JP7548007B2 - Sheet and method for manufacturing the sheet - Google Patents

Description

The present invention relates to a sheet and a method for manufacturing the sheet.

In recent years, materials using renewable natural fibers have been attracting attention due to the need to replace petroleum resources and the growing environmental awareness. Among natural fibers, fibrous cellulose with a fiber width of 10 μm to 50 μm, particularly fibrous cellulose (pulp) derived from wood, has been widely used mainly for paper products.
As the fibrous cellulose, fine fibrous cellulose having a fiber width of 1,000 nm or less is also known. In addition, sheets made of such fine fibrous cellulose, composite sheets containing fine fibrous cellulose and resin, and molded articles have been developed. It is known that in sheets and molded articles containing fine fibrous cellulose, the number of contact points between fibers is significantly increased, and therefore the tensile strength is greatly improved.
Patent Document 1 discloses a sheet containing fibrous cellulose having a fiber width of 1,000 nm or less and a polymer containing units derived from a polyol and units derived from a nitrogen-containing compound, with the aim of providing a fine fibrous cellulose-containing sheet that combines high transparency and water resistance.
Furthermore, Patent Document 2 describes a sheet that contains fibrous cellulose having a fiber width of 1,000 nm or less and a cationic resin, and has a haze of 6% or less, with the aim of obtaining a sheet that is excellent in transparency and water resistance.

Patent Document 3 describes a sheet that is made by blending fine fibrous cellulose having a fiber width of 1,000 nm or less, a resin derived from at least one selected from an aqueous resin emulsion and an aqueous resin dispersion, and a wet strength agent, in which the blending amount of the fine fibrous cellulose in the solid content of the sheet is 50 mass % or less, the water absorption rate of the sheet is 50% or less, the linear thermal expansion coefficient at 100 to 150°C is 60 ppm/K or less, and the haze is 10% or less.

JP 2018-9116 A International Publication No. 2017/138589 JP 2020-193258 A

The sheets described in Patent Documents 1 and 2 have an excellent tensile modulus of elasticity, but have a problem of low water resistance.
Furthermore, although the sheet described in Patent Document 3 has excellent water resistance, there is a need to improve the tensile modulus of elasticity.
An object of the present invention is to provide a sheet having a low linear thermal expansion coefficient, high transparency with suppressed yellowing, and high tensile modulus, and a method for producing the same.

The present inventors have found that the above problems can be solved by providing a sheet obtained by UV curing a sheet composition containing an ultraviolet-polymerizable compound and fine fibrous cellulose.
That is, the present invention relates to the following <1> to <12>.
<1> A sheet obtained by ultraviolet curing a sheet composition containing an ultraviolet-polymerizable compound and fine fibrous cellulose having a fiber width of 1,000 nm or less, the sheet having a linear thermal expansion coefficient of 30 ppm/K or less at 100° C. or more and 150° C. or less and a yellow index (YI value) of 5 or less.
<2> The sheet according to <1>, wherein the content of the fine fibrous cellulose relative to the total amount of the ultraviolet-polymerizable compound and the fine fibrous cellulose is 20% by mass or more and 80% by mass or less.
<3> The sheet according to <1> or <2>, wherein the sheet has a tensile modulus of 3 GPa or more.
<4> The sheet according to any one of <1> to <3>, wherein the sheet has a total light transmittance of 70% or more.
<5> The sheet according to any one of <1> to <4>, wherein the haze of the sheet is 10% or less.
<6> The sheet according to any one of <1> to <5>, wherein the sheet composition contains a polymerization initiator.
<7> The sheet according to any one of <1> to <6>, wherein the ultraviolet polymerizable compound is a radical polymerizable compound.
<8> The sheet according to any one of <1> to <7>, wherein the fine fibrous cellulose has an average fiber width of 2 nm or more and 10 nm or less.
<9> The sheet according to any one of <1> to <8>, wherein the fine fibrous cellulose has an anionic group.
<10> The sheet according to any one of <1> to <9>, wherein the fine fibrous cellulose has a phosphorus oxoacid group or a group derived from a phosphorus oxoacid group.
<11> The sheet according to any one of <1> to <10>, wherein the polymerizable compound contains at least one selected from the group consisting of a polyfunctional urethane acrylate compound, a polyfunctional acrylamide monomer, and a polyfunctional acrylic acid ester compound.
<12> A method for producing a sheet, comprising the steps of preparing a sheet composition containing an ultraviolet-polymerizable compound and fine fibrous cellulose having a fiber width of 1,000 nm or less, applying and drying the sheet composition to form an uncured sheet composition, and irradiating the uncured sheet composition with ultraviolet light to cure it with ultraviolet light, wherein the obtained sheet has a linear thermal expansion coefficient of 30 ppm/K or less at 100° C. or more and a yellow index (YI value) of 5 or less.

The present invention provides a sheet and a method for producing the same that has a low linear thermal expansion coefficient, high transparency with reduced yellowing, and high tensile modulus.

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

[Sheet]
The sheet of the present embodiment is a sheet obtained by ultraviolet curing a sheet composition containing an ultraviolet-polymerizable compound and fine fibrous cellulose having a fiber width of 1,000 nm or less (hereinafter simply referred to as fine fibrous cellulose), and has a linear thermal expansion coefficient of 30 ppm/K or less at 100° C. or more and 150° C. or less and a yellow index (YI value) of 5 or less.
According to this embodiment, a sheet having high transparency, excellent water resistance, a high tensile modulus, and a low linear thermal expansion coefficient can be obtained.
Although the detailed reasons why the above-mentioned effects are obtained are unclear, some of them are thought to be as follows. In this embodiment, since the sheet is obtained by polymerizing a sheet composition containing an ultraviolet-polymerizable compound and fine fibrous cellulose, a crosslinked structure is formed by polymerization. In addition, the sheet containing fine fibrous cellulose has extremely high transparency, and it is thought that the fine fibrous cellulose functions as a filler, and thus, in combination with the polymerization of the ultraviolet-polymerizable compound, a sheet having a low linear thermal expansion coefficient and a high tensile modulus is obtained.
The present invention will now be described in further detail.

The terms used in this specification have the following meanings:
A numerical range expressed by "-" means a range including both ends of the numerical range.
"(Meth)acrylic" is a general term for "acrylic" and "methacrylic", and means acrylic and methacrylic individually. The same applies to "(meth)acryloyl", "(meth)acryloyloxy", "(meth)acrylic acid", "(meth)acrylate", and "(meth)acrylamide".

<Sheet composition>
The sheet of the present embodiment is produced by curing a sheet composition with ultraviolet light, and the sheet composition contains an ultraviolet-polymerizable compound and fine fibrous cellulose.

<Ultraviolet polymerizable compound>
The ultraviolet-polymerizable compound has a property of being polymerized by the action of reactive active species such as radicals and cations generated from a polymerization initiator by irradiation with ultraviolet light. Among these, the ultraviolet-polymerizable compound has a high degree of curability and is From the viewpoint of obtaining a sheet having transparency, excellent water resistance, a high tensile elastic modulus, and a low linear thermal expansion coefficient, it is preferable that the compound is a radical polymerizable compound, and a compound having an ethylenically unsaturated group is preferable. It is preferable that there is.
The ultraviolet-polymerizable compound includes low molecular weight so-called oligomers and monomers, as well as compounds generally called polymers.
When the ultraviolet-polymerizable compound is a polymer compound, examples of the structure serving as the backbone include polyalkylene glycols (polyethylene glycol, polypropylene glycol, etc.), cellulose derivatives (hydroxyethyl cellulose, carboxyethyl cellulose, carboxymethyl cellulose, etc.), casein, Dextrin, polyvinyl alcohol, modified polyvinyl alcohol (acetoacetylated polyvinyl alcohol, ethylene-vinyl alcohol copolymer, ethylene oxide-modified polyvinyl alcohol, etc.), polyalkylene oxide (polyethylene oxide, etc.), polyvinylpyrrolidone, polyvinyl methyl ether, polyacrylates , polyacrylamide, acrylic acid ester copolymer, urethane copolymer, polyhydroxystyrene copolymer, polyalkyleneimine, etc. Among the above, polyethylene glycol, polyethylene oxide, polyvinyl alcohol, modified polyvinyl alcohol, etc. It is particularly preferable to use a structure containing polyacrylamide, an acrylic acid ester copolymer, a urethane copolymer, or a polyalkyleneimine.
The above skeleton is preferred because it has excellent affinity with fine fibrous cellulose.
The ultraviolet-polymerizable compound preferably has a radical-polymerizable group (for example, an ethylenically unsaturated group) or a small heterocyclic group in addition to the above-mentioned backbone structure.

Examples of the small heterocyclic group include an epoxy group and an oxetane group, and it is particularly preferable to use the group in combination with a cationic photopolymerization initiator as the polymerization initiator.
Examples of the ethylenically unsaturated group include an acryloyl group, a methacryloyl group, an acrylamide group, and a methacrylamide group. It is particularly preferable to use a photoradical polymerization initiator in combination with the ethylenically unsaturated group.

As the ultraviolet-polymerizable compound, an oligomer or a monomer may be used.
In particular, the oligomer or monomer is preferably an oligomer or monomer having a polymerizable unsaturated group (hereinafter, these are also collectively referred to as "polymerizable unsaturated compound").
The polymerizable unsaturated compound is an unsaturated compound that is polymerized by irradiating ultraviolet light in the presence of a polymerization initiator. Such a polymerizable unsaturated compound is not particularly limited, but for example, a polyfunctional (meth)acrylic acid ester having two or more functional groups is preferable because it has good polymerizability and improves the strength of the sheet formed.

Examples of the bifunctional (meth)acrylic acid esters include ethylene glycol di(meth)acrylate, dipropylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol dimethacrylate, tetraethylene glycol diacrylate, tetraethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, polyethylene glycol di(meth)acrylate, and the like. Examples of commercially available products include, by trade name, Aronix (registered trademark) M-220 and M-240 (both manufactured by Toagosei Co., Ltd.), KAYARAD (registered trademark) HX-220 and R-604 (both manufactured by Nippon Kayaku Co., Ltd.), Viscoat 260 (manufactured by Osaka Organic Chemical Industry Co., Ltd.), Light Acrylate 1,9-NDA (manufactured by Kyoeisha Chemical Co., Ltd.), NK Ester 2G, 3G, 4G, 9G, 14G, 23G, A200, A400, and A600 (all manufactured by Shin-Nakamura Chemical Co., Ltd.).

Examples of the trifunctional or higher (meth)acrylic acid esters include trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, and dipentaerythritol hexa(meth)acrylate;
A mixture of dipentaerythritol penta(meth)acrylate and dipentaerythritol hexa(meth)acrylate;
Ethylene oxide modified dipentaerythritol hexa(meth)acrylate, tri(2-(meth)acryloyloxyethyl)phosphate, succinic acid modified pentaerythritol tri(meth)acrylate, succinic acid modified dipentaerythritol penta(meth)acrylate, tripentaerythritol hepta(meth)acrylate, tripentaerythritol octa(meth)acrylate, isocyanuric acid EO modified diacrylate, isocyanuric acid EO modified triacrylate;
A mixture of an isocyanuric acid EO-modified diacrylate and an isocyanuric acid EO-modified triacrylate;
Examples include glycerin di(meth)acrylate, glycerin tri(meth)acrylate, glycerin carbonate acrylate, ethoxylated glycerin (meth)acrylate, and ethoxylated glycerin tri(meth)acrylate.

Further, examples of polyfunctional (meth)acrylic acid esters include polyfunctional urethane acrylate compounds obtained by reacting a compound having a linear alkylene group and an alicyclic structure and having two or more isocyanate groups with a compound having one or more hydroxyl groups and three, four or five (meth)acryloyloxy groups in the molecule.
Commercially available products of the above-mentioned trifunctional or higher (meth)acrylic acid esters include, by trade name, for example, ARONIX (registered trademark) M-309, M-400, M-405, and M-450 (all manufactured by Toagosei Co., Ltd.), KAYARAD (registered trademark) TMPTA, KAYARAD DPHA, KAYARAD DPCA-20, KAYARAD DPCA-30, KAYARAD DPCA-60, and KAYARAD DPEA-12 (all manufactured by Nippon Kayaku Co., Ltd.), and Viscoat 295 (manufactured by Osaka Organic Chemical Industry Co., Ltd.). Commercially available products containing multifunctional urethane acrylate compounds include UCECOAT (registered trademark) 7571, 7655, 7770, 7773, 7788, and 7850 (all manufactured by Daicel Chemical Industries, Ltd.). Examples of commercially available products containing polyfunctional glycerin acrylate compounds include ARONIX (registered trademark) M-930 (manufactured by Toagosei Co., Ltd.), NK Ester A-GLY-3E, A-GLY-6E, A-GLY-9E, A-GLY-20E, A-GLY-3P, A-GLY-6P, A-GLY-9P, GLY-3E, GLY-6E, GLY-9E, and GLY-20E (all manufactured by Shin-Nakamura Chemical Co., Ltd.).

Moreover, as the polyfunctional (meth)acrylamide monomer, a polyfunctional acrylamide monomer is preferable, and examples thereof include N,N'-{[(2-acrylamide-2-[(3-acrylamidopropoxy)methyl]propane-1,3-diyl)bis(oxy)]bis(propane-1,3-diyl)}diacrylamide, N,N',N"-triacryloyldiethylenetriamine, N,N'-diacryloyl-4,7,10-trioxa-1,13-tridecanediamine, and N,N'-diacryloylethylenediamine.
Commercially available products of the polyfunctional acrylamide monomer include FOM-03006, FOM-03007, FOM-03008 (all manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), E1086 (manufactured by Tokyo Chemical Industry Co., Ltd.), and the like.

Among these, in particular, 1,9-nonanediol dimethacrylate, polyethylene glycol di(meth)acrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, dipentaerythritol pentaacrylate, and dipentaerythritol hexaacrylate;
A mixture of dipentaerythritol pentaacrylate and dipentaerythritol hexaacrylate;
A mixture of tripentaerythritol hepta(meth)acrylate and tripentaerythritol octa(meth)acrylate;
Ethylene oxide-modified dipentaerythritol hexaacrylate, polyfunctional urethane acrylate compounds, succinic acid-modified pentaerythritol triacrylate, succinic acid-modified dipentaerythritol pentaacrylate;
Ethoxylated glycerin (meth)acrylate, ethoxylated glycerin tri(meth)acrylate;
Polyfunctional acrylamide monomers such as N,N'-{[(2-acrylamido-2-[(3-acrylamidopropoxy)methyl]propane-1,3-diyl)bis(oxy)]bis(propane-1,3-diyl)}diacrylamide, N,N',N"-triacryloyldiethylenetriamine, N,N'-diacryloyl-4,7,10-trioxa-1,13-tridecanediamine, and N,N'-diacryloylethylenediamine; and commercially available products containing these are preferred, with polyfunctional urethane acrylate compounds, polyfunctional acrylamide monomers, and polyfunctional acrylic acid ester compounds being more preferred. Commercially available products containing these may also be used.
The above-mentioned polyfunctional polymerizable unsaturated compounds may be used alone or in combination of two or more kinds.

The molecular weight of the ultraviolet polymerizable compound (weight average molecular weight when it has a molecular weight distribution) is not particularly limited, but may be at least 200, preferably at least 300, and more preferably at least 500. In addition, the molecular weight (weight average molecular weight) of the ultraviolet polymerizable compound is preferably at most 100,000, more preferably at most 50,000, and even more preferably at most 30,000.
The weight average molecular weight can be measured, for example, by using GPC.

From the viewpoint of obtaining a sheet having high transparency, excellent water resistance, a high tensile modulus, and a low linear thermal expansion coefficient, the content of the ultraviolet-polymerizable compound in the solid content of the sheet composition is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, and is preferably 80% by mass or less, more preferably 75% by mass or less, even more preferably 70% by mass or less.

[Photopolymerization initiator]
In the present invention, the sheet composition preferably contains a photopolymerization initiator.
A photopolymerization initiator (hereinafter, simply referred to as a "polymerization initiator") is a component that generates active species capable of initiating polymerization of a polymerizable compound in response to ultraviolet light. Examples of photopolymerization initiators include photoradical polymerization initiators and photocationic polymerization initiators, and the photopolymerization initiator may be selected according to the ultraviolet-polymerizable compound. As the photopolymerization initiator, for example, a photopolymerization initiator that generates the most radicals by ultraviolet light, particularly around 365 nm, is preferable.
Examples of the photoradical polymerization initiator include O-acyloxime compounds, acetophenone compounds, biimidazole compounds, acylphosphine oxide compounds, etc. These compounds can be used alone or in combination of two or more.

Examples of O-acyloxime compounds include 1-[4-(phenylthio)-2-(O-benzoyloxime)], 1,2-octanedione-1-[4-(phenylthio)-2-(O-benzoyloxime)], ethanone-1-[9-ethyl-6-(2-methylbenzoyl)-9H-carbazol-3-yl]-1-(O-acetyloxime), 1-[9-ethyl-6-benzoyl-9H-carbazol-3-yl]-octan-1-one oxime-O-acetate, 1-[9-ethyl-6-(2-methylbenzoyl)-9H-carbazol-3-yl]-ethane-1-one oxime-O-benzoate, 1-[9-n-butyl-6-(2-ethylbenzoyl)-9H-carbazol-3-yl]-ethane-1-one oxime-O-acetate, oxime), ethanone-1-[9-ethyl-6-(2-methyl-4-tetrahydrofuranylbenzoyl)-9H-carbazol-3-yl]-1-(O-acetyloxime), ethanone-1-[9-ethyl-6-(2-methyl-4-tetrahydropyranylbenzoyl)-9H-carbazol-3-yl]-1-(O-acetyloxime), ethanone-1-[9-ethyl-6-(2-methyl-5-tetrahydrofuranylbenzoyl)-9H-carbazol-3-yl]-1-(O-acetyloxime), ethanone-1-[9-ethyl-6-{2-methyl-4-(2,2-dimethyl-1,3-dioxolanyl)methoxybenzoyl}-9H-carbazol-3-yl]-1-(O-acetyloxime), etc.

Examples of the acetophenone compound include an α-aminoketone compound and an α-hydroxyketone compound.
Examples of the α-aminoketone compound include 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one, 2-dimethylamino-2-(4-methylbenzyl)-1-(4-morpholin-4-yl-phenyl)-butan-1-one, and 2-methyl-1-(4-methylthiophenyl)-2-morpholinopropan-1-one.
Examples of the α-hydroxyketone compound include 1-phenyl-2-hydroxy-2-methylpropan-1-one, 1-(4-i-propylphenyl)-2-hydroxy-2-methylpropan-1-one, 4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-propyl)ketone, 1-hydroxycyclohexylphenyl ketone, 2-hydroxy-1-{4-[2-(2-hydroxyethoxy)ethoxy]phenyl}-2-methylpropan-1-one, and 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone.

Examples of the biimidazole compound include 2,2'-bis(2-chlorophenyl)-4,4',5,5'-tetraphenyl-1,2'-biimidazole, 2,2'-bis(2,4-dichlorophenyl)-4,4',5,5'-tetraphenyl-1,2'-biimidazole, and 2,2'-bis(2,4,6-trichlorophenyl)-4,4',5,5'-tetraphenyl-1,2'-biimidazole.
Examples of the acylphosphine oxide compound include 2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, and lithium phenyl(2,4,6-trimethylbenzoyl)phosphineate.

The photocationic polymerization initiator is not particularly limited as long as it generates an acid by light, and examples thereof include B(C ₆ F ₅ ) ₄ ⁻ , PF ₆ ⁻ , AsF ₆ ⁻ , SbF ₆ ⁻ , and CF ₃ SO ₃ ⁻ salts of aromatic onium compounds such as diazonium, ammonium, iodonium, sulfonium, and phosphonium; sulfonates that generate sulfonic acid; halides that photogenerate hydrogen halide; and iron allene complexes.

The content of the photopolymerization initiator in the sheet composition is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and preferably 40 parts by mass or less, more preferably 20 parts by mass or less, and even more preferably 10 parts by mass or less, per 100 parts by mass of the ultraviolet-polymerizable compound. By using the photopolymerization initiator in this range, excellent polymerizability can be imparted. Furthermore, by setting the content of the photopolymerization initiator in this range, the sheet composition of this embodiment can exhibit excellent polymerizability (curability) even at a low exposure dose.

The ultraviolet-polymerizable compound and the photopolymerization initiator are preferably dissolved or dispersed in at least one of water, alcohol, or a mixed solvent thereof, and more preferably dissolved in the solvent, which is preferable because it facilitates the coating step described below.
In addition, the phrase "the ultraviolet polymerizable compound and the photopolymerization initiator are soluble in at least one of water, alcohol, or a mixed solvent thereof" means that 1 g or more of the ultraviolet polymerizable compound and the photopolymerization initiator are soluble in 100 g of water, an alcohol having 1 to 6 carbon atoms (preferably an alcohol having 1 to 4 carbon atoms, more preferably an alcohol having 1 to 3 carbon atoms), or a mixed solvent of water and an alcohol having 1 to 6 carbon atoms at 25° C. and atmospheric pressure.
In addition, it is preferable that the ultraviolet-polymerizable compound and the photopolymerization initiator are dissolved or dispersed in at least one of water, alcohol, or a mixed solvent thereof, since this has a high affinity with the fine fibrous cellulose and allows the preparation of a uniform coating liquid.

<Fine fibrous cellulose>
In the present invention, the sheet composition contains fine fibrous cellulose.
The fine fibrous cellulose is a fibrous cellulose having a fiber width of 1,000 nm or less. The fiber width of the fibrous cellulose can be measured, for example, by observation using an electron microscope.
The fiber width of the fine fibrous cellulose is 1,000 nm or less. The fiber width of the fine fibrous cellulose is, for example, preferably 2 nm or more and 1,000 nm or less, more preferably 2 nm or more and 100 nm or less, even more preferably 2 nm or more and 50 nm or less, and particularly preferably 2 nm or more and 10 nm or less. By making the fiber width of the fine fibrous cellulose 2 nm or more, it is possible to suppress dissolution of the cellulose molecules in water, and to more easily realize the effect of improving the strength, rigidity, and dimensional stability of the fine fibrous cellulose.

The average fiber width of the fine fibrous cellulose is, for example, 1,000 nm or less. The average fiber width of the fine fibrous cellulose is preferably 2 nm or more and 1,000 nm or less, more preferably 2 nm or more and 100 nm or less, even more preferably 2 nm or more and 50 nm or less, and particularly preferably 2 nm or more and 10 nm or less. By making the average fiber width of the fine fibrous cellulose 2 nm or more, it is possible to suppress the dissolution of the cellulose molecules in water, and to more easily realize the effect of improving the strength, rigidity, and dimensional stability of the fine fibrous cellulose. The fine fibrous cellulose is, for example, single-fiber cellulose.

The average fiber width of fine 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 the suspension is cast on a hydrophilized carbon film-coated grid to prepare a sample for TEM observation. When wide fibers are included, an SEM image of the surface cast on glass may be observed. Next, observation is performed using an electron microscope image at a magnification of 1,000x, 5,000x, 10,000x, or 50,000x depending on the width of the fiber to be observed. However, the sample, observation conditions, and magnification are adjusted to satisfy the following conditions.
(1) A straight line X is drawn at an arbitrary location within the observed image, and 20 or more fibers intersect with the straight line X.
(2) Within the same image, a straight line Y is drawn that intersects the straight line perpendicularly, and 20 or more fibers intersect the straight line Y.

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

The fiber length of the fine fibrous cellulose is not particularly limited, but is preferably 0.1 μm or more and 1,000 μm or less, more preferably 0.1 μm or more and 800 μm or less, and even more preferably 0.1 μm or more and 600 μm or less. By setting the fiber length within the above range, destruction of the crystalline regions of the fine fibrous cellulose can be suppressed. It is also possible to set the slurry viscosity of the fine fibrous cellulose to an appropriate range. The fiber length of the fine fibrous cellulose can be determined, for example, by image analysis using a TEM, SEM, or AFM.

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

The axial ratio (fiber length/fiber width) of the fine fibrous cellulose is not particularly limited, but is preferably, for example, 20 to 10,000, and more preferably 50 to 1,000. By setting the axial ratio to the above lower limit or more, it is easy to form a sheet containing the fine fibrous cellulose. In addition, sufficient viscosity is easily obtained when a solvent dispersion is produced. By setting the axial ratio to the above upper limit or less, it is preferable in that, for example, when the fine fibrous cellulose is treated as an aqueous dispersion, handling such as dilution is easy.

The fine fibrous cellulose in this embodiment has, for example, at least one of an ionic group and a nonionic group. From the viewpoint of improving the dispersibility of the fibers in the dispersion medium and increasing the defibration efficiency in the defibration treatment, it is more preferable that the fine fibrous cellulose has an ionic group. The ionic group may include, for example, either one or both of an anionic group and a cationic group. Furthermore, the nonionic group may include, for example, an alkyl group and an acyl group. In this embodiment, it is particularly preferable that the ionic group has an anionic group.
The fine fibrous cellulose does not need to be subjected to a treatment for introducing an ionic group.

Examples of anionic groups as ionic groups include phosphorus oxoacid groups or substituents derived from phosphorus oxoacid groups (sometimes simply referred to as phosphorus oxoacid groups), carboxy groups or substituents derived from carboxy groups (sometimes simply referred to as carboxy groups), sulfur oxoacid groups or substituents derived from sulfur oxoacid groups (sometimes simply referred to as sulfur oxoacid groups), xanthate groups, phosphonic groups, phosphine groups, sulfonic groups, carboxyalkyl groups, etc. Among them, the anionic group is preferably at least one selected from the group consisting of phosphorus oxoacid groups, substituents derived from phosphorus oxoacid groups, carboxy groups, sulfur oxoacid groups, substituents derived from sulfur oxoacid groups, carboxymethyl groups, and sulfonic groups, more preferably at least one selected from the group consisting of phosphorus oxoacid groups, substituents derived from phosphorus oxoacid groups, carboxy groups, sulfur oxoacid groups, and substituents derived from sulfur oxoacid groups, and particularly preferably a phosphorus oxoacid group. By introducing a phosphorus oxoacid group as an anionic group, the dispersibility of the fibrous cellulose can be further increased, for example, even under alkaline or acidic conditions, making it easier to obtain a sheet with high strength and high transparency.
Examples of the cationic group as the ionic group include an ammonium group, a phosphonium group, a sulfonium group, etc. Among these, the cationic group is preferably an ammonium group.

The phosphorus oxoacid group or the substituent derived from the phosphorus oxoacid group is, for example, a substituent represented by the following formula (1). A plurality of types of the substituent represented by the following formula (1) may be introduced into each fibrous cellulose. In this case, the plurality of 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 (wherein a=b×m). At least one of the n α and α' is ^O- , and the rest are R or OR. All of the α and α' may be ^O- . The n α may be the same or different. ^{β b+} is a monovalent or higher cation made of an organic or inorganic substance.

Each R is a hydrogen atom, a saturated linear hydrocarbon group, a saturated branched hydrocarbon group, a saturated cyclic hydrocarbon group, an unsaturated linear hydrocarbon group, an unsaturated branched hydrocarbon group, an unsaturated cyclic hydrocarbon group, an aromatic group, or a group derived from any of these. In formula (1), n is preferably 1.

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

In addition, the derivative group in R may be, but is not limited to, a functional group in which at least one selected from functional groups such as a carboxy group, a carboxylate group (-COO-), a hydroxy group, an amino group, and an ammonium group is added to or substituted for the main chain or side chain of the above-mentioned various hydrocarbon groups. In addition, the number of carbon atoms constituting the main chain of R is not particularly limited, but is preferably 20 or less, and more preferably 10 or less. By setting the number of carbon atoms constituting the main chain of R within the above range, the molecular weight of the phosphorus oxo acid group can be set to an appropriate range, which facilitates penetration into the fiber raw material and increases the yield of fine cellulose fiber. Note that when there are multiple Rs in formula (1) or when multiple types of substituents represented by the above formula (1) are introduced into the fibrous cellulose, the multiple Rs present may be the same or different.

β ^b+ is a monovalent or higher cation made of an organic or inorganic substance. As the monovalent or higher cation made of an organic substance, an organic onium ion can be mentioned. As the organic onium ion, for example, an organic ammonium ion or an organic phosphonium ion can be mentioned. As the organic ammonium ion, for example, an aliphatic ammonium ion or an aromatic ammonium ion can be mentioned, and as the organic phosphonium ion, for example, an aliphatic phosphonium ion or an aromatic phosphonium ion can be mentioned. As the monovalent or higher cation made of an inorganic substance, an ion of an alkali metal such as sodium, potassium, or lithium, an ion of a divalent metal such as calcium or magnesium, a hydrogen ion, an ammonium ion, etc. can be mentioned. In addition, when there are multiple β ^b+ in the formula (1) or when multiple types of substituents represented by the above formula (1) are introduced into the fibrous cellulose, the multiple β ^b + may be the same or different. As the monovalent or higher cation made of an organic or inorganic substance, sodium or potassium ions are preferable because they are less likely to yellow when a fiber raw material containing β ^b+ is heated and are easy to use industrially, but are not particularly limited.

More specifically, examples _of the phosphorus oxoacid group or a substituent derived from a phosphorus oxoacid group include a phosphate group ( _-PO3H2 ), a salt of a phosphate group, a phosphorous acid group ( _phosphonic acid group) ( _-PO2H2 ), and a salt of a phosphite group (phosphonic acid group). In addition, the phosphorus oxoacid group or a substituent derived from a phosphorus oxoacid group may be a group in which a phosphate group is condensed (e.g., a pyrophosphate group), a group in which a phosphonic acid is condensed (e.g., a polyphosphonic acid group), a phosphate ester group (e.g., a monomethyl phosphate group, a polyoxyethylene alkyl phosphate group), an alkyl phosphonic acid group (e.g., a methylphosphonic acid group), etc.

<Sulfur oxoacids>
The sulfur oxoacid group (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 substituents represented by the following formula (2) may be introduced into each fibrous cellulose. In this case, the plurality of 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 any number (wherein 1=b×m). When n is 2 or more, the multiple p's may be the same number or different numbers. In the above structural formula, β ^b+ is a monovalent or higher cation made of an organic or inorganic substance. Examples of the monovalent or higher cation made of an organic substance include organic onium ions. Examples of the organic onium ions include organic ammonium ions and organic phosphonium ions. Examples of the organic ammonium ions include aliphatic ammonium ions and aromatic ammonium ions, and examples of the organic phosphonium ions include aliphatic phosphonium ions and aromatic phosphonium ions. Examples of the monovalent or higher cation made of an inorganic substance include ions of alkali metals such as sodium, potassium, or lithium, ions of divalent metals such as calcium or magnesium, hydrogen ions, ammonium ions, etc. In addition, when multiple types of substituents represented by the above formula (2) are introduced into the fibrous cellulose, the multiple β ^b+ present may be the same or different. As the organic or inorganic cation having a valence of one or more, sodium or potassium ions are preferred, which are less likely to yellow when the fiber raw material containing β ^b+ is heated and are easy to use industrially, but there is no particular limitation.

The amount of ionic groups introduced into the fine fibrous cellulose is preferably 0.10 mmol/g or more per 1 g (mass) of fine fibrous cellulose, more preferably 0.20 mmol/g or more, even more preferably 0.50 mmol/g or more, and particularly preferably 1.00 mmol/g or more. The amount of ionic groups introduced into the fine fibrous cellulose is preferably 5.20 mmol/g or less per 1 g (mass) of fibrous cellulose, more preferably 3.65 mmol/g or less, even more preferably 3.50 mmol/g or less, and even more preferably 3.00 mmol/g or less. By setting the amount of ionic groups introduced within the above range, it is possible to easily refine the fiber raw material and to increase the stability of the fine fibrous cellulose. In addition, by setting the amount of ionic groups introduced within the above range, it is possible to exhibit good properties in various applications such as a thickener for fine fibrous cellulose.
Here, the denominator in the unit mmol/g indicates the mass of the fine fibrous cellulose when the counter ion of the ionic group is a hydrogen ion (H ⁺ ).

The amount of ionic groups introduced into the fibrous cellulose can be measured, for example, by neutralization titration, in which an alkali such as an aqueous sodium hydroxide solution is added to the obtained slurry containing the fibrous cellulose to determine the change in pH, thereby measuring the amount introduced.
FIG. 1 is a graph showing the relationship between the amount of NaOH dropped onto fibrous cellulose having phosphorus oxoacid groups and pH.

1 is a graph showing the relationship between the amount of NaOH dropped onto a slurry containing fibrous cellulose having phosphorus oxo acid groups and pH. The amount of phosphorus oxo acid groups introduced into the fibrous cellulose is measured, for example, as follows.
First, a slurry containing fibrous cellulose is treated with a strongly acidic ion exchange resin. If necessary, the measurement target may be subjected to a defibration treatment similar to the defibration treatment step described below prior to the treatment with the strongly acidic ion exchange resin.
Next, the change in pH is observed while adding the aqueous sodium hydroxide solution, and a titration curve as shown in the upper part of Figure 1 is obtained. In the titration curve shown in the upper part of Figure 1, the measured pH is plotted against the amount of alkali added, and in the titration curve shown in the lower part of Figure 1, the increment (differential value) (1/mmol) of pH against the amount of alkali added is plotted. In this neutralization titration, two points are confirmed in the curve plotting the measured pH against the amount of alkali added, where the increment (differential value of pH against the amount of alkali added) is maximum. Of these, the first maximum point of the increment obtained after starting to add the alkali is called the first endpoint, and the second maximum point of the increment obtained next is called the second endpoint. The amount of alkali required from the start of titration to the first end point is equal to the amount of the first dissociated acid of the fibrous cellulose contained in the slurry used for titration, the amount of alkali required from the first end point to the second end point is equal to the amount of the second dissociated acid of the fibrous cellulose contained in the slurry used for titration, and the amount of alkali required from the start of titration to the second end point is equal to the total amount of dissociated acid of the fibrous cellulose contained in the slurry used for titration. The value obtained by dividing the amount of alkali required from the start of titration to the first end point by the solid content (g) in the slurry to be titrated is the amount of phosphorus oxo acid group introduced (mmol/g). Note that when simply referring to the amount of phosphorus oxo acid group introduced (or the amount of phosphorus oxo acid group), it refers to 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 phosphorus oxoacid group is a phosphate group and this phosphate group undergoes condensation, the amount of weak acid groups in the phosphorus oxoacid group (also referred to as the second dissociated acid amount in this specification) appears to decrease, and the amount of alkali required in the second region becomes smaller than the amount of alkali required in the first region. On the other hand, the amount of strong acid groups in the phosphorus oxoacid group (also referred to as the first dissociated acid amount in this specification) is equal to the amount of phosphorus atoms regardless of the presence or absence of condensation. In addition, when the phosphorus oxoacid group is a phosphorous acid group, the weak acid groups are no longer present in the phosphorus oxoacid group, so the amount of alkali required in the second region becomes smaller, or the amount of alkali required in the second region may become zero. In this case, there is only one point on the titration curve where the pH increment is maximized.
The above-mentioned amount of phosphorus oxoacid groups introduced (mmol/g) indicates the amount of phosphorus oxoacid groups possessed by the acid-type fibrous cellulose (hereinafter referred to as the amount of phosphorus oxoacid groups (acid type)), since the denominator indicates the mass of the acid-type fibrous cellulose. On the other hand, when the counter ion of the phosphorus oxoacid group is replaced with an arbitrary cation C so as to be the charge equivalent, the amount of phosphorus oxoacid groups possessed by the fibrous cellulose in which the cation C is the counter ion (hereinafter referred to as the amount of phosphorus oxoacid groups (C type)) can be obtained by converting the denominator to the mass of the fibrous cellulose when the cation C is the counter ion.
That is, it is calculated according to the following formula.
Amount of phosphorus oxoacid group (C type)=Amount of phosphorus oxoacid group (acid type)/{1+(W-1)×A/1000}
A [mmol/g]: total amount of anions derived from phosphorus oxoacid groups in fibrous cellulose (the sum of the amount of strongly acidic groups and the amount of weakly acidic groups in phosphorus oxoacid groups)
W: Formula weight per valence of cation C (for example, Na is 23, Al is 9)

FIG. 2 is a graph showing the relationship between the amount of NaOH dropped onto fibrous cellulose having a carboxy group and pH.
The amount of carboxyl groups introduced into the fibrous cellulose is measured, for example, as follows.
First, a slurry containing fibrous cellulose is treated with a strongly acidic ion exchange resin. If necessary, the measurement object may be subjected to a defibration treatment similar to the defibration treatment process described below before the treatment with the strongly acidic ion exchange resin. Next, the change in pH is observed while adding an aqueous sodium hydroxide solution, and a titration curve as shown in FIG. 2 is obtained. If necessary, the measurement object may be subjected to a defibration treatment similar to the defibration treatment process described below.
As shown in FIG. 2, in this neutralization titration, a single point is observed where the increment (differential value of pH with respect to the amount of alkali added) is maximum on the curve plotting the measured pH against the amount of alkali added. This maximum point of the 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. The amount of alkali required in the first region of the titration curve (mmol) was then divided by the solid content (g) in the fine fibrous cellulose-containing slurry to be titrated to calculate the amount of carboxyl groups introduced (mmol/g).
The above-mentioned amount of carboxyl groups introduced (mmol/g) indicates the amount of substituents per 1 g of fibrous cellulose mass when the counter ion of the carboxyl group is a hydrogen ion (H ⁺ ) (hereinafter referred to as the amount of carboxyl groups (acid type)).

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

The amount of sulfur oxoacid groups introduced into the fine fibrous cellulose is determined by wet ashing the resulting fibrous cellulose with perchloric acid and concentrated nitric acid, diluting it at an appropriate ratio, and measuring the amount of sulfur by ICP emission spectrometry.
The amount of sulfur divided by the bone dry mass of the fibrous cellulose sample is defined as the amount of sulfur oxoacid groups (unit: mmol/g).

In measuring the amount of substituents by titration, if the amount of sodium hydroxide solution dropped is too large or the titration interval is too short, the amount of substituents may be lower than the actual amount, and an accurate value may not be obtained. As an appropriate amount of drop and titration interval, for example, it is preferable to titrate 10 to 50 μL of 0.1 N sodium hydroxide solution for 5 to 30 seconds. In addition, in order to eliminate the influence of carbon dioxide dissolved in the fibrous cellulose-containing slurry, it is preferable to measure while blowing an inert gas such as nitrogen gas into the slurry from 15 minutes before the start of titration until the end of titration.
The measurement of the amount of ionic groups by the above-mentioned method is applied to fine fibrous cellulose having a fiber width of 1,000 nm or less. When measuring the amount of ionic groups in pulp fibers having a fiber width of more than 1,000 nm, the pulp fibers are first finely divided before measurement.

[Method for producing fine fibrous cellulose]
(cellulose-containing fiber materials)
Fine fibrous cellulose is produced from a fiber raw material containing cellulose.
The fiber raw material containing cellulose is not particularly limited, but pulp is preferably used because it is easily available and inexpensive. Examples of pulp include wood pulp, non-wood pulp, and deinked pulp. Examples of wood pulp include, but are not particularly limited to, chemical pulp such as hardwood kraft pulp (LBKP), softwood kraft pulp (NBKP), sulfite pulp (SP), dissolving pulp (DP), soda pulp (AP), unbleached kraft pulp (UKP), and oxygen bleached kraft pulp (OKP), semi-chemical pulp such as semi-chemical pulp (SCP) and chemi-ground wood pulp (CGP), and mechanical pulp such as groundwood pulp (GP) and thermomechanical pulp (TMP, BCTMP). Examples of non-wood pulp include, but are not particularly limited to, cotton-based pulp such as cotton linters and cotton lint, and non-wood-based pulp such as hemp, straw, bamboo, and bagasse. The deinked pulp is not particularly limited, but may be, for example, deinked pulp made from waste paper. The pulp of this embodiment may be one of the above-mentioned types alone or a mixture of two or more types.
Among the above pulps, for example, wood pulp and deinked pulp are preferred from the viewpoint of availability. Furthermore, among wood pulps, for example, chemical pulp is more preferred, and kraft pulp and sulfite pulp are even more preferred, from the viewpoint of a high cellulose ratio and a high yield of fine fibrous cellulose during defibration treatment, and of obtaining long-fiber fine fibrous cellulose with a large axial ratio due to little decomposition of cellulose in the pulp. Note that the viscosity tends to increase when long-fiber fine fibrous cellulose with a large axial ratio is used.
As a fiber raw material containing cellulose, for example, cellulose contained in sea squirts and bacterial cellulose produced by acetic acid bacteria can be used.
Moreover, instead of fiber raw materials containing cellulose, fibers formed from linear nitrogen-containing polysaccharide polymers such as chitin and chitosan can also be used.

In order to obtain fine fibrous cellulose having ionic groups introduced therein as described above, it is preferable to have an ionic group introduction step for introducing ionic groups into the above-mentioned cellulose-containing fiber raw material, a washing step, an alkali treatment step (neutralization step), and a defibration treatment step in this order, and an acid treatment step may be included instead of or in addition to the washing step. Examples of ionic group introduction steps include a phosphorus oxoacid group introduction step, a carboxyl group introduction step, a sulfur oxoacid group introduction step, a xanthate group introduction step, a phosphonic or phosphine group introduction step, and a sulfonic group introduction step. Each of these steps is described below.

(Ionic Group Introduction Step)
- Phosphorus oxo acid group introduction process -
The phosphorus oxo acid group introduction step is a step of reacting at least one compound (hereinafter also referred to as "compound A") selected from compounds capable of introducing phosphorus oxo acid groups by reacting with hydroxyl groups possessed by a fiber raw material containing cellulose, to the fiber raw material containing cellulose, thereby obtaining a fiber into which a phosphorus oxo acid group has been introduced.
In the phosphate group introduction step according to the present embodiment, the reaction of the cellulose-containing fiber raw material with compound A may be carried out in the presence of at least one selected from urea and its derivatives (hereinafter also referred to as "compound B"). On the other hand, the reaction of the cellulose-containing fiber raw material with compound A may be carried out in the absence of compound B.
An example of a method for acting compound A on a fiber raw material in the presence of compound B includes a method of mixing compound A and compound B with a fiber raw material in a dry state, a wet state, or a slurry state. Of 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 reaction is highly uniform. The form of the fiber raw material is not particularly limited, but it is preferable to use a cotton-like or thin sheet-like form. Compound A and compound B are added to the fiber raw material in a powder form, a solution form dissolved in a solvent, or a melted state by heating to a melting point or higher. Of these, it is preferable to add them in a solution form dissolved in a solvent, especially in an aqueous solution form, because the reaction is highly uniform. Compound A and compound B may be added to the fiber raw material simultaneously, separately, or as a mixture. The method of adding compound A and compound B is not particularly limited, but when compound A and compound B are in a solution form, the fiber raw material may be immersed in the solution to absorb the liquid and then removed, or the solution may be dropped onto the fiber raw material. In addition, the required amounts of compound A and compound B may be added to the fiber raw material, or excess amounts of compound A and compound B may be added to the fiber raw material, respectively, and then the excess compound A and compound B may be removed by squeezing or filtration.

The compound A used in this embodiment may be any compound that has a phosphorus atom and can form an ester bond with cellulose, and may include, but is not limited to, phosphoric acid or its salt, phosphorous acid or its salt, dehydrated condensed phosphoric acid or its salt, and anhydrous phosphoric acid (diphosphorus pentoxide). Phosphoric acid may be used with various purities, for example, 100% phosphoric acid (orthophosphoric acid) or 85% phosphoric acid. Phosphorous acid may be, for example, 99% phosphorous acid (phosphonic acid). Dehydrated condensed phosphoric acid is phosphoric acid condensed by dehydration reaction into two or more molecules, and may include, for example, pyrophosphoric acid, polyphosphoric acid, etc. Phosphates, phosphites, and dehydrated condensed phosphates may include lithium salts, sodium salts, potassium salts, and ammonium salts of phosphoric acid, phosphorous acid, or dehydrated condensed phosphoric acid, and these may be neutralized to various degrees.
Among these, from the viewpoints of high efficiency of introduction of phosphate groups, easier improvement of defibration efficiency in the defibration step described below, low cost, and ease of industrial application, phosphoric acid, sodium salts of phosphoric acid, potassium salts of phosphoric acid, and ammonium salts of phosphoric acid are preferred, and phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, and ammonium dihydrogen phosphate are more preferred.
The amount of compound A added to the fiber raw material is not particularly limited, but for example, when the amount of compound A added is converted into the amount of phosphorus atoms, the amount of phosphorus atoms added to the fiber raw material (bone dry mass) is preferably 0.5% by mass or more and 100% by mass or less, more preferably 1% by mass or more and 50% by mass or less, and even more preferably 2% by mass or more and 30% by mass or less. By setting the amount of phosphorus atoms added to the fiber raw material within the above range, the yield of fine fibrous cellulose can be further improved. On the other hand, by setting the amount of phosphorus atoms added to the fiber raw material to the above upper limit or less, a balance can be achieved between the effect of improving the yield and the cost.

As described above, the compound B used in this embodiment is at least one selected from urea and its derivatives. Examples of the compound B include urea, biuret, 1-phenylurea, 1-benzylurea, 1-methylurea, and 1-ethylurea.
From the viewpoint of improving the uniformity of the reaction, it is preferable to use an aqueous solution of compound B. Also, from the viewpoint of further improving the uniformity of the reaction, it is preferable to use an aqueous solution in which both compound A and compound B are dissolved.
The amount of compound B added relative to the fiber raw material (bone dry mass) is not particularly limited, but is preferably, for example, 1 mass % or more and 500 mass % or less, more preferably 10 mass % or more and 400 mass % or less, and even more preferably 100 mass % or more and 350 mass % or less.

In the reaction of a fiber raw material containing cellulose with 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, and dimethylacetamide. Examples of amines include methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, and hexamethylenediamine. Among these, triethylamine in particular is known to act as a good reaction catalyst.

In the phosphorus oxo acid group introduction step, it is preferable to add or mix compound A or the like to the fiber raw material, and then heat-treat the fiber raw material. It is preferable to select a temperature for the heat treatment that can efficiently introduce phosphorus oxo acid groups while suppressing thermal decomposition and hydrolysis of the fiber. The heat treatment temperature is preferably, for example, 50°C to 300°C, more preferably 100°C to 250°C, and even more preferably 130°C to 200°C. In addition, various devices having heat media can be used for the heat treatment, such as a stirring dryer, a rotary dryer, a disk dryer, a roll-type heater, a plate-type heater, a fluidized bed dryer, a band-type dryer, a filtration dryer, a vibration fluidized dryer, an airflow dryer, a reduced pressure dryer, an infrared heater, a far-infrared heater, a microwave heater, and a high-frequency dryer.

In the heat treatment according to the present embodiment, for example, a method of adding compound A to a thin sheet-like fiber raw material by a method such as impregnation and then heating, or a method of heating while kneading or stirring the fiber raw material and compound A in a kneader or the like can be adopted. This makes it possible to suppress unevenness in the concentration of compound A in the fiber raw material and introduce phosphate groups more uniformly onto the surface of the cellulose fibers contained in the fiber raw material. This is considered to be due to the fact that when water molecules move to the surface of the fiber raw material as it is dried, the dissolved compound A can be suppressed from being attracted to the water molecules by surface tension and moving to the surface of the fiber raw material in the same manner (i.e., causing unevenness in the concentration of compound A).
In addition, the heating device used in the heat treatment is preferably a device that can constantly discharge, to the outside of the device system, for example, moisture held by the slurry and moisture generated in the dehydration condensation (phosphorylation) reaction between compound A and hydroxyl groups contained in cellulose or the like in the fiber raw material. Examples of such heating devices include ovens that use a blowing system. By constantly discharging moisture from within the device system, it is possible to suppress the hydrolysis reaction of phosphate ester bonds, which is the reverse reaction of phosphate esterification, and also to suppress acid hydrolysis of sugar chains in the fibers. This makes it possible to obtain fine fibrous cellulose with a high axial ratio.
The time for the heat treatment is, for example, preferably from 1 second to 300 minutes after the moisture has been substantially removed from the fiber raw material, more preferably from 1 second to 1,000 seconds, and even more preferably from 10 seconds to 800 seconds. In this embodiment, the introduction amount of phosphorus oxo acid groups can be kept within a preferred range by setting the heating temperature and heating time within appropriate ranges.

The phosphorus oxo acid group introduction step may be carried out at least once, but may also be repeated two or more times. By carrying out the phosphorus oxo acid group introduction step two or more times, many phosphorus oxo acid groups can be introduced into the fiber raw material. In this embodiment, one preferred embodiment is where the phosphorus oxo acid group introduction step is carried out two times.

The amount of phosphorus oxoacid groups introduced into the fiber raw material is preferably 0.10 mmol/g or more per 1 g (mass) of fine fibrous cellulose, more preferably 0.20 mmol/g or more, even more preferably 0.50 mmol/g or more, and particularly preferably 1.00 mmol/g or more. The amount of phosphorus oxoacid groups introduced into the fiber raw material is preferably 5.20 mmol/g or less per 1 g (mass) of fine fibrous cellulose, more preferably 3.65 mmol/g or less, and even more preferably 3.00 mmol/g or less. By setting the amount of phosphorus oxoacid groups introduced within the above range, it is possible to easily finely disperse the fiber raw material and increase the stability of the fine fibrous cellulose.

-Carboxy group introduction step-
The carboxyl group introduction process is carried out by subjecting a fiber raw material containing cellulose to an oxidation treatment such as ozone oxidation, oxidation by the Fenton method, or TEMPO oxidation treatment, or by treating the raw material with a compound having a carboxylic acid-derived group or a derivative thereof, or an acid anhydride of a compound having a carboxylic acid-derived group or a derivative thereof.
Examples of compounds having a group derived from carboxylic acid include, but are not limited to, dicarboxylic acid compounds such as maleic acid, succinic acid, phthalic acid, fumaric acid, glutaric acid, adipic acid, and itaconic acid, and tricarboxylic acid compounds such as citric acid and aconitic acid. Examples of derivatives of compounds having a group derived from carboxylic acid include, but are not limited to, imidized products of acid anhydrides of compounds having carboxy groups, and derivatives of acid anhydrides of compounds having carboxy groups. Examples of imidized products of acid anhydrides of compounds having carboxy groups include, but are not limited to, imidized products of dicarboxylic acid compounds such as maleimide, succinimide, and phthalimide.

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

When a TEMPO oxidation treatment is performed in the carboxy group introduction step, it is preferable to perform the treatment under conditions of a pH of 6 or more and 8 or less. Such a treatment is also called a neutral TEMPO oxidation treatment. The neutral TEMPO oxidation treatment can be performed, for example, by adding pulp as a fiber raw material, a nitroxy radical such as TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) as a catalyst, and sodium hypochlorite as a sacrificial reagent to a sodium phosphate buffer solution (pH = 6.8). Furthermore, by allowing sodium chlorite to coexist, aldehydes generated during the oxidation process can be efficiently oxidized to carboxy groups.
The TEMPO oxidation treatment may be carried out under conditions of a pH of 10 to 11. Such a treatment is also called an alkaline TEMPO oxidation treatment. The alkaline TEMPO oxidation treatment can be carried out, for example, by adding a nitroxy radical such as TEMPO as a catalyst, sodium bromide as a co-catalyst, and sodium hypochlorite as an oxidizing agent to pulp as a fiber raw material.
The amount of carboxyl groups introduced into the fiber raw material varies depending on the type of the substituent, but for example, when the carboxyl groups are introduced by TEMPO oxidation, it is preferably 0.10 mmol/g or more per 1 g (mass) of fine fibrous cellulose, more preferably 0.20 mmol/g or more, even more preferably 0.50 mmol/g or more, and particularly preferably 0.90 mmol/g or more. It is also preferably 2.5 mmol/g or less, more preferably 2.20 mmol/g or less, and even more preferably 2.00 mmol/g or less. In addition, when the substituent is a carboxymethyl group, it may be 5.8 mmol/g or less per 1 g (mass) of fine fibrous cellulose.

- Sulfur oxo acid group introduction process -
The process for producing fine fibrous cellulose may include, for example, a sulfur oxoacid group introduction step as an ionic substituent introduction step, in which a hydroxyl group in a fiber raw material containing cellulose reacts with a sulfur oxoacid to obtain cellulose fibers having sulfur oxoacid groups (sulfur oxoacid group-introduced fibers).

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

In the sulfur oxoacid group introduction process, it is preferable to mix the cellulose raw material with an aqueous solution containing sulfur oxoacid and urea and/or a urea derivative, and then heat-treat the cellulose raw material. It is preferable to select a heat-treating temperature that can efficiently introduce sulfur oxoacid groups while suppressing thermal decomposition and hydrolysis reactions of the fibers. The heat-treating temperature is preferably 100°C or higher, more preferably 120°C or higher, and even more preferably 150°C or higher. The heat-treating 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 substantially no moisture is present. For this reason, the heat treatment time varies depending on the amount of moisture contained in the cellulose raw material and the amount of the aqueous solution containing sulfur oxoacid and urea and/or urea derivative added, but is preferably set to, for example, 10 seconds or more and 10,000 seconds or less. For the heat treatment, equipment having various heat media can be used, such as a stirring dryer, a rotary dryer, a disk dryer, a roll-type heater, a plate-type heater, a fluidized bed dryer, a band-type dryer, a filtration dryer, a vibration fluidized dryer, an airflow dryer, a reduced pressure dryer, an infrared heater, a far-infrared heater, a microwave heater, or a high-frequency dryer.

The amount of sulfur oxoacid groups introduced into the cellulose raw material is preferably 0.05 mmol/g or more, more preferably 0.10 mmol/g or more, even more preferably 0.20 mmol/g or more, even more preferably 0.50 mmol/g or more, and particularly preferably 0.90 mmol/g or more. The amount of sulfur oxoacid groups introduced into the cellulose raw material is preferably 5.00 mmol/g or less, more preferably 3.00 mmol/g or less. By setting the amount of sulfur oxoacid groups introduced within the above range, it is possible to easily refine the fiber raw material and increase the stability of the fibrous cellulose.

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

Examples of chlorine-based oxidizing agents include hypochlorous acid, hypochlorites, chlorous acid, chlorites, chloric acid, chlorates, perchloric acid, perchlorates, chlorine dioxide, etc. From the viewpoints of the efficiency of introducing a substituent, and therefore the defibration efficiency, cost, and ease of handling, sodium hypochlorite, sodium chlorite, and chlorine dioxide are preferred.
The chlorine-based oxidizing agent may be added to the fiber raw material as it is, or may be dissolved in a suitable solvent and then added.

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

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

-Xanthate group introduction step (xanthogen acid esterification step)-
The manufacturing process of fine fibrous cellulose may include, for example, a xanthate group introduction step (hereinafter also referred to as a xanthate formation step) as an ionic substituent introduction step. In the xanthate formation step, carbon disulfide and an alkali compound are added to a wet or dry fiber raw material having a hydroxyl group to react with each other, thereby introducing a xanthate group into the fiber raw material. Specifically, carbon disulfide is added to a fiber raw material that has been converted into alkali cellulose by the method described below, and the reaction is carried out.

<Alkaline cellulose>
When introducing ionic functional groups into the fiber raw material, it is preferable to apply an alkaline solution to the cellulose contained in the fiber raw material to convert the cellulose into alkaline cellulose. This treatment causes some of the hydroxyl groups of the cellulose to be ionized, thereby increasing the nucleophilicity (reactivity). The alkaline compound contained in the alkaline solution is not particularly limited, and may be an inorganic alkaline compound or an organic alkaline compound. Because of their high versatility, it is preferable to use, for example, sodium hydroxide, potassium hydroxide, tetraethylammonium hydroxide, or tetrabutylammonium hydroxide. The conversion into alkaline cellulose may be performed simultaneously with the introduction of ionic functional groups, may be performed as a precursor to the introduction, or may be performed at both the same time and the same time.

The solution temperature at the start of alkali cellulose conversion is preferably 0°C or higher and 50°C or lower, more preferably 5°C or higher and 40°C or lower, and even more preferably 10°C or higher and 30°C or lower.

The alkaline solution concentration is preferably 0.01 mol/L or more and 4 mol/L or less, more preferably 0.1 mol/L or more and 3 mol/L or less, and even more preferably 1 mol/L or more and 2.5 mol/L or less. In particular, when the treatment temperature is less than 10°C, it is preferably 1 mol/L or more and 2 mol/L or less.

The processing time for alkali cellulose formation is preferably 1 minute or more, more preferably 10 minutes or more, and even more preferably 30 minutes or more. The alkali treatment time is preferably 6 hours or less, more preferably 5 hours or less, and even more preferably 4 hours or less.

By adjusting the type of alkaline solution, treatment temperature, concentration, and immersion time as described above, it is possible to suppress the penetration of the alkaline solution into the crystalline regions of the cellulose, making it easier to maintain the crystalline structure of cellulose type I, and increasing the yield of fine fibrous cellulose.

When the introduction of ionic functional groups and the conversion to alkali cellulose are not performed simultaneously, it is preferable to separate the alkali cellulose obtained by the alkali treatment into solid and liquid and remove water by a general deliquescence method such as centrifugation or filtration. This improves the reaction efficiency in the subsequent ionic functional group introduction step. The cellulose fiber concentration after solid-liquid separation is preferably 5% or more and 50% or less, more preferably 10% or more and 40% or less, and even more preferably 15% or more and 35% or less.

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

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

Compound E _A may be added to the fiber raw material as a reagent as it is, or may be dissolved in a suitable solvent and added. The fiber raw material is preferably converted into alkali cellulose in advance or simultaneously with the reaction. The method of conversion into alkali cellulose is as described above.

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

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

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

-Sulfonic group introduction step (sulfoalkylation step)-
The process for producing fine fibrous cellulose may include, for example, a sulfone group introduction step (sulfoalkylation step) as an ionic substituent introduction step. In the sulfoalkylation, a compound (compound E _B ) having a reactive group and a sulfone group as an essential component, and an alkali compound and the above-mentioned compound B selected from urea and its derivatives as optional components are added to a wet or dry fiber raw material having a hydroxyl group to carry out a reaction, thereby introducing a sulfone group into the fiber raw material.

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

Compound E _B may be added to the fiber raw material as a reagent as it is, or may be dissolved in a suitable solvent and added. The fiber raw material is preferably converted into alkali cellulose in advance or simultaneously with the reaction. The method of conversion into alkali cellulose is as described above.

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

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

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

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

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

Compound E _C may be added to the fiber raw material as a reagent as it is, or may be dissolved in a suitable solvent and added. The fiber raw material is preferably converted into alkali cellulose in advance or simultaneously with the reaction. The method of conversion into alkali cellulose is as described above.

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

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

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

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

Reactive groups include halogenated alkyl groups, vinyl groups, and epoxy groups (glycidyl groups).

As the compound E _D , glycidyl trimethyl ammonium chloride, 3-chloro-2-hydroxypropyl trimethyl ammonium chloride, etc. are preferred from the viewpoints of the efficiency of introducing the substituent, and therefore the defibration efficiency, cost, and ease of handling.

Furthermore, as an optional component, it is also preferable to use compound B in the above-mentioned <Phosphorus Oxo Acid Group Introduction Step>. The amount added is also preferably as described above.

Compound E _D may be added to the fiber raw material as a reagent as it is, or may be dissolved in a suitable solvent and added. It is preferable that the fiber raw material is converted into alkali cellulose in advance or simultaneously with the reaction. The method of conversion into alkali cellulose is as described above.

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

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

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

(Washing process)
In the method for producing fine fibrous cellulose in this embodiment, a washing step can be carried out on the ionic group-introduced fiber as necessary. The washing step is carried out, for example, by washing the ionic group-introduced fiber with water or an organic solvent. The washing step may be carried out after each step described below, and the number of washing steps carried out in each washing step is not particularly limited.

(Alkaline treatment step)
When producing fine fibrous cellulose, the fiber raw material may be subjected to an alkali treatment between the ionic group introduction step and the defibration step described below. The alkali treatment method is not particularly limited, but may be, for example, a method of immersing the ionic 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 this embodiment, it is preferable to use, for example, sodium hydroxide or potassium hydroxide as the alkaline compound because of its high versatility. 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 a polar solvent including water or a polar organic solvent exemplified by alcohol, and more preferably an aqueous solvent including 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.
The temperature of the alkaline solution in the alkaline treatment step is not particularly limited, but is preferably, for example, from 5° C. to 80° C., and more preferably from 10° C. to 60° C. The immersion time of the ionic group-introduced fiber in the alkaline solution in the alkaline treatment step is not particularly limited, but is, for example, preferably from 5 minutes to 30 minutes, and more preferably from 10 minutes to 20 minutes. The amount of the alkaline solution used in the alkaline treatment is not particularly limited, but is, for example, preferably from 100 mass% to 100,000 mass%, and more preferably from 1,000 mass% to 10,000 mass%, based on the absolute dry mass of the ionic group-introduced fiber.
When the fine fibrous cellulose has anionic groups, the alkali treatment may be a neutralization/ion exchange treatment of the anionic groups. In this case, the temperature of the alkali solution is preferably room temperature.

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

(Acid treatment process)
When producing fine fibrous cellulose, the fiber raw material may be subjected to an acid treatment between the step of introducing an ionic group and the defibration step described below. For example, the steps of introducing an ionic group, the acid treatment, the alkali treatment, and the defibration step may be performed in this order.
The method of acid treatment is not particularly limited, but may be, for example, a method of immersing the fiber raw material in an acidic solution containing an acid. The concentration of the acidic solution used is not particularly limited, but is preferably, for example, 10% by mass or less, and more preferably, 5% by mass or less. The pH of the acidic solution used is not particularly limited, but is preferably, for example, 0 to 4, and more preferably, 1 to 3. The acid contained in the acidic solution may be, for example, an inorganic acid, a sulfonic acid, a carboxylic acid, or the like. Examples of inorganic acids include sulfuric acid, nitric acid, hydrobromic acid, hydroiodic acid, hypochlorous acid, chlorous acid, chloric acid, perchloric acid, phosphoric acid, and boric acid. Examples of sulfonic acids include, for example, methanesulfonic acid, ethanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid, and trifluoromethanesulfonic acid. Examples of 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, but is preferably, for example, 5° C. to 100° C., and more preferably, 20° C. to 90° C. The immersion time in the acid solution in the acid treatment is not particularly limited, but is, for example, preferably, 5 minutes to 120 minutes, and more preferably, 10 minutes to 60 minutes. The amount of the acid solution used in the acid treatment is not particularly limited, but is, for example, preferably, 100 mass% to 100,000 mass%, and more preferably, 1,000 mass% to 10,000 mass%, based on the absolute dry mass of the fiber raw material.
When the fine fibrous cellulose has cationic groups, the acid treatment may be a neutralization/ion exchange treatment of the cationic groups. In this case, the temperature of the acid solution is preferably room temperature.

(Fibrillation process)
By subjecting the ionic group-introduced fiber to defibration treatment in a defibration treatment step, fine fibrous cellulose can be obtained.
In the defibration process, for example, a defibration treatment device can be used. The defibration treatment device is not particularly limited, but for example, a high-speed defibrator, a grinder (stone mill type grinder), a high-pressure homogenizer, an ultra-high-pressure homogenizer, a high-pressure collision type grinder, a ball mill, a bead mill, a disk type refiner, a conical refiner, a biaxial kneader, a vibration mill, a homomixer under high-speed rotation, an ultrasonic disperser, or a beater can be used. Among the above defibration treatment devices, it is more preferable to use a high-speed defibrator, a high-pressure homogenizer, or an ultra-high-pressure homogenizer, which are less affected by the grinding media and have less risk of contamination.

In the fiber defibration process, for example, it is preferable to dilute the ionic group-introduced fiber with a dispersion medium to make it into a slurry state. As the dispersion medium, one or more types selected from water and organic solvents such as polar organic solvents can be used. The polar organic solvent is not particularly limited, but for example, alcohols, polyhydric alcohols, ketones, ethers, esters, aprotic polar solvents, etc. are preferable. Examples of alcohols include methanol, ethanol, isopropanol, n-butanol, isobutyl alcohol, etc. Examples of polyhydric alcohols include ethylene glycol, propylene glycol, glycerin, etc. Examples of ketones include acetone, methyl ethyl ketone (MEK), etc. 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, etc. Examples of esters include ethyl acetate, butyl acetate, etc. Aprotic polar solvents include dimethylsulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), N-methyl-2-pyrrolidinone (NMP), etc.

The solids concentration of the fine fibrous cellulose during the defibration treatment can be appropriately set.
Furthermore, the slurry obtained by dispersing the phosphorus oxo acid group-introduced fiber in a dispersion medium may contain solid matters other than the phosphorus oxo acid group-introduced fiber, such as urea having hydrogen bonding properties.

From the viewpoint of sheet transparency, the content of fine fibrous cellulose in the solid content of the sheet composition is preferably 20% by mass or more, more preferably 25% by mass or more, even more preferably 30% by mass or more, and is preferably 80% by mass or less, more preferably 75% by mass or less, even more preferably 70% by mass or less.
In addition, the content of the fine fibrous cellulose relative to the total amount of the ultraviolet-polymerizable compound and the fine fibrous cellulose is preferably 20% by mass or more, more preferably 25% by mass or more, even more preferably 30% by mass or more, and is preferably 85% by mass or less, more preferably 80% by mass or less, even more preferably 75% by mass or less.

<Other ingredients>
The sheet composition of the present invention may contain other components in addition to the above-mentioned ultraviolet-polymerizable compound, fine fibrous cellulose, and photopolymerization initiator.
Examples of other components include hydrophilic polymers, monofunctional polymerizable unsaturated compounds, wet strength agents, organic ions, crosslinking agents, surfactants, coupling agents, inorganic layered compounds, inorganic compounds, leveling agents, preservatives, defoamers, organic particles, lubricants, antistatic agents, UV absorbers, dyes, pigments, stabilizers, magnetic powders, orientation promoters, plasticizers, and dispersants.

Examples of hydrophilic polymers used in the present invention include polyalkylene glycols (polyethylene glycol, polypropylene glycol, etc.), cellulose derivatives (hydroxyethyl methylcellulose, hydroxypropyl methylcellulose, hydroxyethyl cellulose, methylcellulose, carboxyethyl cellulose, carboxymethyl cellulose, etc.), proteins such as casein, starches (dextrin, cationic starch, raw starch, oxidized starch, etherified starch, esterified starch, and amylose, etc.), polyvinyl alcohol, modified polyvinyl alcohol (acetoacetylated polyvinyl alcohol, ethylene-vinyl alcohol copolymer, ethylene oxide-modified polyvinyl alcohol, etc.), polyalkylene oxides (polyethylene oxy Examples of suitable polysaccharides include polyvinylpyrrolidone, polyvinyl methyl ether, polyacrylates (sodium polyacrylate, etc.), polycations (polyacrylamide, polyethyleneimine, etc.), polyanions, amphoteric ion-type polymers, acrylic acid alkyl ester copolymers, urethane copolymers, thickening polysaccharides (xanthan gum, guar gum, tamarind gum, locust bean gum, quince seed, alginic acid, metal salts of alginic acid, pullulan, carrageenan, sacran, pectin, etc.), polyesters, modified polyesters, modified polyimides, glycerins such as polyglycerin, hyaluronic acid, metal salts of hyaluronic acid, carboxyvinyl polymers, alkyl methacrylates, acrylic acid copolymers, polyacrylates (sodium polyacrylate, etc.), etc. Among the above, polyethylene glycol, polyethylene oxide, polyvinyl alcohol, and modified polyvinyl alcohol are particularly preferred.

The weight-average molecular weight of the hydrophilic polymer is not particularly limited as long as it is 5,000 or more, but from the viewpoint of the shape stability of the sheet and the viewpoint of enabling fine patterning, it is preferably 10,000 or more, more preferably 15,000 or more, and even more preferably 20,000 or more. In addition, the weight-average molecular weight of the hydrophilic polymer is preferably 8 million or less, more preferably 5 million or less, even more preferably 1 million or less, and even more preferably 100,000 or less.
The weight average molecular weight can be measured, for example, by using GPC.

When the sheet of the present invention contains a hydrophilic polymer, the content of the hydrophilic polymer in the solid content of the sheet is preferably 1% by mass or more, more preferably 3% by mass or more, even more preferably 8% by mass or more, from the viewpoint of improving the shape stability, transparency, water resistance, tensile modulus, and linear thermal expansion coefficient of the sheet, and is preferably 60% by mass or less, more preferably 40% by mass or less, even more preferably 20% by mass or less, and even more preferably 15% by mass or less.

Examples of the monofunctional (meth)acrylic acid esters include 2-hydroxyethyl (meth)acrylate, diethylene glycol monoethyl ether (meth)acrylate, (2-(meth)acryloyloxyethyl)(2-hydroxypropyl)phthalate, ω-carboxypolycaprolactone mono(meth)acrylate, and the like. Examples of commercially available products of these, by trade name, include ARONIX (registered trademark) M-101, M-111, M-114, and M-5300 (all manufactured by Toagosei Co., Ltd.); KAYARAD (registered trademark) TC-110S and TC-120S (all manufactured by Nippon Kayaku Co., Ltd.); Viscoat 158 and 2311 (all manufactured by Osaka Organic Chemical Industry Ltd.), and the like.

Wet strength agents are chemicals that prevent the paper from losing strength when it gets wet. By adding them to pulp fibers, which tend to unravel in water, they help maintain the bonds between the pulp fibers even when wet, making them less likely to unravel, and thus increasing their strength.
Examples of wet strength agents include polyamine polyamide epihalohydrin, melamine formaldehyde resin, and urea formaldehyde resin.
Polyamine polyamide epichlorohydrin is synthesized, for example, by adding epichlorohydrin to the main chain formed by condensing polyacid and polyethylene polyamine. Polyamine polyamide epichlorohydrin can be used in a wide pH range, and can provide a high wet strength effect even with a small amount added.
Melamine formaldehyde resin is synthesized by adding formaldehyde to melamine and then subjecting the mixture to acid condensation.
Urea-formaldehyde resin is synthesized by adding formaldehyde to urea and then partially crosslinking the resulting mixture.
Among these, it is preferable to use polyamine polyamide epihalohydrin from the viewpoint of obtaining a sheet having a low water absorption rate and a low linear thermal expansion coefficient and excellent transparency. Among the polyamine polyamide epihalohydrins, polyamine polyamide epichlorohydrin is more preferable.

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

<Sheet manufacturing method>
The sheet manufacturing method of the present invention includes the steps of preparing a sheet composition containing an ultraviolet-polymerizable compound and fine fibrous cellulose having a fiber width of 1,000 nm or less, applying and drying the sheet composition to obtain an uncured sheet composition, and irradiating the uncured sheet composition with ultraviolet light to cure it with ultraviolet light. The sheet obtained has a linear thermal expansion coefficient of 30 ppm/K or less at 100° C. or more and a yellow index (YI value) of 5 or less.
The sheet composition used in the method for producing a sheet of the present invention is the same as the sheet composition used in the above-mentioned sheet, and the preferred embodiments are also the same.
When applying the sheet composition, the composition may be applied to a substrate having no holes (hereinafter, simply referred to as substrate) and dried to obtain an uncured sheet composition, or the composition may be applied to a porous substrate and the uncured sheet composition may be obtained by so-called papermaking, but it is preferable to apply the composition to a substrate and dry it to obtain an uncured sheet composition. In addition, the uncured sheet composition may be continuously obtained by using a coating device and a belt-shaped substrate. Among these, it is more preferable to apply the composition to a substrate and dry it to obtain an uncured sheet composition.
The sheet of the present invention can be obtained by irradiating the uncured sheet-like composition obtained as described above with ultraviolet light. When applying the composition to a substrate, the ultraviolet-curable sheet dried on the substrate can be irradiated with ultraviolet light to cure the composition on the substrate, and then peeled off.

The material of the substrate used for application is not particularly limited, but one that has high wettability with the sheet composition (slurry) is preferred from the viewpoint of suppressing shrinkage of the sheet during drying, and it is also preferable that the sheet formed after drying can be easily peeled off. Preferred examples include resin films and plates, and metal films and plates, but are not particularly limited. Specific examples include resin films and plates such as acrylic, polyethylene terephthalate, vinyl chloride, polystyrene, polypropylene, polycarbonate, and polyvinylidene chloride, metal films and plates such as aluminum, zinc, copper, and iron plates, and those whose surfaces have been oxidized, stainless steel films and plates, and brass films and plates.

If the viscosity of the sheet composition (slurry) is low during application and it spreads on the substrate, a blocking frame may be fixed on the substrate to obtain an uncured sheet composition of a desired thickness and basis weight. The blocking frame is not particularly limited, but it is preferable to select one that allows the end of the sheet to be easily peeled off after drying. From this perspective, a molded resin plate or metal plate is more preferable. In this embodiment, for example, resin plates such as acrylic plates, polyethylene terephthalate plates, polyvinyl chloride plates, polystyrene plates, polypropylene plates, polycarbonate plates, polyvinylidene chloride plates, etc., metal plates such as aluminum plates, zinc plates, copper plates, iron plates, etc., and plates whose surfaces have been oxidized, stainless steel plates, brass plates, etc., can be used.

The coating machine for applying the sheet composition (slurry) to the substrate is not particularly limited, but for example, a roll coater, gravure coater, die coater, curtain coater, air doctor coater, etc. can be used. Die coaters, curtain coaters, and spray coaters are particularly preferred because they can make the sheet thickness more uniform.

The slurry temperature and the ambient temperature when applying the sheet composition (slurry) to the substrate are not particularly limited, but are preferably, for example, 5°C to 80°C, more preferably 10°C to 60°C, even more preferably 15°C to 50°C, and particularly preferably 20°C to 40°C. If the application temperature is equal to or higher than the lower limit, the sheet composition (slurry) can be applied more easily. If the application temperature is equal to or lower than the upper limit, the evaporation of the dispersion medium during application and side reactions due to heat can be suppressed.

The method for drying the sheet composition (slurry) applied to the substrate is not particularly limited, but examples include a non-contact drying method, a method in which the sheet is dried while being restrained, or a combination of these.
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 by vacuum (vacuum drying method) can be applied. The heat drying method and the vacuum drying method may be combined, but the heat drying method is usually applied. Drying by infrared rays, far infrared rays, or near infrared rays can be performed using, for example, an infrared device, a far infrared device, or a near infrared device, but is not particularly limited.
The heating temperature in the heat drying method is not particularly limited, but is preferably 20° C. or higher, more preferably 50° C. or higher, and preferably 150° C. or lower, more preferably 120° C. or lower, and even more preferably 100° C. or lower. If the heating temperature is equal to or higher than the lower limit, the dispersion medium can be quickly volatilized. If the heating temperature is equal to or lower than the upper limit, the cost required for heating can be reduced, and discoloration and side reactions of the fibrous cellulose due to heat can be suppressed.

When making paper, that is, when applying and drying the sheet-shaped composition to a porous substrate, the composition for sheet (slurry) is made into paper by a papermaking machine. The papermaking machine used in the papermaking process is not particularly limited, but may be, for example, a continuous papermaking machine such as a fourdrinier type, a cylinder type, or a tilt type, or a multi-layer papermaking machine combining these. In the papermaking process, a known papermaking method such as handmaking may be used.
The papermaking process is carried out by filtering and dehydrating the sheet composition (slurry) with a wire to obtain a sheet in a wet paper state, and then pressing and drying the sheet. The filter cloth used for filtering and dehydrating the slurry is not particularly limited, but it is more preferable that, for example, fibrous cellulose and ultraviolet polymerizable compounds do not pass through and the filtration speed does not become too slow. Such filter cloth is not particularly limited, but, for example, a sheet, woven fabric, or porous film made of an organic polymer is preferable. The organic polymer is not particularly limited, but, for example, a non-cellulose organic polymer such as polyethylene terephthalate, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), etc. are preferable. In this embodiment, for example, a porous film of polytetrafluoroethylene with a pore size of 0.1 μm to 20 μm, and a woven fabric of polyethylene terephthalate or polyethylene with a pore size of 0.1 μm to 20 μm are mentioned.

In the papermaking process, a method for producing an uncured sheet-like composition from the sheet composition (slurry) can be carried out, for example, by using a production apparatus that includes a water squeezing section in which the slurry is discharged onto the top surface of an endless belt and the dispersion medium is squeezed out of the discharged slurry to produce a web, and a drying section in which the web is dried to produce a sheet. An endless belt is disposed between the water squeezing section and the drying section, and the web produced in the water squeezing section is transported to the drying section while still placed on the endless belt.

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

The irradiation light source for irradiating ultraviolet light is not particularly limited, and examples thereof include known exposure light sources such as a high pressure mercury lamp, a metal halide lamp, an ultra-high pressure mercury lamp, an ultraviolet LED (light emitting diode), etc. A post-exposure bake (PEB) step may be included.
As the ultraviolet rays, ultraviolet rays having a wavelength of 313 nm or more are preferred, and ultraviolet rays including 365 nm ultraviolet rays are particularly preferred, so that as the exposure light source, high pressure mercury lamps, metal halide lamps, and ultra-high pressure mercury lamps that can irradiate radiation having a wavelength of 313 nm or more including 365 nm ultraviolet rays are more preferred. By using these exposure light sources, a relatively high transmittance can be obtained even for an ultraviolet curable sheet having a thickness of 10 μm or more in the wavelength region of 313 nm or more, and the ultraviolet curable sheet can be sufficiently cured up to the back side without reducing the amount of light. In addition, the exposure amount is preferably 3 mJ/cm 2 or more to 1,500 mJ/cm 2 or less, more preferably 100 mJ/cm ² or more to 1,000 mJ/cm ² or less, and even more preferably 500 mJ/cm ² or more to 1,000 mJ/cm ² or ^less , as measured by an illuminometer (manufactured by Ushio Inc., UIT-150- ^A , sensor part is UVD-C365) at a wavelength of 365 nm.

<Characteristics of the sheet>
[Linear thermal expansion coefficient]
The sheet of the present invention has a linear thermal expansion coefficient at 100 to 150° C. of 30 ppm/K or less.
The linear thermal expansion coefficient is preferably low, and from the viewpoint of production, is 20 ppm/K or less, more preferably 15 ppm/K or less, and even more preferably 12 ppm/K or less. The lower limit of the linear thermal expansion coefficient is not particularly limited, and may be, for example, a negative value, but is preferably −20 ppm/K or more, more preferably −15 ppm/K or more.
The linear thermal expansion coefficient of the sheet at 100 to 150° C. is measured by the method described in the examples.

[Yellow Index (YI value)]
The sheet of the present invention is preferably transparent and has no yellowish color, and the yellow index (YI value) of the sheet of the present invention is 5 or less, preferably 3 or less, more preferably 2.5 or less, and further preferably 1.5 or less. The lower limit of the YI value is not particularly limited, and may be 0.

[Tensile modulus]
From the viewpoint of obtaining a sheet having excellent strength, the tensile modulus of the sheet of the present invention is preferably 3 GPa or more, more preferably 5 GPa or more, and even more preferably 8 GPa or more. The upper limit of the tensile modulus of the sheet is not particularly limited, but can be, for example, 50 GPa or less.
Here, the tensile modulus of the sheet is a value measured using a tensile tester Tensilon (manufactured by A & D Co., Ltd.) in accordance with, for example, JIS P 8113: 2006. When measuring the tensile modulus of the sheet, a test piece is prepared by conditioning the sample at 23°C and a relative humidity of 50% for 24 hours, and the measurement is performed under conditions of 23°C and a relative humidity of 50%.

[Total light transmittance]
The sheet of the present invention preferably has excellent light transmittance, and the total light transmittance is preferably 70% or more, more preferably 80% or more, and even more preferably 90% or more, and is 100% or less, and from the viewpoint of ease of production, is preferably 99.5% or less.
The total light transmittance is measured with a haze meter in accordance with JIS K 7361-1:1997.

[Haze]
In addition, the sheet of the present invention preferably has excellent transparency, and the haze is preferably 10% or less, more preferably 3% or less, and even more preferably 2% or less, and is 0% or more, and from the viewpoint of ease of production, is preferably 0.1% or more.
Haze is measured with a haze meter in accordance with JIS K 7136:2000.

[Sheet thickness]
From the viewpoint of shape stability and enabling fine patterning, the thickness of the sheet is preferably 10 μm or more, more preferably 20 μm or more, even more preferably 25 μm or more, and is preferably 1,000 μm or less, more preferably 500 μm or less, even more preferably 100 μm or less.
The coating amount and paper making amount may be appropriately adjusted so that the sheet thickness falls within the above range.

<Water absorption rate>
The sheet of the present invention preferably has a water absorption rate represented by the following formula (1) of 60% or less.
Water absorption rate = (W-Wd)/Wd×100 (1)
(W here indicates the mass of the sheet after immersing it in ion-exchanged water for 24 hours, and Wd indicates the mass of the sheet after conditioning it at 23° C. and a relative humidity of 50% for 24 hours.)
The water absorption rate is preferably low, and from the viewpoint of production, is preferably 50% or less, more preferably 45% or less. The lower limit of the water absorption rate is not particularly limited, and may be, for example, 0%.

<Basis weight of sheet>
The basis weight of the sheet is not particularly limited, but is, for example, preferably 10 g/ ^m2 or more, more preferably 20 g/ ^m2 or more, even more preferably 30 g/ ^m2 or more, and is preferably 200 g/ ^m2 or less, more preferably 150 g/ ^m2 or less. Here, the basis weight of the sheet can be calculated, for example, by conditioning a 50 mm square sheet for 24 hours under conditions of 23°C and 50% RH, and then measuring the mass of the sheet.

<Sheet density>
The density of the sheet is not particularly limited, but is, for example, preferably 0.1 g/cm ^or more, more preferably 0.5 g/cm ^or more, even more preferably 0.8 g/cm ^or more, and preferably 5.0 g/cm ^or less, more preferably 3.0 g/cm ^or less, even more preferably 1.5 g/cm ^or less. Here, the density of the sheet can be calculated by measuring the thickness and mass of a 50 mm square sheet after conditioning the moisture for 24 hours under conditions of 23° C. and 50% RH.

[Sheet Usage]
The sheet of the present invention has a low yellowish color and can be applied to various applications where transparency, thermal expansion, and strength are important. Specifically, for example, it is suitable for use as a light-transmitting substrate for various display devices, solar cells, etc. It can also be applied to applications such as substrates for electronic devices, components for home appliances, window materials for various vehicles and building materials, interior materials, exterior materials, and packaging materials.

The sheet of the present invention may be laminated with other layers to form a laminate. The other layers may be provided on both surfaces of the sheet, or may be provided only on one surface of the sheet, and are not particularly limited. Examples of the other layers include inorganic layers and organic layers.
The organic layer is preferably a layer formed of a resin, and the organic layer is a layer mainly composed of a natural resin or a synthetic resin. Here, the main component refers to a component contained in an amount of 50% by mass or more relative to the total mass of the organic layer. The content of the resin is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more relative to the total mass of the organic layer. 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 ester, and hydrogenated rosin ester.

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

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

The organic layer may be made of one type of resin alone, or a copolymer formed by copolymerization or graft polymerization of multiple resin components. In addition, multiple resin components may be mixed by a physical process to form a blend material.

The method for forming the organic layer is not particularly limited. For example, the organic layer may be formed by coating a resin composition for forming the organic layer on a sheet. Alternatively, a pre-formed organic layer may be laminated on the sheet. In this case, an adhesive layer may be provided between the organic layer and the sheet, and such an adhesive layer is also included in the organic layer. Also, if the substrate during sheet production is a resin, the substrate may be included as part of the organic layer without being peeled off.

An adhesive layer may be provided between the sheet and the organic layer, or the sheet and the organic layer may be directly attached without providing an adhesive layer. When an adhesive layer is provided between the sheet and the organic layer, an adhesive constituting the adhesive layer may be, for example, an acrylic resin. In addition, examples of adhesives other than acrylic resins include vinyl chloride resin, (meth)acrylic ester resin, styrene/acrylic ester copolymer resin, vinyl acetate resin, vinyl acetate/(meth)acrylic ester copolymer resin, urethane resin, silicone resin, epoxy resin, ethylene/vinyl acetate copolymer resin, polyester resin, polyvinyl alcohol resin, ethylene-vinyl alcohol copolymer resin, and rubber emulsions such as SBR and NBR.

When no adhesive layer is provided between the sheet and the organic layer, the organic layer may contain an adhesion aid, and the surface of the organic layer may be subjected to a surface treatment such as hydrophilization.
Examples of the adhesion aid 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, it is preferable that the adhesion aid is at least one selected from compounds containing isocyanate groups (isocyanate compounds) and organosilicon compounds. Examples of the organosilicon compounds include silane coupling agent condensates and silane coupling agents.
Examples of the surface treatment method include corona treatment, plasma discharge treatment, UV irradiation treatment, electron beam irradiation treatment, and flame treatment.

The thickness of the organic layer is not particularly limited, but is preferably 100 nm or more, more preferably 200 nm or more, and even more preferably 500 nm or more. From the viewpoints of transparency and flexibility, the thickness of the organic layer is preferably 10 μm or less, more preferably 5 μm or less, and even more preferably 2 μm or less.

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

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

Atomic Layer Deposition (ALD) can also be used as a method for forming an inorganic layer. The ALD method is a method for forming a thin film in atomic layers by alternately supplying the raw material gases of each element that constitutes the film to be formed to the surface on which the layer is to be formed. Although it has the disadvantage of a slow film formation speed, it has the advantage of being able to cover surfaces with complex shapes neatly and to form a thin film with fewer defects than the plasma CVD method. In addition, the ALD method has the advantage that the film thickness can be controlled to the nano order, and it is relatively easy to cover a large surface. Furthermore, the ALD method is expected to improve the reaction rate, achieve low-temperature processing, and reduce unreacted gas by using plasma.

A laminate including the sheet of the present invention has low yellowness, excellent transparency and strength, and a low linear thermal expansion coefficient, and is expected to be used in a variety of applications where yellowness, transparency, strength, and thermal expansion are problems. Specifically, it is expected to be used in the various applications described above.

The features of the present invention are explained in more detail below with reference to examples and comparative examples. The materials, amounts used, ratios, processing contents, processing procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be interpreted as being limited by the specific examples shown below.

<Production Example 1>
[Production of fine fibrous cellulose dispersion (1)]
As the raw material pulp, softwood kraft pulp (93% solids by mass, 245 g/m ² basis weight, sheet form, Canadian standard freeness (CSF) measured in accordance with JIS P 8121-2:2012 after disintegration is 700 mL) manufactured by Oji Paper Co., Ltd. was used.
This raw pulp was subjected to phosphorus oxo-oxidation treatment as follows.
First, a mixed aqueous solution of ammonium dihydrogen phosphate and urea was added to 100 parts by mass (bone dry mass) of the above raw pulp to adjust the composition to 45 parts by mass of ammonium dihydrogen phosphate, 120 parts by mass of urea, and 150 parts by mass of water, to obtain a chemical-impregnated pulp. The obtained chemical-impregnated pulp was then heated for 250 seconds in a hot air dryer at 165° C. to introduce phosphorus oxo acid groups into the cellulose in the pulp, to obtain phosphorus oxo-oxidized pulp 1.

Next, the resulting phosphorus oxo-oxidized pulp 1 was subjected to a washing treatment.
The washing treatment was carried out by repeatedly pouring 10 L of ion-exchanged water into 100 g (absolute dry weight) of phosphorus oxyoxidized pulp, stirring the resulting pulp dispersion to uniformly disperse the pulp, and then filtering and dehydrating the pulp. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less.

Next, the washed phosphorus oxy-oxidized pulp was subjected to a neutralization treatment as follows.
First, the washed phosphorus oxidized pulp was diluted with 10 L of ion-exchanged water, and then a 1N aqueous sodium hydroxide solution was added little by little while stirring to obtain a phosphorus oxidized pulp slurry having a pH of 12 to 13. The phosphorus oxidized pulp slurry was then dehydrated to obtain a phosphorus oxidized pulp that had been subjected to a neutralization treatment. The neutralized phosphorus oxidized pulp was then subjected to the above-mentioned washing treatment.

The infrared absorption spectrum of the phosphorus oxy-oxidized pulp thus obtained was measured using FT-IR, and as a result, absorption due to the P=O of the phosphate group was observed around 1230 cm ^-1 , confirming that the phosphate group had been added to the pulp.
In addition, the obtained phosphorus oxo-oxidized pulp was analyzed using an X-ray diffraction device, and typical peaks were confirmed at two positions, around 2θ = 14° to 17° and around 2θ = 22° to 23°, confirming that it has a cellulose type I crystal structure.

Ion-exchanged water was added to the obtained phosphorus oxo-oxidized pulp 1 to prepare a slurry having a solid content concentration of 2% by mass. This slurry was treated twice at a pressure of 200 MPa with a wet pulverizer (Starburst, manufactured by Sugino Machine Co., Ltd.) to obtain a fine fibrous cellulose dispersion (1) containing fine fibrous cellulose.
X-ray diffraction confirmed that this fine fibrous cellulose maintained the cellulose I type crystal structure.
The fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 3 to 5 nm. The amount of phosphorus oxoacid groups (amount of first dissociated acid) was 1.45 mmol/g. The total amount of dissociated acid was 2.45 mmol/g.

<Production Example 2>
[Production of fine fibrous cellulose dispersion (2)]
The same procedure as in Production Example 1 was carried out except that 33 parts by mass of phosphorous acid (phosphonic acid) was used instead of ammonium dihydrogen phosphate, to obtain phosphorus oxo-oxidized pulp 2 and fine fibrous cellulose dispersion (2).

The infrared absorption spectrum of the obtained phosphorus oxo-oxidized pulp 2 was measured using FT-IR. As a result, absorption due to P=O of a phosphonic acid group, which is a tautomer of a phosphorous acid group, was observed around 1210 cm ^-1 , confirming that a phosphorous (or phosphorous) group (phosphonic acid group) was added to the pulp.

The obtained phosphorus oxo-oxidized pulp 2 was also analyzed using an X-ray diffraction apparatus. Typical peaks were confirmed at two positions, near 2θ = 14° to 17° and near 2θ = 22° to 23°, and it was confirmed that the pulp had a cellulose type I crystal structure.

X-ray diffraction confirmed that the obtained fine fibrous cellulose maintained the cellulose type I crystal structure. Furthermore, the fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 3 to 5 nm. Furthermore, for the fine fibrous cellulose dispersion (2), the amount of phosphorous acid groups (amount of first dissociated acid) introduced into the cellulose and the total amount of dissociated acid were 1.51 mmol/g and 1.54 mmol/g, respectively.

<Production Example 3>
[Production of fine fibrous cellulose dispersion (3)]
The same procedure as in Production Example 1 was carried out except that 38 parts by mass of amidosulfuric acid was used instead of ammonium dihydrogen phosphate and the heating time in the hot air dryer was 19 minutes, thereby obtaining a sulfated pulp and fine fibrous cellulose dispersion (3).

The obtained sulfated pulp was subjected to infrared absorption spectrum measurement using FT-IR, and as a result, absorption due to sulfate groups was observed in the vicinity of 1220-1260 cm ^-1 , confirming that sulfate groups had been added to the pulp.

The obtained sulfated pulp was also analyzed using an X-ray diffraction device, and typical peaks were confirmed at two positions, near 2θ = 14° to 17° and near 2θ = 22° to 23°, confirming that it contains cellulose type I crystals.

X-ray diffraction confirmed that the obtained fine fibrous cellulose maintained the cellulose type I crystal structure. Furthermore, the fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 3 to 5 nm. Furthermore, the amount of sulfate groups introduced into the cellulose in the fine fibrous cellulose dispersion (3) was 1.12 mmol/g.

<Production Example 4>
[Production of fine fibrous cellulose dispersion (4)]
The raw material pulp used was softwood kraft pulp (undried) manufactured by Oji Paper Co., Ltd. This raw material pulp was subjected to an alkaline TEMPO oxidation treatment as follows.
First, the raw pulp equivalent to 100 parts by dry weight, 1.6 parts by weight of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), and 10 parts by weight of sodium bromide were dispersed in 10,000 parts by weight of water. Next, 13 parts by weight of an aqueous solution of sodium hypochlorite was added to 3.8 mmol per 1.0 g of pulp to start the reaction. During the reaction, a 0.5 M aqueous solution of sodium hydroxide was added dropwise to keep the pH at 10 to 10.5, and the reaction was deemed to have ended when no change in pH was observed.

The resulting TEMPO oxidized pulp was then subjected to a washing treatment.
The washing treatment was carried out by dehydrating the pulp slurry after the TEMPO oxidation to obtain a dehydrated sheet, pouring 5,000 parts by mass of ion-exchanged water into the sheet, stirring to disperse the sheet uniformly, and then filtering and dehydrating the sheet. The washing was completed when the electrical conductivity of the filtrate reached 100 μS/cm or less.

Furthermore, when the obtained TEMPO oxidized pulp was analyzed using an X-ray diffraction device, typical peaks were confirmed at two positions, near 2θ = 14° to 17° and near 2θ = 22° to 23°, confirming that it has a cellulose type I crystal structure.

Ion-exchanged water was added to the obtained TEMPO oxidized pulp to prepare a slurry having a solid content of 2% by mass. This slurry was treated twice with a wet pulverizer (Starburst, manufactured by Sugino Machine Co., Ltd.) at a pressure of 200 MPa to obtain a fine fibrous cellulose dispersion (4) containing fine fibrous cellulose.
For the fine fibrous cellulose dispersion (4), the amount of carboxy groups introduced into the cellulose was 1.30 mmol/g.

<Method for measuring fiber width and ionic group content of fine fibrous cellulose>
[Measurement of fiber width]
The fiber width of the fine fibrous cellulose was measured by the following method. The supernatant of the fine fibrous cellulose dispersion was diluted with water so that the concentration of the fine fibrous cellulose was 0.01% by mass or more and 0.1% by mass or less, and the diluted solution was dropped onto a hydrophilized carbon grid membrane. After drying, the diluted solution was stained with uranyl acetate and observed under a transmission electron microscope (JEOL-2000EX, manufactured by JEOL Ltd.).

[Measurement of phosphorus oxoacid group content]
The amount of phosphorus oxoacid groups in the fine fibrous cellulose was measured by diluting a dispersion containing the target fine fibrous cellulose with ion exchange water to a content of 0.2 mass% to prepare a fine fibrous cellulose-containing slurry, treating it with an ion exchange resin, and then titrating it with an alkali.
The treatment with ion exchange resin was carried out by adding 1/10 by volume of a strongly acidic ion exchange resin (Amberjet 1024; Organo Corporation, conditioned) to the above-mentioned fine fibrous cellulose-containing slurry, shaking the mixture for 1 hour, and then pouring the mixture onto a mesh with 90 μm openings to separate the resin and the slurry.
In addition, the titration using alkali was performed by measuring the change in the pH value of the slurry while adding 10 μL of 0.1N sodium hydroxide aqueous solution to the fine fibrous cellulose-containing slurry after the treatment with ion exchange resin every 5 seconds. The titration was performed while blowing nitrogen gas into the slurry 15 minutes before the start of the titration. In this neutralization titration, two points are observed in which the increment (differential value of pH with respect to the amount of alkali added) is maximized in the curve plotting the measured pH against the amount of alkali added. Of these, the maximum point of the increment obtained first after the addition of alkali 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 the titration to the first end point is equal to the amount of first dissociated acid in the slurry used for the titration. In addition, the amount of alkali required from the start of the titration to the second end point is equal to the total amount of dissociated acid in the slurry used for the 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, and this value was taken as the amount of phosphate groups (mmol/g).

[Measurement of sulfur oxoacid group content]
The amount of sulfur oxoacid groups was determined by wet ashing the obtained fibrous cellulose with perchloric acid and concentrated nitric acid, diluting the cellulose at an appropriate ratio, and then measuring the amount of sulfur by ICP emission spectrometry.
The amount of sulfur was divided by the bone dry mass of the fibrous cellulose sample to obtain the amount of sulfur oxoacid groups (unit: mmol/g).

[Measurement of Carboxy Group Amount]
The amount of carboxyl groups in the fine fibrous cellulose was measured in the same manner as in [Measurement of the amount of phosphorus oxoacid groups], except that 50 μL of 0.1 N aqueous sodium hydroxide solution was added once every 30 seconds to the slurry containing the fine fibrous cellulose after the treatment with the ion exchange resin. The amount of carboxyl groups (mmol/g) was calculated by dividing the amount of alkali (mmol) required in the region corresponding to the first region shown in Figure 2 by the solid content (g) in the slurry to be titrated.

<Production Example 5>
[Production of UV-curable composition (1)]
A mixture of 100 parts by mass of N,N'-{[(2-acrylamide-2-[(3-acrylamidopropoxy)methyl]propane-1,3-diyl)bis(oxy)]bis(propane-1,3-diyl)}diacrylamide (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., FOM-03006) and 5 parts by mass of 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (manufactured by IGM Resins B.V., Omnirad1173) was dissolved in pure water to a concentration of 0.5% by mass, to prepare an ultraviolet-curable composition (1).

<Production Example 6>
[Production of ultraviolet-curable composition (2)]
A mixture of 250 parts by mass of urethane dispersion (UCECOAT7788, manufactured by Daicel-Allnex Corporation, solid content concentration 40%) and 5 parts by mass of 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (Omnirad1173, manufactured by IGM Resins B.V.) was dissolved in pure water to a concentration of 0.5% by mass to prepare an ultraviolet-curable composition (2).

<Production Example 7>
[Production of UV-curable composition (3)]
A mixture of 100 parts by mass of polyethylene glycol dimethacrylate (NK Ester 14G, manufactured by Shin-Nakamura Chemical Co., Ltd.) and 5 parts by mass of 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (Omnirad 1173, manufactured by IGM Resins B.V.) was dissolved in pure water to a concentration of 0.5% by mass, to prepare an ultraviolet-curable composition (3).

<Production Example 8>
[Production of ultraviolet-curable composition (4)]
A mixture of 100 parts by mass of ethoxylated glycerin triacrylate (NK Ester A-GLY-9E, manufactured by Shin-Nakamura Chemical Co., Ltd.) and 5 parts by mass of 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (Omnirad 1173, manufactured by IGM Resins B.V.) was dissolved in pure water to a concentration of 0.5% by mass, to prepare an ultraviolet-curable composition (4).

Example 1
Ion-exchanged water was added to the fine fibrous cellulose dispersion (1) having a solid content concentration of 2.0% by mass to obtain a fine fibrous cellulose dispersion (A) having a solid content concentration of 0.5% by mass.
To 30 parts by mass of the obtained fine fibrous cellulose dispersion (A), 70 parts by mass of the ultraviolet-curable composition (1) solution was added and stirred. Furthermore, the mixed liquid was weighed so that the finished basis weight of the sheet was 65 g/ ^m2 , spread on a commercially available acrylic plate, and dried for 24 hours in a dryer at 50 ° C. In addition, a dam plate of 210 mm x 297 mm was placed on the acrylic plate to obtain a predetermined basis weight. By the above procedure, an uncured ultraviolet-curable sheet composition was obtained.
An illuminance meter (Ushio Inc., UIT-150-A, sensor part UVD-C365, sensitivity wavelength range 310-390 nm) was used in a belt conveyor type exposure device (Igraphics Inc., ECS-401GX, with IR cut filter) equipped with a metal halide lamp (Igraphics Inc., M04-L41) to set the illuminance to 150 mW/cm ² and the light amount to 750 mJ/cm ^2. An uncured ultraviolet-curable sheet-like composition was then placed on the sheet and cured by irradiating it with ultraviolet light to obtain a sheet.
The thickness of the obtained sheet was 50 μm.

Example 2
A sheet was obtained in the same manner as in Example 1, except that the amount of the ultraviolet-curable composition (1) solution was 50 parts by mass per 50 parts by mass of the fine fibrous cellulose dispersion (A).

Example 3
A sheet was obtained in the same manner as in Example 1, except that the amount of the ultraviolet-curable composition (1) solution was 30 parts by mass relative to 70 parts by mass of the fine fibrous cellulose dispersion (A).

Example 4
Ion-exchanged water was added to the fine fibrous cellulose dispersion (2) having a solid content concentration of 2.0% by mass to obtain a fine fibrous cellulose dispersion (B) having a solid content concentration of 0.5% by mass.
A sheet was obtained in the same manner as in Example 2, except that the fine fibrous cellulose dispersion (B) was used instead of the fine fibrous cellulose dispersion (A).

Example 5
Ion-exchanged water was added to the fine fibrous cellulose dispersion (3) having a solid content concentration of 2.0% by mass to obtain a fine fibrous cellulose dispersion (C) having a solid content concentration of 0.5% by mass.
A sheet was obtained in the same manner as in Example 2, except that the fine fibrous cellulose dispersion (C) was used instead of the fine fibrous cellulose dispersion (A).

Example 6
Ion-exchanged water was added to the fine fibrous cellulose dispersion (4) having a solid content concentration of 2.0% by mass to obtain a fine fibrous cellulose dispersion (D) having a solid content concentration of 0.5% by mass.
A sheet was obtained in the same manner as in Example 2, except that the fine fibrous cellulose dispersion (D) was used instead of the fine fibrous cellulose dispersion (A).

Example 7
A sheet was obtained in the same manner as in Example 1, except that the ultraviolet-curable composition (2) was used instead of the ultraviolet-curable composition (1).

Example 8
A sheet was obtained in the same manner as in Example 2, except that the ultraviolet-curable composition (2) was used instead of the ultraviolet-curable composition (1).

<Example 9>
A sheet was obtained in the same manner as in Example 3, except that the ultraviolet-curable composition (2) was used instead of the ultraviolet-curable composition (1).

Example 10
A sheet was obtained in the same manner as in Example 2, except that the ultraviolet-curable composition (3) was used instead of the ultraviolet-curable composition (1).

Example 11
A sheet was obtained in the same manner as in Example 2, except that the ultraviolet-curable composition (4) was used instead of the ultraviolet-curable composition (1).

<Comparative Example 1>
A sheet was obtained in the same manner as in Example 1, except that the amount of the ultraviolet-curable composition (1) solution was 90 parts by mass per 10 parts by mass of the fine fibrous cellulose dispersion (A).

[Measurement method]
<Total light transmittance of sheet>
The total light transmittance was measured using a haze meter (HM-150, manufactured by Murakami Color Laboratory Co., Ltd.) in accordance with JIS K 7361-1: 1997. The results are shown in Table 1.

<Sheet haze>
The haze was measured using a haze meter (HM-150, manufactured by Murakami Color Research Laboratory Co., Ltd.) in accordance with JIS K 7136:2000. The results are shown in Table 1.

<Yellow index (YI value) of sheet>
The yellow index (YI value) was measured using a color computer (Colour Cute i, manufactured by Suga Test Instruments Co., Ltd.) in accordance with JIS K 7373:2006. The results are shown in Table 1.

<Coefficient of linear thermal expansion>
The sheet was cut into a width of 4 mm x length of 30 mm by a laser cutter. This was set in a thermomechanical analyzer (Hitachi High-Tech Science Corporation, TMA7100), and heated from 25 ° C. to 180 ° C. at 5 ° C./min in a tensile mode with a chuck distance of 20 mm, a load of 10 g, and a nitrogen atmosphere, and then cooled from 180 ° C. to 10 ° C. at 10 ° C./min. The linear thermal expansion coefficient (ppm/K) was calculated from the measured value from 100 ° C. to 150 ° C. in the second heating when the temperature was raised from 10 ° C. to 200 ° C. at 5 ° C./min. The results are shown in Table 1.

<Water absorption rate>
A 50 mm square sheet was cut out, and the mass of the sheet after conditioning at 23° C. and 50% relative humidity for 24 hours was taken as W _d (g), and the mass of the sheet after immersion in ion-exchanged water for 24 hours was taken as W (g). The water absorption was calculated using the following formula. The results are shown in Table 1.
Water absorption rate (%) = (W-W _d )/W _d ×100

<Tensile modulus>
The tensile modulus of the sheet was measured using a tensile tester Tensilon (manufactured by A&D Co., Ltd.) in accordance with JIS P 8113:2006, except that the extension speed was changed to 5 mm/min. When measuring the tensile modulus of the sheet, the test piece was conditioned at 23°C and a relative humidity of 50% for 24 hours. The results are shown in Table 1.

The present invention provides a sheet that has a low linear thermal expansion coefficient, high transparency with suppressed yellowing, and high tensile modulus. The sheet of the present invention is expected to be used in a variety of applications where thermal expansion, transparency, and strength are important. Specifically, the sheet is expected to be used as a light-transmitting substrate for various display devices and solar cells, as well as substrates for electronic devices, components for home appliances, window materials for various vehicles and building materials, interior materials, exterior materials, packaging materials, and the like.

Claims (10)

A sheet obtained by curing with ultraviolet light a composition for a sheet, the composition containing an ultraviolet-polymerizable compound and fine fibrous cellulose having a fiber width of 1,000 nm or less,
the fine fibrous cellulose has a phosphorus oxoacid group or a group derived from a phosphorus oxoacid group,
the amount of phosphorus oxoacid groups or groups derived from phosphorus oxoacid groups introduced into the fine fibrous cellulose is 0.50 mmol/g or more and 3.00 mmol/g or less;
In the sheet composition, an ultraviolet-polymerizable compound is dispersed or dissolved, and fine fibrous cellulose is dispersed,
The molecular weight of the ultraviolet polymerizable compound is 500 or more,
A linear thermal expansion coefficient of 30 ppm/K or less at 100° C. or more and 150° C. or less; and
The yellow index (YI value) is 5 or less.
Sheet. The sheet according to claim 1, wherein the content of the fine fibrous cellulose relative to the total amount of the ultraviolet-polymerizable compound and the fine fibrous cellulose is 20% by mass or more and 80% by mass or less. The sheet according to claim 1 or 2, wherein the sheet has a tensile modulus of elasticity of 3 GPa or more. The sheet according to any one of claims 1 to 3, wherein the total light transmittance of the sheet is 70% or more. The sheet according to any one of claims 1 to 4, wherein the haze of the sheet is 10% or less. The sheet according to any one of claims 1 to 5, wherein the sheet composition contains a polymerization initiator. The sheet according to any one of claims 1 to 6, wherein the ultraviolet-polymerizable compound is a radical-polymerizable compound. The sheet according to any one of claims 1 to 7, wherein the average fiber width of the fine fibrous cellulose is 2 nm or more and 10 nm or less. The sheet according to any one of claims 1 to 8, wherein the polymerizable compound contains at least one selected from the group consisting of a polyfunctional urethane acrylate compound, a polyfunctional acrylamide monomer, and a polyfunctional acrylic acid ester compound. A step of preparing a sheet composition containing an ultraviolet-polymerizable compound and fine fibrous cellulose having a fiber width of 1,000 nm or less;
The method for producing a sheet includes a step of applying and drying the sheet composition to obtain an uncured sheet-like composition, and a step of irradiating the uncured sheet-like composition with ultraviolet light to cure the composition with ultraviolet light,
the fine fibrous cellulose has a phosphorus oxoacid group or a group derived from a phosphorus oxoacid group,
the amount of phosphorus oxoacid groups or groups derived from phosphorus oxoacid groups introduced into the fine fibrous cellulose is 0.50 mmol/g or more and 3.00 mmol/g or less;
The molecular weight of the ultraviolet polymerizable compound is 500 or more,
The obtained sheet has a linear thermal expansion coefficient of 30 ppm/K or less at 100° C. or more and a yellow index (YI value) of 5 or less.
A method for manufacturing a sheet.