WO2024253027A1 - Fiber sheet and method for producing same - Google Patents

Abstract

According to the present invention, the strength of a fiber sheet such as a paper is improved.　A fiber sheet according to an embodiment contains cellulose fibers satisfying conditions (A) to (C), and the proportion of the cellulose fibers which satisfy (A) to (C) among all the cellulose fibers contained in the fiber sheet is 50 mass% or greater. (A) Anionic functional groups are comprised, with at least a portion of the same being salt-type. (B) The number average fiber width is 1 μm or greater. (C) When an aqueous suspension with a cellulose fiber concentration of 0.2 mass%, adjusted to 20°C, is filtered using a filter with 60 μm mesh openings, the content of the cellulose fibers in the filtrate is 0.06 mass% or less.

Description

Fiber sheet and method for producing same

The present invention relates to a fiber sheet and a method for producing the same.

Fiber sheets, such as paper, are generally produced by beating pulp and then making paper from a stock containing the beaten pulp. During this papermaking process, bonds form between the pulp fibers during drying, creating voids between the fibers. The presence of these voids reduces the strength of the paper, and the paper turns white due to light scattering in the voids.

It is known that cellulose nanofibers, which are finely divided cellulose fibers, are added to paper to improve its physical properties such as strength. However, cellulose nanofibers have problems such as high production costs. For this reason, it has been proposed to use fibrillated chemically modified cellulose fibers, which have a lower degree of defibration than cellulose nanofibers.

For example, Patent Documents 1 and 2 disclose fibrillated chemically modified cellulose fibers with an average fiber width of 500 nm or more, in which carboxy groups have been introduced as a chemical modification. However, the chemically modified cellulose fibers described in Patent Documents 1 and 2 are used, for example, as papermaking additives, and there is no disclosure of using the chemically modified cellulose fibers as the main cellulose fibers to produce fiber sheets.

Patent Document 3, on the other hand, describes how TEMPO oxidized pulp is added to raw pulp as a chemically modified pulp to prepare a mixed pulp, which is then beaten to produce paper, and how the TEMPO oxidized pulp is refined by the beating. Patent Document 3 describes that there are no particular limitations on the mixture ratio of raw pulp and chemically modified pulp, but only gives a specific example in which a very small amount of chemically modified pulp is added.

International Publication No. 2020/195671 International Publication No. 2021/054274 International Publication No. 2019/189615

The object of one embodiment of the present invention is to provide a fiber sheet with excellent strength that uses anionically modified cellulose fibers as the main cellulose fiber.

The present invention includes the embodiments set forth below.
[1] A fiber sheet containing cellulose fibers that satisfy the following conditions (A) to (C), wherein the proportion of the cellulose fibers that satisfy the conditions (A) to (C) among all the cellulose fibers contained in the fiber sheet is 50 mass% or more.
(A) having anionic functional groups, at least a portion of which are in the salt form; (B) having a number average fiber width of 1 μm or more; and (C) having a cellulose fiber concentration of 0.2% by mass in water adjusted to 20° C., filtered through a filter having an opening of 60 μm, such that the cellulose fiber content in the filtrate is 0.06% by mass or less. [2] The fiber sheet according to [1], having a total light transmittance at a wavelength of 600 nm of 70% or more.
[3] The fiber sheet according to [1] or [2], having a tensile strength of 100 MPa or more. [4] The fiber sheet according to any one of [1] to [3], wherein the amount of anionic functional groups in the cellulose fibers is 1.5 to 2.5 mmol/g.
[5] The fiber sheet according to any one of [1] to [4], wherein the anionic functional group of the cellulose fiber is a carboxy group.
[6] The fiber sheet according to any one of [1] to [5], wherein the cellulose fibers have externally fibrillated fluff on the fiber surface.

[7] A step of preparing a suspension in which cellulose fibers having an anionic functional group are suspended in water;
adjusting the pH of the suspension to 5.0 or more; and
and filtering the suspension having a pH of 5.0 or more to form a sheet.
A method for producing paper, which does not include a step of beating the cellulose fibers under a condition of pH 5.0 or higher.
[8] A step of preparing a suspension in which cellulose fibers having an anionic functional group are suspended in water;
beating the cellulose fibers under a pH condition of less than 5.0;
adjusting the pH of the suspension after beating to 5.0 or more; and
and filtering the suspension having a pH of 5.0 or more to form a sheet.
A method for producing paper, which does not include a step of beating the cellulose fibers under a condition of pH 5.0 or higher.

An embodiment of the present invention can provide a fiber sheet with excellent strength that uses anionically modified cellulose fibers as the main cellulose fiber, and a method for producing the same.

Micrograph of cellulose fibers after neutralization step of Example 1 Micrograph of cellulose fibers after neutralization step of Example 4

The fiber sheet according to the embodiment contains cellulose fibers (hereinafter also referred to as anion-modified cellulose fibers) that satisfy the following conditions (A) to (C) in an amount of 50% by mass or more of the total cellulose fibers.
(A) has anionic functional groups, at least a portion of which is in the salt form; (B) has a number-average fiber width of 1 μm or more; and (C) when an aqueous suspension having a cellulose fiber concentration of 0.2% by mass and adjusted to 20° C. is filtered through a filter having an opening of 60 μm, the cellulose fiber content in the filtrate is 0.06% by mass or less.

By using anion-modified cellulose fibers having anionic functional groups introduced therein as the cellulose fibers constituting the fiber sheet, the void ratio between fibers can be reduced, and the strength and transparency of the fiber sheet can be improved. In this case, the anionic functional groups are in the salt form rather than the acid form, which can reduce the void ratio and is advantageous for increasing strength and transparency. However, when the salt-form anion-modified cellulose fibers are subjected to a beating process, the cellulose fibers are finely divided, and the viscosity of the suspension containing the cellulose fibers increases, making handling difficult, and it has been found that it is difficult to form a fiber sheet in a normal papermaking process. Therefore, when a fiber sheet is produced using anion-modified cellulose fibers as the main cellulose fibers, it is required that the amount of finely divided cellulose fibers is small. In this embodiment, by including anion-modified cellulose fibers that satisfy the above conditions (A) to (C), a fiber sheet that can be produced in a normal papermaking process and has excellent strength and transparency can be obtained.

Anion-modified cellulose fibers that satisfy the above conditions (A) to (C) can be obtained, for example, by suspending cellulose fibers having anionic functional groups in water and beating the suspension under conditions of a pH of less than 5.0, or by adjusting the pH of the suspension to 5.0 or higher without beating. Therefore, by filtering the suspension without further beating and forming it into a sheet, a fiber sheet containing anion-modified cellulose fibers that satisfy the conditions (A) to (C) can be obtained. However, this is not limited to this. Note that Patent Documents 1 to 3 do not disclose beating anion-modified cellulose fibers while they are in the acid form and then neutralizing them to convert them to the salt form, and do not disclose the construction of a fiber sheet with anion-modified cellulose fibers that satisfy the above conditions (A) to (C).

Anionically modified cellulose fibers are cellulose fibers obtained by chemically modifying unmodified cellulose fibers to introduce anionic functional groups. The anionic functional groups are preferably introduced at least to the fiber surface. There are no particular limitations on unmodified cellulose fibers, and examples include those derived from plants, animals, algae, microorganisms, and microbial products, with plant-derived pulp being preferred.

Examples of plant-derived pulps include unbleached softwood kraft pulp (NUKP), bleached softwood kraft pulp (NBKP), unbleached hardwood kraft pulp (LUKP), bleached hardwood kraft pulp (LBKP), unbleached softwood sulfite pulp (NUSP), bleached softwood sulfite pulp (NBSP), thermomechanical pulp (TMP), recycled pulp, and waste paper pulp. These may be used alone or in combination of two or more.

The anionic functional group (A) may be, for example, at least one selected from the group consisting of a carboxy group, a phosphate group, a sulfonic acid group, a nitrate group, a borate group, and a sulfate group. Of these, at least one selected from the group consisting of a carboxy group, a phosphate group, and a sulfate group is preferred. These functional groups may be directly or indirectly bonded to glucose units, which are structural units of cellulose molecules. When bonded indirectly, an alkylene group having 1 to 4 carbon atoms may be present between the glucose units and the anionic functional group. One or more anionic functional groups may be bonded to all glucose units constituting the cellulose molecule, or one or more anionic functional groups may be bonded to some of the glucose units constituting the cellulose molecule.

The concept of anionic functional groups includes not only acid types (for example, -COOH in the case of a carboxy group), but also salt types (for example, -COOX in the case of a carboxy group, where X is a cation that forms a salt with a carboxylic acid). In addition, the functional groups may be reacted with a compound (for example, PAE, described below) that is added when preparing the paper stock. There are no particular limitations on the salts, and examples of the salts include alkali metal salts such as sodium salts and potassium salts, alkaline earth metal salts such as magnesium salts and calcium salts, onium salts such as ammonium salts and phosphonium salts, and amine salts such as primary amines, secondary amines, and tertiary amines.

The amount of anionic functional groups is preferably 1.5 to 2.5 mmol/g, more preferably 1.8 to 2.3 mmol/g, per dry mass of the anion-modified cellulose fiber. The amount of anionic functional groups can be measured by the following method.

For example, in the case of carboxyl groups, 50 mL of anion-modified cellulose fiber-containing slurry adjusted to a concentration of 0.1 to 1% by mass is prepared, and the pH is adjusted to about 2.5 with a 0.1 mol/L aqueous hydrochloric acid solution. Next, a 0.05 mol/L aqueous sodium hydroxide solution is dropped into the slurry, and electrical conductivity measurement is performed, and this is continued until the pH becomes about 11. The amount of anionic functional groups can be calculated according to the following formula from the amount of sodium hydroxide (V) consumed in the neutralization stage of a weak acid in which the electrical conductivity changes slowly. The amount of phosphate groups can also be measured by electrical conductivity measurement. Other anionic functional groups may also be measured by known methods. In this specification, the "dry mass" refers to the mass after drying at 140°C until the mass change rate per minute is 0.05% or less.
Amount of anionic functional group (mmol/g) = V (mL) x [0.05/mass of anion-modified cellulose fiber (g)]

In one embodiment, examples of anion-modified cellulose fibers include oxidized cellulose fibers obtained by oxidizing the hydroxyl groups of the glucose units in the cellulose molecules, and carboxymethylated cellulose fibers obtained by carboxymethylating the hydroxyl groups of the glucose units in the cellulose molecules.

Oxidized cellulose fibers include those in which the hydroxyl group at the C6 position of the glucose unit in the cellulose molecule is selectively oxidized to a carboxyl group. Oxidized cellulose fibers are obtained by oxidizing natural cellulose such as wood pulp using a co-oxidant in the presence of an N-oxyl compound. As the N-oxyl compound, a compound having a nitroxy radical that is generally used as an oxidation catalyst is used, for example, a piperidine nitroxyoxy radical, and in particular, 2,2,6,6-tetramethylpiperidinooxy radical (TEMPO) or 4-acetamide-TEMPO is preferred. The anion-modified cellulose fiber according to a preferred embodiment is a TEMPO-oxidized cellulose fiber oxidized using TEMPO.

As mentioned above in (A), at least a portion of the anionic functional groups exists in the salt form. The anionic functional groups being in the salt form can increase the inter-pulp bonds that occur during drying in the sheet forming process (papermaking process in the case of papermaking), thereby reducing the void ratio between fibers. This can improve the strength of the fiber sheet, and can also improve the transparency of the fiber sheet by suppressing whitening caused by light scattering in the voids.

In the state of the fiber sheet, all of the anionic functional groups may be present in the salt form, and it is preferable that all of them are present in the salt form, but some may remain in the acid form. In addition, some of the anionic functional groups may have reacted with an additive such as PAE. The ratio of the amount of anionic functional groups present in the salt form to the total amount of anionic functional groups is preferably 50 mol % or more, more preferably 70 mol % or more, and may be 100 mol %. This ratio can be measured by using FT-IR to determine the ratio of the areas of the peaks showing the acid or salt functional groups.

As described above in (B), the number average fiber width of the anionically modified cellulose fiber is 1 μm or more. There is no particular upper limit to the number average fiber width, but it is preferably the same as the number average fiber width of unmodified pulp, for example, 60 μm or less. The number average fiber width of the anionically modified cellulose fiber is preferably 10 to 50 μm, more preferably 20 to 45 μm, and even more preferably 25 to 40 μm. The number average fiber width can be measured by the method described in the Examples section.

The number average fiber width of anionically modified cellulose fibers, even when subjected to beating treatment, is preferably approximately the same as that of untreated pulp. That is, in a preferred embodiment, the beating treatment is not performed to refine the anionically modified cellulose fibers to a smaller fiber width, but is a treatment to fluff the fiber surface and soften the fibers while minimizing the refinement of the fibers themselves as much as possible. Therefore, it is preferable that the anionically modified cellulose fibers have externally fibrillated fluff on the fiber surface.

The average aspect ratio (number average fiber length/number average fiber width) of the anion-modified cellulose fibers is not particularly limited, but is preferably, for example, 10 to 400, and more preferably 40 to 100. The average aspect ratio can be measured by the method described in the Examples section.

The above condition (C), in conjunction with condition (B), specifies that the amount of finely divided cellulose fibers is small. That is, the anion-modified cellulose fibers contained in the fiber sheet according to this embodiment satisfy the condition that when an aqueous suspension with a cellulose fiber concentration of 0.2% by mass is adjusted to 20°C and the aqueous suspension is filtered using a filter with a mesh size of 60 μm, the content of cellulose fibers in the filtrate (hereinafter, this content is also referred to as the "fine fiber content") is 0.06% by mass or less. The fine fiber content is preferably 0.04% by mass or less, more preferably 0.02% by mass or less, and may be 0% by mass. Here, the fine fiber content is the concentration of cellulose fibers in the filtrate, and is determined by evaporating water from the filtrate and measuring the dry mass of cellulose fibers contained in the filtrate.

The fiber sheet according to this embodiment contains the above-mentioned anion-modified cellulose fibers as the main cellulose fibers. Here, the fiber sheet is a sheet made by entangling fibers, and can be obtained, for example, by papering a suspension containing the fibers into a sheet shape.

In the fiber sheet according to this embodiment, the proportion of the anion-modified cellulose fibers among all the cellulose fibers contained in the fiber sheet is 50% by mass or more. The cellulose fibers constituting the fiber sheet may be only the anion-modified cellulose fibers, but may also contain other cellulose fibers such as unmodified pulp in addition to the anion-modified cellulose fibers. The amount of the anion-modified pulp in 100% by mass of all the cellulose fibers is preferably 70% by mass or more, more preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and may be 100% by mass.

In one embodiment, the fibers constituting the fiber sheet are preferably only cellulose fibers, but may contain other fibers in addition to the cellulose fibers. Of all the fibers constituting the fiber sheet, the proportion of cellulose fibers is preferably 70% by mass or more, more preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and may be 100% by mass.

In one embodiment, the fiber sheet preferably contains the above-mentioned anionically modified cellulose fiber as a main component. The content of the anionically modified cellulose fiber in 100% by mass of the fiber sheet is preferably 50% by mass or more, more preferably 70% by mass or more, more preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and may be 100% by mass.

The fiber sheet according to this embodiment may be composed of fibers only, but may also contain various additives in addition to the fibers. Examples of additives include water-resistant agents, flame retardants, colorants such as pigments and dyes, paper strength agents, retention aids, drainage aids, sizing agents, bulking agents, etc.

Water-resistant agents are additives for imparting water resistance to fiber sheets, and examples of such agents include polyamide epichlorohydrin (PAE), polyamine epichlorohydrin, urea formaldehyde resin, melamine formaldehyde resin, and polyvinylamine. The content of the water-resistant agent in 100% by mass of the fiber sheet is not particularly limited, and may be, for example, 0.01 to 5% by mass, or 0.05 to 1% by mass.

The flame retardant is an additive that imparts flame retardancy to the fiber sheet, and examples of such additives include aluminum hydroxide, organic or inorganic phosphoric acid, nitrogen-containing compounds, and halogen-based compounds. The content of the flame retardant in 100% by mass of the fiber sheet is not particularly limited, and may be, for example, 0.1 to 80% by mass, or 1 to 50% by mass.

The basis weight (mass per ^m2 ) of the fiber sheet is not particularly limited and may be, for example, 20 to 500 g/ ^m2 , or 30 to 200 g/ ^m2 .

In one embodiment, the fiber sheet preferably has a total light transmittance of 70% or more at a wavelength of 600 nm. The total light transmittance is more preferably 75% or more, more preferably 80% or more, and even more preferably 85% or more. The higher the total light transmittance, the better, so there is no particular upper limit, but it is usually 98% or less, and may be 95% or less. The total light transmittance can be measured by the method described in the Examples section.

In one embodiment, the tensile strength of the fiber sheet is preferably 100 MPa or more. The tensile strength of the fiber sheet is more preferably 120 MPa or more, and even more preferably 150 MPa or more. The higher the tensile strength, the better, so there is no particular upper limit, but it is usually 500 MPa or less. The tensile strength can be measured by the method described in the Examples section.

In one embodiment, the fiber sheet preferably has a porosity of 30% or less, and more preferably 25% or less. Since a lower porosity is preferable, there is no particular lower limit, but it is usually 10% or more, and may be 15% or more. The porosity can be measured by the method described in the Examples section.

In a preferred embodiment, the fiber sheet is paper, more preferably transparent paper with a total light transmittance of 70% or more. Here, paper includes not only single-ply paper made from a single layer, but also multi-ply paperboard (multi-layer paper) made from multiple layers, such as cardboard base paper.

In one embodiment, the fiber sheet may be made into a laminated sheet by providing a clear layer, a colored layer, or the like on the front or back surface of the sheet.

A method for producing a fiber sheet according to one embodiment includes the steps of:
(1) preparing a suspension in which cellulose fibers having anionic functional groups (i.e., anion-modified cellulose fibers) are suspended in water (suspension preparation step);
(2) adjusting the pH of the suspension to 5.0 or more (neutralization step); and
(3) A step of filtering the suspension having a pH of 5.0 or more to form a sheet (sheet forming step), but not including a step of beating the anion-modified cellulose fiber under a condition of a pH of 5.0 or more.

In a preferred embodiment, the method for producing a fiber sheet further includes a step of beating the cellulose fibers under a condition of a pH of less than 5.0, and adjusting the pH of the suspension after beating to 5.0 or more.
(1) the suspension preparation step;
(2) A step of beating the anion-modified cellulose fiber under a condition of pH less than 5.0 (beating step);
(3) a step of adjusting the pH of the suspension after beating to 5.0 or more (neutralization step); and
(4) The sheet forming step,
and does not include a step of beating the anionically modified cellulose fibers under a condition of pH 5.0 or higher.

Although a beating process under conditions of pH less than 5.0 is not essential, the beating process can fluff the fiber surface, thereby further improving the strength and transparency of the fiber sheet.

In the suspension preparation process, the suspension containing anionically modified cellulose fibers can be prepared, for example, by introducing anionic functional groups into the cellulose of unmodified pulp by a known method and suspending the resulting anionically modified cellulose fibers in water. It may also be prepared by suspending commercially available anionically modified cellulose fibers in water. In this case, the suspension may be prepared as a suspension in which cellulose fibers having acid-type anionic functional groups (i.e., acid-type anionically modified cellulose fibers) are suspended in water, and in this case, the pH of the suspension is less than 5.0. Alternatively, if the beating process is not performed, the acid-type anionically modified cellulose fibers may be neutralized by adding an alkali while being suspended in water, or the neutralization process may be performed simultaneously in the suspension preparation process.

The suspension may contain other cellulose fibers, such as unmodified pulp, as well as fibers other than cellulose fibers, in addition to the anionically modified cellulose fibers, as long as the effect is not impaired.

In the beating process, the anion-modified cellulose fibers are beated under conditions where the pH of the suspension is less than 5.0. In the acidic range of less than pH 5.0, the anion-modified cellulose fibers have acid-type anionic functional groups, and the beating process is carried out on such acid-type anion-modified cellulose fibers.

Beating is a process in which cellulose fibers are mechanically treated with water to soften or fibrillate them, and can be carried out, for example, by the beating process used in normal papermaking. For beating, for example, high-speed rotary, colloid mill, high-pressure, roll mill, ultrasonic, and other types of equipment are used. Specific examples include high-pressure homogenizers, refiners, beaters, PFI mills, kneaders, dispersers, and high-speed disintegrators.

The cellulose fiber concentration of the suspension to be subjected to the beating treatment is not particularly limited, but is preferably 1 to 20% by mass, and more preferably 5 to 15% by mass. The temperature of the suspension is preferably 5 to 50°C, and more preferably 10 to 30°C. The H of the suspension is less than 5.0 as described above, and may be, for example, 1.0 to 4.5, or 1.5 to 4.0.

In the neutralization process, an alkali is added to a suspension containing unbeaten or beaten acid-type anion-modified cellulose fibers to adjust the suspension to a neutral to alkaline pH of 5.0 or higher. As a result, at least a portion of the anionic functional groups of the anion-modified cellulose fibers contained in the suspension become salt-type. The alkali is not particularly limited as long as it can adjust the pH of the suspension to 5.0 or higher, and examples of the alkali include hydroxides of alkali metals or alkaline earth metals, ammonia, and amines.

The final pH of the suspension in the neutralization step is preferably 5.0 to 11.0, more preferably 6.0 to 10.5, more preferably 6.5 to 10.0, more preferably 7.0 to 9.0, and even more preferably 7.0 to 8.0. The temperature of the suspension in the neutralization step is not particularly limited, and may be, for example, 5 to 50°C or 10 to 30°C.

In the sheet forming process, the suspension adjusted to pH 5.0 or higher in the neutralization process is filtered to form the cellulose fibers into a sheet. In other words, the suspension is used to make the cellulose fibers into a sheet. Preferably, the suspension is used as a paper stock to make paper.

In the sheet forming process, the above suspension (preferably paper stock) which is the raw material to be formed into sheets may contain various additives such as water-resistant agents, flame retardants, colorants such as pigments and dyes, paper strength agents, retention agents, drainage agents, sizing agents, bulking agents, etc., in addition to the salt-type anion-modified cellulose fibers and water.

The sheeting process can be carried out by a known method, and is not particularly limited. For example, papermaking is a process in which the paper stock is dehydrated by filtration to form a sheet, which is then pressed and dried to produce paper. The sheeting process (preferably the papermaking process) can be carried out using a known papermaking machine, such as a Fourdrinier wet papermaking machine, a twin-wire papermaking machine, a Yankee papermaking machine, a cylinder papermaking machine, or a cylinder-shortened papermaking machine.

The solids concentration of the suspension (preferably the paper stock) in the sheet forming process is not particularly limited, and may be, for example, 0.05 to 10% by mass, or 0.1 to 5% by mass.

The fiber sheet manufacturing method according to this embodiment does not include a step of beating the suspension at a pH of 5.0 or higher, as described above. When anion-modified cellulose fibers containing salt-type anionic functional groups at a pH of 5.0 or higher are subjected to a beating process, the cellulose fibers are finely divided and the viscosity of the suspension increases. This increases the viscosity of the suspension in the sheeting process, making handling difficult and also making it difficult to form a sheet in a normal papermaking process. By not performing a beating process at a pH of 5.0 or higher, the fine fiber content of (C) above can be reduced, making it possible to apply the method to a normal papermaking process and preventing the dehydration time from becoming excessively long.

The following examples are explained in detail together with comparative examples. However, the present invention is not limited to these examples.

The methods for measuring the various physical properties in the examples and comparative examples are as follows.
[Amount of functional groups (amount of carboxy groups)]
A 50 mL aqueous suspension of anion-modified cellulose fibers with a cellulose fiber concentration of 0.1% by mass was prepared, and the pH was adjusted to about 2.5 with a 0.1 mol/L aqueous hydrochloric acid solution. Next, a 0.05 mol/L aqueous sodium hydroxide solution was dropped into the aqueous suspension, and the electrical conductivity was measured until the pH reached about 11. The amount of carboxyl groups was calculated from the amount of sodium hydroxide (V) consumed in the neutralization stage of the weak acid, where the change in electrical conductivity was gradual, according to the following formula.
Amount of carboxyl group (mmol/g) = V (mL) x [0.05/mass of anion-modified cellulose fiber (g)]

[Amount of functional groups (amount of phosphate groups)]
The anion-modified cellulose fiber was diluted with ion-exchanged water to a content of 0.2% by mass to prepare an aqueous suspension, which was then treated with an ion-exchange resin and titrated with an alkali. The treatment with the ion-exchange resin was carried out by adding 1/10 by volume of a strongly acidic ion-exchange resin (Amberjet 1024; Organo Corporation, conditioned) to the aqueous suspension, shaking for 1 hour, and then pouring the mixture onto a mesh with an opening of 90 μm to separate the resin from the aqueous suspension. The titration with the alkali was carried out by adding 50 μL of 0.1 mol/L of sodium hydroxide aqueous solution to the aqueous suspension after the treatment with the ion-exchange resin once every 30 seconds, while measuring the change in the electrical conductivity value of the aqueous suspension. The amount of phosphate groups (mmol/g) was calculated by dividing the amount of alkali (mmol) required in the region corresponding to the first region among the measurement results by the solid content (g) in the aqueous suspension to be titrated.

[Functional group amount (sulfate group amount)]
A predetermined amount of anion-modified cellulose fiber was combusted, and the sulfur content in the combustion product was measured using a combustion ion chromatograph according to a method in accordance with IEC 62321, and calculated in terms of the amount of sulfate groups.

[pH]
The measurement was performed using a pH meter (HM-40P, manufactured by DKK Toa Corporation).

[Number average fiber width and average aspect ratio]
Ten images were taken of an aqueous suspension of cellulose fibers diluted to 0.01% by mass using an optical microscope (RH-2000, manufactured by Hirox Co., Ltd.), from which 25 fibers were selected and their fiber widths and lengths were measured. The fiber widths and fiber lengths were determined by measuring the width (diameter) and length of the fiber body excluding externally fibrillated fluff. The arithmetic means of the obtained fiber width and fiber length data were calculated to determine the number average fiber width [μm] and number average fiber length [μm]. The average aspect ratio was calculated according to the following formula:
Average aspect ratio = number average fiber length [μm] / number average fiber width [μm]

[Fine fiber content]
100 mL of an aqueous suspension of anion-modified cellulose fibers having a cellulose fiber concentration of 0.2% by mass was prepared, and the temperature of the aqueous suspension was adjusted to 20°C. The prepared aqueous suspension was filtered using a nylon mesh filter (diameter 90 mm) with an opening of 60 μm, and the cellulose fiber content in the filtrate was measured. The filtration was performed in an atmosphere of 20°C, and the aqueous suspension was slowly poured onto the filter to allow natural filtration, and the filtrate after 30 minutes was used to measure the above content. The solid content in the filtrate was measured using an infrared heating and drying type moisture meter (MX-50, manufactured by A&D Co., Ltd.) and was taken as the fine fiber content.

[Viscosity of paper material (suspension)]
A paper stock was prepared as an aqueous suspension of anion-modified cellulose fibers with a cellulose fiber concentration of 0.2% by mass, and then allowed to stand for one day, after which the viscosity was measured using a BM type viscometer (25° C., 3 minutes).

[Porosity of paper]
The porosity was calculated from the sheet density ρ at 23° C. and 50% RH and the true density _ρt of the anion-modified cellulose by the following formula.
(Porosity)=1-(ρ×(1-M))/ _ρt
M is the moisture content at 23° C. and 50% RH. The sheet density at 23° C. and 50% RH was calculated from the thickness (PG-02J, measured by Techlock Co., Ltd.), side length (DT-150, measured by Niigata Seiki Co., Ltd.), and mass (HM-202, measured by A&D Co., Ltd.) of a test piece (approximately 5×5 cm). The true density was 1.7 for TEMPO oxidized cellulose, 1.8 for phosphated cellulose, and 1.8 for sulfated cellulose.

[Total light transmittance of paper]
The total light transmittance at a wavelength of 600 nm was measured using a spectrophotometer (UV-Vis V670, manufactured by JASCO Corporation).

[Paper tensile strength and breaking elongation]
Using a tensile tester (EZ-SX, manufactured by Shimadzu Corporation), the paper cut into a size of 6 cm length x 0.5 cm width was subjected to a tensile test under the conditions of a gripper distance of 3 cm, a tensile speed of 3 mm/min, 23°C, and 50% RH. The tensile strength is the maximum tensile force recorded when the paper is pulled until it breaks, divided by the cross-sectional area of the paper before the test. The breaking elongation is the elongation at which the paper breaks, and is the ratio to the length before the test.

[Example 1]
(Preparation of TEMPO-oxidized cellulose fiber suspension)
150 mL of water, 0.25 g of sodium bromide, and 0.025 g of TEMPO were added to 2 g of softwood pulp, thoroughly stirred to disperse, and then a 13% by mass aqueous solution of sodium hypochlorite (co-oxidant) was added so that the amount of sodium hypochlorite was 6.0 mmol/g per 1.0 g of the pulp to initiate the reaction. Since the pH decreased with the progress of the reaction, a 0.5 mol/L aqueous solution of sodium hydroxide was added dropwise to maintain the pH at 10 to 11, and the reaction was continued until no change in pH was observed (reaction time: 120 minutes). After the reaction was completed, 0.1 mol/L hydrochloric acid was added to adjust the pH to 2.0, and then the mixture was purified by repeated filtration and washing with water to obtain cellulose fibers with oxidized fiber surfaces. Pure water was added to the mixture to dilute it to a cellulose fiber concentration of 10% by mass, to prepare a TEMPO-oxidized cellulose fiber suspension.

In the obtained TEMPO-oxidized cellulose fiber suspension, the amount of carboxy groups in the TEMPO-oxidized cellulose fibers was 2.2 mmol/g. The pH (25°C) of the suspension was 4.0, and the carboxy groups were in the acid form.

(Neutralization process)
The TEMPO oxidized cellulose fiber suspension obtained above was diluted to 0.2% by mass with ion-exchanged water, and then neutralized with a 0.5 mol/L aqueous sodium hydroxide solution to a pH (25°C) of 7.0. This resulted in a suspension of TEMPO oxidized cellulose fibers having salt-type carboxy groups. The resulting TEMPO oxidized cellulose fibers had a number-average fiber width of 38 μm, an average aspect ratio of 65, and a fine fiber content of (C) of 0.01% by mass. The viscosity of the suspension after neutralization was 10 mPa·s or less. As shown in Figure 1, the neutralized TEMPO oxidized cellulose fibers had a fiber morphology similar to that of the raw pulp, and no fluff due to external fibrillation was observed on the fiber surface.

(Papermaking process)
The above-obtained 0.2% by mass suspension of TEMPO-oxidized cellulose fibers having a salt-type carboxyl group was used as a paper stock to make paper. The above suspension was filtered using a "Standard Sheet Machine Papermaking Device" manufactured by Kumagai Riki Kogyo Co., Ltd., equipped with a nylon mesh filter having an opening of 59 μm. The obtained sheet-like wet deposit was sandwiched between a flat membrane filter and absorbent paper, pressed at room temperature at 4 MPa using a heat press device, and further heated at 60° C. for 1 hour at 4 MPa to prepare paper of Example 1 having a basis weight of 60 g/m ^2. The filtration time during papermaking was 10 seconds.

[Example 2]
In the preparation method of Example 1, a TEMPO-oxidized cellulose fiber suspension having a cellulose fiber concentration of 10% by mass and a pH of 4.0 was obtained, and then the suspension was subjected to a beating process. In the beating process, the suspension was beaten 10,000 times using a "PFI Mill" manufactured by Kumagai Riki Kogyo Co., Ltd.

The suspension of TEMPO-oxidized cellulose fibers after beating was diluted to 0.2% by mass with ion-exchanged water, and then neutralized with 0.5 mol/L aqueous sodium hydroxide solution to a pH (25°C) of 7.0. The number-average fiber width of the resulting TEMPO-oxidized cellulose fibers was 38 μm, the average aspect ratio was 65, and the fine fiber content of (C) above was 0.02% by mass. In addition, the viscosity of the suspension after neutralization was 10 mPa·s or less.

The resulting TEMPO oxidized cellulose fiber suspension (cellulose fiber concentration: 0.2% by mass) with a pH of 7.0 was used as a paper stock, and paper of Example 2 with a basis weight of 60 g/ ^m2 was prepared in the same manner as in Example 1. The filtration time during papermaking was 30 seconds.

[Example 3]
Paper of Example 3 having a basis weight of 60 g/ ^m2 was prepared in the same manner as in Example 2, except that the number of beatings was 20,000. The number average fiber width of the TEMPO-oxidized cellulose fibers after beating and neutralization was 35 μm, the average aspect ratio was 66, and the fine fiber content of (C) above was 0.04 mass%. The viscosity of the suspension used as the paper stock was 40 mPa·s. The filtration time during papermaking was 900 seconds.

[Example 4]
Paper of Example 4 with a basis weight of 60 g/ ^m2 was prepared in the same manner as in Example 2, except that the number of beatings was 40,000. The number average fiber width of the TEMPO-oxidized cellulose fibers after beating and neutralization was 33 μm, the average aspect ratio was 70, and the fine fiber content of (C) above was 0.05 mass%. The viscosity of the suspension used as the paper stock was 60 mPa·s. The filtration time during papermaking was 360 seconds. As shown in Figure 2, the TEMPO-oxidized cellulose fibers after neutralization had fuzz due to external fibrillation on the fiber surface while maintaining a fiber width in the fiber body that was approximately the same as that of the raw material pulp.

[Example 5]
Paper of Example 5 having a basis weight of 60 g/ ^m2 was prepared in the same manner as in Example 4, except that the amount of sodium hypochlorite added during preparation of the suspension of TEMPO-oxidized cellulose fibers was 4.5 mmol/g. The amount of carboxyl groups in the TEMPO-oxidized cellulose fibers was 1.6 mmol/g. After beating and neutralization, the number-average fiber width of the TEMPO-oxidized cellulose fibers was 36 μm, the average aspect ratio was 68, and the fine fiber content of (C) above was 0.04 mass%. The viscosity of the suspension used as the paper stock was 40 mPa·s. The filtration time during papermaking was 300 seconds.

[Example 6]
(Preparation of Phosphated Cellulose Fiber Suspension)
A mixed aqueous solution of ammonium dihydrogen phosphate and urea was added to 100 parts by mass (bone dry mass) of softwood kraft pulp to adjust the mixture to 45 parts by mass of ammonium dihydrogen phosphate, 120 parts by mass of urea, and 150 parts by mass of water, to obtain a chemical-impregnated pulp. The obtained chemical-impregnated pulp was then heated for 200 seconds in a hot air dryer at 165°C to introduce phosphoric acid groups into the cellulose in the pulp, to obtain phosphoric acid esterified cellulose fibers. After the reaction, 0.1 mol/L hydrochloric acid was added to adjust the pH to 1.0, and then filtration and washing with water were repeated to refine the mixture, to obtain cellulose fibers whose fiber surfaces were phosphoric acid esterified. Pure water was added to the mixture to dilute it to a cellulose fiber concentration of 10% by mass, to prepare a phosphoric acid esterified cellulose fiber suspension.

The amount of phosphate groups in the resulting phosphated cellulose fibers was 2.0 mmol/g. The pH of the suspension was 1.8, and the phosphate groups were in the acid form.

(beating process/neutralization process)
After obtaining a phosphated cellulose fiber suspension having a cellulose fiber concentration of 10% by mass and a pH of 1.8, the suspension was subjected to a beating process. The suspension of the phosphated cellulose fibers after beating was diluted to 0.2% by mass with ion-exchanged water, and then diluted with a 0.5 mol/L aqueous sodium hydroxide solution. The resulting cellulose phosphate fiber had a number average fiber width of 31 μm and an average aspect ratio of 55. The content was 0.05% by mass. The viscosity of the suspension after neutralization was 70 mPa·s.

(Papermaking process)
The 0.2% by mass suspension of phosphoric acid esterified cellulose fibers having salt-type phosphoric acid groups obtained above was used as a paper stock, and paper was made in the same manner as in Example 1 to prepare paper of Example 6 having a basis weight of 60 g/ ^m2 . The filtration time during papermaking was 320 seconds.

[Example 7]
(Preparation of sulfated cellulose fiber suspension)
2 g of softwood kraft pulp, 20 g of sulfamic acid, 50 g of urea, and 100 g of ion-exchanged water were mixed and stirred with a stirrer for 10 minutes. After stirring, the slurry was suction filtered using filter paper (No. 2). Suction filtration was performed until the solution stopped dripping. After suction filtration, the pulp was peeled off from the filter paper and placed in a dryer with a thermostatic bath temperature set to 50°C and reacted for 6 hours. After the reaction, 0.1 mol/L hydrochloric acid was added to adjust the pH to 1.0, and then filtration and washing with water were repeated to refine the mixture, and cellulose fibers with oxidized fiber surfaces were obtained. Pure water was added to the mixture to dilute it to a cellulose fiber concentration of 10% by mass, and a sulfated cellulose fiber suspension was prepared.

The amount of sulfate groups in the obtained sulfated cellulose fibers was 1.8 mmol/g. The pH of the suspension was 1.8, and the sulfate ester groups were in the acid form.

(beating process/neutralization process)
After obtaining a sulfated cellulose fiber suspension having a cellulose fiber concentration of 10% by mass and a pH of 1.8, the suspension was subjected to a beating process. The suspension of sulfated cellulose fibers after beating was diluted to 0.2% by mass with ion-exchanged water, and then neutralized with a 0.5 mol/L aqueous solution of sodium hydroxide. The number average fiber width of the resulting sulfated cellulose fiber was 30 μm, the average aspect ratio was 50, and the fine fiber content of the above (C) was The neutralized suspension had a viscosity of 90 mPa·s.

(Papermaking process)
The 0.2% by mass suspension of sulfated cellulose fibers having salt-type sulfate groups obtained above was used as a paper stock, and paper was made in the same manner as in Example 1 to prepare paper of Example 7 having a basis weight of 60 g/ ^m2 . The filtration time during papermaking was 340 seconds.

[Comparative Example 1]
In the preparation method of Example 1, a TEMPO-oxidized cellulose fiber suspension having a cellulose fiber concentration of 10% by mass and a pH of 4.0 was obtained, and then the suspension was diluted to 0.2% by mass with ion-exchanged water but not neutralized, and paper was made in the same manner as in Example 1 while remaining in an acid form with a pH of 4.0, to obtain paper of Comparative Example 1 having a basis weight of 60 g/ ^m2 . The TEMPO-oxidized cellulose fiber used in the papermaking had a number-average fiber width of 38 μm, an average aspect ratio of 65, and the fine fiber content of (C) above of 0% by mass. The viscosity of the suspension used in the papermaking was 10 mPa·s or less, and the filtration time during papermaking was 10 seconds.

[Comparative Example 2]
In the preparation method of Example 1, a TEMPO oxidized cellulose fiber suspension (cellulose fiber concentration 10% by mass) with a pH of 4.0 was neutralized with a 0.5 mol/L aqueous sodium hydroxide solution to adjust the pH (25°C) to 7.0, and the neutralized suspension was then subjected to a beating step. In the beating step, the suspension was beaten 40,000 times using a "PFI Mill" manufactured by Kumagai Riki Kogyo Co., Ltd.

The beaten suspension was diluted with ion-exchanged water to 0.2% by mass to prepare paper stock. After beating, the number-average fiber width of the TEMPO-oxidized cellulose fibers was 12 μm, the average aspect ratio was 120, and the fine fiber content of (C) was 0.15% by mass. The viscosity of the suspension after dilution to 0.2% by mass was 420 mPa·s. After beating, the TEMPO-oxidized cellulose fibers had external fibrillation of the fiber surface and the cellulose fibers were cut, which is thought to have increased the viscosity and increased the fine fiber content of (C).

The obtained paper stock was used to carry out the papermaking process in the same manner as in Example 1, but the amount of cellulose fiber remaining on the nylon mesh filter was too small to prepare paper.

[Comparative Example 3]
The unmodified softwood pulp was diluted to 0.2% by mass with ion-exchanged water to prepare a cellulose fiber suspension having a pH of 7.0. The suspension was subjected to a beating process. In the beating process, the suspension was beaten 40,000 times using a "PFI mill" manufactured by Kumagai Riki Kogyo Co., Ltd. The number average fiber width of the cellulose fibers in the suspension after beating was 36 μm, the average aspect ratio was 70, and the fine fiber content of (C) above was 0% by mass. In addition, the viscosity of the suspension after beating was 10 mPa·s or less. The cellulose fiber suspension was used as a paper stock to prepare paper of Comparative Example 3 having a basis weight of 60 g/m ² by the same method as in Example 1. The filtration time during papermaking was 5 seconds.

[Comparative Example 4]
150 mL of water, 0.25 g of sodium bromide, and 0.025 g of TEMPO were added to 2 g of softwood pulp, thoroughly stirred to disperse, and then a 13% by mass aqueous solution of sodium hypochlorite (co-oxidant) was added so that the amount of sodium hypochlorite was 6.0 mmol/g per 1.0 g of the pulp to start the reaction. The temperature was kept at 20° C. during the reaction. Since the pH decreased with the progress of the reaction, the reaction was continued until no change in pH was observed while dropping 0.5 mol/L aqueous solution of sodium hydroxide to maintain the pH at 10 to 11 (reaction time: 120 minutes). After the reaction was completed, 0.1 mol/L hydrochloric acid was added to adjust the pH to 2 or less, and then the mixture was purified by repeatedly filtering and washing with water. Pure water was added to the mixture to adjust the solid content concentration to 4% by mass. The pH of the slurry was then adjusted to 10 with a 24% by mass aqueous solution of sodium hydroxide. The slurry was heated to 30°C, and sodium borohydride was added at 0.2 mmol/g relative to the cellulose fibers, and the mixture was reacted for 2 hours to perform reduction treatment. After the reaction, 0.1 mol/L hydrochloric acid was added to adjust the pH to 2 or less, and then the mixture was purified by repeatedly filtering and washing with water. Pure water was added to the purified cellulose fibers to adjust the final concentration to 0.2 mass% of cellulose fibers. A 24 mass% aqueous sodium hydroxide solution was added thereto to adjust the pH to 7. The mixture was treated twice at a pressure of 100 MPa using a high-pressure homogenizer (H11, manufactured by Sanwa Engineering Co., Ltd.) to prepare a cellulose nanofiber suspension.

The obtained cellulose nanofibers had a number average fiber width of 0.003 μm, an average aspect ratio of 250, and a fine fiber content of (C) of 0.2 mass%. The viscosity of the cellulose nanofiber suspension was 1020 mPa·s.

The cellulose nanofiber suspension was used as the paper stock and the papermaking process was carried out in the same manner as in Example 1, but the cellulose nanofibers all passed through the nylon mesh filter, so paper could not be prepared.

The porosity, total light transmittance, tensile strength and breaking elongation were measured for the papers prepared above in Examples 1 to 7 and Comparative Examples 1 and 3. These measurements were carried out after adjusting the condition of the paper in an atmosphere with a temperature of 23°C and a relative humidity of 50%. The results are shown in Tables 1 and 2 below.

The results are shown in Tables 1 and 2. In Comparative Example 3, unmodified pulp was beaten to produce paper. In Comparative Example 3, the paper had a high porosity, which resulted in low total light transmittance and a white color similar to that of ordinary paper, and the tensile strength was also low. In Comparative Example 4, cellulose nanofibers were used as the paper stock. In Comparative Example 4, the cellulose fibers were so finely divided that they all passed through the filter mesh, making it impossible to prepare paper using the normal papermaking process.

On the other hand, in Comparative Example 1, the suspension of TEMPO-oxidized cellulose fibers was not neutralized and was made in an acid form. In Comparative Example 1, anionic functional groups were introduced to the fiber surface of the cellulose fibers, so the porosity of the paper was lower than that of unmodified Comparative Example 3, and therefore the total light transmittance was improved and the paper was made transparent, and the tensile strength was also improved. In Example 1, the suspension of TEMPO-oxidized cellulose fibers was neutralized to make it a salt form and then made into paper. In this case, the average fiber width and fine fiber content were the same as those of Comparative Example 1, but since the anionic functional groups were in the salt form, the porosity of the paper was lower than that of Comparative Example 3, and the total light transmittance was significantly improved, and the tensile strength was also significantly improved, not only compared to Comparative Example 3, but also to Comparative Example 1. In Example 1, the breaking elongation of the paper was significantly improved compared to Comparative Example 1. The thickness variation of the paper of Example 1, which was made with the anionic functional groups in the salt form, was smaller than that of Comparative Example 1, which was made with the anionic functional groups in the acid form, and this is thought to have contributed to the improvement of the breaking elongation.

In Comparative Example 2, a suspension of TEMPO-oxidized cellulose fibers was neutralized and then beaten. Beating is thought to fibrillate the cellulose fibers, reducing the porosity of the paper and promoting strength and transparency. However, in Comparative Example 2, the TEMPO-oxidized cellulose fibers were converted to a salt form before being beaten, which caused the cellulose fibers to be partially refined and the viscosity to increase, with many cellulose fibers passing through the filter mesh. In addition, the cellulose fibers remaining on the filter mesh were of low concentration and in a gel-like state, making it impossible to prepare paper using the normal papermaking process.

In contrast, in Examples 2 to 7, the anionic functional groups were beaten while still in the acid form, and then neutralized to form the salt form for papermaking, which made it possible to promote the formation of fluff through external fibrillation while suppressing the fineness of the cellulose fibers. This prevented the viscosity of the anion-modified cellulose fiber suspension from increasing, and made it possible to make the paper using the normal papermaking process while reducing the porosity of the paper and significantly improving the total light transmittance and tensile strength. The results of Examples 2 to 4 showed that the greater the number of beating cycles, the smaller the porosity of the paper and the higher the total light transmittance and tensile strength of the paper.

The various numerical ranges described in the specification can be arbitrarily combined with their respective upper and lower limit values, and all such combinations are considered to be preferred numerical ranges described in this specification. In addition, a numerical range described as "X to Y" means greater than or equal to X and less than or equal to Y.

Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the gist of the invention. These embodiments and their omissions, substitutions, modifications, etc. are included within the scope of the invention and its equivalents as set forth in the claims, as well as within the scope and gist of the invention.

Claims (8)

A fiber sheet containing cellulose fibers that satisfy the following conditions (A) to (C), wherein the proportion of the cellulose fibers that satisfy the conditions (A) to (C) among all the cellulose fibers contained in the fiber sheet is 50 mass% or more.
(A) has anionic functional groups, at least a portion of which is in the salt form; (B) has a number-average fiber width of 1 μm or more; and (C) when an aqueous suspension having a cellulose fiber concentration of 0.2% by mass and adjusted to 20° C. is filtered through a filter having an opening of 60 μm, the cellulose fiber content in the filtrate is 0.06% by mass or less. The fiber sheet according to claim 1, having a total light transmittance of 70% or more at a wavelength of 600 nm. The fiber sheet according to claim 1, having a tensile strength of 100 MPa or more. The fiber sheet according to claim 1, wherein the amount of anionic functional groups in the cellulose fibers is 1.5 to 2.5 mmol/g. The fiber sheet according to claim 1, wherein the anionic functional group of the cellulose fiber is a carboxyl group. The fiber sheet according to any one of claims 1 to 5, wherein the cellulose fibers have externally fibrillated fluff on the fiber surface. A step of preparing a suspension in which cellulose fibers having anionic functional groups are suspended in water;
adjusting the pH of the suspension to 5.0 or more; and
and filtering the suspension having a pH of 5.0 or more to form a sheet.
The method for producing a fiber sheet does not include a step of beating the cellulose fibers under a condition of pH 5.0 or higher. A step of preparing a suspension in which cellulose fibers having anionic functional groups are suspended in water;
beating the cellulose fibers under a pH condition of less than 5.0;
adjusting the pH of the suspension after beating to 5.0 or more; and
and filtering the suspension having a pH of 5.0 or more to form a sheet.
The method for producing a fiber sheet does not include a step of beating the cellulose fibers under a condition of pH 5.0 or higher.