JP2018199753A - Hydrogel and production method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength hydrogel containing anion-modified cellulose nanofibers and a method for producing the same. SOLUTION: A modification step of chemically modifying a cellulose raw material to obtain anion-modified cellulose, a defibration step of defibrating anion-modified cellulose to obtain anion-modified cellulose nanofibers, and anion-modified cellulose nanofibers in the presence of water. Hydrogel having a breaking strength of 2.0 to 8.0 kPa in a method for producing hydrogel obtained by high-temperature and high-pressure treatment and a gel concentration of 0.1 to 3.0% by mass produced by the method. gel. [Selection diagram] None

Description

  The present invention relates to a hydrogel containing anion-modified cellulose nanofibers and a method for producing the hydrogel.

  When a cellulose raw material is treated in the presence of 2,2,6,6-tetramethyl-1-piperidine-N-oxy radical (hereinafter also referred to as “TEMPO”) and an inexpensive oxidizing agent, sodium hypochlorite. In addition, carboxyl groups can be efficiently introduced onto the surface of cellulose microfibrils. When cellulose introduced with a carboxyl group is treated in water with a mixer or the like, a highly viscous and transparent aqueous dispersion of cellulose nanofibers is obtained.

  As described above, since a carboxyl group is introduced on the surface of the cellulose nanofiber, it can be freely modified starting from the carboxyl group. In addition, since the cellulose nanofiber is in the form of a dispersion, it can be blended with a water-soluble polymer, or can be modified by being combined with an organic / inorganic pigment. Furthermore, cellulose nanofibers can be formed into sheets or fibers. Due to such characteristics, development of new high-functional products in which cellulose nanofibers are applied to high-functional packaging materials, transparent organic substrate members, high-functional fibers, separation membranes, regenerative medical materials, and the like has been studied.

Attempts have been made to gel cellulose nanofibers when applied to the above-described novel high-functional products. For example, Patent Documents 1 and 2 disclose a method of gelation by subjecting to acidic conditions. Patent Document 3 discloses that acid treatment and subsequent base treatment provide excellent gelling properties. Furthermore, Patent Document 4 describes treatment with a strong alkaline aqueous solution, addition to liquid paraffin containing a surfactant, addition of acid and organic solvent, neutralization, removal of liquid paraffin, and immersion in a NaIO ₄ aqueous solution. A method for producing a biodegradable cellulose nanofiber gel through a series of steps for imparting degradability is disclosed.

JP 2010-235687 A JP 2012-001626 A JP-T-2015-507181 Japanese Patent Laid-Open No. 2016-210893

In the methods described in Patent Documents 1 to 4, it is essential to use various additives, and there is a concern about cost increase in commercialization.
Therefore, it is desired to more easily produce a high-strength hydrogel containing cellulose nanofibers.

  The subject of this invention is providing the high intensity | strength hydrogel containing an anion modified cellulose nanofiber.

As a result of intensive investigations on the above problems, the present inventors have found that the above problems can be solved by including anion-modified cellulose nanofibers obtained by treating a salt of anion-modified cellulose nanofibers in the presence of water at high temperature and high pressure. The headline and the present invention were completed. Moreover, it discovered that it could decolorize by immersing the anion modified cellulose nanofiber colored by the high temperature and high pressure process in water.
That is, the present inventors provide the following [1] to [9].
[1] A hydrogel containing anion-modified cellulose nanofibers and having a breaking strength of 2.0 to 8.0 kPa at a gel concentration of 0.1 to 3.0% by mass.
[2] The hydrogel according to [1], wherein the compression elastic modulus is 0.5 to 8.0 kPa at a gel concentration of 0.1 to 3.0% by mass.
[3] The hydrogel as described in [1] or [2] above, wherein the light transmittance at a wavelength of 600 nm is 15 to 95% at a gel concentration of 0.1 to 3.0% by mass.
[4] The hydrogel according to any one of [1] to [3], wherein the anion-modified cellulose nanofiber is a carboxylated cellulose nanofiber.
[5] The hydrogel according to any one of [1] to [3], wherein the anion-modified cellulose nanofiber is a carboxymethylated cellulose nanofiber.
[6] A modification step of chemically modifying a cellulose raw material to obtain anion-modified cellulose, a defibration step of defibrating the anion-modified cellulose to obtain anion-modified cellulose nanofibers, and the anion-modified cellulose nanofibers in the presence of water, The manufacturing method of the hydrogel which has the temperature of 80-400 degreeC, the pressure of 0.1-50 MPa, and the gelling process which heats on the conditions for 0.1 to 6000 minutes, and obtains hydrogel.
[7] The modification step is a step of obtaining oxidized cellulose by oxidizing the cellulose raw material using an oxidizing agent in the presence of an N-oxyl compound and bromide, iodide or a mixture thereof. ] The manufacturing method of the hydrogel of description.
[8] The production of the hydrogel according to [6], wherein the modification step is a step of subjecting the cellulose raw material to a mercerization treatment with a mercerizing agent and then reacting with a carboxymethylating agent to obtain carboxymethylated cellulose. Method.
[9] The method for producing a hydrogel according to any one of [6] to [8], further including a decolorization step of decolorizing the hydrogel by immersing it in water.

  ADVANTAGE OF THE INVENTION According to this invention, the high intensity | strength hydrogel containing an anion modified cellulose nanofiber can be provided.

FIG. 1 is a photograph showing a state where the hydrogel is self-supporting. FIG. 2 is a scatter diagram showing the relationship between the heating time and gel concentration of the hydrogels of Examples and Comparative Examples. FIG. 3 is a scatter diagram showing the relationship between the heating time, the compression modulus, and the breaking strength. FIG. 4 is a chart showing the relationship between the heating time and the light transmittance of the hydrogel before decolorization. FIG. 5 is a chart showing the relationship between the carboxylated CNF and the light transmittance of the hydrogel before and after decoloring (Example 4).

Hereinafter, the present invention will be described in detail with reference to preferred embodiments thereof.
The “hydrogel” generally refers to a gel using water as a dispersion medium. In the present specification, “high strength” means having strength capable of supporting itself while maintaining its shape. The term “self-supporting” means that a hydrogel column having a bottom area of 0.5 cm ² and a height of 0.6 cm is self-supporting for at least 24 hours. FIG. 1 shows a photograph showing a state where the hydrogel is self-supporting.

[1. Hydrogel]
The hydrogel of the present invention is a hydrogel containing anion-modified cellulose nanofibers and having a breaking strength of 2.0 to 8.0 kPa at a gel concentration of 0.1 to 3.0% by mass. The breaking strength of the hydrogel of the present invention is preferably 2.5 to 7.5 kPa, more preferably 3.0 to 7.0 kPa. Since it has such a breaking strength, the hydrogel of the present invention has a self-supporting strength.
The breaking strength can be measured with an EZ-test (compression tester) manufactured by Shimadzu Corporation.

One of the characteristics of the hydrogel of the present invention is that the anion-modified cellulose nanofiber is neutral to weakly alkaline. In other words, the anionic functional group introduced into the cellulose nanofibers, acid type (e.g., -COOH groups) rather than in the form of a salt type (e.g., -COO ^- group) be in the form of. Therefore, the hydrogel of the present invention has biodegradability, and is preferable for use as a material in paper diapers, moisturizing cosmetics, and agriculture.

Conventional gelling, anionic functional groups introduced into cellulose nanofibers (e.g., -COO ^- group) is performed, acid form by acid treatment (e.g., -COOH group) is converted to. The present inventors infer the reason why anion-modified cellulose nanofibers are gelated by acid treatment as follows. The anion-modified cellulose nanofibers converted to the acid form have a reduced zeta potential because the charge present on the surface is reduced. Therefore, the cellulose nanofibers self-assemble by van der Waals forces between the cellulose nanofibers to form a polymer-like aggregate and gel.
When the obtained gel is neutralized, it is considered that the charge existing on the surface increases, the zeta potential increases, and the gelled structure collapses. Therefore, it is considered that the salt type anion-modified cellulose nanofiber cannot be gelled.

On the other hand, in the hydrogel of the present invention, the anionic functional group introduced into the cellulose nanofibers, salt type (e.g., -COO ^- group) is in the form of. The present inventors infer the reason why the hydrogel of the present invention containing salt-type anion-modified cellulose nanofibers is gelled as follows. As will be described later, the hydrogel of the present invention is obtained by heating anion-modified cellulose nanofibers in the presence of water at a temperature of 80 to 400 ° C., a pressure of 0.1 to 50 MPa, and a time of 0.1 to 6000 minutes. . In the heat treatment, the anionic functional group (e.g., -COO ^- group) part of the glucuronic acid residue with the desorbed charge present on the surface of the anionically modified cellulose nanofibers is reduced. Therefore, the zeta potential decreases, and the cellulose nanofibers self-assemble due to van der Waals forces between cellulose nanofibers and hydrogen bonds due to hydroxyl groups at the detached sites, forming a polymer-like aggregate and gelling. To do.

The compression elastic modulus of the hydrogel of the present invention is preferably 0.5 to 8.0 kPa, more preferably 0.6 to 7.5 kPa, and 0.7 to 3.0 at a gel concentration of 0.1 to 3.0% by mass. 7.0 kPa is more preferable. Since it has such a compression elastic modulus, the hydrogel of the present invention can maintain its shape when it is self-supporting.
The compression modulus can be measured with a compression tester (EZ-test from Shimadzu Corporation).

The light transmittance of the hydrogel of the present invention at a wavelength of 600 nm is preferably 15 to 95%, more preferably 50 to 95%, and more preferably 70 to 95% at a gel concentration of 0.1 to 3.0% by mass. Is more preferable. Since it has such light transmittance, when the hydrogel of the present invention is used as a material, it is difficult for the material to be colored due to the hydrogel. Therefore, the hydrogel of the present invention can be used for a wide range of materials.
The light transmittance can be measured with a UV-visible spectrophotometer (JASCO V530).

  The gel concentration can be calculated as follows. Measure the mass of the hydrogel. Next, the mass of the dried product obtained by drying the hydrogel whose mass was measured is measured. The gel concentration can be calculated by dividing the mass of the dried product by the mass of the hydrogel and multiplying by 100.

(Anion-modified cellulose nanofiber)
The anion-modified cellulose nanofibers contained in the hydrogel of the present invention are those obtained by treating anion-modified cellulose nanofibers obtained by defibrating anion-modified cellulose in the presence of water at high temperature and high pressure. A part of the glucuronic acid residue having an anionic functional group is eliminated by high temperature and high pressure treatment in the presence of water. Therefore, the anion-modified cellulose nanofibers contained in the hydrogel of the present invention are different from anion-modified cellulose nanofibers obtained by defibrating anion-modified cellulose in terms of physical properties.
Hereinafter, in the present specification, when only cellulose nanofibers obtained by fibrillating anion-modified cellulose are meant, they are abbreviated as “CNF”.

  Examples of the anion-modified cellulose nanofiber include carboxylated cellulose nanofiber, carboxymethylated cellulose nanofiber, and cellulose nanofiber treated with a compound having a phosphate group. Among these, carboxylated cellulose nanofiber or carboxymethylated cellulose nanofiber is preferable.

The average fiber length of the anion-modified CNF is preferably 50 to 5000 nm, and more preferably 100 to 5000 nm. Moreover, 1-1000 nm is preferable, as for the average fiber diameter of anion modified CNF, 2-300 nm is more preferable, and 2.5-7 nm is further more preferable. For anion-modified CNF, it is important for gelation to use CNF, which is a single microfibril of cellulose.
The average fiber length of anion-modified CNF can be calculated as follows. Anion-modified CNF can be fixed on a mica section, 200 fiber lengths can be measured using an atomic force microscope (AFM), and a length (weighted) average fiber length can be calculated. In addition, the measurement of fiber length is performed in the range of arbitrary length using image analysis software WinROOF (made by Mitani Corporation).
The average fiber diameter of anion-modified CNF can be calculated as follows. An anion-modified CNF aqueous dispersion diluted so that the concentration of the anion-modified CNF is 0.001% by mass is prepared. This diluted dispersion is thinly spread on a mica sample stage and heated and dried at 50 ° C. to prepare an observation sample. The cross-sectional height of the shape image observed with an atomic force microscope (AFM) can be measured to calculate the weighted average fiber diameter.

(Carboxylated cellulose nanofiber)
In the carboxylated cellulose nanofiber, at least a part of the cellulose molecular chain is preferably composed of a structural unit having a carboxyl group in which a carbon atom having a primary hydroxyl group at the C6 position of a glucopyranose unit is selectively oxidized. .
Here, the glucopyranose unit refers to a structural unit represented by the following formula (0).

  The carboxyl group amount of carboxylated CNF is preferably 0.6 to 2.0 mmol / g, more preferably 1.0 to 2.0 mmol / g, and 1.4 to 1 with respect to the absolute dry mass of carboxylated CNF. More preferably, 6 mmol / g. When the carboxyl group amount is 0.6 mmol / g or more, the carboxyl group is sufficiently introduced to the surface of the cellulose molecular chain, and can have an electrostatic repulsion effect, and can be easily converted into nanofibers by defibration. Moreover, the excessive swelling by a water | moisture content can be suppressed as it is 2.0 mmol / g or less.

The amount of carboxyl groups can be measured as follows. 60 ml of a 0.5% by weight slurry (aqueous dispersion) of carboxylated cellulose is prepared. A 0.1 M hydrochloric acid aqueous solution is added to the prepared slurry to adjust the pH to 2.5, and then 0.05 N sodium hydroxide aqueous solution is dropped to measure the electric conductivity until the pH becomes 11. The amount of carboxyl groups can be calculated from the amount of sodium hydroxide consumed (a) in the neutralization step of the weak acid with a gradual change in electrical conductivity using the following formula:
Amount of carboxyl group [mmol / g carboxylated cellulose] = a [ml] × 0.05 / carboxylated cellulose mass [g]
In addition, the carboxyl group amount of carboxylated CNF and the carboxyl group amount of carboxylated cellulose are usually the same value.

(Carboxymethylated cellulose nanofiber)
The partial structure of carboxymethylated cellulose nanofiber is shown in general formula (1).

(In general formula (1), R represents a hydrogen atom, an alkali metal, or a group represented by general formula (2).)

(In the general formula (2), M ₂ represents a hydrogen atom or an alkali metal.)

Examples of the alkali metal represented by R in the general formula (1) and M ₂ in the general formula (2) include sodium and potassium. Of these, sodium is preferable.

  The degree of carboxymethyl substitution of carboxymethylated CNF is 0.01 to 0.50, preferably 0.01 to 0.40, and more preferably 0.05 to 0.35. If the degree of carboxymethyl substitution per glucose unit is 0.01 or more, celluloses repel each other electrically, so that they can be easily fibrillated to nano-order fiber diameters. On the other hand, when the degree of carboxymethyl substitution per glucose unit is less than 0.50, the carboxymethylated CNF swells or dissolves, so that it is impossible to maintain the fiber form and it is possible to suppress the fiber from being obtained.

The degree of carboxymethyl substitution per glucose unit can be calculated by the following method. About 2.0 g of carboxymethylated cellulose (absolutely dry) is precisely weighed and placed in an Erlenmeyer flask with a 300 mL stopper. Add 100 mL of a solution of 100 mL of special concentrated nitric acid to 1000 mL of methanol and shake for 3 hours to convert the salt type carboxymethylated cellulose (hereinafter also referred to as “salt type CMized cellulose”) into the acid type carboxymethylated cellulose. (Hereinafter also referred to as “H-type CMized cellulose”). H-type CMized cellulose (absolutely dried) is accurately weighed in an amount of 1.5 to 2.0 g, and placed in a 300 mL conical flask with a stopper. Wet H-type CM cellulose with 15 mL of 80% methanol, add 100 mL of 0.1 N NaOH, and shake at room temperature for 3 hours. Excess NaOH can be back titrated with 0.1N H ₂ SO ₄ using phenolphthalein as an indicator and the degree of carboxymethyl substitution (DS) can be calculated by the following formula:
A = [(100 × F− (0.1 N H ₂ SO ₄ (mL)) × F ′) × 0.1] / (absolute dry mass (g) of H-type CMized cellulose)
DS = 0.162 × A / (1-0.058 × A)
A: 1N NaOH amount (mL) required to neutralize 1 g of H-type CMized cellulose
F ′: 0.1N H ₂ SO ₄ factor F: 0.1N NaOH factor The carboxymethyl substitution degree of carboxymethylated CNF and the carboxymethyl substitution degree of carboxymethylated cellulose are usually the same. is there.

(Phosphate group-containing cellulose nanofiber)
Phosphate group-containing cellulose nanofibers are cellulose nanofibers treated with a compound having a phosphate group. Examples of the compound having a phosphoric acid group include phosphoric acid, polyphosphoric acid, phosphorous acid, phosphonic acid, polyphosphonic acid, and esters and salts thereof. These compounds are low in cost, easy to handle, and can improve the fibrillation efficiency by the anionic functional group introduced into the cellulose raw material.

As the compound having a phosphate group, in detail, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium pyrophosphate, sodium metaphosphate, potassium dihydrogen phosphate, dihydrogen phosphate Examples include potassium, tripotassium phosphate, potassium pyrophosphate, potassium metaphosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, ammonium pyrophosphate, ammonium metaphosphate, and the like. Among them, phosphoric acid, phosphoric acid sodium salt, phosphoric acid potassium salt, phosphoric acid ammonium salt are preferable because of the high introduction efficiency of phosphoric acid groups, easy defibration in the defibrating process, and industrial application. A salt is preferable, and sodium dihydrogen phosphate and disodium hydrogen phosphate are more preferable.
In addition, the compound which has a phosphate group may be used individually by 1 type, and may be used in combination of 2 or more type.

In CNF treated with a compound having a phosphoric acid group, the lower limit of the substitution degree of the phosphoric acid group per glucose unit is preferably 0.001 or more. By being in such a range, sufficient defibration (for example, nano defibration) can be implemented. Further, the upper limit of the substitution degree of the phosphate group is preferably 0.60 or less. By being in such a range, swelling or dissolution of cellulose into which phosphoric acid has been introduced can be prevented, and a situation where nanofibers cannot be obtained can be prevented.
The degree of substitution of phosphate groups per glucose unit is preferably 0.001 to 0.60.

[2. Method for producing hydrogel]
The hydrogel production method of the present invention includes a modification step (hereinafter also referred to as “denaturation step”) in which a cellulose raw material is chemically modified to obtain an anion-modified cellulose, and an anion-modified cellulose is defibrated to obtain an anion-modified CNF. Step (hereinafter also referred to as “defibration step”), anion-modified CNF is heated in the presence of water under conditions of a temperature of 80 to 400 ° C., a pressure of 0.1 to 50 MPa, and a time of 0.1 to 6000 minutes. A gelling step (hereinafter also referred to as “gelling step”). Moreover, it is preferable that the manufacturing method of the hydrogel of this invention further has the decoloring process (henceforth a "decoloring process") which decolors by immersing hydrogel in water.
In addition, you may perform shortening process, such as alkali hydrolysis, after a denaturation process and before a defibration process.

[2-1. Denaturation process]
The modification step is a step of obtaining anion-modified cellulose by chemically modifying a cellulose raw material. Examples of the modification step include an oxidation step (hereinafter, also referred to as “oxidation step”) in which cellulose raw material is prepared by oxidizing a cellulose raw material using an oxidizing agent, a cellulose raw material is mercerized with a mercerizing agent, and then carboxylated. A step of reacting with a methylating agent to obtain carboxymethylated cellulose (hereinafter also referred to as “CM conversion step”), a step of preparing a phosphate group-containing cellulose by reacting a cellulose raw material and a compound having a phosphate group (hereinafter referred to as “CM”) , Also referred to as “phosphate group introduction step”). Among them, the modification step is preferably an oxidation step or a CM conversion step.

  The cellulose raw materials include wood-derived kraft pulp or sulfite pulp, powdered cellulose obtained by pulverizing them with a high-pressure homogenizer, a mill, or the like, or microcrystalline cellulose powder obtained by purifying them by chemical treatment such as acid hydrolysis. In addition, plant-derived cellulose raw materials such as kenaf, hemp, rice, bagasse and bamboo can also be used. From the viewpoint of mass production and cost, it is preferable to use powdered cellulose, microcrystalline cellulose powder, or chemical pulp such as kraft pulp or sulfite pulp. When using chemical pulp, it is preferable to perform a known bleaching treatment to remove lignin. As the bleached pulp, for example, bleached kraft pulp or bleached sulfite pulp having a whiteness (ISO 2470) of 80% or more can be used.

  Powdered cellulose is a rod-like particle made of microcrystalline or crystalline cellulose obtained by removing an amorphous portion of wood pulp by acid hydrolysis, and then pulverizing and sieving. In powdered cellulose, the degree of polymerization of cellulose is about 100 to 500, the degree of crystallinity of powdered cellulose by X-ray diffraction method is 70 to 90%, and the volume average particle size by a laser diffraction type particle size distribution device is usually 100 μm or less. Preferably, it is 50 μm or less. Such powdered cellulose may be prepared by purifying and drying an undegraded residue obtained after acid hydrolysis of a selected pulp, pulverizing and sieving, or KC Flock (registered trademark) (Nippon Paper Industries Co., Ltd.). Manufactured products), Theolas (registered trademark) (manufactured by Asahi Kasei Chemicals), Avicel (registered trademark) (manufactured by FMC), and the like may be used.

  Bleaching methods include chlorine process (C), chlorine dioxide bleach (D), alkali extraction (E), hypochlorite bleach (H), hydrogen peroxide bleach (P), alkaline hydrogen peroxide process stage (Ep ), Alkaline hydrogen peroxide / oxygen process stage (Eop), ozone process (Z), chelate process (Q) and the like. For example, C / D-E-H-D, Z-E-D-P, Z / D-Ep-D, Z / D-Ep-D-P, D-Ep-D, D-Ep-D- P, D-Ep-PD, Z-Eop-DD, Z / D-Eop-D, Z / D-Eop-D-ED, and the like can be performed. Note that “/” in the sequence means that the steps before and after “/” are continuously performed without cleaning.

  In addition, the above cellulose raw material that has been refined by a high-speed rotating type, colloid mill type, high-pressure type, roll mill type, ultrasonic type dispersing device, wet high-pressure or ultra-high pressure homogenizer, etc. should be used as the cellulose raw material. You can also.

[2-1-1. Oxidation process]
The oxidation process is a process for preparing oxidized cellulose by oxidizing a cellulose raw material using an oxidizing agent. Among these, the oxidation step is more preferably a step of preparing oxidized cellulose by oxidizing a cellulose raw material using an oxidizing agent in the presence of an N-oxyl compound and bromide, iodide or a mixture thereof. When the cellulose raw material is oxidized by this step, a structural unit having a carboxyl group in which a carbon atom having a primary hydroxyl group at the C6 position of a glucopyranose unit constituting a cellulose molecular chain is selectively oxidized can be obtained.
The partial structure of oxidized cellulose obtained by this step is shown in the following general formula (3).

(In General Formula (3), M ₁ represents a cation salt.)

Examples of the cation salt represented as M ₁ in the general formula (3) include alkali metal salts such as sodium salt and potassium salt, phosphonium salt, imidazolinium salt, ammonium salt and sulfonium salt.

  Natural cellulose has a microfibril structure in which a large number of linear cellulose molecular chains are converged by hydrogen bonds. When the cellulose raw material is oxidized using an N-oxyl compound, as described above, the carbon atom having the primary hydroxyl group at the C6 position of the glucopyranose unit constituting the cellulose molecular chain is selectively oxidized to a carboxyl group via an aldehyde group. The Therefore, carboxyl groups are introduced at a high density on the surface of the microfibril structure. The introduced carboxyl group has a repulsive action, and nanofibers that are separated one by one by defibration can be obtained.

  N-oxyl compounds are compounds that can generate nitroxy radicals. As the N-oxyl compound, any compound can be used as long as it is a compound that performs the target oxidation reaction. Examples of the N-oxyl compound include compounds represented by the following general formulas (4) to (7) and (9) and compounds represented by the following formula (8).

(In General Formula (4), R ^{1 to} R ⁴ represent the same or different alkyl groups having 1 to 4 carbon atoms, and R ⁵ represents a hydrogen atom or a hydroxyl group.)

(In General Formulas (5) to (7), R ⁶ represents a linear or branched hydrocarbon group having 1 to 4 carbon atoms.)

(In General Formula (9), R ^{7 to} R ⁸ represent a hydrogen atom or a linear or branched alkyl group having 1 to 6 carbon atoms, which may be the same or different.)

In the general formula (4), examples of the alkyl group having 1 to 4 carbon atoms represented by R ^{1 to} R ⁴ include a methyl group, an ethyl group, a propyl group, and a butyl group. Of these, a methyl group or an ethyl group is preferable.
In general formulas (5) to (7), examples of the linear or branched hydrocarbon group having 1 to 4 carbon atoms represented by R ⁶ include a methyl group, an ethyl group, an n-propyl group, Examples include isopropyl group, n-butyl group, s-butyl group, and t-butyl group. Of these, a methyl group or an ethyl group is preferable.
In the general formula (9), examples of the linear or branched alkyl group having 1 to 6 carbon atoms represented by R ^{7 to} R ⁸ include a methyl group, an ethyl group, an n-propyl group, and an isopropyl group. , N-butyl group, s-butyl group, t-butyl group, n-pentyl group, and n-hexyl group. Of these, a methyl group or an ethyl group is preferable.

Examples of the compound represented by the general formula (4) include 2,2,6,6-tetramethyl-1-piperidine-N-oxy radical (hereinafter also referred to as “TEMPO”), or 4-hydroxy-2. 2,6,6-tetramethyl-1-piperidine-N-oxy radical (hereinafter also referred to as “4-hydroxy TEMPO”).
The N-oxyl compound may be TEMPO or a derivative of 4-hydroxy TEMPO. As a derivative of 4-hydroxy TEMPO, for example, the compound represented by the general formula (5), that is, the hydroxyl group of 4-hydroxy TEMPO has a linear or branched hydrocarbon group having 4 or less carbon atoms. Derivatives obtained by etherification with alcohol and compounds represented by general formula (6) or (7), that is, derivatives obtained by esterification with carboxylic acid or sulfonic acid can be mentioned.
When 4-hydroxy TEMPO is etherified, if an alcohol having 4 or less carbon atoms is used, the resulting derivative becomes water-soluble regardless of the presence or absence of a saturated or unsaturated bond in the alcohol, and is a good oxidation catalyst. To work.

  When the N-oxyl compound is a compound represented by the formula (8), that is, a compound in which the amino group of 4-amino TEMPO is acetylated, moderate hydrophobicity is imparted, and it is inexpensive and uniform oxidation. Since cellulose can be obtained, it is preferable. The N-oxyl compound is preferably a compound represented by the general formula (9), that is, an azaadamantane type nitroxy radical, because uniform oxidized cellulose can be obtained in a short time.

  The amount of the N-oxyl compound used is not particularly limited as long as it is a catalytic amount that can oxidize the cellulose raw material sufficiently to make the obtained oxidized cellulose nanofiber. For example, it is preferably 0.01 to 10 mmol, more preferably 0.01 to 1 mmol, and still more preferably 0.01 to 0.5 mmol with respect to 1 g of the cellulose raw material.

The bromide used in the oxidation of the cellulose raw material is a compound containing bromine, and examples thereof include an alkali metal bromide that can be dissociated and ionized in water. Further, an iodide is a compound containing iodine, and examples thereof include alkali metal iodide.
The amount of bromide or iodide used can be adjusted as long as the target oxidation reaction can be promoted. The total amount of bromide and iodide is, for example, preferably 0.1 to 100 mmol, more preferably 0.1 to 10 mmol, and still more preferably 0.5 to 5 mmol, with respect to 1 g of the absolutely dry dry cellulose raw material.

As the oxidizing agent, for example, known oxidizing agents such as halogen, hypohalous acid, halous acid, perhalogen acid or salts thereof, halogen oxide, peroxide, and the like can be used. Of these, sodium hypochlorite is preferable because it is inexpensive and has a low environmental impact.
The amount of the oxidizing agent used may be an amount for performing an oxidation reaction. For example, it is preferably 0.5 to 500 mmol, more preferably 0.5 to 50 mmol, and still more preferably 2 with respect to 1 g of the cellulose raw material. .5 to 25 mmol.

  Since the oxidation reaction of the cellulose raw material proceeds efficiently even under relatively mild conditions, the reaction temperature may be a room temperature of about 15 to 30 ° C. As the reaction proceeds, carboxyl groups are generated in the cellulose, so that the pH value of the reaction solution decreases. In order to advance the oxidation reaction efficiently, an alkaline solution such as an aqueous sodium hydroxide solution is added to the reaction system in a timely manner so that the pH value of the reaction solution is maintained at 9 to 12, preferably about 10 to 11. preferable. The reaction medium is preferably water because it is easy to handle and hardly causes side reactions.

  The reaction time in the oxidation reaction can be appropriately set according to the progress of oxidation, and is usually about 0.5 to 6 hours, preferably about 0.5 to 4 hours.

  The oxidation reaction may be performed in two stages. For example, by oxidizing the cationic salt of oxidized cellulose obtained by filtration after completion of the first-stage reaction again under the same or different reaction conditions, the reaction is inhibited by the salt produced as a by-product in the first-stage reaction. And a carboxyl group can be efficiently introduced into the cellulose raw material.

  In the oxidized cellulose obtained in the oxidation step, the carboxyl group introduced into the cellulose raw material is usually an alkyl metal salt such as a sodium salt (that is, a carboxylate group). Prior to the defibrating step, the alkali metal salt of oxidized cellulose may be replaced with another cation salt such as a phosphonium salt, an imidazolinium salt, an ammonium salt, or a sulfonium salt. The substitution can be performed by a known method.

  As another example of the oxidation method, a method of oxidizing by bringing a gas containing ozone and a cellulose raw material into contact with each other can be mentioned. By this oxidation reaction, the carbon atom having at least the 2-position and 6-position hydroxyl groups of the glucopyranose ring is oxidized and the cellulose chain is decomposed.

Ozone concentration in the ozone containing gas is preferably 50 to 250 g / ^{m 3,} more preferably 50~220g / ^{m 3.} The amount of ozone added to the cellulose raw material is preferably 0.1 to 30 parts by mass, and more preferably 5 to 30 parts by mass when the solid content of the cellulose raw material is 100 parts by mass. The ozone treatment temperature is preferably 0 to 50 ° C, and more preferably 20 to 50 ° C. The ozone treatment time is not particularly limited, but is usually about 1 to 360 minutes, and preferably about 30 to 360 minutes. When the conditions for the ozone treatment are within these ranges, the cellulose can be prevented from being excessively oxidized and decomposed, and the yield of oxidized cellulose is improved.

  After the ozone treatment, an additional oxidation treatment may be performed using an oxidizing agent. The oxidizing agent used for the additional oxidation treatment is not particularly limited, and examples thereof include chlorine compounds such as chlorine dioxide and sodium chlorite, oxygen, hydrogen peroxide, persulfuric acid, peracetic acid and the like. For example, these oxidizing agents can be dissolved in a polar organic solvent such as water or alcohol to prepare an oxidizing agent solution, and a cellulose raw material can be immersed in the solution for additional oxidation treatment.

  The oxidized cellulose obtained in the oxidation step is preferably washed from the viewpoint of avoiding side reactions. The washing method is not particularly limited, and can be performed by a known method.

[2-1-2. CM conversion process]
The CM conversion step is a step of obtaining a carboxymethylated cellulose by subjecting a cellulose raw material to a mercerization treatment with a mercerizing agent and then reacting with a carboxymethylating agent.

A mercerization process can be normally performed by mixing a cellulose raw material, a solvent, and a mercerizing agent.
The solvent is preferably water and / or a lower alcohol, and more preferably water. Moreover, it is preferable that the usage-amount of a solvent is 3-20 times of a cellulose raw material in mass conversion.

Examples of the lower alcohol include alcohols such as methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, and tertiary butyl alcohol.
In addition, a lower alcohol may be used individually by 1 type and may be used as a mixed medium which combined 2 or more types.
When the solvent contains a lower alcohol, the mixing ratio is preferably 60 to 95% by mass.

  As the mercerizing agent, an alkali metal hydroxide is preferable, and sodium hydroxide or potassium hydroxide is more preferable. Moreover, it is preferable that the usage-amount of a mercerizing agent is 0.5-20 times per anhydroglucose residue of a cellulose raw material in molar conversion.

  The reaction temperature of mercerization is usually 0 to 70 ° C, preferably 10 to 60 ° C. The reaction time for mercerization is usually 15 minutes to 8 hours, preferably 30 minutes to 7 hours. The mercerization process may be performed under stirring.

  After the mercerization treatment, a carboxymethylating agent is added to the reaction system to introduce carboxymethyl groups into the cellulose. As the carboxymethylating agent, a compound represented by the following general formula (10) is preferable, and monochloroacetic acid and sodium monochloroacetate are more preferable. Moreover, it is preferable that the addition amount of a carboxymethylating agent is 0.05-10.0 times per glucose residue of a cellulose raw material in molar conversion.

(In general formula (10), X represents a halogen atom, and M ₃ represents a hydrogen atom or an alkali metal.)

As a halogen atom represented as X in General formula (10), a chlorine atom, a bromine atom, and an iodine atom are mentioned, for example. Among these, a chlorine atom is preferable.
Examples of the alkali metal represented as M ₃ in the general formula (10) include sodium and potassium. Of these, sodium is preferable.

  The reaction temperature of the carboxymethylation reaction is usually 30 to 90 ° C, preferably 40 to 80 ° C. The reaction time is usually 30 minutes to 10 hours, preferably 1 hour to 4 hours.

[2-1-3. Phosphate group introduction process]
The phosphate group introduction step is a step of preparing a phosphate group-containing cellulose by reacting, for example, a cellulose raw material with a compound having a phosphate group. Examples of a method of reacting a cellulose raw material with a compound having a phosphate group include, for example, a method of mixing a powder or an aqueous solution of a compound having a phosphate group with a cellulose raw material, and an aqueous solution of a compound having a phosphate group in a slurry of a cellulose raw material. The method of adding etc. is mentioned.
Among these, a method of mixing an aqueous solution of a compound having a phosphate group with a cellulose raw material or a slurry thereof is preferable because the uniformity of the reaction is increased and the introduction efficiency of the phosphate group is increased. The pH of the aqueous solution of the compound having a phosphoric acid group is preferably 7 or less from the viewpoint of increasing the efficiency of introduction of the phosphoric acid group, and more preferably 3 to 7 from the viewpoint of suppressing hydrolysis.

Examples of the compound having a phosphate group include the compounds described in “(Phosphate group-containing cellulose nanofiber)”.
The lower limit of the addition amount of the compound having a phosphoric acid group is preferably 0.2 parts by mass or more and more preferably 1 part by mass or more with respect to 100 parts by mass of the cellulose raw material in terms of phosphorus atoms. By being in such a range, the yield of the cellulose which introduce | transduced the phosphate group can be improved. On the other hand, the upper limit is preferably 500 parts by mass or less, and more preferably 400 parts by mass or less. By being in such a range, the yield corresponding to the addition amount of the compound which has a phosphate group can be obtained efficiently.
0.2 mass part-500 mass parts are preferable, and, as for the addition amount of the compound which has a phosphoric acid group, 1 mass part-400 mass parts are more preferable.

When the cellulose raw material and the compound having a phosphate group are reacted, a basic compound may be further added to the reaction system. Examples of the method of adding the basic compound to the reaction system include a method of adding to a slurry of a cellulose raw material, an aqueous solution of a compound having a phosphoric acid group, or a slurry of a cellulose raw material and a compound having a phosphoric acid group.
Although a basic compound is not specifically limited, The nitrogen-containing compound which shows basicity is preferable. “Show basic” usually means that the aqueous solution of the basic compound is pink to red in the presence of the phenolphthalein indicator, or the pH of the aqueous solution of the basic compound is greater than 7.

The nitrogen-containing compound exhibiting basicity is not particularly limited as long as the effects of the present invention are exhibited. Of these, compounds having an amino group are preferred. For example, urea, methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, hexamethylenediamine and the like can be mentioned. Among these, urea is preferable because it is easy to handle at low cost.
The amount of the basic compound added is preferably 2 to 1000 parts by mass, and more preferably 100 to 700 parts by mass. The reaction temperature is preferably 0 ° C to 95 ° C, more preferably 30 ° C to 90 ° C. Although reaction time is not specifically limited, Usually, it is about 1 minute-600 minutes, and 30 minutes-480 minutes are preferable. When the reaction conditions are in any of these ranges, it is possible to prevent the phosphoric acid group from being excessively introduced into the cellulose to facilitate dissolution, and to improve the yield of the cellulose into which the phosphoric acid group has been introduced. .

  After reacting the cellulose raw material with a compound having a phosphate group, a suspension is usually obtained. The suspension is dehydrated if necessary. Heat treatment is preferably performed after dehydration. Thereby, hydrolysis of a cellulose raw material can be suppressed. The heating temperature is preferably 100 ° C. to 170 ° C., and is heated at 130 ° C. or lower (more preferably 110 ° C. or lower) while water is contained in the heat treatment, and after removing water, 100 ° C. to 170 ° C. More preferably, the heat treatment is performed at a temperature of ° C.

  The phosphate group-containing cellulose is preferably subjected to a washing treatment such as washing with cold water after boiling. By performing the cleaning treatment, defibration can be performed efficiently.

[2-2. Defibration process]
The defibrating step is a step of defibrating anion-modified cellulose. Since an anionic functional group is introduced into the anion-modified cellulose by the modification step, it can be easily made into nanofibers by the defibration step.

  As the defibrating step, for example, after sufficiently washing the anion-modified cellulose, it can be carried out using a known apparatus such as a high-speed shear mixer or a high-pressure homogenizer. Examples of the type of defibrating apparatus include a high-speed rotation type, a colloid mill type, a high-pressure type, a roll mill type, and an ultrasonic type. These devices may be used alone or in combination of two or more.

When a high-speed shear mixer is used, the shear rate is preferably 1000 sec ⁻¹ or more. When the shear rate is 1000 sec ⁻¹ or more, there are few aggregated structures, and nanofibers can be formed uniformly.
When using a high-pressure homogenizer, the applied pressure is preferably 50 MPa or more, more preferably 100 MPa or more, and further preferably 140 MPa or more. When treated with a wet high-pressure or ultrahigh-pressure homogenizer at the pressure, nanofibrosis proceeds efficiently, and anion-modified CNF can be obtained efficiently.

  Anion-modified cellulose is subjected to a defibrating step as an aqueous dispersion such as water. If the concentration of the anion-modified cellulose in the aqueous dispersion is high, the viscosity may increase excessively during the defibrating step and the defibration may not be performed uniformly, or the apparatus may stop. Accordingly, the concentration of the anion-modified cellulose needs to be appropriately set according to the processing conditions of oxidized cellulose. As an example, the concentration of oxidized cellulose is preferably 0.3 to 50% (w / v), more preferably 0.5 to 10% (w / v), and 1.0 to 5% (w / v). Further preferred.

[2-3. Gelation process]
In the gelation step, the anion-modified CNF obtained in the defibration step is heated in the presence of water at a temperature of 80 to 400 ° C., a pressure of 0.1 to 50 MPa, and a time of 0.1 to 6000 minutes. It is a process to obtain.
By passing through this step, a high-strength hydrogel containing anion-modified cellulose nanofibers can be produced. The present inventors have presumed the reason why a self-supporting hydrogel can be produced by using the anion-modified cellulose nanofibers that have undergone the above process as follows.

The anion-modified CNF, in the presence of water, when heated under the above conditions, the anionic functional group (e.g., -COO ^- group) part of the glucuronic acid residue with is eliminated, anionically modified cellulose nanofibers on the surface The charge present in the is reduced. Therefore, the zeta potential decreases, and the cellulose nanofibers self-assemble due to van der Waals forces between cellulose nanofibers and hydrogen bonds due to hydroxyl groups at the detached sites, forming a polymer-like aggregate and gelling. To do.

  Water is not particularly limited. Examples include pure water, ultrapure water, tap water, well water, natural water, mineral spring water, mineral water, hot spring water, spring water, fresh water, distilled water, purified water, sterilized water, hot water, and ion exchange water. Among these, pure water, ultrapure water, distilled water, and ion exchange water are preferable from the viewpoint of suppressing undesirable side reactions.

The heating conditions are a temperature of 80 to 400 ° C., a pressure of 0.1 to 50 MPa, and a time of 0.1 to 6000 minutes.
The heating temperature is 80 to 400 ° C, preferably 120 to 250 ° C, and more preferably 140 to 200 ° C.
The heating pressure is 0.1 to 50 MPa, preferably 0.2 to 4.0 MPa, and more preferably 0.3 to 1.6 MPa.
The heating time is 0.1 to 6000 minutes, preferably 1 to 240 minutes, and more preferably 5 to 180 minutes.

[2-4. Decolorization process]
The decoloring step is a step of decoloring by immersing the hydrogel in water. By passing through the decoloring step, the light transmittance of the hydrogel is improved. The reason why the light transmittance is improved by immersing the hydrogel in water is presumed as follows. As described above, a part of the glucuronic acid residue of the anion-modified CNF is decomposed in the gelation step. Therefore, a hydrogel contains low molecular compounds, such as a glucose residue, in addition to an anion modified cellulose nanofiber. When the hydrogel is immersed in water, the low molecular weight compound is diffused and dissolved in water. Accordingly, the light transmittance of the hydrogel is improved.

  Water is not particularly limited. Examples include pure water, ultrapure water, tap water, well water, natural water, mineral spring water, mineral water, hot spring water, spring water, fresh water, distilled water, purified water, sterilized water, hot water, and ion exchange water. Among these, pure water, ultrapure water, distilled water, and ion exchange water are preferable from the viewpoint of dissolving the low molecular weight compound.

  The time for immersing in water can be appropriately set according to the light transmittance of the hydrogel. For example, it is about 1 to 3 days.

[2-5. Short fiber treatment]
When preparing CNF, you may perform a fiber shortening process after a denaturation process and before a defibration process. By performing such treatment, an anion-modified CNF having a shorter fiber length can be prepared.
The shortening treatment refers to a treatment for appropriately shortening the fiber by appropriately cutting the cellulose chain of anion-modified cellulose. For example, ultraviolet irradiation treatment, oxidative decomposition treatment, alkaline hydrolysis treatment, and acid hydrolysis treatment can be mentioned. Among these, alkali hydrolysis treatment is preferable.
In addition, said process may be 1 type of single process, and may combine 2 or more types of processes.

[3. Application]
The hydrogel of the present invention is a spray composition; a rubber reinforcing material such as NR, SBR, EPDM, or NBR; a resin reinforcing material such as a polyolefin resin, acrylic resin, urethane resin, PVC resin, polyamide resin, or PC resin. ; Cosmetics such as creams, lotions, gels, sticks, pump sprays, aerosols, roll-on antiperspirants, deodorants, fragrance-releasing gels, lipsticks, lip gloss, liquid cosmetic products applied to the epithelium; , Sustained or delayed additives, disintegrants for tablets, liquid retention agents in wound care products, medical products such as rheology modifiers; chemical and biological materials such as cell culture bases, catalyst carriers, separation agents; Rheology modifiers, stabilizers that suppress creaming and precipitation in suspension, foods such as indigestible dietary fiber; beverages; paints; Metal adsorbents that are unfavorable to the environment contained in soil and wastewater; absorbents used for disposable diapers, urine pads, light incontinence pads, etc., moisturizers, heat insulating materials, soundproofing materials, buffering agents, air in the agricultural field It can be used for household materials such as filters.
Among these, since it has biodegradability, cosmetics such as liquid cosmetics, absorbents, and moisturizers in the agricultural field are preferable.

  Hereinafter, the present invention will be described in detail with reference to examples. The following examples are for explaining the present invention preferably and are not intended to limit the present invention. In addition, unless otherwise indicated, the measuring methods, such as a physical-property value, are the measuring methods described above.

  [Break strength (kPa)]: Using a compression tester (EZ-test, manufactured by Shimadzu Corporation), a sample having a diameter of about 8 mm and a height of about 6 mm was compressed at a compression rate of 1 mm / min using a 50 N load cell. Compression was performed at a speed, and the breaking strength was measured from the strain and stress immediately before breaking.

  [Compression modulus (kPa)]: Using a compression tester (EZ-test, manufactured by Shimadzu Corporation), a sample having a diameter of about 8 mm and a height of about 6 mm is compressed using a 50N load cell at a compression rate of 1 mm / min. The compression elastic modulus was measured from the stress between strains of 4-8%.

  [Light transmittance (%)]: 200-700 nm with respect to an optical path length of 1 cm of a sample obtained by pulverizing a hydrogel with a mortar and a pestle using an ultraviolet-visible spectrophotometer (V530, manufactured by JASCO Corporation). The light transmittance was measured by entering light having a wavelength of.

  [Gel yield (%)]: It was calculated by dividing the mass of the dried hydrogel by the product of the mass and concentration of TEMPO-oxidized CNF introduced into the reaction tube and multiplying by 100.

  [Gel concentration (%)]: Calculated by dividing the mass of the dried hydrogel by the mass of the recovered hydrogel and multiplying by 100.

  [Average fiber length (nm)]: TEMPO-oxidized CNF was fixed on a mica section, 200 fiber lengths were measured using an atomic force microscope (AFM), and a length (weighted) average fiber length was calculated. . In addition, the measurement of fiber length was performed in the range of arbitrary length using image analysis software WinROOF (made by Mitani Corporation).

  [Average fiber diameter (nm)]: A TEMPO-oxidized CNF aqueous dispersion diluted so that the concentration of TEMPO-oxidized CNF was 0.001% by mass was prepared. This diluted dispersion was thinly spread on a mica sample stage and heated and dried at 50 ° C. to prepare an observation sample. The cross-sectional height of the shape image observed with an atomic force microscope (AFM) was measured, and the weighted average fiber diameter was calculated.

[Amount of carboxyl group (mmol / g carboxylated cellulose)]: 60 ml of a 0.5 mass% slurry (aqueous dispersion) of carboxylated cellulose was prepared. A 0.1 M hydrochloric acid aqueous solution was added to the prepared slurry to adjust the pH to 2.5, and then 0.05 N sodium hydroxide aqueous solution was added dropwise to measure the electric conductivity until the pH reached 11. The amount of carboxyl groups was calculated from the amount of sodium hydroxide consumed (a) in the weak acid neutralization stage where the change in electrical conductivity was gradual using the following formula:
Amount of carboxyl group [mmol / g carboxylated cellulose] = a [ml] × 0.05 / carboxylated cellulose mass [g]

(Examples 1-5)
5 g (absolutely dried) of bleached softwood unbeaten pulp (Nippon Paper Industries) was added to 500 ml of an aqueous solution in which 78 mg (0.5 mmol) of TEMPO (manufactured by Sigma Aldrich) and 754 mg (7.4 mmol) of sodium bromide were dissolved. Stir until the pulp is uniformly dispersed. After adding 14 ml of 2M aqueous sodium hypochlorite solution to the reaction system, the pH was adjusted to 10.3 with 0.5N aqueous hydrochloric acid solution to initiate the oxidation reaction. Since the pH in the system was lowered during the reaction, a 0.5N sodium hydroxide aqueous solution was sequentially added to maintain the pH at 10. After reacting for 2 hours, the mixture was filtered through a glass filter and washed thoroughly with water to obtain oxidized cellulose having a carboxyl group amount of 1.6 mmol / g.

Next, the obtained slurry of oxidized cellulose was adjusted to 1% (w / v) with water, treated with an ultra-high pressure homogenizer (20 ° C., 140 MPa) three times, and a transparent gel-like carboxylated cellulose nanofiber salt was obtained. A dispersion (1% (w / v)) was obtained.
The obtained carboxylated cellulose nanofibers had an average fiber length of 446 nm and an average fiber diameter of 2.83 nm.

The obtained dispersion of carboxylated CNF was introduced into a reaction tube made of SUS316 and having an internal volume of 6 mL. The reaction tube was placed in a molten salt furnace and heated at a temperature of 160 ° C. for a predetermined time. At this time, the pressure inside the reaction tube was 0.9 MPa. After heating, the reaction tube was cooled with tap water and filtered to produce a hydrogel. Moreover, the obtained hydrogel was immersed in distilled water for 3 days and decolored.
The heating time is 15 minutes in Example 1, 20 minutes in Example 2, 30 minutes in Example 3, 60 minutes in Example 4, and 120 minutes in Example 5. The physical property values of Examples 1 to 5 are shown in Table 1 below.

(Comparative Example 1)
A hydrogel was produced in the same manner as in Examples 1 to 5 except that the heating time was 10 minutes.
The physical property values of Comparative Example 1 are shown in Table 1 below.

  About the obtained hydrogel, the scatter diagram which shows the relationship between heating time and a gel density | concentration is shown in FIG. 2, and the scatter diagram which shows the relationship between a heating time, a compression elastic modulus, and breaking strength is shown in FIG. Moreover, the chart which shows the relationship between heating time and the light transmittance of the hydrogel before decoloring is shown in FIG. Furthermore, the chart which shows the relationship between the light transmittance of TEMPO oxidation CNF and the hydrogel before and after decoloring (Example 4) is shown in FIG.

  As apparent from FIG. 3 and Table 1, the breaking strength and the compressive elastic modulus were improved by increasing the heating time during the production of the hydrogel. Further, as apparent from FIG. 2, the gel concentration was decreased by decolorization, which can be attributed to the elution of the low molecular weight compound. Further, as apparent from FIG. 4 and Table 1, when the heating time was increased, the light transmittance decreased. This is presumably because the proportion of low-molecular compounds increases.

  As is apparent from FIG. 5, it is recognized that the hydrogel is decolored and the light transmittance is improved by being immersed in distilled water.

Claims (9)

Including anion-modified cellulose nanofibers,
A hydrogel having a breaking strength of 2.0 to 8.0 kPa at a gel concentration of 0.1 to 3.0% by mass.   The hydrogel according to claim 1, wherein the compression elastic modulus is 0.5 to 8.0 kPa at a gel concentration of 0.1 to 3.0 mass%.   The hydrogel according to claim 1 or 2, wherein the light transmittance at a wavelength of 600 nm is 15 to 95% at a gel concentration of 0.1 to 3.0 mass%.   The hydrogel according to any one of claims 1 to 3, wherein the anion-modified cellulose nanofiber is a carboxylated cellulose nanofiber.   The hydrogel according to any one of claims 1 to 3, wherein the anion-modified cellulose nanofiber is a carboxymethylated cellulose nanofiber.   A modification step of chemically modifying a cellulose raw material to obtain anion-modified cellulose, a defibration step of defibrating the anion-modified cellulose to obtain anion-modified cellulose nanofibers, and the anion-modified cellulose nanofibers in the presence of water at a temperature of 80 to A method for producing a hydrogel comprising: a gelation step of obtaining a hydrogel by heating at 400 ° C, a pressure of 0.1 to 50 MPa, and a time of 0.1 to 6000 minutes.   The said modification | denaturation process is a process of oxidizing the said cellulose raw material using an oxidizing agent in presence of a N-oxyl compound, a bromide, iodide, or these mixtures, and obtaining an oxidized cellulose. Production method of hydrogel.   The method for producing a hydrogel according to claim 6, wherein the modification step is a step of obtaining a carboxymethylated cellulose by reacting with a carboxymethylating agent after the cellulose raw material is mercerized with a mercerizing agent.   The manufacturing method of the hydrogel of any one of Claims 6-8 which further has the decoloring process of immersing the said hydrogel in water and decoloring.