Ionic functional group-introduced cellulose nanocrystals and method of producing the same, and dispersion liquids, sheets, and powders containing the same

Abstract

To provide cellulose nanocrystals which, when made into aqueous dispersion liquids, have low viscosity, are highly transparent, have excellent salinity tolerance, and have a large amount of ionic functional groups introduced thereinto, and a method of producing the same, and further to provide dispersion liquids, sheets, and powders containing the cellulose nanocrystals.SOLUTION: Provided is a method of producing an ionic functional group-introduced cellulose nanocrystal, comprising the following steps A to C in this order. Step A: Ionic functional group introduction step of introducing ionic functional groups into cellulose fibers. Step B: Hydrolysis step of hydrolyzing the cellulose fibers, into which ionic functional groups are introduced, with an acidic aqueous solution having a concentration of 5 mol / L or more. C: Fibrillation step of fibrillating cellulose fibers into which hydrolyzed ionic functional groups are introduced.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to cellulose nanocrystals having ionic functional groups introduced therein and a method for producing the same, as well as a dispersion, a sheet, and a powder containing the cellulose nanocrystals having ionic functional groups introduced therein. [Background technology]

[0002] 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, especially wood-derived fibrous cellulose (pulp), has been widely used mainly in paper products.

[0003] 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. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2011 / 118746 [Non-patent literature]

[0005] [Non-Patent Document 1] Biomacromolecules, vol.14, 1223-1230, 2013 [Non-patent document 2] Carbohydrate Polymers, vol.125, 301-313, 2015 Summary of the Invention [Problem to be solved by the invention]

[0006] As described in Patent Document 1, the shortened nanocellulose fibers did not sufficiently reduce the viscosity when dispersed in water. Although cellulose nanocrystals exhibit 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. 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) introduces more ionic functional groups than method (i), it is believed that a large amount of unintended functional groups (carbamide groups) is also present. Furthermore, both methods (i) and (ii) require the introduction of ionic functional groups into the cellulose nanocrystals, which 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. [Means for solving the problem]

[0007] 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 provides the following <1> ~ <19> Regarding. <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: Ionic functional group introduction step for introducing ionic functional groups into cellulose fibers Step B: A hydrolysis step in which the cellulose fiber to which ionic functional groups have been introduced is hydrolyzed using an acidic aqueous solution with a concentration of 5 mol / L or more. Step C: A defibration step for defibrating the cellulose fibers to which hydrolyzed ionic functional groups have been introduced. <2> 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; <1> The manufacturing method described in <3> 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; <1> or <2> The manufacturing method described in <4> In step A, the amount of ionic functional groups introduced into the cellulose fibers is 0.5 mmol / g or more. <1> ~ <3> 1. The manufacturing method according to any one of the preceding claims. <5> The amount of ionic functional groups introduced into the ionic functional group-introduced cellulose nanocrystal is 0.1 mmol / g or more. <1> ~ <4> 1. The manufacturing method according to any one of the preceding claims. <6> The amount of carbamide groups in the cellulose nanocrystals having ionic functional groups introduced therein is 0.06 mmol / g or less. <1> ~ <5> 1. The manufacturing method according to any one of the preceding claims. <7> 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. <1> ~ <6> 1. The manufacturing method according to any one of the preceding claims. <8> Ionically functionalized cellulose nanocrystals with 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 carboxyl group has been introduced). <9> 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; <8> The ionic functionalized cellulose nanocrystals according to claim 1. <10> 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; <8> or <9> The ionic functionalized cellulose nanocrystals according to claim 1. <11> The amount of the ionic functional group introduced is 0.5 mmol / g or more. <8> ~ <10> 1. An ionic functionalized cellulose nanocrystal according to any one of claims 1 to 9. <12> The ionic functional group is introduced into at least one of the carbon atoms at the 2nd and 3rd positions of cellulose. <8> ~ <11> 1. An ionic functionalized cellulose nanocrystal according to any one of claims 1 to 9. <13> The amount of carbamide groups in the cellulose nanocrystals having ionic functional groups introduced therein is 0.06 mmol / g or less. <8> ~ <12> 1. An ionic functionalized cellulose nanocrystal according to any one of claims 1 to 9. <14> The viscosity of a 1.5 mass% aqueous dispersion of the ionic functional group-introduced cellulose nanocrystal is 100 mPa s or less. <8> ~ <13> 1. An ionic functionalized cellulose nanocrystal according to any one of claims 1 to 9. <15> The total light transmittance of a 0.2 mass% aqueous dispersion of the ionic functional group-introduced cellulose nanocrystal is 95.5% or more. <8> ~ <14> 1. An ionic functionalized cellulose nanocrystal according to any one of claims 1 to 9. <16> When sodium chloride is added to a 0.2 mass% aqueous dispersion of the ionic functional group-introduced cellulose nanocrystal so as to give a concentration of 100 mmol / L, the total light transmittance after the addition of sodium chloride is T and the total light transmittance before the addition of sodium chloride is T0, and T / T0 is 0.98 or more. <8> ~ <15> 1. An ionic functionalized cellulose nanocrystal according to any one of claims 1 to 9. <17> <8> ~ <16> A dispersion containing ionically functionalized cellulose nanocrystals according to any one of the above. <18> <8> ~ <16> A sheet containing the ionically functionalized cellulose nanocrystals described in any one of the above. <19> <8> ~ <16> A powder containing the ionically functionalized cellulose nanocrystals described in any one of the above. [Effects of the Invention]

[0008] 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 therein, as well as a method for producing the same.Furthermore, the present invention also provides dispersions, sheets, and powders containing the cellulose nanocrystals. [Brief explanation of the drawings]

[0009] [Figure 1] 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. [Figure 2] 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. DETAILED DESCRIPTION OF THE INVENTION

[0010] [Method for producing cellulose nanocrystals with ionic functional groups] The method for producing ionic functionalized cellulose nanocrystals of the present invention (hereinafter, "ionic functionalized cellulose nanocrystals" may also be simply referred to as "ionic functionalized CNCs" or "CNCs") comprises the following steps A to C in this order: Step A: Ionic functional group introduction step for introducing ionic functional groups into cellulose fibers Step B: A hydrolysis step in which the cellulose fiber to which ionic functional groups have been introduced is hydrolyzed using an acidic aqueous solution with a concentration of 5 mol / L or more. Step C: A defibration step for defibrating the cellulose fibers to which hydrolyzed ionic functional groups have been introduced. According to the production method of the present invention, cellulose nanocrystals are provided which, when dispersed in water, have low viscosity, high transparency, excellent salt resistance, and a high amount of ionic functional groups introduced therein. 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-mentioned 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 it has been difficult to sufficiently reduce the viscosity of the resulting aqueous dispersion by shortening the cellulose nanofibers. 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 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 low cellulose nanocrystal dispersion concentrations, making chemical treatment at high concentrations difficult. This, in turn, results in low reaction efficiency, leading to limited introduction of ionic functional groups and failure to sufficiently improve transparency and salt tolerance. 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 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.

[0011] <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 minimal 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.

[0012] [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.

[0013] 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 phosphorus oxoacid groups, groups derived from phosphorus oxoacid groups, carboxy groups, sulfonic groups, sulfur oxoacid groups, groups derived from sulfur oxoacid groups, carboxymethyl groups, carboxyethyl groups, and cationic functional groups, more preferably at least one selected from the group consisting of phosphorus oxoacid groups, groups derived from phosphorus oxoacid groups, carboxy groups, sulfur oxoacid groups, and groups derived from sulfur oxoacid groups, even more preferably at least one selected from the group consisting of phosphorus oxoacid groups and groups derived from phosphorus oxoacid groups, and even more preferably a phosphorus oxoacid group. Introducing a phosphorus oxoacid group as the anionic functional group allows for the production of cellulose nanocrystals with excellent transparency and salt resistance.

[0014] The phosphorus oxo acid group or a group derived from the phosphorus oxo acid group is, for example, a substituent represented by the following formula (1). A plurality of substituents represented by the following formula (1) may be introduced into each cellulose fiber. In this case, the plurality of introduced substituents represented by the following formula (1) may be the same or different.

[0015] [ka]

[0016] 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. Note that all of α and α' are O - The n α's may all be the same or may be different. b+ is a cation of one or more valences consisting of organic or inorganic substances.

[0017] 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 group derived therefrom. In formula (1), n ​​is preferably 1.

[0018] Examples of saturated linear hydrocarbon groups include, but are not limited to, methyl, ethyl, n-propyl, or n-butyl groups. Examples of saturated branched hydrocarbon groups include, but are not limited to, i-propyl or t-butyl groups. Examples of saturated cyclic hydrocarbon groups include, but are not limited to, cyclopentyl or cyclohexyl groups. Examples of unsaturated linear hydrocarbon groups include, but are not limited to, vinyl or allyl groups. Examples of unsaturated branched hydrocarbon groups include, but are not limited to, i-propenyl or 3-butenyl groups. Examples of unsaturated cyclic hydrocarbon groups include, but are not limited to, cyclopentenyl or cyclohexenyl groups. Examples of aromatic groups include, but are not limited to, phenyl or naphthyl groups.

[0019] Furthermore, examples of the derivative group in R include, but are 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 keeping the number of carbon atoms constituting the main chain of R 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. When multiple Rs are present 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.

[0020] β b+is a monovalent or higher cation made of an organic or inorganic substance. Examples of the monovalent or higher cation made 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 made 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, ammonium ions, etc. It should be noted that in formula (1), β b+ When a plurality of β b+ may be the same or different. The monovalent or higher cations made of organic or inorganic substances include β b+ Sodium or potassium ions are preferred because they are less likely to yellow when cellulose fibers containing sodium or potassium ions are heated and are easily available industrially, but there is no particular limitation.

[0021] More specifically, examples of the phosphorus oxo acid group or a substituent derived from a phosphorus oxo acid group include a phosphate group (-POH), a salt of a phosphate group, a phosphorous acid (phosphonic acid) group (-POH), and a salt of a phosphite (phosphonic acid) group. The phosphorus oxo acid group or a substituent derived from a phosphorus oxo acid group may also 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), or an alkyl phosphonic acid group (e.g., a methylphosphonic acid group).

[0022] The sulfur oxoacid group (a 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.

[0023] [ka]

[0024] In the above structural formula, b and n are natural numbers, p is 0 or 1, and m is an arbitrary number (where 1 = b × m). When n is 2 or more, multiple p's may be the same number or different numbers. In the above 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, ammonium ions, etc. Note that 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. The monovalent or higher cations made of organic or inorganic substances include β b+ Sodium or potassium ions are preferred because they are less likely to yellow when cellulose fibers containing sodium or potassium ions are heated and are easily available industrially, but there is no particular limitation.

[0025] The amount of ionic functional groups introduced into the cellulose fibers is, for example, preferably 0.30 mmol / g or more per gram (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 gram (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 is the value of the counter ion of the ionic functional group being a hydrogen ion (H + ) indicates the mass of fibrous cellulose when

[0026] The amount of ionic functional groups introduced into cellulose fibers and cellulose nanocrystals with ionic functional groups introduced therein (described later) can be measured by, for example, neutralization titration or elemental analysis. In the measurement by neutralization titration, the amount introduced is measured by measuring the change in pH while adding an alkali such as sodium hydroxide solution to a slurry containing the obtained cellulose fibers or cellulose nanocrystals with ionic functional groups introduced therein.

[0027] Figure 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 way. 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 like the one 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 of maximum increment (derivative value of pH with respect to the amount of alkali added) are confirmed on the curve plotting the measured pH against the amount of alkali added. Of these, the first maximum increment obtained after starting to add alkali is called the first endpoint, and the next maximum increment obtained 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 from 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 from 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 from the fibrous cellulose contained in the slurry used for titration. The amount of alkali required from the start of titration to the first endpoint divided by the solids content (g) in the slurry to be titrated is the amount of phosphorus oxo acid groups introduced (mmol / g). Note that the term "amount of phosphorus oxo acid groups introduced" (or "amount of phosphorus oxo acid groups") simply refers to the amount of first dissociated acid. In Figure 1, the region from the start of titration to the first endpoint is referred to as Region 1, and the region from the first endpoint to the second endpoint is referred to as Region 2. For example, if the phosphorus oxoacid group is a phosphate group and this phosphate group undergoes condensation, the apparent amount of weakly acidic groups in the phosphorus oxoacid group (also referred to herein as the second dissociated acid amount) decreases, and the amount of alkali required in Region 2 is less than the amount required in Region 1. 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, if the phosphorus oxoacid group is a phosphite group, the phosphorus oxoacid group no longer contains weakly acidic groups, and the amount of alkali required in Region 2 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 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 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 strong acid groups and weak acid groups in phosphorus oxoacid groups) W: Formula weight per valence of cation C (e.g., Na is 23, Al is 9)

[0028] 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 process 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 Figure 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) reaches a maximum on the curve plotting the measured pH against the amount of alkali added. This maximum increment is called the first endpoint. Here, the region from the start of the titration to the first endpoint 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 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 being titrated. The amount of carboxyl groups introduced (mmol / g) is calculated based on the amount of carboxyl groups introduced (mmol / g) when the counter ions of the carboxyl groups are hydrogen ions (H + ) (hereinafter referred to as the amount of carboxyl groups (acid type)) per 1 g of cellulose fiber.

[0029] The above-mentioned amount of carboxyl groups introduced (mmol / g) indicates the amount of carboxyl groups possessed by acid-form cellulose fibers (hereinafter referred to as the amount of carboxyl 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 carboxyl groups are substituted with an arbitrary cation C so as to be charge equivalent, the amount of carboxyl groups possessed by cellulose fibers with the cation C as the counter ion (hereinafter referred to as the amount of carboxyl groups (C form)) (mmol / g) can be calculated 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 (e.g., Na is 23, Al is 9)

[0030] The amount of sulfur oxoacid and sulfonic acid groups introduced into the cellulose fibers and cellulose nanocrystals with ionic functional groups was measured by wet ashing the resulting cellulose fibers or cellulose nanocrystals with ionic functional groups using perchloric acid and concentrated nitric acid, diluting them at an appropriate ratio, and measuring the amount of sulfur by ICP atomic emission spectrometry. 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 and sulfonic acid groups (unit: mmol / g).

[0031] When measuring the amount of substituents using titration, adding too much sodium hydroxide solution or titrating too quickly can result in lower than expected amounts of substituents, leading to inaccurate values. An appropriate amount and interval is, for example, titrating 10 to 50 μL of 0.1 N sodium hydroxide solution over a 5 to 30 second period. To eliminate the influence of carbon dioxide dissolved in the cellulose fiber-containing slurry, it is recommended to measure the amount of substituents while blowing an inert gas such as nitrogen gas into the slurry from 15 minutes before the start of titration until the end of titration.

[0032] The cellulose fibers may contain carbamide groups derived from urea and / or urea derivatives added during the production process of ionic functional group-introduced cellulose fibers, as 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 gram (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 oxo acid groups are introduced by reaction with hydroxyl groups of cellulose, the amount of phosphorus oxo acid groups introduced decreases as the amount of carbamide groups introduced increases. Therefore, by setting the amount of carbamide groups within the above range, the amount of phosphorus oxo acid groups introduced can be increased and maintained within an appropriate range. Since the carbamide group itself is not electrically conductive, the introduction of carbamide groups does not result in cellulose having ionic functional groups introduced therein. Therefore, by adjusting the amount of carbamide groups introduced within the above range, the amount of phosphorus oxoacid groups introduced therein is increased, thereby improving the transparency and salt tolerance of the resulting cellulose nanocrystals having ionic functional groups introduced therein.

[0033] 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 by trace nitrogen analysis. The liberation of ionic nitrogen (ammonium ions) is carried out 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 Trace Analyzer manufactured by Mitsubishi Chemical Analytech Corporation. Prior to 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 fibers or cellulose nanocrystals with ionic functional groups introduced is calculated by dividing the nitrogen content per unit mass (g / g) of cellulose fibers or cellulose nanocrystals with ionic functional groups introduced, obtained by nitrogen trace analysis, by the atomic weight of nitrogen.

[0034] [Ionic Functional Group Introduction Step] 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 with a chlorine-based oxidizing agent, a xanthate group introduction step, a sulfone group introduction step, and a carboxyalkylation step. Each of these steps is described below.

[0035] -Phosphorus oxoacid group introduction process- 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 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 compound A may be carried out in the absence of compound B. One example of a method for reacting compound A with cellulose fibers in the presence of compound B is to mix compound A and compound B with dry, wet, or slurry-like cellulose fibers. Among these methods, using dry or wet cellulose fibers is preferred because of the high uniformity of the reaction, and dry cellulose fibers are particularly preferred. The form of the cellulose fibers is not particularly limited, but is preferably, for example, in the form of a cotton or thin sheet. Compound A and compound B may be added to the cellulose fibers in the form of a powder, a solution dissolved in a solvent, or a melted state obtained by heating 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 the 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.

[0036] The compound A used in this embodiment may be any compound that has a phosphorus atom and can form an ester bond with cellulose, and examples thereof include, but are 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 formed by condensing two or more molecules of phosphoric acid through 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, and these 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. 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 to 100% by mass, more preferably 1% by mass to 50% by mass, and even more preferably 2% by mass to 30% by mass. By setting the amount of phosphorus atoms added to the cellulose fibers within the above range, the transparency and salt tolerance of the resulting 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.

[0037] 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% by mass or more and 500% by mass or less, more preferably 10% by mass or more and 400% by mass or less, and even more preferably 100% by mass or more and 350% by mass or less.

[0038] In the reaction between cellulose fibers and compound A, the reaction system may contain, in addition to compound B, amides or amines, for example. Examples of amides include formamide, dimethylformamide, acetamide, and dimethylacetamide. Examples of amines include methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, and hexamethylenediamine. Among these, triethylamine is known to act as a particularly good reaction catalyst.

[0039] In the phosphorus oxo acid group introduction step, it is preferable to add or mix compound A or the like to 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, for example, 50°C to 300°C, more preferably 100°C to 250°C, and even more preferably 130°C to 200°C. Various types of equipment having 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.

[0040] The heat treatment according to this embodiment can employ, for example, a method in which compound A is added to a thin sheet of cellulose fiber by a method such as impregnation, followed by heating, or a method in which the cellulose fiber and compound A are heated while being kneaded or stirred using a kneader or the like. This makes it possible to suppress unevenness in the concentration of compound A in the cellulose fiber and to introduce phosphate groups more uniformly onto the surface of the cellulose fiber. This is thought to be because, when water molecules move to the surface of the cellulose fiber as it dries, the dissolved compound A is attracted to the water molecules by surface tension, preventing it from migrating to the surface of the cellulose fiber 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. 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, the amount of phosphorus oxo acid groups introduced can be kept within a preferred range by setting the heating temperature and heating time within appropriate ranges.

[0041] The phosphorus oxo acid group introduction step may be carried out at least once, but may also be carried out twice or more. By carrying out the phosphorus oxo acid group introduction step twice or more, a large number of phosphorus oxo acid groups can be introduced into the cellulose fiber. In the present embodiment, a preferred example is when the phosphorus oxo acid group introduction step is carried out twice.

[0042] The amount of phosphorus oxoacid groups introduced into the cellulose fibers is, for example, preferably 0.30 mmol / g or more per gram (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. The amount of phosphorus oxoacid groups introduced into the cellulose fibers is, for example, preferably 5.20 mmol / g or less per gram (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.

[0043] -Carboxy group introduction process- The carboxyl group introduction process is carried out by treating the cellulose fibers with an oxidation treatment such as ozone oxidation, oxidation by the Fenton method, or TEMPO oxidation treatment, or with a compound having a carboxylic acid-derived group or its derivative, or an acid anhydride of a compound having a carboxylic acid-derived group or its derivative. 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.

[0044] The acid anhydride of a compound having a group derived from carboxylic acid is not particularly limited, but examples thereof include acid anhydrides of dicarboxylic acid compounds such as maleic anhydride, succinic anhydride, phthalic anhydride, glutaric anhydride, adipic anhydride, itaconic anhydride, etc. Furthermore, the derivative of an acid anhydride of a compound having a group derived from carboxylic acid is not particularly limited, but examples thereof include acid anhydrides of compounds having carboxy groups such as dimethyl maleic anhydride, diethyl maleic anhydride, diphenyl maleic anhydride, etc., in which at least some of the hydrogen atoms are substituted with substituents such as alkyl groups or phenyl groups.

[0045] When TEMPO oxidation treatment is performed in the carboxyl group introduction step, it is preferable to perform the treatment under conditions of, for example, a pH of 6 or higher and 8 or lower. This type of treatment is also called neutral TEMPO oxidation treatment. Neutral TEMPO oxidation treatment can be performed, for example, by adding pulp as 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 also be carried out under conditions of a pH of 10 to 11. This type of 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 per gram (mass) of cellulose fiber, 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. 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.

[0046] -Sulfur oxoacid group introduction process- 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).

[0047] In the sulfur oxo acid group introduction step, instead of compound A in the above-described <Phosphorus oxo acid group introduction step>, 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, for example, 96% sulfuric acid (concentrated sulfuric acid). Sulfurous acid can be 5% aqueous sulfurous acid. Sulfates or sulfites can include lithium, sodium, potassium, and ammonium salts of sulfates or sulfites, which can be neutralized to various degrees. Sulfamic acid or the like can be used as the sulfuric acid amide. In the sulfur oxo acid group introduction step, it is preferable to use compound B in the above-described <Phosphorus oxo acid group introduction step> as well.

[0048] In the sulfur oxoacid group introduction step, cellulose fibers are preferably mixed with an aqueous solution containing sulfur oxoacid and urea and / or a urea derivative, and then the cellulose fibers are subjected to a heat treatment. 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.

[0049] In the heat treatment step, heating is preferably carried out 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 to 10,000 seconds. For the heat treatment, various devices having a heat medium can be used, such as an agitator dryer, rotary dryer, disk dryer, roll-type heater, plate-type heater, fluidized-bed dryer, band-type dryer, filtration dryer, vibration fluidized dryer, flash dryer, reduced-pressure dryer, infrared heater, far-infrared heater, microwave heater, and high-frequency dryer.

[0050] 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 properties and obtain cellulose nanocrystals with ionic functional groups introduced therein and with a narrower fiber width.

[0051] -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.

[0052] 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.

[0053] 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 from 1% by mass to 1,000% by mass, more preferably from 5% by mass to 500% by mass, and even more preferably from 10% by mass to 100% by mass. The amount of 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.

[0054] 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, it is preferable to maintain the pH 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 and removed with water by filtration or the like.

[0055] -Xanthate group introduction process (xanthogen acid esterification process)- 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 cause a reaction, 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.

[0056] <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 treating the cellulose with an alkaline solution. This treatment causes ionic dissociation of some of the hydroxyl groups in the cellulose, thereby increasing the 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 carried out simultaneously with the introduction of ionic functional groups, before the introduction, or at both the same time.

[0057] 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.

[0058] The alkaline solution concentration is preferably 0.01 mol / L to 4 mol / L in molar concentration, more preferably 0.1 mol / L to 3 mol / L in molar concentration, and even more preferably 1 mol / L to 2.5 mol / L in molar concentration. In particular, when the treatment temperature is less than 10° C., the concentration is preferably 1 mol / L to 2 mol / L in molar concentration.

[0059] 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, and the alkali treatment time is preferably 6 hours or less, more preferably 5 hours or less, and even more preferably 4 hours or less.

[0060] 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 crystal structure and increasing the yield of cellulose nanocrystals with introduced ionic functional groups.

[0061] When the introduction of ionic functional groups and the conversion to alkali cellulose are not carried out simultaneously, the alkali cellulose obtained by the alkali treatment is preferably subjected to solid-liquid separation and water removal 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% to 50%, more preferably 10% to 40%, and even more preferably 15% to 35%.

[0062] -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) is used as an essential component. A ), an optional alkali compound, and a compound B selected from the aforementioned urea and its derivatives are added to wet or dry cellulose fibers having hydroxyl groups and reacted to introduce phosphonic or phosphine groups into the cellulose fibers.

[0063] Examples of the reactive group include a halogenated alkyl group, a vinyl group, and an epoxy group (glycidyl group). Compound E AExamples of suitable cellulose nanocrystals include vinyl phosphonic acid, phenyl vinyl phosphonic acid, and phenyl vinyl phosphinic acid. From the viewpoints of the efficiency of introducing substituents, transparency and salt tolerance of the resulting cellulose nanocrystals having ionic functional groups introduced therein, cost, and ease of handling, Compound E A is preferably 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.

[0064] Compound E A When adding the reagent, it may be added to the cellulose fiber as is (solid or liquid) or dissolved in an appropriate solvent. 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.

[0065] The temperature during the reaction is, for example, 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.

[0066] Compound E A The amount 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.

[0067] 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 20 minutes to 400 minutes. After the reaction, excess reaction reagents, by-products, etc. may be washed and removed with water by filtration or the like.

[0068] -Sulfonic acid group introduction step (sulfoalkylation step)- The ionic functional group introduction step may include, for example, a sulfonic acid group introduction step (sulfoalkylation step). In the sulfoalkylation, a compound having a reactive group and a sulfonic acid group (compound E) is used as an essential component. B ) and, as an optional component, an alkaline compound, 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 sulfonic groups into the cellulose fibers.

[0069] Examples of the reactive group include a halogenated alkyl group, a vinyl group, and an epoxy group (glycidyl group). Compound E B Examples of suitable ionic functional groups 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 substituents, and hence the transparency and salt resistance of the resulting ionic functional group-introduced cellulose nanocrystals, 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.

[0070] Compound E B The reagent may be added to the cellulose fiber directly 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.

[0071] The temperature during the reaction is, for example, 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.

[0072] Compound E BThe amount 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.

[0073] 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 with water by filtration or the like.

[0074] -Carboxyalkylation step (third carboxy group introduction step)- The ionic functional group introduction step may include, for example, a carboxyalkylation step. As an essential component, a compound having a reactive group and a carboxy group (compound E C ), an optional alkaline compound, 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 carboxyl groups into the cellulose fibers.

[0075] Examples of the reactive group include a halogenated alkyl group, a vinyl group, and an epoxy group (glycidyl group). Compound E C As the substituent, 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 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 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.

[0076] Compound E CThe reagent may be added to the cellulose fiber directly 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.

[0077] The temperature during the reaction is, for example, 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.

[0078] Compound E C The amount 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.

[0079] 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 with water by filtration or the like.

[0080] -Cationic group introduction step (cationization step)- As an essential component, a compound having a reactive group and a cationic group (compound E D ), an optional alkaline compound, 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.

[0081] Examples of the reactive group include a halogenated alkyl group, a vinyl group, and an epoxy group (glycidyl group).

[0082] Compound E DAs the substituent group, glycidyl trimethyl ammonium chloride, 3-chloro-2-hydroxypropyl trimethyl ammonium chloride, etc. are preferred from the viewpoints of the efficiency of introducing the substituent group, 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.

[0083] Furthermore, it is also preferable to use the compound B in the above-mentioned <Phosphorus oxo acid group introduction step> as an optional component in the same manner. The amount added is also preferably as described above.

[0084] Compound E D The reagent may be added to the cellulose fiber directly 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.

[0085] The temperature during the reaction is, for example, 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.

[0086] Compound E D The amount 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.

[0087] 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 with water by filtration or the like.

[0088] [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 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.

[0089] [Alkali treatment step] The cellulose fiber having 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 ionic functional groups introduced therein in an alkaline 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% to 100,000% by mass, and more preferably from 1,000% 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 ion exchange treatment of the anionic groups. In this case, the temperature of the alkali solution is preferably room temperature.

[0090] In order to reduce the amount of alkaline solution used in the alkali 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 and before the alkali treatment step. From the viewpoint of improving handleability, it is preferable to wash the alkali-treated ionic functional group-introduced cellulose fiber with water or an organic solvent after the alkali treatment step and before step B.

[0091] [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 cellulose fibers having ionic functional groups introduced therein 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% to 100,000% by mass, and more preferably from 1,000% 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.

[0092] <Process B> Step B is a hydrolysis step in which the cellulose fibers to which ionic functional groups have been introduced 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, reducing or equalizing the difference between the first and second dissociation amounts in the resulting cellulose nanocrystals having ionic functional groups introduced therein. 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.

[0093] [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.

[0094] 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, and from the viewpoint of ease of operation, is preferably 30 mol / L or less, more preferably 25 mol / L or less, and even more preferably 20 mol / L or less.

[0095] [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 shortening the production time, and is preferably 12 hours or shorter, more preferably 6 hours or shorter, even more preferably 3 hours or shorter.

[0096] 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.

[0097] <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.

[0098] [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, grinder (stone mill-type grinder), high-pressure homogenizer, ultra-high-pressure homogenizer, high-pressure collision grinder, ball mill, bead mill, disk-type refiner, conical refiner, twin-screw kneader, vibration mill, homomixer under high-speed rotation, ultrasonic disperser, or beater. Among the above defibration treatment devices, it is more preferable to use a high-speed defibrator, high-pressure homogenizer, or ultra-high-pressure homogenizer, which are less affected by the grinding media and have less risk of contamination.

[0099] In the defibration process, for example, hydrolyzed cellulose fibers with introduced ionic functional groups 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.

[0100] From the viewpoint of efficient defibration, the concentration of the dispersion liquid (slurry) during the defibration treatment 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.

[0101] <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 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.

[0102] 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.

[0103] The fiber width of the cellulose nanocrystals into which ionic functional groups have been introduced is 1000 nm or less, and 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.

[0104] 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 can be suppressed, making it easier to achieve the effect of reducing the viscosity of the ionic functionalized cellulose nanocrystal dispersion.

[0105] 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 hydrophilized carbon film-coated grid to prepare a sample for TEM observation. If the sample contains wide fibers, an SEM image 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 must be adjusted to meet the following conditions. (1) Draw a line X 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.

[0106] For observation images that satisfy the above conditions, the widths of the fibers intersecting with lines X and Y are visually read. In this way, three or more sets of observation images of at least the surface portions that do not overlap each other are obtained. Next, for each image, the widths of the fibers intersecting 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 value of the read fiber widths is then taken as the average fiber width of the cellulose nanocrystals.

[0107] 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, AFM observation is performed 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.

[0108] The fiber length of the ionic functionalized cellulose nanocrystals is not particularly limited, but is preferably 30 nm to 1,000 nm, more preferably 40 nm to 800 nm, and even more preferably 50 nm to 500 nm. By setting the fiber length within the above range, it is also possible to set the slurry viscosity of the ionic functionalized cellulose nanocrystals within an appropriate range. The fiber length of the ionic functionalized cellulose nanocrystals can be determined, for example, by image analysis using TEM, SEM, or AFM.

[0109] 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, the structure 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 cellulose nanocrystals with ionic functional groups introduced therein is preferably 30% or more, more preferably 40% or more, and even more preferably 50% or more. This is expected to reduce the viscosity of the resulting aqueous dispersion. The degree of crystallinity can be determined by measuring the X-ray diffraction profile and using standard methods from the pattern (Seagal et al., Textile Research Journal, Vol. 29, p. 786, 1959).

[0110] The axial ratio (fiber length / fiber width) of the cellulose nanocrystals into which ionic functional groups have been introduced is not particularly limited, but is preferably, for example, from 5 to 500, and more preferably from 10 to 250. By keeping the axial ratio within the above range, a low-viscosity dispersion is likely to be obtained when a solvent dispersion is prepared.

[0111] 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. When 100 cellulose nanocrystals are randomly observed with an atomic force microscope, the number of fibers that are bent by more than 30° 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.

[0112] [Cellulose nanocrystals with ionic functional groups] 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 cellulose nanocrystals introduced with carboxyl groups 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 carbon atom at the 2nd or 3rd position. This limits the flexibility of introducing ionic functional groups. 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 cellulose nanocrystals were prepared by hydrolysis and defibration treatment. However, with other ionic functional groups, it was difficult to introduce a sufficient amount of ionic functional groups.

[0113] 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. Introducing a phosphorus oxoacid group as the anionic functional group allows for the production of cellulose nanocrystals with excellent transparency and salt resistance.

[0114] In the ionic functional group-introduced cellulose nanocrystals of this embodiment, the amount of ionic functional group 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, even more preferably 1.20 mmol / g or more. Although the upper limit is not particularly limited, since excessive introduction amount causes destruction and dissolution of cellulose crystals, it 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.

[0115] 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 the cellulose nanocrystals of 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.

[0116] 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, the density of ionic functional groups is increased in cellulose nanocrystals with ionic functional groups introduced, which is thought to result in cellulose nanocrystals with better dissociation properties and smaller fiber diameters. The fact that an ionic functional group is introduced 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 is 13 C NMR, e.g., 13 It can be detected by C CP / MAS NMR (Cross Polarization Magic Angle Spinning-NMR (Nuclear Magic Resonance)).

[0117] The ionic functional group-introduced cellulose nanocrystals of this embodiment are preferably obtained by the above-mentioned method for producing ionic functional group-introduced cellulose nanocrystals, which comprises, in this order, steps A to C. However, the method for producing ionic functional group-introduced nanocrystals of this embodiment is not limited thereto, and they may be produced by other methods.

[0118] 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 functionalized 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.

[0119] The cellulose nanocrystals of the present invention, which have been introduced with ionic functional groups, have excellent transparency. Specifically, the total light transmittance of a 0.2% by mass aqueous dispersion of the cellulose nanocrystals containing 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.).

[0120] 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 cellulose nanocrystals containing ionic functional groups 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.

[0121] 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, it is evaluated by visually determining the degree of coloration when a 1% by mass dispersion is heated at 95°C for 10 hours.

[0122] 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 more than 30° 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.

[0123] [Dispersion] 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.

[0124] 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.

[0125] [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.

[0126] [Applications of cellulose nanocrystals with ionic functional groups] The ionic functionalized 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 growth promoters, antifreeze agents, paint strippers, insecticides and insect repellents, agricultural chemicals, mold forming compositions, lubricants, piping friction resistance reducers, and fragrances and deodorizers. The ionic functionalized 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. [Example]

[0127] The features of the present invention will be explained in more detail below with reference to examples and comparative examples. The materials, amounts used, ratios, treatment contents, treatment procedures, etc. shown in the following examples can be appropriately changed 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.

[0128] Example A1 The raw pulp (cellulose fiber) was softwood bleached kraft pulp (NBKP, solid content 93% by mass, basis weight 245 g / m) manufactured by Oji Paper Co., Ltd. 2 Sheet-type pulp with a Canadian Standard Freeness (CSF) of 700 mL when disintegrated and measured in accordance with JIS P 8121-2:2012 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 total weight 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. 100 g (bone dry mass) of phosphorylated pulp was mixed with 10 L of ion-exchanged water to obtain a pulp dispersion. The pulp was stirred to uniformly disperse the pulp, and then repeatedly filtered and dehydrated. 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 with a pH of 12 to 13. Next, the phosphorylated pulp slurry was dehydrated and washed to obtain a neutralized phosphorylated pulp.

[0129] 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 an equal 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 contacted with a strongly basic ion-exchange resin (Amberjet 4400; Organo Corporation, conditioned). This treatment was continued until the electrical conductivity and pH of the slurry stabilized. 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 pulverization device (Beryu-Mini, high-pressure homogenizer, manufactured by Biryu Co., Ltd.), to obtain a dispersion containing phosphorylated cellulose nanocrystals.

[0130] The infrared absorption spectra of the obtained phosphorylated pulp and phosphorylated cellulose nanocrystals were measured using FT-IR. -1The absorption peaks due to the P=O phosphate groups were observed near the 2θ angle, 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 phosphoric acid groups (first dissociated acid amount), measured by the method described below in [Measurement of phosphorus oxoacid group amount], was 1.45 mmol / g for phosphorylated pulp and 0.60 mmol / g for phosphorylated cellulose nanocrystals. The total amount of dissociated acid was 2.45 mmol / g (second dissociated acid amount: 1.00 mmol / g) for phosphorylated pulp and 1.20 mmol / g (second dissociated acid amount: 0.60 mmol / g) for phosphorylated cellulose nanocrystals. Furthermore, the fiber width of the cellulose nanocrystals was measured using an atomic force microscope and found to be 3 to 20 nm.

[0131] 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 obtained phosphorylated pulp and phosphorylated cellulose nanocrystals were measured using FT-IR. -1Absorption due to the P=O of phosphate groups was observed near the nucleus, 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 method described below in [Measurement of phosphorus oxoacid group amount] was 2.00 mmol / g for phosphorylated pulp and 0.85 mmol / g for phosphorylated cellulose nanocrystals. The total amount of dissociated acid was 3.30 mmol / g for phosphorylated pulp (amount of second dissociated acid: 1.30 mmol / g) and 1.65 mmol / g for 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.

[0132] 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.

[0133] The infrared absorption spectra of the obtained phosphorylated pulp and phosphorylated cellulose nanocrystals were measured using FT-IR. -1Absorption due to the P=O of phosphate groups was observed near the nucleus, 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 method described below in "Measurement of phosphorus oxoacid group amount" was 2.53 mmol / g for phosphorylated pulp and 1.25 mmol / g for phosphorylated cellulose nanocrystals. The total amount of dissociated acid was 4.22 mmol / g for phosphorylated pulp (amount of second dissociated acid: 1.69 mmol / g) and 2.39 mmol / g for 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.

[0134] 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.

[0135] The infrared absorption spectrum of the obtained phosphorylated cellulose nanocrystals was measured using FT-IR. -1 Absorption due to P=O of phosphate groups was observed near the center, 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 groups] 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). The fiber width was measured using an atomic force microscope and was found to be 3 to 20 nm.

[0136] 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 pentoxide, indicating that regenerated diphosphorus pentoxide 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.

[0137] The infrared absorption spectrum of the obtained phosphorylated cellulose nanocrystals was measured using FT-IR. -1 Absorption due to P=O of phosphate groups was observed near the center, 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 groups] 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). The fiber width was measured using an atomic force microscope and was found to be 3 to 20 nm.

[0138] 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.

[0139] The infrared absorption spectrum of the obtained phosphorylated cellulose nanocrystals was measured using FT-IR. -1Absorption due to P=O of phosphate groups was observed near the center, 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 groups] 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). The fiber width was measured using an atomic force microscope and was found to be 3 to 20 nm.

[0140] 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.

[0141] The infrared absorption spectrum of the obtained phosphorylated cellulose nanocrystals was measured using FT-IR. -1 Absorption due to P=O of phosphate groups was observed near the center, 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 groups] 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). The fiber width was measured using an atomic force microscope and was found to be 3 to 20 nm.

[0142] 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.

[0143] The infrared absorption spectrum of the obtained phosphorylated cellulose nanocrystals was measured using FT-IR. -1Absorption due to P=O of phosphate groups was observed near the center, 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 groups] 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). The fiber width was measured using an atomic force microscope and was found to be 3 to 20 nm.

[0144] 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.

[0145] The infrared absorption spectra of the obtained phosphited pulp and phosphited cellulose nanocrystals were measured using FT-IR. -1 Absorption due to P=O of the phosphonic acid group, a tautomer of the phosphorous acid group, was observed near the center, confirming the addition of a phosphorous acid 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 phosphorous acid groups (first dissociated acid amount) measured using the method described below in [Measurement of phosphorous oxoacid 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 (second dissociated acid amount: 0.03 mmol / g) and 0.68 mmol / g for the phosphite cellulose nanocrystals (second dissociated acid amount: 0.01 mmol / g). The fiber width of the phosphite 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, the total amount of dissociated acid is thought to be larger than the first amount of dissociated acid due to slight oxidation of phosphorous acid and carboxyl groups derived from amorphous hemicellulose.

[0146] 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.

[0147] The obtained sulfated pulp and sulfated cellulose nanocrystals were subjected to infrared absorption spectroscopy using FT-IR. -1 Absorption due to the S=O of sulfate ester groups was observed near the center, confirming the addition of sulfate ester groups. X-ray diffraction confirmed that the resulting sulfated pulp and sulfated cellulose nanocrystals maintained cellulose type I crystallinity. The amount of sulfate ester groups measured using the method described below in [Measurement of sulfur oxoacid and sulfonic acid groups] 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.

[0148] Example D1 The raw pulp used was softwood bleached kraft pulp (NBKP, never dried, Canadian Standard Freeness (CSF) measured according to JIS P 8121-2:2012: 700 mL) manufactured by Oji Paper Co., Ltd. This raw pulp was subjected to alkaline TEMPO oxidation treatment as follows. First, 100 parts by weight of the raw pulp (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 10 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. The reaction was considered complete when no further 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 added, and the sheet was stirred to uniformly disperse the pulp. This process was repeated until the electrical conductivity of the filtrate reached 100 μS / cm or less, marking the end of the washing process.

[0149] 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 the mixture was stirred at 500 rpm using a magnetic stirrer while reacting at room temperature for 48 hours to obtain a pulp slurry. The resulting TEMPO-oxidized pulp was then washed. The pulp slurry after the additional oxidation was dehydrated to obtain a dehydrated sheet, to which 5,000 parts by mass of ion-exchanged water was added, and the sheet was stirred to uniformly disperse the pulp. This process was repeated until the electrical conductivity of the filtrate reached 100 μS / cm or less, marking the end of the washing process. 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.

[0150] X-ray diffraction confirmed that the resulting TEMPO-oxidized pulp and TEMPO-oxidized cellulose nanocrystals maintained cellulose type I crystal structure. The carboxyl group content, 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 3-20 nm.

[0151] Example E1 [Hypochlorous acid oxidation] A sheet (solids concentration 90% by mass) made from softwood bleached kraft pulp (NBKP) was mixed in a hand mixer (Lab Millser PLUS, manufactured by Osaka Chemical Co., Ltd.) 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% sodium hypochlorite aqueous solution was added to 100 parts by mass of the fluffy fluffed pulp, and the mixture was reacted for 2 hours in a warm bath at 30°C to obtain carboxylated pulp. During the reaction, the pH was maintained at 11 by adding 1N aqueous sodium hydroxide solution as needed. The resulting carboxylated pulp was then washed. The washing process involved pouring ion-exchanged water over the resulting carboxylated 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 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.

[0152] X-ray diffraction confirmed that the obtained carboxylated pulp and carboxylated cellulose nanocrystals maintained cellulose type I crystal structure. The amount of carboxyl groups measured by 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.

[0153] Example F1 [Maleic acid esterification] A sheet (solids concentration 90% by mass) made from bleached softwood (NBKP) was mixed for 15 seconds at 20,000 rpm using a hand mixer (Lab Millser PLUS, manufactured by Osaka Chemical Co., Ltd.) to produce a fluffy pulp (solids concentration 90% by mass). 100 parts by mass of the fluffy 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 process involved pouring ion-exchanged water over the resulting carboxylated 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 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.

[0154] X-ray diffraction confirmed that the obtained carboxylated pulp and carboxylated cellulose nanocrystals maintained cellulose type I crystal structure. The amount of carboxyl groups measured by 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.

[0155] Example G1 [Carboxyethylated] The raw pulp was softwood bleached kraft pulp (NBKP, solid content 93% by mass, basis weight 245 g / m) manufactured by Oji Paper Co., Ltd. 2 Sheet-type pulp with a Canadian Standard Freeness (CSF) of 700 mL when disintegrated and measured in accordance with JIS P 8121-2:2012 was used. To 100 parts by mass (bone dry mass) of this raw pulp, a chemical solution consisting of 250 parts by mass of 12N NaOH aqueous solution, 163 parts by mass of 2-chloropropionic acid, and 140 parts by mass of ion-exchanged water (total 553 parts by mass) was added to obtain a chemical-impregnated pulp. The obtained 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, thereby obtaining a carboxy-introduced pulp. The resulting carboxylated pulp was then washed. The washing process involved pouring ion-exchanged water over the resulting carboxylated 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. 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 sodium hydroxide solution was added little by little while stirring to obtain a carboxylated pulp slurry with 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.

[0156] X-ray diffraction confirmed that the obtained carboxylated pulp and carboxylated cellulose nanocrystals maintained cellulose type I crystal structure. The amount of carboxyl groups measured by 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.

[0157] <Example H1> [Carboxymethylation] The raw pulp used was softwood kraft pulp (solid content 93% by mass, basis weight 245 g / m) manufactured by Oji Paper Co., Ltd. 2 Sheet-type pulp with a Canadian Standard Freeness (CSF) of 700 mL when disintegrated and measured in accordance with JIS P 8121-2:2012 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 (total 571 parts by mass) 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 process involved pouring ion-exchanged water over the resulting carboxylated 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 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.

[0158] X-ray diffraction confirmed that the obtained carboxylated pulp and carboxylated cellulose nanocrystals maintained cellulose type I crystal structure. The amount of carboxyl groups measured by the method described below was 1.21 mmol / g for the carboxylated pulp and 0.41 mmol / g for the carboxylated cellulose nanocrystals. The fiber width of the carboxylated cellulose nanocrystals was measured using an atomic force microscope and was found to be 3 to 20 nm.

[0159] Example J1 [Sulfoethylation] The raw pulp was softwood bleached kraft pulp (NBKP, solid content 93% by mass, basis weight 245 g / m) manufactured by Oji Paper Co., Ltd. 2 Sheet-type pulp with a Canadian Standard Freeness (CSF) of 700 mL when disintegrated and measured in accordance with JIS P 8121-2:2012 was used. 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 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 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 process 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.

[0160] X-ray diffraction confirmed that the obtained sulfoethyl-group-introduced pulp and sulfoethyl-group-introduced cellulose nanocrystals maintained cellulose type I crystals. The sulfoethyl group content (sulfonic acid content), measured by 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.

[0161] <Example K1> [Cationization treatment] The raw pulp was softwood bleached kraft pulp (NBKP, solid content 93% by mass, basis weight 245 g / m) manufactured by Oji Paper Co., Ltd. 2 Sheet-type pulp with a Canadian Standard Freeness (CSF) of 700 mL when disintegrated and measured in accordance with JIS P 8121-2:2012 was used. 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, yielding a cationic group-introduced pulp. The resulting cation-group-introduced pulp was then washed. The washing process involved pouring ion-exchanged water over the resulting cation-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. 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 with 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.

[0162] Concentrated hydrochloric acid (11.2 mol / L hydrochloric acid) was added little by little to the obtained cationized pulp 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, 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.

[0163] 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 by 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.

[0164] <Comparative Example 1> Instead of phosphorylated pulp, softwood bleached kraft pulp (NBKP, solid content 93% by mass, basis weight 245 g / m) manufactured by Oji Paper Co., Ltd. was used.2 A dispersion containing cellulose nanocrystals was obtained in the same manner as in Example A1, except that a sheet-like cellulose nanocrystal (disintegrated and having a Canadian Standard Freeness (CSF) of 700 mL as measured in accordance with JIS P 8121-2:2012) was used. The amount of phosphate groups in the obtained cellulose nanocrystals was less than 0.01 mmol / g.The fiber width was measured using an atomic force microscope and was found to be 10 to 30 nm.

[0165] <Comparative Example 2> To the dispersion containing cellulose nanocrystals (solids concentration 2%) obtained in Comparative Example 1, 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. 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 an equal volume of ion-exchanged water was added. The precipitate was thoroughly mixed and dispersed, and the resulting mixture was centrifuged again. This procedure 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). 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. This slurry was treated six times at a pressure of 100 MPa in a wet atomization device (Beryu-Mini, manufactured by Biryu Co., Ltd.) to obtain a dispersion containing phosphorylated cellulose nanocrystals.

[0166] The infrared absorption spectrum of the obtained phosphorylated cellulose nanocrystals was measured using FT-IR. -1Absorption due to the P=O of phosphate groups was observed near the nucleus, 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 oxoacid groups] 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.

[0167] <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 set 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.

[0168] The infrared absorption spectrum of the obtained phosphorylated cellulose nanocrystals was measured using FT-IR. -1 Absorption due to the P=O of phosphate groups was observed near the nucleus, 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 method described below in [Measurement of amount of phosphorus oxoacid groups] was 1.03 mmol / g. The total amount of dissociated acid was 1.77 mmol / g (amount of second dissociated acid was 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.

[0169] <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. 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.

[0170] 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 3-20 nm.

[0171] <Comparative Example 4> The raw pulp was softwood bleached kraft pulp (NBKP, solid content 93% by mass, basis weight 245 g / m) manufactured by Oji Paper Co., Ltd. 2 Sheet-type pulp with a Canadian Standard Freeness (CSF) of 700 mL when disintegrated and measured in accordance with JIS P 8121-2:2012 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 total weight 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. 100 g (bone dry mass) of phosphorylated pulp was mixed with 10 L of ion-exchanged water to obtain a pulp dispersion. The pulp was stirred to uniformly disperse the pulp, and then repeatedly filtered and dehydrated. The washing was completed when the electrical conductivity of the filtrate reached 100 μS / cm or less.

[0172] Ion-exchanged water and 85% phosphoric acid were added to the washed phosphorylated pulp to adjust the solids concentration to 2% and the phosphoric acid concentration to 1 mol / L, and the mixture was subjected to thermal hydrolysis treatment in a hot bath at 95°C for 90 minutes while stirring (dilute acid hydrolysis treatment).

[0173] The resulting hydrolyzed phosphorylated pulp was then washed. The washing process consisted of pouring 10 L of ion-exchanged water over 100 g (bone dry weight) of phosphorylated pulp to obtain a pulp dispersion. The pulp was stirred to uniformly disperse, and then repeatedly filtered and dehydrated. The washing was completed when the electrical conductivity of the filtrate reached 100 μS / cm or less.

[0174] 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 with 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.

[0175] The infrared absorption spectrum of the obtained phosphorylated cellulose nanofibers was measured using FT-IR. -1 Absorption due to the P=O of phosphate groups was observed near the α-axis, 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, 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 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.

[0176] <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.

[0177] The infrared absorption spectrum of the obtained phosphorylated cellulose nanofibers was measured using FT-IR. -1Absorption due to the P=O of phosphate groups was observed near the α-axis, 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, 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 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.

[0178] <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 pulverization apparatus (Beryu-Mini, manufactured by Biryu Co., Ltd.) at a pressure of 100 MPa to obtain a dispersion containing phosphorylated cellulose nanofibers.

[0179] 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 increased to 100°C, and the enzyme was thermally inactivated. The slurry was then treated six times at a pressure of 100 MPa in a wet pulverization 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 oxo acid group amount] was 1.45 mmol / g. The total amount of dissociated acid was 2.45 mmol / g.

[0180] [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 finished basis weight of the sheet was 70 g / m 2 The mixture was weighed out so that the weight was 100g and spread on a commercially available acrylic plate. A damming frame (inner dimensions 250mm x 250mm, height 5cm) was placed on the acrylic plate to achieve the specified basis weight. The plate was 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 type of cellulose nanocrystal. The sheet thickness was 50µm.

[0181] [Powder production] 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 powder was 20 μm or less, and the moisture content was 20% or less.

[0182] [evaluation] <Measurement of phosphorus oxoacid group content> The amount of phosphorus oxoacid groups in cellulose nanocrystals, cellulose nanofibers, and cellulose fibers with ionic functional groups introduced therein 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 ionic functional groups introduced therein) to make the content 0.2 mass%, treating the dispersion with ion-exchange resin, and then titrating it with alkali. 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 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 measuring the change in the pH of various cellulose-containing slurries after ion exchange resin treatment while adding 10 μL of 0.1 N sodium hydroxide solution every 5 seconds. Nitrogen gas was bubbled through the slurry 15 minutes before the start of the titration. In this neutralization titration, two maximum points of increment (the derivative of pH with respect to the amount of alkali added) were observed on the plot of pH versus the amount of alkali added. The first maximum point of increment after starting the alkali addition is called the first endpoint, and the second maximum point is called the second endpoint (Figure 1). The amount of alkali required from the start of the titration to the first endpoint is equal to the amount of first dissociated acid in the slurry used for titration. The amount of alkali required from the start of the titration to the second endpoint is equal to the total amount of dissociated acid in the slurry used for titration. The amount of alkali (mmol) required from the start of titration to the first endpoint divided by the solid 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 solid 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.

[0183] <Measurement of carboxyl 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. 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, alkali titration was performed by measuring the change in pH of the fibrous cellulose-containing slurry after treatment with an ion exchange resin while adding 0.1 N aqueous sodium hydroxide. Observing the change in pH while adding aqueous sodium hydroxide yielded the titration curve shown in Figure 2. As shown in Figure 2, in this neutralization titration, a single point was observed where the increment (the differential value of pH with respect to the amount of alkali added) reached a maximum on the curve plotting the measured pH against the amount of alkali added. This maximum increment was called the first endpoint. The region from the start of the titration to the first endpoint in Figure 2 is called Region 1. The amount of alkali required in Region 1 was equal to the amount of carboxyl groups in the slurry used for titration. The amount of alkali required in Region 1 of the titration curve (mmol) was then divided by the solids content (g) of the cellulose-containing slurry being titrated to calculate the amount of carboxyl groups introduced (mmol / g). The amount of carboxyl groups introduced (mmol / g) is calculated based on the amount of carboxyl groups introduced (mmol / g) when the counter ions of the carboxyl groups are hydrogen ions (H + ) (hereinafter referred to as the amount of carboxy groups (acid type)) per 1 g of fibrous cellulose.

[0184] <Measurement of sulfur oxoacid and sulfonic acid groups> The amounts of sulfur oxoacid and sulfonic acid groups were measured for the pulp and cellulose nanocrystals obtained in Examples C1 and J1. Specifically, the various 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 and sulfonic acid groups (unit: mmol / g).

[0185] <Measurement of the amount of cationic groups> The amount of cationic groups was measured for the pulp and cellulose nanocrystals obtained in Example K1. Specifically, the freeze-dried and pulverized samples were measured using a TN-110 trace total nitrogen analyzer manufactured by Mitsubishi Chemical Analytech Co., Ltd. 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.

[0186] <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, 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.

[0187] <Measurement of dispersion viscosity> 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 recorded as the viscosity of the dispersion. 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 temperature of the dispersions during measurement was 23°C.

[0188] <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 a 1 cm optical path length (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%. 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.

[0189] <Salt tolerance> The total light transmittance was measured when a predetermined amount of sodium chloride was added to the resulting 0.2% by mass dispersion of cellulose nanocrystals and cellulose nanofibers, and those dispersions whose 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: At a sodium chloride concentration of 300 mmol / L, T / T0 is 0.98 or higher. B: When the sodium chloride concentration is 300 mmol / L, T / T0 is less than 0.98, but when the sodium chloride concentration is 100 mmol / L, T / T0 is 0.98 or more. C: At a sodium chloride concentration of 100 mmol / L, T / T0 is less than 0.98 where T is the total light transmittance after adding sodium chloride, and T0 is the 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) Stir by hand with a Teflon rod until the sodium chloride dissolves. (3) After dissolving sodium chloride, stir in a rotary disperser at 4000 rpm for 1 minute. Measure the total light transmittance by the method described in (4) <Measurement of total light transmittance of the dispersion liquid>

[0190] <Presence or absence of modification other than the C6 position> The obtained samples of the cellulose nanocrystal and cellulose nanofiber dispersions freeze-dried were 13 Analyzed by C CP / MAS NMR. When compared with the results of the sample of Comparative Example 1, if changes were observed in the peaks at the chemical shifts of 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 considered that modification had occurred not only at the C6 position.

[0191] <Purity> The purity of the obtained cellulose nanocrystal and cellulose nanofiber was determined from whether it contained functional groups other than ionic functional groups and the coloring during wet heating. A: Does not contain functional groups other than ionic functional groups, or contains functional groups other than ionic functional groups but hardly colors upon wet heating B: Contains functional groups other than ionic functional groups and has a slightly yellowish color upon wet heating C: Contains functional groups other than ionic functional groups and becomes strong yellow to light brown upon wet heating Note that the coloring during wet heating was visually judged by the degree of coloring when a 1 mass% concentration cellulose nanocrystal or cellulose nanofiber dispersion liquid was heated at 95 °C for 10 hours. A nitrogen-containing structure that can release ammonia by hydrolysis such as a carbamide group is considered to promote the formation of the coloring structure.

[0192] <Flexure of fibers> Take images of the cellulose nanocrystal and cellulose nanofiber with an atomic force microscope, and count the number of fibers among 100 randomly selected fibers that have one or more bends (angle 30° or more) in the middle, and judge according to the following criteria. A: The number of fibers with bends is less than 10 B: The number of fibers with bends is 10 or more and less than 20 C: There are 20 or more bent fibers.

[0193] [Table 1-1]

[0194] [Table 1-2]

[0195] For the ionically functionalized cellulose fibers, ionically functionalized cellulose nanofibers, and ionically functionalized cellulose nanocrystals listed in Tables 1-1 and 1-2, when the amount of ionically functional groups introduced was measured by the above-mentioned <Measurement of the amount of phosphorus oxo acid groups>, the results of the measurement were entered in the respective columns for the amount of first dissociated acid and the amount of second dissociated acid. Furthermore, when the amount of ionically 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 ionically 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.

[0196] 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 tolerance when dispersed in water, and 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. Also, 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 inferior transparency and salt resistance (Comparative Example 2) or inferior 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

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: Ionic functional group introduction step of introducing ionic functional groups into cellulose fibers Step B: A hydrolysis step in which the cellulose fibers having ionic functional groups introduced therein are hydrolyzed using an acidic aqueous solution having a concentration of 5 mol / L or more. Step C: Defibration step of defibrating the cellulose fibers into which hydrolyzed ionic functional groups have been introduced

2. 2. The 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.

3. 3. The method according to claim 1, 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 method according to any one of claims 1 to 3, wherein in step A, the amount of ionic functional groups introduced into the cellulose fibers is 0.50 mmol / g or more.

5. The method according to any one of claims 1 to 4, wherein the amount of ionic functional groups introduced into the cellulose nanocrystals having ionic functional groups introduced therein is 0.10 mmol / g or more.

6. The method according to any one of claims 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 method according to any one of claims 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. An ionic functional group-introduced cellulose nanocrystal having an ionic functional group introduction amount of 0.10 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 claim 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. 10. The cellulose nanocrystal having an ionically functionalized group introduced therein according to claim 8, 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 ionic functional group-introduced cellulose nanocrystal according to any one of claims 8 to 10, wherein the amount of the ionic functional group introduced is 0.50 mmol / g or more.

12. The ionic functional group-introduced cellulose nanocrystal according to any one of claims 8 to 11, wherein the ionic functional group is introduced into at least one of the 2- and 3-position carbon atoms of cellulose.

13. The ionic functional group-introduced cellulose nanocrystal according to any one of claims 8 to 12, wherein the amount of carbamide groups in the ionic functional group-introduced cellulose nanocrystal is 0.06 mmol / g or less.

14. The ionic functional group-introduced cellulose nanocrystal according to any one of claims 8 to 13, wherein the viscosity of a 1.5 mass% aqueous dispersion of the ionic functional group-introduced cellulose nanocrystal is 100 mPa s or less.

15. The ionic functional group-introduced cellulose nanocrystal according to any one of claims 8 to 14, wherein the total light transmittance of a 0.2% by mass aqueous dispersion of the ionic functional group-introduced cellulose nanocrystal is 95.5% or more.

16. When sodium chloride was added to a 0.2% by mass aqueous dispersion of the ionic functional group-introduced cellulose nanocrystal so as to give a concentration of 100 mmol / L, the total light transmittance after the addition of sodium chloride was T and the total light transmittance before the addition of sodium chloride was T. ０ When this is the case, T / T ０ The ionic functional group-introduced cellulose nanocrystal according to any one of claims 8 to 15, wherein the ionic functional group-introduced cellulose nanocrystal has a molecular weight of 0.98 or more.

17. A dispersion containing the ionic functional group-introduced cellulose nanocrystals according to any one of claims 8 to 16.

18. A sheet containing the ionic functional group-introduced cellulose nanocrystal according to any one of claims 8 to 16.

19. A powder containing the ionic functional group-introduced cellulose nanocrystal according to any one of claims 8 to 16.

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