JP7767751B2 - Dispersion and Sheet - Google Patents

Description

The present invention relates to a dispersion and a sheet. Specifically, the present invention relates to a dispersion and a sheet containing fine fibrous cellulose.

Cellulose fibers have traditionally been widely used in clothing, absorbent articles, paper products, and more. In addition to fibrous cellulose with a fiber diameter of 10 μm to 50 μm, fine fibrous cellulose with a fiber diameter of 1 μm or less is also known as a cellulose fiber. Fine fibrous cellulose has attracted attention as a new material, and its applications are diverse. For example, development is underway for sheets, resin composites, and thickeners containing fine fibrous cellulose.

Fine fibrous cellulose can be produced by defibrating cellulose fibers. However, because cellulose fibers are strongly bonded to each other by hydrogen bonds, simply defibrating the fibers requires a huge amount of energy to obtain fine fibrous cellulose. For this reason, it is known that in order to produce fine fibrous cellulose with less defibration energy, it is effective to combine defibration with pretreatment such as chemical or enzymatic treatment. For example, it is known that introducing ionic substituents into cellulose fibers through chemical treatment of cellulose fibers can easily finely reduce the cellulose fibers, and further improve dispersion stability after finely reducing the fibers.

When fine fibrous cellulose is made into a dispersion, it is often in the form of a low-concentration dispersion to ensure its dispersion stability. However, in such cases, most of the material used for transportation and storage is a solvent such as water, which tends to increase the transportation and storage costs per unit quantity of fine fibrous cellulose. For this reason, in order to reduce the transportation and storage costs per unit quantity of fine fibrous cellulose, one approach that has been considered is to add a flocculant to the fine fibrous cellulose dispersion to agglomerate the fine fibrous cellulose, and then transport and store the resulting agglomerates. When using these agglomerates, they are redispersed in a solvent such as water, and the resulting redispersion is used for various purposes.

For example, Patent Documents 1 and 2 disclose fine fibrous cellulose-containing materials that contain polyvalent metals as flocculants. In the examples of these documents, improving redispersibility is studied by redispersing the fine fibrous cellulose-containing materials under alkaline conditions. Furthermore, Patent Document 3 discloses a process for adding a certain amount of monovalent or divalent metal ions to an anion-modified cellulose nanofiber dispersion, thereby improving the fluidity of the cellulose nanofiber dispersion.

Patent Document 4 discloses a sheet containing fine fibers having ionic substituents and a divalent or higher metal. It is studied here how to provide a sheet that maintains high transparency, does not swell or break even under humid conditions, and has a certain level of strength by treating a substituted fine fiber-containing sheet with a divalent or higher metal ion. Furthermore, Patent Document 5 discloses a cellulose film in which at least some of the carboxyl groups have been substituted with a metal. In Patent Documents 4 and 5, a metal ion treatment (ion exchange treatment) is carried out after the formation of a fine fiber-containing sheet or film.

Japanese Patent Application Laid-Open No. 2020-105471 International Publication No. 2014/024876 WO 2013/137140 Japanese Patent Application Laid-Open No. 2017-31548 JP 2016-210830 A

When a flocculant such as a polyvalent metal is added to a fine fibrous cellulose dispersion to obtain a fine fibrous cellulose aggregate, the aggregate is required to exhibit excellent redispersibility during use. In prior art, attempts have been made to improve the redispersibility of fine fibrous cellulose by making the solvent used for redispersion alkaline. However, in such cases, the resulting redispersion also becomes alkaline, posing problems such as limited uses for the redispersion and reduced ease of handling. Furthermore, when a sheet is formed from such a redispersion, the sheet yellows, which is an issue. On the other hand, when the redispersibility of the fine fibrous cellulose is insufficient, the transparency of the redispersion is poor, and the transparency of the sheet formed from such a redispersion is also insufficient.

In addition, as disclosed in Patent Documents 4 and 5, a technique is known in which polyvalent metal ions are introduced into a fine fibrous cellulose sheet (film) after it has been formed to improve the gas barrier properties of the sheet. However, when the fine fibrous cellulose sheet (film) is immersed in a solution for ion exchange treatment, the ion exchange may not proceed sufficiently, causing the sheet to yellow, and improvements have been sought.

In order to solve these problems of the prior art, the inventors have conducted research with the aim of providing a dispersion obtained by redispersing fine fibrous cellulose aggregates containing polyvalent metal ions, which can be used to form a sheet that is highly transparent and has excellent yellowing resistance.

Specifically, the present invention has the following configuration:

[1] A dispersion containing fibrous cellulose having an anionic group and a fiber width of 1000 nm or less,
the amount of the anionic group introduced into the fibrous cellulose is less than 0.5 mmol/g;
the fibrous cellulose has a polyvalent metal ion as a counter ion of the anionic group,
the electrical conductivity of the dispersion containing 0.4% by mass of the fibrous cellulose is 20 mS/m or less;
A dispersion having a haze of 10% or less when the dispersion contains 0.2% by mass of the fibrous cellulose.
[2] The dispersion according to [1], wherein the dispersion contains 0.2% by mass of the fibrous cellulose and has a total light transmittance of 85% or more.
[3] The dispersion according to [1] or [2], wherein the pH of the dispersion when containing 0.4% by mass of the fibrous cellulose is 4 to 9.
[4] The anionic group is at least one selected from the group consisting of a phosphorus oxoacid group, a substituent derived from a phosphorus oxoacid group, a sulfur oxoacid group, a substituent derived from a sulfur oxoacid group, a xanthate group, a substituent derived from a xanthate group, a carboxy group, and a substituent derived from a carboxy group. [1] The dispersion liquid according to any one of [3] to [4].
[5] The dispersion liquid according to any one of [1] to [4], wherein the anionic group is a phosphorus oxo acid group or a substituent derived from a phosphorus oxo acid group.
[6] The dispersion liquid according to any one of [1] to [5], wherein the polyvalent metal ion is at least one selected from the group consisting of calcium ions, magnesium ions, zinc ions, and aluminum ions.
[7] The dispersion according to any one of [1] to [6], wherein the fiber width of the fibrous cellulose is 10 nm or less.
[8] A sheet formed from the dispersion according to any one of [1] to [7].
[9] The sheet according to [8], having a thickness of 40 μm or more.
[10] The sheet according to [8] or [9], wherein when the sheet is heated at 160°C for 6 hours, the YI increase rate calculated by the following formula is 1600% or less;
YI increase rate (%)=(yellowing index of sheet after heating−yellowing index of sheet before heating)/yellowing index of sheet before heating×100
In the above formula, the yellowness of the sheet is the yellowness measured in accordance with JIS K 7373:2006.
[11] The sheet according to any one of [8] to [10], which has a total light transmittance of 70% or more.
[12] The sheet according to any one of [8] to [11], having a haze of 10% or less.

According to the present invention, a dispersion can be obtained by redispersing fine fibrous cellulose aggregates containing polyvalent metal ions, which can be used to form a sheet that is highly transparent and has excellent yellowing resistance.

FIG. 1 is a graph showing the relationship between the amount of NaOH dropped onto a slurry containing fibrous cellulose having phosphorus oxo acid groups and pH. FIG. 2 is a graph showing the relationship between the amount of NaOH dropped onto a slurry containing fibrous cellulose having a carboxy group and the pH.

The present invention will be described in detail below. The following explanation of the components may be based on representative embodiments and specific examples, but the present invention is not limited to such embodiments.

(Dispersion liquid)
This embodiment relates to a dispersion containing fibrous cellulose having anionic groups and a fiber width of 1000 nm or less. Here, the amount of anionic groups introduced into the fibrous cellulose is less than 0.5 mmol/g, and the fibrous cellulose has polyvalent metal ions as counterions of the anionic groups. The dispersion containing 0.4% by mass of fibrous cellulose has an electrical conductivity of 20 mS/m or less, and the dispersion containing 0.2% by mass of fibrous cellulose has a haze of 10% or less. In this specification, fibrous cellulose having a fiber width of 1000 nm or less is also referred to as fine fibrous cellulose or CNF.

The present embodiment has the above-described configuration, making it possible to form a sheet that is highly transparent and has excellent yellowing resistance. For example, when a sheet containing fine fibrous cellulose is formed from the dispersion of the present embodiment, a highly transparent sheet can be obtained. Furthermore, according to the present embodiment, yellowing of the sheet can be suppressed even when the sheet is heated. Specifically, in the present embodiment, when a sheet containing fine fibrous cellulose is formed and heated at 160°C for 6 hours, the YI increase rate can be kept low. In this way, by using the dispersion of the present embodiment, a sheet that is highly transparent and has excellent yellowing resistance can be formed.

When the dispersion contains 0.4% by mass of fibrous cellulose, the electrical conductivity of the dispersion may be 20 mS/m or less, preferably 15 mS/m or less, more preferably 10 mS/m or less, and even more preferably 5 mS/m or less. The lower limit of the electrical conductivity of the dispersion when the dispersion contains 0.4% by mass of fibrous cellulose is not particularly limited and may be 0 mS/m. By ensuring that the electrical conductivity of the dispersion is within the above range, a sheet with high transparency and excellent yellowing resistance can be formed, and in particular, a sheet that can exhibit excellent yellowing resistance even under high-temperature conditions can be formed. Furthermore, by ensuring that the electrical conductivity of the dispersion is within the above range, the ease of handling of the dispersion can be improved.

The electrical conductivity of a dispersion containing 0.4% by mass of fibrous cellulose is measured using an electrical conductivity meter. The temperature of the dispersion during measurement is 23°C. A CT-57101B manufactured by DKK-TOA Corporation can be used as the electrical conductivity meter. When diluting the dispersion to a concentration of 0.4% by mass, the electrical conductivity is measured after dilution with ion-exchanged water. When concentrating the dispersion to a concentration of 0.4% by mass, the electrical conductivity is measured after the dispersion is cross-flowed through a hollow fiber membrane with 0.1 μm openings at room temperature.

When the dispersion contains 0.4% by mass of fibrous cellulose, the electrical conductivity of the dispersion is equal to or less than a predetermined value, which indicates, for example, that there is a small amount of H ⁺ and OH- ^, which promote the discoloration of the fibrous cellulose. Furthermore, even if the pH of the dispersion is the same, a dispersion with low electrical conductivity has a low absolute amount of ionic substances and a low mobility in water. Thus, by setting the electrical conductivity of the dispersion to a predetermined value or less, the mobility of the ionic substances can be reduced, which makes it difficult for them to catalyze the polymerization reaction of the coloring component, thereby presumably achieving coloration suppression.

When the dispersion contains 0.2% by mass of fibrous cellulose, the haze of the dispersion may be 10% or less, preferably 8% or less, more preferably 7% or less, and even more preferably 6% or less. The lower limit of the haze of the dispersion when the dispersion contains 0.2% by mass of fibrous cellulose is not particularly limited and may be 0%. By maintaining the haze of the dispersion within the above range, a sheet with high transparency and excellent yellowing resistance can be formed. Having the haze of the dispersion within the above range means that the fibrous cellulose is well dispersible in the dispersion. Therefore, when the haze of the dispersion is within the above range, a sheet with even better transparency can be formed.

The total light transmittance of a dispersion containing 0.2% by mass of fibrous cellulose is preferably 85% or more, more preferably 87% or more, even more preferably 90% or more, and particularly preferably 95% or more. There is no particular upper limit to the total light transmittance of a dispersion containing 0.2% by mass of fibrous cellulose, and it may be 100%.

The haze of a dispersion containing 0.2% by mass of fibrous cellulose is measured using a haze meter and a glass liquid cell with a 1-cm optical path length in accordance with JIS K 7136:2000. The total light transmittance of a dispersion containing 0.2% by mass of fibrous cellulose is measured using a haze meter and a glass liquid cell with a 1-cm optical path length in accordance with JIS K 7361-1:1997. Zero-point measurements are performed using ion-exchanged water placed in the same glass cell, with the dispersion temperature at 23°C during measurement. The haze meter used can be the HM-150 manufactured by Murakami Color Research Laboratory Co., Ltd., and the glass liquid cell used can be the MG-40 (reverse optical path) manufactured by Fujiwara Seisakusho Co., Ltd. When diluting the dispersion to a 0.2% by mass concentration, the haze and total light transmittance are measured after dilution with ion-exchanged water. Furthermore, when the dispersion is concentrated to a concentration of 0.2% by mass, the dispersion is passed through a hollow fiber membrane with a 0.1 μm opening at room temperature via crossflow, and then the haze and total light transmittance are measured.

When the dispersion contains 0.4% by mass of fibrous cellulose, the pH of the dispersion is preferably 4 or higher, and more preferably 5 or higher. When the dispersion contains 0.4% by mass of fibrous cellulose, the pH of the dispersion is preferably 9 or lower, and more preferably 8 or lower. By maintaining the pH of the dispersion within the above range, a sheet with high transparency and excellent yellowing resistance can be formed, and in particular, a sheet that exhibits excellent yellowing resistance even under high temperature conditions can be formed. The pH of the dispersion containing 0.4% by mass of fibrous cellulose is measured using a pH meter, and the temperature of the dispersion during measurement is 23°C. When diluting the dispersion to a concentration of 0.4% by mass, the pH is measured after dilution with ion-exchanged water. When concentrating the dispersion to a concentration of 0.4% by mass, the pH is measured after cross-flowing the dispersion through a hollow fiber membrane with 0.1 μm openings at room temperature.

When the dispersion contains 0.4% by mass of fibrous cellulose, the viscosity is preferably 7,000 mPa·s or more, more preferably 7,500 mPa·s or more, and even more preferably 8,000 mPa·s or more. When the dispersion contains 0.4% by mass of fibrous cellulose, the viscosity is preferably 200,000 mPa·s or less, more preferably 100,000 mPa·s or less, and even more preferably 50,000 mPa·s or less. By maintaining the viscosity of the dispersion within the above range, it is possible to form a sheet with high transparency and excellent yellowing resistance. Furthermore, by maintaining the viscosity of the dispersion within the above range, the dispersion can be used for various applications and can also improve ease of handling when forming a sheet from the dispersion. The viscosity of the dispersion containing 0.4% by mass of fibrous cellulose is measured using a Brookfield viscometer after stirring in a disperser at 1,500 rpm for 5 minutes. The measurement is performed at a rotation speed of 3 rpm, and the viscosity value 3 minutes after the start of measurement is taken as the viscosity of the dispersion. The temperature of the dispersion to be measured is 23°C. A Brookfield T-LVT analog viscometer can be used as the Brookfield viscometer. When diluting the dispersion to a 0.4% by mass concentration, the viscosity is measured after diluting it with ion-exchanged water. When concentrating the dispersion to a 0.4% by mass concentration, the viscosity is measured after cross-flowing the dispersion through a hollow fiber membrane with 0.1 μm openings at room temperature.

The content of fibrous cellulose in the dispersion is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, and even more preferably 0.2% by mass or more, relative to the total mass of the dispersion. There is no particular upper limit to the content of fibrous cellulose in the dispersion, but it can be, for example, 20% by mass.

The solvent content in the dispersion is preferably 99.99% by mass or less, more preferably 99.9% by mass or less, and even more preferably 99.8% by mass or less, based on the total mass of the dispersion. Furthermore, the solvent content in the dispersion is preferably 80% by mass or more, based on the total mass of the dispersion. The solvent for the dispersion is preferably water, and the dispersion in this embodiment is preferably an aqueous dispersion.

The dispersion may contain an organic solvent. Examples of organic solvents include alcohols, polyhydric alcohols, ketones, ethers, esters, and aprotic polar solvents. Examples of alcohols include methanol, ethanol, isopropanol, n-butanol, and isobutyl alcohol. Examples of polyhydric alcohols include ethylene glycol, propylene glycol, and glycerin. Examples of ketones include acetone and methyl ethyl ketone (MEK). Examples of ethers include diethyl ether, tetrahydrofuran, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-butyl ether, and propylene glycol monomethyl ether. Examples of esters include ethyl acetate and butyl acetate. Examples of aprotic polar solvents include dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), and N-methyl-2-pyrrolidinone (NMP). When the dispersion contains an organic solvent, it is preferable that the total content of water and the organic solvent be within the above range.

The dispersion of this embodiment is a redispersion obtained by redispersing fine fibrous cellulose aggregates containing polyvalent metal ions. Specifically, fibrous cellulose aggregates can be obtained by adding a flocculant containing a polyvalent metal to a dispersion of fine fibrous cellulose, and a redispersion can be obtained by redispersing the fibrous cellulose aggregates in a solvent such as water. Examples of solvents for redispersing fibrous cellulose aggregates include the above-mentioned solvents, but water is preferably used as the solvent.

(Fine fibrous cellulose)
The dispersion of this embodiment contains fibrous cellulose having anionic groups and a fiber width of 1000 nm or less. The fiber width of the fibrous cellulose is preferably 100 nm or less, more preferably 50 nm or less, even more preferably 20 nm or less, and particularly preferably 10 nm or less. The fiber width of the fibrous cellulose is preferably 2 nm or more. The amount of anionic groups introduced into the fibrous cellulose is less than 0.5 mmol/g.

The average fiber width of fibrous cellulose is, for example, 1000 nm or less. The average fiber width of fibrous cellulose is, for example, preferably 2 nm or more and 1000 nm or less, more preferably 2 nm or more and 100 nm or less, even more preferably 2 nm or more and 50 nm or less, even more preferably 2 nm or more and 20 nm or less, and particularly preferably 2 nm or more and 10 nm or less. Note that fibrous cellulose is, for example, monofilament cellulose.

The fiber width of fibrous cellulose is measured, for example, using an electron microscope as follows. First, an aqueous suspension of fibrous cellulose with a concentration of 0.05% by mass or more and 0.1% by mass or less is prepared, and this suspension is cast onto a hydrophilically treated carbon film-coated grid to prepare a sample for TEM observation. If the sample contains wide fibers, an SEM image of the surface cast onto glass can be observed. Next, electron microscope images are observed at magnifications of 1000x, 5000x, 10000x, or 50000x, depending on the width of the fibers to be observed. However, the sample, observation conditions, and magnification should be adjusted to meet the following conditions.

(1) A line X is drawn at any point in the observed image, and 20 or more fibers intersect with the line X.
(2) Draw a line Y that intersects the line perpendicularly within the same image, and 20 or more fibers intersect the line Y.

For observation images that satisfy the above conditions, the widths of the fibers that intersect with lines X and Y are visually read. In this way, three or more sets of observation images are obtained of at least the surface portions that do not overlap each other. Next, for each image, the widths of the fibers that intersect with lines X and Y are read. In this way, the widths of at least 20 fibers x 2 x 3 = 120 fibers are read. The average of the read fiber widths is then taken as the average fiber width of the fibrous cellulose.

The fiber length of the fibrous cellulose is not particularly limited, but is preferably 0.1 μm or more and 1000 μm or less, more preferably 0.1 μm or more and 800 μm or less, and even more preferably 0.1 μm or more and 600 μm or less. By keeping the fiber length within the above range, destruction of the crystalline regions of the fibrous cellulose can be suppressed. It also becomes possible to keep the slurry viscosity of the fibrous cellulose within an appropriate range. The fiber length of the fibrous cellulose can be determined, for example, by image analysis using TEM, SEM, or AFM.

Fibrous cellulose preferably has a type I crystal structure. The presence of type I crystal structure in fibrous cellulose can be identified by a diffraction profile obtained from a wide-angle X-ray diffraction photograph using CuKα (λ=1.5418 Å) monochromated with graphite. Specifically, it can be identified by the presence of two typical peaks at two positions: 2θ=14° to 17° and 2θ=22° to 23°. The proportion of type I crystal structure in the fibrous cellulose is preferably 30% or more, more preferably 40% or more, and even more preferably 50% or more. This can be expected to provide even better performance in terms of heat resistance and low linear thermal expansion. The degree of crystallinity can be determined by measuring the X-ray diffraction profile and using the pattern in a conventional manner (Seagal et al., Textile Research Journal, Vol. 29, p. 786, 1959).

The axial ratio (fiber length/fiber width) of the fibrous cellulose is not particularly limited, but is preferably, for example, 20 to 10,000, and more preferably 50 to 1,000. By setting the axial ratio at or above the lower limit, it becomes easier to form a sheet containing fibrous cellulose. Furthermore, by setting the axial ratio at or below the upper limit, it becomes easier to handle the fibrous cellulose, for example, by diluting it when handling it as a dispersion.

For example, fibrous cellulose has both crystalline and amorphous regions. Fibrous cellulose having both crystalline and amorphous regions and an axial ratio within the above range can be achieved by the method for producing fine fibrous cellulose described below.

The fibrous cellulose has anionic groups. The anionic groups are preferably groups that are linked to the fibrous cellulose via ester bonds. In this case, the ester bonds are formed by dehydration condensation between the hydroxyl groups of the fibrous cellulose and a compound that becomes the anionic group.

Examples of anionic groups include phosphorus oxoacid groups or substituents derived from phosphorus oxoacid groups (sometimes simply referred to as phosphorus oxoacid groups), carboxy groups or substituents derived from carboxy groups (sometimes simply referred to as carboxy groups), sulfur oxoacid groups or substituents derived from sulfur oxoacid groups (sometimes simply referred to as sulfur oxoacid groups), xanthate groups or substituents derived from xanthate groups (sometimes simply referred to as xanthate groups), phosphonic groups or substituents derived from phosphonic groups, phosphine groups or substituents derived from phosphine groups, sulfonic groups or substituents derived from sulfonic groups, and carboxyalkyl groups (including carboxymethyl groups). Among these, the anionic group is preferably at least one selected from the group consisting of a phosphorus oxoacid group, a substituent derived from a phosphorus oxoacid group, a carboxy group, a substituent derived from a carboxy group, a sulfur oxoacid group, a substituent derived from a sulfur oxoacid group, a xanthate group, and a substituent derived from a xanthate group; more preferably at least one selected from the group consisting of a phosphorus oxoacid group, a substituent derived from a phosphorus oxoacid group, a carboxy group, a substituent derived from a carboxy group, a sulfur oxoacid group, and a substituent derived from a sulfur oxoacid group; and particularly preferably a phosphorus oxoacid group or a substituent derived from a phosphorus oxoacid group. By appropriately selecting the anionic group, a sheet with excellent transparency and yellowing resistance can be obtained.

The phosphorus oxoacid group or the substituent derived from the phosphorus oxoacid group is, for example, a substituent represented by the following formula (1). Multiple types of substituents represented by the following formula (1) may be introduced into each fibrous cellulose. In this case, the multiple introduced substituents represented by the following formula (1) may be the same or different.

In formula (1), a, b, and n are natural numbers, and m is an arbitrary number (where a = b × m). At least one of the n α and α' is ^O- , and the rest are R or OR. All of the α and α' may be ^O- . The n α may all be the same or different. ^{β b+} is a monovalent or higher cation made of an organic or inorganic substance.

Each R is a hydrogen atom, a saturated linear hydrocarbon group, a saturated branched hydrocarbon group, a saturated cyclic hydrocarbon group, an unsaturated linear hydrocarbon group, an unsaturated branched hydrocarbon group, an unsaturated cyclic hydrocarbon group, an aromatic group, or a derivative thereof. Furthermore, in formula (1), n is preferably 1.

Saturated linear hydrocarbon groups include, but are not limited to, methyl, ethyl, n-propyl, or n-butyl groups. Saturated branched hydrocarbon groups include, but are not limited to, i-propyl or t-butyl groups. Saturated cyclic hydrocarbon groups include, but are not limited to, cyclopentyl or cyclohexyl groups. Unsaturated linear hydrocarbon groups include, but are not limited to, vinyl or allyl groups. Unsaturated branched hydrocarbon groups include, but are not limited to, i-propenyl or 3-butenyl groups. Unsaturated cyclic hydrocarbon groups include, but are not limited to, cyclopentenyl or cyclohexenyl groups. Aromatic groups include, but are not limited to, phenyl or naphthyl groups.

Furthermore, examples of the derivative group in R include, but are not limited to, functional groups in which at least one functional group selected from the group consisting of a carboxy group, a carboxylate group (—COO ⁻ ), a hydroxy group, an amino group, and an ammonium group is added to or substituted on the main chain or side chain of the above-mentioned hydrocarbon groups. The number of carbon atoms constituting the main chain of R is not particularly limited, but is preferably 20 or less, and more preferably 10 or less. By setting the number of carbon atoms constituting the main chain of R within the above range, the molecular weight of the phosphorus oxoacid group can be set within an appropriate range, facilitating penetration into the fiber raw material and increasing the yield of finely divided cellulose fibers. When multiple R groups are present in formula (1) or when multiple types of substituents represented by formula (1) are introduced into the fibrous cellulose, the multiple R groups may be the same or different.

β ^b+ is a monovalent or higher cation composed of an organic or inorganic substance. Examples of the monovalent or higher cation composed of an organic substance include organic onium ions. Examples of the organic onium ions include organic ammonium ions and organic onium ions. Examples of the organic ammonium ions include aliphatic ammonium ions and aromatic ammonium ions, and examples of the organic onium ions include aliphatic phosphonium ions and aromatic phosphonium ions. Examples of the monovalent or higher cation composed of an inorganic substance include ions of alkali metals such as sodium, potassium, or lithium, ions of divalent metals such as calcium or magnesium, hydrogen ions, and ammonium ions. When multiple β ^b+ groups exist in formula (1) or when multiple types of substituents represented by formula (1) are introduced into the fibrous cellulose, the multiple β ^b + groups may be the same or different. In this embodiment, at least a portion of the β ^{b+ groups} is a polyvalent metal ion.

More specific examples of the phosphorus oxo acid group or substituent derived from a phosphorus oxo acid group include a phosphate group (-PO ₃ H ₂ ), a salt of a phosphate group, a phosphorous acid (phosphonic acid) group (-PO ₂ H ₂ ), and a salt of a phosphite (phosphonic acid) group. Furthermore, the phosphorus oxo acid group or the substituent derived from a phosphorus oxo acid group may be a group in which a phosphate group is condensed (e.g., a pyrophosphate group), a group in which a phosphonic acid is condensed (e.g., a polyphosphonic acid group), a phosphate ester group (e.g., a monomethyl phosphate group, a polyoxyethylene alkyl phosphate group), an alkyl phosphonic acid group (e.g., a methylphosphonic acid group), or the like.

Furthermore, the sulfur oxoacid group (a sulfur oxoacid group or a substituent derived from a sulfur oxoacid group) is, for example, a substituent represented by the following formula (2). Multiple types of substituents represented by the following formula (2) may be introduced into each fibrous cellulose. In this case, the multiple introduced substituents represented by the following formula (2) may be the same or different.

In the structural formula, b and n are natural numbers, p is 0 or 1, and m is any number (wherein 1 = b × m). When n is 2 or greater, the multiple p's may be the same or different. In the structural formula, β ^b+ is a monovalent or higher cation composed of an organic or inorganic substance. Examples of the monovalent or higher cation composed of an organic substance include organic onium ions. Examples of the organic onium ions include organic ammonium ions and organic onium ions. Examples of the organic ammonium ions include aliphatic ammonium ions and aromatic ammonium ions, and examples of the organic onium ions include aliphatic phosphonium ions and aromatic phosphonium ions. Examples of the monovalent or higher cation composed of an inorganic substance include ions of alkali metals such as sodium, potassium, or lithium, ions of divalent metals such as calcium or magnesium, hydrogen ions, and ammonium ions. When multiple types of substituents represented by the above formula (2) are introduced into the fibrous cellulose, the multiple β ^b+ may be the same or different. In this embodiment, at least a portion of β ^b+ is a polyvalent metal ion.

The content (introduction amount) of the anionic group may be, for example, less than 0.50 mmol/g per 1 g (mass) of fibrous cellulose, preferably 0.40 mmol/g or less, more preferably 0.30 mmol/g or less, even more preferably 0.25 mmol/g or less, and particularly preferably 0.15 mmol/g or less. The amount of the anionic group introduced into the fibrous cellulose is preferably 0.01 mmol/g or more, more preferably 0.03 mmol/g or more, even more preferably 0.04 mmol/g or more, even more preferably 0.05 mmol/g or more, and particularly preferably 0.07 mmol/g or more. Here, the denominator in the unit mmol/g indicates the mass of the fibrous cellulose when the counter ion of the anionic group is a hydrogen ion (H ⁺ ). By setting the amount of the anionic group introduced within the above range, it becomes easier to obtain a sheet with excellent transparency and better yellowing resistance.

Fibrous cellulose having an anionic group content (introduced amount) within the above range is obtained, for example, through a substituent removal treatment process as described below. In other words, the fibrous cellulose in this embodiment is fibrous cellulose after substituent removal treatment.

The amount of anionic groups introduced into fibrous cellulose can be measured, for example, by neutralization titration. Measurement by neutralization titration involves adding an alkali, such as aqueous sodium hydroxide solution, to a slurry containing the obtained fibrous cellulose, and measuring the change in pH to determine the amount introduced.

1 is a graph showing the relationship between the amount of NaOH added dropwise to a slurry containing fibrous cellulose having phosphorus oxo acid groups and pH. The amount of phosphorus oxo acid groups introduced into the fibrous cellulose is measured, for example, as follows.
First, a slurry containing fibrous cellulose is treated with a strongly acidic ion exchange resin. If necessary, the measurement object may be subjected to a defibration treatment similar to the defibration treatment step described below before the treatment with the strongly acidic ion exchange resin.
Next, the change in pH is observed while adding aqueous sodium hydroxide solution, and a titration curve such as that shown in the upper part of Figure 1 is obtained. The titration curve shown in the upper part of Figure 1 plots the measured pH against the amount of alkali added, while the titration curve shown in the lower part of Figure 1 plots the pH increment (derivative value) (1/mmol) against the amount of alkali added. In this neutralization titration, two points are identified in the curve plotting the measured pH against the amount of alkali added, where the increment (derivative value of pH with respect to the amount of alkali added) is maximized. Of these, the first maximum point of the increment after starting to add the alkali is called the first endpoint, and the next maximum point of the increment is called the second endpoint. The amount of alkali required from the start of titration to the first endpoint is equal to the amount of first dissociated acid of the fibrous cellulose contained in the slurry used for titration, the amount of alkali required from the first endpoint to the second endpoint is equal to the amount of second dissociated acid of the fibrous cellulose contained in the slurry used for titration, and the amount of alkali required from the start of titration to the second endpoint is equal to the total amount of dissociated acid of the fibrous cellulose contained in the slurry used for titration.The value obtained by dividing the amount of alkali required from the start of titration to the first endpoint by the solids content (g) in the slurry to be titrated is the amount of phosphorus oxo acid group introduced (mmol/g).Note that when simply referring to the amount of phosphorus oxo acid group introduced (or amount of phosphorus oxo acid group), it refers to the amount of first dissociated acid.
In FIG. 1 , the region from the start of titration to the first endpoint is referred to as the first region, and the region from the first endpoint to the second endpoint is referred to as the second region. For example, when the phosphorus oxoacid group is a phosphate group and this phosphate group undergoes condensation, the amount of weakly acidic groups in the phosphorus oxoacid group (also referred to herein as the second dissociated acid amount) appears to decrease, and the amount of alkali required in the second region is smaller than the amount of alkali required in the first region. On the other hand, the amount of strongly acidic groups in the phosphorus oxoacid group (also referred to herein as the first dissociated acid amount) corresponds to the amount of phosphorus atoms regardless of whether condensation occurs. Furthermore, when the phosphorus oxoacid group is a phosphite group, the phosphorus oxoacid group no longer contains weakly acidic groups, so the amount of alkali required in the second region is reduced or may even be zero. In this case, there is only one point on the titration curve where the pH increment is maximized.

The above-mentioned amount of introduced phosphorus oxoacid groups (mmol/g) indicates the amount of phosphorus oxoacid groups possessed by the acid-form fibrous cellulose (hereinafter referred to as the amount of phosphorus oxoacid groups (acid form)), since the denominator indicates the mass of the acid-form fibrous cellulose. On the other hand, when the counter ions of the phosphorus oxoacid groups are substituted with any cation C so as to be charge equivalent, the amount of phosphorus oxoacid groups possessed by the fibrous cellulose with the cation C as the counter ion (hereinafter referred to as the amount of phosphorus oxoacid groups (C form)) can be determined by converting the denominator to the mass of the fibrous cellulose when the cation C is the counter ion.
That is, it is calculated using the following formula.
Amount of phosphorus oxoacid group (C type)=Amount of phosphorus oxoacid group (acid type)/{1+(W−1)×A/1000}
A [mmol/g]: total amount of anions derived from phosphorus oxoacid groups in the fibrous cellulose (total amount of dissociated acids from phosphorus oxoacid groups)
W: Formula weight per valence of cation C (for example, Na is 23, Al is 9)

2 is a graph showing the relationship between the amount of NaOH added dropwise to a dispersion containing fibrous cellulose having carboxy groups as anionic groups and pH. The amount of carboxy groups introduced into the fibrous cellulose is measured, for example, as follows.
First, a dispersion containing fibrous cellulose is treated with a strongly acidic ion exchange resin. If necessary, the measurement object may be subjected to a defibration treatment similar to the defibration treatment step described below before the treatment with the strongly acidic ion exchange resin.
Next, the change in pH was observed while adding aqueous sodium hydroxide solution, and a titration curve such as that shown in the upper part of Figure 2 was obtained. The titration curve shown in the upper part of Figure 2 plots the measured pH against the amount of alkali added, while the titration curve shown in the lower part of Figure 2 plots the pH increment (differential value) (1/mmol) against the amount of alkali added. In this neutralization titration, a single point was identified in the curve plotting the measured pH against the amount of alkali added, where the increment (differential value of pH with respect to the amount of alkali added) was maximized. This maximum point is referred to as the first endpoint. The region from the start of titration to the first endpoint in Figure 2 is referred to as the first region. The amount of alkali required in the first region is equal to the amount of carboxy groups in the dispersion used for titration. The amount of alkali introduced (mmol/g) was then calculated by dividing the amount of alkali required in the first region of the titration curve (mmol) by the solids content (g) in the dispersion containing the fibrous cellulose to be titrated.

The above-mentioned amount of carboxy groups introduced (mmol/g) indicates the amount of carboxy groups in the acid-form fibrous cellulose (hereinafter referred to as the amount of carboxy groups (acid form)), since the denominator is the mass of the acid-form fibrous cellulose. On the other hand, when the counter ions of the carboxy groups are substituted with any cation C so as to be the charge equivalent, the amount of carboxy groups in the fibrous cellulose in which the cation C is the counter ion (hereinafter referred to as the amount of carboxy groups (C form)) can be determined by converting the denominator to the mass of the fibrous cellulose when the cation C is the counter ion. That is, it is calculated using the following formula:
Amount of carboxyl group (C type)=Amount of carboxyl group (acid type)/{1+(W-1)×(Amount of carboxyl group (acid type))/1000}
W: Formula weight per valence of cation C (for example, Na is 23, Al is 9)

When measuring the amount of anionic groups using titration, adding too much sodium hydroxide solution or adding too little time between titrations can result in an inaccurate value, resulting in a lower anionic group content than expected. An appropriate amount and titration interval is, for example, titrating 10 to 50 μL of 0.1 N sodium hydroxide solution every 5 to 30 seconds. Additionally, to eliminate the effects of carbon dioxide dissolved in the fibrous cellulose-containing slurry, it is advisable to perform measurements while blowing an inert gas such as nitrogen gas into the slurry, for example, from 15 minutes before the start of titration until the end of titration.

The amount of sulfur oxoacid groups or sulfonic acid groups introduced into fibrous cellulose can be calculated by wet ashing the fibrous cellulose using perchloric acid and concentrated nitric acid, diluting it at an appropriate ratio, and measuring the amount of sulfur by ICP atomic emission spectrometry. The amount of sulfur divided by the bone-dry mass of the fibrous cellulose used is the amount of sulfur oxoacid groups or sulfonic acid groups (unit: mmol/g).

The amount of xanthate group introduced into fibrous cellulose can be measured by the Breedee method using the following method. First, 40 mL of saturated ammonium chloride solution was added to 1.5 parts by mass (bone dry mass) of fibrous cellulose, and the sample was crushed with a glass rod while mixing thoroughly. After leaving for about 15 minutes, the sample was filtered through GFP filter paper (GS-25 manufactured by ADVANTEC Co., Ltd.) and thoroughly washed with saturated ammonium chloride solution. Next, the sample was placed in a 500 mL tall beaker with the GFP filter paper, and 50 mL of 0.5 M sodium hydroxide solution (5 ° C.) was added and stirred, and the mixture was left for 15 minutes. After adding phenolphthalein solution until the solution turned pink, 1.5 M acetic acid was added, and the point at which the solution changed from pink to colorless was taken as the neutralization point. After neutralization, 250 mL of distilled water was added and stirred thoroughly, and 10 mL of 1.5 M acetic acid and 10 mL of 0.05 mol / L iodine solution were added using a whole pipette. Then, this solution is titrated with a 0.05 mol/L sodium thiosulfate solution, and the amount of xanthate groups is calculated using the following formula from the titration amount of sodium thiosulfate and the bone dry mass of the fibrous cellulose.
Amount of xanthate group (mmol/g) = (0.05 × 10 × 2 - 0.05 × sodium thiosulfate titration amount (mL)) / 1000 / bone dry mass of fibrous cellulose (g)

(polyvalent metal ions)
The fibrous cellulose has polyvalent metal ions as counter ions of the anionic groups. In this specification, the polyvalent metal ions are metal ions with a valence of two or more, and the polyvalent metal ions are preferably divalent metal ions. In the dispersion of this embodiment, at least a portion of the polyvalent metal ions preferably form ionic bonds with the anionic groups through electrostatic interaction. In addition, free polyvalent metal elements or polyvalent metal ions may be present in the dispersion.

The polyvalent metal ion is preferably at least one selected from the group consisting of calcium ions, magnesium ions, zinc ions, aluminum ions, nickel ions, copper ions, and cobalt ions, more preferably at least one selected from the group consisting of calcium ions, magnesium ions, zinc ions, and aluminum ions, and even more preferably at least one selected from the group consisting of calcium ions, magnesium ions, and zinc ions. By selecting calcium ions, magnesium ions, zinc ions, or aluminum ions as the polyvalent metal ion, it is possible to obtain a dispersion or sheet with more suppressed coloration.

The polyvalent metal ions are derived from components added as metal salts (polyvalent metal salts) during the ion exchange process in the production process of the microfibrous cellulose dispersion, described below. Such polyvalent metal salts can also be referred to as flocculants because they function to flocculate the microfibrous cellulose. The polyvalent metal salts are preferably sulfates, chlorides, nitrates, carbonates, or phosphates of the aforementioned polyvalent metal ions, and are water-soluble. Specific examples of polyvalent metal salts include aluminum sulfate (aluminum sulfate), magnesium sulfate, iron sulfate, copper sulfate, zinc sulfate, aluminum chloride, polyaluminum chloride, calcium chloride, magnesium chloride, nickel chloride, cobalt chloride, copper chloride, iron chloride, lead chloride, calcium nitrate, magnesium nitrate, and calcium bicarbonate. Metal salts containing divalent or higher metal ions may be added singly or in combination. Among these, it is preferable to use at least one metal salt selected from the group consisting of magnesium sulfate, zinc sulfate, copper sulfate, calcium chloride, nickel chloride, and cobalt chloride as the metal salt containing a divalent or higher metal ion, and it is more preferable to use at least one metal salt selected from the group consisting of magnesium sulfate, zinc sulfate, and calcium chloride.

(optional ingredient)
The dispersion may contain optional components (additives) in addition to the fibrous cellulose and polyvalent metal ions. Examples of optional components include surfactants, coupling agents, inorganic layered compounds, inorganic compounds, leveling agents, preservatives, antifoaming agents, organic particles, lubricants, antistatic agents, UV protection agents, dyes, pigments, stabilizers, magnetic powders, alignment promoters, plasticizers, dispersants, and crosslinking agents. The dispersion may also contain optional components such as hydrophilic polymers, hydrophilic low-molecular-weight compounds, and organic ions.

Hydrophilic polymers include, for example, carboxyvinyl polymers; polyvinyl alcohol; alkyl methacrylate-acrylic acid copolymers; polyvinylpyrrolidone; polyvinyl methyl ether; polyacrylates such as sodium polyacrylate; alkyl acrylate copolymers; urethane copolymers; modified polyesters; modified polyimides; polyalkylene glycols such as polyethylene glycol and polypropylene glycol; polycations such as polyacrylamide and polyethyleneimine; polyanions; amphoteric polymers; xanthan gum, guar gum, tamarind gum, carrageenan, locust bean gum, and quince seeds. Examples of suitable hydrophilic polymers include thickening polysaccharides such as alginic acid, metal salts of alginic acid, pullulan, sacran, and pectin; cellulose derivatives such as carboxymethyl cellulose, carboxyethyl cellulose, methyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, and hydroxyethyl cellulose; starches such as cationized starch, raw starch, oxidized starch, etherified starch, esterified starch, dextrin, and amylose; glycerins such as polyglycerin; hyaluronic acid, metal salts of hyaluronic acid; and proteins such as casein. Copolymers of these hydrophilic polymers may also be used.

The hydrophilic low-molecular-weight compound is preferably a hydrophilic oxygen-containing organic compound, and more preferably a polyhydric alcohol. Examples of polyhydric alcohols include glycerin, sorbitol, and ethylene glycol.

Examples of organic ions include tetraalkylammonium ions and tetraalkylphosphonium ions. Examples of tetraalkylammonium ions include tetramethylammonium ion, tetraethylammonium ion, tetrapropylammonium ion, tetrabutylammonium ion, tetrapentylammonium ion, tetrahexylammonium ion, tetraheptylammonium ion, tributylmethylammonium ion, lauryltrimethylammonium ion, cetyltrimethylammonium ion, stearyltrimethylammonium ion, octyldimethylethylammonium ion, lauryldimethylethylammonium ion, didecyldimethylammonium ion, lauryldimethylbenzylammonium ion, and tributylbenzylammonium ion. Examples of tetraalkylphosphonium ions include tetramethylphosphonium ion, tetraethylphosphonium ion, tetrapropylphosphonium ion, tetrabutylphosphonium ion, and lauryltrimethylphosphonium ion. Examples of tetrapropylonium ions and tetrabutylonium ions include tetra-n-propylonium ion and tetra-n-butylonium ion, respectively.

(Method for producing fine fibrous cellulose dispersion)
The method for producing a fine fibrous cellulose dispersion preferably comprises, in this order: an anionic group introduction step for introducing anionic groups into a cellulose-containing fiber raw material; a washing step; an alkali treatment step (neutralization step); and a defibration treatment step. After the defibration treatment step, the method preferably comprises step (A) for removing some of the anionic groups from fine fibrous cellulose having anionic groups and having a fiber width of 1000 nm or less. After step (A), the method preferably further comprises step (B) for uniform dispersion treatment, an ion exchange step, and a redispersion (uniform dispersion) step. The method for producing a fine fibrous cellulose dispersion in this embodiment preferably comprises, in this order: step (A) for removing some of the anionic groups from fine fibrous cellulose having anionic groups and having a fiber width of 1000 nm or less; step (B) for uniform dispersion treatment; an ion exchange step, and a redispersion (uniform dispersion) step. Note that an acid treatment step may be included instead of or in addition to the washing step. Examples of the anionic group introduction step include a phosphorus oxoacid group introduction step, a carboxyl group introduction step, a sulfur oxoacid group introduction step, and a xanthate group introduction step. In the uniform dispersion treatment step (B) and the redispersion (uniform dispersion) step, it is preferable to redisperse the aggregates using a high-speed defibrator or a high-pressure homogenizer.

Step (A) is a step of removing some of the anionic groups from fine fibrous cellulose that has anionic groups and a fiber width of 1000 nm or less. Below, we will first explain the method for producing fine fibrous cellulose that has anionic groups and a fiber width of 1000 nm or less (the fine fibrous cellulose that is subjected to step (A)), and then explain step (A), step (B) of uniform dispersion treatment, the ion exchange step, and the redispersion (uniform dispersion) step.

<Fiber raw materials>
Fine fibrous cellulose is produced from a cellulose-containing fiber raw material. The cellulose-containing fiber raw material is not particularly limited, but pulp is preferably used because it is readily available and inexpensive. Examples of pulp include wood pulp, non-wood pulp, and deinked pulp. Examples of wood pulp include, but are not limited to, chemical pulps such as hardwood kraft pulp (LBKP), softwood kraft pulp (NBKP), sulfite pulp (SP), dissolving pulp (DP), soda pulp (AP), unbleached kraft pulp (UKP), and oxygen-bleached kraft pulp (OKP), semi-chemical pulps such as semi-chemical pulp (SCP) and chemi-groundwood pulp (CGP), and mechanical pulps such as groundwood pulp (GP) and thermomechanical pulp (TMP, BCTMP). Non-wood pulps include, but are not limited to, cotton-based pulps such as cotton linters and cotton lint, and non-wood pulps such as hemp, wheat straw, and bagasse. Deinked pulps include, but are not limited to, deinked pulp made from recycled paper. The pulp of this embodiment may be used alone or in combination with two or more of the above. Among the above pulps, wood pulp and deinked pulp are preferred from the viewpoint of ease of availability. Furthermore, among wood pulps, chemical pulps are more preferred, and kraft pulp and sulfite pulp are even more preferred, from the viewpoints of a high cellulose content, a high yield of fine fibrous cellulose during defibration treatment, and the small amount of cellulose decomposition in the pulp, allowing for the production of long-fiber fine fibrous cellulose with a large axial ratio. The use of long-fiber fine fibrous cellulose with a large axial ratio tends to increase viscosity.

Cellulose-containing fiber raw materials include, for example, cellulose contained in sea squirts and bacterial cellulose produced by acetic acid bacteria. Furthermore, instead of cellulose-containing fiber raw materials, fibers formed from linear nitrogen-containing polysaccharide polymers such as chitin and chitosan can also be used.

<Phosphorus oxoacid group introduction step>
The fine fibrous cellulose subjected to step (A) has anionic groups. Therefore, the process for producing the fine fibrous cellulose subjected to step (A) preferably includes an anionic group introduction step. Examples of the anionic group introduction step include a phosphorus oxo acid group introduction step. The phosphorus oxo acid group introduction step is a step of reacting a cellulose-containing fiber raw material with at least one compound (hereinafter also referred to as "compound A") selected from compounds capable of introducing phosphorus oxo acid groups by reacting with hydroxyl groups possessed by the cellulose-containing fiber raw material. This step results in the production of phosphorus oxo acid group-introduced fibers.

In the phosphorus oxoacid group introduction process according to this embodiment, the reaction of the cellulose-containing fiber raw material with compound A may be carried out in the presence of at least one selected from urea and its derivatives (hereinafter also referred to as "compound B"). Alternatively, the reaction of the cellulose-containing fiber raw material with compound A may be carried out in the absence of compound B.

One example of a method for reacting compound A with fiber raw materials in the presence of compound B is to mix compound A and compound B with dry, wet, or slurried fiber raw materials. Among these, using dry or wet fiber raw materials is preferred due to the high degree of reaction uniformity, and dry fiber raw materials are particularly preferred. The form of the fiber raw materials is not particularly limited, but is preferably in the form of a cotton or thin sheet. Compound A and compound B can be added to the fiber raw materials in powder form, in solution form in a solvent, or in a melted state by heating above their melting point. Among these, adding compound A and compound B in solution form in a solvent, particularly in aqueous solution form, is preferred due to the high degree of reaction uniformity. Compound A and compound B can be added to the fiber raw materials simultaneously, separately, or as a mixture. The method of adding compound A and compound B is not particularly limited. When compound A and compound B are in solution form, the fiber raw materials can be immersed in the solution and allowed to absorb the liquid before being removed, or the solution can be added dropwise to the fiber raw materials. Alternatively, the required amounts of compound A and compound B may be added to the fiber raw material, or excess amounts of compound A and compound B may be added to the fiber raw material, and then the excess compound A and compound B may be removed by squeezing or filtration.

Compound A used in this embodiment may be any compound that contains a phosphorus atom and is capable of forming an ester bond with cellulose. Examples include, but are not limited to, phosphoric acid or its salts, phosphorous acid or its salts, dehydrated condensed phosphoric acid or its salts, and phosphoric anhydride (diphosphorus pentoxide). Phosphoric acids of various purities can be used, such as 100% phosphoric acid (orthophosphoric acid) or 85% phosphoric acid. Phosphorous acids include 99% phosphorous acid (phosphonic acid). Dehydrated condensed phosphoric acids are formed by the condensation of two or more molecules of phosphoric acid through a dehydration reaction, and examples include pyrophosphoric acid and polyphosphoric acid. Phosphates, phosphites, and dehydrated condensed phosphates include lithium salts, sodium salts, potassium salts, and ammonium salts of phosphoric acid, phosphorous acid, or dehydrated condensed phosphoric acid, which can be neutralized to various degrees. Of these, from the viewpoints of high efficiency of introduction of phosphorus oxoacid groups, ease of improving defibration efficiency in the defibration step described below, low cost, and ease of industrial application, phosphoric acid, sodium salt of phosphoric acid, potassium salt of phosphoric acid, ammonium salt of phosphoric acid, or phosphorous acid, sodium salt of phosphorous acid, potassium salt of phosphorous acid, or ammonium salt of phosphorous acid are preferred, and phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, or phosphorous acid, sodium phosphite are more preferred.

The amount of compound A added to the fiber raw material is not particularly limited. However, for example, when the amount of compound A added is converted into the amount of phosphorus atoms, the amount of phosphorus atoms added to the fiber raw material (bone dry mass) is preferably 0.5% by mass or more and 100% by mass or less, more preferably 1% by mass or more and 50% by mass or less, and even more preferably 2% by mass or more and 30% by mass or less. By setting the amount of phosphorus atoms added to the fiber raw material within the above range, the yield of fine fibrous cellulose can be further improved. On the other hand, by setting the amount of phosphorus atoms added to the fiber raw material at or below the above upper limit, a balance can be achieved between the yield improvement effect and costs.

As described above, compound B used in this embodiment is at least one selected from urea and its derivatives. Examples of compound B include urea, biuret, 1-phenylurea, 1-benzylurea, 1-methylurea, and 1-ethylurea. From the perspective of improving the uniformity of the reaction, compound B is preferably used as an aqueous solution. Furthermore, from the perspective of further improving the uniformity of the reaction, it is preferable to use an aqueous solution in which both compound A and compound B are dissolved.

The amount of compound B added relative to the fiber raw material (bone dry mass) is not particularly limited, but is preferably 1% by mass or more and 500% by mass or less, more preferably 10% by mass or more and 400% by mass or less, and even more preferably 100% by mass or more and 350% by mass or less.

In the reaction of cellulose-containing fiber raw materials with compound A, in addition to compound B, the reaction system may contain, for example, amides or amines. Examples of amides include formamide, dimethylformamide, acetamide, and dimethylacetamide. Examples of amines include methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, and hexamethylenediamine. Among these, triethylamine in particular is known to act as a good reaction catalyst.

In the phosphorus oxo acid group introduction step, compound A or the like is preferably added to or mixed with the fiber raw material, and then the fiber raw material is subjected to a heat treatment. The heat treatment temperature is preferably selected so that the phosphorus oxo acid group can be efficiently introduced while suppressing thermal decomposition and hydrolysis of the fiber. The heat treatment temperature is preferably, for example, 50°C to 300°C, more preferably 100°C to 250°C, and even more preferably 130°C to 200°C. Furthermore, various types of equipment with heat transfer media can be used for the heat treatment, including hot air dryers, agitator dryers, rotary dryers, disk dryers, roll-type heaters, plate-type heaters, fluidized bed dryers, band dryers, filtration dryers, vibration fluidized dryers, flash dryers, reduced-pressure dryers, infrared heaters, far-infrared heaters, microwave heaters, and high-frequency dryers.

The heat treatment according to this embodiment can employ, for example, a method in which compound A is added to a thin sheet-like fiber raw material by a method such as impregnation, followed by heating, or a method in which the fiber raw material and compound A are heated while being kneaded or stirred using a kneader or the like. This makes it possible to suppress unevenness in the concentration of compound A in the fiber raw material and more uniformly introduce phosphorus oxoacid groups onto the surface of the cellulose fibers contained in the fiber raw material. This is thought to be because, when water molecules move to the surface of the fiber raw material as it dries, dissolved compound A is attracted to the water molecules by surface tension, preventing it from migrating to the surface of the fiber raw material in the same way (i.e., causing unevenness in the concentration of compound A).

The heating device used for the heat treatment is preferably one that can constantly discharge, to the outside of the system, moisture contained in the slurry and moisture generated in the dehydration condensation (phosphate esterification) reaction between compound A and hydroxyl groups contained in cellulose or other components in the fiber raw material. An example of such a heating device is an oven that uses a blower. By constantly discharging moisture from within the system, it is possible to suppress the hydrolysis of phosphate ester bonds, which is the reverse reaction of phosphate esterification, as well as the acid hydrolysis of sugar chains in the fibers. This makes it possible to obtain fine fibrous cellulose with a high axial ratio.

The heat treatment time is preferably from 1 second to 300 minutes after the moisture has been substantially removed from the fiber raw material, more preferably from 1 second to 1,000 seconds, and even more preferably from 10 seconds to 800 seconds. In this embodiment, by setting the heating temperature and heating time within appropriate ranges, the amount of phosphorus oxo acid groups introduced can be kept within the preferred range.

The phosphorus oxo acid group introduction process needs to be carried out at least once, but can also be repeated two or more times. By carrying out the phosphorus oxo acid group introduction process two or more times, it is possible to introduce many phosphorus oxo acid groups into the fiber raw material.

The amount of phosphorus oxoacid groups introduced into the fiber raw material is, for example, preferably 0.60 mmol/g or more per 1 g (mass) of fiber raw material, more preferably 0.70 mmol/g or more, even more preferably 0.80 mmol/g or more, even more preferably 1.00 mmol/g or more, and particularly preferably 1.20 mmol/g or more. The amount of phosphorus oxoacid groups introduced is, for example, preferably 5.20 mmol/g or less per 1 g (mass) of fiber raw material, more preferably 3.65 mmol/g or less, and even more preferably 3.00 mmol/g or less. Note that the amount of phosphorus oxoacid groups introduced in the phosphorus oxoacid group introduction step falling within the above range means that the amount of substituents introduced into the fine fibrous cellulose subjected to step (A) is within the above range. By keeping the amount of phosphorus oxoacid groups introduced within the above range, the amount of anionic groups introduced into the fine fibrous cellulose subjected to step (A) can be kept within the above range, which results in the production of fine fibrous cellulose with a small final fiber width (e.g., a fiber width of 10 nm or less). This makes it easier to obtain sheets with excellent transparency and better resistance to yellowing.

<Carboxy group introduction step>
The process for producing fine fibrous cellulose subjected to step (A) may include a carboxyl group introduction step as an anionic group introduction step. The carboxyl group introduction step is carried out by treating with an acid anhydride or a derivative thereof of a compound having a group derived from carboxylic acid.

Compounds having a carboxylic acid-derived group include, but are not limited to, dicarboxylic acid compounds such as maleic acid, succinic acid, phthalic acid, fumaric acid, glutaric acid, adipic acid, and itaconic acid, and tricarboxylic acid compounds such as citric acid and aconitic acid. Derivatives of compounds having a carboxylic acid-derived group include, but are not limited to, imidized products of acid anhydrides of compounds having a carboxy group, and derivatives of acid anhydrides of compounds having a carboxy group. Imidized products of acid anhydrides of compounds having a carboxy group include, but are not limited to, imidized products of dicarboxylic acid compounds such as maleimide, succinimide, and phthalimide.

Acid anhydrides of compounds having a group derived from carboxylic acid include, but are not limited to, acid anhydrides of dicarboxylic acid compounds such as maleic anhydride, succinic anhydride, phthalic anhydride, glutaric anhydride, adipic anhydride, and itaconic anhydride. Furthermore, derivatives of acid anhydrides of compounds having a group derived from carboxylic acid include, but are not limited to, acid anhydrides of compounds having carboxy groups such as dimethylmaleic anhydride, diethylmaleic anhydride, and diphenylmaleic anhydride, in which at least some of the hydrogen atoms have been substituted with substituents such as alkyl groups or phenyl groups.

The amount of carboxy groups introduced into the fiber raw material is, for example, preferably 0.60 mmol/g or more per 1 g (mass) of fiber raw material, more preferably 0.70 mmol/g or more, even more preferably 0.80 mmol/g or more, even more preferably 1.00 mmol/g or more, and particularly preferably 1.20 mmol/g or more. The amount of carboxy groups introduced is, for example, preferably 3.65 mmol/g or less per 1 g (mass) of fiber raw material, more preferably 3.00 mmol/g or less, and even more preferably 2.50 mmol/g or less. Note that the amount of carboxy groups introduced in the carboxy group introduction step falling within the above range means that the amount of anionic groups introduced into the fine fibrous cellulose used in step (A) is within the above range. By ensuring that the amount of carboxy groups introduced falls within the above range, the amount of anionic groups introduced into the fine fibrous cellulose used in step (A) can be kept within the above range, making it easier to produce fine fibrous cellulose with a fiber width of 10 nm or less. This makes it easier to obtain dispersions and sheets with excellent transparency and reduced coloration.

<Sulfur oxoacid group introduction step>
The process for producing fine fibrous cellulose subjected to step (A) may include a sulfur oxoacid group introduction step as an anionic group introduction step, in which hydroxyl groups in a cellulose-containing fiber raw material react with sulfur oxoacids to obtain cellulose fibers having sulfur oxoacid groups (sulfur oxoacid group-introduced fibers).

In the sulfur oxo acid group introduction step, instead of compound A in the above-described <Phosphorus oxo acid group introduction step>, at least one compound (hereinafter also referred to as "compound C") selected from compounds capable of introducing sulfur oxo acid groups by reacting with hydroxyl groups in cellulose-containing fiber raw materials is used. Compound C may be any compound containing a sulfur atom and capable of forming an ester bond with cellulose, including, but not limited to, sulfuric acid or its salts, sulfurous acid or its salts, and sulfuric acid amides. Sulfuric acid of various purities can be used, for example, 96% sulfuric acid (concentrated sulfuric acid). Sulfurous acid can be 5% aqueous sulfurous acid. Sulfates or sulfites include lithium, sodium, potassium, and ammonium salts of sulfates or sulfites, which can be neutralized to various degrees. Sulfamic acid or the like can be used as the sulfuric acid amide. In the sulfur oxo acid group introduction step, it is preferable to use compound B in the above-described <Phosphorus oxo acid group introduction step> as well.

In the sulfur oxoacid group introduction process, it is preferable to mix the cellulose raw material with an aqueous solution containing sulfur oxoacid and urea and/or a urea derivative, and then heat-treat the cellulose raw material. The heat-treatment temperature is preferably selected so that sulfur oxoacid groups can be efficiently introduced while suppressing thermal decomposition and hydrolysis of the fiber. The heat-treatment temperature is preferably 100°C or higher, more preferably 120°C or higher, and even more preferably 150°C or higher. The heat-treatment temperature is also preferably 300°C or lower, more preferably 250°C or lower, and even more preferably 200°C or lower.

In the heat treatment step, it is preferable to heat until substantially all moisture is removed. Therefore, the heat treatment time varies depending on the amount of moisture contained in the cellulose raw material and the amount of aqueous solution containing sulfur oxoacid and urea and/or a urea derivative added, but is preferably, for example, 10 to 10,000 seconds. Heat treatment can be performed using equipment with a variety of heat media, including hot air dryers, agitator dryers, rotary dryers, disk dryers, roll-type heaters, plate-type heaters, fluidized bed dryers, band dryers, filtration dryers, vibration fluidized dryers, pneumatic dryers, reduced-pressure dryers, infrared heaters, far-infrared heaters, microwave heaters, and high-frequency dryers.

The amount of sulfur oxoacid groups introduced into the cellulose raw material is preferably 0.60 mmol/g or more, more preferably 0.70 mmol/g or more, even more preferably 0.80 mmol/g or more, even more preferably 1.00 mmol/g or more, and particularly preferably 1.20 mmol/g or more per gram (mass) of fibrous cellulose. Furthermore, the amount of sulfur oxoacid groups introduced is preferably 5.00 mmol/g or less, more preferably 3.00 mmol/g or less, per gram (mass) of fibrous raw material. Note that the amount of sulfur oxoacid groups introduced in the sulfur oxoacid group introduction step falling within the above range means that the amount of anionic groups introduced into the fine fibrous cellulose used in step (A) falls within the above range. By keeping the amount of sulfur oxoacid groups introduced within the above range, the amount of anionic groups introduced into the fine fibrous cellulose used in step (A) can be kept within the above range, thereby facilitating the production of fine fibrous cellulose with a fiber width of 10 nm or less. This facilitates the production of dispersions and sheets with excellent transparency and reduced discoloration.

<Xanthate group introduction step (xanthogenic acid esterification step)>
The process for producing fine fibrous cellulose subjected to step (A) may include a xanthate group introduction step as an anionic group introduction step. The xanthate group introduction step is performed by substituting the hydroxyl groups of the cellulose-containing fiber raw material with the xanthate groups represented by the following formula (3), thereby obtaining cellulose fibers having xanthate groups (xanthate group-introduced fibers).
-OCSS ^- M ⁺ ...(3)
Here, M ⁺ is at least one selected from the group consisting of a hydrogen ion, a monovalent metal ion, an ammonium ion, and an aliphatic or aromatic ammonium ion.

In the xanthate group introduction process, the cellulose-containing fiber raw material is first treated with an alkaline solution to obtain alkali cellulose. Examples of alkaline solutions include aqueous alkali metal hydroxides and aqueous alkaline earth metal hydroxides. Among these, the alkaline solution is preferably an aqueous alkali metal hydroxide solution such as sodium hydroxide or potassium hydroxide, with sodium hydroxide being particularly preferred. When the alkaline solution is an aqueous alkali metal hydroxide solution, the alkali metal hydroxide concentration in the aqueous alkali metal hydroxide solution is preferably 4% by mass or more, more preferably 5% by mass or more. Furthermore, the alkali metal hydroxide concentration in the aqueous alkali metal hydroxide solution is preferably 9% by mass or less. By maintaining the alkali metal hydroxide concentration at or above the lower limit, cellulose mercerization can be sufficiently promoted, reducing the amount of by-products generated during the subsequent xanthation process and, as a result, increasing the yield of xanthate group-introduced fiber. This allows for more effective defibration, as described below. Furthermore, by setting the alkali metal hydroxide concentration to the above upper limit or less, it is possible to prevent the aqueous alkali metal hydroxide solution from penetrating into the crystalline regions of the cellulose while allowing mercerization to proceed, which makes it easier to maintain the cellulose type I crystal structure and further increases the yield of fine fibrous cellulose.

The alkali treatment time is preferably 30 minutes or more, and more preferably 1 hour or more. Furthermore, the alkali treatment time is preferably 6 hours or less, and more preferably 5 hours or less. By keeping the alkali treatment time within the above range, the final yield can be increased, and productivity can be improved.

The alkali cellulose obtained by the above alkali treatment is preferably subsequently subjected to solid-liquid separation to remove as much of the aqueous solution as possible. This reduces the water content during the subsequent xanthate formation treatment and promotes the reaction. Common dehydration methods such as centrifugation and filtration can be used as the solid-liquid separation method. Furthermore, the concentration of alkali metal hydroxide contained in the alkali cellulose after solid-liquid separation is preferably 3% by mass or more and 8% by mass or less, based on the total mass of the alkali cellulose after solid-liquid separation.

In the xanthate group introduction process, a xanthate treatment process is carried out after alkali treatment. In the xanthate treatment process, alkali cellulose is reacted with carbon disulfide (CS ₂ ) to convert the (-O ^- Na ⁺ ) group to a (-OCSS ^- Na ⁺ ) group, resulting in xanthate group-introduced fiber. In the above, the metal ion introduced into the alkali cellulose is described as Na ⁺ as a representative, but the same reaction also occurs with other alkali metal ions.

In the xanthation treatment, it is preferable to supply 10% by mass or more of carbon disulfide relative to the bone dry mass of cellulose in the alkali cellulose. Furthermore, in the xanthation treatment, the contact time between carbon disulfide and alkali cellulose is preferably 30 minutes or more, and more preferably 1 hour or more. Xanthation proceeds quickly when carbon disulfide comes into contact with alkali cellulose, but it takes time for carbon disulfide to penetrate into the interior of the alkali cellulose, so it is preferable to set the reaction time within the above range. On the other hand, the contact time between carbon disulfide and alkali cellulose only needs to be 6 hours or less, which allows sufficient penetration into the alkali cellulose mass after dehydration and allows reactive xanthation to be almost completed.

The reaction temperature in the xanthate treatment is preferably 46°C or lower. Keeping the reaction temperature within this range helps to suppress decomposition of the alkali cellulose. Furthermore, keeping the reaction temperature within this range facilitates uniform reaction, suppressing the generation of by-products and further suppressing the removal of the generated xanthate groups.

The amount of xanthate groups introduced into the cellulose raw material is preferably 0.60 mmol/g or more per 1 g (mass) of fiber raw material, more preferably 0.70 mmol/g or more, even more preferably 0.80 mmol/g or more, even more preferably 1.00 mmol/g or more, and particularly preferably 1.20 mmol/g or more. The amount of xanthate groups introduced is preferably 5.00 mmol/g or less per 1 g (mass) of fiber raw material, more preferably 3.00 mmol/g or less. Note that the amount of xanthate groups introduced in the xanthate group introduction step within the above range means that the amount of anionic groups introduced into the fine fibrous cellulose used in step (A) is within the above range. By ensuring that the amount of xanthate groups introduced within the above range, the amount of anionic groups introduced into the fine fibrous cellulose used in step (A) can be kept within the above range, making it easier to produce fine fibrous cellulose with a fiber width of 10 nm or less. This makes it easier to obtain dispersions and sheets with excellent transparency and reduced coloration.

<Cleaning process>
In the production process of fine fibrous cellulose to be subjected to step (A), a washing step can be carried out on the anionic group-introduced fibers, if necessary. The washing step is carried out by washing the anionic group-introduced fibers with water or an organic solvent, for example. The washing step may be carried out after each step described below, and the number of washings carried out in each washing step is not particularly limited.

<Alkali treatment step>
In the process for producing fine fibrous cellulose to be subjected to step (A), an alkali treatment may be carried out on the fibrous raw material between the anionic group introduction step and the defibration treatment step described below. The alkali treatment method is not particularly limited, but examples thereof include a method of immersing the anionic group-introduced fibers in an alkali solution.

The alkaline compound contained in the alkaline solution is not particularly limited and may be an inorganic alkaline compound or an organic alkaline compound. In this embodiment, it is preferable to use, for example, sodium hydroxide or potassium hydroxide as the alkaline compound due to their high versatility. The solvent contained in the alkaline solution may be either water or an organic solvent. In particular, the solvent contained in the alkaline solution is preferably a polar solvent including water or a polar organic solvent such as alcohol, and more preferably an aqueous solvent containing at least water. As the alkaline solution, for example, an aqueous sodium hydroxide solution or an aqueous potassium hydroxide solution is preferable due to their high versatility.

The temperature of the alkaline solution in the alkaline treatment step is not particularly limited, but is preferably from 5°C to 80°C, and more preferably from 10°C to 60°C. The immersion time of the anionic group-introduced fiber in the alkaline solution in the alkaline treatment step is not particularly limited, but is preferably from 5 minutes to 30 minutes, and more preferably from 10 minutes to 20 minutes. The amount of alkaline solution used in the alkaline treatment is not particularly limited, but is preferably from 100% to 100,000% by mass, and more preferably from 1,000% to 10,000% by mass, based on the absolute dry mass of the anionic group-introduced fiber.

In order to reduce the amount of alkaline solution used in the alkaline treatment step, the anionic group-introduced fiber may be washed with water or an organic solvent after the anionic group introduction step but before the alkaline treatment step. From the perspective of improving handleability, it is preferable to wash the alkaline-treated anionic group-introduced fiber with water or an organic solvent after the alkaline treatment step but before the defibration treatment step.

<Acid treatment step>
In the production process of the fine fibrous cellulose subjected to step (A), the fibrous raw material may be subjected to an acid treatment between the step of introducing anionic groups and the defibration treatment step described below. For example, the anionic group introduction step, acid treatment, alkali treatment, and defibration treatment may be performed in this order.

The acid treatment method is not particularly limited, but examples include immersing the fiber raw material in an acidic solution containing an acid. The concentration of the acidic solution used is not particularly limited, but is preferably 10% by mass or less, and more preferably 5% by mass or less. The pH of the acidic solution used is also not particularly limited, but is preferably 0 to 4, and more preferably 1 to 3. Examples of acids that can be used in the acidic solution include inorganic acids, sulfonic acids, and carboxylic acids. Examples of inorganic acids include sulfuric acid, nitric acid, hydrobromic acid, hydroiodic acid, hypochlorous acid, chlorous acid, chloric acid, perchloric acid, phosphoric acid, and boric acid. Examples of sulfonic acids include methanesulfonic acid, ethanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid, and trifluoromethanesulfonic acid. Examples of carboxylic acids include formic acid, acetic acid, citric acid, gluconic acid, lactic acid, oxalic acid, and tartaric acid. Among these, hydrochloric acid or sulfuric acid is particularly preferred.

The temperature of the acid solution used in the acid treatment is not particularly limited, but is preferably between 5°C and 100°C, and more preferably between 20°C and 90°C. The immersion time in the acid solution used in the acid treatment is not particularly limited, but is preferably between 5 and 120 minutes, and more preferably between 10 and 60 minutes. The amount of acid solution used in the acid treatment is not particularly limited, but is preferably between 100% and 100,000% by mass, and more preferably between 1,000% and 10,000% by mass, based on the absolute dry mass of the fiber raw material.

<Nitrogen removal treatment>
The process for producing fine fibrous cellulose subjected to step (A) may further include a step of reducing the nitrogen content (nitrogen removal treatment step). Reducing the nitrogen content in the dispersion liquid can further improve the yellowing resistance of the sheet. The nitrogen removal treatment step may be carried out after the uniform dispersion treatment step in step (B) described below, but is preferably carried out before the uniform dispersion treatment step in step (B) described below. It is also preferably carried out before the defibration treatment step.

In the nitrogen removal treatment step, it is preferable to adjust the pH of the slurry containing the anionic group-introduced fibers to 10 or higher and then perform a heat treatment. In the heat treatment, the liquid temperature of the slurry is preferably 50°C or higher and 100°C or lower, and the heating time is preferably 15 minutes or higher and 180 minutes or lower. When adjusting the pH of the slurry containing the anionic group-introduced fibers, it is preferable to add an alkaline compound that can be used in the alkaline treatment step described above to the slurry.

After the nitrogen removal treatment step, the anionic group-introduced fiber can be subjected to a washing step, if necessary. The washing step is carried out by washing the anionic group-introduced fiber with, for example, water or an organic solvent. Furthermore, there is no particular limit to the number of washings performed in each washing step.

<Defibrillation process>
The production process of fine fibrous cellulose subjected to step (A) includes a defibration treatment step. This produces fine fibrous cellulose having anionic groups and a fiber width of 1000 nm or less. In the defibration treatment step, for example, a defibration treatment device can be used. The defibration treatment device is not particularly limited, but examples that can be used include a high-speed defibrator, a grinder (stone mill), a high-pressure homogenizer, an ultra-high-pressure homogenizer, a high-pressure collision grinder, a ball mill, a bead mill, a disk refiner, a conical refiner, a twin-screw kneader, a vibration mill, a homomixer under high-speed rotation, an ultrasonic disperser, or a beater. Among the above defibration treatment devices, it is more preferable to use a high-speed defibrator, a high-pressure homogenizer, or an ultra-high-pressure homogenizer, which are less affected by the grinding media and have less risk of contamination.

The processing conditions for the defibration process are not particularly limited, but for example, when using a high-pressure homogenizer, the processing pressure is preferably 1 MPa or more and 350 MPa or less, more preferably 10 MPa or more and 300 MPa or less, and even more preferably 50 MPa or more and 250 MPa or less.

In the defibration process, for example, the anionic group-introduced fibers are preferably diluted with a dispersion medium to form a slurry. The dispersion medium can be one or more selected from water and organic solvents such as polar organic solvents. Polar organic solvents are not particularly limited, but preferred examples include alcohols, polyhydric alcohols, ketones, ethers, esters, and aprotic polar solvents. Examples of alcohols include methanol, ethanol, isopropanol, n-butanol, and isobutyl alcohol. Examples of polyhydric alcohols include ethylene glycol, propylene glycol, and glycerin. Examples of ketones include acetone and methyl ethyl ketone (MEK). Examples of ethers include diethyl ether, tetrahydrofuran, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-butyl ether, and propylene glycol monomethyl ether. Examples of esters include ethyl acetate and butyl acetate. Aprotic polar solvents include dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), N-methyl-2-pyrrolidinone (NMP), etc.

The solids concentration of the fine fibrous cellulose during the defibration process can be set as appropriate. Furthermore, the slurry obtained by dispersing the anionic group-introduced fibers in a dispersion medium may contain solids other than the anionic group-introduced fibers, such as urea, which has hydrogen bonding properties.

The fiber width of the fine fibrous cellulose after defibration treatment in step (A) is preferably 2 to 50 nm, more preferably 2 to 25 nm, even more preferably 2 to 15 nm, and particularly preferably 2 to 10 nm.

Furthermore, when the fine fibrous cellulose to be subjected to step (A) is prepared as an aqueous dispersion with a concentration of 0.1% by mass and the nanofiber yield is calculated, the nanofiber yield is preferably 90% by mass or more, more preferably 93% by mass or more, and even more preferably 96% by mass or more. The nanofiber yield may be 100% by mass. Here, the nanofiber yield is a value measured based on the cellulose concentration of the supernatant obtained by centrifuging a 0.1% by mass fine fibrous cellulose dispersion using a refrigerated high-speed centrifuge (Kokusan Co., Ltd., H-2000B) at 12,000 G for 10 minutes, using the following formula:
Nanofiber yield (mass%) = cellulose concentration in supernatant (mass%) / 0.1 × 100

Furthermore, when the fine fibrous cellulose used in step (A) is prepared as an aqueous dispersion with a concentration of 0.2% by mass, the haze of the aqueous dispersion is preferably 10% or less, more preferably 5.0% or less, and even more preferably 3.0% or less. The haze of the aqueous dispersion may be 0%. The haze of the aqueous dispersion of fine fibrous cellulose is measured in accordance with JIS K 7136:2000 using a haze meter (HM-150, manufactured by Murakami Color Research Laboratory Co., Ltd.). A glass liquid cell with a 1 cm optical path length (MG-40, reverse optical path, manufactured by Fujiwara Seisakusho Co., Ltd.) is used for the measurement. Zero-point measurement is performed using ion-exchanged water placed in the glass cell. The dispersion to be measured is allowed to stand for 24 hours in an environment of 23°C and 50% relative humidity before measurement, bringing the dispersion temperature to 23°C.

By ensuring that the fiber width of the fine fibrous cellulose used in step (A) and the nanofiber yield and haze of the fine fibrous cellulose dispersion are within the above ranges, the transparency of the dispersion and sheet containing the fine fibrous cellulose can be more effectively improved.

<Substituent Removal Treatment Step (Step A)>
The method for producing a fine fibrous cellulose dispersion preferably includes a step (A) of removing a portion of the anionic groups from fine fibrous cellulose having anionic groups and having a fiber width of 1,000 nm or less. In this specification, the step of removing a portion of the anionic groups from the fine fibrous cellulose is also referred to as a substituent removal treatment step.

Substituent removal treatment processes include heat treatment, enzyme treatment, acid treatment, and alkali treatment of fine fibrous cellulose having anionic groups and a fiber width of 1000 nm or less. These processes may be performed alone or in combination. Of these, the substituent removal treatment process is preferably a heat treatment or enzyme treatment. By undergoing the above treatment processes, a portion of the anionic groups is removed from fine fibrous cellulose having anionic groups and a fiber width of 1000 nm or less, resulting in fine fibrous cellulose having an anionic group introduction amount of less than 0.5 mmol/g.

The substituent removal treatment step is preferably carried out in the form of a slurry. That is, the substituent removal treatment step is preferably a step of subjecting a slurry containing fine fibrous cellulose having anionic groups and a fiber width of 1000 nm or less to a heat treatment, enzyme treatment, acid treatment, alkali treatment, or the like. By carrying out the substituent removal treatment step in the form of a slurry, it is possible to prevent the residue of colored substances caused by heating or other factors during the substituent removal treatment, as well as added or generated acids, alkalis, salts, and the like. This makes it possible to suppress coloration when the fine fibrous cellulose obtained through step (B) is made into a dispersion or sheet. Furthermore, when a treatment is carried out to remove salts derived from the substituents removed after the substituent removal treatment, it is also possible to increase the salt removal rate.

When a substituent removal treatment is performed on a slurry containing fine fibrous cellulose having anionic groups and a fiber width of 1000 nm or less, the concentration of the fine fibrous cellulose in the slurry is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, and even more preferably 0.2% by mass or more. The concentration of the fine fibrous cellulose in the slurry is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less. By maintaining the concentration of the fine fibrous cellulose in the slurry within the above range, the substituent removal treatment can be performed more efficiently. Furthermore, maintaining the concentration of the fine fibrous cellulose in the slurry within the above range can prevent the residue of coloring substances caused by heating during the substituent removal treatment, as well as added or generated acids, alkalis, salts, etc. This can suppress coloration when the fine fibrous cellulose obtained through step (B) is made into a dispersion or sheet. Furthermore, when a treatment is performed to remove salts derived from the removed substituents after the substituent removal treatment, the salt removal rate can be increased.

When the substituent removal treatment step is a step of heat-treating fine fibrous cellulose having anionic groups and a fiber width of 1000 nm or less, the heating temperature in the heat treatment step is preferably 40°C or higher, more preferably 50°C or higher, and even more preferably 60°C or higher. Furthermore, the heating temperature in the heat treatment step is preferably 250°C or lower, more preferably 230°C or lower, and even more preferably 200°C or lower. In particular, when the substituents on the fine fibrous cellulose subjected to the substituent removal treatment step are phosphorus oxoacid groups, sulfur oxoacid groups, or maleic acid groups, the heating temperature in the heat treatment step is preferably 80°C or higher, more preferably 100°C or higher, and even more preferably 120°C or higher.

When the substituent removal treatment step is a heat treatment step, the heating device that can be used in the heat treatment step is not particularly limited, but examples include hot air heating devices, steam heating devices, electric heating devices, hydrothermal heating devices, thermal heating devices, infrared heating devices, far-infrared heating devices, microwave heating devices, high-frequency heating devices, agitator drying devices, rotary drying devices, disk drying devices, roll-type heating devices, plate-type heating devices, fluidized bed drying devices, band-type drying devices, filtration drying devices, vibration fluidized drying devices, flash drying devices, and reduced-pressure drying devices. To prevent evaporation, heating is preferably carried out in a closed system, and to increase the heating temperature, heating is preferably carried out in a pressure-resistant device or container. Heat treatment may be batch processing, continuous batch processing, or continuous processing.

When the substituent removal treatment step is a step of enzymatically treating fine fibrous cellulose having anionic groups and a fiber width of 1000 nm or less, it is preferable to use a phosphate ester hydrolase, a sulfate ester hydrolase, or the like in the enzymatic treatment step, depending on the type of substituent.

In the enzyme treatment step, the enzyme is preferably added so that the enzymatic activity per 1 g of fine fibrous cellulose is 0.1 nkat or more, more preferably 1.0 nkat or more, and even more preferably 10 nkat or more. Furthermore, the enzyme is preferably added so that the enzymatic activity per 1 g of fine fibrous cellulose is 100,000 nkat or less, more preferably 50,000 nkat or less, and even more preferably 10,000 nkat or less. After adding the enzyme to the fine fibrous cellulose dispersion (slurry), it is preferable to treat the dispersion (slurry) at a temperature of 0°C or higher but less than 50°C for 1 minute to 100 hours.

After the enzymatic reaction, a step of deactivating the enzyme may be carried out. Methods for deactivating the enzyme include adding an acid or alkaline component to the enzymatically treated slurry, and raising the temperature of the enzymatically treated slurry to 90°C or higher.

When the substituent removal treatment step is a step of acid-treating fine fibrous cellulose having anionic groups and a fiber width of 1000 nm or less, it is preferable to add an acid compound that can be used in the acid treatment step described above to the slurry in the acid treatment step.

When the substituent removal treatment step is a step of alkali treating fine fibrous cellulose having anionic groups and a fiber width of 1000 nm or less, it is preferable to add an alkali compound that can be used in the alkali treatment step described above to the slurry in the alkali treatment step.

In the substituent removal treatment step, it is preferable that the substituent removal reaction proceeds uniformly. To ensure uniform reaction, for example, the slurry containing the fine fibrous cellulose may be stirred, or the specific surface area of the slurry in contact with the heating medium may be increased. The slurry may be stirred by applying external mechanical shear, or by increasing the slurry flow rate during the reaction to promote self-stirring.

Spacer molecules may be added in the substituent removal treatment step. The spacer molecules penetrate between adjacent fine fibrous cellulose particles, thereby acting as spacers to create fine spaces between the fine fibrous cellulose particles. Adding such spacer molecules in the substituent removal treatment step can suppress aggregation of the fine fibrous cellulose after the substituent removal treatment. This can more effectively increase the transparency of dispersions and sheets containing fine fibrous cellulose.

The spacer molecule is preferably a water-soluble organic compound. Examples of water-soluble organic compounds include sugars, water-soluble polymers, and urea. Specific examples include trehalose, urea, polyethylene glycol (PEG), polyethylene oxide (PEO), carboxymethyl cellulose, and polyvinyl alcohol (PVA). Additionally, water-soluble organic compounds that can be used include alkyl methacrylate-acrylic acid copolymer, polyvinylpyrrolidone, sodium polyacrylate, propylene glycol, dipropylene glycol, polypropylene glycol, isoprene glycol, hexylene glycol, 1,3-butylene glycol, polyacrylamide, xanthan gum, guar gum, tamarind gum, carrageenan, locust bean gum, quince seed, alginic acid, pullulan, carrageenan, pectin, starches such as cationized starch, raw starch, oxidized starch, etherified starch, esterified starch, and amylose, glycerin, diglycerin, polyglycerin, hyaluronic acid, and metal salts of hyaluronic acid.

In addition, known pigments can be used as spacer molecules. Examples include kaolin (containing clay), calcium carbonate, titanium oxide, zinc oxide, amorphous silica (containing colloidal silica), aluminum oxide, zeolite, sepiolite, smectite, synthetic smectite, magnesium silicate, magnesium carbonate, magnesium oxide, diatomaceous earth, styrene-based plastic pigments, hydrotalcite, urea resin-based plastic pigments, and benzoguanamine-based plastic pigments.

<pH adjustment process>
When the substituent removal treatment step is performed in the form of a slurry, a step of adjusting the pH of the slurry containing the fine fibrous cellulose may be provided before the substituent removal treatment step. For example, when anionic groups are introduced into cellulose fibers and the counter ions of these anionic groups are Na ⁺ , the slurry containing the defibrated fine fibrous cellulose exhibits weak alkalinity. If heating is performed in this state, monosaccharides, which are one of the causes of coloration, may be generated due to decomposition of cellulose, so it is preferable to adjust the pH of the slurry to 8 or less. Furthermore, since monosaccharides may also be generated under acidic conditions, it is preferable to adjust the pH of the slurry to 3 or more.

Furthermore, when the substituted fine fibrous cellulose is a fine fibrous cellulose having phosphate groups, from the viewpoint of improving the efficiency of removing the substituents, it is preferable that the phosphorus of the phosphate groups is in a state susceptible to nucleophilic attack. The state susceptible to nucleophilic attack is a state with a neutralization degree of 1, expressed as cellulose-O-P(=O)(-O-H ⁺ )(-O-Na ⁺ ). To achieve this state, it is preferable to adjust the pH of the slurry to 3 or more and 8 or less, and more preferably to adjust the pH to 4 or more and 6 or less.

The means for adjusting the pH is not particularly limited. For example, an acid or alkaline component may be added to a slurry containing fine fibrous cellulose. The acid component may be either an inorganic or organic acid. Inorganic acids include sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid. Organic acids include formic acid, acetic acid, citric acid, malic acid, lactic acid, adipic acid, sebacic acid, stearic acid, maleic acid, succinic acid, tartaric acid, fumaric acid, and gluconic acid. The alkaline component may be an inorganic or organic alkaline compound. Inorganic alkaline compounds include lithium hydroxide, sodium hydroxide, potassium hydroxide, lithium carbonate, lithium bicarbonate, potassium carbonate, potassium bicarbonate, sodium carbonate, and sodium bicarbonate. Examples of organic alkaline compounds include ammonia, hydrazine, methylamine, ethylamine, diethylamine, triethylamine, propylamine, dipropylamine, butylamine, diaminoethane, diaminopropane, diaminobutane, diaminopentane, diaminohexane, cyclohexylamine, aniline, tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, benzyltrimethylammonium hydroxide, pyridine, and N,N-dimethyl-4-aminopyridine.

In addition, in the pH adjustment step, ion exchange treatment may be performed to adjust the pH. A strong acid cation exchange resin or a weak acid ion exchange resin can be used for the ion exchange treatment. By treating with an appropriate amount of cation exchange resin for a sufficient period of time, a slurry containing fine fibrous cellulose with the desired pH can be obtained. Furthermore, in the pH adjustment step, the addition of an acid component or an alkali component may be combined with the ion exchange treatment.

<Salt removal treatment>
After the substituent removal treatment step, it is preferable to carry out a treatment to remove salts derived from the removed substituents. Removing the salts derived from the substituents makes it easier to obtain fine fibrous cellulose that can suppress coloration. The means for removing the salts derived from the substituents is not particularly limited, and examples thereof include washing treatment and ion exchange treatment. The washing treatment is carried out by washing the fine fibrous cellulose that has aggregated in the substituent removal treatment with, for example, water or an organic solvent. In the ion exchange treatment, an ion exchange resin can be used.

<Uniform Dispersion Treatment Step (Step (B))>
The method for producing a fine fibrous cellulose dispersion preferably further comprises a step (B) of performing a uniform dispersion treatment after the step (A). The step (B) of performing a uniform dispersion treatment is a step of uniformly dispersing the fine fibrous cellulose obtained through the substituent removal treatment of the step (A). In the step (A), the fine fibrous cellulose is subjected to the substituent removal treatment, whereby at least a portion of the fine fibrous cellulose is aggregated. The step (B) is a step of uniformly dispersing the aggregated fine fibrous cellulose. Thus, the fine fibrous cellulose obtained in this embodiment has a fiber width of 1000 nm or less (preferably 10 nm or less), despite having a low substituent introduction amount of less than 0.5 mmol/g.

In step (B) of the uniform dispersion treatment, for example, a high-speed defibrator, grinder (stone mill type grinder), high-pressure homogenizer, high-pressure collision type grinder, ball mill, bead mill, disk type refiner, conical refiner, twin-screw kneader, vibration mill, homomixer under high-speed rotation, ultrasonic disperser, or beater can be used. Of the above-mentioned uniform dispersion treatment devices, it is more preferable to use a high-speed defibrator or high-pressure homogenizer.

The processing conditions for the uniform dispersion process (B) are not particularly limited, but it is preferable to increase the maximum movement speed of the fine fibrous cellulose during processing and the pressure during processing. In the case of a high-speed fiberizer, the peripheral speed is preferably 20 m/sec or more, more preferably 25 m/sec or more, and even more preferably 30 m/sec or more. A high-pressure homogenizer is more preferably used than a high-speed fiberizer because it has a higher maximum movement speed of the fine fibrous cellulose during processing and a higher pressure during processing. In high-pressure homogenizer processing, the pressure during processing is preferably 1 MPa or more and 350 MPa or less, more preferably 10 MPa or more and 300 MPa or less, and even more preferably 50 MPa or more and 250 MPa or less.

In step (B), the above-mentioned spacer molecules may be added. By adding such spacer molecules in the uniform dispersion treatment step of step (B), uniform dispersion of the fine fibrous cellulose can be achieved more smoothly. This makes it possible to more effectively improve the transparency of the dispersion liquid and sheet containing the fine fibrous cellulose.

<Ion exchange process>
After step (A) of removing a portion of the anionic groups from fine fibrous cellulose having an anionic group and a fiber width of 1000 nm or less, a step of adding a polyvalent metal salt to a dispersion containing the fine fibrous cellulose is preferably included. In the step of adding the polyvalent metal salt, the counter ions of the fine fibrous cellulose are exchanged for polyvalent metal ions, and at least a portion of the fine fibrous cellulose aggregates, so this step is also called an ion exchange step or aggregation step. Note that the ion exchange step may be carried out after step (A), but is more preferably carried out after step (B) following step (A). In this embodiment, since a polyvalent metal salt is added to a dispersion containing fine fibrous cellulose obtained after the defibration treatment and the substituent removal treatment, the redispersibility of the fine fibrous cellulose can be further improved in the redispersion step described below. This makes it possible to obtain a redispersion liquid or sheet with excellent transparency.

The amount of polyvalent metal salt added in the ion exchange process is preferably 0.01 mmol or more, and more preferably 0.02 mmol or more, per 1 g of fine fibrous cellulose contained in the dispersion. Furthermore, the amount of polyvalent metal salt added in the ion exchange process is preferably 30 mmol or less, and more preferably 20 mmol or less, per 1 g of fine fibrous cellulose contained in the dispersion. When adding the polyvalent metal salt, it is preferable to add an aqueous solution containing the polyvalent metal salt.

In the ion exchange process, when agglomerates are generated by adding a polyvalent metal salt, it is preferable to further include a filtration process and a washing process after stirring. The washing process is performed by washing the agglomerates with, for example, water or an organic solvent. Then, by providing a filtration process after the washing process, a concentrate can be obtained. The filter material used in the filtration process is not particularly limited, but filter materials made of stainless steel, filter paper, polypropylene, nylon, polyethylene, polyester, etc. can be used. Since acid may be used, polypropylene filter materials are preferred. The lower the air permeability of the filter material, the higher the yield, so it is 30 cm ³ /cm ² ·sec or less, more preferably 10 cm ³ /cm ² ·sec or less, and even more preferably 1 cm ³ /cm ² ·sec or less.

The filtration step may further include a compression step. A compression device can be used in the compression step. Common press devices such as belt presses, screw presses, and filter presses can be used as such devices, and the device is not particularly limited. The pressure during compression is preferably 0.2 MPa or higher, and more preferably 0.4 MPa or higher.

The content of fine fibrous cellulose in the aggregate obtained in the ion exchange step is preferably 40% by mass or less, more preferably 20% by mass or less, and even more preferably less than 6% by mass, based on the total mass of the aggregate including the solvent. Furthermore, the content of fine fibrous cellulose in the aggregate is preferably 0.1% by mass or less, based on the total mass of the aggregate. By keeping the content of the aggregate within this range, it is possible to more effectively improve redispersibility in the redispersion step described below.

In the ion exchange process, a drying process may be included to ensure that the content of fine fibrous cellulose in the aggregate falls within a predetermined range. Furthermore, after the ion exchange process, a process of powdering the aggregate may be included.

Furthermore, in the ion exchange process, the step of adding a polyvalent metal salt may be carried out multiple times. By carrying out the step of adding a polyvalent metal salt multiple times, the counter ions of the fine fibrous cellulose can be efficiently exchanged with polyvalent metal ions.

<Re-dispersion (uniform dispersion) process>
After the ion exchange step, it is preferable to provide a step of redispersing the obtained aggregates and contained components in a solvent. In this embodiment, the redispersion step is sometimes referred to as a uniform dispersion treatment step because the fine fibrous cellulose in the aggregates is uniformly dispersed in the redispersion step. Through such a redispersion step, a dispersion liquid (redispersion liquid) in which the fine fibrous cellulose is uniformly dispersed is obtained. Note that the redispersion (uniform dispersion) step provided after the ion exchange step may be a step similar to the uniform dispersion treatment step (step (B)) provided after the substituent removal treatment step (step (A)).

The type of solvent is not particularly limited, but examples include water, organic solvents, and mixtures of water and organic solvents. Examples of organic solvents include alcohols, polyhydric alcohols, ketones, ethers, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and dimethylacetamide (DMAc). Examples of alcohols include methanol, ethanol, n-propanol, isopropanol, n-butanol, and t-butyl alcohol. Examples of polyhydric alcohols include ethylene glycol and glycerin. Examples of ketones include acetone and methyl ethyl ketone. Examples of ethers include diethyl ether, tetrahydrofuran, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-butyl ether, and ethylene glycol mono-t-butyl ether.

In the redispersion step, for example, a high-speed defibrator, grinder (stone mill), high-pressure homogenizer, high-pressure collision grinder, ball mill, bead mill, disk refiner, conical refiner, twin-screw kneader, vibration mill, homomixer under high-speed rotation, ultrasonic disperser, or beater can be used. Of the above-mentioned uniform dispersion treatment devices, it is more preferable to use a high-speed defibrator or high-pressure homogenizer.

The processing conditions for the redispersion step are not particularly limited, but it is preferable to increase the maximum movement speed of the fine fibrous cellulose during processing and the pressure during processing. In the case of a high-speed fiberizer, the peripheral speed is preferably 20 m/sec or more, more preferably 25 m/sec or more, and even more preferably 30 m/sec or more. A high-pressure homogenizer is more preferably used than a high-speed fiberizer because it has a higher maximum movement speed of the fine fibrous cellulose during processing and a higher pressure during processing. In high-pressure homogenizer processing, the pressure during processing is preferably 1 MPa or more and 350 MPa or less, more preferably 10 MPa or more and 300 MPa or less, and even more preferably 50 MPa or more and 250 MPa or less.

As described above, the method for producing a fine fibrous cellulose dispersion of the present invention preferably includes, in this order, a step (A) of removing a portion of the anionic groups from fine fibrous cellulose having an anionic group and a fiber width of 1000 nm or less, a step (B) of performing a uniform dispersion treatment, an ion exchange step, and a redispersion (uniform dispersion) step. In this embodiment, particularly in the step of redispersing (uniformly dispersing) the aggregates and components obtained in the ion exchange step in a solvent, a high-speed defibrator or high-pressure homogenizer is used to uniformly disperse the fine fibrous cellulose, thereby obtaining a dispersion that is highly transparent and has a high viscosity recovery rate. Furthermore, by forming a sheet using the fine fibrous cellulose dispersion obtained through these steps, a sheet that is highly transparent and has excellent yellowing resistance can be obtained.

(seat)
This embodiment also relates to a sheet formed from the above-described dispersion. The sheet of this embodiment contains fibrous cellulose having anionic groups and a fiber width of 1000 nm or less, in which the amount of anionic groups introduced into the fibrous cellulose is less than 0.5 mmol/g, and the fibrous cellulose has polyvalent metal ions as counter ions of the anionic groups.

The content of fibrous cellulose relative to the total mass of solids in the sheet is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more. Furthermore, the content of fibrous cellulose relative to the total mass of solids in the sheet is preferably 90% by mass or less, more preferably 85% by mass or less, and even more preferably 80% by mass or less. By keeping the content of fibrous cellulose within the above range, it becomes easier to obtain a sheet that has excellent transparency and yellowing resistance.

The monovalent metal element content in the sheet is preferably 40% or less, more preferably 30% or less, even more preferably 25% or less, even more preferably 20% or less, even more preferably 15% or less, and particularly preferably 10% or less. The lower limit of the monovalent metal element content in the sheet is not particularly limited and may be 0%. Here, the monovalent metal element content in the sheet is a value calculated as follows using X-ray fluorescence analysis. First, for creating a calibration curve, filter papers with known elemental contents of sodium, calcium, magnesium, zinc, nickel, copper, and cobalt are prepared, and the X-ray intensity of each filter paper is measured by X-ray fluorescence analysis. Next, a calibration curve is created based on the obtained X-ray intensity and the known content of the analyzed element. Next, the X-ray intensity of the measurement target sheet (sheets obtained in the Examples and Comparative Examples) is measured by X-ray fluorescence analysis. From the obtained X-ray intensity and the above calibration curve, the content (mmol) of each analyzed element in the measurement target sheet is determined, and the monovalent metal element content is calculated using the following formula:
Monovalent metal element content (%) = Monovalent metal element content (mmol) / Total metal element content (mmol) × 100
By setting the content of the monovalent metal element within the above range, the yellowing resistance and water resistance of the sheet can be more effectively improved.

The yellowness index (YI) of the sheet is preferably 2.0 or less, more preferably 1.5 or less, and even more preferably 1.0 or less. The lower limit of the yellowness index (YI) of the sheet is not particularly limited, but may be 0.0. Note that the above yellowness index (YI) of the sheet is the yellowness index (YI) before the sheet is heated.

Furthermore, when the sheet is heated at 160°C for 6 hours, the YI increase rate is preferably 1600% or less, more preferably 1580% or less, even more preferably 1560% or less, and particularly preferably 1540% or less. The lower limit of the YI increase rate is not particularly limited, and may be 0%. Here, the YI increase rate is a value calculated by the following formula.
YI increase rate (%)=(yellowing index of sheet after heating−yellowing index of sheet before heating)/yellowing index of sheet before heating×100
In the above formula, the yellowness of the sheet is the yellowness measured in accordance with JIS K 7373:2006, the yellowness of the sheet after heating is the yellowness after the sheet is heated at 160°C for 6 hours, and the yellowness of the sheet before heating is the yellowness before the sheet is heated at 160°C for 6 hours.

The total light transmittance of the sheet is preferably 70% or more, more preferably 75% or more, even more preferably 80% or more, even more preferably 85% or more, and particularly preferably 90% or more. There is no particular upper limit to the total light transmittance of the sheet, and it may be 100%. Here, the total light transmittance of the sheet is a value measured in accordance with JIS K 7361-1:1997. An instrument for measuring haze can be, for example, a haze meter (HM-150) manufactured by Murakami Color Research Laboratory Co., Ltd.

The haze of the sheet is preferably 10% or less, more preferably 5% or less, even more preferably 4% or less, even more preferably 3% or less, and particularly preferably 2.5% or less. There is no particular limit to the lower limit of the sheet haze, and it may be 0%. Here, the haze of the sheet is a value measured in accordance with JIS K 7136:2000. An instrument for measuring haze can be, for example, a haze meter (HM-150) manufactured by Murakami Color Research Laboratory Co., Ltd.

The thickness of the sheet is not particularly limited, but is preferably 5 μm or more, more preferably 10 μm or more, even more preferably 20 μm or more, even more preferably 30 μm or more, and particularly preferably 40 μm or more. Furthermore, the thickness of the sheet is preferably 200 μm or less. In this embodiment, polyvalent metal ions are introduced as counterions to the anionic groups before the sheet is formed, so that even thick sheets exhibit excellent yellowing resistance.

The basis weight of the sheet is not particularly limited, but is preferably 10 g/m2 or ^more , and more preferably 20 g/m2 or ^more . The basis weight of the sheet is preferably 200 g/m2 or ^less , and more preferably 150 g/m2 ^{or less} . The basis weight of the sheet is a value calculated according to the following method. A sheet cut into a size of 50 mm square or more is conditioned at 23°C and a relative humidity of 50% for 24 hours, and then the weight is measured, and this weight is divided by the area of the cut sheet to calculate the basis weight of the sheet.

The density of the sheet is not particularly limited, but is preferably 0.5 g/cm or ^more , more preferably 1 g/cm or ^more , and even more preferably 1.2 g/cm or more. The density of the sheet is preferably 3 g/cm ^or less, more preferably 2 g/cm or ^less , and even ^more preferably 1.8 g/cm ^or less. The density of the sheet is calculated by dividing the basis weight of the sheet by the thickness.

The water absorption rate of the sheet is preferably 600% or less, more preferably 580% or less, and even more preferably 550% or less. The water absorption rate of the sheet is preferably 0% or more. By setting the water absorption rate of the sheet within the above range, a sheet with excellent water resistance can be obtained. Here, the water absorption rate of the sheet is a value calculated by the following method. First, a sheet is cut into 50 mm square sheets and immersed in ion-exchanged water for 24 hours. The water absorption rate of the sheet is then calculated using the following formula. The bone dry mass of the sheet is the mass measured after drying the sheet at 105°C for 24 hours.
Water absorption rate of sheet (%) = Mass (g) of sheet after immersion in ion-exchanged water / Bone-dry mass (g) of sheet × 100

In this embodiment, the sheet preferably contains a resin in addition to the above-mentioned fibrous cellulose. The type of resin is not particularly limited, but examples include thermoplastic resins and thermosetting resins.

Examples of resins include acrylic resins, polystyrene resins, polycarbonate resins, polyester resins, polyamide resins, silicone resins, fluorine-based resins, chlorine-based resins, epoxy resins, melamine resins, phenolic resins, polyurethane resins, diallyl phthalate resins, alcohol-based resins, cellulose derivatives, and precursors of these resins. Examples of cellulose derivatives include carboxymethyl cellulose, methyl cellulose, and hydroxyethyl cellulose. The type of resin precursor is not particularly limited, but examples include precursors of thermoplastic resins and thermosetting resins. A precursor of a thermoplastic resin refers to a monomer or a relatively low-molecular-weight oligomer used to produce a thermoplastic resin. A precursor of a thermosetting resin refers to a monomer or a relatively low-molecular-weight oligomer that can undergo a polymerization or crosslinking reaction under the action of light, heat, or a curing agent to form a thermosetting resin. If the sheet contains a resin precursor, the polymerization or crosslinking reaction of the resin precursor may be promoted by the action of light, heat, or a curing agent in a subsequent process or during use, depending on the intended use.

The resin content relative to the total mass of solids in the sheet is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more. Furthermore, the resin content relative to the total mass of solids in the sheet is preferably 90% by mass or less, more preferably 80% by mass or less, and even more preferably 70% by mass or less. By keeping the resin content within the above range, it becomes easier to obtain a sheet with excellent transparency and suppressed discoloration.

In addition, in this embodiment, the sheet may contain optional components in addition to the above-mentioned fibrous cellulose and resin. Examples of optional components include the optional components that may be contained in the dispersion liquid.

(Laminate)
This embodiment may also relate to a laminate having a structure in which another layer is further laminated on the above-mentioned sheet. Such another layer may be provided on both surfaces of the sheet, or may be provided only on one surface of the sheet. Examples of the other layer laminated on at least one surface of the sheet include a resin layer and an inorganic layer.

<Resin layer>
The resin layer is a layer whose main component is a natural resin or a synthetic resin. Here, the main component refers to a component that is contained in an amount of 50% by mass or more relative to the total mass of the resin layer. The resin content is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more relative to the total mass of the resin layer. The resin content may be 100% by mass or may be 95% by mass or less.

Examples of natural resins include rosin-based resins such as rosin, rosin ester, and hydrogenated rosin ester.

Examples of synthetic resins include polyolefin resins, cyclic olefin resins, polycarbonate resins, polyethylene terephthalate resins, polyethylene naphthalate resins, polyimide resins, polystyrene resins, and acrylic resins. Among these, the synthetic resin is preferably a polyolefin resin, and preferably contains at least one resin selected from polyethylene resins and polypropylene resins.

Examples of the polycarbonate resin that constitutes the resin layer include aromatic polycarbonate resins and aliphatic polycarbonate resins. Specific examples of these polycarbonate resins are known, such as the polycarbonate resins described in JP 2010-023275 A.

The resin layer may be made of a single resin, or a copolymer formed by copolymerization or graft polymerization of multiple resin components. Alternatively, multiple resin components may be mixed using a physical process to form a blend material.

An adhesive layer may be provided between the sheet and the resin layer, or no adhesive layer may be provided and the sheet and resin layer may be directly adhered to each other. When an adhesive layer is provided between the sheet and the resin layer, the adhesive constituting the adhesive layer may be, for example, an acrylic resin. Examples of adhesives other than acrylic resins include vinyl chloride resin, (meth)acrylic ester resin, styrene/acrylic ester copolymer resin, vinyl acetate resin, vinyl acetate/(meth)acrylic ester copolymer resin, urethane resin, silicone resin, epoxy resin, ethylene/vinyl acetate copolymer resin, polyester resin, polyvinyl alcohol resin, ethylene-vinyl alcohol copolymer resin, and rubber emulsions such as SBR and NBR.

When no adhesive layer is provided between the sheet and the resin layer, the resin layer may contain an adhesion aid, and the surface of the resin layer may be subjected to a surface treatment such as hydrophilization.
Examples of the adhesion aid include a compound containing at least one selected from an isocyanate group, a carbodiimide group, an epoxy group, an oxazoline group, an amino group, and a silanol group, and an organosilicon compound. Among them, the adhesion aid is preferably at least one selected from a compound containing an isocyanate group (isocyanate compound) and an organosilicon compound. Examples of the organosilicon compound include a silane coupling agent condensate and a silane coupling agent.
Examples of the surface treatment method include corona treatment, plasma discharge treatment, UV irradiation treatment, electron beam irradiation treatment, and flame treatment.

<Inorganic layer>
The material constituting the inorganic layer is not particularly limited, and examples thereof include aluminum, silicon, magnesium, zinc, tin, nickel, and titanium; their oxides, carbides, nitrides, oxycarbides, oxynitrides, and oxycarbonitrides; and mixtures thereof. From the viewpoint of stably maintaining high moisture resistance, silicon oxide, silicon nitride, silicon oxide carbide, silicon oxynitride, silicon oxycarbonitride, aluminum oxide, aluminum nitride, aluminum oxide carbide, aluminum oxynitride, and mixtures thereof are preferred.

The method for forming the inorganic layer is not particularly limited. Generally, methods for forming thin films can be broadly divided into chemical vapor deposition (CVD) and physical vapor deposition (PVD), but either method may be used. Specific examples of CVD methods include plasma CVD, which uses plasma, and catalytic chemical vapor deposition (Cat-CVD), which uses a heated catalyst to catalytically decompose a material gas. Specific examples of PVD methods include vacuum deposition, ion plating, and sputtering.

Atomic layer deposition (ALD) can also be used to form inorganic layers. ALD is a method for forming thin films atomically by alternately supplying the source gases of each element that make up the film to be formed to the surface on which the layer is to be formed. While it has the disadvantage of a slow film formation speed, it has the advantage of being able to coat even complex surfaces cleanly, more so than plasma CVD, and to deposit thin films with fewer defects. ALD also has the advantage of being able to control film thickness at the nanometer level, making it relatively easy to cover large surfaces. Furthermore, the use of plasma in ALD is expected to improve reaction speed, enable lower processing temperatures, and reduce unreacted gases.

(Sheet manufacturing method)
The sheet manufacturing method preferably includes a step of forming a sheet from the dispersion obtained by the above-described method. The step of forming a sheet from the dispersion preferably includes a coating step of coating the dispersion onto a substrate, or a papermaking step of making paper from the dispersion.

<Coating process>
In the coating step, the dispersion obtained in the dispersion obtaining step is coated on a substrate, and the coated substrate is dried to form a sheet, which can be peeled off from the substrate to obtain a sheet. In addition, by using a coating device and a long substrate, sheets can be produced continuously.

The material of the substrate used in the coating process is not particularly limited, but a substrate with high wettability to the dispersion liquid can suppress shrinkage of the sheet during drying, but it is preferable to select a substrate that allows the formed sheet to be easily peeled off after drying. Of these, resin films or plates or metal films or plates are preferred, but there are no particular limitations. For example, resin films or plates such as acrylic, polyethylene terephthalate, vinyl chloride, polystyrene, and polyvinylidene chloride, metal films or plates such as aluminum, zinc, copper, and iron plates, and those with oxidized surfaces, stainless steel films or plates, and brass films or plates can be used.

If the viscosity of the dispersion is low and it spreads on the substrate during the coating process, a blocking frame may be fixed to the substrate to obtain a sheet of the desired thickness and basis weight. There are no particular limitations on the blocking frame, but it is preferable to select one that allows the edges of the sheet to be easily peeled off after drying. From this perspective, molded resin or metal plates are more preferable. In this embodiment, for example, resin plates such as acrylic plates, polyethylene terephthalate plates, polyvinyl chloride plates, polystyrene plates, and polyvinylidene chloride plates, metal plates such as aluminum plates, zinc plates, copper plates, and iron plates, and plates with their surfaces oxidized, as well as molded stainless steel plates, brass plates, etc. can be used.

The coating machine used to apply the dispersion to the substrate is not particularly limited, but examples include roll coaters, gravure coaters, die coaters, curtain coaters, and air doctor coaters. Die coaters, curtain coaters, and spray coaters are particularly preferred because they can produce a more uniform sheet thickness.

The dispersion temperature and ambient temperature when applying the dispersion to the substrate are not particularly limited, but are preferably from 5°C to 80°C, more preferably from 10°C to 60°C, even more preferably from 15°C to 50°C, and particularly preferably from 20°C to 40°C. If the application temperature is above the lower limit, the dispersion can be applied more easily. If the application temperature is below the upper limit, evaporation of the dispersion medium during application can be suppressed.

In the coating step, the dispersion is preferably applied to the substrate so that the finished basis weight of the sheet is preferably from 10 g/m ² to 200 g/m ² , more preferably from 20 g/m ² to 150 g/m ^2. By coating so that the basis weight falls within the above range, a sheet with excellent strength can be obtained.

As described above, the coating process includes drying the dispersion liquid coated on the substrate. The drying process for the dispersion liquid is not particularly limited, but can be performed by, for example, a non-contact drying method, a method of drying while restraining the sheet, or a combination of these. Non-contact drying methods include, but are not limited to, a method of drying by heating with hot air, infrared rays, far-infrared rays, or near-infrared rays (heat drying method), or a method of drying in a vacuum (vacuum drying method). While heat drying and vacuum drying may be combined, heat drying is typically used. Drying with infrared rays, far-infrared rays, or near-infrared rays can be performed using, for example, an infrared device, a far-infrared device, or a near-infrared device. The heating temperature in the heat drying method is not particularly limited, but is preferably 20°C to 150°C, and more preferably 25°C to 105°C. Setting the heating temperature above the lower limit allows for rapid volatilization of the dispersion medium. Setting the heating temperature below the upper limit reduces heating costs and suppresses heat-induced discoloration of the fibrous cellulose.

<Paper making process>
The papermaking process is carried out by making paper from the dispersion using a papermaking machine. The papermaking machine used in the papermaking process is not particularly limited, but examples thereof include continuous papermaking machines such as Fourdrinier, cylinder, and tilting types, and multi-layer papermaking machines that combine these. In the papermaking process, known papermaking methods such as handmaking may also be used.

The papermaking process involves filtering and dehydrating the dispersion through a wire to obtain a wet sheet, which is then pressed and dried. The filter cloth used to filter and dehydrate the dispersion is not particularly limited, but it is preferable that it does not allow fibrous cellulose to pass through and that the filtration rate does not become too slow. Such filter cloths are not particularly limited, but are preferably sheets, woven fabrics, or porous membranes made of organic polymers. The organic polymer is not particularly limited, but is preferably a non-cellulose organic polymer such as polyethylene terephthalate, polyethylene, polypropylene, or polytetrafluoroethylene (PTFE). In this embodiment, examples include porous membranes of polytetrafluoroethylene with a pore size of 0.1 μm to 20 μm, and woven polyethylene terephthalate or polyethylene with a pore size of 0.1 μm to 20 μm.

In the sheet-forming process, a method for producing a sheet from a dispersion can be carried out using a production apparatus equipped with a water-squeezing section in which a dispersion containing fine fibrous cellulose is discharged onto the upper surface of an endless belt and the dispersion medium is squeezed out of the discharged dispersion to produce a web, and a drying section in which the web is dried to produce a sheet. An endless belt is disposed between the water-squeezing section and the drying section, and the web produced in the water-squeezing section is transported to the drying section while still on the endless belt.

The dehydration method used in the papermaking process is not particularly limited, but examples include dehydration methods commonly used in paper manufacturing. Among these, methods of dehydrating using a Fourdrinier, cylinder, or inclined wire, followed by further dehydration using a roll press, are preferred. Furthermore, the drying method used in the papermaking process is not particularly limited, but examples include methods used in paper manufacturing. Among these, drying methods using a cylinder dryer, Yankee dryer, hot air dryer, near-infrared heater, infrared heater, etc. are more preferred.

(Application)
The fibrous cellulose dispersion of this embodiment can be used as an additive for, for example, foods, cosmetics, cement, paints (for painting vehicles such as automobiles, ships, and aircraft, for building materials, for daily necessities, etc.), inks, pharmaceuticals, etc. Furthermore, the fine fibrous cellulose of this embodiment can also be applied to daily necessities by adding it to resin-based materials or rubber-based materials.

The sheet containing the fibrous cellulose of this embodiment may also be used for optical components. For example, it can be used as a light-transmitting substrate for various display devices, various solar cells, and the like. The laminate sheet of the present invention is also suitable for applications such as substrates for electronic devices, components for home appliances, window materials for various vehicles and buildings, interior and exterior materials, and packaging materials.

The following examples and comparative examples further illustrate the features of the present invention. The materials, amounts used, ratios, processing details, processing procedures, etc. shown in the following examples can be modified as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the specific examples shown below.

<Production Example 1>
[Phosphorylation]
Hardwood dissolving pulp (dry sheet) manufactured by Oji Paper Co., Ltd. was used as the raw material pulp. This raw material pulp was subjected to a phosphating treatment as follows. First, a mixed aqueous solution of ammonium dihydrogen phosphate and urea was added to 100 parts by mass (bone dry mass) of the raw material pulp to adjust the composition to 45 parts by mass of ammonium dihydrogen phosphate, 120 parts by mass of urea, and 150 parts by mass of water, thereby obtaining a chemical-impregnated pulp. Next, the obtained chemical-impregnated pulp was heated in a hot air dryer at 165°C for 200 seconds to introduce phosphate groups into the cellulose in the pulp, thereby obtaining a phosphorylated pulp.

[Cleaning process]
The resulting phosphorylated pulp was then washed. The washing process consisted of pouring 10 L of ion-exchanged water into 100 g (bone dry mass) of phosphorylated pulp to obtain a pulp dispersion, stirring the resulting solution to uniformly disperse the pulp, and then repeatedly filtering and dehydrating the pulp. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less.

[Neutralization treatment]
Next, the washed phosphorylated pulp was neutralized as follows: First, the washed phosphorylated pulp was diluted with 10 L of ion-exchanged water, and then a 1 N aqueous solution of sodium hydroxide was added little by little while stirring to obtain a phosphorylated pulp slurry having a pH of 12 to 13. Next, the phosphorylated pulp slurry was dehydrated to obtain a neutralized phosphorylated pulp. Next, the neutralized phosphorylated pulp was subjected to the above-mentioned washing treatment.

The infrared absorption spectrum of the phosphorylated pulp obtained was measured using FT-IR, and as a result, absorption due to the phosphate group was observed around 1230 cm ⁻¹ , confirming that the phosphate group had been added to the pulp.

[Defibrillation process]
Ion-exchanged water was added to the obtained phosphorylated pulp to prepare a slurry with a solids concentration of 2% by mass. This slurry was treated six times with a high-pressure homogenizer (Starburst, manufactured by Sugino Machine Co., Ltd.) at a pressure of 200 MPa to obtain a fine fibrous cellulose dispersion containing fine fibrous cellulose. X-ray diffraction confirmed that this fine fibrous cellulose maintained cellulose type I crystallinity. The amount of phosphate groups (amount of first dissociated acid) measured by the measurement method described below in [Measurement of Phosphorus Oxo Acid Group Amount] was 1.45 mmol/g. The total amount of dissociated acid was 2.45 mmol/g.

<Production Example 2>
[Phosphorous Treatment]
The same procedure as in Production Example 1 was carried out, except that 33 parts by mass of phosphorous acid (phosphonic acid) was used instead of ammonium dihydrogen phosphate in the phosphite treatment, to obtain a fine fibrous cellulose dispersion containing phosphite pulp and fine fibrous cellulose.

The infrared absorption spectrum of the resulting phosphite pulp was measured using FT-IR. As a result, absorption due to P=O of the phosphonic acid group, which is a tautomer of the phosphite group, was observed around 1210 cm ^-1 , confirming that phosphite groups (phosphonic acid groups) had been added to the pulp. Furthermore, X-ray diffraction confirmed that the resulting fine fibrous cellulose maintained cellulose type I crystal structure. The amount of phosphite groups (amount of first dissociated acid), measured by the method described in [Measurement of phosphorus oxo acid group amount] below, was 1.51 mmol/g, and the total amount of dissociated acid was 1.54 mmol/g.

<Production Example 3>
The same operation as in Production Example 1 was performed except that the following xanthation treatment was performed instead of the phosphate treatment, to obtain a fine fibrous cellulose dispersion containing xanthated pulp and fine fibrous cellulose.

[Xanthate treatment]
To 100 parts by mass (bone dry mass) of raw pulp (hardwood dissolving pulp (dry sheet) manufactured by Oji Paper Co., Ltd.), 2500 parts by mass of an 8.5% by mass aqueous solution of sodium hydroxide was added, and the mixture was stirred at room temperature for 3 hours to perform an alkali treatment. The pulp after the alkali treatment was centrifuged (filter cloth 400 mesh, 3000 rpm for 5 minutes) to obtain a dehydrated alkali cellulose. 3.5 parts by mass of carbon disulfide was added to 10 parts by mass (bone dry mass) of the obtained alkali cellulose, and the sulfurization reaction was allowed to proceed at room temperature for 4.5 hours to perform a xanthate treatment.

X-ray diffraction confirmed that the resulting fine fibrous cellulose maintained cellulose type I crystal structure. The amount of xanthate groups measured using the method described below in "Measurement of xanthate group amount" was 1.73 mmol/g.

<Production Example 4>
[Sulfation treatment]
The same operations as in Production Example 1 were carried out, except that 38 parts by mass of amidosulfuric acid (sulfamic acid) was used instead of ammonium dihydrogen phosphate in the phosphorylation treatment and the heating time was extended to 20 minutes, to obtain a fine fibrous cellulose dispersion containing sulfated pulp and fine fibrous cellulose.

The infrared absorption spectrum of the sulfated pulp obtained was measured using FT-IR. As a result, absorption due to sulfate groups was observed around 1220-1260 cm ^-1 , confirming that sulfate groups had been added to the pulp. Furthermore, X-ray diffraction confirmed that the obtained fine fibrous cellulose maintained cellulose type I crystal structure. The amount of sulfate groups measured by the measurement method described below in [Measurement of sulfur oxoacid group amount] was 1.47 mmol/g.

<Production Example 5>
The same procedure as in Production Example 1 was carried out, except that the following maleic oxidation treatment was carried out instead of the phosphating treatment, to obtain a fine fibrous cellulose dispersion containing maleic pulp and fine fibrous cellulose.

[Maleic oxidation treatment]
Raw material pulp (hardwood dissolving pulp (dry sheet) manufactured by Oji Paper Co., Ltd.) was dried at 105°C for 3 hours to obtain dry pulp with a moisture content of 3% by mass or less. Next, 100 parts by mass of the dry pulp and 50 parts by mass of maleic anhydride were charged into an autoclave and treated at 150°C for 2 hours to carry out maleic oxidation treatment.

The infrared absorption spectrum of the maleated pulp thus obtained was measured using FT-IR. As a result, absorption due to carboxy groups was observed around 1580 and 1720 cm ⁻¹ , confirming that carboxy group-containing substituents (maleic acid groups) had been added to the pulp. Furthermore, X-ray diffraction confirmed that the obtained fine fibrous cellulose maintained cellulose type I crystal structure. The amount of carboxy groups measured by the measurement method described in [Measurement of Carboxy Group Amount] below was 1.22 mmol/g.

[Measurement of phosphorus oxoacid group content]
In measuring the amount of phosphorus oxoacid groups (amount of phosphate groups or phosphite groups), ion-exchanged water was first added to the target fine fibrous cellulose to prepare a slurry with a solids concentration of 0.2% by mass. The resulting slurry was treated with an ion-exchange resin and then titrated with an alkali to measure the amount of phosphorus oxoacid groups.
The treatment with ion exchange resin was carried out by adding 1/10 by volume of a strongly acidic ion exchange resin (Amberjet 1024; manufactured by Organo Corporation, conditioned) to the above-mentioned fine fibrous cellulose-containing slurry, shaking the mixture for 1 hour, and then pouring the mixture onto a mesh with 90 μm openings to separate the resin from the slurry.
In addition, the alkali titration was performed by adding 10 μL of 0.1 N aqueous sodium hydroxide solution to the fine fibrous cellulose-containing slurry after treatment with an ion exchange resin at 5-second intervals, while measuring the change in the pH value of the slurry. Nitrogen gas was introduced into the slurry 15 minutes before the start of the titration. In this neutralization titration, two maximum points of increment (the differential pH value with respect to the amount of alkali added) were observed on the curve plotting the measured pH against the amount of alkali added. Of these, the maximum point of increment obtained first after starting the addition of alkali is called the first endpoint, and the maximum point of increment obtained next is called the second endpoint (FIG. 1). The amount of alkali required from the start of the titration to the first endpoint is equal to the amount of first dissociated acid in the slurry used in the titration. Furthermore, the amount of alkali required from the start of the titration to the second endpoint is equal to the total amount of dissociated acid in the slurry used in the titration. The amount of alkali (mmol) required from the start of titration to the first endpoint was divided by the solid content (g) in the slurry to be titrated, and the value was taken as the amount of phosphorus oxo acid groups (mmol/g).

[Measurement of sulfur oxoacid group content]
The amount of sulfur oxoacid groups was measured as follows. The obtained fibrous cellulose was wet ashed using perchloric acid and concentrated nitric acid, then diluted appropriately and the amount of sulfur was measured by ICP atomic emission spectrometry. The amount of sulfur was divided by the bone dry mass of the fibrous cellulose tested, and the value was taken as the amount of sulfur oxoacid groups (unit: mmol/g).

[Measurement of xanthate group amount]
The amount of xanthate groups was measured by the Breedee method. Specifically, 40 mL of saturated ammonium chloride solution was added to 1.5 parts by mass (bone dry mass) of fibrous cellulose, and the sample was crushed with a glass rod while mixing thoroughly. After leaving it for about 15 minutes, it was filtered with GFP filter paper (GS-25 manufactured by ADVANTEC) and thoroughly washed with saturated ammonium chloride solution. The sample was placed in a 500 mL tall beaker together with the GFP filter paper, and 50 mL of 0.5 M sodium hydroxide solution (5 ° C.) was added and stirred. After leaving it for 15 minutes, phenolphthalein solution was added until the solution turned pink, and then 1.5 M acetic acid was added, and the point at which the solution changed from pink to colorless was taken as the neutralization point. After neutralization, 250 mL of distilled water was added and stirred well, and 10 mL of 1.5 M acetic acid and 10 mL of 0.05 mol / L iodine solution were added using a whole pipette. This solution was titrated with 0.05 mol / L sodium thiosulfate solution. The amount of xanthate groups was calculated using the following formula from the titration amount of sodium thiosulfate and the bone dry mass of fibrous cellulose.
Amount of xanthate group (mmol/g) = (0.05 × 10 × 2 - 0.05 × sodium thiosulfate titration amount (mL)) / 1000 / bone dry mass of fibrous cellulose (g)

[Measurement of Carboxy Group Amount]
The amount of carboxy groups in the fine fibrous cellulose was measured by adding ion-exchanged water to a fine fibrous cellulose dispersion containing the target (maleated) fine fibrous cellulose to adjust the content to 0.2 mass%, treating the dispersion with an ion-exchange resin, and then titrating the dispersion with an alkali.
The treatment with ion exchange resin was carried out by adding 1/10 by volume of a strongly acidic ion exchange resin (Amberjet 1024; manufactured by Organo Corporation, conditioned) to a 0.2% by mass slurry containing fine fibrous cellulose, shaking the mixture for 1 hour, and then pouring the mixture onto a mesh with 90 μm openings to separate the resin from the slurry.
In addition, the titration using alkali was performed by adding 0.1 N aqueous sodium hydroxide to the fibrous cellulose-containing slurry after treatment with an ion exchange resin, while measuring the change in the pH value of the slurry. Observing the change in pH while adding aqueous sodium hydroxide yields a titration curve such as that shown in Figure 2. As shown in Figure 2, in this neutralization titration, a single point is observed in the curve plotting the measured pH against the amount of alkali added, where the increment (the differential value of pH with respect to the amount of alkali added) is maximized. This maximum increment is called the first end point. Here, the region from the start of titration to the first end point in Figure 2 is called the first region. The amount of alkali required in the first region is equal to the amount of carboxyl groups in the slurry used for titration. The amount of alkali (mmol) required in the first region of the titration curve was divided by the solids content (g) in the fine fibrous cellulose-containing slurry to be titrated to calculate the amount of carboxyl groups introduced (mmol/g).
The above-mentioned amount of carboxyl groups introduced (mmol/g) indicates the amount of substituents per 1 g of fibrous cellulose mass when the counter ion of the carboxyl group is a hydrogen ion (H ⁺ ) (hereinafter referred to as the amount of carboxyl groups (acid type)).

[Measurement of sulfur oxoacid group content]
The amount of sulfur oxoacid groups was calculated by freeze-drying a dispersion containing fibrous cellulose and then pulverizing the sample to measure the amount of sulfur. Specifically, a dispersion containing fibrous cellulose was freeze-dried and then pulverized. The resulting sample was subjected to pressure-heat decomposition using nitric acid in a sealed container, appropriately diluted, and the amount of sulfur was measured by ICP-OES. The value calculated by dividing the value by the bone-dry mass of the fibrous cellulose tested was taken as the amount of sulfur oxoacid groups (unit: mmol/g) of the fibrous cellulose.

Example 1
[Substituent removal treatment (high-temperature heat treatment)]
A 20% by mass aqueous citric acid solution was added to the fine fibrous cellulose dispersion obtained in Production Example 1, and the pH of the dispersion was adjusted to 5.5. The resulting slurry was placed in a pressure-resistant container and heated at a liquid temperature of 160°C for 15 minutes until the amount of phosphate groups reached 0.08 mmol/g. The production of fine fibrous cellulose aggregates was confirmed by this operation.

[Cleaning treatment of slurry after removal of substituents]
Ion-exchanged water was added to the obtained aggregate to prepare a slurry with a solids concentration of approximately 1% by mass. The slurry was stirred and then filtered and dehydrated, and the slurry was washed by repeating this process. When the electrical conductivity of the filtrate reached 10 μS/cm or less, ion-exchanged water was added again to prepare a slurry with a solids concentration of approximately 1% by mass, and the mixture was allowed to stand for 24 hours. The filtration and dehydration process was then repeated, and the washing end point was determined when the electrical conductivity of the filtrate again reached 10 μS/cm or less. Ion-exchanged water was added to the obtained fine fibrous cellulose aggregate, and a slurry was obtained after removing the substituents. The solids concentration of this slurry was 1.7% by mass.

[Uniform dispersion of slurry after removal of substituents]
Ion-exchanged water was added to the obtained slurry after removal of substituents to make a slurry with a solids concentration of 1.0 mass%, which was then treated three times at a pressure of 200 MPa in a wet atomization apparatus (Starburst, manufactured by Sugino Machine Co., Ltd.) to obtain a dispersion of substituent-removed fine fibrous cellulose containing substituent-removed fine fibrous cellulose. The number-average fiber width of the substituent-removed fine fibrous cellulose measured in [Measurement of fiber width] described below was 4 nm.

[Polyvalent metal ion substitution]
To 100 parts by mass of the obtained dispersion of the substituent-removed fine fibrous cellulose, 100 parts by mass of a 0.036% by mass aqueous calcium chloride solution was added, stirred, and allowed to stand for 30 minutes. This operation confirmed the formation of fine fibrous cellulose aggregates. Subsequently, washing was performed by repeatedly dehydrating by filtration and adding ion-exchanged water. The washing endpoint was determined when the electrical conductivity of the filtrate reached 10 μS/cm or less. Ion-exchanged water was added to the obtained fine fibrous cellulose aggregates to prepare a dispersion with a solids concentration of 1.0% by mass, and 100 parts by mass of a 0.036% by mass aqueous calcium chloride solution was added again to 100 parts by mass of the dispersion, followed by the same filtration, dehydration, and washing operations as above. Ion-exchanged water was added to the obtained fine fibrous cellulose aggregates to prepare a dispersion with a solids concentration of 1.0% by mass, and the dispersion was stirred at 1000 rpm for 3 minutes in a disperser to obtain a polyvalent metal ion-containing fine fibrous cellulose dispersion.

[Uniform Dispersion of Polyvalent Metal Ion-Containing Fine Fibrous Cellulose Dispersion]
The obtained dispersion of polyvalent metal ions-containing fine fibrous cellulose was treated five times at a pressure of 100 MPa with a high-pressure homogenizer (Beryu-Mini, manufactured by Biryu Co., Ltd.) to obtain a dispersion of polyvalent metal ions-containing fine fibrous cellulose. The number-average fiber width of the fine fibrous cellulose measured in the "Fiber Width Measurement" section described below was 4 nm.

[Sheet production]
Acetoacetyl group-modified polyvinyl alcohol (Gohsenex Z-200, manufactured by Mitsubishi Chemical Corporation) was added to ion-exchanged water so as to have a concentration of 12% by mass, and the mixture was stirred at 95° C. for 1 hour to dissolve. By the above procedure, an aqueous polyvinyl alcohol solution was obtained.
The polyvalent metal ion-containing fine fibrous cellulose dispersion and the polyvinyl alcohol aqueous solution were each diluted with ion-exchanged water to a solids concentration of 0.6% by mass. Next, 70 parts by mass of the diluted polyvalent metal ion-containing fine fibrous cellulose dispersion were mixed with 30 parts by mass of the diluted polyvinyl alcohol aqueous solution to obtain a mixed solution. The mixed solution was then weighed so that the finished sheet had a basis weight of 70 g/m ² and spread on a commercially available acrylic plate. A damming frame (inner dimensions 250 mm x 250 mm, height 5 cm) was placed on the acrylic plate to obtain the desired basis weight. The sheet was then dried in a dryer at 70 ° C. for 24 hours and peeled off from the acrylic plate to obtain a polyvalent metal ion-containing fine fibrous cellulose sheet. The sheet had a thickness of 50 μm.

Example 2
A polyvalent metal ion-containing fine fibrous cellulose dispersion and a polyvalent metal ion-containing fine fibrous cellulose sheet were obtained in the same manner as in Example 1, except that in the [polyvalent metal ion substitution], a 0.039 mass% magnesium sulfate aqueous solution was used instead of the 0.036 mass% calcium chloride aqueous solution.

Example 3
A polyvalent metal ion-containing fine fibrous cellulose dispersion and a polyvalent metal ion-containing fine fibrous cellulose sheet were obtained in the same manner as in Example 1, except that in the [polyvalent metal ion substitution], a 0.052 mass% zinc sulfate aqueous solution was used instead of the 0.036 mass% calcium chloride aqueous solution.

Example 4
A polyvalent metal ion-containing fine fibrous cellulose dispersion and a polyvalent metal ion-containing fine fibrous cellulose sheet were obtained in the same manner as in Example 1, except that in the [polyvalent metal ion substitution], a 0.041 mass % nickel chloride aqueous solution was used instead of the 0.036 mass % calcium chloride aqueous solution.

Example 5
A polyvalent metal ion-containing fine fibrous cellulose dispersion and a polyvalent metal ion-containing fine fibrous cellulose sheet were obtained in the same manner as in Example 1, except that in the [polyvalent metal ion substitution], a 0.051 mass % copper sulfate aqueous solution was used instead of the 0.036 mass % calcium chloride aqueous solution.

Example 6
A polyvalent metal ion-containing fine fibrous cellulose dispersion and a polyvalent metal ion-containing fine fibrous cellulose sheet were obtained in the same manner as in Example 1, except that in the [polyvalent metal ion substitution], a 0.042 mass% cobalt chloride aqueous solution was used instead of the 0.036 mass% calcium chloride aqueous solution.

<Comparative Example 1>
A fine fibrous cellulose sheet was obtained by the same procedure as in Example 1, except that the polyvalent metal ion substitution and the uniform dispersion of the polyvalent metal ion-containing fine fibrous cellulose dispersion were not carried out. The fine fibrous cellulose dispersion of Comparative Example 1 was the fine fibrous cellulose dispersion obtained in Production Example 1.

<Comparative Example 2>
The same operations as in Example 1 were carried out, except that the "uniform dispersion of polyvalent metal ion-containing fine fibrous cellulose dispersion" was changed to the "dispersion of dispersion under alkali" below, to obtain a polyvalent metal ion-containing fine fibrous cellulose dispersion and a polyvalent metal ion-containing fine fibrous cellulose sheet.

[Dispersion treatment of dispersion liquid under alkaline conditions]
The obtained dispersion of polyvalent metal ions-containing fine fibrous cellulose was stirred and then added with 1N aqueous sodium hydroxide solution to adjust the pH to 11.5. The dispersion was treated in a disperser at 4000 rpm for 3 minutes to obtain a dispersion of polyvalent metal ions-containing fine fibrous cellulose.

<Comparative Example 3>
In the step of [Uniform dispersion of polyvalent metal ion-containing fine fibrous cellulose dispersion], the same procedure as in Example 1 was carried out except that a rotary homogenizer was used instead of the high-pressure homogenizer, thereby obtaining a polyvalent metal ion-containing fine fibrous cellulose dispersion and a polyvalent metal ion-containing fine fibrous cellulose sheet.

<Comparative Example 4>
In the step of [Uniform dispersion of polyvalent metal ion-containing fine fibrous cellulose dispersion], the same procedure as in Example 1 was carried out except that an ultrasonic homogenizer was used instead of the high-pressure homogenizer, thereby obtaining a polyvalent metal ion-containing fine fibrous cellulose dispersion and a polyvalent metal ion-containing fine fibrous cellulose sheet.

Comparative Example 5
A fine fibrous cellulose sheet was obtained by the same procedure as in Example 1, except that the steps of "polyvalent metal ion substitution" and "uniform dispersion of polyvalent metal ion-containing fine fibrous cellulose dispersion" were not performed. The obtained fine fibrous cellulose-containing sheet was then subjected to polyvalent metal ion substitution by the following method, to obtain a polyvalent metal-containing fine fibrous cellulose sheet. The fine fibrous cellulose dispersion of Comparative Example 5 was the fine fibrous cellulose dispersion obtained in Production Example 1.

[Polyvalent metal ion substitution (sheet form)]
The obtained fine fibrous cellulose-containing sheet was immersed in a 5% by mass aqueous calcium chloride solution for 30 minutes to exchange the sodium counterion of the fine fibrous cellulose for calcium (ion exchange). The sheet was then immersed in ion-exchanged water for 15 minutes and washed. After repeating this washing process twice, the sheet was attached to an acrylic plate and dried in a chamber at 35°C and 15% relative humidity.

[evaluation]
The dispersions and sheets obtained in the examples and comparative examples were evaluated by the following methods.

[Measurement of fiber width]
The fiber width of the fibrous cellulose contained in the dispersion was measured using the following method. Each fibrous cellulose dispersion was diluted with water to a cellulose concentration of 0.01% by mass or more and 0.1% by mass or less and cast onto a hydrophilized carbon film-coated grid. After drying, the grid was stained with uranyl acetate and observed under a transmission electron microscope (TEM, JEOL-2000EX, manufactured by JEOL Ltd.). The magnification was adjusted so that 20 or more fibers intersected the vertical and horizontal axes of the image width. After obtaining observation images satisfying these conditions, two random axes were drawn vertically and horizontally per image, and the fiber widths of the fibers intersecting the axes were visually determined. Three unique observation images were taken for each dispersion, and the fiber widths of the fibers intersecting the two axes were read (20 or more × 2 × 3 = 120 or more). The number-average fiber width was calculated from the fiber widths obtained in this manner.

[Measurement of Haze of Dispersion]
The haze of the dispersions obtained in the examples and comparative examples was measured by diluting each dispersion with ion-exchanged water to 0.2% by mass, and then measuring the haze in accordance with JIS K 7136:2000 using a haze meter (HM-150, manufactured by Murakami Color Research Laboratory Co., Ltd.) and a glass cell for liquids with an optical path length of 1 cm (MG-40, reverse optical path, manufactured by Fujiwara Seisakusho Co., Ltd.). Zero-point measurement was performed using ion-exchanged water placed in the same glass cell. Furthermore, the dispersions to be measured were allowed to stand for 24 hours in an environment of 23°C and a relative humidity of 50% before measurement. The liquid temperature of the dispersions during measurement was 23°C.

[Measurement of total light transmittance of dispersion]
The total light transmittance of the dispersions obtained in the Examples and Comparative Examples was measured by diluting each dispersion with ion-exchanged water to 0.2% by mass, and then measuring with a haze meter (HM-150, manufactured by Murakami Color Research Laboratory Co., Ltd.) using a glass cell for liquids with an optical path length of 1 cm (MG-40, reverse optical path, manufactured by Fujiwara Seisakusho Co., Ltd.) in accordance with 7361-1:1997. Zero-point measurement was performed using ion-exchanged water placed in the same glass cell. Furthermore, the dispersions to be measured were allowed to stand for 24 hours in an environment of 23°C and a relative humidity of 50% before measurement. The liquid temperature of the dispersions during measurement was 23°C.

[pH measurement]
The dispersions obtained in the examples and comparative examples were diluted with ion-exchanged water to a solid content of 0.4% by mass, and the pH was measured using a pH meter. The temperature of the dispersions during the measurement was 23°C.

[Electrical conductivity measurement]
The dispersions obtained in the examples and comparative examples were diluted with ion-exchanged water to a solid content of 0.4% by mass, and the electrical conductivity was measured using an electrical conductivity meter (CT-57101B, manufactured by DKK-TOA Corporation). The temperature of the dispersions during the measurement was 23°C.

[Measurement of Dispersion Viscosity]
The dispersions obtained in the Examples and Comparative Examples were diluted with ion-exchanged water to a solids concentration of 0.4% by mass and stirred in a disperser at 1500 rpm for 5 minutes. The viscosity of the resulting dispersions was then measured using a Brookfield analog viscometer (T-LVT). The measurement was performed at a rotation speed of 3 rpm, and the viscosity value 3 minutes after the start of measurement was recorded as the viscosity of the dispersion. The dispersions to be measured were allowed to stand for 24 hours in an environment of 23°C and 50% relative humidity before measurement. The temperature of the dispersions during measurement was 23°C.

[Monovalent metal element content]
The monovalent metal element content (%) in the sheets obtained in the examples and comparative examples was measured using fluorescent X-ray analysis as follows.
First, for the purpose of creating a calibration curve, filter papers with known elemental contents of sodium, calcium, magnesium, zinc, nickel, copper, and cobalt were prepared, and the X-ray intensity of each filter paper was measured by X-ray fluorescence analysis. Next, a calibration curve was created based on the X-ray intensity thus obtained and the known contents of the analyzed elements. Next, the X-ray intensity of the measurement target sheets (sheets obtained in the Examples and Comparative Examples) was measured by X-ray fluorescence analysis. From the X-ray intensity thus obtained and the calibration curve, the content (mmol) of each analyzed element in the measurement target sheets was determined, and the monovalent metal element content was calculated using the following formula.
Monovalent metal element content (%) = Monovalent metal element content (mmol) / Total metal element content (mmol) × 100

[YI increase rate]
The yellowness index of the sheets obtained in the examples and comparative examples was measured before and after heating using a Colour Cute i (manufactured by Suga Test Instruments Co., Ltd.) in accordance with JIS K 7373:2006. The yellowness index after heating was the yellowness index of the sheet heated at 160°C for 6 hours. The YI increase rate was calculated using the following method.
YI increase rate (%)=(yellowing index of sheet after heating−yellowing index of sheet before heating)/yellowing index of sheet before heating×100

[Sheet haze]
The haze of the sheets obtained in the examples and comparative examples was measured in accordance with JIS K 7136:2000 using a haze meter ("HM-150" manufactured by Murakami Color Research Laboratory Co., Ltd.).

[Total light transmittance of sheet]
The total light transmittance of the sheets obtained in the examples and comparative examples was measured in accordance with JIS K 7361-1: 1997 using a haze meter ("HM-150" manufactured by Murakami Color Research Laboratory Co., Ltd.).

[Water absorption rate of sheet]
The sheets obtained in the examples and comparative examples were cut into 50 mm square sheets, immersed in ion-exchanged water for 24 hours, and the water absorption of the sheets was calculated using the following formula: The bone dry mass of the sheet was the mass measured after drying the sheet at 105°C for 24 hours.
Water absorption rate of sheet (%) = Mass (g) of sheet after immersion in ion-exchanged water / Bone-dry mass (g) of sheet × 100

Example 7
A polyvalent metal ion-containing fine fibrous cellulose dispersion and a polyvalent metal ion-containing fine fibrous cellulose sheet were obtained in the same manner as in Example 1, except that the fine fibrous cellulose dispersion obtained in Production Example 2 was used instead of the fine fibrous cellulose dispersion obtained in Production Example 1, and the calcium chloride aqueous solution concentration was changed to 0.018 mass% in the [polyvalent metal ion substitution] step.

<Comparative Example 6>
The same procedures as in Example 7 were carried out except that the polyvalent metal ion substitution treatment was not carried out, to obtain a fine fibrous cellulose dispersion and a fine fibrous cellulose sheet.

Example 8
The fine fibrous cellulose dispersion obtained in Production Example 3 was used instead of the fine fibrous cellulose dispersion obtained in Production Example 1, and further, the "substituent removal treatment (high-temperature heat treatment)" described below was replaced with the "substituent removal (low-temperature heat treatment)." In addition, in the "polyvalent metal ion substitution," the calcium chloride aqueous solution concentration was changed to 0.018 mass%. Otherwise, a polyvalent metal ion-containing fine fibrous cellulose dispersion and a polyvalent metal ion-containing fine fibrous cellulose sheet were obtained in the same manner as in Example 1.

[Removal of Substituents (Low-Temperature Heat Treatment)]
The obtained fine fibrous cellulose dispersion was heated at a liquid temperature of 40°C for 45 minutes until the amount of xanthate groups reached 0.08 mmol/g.

Comparative Example 7
The same procedure as in Example 8 was carried out except that the polyvalent metal ion substitution treatment was not carried out, to obtain a fine fibrous cellulose dispersion and a fine fibrous cellulose sheet.

Example 9
A polyvalent metal ion-containing fine fibrous cellulose dispersion and a polyvalent metal ion-containing fine fibrous cellulose sheet were obtained in the same manner as in Example 1, except that the fine fibrous cellulose dispersion obtained in Production Example 4 was used instead of the fine fibrous cellulose dispersion obtained in Production Example 1, and the calcium chloride aqueous solution concentration in [polyvalent metal ion substitution] was changed to 0.018 mass%.

<Comparative Example 8>
The same procedures as in Example 9 were carried out except that the polyvalent metal ion substitution treatment was not carried out, to obtain a fine fibrous cellulose dispersion and a fine fibrous cellulose sheet.

Example 10
A polyvalent metal ion-containing fine fibrous cellulose dispersion and a polyvalent metal ion-containing fine fibrous cellulose sheet were obtained in the same manner as in Example 1, except that the fine fibrous cellulose dispersion obtained in Production Example 5 was used instead of the fine fibrous cellulose dispersion obtained in Production Example 1, and the calcium chloride aqueous solution concentration in [polyvalent metal ion substitution] was changed to 0.018 mass%.

<Comparative Example 9>
The same procedure as in Example 10 was carried out except that the polyvalent metal ion substitution treatment was not carried out, to obtain a fine fibrous cellulose dispersion and a fine fibrous cellulose sheet.

The sheets obtained in the Examples were highly transparent and inhibited from yellowing. On the other hand, the sheets obtained in the Comparative Examples did not achieve both transparency and inhibition of yellowing. Furthermore, the sheets obtained in the Examples also exhibited inhibited water absorption.

Claims (8)

A sheet formed from a dispersion containing fibrous cellulose having anionic groups and a fiber width of 1000 nm or less,
the amount of the anionic group introduced into the fibrous cellulose is 0.07 mmol/g or more and 0.15 mmol/g or less,
the fibrous cellulose has a polyvalent metal ion as a counter ion of the anionic group,
the anionic group is at least one selected from the group consisting of a phosphorus oxoacid group, a substituent derived from a phosphorus oxoacid group, a sulfur oxoacid group, a substituent derived from a sulfur oxoacid group, a xanthate group, a substituent derived from a xanthate group, a carboxy group, and a substituent derived from a carboxy group;
the polyvalent metal ion is at least one selected from the group consisting of calcium ions, magnesium ions, zinc ions, and aluminum ions;
the electrical conductivity of the dispersion containing 0.4% by mass of the fibrous cellulose is 20 mS/m or less;
the haze of the dispersion containing 0.2% by mass of the fibrous cellulose is 10% or less;
a sheet in which, when heated at 160°C for 6 hours, the YI increase rate calculated by the following formula is 1600% or less;
YI increase rate (%)=(yellowing index of sheet after heating−yellowing index of sheet before heating)/yellowing index of sheet before heating×100
In the above formula, the yellowness of the sheet is the yellowness measured in accordance with JIS K 7373:2006. The sheet according to claim 1, wherein the dispersion containing 0.2% by mass of the fibrous cellulose has a total light transmittance of 85% or more. The sheet according to claim 1 or 2, wherein the pH of the dispersion when containing 0.4% by mass of the fibrous cellulose is 4 to 9. The sheet according to any one of claims 1 to 3, wherein the anionic group is a phosphorus oxoacid group or a substituent derived from a phosphorus oxoacid group. The sheet according to any one of claims 1 to 4, wherein the fiber width of the fibrous cellulose is 10 nm or less. The sheet according to any one of claims 1 to 5, having a thickness of 40 μm or more. The sheet described in any one of claims 1 to 6 has a total light transmittance of 70% or more. The sheet according to any one of claims 1 to 7, having a haze of 10% or less.