Aggregate of acylated cellulose nanofibers

Abstract

To provide an aggregate of cellulose nanofibers predominantly composed of acylated single cellulose nanofibers with a fiber diameter of 3-10 nm and a method for producing the same.SOLUTION: The present invention discloses an aggregate of cellulose nanofibers in which the proportion of acylated hydroxy groups of cellulose of cellulose nanofibers with a fiber diameter of 3-100 nm (an average level of substitution of acyl groups) is 0.05-1.8. The aggregate of cellulose nanofibers has a cellulose Iβcrystal structure. The number content of the 3-10 nm-diameter single cellulose nanofibers forming the aggregate is 98% or more. The single cellulose nanofibers has an aspect ratio of 60-300.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention provides an aggregate of acylated modified cellulose nanofibers (average acyl group substitution degree of 0.05 or more) in which the number content of single cellulose nanofibers having a fiber diameter of 3 to 10 nm is 98% or more. [Background technology]

[0002] Cellulose fibers make up approximately 40% of plant cell walls and are the most abundant organic resource on Earth. In addition to having excellent elastic modulus, strength, and dimensional stability, they are also environmentally friendly, making them highly anticipated as a replacement for petroleum-derived plastics.

[0003] Cellulose is a natural polymer formed by the linear polymerization of β-glucose molecules via glycosidic bonds. Cellulose fibers are formed through structural stages such as cellulose molecular chains, elementary fibrils (approximately 3-10 nm), microfibrils (20-50 nm), and lamellae. Depending on the manufacturing method, cellulose nanofibers include elementary fibrils (single cellulose nanofibers or monofilaments) with a width of 3 to 10 nm, which are the most basic units; cellulose microfibril bundles (cellulose nanofibers) with a width of 20 to 50 nm, which are loose bundles of several fibrils and exist as basic units in the cell wall; and microfibrillated cellulose (MFC), in which microfibril bundles are further bundled together to form a spider web-like network of several tens to several hundred nanometers. That is, when cellulose fibers are broken down into microfibrils, they become the conventionally-conceived cellulose nanofibers (CNFs, hereinafter cellulose nanofibers may be abbreviated as "CNFs"), which are several tens of nanometers wide. On the other hand, when they are broken down into elementary fibrils, they become single nanofibers (single CNFs), which are several nanometers wide.

[0004] CNFs have attracted particular attention as a novel reinforcing material due to their high recyclability and environmental friendliness compared to inorganic reinforcing materials such as carbon fiber and glass fiber. Furthermore, single CNFs are expected to be used as reinforcing materials for optical plastics. Typically, only electrostatic repulsion methods can be used to disaggregate single CNFs. For example, anionic CNFs such as TEMPO-oxidized CNF (Non-Patent Document 1), succinic acid CNF or maleic acid CNF (Patent Document 1, Non-Patent Document 2, Non-Patent Document 3), phosphated CNF (Patent Document 2), and sulfated CNF (Patent Document 3) have anionic functional groups introduced onto the surface of elementary fibrils or microfibrils. The anionic groups undergo electrostatic repulsion in water, and cellulose fibers are disaggregated into elementary fibrils to obtain single nanofibers. However, anionic single CNF has high hydrophilicity and a low thermal decomposition temperature, limiting its application as a reinforcing material.

[0005] The applicant has been developing acylated-modified cellulose nanofibers to solve the problems associated with anionic cellulose nanofibers, but has not been able to obtain acylated-modified cellulose nanofibers that are primarily composed of acylated-modified single cellulose nanofibers from acylated cellulose nanofibers modified with acyl groups (Patent Documents 4, 5, 6, and 7). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2017-082188 [Patent Document 2] Japanese Patent Application Publication No. 2017-025468 [Patent Document 3] International Publication No. 2018 / 131721 [Patent Document 4] Patent No. 5676860 [Patent Document 5] Patent No. 5875323 [Patent Document 6] Patent No. 6454427 [Patent Document 7] Patent No. 6633182 [Non-patent literature]

[0007] [Non-Patent Document 1] Nanoscale,2011,3,71-85. [Non-patent document 2] ACS Macro Lett.2015,4,80-83 [Non-patent document 3] Chem.Soc.Rev.,2011,40,3941-3994 Summary of the Invention [Problem to be solved by the invention]

[0008] An object of the present invention is to provide an aggregate of cellulose nanofibers composed mainly of acylated modified single cellulose nanofibers (average acyl group substitution degree of 0.05 or more) having a fiber diameter of 3 to 10 nm. [Means for solving the problem]

[0009] In view of the above problems, the inventors of the present invention conducted a variety of trial and error experiments and discovered a cellulose nanofiber aggregate whose main component is acylated modified single cellulose nanofibers with a fiber diameter of 3 to 10 nm, as well as a method for producing the same.

[0010] That is, the present invention is characterized by the following configuration. [1] An aggregate of acylated modified cellulose nanofibers having a fiber diameter of 3 to 100 nm, in which the proportion of hydroxyl groups of the cellulose in the cellulose nanofibers that have been acylated (average degree of acyl group substitution) is 0.05 to 1.8, the aggregate having a cellulose Iβ crystal structure, the number content of single cellulose nanofibers having a fiber diameter of 3 to 10 nm that make up the aggregate being 98% or more, and the aspect ratio of the single cellulose nanofibers being 60 to 300. [2] The assembly of cellulose nanofibers according to [1], characterized in that the assembly of cellulose nanofibers is an assembly of acylated modified cellulose nanofibers. [3] The cellulose nanofiber aggregate described in [1] or [2], characterized in that a 0.4 wt% dimethylacetamide dispersion of the cellulose nanofiber aggregate has a visible light transmittance of 40% or more. (4) A method for producing an aggregate of cellulose nanofibers in which the proportion of hydroxyl groups of cellulose that have been acylated (average degree of acyl group substitution) is 0.05 to 1.8, comprising: a first step of modifying the hydroxyl groups of cellulose with anionic groups; a second step of defibrating the anionic group-modified cellulose into an aggregate of modified cellulose nanofibers having a fiber diameter of 3 to 100 nm, in which single cellulose nanofibers having a fiber diameter of 3 to 10 nm account for 98% or more by number; and a third step of hydrolyzing the aggregate of cellulose nanofibers obtained in the first step to convert the modified anionic groups into hydroxyl groups to obtain an aggregate of unmodified cellulose nanofibers. The method for producing an aggregate of acylated modified cellulose nanofibers includes a fourth step of acylation-modifying the aggregate of unmodified cellulose nanofibers obtained in the previous step, wherein the second step is to neutralize the anionic group-modified cellulose obtained in the first step, disperse it in water or an aqueous solution of 50% or less polar solvent, and stir to defibrate it, and the fourth step is to replace the hydrolyzed solution of the aggregate of unmodified cellulose nanofibers obtained in the third step with an aprotic solvent to prepare a dispersion of the aggregate of unmodified cellulose nanofibers in an aprotic solvent, add an acylation reaction agent, and stir.

[0011] Here, the aggregate of acylated modified cellulose nanofibers is a mixture containing single cellulose nanofibers with a fiber diameter of 3 to 10 nm and cellulose nanofibers with a fiber diameter of 10 to 100 nm. Preferably, it is a mixture containing single cellulose nanofibers with a fiber diameter of 3 to 10 nm and cellulose nanofibers with a fiber diameter of 10 to 50 nm. The form of the mixture is not particularly limited and can be selected depending on the application. Examples include a dispersion in an organic solvent, a paste, a gel, a dry state, or a state dispersed in a resin.

[0012] The average degree of acyl group substitution is the proportion of hydroxyl groups in cellulose that are modified with acyl groups, and more specifically, it is the average value (average degree of substitution) of the number of hydroxyl groups (number of substituents) modified with acyl groups per repeating unit of cellulose.

[0013] The content of the number of acylated modified single cellulose nanofibers described in [1] above was determined by observing a sample taken from the aggregate of acylated modified cellulose nanofibers using a TEM, counting the number of large CNFs (crude CNFs) with a fiber diameter of more than 10 nm that appeared in randomly selected images, and confirming that the number of single cellulose nanofibers with a fiber diameter of 3 to 10 nm was 50 times or more that number. [Effects of the Invention]

[0014] The aggregates of acylated modified CNFs of the present invention contain 98% or more single cellulose nanofibers with fiber diameters of 3 to 10 nm, which allows them to maintain high resin transparency when composited with resins compared to conventional acylated modified cellulose nanofibers, making them promising as reinforcing materials for optical materials. Furthermore, compared to anionic celluloses such as TEMPO-oxidized cellulose, they have higher heat resistance (thermal decomposition temperature) and affinity with resins, overcoming problems such as discoloration and deterioration of composites caused by thermal decomposition of cellulose nanofibers during melt blending. Furthermore, their enhanced dispersibility in resins promises to provide a high reinforcing effect. The surface hydroxyl groups of CNF are acylated to further increase their thermal stability, and therefore acylated CNF can have even greater heat resistance than unmodified CNF. [Brief explanation of the drawings]

[0015] [Figure 1] IR spectra of CNF obtained in Examples 1 and 2 [Figure 2] SEM image of the acetylated modified CNF obtained in Example 1 [Figure 3] TEM image of the acetylated modified CNF obtained in Example 1 [Figure 4] UV spectra of CNF dispersions obtained in Examples 1 and 3 [Figure 5] SEM photograph of butyrylated modified CNF in Example 2 [Figure 6] TEM image of butyrylated modified CNF in Example 2 [Figure 7] IR spectrum of CNF obtained in Example 3 [Figure 8] SEM photograph of benzoyl CNF obtained in Example 3 [Figure 9] TEM photograph of benzoyl CNF obtained in Example 3 [Figure 10] SEM photograph of acetylated modified CNF in Comparative Example 1 [Figure 11] SEM photograph of acetylated modified CNF in Comparative Example 2 DETAILED DESCRIPTION OF THE INVENTION

[0016] The aggregate of acylated modified cellulose nanofibers of the present invention is an aggregate of cellulose nanofibers having a fiber diameter of 3 to 100 nm and an average acyl group substitution degree of 0.05 to 1.8 in the acylation modification, characterized in that it has a cellulose Iβ crystal structure, the number content of single cellulose nanofibers having a fiber diameter of 3 to 10 nm constituting the aggregate is 98% or more, and the aspect ratio of the single cellulose nanofibers is 60 to 300.

[0017] Acylated cellulose nanofibers preferably do not have anionic functional groups. However, depending on the production method, CNFs with anionic functional groups may be mixed in. Even if they have anionic functional groups, the average substitution degree is preferably 0.01 or less. More preferably, it is 0.005 or less, and most preferably, the cellulose nanofibers have little or no anionic functional groups. The presence of anionic functional groups reduces the thermal stability of CNF and may lead to decomposition during application, so it is preferable to remove most or all of them as much as possible.

[0018] The average acyl group substitution degree of CNF is preferably 0.05 to 1.8, more preferably 0.1 to 1.7, and most preferably 0.15 to 1.6. An average acyl group substitution degree of 0.05 or less is undesirable because it results in low hydrophobicity and insufficient dispersibility in resins. However, an average acyl group substitution degree of 1.8 or more is undesirable because it may reduce the crystallinity of CNF or cause the loss of type I crystal structure. The type of acyl group is not particularly limited, but an acyl group represented by the following formula (1), in which R is an alkyl group, an allyl group, or an aryl group having 2 to 18 carbon atoms, is particularly preferred. Modification with these acyl groups is preferred because they can be dispersed in resins and organic solvents. -C(=O)-R...Formula (1)

[0019] The aggregate of acylated modified cellulose nanofibers of the present invention having a fiber diameter of 3 to 100 nm preferably has an Iβ crystal structure. Loss of the Iβ crystal structure is undesirable because it reduces heat resistance and reinforcing effects. The Iβ crystallinity is 20 to 90%, more preferably 30 to 88%, and even more preferably 35 to 85%. The crystallinity depends on the raw pulp, defibration conditions, and fiber diameter. By controlling these influencing factors, the crystallinity can be controlled within these ranges.

[0020] The aggregate of acylated modified cellulose nanofibers has a content of single cellulose nanofibers with a fiber diameter of 3 to 10 nm of 98% or more, in terms of the number of cellulose nanofibers. More preferably, it is 99% or more. Even more preferably, it is an aggregate of CNFs that contains almost no cellulose nanofibers of 10 nm or more. If the content of single CNFs is lower than this, transparency decreases significantly, and even adding only a small amount (5 wt%) to the resin increases the haze to 10% or more, making it undesirable as an optical material. Furthermore, CNFs with a fiber diameter of 50 nm or more are present only as impurities, and CNFs with a fiber diameter of 100 nm or more are substantially absent.

[0021] The average fiber length of single cellulose nanofibers with a fiber diameter of 3 to 10 nm contained in the aggregate of acylated modified cellulose nanofibers is preferably 0.3 μm to 2.0 μm, more preferably 0.5 to 1.5 μm, and even more preferably 0.6 to 1.0 μm. If the fiber length is shorter than this range, the aspect ratio will be less than 60, which is undesirable as it reduces the reinforcing effect of the cellulose nanofibers. On the other hand, if the fiber length is longer than this range, it is undesirable as it may easily aggregate when composited with a resin or the viscosity may increase dramatically.

[0022] The aspect ratio of single cellulose nanofibers with a fiber diameter of 3 to 10 nm contained in the aggregate of acylated modified cellulose nanofibers of the present invention is 60 to 300, more preferably 70 to 250, and even more preferably 80 to 200. If the aspect ratio is smaller than this range, the reinforcing effect cannot be exerted, which is not preferred. On the other hand, if the aspect ratio is larger than this range, the cellulose nanofibers may become difficult to disperse, which is also not preferred.

[0023] The thermal decomposition temperature of the aggregate of acylated modified cellulose nanofibers of the present invention is 250 to 350°C, more preferably 260 to 340°C, and even more preferably 265 to 330°C. While there is a risk of thermal decomposition when melt-kneaded with resin at temperatures below 250°C, the acylated modified CNF of the present invention contains almost no ionic functional groups and therefore typically has a thermal decomposition temperature of 250°C or higher. The thermal decomposition temperature of acylated modified CNF depends on the raw material pulp and the presence or absence of residual ionic functional groups. When linter pulp, a cotton-based pulp, is used, the resulting CNF has a higher thermal decomposition temperature than wood-derived CNF. Furthermore, the presence of ionic functional groups decreases the thermal decomposition temperature. By selecting wood pulp, linter pulp, or other plant-derived cellulose pulp, the thermal decomposition temperature can be controlled within a certain range depending on the application.

[0024] Furthermore, the acylated modified cellulose nanofibers of the present invention have good transparency because they contain 98% or more single cellulose nanofibers in the CNF aggregate. This effect is particularly pronounced when dispersed in an organic solvent, and the visible light transmittance of a 0.4 wt% dimethylacetamide dispersion of an aggregate of acylated modified cellulose nanofibers is preferably 40% or more, more preferably 50% or more, and most preferably 55% or more. A visible light transmittance of 40% or more ensures good transparency when mixed with a resin.

[0025] The optical transmittance of a DMAc dispersion of an aggregate of acylated cellulose nanofibers depends on the fiber diameter and the change in refractive index due to acylation. The aggregate of acylated cellulose nanofibers was washed and then redispersed in dimethylacetamide to prepare a 0.4 wt% dimethylacetamide dispersion of acylated cellulose nanofibers. The visible light transmittance was evaluated by measuring the UV spectrum.

[0026] Next, the method for producing an aggregate of cellulose nanofibers of the present invention will be described. The method for producing an aggregate of cellulose nanofibers of the present invention includes the steps of: (1) a first step of modifying the hydroxyl groups of cellulose with anionic groups; (2) a second step of defibrating the anion-modified cellulose into an aggregate of anion-modified cellulose nanofibers having a fiber diameter of 3 to 100 nm, in which single cellulose nanofibers having a fiber diameter of 3 to 10 nm account for 98% or more by number; (3) a third step of hydrolyzing the cellulose nanofiber aggregate obtained in the previous step to convert the modified anion groups into hydroxyl groups; (4) a fourth step of acylation-modifying the aggregate of unmodified cellulose nanofibers obtained in the previous step; Through the above four steps, an aggregate of cellulose nanofibers can be produced in which the proportion of hydroxyl groups of cellulose that have been acylated (average degree of acyl group substitution) is 0.05 to 1.8.

[0027] The cellulose raw material for producing the acylated modified cellulose nanofiber aggregates of the present invention is not particularly limited as long as it is cellulose with a type I crystal structure, but examples include wood-derived pulp, wood, bamboo, linder pulp, cotton, and substances containing cellulose powder. Cellulose may be in the form of a single substance, or may be in a mixed form containing non-cellulose components such as lignin or hemicellulose. When a small fiber diameter is important, wood pulp is preferred. On the other hand, when heat resistance and crystallinity are important, linter pulp is preferred.

[0028] In the first step, the hydroxyl groups of cellulose are modified with anionic groups, and the anionic groups are bonded to the hydroxyl groups of cellulose via ester bonds to produce modified cellulose. By making the bond with the hydroxyl group of cellulose an esterified bond, the anionic group can be easily cleaved by hydrolysis in the third step, allowing conversion to unmodified cellulose nanofibers.

[0029] The method for modifying the hydroxyl groups of cellulose with anionic groups in the first step is not particularly limited, but can be obtained by reacting cellulose with a dicarboxylic acid anhydride (dibasic carboxylic acid anhydride), a dicarboxylic acid divinyl ester, a dicarboxylic acid dichloride, sulfuric acid, phosphoric acid, or the like, and known methods can be applied to these reactions.

[0030] The reaction of cellulose with a dibasic carboxylic acid anhydride can be carried out, for example, in accordance with the method described in JP 2017-82188 A. A base catalyst is added to a mixture of a dibasic carboxylic acid anhydride and a polar aprotic solvent to form a reaction solution. Cellulose pulp is added to the reaction solution, and the mixture is stirred at a predetermined temperature for a predetermined time. Dibasic carboxylic acid-modified cellulose is then obtained by washing with water or alcohol. The polar aprotic solvent used in the reaction is preferably pyridine, dimethyl sulfoxide, or a mixture of pyridine and dimethyl sulfoxide. Such solvents are preferred because they can penetrate into the gaps between fibrils in cellulose fibers, resulting in few or no fibers with a diameter of 50 nm or more in the resulting dibasic carboxylic acid-modified cellulose nanofibers.

[0031] The reaction of cellulose with a dibasic carboxylic acid anhydride can also be carried out according to the method described in Non-Patent Document 2. For example, cellulose and a dibasic carboxylic acid anhydride are kneaded together under heating to obtain a dibasic carboxylic acid-modified cellulose.

[0032] Dibasic carboxylic acid anhydrides are esterifying agents, and various dibasic carboxylic acid anhydrides such as malonic anhydride, succinic anhydride, maleic anhydride, and phthalic anhydride can be used.

[0033] Cellulose can be modified by sulfate esterification by impregnating cellulose with a solution containing dimethyl sulfoxide, a carboxylic acid anhydride, and sulfuric acid. For details, sulfated cellulose nanofibers can be produced by the method and conditions described in Examples 1 to 3 or 9 to 19 of International Publication WO 2018 / 131721. Furthermore, the method of modifying cellulose by phosphorylation can be carried out by the method described in Patent Document 2.

[0034] The cellulose modified with anionic groups produced as described above preferably has an average degree of substitution of anionic groups of 0.10 or more, more preferably 0.12 or more, and even more preferably 0.15 or more, in order to defibrate the modified cellulose into single CNFs. If the average degree of substitution of anionic groups is less than 0.10, the resulting anionic cellulose will have insufficient electrostatic repulsion in water, resulting in a low degree of defibration, and this is undesirable because there is a risk of a high content of crude CNF with a fiber diameter exceeding 10 nm after defibration.

[0035] The form of the anionic cellulose prepared in the first step is not particularly limited. For example, it may be in a wet state containing a protic solvent such as water after washing, or in a dry state. Even if dried, it can be redispersed by adding water, causing electrostatic repulsion.

[0036] The shape of the anionic cellulose obtained in the first step varies depending on the reaction and stirring conditions during the modification reaction, but is typically fibrous with a fiber diameter of several microns to tens of microns, fine fibers with a diameter of several hundred nanometers to 1,000 nm, or cellulose nanofibers with a diameter of 3 to 100 nm. The faster the stirring speed, the lower the content of fibers larger than the micron order in the resulting anionic cellulose, as defibration occurs during the reaction. However, by combining electrostatic repulsion and mechanical defibration in the second step, it is possible to nanosize the fibers to 3 to 10 nm regardless of their shape.

[0037] Next, the second step will be described. In the second step, the anionic group-modified cellulose obtained in the first step is neutralized, then dispersed in water or an aqueous solution of 50% or less polar solvent and stirred to defibrate it into an aggregate of modified cellulose nanofibers with a fiber diameter of 3 to 100 nm, with 98% or more being single cellulose nanofibers with a fiber diameter of 3 to 10 nm. Although it is possible to defibrate to a fiber diameter of 3 to 10 nm without neutralization, the electrostatic repulsion is weaker than with neutralization, which may reduce the efficiency of defibration, so it is preferable to defibrate after neutralization.

[0038] The anionic group-modified cellulose obtained in the first step is first dispersed in water, an aqueous alcohol solution, or alcohol and neutralized with a neutralizing alkali. The neutralizing alkali is not particularly limited, but examples include alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, and lithium hydroxide; alkali metal carbonates such as sodium carbonate, potassium carbonate, and lithium carbonate; and amine compounds such as pyridine. It is particularly preferred to use the same alkali compound as the catalyst for hydrolysis in the third step. For example, in the case of anionic cellulose modified with a dibasic carboxylic acid, sodium hydroxide is particularly preferred. On the other hand, in the case of cellulose modified by sulfate esterification, pyridine or an amine is particularly preferred.

[0039] Thereafter, the mixture is dispersed in water or an aqueous solution containing 50% or less of a polar solvent and stirred. Due to the electrostatic repulsion effect, water (distilled water) is the most preferable dispersion solvent, but for the convenience of concentrating the CNF after nano-fibrillation, an aqueous solution of 50% or less of a polar solvent such as alcohol may also be used. By adjusting the solid content of the modified cellulose fiber in the dispersion solvent to approximately 0.1 to 0.8% and stirring for 3 to 20 minutes using a stirring device such as a mixer or homogenizer, an aggregate of modified CNF with a fiber diameter of 3 to 100 nm, mainly composed of single CNF with a fiber diameter of 3 to 10 nm, is obtained. Because the hydroxyl groups of CNF are replaced with anionic groups, electrostatic repulsion in the dispersion solvent allows it to be defibrillated down to single CNFs with fiber diameters of 3 to 10 nm.

[0040] In some cases, remaining coarse fibers can be removed by further filtering using a filter such as a nylon mesh.Further processing using a Clearmix or homogenizer can improve the degree of defibration or shorten the fiber length to the required range.

[0041] In the third step, the CNF aggregate obtained in the second step is hydrolyzed to convert the modified anionic groups into hydroxyl groups, thereby producing a CNF aggregate with a modification rate (average substitution degree) of cellulose hydroxyl groups of 0.02 or less. Complete hydrolysis can produce completely unmodified CNF that does not contain anionic groups. However, if the intended use does not strictly require unmodified CNF, an aggregate of CNF with a cellulose hydroxyl group modification rate (average degree of substitution) of 0.02 or less can exhibit effects almost identical to those of unmodified CNF.

[0042] The hydrolysis in the third step is described in detail below. A hydrolysis catalyst is added to the aqueous dispersion of anionic CNF obtained in the second step to carry out hydrolysis. The hydrolysis method varies depending on the type of anionic group, but can be carried out using a general hydrolysis method suitable for each. Below, the hydrolysis methods for when the anionic group is a carboxylic acid group and when it is a sulfate ester group are described in detail.

[0043] First, we will explain the method for hydrolyzing dibasic carboxylic acid-modified CNF. A hydration catalyst is added to the aqueous dispersion of dibasic carboxylic acid-modified single CNF obtained in the second step, and the dibasic carboxylic acid groups are removed by hydrolysis, converting the CNF into an aggregate of unmodified CNF.

[0044] The catalyst used for hydrolysis is not particularly limited, but to avoid decomposition of CNF, an alkaline catalyst is preferred over an acid catalyst. Examples include sodium hydroxide, potassium hydroxide, and lithium hydroxide. The required concentration of the alkaline catalyst in the aqueous dispersion of dibasic carboxylic acid-modified cellulose nanofibers can be adjusted depending on the reaction temperature. For example, it is 0.05% to 10%, more preferably 0.1% to 8%, and most preferably 0.3 to 5%. If the concentration is too low, the reaction will be slow and the efficiency will be low. On the other hand, if the concentration is too high, it is undesirable because the CNF may decompose or lose its crystalline structure.

[0045] Rather than adding the alkali catalyst directly to the aqueous dispersion of dibasic carboxylic acid-modified single cellulose nanofibers, it is preferable to dissolve the alkali catalyst in water to form an aqueous solution of the alkali catalyst and then add it. Direct addition of a solid alkali catalyst is not preferred because it can cause a localized rapid increase in alkali concentration, which can lead to decomposition of the cellulose components.

[0046] The hydrolysis temperature is not particularly limited and may be adjusted depending on the strength and concentration of the alkaline catalyst. For example, it is 15°C to 80°C, more preferably 20°C to 60°C, and most preferably 23°C to 50°C. Temperatures below 15°C are undesirable because the reaction is slow. On the other hand, temperatures above 80°C are undesirable because the cellulose component may also decompose.

[0047] The reaction time can be adjusted based on the temperature and alkalinity of the catalyst, with the anionic groups being used as a guideline, and is, for example, 0.5 to 10 hours, more preferably 1 to 8 hours, and most preferably 2 to 6 hours. Strong reaction conditions and a short reaction time are undesirable because they reduce uniformity, leave anionic groups, or cause the cellulose nanofibers to decompose. Weak reaction conditions and a reaction time of 10 hours or longer are also undesirable because they result in poor efficiency. Furthermore, it is preferable to carry out the reaction while stirring in order to efficiently hydrolyze the anionic groups.

[0048] Next, we will explain the method for hydrolyzing sulfated CNF. The sulfate groups of the sulfated single CNF obtained in the second step can be removed by referring to the hydrolysis method described in the non-patent document (Trends in Glycoscience and Glycotechnology, Vol. 14, No. 80 (November 2002) pp. 343-351).

[0049] For example, a dispersion of sulfated single CNF is replaced with a pyridine / DMSO mixed solvent to prepare a pyridine / DMSO dispersion. A certain amount of distilled water is added to the dispersion, and hydrolysis is carried out while stirring at a constant temperature to obtain unmodified CNF. The weight ratio of the pyridine / DMSO mixed solvent is preferably 10 / 90 to 90 / 10. A ratio lower than 10 / 90 is undesirable because hydrolysis is insufficient, leaving sulfate ester groups behind. On the other hand, a ratio higher than 90 / 10 is undesirable because the sulfated CNF aggregates in the mixed solvent, causing the hydrolysis reaction to become uniform, leaving sulfate ester groups behind. The most preferable weight ratio is 20 / 80 to 70 / 30. The hydrolysis temperature is preferably 20°C to 80°C, and more preferably 30 to 70°C. A temperature below 20°C is not preferred because the reaction slows down. On the other hand, a temperature above 80°C is not preferred because cellulose may decompose. The hydrolysis time is not particularly limited and can be adjusted by the reaction temperature and the weight ratio of pyridine / DMSO, and can be, for example, 3 to 10 hours.

[0050] When producing unmodified CNF by hydrolyzing anionic CNF to remove anionic groups, if the alkaline substance concentration of the alkaline aqueous solution used for hydrolysis is low, the hydrolysis time is short, or the stirring is uneven, hydrolysis is insufficient and some of the surface hydroxyl groups remain in an anion-modified state. If a large number of anionic groups remain, the heat resistance of the CNF, and in particular, the thermal decomposition temperature, decreases, which is undesirable. To maintain a high thermal decomposition temperature, it is preferable that the hydroxyl groups on the surface of the cellulose nanofibers are not anion-modified.

[0051] After hydrolysis, the unmodified CNF aggregates are recovered by washing with water or alcohol. The washing method is not particularly limited, but examples include centrifugation, filtration, pressure filtration, and squeezing. When using centrifugation, washing is preferably repeated three or more times. In particular, when dibasic carboxylic acid-modified CNFs are hydrolyzed, insufficient washing is not preferred because the alkali metal hydroxide, which serves as the hydrolysis catalyst, and the dibasic carboxylic acid produced by hydrolysis may remain, which may lead to a decrease in the performance of the acylated modified CNF aggregates. The washing solvent is not particularly limited, but water or a mixture of water and alcohol is preferred.

[0052] Next, the fourth step will be described. In the fourth step, the unmodified cellulose nanofibers obtained in the third step are washed with an aprotic solvent to prepare a dispersion of unmodified cellulose nanofibers in an aprotic solvent, and then an acylation modifying agent and a catalyst are added and stirred to obtain an aggregate of acylated modified cellulose nanofibers in which acylated modified single cellulose nanofibers with a fiber diameter of 3 to 10 nm account for 98% or more of the total.

[0053] The aprotic solvent used in the acylation modification reaction is not particularly limited, but a solvent in which unmodified cellulose nanofibers can be well dispersed is preferred. Examples include aprotic solvents with an SP value of 9 or higher, such as dimethylacetamide (DMAc), dimethylformacetate (DMF), N-methylpyrrolidone (NMP), pyridine, 1,4-dioxane, and tetrahydrofuran (THF). Particularly preferred are pyridine, DMSO, DMAc, DMF, and NMP, either alone or in combination.

[0054] The concentration of unmodified cellulose nanofiber aggregates in the aprotic solvent is preferably 0.05 to 10 wt %, more preferably 0.1 to 8 wt %, even more preferably 0.12 to 6 wt %, and most preferably 0.15 to 5 wt %. Concentrations below this range are undesirable because of low efficiency. Concentrations above this range are undesirable because the viscosity of the dispersion is too high, which may reduce the uniformity of modification.

[0055] The acylating agent is not particularly limited, but is preferably one or more of monobasic carboxylic acid anhydrides, monobasic vinyl carboxylates, monobasic carboxylic acid halide salts, and monobasic carboxylic acids, of which monobasic carboxylic acid anhydrides and monobasic vinyl carboxylate salts are particularly preferred.

[0056] Examples of carboxylic acid anhydrides include acetic anhydride, propionic anhydride, butyric anhydride, benzoic anhydride, cinnamic anhydride, and lauric anhydride.

[0057] Examples of vinyl carboxylate include vinyl acetate, vinyl propionate, vinyl butyrate, vinyl caproate, vinyl cyclohexanecarboxylate, vinyl caprylate, vinyl caprate, vinyl laurate, vinyl myristate, vinyl palmitate, vinyl stearate, vinyl pivalate, vinyl octylate, divinyl adipate, vinyl methacrylate, vinyl crotonate, vinyl pivalate, vinyl octylate, vinyl benzoate, and vinyl cinnamate.

[0058] Examples of the carboxylic acid halide include acetyl chloride, propionyl chloride, butyryl chloride, octanoyl chloride, stearoyl chloride, benzoyl chloride, and paratoluenesulfonic acid chloride.

[0059] When a carboxylic acid is used as the acylating agent, an aliphatic carboxylic acid or aromatic carboxylic acid having a boiling point of 150° C. or higher is preferred, and examples thereof include butyric acid, pivalic acid, methacrylic acid, lauric acid, cinnamic acid, crotonic acid, and benzoic acid.

[0060] The amount of acylating agent added is not particularly limited, but 0.05 to 15 moles per mole of anhydroglucan of CNF is preferred. Adding too little is undesirable because it slows the reaction rate and reduces the degree of modification. Adding too much acylating agent is undesirable because it can lead to over-modification, reducing the crystallinity and physical properties of the cellulose nanofibers. The amount is preferably 0.1 to 10 moles, more preferably 0.3 to 9 moles, and even more preferably 0.5 to 8 moles.

[0061] When using a carboxylic acid anhydride or vinyl carboxylate as the acylating agent, the addition of a base catalyst is preferred because it increases the reaction rate. The type of base catalyst is not particularly limited, but examples include hydroxides, carbonates, hydrogencarbonates, and carboxylates of alkali metals or alkaline earth metals, as well as pyridines, imidazoles, and amines. These base catalysts may be used alone or in combination of two or more.

[0062] When a carboxylic acid halide is used as the acylating agent, the reaction proceeds vigorously without the use of a catalyst, but a catalyst may be added. The catalyst to be added is preferably the above-mentioned base catalyst, and among them, weakly basic base catalysts such as amines are more preferred. When a carboxylic acid is used as the acylating agent, an acid catalyst is preferred over a base catalyst. Examples include sulfuric acid and p-toluenesulfonic acid. To promote the acylation reaction, it is preferable to include a dehydration condensation agent in the reaction system. The dehydration condensation agent is not particularly limited as long as it is a commonly used esterification condensation agent. Examples include 4-dimethylaminopyridine (DMAP) and N,N-dicyclohexylcarbodiimide (DCC). Furthermore, it is preferable to remove water generated during ester formation from the system by heating or reducing pressure.

[0063] The temperature of the acylation modification reaction can be adjusted appropriately depending on the type of modifying agent and catalyst. For example, in the case of carboxylic acid anhydrides or vinyl carboxylates, room temperature to 150°C is preferred. If the reaction temperature is too low, the reaction rate will be slow, and if it is too high, the cellulose nanofibers may be damaged, which is undesirable. The temperature is more preferably room temperature to 120°C, even more preferably 25 to 100°C, and even more preferably 30 to 90°C.

[0064] The reaction time is not particularly limited, but can be adjusted appropriately by adjusting the reaction temperature and the type and amount of catalyst added. If the reaction time is too short, the degree of modification will be low, and if it is too long, over-modification may result in a decrease in the crystallinity and yield of cellulose, which is undesirable. For example, the reaction time is 20 to 240 minutes.

[0065] After the reaction is complete, an organic solvent is added to dilute the reaction and then the reaction is stopped and washed. While there are no particular limitations on the solvent used to stop the reaction or the washing solvent, water or alcoholic solvents such as methanol, ethanol, and IPA are preferred in terms of stopping the reaction. However, in the case of hydrophobic acyl groups, alcohols or ketones or amide solvents are preferred over water. The washing method is not particularly limited, but examples include pressure filtration, vacuum filtration, squeezing, and centrifugation. By washing, the reagents and solvents used in the reaction and by-products can be removed, and the cellulose nanofibers to be acylated can be recovered.

[0066] The state of the recovered cellulose nanofibers is not particularly limited, and may be prepared as appropriate depending on the application. Examples include a dispersion containing a solvent, a paste or slurry, or a powder from which most of the solvent has evaporated.

[0067] The aggregate of acylated modified cellulose nanofibers of the present invention can be produced by the four steps described above. [Example]

[0068] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples. Details of the raw materials used are as follows, and the properties of the obtained modified cellulose nanofibers were measured as follows.

[0069] (Materials and solvents used) Cellulose pulp: Commercially available softwood pulp (manufactured by Georgia Pacific, product name: Fluff Pulp ARC48000GP) was used. It was shredded into 1 cm cubes before use. Unmodified cellulose nanocrystals (CNC): CNC manufactured by Alberta Innovates, Canada was used. Other reagents used were those manufactured by Nacalai Tesque, Inc.

[0070] (Devices used for defibration in the examples) Mixer: Panasonic MX-X701-T mixer.

[0071] (IR analysis of cellulose nanofibers) The IR spectrum of the cellulose nanofibers was measured using a Fourier transform infrared spectrophotometer (FT-IR) in ATR mode using a NICOLET MAGNA-IR760 Spectrometer manufactured by NICOLET.

[0072] (Quantification of succinic acid modification rate of cellulose or CNF) The degree of succinic acid modification of cellulose or CNF was expressed as the average degree of substitution and was quantified by conductometric titration. Specifically, 100 g of a 0.3% aqueous dispersion of succinic acid-modified cellulose or CNF (unneutralized) was prepared, and the pH was adjusted to 2.5 by adding 0.1 N hydrochloric acid. Then, 0.05 N sodium hydroxide was added dropwise until the pH reached 11, and the electrical conductivity was recorded. The electrical conductivity and pH were plotted, and the number of moles of succinic acid groups, Q, was calculated using the following formula, based on the amount of sodium hydroxide (a) consumed during the neutralization stage of the weak acid, where the electrical conductivity change was gradual. Q (mol) = a [ml] × 0.05 / 1000 The average degree of substitution was calculated from the number of moles of this substituent, Q, and the molecular weight (162) of anhydroglucan, the structural unit of cellulose. Average degree of substitution=162×Q / (100×0.3%-118×Q+18×Q) (In the formula, 100 is the amount of the aqueous dispersion of succinic acid-modified cellulose or CNF, 118 is the molecular weight of succinic acid, and 18 is the molecular weight of water produced by the succinic acid modification.)

[0073] (Quantitative determination of sulfate modification rate of cellulose or CNF) The sulfate ester modification rate of cellulose or CNF was expressed as the average degree of substitution, and the sulfur content was quantified using the combustion absorption-IC method. Specifically, dried sulfate ester-modified CNF compound (0.01 g) was placed on a magnetic board and burned in a circular furnace (1350 °C) in an oxygen atmosphere (flow rate: 1.5 L / min). The generated gas components were absorbed in 3% hydrogen peroxide solution (20 ml). The resulting absorbed solution was diluted to 100 ml with pure water, and the sulfate ion concentration (wt%) was calculated from the results of ion chromatography of the diluted solution. The sulfur content was converted from the sulfate ion concentration using the following formula. A Thermo Fisher Scientific ICS-1500 ion chromatograph was used for the analysis. Sulfate ester modification rate (average substitution degree) = [(sulfate ion concentration / 96) × 162] / [0.01 × (1 - sulfate ion concentration)] (In the formula, 162 is the molecular weight of anhydroglucan, a building block of cellulose.)

[0074] (Quantification of the average degree of substitution (DS) of acylated modified CNF) A specified amount of acylated CNF is dispersed in a mixture of NaOH / EtOH / HO and stirred at room temperature for 4 hours to hydrolyze the ester bonds, removing the acyl groups from the hydroxyl groups of the CNF, converting the acylated CNF into unmodified CNF. Meanwhile, the removed acyl groups combine with sodium hydroxide to convert into sodium carboxylate, the number of moles of which can be quantified using the titration method shown below. After hydrolysis, the reaction solution (solvent, sodium carboxylate, and sodium hydroxide) and the residue (unmodified CNF) were separated by filtration. The residue was dried and weighed. The amount of residual sodium hydroxide was determined by titrating the solution with aqueous hydrochloric acid. The number of moles of acyl groups (C) and the average degree of substitution of acyl groups (DS) were calculated using the following formula: A = the equivalent number of sodium hydroxide added to the hydrolysis solution, W = the weight of the CNF recovered and dried after hydrolysis, and B = the equivalent number of hydrochloric acid consumed in titration. Number of moles of cellulose (anhydroglucan) (M) = W / 162 Number of moles of acyl groups C=AB Average degree of substitution DS=C / M

[0075] (SEM observation) The shape of the cellulose nanofibers was observed using FE-SEM (JEOL Ltd. "JSM-6700F", measurement conditions: 20 mA, 60 seconds).

[0076] (Preparation of TEM observation sample) A sample for TEM observation was prepared by diluting the acylated cellulose nanofiber aggregates with DMAc to 0.003% and then mixing them with an ionic liquid for electron microscopy (Hitachi High-Technologies HILEM IL1000) to a final concentration of 0.0015% or less. The mixture was then dropped onto a carbon support film for TEM observation (Oken Shoji Super Hi-Res Carbon SHR-C075) and allowed to dry. To enhance image contrast, the sample was subjected to electron staining (negative staining) using a gadolinium acetate-based electron microscope stain (Nissin EM EM Stainer).

[0077] (TEM observation) TEM observation samples were observed using a JEOL JEM-2100F field-emission transmission electron microscope (FE-TEM) at an accelerating voltage of 120 kV in a TEM bright field. The average fiber diameter and average fiber length were measured by randomly selecting 50 or more fibers from the TEM photograph. For example, the fiber diameter of each CNF was measured at a magnification of 400,000 times or more using a combination of optical and digital magnification, and the actual fiber diameter was calculated based on the ratio to the magnification of the image. Repeated measurements were performed on 50 CNFs and the average was calculated. Meanwhile, when measuring fiber length, the CNF dispersion for observation was prepared by diluting it as much as possible to reduce CNF overlap. Using a magnification of 400,000 times or more, both ends of the fiber were found along the fiber curvature, and the linear distance between each curvature was measured and added together to determine the fiber length.

[0078] (Method for measuring crystallinity) The acylated cellulose nanofiber dispersion was dried and the crystallinity was measured using powder X-ray crystal diffraction (XRD). The analysis was performed using an X-ray diffractometer (Ultima IV, manufactured by Rigaku Corporation). The measurement conditions are as follows: ·X-ray:Cu / 40kV / 40mA Scan speed: 10° / min Scanning range: 2θ=5~70° The crystallinity was calculated using the following formula (see Textile Res. J. 29:786-794, 1959). Crystallinity (%)=[(I200-IAM) / I200]×100 I200: Diffraction intensity at 2θ=22.6° IAM: Diffraction intensity of amorphous part at 2θ=18.5°

[0079] (Method for measuring thermal decomposition temperature) The acylated cellulose nanofiber aggregate was dried at 105°C for 2 hours, and its thermal decomposition behavior was analyzed using a thermogravimetric and differential thermal analyzer (STA7200, Hitachi High-Tech Science Corporation). The measurement conditions are as follows: The 5% weight loss temperature was measured and used as the thermal decomposition temperature. Atmosphere: Argon gas (flow rate 300 mL / min) Temperature range: 30~400℃ Heating rate: 10℃ / min

[0080] (Measurement of visible light transmittance) A 0.4 wt% dimethylacetamide dispersion of the acylated modified cellulose nanofiber aggregate was prepared and measured using UV spectroscopy to measure the visible light transmittance at a wavelength of 589 nm (D-ray).

[0081] [Example 1] 18 g of succinic anhydride, 25 g of dimethyl sulfoxide, and 145 g of pyridine were placed in a 300 mL Erlenmeyer flask and stirred with a magnetic stirrer until the succinic anhydride was completely dissolved. Next, 5 g of cellulose pulp was added, and the flask was placed in a frosted glass lid and stirred at room temperature for 22 hours at 23 °C. After this, distilled water or a distilled water / methanol (70 / 30 wt) mixture was added and washed until the residual succinic anhydride, by-product succinic acid, dimethyl sulfoxide, and pyridine were completely removed. The washed succinic acid-modified cellulose fibers were dispersed in 500 mL of a 1% aqueous sodium carbonate dispersion and stirred for 10 minutes. After further washing, the fibers were added to distilled water to prepare a 0.3% solids succinic acid-modified cellulose fiber dispersion, which was then placed in a mixer and stirred for 10 minutes. The resulting mixture was then filtered through a nylon mesh (T-No. 380T) to obtain a sodium succinate-modified cellulose nanofiber dispersion. The production of sodium succinate-modified cellulose nanofibers was confirmed by IR spectroscopy (Figure 1). Furthermore, the succinic acid modification rate (average substitution degree) of a 0.3% aqueous dispersion of the obtained succinic acid-modified single CNF was confirmed to be 0.29 by electrical conductance titration.

[0082] 200 ml of the resulting 0.3% aqueous dispersion of sodium succinate-modified single CNF was added to a 500 ml Erlenmeyer flask, and while stirring with a magnetic stirrer, 65 ml of 0.5 N aqueous sodium hydroxide was added and hydrolysis (de-succination reaction) was carried out at room temperature for 2 hours. Next, succinic acid or sodium succinate and sodium hydroxide were removed by filtration and washing with distilled water to obtain an aggregate of unmodified cellulose nanofibers.

[0083] The resulting succinic acid-modified cellulose nanofibers and unmodified cellulose nanofibers after hydrolysis were dried and measured using an ATR FT-IR. The resulting IR spectrum is shown in Figure 1. As shown in Figure 1, the absorption band of the esterified bond near 1730 cm-1 was not detected compared to before hydrolysis, confirming that unmodified cellulose nanofibers had been formed. The resulting unmodified cellulose nanofibers (0.2 g in terms of solids) were substituted with pyridine to prepare 100 g of a pyridine dispersion. The concentration of unmodified cellulose nanofibers in the dispersion was approximately 0.2 wt%. The pyridine dispersion was added to a 300 ml Erlenmeyer flask, and 6 g of acetic anhydride was added while stirring. The flask was placed on a heated magnetic stirrer and stirred for 3 hours. The solid cellulose nanofibers were then collected by centrifugation. The collected cellulose nanofibers were further washed with IPA to obtain acetylated cellulose nanofibers. The resulting acetylated cellulose nanofibers were dried and analyzed by ATR FT-IR. The resulting IR spectrum is shown in Figure 1. An absorption band related to esterification bonds was detected at a frequency of approximately 1730 cm-1. The average substitution was evaluated and confirmed to be 0.53.

[0084] An SEM photograph of the obtained acetylated cellulose nanofibers is shown in Figure 2. No fine fibers were observed even at a magnification of 10,000 times, but a nanofiber skeleton with a fiber diameter of approximately 10 nm or less was observed at a magnification of 50,000 times. Next, the nanofibers were observed using a TEM, and the resulting images are shown in Figure 3. 50 cellulose nanofibers were randomly selected from the TEM image, and their fiber diameter and length were measured. No fiber diameters exceeding 10 nm were observed, and the number content of cellulose nanofibers between 3 and 10 nm was nearly 100%. It was also confirmed that the fiber lengths were distributed between 500 and 1,000 nm. It was confirmed that a single cellulose nanofiber had several bending points, and that the linear distance between the bending points was approximately 200 nm. It is believed that the bending points are due to the softness of the amorphous zones that connect the cellulose crystalline zones. The measurement results of fiber diameter and fiber length are shown in Table 1 together with other evaluation results.

[0085] The crystallinity of the acetylated cellulose nanofibers was evaluated by XRD and found to be 72%. Furthermore, TG-DTA analysis revealed that the thermal decomposition temperature was 296°C. Figure 4 shows the UV spectra of a 0.4% dimethylacetamide dispersion and an aqueous dispersion of the same acetylated cellulose nanofiber aggregate. The visible light transmittance (D-line, wavelength 589 nm) of the DMAc dispersion was 68%.

[0086] Example 2 100 g of the pyridine dispersion of the unmodified cellulose nanofibers (0.2 g) obtained in Example 1 and 10 g of butyric anhydride were added to a 300 ml Erlenmeyer flask and stirred for 3 hours using a heated magnetic stirrer set to 70°C. After this, butyrylated-modified cellulose nanofibers were obtained by washing with IPA in the same manner as in the previous examples. The IR spectrum, average degree of substitution, crystallinity, and thermal decomposition temperature of the obtained butyrylated-modified cellulose nanofibers were evaluated in the same manner as the acetylated-modified cellulose nanofibers in Example 1, and the results are shown in Figure 1 and Table 1. The average substitution, crystallinity, and thermal decomposition temperature were evaluated and found to be 0.36, 69%, and 313°C, respectively. SEM and TEM observations were performed, and the corresponding images are shown in Figures 5 and 6. TEM measurements of fiber diameter and length revealed no cellulose nanofibers with a diameter of 10 nm or greater. Fiber lengths were found to be distributed within the range of 500 to 1200 nm. Furthermore, a 0.4 wt% DMAc dispersion of the obtained butyrylated modified cellulose nanofibers was prepared, and UV spectra were measured, confirming a transmittance of 70%.

[0087] Example 3: 90 g of dimethyl sulfoxide, 10 g of acetic anhydride, and 1.3 g of sulfuric acid were placed in a 200 ml Erlenmeyer flask and stirred with a magnetic stirrer for 3 minutes. 3 g of cellulose pulp was then added, and the mixture was stirred at room temperature of 23°C for 2.5 hours. After that, the mixture was washed with distilled water to obtain sulfated modified cellulose. The wet sulfated modified cellulose was added to a pyridine / distilled water (1 / 9, weight ratio) mixed solvent to prepare a dispersion with a solids content of 0.3%. This dispersion was then stirred in a mixer for 10 minutes to obtain a dispersion of sulfated modified single CNF. The resulting dispersion of sulfate-esterified modified cellulose nanofibers was replaced with a mixture of pyridine / DMSO (70 / 30, weight ratio) to prepare a pyridine / DMSO (70 / 30, wt) dispersion with a solids content of 0.3 wt%. 3 wt% distilled water was added to the dispersion, which was then placed in a three-neck flask and stirred in an oil bath at 60°C for 5 hours. After the reaction, water was added and mixed, and the unmodified cellulose nanofibers were collected by centrifugation. The same process was repeated three times to obtain an aggregate of unmodified cellulose nanofibers. The resulting aggregates of sulfate-modified and unmodified cellulose nanofibers were dried and analyzed using FT-IR in ATR mode. The IR spectra are shown in Figure 7. In the spectrum before hydrolysis, an absorption band derived from sulfate ester bonds was detected near 1250 cm-1, but in the spectrum after hydrolysis, the absorption band near 1250 cm-1 was so reduced that it was no longer detectable. The resulting unmodified cellulose nanofibers (0.2 g solids) were substituted with DMSO to prepare 70 g of a DMSO dispersion. The DMSO dispersion was placed in an Erlenmeyer flask, to which 30 g of acetone, 10 g of vinyl benzoate, and 2 g of DBU were added. The mixture was then stirred for 2 hours on a heated magnetic stirrer set at 50 °C. The mixture was then washed with IPA to obtain benzoic acid-modified cellulose. The IR spectrum of the resulting benzoic acid-modified cellulose nanofibers is shown in Figure 7. The average degree of substitution, crystallinity, and thermal decomposition temperature were determined to be 0.58, 70%, and 293 °C, respectively. The samples were observed using SEM and TEM, and the corresponding images are shown in Figures 8 and 9. TEM observation and measurement of the fiber diameter and length revealed that no CNFs with a fiber diameter of 10 nm or more were observed. The fiber lengths were confirmed to be distributed within the range of 550 to 1100 nm. In addition, a 0.4 wt % DMAc dispersion of the obtained benzoyl-modified cellulose nanofibers was prepared, and the UV spectrum was measured, as shown in Figure 4. The transmittance was 75%.

[0088] [Comparative Example 1] (Acetylated modified CNF of Example 3 of Patent Document 7) 1 g of vinyl acetate, 9 g of DMSO, and 0.01 g of potassium carbonate were placed in a 20 ml sample bottle and stirred with a magnetic stirrer until the mixture was uniform. Next, 0.3 g of cellulose pulp was added, and the mixture was stirred for an additional 3 hours. Afterwards, the mixture was washed with a mixed solvent of distilled water and ethanol to obtain cellulose microfibers. The average degree of substitution, crystallinity, and morphology of the obtained acetylated CNF were evaluated, and the results are shown in Figure 10 and Table 1. It was confirmed that the fiber diameter of the CNF was mostly distributed within the range of 20 to 100 nm. UV spectra of a 0.4% aqueous dispersion and a DMAc dispersion were measured, and the transmittance was less than 1%.

[0089] [Comparative Example 2] (Example 3 of Patent Document 6) 7 g of pyridine, 3 g of DMSO, and 1 g of acetic anhydride were placed in a 20 ml sample bottle and stirred with a stirrer until the mixture was uniform. Next, 0.3 g of cellulose pulp was added, and the mixture was stirred for an additional 3 hours. After that, the mixture was washed with a mixed solution of acetone and water to obtain acetylated modified CNF. The average degree of substitution, crystallinity, and shape of the obtained acetylated CNF were evaluated, and the evaluation results are shown in Figure 11 and Table 1. The average diameter of the CNF was 110 nm, and the average fiber length was 13.6 μm. The UV spectra of the 0.4% aqueous dispersion and the DMAc dispersion were measured, and the values ​​were less than 1%.

[0090] [Table 1] [Industrial Applicability]

[0091] The aggregates of acylated modified CNFs of the present invention are heat-resistant and highly transparent, and are therefore expected to be applied to optical materials such as lens resins, polycarbonate, PMMA, and epoxy resins as reinforcements.

Claims

[Claim 1] The cellulose nanofiber assembly is an aggregate of acylated modified cellulose nanofibers having a fiber diameter of 3 to 100 nm, in which the proportion of hydroxyl groups of the cellulose in the cellulose nanofibers that have been acylated (average degree of acyl group substitution) is 0.05 to 1.8, and is characterized in that the cellulose nanofiber assembly has a cellulose Iβ crystal structure, the number content of single cellulose nanofibers having a fiber diameter of 3 to 10 nm that constitute the assembly is 98% or more, and the aspect ratio of the single cellulose nanofibers is 60 to 300.

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