JP7629282B2 - Dicarboxylic acid monoester modified cellulose nanofiber - Google Patents

Description

We provide a method for producing cellulose nanofibers modified with dicarboxylic acid monoesters using a reaction solution in a new solvent, and cellulose nanofibers modified with dicarboxylic acid monoesters from single cellulose nanofibers.

Cellulose fibers make up about 40% of plant cell walls and are the most abundant organic resource on Earth. In addition to having excellent elasticity, strength, and dimensional stability, they are also environmentally friendly, making them a promising alternative to petroleum-derived plastics.

Cellulose is a natural polymer in which β-glucose molecules are polymerized in a linear chain through glycosidic bonds. Cellulose fibers are formed through structural stages such as cellulose molecular chains, elementary fibrils (width: about 3-10 nm), microfibrils (width: 20-50 nm), fibrils (width: several hundred nm), and lamellae.
Depending on the production method, cellulose nanofibers include elementary fibril fibrils (single cellulose nanofibers or monofilaments) with a width of 3 to 10 nm, which are the most basic unit; cellulose microfibril bundles (cellulose nanofibers) with a width of 20 to 50 nm, which exist as basic units in cell walls and are formed by loosely bundling several of these; and microfibrillated cellulose (MFC), in which microfibril bundles are further bundled to several tens to several hundreds of nm to form a spider web-like network.
That is, when cellulose fibers are finely decomposed to the microfibril level, they become cellulose nanofibers (CNFs) with a width of several tens of nm as generally understood. When cellulose fibers are further decomposed to the elementary fibrils, they become single cellulose nanofibers (single CNFs) with a width of several nm.

Depending on the production method, cellulose nanofibers have been developed, including unmodified CNF, acylated modified CNF, and anionic CNF such as dicarboxylic acid modified CNF.
Regarding dicarboxylic acid modified CNF, a solventless method (Non-Patent Document 1) and a solvent method (Patent Documents 1 and 2) have been known so far.

However, in the solvent-free method, the dicarboxylic anhydride does not easily penetrate between the cellulose fibrils, so the degree of defibration of the resulting CNF and the uniformity of the distribution of modification groups are low. In addition, because no catalyst is used and high temperatures are used, not only dicarboxylic acid monoesters but also dicarboxylic acid diesters are formed, which may lead to cross-linking between fibrils.

On the other hand, Patent Documents 1 and 2, which are related to the solvent method, describe dicarboxylic acid-modified CNF, which is chemically defibrated by permeating and reacting cellulose with a defibration solution containing a catalyst and a dicarboxylic acid anhydride (claim 1) or a defibration solution containing a catalyst, a dicarboxylic acid anhydride, and an aprotic solvent (claim 3). However, as shown in Table 1 of the examples of Patent Document 1, in order to perform good defibration indicated by the degree of defibration marked with ◎, the reaction time required is 24 hours or more in both cases where a mixture of succinic anhydride or phthalic anhydride with pyridine, which also serves as a solvent, is used as a catalyst (Examples 1 to 3), and where pyridine, a solvent (DMAc, DMSO, etc.), and a dicarboxylic acid such as succinic anhydride (Examples 4 to 6, 10) are used.

The solvents for the defibration liquid shown in Patent Documents 1 and 2 are aprotic solvents with a donor number of 25 or more, including pyridine, which is a solvent that also serves as a catalyst, and each has the following problems.
Pyridine is a solvent with a low boiling point and high flammability, and has a high toxicity and a unique strong odor, which is harmful to the human body and the environment, making it difficult to handle. When aprotic solvents other than pyridine are used in combination with a basic catalyst, side reactions occur with cellulose when heated, causing discoloration of the cellulose and the solvent, making it impossible to heat the solution and making it impossible to shorten the reaction time. In addition, aprotic solvents other than DMSO have a problem of low permeability to cellulose.
Furthermore, in Patent Documents 1 and 2, single cellulose nanofibers modified with dicarboxylic acid were not obtained.

JP 2017-082188 A JP 2017-190437 A

Chem. Soc. Rev. , 2011, 10, 3941-3994

The present invention provides a method for producing dicarboxylic acid monoesterified modified cellulose nanofibers by reacting a dicarboxylic acid anhydride with cellulose in the absence of a catalyst, preferably with the addition of a base catalyst, using a protic amide solvent as the solvent.
The present invention also provides an aggregate of cellulose nanofibers composed primarily of dicarboxylic acid monoester modified single cellulose nanofibers.

As a result of extensive investigations aimed at achieving the above object, the present inventors have completed the invention described below.
[1] A method for producing dicarboxylic acid monoesterified modified cellulose, comprising penetrating a reaction solution containing a dicarboxylic acid anhydride and a protic amide solvent represented by the following formula (1) between cellulose fibrils to chemically disintegrate the hydroxyl groups on the surface of the cellulose fibrils through an esterification reaction:
R1-C(=O)-NH(R2) (1)
(In the formula, R1 and R2 represent a proton or an alkyl group having 1 to 3 carbon atoms. R1 and R2 may be the same or different.)
[2] The method according to [1] above, wherein the protic amide solvent is at least one of formamide, methylformamide, acetamide, and N-methylacetamide.
[3] The method according to [1] or [2] above, wherein the dicarboxylic acid anhydride is at least one of maleic anhydride, an aliphatic dicarboxylic acid anhydride having 4 to 8 carbon atoms, and phthalic anhydride.
[4] The method according to any one of [1] to [3] above, wherein the reaction solution further contains a base catalyst.
[5] A method for producing dicarboxylic acid monoesterified modified cellulose nanofibers, comprising: a first step of producing a dicarboxylic acid monoesterified modified cellulose by the production method according to any one of [1] to [4] above; and a second step of dispersing the dicarboxylic acid monoesterified modified cellulose produced in the first step in water or an aqueous solution of a polar solvent at a concentration of 50% or less, and defibrating the resulting cellulose nanofibers into dicarboxylic acid monoesterified modified single cellulose nanofibers by electrostatic repulsion and shear force.
[6] An aggregate of dicarboxylic acid monoesterified modified cellulose nanofibers having a fiber diameter of 3 to 100 nm, wherein the cellulose nanofibers constituting the aggregate have a cellulose Iβ crystal structure, an average degree of substitution of dicarboxylic acid monoester groups is 0.1 to 1.0, and a 0.3 wt % aqueous dispersion of the aggregate has a visible light transmittance of 65% or more.
[7] An aggregate of dicarboxylic acid monoester modified cellulose nanofibers having a fiber diameter of 3 to 100 nm, wherein the cellulose nanofibers constituting the aggregate have a cellulose Iβ crystal structure and an average degree of substitution of dicarboxylic acid monoester groups of 0.1 to 1.0, and the number content of single cellulose nanofibers having a fiber diameter of 3 to 10 nm constituting the aggregate is 98% or more.
[8] An aggregate of dicarboxylic acid monoesterified modified cellulose nanofibers according to [6] or [7], characterized in that the average fiber length of the single cellulose nanofibers constituting the aggregate is 200 nm or more and less than 1000 nm.

Here, the cellulose nanofiber aggregate 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. There are no particular limitations on the form of the mixture. It may be selected according to the application. For example, it may be in the form of a dispersion in water or an organic solvent, a paste, a gel, a dry state, or a state dispersed in a resin.

The average degree of substitution is the proportion of modified hydroxyl groups in cellulose, and more specifically, it is the average number of modified hydroxyl groups (number of substituents) per repeating unit of cellulose (average degree of substitution).

The number content of the dicarboxylic acid monoester modified single cellulose nanofibers described in [7] above was determined by observing a sample sampled from the aggregate of the dicarboxylic acid monoester modified cellulose nanofibers using TEM images at a magnification of 400,000 or more, counting the number of large CNFs (crude CNFs) with fiber diameters exceeding 10 nm in randomly selected images, and confirming that the number of single cellulose nanofibers with fiber diameters of 3 to 10 nm was 49 times or more that number.

The average fiber length of the dicarboxylic acid monoester modified single cellulose nanofibers was measured as follows.
To measure the fiber length of single cellulose nanofibers with a fiber diameter of 3 to 10 nm, the CNF dispersion for observation was diluted as much as possible to reduce CNF overlap, and a TEM observation sample was prepared, and measurements were taken using TEM images at a magnification of 400,000 or more. If the CNF was curved, both ends of the fiber were found along the fiber curvature, and the linear distance between each curvature was measured and added to obtain the fiber length.

The manufacturing method of the present invention uses a reaction solution (defibration solution) containing a protic amide solvent and a dicarboxylic anhydride, which causes few side reactions and has excellent permeability and swelling properties for cellulose, and can produce cellulose nanofibers by modifying cellulose into dicarboxylic acid monoesters without a catalyst.

The manufacturing method of the present invention not only avoids side reactions, but also significantly shortens the reaction time, and allows the dicarboxylic acid monoesterification modification reaction to be carried out by heating without the use of a catalyst.

The reaction solution of the present invention penetrates between cellulose fibrils to modify the hydroxyl groups on the surface of the cellulose fibrils with dicarboxylic acid monoesterification, and then electrostatic repulsion and shear force are applied to produce an aggregate of cellulose nanofibers whose main component is single cellulose nanofibers modified with dicarboxylic acid monoesterification.

In the manufacturing method of the present invention, the esterification reaction between the dicarboxylic anhydride and the hydroxyl groups of cellulose is promoted by adding a base catalyst to the reaction solution (defibration solution), making it possible to shorten the reaction time and lower the reaction temperature.

The cellulose nanofiber aggregates of the present invention, whose main component is the dicarboxylic acid monoester modified single cellulose nanofiber, have a small average fiber diameter of CNF and a narrow distribution of fiber diameters, and therefore are highly transparent. When composited with a resin as a reinforcing material, the transparency of the resin can be ensured. Furthermore, despite the fiber length being less than 1 μm, the aspect ratio is 100 or more, so that both dispersibility and reinforcing effect can be achieved.

Photographs of 0.3% aqueous dispersions of dicarboxylic acid monoesterified modified CNF obtained in Examples 1, 3 to 7, 9 and 10, and Comparative Example 1 IR spectra of dicarboxylic acid monoesterified modified CNF obtained in Example 1, Examples 3 to 7, and Example 10 UV spectra of 0.3% aqueous dispersions of dicarboxylic acid monoesterified modified CNF obtained in Examples 1, 3 to 7, 9 and 10, and Comparative Example 1

The method for producing dicarboxylic acid monoester-modified cellulose nanofibers of the present invention is characterized in that a reaction solution containing a dicarboxylic acid anhydride and the protic amide-based solvent of formula (1) is infiltrated between cellulose fibrils, and the hydroxyl groups of the cellulose fibrils are subjected to a dicarboxylic acid monoesterification reaction to modify them into dicarboxylic acid monoesters, thereby chemically defibrating the cellulose fibrils.

The cellulose used as the raw material in the present invention may be in the form of cellulose alone or in a mixed form containing non-cellulose components such as lignin and hemicellulose.
Examples of cellulose in a pure form (or cellulose with a small content of non-cellulose components) include pulp (e.g., wood pulp, bamboo pulp, straw pulp, bagasse pulp, linter pulp, flax pulp, hemp pulp, paper mulberry pulp, mitsumata pulp, etc.), sea squirt cellulose, bacterial cellulose, cellulose powder, crystalline cellulose, etc.
Examples of mixed forms of cellulose (cellulose compositions) include wood (e.g., conifers (pine, fir, spruce, hemlock, cedar, etc.), broad-leaved trees (beech, birch, poplar, maple, etc.)), herbaceous plants (hemp (hemp, flax, Manila hemp, ramie, etc.), straw, bagasse, Mitsumata, etc.), seed hair fibers (cotton linters, bombax cotton, kapok, etc.), bamboo, sugar cane, paper, etc.

These celluloses can be used alone or in combination. In mixed celluloses, the proportion of non-cellulose components may be 90% by weight or less, for example, 1 to 90% by weight, preferably 3 to 80% by weight, and more preferably 5 to 70% by weight. If the proportion of other components is too high, it may be difficult to produce dicarboxylic acid monoester modified CNF.

The cellulose preferably contains crystalline cellulose (particularly type I crystalline cellulose), or may be a combination of crystalline cellulose and non-crystalline cellulose (such as amorphous cellulose). The proportion of crystalline cellulose (particularly type I crystalline cellulose) may be 10% by weight or more of the total cellulose, for example, 30 to 99% by weight, preferably 50 to 98.5% by weight, and more preferably about 60 to 98% by weight. If the proportion of crystalline cellulose is too low, the heat resistance and strength of the dicarboxylic acid monoester modified CNF may decrease.

Among these, wood pulp (e.g., softwood pulp, hardwood pulp, etc.), seed hair fiber pulp (e.g., cotton linter pulp), cellulose powder, etc. are commonly used because they are easy to modify and defibrate. The pulp may be mechanical pulp made by mechanically processing pulp material, but chemical pulp made by chemically processing pulp material is preferred because it contains less non-cellulose components.

The moisture content of cellulose (weight ratio of moisture to dry cellulose) may be 1% by weight or more, for example, about 1 to 100% by weight, preferably about 2 to 80% by weight, and more preferably about 3 to 60% by weight (particularly about 5 to 50% by weight). In the present invention, from the viewpoint of the permeability or swelling efficiency of the reaction solution, it is preferable that the cellulose contains moisture in such a range, and for example, in the case of commercially available cellulose pulp, the cellulose pulp may be used as it is without drying. If the moisture content is too low, there is a risk that the defibration property of the cellulose may decrease.
Regarding the pretreatment of the cellulose raw material, it is preferable to crush the cellulose raw material (especially when pulp is used) into a size that can fit into the defibration reaction device or container by tearing or shredding it depending on the size of the defibration reaction device, and then subject it to the defibration reaction process.

Dicarboxylic acid anhydrides include aliphatic dicarboxylic acid anhydrides, alicyclic dicarboxylic acid anhydrides, and aromatic dicarboxylic acid anhydrides. Preferred dicarboxylic acid anhydrides are maleic anhydride, aliphatic dicarboxylic acid anhydrides having 4 to 8 carbon atoms, and phthalic anhydride. Examples of aliphatic dicarboxylic acid anhydrides having 4 to 8 carbon atoms include succinic anhydride, glutaric anhydride, adipic anhydride, pimelic anhydride, suberic anhydride, azelaic anhydride, and sebacic anhydride.

Furthermore, preferred dicarboxylic acid anhydrides are succinic anhydride, maleic anhydride and glutaric anhydride. These anhydrides are preferred because they are inexpensive and readily available commercially.

As the reaction solvent, a protic amide solvent represented by the following formula (1) is used.
R1-C(=O)-NH(R2) (1)
(In the formula, R1 and R2 represent a proton or an alkyl group having 1 to 3 carbon atoms. R1 and R2 may be the same or different.)

Preferred protic amide solvents are those with a dielectric constant of 45 or more at room temperature and an acceptor number of 20 or more. Examples include formamide (FA), N-methylformamide (MFA), N-ethylformamide, acetamide, N-methylacetamide, and N-ethylacetamide. Preferred are formamide, methylformamide, acetamide, and N-methylacetamide, and most preferred are formamide and methylformamide.

The solvent used in the production of modified cellulose nanofibers must be able to dissolve the reactant, and the four most important properties are the rate of penetration into cellulose, the degree of swelling of cellulose, and the ability to promote the modification reaction. These properties are thought to govern the fiber diameter and uniformity of the modified cellulose nanofibers.
Protic amide solvents, particularly formamide, N-methylformamide, acetamide and N-methylacetamide, are preferred because they combine these effects and are suitable for heating reactions due to their high solvent boiling points. In addition, protic amide solvents are preferred because they are believed to have a high effect of stabilizing anions by solvation.
Furthermore, protic amide solvents differ from general protic polar solvents in that they have a low ability to donate or release protons from the solvent itself, and therefore competitive reactions are unlikely to occur.

As long as the reaction solution does not reduce the permeability of the cellulose or does not cause side reactions, it may contain an aprotic solvent such as ketones such as DMAc, DMF, NMP, DMSO, acetone, etc., or ethers such as THF, etc. The proportion of the aprotic solvent added to the protic amide solvent is 50 wt % or less, preferably 35 wt % or less, more preferably 30 wt % or less of the total solvent.
If the content of the aprotic solvent exceeds this range, the permeability decreases and side reactions occur, which is undesirable.

The concentration of the dicarboxylic anhydride contained in the protic amide solvent may be adjusted appropriately depending on the type or reactivity of the anhydride and the reaction conditions (temperature, time, pulp concentration, presence or absence of catalyst). For example, it is 1 to 50% by weight, more preferably 1.5 to 40% by weight, even more preferably 2 to 35% by weight, and most preferably 3 to 30% by weight. Concentrations lower than this range are not preferred because they may slow the reaction rate or reduce the modification rate. Conversely, concentrations higher than this range are not preferred because they may not completely dissolve the dicarboxylic anhydride or the reaction solution may have reduced permeability between cellulose fibrils.

The weight ratio of cellulose to the reaction solution can be selected from the range of about 1/99 to 35/65 (former/latter), for example about 1.2/98.8 to 30/70, preferably 1.5/98.5 to 25/75, and more preferably about 2/98 to 20/80. If the cellulose ratio is too low, the production efficiency of dicarboxylic acid monoester modified CNF will be low, and if it is too high, the reaction time will be long, so in either case there is a risk of productivity decreasing. Furthermore, if the cellulose ratio is too high, there is a risk of the uniformity of the size and modification rate of the obtained fine fibers decreasing.

A base catalyst may further be added to the reaction solution.
The reaction solution preferably contains a base catalyst, since it can lower the reaction temperature and shorten the reaction time. The base catalyst is not particularly limited, but examples thereof include alkali metal carbonates such as sodium carbonate, potassium carbonate, and lithium carbonate, alkali metal hydrogen carbonates such as sodium hydrogen carbonate, alkyl metal acetates such as sodium acetate and potassium acetate, alkyl metal hydroxides, and amines. Among these, alkali metal carbonates and alkyl metal acetates are preferred, and sodium carbonate, potassium carbonate, lithium carbonate, sodium acetate, and potassium acetate are more preferred. On the other hand, alkyl metal hydroxides are highly basic, and may cause hydrolysis of dicarboxylic acid anhydrides, so their use has a low priority.

The concentration of the base catalyst in the reaction solution may be appropriately controlled depending on the type of dicarboxylic anhydride and the reaction temperature. For example, it is 0.01 to 10% by weight, preferably 0.05 to 5% by weight, and more preferably 0.1 to 4% by weight. It is most preferably 0.2 to 3% by weight. If it is lower than this range, the catalytic action cannot be expected, which is not preferable. On the other hand, if it is higher than this range, it is not preferable because it may lead to decomposition of the cellulose and dicarboxylic anhydride.

The method of modifying cellulose with dicarboxylic acid monoesterification can be carried out by penetrating a reaction solution between cellulose fibrils and chemically defibrating the hydroxyl groups on the surface of the cellulose fibrils through an esterification reaction.
The method for chemical defibration is not particularly limited, and may be carried out by mixing the reaction solution with the raw cellulose.

The reaction solution is prepared by mixing a dicarboxylic anhydride and a reaction solution containing the protic amide solvent shown in formula (1) above, and optionally adding a base catalyst to prepare the reaction solution. A typical method for preparing the reaction solution is to mix and stir the components together to dissolve the other components uniformly in the solvent.

The reaction solution and raw cellulose can be mixed by adding the raw cellulose to the reaction solution in the reaction vessel, or by adding the reaction solution to the raw cellulose in the reaction vessel. On a laboratory scale, a sample bottle or flask can be used as the reaction vessel. For scale-up, a reaction apparatus normally used in organic synthesis is recommended. When the cellulose concentration is high, a kneader such as a twin-screw extruder can also be used.

By stirring the mixture at a predetermined temperature for a predetermined time, the reaction solution is allowed to penetrate between the cellulose fibrils, and the hydroxyl groups on the surface of the cellulose fibrils are esterified to chemically disintegrate them, yielding a reaction product containing dicarboxylic acid monoesterified modified cellulose.

The reaction temperature is preferably 15°C to 250°C. Temperatures lower than 15°C are not preferred because the reaction rate is slow and the required reaction time is long. Temperatures higher than 250°C are not preferred because the cellulose or dicarboxylic anhydride may decompose, or the dicarboxylic monoester group may further dehydrate with the hydroxyl group of the cellulose, resulting in loss of anionicity. More preferred is 20°C to 200°C, and most preferred is 23°C to 180°C.

The reaction can be carried out in an air atmosphere, but in the case of a reaction involving high temperature heating, it is preferable to carry out the reaction under an inert gas in order to reduce the oxidation reaction of the reactant and cellulose. By carrying out the reaction under an inert gas, side reactions can be suppressed, which is preferable. Either nitrogen or argon gas can be used as the inert gas.

The stirring may be carried out by a method selected from various commonly used stirring methods.
The shape of the cellulose modified by dicarboxylic acid monoesterification varies depending on the stirring method and the shear force. For example, when mild stirring such as a magnetic stirrer or mechanical stirring is used, the fiber diameter obtained is on the order of microns to tens of microns. Because the shape is large, it is easy to wash and can be easily washed by conventional filtration, pressure filtration, squeezing, etc.
On the other hand, if a strong stirring method such as a paint shaker is used, most of the cellulose fibers after modification will have a nanoscale diameter. Therefore, the reaction method should be appropriately selected depending on the application and purpose.

The reaction time may be appropriately adjusted depending on the temperature, the dicarboxylic anhydride concentration, the catalyst concentration and the catalyst type.
For example, in the case of reaction at room temperature without the addition of a catalyst, the reaction time is preferably 6 to 48 hours.
The reaction time can be shortened by the addition of heat or a base catalyst.
In the case of a heating reaction without adding a catalyst, the reaction time is preferably 1 hour to 24 hours.
When a base catalyst is added, the reaction time can be shortened to 3 to 10 hours at room temperature, and to 30 minutes to 6 hours when heated.

If the reaction time is too short, the modification rate will be low or the distribution of the dicarboxylic acid monoester groups will be uneven, which is undesirable since it may result in a low transparency of the resulting dispersion of dicarboxylic acid monoester modified cellulose nanofibers. On the other hand, if the reaction time is too long, it is undesirable since the dicarboxylic acid monoester groups may further react with other cellulose hydroxyl groups to produce dicarboxylic acid diester modified cellulose, which may inhibit defibration.

Next, the dicarboxylic acid monoesterified modified cellulose is separated and washed from the reaction solution containing the dicarboxylic acid monoesterified modified cellulose obtained by the reaction, thereby removing the protic amide solvent, dicarboxylic acid anhydride, dicarboxylic acid, base catalyst, etc. in the reaction solution, and the dicarboxylic acid monoesterified modified cellulose is recovered.
The washing solvent is not particularly limited, but may be any solvent capable of dissolving each component of the reaction solution, and is preferably, for example, water, alcohol, or a mixture of water and alcohol.
The washing method is not particularly limited, but possible methods include squeezing, pressure filtration, reduced pressure filtration, centrifugation, and precipitation.

Depending on the reaction conditions, it may be preferable to neutralize the reaction mixture using a neutralizing alkali during the washing process. There are no particular limitations on the neutralizing alkali, 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.

The state of the recovered dicarboxylic acid monoester modified cellulose is not particularly limited. For example, after washing, it may be in a wet state still containing a protic solvent such as water, or in a dry state. Even if the recovered modified cellulose is dried, it will disperse again when water is added.

The shape of the recovered dicarboxylic acid monoester modified cellulose varies depending on the reaction conditions and stirring conditions during the modification reaction, but is usually fibrous with a fiber diameter of several μm to several tens of μm, fine fiber with a fiber diameter of several hundred nm to 1000 nm, or cellulose nanofiber with a fiber diameter of 3 nm to 100 nm. The faster the stirring speed, the more fiber is defibrated during the reaction, and the lower the content of fibers on the order of microns or larger in the resulting anionic cellulose. However, if the stirring is too strong, the cellulose is decomposed to the nanoscale during the reaction, which reduces water release during washing, increases the amount of washing solvent required for washing, or increases the washing time, which may lead to a decrease in production efficiency.

A dicarboxylic acid monoester modified cellulose nanofiber mainly composed of single CNF can be produced by a production method comprising the steps of: a first step of producing dicarboxylic acid monoester modified cellulose by any one of the production methods [1] to [4] above, in which a reaction solution containing a dicarboxylic acid anhydride and a protic amide solvent shown in formula (1) is infiltrated between cellulose fibrils to modify the hydroxyl groups on the surface of the cellulose fibrils with dicarboxylic acid monoester; and a second step of dispersing the dicarboxylic acid monoester modified cellulose produced in the first step in an aqueous solution of water or a polar solvent of 50% or less and nanoizing it by electrostatic repulsion and shear force.

In order to defibrate the modified cellulose into single CNF in the second step, the dicarboxylic acid monoester modified cellulose produced in the first step preferably has an average degree of substitution of dicarboxylic acid monoester groups of 0.10 or more, more preferably 0.12 or more, and even more preferably 0.15 or more.
If the average degree of substitution is less than 0.10, the obtained anionic cellulose will have insufficient electrostatic repulsion in water, resulting in a low degree of defibration, and there is a risk that the content of crude CNF with a fiber diameter exceeding 10 nm after defibration will be high, which is not preferable.

In the second step, by combining electrostatic repulsion and mechanical defibration (shear force), cellulose can be nano-sized to 3 to 10 nm regardless of the shape of the fiber.In the second step, the dicarboxylic acid monoester modified cellulose produced in the first step is dispersed in water or an aqueous solution of 50% or less polar solvent, and defibrated by electrostatic repulsion and the application of mechanical or physical shear force.
Due to the electrostatic repulsion effect, water (distilled water) is most preferable as the 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 be used.
Because the hydroxyl groups of CNF are replaced with anionic groups, they can be defibrated down to single CNFs with fiber diameters of 3 to 10 nm due to electrostatic repulsion in the dispersion solvent.

A preferred method for obtaining 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 to adjust the solid content of the modified cellulose fiber in the dispersion solvent to about 0.1 to 0.8%, and to stir the mixture for 3 to 20 minutes using a stirring device such as a mixer or homogenizer.

If coarse fibers remain in the second step, they can be removed by filtering using a filter such as a nylon mesh. In addition, further processing with a Clearmix or homogenizer can improve the degree of defibration or shorten the fiber length to the required range.

The first and second steps produce an aggregate of dicarboxylic acid monoester modified cellulose nanofibers in which the average substitution degree of the dicarboxylic acid monoester group is 0.1 to 1.0 and the number content of single cellulose nanofibers with fiber diameters of 3 to 10 nm that make up the aggregate is 98% or more.

The cellulose nanofibers constituting the aggregate of the dicarboxylic acid monoester modified cellulose nanofibers of the present invention preferably have a cellulose Iβ crystal structure. By having an Iβ crystal structure, the aggregate of the dicarboxylic acid monoester modified cellulose nanofibers is strong and is insoluble in general organic solvents, which is preferable.

The degree of crystallinity of the cellulose nanofibers constituting the aggregate of the present invention is 30% to 75%. If the degree of crystallinity is higher than 75%, the content of the number of nanofibers with a fiber diameter of 20 nm or more is high, and the transparency of the aggregate is low, which is not preferable. Usually, the degree of crystallinity of cellulose nanofibers depends on the fiber diameter and reflects the fiber diameter. It is preferable to control the degree of crystallinity to less than 70%, because it is possible to ensure that the content of single cellulose nanofibers with a fiber diameter of 3 to 10 in the aggregate is 98% or more. On the other hand, if the degree of crystallinity is 30% or less, the content of amorphous cellulose increases, which is not preferable because there is a risk of a decrease in the reinforcing effect and thickening effect. A more preferable degree of crystallinity is 35 to 69%, and most preferably 40 to 68%.

The number of hydroxyl groups modified with dicarboxylic acid monoester in the cellulose nanofibers constituting the aggregate of the present invention (average degree of substitution) is preferably 0.1 to 1.0. More preferably, it is 0.12 to 0.9, and most preferably 0.15 to 0.8. If it is lower than 0.1, the content of CNF with a fiber diameter of 10 nm or more will increase, and the transparency of the single cellulose nanofiber aggregate may decrease. On the other hand, an average degree of substitution of 1.0 or more is not preferable because it may significantly decrease the degree of crystallinity.

The visible light transmittance of a 0.3 wt% aqueous dispersion of the aggregate of the dicarboxylic acid monoester modified cellulose nanofibers of the present invention is 65% or more. If it is less than 65%, the transparency of the reinforced composite and dispersion is low, which is not preferable. If the visible light transmittance is 65% or more, it is possible to ensure that the number content of single cellulose nanofibers with a fiber diameter of 3 to 10 nm is 98% or more. The visible light transmittance is more preferably 75% or more, and most preferably 80% or more.

The number content of single cellulose nanofibers having a fiber diameter of 3 to 10 nm constituting the aggregate of the present invention is 98% or more. If the number content of single cellulose nanofibers having a fiber diameter of 3 to 10 nm is lower than 98%, it is not preferable because the transparency of the reinforced composite or dispersion is low.

The average fiber length of the single cellulose nanofibers having a fiber diameter of 3 to 10 nm that constitute the aggregate is 200 nm or more and less than 1000 nm. If it is shorter than this range, the reinforcing effect, thickening effect, and water retention effect are reduced, which is not preferable. On the other hand, if it is 1 μm or more, it is not preferable because it is prone to aggregation when compounded with a polymer and the reinforcing effect cannot be expressed. Furthermore, if the fiber length is too long, it is not preferable because the viscosity increases significantly when added to a paint or adhesive, and there is a risk of the paint or adhesive losing its fluidity and processability.
The average fiber length of the cellulose nanofibers constituting the aggregate is more preferably 250 nm to 900 nm, and most preferably 300 to 850 nm.

The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples. Table 1 shows a list of the composition of the reaction solution and the reaction conditions in the examples and comparative examples. The details of the raw materials used are as follows, and the properties of the obtained modified cellulose nanofibers were measured as follows.

(Materials and solvents used)
Cellulose pulp: Commercially available coniferous wood pulp (manufactured by Georgia Pacific, product name: Fluff Pulp ARC48000GP) was used. It was used after being torn into 1 cm cubes.
Powdered cellulose: KC Flock W-100G manufactured by Nippon Paper Industries Co., Ltd., with an average degree of polymerization of 450, was used.
Other reagents were purchased from Nacalai Tesque.

(Equipment used for defibration)
Mixer: Panasonic MX-X701-T mixer.
Clearmix: Clearmix CLM manufactured by M TECHNIQUE.

(IR analysis of cellulose nanofibers)
The dicarboxylic acid monoesterification modification was confirmed by IR spectroscopy. The IR spectrum of the cellulose nanofiber was measured using a Fourier transform infrared spectrophotometer (FT-IR). The measurement was performed using a NICOLET MAGNA-IR760 Spectrometer manufactured by NICOLET Corporation, and analysis was performed in ATR mode.

(Quantitative Determination of Dicarboxylic Acid Monoester Group Modification Rate of Cellulose or CNF)
The dicarboxylic acid monoester group modification rate of cellulose or CNF was expressed as an average substitution degree, and was quantified by electrical conductivity titration. That is, 100 g of a 0.3% aqueous dispersion of cellulose modified by dicarboxylic acid monoesterification or CNF modified by dicarboxylic acid monoesterification was prepared, and a 0.1 N hydrochloric acid aqueous solution was added thereto to adjust the pH to 2.5, and then a 0.05 N sodium hydroxide aqueous solution was added dropwise until the pH reached 11, while recording the electrical conductivity value. The electrical conductivity and pH were plotted, and the mole number Q of the carboxylic acid group was calculated by the following formula from the amount (a) of the aqueous sodium hydroxide solution consumed in the neutralization stage of the weak acid where the change in electrical conductivity was gradual.
Q (mol) = a [ml] x 0.05/1000
The average degree of substitution was calculated from the number of moles Q of the substituent and the molecular weight (162) of anhydroglucan, which is a structural unit of cellulose.
Average degree of substitution = 162 x Q/(100 x 0.3% - M x Q + 18 x Q)
(In the formula, 100 is the amount of the aqueous dispersion of dicarboxylic acid monoesterified modified cellulose or dicarboxylic acid monoesterified modified CNF, M is the molecular weight of the dicarboxylic acid anhydride, and 18 is the molecular weight of water produced by dehydration through the reaction of the dicarboxylic acid anhydride with a hydroxyl group.)

(SEM Observation)
The shape of the dicarboxylic acid monoesterified modified cellulose nanofibers was observed using an FE-SEM ("JSM-6700F" manufactured by JEOL Ltd., measurement conditions: 20 mA, 60 seconds).

(Preparation of TEM Observation Sample)
The aggregate of dicarboxylic acid monoesterified modified cellulose nanofibers was diluted with water to 0.003%, then mixed with an ionic liquid for electron microscopes (Hitachi High-Technologies HILEM IL1000) so that the final concentration was 0.0015% or less, and the mixture was dropped onto a carbon support film for TEM observation (Oken Shoji Super Hi-Reso Carbon SHR-C075) and dried to prepare a sample for TEM observation. To enhance the image contrast, the above sample was subjected to electron staining (negative staining) using an electron microscope staining agent (Nissin EM EM Stainer) mainly composed of gadolinium acetate.

(TEM Observation)
The TEM observation sample was observed in a TEM bright field at an acceleration voltage of 120 kV using a field emission transmission electron microscope (FE-TEM) JEM-2100F manufactured by JEOL Ltd.
CNFs with fiber diameters exceeding 10 nm were randomly found from the TEM photograph images, and it was confirmed that there were 49 or more CNFs around them with fiber diameters of 10 nm or less.
The average fiber diameter and average fiber length of single cellulose nanofibers with a fiber diameter of 3 to 10 nm were measured by observing TEM images at a magnification of 400,000 times or more. 50 or more fibers were randomly selected from the TEM photograph, and the fiber diameter and fiber length of each were measured.
When the CNF was curved, both ends of the fiber were found along the fiber curvature, and the straight-line distance between each curvature was measured and added up to determine the fiber length.

(Method of measuring crystallinity)
The dispersion of the dicarboxylic acid monoester modified cellulose nanofiber was dried and the crystallinity was measured by 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. Scan range: 2θ=5 to 70°.
The crystallinity was calculated by the following formula (see Textile Res. J. 29:786-794, 1959).
Crystallinity (%) = [(I200-IAM)/I200] x 100
I200: Diffraction intensity at 2θ = 22.6° IAM: Diffraction intensity of the amorphous part at 2θ = 18.5°

(Method of measuring thermal decomposition temperature)
The aggregate of the dicarboxylic acid monoesterified modified cellulose nanofibers 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 Co., Ltd.). The measurement conditions are as follows. The 5% weight loss temperature was measured and defined as the thermal decomposition temperature.
Atmosphere: Argon gas (flow rate 300 mL/min)
Temperature range: 30 to 400°C
Heating rate: 10° C./min

(Measurement of visible light transmittance)
A 0.3 wt% aqueous dispersion of the aggregate of dicarboxylic acid monoesterified modified cellulose nanofibers was prepared and measured using a UV-visible spectrophotometer to measure the visible light transmittance at a wavelength of 589 nm (D-ray). The device used for the measurement was Shimadzu UV-3600, and the measurement wavelength range was 300 nm to 800 nm.

[Example 1]
A mixture of 5 g of maleic anhydride and 45 g of formamide and 1.5 g of cellulose pulp were placed in a 100 ml round-bottom three-neck flask, a PTFE blade-type stirring rod was attached, and the mixture was stirred at room temperature for 30 minutes, then placed in an oil bath at 60° C. and stirred for 6 hours while argon gas was continuously flowing into the flask. After cooling, the reaction mixture was squeezed through a nylon mesh (T380) to remove the reaction solution, and the dicarboxylic acid monoesterified modified cellulose component and the reaction solution were collected.
The recovered modified cellulose was dispersed in 150 g of distilled water and squeezed again to recover the dicarboxylic acid monoester modified cellulose component. After repeating dispersion and squeezing twice, the squeezed dicarboxylic acid monoester modified cellulose was dispersed in a solution of 0.4 g of sodium carbonate dissolved in 250 ml of water, squeezed again, and washed twice with 250 ml of distilled water to obtain maleic acid monoester modified cellulose. The washed maleic acid monoester modified cellulose and distilled water were combined to prepare a 500 ml dispersion, which was then placed in a mixer and stirred for 10 minutes to obtain an aqueous dispersion of maleic acid monoester modified cellulose nanofibers.

A photograph of the appearance of the aqueous dispersion is shown in Figure 1. The maleic acid monoesterification modification was confirmed by IR spectroscopy (Figure 2). The absorption of the esterified bond was detected at around 1730 cm ^-1 , and the absorption of the terminal sodium carboxylate was detected at around 1600 cm ^-1 .
A 0.3% aqueous dispersion of the obtained maleic acid monoester modified CNF was confirmed by conductometric titration to have a maleic acid monoester modification rate (average substitution degree) of 0.46.
The ultraviolet-visible transmission spectrum of a 0.3% aqueous dispersion of the obtained maleic acid monoesterified modified CNF was measured, and the results are shown in Figure 3. The visible light transmittance was 67%.
The crystallinity of the resulting aggregate of maleic acid monoester modified CNF was evaluated by XRD and found to be 69%, and the thermal decomposition temperature was found to be 260°C by TG-DTA analysis.
The resulting aggregate of maleic acid monoester modified CNF was observed by FE-SEM. When magnified up to 50,000 times, a nanofiber skeleton with a fiber diameter of about 10 nm or less was observed, but the exact fiber diameter and fiber length could not be clearly measured.
Next, observation was performed using a TEM. 50 cellulose nanofibers were randomly selected from the image obtained by TEM, and the fiber diameter and fiber length were measured. No fiber diameters exceeding 10 nm were observed, the number content of cellulose nanofibers between 3 and 10 nm was 98% or more, and the average fiber diameter of single CNF was 8 nm. It was also confirmed that the fiber lengths were distributed between 500 nm and 1000 nm, and the average fiber length was 945 nm.
The evaluation results of the obtained maleic acid monoester modified CNF are shown in Table 2.

[Example 2]
Except for further treating with Clearmix for 20 minutes after the mixer treatment, the same procedure was followed as in Example 1. That is, the same procedure and conditions as in Example 1 were followed except that the aqueous dispersion (0.3%) of maleic acid monoesterified modified cellulose nanofiber obtained by mixer treatment as in Example 1 was further treated with Clearmix CLM at a rotation speed of 18,000 rpm on a 100 ml scale for 20 minutes.
The recovered maleic acid monoesterified modified CNF was evaluated in the same manner as in Example 1, and the evaluation results are shown in Table 2.

[Example 3]
Except for changing the reaction temperature to room temperature (23° C.) and the reaction time to 24 hours, the maleic acid monoesterified modified CNF was prepared in the same manner as in Example 1. The evaluation results are shown in Table 2 and Figures 1, 2 and 3.

[Example 4]
Succinic acid monoesterified modified CNF was prepared in the same manner as in Example 3, except that succinic anhydride was used instead of maleic anhydride as the modifying agent, N-methylformamide was used instead of formamide as the solvent, 0.75 g of sodium carbonate was added as a catalyst, and the reaction time was changed to 6 hours. The evaluation results are shown in Table 2 and Figures 1, 2 and 3.

[Example 5]
Except for using formamide instead of N-methylformamide as the solvent, succinic acid monoesterified modified CNF was prepared in the same manner as in Example 4. The evaluation results are shown in Table 2 and Figures 1, 2 and 3.

[Example 6]
Except for changing the reaction time from 6 hours to 4 hours, succinic acid monoesterified modified CNF was prepared in the same manner as in Example 5. The evaluation results are shown in Table 2 and Figures 1, 2 and 3.

[Example 7]
The modified CNF modified with succinic acid monoester was prepared in the same manner as in Example 6, except that the amount of succinic anhydride added was changed from 5 g to 2.5 g and the amount of formamide added was changed from 45 g to 47.5 g. The evaluation results are shown in Table 2 and Figures 1, 2 and 3.

[Example 8]
Except for using glutaric anhydride instead of succinic anhydride, glutaric acid monoester modified CNF was prepared in the same manner as in Example 5. The evaluation results are shown in Table 2.

[Example 9]
Maleic acid monoesterified modified CNF was prepared in the same manner as in Example 5, except that maleic anhydride was used instead of succinic anhydride and sodium bicarbonate was used instead of sodium carbonate. The evaluation results are shown in Table 2 and Figures 1 and 3.

[Example 10]
Except for using powdered cellulose instead of cellulose pulp, succinic acid monoesterified modified CNF was prepared in the same manner as in Example 5. The measurement results of the modification rate, crystallinity, visible light transmittance of the dispersion, etc. are shown in Table 2 and Figures 1, 2 and 3.

[Comparative Example 1]
Except for using pyridine instead of formamide as the solvent and using succinic anhydride instead of maleic anhydride as the modifying agent, succinic acid monoester modified CNF was prepared in the same manner as in Example 3. The measurement results of the modification rate, crystallinity, visible light transmittance of the dispersion, etc. are shown in Table 2 and Figures 1 and 3.
The obtained aqueous dispersion of the succinic acid monoester modified CNF was cloudy, and the visible light transmittance of the 0.3% aqueous dispersion was as low as 38%. Although single CNF with fiber diameters of 3 to 10 nm were present, the presence of CNF with fiber diameters of 10 nm or more was noticeable.

[Comparative Example 2]
Succinic acid modified cellulose nanofibers were synthesized in the same manner as in Examples 4 and 5, except that DMAc was used instead of N-methylformamide (Example 4) or formamide (Example 5) as the solvent. When sodium carbonate was added to a DMAc solution of succinic anhydride, the solution immediately changed color to red, then purple, and then squid ink. Pulp was added and stirred for 6 hours, after which it was washed and defibrated in the same manner as in Examples 4 and 5. The resulting aqueous dispersion of succinic acid monoester modified CNF was cloudy, and the visible light transmittance of a 0.3% aqueous dispersion was low at 9%. Furthermore, single CNF with fiber diameters of 3 to 10 nm was hardly present.

[Comparative Example 3]
Succinic acid modified cellulose nanofibers were synthesized in the same manner as in Example 5, except that methanol was used instead of formamide as the solvent. After stirring for 6 hours, the pulp maintained its sheet shape and was not broken down even when defibrated in a mixer. After drying, the IR spectrum was measured, and no carbonyl groups were detected. This result demonstrated that methanol could hardly penetrate into the interior of the cellulose pulp, and no reaction occurred between cellulose and succinic anhydride.

[Comparative Example 4]
Succinic acid-modified cellulose nanofibers were synthesized in the same manner as in Example 1, except that dimethyl sulfoxide (DMSO) was used instead of formamide as the solvent. Thirty minutes after the start of the reaction, the reaction solution turned brown. The cellulose obtained after washing was yellow. The appearance did not change much even after defibrating with a mixer.
These results indicate that the esterification reaction in DMSO is slow, and that side reactions occur, causing the cellulose fibers to become discolored and the fibers to not be defibrated.

Based on the results of the above examples and comparative examples, it has become clear that the reaction solution of the technology of the present application has excellent permeability into cellulose, and also has few side reactions, so that the dicarboxylic acid monoester modified CNF obtained does not discolor and the dispersion liquid has high transparency. Furthermore, no side reactions occurred whether a catalyst was added or not, and whether the mixture was heated at room temperature or not, and dicarboxylic acid modified cellulose nanofibers were obtained.

The aggregate of the dicarboxylic acid monoesterified modified cellulose nanofibers of the present invention can be used as a reinforcing material for resins, composite materials with inorganic particles, coating agents, and can also be formed into sheets or films for use. Furthermore, since it has functions such as emulsifying action (surfactant), thickening action, water retention or moisturizing action, it is expected to be applied in the fields of medical materials and pharmaceuticals, cosmetics, and food. Furthermore, since the aggregate of the dicarboxylic acid monoesterified modified cellulose nanofibers of the present invention has high transparency, it is particularly suitable as a reinforcing material for transparent resins.

Claims (5)

A method for producing dicarboxylic acid monoesterified modified cellulose, comprising penetrating a reaction solution containing a dicarboxylic acid anhydride and a protic amide-based solvent represented by the following formula (1) between cellulose fibrils to cause an esterification reaction of hydroxyl groups on the surface of the cellulose fibrils, thereby chemically defibrating the cellulose fibrils.
R1-C(=O)-NH(R2) (1)
(In the formula, R1 and R2 represent a proton or an alkyl group having 1 to 3 carbon atoms. R1 and R2 may be the same or different.) The method according to claim 1, wherein the protic amide solvent is one or more of formamide, methylformamide, acetamide, and N-methylacetamide. The method according to claim 1 or 2, wherein the dicarboxylic anhydride is at least one of maleic anhydride, an aliphatic dicarboxylic anhydride having 4 to 8 carbon atoms, and phthalic anhydride. The method according to any one of claims 1 to 3, wherein the reaction solution further contains a base catalyst. A method for producing dicarboxylic acid monoester modified cellulose nanofibers, comprising a first step of producing dicarboxylic acid monoester modified cellulose by any one of the production methods described in claims 1 to 4, and a second step of dispersing the dicarboxylic acid monoester modified cellulose produced in the first step in water or an aqueous solution of 50% or less polar solvent, and defibrating it into dicarboxylic acid monoester modified single cellulose nanofibers by electrostatic repulsion and shear force.