JP6877136B2 - Manufacturing method of carboxylated cellulose nanofibers - Google Patents

Description

The present invention relates to a method for producing carboxylated cellulose nanofibers.

Cellulose-based raw materials are treated in the coexistence of 2,2,6,6-tetramethyl-1-piperidin-N-oxyradical (hereinafter, also referred to as "TEMPO") and sodium hypochlorite, which is an inexpensive oxidizing agent. Then, the carboxyl group can be efficiently introduced into the surface of the cellulose microfibrils. When cellulose having a carboxyl group introduced is treated in water with a mixer or the like, a highly viscous and transparent aqueous dispersion of cellulose nanofibers can be obtained.

As described above, since the carboxyl group is introduced on the surface of the cellulose nanofiber, it can be freely modified starting from the carboxyl group. Further, since the cellulose nanofibers are in the form of a dispersion liquid, they can be blended with a water-soluble polymer or compounded with an organic / inorganic pigment to be modified. Furthermore, cellulose nanofibers can be made into sheets or fibers. Due to these characteristics, the development of new high-performance products in which cellulose nanofibers are applied to high-performance packaging materials, transparent organic substrate members, high-performance fibers, separation membranes, regenerative medicine materials, etc. is being studied.

By the way, the dispersion liquid of cellulose nanofibers obtained by oxidizing a cellulosic raw material derived from wood (hardwood pulp, softwood pulp, etc.) with TEMPO and then defibrating it with a mixer is 0.3 to 0.5. The B-type viscosity (60 rpm, 20 ° C.) shows a value of 800 to 4000 mPa · s even at a concentration as low as% (w / v). Therefore, the cellulose nanofibers having a carboxyl group introduced have a problem in handleability, and there is a problem in application to the above-mentioned applications.

In view of such problems, various techniques for improving the fluidity and lowering the viscosity of cellulose nanofibers having a carboxyl group introduced have been proposed (see, for example, Patent Documents 1 to 5).
In Patent Document 1, by using pulp (DKP) obtained by performing kraft cooking after hydrolysis treatment as a cellulose-based raw material, cellulose having a low viscosity even at a high concentration and excellent fluidity is used. A method for producing nanofibers is disclosed.
Patent Document 2 discloses a method for producing cellulose nanofibers having excellent fluidity by oxidizing a cellulosic raw material and then alkaline hydrolysis.
Patent Document 3 describes a method for producing cellulose nanofibers having a low viscosity by treating a cellulosic raw material in water having a hydroxide ion concentration of 0.75 to 3.75 mol / L before oxidizing the cellulosic raw material. It is disclosed.
Patent Document 4 discloses a method for producing low-viscosity cellulose nanofibers by reducing the hemicellulose content contained in a cellulosic raw material derived from broad-leaved tree to less than a predetermined amount.
Patent Document 5 discloses a method for producing cellulose nanofibers having excellent fluidity by oxidizing a cellulosic raw material with a subhalide salt without filtering or washing. ..

International Publication No. 2013/047218 Japanese Unexamined Patent Publication No. 2012-214717 Japanese Unexamined Patent Publication No. 2012-207135 Japanese Unexamined Patent Publication No. 2012-207133 International Publication No. 2011/0743301

However, in the cellulose nanofibers obtained by the above method, the effect of having excellent fluidity or low viscosity is that before the acid treatment, that is, the introduced carboxyl group or the like becomes a metal salt such as a sodium salt. This is the effect when there is. When cellulose nanofibers are acid-treated with hydrochloric acid or the like to replace metal salts with protons, the viscosity may increase, and there is a problem that the yield is particularly significantly decreased.
Therefore, there is room for improvement in applying cellulose nanofibers in which a metal salt is replaced with a proton after introducing a carboxyl group into a new high-performance product.

An object of the present invention is to provide a method for producing low-viscosity carboxylated cellulose nanofibers in a high yield when prepared as a dispersion liquid.

As a result of diligent studies on the above problems, the present inventors have found that the above problems can be solved by performing the acid treatment with a mineral acid such as hydrochloric acid with a cation exchange resin, and complete the present invention. It came to.
That is, the present inventors provide the following [1] to [4].
[1] An oxidation step of oxidizing a cellulosic raw material with an oxidizing agent in the presence of an N-oxyl compound and a bromide, iodide or a mixture thereof to obtain oxidized cellulose, and defibrating the oxidized cellulose. A method for producing carboxylated cellulose nanofibers, which comprises a defibration step and a desalting step of desalting by a cation exchange reaction, wherein the desalting step is a step of desalting with a cation exchange resin.
[2] The method for producing carboxylated cellulose nanofibers according to the above [1], further comprising a low viscosity step of hydrolyzing the oxidized cellulose in an alkaline solution.
[3] The method for producing carboxylated cellulose nanofibers according to the above [2], wherein the low viscosity step is a step of hydrolyzing the oxidized cellulose in an alkaline solution containing an oxidizing agent or a reducing agent.
[4] The defibration step is a step of defibrating the oxidized cellulose to obtain a carboxylated cellulose nanofiber salt, and the desalting step is a step of removing the carboxylated cellulose nanofiber salt with the cation exchange resin. The method for producing carboxylated cellulose nanofibers according to any one of the above [1] to [3], which is a step of salt treatment.

According to the present invention, it is possible to provide a method for producing carboxylated cellulose nanofibers, which can produce carboxylated cellulose nanofibers having a low viscosity when used as a dispersion liquid in a high yield.

Hereinafter, the present invention will be described in detail according to the preferred embodiment thereof.

In the method for producing carboxylated cellulose nanofibers of the present invention, oxidation of a cellulosic raw material using an oxidizing agent in the presence of an N-oxyl compound and a bromide, iodide or a mixture thereof to obtain oxidized cellulose is obtained. It has a step, a defibration step of defibrating cellulose oxide, and a desalting step of desalting by a cation exchange reaction. The desalting step is a step of desalting with a cation exchange resin.
Hereinafter, after the defibration step of defibrating the oxidized cellulose to obtain the carboxylated cellulose nanofiber salt, the carboxylated cellulose nanofiber salt is brought into contact with the cation exchange resin to perform the desalting step to obtain the carboxylated cellulose nanofiber. The form obtained is referred to as "one embodiment". In addition, after the desalting step is performed by bringing the oxidized cellulose into contact with a cation exchange resin, the desalted cellulose is defibrated and the defibration step is performed to obtain carboxylated cellulose nanofibers. It is referred to as "embodiment".

In the conventional method for producing carboxylated cellulose nanofibers, the metal salt of cellulose nanofibers into which a carboxyl group has been introduced has been acid-treated with a mineral acid such as hydrochloric acid to replace the metal salt with a proton. When acid treatment is performed using mineral acid, by-products such as sodium chloride are produced, so it is necessary to remove the by-products by washing. In addition, it was necessary to dehydrate and remove the water used for washing, and after dehydration, it was necessary to pass through a filter cloth to collect the remaining filter medium and re-defiber the aggregated filter medium. Carboxylated cellulose nanofibers with extremely short fiber lengths pass through the filter cloth, which is presumed to cause a large decrease in yield and an increase in viscosity. In addition, it is presumed that the aggregation of the carboxylated cellulose nanofibers due to dehydration or filtration also causes the viscosity to increase.

On the other hand, in one embodiment of the method for producing carboxylated cellulose nanofibers of the present invention, the cationic salt of the cellulose nanofibers into which a carboxyl group has been introduced is replaced with protons by acid treatment using a cation exchange resin. When acid treatment is performed using a cation exchange resin, unnecessary by-products such as sodium chloride are not produced. Therefore, after acid treatment with a cation exchange resin, carboxylated cellulose nanofibers can be obtained simply by filtering and removing the cation exchange resin with a metal mesh or the like.
The target to be removed as a filter medium by a metal mesh or the like is a cation exchange resin, and carboxylated cellulose nanofibers are difficult to remove with a diameter of a metal mesh or the like. Therefore, it is presumed that the decrease in yield is extremely small.
Further, since the filtrate contains a large amount of carboxylated cellulose nanofibers having a short fiber length and the filtrate does not need to be washed or dehydrated, the carboxylated cellulose nanofibers are less likely to aggregate and the increase in viscosity is suppressed. It is presumed to get it.

（一般式（１）中、Ｍ_１はカチオン塩を示す。） (1) Oxidation process:
The oxidation step is a step of oxidizing a cellulosic raw material with an oxidizing agent in the presence of an N-oxyl compound and a bromide, an iodide or a mixture thereof to obtain oxidized cellulose. The oxidation step is a step of selectively oxidizing and carboxylating the 6-position primary hydroxyl group of cellulose. The partial structure of the oxidized cellulose obtained by this step is shown in the following general formula (1).

(In the general formula (1), M ₁ represents a cationic salt.)

_{Examples of the cationic salt represented by M 1} in the general formula (1) 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 molecules are converged by hydrogen bonds. When cellulose is oxidized using an N-oxyl compound, as described above, the primary hydroxyl group at the 6-position of cellulose is selectively oxidized to a carboxyl group via an aldehyde group. Therefore, a carboxyl group is introduced at a high density on the surface of the microfibril structure. The introduced carboxyl group has a repulsive action, and cellulose nanofibers separated one by one by defibration can be obtained.

(Cellulose-based raw material)
Cellulose-based raw materials include wood-derived kraft pulp or sulfite pulp, powdered cellulose obtained by crushing 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 cellulosic 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. Powdered cellulose or microcrystalline cellulose powder can provide cellulose nanofibers that provide dispersions with low viscosities even at high concentrations. When chemical pulp is used, 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 rod-axis particles made of microcrystalline or crystalline cellulose obtained by removing the non-crystalline portion of wood pulp by acid hydrolysis, 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 the X-ray diffraction method is 70 to 90%, and the volume average particle size by the laser explanatory particle size distribution device is usually 100 μm or less. It is preferably 50 μm or less. When the volume average particle size is 100 μm or less, cellulose nanofibers that provide a dispersion liquid having excellent fluidity can be provided. Such powdered cellulose may be prepared by purifying and drying the undecomposed residue obtained after acid hydrolysis of the selected pulp, pulverizing and sieving, or KC Flock (registered trademark) (Nippon Paper Industries, Ltd.). ], Theoras (registered trademark) (manufactured by Asahi Kasei Chemicals Co., Ltd.), Abyssel (registered trademark) (manufactured by FMC) and the like may be used.

As the bleaching treatment method, chlorine treatment (C), chlorine dioxide bleaching (D), alkali extraction (E), hypochlorite bleaching (H), hydrogen peroxide bleaching (P), alkaline hydrogen peroxide treatment stage ( Ep), alkaline hydrogen peroxide / oxygen treatment stage (Eop), ozone treatment (Z), chelate treatment (Q) and the like can be combined. For example, C / D-E-HD, Z-E-D-P, Z / D-Ep-D, Z / D-Ep-D-P, D-Ep-D, D-Ep-D- It can be performed in a sequence such as P, D-Ep-PD, Z-Eop-DD, Z / D-Eop-D, Z / D-Eop-D-ED. In addition, "/" in the sequence means that the processing before and after "/" is continuously performed without cleaning.

Further, the above-mentioned cellulose-based raw material is used as a cellulose-based raw material by using a dispersion device such as a high-speed rotary type, a colloid mill type, a high-pressure type, a roll mill type, an ultrasonic type, or a wet high-pressure or ultra-high pressure homogenizer. You can also do it.

（一般式（２）中、Ｒ^１〜Ｒ^４は、同一又は異なっていてもよい炭素原子数１〜４のアルキル基を示し、Ｒ^５は、水素原子又はヒドロキシル基を示す。） (N-oxyl compound)
The N-oxyl compound is a compound capable of generating a nitroxy radical. As the N-oxyl compound used in the present invention, any compound can be used as long as it is a compound that carries out the desired oxidation reaction. Examples of the N-oxyl compound include compounds represented by the following general formulas (2) to (5) and (7) and compounds represented by the following formula (6).

(In the general formula (2), R ^{1 to} R ⁴ represent an alkyl group having 1 to 4 carbon atoms which may be the same or different, and R ⁵ represents a hydrogen atom or a hydroxyl group.)

（一般式（３）〜（５）中、Ｒ^６は、炭素原子数１〜４の直鎖状又は分岐状の炭化水素基を示す。）

(In the general formulas (3) to (5), R ⁶ represents a linear or branched hydrocarbon group having 1 to 4 carbon atoms.)

（一般式（７）中、Ｒ^７〜Ｒ^８は、同一若しくは異なっていてもよい、水素原子又は炭素原子数１〜６の直鎖状若しくは分岐状のアルキル基を示す。）

(In the general formula (7), R ^{7 to} R ⁸ indicate 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 (2) ^{, 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 the general formulas (3) to (5), 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 and an n-propyl group.} Examples thereof include an isopropyl group, an n-butyl group, an s-butyl group and a t-butyl group. Of these, a methyl group or an ethyl group is preferable.
In the general formula (7) ^{, 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, n-Hexyl group. Of these, a methyl group or an ethyl group is preferable.

Examples of the compound represented by the general formula (2) 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-piperidin-N-oxy radical (hereinafter, also referred to as "4-hydroxy TEMPO").
The N-oxyl compound may be a derivative of TEMPO or 4-hydroxy TEMPO. As the derivative of 4-hydroxy TEMPO, for example, the compound represented by the general formula (3), that is, the hydroxyl group of 4-hydroxy TEMPO has a linear or branched hydrocarbon group having 4 or less carbon atoms. Examples thereof include derivatives obtained by etherification with alcohol and compounds represented by the general formula (4) or (5), that is, derivatives obtained by esterification with carboxylic acid or sulfonic acid.
When 4-hydroxy TEMPO is etherified, if an alcohol having 4 or less carbon atoms is used, the obtained derivative becomes water-soluble regardless of the presence or absence of saturated or unsaturated bonds in the alcohol, which is good as an oxidation catalyst. Works for.

When the N-oxyl compound is a compound represented by the formula (6), that is, a compound in which the amino group of 4-aminoTEMPO is acetylated, appropriate hydrophobicity is imparted, the cost is low, and uniform oxidation is performed. It is preferable because cellulose can be obtained. Further, the N-oxyl compound is preferably a compound represented by the general formula (7), that is, an azaadamantane type nitroxy radical, because a uniform cellulose oxide 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 catalyst amount capable of oxidizing the cellulosic raw material sufficiently enough to convert the obtained cellulose oxide into nanofibers. For example, it is preferably 0.01 to 10 mmol, more preferably 0.01 to 1 mmol, and further preferably 0.01 to 0.5 mmol with respect to 1 g of an absolute dry cellulosic raw material.

(Bromide, iodide or a mixture thereof)
The bromide used in the oxidation of the cellulosic raw material is a compound containing bromine, and an example thereof includes an alkali metal bromide that can be dissociated and ionized in water. Further, the iodide is a compound containing iodine, and an example thereof includes an alkali metal iodide.
The amount of bromide or iodide used can be adjusted to the extent that the desired 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 even more preferably 0.5 to 5 mmol with respect to 1 g of an absolute dry cellulosic raw material.

(Oxidant)
As the oxidizing agent, for example, known oxidizing agents such as halogen, hypochlorous acid, hypochlorous acid, perhalogen acid or salts thereof, halogen oxides, and peroxides can be used. Of these, sodium hypochlorite, which is inexpensive and has a low environmental impact, is preferable.
The amount of the oxidizing agent to be used may be any amount as long as it undergoes an oxidation reaction. It is 2.5 to 25 mmol.

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

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

Further, the oxidation reaction may be carried out in two steps. For example, by oxidizing the cationic salt of cellulose oxide obtained by filtration after the 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. It is possible to efficiently introduce a carboxyl group into a cellulosic raw material.

In the oxidized cellulose obtained in the above step, the carboxyl group introduced into the cellulosic raw material is usually an alkyl metal salt such as a sodium salt. Prior to the defibration step, the alkali metal salt of cellulose oxide may be replaced with other cationic salts such as phosphonium salt, imidazolinium salt, ammonium salt and sulfonium salt. The substitution can be carried out by a known method.

(2) Defibering process:
In one embodiment, the defibration step is a step of defibrating the oxidized cellulose to obtain a carboxylated cellulose nanofiber salt. Since the oxidation reaction of cellulose oxide is sufficiently advanced and the content of carboxyl groups is high, it can be easily converted into nanofibers by defibration treatment.
In another embodiment, the defibration step is a step of defibrating desalted cellulose oxide to obtain carboxylated cellulose nanofibers. Since the surface of the oxidized cellulose having a carboxyl group introduced and proton-substituted by desalting treatment has a high content of carboxyl groups, it can be easily converted into nanofibers by defibration treatment.

The defibration treatment can be performed, for example, by thoroughly washing the oxidized cellulose or the desalted cellulose with water, and then using a known device such as a high-speed shear mixer or a high-pressure homogenizer. Examples of the type of defibrating device include a high-speed rotary 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, the aggregated structure is small and the nanofibers can be uniformly formed.
When a high-pressure homogenizer is used, the applied pressure is preferably 50 MPa or more, more preferably 100 MPa or more, and even more preferably 140 MPa or more. When treated with a wet high-pressure or ultra-high-pressure homogenizer at the pressure, nanofiber formation proceeds efficiently, and low-viscosity carboxylated cellulose nanofibers can be efficiently obtained when used as an aqueous dispersion.

Cellulose oxide or desalted cellulose oxide is subjected to defibration treatment as an aqueous dispersion such as water. If the concentration of the oxidized cellulose or the desalted cellulose oxide in the aqueous dispersion is high, the viscosity may be excessively increased during the defibration treatment to prevent uniform defibration, or the apparatus may stop. Therefore, it is necessary to appropriately set the concentration of the oxidized cellulose or the desalted cellulose oxide according to the treatment conditions of the oxidized cellulose or the desalted cellulose oxide. As an example, the concentration of the oxidized cellulose or the desalted cellulose oxide is preferably 0.3 to 50% (w / v), more preferably 0.5 to 10% (w / v), and 1.0 to 5%. % (W / v) is more preferable.

(3) Desalting process:
In one embodiment, the desalting step is a step of contacting the carboxylated cellulose nanofiber salt with a cation exchange resin to obtain carboxylated cellulose nanofibers. In the carboxylated cellulose nanofiber salt, the cationic salt is replaced with a proton by contacting with a cation exchange resin. Since a cation exchange resin is used, unnecessary by-products such as sodium chloride are not generated. After acid treatment with the cation exchange resin, the cation exchange resin is simply filtered through a metal mesh or the like to be removed. Then, an aqueous dispersion of carboxylated cellulose nanofibers is obtained as the filtrate.
In another embodiment, the desalting step is a step of bringing the oxidized cellulose into contact with a cation exchange resin. In the cellulose oxide, the cationic salt is replaced with a proton by contacting with the cation exchange resin. Since a cation exchange resin is used, unnecessary by-products such as sodium chloride are not generated. After acid treatment with the cation exchange resin, the cation exchange resin is simply filtered through a metal mesh or the like to be removed. Then, an aqueous dispersion of ion-substituted cellulose oxide is obtained as the filtrate.

The target to be removed as a filter medium by a metal mesh or the like is a cation exchange resin, and carboxylated cellulose nanofibers or desalted cellulose oxide is difficult to remove with a diameter of a metal mesh or the like, and almost the entire amount is contained in the filtrate. .. Therefore, it is presumed that the decrease in yield is extremely small.
The filtrate contains a large amount of carboxylated cellulose nanofibers having a short fiber length or desalted cellulose oxide. Further, since the filtrate does not need to be washed or dehydrated, the carboxylated cellulose nanofibers or the desalted cellulose oxide are less likely to aggregate. Therefore, it is presumed that the dispersion liquid of the carboxylated cellulose nanofibers can suppress the increase in viscosity.

As for the carboxylated cellulose nanofiber salt, the aqueous dispersion obtained in the defibration step can be used as it is in the desalting step. Further, as for the oxidized cellulose, the aqueous dispersion obtained in the oxidation step can be used as it is in the desalting step. If necessary, water can be added to reduce the concentration.

As the cation exchange resin, either a strongly acidic ion exchange resin or a weakly acidic ion exchange resin can be used as long as the counter ion is H ^+. Above all, it is preferable to use a strongly acidic ion exchange resin. Examples of the strongly acidic ion exchange resin and the weakly acidic ion exchange resin include those in which a sulfonic acid group or a carboxy group is introduced into a styrene resin or an acrylic resin.
The shape of the cation exchange resin is not particularly limited, and various shapes such as fine particles (granular), film-like, and fibers can be used. Of these, granules are preferable from the viewpoint of efficiently treating the carboxylated cellulose nanofiber salt or oxidized cellulose and facilitating separation after the treatment. A commercially available product can be used as such a cation exchange resin. Commercially available products include, for example, Amber Jet 1020, 1024, 1060, 1220 (above, manufactured by Organo Corporation), Amberlite IR-200C, IR-120B (above, manufactured by Tokyo Organic Chemistry Co., Ltd.), Rebatit SP 112. , S100 (all manufactured by Bayer), GEL CK08P (manufactured by Mitsubishi Chemical Co., Ltd.), Dowex 50W-X8 (manufactured by Dow Chemical Co., Ltd.) and the like.

For contact between the carboxylated cellulose nanofiber salt or the oxidized cellulose and the cation exchange resin, for example, a granular cation exchange resin and an aqueous dispersion of the carboxylated cellulose nanofiber salt or the oxidized cellulose are mixed, and stirred and shaken as necessary. This can be done by contacting the carboxylated cellulose nanofiber salt or the oxidized cellulose with the cation exchange resin for a certain period of time, and then separating the cation exchange resin and the aqueous dispersion.

The concentration of the aqueous dispersion and the ratio with the cation exchange resin are not particularly limited, and those skilled in the art can appropriately set the concentration from the viewpoint of efficiently performing proton substitution. As an example, the concentration of the aqueous dispersion is preferably 0.05 to 10% by mass. If the concentration of the aqueous dispersion is less than 0.05% by mass, the time required for proton substitution may be too long. If the concentration of the aqueous dispersion is more than 10% by mass, the effect of sufficient proton substitution may not be obtained.
The contact time is not particularly limited, and a person skilled in the art can appropriately set it from the viewpoint of efficiently performing proton substitution. For example, it can be carried out by contact for 0.2 to 4 hours.

At this time, the carboxylated cellulose nanofiber salt or the oxidized cellulose is contacted with an appropriate amount of the cation exchange resin for a sufficient time, and then the cation exchange resin is removed as a filter medium by a metal mesh or the like to remove the cation exchange resin. Salt treatment can be performed.

(4) Low viscosity step:
It is preferable that the method for producing carboxylated cellulose nanofibers of the present invention further has a viscosity reducing step in both one embodiment and the other embodiments. The low-viscosity step is a step of appropriately cutting the cellulose chain of oxidized cellulose to reduce the viscosity. The step may be any step as long as the viscosity of the oxidized cellulose is lowered, and examples thereof include an ultraviolet irradiation treatment, an oxidative decomposition treatment, and a hydrolysis treatment. Above all, hydrolysis treatment is preferable.
In addition, the above-mentioned processing may be the processing of 1 type alone or may be the combination of 2 or more types of processing.

The cellulose oxide obtained in the oxidation step is preferably washed before being subjected to the low viscosity step from the viewpoint of avoiding side reactions. The cleaning method is not particularly limited, and a known method can be used.

(Hydrolyzed treatment)
The hydrolysis treatment is a treatment for hydrolyzing the oxidized cellulose in an alkaline solution or an acidic solution. Water is preferable as the reaction medium for the hydrolysis treatment from the viewpoint of suppressing side reactions.
By hydrolyzing in an alkaline solution, the viscosity of the dispersion of carboxylated cellulose nanofibers can be reduced. The reason for this can be inferred as follows. Carboxylic groups are scattered in the amorphous region of cellulose oxide oxidized using the N-oxyl compound, and the hydrogen at the C5 position adjacent to the carboxyl group is attracted by the carboxyl group. It is in a state of lack of charge. Therefore, under alkaline conditions of pH 8 to 14, the hydrogen is easily extracted by hydroxide ions, and the β-elimination reaction causes cleavage of the glucoside bond. As a result, since the oxidized cellulose is shortened, the proportion of the carboxylated cellulose nanofibers having a short fiber length is also increased, and the viscosity of the dispersion liquid of the carboxylated cellulose nanofibers can be lowered.

When hydrolyzing in an alkaline solution, the pH value of the reaction solution in the reaction is preferably 8 to 14, more preferably 9 to 13, and even more preferably 10 to 12. If the pH value is less than 8, sufficient hydrolysis may not occur and the shortening of the oxidized cellulose may be insufficient. On the other hand, when the pH value is more than 14, the hydrolysis proceeds, but the oxidized cellulose after hydrolysis is colored, and the obtained cellulose nanofibers are also colored, so that the transparency is lowered and the application technique is limited. May occur. The alkali used for adjusting the pH value may be water-soluble, and sodium hydroxide is preferable from the viewpoint of production cost.

When cellulose oxide is hydrolyzed in an alkaline solution, the cellulose oxide is colored yellow due to the formation of double bonds during β-elimination, and the resulting cellulose nanofibers are also colored, resulting in transparency. It may be reduced and the applicable technology may be limited. Therefore, the hydrolysis step is preferably carried out using an oxidizing agent or a reducing agent as an auxiliary agent in order to suppress the formation of double bonds. When an oxidizing agent or a reducing agent is used during the hydrolysis treatment in an alkaline solution having a pH value of 8 to 14, the oxidized cellulose can be shortened while oxidizing or reducing the double bond. As the oxidizing agent or reducing agent, those having activity in the alkaline region can be used.
From the viewpoint of reaction efficiency, the amount of the auxiliary agent added is preferably 0.1 to 10% by mass, more preferably 0.3 to 5% by mass, and 0.5 to 2% by mass with respect to the absolutely dried cellulose oxide. More preferred.

Examples of the oxidizing agent include oxygen, ozone, hydrogen peroxide, and hypochlorite. Among them, as the oxidizing agent, oxygen, hydrogen peroxide, and hypochlorite, which are less likely to generate radicals, are preferable, and hydrogen peroxide is more preferable.
The oxidizing agent may be used alone or in combination of two or more.

Examples of the reducing agent include sodium borohydride, hydrosulfite, and sulfites.
The reducing agent may be used alone or in combination of two or more.

The reaction temperature for hydrolysis is preferably 40 to 120 ° C., more preferably 50 to 100 ° C., and even more preferably 60 to 90 ° C. from the viewpoint of reaction efficiency. When the temperature is low, sufficient hydrolysis does not occur, and the decrease in viscosity of the dispersion liquid of cellulose oxide or carboxylated cellulose nanofibers may be insufficient. On the other hand, when the temperature is high, hydrolysis proceeds, but the oxidized cellulose may be colored after hydrolysis.
The hydrolysis reaction time is preferably 0.5 to 24 hours, more preferably 1 to 10 hours, still more preferably 2 to 6 hours.
From the viewpoint of reaction efficiency, the concentration of cellulose oxide in the alkaline solution is preferably 1 to 20% by mass, more preferably 3 to 15% by mass, still more preferably 5 to 10% by mass.

By hydrolyzing in an acidic aqueous solution, the viscosity of the dispersion of carboxylated cellulose nanofibers can be reduced. The reason for this can be inferred as follows. A carboxyl group is localized on the surface of the cellulosic raw material oxidized using the N-oxyl compound, and a hydrated layer is formed. Therefore, the oxidized celluloses exist in close proximity to each other and form a network. When an acid is added to the oxidized cellulose to hydrolyze it, the charge balance in the network is lost and the strong network of cellulose molecules is broken. As a result, the specific surface area of the oxidized cellulose is increased, and the shortening of the fiber of the oxidized cellulose is promoted. Therefore, since the oxidized cellulose is shortened, the proportion of the carboxylated cellulose nanofibers having a short fiber length is also increased, and the viscosity of the dispersion liquid of the carboxylated cellulose nanofibers can be lowered.

When hydrolyzing in an acidic solution, it is preferable to use a mineral acid such as sulfuric acid, hydrochloric acid, nitric acid, or phosphoric acid as the acid. Further, in order to carry out the reaction efficiently, it is preferable to use a dispersion liquid in which cellulose oxide is dispersed in a dispersion medium such as water.
The conditions for hydrolysis in an acidic solution may be such that the acid acts on the amorphous part of cellulose. For example, the amount of the acid added is preferably 0.01 to 0.5% by mass, more preferably 0.1 to 0.5% by mass, based on the absolute dry mass of the oxidized cellulose. When the amount of the acid added is 0.01% by mass or more, hydrolysis of cellulose proceeds and the treatment efficiency in the defibration step is improved, which is preferable. Further, when the addition amount is 0.5% by mass or less, excessive hydrolysis of cellulose can be prevented, and a decrease in the yield of cellulose nanofibers can be prevented.

When hydrolyzing in an acidic solution, the pH value of the reaction solution in the reaction is preferably 2.0 to 4.0, more preferably 2.0 or more and less than 3.0. From the viewpoint of reaction efficiency, the reaction temperature is preferably 70 to 120 ° C. and the reaction time is preferably 1 to 10 hours.

When hydrolyzed in an acidic solution, it is usually neutralized by adding an alkali such as sodium hydroxide in order to efficiently carry out the defibration step.

(Ultraviolet irradiation treatment)
The ultraviolet irradiation treatment is a process of irradiating the oxidized cellulose with ultraviolet rays. By irradiating with ultraviolet rays, the viscosity of the dispersion liquid of the carboxylated cellulose nanofibers can be reduced. The reason for this can be inferred as follows. Ultraviolet rays act directly on cellulose and hemicellulose to cause low molecular weight, and can shorten the cellulose chains in oxidized cellulose. Therefore, the proportion of the carboxylated cellulose nanofibers having a short fiber length also increases, and the viscosity of the dispersion liquid of the carboxylated cellulose nanofibers can be lowered.

When irradiating the oxidized cellulose with ultraviolet rays in the viscosity reducing step, the wavelength of the ultraviolet rays used is preferably 100 to 400 nm, more preferably 100 to 300 nm. Of these, ultraviolet rays having a wavelength of 135 to 260 nm are preferable because they act directly on cellulose and hemicellulose to cause low molecular weight and shorten the cellulose chains in the oxidized cellulose.

As the light source for irradiating ultraviolet rays, a light source having light in a wavelength region of 100 to 400 nm can be used. Examples thereof include xenon short arc lamps, ultra-high pressure mercury lamps, high pressure mercury lamps, low pressure mercury lamps, deuterium lamps, and metal halide lamps.
It should be noted that these light sources may be used individually by 1 type, or may be used in any combination of 2 or more types. It is preferable to use a combination of a plurality of light sources having different wavelength characteristics because the simultaneous irradiation of ultraviolet rays having different wavelengths increases the number of cut points in the cellulose chain or hemicellulose chain and promotes shortening of fibers.

As a container for accommodating cellulose oxide when irradiating with ultraviolet rays, for example, when ultraviolet rays of 300 to 400 nm are used, a container made of hard glass can be used. When ultraviolet rays having a wavelength shorter than 300 nm are used, it is preferable to use a quartz glass container that transmits ultraviolet rays more. The material of the portion of the container that is not involved in the light transmission reaction may be appropriately selected from the materials that are less deteriorated with respect to the wavelength of the ultraviolet rays used.

The concentration of cellulose oxide when irradiated with ultraviolet rays is preferably 0.1 to 12% by mass, more preferably 0.5 to 5% by mass, and even more preferably 1 to 3% by mass. When the concentration of cellulose oxide is 0.1% by mass or more, energy efficiency is increased, which is preferable. When the concentration of cellulose oxide is 12% by mass or less, the fluidity of cellulose oxide in the ultraviolet irradiation device is good and the reaction efficiency is increased, which is preferable.

The temperature at the time of irradiation with ultraviolet rays is preferably 20 to 95 ° C, more preferably 20 to 80 ° C, and even more preferably 20 to 50 ° C. When the temperature is 20 ° C. or higher, the efficiency of the photooxidation reaction is increased, which is preferable. When the temperature is 95 ° C. or lower, there is no risk of adverse effects such as deterioration of the quality of cellulose oxide, and there is no risk of the pressure inside the reactor exceeding atmospheric pressure, so it is necessary to design the device in consideration of pressure resistance. It is preferable because it disappears.

The pH value when irradiating with ultraviolet rays is not particularly limited, but a neutral region, for example, a pH value of about 6.0 to 8.0 is preferable in consideration of process simplification.

The degree of irradiation received by the cellulose oxide during ultraviolet irradiation can be arbitrarily set by adjusting the residence time of the cellulose oxide in the irradiation reaction apparatus, adjusting the amount of energy of the irradiation light source, and the like. In addition, the concentration of cellulose oxide in the irradiation device can be adjusted by diluting with water, or the concentration of cellulose oxide can be adjusted by blowing an inert gas such as air or nitrogen into the cellulose oxide. The amount of ultraviolet radiation received by the oxidized cellulose can be arbitrarily controlled. These conditions such as residence time and concentration can be appropriately set according to the target quality of oxidized cellulose after irradiation with ultraviolet rays (fiber length, degree of polymerization of cellulose, etc.).

When the ultraviolet irradiation treatment is performed in the presence of an auxiliary agent such as oxygen, ozone, or peroxide (hydrogen peroxide, peracetic acid, sodium percarbonate, sodium perborate, etc.), the efficiency of the photooxidation reaction increases. preferable.

When irradiating ultraviolet rays in the wavelength range of 135 to 242 nm, ozone is generated from the air existing in the gas phase around the light source. While continuously supplying air to the periphery of this light source, the generated ozone is continuously extracted, and the extracted ozone is injected into the oxidized cellulose, so that light can be obtained without supplying ozone from outside the system. Ozone can also be used as an auxiliary agent for the oxidation reaction. Further, by supplying oxygen to the gas phase portion around the light source, a larger amount of ozone can be generated in the system, and the generated ozone can be used as an auxiliary agent for the photooxidation reaction. In this way, ozone secondarily generated by the ultraviolet irradiation reaction device can also be used.

The ultraviolet irradiation treatment may be repeated a plurality of times. The number of repetitions is not particularly limited, but can be appropriately set according to the relationship such as the quality of the target oxidized cellulose. For example, ultraviolet rays of preferably 100 to 400 nm, more preferably 135 to 260 nm, preferably 1 to 10 times, more preferably 2 to 5 times, and the irradiation time per one time are preferably 0.5 to 10 hours. More preferably, it can be carried out by irradiating with ultraviolet rays for 0.5 to 3 hours.

(Oxidative decomposition treatment)
When oxidatively decomposing cellulose oxide in the low viscosity step, hydrogen peroxide and ozone are used in combination.
The reason why the viscosity of oxidized cellulose can be efficiently reduced by using hydrogen peroxide and ozone in combination is presumed as follows. A carboxyl group is localized on the surface of the oxidized cellulose produced by oxidation using an N-oxyl compound, and a hydrated layer is formed. Therefore, it is considered that there are microscopic gaps between the cellulose chains of the oxidized cellulose, which are not seen in ordinary cellulose, due to the action of the charge repulsive force between the carboxyl groups. Then, when the oxidized cellulose 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 oxidized cellulose are efficiently oxidatively decomposed, and finally the oxidized cellulose is produced. Shorten the fiber. Therefore, the proportion of the carboxylated cellulose nanofibers having a short fiber length also increases, and the viscosity of the dispersion liquid of the carboxylated cellulose nanofibers can be lowered.

Ozone can be generated by a known method using an ozone generator using air or oxygen as a raw material. The amount of ozone added (mass equivalent) is preferably 0.1 to 3 times, more preferably 0.3 to 2.5 times, and 0.5 to 1.5 times the absolute dry mass of the oxidized cellulose. More preferred. When the amount of ozone added is 0.1 times or more the absolute dry mass of the oxidized cellulose, the amorphous portion of the cellulose can be sufficiently decomposed. When the amount of ozone added is 3 times or less the absolute dry mass of cellulose oxide, excessive decomposition of cellulose can be suppressed and a decrease in the yield of cellulose oxide can be prevented.

The amount of hydrogen peroxide added (in terms of mass) is preferably 0.001 to 1.5 times, more preferably 0.1 to 1.0 times the absolute dry mass of the oxidized cellulose. When the amount of hydrogen peroxide added is 0.001 times or more the absolute dry mass of cellulose oxide, the synergistic action of ozone and hydrogen peroxide is exhibited. Further, when decomposing cellulose oxide, it is sufficient that the amount of hydrogen peroxide added is 1.5 times or less that of cellulose oxide, and adding more than 1.5 times is not preferable because it leads to an increase in cost.

As conditions for the oxidative decomposition treatment with ozone and hydrogen peroxide, the pH value is preferably 2 to 12, more preferably 4 to 10, still more preferably 6 to 8, and the temperature is preferably 10 to 90 ° C. The reaction efficiency is preferably 20 to 70 ° C., more preferably 30 to 50 ° C., and the reaction time is preferably 1 to 20 hours, more preferably 2 to 10 hours, still more preferably 3 to 6 hours. It is preferable from the viewpoint of.

The device for performing the treatment with ozone and hydrogen peroxide is not particularly limited, and a known device can be used. For example, a conventional reactor equipped with a reaction chamber, a stirrer, a chemical injection device, a heater, and a pH electrode can be used.

The ozone and hydrogen peroxide remaining in the aqueous solution after the treatment with ozone and hydrogen peroxide can effectively act in the defibration step and further promote the lowering of the viscosity of the dispersion liquid of the carboxylated cellulose nanofibers.

Hereinafter, the present invention will be described in detail with reference to Examples. The following examples are for the purpose of preferably explaining the present invention, and do not limit the present invention. Unless otherwise specified, the method for measuring the physical property value or the like is the measurement method described above.

[B-type viscosity (mPa · s)]: Using a TV-10 type viscometer (Toki Sangyo Co., Ltd.), the B-type viscosity of the aqueous dispersion of 1% by mass of carboxylated cellulose nanofibers was adjusted to 20 ° C. and 60 rpm. Alternatively, it was measured under the condition of 6 rpm.

[Yield (%)]: The yield in the desalting step of acid-treating the carboxylated cellulose nanofiber salt into the carboxylated cellulose nanofiber.

[Carboxylic acid group amount]: The carboxyl group amount was measured as follows. 60 ml of a 0.5 mass% slurry (aqueous dispersion) of carboxylated cellulose was prepared, and a 0.1 M hydrochloric acid aqueous solution was added to adjust the pH to 2.5, and then a 0.05 N sodium hydroxide aqueous solution was added dropwise to adjust the pH to 11. The electrical conductivity was measured until. From the amount of sodium hydroxide (a) consumed in the neutralization step of a weak acid with a gradual change in electrical conductivity, the amount of carboxyl groups was calculated using the following formula:
Amount of carboxyl groups [mmol / g carboxylated cellulose] = a [ml] x 0.05 / mass of carboxylated cellulose [g]

(Example 1)
5 g (absolutely dried) of bleached softwood unbeaten pulp (manufactured by Nippon Paper Industries, Ltd.) was added to 500 ml of an aqueous solution prepared by dissolving 78 mg (0.5 mmol) of TEMPO (manufactured by Sigma Aldrich) and 754 mg (7.4 mmol) of sodium bromide. The mixture was stirred until the pulp was uniformly dispersed. After adding 18 ml of a 2M sodium hypochlorite aqueous solution to the reaction system, the pH was adjusted to 10.3 with a 0.5N hydrochloric acid aqueous solution, and the oxidation reaction was started (oxidation step). Since the pH in the system decreased during the reaction, a 0.5 N aqueous sodium hydroxide 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 1.7 mmol / g cellulose oxide.

Next, the obtained slurry of cellulose oxide was adjusted to 1% (w / v) with water and treated 5 times with an ultra-high pressure homogenizer (20 ° C., 140 MPa) to obtain a transparent gel-like carboxylated cellulose nanofiber salt. A dispersion (1% (w / v)) was obtained (defibration step).

A cation exchange resin (manufactured by Organo Corporation, "Amber Jet 1024") was added to the obtained dispersion of carboxylated cellulose nanofiber salt, and the mixture was contacted with stirring at 20 ° C. for 0.3 hours. Then, the cation exchange resin and the aqueous dispersion were separated by a metal mesh (opening 100 mesh) to obtain carboxylated cellulose nanofibers in a high yield of 95% (desalting step).

The B-type viscosity of the 1% by mass aqueous dispersion of the obtained carboxylated cellulose nanofibers was 3699 mPa · s under the condition of (60 rpm, 20 ° C.) and 21799 mPa · s under the condition of (6 rpm, 20 ° C.). there were. The results are shown in Table 1 along with the yields.

(Comparative Example 1)
Carboxylated cellulose nanofibers were obtained in the same manner as in Example 1 except that the desalting step was changed as follows.
A 10% aqueous hydrochloric acid solution was added to the dispersion of the carboxylated cellulose nanofiber salt until the pH reached 2.4, and the mixture was contacted with stirring at 20 ° C. for 0.4 hours. Then, washing and dehydration treatment were repeated three times, and then filtration was performed. Water was added to the filter medium to adjust the yield to 1.0% (w / v), and the mixture was treated twice with an ultra-high pressure homogenizer (20 ° C., 140 MPa) to obtain a transparent gel-like dispersion of carboxylated cellulose nanofibers. 1% (w / v)) was obtained in a yield of 67%.

The B-type viscosity of the 1% by mass aqueous dispersion of the obtained carboxylated cellulose nanofibers was 3909 mPa · s under the condition of (60 rpm, 20 ° C.) and 31593 mPa · s under the condition of (6 rpm, 20 ° C.). there were. The results are shown in Table 1 along with the yields.

(Example 2)
5 g (absolutely dried) of bleached softwood unbeaten pulp (manufactured by Nippon Paper Industries, Ltd.) was added to 500 ml of an aqueous solution prepared by dissolving 78 mg (0.5 mmol) of TEMPO (Sigma Aldrich) and 754 mg (7.4 mmol) of sodium bromide. The mixture was stirred until the pulp was uniformly dispersed. After adding 16 ml of a 2M sodium hypochlorite aqueous solution to the reaction system, the pH was adjusted to 10.3 with a 0.5N hydrochloric acid aqueous solution, and the oxidation reaction was started (oxidation step). Although the pH in the system decreased during the reaction, a 0.5 N aqueous sodium hydroxide solution was sequentially added to adjust the pH to 10. After reacting for 2 hours, the mixture was filtered through a glass filter and washed thoroughly with water to obtain cellulose oxide having a carboxyl group amount of 1.60 mmol / g.

Next, 1% (w / v) of hydrogen peroxide was added to the 5% (w / v) slurry of cellulose oxide with respect to the cellulose oxide, and the pH was adjusted to 12 with 1 M sodium hydroxide. This slurry was hydrolyzed at 80 ° C. for 2 hours (low viscosity step). Then, it was filtered with a glass filter and washed thoroughly with water.

When the 2% (w / v) oxidized cellulose slurry subjected to the low viscosity step was treated 5 times with an ultra-high pressure homogenizer (20 ° C., 140 MPa), a transparent gel-like dispersion of carboxylated cellulose nanofiber salt (1%) was applied. (W / v)) was obtained (defibration step).

A cation exchange resin (manufactured by Organo Corporation, "Amber Jet 1024") was added to the obtained dispersion of carboxylated cellulose nanofiber salt, and the mixture was contacted with stirring at 20 ° C. for 0.3 hours. Then, the cation exchange resin and the aqueous dispersion were separated by a metal mesh (opening 100 mesh) to obtain carboxylated cellulose nanofibers in a high yield of 92% (desalting step).

The B-type viscosity of the 1% by mass aqueous dispersion of the obtained carboxylated cellulose nanofibers was 53 mPa · s under the condition of (60 rpm, 20 ° C.) and 135 mPa · s under the condition of (6 rpm, 20 ° C.). there were. The results are shown in Table 1 along with the yields.

(Comparative Example 2)
When the same procedure as in Example 2 was carried out except that the desalting step was changed as follows, carboxylated cellulose nanofibers could not be obtained.
A 10% aqueous hydrochloric acid solution was added to the dispersion of the carboxylated cellulose nanofiber salt until the pH reached 2.4, and the mixture was contacted with stirring at 20 ° C. for 0.6 hours. After that, it was filtered for washing, but no filter medium was obtained.

As can be seen from Table 1, when the desalting step was carried out using a cation exchange resin, carboxylated cellulose nanofibers were obtained in high yields exceeding 90% in both Examples 1 and 2. On the other hand, when the desalting step was carried out by hydrochloric acid treatment, the yield of the carboxylated cellulose nanofibers decreased to 67% in Comparative Example 1, and the carboxylated cellulose nanofibers could be obtained as a product in Comparative Example 2. There wasn't. Therefore, the method for producing cellulose nanofibers of the present invention not only obtains cellulose nanofibers in high yield (see Example 1 and Comparative Example 1), but also produces carboxylated cellulose nanofibers that cannot be obtained by hydrochloric acid treatment. It could be obtained as a product (see Example 2 and Comparative Example 2).
Further, comparing the B-type viscosities of the cellulose nanofibers obtained in Example 1 and Comparative Example 1, under the condition of (60 rpm, 20 ° C.), 3699 mPa · s (Example 1) and 3909 mPa · s (Comparative Example 1). ), No significant difference was observed. However, under the conditions of (6 rpm, 20 ° C.), there is a significant difference between 21799 mPa · s (Example 2) and 31593 mPa · s (Comparative Example 2), and when the desalting step is performed using a cation exchange resin, It can be seen that carboxylated cellulose nanofibers can be obtained with low viscosity.

Claims (4)

An oxidation step of oxidizing a cellulosic raw material with an oxidizing agent in the presence of an N-oxyl compound and a bromide, iodide or a mixture thereof to obtain oxidized cellulose (however, undried by 100 parts by mass of dry mass). 1.25 parts by mass of coniferous bleached kraft pulp, 2,2,6,6-tetramethylpiperidin 1-oxyl and 12.5 parts by mass of sodium hydroxide were dispersed in 10000 parts by mass of water, and then 13% by mass. Sodium hypochlorite aqueous solution was added to 1.0 g of pulp so that the amount of sodium hypochlorite was 8.0 mmol to start the reaction, and 0.5 M sodium hydroxide aqueous solution was added dropwise during the reaction. The pH is kept at 10 or more and 11 or less, and when no change in pH is observed, the reaction is considered to be completed to obtain cellulose oxide) .
The defibration process for defibrating the oxidized cellulose and
It has a desalting step of desalting the oxidized cellulose by a cation exchange reaction before or after the defibration step.
A method for producing carboxylated cellulose nanofibers, wherein the desalting step is a step of desalting with a cation exchange resin. The method for producing carboxylated cellulose nanofibers according to claim 1, further comprising a low viscosity step of hydrolyzing the oxidized cellulose in an alkaline solution before the defibration step. The method for producing carboxylated cellulose nanofibers according to claim 2, wherein the low viscosity step is a step of hydrolyzing the oxidized cellulose in an alkaline solution containing an oxidizing agent or a reducing agent. The defibration step is a step of defibrating the oxidized cellulose to obtain a carboxylated cellulose nanofiber salt.
The method for producing carboxylated cellulose nanofibers according to any one of claims 1 to 3, wherein the desalting step is a step of desalting the carboxylated cellulose nanofiber salt with the cation exchange resin.