JP7735894B2 - Ionic functionalized cellulose nanocrystals, their production method, and dispersions, sheets, and powders - Google Patents

Description

The present invention relates to cellulose nanocrystals with ionic functional groups introduced therein, a method for producing the same, and dispersions, sheets, and powders containing the cellulose nanocrystals with ionic functional groups introduced therein.

In recent years, materials made from renewable natural fibers have been attracting attention due to the need to replace petroleum resources and growing environmental awareness. Among natural fibers, fibrous cellulose with a fiber diameter of 10 μm to 50 μm, particularly wood-derived fibrous cellulose (pulp), has been widely used, primarily in paper products.

Patent Document 1 describes a method for producing cellulose nanofibers, in which a cellulosic material is oxidized using an oxidizing agent, and the oxidized cellulosic material is treated with a homogenizer in the presence of cellulase and/or hemicellulase to defibrate and disperse the material into cellulose nanofibers, with the aim of obtaining cellulose nanofibers that have low viscosity even at high concentrations.
Non-Patent Document 1 describes the production of phosphorylated cellulose nanocrystals by hydrolyzing unmodified cotton pulp with a high concentration of phosphoric acid.
Furthermore, Non-Patent Document 2 describes the introduction of phosphate groups into cellulose nanocrystals by (i) phosphorylation in an aqueous system or (ii) phosphorylation in a molten urea system.

International Publication No. 2011/118746

Biomacromolecules, vol.14, 1223-1230, 2013 Carbohydrate Polymers, vol.125, 301-313, 2015

As described in Patent Document 1, nanocellulose fibers that have been shortened to short fibers do not sufficiently reduce the viscosity when dispersed in water.
Although cellulose nanocrystals have a lower viscosity when dispersed in water, the introduction of a small amount of ionic functional groups poses problems such as insufficient transparency and salt tolerance. The method described in Non-Patent Document 1 introduces phosphate groups during hydrolysis, which prevents the introduction of sufficient ionic functional groups into the cellulose nanocrystals. Furthermore, the method described in Non-Patent Document 2 also introduces phosphate groups into the cellulose nanocrystals. However, method (i) does not allow the introduction of sufficient ionic functional groups. While method (ii) can introduce more ionic functional groups than method (i), it is estimated that a large amount of unintended functional groups (carbamide groups) is also present. Furthermore, in both methods (i) and (ii), the introduction of ionic functional groups into the cellulose nanocrystals necessitates the use of large amounts of reagents in a low-concentration cellulose nanocrystal dispersion, leaving room for improvement in terms of industrialization.
An object of the present invention is to provide cellulose nanocrystals that, when dispersed in water, have low viscosity, high transparency, excellent salt tolerance, and a high amount of ionic functional groups introduced therein, and a method for producing the same.A further object of the present invention is to provide dispersions, sheets, and powders containing the cellulose nanocrystals.

The present inventors have discovered that the above problems can be solved by producing cellulose nanocrystals using a specific production method, or by using cellulose nanocrystals having an amount of ionic functional groups introduced that is equal to or greater than a specific value.
The present invention relates to the following items <1> to <19>.
<1> A method for producing cellulose nanocrystals having ionic functional groups introduced therein, comprising the following steps A to C in this order:
Step A: an ionic functional group introduction step of introducing ionic functional groups into cellulose fibers; Step B: a hydrolysis step of hydrolyzing the cellulose fibers having the ionic functional groups introduced therein with an acidic aqueous solution having a concentration of 5 mol/L or more; and Step C: a defibration step of defibrating the hydrolyzed cellulose fibers having the ionic functional groups introduced therein. <2> The production method according to <1>, wherein the ionic functional group is at least one selected from the group consisting of a phosphorus oxoacid group, a group derived from a phosphorus oxoacid group, a carboxy group, a sulfone group, a sulfur oxoacid group, a group derived from a sulfur oxoacid group, and a cationic group.
<3> The method according to <1> or <2>, wherein the ionic functional group is at least one selected from the group consisting of a phosphorus oxo acid group and a group derived from a phosphorus oxo acid group.
<4> The production method according to any one of <1> to <3>, wherein in step A, the amount of ionic functional groups introduced into the cellulose fiber is 0.5 mmol/g or more.
<5> The method according to any one of <1> to <4>, wherein the amount of ionic functional groups introduced into the cellulose nanocrystals having ionic functional groups introduced therein is 0.1 mmol/g or more.
<6> The method according to any one of <1> to <5>, wherein the amount of carbamide groups in the cellulose nanocrystals into which ionic functional groups have been introduced is 0.06 mmol/g or less.
<7> The production method according to any one of <1> to <6>, wherein the acidic aqueous solution used in step B is an aqueous solution of an acid compound selected from the group consisting of phosphoric acid, sulfuric acid, diphosphorus pentoxide, and hydrochloric acid.
<8> Cellulose nanocrystals having ionic functional groups introduced therein, in which the amount of ionic functional groups introduced is 0.1 mmol/g or more (excluding cellulose nanocrystals in which only the carbon atom at the 6th position of cellulose is oxidized and a carboxy group is introduced).
<9> The cellulose nanocrystal having an ionic functional group introduced therein according to <8>, wherein the ionic functional group is at least one selected from the group consisting of a phosphorus oxoacid group, a group derived from a phosphorus oxoacid group, a carboxy group, a sulfone group, a sulfur oxoacid group, a group derived from a sulfur oxoacid group, and an ammonium group.
<10> The cellulose nanocrystal having an ionic functional group introduced therein according to <8> or <9>, wherein the ionic functional group is at least one selected from the group consisting of a phosphorus oxo acid group and a group derived from a phosphorus oxo acid group.
<11> The cellulose nanocrystal having an ionic functional group introduced therein according to any one of <8> to <10>, wherein the amount of the ionic functional group introduced therein is 0.5 mmol/g or more.
<12> The cellulose nanocrystal having an ionic functional group introduced therein according to any one of <8> to <11>, wherein the ionic functional group is introduced into at least one of the carbon atoms at the 2nd and 3rd positions of the cellulose.
<13> The cellulose nanocrystal having an ionic functional group introduced therein according to any one of <8> to <12>, wherein the amount of carbamide groups in the cellulose nanocrystal having an ionic functional group introduced therein is 0.06 mmol/g or less.
<14> The cellulose nanocrystal having introduced therein an ionic functional group according to any one of <8> to <13>, wherein a 1.5% by mass aqueous dispersion of the cellulose nanocrystal having introduced therein an ionic functional group has a viscosity of 100 mPa·s or less.
<15> The ionically functionalized cellulose nanocrystal according to any one of <8> to <14>, wherein a 0.2% by mass aqueous dispersion of the ionically functionalized cellulose nanocrystal has a total light transmittance of 95.5% or more.
<16> The cellulose nanocrystal having an ionically functionalized group introduced therein according to any one of <8> to <15>, wherein when sodium chloride is added to a 0.2 mass% aqueous dispersion of the cellulose nanocrystal having an ionically functionalized group introduced therein so as to give a concentration of 100 mmol/L, T/ _T0 is 0.98 or more, where T is the total light transmittance after the _addition of sodium chloride and T0 is the total light transmittance before the addition of sodium chloride.
<17> A dispersion liquid containing the cellulose nanocrystal having an ionic functional group introduced therein according to any one of <8> to <16>.
<18> A sheet containing the cellulose nanocrystal having an ionic functional group introduced therein according to any one of <8> to <16>.
<19> A powder containing the cellulose nanocrystal having an ionic functional group introduced therein according to any one of <8> to <16>.

The present invention provides cellulose nanocrystals that, when dispersed in water, have low viscosity, high transparency, excellent salt tolerance, and a high amount of ionic functional groups introduced, as well as a method for producing the same. Furthermore, the present invention can provide dispersions, sheets, and powders containing the cellulose nanocrystals.

FIG. 1 is a graph showing the relationship between the amount of NaOH dropped onto a slurry containing cellulose fibers having phosphorus oxo acid groups and pH. FIG. 2 is a graph showing the relationship between the amount of NaOH dropped onto a slurry containing cellulose fibers having carboxy groups and the pH.

[Method for producing cellulose nanocrystals with ionic functional groups introduced]
The method for producing ionic functional group-introduced cellulose nanocrystals of the present invention (hereinafter, "ionic functional group-introduced cellulose nanocrystals" may also be simply referred to as "ionic functional group-introduced CNCs" or "CNCs") comprises the following steps A to C in this order:
Step A: An ionic functional group introduction step for introducing ionic functional groups into cellulose fibers. Step B: A hydrolysis step for hydrolyzing the cellulose fibers into which ionic functional groups have been introduced using an acidic aqueous solution having a concentration of 5 mol/L or more. Step C: A defibration step for defibrating the hydrolyzed cellulose fibers into which ionic functional groups have been introduced. According to the production method of the present invention, cellulose nanocrystals are provided which, when made into an aqueous dispersion, have low viscosity, high transparency, excellent salt tolerance, and a high amount of ionic functional groups introduced.
Cellulose nanocrystals are defibrated to the nano level with a fiber width of 1000 nm or less, and are highly crystallized by removing amorphous regions through acid hydrolysis.
The reason why the above-described effects are obtained by the production method of this embodiment is thought to be as follows: As described in Patent Document 1, attempts have been made to shorten cellulose nanofibers to obtain nanocellulose with low viscosity when dispersed in water, but shortening cellulose nanofibers has proven difficult to achieve a sufficient viscosity reduction when dispersed in water.
To achieve a sufficiently low viscosity when prepared as an aqueous dispersion, cellulose nanocrystals have been produced by acid hydrolysis of the amorphous portion of cellulose followed by defibration. While this method achieves a low viscosity when prepared as an aqueous dispersion, the resulting cellulose nanocrystals suffer from problems with the transparency and salt tolerance of the aqueous dispersion. To address this issue, attempts have been made to introduce ionic functional groups into the cellulose nanocrystals obtained by acid hydrolysis and defibration. However, this method of introducing ionic functional groups into cellulose nanocrystals results in a low concentration of the cellulose nanocrystal dispersion, making chemical treatment at high concentrations difficult and reducing the reaction efficiency. Consequently, the amount of ionic functional groups that can be introduced is limited, and transparency and salt tolerance cannot be sufficiently improved. Another problem is that functional groups other than the ionic functional groups remain as impurities. Furthermore, when phosphorus oxoacid groups are introduced after cellulose nanocrystals are obtained, condensation between phosphorus oxoacid groups or the reaction of a single phosphorus oxoacid molecule with multiple cellulose hydroxyl groups can lead to differences in the amount of strongly acidic and weakly acidic groups introduced.
In the present invention, by introducing ionic functional groups into cellulose fibers and then carrying out hydrolysis and defibration, it is possible to increase the amount of ionic functional groups introduced into the resulting cellulose nanocrystals, which is thought to result in improved transparency and salt tolerance. Furthermore, by carrying out acid hydrolysis after introducing ionic functional groups into cellulose fibers, it is possible to reduce functional groups other than ionic functional groups, and it is thought that ionic functional group-introduced cellulose nanocrystals with higher purity can be obtained. Furthermore, by carrying out hydrolysis after introducing ionic functional groups, it is possible to obtain ionic functional group-introduced cellulose nanocrystals with little or no difference in the amount of strongly acidic groups and weakly acidic groups introduced, even when phosphorus oxo acid groups are introduced.
Furthermore, by hydrolyzing cellulose fibers into which ionic functional groups have been introduced and then defibrating them, the electrical repulsive force of the ionic functional groups possessed by the hydrolyzed cellulose fibers increases the defibration efficiency, resulting in cellulose nanocrystals with smaller fiber widths.
The present invention will be described in further detail below.

<Process A>
Step A is an ionic functional group introduction step in which ionic functional groups are introduced into cellulose fibers.
[Cellulose fiber]
Cellulose fibers are fiber materials containing cellulose. While not particularly limited, pulp is preferred due to its availability and low cost. Examples of pulp include wood pulp, non-wood pulp, and deinked pulp. Examples of wood pulp include, but are not limited to, chemical pulps such as hardwood kraft pulp (LBKP), softwood kraft pulp (NBKP), sulfite pulp (SP), dissolving pulp (DP), soda pulp (AP), unbleached kraft pulp (UKP), and oxygen-bleached kraft pulp (OKP); semi-chemical pulps such as semi-chemical pulp (SCP) and chemi-ground wood pulp (CGP); and mechanical pulps such as groundwood pulp (GP) and thermomechanical pulp (TMP, BCTMP). Examples of non-wood pulp include, but are not limited to, cotton-based pulps such as cotton linters and cotton lint, and non-wood pulps such as hemp, straw, bamboo, and bagasse. The deinked pulp is not particularly limited, but may be, for example, deinked pulp made from recycled paper. The pulp of this embodiment may be one of the above types alone or a mixture of two or more types.
Among the above pulps, for example, wood pulp and deinked pulp are preferred from the viewpoint of availability. Furthermore, among wood pulps, for example, chemical pulp is more preferred, and kraft pulp and sulfite pulp are even more preferred, from the viewpoint of a high cellulose content, a high CNC yield during defibration treatment, and small decomposition of cellulose in the pulp.
As the cellulose fiber, for example, cellulose contained in sea squirts or bacterial cellulose produced by acetic acid bacteria can also be used.
Moreover, instead of cellulose fibers, fibers formed from linear nitrogen-containing polysaccharide polymers such as chitin and chitosan can also be used.

[Ionic Functional Group]
In step A, ionic functional groups are introduced into cellulose fibers. By introducing ionic functional groups into cellulose fibers, the defibration efficiency in the defibration treatment can be increased, and the transparency and salt resistance of the resulting cellulose nanocrystals can be improved.
The ionic functional group may include, for example, either or both of an anionic functional group and a cationic functional group. In this embodiment, it is particularly preferable that the ionic functional group is an anionic functional group.

Examples of anionic functional groups as ionic functional groups include phosphorus oxo acid groups or groups derived from phosphorus oxo acid groups (sometimes simply referred to as phosphorus oxo acid groups), carboxy groups or groups derived from carboxy groups (sometimes simply referred to as carboxy groups), sulfur oxo acid groups or groups derived from sulfur oxo acid groups (sometimes simply referred to as sulfur oxo acid groups), xanthate groups, phosphonic groups, phosphine groups, sulfonic groups, and carboxyalkyl groups.
Examples of cationic functional groups as ionic functional groups include ammonium groups, phosphonium groups, sulfonium groups, etc. Among these, the cationic functional group is preferably an ammonium group.
Among these, the ionic functional group is preferably at least one selected from the group consisting of a phosphorus oxoacid group, a group derived from a phosphorus oxoacid group, a carboxy group, a sulfonic acid group, a sulfur oxoacid group, a group derived from a sulfur oxoacid group, a carboxymethyl group, a carboxyethyl group, and a cationic functional group, more preferably at least one selected from the group consisting of a phosphorus oxoacid group, a group derived from a phosphorus oxoacid group, a carboxy group, a sulfur oxoacid group, and a group derived from a sulfur oxoacid group, even more preferably at least one selected from the group consisting of a phosphorus oxoacid group and a group derived from a phosphorus oxoacid group, and even more preferably a phosphorus oxoacid group. By introducing a phosphorus oxoacid group as the anionic functional group, cellulose nanocrystals with excellent transparency and salt resistance can be obtained.

The phosphorus oxoacid group or a group derived from the phosphorus oxoacid group is, for example, a substituent represented by the following formula (1). Multiple substituents represented by the following formula (1) may be introduced into each cellulose fiber. In this case, the multiple introduced substituents represented by the following formula (1) may be the same or different.

In formula (1), a, b, and n are natural numbers, and m is an arbitrary number (where a = b × m). At least one of the n α and α' is ^O- , and the rest are R or OR. All of the α and α' may be ^O- . The n α may all be the same or different. ^{β b+} is a monovalent or higher cation made of an organic or inorganic substance.

Each R is a hydrogen atom, a saturated linear hydrocarbon group, a saturated branched hydrocarbon group, a saturated cyclic hydrocarbon group, an unsaturated linear hydrocarbon group, an unsaturated branched hydrocarbon group, an unsaturated cyclic hydrocarbon group, an aromatic group, or a derivative thereof. Furthermore, in formula (1), n is preferably 1.

Saturated linear hydrocarbon groups include, but are not limited to, methyl, ethyl, n-propyl, and n-butyl groups. Saturated branched hydrocarbon groups include, but are not limited to, i-propyl and t-butyl groups. Saturated cyclic hydrocarbon groups include, but are not limited to, cyclopentyl and cyclohexyl groups. Unsaturated linear hydrocarbon groups include, but are not limited to, vinyl and allyl groups. Unsaturated branched hydrocarbon groups include, but are not limited to, i-propenyl and 3-butenyl groups. Unsaturated cyclic hydrocarbon groups include, but are not limited to, cyclopentenyl and cyclohexenyl groups. Aromatic groups include, but are not limited to, phenyl and naphthyl groups.

Furthermore, the derivative group in R includes, but is not limited to, functional groups in which at least one functional group selected from the group consisting of a carboxy group, a carboxylate group (-COO-), a hydroxy group, an amino group, and an ammonium group is added to or substituted on the main chain or side chain of the above-mentioned hydrocarbon groups. The number of carbon atoms constituting the main chain of R is not particularly limited, but is preferably 20 or less, and more preferably 10 or less. By ensuring that the number of carbon atoms constituting the main chain of R falls within the above range, the molecular weight of the phosphorus oxoacid group can be kept within an appropriate range, facilitating penetration into cellulose fibers and increasing the yield of cellulose nanocrystals. Note that when there are multiple Rs in formula (1) or when multiple types of substituents represented by formula (1) are introduced into cellulose fibers, the multiple Rs may be the same or different.

β ^b+ is a monovalent or higher cation composed of an organic or inorganic substance. Examples of the monovalent or higher cation composed of an organic substance include organic onium ions. Examples of the organic onium ions include organic ammonium ions and organic phosphonium ions. Examples of the organic ammonium ions include aliphatic ammonium ions and aromatic ammonium ions, and examples of the organic phosphonium ions include aliphatic phosphonium ions and aromatic phosphonium ions. Examples of the monovalent or higher cation composed of an inorganic substance include ions of alkali metals such as sodium, potassium, or lithium, ions of divalent metals such as calcium or magnesium, hydrogen ions, and ammonium ions. Note that when multiple β ^b+ groups exist in formula (1) or when multiple types of substituents represented by formula (1) are introduced into cellulose fibers, the multiple β ^b+ groups may be the same or different. Examples of the monovalent or higher cation composed of an organic or inorganic substance include, but are not limited to, sodium or potassium ions, which are resistant to yellowing when cellulose fibers containing β ^b+ are heated and are easy to use industrially.

More specific examples of the phosphorus oxo acid group or substituent derived from a phosphorus oxo acid group include a phosphate group ( _-PO3H2 ), a salt of a phosphate group, a phosphorous acid ( _phosphonic acid) group ( _-PO2H2 ), and a salt of a _phosphite (phosphonic acid) group. Furthermore, the phosphorus oxo acid group or the substituent derived from a phosphorus oxo acid group may be a group in which a phosphate group is condensed (e.g., a pyrophosphate group), a group in which a phosphonic acid is condensed (e.g., a polyphosphonic acid group), a phosphate ester group (e.g., a monomethyl phosphate group, a polyoxyethylene alkyl phosphate group), an alkyl phosphonic acid group (e.g., a methylphosphonic acid group), or the like.

Furthermore, the sulfur oxoacid group (sulfur oxoacid group or a substituent derived from a sulfur oxoacid group) is, for example, a substituent represented by the following formula (2). Each cellulose fiber may contain multiple substituents represented by the following formula (2). In this case, the multiple introduced substituents represented by the following formula (2) may be the same or different.

In the structural formula, b and n are natural numbers, p is 0 or 1, and m is any number (where 1 = b × m). When n is 2 or greater, the multiple p's may be the same or different. In the structural formula, β ^b+ is a monovalent or higher cation composed of an organic or inorganic substance. Examples of the monovalent or higher cation composed of an organic substance include organic onium ions. Examples of the organic onium ions include organic ammonium ions and organic phosphonium ions. Examples of the organic ammonium ions include aliphatic ammonium ions and aromatic ammonium ions, and examples of the organic phosphonium ions include aliphatic phosphonium ions and aromatic phosphonium ions. Examples of the monovalent or higher cation composed of an inorganic substance include ions of alkali metals such as sodium, potassium, or lithium, ions of divalent metals such as calcium or magnesium, hydrogen ions, and ammonium ions. When multiple types of substituents represented by the above formula (2) are introduced into the fibrous cellulose, the multiple β ^b+ may be the same or different. As the organic or inorganic cations of monovalent or higher, sodium or potassium ions are preferred, as they are less likely to yellow when β ^b+ -containing cellulose fibers are heated and are easy to use industrially, but there is no particular limitation.

The amount of ionic functional groups introduced into the cellulose fibers is, for example, preferably 0.30 mmol/g or more per 1 g (mass) of fibrous cellulose, more preferably 0.50 mmol/g or more, even more preferably 0.80 mmol/g or more, and even more preferably 1.00 mmol/g or more. Furthermore, the amount of ionic functional groups introduced into the cellulose fibers is, for example, preferably 5.20 mmol/g or less per 1 g (mass) of fibrous cellulose, more preferably 3.65 mmol/g or less, even more preferably 3.50 mmol/g or less, and even more preferably 3.00 mmol/g or less. By keeping the amount of ionic functional groups introduced within the above ranges, the transparency and salt resistance of the resulting cellulose nanocrystals having ionic functional groups introduced therein are improved.
Here, the denominator in the unit mmol/g indicates the mass of the fibrous cellulose when the counter ion of the ionic functional group is a hydrogen ion (H ⁺ ).

The amount of ionic functional groups introduced into cellulose fibers and cellulose nanocrystals with ionic functional groups introduced, described below, can be measured, for example, by neutralization titration or elemental analysis. When measuring by neutralization titration, the amount introduced is measured by measuring the change in pH while adding an alkali such as aqueous sodium hydroxide solution to a slurry containing the obtained cellulose fibers or cellulose nanocrystals with ionic functional groups introduced.

1 is a graph showing the relationship between the amount of NaOH added dropwise to a slurry containing cellulose fibers having phosphorus oxo acid groups and pH. The amount of phosphorus oxo acid groups introduced into the cellulose fibers is measured, for example, as follows. The amount of phosphorus oxo acid groups in cellulose nanocrystals into which ionic functional groups have been introduced is also measured in the same manner.
First, a slurry containing cellulose fibers is treated with a strongly acidic ion exchange resin. If necessary, the measurement object may be subjected to a defibration treatment similar to the defibration treatment step described below before the treatment with the strongly acidic ion exchange resin.
Next, the change in pH is observed while adding aqueous sodium hydroxide solution, and a titration curve such as that shown in the upper part of Figure 1 is obtained. The titration curve shown in the upper part of Figure 1 plots the measured pH against the amount of alkali added, while the titration curve shown in the lower part of Figure 1 plots the pH increment (derivative value) (1/mmol) against the amount of alkali added. In this neutralization titration, two points are identified in the curve plotting the measured pH against the amount of alkali added, where the increment (derivative value of pH with respect to the amount of alkali added) is maximized. Of these, the first maximum point of the increment after starting to add the alkali is called the first endpoint, and the next maximum point of the increment is called the second endpoint. The amount of alkali required from the start of titration to the first endpoint is equal to the amount of first dissociated acid of the fibrous cellulose contained in the slurry used for titration, the amount of alkali required from the first endpoint to the second endpoint is equal to the amount of second dissociated acid of the fibrous cellulose contained in the slurry used for titration, and the amount of alkali required from the start of titration to the second endpoint is equal to the total amount of dissociated acid of the fibrous cellulose contained in the slurry used for titration.The value obtained by dividing the amount of alkali required from the start of titration to the first endpoint by the solids content (g) in the slurry to be titrated is the amount of phosphorus oxo acid group introduced (mmol/g).Note that when simply referring to the amount of phosphorus oxo acid group introduced (or amount of phosphorus oxo acid group), it refers to the amount of first dissociated acid.
In FIG. 1 , the region from the start of titration to the first endpoint is referred to as the first region, and the region from the first endpoint to the second endpoint is referred to as the second region. For example, when the phosphorus oxoacid group is a phosphate group and this phosphate group undergoes condensation, the amount of weakly acidic groups in the phosphorus oxoacid group (also referred to herein as the second dissociated acid amount) appears to decrease, and the amount of alkali required in the second region is smaller than the amount of alkali required in the first region. On the other hand, the amount of strongly acidic groups in the phosphorus oxoacid group (also referred to herein as the first dissociated acid amount) corresponds to the amount of phosphorus atoms regardless of whether condensation occurs. Furthermore, when the phosphorus oxoacid group is a phosphite group, the phosphorus oxoacid group no longer contains weakly acidic groups, so the amount of alkali required in the second region is reduced or may even be zero. In this case, there is only one point on the titration curve where the pH increment is maximized.
The above-mentioned amount of introduced phosphorus oxoacid groups (mmol/g) indicates the amount of phosphorus oxoacid groups in the acid-form cellulose fibers (hereinafter referred to as the amount of phosphorus oxoacid groups (acid form)), since the denominator indicates the mass of the acid-form cellulose fibers. On the other hand, when the counter ions of the phosphorus oxoacid groups are substituted with an arbitrary cation C so as to be the charge equivalent, the amount of phosphorus oxoacid groups in the cellulose fibers with the cation C as the counter ion (hereinafter referred to as the amount of phosphorus oxoacid groups (C-form)) can be determined by converting the denominator to the mass of the cellulose fibers when the cation C is the counter ion.
That is, it is calculated using the following formula.
Amount of phosphorus oxoacid group (C type)=Amount of phosphorus oxoacid group (acid type)/{1+(W−1)×A/1000}
A [mmol/g]: total amount of anions derived from phosphorus oxoacid groups in cellulose fibers (the sum of the amount of strongly acidic groups and the amount of weakly acidic groups in phosphorus oxoacid groups)
W: Formula weight per valence of cation C (for example, Na is 23, Al is 9)

FIG. 2 is a graph showing the relationship between the amount of NaOH dropped onto cellulose fibers having carboxy groups and pH.
The amount of carboxyl groups introduced into cellulose fibers is measured, for example, as follows: The amount of carboxyl groups in cellulose nanocrystals into which ionic functional groups have been introduced is also measured in the same manner.
First, a slurry containing cellulose fibers is treated with a strongly acidic ion exchange resin. If necessary, the measurement target may be subjected to a defibration treatment similar to the defibration treatment step described below before treatment with the strongly acidic ion exchange resin. Next, a sodium hydroxide solution is added, and the change in pH is observed to obtain a titration curve as shown in FIG. 2.
As shown in Figure 2, in this neutralization titration, a single point is observed where the increment (the differential value of pH with respect to the amount of alkali added) is maximum on the curve plotting the measured pH against the amount of alkali added. This maximum increment is called the first end point. Here, the region from the start of titration to the first end point in Figure 2 is called the first region. The amount of alkali required in the first region is equal to the amount of carboxy groups in the slurry used for titration. The amount of carboxy groups introduced (mmol/g) was calculated by dividing the amount of alkali (mmol) required in the first region of the titration curve by the solids content (g) in the cellulose fiber-containing slurry to be titrated.
The above-mentioned amount of carboxyl groups introduced (mmol/g) indicates the amount of substituents per 1 g of cellulose fiber mass when the counter ion of the carboxyl group is a hydrogen ion (H ⁺ ) (hereinafter referred to as the amount of carboxyl groups (acid type)).

The above-mentioned amount of carboxy groups introduced (mmol/g) indicates the amount of carboxy groups possessed by acid-form cellulose fibers (hereinafter referred to as the amount of carboxy groups (acid form)), since the denominator is the mass of the acid-form cellulose fibers. On the other hand, when the counter ions of the carboxy groups are substituted with an arbitrary cation C so as to be the charge equivalent, the amount of carboxy groups possessed by the cellulose fibers with the cation C as the counter ion (hereinafter referred to as the amount of carboxy groups (C form)) (mmol/g) can be determined by converting the denominator to the mass of the cellulose fibers when the cation C is the counter ion.
That is, it is calculated using the following formula.
Amount of carboxyl group (C type)=Amount of carboxyl group (acid type)/{1+(W−1)×(Amount of carboxyl group (acid type))/1000}
W: Formula weight per valence of cation C (for example, Na is 23, Al is 9)

Furthermore, the amount of sulfur oxoacid groups and sulfonic acid groups introduced into the cellulose fibers and cellulose nanocrystals with ionic functional groups introduced therein is measured by wet ashing the obtained cellulose fibers or cellulose nanocrystals with ionic functional groups introduced therein using perchloric acid and concentrated nitric acid, diluting them at an appropriate ratio, and then measuring the amount of sulfur by ICP optical emission analysis.
The amount of sulfur divided by the bone dry mass of the cellulose fiber or cellulose nanocrystals to which ionic functional groups have been introduced is taken as the amount of sulfur oxoacid groups or sulfonic acid groups (unit: mmol/g).

When measuring the amount of substituents using titration, if too much sodium hydroxide solution is added or if the titration interval is too short, the amount of substituents may be lower than expected and an accurate value may not be obtained. An appropriate amount and titration interval, for example, is to titrate 10 to 50 μL of 0.1 N sodium hydroxide solution over a period of 5 to 30 seconds. To eliminate the influence of carbon dioxide dissolved in the cellulose fiber-containing slurry, it is also recommended to measure while blowing an inert gas such as nitrogen gas into the slurry, for example, from 15 minutes before the start of titration until the end of titration.

The cellulose fibers may contain carbamide groups derived from urea and/or urea derivatives added during the manufacturing process of the ionic functional group-introduced cellulose fibers described below. In this case, the amount of carbamide groups introduced into the cellulose fibers (carbamide group amount) is, for example, preferably 1.50 mmol/g or less per 1 g (mass) of cellulose fiber, more preferably 1.00 mmol/g or less, even more preferably 0.30 mmol/g or less, and particularly preferably 0.20 mmol/g or less. The amount of carbamide groups introduced into the cellulose fibers (carbamide group amount) may be 0.00 mmol/g. Because carbamide groups and phosphorus oxoacid groups are introduced by reaction with hydroxyl groups of cellulose, the amount of phosphorus oxoacid groups introduced decreases as the amount of carbamide groups introduced increases. Therefore, by setting the amount of carbamide groups introduced within the above range, the amount of phosphorus oxoacid groups introduced can be increased and maintained within an appropriate range. Note that carbamide groups themselves are not electrically conductive, so the introduction of carbamide groups does not result in cellulose with ionic functional groups. Therefore, by keeping the amount of carbamide groups introduced within the above range, the amount of phosphorus oxoacid groups introduced can be increased, thereby improving the transparency and salt tolerance of the resulting cellulose nanocrystals with ionic functional groups introduced.

The amount of carbamide groups introduced into cellulose fibers and ionic-functionalized cellulose nanocrystals is determined by measuring the amount of nitrogen covalently bonded to cellulose. Specifically, ionic nitrogen (ammonium ions) is liberated and removed from the measurement target, including cellulose fibers or ionic-functionalized cellulose nanocrystals, and then the amount of nitrogen is measured using trace nitrogen analysis. The liberation of ionic nitrogen (ammonium ions) is performed under conditions that do not substantially remove the nitrogen covalently bonded to cellulose. For example, after the phosphorus oxoacid group introduction step, ammonium ions can be liberated by alkali treatment and washed away, followed by step B, as described below. Alternatively, ammonium ions can be adsorbed and removed using a strongly acidic ion exchange resin after steps B and C. An example of a device for measuring nitrogen content using trace nitrogen analysis is the TN-110 Total Nitrogen Analyzer manufactured by Mitsubishi Chemical Analytech Corporation. Before measurement, the cellulose fibers or ionic-functionalized cellulose nanocrystals are dried at low temperature (e.g., in a vacuum oven at 40°C for 24 hours) until completely dry. The amount of carbamide groups introduced per unit mass (mmol/g) of cellulose fiber or cellulose nanocrystals with ionic functional groups introduced therein is calculated by dividing the nitrogen content (g/g) per unit mass of cellulose fiber or cellulose nanocrystals with ionic functional groups introduced therein obtained by nitrogen trace analysis by the atomic weight of nitrogen.

[Ionic Functional Group Introduction Step]
In order to introduce ionic functional groups into cellulose fibers, the method preferably includes the ionic functional group introduction step for introducing ionic functional groups into the cellulose fibers described above, and further includes a washing step and an alkali treatment step in this order before step B. Alternatively, an acid treatment step may be included instead of or in addition to the washing step. Examples of ionic functional group introduction steps include a phosphorus oxoacid group introduction step, a carboxyl group introduction step, a sulfur oxoacid group introduction step, an oxidation step using a chlorine-based oxidizing agent, a xanthate group introduction step, a sulfone group introduction step, and a carboxyalkylation step. Each of these steps will be described below.

- Phosphorus oxo acid group introduction step -
The phosphorus oxo acid group introduction step is a step in which at least one compound (hereinafter also referred to as "compound A") selected from compounds capable of introducing phosphorus oxo acid groups by reacting with hydroxyl groups in cellulose fibers is allowed to act on the cellulose fibers, thereby obtaining fibers into which phosphorus oxo acid groups have been introduced.
In the phosphate group introduction step according to this embodiment, the reaction between the cellulose fibers and the compound A may be carried out in the presence of at least one selected from urea and its derivatives (hereinafter also referred to as "compound B"). Alternatively, the reaction between the cellulose fibers and the compound A may be carried out in the absence of compound B.
One example of a method for allowing compound A to act on cellulose fibers in the presence of compound B is a method in which compound A and compound B are mixed with cellulose fibers in a dry, wet, or slurry state. Among these methods, using cellulose fibers in a dry or wet state is preferred because of the high uniformity of the reaction, and using cellulose fibers in a dry state is particularly preferred. The form of the cellulose fibers is not particularly limited, but a cotton-like or thin sheet-like form is preferred. Examples include methods in which compound A and compound B are added to cellulose fibers in the form of a powder, a solution dissolved in a solvent, or a melted state obtained by heating to or above their melting point. Among these methods, adding compound A and compound B in the form of a solution dissolved in a solvent, particularly an aqueous solution, is preferred because of the high uniformity of the reaction. Compound A and compound B may be added to cellulose fibers simultaneously, separately, or as a mixture. The method for adding compound A and compound B is not particularly limited. When compound A and compound B are in the form of a solution, the cellulose fibers may be immersed in the solution, absorbed, and then removed, or the solution may be dripped onto the cellulose fibers. Alternatively, the required amounts of compound A and compound B may be added to the cellulose fibers, or excess amounts of compound A and compound B may be added to the cellulose fibers, and then the excess compound A and compound B may be removed by squeezing or filtration.

The compound A used in this embodiment may be any compound that has a phosphorus atom and can form an ester bond with cellulose, including, but not limited to, phosphoric acid or a salt thereof, phosphorous acid or a salt thereof, dehydrated condensed phosphoric acid or a salt thereof, and phosphoric anhydride (diphosphorus pentoxide). Phosphoric acid can be used with various purities, such as 100% phosphoric acid (orthophosphoric acid) or 85% phosphoric acid. Phosphorous acid can be used with, for example, 99% phosphorous acid (phosphonic acid). Dehydrated condensed phosphoric acid is a compound in which two or more molecules of phosphoric acid are condensed by a dehydration reaction, and examples thereof include pyrophosphoric acid and polyphosphoric acid. Phosphates, phosphites, and dehydrated condensed phosphates include lithium salts, sodium salts, potassium salts, and ammonium salts of phosphoric acid, phosphorous acid, or dehydrated condensed phosphoric acid, which can be neutralized to various degrees.
Among these, from the viewpoints of high efficiency in introducing a phosphate group, low cost, and ease of industrial application, phosphoric acid, sodium salt of phosphoric acid, potassium salt of phosphoric acid, and ammonium salt of phosphoric acid are preferred, and phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, and ammonium dihydrogen phosphate are more preferred.
The amount of compound A added to the cellulose fibers is not particularly limited, but for example, when the amount of compound A added is converted into the amount of phosphorus atoms, the amount of phosphorus atoms added to the cellulose fibers (bone dry mass) is preferably 0.5% by mass or more and 100% by mass or less, more preferably 1% by mass or more and 50% by mass or less, and even more preferably 2% by mass or more and 30% by mass or less. By setting the amount of phosphorus atoms added to the cellulose fibers within the above range, the transparency and salt tolerance of the obtained cellulose nanocrystals having ionic functional groups introduced therein can be further improved. On the other hand, by setting the amount of phosphorus atoms added to the cellulose fibers to the above upper limit or less, a balance can be achieved between the effect of improving yield and costs.

As described above, the compound B used in this embodiment is at least one selected from urea and its derivatives. Examples of the compound B include urea, biuret, 1-phenylurea, 1-benzylurea, 1-methylurea, and 1-ethylurea.
From the viewpoint of improving the uniformity of the reaction, it is preferable to use an aqueous solution of compound B. Furthermore, from the viewpoint of further improving the uniformity of the reaction, it is preferable to use an aqueous solution in which both compound A and compound B are dissolved.
The amount of compound B added relative to the cellulose fiber (bone dry mass) is not particularly limited, but is preferably, for example, 1 mass% or more and 500 mass% or less, more preferably 10 mass% or more and 400 mass% or less, and even more preferably 100 mass% or more and 350 mass% or less.

In the reaction between cellulose fibers and compound A, in addition to compound B, the reaction system may contain, for example, amides or amines. Examples of amides include formamide, dimethylformamide, acetamide, and dimethylacetamide. Examples of amines include methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, and hexamethylenediamine. Of these, triethylamine in particular is known to act as a good reaction catalyst.

In the phosphorus oxo acid group introduction step, it is preferable to add or mix compound A or the like with the cellulose fibers and then heat-treat the cellulose fibers. The heat treatment temperature is preferably selected so that phosphorus oxo acid groups can be efficiently introduced while suppressing thermal decomposition and hydrolysis of the cellulose fibers. The heat treatment temperature is preferably 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower. Various types of equipment with heat transfer media can be used for the heat treatment, including, for example, agitator dryers, rotary dryers, disk dryers, roll-type heaters, plate-type heaters, fluidized-bed dryers, band dryers, filtration dryers, vibration fluidized-bed dryers, flash dryers, reduced-pressure dryers, infrared heaters, far-infrared heaters, microwave heaters, and high-frequency dryers.

In the heat treatment according to this embodiment, for example, a method in which compound A is added to a thin sheet of cellulose fibers by a method such as impregnation, followed by heating, or a method in which the cellulose fibers and compound A are heated while being kneaded or stirred using a kneader or the like, can be employed. This makes it possible to suppress unevenness in the concentration of compound A in the cellulose fibers and to introduce phosphate groups more uniformly onto the surface of the cellulose fibers. This is thought to be because, when water molecules move to the surface of the cellulose fibers as they are dried, the dissolved compound A is attracted to the water molecules by surface tension, and is prevented from migrating to the surface of the cellulose fibers in the same way (i.e., causing unevenness in the concentration of compound A).
Furthermore, the heating device used for the heat treatment is preferably one that can constantly discharge, to the outside of the device system, for example, the moisture retained in the slurry and the moisture generated in the dehydration condensation (phosphorylation) reaction between compound A and hydroxyl groups contained in cellulose or the like in the cellulose fibers. Examples of such a heating device include an oven using a blower system. By constantly discharging moisture from the device system, it is possible to suppress the hydrolysis reaction of phosphate ester bonds, which is the reverse reaction of phosphate esterification, and also to suppress acid hydrolysis of sugar chains in the cellulose fibers.
The heat treatment time is, for example, preferably from 1 second to 300 minutes after the water content has been substantially removed from the cellulose fibers, more preferably from 1 second to 1,000 seconds, and even more preferably from 10 seconds to 800 seconds. In this embodiment, by setting the heating temperature and heating time within appropriate ranges, the amount of phosphorus oxo acid groups introduced can be kept within a preferred range.

The phosphorus oxo acid group introduction process needs to be performed at least once, but can also be repeated two or more times. By performing the phosphorus oxo acid group introduction process two or more times, a large number of phosphorus oxo acid groups can be introduced into the cellulose fibers. In this embodiment, a preferred example is when the phosphorus oxo acid group introduction process is performed two times.

The amount of phosphorus oxoacid groups introduced into the cellulose fibers is, for example, preferably 0.30 mmol/g or more per 1 g (mass) of cellulose fiber, more preferably 0.50 mmol/g or more, even more preferably 0.80 mmol/g or more, even more preferably 1.00 mmol/g or more, and particularly preferably 1.25 mmol/g or more. Furthermore, the amount of phosphorus oxoacid groups introduced into the cellulose fibers is, for example, preferably 5.20 mmol/g or less per 1 g (mass) of cellulose fiber, more preferably 3.65 mmol/g or less, and even more preferably 3.00 mmol/g or less. By keeping the amount of phosphorus oxoacid groups introduced within the above range, the transparency and salt tolerance of the resulting cellulose nanocrystals having ionic functional groups introduced therein can be improved.

-Carboxy group introduction step-
The carboxyl group introduction process is carried out by subjecting the cellulose fibers to an oxidation treatment such as ozone oxidation, oxidation by the Fenton method, or TEMPO oxidation treatment, or by treating the fibers with a compound having a carboxylic acid-derived group or a derivative thereof, or an acid anhydride of a compound having a carboxylic acid-derived group or a derivative thereof.
Examples of compounds having a group derived from carboxylic acid include, but are not limited to, dicarboxylic acid compounds such as maleic acid, succinic acid, phthalic acid, fumaric acid, glutaric acid, adipic acid, and itaconic acid, and tricarboxylic acid compounds such as citric acid and aconitic acid. Examples of derivatives of compounds having a group derived from carboxylic acid include, but are not limited to, imidized products of acid anhydrides of compounds having carboxy groups, and derivatives of acid anhydrides of compounds having carboxy groups. Examples of imidized products of acid anhydrides of compounds having carboxy groups include, but are not limited to, imidized products of dicarboxylic acid compounds such as maleimide, succinimide, and phthalimide.

Acid anhydrides of compounds having a group derived from carboxylic acid include, but are not limited to, acid anhydrides of dicarboxylic acid compounds such as maleic anhydride, succinic anhydride, phthalic anhydride, glutaric anhydride, adipic anhydride, and itaconic anhydride. Furthermore, derivatives of acid anhydrides of compounds having a group derived from carboxylic acid include, but are not limited to, acid anhydrides of compounds having carboxy groups such as dimethylmaleic anhydride, diethylmaleic anhydride, and diphenylmaleic anhydride, in which at least some of the hydrogen atoms have been substituted with substituents such as alkyl groups or phenyl groups.

When TEMPO oxidation treatment is performed in the carboxyl group introduction step, it is preferable to perform the treatment under conditions of a pH of 6 or higher and 8 or lower. This type of treatment is also called a neutral TEMPO oxidation treatment. The neutral TEMPO oxidation treatment can be performed, for example, by adding pulp as the cellulose fiber, a nitroxy radical such as TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) as a catalyst, and sodium hypochlorite as a sacrificial reagent to a sodium phosphate buffer solution (pH = 6.8). Furthermore, by adding sodium chlorite, aldehydes generated during the oxidation process can be efficiently oxidized to carboxyl groups.
The TEMPO oxidation treatment may be carried out under conditions of a pH of 10 to 11. Such a treatment is also called alkaline TEMPO oxidation treatment. The alkaline TEMPO oxidation treatment can be carried out, for example, by adding a nitroxy radical such as TEMPO as a catalyst, sodium bromide as a co-catalyst, and sodium hypochlorite as an oxidizing agent to pulp as cellulose fibers.
The amount of carboxyl groups introduced into cellulose fibers varies depending on the type of substituent. For example, when carboxyl groups are introduced by TEMPO oxidation, the amount is preferably 0.30 mmol/g or more, more preferably 0.50 mmol/g or more, even more preferably 0.80 mmol/g or more, even more preferably 0.90 mmol/g or more, and particularly preferably 1.00 mmol/g or more per gram (mass) of cellulose fibers. Also, the amount is preferably 2.5 mmol/g or less, more preferably 2.20 mmol/g or less, and even more preferably 2.00 mmol/g or less. Furthermore, when the substituent is a carboxymethyl group, the amount may be 5.8 mmol/g or less per gram (mass) of fibrous cellulose.

- Sulfur oxo acid group introduction step -
The process for producing cellulose nanocrystals having ionic functional groups introduced therein may include, for example, a sulfur oxoacid group introduction step, in which hydroxyl groups in cellulose fibers react with sulfur oxoacids to obtain cellulose fibers having sulfur oxoacid groups (sulfur oxoacid group-introduced fibers).

In the sulfur oxo acid group introduction process, instead of compound A in the above-described <Phosphorus oxo acid group introduction process>, at least one compound (hereinafter also referred to as "compound C") selected from compounds capable of introducing sulfur oxo acid groups by reacting with hydroxyl groups in cellulose fibers is used. Compound C may be any compound containing a sulfur atom and capable of forming an ester bond with cellulose, including, but not limited to, sulfuric acid or its salts, sulfurous acid or its salts, and sulfuric acid amides. Sulfuric acid of various purities can be used, such as 96% sulfuric acid (concentrated sulfuric acid). Sulfurous acid can be 5% aqueous sulfurous acid. Sulfates or sulfites include lithium, sodium, potassium, and ammonium salts of sulfates or sulfites, which can be neutralized to various degrees. Sulfamic acid or other sulfuric acid amides can be used. In the sulfur oxo acid group introduction process, it is preferable to use compound B in the above-described <Phosphorus oxo acid group introduction process>.

In the sulfur oxoacid group introduction process, it is preferable to mix cellulose fibers with an aqueous solution containing sulfur oxoacid and urea and/or a urea derivative, and then heat-treat the cellulose fibers. The heat-treatment temperature is preferably selected so that sulfur oxoacid groups can be efficiently introduced while suppressing thermal decomposition and hydrolysis of the fibers. The heat-treatment temperature is preferably 100°C or higher, more preferably 120°C or higher, and even more preferably 150°C or higher. The heat-treatment temperature is preferably 300°C or lower, more preferably 250°C or lower, and even more preferably 200°C or lower.

In the heat treatment step, it is preferable to heat until substantially all moisture is removed. Therefore, the heat treatment time varies depending on the amount of moisture contained in the cellulose fiber and the amount of aqueous solution containing sulfur oxoacid and urea and/or a urea derivative added, but is preferably, for example, 10 seconds or more and 10,000 seconds or less. Heat treatment can be performed using equipment with various heat media, such as agitator dryers, rotary dryers, disk dryers, roll-type heaters, plate-type heaters, fluidized bed dryers, band dryers, filtration dryers, vibration fluidized dryers, pneumatic dryers, reduced-pressure dryers, infrared heaters, far-infrared heaters, microwave heaters, and high-frequency dryers.

The amount of sulfur oxoacid groups introduced into cellulose fibers is preferably 0.05 mmol/g or more, more preferably 0.10 mmol/g or more, even more preferably 0.20 mmol/g or more, even more preferably 0.50 mmol/g or more, even more preferably 0.80 mmol/g or more, and particularly preferably 0.90 mmol/g or more. Furthermore, the amount of sulfur oxoacid groups introduced into cellulose fibers is preferably 5.00 mmol/g or less, more preferably 3.00 mmol/g or less. By keeping the amount of sulfur oxoacid groups introduced within the above range, it is possible to improve defibration ability and obtain cellulose nanocrystals with ionic functional groups introduced and smaller fiber widths.

- Oxidation step using a chlorine-based oxidizing agent (second carboxyl group introduction step) -
The ionic functional group introduction step may include, for example, an oxidation step using a chlorine-based oxidizing agent, in which the chlorine-based oxidizing agent is added to wet or dry cellulose fibers having hydroxyl groups to cause a reaction, thereby introducing carboxyl groups into the cellulose fibers.

Examples of chlorine-based oxidizing agents include hypochlorous acid, hypochlorites, chlorous acid, chlorites, chloric acid, chlorates, perchloric acid, perchlorates, chlorine dioxide, etc. Sodium hypochlorite, sodium chlorite, and chlorine dioxide are preferred from the standpoints of the efficiency of introducing ionic functional groups, and thus the transparency, salt resistance, cost, and ease of handling of the resulting cellulose nanocrystals having ionic functional groups introduced therein.
The chlorine-based oxidizing agent may be added to the cellulose fibers as a reagent, or may be dissolved in a suitable solvent and then added.

The concentration of the chlorine-based oxidizing agent in the solution in the oxidation step using the chlorine-based oxidizing agent is, for example, converted into an effective chlorine concentration, preferably 1% by mass or more and 1,000% by mass or less, more preferably 5% by mass or more and 500% by mass or less, and even more preferably 10% by mass or more and 100% by mass or less.
The amount of the chlorine-based oxidizing agent added per 100 parts by mass of cellulose fibers is preferably 1 part by mass or more and 100,000 parts by mass or less, more preferably 10 parts by mass or more and 10,000 parts by mass or less, and even more preferably 100 parts by mass or more and 5,000 parts by mass or less.

The reaction time with the chlorine-based oxidizing agent in the oxidation step using the chlorine-based oxidizing agent may vary depending on the reaction temperature, but is preferably, for example, from 1 minute to 1,000 minutes, more preferably from 10 minutes to 500 minutes, and even more preferably from 20 minutes to 400 minutes.
The pH during the reaction is preferably 5 or more and 15 or less, more preferably 7 or more and 14 or less, and even more preferably 9 or more and 13 or less. At the start of the reaction and during the reaction, the pH is preferably maintained constant (for example, pH 11) by appropriately adding hydrochloric acid or sodium hydroxide. After the reaction, excess reaction reagents, by-products, etc. may be washed with water and removed by filtration or the like.

-Xanthate group introduction step (xanthogen acid esterification step)-
The ionic functional group introduction process may include, for example, a xanthate group introduction process (hereinafter also referred to as a xanthation process). In the xanthation process, carbon disulfide and an alkali compound are added to wet or dry cellulose fibers having hydroxyl groups to react with each other, thereby introducing xanthate groups into the cellulose fibers. Specifically, carbon disulfide is added to cellulose fibers that have been converted into alkali cellulose by the method described below, and the reaction is carried out.

<Alkali cellulose>
When introducing ionic functional groups into cellulose fibers, it is preferable to convert the cellulose contained in the cellulose fibers into alkali cellulose by applying an alkaline solution to the cellulose. This treatment ionizes some of the hydroxyl groups of the cellulose, increasing its nucleophilicity (reactivity). The alkaline compound contained in the alkaline solution is not particularly limited and may be an inorganic alkaline compound or an organic alkaline compound. Due to their high versatility, it is preferable to use, for example, sodium hydroxide, potassium hydroxide, tetraethylammonium hydroxide, or tetrabutylammonium hydroxide. The conversion into alkali cellulose may be performed simultaneously with the introduction of ionic functional groups, before the introduction, or at both stages.

The solution temperature at the start of alkali cellulose formation is preferably 0°C or higher and 50°C or lower, more preferably 5°C or higher and 40°C or lower, and even more preferably 10°C or higher and 30°C or lower.

The alkaline solution concentration is preferably 0.01 mol/L or more and 4 mol/L or less, more preferably 0.1 mol/L or more and 3 mol/L or less, and even more preferably 1 mol/L or more and 2.5 mol/L or less. In particular, when the treatment temperature is less than 10°C, a concentration of 1 mol/L or more and 2 mol/L or less is preferable.

The treatment time for alkali cellulose formation is preferably 1 minute or more, more preferably 10 minutes or more, and even more preferably 30 minutes or more. The alkali treatment time is preferably 6 hours or less, more preferably 5 hours or less, and even more preferably 4 hours or less.

By adjusting the type of alkaline solution, treatment temperature, concentration, and immersion time as described above, it is possible to suppress the penetration of the alkaline solution into the crystalline regions of cellulose, making it easier to maintain the cellulose type I crystalline structure and increasing the yield of cellulose nanocrystals with introduced ionic functional groups.

If the introduction of ionic functional groups and the conversion to alkali cellulose are not carried out simultaneously, it is preferable to remove water from the alkali cellulose obtained by alkali treatment by solid-liquid separation using a common deliquoring method such as centrifugation or filtration. This improves the reaction efficiency in the subsequent ionic functional group introduction step. The cellulose fiber concentration after solid-liquid separation is preferably 5% or more and 50% or less, more preferably 10% or more and 40% or less, and even more preferably 15% or more and 35% or less.

-Phosphonic or phosphine group introduction step (phosphoalkylation step)-
The ionic functional group introduction step may include a phosphonic or phosphine group introduction step (phosphoalkylation step). In the phosphoalkylation step, a compound having a reactive group and a phosphonic or phosphine group (compound E _A ) as an essential component, and an alkali compound and a compound B selected from the above-mentioned urea and its derivatives as optional components, are added to wet or dry cellulose fibers having hydroxyl groups, and a reaction is carried out to introduce phosphonic or phosphine groups into the cellulose fibers.

Examples of the reactive group include a halogenated alkyl group, a vinyl group, and an epoxy group (glycidyl group).
Examples of the compound _EA include vinylphosphonic acid, phenylvinylphosphonic acid, phenylvinylphosphinic acid, etc. From the viewpoints of the efficiency of introducing a substituent, and thus the transparency, salt resistance, cost, and ease of handling of the resulting cellulose nanocrystals having an ionic functional group introduced therein, it is preferable that the compound _EA is vinylphosphonic acid.
Furthermore, as an optional component, it is also preferable to use the compound B in the above-mentioned <Phosphorus oxo acid group introduction step> in the same manner, and the amount added is also preferably as described above.

When compound _EA is added, it may be added to the cellulose fiber as a reagent (solid or liquid) as is, or may be dissolved in an appropriate solvent and then added. It is preferable that the cellulose fiber is converted into alkali cellulose in advance or simultaneously with the reaction. The method for converting the cellulose fiber into alkali cellulose is as described above.

The reaction temperature is preferably, for example, 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower.

The amount of compound E _A added per 100 parts by mass of cellulose fibers is preferably 1 part by mass or more and 100,000 parts by mass or less, more preferably 2 parts by mass or more and 10,000 parts by mass or less, and even more preferably 5 parts by mass or more and 1,000 parts by mass or less.

The reaction time may vary depending on the reaction temperature, but is preferably between 1 minute and 1,000 minutes, more preferably between 10 minutes and 500 minutes, and even more preferably between 20 minutes and 400 minutes. After the reaction, excess reaction reagents, by-products, etc. may be washed away with water or removed by filtration, etc.

-Sulfonic acid group introduction step (sulfoalkylation step)-
The ionic functional group introduction step may include, for example, a sulfonic group introduction step (sulfoalkylation step). In the sulfoalkylation, a compound having a reactive group and a sulfonic group (compound E _B ) as an essential component, and an alkali compound and the aforementioned compound B selected from urea and its derivatives as optional components, are added to wet or dry cellulose fibers having hydroxyl groups, and a reaction is carried out to introduce sulfonic groups into the cellulose fibers.

Examples of the reactive group include a halogenated alkyl group, a vinyl group, and an epoxy group (glycidyl group).
Examples of compound E _B include sodium 2-chloroethanesulfonate, sodium vinylsulfonate, sodium p-styrenesulfonate, 2-acrylamido-2-methylpropanesulfonic acid, etc. Among these, sodium vinylsulfonate is preferred from the viewpoints of the efficiency of introducing a substituent, and hence the transparency and salt resistance, cost, and ease of handling of the resulting cellulose nanocrystals having an ionic functional group introduced therein.
Furthermore, as an optional component, it is also preferable to use the compound B in the above-mentioned <Phosphorus oxo acid group introduction step> in the same manner, and the amount added is also preferably as described above.

Compound E _B may be added to the cellulose fiber as a reagent or may be dissolved in a suitable solvent and then added. The cellulose fiber is preferably converted into alkali cellulose in advance or simultaneously with the reaction. The method for converting the cellulose fiber into alkali cellulose is as described above.

The reaction temperature is preferably, for example, 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower.

The amount of compound E _B added per 100 parts by mass of cellulose fibers is preferably 1 part by mass or more and 100,000 parts by mass or less, more preferably 2 parts by mass or more and 10,000 parts by mass or less, and even more preferably 5 parts by mass or more and 1,000 parts by mass or less.

The reaction time may vary depending on the reaction temperature, but is preferably, for example, from 1 minute to 1,000 minutes, more preferably from 10 minutes to 500 minutes, and even more preferably from 15 minutes to 400 minutes.
After the reaction, excess reaction reagents, by-products, etc. may be washed and removed by filtration or the like.

-Carboxyalkylation step (third carboxy group introduction step)-
The ionic functional group introduction step may include, for example, a carboxyalkylation step, in which a compound having a reactive group and a carboxy group (compound E _C ) as an essential component, an alkali compound as an optional component, and the aforementioned compound B selected from urea and its derivatives are added to wet or dry cellulose fibers having hydroxyl groups and reacted to introduce carboxy groups into the cellulose fibers.

Examples of the reactive group include a halogenated alkyl group, a vinyl group, and an epoxy group (glycidyl group).
As the compound E _C , monochloroacetic acid, sodium monochloroacetate, 2-chloropropionic acid, 3-chloropropionic acid, sodium 2-chloropropionate, and sodium 3-chloropropionate are preferred from the viewpoints of the efficiency of introducing a substituent, and therefore the transparency and salt resistance of the resulting cellulose nanocrystal having an ionic functional group introduced therein, cost, and ease of handling.
Furthermore, as an optional component, it is also preferable to use the compound B in the above-mentioned <Phosphorus oxo acid group introduction step> in the same manner, and the amount added is also preferably as described above.

Compound E and _C may be added to the cellulose fiber as a reagent or dissolved in a suitable solvent. The cellulose fiber is preferably converted into alkali cellulose in advance or simultaneously with the reaction. The method for converting the cellulose fiber into alkali cellulose is as described above.

The reaction temperature is preferably, for example, 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower.

The amount of compound E _C added per 100 parts by mass of cellulose fibers is preferably 1 part by mass or more and 100,000 parts by mass or less, more preferably 2 parts by mass or more and 10,000 parts by mass or less, and even more preferably 5 parts by mass or more and 1,000 parts by mass or less.

The reaction time may vary depending on the reaction temperature, but is preferably, for example, from 1 minute to 1,000 minutes, more preferably from 3 minutes to 500 minutes, and even more preferably from 5 minutes to 400 minutes.
After the reaction, excess reaction reagents, by-products, etc. may be washed and removed by filtration or the like.

-Cationic group introduction step (cationization step)-
A compound having a reactive group and a cationic group (compound E _D ) as an essential component, an alkaline compound as an optional component, and the aforementioned compound B selected from urea and its derivatives are added to wet or dry cellulose fibers having hydroxyl groups and reacted to introduce cationic groups into the cellulose fibers.

Reactive groups include halogenated alkyl groups, vinyl groups, and epoxy groups (glycidyl groups).

As the compound E _D , glycidyl trimethyl ammonium chloride, 3-chloro-2-hydroxypropyl trimethyl ammonium chloride, etc. are preferred from the viewpoints of the efficiency of introducing the substituent, and therefore the transparency and salt resistance of the resulting cellulose nanocrystals into which the ionic functional groups have been introduced, cost, and ease of handling.

Furthermore, as an optional component, it is also preferable to use compound B in the above-mentioned <Phosphorus Oxo Acid Group Introduction Step>. It is also preferable to add it in the amount described above.

Compounds E and _D may be added to the cellulose fibers as reagents or dissolved in a suitable solvent. The cellulose fibers are preferably converted into alkali cellulose in advance or simultaneously with the reaction. The method for converting the cellulose fibers into alkali cellulose is as described above.

The reaction temperature is preferably, for example, 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower.

The amount of compound E _D added per 100 parts by mass of cellulose fibers is preferably 1 part by mass or more and 100,000 parts by mass or less, more preferably 2 parts by mass or more and 10,000 parts by mass or less, and even more preferably 5 parts by mass or more and 1,000 parts by mass or less.

The reaction time may vary depending on the reaction temperature, but is preferably, for example, from 1 minute to 1,000 minutes, more preferably from 5 minutes to 500 minutes, and even more preferably from 10 minutes to 400 minutes.
After the reaction, excess reaction reagents, by-products, etc. may be washed and removed by filtration or the like.

[Cleaning process]
In the manufacturing method of this embodiment, if necessary, a washing step can be performed on the ionic functional group-introduced fiber before the step B described below. The washing step is performed by washing the ionic functional group-introduced cellulose fiber with, for example, water or an organic solvent. Furthermore, the washing step may be performed after each of the steps described below, and the number of washings performed in each washing step is not particularly limited.

[Alkali treatment step]
The cellulose fiber having the ionic functional groups introduced therein may be subjected to an alkali treatment between step A (the step of introducing ionic functional groups) and step B (the hydrolysis step) described later. The alkali treatment method is not particularly limited, but may be, for example, a method of immersing the cellulose fiber having the ionic functional groups introduced therein in an alkali solution.
The alkaline compound contained in the alkaline solution is not particularly limited and may be an inorganic alkaline compound or an organic alkaline compound. In this embodiment, it is preferable to use, for example, sodium hydroxide or potassium hydroxide as the alkaline compound because of their high versatility. The solvent contained in the alkaline solution may be either water or an organic solvent. Among these, the solvent contained in the alkaline solution is preferably water or a polar solvent including a polar organic solvent such as an alcohol, and more preferably an aqueous solvent including at least water. As the alkaline solution, for example, an aqueous sodium hydroxide solution or an aqueous potassium hydroxide solution is preferable because of their high versatility.
The temperature of the alkaline solution in the alkaline treatment step is not particularly limited, but is preferably from 5° C. to 80° C., and more preferably from 10° C. to 60° C. The immersion time of the ionic functional group-introduced fiber in the alkaline solution in the alkaline treatment step is not particularly limited, but is preferably from 5 minutes to 30 minutes, and more preferably from 10 minutes to 20 minutes. The amount of alkaline solution used in the alkaline treatment is not particularly limited, but is preferably from 100% by mass to 100,000% by mass, and more preferably from 1,000% by mass to 10,000% by mass, based on the absolute dry mass of the ionic functional group-introduced fiber.
When the cellulose fiber having ionic functional groups introduced therein has anionic groups, the alkali treatment may be a neutralization treatment or an ion exchange treatment of the anionic groups. In this case, the temperature of the alkali solution is preferably room temperature.

In order to reduce the amount of alkaline solution used in the alkaline treatment step, the ionic functional group-introduced cellulose fiber may be washed with water or an organic solvent after the ionic functional group-introducing step but before the alkaline treatment step. From the perspective of improving handleability, it is preferable to wash the alkaline-treated ionic functional group-introduced cellulose fiber with water or an organic solvent after the alkaline treatment step but before step B.

[Acid treatment step]
The cellulose fiber having the ionic functional groups introduced therein may be subjected to an acid treatment between the ionic functional group introduction step and the hydrolysis step described later. For example, the ionic functional group introduction step, the acid treatment step, the alkali treatment step, step B, and step C may be performed in this order.
The acid treatment method is not particularly limited, but examples include a method of immersing ionic functional group-introduced cellulose fibers in an acidic solution containing an acid. The concentration of the acidic solution used is not particularly limited, but is preferably 10% by mass or less, and more preferably 5% by mass or less. The pH of the acidic solution used is also not particularly limited, but is preferably 0 to 4, and more preferably 1 to 3. Examples of acids that can be used in the acidic solution include inorganic acids, sulfonic acids, and carboxylic acids. Examples of inorganic acids include sulfuric acid, nitric acid, hydrobromic acid, hydroiodic acid, hypochlorous acid, chlorous acid, chloric acid, perchloric acid, phosphoric acid, and boric acid. Examples of sulfonic acids include methanesulfonic acid, ethanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid, and trifluoromethanesulfonic acid. Examples of carboxylic acids include formic acid, acetic acid, citric acid, gluconic acid, lactic acid, oxalic acid, and tartaric acid. Among these, hydrochloric acid or sulfuric acid is particularly preferred.
The temperature of the acid solution in the acid treatment is not particularly limited, but is preferably from 5° C. to 100° C., and more preferably from 20° C. to 90° C. The immersion time in the acid solution in the acid treatment is not particularly limited, but is preferably from 5 minutes to 120 minutes, and more preferably from 10 minutes to 60 minutes. The amount of the acid solution used in the acid treatment is not particularly limited, but is preferably from 100% by mass to 100,000% by mass, and more preferably from 1,000% by mass to 10,000% by mass, based on the absolute dry mass of the cellulose fiber having ionic functional groups introduced therein.
When the cellulose fiber having ionic functional groups introduced therein has cationic groups, the acid treatment may be a neutralization treatment or an ion exchange treatment of the cationic groups. In this case, the temperature of the acid solution is preferably room temperature.

<Process B>
Step B is a hydrolysis step in which the cellulose fibers having ionic functional groups introduced therein are hydrolyzed with an acidic aqueous solution having a concentration of 5 mol/L or more.
In the above-mentioned case, it is preferable that the cellulose fibers into which ionic functional groups have been introduced (ionic functional group-introduced cellulose fibers) have a washing step after step A and before step B.
The hydrolysis step removes amorphous portions in the cellulose fibers having ionic functional groups introduced therein, improving crystallinity. Furthermore, the hydrolysis step hydrolyzes, for example, condensed portions in phosphorus oxoacids, and the difference between the first dissociation amount and the second dissociation amount in the resulting cellulose nanocrystals having ionic functional groups introduced therein becomes smaller or equal.
In the hydrolysis step, the ionic functional groups introduced into the amorphous regions of the cellulose fibers are removed as the cellulose fibers are hydrolyzed, and therefore, the amount of ionic functional groups introduced into the ionic functional group-introduced cellulose nanocrystals obtained through steps B and C is generally smaller than the amount of ionic functional groups introduced into the ionic functional group-introduced cellulose fibers obtained through step A. Furthermore, carbamide groups and the like introduced into the amorphous regions of the cellulose fibers are also removed by the hydrolysis step, resulting in ionic functional group-introduced cellulose nanocrystals of higher purity.

[Acidic aqueous solution]
In step B, an acidic aqueous solution having a concentration of 5 mol/L or more is used.
The acid contained in the acidic aqueous solution may be, for example, an inorganic acid or an organic acid such as a sulfonic acid or a carboxylic acid.
Examples of inorganic acids include sulfuric acid, nitric acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, hypochlorous acid, chlorous acid, chloric acid, perchloric acid, phosphoric acid, and boric acid.
Examples of sulfonic acids include methanesulfonic acid, ethanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid, trifluoromethanesulfonic acid, etc. Examples of carboxylic acids include formic acid, acetic acid, citric acid, gluconic acid, lactic acid, oxalic acid, tartaric acid, etc.
The aqueous phosphoric acid solution may be an aqueous solution of diphosphorus pentoxide. The use of diphosphorus pentoxide is preferable because it allows a highly concentrated acidic aqueous solution to be obtained, and the resulting solution can be recovered and reused as regenerated diphosphorus pentoxide after use in the hydrolysis treatment.
Among these, the acidic aqueous solution used in step B is preferably an aqueous solution of an acid compound selected from the group consisting of phosphoric acid, sulfuric acid, diphosphorus pentoxide, and hydrochloric acid.

The concentration of the acidic aqueous solution used in step B is 5 mol/L or more, preferably 7 mol/L or more, more preferably 9 mol/L or more, and even more preferably 10 mol/L or more. From the standpoint of ease of operation, the concentration is preferably 30 mol/L or less, more preferably 25 mol/L or less, and even more preferably 20 mol/L or less.

[Hydrolysis process]
The concentration of the cellulose fiber having ionic functional groups introduced therein in step B is preferably 1% by mass or more, more preferably 1.5% by mass or more, and preferably 10% by mass or less, more preferably 7.5% by mass or less, and even more preferably 5% by mass or less, from the viewpoint of the reactivity of the hydrolysis reaction.
The reaction temperature in step B is preferably 50°C or higher, more preferably 70°C or higher, even more preferably 90°C or higher, and preferably 100°C or lower, from the viewpoint of the reactivity of the hydrolysis reaction.
The reaction time in step B is preferably 5 minutes or longer, more preferably 15 minutes or longer, even more preferably 30 minutes or longer, from the viewpoint of the reactivity of the hydrolysis reaction and the viewpoint of shortening the production time, and is preferably 12 hours or shorter, more preferably 6 hours or shorter, even more preferably 3 hours or shorter.

It is preferable to have a purification step after step B and before step C.
In the purification step, it is preferable to remove the acidic aqueous solution used in the hydrolysis by repeating solid-liquid separation and re-dispersion in water.
After the above treatment, it is preferable to purify the slurry by contacting it with a strongly basic ion exchange resin and continuing the treatment until the electrical conductivity and pH of the slurry become stable.

<Process C>
Step C is a defibration step in which the hydrolyzed ionic functional group-introduced fibers obtained in step B are defibrated.
In the defibration step, it is preferable to make the hydrolyzed ionic functional group-introduced fibers obtained in step B into a dispersion (slurry) and subject it to defibration.

[Fibrillation process]
The hydrolyzed cellulose fibers having ionic functional groups introduced therein are defibrated in a defibration treatment step to obtain cellulose nanocrystals having ionic functional groups introduced therein.
In the defibration treatment step, for example, a defibration treatment device can be used. The defibration treatment device is not particularly limited, but examples that can be used include a high-speed defibrator, a grinder (stone mill-type grinder), a high-pressure homogenizer, an ultra-high-pressure homogenizer, a high-pressure collision grinder, a ball mill, a bead mill, a disk-type refiner, a conical refiner, a twin-screw kneader, a vibration mill, a homomixer under high-speed rotation, an ultrasonic disperser, or a beater. Among the above defibration treatment devices, it is more preferable to use a high-speed defibrator, a high-pressure homogenizer, or an ultra-high-pressure homogenizer, which are less affected by the grinding media and have less risk of contamination.

In the defibration process, for example, hydrolyzed cellulose fibers with ionic functional groups introduced therein are preferably diluted with a dispersion medium to form a slurry. The dispersion medium can be one or more selected from water and organic solvents such as polar organic solvents. Polar organic solvents are not particularly limited, but examples of preferred polar organic solvents include alcohols, polyhydric alcohols, ketones, ethers, esters, and aprotic polar solvents. Examples of alcohols include methanol, ethanol, isopropanol, n-butanol, and isobutyl alcohol. Examples of polyhydric alcohols include ethylene glycol, propylene glycol, and glycerin. Examples of ketones include acetone and methyl ethyl ketone (MEK). Examples of ethers include diethyl ether, tetrahydrofuran, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-butyl ether, and propylene glycol monomethyl ether. Examples of esters include ethyl acetate and butyl acetate. Examples of aprotic polar solvents include dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), and N-methyl-2-pyrrolidinone (NMP).
Of these, water is preferred as the dispersion medium.

From the viewpoint of efficient defibration, the concentration of the dispersion liquid (slurry) during the defibration process is preferably 0.3% by mass or more, more preferably 1.0% by mass or more, even more preferably 1.5% by mass or more, and preferably 10% by mass or less, more preferably 6% by mass or less, even more preferably 3% by mass or less.

<Cellulose nanocrystals with ionic functional groups>
The cellulose nanocrystals having ionic functional groups introduced therein obtained through steps A to C have ionic functional groups introduced therein and are highly crystallized.
From the viewpoints of transparency and salt resistance, the amount of ionic functional groups introduced into cellulose nanocrystals having ionic functional groups introduced therein is preferably 0.10 mmol/g or more, more preferably 0.20 mmol/g or more, even more preferably 0.30 mmol/g or more, even more preferably 0.40 mmol/g or more, and particularly preferably 0.50 mmol/g or more, and the upper limit is not particularly limited, and is, for example, 3.00 mmol/g or less.

From the viewpoint of improving the purity of the cellulose nanocrystals into which ionic functional groups have been introduced, the amount of carbamide groups in the cellulose nanocrystals into which ionic functional groups have been introduced is preferably low, and is preferably 0.06 mmol/g or less, more preferably 0.05 mmol/g or less, even more preferably 0.04 mmol/g or less, and even more preferably 0.03 mmol/g or less. The lower limit is not particularly limited, and may be 0.00 mmol/g.
As described above, carbamide groups may be introduced when ionic functional groups are introduced into cellulose fibers using urea and/or urea derivatives, and the carbamide groups introduced into at least the amorphous portions of the cellulose fibers in step B are removed together with the amorphous portions.

The fiber width of the cellulose nanocrystals into which ionic functional groups have been introduced is 1000 nm or less. The fiber width of the cellulose nanocrystals can be measured, for example, by observation using an electron microscope.
The fiber width of the ionic functionalized cellulose nanocrystals is 1,000 nm or less, preferably 2 nm to 1,000 nm, more preferably 2 nm to 100 nm, even more preferably 2 nm to 50 nm, and particularly preferably 2 nm to 30 nm. By keeping the fiber width of the ionic functionalized cellulose nanocrystals within this range, dissolution of the cellulose molecules in water can be suppressed, making it easier to achieve the effect of reducing the viscosity of the ionic functionalized cellulose nanocrystal dispersion.

The average fiber width of the ionic functionalized cellulose nanocrystals is, for example, 1,000 nm or less. The average fiber width of the ionic functionalized cellulose nanocrystals is preferably 2 nm or more and 1,000 nm or less, more preferably 2 nm or more and 100 nm or less, even more preferably 2 nm or more and 50 nm or less, and particularly preferably 2 nm or more and 30 nm or less. By keeping the average fiber width of the ionic functionalized cellulose nanocrystals within the above range, dissolution of the cellulose molecules in water is suppressed, making it easier to achieve the effect of reducing the viscosity of the ionic functionalized cellulose nanocrystal dispersion.

The average fiber width of cellulose nanocrystals is measured, for example, using an electron microscope as follows. First, an aqueous suspension of cellulose nanocrystals with a concentration of 0.05% by mass or more and 0.1% by mass or less is prepared, and this suspension is cast onto a hydrophilically treated carbon film-coated grid to prepare a sample for TEM observation. When wide fibers are included, SEM images of the surface cast onto glass may be observed. Next, electron microscope images are observed at magnifications of 1,000x, 5,000x, 10,000x, or 50,000x, depending on the width of the fibers to be observed. However, the sample, observation conditions, and magnification are adjusted to satisfy the following conditions.
(1) A line X is drawn at any point in the observed image, and 20 or more fibers intersect with the line X.
(2) Draw a line Y that intersects the line perpendicularly within the same image, and 20 or more fibers intersect the line Y.

For observation images that satisfy the above conditions, the widths of the fibers that intersect with lines X and Y are visually read. In this way, three or more sets of observation images are obtained of at least the surface portions that do not overlap each other. Next, for each image, the widths of the fibers that intersect with lines X and Y are read. In this way, the widths of at least 20 fibers x 2 x 3 = 120 fibers are read. The average of the read fiber widths is then used as the average fiber width of the cellulose nanocrystals.

The fiber diameter and average fiber width of cellulose nanocrystals may also be measured using an atomic force microscope (AFM) as follows. First, an aqueous suspension of cellulose nanocrystals with a concentration of 0.0001% by mass or more and 0.1% by mass or less is prepared, and this suspension is cast onto mica cleaved with tape to prepare a sample for AFM observation. Next, observation is performed using the AFM at a magnification of 5 μm, 2 μm, or 1 μm square field of view, depending on the width of the fibers to be observed. Three or more sets of observation images of at least non-overlapping surface portions are obtained, and non-overlapping fibers are randomly selected. The height of the fibers in the Z-axis direction is read and used as the fiber width. The widths of at least 100 fibers are measured, and the average of the obtained fiber widths is used as the average fiber width of the cellulose nanocrystals.

The fiber length of ionic functionalized cellulose nanocrystals is not particularly limited, but is preferably 30 nm or more and 1,000 nm or less, more preferably 40 nm or more and 800 nm or less, and even more preferably 50 nm or more and 500 nm or less. By keeping the fiber length within the above range, it is also possible to set the slurry viscosity of ionic functionalized cellulose nanocrystals within an appropriate range. The fiber length of ionic functionalized cellulose nanocrystals can be determined, for example, by image analysis using TEM, SEM, or AFM.

The cellulose nanocrystals having ionic functional groups preferably have a type I crystal structure. The fact that the cellulose nanocrystals having ionic functional groups have a type I crystal structure can be identified from a diffraction profile obtained from a wide-angle X-ray diffraction photograph using CuKα (λ=1.5418 Å) monochromated with graphite. Specifically, this can be identified from the presence of typical peaks at two positions: 2θ=14° to 17° and 2θ=22° to 23°.
The proportion of type I crystal structure in the cellulose nanocrystals containing ionic functional groups is preferably 30% or more, more preferably 40% or more, and even more preferably 50% or more. This is expected to result in a low viscosity when the nanocrystals are dispersed in water. The degree of crystallinity can be determined by measuring the X-ray diffraction profile and then using standard methods from the pattern (Seagal et al., Textile Research Journal, Vol. 29, p. 786, 1959).

The axial ratio (fiber length/fiber width) of cellulose nanocrystals with ionic functional groups introduced therein is not particularly limited, but is preferably 5 to 500, and more preferably 10 to 250. By keeping the axial ratio within the above range, it is easier to obtain a low-viscosity dispersion when a solvent dispersion is prepared.

Cellulose nanocrystals, unlike cellulose nanofibers, typically have a needle-like or spindle-like shape and are less bent.
This is thought to be because the crystallinity increases due to the hydrolysis of the amorphous portion in step B, and those skilled in the art can distinguish between cellulose nanocrystals and cellulose nanofibers by observing their shapes as well.
When 100 cellulose nanocrystals are randomly observed with an atomic force microscope, the number of fibers that are bent by 30° or more along the way is preferably 20 or less, more preferably 10 or less, even more preferably 5 or less, and even more preferably 3 or less.

[Cellulose nanocrystals with ionic functional groups introduced]
The cellulose nanocrystals of this embodiment into which ionic functional groups have been introduced have an amount of ionic functional groups introduced of 0.1 mmol/g or more (excluding cellulose nanocrystals in which only the carbon atom at the 6th position of cellulose has been oxidized and a carboxy group has been introduced).
The cellulose nanocrystals introduced with ionic functional groups according to this embodiment exclude cellulose nanocrystals in which only the carbon atom at the 6th position of cellulose has been oxidized and a carboxyl group has been introduced, as described above. Specifically, in carboxyl group-introduced cellulose nanocrystals obtained by TEMPO oxidation, a carboxyl group is introduced only at the 6th position, and an ionic functional group cannot be introduced at least at either the 2nd or 3rd carbon atom. Therefore, the degree of freedom for introducing ionic functional groups is low. Furthermore, in the past, when carboxyl groups were introduced by TEMPO oxidation, a relatively high amount of carboxyl groups was obtained even when the carboxyl groups were introduced after the cellulose nanocrystals were prepared by hydrolysis and defibration treatment. However, in the case of other ionic functional groups, it was difficult to introduce a sufficient amount of ionic functional groups.

Examples of the ionic functional groups introduced into the ionic functional group-introduced cellulose nanocrystals of this embodiment include the ionic functional groups exemplified in the above-mentioned method for producing ionic functional group-introduced cellulose nanocrystals, and the preferred ranges are also the same.
The ionic functional group may include, for example, either or both of an anionic functional group and a cationic functional group. In this embodiment, it is particularly preferable that the ionic functional group is an anionic functional group.
Examples of anionic functional groups as ionic functional groups include phosphorus oxo acid groups or groups derived from phosphorus oxo acid groups (sometimes simply referred to as phosphorus oxo acid groups), carboxy groups or groups derived from carboxy groups (sometimes simply referred to as carboxy groups), sulfur oxo acid groups or groups derived from sulfur oxo acid groups (sometimes simply referred to as sulfur oxo acid groups), xanthate groups, phosphonic groups, phosphine groups, sulfonic groups, and carboxyalkyl groups.
Examples of cationic functional groups as ionic functional groups include ammonium groups, phosphonium groups, sulfonium groups, etc. Among these, the cationic functional group is preferably an ammonium group.
Among these, the ionic functional group is preferably at least one selected from the group consisting of a phosphorus oxoacid group, a group derived from a phosphorus oxoacid group, a carboxy group, a sulfone group, a sulfur oxoacid group, a group derived from a sulfur oxoacid group, and a cationic functional group, more preferably at least one selected from the group consisting of a phosphorus oxoacid group, a group derived from a phosphorus oxoacid group, a carboxy group, a sulfur oxoacid group, and a group derived from a sulfur oxoacid group, even more preferably at least one selected from the group consisting of a phosphorus oxoacid group and a group derived from a phosphorus oxoacid group, and even more preferably a phosphorus oxoacid group. By introducing a phosphorus oxoacid group as the anionic functional group, cellulose nanocrystals with excellent transparency and salt resistance can be obtained.

In the ionic functional group-introduced cellulose nanocrystals of this embodiment, the amount of ionic functional groups introduced is 0.20 mmol/g or more, preferably 0.40 mmol/g or more, more preferably 0.50 mmol/g or more, even more preferably 0.60 mmol/g or more, even more preferably 0.70 mmol/g or more, even more preferably 0.80 mmol/g or more, even more preferably 0.90 mmol/g or more, even more preferably 1.00 mmol/g or more, even more preferably 1.10 mmol/g or more, and even more preferably 1.20 mmol/g or more. While there are no particular upper limits, excessive amounts can cause destruction and dissolution of the cellulose crystals, so the amount is preferably 5.00 mmol/g or less, more preferably 4.00 mmol/g or less, and even more preferably 3.00 mmol/g or less.

From the viewpoint of improving the purity of the cellulose nanocrystals having ionic functional groups introduced therein, the amount of carbamide groups in the cellulose nanocrystals having ionic functional groups introduced therein is preferably low, and is preferably 0.06 mmol/g or less, more preferably 0.05 mmol/g or less, even more preferably 0.04 mmol/g or less, and even more preferably 0.03 mmol/g or less. The lower limit is not particularly limited, and may be 0.00 mmol/g.
As described above, ionic functional groups may be introduced into cellulose fibers using urea and/or urea derivatives. When cellulose nanocrystals having ionic functional groups introduced therein are produced by the method for producing cellulose nanocrystals having ionic functional groups introduced therein according to the present embodiment, the carbamide groups introduced into at least the amorphous parts of the cellulose fibers in step B are removed together with the amorphous parts, resulting in a low amount of carbamide groups, which is preferable.

In the cellulose nanocrystals of this embodiment having an ionic functional group introduced therein, it is preferable that an ionic functional group be introduced to at least one of the carbon atoms at the 2nd and 3rd positions of the cellulose, in addition to the carbon atom at the 6th position of the cellulose.
By introducing an ionic functional group into the carbon atom at the 6th position of cellulose as well as into at least one carbon atom selected from the group consisting of the carbon atom at the 2nd position and the carbon atom at the 3rd position, it is possible to increase the amount of ionic functional group introduced and improve the degree of freedom in the amount of ionic functional group introduced.
Furthermore, compared to when ionic functional groups are introduced only at the 6-position, it is thought that the density of ionic functional groups is increased in cellulose nanocrystals with ionic functional groups introduced, resulting in cellulose nanocrystals with better dissociation properties and smaller fiber diameters.
The introduction of an ionic functional group into at least one carbon atom selected from the group consisting of the carbon atom at the 2-position and the carbon atom at the 3-position can be detected by ^13C NMR, for example, by ^13C CP/MAS NMR (Cross Polarization Magic Angle Spinning-NMR (Nuclear Magic Resonance)).

The ionic functionalized cellulose nanocrystals of this embodiment are preferably obtained by the above-described method for producing ionic functionalized cellulose nanocrystals, which comprises steps A to C in this order. However, the method for producing ionic functionalized cellulose nanocrystals of this embodiment is not limited to this, and they may also be produced by other methods.

The cellulose nanocrystals having ionic functional groups introduced therein according to this embodiment have a low viscosity when dispersed in water.
The viscosity of a 1.5% by mass aqueous dispersion of the ionic functional group-introduced cellulose nanocrystal of this embodiment at 23° C. is preferably 100 mPa·s or less, more preferably 50 mPa·s or less, even more preferably 30 mPa·s or less, and even more preferably 15 mPa·s or less. The lower limit is not particularly limited, but from the viewpoint of ease of production, it is preferably 1 mPa·s or more, more preferably 2 mPa·s or more.
The viscosity of the aqueous dispersion of ionic functionalized cellulose nanocrystals is measured by the method described in the Examples.

The cellulose nanocrystals having ionic functional groups introduced therein exhibit excellent transparency. Specifically, the total light transmittance of a 0.2% by mass aqueous dispersion of the cellulose nanocrystals having ionic functional groups at 23°C is preferably 95.5% or more, more preferably 96.0% or more, and even more preferably 96.5% or more, with no particular upper limit, which may be 100%.
Here, the total light transmittance is a value measured in accordance with, for example, JIS K 7361-1:1997 using a haze meter (HM-150, manufactured by Murakami Color Research Laboratory Co., Ltd.).

The cellulose nanocrystals introduced with ionic functional groups according to this embodiment have excellent salt tolerance.
Specifically, it is preferable that the change in the total light transmittance of a 0.2 mass% aqueous dispersion of ionic functional group-containing cellulose nanocrystals at 23°C due to the addition of sodium chloride is suppressed, and when the total light transmittance before the addition of sodium chloride _is T0 and the total light transmittance after the addition of sodium chloride is T, the sodium chloride concentration at which T/ _T0 is 0.98 or less is preferably 100 mmol/L or more, more preferably 300 mmol/L or more.

The cellulose nanocrystals having ionic functional groups introduced therein according to the present invention preferably have excellent purity and are evaluated by coloration due to wet heating. Note that the nitrogen-containing structure such as the carbamide group is thought to promote coloration.
Specifically, the coloring degree is evaluated by visually determining the coloring degree when a 1% by mass dispersion is heated at 95° C. for 10 hours.

The ranges of fiber width and fiber length of the ionic functional group-introduced cellulose nanocrystals of this embodiment are the same as those of the ionic functional group-introduced cellulose nanocrystals obtained by the production method of this embodiment, and the preferred ranges are also the same.
Furthermore, as described above, unlike cellulose nanofibers, cellulose nanocrystals typically have a needle-like or spindle-like shape with few bends.
When 100 cellulose nanocrystals are randomly observed with an atomic force microscope, the number of fibers that are bent by 30° or more along the way is preferably 20 or less, more preferably 10 or less, even more preferably 5 or less, and even more preferably 3 or less.

[Dispersion liquid]
The dispersion of this embodiment contains cellulose nanocrystals to which ionic functional groups have been introduced.
The cellulose nanocrystals having ionic functional groups introduced therein according to this embodiment have low viscosity even when prepared as a dispersion, and can be used as a dispersion at a higher concentration than conventional cellulose nanofibers.

[Sheet]
The sheet of this embodiment contains cellulose nanocrystals to which ionic functional groups have been introduced.
In this embodiment, the sheet can be obtained by carrying out a sheet forming process using, for example, a liquid composition containing the above-mentioned ionic functional group-introduced cellulose nanocrystals, and optionally a hydrophilic polymer and other components.
The sheet production process preferably includes at least a coating step of coating the composition onto a substrate or a papermaking step of making the composition into paper, and more preferably includes a coating step of coating the composition onto a substrate, thereby obtaining a sheet containing cellulose nanocrystals having ionic functional groups introduced therein.
The thickness of the sheet is not particularly limited, but is, for example, 5 μm or more, more preferably 10 μm or more, even more preferably 20 μm or more, and is preferably 500 μm or less, more preferably 300 μm or less, even more preferably 200 μm or less.

[powder]
The powder of this embodiment contains cellulose nanocrystals to which ionic functional groups have been introduced.
In this embodiment, the powder is obtained by drying a dispersion of cellulose nanocrystals having ionic functional groups introduced therein. Specifically, a method in which a dispersion containing cellulose nanocrystals having ionic functional groups introduced therein is spray-dried using a spray dryer is exemplified.
The moisture content of the powder is preferably 50% or less, more preferably 30% or less, even more preferably 25% or less, and is preferably 5% or more, more preferably 10% or more.
Furthermore, from the viewpoint of ease of production, the particle size of the powder is preferably 1 μm or more, more preferably 3 μm or more, even more preferably 5 μm or more, and is preferably 1000 μm or less, more preferably 300 μm or less, even more preferably 100 μm or less.

[Applications of cellulose nanocrystals with ionic functional groups introduced]
The ionic functional group-introduced cellulose nanocrystals of this embodiment can be used as additives for, for example, cosmetics, medical compositions, paints, metal surface treatment agents, abrasives, foods, rubber, resins, excavation compositions, concrete hydration compositions, concrete precursors, dental materials, cell proliferation promoters, antifreeze agents, paint film strippers, insecticides/insect repellents, pesticides, mold molding compositions, lubricants, piping friction resistance reducers, and fragrances/deodorizers. The ionic functional group-introduced cellulose nanocrystals of this embodiment have excellent transparency and salt resistance and are inhibited from yellowing due to wet heating, making them suitable for applications requiring transparency and salt resistance or applications requiring heating.
In addition, when made into a sheet form, it is suitable for use as optical components in display devices, various solar cells, etc. Furthermore, it is also suitable for applications such as substrates for electronic devices, separators for electrochemical elements, components for home appliances, window materials for various vehicles and buildings, interior and exterior materials, and packaging materials.

The following examples and comparative examples will further illustrate the features of the present invention. The materials, amounts used, ratios, processing details, processing procedures, etc. shown in the following examples can be modified as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the specific examples shown below.

<Example A1>
As the raw material pulp (cellulose fiber), softwood bleached kraft pulp (NBKP, solids content 93% by mass, basis weight 245 g/ ^m2 , sheet form, Canadian Standard Freeness (CSF) measured after disintegration in accordance with JIS P 8121-2:2012 of 700 mL) manufactured by Oji Paper Co., Ltd. was used.
This raw pulp was subjected to phosphorus oxo-oxidation treatment as follows. First, a mixed aqueous solution of ammonium dihydrogen phosphate and urea was added to 100 parts by mass (bone dry mass) of the raw pulp to adjust the composition to 45 parts by mass of ammonium dihydrogen phosphate, 120 parts by mass of urea, and 150 parts by mass of water, to obtain a chemical-impregnated pulp. Next, the obtained chemical-impregnated pulp was heated in a hot air dryer at 165°C for 250 seconds to introduce phosphate groups into the cellulose in the pulp, thereby obtaining a phosphorylated pulp.
The resulting phosphorylated pulp was then washed. The washing process consisted of pouring 10 L of ion-exchanged water into 100 g (bone dry mass) of phosphorylated pulp to obtain a pulp dispersion, stirring the resulting solution to uniformly disperse the pulp, and then repeatedly filtering and dehydrating the pulp. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less.
Next, the washed phosphorylated pulp was neutralized as follows: First, the washed phosphorylated pulp was diluted with 10 L of ion-exchanged water, and then a 1 N aqueous solution of sodium hydroxide was added little by little while stirring to obtain a phosphorylated pulp slurry having a pH of 12 to 13. Next, the phosphorylated pulp slurry was dehydrated and washed to obtain a neutralized phosphorylated pulp.

The resulting phosphorylated pulp was subjected to high-concentration acid hydrolysis.
Specifically, 85% phosphoric acid (14.7 mol/L phosphoric acid) was added little by little to the obtained phosphorylated pulp to obtain a slurry in which the phosphorylated pulp was suspended at 4% by mass in 10.7 mol/L phosphoric acid. This suspension slurry was a viscous liquid with spinnability. The obtained suspension slurry was placed in a glass container and, while stirring, was subjected to thermal hydrolysis treatment in a hot bath at 95°C for 90 minutes.
The resulting slurry after high-concentration acid hydrolysis was light brown in color, had reduced viscosity, and exhibited no spinnability. The resulting post-reaction slurry was purified by removing dissolved components and acid using the following procedure. Specifically, the resulting post-reaction slurry was ice-cooled and centrifuged (12,000 G, 5 minutes) for solid-liquid separation. After centrifugation, the supernatant was removed, and the same volume of ion-exchanged water was added. The precipitate was thoroughly mixed and dispersed, and then centrifuged again. This procedure was repeated three times, after which the precipitate was again suspended in ion-exchanged water and brought into contact with a strongly basic ion-exchange resin (Amberjet 4400; Organo Corporation, conditioned). Treatment was continued until the electrical conductivity and pH of the slurry no longer fluctuated.
Ion-exchanged water was added to the resulting purified slurry to prepare a slurry with a solids concentration of 2% by mass, which was then subjected to the following defibration treatment: Specifically, the slurry was defibrated by treating it six times at a pressure of 100 MPa in a wet atomization device (Beryu-Mini, high-pressure homogenizer, manufactured by Biryu Co., Ltd.), to obtain a dispersion containing phosphorylated cellulose nanocrystals.

The obtained phosphorylated pulp and phosphorylated cellulose nanocrystals were subjected to infrared absorption spectroscopy using FT-IR. As a result, absorption due to the P=O of the phosphate group was observed near 1230 cm ⁻¹ , confirming the addition of phosphate groups. Furthermore, when the obtained phosphorylated pulp and phosphorylated cellulose nanocrystals were analyzed using an X-ray diffractometer, typical peaks were observed at two positions, near 2θ = 14° to 17° and near 2θ = 22° to 23°, confirming the presence of cellulose type I crystals. The amount of phosphate groups (amount of first dissociated acid), measured by the measurement method described below in [Measurement of phosphorus oxo acid group amount], was 1.45 mmol/g for the phosphorylated pulp and 0.60 mmol/g for the phosphorylated cellulose nanocrystals. The total amount of dissociated acid in the phosphorylated pulp was 2.45 mmol/g (the amount of the second dissociated acid was 1.00 mmol/g), and that in the phosphorylated cellulose nanocrystals was 1.20 mmol/g (the amount of the second dissociated acid was 0.60 mmol/g). Furthermore, the fiber width of the cellulose nanocrystals was measured using an atomic force microscope and found to be 3 to 20 nm.

<Example A2>
A dispersion containing phosphorylated pulp and phosphorylated cellulose nanocrystals was obtained in the same manner as in Example A1, except that the washed phosphorylated pulp was further subjected to the above phosphorylation treatment and the above washing treatment, in that order, twice each.
The infrared absorption spectra of the resulting phosphorylated pulp and phosphorylated cellulose nanocrystals were measured using FT-IR. As a result, absorption due to the P=O of the phosphate group was observed around 1230 cm ⁻¹ , confirming the addition of phosphate groups. X-ray diffraction confirmed that the resulting phosphorylated pulp and phosphorylated cellulose nanocrystals maintained cellulose type I crystallinity. The amount of phosphate groups (amount of first dissociated acid) measured using the measurement method described below in [Measurement of phosphorus oxoacid group amount] was 2.00 mmol/g for the phosphorylated pulp and 0.85 mmol/g for the phosphorylated cellulose nanocrystals. The total amount of dissociated acid was 3.30 mmol/g for the phosphorylated pulp (amount of second dissociated acid: 1.30 mmol/g) and 1.65 mmol/g for the phosphorylated cellulose nanocrystals (amount of second dissociated acid: 0.80 mmol/g). Furthermore, the fiber width of the phosphorylated cellulose nanocrystals was measured using an atomic force microscope and found to be 3 to 20 nm.

<Example A3>
In Example A1, the chemical-impregnated pulp was heated in a hot air dryer at 165°C for 750 seconds instead of 250 seconds in a hot air dryer at 165°C. A dispersion containing phosphorylated pulp and phosphorylated cellulose nanocrystals was obtained in the same manner as in Example A1, except that the chemical-impregnated pulp was heated in a hot air dryer at 165°C for 750 seconds.

The infrared absorption spectra of the resulting phosphorylated pulp and phosphorylated cellulose nanocrystals were measured using FT-IR. As a result, absorption due to the P=O of the phosphate group was observed around 1230 cm ⁻¹ , confirming the addition of phosphate groups. X-ray diffraction confirmed that the resulting phosphorylated pulp and phosphorylated cellulose nanocrystals maintained cellulose type I crystallinity. The amount of phosphate groups (amount of first dissociated acid) measured using the measurement method described below in [Measurement of phosphorus oxoacid group amount] was 2.53 mmol/g for the phosphorylated pulp and 1.25 mmol/g for the phosphorylated cellulose nanocrystals. The total amount of dissociated acid was 4.22 mmol/g for the phosphorylated pulp (amount of second dissociated acid: 1.69 mmol/g) and 2.39 mmol/g for the phosphorylated cellulose nanocrystals (amount of second dissociated acid: 1.14 mmol/g). Furthermore, the fiber width of the phosphorylated cellulose nanocrystals was measured using an atomic force microscope and found to be 3 to 20 nm.

<Example A4>
A dispersion containing phosphorylated cellulose nanocrystals was obtained in the same manner as in Example A1, except that diphosphorus pentoxide was used instead of 85% phosphoric acid.

The infrared absorption spectrum of the obtained phosphorylated cellulose nanocrystals was measured using FT-IR. As a result, absorption due to the P=O of the phosphate group was observed around 1230 cm ^-1 , confirming the addition of phosphate groups. X-ray diffraction confirmed that cellulose type I crystals were maintained. The amount of phosphate groups (amount of first dissociated acid) measured by the measurement method described below in [Measurement of amount of phosphorus oxo acid group] was 0.60 mmol/g. The total amount of dissociated acid was 1.20 mmol/g (amount of second dissociated acid was 0.60 mmol/g). Furthermore, the fiber width was measured using an atomic force microscope and was found to be 3 to 20 nm.

<Example A5>
The slurry obtained in Example A1 after high-concentration acid hydrolysis was centrifuged, and the resulting supernatant was collected and dried overnight in a hot air oven at 105°C. The resulting dried material was placed in a hot air drying furnace at 650°C to combust and remove organic matter, and the sublimated phosphorus component was collected. The X-ray diffraction pattern of the resulting phosphorus component was similar to that of commercially available diphosphorus pentaoxide, indicating that regenerated diphosphorus pentaoxide had been obtained.
A dispersion containing phosphorylated cellulose nanocrystals was obtained in the same manner as in Example A4, except that this regenerated diphosphorus pentoxide was used.

The infrared absorption spectrum of the obtained phosphorylated cellulose nanocrystals was measured using FT-IR. As a result, absorption due to the P=O of the phosphate group was observed around 1230 cm ^-1 , confirming the addition of phosphate groups. X-ray diffraction confirmed that cellulose type I crystals were maintained. The amount of phosphate groups (amount of first dissociated acid) measured by the measurement method described below in [Measurement of amount of phosphorus oxo acid group] was 0.60 mmol/g. The total amount of dissociated acid was 1.20 mmol/g (amount of second dissociated acid was 0.60 mmol/g). Furthermore, the fiber width was measured using an atomic force microscope and was found to be 3 to 20 nm.

<Example A6>
A dispersion containing phosphorylated cellulose nanocrystals was obtained in the same manner as in Example A1, except that concentrated sulfuric acid (18.7 mol/L sulfuric acid) was used instead of 85% phosphoric acid to obtain a slurry in which phosphorylated pulp was suspended at 4% by mass in 10.7 mol/L sulfuric acid.

The infrared absorption spectrum of the obtained phosphorylated cellulose nanocrystals was measured using FT-IR. As a result, absorption due to the P=O of the phosphate group was observed around 1230 cm ^-1 , confirming the addition of phosphate groups. X-ray diffraction confirmed that cellulose type I crystals were maintained. The amount of phosphate groups (amount of first dissociated acid) measured by the measurement method described below in [Measurement of amount of phosphorus oxo acid group] was 0.53 mmol/g. The total amount of dissociated acid was 1.06 mmol/g (amount of second dissociated acid was 0.53 mmol/g). Furthermore, the fiber width was measured using an atomic force microscope and was found to be 3 to 20 nm.

<Example A7>
A dispersion containing phosphorylated cellulose nanocrystals was obtained in the same manner as in Example A1, except that concentrated hydrochloric acid (11.2 mol/L hydrochloric acid) was used instead of 85% phosphoric acid to obtain a slurry in which phosphorylated pulp was suspended at 2% by mass in 10.7 mol/L hydrochloric acid.

The infrared absorption spectrum of the obtained phosphorylated cellulose nanocrystals was measured using FT-IR. As a result, absorption due to the P=O of the phosphate group was observed around 1230 cm ^-1 , confirming the addition of phosphate groups. X-ray diffraction confirmed that cellulose type I crystals were maintained. The amount of phosphate groups (amount of first dissociated acid) measured by the measurement method described below in [Measurement of amount of phosphorus oxo acid group] was 0.66 mmol/g. The total amount of dissociated acid was 1.32 mmol/g (amount of second dissociated acid was 0.66 mmol/g). Furthermore, the fiber width was measured using an atomic force microscope and was found to be 3 to 20 nm.

<Example A8>
A dispersion containing phosphorylated cellulose nanocrystals was obtained in the same manner as in Example A4, except that a slurry in which phosphorylated pulp was suspended at 4 mass % in 16 mol/L phosphoric acid was obtained.

The infrared absorption spectrum of the obtained phosphorylated cellulose nanocrystals was measured using FT-IR. As a result, absorption due to the P=O of the phosphate group was observed around 1230 cm ^-1 , confirming the addition of phosphate groups. X-ray diffraction confirmed that cellulose type I crystals were maintained. The amount of phosphate groups (amount of first dissociated acid) measured by the measurement method described below in [Measurement of amount of phosphorus oxo acid group] was 0.48 mmol/g. The total amount of dissociated acid was 0.96 mmol/g (amount of second dissociated acid was 0.48 mmol/g). Furthermore, the fiber width was measured using an atomic force microscope and was found to be 3 to 20 nm.

<Example B1>
The same procedure as in Example A1 was carried out except that 33 parts by mass of phosphorous acid (phosphonic acid) was used instead of ammonium dihydrogen phosphate, to obtain a dispersion containing phosphited pulp and phosphited cellulose nanocrystals.

The infrared absorption spectra of the resulting phosphite pulp and phosphite cellulose nanocrystals were measured using FT-IR. As a result, absorption due to the P=O of the phosphonic acid group, a tautomer of the phosphite group, was observed around 1210 cm ⁻¹ , confirming the addition of a phosphite group (phosphonic acid group). X-ray diffraction confirmed that the resulting phosphite pulp and phosphite cellulose nanocrystals maintained cellulose type I crystallinity. The amount of phosphite groups (amount of first dissociated acid), measured using the measurement method described below in [Measurement of phosphorus oxo acid group amount], was 1.51 mmol/g for the phosphite pulp and 0.67 mmol/g for the phosphite cellulose nanocrystals. The total amount of dissociated acid was 1.54 mmol/g for the phosphite pulp (amount of second dissociated acid: 0.03 mmol/g) and 0.68 mmol/g for the phosphite cellulose nanocrystals (amount of second dissociated acid: 0.01 mmol/g). Furthermore, the fiber width of the phosphited cellulose nanocrystals was measured using an atomic force microscope and found to be 3 to 20 nm.
In addition, in phosphite pulp and phosphite cellulose nanocrystals, it is thought that the total amount of dissociated acid is larger than the first amount of dissociated acid due to slight oxidation of phosphorous acid and carboxyl groups derived from amorphous hemicellulose.

<Example C1>
The same procedure as in Example A1 was carried out, except that 38 parts by mass of amidosulfonic acid (sulfamic acid) was used instead of ammonium dihydrogen phosphate and the heating time was extended to 20 minutes, to obtain a dispersion containing sulfated pulp and sulfated cellulose nanocrystals.

The infrared absorption spectra of the obtained sulfated pulp and sulfated cellulose nanocrystals were measured using FT-IR. As a result, absorption due to the S=O of sulfate ester groups was observed around 1220-1260 cm ^-1 , confirming the addition of sulfate ester groups. X-ray diffraction confirmed that the obtained sulfated pulp and sulfated cellulose nanocrystals maintained cellulose type I crystal structure. The amount of sulfate ester groups measured using the measurement method described below in "Measurement of sulfur oxoacid group and sulfonic acid group amounts" was 1.47 mmol/g for sulfated pulp and 0.68 mmol/g for sulfated cellulose nanocrystals. Furthermore, the fiber width of the sulfated cellulose nanocrystals was measured using an atomic force microscope and found to be 3-20 nm.

<Example D1>
The raw material pulp used was softwood bleached kraft pulp (NBKP, never dried, Canadian Standard Freeness (CSF) measured in accordance with JIS P 8121-2:2012: 700 mL) manufactured by Oji Paper Co., Ltd. This raw material pulp was subjected to an alkaline TEMPO oxidation treatment as follows.
First, 100 parts by mass of the above raw material pulp (dry mass equivalent), 1.6 parts by mass of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), and 10 parts by mass of sodium bromide were dispersed in 10,000 parts by mass of water. Next, a 13% by mass aqueous solution of sodium hypochlorite was added to 10 mmol per 1.0 g of pulp to initiate the reaction. During the reaction, a 0.5 M aqueous solution of sodium hydroxide was added dropwise to maintain the pH at 10 to 10.5, and the reaction was considered to have ended when no change in pH was observed.
The resulting TEMPO-oxidized pulp was then washed. The pulp slurry after TEMPO oxidation was dehydrated to obtain a dehydrated sheet, to which 5,000 parts by mass of ion-exchanged water was poured, stirred to uniformly disperse the pulp, and then filtered and dehydrated. This process was repeated. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less.

The remaining aldehyde groups in this dehydrated sheet were further oxidized as follows: 100 parts by weight of the dehydrated sheet (dry mass equivalent) was dispersed in 10,000 parts by weight of 0.1 mol/L acetate buffer (pH 4.8). 113 parts by weight of 80% by weight sodium chlorite was then added. The container was immediately sealed and then stirred at 500 rpm using a magnetic stirrer for 48 hours at room temperature to obtain a pulp slurry.
The resulting TEMPO-oxidized pulp was then washed. The washing process consisted of dehydrating the pulp slurry after the additional oxidation to obtain a dehydrated sheet, pouring 5,000 parts by mass of ion-exchanged water into the sheet, stirring to uniformly disperse the pulp, and then filtering and dehydrating the sheet. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less.
Except for using the obtained TEMPO-oxidized pulp, the high-concentration acid hydrolysis treatment and subsequent treatments were carried out in the same manner as in Example A1, to obtain a dispersion containing TEMPO-oxidized cellulose nanocrystals.

X-ray diffraction confirmed that the resulting TEMPO-oxidized pulp and TEMPO-oxidized cellulose nanocrystals maintained cellulose type I crystal structure. The amount of carboxyl groups measured using the method described below was 1.80 mmol/g for the TEMPO-oxidized pulp and 0.86 mmol/g for the TEMPO-oxidized cellulose nanocrystals. Furthermore, the fiber width of the TEMPO-oxidized cellulose nanocrystals was measured using an atomic force microscope and found to be 3 to 20 nm.

<Example E1>
[Hypochlorous acid oxidation]
A sheet (solids concentration 90% by mass) made from softwood bleached kraft pulp (NBKP) was mixed with a hand mixer (Osaka Chemical Co., Ltd., Labo Millser PLUS) at 20,000 rpm for 15 seconds to produce a fluffy fluffed pulp (solids concentration 90% by mass). Sodium hypochlorite pentahydrate was then added to ion-exchanged water to prepare an aqueous solution with a sodium hypochlorite solids concentration of 22% by mass. 9,000 parts by mass of a 22% by mass aqueous sodium hypochlorite solution was added to 100 parts by mass of the fluffy fluffed pulp, and the mixture was allowed to react for 2 hours while adjusting the temperature in a warm bath to 30°C, yielding a carboxyl-introduced pulp. During the reaction, a 1N aqueous sodium hydroxide solution was added as needed to maintain the pH at 11.
The resulting carboxylated pulp was then washed. The washing was performed by pouring ion-exchanged water over the resulting carboxylated pulp to obtain a pulp dispersion, stirring the resulting dispersion to uniformly disperse the pulp, and then repeatedly filtering and dehydrating the pulp. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less.
Except for using the obtained carboxyl group-introduced pulp, the high concentration acid hydrolysis treatment and subsequent treatments were carried out in the same manner as in Example A1 to obtain a dispersion containing carboxyl group-introduced cellulose nanocrystals.

X-ray diffraction confirmed that the resulting carboxylated pulp and carboxylated cellulose nanocrystals maintained cellulose type I crystal structure. The amount of carboxyl groups measured using the method described below was 0.70 mmol/g for the carboxylated pulp and 0.33 mmol/g for the carboxylated cellulose nanocrystals. Furthermore, the fiber width of the carboxylated cellulose nanocrystals was measured using an atomic force microscope and found to be 3 to 20 nm.

<Example F1>
[Maleic acid esterification]
A sheet (solids concentration 90% by mass) made from bleached softwood (NBKP) was mixed with a hand mixer (Labo Millser PLUS, manufactured by Osaka Chemical Co., Ltd.) at 20,000 rpm for 15 seconds to produce a cotton-like fluffing pulp (solids concentration 90% by mass). 100 parts by mass of the cotton-like fluffing pulp and 50 parts by mass of maleic anhydride were placed in an autoclave and mixed at 150°C for 2 hours to obtain a carboxyl-introduced pulp.
The resulting carboxylated pulp was then washed. The washing was performed by pouring ion-exchanged water over the resulting carboxylated pulp to obtain a pulp dispersion, stirring the resulting dispersion to uniformly disperse the pulp, and then repeatedly filtering and dehydrating the pulp. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less.
Except for using the obtained carboxyl group-introduced pulp, the high concentration acid hydrolysis treatment and subsequent treatments were carried out in the same manner as in Example A1 to obtain a dispersion containing carboxyl group-introduced cellulose nanocrystals.

X-ray diffraction confirmed that the resulting carboxylated pulp and carboxylated cellulose nanocrystals maintained cellulose type I crystal structure. The amount of carboxyl groups measured using the method described below was 1.22 mmol/g for the carboxylated pulp and 0.49 mmol/g for the carboxylated cellulose nanocrystals. Furthermore, the fiber width of the carboxylated cellulose nanocrystals was measured using an atomic force microscope and found to be 3 to 20 nm.

<Example G1>
[Carboxyethylation]
The raw material pulp used was softwood bleached kraft pulp (NBKP, solids content 93% by mass, basis weight 245 g/ ^m2 , sheet form, Canadian Standard Freeness (CSF) of 700 mL after disintegration measured in accordance with JIS P 8121-2:2012) manufactured by Oji Paper Co., Ltd.
To 100 parts by mass (bone dry mass) of this raw pulp, a chemical solution consisting of 250 parts by mass of a 12N NaOH aqueous solution, 163 parts by mass of 2-chloropropionic acid, and 140 parts by mass of ion-exchanged water (553 parts by mass in total) was added, yielding a chemical-impregnated pulp. The resulting chemical-impregnated pulp was then heated in a hot air dryer at 165°C for 10 minutes to introduce carboxyethyl groups (carboxy groups) into the cellulose in the pulp, yielding a carboxy-introduced pulp.
The resulting carboxylated pulp was then washed. The washing was performed by pouring ion-exchanged water over the resulting carboxylated pulp to obtain a pulp dispersion, stirring the resulting dispersion to uniformly disperse the pulp, and then repeatedly filtering and dehydrating the pulp. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less.
Next, the washed carboxylated pulp was neutralized as follows: First, the washed carboxylated pulp was diluted with 10 L of ion-exchanged water, and then a 1 N aqueous solution of sodium hydroxide was added little by little while stirring to obtain a carboxylated pulp slurry having a pH of 12 to 13. Next, the carboxylated pulp slurry was dehydrated and washed to obtain a neutralized carboxylated pulp.
Except for using the obtained carboxyl group-introduced pulp, the high concentration acid hydrolysis treatment and subsequent treatments were carried out in the same manner as in Example A1 to obtain a dispersion containing carboxyl group-introduced cellulose nanocrystals.

X-ray diffraction confirmed that the resulting carboxylated pulp and carboxylated cellulose nanocrystals maintained cellulose type I crystal structure. The amount of carboxyl groups measured using the method described below was 1.41 mmol/g for the carboxylated pulp and 0.65 mmol/g for the carboxylated cellulose nanocrystals. Furthermore, the fiber width of the carboxylated cellulose nanocrystals was measured using an atomic force microscope and found to be 3 to 20 nm.

<Example H1>
[Carboxymethylation]
As the raw material pulp, softwood kraft pulp (solid content 93% by mass, basis weight 245 g/ ^m2 sheet form, Canadian Standard Freeness (CSF) measured after disintegration in accordance with JIS P 8121-2:2012 of 700 mL) manufactured by Oji Paper Co., Ltd. was used.
To 100 parts by mass (bone dry mass) of this raw pulp, a chemical solution consisting of 83 parts by mass of 12N NaOH aqueous solution, 175 parts by mass of sodium monochloroacetate, and 313 parts by mass of ion-exchanged water (571 parts by mass in total) was added to obtain a chemical-impregnated pulp. The obtained chemical-impregnated pulp was then placed in a polyethylene container and heated in a warm bath at 95°C for 60 minutes to introduce carboxymethyl groups (carboxy groups) into the cellulose in the pulp, thereby obtaining a carboxy-introduced pulp.
The resulting carboxylated pulp was then washed. The washing was performed by pouring ion-exchanged water over the resulting carboxylated pulp to obtain a pulp dispersion, stirring the resulting dispersion to uniformly disperse the pulp, and then repeatedly filtering and dehydrating the pulp. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less.
Except for using the obtained carboxyl group-introduced pulp, the high concentration acid hydrolysis treatment and subsequent treatments were carried out in the same manner as in Example A1 to obtain a dispersion containing carboxyl group-introduced cellulose nanocrystals.

X-ray diffraction confirmed that the resulting carboxylated pulp and carboxylated cellulose nanocrystals maintained cellulose type I crystal structure. The amount of carboxyl groups measured using the method described below was 1.21 mmol/g for the carboxylated pulp and 0.41 mmol/g for the carboxylated cellulose nanocrystals. Furthermore, the fiber width of the carboxylated cellulose nanocrystals was measured using an atomic force microscope and found to be 3 to 20 nm.

<Example J1>
[Sulfoethylation]
The raw material pulp used was softwood bleached kraft pulp (NBKP, solids content 93% by mass, basis weight 245 g/ ^m2 , sheet form, Canadian Standard Freeness (CSF) of 700 mL after disintegration measured in accordance with JIS P 8121-2:2012) manufactured by Oji Paper Co., Ltd.
To 100 parts by mass (bone dry mass) of this raw pulp, a chemical solution consisting of 180 parts by mass of a 2N NaOH aqueous solution and 780 parts by mass of a 25% by mass sodium vinyl sulfonate aqueous solution (total 960 parts by mass) was added, yielding a chemical solution-impregnated pulp. The resulting chemical solution-impregnated pulp was then heated in a hot air dryer at 165°C for 16 minutes to introduce sulfoethyl groups (sulfonic groups) into the cellulose in the pulp, yielding a sulfoethyl group-introduced pulp (sulfonic group-introduced pulp).
The resulting sulfoethyl group-introduced pulp was then washed. The washing treatment involved pouring ion-exchanged water over the resulting sulfoethyl group-introduced pulp to obtain a pulp dispersion, which was then stirred to uniformly disperse the pulp, followed by filtration and dehydration. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less.
Except for using the obtained sulfoethyl group-introduced pulp, the high concentration acid hydrolysis treatment and subsequent treatments were carried out in the same manner as in Example A1 to obtain a dispersion containing sulfoethyl group-introduced cellulose nanocrystals.

X-ray diffraction confirmed that the resulting sulfoethyl-group-introduced pulp and sulfoethyl-group-introduced cellulose nanocrystals maintained cellulose type I crystal structure. The sulfoethyl group content (sulfonic acid content), measured using the method described below, was 1.48 mmol/g for the sulfoethyl-group-introduced pulp and 0.71 mmol/g for the sulfoethyl-group-introduced cellulose nanocrystals. Furthermore, the fiber width of the sulfoethyl-group-introduced cellulose nanocrystals was measured using an atomic force microscope and found to be 3 to 20 nm.

<Example K1>
[Cationization treatment]
The raw material pulp used was softwood bleached kraft pulp (NBKP, solids content 93% by mass, basis weight 245 g/ ^m2 , sheet form, Canadian Standard Freeness (CSF) of 700 mL measured after disintegration in accordance with JIS P 8121-2:2012) manufactured by Oji Paper Co., Ltd.
To 100 parts by mass (bone dry mass) of this raw pulp, a chemical solution consisting of 180 parts by mass of a 1N NaOH aqueous solution and 325 parts by mass of a cationizing agent (Catiomaster G, manufactured by Yokkaichi Synthetic Co., Ltd., glycidyl trimethylammonium chloride, purity 73.1% by mass, moisture content 20.2% by mass) (total 505 parts by mass) was added to obtain a chemical solution-impregnated pulp. The obtained chemical solution-impregnated pulp was then heated in a hot air dryer at 165°C for 12 minutes to introduce cationic groups into the cellulose in the pulp, thereby obtaining a cationic group-introduced pulp.
The resulting cation-group-introduced pulp was then washed. The resulting cation-group-introduced pulp was poured into ion-exchanged water to obtain a pulp dispersion. The resulting pulp was stirred to uniformly disperse the pulp, followed by filtration and dehydration. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less.
Next, the washed cationic group-introduced pulp was neutralized as follows: First, the washed cationic group-introduced pulp was diluted with 10 L of ion-exchanged water, and then 1 N hydrochloric acid was added little by little while stirring to obtain a cationic group-introduced pulp slurry having a pH of 1 to 2. Next, the cationic group-introduced pulp slurry was dehydrated and washed to obtain a cationic group-introduced pulp that had been subjected to a neutralization treatment.

To the obtained cationized pulp, concentrated hydrochloric acid (11.2 mol/L hydrochloric acid) was added little by little to obtain a slurry in which the cationized pulp was suspended at 2% by mass in 10.7 mol/L hydrochloric acid. This suspension slurry was a viscous liquid with spinnability. The obtained suspension slurry was placed in a glass container and, while stirring, was subjected to thermal hydrolysis treatment in a 95°C hot bath for 90 minutes. (High-concentration acid hydrolysis treatment)
The treatments after the high-concentration acid hydrolysis treatment were carried out in the same manner as in Example A1, to obtain a dispersion containing cellulose nanocrystals with introduced cationic groups.

X-ray diffraction confirmed that the resulting cationic group-introduced pulp and cationic group-introduced cellulose nanocrystals maintained cellulose type I crystal structure. The amount of cationic groups measured using the method described below was 1.45 mmol/g for the cationic group-introduced pulp and 0.71 mmol/g for the cationic group-introduced cellulose nanocrystals. Furthermore, the fiber width of the cationic group-introduced cellulose nanocrystals was measured using an atomic force microscope and found to be 3 to 20 nm.

<Comparative Example 1>
A dispersion containing cellulose nanocrystals was obtained in the same manner as in Example A1, except that softwood bleached kraft pulp (NBKP, solids content 93% by mass, basis weight 245 g/m, ^sheet form, Canadian Standard Freeness (CSF) of 700 mL after disintegration measured in accordance with JIS P 8121-2:2012) manufactured by Oji Paper Co., Ltd. was used instead of phosphorylated pulp.
The amount of phosphate groups in the resulting cellulose nanocrystals was less than 0.01 mmol/g, and the fiber width was measured using an atomic force microscope to be 10 to 30 nm.

<Comparative Example 2>
To the dispersion containing cellulose nanocrystals obtained in Comparative Example 1 (solid content concentration 2%), 85% phosphoric acid was added until the phosphoric acid concentration reached 10.7 mol/L, and then the mixture was treated in a warm bath at 95 °C for 90 minutes. The resulting post-reaction slurry was purified by removing dissolved components and acid using the following procedure. That is, the resulting post-reaction slurry was ice-cooled and subjected to solid-liquid separation by centrifugation (12,000 G, 5 minutes). After centrifugation, the supernatant was removed, and the same volume of ion-exchanged water was added. The precipitate was thoroughly mixed and dispersed, and then centrifuged again. This operation was repeated three times, and the precipitate was again suspended in ion-exchanged water and contacted with a strongly basic ion-exchange resin (Amberjet 4400; Organo Corporation, conditioned). The treatment was continued until the electrical conductivity and pH of the slurry stabilized.
Ion-exchanged water was added to the purified slurry to prepare a slurry with a solids concentration of 2% by mass, which was then processed six times at a pressure of 100 MPa in a wet atomizer (Beryu-Mini, manufactured by Biryu Co., Ltd.) to obtain a dispersion containing phosphorylated cellulose nanocrystals.

The infrared absorption spectrum of the obtained phosphorylated cellulose nanocrystals was measured using FT-IR. As a result, absorption due to the P=O of the phosphate group was observed around 1230 cm ^-1 , confirming the addition of phosphate groups. X-ray diffraction confirmed that cellulose type I crystals were maintained. The amount of phosphate groups (amount of first dissociated acid) measured by the measurement method described below in [Measurement of amount of phosphorus oxo acid group] was 0.24 mmol/g. The total amount of dissociated acid was 0.40 mmol/g (amount of second dissociated acid was 0.16 mmol/g). The fiber width of the cellulose nanocrystals was measured using an atomic force microscope and was found to be 10 to 20 nm.

<Comparative Example 3>
The dispersion containing cellulose nanocrystals (solids concentration 2%) obtained in Comparative Example 1 was left to stand in a constant temperature and humidity chamber (23°C, 50% RH) and air-dried until the solids concentration reached 42%. 24 parts by mass of the resulting cellulose nanocrystals with a solids concentration of 42% and 51 parts by mass of 85% phosphoric acid were added to 64 parts by mass of urea melted in a glass flask placed in an oil bath at 140°C. The mixture was then heated to 150°C and heat-treated for 30 minutes.
After the heat treatment, 500 parts by mass of water was added to obtain a post-reaction slurry. This slurry was subjected to the same purification procedure as in Comparative Example 2 and treatment with a wet atomizer to obtain a dispersion containing phosphorylated cellulose nanocrystals.

The infrared absorption spectrum of the obtained phosphorylated cellulose nanocrystals was measured using FT-IR. As a result, absorption due to the P=O of the phosphate group was observed around 1230 cm ^-1 , confirming the addition of phosphate groups. X-ray diffraction confirmed that cellulose type I crystals were maintained. The amount of phosphate groups (amount of first dissociated acid) measured by the measurement method described below in [Measurement of amount of phosphorus oxo acid group] was 1.03 mmol/g. The total amount of dissociated acid was 1.77 mmol/g (amount of second dissociated acid 0.74 mmol/g). The fiber width of the cellulose nanocrystals was measured using an atomic force microscope and was found to be 3 to 20 nm.

<Reference example 1>
The dispersion containing cellulose nanocrystals obtained in Comparative Example 1 (solid content concentration: 2%) was subjected to alkaline TEMPO oxidation treatment as follows.
First, 100 parts by weight of the above cellulose nanocrystals (dry mass equivalent), 1.6 parts by weight of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), and 10 parts by weight of sodium bromide were dispersed in 10,000 parts by weight of water. Next, a 13% by weight aqueous solution of sodium hypochlorite was added to 1.0 g of pulp to give a concentration of 2 mmol, to initiate the reaction. During the reaction, a 0.5 M aqueous solution of sodium hydroxide was added dropwise to maintain the pH at 10 to 10.5, and the reaction was considered complete when no further change in pH was observed.
The resulting slurry was subjected to the same purification procedure as in Comparative Example 2 and treatment with a wet atomizer to obtain a dispersion containing TEMPO-oxidized cellulose nanocrystals.

X-ray diffraction confirmed that the resulting TEMPO-oxidized pulp and TEMPO-oxidized cellulose nanocrystals maintained cellulose type I crystal structure. The amount of carboxyl groups in the TEMPO-oxidized cellulose nanocrystals, as measured by the method described below, was 0.86 mmol/g. Furthermore, the fiber width of the TEMPO-oxidized cellulose nanocrystals was measured using an atomic force microscope and found to be 3 to 20 nm.

<Comparative Example 4>
The raw material pulp used was softwood bleached kraft pulp (NBKP, solids content 93% by mass, basis weight 245 g/ ^m2 , sheet form, Canadian Standard Freeness (CSF) of 700 mL after disintegration measured in accordance with JIS P 8121-2:2012) manufactured by Oji Paper Co., Ltd.
This raw pulp was subjected to phosphorus oxo-oxidation treatment as follows. First, a mixed aqueous solution of ammonium dihydrogen phosphate and urea was added to 100 parts by mass (bone dry mass) of the raw pulp to adjust the composition to 45 parts by mass of ammonium dihydrogen phosphate, 120 parts by mass of urea, and 150 parts by mass of water, to obtain a chemical-impregnated pulp. Next, the obtained chemical-impregnated pulp was heated in a hot air dryer at 165°C for 250 seconds to introduce phosphate groups into the cellulose in the pulp, thereby obtaining a phosphorylated pulp.
The resulting phosphorylated pulp was then washed. The washing process consisted of pouring 10 L of ion-exchanged water into 100 g (bone dry mass) of phosphorylated pulp to obtain a pulp dispersion, stirring the resulting solution to uniformly disperse the pulp, and then repeatedly filtering and dehydrating the pulp. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less.

The washed phosphorylated pulp was mixed with ion-exchanged water and 85% phosphoric acid to adjust the solids concentration to 2% and the phosphoric acid concentration to 1 mol/L. The mixture was then heated and hydrolyzed in a 95°C hot bath for 90 minutes while stirring. (Dilute Acid Hydrolysis Treatment)

Next, the resulting hydrolyzed phosphorylated pulp was washed. The washing process was carried out by pouring 10 L of ion-exchanged water over 100 g (bone dry mass) of phosphorylated pulp to obtain a pulp dispersion, stirring the resulting solution to uniformly disperse the pulp, and then repeatedly filtering and dehydrating the pulp. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less.

Next, the washed phosphorylated pulp was neutralized as follows: First, the washed phosphorylated pulp was diluted with 10 L of ion-exchanged water, and then a 1 N aqueous solution of sodium hydroxide was added little by little while stirring to obtain a phosphorylated pulp slurry having a pH of 12 to 13. Next, the phosphorylated pulp slurry was dehydrated and washed to obtain a neutralized phosphorylated pulp.
Ion-exchanged water was added to the resulting phosphorylated pulp to prepare a slurry with a solids concentration of 2% by mass. This slurry was processed six times at a pressure of 100 MPa in a wet atomizer (Beryu-Mini, manufactured by Biryu Co., Ltd.), yielding a dispersion primarily containing phosphorylated cellulose nanofibers.

The infrared absorption spectrum of the obtained phosphorylated cellulose nanofibers was measured using FT-IR. As a result, absorption due to the P=O of the phosphate group was observed around 1230 cm ^-1 , confirming the addition of phosphate groups. Furthermore, when the obtained phosphorylated cellulose nanofibers were analyzed using an X-ray diffractometer, typical peaks were observed at two positions, around 2θ = 14° to 17° and around 2θ = 22° to 23°, confirming the presence of cellulose type I crystals. The amount of phosphate groups (amount of first dissociated acid) measured using the measurement method described below in [Measurement of phosphorus oxoacid group amount] was 0.93 mmol/g. The total amount of dissociated acid was 1.56 mmol/g (amount of second dissociated acid was 0.63 mmol/g). Furthermore, the fiber width of the cellulose nanofibers was measured using an atomic force microscope and found to be 3 to 5 nm.

Comparative Example 5
A dispersion mainly containing phosphorylated cellulose nanofibers was obtained in the same manner as in Comparative Example 4, except that the dilute acid hydrolysis treatment and the washing treatment after the dilute acid hydrolysis treatment were performed on softwood bleached kraft pulp before phosphorylation, rather than on phosphorylated pulp after washing.

The infrared absorption spectrum of the obtained phosphorylated cellulose nanofibers was measured using FT-IR. As a result, absorption due to the P=O of the phosphate group was observed around 1230 cm ^-1 , confirming the addition of phosphate groups. Furthermore, when the obtained phosphorylated cellulose nanofibers were analyzed using an X-ray diffractometer, typical peaks were observed at two positions, around 2θ = 14° to 17° and around 2θ = 22° to 23°, confirming the presence of cellulose type I crystals. The amount of phosphate groups (amount of first dissociated acid) measured using the measurement method described below in [Measurement of phosphorus oxoacid group amount] was 1.45 mmol/g. The total amount of dissociated acid was 2.45 mmol/g (amount of second dissociated acid was 1.00 mmol/g). Furthermore, the fiber width of the cellulose nanofibers was measured using an atomic force microscope and found to be 3 to 5 nm.

<Comparative Example 6>
Ion-exchanged water was added to the neutralized phosphorylated pulp obtained in Example A1 to prepare a slurry with a solids concentration of 2% by mass. This slurry was processed six times in a wet atomization apparatus (Beryu-Mini, manufactured by Biryu Co., Ltd.) at a pressure of 100 MPa to obtain a dispersion containing phosphorylated cellulose nanofibers.

An enzyme (cellulase) was added to the resulting phosphorylated cellulose nanofiber dispersion at a concentration of 550 nkat per 1 g of phosphorylated cellulose nanofiber, and the mixture was treated with the enzyme at 50°C (Step D). The temperature of the resulting dispersion was raised to 100°C to thermally inactivate the enzyme. This slurry was then treated six times at a pressure of 100 MPa in a wet micronization device (Beryu-Mini, manufactured by Biryu Co., Ltd.) to obtain a dispersion containing enzyme-treated phosphorylated cellulose nanofibers. The amount of phosphate groups (amount of first dissociated acid) measured using the method described below in [Measurement of phosphorus oxoacid group amount] was 1.45 mmol/g. The total amount of dissociated acid was 2.45 mmol/g.

[Sheet production]
The various cellulose nanocrystals obtained in Examples A1 to A8, B1, C1, D1, E1, F1, G1, H1, J1, and K1 were diluted with ion-exchanged water to a solids concentration of 0.6% by mass. The mixed solution was weighed so that the finished sheet would have a basis weight of 70 g/ ^m² and spread on a commercially available acrylic plate. A damming frame (inner dimensions 250 mm x 250 mm, height 5 cm) was placed on the acrylic plate to achieve the desired basis weight. The cellulose nanocrystals were then dried in a dryer at 70°C for 24 hours and peeled off from the acrylic plate. Sheets containing cellulose nanocrystals were obtained from each of the cellulose nanocrystals. The sheet thickness was 50 μm.

[Powder preparation]
The 2% by mass dispersions of the various cellulose nanocrystals obtained in Examples A1 to A8, B1, C1, D1, E1, F1, G1, H1, J1, and K1 were spray-dried using a spray dryer set at an outlet temperature of 150°C. Powders containing cellulose nanocrystals were obtained from all of the cellulose nanocrystals. The particle size of the powders was 20 μm or less, and the moisture content was 20% or less.

[evaluation]
<Measurement of phosphorus oxoacid group content>
The amount of phosphorus oxoacid groups in cellulose nanocrystals, cellulose nanofibers, and cellulose fibers with introduced ionic functional groups was measured by adding ion-exchanged water to a dispersion containing the various celluloses of interest (various celluloses are a general term for cellulose nanocrystals, cellulose nanofibers, and cellulose fibers with introduced ionic functional groups) to make the content 0.2 mass%, treating the dispersion with ion-exchange resin, and then titrating with alkali.
The treatment with ion exchange resin was carried out by adding 1/10 by volume of a strongly acidic ion exchange resin (Amberjet 1024; manufactured by Organo Corporation, conditioned) to the above-mentioned various celluloses, shaking the mixture for 1 hour, and then pouring the mixture onto a mesh with 90 μm openings to separate the resin from the slurry.
In addition, alkali titration was performed by adding 10 μL of 0.1 N aqueous sodium hydroxide solution every 5 seconds to various cellulose-containing slurries after treatment with ion exchange resin, while measuring the change in the pH value of the slurry. Nitrogen gas was blown into the slurry 15 minutes before the start of the titration. In this neutralization titration, two maximum points of increment (differential value of pH with respect to the amount of alkali added) were observed on the curve plotting the measured pH against the amount of alkali added. Of these, the maximum point of increment obtained first after starting the addition of alkali is called the first endpoint, and the maximum point of increment obtained next is called the second endpoint (Figure 1). The amount of alkali required from the start of titration to the first endpoint is equal to the amount of first dissociated acid in the slurry used in the titration. Furthermore, the amount of alkali required from the start of titration to the second endpoint is equal to the total amount of dissociated acid in the slurry used in the titration. The amount of alkali (mmol) required from the start of titration to the first endpoint divided by the solids content (g) in the titrated slurry was used to determine the amount of phosphorus oxo acid group (first dissociated acid amount) (mmol/g). The amount of alkali (mmol) required from the start of titration to the second endpoint divided by the solids content (g) in the titrated slurry was used to determine the total dissociated acid amount (mmol/g). The second dissociated acid amount was determined by subtracting the first dissociated acid amount from the total dissociated acid amount.

<Measurement of Carboxy Group Amount>
The amount of carboxyl groups in cellulose nanocrystals, cellulose nanofibers, and cellulose fibers with ionic functional groups introduced therein was measured by adding ion-exchanged water to various cellulose dispersions containing the target cellulose to adjust the content to 0.2 mass%, treating the dispersions with ion-exchange resin, and then titrating them with alkali.
The treatment with ion exchange resin was carried out by adding 1/10 by volume of a strongly acidic ion exchange resin (Amberjet 1024; manufactured by Organo Corporation, conditioned) to 0.2% by mass of each cellulose-containing slurry, shaking for 1 hour, and then pouring the mixture onto a mesh with 90 μm openings to separate the resin from the slurry.
In addition, the alkali titration was performed by adding 0.1 N aqueous sodium hydroxide to the fibrous cellulose-containing slurry after treatment with an ion exchange resin, while measuring the change in the pH value of the slurry. Observing the change in pH while adding aqueous sodium hydroxide yields a titration curve as shown in Figure 2. As shown in Figure 2, in this neutralization titration, a single point is observed in the curve plotting the measured pH against the amount of alkali added, where the increment (the differential value of pH with respect to the amount of alkali added) is maximized. This maximum increment is called the first end point. Here, the region from the start of titration to the first end point in Figure 2 is called the first region. The amount of alkali required in the first region is equal to the amount of carboxyl groups in the slurry used for titration. The amount of alkali (mmol) required in the first region of the titration curve was divided by the solids content (g) in the various cellulose-containing slurries being titrated to calculate the amount of carboxyl groups introduced (mmol/g).
The above-mentioned amount of carboxyl groups introduced (mmol/g) indicates the amount of substituents per 1 g of fibrous cellulose mass when the counter ion of the carboxyl group is a hydrogen ion (H ⁺ ) (hereinafter referred to as the amount of carboxyl groups (acid type)).

<Measurement of sulfur oxoacid group and sulfonic acid group content>
The amounts of sulfur oxoacid groups and sulfonic groups were measured for the pulp and cellulose nanocrystals obtained in Examples C1 and J1. Specifically, the obtained cellulose samples were wet incinerated using perchloric acid and concentrated nitric acid, diluted appropriately, and then subjected to ICP atomic emission spectrometry to measure the amount of sulfur. The sulfur amount was divided by the bone-dry mass of each cellulose sample to determine the amount of sulfur oxoacid groups and sulfonic groups (unit: mmol/g).

<Measurement of Amount of Cationic Groups>
The amount of cationic groups was measured for the pulp and cellulose nanocrystals obtained in Example K1. Specifically, the samples after freeze-drying and pulverization were subjected to measurement using a TN-110 trace total nitrogen analyzer manufactured by Mitsubishi Chemical Analytech Corporation. Note that ionic nitrogen was removed during the neutralization and washing processes. The amount of cationic groups introduced per unit mass of each cellulose (mmol/g) was calculated by dividing the nitrogen content per unit mass of each cellulose (g/g) obtained by trace nitrogen analysis by the atomic weight of nitrogen.

<Measurement of carbamide group amount>
The amount of carbamide groups was measured for the pulp, cellulose nanocrystals, and cellulose nanofibers obtained in Examples A1 to A8, B1, and C1 and Comparative Examples 3 to 6. Specifically, the samples after freeze-drying and pulverization were subjected to measurement using a TN-110 trace total nitrogen analyzer manufactured by Mitsubishi Chemical Analytech Corporation. Note that ionic nitrogen was removed during the neutralization and washing processes. The amount of carbamide groups introduced per unit mass of each cellulose (mmol/g) was calculated by dividing the nitrogen content (g/g) per unit mass of each cellulose obtained by trace nitrogen analysis by the atomic weight of nitrogen.

<Measurement of Viscosity of Dispersion>
First, various cellulose nanocrystal and cellulose nanofiber dispersions were diluted with ion-exchanged water to a solids concentration of 1.5% by mass, and then stirred in a disperser at 1500 rpm for 5 minutes. The viscosity of the resulting dispersion was then measured using a Brookfield analog viscometer (T-LVT). The measurement conditions were a rotation speed of 3 rpm, and the viscosity value 3 minutes after the start of measurement was taken as the viscosity of the dispersion. Furthermore, the dispersions to be measured were allowed to stand for 24 hours in an environment of 23°C and 50% relative humidity before measurement. The liquid temperature of the dispersions during measurement was 23°C.

<Measurement of total light transmittance of dispersion>
The total light transmittance of the dispersion was measured by diluting various cellulose dispersions with ion-exchanged water to 0.2% by mass, and then measuring using a haze meter (HM-150, manufactured by Murakami Color Research Laboratory Co., Ltd.) and a glass cell for liquids with an optical path length of 1 cm (MG-40, reverse optical path, manufactured by Fujiwara Seisakusho Co., Ltd.) in accordance with 7361-1:1997. Zero-point measurement was performed using ion-exchanged water placed in the same glass cell. In other words, the total light transmittance measured using ion-exchanged water alone was taken as 100%. Furthermore, the dispersions to be measured were allowed to stand for 24 hours in an environment of 23°C and 50% relative humidity before measurement. The liquid temperature of the dispersions during measurement was 23°C.

<Salt tolerance>
A predetermined amount of sodium chloride was added to the resulting 0.2% by mass dispersion of cellulose nanocrystals and cellulose nanofibers, and the total light transmittance was measured. Dispersions in which the total light transmittance was least affected by the addition of sodium chloride were determined to have high salt tolerance. Specifically, the evaluation was based on the following criteria:
A: When the sodium chloride concentration is 300 mmol/L, T/T ₀ is 0.98 or more. B: When the sodium chloride concentration is 300 mmol/L, T/T ₀ is less than 0.98, but when the sodium chloride concentration is 100 mmol/L, T/T ₀ is 0.98 or more. C: When the sodium chloride concentration is 100 mmol/L, T/T ₀ is less than 0.98. Here, T: total light transmittance after adding sodium chloride, T ₀ : total light transmittance before adding sodium chloride.
The specific procedure for measuring the total light transmittance after adding sodium chloride is as follows.
(1) A predetermined amount of solid sodium chloride is added to a 0.2% by mass dispersion of cellulose nanocrystals and cellulose nanofibers. (2) The mixture is manually stirred with a Teflon (registered trademark) rod until the sodium chloride is dissolved. (3) After dissolving the sodium chloride, the mixture is stirred at 4000 rpm for 1 minute in a rotary disperser. (4) The total light transmittance is measured using the method described in <Measurement of the total light transmittance of the dispersion>.

<Presence or absence of alterations other than at C6 position>
The resulting cellulose nanocrystal and cellulose nanofiber dispersions were freeze-dried and the resulting samples were analyzed by ^13C CP/MAS NMR. When compared with the results of the sample of Comparative Example 1, if changes were observed in the chemical shift peaks at 68-80 ppm corresponding to the C2, C3, and C5 positions, 80-91 ppm corresponding to the C4 position adjacent to the C3 position, and 101-109 ppm corresponding to the C1 position adjacent to the C2 position, it was deemed that modifications had occurred at positions other than the C6 position.

<Purity>
The purity of the obtained cellulose nanocrystals and cellulose nanofibers was judged based on whether or not they contained functional groups other than ionic functional groups, and on the coloration that occurred upon wet heating.
A: Contains no functional groups other than ionic functional groups, or contains functional groups other than ionic functional groups but is barely discolored by wet heating B: Contains functional groups other than ionic functional groups and becomes slightly yellowish by wet heating C: Contains functional groups other than ionic functional groups and becomes strong yellow to light brown by wet heating Note that discoloration during wet heating was determined by visually observing the degree of discoloration when a 1% by mass concentration of cellulose nanocrystal or cellulose nanofiber dispersion was heated at 95°C for 10 hours. Nitrogen-containing structures that can liberate ammonia by hydrolysis of carbamide groups, etc. are thought to promote the formation of colored structures.

<Bending of fibers>
Images of cellulose nanocrystals and cellulose nanofibers were taken using an atomic force microscope, and 100 randomly selected fibers were counted to see if they had one or more bends (angles of 30° or more) along the fiber. The results were evaluated based on the following criteria.
A: There are less than 10 bent fibers. B: There are 10 or more but less than 20 bent fibers. C: There are 20 or more bent fibers.

For the ionic functional group-introduced cellulose fibers, ionic functional group-introduced cellulose nanofibers, and ionic functional group-introduced cellulose nanocrystals listed in Tables 1-1 and 1-2, when the amount of ionic functional groups introduced was measured by the above-mentioned <Measurement of the amount of phosphorus oxo acid groups>, the results of the measurement for the amount of first dissociated acid and the amount of second dissociated acid were entered in the respective columns. Furthermore, when the amount of ionic functional groups was measured by the above-mentioned <Measurement of the amount of carboxyl groups> or <Measurement of the amount of sulfur oxo acid groups and sulfonic groups>, the results of the measurement were entered in the column for the amount of first dissociated acid. Furthermore, when the amount of ionic functional groups introduced was measured by the above-mentioned <Measurement of the amount of cationic groups>, the results of the measurement were entered in the column for the amount of cationic groups.

According to Tables 1-1 and 1-2, the cellulose nanocrystals into which ionic functional groups have been introduced, obtained by the production method having steps A to C in this order, have low viscosity, high transparency, and excellent salt resistance when dispersed in water. They also have high purity.
On the other hand, as shown in the comparative examples, Comparative Example 1, which did not include the ionic functional group introduction step, was inferior in transparency and salt resistance. Furthermore, as shown in Comparative Examples 2 and 3, when ionic functional groups were introduced after defibration, the amount of ionic functional groups introduced was low, resulting in poor transparency and salt resistance (Comparative Example 2) or poor purity (Comparative Example 3).
Furthermore, as shown in Comparative Examples 4 and 5, when the concentration of the acidic aqueous solution used for hydrolysis in step B was less than 5 mol/L, the viscosity of the aqueous solution was high. Furthermore, as shown in Comparative Example 6, when step B was not performed, the viscosity of the aqueous solution was high even after enzyme treatment.

Claims (6)

A method for producing ionic functional group-introduced cellulose nanocrystals, comprising the following steps A to C in this order:
Step A: An ionic functional group introduction step for introducing ionic functional groups into cellulose fibers. Step B: A hydrolysis step for hydrolyzing the cellulose fibers into which ionic functional groups have been introduced using an acidic aqueous solution having a concentration of 5 mol/L or more and 30 mol/L or less. Step C: A defibration step for defibrating the hydrolyzed cellulose fibers into which ionic functional groups have been introduced.
In step A, the amount of ionic functional groups introduced into the cellulose fibers is 0.50 mmol/g or more and 5.20 mmol/g or less,
The amount of ionic functional groups introduced into the ionic functional group-introduced cellulose nanocrystal is 0.10 mmol/g or more and 5.00 mmol/g or less.
A method for producing cellulose nanocrystals with ionic functional groups . The manufacturing method according to claim 1, wherein the ionic functional group is at least one selected from the group consisting of a phosphorus oxoacid group, a group derived from a phosphorus oxoacid group, a carboxy group, a sulfone group, a sulfur oxoacid group, a group derived from a sulfur oxoacid group, and a cationic functional group. The manufacturing method described in claim 1 or 2, wherein the ionic functional group is at least one selected from the group consisting of a phosphorus oxoacid group and a group derived from a phosphorus oxoacid group. The method according to any one of claims 1 to 3, wherein step A is carried out in the presence of at least one selected from urea and its derivatives. The method according to any one of claims 1 to 4 , wherein the amount of carbamide groups in the cellulose nanocrystals into which ionic functional groups have been introduced is 0.06 mmol/g or less. The production method according to any one of claims 1 to 5 , wherein the acidic aqueous solution used in step B is an aqueous solution of an acid compound selected from the group consisting of phosphoric acid, sulfuric acid, diphosphorus pentoxide, and hydrochloric acid.