JP2021046541A - Sulfonated fine cellulose fiber and method for producing the same - Google Patents

Abstract

To provide a sulfonated fine cellulose fiber having excellent viscosity, and a method for producing a sulfonated fine cellulose fiber capable of efficiently producing the sulfonated fine cellulose fiber.SOLUTION: In a fine cellulose fiber in which cellulose fibers are micronized, a part of hydroxyl groups are substituted with a sulfo group, a sulfur introduction amount due to the sulfo group is larger than 0.42 mmol/g, the fine cellulose fiber is a fiber composed of a plurality of unit fibers and has an average fiber width of 30 mm or less, and viscosity of a dispersion liquid at 25°C in a state where the fine cellulose fibers are dispersed in a water-soluble solvent so that solid content concentration is 0.5 mass% is 5,000 mPa s or more. Therefore, desired viscosity when the fine cellulose fibers are dispersed in the dispersion liquid can be exhibited.SELECTED DRAWING: Figure 1

Description

The present invention relates to sulfonated fine cellulose fibers and a method for producing the same.

Cellulose nanofibers (CNFs) are expected to exert superior thickening action (high viscosity, thixotropy) compared to cellulose fibers, and are therefore attracting attention as a new thickener with less impact on the environment. ing.
In general, it is considered necessary to increase the fiber length of CNF in order for CNF to exhibit high viscosity, and it is considered that CNF having high viscosity by preventing shortening of pulp during chemical treatment and defibration. The development of a technique for producing the above is proposed (for example, Patent Documents 1 and 2).

Patent Document 1 discloses a technique relating to phosphorylated CNF, and by setting the degree of polymerization to 400 or more, the viscosity of the slurry in which this phosphorylated CNF is dispersed is adjusted to 1000 to 20000 mPa · s. It is stated that it can be done.
However, the technique of Patent Document 1 has a problem that the production efficiency is very poor and it is not practical because it is necessary to go through a very complicated and a plurality of steps in order to prevent the fiber from being shortened in the chemical treatment step. There is.

Here, when only the viewpoint of improving the productivity of CNF is considered, the short fiber pulp may be treated with a high defibration pressure. However, the obtained CNF has a problem that the fiber length is short and it is difficult to produce a CNF having a desired viscosity. On the other hand, if the defibration treatment is performed slowly while suppressing the defibration pressure during defibration, the shortening of fibers during defibration can be suppressed to some extent, so that a CNF having a certain fiber length can be obtained. .. However, this manufacturing method causes a problem that the productivity drops sharply and it cannot be used as a practical manufacturing method.

Therefore, the technique of Patent Document 2 has been proposed as a technique for solving the above-mentioned problems.
Patent Document 2 discloses a technique for producing a sulfated CNF, and describes that by defibrating at a defibration pressure of 150 MPa, a sulfated CNF having a degree of polymerization of 350 or more can be produced while maintaining productivity. Has been done. It is described that the obtained sulfated CNF has a viscosity of 5400 to 8000 mPa · s.

International Publication No. 2017/170908 Japanese Unexamined Patent Publication No. 2019-11411

However, with the technique of Patent Document 2, although it is possible to practically produce a CNF having a certain degree of viscosity (up to 8,100 mPa · s), the high viscosity required as the current market needs (for example). CNF of 10,000 mPa · s or more) has not been obtained, and the defibration treatment must be performed with a high energy of 150 MPa or more. Further, Patent Document 2 does not disclose a description regarding biodegradability or a problem due to coloring.

In view of the above circumstances, it is an object of the present invention to provide a sulfonated fine cellulose fiber having characteristics such as excellent viscosity and a method for producing a sulfonated fine cellulose fiber capable of efficiently producing the sulfonated fine cellulose fiber. ..

(Sulfonized fine cellulose fiber)
The sulfonated fine cellulose fiber of the first invention is a fine cellulose fiber in which the cellulose fiber is finely divided, and a part of the hydroxyl groups of the fine cellulose fiber is substituted with a sulfo group, and the sulfo group is used. The resulting amount of sulfur introduced is adjusted to be higher than 0.42 mmol / g, and the fine cellulose fiber is a fiber composed of a plurality of unit fibers and having an average fiber width of 30 nm or less, and the fine cellulose fiber. It is characterized in that the viscosity of the dispersion liquid at 25 ° C. in a state where the fibers are dispersed in a water-soluble solvent so that the solid content concentration is 0.5% by mass is 5,000 mPa · s or more.
The sulfonated fine cellulose fiber of the second invention is the fine cellulose fiber of the first invention in a state where the fine cellulose fiber is dispersed in a water-soluble solvent so as to have a solid content concentration of 0.5% by mass. Using a B-type viscometer of the dispersion, measurements were taken at a temperature of 25 ° C., a rotation speed of 6 rpm and a rotation speed of 60 rpm, and each viscosity was calculated, and each viscosity ratio (viscosity at a rotation speed of 6 rpm / rotation speed of 60 rpm) was calculated. The thixotropy index calculated from (viscosity in) is 5 or more.
The sulfonated fine cellulose fiber of the third invention is the state in which the fine cellulose fiber is dispersed in a water-soluble solvent so as to have a solid content concentration of 0.5% by mass in the first invention. Using a B-type viscometer of the dispersion, measurements were taken at a temperature of 25 ° C., a rotation speed of 6 rpm and a rotation speed of 60 rpm, and each viscosity was calculated, and each viscosity ratio (viscosity at a rotation speed of 6 rpm / rotation speed of 60 rpm) was calculated. The thixotropy index calculated from (viscosity in) is lower than 5.0.
The sulfonated fine cellulose fiber of the fourth invention is characterized in that, in any one of the first invention, the second invention or the third invention, the unit fiber has an average degree of polymerization of 350 or less.
The sulfonated fine cellulose fiber of the fifth invention is the invention of any one of the first invention, the second invention, the third invention or the fourth invention, and the fine cellulose fiber has a solid content of the fine cellulose fiber in a water-soluble solvent. The haze value of the dispersion liquid in a state of being dispersed so as to have a concentration of 0.5% by mass is 10% or less.
The sulfonated fine cellulose fiber of the sixth invention is the invention of any one of the first invention, the second invention, the third invention, the fourth invention or the fifth invention, and the fine cellulose fiber makes the fine cellulose fiber water-soluble. It is characterized in that the total light transmittance of the dispersion liquid in a state of being dispersed in a solvent so that the solid content concentration is 0.5% by mass is 90% or more.
The sulfonated fine cellulose fiber of the seventh invention is the invention of any one of the first to sixth inventions, wherein the fine cellulose fiber has a solid content concentration of 0.5% by mass using the fine cellulose fiber as a water-soluble solvent. In the dispersion liquid in such a dispersed state, the L ^* a ^* b ^* color space calculated in accordance with JIS Z 8781-4: 2013 is among the following L ^* , a ^* and b ^* . It is characterized by satisfying at least one.
L ^* is 95% or more.
a ^* is -5 or more and +5 or less.
b ^* is -5 or more and +5 or less.
(Manufacturing method of sulfonated fine cellulose fiber)
The method for producing sulfonated fine cellulose fibers of the eighth invention is a method for producing sulfonated fine cellulose fibers according to any one of the first to seventh inventions from pulp, and the pulp is chemically treated. It is a method of sequentially performing a chemical treatment step and a micronization treatment step of refining the pulp after the chemical treatment step, and includes a polymerization degree lowering treatment step of depolymerizing the pulp before the chemical treatment step. It is characterized by that.
In the method for producing sulfonated fine cellulose fibers of the ninth invention, in the eighth invention, in the degree of polymerization reduction treatment step, the pulp is contacted or immersed in an acid solution or an alkaline solution to prepare a depolymerized pulp. It is characterized by that.
In the method for producing sulfonated fine cellulose fibers of the tenth invention, in the eighth invention or the ninth invention, in the micronization treatment step, the pulp after the chemical treatment step is supplied to the defibration apparatus, and the solution is 5 MPa to 60 MPa. It is characterized by being miniaturized by fiber pressure.
In the method for producing sulfonated fine cellulose fibers of the eleventh invention, in the tenth invention, the number of times of repeatedly defibrating the pulp after the chemical treatment step with a defibration apparatus in the micronization treatment step is 30 passes or less. It is characterized by that.
The method for producing a sulfonated fine cellulose fiber of the twelfth invention is the method for producing a sulfonated fine cellulose fiber according to any one of the eighth invention, the ninth invention, the tenth or the eleventh invention. On the other hand, it is characterized in that defibration energy of 1000 J or less is applied.

(Sulfonized fine cellulose fiber)
According to the first invention, when the fine cellulose fibers are dispersed in the dispersion liquid, a desired viscosity can be exhibited.
According to the second and third inventions, since the thixotropy index is a predetermined value, the degree of freedom in handleability can be improved.
According to the fourth invention, since the fine cellulose fibers are formed from a plurality of fibers having a low average degree of polymerization, excellent viscosity can be exhibited more appropriately. Moreover, since it can exhibit excellent biodegradability, it can be used as a material having a low environmental load.
According to the fifth, sixth and seventh inventions, the transparency of the dispersion liquid in which the fine cellulose fibers are dispersed in a water-soluble solvent can be improved.
(Manufacturing method of sulfonated fine cellulose fiber)
According to the eighth invention, the degree of polymerization of pulp fibers can be reduced before the chemical treatment by including the step of reducing the degree of polymerization. Therefore, since it is possible to prepare fine cellulose fibers whose dispersion form after miniaturization is liable to change, it is possible to provide sulfonated fine cellulose fibers with improved handleability.
According to the ninth aspect of the present invention, the degree of polymerization reduction treatment can be performed more appropriately, and the colored substances during the chemical treatment can be easily removed, so that highly transparent sulfonated fine cellulose fibers can be produced. ..
According to the tenth invention, since the miniaturization treatment can be appropriately performed even at a low defibration pressure, the operability of the miniaturization treatment step can be improved.
According to the eleventh invention, the viscosity of the sulfonated fine cellulose fiber can be controlled by adjusting the defibration treatment.
According to the twelfth invention, the desired sulfonated fine cellulose fiber can be obtained more appropriately by adjusting the defibration energy.

It is a schematic flow chart of the manufacturing method of the sulfonated fine cellulose fiber of this embodiment. It is a figure which showed the experimental result. It is the figure which showed the experimental result, and is the photograph of the sulfonated fine cellulose fiber with different viscosity. It is the schematic explanatory drawing of the sulfonated fine cellulose fiber of this embodiment. It is a figure which showed the experimental result of the biodegradability test. It is a figure which showed the experimental result, and is the figure which showed the relationship between the defibration strength, the viscosity and the TI value. It is a figure which showed the experimental result, and is the figure which showed the relationship between the defibration strength, the viscosity and the TI value of the comparative example. It is a figure which showed the experimental result, and is the figure which showed the relationship between the defibration energy and the haze value.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
The sulfonated fine cellulose fiber of the present embodiment (hereinafter, simply referred to as sulfonated fine cellulose fiber) is a fine cellulose fiber in which the cellulose fiber is finely divided, and is capable of exhibiting excellent viscosity characteristics. It has the characteristics of. Then, the method for producing the sulfonated fine cellulose fiber of the present embodiment (hereinafter referred to as the sulfonated fine cellulose fiber manufacturing method), which is the method for producing the sulfonated fine cellulose fiber, provides the sulfonated fine cellulose fiber exhibiting excellent viscosity. It is characterized by being able to be manufactured appropriately.
The "viscosity characteristic" in the present specification means a property that exhibits high viscosity and a desired TI value (low TI value, high TI value, etc.).

Hereinafter, embodiments of the sulfonated fine cellulose fiber will be described, and then embodiments of the sulfonated fine cellulose fiber manufacturing method will be described in order.

<Sulfonized fine cellulose fiber>
The sulfonated fine cellulose fiber is a fine cellulose fiber in which the cellulose fiber is refined. The sulfonated fine cellulose fibers contain a plurality of finer cellulose fibers (hereinafter referred to as unit fibers). Specifically, the sulfonated fine cellulose fiber is a fiber formed by connecting a plurality of unit fibers. This unit fiber is at least a part of the hydroxyl group (-OH group) of the cellulose (a chain polymer in which D-glucose is β (1 → 4) glycosidic bonded, sometimes simply a cellulose molecule) constituting the fiber. Is sulfonated with a sulfo group represented by the following formula (1). That is, in the sulfonated fine cellulose fiber, a part of the hydroxyl group of the fine cellulose fiber is replaced with a sulfo group.

(-SO3-) r · Zr + (1)
(Here, r is an independent natural number of 1 to 3, and Zr + is hydrogen ion, alkali metal cation, ammonium ion, aliphatic ammonium ion, aromatic ammonium ion, and cationic when r = 1. It is at least one selected from the group consisting of polymers. When r = 2 or 3, a cationic functional group such as an alkali earth metal cation, a polyvalent metal cation, or a diamine is incorporated into the molecule. It is at least one selected from the group consisting of compounds containing two or more of the compounds.)

The sulfonated fine cellulose fiber may have a functional group other than the sulfo group bonded to a part of the hydroxyl group of the fine cellulose fiber, and particularly contains a functional group (substituent) containing sulfur in addition to the sulfo group. You may.
In the following description, a case where only a sulfo group is introduced into the hydroxyl group of the cellulose fiber constituting the sulfonated fine cellulose fiber will be described as a representative.

(Introduction amount of sulfo group in sulfonated fine cellulose fiber)
The amount of sulfo group introduced in the sulfonated fine cellulose fiber can be expressed by the amount of sulfur caused by the sulfo group.
The amount of the sulfo group introduced is not particularly limited. For example, the amount of sulfur introduced due to the sulfo group per 1 g (mass) of the sulfonated fine cellulose fiber is preferably adjusted to be higher than 0.42 mmol / g, and more preferably 0.42 mmol / g to. It is 9.9 mmol / g, more preferably 0.5 mmol / g to 9.9 mmol / g, and even more preferably 0.6 mmol / g to 9.9 mmol / g.

Since the number of sulfur atoms in the sulfo group is 1, the amount of sulfur introduced: the amount of sulfo group introduced = 1: 1. For example, when the sulfur introduction per 1 g (mass) of the sulfonated fine cellulose fiber is 0.42 mmol / g, the amount of the sulfo group introduced is naturally 0.42 mmol / g.

When the amount of sulfur introduced due to the sulfo group per 1 g (mass) of the sulfonated fine cellulose fiber is 0.42 mmol / g or less, the hydrogen bond between the fibers is strong and the dispersibility tends to decrease. On the contrary, if the amount of sulfur introduced is higher than 0.42 mmol / g, the dispersibility can be easily improved, and if it is 0.5 mmol / g or more, the electronic resilience can be further strengthened. It becomes easier to maintain a stable and dispersed state. That is, in order to homogenize the viscosity of the dispersion liquid in which the sulfonated fine cellulose fibers described later are dispersed at a predetermined concentration, the amount of sulfur introduced is preferably higher than 0.42 mmol / g, more preferably 0.5 mmol. It is better to set it to / g or more. On the other hand, as the amount of sulfur introduced approaches 9.9 mmol / g, there is a concern that the crystallinity will decrease, and the cost of introducing sulfur tends to increase.
Therefore, the amount of the sulfo group introduced into the sulfonated fine cellulose fiber, that is, the amount of sulfur introduced due to the sulfo group is preferably adjusted to be higher than 0.42 mmol / g and 3.0 mmol / g or less. More preferably 0.5 mmol / g to 3.0 mmol / g, even more preferably 0.5 mmol / g to 2.0 mmol / g, even more preferably 0.5 mmol / g to 1.7 mmol / g. Yes, more preferably 0.5 mmol / g to 1.5 mmol / g.

From the viewpoint of transparency of the sulfonated fine cellulose fiber, it is preferable to adjust the amount of sulfur introduced due to the sulfo group so as to be in the same range as the above range.

(Measuring method of introduction amount of sulfo group)
The amount of sulfo group introduced into the sulfonated fine cellulose fiber can be evaluated by the amount of sulfur introduced due to the sulfo group, or by directly measuring the sulfo group.
For example, a predetermined amount of sulfonated fine cellulose fiber is burned, and the sulfur content in the combustible product is measured by a method compliant with IEC 62321 using a combustion ion chromatograph and calculated based on the obtained value. Alternatively, it may be calculated based on the value obtained by measuring the electrical conductivity while dropping an aqueous solution of sodium hydroxide after treating the sulfonated fine cellulose fiber with an ion exchange resin.

For example, the latter method will be described as the amount of sulfo groups introduced. Details can be calculated by measuring the electrical conductivity described in Examples described later.
First, a strongly acidic ion exchange resin (manufactured by Organo Corporation, Amber Jet 1024; conditioned) with a volume ratio of 1/10 is added to 0.2% by mass of sulfonated fine cellulose fibers, and shaken for 1 hour or more. (Treatment with ion exchange resin). Then, it is poured onto a mesh having an opening of about 90 μm to 200 μm to separate the resin and the slurry. In the subsequent titration using an alkali, the change in the value of electrical conductivity is measured while adding a 0.5 N aqueous solution of sodium hydroxide to the sulfonated fine cellulose fiber-containing slurry after the treatment with the ion exchange resin. The obtained measurement data can be plotted with electrical conductivity on the vertical axis and sodium hydroxide titration on the horizontal axis to obtain a curve, and the inflection point can be confirmed. The amount of sodium hydroxide droplets at this inflection point corresponds to the amount of sulfo groups, and by dividing the amount of sodium hydroxide at this inflection point by the amount of sulfonated fine cellulose fiber solids used for measurement, the amount of sulfo groups, that is, The amount of sulfur introduced due to the sulfo group can be determined.

When the sulfonated pulp fiber which has been chemically treated is finely treated to prepare the sulfonated fine cellulose fiber as described later, it may be obtained from the amount of sulfur introduced in the sulfonated pulp fiber before the fineness.

(Viscosity of sulfonated fine cellulose fiber)
The sulfonated fine cellulose fibers are prepared so that they can exhibit appropriate viscosity in a state of being dispersed in a dispersion liquid.
Specifically, the viscosity of the dispersion prepared so that the solid content concentration of the sulfonated fine cellulose fibers becomes a predetermined value (for example, 0.5% by mass) is adjusted to be 5,000 mPa · s or more. It may be adjusted so as to be preferably 10,000 mPa · s or more, more preferably 15,000 mPa · s or more, and further preferably 20,000 mPa · s or more.

(Viscosity measurement method)
The viscosity can be measured using, for example, the B-type viscometer described in Examples described later.
For example, if a dispersion prepared so that the solid content concentration of the sulfonated fine cellulose fibers is 0.5% by mass is measured using a B-type viscometer at a rotation speed of 6 rpm, 25 ° C., and 3 minutes, sulfone is obtained. The viscosity of the chemical fine cellulose fiber can be measured.
The solvent used in this dispersion may be a water-soluble solvent (water-soluble solvent). For example, in addition to water alone, alcohols, ketones, amines, carboxylic acids, ethers, amides, etc., and mixtures thereof, etc. Can be adopted.

(Average fiber width of sulfonated fine cellulose fibers)
The sulfonated fine cellulose fiber is a fine cellulose fiber in which the cellulose fiber is finely divided as described above, and the fiber is very fine. Specifically, the average fiber width of the sulfonated fine cellulose fibers is adjusted to be 1 nm to 30 nm when observed with a device capable of observing on a nanoscale such as an electron microscope or an atomic force microscope. It is preferably adjusted to 2 nm to 30 nm.
When the average fiber width of the sulfonated fine cellulose fibers is larger than 30 nm, the aspect ratio tends to decrease, and as a result, the entanglement between the fibers may decrease and the viscosity may decrease.
Therefore, the average fiber width of the sulfonated fine cellulose fibers is preferably 2 nm to 30 nm, more preferably 2 nm to 20 nm, and further preferably 2 nm to 10 nm in order to improve the viscosity.

Further, when the average fiber width becomes larger than 30 nm, it approaches 1/10 of the wavelength of visible light, and when it is combined with a matrix material, visible light is likely to be refracted and scattered at the interface, and visible light is scattered. Therefore, the transparency, which will be described later, tends to decrease. Therefore, from the viewpoint of transparency described later, it is preferable that the average fiber width of the sulfonated fine cellulose fibers is 20 nm or less, and more preferably 10 nm or less. In particular, if the average fiber width is adjusted to 10 nm or less, the scattering of visible light can be further reduced, so that sulfonated fine cellulose fibers having high transparency can be obtained.

(Measuring method of average fiber width)
The average fiber width of the sulfonated fine cellulose fibers can be measured using a known technique.
For example, it can be measured by a method using a scanning probe microscope described in Examples described later. Specifically, the sulfonated fine cellulose fibers are dispersed in a solvent such as pure water, and the mixed solution is adjusted so as to have a predetermined mass%. Then, this mixed solution is spin-coated on a silica substrate coated with PEI (polyethyleneimine), and sulfonated fine cellulose fibers on the silica substrate are observed. As an observation method, for example, a scanning probe microscope (for example, manufactured by Shimadzu Corporation; SPM-9700) can be used. Twenty sulfonated fine cellulose fibers in the obtained observation image are randomly selected, and the average fiber width of the sulfonated fine cellulose fibers can be obtained by measuring and averaging each fiber width.

As described above, since the sulfonated fine cellulose fibers have the above-mentioned structure, excellent viscosity can be exhibited in a state of being dispersed in the dispersion liquid.

(Fiber length of sulfonated fine cellulose fiber and average degree of polymerization of unit fiber)
The sulfonated fine cellulose fiber is a fiber in which a plurality of unit fibers are connected as described above. The number of connected unit fibers is not particularly limited as long as they are connected so as to have a length having the viscosity as described above.
On the other hand, the average fiber length of the unit fibers constituting the sulfonated fine cellulose fibers, that is, the average degree of polymerization of the unit fibers is adjusted to be close to the level-off degree of polymerization of cellulose described later. More specifically, the unit fibers are preferably prepared so that the average degree of polymerization is 100 to 350, more preferably 200 to 350, and even more preferably 250 to 350. There is.
The average fiber length of the unit fiber can be calculated from the average degree of polymerization, and when the axial length of one glucose molecule is about 5 Å (about 0.5 nm), the degree of polymerization of the sulfonated fine cellulose fiber is 350. In the case of, theoretically, a sulfonated fine cellulose fiber having a fiber length of about 175 nm (about 0.175 μm) can be prepared. In other words, when the average degree of polymerization of the unit fibers is 350, the sulfonated fine cellulose fibers naturally have an average fiber length of 175 nm or more, which is the average fiber length of the unit fibers.

When the degree of polymerization of the unit fiber is within the above-mentioned predetermined range, it can be easily decomposed by microorganisms and the like, so that the biodegradability can be improved. Specifically, when a microorganism or the like is brought into contact with the sulfonated fine cellulose fiber, a cellulolytic enzyme having endoglucanase activity or exoglucanase activity that cleaves the cellulose molecular chain of the unit fiber constituting the sulfonated fine cellulose fiber. The decomposition efficiency of (cellulase) is improved. In particular, it is possible to increase the points at which a cellulose-degrading enzyme having an exoglucanase activity that cleaves a cellulose molecular chain from the terminal side of the cellulose molecular chain can attack. Therefore, the enzymatic decomposition can be promoted more efficiently by microorganisms and the like, so that the biodegradability of the sulfonated fine cellulose fibers can be improved.

As an index of biodegradability, for example, the above-mentioned cellulolytic enzyme (cellulase) can be used. Examples of this evaluation method include the decomposition measurement method by enzyme treatment described in Examples described later. Further, the method is not limited to this method, and examples thereof include a method of measuring a biochemical oxygen demand (BOD).
As an evaluation method using a cellulolytic enzyme (cellulase), for example, after suspending sulfonated fine cellulose fibers in an acetate buffer (pH 5.0), a commercially available cellulase (for example, cellulase manufactured by Nacalai Tesque) is used. ) Is added and decomposed for a predetermined period. Then, it can be evaluated by measuring the reduction rate of the weight of the fiber recovered by filtering the decomposition product. Further, the decomposition products such as glucose contained in the filtrate when filtered may be evaluated by quantification by liquid chromatography such as high performance liquid chromatography (HPLC).

Therefore, by setting the degree of polymerization of the unit fibers within the above-mentioned predetermined range, it is possible to appropriately exhibit the biodegradability while making the fine cellulose fibers function as a thickener, so that the fine cellulose fibers can be handled easily. It will be possible to further improve the degree of freedom of.
For example, by making it contained in cosmetics, paint products, foods, etc. as a thickener, it is possible to improve the viscosity after use and improve the biodegradability after use, so that the environmental load can be reduced. Can be improved.

The biodegradability in the present specification means that soluble monosaccharides can be detected by enzymatic decomposition.

Here, when improving the viscosity of the fiber, it is generally adjusted so that the fiber length becomes long. In other words, conventional techniques aim to increase the degree of polymerization of a single fiber as much as possible in order to improve its viscosity. On the other hand, the present inventors have found for the first time that the viscosity of sulfonated fine cellulose fibers can be improved by connecting a plurality of fine fibers (unit fibers) having a short fiber length. That is, the present inventors have found the present invention based on an idea completely opposite to the idea of increasing the length of one fiber in order to improve the viscosity of a general fiber. In other words, the present inventors have found for the first time that the viscosity of the dispersion liquid in which the sulfonated fine cellulose fibers are dispersed depends not on the degree of polymerization but on the fiber length in which a large number of unit fibers are connected. Based on this finding, the present invention has been completed.
Moreover, the present inventors are the first to be able to control, that is, control the viscosity of the sulfonated fine cellulose fibers by adjusting the amount of connected fine fibers (unit fibers) constituting the sulfonated fine cellulose fibers. It was found, and the present invention was completed based on such findings.
Furthermore, the present inventors have first seen that by preparing unit fibers of sulfonated fine cellulose fibers near the level-off degree of polymerization, different thixotropic properties are exhibited even though the viscosities of the dispersions are substantially the same. It was issued, and the present invention was completed based on such findings. That is, the present inventors have found for the first time that by preparing unit fibers of sulfonated fine cellulose fibers near the level-off degree of polymerization, predetermined viscosity characteristics (high viscosity, predetermined thixotropy) are exhibited. Based on such findings, the present invention has been completed (see FIGS. 6 (A) and 6 (B)).
Further, by preparing the unit fiber of the sulfonated fine cellulose fiber near the level-off degree of polymerization, high transparency is exhibited even if the energy required for the defibration treatment of the sulfonated pulp fiber is lowered, and a predetermined viscosity is exhibited. For the first time, the present inventors have found that the productivity of sulfonated fine cellulose fibers having characteristics can be maintained, and based on such findings, the production method of the present invention has been completed (see FIG. 8).

(Method of measuring the average degree of polymerization of unit fibers)
The average degree of polymerization of the unit fibers constituting the sulfonated fine cellulose fibers can be measured by the following known measuring methods (for example, the copper ethylenediamine method). For example, the measurement can be performed by a method based on JIS P 8215 described later or a method referring to JIS P 8215 described in Examples.
Further, for example, in the case of measurement by the copper ethylenediamine method, the sulfonated fine cellulose fiber is dissolved in a 0.5 M copper ethylenediamine solution. Then, if the viscosity of this solution is measured by the viscosity method, the average degree of polymerization of the unit fibers in a state where the unit fibers constituting the sulfonated fine cellulose fibers are disconnected can be obtained.

The above-mentioned level-off degree of polymerization means that when cellulose is hydrolyzed, a region other than the crystal in which an acid can permeate, a so-called amorphous region, is selectively depolymerized, and the degree of polymerization is lowered. At this time, it is known that the degree of polymerization rapidly decreases at the initial stage of decomposition, and when a certain degree of polymerization is reached, the decrease becomes gradual (INDUSTRIAL AND ENGINEERING CHEMISTRY, Vol. 42, No. 3, p. 502). -507 (1950)). This constant average degree of polymerization is called the level-off degree of polymerization, and it is said that the degree of polymerization does not fall below the level-off degree of polymerization even if the hydrolysis time is extended.
For example, if NBKP (coniferous kraft pulp) is hydrolyzed under 2.5N hydrochloric acid, boiling temperature, and 15 minutes, and then measured by the viscosity method (copper ethylenediamine method), the level-off degree of polymerization of wood-based cellulose fibers can be determined. Can be sought. The level-off degree of polymerization is determined when the degree of polymerization is about 200 to 300 under the above conditions.

(Thixotropy index (TI value) of sulfonated fine cellulose fiber)
Further, the sulfonated fine cellulose fiber is prepared so as to exhibit the viscosity as described above and also to exhibit an appropriate thixotropy index (TI value) in a state of being dispersed in the dispersion liquid. Is preferable. That is, it is preferable that the sulfonated fine cellulose fibers are prepared so as to exhibit predetermined viscosity characteristics (high viscosity, predetermined TI value) in a state of being dispersed in the dispersion liquid.

The TI value is evaluated by the viscosity generated when an external stress is applied to the dispersion liquid, and a high TI value means that the dispersion liquid flows when a predetermined force (energy) is applied. It means that it has the property of easily demonstrating its properties. Further, a low TI value means that the dispersion liquid has a property that it is difficult to exhibit fluidity when a predetermined force (energy) is applied to the dispersion liquid.
Based on this property, sulfonated fine cellulose fibers having the same viscosity, that is, sulfonated fine cellulose fibers having the same fiber length (in other words, unit fibers having the same degree of polymerization are linked (for example, linearly). In the sulfonated fine cellulose fiber) (linked), the following can be inferred from the viewpoint of the fiber state.
In the stationary state of the dispersion liquid, the sulfonated fine cellulose fibers are in an entangled state. This entanglement develops a high viscosity.
A predetermined force is applied to this dispersion. At this time, it is presumed that the sulfonated fine cellulose fiber having a property of having a low TI value has a structure in which a decrease in viscosity is unlikely to occur even when a predetermined energy is applied. That is, it can be said that the sulfonated fine cellulose fibers exhibiting high viscosity and low TI value have a structure in which the entanglement of the fibers is hard to be broken (the entanglement of the fibers is hard to be unraveled). On the other hand, it is presumed that the sulfonated fine cellulose fiber having a property of having a high TI value has a structure in which a decrease in viscosity is likely to occur when a predetermined energy is applied. That is, it can be said that the sulfonated fine cellulose fibers exhibiting high viscosity and high TI value have a structure in which the entanglement of the fibers is easily broken (the entanglement of the fibers is easily disentangled).
That is, it is presumed that the TI value can be used as one of the structural indexes of the sulfonated fine cellulose fiber exhibiting substantially the same viscosity.

For example, sulfonated fine cellulose fibers (those with a low TI value) whose entanglement between fibers does not easily break even when a predetermined force is applied to the dispersion, that is, the fluidity of the dispersion is small even when a predetermined force is applied. Contains a TI value of 1.0 to 4.0 (4.0) in a dispersion prepared so that the solid content concentration of the sulfonated fine cellulose fibers becomes a predetermined value (for example, 0.5% by mass). It is preferably 1.0 to 3.5 or less, and more preferably 1.0 to 3.0 or less. Those having a TI value in this range can be said to be sulfonated fine cellulose fibers having a structure in which the entangled state is strongly maintained.
Further, for example, when a predetermined force is applied to the dispersion liquid, the entanglement of the fibers is easily broken, that is, the fluidity of the dispersion liquid is larger than that of the sulfonated fine cellulose fiber exhibiting the above-mentioned low TI value. The sulfonated fine cellulose fiber (those having a high TI value) has a TI value of 4 in a dispersion liquid adjusted so that the solid content concentration of the sulfonated fine cellulose fiber becomes a predetermined value (for example, 0.5% by mass). It is 0.0 or more, preferably 5.0 or more, and more preferably 7.0 or more. Those having a TI value in this range can be said to be sulfonated fine cellulose fibers having a structure in which the entangled state is easily unwound.

By adjusting the TI value of the sulfonated fine cellulose fiber to be equal to or higher than the above value, the fluidity of the dispersion liquid can be improved when an external stress is applied to the dispersion liquid. Therefore, the dispersibility of the sulfonated fine cellulose fibers in the dispersion liquid and the dilution of the dispersion liquid become easier, so that the handleability can be improved.

(Measuring method of thixotropy index (TI value))
The thixotropy index (TI value) can be measured using, for example, the B-type viscometer described in Examples described later. As a general rule, a dispersion prepared so that the solid content concentration of the sulfonated fine cellulose fibers is 0.5% by mass is measured using a B-type viscometer at a temperature of 25 ° C., a rotation speed of 6 rpm, and a rotation speed of 60 rpm. , Each viscosity is calculated, and can be calculated from each viscosity ratio (viscosity at 6 rpm / viscosity at 60 rpm).

(About the transparency of sulfonated fine cellulose fibers)
The sulfonated fine cellulose fiber can exhibit excellent transparency by adjusting the average fiber width so as to have the above-mentioned constitution. The above-mentioned transparency when the sulfonated fine cellulose fibers are dispersed in the dispersion liquid can be evaluated as follows.

The transparency in the present specification includes the properties of both transparency and / or turbidity of the liquid. Specifically, in the evaluation of transparency, the haze value can more appropriately evaluate the turbidity of the liquid, and the total light transmittance can more appropriately evaluate the transparency of the liquid. Hereinafter, each evaluation method will be specifically described.

(Haze value)
The haze value of the dispersion liquid in which the sulfonated fine cellulose fibers are dispersed to a predetermined concentration can be measured as follows.
First, a dispersion liquid in which sulfonated fine cellulose fibers are dispersed in a water-soluble solvent so that the solid content concentration is 0.1% by mass to 20% by mass is prepared. When the haze value of the dispersion is 20% or less, it can be said that the sulfonated fine cellulose fibers can exhibit transparency with less turbidity. Conversely, when the haze value of the dispersion prepared so that the solid content concentration is within the above range is higher than 20%, it can be said that it is difficult to exhibit appropriate transparency.

For example, when the solid content concentration of the dispersion liquid in which the sulfonated fine cellulose fibers are dispersed is adjusted to 0.2% by mass to 0.5% by mass, the haze value of the dispersion liquid is 20% or less. It can be said that it is in a state where it can properly exert transparency with less turbidity. In particular, in order to appropriately suppress the turbidity of the dispersion liquid when the sulfonated fine cellulose fibers are dispersed at a predetermined concentration, the haze value is more preferably 15% or less, still more preferably 10% or less. It is better to prepare.

Therefore, in order to exhibit a transparent state with less turbidity, the haze value of the dispersion prepared so that the solid content concentration of the sulfonated fine cellulose fiber is 0.1% by mass to 20% by mass is 20% or less. It is preferable that the haze value of the dispersion liquid is 15% or less, more preferably 15% by mass or less, when the solid content concentration is adjusted to 0.2% by mass to 0.5% by mass. It is better to prepare so that it is 10% or less.

(Measurement method of haze value)
The haze value can be measured, for example, by using a spectroscopic haze meter or a spectrophotometer based on JIS K 7105 described in Examples described later. As a general rule, the sulfonated fine cellulose fibers are dispersed in the above-mentioned dispersion liquid so as to have a predetermined solid content concentration. Then, if this dispersion is measured using a spectroscopic haze meter or a spectrophotometer conforming to JIS K 7105, the haze value of the sulfonated fine cellulose fibers can be obtained.

(Total light transmittance)
Further, the total light transmittance of the dispersion liquid in which the sulfonated fine cellulose fibers are dispersed so as to have a predetermined concentration can be measured as follows.
First, a dispersion liquid in which sulfonated fine cellulose fibers are dispersed in a water-soluble solvent so that the solid content concentration is 0.1% by mass to 20% by mass is prepared. When the total light transmittance of the dispersion is 90% or more, it can be said that the sulfonated fine cellulose fibers can exhibit clear transparency. Conversely, when the total light transmittance of the dispersion prepared so that the solid content concentration is within the above range is lower than 90%, it can be said that it is difficult to exhibit appropriate transparency.

For example, when the solid content concentration of the dispersion liquid in which the sulfonated fine cellulose fibers are dispersed is adjusted to 0.2% by mass to 0.5% by mass, the total light transmittance of this dispersion liquid is 90% or more. If there is, it can be said that the state is in a state where clear transparency can be properly exhibited. In particular, in order to appropriately exhibit the transparency of the dispersion liquid when the sulfonated fine cellulose fibers are dispersed at a predetermined concentration, it is preferable to adjust the total light transmittance to 95% or more.

Therefore, in order to exhibit clear transparency, the sulfonated fine cellulose fiber has a total light transmittance of 90% or more in the dispersion prepared so that the solid content concentration is 0.1% by mass to 20% by mass. It is preferable that the total light transmittance of the dispersion liquid is 90% or more when the solid content concentration is adjusted to 0.2% by mass to 0.5% by mass, more preferably 90% or more. It is preferable to adjust the content to 95% or more.

(Measurement method of total light transmittance)
The total light transmittance can be measured, for example, by using a spectrophotometer or a spectrophotometer based on JIS K 7105 described in Examples described later. As a general rule, first, the sulfonated fine cellulose fibers are dispersed in the above-mentioned dispersion liquid so as to have a predetermined solid content concentration. Then, if this dispersion is measured using a spectrophotometer or a spectrophotometer in accordance with JIS K 7105, the total light transmittance of the sulfonated fine cellulose fibers can be determined.

The solvent of the dispersion liquid used for measuring the haze value and the total light transmittance is not particularly limited as long as it is a water-soluble solvent (water-soluble solvent) as in the case of the above-mentioned water-based solvent for viscosity measurement. For example, as the water-soluble solvent, not only water alone, but also alcohol, ketone, amine, carboxylic acid, ether, amide and the like may be used alone or in combination of two or more.

(About the color of sulfonated fine cellulose fibers)
^{The color of the sulfonated fine cellulose fibers is determined by, for example, L *} , a in which a dispersion liquid in which the sulfonated fine cellulose fibers are dispersed at a predetermined concentration is defined by JIS Z 8781-4 as described in Examples described later. It can be evaluated by measuring ^* and b ^*.
Specifically, first, a dispersion liquid in which sulfonated fine cellulose fibers are dispersed in a water-soluble solvent so that the solid content concentration is 0.1% by mass to 20% by mass is prepared. Then, the L ^* a ^* b ^* color space of the prepared dispersion is measured.
If the measured L ^* a ^* b ^* color space satisfies at least one of the following numerical ranges, it can be said that a clearer transparency without color is exhibited.
L ^* is 95% or more a ^* is -5 or more +5 or less b ^* is -5 or more +5 or less In particular, a ^* is -2 or more +2 or less and b ^* is -2 or more +2 or less. If so, it can be said that it is in a state where it can exhibit clearer transparency.

In addition, in this specification, transparency includes the property of both transparency and / or turbidity of a liquid. In the evaluation of transparency, the haze value can more appropriately evaluate the turbidity of the liquid, and the total light transmittance can more appropriately evaluate the transparency. Then, the clearness in transparency, that is, the tint, can be appropriately evaluated by the ^{L *} a ^* b ^{* color space.}

<Manufacturing method of sulfonated fine cellulose fiber>

The method for producing sulfonated fine cellulose fibers is characterized by depolymerizing pulp as described above.

Here, since the cellulose fibers composed of pulp are strongly bonded to each other by hydrogen bonds, in order to obtain fine cellulose fibers, in general, in the micronization process, a large amount is required at a high defibration pressure. Since it is necessary to perform the number of times of defibration, a large amount of energy and labor are required.
On the other hand, in the obtained fine cellulose fibers, the fiber length is generally shortened by performing a large number of defibration times at a high defibration pressure. It is considered that when the fiber length is short, that is, the degree of polymerization of the fine cellulose fibers is low, the fibers are severely damaged and the viscosity of the obtained fine cellulose fibers is also lowered.
Therefore, in the conventional technique, when trying to improve the viscosity of the fine cellulose fiber, a manufacturing method has been proposed in which a low defibration pressure and a larger number of defibration are performed with a small amount of processing in the miniaturization step. There is.
However, such a conventional manufacturing method cannot be said to be a practical manufacturing method because the amount of fine cellulose fibers obtained is extremely small and the treatment efficiency in the miniaturization treatment step is extremely low.
In other words, in the conventional technology, there is a trade-off relationship between preparing highly viscous fibers and improving productivity, and a technique for efficiently and appropriately producing highly viscous fibers has not been developed. That is the reality.
On the other hand, as described above, the present inventors have an idea completely opposite to the conventional idea, that is, in a state where the pulp before being subjected to the micronization treatment step is depolymerized to intentionally lower the degree of polymerization of the fiber. For the first time, we have found that sulfonated fine cellulose fibers that exhibit excellent viscosity, which is not expected by conventional techniques, can be easily and efficiently produced with less energy by being subjected to the micronization treatment process (see Tables 2 and 2). ). Moreover, it was also found for the first time that the prepared sulfonated fine cellulose fiber can be obtained in a state where the damage of the fiber is suppressed. That is, the method for producing the sulfonated fine cellulose fiber of the present invention has been reached based on a finding different from the conventional idea that the fiber length of the fine cellulose fiber depends on the degree of polymerization.
Then, in order to solve the above-mentioned problems, CNF (sulfonated fine cellulose fiber) that exhibits high viscosity with low energy, which has not been expected in the past, is produced by performing a treatment on pulp to reduce the degree of polymerization. For the first time, we have found that it can be prepared without reducing productivity. The treatment for lowering the degree of polymerization of pulp is a treatment for lowering the degree of polymerization, which will be described later.
Moreover, the present inventors have also found for the first time that sulfonated fine cellulose fibers having different structures can be prepared by adjusting the conditions for the defibration treatment by subjecting the pulp to a degree of polymerization reduction treatment described later. It is a thing.

As shown in FIG. 1, the outline of the method for producing the sulfonated fine cellulose fiber is described in that the fiber raw material containing cellulose (for example, pulp) is subjected to the polymerization degree lowering treatment step S1 for depolymerizing, and then the pulp is subjected to the chemical treatment step S2. This is a method of producing sulfonated fine cellulose fibers by subjecting it to a micronization treatment step S3 for refining pulp containing chemically treated sulfonated pulp fibers. Hereinafter, each step will be specifically described.

In the present specification, the fiber raw material refers to a fibrous raw material such as pulp containing a cellulose molecule, and the pulp refers to an aggregate of a plurality of pulp fibers. Further, the pulp fiber is an aggregate of a plurality of cellulose fibers, and the cellulose fiber is an aggregate of a plurality of fine fibers (for example, microfibrils). The fine fibers are a collection of a plurality of cellulose molecules (hereinafter, also simply referred to as cellulose) which are chain polymers in which D-glucose is β (1 → 4) glycosidic bonded. The details of the fiber raw material will be described later.

(Polymerization Degree Reduction Treatment Step S1)
The degree of polymerization reduction treatment step S1 is a step of depolymerizing the fiber raw material containing cellulose. Specifically, the degree of polymerization lowering treatment step S1 includes a contact step of bringing the degree of polymerization lowering agent into contact with a fiber raw material such as pulp, and a reaction step of lowering the degree of polymerization of the fiber raw material after the contact step. It is a process.

Depolymerization refers to a phenomenon in which the degree of polymerization of pulp is lowered or the molecular weight of pulp is lowered.

(Contact step in step S1 for reducing the degree of polymerization)
The contact step in the polymerization degree reducing treatment step S1 is a step of bringing the polymerization degree reducing agent into contact with the fiber raw material. This contact step is not particularly limited as long as it is a method capable of bringing the polymerization degree reducing agent into contact with the fiber raw material. For example, the fiber raw material (wood pulp or the like) may be immersed in a treatment liquid containing a polymerization degree lowering agent to lower the degree of polymerization to impregnate the polymerization degree lowering agent, or the treatment liquid may be applied to the fiber raw material. You may.
Among these methods, if the method of immersing the fiber raw material in the above-mentioned treatment liquid and impregnating the fiber raw material with the treatment liquid containing the polymerization degree reducing agent is adopted, the polymerization degree reducing treatment chemical is uniformly contacted with the fiber raw material. It is preferable because it can be polymerized.

(Treatment solution in step S1 for reducing the degree of polymerization)
The treatment liquid in the degree of polymerization lowering treatment S1 is not particularly limited as long as it contains a degree of polymerization lowering agent capable of lowering the degree of polymerization.
Examples of the degree-reducing agent include chemicals such as sodium hydroxide, potassium hydroxide and sodium hydrogencarbonate which show alkalinity when made into an aqueous solution, and chemicals such as hydrochloric acid and nitric acid which show acidity, and are prepared. Depending on the use of the sulfonated fine cellulose fiber, it may be used properly.
In particular, if an alkaline chemical is used as the degree of polymerization lowering agent, the hemicellulose component contained in these fiber raw materials can be easily removed when NBKP or LBKP described later is used as the fiber raw material. Therefore, it is preferable because it makes it easier to prepare sulfonated fine cellulose fibers that are less colored, have a low color such as yellow, and have high transparency.

(Contact amount of treatment liquid)
The contact amount of the treatment liquid is not particularly limited as long as it can be contacted to such an extent that the degree of polymerization can be lowered.
For example, in the case of a treatment liquid in which a degree of polymerization reducing agent is dissolved in water and the concentration thereof is 1 M, the dry weight of pulp is 1 part by mass to 500 parts by mass with respect to 100 parts by mass of the treatment liquid. Can be prepared.

(Reaction step in the polymerization degree reduction treatment step S1)
The reaction step in the degree of polymerization reduction treatment step S1 is a step of depolymerizing the fiber raw material in contact with the treatment liquid.
This step is not particularly limited as long as the fiber raw material can be depolymerized by the treatment liquid. For example, the fiber raw material in contact with the treatment liquid may be left to stand under normal temperature and pressure conditions as it is to be depolymerized, or may be heated at a predetermined temperature to be depolymerized. The former method is desirable from the viewpoint of preventing excessive depolymerization, and the latter method is preferable in increasing the efficiency of the degree of polymerization reduction treatment.

(Washing step after polymerization degree reduction treatment step S1)
A cleaning step of cleaning the fiber raw material after the polymerization degree reduction treatment step S1 may be included.
When an alkaline chemical is used as the polymerization degree reducing agent of the treatment liquid in the polymerization degree reducing treatment step S1, if an excessively applied alkaline chemical remains, the sulfonate used in the chemical treatment step S2 described later contains the alkaline chemical. It is wasted to reconcile and the chemical treatment step S2 may not be performed properly. Therefore, when an alkaline chemical is used as the degree of polymerization reducing agent, it is desirable to supply it to the chemical treatment step S2 after cleaning. In this case, the efficiency of contacting the sulfonate with the fiber raw material supplied to the chemical treatment step S2 can be improved, so that the treatment can proceed appropriately.
The method of this washing step is not particularly limited as long as the fiber raw material after the degree of polymerization reduction treatment step S1 can be washed so as to be substantially neutral. For example, a method of washing with pure water until the fiber raw material after the polymerization degree reduction treatment step S1 becomes substantially neutral can be adopted.

(Chemical treatment step S2)
The chemical treatment step S2 includes a contact step of bringing urea into contact with a sulfonate having a sulfo group on the cellulose fiber of the fiber raw material containing cellulose such as depolymerized pulp, and the cellulose fiber of the pulp after this contact step. It includes a reaction step of introducing a sulfo group into at least a part of the hydroxyl group.

(Contact step in chemical treatment step S2)
The contact step in the chemical treatment step S2 is a step of bringing the sulfonate and urea into contact with the fiber raw material containing cellulose. This contact step is not particularly limited as long as it is a method capable of causing the contact.
For example, a fiber raw material (depolymerized pulp, the same applies hereinafter) may be immersed in a reaction solution in which a sulfonate agent and urea coexist to impregnate the reaction solution, or the reaction solution may be applied to the fiber raw material. It may be applied, or the fiber raw material may be separately applied or impregnated with the sulfonate and urea.
Among these methods, if the method of immersing the fiber raw material in the reaction solution and impregnating the fiber raw material with the reaction solution is adopted, the sulfonate and urea can be uniformly brought into contact with the fiber raw material, which is preferable. ..

(Mixing ratio of reaction solution)
When the method of immersing the fiber raw material in the reaction solution and impregnating the fiber raw material with the reaction solution is adopted, the mixing ratio of the sulfonate and urea contained in the reaction solution is not particularly limited.
For example, the sulfonate and urea are in a concentration ratio (g / L) of 4: 1 (1: 0.25), 2: 1 (1: 0.5), 1: 1, 1: 2.5. Can be adjusted to be.

When the sulfonate and urea were dissolved in water and the respective concentrations were adjusted to 200 g / L and 100 g / L, the sulfonate: urea was 2: in the concentration ratio (g / L). The reaction solution of 1 can be prepared. In other words, if urea is prepared so as to be 50 parts by weight with respect to 100 parts by weight of sulfamic acid, a reaction solution having a sulfonate: urea of 2: 1 can be prepared.
Further, if the sulfonate and urea are dissolved in water and the respective concentrations are adjusted to 200 g / L and 500 g / L, the sulfonate: urea is 1: in the concentration ratio (g / L). A reaction solution of 2.5 can be prepared.

(Contact amount of reaction solution)
The amount of the reaction solution to be brought into contact with the fiber raw material is also not particularly limited.
For example, in a state where the reaction solution and the fiber raw material are in contact with each other, the sulfone agent contained in the reaction solution is 1 part by weight to 20,000 parts by weight with respect to 100 parts by weight of the dry weight of the fiber raw material. The urea and / or its derivative contained in the above can be prepared so as to be 1 part by weight to 100,000 parts by weight with respect to 100 parts by weight of the dry weight of the fiber raw material.

(Reaction step in chemical treatment step S2)
In the reaction step in the chemical treatment step S2, as described above, the sulfo group of the sulfonizing agent brought into contact with the hydroxyl group of the cellulose fiber contained in the fiber raw material is replaced, and the sulfo group is introduced into the cellulose fiber contained in the fiber raw material. It is a process.
This reaction step is not particularly limited as long as it is a method capable of a sulfonation reaction in which the hydroxyl group of the cellulose fiber is substituted with a sulfo group.

For example, if the fiber raw material in which the above reaction solution is contacted (for example, impregnated with the reaction solution) is supplied to the reaction step as it is and heated at a predetermined temperature, the cellulose fibers contained in the fiber raw material are provided with sulfo groups. Can be properly introduced.
Specifically, in the reaction step, the sulfonation reaction can proceed while supplying heat to the supplied fiber raw material. That is, if heating is performed in the reaction step, a sulfonation reaction is carried out in which a sulfo group is introduced into the hydroxyl groups of the cellulose fibers contained in the fiber raw material while drying the fiber raw material in which the reaction solution is impregnated and / or adhered. Can be made. In this case, since the amount of water in the sulfonate reaction (the amount of water in the reaction solution adhering to the fiber raw material) can be reduced, sulfonation is performed while the influence of such water (for example, reaction failure, hydrolysis, etc.) is suppressed. You will be able to make a reaction.
Therefore, in the reaction step, by carrying out the reaction while heating at a predetermined temperature, the efficiency of introducing the sulfo group into the hydroxyl group of the cellulose fiber is improved while suppressing the damage to the cellulose fiber contained in the supplied fiber raw material. be able to. Moreover, as will be described later, the sulfo group can be appropriately introduced into a predetermined hydroxyl group of the cellulose fiber in a short time.

The fiber raw material to be supplied to the reaction step is not particularly limited as long as it contains water as described above. For example, the fiber raw material to be supplied to the reaction step may have a high water content after the contact step, or may be dehydrated or the like to lower the water content before being supplied to the reaction step. Alternatively, it may be in a state where the water content is further lowered by a drying treatment or the like. In other words, the fiber raw material to be supplied to the reaction step is not particularly limited as long as it is in a state other than that it does not contain water, that is, in a non-absolute state having a water content of 1% or more.

In the present specification, such a non-dehydrated fiber raw material having a water content of 1% or more is referred to as a state containing water (that is, a wet state), and is in a state of being impregnated with a reaction solution or the like. Not only those that have been dehydrated to some extent, but also those that have been dried to some extent are referred to as wet states in the present specification.
In the present specification, the term "absolutely dry" means a state in which the moisture content is lower than 1%. The drying treatment step of making the product absolutely dry means, for example, a step of reducing the pressure with a desiccator containing a desiccant such as calcium chloride or diphosphorus pentoxide. That is, the step of drying the fiber raw material before supplying it to the reaction step in the specification of the present application is not a drying treatment step of making it absolutely dry.

The state in which the fiber raw material is impregnated with the reaction solution when supplied to the reaction step is, for example, a state in which the reaction solution is brought into contact with the fiber material so that water drips, that is, a state in which the water content is close to 100%. It may be.
Further, a state in which the fiber raw material supplied to the reaction step has water removed from the fiber raw material after the contact step to some extent means that the fiber is in an absolutely dry state when supplied to the reaction step (fibers when supplied to the reaction step). It refers to a state in which the water content of the raw material is not lower than 1%), and is not particularly limited as long as the water content is 1% or more. In such a method of removing water from the fiber raw material after the contact step to some extent, the fiber raw material after the contact step may be dehydrated or the like as described above, or may be dehydrated and then dried. Or the fiber raw material after the contact step may be directly dried.

In this case, since the water is removed to some extent, there is an advantage that the handleability and operability are improved. As the dehydration treatment, for example, a general treatment method such as suction dehydration or centrifugal dehydration can be adopted, and the water content of the fiber raw material when supplied to the reaction step is adjusted to be about 20% to 80%. can do. Further, as the drying treatment, for example, a drying treatment using a general device such as a dryer can be adopted so that the water content of the fiber raw material after the drying treatment is 1% or more and smaller than 20%. Can be adjusted.

The water content in the present specification can be calculated by the following formula.

Moisture content (%) = 100- (solid content weight of fiber raw material (g) / weight of fiber raw material at the time of water content measurement after contact step (g)) x 100

The solid content weight (g) of the fiber raw material is the dry weight of the fiber raw material itself to be measured. For example, a fiber raw material having a water content of about 50% and containing the same amount of water is dried in advance at 105 ° C. for 2 hours with respect to a dry weight of 2 g of the fiber raw material supplied to the reaction step. The value obtained by measuring can be used.
The weight (g) of the fiber raw material at the time of measuring the water content after the contact step is the weight of the fiber raw material at the time of measuring after the contact step. For example, the weight of the fiber raw material when supplied to the above-mentioned reaction step, the weight of the fiber raw material after the dehydration treatment after the contact step, the weight of the fiber raw material after the drying treatment, and the like can be mentioned.

(Reaction temperature in the reaction process)
The reaction temperature in the reaction step is not particularly limited as long as it is a temperature at which a sulfo group can be introduced into the cellulose fibers constituting the fiber raw material while suppressing the thermal decomposition and hydrolysis reactions of the fibers.
Specifically, the fiber raw material after the contact step may be directly or indirectly heated while satisfying the above requirements. Examples of such a method include a hot press method using a known dryer, a vacuum dryer, a microwave heating device, an autoclave, an infrared heating device, and a heat press machine (for example, AH-2003C manufactured by AS ONE Corporation). Etc. can be adopted. In particular, since gas may be generated in the reaction step, it is preferable to use a circulation blower type dryer.

The shape of the fiber raw material after the contact step is not particularly limited, but for example, it is preferable to heat the fiber raw material in a sheet shape or in a state of being loosened to some extent using the above-mentioned equipment, because the reaction can be promoted uniformly. ..

The reaction temperature in the reaction step is not particularly limited as long as the above requirements are satisfied.
For example, the ambient temperature is preferably 250 ° C. or lower, more preferably 200 ° C. or lower, and even more preferably 180 ° C. or lower.
If the ambient temperature during heating is higher than 250 ° C., thermal decomposition occurs as described above, and the progress of discoloration of the fibers becomes faster. On the other hand, when the reaction temperature is lower than 100 ° C., the reaction time tends to be long.
Therefore, from the viewpoint of workability, the reaction temperature (specifically, the ambient temperature) during heating is 100 ° C. or higher and 250 ° C. or lower, more preferably 100 ° C. or higher and 200 ° C. or lower, and further preferably 100 ° C. or higher and 180 ° C. or lower. Adjust so that.

(Reaction time in reaction process)
The heating time (that is, reaction time) when the above heating method is adopted as the reaction step is not particularly limited as long as the sulfo group can be appropriately introduced into the cellulose fiber as described above.

For example, the reaction time in the reaction step is adjusted to be 1 minute or more when the reaction temperature is adjusted to be within the above range. More specifically, it is preferably 5 minutes or longer, more preferably 10 minutes or longer, and even more preferably 20 minutes or longer.
When the reaction time is shorter than 1 minute, it is presumed that the substitution reaction of the sulfo group with respect to the hydroxyl group of the cellulose fiber has hardly proceeded. On the other hand, even if the heating time is too long, the amount of sulfo groups introduced tends not to be improved.

That is, for the reaction time in the reaction step, it is sufficient that the reaction solution can be brought into contact with the fiber raw material to replace a part of the hydroxyl groups of the cellulose fibers in the fiber raw material with sulfo groups. By reacting the sulfonated pulp fiber prepared through this reaction step with the above reaction solution, the fiber length retention rate (%) before and after the reaction can be maintained high.
Therefore, the reaction time when the above heating method is adopted as the reaction step is not particularly limited, but is preferably 5 minutes or more and 300 minutes or less, more preferably 5 minutes or more and 120 minutes or less, from the viewpoint of reaction time and operability. It is better to do it.

(Sulfonizing agent)
The sulfonate agent in the reaction step is not particularly limited as long as it is a compound having a sulfo group.
For example, sulfamic acid, sulfamate, sulfyl compounds having a sulfonyl group having two oxygen covalently bonded to sulfur, and the like can be mentioned. As the sulfonate agent, these compounds may be used alone, or two or more kinds may be mixed and used.
The sulfonate agent is not particularly limited as long as it is a compound as described above, but it has a lower acidity than sulfuric acid or the like, a high efficiency of introducing a sulfo group, a low cost, and a high safety, so from the viewpoint of handleability. , Sulfamic acid is preferably used.

(Fiber raw material)
The fiber raw material used in the method for producing a sulfonated fine cellulose fiber is not particularly limited as long as it contains cellulose as described above.
For example, what is generally called pulp may be used, or a material containing cellulose isolated from sea squirts, seaweed, etc. can be used as a fiber raw material, but any material composed of cellulose molecules can be used. , Anything can be used.
Examples of the above-mentioned pulp include wood-based pulp (hereinafter simply referred to as wood pulp), melted pulp, cotton-based pulp such as cotton linter, straw, bagus, 楮, sansho, hemp, kenaf, and fruits. Non-wood pulp, used newspaper pulp, used paper pulp produced from used magazine paper, used cardboard, etc., but are not limited to these. From the viewpoint of availability, wood pulp is easy to use as a fiber raw material.

There are various types of this wood pulp, but the use is not particularly limited. For example, papermaking pulp such as softwood kraft pulp (NBKP), hardwood kraft pulp (LBKP), and thermomechanical pulp (TMP) can be mentioned.
When the above-mentioned pulp is used as the fiber raw material, one of the above-mentioned types of pulp may be used alone, or two or more of them may be mixed and used.

(Drying process)
As described above, the state of the fiber raw material when it is supplied to the reaction step may be a state in which water is contained when it is supplied to the reaction step, and if such a state is maintained, the contact step and the reaction step. A drying step may be included between the two.
This drying step is a step that functions as a pretreatment step of the reaction step, and is a step of reducing the water content of the fiber raw material after the contact step.

Here, it is desirable that the fiber raw material after the drying step is adjusted so that the water content is 1% or more. When the fiber raw material in a state where the water content of the fiber raw material after the drying step is lower than 1%, that is, in a state of being dried until it becomes an absolutely dry state is supplied to the reaction step, the hydrogen bond between the fibers in the fiber raw material is formed due to the absolute dry state. It may act strongly and make it difficult for the sulfonated reaction in the reaction step to proceed properly. Therefore, in the drying step, it is desirable to adjust the water content of the fiber raw material after the drying step to be 1% or more.

Further, if the drying step is provided, the following advantages can be obtained.
If the water content of the fiber raw material when it is supplied to the reaction step is high, some of the sulfamic acid, urea, etc. may be hydrolyzed during the heating reaction in the reaction step, and the reaction efficiency may decrease. In some cases, an undesired reaction may occur as a fiber raw material for micronization. Therefore, from the viewpoint of suppressing the possibility of damage during the heating reaction, a drying step of lowering the water content of the fiber raw material may be provided before the reaction step.
By providing the drying step, in the reaction step of the next step, the possibility of the above-mentioned obstacles and the like can be suppressed, the reaction time of the esterification reaction can be shortened, and the operability can be improved. That is, by providing the drying step, the water in the reaction liquid adhering to the fiber raw material after the contact step can be appropriately removed when it is supplied to the reaction step, so that the fiber when it is supplied to the reaction step can be obtained. The concentration of sulfamic acid in the raw material can be increased. Therefore, in the reaction step, there is an advantage that the esterification reaction between the hydroxyl group in the fiber raw material and sulfamic acid can be more easily promoted.

In the drying step, the method is not particularly limited as long as the water content of the fiber raw material after the drying step is adjusted to be 1% or more as described above. For example, a dryer may be used to adjust the water content of the fiber raw material after the drying step so that it is in an equilibrium state with the water content in the atmosphere, or the water content is 15% or less, 10% or less, and further. May be adjusted to be up to about 5%.

Here, the equilibrium state in the present specification means a state in which the moisture in the atmosphere and the moisture in the raw material in the treatment facility apparently do not enter and exit. Specifically, it means a state in which after drying for a certain period of time (for example, 2 hours), the amount of change in weight measured twice in succession is within 1% of the weight at the start of drying (however). Second, the second weight measurement shall be at least half of the drying time required for the first time).

The drying temperature in the drying step is not particularly limited, but it is desirable to dry at a temperature lower than the heating temperature of the heating reaction in the reaction step. If the ambient temperature during heating is higher than 100 ° C., decomposition of the sulfonate and the like may occur. On the other hand, if the ambient temperature during heating is lower than 20 ° C., it takes time to dry. Therefore, the drying temperature in the drying step is, for example, preferably an atmospheric temperature of 100 ° C. or lower, more preferably 20 ° C. or higher and 100 ° C. or lower, and further preferably 50 ° C. or higher and 100 ° C. or lower. That is, the atmospheric temperature during drying in the drying step is preferably 100 ° C. or lower, and is preferably adjusted to 20 ° C. or higher from the viewpoint of operability.

(Washing process)
After the reaction step in the chemical treatment step S2, a washing step of washing the sulfonated pulp fibers after introducing the sulfo group may be included.
The surface of the sulfonated pulp fiber after the introduction of the sulfo group is acidified due to the influence of the sulfonate agent. In addition, an unreacted reaction solution is also present. Therefore, if a cleaning step is provided to ensure that the reaction is terminated and the excess reaction solution is removed to bring the reaction solution into a neutral state, the handleability can be improved.

This washing step is not particularly limited as long as the sulfonated pulp fiber after introducing the sulfo group can be made substantially neutral. For example, a method of washing with pure water or the like until the sulfonated pulp fiber after introducing the sulfo group becomes neutral can be adopted. Further, neutralization cleaning using alkali or the like may be performed. When such neutralization cleaning is performed, examples of the alkaline compound contained in the alkaline solution include an inorganic alkaline compound and an organic alkaline compound. Examples of the inorganic alkali compound include alkali metal hydroxides, carbonates, phosphates and the like. Examples of the organic alkaline compound include ammonia, an aliphatic amine, an aromatic amine, an aliphatic ammonium, an aromatic ammonium, a heterocyclic compound, and a hydroxide of a heterocyclic compound.

(Miniaturization processing step S3)
The micronization treatment step S3 is a step of refining the sulfonated pulp fiber obtained in the chemical treatment step S2 into fine fibers having a predetermined size (for example, nano level).
The processing apparatus (defibration equipment) used in the miniaturization processing step S3 is not particularly limited as long as it has the above functions.
For example, a low pressure homogenizer, a high pressure homogenizer, a grinder (stone mill type crusher), a ball mill, a cutter mill, a jet mill, a short shaft extruder, a twin shaft extruder, an ultrasonic stirrer, a household mixer, etc. can be used. , The processing device is not limited to these devices. Of these, it is desirable to use a high-pressure homogenizer in that the force can be applied evenly to the material and the homogenization is excellent, but the device is not limited to this.

When a high-pressure homogenizer is used in the miniaturization treatment step S3, the sulfonated pulp fibers obtained by the above-mentioned production method are supplied in a state of being dispersed in a water-soluble solvent such as water. In the following, a solution in which sulfonated pulp fibers are dispersed is referred to as a slurry.

The solid content concentration of the sulfonated pulp fiber of this slurry is not particularly limited. For example, a solution adjusted so that the solid content concentration of the sulfonated pulp fiber of this slurry is 0.1% by mass to 20% by mass may be supplied to a processing apparatus such as a high-pressure homogenizer.
For example, when a slurry adjusted so that the solid content concentration of the sulfonated pulp fiber is 0.5% by mass is supplied to a processing device such as a high-pressure homogenizer, the sulfonated fine cellulose fiber having the same solid content concentration becomes a water-soluble solvent. A dispersion liquid in a dispersed state can be obtained. That is, in this case, it is possible to obtain a dispersion liquid adjusted so that the solid content concentration of the sulfonated fine cellulose fibers is 0.5% by mass.

In the miniaturization treatment step S3, when a high-pressure homogenizer is used as the treatment apparatus, the defibration pressure and the number of defibrations are determined if sulfonated fine cellulose fibers having desired viscosity characteristics (viscosity, TI value) can be appropriately prepared. , Not particularly limited.

(When adjusting by defibration pressure)
For example, when preparing by the defibration pressure of a high-pressure homogenizer, it is preferable to adjust the defibration pressure to be 100 MPa or less when the solid content concentration of the sulfonated pulp fiber of the slurry is 0.5% by mass. It is preferably 60 MPa or less.
If the sulfonated pulp fiber is defibrated with a defibration force having a defibration pressure higher than 100 MPa, the number of connected unit fibers tends to decrease and the fiber length of the sulfonated fine cellulose fiber tends to be shortened. It is presumed that the reason for this is that in the method for producing the sulfonated fine cellulose fiber, the sulfonated pulp fiber to be subjected to the micronization treatment step S3 is in a state of being depolymerized by the degree of polymerization reduction treatment step S1. To.
Therefore, the fiber length of the sulfonated fine cellulose fiber obtained by the defibrating force higher than 100 MPa, that is, the number of connected unit fibers tends to decrease. Then, the decrease in the number of connected unit fibers tends to decrease the fiber length of the sulfonated fine cellulose fibers, and the viscosity of the dispersion liquid also tends to decrease.
Therefore, when a high-pressure homogenizer is used as a processing device (defibration equipment) in the miniaturization processing step S3 and the viscosity of the sulfonated fine cellulose fibers is adjusted by the defibration pressure, the defibration pressure is 100 MPa or less. It is preferable to adjust as such, and more preferably 60 MPa or less.
Further, since the defibration pressure and the viscosity of the sulfonated fine cellulose fiber tend to have a correlation, when preparing a sulfonated fine cellulose fiber having a high viscosity, the defibration pressure should be adjusted appropriately while being lowered. Just do it. For example, the defibration pressure can be adjusted to be 30 MPa or less and 15 MPa or less. That is, the viscosity characteristics of the sulfonated fine cellulose fibers can be adjusted by adjusting the defibration pressure.

(When adjusting according to the number of defibration)
Further, for example, when the high-pressure homogenizer is prepared by the number of defibration (number of passes) while the defibration pressure is fixed, the number of defibration is performed when the solid content concentration of the sulfonated pulp fiber of the slurry is 0.5% by mass. It is preferable to adjust so that the number of passes is 30 or less. From the viewpoint of operability and the like, 25 passes or less is preferable, 20 passes or less is more preferable, 15 passes or less is more preferable, and 10 passes or less is more preferable.
When the number of defibration is more than 30 passes, the number of connected unit fibers tends to decrease and the fiber length of the sulfonated fine cellulose fibers tends to be shortened. The reason for this is the same as in the case of preparing with the above-mentioned defibration pressure.
Therefore, when a high-pressure homogenizer is used as a processing device (defibration device) in the miniaturization processing step S3 and the viscosity of the sulfonated fine cellulose fiber is adjusted by the number of defibration (number of passes), the number of defibration is It is preferable to adjust so as to be within the above range.

Further, since the number of defibration (number of passes) and the viscosity (that is, viscosity) of the sulfonated fine cellulose fiber tend to have a correlation, defibration is performed when preparing a sulfonated fine cellulose fiber having a high viscosity. The number of times (number of passes) may be reduced, and when it is desired to reduce the viscosity (viscosity), the number of defibration (number of passes) may be increased and adjusted appropriately. That is, the viscosity characteristics of the sulfonated fine cellulose fibers can be adjusted by adjusting the number of defibration (number of passes).

On the other hand, regarding the number of defibration (number of passes), from the viewpoint of productivity, the smaller the number of defibration, the smaller the number of operations, the more the processing amount can be increased, and the workability can be improved. It is preferable because the cost can be reduced. From this point of view, for example, the number of defibration (number of passes) is preferably 20 passes or less, more preferably 15 passes or less, still more preferably 10 passes or less, and particularly preferably 5 passes or less.

(When adjusting according to the defibration pressure and the number of defibration)
Further, the sulfonated fine cellulose fiber can be prepared so as to exhibit desired viscosity characteristics (viscosity, TI value) by adjusting the defibration strength based on the defibration pressure and the number of defibration.

For example, the defibration strength can be adjusted by fixing the defibration pressure at 30 MPa or less and increasing the number of defibration (number of passes).
This defibration strength can also be defined as the defibration energy (J) applied to the sulfonated pulp fiber during the defibration treatment.
For example, the flow rate at each defibration pressure is calculated based on the water flowing in the piping of the defibration equipment used for the defibration treatment and the defibration pressure during the defibration treatment. Then, the flow velocity of water is calculated from this flow rate. Then, the kinetic energy (J) in each defibration treatment can be calculated based on the flow rate at this time and the defibration pressure in each defibration treatment. Specific calculation examples of the method of expressing the defibration strength by the defibration energy (J) will be described in Examples described later.

For example, in order to prepare a sulfonated fine cellulose fiber exhibiting transparency (for example, a haze value of 20% or less), the defibration energy (J) is adjusted to 10 J or more. The defibration energy (J) is preferably 20 J or more, more preferably 40 J or more, and further preferably 50 J or more (see FIG. 8). On the other hand, from the viewpoint of productivity, the defibration energy (J) may be 2000 J or less, preferably 1000 J or less, more preferably 500 J or less, still more preferably 250 J or less, still more preferably 100 J. It is as follows.
Further, for example, when preparing a sulfonated fine cellulose fiber in which the dispersion liquid exhibits viscosity and transparency, the lower limit value of the defibration energy (J) is the same as the above value, and the upper limit value is 500 J or less. It is good, preferably 100 J or less.

Further, for example, in order to prepare a sulfonated fine cellulose fiber exhibiting a high viscosity and a low TI value while the dispersion liquid has transparency, the lower limit value of the defibration energy (J) is the same as the above value. The upper limit may be 200 J or less, preferably 150 J or less, and more preferably 100 J or less (CNF including the first maximum point described in the examples in FIG. 6, first, FIG. 8, See Table 1, Tables 5-9).
On the other hand, in order to prepare sulfonated fine cellulose fibers in which the dispersion liquid exhibits high viscosity and high TI value while the dispersion liquid has transparency, the lower limit value of the defibration energy (J) is the same as the above value. The upper limit may be 150 J or more, preferably 200 J or more (see CNF including the second maximum point described in the example in FIG. 6, FIG. 8, Table 1, Tables 5 to 9). ..

The relationship between the defibration treatment and the viscosity of the sulfonated fine cellulose fibers is summarized below.
Conventionally, when a high-pressure homogenizer is used as a processing device (defibration equipment) for defibration processing, there is a problem that the processing capacity is greatly reduced as the defibration pressure of the high-pressure homogenizer becomes higher. For example, when a general general-purpose machine that performs defibration processing at a defibration pressure of 100 MPa or more is used in order to produce fibers of a desired size, the processing amount per hour is as high as several hundred liters. The manufacturing efficiency is poor. On the other hand, if this defibration pressure is applied at several tens of MPa, the amount of processing per hour can be several thousand L, which is an order of magnitude higher. However, since the defibration pressure is set low, there is a problem that the fibers cannot be defibrated to a desired size.

On the other hand, in the method for producing the sulfonated fine cellulose fiber of the present embodiment, the desired viscosity characteristics (viscosity (viscosity)) are obtained even if the defibration pressure and the number of defibration (number of passes) in the pulverization treatment step S3 are within the above ranges as described above. And TI value) can be appropriately produced to produce sulfonated fine cellulose fibers.
The reason for this is that, as described above, the sulfonated pulp fibers used in the micronization treatment step S3 are in a state of being depolymerized in the degree of polymerization reduction treatment step S1. Therefore, it is presumed that the sulfonated pulp fibers are in a state where the fibers are easily loosened because the hydrogen bonds between the fibers are loosened.

As described above, if this production method is used, smooth miniaturization (defibration) can be performed, so that the time required for the miniaturization processing step can be further shortened. Therefore, a sulfone having the above-mentioned function can be obtained. The productivity of the refined fine cellulose fiber can be improved. For example, in the conventional technique, the defibration pressure during the defibration treatment is high and a large number of defibration times are required. However, if the method for producing sulfonated fine cellulose fibers of the present embodiment is used, the defibration pressure is lower. , It will be possible to process with a smaller number of defibration.

Therefore, by using this production method, it is possible to easily produce a sulfonated fine cellulose fiber capable of exhibiting the above-mentioned excellent viscosity characteristics even at a low defibration pressure which is not expected in the past. Moreover, since the number of times of defibration can be reduced, the efficiency of the processing operation can be improved. Then, for example, as shown in Tables 5 and 6, sulfonated fine cellulose fibers exhibiting desired viscosity characteristics with less energy can be produced without lowering the productivity.
Further, by adjusting the defibration treatment conditions (for example, defibration pressure and / or number of defibration (number of passes)) in the defibration treatment step, the viscosity (viscosity) and TI value of the sulfonated fine cellulose fibers can be adjusted. Viscosity characteristics can be controlled.
Therefore, by using the method for producing a sulfonated fine cellulose fiber of the present embodiment, it is possible to easily, efficiently and appropriately produce a sulfonated fine cellulose fiber capable of exhibiting excellent viscosity characteristics.

It was confirmed that the sulfonated fine cellulose fiber of the present invention can be produced by using the method for producing the sulfonated fine cellulose fiber of the present invention. Moreover, it was confirmed that the prepared sulfonated fine cellulose fiber of the present invention had a predetermined property.

<Experiment 1>
As a fiber raw material, softwood kraft pulp (NBKP) manufactured by Marusumi Paper Co., Ltd. (average fiber length of 2.6 mm) was used. In the following, NBKP used in the experiment will be simply described as pulp.
The pulp was washed with a large amount of pure water, drained with a 200-mesh sieve, the solid content concentration was measured, and the pulp was subjected to an experiment without being dried.

(Polymerization degree reduction treatment step)
A treatment was performed to reduce the degree of polymerization of the pulp washed with pure water.
Sodium hydroxide (concentration: about 48%, manufactured by Sumitomo Chemical Co., Ltd.) was used as the reagent used in the polymerization degree reduction treatment step. These reagents were adjusted to a concentration of 1 mol / L and used as a treatment liquid for the degree of polymerization reduction treatment.
Pulp was added to the prepared aqueous sodium hydroxide solution. At this time, the solid content concentration of the pulp was adjusted to 10% with respect to 1 mol of an aqueous solution of sodium hydroxide, and the pulp was allowed to stand overnight at room temperature to prepare a pulp having a degree of polymerization reduced.
For example, 100 g (dry weight) of pulp was added to 900 g of the above reagent, the reagent and pulp were uniformly mixed, and then allowed to stand at room temperature for 24 hours at normal temperature and pressure to reduce the degree of polymerization.

The degree of polymerization reduction treatment corresponds to the contact step of the degree of polymerization reduction treatment step in the method for producing sulfonated fine cellulose fibers of the present embodiment, and the reagent used corresponds to the degree of polymerization reducing agent.

(Washing step after polymerization degree reduction treatment)
It was confirmed that the pulp reduced in degree of polymerization after the step of reducing the degree of polymerization was neutralized with 0.5 M sulfuric acid to neutralize the pH. Then, it was washed with a large amount of pure water and subjected to a chemical treatment step.

(Chemical treatment process)
The supplied pulp for reducing the degree of polymerization was added to the reaction solution prepared as follows, and impregnated with the chemical solution to form a slurry.
The step of adding the polymerized pulp to the reaction solution to form a slurry corresponds to the contact step in the chemical treatment step of the method for producing sulfonated fine cellulose fibers of the present embodiment.

(Preparation of reaction solution)
The sulfonate and urea and / or its derivatives were prepared to have the following concentrations.
In the experiment, sulfamic acid (purity 98.5%, manufactured by Fuso Chemical Industries) was used as the sulfonate, and urea solution (purity 99%, manufactured by Wako Pure Chemical Industries, Ltd., model number; special grade reagent) was used as urea or a derivative thereof. used.
The mixing ratio of the two was adjusted so that the concentration ratio (g / L) was 2: 1 (1: 0.5) to prepare an aqueous solution.
Sulfamic acid and urea were mixed as follows. Sulfamic acid / urea ratio ((g / L) / (g / L)) = 200/100

An example of the method for preparing the reaction solution is shown below.
100 ml of water was added to the container. Then, 20 g of sulfamic acid and 10 g of urea were added to this container to prepare a reaction solution having a sulfamic acid / urea ratio ((g / L) / (g / L)) of 200/100 (1: 0.5). .. That is, urea was added so as to be 50 parts by weight with respect to 100 parts by weight of sulfamic acid.

(Method of contact between reaction solution and pulp with reduced degree of polymerization)
The polymerized pulp (dry weight) was added to the prepared reaction solution. At this time, the slurry was adjusted so that the ratio of the chemicals in the reaction solution to the polymerized pulp (dry weight) was pulp: chemicals = 1: 1.5.
In the experiment, in the case of a reaction solution having a sulfamic acid / urea ratio ((g / L) / (g / L)) of 200/100 (1: 0.5), the degree of polymerization was reduced with respect to 1 g (dry weight) of the treated pulp. Then, about 6.5 g of the reaction solution was added and impregnated with the chemical.

The slurry prepared by adding the polymerized pulp to the reaction solution was loosened by hand for 30 minutes, and the polymerized pulp was uniformly impregnated with the reaction solution. The polymerized pulp impregnated with this reaction solution was placed in a dryer (manufactured by Isuzu Seisakusho, model number; VTR-115). The temperature of the constant temperature bath of the dryer was set to 50 ° C. The polymerized pulp was dried until the water content reached an equilibrium state. Then, the pulp reduced in degree of polymerization, which had been dried before the heating reaction, was supplied to the next heating reaction step.

The water content of the polymerized pulp reduced in degree of polymerization, which was brought into contact with the reaction solution when it was supplied to the heating reaction step, was calculated using the following formula.
The "water content is in equilibrium" was evaluated as follows.
First, it was dried for 1 hour in the above-mentioned dryer in which the temperature of the constant temperature bath was set to 50 ° C. After that, the state in which the amount of change in weight measured twice in succession was within 1% of the weight at the start of drying was regarded as an equilibrium state (however, the second weight measurement was performed the first time). More than half of the drying time required).

Moisture content (%) = 100- (Pulp solid content weight (g) / Pulp weight at the time of water content measurement after contact with reaction solution (g)) x 100

The solid content weight (g) of the pulp means the dry weight of the pulp itself to be measured, and in this experiment, 2 g of the dry pulp used in the experiment corresponds to it. The weight of the dried pulp was measured by using the above-mentioned dryer to dry the pulp at a temperature of 105 ° C. for 2 hours.
In this experiment, the weight of the pulp after drying, which was dried using a dryer before being supplied to the heating reaction, corresponds to the weight of the pulp (g) at the time of measuring the water content after contacting with the reaction solution.
In the experiment, the water content of the pulp after drying was about several% to 10% (drying temperature 50 ° C.). That is, in this experiment, in this drying step, the water content of the dried pulp was adjusted so as not to be 1% or less (so-called absolute dry state).

The step of drying the polymerized pulp impregnated with the reaction solution corresponds to the drying step in the chemical treatment step of the method for producing sulfonated fine cellulose fibers of the present embodiment.

(Heating reaction)
Then, the dried pulp obtained by drying the polymerized pulp impregnated with the reaction solution was subjected to the heating reaction step of the next step to carry out the heating reaction.
A dryer (manufactured by Isuzu Seisakusho, model number; VTR-115) was used for the heating reaction.
The reaction conditions of the heating reaction were as follows.
Constant temperature bath temperature: 120 ° C, heating time: 25 minutes

This heating reaction corresponds to the reaction step in the chemical treatment step of the method for producing the sulfonated fine cellulose fiber of the present embodiment.
The temperature of the reaction conditions in this heating reaction corresponds to the reaction temperature of the reaction step in the chemical treatment step of the method for producing the sulfonated fine cellulose fiber of the present embodiment.
The heating time of the reaction conditions in this heating reaction corresponds to the reaction time of the reaction step in the chemical treatment step of the method for producing the sulfonated fine cellulose fiber of the present embodiment.

After the heating reaction, it was confirmed that the reacted pulp was neutralized with sodium hydrogen carbonate and the pH became neutral. Then, it was washed with a large amount of pure water to prepare a sulfamic acid / urea-treated pulp.

The step of washing the reacted pulp with pure water until it becomes neutral corresponds to the washing step in the chemical treatment step of the method for producing sulfonated fine cellulose fibers of the present embodiment.
The reactive pulp fibers constituting the prepared sulfamic acid / urea-treated pulp correspond to the sulfonated pulp fibers referred to in the present embodiment.

(Miniaturization process)
Then, the prepared sulfamic acid / urea-treated pulp was prepared so that the solid content concentration was 0.5% by mass, and a high-pressure homogenizer (manufactured by Kos Nijuichi Co., Ltd., model number; N2000-2C-045 type) was used. Nanocellulose fibers were prepared by performing micronization (nanogenization).
The treatment conditions for the high-pressure homogenizer were as follows.
First, preliminary defibration was performed using a high-pressure homogenizer. The preliminary defibration was performed at a defibration pressure of 10 MPa with two defibration times (2 passes).
Then, the pre-defibrated slurry was subjected to a high-pressure homogenizer and subjected to a defibration treatment at a defibration pressure of 15 MPa to prepare nanocellulose fibers.
Each nanocellulose fiber sample (sample A; 3 passes, sample B; 5 passes, sample C; 7 passes, sample D; 9 passes, sample E; 11 passes) according to the number of defibration (number of passes) of the high-pressure homogenizer. , Sample F; 15 passes).

The defibration operation using this high-pressure homogenizer corresponds to the miniaturization treatment step in the method for producing sulfonated fine cellulose fibers of the present embodiment. The prepared nanocellulose fiber corresponds to the sulfonated fine cellulose fiber of the present embodiment.

(Measurement of sulfur introduction amount by measuring electrical conductivity)
The amount of sulfur introduced due to the sulfo group was measured by treating the prepared nanocellulose fiber with an ion exchange resin and then titrating with an aqueous sodium hydroxide solution.
In the treatment with an ion exchange resin, a strongly acidic ion exchange resin (manufactured by Organo Corporation, Amber Jet 1024; conditioned) with a volume ratio of 1/10 was added to 100 mL of a slurry containing 0.2% by mass of nanocellulose fibers for 1 hour. Shaking processing was performed. Then, it was poured onto a mesh having a mesh size of 200 μm to separate the resin and the slurry.
In titration using an aqueous sodium hydroxide solution, the change in the value of electrical conductivity is measured while adding 10 μL to 50 μL each of 0.5 N aqueous sodium hydroxide solution to the nanocellulose fiber-containing slurry treated with an ion exchange resin. , The vertical axis was plotted as electrical conductivity and the horizontal axis was titrated with sodium hydroxide titration to obtain a curve, and the inflection point was confirmed from the obtained curve.
The titration of sodium hydroxide at this inflection point corresponds to the amount of sulfo groups. Therefore, the amount of sulfo groups in the nanocellulose fibers, that is, the amount of sulfur introduced due to the sulfo groups was measured by dividing the amount of sodium hydroxide at the inflection point by the solid content of the nanocellulose fibers used for the measurement.

(Observation of fiber morphology and measurement of fiber width using SPM)
The nanocellulose fibers treated with the high-pressure homogenizer were prepared with pure water to a solid content concentration of 0.0005 to 0.001% by mass suitable for observation. Then, spin coating was performed on a silica substrate coated with PEI (polyethyleneimine).
Observation of the nanocellulose fibers on the silica substrate was performed using a scanning probe microscope (manufactured by Shimadzu Corporation, model number; SPM-9700).
The fiber width and fiber length were measured by randomly selecting 20 fibers in the observation image.

(Measurement of haze value and measurement of total light transmittance)
For the measurement of the haze value and the measurement of the total light transmittance, a measurement solution prepared by using pure water for nanocellulose fibers and having a solid content concentration of 0.5 to 1.0% by mass was used.
In the micronization treatment step, those prepared so that the solid content concentration (processed pulp solid content concentration) of the sulfamic acid / urea-treated pulp in the slurry supplied to the high-pressure homogenizer is 0.5% by mass are micro-treated. The solid content concentration of the nanocellulose fibers in the dispersion obtained later is also 0.5% by mass, which is the same. Therefore, this dispersion was used for measurement as it was without adjusting the solid content concentration of the nanocellulose fibers as 0.5% by mass.

A predetermined amount was separated from the prepared measurement solution to prepare a CNF slurry for measurement.
For the measurement of haze value and total light transmittance, a spectrophotometer (manufactured by JASCO Corporation, model number 0V-570) is attached with an integrating sphere (ISN-470 manufactured by JASCO Corporation) to measure the haze value. And the total light transmittance was measured as follows with reference to JIS K 7105.
A quartz cell containing pure water was used as a blank measurement value, and the light transmittance of 0.5% by mass of the nanocellulose dispersion was measured. The measurement wavelength range was 380 to 780 nm.
The total light transmittance (%) and the haze value (%) were calculated from the numerical values obtained by the spectrophotometer using the attached calculation software.

The measurement solution used in the spectrophotometer corresponds to the dispersion in the method for producing sulfonated fine cellulose fibers of the present embodiment.

(Measurement of degree of polymerization)
The intrinsic viscosity (intrinsic viscosity) of the nanocellulose fiber was measured according to JIS P 8215 (1998).
For the measurement of the ultimate viscosity of this JIS standard, the A method and the B method are described. In this experiment, it was measured according to "Method A-Measurement of ultimate cellulose viscosity at dilute concentration". The changes at the time of measurement were that the raw material pulp and nanocellulose fibers were separated so that the solid content was 0.1 g and used for the measurement. As the measured value, the average value of the results of three measurements was used.
The average degree of polymerization (DP) of the nanocellulose fiber was calculated from each intrinsic viscosity (η) by the following formula. Since this average degree of polymerization is the average degree of polymerization measured by the viscosity method, it is sometimes called "viscosity average degree of polymerization".

DP = 1.75 × [η]

(Measurement of viscosity)
The viscosity was measured by using a sealed one that had been allowed to stand at room temperature for 24 hours. After standing, 100 g was dispensed into a 110 mL tall beaker, and the viscosity was measured using a B-type viscometer (LVDV-I Prime, manufactured by Eiko Seiki Co., Ltd.).
The measurement conditions for viscosity measurement were as follows.
Measurement conditions: Rotation speed 6 rpm, measurement temperature 25 ° C., measurement time 3 minutes, spindle No. 64, data recording method is single point

(Measurement of TI value)
The thixotropy index (TI value) was measured as follows.
The TI value was calculated using the above-mentioned B-type viscometer at rotation speeds of 6 rpm and 60 rpm, and the respective viscosities were calculated by the following formulas. Other conditions are as described above.

TI value = (viscosity at 6 rpm) / (viscosity at 60 rpm)

(Experimental result)
Amount of sulfur introduced due to sulfo groups in nanocellulose fibers (Sample A; 3 passes, Sample B; 5 passes, Sample C; 7 passes, Sample D; 9 passes, Sample E; 11 passes, Sample F; 15 passes) Was 1.0 to 1.5 mmol / g, with an average of 1.3 mmol / g.
The average degree of polymerization of the sulfamic acid / urea-treated pulp fibers before the micronization treatment was 404. The average degree of polymerization of each nanocellulose fiber after the micronization treatment was 300 to 314, and the average of these was 308.
The average fiber width of each nanocellulose fiber was 30 nm or less.

The total light transmittance (%), haze value (%), viscosity (mPa · s), TI value and average degree of polymerization of each nanocellulose fiber were as shown in the table below.

FIG. 2 shows the relationship between the number of defibration, the average degree of polymerization, and the viscosity (mPa · s).
As shown in FIG. 2, it was confirmed that the viscosity can be controlled by controlling the number of defibration. On the other hand, the average degree of polymerization for each number of defibration was almost within the range of variation, and all of them were near the level-off degree of polymerization of cellulose.

From the experimental results, the pulp after the degree of polymerization reduction treatment was in a depolymerized state by the degree of polymerization reduction treatment. Then, it is presumed that by chemically treating the pulp in this state, the hydrogen bonds between the fibers of the sulfamic acid / urea-treated pulp after the chemical treatment were loosened, and the fibers were in a state of being easily loosened.
Therefore, it was presumed that the nanocellulose fibers having the desired viscosity and transparency could be prepared even with a low defibration pressure and a small number of defibrations (number of passes). In other words, the degree of polymerization reduction treatment shortened the cellulose molecular chains and increased the number of fiber breaks. Then, it is presumed that the above phenomenon occurs because the crystal region in which the fibers are lined up is reduced and the hydrogen bond is loosened.
Then, if the number of defibration (number of passes) is changed while the defibration pressure is constant, the number of defibration (number of passes) even though the degree of polymerization of each sample of nanocellulose fiber is almost the same level. The viscosity tends to decrease when the number of passes exceeds 5 passes. Therefore, it is presumed that the average degree of polymerization of each nanocellulose fiber represents the average degree of polymerization of the finer cellulose fibers (corresponding to the unit fibers in the present embodiment) constituting the nanocellulose fibers.
That is, from the experimental result that the viscosity of each nanocellulose fiber was different even though the average degree of polymerization was almost the same, the fine cellulose fibers (unit fibers) constituting the nanocellulose fibers were adjacent unit fibers. The connection between the fibers is broken according to the number of fibers (number of passes). As a result, it was presumed that the viscosity decreased as the number of defibration (number of passes) increased.

From the above results, it was confirmed that by using the method for producing a sulfonated fine cellulose fiber of the present invention, a sulfonated fine cellulose fiber having a desired excellent viscosity can be appropriately and efficiently produced. For example, it was confirmed that the viscosity of the sulfonated fine cellulose fiber can be controlled by adjusting the number of defibration.
Moreover, it was confirmed that the obtained sulfonated fine cellulose fibers were in a state in which a plurality of unit fibers having an average degree of polymerization near the level-off degree of polymerization of cellulose were linked.

<Experiment 2>
(Viscosity and fiber morphology)
Nanocellulose fibers (hereinafter referred to as CNF) having different degrees of polymerization were prepared by the treatment for lowering the degree of polymerization, and the influence of the degree of polymerization on the viscosity and fiber morphology was confirmed. That is, focusing on the relationship between the degree of polymerization and the fiber morphology and viscosity, it was confirmed that a CNF having a low degree of polymerization but a high viscosity can be prepared.

As Sample 1, Sample B (viscosity: 17396 mPa · s) of Experiment 1 was used.
As Sample 2, Sample G (viscosity: 3000 mPa · s) prepared by further defibrating Sample F with high strength was used. Sample G was further defibrated at 100 MPa five times using the same defibrator for sample F (15 passes).

(Preparation of Comparative Sample 1)
As Comparative Sample 1, a sulfonated pulp which had not been subjected to the degree of polymerization reduction treatment was prepared. It was carried out in the same manner as in Example 1 except that the following conditions and the treatment for lowering the degree of polymerization were not carried out.
The mixing ratio of sulfamic acid and urea was such that the concentration ratio (g / L) was 2: 5 (1: 2.5). Specifically, 100 ml of water is added to the container, and then 20 g of sulfamic acid and 50 g of urea are added to this container, so that the sulfamic acid / urea ratio ((g / L) / (g / L)) is 200/500 (. The reaction solution of 1: 2.5) was prepared. That is, urea was added so as to be 50 parts by weight with respect to 100 parts by weight of sulfamic acid.
The slurry was adjusted so that the ratio of the chemicals in the reaction solution to each pulp (dry weight) was pulp: chemicals = 1: 2.5 with respect to the prepared reaction solution.
In the experiment, in the case of a reaction solution having a sulfamic acid / urea ratio ((g / L) / (g / L)) of 200/500 (1: 2.5), the reaction solution was compared with 1 g of pulp in terms of absolute dryness. Was added in an amount of about 3.6 g and impregnated with a chemical.
The defibration treatment was carried out using NanoVatter (manufactured by Yoshida Kikai Kogyo Co., Ltd., L-ES008-D10) twice at a defibration pressure of 10 MPa, once at a defibration pressure of 50 MPa, and five times at a defibration pressure of 60 MPa. Hereinafter, when the pressure MPa is simply described, it means the defibration pressure of the defibration processing device used for the defibration treatment.
The viscosity was measured using a B-type viscometer (Eiko Seiki Co., Ltd., model number; RV-DV2T) after sealing and leaving it at room temperature for 24 hours, and then putting 50 g into a polypropylene 50 mL falcon tube (manufactured by Corning Inc.). ..
The measurement conditions for viscosity measurement were as follows.
Measurement conditions: Rotation speed 6 rpm, measurement temperature 25 ° C, measurement time 3 minutes, spindle RV-06

(Result of Experiment 2)
The amount of sulfur introduced due to the sulfo group of the sulfonated fine cellulose fiber of the prepared comparative sample 1 was 1.0 mmol / g.
Conventionally, it has been considered that a CNF obtained when the degree of polymerization is low has a short fiber length, and a high-viscosity CNF cannot be obtained. Therefore, in the conventional technique, in order to obtain a CNF having a high viscosity like a gel, a chemical treatment is performed so that the degree of polymerization does not decrease, and then a defibration treatment is performed. By performing such a treatment, a transparent and highly viscous CNF is obtained.
Considering such common general knowledge, it is considered necessary to maintain a high degree of polymerization in the preparation of a transparent and highly viscous CNF.

On the other hand, in the present invention, focusing on the relationship between the degree of polymerization and the fiber morphology and viscosity, it was examined whether or not it is possible to produce a CNF having a low degree of polymerization and a high viscosity.
As a sample, pulps having different degrees of polymerization were prepared depending on the presence or absence of the degree of polymerization reduction treatment. Each of the prepared pulps was defibrated to examine the degree of polymerization, fiber morphology, and viscosity for each defibration condition.
Table 2 shows the characteristics of the sample and various measurement results.

As the definition of CNF in the present invention, it was considered that the preparation of CNF (that is, defibration) was completed when the haze was 20% or less.
From the results of Table 1 of Experiment 1 and Table 2 of this experiment, it is possible to prepare a CNF having a degree of polymerization of around 300 after defibration by performing a degree of polymerization reduction treatment (alkaline treatment) before the sulfonation step. did it. Looking at the degree of polymerization and the change in viscosity of each of these pulps in each defibration state, the degree of polymerization hardly changed in each defibration state, but the viscosity once reached a maximum value and then decreased.
Further, from Table 1 of Experiment 1 and Table 2 of this experiment, the maximum value of the viscosity of CNF exceeds 15,000 mPa · s regardless of the presence or absence of the degree of polymerization reduction treatment, and the degree of polymerization of Comparative Sample 1 is high. It turns out that it is similar to 400 CNFs. From these facts, it was suggested that the viscosity does not completely depend on the degree of polymerization.

FIG. 3 shows the observation results of the fiber morphology of CNFs having almost the same degree of polymerization and different viscosities.
The photograph of FIG. 3 (A) is sample 1, and the photograph of FIG. 3 (B) is sample 2.
In Sample 1 having a viscosity of 10000 mPa · s or more, a state in which fibers having a width of several nm and a fiber length of 1 μm to 2 μm were often entangled was observed ((A) in FIG. 3). On the other hand, in Sample 2 having a viscosity of about 3000 mPa · s, there was little entanglement between the fibers, and many fibers having a width of several nm and a length of about 500 nm were observed ((B) in FIG. 3).
From this result, it was confirmed that the influence on the viscosity is not the degree of polymerization, but how to prepare a CNF having a long fiber length and a lot of entanglement between fibers in the defibration process is important.

From the above results, using the production method of the present invention, the sulfonated fine cellulose fiber of the present invention corresponds to the cellulose molecule (the unit fiber of the present embodiment) which has been subjected to the degree of polymerization reduction treatment, as shown in the image diagram of FIG. It was confirmed that a CNF having a width of several nm and a length of several μm, which is composed of (the left figure of FIG. 4), can be prepared.
That is, in the above results, it is considered that CNFs having different lengths of cellulose molecules constituting CNF can be prepared even if the viscosities are the same (the lengths of CNFs are the same). Since it is a CNF having these structures, it was confirmed that a high viscosity can be developed in spite of a low degree of polymerization.

<Experiment 3>
(Color test)
In Experiment 3, the effect of the degree of polymerization reduction treatment on the color was confirmed.
In this experiment, sulfonated fine cellulose fibers (Sample 3) were prepared in the same manner as in Experiment 1 except for the conditions described below.
A high-pressure homogenizer (model number N2000-2C-045 type) manufactured by Kos Nijuichi Co., Ltd. was used for the defibration treatment.
The solid content concentration of the sulfonated pulp was adjusted to 1.0%.
The defibration treatment was performed twice at a defibration pressure of 10 MPa, once at 50 MPa, and three times at 60 MPa.
The color of Sample 3 was measured using a spectroscopic haze meter.

(Measurement of color)
The tint was measured as follows using a spectroscopic haze meter (manufactured by Nippon Denshoku Kogyo Co., Ltd., model number; SH-7000).
An optional glass cell (part number: 2277, square cell, optical path length 10 mm × width 40 × height 55) of the spectroscopic haze meter was used for measuring the light transmittance of the pulp slurry for measurement. For the blank measurement, the value obtained by adding pure water to the glass cell of the spectroscopic haze meter was used as the blank measurement value.
Measurement conditions: Light source D65, viewing angle 2 °, measurement wavelength range 380 to 780 nm
JIS set: JIS K 7105
For the L *, a *, and b * values, the values obtained by the control unit of the spectroscopic haze meter (model number CU-II, Ver2.00.02) were used.

(Measurement of degree of polymerization)
The degree of polymerization was measured in the same manner as in Experiment 1.

(Sample 4)
In this experiment, sample 4 was further prepared and the same analysis as sample 3 was performed.
Sample 4 is sample 3 except that the sulfamic acid / urea ratio ((g / L) / (g / L)) was set to 200/200 under the preparation conditions of the sulfonated pulp of Experiment 1 to prepare the sulfonated pulp. It was prepared in the same manner as above. In Sample 4, the amount of sulfur introduced due to the sulfo group was 1.5 mmol / g.

(Comparison sample 2)
Comparative sample 2 was prepared in the same manner as sample 3 except that the degree of polymerization reduction treatment was not performed. In Comparative Sample 2, the amount of sulfur introduced due to the sulfo group was 1.3 mmol / g.

(Result of Experiment 3)
The experimental results are shown in Table 3.

In addition to being highly transparent, CNF may be required to be colorless. Therefore, the effect of the degree of polymerization reduction treatment on the color of CNF was investigated.
As a result, the degree of polymerization of each of Samples 3 and 4 was around 300, and the degree of polymerization of Comparative Sample 2 was about 400.
Table 3 shows the effect of CNF on the tint depending on the presence or absence of the degree of polymerization reduction treatment.
Samples 3 and 4 that had been subjected to the degree of polymerization reduction treatment tended to have higher L * values than the comparative sample 2. Further, since the a * value and the b * value tend to be close to 0, it can be seen that both are colorless. It was presumed that this was because the components that are the source of color were removed when the pulp that had been subjected to the degree of polymerization reduction treatment was sulfonated.

<Experiment 4>
(Biodegradability test)
CNFs produced by extracting cellulose from wood and mechanically treating them are said to be biodegradable, but those that have been chemically treated are often unclear. Biodegradability is degradation by bacteria and enzymes in nature, and CNF is degraded if there is no problem with the substrate specificity of the degrading enzyme. However, if this is not the case, there are problems, for example, in the biodegradability of chemically modified cellulose. As a specific example, cellulose acetate obtained by esterifying cellulose has biodegradability, but it is said that the decomposition rate is slower than that of cellulose.
Considering the use of CNF in terms of environmental load, it is desirable that CNF has excellent biodegradability as much as possible.
Therefore, in this experiment, it was investigated whether the sulfonated fine cellulose fiber had biodegradability, and based on the result, the effect of the degree of polymerization on biodegradability was confirmed.

(Sample 5)
Sulfonized pulp was prepared by the same method as in Experiment 1, and the obtained sulfonated pulp was defibrated to prepare sulfonated fine cellulose fibers (Sample 5).
The defibration treatment was performed using a high-pressure homogenizer NanoVator (manufactured by Yoshida Kikai Kogyo Co., Ltd., model number: L-ES008-D10).
The solid content concentration of the sulfonated pulp subjected to the defibration treatment was 0.5%.
First, it was carried out twice at 10 MPa and once at 50 MPa. Then, it was treated three times at 60 MPa. The prepared sample 5 was subjected to the following enzyme treatment.

(Measurement of Degradability of Sample 5 by Enzyme Treatment)
20 g of sample 5 prepared at 0.5% and 30 g of 0.1 M acetate buffer were placed in a polypropylene 50 mL falcon tube (manufactured by Corning), and cellulase derived from Aspergillus niger, Nacalai Tesque product code: 07550-74) 10 mg was added, and the mixture was placed in a constant temperature bath set at 50 ° C. and reacted for 24 hours.
At each reaction time, the solution was appropriately recovered, filtered through a 0.1 μm filter, and then sugar analysis was performed by HPLC.

A high performance liquid chromatograph (10A series) manufactured by Shimadzu Corporation was used for sugar analysis. As the column, a sugar analysis column (Aminex sugar analysis column: HPX-87P) manufactured by Bio-Rad Laboratories was used. A differential refractometer was used for detection. Ultrapure water was used as the mobile phase.
The detection time was confirmed using glucose as a reference substance.
The peak obtained after the enzyme treatment of sample 5 and the HPLC detection peak of glucose were approximately the same time. Therefore, it was presumed that glucose was obtained from sample 5 by the enzyme treatment. The quantification was calculated by preparing a calibration curve with glucose of known concentration.

(Comparison sample 3)
The comparative sample 3 was prepared by the same production method as that of the comparative sample 1 of the experiment 2 except that the defibration treatment was set as follows.
The solid content concentration of the sulfonated pulp subjected to the defibration treatment was 0.5%.
First, it was carried out twice at 10 MPa and once at 50 MPa. Then, it was treated 5 times at 60 MPa.
The prepared comparative sample 3 was subjected to enzyme treatment in the same manner as in sample 5, and sugar analysis was carried out in the same manner.
As the blank sample, NBKP manufactured by Marusumi Paper Co., Ltd. (indicated as NBKP in FIG. 5, hereinafter simply referred to as NBKP) was used.

(Result of Experiment 4)
To confirm the biodegradability, cellulase having a cellulolytic activity was used.
The degree of polymerization of sample 5 was around 300, the degree of polymerization of comparative sample 3 was around 400, and the degree of polymerization of NBKP was around 900.
FIG. 5 shows the results of the enzymatic decomposition rate.
CNF was converted to glucose over time, regardless of the degree of polymerization.
From the results of this experiment, it was confirmed that the sulfonated cellulose can be decomposed by cellulase and has biodegradability.
Comparing the decomposition rates in the degree of polymerization, the glucose yield of sample 5 (triangle in FIG. 5) was 1.6% → 4.1% → 10.9%. The yield of Comparative Sample 3 (square in FIG. 5) is 1.6% → 4.4% → 9.9%, and the yield of NBKP (diamond in FIG. 5) is 1.5% → 3. It was 7% → 7.7%.
From this result, it was confirmed that CNF having a lower degree of polymerization has better biodegradability than CNF having a high degree of polymerization.
In the experiment in which the amount of enzyme was increased 10 times, the glucose yield of sample 5 exceeded 80% after 24 hours.

<Experiment 5>
(Dispersibility test)
Generally, it is considered that the structure and chemical properties of CNF are derived from the viscosity and TI value. Therefore, if such viscosity properties can be controlled, the application development of CNF can be further expanded. You can expect it. In this experiment, it was confirmed that CNF having a predetermined viscosity characteristic can be controlled by performing the degree of polymerization reduction treatment.
From the results of Experiment 2, it was clarified that as the defibration strength was increased, the sulfonated pulp once reached a maximum viscosity and then decreased.
Therefore, in this experiment, we tried to confirm the change in the dispersion state (viscosity change) of the fibers at a more delicate level by setting the defibration strength to a lower value.
In this experiment, the effect of the degree of polymerization reduction treatment on the dispersibility was judged from the viscosity characteristics of each defibration condition.

(Sample 6)
The sulfonated fine cellulose fiber (Sample 6) of this experiment was prepared by the same method as in Experiment 1 except as shown below.
Under the conditions of Sample B in Experiment 1, the degree of polymerization reduction treatment step was prepared so that the solid content concentration of the pulp was 10% by mass in a 25% by mass sulfuric acid solution. Then, it was neutralized with 0.5 M aqueous sodium hydroxide solution. Then, the sulfamic acid / urea ratio ((g / L) / (g / L)) is set to 200/200, and the ratio of the chemicals in the reaction solution to each pulp (dry weight) is pulp: chemicals = 1:10. The slurry was adjusted in this manner so that the reaction temperature was 140 ° C. and the reaction time was 30 minutes.

The defibration treatment was performed using a high-pressure homogenizer (NanoVatter, manufactured by Yoshida Kikai Kogyo Co., Ltd., model number L-ES008-D10).
As for the defibration conditions, the solid content concentration of the pulp was set to 0.5%, the defibration strength was set to 1 to 7 steps, and defibration was performed. The defibration strength was set as follows.
Defibering strength 1:10 MPa once,
Twice at a defibration strength of 2:10 MPa,
Defibering strength 3:10 MPa 3 times,
Defibering strength 4:10 MPa 4 times,
Defibering strength 5:10 MPa 3 times + 30 MPa twice,
Defibering strength 6: 10 MPa 3 times + 30 MPa 2 times + 60 MPa 2 times,
Defibering strength 7: 7 times at 100 MPa,
The higher the set value of the defibration strength, the stronger the defibration strength.
The viscosity was measured in the same manner as in Comparative Sample 1 of Experiment 2.
The degree of polymerization was measured in the same manner as in Experiment 1. CNF prepared with a defibration strength of 6 was used.
The total light transmittance and the haze value were measured in the same manner as in Experiment 3 except that the solid content concentration of CNF was set to 0.5%.

(Comparison sample 4)
The comparative sample 4 was prepared by the same production method as that of the comparative sample 1 of the experiment 2 except that the defibration treatment was made under the following conditions.

As for the defibration conditions, the solid content concentration of the pulp was set to 0.5%, the defibration strength was set in 1 to 9 steps, and defibration was performed.
The defibration strength was set as follows.
Disintegration strength 1: The 200 ml capacity was set to 2 minutes by setting the strength of a Panasonic mixer (model number: MX-X701).
The defibration strengths 2 to 9 were set as follows in addition to the treatment of the defibration strength 1.
Once at a defibration strength of 2:10 MPa,
Defibering strength 3 times at 3:10 MPa + 2 times at 30 MPa,
Defibering strength 4:10 MPa 3 times + 30 MPa 2 times + 60 MPa 1 time,
Defibering strength 5:10 MPa 3 times + 30 MPa 2 times + 60 MPa 2 times,
Defibering strength 6: 10 MPa 3 times + 30 MPa 2 times + 60 MPa 4 times,
Defibering strength 7:10 MPa 3 times + 30 MPa 2 times + 60 MPa 2 times + 100 MPa 2 times,
Defibering strength 8:10 MPa 3 times + 30 MPa 2 times + 60 MPa 2 times + 100 MPa 5 times,
Defibering strength 9:10 MPa 3 times + 30 MPa 2 times + 60 MPa 2 times + 100 MPa 10 times,
The higher the set value of the defibration strength, the stronger the defibration strength. That is, the comparative sample 4 was defibrated with a defibration strength stronger than that of the sample 6.
The viscosity, transparency, and degree of polymerization of the obtained comparative sample 4 were measured in the same manner as in sample 6.
A sulfonated fine cellulose fiber obtained with a defibration strength of 5 was used for measuring the degree of polymerization.

(Experimental result of Experiment 5)
Table 4 shows the relationship between the degree of polymerization of the sulfonated pulp before defibration and the degree of polymerization of the sulfonated fine cellulose fibers (hereinafter, simply referred to as CNF) after the defibration treatment. In sample 6, the amount of sulfur introduced due to the sulfo group was 1.6 mmol / g.

As shown in Table 4, the degree of polymerization of the sulfonated pulp subjected to the degree of polymerization reduction treatment was 420, and the degree of polymerization of the sulfonated pulp not subjected to the degree of polymerization reduction treatment was 860.

The fact that CNF exhibits thickening (high viscosity) and high TI value means that the fibers are entangled with each other in a stationary state to develop viscosity, but when a predetermined force is applied, the entanglement of the fibers is broken. That's what it means. At this time, the entanglement of the fibers that contributes to the viscosity is considered to be stronger as the fiber width is narrower and longer. On the other hand, in order to prepare a CNF that has a high viscosity (high viscosity) and exhibits a low TI value, a dispersed state in which the entanglement of fibers does not break even when a predetermined force is applied as compared with a CNF having a high TI value. Must be formed.
Therefore, in this experiment, the dispersed state of CNFs having different degrees of polymerization was judged from the viscosity and the TI value.

Table 5 shows changes in viscosity, TI value, and transparency (total light transmittance and haze value) at each defibration strength. FIG. 6 shows the relationship between the defibration strength setting and the viscosity and TI value.

In the sample 6 subjected to the low polymerization degree treatment, the haze when the defibration treatment was performed in the defibration setting 2 was 20% or less, so that it was confirmed that CNF was appropriately obtained. The degree of polymerization of CNF after defibration was about 330. No significant change in the degree of polymerization was confirmed after the coarse fibers were no longer observed in the visual observation of the CNF dispersion.
As shown in FIG. 6A, it was clarified that in the defibration treatment in which the defibration pressure was set to 10 MPa, two maximum viscosity points were generated as the viscosity behavior of the sample 6. The values of viscosity at these two maximum points showed similar values. As shown in Table 5 and FIG. 6 (B), the range of the TI value when the viscosity takes the maximum point is 1.5 to 4 for the first maximum point (first maximum point), and two. It was found that the TI value at the second maximum point (second maximum point) was 4 or more. That is, since different TI values are exhibited in the CNF exhibiting the same viscosity, the dispersed state of the fibers is different between the CNF in the range including the first maximum point and the CNF in the range including the second maximum point. It was speculated that there was.

Table 6 shows changes in viscosity, TI value, and transparency (total light transmittance and haze value) at each defibration intensity of Comparative Sample 4.
FIG. 7 shows the relationship between the defibration strength setting of the comparative sample 4 and the viscosity and TI value.

In Comparative Sample 4, the degree of polymerization of the pulp before defibration was high (800 or more), and CNF could not be obtained with the same defibration strength setting as in Sample 6. Therefore, in Comparative Sample 4, defibration was performed with a stronger strength setting.
As shown in FIG. 7, in the comparative sample 4 using the pulp having a high degree of polymerization, the haze was 20% or less after the defibration strength setting was 3 (corresponding to the defibration strength setting 5 of the sample 6). That is, in Comparative Sample 4, it was confirmed that CNF was obtained for the first time at this set intensity. The obtained comparative sample 4 had a degree of polymerization of about 400 and a TI value of about 3.5, which tended to be higher than that of sample 6.

From the above results, it was confirmed that in this production method, CNF can be prepared with a lower defibration strength setting than in the conventional production method. That is, it was confirmed that when this production method is used, a transparent CNF can be obtained with lower energy than the conventional production method.
Further, in this production method, it was confirmed that a CNF having a high viscosity and a low TI value can be prepared in a region having a low defibration strength, and a CNF having a high viscosity and a high TI value can be prepared in a region having a high defibration strength. It is considered that the CNF having a high TI value has a structure in which the entanglement of the CNF is easily disentangled when a force is applied to the dispersion of the CNF. On the contrary, the CNF having a low TI value is considered to have a structure in which the entanglement of the CNF is difficult to be disentangled when a force is applied to the dispersion.
Therefore, it was confirmed that in this production method, CNF can be prepared with lower energy than the conventional production method, and moreover, CNF having a different structure can be prepared while being controlled. Furthermore, it was also confirmed that the CNF obtained by this production method can exhibit high transparency when dispersed at a predetermined concentration. Further, it was confirmed that by using this production method, it is possible to prepare while controlling CNF exhibiting a low TI value or a high TI value while having a high viscosity (viscosity) despite a low degree of polymerization.

<Experiment 6>
(Deflexibility)
The effect of the degree of polymerization reduction treatment on the defibration of the sulfonated pulp was confirmed.
In this experiment, the effect of sulfonated pulp on the defibration property was evaluated from the relationship between the defibration energy and the degree of polymerization in the defibration treatment.
In this experiment, the defibration energy (J) required for the defibration treatment was determined based on the equipment used for the defibration treatment and the prepared sample 6 (sample 6 of the experiment 5).

(Calculation of defibration energy (J))
First, the completion of the pulp defibration treatment was defined as when the haze value of the CNF dispersion (CNF slurry) obtained after the defibration treatment became 20% or less, as in other experiments. This is because if the haze value of the dispersion is 20% or less, the fiber width of the obtained CNF is about 20 nm or less, and when the slurry is visually evaluated, it can be judged to be transparent. ..

Next, the concept of the energy applied to the pulp at each pressure of the high-pressure homogenizer used for defibration is shown below.
The nozzle of the defibrating equipment used in the experiment (manufactured by Yoshida Kikai Kogyo Co., Ltd., nozzle model number: cross type XT260, groove width 160 μm) was constant, and water was used as the liquid used. Therefore, at each pressure, the kinetic energy was obtained from the flow velocity of the water flowing in the pipe, this kinetic energy was regarded as the defibration energy, and each defibration state was compared based on this defibration energy.
In this calculation method, when the solvent is water, the flow path, etc. is constant, the theoretical flow velocity change due to the pressure change of the water flowing in the horizontal pipe is considered, so there is no influence of viscosity or pipe resistance. ..

The flow rate and flow velocity of the liquid flowing in the pipe can be calculated from the following equations.

Q = C × A × V
(Q: flow rate, C: flow coefficient, V: flow rate, A: flow path area)
From the application of Bernoulli's theorem, the flow velocity is calculated by the following formula.
V = (2 × P ÷ ρ) ^ 0.5
(P: Pressure differential pressure, ρ: Fluid density)

From this equation, it can be seen that the flow rate is proportional to (2 × P ÷ ρ) ^ 0.5 (that is, proportional to the square root of the pressure P) and inversely proportional to the square root of the density ρ.
When resistance is not taken into consideration, this pressure P can be considered to be proportional to the change in the set pressure of the defibrating equipment (high pressure homogenizer). That is, when the set pressure changes from 10 MPa to 30 MPa, the flow velocity V triples the pressure from the application of Bernoulli's theorem of V = (2 × P ÷ ρ) ^ 0.5. Therefore, the flow velocity V becomes √3 times (that is, the flow velocity is 1.73 times) (at this time, the fluid density and the flow coefficient are constant). Then, if the energy of this fluid is calculated based on the equation of kinetic energy (kinetic energy = 1/2 mv ² ), the energy of this fluid can be obtained as the energy given to the pulp of this experiment as the defibrating force.

Next, the concept of flow velocity and kinetic energy at each pressure will be described.
The maximum set pressure of the defibrator (high pressure homogenizer) used in this experiment is 200 MPa, and the discharge flow rate at that time is 7.8 mL / s. Since the flow rate is proportional to the square root of the pressure P, the flow rate at 10 MPa is 7.8 / √20≈1.74 cm ³ / s. Table 7 shows each pressure and the flow rate at that time.

The nozzle groove width (nozzle diameter) of the defibrating equipment used in the experiment was 160 μm. Assuming that this nozzle is a circular pipe, the flow path area is 0.0002096 cm ² . Then, the flow velocity can be calculated from the flow velocity (cm / s) = (flow rate (cm ³ / s)) / (flow path area (cm ^2)).
Table 8 shows the flow velocity of each pressure.

From the values in Table 8, when the pressure setting is 10 MPa and the one-shot set amount of the device is 3.9 mL, 3.9 g of water (water density is 1 g / cm ³ ) is 8679 cm / s in the pipe with a nozzle diameter of 160 μm. It is moving in. When the kinetic energy at each pressure under these conditions is calculated from the following formula, the kinetic energy (J) at each pressure is as shown in Table 9.

Kinetic energy (J) = 1/2 mv ²
(M: mass (kg), v: velocity (m / s))

The kinetic energy (J) shown in Table 9 is the defibration energy (J) given to the pulp when the pulp is defibrated.
Then, based on the relationship between the pressure and the kinetic energy (J) shown in Table 9, the defibration energy (J) at each defibration set intensity in this experiment is calculated. The effect of the number of treatments on each defibration set strength was calculated by multiplying the defibration energy (J) value per treatment at each pressure by the number of treatments. The calculation result is shown in FIG.

FIG. 8 shows the relationship between the defibration energy required to prepare each CNF and the degree of polymerization.
The graph of FIG. 8 (B) is an enlargement of the range surrounded by the dotted line of the graph of FIG. 8 (A). The arrow in FIG. 8 indicates a line with a haze value of 20%. Above this line, the pulp is in an unfibered state, and below this line, defibration is completed and appropriate. CNF has been obtained.
In sample 6 (square in FIG. 8), the defibration energy was around 20 J, and the haze value was already below 20%. On the other hand, in the comparative sample 4 (circled in FIG. 8), a defibration energy of 130 J or more was required for the haze value to be 20% or less.
From the above results, it can be confirmed that the sample 6 has excellent defibration property as compared with the comparative sample 4 because the energy required for defibration can be set to be much lower than that of the comparative sample 4. It was.

Therefore, if the production method of the present invention is used, the sulfonated fine cellulose fiber of the present invention (hereinafter, simply referred to as CNF of the present invention) exhibits a low TI value that is difficult to prepare from pulp having a high degree of polymerization by the treatment for reducing the degree of polymerization. Can be obtained. Further, it was confirmed that the CNF of the present invention exhibiting such high transparency and high viscosity can be prepared with much less energy than the conventional production method.
Further, by using the production method of the present invention, the obtained CNF of the present invention can form various dispersed forms because the fiber becomes supple due to the low degree of polymerization. Moreover, the manufacturing energy is very low.
The CNF of the present invention exhibits viscosity characteristics that cannot be obtained by conventional techniques. Specifically, the CNF of the present invention is one that exhibits high transparency, high viscosity, and low TI value, or one that exhibits high transparency, high viscosity, and high TI value. The former CNF of the present invention has a fiber structure that maintains the entanglement of fibers even when an external stress is applied, and the latter CNF of the present invention breaks the entanglement of fibers due to external stress. It has an easy fiber structure.
Therefore, the CNF of the present invention obtained by the production method of the present invention can be used in various fields. For example, it can be applied to drug delivery systems and can maintain the morphology of fibers, so it can be used in industries such as high-viscosity products such as jelly, food viscosity adjusters, cosmetic thickeners, paints and hydraulic oils for hydraulic equipment. It can be used for products, experimental devices such as cells and microorganisms, virus media, crystallization nucleating agents, soft materials such as contact lenses, and seismic isolation devices such as oil damping members and seismic isolation devices.
Further, in the production method of the present invention, the TI value (that is, the fiber structure) of the CNF of the present invention obtained can be controlled by adjusting the defibration energy (J) as described above. Therefore, it can be expected to be used as a thickening control agent such as grease.

The method for producing a sulfonated fine cellulose fiber and a sulfonated fine cellulose fiber of the present invention can be suitably used for many uses in various fields such as an industrial field, a food field, a medical field, and a cosmetics field, and these fields It can also be suitably used as a raw material for a composite material used in the above.

S1 Degree of polymerization reduction treatment step S2 Chemical treatment step S3 Fine treatment step

Claims (12)

Cellulose fiber is a finely divided fine cellulose fiber,
A part of the hydroxyl group of the fine cellulose fiber is substituted with a sulfo group.
The amount of sulfur introduced due to the sulfo group was adjusted to be higher than 0.42 mmol / g.
The fine cellulose fiber is
It is a fiber consisting of a plurality of unit fibers and having an average fiber width of 30 nm or less.
The fine cellulose fibers are dispersed in a water-soluble solvent so that the solid content concentration is 0.5% by mass, and the viscosity of the dispersion liquid at 25 ° C. is 5,000 mPa · s or more. Sulfated fine cellulose fibers. The fine cellulose fiber is
In a state where the fine cellulose fibers are dispersed in a water-soluble solvent so that the solid content concentration is 0.5% by mass, the dispersion liquid is rotated at a temperature of 25 ° C. and a rotation speed of 6 rpm using a B-type viscometer. It is characterized in that the thixotropy index calculated from each viscosity ratio (viscosity at 6 rpm / viscosity at 60 rpm) is 5.0 or more after measurement is performed at several 60 rpm and each viscosity is calculated. The sulfonated fine cellulose fiber according to claim 1. The fine cellulose fiber is
In a state where the fine cellulose fibers are dispersed in a water-soluble solvent so that the solid content concentration is 0.5% by mass, the dispersion liquid is rotated at a temperature of 25 ° C. and a rotation speed of 6 rpm using a B-type viscometer. The thixotropy index calculated from each viscosity ratio (viscosity at 6 rpm / viscosity at 60 rpm) is lower than 5.0 by measuring at several 60 rpm and calculating each viscosity. The sulfonated fine cellulose fiber according to claim 1, wherein the sulfonated fine cellulose fiber. The sulfonated fine cellulose fiber according to claim 1, 2 or 3, wherein the unit fiber has an average degree of polymerization of 350 or less. The fine cellulose fiber is
Claims 1, 2 and 1. The haze value of the dispersion liquid in a state where the fine cellulose fibers are dispersed in a water-soluble solvent so that the solid content concentration is 0.5% by mass is 10% or less. 3 or 4 sulfonated fine cellulose fibers. The fine cellulose fiber is
Claim 1, characterized in that the total light transmittance of the dispersion liquid in a state where the fine cellulose fibers are dispersed in a water-soluble solvent so that the solid content concentration is 0.5% by mass is 90% or more. 2, 3, 4 or 5 sulfonated fine cellulose fibers. The fine cellulose fiber is
In the dispersion liquid in which the fine cellulose fibers are dispersed in a water-soluble solvent so that the solid content concentration is 0.5% by mass.
^{The L *} a ^* b ^* color space calculated in accordance with JIS Z 8781-4: 2013 is the following L ^* , a ^* and b ^* :
L ^* is 95% or more,
a ^* is -5 or more and +5 or less,
b ^* is -5 or more and +5 or less,
The sulfonated fine cellulose fiber according to any one of claims 1 to 6, wherein the sulfonated fine cellulose fiber satisfies at least one of the above. The method for producing a sulfonated fine cellulose fiber according to any one of claims 1 to 7 from pulp.
It is a method of sequentially performing a chemical treatment step of chemically treating the pulp and a miniaturization treatment step of refining the pulp after the chemical treatment step.
A method for producing a sulfonated fine cellulose fiber, which comprises a degree-reducing treatment step of depolymerizing the pulp before the chemical treatment step. In the polymerization degree reduction treatment step,
The method for producing a sulfonated fine cellulose fiber according to claim 8, wherein the pulp is brought into contact with or immersed in an acid solution or an alkaline solution to depolymerize the pulp. In the miniaturization processing step
The method for producing a sulfonated fine cellulose fiber according to claim 8 or 9, wherein the pulp after the chemical treatment step is supplied to a defibration apparatus and pulverized at a defibration pressure of 5 MPa to 60 MPa. In the miniaturization processing step
The method for producing a sulfonated fine cellulose fiber according to claim 10, wherein the number of times of repeatedly defibrating the pulp after the chemical treatment step with a defibration apparatus is 30 passes or less. In the miniaturization processing step
The method for producing a sulfonated fine cellulose fiber according to claim 8, 9, 10 or 11, wherein 1000 J or less of defibration energy is applied to the pulp after the chemical treatment step.

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