Method for heat treatment of wood and method for producing wood having dimensional stability and durability

Abstract

To provide a heat treatment method for lumber that allows lumber not only not to be decomposed by wood decay fungi but to have dimensional stability and high durability by producing a compound such as furan exhibiting an antibacterial property by itself as exhibiting more resistivity more than that of lignin through decomposition and transformation of hemicellulouse susceptible to decomposition by wood decay fungi and like and an amorphous region of cellulose.SOLUTION: A heat treatment method comprises: a step for immersing lumber in a diluted solution of salt generating inorganic acid by decomposition through decomposition and sublimation treatments of all amount heated at 200°C or less, or coating, spraying, and impregnating the solution on the lumber; and a step for heating the lumber immersed in the solution, or coated, sprayed, and impregnated with the solution at 120 to 200°C for 72 hours or less, wherein the solution is an ammonium chloride solution with a concentration of 0.1 to 5.0 wt.%, in which a weight loss after the heat-treatment step is 3% or more to 20% or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a heat treatment for imparting dimensional stability and durability to wood, and to the wood subjected to this heat treatment.

Background Art

[0002] When using wood in a harsh environment such as outdoors, drawbacks such as "rotting (deterioration by wood-decaying fungi)" and "being damaged by termites" become problems. Conventionally, it has been common to apply and impregnate a wood preservative to wood to deal with deterioration by wood-decaying fungi and damage by termites. However, due to the increasing awareness of the user side regarding the environment and health, the development of technologies for enhancing the durability of wood without relying on chemicals is desired. Furthermore, in a harsh environment such as outdoors, wood may develop large warps or cracks, which often necessitate repair or replacement in a short period of time.

[0003] In recent years, in Europe such as the Netherlands and Finland, heat treatment technologies for the purpose of improving dimensional stability, durability of wood, etc. have been developed and are sold under the product names Plato wood and Thermo wood (registered trademark), respectively. Furthermore, in Japan as well, a heat treatment technology called Estek treatment has been developed and put into practical use.

[0004] The main components constituting wood are cellulose, hemicellulose, and lignin. In heat treatment, among these wood components, the hemicellulose with low durability (easily decomposed by wood-decaying fungi and termites) and the non-crystallized part (amorphous region) of cellulose are decomposed and removed, and at the same time, a part of them is transformed into a component with high durability, so it is considered that the durability is improved. In addition, to prevent warping and cracking, it is essential to reduce dimensional changes (swelling and shrinkage) associated with moisture content fluctuations, that is, to impart dimensional stability to the wood. It is considered that the swelling and shrinkage of wood are largely due to the amorphous regions of hemicellulose and cellulose. By decomposing and removing them, warping and cracking can be prevented. Furthermore, the modified product is known to be hydrophobic, and the modification caused by heat also contributes to dimensional stabilization. From a practical perspective, in order to decompose and remove the amorphous regions of hemicellulose and cellulose while simultaneously modifying them, heating at 200°C or higher and 300°C or lower, preferably 220°C or higher and 250°C or lower, was essential for any heat treatment technology developed so far. Since there is a risk of ignition in that temperature range, it is essential to prevent combustion, such as heating under an inert gas with air excluded. Incidentally, the upper limit of the treatment temperature was specified because, at temperatures higher than that, not only the amorphous regions of hemicellulose and cellulose but also the crystalline regions of cellulose and lignin that guarantee the strength of the wood are significantly decomposed, and the wood becomes extremely vulnerable. Therefore, in order to perform a more preferable heat treatment with the prior art, it has been required to perform fairly strict temperature control in the presence of an inert gas.

[0005] Treatment using nitrogen as the inert gas is disclosed in Japanese Patent Application Laid-Open No. 56-135004, treatment using superheated steam is disclosed in Japanese Patent Application Laid-Open No. 09-502508, and heat treatment in supercritical carbon dioxide is disclosed in Japanese Patent Application Laid-Open No. 2013-180460, respectively.

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Disclosure of the Invention

Problems to be Solved by the Invention

[0007] The inventor used sugi sapwood as a test material and conducted heat treatment using superheated steam described in Patent Document 2, and evaluated the resistance to wood-decaying fungi based on JIS K 1571. As a result, to exhibit high performance comparable to treatment with preservatives (in other words, to satisfy "the weight loss rate due to decomposition by wood-decaying fungi is 3% or less" described in the standard), a considerable proportion of the hemicellulose contained in the wood had to be decomposed and removed, and a part of it had to be transformed, reducing the weight of the wood by 15% or more, desirably about 18% on a dry weight basis. For this, treatment at 240°C for 8 hours or more was essential. At 200°C for 72 hours or 220°C for 24 hours, this level could not be reached, and further extension of the treatment time was required. This was not only true for superheated steam treatment, but also for any of the above-mentioned treatments, a treatment at a temperature exceeding 200°C for a considerable time was essential for the expression of high durability. That is, in addition to the excessive energy required for the treatment and the difficulty of temperature control to keep the temperature in the apparatus uniform or maintain a predetermined material temperature, special equipment was required because ignition would occur unless the treatment was carried out in a state filled with an inert gas such as nitrogen or steam. For this reason, heat-treated wood by the conventional method had to be expensive. Furthermore, for any of the above-mentioned treatments, there was concern about a significant decrease in physical properties such as strength due to heat treatment.

[0008] The outline of an indoor test (hereinafter referred to as "indoor bottle test") using a culture bottle based on JIS K 1571 "Wood preservatives - Performance and test methods thereof" is shown. This standard is a method for evaluating wood preservatives. In order to observe the reduction in antiseptic efficacy due to the leaching of the chemical from the wood, a "weathering operation" consisting of repeating the process of immersing a wood test piece in water and the drying process 10 times is carried out. After that, the wood test specimens are forcibly decomposed by decay using Oudemansiella mucida and Trametes versicolor as test fungi over a period of 12 weeks, and the weight loss rate at that time is examined. If the weight loss rate is 3% or less, it is judged that the durability is sufficient and it is determined that "the antiseptic efficacy is present". This standard is considered to be the optimal method as a way to confirm that durability, particularly resistance to wood decay fungi (hereinafter referred to as "decay resistance"), has been manifested by heat treatment.

[0009] When the present inventors used sugi sapwood as the wood to be treated and performed heat treatment under various temperature and time conditions according to Patent Document 2 and evaluated its decay resistance in an indoor bin test, a clear relationship was recognized between the weight loss rate caused by the heat treatment and the decay resistance (weight loss rate due to decay in the indoor bin test). If the weight loss rate due to the heat treatment is 15% or more in the case of Trametes versicolor and 18% or more in the case of Oudemansiella mucida, the weight loss rate due to decay in the indoor bin test becomes 3% or less (see Figure 1). However, this criterion (3% or less) is an essential condition for using wood over a long period of time in places where rain directly falls and water accumulates, such as a wood deck. Under eaves or on the outer walls of buildings where water does not accumulate, if the weight loss rate due to wood decay fungi is generally 10% or less, a sufficient service life can be ensured. Therefore, when the inventors proceed with research aimed at imparting decay resistance to wood, the target values for the weight loss rates of Oudemansiella mucida and Trametes versicolor in the indoor bin test are set at 10% or less, preferably 3% or less. On the one hand, the heat treatment in Patent Document 2 is effective for imparting dimensional stability. As shown in Fig. 2, as the weight loss rate associated with the heat treatment increases, the dimensional change associated with moisture absorption decreases. The swelling rate of untreated wood is approximately 10%, but based on the inventor's experience, if the swelling rate can be suppressed to 7%, preferably 5%, the occurrence of warping and cracking during outdoor use will be eliminated or, if they do occur, will be minor. According to Fig. 2, to suppress the swelling rate of untreated wood to 7% or 5%, the weight loss rate associated with the heat treatment needs to be 3% or 5% respectively.

[0010] The present inventor has been conducting research on the heat treatment of wood with superheated steam for many years. During that time, when attempting to reduce the heat treatment temperature under various conditions, it was found that by impregnating a dilute aqueous solution of a salt that exhibits almost neutral to weakly acidic properties at room temperature before heating, a weight loss rate of 15% or more can be obtained even when heating at 200°C or lower (see Japanese Patent Application Laid-Open No. 2018-161802).

[0011] Now, aiming at the practical application of the technology disclosed in Japanese Patent Application Laid-Open No. 2018-161802, as a result of conducting detailed research under the conditions shown in the same published gazette, it was suggested that in this technology, when the cross-sectional dimension of the wood to be treated becomes larger than a certain size, the following problems occur and practical application is difficult. Speculating from Fig. 1, aiming at a weight loss rate of 13% or more by superheated steam treatment, which is the minimum required for the manifestation of decay resistance, Japanese cedar sapwood with a cross-sectional dimension of 30 mm square was used as the test material, and an aqueous solution of magnesium chloride with a concentration of 0.5 to 1.5% described in the same published gazette was used. When heat treatment was carried out at 160 to 180°C for 24 to 72 hours, it became clear that there was a significant difference in the degree of heat decomposition and transformation between the surface and the center of the material. That is, near the surface, the decomposition and transformation of the wood due to heating progressed excessively, and a brittle surface layer like "charcoal" was observed in the treated wood. On the other hand, in the center of the wood, the decomposition and transformation to the extent that decay resistance is improved did not progress.

[0012] As a result of various studies on the causes of such non-uniform decomposition and transformation by heat treatment, it was found that even if the wood is pre-impregnated with a dilute aqueous solution of the above-mentioned salt (an aqueous solution of magnesium chloride with a concentration of 0.5 to 1.5%), when drying, magnesium chloride dissolved in water also moves as water moves from the inside to the surface of the wood. As a result, the amount of magnesium chloride remaining in the central part decreases, while on the other hand, an excessive amount of magnesium chloride accumulates near the surface, leading to non-uniformity. This led to the conclusion that this is the cause of the non-uniformity.

[0013] As a result of intensive research on a method for reducing the deviation of salt concentration in the wood to be treated and preventing non-uniform decomposition and transformation by heat treatment, the present invention has been achieved. According to the present invention, since a special device for using an inert gas is not required and temperature control is easy, deterioration of physical properties such as strength due to heat is reduced. As a result, heat-treated wood of uniform quality can be provided at low cost.

[0014] The present invention was devised in view of the above circumstances, and at a heat treatment temperature of 200 ° C or lower, preferably 180 ° C or lower, while suppressing deterioration of physical properties such as strength compared to conventional heat-treated wood, dimensional stability and high Durability to Cause to appear can be achieved, and moreover, reduction of energy required for heat treatment is achieved, temperature control is facilitated, and treatment under conditions where oxygen is present is enabled, and a heat treatment method for wood that does not require the use of a special device, Method for producing wood having dimensional stability and durability and is provided.

Means for Solving the Problems

[0015] The heat treatment method for wood according to the present invention is a solution in which 0.1 to 5.0% by weight of ammonium chloride as a salt that decomposes and sublimes in total amount by heating at 200 ° C or lower and generates an inorganic acid by the decomposition is dissolved in Ammonium chloride aqueous solution immersing the wood, or applying, spraying, or impregnating the wood, and the above-mentioned Ammonium chloride aqueous solution heating treatment is performed on the wood immersed, or applied, sprayed, or impregnated, 140℃~180℃ and Be equipped with is carried out, The inorganic acid generated by the heat treatment decomposes and transforms the amorphous regions of hemicellulose and cellulose in the wood to generate furan compounds.

[0016] The dilute aqueous solution of salt used in the heat treatment method of wood according to the present invention is an aqueous ammonium chloride solution with a concentration of 0.1 to 5.0% by weight.

[0017] The wood treated by the heat treatment method of wood according to the present invention has a weight loss rate after the heating step of 3% or more and 20% or less.

[0018] In addition, the treatment time of the heat treatment method of wood according to the present invention is 140~180℃ within 72 hours after reaching

Advantages of the Invention

[0019] According to the heat treatment method of wood according to the present invention, hemicellulose, which is easily decomposed by wood-decaying fungi and the like, and the amorphous region of cellulose are decomposed and transformed to show resistance higher than that of lignin. It is not only not decomposed by wood-decaying fungi, but also becomes a compound such as furan that exhibits antibacterial properties by itself. As a result, the wood can be made into highly durable wood with high decay resistance. In addition, these furan compounds are hydrophobic and do not swell due to water, so they also contribute to the improvement of dimensional stability. Moreover, for improving the performance of wood by heat treatment, it has the merit that it does not require a high temperature or a complicated apparatus that was necessary in the conventional method.

Brief Description of the Drawings

[0020]

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Embodiments for Carrying Out the Invention

[0021] The heat treatment method for wood according to an embodiment of the present invention will show the embodiments for carrying out the present invention below, but this is only an example and does not limit the scope of the present invention. As a salt that decomposes and sublimes in its entirety and generates an inorganic acid by heating at 200°C or lower, ammonium chloride can be cited as a representative. According to the Shukusho Kagaku Jiten (published by Kyoritsu Shuppan Co., Ltd., 4-6-19 Kojimachi, Bunkyo-ku, Tokyo, issued on February 15, 1987), ammonium chloride is said to decompose and sublime around 335°C and dissociate into ammonia and hydrogen chloride. However, when the inventor investigated using a thermogravimetric differential thermal analyzer, it was found that the entire amount of ammonium chloride used in the test decomposed and sublimed in 10 hours at 160°C and in 3 hours at 180°C (see Figure 3).

[0022] When modifying wood by heat treatment at 120 to 200°C, no significant decomposition, sublimation, or evaporation occurs until reaching that temperature range. By using a chemical agent that decomposes, sublimes, or evaporates in its entirety within the range of the treatment time after reaching that temperature range, it is possible to prevent the accumulation of the chemical agent near the wood surface as described above. That is, when drying wood impregnated with an aqueous solution of a chemical agent that sublimes, decomposes, or evaporates at 200°C or lower, part of the chemical agent moves near the surface along with the movement of water, and the concentration near the surface increases. However, during the heat treatment at 200°C or lower for the purpose of subsequent modification, due to decomposition and sublimation, the concentration near the surface decreases, preventing the chemical agent concentration from rising more than necessary. As a result, non-uniform decomposition and transformation due to heat treatment between the inside and near the surface of the wood are reduced, enabling uniform heat treatment.

[0023] Based on such a development concept, differential thermal analysis was carried out using sugi sapwood impregnated with a dilute aqueous solution of ammonium chloride, which was found to decompose and sublime completely at a temperature of 200°C or lower, as the test material. As a result, it became clear that in the temperature range of 200°C or lower, its weight decreased and thermal decomposition was progressing. This is considered to be the result of the promotion of the thermal decomposition of wood by hydrogen chloride generated by the thermal decomposition of ammonium chloride. On the other hand, in the control group (untreated sugi sapwood test specimen) that was not impregnated with such an aqueous solution, almost no weight loss was observed, and as conventionally stated, almost no thermal decomposition occurred in the temperature range of 200°C or lower (see Figure 4).

[0024] As described above, it became clear that thermal decomposition was progressing even in the region of 200°C or lower in the differential thermal analysis of sugi sapwood impregnated with a dilute aqueous solution of ammonium chloride. As a characteristic at that time, no heat balance was observed (see Figure 4). Generally, when wood thermally decomposes, heat is generated. In the heat treatment at 200°C or higher carried out in the previous Patent Document 2, if the amount of heat energy added becomes slightly excessive, self-heating will cause the temperature control to become impossible, and the temperature will rise to a region where the decomposition of the entire wood component is involved, and carbonization will progress, making the wood extremely vulnerable. On the other hand, when endothermic occurs, additional energy is required to maintain the temperature at which decomposition and transformation occur. Furthermore, it goes without saying that temperature control becomes difficult when there is a heat balance. The fact that decomposition and transformation that can impart high decay resistance and dimensional stability to wood can be achieved without a heat balance also means that treatment in the atmosphere in which oxygen is present becomes possible, which is very significant.

[0025] Specific names of salts that decompose by heating to generate inorganic acids are described in the previous Japanese Patent Application Laid-Open No. 2018-161802. For example, magnesium chloride, copper sulfate, ammonium sulfate, etc. Although heat treatment using magnesium chloride, copper sulfate, and ammonium sulfate can also promote the decomposition and transformation of wood, as a result of differential thermal analysis, it was confirmed that the entire amount did not decompose and sublime at temperatures below 200°C, and a considerable amount remained. In addition, with these salts, when the material size becomes large, the degree of decomposition and transformation differs greatly between the vicinity of the surface and the vicinity of the center, and as previously described, the treatment becomes non-uniform.

[0026] As described so far, the degree of decomposition by heat treatment can be known from the magnitude of the weight loss rate accompanying the heat treatment of wood. On the other hand, which components of wood are decomposing can be known from the analysis results of the components of wood and differential heat. In order to impart high decay resistance and dimensional stability to wood while minimizing the decrease in physical properties such as strength, it is desirable to cause decomposition mainly in the amorphous regions of hemicellulose and cellulose among lignin, cellulose, and hemicellulose, which are the main components of wood. As a result of heat-treating sugi sapwood impregnated with a dilute aqueous solution of ammonium chloride at 150 to 180°C, while there was almost no change in lignin, it became clear that hemicellulose, which is easily decomposed by wood-decaying fungi and the like and is involved in the swelling and shrinkage of wood, and the amorphous region of cellulose decomposed. The amorphous regions of hemicellulose and cellulose are components that exhibit the flexibility of wood, and although it is inevitable that the amount of deflection of the wood decreases (a little flexibility is lost) due to the heat treatment of the present invention, on the other hand, since these components do not have a great influence on the breaking strength, it is possible to minimize the decrease in physical properties such as strength.

[0027] Regarding transformation by heat, it can be estimated from the component analysis of wood and the analysis results by FT-IR etc. Lignin contained in wood can be quantified by the Klason method. That is, the test wood powder is put into sulfuric acid with a concentration of 72%, stirred at room temperature for 4 hours, then water is added thereto and diluted to a concentration of 3%, boiled for 4 hours, and the insoluble part is filtered under reduced pressure with a glass filter, dried and quantified. In this method, among the wood components, cellulose and hemicellulose are hydrolyzed by acid and become soluble in water, while lignin is condensed by acid and insolubilized.

[0028] When the Japanese cedar sapwood impregnated with a dilute aqueous solution of ammonium chloride was heat-treated at 150 to 180 °C and then the Klason method was carried out, the components insoluble in acid increased sharply. Also, when a cellulose filter paper impregnated with a dilute aqueous solution of ammonium chloride was heat-treated at 170 °C and the Klason method was carried out, the amorphous region of cellulose was decomposed and transformed, and components insoluble in acid were observed, and as a result of FT-IR analysis, they were suggested to be furan compounds. Even in the case of the heat-treated wood specimen, among the acid-insoluble components quantified as Klason lignin, the increase due to heat treatment is considered to be furan compounds transformed from hemicellulose and the amorphous region of cellulose. It has been a common theory that furan compounds are generated by heat-treating wood at a temperature exceeding 200 °C, but it has been shown that similar transformation also occurs even with a treatment at 200 °C or lower by using an aqueous ammonium chloride solution. Furthermore, in the conventional heat treatment without using salt, the increase in the acid-insoluble components, that is, the furan compounds generated by heat treatment, was at most 4% even when estimated highly, whereas in the heat treatment at 150 to 180 °C using the aqueous ammonium chloride solution according to the present invention, the increase rate sometimes reached 10% or more. It was suggested that using the aqueous ammonium chloride solution promotes decomposition by heating and also promotes transformation into furan compounds.

[0029] Among the main components of wood (cellulose, hemicellulose, and lignin), the component most resistant to wood-decaying fungi is lignin. However, according to the present invention, a large amount of furan compounds generated by the transformation from the amorphous region of cellulose and hemicellulose during the heat treatment exhibit resistance superior to that of lignin. They are not only not decomposed by wood-decaying fungi but are also said to exhibit antibacterial properties. That is, decomposing the hemicellulose and the amorphous region of cellulose, which are weak against wood-decaying fungi among the wood components, and simultaneously generating as many furan compounds as possible from them can be said to be very effective for enhancing the durability of wood. As described above, in the case of heat-treated wood produced by the method of Patent Document 2, which is a conventional heat treatment, in order to impart decay resistance satisfying the JIS K 1571 standard to sugi sapwood, it was necessary to reduce its weight by 18% by heat treatment, whereas in the present invention, decay resistance satisfying the same standard was obtained with a weight reduction of about 13%. Also, in order to suppress the weight loss rate by the indoor bottle test using Pleurotus ostreatus to about 10%, a weight loss rate of 13% was required in the conventional heat treatment without using inorganic salts, whereas in the present invention, it was found that equivalent decay resistance was exhibited with a weight loss rate of about 10%. On the other hand, as will be described later, regarding the imparting of dimensional stability, almost the same results as the relationship between the weight loss rate and the swelling rate when heat treatment was performed without using the conventional inorganic salts shown in FIG. 2 were also obtained in sugi sapwood specimens impregnated with an aqueous ammonium chloride solution having a concentration of 0.2 to 2.5% and heat-treated at 120 to 170°C for 1 to 24 hours.

[0030] Analysis by the Klason method was also performed after performing the same heat treatment on sugi sapwood impregnated with a dilute aqueous solution of salts described in the previous Japanese Unexamined Patent Application Publication No. 2018-161802, such as magnesium chloride and copper sulfate. However, no significant increase in furan compounds was confirmed as in the heat treatment using ammonium chloride according to the present invention.

[0031] Hereinafter, the procedure for actually treating wood according to the present invention will be described. An aqueous ammonium chloride solution adjusted to a concentration of 0.1 to 5% is applied to wood by coating, spraying, impregnation, or immersion in an aqueous salt solution to penetrate into the wood. If the concentration of the aqueous ammonium chloride solution used at this time is less than 0.1%, it is too dilute to expect its effect. On the other hand, it is clear from the research results that using an aqueous ammonium chloride solution with a concentration higher than 5% does not enhance the effect of promoting decomposition and transformation, or there is a risk that ammonium chloride will remain in the wood after treatment. As a method for reliably impregnating an aqueous ammonium chloride solution into wood, the pressure injection method is effective. To give an example, place the wood to be treated in a pressure-resistant container made of stainless steel, and tie and fix it with a weight or a rope so that it does not float. Close the lid of the pressure-resistant container, and reduce the pressure inside the container with a vacuum pump. For example, maintain a reduced pressure state of about 50 to 100 hPa for 30 minutes to 4 hours. Then, using the pressure difference between the inside and outside of the pressure-resistant container, pour the aqueous ammonium chloride solution into the container, and further fill the inside of the container with the aqueous ammonium chloride solution as much as possible by using a liquid feed pump or the like. Subsequently, use a pressure-resistant pump such as a plunger pump to send the aqueous ammonium chloride solution into the container to create a pressurized state of 0.5 to 1.5 MPa, maintain this state for 1 to 24 hours, and then decompress. Reverse the rotation of the liquid feed pump to recover the excess aqueous ammonium chloride solution, and finish the injection operation. After the liquid recovery, it may also be necessary to perform an operation of reducing the pressure with a vacuum pump to recover the excess aqueous ammonium chloride solution in the permanent voids in the wood.

[0032] Another example of the pressure injection method is as follows. Put the wood to be treated into a stainless steel box-shaped container, tie and fix it with weights or ropes so that it does not float, and then pour an ammonium chloride aqueous solution until the wood is fully immersed in the ammonium chloride aqueous solution. Then, put the box-shaped container into a pressure-resistant container, close the lid of the pressure-resistant container, and reduce the pressure inside the container with a vacuum pump. For example, maintain a reduced pressure state of about 50 to 100 hPa for 30 minutes to 4 hours. Then, use a compressor or the like to put compressed air into the pressure-resistant container to bring it to a pressurized state of 0.5 to 1.5 MPa, maintain that state for 1 to 24 hours, and then decompress. After recovering the ammonium chloride aqueous solution, or taking out the wood directly from the liquid, the injection operation is completed.

[0033] Dry the wood impregnated with the ammonium chloride aqueous solution by a general wood drying method. Natural drying is also one of the means, but it is more desirable to use artificial drying using steam or the like to make the moisture content as low as possible and then subject it to the subsequent heat treatment process, as it is more efficient. Here, dry the wood until there is no free water in it or below that state. Also, it is possible to perform this drying process and the subsequent heating process using the same device.

[0034] For the heating process, it is possible to use a wood drying device capable of high-temperature drying, or a dedicated heat treatment device that can maintain a state close to superheated steam, nitrogen gas replacement, or vacuum, and can create an oxygen-free or low-oxygen state. After putting the wood to be treated that has completed the above-mentioned drying process into the device, it is placed for a certain period of time, for example, 1 hour to 72 hours, in an environment of 120°C or higher and 200°C or lower, preferably from 140°C to 180°C, to promote the decomposition and transformation of the amorphous regions of hemicellulose and cellulose among the components of the wood. When exceeding 200°C, as described above, there is a risk of ignition unless it is treated in a state filled with an inert gas. On the other hand, when it is less than 120°C, almost no decomposition occurs. The temperature range where there is no risk of ignition even without using an inert gas and the decomposition proceeds moderately is from 140°C to 180°C. When the main purpose is the expression of decay resistance, the weight loss that occurs at this time is preferably in the range of 10% or more and 20% or less, and more preferably 12% or more and 15% or less. As will be described later, in the heat treatment using ammonium chloride, decomposition and transformation occur soon after reaching a certain temperature. However, after the ammonium chloride in the wood decomposes and disappears even if the treatment is carried out for 72 hours or more, the effect cannot be expected much. Also, when the main purpose is to impart dimensional stability, the weight loss rate associated with the treatment is preferably 3% or more and 20% or less, and more preferably 5% or more and 15% or less. The upper limit is set here in consideration of the deterioration of the mechanical properties of the wood associated with the treatment.

[0035] Using 5 mg of ammonium chloride (Wako Pure Chemical Reagent Special Grade), the weight loss behavior was examined with a thermogravimetric differential thermal analyzer at a constant temperature of 160°C, 170°C, and 180°C. It was revealed that all the tested ammonium chloride decomposed and sublimated in 10 hours at 160°C, 5 hours at 170°C, and 3 hours at 180°C (see Figure 3).

[0036] For the wood (test specimen) to be treated, sugi sapwood cut into a size of 20 mm in the tangential direction (T) × 20 mm in the radial direction (R) × 10 mm in the longitudinal direction (L) in the air-dried state was dried to a completely dry state at 105°C, and 24 pieces were used for each test condition. An ammonium chloride aqueous solution with a concentration of 0 to 2.5% was used as an aqueous salt solution. The specimen was placed in a stainless-steel vat, and after placing a stainless-steel weight on it, a sufficient amount of the above ammonium chloride aqueous solution was poured and submerged in the ammonium chloride aqueous solution. The vat was placed in a pressure injection can, degassed with a vacuum pump, and left under a reduced pressure of approximately 50 hPa for 1 hour, then under a pressure of 1.3 MPa for 2 hours using a compressor, and further left in the liquid for one day and night after decompression. Then, the specimen was taken out from the ammonium chloride aqueous solution, the impregnation amount was measured, and after that, it was dried in a forced-air dryer at 60 °C for one day, and then the temperature was raised to 105 °C and dried for one day to obtain a completely dry state. After measuring the total dry weight (W1), it was placed in a heat treatment device filled with superheated steam, and the device was adjusted so that the material temperature became 150 °C, 160 °C, 170 °C, and 180 °C, and a 24-hour treatment was performed. After that, it was taken out when the temperature reached 150 °C or lower, and the weight (W2) in the completely dry state was measured. The weight loss rate due to heat treatment was determined by the formula (W1 - W2) / W1 × 100. As shown in Fig. 5, the weight loss rate when heat treatment was performed without using an ammonium chloride aqueous solution was about 2 to 3%, while the weight loss rate clearly increased by using an ammonium chloride aqueous solution. By treating an impregnating material with a concentration of 1.5% or more at 160 to 180 °C, a weight loss rate of 12 to 15% could be obtained. Furthermore, using Japanese cedar sapwood impregnated with an ammonium chloride aqueous solution with a concentration of 0.2 to 1.0% as a specimen in the same way, Fig. 6 shows the relationship between the treatment time and the weight loss rate due to heat treatment when heat treatment was performed at 170 °C for 24 to 72 hours. For the 1.0% concentration impregnating material, the decomposition of ammonium chloride occurred rapidly within 24 hours, and the weight did not decrease further even if the treatment time was extended. Also, for the more dilute 0.2% and 0.5% concentration impregnating materials, the weight decreased to some extent until 48 hours, but then stopped at a weight loss rate of about 1% even if the treatment was extended to 72 hours, and it was shown that no further weight loss could be obtained even if heat treatment was performed for more than 72 hours. Also, it was clarified that ammonium chloride did not exist in the specimens subjected to 72-hour heat treatment and had decomposed and sublimated within 72 hours.

[0037] Six of the 24 specimens that had undergone the above-described 24-hour heat treatment were pulverized. After extraction with cold water, the acid-insoluble content was quantified by the Klason method. As shown in Fig. 7, in the treatment without using an aqueous ammonium chloride solution, the increase in the acid-insoluble content was less than 3%, whereas when an aqueous ammonium chloride solution was used, an increase of up to 10% or more was observed.

[0038] An indoor bottle test was conducted using the remaining 18 specimens out of the above-described 24 specimens (9 specimens each for Pleurotus ostreatus and Trametes versicolor). Fig. 8 shows the weight loss rate after the indoor bottle test. For Trametes versicolor, when only the specimens treated with an aqueous ammonium chloride solution at a concentration of 1.5% or more were used, the weight loss of the untreated specimens was 44%, whereas the weight loss rate of the heat-treated specimens was less than 0.5% under any conditions, indicating that high decay resistance was imparted to Trametes versicolor. When this result is considered together with Fig. 5, for Trametes versicolor, if the weight loss rate associated with the heat treatment was generally 10% or more, the performance standard specified in JIS K 1571, "the weight loss rate due to decomposition by wood-decaying fungi is 3% or less," was satisfied. When heat treatment was performed according to Patent Document 2, a weight loss rate of about 15% was required, and in the heat treatment using ammonium chloride, high decay resistance was exhibited in a region where the weight loss rate associated with the heat treatment was about 5% lower than when it was not used. On the other hand, for the Japanese common mushroom, the weight loss rate exceeded 55% in the untreated specimens, whereas sufficient decay resistance to meet the performance criteria was observed in specimens impregnated with an aqueous ammonium chloride solution with a concentration of 2.5% in heat treatment at 150 to 160°C and in specimens impregnated with an impregnating material with a concentration of 1.5% in heat treatment at 170 to 180°C. Combining this result with the result of Fig. 5, it was confirmed that high decay resistance to the Japanese common mushroom is surely exhibited if the weight loss rate due to heat treatment is 13% or more. When heat treatment was performed according to Patent Document 2, a weight loss rate of about 18% was required, and for the Japanese common mushroom as well, the weight loss rate associated with heat treatment could be reduced by about 5%. Thus, by using a dilute aqueous ammonium chloride solution, decay resistance was exhibited where the weight loss rate of the wood was small, in other words, where the deterioration of the wood due to heat was small.

[0039] Using Japanese cedar sapwood specimens with a cross-sectional dimension of 30 mm square and a length of 70 mm, after impregnating them with an aqueous ammonium chloride solution with a concentration of 1.5 to 2.5% by a pressure injection method, drying was carried out at 105°C, and heat treatment was performed at 170°C for 24 hours. Along with the measurement of the weight loss rate, visual observation and palpation of the surface and cross section were performed. As a result, the weight loss rate associated with heat treatment was about 15%, and no particularly weakened layer was observed by visual inspection or palpation. As a result of cutting near the center and visually observing the difference between the center part and the vicinity of the surface, the difference in the wood color between the center part and the surface layer was slight, indicating that the heat treatment was performed almost uniformly (see Fig. 9).

[0040] Filter paper with more than 99% α-cellulose was immersed in an aqueous ammonium chloride solution with a concentration of 2.5%. After gently wiping the surface of the filter paper to remove excess liquid, it was dried at 105 °C and then heat-treated at 170 °C for 2 hours using a general air dryer. The weight loss rate at that time was approximately 8%. When the same filter paper was heated at 170 °C for 2 hours without being immersed in the aqueous ammonium chloride solution, the weight loss rate was about 1%. It was revealed that the thermal decomposition progressed by using the aqueous ammonium chloride solution. Also, as a result of analyzing it by the Klason method, the acid-insoluble content was 7%. Originally, cellulose is acid-soluble. The acid-insoluble content of cellulose filter paper by the Klason method is less than 1%. It was found that a considerable amount of components that become insoluble in acid are generated by thermal decomposition from cellulose. As a result of analysis by FT-IR, the insoluble matter was suggested to be a furan compound.

[0041] In the detailed study of the thermal decomposition and transformation of wood by an aqueous ammonium chloride solution, the results of thermogravimetric differential thermal analysis using sugi sapwood impregnated with an aqueous ammonium chloride solution with a concentration of 2.5% and sugi sapwood not impregnated as specimens are shown in Fig. 10. This figure shows the relationship between the weight loss of the specimen and the temperature when the temperature is raised from room temperature to 500 °C at a rate of 10 °C / min. As a result, in the specimen impregnated with the aqueous ammonium chloride solution, weight loss considered to be due to the decomposition of hemicellulose and a part (amorphous region) of cellulose was observed in the temperature range of 150 to 200 °C where almost no thermal decomposition occurs in the non-impregnated specimen. On the other hand, the crystalline region and lignin that occupy most of the cellulose are not decomposed in this temperature range, similar to the non-impregnated specimen. Also, the thermal decomposition promoted by ammonium chloride is limited to hemicellulose and cellulose that are easily decomposed by wood-decaying fungi. Amorphous region This suggests that it is one of the factors contributing to the manifestation of decay resistance even in the temperature range with a small weight loss due to heat treatment. On the other hand, with respect to magnesium chloride, ammonium sulfate, and ammonium phosphate, which is similar to them and is described in the previously mentioned Japanese Patent Application Laid-Open No. 2018-161802 and has the potential to accelerate the thermal decomposition of wood, the thermal decomposition of the entire hemicellulose and cellulose is promoted, and there is a risk of considerable thermal deterioration in imparting decay resistance to wood.

[0042] Regarding magnesium chloride, ammonium sulfate, and ammonium phosphate, which are described by specific names in the previously mentioned Japanese Patent Application Laid-Open No. 2018-161802 and have the potential to accelerate the thermal decomposition of wood, a thermogravimetric differential thermal analyzer was used to raise the temperature from room temperature to 500°C at a heating rate of 10°C / min to examine the weight loss due to thermal decomposition. The results are shown in Fig. 11. Even when the temperature was raised to 500°C, the weight loss of ammonium phosphate remained at about 30%, and that of magnesium chloride remained at about 70%. None of the chemicals decomposed, sublimated, and disappeared completely. Although all of ammonium sulfate sublimated, the temperature range was much higher than that of ammonium chloride. On the other hand, ammonium chloride started to decompose at around 120°C and completely decomposed at around 275°C. Fig. 12 shows the weight loss rate of the filter paper itself at 275°C when a filter paper containing more than 99% α-cellulose was impregnated with an aqueous ammonium chloride solution with a concentration of 0 to 5.0%, the surface of the filter paper was gently wiped to remove the excess liquid, and then dried at 105°C using a forced-air dryer, and the temperature was raised from room temperature to 275°C at a heating rate of 10°C / min using a thermogravimetric differential thermal analyzer. From this result, it was found that even when impregnated with ammonium chloride at a concentration exceeding 5.0%, no further effect of promoting decomposition was obtained.

[0043] As the wood (specimen) to be treated, sugi sapwood cut into a size of 20 mm in the tangential direction (T) × 20 mm in the radial direction (R) × 10 mm in the longitudinal direction (L) in the air-dried state was used after being dried to a completely dry state at 105°C. An ammonium chloride aqueous solution with a concentration of 0 to 2.5% was used as an aqueous solution of salt. The test specimen was placed in a stainless-steel vat, and after placing a stainless-steel weight on it, a sufficient amount of the above ammonium chloride aqueous solution was poured and immersed in the ammonium chloride aqueous solution. The vat was placed in a pressure injection can, degassed with a vacuum pump, kept under a reduced pressure of approximately 50 hPa for 1 hour, then under a pressure of 1.3 MPa for 2 hours using a compressor, and further left in the liquid for one day and night after decompression. After taking out the test specimen from the ammonium chloride aqueous solution, measuring the impregnation amount, it was dried in a forced-air dryer at 60°C for one day, and then the temperature was raised to 105°C and dried for one day to obtain a completely dry state. Figure 13 shows the relationship between the concentration and the weight loss rate when the ammonium chloride-impregnated test specimens prepared in this way and the test specimens impregnated with a magnesium chloride aqueous solution (concentration 0 to 1.0%) in the same method were subjected to superheated steam treatment at 180°C. In both cases, the weight loss rate increased as the concentration increased. Also, the weight loss rates at their respective concentrations were of the same degree, and thermal decomposition occurred equally.

[0044] Among the test specimens subjected to the above 24-hour superheated steam treatment, test specimens with an ammonium chloride aqueous solution concentration of 0.2 to 1.0%, a heat treatment temperature of 160°C, and a weight loss rate due to heat treatment in the range of 3 to 10%, and untreated sugi sapwood as a control material were used to determine the swelling rate in a saturated water state. The weight loss rates due to heat treatment of the test specimens for which the swelling rate was determined were as shown in Figure 5, approximately 4% for the aqueous solution with a concentration of 0.2%, 6% for 0.5%, and 9% for 1.0%. As a result, while the swelling rate of the untreated sugi wood at the end grain surface was approximately 10%, the swelling rates of the test specimens with weight loss rates due to heat treatment of 4%, 7%, and 9% were 5.0%, 4.7%, and 4.5% respectively, and a dimensionally stable effect sufficient for practical use was obtained.

[0045] In addition to the above example (heat treatment of the injection material with a concentration of 0.2% at 160°C for 24 hours), as an example of the conditions satisfying a weight loss rate of 3 to 5% by heat treatment, in the sugi sapwood cut and processed in the tangential direction (T) of 30 mm × radial direction (R) of 30 mm × longitudinal direction (L) of 6 mm, after impregnation with an aqueous ammonium chloride solution with a concentration of 0.2%, it was 170°C for 4 hours, 0.5%, 170°C for 1 hour, 2.5%, 140°C for 1 hour, 2.0%, 120°C for 24 hours, etc. (see Table 1). Here, the purpose and method of the experiment in Table 1 will be explained. The heat treatment using wood specimens other than this test was carried out under superheated steam filling without the presence of oxygen. The reason is that there was a concern that the specimen would catch fire and cause a fire during the heat treatment. However, as is clear from Figure 4, when ammonium chloride was used, no heat generation due to thermal decomposition occurred during heating at 200°C or lower. Therefore, the first purpose of this experiment was to confirm that heating was carried out in the atmosphere with the presence of oxygen using a forced-air dryer without catching fire while ensuring safety. Four specimens of sugi sapwood with the dimensions as described above were used per condition. The concentration of ammonium chloride used for impregnation was set to 0 to 2.5%. The heating temperatures were 120°C, 140°C, and 170°C, and heating was carried out for 1 to 24 hours. The second purpose of this experiment was to determine the lower limit value of the concentration of the aqueous solution used and the lower limit value of the temperature from the perspective of weight loss associated with the treatment. As a result, it was confirmed that no ignition occurred even during heating in the atmosphere. In addition, even with a very short treatment time of 1 to 4 hours, a weight loss rate sufficient for the manifestation of dimensional stability was obtained, and furthermore, depending on the concentration of ammonium chloride, a weight loss reaching the region where decay resistance was manifested was found. As described above, it became clear that after heating, the decomposition and transformation of wood components proceed extremely rapidly, and 120°C is the lower limit value at which decomposition and transformation proceed. After impregnating the ammonium chloride aqueous solution described in Table 1 and then heat-treating the specimen in the atmosphere, a confirmation test of the dimensional stability effect by heat treatment in the atmosphere was conducted. For the heat-treated specimen and the untreated specimen that had not been subjected to any treatment, the dimensions in the radial direction (R) and the tangential direction (T) in the completely dry state were measured with a digital caliper, and then the specimens were immersed in deionized water, kept under a reduced pressure of 50 hPa or less for 2 hours, returned to normal pressure, and left standing in water for 24 hours to reach a saturated state. After gently wiping the surface of the specimen taken out of the water with a paper towel, the dimensions in the radial direction (R) and the tangential direction (T) were measured, and the total swelling ratio of the cross-sectional area was calculated from these values. Fig. 14 shows the relationship between the weight loss rate caused by heat treatment and the swelling ratio of the cross-sectional area of untreated sugi wood and heat-treated sugi wood. Although the conditions for obtaining the swelling ratio are slightly different, the results are very similar to those of the heat treatment using superheated steam at a temperature of 200 °C or higher without using the salt shown in Fig. 2. The swelling ratio, which was more than 10% in the untreated specimen, was about 6 - 7% when the weight loss rate due to heat treatment was about 3%, and the swelling ratio could be suppressed to about 5% when the weight loss rate reached 5%.

[0046]

Table 1

[0047] After impregnating the ammonium chloride and magnesium chloride aqueous solutions described in paragraph number 0043 and then heat-treating the specimen using superheated steam at 180 °C, the specimen was pulverized, cold water extraction was performed, and then the quantification of acid-insoluble components was carried out by the Klason method. The acid-insoluble component ratio at each concentration is shown in Fig. 15. The weight loss rate due to superheated steam treatment was similar for both ammonium chloride and magnesium chloride (see Fig. 13), but the acid-insoluble components increased as the concentration increased in the ammonium chloride-treated material, while almost no increase was observed in the magnesium chloride-treated material.

[0048] After subjecting the test specimens that had been treated with superheated steam at 180°C to weathering operations, an indoor bottle test was conducted using Phellinus linteus. The weight loss rates in the indoor bottle test at each concentration are shown in Fig. 16. As described above, the weight loss rates due to superheated steam treatment were similar for both ammonium chloride and magnesium chloride, but there was a significant difference in the results of the indoor bottle test, and the treated material using magnesium chloride clearly showed higher decay resistance. This was more pronounced for materials treated at higher concentrations.

[0049] Fig. 17 shows cross-sectional photographs of test specimens of Japanese cedar sapwood with a cross-sectional dimension of 30 mm square and a length of 70 mm after impregnation with aqueous ammonium sulfate and ammonium phosphate solutions with a concentration of 1.5 to 2.5% and then subjected to superheated steam treatment at 170°C for 24 hours. As shown in Fig. 9, there was no uneven treatment in the specimens heat-treated after impregnation with an aqueous ammonium chloride solution, whereas it was clearly found that there was significant uneven treatment inside and outside the test specimens in those heat-treated after impregnation with aqueous ammonium sulfate and ammonium phosphate solutions.

[0050] Fig. 18 shows the weight loss behavior when specimens obtained by impregnating filter paper with more than 99% α-cellulose with aqueous ammonium chloride, ammonium sulfate, and ammonium phosphate solutions with a concentration of 2.5%, gently wiping the surface of the filter paper to remove excess liquid, and drying at 105°C using a forced-air dryer were heated from room temperature to 500°C at a heating rate of 10°C / min using a thermogravimetric analyzer. The aqueous ammonium chloride solution selectively decomposes the amorphous region of cellulose from around 180°C, while the crystalline region of cellulose is not decomposed until almost the same temperature range as that of the untreated filter paper. In contrast, for the aqueous ammonium sulfate and ammonium phosphate solutions, the decomposition start temperature is higher, and once decomposition begins, the entire cellulose including the crystalline region is uniformly decomposed.

[0051] A sugi sapwood specimen impregnated with a 2.5% aqueous solution of ammonium phosphate and ammonium sulfate was used as the test material, and a specimen impregnated with an ammonium chloride aqueous solution of the same concentration was used as the control material. Thermogravimetric differential thermal analysis was performed. The results are shown in Fig. 19. The temperature was raised from room temperature to 500 °C at a rate of 10 °C / min, and the relationship between the weight loss and the temperature was examined. As a result, for the ammonium chloride-treated material, a weight loss considered to be due to the decomposition of hemicellulose and a part of cellulose (amorphous region) was observed from around 150 °C and above. On the other hand, ammonium phosphate and ammonium sulfate had a higher decomposition start temperature than that, and when the decomposition started, it was not a selective decomposition, but it was suggested that it decomposed uniformly including not only the amorphous region of hemicellulose and cellulose but also the crystalline region of cellulose.

[0052] As described above, all of ammonium chloride decomposes and sublimes at a temperature of 200 °C or lower. By using a salt such as ammonium chloride that decomposes and sublimes completely at 200 °C or lower and performing heat treatment at 140 to 180 °C, uniform decomposition and transformation become possible, and the weight loss due to heating is less than when using the conventional technology, that is, in a region with less heat deterioration, the effect of imparting dimensional stability, high durability, particularly decay resistance to wood was recognized.

[0053] In the above description, only the experiment of impregnating a sugi sapwood specimen with an ammonium chloride aqueous solution etc. was given as an example, but it goes without saying that the same effect can be obtained even if it is applied, sprayed or immersed on the sugi sapwood specimen instead of impregnation.

Claims

1. A step of immersing, applying, spraying, or impregnating wood into an aqueous ammonium chloride solution in which ammonium chloride as a salt that decomposes and sublimes in its entirety upon heating at 200°C or lower and generates an inorganic acid by the decomposition is dissolved at 0.1 to 5.0% by weight; and a step of performing a heat treatment on the wood immersed, applied, sprayed, or impregnated with the aqueous ammonium chloride solution at 140°C to 180°C. The inorganic acid generated by the heat treatment decomposes and transforms the amorphous regions of hemicellulose and cellulose in the wood to generate furan compounds. A heat treatment method for wood, characterized in that.

2. The heat treatment method for wood according to Claim 1, characterized in that the weight loss rate after the heat treatment step is 3% or more and 20% or less.

3. The heat treatment method for wood according to Claim 1 or 2, characterized in that the treatment time after reaching 140°C to 180°C in the heat treatment step is within 72 hours.

4. A step of immersing, applying, spraying, or impregnating wood into an aqueous solution in which ammonium chloride as a salt that decomposes and sublimes in its entirety upon heating at 200°C or lower and generates an inorganic acid by the decomposition is dissolved at 0.1 to 5.0% by weight; and a step of performing a heat treatment on the wood immersed, sprayed, applied, or impregnated with the aqueous ammonium chloride solution at 140°C to 180°C. The inorganic acid generated by the heat treatment decomposes and transforms the amorphous regions of hemicellulose and cellulose in the wood to generate furan compounds. A method for manufacturing wood having dimensional stability and durability, characterized in that.

5. The method for manufacturing wood having dimensional stability and durability according to Claim 4, characterized in that the weight loss rate after the heat treatment step is 3% or more and 20% or less.

6. The method for manufacturing wood having dimensional stability and durability according to Claim 4 or 5, characterized in that the treatment time after reaching 140°C to 180°C in the heat treatment step is within 72 hours.

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