JP2023007000A - dispersion and sheet - Google Patents

Abstract

To provide a dispersion obtained by re-dispersing a microfibrous cellulose aggregate containing a polyvalent metal ion, which can form a sheet having high transparency and excellent resistance to yellowing.SOLUTION: There is provided a dispersion containing a fibrous cellulose having an anionic group and a fiber width of 1000 nm or less, wherein the amount introduced of the anionic group in the fibrous cellulose is less than 0.5 mmol/g, the fibrous cellulose has a polyvalent metal ion as a counter ion of an anionic group, the electrical conductivity of the dispersion containing 0.4 mass% of the fibrous cellulose is 20 mS/m or less and the haze of the dispersion containing 0.2 mass% of the fibrous cellulose is 10% or less.SELECTED DRAWING: None

Description

The present invention relates to dispersions and sheets. Specifically, the present invention relates to dispersions and sheets comprising fine fibrous cellulose.

Conventionally, cellulose fibers have been widely used in clothing, absorbent articles, paper products and the like. As cellulose fibers, in addition to fibrous cellulose with a fiber diameter of 10 μm or more and 50 μm or less, fine fibrous cellulose with a fiber diameter of 1 μm or less is also known. Fine fibrous cellulose is attracting attention as a new material, and its uses are wide-ranging. For example, development of sheets, resin composites, and thickeners containing fine fibrous cellulose is underway.

Fine fibrous cellulose can be produced by defibrating cellulose fibers. However, since cellulose fibers are strongly bonded to each other by hydrogen bonding, a simple defibration treatment requires a huge amount of energy to obtain fine fibrous cellulose. Therefore, in order to produce fine fibrous cellulose with less defibration treatment energy, it is known to be effective to perform pretreatment such as chemical treatment or enzyme treatment together with defibration treatment. For example, it is known that chemical treatment of cellulose fibers to introduce ionic substituents into the cellulose fibers facilitates miniaturization of the cellulose fibers, and further increases dispersion stability after miniaturization.

When fine fibrous cellulose is used as a dispersion, it is often in a state of a low-concentration dispersion in order to ensure its dispersion stability. However, in such cases, most of the material to be transported and stored is a solvent such as water, and the transport and storage costs per unit quantity of fine fibrous cellulose tend to be high. For this reason, in order to reduce the transportation cost and storage cost per unit quantity of fine fibrous cellulose, a flocculant or the like is added to the fine fibrous cellulose dispersion to aggregate the fine fibrous cellulose, and the obtained flocculated It is also being considered to transport and store things. When using this aggregate, the aggregate is re-dispersed in a solvent such as water, and the obtained re-dispersed liquid is used for various purposes.

For example, Patent Documents 1 and 2 disclose fine fibrous cellulose inclusions containing polyvalent metals as flocculants. In the examples of these documents, redispersion of fine fibrous cellulose inclusions under alkaline conditions is considered to enhance redispersibility. In addition, Patent Document 3 discloses a process of adding a certain amount of monovalent or divalent metal ions to an anion-modified cellulose nanofiber dispersion, thereby improving the fluidity of the cellulose nanofiber dispersion. It is being considered to have

Patent Document 4 discloses a sheet containing fine fibers having an ionic substituent and a metal having a valence of 2 or higher. Here, by treating a fine fiber-containing sheet having a substituent with divalent or higher metal ions, it is possible to provide a sheet that maintains high transparency, does not swell or break even under wet conditions, and has a certain level of strength. is being considered. Moreover, Patent Document 5 discloses a cellulose film in which at least a portion of the carboxy groups are substituted with a metal. In Patent Documents 4 and 5, a metal ion treatment (ion exchange treatment) is performed after forming a fine fiber-containing sheet or film.

JP 2020-105471 A WO2014/024876 WO2013/137140 Japanese Unexamined Patent Application Publication No. 2017-31548 Japanese Patent Application Laid-Open No. 2016-210830

When a flocculant such as a polyvalent metal is added to a fine fibrous cellulose dispersion to obtain fine fibrous cellulose aggregates, it is required that the aggregates exhibit excellent redispersibility during use. It is In the prior art, it has been considered to increase the redispersibility of fine fibrous cellulose by setting the solvent used for redispersion under alkaline conditions. However, in such a case, the resulting redispersion liquid is also alkaline, and there are problems such as limited applications of the redispersion liquid and poor ease of handling. Moreover, when a sheet is formed from such a redispersion liquid, there is also a problem that the sheet yellows. On the other hand, when the redispersibility of the fine fibrous cellulose is insufficient, the transparency of the redispersed liquid is inferior, and the transparency of the sheet formed from such a redispersed liquid is also insufficient. .

In addition, as disclosed in Patent Documents 4 and 5, a technique is also known in which polyvalent metal ions are introduced after forming a fine fibrous cellulose sheet (film) to improve gas barrier properties of the sheet. However, when a fine fibrous cellulose sheet (film) is immersed in a solution and subjected to ion exchange treatment, the ion exchange may not proceed sufficiently and the sheet may turn yellow.

Therefore, in order to solve such problems of the prior art, the present inventors have developed a dispersion obtained by redispersing fine fibrous cellulose aggregates containing polyvalent metal ions, which has high transparency and A study was carried out with the aim of providing a dispersion capable of forming a sheet having excellent yellowing resistance.

Specifically, the present invention has the following configurations.

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

According to the present invention, a dispersion obtained by redispersing fine fibrous cellulose aggregates containing polyvalent metal ions, which is capable of forming a sheet having high transparency and excellent yellowing resistance is obtained. be able to.

FIG. 1 is a graph showing the relationship between the amount of dropped NaOH and the pH for a fibrous cellulose-containing slurry having a phosphorous acid group. FIG. 2 is a graph showing the relationship between the amount of dropped NaOH and pH for a fibrous cellulose-containing slurry having carboxyl groups.

The present invention will be described in detail below. Although the constituent elements described below may be described based on representative embodiments and specific examples, the present invention is not limited to such embodiments.

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

Since the present embodiment has the above configuration, it is possible to form a sheet having high transparency and excellent yellowing resistance. For example, when a sheet containing fine fibrous cellulose is formed from the dispersion liquid of the present embodiment, a highly transparent sheet can be obtained. Further, according to the present embodiment, yellowing of the sheet can be suppressed even when the sheet is heated. Specifically, in the present embodiment, when a sheet containing fine fibrous cellulose is formed and the sheet is heated at 160° C. for 6 hours, the YI increase rate can be kept low. Thus, by using the dispersion liquid of the present embodiment, a sheet having high transparency and excellent yellowing resistance can be formed.

The electrical conductivity of the dispersion containing 0.4% by mass of fibrous cellulose may be 20 mS/m or less, preferably 15 mS/m or less, more preferably 10 mS/m or less, and 5 mS. /m or less is more preferable. The lower limit of the electrical conductivity of the dispersion containing 0.4% by mass of fibrous cellulose is not particularly limited, and may be 0 mS/m. By setting the electrical conductivity of the dispersion liquid within the above range, it is possible to form a sheet with high transparency and excellent yellowing resistance, and in particular, exhibits excellent yellowing resistance even under high temperature conditions. It is possible to form a sheet that can be Further, by setting the electrical conductivity of the dispersion within the above range, the ease of handling of the dispersion can be enhanced.

The electrical conductivity of the dispersion containing 0.4% by mass of fibrous cellulose is measured by an electrical conductivity meter. The liquid temperature of the dispersion during the measurement shall be 23°C. As the electrical conductivity meter, CT-57101B manufactured by Toa DKK Co., Ltd. can be used. When the dispersion is diluted to a concentration of 0.4% by mass, the electrical conductivity is measured after dilution with ion-exchanged water. In addition, when the dispersion is concentrated to a concentration of 0.4% by mass, the electrical conductivity is measured after cross-flowing the dispersion through a hollow fiber membrane with an opening of 0.1 μm under room temperature conditions. .

The fact that the electric conductivity value of the dispersion containing 0.4% by mass of fibrous cellulose is equal to or less than a predetermined value indicates, for example, that H ^{2 +} and OH ⁻ that promote coloring of fibrous cellulose are small. Further, even if the pH of the dispersion liquid is the same, the absolute amount of the ionic substance and the mobility in water are low in the dispersion liquid having low electrical conductivity. Thus, by setting the electrical conductivity of the dispersion liquid to a predetermined value or less, the mobility of the ionic substance can be reduced. presumed to be achieved.

The haze of the dispersion containing 0.2% by mass of fibrous cellulose may be 10% or less, preferably 8% or less, more preferably 7% or less, and 6% or less. is more preferred. The lower limit of the haze of the dispersion containing 0.2% by mass of fibrous cellulose is not particularly limited, and may be 0%. By setting the haze of the dispersion within the above range, a sheet having high transparency and excellent yellowing resistance can be formed. The fact that the haze of the dispersion is within the above range means that the fibrous cellulose has good dispersibility in the dispersion. Therefore, if the haze of the dispersion is within the above range, a sheet with more excellent transparency can be formed.

When the dispersion contains 0.2% by mass of fibrous cellulose, the total light transmittance of the dispersion is preferably 85% or more, more preferably 87% or more, further preferably 90% or more, and 95% or more. % or more is particularly preferable. The upper limit of the total light transmittance of the dispersion containing 0.2% by mass of fibrous cellulose is not particularly limited, and may be 100%.

The haze of a dispersion containing 0.2% by mass of fibrous cellulose is measured with a haze meter using a glass cell for liquids with an optical path length of 1 cm according to JIS K 7136:2000. Further, the total light transmittance of the dispersion containing 0.2% by mass of fibrous cellulose was determined by a haze meter using a liquid glass cell with an optical path length of 1 cm, in accordance with JIS K 7361-1: 1997. Measure. The zero point measurement is performed with ion-exchanged water placed in the same glass cell, and the liquid temperature of the dispersion during measurement is 23°C. HM-150 manufactured by Murakami Color Research Laboratory Co., Ltd. can be used as the haze meter, and MG-40 (reverse optical path) manufactured by Fujiwara Seisakusho Co., Ltd. can be used as the liquid glass cell. When the dispersion is diluted to a concentration of 0.2% by mass, the haze and total light transmittance are measured after dilution with ion-exchanged water. In addition, when the dispersion is concentrated to a concentration of 0.2% by mass, the haze and total light transmittance are reduced after cross-flowing the dispersion through a hollow fiber membrane having an opening of 0.1 μm under room temperature conditions. take measurements.

The pH of the dispersion containing 0.4% by mass of fibrous cellulose is preferably 4 or higher, more preferably 5 or higher. The pH of the dispersion containing 0.4% by mass of fibrous cellulose is preferably 9 or less, more preferably 8 or less. By setting the pH of the dispersion liquid within the above range, a sheet having high transparency and excellent yellowing resistance can be formed, and in particular, excellent yellowing resistance can be exhibited even under high temperature conditions. A sheet can be formed. The pH of the dispersion containing 0.4% by mass of fibrous cellulose is a value measured with a pH meter, and the liquid temperature of the dispersion during measurement is 23°C. When the dispersion is diluted to a concentration of 0.4% by mass, the pH is measured after dilution with ion-exchanged water. When the dispersion is concentrated to a concentration of 0.4% by mass, the pH is measured after cross-flowing the dispersion through a hollow fiber membrane having an opening of 0.1 μm under room temperature conditions.

The viscosity of the dispersion containing 0.4% by mass of fibrous cellulose is preferably 7000 mPa·s or more, more preferably 7500 mPa·s or more, and even more preferably 8000 mPa·s or more. The viscosity of the dispersion containing 0.4% by mass of fibrous cellulose is preferably 200000 mPa s or less, more preferably 100000 mPa s or less, and further preferably 50000 mPa s or less. preferable. By setting the viscosity of the dispersion liquid within the above range, a sheet having high transparency and excellent yellowing resistance can be formed. Further, by setting the viscosity of the dispersion liquid within the above range, the dispersion liquid can be used for various purposes, and the ease of handling when forming a sheet from the dispersion liquid can be enhanced. The viscosity of the dispersion containing 0.4% by mass of fibrous cellulose is measured using a Brookfield viscometer after stirring at 1500 rpm for 5 minutes with a disperser. In the measurement, the rotational speed is 3 rpm, and the viscosity value after 3 minutes from the start of the measurement is taken as the viscosity of the dispersion. The liquid temperature of the dispersion liquid to be measured is set to 23°C. As the Brookfield viscometer, an analog viscometer T-LVT manufactured by BLOOKFIELD can be used. When the dispersion is diluted to a concentration of 0.4% by mass, the viscosity is measured after dilution with ion-exchanged water. When the dispersion is concentrated to a concentration of 0.4% by mass, the viscosity is measured after cross-flowing the dispersion through a hollow fiber membrane having an opening of 0.1 μm under room temperature conditions.

The content of fibrous cellulose contained in the dispersion is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, and 0.01% by mass or more, based on the total mass of the dispersion. More preferably, it is 2% by mass or more. Although the upper limit of the content of fibrous cellulose contained in the dispersion is not particularly limited, it can be, for example, 20% by mass.

The solvent content in the dispersion is preferably 99.99% by mass or less, more preferably 99.9% by mass or less, and 99.8% by mass, based on the total mass of the dispersion. The following are more preferable. Moreover, the content of the solvent contained in the dispersion is preferably 80% by mass or more with respect to the total mass of the dispersion. The solvent of the dispersion is preferably water, and the dispersion in this embodiment is preferably an aqueous dispersion.

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

The dispersion of the present embodiment is a redispersion obtained by redispersing fine fibrous cellulose aggregates containing polyvalent metal ions. Specifically, a fibrous cellulose aggregate can be obtained by adding a flocculant containing a polyvalent metal to a dispersion of fine fibrous cellulose, which is then redispersed by redispersing it in a solvent such as water. You can get the liquid. Examples of the solvent for redispersing the fibrous cellulose aggregates include the above solvents, and the solvent is preferably water.

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

The average fiber width of fibrous cellulose is, for example, 1000 nm or less. The average fiber width of the 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 2 nm or more and 20 nm or less. More preferably, it is particularly preferably 2 nm or more and 10 nm or less. The fibrous cellulose is, for example, monofibrous 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 on a hydrophilized carbon film-coated grid to form a sample for TEM observation. and SEM images of surfaces cast on glass may be observed if they contain wide fibers. Then, an electron microscope image is observed at a magnification of 1,000, 5,000, 10,000, or 50,000 times depending on the width of the fiber to be observed. However, the sample, observation conditions and magnification are adjusted so as to satisfy the following conditions.

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

Widths of fibers intersecting the straight lines X and Y are visually read from the observation image satisfying the above conditions. In this way, at least three sets of observed images of surface portions that do not overlap each other are obtained. Then, for each image, the width of the fiber that crosses straight line X and straight line Y is read. This gives at least 20 x 2 x 3 = 120 fiber width readings. Then, the average value of the read fiber widths is taken as the average fiber width of the fibrous cellulose.

Although the fiber length of the fibrous cellulose is not particularly limited, it 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 further preferably 0.1 μm or more and 600 μm or less. preferable. By setting the fiber length within the above range, destruction of the crystalline regions of the fibrous cellulose can be suppressed. Moreover, it becomes possible to make the slurry viscosity of fibrous cellulose into an appropriate range. The fiber length of fibrous cellulose can be determined by image analysis using TEM, SEM, or AFM, for example.

Preferably, the fibrous cellulose has a Type I crystal structure. Here, the fact that the fibrous cellulose has a type I crystal structure can be identified in 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 having typical peaks at two positions near 2θ=14° or more and 17° or less and 2θ=22° or more and 23° or less. The proportion of type I crystal structure in the fibrous cellulose is, for example, preferably 30% or more, more preferably 40% or more, even more preferably 50% or more. As a result, even better performance can be expected in terms of heat resistance and low coefficient of linear thermal expansion. The degree of crystallinity is determined by a conventional method from the pattern of X-ray diffraction profiles (Seagal et al., Textile Research Journal, vol. 29, p. 786, 1959).

Although the axial ratio (fiber length/fiber width) of fibrous cellulose is not particularly limited, it is preferably 20 or more and 10,000 or less, and more preferably 50 or more and 1,000 or less. A sheet containing fibrous cellulose can be easily formed by making the axial ratio equal to or higher than the above lower limit. Further, by setting the axial ratio to the above upper limit or less, handling such as dilution becomes easier when fibrous cellulose is treated as a dispersion liquid, which is preferable.

Fibrous cellulose, for example, has both crystalline and amorphous regions. Fibrous cellulose having both crystalline regions and non-crystalline regions and having an axial ratio within the above range is realized by a method for producing fine fibrous cellulose, which will be described later.

Fibrous cellulose has anionic groups. The anionic group is preferably a group that links to the fibrous cellulose through an ester bond. In this case, the ester bond is formed by dehydration condensation between the hydroxyl group of the fibrous cellulose and the anionic group compound.

The anionic group includes, for example, a phosphate group or a substituent derived from a phosphate group (sometimes simply referred to as a phosphate group), a carboxy group or a substituent derived from a carboxy group (sometimes simply referred to as a carboxy group), a sulfur oxoacid group or a substituent derived from a sulfur oxoacid group (sometimes simply referred to as a sulfur oxoacid group), a xanthate group or a substituent derived from a xanthate group (sometimes simply referred to as a xanthate group), a phosphonic group or a phosphonic group groups, phosphine groups or substituents derived from phosphine groups, sulfone groups or substituents derived from sulfone groups, carboxyalkyl groups (including carboxymethyl groups), and the like. Among them, the anionic group is a phosphorus oxo acid group, a substituent derived from a phosphorus oxo acid group, a carboxy group, a substituent derived from a carboxy group, a sulfur oxo acid group, a substituent derived from a sulfur oxo acid group, a xanthate group and a xanthate It is preferably at least one selected from the group consisting of substituents derived from groups such as a phosphorous oxoacid group, a substituent derived from a phosphorous oxoacid group, a carboxyl group, a substituent derived from a carboxyl group, and a sulfur oxoacid group. and a substituent derived from a sulfur oxoacid group, more preferably at least one selected from the group consisting of a phosphorous oxoacid group or a substituent derived from a phosphorous oxoacid group. By appropriately selecting the anionic group, it is possible to obtain a sheet having excellent transparency and excellent resistance to yellowing.

A phosphorous acid group or a substituent derived from a phosphorous acid group is, for example, a substituent represented by the following formula (1). A plurality of types of substituents represented by the following formula (1) may be introduced into each fibrous cellulose. In this case, the plural 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 n α and α' is O 2 ^- , and the rest are R or OR. Note that all of α and α' may be O 2 ⁻ . All of the n αs may be the same or different. β ^b+ is a monovalent or higher cation composed of an organic substance or an 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 A hydrogen group, an unsaturated-cyclic hydrocarbon group, an aromatic group, or a derivative group thereof. Also, in formula (1), n is preferably 1.

Examples of the saturated straight-chain hydrocarbon group include, but are not limited to, methyl group, ethyl group, n-propyl group, n-butyl group, and the like. The saturated-branched hydrocarbon group includes i-propyl group, t-butyl group and the like, but is not particularly limited. The saturated cyclic hydrocarbon group includes, but is not particularly limited to, a cyclopentyl group, a cyclohexyl group, and the like. The unsaturated straight-chain hydrocarbon group includes, but is not particularly limited to, a vinyl group, an allyl group, and the like. The unsaturated-branched hydrocarbon group includes i-propenyl group, 3-butenyl group and the like, but is not particularly limited. The unsaturated-cyclic hydrocarbon group includes, but is not limited to, a cyclopentenyl group, a cyclohexenyl group, and the like. The aromatic group includes, but is not particularly limited to, a phenyl group, a naphthyl group, or the like.

In addition, the derivative group in R is selected from functional groups such as carboxy group, carboxylate group (—COO ⁻ ), hydroxy group, amino group and ammonium group for the main chain or side chain of the above various hydrocarbon groups. functional groups to which at least one type is added or substituted, but are not particularly limited. Although the number of carbon atoms constituting the main chain of R is not particularly limited, it is preferably 20 or less, 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 phosphorous acid group can be set within an appropriate range, facilitating penetration into the fiber raw material and increasing the yield of fine cellulose fibers. can also In addition, when a plurality of R are present in the formula (1) or when a plurality of types of substituents represented by the above formula (1) are introduced into the fibrous cellulose, the plurality of Rs present are the same. There may be or may be different.

β ^b+ is a monovalent or higher cation composed of an organic substance or an inorganic substance. An organic onium ion can be mentioned as a cation having a valence of 1 or more composed of an organic substance. Examples of organic onium ions include organic ammonium ions and organic onium ions. Examples of organic ammonium ions include aliphatic ammonium ions and aromatic ammonium ions, and examples of organic onium ions include aliphatic phosphonium ions and aromatic phosphonium ions. Examples of monovalent or higher-valent cations composed of inorganic substances 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. In addition, when a plurality of β ^b+ are present in the formula (1) or when a plurality of types of substituents represented by the above formula (1) are introduced into the fibrous cellulose, the plurality of β ^b+ are each They may be the same or different. In this embodiment, at least a portion of the β ^b+ are polyvalent metal ions.

More specifically, the phosphoric acid group or the substituent derived from the phosphoric acid group includes a phosphoric acid group (—PO ₃ H ₂ ), a salt of a phosphoric acid group, a phosphorous acid group (phosphonic acid group) (—PO ₂ H ₂ ), salts of phosphite group (phosphonic acid group). In addition, the phosphoric acid group or the substituent derived from the phosphoric acid group includes a condensed phosphoric acid group (e.g., pyrophosphate group), a condensed phosphonic acid group (e.g., polyphosphonic acid group), a phosphoric acid ester group ( For example, it may be a monomethyl phosphate group, a polyoxyethylene alkyl phosphate group), an alkylphosphonic acid group (eg, a methylphosphonic acid group), or the like.

Further, 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). A plurality of types of substituents represented by the following formula (2) may be introduced into each fibrous cellulose. In this case, the plural introduced substituents represented by the following formula (2) may be the same or different.

In the above structural formula, b and n are natural numbers, p is 0 or 1, and m is an arbitrary number (where 1=b×m). In addition, when n is 2 or more, multiple p may be the same number or may be different numbers. In the above structural formula, β ^b+ is a monovalent or higher cation composed of an organic substance or an inorganic substance. An organic onium ion can be mentioned as a cation having a valence of 1 or more composed of an organic substance. Examples of organic onium ions include organic ammonium ions and organic onium ions. Examples of organic ammonium ions include aliphatic ammonium ions and aromatic ammonium ions, and examples of organic onium ions include aliphatic phosphonium ions and aromatic phosphonium ions. Examples of monovalent or higher-valent cations composed of inorganic substances 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 fibrous cellulose, the multiple β ^b+ may be the same or different. In this embodiment, at least a portion of the β ^b+ are polyvalent metal ions.

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

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

The amount of anionic groups introduced into fibrous cellulose can be measured, for example, by a neutralization titration method. In the measurement by the neutralization titration method, the introduced amount is measured by determining the pH change while adding an alkali such as an aqueous sodium hydroxide solution to the obtained slurry containing the fibrous cellulose.

FIG. 1 is a graph showing the relationship between the amount of dropped NaOH and the pH for a fibrous cellulose-containing slurry having a phosphorous acid group. The amount of phosphorus oxoacid groups introduced into fibrous cellulose is measured, for example, as follows.
First, a slurry containing fibrous cellulose is treated with a strongly acidic ion exchange resin. In addition, before the treatment with the strongly acidic ion exchange resin, a defibration treatment similar to the fibrillation treatment step described below may be performed on the object to be measured, if necessary.
Next, the change in pH is observed while adding sodium hydroxide aqueous solution, and a titration curve as shown in the upper part of FIG. 1 is obtained. The titration curve shown in the upper part of FIG. 1 plots the measured pH against the amount of alkali added, and the titration curve shown in the lower part of FIG. 1 plots the pH against the amount of alkali added. The increment (differential value) (1/mmol) is plotted. In this neutralization titration, two points where the increment (the differential value of the pH with respect to the amount of alkali dropped) are maximized are confirmed in the curve obtained by plotting the measured pH against the amount of alkali added. Among these, the maximum point of the increment obtained first when the alkali is first added is called the first end point, and the maximum point of the increment obtained next is called the second end point. The amount of alkali required from the start of titration to the first end point is equal to the first dissociated acid amount of fibrous cellulose contained in the slurry used for titration, and the amount of alkali required from the first end point to the second end point The amount is equal to the second dissociated acid amount of fibrous cellulose contained in the slurry used for titration, and the amount of alkali required from the start of titration to the second end point is equal to the amount of fibrous cellulose contained in the slurry used for titration. equal to the total dissociated acid content of A value obtained by dividing the amount of alkali required from the start of titration to the first end point by the solid content (g) in the slurry to be titrated is the amount of phosphorus oxoacid group introduced (mmol/g). In addition, when simply referring to the amount of phosphorus oxoacid groups introduced (or the amount of phosphorus oxoacid groups), it means the amount of the first dissociated acid.
In FIG. 1, the region from the start of titration to the first end point is called the first region, and the region from the first end point to the second end point is called the second region. For example, when the phosphoric acid group is a phosphoric acid group, and the phosphoric acid group causes condensation, it appears that the amount of weakly acidic groups in the phosphoric acid group (also referred to herein as the second dissociated acid amount) is The amount of alkali required in the second region is less than that 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 amount of first dissociated acid) matches the amount of phosphorus atoms regardless of the presence or absence of condensation. In addition, when the phosphorous acid group is a phosphorous acid group, the weakly acidic group does not exist in the phosphorous acid group, so the amount of alkali required for the second region is reduced, or the amount of alkali required for the second region is may be zero. In this case, the titration curve has one point at which the pH increment is maximum.

In addition, since the denominator of the amount of introduced phosphate groups (mmol/g) indicates the mass of the acid-form fibrous cellulose, the amount of phosphate groups possessed by the acid-form fibrous cellulose (hereinafter referred to as the phosphate group amount (acid form)). On the other hand, when the counter ion of the phosphooxy acid group is substituted with an arbitrary cation C so as to have a charge equivalent, the denominator is converted to the mass of fibrous cellulose when the cation C is the counter ion. Thus, the amount of phosphate groups (hereinafter referred to as the amount of phosphate groups (type C)) possessed by fibrous cellulose whose counter ion is cation C can be determined.
That is, it is calculated by the following formula.
Phosphorus oxo acid group amount (C type) = Phosphorus oxo acid group amount (acid type) / {1 + (W-1) × A / 1000}
A [mmol/g]: Total amount of anions derived from phosphate groups possessed by fibrous cellulose (total dissociated acid amount of phosphate groups)
W: Formula weight per valence of cation C (for example, 23 for Na and 9 for Al)

FIG. 2 is a graph showing the relationship between the amount of dropped NaOH and pH for a dispersion containing fibrous cellulose having a carboxyl group as an anionic group. The amount of carboxyl groups introduced into fibrous cellulose is measured, for example, as follows.
First, a dispersion containing fibrous cellulose is treated with a strongly acidic ion exchange resin. In addition, before the treatment with the strongly acidic ion exchange resin, a defibration treatment similar to the fibrillation treatment step described below may be performed on the object to be measured, if necessary.
Next, while adding sodium hydroxide aqueous solution, the change in pH is observed to obtain a titration curve as shown in the upper part of FIG. The titration curve shown in the upper part of FIG. 2 plots the measured pH against the amount of alkali added, and the titration curve shown in the lower part of FIG. 2 plots the pH against the amount of alkali added. The increment (differential value) (1/mmol) is plotted. In this neutralization titration, in the curve plotting the measured pH against the amount of alkali added, one point where the increment (the differential value of pH with respect to the amount of alkali dropped) is maximum was confirmed. 1 end point. Here, the region from the start of titration to the first end point in FIG. 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 dispersion used for titration. Then, the amount of alkali (mmol) required in the first region of the titration curve is divided by the solid content (g) in the dispersion containing the fibrous cellulose to be titrated to obtain the amount of carboxyl groups introduced (mmol / g).

In addition, since the denominator of the amount of introduced carboxy groups (mmol/g) is the mass of the acid-type fibrous cellulose, the amount of carboxy groups possessed by the acid-type fibrous cellulose (hereinafter referred to as the amount of carboxy groups (acid-type )). On the other hand, when the counterion of the carboxy group is substituted with an arbitrary cation C so as to have a charge equivalent, the denominator is converted to the mass of fibrous cellulose when the cation C is the counterion. , the amount of carboxy groups (hereinafter referred to as the amount of carboxy groups (C type)) possessed by fibrous cellulose whose counter ion is cation C can be obtained. That is, it is calculated by the following formula.
Carboxy group amount (C type) = carboxy group amount (acid form) / {1 + (W-1) x (carboxy group amount (acid form)) / 1000}
W: Formula weight per valence of cation C (for example, 23 for Na and 9 for Al)

When measuring the amount of anionic groups by the titration method, if the amount of one drop of aqueous sodium hydroxide solution is too large, or if the titration interval is too short, the amount of anionic groups will be lower than it should be, and accurate values cannot be obtained. sometimes not. As a suitable drop amount and titration interval, for example, it is desirable to titrate 10 to 50 μL of 0.1N sodium hydroxide aqueous solution every 5 to 30 seconds. In order to eliminate the influence of carbon dioxide dissolved in the fibrous cellulose-containing slurry, for example, it is desirable to measure while blowing an inert gas such as nitrogen gas into the slurry from 15 minutes before the start of titration to the end of titration.

The amount of sulfur oxoacid groups or sulfone groups introduced into fibrous cellulose was determined by wet ashing fibrous cellulose with perchloric acid and concentrated nitric acid, diluting the fibrous cellulose at an appropriate ratio, and measuring the amount of sulfur by ICP emission spectrometry. can be calculated by measuring A sulfur oxoacid group or a sulfone group (unit: mmol/g) is obtained by dividing the amount of sulfur by the absolute dry mass of the fibrous cellulose tested.

The introduction amount of the xanthate group into the fibrous cellulose can be measured by the Bredee method as follows. First, 40 mL of a saturated ammonium chloride solution was added to 1.5 parts by mass (absolute dry mass) of fibrous cellulose, mixed well while crushing the sample with a glass rod, and left for about 15 minutes. 25) and wash thoroughly with saturated ammonium chloride solution. Next, the sample is placed in a 500 mL tall beaker together with the GFP filter paper, 50 mL of 0.5 M sodium hydroxide solution (5° C.) is added, stirred, and allowed to stand for 15 minutes. After adding the phenolphthalein solution until the solution turns pink, 1.5 M acetic acid is added and the point at which the solution turns from pink to colorless is taken as the neutralization point. After neutralization, 250 mL of distilled water is added and well stirred, and 10 mL of 1.5 M acetic acid and 10 mL of 0.05 mol/L iodine solution are 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 from the following equation from the titration amount of sodium thiosulfate and the absolute dry mass of fibrous cellulose.
Xanthate group amount (mmol/g) = (0.05 x 10 x 2-0.05 x sodium thiosulfate titer (mL))/1000/absolute dry mass of fibrous cellulose (g)

(Polyvalent metal ion)
Fibrous cellulose has polyvalent metal ions as counterions for the anionic groups described above. In the present specification, polyvalent metal ions are metal ions with a valence of 2 or higher, and preferably polyvalent metal ions are divalent metal ions. In the dispersion liquid of the present embodiment, at least part of the polyvalent metal ions preferably form ionic bonds through electrostatic interactions with the anionic groups. In addition, free polyvalent metal elements and polyvalent metal ions may be present in the dispersion.

Polyvalent metal ions are preferably at least one selected from the group consisting of calcium ions, magnesium ions, zinc ions, aluminum ions, nickel ions, copper ions and cobalt ions, and are calcium ions, magnesium ions and zinc ions. and aluminum ions, and more preferably at least one selected from the group consisting of calcium ions, magnesium ions and zinc ions. By selecting calcium ions, magnesium ions, zinc ions, or aluminum ions as the polyvalent metal ions, it is possible to obtain dispersions and sheets in which coloration is further suppressed.

The polyvalent metal ion is derived from a component added as a metal salt (polyvalent metal salt) in the ion exchange step of the manufacturing process of the fine fibrous cellulose dispersion, which will be described later. Such polyvalent metal salts can also be referred to as flocculants because they serve to flocculate fine fibrous cellulose. The polyvalent metal salt is preferably a sulfate, chloride, nitrate, carbonate or phosphate of the polyvalent metal ion described above and is soluble in water. Specific examples of polyvalent metal salts include aluminum sulfate (aluminum sulfate), magnesium sulfate, iron sulfate, copper sulfate, zinc sulfate, aluminum chloride, polyaluminum chloride, calcium chloride, magnesium chloride, nickel chloride, and cobalt chloride. , copper chloride, iron chloride, lead chloride, calcium nitrate, magnesium nitrate, calcium hydrogen carbonate and the like. As the metal salt containing a divalent or higher valent metal ion, only one of the above salts may be added, or two or more may be added. Among them, as the metal salt containing a divalent or higher metal ion, it is preferable to use at least one selected from the group consisting of magnesium sulfate, zinc sulfate, copper sulfate, calcium chloride, nickel chloride and cobalt chloride. , zinc sulfate and calcium chloride.

(Optional component)
The dispersion may contain optional components (additives) in addition to the fibrous cellulose and polyvalent metal ions. Examples of optional components include surfactants, coupling agents, inorganic stratiform compounds, inorganic compounds, leveling agents, preservatives, antifoaming agents, organic particles, lubricants, antistatic agents, UV protection agents, dyes, and pigments. , stabilizers, magnetic powders, orientation promoters, plasticizers, dispersants, cross-linking agents, and the like. Moreover, the dispersion liquid may contain a hydrophilic polymer, a hydrophilic low molecular weight, an organic ion, etc. as optional components.

Hydrophilic polymers include, for example, carboxyvinyl polymer; polyvinyl alcohol; alkyl methacrylate/acrylic acid copolymer; polyvinylpyrrolidone; polyvinyl methyl ether; polyacrylates such as sodium polyacrylate; modified polyesters; modified polyimides; polyalkylene glycols such as polyethylene glycol and polypropylene glycol; polycations such as polyacrylamide and polyethyleneimine; polyanions; Thickening polysaccharides exemplified by bean gum, quince seed, alginic acid, metal salts of alginic acid, pullulan, sacran, and pectin; Cellulose derivatives exemplified; starches exemplified by cationic starch, raw starch, oxidized starch, etherified starch, esterified starch, dextrin, and amylose; glycerins such as polyglycerin; hyaluronic acid, metals of hyaluronic acid salts; proteins such as casein; Copolymers of these hydrophilic polymers may also be used.

The hydrophilic small molecule is preferably a hydrophilic oxygen-containing organic compound, more preferably a polyhydric alcohol. Examples of polyhydric alcohols include glycerin, sorbitol, ethylene glycol and the like.

Examples of organic ions include tetraalkylammonium ions and tetraalkylphosphonium ions. Tetraalkylammonium ions include, for example, tetramethylammonium ion, tetraethylammonium ion, tetrapropylammonium ion, tetrabutylammonium ion, tetrapentylammonium ion, tetrahexylammonium ion, tetraheptylammonium ion, tributylmethylammonium ion, lauryltrimethyl ammonium ion, cetyltrimethylammonium ion, stearyltrimethylammonium ion, octyldimethylethylammonium ion, lauryldimethylethylammonium ion, didecyldimethylammonium ion, lauryldimethylbenzylammonium ion, tributylbenzylammonium ion; Tetraalkylphosphonium ions include, for example, tetramethylphosphonium ion, tetraethylphosphonium ion, tetrapropylphosphonium ion, tetrabutylphosphonium ion, and lauryltrimethylphosphonium ion. Moreover, tetra-n-propylonium ion and tetra-n-butylonium ion can also be mentioned as examples of tetrapropylonium ion and tetrabutylonium ion, respectively.

(Method for producing fine fibrous cellulose dispersion)
A method for producing a fine fibrous cellulose dispersion comprises an anionic group introduction step of introducing an anionic group into a fibrous raw material containing cellulose, a washing step, an alkali treatment step (neutralization step), and a fibrillation treatment step in this order. Preferably, after the fibrillation treatment step, a step (A) of removing a part of the anionic group from the fine fibrous cellulose having an anionic group and a fiber width of 1000 nm or less is included, and a step ( After A), it is preferable to further include a step (B) of uniform dispersion treatment, an ion exchange step and a re-dispersion (uniform dispersion) step. In the present embodiment, the method for producing a fine fibrous cellulose dispersion comprises a step (A) of removing part of the anionic groups from the fine fibrous cellulose having an anionic group and having a fiber width of 1000 nm or less; It is preferable to have the step (B) of uniform dispersion treatment, the ion exchange step and the re-dispersion (uniform dispersion) step in this order. An acid treatment step may be provided instead of or in addition to the washing step. Examples of the anionic group-introducing step include a phosphorus oxoacid group-introducing step, a carboxyl group-introducing step, a sulfur oxoacid group-introducing step, and a xanthate group-introducing step. In the step (B) of uniformly dispersing treatment and the step of re-dispersing (uniformly dispersing), it is preferable to re-disperse aggregates using a high-speed fibrillating machine or a high-pressure homogenizer.

Step (A) is a step of removing part of the anionic groups from fine fibrous cellulose having anionic groups and having a fiber width of 1000 nm or less. In the following, first, a method for producing fine fibrous cellulose having an anionic group and a fiber width of 1000 nm or less (fine fibrous cellulose to be subjected to step (A)) will be described. ), the step (B) of uniform dispersion treatment, the ion exchange step and the re-dispersion (uniform dispersion) step.

<Textile raw material>
Microfibrous cellulose is produced from fibrous raw materials containing cellulose. The fibrous raw material containing cellulose is not particularly limited, but pulp is preferably used because it is readily available and inexpensive. Pulp includes, for example, wood pulp, non-wood pulp, and deinked pulp. Examples of wood pulp include, but are not limited to, hardwood kraft pulp (LBKP), softwood kraft pulp (NBKP), sulfite pulp (SP), dissolving pulp (DP), soda pulp (AP), unbleached kraft pulp (UKP). ) and chemical pulp such as oxygen bleached kraft pulp (OKP), semi-chemical pulp such as semi-chemical pulp (SCP) and chemigroundwood pulp (CGP), groundwood pulp (GP) and thermomechanical pulp (TMP, BCTMP) A mechanical pulp etc. are mentioned. Examples of non-wood pulp include, but are not limited to, cotton pulp such as cotton linters and cotton lints, and non-wood pulp such as hemp, straw and bagasse. The deinked pulp is not particularly limited, but includes, for example, deinked pulp made from waste paper. The pulp of this embodiment may be used alone or in combination of two or more. Among the above pulps, wood pulp and deinked pulp are preferable from the viewpoint of easy availability. In addition, among wood pulps, the cellulose ratio is high and the yield of fine fibrous cellulose at the time of defibration is high, and the decomposition of cellulose in the pulp is small, and fine fibrous cellulose of long fibers with a large axial ratio can be obtained. From the point of view, for example, chemical pulp is more preferable, and kraft pulp and sulfite pulp are more preferable. The use of fine fibrous cellulose of long fibers with a large axial ratio tends to increase the viscosity.

As the fiber raw material containing cellulose, for example, cellulose contained in sea squirts and bacterial cellulose produced by acetic acid bacteria can be used. Fibers formed by straight-chain nitrogen-containing polysaccharide polymers such as chitin and chitosan can also be used instead of fiber raw materials containing cellulose.

<Phosphorus oxoacid group introduction step>
The fine fibrous cellulose provided for step (A) has an anionic group. For this reason, it is preferable that the step of producing the fine fibrous cellulose used in step (A) includes an anionic group introduction step. Examples of the anionic group-introducing step include a phosphorus oxoacid group-introducing step. In the phosphorus oxoacid group-introducing step, at least one compound selected from compounds capable of introducing a phosphorus oxoacid group (hereinafter also referred to as "compound A") is added to cellulose by reacting with a hydroxyl group possessed by a fiber raw material containing cellulose. It is a step of acting on a fiber raw material containing. Through this step, the phosphate group-introduced fiber is obtained.

In the phosphorus oxoacid group-introducing step according to the present embodiment, the reaction between the fiber material containing cellulose and compound A is performed in the presence of at least one selected from urea and derivatives thereof (hereinafter also referred to as "compound B"). may On the other hand, the reaction between the cellulose-containing fiber raw material and the compound A may be carried out in the absence of the compound B.

An example of the method of allowing the compound A to act on the fiber raw material in the presence of the compound B includes a method of mixing the compound A and the compound B with the fiber raw material in a dry state, a wet state, or a slurry state. Among these, it is preferable to use a fiber raw material in a dry state or a wet state, and it is particularly preferable to use a fiber raw material in a dry state, because the uniformity of the reaction is high. Although the form of the fiber raw material is not particularly limited, it is preferably, for example, cotton-like or thin sheet-like. The compound A and the compound B can be added to the fiber raw material in the form of a powder, a solution dissolved in a solvent, or a melted state by heating to a melting point or higher. Among these, it is preferable to add in the form of a solution dissolved in a solvent, particularly in the form of an aqueous solution, since the uniformity of the reaction is high. Moreover, the compound A and the compound B may be added simultaneously to the fiber raw material, may be added separately, or may be added as a mixture. The method for adding the compound A and the compound B is not particularly limited, but when the compound A and the compound B are in the form of a solution, the fiber raw material may be immersed in the solution to absorb the liquid and then taken out, or the fiber raw material may be taken out. The solution may be added dropwise to the Further, the necessary amount of compound A and compound B may be added to the fiber raw material, or after adding excessive amounts of compound A and compound B to the fiber raw material, the excess compound A and compound B are removed by pressing or filtering. may be removed.

The compound A used in the present embodiment may be any compound having a phosphorus atom and capable of forming an ester bond with cellulose. Salts, phosphoric anhydride (diphosphorus pentoxide) and the like can be mentioned, but are not particularly limited. Phosphoric acid of various purities can be used, for example, 100% phosphoric acid (orthophosphoric acid) or 85% phosphoric acid can be used. Phosphorous acid includes 99% phosphorous acid (phosphonic acid). Dehydration-condensed phosphoric acid is obtained by condensing two or more molecules of phosphoric acid by dehydration reaction, and examples thereof 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 acids. It can be a degree of harmony. Among these, phosphoric acid and sodium phosphate have a high introduction efficiency of a phosphate group, are easy to improve the defibration efficiency in the fibrillation step described later, are low in cost, and are easy to apply industrially. salt, potassium phosphate, ammonium phosphate or phosphorous acid, sodium phosphite, potassium phosphite, ammonium phosphite, phosphoric acid, sodium dihydrogen phosphate, More preferred are disodium hydrogen phosphate, ammonium dihydrogen phosphate, phosphorous acid, and sodium phosphite.

The amount of compound A added to the fiber raw material is not particularly limited, but for example, when the added amount of compound A is converted to the phosphorus atomic weight, the amount of phosphorus atoms added to the fiber raw material (absolute dry mass) is 0.5% by mass or more. It is preferably 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 to be equal to or less than the above upper limit, it is possible to balance the effect of improving the yield and the cost.

Compound B used in this embodiment is at least one selected from urea and derivatives thereof as described above. Compound B includes, for example, urea, biuret, 1-phenylurea, 1-benzylurea, 1-methylurea, 1-ethylurea and the like. From the viewpoint of improving the uniformity of the reaction, compound B is preferably used as an aqueous solution. 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 to the fiber raw material (absolute 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, More preferably, it is 100% by mass or more and 350% by mass or less.

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

In the phosphorus oxoacid group-introducing step, it is preferable to heat-treat the fiber raw material after adding or mixing the compound A or the like to the fiber raw material. As the heat treatment temperature, it is preferable to select a temperature that can efficiently introduce phosphorus oxoacid groups while suppressing thermal decomposition and hydrolysis reaction of the fiber. The heat treatment temperature is, for example, preferably 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. In addition, for the heat treatment, equipment having various heat media can be used, such as hot air dryers, stirring dryers, rotary dryers, disc dryers, roll heaters, plate heaters, and fluidized beds. A drying device, a band-type drying device, a filter drying device, a vibrating fluidized drying device, a stream drying device, a vacuum drying device, an infrared heating device, a far infrared heating device, a microwave heating device, and a high frequency drying device can be used.

In the heat treatment according to the present embodiment, for example, the compound A is added to a thin sheet-like fiber raw material by a method such as impregnation, and then heated, or the fiber raw material and the compound A are heated while kneading or stirring with a kneader or the like. method can be adopted. This makes it possible to suppress unevenness in the concentration of the compound A in the fiber raw material, and to more uniformly introduce the phosphorous acid groups to the surface of the cellulose fibers contained in the fiber raw material. This is because when water molecules move to the surface of the fiber material during drying, the dissolved compound A is attracted to the water molecules by surface tension and similarly moves to the surface of the fiber material (that is, the concentration unevenness of the compound A is It is thought that this is due to the fact that it is possible to suppress the occurrence of

In addition, the heating device used for the heat treatment always removes, for example, the moisture retained by the slurry and the moisture generated due to the dehydration condensation (phosphoric acid esterification) reaction between the compound A and the hydroxyl group or the like contained in the cellulose or the like in the fiber raw material. It is preferable that the device can be discharged to the outside of the device system. As such a heating device, for example, an air-blowing oven can be used. By constantly draining the water in the device system, it is possible to suppress the hydrolysis reaction of the phosphate ester bond, which is the reverse reaction of phosphorylation, and to suppress the acid hydrolysis of the sugar chains in the fiber. can. Therefore, it is possible to obtain fine fibrous cellulose having a high axial ratio.

The heat treatment time is, for example, preferably 1 second or more and 300 minutes or less, more preferably 1 second or more and 1000 seconds or less, and 10 seconds or more and 800 seconds or less, after water is substantially removed from the fiber raw material. is more preferable. In the present embodiment, the amount of phosphorus oxoacid groups to be introduced can be set within a preferable range by setting the heating temperature and the heating time within appropriate ranges.

The phosphorus oxoacid group-introducing step may be performed at least once, but may be performed repeatedly two or more times. By performing the phosphorus oxoacid group-introducing step two or more times, many phosphorus oxoacid groups can be introduced into the fiber raw material.

The amount of phosphorus oxoacid groups introduced into the fiber raw material is, for example, preferably 0.60 mmol/g or more, more preferably 0.70 mmol/g or more, and 0.80 mmol/g or more per 1 g (mass) of the fiber raw material. is more preferably 1.00 mmol/g or more, and particularly preferably 1.20 mmol/g or more. In addition, the amount of phosphorus oxoacid groups introduced is, for example, preferably 5.20 mmol/g or less, more preferably 3.65 mmol/g or less, and 3.00 mmol/g or less per 1 g (mass) of fiber raw material. It is even more preferable to have The fact that the amount of phosphorus oxoacid groups introduced in the step of introducing phosphorus oxoacid groups is within the above range means that the amount of substituents introduced into the fine fibrous cellulose supplied to step (A) is within the above range. do. By setting the amount of phosphorus oxoacid groups to be introduced within the above range, the amount of anionic groups introduced into the fine fibrous cellulose to be subjected to the step (A) can be set within the above range. It becomes easy to produce fine fibrous cellulose with a small width (for example, a fiber width of 10 nm or less). This makes it easier to obtain a sheet with excellent transparency and excellent yellowing resistance.

<Carboxy Group Introduction Step>
The process for producing fine fibrous cellulose to be subjected to step (A) may include a carboxyl group-introducing step as the anionic group-introducing step. The carboxy group-introducing step is performed by treating with an acid anhydride of a compound having a carboxylic acid-derived group or a derivative thereof.

The compound having a carboxylic acid-derived group is not particularly limited. Examples include tricarboxylic acid compounds. Derivatives of compounds having a carboxylic acid-derived group are not particularly limited, but include, for example, imidized acid anhydrides of compounds having a carboxy group and derivatives of acid anhydrides of compounds having a carboxy group. Examples of imidized acid anhydrides of compounds having a carboxy group include, but are not particularly limited to, imidized dicarboxylic acid compounds such as maleimide, succinimide and phthalimide.

The acid anhydride of the compound having a group derived from carboxylic acid is not particularly limited. acid anhydrides; In addition, the acid anhydride derivative of the compound having a group derived from carboxylic acid is not particularly limited. Acid anhydrides in which at least some of the hydrogen atoms are substituted with substituents such as alkyl groups and phenyl groups can be mentioned.

The amount of carboxy groups introduced into the fiber raw material is, for example, preferably 0.60 mmol/g or more, more preferably 0.70 mmol/g or more, and more preferably 0.80 mmol/g or more per 1 g (mass) of the fiber raw material. more preferably 1.00 mmol/g or more, particularly preferably 1.20 mmol/g or more. The amount of carboxyl groups introduced is, for example, preferably 3.65 mmol/g or less, more preferably 3.00 mmol/g or less, and 2.50 mmol/g or less per 1 g (mass) of fiber raw material. is more preferred. The fact that the amount of carboxy groups introduced in the step of introducing carboxy groups is within the above range means that the amount of anionic groups introduced into the fine fibrous cellulose to be subjected to step (A) is within the above range. . By setting the amount of carboxyl groups to be introduced within the above range, the amount of anionic groups introduced into the fine fibrous cellulose supplied to step (A) can be within the above range, resulting in a fiber width of 10 nm or less. It becomes easy to produce fine fibrous cellulose of. This makes it easier to obtain a dispersion or a sheet with excellent transparency and further suppressed coloring.

<Sulfur oxoacid group introduction step>
The step of producing fine fibrous cellulose to be subjected to step (A) may include a sulfur oxoacid group introduction step as the anionic group introduction step. In the step of introducing sulfur oxo acid groups, cellulose fibers having sulfur oxo acid groups (sulfur oxo acid group-introduced fibers) can be obtained by reacting hydroxyl groups of the fiber raw material containing cellulose with sulfur oxo acids.

In the sulfur oxoacid group introduction step, in place of compound A in the <phosphorus oxoacid group introduction step> described above, a compound that can introduce a sulfur oxoacid group by reacting with a hydroxyl group possessed by a fiber raw material containing cellulose is selected. At least one compound (hereinafter also referred to as "compound C") is used. Compound C is not particularly limited as long as it has a sulfur atom and is capable of forming an ester bond with cellulose, and includes sulfuric acid or its salts, sulfurous acid or its salts, and sulfate amides. Sulfuric acid of various purities can be used, for example, 96% sulfuric acid (concentrated sulfuric acid) can be used. Sulfurous acid includes 5% sulfurous acid water. Sulfates or sulfites include lithium, sodium, potassium, ammonium salts, etc. of sulfates or sulfites, which can be of varying degrees of neutralization. As the sulfate amide, sulfamic acid or the like can be used. In the sulfur oxoacid group-introducing step, it is preferable to use compound B in the same manner as in <phosphorus oxoacid group-introducing step>.

In the step of introducing sulfur oxoacid groups, it is preferable to heat-treat the cellulose raw material after mixing the cellulose raw material with an aqueous solution containing sulfur oxoacid and urea and/or a urea derivative. As the heat treatment temperature, it is preferable to select a temperature at which sulfur oxoacid groups can be efficiently introduced while suppressing thermal decomposition and hydrolysis reaction 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 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 the moisture is substantially removed. For this reason, the heat treatment time varies depending on the amount of water contained in the cellulose raw material, sulfur oxo acid, and the added amount of the aqueous solution containing urea and/or urea derivatives, but for example, 10 seconds or more and 10000 seconds or less. preferably. For the heat treatment, equipment having various heat media can be used, such as hot air dryers, stirring dryers, rotary dryers, disk dryers, roll type heaters, plate type heaters, and fluidized bed dryers. , a band-type drying device, a filter drying device, a vibrating fluidized drying device, a flash drying device, a vacuum drying device, an infrared heating device, a far infrared heating device, a microwave heating device, and a high frequency drying device can be used.

The amount of sulfur oxoacid groups introduced into the cellulose raw material is preferably 0.60 mmol/g or more, more preferably 0.70 mmol/g or more, more preferably 0.80 mmol/g per 1 g (mass) of fibrous cellulose. It is more preferably 1.00 mmol/g or more, particularly preferably 1.20 mmol/g or more. The amount of sulfur oxoacid groups introduced is, for example, preferably 5.00 mmol/g or less, more preferably 3.00 mmol/g or less per 1 g (mass) of fiber raw material. The fact that the amount of sulfur oxo acid groups introduced in the step of introducing sulfur oxo acid groups is within the above range means that the amount of anionic groups introduced into the fine fibrous cellulose supplied to the step (A) is within the above range. means that By setting the amount of sulfur oxoacid groups to be introduced within the above range, the amount of anionic groups introduced into the fine fibrous cellulose to be subjected to the step (A) can be within the above range, and as a result, the fiber width is increased. It becomes easy to produce fine fibrous cellulose of 10 nm or less. This makes it easier to obtain a dispersion or a sheet with excellent transparency and further suppressed coloring.

<Xanthate group introduction step (xanthate esterification step)>
The step of producing fine fibrous cellulose to be subjected to step (A) may include a step of introducing a xanthate group as an anionic group-introducing step. In the xanthate group-introducing step, a cellulose fiber having a xanthate group (a xanthate group-introduced fiber) is obtained by substituting a xanthate group represented by the following formula (3) for a hydroxyl group of a fiber raw material containing cellulose.
- OCSS ^- M ⁺ (3)
Here, M ⁺ is at least one selected from hydrogen ions, monovalent metal ions, ammonium ions, and aliphatic or aromatic ammonium ions.

In the xanthate group-introducing step, first, alkali treatment is performed to treat the fiber raw material containing cellulose with an alkali solution to obtain alkali cellulose. Examples of the alkaline solution include an aqueous alkali metal hydroxide solution and an aqueous alkaline earth metal hydroxide solution. Among them, the alkaline solution is preferably an aqueous alkali metal hydroxide solution such as sodium hydroxide or potassium hydroxide, and particularly preferably an aqueous sodium hydroxide solution. When the alkaline solution is an aqueous alkali metal hydroxide solution, the concentration of the alkali metal hydroxide in the aqueous alkali metal hydroxide solution is preferably 4% by mass or more, more preferably 5% by mass or more. Further, the alkali metal hydroxide concentration in the aqueous alkali metal hydroxide solution is preferably 9% by mass or less. By setting the alkali metal hydroxide concentration to the above lower limit or more, cellulose mercerization can be sufficiently advanced, and the amount of by-products generated during the subsequent xanthate formation can be reduced. As a result, The yield of the xanthate group-introduced fiber can be increased. Thereby, the fibrillation process mentioned later can be performed more effectively. In addition, by setting the alkali metal hydroxide concentration to the above upper limit or less, it is possible to suppress the penetration of the aqueous alkali metal hydroxide solution into the crystalline region of cellulose while promoting mercerization. The crystalline structure of the mold is easily maintained, and the yield of fine fibrous cellulose can be further increased.

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

It is preferable that the alkali cellulose obtained by the alkali treatment is then subjected to solid-liquid separation to remove as much of the aqueous solution as possible. As a result, the water content during the subsequent xanthate treatment can be reduced, and the reaction can be promoted. As a solid-liquid separation method, a general dehydration method such as centrifugation or filtration can be used. The concentration of the 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 with respect to the total mass of the alkali cellulose after solid-liquid separation.

In the xanthate group-introducing step, the xanthate-forming treatment step is performed after the alkali treatment. In the xanthate treatment step, alkali cellulose is reacted with carbon disulfide (CS ₂ ) to convert (-O ^- Na ⁺ ) groups to (-OCSS ^- Na ⁺ ) groups to obtain xanthate group-introduced fibers. In the above description, the metal ions introduced into the alkali cellulose are represented by Na ⁺ , but similar reactions proceed with other alkali metal ions.

In the xanthate treatment, it is preferable to supply 10% by mass or more of carbon disulfide with respect to the absolute dry mass of cellulose in the alkali cellulose. In addition, in the xanthate-forming treatment, the contact time between carbon disulfide and alkali cellulose is preferably 30 minutes or longer, more preferably 1 hour or longer. Xanthate formation proceeds rapidly when carbon disulfide comes into contact with alkali cellulose, but it takes time for carbon disulfide to penetrate into the interior of alkali cellulose, so the reaction time is preferably within the above range. On the other hand, the contact time between the carbon disulfide and the alkali cellulose should be 6 hours or less, whereby the alkali cellulose mass after dehydration is sufficiently permeated, and the reactable xanthate is almost completely formed. can be completed.

The reaction temperature in the xanthate-forming treatment is preferably 46° C. or lower. By setting the reaction temperature within the above range, decomposition of alkali cellulose can be easily suppressed. Further, by setting the reaction temperature within the above range, uniform reaction is facilitated, so that the generation of by-products can be suppressed, and furthermore, the removal of the generated xanthate groups can be suppressed.

The amount of xanthate groups introduced into the cellulose raw material is preferably 0.60 mmol/g or more, more preferably 0.70 mmol/g or more, and 0.80 mmol/g or more per 1 g (mass) of the fiber raw material. more preferably 1.00 mmol/g or more, and particularly preferably 1.20 mmol/g or more. The amount of the xanthate group introduced is, for example, preferably 5.00 mmol/g or less, more preferably 3.00 mmol/g or less per 1 g (mass) of the fiber raw material. The fact that the amount of xanthate groups introduced in the xanthate group-introducing step is within the above range means that the amount of anionic groups introduced into the fine fibrous cellulose supplied to the step (A) is within the above range. . By setting the amount of xanthate groups introduced within the above range, the amount of anionic groups introduced into the fine fibrous cellulose supplied to step (A) can be within the above range, resulting in a fiber width of 10 nm or less. It becomes easy to produce fine fibrous cellulose of. This makes it easier to obtain a dispersion or a sheet with excellent transparency and further suppressed coloring.

<Washing process>
In the step of producing the fine fibrous cellulose used in step (A), the anionic group-introduced fiber can be subjected to a washing step, if necessary. The washing step is performed by washing the anionic group-introduced fiber with water or an organic solvent, for example. Further, the washing step may be performed after each step described later, and the number of washings performed in each washing step is not particularly limited.

<Alkali treatment process>
In the step of producing fine fibrous cellulose used in step (A), the fiber raw material may be subjected to alkali treatment between the anionic group introduction step and the defibration treatment step described below. The method of alkali treatment is not particularly limited, but includes, for example, a method of immersing the anionic group-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 the present embodiment, it is preferable to use, for example, sodium hydroxide or potassium hydroxide as the alkaline compound because of its high versatility. Moreover, the solvent contained in the alkaline solution may be either water or an organic solvent. Among them, the solvent contained in the alkaline solution is preferably water or a polar solvent including a polar organic solvent exemplified by 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 because of its high versatility.

Although the temperature of the alkaline solution in the alkaline treatment step is not particularly limited, it is preferably 5° C. or higher and 80° C. or lower, and more preferably 10° C. or higher and 60° C. or lower. The immersion time of the anionic group-introduced fiber in the alkali solution in the alkali treatment step is not particularly limited, but is preferably 5 minutes or more and 30 minutes or less, more preferably 10 minutes or more and 20 minutes or less. The amount of the alkaline solution used in the alkaline treatment is not particularly limited, but for example, it is preferably 100% by mass or more and 100000% by mass or less, and 1000% by mass or more and 10000% by mass or less relative to the absolute dry mass of the anionic group-introduced fiber. is more preferable.

In order to reduce the amount of alkaline solution used in the alkaline treatment step, the anionic group-introduced fiber may be washed with water or an organic solvent after the anionic group-introducing step and before the alkaline treatment step. After the alkali treatment step and before the fibrillation treatment step, it is preferable to wash the alkali-treated anionic group-introduced fibers with water or an organic solvent from the viewpoint of improving handleability.

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

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

The temperature of the acid solution in the acid treatment is not particularly limited. The immersion time in the acid solution in the acid treatment is not particularly limited, but is preferably 5 minutes or more and 120 minutes or less, more preferably 10 minutes or more and 60 minutes or less. The amount of the acid solution used in the acid treatment is not particularly limited. is more preferred.

<Nitrogen removal treatment>
The step of producing fine fibrous cellulose to be subjected to step (A) may further include a step of reducing the nitrogen content (nitrogen removal treatment step). By reducing the amount of nitrogen contained in the dispersion, the yellowing resistance of the sheet can be further enhanced. The nitrogen removal treatment step may be provided after the uniform dispersion treatment step in step (B) described below, but is preferably provided before the uniform dispersion treatment step in step (B) described below. Moreover, it is preferable that it is provided before the fibrillation treatment process.

In the nitrogen removal treatment step, it is preferable to adjust the pH of the slurry containing the anionic group-introduced fiber to 10 or more and perform 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 longer and 180 minutes or shorter. When adjusting the pH of the slurry containing the anionic group-introduced fiber, it is preferable to add an alkali compound that can be used in the alkali treatment step described above to the slurry.

After the nitrogen removal treatment step, the anionic group-introduced fiber can be subjected to a washing step, if necessary. The washing step is performed by washing the anionic group-introduced fiber with water or an organic solvent, for example. Moreover, the number of washings performed in each washing step is not particularly limited.

<Fibrillation treatment process>
The manufacturing process of the fine fibrous cellulose used in the step (A) includes a fibrillation treatment process. As a result, fine fibrous cellulose having an anionic group and a fiber width of 1000 nm or less can be obtained. In the defibration treatment process, for example, a fibrillation treatment device can be used. The fibrillation treatment device is not particularly limited, but for example, a high-speed fibrillator, a grinder (stone mill type pulverizer), a high-pressure homogenizer, an ultra-high pressure homogenizer, a high-pressure collision type pulverizer, a ball mill, a bead mill, a disk-type refiner, a conical refiner, and a biaxial refiner. A kneader, a vibration mill, a homomixer under high-speed rotation, an ultrasonic disperser, a beater, or the like can be used. Among the above defibration equipment, it is more preferable to use a high-speed fibrillation machine, a high-pressure homogenizer, and an ultrahigh-pressure homogenizer, which are less affected by the grinding media and less likely to cause contamination.

The treatment conditions in the fibrillation treatment step are not particularly limited. For example, when a high-pressure homogenizer is used, the pressure during treatment 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 fibrillation treatment step, for example, the anionic group-introduced fibers are preferably diluted with a dispersion medium to form a slurry. As the dispersion medium, one or more selected from organic solvents such as water and polar organic solvents can be used. The polar organic solvent is not particularly limited, but alcohols, polyhydric alcohols, ketones, ethers, esters, aprotic polar solvents and the like are preferable. Examples of alcohols include methanol, ethanol, isopropanol, n-butanol, isobutyl alcohol and the like. Examples of polyhydric alcohols include ethylene glycol, propylene glycol and glycerin. Ketones include acetone, methyl ethyl ketone (MEK), and the like. Examples of ethers include diethyl ether, tetrahydrofuran, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-butyl ether, propylene glycol monomethyl ether and the like. Examples of esters include ethyl acetate and butyl acetate. Aprotic polar solvents include dimethylsulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), N-methyl-2-pyrrolidinone (NMP) and the like.

The solid content concentration of the fine fibrous cellulose at the defibration treatment can be appropriately set. Further, the slurry obtained by dispersing the anionic group-introduced fiber in the dispersion medium may contain solids other than the anionic group-introduced fiber, such as urea having hydrogen bonding properties.

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

Further, when the nanofiber yield is calculated by making the fine fibrous cellulose to be subjected to the step (A) into an aqueous dispersion having a concentration of 0.1% by mass, the nanofiber yield is preferably 90% by mass or more. , more preferably 93% by mass or more, and even more preferably 96% by mass or more. Note that the nanofiber yield may be 100% by mass. Here, the nanofiber yield was obtained by centrifuging a fine fibrous cellulose dispersion having a concentration of 0.1% by mass using a cooling high-speed centrifuge (H-2000B, manufactured by Kokusan Co., Ltd.) at 12000 G for 10 minutes. , is a value measured based on the following formula from the cellulose concentration of the obtained supernatant.
Nanofiber yield (mass%) = cellulose concentration in supernatant (mass%)/0.1 x 100

Furthermore, when the fine fibrous cellulose to be subjected to the step (A) is made into an aqueous dispersion having a concentration of 0.2% by mass, the haze of the aqueous dispersion is preferably 10% or less, more preferably 5.0% or less. is more preferably 3.0% or less. The haze of the aqueous dispersion may be 0%. Here, the haze of the aqueous dispersion of fine fibrous cellulose is a value measured according to JIS K 7136:2000 using a haze meter (HM-150, manufactured by Murakami Color Research Laboratory Co., Ltd.). For measurement, a glass cell for liquids with an optical path length of 1 cm (manufactured by Fujiwara Seisakusho Co., Ltd., MG-40, reverse optical path) is used. The zero point measurement is performed with ion-exchanged water in the same glass cell. is 23°C.

By setting the fiber width of the fine fibrous cellulose and the nanofiber yield and haze of the fine fibrous cellulose dispersion provided in the step (A) within the above ranges, the dispersion and sheet containing fine fibrous cellulose can be made transparent. It is possible to enhance sexuality more effectively.

<Substituent removal treatment step (step A)>
The method for producing a fine fibrous cellulose dispersion liquid preferably includes a step (A) of removing part of the anionic groups from the fine fibrous cellulose having an anionic group and having a fiber width of 1000 nm or less. In the present specification, the step of removing some of the anionic groups from the fine fibrous cellulose is also referred to as a substituent removal treatment step.

Examples of the substituent-removing treatment step include a step of heat-treating fine fibrous cellulose having an anionic group and a fiber width of 1000 nm or less, an enzyme treatment step, an acid treatment step, an alkali treatment step, and the like. . These may be carried out singly or in combination. Among them, the substituent removal treatment step is preferably a heat treatment step or an enzyme treatment step. By going through the above treatment step, a part of the anionic groups is removed from the fine fibrous cellulose having an anionic group and a fiber width of 1000 nm or less, and the introduction amount of the anionic group is 0.5 mmol/g. of fine fibrous cellulose can be obtained.

The substituent removal treatment step is preferably performed in a slurry state. That is, in the substituent removal treatment step, a slurry containing fine fibrous cellulose having an anionic group and a fiber width of 1000 nm or less is subjected to heat treatment, enzyme treatment, acid treatment, and alkali treatment. A process or the like is preferable. By carrying out the substituent removal treatment step in a slurry form, it is possible to prevent residual coloring substances caused by heating or the like during the substituent removal treatment, and added or generated acids, alkalis, salts, and the like. This makes it possible to suppress coloring when the fine fibrous cellulose obtained through the step (B) is made into a dispersion or a sheet. In addition, when the salt derived from the removed substituent is removed after the substituent removal treatment, it is possible to increase the salt removal rate.

When a slurry containing fine fibrous cellulose having an anionic group and a fiber width of 1000 nm or less is subjected to substituent removal treatment, the concentration of fine fibrous cellulose in the slurry is 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 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 setting 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 setting the concentration of the fine fibrous cellulose in the slurry within the above range, it is possible to prevent the residual coloring matter caused by heating during the substituent removal treatment, and the acid, alkali, salt, etc. added or generated. can. This makes it possible to suppress coloring when the fine fibrous cellulose obtained through the step (B) is made into a dispersion or a sheet. In addition, when the salt derived from the removed substituent is removed after the substituent removal treatment, it is possible to increase the salt removal rate.

When the substituent-removing treatment step is a step of heat-treating fine fibrous cellulose having an anionic group and a fiber width of 1000 nm or less, the heating temperature in the heat-treating step may be 40° C. or higher. It is preferably 50° C. or higher, more preferably 60° C. or higher. 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. Among them, when the substituent of the fine fibrous cellulose to be subjected to the substituent removal treatment step is a phosphorous oxoacid group, a sulfur oxoacid group or a maleic acid group, the heating temperature in the heat treatment step is preferably 80° C. or higher. It is preferably 100° C. or higher, 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 hot air heating device, steam heating device, electric heating device, hydrothermal heating device, thermal heating device. , infrared heating device, far infrared heating device, microwave heating device, high frequency heating device, stirring drying device, rotary drying device, disk drying device, roll type heating device, plate type heating device, fluidized bed drying device, band type drying device , a filtration drying apparatus, a vibrating fluidized drying apparatus, a flash drying apparatus, and a vacuum drying apparatus can be used. From the viewpoint of preventing evaporation, the heating is preferably performed in a closed system, and from the viewpoint of increasing the heating temperature, it is preferably performed in a pressure-resistant device or container. The heat treatment may be batch treatment, batch continuous treatment, or continuous treatment.

When the substituent removal treatment step is a step of enzymatically treating fine fibrous cellulose having an anionic group and a fiber width of 1000 nm or less, in the enzymatic treatment step, depending on the type of substituent, phosphoric acid Ester hydrolase, sulfate ester hydrolase, etc. are preferably used.

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

A step of deactivating the enzyme may be provided after the enzymatic reaction. As a method for deactivating the enzyme, an acid component or an alkaline component is added to the enzyme-treated slurry to deactivate the enzyme. A method of deactivation can be mentioned.

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

When the substituent-removing treatment step is a step of treating with an alkali fine fibrous cellulose having an anionic group and a fiber width of 1000 nm or less, the alkali treatment step can be used in the alkali treatment step described above. It is preferred to add an alkaline compound to the slurry.

In the substituent-removing treatment step, it is preferable that the substituent-removing reaction proceed uniformly. In order to proceed the reaction uniformly, for example, the slurry containing fine fibrous cellulose may be stirred, or the specific surface area of the slurry in contact with the heating medium may be increased. As a method for agitating the slurry, a mechanical shear may be applied from the outside, or the self-agitation may be promoted by increasing the feeding speed of the slurry during the reaction.

A spacer molecule may be added in the substituent removal treatment step. Spacer molecules act as spacers to get between adjacent fibrous celluloses, thereby providing fine spaces between the fine fibrous celluloses. By adding such a spacer molecule in the substituent-removing treatment step, aggregation of the fine fibrous cellulose after the substituent-removing treatment can be suppressed. As a result, the transparency of the dispersion or sheet containing fine fibrous cellulose can be more effectively enhanced.

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

Also, known pigments can be used as spacer molecules. For example, kaolin (including clay), calcium carbonate, titanium oxide, zinc oxide, amorphous silica (including colloidal silica), aluminum oxide, zeolite, sepiolite, smectite, synthetic smectite, magnesium silicate, magnesium carbonate, magnesium oxide, diatomaceous earth, Examples include styrene plastic pigments, hydrotalcite, urea resin plastic pigments, benzoguanamine plastic pigments, and the like.

<pH adjustment step>
When the substituent-removing treatment step is performed in the form of a slurry, a step of adjusting the pH of the slurry containing fine fibrous cellulose may be provided before the substituent-removing treatment step. For example, when an anionic group is introduced into cellulose fibers and the counter ion of this anionic group is Na ⁺ , the slurry containing fine fibrous cellulose after defibration exhibits weak alkalinity. If the slurry is heated in this state, monosaccharides, which are one of the factors of coloration, may be generated due to the decomposition of cellulose. Therefore, it is preferable to adjust the pH of the slurry to 8 or less. In addition, since monosaccharides may also be generated under acidic conditions, it is preferable to adjust the pH of the slurry to 3 or higher.

Further, when the fine fibrous cellulose having a substituent is a fine fibrous cellulose having a phosphate group, from the viewpoint of improving the efficiency of removing the substituent, phosphorus of the phosphate group should be in a state of being susceptible to nucleophilic attack. is preferred. Cellulose -OP (=O) (-OH ⁺ ) (-O-Na ⁺ ) with a degree of neutralization of 1 is susceptible to nucleophilic attack. Preferably, the pH of the slurry is adjusted to 3 or more and 8 or less, more preferably 4 or more and 6 or less.

Means for adjusting the pH are not particularly limited, but for example, an acid component or an alkali component may be added to the slurry containing fine fibrous cellulose. The acid component may be either an inorganic acid or an organic acid. Examples of 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, gluconic acid and the like. The alkali component may be an inorganic alkali compound or an organic alkali compound. Examples of inorganic alkali compounds include lithium hydroxide, sodium hydroxide, potassium hydroxide, lithium carbonate, lithium hydrogen carbonate, potassium carbonate, potassium hydrogen carbonate, sodium carbonate, and sodium hydrogen carbonate. Organic alkaline compounds include ammonia, hydrazine, methylamine, ethylamine, diethylamine, triethylamine, propylamine, dipropylamine, butylamine, diaminoethane, diaminopropane, diaminobutane, diaminopentane, diaminohexane, cyclohexylamine, aniline, tetramethyl ammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, benzyltrimethylammonium hydroxide, pyridine, N,N-dimethyl-4-aminopyridine and the like.

Moreover, in the pH adjustment step, an ion exchange treatment may be performed in order to adjust the pH. For the ion exchange treatment, a strongly acidic cation exchange resin or a weakly acidic ion exchange resin can be used. By treating with an appropriate amount of cation exchange resin for a sufficient period of time, a slurry containing fine fibrous cellulose at a desired pH can be obtained. Furthermore, in the pH adjustment step, the addition of an acid component or an alkali component and the ion exchange treatment may be combined.

<Salt Removal Treatment>
After the substituent-removing treatment step, it is preferable to perform a treatment for removing salts derived from the removed substituents. By removing the salt derived from the substituent, it becomes easier to obtain fine fibrous cellulose capable of suppressing coloration. Although the means for removing the salt derived from the substituent is not particularly limited, examples thereof include washing treatment and ion exchange treatment. The washing treatment is performed, for example, by washing the fine fibrous cellulose aggregated by the substituent removal treatment with water or an organic solvent. An ion exchange resin can be used in the ion exchange treatment.

<Uniform dispersion treatment step (step (B))>
It is preferable that the method for producing a fine fibrous cellulose dispersion further includes a step (B) of performing a uniform dispersion treatment after the step (A). The step (B) of uniform dispersion treatment is a step of uniform dispersion treatment of the fine fibrous cellulose obtained through the substituent removal treatment of step (A). In the step (A), at least a part of the fine fibrous cellulose aggregates by subjecting the fine fibrous cellulose to the substituent removing treatment. Step (B) is a step of uniformly dispersing the aggregated fine fibrous cellulose. Thus, the fine fibrous cellulose obtained in the present embodiment has a fiber width of 1000 nm or less (preferably 10 nm below).

In the step (B) for uniformly dispersing treatment, for example, high-speed fibrillation machine, grinder (stone mill type crusher), high pressure homogenizer high pressure collision type crusher, ball mill, bead mill, disc refiner, conical refiner, twin shaft kneader, vibration mill, A homomixer under high-speed rotation, an ultrasonic disperser, a beater, or the like can be used. Among the uniform dispersion treatment apparatuses, it is more preferable to use a high-speed defibrator and a high-pressure homogenizer.

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

In step (B), the spacer molecules described above may be newly added. By adding such a spacer molecule in the uniform dispersion treatment step (B), uniform dispersion of the fine fibrous cellulose can be carried out more smoothly. As a result, the transparency of the dispersion or sheet containing fine fibrous cellulose can be more effectively enhanced.

<Ion exchange process>
After the step (A) of removing part of the anionic groups from the fine fibrous cellulose having an anionic group and having a fiber width of 1000 nm or less, a polyvalent metal salt is added to the dispersion containing the fine fibrous cellulose. It is preferable to include the step of adding In the step of adding a polyvalent metal salt, the counterions of the fine fibrous cellulose are exchanged for polyvalent metal ions, and at least part of the fine fibrous cellulose is aggregated, so such a step is an ion exchange step or an aggregation step. Also called The ion exchange step may be provided after step (A), but is more preferably provided after step (B) following step (A). In the present embodiment, since the polyvalent metal salt is added to the dispersion containing fine fibrous cellulose obtained after the disentanglement treatment and after the substituent removal treatment, in the redispersion step described later, the fine fibers The redispersibility of the cellulose can be further enhanced. This makes it possible to obtain a redispersion liquid or a sheet having excellent transparency.

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

In the ion exchange step, when aggregates are generated by adding the polyvalent metal salt, it is preferable to further include a filtration step and a washing step after stirring. The washing step is performed by washing the aggregate with water or an organic solvent, for example. And a concentrate can be obtained by providing a filtration process after a washing process. The filter medium used in the filtration step is not particularly limited, but filter mediums such as those made of stainless steel, filter paper, polypropylene, nylon, polyethylene, and polyester can be used. Polypropylene filter media are preferred due to the possible use of acids. ^{The lower} the ^{air permeability} of the filter medium ^, the higher the ^yield .

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

The content of fine fibrous cellulose contained in the aggregates obtained in the ion exchange step is preferably 40% by mass or less, and preferably 20% by mass or less, relative to the total mass of the aggregates containing the solvent. is more preferred, and less than 6% by mass is even more preferred. Moreover, the content of fine fibrous cellulose contained in the aggregate is preferably 0.1% by mass or less with respect to the total mass of the aggregate. By setting the content of the aggregates within the above range, the redispersibility in the redispersion step, which will be described later, can be more effectively enhanced.

In addition, in the ion exchange step, a drying step may be provided in order to keep the content of the fine fibrous cellulose contained in the aggregate within a predetermined range. Moreover, after the ion exchange step, a step of pulverizing the aggregates may be provided.

Moreover, in the ion exchange step, the step of adding the polyvalent metal salt may be provided a plurality of times. By providing the step of adding the polyvalent metal salt a plurality of times, the counter ions of the fine fibrous cellulose can be efficiently exchanged for polyvalent metal ions.

<Redispersion (uniform dispersion) step>
After the ion exchange step, it is preferable to provide a step of redispersing the obtained aggregates and ingredients in a solvent. In the present embodiment, in the redispersion step, the fine fibrous cellulose in the aggregates is uniformly dispersed, so the redispersion step may also be referred to as a uniform dispersion treatment step. Through such a redispersion step, a dispersion (redispersion) in which fine fibrous cellulose is uniformly dispersed is obtained. The re-dispersion (uniform dispersion) step provided after the ion exchange step may be the same step as the uniform dispersion treatment step (step (B)) provided after the substituent removal treatment step (step (A)). .

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

In the re-dispersion process, for example, high-speed fibrillation machine, grinder (stone mill type crusher), high pressure homogenizer high pressure collision type crusher, ball mill, bead mill, disk refiner, conical refiner, twin screw kneader, vibration mill, high speed rotation A homomixer, an ultrasonic disperser, a beater, or the like can be used. Among the uniform dispersion treatment apparatuses, it is more preferable to use a high-speed defibrator and a high-pressure homogenizer.

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

As described above, in the method for producing a fine fibrous cellulose dispersion of the present invention, the step (A ), a step (B) of uniform dispersion treatment, an ion exchange step and a re-dispersion (uniform dispersion) step in this order. In the present embodiment, in particular, in the step of redispersing (uniformly dispersing) aggregates and components obtained in the ion exchange step in a solvent, a high-speed defibrator or a high-pressure homogenizer is used to uniformly disperse fine fibrous cellulose. By doing so, it is possible to obtain a dispersion having high transparency and a high viscosity recovery rate. By forming a sheet using the fine fibrous cellulose dispersion obtained through such steps, a sheet having high transparency and excellent yellowing resistance can be formed.

(sheet)
This embodiment also relates to a sheet formed from the dispersion described above. The sheet of the present embodiment contains fibrous cellulose having an anionic group and a fiber width of 1000 nm or less, the introduction amount of the anionic group in the fibrous cellulose is less than 0.5 mmol/g, and the fibrous cellulose have polyvalent metal ions as counter ions for the anionic groups.

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

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

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

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

The total light transmittance of the sheet is preferably 70% or more, more preferably 75% or more, still more preferably 80% or more, still more preferably 85% or more, and 90% or more. is particularly preferred. The upper limit of the total light transmittance of the sheet is not particularly limited, and may be 100%. Here, the total light transmittance of the sheet is a value measured according to JIS K 7361-1:1997. As a device for measuring haze, for example, a haze meter (HM-150) manufactured by Murakami Color Research Laboratory Co., Ltd. can be used.

The haze of the sheet is preferably 10% or less, more preferably 5% or less, still more preferably 4% or less, even more preferably 3% or less, and 2.5% or less. It is particularly preferred to have The lower limit of the haze of the sheet is not particularly limited, and may be 0%. Here, the sheet haze is a value measured according to JIS K 7136:2000. As a device for measuring haze, for example, a haze meter (HM-150) manufactured by Murakami Color Research Laboratory Co., Ltd. can be used.

The thickness of the sheet is not particularly limited, but is preferably 5 μm or more, more preferably 10 μm or more, still more preferably 20 μm or more, even more preferably 30 μm or more, and 40 μm. It is particularly preferable that it is above. Also, the thickness of the sheet is preferably 200 μm or less. In this embodiment, polyvalent metal ions are introduced as counter ions for the anionic groups before the sheet is formed, so that even a thick sheet exhibits excellent yellowing resistance.

Although the basis weight of the sheet is not particularly limited, it is preferably 10 g/m ² or more, more preferably 20 g/m ² or more. Also, the basis weight of the sheet is preferably 200 g/m ² or less, more preferably 150 g/m ² or less. The basis weight of the sheet is a value calculated according to the following method. A sheet cut into a size of 50 mm square or larger is conditioned at 23° C. and a relative humidity of 50% for 24 hours, then weighed and divided by the area of the cut sheet to calculate the basis weight of the sheet. .

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

The water absorption rate of the sheet is preferably 600% or less, more preferably 580% or less, even more preferably 550% or less. Moreover, the water absorption rate of the sheet is preferably 0% or more. By setting the water absorption rate of the sheet within the above range, a sheet having excellent water resistance can be obtained. Here, the water absorption rate of the sheet is a value calculated by the following method. First, a sheet of 50 mm square is immersed in deionized water for 24 hours. Then, the water absorption rate of the sheet is obtained from the following formula. The absolute dry mass of the sheet is the mass measured after drying the sheet at 105° C. for 24 hours.
Water absorption rate of sheet (%) = weight of sheet after immersion in deionized water (g)/absolute dry weight of sheet (g) x 100

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

Resins include acrylic resins, polystyrene resins, polycarbonate resins, polyester resins, polyamide resins, silicone resins, fluorine resins, chlorine resins, epoxy resins, melamine resins, phenol resins, and polyurethane resins. Examples include resins, diallyl phthalate-based resins, alcohol-based resins, cellulose derivatives, and precursors of these resins. Examples of cellulose derivatives include carboxymethylcellulose, methylcellulose, and hydroxyethylcellulose. The type of resin precursor is not particularly limited, but examples thereof include precursors of thermoplastic resins and thermosetting resins. A precursor of a thermoplastic resin means a monomer or oligomer having a relatively low molecular weight used to produce the thermoplastic resin. Also, the thermosetting resin precursor means a monomer or an oligomer having a relatively low molecular weight that can form a thermosetting resin by causing a polymerization reaction or a cross-linking reaction by the action of light, heat, or a curing agent. When the sheet contains a resin precursor, the resin precursor may be polymerized or cross-linked by the action of light, heat, or a curing agent in a post-process or during use depending on the mode of use.

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

Further, in the present embodiment, the sheet may contain optional components in addition to the fibrous cellulose and resin described above. Examples of optional components include optional components that the dispersion may contain.

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

<Resin layer>
The resin layer is a layer containing natural resin or synthetic resin as a main component. Here, the main component refers to a component that is contained in an amount of 50% by mass or more with respect to the total mass of the resin layer. The content of the resin is preferably 60% by mass or more, more preferably 70% by mass or more, further preferably 80% by mass or more, and 90% by mass with respect to the total mass of the resin layer. It is particularly preferable that it is above. In addition, the content of the resin may be 100% by mass, or may be 95% by mass or less.

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

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

Examples of the polycarbonate resin constituting the resin layer include aromatic polycarbonate resins and aliphatic polycarbonate resins. Specific polycarbonate-based resins for these are known, and include, for example, the polycarbonate-based resins described in JP-A-2010-023275.

The resin constituting the resin layer may be used singly, or a copolymer obtained by copolymerizing or graft-polymerizing a plurality of resin components may be used. Also, it may be used as a blend material in which a plurality of resin components are mixed by a physical process.

An adhesive layer may be provided between the sheet and the resin layer, or the sheet and the resin layer may be in direct contact without an adhesive layer. When an adhesive layer is provided between the sheet and the resin layer, examples of the adhesive constituting the adhesive layer include acrylic resins. Examples of adhesives other than acrylic resins include vinyl chloride resins, (meth)acrylic acid ester resins, styrene/acrylic acid ester copolymer resins, vinyl acetate resins, and vinyl acetate/(meth)acrylic acid ester copolymer resins. Polymer resins, urethane resins, silicone resins, epoxy resins, ethylene/vinyl acetate copolymer resins, polyester resins, polyvinyl alcohol resins, ethylene vinyl alcohol copolymer resins, and rubber emulsions such as SBR and NBR. be done.

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 surface treatment such as hydrophilization.
Examples of adhesion promoters include compounds containing at least one selected from isocyanate groups, carbodiimide groups, epoxy groups, oxazoline groups, amino groups and silanol groups, and organosilicon compounds. Among them, the adhesion aid is preferably at least one selected from compounds containing an isocyanate group (isocyanate compounds) and organosilicon compounds. Examples of organic silicon compounds include condensates of silane coupling agents and silane coupling agents.
Examples of surface treatment methods include corona treatment, plasma discharge treatment, UV irradiation treatment, electron beam irradiation treatment, and flame treatment.

<Inorganic layer>
The material constituting the inorganic layer is not particularly limited, but for example, aluminum, silicon, magnesium, zinc, tin, nickel, titanium; or mixtures thereof. From the viewpoint of stably maintaining high moisture resistance, silicon oxide, silicon nitride, silicon oxycarbide, silicon oxynitride, silicon oxycarbonitride, aluminum oxide, aluminum nitride, aluminum oxycarbide, aluminum oxynitride, or these Mixtures are preferred.

A method for forming the inorganic layer is not particularly limited. In general, methods for forming thin films are roughly classified into chemical vapor deposition (CVD) and physical vapor deposition (PVD), and either method may be adopted. . Specific examples of the CVD method include plasma CVD using plasma, catalytic chemical vapor deposition (Cat-CVD) in which a material gas is catalytically thermally decomposed using a heated catalyst, and the like. Specific examples of the PVD method include vacuum deposition, ion plating, and sputtering.

Moreover, as a method for forming the inorganic layer, an atomic layer deposition (ALD) method can be employed. The ALD method is a method of forming a thin film in units of atomic layers by alternately supplying raw material gases of elements constituting a film to be formed to a surface on which a layer is to be formed. Although it has the disadvantage of a slow deposition rate, it has the advantage over the plasma CVD method that even surfaces with complicated shapes can be covered cleanly, and a thin film with few defects can be deposited. In addition, the ALD method has the advantage that the film thickness can be controlled on the order of nanometers, and that it is relatively easy to cover a wide surface. Furthermore, the ALD method is expected to improve the reaction rate, lower the temperature of the process, and reduce unreacted gas by using plasma.

(Manufacturing method of sheet)
A method for manufacturing a sheet preferably includes a step of forming a sheet from the dispersion liquid obtained by the method described above. The step of forming a sheet from the dispersion preferably includes a coating step of applying the dispersion onto a substrate or a papermaking step of papermaking the dispersion.

<Coating process>
In the coating step, a sheet can be obtained by applying the dispersion obtained in the step of obtaining the dispersion onto a base material, drying it, and peeling the formed sheet from the base material. In addition, sheets can be continuously produced by using a coating device and a long base material.

The material of the base material used in the coating step is not particularly limited, but a material with high wettability to the dispersion liquid may suppress shrinkage of the sheet during drying, but the sheet formed after drying is It is preferable to select a material that can be easily peeled off. Among them, a resin film or plate or a metal film or plate is preferable, but is not particularly limited. For example, resin films and plates such as acrylic, polyethylene terephthalate, vinyl chloride, polystyrene, and polyvinylidene chloride, metal films and plates such as aluminum, zinc, copper, and iron plates, and those whose surfaces have been oxidized, and stainless steel films. A plate, brass film or plate, etc. can be used.

In the coating process, when the viscosity of the dispersion is low and it spreads on the base material, a dam frame is fixed on the base material in order to obtain a sheet with a predetermined thickness and basis weight. You may The damming frame is not particularly limited, but it is preferable to select, for example, one that allows easy peeling of the edge of the sheet that adheres after drying. From such a point of view, a molded resin plate or metal plate is more preferable. In the present 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 their surfaces An oxidized one, a stainless steel plate, a brass plate, or the like can be used.

The coating machine for coating the base material with the dispersion is not particularly limited, but for example, a roll coater, a gravure coater, a die coater, a curtain coater, an air doctor coater and the like can be used. A die coater, a curtain coater, and a spray coater are particularly preferable because they can make the thickness of the sheet more uniform.

The dispersion temperature and the ambient temperature when the dispersion is applied to the substrate are not particularly limited, but are preferably 5° C. or higher and 80° C. or lower, more preferably 10° C. or higher and 60° C. or lower. It is more preferably 15°C or higher and 50°C or lower, and particularly preferably 20°C or higher and 40°C or lower. If the coating temperature is at least the above lower limit, the dispersion can be more easily coated. When the coating temperature is equal to or lower than the above upper limit, volatilization of the dispersion medium during coating can be suppressed.

In the coating step, the dispersion liquid is added 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. Coating on a substrate is preferred. By coating so that the basis weight is within the above range, a sheet having excellent strength can be obtained.

The coating step includes the step of drying the dispersion applied onto the substrate, as described above. The step of drying the dispersion is not particularly limited, but may be performed by, for example, a non-contact drying method, a method of drying while restraining the sheet, or a combination thereof. The non-contact drying method is not particularly limited, but for example, a method of drying by heating with hot air, infrared rays, far infrared rays, or near infrared rays (heat drying method), or a method of drying in a vacuum (vacuum drying method) is applied. can do. Although the heat drying method and the vacuum drying method may be combined, the heat drying method is usually applied. Drying with infrared rays, far-infrared rays, or near-infrared rays is not particularly limited. The heating temperature in the heat drying method is not particularly limited, but is preferably 20° C. or higher and 150° C. or lower, more preferably 25° C. or higher and 105° C. or lower. If the heating temperature is equal to or higher than the above lower limit, the dispersion medium can be rapidly volatilized. Further, if the heating temperature is equal to or lower than the above upper limit, it is possible to suppress the cost required for heating and suppress discoloration of fibrous cellulose due to heat.

<Papermaking process>
A papermaking process is performed by papermaking a dispersion liquid with a paper machine. The paper machine used in the papermaking process is not particularly limited, but examples thereof include continuous paper machines such as fourdrinier, cylinder, and inclined paper machines, and multi-layer paper machines combining these. In the paper-making process, a known paper-making method such as hand-making may be employed.

The papermaking process is carried out by filtering the dispersion with a wire and dehydrating it to obtain a wet paper sheet, which is then pressed and dried. The filter cloth used for filtering and dehydrating the dispersion is not particularly limited, but it is more preferable that, for example, fibrous cellulose does not pass through and the filtration rate does not become too slow. Such a filter cloth is not particularly limited, but preferably includes, for example, a sheet, a fabric, or a porous membrane made of an organic polymer. Although the organic polymer is not particularly limited, non-cellulose organic polymers such as polyethylene terephthalate, polyethylene, polypropylene, and polytetrafluoroethylene (PTFE) are preferred. In this embodiment, for example, a polytetrafluoroethylene porous film having a pore size of 0.1 μm or more and 20 μm or less, or a polyethylene terephthalate or polyethylene fabric having a pore size of 0.1 μm or more and 20 μm or less can be used.

In the sheeting process, a method of producing a sheet from a dispersion liquid includes, for example, discharging a dispersion liquid containing fine fibrous cellulose onto the upper surface of an endless belt, and squeezing the dispersion medium from the discharged dispersion liquid to form a web. It can be done using manufacturing equipment that includes a water squeeze section and a drying section that dries the web to produce a sheet. An endless belt is provided from the water squeezing section to the drying section, and the web produced in the water squeezing section is conveyed to the drying section while being placed on the endless belt.

The dehydration method used in the papermaking process is not particularly limited, but includes, for example, dehydration methods commonly used in the manufacture of paper. Among these, the method of dehydrating with a fourdrinier, a circular net, an inclined wire, or the like and then further dehydrating with a roll press is preferable. Moreover, the drying method used in the papermaking process is not particularly limited, but includes, for example, methods used in the manufacture of paper. Among these, a drying method using a cylinder dryer, Yankee dryer, hot air drying, near-infrared heater, infrared heater, or the like is more preferable.

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

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

EXAMPLES The characteristics of the present invention will be described more specifically below with reference to examples and comparative examples. The materials, amounts used, proportions, treatment details, treatment procedures, etc. shown in the following examples can be changed as appropriate without departing from the gist of the present invention. Therefore, the scope of the present invention should not be construed to be limited by the specific examples shown below.

<Production Example 1>
[Phosphorylation treatment]
As raw material pulp, hardwood dissolving pulp (dry sheet) manufactured by Oji Paper Co., Ltd. was used. The raw material pulp was subjected to a phosphorylation treatment as follows. First, a mixed aqueous solution of ammonium dihydrogen phosphate and urea is added to 100 parts by mass (absolute dry mass) of the raw material pulp to obtain 45 parts by mass of ammonium dihydrogen phosphate, 120 parts by mass of urea, and 150 parts by mass of water. A chemical-impregnated pulp was obtained by adjusting as follows. Next, the resulting chemical solution-impregnated pulp was heated in a hot air dryer at 165° C. for 200 seconds to introduce phosphoric acid groups into cellulose in the pulp to obtain phosphorylated pulp.

[Washing process]
Then, the obtained phosphorylated pulp was washed. In the washing treatment, a pulp dispersion liquid obtained by pouring 10 L of ion-exchanged water into 100 g (absolute dry mass) of phosphorylated pulp is stirred so that the pulp is uniformly dispersed, and then filtration and dehydration are repeated. went. The washing was finished when the electric conductivity of the filtrate became 100 μS/cm or less.

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

The phosphorylated pulp thus obtained was subjected to infrared absorption spectrum measurement using FT-IR. As a result, absorption due to phosphate groups was observed near 1230 cm ⁻¹ , confirming that phosphate groups were added to the pulp.

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

<Production Example 2>
[Phosphating treatment]
Except for using 33 parts by mass of phosphorous acid (phosphonic acid) instead of ammonium dihydrogen phosphate in the phosphorylation treatment, the same operation as in Production Example 1 was performed to produce fine particles containing phosphorous pulp and fine fibrous cellulose. A fibrous cellulose dispersion was obtained.

The infrared absorption spectrum of the obtained phosphorous acid pulp was measured using FT-IR. As a result, an absorption based on P=O of a phosphonic acid group, which is a tautomer of a phosphite group, was observed near 1210 cm ⁻¹ , indicating that a phosphite group (phosphonic acid group) was added to the pulp. was confirmed. Moreover, it was confirmed by X-ray diffraction that the obtained fine fibrous cellulose maintained cellulose type I crystals. The amount of phosphite groups (first dissociated acid amount) measured by the measuring method described later in [Measurement of Phosphorus Acid Group Amount] was 1.51 mmol/g, and the total dissociated acid amount was 1.51 mmol/g. It was 54 mmol/g.

<Production Example 3>
A fine fibrous cellulose dispersion containing xanthate pulp and fine fibrous cellulose was obtained in the same manner as in Production Example 1 except that the following xanthate treatment was performed instead of the phosphorylation treatment.

[Xanthate treatment]
2500 parts by mass of 8.5% by mass sodium hydroxide aqueous solution is added to 100 parts by mass (absolute dry mass) of raw material pulp (hardwood dissolving pulp (dry sheet) manufactured by Oji Paper Co., Ltd.) and stirred at room temperature for 3 hours. Then, alkali treatment was performed. The alkali-treated pulp was subjected to solid-liquid separation by centrifugation (filter cloth 400 mesh, 3000 rpm for 5 minutes) to obtain dehydrated alkali cellulose. 3.5 parts by mass of carbon disulfide was added to 10 parts by mass (absolute dry mass) of the obtained alkali cellulose, and a sulfurization reaction was allowed to proceed at room temperature for 4.5 hours to perform a xanthate treatment.

It was confirmed by X-ray diffraction that the resulting fine fibrous cellulose maintained cellulose type I crystals. The xanthate group content measured by the measuring method described in [Measurement of xanthate group content] described later was 1.73 mmol/g.

<Production Example 4>
[Sulfation treatment]
In the phosphorylation treatment, 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. A fine fibrous cellulose dispersion containing fine fibrous cellulose was obtained.

The sulfated pulp thus obtained was subjected to infrared absorption spectrum measurement using FT-IR. As a result, absorption due to sulfate groups was observed near 1220-1260 cm ⁻¹ , confirming that sulfate groups were added to the pulp. Moreover, it was confirmed by X-ray diffraction that the obtained fine fibrous cellulose maintained cellulose type I crystals. The sulfuric acid group content measured by the measurement method described in [Measurement of sulfur oxoacid group content] described later was 1.47 mmol/g.

<Production Example 5>
A fine fibrous cellulose dispersion containing maleated pulp and fine fibrous cellulose was obtained in the same manner as in Production Example 1, except that the following maleation treatment was performed instead of the phosphorylation treatment.

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

The infrared absorption spectrum of the maleated pulp thus obtained was measured using FT-IR. As a result, absorption due to carboxy groups was observed near 1580 and 1720 cm ⁻¹ , confirming that a carboxy group-containing substituent (maleic acid group) was added to the pulp. Moreover, it was confirmed by X-ray diffraction that the obtained fine fibrous cellulose maintained cellulose type I crystals. The carboxy group content measured by the measurement method described in [Measurement of carboxy group content] described later was 1.22 mmol/g.

[Measurement of Phosphorus Acid Group Amount]
In the measurement of the phosphate group content (phosphoric acid group or phosphite group content), ion-exchanged water is first added to the target fine fibrous cellulose to prepare a slurry having a solid content concentration of 0.2% by mass. bottom. The obtained slurry was treated with an ion-exchange resin and then titrated with an alkali for measurement.
The ion-exchange resin treatment was carried out by adding 1/10 by volume of a strongly acidic ion-exchange resin (Amberjet 1024; manufactured by Organo Co., Ltd., conditioned) to the fine fibrous cellulose-containing slurry and shaking for 1 hour. After that, the slurry was separated from the resin by pouring it onto a mesh with an opening of 90 μm.
In the titration with alkali, 0.1 N sodium hydroxide aqueous solution is added to the fine fibrous cellulose-containing slurry after treatment with the ion exchange resin, and 10 μL of 0.1 N sodium hydroxide solution is added every 5 seconds. was measured. The titration was performed while nitrogen gas was blown into the slurry from 15 minutes before the start of the titration. In this neutralization titration, two points where the increment (the differential value of the pH with respect to the amount of alkali added) are maximized are observed in the curve obtained by plotting the measured pH against the amount of alkali added. Among these, the maximum point of the increment obtained first when the alkali is first added is called the first end point, and the maximum point of the increment obtained next is called the second end point (Fig. 1). The amount of alkali required from the start of titration to the first end point is equal to the amount of first dissociated acid in the slurry used for titration. Also, the amount of alkali required from the start of titration to the second end point is equal to the total amount of dissociated acid in the slurry used for titration. The amount of alkali (mmol) required from the start of titration to the first end point was divided by the solid content (g) in the slurry to be titrated to obtain the amount of phosphate group (mmol/g).

[Measurement of sulfur oxoacid group content]
The amount of sulfur oxoacid groups was measured as follows. The obtained fibrous cellulose was subjected to wet incineration using perchloric acid and concentrated nitric acid, diluted at an appropriate ratio, and the amount of sulfur was measured by ICP emission spectrometry. The sulfur oxoacid group content (unit: mmol/g) was obtained by dividing this sulfur content by the absolute dry weight of the fibrous cellulose tested.

[Measurement of xanthate base amount]
The xanthate group content was measured by the Bredee method. Specifically, 40 mL of a saturated ammonium chloride solution was added to 1.5 parts by mass of fibrous cellulose (absolute dry mass), mixed well while crushing the sample with a glass rod, and left to stand for about 15 minutes. GS-25) and washed thoroughly with saturated ammonium chloride solution. The sample was placed in a 500 mL tall beaker together with the GFP filter paper, and 50 mL of 0.5 M sodium hydroxide solution (5°C) was added and stirred. After allowing to stand for 15 minutes, the phenolphthalein solution was added until the solution turned pink, and then 1.5 M acetic acid was added. After neutralization, 250 mL of distilled water was added and well stirred, and 10 mL of 1.5 M acetic acid and 10 mL of 0.05 mol/L iodine solution were added using a whole pipette. This solution was titrated with a 0.05 mol/L sodium thiosulfate solution. The amount of xanthate group was calculated from the titration amount of sodium thiosulfate and the absolute dry mass of fibrous cellulose according to the following formula.
Xanthate group amount (mmol/g) = (0.05 x 10 x 2-0.05 x sodium thiosulfate titer (mL))/1000/absolute dry mass of fibrous cellulose (g)

[Measurement of carboxy group content]
The amount of carboxy groups in the fine fibrous cellulose is determined by adding ion-exchanged water to a fine fibrous cellulose dispersion containing the target (maleated) fine fibrous cellulose to make the content 0.2% by mass, and performing ion exchange. It was measured by titration with alkali after treatment with resin.
The treatment with an ion-exchange resin is carried out by adding 1/10 by volume of a strongly acidic ion-exchange resin (Amberjet 1024; manufactured by Organo Co., Ltd., conditioned) to a slurry containing 0.2% by mass of fine fibrous cellulose, followed by shaking for 1 hour. After being treated, the slurry was separated from the resin by pouring it onto a mesh with an opening of 90 μm.
In addition, the titration using an alkali was performed by measuring the change in the pH value of the slurry while adding a 0.1N sodium hydroxide aqueous solution to the fibrous cellulose-containing slurry after treatment with the ion-exchange resin. . Observing the change in pH while adding an aqueous sodium hydroxide solution yields a titration curve as shown in FIG. As shown in FIG. 2, in this neutralization titration, in the curve plotting the measured pH against the amount of alkali added, there is one point where the increment (the differential value of pH with respect to the amount of alkali added) is maximum. Observed. The maximum point of this increment is called the first end point. Here, the region from the start of titration to the first end point in FIG. 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. Then, by dividing the alkali amount (mmol) required in the first region of the titration curve by the solid content (g) in the fine fibrous cellulose-containing slurry to be titrated, the introduction amount of the carboxy group (mmol/g ) was calculated.
The amount of carboxyl groups introduced (mmol/g) described above is the amount of substituents per 1 g of mass of fibrous cellulose (hereinafter referred to as the amount of carboxyl groups ( ^acid type)).

[Measurement of sulfur oxoacid group content]
The amount of sulfur oxoacid groups was calculated by freeze-drying a dispersion containing fibrous cellulose and measuring the amount of sulfur in a pulverized sample. Specifically, a dispersion liquid containing fibrous cellulose is freeze-dried, and the ground sample is decomposed under pressure and heat using nitric acid in a closed container, diluted as appropriate, and the sulfur content is measured by ICP-OES. bottom. The value calculated by dividing by the absolute dry mass of the fibrous cellulose tested was defined as the sulfur oxoacid group amount (unit: mmol/g) of the fibrous cellulose.

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

[Washing treatment of slurry after removal of substituents]
Ion-exchanged water was added to the obtained flocculate to obtain a slurry having a solid content concentration of about 1% by mass, and after stirring the slurry, the slurry was washed by repeating an operation of filtering and dehydrating. When the electric conductivity of the filtrate became 10 μS/cm or less, ion-exchanged water was added again to obtain a slurry of about 1% by mass, and the slurry was allowed to stand for 24 hours. From there, the operation of filtering and dehydrating was repeated, and the washing was finished when the electrical conductivity of the filtrate became 10 μS/cm or less. Ion-exchanged water was added to the obtained fine fibrous cellulose aggregate to obtain a slurry after removal of substituents. The solid content concentration of this slurry was 1.7% by mass.

[Uniform dispersion of slurry after removal of substituents]
Ion-exchanged water was added to the obtained slurry after removing the substituents to obtain a slurry having a solid content concentration of 1.0% by mass, and then a wet atomization apparatus (Starburst manufactured by Sugino Machine Co., Ltd.) was used at a pressure of 200 MPa. After three treatments, a dispersion of substituent-removed fine fibrous cellulose containing substituent-removed fine fibrous cellulose was obtained. The number average fiber width of the substituent-removed fine fibrous cellulose measured in [Measurement of Fiber Width] described later was 4 nm.

[Polyvalent metal ion replacement]
To 100 parts by mass of the resulting dispersion of substituent-removed fine fibrous cellulose, 100 parts by mass of a 0.036% by mass calcium chloride aqueous solution was added, stirred, and allowed to stand for 30 minutes. This operation confirmed the formation of fine fibrous cellulose aggregates. After that, washing was carried out by repeating dehydration by filtration and addition of ion-exchanged water. The end point of washing was defined as the time when the electric conductivity of the filtrate became 10 μS/cm or less. Deionized water was added to the resulting fine fibrous cellulose aggregates to obtain a dispersion having a solid concentration of 1.0% by mass, and 100 parts by mass of an aqueous calcium chloride solution having a concentration of 0.036% by mass was again added to 100 parts by mass of the dispersion. was added, and the same filtration, dehydration and washing operations were performed. Ion-exchanged water was added to the obtained fine fibrous cellulose aggregate to form a dispersion having a solid content concentration of 1.0% by mass, and the mixture was stirred with a disperser at 1000 rpm for 3 minutes to obtain polyvalent metal ion-containing fine fibrous cellulose. A dispersion was obtained.

[Uniform dispersion of polyvalent metal ion-containing fine fibrous cellulose dispersion]
The obtained polyvalent metal ion-containing fine fibrous cellulose dispersion was treated with a high-pressure homogenizer (Beryu-Mini, manufactured by Miryu Co., Ltd.) five times at a pressure of 100 MPa to obtain a polyvalent metal ion-containing fine fibrous cellulose dispersion. A cellulose dispersion was obtained. The number average fiber width of the fine fibrous cellulose measured in [Measurement of fiber width] described later was 4 nm.

[Preparation of sheet]
Acetoacetyl group-modified polyvinyl alcohol (manufactured by Mitsubishi Chemical Corporation, Gohsenex Z-200) was added to ion-exchanged water so as to make 12% by mass, and dissolved by stirring at 95° C. for 1 hour. A polyvinyl alcohol aqueous solution was obtained by the above procedure.
The polyvalent metal ion-containing fine fibrous cellulose dispersion and the polyvinyl alcohol aqueous solution were each diluted with ion-exchanged water so that the solid content concentration was 0.6% by mass. Next, 30 parts by mass of the diluted polyvinyl alcohol aqueous solution was mixed with 70 parts by mass of the diluted polyvalent metal ion-containing fine fibrous cellulose dispersion to obtain a mixture. Further, the mixed liquid was measured so that the finished basis weight of the sheet was 70 g/m ² and spread on a commercially available acrylic plate. A damming frame (inner dimensions: 250 mm×250 mm, height: 5 cm) was arranged on the acrylic plate so as to obtain a predetermined basis weight. After that, it was dried in a drier at 70° C. for 24 hours and separated from the acrylic plate to obtain a polyvalent metal ion-containing fine fibrous cellulose sheet. The thickness of the sheet was 50 µm.

<Example 2>
In [Replacement of polyvalent metal ions], fine particles containing polyvalent metal ions were prepared in the same manner as in Example 1, except that an aqueous magnesium sulfate solution with a concentration of 0.039% by mass was used in place of the aqueous solution of calcium chloride with a concentration of 0.036% by mass. A fibrous cellulose dispersion and a fine fibrous cellulose sheet containing polyvalent metal ions were obtained.

<Example 3>
In [Replacement of polyvalent metal ions], fine particles containing polyvalent metal ions were prepared in the same manner as in Example 1, except that an aqueous solution of zinc sulfate with a concentration of 0.052% by mass was used instead of the aqueous solution of calcium chloride with a concentration of 0.036% by mass. A fibrous cellulose dispersion and a fine fibrous cellulose sheet containing polyvalent metal ions were obtained.

<Example 4>
In [Replacement of polyvalent metal ions], fine particles containing polyvalent metal ions were prepared in the same manner as in Example 1, except that an aqueous nickel chloride solution with a concentration of 0.041% by mass was used in place of the aqueous solution of calcium chloride with a concentration of 0.036% by mass. A fibrous cellulose dispersion and a fine fibrous cellulose sheet containing polyvalent metal ions were obtained.

<Example 5>
In [Replacement of polyvalent metal ions], fine particles containing polyvalent metal ions were prepared in the same manner as in Example 1, except that an aqueous solution of copper sulfate with a concentration of 0.051% by mass was used instead of the aqueous solution of calcium chloride with a concentration of 0.036% by mass. A fibrous cellulose dispersion and a fine fibrous cellulose sheet containing polyvalent metal ions were obtained.

<Example 6>
In [Replacement of polyvalent metal ions], fine particles containing polyvalent metal ions were prepared in the same manner as in Example 1, except that an aqueous solution of cobalt chloride with a concentration of 0.042% by mass was used instead of the aqueous solution of calcium chloride with a concentration of 0.036% by mass. A fibrous cellulose dispersion and a fine fibrous cellulose sheet containing polyvalent metal ions were obtained.

<Comparative Example 1>
A fine fibrous cellulose sheet was obtained in the same manner as in Example 1 except that [substitution with polyvalent metal ions] and [uniform dispersion of polyvalent metal ion-containing fine fibrous cellulose dispersion] were not carried out. The fine fibrous cellulose dispersion of Comparative Example 1 is the fine fibrous cellulose dispersion obtained in Production Example 1.

<Comparative Example 2>
The same operation as in Example 1 was performed except that [uniform dispersion of polyvalent metal ion-containing fine fibrous cellulose dispersion] was changed to the following [dispersion of dispersion under alkali]. A fine fibrous cellulose dispersion and a fine fibrous cellulose sheet containing polyvalent metal ions were obtained.

[Dispersion treatment of dispersion liquid under alkali]
A 1N sodium hydroxide aqueous solution was added to the resulting polyvalent metal ion-containing fine fibrous cellulose dispersion with stirring to adjust the pH to 11.5. This dispersion was treated with a disperser at 4000 rpm for 3 minutes to obtain a polyvalent metal ion-containing fine fibrous cellulose dispersion.

<Comparative Example 3>
In [Uniform dispersion of polyvalent metal ion-containing fine fibrous cellulose dispersion], the same operation as in Example 1 was performed except that a rotary homogenizer was used instead of the high-pressure homogenizer. A dispersion and a fine fibrous cellulose sheet containing polyvalent metal ions were obtained.

<Comparative Example 4>
In [Uniform dispersion of polyvalent metal ion-containing fine fibrous cellulose dispersion], the same operation as in Example 1 was performed except that an ultrasonic homogenizer was used instead of the high-pressure homogenizer. A dispersion and a fine fibrous cellulose sheet containing polyvalent metal ions were obtained.

<Comparative Example 5>
A fine fibrous cellulose sheet was obtained in the same manner as in Example 1 except that [substitution with polyvalent metal ions] and [uniform dispersion of polyvalent metal ion-containing fine fibrous cellulose dispersion] were not carried out. Next, the resulting sheet containing fine fibrous cellulose was subjected to substitution with polyvalent metal ions by the following method to obtain a fine fibrous cellulose sheet containing polyvalent metal. The fine fibrous cellulose dispersion of Comparative Example 5 is the fine fibrous cellulose dispersion obtained in Production Example 1.

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

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

[Measurement of fiber width]
The fiber width of fibrous cellulose contained in the dispersion was measured by the following method. Each fibrous cellulose dispersion was diluted with water so that the cellulose concentration was 0.01% by mass or more and 0.1% by mass or less, and cast on a hydrophilized carbon film-coated grid. After drying, it was stained with uranyl acetate and observed with a transmission electron microscope (TEM, JEOL-2000EX, manufactured by JEOL Ltd.). At that time, an arbitrary image width axis was assumed in the obtained image, and the magnification was adjusted so that 20 or more fibers intersected the axis. After obtaining an observation image that satisfies this condition, two random axes were drawn vertically and horizontally from each image, and the fiber widths of the fibers intersecting the axes were visually read. Three non-overlapping observation images were taken for each dispersion, and the fiber width values of the fibers crossing each of the two axes were read (20 or more x 2 x 3 = 120 or more). The number average fiber width was calculated from the fiber width thus obtained.

[Measurement of haze of dispersion]
The haze of the dispersions obtained in Examples and Comparative Examples was measured by diluting each dispersion with ion-exchanged water to a concentration of 0.2% by mass, followed by a haze meter (manufactured by Murakami Color Research Laboratory Co., Ltd., HM-150) using a glass cell for liquids with an optical path length of 1 cm (manufactured by Fujiwara Seisakusho Co., Ltd., MG-40, reverse optical path), and measured according to JIS K 7136:2000. The zero point measurement was performed with ion-exchanged water in the same glass cell. In addition, the dispersion liquid to be measured was allowed to stand in an environment of 23° C. and 50% relative humidity for 24 hours before the measurement. The liquid temperature of the dispersion liquid at the time of measurement was 23°C.

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

[pH measurement]
The dispersions obtained in Examples and Comparative Examples were diluted with ion-exchanged water so that the solid content concentration was 0.4% by mass, and the pH was measured with a pH meter. The liquid temperature of the dispersion liquid at the time of measurement was 23°C.

[Electrical conductivity measurement]
The dispersions obtained in Examples and Comparative Examples were diluted with ion-exchanged water so that the solid content concentration was 0.4% by mass, and the electrical conductivity was measured by an electrical conductivity meter (CT-57101B, manufactured by Toa DKK Co., Ltd.). degree was measured. The liquid temperature of the dispersion liquid at the time of measurement was 23°C.

[Measurement of viscosity of dispersion]
The dispersions obtained in Examples and Comparative Examples were diluted with ion-exchanged water so that the solid content concentration was 0.4% by mass, and stirred at 1500 rpm for 5 minutes with a disperser. Then, the viscosity of the resulting dispersion was measured using a Brookfield viscometer (manufactured by BLOOKFIELD, analog viscometer T-LVT). During the measurement, the rotational speed was set to 3 rpm, and the viscosity value after 3 minutes from the start of the measurement was taken as the viscosity of the dispersion. In addition, the dispersion liquid to be measured was allowed to stand in an environment of 23° C. and 50% relative humidity for 24 hours before the measurement. The liquid temperature of the dispersion liquid at the time of measurement was 23°C.

[Monovalent metal element content]
The contents (%) of monovalent metal elements in the sheets obtained in Examples and Comparative Examples were measured using fluorescent X-ray analysis as follows.
First, for creating a calibration curve, filter papers with known element contents of sodium, calcium, magnesium, zinc, nickel, copper and cobalt are prepared, and the X-ray intensity of each filter paper is measured by fluorescent X-ray analysis. bottom. Next, a calibration curve was created based on the X-ray intensities thus obtained and the known contents of the analytical elements. Then, the X-ray intensity of the sheets to be measured (the sheets obtained in Examples and Comparative Examples) was measured by fluorescent X-ray analysis. From the obtained X-ray intensity and the calibration curve, the content (mmol) of each analytical element in the sheet to be measured was obtained, and the monovalent metal element content was calculated from the following formula.
Monovalent metal element content (%) = content of monovalent metal element (mmol) / content of all metal elements (mmol) x 100

[YI increase rate]
Based on JIS K 7373:2006, the yellowness of the sheets obtained in Examples and Comparative Examples before and after heating was measured using Color Cute i (manufactured by Suga Test Instruments Co., Ltd.). The degree of yellowness after heating was the degree of yellowness of the sheet heated at 160° C. for 6 hours. Moreover, the YI increase rate was calculated by the following method.
YI increase rate (%) = (yellowness of sheet after heating - yellowness of sheet before heating) / yellowness of sheet before heating x 100

[Sheet Haze]
The haze of the sheets obtained in Examples and Comparative Examples was measured according to JIS K 7136:2000 using a haze meter ("HM-150" manufactured by Murakami Color Research Laboratory).

[Total light transmittance of sheet]
The total light transmittance of the sheets obtained in Examples and Comparative Examples was measured according to JIS K 7361-1: 1997 using a haze meter ("HM-150" manufactured by Murakami Color Research Laboratory).

[Water absorption rate of sheet]
The sheets obtained in Examples and Comparative Examples were made into 50 mm square sheets, immersed in deionized water for 24 hours, and the water absorption of the sheets was obtained from the following formula. The absolute dry weight of the sheet is the weight measured after drying the sheet at 105° C. for 24 hours.
Water absorption rate of sheet (%) = weight of sheet after immersion in deionized water (g)/absolute dry weight of sheet (g) x 100

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

<Comparative Example 6>
A fine fibrous cellulose dispersion and a fine fibrous cellulose sheet were obtained in the same manner as in Example 7 except that the polyvalent metal ion substitution treatment was not performed.

<Example 8>
[ Substituent removal (low temperature heat treatment)] was performed. In [Replacement with polyvalent metal ion], the concentration of the calcium chloride aqueous solution was changed to 0.018% by mass. Other conditions were the same as in Example 1 to obtain a polyvalent metal ion-containing fine fibrous cellulose dispersion and a polyvalent metal ion-containing fine fibrous cellulose sheet.

[Substituent removal (low temperature heat treatment)]
The obtained fine fibrous cellulose dispersion was heated at a liquid temperature of 40° C. for 45 minutes until the xanthate group amount reached 0.08 mmol/g.

<Comparative Example 7>
A fine fibrous cellulose dispersion and a fine fibrous cellulose sheet were obtained in the same manner as in Example 8, except that the polyvalent metal ion substitution treatment was not performed. "

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

<Comparative Example 8>
A fine fibrous cellulose dispersion and a fine fibrous cellulose sheet were obtained in the same manner as in Example 9 except that the polyvalent metal ion substitution treatment was not performed.

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

<Comparative Example 9>
A fine fibrous cellulose dispersion and a fine fibrous cellulose sheet were obtained in the same manner as in Example 10 except that the polyvalent metal ion substitution treatment was not performed.

The sheets obtained in Examples had high transparency and suppressed yellowing. On the other hand, the sheet obtained in the comparative example did not achieve both transparency and suppression of yellowing. In addition, the sheets obtained in Examples had a suppressed water absorption rate.

Claims (12)

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

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