Sheets and laminates

Abstract

The purpose of the present invention is to provide: a sheet which is highly transparent, for which yellowing due to heating is suppressed, which has a high tensile modulus, and which has excellent flexibility; and a layered body having this sheet.　This sheet contains: microfibrous cellulose having a fiber width of 10 nm or less; and a cellulose derivative having a weight-average molecular weight of 1.0 x 104 to 3.0 x 105.

Description

[Technical Field]

[0001] The present invention relates to a sheet and a laminate having the sheet. [Background technology]

[0002] In recent years, materials made from renewable natural fibers have been attracting attention due to the need to replace petroleum resources and growing environmental awareness. Among natural fibers, fibrous cellulose with a fiber diameter of 10 μm to 50 μm, especially wood-derived fibrous cellulose (pulp), has been widely used mainly in paper products.

[0003] As fibrous cellulose, fine fibrous cellulose with a fiber diameter of 1 μm or less is also known. Sheets made from such fine fibrous cellulose and sheets containing fine fibrous cellulose and resin have also been developed. These sheets are known to have significantly increased fiber-to-fiber contact points, resulting in significantly improved tensile strength.

[0004] For example, Patent Documents 1 to 4 disclose sheets containing fine fibrous cellulose and a resin. Patent Document 1 describes a sheet containing fibrous cellulose having an anionic functional group and a fiber width of 1000 nm or less, which has a YI increase rate of 1500% or less. Patent Document 2 also describes a cellulose fiber composite that includes cellulose fibers having a number average fiber diameter of 4 to 100 nm and a matrix, and has a haze of 2 or less and a YI value of 25 or less after four cycles of heat treatment at 190°C for four hours. Furthermore, Patent Document 3 describes a modified cellulose fiber composite polyvinyl alcohol film containing a polyvinyl alcohol resin and modified cellulose fibers. Furthermore, Patent Document 4 discloses a resin composition containing modified cellulose fibers and one or more resins selected from the group consisting of a thermoplastic resin and a curable resin selected from epoxy resin, (meth)acrylic resin, phenolic resin, unsaturated polyester resin, polyurethane resin, or polyimide resin. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2019-108488 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-144363 [Patent Document 3] Japanese Patent Application Publication No. 2017-052840 [Patent Document 4] Japanese Patent Application Laid-Open No. 2017-052940 Summary of the Invention [Problem to be solved by the invention]

[0006] When a sheet containing fine fibrous cellulose is heated, yellowing due to heating may occur. As described in Patent Documents 1 and 2, various attempts have been made to inhibit yellowing. Furthermore, the sheets containing fine fibrous cellulose described in Patent Documents 1 to 4 have high rigidity and a high tensile modulus, but they have low flexibility and poor formability. An object of the present invention is to provide a sheet that has high transparency, is inhibited from yellowing due to heating, has a high tensile modulus, and is also excellent in flexibility, and a laminate including the sheet. [Means for solving the problem]

[0007] The present inventors have found that the above problems can be solved by a sheet containing fine fibrous cellulose and a specific cellulose derivative. The present invention provides the following <1> ~ <20> Regarding. <1> Fine fibrous cellulose with a fiber width of 10 nm or less and a weight-average molecular weight of 1.0 × 10 4 Over 3.0 x 10 5 A sheet comprising: a cellulose derivative: <2> The fine fibrous cellulose has anionic groups. <1> The sheet described in <3> The fine fibrous cellulose has a phosphorus oxo acid group or a group derived from a phosphorus oxo acid group. <1> or <2> The sheet described in <4> The amount of anionic groups in the fine fibrous cellulose is less than 0.50 mmol / g. <1> ~ <3> A sheet according to any one of the above. <5> The amount of anionic groups in the fine fibrous cellulose is 0.80 mmol / g or more. <2> or <3> The sheet described in <6> The fine fibrous cellulose contains carbamide groups. <1> ~ <5> A sheet according to any one of the above. <7> The cellulose derivative is a water-soluble cellulose ether. <1> ~ <6> A sheet according to any one of the above. <8> The water-soluble cellulose ether is nonionic. <7> The sheet described in <9> The water-soluble cellulose ether has at least one functional group selected from the group consisting of a methoxy group and a hydroxypropoxy group. <7> or <8> The sheet described in <10> The water-soluble cellulose ether is selected from the group consisting of methylcellulose and hydroxypropylmethylcellulose. <7> ~ <9> A sheet according to any one of the above. <11> The total content of fine fibrous cellulose and cellulose derivatives in the solid content of the sheet is 90% by mass or more. <1> ~ <10> A sheet according to any one of the above. <12> The content of fine fibrous cellulose in the solid content of the sheet is 50% by mass or more. <1> ~ <11> A sheet according to any one of the above. <13> The change in yellow index (YI value) of the sheet before and after heating at 160°C for 6 hours is 1.5 or less. <1> ~ <12> A sheet according to any one of the above. <14> The haze of the sheet is 5% or less. <1> ~ <13> A sheet according to any one of the above. <15> The total light transmittance of the sheet is 90% or more. <1> ~ <14> A sheet according to any one of the above. <16> The sheet has a tensile modulus of 6.5 GPa or more. <1> ~ <15> A sheet according to any one of the above. <17> The tensile elongation of the sheet is 3% or more. <1> ~ <16> A sheet according to any one of the above. <18> <1> ~ <17> 1. A laminate comprising the sheet according to any one of 1 to 8 above and a resin layer on at least one surface of the sheet. <19> For optical components, <1> ~ <17> A sheet according to any one of the above. <20> for food containers, cutlery, or straws; <1> ~ <17> A sheet according to any one of the above. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a sheet that has high transparency, is inhibited from yellowing due to heating, has a high tensile modulus, and is also excellent in flexibility, and a laminate including the sheet. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a graph showing the relationship between the amount of NaOH dropped onto a slurry containing fine fibrous cellulose having phosphorus oxo acid groups and pH. [Figure 2] FIG. 2 is a graph showing the relationship between the amount of NaOH dropped onto a slurry containing fine fibrous cellulose having carboxy groups and pH. [Figure 3] FIG. 1 is a schematic diagram showing a method for evaluating the formability of a sheet. DETAILED DESCRIPTION OF THE INVENTION

[0010] Sheet The sheet of the present invention is made of fine fibrous cellulose having a fiber width of 10 nm or less (hereinafter simply referred to as "fine fibrous cellulose") and a cellulose ester having a weight-average molecular weight of 1.0 × 10 4 Over 3.0 x 10 5 and a cellulose derivative as follows: According to the present invention, there is provided a sheet which has high transparency, is inhibited from yellowing due to heating, has a high tensile modulus, and is excellent in flexibility. Microfibrous cellulose is cellulose defibrated to the nano-level, with a fiber width of 10 nm or less. Sheets containing microfibrous cellulose are highly transparent and are used in a variety of applications where transparency is required. However, sheets made from commonly available microfibrous cellulose have the problem of yellowing when heated. Furthermore, while sheets containing microfibrous cellulose have excellent stiffness and a high tensile modulus, they have the problem of low flexibility. Previously, attempts have been made to solve the above problems by modifying the fine fibrous cellulose or by incorporating other components, but it has not been possible to solve all of the above problems. As a result of intensive research by the present inventors, it has become clear that by containing fine fibrous cellulose and a cellulose derivative having a specific weight-average molecular weight, yellowing upon heating is suppressed compared to conventional products, and further, excellent flexibility is achieved while maintaining rigidity. The reason for the above effect is unclear, but it is presumed that the high affinity between fine fibrous cellulose and cellulose derivatives and the low amount of functional groups in the cellulose derivatives that cause yellowing contribute to the above effect. The present invention will be described in further detail below.

[0011] <Fine fibrous cellulose> The sheet of the present invention contains fine fibrous cellulose. Fine fibrous cellulose is fibrous cellulose having a fiber width of 10 nm or less. The fiber width of fibrous cellulose can be measured, for example, by observation using an electron microscope. The fiber width of the fine fibrous cellulose is 10 nm or less. From the viewpoint of suppressing dissolution of cellulose molecules in water and more easily achieving the effects of the fine fibrous cellulose, such as improved strength, rigidity, and dimensional stability, the fiber width of the fine fibrous cellulose is, for example, 1 nm or more, preferably 2 nm or more, and 10 nm or less, preferably 8 nm or less, more preferably 6 nm or less, and even more preferably 5 nm or less.

[0012] The average fiber width of the fine fibrous cellulose is preferably 2 nm or more and 10 nm or less, more preferably 2 nm or more and 5 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 more easily realize the effects of the fine fibrous cellulose, such as improved strength, rigidity, and dimensional stability. The fine fibrous cellulose is, for example, monofilament cellulose.

[0013] 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 this suspension is cast onto a hydrophilically treated carbon film-coated grid to prepare a sample for TEM observation. When wide fibers are included, SEM images of the surface cast onto glass may be observed. Next, electron microscope images are observed at magnifications of 1,000x, 5,000x, 10,000x, or 50,000x, depending on the width of the fibers to be observed. However, the sample, observation conditions, and magnification are adjusted to satisfy the following conditions. (1) Draw a line X at any point in the observed image, and 20 or more fibers intersect with the line X. (2) Draw a line Y that intersects the line perpendicularly within the same image, and 20 or more fibers intersect the line Y.

[0014] For observation images that satisfy the above conditions, the widths of the fibers intersecting with lines X and Y are 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 widths of the fibers intersecting with lines X and Y are read. In this way, the widths of at least 20 fibers x 2 x 3 = 120 fibers are read. The average value of the read fiber widths is then taken as the average fiber width of the fibrous cellulose.

[0015] The fiber length of the fine fibrous cellulose is not particularly limited, but is preferably 0.1 μm to 1,000 μm, more preferably 0.1 μm to 800 μm, and even more preferably 0.1 μm to 600 μm. By setting the fiber length within the above range, destruction of the crystalline regions of the fine fibrous cellulose can be suppressed. It also becomes possible to set the slurry viscosity of the fine fibrous cellulose within an appropriate range. The fiber length of the fine fibrous cellulose can be determined, for example, by image analysis using TEM, SEM, or AFM.

[0016] 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) monochromated with graphite. Specifically, the cellulose 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 type I 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 allows for the expectation of even better performance in terms of heat resistance and low linear thermal expansion coefficient. The degree of crystallinity can be determined by measuring an X-ray diffraction profile and using the pattern in a conventional manner (Seagal et al., Textile Research Journal, Vol. 29, p. 786, 1959).

[0017] 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, 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 prepared. 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 used as an aqueous dispersion, handling such as dilution is easier.

[0018] The fine fibrous cellulose in this embodiment preferably has at least one of, for example, 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. Note that it is preferable that the cellulose has an ionic group, preferably an anionic group, at least during the defibration treatment, and the ionic group may be removed after the defibration treatment. Furthermore, the fine fibrous cellulose does not need to be subjected to a treatment for introducing ionic groups.

[0019] 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, and carboxyalkyl groups. Among these, 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, carboxyethyl 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 fibrous cellulose can be further improved, for example, even under alkaline or acidic conditions, making it easier to obtain a high-strength, highly transparent sheet. Examples of cationic groups as ionic groups include ammonium groups, phosphonium groups, sulfonium groups, etc. Among these, the cationic group is preferably an ammonium group.

[0020] The phosphorus oxo acid group or the substituent derived from the phosphorus oxo acid group is, for example, a substituent represented by the following formula (1). A plurality of types of substituents 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.

[0021] [ka]

[0022] In formula (1), a, b, and n are natural numbers, and m is an arbitrary number (where a=b×m). At least one of n α and α' is O. - and the rest are R or OR. Note that all of α and α' are O - The n α's may all be the same or may be different. b+ is a cation of one or more valences consisting of organic or inorganic substances.

[0023] 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 therefrom. In formula (1), n ​​is preferably 1.

[0024] 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.

[0025] Furthermore, examples of the derivative group in R include, but are not limited to, functional groups in which at least one functional group selected from the group consisting of a carboxy group, a carboxylate group (—COO—), a hydroxy group, an amino group, and an ammonium group is added to or substituted on the main chain or side chain of the above-mentioned hydrocarbon groups. The number of carbon atoms constituting the main chain of R is not particularly limited, but is preferably 20 or less, and more preferably 10 or less. By setting the number of carbon atoms constituting the main chain of R within the above range, the molecular weight of the phosphorus oxoacid group can be set within an appropriate range, facilitating penetration into the fiber raw material and increasing the yield of finely divided cellulose fibers. When multiple Rs are present in formula (1) or when multiple types of substituents represented by formula (1) are introduced into the fibrous cellulose, the multiple Rs may be the same or different.

[0026] 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. It should be noted that in formula (1), β b+ When a plurality of β b+ may be the same or different. The monovalent or higher cations made of organic or inorganic substances include β b+ Sodium or potassium ions are preferred because they are less likely to yellow when the fiber material containing the cation is heated and are easy to use industrially, but there is no particular limitation.

[0027] More specifically, examples of the phosphorus oxo acid group or a substituent derived from a phosphorus oxo acid group include a phosphate group (-POH), a salt of a phosphate group, a phosphorous acid (phosphonic acid) group (-POH), and a salt of a phosphite (phosphonic acid) group. The phosphorus oxo acid group or a substituent derived from a phosphorus oxo acid group may also 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), or an alkyl phosphonic acid group (e.g., a methylphosphonic acid group).

[0028] The sulfur oxoacid group (a sulfur oxoacid group or a substituent derived from a sulfur oxoacid group) is, for example, a substituent represented by the following formula (2). A plurality of types of substituents represented by the following formula (2) may be introduced into each fibrous cellulose. In this case, the plurality of introduced substituents represented by the following formula (2) may be the same or different.

[0029] [ka]

[0030] In the above structural formula, b and n are natural numbers, p is 0 or 1, and m is an arbitrary number (where 1 = b × m). When n is 2 or more, the multiple p's may be the same or different numbers. In the above structural formula, β b+is a monovalent or higher cation composed of an organic or inorganic substance. Examples of the monovalent or higher cation composed of an organic substance include organic onium ions. Examples of the organic onium ions include organic ammonium ions and organic phosphonium ions. Examples of the organic ammonium ions include aliphatic ammonium ions and aromatic ammonium ions, and examples of the organic phosphonium ions include aliphatic phosphonium ions and aromatic phosphonium ions. Examples of the monovalent or higher cation composed of an inorganic substance include ions of alkali metals such as sodium, potassium, or lithium, ions of divalent metals such as calcium or magnesium, hydrogen ions, ammonium ions, etc. Note that when multiple types of substituents represented by the above formula (2) are introduced into the fibrous cellulose, the multiple β b+ may be the same or different. The monovalent or higher cations made of organic or inorganic substances include β b+ Sodium or potassium ions are preferred because they are less likely to yellow when the fiber material containing the cation is heated and are easy to use industrially, but there is no particular limitation.

[0031] The amount of ionic groups introduced into the fibrous cellulose is, for example, preferably 0.10 mmol / g or more per 1 g (mass) of fibrous cellulose, more preferably 0.20 mmol / g or more, even more preferably 0.50 mmol / g or more, even more preferably 0.80 mmol / g or more, and particularly preferably 1.00 mmol / g or more. The amount of ionic groups introduced into the fibrous cellulose is, for example, 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 controlling the amount of ionic groups introduced within the above range, it is possible to easily refine the fiber raw material and improve the stability of the fine fibrous cellulose. Furthermore, by controlling 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 is the value of the counter ion of the ionic group being a hydrogen ion (H + ) indicates the mass of fibrous cellulose when The ionic groups introduced into the fibrous cellulose may remain in the fine fibrous cellulose as they are, or may be removed after the fibrous cellulose is formed, as described below. From the viewpoint of suppressing yellowing of the sheet due to heating, it is preferable to remove the ionic groups.

[0032] The amount of ionic groups introduced into the fibrous cellulose can be measured, for example, by neutralization titration, which involves adding an alkali such as an aqueous sodium hydroxide solution to a slurry containing the obtained fibrous cellulose and measuring the change in pH to measure the amount introduced. FIG. 1 is a graph showing the relationship between the amount of NaOH added dropwise to fibrous cellulose having phosphorus oxo acid groups and pH.

[0033] 1 is a graph showing the relationship between the amount of NaOH added dropwise to a slurry containing fibrous cellulose having phosphorus oxo acid groups and pH. The amount of phosphorus oxo acid groups introduced into the fibrous cellulose is measured, for example, as follows. First, a slurry containing fibrous cellulose is treated with a strongly acidic ion exchange resin. If necessary, the measurement object may be subjected to a defibration treatment similar to the defibration treatment step described below before the treatment with the strongly acidic ion exchange resin. Next, the change in pH is observed while adding aqueous sodium hydroxide solution, and a titration curve like the one shown in the upper part of Figure 1 is obtained. The titration curve shown in the upper part of Figure 1 plots the measured pH against the amount of alkali added, while the titration curve shown in the lower part of Figure 1 plots the pH increment (derivative value) (1 / mmol) against the amount of alkali added. In this neutralization titration, two points of maximum increment (derivative value of pH with respect to the amount of alkali added) are confirmed on the curve plotting the measured pH against the amount of alkali added. Of these, the first maximum increment obtained after starting to add alkali is called the first endpoint, and the next maximum increment obtained is called the second endpoint. The amount of alkali required from the start of titration to the first endpoint is equal to the amount of first dissociated acid from the fibrous cellulose contained in the slurry used for titration; the amount of alkali required from the first endpoint to the second endpoint is equal to the amount of second dissociated acid from the fibrous cellulose contained in the slurry used for titration; and the amount of alkali required from the start of titration to the second endpoint is equal to the total amount of dissociated acid from the fibrous cellulose contained in the slurry used for titration. The amount of alkali required from the start of titration to the first endpoint divided by the solids content (g) in the slurry to be titrated is the amount of phosphorus oxo acid groups introduced (mmol / g). Note that the term "amount of phosphorus oxo acid groups introduced" (or "amount of phosphorus oxo acid groups") simply refers to the amount of first dissociated acid. In Figure 1, the region from the start of titration to the first endpoint is referred to as Region 1, and the region from the first endpoint to the second endpoint is referred to as Region 2. For example, if the phosphorus oxoacid group is a phosphate group and this phosphate group undergoes condensation, the apparent amount of weakly acidic groups in the phosphorus oxoacid group (also referred to herein as the second dissociated acid amount) decreases, and the amount of alkali required in Region 2 is less than the amount required in Region 1. On the other hand, the amount of strongly acidic groups in the phosphorus oxoacid group (also referred to herein as the first dissociated acid amount) corresponds to the amount of phosphorus atoms regardless of whether condensation occurs. Furthermore, if the phosphorus oxoacid group is a phosphite group, the phosphorus oxoacid group no longer contains weakly acidic groups, and the amount of alkali required in Region 2 is reduced or may even be zero. In this case, there is only one point on the titration curve where the pH increment is maximized. The above-mentioned amount of introduced phosphorus oxoacid groups (mmol / g) indicates the amount of phosphorus oxoacid groups in the acid-form fibrous cellulose (hereinafter referred to as the amount of phosphorus oxoacid groups (acid form)) because the denominator indicates the mass of the acid-form fibrous cellulose. On the other hand, when the counter ions of the phosphorus oxoacid groups are substituted with an arbitrary cation C so as to be charge equivalent, the amount of phosphorus oxoacid groups in the fibrous cellulose with the cation C as the counter ion (hereinafter referred to as the amount of phosphorus oxoacid groups (C form)) can be determined by converting the denominator to the mass of the fibrous cellulose when the cation C is the counter ion. That is, it is calculated using the following formula. Amount of phosphorus oxoacid group (C type) = Amount of phosphorus oxoacid group (acid type) / {1 + (W - 1) × A / 1000} A [mmol / g]: total amount of anions derived from phosphorus oxoacid groups in fibrous cellulose (sum of the amount of strong acidic groups and weak acidic groups in phosphorus oxoacid groups) W: Formula weight per valence of cation C (e.g., Na is 23, Al is 9)

[0034] FIG. 2 is a graph showing the relationship between the amount of NaOH added dropwise to fibrous cellulose having carboxy groups and pH. The amount of carboxyl groups introduced into the fibrous cellulose can be measured, for example, as follows. First, a slurry containing fibrous cellulose is treated with a strongly acidic ion exchange resin. If necessary, the measurement object may be subjected to a defibration treatment similar to the defibration treatment step described below before treatment with the strongly acidic ion exchange resin. Next, a sodium hydroxide aqueous solution is added while observing the change in pH, and a titration curve such as that shown in FIG. 2 is obtained. If necessary, the measurement object may be subjected to a defibration treatment similar to the defibration treatment step described below. As shown in Figure 2, in this neutralization titration, a single point is observed where the increment (the differential value of pH with respect to the amount of alkali added) reaches a maximum on the curve plotting the measured pH against the amount of alkali added. This maximum increment is called the first endpoint. Here, the region from the start of the titration to the first endpoint in Figure 2 is called the first region. The amount of alkali required in the first region is equal to the amount of carboxyl groups in the slurry used for titration. The amount of alkali introduced (mmol / g) was calculated by dividing the amount of alkali (mmol) required in the first region of the titration curve by the solids content (g) in the fine fibrous cellulose-containing slurry to be titrated. The amount of carboxyl groups introduced (mmol / g) is calculated based on the amount of carboxyl groups introduced (mmol / g) when the counter ions of the carboxyl groups are hydrogen ions (H + ) (hereinafter referred to as the amount of carboxy groups (acid type)) per 1 g of fibrous cellulose.

[0035] The above-mentioned amount of carboxy groups introduced (mmol / g) indicates the amount of carboxy groups in the acid-form fibrous cellulose (hereinafter referred to as the amount of carboxy groups (acid form)) because the denominator is the mass of the acid-form fibrous cellulose. On the other hand, when the counter ions of the carboxy groups are substituted with any cation C so as to be 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 form)) (mmol / g) can be determined by converting the denominator to the mass of the fibrous cellulose when the cation C is the counter ion. That is, it is calculated using the following formula. Amount of carboxyl group (C type) = Amount of carboxyl group (acid type) / {1 + (W - 1) × (Amount of carboxyl group (acid type)) / 1000} W: Formula weight per valence of cation C (e.g., Na is 23, Al is 9)

[0036] The amount of sulfur oxoacid and sulfonic acid groups introduced into the fine fibrous cellulose can be determined by wet ashing the resulting fibrous cellulose using perchloric acid and concentrated nitric acid, then diluting it at an appropriate ratio and measuring the amount of sulfur by ICP atomic emission spectrometry. The amount of sulfur divided by the bone-dry mass of the fibrous cellulose used is taken as the amount of sulfur oxoacid groups and sulfonic acid groups (unit: mmol / g).

[0037] In measuring the amount of substituents by titration, adding too much sodium hydroxide solution or titrating too quickly can result in lower than expected amounts of substituents, leading to inaccurate values. For example, an appropriate amount and interval is desirable: titrating 10 to 57 μL of 0.1 N sodium hydroxide solution over 5 to 30 seconds. To eliminate the influence of carbon dioxide dissolved in the fibrous cellulose-containing slurry, it is also desirable to measure the amount of substituents while blowing an inert gas such as nitrogen gas into the slurry, for example, from 15 minutes before the start of titration until the end of titration. The measurement of the amount of ionic groups by the above-mentioned method is applied to fine fibrous cellulose with a fiber width of 1,000 nm or less. When measuring the amount of ionic groups in pulp fibers with a fiber width of more than 1,000 nm, the pulp fibers are first refined before measurement.

[0038] In the present invention, as described above, the fine fibrous cellulose may be obtained by removing at least a portion of the ionic groups from the fine fibrous cellulose defibrated by introducing the above-mentioned ionic groups, and from the viewpoints of lowering the YI value before heating and suppressing yellowing due to heating, it is preferable to remove the ionic groups. Note that the ionic groups do not need to be completely removed, and it is preferable to remove the ionic groups so that, for example, the amount of ionic groups is less than 0.50 mmol / g. In this case, it is preferable to remove the ionic groups from the defibrated fine fibrous cellulose and then carry out a uniform dispersion treatment to obtain a fine fibrous cellulose dispersion. By removing the ionic groups, a sheet with a low YI value and reduced yellowing due to heating can be obtained, which is preferable.

[0039] The fine fibrous cellulose may contain carbamide groups derived from urea and / or urea derivatives added during the production process of the fine fibrous cellulose, as described below. In this case, the amount of carbamide groups introduced into the fine fibrous cellulose (carbamide group amount) is, for example, preferably 1.50 mmol / g or less, more preferably 1.00 mmol / g or less, even more preferably 0.30 mmol / g or less, and particularly preferably 0.20 mmol / g or less per gram (mass) of fine fibrous cellulose. The amount of carbamide groups introduced into the fibrous cellulose (carbamide group amount) may be 0.00 mmol / g. Because carbamide groups and phosphorus oxo acid groups are introduced by reaction with hydroxyl groups of cellulose, the greater the amount of carbamide groups introduced, the less phosphorus oxo acid groups introduced. Therefore, by setting the amount of carbamide groups within the above range, the amount of phosphorus oxo acid groups introduced can be increased and maintained within an appropriate range. Since the carbamide group itself is not electrically conductive, the introduction of the carbamide group does not provide a charge repulsion effect (the effect of making the fibrous cellulose finer). Therefore, by setting the amount of carbamide group introduced within the above range and increasing the amount of phosphorus oxoacid group introduced, the dispersibility of the fibrous cellulose in the solvent can be more effectively improved, making it easier to obtain a highly transparent dispersion containing fine fibrous cellulose.

[0040] The amount of carbamide groups introduced is determined by measuring the amount of nitrogen covalently bonded to the fibrous cellulose. Specifically, after liberating and removing ionic nitrogen (ammonium ions) from a sample containing fibrous cellulose, the amount of nitrogen is measured by trace nitrogen analysis. The liberation of ionic nitrogen (ammonium ions) is carried out under conditions that do not substantially remove the nitrogen covalently bonded to the cellulose. For example, after the phosphorus oxoacid group introduction step, ammonium ions may be liberated by alkali treatment, washed, and then defibrated. Alternatively, the ammonium ions may be adsorbed and removed using a strongly acidic ion exchange resin after the defibration step. An example of a device for measuring the amount of nitrogen using trace nitrogen analysis is the TN-110 Total Nitrogen Trace Analyzer manufactured by Mitsubishi Chemical Analytech Corporation. Before measurement, the fibrous cellulose is dried at low temperature (e.g., in a vacuum oven at 40°C for 24 hours) until completely dry. The amount of carbamide groups introduced per unit mass of fibrous cellulose (mmol / g) is calculated by dividing the nitrogen content per unit mass of fibrous cellulose (g / g) obtained by trace nitrogen analysis by the atomic weight of nitrogen.

[0041] [Method for producing fine fibrous cellulose] (cellulose-containing fiber materials) Fine fibrous cellulose is produced from a fiber raw material containing cellulose. Although the cellulose-containing fiber raw material is not particularly limited, pulp is preferably used because of its ease of availability and low cost. Examples of pulp include wood pulp, non-wood pulp, and deinked pulp. Examples of wood pulp include, but are not limited to, chemical pulps such as hardwood kraft pulp (LBKP), softwood kraft pulp (NBKP), sulfite pulp (SP), dissolving pulp (DP), soda pulp (AP), unbleached kraft pulp (UKP), and oxygen-bleached kraft pulp (OKP); semi-chemical pulps such as semi-chemical pulp (SCP) and chemi-ground wood pulp (CGP); and mechanical pulps such as groundwood pulp (GP) and thermomechanical pulp (TMP, BCTMP). Examples of non-wood pulp include, but are not limited to, cotton-based pulps such as cotton linters and cotton lint, and non-wood pulps such as hemp, straw, bamboo, and bagasse. The deinked pulp is not particularly limited, but may be, for example, deinked pulp made from recycled paper. The pulp of this embodiment may be one of the above 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 ease 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 having a high cellulose content 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 use of long-fiber fine fibrous cellulose with a large axial ratio tends to increase viscosity. 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. Furthermore, instead of fiber raw materials containing cellulose, fibers formed from linear nitrogen-containing polysaccharide polymers such as chitin and chitosan can also be used.

[0042] To obtain the above-mentioned fine fibrous cellulose having ionic groups introduced therein, 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 the ionic group introduction step include a phosphorus oxo acid group introduction step, a carboxy group introduction step, and a sulfur oxo acid group introduction step. Each of these steps will be explained below.

[0043] (Ionic group introduction step) -Phosphorus oxoacid group introduction process- The phosphorus oxo acid group introduction step involves reacting a cellulose-containing fiber raw material with at least one compound (hereinafter also referred to as "compound A") selected from compounds capable of introducing phosphorus oxo acid groups by reacting with hydroxyl groups in the cellulose-containing fiber raw material, thereby obtaining a phosphorus oxo acid group-introduced fiber. In the phosphate group introduction step according to this embodiment, the reaction of the cellulose-containing fiber raw material with compound A may be carried out in the presence of at least one selected from urea and its derivatives (hereinafter also referred to as "compound B"). Alternatively, the cellulose-containing fiber raw material with compound A may be reacted in the absence of compound B. One example of a method for reacting compound A with a fiber raw material in the presence of compound B is to mix compound A and compound B with a fiber raw material in a dry, wet, or slurry state. Among these methods, using a dry or wet fiber raw material is preferred because of the high uniformity of the reaction, and using a dry fiber raw material is particularly preferred. The form of the fiber raw material is not particularly limited, but is preferably a cotton-like or thin sheet form. Compound A and compound B may be added to the fiber raw material in the form of a powder, a solution dissolved in a solvent, or a melted state heated to or above their melting point. Among these methods, adding compound A and compound B in the form of a solution dissolved in a solvent, particularly an aqueous solution, is preferred because of the high uniformity of the reaction. Compound A and compound B may be added to the fiber raw material simultaneously, separately, or as a mixture. The method for adding compound A and compound B is not particularly limited, but when compound A and compound B are in solution form, the fiber raw material may be immersed in the solution to absorb the liquid and then removed, or the solution may be added dropwise to the fiber raw material. Alternatively, the required amounts of compound A and compound B may be added to the fiber raw material, or excess amounts of compound A and compound B may be added to the fiber raw material, and then the excess compound A and compound B may be removed by squeezing or filtration.

[0044] 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 examples thereof include, but are not limited to, phosphoric acid or a salt thereof, phosphorous acid or a salt thereof, dehydrated condensed phosphoric acid or a salt thereof, and phosphoric anhydride (diphosphorus pentoxide). Phosphoric acid can be used with various purities, such as 100% phosphoric acid (orthophosphoric acid) or 85% phosphoric acid. Phosphorous acid can be used with, for example, 99% phosphorous acid (phosphonic acid). Dehydrated condensed phosphoric acid is formed by condensing two or more molecules of phosphoric acid through a dehydration reaction, and examples thereof include pyrophosphoric acid and polyphosphoric acid. Phosphates, phosphites, and dehydrated condensed phosphates include lithium salts, sodium salts, potassium salts, and ammonium salts of phosphoric acid, phosphorous acid, or dehydrated condensed phosphoric acid, and these can be neutralized to various degrees. Among these, 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, from the viewpoints of high efficiency in introducing phosphate groups, easier improvement of defibration efficiency in the defibration step described below, low cost, and ease of industrial application. 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 yield improvement effect and costs.

[0045] As described above, the compound B used in this embodiment is at least one selected from urea and its derivatives. Examples of the compound B include urea, biuret, 1-phenylurea, 1-benzylurea, 1-methylurea, and 1-ethylurea. From the viewpoint of improving the uniformity of the reaction, it is preferable to use an aqueous solution of compound B. Furthermore, from the viewpoint of further improving the uniformity of the reaction, it is preferable to use an aqueous solution in which both compound A and compound B are dissolved. The amount of compound B added relative to the fiber raw material (bone dry mass) is not particularly limited, but is preferably, for example, 1% by mass or more and 500% by mass or less, more preferably 10% by mass or more and 400% by mass or less, and even more preferably 100% by mass or more and 350% by mass or less.

[0046] In the reaction of a fiber raw material containing cellulose with compound A, the reaction system may contain, in addition to compound B, for example, amides or amines. Examples of amides include formamide, dimethylformamide, acetamide, and dimethylacetamide. Examples of amines include methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, and hexamethylenediamine. Among these, triethylamine is known to function as a particularly good reaction catalyst.

[0047] 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. The heat treatment temperature is preferably selected so that the phosphorus oxo acid group can be efficiently introduced while suppressing thermal decomposition and hydrolysis of the fiber. The heat treatment temperature is preferably, for example, 50°C to 300°C, more preferably 100°C to 250°C, and even more preferably 130°C to 200°C. Furthermore, various types of equipment having heat transfer media can be used for the heat treatment, including, for example, a stirring dryer, rotary dryer, disk dryer, roll-type heater, plate-type heater, fluidized-bed dryer, band-type dryer, filtration dryer, vibration fluidized dryer, flash dryer, reduced-pressure dryer, infrared heater, far-infrared heater, microwave heater, and high-frequency dryer.

[0048] In the heat treatment according to this embodiment, for example, compound A may be added to a thin sheet-like fiber raw material by impregnation or other methods, followed by heating, or heating while kneading or stirring the fiber raw material and compound A in a kneader or the like. This makes it possible to suppress unevenness in the concentration of compound A in the fiber raw material and more uniformly introduce phosphate groups onto the surface of the cellulose fibers contained in the fiber raw material. This is thought to be because, when water molecules move to the surface of the fiber raw material as it dries, the dissolved compound A is attracted to the water molecules by surface tension, preventing it from migrating to the surface of the fiber raw material in the same way (i.e., causing unevenness in the concentration of compound A). Furthermore, the heating device used for the heat treatment is preferably one that can constantly discharge, to the outside of the system, moisture contained in the slurry and moisture generated in the dehydration condensation (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 an oven with a blower system. Constantly discharging moisture from the system can suppress the hydrolysis of phosphate ester bonds, which is the reverse reaction of phosphate esterification, as well as the acid hydrolysis of sugar chains in the fiber. This makes it possible to obtain fine fibrous cellulose with a high axial ratio. The heat treatment time is, for example, preferably from 1 second to 300 minutes after the water content 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 amount of phosphorus oxo acid groups introduced can be kept within a preferred range by setting the heating temperature and heating time within appropriate ranges.

[0049] The phosphorus oxo acid group introduction step may be carried out at least once, but may also be carried out twice or more. By carrying out the phosphorus oxo acid group introduction step twice or more, a large number of phosphorus oxo acid groups can be introduced into the fiber raw material. In the present embodiment, a preferred example is when the phosphorus oxo acid group introduction step is carried out twice.

[0050] The amount of phosphorus oxoacid groups introduced into the fiber raw material is, for example, preferably 0.10 mmol / g or more per gram (mass) of fibrous cellulose, more preferably 0.20 mmol / g or more, even more preferably 0.50 mmol / g or more, even more preferably 0.80 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, for example, preferably 5.20 mmol / g or less per gram (mass) of fibrous cellulose, more preferably 3.65 mmol / g or less, and even more preferably 3.00 mmol / g or less. By keeping the amount of phosphorus oxoacid groups introduced within the above range, it is possible to facilitate the microfibrillation of the fiber raw material and improve the stability of the fine fibrous cellulose.

[0051] -Carboxy group introduction process- The carboxyl group introduction process is carried out by treating a fiber raw material containing cellulose with an oxidation treatment such as ozone oxidation, oxidation by the Fenton method, or TEMPO oxidation treatment, or 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.

[0052] The acid anhydride of a compound having a group derived from carboxylic acid is not particularly limited, but examples thereof include acid anhydrides of dicarboxylic acid compounds such as maleic anhydride, succinic anhydride, phthalic anhydride, glutaric anhydride, adipic anhydride, itaconic anhydride, etc. Furthermore, the derivative of an acid anhydride of a compound having a group derived from carboxylic acid is not particularly limited, but examples thereof include acid anhydrides of compounds having carboxy groups such as dimethyl maleic anhydride, diethyl maleic anhydride, diphenyl maleic anhydride, etc., in which at least some of the hydrogen atoms are substituted with substituents such as alkyl groups or phenyl groups.

[0053] When TEMPO oxidation treatment is performed in the carboxyl group introduction step, it is preferable to perform the treatment under conditions of, for example, a pH of 6 or higher and 8 or lower. This type of treatment is also called neutral TEMPO oxidation treatment. Neutral TEMPO oxidation treatment can be performed, for example, by adding pulp as the fiber raw material, a nitroxy radical such as TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) as the catalyst, and sodium hypochlorite as the sacrificial reagent to a sodium phosphate buffer solution (pH = 6.8). Furthermore, by adding sodium chlorite, aldehydes generated during the oxidation process can be efficiently oxidized to carboxyl groups. The TEMPO oxidation treatment may also be carried out under conditions of a pH of 10 to 11. This type of treatment is also called 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 substituent. For example, when carboxyl groups are introduced by TEMPO oxidation, the amount is preferably 0.10 mmol / g or more per gram (mass) of fibrous cellulose, more preferably 0.20 mmol / g or more, even more preferably 0.50 mmol / g or more, even more preferably 0.80 mmol / g or more, and particularly preferably 0.90 mmol / g or more. Also, the amount is 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. Furthermore, when the substituent is a carboxymethyl group, the amount may be 5.8 mmol / g or less per gram (mass) of fibrous cellulose.

[0054] -Sulfur oxoacid 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 hydroxyl groups in a cellulose-containing fiber raw material react with sulfur oxoacid to obtain cellulose fibers having sulfur oxoacid groups (sulfur oxoacid group-introduced fibers).

[0055] In the sulfur oxo acid group introduction step, instead of compound A in the above-described <Phosphorus oxo acid group introduction step>, at least one compound (hereinafter also referred to as "compound C") selected from compounds capable of introducing sulfur oxo acid groups by reacting with hydroxyl groups in cellulose-containing fiber raw materials is used. Compound C may be any compound containing a sulfur atom and capable of forming an ester bond with cellulose, including, but not limited to, sulfuric acid or its salts, sulfurous acid or its salts, and sulfuric acid amides. Sulfuric acid of various purities can be used, for example, 96% sulfuric acid (concentrated sulfuric acid). Sulfurous acid can be 5% aqueous sulfurous acid. Sulfates or sulfites include lithium, sodium, potassium, and ammonium salts of sulfates or sulfites, which can be neutralized to various degrees. Sulfamic acid or the like can be used as the sulfuric acid amide. In the sulfur oxo acid group introduction step, it is preferable to use compound B in the above-described <Phosphorus oxo acid group introduction step> as well.

[0056] In the sulfur oxoacid group introduction step, the cellulose raw material is preferably mixed with an aqueous solution containing a sulfur oxoacid and urea and / or a urea derivative, and then the cellulose raw material is subjected to a heat treatment. The heat treatment temperature is preferably selected so that the sulfur oxoacid groups can be efficiently introduced while suppressing thermal decomposition and hydrolysis of the fiber. The heat treatment temperature is preferably 100°C or higher, more preferably 120°C or higher, and even more preferably 150°C or higher. The heat treatment temperature is preferably 300°C or lower, more preferably 250°C or lower, and even more preferably 200°C or lower.

[0057] In the heat treatment step, heating is preferably performed until substantially all moisture is removed. Therefore, the heat treatment time varies depending on the amount of moisture contained in the cellulose raw material and the amount of aqueous solution containing sulfur oxoacid and urea and / or a urea derivative added, but is preferably set to, for example, 10 seconds or more and 10,000 seconds or less. For the heat treatment, various devices having a heat medium can be used, such as an agitator dryer, rotary dryer, disk dryer, roll-type heater, plate-type heater, fluidized-bed dryer, band-type dryer, filtration dryer, vibration fluidized dryer, flash dryer, reduced-pressure dryer, infrared heater, far-infrared heater, microwave heater, and high-frequency dryer.

[0058] 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, even more preferably 0.80 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 keeping the amount of sulfur oxoacid groups introduced within the above range, it is possible to easily refine the fiber raw material and improve the stability of the fibrous cellulose.

[0059] -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.

[0060] 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 efficiency of introducing substituents, and therefore 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 a reagent as it is, or may be dissolved in an appropriate solvent and then added.

[0061] The concentration of the chlorine-based oxidizing agent in the solution in the oxidation step using the chlorine-based oxidizing agent is, for example, converted into an effective chlorine concentration, preferably from 1% by mass to 1,000% by mass, more preferably from 5% by mass to 500% by mass, and even more preferably from 10% by mass to 100% by mass. The amount of chlorine-based oxidizing agent added per 100 parts by mass of fiber raw material is preferably 1 part by mass or more and 100,000 parts by mass or less, more preferably 10 parts by mass or more and 10,000 parts by mass or less, and even more preferably 100 parts by mass or more and 5,000 parts by mass or less.

[0062] The reaction time with the chlorine-based oxidizing agent in the oxidation step using the 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 or more and 15 or less, more preferably 7 or more and 14 or less, and even more preferably 9 or more and 13 or less. At the start of the reaction and during the reaction, it is preferable to maintain the pH 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 and removed with water by filtration or the like.

[0063] -Xanthate group introduction process (xanthogen acid esterification process)- The process for producing fine fibrous cellulose may include, for example, a xanthate group introduction step (hereinafter also referred to as a xanthation step) as an ionic substituent introduction step. In the xanthation step, carbon disulfide and an alkali compound are added to a wet or dry fiber raw material having a hydroxyl group to allow the reaction to occur, thereby introducing xanthate groups 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 allowed to occur.

[0064] <Alkali cellulose> When introducing ionic functional groups into a fiber raw material, it is preferable to convert the cellulose contained in the fiber raw material into alkali cellulose by treating the cellulose with an alkaline solution. This treatment causes ionic dissociation of some of the hydroxyl groups in the cellulose, thereby increasing its 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. Due to their high versatility, it is preferable to use, for example, sodium hydroxide, potassium hydroxide, tetraethylammonium hydroxide, or tetrabutylammonium hydroxide. The conversion into alkali cellulose may be carried out simultaneously with the introduction of ionic functional groups, before the introduction, or at both the same time.

[0065] The solution temperature at the start of alkali cellulose formation 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.

[0066] The alkaline solution concentration is preferably 0.01 mol / L to 4 mol / L in molar concentration, more preferably 0.1 mol / L to 3 mol / L in molar concentration, and even more preferably 1 mol / L to 2.5 mol / L in molar concentration. In particular, when the treatment temperature is less than 10° C., the concentration is preferably 1 mol / L to 2 mol / L in molar concentration.

[0067] The treatment 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, and the alkali treatment time is preferably 6 hours or less, more preferably 5 hours or less, and even more preferably 4 hours or less.

[0068] 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 cellulose, making it easier to maintain the crystalline structure of cellulose type I, and increasing the yield of fine fibrous cellulose.

[0069] When the introduction of ionic functional groups and the conversion to alkali cellulose are not carried out simultaneously, the alkali cellulose obtained by the alkali treatment is preferably subjected to solid-liquid separation and water removal using a common deliquoring 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% to 50%, more preferably 10% to 40%, and even more preferably 15% to 35%.

[0070] -Phosphonic or phosphine group introduction step (phosphoalkylation step)- The ionic substituent introduction step may include a phosphonic or phosphine group introduction step (phosphoalkylation step). In the phosphoalkylation step, a compound having a reactive group and a phosphonic or phosphine group (compound E) is used as an essential component. A ), an optional alkali compound, and a compound B selected from the above-mentioned urea and its derivatives are added to a wet or dry fiber raw material having hydroxyl groups and reacted to introduce phosphonic or phosphine groups into the fiber raw material.

[0071] Examples of the reactive group include a halogenated alkyl group, a vinyl group, and an epoxy group (glycidyl group). Compound E AExamples of suitable compounds include vinyl phosphonic acid, phenyl vinyl phosphonic acid, and phenyl vinyl phosphinic acid. From the viewpoints of the efficiency of introducing substituents, the defibration efficiency, cost, and ease of handling, Compound E A is preferably vinylphosphonic acid. Furthermore, as an optional component, it is also preferable to use the compound B in the above-mentioned <Phosphorus oxo acid group introduction step> in the same manner, and the amount added is also preferably as described above.

[0072] Compound E A When adding, it may be added to the fiber raw material as a reagent (solid or liquid) as is, or it may be dissolved in an appropriate solvent and added. The fiber raw material is preferably converted into alkali cellulose in advance or simultaneously with the reaction. The method for converting into alkali cellulose is as described above.

[0073] The temperature during the reaction is, for example, preferably 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower.

[0074] Compound E A The amount 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.

[0075] The reaction time may vary depending on the reaction temperature, but is preferably from 1 minute to 1,000 minutes, more preferably from 10 minutes to 500 minutes, and even more preferably from 15 minutes to 400 minutes. After the reaction, excess reaction reagents, by-products, etc. may be washed and removed with water by filtration or the like.

[0076] -Sulfonic acid group introduction step (sulfoalkylation step)- The process for producing fine fibrous cellulose may include, for example, a sulfonic acid group introduction step (sulfoalkylation step) as an ionic substituent introduction step. In the sulfoalkylation, a compound having a reactive group and a sulfonic acid group (compound E) is used as an essential component. B ) and, as an optional component, an alkaline compound, and the aforementioned 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 sulfonic acid group into the fiber raw material.

[0077] Examples of the reactive group include a halogenated alkyl group, a vinyl group, and an epoxy group (glycidyl group). Compound E B Examples of such sulfonates include sodium 2-chloroethanesulfonate, sodium vinylsulfonate, sodium p-styrenesulfonate, 2-acrylamido-2-methylpropanesulfonic acid, etc. Among these, sodium vinylsulfonate is preferred from the standpoints of the efficiency of introducing substituents, and therefore defibration efficiency, cost, and ease of handling. Furthermore, as an optional component, it is also preferable to use the compound B in the above-mentioned <Phosphorus oxo acid group introduction step> in the same manner, and the amount added is also preferably as described above.

[0078] Compound E B The reagent may be added to the fiber raw material as is, or may be dissolved in an appropriate solvent and then added. The fiber raw material is preferably converted into alkali cellulose in advance or simultaneously with the reaction. The method for converting into alkali cellulose is as described above.

[0079] The temperature during the reaction is, for example, preferably 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower.

[0080] Compound E BThe amount 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.

[0081] 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 and removed with water by filtration or the like.

[0082] -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. C ), an optional alkaline compound, and the aforementioned 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 carboxyl group into the fiber raw material.

[0083] Examples of the reactive group include a halogenated alkyl group, a vinyl group, and an epoxy group (glycidyl group). Compound E C As the chloroisothiazolinone, monochloroacetic acid, sodium monochloroacetate, 2-chloropropionic acid, 3-chloropropionic acid, sodium 2-chloropropionate, and sodium 3-chloropropionate are preferred from the standpoints of efficiency in introducing substituents, and therefore defibration efficiency, cost, and ease of handling. Furthermore, as an optional component, it is also preferable to use the compound B in the above-mentioned <Phosphorus oxo acid group introduction step> in the same manner, and the amount added is also preferably as described above.

[0084] Compound E CThe reagent may be added to the fiber raw material as is, or may be dissolved in an appropriate solvent and then added. The fiber raw material is preferably converted into alkali cellulose in advance or simultaneously with the reaction. The method for converting into alkali cellulose is as described above.

[0085] The temperature during the reaction is, for example, preferably 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower.

[0086] Compound E C The amount 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.

[0087] 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 3 minutes to 500 minutes, and even more preferably from 5 minutes to 400 minutes. After the reaction, excess reaction reagents, by-products, etc. may be washed and removed with water by filtration or the like.

[0088] -Cationic group introduction step (cationization step)- As an essential component, a compound having a reactive group and a cationic group (compound E D ), an optional alkaline compound, and the aforementioned compound B selected from urea and its derivatives are added to a wet or dry fiber raw material having hydroxyl groups and reacted to introduce cationic groups into the fiber raw material.

[0089] Examples of the reactive group include a halogenated alkyl group, a vinyl group, and an epoxy group (glycidyl group).

[0090] Compound E DAs the alkyl group, glycidyl trimethyl ammonium chloride, 3-chloro-2-hydroxypropyl trimethyl ammonium chloride, etc. are preferred from the viewpoints of the efficiency of introducing substituents, and therefore the defibration efficiency, cost, and ease of handling.

[0091] Furthermore, it is also preferable to use the compound B in the above-mentioned <Phosphorus oxo acid group introduction step> as an optional component in the same manner. The amount added is also preferably as described above.

[0092] Compound E D The reagent may be added to the fiber raw material as is, or may be dissolved in an appropriate solvent and then added. The fiber raw material is preferably converted into alkali cellulose in advance or simultaneously with the reaction. The method for converting into alkali cellulose is as described above.

[0093] The temperature during the reaction is, for example, preferably 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower.

[0094] Compound E D The amount 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.

[0095] 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 5 minutes to 500 minutes, and even more preferably from 10 minutes to 400 minutes. After the reaction, excess reaction reagents, by-products, etc. may be washed and removed with water by filtration or the like.

[0096] (Cleaning process) In the method for producing fine fibrous cellulose in this embodiment, a washing step can be carried out on the ionic group-introduced fibers as needed. The washing step is carried out by washing the ionic group-introduced fibers with water or an organic solvent, for example. The washing step may be carried out after each step described below, and the number of washings carried out in each washing step is not particularly limited.

[0097] (Alkali treatment process) 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 an example thereof is 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 their high versatility. The solvent contained in the alkaline solution may be either water or an organic solvent. Among these, the solvent contained in the alkaline solution is preferably water or a polar solvent including a polar organic solvent such as an 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 their high versatility. The temperature of the alkaline solution in the alkaline treatment step is not particularly limited, but is preferably from 5°C to 80°C, and more preferably from 10°C to 60°C. The immersion time of the ionic group-introduced fiber in the alkaline solution in the alkaline treatment step is not particularly limited, but is preferably from 5 minutes to 30 minutes, and more preferably from 10 minutes to 20 minutes. The amount of alkaline solution used in the alkaline treatment is not particularly limited, but is preferably from 100% by mass to 100,000% by mass, and more preferably from 1,000% by mass to 10,000% by 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 treatment or ion exchange treatment of the anionic groups. In this case, the temperature of the alkali solution is preferably room temperature.

[0098] 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 defibrating treatment step.

[0099] (Acid treatment process) When producing fine fibrous cellulose, the fiber raw material may be subjected to an acid treatment between the step of introducing ionic groups and the defibration treatment step described below. For example, the ionic group introduction step, acid treatment step, alkali treatment step, and defibration treatment step may be performed in this order. The acid treatment method is not particularly limited, but examples include a method of immersing the fiber raw material in an acid-containing acid solution. The concentration of the acid solution used is not particularly limited, but is preferably 10% by mass or less, and more preferably 5% by mass or less. The pH of the acid solution used is also not particularly limited, but is preferably 0 to 4, and more preferably 1 to 3. Examples of the acid contained in the acid solution include inorganic acids, sulfonic acids, and carboxylic acids. Examples of inorganic acids include sulfuric acid, nitric acid, hydrobromic acid, hydroiodic acid, hypochlorous acid, chlorous acid, chloric acid, perchloric acid, phosphoric acid, and boric acid. Examples of sulfonic acids include methanesulfonic acid, ethanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid, and trifluoromethanesulfonic acid. Examples of carboxylic acids include formic acid, acetic acid, citric acid, gluconic acid, lactic acid, oxalic acid, and tartaric acid. Among these, hydrochloric acid or sulfuric acid is particularly preferred. The temperature of the acid solution in the acid treatment is not particularly limited, but is preferably from 5°C to 100°C, and more preferably from 20°C to 90°C. The immersion time in the acid solution in the acid treatment is not particularly limited, but is preferably from 5 minutes to 120 minutes, and more preferably from 10 minutes to 60 minutes. The amount of the acid solution used in the acid treatment is not particularly limited, but is preferably from 100% to 100,000% by mass, and more preferably from 1,000% to 10,000% by mass, based on the absolute dry mass of the fiber raw material. When the fine fibrous cellulose has cationic groups, the acid treatment may be a neutralization treatment or ion exchange treatment of the cationic groups. In this case, the temperature of the acid solution is preferably room temperature.

[0100] (Defibrillation process) By subjecting the ionic group-introduced fibers to defibration treatment in a defibration treatment step, fine fibrous cellulose can be obtained. In the defibration treatment step, for example, a defibration treatment device can be used. The defibration treatment device is not particularly limited, but examples that can be used include a high-speed defibrator, grinder (stone mill-type grinder), high-pressure homogenizer, ultra-high-pressure homogenizer, high-pressure collision grinder, ball mill, bead mill, disk-type refiner, conical refiner, twin-screw kneader, vibration mill, homomixer under high-speed rotation, ultrasonic disperser, or beater. Among the above defibration treatment devices, it is more preferable to use a high-speed defibrator, high-pressure homogenizer, or ultra-high-pressure homogenizer, which are less affected by the grinding media and have less risk of contamination.

[0101] In the defibration process, for example, the ionic group-introduced fibers are preferably diluted with a dispersion medium to form a slurry. The dispersion medium can be one or more selected from water and organic solvents such as polar organic solvents. The polar organic solvent is not particularly limited, but examples thereof include alcohols, polyhydric alcohols, ketones, ethers, esters, and aprotic polar solvents. Examples of alcohols include methanol, ethanol, isopropanol, n-butanol, and isobutyl alcohol. Examples of polyhydric alcohols include ethylene glycol, propylene glycol, and glycerin. Examples of ketones include acetone and methyl ethyl ketone (MEK). Examples of ethers include diethyl ether, tetrahydrofuran, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-butyl ether, and propylene glycol monomethyl ether. Examples of esters include ethyl acetate and butyl acetate. Examples of aprotic polar solvents include dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), and N-methyl-2-pyrrolidinone (NMP).

[0102] The solid content concentration of the fine fibrous cellulose during the defibration treatment can be appropriately set. The slurry obtained by dispersing the phosphorus oxo acid group-introduced fibers in a dispersion medium may contain solid components other than the phosphorus oxo acid group-introduced fibers, such as urea having hydrogen bonding properties.

[0103] (Method of producing fine fibrous cellulose from which substituents have been removed) In the present invention, the fine fibrous cellulose may be obtained by removing the ionic groups as substituents from the fine fibrous cellulose that has been defibrated by introducing the above-mentioned ionic groups. From the viewpoints of lowering the YI value of the sheet and suppressing yellowing due to heating, it is preferable to remove the ionic groups. In this case, it is preferable to include a step (Step I) of removing at least a portion of the substituents from fine fibrous cellulose having a substituent, preferably an ionic group, and a fiber width of 1000 nm or less, and a step II of uniformly dispersing the cellulose after Step I. Furthermore, the fine fibrous cellulose to be subjected to step I preferably undergoes a step of reducing the nitrogen content (nitrogen removal treatment step) before the defibration treatment.

[0104] -Nitrogen removal treatment process- The process for producing the fine fibrous cellulose subjected to Step I may further include a step of reducing the nitrogen content (nitrogen removal treatment step). By reducing the nitrogen content, it is possible to obtain fine fibrous cellulose that can further suppress discoloration. The nitrogen removal treatment step may be carried out after the uniform dispersion treatment step in Step II described below, but is preferably carried out before the uniform dispersion treatment step in Step II described below. It is also preferably carried out before the above-mentioned defibration treatment step.

[0105] In the nitrogen removal treatment step, it is preferable to adjust the pH of the slurry containing the anionic group-introduced fiber to 10 or more and then perform a heat treatment. In the heat treatment, the liquid temperature of the slurry is preferably 50°C or more and 100°C or less, and the heating time is preferably 15 minutes or more and 180 minutes or less. When adjusting the pH of the slurry containing the anionic group-introduced fiber, it is preferable to add an alkali compound that can be used in the above-mentioned alkali treatment step to the slurry.

[0106] After the nitrogen removal treatment step, the anionic group-introduced fiber may be subjected to a washing step, if necessary. The washing step is carried out by washing the anionic group-introduced fiber with, for example, water or an organic solvent. The number of washing steps to be carried out in each washing step is not particularly limited.

[0107] -Process I- In the present invention, the method for producing fine fibrous cellulose may include a step (Step I) of removing at least a portion of the substituents from fine fibrous cellulose having substituents and having a fiber width of 10 nm or less. In this specification, the step (Step I) of removing at least a portion of the substituents from the fine fibrous cellulose is also referred to as a substituent removal treatment step.

[0108] Examples of the substituent removal treatment step include a step of heat treating, enzyme treating, acid treating, alkali treating, etc., fine fibrous cellulose having substituents and a fiber width of 10 nm or less. These may be performed alone or in combination. Among these, the substituent removal treatment step is preferably a heat treating step or an enzyme treating step. By undergoing the above treatment steps, at least a portion of the substituents is removed from the fine fibrous cellulose having substituents and a fiber width of 10 nm or less, and, for example, fine fibrous cellulose having an introduced amount of substituents of less than 0.5 mmol / g can be obtained. The above-mentioned substituent removal treatment step is suitable when the substituent is a phosphorus oxo acid group or a sulfur oxo acid group. The amount of introduced substituents after the substituent removal treatment step is preferably 0.3 mmol / g or less, more preferably 0.2 mmol / g or less, and even more preferably 0.1 mmol / g or less.

[0109] The substituent removal treatment step is preferably carried out in the form of a slurry. That is, the substituent removal treatment step is preferably a step of subjecting a slurry containing a substituent-containing fine fibrous cellulose having a fiber width of 10 nm or less to a heat treatment, an enzyme treatment, an acid treatment, an alkali treatment, or the like. By carrying out the substituent removal treatment step in the form of a slurry, it is possible to prevent the residue of colored substances generated by heating or the like during the substituent removal treatment, as well as acids, alkalis, salts, and the like that are added or generated. This makes it possible to suppress coloration when the fine fibrous cellulose obtained through step II is made into a slurry or sheet. Furthermore, when a treatment is carried out to remove salts derived from the substituents removed after the substituent removal treatment, it is also possible to increase the efficiency of salt removal.

[0110] When a substituent removal treatment is performed on a slurry containing a fine fibrous cellulose having a substituent and a fiber width of 10 nm or less, the concentration of the fine fibrous cellulose in the slurry is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, and even more preferably 0.2% by mass or more. The concentration of the fine fibrous cellulose in the slurry is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less. By controlling the concentration of the fine fibrous cellulose in the slurry within the above range, the substituent removal treatment can be performed more efficiently. Furthermore, by controlling the concentration of the fine fibrous cellulose in the slurry within the above range, it is possible to prevent the residue of colored substances caused by heating or the like during the substituent removal treatment, as well as added or generated acids, alkalis, salts, etc. This can suppress coloration when the fine fibrous cellulose obtained through Step II is made into a slurry or sheet. Furthermore, when a treatment is performed to remove salts derived from the substituents removed after the substituent removal treatment, it is also possible to increase the efficiency of salt removal.

[0111] When the substituent removal treatment step is a step of heat-treating fine fibrous cellulose having a substituent and a fiber width of 10 nm or less, the heating temperature in the heat treatment step is preferably 40° C. or higher, more preferably 50° C. or higher, and even more preferably 60° C. or higher. The heating temperature in the heat treatment step is preferably 250° C. or lower, more preferably 230° C. or lower, and even more preferably 200° C. or lower. In particular, when the substituent on the fine fibrous cellulose subjected to the substituent removal treatment step is a phosphorus oxo acid group or a sulfone group, the heating temperature in the heat treatment step is preferably 80° C. or higher, more preferably 100° C. or higher, and even more preferably 120° C. or higher.

[0112] When the substituent removal treatment step is a heat treatment step, the heating device that can be used in the heat treatment step is not particularly limited, and examples include a hot air heater, a steam heater, an electric heater, a hydrothermal heater, a thermal heater, an infrared heater, a far-infrared heater, a microwave heater, a high-frequency heater, 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, a flash dryer, and a reduced-pressure dryer. From the viewpoint of preventing evaporation, the heating is preferably carried out in a closed system, and from the viewpoint of further increasing the heating temperature, it is preferably carried out in a pressure-resistant device or container. The heat treatment may be a batch process, a batch continuous process, or a continuous process.

[0113] When the substituent removal treatment step is a step of enzymatically treating fine fibrous cellulose having substituents and a fiber width of 10 nm or less, it is preferable to use a phosphate ester hydrolase, a sulfate ester hydrolase, or the like in the enzymatic treatment step depending on the type of substituent. In the enzyme treatment step, the enzyme is added so that the enzymatic activity per 1 g of fine fibrous cellulose is preferably 0.1 nkat or more, more preferably 1.0 nkat or more, and even more preferably 10 nkat or more. Furthermore, the enzyme is added so that the enzymatic activity per 1 g of fine fibrous cellulose is preferably 100,000 nkat or less, more preferably 50,000 nkat or less, and even more preferably 10,000 nkat or less. After adding the enzyme to the fine fibrous cellulose dispersion (slurry), it is preferable to treat it at a temperature of 0°C or higher but lower than 50°C for 1 minute to 100 hours. After the enzymatic reaction, a step of deactivating the enzyme may be carried out. Examples of methods for deactivating the enzyme include adding an acid or alkali component to the enzymatically treated slurry to deactivate the enzyme, and raising the temperature of the enzymatically treated slurry to 90°C or higher to deactivate the enzyme.

[0114] When the substituent removal treatment step is a step of acid treating fine fibrous cellulose having a substituent and a fiber width of 10 nm or less, it is preferable to add an acid compound that can be used in the acid treatment step described above to the slurry in the acid treatment step.

[0115] When the substituent removal treatment step is a step of alkali treating fine fibrous cellulose having a substituent and a fiber width of 10 nm or less, it is preferable to add an alkali compound that can be used in the alkali treatment step described above to the slurry in the alkali treatment step.

[0116] In the substituent removal treatment step, it is preferable that the substituent removal reaction proceeds uniformly. To proceed with the reaction uniformly, for example, the slurry containing the fine fibrous cellulose may be stirred, or the specific surface area of ​​the slurry may be increased. As a method for stirring the slurry, external mechanical shear may be applied, or self-stirring may be promoted by increasing the liquid feed rate of the slurry during the reaction.

[0117] In the substituent removal treatment step, spacer molecules may be added. The spacer molecules penetrate between adjacent fine fibrous cellulose particles, thereby acting as spacers to create fine spaces between the fine fibrous cellulose particles. Adding such spacer molecules in the substituent removal treatment step can suppress aggregation of the fine fibrous cellulose after the substituent removal treatment. This can more effectively improve the transparency of dispersions and sheets containing fine fibrous cellulose. The spacer molecule is preferably a water-soluble organic compound. Examples of water-soluble organic compounds include sugars, water-soluble polymers, and urea. Specific examples include trehalose, urea, polyethylene glycol (PEG), polyethylene oxide (PEO), carboxymethyl cellulose, and polyvinyl alcohol (PVA). Additionally, water-soluble organic compounds that can be used include alkyl methacrylate-acrylic acid copolymer, polyvinylpyrrolidone, sodium polyacrylate, propylene glycol, dipropylene glycol, polypropylene glycol, isoprene glycol, hexylene glycol, 1,3-butylene glycol, polyacrylamide, xanthan gum, guar gum, tamarind gum, carrageenan, locust bean gum, quince seed, alginic acid, pullulan, carrageenan, pectin, cationic starch, raw starch, oxidized starch, etherified starch, esterified starch, starches such as amylose, glycerin, diglycerin, polyglycerin, hyaluronic acid, and metal salts of hyaluronic acid.

[0118] Also, known pigments can be used as spacer molecules, such as kaolin (containing clay), calcium carbonate, titanium oxide, zinc oxide, amorphous silica (containing colloidal silica), aluminum oxide, zeolite, sepiolite, smectite, synthetic smectite, magnesium silicate, magnesium carbonate, magnesium oxide, diatomaceous earth, styrene-based plastic pigments, hydrotalcite, urea resin-based plastic pigments, and benzoguanamine-based plastic pigments.

[0119] -pH adjustment process- When the substituent removal treatment step is carried out in the form of a slurry, a step of adjusting the pH of the slurry containing the fine fibrous cellulose may be carried out before the substituent removal treatment step. For example, anionic groups are introduced into the cellulose fibers, and the counter ions of the anionic groups are Na. +In this case, the slurry containing the defibrated fine fibrous cellulose exhibits a weak alkaline pH. If the slurry is heated in this state, the decomposition of cellulose may produce monosaccharides, which are one of the causes of discoloration, so it is preferable to adjust the pH of the slurry to 8 or less. Similarly, monosaccharides may be produced under acidic conditions, so it is preferable to adjust the pH of the slurry to 3 or more.

[0120] Furthermore, when the substituted fine fibrous cellulose is a phosphate-containing fine fibrous cellulose, it is preferable that the phosphorus of the phosphate group is susceptible to nucleophilic attack in order to improve the efficiency of removing the substituent. The phosphorus susceptible to nucleophilic attack is cellulose-OP(=O)(-O-H+)(-O-Na + To achieve this state, the pH of the slurry is preferably adjusted to 3 or more and 8 or less, and more preferably adjusted to 4 or more and 6 or less.

[0121] The means for adjusting the pH is not particularly limited, and for example, an acid component or an alkali component may be added to a slurry containing fine fibrous cellulose. The acid component may be either an inorganic acid or an organic acid. Examples of inorganic acids include sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid. Examples of organic acids include formic acid, acetic acid, citric acid, malic acid, lactic acid, adipic acid, sebacic acid, stearic acid, maleic acid, succinic acid, tartaric acid, fumaric acid, and gluconic acid. The alkali component may be an inorganic alkali compound or an organic alkali compound. Examples of inorganic alkali compounds include lithium hydroxide, sodium hydroxide, potassium hydroxide, lithium carbonate, lithium bicarbonate, potassium carbonate, potassium bicarbonate, sodium carbonate, and sodium bicarbonate. Examples of organic alkali compounds include ammonia, hydrazine, methylamine, ethylamine, diethylamine, triethylamine, propylamine, dipropylamine, butylamine, diaminoethane, diaminopropane, diaminobutane, diaminopentane, diaminohexane, cyclohexylamine, aniline, tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, benzyltrimethylammonium hydroxide, pyridine, and N,N-dimethyl-4-aminopyridine.

[0122] In addition, in the pH adjustment step, an ion exchange treatment may be performed to adjust the pH. A strong acid cation exchange resin or a weak acid ion exchange resin can be used in the ion exchange treatment. By treating with an appropriate amount of cation exchange resin for a sufficient time, a slurry containing fine fibrous cellulose with the desired pH can be obtained. Furthermore, in the pH adjustment step, the addition of an acid component or an alkali component may be combined with the ion exchange treatment.

[0123] <Salt removal treatment> After the substituent removal treatment step, it is preferable to carry out a treatment to remove salts derived from the removed substituents. Removing the salts derived from the substituents makes it easier to obtain fine fibrous cellulose that can suppress coloration. The means for removing the salts derived from the substituents is not particularly limited, but examples include washing treatment and ion exchange treatment. The washing treatment is carried out by washing the fine fibrous cellulose that has aggregated in the substituent removal treatment with, for example, water or an organic solvent. In the ion exchange treatment, an ion exchange resin can be used.

[0124] -Process II- In this embodiment, the method for producing fine fibrous cellulose may include a step (Step I) of removing at least a portion of the substituents from fine fibrous cellulose having a substituent and a fiber width of 1,000 nm or less, and a step (Step II) of performing a uniform dispersion treatment after Step I. The uniform dispersion treatment step (Step II) is a step of uniformly dispersing the fine fibrous cellulose obtained through the substituent removal treatment in Step I. In Step I, the fine fibrous cellulose is subjected to the substituent removal treatment, thereby causing at least a portion of the fine fibrous cellulose to aggregate. Step II is a step of uniformly dispersing the aggregated fine fibrous cellulose. In Step II, the state in which the fine fibrous cellulose is uniformly dispersed refers to a state in which the fiber width of the fine fibrous cellulose is 10 nm or less. Thus, the fine fibrous cellulose obtained by the production method of this embodiment has a fiber width of 10 nm or less, even when the amount of introduced substituents is as low as less than 0.50 mmol / g.

[0125] In the uniform dispersion treatment step (step II), for example, a high-speed defibrator, grinder (stone mill type grinder), high-pressure homogenizer, high-pressure collision type grinder, ball mill, bead mill, disk type refiner, conical refiner, twin-screw kneader, vibration mill, homomixer under high-speed rotation, ultrasonic disperser or beater can be used. Among the above-mentioned uniform dispersion treatment devices, it is more preferable to use a high-speed defibrator or high-pressure homogenizer. The treatment conditions for the uniform dispersion treatment step (Step II) are not particularly limited, but it is preferable to increase the maximum movement speed of the fine fibrous cellulose during treatment and the pressure during treatment. In a high-speed defibrator, the peripheral speed is preferably 20 m / sec or more, more preferably 25 m / sec or more, and even more preferably 30 m / sec or more. A high-pressure homogenizer is more preferably used because it has a higher maximum movement speed of the fine fibrous cellulose during treatment and a higher pressure during treatment than a high-speed defibrator. In high-pressure homogenizer treatment, the pressure during treatment is preferably 1 MPa or more and 350 MPa or less, more preferably 10 MPa or more and 300 MPa or less, and even more preferably 50 MPa or more and 250 MPa or less.

[0126] In Step II, the above-mentioned spacer molecules may be added. By adding such spacer molecules in the uniform dispersion treatment step of Step II, the fine fibrous cellulose can be dispersed more smoothly and uniformly. This makes it possible to more effectively improve the transparency of the dispersion or sheet containing the fine fibrous cellulose.

[0127] From the viewpoint of obtaining transparency of the sheet, suppression of yellowing due to heating, high tensile modulus, and high flexibility, the content of fine fibrous cellulose in the solid content of the sheet is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 25% by mass or more, even more preferably 50% by mass or more, particularly preferably 65% ​​by mass or more, and preferably 95% by mass or less, more preferably 90% by mass or less, even more preferably 85% by mass or less, even more preferably 80% by mass or less, particularly preferably 75% by mass or less. As the fine fibrous cellulose, a fine fibrous cellulose containing an ionic group and an unmodified fine fibrous cellulose may be used in combination.

[0128] <Cellulose derivatives> In the present invention, the sheet contains, in addition to the above-mentioned fine fibrous cellulose, a cellulose ester having a weight average molecular weight of 1.0 × 10 4 Over 3.0 x 10 5The sheet contains the following cellulose derivative: By forming a sheet containing fine fibrous cellulose and a cellulose derivative having a specific weight-average molecular weight, it is possible to obtain a sheet that has high transparency, is inhibited from yellowing due to heating, has a high tensile modulus, and is also excellent in flexibility. The weight average molecular weight of the cellulose derivative is preferably 2.5 × 10 from the viewpoint of achieving shape stability as a sheet, suppressing gelation while forming a sheet, achieving both a high tensile modulus and a high tensile elongation, and suppressing yellowing before and after heating. 4 More preferably, 5.0 × 10 4 More preferably, 1.0 × 10 5 and preferably 2.8 x 10 5 Less than or equal to 2.6 × 10 5 The following is the result. The weight-average molecular weight of the cellulose derivative is measured by gel permeation chromatography using light scattering (GPC-MALLS method).

[0129] The cellulose derivative is preferably a water-soluble cellulose ether from the viewpoint of enhancing affinity with fine fibrous cellulose and facilitating addition to a slurry of fine fibrous cellulose (fine fibrous cellulose dispersion). Here, water-soluble means that 1 g or more of the cellulose derivative dissolves in 100 g of water at 20°C. Cellulose ether is a general term for cellulose derivatives in which the hydroxyl groups of cellulose are etherified. Preferred examples of the water-soluble cellulose ether include methyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, and carboxyethyl cellulose. In order to prevent the sheet from yellowing due to heating, the water-soluble cellulose is preferably a non-ionic water-soluble cellulose ether. Examples of non-ionic water-soluble cellulose ethers include methyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, and hydroxypropyl cellulose. The nonionic water-soluble cellulose ether preferably has at least one functional group selected from the group consisting of a methoxy group and a hydroxypropoxy group, more preferably selected from the group consisting of methylcellulose and hydroxypropylmethylcellulose, and even more preferably hydroxypropylmethylcellulose.

[0130] When the cellulose derivative is methylcellulose, the degree of substitution of the methoxy group is preferably 0.5 or more, more preferably 0.8 or more, even more preferably 1.0 or more, still more preferably 1.2 or more, particularly preferably 1.5 or more, and preferably 3.0 or less, more preferably 2.6 or less, even more preferably 2.2 or less, and still more preferably 2.0 or less. When the cellulose derivative is hydroxypropylmethylcellulose, the preferred range of the degree of substitution of the methoxy group is the same as that of the methoxy group in the above-mentioned methylcellulose. The degree of substitution of the hydroxypropoxy group is preferably 0.08 or more, more preferably 0.10 or more, even more preferably 0.12 or more, even more preferably 0.15 or more, particularly preferably 0.18 or more, and is preferably 0.50 or less, more preferably 0.40 or less, even more preferably 0.35 or less, and even more preferably 0.30 or less.

[0131] From the viewpoint of obtaining transparency of the sheet, suppression of yellowing due to heating, high tensile modulus, and high flexibility, the content of the cellulose derivative in the solid content of the sheet is preferably 95% by mass or less, more preferably 90% by mass or less, even more preferably 75% by mass or less, even more preferably 50% by mass or less, and particularly preferably 35% by mass or less, and is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, even more preferably 20% by mass or more, and particularly preferably 25% by mass or more.

[0132] In this embodiment, the total content of fine fibrous cellulose and cellulose derivatives in the solid content of the sheet is preferably 60% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and even more preferably 95% by mass or more, and is 100% by mass or less, and 100% by mass is particularly preferred, from the viewpoint of obtaining transparency of the sheet, suppression of yellowing due to heating, high tensile modulus, and high flexibility.

[0133] <Optional ingredients> In addition to the fine fibrous cellulose and cellulose derivatives, the sheet may contain optional components such as hydrophilic polymers (excluding cellulose derivatives), hydrophilic low-molecular-weight polymers, paper strength agents, thermoplastic resins, surfactants, organic ions, coupling agents, inorganic layered compounds, inorganic compounds, leveling agents, preservatives, antifoaming agents, organic particles, lubricants, antistatic agents, UV protection agents, dyes, pigments, stabilizers, magnetic powders, alignment promoters, plasticizers, dispersants, color inhibitors, polymerization inhibitors, pH adjusters, and crosslinking agents.

[0134] Examples of hydrophilic polymers include polyethylene glycol, polyethylene oxide, polyvinyl alcohol, modified polyvinyl alcohol (such as acetoacetylated polyvinyl alcohol), polyvinylpyrrolidone, polyvinyl methyl ether, polyacrylates, polyacrylamide, alkyl acrylate copolymers, and urethane copolymers. Examples of hydrophilic low-molecular-weight compounds include glycerin, sorbitol, and ethylene glycol. Examples of organic ions include tetraalkylammonium ions and tetraalkylphosphonium ions. Examples of tetraalkylammonium ions include tetramethylammonium ions, tetraethylammonium ions, tetrapropylammonium ions, tetrabutylammonium ions, tetrapentylammonium ions, tetrahexylammonium ions, tetraheptylammonium ions, tributylmethylammonium ions, lauryltrimethylammonium ions, cetyltrimethylammonium ions, stearyltrimethylammonium ions, octyldimethylethylammonium ions, lauryldimethylethylammonium ions, didecyldimethylammonium ions, lauryldimethylbenzylammonium ions, and tributylbenzylammonium ions. Examples of the tetraalkylphosphonium ion include the tetramethylphosphonium ion, the tetraethylphosphonium ion, the tetrapropylphosphonium ion, the tetrabutylphosphonium ion, and the lauryltrimethylphosphonium ion. Examples of the tetrapropylonium ion and the tetrabutylonium ion include the tetra-n-propylonium ion and the tetra-n-butylonium ion, respectively.

[0135] <Sheet characteristics> [Yellow index] When the sheet of this embodiment is heated at 160°C for 6 hours, the change in yellow index (YI value) before and after heating is preferably 5.0 or less, more preferably 4.5 or less, even more preferably 3.5 or less, even more preferably 2.5 or less, even more preferably 1.5 or less, even more preferably 1.2 or less, and even more preferably 1.0, with no particular lower limit. If the change in YI value before and after heating is within the above range, yellowing due to heating is suppressed, which is preferable. The YI value before heating is preferably 1.2 or less, more preferably 1.0 or less, even more preferably 0.8 or less, and even more preferably 0.5 or less. There is no particular lower limit. The YI value after heating is preferably 5.0 or less, more preferably 4.0 or less, even more preferably 3.0 or less, even more preferably 2.5 or less, even more preferably 2.0 or less, and even more preferably 1.5 or less. The YI value is measured in accordance with JIS K 7373:2006.

[0136] [Haze] The sheet of the present embodiment preferably has a haze of 5% or less, which is preferable since transparency is excellent when the haze is 5% or less. The haze of the sheet is preferably 5% or less, more preferably 4% or less, even more preferably 3% or less, and even more preferably 2% or less. The haze of the sheet is measured in accordance with JIS K 7136:2000 using a haze meter (HM-150, manufactured by Murakami Color Research Laboratory Co., Ltd.).

[0137] [Total light transmittance] The total light transmittance of the sheet of this embodiment is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more. The upper limit of the total light transmittance of the sheet is not particularly limited and may be, for example, 100%. If the total light transmittance of the sheet is within the above range, it is preferable because the sheet has excellent transparency. Here, the total light transmittance of the sheet is a value measured in accordance with, for example, JIS K 7361-1:1997 using a haze meter (HM-150, manufactured by Murakami Color Research Laboratory Co., Ltd.).

[0138] The tensile modulus of the sheet of this embodiment is preferably 4.0 GPa or more, more preferably 5.5 GPa or more, even more preferably 6.5 GPa or more, even more preferably 7.5 GPa or more, and particularly preferably 8.0 GPa or more, and although there are no particular upper limits, from the viewpoint of compatibility with flexibility, it is preferably 15 GPa or less, more preferably 12 GPa or less, and even more preferably 10 GPa or less. When the tensile modulus of the sheet is within the above range, it is preferable because it has excellent rigidity. 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, a test specimen is prepared by conditioning the specimen at 23°C and 50% relative humidity for 24 hours, and the measurement is carried out under conditions of 23°C and 50% relative humidity.

[0139] The tensile elongation of the sheet of this embodiment is preferably 3% or more, more preferably 4% or more, even more preferably 5% or more, and even more preferably 7% or more, and although there are no particular upper limits, from the viewpoint of achieving both rigidity and flexibility, it is preferably 50% or less, more preferably 40% or less, even more preferably 30% or less, and even more preferably 20% or less. A tensile elongation within the above range is preferable because it provides excellent flexibility. The tensile elongation of the sheet is a value measured using a tensile tester Tensilon (manufactured by A&D Co., Ltd.) in accordance with JIS K 7127: 1999. When measuring the tensile modulus, the test specimen is conditioned at 23°C and 50% relative humidity for 24 hours, and the measurement is carried out under conditions of 23°C and 50% relative humidity.

[0140] The tensile strength of the sheet of this embodiment is preferably 60 MPa or more, more preferably 70 MPa or more, even more preferably 80 MPa or more, and even more preferably 85 MPa or more.The upper limit is not particularly limited, but from the viewpoint of achieving both rigidity and flexibility, it is preferably 200 MPa or less, more preferably 180 MPa or less, even more preferably 160 MPa or less, and even more preferably 150 MPa or less. The tensile strength of the sheet is measured using a tensile tester Tensilon (manufactured by A&D Co., Ltd.) in accordance with JIS K 7127: 1999. When measuring tensile strength, test specimens are prepared by conditioning the specimens at 23°C and 50% relative humidity for 24 hours, and measurements are carried out under conditions of 23°C and 50% relative humidity.

[0141] The surface pH of the sheet of the present invention is preferably 5.00 or higher, more preferably 5.20 or higher, and even more preferably 5.40 or higher. The surface pH of the sheet is preferably 7.0 or lower. The surface pH of the sheet is measured, for example, with a calibrated pH meter (F-53, manufactured by Horiba, Ltd.). The surface pH of the sheet is measured by wetting the sheet with a small amount of water and then contacting the water with a flat pH composite electrode (6261-10C, manufactured by Horiba, Ltd.).

[0142] [Thickness] The thickness of the sheet of this embodiment is not particularly limited, but is preferably 5 μm or more, more preferably 10 μm or more, and even more preferably 20 μm or more. The thickness of the sheet is preferably 500 μm or less, more preferably 300 μm or less, and even more preferably 200 μm or less. The thickness of the sheet is preferably adjusted appropriately depending on the intended use. The thickness of the sheet can be measured using a constant pressure thickness gauge (PG-02, manufactured by TECLOCK CORPORATION). The thickness of the sheet is measured according to the following method. A sheet cut into a size of 50 mm square or more is conditioned at 23°C and a relative humidity of 50% for 24 hours, and the thickness is measured at four random points, and the average value is taken as the sheet thickness.

[0143] [Basic weight] The basis weight of the sheet is 10 g / m 2 It is preferable that the weight is 20 g / m or more. 2 More preferably, it is 30 g / m or more. 2 The basis weight of the sheet is not particularly limited, but is preferably 1000 g / m or more. 2 Preferably, it is 500 g / m or less. 2 More preferably, it is 200 g / m or less. 2 More preferably, it is 100 g / m or less. 2 It is particularly preferred that: The basis weight of the sheet is a value calculated according to the following method: a sheet cut into a size of 50 mm square or larger is conditioned at 23°C and a relative humidity of 50% for 24 hours, and then the mass is measured and divided by the area of ​​the cut sheet to calculate the basis weight of the sheet.

[0144] (density) The density of the sheet is 1.00 g / cm 3 It is preferable that the concentration is 1.10 g / cm or more. 3 More preferably, it is 1.20 g / cm or more. 3 The density of the sheet is not particularly limited, but is preferably 3.00 g / cm or more. 3 Preferably, it is 1.70 g / cm or less. 3 More preferably, it is 1.50 g / cm or less. 3 It is more preferable that the density of the sheet is calculated by dividing the basis weight of the sheet by the thickness.

[0145] The sheet of this embodiment is preferably not a porous sheet, i.e., the porosity and void ratio are preferably 10% or less, more preferably 3% or less, even more preferably 1% or less, and even more preferably 0.1% or less.

[0146] [Sheet manufacturing method] A sheet containing fine fibrous cellulose and cellulose derivatives with a fiber width of 10 nm or less is described. In this embodiment, the sheet can be obtained by carrying out the sheet forming step described below using a liquid composition containing the above-mentioned fine fibrous cellulose and cellulose derivative, as well as other components. The sheet production process preferably includes at least a coating step of coating the composition onto a substrate or a papermaking step of making paper from the slurry, thereby obtaining a sheet containing fine fibrous cellulose and a cellulose derivative.

[0147] - Coating process - In the coating process, for example, a slurry containing fine fibrous cellulose and a cellulose derivative is coated on a substrate, dried, and the resulting sheet is peeled off from the substrate to obtain a sheet. Furthermore, by using a coating device and a long or continuous substrate, sheets can be produced continuously. The material of the substrate used in the coating process is not particularly limited, but a substrate with high wettability with the composition (slurry) is preferable because it can suppress shrinkage of the sheet during drying, but it is preferable to select a substrate that allows the sheet formed after drying to be easily peeled off. Among these, resin films or plates or metal films or plates are preferred, but there are no particular limitations. For example, resin films or plates such as acrylic, polyethylene terephthalate, vinyl chloride, polystyrene, polypropylene, polycarbonate, and polyvinylidene chloride, metal films or plates such as aluminum, zinc, copper, and iron plates, and those with oxidized surfaces, stainless steel films or plates, brass films or plates, etc. can be used. In the coating process, if the viscosity of the slurry is low and it spreads on the substrate, a blocking frame may be fixed to the substrate to obtain a sheet of the desired thickness and basis weight. The blocking frame is not particularly limited, but it is preferable to select one that allows the edge of the sheet to be easily peeled off after drying. From this perspective, molded resin or metal plates are more preferable. In this embodiment, for example, resin plates such as acrylic plates, polyethylene terephthalate plates, vinyl chloride plates, polystyrene plates, polypropylene plates, polycarbonate plates, and polyvinylidene chloride plates, metal plates such as aluminum plates, zinc plates, copper plates, and iron plates, and plates with their surfaces oxidized, as well as molded stainless steel plates, brass plates, etc., can be used. The coating machine for coating the substrate with the slurry is not particularly limited, and examples thereof include a roll coater, gravure coater, die coater, curtain coater, air doctor coater, etc. Die coaters, curtain coaters, and spray coaters are particularly preferred because they can make the thickness of the sheet more uniform.

[0148] The slurry temperature and ambient temperature when applying the slurry to the substrate (hereinafter, the slurry temperature and ambient temperature are collectively referred to as "application temperature") are not particularly limited, but are preferably, for example, from 5°C to 80°C, more preferably from 10°C to 60°C, even more preferably from 15°C to 50°C, and particularly preferably from 20°C to 40°C. If the application temperature is above the lower limit, the slurry can be applied more easily. If the application temperature is below the upper limit, evaporation of the dispersion medium during application can be suppressed. In the coating step, it is preferable to coat the substrate with the slurry so that the finished basis weight and thickness of the sheet are within the above-mentioned preferred ranges. By coating so that the basis weight and thickness are within the above-mentioned ranges, a sheet with better transparency, rigidity, and flexibility can be obtained.

[0149] As described above, the coating step includes a step of drying the slurry coated on the substrate. The step of drying the slurry is not particularly limited, but may be performed by, for example, a non-contact drying method, a method of drying while fixing the sheet, 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. Although the heat drying method and the vacuum drying method can be combined, the heat drying method is usually applied. Drying by infrared rays, far infrared rays, or near infrared rays can be carried out 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 and 150°C or lower, and more preferably 25°C or higher and 105°C or lower. If the heating temperature is equal to or higher than the lower limit, the dispersion medium can be quickly volatilized. Furthermore, if the heating temperature is equal to or lower than the upper limit, it is possible to reduce the cost required for heating and to suppress discoloration of the fibrous cellulose due to heat.

[0150] -Paper making process- The papermaking process is carried out by making paper from the slurry using a papermaking machine. The papermaking machine used in the papermaking process is not particularly limited, but examples include continuous papermaking machines such as Fourdrinier, cylinder, and tilting types, and multi-layer papermaking machines that combine these. In the papermaking process, known papermaking methods such as handmaking may also be used. The papermaking process involves filtering and dewatering the slurry with a wire to obtain a wet sheet, which is then pressed and dried. The filter cloth used to filter and dewater the slurry is not particularly limited, but it is preferable that it does not allow fibrous cellulose to pass through and does not slow the filtration rate too much. Such filter cloths are not particularly limited, but are preferably sheets, woven fabrics, or porous membranes made of organic polymers. The organic polymer is not particularly limited, but is preferably a non-cellulose organic polymer such as polyethylene terephthalate, polyethylene, polypropylene, or polytetrafluoroethylene (PTFE). In this embodiment, examples include porous membranes made of polytetrafluoroethylene with a pore size of 0.1 μm to 20 μm, and woven fabrics made of polyethylene terephthalate or polyethylene with a pore size of 0.1 μm to 20 μm.

[0151] In the papermaking process, a method for producing a sheet from a slurry can be carried out using a production apparatus including a water squeezing section in which a slurry containing fine fibrous cellulose is discharged onto the upper surface of an endless belt and the dispersion medium is squeezed out of the discharged slurry to produce a web, and a drying section in which the web is dried to produce a sheet. An endless belt is disposed between the water squeezing section and the drying section, and the web produced in the water squeezing section is transported to the drying section while still on the endless belt.

[0152] The dehydration method used in the papermaking process is not particularly limited, but examples thereof include dehydration methods commonly used in paper manufacturing. Among these, methods of dehydrating using a Fourdrinier, cylinder, or inclined wire, followed by further dehydration using a roll press, are preferred. Furthermore, the drying method used in the papermaking process is not particularly limited, but examples thereof 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.

[0153] The thickness, basis weight, and density of the sheet may be appropriately set according to the desired thickness, basis weight, and density of the sheet.

[0154] [Laminate] The present invention may also relate to a laminate having a structure in which another layer is laminated on the above-mentioned sheet. Such another layer may be provided on both surfaces of the sheet, or may be provided only on one surface of the sheet. Examples of the other layer laminated on at least one surface of the sheet include a resin layer and an inorganic layer, with a resin layer being preferred. Furthermore, a laminate may be constructed by laminating another layer on the side of the resin layer that is not in contact with the sheet. Examples of the other layer in this case include a polyethylene film, a polypropylene film, a cycloolefin polymer film, a polyimide film, etc.

[0155] The thickness of the other layers in the laminate is not particularly limited, but is preferably 20 μm or more, more preferably 50 μm or more, and even more preferably 100 μm or more. Also, it is preferably 5000 μm or less, more preferably 1000 μm or less, and even more preferably 500 μm or less. However, for example, when the resin layer is a coated layer formed by coating, the thickness of the resin layer may be 1 μm or more, 2 μm or more, or 3 μm or more. Furthermore, the thickness of the resin layer is preferably 30 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less.

[0156] <Resin layer> The resin layer is a layer whose main component is a natural resin or a synthetic resin. Here, the main component refers to a component that is contained in an amount of 50% by mass or more relative to the total mass of the resin layer. The resin content is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more relative to the total mass of the resin layer. The resin content may be 100% by mass or may be 95% by mass or less.

[0157] Examples of natural resins include rosin-based resins such as rosin, rosin ester, and hydrogenated rosin ester. The synthetic resin is preferably at least one selected from, for example, polycarbonate resin, polyethylene terephthalate resin, polyethylene naphthalate resin, polyethylene resin, polypropylene resin, polyimide resin, polystyrene resin, and acrylic resin. Among these, the synthetic resin is preferably at least one selected from polycarbonate resin, acrylic resin, and polypropylene resin, and more preferably polycarbonate resin.

[0158] Examples of the polycarbonate resin constituting the resin layer include aromatic polycarbonate resins and aliphatic polycarbonate resins. Specific examples of these polycarbonate resins are known, such as the polycarbonate resins described in JP-A-2010-023275.

[0159] Examples of polypropylene resins constituting the resin layer include acid-modified polypropylene resins and chlorinated polypropylene resins. Among these, acid-modified polypropylene resins are preferred, and maleated polypropylene resins or maleic anhydride-modified polypropylene resins are more preferred.

[0160] The resin layer may be made of a single resin, a copolymer obtained by copolymerization or graft polymerization of multiple resin components, or a blend material obtained by mixing multiple resin components by a physical process.

[0161] An adhesive layer may be provided between the sheet and the resin layer, or the sheet and the resin layer may be directly adhered to each other without providing an adhesive layer. When an adhesive layer is provided between the sheet and the resin layer, an acrylic resin may be used as the adhesive. Examples of adhesives other than acrylic resins include vinyl chloride resins, (meth)acrylic ester resins, styrene / acrylic ester copolymer resins, vinyl acetate resins, vinyl acetate / (meth)acrylic ester copolymer resins, urethane resins, silicone resins, epoxy resins, ethylene / vinyl acetate copolymer resins, polyester resins, polyvinyl alcohol resins, ethylene-vinyl alcohol copolymer resins, and rubber emulsions such as SBR and NBR.

[0162] When no adhesive layer is provided between the sheet and the resin layer, the resin layer may contain an adhesion aid, and the surface of the resin layer may be subjected to a surface treatment such as hydrophilization. Examples of the adhesion aid include compounds containing at least one selected from an isocyanate group, a carbodiimide group, an epoxy group, an oxazoline group, an amino group, and a silanol group, and organosilicon compounds. Among them, the adhesion aid is preferably at least one selected from compounds containing an isocyanate group (isocyanate compounds) and organosilicon compounds. Examples of the organosilicon compound 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.

[0163] <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.

[0164] The method for forming the inorganic layer is not particularly limited. Generally, methods for forming thin films are roughly divided into chemical vapor deposition (CVD) and physical vapor deposition (PVD), and either method may be employed. 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.

[0165] Atomic layer deposition (ALD) can also be used to form inorganic layers. ALD is a method for forming thin films atomically by alternately supplying the source gases of each element that make up the film to be formed to the surface on which the layer is to be formed. While it has the drawback of a slow film formation speed, it has the advantage of being able to coat even complex surfaces more cleanly than plasma CVD, and to deposit thin films with fewer defects. ALD also has the advantage of being able to control film thickness at the nanometer level, making it relatively easy to cover large surfaces. Furthermore, the use of plasma in ALD is expected to improve reaction speed, enable lower processing temperatures, and reduce unreacted gases.

[0166] <Use of the sheet> The sheet of this embodiment is suitable for optical components such as various display devices and various solar cells. It is also suitable for applications such as substrates for electronic devices, separators for electrochemical devices, components for home appliances, window materials for various vehicles and buildings, interior and exterior materials, and packaging materials. It is also suitable for applications such as threads, filters, fabrics, cushioning materials, sponges, and abrasives, as well as for using the sheet itself as a reinforcing material. The sheet of the present invention is also suitable for use in food containers such as plates, cups, and trays, cutlery such as knives, spoons, and forks, and straws. The sheet of this embodiment has excellent transparency and suppressed yellowing due to heating, making it suitable for use in optical components where transparency and yellowing are important. Furthermore, because it has excellent tensile modulus and tensile elongation, it is also suitable for various molded products such as food containers, cutlery, and straws, which have traditionally been difficult to use due to cracking during processing. In the above-mentioned applications, the sheet itself may be used, or a laminate in which a resin layer or an inorganic layer is laminated on the sheet may be used. [Example]

[0167] The features of the present invention will be explained in more detail below with reference to examples and comparative examples. The materials, amounts used, ratios, treatment contents, treatment procedures, etc. shown in the following examples can be appropriately changed without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the specific examples shown below.

[0168] <Production Example 1> [Phosphorylation] Hardwood dissolving pulp (dry sheet) manufactured by Oji Paper Co., Ltd. was used as the raw material pulp. This raw material pulp was subjected to a phosphating treatment as follows. First, a mixed aqueous solution of ammonium dihydrogen phosphate and urea was added to 100 parts by mass (bone dry mass) of the raw material pulp to adjust the composition to 45 parts by mass of ammonium dihydrogen phosphate, 120 parts by mass of urea, and 150 parts by mass of water, thereby obtaining a chemical-impregnated pulp. Next, the obtained chemical-impregnated pulp was heated in a hot air dryer at 165°C for 250 seconds to introduce phosphate groups into the cellulose in the pulp, thereby obtaining a phosphorylated pulp.

[0169] [Cleaning process] The resulting phosphorylated pulp was then washed. 100 g (bone dry mass) of phosphorylated pulp was mixed with 10 L of ion-exchanged water to obtain a pulp dispersion. The pulp was stirred to uniformly disperse the pulp, and then repeatedly filtered and dehydrated. The washing was completed when the electrical conductivity of the filtrate reached 100 μS / cm or less.

[0170] [Neutralization treatment] Next, the washed phosphorylated pulp was neutralized as follows. First, the washed phosphorylated pulp was diluted with 10 L of ion-exchanged water, and then a 1 N aqueous solution of sodium hydroxide was added little by little while stirring to obtain a phosphorylated pulp slurry with a pH of 12 to 13. Next, the phosphorylated pulp slurry was dehydrated to obtain a neutralized phosphorylated pulp. Next, the neutralized phosphorylated pulp was subjected to the above-mentioned washing treatment.

[0171] [Nitrogen removal treatment] Deionized water was added to phosphorylated pulp to prepare a slurry with a solids concentration of 4% by mass. A 48% by mass aqueous solution of sodium hydroxide was added to the slurry to adjust the pH to 13.4 and heated at 85°C for 1 hour. The pulp slurry was then dehydrated, and 10 L of deionized water was added to 100 g of phosphorylated pulp (bone dry mass) to obtain a pulp dispersion. The pulp was stirred to uniformly disperse, and the filtration and dehydration process was repeated to remove excess sodium hydroxide. The removal was terminated when the electrical conductivity of the filtrate reached 100 μS / cm or less. The amount of carbamide groups introduced, calculated from the nitrogen content measured by the measurement method described below, was 0.01 mmol / g.

[0172] The infrared absorption spectrum of the obtained phosphorus oxyoxidized pulp was measured using FT-IR. -1 Absorption due to the P=O of phosphate groups was observed around 2θ = 14° to 17° and 2θ = 22° to 23°, confirming that phosphate groups had been added to the pulp. Furthermore, when the obtained phosphorylated pulp was analyzed using an X-ray diffractometer, typical peaks were observed at two positions, around 2θ = 14° to 17° and around 2θ = 22° to 23°, confirming that cellulose type I crystals had been maintained. The amount of phosphate groups (amount of first dissociated acid) measured by the measurement method described below was 1.45 mmol / g. The total amount of dissociated acid was 2.45 mmol / g.

[0173] [Fiber defibration processing] Ion-exchanged water was added to the obtained phosphorylated pulp to prepare a slurry with a solid content of 2% by mass. This slurry was treated six times at a pressure of 200 MPa in a wet pulverizer (Starburst, manufactured by Sugino Machine Co., Ltd.) to obtain a fine fibrous cellulose dispersion containing fine fibrous cellulose.

[0174] [Substituent removal treatment (high temperature heat treatment)] The fine fibrous cellulose dispersion was placed in a pressure vessel and heated at 160°C for 15 minutes until the amount of phosphate groups reached 0.08 mmol / g. This confirmed the formation of fine fibrous cellulose aggregates.

[0175] [Cleaning of slurry after removing substituents] After heating, the slurry was washed by adding an equal amount of ion-exchanged water to the slurry to obtain a slurry with a solids concentration of approximately 1% by mass. The slurry was then stirred and then filtered and dehydrated. When the electrical conductivity of the filtrate reached 10 μS / cm or less, ion-exchanged water was added again to obtain a slurry with a solids concentration of approximately 1% by mass, which was then allowed to stand for 24 hours. The filtration and dehydration process was then repeated, and the washing endpoint was reached when the electrical conductivity of the filtrate again reached 10 μS / cm or less. Ion-exchanged water was added to the resulting fine fibrous cellulose aggregates, and a slurry was obtained after removing the substituents. The solids concentration of this slurry was 1.7% by mass.

[0176] [Uniform dispersion of slurry after removing substituents] Ion-exchanged water was added to the resulting slurry after the removal of substituents to give a slurry with a solids concentration of 1.0% by mass, which was then treated three times at a pressure of 200 MPa in a wet atomizer (Starburst, manufactured by Sugino Machine Co., Ltd.) to obtain a dispersion of substituent-removed fine fibrous cellulose (A) containing the substituent-removed fine fibrous cellulose. The fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 4 nm.

[0177] <Production Example 2> The washed and neutralized phosphorylated pulp obtained in Production Example 1 was subjected to the following treatment to obtain a fine fibrous cellulose dispersion (B) containing fine fibrous cellulose.

[0178] [Fiber defibration processing] Ion-exchanged water was added to the obtained phosphorylated pulp to prepare a slurry with a solids concentration of 2% by mass. This slurry was treated six times at a pressure of 200 MPa using a wet pulverizer (Starburst, manufactured by Sugino Machine Co., Ltd.) to obtain a fine fibrous cellulose dispersion containing fine fibrous cellulose. X-ray diffraction confirmed that this fine fibrous cellulose maintained cellulose type I crystal structure. Furthermore, the fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 3 to 5 nm. The amount of phosphate groups (first dissociated amount) measured by the method described below in the measurement of phosphorus oxoacid group amount was 1.45 mmol / g. The total dissociated acid amount was 2.45 mmol / g.

[0179] <Production Example 3> [Phosphorous Treatment] A phosphite pulp was obtained by the same procedure as in Production Example 2, except that 33 parts by mass of phosphorous acid (phosphonic acid) was used instead of ammonium dihydrogen phosphate in the phosphating treatment.

[0180] The infrared absorption spectrum of the obtained phosphorous pulp was measured using FT-IR. -1 The absorption due to P=O of the phosphonic acid group, which is a tautomer of the phosphorous acid group, was observed near the α-axis, confirming that the phosphorous acid group (phosphonic acid group) had been added to the pulp. Furthermore, when the obtained phosphorous-oxidized pulp was analyzed using an X-ray diffractometer, typical peaks were observed at two positions, around 2θ=14° to 17° and around 2θ=22° to 23°, confirming the presence of cellulose type I crystals.

[0181] [Fiber defibration processing] Ion-exchanged water was added to the obtained phosphite pulp to prepare a slurry with a solids concentration of 2% by mass. This slurry was treated six times at a pressure of 200 MPa using a wet pulverizer (Starburst, manufactured by Sugino Machine Co., Ltd.) to obtain a fine fibrous cellulose dispersion (C) containing fine fibrous cellulose. X-ray diffraction confirmed that the fine fibrous cellulose maintained cellulose type I crystal structure. Furthermore, the fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 3 to 5 nm. The amount of phosphite groups (amount of first dissociated acid), as measured by the method for measuring the amount of phosphorus oxo acid groups described below, was 1.51 mmol / g. The total amount of dissociated acid was 1.54 mmol / g.

[0182] <Production Example 4> [TEMPO oxidation treatment] The raw material pulp used was hardwood dissolving pulp (dry sheet) manufactured by Oji Paper Co., Ltd. This raw material pulp was subjected to alkaline TEMPO oxidation treatment as follows. First, 100 parts by weight of the raw pulp (dry mass equivalent), 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, a 13% by weight aqueous solution of sodium hypochlorite was added to 1.0 g of pulp to give a concentration of 3.8 mmol to initiate the reaction. During the reaction, a 0.5 M aqueous solution of sodium hydroxide was added dropwise to maintain the pH at 10 to 10.5. The reaction was considered complete when no further change in pH was observed.

[0183] [Cleaning process] The resulting TEMPO-oxidized pulp was then washed. The pulp slurry after TEMPO oxidation was dehydrated to obtain a dehydrated sheet, to which 5,000 parts by mass of ion-exchanged water was added, and the sheet was stirred to uniformly disperse the pulp. This process was repeated until the electrical conductivity of the filtrate reached 100 μS / cm or less, marking the end of the washing process.

[0184] [Additional oxidation treatment] The remaining aldehyde groups in this dehydrated sheet were further oxidized as follows: 100 parts by weight of the dehydrated sheet (dry mass equivalent) was dispersed in 10,000 parts by weight of 0.1 mol / L acetate buffer (pH 4.8). 113 parts by weight of 80% sodium chlorite was then added, the container was immediately sealed, and the mixture was stirred at 500 rpm using a magnetic stirrer for 48 hours at room temperature to produce a pulp slurry.

[0185] [Cleaning process] The resulting TEMPO-oxidized pulp was then washed. The pulp slurry after the additional oxidation was dehydrated to obtain a dehydrated sheet, to which 5,000 parts by mass of ion-exchanged water was added, and the sheet was stirred to uniformly disperse the pulp. This process was repeated until the electrical conductivity of the filtrate reached 100 μS / cm or less, marking the end of the washing process.

[0186] The carboxyl group content of the resulting TEMPO-oxidized pulp was measured using the method described below and was found to be 1.30 mmol / g. Furthermore, when the resulting TEMPO-oxidized pulp was analyzed using an X-ray diffractometer, typical peaks were observed at two positions, around 2θ = 14° to 17° and around 2θ = 22° to 23°, confirming the presence of cellulose type I crystals.

[0187] [Fiber defibration processing] Ion-exchanged water was added to the obtained TEMPO-oxidized pulp to prepare a slurry with a solids concentration of 2% by mass. This slurry was treated six times at a pressure of 200 MPa using a wet pulverizer (Starburst, manufactured by Sugino Machine Co., Ltd.) to obtain a fine fibrous cellulose dispersion (D) containing fine fibrous cellulose. X-ray diffraction confirmed that the fine fibrous cellulose maintained cellulose type I crystal structure. Furthermore, the fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 3 to 5 nm. The carboxyl group content, as measured by the measurement method described below, was 1.30 mmol / g.

[0188] <Production Example 5> [Sulfur oxo-oxidation treatment] A sulfated pulp was obtained in the same manner as in Production Example 1, except that 38 parts by mass of amidosulfuric acid was used instead of ammonium dihydrogen phosphate, except that the heating time in the hot air dryer was 20 minutes.

[0189] The infrared absorption spectrum of the sulfated pulp obtained was measured using FT-IR. -1 Absorption due to sulfate groups was observed in the vicinity, confirming that sulfate groups had been added to the pulp.

[0190] [Fiber defibration processing] Ion-exchanged water was added to the obtained sulfated pulp, followed by stirring to form a 2% by mass slurry. This slurry was treated six times at a pressure of 200 MPa using a wet pulverizer (Starburst, manufactured by Sugino Machine Corporation) to obtain a fine fibrous cellulose dispersion (E) containing fine fibrous cellulose. X-ray diffraction confirmed that the fine fibrous cellulose maintained cellulose type I crystal structure. Furthermore, the fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 2 to 5 nm. The amount of sulfate groups measured using the method for measuring the amount of sulfur oxoacid groups described below was 1.47 mmol / g.

[0191] <Measurement> [Measurement of phosphorus oxoacid group content] The amount of phosphorus oxo acid groups in the fine fibrous cellulose (equivalent to the amount of phosphorus oxo acid groups in the phosphorus oxo-oxidized pulp) was measured by adding ion-exchanged water to a fine fibrous cellulose dispersion containing the target fine fibrous cellulose to adjust the content to 0.2 mass%, treating the dispersion with ion-exchange resin, and then titrating it with alkali. Treatment with ion exchange resin was carried out by adding 1 / 10 by volume of a strongly acidic ion exchange resin (Amberjet 1024; manufactured by Organo Corporation, conditioned) to the above-mentioned fine fibrous cellulose-containing slurry, shaking for 1 hour, and then pouring it onto a mesh with 90 μm openings to separate the resin from the slurry. In addition, alkali titration was performed by measuring the change in the pH of a slurry containing fine fibrous cellulose after ion exchange resin treatment while adding 10 μL of 0.1 N sodium hydroxide solution every 5 seconds. Nitrogen gas was bubbled through the slurry for 15 minutes before the start of the titration. In this neutralization titration, two maximum points of increment (the derivative of pH with respect to the amount of alkali added) were observed on the plot of pH versus the amount of alkali added. The first maximum point of increment after starting the alkali addition is called the first endpoint, and the second maximum point is called the second endpoint (Figure 1). The amount of alkali required from the start of the titration to the first endpoint is equal to the amount of first dissociated acid in the slurry used for titration. The amount of alkali required from the start of the titration to the second endpoint is equal to the total amount of dissociated acid in the slurry used for titration. The amount of alkali (mmol) required from the start of titration to the first endpoint divided by the solid content (g) in the titrated slurry was defined as the amount of phosphorus oxo acid group (first dissociated acid amount) (mmol / g). The amount of alkali (mmol) required from the start of titration to the second endpoint divided by the solid content (g) in the titrated slurry was defined as the total dissociated acid amount (mmol / g).

[0192] [Measurement of carboxyl group content] The amount of carboxyl groups in the fine fibrous cellulose (equivalent to the amount of carboxyl groups in the TEMPO-oxidized pulp) was measured by adding ion-exchanged water to a fine fibrous cellulose dispersion containing the target fine fibrous cellulose to adjust the content to 0.2 mass%, treating the dispersion with ion-exchange resin, and then titrating it with alkali. Treatment with ion exchange resin was carried out by adding 1 / 10 by volume of a strongly acidic ion exchange resin (Amberjet 1024; manufactured by Organo Corporation, conditioned) to a 0.2% by mass slurry containing fine fibrous cellulose, shaking for 1 hour, and then pouring the mixture onto a mesh with 90 μm openings to separate the resin from the slurry. In addition, alkali titration was performed by measuring the change in pH of the fibrous cellulose-containing slurry after treatment with an ion exchange resin while adding 0.1 N aqueous sodium hydroxide. Observing the change in pH while adding aqueous sodium hydroxide yielded a titration curve like the one shown in Figure 2. As shown in Figure 2, in this neutralization titration, a single point was observed where the increment (the differential value of pH with respect to the amount of alkali added) reached a maximum on the curve plotting the measured pH against the amount of alkali added. This maximum increment was called the first endpoint. The region from the start of the titration to the first endpoint in Figure 2 is called the first region. The amount of alkali required in the first region was equal to the amount of carboxyl groups in the slurry used for titration. The amount of alkali (mmol) required in the first region of the titration curve was divided by the solids content (g) of the fine fibrous cellulose-containing slurry to be titrated to calculate the amount of carboxyl groups introduced (mmol / g). The amount of carboxyl groups introduced (mmol / g) is calculated based on the amount of carboxyl groups introduced (mmol / g) when the counter ions of the carboxyl groups are hydrogen ions (H + ) (hereinafter referred to as the amount of carboxy groups (acid type)) per 1 g of fibrous cellulose.

[0193] [Measurement of sulfur oxoacid group content] The obtained fibrous cellulose was wet ashed using perchloric acid and concentrated nitric acid, then diluted appropriately and the sulfur content was measured by ICP emission analysis. The sulfur content was divided by the bone dry mass of the fibrous cellulose used as the test sample, and the value was taken as the amount of sulfur oxoacid groups (unit: mmol / g).

[0194] [Measurement of carbamide group content] The amount of carbamide groups in the fine fibrous cellulose was measured by subjecting freeze-dried and pulverized samples to a Mitsubishi Chemical Analytech TN-110 trace total nitrogen analyzer. Note that ionic nitrogen was removed during the neutralization and washing processes. The amount of carbamide groups introduced per unit mass of fine fibrous cellulose (mmol / g) was calculated by dividing the nitrogen content (g / g) per unit mass of fine fibrous cellulose obtained by trace nitrogen analysis by the atomic weight of nitrogen.

[0195] Example 1 [Dissolution of cellulose ether] Methylcellulose (Metolose SM-25, manufactured by Shin-Etsu Chemical Co., Ltd., weight average molecular weight: 6.0 × 10 4 The resulting mixture was stirred at room temperature for 1 hour to dissolve the cellulose ether in an aqueous solution (A).

[0196] [Sheet production] The fine fibrous cellulose dispersion (A) and the cellulose ether aqueous solution (A) were each diluted with ion-exchanged water to a solid content of 0.5% by mass. Then, 70 parts by mass of the diluted fine fibrous cellulose dispersion and 30 parts by mass of the diluted cellulose ether aqueous solution were mixed to obtain a mixed solution. Furthermore, the finished sheet weight is 32 g / m 2 The mixture was weighed out so that the weight was 25 μm, and spread on a commercially available acrylic plate. A damming frame (inner dimensions 250 mm × 250 mm, height 5 cm) was placed on the acrylic plate to achieve the specified basis weight. The sheet was then dried in a dryer at 100 °C for 1 hour and peeled off from the acrylic plate to obtain a sheet containing fine fibrous cellulose. The sheet had a thickness of 25 μm.

[0197] <Example 2> In Example 1 [Dissolution of cellulose ether], methylcellulose (Metolose SM-400, manufactured by Shin-Etsu Chemical Co., Ltd., weight average molecular weight: 1.4 × 10 5 A cellulose ether aqueous solution (B) was obtained in which cellulose ether (substitution degree (methoxy group): 1.8) was dissolved. Except for this, a fine fibrous cellulose-containing sheet was obtained in the same manner as in Example 1.

[0198] Example 3 In Example 1 [Dissolution of cellulose ether], hydroxypropyl methylcellulose (Metolose 65SH-50, manufactured by Shin-Etsu Chemical Co., Ltd., weight average molecular weight: 7.5 × 10 4A cellulose ether aqueous solution (D) was obtained in which cellulose ether (a cellulose ether having a substitution degree (methoxy group): 1.8, a molar substitution number (hydroxypropoxy group): 0.15) was dissolved. Except for this, a fine fibrous cellulose-containing sheet was obtained in the same manner as in Example 1.

[0199] Example 4 In Example 1 [Dissolution of cellulose ether], hydroxypropyl methylcellulose (Metolose 65SH-400, manufactured by Shin-Etsu Chemical Co., Ltd., weight average molecular weight: 1.4 × 10 5 A cellulose ether aqueous solution (E) was obtained in which cellulose ether (a cellulose ether having a substitution degree (methoxy group): 1.8, a molar substitution number (hydroxypropoxy group): 0.15) was dissolved. Except for this, a fine fibrous cellulose-containing sheet was obtained in the same manner as in Example 1.

[0200] <Example 5> In Example 1 [Dissolution of cellulose ether], hydroxypropyl methylcellulose (Metolose 65SH-1500, manufactured by Shin-Etsu Chemical Co., Ltd., weight average molecular weight: 2.2 × 10 5 A cellulose ether aqueous solution (F) was obtained in which cellulose ether (a hydroxypropoxy group): 0.15, a substitution degree (methoxy group): 1.8, and a substitution molar number (hydroxypropoxy group): 0.15) was dissolved. Except for this, a fine fibrous cellulose-containing sheet was obtained in the same manner as in Example 1.

[0201] Example 6 In Example 5, the same procedure as in Example 5 was carried out except that the amount of diluted fine fibrous cellulose dispersion (A) mixed was changed to 50 parts by mass, and the amount of diluted cellulose ether aqueous solution (F) mixed was changed to 50 parts by mass, to obtain a fine fibrous cellulose-containing sheet.

[0202] Example 7 In Example 5, a fine fibrous cellulose-containing sheet was obtained in the same manner as in Example 5, except that the amount of diluted fine fibrous cellulose dispersion (A) mixed was changed to 30 parts by mass and the amount of diluted cellulose ether aqueous solution (F) mixed was changed to 70 parts by mass.

[0203] Example 8 In Example 5, the same procedure as in Example 5 was carried out except that the amount of diluted fine fibrous cellulose dispersion (A) mixed was changed to 10 parts by mass and the amount of diluted cellulose ether aqueous solution (F) mixed was changed to 90 parts by mass, and a fine fibrous cellulose-containing sheet was obtained.

[0204] Example 9 In Example 5, the finished basis weight was 180 g / m 2 A fine fibrous cellulose-containing sheet having a thickness of 150 μm was obtained in the same manner as in Example 5, except that the above-mentioned conditions were changed.

[0205] Example 10 In Example 6, the finished basis weight was 180 g / m 2 A fine fibrous cellulose-containing sheet having a thickness of 150 μm was obtained in the same manner as in Example 5, except that the above-mentioned conditions were changed.

[0206] Example 11 A fine fibrous cellulose-containing sheet was obtained in the same manner as in Example 5, except that the fine fibrous cellulose dispersion (B) was used.

[0207] Example 12 A fine fibrous cellulose-containing sheet was obtained in the same manner as in Example 6, except that the fine fibrous cellulose dispersion (B) was used.

[0208] Example 13 A fine fibrous cellulose-containing sheet was obtained in the same manner as in Example 5, except that the fine fibrous cellulose dispersion (C) was used.

[0209] Example 14 A fine fibrous cellulose-containing sheet was obtained in the same manner as in Example 5, except that the fine fibrous cellulose dispersion (D) was used.

[0210] Example 15 A fine fibrous cellulose-containing sheet was obtained in the same manner as in Example 5, except that the fine fibrous cellulose dispersion (E) was used.

[0211] Example 16 A laminate in which resin layers were laminated on both sides of the fine fibrous cellulose-containing sheet was obtained in the same manner as in Example 9, except that the fine fibrous cellulose-containing sheet obtained in Example 9 was subjected to the following treatment.

[0212] [Formation of resin layer] A resin coating solution was prepared by mixing 8.5 parts by mass of a modified polycarbonate resin (Iupizeta FPC-2136, manufactured by Mitsubishi Gas Chemical Company, Inc.), 60 parts by mass of toluene, and 30 parts by mass of methyl ethyl ketone. Next, 1.5 parts by mass of an isocyanate compound (Duranate TPA-100, manufactured by Asahi Kasei Chemicals Corporation) was added as an adhesion aid to the resin coating solution and mixed. This resin coating solution was applied to one side of a fine fibrous cellulose-containing sheet (the side that had been in contact with the acrylic plate) using a bar coater. The resin coating solution was then cured by heating at 100°C for 1 hour, forming a resin layer. A resin layer was then formed on the opposite side of the fine fibrous cellulose in the same manner, yielding a laminate. The resin layer had a thickness of 3 μm per side.

[0213] <Comparative Example 1> In Example 1 [Dissolution of cellulose ether], methylcellulose (Metolose SM-8000, manufactured by Shin-Etsu Chemical Co., Ltd., weight average molecular weight: 3.6 × 10 5 A cellulose ether aqueous solution (C) was obtained in which cellulose ether (substitution degree (methoxy group): 1.8) was dissolved. Except for this, a fine fibrous cellulose-containing sheet was obtained in the same manner as in Example 1.

[0214] <Comparative Example 2> In Example 1 [Dissolution of cellulose ether], hydroxypropyl methylcellulose (Metolose 65SH-15000, manufactured by Shin-Etsu Chemical Co., Ltd., weight average molecular weight: 4.3 × 10 5 A cellulose ether aqueous solution (G) was obtained by dissolving cellulose ether (a cellulose ether having a substitution degree (methoxy group): 1.8, and a molar substitution number (hydroxypropoxy group): 0.15). Except for the above, a fine fibrous cellulose-containing sheet was obtained in the same manner as in Example 1.

[0215] <Comparative Example 3> A fine fibrous cellulose-containing sheet was obtained in the same manner as in Example 1, except that the dissolution of cellulose ether in Example 1 was changed as follows.

[0216] [Dissolution of polyvinyl alcohol] Polyvinyl alcohol (Poval 5-74LLA, manufactured by Kuraray Co., Ltd., polymerization degree: 500, saponification degree: 74 mol%) was added to ion-exchanged water to a concentration of 12 mass%, and the mixture was stirred at 95°C for 1 hour to dissolve. By the above procedure, an aqueous polyvinyl alcohol solution (A) was obtained.

[0217] <Comparative Example 4> A fine fibrous cellulose-containing sheet was obtained in the same manner as in Example 1, except that the dissolution of cellulose ether in Example 1 was changed as follows.

[0218] [Dissolution of polyvinyl alcohol] Polyvinyl alcohol (Poval 5-98, manufactured by Kuraray Co., Ltd., polymerization degree: 500, saponification degree: 99 mol%) was added to ion-exchanged water to a concentration of 12% by mass, and the mixture was stirred at 95°C for 1 hour to dissolve. By the above procedure, an aqueous polyvinyl alcohol solution (B) was obtained.

[0219] <Comparative Example 5> A fine fibrous cellulose-containing sheet was obtained in the same manner as in Example 1, except that the dissolution of cellulose ether in Example 1 was changed as follows.

[0220] [Dissolution of polyethylene oxide] Polyethylene oxide (Sumitomo Seika Chemicals Co., Ltd., PEO-18, viscosity average molecular weight 4.3 × 10 6 ) was added so that the concentration was 2% by mass, and the mixture was stirred at room temperature for 1 hour to dissolve. By the above procedure, an aqueous polyethylene oxide solution was obtained.

[0221] <Comparative Example 6> A fine fibrous cellulose-containing sheet was obtained in the same manner as in Example 11, except that the dissolution of cellulose ether in Example 11 was changed as follows.

[0222] [Dissolution of polyvinyl alcohol] Polyvinyl alcohol (Poval 5-98, manufactured by Kuraray Co., Ltd., polymerization degree: 500, saponification degree: 99 mol%) was added to ion-exchanged water to a concentration of 12% by mass, and the mixture was stirred at 95°C for 1 hour to dissolve. By the above procedure, an aqueous polyvinyl alcohol solution (B) was obtained.

[0223] <Evaluation> [Measurement of total light transmittance of sheet] The total light transmittance of the sheet was measured using a haze meter (HM-150, manufactured by Murakami Color Research Laboratory Co., Ltd.) in accordance with JIS K 7361-1:1997.

[0224] [Sheet haze measurement] The haze of the sheet was measured in accordance with JIS K 7136:2000 using a haze meter (HM-150, manufactured by Murakami Color Research Laboratory Co., Ltd.).

[0225] [Sheet tensile properties] The tensile modulus, tensile strength, and tensile elongation were measured using a Tensilon tensile tester (manufactured by A&D Corporation) in accordance with JIS P 8113:2006. Test specimens were conditioned at 23°C and 50% relative humidity for 24 hours. The tensile modulus was calculated from the maximum positive slope of the SS curve.

[0226] [Yellowness measurement of sheet before and after heating] The yellowness index (YI) of the sheet was measured before and after heating using Colour Cute i (manufactured by Suga Test Instruments Co., Ltd.) in accordance with JIS K 7373:2006. The YI after heating was the YI of the sheet heated at 160°C for 6 hours. The change in YI before and after heating (ΔYI) was calculated using the following method. Change in YI before and after heating (ΔYI) = (yellowness of sheet after heating) - (yellowness of sheet before heating)

[0227] Sheet Appearance The appearance of the sheet was evaluated based on the YI before and after heating according to the following criteria. A: The change in YI before and after heating (ΔYI) is less than 1.5 B: Change in YI before and after heating (ΔYI) is 1.5 or more and less than 5.0 C: Change in YI before and after heating (ΔYI) is 5.0 or more

[0228] [Sheet formability] The sheet was cut into 5 cm x 5 cm test pieces, which were then conditioned at 23°C and 50% relative humidity for 24 hours. As shown in Figure 3, the test pieces were bent until θ reached 0°, and evaluated according to the following criteria. A: The sheet does not crack when bent. B: The sheet breaks when bent.

[0229] [Table 1-1]

[0230] [Table 1-2]

[0231] The sheets obtained in Examples 1 to 16 had low haze, high total light transmittance, and excellent transparency. Furthermore, the YI value before heating was low, and the increase in YI due to heating was also suppressed. Furthermore, the sheets had high tensile modulus and tensile strength, excellent rigidity, and high tensile elongation, excellent flexibility. On the other hand, the weight average molecular weight of the cellulose derivative is 3.0 × 10 5In Comparative Examples 1 and 2, where the temperature exceeded 100°C, gelation occurred, and it was not possible to form a sheet of uniform thickness. Furthermore, in Comparative Examples 3 to 6, where polyvinyl alcohol or polyethylene oxide was used instead of a cellulose derivative, the increase in YI due to heating was greater than in the cases where a cellulose derivative was used. Furthermore, the tensile elongation was small, and the sheet cracked when bent, resulting in poor moldability.

Claims

1. Fine fibrous cellulose having a fiber width of 10 nm or less; Weight average molecular weight is 1.0 × 10 ４ Above 3.0 x 10 ５ and a cellulose derivative, At least one selected from the group consisting of the following conditions A and B is satisfied: Sheet. Condition A: The amount of anionic groups in the fine fibrous cellulose is less than 0.50 mmol / g. Condition B: The cellulose derivative is a nonionic water-soluble cellulose ether.

2. A sheet as described in claim 1, which satisfies condition B and wherein the fine fibrous cellulose has anionic groups.

3. 3. The sheet according to claim 1, wherein the fine fibrous cellulose has a phosphorus oxo acid group or a group derived from a phosphorus oxo acid group.

4. The sheet according to claim 1, wherein the weight average molecular weight of the cellulose derivative is 6.0×10 4 or more and 3.0×10 5 or less.

5. A sheet described in any one of claims 2 to 4, which satisfies only condition B and has an amount of anionic groups in the fine fibrous cellulose of 0.80 mmol / g or more.

6. The sheet according to any one of claims 1 to 5, wherein the fine fibrous cellulose contains carbamide groups.

7. 7. The sheet according to claim 1, wherein the nonionic water-soluble cellulose ether has at least one functional group selected from the group consisting of a methoxy group and a hydroxypropoxy group.

8. The sheet according to any one of claims 1 to 7, wherein the non-ionic water-soluble cellulose ether is selected from the group consisting of methylcellulose and hydroxypropylmethylcellulose.

9. The sheet according to any one of claims 1 to 8, wherein the total content of the fine fibrous cellulose and the cellulose derivative in the solid content of the sheet is 90 mass% or more.

10. The sheet according to any one of claims 1 to 9, wherein the content of fine fibrous cellulose in the solid content of the sheet is 50 mass% or more.

11. The sheet according to any one of claims 1 to 10, wherein the change in yellow index (YI value) before and after heating the sheet at 160°C for 6 hours is 1.5 or less.

12. The sheet according to any one of claims 1 to 11, wherein the haze of the sheet is 5% or less.

13. The sheet according to any one of claims 1 to 12, wherein the sheet has a total light transmittance of 90% or more.

14. The sheet according to any one of claims 1 to 13, wherein the sheet has a tensile modulus of 6.5 GPa or more.

15. The sheet according to any one of claims 1 to 14, wherein the sheet has a tensile elongation of 3% or more.

16. A laminate comprising the sheet according to any one of claims 1 to 15 and a resin layer on at least one surface of the sheet.

17. The sheet according to any one of claims 1 to 15, which is for an optical member.

18. The sheet according to any one of claims 1 to 15, which is for food containers, cutlery, or straws.

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