JP5329279B2 - Method for producing cellulose nanofiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method capable of efficiently manufacturing a cellulose nano-fiber dispersion liquid of a high concentration, excellent in flowability and transparency at a low energy. <P>SOLUTION: In the manufacturing method, a cellulose-based material is oxidized in the presence of (1) an N-oxyl compound and (2) a bromide, an iodide or the mixture thereof in water using an oxidizing agent, hydrogen peroxide and ozone are added thereto to perform oxidative decomposition treatment, and subsequently, it is subjected to defibering-dispersion treatment. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a method capable of producing a cellulose nanofiber dispersion liquid having a lower energy and higher concentration than conventional from a cellulose-based raw material oxidized with an N-oxyl compound.

  Cellulose-based raw materials in the presence of a catalytic amount of 2,2,6,6-tetramethyl-1-piperidine-N-oxy radical (hereinafter referred to as TEMPO) and sodium hypochlorite which is an inexpensive oxidizing agent. When treated, carboxyl groups can be efficiently introduced onto the surface of cellulose microfibrils. Cellulosic raw materials into which these carboxyl groups have been introduced are highly viscous and transparent by performing a simple mechanical treatment with a mixer in water. It is known that it can be prepared into an aqueous cellulose nanofiber dispersion (Non-Patent Document 1).

  Cellulose nanofibers are a novel biodegradable water-dispersed material. Since the carboxyl group is introduced into the surface of the cellulose nanofiber by an oxidation reaction, the cellulose nanofiber can be freely modified with the carboxyl group as a base point. In addition, since the cellulose nanofibers obtained by the above method are in the form of a dispersion, the quality can be modified by blending with various water-soluble polymers or by combining with organic / inorganic pigments. it can. Furthermore, cellulose nanofibers can be made into sheets or fibers. Taking advantage of these characteristics of cellulose nanofibers, it is envisaged to be applied to highly functional packaging materials, transparent organic base members, high performance fibers, separation membranes, regenerative medical materials, and the like. In the future, it is expected to develop new high-functional products that are essential for the formation of a recycling-type safety and security society by making the best use of the characteristics of cellulose nanofibers.

Saito, T., et al., Cellulose Commun., 14 (2), 62 (2007)

  However, the cellulose nanofiber dispersion obtained by oxidizing the cellulose raw material using TEMPO and defibrating with a mixer is 0.3 to 0.5% (w / v). Even at low concentrations, the B-type viscosity (60 rpm, 20 ° C.) has a very high viscosity, such as about 800 to 4000 mPa · s, is not easy to handle, and its application range is actually limited. It was. For example, when a cellulose nanofiber dispersion is applied to a substrate to form a film on the substrate, the dispersion cannot be uniformly applied if the viscosity of the dispersion is too high, so the B-type viscosity of the dispersion (60 rpm, 20 ° C.) must be adjusted to about 500 to 3000 mPa · s, and for that purpose, the concentration of cellulose nanofibers in the dispersion is reduced to a low concentration of about 0.2 to 0.4% (w / v). I had to set it. However, when such a low-concentration dispersion is used, there is a problem that the application and drying must be repeated many times until the desired film thickness is achieved, resulting in poor efficiency.

  In addition, when the cellulose nanofiber dispersion is mixed with a paint containing a pigment and a binder and applied to paper or the like, the dispersion cannot be uniformly mixed if the viscosity of the dispersion is too high. However, if such a low-concentration dispersion liquid is used, the concentration of the paint becomes dilute, making it difficult to apply the sufficient viscosity necessary for application and increasing the drying load. In addition, there is also a problem that the desired function expected for the coating film such as gloss development, surface strength, and suppression of printing unevenness does not appear because the effective coating film becomes thin by the penetration of the paint into the base paper. .

  Thus, in the conventional method in which the cellulose-based raw material obtained by oxidation using TEMPO is defibrated using a mixer, the viscosity of the obtained dispersion becomes very high, causing various problems. . Further, if the viscosity is too high, the dispersion proceeds only around the stirring blades, resulting in non-uniform dispersion, resulting in a dispersion with low transparency.

  In addition, if the oxidized cellulosic material is defibrated using a homogenizer with higher defibration / dispersion power than the mixer, the cellulosic material will thicken significantly in the initial stage of dispersion and fluidity will deteriorate. There is a problem that the amount of power consumption required sometimes increases significantly, the cellulose nanofiber dispersion liquid adheres to the inside of the apparatus and the dispersion is not sufficiently performed, and operations such as taking out the dispersion liquid from the apparatus are performed. There is also a problem that the yield of the dispersion is lowered due to difficulty.

  In view of the above problems, an object of the present invention is to provide a method capable of producing a high-concentration cellulose nanofiber dispersion excellent in fluidity and transparency with low energy.

  As a result of intensive studies to solve the problems of the prior art, the present inventors treated cellulose raw materials obtained by oxidation with TEMPO with hydrogen peroxide and ozone, and then defibrated and dispersed. As a result, it was found that a cellulose nanofiber dispersion excellent in fluidity and transparency can be efficiently prepared even at a high concentration of 1 to 3% (w / v), and the present invention was completed. It was.

  According to the present invention, a cellulosic raw material is oxidized in the presence of an N-oxyl compound and bromide, iodide, or a mixture thereof, and the resulting oxidized cellulosic raw material is treated with hydrogen peroxide and ozone. Then, by performing defibration / dispersion treatment, a cellulose nanofiber dispersion that is excellent in fluidity, easy to handle, and excellent in transparency even at high concentrations can be efficiently consumed with low power consumption. Can be manufactured. The cellulose nanofiber dispersion obtained by the present invention is excellent in fluidity even at a high concentration.For example, when a cellulose nanofiber is applied to a substrate to form a film on the substrate, A paint containing cellulose nanofibers at a high concentration of 1 to 3% (w / v) can be prepared at a low viscosity of 500 to 3000 mPa · s (B-type viscosity, 60 rpm, 20 ° C.). There is an advantage that a film having a thickness of about 5 to 30 μm can be formed only by coating. Conventionally, in order to prepare a paint having a viscosity of about 500 to 3000 mPa · s (B-type viscosity, 60 rpm, 20 ° C.), the concentration of cellulose nanofiber is 0.2 to 0.4% (w / v). In order to produce a film having a thickness of about 5 to 30 μm, it has been necessary to repeat coating and drying many times. The feature of the cellulose nanofiber dispersion obtained by the present invention having a high fluidity at a high concentration is very excellent.

    The present invention comprises (1) an N-oxyl compound and (2) an oxidized cellulosic material by oxidizing the cellulosic material in water using an oxidizing agent in the presence of bromide, iodide or a mixture thereof. It is characterized by treatment with hydrogen peroxide solution and ozone, followed by defibration / dispersion treatment, which can reduce power consumption in defibration / dispersion treatment, and make cellulose nanofibers efficient with low energy Can be manufactured well.

(N-oxyl compound)
As the N-oxyl compound used in the present invention, any compound can be used as long as it promotes the target oxidation reaction. For example, the N-oxyl compound used in the present invention includes a substance represented by the following general formula (Formula 1).

(In Formula 1, R ^{1 to} R ⁴ represent the same or different alkyl groups having about 1 to 4 carbon atoms.)
Among the compounds represented by Formula 1, 2,2,6,6-tetramethyl-1-piperidine-oxy radical (hereinafter referred to as TEMPO) and 4-hydroxy-2,2,6,6-tetramethyl-1 A compound that generates a piperidine-oxy radical (hereinafter referred to as 4-hydroxy TEMPO) is preferable. In addition, the radical of the N-oxyl compound represented by any one of the following formulas 2 to 4, that is, the hydroxyl group of 4-hydroxy TEMPO is etherified with alcohol, or esterified with carboxylic acid or sulfonic acid, and has an appropriate hydrophobicity. The imparted 4-hydroxy TEMPO derivative is particularly preferable because it is inexpensive and can provide uniform oxidized cellulose.

(In formulas 2 to 4, R is a linear or branched carbon chain having 4 or less carbon atoms.)
Furthermore, the radical of the N-oxyl compound represented by the following formula 5, that is, an azaadamantane type nitroxy radical, is particularly preferable because it can produce uniform cellulose nanofibers in a short time.

(In Formula 5, R ⁵ and R ⁶ represent the same or different hydrogen or a C _{1 to} C ₆ linear or branched alkyl group.)
The amount of the N-oxyl compound used is not particularly limited as long as it is a catalyst amount that can make the cellulose-based raw material into nanofibers. For example, about 0.01 to 10 mmol, preferably 0.01 to 1 mmol, and more preferably about 0.05 to 0.5 mmol can be used with respect to 1 g of the cellulosic raw material.

(Bromide or iodide)
As the bromide or iodide used in oxidizing the cellulosic raw material, a compound that can be dissociated and ionized in water, such as an alkali metal bromide or an alkali metal iodide, can be used. The amount of bromide or iodide used can be selected as long as the oxidation reaction can be promoted. For example, 0.1 to 100 mmol, preferably 0.1 to 10 mmol, and more preferably about 0.5 to 5 mmol can be used for 1 g of cellulosic raw material.

(Oxidant)
As the oxidizing agent used for oxidizing the cellulosic raw material, the target oxidation reaction such as halogen, hypohalous acid, halous acid, perhalogen acid or salts thereof, halogen oxide, peroxide can be promoted. Any oxidizing agent can be used as long as it is an oxidizing agent. Among these, from the viewpoint of nanofiber production costs, sodium hypochlorite, which is currently most widely used in industrial processes and has low environmental impact, is particularly suitable. The amount of the oxidizing agent used can be selected within a range that can promote the oxidation reaction. For example, 0.5 to 500 mmol, preferably 0.5 to 50 mmol, and more preferably about 2.5 to 25 mmol can be used for 1 g of cellulosic raw material.

(Cellulosic material)
The cellulose-based raw material used in the present invention is not particularly limited, and kraft pulp or sulfite pulp derived from various woods, powdered cellulose obtained by pulverizing them with a high-pressure homogenizer or a mill, or chemical treatment such as acid hydrolysis. In addition to the microcrystalline cellulose powder purified by the above, plants such as kenaf, hemp, rice, bacus, and bamboo can also be used. Of these, bleached kraft pulp, bleached sulfite pulp, powdered cellulose, or microcrystalline cellulose powder is preferably used from the viewpoint of mass production and cost. In addition, it is particularly preferable to use powdered cellulose and microcrystalline cellulose powder because a cellulose nanofiber dispersion having a lower viscosity can be produced even at a high concentration.

Powdered cellulose is a rod-like particle made of microcrystalline cellulose obtained by removing a non-crystalline part of wood pulp by acid hydrolysis and then pulverizing and sieving. The degree of polymerization of cellulose in the powdered cellulose is preferably about 100 to 500, the degree of crystallinity of the powdered cellulose by X-ray diffraction is preferably 70 to 90%, and the volume average particle size by a laser diffraction type particle size distribution measuring device. Is preferably 100 μm or less, more preferably 50 μm or less. When the volume average particle size is 100 μm or less, a cellulose nanofiber dispersion having excellent fluidity can be obtained. As the powdered cellulose used in the present invention, for example, a fixed particle size in the form of a rod shaft produced by a method of purifying and drying an undegraded residue obtained after acid hydrolysis of a selected pulp, pulverizing and sieving. it may be a crystalline cellulose powder having a distribution, (manufactured by Nippon Paper Chemicals Co.) KC flock ^R, Ceolus ^TM (manufactured by Asahi Kasei Chemicals Corporation), a commercially available product may be used, such as Avicel ^R (FMC Corp.) .

(Oxidation reaction conditions)
The method of the present invention is characterized in that the oxidation reaction can proceed smoothly even under mild conditions. Therefore, the reaction temperature may be a room temperature of about 15 to 30 ° C. In addition, since a carboxyl group produces | generates in a cellulose with progress of reaction, the fall of pH of a reaction liquid is recognized. In order to advance the oxidation reaction efficiently, it is desirable to maintain the pH of the reaction solution at about 9 to 12, preferably about 10 to 11, by adding an alkaline solution such as an aqueous sodium hydroxide solution. The reaction time in the oxidation reaction can be appropriately set and is not particularly limited, but is, for example, about 0.5 to 4 hours.

(Oxidative decomposition treatment with hydrogen peroxide and ozone)
In the present invention, hydrogen peroxide and ozone are added to an oxidized cellulose raw material obtained using an N-oxyl compound, and the raw material is oxidatively decomposed. The ozone used in the present invention can be generated by a known method using an ozone generator using air or oxygen as a raw material. The addition amount (mass) of ozone in the present invention is preferably 0.1 to 3 times the absolute dry mass of the cellulosic material. If the amount of ozone added is at least 0.1 times the absolute dry mass of the cellulosic material, the amorphous part of the cellulose can be sufficiently decomposed, greatly increasing the energy required for the defibration / dispersion treatment in the next step. Can be reduced. Moreover, if it is 3 times or less, the excessive decomposition | disassembly of a cellulose can be suppressed and the fall of the yield of a cellulose raw material can be prevented. The amount of ozone added is more preferably 0.3 to 2.5 times the absolute dry mass of the cellulosic material, and even more preferably 0.5 to 1.5 times.

  The addition amount (mass) of hydrogen peroxide used in the present invention is preferably 0.001 to 1.5 times the absolute dry mass of the cellulosic material. When hydrogen peroxide is used in an amount of 0.001 times or more of the addition amount of the cellulosic material, a synergistic effect between ozone and hydrogen peroxide is exhibited. In addition, it is sufficient to use hydrogen peroxide in an amount about 1.5 times or less that of the cellulosic raw material for decomposing the cellulosic raw material, and it is thought that a larger addition amount leads to an increase in cost. The amount of hydrogen peroxide added is more preferably 0.1 to 1.0 times the absolute dry mass of the cellulosic material.

  The oxidative decomposition treatment with ozone and hydrogen peroxide is pH 2 to 12, preferably pH 4 to 10, more preferably pH 6 to 8, and the temperature is 10 to 90 ° C, preferably 20 to 70 ° C, more preferably 30. It is preferable from about oxidative decomposition reaction efficiency to carry out at -50 degreeC for 1 to 20 hours, Preferably it is 2 to 10 hours, More preferably, it is about 3 to 6 hours.

  As a device for performing treatment with ozone and hydrogen peroxide, a device commonly used by those skilled in the art can be used. For example, a normal chamber equipped with a reaction chamber, a stirrer, a chemical injection device, a heater, and a pH electrode. A reactor can be used.

  After the treatment with ozone and hydrogen peroxide, ozone and hydrogen peroxide remaining in the aqueous solution can effectively work in the defibration / dispersion treatment in the next step, and can further promote the lowering of the viscosity of the cellulose nanofiber dispersion. .

(Defibration / dispersion processing)
In the present invention, the oxidized cellulose-based raw material is irradiated with ultraviolet rays and then subjected to defibration and dispersion treatment. Examples of types of defibrating / dispersing devices include high-speed rotation type, colloid mill type, high pressure type, roll mill type, ultrasonic type, etc. Cellulose nanofiber dispersion liquid with excellent transparency and fluidity can be efficiently used. In order to obtain, it is preferable to treat with a wet high pressure or ultra high pressure homogenizer that can be dispersed under conditions of 50 MPa or more, preferably 100 MPa or more, and more preferably 140 MPa or more.

(Cellulose nanofiber)
The cellulose nanofiber produced by the present invention is a single microfibril of cellulose having a width of 2 to 5 nm and a length of about 1 to 5 μm. In the present invention, “to form a nanofiber” means that a cellulose-based raw material is processed into cellulose nanofiber which is a single microfibril of cellulose having a width of about 2 to 5 nm and a length of about 1 to 5 μm.

  The cellulose nanofiber dispersion obtained by the present invention has a B-type viscosity (60 rpm, 20 ° C.) at a concentration of 1.0% by mass (w / v) of 1000 mPa · s or less, preferably 800 mPa · s or less, more preferably It is 500 mPa · s or less, and the light transmittance (660 nm) at a concentration of 0.1% by mass (w / v) is 90% or more, preferably 95% or more. Cellulose nanofibers produced according to the present invention are excellent in fluidity and transparency, and are also excellent in barrier properties and heat resistance, and thus can be used for various applications such as packaging materials.

  In the present invention, the B-type viscosity of the cellulose nanofiber dispersion can be measured using a normal B-type viscometer commonly used by those skilled in the art, for example, TV-10 type viscosity of Toki Sangyo Co., Ltd. Using a meter, it can be measured at 20 ° C. and 60 rpm.

  The light transmittance of the cellulose nanofiber dispersion can be measured with an ultraviolet / visible spectrophotometer.

(Operation of the present invention)
In the present invention, the cellulosic raw material oxidized using an N-oxyl compound is treated with ozone and hydrogen peroxide, and then defibrated and dispersed to provide excellent fluidity and transparency even at high concentrations. The cellulose nanofiber dispersion can be obtained with low power consumption. The reason is guessed as follows. Carboxyl groups are localized on the surface of the cellulose-based raw material oxidized using the N-oxyl compound, and a hydrated layer is formed. Therefore, it is considered that there is a microscopic gap between the raw materials which is not found in ordinary pulp due to the action of the charge repulsive force between the carboxyl groups. Then, when the raw material is treated with ozone and hydrogen peroxide, hydroxy radicals having excellent oxidizing power are generated from ozone and hydrogen peroxide, and the cellulose chains in the raw material are efficiently oxidized and decomposed, and finally the cellulose-based raw material Is considered to be a short fiber. By shortening the cellulose-based raw material, it is considered that the B-type viscosity of the dispersion in the defibration / dispersion treatment in the next step is significantly reduced, and the fluidity of the dispersion is improved. Moreover, it is thought that the cellulose nanofiber dispersion liquid excellent in transparency is obtained by shortening the fiber of a cellulose raw material.

  Next, based on an Example, this invention is demonstrated still in detail.

  5 g (absolutely dried) bleached unbeaten sulfite pulp (Nippon Paper Chemical Co., Ltd.) derived from coniferous tree was added to 500 ml of an aqueous solution in which 78 mg (0.5 mmol) of TEMPO (Sigma Aldrich) and 755 mg (7 mmol) of sodium bromide were dissolved, Stir until the pulp is uniformly dispersed. After adding 18 ml of an aqueous sodium hypochlorite solution (effective chlorine 5%) to the reaction system, the pH was adjusted to 10.3 with an aqueous 0.5N hydrochloric acid solution to initiate the oxidation reaction. During the reaction, the pH in the system was lowered, but a 0.5N aqueous sodium hydroxide solution was successively added to adjust the pH to 10. After reacting for 2 hours, it was filtered through a glass filter and washed thoroughly with water to obtain oxidized pulp. Ozone concentration 6g / L (corresponding to 0.6 times the absolute dry mass of cellulosic raw material), hydrogen peroxide concentration 3g / L (absolutely dry cellulosic raw material) in 2L of 1% (w / v) slurry of oxidized pulp (Corresponding to 0.3 times the mass) and treated at room temperature for 6 hours. When the oxidized pulp slurry treated with ozone and hydrogen peroxide was treated 10 times with an ultra-high pressure homogenizer (treatment pressure 140 MPa), a transparent gel dispersion was obtained. The B-type viscosity (60 rpm, 20 ° C.) of the obtained 1% (w / v) cellulose nanofiber dispersion was measured using a TV-10 viscometer (Toki Sangyo Co., Ltd.). In addition, the transparency (660 nm light transmittance) of a 0.1% (w / v) cellulose nanofiber dispersion was measured using a UV-VIS spectrophotometer UV-265FS (Shimadzu Corporation). The power consumption required for the defibration / dispersion processing was determined by (power during processing) × (processing time) ÷ (amount of sample processed). The results are shown in Table 1.

  A nanofiber dispersion was obtained in the same manner as in Example 1 except that the treatment pressure of the ultrahigh pressure homogenizer was 100 MPa. The results are shown in Table 1.

  A nanofiber dispersion was obtained in the same manner as in Example 1 except that the treatment pressure of the ultrahigh pressure homogenizer was 50 MPa. The results are shown in Table 1.

  A nanofiber dispersion was obtained in the same manner as in Example 1 except that the treatment pressure of the ultrahigh pressure homogenizer was 30 MPa. The results are shown in Table 1.

  A nanofiber dispersion was obtained in the same manner as in Example 1 except that powdered cellulose (Nippon Paper Chemical Co., Ltd., particle size 75 μm) was used as the cellulose-based material. The results are shown in Table 1.

  A nanofiber dispersion was obtained in the same manner as in Example 3 except that powdered cellulose (Nippon Paper Chemical Co., Ltd., particle size 75 μm) was used as the cellulose-based material. The results are shown in Table 1.

  A nanofiber dispersion was obtained in the same manner as in Example 4 except that powdered cellulose (Nippon Paper Chemical Co., Ltd., particle size 75 μm) was used as the cellulose-based material. The results are shown in Table 1.

  A nanofiber dispersion was obtained in the same manner as in Example 1 except that a high shear mixer (circumferential speed 37 m / s, Nippon Seiki Seisakusho) equipped with a rotary blade was used as a dispersing device. The results are shown in Table 1.

  The ozone concentration is 10 g / L (corresponding to 1.0 times the absolute dry mass of the cellulosic material), and the hydrogen peroxide concentration is 3 g / L (corresponding to 0.3 times the absolute dry mass of the cellulosic material). The nanofiber dispersion was obtained in the same manner as in Example 1 except that the addition was performed so that The results are shown in Table 1.

[Comparative Example 1]
A nanofiber dispersion was obtained in the same manner as in Example 1 except that the treatment with ozone and hydrogen peroxide was not performed. The results are shown in Table 1.

[Comparative Example 2]
A nanofiber dispersion was obtained in the same manner as in Example 1 except that treatment with ozone alone was performed. The results are shown in Table 1.

[Comparative Example 3]
A nanofiber dispersion was obtained in the same manner as in Example 1 except that treatment with hydrogen peroxide alone was performed. The results are shown in Table 1.

[Comparative Example 4]
A nanofiber dispersion was obtained in the same manner as in Example 8 except that treatment with ozone and hydrogen peroxide was not performed. The results are shown in Table 1.

  The 1% (w / v) cellulose nanofiber dispersion produced in Example 1 was coated on one side of a polyethylene terephthalate film (thickness 20 μm) with a bar exclusively for hand-coating (bar No. 16), and at 50 ° C. Dried to form a film. The thickness of the film was about 5.3 μm.

  The 1% (w / v) cellulose nanofiber dispersion produced in Example 3 was applied to one side of a polyethylene terephthalate film (thickness 20 μm) with a bar (bar No. 16) dedicated to hand coating, at 50 ° C. Dried to form a film. The thickness of the film was about 7.1 μm.

  The 1% (w / v) cellulose nanofiber dispersion produced in Example 5 was applied to one side of a polyethylene terephthalate film (thickness 20 μm) with a bar (bar No. 16) dedicated to hand coating, at 50 ° C. Dried to form a film. The film thickness was about 3.6 μm.

  The 1% (w / v) cellulose nanofiber dispersion produced in Example 9 was applied to one side of a polyethylene terephthalate film (thickness 20 μm) with a hand-painted bar (bar No. 16) at 50 ° C. Dried to form a film. The thickness of the film was about 5.7 μm.

[Comparative Example 5]
The concentration of the 1% (w / v) cellulose nanofiber dispersion produced in Comparative Example 1 was adjusted to have a B-type viscosity of 600 mPa · s (60 rpm, 20 ° C.). The cellulose nanofiber concentration at this time was 0.4% (w / v). This dispersion was applied to one side of a polyethylene terephthalate film (thickness: 20 μm) with a hand-painted bar (bar No. 16) and dried at 50 ° C. to form a film. The film thickness was about 2.0 μm. In order to form a 5.3 μm film having the same thickness as in Example 10, it was necessary to repeat coating and drying at least three times.

Claims (4)

(A) (1) N-oxyl compound, and (2) oxidizing a cellulosic raw material using an oxidizing agent in the presence of a compound selected from the group consisting of bromide, iodide or a mixture thereof,
(B) Addition of hydrogen peroxide and ozone to the cellulosic material from (A) for oxidative decomposition treatment; and
(C) Nanofiberization by defibrating and dispersing the cellulosic material from (B),
The manufacturing method of the cellulose nanofiber characterized by including.   The manufacturing method of the cellulose nanofiber of Claim 1 whose addition amount of the said ozone is 0.1 to 3 times with respect to the absolute dry mass of the cellulose raw material from said (A).   The production of cellulose nanofibers according to claim 1 or 2, wherein the amount of hydrogen peroxide added is 0.001 to 1.5 times the absolute dry mass of the cellulosic material from (A). Method. The method for producing cellulose nanofiber according to any one of claims 1 to 3, wherein the fibrillation / dispersion treatment is performed under a pressure of 50 MPa or more.