Ethyl cellulose derivative

Abstract

In this ethyl cellulose derivative, some of hydroxy groups of ethyl cellulose are substituted with one or more groups selected from an acetyl group, a propionyl group, and a butyryl group. A value obtained by dividing the Young's modulus MPa of the ethyl cellulose derivative by the specific surface area m2 / g is 50.0 or less.

Description

Ethyl cellulose derivatives

[0001] This disclosure relates to ethyl cellulose derivatives.

[0002] Patent Document 1 discloses a method for producing a cyclic dinucleotide derivative and a desiccant used in the method. The desiccant is a molecular sieve in powder or pellet form. In one step of the method for producing the cyclic dinucleotide derivative, the molecular sieve is added to a solution. The molecular sieve in the solution is then dehydrated by stirring. After that, the molecular sieve is removed from the solution by filtration.

[0003] Japanese Patent Publication No. 2023-73344

[0004] For example, when stirring a solution with molecular sieves in it, the molecular sieve particles may collide with each other in the solution and break apart. If the broken molecular sieve fragments are small, it can be difficult to remove them from the solution.

[0005] To solve the above problems, this disclosure provides a method in which some of the hydroxyl groups of ethylcellulose are substituted with one or more selected from acetyl groups, propionyl groups, and butyryl groups, thereby improving the Young's modulus (MPa) and specific surface area (m²). ２ It is an ethylcellulose derivative whose value when divided by ( / g) is 50.0 or less.

[0006] Furthermore, in order to solve the above problems, this disclosure provides that some of the carbon atoms at positions 2, 3, and 6 of ethylcellulose are bonded via oxygen atoms to one or more selected groups from acetyl groups, propionyl groups, and butyryl groups, thereby improving the Young's modulus (MPa) and specific surface area (m²). ２ It is an ethylcellulose derivative whose value when divided by ( / g) is 50.0 or less.

[0007] Because the particles are less likely to break even when they collide with each other, it is less likely to produce small fragments that are difficult to remove.

[0008] Figure 1 is a diagram illustrating the method for calculating sphericity. Figure 2 is an explanatory diagram showing the steps of the manufacturing method for ethyl cellulose derivatives.

[0009] <Ethylcellulose Derivatives> The following describes one embodiment of an ethylcellulose derivative. An ethylcellulose derivative is obtained in which some of the hydroxyl groups of ethylcellulose are replaced with other functional groups. Specifically, an ethylcellulose derivative is obtained in which some of the hydrogen atoms of some of the hydroxyl groups of ethylcellulose are replaced with one or more selected from acetyl groups, propionyl groups, and butyryl groups. In other words, an ethylcellulose derivative is obtained in which some of the carbon atoms at positions 2, 3, and 6 of ethylcellulose are bonded to one or more selected from acetyl groups, propionyl groups, and butyryl groups via oxygen atoms.

[0010] The basic structure of the ethylcellulose derivative is ethylcellulose as shown in Formula 1 below. In Formula 1, m is an integer of 2 or more. In this embodiment, the degree of ethoxylation of ethylcellulose is 48 wt%. Here, "degree of ethoxylation" refers to the mass percentage of hydroxyl groups substituted with ethoxyl groups out of the total hydroxyl groups in cellulose, the precursor of ethylcellulose. Hereafter, unless otherwise specified, ethylcellulose refers to ethylcellulose with a degree of ethoxylation of 48 wt%.

[0011]

[0012] The acetyl group is a functional group that can be substituted by reacting ethylcellulose with acetyl chloride, as shown in Chemical Formula 2 below. More specifically, the chemical structure of the acetyl group is shown in Chemical Formula 3 below. In Chemical Formula 3, "*" indicates the bonding site with the ethylcellulose that forms the basic skeleton.

[0013]

[0014]

[0015] The propionyl group is a functional group that can be substituted by reacting ethylcellulose with propionyl chloride, as shown in formula 4 below. More specifically, the chemical structure of the propionyl group is shown in formula 5 below. In formula 5, "*" indicates the bonding site with the ethylcellulose that forms the basic skeleton.

[0016]

[0017]

[0018] The butyryl group is a functional group that can be substituted by reacting ethylcellulose with the butyryl chloride shown in formula 6 below. More specifically, the chemical structure of the butyryl group is shown in formula 7 below. In formula 7, "*" indicates the bonding site with the ethylcellulose that forms the basic skeleton.

[0019]

[0020]

[0021] Therefore, ethylcellulose derivatives are represented by the general formula shown in Chemical Formula 8 below. In Chemical Formula 8, m is an integer of 2 or more.

[0022]

[0023] Hereinafter, the "degree of substitution" of a particular substituent is defined as the average number of substituents that replace the hydroxyl group in the monomer unit of ethylcellulose. In this embodiment, the ethoxy group in ethylcellulose is not substituted with any of the acetyl, propionyl, or butyryl groups. Therefore, the higher the degree of ethoxylation of ethylcellulose, the lower the upper limit of the degree of substitution of the acetyl, propionyl, and butyryl groups. Note that the degree of substitution is, for example, １ The degree of substitution is calculated from the integral value of the characteristic proton peaks of the substituents using 1H-NMR. Specifically, the degree of substitution can be measured using the Bruker ADVANCENEO 600 instrument. For measurement using this instrument, a 1% by weight sample solution was prepared using deuterated chloroform. １ Measurements were performed using 1H-NMR (single-pulse method) with 8 integration cycles and a pulse waiting time of 11 seconds. The chemical shift was based on tetramethylsilane added in trace amounts to the solution.

[0024] The ethylcellulose derivative of this embodiment is particulate. More specifically, it has a median diameter of 50 μm or more and is substantially spherical. The sphericity of the ethylcellulose derivative of this embodiment is 0.7 or more and 1.0 or less. Here, "sphericity" refers to the aspect ratio of the ethylcellulose derivative.

[0025] The specific method for measuring sphericity is as follows: As shown in Figure 1, in any cross-section of a particle P of each ethylcellulose derivative, the longest line segment connecting the outer circumference of the cross-section is designated as the first line segment L1. Next, the longest line segment connecting the outer circumference of the second line segment L2 that is perpendicular to the first line segment L1 is designated as the second line segment L2. The ratio of the length of the second line segment L2 to the length of the first line segment L1 is then defined as the sphericity.

[0026] <Method for producing ethylcellulose derivatives> An example of a method for producing ethylcellulose derivatives is described below. As shown in Figure 2, the method for producing ethylcellulose derivatives comprises a solution preparation step S1, a mixing step S2, and a dropwise addition step S3.

[0027] First, in solution preparation step S1, ethyl cellulose and acetone are prepared. Specifically, in solution preparation step S1, ethyl cellulose with an ethoxylation degree of 48 wt% and super-dehydrated acetone are prepared. Then, the ethyl cellulose and super-dehydrated acetone are stirred at a temperature of 50 degrees Celsius. As a result, the ethyl cellulose is dispersed in the super-dehydrated acetone. The super-dehydrated acetone in which the ethyl cellulose is dispersed will henceforth be referred to as the reaction dispersion.

[0028] Next, the mixing step S2 is performed. In the mixing step S2, an additive is mixed into the reaction dispersion prepared in the solution preparation step S1. Specifically, in the mixing step S2, the reaction dispersion prepared in the solution preparation step S1 is put into the container of a stirrer. The stirrer is, for example, a planetary mixer. Then, one or more selected from acetyl chloride, propionyl chloride, and butyryl chloride are put into the above container as an additive. Further, the reaction dispersion and the additive are mixed at 50 degrees for 3 hours. Thereby, an ethyl cellulose derivative in which a part of the hydrogen of the hydroxy group of ethyl cellulose in the reaction dispersion is substituted with one or more selected from an acetyl group, a propionyl group, and a butyryl group is produced.

[0029] Since ethyl cellulose is dispersed in super dehydrated acetone in the reaction dispersion, the bias in the probability of substitution with one or more functional groups selected from an acetyl group, a propionyl group, and a butyryl group among the monomers of the ethyl cellulose derivative is small. Therefore, the bias in the distribution of the acetyl group, the propionyl group, and the butyryl group among the monomers of the ethyl cellulose derivative is small.

[0030] Next, the dropping step S3 is performed. First, in the dropping step S3, the reaction dispersion containing the ethyl cellulose derivative that has undergone the mixing step S2 is collected with, for example, a dropper. Then, the solution containing the ethyl cellulose derivative is dropped one by one into pure water. The ethyl cellulose derivative dropped into the pure water becomes spherical by aggregation.

[0031] Although not shown in FIG. 2, particulate ethyl cellulose derivatives can be obtained by filtering the solution after the dropping step S3. Further, by drying the ethyl cellulose derivative, the ethyl cellulose derivative can be used as a drying material.

[0032] <Regarding Test Results> A comparative test was conducted on the sphericity and elasticity indices of ethyl cellulose derivatives. Hereinafter, the test results of the ethyl cellulose derivatives of Examples 1 to 4 and the ethyl cellulose derivatives of Comparative Examples 1 to 5 will be described. All of the ethyl cellulose derivatives used in this comparative test have an ethyl cellulose with an ethoxylation degree of 48 wt% as the starting material. Unless otherwise specified, the ethyl cellulose derivatives of Examples 1 to 4 and the ethyl cellulose derivatives of Comparative Examples 1 to 5 were manufactured according to the above-mentioned manufacturing method of ethyl cellulose derivatives.

[0033]

[0034] As shown in Table 1, the ethyl cellulose derivative of Example 1 was manufactured by mixing a reaction dispersion with butyryl chloride as an additive in the above-mentioned mixing step S2. That is, in the ethyl cellulose derivative of Example 1, a part of the hydrogen of the hydroxy group of ethyl cellulose is substituted with a butyryl group. In the ethyl cellulose derivative of Example 1, the substitution degree of the butyryl group is 0.46. Also, the median diameter of the manufactured ethyl cellulose derivative of Example 1 is 100 μm.

[0035] The ethyl cellulose derivative of Comparative Example 1 was manufactured without mixing an additive in the above-mentioned mixing step S2. Therefore, in the ethyl cellulose derivative of Comparative Example 1, the substitution degrees of the acetyl group, propionyl group, and butyryl group are 0.0.

[0036] The ethyl cellulose derivative of Example 2 was manufactured by mixing a reaction dispersion with acetyl chloride as an additive in the above-mentioned mixing step S2. That is, in the ethyl cellulose derivative of Example 2, a part of the hydrogen of the hydroxy group of ethyl cellulose is substituted with an acetyl group. In the ethyl cellulose derivative of Example 2, the substitution degree of the acetyl group is 0.46. Also, the median diameter of the manufactured ethyl cellulose derivative of Example 2 is 60 μm.

[0037] The ethylcellulose derivative of Example 3 was produced by mixing propionyl chloride and the reaction dispersion as additives in the mixing step S2 described above. That is, in the ethylcellulose derivative of Example 3, some of the hydrogen atoms of the hydroxyl group of ethylcellulose are substituted with propionyl groups. In the ethylcellulose derivative of Example 3, the degree of substitution of the propionyl group is 0.46. The median diameter of the produced ethylcellulose derivative of Example 3 is 80 μm.

[0038] The ethylcellulose derivative of Comparative Example 2 was produced by mixing pentyryl chloride and the reaction dispersion as additives in the mixing step S2 described above. That is, in the ethylcellulose derivative of Comparative Example 2, some of the hydrogen atoms of the hydroxyl group of ethylcellulose are substituted with pentyryl groups. Although the chemical structural formula is omitted, pentyryl chloride has one more carbon atom than butyryl chloride. In the ethylcellulose derivative of Comparative Example 2, the degree of substitution of the pentyryl group is 0.46. Furthermore, the ethylcellulose derivative of Comparative Example 2 that was produced gelled. That is, the ethylcellulose derivative of Comparative Example 2 does not take the form of particulate matter.

[0039] The ethylcellulose derivative of Comparative Example 3 was produced in the mixing step S2 described above by mixing butyryl chloride and the reaction dispersion as additives. In other words, in the ethylcellulose derivative of Comparative Example 3, some of the hydrogen atoms of the hydroxyl group of ethylcellulose are substituted with butyryl groups. In the ethylcellulose derivative of Comparative Example 3, the degree of substitution of the butyryl group is 0.46. Furthermore, the ethylcellulose derivative of Comparative Example 3 was produced in the dropping step S3 by continuously extruding it with a syringe instead of dropping it with a dropper. In other words, in the dropping step S3, the reaction dispersion containing the ethylcellulose derivative was continuously mixed with pure water without interruption. Therefore, the ethylcellulose derivative of Comparative Example 3 is in a thread-like shape rather than a particulate shape.

[0040] The ethylcellulose derivative of Comparative Example 4 was produced by mixing butyryl chloride and the reaction dispersion as additives in the mixing step S2 described above. In other words, in the ethylcellulose derivative of Comparative Example 4, some of the hydrogen atoms of the hydroxyl group of ethylcellulose are substituted with butyryl groups. In the ethylcellulose derivative of Comparative Example 4, the degree of substitution of the butyryl group is 0.49. Furthermore, the ethylcellulose derivative of Comparative Example 4 that was produced gelled. That is, the ethylcellulose derivative of Comparative Example 4 does not take the form of particulate matter.

[0041] The ethylcellulose derivative of Comparative Example 5 was produced by mixing butyryl chloride and the reaction dispersion as additives in the mixing step S2 described above. In other words, in the ethylcellulose derivative of Comparative Example 5, some of the hydrogen atoms of the hydroxyl group of ethylcellulose are substituted with butyryl groups. The degree of substitution of the butyryl group in the ethylcellulose derivative of Comparative Example 5 is 0.15. The median diameter of the produced ethylcellulose derivative of Comparative Example 5 was 85 μm.

[0042] The ethylcellulose derivative of Example 4 was produced by mixing butyryl chloride and the reaction dispersion as additives in the mixing step S2 described above. That is, in the ethylcellulose derivative of Example 6, some of the hydrogen atoms of the hydroxyl group of ethylcellulose are substituted with butyryl groups. In the ethylcellulose derivative of Example 4, the degree of substitution of the butyryl group is 0.18. The median diameter of the ethylcellulose derivative of Example 4 produced was 90 μm.

[0043] The results of the comparative sphericity tests for the ethylcellulose derivatives of Examples 1-4 and Comparative Examples 1-5 will be described below. Note that the ethylcellulose derivatives of Comparative Examples 1-4 did not take the shape of particles, so their sphericity was considered to be 0.0.

[0044] The sphericity of the ethyl cellulose derivative of Example 1 was 0.9. The sphericity of the ethyl cellulose derivative of Example 2 was 0.7. The sphericity of the ethyl cellulose derivative of Example 3 was 0.8. The sphericity of the ethyl cellulose derivative of Comparative Example 5 was 0.8. The sphericity of the ethyl cellulose derivative of Example 4 was 0.8.

[0045] In the "Judgment" column of "Sphericity" in Table 1, "G" indicates that the sphericity is 0.7 or more and 1.0 or less. And "B" indicates that the sphericity is less than 0.7. If the sphericity is 0.7 or more, the particles of the ethyl cellulose derivative are generally spherical and do not have extremely sharp parts. Therefore, the closer the sphericity of the ethyl cellulose derivative is to 1, for example, when the particles of the ethyl cellulose derivative collide with each other, it becomes less likely to break. Therefore, it is preferable that the sphericity of the ethyl cellulose derivative is 0.7 or more and 1.0 or less.

[0046] A comparative test of the elastic indexes of the ethyl cellulose derivatives of Examples 1 to 4 and Comparative Examples 1 to 5 will be described. The "elastic index" of the ethyl cellulose derivative referred to here is the value obtained by dividing the Young's modulus (MPa) of the ethyl cellulose derivative by the specific surface area (m ２ / g). The specific surface area (m ２ / g) was measured using krypton gas in accordance with JIS Z8830. Specifically, the specific surface area (m ２ / g) was measured using a high-performance type electric measurement stand (Imada Co., Ltd., EMX-1000N-L).

[0047] For the ethyl cellulose derivatives of Comparative Examples 2 and 4, the specific surface area (m ２ / g) does not become constant due to gelation. In addition, since the ethyl cellulose derivative of Comparative Example 3 has a filamentous shape, the Young's modulus (MPa) cannot be measured. Therefore, the ethyl cellulose derivatives of Comparative Examples 2 to 4 were excluded from the measurement of the elastic index.

[0048] The specific surface area (m ２The elastic index (MPa / g) of the ethylcellulose derivative of Example 1 was 10. On the other hand, the Young's modulus (MPa) of the ethylcellulose derivative of Example 1 was 173. Therefore, the elastic index (MPa·g / m) of the ethylcellulose derivative of Example 1 was 173. ２ The value was 17.3.

[0049] Specific surface area (m²) of the ethyl cellulose derivative of Comparative Example 1 ２ The elastic index (MPa / g) of the ethylcellulose derivative of Comparative Example 1 was 3. On the other hand, the Young's modulus (MPa) of the ethylcellulose derivative of Comparative Example 1 was 482. Therefore, the elastic index (MPa·g / m) of the ethylcellulose derivative of Comparative Example 1 was 3. ２ The value was 160.7.

[0050] Specific surface area (m²) of the ethyl cellulose derivative of Example 2 ２ The elastic index (MPa / g) of the ethylcellulose derivative of Example 2 was 6. On the other hand, the Young's modulus (MPa) of the ethylcellulose derivative of Example 2 was 210. Therefore, the elastic index (MPa·g / m) of the ethylcellulose derivative of Example 2 was 6. ２ The value was 35.0.

[0051] Specific surface area (m²) of the ethylcellulose derivative of Example 3 ２ The elastic index (MPa / g) of the ethylcellulose derivative of Example 3 was 7. On the other hand, the Young's modulus (MPa) of the ethylcellulose derivative of Example 3 was 197. Therefore, the elastic index (MPa·g / m) of the ethylcellulose derivative of Example 3 was 7. ２ The value was 28.1.

[0052] Specific surface area (m²) of the ethyl cellulose derivative of Comparative Example 5 ２ The elastic index (MPa / g) of the ethylcellulose derivative of Comparative Example 5 was 5. On the other hand, the Young's modulus (MPa) of the ethylcellulose derivative of Comparative Example 5 was 374. Therefore, the elastic index (MPa·g / m) of the ethylcellulose derivative of Comparative Example 5 was 5. ２ The score was 74.8.

[0053] Specific surface area (m²) of the ethylcellulose derivative of Example 4 ２ The elastic index (MPa / g) of the ethylcellulose derivative of Example 3 was 5. On the other hand, the Young's modulus (MPa) of the ethylcellulose derivative of Example 3 was 245. Therefore, the elastic index (MPa·g / m) of the ethylcellulose derivative of Example 3 was 5. ２ The score was 49.0.

[0054] In the "Equilibrium Index" column of Table 1, "G" indicates that the elasticity index is 50.0 or less. "B" indicates that the elasticity index is greater than 50.0. If the elasticity index is 50.0 or less, for example, when particles of ethylcellulose derivative collide with each other, they are less likely to break. In other words, it is preferable that the elasticity index of the ethylcellulose derivative is 50.0 or less.

[0055] In the "Overall Judgment" column of Table 1, samples where both the "Judgment" for "Sphericity" and the "Judgment" for "Elasticity Index" were "G" were marked as "G". All other samples were marked as "B". As a result, all of Examples 1 to 4 had an overall judgment of "G". In contrast, all of Comparative Examples 1 to 5 had an overall judgment of "B". From the above, it can be concluded that when the sum of the degree of substitution of one or more groups selected from acetyl groups, propionyl groups, and butyryl groups is between 0.18 and 0.46, an ethyl cellulose derivative with a sphericity of 0.7 to 1.0 and an elasticity index of 50.0 or less can be produced.

[0056] <Effects> (1) In the above embodiment, some of the hydrogen atoms of the hydroxyl groups of ethylcellulose are replaced with acetyl groups, propionyl groups, or butyryl groups. As a result, the hydroxyl groups that would normally be able to form hydrogen bonds are replaced with acetyl groups, propionyl groups, or butyryl groups. Due to the presence of acetyl groups, propionyl groups, or butyryl groups, the distance between the main skeletons of the ethylcellulose derivative becomes longer than the distance between the main skeletons of ethylcellulose. Consequently, the molecular density of the ethylcellulose derivative becomes smaller compared to ethylcellulose. As a result, the elastic index of the ethylcellulose derivative becomes smaller. And, as the elastic index becomes smaller, the ethylcellulose derivative becomes elastically deformable, so even if particles of the ethylcellulose derivative collide with each other, the particles are less likely to crack or chip.

[0057] (2) In the above formulation, the sum of the degree of substitution of the acetyl group, the degree of substitution of the propionyl group, and the degree of substitution of the butyryl group is 0.18 or more and 0.46 or less. If the degree of substitution of each functional group is within this range, the ethylcellulose derivative is likely to have an elastic index of 50.0 or less, even while taking a particulate shape.

[0058] (3) In the above embodiment, the sphericity of the ethylcellulose derivative is 0.7 to 1.0. When the sphericity of the ethylcellulose derivative is large in this way, it is less likely that a part of the particle will chip or break. In other words, the ethylcellulose derivative of the above embodiment can prevent cracking and chipping in terms of shape as well.

[0059] (4) In the above embodiment, the median diameter of the ethylcellulose derivative is 50 μm or more. This allows for the recovery of the ethylcellulose derivative during the filtration step in the manufacturing process. Furthermore, for example, the ethylcellulose derivative can be recovered by filtration after use as a desiccant. In other words, the above particle size is suitable for elimination by filtration or the like.

[0060] (5) By adopting the manufacturing method in the above embodiment, the target ethylcellulose derivative can be produced in relatively few steps. <Modification Example> This embodiment can be implemented with the following modifications. This embodiment and the following modifications can be combined with each other to the extent that they are not technically contradictory.

[0061] - The degree of ethoxylation of the ethylcellulose used as a raw material does not have to be 48 wt%. The elastic index of the ethylcellulose derivative produced is sufficient to be 50.0 or less. - The ethylcellulose derivative may have two or more groups selected from acetyl groups, propionyl groups, and butyryl groups. Preferably, the sum of the degrees of substitution of the acetyl group, propionyl group, and butyryl group in the ethylcellulose derivative is 0.18 or more and 0.46 or less.

[0062] The degree of substitution of the ethylcellulose derivative in this embodiment may be less than 0.18 or greater than 0.46. The ethylcellulose derivative of this application may take particulate form during the manufacturing process and have an elastic index of 50.0 or less.

[0063] The shape of the ethylcellulose derivative particles is not restricted. They may be polyhedra or solids consisting of multiple curved surfaces, or solids consisting of planes and curved surfaces. Furthermore, the sphericity of the ethylcellulose derivative does not need to be less than 0.7. Even if the sphericity of the ethylcellulose derivative is less than 0.7, if the elasticity index is 50.0 or less, the cracking or chipping of the ethylcellulose derivative particles can be suppressed.

[0064] The median diameter of the ethylcellulose derivative may be less than 50 μm. Even if the median diameter of the ethylcellulose derivative is small, it is still possible to recover the ethylcellulose derivative, for example, by centrifugation.

[0065] The manufacturing method is not limited to the example of this embodiment. For example, conditions such as the temperature and stirring time in the mixing step S2 may be changed as appropriate. The same applies to the solution preparation step S1 and the dropping step S3.

[0066] L1...First line segment L2...Second line segment P...Particle S1...Solution preparation process S2...Mixing process S3...Dripping process

Claims

1. Some of the hydroxyl groups of ethylcellulose are substituted with one or more selected from acetyl groups, propionyl groups, and butyryl groups, and the Young's modulus (MPa) is determined by the specific surface area (m²). ２ Ethyl cellulose derivatives whose value when divided by ( / g) is 50.0 or less.

2. The ethylcellulose derivative according to claim 1, wherein the degree of substitution of the acetyl group, propionyl group, or butyryl group substituted with the hydroxyl group of the ethylcellulose monomer is 0.18 or more and 0.46 or less.

3. The ethyl cellulose derivative according to claim 1 or claim 2, wherein, in any cross-section, the longest of the line segments connecting the outer circumference of the cross-section is designated as the first line segment, and the longest of the line segments connecting the outer circumference that are perpendicular to the first line segment is designated as the second line segment, and the ratio of the length of the second line segment to the length of the first line segment is 0.7 or more and 1.0 or less.

4. An ethylcellulose derivative according to any one of claims 1 to 3, wherein the median diameter is 50 μm or more.

5. Some of the carbon atoms at positions 2, 3, and 6 of ethylcellulose are bonded via oxygen atoms to one or more selected groups from acetyl, propionyl, and butyryl groups, and the Young's modulus (MPa) is defined by the specific surface area (m²). ２ Ethyl cellulose derivatives whose value when divided by ( / g) is 50.0 or less.

6. A method for producing an ethylcellulose derivative, comprising: a solution preparation step of preparing a reaction dispersion by adding the ethylcellulose to acetone and stirring; a mixing step of mixing one or more selected from acetyl chloride, propionyl chloride, and butyryl chloride into the reaction dispersion; and a dropping step of dropping the reaction dispersion after the mixing step into pure water.

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