Patent Publication Number: US-2021180214-A1

Title: Carboxymethylated cellulose nanofibers

Description:
TECHNICAL FIELD 
     The present invention relates to a carboxymethylated cellulose nanofiber. Specifically, the present invention relates to a carboxymethylated cellulose nanofiber which exhibits a high transparency when dispersed in water to form a water dispersion, and is high in degree of carboxymethyl substitution and high in degree of crystallization of cellulose I type. 
     BACKGROUND ART 
     Carboxymethylated celluloses are cellulose derivatives, and are obtained by linking carboxymethyl groups to some of hydroxyl groups in glucose residues constituting cellulose backbones to form ether linkages. Increase in the number of carboxymethyl groups (namely, increase in the degree of carboxymethyl substitution) allows carboxymethylated celluloses to be dissolved in water. On the other hand, adjusting the degree of carboxymethyl substitution to a proper range allows carboxymethylated celluloses to maintain fibrous shapes in water. A carboxymethylated cellulose having a fibrous shape can be mechanically defibrated and thus converted into a nanofiber having a nanoscale fiber diameter (PTL 1). 
     A commonly known method for producing carboxymethylated cellulose is a method including subjecting cellulose to an alkaline treatment (mercerization) and then a treatment with an etherifying agent (also referred to as “carboxymethylation agent”) (carboxymethylation, also called “etherification”). In the method, there are two known processes, one of which includes performing both mercerization and carboxymethylation by using water as a solvent and another of which includes performing both mercerization and carboxymethylation in an organic solvent or a mixed solvent of an organic solvent and water (PTL 2), and the former is called “water mediated method” and the latter is called “solvent mediated method”. 
     A known method for producing a cellulose nanofiber having a nanoscale fiber diameter is, for example, not only mechanical defibration of carboxymethylated cellulose, but also mechanical defibration of a cellulose into which carboxyl groups have been introduced (PTL 3). A water dispersion of a cellulose nanofiber obtained by defibration of such a carboxylated cellulose is known to be high in transparency. On the other hand, a water dispersion of a cellulose nanofiber obtained by defibration of a carboxymethylated cellulose obtained by a solvent mediated method is low in transparency as compared with a water dispersion of a cellulose nanofiber obtained by defibration of a carboxylated cellulose. For obtaining a water dispersion of a cellulose nanofiber with enhanced transparency by defibration of a carboxymethylated cellulose obtained by a water mediated method, a large amount of chemical agents such as a mercerizing agent and/or a carboxymethylation agent is needed, which has large problems with production and economy. Transparent materials are suitable for various applications and therefore it is demanded that cellulose nanofibers are transparent. In particular, carboxymethylated celluloses are materials high in safety and thus it is demanded to obtain a cellulose nanofiber high in transparency by use of carboxymethylated cellulose in an economic manner. 
     CITATION LIST 
     Patent Literature 
     PTL 1: International Publication No. WO 2014/088072 
     PTL 2: Japanese Patent Laid-Open No. 2017-149901 
     PTL 3: Japanese Patent Laid-Open No. 2008-1728 
     SUMMARY OF INVENTION 
     Technical Problem 
     An object of the present invention is to provide a carboxymethylated cellulose nanofiber which can be used to form a water dispersion high in transparency. 
     Solution to Problem 
     The present inventors have made intensive studies to achieve the above object, and as a result, have found that when mercerization (alkaline treatment of cellulose) in a solvent containing mainly water is followed by carboxymethylation (also referred to as “etherification”) in a mixed solvent of water and an organic solvent in carboxymethylation of cellulose, a carboxymethylated cellulose which can be defibrated to form a cellulose nanofiber dispersion very high in transparency can be produced at a high rate of effective utilization of a carboxymethylation agent in an economic manner, as compared with any carboxymethylated cellulose obtained according to a conventional water mediated method (method including performing both mercerization and carboxymethylation in water as a solvent) or solvent mediated method (method including performing both mercerization and carboxymethylation in an organic solvent or in a mixed solvent of water and an organic solvent). Such a carboxymethylated cellulose nanofiber is a novel carboxymethylated cellulose nanofiber which has not only a certain degree of carboxymethyl substitution, but also a high degree of crystallization of cellulose I type, and which exhibits a higher transparency than ever before in the form of a water dispersion. 
     The present invention provides the following, but is not limited thereto. 
     (1) A carboxymethylated cellulose nanofiber having a degree of carboxymethyl substitution of more than 0.30 and 0.60 or less and a degree of crystallization of cellulose I type of 60% or more, and exhibiting a transmittance to light at 660 nm of 60% or more when the carboxymethylated cellulose nanofiber is dispersed in water to form a water dispersion having a solid content of 1% (w/v). 
     (2) The carboxymethylated cellulose nanofiber according to (1), having an average fiber diameter of 3 nm to 500 nm. 
     (3) The carboxymethylated cellulose nanofiber according to (1) or (2), having an aspect ratio of 50 or more. 
     (4) The carboxymethylated cellulose nanofiber according to any one of (1) to (3), having a structure in which carboxymethyl groups are linked to hydroxyl groups of cellulose via ether linkages. 
     Advantageous Effects of Invention 
     The present invention provides a carboxymethylated cellulose nanofiber which can be used to form a water dispersion high in transparency. The carboxymethylated cellulose nanofiber of the present invention is characterized by having not only a high degree of carboxymethyl substitution (more than 0.30 and 0.60 or less), but also a high degree of crystallization of cellulose I type (60% or more). A high degree of carboxymethyl substitution, namely, a large number of carboxymethyl groups introduced into a cellulose fiber, enables electrical repulsion of cellulose fibers to thereby facilitate defibration of cellulose into a nanofiber, resulting in an increase in transparency of the nanofiber dispersion. When the degree of carboxymethyl substitution is high, the polarity is also high, and this brings about an advantage of, for example, but not limited to, enhanced compatibility with another high-polarity compound when added to medicines, foods, or the like. When a degree of crystallization of cellulose I type is high, the proportion of cellulose which maintains a crystal structure without dissolution in a solvent such as water is high, and this brings about advantages that such a nanofiber can impart high thixotropic properties and therefore is suitable for an application in viscosity modification such as a thickener, and also that such a nanofiber enhances ability to retain a shape of a gel-like substance (foods, medicines, and/or the like) when added thereto. 
    
    
     DESCRIPTION OF EMBODIMENTS 
     &lt;Carboxymethylated Cellulose Nanofiber&gt; 
     The present invention relates to a carboxymethylated cellulose nanofiber. The carboxymethylated cellulose has a structure formed by linking carboxymethyl groups to some of hydroxyl groups in glucose residues constituting cellulose. 
     The carboxymethylated cellulose nanofiber refers to a product obtained by converting the carboxymethylated cellulose having the above structure into a nanofiber having a nanoscale fiber diameter. The carboxymethylated cellulose may be in the form of a salt including a metal salt such as a carboxymethylated cellulose sodium salt, and the carboxymethylated cellulose nanofiber may also be in the form of a salt. 
     The carboxymethylated cellulose nanofiber of the present invention maintains at least a portion of a fibrous shape, even when dispersed in water. Specifically, the carboxymethylated cellulose nanofiber of the present invention is such that a fibrous substance is found in observation of a water dispersion of the carboxymethylated cellulose nanofiber with an electron microscope. It is also possible to observe a peak of a cellulose I type crystal in observation of the carboxymethylated cellulose nanofiber with X-ray diffraction. 
     &lt;Degree of Carboxymethyl Substitution&gt; 
     The carboxymethylated cellulose nanofiber of the present invention has a degree of carboxymethyl substitution per anhydrous glucose unit of cellulose of more than 0.30 and 0.60 or less, preferably more than 0.30 and less than 0.50. If the degree of substitution is more than 0.60, dissolution in water occurs to fail to maintain a fibrous form. The degree of substitution is further preferably more than 0.30 and 0.40 or less in consideration of operationability. Introduction of carboxymethyl groups to cellulose enables electrical repulsion of cellulose to thereby make defibration into a nanofiber possible. In a case where the degree of carboxymethyl substitution is more than 0.30, defibration into a nanofiber is facilitated and dispersibility is enhanced, thereby obtaining a cellulose nanofiber dispersion high in transparency. In a case where the degree of carboxymethyl substitution is more than 0.30, the cellulose nanofiber is enhanced in polarity, and this brings about an advantage of, for example, but not limited to, enhanced compatibility with another high-polarity compound when added to and use in foods, cosmetics, and/or the like. The degree of carboxymethyl substitution can be adjusted by controlling the amount of a carboxymethylation agent for reaction to be added, the amount of a mercerizing agent, and the compositional ratio between water and an organic solvent. 
     The anhydrous glucose unit in the present invention means individual anhydrous glucose (glucose residue) constituting cellulose. The degree of carboxymethyl substitution (also referred to as “degree of etherification”) herein represents the proportion of hydroxyl groups replaced with carboxymethyl ether groups among hydroxyl groups of the glucose residue constituting cellulose (the number of carboxymethyl ether groups per glucose residue). The degree of carboxymethyl substitution may be abbreviated as DS. 
     The method for measuring the degree of carboxymethyl substitution is as follows: 
     About 2.0 g of a sample is precisely weighed and is put in a 300-mL stoppered conical flask. 100 mL of a liquid obtained by adding 100 mL of nitric acid (special grade) to 1000 mL of nitric acid/methanol is added thereto and the resulting flask is shaken for 3 hours, thereby converting the salt of the carboxymethylated cellulose nanofiber (CMC) into H-CMC (hydrogen-type carboxymethylated cellulose nanofiber). 1.5 to 2.0 g of the absolute dry H-CMC is precisely weighed, and put in a 300-mL stoppered conical flask. The H-CMC is wetted with 15 mL of 80% methanol. 100 mL of 0.1 N—NaOH is added thereto, and the resulting flask was shaken at room temperature for 3 hours. 
     Phenolphthalein is used as an indicator to reversely titrate excess NaOH by 0.1 N—H 2 SO 4 , and the degree of carboxymethyl substitution (DS value) is calculated according to the following expressions. 
       A=[(100×F′−0.1N—H 2 SO 4  (mL)×F)×0.1]/(Absolute dry mass(g) of H-CMC)
 
     Degree of carboxymethyl substitution=0.162×A/(1−0.058×A)
 
F′: factor of 0.1 N—H 2 SO 4  
 
F: factor of 0.1 N—NaOH.
 
     The degree of carboxymethyl substitution of the carboxymethylated cellulose nanofiber is generally the same as the degree of carboxymethyl substitution of the carboxymethylated cellulose before formation of the nanofiber. 
     &lt;Degree of Crystallization of Cellulose I Type&gt; 
     The degree of crystallization of cellulose in the carboxymethylated cellulose nanofiber of the present invention is 60% or more, with respect to crystal I type. Although it is known that a higher degree of carboxymethyl substitution generally results in an increase in solubility of fiber and a decrease in degree of crystallization, the carboxymethylated cellulose nanofiber of the present invention is characterized by not only having a degree of carboxymethyl substitution of more than 0.3 and 0.6 or less, but also maintaining a degree of crystallization of 60% or more. A carboxymethylated cellulose nanofiber having such a degree of carboxymethyl substitution and a degree of crystallization of cellulose I type can be produced by a method described below. Crystallinity of cellulose can be controlled by the concentration of a mercerizing agent and the temperature in treatment, as well as the degree of carboxymethylation. An alkali at a high concentration is used in mercerization and carboxymethylation to thereby allow a I type crystal of cellulose to be easily converted into a II type crystal; however, for example, the amount of the alkali (mercerizing agent) used can be adjusted to adjust the degree of denaturation, thereby allowing desired crystallinity to be maintained. The upper limit of the degree of crystallization of cellulose I type is not particularly limited. The upper limit is considered to be actually about 90%. 
     The method for measuring the degree of crystallization of cellulose I type of the carboxymethylated cellulose nanofiber is as follows: 
     A sample is placed on a glass cell, and subjected to measurement with an X-ray diffractometer (LabX XRD-6000, manufactured by Shimadzu Corporation). The degree of crystallization is calculated according to a procedure of Segal, et al., and is calculated from the diffraction intensity of the 002 plane at 2θ=22.6° and the diffraction intensity of an amorphous portion at 2θ=18.5° with the diffraction intensity at 2θ=10° to 30° as the baseline in an X-ray diffraction diagram, according to the following expressions. 
         Xc =( I 002 c−Ia )/ I 002 c× 100 
     Xc=degree (%) of crystallization of cellulose I type 
     I002c: diffraction intensity of 002 plane at 20=22.6° 
     Ia: diffraction intensity of amorphous portion at 20=18.5°. 
     In a case where the degree of crystallization of cellulose I type is 60% or more, the proportion of cellulose which maintains a crystal structure without dissolution in a solvent such as water is high, which results in enhanced thixotropic properties (thixotropy), and such a nanofiber is suitable for an application in viscosity modification such as a thickener. Moreover, such a nanofiber has an advantage of, for example, but is not limited to, imparting an excellent ability to retain a shape to a gel-like substance (for example, foods and/or cosmetics) when added thereto. 
     The proportion of the I type crystal of the carboxymethylated cellulose nanofiber is generally the same as that of the carboxymethylated cellulose before formation of the nanofiber. 
     &lt;Transparency in Water Dispersion&gt; 
     The carboxymethylated cellulose nanofiber of the present invention is characterized by exhibiting a high transparency when dispersed in water as a dispersion medium to form a dispersion (water dispersion). The transparency herein refers to the transmittance to light at 660 nm (length of light path: 10 mm) when the carboxymethylated cellulose nanofiber is dispersed in water to form a water dispersion having a solid content of 1% (w/v). The transparency of the carboxymethylated cellulose nanofiber of the present invention is 60% or more, more preferably 70% or more, further preferably 80% or more, further preferably 90% or more. The upper limit of the transparency is not particularly limited, and is, for example, 100%. Such a cellulose nanofiber can be optimally used in an application which requires transparency. 
     &lt;Fiber Diameter and Aspect Ratio&gt; 
     The carboxymethylated cellulose nanofiber has a nanoscale fiber diameter. The average fiber diameter is preferably 3 nm to 500 nm, further preferably 3 nm to 150 nm, further preferably 3 nm to 20 nm, further preferably 5 nm to 19 nm, further preferably 5 nm to 15 nm. The aspect ratio is preferably 50 or more, further preferably 70 or more. The upper limit of the aspect ratio is not particularly limited, and is about 500 or less, preferably less than 400. In a case where the aspect ratio is 50 or more, the nanofiber has a fibrous shape, which exhibits the effect of enhancing thixotropic properties. In a case where the aspect ratio is less than 400, the fiber length is not too long and fiber entwining is thus decreased, thereby enabling the occurrence of an aggregate (clump) of the cellulose nanofiber to be reduced. 
     The average fiber diameter and the average fiber length of the carboxymethylated cellulose nanofiber can be each determined by analyzing 200 fibers randomly selected, with an atomic force microscope (AFM) in the case of a diameter of 20 nm or less or with a field emission scanning electron microscope (FE-SEM) in the case of a diameter of 20 nm or more, and calculating the average. The aspect ratio can be calculated according to the following expression: 
       Aspect ratio=Average fiber length/Average fiber diameter. 
     &lt;Others&gt; 
     The carboxymethylated cellulose nanofiber may be in the state of a dispersion obtained after production, or may be, if necessary, dried or re-dispersed in water. The drying method is not limited, and, for example, any known method such as a freeze-drying method, a spray-drying method, a shelf-type drying method, a drum drying method, a belt drying method, a drying method including thinly extending on a glass plate or the like, a fluid-bed drying method, a microwave drying method, or a drying method including using heat generating fan under reduced pressure can be used. After drying, the resultant may be, if necessary, pulverized by a cutter mill, a hammer mill, a pin mill, a jet mill, or the like. The method for re-dispersing in water is also not particularly limited, and any known dispersing apparatus can be used. 
     The application of the carboxymethylated cellulose nanofiber is not particularly limited. It is considered that the carboxymethylated cellulose nanofiber can be used as a thickener, a gelling agent, a pasting agent, a food additive, an excipient, an additive for paints, an additive for adhesives, an additive for papermaking, a polishing agent, a compounding material for rubber or plastics, a water retention agent, a shape retention agent, a mud adjuster, a filtration aid, a mud overflow inhibitor, or the like in various fields in which an additive is generally used, for example, foods, beverages, cosmetics, medicines, papermaking, various chemical goods, paints, spray, agricultural chemicals, civil engineering, architecture, electronic materials, flame retardants, household products, adhesives, detergents, aromatic substances, and lubricant compositions. 
     &lt;Method for Producing Carboxymethylated Cellulose Nanofiber&gt; 
     A carboxymethylated cellulose nanofiber having a degree of carboxymethyl substitution of more than 0.30 and 0.60 or less and a degree of crystallization of cellulose I type of 60% or more, and exhibiting a transmittance to light at 660 nm of 60% or more when the carboxymethylated cellulose is dispersed in water to form a water dispersion having a solid content of 1% (w/v) can be produced by defibrating a carboxymethylated cellulose produced by the following method, but the method being not limited thereto. 
     Carboxymethylated celluloses can be commonly produced by subjecting cellulose to an alkaline treatment (mercerization), and thereafter allowing the resulting mercerized cellulose (also referred to as “alkaline cellulose”) to react with a carboxymethylation agent (also referred to as “etherifying agent”). A carboxymethylated cellulose which can form the nanofiber having the above characteristics of the present invention can be produced by performing mercerization (alkaline treatment of cellulose) in a solvent containing mainly water, and thereafter performing carboxymethylation (also referred to as “etherification”) in a mixed solvent of water and an organic solvent. The carboxymethylated cellulose thus obtained can be converted into a cellulose nanofiber dispersion high in transparency by defibration while having achieved a high rate of effective utilization of a carboxymethylation agent, as compared with any carboxymethylated cellulose obtained according to a conventional water mediated method (method including performing both mercerization and carboxymethylation with water as a solvent) or solvent mediated method (method including performing both mercerization and carboxymethylation in a solvent containing mainly an organic solvent). 
     &lt;Cellulose&gt; 
     The cellulose in the present invention means polysaccharide having a structure in which D-glucopyranoses (D-glucopyranose is also simply referred to as “glucose residue” or “anhydrous glucose”) are connected by β-1,4 linkages. Celluloses are commonly classified to, for example, native cellulose, regenerated cellulose, fine cellulose, and microcrystalline cellulose, from which an amorphous region is removed, depending on the source, the production method, and the like. Any of such celluloses can be used as a raw material of the mercerized cellulose in the present invention. 
     Examples of the native cellulose include bleached pulp and unbleached pulp (bleached wood pulp or unbleached wood pulp); linter and refined linter; and cellulose produced by microorganisms such as acetic acid bacteria. The raw material of the bleached pulp or unbleached pulp is not limited, and examples thereof include wood, cotton, straw, bamboo, hemp, jute, and kenaf. The method for producing the bleached pulp or unbleached pulp is also not particularly limited, and may be a mechanical method, a chemical method, or a combined intermediate method between these two methods. Examples of the bleached pulp or unbleached pulp classified according to the production method include mechanical pulp (thermomechanical pulp (TMP), groundwood pulp), chemical pulp (sulfite pulp such as needle (softwood) unbleached sulfite pulp (NUSP), and needle bleached sulfite pulp (NBSP), and kraft pulp such as needle unbleached kraft pulp (NUKP), needle bleached kraft pulp (NBKP), leaf (hardwood) unbleached kraft pulp (LUKP), and leaf bleached kraft pulp (LBKP)). Dissolving pulp may also be used, besides papermaking pulp. Dissolving pulp is pulp chemically refined, is mainly used in a dissolved state in chemicals, and serves as a main raw material of an artificial fiber, cellophane, or the like. 
     Examples of the regenerated cellulose include one obtained by dissolving cellulose in a solvent such as a cuprammonium solution, a cellulose xanthate solution, or a morpholine derivative, and anew subjecting the resultant to spinning. Examples of the fine cellulose include one obtained by subjecting a cellulose material such as the native cellulose or regenerated cellulose to a depolymerization treatment (for example, acid hydrolysis, alkali hydrolysis, enzymatic degradation, a blasting treatment, or a vibration ball mill treatment), and one obtained by mechanically treating the cellulose-based material. 
     &lt;Mercerization&gt; 
     The mercerized cellulose (also referred to as “alkaline cellulose”) is obtained by using the above cellulose as a raw material and adding a mercerizing agent (alkali) thereto. A carboxymethylated cellulose which can be defibrated to form a cellulose nanofiber dispersion having a very high transparency can be obtained in an economic manner by using mainly water as a solvent for a mercerization reaction and a mixed solvent of an organic solvent and water for the next carboxymethylation, according to the method described herein. 
     “Using mainly water as a solvent (solvent containing mainly water)” refers to using a solvent containing water in an amount of higher than 50% by mass. The solvent containing mainly water preferably contains 55% by mass or more, more preferably 60% by mass or more, more preferably 70% by mass or more, more preferably 80% by mass or more, further preferably 90% by mass or more, further preferably 95% by mass or more of water. The solvent containing mainly water particularly preferably contains 100% by mass of water (namely, is water). As the proportion of water in mercerization is higher, the transparency of a cellulose nanofiber dispersion obtained by defibrating the carboxymethylated cellulose is higher. Examples of the solvent (which is mixed with water when use) other than water in the solvent containing mainly water include an organic solvent used as a solvent in carboxymethylation of the later stage. Examples can include alcohols such as methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butanol, isobutanol, and tert-butanol, ketones such as acetone, diethyl ketone, and methyl ethyl ketone, and dioxane, diethyl ether, benzene and dichloromethane. These can be used singly or in the form of a mixture of two or more thereof, and be added to water in an amount of less than 50% by mass, as a solvent for mercerization. The solvent containing mainly water preferably contains 45% by mass or less, further preferably 40% by mass or less, further preferably 30% by mass or less, further preferably 20% by mass or less, further preferably 10% by mass or less, further preferably 5% by mass or less, more preferably 0% by mass of the organic solvent. 
     Examples of the mercerizing agent include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide, and these can be used singly or in combinations of any two or more thereof. The mercerizing agent is not limited thereto, but such an alkali metal hydroxide can be added to a reactor, for example, in the form of an aqueous solution having a content of the alkali metal hydroxide of 1 to 60% by mass, preferably 2 to 45% by mass, more preferably 3 to 25% by mass. The amount of the mercerizing agent used is preferably 0.1 mol or more and 2.5 mol or less, more preferably 0.3 mol or more and 2.0 mol or less, further preferably 0.4 mol or more and 1.5 mol or less, per 100 g of cellulose (absolute dry), in one embodiment. 
     The amount of the solvent containing mainly water in mercerization may be any amount as long as a raw material can be stirred and mixed, and is not particularly limited, and the amount thereof is preferably 1.5 to 20 times, more preferably 2 to 10 times as large as that of cellulose as a raw material, in terms of mass. 
     A mercerization treatment is performed by mixing a starting raw material (cellulose) and the solvent containing mainly water, adjusting the temperature of a reactor to 0 to 70° C., preferably 10 to 60° C., more preferably 10 to 40° C., adding an aqueous solution of a mercerizing agent, and stirring the resultant for 15 minutes to 8 hours, preferably 30 minutes to 7 hours, more preferably 30 minutes to 3 hours. Thus, the mercerized cellulose (alkaline cellulose) is obtained. 
     The pH in mercerization is preferably 9 or more, and thus the mercerization reaction can progress. The pH is more preferably 11 or more, further preferably 12 or more, and may be 13 or more. The upper limit of the pH is not particularly limited. 
     Such mercerization can be performed by use of a reactor in which the above respective components can be mixed and stirred while controlling the temperature, and any of various reactor conventionally used for a mercerization reaction can be used. For example, a batch type stirred reactor in which two screws are used to stir and mix the components is preferable in view of uniform mixing ability and also productivity. 
     &lt;Carboxymethylation&gt; 
     The carboxymethylated cellulose is obtained by adding a carboxymethylation agent (also referred to as “etherifying agent”) to the mercerized cellulose. A carboxymethylated cellulose which can be defibrated to form a cellulose nanofiber dispersion very high in transparency can be obtained in an economic manner, by use of a solvent containing mainly water in mercerization and a mixed solvent of water and an organic solvent in carboxymethylation, according to the method described herein. 
     Examples of the carboxymethylation agent include monochloroacetic acid, sodium monochloroacetate, methyl monochloroacetate, ethyl monochloroacetate, and isopropyl monochloroacetate. In particular, monochloroacetic acid or sodium monochloroacetate is preferable in terms of their availability. The carboxymethylation agent is preferably added in an amount ranging from 0.5 to 1.5 mol per anhydrous glucose unit of cellulose. The lower limit of the range is more preferably 0.6 mol or more, further preferably 0.7 mol or more, and the upper limit in the range is more preferably 1.3 mol or less, further preferably 1.1 mol or less. The carboxymethylation agent can be added to a reactor, for example, but not limited to, in the form of an aqueous solution having a content of the carboxymethylation agent of 5 to 80% by mass, more preferably 30 to 60% by mass, and can also be added in the form of a powder without any dissolution. 
     The molar ratio of the mercerizing agent to the carboxymethylation agent (mercerizing agent/carboxymethylation agent) generally adopted is 0.90 to 2.45 in a case where monochloroacetic acid or sodium monochloroacetate is used as the carboxymethylation agent. The reason for this is as follows: a molar ratio of less than 0.90 can cause a carboxymethylation reaction to insufficiently progress, resulting in remaining of the unreacted monochloroacetic acid or sodium monochloroacetate and thus disecomomy, and a molar ratio of more than 2.45 may cause a side reaction of an excess of the mercerizing agent with monochloroacetic acid or sodium monochloroacetate to progress, resulting in production of a glycolic acid alkali metal salt and thus diseconomy. 
     The rate of effective utilization of the carboxymethylation agent in carboxymethylation is preferably 15% or more, more preferably 20% or more, further preferably 25% or more, particularly preferably 30% or more. The rate of effective utilization of the carboxymethylation agent refers to the proportion of carboxymethyl groups introduced into cellulose to carboxymethyl groups in the carboxymethylation agent. A carboxymethylated cellulose which can be defibrated to form a cellulose nanofiber dispersion high in transparency can be produced, at a high rate of effective utilization of the carboxymethylation agent (namely, in an economic manner without use of large amount of the carboxymethylation agent), by use of a solvent containing mainly water in mercerization and a mixed solvent of water and an organic solvent in carboxymethylation. The upper limit of the rate of effective utilization of the carboxymethylation agent is not particularly limited, and the upper limit is actually about 80%. The rate of effective utilization of the carboxymethylation agent may be herein abbreviated as AM. 
     The method for calculating the rate of effective utilization of the carboxymethylation agent is as follows: 
         AM =( DS ×Number of moles of cellulose)/Number of moles of carboxymethylation agent
 
     DS: Degree of carboxymethyl substitution (The measurement method is as described above)
 
Number of moles of cellulose: Mass of pulp (Dry mass after drying at 100° C. for 60 minutes)/162
 
(162 means the molecular weight per glucose unit of cellulose).
 
     The concentration of the raw material of cellulose in the carboxymethylation reaction is not particularly limited, and is preferably 1 to 40% (w/v) in view of an enhancement in rate of effective utilization of the carboxymethylation agent. 
     The carboxymethylation reaction is allowed to progress in a mixed solvent of water and an organic solvent, the mixed solvent being formed by appropriately adding an organic solvent or an aqueous solution of an organic solvent to the reactor, or alternatively, appropriately reducing the organic solvent other than water for the mercerization treatment, for example, under reduced pressure, at the same time as of addition of the carboxymethylation agent or before or immediately after addition of the carboxymethylation agent, to thereby form the mixed solvent of water and an organic solvent. The timing of addition or reduction of the organic solvent is not particularly limited as long as it is within the time from completion of the mercerization reaction to immediately after addition of the carboxymethylation agent, and is preferably, for example, within 30 minutes before or after addition of the carboxymethylation agent. 
     Examples of the organic solvent include alcohols such as methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butanol, isobutanol, and tert-butanol, ketones such as acetone, diethyl ketone, and methyl ethyl ketone, and dioxane, diethyl ether, benzene and dichloromethane. These can be used singly or in the form of a mixture of two or more thereof, and be added to water as a solvent for carboxymethylation. In particular, a monohydric alcohol having 1 to 4 carbon atoms is preferable, and a monohydric alcohol having 1 to 3 carbon atoms is further preferable, because compatibility with water is excellent. 
     The content of the organic solvent in the mixed solvent for carboxymethylation is preferably 20 to 99% by mass, more preferably 30 to 99% by mass, further preferably 40 to 99% by mass, further preferably 45 to 99% by mass based on the total of water and the organic solvent. 
     The reaction medium (the mixed solvent of water and the organic solvent, containing no cellulose) for carboxymethylation preferably has a lower water content (in other words, a higher organic solvent content) than the reaction medium for mercerization. When the above range is satisfied, the degree of crystallization of the resulting carboxymethylated cellulose can be easily maintained while the degree of carboxymethyl substitution is also easily increased, and thus a carboxymethylated cellulose which can be defibrated to form a cellulose nanofiber dispersion high in transparency by defibration can be more efficiently obtained. In a case where the reaction medium for carboxymethylation has a lower water content (a higher organic solvent content) than the reaction medium for mercerization, the mixed solvent for the carboxymethylation reaction can be advantageously formed by a simple procedure in which a desired amount of the organic solvent is added to the reaction system after completion of the mercerization reaction, in transferring from the mercerization reaction to the carboxymethylation reaction. 
     The mixed solvent of water and an organic solvent is thus formed, the carboxymethylation agent is added to the mercerized cellulose, and then the resultant is stirred for about 15 minutes to 4 hours, preferably about 15 minutes to 1 hour while the temperature is preferably kept constantly in the range from 10 to 40° C. Mixing of a liquid containing the mercerized cellulose with the carboxymethylation agent is preferably performed by adding the carboxymethylation agent in portions or dropwise, in view of preventing the reaction mixture from being at a high temperature. After the carboxymethylation agent is added, the resulting mixture was stirred for a certain time, and the temperature thereof is, if necessary, raised. An etherification (carboxymethylation) reaction is then performed at a reaction temperature of 30 to 90° C., preferably 40 to 90° C., further preferably 60 to 80° C. for 30 minutes to 10 hours, preferably 1 hour to 4 hours, thereby obtaining the carboxymethylated cellulose. 
     The reactor which has been used in mercerization may be used as it is, or another reactor in which the components can be mixed and stirred while controlling the temperature may be used, in carboxymethylation. 
     After completion of the reaction, the remaining alkali metal salt may be neutralized with a mineral acid or organic acid so that the pH is 6 to 8. If necessary, an inorganic salt, an organic acid salt, and others as by-product(s) may be removed by washing with water-containing methanol, and the resultant may be dried, pulverized and classified to form the carboxymethylated cellulose or the salt thereof. For washing for removal of by-product(s), the carboxymethylated cellulose may be converted into an acid form before the washing, and the acid form may be returned to a salt form after the washing. Examples of an apparatus for use in dry pulverization include impact mills such as a hammer mill and a pin mill, medium mills such as a ball mill and a tower mill, and jet mills. Examples of an apparatus for use in wet pulverization include apparatuses such as a homogenizer, a masscolloider, and a pearl mill. 
     &lt;Defibration into Nanofiber&gt; 
     A carboxymethylated cellulose obtained by the above method can be defibrated and thus converted into a cellulose nanofiber having a nanoscale fiber diameter. 
     A dispersion of the carboxymethylated cellulose obtained by the above method is prepared for defibration. A dispersion medium is preferably water in terms of ease of handling. The concentration of the carboxymethylated cellulose in the dispersion in defibration is preferably 0.01 to 10% (w/v) in consideration of defibration and dispersing efficiencies. 
     The apparatus for use in defibration of the carboxymethylated cellulose is not particularly limited, and, for example, a high-speed rotation type, colloide mill type, high pressure type, roll mill type, or ultrasound type apparatus can be used. It is preferable to apply a strong shear force to the dispersion of the carboxymethylated cellulose in defibration. Particularly, it is preferable for efficient defibration to use a wet high-pressure or ultra-high-pressure homogenizer which can apply a pressure of 50 MPa or more to the dispersion and can apply a strong shear force. The pressure is more preferably 100 MPa or more, further preferably 140 MPa or more. The dispersion may be subjected to, if necessary, a pre-treatment with known mixing, stirring, emulsifying, and/or dispersing apparatus(es), such as a high-speed shear mixer, prior to a defibration and dispersing treatment by a high-pressure homogenizer. 
     The high-pressure homogenizer is an apparatus which performs emulsifying, dispersing, defibrating, pulverizing, and ultra-fining with total energy of particle collision, a shear force due to the difference in pressure, and the like by subjecting a fluid to pressurizing (high pressure) by a pump and thus ejecting the fluid through a very fine gap provided in a passage. 
     While the reason why the above method can provide a cellulose nanofiber high in transparency in an economic manner is not clear, the present inventors have confirmed that the above method enables maintaining a high degree of crystallization of cellulose I type and thus enables maintaining a fibrous shape of the carboxymethylated cellulose even at a high degree of carboxymethyl substitution. It is considered that a high degree of carboxymethyl substitution (namely, introduction of many carboxymethyl groups) with the fibrous shape being maintained leads to an enhancement in defibrating properties of carboxymethylated cellulose, and this is probably one reason why a nanofiber dispersion high in transparency is obtained. However, reasons other than the above are not excluded. 
     EXAMPLES 
     Hereinafter, the present invention will be more specifically described with reference to Examples and Comparative Examples, but the present invention is not intended to be limited thereto. Unless particularly noted, “part(s)” and “%” represent “part(s) by mass” and “% by mass”. 
     Example 1 
     To a twin-screw kneader whose rotational speed was modulated to 100 rpm, were added 130 parts of water and a solution of 20 parts of sodium hydroxide in 100 parts of water, and leaf pulp (LBKP manufactured by Nippon Paper Industries Co., Ltd.) was added thereto in an amount of 100 parts in terms dry mass when dried at 100° C. for 60 minutes. The resultant was stirred and mixed at 30° C. for 90 minutes, to prepare mercerized cellulose. Furthermore, 230 parts of isopropanol (IPA) and 60 parts of sodium monochloroacetate were added thereto while stirring. The resultant was stirred for 30 minutes, and the temperature was raised to 70° C. to allow a carboxymethylation reaction to occur for 90 minutes. The concentration of IPA in a reaction medium in the carboxymethylation reaction was 50%. After completion of the reaction, the resultant was subjected to neutralization to a pH of about 7 with acetic acid, washing with water-containing methanol, liquid removal, and drying, and the resulting dried product was pulverized, thereby obtaining a carboxymethylated cellulose sodium salt having a degree of carboxymethyl substitution of 0.31 and a degree of crystallization of cellulose I type of 67%. The rate of effective utilization of the carboxymethylation agent was 37%. The methods for measuring the degree of carboxymethyl substitution and the degree of crystallization of cellulose I type, and the method for calculating the rate of effective utilization of the carboxymethylation agent are as described above. 
     The resulting carboxymethylated cellulose sodium salt was dispersed in water to form a 1% (w/v) water dispersion. The water dispersion was treated by a high-pressure homogenizer at 150 MPa three times, thereby obtaining a carboxymethylated cellulose nanofiber dispersion. The transparency and the viscosity of the resulting dispersion were measured according to the following methods. 
     &lt;Measurement of Transparency of Cellulose Nanofiber Dispersion&gt; 
     The transparency (transmittance to light at 660 nm) of the cellulose nanofiber dispersion (solid content: 1% (w/v), dispersion medium: water) was measured with a square cell having a length of light path of 10 mm and UV-VIS spectrophotometer UV-1800 (Shimadzu Corporation). 
     &lt;Measurement of Viscosity&gt; 
     The cellulose nanofiber dispersion (solid content: 1% (w/v), dispersing medium: water) was left to still stand at 25° C. for 16 hours, and thereafter stirred using a stirring machine at 3000 rpm for 1 minute, and the viscosity was measured with a No. 4 rotor at a rotational speed of 60 rpm or 6 rpm after 3 minutes by use of a B-type viscometer (manufactured by Toki Sangyo Co., Ltd). 
     Example 2 
     A carboxymethylated cellulose sodium salt was obtained in the same manner as in Example 1 except that the amount of IPA added was changed to thereby change the concentration of IPA in the reaction liquid in the carboxymethylation reaction to 90%. The degree of carboxymethyl substitution was 0.47, the degree of crystallization of cellulose I type was 63%, and the rate of effective utilization of the carboxymethylation agent was 56%. 
     The resulting carboxymethylated cellulose sodium salt was defibrated in the same manner as in Example 1, thereby obtaining a carboxymethylated cellulose nanofiber dispersion. 
     Example 3 
     A carboxymethylated cellulose sodium salt was obtained in the same manner as in Example 1 except that a solution of 40 parts of sodium hydroxide in 100 parts of water was used instead of the solution of 20 parts of sodium hydroxide in 100 parts of water for the mercerization reaction and 50 parts of monochloroacetic acid was used instead of 60 parts of sodium monochloroacetate as the carboxymethylation agent in the carboxymethylation reaction. The degree of carboxymethyl substitution was 0.31, the degree of crystallization of cellulose I type was 60%, and the rate of effective utilization of the carboxymethylation agent was 36%. The resulting carboxymethylated cellulose sodium salt was defibrated in the same manner as in Example 1, thereby obtaining a carboxymethylated cellulose nanofiber dispersion. 
     Comparative Example 1 
     A carboxymethylated cellulose sodium salt was obtained in the same manner as in Example 1 except that the solvent for the carboxymethylation reaction was of 100% of water. The degree of carboxymethyl substitution was 0.11, the degree of crystallization of cellulose I type was 72%, and the rate of effective utilization of the carboxymethylation agent was 13%. The resulting carboxymethylated cellulose sodium salt was defibrated in the same manner as in Example 1, thereby obtaining a carboxymethylated cellulose nanofiber dispersion. 
     Comparative Example 2 
     A carboxymethylated cellulose sodium salt was obtained in the same manner as in Comparative Example 1 except that a solution of 45 parts of sodium hydroxide in 100 parts of water was used instead of the solution of 20 parts of sodium hydroxide in 100 parts of water for the mercerization reaction and that 150 parts of sodium monochloroacetate was used instead of 60 parts of sodium monochloroacetate as the carboxymethylation agent in the carboxymethylation reaction. The degree of carboxymethyl substitution was 0.28, the degree of crystallization of cellulose I type was 45%, and the rate of effective utilization of the carboxymethylation agent was 13%. The resulting carboxymethylated cellulose sodium salt was defibrated in the same manner as in Example 1, thereby obtaining a carboxymethylated cellulose nanofiber dispersion. 
     Comparative Example 3 
     A carboxymethylated cellulose sodium salt was obtained in the same manner as in Comparative Example 1, except that a solution of 50 parts of sodium hydroxide in 100 parts of water was used instead of the solution of 20 parts of sodium hydroxide in 100 parts of water for the mercerization reaction and that 160 parts of sodium monochloroacetate was used instead of 60 parts of sodium monochloroacetate as the carboxymethylation agent in the carboxymethylation reaction. The degree of carboxymethyl substitution was 0.30, the degree of crystallization of cellulose I type was 40%, and the rate of effective utilization of a carboxymethylation agent was 13%. The resulting carboxymethylated cellulose sodium salt was defibrated in the same manner as in Example 1, thereby obtaining a carboxymethylated cellulose nanofiber dispersion. 
     Comparative Example 4 
     A carboxymethylated cellulose sodium salt was obtained in the same manner as in Example 1, except that the solvent for the mercerization reaction contained 10% of water and 90% of IPA and that a solvent having the same composition was also used in the carboxymethylation reaction. The degree of carboxymethyl substitution was 0.27, the degree of crystallization of cellulose I type was 64%, and the rate of effective utilization of a carboxymethylation agent was 32%. The resulting carboxymethylated cellulose sodium salt was defibrated in the same manner as in Example 1, thereby obtaining a carboxymethylated cellulose nanofiber dispersion. 
     Comparative Example 5 
     A carboxymethylated cellulose sodium salt was obtained in the same manner as in Example 1, except that the solvent for the mercerization reaction contained 5% of water and 95% of IPA and that a solvent having the same composition was also used in the carboxymethylation reaction. The degree of carboxymethyl substitution was 0.31, the degree of crystallization of cellulose I type was 61%, and the rate of effective utilization of a carboxymethylation agent was 37%. The resulting carboxymethylated cellulose sodium salt was defibrated in the same manner as in Example 1, thereby obtaining a carboxymethylated cellulose nanofiber dispersion. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 Carboxymethylated cellulose 
               
            
           
           
               
               
               
               
            
               
                   
                 Mercerization 
                 Carboxymethylation 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Solvent 
                   
                 Solvent 
                 Degree of 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Mercerizing 
                   
                 Organic 
                 Carboxymethylation 
                   
                 Organic 
                 carboxymethyl 
               
               
                   
                 agent 
                 Water 
                 solvent 
                 agent 
                 Water 
                 solvent 
                 substitution 
               
               
                   
               
               
                 Example 1 
                 NaOH 
                 100% 
                 — 
                 Na monochloroacetate 
                 50% 
                 IPA50% 
                 0.31 
               
               
                 Example 2 
                 NaOH 
                 100% 
                 — 
                 Na monochloroacetate 
                 10% 
                 IPA90% 
                 0.47 
               
               
                 Example 3 
                 NaOH 
                 100% 
                 — 
                 Monochloroacetic acid 
                 50% 
                 IPA50% 
                 0.31 
               
               
                 Comparative 
                 NaOH 
                 100% 
                 — 
                 Na monochloroacetate 
                 100%  
                 — 
                 0.11 
               
               
                 Example 1 
               
               
                 Comparative 
                 NaOH 
                 100% 
                 — 
                 Na monochloroacetate 
                 100%  
                 — 
                 0.28 
               
               
                 Example 2 
               
               
                 Comparative 
                 NaOH 
                 100% 
                 — 
                 Na monochloroacetate 
                 100%  
                 — 
                 0.30 
               
               
                 Example 3 
               
               
                 Comparative 
                 NaOH 
                  10% 
                 IPA90% 
                 Na monochloroacetate 
                 10% 
                 IPA90% 
                 0.27 
               
               
                 Example 4 
               
               
                 Comparative 
                 NaOH 
                  5% 
                 IPA95% 
                 Na monochloroacetate 
                  5% 
                 IPA95% 
                 0.31 
               
               
                 Example 5 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Carboxymethylated cellulose 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Degree of 
                 Rate of effective 
                   
               
               
                   
                 crystallization of 
                 utilization of 
                 Cellulose nanofiber dispersion 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 cellulose I 
                 carboxymethylation 
                 Viscosity 
                 Viscosity 
                   
               
               
                   
                   
                 type 
                 agent 
                 (60 pm) 
                 (6 rpm) 
                 Transparency 
               
               
                   
                   
               
               
                   
                 Example 1 
                 67% 
                 37% 
                 3700 
                 22000 
                 94% 
               
               
                   
                 Example 2 
                 63% 
                 56% 
                 2800 
                 20500 
                 92% 
               
               
                   
                 Example 3 
                 60% 
                 36% 
                 N.D. 
                 N.D. 
                 90% 
               
               
                   
                 Comparative 
                 72% 
                 13% 
                 2350 
                 14000 
                  7% 
               
               
                   
                 Example 1 
               
               
                   
                 Comparative 
                 45% 
                 13% 
                 N.D. 
                 N.D. 
                 90% 
               
               
                   
                 Example 2 
               
               
                   
                 Comparative 
                 40% 
                 13% 
                 N.D. 
                 N.D. 
                 91% 
               
               
                   
                 Example 3 
               
               
                   
                 Comparative 
                 64% 
                 32% 
                 N.D. 
                 N.D. 
                 10% 
               
               
                   
                 Example 4 
               
               
                   
                 Comparative 
                 61% 
                 37% 
                 N.D. 
                 N.D. 
                 10% 
               
               
                   
                 Example 5 
               
               
                   
                   
               
            
           
         
       
     
     It was found as shown in Table 1 that in each of Examples 1 to 3, in which mercerization and carboxymethylation were performed in the solvent containing mainly water and the mixed solvent of water and an organic solvent respectively, a carboxymethylated cellulose nanofiber having a degree of carboxymethyl substitution of more than 0.30 and 0.60 or less and a degree of crystallization of cellulose I type of 60% or more, and exhibiting a transparency of 60% or more was produced. On the other hand, it was found that in Comparative Examples 1 to 3, in which a water mediated method as a conventional method was used, an increase in the degree of carboxymethyl substitution caused the degree of crystallization of cellulose I type to be decreased. It was also found that in Comparative Examples 4 and 5, in which a solvent mediated method as a conventional method was used, the transparency was extremely low though the degree of carboxymethyl substitution was high with a degree of crystallization of cellulose I type of 60% or more. The carboxymethylated cellulose nanofiber of the present invention, which has a degree of carboxymethyl substitution of more than 0.3 and 0.60 or less and a degree of crystallization of cellulose I type of 60% or more and exhibits a transparency of 60% or more, is a novel material having unprecedented characteristics, and is expected to be a new option in an application for which a high degree of transparency is demanded, an application for which thixotropic properties are required, and the like.