JP7702091B2 - Method for heat treating wood and method for producing wood with dimensional stability and durability - Google Patents

Description

The present invention relates to a heat treatment for imparting dimensional stability and durability to wood, and to wood that has been subjected to this heat treatment.

When wood is used in harsh environments such as outdoors, it can suffer from problems such as rotting (deterioration caused by wood-decaying fungi) and being eaten away by termites. Previously, wood was generally treated with wood preservatives by coating or impregnating it with them, but with users becoming more environmentally and health-conscious, there is a demand for technology to increase the durability of wood without relying on chemicals.
Furthermore, in harsh environments such as outdoors, wood can suffer significant warping and cracking, which often requires repairs or replacement in a short period of time.

In recent years, heat treatment technologies aimed at improving the dimensional stability and durability of wood have been developed in Europe, including the Netherlands and Finland, and are sold under the product names Plato wood and Thermo wood (registered trademark), respectively. Furthermore, in Japan, a heat treatment technology called Estech treatment has been developed and is in practical use.

The main components of wood are cellulose, hemicellulose, and lignin. Heat treatment decomposes and removes the non-crystallized parts (amorphous regions) of hemicellulose and cellulose, which are less durable among these wood components (easily decomposed by wood-rotting fungi and termites), while also transforming some of them into more durable components, which is thought to improve durability.
In order to prevent warping and cracking, it is essential to reduce the dimensional changes (swelling and shrinkage) that accompany changes in moisture content, i.e., to give dimensional stability to wood, but it is believed that the swelling and shrinkage of wood is largely due to the amorphous regions of hemicellulose and cellulose, and warping and cracking can be prevented by decomposing and removing them. Furthermore, it is known that the modified products are hydrophobic, and the modification caused by heat also contributes to dimensional stabilization.
From a practical standpoint, in order to decompose and remove the amorphous regions of hemicellulose and cellulose and at the same time modify them, all heat treatment techniques developed so far have required heating at 200° C. to 300° C., preferably 220° C. to 250° C. Since there is a risk of ignition in this temperature range, it is essential to prevent combustion, for example by heating under an inert gas from which air has been excluded.
Incidentally, the reason why an upper limit was set for the treatment temperature is that at temperatures above this limit, not only the amorphous regions of hemicellulose and cellulose but also the crystalline regions of cellulose and lignin that ensure the strength of wood are significantly decomposed, causing the wood to become significantly weaker. Therefore, in order to perform a more preferable heat treatment using conventional techniques, it was necessary to control the temperature quite strictly in the presence of an inert gas.

Treatment using nitrogen as an inert gas is disclosed in JP 56-135004 A, treatment using superheated steam is disclosed in JP 09-502508 A, and heat treatment in supercritical carbon dioxide is disclosed in JP 2013-180460 A.

Japanese Patent Application Publication No. 56-135004 Special Publication No. 09-502508 JP 2013-180460 A

The inventors carried out heat treatment using superheated steam as described in Patent Document 2 on cedar sapwood as a test material, and evaluated its resistance to wood-decaying fungi according to JIS K 1571. In order to achieve high performance comparable to that achieved by treatment with preservatives (in other words, to meet the "weight loss rate due to decomposition by wood-decaying fungi of 3% or less" as described in the same standard), a significant proportion of the hemicellulose contained in the wood must be decomposed and removed, and part of it must be transformed to reduce the weight of the wood by 15% or more, preferably about 18%, on a dry weight basis, and treatment at 240°C for 8 hours or more is essential for this. This range could not be reached with 72 hours at 200°C or 24 hours at 220°C, and further extension of the treatment time was required.
This is not only true for superheated steam treatment, but for all of the above treatments, a considerable amount of time at a temperature above 200°C was necessary to achieve high durability. In other words, the energy required for treatment was excessive, and it was difficult to control the temperature inside the device to keep it uniform or to maintain the specified material temperature. In addition, special equipment was required because the wood would ignite if it was not treated in a state filled with inert gas such as nitrogen or steam. For this reason, wood heat-treated using conventional methods was inevitably expensive.
Furthermore, there is concern that any of the above-mentioned treatments may result in a significant decrease in physical properties such as strength due to the heat treatment.

In addition, an outline of the laboratory test using culture bottles based on JIS K 1571 "Wood preservatives - Performance and test methods" (hereinafter referred to as "laboratory bottle test") is provided below.
This standard is a test method for evaluating wood preservatives, and involves a "weathering procedure" in which a wood test piece is immersed in water and dried 10 times to observe the loss of preservative efficacy due to the agent leaching out of the wood. The wood test piece is then forcibly decayed and decomposed for 12 weeks using test fungi, Pleurotus versicolor and Pleurotus versicolor, and the weight loss rate is examined. If the weight loss rate is 3% or less, the product is deemed to have sufficient durability and is judged to have "preservative efficacy." This standard is considered to be the optimal method for confirming that durability, especially resistance to wood-decaying fungi (hereinafter referred to as "decay resistance"), has been achieved through heat treatment.

The inventors used cedar sapwood as the treated wood, subjected it to 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 found between the weight loss rate caused by heat treatment and decay resistance (weight loss rate due to decay in the indoor bin test). If the weight loss rate due to heat treatment was set to 15% or more for Coriolus versicolor and 18% or more for P. palustris, the weight loss rate due to decay in the indoor bin test was 3% or less (see Figure 1).
However, this standard (3% or less) is an essential condition for long-term use of wood in places where rainwater directly falls and water accumulates, such as wood decks. In places where water does not accumulate, such as under eaves or on the exterior walls of buildings, a sufficient service life can be ensured if the weight loss rate due to wood-decaying fungi is roughly 10% or less. Therefore, in conducting research aimed at imparting decay resistance to wood, the inventors have set the target weight loss rate for P. palustris and P. versicolor in indoor bottle tests to be 10% or less, preferably 3% or less.
On the other hand, the heat treatment of Patent Document 2 is also effective in imparting dimensional stability, and as shown in Figure 2, the dimensional change due to moisture absorption decreased as the weight loss rate due to heat treatment increased. The swelling rate of untreated wood is approximately 10%, but from the inventor's experience, if the swelling rate could be suppressed to 7%, preferably 5%, warping and cracking that occur during outdoor use would either be eliminated or would be minor, if any. According to Figure 2, in order to suppress the swelling rate of untreated wood to 7% or 5%, the weight loss rate due to heat treatment needs to be 3% or 5%, respectively.

The inventor has been conducting research into the heat treatment of wood with superheated steam for many years, during which he has attempted to reduce the heat treatment temperature under various conditions. He has discovered that by pre-impregnating the wood with a dilute aqueous solution of a salt that is nearly neutral to weakly acidic at room temperature before heating, a weight loss rate of 15% or more can be achieved even when heated to 200°C or less (see JP 2018-161802 A).

Recently, with the aim of commercializing the technology disclosed in JP 2018-161802 A, detailed research was conducted under the conditions set out in the publication. As a result, it was suggested that the technology would be difficult to commercialize due to the following problems occurring when the cross-sectional dimensions of the wood to be treated exceed a certain size.
Inferring from Figure 1, aiming for a minimum weight loss rate of 13% or more by superheated steam treatment, which is the minimum required for decay resistance, cedar sapwood with a cross-sectional dimension of 30 mm square was used as the test material, and heat treatment was performed for 24 to 72 hours at 160 to 180°C using an aqueous solution of magnesium chloride with a concentration of 0.5 to 1.5% as described in the same publication. It was found that there was a significant difference in the degree of decomposition and transformation due to heat between the surface and the center of the material. In other words, decomposition and transformation of the wood due to heat progressed excessively near the surface, and a weakened surface layer like "charcoal" was observed on the wood after treatment. On the other hand, decomposition and transformation did not progress to a degree that would improve decay resistance in the center of the wood.

After extensive research into the causes of this uneven decomposition and transformation caused by heat treatment, it was found that even if the wood is pre-impregnated with a dilute salt solution like the one mentioned above (a magnesium chloride solution with a magnesium chloride concentration of 0.5-1.5%), when the wood is dried, the magnesium chloride dissolved in the water also moves as water moves from the inside of the wood to the surface. As a result, less magnesium chloride remains in the center, while more magnesium chloride than necessary accumulates near the surface, which is the cause of the unevenness.

As a result of extensive research into methods for reducing the bias in salt concentration in treated wood and preventing uneven decomposition and transformation caused by heat treatment, the present invention was developed. This invention eliminates the need for special equipment for using inert gas and makes temperature control easier, reducing deterioration of physical properties such as strength due to heat. This makes it possible to provide heat-treated wood of uniform quality at low cost.

The present invention has been devised in light of the above circumstances, and aims to provide a method for heat treating wood which can exhibit dimensional stability and high durability while suppressing deterioration of physical properties such as strength compared to conventional heat-treated wood, at a heat treatment temperature of 200°C or less, preferably 180°C or less, while reducing the energy required for heat treatment, facilitating temperature control, and enabling treatment under conditions in which oxygen is present, without the need for special equipment, and a method for producing lumber having dimensional stability and durability .

The method for heat treating wood according to the present invention comprises the steps of immersing, coating, spraying or impregnating wood with an aqueous solution of ammonium chloride in which 0.1 to 5.0% by weight of ammonium chloride is dissolved as a salt that is completely decomposed and sublimated when heated at 200° C. or less and generates an inorganic acid through its decomposition, and heat treating the wood that has been immersed, coated, sprayed or impregnated with the aqueous solution of ammonium chloride at 140° C. to 180° C., where the inorganic acid generated by the heat treatment decomposes and modifies amorphous regions of hemycellulose and cellulose in the wood to generate furan compounds.

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

The weight loss rate of wood treated with the wood heat treatment method according to the present invention after the heating process is 3% or more and 20% or less.

Furthermore, the treatment time for the heat treatment method for wood according to the present invention is within 72 hours after the temperature reaches 140-180°C .

According to the heat treatment method for wood of the present invention, hemicellulose, which is susceptible to decomposition by wood-decaying fungi, and the amorphous regions of cellulose are decomposed and modified to produce compounds such as furans, which are more resistant than lignin and not only cannot be decomposed by wood-decaying fungi but also have antibacterial properties. As a result, wood can be made into highly durable wood with high decay resistance. In addition, these furan compounds are hydrophobic and do not swell with water, which also contributes to improving dimensional stability.
Furthermore, this method has the advantage that it does not require the high temperatures or complex equipment required in conventional methods to improve the performance of wood through heat treatment.

This is a graph showing the relationship between the weight loss rate due to heat treatment (superheated steam treatment) and decay resistance (weight loss rate in indoor bin tests), where (A) shows the weight loss rate for M. palustris, and (B) shows the weight loss rate for M. versicolor. This is a graph showing the relationship between the weight loss rate caused by heat treatment (superheated steam treatment) and the swelling rate of the end surface of untreated cedar wood and heat-treated cedar wood. The swelling rate in the figure shows the value when the equilibrium moisture content is reached in an environment with a temperature of 20°C and a relative humidity of 95%. 1 is a graph showing the relationship between heating time and the weight loss rate of ammonium chloride when the heating temperature is kept constant and the temperature is heated at each temperature using a thermogravimetric differential thermal analyzer. This is a graph showing the weight loss behavior and differential thermal behavior due to thermal decomposition when a cedar sapwood specimen impregnated with a 2.5% ammonium chloride aqueous solution and a cedar sapwood specimen not impregnated with the same are heated from room temperature to 500°C at a heating rate of 10°C/min using a thermogravimetric differential thermal analyzer. This graph shows the relationship between the ammonium chloride concentration and the weight loss rate when cedar sapwood cut in an air-dried state to a size of 20 mm in the tangential direction (T) × 20 mm in the radial direction (R) × 10 mm in the longitudinal direction (L) was dried completely at 105°C, impregnated with an aqueous ammonium chloride solution with a concentration of 0 to 2.5%, dried, and then heat-treated for 24 hours using superheated steam. This graph shows the relationship between the treatment time and the weight loss rate due to heat treatment when cedar sapwood cut in an air-dried state to a size of 20 mm in the tangential direction (T) × 20 mm in the radial direction (R) × 10 mm in the longitudinal direction (L) was completely dried at 105°C, impregnated with an aqueous ammonium chloride solution with a concentration of 0.2 to 1.0%, dried, and then heat-treated using superheated steam at 170°C for 24 to 72 hours. This graph shows the relationship between the ammonium chloride concentration and the acid insoluble content when cedar sapwood was cut in an air-dried state to a size of 20 mm in the tangential direction (T) × 20 mm in the radial direction (R) × 10 mm in the longitudinal direction (L) and then dried at 105°C. The cedar sapwood was impregnated with an aqueous ammonium chloride solution having a concentration of 0 to 2.5%, dried, and then heat-treated for 24 hours using superheated steam and untreated test specimens were crushed and extracted with cold water, and the acid insoluble content was quantified by the Clason method. This graph shows the relationship between the ammonium chloride concentration and the weight loss rate in an indoor bin test for wood (cedar sapwood cut to 20 mm tangential direction (T) x 20 mm radial direction (R) x 10 mm longitudinal direction (L)) that was heat-treated for 24 hours with different impregnating ammonium chloride concentrations and treatment temperatures. Figure 1A shows the weight loss rate for M. palustris, and Figure 1B shows the weight loss rate for M. versicolor. This photograph shows the cross section of a cedar sapwood specimen with cross-sectional dimensions of 30 mm square and 70 mm long, which was impregnated with an aqueous solution of ammonium chloride with a concentration of 1.5 to 2.5%, dried at 105°C, and then heat-treated at 170°C for 24 hours. This is a graph showing the weight loss rate due to thermal decomposition when a cedar sapwood test specimen impregnated with a 2.5% ammonium chloride aqueous solution and an untreated cedar sapwood test specimen that has not been impregnated are heated from room temperature to 500°C at a heating rate of 10°C/min. 1 is a graph showing the weight loss rate due to thermal decomposition when magnesium chloride, ammonium sulfate, ammonium phosphate, and ammonium chloride are heated from room temperature to 500° C. at a heating rate of 10° C./min using a thermogravimetric differential thermal analyzer. This graph shows the weight loss rate of the filter paper itself, which was calculated by impregnating filter paper containing 99% or more α-cellulose with an aqueous ammonium chloride solution having a concentration of 0 to 5.0%, lightly wiping the surface of the filter paper to remove excess liquid, drying the paper at 105°C using a blower dryer, and then heating the paper from room temperature to 275°C at a heating rate of 10°C/min using a thermogravimetric differential thermal analyzer, and subtracting the weight increase rate when the filter paper was impregnated with ammonium chloride from the weight loss rate at 275°C. This graph shows the relationship between the concentration of chemicals in each aqueous solution and the weight loss rate when cedar sapwood cut to a size of 20 mm tangentially (T) × 20 mm radially (R) × 10 mm longitudinally (L) in an air-dried state was impregnated with aqueous solutions of ammonium chloride and magnesium chloride at concentrations of 0 to 1.0%, and then heat-treated using superheated steam at a temperature of 180°C for 24 hours. This is a graph showing the relationship between the weight loss rate caused by heat treatment of cedar sapwood (tangential direction (T) 30 mm x radial direction (R) 30 mm x length direction (L) 6 m) that was impregnated with 0-2.5% ammonium chloride aqueous solution and then heat-treated in air at 120°C, 140°C, and 170°C for 24 hours, and the swelling rate of the end surface of untreated cedar wood and heat-treated cedar wood. The swelling rate in the figure shows the value when saturated with water. This graph shows the relationship between the concentration and the acid insoluble content when the acid insoluble content was quantified by the Clason method after cedar sapwood was cut to 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 an air-dried state after being impregnated with an ammonium chloride aqueous solution or a magnesium chloride aqueous solution having a concentration of 0 to 1.0% and treated with superheated steam at a temperature of 180°C for 24 hours and an untreated specimen was crushed and extracted with cold water. This graph shows the relationship between the concentration and the weight loss rate in an indoor bin test using Mushroom Palustris after weathering operations on test specimens made of cedar sapwood that was cut to a tangential direction (T) of 20 mm, radial direction (R) of 20 mm, and length direction (L) of 10 mm in an air-dried state after being impregnated with ammonium chloride aqueous solution or magnesium chloride aqueous solution of 0 to 1.0% concentration. The test specimens were treated with superheated steam at a temperature of 180°C for 24 hours. After the test specimens were weathered, the test specimens were subjected to an indoor bin test using Mushroom Palustris. This is a cross-sectional photograph of a cedar sapwood specimen with a cross-sectional dimension of 30 mm square and 70 mm long that was impregnated with an aqueous solution of ammonium phosphate and ammonium sulfate at a concentration of 1.5 to 2.5% and then heat-treated with superheated steam at 170°C for 24 hours. This is a graph showing the weight loss behavior when filter paper containing 99% or more α-cellulose is impregnated with an aqueous solution of ammonium chloride, ammonium sulfate, or ammonium phosphate at a concentration of 2.5%, and then the excess liquid is wiped off and the filter paper alone is dried at 105° C. using a blower dryer, and then the temperature is raised from room temperature to 500° C. at a heating rate of 10° C./min using a thermogravimetric differential thermal analyzer. 1 is a graph showing the weight loss behavior of a cedar sapwood specimen impregnated with a 2.5% aqueous solution of ammonium chloride, ammonium phosphate, and ammonium sulfate, and an untreated specimen as a control that was not impregnated, when the specimens were heated from room temperature to 500° C. at a heating rate of 10° C./min using a thermogravimetric differential thermal analyzer.

The heat treatment method for wood according to the embodiment of the present invention will be described below with reference to an embodiment for carrying out the present invention, but this is merely an example and does not limit the scope of the present invention.
Ammonium chloride is a representative example of a salt that decomposes and sublimes entirely when heated at 200° C. or less to generate an inorganic acid. According to the Compact Chemistry Dictionary (Kyoritsu Shuppan Co., Ltd., 4-6-19 Kohinata, Bunkyo-ku, Tokyo, published February 15, 1987), ammonium chloride decomposes and sublimes at around 335° C., dissociating into ammonia and hydrogen chloride. However, when the inventors investigated using a thermogravimetric differential thermal analyzer, they found that the entire amount of ammonium chloride used in the test decomposed and sublimated in 10 hours at 160° C. and in 3 hours at 180° C. (see FIG. 3).

When modifying wood by heat treatment at 120-200°C, no significant decomposition, sublimation or evaporation occurs until the temperature reaches that range, and by using a chemical that decomposes, sublimes or evaporates in its entirety within the treatment time after the temperature reaches that range, it is possible to prevent the accumulation of chemicals near the surface of the wood as described above. In other words, when drying wood impregnated with an aqueous solution of a chemical that sublimes, decomposes or evaporates at 200°C or less, some of the chemical moves to the surface area as the water moves, increasing the concentration near the surface, but when the subsequent heat treatment is performed at 200°C or less for the purpose of modification, the concentration near the surface decreases due to decomposition and sublimation, preventing the chemical concentration from increasing more than necessary. As a result, uneven decomposition and transformation due to heat treatment between the inside and near the surface of the wood is reduced, and uniform heat treatment can be performed.

Based on this development concept, a differential thermal analysis was carried out on test material made of cedar sapwood impregnated with a dilute aqueous solution of ammonium chloride, which was found to completely decompose and sublime below 200°C. It was found that the material lost weight and thermal decomposition progressed at temperatures below 200°C. This is thought to be the result of the hydrogen chloride produced by the thermal decomposition of ammonium chloride accelerating the thermal decomposition of the wood. On the other hand, in the control section (untreated cedar sapwood test specimen) which was not impregnated with such an aqueous solution, there was almost no weight loss and, as has been conventionally said, almost no thermal decomposition progressed at temperatures below 200°C (see Figure 4).

As mentioned above, differential thermal analysis of cedar sapwood impregnated with a dilute aqueous solution of ammonium chloride revealed that thermal decomposition was occurring even at temperatures below 200°C, but a characteristic of this was that no heat balance was observed at all (see Figure 4).
Generally, the thermal decomposition of wood is accompanied by heat generation, and in the heat treatment of 200°C or higher as performed in Patent Document 2, if the amount of thermal energy applied is even slightly excessive, the temperature becomes uncontrollable due to self-heating, and the temperature rises to a range where all of the wood components decompose, progressing to carbonization and significantly weakening the wood.
On the other hand, if endothermic reactions occur, additional energy is required to maintain the temperature at which decomposition and transformation occur. Furthermore, it is self-evident that temperature control becomes difficult when there is a heat balance. Being able to achieve decomposition and transformation that gives wood high decay resistance and dimensional stability without a heat balance also makes it possible to carry out the treatment in an atmosphere containing oxygen, which is extremely meaningful.

Specific names of salts that decompose upon heating to generate inorganic acids are described in JP 2018-161802 A, such as magnesium chloride, copper sulfate, and ammonium sulfate.
Heat treatment using magnesium chloride, copper sulfate, or ammonium sulfate can also accelerate the decomposition and transformation of wood, but differential thermal analysis showed that at temperatures below 200°C, the entire amount was not decomposed and sublimated, and a considerable amount remained. As mentioned above, when the dimensions of the material were large, the degree of decomposition and transformation of these salts differed greatly between the surface and the center, resulting in uneven treatment.

As mentioned above, the degree of decomposition due to heat treatment can be known from the weight loss rate of wood that accompanies the heat treatment. On the other hand, the components of wood that are decomposed can be known from the results of analysis of the wood components and differential thermal analysis.
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 decompose mainly the amorphous regions of hemicellulose and cellulose among the main components of wood, lignin, cellulose, and hemicellulose. As a result of heat treatment at 150 to 180°C of cedar sapwood impregnated with a dilute aqueous solution of ammonium chloride, it was found that while there was almost no change in lignin, hemicellulose, which is susceptible to decomposition by wood-rotting fungi and the like and is involved in the swelling and shrinkage of wood, and the amorphous regions of cellulose were decomposed. The amorphous regions of hemicellulose and cellulose are components that express the flexibility of wood, and although it is inevitable that the amount of bending of wood will decrease (some loss of flexibility) due to the heat treatment of the present invention, on the other hand, these components do not have a significant effect on the breaking strength, so the decrease in physical properties such as strength can be minimized.

Thermal transformation can be estimated from the results of wood component analysis and FT-IR analysis. The lignin contained in wood can be quantified by the Clason method. That is, the test wood flour is placed in 72% sulfuric acid and stirred at room temperature for 4 hours, then water is added to dilute it to a 3% concentration and boiled for 4 hours, after which the insoluble portion is filtered under reduced pressure through a glass filter, dried and quantified. This method makes use of the property of lignin condensing in acid, which becomes insoluble, while cellulose and hemicellulose, which are wood components, are hydrolyzed by acid and become soluble in water.

When cedar sapwood impregnated with a dilute aqueous solution of ammonium chloride was heat-treated at 150-180°C and then the Klason method was carried out, the amount of components insoluble in acid increased dramatically. When 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 regions of the cellulose were decomposed and denatured, and components insoluble in acid were observed, and FT-IR analysis suggested that these were furan compounds.
Even in the case of heat-treated wood specimens, the increase in the acid-insoluble components quantified as Klason lignin due to heat treatment is thought to be furan compounds transformed from the amorphous regions of hemicellulose and cellulose.
It was a common belief that furan compounds are produced by heat treating wood at a temperature exceeding 200°C, but it was shown that similar transformation occurs even in treatment at 200°C or less by using an aqueous ammonium chloride solution. Furthermore, in conventional heat treatments that do not use salt, the increase in acid-insoluble components, i.e., the amount of furan compounds produced by heat treatment, was estimated at 4% or less at most, whereas in heat treatment at 150 to 180°C using an aqueous ammonium chloride solution according to the present invention, the increase rate sometimes reached 10% or more. It was suggested that the use of an aqueous ammonium chloride solution promotes decomposition by heating and also promotes transformation to furan compounds.

Among the main components of wood (cellulose, hemicellulose, and lignin), lignin is the most resistant to wood-decaying fungi, but the large amount of furan compounds produced by the transformation of the amorphous regions of cellulose and hemicellulose in the heat treatment of the present invention are more resistant than lignin, and are not only not decomposed by wood-decaying fungi, but are also said to exhibit antibacterial properties. In other words, it can be said that decomposing the hemicellulose and amorphous regions of cellulose, which are wood components that are vulnerable to wood-decaying fungi, and at the same time producing as many furan compounds as possible by transformation from them is very effective in increasing the durability of wood. As mentioned above, in order to impart the decay resistance of cedar sapwood to the standard of JIS K 1571 using heat-treated wood produced by the method of Patent Document 2, which is the previous heat treatment, it was necessary to reduce the weight by 18% by heat treatment, whereas in the present invention, decay resistance that meets the standard was obtained with a weight reduction of about 13%.
Furthermore, in order to limit the weight loss rate in indoor bin tests using M. palustris to approximately 10%, a weight loss rate of 13% was required using conventional heat treatment without using inorganic salts, whereas it was found that with the present invention, the same decay resistance was achieved with a weight loss rate of approximately 10%.
On the other hand, as will be described later, in terms of providing dimensional stability, results almost identical to the relationship between weight loss rate and swelling rate when heat treatment was performed without using conventional inorganic salts, as shown in Figure 2, were obtained for cedar sapwood specimens that were impregnated with an aqueous solution of ammonium chloride with a concentration of 0.2 to 2.5% and heat-treated at 120 to 170°C for 1 to 24 hours.

A similar heat treatment was also performed on cedar sapwood impregnated with dilute aqueous solutions of salts such as magnesium chloride and copper sulfate described in the above-mentioned JP 2018-161802 A, and then analysis was performed using the Klason method. However, unlike the heat treatment using ammonium chloride according to the present invention, no significant increase in furan compounds was confirmed.

The procedure for actually treating wood according to the present invention will now be described.
The ammonium chloride solution, which has been adjusted to a concentration of 0.1-5%, is applied, sprayed, or impregnated onto wood, or the wood is immersed in an aqueous salt solution to allow it to penetrate into the wood. If the concentration of the ammonium chloride solution used here is less than 0.1%, it is too dilute to be effective. On the other hand, research has shown that using an ammonium chloride solution with a concentration of more than 5% does not increase the effect of promoting decomposition and transformation, and there is also a risk that ammonium chloride will remain in the wood after treatment.
A pressurized injection method is an effective method for ensuring that the aqueous solution of ammonium chloride is impregnated into the wood. For example, the wood to be treated is placed in a stainless steel pressure vessel and secured with weights or ropes to prevent it from floating. The lid of the pressure vessel is closed and the inside of the vessel is depressurized with a vacuum pump. For example, a depressurized state of about 50 to 100 hPa is maintained for 30 minutes to 4 hours. After that, the aqueous solution of ammonium chloride is poured into the vessel using the pressure difference between the inside and outside of the pressure vessel, and the vessel is filled with the aqueous solution of ammonium chloride as much as possible by using a liquid feed pump. Next, a pressure-resistant pump such as a plunger pump is used to feed the aqueous solution of ammonium chloride into the vessel, creating a pressurized state of 0.5 to 1.5 MPa, and this state is maintained for 1 to 24 hours, after which the pressure is released. The liquid feed pump is rotated in the reverse direction to recover the excess aqueous solution of ammonium chloride, completing the injection operation. After the liquid recovery, the vessel may be further depressurized with a vacuum pump to recover the excess aqueous solution of ammonium chloride in the permanent voids in the wood.

Another example of the pressurized injection method is as follows. The wood to be treated is placed in a stainless steel box-shaped container and secured with weights or rope to prevent it from floating, and then the ammonium chloride solution is poured in until the wood is fully submerged in the solution. The box-shaped container is then placed in a pressure-resistant container, the lid of the pressure-resistant container is closed, and the container is depressurized with a vacuum pump. A reduced pressure of, for example, 50 to 100 hPa is maintained for 30 minutes to 4 hours. After that, compressed air is introduced into the pressure-resistant container using a compressor or the like to create a pressurized state of 0.5 to 1.5 MPa, and this state is maintained for 1 to 24 hours before the pressure is released. The ammonium chloride solution is then collected, or the wood is removed directly from the liquid, and the injection process is completed.

The wood that has been soaked in the ammonium chloride solution is dried using standard wood drying methods. Natural drying is one method, but it is more efficient to reduce the moisture content as much as possible before subjecting it to the subsequent heat treatment process, so artificial drying using steam or other methods is more preferable. Here, the wood is dried to a state where there is no free water in it or even less. It is also possible to use the same equipment for this drying process and the subsequent heating process.

For the heating step, a wood drying device capable of high-temperature drying can be used, or a dedicated heat treatment device capable of maintaining a vacuum or a state close to it and creating an oxygen-free or low-oxygen state can be used. The wood to be treated that has been subjected to the above-mentioned drying step is placed in the device and placed in an environment of 120°C to 200°C, preferably 140°C to 180°C, for a certain period of time, for example, 1 hour to 72 hours, to promote decomposition and denaturation of the amorphous regions of hemicellulose and cellulose among the wood components. As mentioned above, if the temperature exceeds 200°C, there is a risk of fire unless the wood is treated in a state filled with inert gas, while at temperatures below 120°C, almost no decomposition occurs. The temperature range in which there is no risk of fire even without using inert gas and the decomposition proceeds moderately is 140°C to 180°C. If the main purpose is to develop decay resistance, the weight loss that occurs at this time is preferably in the range of 10% to 20%, and more preferably 12% to 15%. As will be described later, heat treatment using ammonium chloride causes decomposition and transformation soon after a certain temperature is reached, but even if the treatment is carried out for 72 hours or more, the ammonium chloride in the wood decomposes and disappears, and little effect can be expected.
In addition, when the main purpose is to provide dimensional stability, the weight loss rate due to the treatment is preferably 3% to 20%, more preferably 5% to 15%. The upper limit is set in consideration of the deterioration of the mechanical properties of wood due to the treatment.

Five mg of ammonium chloride (Wako Pure Chemicals special grade reagent) was used in a thermogravimetric differential thermal analyzer to examine its weight loss behavior at constant temperatures of 160°C, 170°C, and 180°C. It was found that all of the ammonium chloride tested decomposed and sublimated within 10 hours at 160°C, 5 hours at 170°C, and 3 hours at 180°C (see Figure 3).

The wood (test specimens) to be treated were cedar sapwood cut in an air-dried state to a size of 20 mm tangentially (T) × 20 mm radially (R) × 10 mm longitudinally (L), and then dried at 105°C. 24 specimens were used per test condition.
As the salt solution, an aqueous solution of ammonium chloride with a concentration of 0 to 2.5% was used. The test specimen was placed in a stainless steel tray, a stainless steel weight was placed on top, and then a sufficient amount of the aqueous solution of ammonium chloride was poured in to submerge the specimen in the aqueous solution of ammonium chloride.
The tray was placed in a pressurized injection canister and degassed using a vacuum pump. After being placed under a reduced pressure of approximately 50 hPa for 1 hour, and then under a pressure of 1.3 MPa using a compressor for 2 hours, and after the pressure was released, the tray was left in the liquid for a whole day and night. After that, the test specimen was removed from the aqueous ammonium chloride solution and the amount of impregnation was measured. The test specimen was then dried in a 60°C air dryer for 1 day, and then heated to 105°C and dried for 1 day until it was completely dry.
After measuring the total dry weight (W1), the material was placed in a heat treatment device, which was filled with superheated steam, and the device was adjusted so that the material temperatures were 150°C, 160°C, 170°C, and 180°C, and the material was treated for 24 hours.
Thereafter, when the temperature dropped to 150° C. or lower, the sample was taken out and its weight (W2) in a completely dry state was measured. The weight loss rate due to the heat treatment was calculated using the formula (W1−W2)/W1×100.
As shown in FIG. 5, when heat treatment was performed without using an aqueous ammonium chloride solution, the weight loss rate was about 2-3%, whereas by using an aqueous ammonium chloride solution, the weight loss rate was clearly higher; by treating an injection material with a concentration of 1.5% or more at 160-180°C, a weight loss rate of 12-15% was achieved.
Furthermore, the relationship between the treatment time and the weight loss rate due to heat treatment when cedar sapwood impregnated with 0.2-1.0% ammonium chloride aqueous solution was used as a test specimen and heat treated at 170°C for 24-72 hours is shown in Figure 6. The 1.0% concentration injection material was rapidly decomposed by the action of ammonium chloride within 24 hours, and no further weight loss was observed even if the treatment time was extended. In addition, the more dilute impregnated materials with concentrations of 0.2% and 0.5% lost some weight up to 48 hours, but even if the treatment was extended to 72 hours, the weight loss rate remained at about 1%, and no further weight loss was obtained even if the heat treatment was performed for more than 72 hours. It was also revealed that no ammonium chloride was present in the test specimens that were heat treated for 72 hours, and that the decomposition and sublimation occurred within 72 hours.

Of the 24 test specimens that had been heat-treated for 24 hours, six were crushed and extracted with cold water, after which the acid-insoluble matter was quantified using the Clason method. As shown in Figure 7, when no ammonium chloride aqueous solution was used, the increase in acid-insoluble matter was less than 3%, whereas when an ammonium chloride aqueous solution was used, an increase of up to 10% or more was observed.

The remaining 18 of the 24 specimens mentioned above were used for laboratory bin tests (9 each for C. palustris and C. versicolor). The weight loss rate after the laboratory bin test is shown in Figure 8. For C. versicolor, only specimens treated with an ammonium chloride solution with a concentration of 1.5% or higher were used. While the untreated specimens lost 44% weight, the weight loss rate of the heat-treated specimens was less than 0.5% under all conditions, indicating that C. versicolor is highly resistant to decay.
Looking at these results together with Figure 5, for Coriolus versicolor, if the weight loss rate due to heat treatment is approximately 10% or more, it meets the performance standard "weight loss rate due to decomposition by wood-decaying fungi is 3% or less" stipulated in JIS K 1571. When heat treatment was performed according to Patent Document 2, a weight loss rate of about 15% was required, and when ammonium chloride was used for heat treatment, high decay resistance was expressed in a range where the weight loss rate due to heat treatment was about 5% lower than when ammonium chloride was not used.
On the other hand, for the untreated specimens of the oyster mushroom, the weight loss rate exceeded 55%, whereas impregnation with an aqueous ammonium chloride solution of 2.5% concentration at 150-160°C and impregnation with an aqueous ammonium chloride solution of 1.5% concentration at 170-180°C demonstrated sufficient decay resistance to meet the performance standards. Combining these results with the results of FIG. 5, it can be confirmed that a weight loss rate of 13% or more due to heat treatment will certainly demonstrate high decay resistance for the oyster mushroom. When heat treatment is performed according to Patent Document 2, a weight loss rate of about 18% is required, and for the oyster mushroom, the weight loss rate due to heat treatment could be reduced by about 5%. In this way, by using a dilute aqueous ammonium chloride solution, decay resistance was demonstrated where the weight loss rate of the wood was small, in other words, where the deterioration of the wood due to heat was small.

Cedar sapwood specimens with cross-sectional dimensions of 30 mm square and 70 mm long were used, and were impregnated with an aqueous solution of ammonium chloride with a concentration of 1.5 to 2.5% using a pressurized injection method. After drying at 105°C, they were heat-treated at 170°C for 24 hours, and the weight loss rate was measured, as well as the surface and cross section were visually observed and palpated.
As a result, the weight loss rate due to heat treatment was about 15%, and no particularly weakened layers were found by visual inspection or palpation. When the specimen was cut near the center and the difference between the center and the surface was visually observed, the difference in material color between the center and the surface was slight, and it was found that the heat treatment was almost uniform (see Figure 9).

After soaking a filter paper containing 99% or more α-cellulose in a 2.5% ammonium chloride aqueous solution, the surface of the filter paper was lightly wiped to remove excess liquid, and the filter paper was dried at 105°C. It was then heat-treated for 2 hours at 170°C using a general air dryer. The weight loss rate was approximately 8%. When the same filter paper was heated at 170°C for 2 hours without being soaked in the ammonium chloride aqueous solution, the weight loss rate was about 1%, making it clear that the use of the ammonium chloride aqueous solution caused thermal decomposition. In addition, analysis of the filter paper using the Clason method revealed that the acid-insoluble content was 7%. Cellulose is inherently acid-soluble. The acid-insoluble content of the cellulose filter paper using the Clason method was less than 1%, and it was found that a considerable amount of components that become insoluble in acid were produced from the cellulose as a result of decomposition by heat treatment. Analysis using FT-IR suggested that the insoluble matter was a furan compound.

In the course of carrying out detailed studies on the thermal decomposition and transformation of wood by an ammonium chloride solution, we carried out a thermogravimetric differential thermal analysis on test specimens of cedar sapwood impregnated with a 2.5% ammonium chloride solution and unimpregnated cedar sapwood, and the results are shown in Figure 10. This figure shows the relationship between the weight loss of the test specimens and the temperature when the temperature was raised from room temperature to 500°C at a rate of 10°C/min.
As a result, the test specimens impregnated with the ammonium chloride aqueous solution showed a weight loss thought to be due to the decomposition of hemicellulose and a part of the cellulose (amorphous region) in the 150-200°C range, where thermal decomposition hardly occurs in the unimpregnated test specimens. On the other hand, the crystalline region that makes up the majority of cellulose and lignin do not decompose in this temperature range, just like the unimpregnated test specimens. Another characteristic is that the thermal decomposition promoted by ammonium chloride is limited to the amorphous regions of hemicellulose and cellulose, which are easily decomposed by wood-rotting fungi. This suggests that this is one of the factors that causes decay resistance to be exhibited even in a temperature range where weight loss due to heat treatment is small.
On the other hand, magnesium chloride, ammonium sulfate, and ammonium phosphate, which are described in JP 2018-161802 A below, have the potential to promote the thermal decomposition of wood in the same way as these substances, promote the thermal decomposition of hemicellulose and cellulose as a whole, and there is a risk that imparting decay resistance to wood will be accompanied by considerable thermal deterioration.

Magnesium chloride, ammonium sulfate, and ammonium phosphate, which may promote the thermal decomposition of wood, as described in the above JP 2018-161802 A, were heated from room temperature to 500°C at a heating rate of 10°C/min using a thermogravimetric differential thermal analyzer to examine the weight loss due to thermal decomposition. The results are shown in FIG. 11. Even when heated to 500°C, the weight loss of ammonium phosphate was only about 30% and that of magnesium chloride was about 70%, and the agent was not completely decomposed, sublimated, and disappeared. In addition, although all of the ammonium sulfate was sublimated, the temperature range was much higher than that of ammonium chloride. On the other hand, ammonium chloride began to decompose at around 120°C and was completely decomposed at around 275°C.
12 shows the weight loss rate of the filter paper itself at 275°C when the filter paper was heated from room temperature to 275°C at a heating rate of 10°C/min using a thermogravimetric differential thermal analyzer after impregnating filter paper containing 99% or more α-cellulose with an aqueous solution of ammonium chloride with a concentration of 0 to 5.0%, lightly wiping the surface of the filter paper to remove excess liquid, and drying at 105°C using a blower dryer. It was found from this result that impregnation with ammonium chloride with a concentration of more than 5.0% did not have the effect of promoting further decomposition.

The wood (test specimen) to be treated was made of cedar sapwood that had been cut in an air-dried state to a size of 20 mm tangentially (T) × 20 mm radially (R) × 10 mm longitudinally (L) and then completely dried at 105°C.
As the salt solution, an aqueous solution of ammonium chloride with a concentration of 0 to 2.5% was used. The test specimen was placed in a stainless steel tray, a stainless steel weight was placed on top, and then a sufficient amount of the aqueous solution of ammonium chloride was poured in to submerge the specimen in the aqueous solution of ammonium chloride.
The tray was placed in a pressurized injection canister and degassed using a vacuum pump. After being placed under a reduced pressure of approximately 50 hPa for 1 hour, and then under a pressure of 1.3 MPa using a compressor for 2 hours, and after the pressure was released, the tray was left in the liquid for a whole day and night. After that, the test specimen was removed from the aqueous ammonium chloride solution and the amount of impregnation was measured. The test specimen was then dried in a 60°C air dryer for 1 day, and then heated to 105°C and dried for 1 day until it was completely dry.
The relationship between the concentration and the weight loss rate when the thus-prepared ammonium chloride-impregnated test specimens and test specimens impregnated with magnesium chloride aqueous solutions (concentrations of 0 to 1.0%) in the same manner were treated with superheated steam at 180°C is shown in Figure 13. For both, the weight loss rate increased as the concentration increased. Furthermore, the weight loss rate at each concentration was about the same for both, indicating that thermal decomposition occurred to the same extent.

Among the specimens that underwent the above-mentioned 24-hour superheated steam treatment, the swelling ratios in a water-saturated state were determined for specimens with a heat treatment temperature of 160°C and a weight loss ratio of 3-10% with an ammonium chloride aqueous solution concentration of 0.2-1.0%, and for untreated cedar sapwood as a control. As shown in Figure 5, the weight loss ratios due to heat treatment for the specimens for which the swelling ratios were determined were approximately 4% for the 0.2% aqueous solution, 6% for the 0.5% solution, and 9% for the 1.0% solution. As a result, while the swelling ratio at the end of the untreated cedar wood was approximately 10%, the swelling ratios of the specimens with a weight loss ratio of 4%, 7%, and 9% due to heat treatment were 5.0%, 4.7%, and 4.5%, respectively, and sufficient dimensional stability was obtained for practical use.

In addition to the above example (heat treatment of 0.2% injection material at 160°C for 24 hours), an example of the condition that satisfies the weight loss rate due to heat treatment of 3 to 5% is as follows: for cedar sapwood cut to 30 mm tangential direction (T) × 30 mm radial direction (R) × 6 mm longitudinal direction (L), after impregnation with 0.2% ammonium chloride aqueous solution, 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 experiments in Table 1 will be explained. Heat treatment using wood specimens other than this test was performed under superheated steam without oxygen. The reason was that there was a concern that the specimens would burn and cause a fire during heat treatment. However, as is clear from Figure 4, when ammonium chloride was used, no heat was generated due to thermal decomposition when heated at 200°C or less. Therefore, the primary purpose of this experiment was to ensure safety by heating in an oxygen-containing atmosphere using a blower dryer and confirming that the specimens would not ignite. The specimens were cedar sapwood of the dimensions described above, four pieces per condition. The concentration of ammonium chloride used for impregnation was 0 to 2.5%. The heating temperatures were 120°C, 140°C, and 170°C, and the specimens were heated for 1 to 24 hours.
The second objective of this experiment was to determine the lower limit of the concentration of the aqueous solution to be used and the lower limit of the temperature from the viewpoint of the weight loss during the treatment.
As a result, it was confirmed that the wood would not ignite even when heated in the air, and that even in a very short period of time, such as 1 to 4 hours, a sufficient weight loss rate was obtained to achieve dimensional stability, and that depending on the concentration of ammonium chloride, the weight loss could reach a level at which decay resistance would be achieved. As described above, it was made clear that decomposition and transformation of wood components progresses in an extremely short period of time after heating, and that 120°C is the lower limit at which decomposition and transformation progress.
Tests were conducted to confirm the effect of heat treatment on dimensional stability in air using specimens that had been heat-treated in air after impregnation with the aqueous ammonium chloride solution shown in Table 1. The dimensions of the heat-treated specimens and untreated specimens in the completely dry state were measured with a digital caliper in the radial (R) and tangential (T) directions, and then the specimens were submerged in deionized water and left under reduced pressure of 50 hPa or less for 2 hours, and then returned to normal pressure and left in water for 24 hours to be saturated with water. The surface of the specimens taken out of the water was lightly wiped with a paper towel, and the dimensions of the radial (R) and tangential (T) directions were measured, and the total swelling rate at the end grain area was calculated from these values. Figure 14 shows the relationship between the weight loss rate caused by heat treatment and the swelling rate of the end grain surface of untreated cedar wood and heat-treated cedar wood. Although the conditions for determining the swelling rate are slightly different, the results shown in this figure are very similar to the results of heat treatment using superheated steam at a temperature of 200 ° C or higher without using salt shown in Figure 2. The swelling rate, which was just over 10% for the untreated test specimen, was suppressed to 6-7% when the weight loss rate due to heat treatment was about 3%, and to about 5% when the weight loss rate was 5%.

After impregnation with the aqueous solutions of ammonium chloride and magnesium chloride described in paragraph 0043, the test specimens were heat-treated using superheated steam at 180°C, crushed, extracted with cold water, and the acid-insoluble content was quantified using the Clason method. Figure 15 shows the acid-insoluble content at each concentration. The weight loss rate due to superheated steam treatment was similar for both ammonium chloride and magnesium chloride (see Figure 13), but while the acid-insoluble content increased as the concentration increased in the ammonium chloride-treated material, there was almost no increase in the magnesium chloride-treated material.

After weathering the test specimens that had been treated with superheated steam at 180°C as described above, an indoor bin test was conducted using M. palustris. The weight loss rate in the indoor bin test at each concentration is shown in Figure 16. As mentioned above, the weight loss rate due to superheated steam treatment was similar for both ammonium chloride and magnesium chloride, but there was a large difference in the results of the indoor bin test, with the material treated with magnesium chloride clearly exhibiting higher resistance to decay. This was more noticeable in specimens treated at higher concentrations.

Figure 17 shows a cross-sectional photograph of a cedar sapwood specimen with cross-sectional dimensions of 30 mm square and 70 mm long, which was impregnated with 1.5-2.5% aqueous ammonium sulfate and ammonium phosphate solutions and then treated with superheated steam at 170°C for 24 hours. As shown in Figure 9, the specimens impregnated with an aqueous ammonium chloride solution and then heat-treated did not show any unevenness in the treatment, whereas the specimens impregnated with an aqueous ammonium sulfate and ammonium phosphate solution and then heat-treated showed large unevenness in the treatment both inside and outside the specimen.

Filter paper containing 99% or more of α-cellulose was impregnated with an aqueous solution of ammonium chloride, ammonium sulfate, or ammonium phosphate at a concentration of 2.5%, and then the surface of the filter paper was lightly wiped to remove excess liquid. The filter paper was then dried at 105°C using a blower dryer. The weight loss behavior of the samples when they were heated from room temperature to 500°C at a heating rate of 10°C/min using a differential gravimetric thermal analyzer is shown in FIG. 18.
In the case of an aqueous ammonium chloride solution, the amorphous regions of cellulose were selectively decomposed from around 180°C, while the crystalline regions of cellulose were not decomposed until the temperature range was almost the same as that of untreated filter paper. In contrast, in the case of an aqueous ammonium sulfate solution and an aqueous ammonium phosphate solution, the decomposition starting temperature was higher, and once decomposition began, the entire cellulose, including the crystalline regions, was decomposed uniformly.

A thermogravimetric differential thermal analysis was carried out on a test specimen of cedar sapwood impregnated with a 2.5% aqueous solution of ammonium phosphate and ammonium sulfate as a test material, and a test specimen impregnated with an aqueous solution of ammonium chloride of the same concentration as the control material. The results are shown in Figure 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 temperature was examined. As a result, for the ammonium chloride-treated material, a weight loss thought to be due to the decomposition of hemicellulose and a part of the cellulose (amorphous region) was observed from about 150°C, whereas for ammonium phosphate and ammonium sulfate, the decomposition starting temperature was higher than that, and it was suggested that once decomposition started, it was not selective, but rather decomposed uniformly, including not only the amorphous regions of hemicellulose and cellulose, but also the crystalline regions of cellulose.

As mentioned above, ammonium chloride completely decomposes and sublimes at temperatures below 200°C. By using a salt such as ammonium chloride that completely decomposes and sublimes at temperatures below 200°C and carrying out heat treatment at 140-180°C, uniform decomposition and transformation becomes possible, and the weight loss due to heating is less than when previous technology was used, i.e., in the range where thermal degradation is small, the effect of imparting dimensional stability and high durability, especially decay resistance, to wood has been confirmed.

In the above explanation, only the experiments in which ammonium chloride solution etc. was impregnated into cedar sapwood specimens were given as examples, but it goes without saying that the same effect can be achieved by coating, spraying or immersing the cedar sapwood specimens in the solution rather than by impregnation.

Claims (6)

The method for heat treating wood comprises the steps of: immersing wood in, or coating, spraying or impregnating wood with, an aqueous solution of ammonium chloride in which 0.1 to 5.0% by weight of ammonium chloride is dissolved as a salt that is completely decomposed and sublimated when heated at 200°C or less and generates an inorganic acid through its decomposition; and heat treating the wood that has been immersed in, coated with, sprayed or impregnated with the aqueous solution of ammonium chloride at 140°C to 180°C, wherein the inorganic acid generated by the heat treatment decomposes and modifies amorphous regions of hemycellulose and cellulose in the wood to generate furan compounds . The wood heat treatment method 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 method for heat treating wood according to claim 1, wherein the heat treatment step is carried out for a treatment time of 72 hours or less after the temperature reaches 140° C. to 180° C. A method for producing dimensionally stable and durable lumber, comprising the steps of: immersing wood in, or coating, spraying or impregnating wood with, an aqueous solution containing 0.1 to 5.0% by weight of ammonium chloride, a salt that is completely decomposed and sublimated when heated at 200°C or less and generates an inorganic acid through its decomposition; and subjecting the wood that has been immersed, sprayed, coated or impregnated with the aqueous ammonium chloride solution to a heat treatment at 140°C to 180°C, wherein the inorganic acid generated by the heat treatment decomposes and modifies amorphous regions of hemycellulose and cellulose in the wood to generate furan compounds . 5. The method for producing lumber having dimensional stability and durability according to claim 4, wherein the weight loss rate after the heat treatment step is 3% or more and 20% or less. 6. The method for producing lumber having dimensional stability and durability according to claim 4 or 5, characterized in that in the heat treatment step, the treatment time from when the temperature reaches 140° C. to 180° C. is within 72 hours.