured. In particular, in the case of sample 17, although the polyol contained in the modified aqueous solution was 1,3-propanediol, the long-term performance stability was unsatisfactory. This result indicates that when citric acid and alkanediol are combined as the modified aqueous solution, both dimensional stability during water absorption and long-term performance stability can be improved.

[0051] [Exam 2] To confirm the effect of the concentration of the modified aqueous solution, samples 19-30 and 44-55 were prepared by varying the concentration of the modified aqueous solution from 10% to 40 wt. The polyols included in the modified aqueous solution were 1,3-propanediol, 1,5-pentanediol, or 3-methyl-1,5-pentanediol. The contents and results are shown in the table in Figure 2 and the graph in Figure 3.

[0052] All samples 19-30 and 44-55 passed the evaluation of dimensional stability during water absorption and long-term performance stability. However, cracks appeared on the surface of samples 47, 48, and 55, which had concentrations of 50%, 60%, and 70%. Furthermore, as shown in the graph in Figure 3, there was a tendency for the rate of dimensional change to worsen (the value increased) as the concentration decreased, and it is expected that the rate of dimensional change will fail (above 3.00%) when the concentration falls below 10%. Based on this Test 2, it is considered that dimensional stability during water absorption and long-term performance stability can be ensured if the concentration of the modified aqueous solution is between 10% and 40%.

[0053] [Exam 3] To investigate the effect of the mixing ratio of citric acid and alkanediol in the modified aqueous solution, samples 31-43 were prepared with different mixing ratios. The polyols included in the modified aqueous solution were 1,3-propanediol or 3-methyl-1,5-pentanediol. The contents and results are shown in the table in Figure 4.

[0054] All samples 31-43 passed the evaluation of dimensional stability upon water absorption and long-term performance stability. This test 3 suggests that dimensional stability upon water absorption and long-term performance stability can be ensured if the ratio of carboxyl groups of citric acid to hydroxyl groups of the alkanediol is 1:2.5 to 2.5:1. In other words, if the number of hydroxyl groups in the alkanediol is between 0.4 and 2.5, relative to the number of carboxyl groups in the citric acid (assuming 1 carboxyl group), then dimensional stability upon water absorption and long-term performance stability can be ensured.

[0055] [Test 4] In this invention, it is thought that the carboxyl group of citric acid is undergoing dehydration condensation with an alkanediol or a hydroxyl group of wood. To evaluate the water resistance of the portion where the carboxyl group of citric acid and the hydroxyl group of the alkanediol have undergone dehydration condensation, an aluminum cup test was performed on the modified aqueous solution. Details of the treatment and measurement are shown below.

[0056] (Step 11) 20g of a 20% modified aqueous solution was placed in an aluminum cup and used as a sample.

[0057] (Step 12) Drying was performed at 40°C for 1 day.

[0058] (Step 13) A heat treatment was performed at 120°C for one day. After the heat treatment was completed, the mass was measured (this measurement result will hereafter be referred to as "mass after heat treatment").

[0059] (Step 14) Boiled for 30 minutes.

[0060] (Step 15) Drying was performed at 60°C for one day. After drying was complete, the mass was measured (this measurement result will hereafter be referred to as "post-drying mass").

[0061] To evaluate water resistance, the residual drug rate was calculated using the following formula. The results are shown in the table in Figure 5. Drug retention rate = (mass after drying) / (mass after heat treatment)

[0062] If the residual agent value is low, it means that the agent was leached out by boiling in step 14. In this case, there are concerns about the water resistance (long-term performance stability) of the modified wood material. The water resistance was evaluated using the residual agent value according to the following criteria. The results are shown in the table in Figure 1. Greater than 80.0%: Pass (indicated by "○" in the table) Below 80.0%: Fail (marked with "×" in the table)

[0063] As shown in the table in Figure 5, the results of Test 4 indicate that samples C01 to C11 passed, while samples C12 and C13 failed. Therefore, Test 4 suggests that dimensional stability during water absorption and long-term performance stability can be ensured if the ratio of carboxyl groups of citric acid to hydroxyl groups of the alkanediol is between 1:2.5 and 1.5:1. In other words, it is considered that dimensional stability during water absorption and long-term performance stability can be ensured if the number of hydroxyl groups of the alkanediol is between 0.6 and 2.5, relative to the number of carboxyl groups of citric acid (which is considered to be 1).

[0064] As shown in the table in Figure 5, the results of Test 4 indicate that sample C17 passed, while samples C14-C16 failed. Therefore, Test 4 suggests that water resistance cannot be ensured when the carboxylic acid consists only of tartaric acid, succinic acid, and malic acid.

[0065] As shown in the table in Figure 5, the results of Test 4 indicate that samples C19 and C20 passed, while sample C18 failed. Therefore, Test 4 suggests that even when some of the citric acid is replaced with succinic acid, water resistance can be ensured if the ratio of citric acid is relatively high, specifically if the ratio of citric acid to succinic acid is 2:1 to 3:1.

[0066] [Exam 5] As the wood material, sapwood of Japanese cedar measuring 30 mm in length, 15 mm in thickness, and 105 mm in width was prepared, and the measurements and treatments described below were performed. For these two samples, 101 and 102, in order to evaluate the longer-term water resistance (ultra-long-term performance stability) of the modified wood material, tests were conducted under more stringent conditions than in Test 1, and the residual chemical rate was calculated. Details of the treatment and measurements are shown below. Test 5 is conducted under more stringent conditions than Test 1, in that moisture absorption at a temperature of 85°C and 85% humidity is performed before water absorption (step 6 of Test 1, step 22 of Test 5). The polyols included in the modified aqueous solution were 1,3-propanediol (Sample 101) and 3-methyl-1,5-pentanediol (Sample 102). The carboxylic acid included in the modified aqueous solution for both samples was citric acid.

[0067] (Step 16) The initial total dry mass was measured.

[0068] (Step 17) An impregnation process with a modified aqueous solution was performed. The concentration of the modified aqueous solution was 20%. The impregnation process was carried out under the conditions of 30 minutes of reduced pressure impregnation at -0.08 MPa followed by 1 hour of pressurized impregnation at 1 MPa.

[0069] (Step 18) The drying process was carried out. The drying process was performed under the conditions of drying at 40°C for 1 day and drying at 60°C for 1 day.

[0070] (Step 19) A heat treatment process was carried out. The heat treatment process was performed at 120°C for 1 day.

[0071] (Step 20) The total dry mass was measured.

[0072] (Step 21) The sample was placed under conditions of 85°C and 85% humidity for 120 hours.

[0073] (Step 22) Water impregnation was performed. Water impregnation was carried out under the conditions of 30 minutes of reduced pressure impregnation at -0.08 MPa followed by 1 hour of pressurized impregnation at 1 MPa.

[0074] (Step 23) Boil for 3 hours.

[0075] (Step 24) Drying at 100°C was carried out until a constant weight was reached.

[0076] (Step 25) Mass measurement was performed (this measurement result will hereafter be referred to as "total dry mass after ultra-long-term performance stability test").

[0077] To evaluate water resistance, the residual drug rate was calculated using the following formula. The results are shown in Figure 6. Drug retention rate = (Total dry mass after ultra-long-term performance stability test - Initial total dry mass) / (Total dry mass - Initial total dry mass)

[0078] As already mentioned, the present invention is thought to involve the dehydration condensation of the carboxyl group of citric acid with an alkanediol or the hydroxyl group of wood. Therefore, under high humidity conditions, hydrolysis may progress, causing the cross-linked structure formed within the wood material to collapse.

[0079] In other words, if the residual drug value is low, it means that the degree of moisture absorption was high under the high humidity conditions of step 21 (in other words, the degree of hydrolysis was high), and the drug leached out of the wood material due to boiling in step 23. In this case, concerns arise regarding the long-term stability of the quality.

[0080] As shown in the table in Figure 6, the results of Test 5 indicate that sample 102 showed a higher drug retention rate. Therefore, Test 5 suggests that using 3-methyl-1,5-pentanediol in the modified aqueous solution ensures higher water resistance.

[0081] [Exam 6] To evaluate the preservative performance of modified wood materials, a preservative performance test was conducted. The polyols included in the modified aqueous solution were 1,3-propanediol (Sample 201) and 3-methyl-1,5-pentanediol (Sample 202). The carboxylic acid included in the modified aqueous solution was citric acid in both samples. As a comparison, an untreated sample (Sample 200) that did not undergo steps 26 to 28 was also subjected to the same test. Cedar sapwood was prepared as the wood material, and the measurements and treatments described below were performed.

[0082] (Step 26) An impregnation process with a modified aqueous solution was performed. The concentration of the modified aqueous solution was 30%. The impregnation process was carried out under the conditions of 30 minutes of reduced pressure impregnation at -0.08 MPa followed by 1 hour of pressurized impregnation at 1 MPa.

[0083] (Step 27) A drying process was carried out. The drying process was performed under the conditions of drying at 40°C for 1 day and drying at 60°C for 1 day.

[0084] (Step 28) A heat treatment process was carried out. The heat treatment process was performed at 120°C for 1 day. After that, it was left at room temperature for 3 weeks.

[0085] (Step 29) Indoor tests were conducted for the antiseptic performance specified in JIS K 1571, and the mass loss rate was determined. The treated samples underwent weathering treatment followed by antibacterial treatment.

[0086] As shown in the table in Figure 7, the results of Test 6 indicate that samples 201 and 202 showed smaller mass loss rates compared to the comparative sample 200. Therefore, it is recognized that preservative performance is exhibited when the modified aqueous solution contains 1,3-propanediol and citric acid (sample 201) or 3-methyl-1,5-pentanediol and citric acid (sample 202). Furthermore, when using the giant quail mushroom strain, sample 202 showed an even lower mass loss rate. Therefore, Test 6 suggests that higher preservative performance can be ensured when 3-methyl-1,5-pentanediol is used in the modified aqueous solution.

[0087] [Exam 7] A weathering test was conducted to evaluate the weather resistance performance of the modified wood material. Weather resistance refers to the property of being resistant to changes caused by natural elements such as sunlight and wind and rain outdoors. As the wood material, sapwood of Japanese cedar measuring 150 mm in length, 15 mm in thickness, and 70 mm in width was prepared, and the measurements and treatments described below were performed. Details of the treatment and measurements are shown below. The polyol contained in the modified aqueous solution was 3-methyl-1,5-pentanediol (Sample 301). As a comparison, the same test was performed on an untreated sample (Sample 300) in which steps 30 to 32 were not performed.

[0088] (Step 30) An impregnation process with a modified aqueous solution was performed. The concentration of the modified aqueous solution was 20%. The impregnation process was carried out under the conditions of 30 minutes of reduced pressure impregnation at -0.08 MPa followed by 1 hour of pressurized impregnation at 1 MPa.

[0089] (Step 31) A drying process was carried out. The drying process was performed under the conditions of drying at 40°C for 1 day and drying at 60°C for 2 days.

[0090] (Step 32) A heat treatment process was carried out. The heat treatment process was performed at 120°C for 1 day. After that, it was left at room temperature for 1 week.

[0091] (Step 33) Apply urethane paint at a rate of 180g / m² 2 I painted it.

[0092] (Step 34) After drying, a weather resistance test was performed using a xenon weather meter (Suga Test Instruments NX75). The weather resistance test consisted of repeated cycles of water spraying and irradiation on the specimen surface for 36 minutes, followed by irradiation only for 84 minutes, for a total of 2000 hours. The irradiance was 120 W / m². 2 (300nm~400nm), black panel temperature set to 63°C.

[0093] (Step 35) The surface of the test specimen was measured for lightness L*, chromaticity a*, and b* using a colorimeter (Konica Minolta CR-410), and the color difference ΔE before and after the test was calculated using the following formula. The differences in L*, a*, and b* before and after the test are denoted as ΔL*, Δa*, and Δb*. ΔE=((ΔL*)^2+(Δa*)^2+(Δb*)^2)^1 / 2

[0094] A large value of ΔE indicates that the appearance of the wood material has changed significantly due to the watering and irradiation in step 34. The weather resistance was evaluated using the value of ΔE according to the following criteria. The results are shown in the table in Figure 8. Less than 5.00: Pass (Excellent, indicated by "◎" in the table) 5.00 or higher and less than 10.00: Pass (Good, indicated by "○" in the table) 10.00 or higher but less than 15.00: Fail (Poor quality, indicated by "△" in the table) 15.00 or higher: Fail (Poor quality, indicated by "×" in the table)

[0095] From the results of Test 7 shown in the tables in Figures 8 and 9, the ΔE of sample 300 is 15 or higher, resulting in a judgment of "poor." The ΔE of sample 301 is less than 5, resulting in a judgment of "good." Therefore, it was shown that sample 301 showed less discoloration compared to sample 300. Based on the above, Test 7 suggests that weather resistance can be imparted by using 3-methyl-1,5-pentanediol in the modified aqueous solution. [Industrial applicability]

[0096] This invention is useful for a method of producing modified wood materials.

Claims

1. An impregnation process in which a modified aqueous solution is impregnated into the wood material, A drying step for drying the wood material that has undergone the impregnation step, The process includes a heat treatment step of heat-treating and hardening the wood material that has undergone the drying step, The modified aqueous solution contains citric acid and an alkanediol having 3 to 8 carbon atoms and having only two primary hydroxyl groups.

2. The method for producing a modified woody material according to claim 1, wherein the alkanediol is 1,3-propanediol or 1,4-butanediol.

3. The method for producing a modified wood material according to claim 1, wherein the alkanediol is 1,5-pentanediol, 3-methyl1,5-pentanediol, or 1,6-hexanediol.

4. The method for producing a modified wood material according to claim 1, wherein the concentration of the modified aqueous solution is 10% or more and 40% or less.

5. A method for producing a modified wood material according to any one of claims 1 to 4, wherein the number of hydroxyl groups in the alkanediol in the modified aqueous solution is 0.6 or more and 2.5 or less, with the number of carboxyl groups in the citric acid being 1.

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