JP7793907B2 - Liquid composition, sheet and laminate - Google Patents

Description

The present invention relates to a liquid composition, a sheet, and a laminate.

In recent years, materials made from renewable natural fibers have been attracting attention due to the need to replace petroleum resources and growing environmental awareness. Among natural fibers, fibrous cellulose with a fiber diameter of 10 μm to 50 μm, particularly wood-derived fibrous cellulose (pulp), has been widely used, primarily in paper products.

Fine fibrous cellulose, with a fiber diameter of 1 μm or less, is also known as fibrous cellulose. Fine fibrous cellulose is attracting attention as a new material with a wide range of applications. For example, development of dispersions and composites containing fine fibrous cellulose is underway.

For example, Patent Document 1 discloses cellulose fibers obtained by reducing oxidized cellulose fibers in a reaction solution containing a reducing agent to obtain reduced oxidized cellulose fibers, which are then dried. These cellulose fibers can be finely dispersed in a solvent, and it has also been studied to form molded articles from these cellulose fibers. Patent Document 2 discloses a method for producing a composite in which a metal salt and a reducing agent are added to a dispersion of finely divided oxidized cellulose to reduce and precipitate flat metal microparticles, and form a composite of the flat metal microparticles and fine cellulose fibers. Patent Document 3 further discloses a composition containing cellulose fibers with an average fiber diameter of 1 to 1,000 nm and to which an antioxidant has been introduced.

JP 2015-113376 A JP 2015-221844 A JP 2011-127075 A

As mentioned above, compositions and sheets containing fine fibrous cellulose are known. However, when sheets are formed from conventional compositions, there is a problem in that the sheets can yellow when heated in the usage environment or during the manufacturing process.

In order to solve these problems of the prior art, the inventors have conducted research with the aim of providing a sheet containing fine fibrous cellulose that is inhibited from yellowing when heated.

As a result of intensive research to solve the above problems, the inventors discovered that by adding a resin and a reducing agent to a liquid composition containing fibrous cellulose having a fiber width of 1000 nm or less, a sheet that is inhibited from yellowing when heated can be obtained.
Specifically, the present invention has the following configuration.

[1] Fibrous cellulose having a fiber width of 1000 nm or less;
Resin and
A liquid composition comprising:
[2] The liquid composition according to [1], wherein the amount of ionic substituents introduced into the fibrous cellulose is less than 0.5 mmol/g.
[3] The liquid composition according to [2], wherein the ionic substituent is an anionic group.
[4] The liquid composition according to [3], wherein 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 carboxy group, and a substituent derived from a carboxy group.
[5] The liquid composition according to any one of [1] to [4], wherein the reducing agent is at least one selected from the group consisting of sodium sulfite, sodium thiosulfate, sodium borohydride, sodium hydrogensulfite, sodium cyanoborohydride, and sodium hyposulfite.
[6] The liquid composition according to any one of [1] to [5], wherein the resin is a water-soluble polymer.
[7] The liquid composition according to any one of [1] to [6], further comprising a polymerization inhibitor.
[8] The liquid composition according to any one of [1] to [7], wherein the content of the reducing agent is 0.1 to 5 parts by mass per 100 parts by mass of the fibrous cellulose.
[9] Fibrous cellulose having a fiber width of 1000 nm or less;
Resin and
A sheet containing a reducing agent.
[10] The sheet according to [9], wherein the amount of ionic substituents introduced into the fibrous cellulose is less than 0.5 mmol/g.
[11] The sheet according to [10], wherein the ionic substituent is an anionic group.
[12] The sheet according to [11], wherein 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 carboxy group, and a substituent derived from a carboxy group.
[13] The sheet according to any one of [9] to [12], wherein the reducing agent is at least one selected from the group consisting of sodium sulfite, sodium thiosulfate, sodium borohydride, sodium hydrogensulfite, sodium cyanoborohydride, and sodium hyposulfite.
[14] The sheet according to any one of [9] to [13], wherein the resin is a water-soluble polymer.
[15] The sheet according to any one of [9] to [14], further containing a polymerization inhibitor.
[16] The sheet according to any one of [9] to [15], wherein the content of the reducing agent is 0.1 to 5 parts by mass per 100 parts by mass of the fibrous cellulose.
[17] The sheet according to any one of [9] to [16], having a YI value of 1.5 or less.
[18] The sheet according to any one of [9] to [17], having a haze of 3.5% or less.
[19] The sheet according to any one of [9] to [18], which is for optical members.
[20] A laminate comprising the sheet according to any one of [9] to [19] and a resin layer laminated on at least one surface of the sheet.

According to the present invention, it is possible to obtain a sheet that is inhibited from yellowing when heated.

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.

(Liquid composition)
The present invention relates to a liquid composition containing fibrous cellulose having a fiber width of 1000 nm or less, a resin, and a reducing agent. In this specification, the fibrous cellulose having a fiber width of 1000 nm or less is also referred to as fine fibrous cellulose or CNF.

The liquid composition of the present invention, having the above-described configuration, can form a molded article that exhibits reduced yellowing upon heating. The shape of the molded article is not particularly limited, but in this embodiment, a sheet that exhibits reduced yellowing upon heating can be formed. In this specification, yellowing upon heating can be evaluated by the YI value after heating at 160°C for 6 hours. If the YI value of the sheet after heating at 160°C for 6 hours is 5.0 or less, it can be determined that yellowing upon heating is suppressed.

Furthermore, because the liquid composition of the present invention has the above-described configuration, it is possible to form a molded article (sheet) with excellent transparency. Furthermore, the molded article (sheet) formed from the liquid composition of the present invention is highly transparent and also has excellent design properties. For example, in the sheet of this embodiment, aggregation of the fibrous cellulose is suppressed, so there is no foreign body feel and the design properties are excellent.

In this embodiment, the pH of the liquid composition is preferably 2.0 or higher, more preferably 3.0 or higher, and even more preferably 4.0 or higher. Furthermore, the pH of the liquid composition is preferably 11.0 or lower, and more preferably 10.0 or lower. By keeping the pH of the liquid composition within the above range, it is possible to prevent the decomposition of fibrous cellulose when the liquid composition is heat-treated, thereby preventing the generation of monosaccharides, which are one of the causes of coloration.

In this embodiment, when the solids concentration is 0.4% by mass, the viscosity of the liquid composition is preferably 100 mPa·s or more, more preferably 300 mPa·s or more, and even more preferably 500 mPa·s or more. Furthermore, when the solids concentration is 0.4% by mass, the viscosity of the liquid composition is preferably 20,000 mPa·s or less, and more preferably 10,000 mPa·s or less. Here, the viscosity of the liquid composition is measured using a Brookfield viscometer. The measurement conditions are a rotation speed of 3 rpm, and the viscosity value 3 minutes after the start of measurement is taken as the viscosity of the liquid composition. Furthermore, the liquid composition to be measured is allowed to stand for 24 hours in an environment of 23°C and 50% relative humidity before measurement, and the liquid temperature of the liquid composition during measurement is 23°C. As a Brookfield viscometer, for example, a T-LVT analog viscometer manufactured by Brookfield can be used.

In this embodiment, the haze of the liquid composition is preferably 15% or less, more preferably 10% or less, and even more preferably 5% or less. The haze of the liquid composition may be 0.0%. The haze of the liquid composition is measured in accordance with JIS K 7136:2000 using a glass cell with a 1 cm optical path length. Specifically, the measurement is performed using a haze meter (HM-150, manufactured by Murakami Color Research Laboratory Co., Ltd.). For example, a 1 cm optical path length glass cell for liquids (MG-40, reverse optical path, manufactured by Fujiwara Seisakusho Co., Ltd.) can be used as the glass cell with a 1 cm optical path length. Furthermore, the liquid composition to be measured is allowed to stand for 24 hours in an environment of 23°C and 50% relative humidity before measurement, and the liquid temperature of the liquid composition during measurement is set to 23°C. Zero-point measurement is performed using ion-exchanged water placed in the same glass cell.

(reducing agent)
The liquid composition of the present invention contains a reducing agent. In this specification, a reducing agent refers to a compound that has the effect of reducing aldehyde groups (reducing ends) originally contained in fibrous cellulose, aldehyde groups and/or ketone groups generated as a by-product when ionic functional groups are introduced into fibrous cellulose, and aldehyde groups and/or ketone groups generated when a sheet containing fibrous cellulose is heated. The reducing agent is also a compound that has the effect of reducing oxygen in the air, which can oxidize fibrous cellulose, and other oxidizing agents present in the system. For example, when a reducing agent acts on aldehyde groups and/or ketone groups generated in fibrous cellulose, alcohols (hydroxyl groups) are generated by the reduction action.

Among these, the reducing agent is preferably at least one selected from the group consisting of sodium sulfite, sodium thiosulfate, sodium borohydride, sodium bisulfite, sodium cyanoborohydride, and sodium hyposulfite, more preferably at least one selected from sodium sulfite and sodium thiosulfate, and particularly preferably sodium thiosulfate. By using this compound as a reducing agent, yellowing of the sheet formed from the liquid composition upon heating can be more effectively suppressed. The mechanism by which sodium thiosulfate specifically suppresses heat yellowing is unclear. However, in addition to its effectiveness as a reducing agent, it is presumed that the pH of the sodium thiosulfate solution is in the neutral range, making it difficult to change the pH of the fibrous cellulose dispersion. For example, when fibrous cellulose has anionic substituents, hydrolysis generates hydrogen ions or hydroxide ions, promoting the production of monosaccharides, which are yellowing substances. However, the use of sodium thiosulfate, which does not easily change the pH of the fibrous cellulose, suppresses the generation of monosaccharides and thus heat yellowing.

The content of the reducing agent is preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, and even more preferably 0.3 parts by mass or more, per 100 parts by mass of fibrous cellulose. Furthermore, the content of the reducing agent is preferably 5 parts by mass or less, more preferably 4 parts by mass or less, and even more preferably 3 parts by mass or less, per 100 parts by mass of fibrous cellulose. By keeping the content of the reducing agent within the above range, yellowing of the sheet after heating can be more effectively suppressed.

(resin)
The liquid composition of the present invention contains a resin. In the present invention, the resin functions as a binder resin in the sheet. For example, a water-soluble polymer is used as the resin. Examples of water-soluble polymers include polyvinyl alcohol and its derivatives, polyethylene oxide, polyethylene glycol, starch and its derivatives, cellulose derivatives such as carboxymethyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, and ethyl cellulose, sodium polyacrylate, polyvinylpyrrolidone, casein, gelatin, and derivatives thereof.

Resins other than water-soluble polymers can also be used. Examples of resins other than water-soluble polymers include acrylamide-acrylic acid ester copolymers, acrylamide-acrylic acid ester-methacrylic acid ester copolymers, styrene-maleic anhydride copolymers, isobutylene-maleic anhydride copolymers, polyvinyl acetate, polyurethane, polyacrylic acid, polyacrylic acid esters, vinyl chloride-vinyl acetate copolymers, polybutyl methacrylate, emulsions of ethylene-vinyl acetate copolymers, styrene-butadiene copolymers, and styrene-butadiene-acrylic copolymers. These resins may also be used as latexes dispersed in water.

Among these, from the viewpoints of ease of sheet formation, transparency, and suppression of heat discoloration, the resin is preferably a water-soluble polymer, more preferably at least one selected from the group consisting of polyethylene glycol (PEG), polyethylene oxide (PEO), and polyvinyl alcohol (PVA), and particularly preferably polyvinyl alcohol (PVA). Using polyvinyl alcohol (PVA) as the resin makes it easier to form a sheet from the liquid composition and also makes it easier to obtain a sheet that is highly transparent and suppresses heat yellowing. Furthermore, using polyvinyl alcohol (PVA) as the resin also makes it possible to achieve high elasticity in the sheet when formed from the liquid composition.

The weight-average molecular weight of the polyvinyl alcohol (PVA) used as the resin is preferably 300 or more, more preferably 500 or more, and particularly preferably 1000 or more. Furthermore, the weight-average molecular weight of the polyvinyl alcohol (PVA) is preferably 4500 or less, more preferably 4000 or less, and particularly preferably 3500 or less. By keeping the weight-average molecular weight of the polyvinyl alcohol (PVA) within the above range, it becomes easier to form the liquid composition into a sheet.

The resin content is preferably 10 parts by mass or more, more preferably 20 parts by mass or more, and even more preferably 30 parts by mass or more, per 100 parts by mass of fibrous cellulose. Furthermore, the resin content is preferably 500 parts by mass or less, more preferably 400 parts by mass or less, and even more preferably 300 parts by mass or less, per 100 parts by mass of fibrous cellulose. By keeping the resin content within the above range, yellowing of the sheet after heating can be more effectively suppressed.

(Fine fibrous cellulose)
The liquid composition of the present invention contains fibrous cellulose (fine fibrous cellulose) having a fiber width of 1,000 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, even more preferably 10 nm or less, and particularly preferably 8 nm or less.

The fiber width of fibrous cellulose can be measured, for example, by observation using an electron microscope. 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, and particularly preferably 2 nm or more and 10 nm or less. By making the average fiber width of fibrous cellulose 2 nm or more, it is possible to suppress the dissolution of cellulose molecules in water, making it easier to achieve the effects of fibrous cellulose, such as improved strength, rigidity, and dimensional stability. Note that the 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 is easy to form a sheet containing fibrous cellulose. In addition, sufficient viscosity is easily obtained when a solvent dispersion is prepared. Setting the axial ratio at or below the upper limit is preferable in that it makes handling, such as dilution, easier when treating the fibrous cellulose as an aqueous dispersion, for example.

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 cellulose components in fibrous cellulose can be classified into α-cellulose components and hemicellulose components. A lower hemicellulose ratio is preferable because it is more likely to suppress yellowing over time and heat. The hemicellulose ratio in fibrous cellulose is preferably less than 30%, more preferably less than 25%, and even more preferably less than 20%.

The amount of nitrogen contained in the fibrous cellulose is preferably 0.08 mmol/g or less, more preferably 0.04 mmol/g or less, and even more preferably 0.02 mmol/g or less. The amount of nitrogen contained in the fibrous cellulose is preferably 0.001 mmol/g or more. The amount of nitrogen in the fibrous cellulose is measured by the following method. First, a dispersion containing fibrous cellulose is adjusted to a solids concentration of 1% by mass and decomposed by the Kjeldahl method (JIS K 0102:2016 44.1). After decomposition, the amount of ammonium ions (mmol) is measured by cation chromatography and divided by the amount of cellulose (g) used in the measurement to calculate the nitrogen content (mmol/g).

In this embodiment, the fibrous cellulose may have an ionic substituent. The ionic substituent may include, for example, either an anionic group or a cationic group, or both. In this embodiment, when the fibrous cellulose has an ionic substituent, the ionic substituent is preferably an anionic group. Furthermore, the ionic substituent is preferably a group that is introduced into the fibrous cellulose via an ester bond or an ether bond, and more preferably a group that is introduced into the fibrous cellulose via an ester bond. In this case, the ester bond is preferably formed by dehydration condensation between the fibrous cellulose and a compound that becomes the ionic substituent.

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 (sometimes simply referred to as phosphonic groups), phosphine groups or substituents derived from phosphine groups (sometimes simply referred to as phosphine groups), sulfonic groups or substituents derived from sulfonic groups (sometimes simply referred to as sulfonic groups), and carboxyalkyl groups (including carboxymethyl and carboxyethyl 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 sulfur oxoacid group, a substituent derived from a sulfur oxoacid group, a carboxy group, and a substituent derived from a carboxy group; more preferably at least one selected from a phosphorus oxoacid group, a substituent derived from a phosphorus oxoacid 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. Introducing a phosphorus oxoacid group as the anionic group can further enhance the dispersibility of the fibrous cellulose, for example, even under alkaline or acidic conditions, making it easier to obtain a highly transparent sheet.

The phosphorus oxo acid group or the substituent derived from the phosphorus oxo acid group is, for example, a substituent represented by the following formula (1). Multiple 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 phosphonium ions. Examples of the organic ammonium ions include aliphatic ammonium ions and aromatic ammonium ions, and examples of the organic phosphonium 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. Examples of the monovalent or higher cation composed of an organic or inorganic substance include, but are not limited to, sodium or potassium ions, which are resistant to yellowing when a fiber raw material containing β ^b+ is heated and are easy to use industrially.

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 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 phosphonium ions. Examples of the organic ammonium ions include aliphatic ammonium ions and aromatic ammonium ions, and examples of the organic phosphonium 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. As the monovalent or higher cations consisting of organic or inorganic substances, sodium or potassium ions are preferred, but are not particularly limited, because they are less likely to yellow when the fiber raw material containing β ^b+ is heated and are easy to use industrially.

When the fibrous cellulose has an ionic substituent, the amount of the ionic substituent introduced is, for example, preferably 0.05 mmol/g or more per 1 g (mass) of the fibrous cellulose, more preferably 0.10 mmol/g or more, even more preferably 0.20 mmol/g or more, even more preferably 0.40 mmol/g or more, and particularly preferably 0.60 mmol/g or more. Furthermore, the amount of the ionic substituent introduced into the fibrous cellulose is, for example, preferably 5.20 mmol/g or less per 1 g (mass) of the fibrous cellulose, more preferably 3.65 mmol/g or less, even more preferably 3.00 mmol/g or less, even more preferably 2.50 mmol/g or less, and particularly preferably 2.00 mmol/g or less. Here, the denominator in the unit mmol/g indicates the mass of the fibrous cellulose when the counter ion of the ionic substituent is a hydrogen ion (H ⁺ ). By setting the amount of the ionic substituent introduced within the above range, it is possible to easily refine the fiber raw material and improve the stability of the fibrous cellulose. Furthermore, by adjusting the amount of ionic substituent introduced to fall within the above range, a highly transparent sheet can be easily obtained.

In this embodiment, the amount of ionic substituents introduced into the fibrous cellulose may be less than 0.5 mmol/g. Such fibrous cellulose is preferably obtained by subjecting fibrous cellulose having ionic substituents to a substituent removal treatment to reduce the amount of ionic substituents introduced to less than 0.5 mmol/g. In other words, the fibrous cellulose may be substituent-removed fine fibrous cellulose. In this way, by using substituent-removed fine fibrous cellulose having an ionic substituent introduction amount of less than 0.5 mmol/g, it is possible to more effectively suppress yellowing of the sheet after heating.

The amount of ionic substituents 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 ionic substituents and the 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 ionic substituents using titration, adding too much sodium hydroxide solution or titrating too quickly can result in a lower amount of ionic substituents than expected, making it difficult to obtain accurate values. An appropriate amount and titration interval, for example, is to titrate 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 measure 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 groups introduced into fibrous cellulose can be calculated by freeze-drying a slurry containing fibrous cellulose and then pulverizing the sample to measure the amount of sulfur. Specifically, a slurry containing fibrous cellulose is freeze-dried, and the resulting pulverized sample is subjected to pressure-heat decomposition using nitric acid in a sealed container, after which the sample is appropriately diluted and the amount of sulfur measured using ICP-OES. The value calculated by dividing this value by the bone-dry mass of the fibrous cellulose tested is taken as the amount of sulfur oxoacid groups or sulfonic groups (unit: mmol/g) of the fibrous cellulose.

The amount of xanthate groups 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)

The amount of cationic groups introduced into the fibrous cellulose can be calculated by the following formula after performing a trace nitrogen analysis.
(Amount of cationic groups) [mmol/g] = (Amount of nitrogen) [g] / 14 × 1000 / (Amount of fine fibrous cellulose tested) [g]

(Method for producing fine fibrous cellulose)
<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 process for producing fine fibrous cellulose preferably includes a step of introducing an ionic substituent, and examples of the step of introducing an ionic substituent include a step of introducing a phosphorus oxo acid group. The step of introducing a phosphorus oxo acid group 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 a phosphorus oxo acid group by reacting with a hydroxyl group possessed by the cellulose-containing fiber raw material. This step results in the production of a fiber into which a phosphorus oxo acid group has been introduced.

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, the compound B used in this embodiment is at least one selected from urea and its derivatives. Examples of the compound B include urea, biuret, 1-phenylurea, 1-benzylurea, 1-methylurea, and 1-ethylurea.
From the viewpoint of improving the uniformity of the reaction, it is preferable to use an aqueous solution of compound B. Furthermore, from the viewpoint 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 0.05 mmol/g or more per 1 g (mass) of fiber raw material, more preferably 0.10 mmol/g or more, even more preferably 0.20 mmol/g or more, even more preferably 0.40 mmol/g or more, and particularly preferably 0.60 mmol/g or more. The amount of phosphorus oxoacid groups introduced into the fiber raw material 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, even more preferably 3.00 mmol/g or less, even more preferably 2.50 mmol/g or less, and particularly preferably 2.00 mmol/g or less. By keeping the amount of phosphorus oxoacid groups introduced within the above range, it is possible to easily refine the fiber raw material and improve the stability of the fine fibrous cellulose. Furthermore, by keeping the amount of phosphorus oxoacid groups introduced within the above range, it is easier to obtain a sheet that exhibits reduced yellowing upon heating.

<Carboxy group introduction step>
The process for producing fine fibrous cellulose may include, for example, a carboxyl group introduction step as an ionic substituent introduction step. The carboxyl group introduction step is carried out by subjecting a cellulose-containing fiber raw material to an oxidation treatment such as ozone oxidation, oxidation by the Fenton method, or TEMPO oxidation treatment, or by treating the cellulose-containing fiber raw material with a compound having a carboxylic acid-derived group or a derivative thereof, or an acid anhydride of a compound having a carboxylic acid-derived group or a derivative thereof.

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.

When TEMPO oxidation treatment is performed in the carboxyl group introduction process, it is preferable to perform the treatment under conditions of pH 6 or higher and 8 or lower. This type of treatment is also known as neutral TEMPO oxidation treatment. Neutral TEMPO oxidation treatment can be performed, for example, by adding pulp as the fiber raw material, a nitroxy radical such as TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) as a catalyst, and sodium hypochlorite as a sacrificial reagent to a sodium phosphate buffer solution (pH = 6.8). Furthermore, by adding sodium chlorite, aldehydes generated during the oxidation process can be efficiently oxidized to carboxyl groups. TEMPO oxidation treatment may also be performed under conditions of pH 10 or higher and 11 or lower. This type of treatment is also known as alkaline TEMPO oxidation treatment. Alkaline TEMPO oxidation treatment can be performed, for example, by adding a nitroxy radical such as TEMPO as a catalyst, sodium bromide as a co-catalyst, and sodium hypochlorite as an oxidizing agent to pulp as the fiber raw material.

The amount of carboxyl groups introduced into the fiber raw material varies depending on the type of substituent. For example, when carboxyl groups are introduced by TEMPO oxidation, the amount is preferably 0.05 mmol/g or more, more preferably 0.10 mmol/g or more, even more preferably 0.20 mmol/g or more, even more preferably 0.40 mmol/g or more, and particularly preferably 0.60 mmol/g or more per gram (mass) of fiber raw material. Furthermore, the amount of carboxyl groups introduced into the fibrous cellulose is preferably 3.65 mmol/g or less, more preferably 3.00 mmol/g or less, even more preferably 2.50 mmol/g or less, and even more preferably 2.00 mmol/g or less. Furthermore, when the substituent is a carboxymethyl group, the amount of carboxyl groups introduced may be 5.8 mmol/g or less per gram (mass) of fine fibrous cellulose. By keeping the amount of carboxyl groups introduced within the above range, it is possible to easily refine the fiber raw material and improve the stability of the fibrous cellulose. Furthermore, by keeping the amount of carboxyl groups introduced within the above range, it is easier to obtain a sheet that exhibits reduced yellowing upon heating.

<Sulfur oxoacid group introduction step>
The process for producing fine fibrous cellulose may include, for example, a sulfur oxoacid group introduction step as an ionic substituent introduction step, in which hydroxyl groups in a cellulose-containing fiber raw material react with sulfur oxoacids to produce 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 0.05 mmol/g or more, more preferably 0.10 mmol/g or more, even more preferably 0.20 mmol/g or more, even more preferably 0.40 mmol/g or more, and particularly preferably 0.50 mmol/g or more. Furthermore, the amount of sulfur oxoacid groups introduced into the cellulose raw material is preferably 5.00 mmol/g or less, more preferably 3.00 mmol/g or less. By keeping the amount of sulfur oxoacid groups introduced within the above range, it is possible to easily refine the fiber raw material and increase the stability of the fibrous cellulose. Furthermore, by keeping the amount of sulfur oxoacid groups introduced within the above range, it becomes easier to obtain a sheet that is suppressed from yellowing when heated.

<Xanthate group introduction process>
The process for producing fine fibrous cellulose may include a xanthate group introduction step as an ionic substituent introduction step. The xanthate group introduction step is performed by substituting the hydroxyl groups of a cellulose-containing fiber raw material with 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 in the xanthate group introduction step is preferably 0.05 mmol/g or more per 1 g (mass) of fiber raw material, more preferably 0.10 mmol/g or more, even more preferably 0.20 mmol/g or more, even more preferably 0.40 mmol/g or more, and particularly preferably 0.60 mmol/g or more. Furthermore, 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. By keeping the amount of xanthate groups introduced within the above range, it is possible to easily refine the fiber raw material and improve the stability of the fibrous cellulose. Furthermore, by keeping the amount of xanthate groups introduced within the above range, it becomes easier to obtain a sheet that suppresses yellowing when heated.

<Oxidation step using a chlorine-based oxidizing agent (second carboxyl group introduction step)>
The process for producing fine fibrous cellulose may include an oxidation step using a chlorine-based oxidizing agent as an ionic substituent introduction step. In the oxidation step using a chlorine-based oxidizing agent, the chlorine-based oxidizing agent is added to a wet or dry fiber raw material having a hydroxyl group to cause a reaction, thereby introducing a carboxyl group into the fiber raw material.

Examples of chlorine-based oxidizing agents include hypochlorous acid, hypochlorites, chlorous acid, chlorites, chloric acid, chlorates, perchloric acid, perchlorates, and chlorine dioxide. From the standpoints of substituent introduction efficiency, defibration efficiency, cost, and ease of handling, sodium hypochlorite, sodium chlorite, and chlorine dioxide are preferred chlorine-based oxidizing agents. When adding a chlorine-based oxidizing agent, it may be added directly to the fiber raw material as a reagent (solid or liquid), or it may be dissolved in an appropriate solvent and added.

In the oxidation process using a chlorine-based oxidizing agent, the concentration of the chlorine-based oxidizing agent in the solution, converted into effective chlorine concentration, is preferably 1% by mass or more and 1,000% by mass or less, more preferably 5% by mass or more and 500% by mass or less, and even more preferably 10% by mass or more and 100% by mass or less. The amount of chlorine-based oxidizing agent added per 100 parts by mass of fiber raw material is preferably 1 part by mass or more and 100,000 parts by mass or less, more preferably 10 parts by mass or more and 10,000 parts by mass or less, and even more preferably 100 parts by mass or more and 5,000 parts by mass or less.

The reaction time with the chlorine-based oxidizing agent in the oxidation step using the chlorine-based oxidizing agent varies depending on the reaction temperature, but is preferably from 1 minute to 1,000 minutes, more preferably from 10 minutes to 500 minutes, and even more preferably from 20 minutes to 400 minutes. The pH during the reaction is preferably from 5 to 15, more preferably from 7 to 14, and even more preferably from 9 to 13. At the start of the reaction, the pH is preferably maintained constant (for example, at pH 11) during the reaction by adding hydrochloric acid or sodium hydroxide as appropriate. After the reaction, excess reaction reagents, by-products, etc. may be washed away with water by filtration or other methods.

<Phosphonic or Phosphine Group Introduction Step (Phosphoalkylation Step)>
The process for producing fine fibrous cellulose may include a phosphonic or phosphine group introduction step (phosphoalkylation step) as an ionic substituent introduction step. In the phosphoalkylation step, a compound having a reactive group and a phosphonic or phosphine group (compound E _A ) as an essential component, and an alkali compound and a compound B selected from the above-mentioned urea and its derivatives as optional components, are added to a wet or dry fiber raw material having a hydroxyl group and reacted to introduce the phosphonic or phosphine group into the fiber raw material.

Examples of the reactive group include a halogenated alkyl group, a vinyl group, and an epoxy group (glycidyl group).
Examples of the compound _EA include vinylphosphonic acid, phenylvinylphosphonic acid, phenylvinylphosphinic acid, etc. From the viewpoints of the efficiency of introducing a substituent, and therefore the defibration efficiency, cost, and ease of handling, it is preferable that the compound _EA is vinylphosphonic acid.
Furthermore, as an optional component, it is also preferable to use the compound B in the above-mentioned <Phosphorus oxo acid group introduction step> in the same manner, and the amount added is also preferably as described above.

When compound _EA is added, it may be added to the fiber raw material as a reagent (solid or liquid) as it is, or may be dissolved in an appropriate solvent and added. The fiber raw material is preferably converted into alkali cellulose in advance or simultaneously with the reaction. The method for converting into alkali cellulose is as described above.

The reaction temperature is preferably, for example, 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower.

The amount of compound E _A added per 100 parts by mass of the fiber raw material is preferably 1 part by mass or more and 100,000 parts by mass or less, more preferably 2 parts by mass or more and 10,000 parts by mass or less, and even more preferably 5 parts by mass or more and 1,000 parts by mass or less.

The reaction time may vary depending on the reaction temperature, but is preferably between 1 minute and 1,000 minutes, more preferably between 10 minutes and 500 minutes, and even more preferably between 20 minutes and 400 minutes. After the reaction, excess reaction reagents, by-products, etc. may be washed away with water or removed by filtration, etc.

<Sulfone Group Introduction Step (Sulfoalkylation Step) (Second Sulfone Group Introduction Step)>
The ionic substituent introduction step may include a sulfonic group introduction step (sulfoalkylation step). In the sulfoalkylation, a compound (compound E _B ) having a reactive group and a sulfonic group as an essential component, and an optional component, an alkali compound and a compound B selected from the above-mentioned urea and its derivatives, are added to a wet or dry fiber raw material having a hydroxyl group and reacted to introduce a sulfonic group into the fiber raw material.

Examples of the reactive group include a halogenated alkyl group, a vinyl group, and an epoxy group (glycidyl group).
Examples of compound E _B include sodium 2-chloroethanesulfonate, sodium vinylsulfonate, sodium p-styrenesulfonate, 2-acrylamido-2-methylpropanesulfonic acid, etc. Among these, from the viewpoints of the efficiency of introducing substituents, and therefore the defibration efficiency, cost, and ease of handling, it is preferable that compound E _B be sodium vinylsulfonate.
Furthermore, as an optional component, it is also preferable to use the compound B in the above-mentioned <Phosphorus oxo acid group introduction step> in the same manner, and the amount added is also preferably as described above.

When compound E _B is added, it may be added to the fiber raw material as a reagent (solid or liquid) as it is, or may be dissolved in an appropriate solvent and added. The fiber raw material is preferably converted into alkali cellulose in advance or simultaneously with the reaction. The method for converting into alkali cellulose is as described above.

The reaction temperature is preferably, for example, 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower.

The amount of compound E _B added per 100 parts by mass of the fiber raw material is preferably 1 part by mass or more and 100,000 parts by mass or less, more preferably 2 parts by mass or more and 10,000 parts by mass or less, and even more preferably 5 parts by mass or more and 1,000 parts by mass or less.

The reaction time may vary depending on the reaction temperature, but is preferably between 1 minute and 1,000 minutes, more preferably between 10 minutes and 500 minutes, and even more preferably between 15 minutes and 400 minutes. After the reaction, excess reaction reagents, by-products, etc. may be washed away with water or removed by filtration, etc.

<Carboxyalkylation step (third carboxy group introduction step)>
The process for producing fine fibrous cellulose may include a carboxyalkylation step as a step for introducing an ionic substituent. A compound having a reactive group and a carboxy group (compound E _C ) as an essential component, and an alkali compound and a compound B selected from the aforementioned urea and its derivatives as optional components, are added to a wet or dry fiber raw material having a hydroxyl group and reacted to introduce a carboxy group into the fiber raw material.

Examples of the reactive group include a halogenated alkyl group, a vinyl group, and an epoxy group (glycidyl group).
As the compound E _C , monochloroacetic acid, sodium monochloroacetate, 2-chloropropionic acid, 3-chloropropionic acid, sodium 2-chloropropionate, and sodium 3-chloropropionate are preferred from the standpoints of the efficiency of introducing substituents, and therefore the defibration efficiency, cost, and ease of handling.
Furthermore, as an optional component, it is also preferable to use the compound B in the above-mentioned <Phosphorus oxo acid group introduction step> in the same manner, and the amount added is also preferably as described above.

When compound E or _C is added, it may be added to the fiber raw material as a reagent (solid or liquid) as is, or may be dissolved in an appropriate solvent and added. The fiber raw material is preferably converted into alkali cellulose in advance or simultaneously with the reaction. The method for converting into alkali cellulose is as described above.

The reaction temperature is preferably, for example, 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower.

The amount of compound E _C added per 100 parts by mass of the fiber raw material is preferably 1 part by mass or more and 100,000 parts by mass or less, more preferably 2 parts by mass or more and 10,000 parts by mass or less, and even more preferably 5 parts by mass or more and 1,000 parts by mass or less.

The reaction time may vary depending on the reaction temperature, but is preferably between 1 minute and 1,000 minutes, more preferably between 3 minutes and 500 minutes, and even more preferably between 5 minutes and 400 minutes. After the reaction, excess reaction reagents, by-products, etc. may be washed away with water and removed by filtration or other methods.

<Cationic group introduction step (cationization step)>
A compound having a reactive group and a cationic group (compound E _D ) as an essential component, and an alkaline compound and a compound B selected from the above-mentioned urea and its derivatives as optional components, are added to a wet or dry fiber raw material having a hydroxyl group and reacted to introduce a cationic group into the fiber raw material.

Examples of the reactive group include a halogenated alkyl group, a vinyl group, and an epoxy group (glycidyl group).
Examples of the cationic group include an ammonium group, a phosphonium group, a sulfonium group, etc. Among these, the cationic group is preferably an ammonium group.
As the compound E _D , glycidyl trimethyl ammonium chloride, 3-chloro-2-hydroxypropyl trimethyl ammonium chloride, etc. are preferred from the viewpoints of the efficiency of introducing substituents, and hence the defibration efficiency, cost, and ease of handling.
Furthermore, it is also preferable to use the compound B in the above-mentioned <Phosphorus Oxo Acid Group Introduction Step> as an optional component in the same manner. The amount added is also preferably as described above.

When compound E or _D is added, it may be added to the fiber raw material as a reagent (solid or liquid) as is, or may be dissolved in an appropriate solvent and added. The fiber raw material is preferably converted into alkali cellulose in advance or simultaneously with the reaction. The method for converting into alkali cellulose is as described above.

The reaction temperature is preferably, for example, 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower.

The amount of compound E _D added per 100 parts by mass of the fiber raw material is preferably 1 part by mass or more and 100,000 parts by mass or less, more preferably 2 parts by mass or more and 10,000 parts by mass or less, and even more preferably 5 parts by mass or more and 1,000 parts by mass or less.

The reaction time may vary depending on the reaction temperature, but is preferably between 1 minute and 1,000 minutes, more preferably between 10 minutes and 500 minutes, and even more preferably between 20 minutes and 400 minutes. After the reaction, excess reaction reagents, by-products, etc. may be washed away with water or removed by filtration, etc.

<Cleaning process>
In the method for producing fine fibrous cellulose according to the present embodiment, a washing step can be carried out on the ionic substituent-introduced fibers as needed. The washing step is carried out by washing the ionic substituent-introduced fibers with, for example, water or an organic solvent. The washing step may be carried out after each step described below, and the number of washing steps carried out in each washing step is not particularly limited.

<Alkali treatment step>
When producing fine fibrous cellulose, the fiber raw material may be subjected to an alkali treatment between the step of introducing an ionic substituent and the defibration treatment step described below. The alkali treatment method is not particularly limited, but examples thereof include a method of immersing the ionic substituent-introduced fiber 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 ionic substituent-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 ionic substituent-introduced fiber.

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

<Acid treatment step>
When producing fine fibrous cellulose, the fiber raw material may be subjected to an acid treatment between the step of introducing an ionic substituent and the defibration treatment step described below. For example, the step of introducing an ionic substituent, the acid treatment, the alkali treatment, and the 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.

<Defibrillation processing>
The ionic substituent-introduced fibers are defibrated in a defibration treatment step to obtain fine fibrous cellulose. 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-type grinder), a high-pressure homogenizer, an ultra-high-pressure homogenizer, a high-pressure collision grinder, a ball mill, a bead mill, a disk-type 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.

In the defibration process, it is preferable to dilute the ionic substituent-introduced fibers 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 examples of preferred polar 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. 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 ionic substituent-introduced fibers in a dispersion medium may contain solids other than the ionic substituent-introduced fibers, such as urea, which has hydrogen bonding properties.

<Nitrogen removal treatment>
The process for producing fine fibrous cellulose may further include a step of reducing the nitrogen content (nitrogen removal treatment step). By reducing the nitrogen content, fine fibrous cellulose that can further suppress discoloration can be obtained. The nitrogen removal treatment step is preferably performed before the defibration treatment step.

In the nitrogen removal treatment step, it is preferable to adjust the pH of the slurry containing the substituted fiber 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 substituted fiber, 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 substituted fiber can be subjected to a washing step, if necessary. The washing step is carried out by washing the ionic substituted fiber with, for example, water or an organic solvent. There is no particular limit to the number of washings carried out in each washing step.

<Substituent Removal Treatment>
The method for producing fine fibrous cellulose may include a step of removing at least a portion of the substituents from fine fibrous cellulose having a substituent and a fiber width of 1000 nm or less. By undergoing such a step, it is possible to obtain fine fibrous cellulose having a small fiber width but a low amount of introduced substituents. In this specification, the step of removing at least a portion of the substituents from the fine fibrous cellulose is also referred to as a substituent removal treatment step.

Substituent removal treatment steps include heat treatment, enzyme treatment, acid treatment, alkali treatment, etc. of fine fibrous cellulose having substituents and a fiber width of 1000 nm or less. These may be performed alone or in combination. Of these, the substituent removal treatment step is preferably a heat treatment or enzyme treatment. By undergoing the above treatment steps, at least a portion of the substituents are removed from fine fibrous cellulose having substituents and a fiber width of 1000 nm or less, and fine fibrous cellulose with an introduced substituent amount of less than 0.5 mmol/g can be obtained.

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 a fine fibrous cellulose having substituents 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 coloring substances that are generated by heating or the like during the substituent removal treatment, as well as acids, alkalis, salts, etc. that are added or generated. This makes it possible to suppress discoloration of the liquid composition 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 efficiency of salt removal.

When a substituent removal treatment is performed on a slurry containing fine fibrous cellulose having a substituent 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, by maintaining the concentration of the fine fibrous cellulose in the slurry within the above range, it is possible to 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 the discoloration of the liquid composition or sheet. Furthermore, when a treatment is performed to remove salts derived from the substituents removed after the substituent removal treatment, it is also possible to improve the efficiency of salt removal.

When the substituent removal treatment step is a step of heat-treating fine fibrous cellulose having a substituent 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 substituent on the fine fibrous cellulose subjected to the substituent removal treatment step is a phosphorus oxo acid group, 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 substituents 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 substituents 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 substituents 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 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 particles after the substituent removal treatment. This can more effectively improve the transparency of the liquid composition and sheet.

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 above-mentioned 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 an ionic substituent is introduced into cellulose fibers and the counter ion of this ionic substituent is 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 a washing treatment and an 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>
The substituent removal treatment step may be followed by a step of uniformly dispersing the fine fibrous cellulose obtained through the substituent removal treatment. By subjecting the fine fibrous cellulose to the substituent removal treatment, at least a portion of the fine fibrous cellulose is aggregated. The uniform dispersion treatment step is a step of uniformly dispersing the aggregated fine fibrous cellulose.

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

(optional ingredient)
The liquid composition of the present invention may further contain a polymerization inhibitor as an optional component. In this specification, the polymerization inhibitor has the effect of reacting with radical species that cause radical polymerization and converting them into inactive radicals or stable compounds that do not cause radical polymerization (radical scavenging effect). Examples of polymerization inhibitors include phenothiazine, dibutylhydroxytoluene, 2,2'-methylenebis(4-ethyl-6-t-butylphenol), tris(nonylphenyl)phosphite, 4,4'-thiobis(3-methyl-6-t-butylphenol), N-phenyl-1-naphthylamine, 2,2'-methylenebis(4-methyl-6-t-butylphenol), 2-mercaptobenzimidazole, hydroquinone, and N,N-diethylhydroxylamine. When the liquid composition contains a polymerization inhibitor, the content of the polymerization inhibitor is preferably 0.1 parts by mass or more and 5 parts by mass or less per 100 parts by mass of the fibrous cellulose. By setting the content of the polymerization inhibitor within the above range, yellowing of the sheet after heating can be more effectively suppressed.

Optional ingredients include fillers, pigments, dyes, stabilizers, surfactants, pH adjusters, UV absorbers, etc.

The liquid composition may also contain, as an optional component, a water-soluble organic compound other than the resins described above. Examples of water-soluble organic compounds include sugars, water-soluble polymers (excluding the resins described above), and urea. Specific examples include trehalose, urea, and carboxymethyl cellulose. Other water-soluble organic compounds that can be used include alkyl methacrylate-acrylic acid copolymers, 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, pectin, cationic starch, raw starch, oxidized starch, etherified starch, esterified starch, and starches such as amylose; glycerin, diglycerin, polyglycerin, hyaluronic acid, and metal salts of hyaluronic acid.

(seat)
The present invention also relates to a sheet formed from the liquid composition described above. Specifically, the sheet of the present invention contains fibrous cellulose having a fiber width of 1000 nm or less, a resin, and a reducing agent. The fibrous cellulose, resin, and reducing agent contained in the sheet are the same as those contained in the liquid composition, as described above.

In this embodiment, the YI value (yellowness index) at a sheet thickness of 25 μm is preferably 1.5 or less, more preferably 1.3 or less, and even more preferably 1.2 or less. The lower limit of the YI value (yellowness index) at a sheet thickness of 25 μm is not particularly limited and may be 0.0. Here, the YI value of the sheet is the yellowness index measured in accordance with JIS K 7373:2006. For example, a Colour Cute i (manufactured by Suga Test Instruments Co., Ltd.) can be used as a YI value measuring device. The yellowness index of the above sheet is a value proportional to a sheet thickness of 25 μm, as shown in the following formula:
Yellowness index = measured yellowness index of sheet × (25/sheet thickness)
The above-mentioned YI value is a YI value measured before the sheet is heated as described below, and is therefore sometimes called an initial YI value.

In this embodiment, the YI value of the sheet after heating at 160°C for 6 hours is preferably 5.5 or less, more preferably 4.5 or less, and even more preferably 3.6 or less. The lower limit of the YI value of the sheet after heating at 160°C for 6 hours is not particularly limited and may be 0.0. This YI value is sometimes referred to as the post-heating YI value. The method for measuring the post-heating YI value is the same as the method described above.

In this embodiment, the YI increase rate of the sheet is preferably 2700% or less, more preferably 2500% or less, even more preferably 2300% or less, even more preferably 2000% or less, and particularly preferably 1500% or less. The lower limit of the YI increase rate of the sheet is not particularly limited and may be 0%. Here, the YI increase rate of the sheet refers to the increase rate of the YI value of the sheet before and after heating the sheet at 160°C for 6 hours. Specifically, 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 measured in accordance with JIS K 7373:2006.

The haze of the sheet of this embodiment is preferably 3.5% or less, more preferably 3.0% or less, even more preferably 2.5% or less, and particularly preferably 2.0% or less. If the haze of the sheet is within the above range, it can be determined that a highly transparent sheet has been formed by using the fine fibrous cellulose of the present invention. The haze of the sheet is a value measured using a haze meter in accordance with JIS K 7136:2000.

The total light transmittance of the sheet of this embodiment is preferably 75% or more, more preferably 80% or more, and even more preferably 85% or more. The haze of the sheet is a value measured using a haze meter in accordance with JIS K 7361-1:1997.

The content of fibrous cellulose relative to the total solid mass in the sheet of this embodiment is preferably 1% by mass or more, more preferably 10% by mass or more, even more preferably 20% by mass or more, and particularly preferably 30% by mass or more. Furthermore, the content of fibrous cellulose relative to the total solid mass in the sheet is preferably 99% by mass or less, more preferably 90% by mass or less.

The thickness of the sheet in this embodiment is not particularly limited, but is preferably 5 μm or more, more preferably 10 μm or more, and even more preferably 20 μm or more. The upper limit of the sheet thickness is also not particularly limited, but is preferably 1000 μm or less. The sheet thickness can be measured, for example, using a constant pressure thickness gauge (PG-02, manufactured by TECLOCK C ORPORATION).

The basis weight of the sheet of this embodiment is not particularly limited, but is preferably 10 g/ ^m2 or more, more preferably 20 g/ ^m2 or more, and even more preferably 30 g/ ^m2 or more. The basis weight of the sheet is also not particularly limited, but is preferably 200 g/ ^m2 or less, and more preferably 150 g/ ^m2 or less. Here, the basis weight of the sheet can be calculated, for example, in accordance with JIS P 8124:2011.

The density of the sheet of this embodiment is not particularly limited, but is preferably 0.1 g/cm ^or more, more preferably 0.5 g/cm or ^more , and even more preferably 1.0 g/cm or ^more . The density of the sheet is also not particularly limited, but is preferably 5.0 g/cm ^or less, and more preferably 3.0 g/cm or ^less . Here, the density of the sheet can be calculated by conditioning a 50 mm square sheet at 23°C and a relative humidity of 50% for 24 hours, and then measuring the thickness and mass of the sheet.

The elastic modulus of the sheet of this embodiment is not particularly limited, but is preferably 2 GPa or more, more preferably 4 GPa or more, and even more preferably 6 GPa or more. Here, the elastic modulus of the sheet can be calculated, for example, in accordance with JIS P 8113:2006.

(Sheet manufacturing method)
The method for producing a sheet of the present invention includes a step of obtaining a slurry containing fibrous cellulose having a fiber width of 1000 nm or less, a resin, and a reducing agent, and a coating step of coating the slurry onto a substrate or a papermaking step of making paper from the slurry, thereby obtaining the above-mentioned sheet.

<Coating process>
In the coating process, a slurry containing fibrous cellulose having a fiber width of 1000 nm or less, a resin, and a reducing agent is coated onto a substrate, dried, and the resulting sheet is peeled off from the substrate to obtain a sheet. Furthermore, 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 slurry can prevent shrinkage of the sheet during drying. However, it is preferable to select a substrate that allows the formed sheet to be easily peeled off after drying. 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 slurry is low during the coating process and it spreads on the substrate, 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 type of 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, vinyl 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 slurry 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 slurry temperature and ambient temperature when applying the slurry to the substrate are not particularly limited, but are preferably, for example, 5°C to 80°C, more preferably 10°C to 60°C, even more preferably 15°C to 50°C, and particularly preferably 20°C to 40°C. If the application temperature is above the lower limit, the slurry 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 slurry is preferably applied to the substrate so that the finished basis weight of the sheet is preferably 10 g/m ² or ^more and 200 g/m ² or less, more preferably 20 g/m 2 or more and 150 g/m ² or less. By coating so that the basis weight is within the above range, a sheet with excellent strength can be obtained.

As described above, the coating process includes drying the slurry coated on the substrate. The drying process can be performed by, but is not limited to, a non-contact drying method, a method of drying while restraining the sheet, or a combination of these. Non-contact drying methods can include, but are not limited to, methods of drying by heating with hot air, infrared rays, far-infrared rays, or near-infrared rays (heat drying), or methods of drying in a vacuum (vacuum drying). While heat drying and vacuum drying can be combined, heat drying is typically used. Drying with infrared rays, far-infrared rays, or near-infrared rays can be performed using, but is not limited to, 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 slurry using a papermaking machine. The papermaking machine used in the papermaking process is not particularly limited, but examples 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 dewatering the slurry using a wire to obtain a wet sheet, which is then pressed and dried. The filter cloth used to filter and dewater the slurry 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 sheeting process, a method for producing a sheet from a slurry can be carried out using a production device equipped with a water squeezing section in which a slurry containing fine fibrous cellulose is discharged onto the upper surface of an endless belt and the dispersion medium is squeezed out of the discharged slurry 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.

(Laminate)
The present invention may also relate to a laminate having the above-mentioned sheet and a resin layer laminated on at least one surface of the sheet.The present invention may also relate to a laminate having resin layers on both surfaces of the sheet.

The resin layer is a layer whose main component is a natural resin or synthetic resin. Here, "main component" refers to a component that accounts for 50% or more by weight of the total weight of the resin layer. The resin content is preferably 60% or more by weight, more preferably 70% or more by weight, even more preferably 80% or more by weight, and particularly preferably 90% or more by weight of the total weight of the resin layer. The resin content may be 100% by weight, or may be 95% or less by weight.

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

The synthetic resin is preferably at least one selected from, for example, polycarbonate resin, polyethylene terephthalate resin, polyethylene naphthalate resin, polyethylene resin, polypropylene resin, polyimide resin, polystyrene resin, and acrylic resin. Among these, the synthetic resin is preferably at least one selected from polycarbonate resin and acrylic resin, and more preferably polycarbonate resin. The acrylic resin is preferably at least one selected from polyacrylonitrile and poly(meth)acrylate.

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.

(Application)
The sheet of the present invention is preferably used for optical members, such as light-transmitting substrates for various display devices and various solar cells.

The sheet of the present invention can also be used 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 features of the present invention will be explained in more detail below with reference to examples and comparative examples. The materials, amounts used, ratios, treatment details, treatment procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be interpreted as being limited by the specific examples shown below. In the following, Examples 15 and 21 to 26 should be read as Reference Examples 15 and 21 to 26, respectively.

<Production Example 1>
[Phosphorylation treatment]
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 250 seconds to introduce phosphate groups into the cellulose in the pulp, thereby obtaining phosphorylated pulp 1.

[Cleaning process]
The resulting phosphorylated pulp 1 was then washed. The washing was performed by repeatedly adding 10 L of ion-exchanged water to 100 g (bone dry mass) of phosphorylated pulp to obtain a pulp dispersion, stirring the resulting solution to uniformly disperse the pulp, and then 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 1 was subjected to a neutralization treatment as follows. First, the washed phosphorylated pulp was diluted with 10 L of ion-exchanged water, and then a 1 N aqueous sodium hydroxide solution 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 1. Next, the neutralized phosphorylated pulp 1 was subjected to the above-mentioned washing treatment.

The infrared absorption spectrum of the phosphorus oxo-oxidized pulp 1 thus obtained was measured using FT-IR. As a result, absorption due to the P=O of the phosphate group was observed around 1230 cm ^-1 , confirming that phosphate groups had been added to the pulp. Furthermore, when the obtained phosphorylated pulp was analyzed using an X-ray diffractometer, typical peaks were confirmed at two positions, around 2θ = 14° to 17° and around 2θ = 22° to 23°, confirming the presence of cellulose type I crystals.

[Defibrillation process]
Ion-exchanged water was added to the resulting phosphorylated pulp to prepare a slurry with a solids concentration of 2% by mass. This slurry was processed six times at a pressure of 200 MPa using a high-pressure homogenizer (Starburst, manufactured by Sugino Machine Co., Ltd.) to obtain a fine fibrous cellulose dispersion (a dispersion containing phosphorylated CNF1) containing fine fibrous cellulose. X-ray diffraction confirmed that this fine fibrous cellulose maintained cellulose type I crystals. Furthermore, the fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 3 to 5 nm. The amount of phosphate groups (amount of first dissociated acid) measured by the measurement method described below for the measurement of [amount of phosphorus oxo acid group] was 1.45 mmol/g. The total amount of dissociated acid was 2.45 mmol/g.

<Production Example 2>
The fine fibrous cellulose dispersion obtained in Production Example 1 (dispersion containing phosphorylated CNF1) was subjected to the following treatment to obtain a fine fibrous cellulose dispersion containing fine fibrous cellulose (dispersion containing phosphorylated CNF2).

[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 slurry was adjusted to pH 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]
After heating, the same amount of ion-exchanged water was added to the slurry to obtain a slurry with a solids concentration of approximately 1.0% by mass. The slurry was then stirred and 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 obtain a slurry with a solids concentration of approximately 1.0% by mass, and the mixture was allowed to stand for 24 hours. The filtration and dehydration process was then repeated, and the washing endpoint was reached 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 aggregates, 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 resulting slurry after removal of the substituents to give a slurry with a solids concentration of 1.0% by 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 the substituent-removed fine fibrous cellulose. The fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 3 to 5 nm.

<Production Example 3>
[Phosphite 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 (a dispersion containing phosphorous-containing CNF) containing phosphorous-containing 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. Furthermore, the fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 3 to 5 nm. 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 4>
[TEMPO oxidation treatment]
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 an alkaline TEMPO oxidation treatment as follows. First, 100 parts by mass of the raw material pulp (dry mass equivalent), 1.6 parts by mass of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), and 10 parts by mass of sodium bromide were dispersed in 10,000 parts by mass of water. Next, a 13% by mass aqueous solution of sodium hypochlorite was added to 1.0 g of pulp to give a concentration of 3.8 mmol to initiate the reaction. During the reaction, a 0.5 M aqueous solution of sodium hydroxide was added dropwise to maintain the pH at 10 to 10.5, and the reaction was considered complete when no further change in pH was observed.

[Cleaning process]
The resulting TEMPO-oxidized pulp was then washed. The pulp slurry after TEMPO oxidation was dehydrated to obtain a dehydrated sheet, to which 5,000 parts by mass of ion-exchanged water was poured, stirred to uniformly disperse the pulp, and then filtered and dehydrated. This process was repeated. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less.

[Additional oxidation treatment]
The remaining aldehyde groups in this dehydrated sheet were further oxidized as follows: 100 parts by weight of the dehydrated sheet (dry mass equivalent) was dispersed in 10,000 parts by weight of 0.1 mol/L acetate buffer (pH 4.8). 113 parts by weight of 80% sodium chlorite was then added. The container was immediately sealed and allowed to react at room temperature for 48 hours while stirring at 500 rpm using a magnetic stirrer to obtain a pulp slurry.

The resulting TEMPO-oxidized pulp was then washed. The washing process involved dehydrating the pulp slurry after the oxidation to obtain a dehydrated sheet, pouring 5,000 parts by mass of ion-exchanged water into the sheet, stirring to uniformly disperse the material, and then repeatedly filtering and dehydrating the sheet. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less. The resulting TEMPO-oxidized pulp was analyzed using an X-ray diffractometer. Typical peaks were observed at two positions, near 2θ = 14° to 17° and near 2θ = 22° to 23°, confirming the presence of cellulose type I crystals.

[Defibrillation process]
Ion-exchanged water was added to the resulting TEMPO-oxidized pulp to prepare a slurry with a solids concentration of 2% by mass. This slurry was processed four times at a pressure of 200 MPa using a wet pulp mill (Starburst, manufactured by Sugino Machine Co., Ltd.) to obtain a fine fibrous cellulose dispersion (a dispersion containing TEMPO-oxidized CNF). X-ray diffraction confirmed that the fine fibrous cellulose maintained cellulose type I crystal structure. Furthermore, the fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 3 to 5 nm. The carboxyl group content, as measured by the method described below, was 1.30 mmol/g.

<Production Example 5>
[Sulfuric acid esterification treatment]
The same procedure as in Production Example 1 was carried out, except that 38 parts by mass of amidosulfuric acid (sulfamic acid) was used instead of ammonium dihydrogen phosphate and the heating time was extended to 20 minutes, to obtain a fine fibrous cellulose dispersion containing sulfated pulp and fine cellulose.

The obtained sulfated pulp was subjected to infrared absorption spectroscopy using FT-IR. As a result, absorption due to S=O of sulfate ester groups was observed around 1220-1260 cm ^-1 , confirming that sulfate ester groups had been added to the pulp. Furthermore, when the obtained sulfated pulp was analyzed using an X-ray diffractometer, typical peaks were confirmed at two positions, around 2θ = 14° to 17° and around 2θ = 22° to 23°, confirming the presence of cellulose type I crystals.

X-ray diffraction confirmed that the resulting fine fibrous cellulose maintained cellulose type I crystal structure. Furthermore, the fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 3 to 5 nm. The amount of sulfate ester groups in the resulting fine fibrous cellulose, as measured using the method described below in (Measurement of sulfur oxoacid group amount and sulfonic acid group amount), was 1.47 mmol/g.

<Production Example 6>
[Hypochlorous acid oxidation treatment]
A sheet (solids concentration 90% by mass) made from softwood bleached kraft pulp (NBKP) was mixed with a hand mixer (Osaka Chemical Co., Ltd., Labo Millser PLUS) at 20,000 rpm for 15 seconds to produce a fluffy fluffed pulp (solids concentration 90% by mass). Sodium hypochlorite pentahydrate was then added to ion-exchanged water to prepare an aqueous solution with a sodium hypochlorite solids concentration of 22% by mass. 9,000 parts by mass of a 22% by mass aqueous sodium hypochlorite solution was added to 100 parts by mass of the fluffy fluffed pulp, and the mixture was allowed to react for 2 hours while adjusting the temperature in a warm bath to 30°C, yielding a carboxyl-introduced pulp. During the reaction, a 1N aqueous sodium hydroxide solution was added as needed to maintain the pH at 11.

[Cleaning process]
The resulting carboxylated pulp was then washed. The washing was performed by pouring ion-exchanged water over the resulting carboxylated pulp to obtain a pulp dispersion, stirring the resulting dispersion 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.

Furthermore, when the resulting carboxylated pulp was analyzed using an X-ray diffractometer, typical peaks were confirmed at two positions, near 2θ = 14° to 17° and near 2θ = 22° to 23°, confirming the presence of cellulose type I crystals.

[Defibrillation process]
Ion-exchanged water was added to the obtained carboxyl-introduced pulp to prepare a slurry with a solids concentration of 2% by mass. This slurry was treated six times in a wet pulverization apparatus (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 the resulting fine fibrous cellulose maintained cellulose type I crystal structure. Furthermore, the fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 3 to 5 nm. The amount of carboxyl groups in the resulting fine fibrous cellulose, as measured by the method described below, was 0.70 mmol/g.

<Production Example 7>
[Maleic acid esterification treatment]
A sheet (solids concentration 90% by mass) made from bleached softwood kraft pulp (NBKP) was mixed with a hand mixer (Labo Millser PLUS, manufactured by Osaka Chemical Co., Ltd.) at 20,000 rpm for 15 seconds to produce a fluffy fluffing pulp (solids concentration 90% by mass). 100 parts by mass of the fluffy fluffing pulp and 50 parts by mass of maleic anhydride were placed in an autoclave and mixed at 150°C for 2 hours to obtain a carboxyl-introduced pulp.

[Cleaning process]
The resulting carboxylated pulp was then washed. The washing was performed by pouring ion-exchanged water over the resulting carboxylated pulp to obtain a pulp dispersion, stirring the resulting dispersion 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.

The infrared absorption spectrum of the obtained carboxylated pulp was measured using FT-IR. As a result, absorption due to carboxyl groups was observed near 1580 and 1720 cm ^-1 , confirming that maleic acid esterification had occurred. The amount of carboxyl groups in the obtained carboxylated pulp, measured using the measurement method described below, was 1.22 mmol/g. Furthermore, when the carboxylated pulp was analyzed using an X-ray diffractometer, typical peaks were confirmed at two positions, near 2θ = 14° to 17° and near 2θ = 22° to 23°, confirming the presence of cellulose type I crystals.

[Defibrillation process]
Ion-exchanged water was added to the obtained carboxyl-introduced pulp to prepare a slurry with a solids concentration of 2% by mass. This slurry was treated six times in a wet pulverization apparatus (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 the resulting fine fibrous cellulose maintained cellulose type I crystal structure. Furthermore, the fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 3 to 5 nm. The amount of carboxyl groups in the resulting fine fibrous cellulose, as measured by the method described below, was 1.22 mmol/g.

<Production Example 8>
[Carboxymethylation treatment]
As the raw material pulp, softwood kraft pulp (solid content 93% by mass, basis weight 245 g/ ^m2 sheet form, Canadian Standard Freeness (CSF) measured in accordance with JIS P 8121-2:2012 after disintegration 700 ml) manufactured by Oji Paper Co., Ltd. was used.

To 100 parts by mass (bone dry mass) of this raw pulp, a chemical solution consisting of 83 parts by mass of 12N NaOH aqueous solution, 175 parts by mass of sodium monochloroacetate, and 313 parts by mass of ion-exchanged water (total 571 parts by mass) was added to obtain chemical-impregnated pulp. The resulting chemical-impregnated pulp was then heated in a 95°C water bath for 60 minutes to introduce carboxymethyl groups (carboxy groups) into the cellulose in the pulp, yielding carboxy-introduced pulp.

The resulting carboxylated pulp was then washed. The washing process involved pouring ion-exchanged water over the resulting carboxylated pulp to obtain a pulp dispersion. The pulp was then stirred to uniformly disperse the pulp, followed by repeated filtration and dehydration. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less.

The amount of carboxy groups in the resulting carboxylated pulp, measured using the method described below, was 1.21 mmol/g. Furthermore, when the carboxylated pulp was analyzed using an X-ray diffractometer, typical peaks were confirmed at two positions, near 2θ = 14° to 17° and near 2θ = 22° to 23°, confirming the presence of cellulose type I crystals.

Ion-exchanged water was added to the resulting carboxyl-introduced pulp to prepare a slurry with a solids concentration of 2% by mass. This slurry was processed six times at a pressure of 200 MPa in a wet atomization device (Starburst, manufactured by Sugino Machine Co., Ltd.) to obtain a fine fibrous cellulose dispersion containing fine fibrous cellulose.

X-ray diffraction confirmed that the resulting fine fibrous cellulose maintained cellulose type I crystal structure. Furthermore, the fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 3 to 5 nm. The amount of carboxyl groups in the resulting fine fibrous cellulose, as measured by the method described below, was 1.21 mmol/g.

<Production Example 9>
[Carboxyethylation Treatment]
As the raw material pulp, softwood kraft pulp (solid content 93% by mass, basis weight 245 g/ ^m2 sheet form, Canadian Standard Freeness (CSF) measured in accordance with JIS P 8121-2:2012 after disintegration 700 ml) manufactured by Oji Paper Co., Ltd. was used.

To 100 parts by mass (bone dry mass) of this raw pulp, a chemical solution consisting of 250 parts by mass of 12N NaOH aqueous solution, 163 parts by mass of 2-chloropropionic acid, and 140 parts by mass of ion-exchanged water (total 553 parts by mass) was added to obtain chemical-impregnated pulp. The resulting chemical-impregnated pulp was then heated in a hot air dryer at 165°C for 10 minutes to introduce carboxyethyl groups (carboxy groups) into the cellulose in the pulp, yielding carboxy-group-introduced pulp.

[Cleaning process]
The resulting carboxylated pulp was then washed. The washing was performed by pouring ion-exchanged water over the resulting carboxylated pulp to obtain a pulp dispersion, stirring the resulting dispersion 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.

Next, the washed carboxylated pulp was neutralized as follows: First, the washed carboxylated pulp was diluted with 10 L of ion-exchanged water, and then a 1N aqueous solution of sodium hydroxide was added little by little while stirring to obtain a carboxylated pulp slurry with a pH of 12 to 13. Next, the carboxylated pulp slurry was dehydrated and washed to obtain neutralized carboxylated pulp.

Furthermore, when the carboxyl-introduced pulp was analyzed using an X-ray diffractometer, typical peaks were confirmed at two positions, near 2θ = 14° to 17° and near 2θ = 22° to 23°, confirming the presence of cellulose type I crystals.

[Defibrillation process]
Ion-exchanged water was added to the obtained carboxyl-introduced pulp to prepare a slurry with a solids concentration of 2% by mass. This slurry was treated six times in a wet pulverization apparatus (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 the resulting fine fibrous cellulose maintained cellulose type I crystal structure. Furthermore, the fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 3 to 5 nm. The amount of carboxyl groups in the resulting fine fibrous cellulose, as measured by the method described below, was 1.41 mmol/g.

<Production Example 10>
[Sulfoethylation treatment]
As the raw material pulp, softwood kraft pulp (solid content 93% by mass, basis weight 245 g/ ^m2 sheet form, Canadian Standard Freeness (CSF) measured in accordance with JIS P 8121-2:2012 after disintegration 700 ml) manufactured by Oji Paper Co., Ltd. was used.

To 100 parts by mass (bone dry mass) of this raw pulp, a chemical solution consisting of 180 parts by mass of 2N NaOH aqueous solution and 780 parts by mass of a 25% sodium vinyl sulfonate aqueous solution (total 960 parts by mass) was added to obtain chemical-impregnated pulp. The resulting chemical-impregnated pulp was then heated in a hot air dryer at 165°C for 16 minutes to introduce sulfoethyl groups (sulfonic groups) into the cellulose in the pulp, yielding sulfoethyl-group-introduced pulp (sulfonic group-introduced pulp).

The resulting sulfoethyl group-introduced pulp was then washed. The washing process involved pouring ion-exchanged water over the resulting sulfoethyl group-introduced pulp to obtain a pulp dispersion. The pulp was then stirred to uniformly disperse the pulp, and the pulp was then filtered and dehydrated. This process was repeated until the electrical conductivity of the filtrate reached 100 μS/cm or less, marking the end point of the washing process.

Furthermore, when sulfoethyl group-introduced pulp was tested and analyzed using an X-ray diffractometer, typical peaks were confirmed at two positions, near 2θ = 14° to 17° and near 2θ = 22° to 23°, confirming the presence of cellulose type I crystals.

[Defibrillation process]
Ion-exchanged water was added to the obtained sulfoethyl group-introduced pulp to prepare a slurry with a solids concentration of 2% by mass. This slurry was treated six times in a wet atomization apparatus (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 the resulting fine fibrous cellulose maintained cellulose type I crystal structure. Furthermore, the fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 3 to 5 nm. The amount of sulfoethyl groups (sulfonic acid groups) of the resulting fine fibrous cellulose, as measured by the method described below, was 1.48 mmol/g.

<Production Example 11>
[Cationization treatment]
The raw material pulp used was softwood kraft pulp manufactured by Oji Paper Co., Ltd. (solids content 93% by mass, basis weight 245 g/ ^m2 sheet form, Canadian Standard Freeness (CSF) measured after disintegration in accordance with JIS P 8121-2:2012 of 700 ml).

To 100 parts by mass (bone dry mass) of this raw pulp, a chemical solution consisting of 180 parts by mass of 1N NaOH aqueous solution and 325 parts by mass of a cationizing agent (Catiomaster G, manufactured by Yokkaichi Synthetic Co., Ltd., glycidyl trimethylammonium chloride, purity 73.1% by mass, moisture content 20.2% by mass) (total 505 parts by mass) was added to obtain chemical-impregnated pulp. The resulting chemical-impregnated pulp was then heated in a hot air dryer at 165°C for 12 minutes to introduce cationic groups into the cellulose in the pulp, yielding cationic group-introduced pulp.

The resulting cation-group-introduced pulp was then washed. The washing process involved pouring ion-exchanged water over the resulting cation-group-introduced pulp to obtain a pulp dispersion. The pulp was then stirred to uniformly disperse the pulp, and the pulp was then filtered and dehydrated. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less.

The washed cation group-introduced pulp was then neutralized as follows: First, the washed cation group-introduced pulp was diluted with 10 L of ion-exchanged water, and then 1 N hydrochloric acid was added little by little while stirring to obtain a cation group-introduced pulp slurry with a pH of 1 to 2. The cation group-introduced pulp slurry was then dehydrated and washed to obtain neutralized cation group-introduced pulp.

Analysis using an X-ray diffractometer confirmed typical peaks at two positions, near 2θ = 14° to 17° and near 2θ = 22° to 23°, confirming the presence of cellulose type I crystals.

[Defibrillation process]
Ion-exchanged water was added to the resulting cationic group-introduced pulp to prepare a slurry with a solids concentration of 2% by mass. This slurry was treated six times in a wet pulverization apparatus (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 the resulting fine fibrous cellulose maintained cellulose type I crystal structure. Furthermore, the fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 3 to 5 nm. The amount of cationic groups, as measured using the cationic group measurement method described below, was 1.45 mmol/g.

<Measurement>
(Measurement of phosphorus oxoacid group content)
The amount of phosphorus oxo acid groups in the fine fibrous cellulose (equivalent to the amount of phosphorus oxo acid groups in the phosphorus oxo-oxidized pulp) was measured by adding ion-exchanged water to a fine fibrous cellulose dispersion containing the target fine fibrous cellulose to adjust the content to 0.2 mass%, treating the dispersion with ion-exchange resin, and then titrating it 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 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 value of pH 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 divided by the solid content (g) in the slurry to be titrated was defined as the amount of phosphorus oxo acid group (first dissociated acid amount) (mmol/g). The amount of alkali (mmol) required from the start of titration to the second endpoint divided by the solid content (g) in the slurry to be titrated was defined as the total dissociated acid amount (mmol/g).

(Measurement of Carboxy Group Amount)
The amount of carboxy groups in the fine fibrous cellulose (equivalent to the amount of carboxy groups in carboxylated pulp such as TEMPO-oxidized pulp) was measured by adding ion-exchanged water to a fine fibrous cellulose dispersion containing the target fine fibrous cellulose to adjust the content to 0.2 mass%, treating the dispersion with ion-exchange resin, and then titrating the dispersion with 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 amount and sulfonic acid group amount)
The amount of sulfur oxoacid groups or sulfonic groups in the fine fibrous cellulose was determined by subjecting a freeze-dried and pulverized sample to pressure-heat decomposition with sulfuric acid in a sealed container, diluting it appropriately, and measuring the amount of sulfur by ICP-OES. The value calculated by dividing by the bone dry mass of the fine fibrous cellulose used was taken as the amount of sulfur oxoacid groups or sulfonic groups (mmol/g) in the fine fibrous cellulose.

(Measurement of Amount of Cationic Groups)
The amount of cationic groups in the fine fibrous cellulose was determined by carrying out a trace nitrogen analysis and calculating the value according to the following formula, which was taken as the amount of cationic groups in the fine fibrous cellulose (mmol/g).
(Amount of cationic groups) [mmol/g] = (amount of nitrogen) [g] / 14 × 1000 / (amount of cationic group-introduced fine fibrous cellulose tested) [g]

Example 1
[Preparation of Liquid Composition A]
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 fine fibrous cellulose dispersion obtained in Production Example 1, the polyvinyl alcohol aqueous solution, and sodium sulfite (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were each diluted with ion-exchanged water to a solids concentration of 0.5% by mass. Next, 100 parts by mass of the diluted fine fibrous cellulose dispersion were added as a resin to 100 parts by mass of the diluted polyvinyl alcohol aqueous solution, and 0.4 parts by mass of the diluted sodium sulfite aqueous solution as a reducing agent to obtain a liquid composition A.

[Sheet production]
Liquid composition A was weighed out so that the finished sheet had a basis weight of 34 g/ ^m² 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 achieve the desired basis weight. The sheet was then dried in a dryer at 100°C for 1 hour and peeled off from the acrylic plate to obtain a fine fibrous cellulose-containing sheet. The sheet had a thickness of 25 µm.

Example 2
A liquid composition and a sheet containing fine fibrous cellulose were obtained in the same manner as in Example 1, except that sodium thiosulfate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the reducing agent.

Example 3
A liquid composition and a sheet containing fine fibrous cellulose were obtained in the same manner as in Example 1, except that sodium borohydride (manufactured by Sigma-Aldrich Japan LLC) was used as the reducing agent.

Example 4
A liquid composition and a sheet containing fine fibrous cellulose were obtained in the same manner as in Example 1, except that sodium hydrogen sulfite (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the reducing agent.

Example 5
A liquid composition and a sheet containing fine fibrous cellulose were obtained in the same manner as in Example 1, except that sodium cyanoborohydride (manufactured by Sigma-Aldrich Japan LLC) was used as the reducing agent.

Example 6
A liquid composition and a sheet containing fine fibrous cellulose were obtained in the same manner as in Example 1, except that sodium hyposulfite (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the reducing agent.

Example 7
A liquid composition and a sheet containing fine fibrous cellulose were obtained in the same manner as in Example 2, except that the amount of diluted aqueous sodium thiosulfate solution added was changed to 1 part by mass.

Example 8
A liquid composition and a sheet containing fine fibrous cellulose were obtained in the same manner as in Example 2, except that the amount of diluted aqueous sodium thiosulfate solution added was changed to 4 parts by mass.

Example 9
A liquid composition was obtained in the same manner as in Example 1, except that 43 parts by mass of the diluted aqueous polyvinyl alcohol solution as a resin and 0.4 parts by mass of the diluted aqueous sodium thiosulfate solution as a reducing agent were added to 100 parts by mass of the diluted fine fibrous cellulose dispersion. A fine fibrous cellulose-containing sheet was obtained in the same manner as in Example 1, except that the obtained liquid composition was weighed out so that the finished basis weight of the sheet was 35 g/ ^m2 . The thickness of the sheet was 25 μm.

Example 10
A liquid composition and a sheet containing fine fibrous cellulose were obtained in the same manner as in Example 2, except that the fine fibrous cellulose dispersion obtained in Production Example 2 was used.

Example 11
A liquid composition and a sheet containing fine fibrous cellulose were obtained in the same manner as in Example 9, except that the fine fibrous cellulose dispersion obtained in Production Example 2 was used.

Example 12
A liquid composition and a sheet containing fine fibrous cellulose were obtained in the same manner as in Example 7, except that the fine fibrous cellulose dispersion obtained in Production Example 2 was used.

Example 13
A liquid composition and a sheet containing fine fibrous cellulose were obtained in the same manner as in Example 1, except that the fine fibrous cellulose dispersion obtained in Production Example 2 was used.

Example 14
A liquid composition and a sheet containing fine fibrous cellulose were obtained in the same manner as in Example 2, except that the fine fibrous cellulose dispersion obtained in Production Example 3 was used.

Example 15
A liquid composition and a sheet containing fine fibrous cellulose were obtained in the same manner as in Example 2, except that the fine fibrous cellulose dispersion obtained in Production Example 4 was used.

Example 16
A liquid composition and a sheet containing fine fibrous cellulose were obtained in the same manner as in Example 1, except that 0.2 parts by mass of dibutylhydroxytoluene (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was further added to liquid composition A as a polymerization inhibitor.

Example 17
A liquid composition and a fine fibrous cellulose-containing sheet were obtained in the same manner as in Example 1, except that 0.2 parts by mass of phenothiazine (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was further added to liquid composition A as a polymerization inhibitor.

Example 18
A liquid composition and a fine fibrous cellulose-containing sheet were obtained in the same manner as in Example 9, except that polyethylene oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.: molecular weight 1 million) was used as the resin instead of acetoacetyl-modified polyvinyl alcohol.

Example 19
A liquid composition and a fine fibrous cellulose-containing sheet were obtained in the same manner as in Example 9, except that a polyurethane resin (Superflex 170, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) was used instead of the acetoacetyl-modified polyvinyl alcohol as the resin.

<Examples 20 to 26>
A liquid composition and a fine fibrous cellulose-containing sheet were obtained in the same manner as in Example 2, except that the fine fibrous cellulose dispersions obtained in Production Examples 5 to 11 were used instead of the fine fibrous cellulose dispersion obtained in Production Example 1.

<Comparative Example 1>
A liquid composition and a sheet containing fine fibrous cellulose were obtained in the same manner as in Example 1, except that no reducing agent was added.

<Comparative Example 2>
A liquid composition and a sheet containing fine fibrous cellulose were obtained in the same manner as in Example 16, except that no reducing agent was added.

<Comparative Example 3>
A liquid composition and a sheet containing fine fibrous cellulose were obtained in the same manner as in Example 17, except that no reducing agent was added.

<Comparative Example 4>
A liquid composition was obtained in the same manner as in Example 2, except that no aqueous polyvinyl alcohol solution was added to the diluted fine fibrous cellulose dispersion. An attempt was made to produce a sheet using the obtained liquid composition in the same manner as in Example 1, but the sheet could not be formed due to significant shrinkage due to drying.

<Comparative Example 5>
A liquid composition was obtained in the same manner as in Example 15, except that no aqueous polyvinyl alcohol solution was added to the diluted fine fibrous cellulose dispersion. An attempt was made to produce a sheet using the obtained liquid composition in the same manner as in Example 1, but the sheet could not be formed due to significant shrinkage due to drying.

[evaluation]
The liquid compositions and fine fibrous cellulose-containing sheets obtained in the examples and comparative examples were evaluated by the following methods.

[Measurement of pH of liquid composition]
The pH of the liquid composition was measured using a calibrated handheld pH meter (D-51S, manufactured by Horiba Ltd.).

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

[Yellowness measurement of sheet before and after heating]
The yellowness of the sheet before and after heating was measured in accordance with JIS K 7373:2006 using Colour Cute i (manufactured by Suga Test Instruments Co., Ltd.) The yellowness after heating was the yellowness of the sheet heated at 160°C for 6 hours.
The yellowness of the sheet was calculated as a value proportional to a sheet thickness of 25 μm according to the following formula.
Yellowness index = measured yellowness index of sheet × (25/sheet thickness)

[Sheet thickness measurement]
A sheet cut into a size of 50 mm square or more was conditioned at 23°C and a relative humidity of 50% for 24 hours, and then the thickness of the sheet was measured at four arbitrary points using a constant pressure thickness measuring device (PG-02, manufactured by TECLOCK C ORPORATION), and the average value was taken as the thickness of the sheet.

The sheets obtained in the examples had a suppressed YI value after heating. Furthermore, the sheets obtained in the examples had low haze and excellent transparency.

Furthermore, a laminate was produced in which resin layers were laminated on both sides of a fine fibrous cellulose-containing sheet using the following method.

Example 50
(Formation of resin layer)
A resin coating liquid was obtained by mixing 15 parts by mass of a modified polycarbonate resin (Iupizeta FPC-2136, manufactured by Mitsubishi Gas Chemical Company, Inc.), 57 parts by mass of toluene, and 28 parts by mass of methyl ethyl ketone. Next, 2.25 parts by mass of an isocyanate compound (Duranate TPA-100, manufactured by Asahi Kasei Chemicals Corporation) was added as an adhesion aid to the resin coating liquid and mixed.
This resin coating liquid was applied to one side of the fine fibrous cellulose-containing sheet obtained in Example 2 using a bar coater. The resin coating liquid was then cured by heating at 100°C for 1 hour to form a resin layer. A resin layer was then formed on the opposite side of the fine fibrous cellulose-containing sheet using the same procedure. This procedure resulted in a laminate in which resin layers were laminated on both sides of the fine fibrous cellulose-containing sheet. The thickness of the laminate was 30 µm.

The haze of the resulting laminate was 0.2%, the YI value before heating was 0.2, and the YI value after heating was 3.4. Thus, the YI value after heating was suppressed in the laminate, and a highly transparent laminate with low haze was obtained.

Claims (14)

Fibrous cellulose having a fiber width of 1000 nm or less and having an ionic substituent;
Resin and
a reducing agent,
the ionic substituent 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, and a substituent derived from a sulfur oxoacid group;
A liquid composition in which the amount of ionic substituents introduced into the fibrous cellulose is 0.05 mmol/g or more and less than 0.5 mmol/g . 2. The liquid composition according to claim 1 , wherein the reducing agent is at least one selected from the group consisting of sodium sulfite, sodium thiosulfate, sodium borohydride, sodium hydrogensulfite, sodium cyanoborohydride, and sodium hyposulfite. The liquid composition according to claim 1 or 2 , wherein the resin is a water-soluble polymer. The liquid composition according to any one of claims 1 to 3 , further comprising a polymerization inhibitor. The liquid composition according to any one of claims 1 to 4 , wherein the content of the reducing agent is 0.1 to 5 parts by mass per 100 parts by mass of the fibrous cellulose. Fibrous cellulose having a fiber width of 1000 nm or less and having an ionic substituent;
Resin and
a reducing agent,
the ionic substituent 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, and a substituent derived from a sulfur oxoacid group;
The sheet has an amount of ionic substituents introduced into the fibrous cellulose of 0.05 mmol/g or more and less than 0.5 mmol/g . 7. The sheet according to claim 6 , wherein the reducing agent is at least one selected from the group consisting of sodium sulfite, sodium thiosulfate, sodium borohydride, sodium hydrogen sulfite, sodium cyanoborohydride, and sodium hyposulfite. The sheet according to claim 6 or 7 , wherein the resin is a water-soluble polymer. The sheet according to any one of claims 6 to 8 , further comprising a polymerization inhibitor. The sheet according to any one of claims 6 to 9 , wherein the content of the reducing agent is 0.1 to 5 parts by mass per 100 parts by mass of the fibrous cellulose. The sheet according to any one of claims 6 to 10 , which has a YI value of 1.5 or less. The sheet according to any one of claims 6 to 11 , having a haze of 3.5% or less. The sheet according to any one of claims 6 to 12 , which is for an optical member. A laminate comprising the sheet according to any one of claims 6 to 13 and a resin layer laminated on at least one surface of the sheet.