Process for the production of wetted sheets and shaped bodies of cellulose fibers

Abstract

The present invention provides a wet sheet which is not easily broken in the aspect of manufacturing a molded body, is stable in shape, and is easily handled in the aspect of processing, and a manufacturing method of a molded body manufactured from the wet sheet. The above-described problem is solved by a wet sheet and a manufacturing method of a molded body. The wet sheet is characterized in that the wet sheet has pulp and cellulose microfine fibers having an average fiber diameter of 10,000 nm or less, the moisture content of the wet sheet is 60% by mass or more, and the thickness of the wet sheet is 0.5 mm or more. The manufacturing method of the molded body is characterized in that the manufacturing method of the molded body includes a heating and pressing step of obtaining a molded body by heating and pressing a wet sheet, the wet sheet has pulp and cellulose microfine fibers having an average fiber diameter of 10,000 nm or less, the moisture content of the wet sheet is 60% by mass or more, and the thickness of the wet sheet is 0.5 mm or more.

Description

Technical Field

[0001] This invention relates to a method for manufacturing wet sheets and molded articles of cellulose fibers. Background Technology

[0002] In recent years, nanotechnology, which aims to refine materials to the nanoscale and obtain new physical properties different from those of the monomers of the materials, has attracted attention. Nanotechnology is also being applied to cellulose-based raw materials. Cellulose microfibers obtained by chemically treating and pulverizing pulp to break it down into nanoscale fibers exhibit excellent strength and elasticity, and are expected to be used in various applications. In particular, molded articles manufactured by pulping and drying cellulose microfibers possess high strength and are reusable organic resources, making them useful as versatile materials. For example, Patent Document 1 proposes "a method for molding CNF, characterized in that a slurry containing cellulose nanofibers (CNF) is filled into a mold formed using a steam permeation mechanism, and a load is applied to the CNF-containing slurry to concentrate it." The purpose of this document is "to provide a method for molding CNF and a CNF molded article obtained by this method, in which the adjustment of drying conditions becomes easy, there is no shrinkage or cracking, and CNF molded articles with a high degree of productivity can be stably obtained."

[0003] However, when using the method described in this paper to manufacture 3D molded bodies, if there are areas with varying thicknesses during the slurry drying process, the load cannot be applied uniformly. This can lead to uneven drying at these areas, resulting in breakage. Furthermore, the shape of the slurry is unstable and difficult to handle.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2016-94683 Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] The main problem to be solved by the present invention is to provide a wetted sheet and a method for manufacturing a molded body made from the wetted sheet, wherein the wetted sheet is not prone to breakage, has stable shape, and is easy to process in terms of manufacturing the molded body.

[0009] Methods for solving problems

[0010] Because the slurry contains a large amount of water, it is difficult to shape. In Patent Document 1, a method is used to dry the slurry as follows: the slurry is placed on a porous material, and the top of the slurry is covered with another porous material. The slurry is then dried while being flattened by applying pressure to it using these two porous materials. In this method, when pressure is applied to the slurry in the direction of gravity, a molded body with a small thickness difference is formed. However, when pressure is applied in directions other than gravity, a concentration deviation occurs in the slurry due to its own weight, resulting in a molded body with a considerable thickness difference. Furthermore, if the slurry with the concentration deviation is dried, breakage occurs due to the difference in shrinkage rate.

[0011] The inventors conducted repeated and in-depth research and conceived the following solution: To produce a molded body that is not prone to breakage, it is preferable to prepare the raw material for the molded body in sheet form. In sheet form, it is less likely to produce concentration deviations like those found in slurries, and regardless of the direction of applied pressure, there is almost no difference in shrinkage rate, thus enabling the molding of a molded body that is not prone to breakage. Considering the above aspects, the invention that solves the aforementioned problem is as follows.

[0012] (Method 1)

[0013] A wetted sheet, characterized in that,

[0014] The wetted sheet contains pulp and cellulose microfibers with an average fiber diameter of less than 10,000 nm.

[0015] The moisture content of the wetted sheet is 60% by mass or more, and the thickness of the wetted sheet is 0.5 mm or more and 10 mm or less.

[0016] Sometimes, it is impossible to obtain a molded article with sufficient dehydration properties from a wet sheet composed solely of cellulose microfibers. However, if the wet sheet contains both pulp and cellulose microfibers, the molded article exhibits sufficient dehydration properties. Furthermore, this wet sheet is a tangible sheet with a moisture content of 60% by mass or more and a thickness within the aforementioned range, making it easily deformable and capable of being processed into a stable shape. Moreover, the wet sheet is such that the pulp and cellulose microfibers are fixed within the wet sheet rather than moving freely like pulp. Additionally, during the manufacturing of the molded article, the wet sheet is less prone to breakage caused by pulp flow, and volume increase is relatively minor, thus simplifying processing.

[0017] (Method 2)

[0018] According to the wetted sheet of the first embodiment, the cellulose microfibers are composed of at least one of cellulose nanofibers and microfibrillated cellulose with an average fiber diameter larger than that of the cellulose nanofibers.

[0019] The cellulose microfibers used for wet sheets can be cellulose nanofibers, microfibrillated cellulose, or a mixture of cellulose nanofibers and microfibrillated cellulose. Furthermore, regarding wet sheets, pulp alone does not provide sufficient water retention; however, by containing at least one of cellulose nanofibers and microfibrillated cellulose, which have excellent water retention properties, water retention is achieved.

[0020] (Method 3)

[0021] According to the first or second method of wetting the sheet, wherein,

[0022] The thickness variation rate calculated according to Equation 1 below is less than 0.4.

[0023] [Formula 1]

[0024] Thickness change rate = ((thickness of the wet sheet after applying a pressure of 100 kPa in the thickness direction for 1 second) - (thickness of the wet sheet after applying a pressure of 100 kPa in the thickness direction for 5 seconds)) ÷ (thickness of the wet sheet after applying a pressure of 100 kPa in the thickness direction for 1 second).

[0025] Building upon the first method, since the thickness variation rate is less than 0.4, the wetted sheet in this method is less prone to deformation in the thickness direction. This reduces the likelihood of breakage associated with deformation in the thickness direction when manufacturing the molded body. Furthermore, because the wetted sheet is less prone to deformation in the thickness direction, localized unevenness is less likely to form even under pressure and heat, enabling the production of homogeneous molded bodies.

[0026] (Method 4)

[0027] The wetted sheet according to any one of the methods 1 to 3, wherein the solid content concentration of the cellulose microfibers is 10% by mass or more.

[0028] Molded articles containing cellulose microfibers have relatively high strength. The wetted sheet of this method contains the above-mentioned concentration of cellulose microfibers, thus enabling the manufacture of molded articles with sufficient strength from the wetted sheet.

[0029] (Method 5)

[0030] A method for manufacturing a molded body, characterized in that,

[0031] The method for manufacturing the molded body includes a heating and pressurizing step of heating and pressurizing a wet sheet to obtain the molded body.

[0032] The wetted sheet has pulp and cellulose microfibers with an average fiber diameter of less than 10,000 nm, the moisture content of the wetted sheet is more than 60% by mass, and the thickness of the wetted sheet is more than 0.5 mm and less than 10 mm.

[0033] Pulp and cellulose microfibers are the materials that form the wet sheet, and unlike the materials that form the pulp, they do not move freely. In the case of pulp, the overall shape of the pulp deforms due to its own weight during processing. However, in this method, because it is a wet sheet, the pulp and cellulose microfibers that form the wet sheet are fixed within the wet sheet, and the concentration is less likely to deviate. Therefore, it is less likely to break during processing, and a homogeneous molded body can be produced.

[0034] (Method 6)

[0035] A method for manufacturing a molded body, characterized in that,

[0036] The method for manufacturing the molded body includes:

[0037] The preparation process involves mixing pulp with cellulose microfibers with an average fiber diameter of less than 10,000 nm to prepare a pulp.

[0038] In the forming process, the slurry is sandwiched between two opposing mesh sheets and dehydrated under pressure, forming a wet sheet; and

[0039] The heating and pressurizing process involves heating and pressing the wet sheet to obtain a molded body.

[0040] The moisture content of the wetted sheet is 60% by mass or more, and the thickness of the wetted sheet is 0.5 mm or more and 10 mm or less.

[0041] In this method, the pulp is processed into sheets during the processing steps to obtain wet sheets. Since the wet sheets are sheet-like, their shape is not easily changed due to their own weight. In addition, there is little risk of raw material loss due to leakage or damage of pulp and cellulose microfibers. Furthermore, the operation is easy.

[0042] (Method 7)

[0043] According to the manufacturing method of the molded body according to the fifth or sixth method, wherein,

[0044] The cellulose microfibers are composed of at least one of cellulose nanofibers and microfibrillated cellulose with an average fiber diameter larger than that of the cellulose nanofibers.

[0045] This method achieves the same effect as the second method.

[0046] (Method 8)

[0047] The method for manufacturing a molded article according to any one of methods 5 to 7, wherein,

[0048] The thickness variation rate calculated according to Equation 1 below is less than 0.4.

[0049] [Formula 1]

[0050] Thickness change rate = ((thickness of the wet sheet after applying a pressure of 100 kPa in the thickness direction for 1 second) - (thickness of the wet sheet after applying a pressure of 100 kPa in the thickness direction for 5 seconds)) ÷ (thickness of the wet sheet after applying a pressure of 100 kPa in the thickness direction for 1 second).

[0051] This method achieves the same effect as method 3.

[0052] (Method 9)

[0053] The method for manufacturing a molded article according to any one of the methods 5 to 8, wherein the solid component concentration of the cellulose microfibers is 10% by mass or more.

[0054] This method achieves the same effect as method 4.

[0055] (Method 10)

[0056] The method for manufacturing a molded body according to any one of the methods 5 to 9, wherein the dehydration process is carried out without heating.

[0057] When moisture is vaporized by heating, the moisture content within the wet sheet changes locally, sometimes resulting in uneven moisture content as a whole. By substantially avoiding heating, vaporization associated with moisture evaporation is less likely to occur, thus preventing deviations in the concentration of cellulose microfibers.

[0058] The effects of the invention

[0059] According to the present invention, a method for manufacturing a wet sheet and a molded body made from the wet sheet is provided, wherein the wet sheet is not easily broken in terms of manufacturing the molded body, has stable shape, and is easy to process. Attached Figure Description

[0060] Figure 1 This is an illustrative diagram illustrating the manufacturing method of wet sheets.

[0061] Figure 2 This is an illustrative diagram illustrating the manufacturing method of wet sheets and molded articles. Detailed Implementation

[0062] Next, the method for implementing the present invention will be described. Furthermore, this embodiment is only one example of the present invention. The scope of the present invention is not limited to the scope of this embodiment.

[0063] Regarding the wetted sheet of this method, it comprises pulp and cellulose microfibers with an average fiber diameter of 10,000 nm or less, has a moisture content of 60% by mass or more, and a thickness of 0.5 mm or more and 10 mm or less. The cellulose microfibers are composed of at least one of cellulose nanofibers (hereinafter, sometimes referred to as "CNF") and microfibrillated cellulose (hereinafter, sometimes referred to as "MFC") with an average fiber diameter larger than that of the cellulose nanofibers. The pulp, cellulose nanofibers, microfibrillated cellulose, and wetted sheet will be described below.

[0064] (pulp)

[0065] The presence of pulp in wet sheets enhances their dehydration properties. By adjusting the amount of pulp in the wet sheet, the moisture content can be kept within a desired range. Furthermore, by adjusting the ratio of pulp to cellulose microfibers in the wet sheet, the strength of the molded body can be kept within a desired range.

[0066] The pulp used in this method can be selected from one or more of the raw material pulps for cellulose nanofibers described later. Among these, lignin-containing pulp is particularly preferred, mechanical pulp is more preferred, and BTMP is particularly preferred. Using these pulps further improves the dewatering properties of the cellulose fiber pulp. Furthermore, it is preferable to use the same pulp for this method as for the cellulose microfibers. If the starting materials are the same, they have good affinity, and during the process of pressurizing the pulp to obtain a wet sheet, the outflow of cellulose microfibers can be suppressed, dewatering becomes easier, and the time required for this process is shorter.

[0067] The pulp mentioned above can be either unrefined pulp or refined pulp. Using unrefined pulp can improve dewatering efficiency. Using refined pulp allows cellulose microfibers to easily entangle in the pulp, thereby inhibiting the outflow of cellulose nanofibers or microfibrillated cellulose, and also results in a relatively large number of hydrogen bonding points, thus enabling the formation of high-strength molded articles.

[0068] The degree of pulp beating can be measured by its freeness. The freeness of pulp is, for example, 200–800 ml, preferably 350–780 ml, and more preferably 400–750 ml. If the freeness of pulp exceeds 800 ml, although the dehydration of the wet sheet is improved, it is prone to breakage when processed into molded parts, etc. In addition, there are concerns that the fibers become rigid, the pulp and cellulose microfibers are not integrated, and thus the density cannot be increased.

[0069] On the other hand, if the pulp freeness is less than 200ml, the dehydration of the wet sheet may not be sufficiently improved. In addition, the rigidity of the pulp fibers themselves is reduced, and the wet sheet may not be able to maintain its sheet shape.

[0070] The degree of pulp freeness is measured according to JIS P8121-2 (2012).

[0071] For pulp, the average fiber diameter can be adjusted depending on the type of pulp selected and the degree of defiberization, etc.

[0072] The average fiber diameter (average fiber width, the average diameter of a single fiber) of the pulp is preferably greater than 10 μm to 100 μm, more preferably greater than 10 μm to 80 μm, and particularly preferably greater than 10 μm to 60 μm. If the average fiber diameter of the pulp is within this range, the dewatering properties of the wetted sheet are further improved by keeping the pulp content within the range described later.

[0073] The average fiber diameter of pulp can be measured using the Valment FS5 fiber analyzer. The FS5 fiber analyzer can measure the length and width of cellulose fibers with high precision by analyzing images of diluted cellulose fibers passing through the measurement unit inside the analyzer.

[0074] When the average fiber diameter of the pulp and cellulose microfibers is set within the above-mentioned range, the pulp content (solids concentration) in the wet sheet is preferably 0.1 to 20% by mass, more preferably 0.5 to 12% by mass, and particularly preferably 1.0 to 8% by mass. If this content is less than 0.1% by mass, there is a concern that the dehydration of the wet sheet will take longer, resulting in reduced productivity. In addition, if this content (solids concentration) exceeds 20% by mass, the content of cellulose microfibers, etc., will be relatively reduced when manufacturing molded articles, etc., from the wet sheet, which may make it impossible to ensure the strength of the molded articles, etc.

[0075] (cellulose nanofibers)

[0076] Next, the cellulose nanofibers contained in the moistened sheet will be described in detail. Cellulose nanofibers possess multiple hydrogen bonding sites and exhibit the property of dispersing and forming a three-dimensional network structure when mixed with media such as water and organic solvents. This three-dimensional network structure is formed by the cellulose nanofibers forming a framework for the network. Although difficult to describe precisely, it can be inferred that it resembles a three-dimensional lattice structure, similar to a climbing frame (wherein the lattice can be arranged regularly or irregularly). The interior of this lattice formed by the cellulose nanofibers is called voids.

[0077] Cellulose nanofibers can be obtained, for example, by defibrillating (micro-refining) plant-based pulp. The pulp used as raw material for cellulose nanofibers can be selected from one or more of the following: wood pulp made from broadleaf or coniferous trees; non-wood pulp made from wheat straw, bagasse, cotton, hemp, or bast fibers; and waste paper pulp (DIP) made from waste tea paper, envelope paper, magazine paper, flyer paper, corrugated paper, white paper, imitation paper, old paper, recycled paper, and damaged paper. It should be noted that the above-mentioned raw materials can be in the form of pulverized materials, such as cellulose-based powders.

[0078] However, to minimize the introduction of impurities, wood pulp is preferred. As wood pulp, one or more types can be selected, such as hardwood kraft pulp (LKP), softwood kraft pulp (NKP), and mechanical pulp (TMP).

[0079] Broadleaf kraft pulp can be bleached broadleaf kraft pulp, unbleached broadleaf kraft pulp, or semi-bleached broadleaf kraft pulp. Similarly, coniferous kraft pulp can be bleached coniferous kraft pulp, unbleached coniferous kraft pulp, or semi-bleached coniferous kraft pulp.

[0080] As mechanical pulp, one or more of the following can be selected: stone groundwood pulp (SGP), pressurized mechanical pulp (PGW), refined groundwood pulp (RGP), chemi-groundwood pulp (CGP), thermo-groundwood pulp (TGP), groundwood pulp (GP), thermo-mechanical pulp (TMP), chemi-thermal-mechanical pulp (CTMP), refined thermo-mechanical pulp (RMP), bleached thermo-mechanical pulp (BTMP).

[0081] In the manufacture of wet sheets and molded articles, from the viewpoint of having the best high strength, chemical pulp is preferred, and LKP and NKP are even more preferred.

[0082] Different pulps can be used for raw material pulp of cellulose nanofibers and raw material pulp of microfibrillated cellulose, but it is preferable to use the same raw material pulp to reduce raw material costs.

[0083] For cellulose nanofibers, pretreatment can also be performed before defibrillation. For example, as pretreatment, the raw pulp can be mechanically pre-pulped or chemically modified. There are no particular limitations on the pre-pulping method; well-known methods can be used.

[0084] Examples of pretreatments for raw pulp using chemical methods include: hydrolysis of polysaccharides using acids (e.g., sulfuric acid), hydrolysis of polysaccharides using enzymes, swelling of polysaccharides using alkalis, oxidation of polysaccharides using oxidants (e.g., ozone), reduction of polysaccharides using reducing agents, oxidation using TEMPO catalysts, anionization using phosphorylation or carbamate, and cationization.

[0085] During alkali treatment, the hydroxyl groups of hemicellulose or cellulose in the pulp partially dissociate, and the molecules become anionized. As a result, the intramolecular and intermolecular hydrogen bonds weaken, making it easier to de-fibrillate and promoting the dispersion of cellulose fibers.

[0086] Examples of alkalis used in alkaline treatment include sodium hydroxide, lithium hydroxide, potassium hydroxide, ammonia solution, tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrabutylammonium hydroxide, benzyltrimethylammonium hydroxide, and other organic alkalis. From a manufacturing cost perspective, sodium hydroxide is preferred.

[0087] Enzyme treatment, acid treatment, and oxidation treatment can reduce the water retention of cellulose nanofibers, increase crystallinity, and improve homogeneity. In this respect, if the water retention of cellulose nanofibers is low, they become easier to dehydrate, thus improving the dehydration properties of wet sheets.

[0088] If the raw pulp is subjected to enzymatic treatment, acid treatment, or oxidation treatment, the amorphous regions of hemicellulose or cellulose in the pulp are decomposed. As a result, the energy required for micronization can be reduced, and the uniformity and dispersibility of cellulose microfibers can be improved. For example, in the case of manufacturing molded articles from cellulose fiber pulp, the dispersibility of cellulose fibers contributes to the homogeneity of the molded articles. However, if pretreatment is performed, the average fiber diameter of the cellulose nanofibers decreases, resulting in a reduction in the aspect ratio of the cellulose nanofibers. Therefore, excessive pretreatment is preferable to avoid.

[0089] Examples of cellulose nanofibers that are modified by introducing anionic functional groups through anionization include cellulose nanofibers that have been esterified with phosphoric oxyacids or cellulose nanofibers that have been urethane-esterified, and cellulose nanofibers in which the hydroxyl groups of the pyranose ring are directly oxidized to carboxyl groups.

[0090] Cellulose nanofibers modified with anionic functional groups exhibit relatively high dispersibility. This is presumably because the localized charge deviation caused by the anionic functional groups facilitates the formation of hydrogen bonds with water or organic solvents in the dispersion.

[0091] If esterification based on phosphorus-containing oxyacids is performed on cellulose fibers as an example of anionization, the fiber raw material can be miniaturized, and the resulting cellulose nanofibers have a high aspect ratio, excellent strength, and high light transmittance and viscosity. Esterification based on phosphorus-containing oxyacids can be carried out, for example, by the method disclosed in Japanese Patent Application Publication No. 2019-199671.

[0092] The esterification reaction based on phosphorus oxyacids is carried out by adding a solution with a pH less than 3.0, consisting of an additive containing at least one of phosphorus oxyacids and phosphorus oxyacid metal salts, to cellulose fibers and heating the solution.

[0093] As additives, phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, ammonium pyrophosphate, ammonium polyphosphate, lithium dihydrogen phosphate, trilithium phosphate, dilithium hydrogen phosphate, lithium pyrophosphate, lithium polyphosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium pyrophosphate, sodium polyphosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, tripotassium phosphate, potassium pyrophosphate, potassium polyphosphate, phosphorous acid, sodium hydrogen phosphate, ammonium hydrogen phosphate, potassium hydrogen phosphate, sodium dihydrogen phosphate, sodium phosphite, lithium phosphite, potassium phosphite, magnesium phosphite, calcium phosphite, triethyl phosphite, triphenyl phosphite, pyrophosphate, and other phosphorous acid compounds can be used. These additives can be used individually or in combination. Among them, phosphonic acids are preferred as part or all of the phosphorus oxyacids. Using phosphonic acids can prevent yellowing of cellulose fibers, thus minimizing their impact on the color of the molded article, making them a preferred choice.

[0094] Cellulose fibers are structures composed of multiple glucose molecules, with glucose as a building block. Within a single polymerized cellulose fiber, the ester group of a phosphorus oxyacid may or may not be replaced by a specific glucose molecule. Furthermore, within a specific glucose molecule, the ester group of the phosphorus oxyacid may be substituted at multiple sites.

[0095] Cellulose nanofibers that have been introduced with cationic functional groups through cationic treatment can be exemplified by cellulose nanofibers with cationic groups such as ammonium (e.g., quaternary ammonium), phosphonium, and sulfonium, but are not limited thereto.

[0096] Regarding methods for introducing cationic groups, one example is reacting the reactants and catalyst with cellulose fibers in a solvent. Introduction is promoted when the reaction temperature is 10°C or higher and 90°C or lower, and the reaction time is 10 minutes or higher and 10 hours or lower. Examples of reactants include glycidyltrimethylammonium chloride, 3-chloro-2-hydroxypropyltrialkylammonium hydride, or their haloalcohol forms. Examples of catalysts include sodium hydroxide and potassium hydroxide. Solvents can be water or alcohols; examples of alcohols include those with 4 or fewer carbon atoms.

[0097] The reactants are preferably 5% or more by mass, more preferably 10% or more by mass, relative to 100% by mass of cellulose fibers. The catalyst is preferably 0.5% or more by mass, more preferably 1% or more by mass, relative to 100% by mass of cellulose fibers.

[0098] The amount of cationic substituents introduced into cellulose fibers can be adjusted based on the presence or absence of reactants or catalysts, and the type of solvent. If the glucose (e.g., pyranose ring) of the cellulose fiber is used as a building block, then 0.01 to 0.4 cationic substituents are introduced into each building block. If the amount is below this range, the effect of introducing cationic functional groups, i.e., the ease of fiber disintegration, will be weaker. If the amount is exceeded, excessive swelling and dissolution of the cellulose nanofibers may occur.

[0099] Cellulose fibers can be defibriled using the following devices and methods. For example, one or more methods can be selected from homogenizers such as high-pressure homogenizers and high-pressure homogenizing devices, stone mills such as grinders and pulverizers, conical refiners, disc refiners, and various bacteria. However, defibrilation of cellulose fibers is preferably carried out using devices and methods that utilize water flow, especially high-pressure water flow, for micronization. According to this device and method, the obtained cellulose nanofibers exhibit very high size uniformity and dispersion uniformity. In contrast, for example, if a grinder is used to grind the fibers between rotating grinding stones, it is difficult to uniformly micronize the cellulose fibers, and depending on the situation, undisintegrated fiber clumps may remain in certain areas.

[0100] Examples of milling machines used in the defiberization of cellulose fibers include the Masscolloder from Masukuni Sangyo Co., Ltd. Additionally, devices that utilize high-pressure water jets for micronization include the Star Burst (registered trademark) from SuginoMachine Co., Ltd., and the Nanovater (registered trademark) from Yoshida Machinery Kogyo Co., Ltd. Furthermore, examples of high-speed rotary homogenizers used in the defiberization of cellulose fibers include the CLEARMIX-11S from M-technique Co., Ltd.

[0101] The inventors have discovered that by grinding cellulose fibers in a rotating grinding stone and by micronizing them using high-pressure water flow, respectively, the fibers obtained by micronizing them using high-pressure water flow have a uniform fiber width when observed under a microscope.

[0102] The defiberization based on high-pressure water flow is preferably performed in the following manner (under reduced pressure conditions): a dispersion of cellulose fibers is pressurized using a booster to, for example, 30 MPa or more, preferably 100 MPa or more, more preferably 150 MPa or more, particularly preferably 220 MPa or more (high-pressure conditions), and ejected from a nozzle with a fine orifice diameter of 50 μm or more, under reduced pressure conditions with a pressure difference of, for example, 30 MPa or more, preferably 80 MPa or more, more preferably 90 MPa or more. The pulp fibers are defiberized through the cleavage phenomenon generated by this pressure difference. When the pressure under high-pressure conditions is low, or when the pressure difference between high-pressure and reduced-pressure conditions is small, the defiberization efficiency decreases, and repeated defiberization (ejection from the nozzle) is required to achieve the desired fiber width.

[0103] As a device for defibrating cellulose fibers using high-pressure water flow, a high-pressure homogenizer is preferred. A high-pressure homogenizer is a homogenizer capable of ejecting a slurry of cellulose fibers at a pressure of, for example, 10 MPa or higher, preferably 100 MPa or higher. When cellulose fibers are treated using a high-pressure homogenizer, the collisions between the cellulose fibers, pressure differences, and micro-cavitation all play a role, thereby effectively defibrating the cellulose fibers. Therefore, the number of defibrating processes can be reduced, thereby improving the manufacturing efficiency of cellulose nanofibers.

[0104] As a high-pressure homogenizer, it is preferable to use a high-pressure homogenizer that causes the cellulose fiber slurry to collide opposite each other in a straight line. Specifically, for example, an opposite-collision type high-pressure homogenizer (MICROFLUIDIZER (registered trademark), wet jet mill). In this device, two upstream flow paths are formed by the pressurized cellulose fiber slurry colliding opposite each other at the confluence. In addition, the cellulose fiber slurry collides at the confluence, and the colliding cellulose fiber slurry flows out from the downstream flow path. The downstream flow path is set perpendicular to the upstream flow path, and the upstream and downstream flow paths form a T-shaped flow path. When such an opposite-collision type high-pressure homogenizer is used, the energy provided by the high-pressure homogenizer is converted into collision energy to the maximum extent, thus enabling more efficient defiberization of cellulose fibers.

[0105] Cellulose nanofibers obtained through defibrillation can be pre-dispersed in an aqueous medium to form a dispersion before being mixed with microfibrillated cellulose or pulp. The aqueous medium is particularly preferably entirely water (aqueous dispersion). However, the aqueous medium can also be a portion of other liquids that are miscible with water. Examples of other liquids include lower alcohols with 3 or fewer carbon atoms.

[0106] The defibrillation of the raw pulp is preferably carried out in a manner that ensures the physical properties of the resulting cellulose nanofibers are the desired values ​​or evaluation methods shown below.

[0107] <Average fiber diameter>

[0108] The lower limit of the average fiber diameter (average fiber width, average diameter of a single fiber) of cellulose nanofibers is 10 nm or more, preferably 15 nm or more, and more preferably 20 nm or more. The upper limit of the average fiber diameter of cellulose nanofibers is 100 nm or less, preferably 90 nm or less, and more preferably 80 nm or less. If the average fiber diameter of cellulose nanofibers is less than 10 nm (the lower limit), the dehydration properties of the wetted sheet may decrease. If the average fiber diameter of cellulose nanofibers is 100 nm or less (the upper limit), the miniaturization of cellulose fibers will be sufficient, resulting in a dense structure and excellent physical properties in the wetted sheet.

[0109] The cellulose microfibers contained in the moist sheet can be cellulose nanofibers alone, microfibrillated cellulose alone, or both. Regarding the dehydration properties of cellulose microfibers, microfibrillated cellulose is superior to cellulose nanofibers. By adjusting the mixing ratio of cellulose nanofibers and microfibrillated cellulose, a moist sheet with the desired dehydration properties can be produced. To achieve relatively high dehydration properties, the mixing ratio of microfibrillated cellulose can be increased (in which case, the mixing ratio of cellulose nanofibers can be 0); to achieve relatively low dehydration properties, the mixing ratio of microfibrillated cellulose can be decreased (in which case, the mixing ratio of microfibrillated cellulose can be 0).

[0110] The average fiber diameter of cellulose nanofibers can be adjusted, for example, by selecting the raw pulp, pretreatment, and defiberization.

[0111] The method for measuring the average fiber diameter of cellulose nanofibers is described below.

[0112] First, 100 ml of an aqueous dispersion of cellulose nanofibers with a solid content of 0.01–0.1% by mass was filtered through a Teflon (registered trademark) membrane filter. The solution was then subjected to one solvent exchange with 100 ml of ethanol and three solvent exchanges with 20 ml of tert-butanol. Next, the solution was freeze-dried and coated with osmium to prepare a sample. This sample was then observed using a SEM (Self-Electron Microscopy) image at any magnification between 3000x and 30000x, depending on the width of the fibers. Specifically, two diagonal lines were drawn on the image, and three straight lines were arbitrarily drawn passing through the intersection of these diagonal lines. The width of a total of 100 fibers intersecting these three straight lines was then visually measured. The median diameter of the measured values ​​was then set as the average fiber diameter.

[0113] <Average fiber length>

[0114] The average fiber length (average length of a single fiber) of cellulose nanofibers is, for example, 0.3–2000 μm, preferably 0.4–200 μm, and more preferably 0.5–20 μm. If the average fiber length is less than 0.3 μm, the water-filtration and drying properties decrease, and it becomes difficult to form a three-dimensional network structure of cellulose nanofibers, thus potentially reducing the reinforcing effect. If the average fiber length exceeds 2000 μm, the entanglement of cellulose fibers increases, making it difficult to form a homogeneous three-dimensional network structure.

[0115] The average fiber length can be adjusted arbitrarily, for example, by selecting the raw pulp, pretreatment, and defiberization.

[0116] The average fiber length of cellulose nanofibers is measured visually, similar to the method used for the average fiber diameter. The median length of the measurements is set as the average fiber length.

[0117] Aspect Ratio

[0118] When manufacturing molded articles from wet sheets, it is preferable to maintain the ductility of the molded article to a certain extent while improving its strength. From this point of view, the aspect ratio of cellulose nanofibers is preferably 3 or more, more preferably 6 or more, and more preferably 10 or more, and the upper limit is preferably 150,000 or less, more preferably 120,000 or less, and more preferably 100,000 or less. If the aspect ratio of cellulose nanofibers is less than 3, the cellulose nanofibers cannot be expected to have fibrous properties. If the aspect ratio of cellulose nanofibers exceeds 150,000, the prepared cellulose fiber slurry has high viscosity, and the manufacture of wet sheets may become difficult.

[0119] It should be noted that the aspect ratio is the value obtained by dividing the average fiber length of the cellulose nanofiber by the average fiber width of the cellulose nanofiber. The larger the aspect ratio, the more hooking points occur in the fiber, and therefore the stronger the reinforcing effect can be. However, on the other hand, it can be expected that with more hooking, the ductility of the molded body, etc., will decrease.

[0120] <Simulated particle size distribution curve>

[0121] The simulated particle size distribution curve of cellulose nanofibers preferably shows only one peak. With only one peak, the cellulose nanofibers exhibit high uniformity in fiber length and diameter, readily forming a dense three-dimensional structure, resulting in superior physical properties of the manufactured molded articles. Furthermore, the cellulose fiber slurry demonstrates excellent drying and dehydration properties.

[0122] When cellulose nanofibers exhibit a single peak in the simulated particle size distribution curve, it is preferable that the smaller the deviation (dispersion) in fiber length and / or fiber diameter, the easier it is to form a three-dimensional network structure. When cellulose nanofibers exhibit a single peak in the simulated particle size distribution curve, the full width at half maximum (FWHM) of this peak is, for example, 250 μm or less, preferably 200 μm or less, and particularly preferably 150 μm. If the FWHM of this peak exceeds 250 μm, sufficient micronization of the cellulose fibers may not be achieved, and the molded article may not possess a dense three-dimensional network structure, potentially leading to a reduction in physical properties. To achieve a FWHM of this peak of 250 μm or less, methods such as increasing the number of micronization processes can be employed.

[0123] Regarding the peak value of cellulose nanofibers, for example, the lower limit is 1 μm or more, preferably 3 μm or more, and more preferably 5 μm or more. If the peak value is below 1 μm, there is a possibility that the fibers are excessively defibrinated, and the water permeability and drying properties of wet sheets or molded articles become unsatisfactory.

[0124] Regarding the peak value of cellulose nanofibers, for example, the upper limit is below 100 μm, preferably below 80 μm, and more preferably below 60 μm. When the peak value exceeds 100 μm, there is a possibility of insufficient fiber disintegration, resulting in poor uniformity of fiber diameter and fiber length.

[0125] The peak value in the simulated particle size distribution curve of cellulose nanofibers is the value measured according to ISO-13320 (2009). As an example of the measurement, firstly, the volume-based particle size distribution of the aqueous dispersion of cellulose nanofibers was investigated using a particle size distribution measuring device (laser diffraction / scattering particle size distribution measuring instrument from SEISHIN Corporation). Next, the median diameter of the cellulose nanofibers was measured based on this distribution. This median diameter was taken as the peak value.

[0126] The peak value and median diameter of the simulated particle size distribution curve of cellulose nanofibers can be adjusted, for example, by selecting the raw pulp, pretreatment, and defiberization.

[0127] <Pulp Viscosity>

[0128] The pulp viscosity of the defibrinated cellulose nanofibers is preferably 1 cP or higher, more preferably 2 cP or higher. If the pulp viscosity is less than 1 cP, it may not be able to sufficiently inhibit the aggregation of cellulose nanofibers.

[0129] <Type B viscosity>

[0130] After defibrillation, cellulose nanofibers can be pre-mixed with water to form an aqueous dispersion. This cellulose nanofiber aqueous dispersion has a viscosity, which can be evaluated using B-type viscosity. Regarding B-type viscosity, even for dispersions of cellulose nanofibers obtained from specific raw materials and through the same manufacturing process, the viscosity varies depending on the concentration of cellulose nanofibers; the higher the concentration, the higher the viscosity. The B-type viscosity of the aqueous dispersion of cellulose nanofibers (solid component concentration of 1% (w / w)) is preferably 10–4000 cP, more preferably 80–3000 cP, and particularly preferably 100–2000 cP. For aqueous dispersions with a B-type viscosity below 10 cP, the dispersibility of cellulose nanofibers is lacking, and even when mixed with microfibrillated cellulose or pulp, it may not mix sufficiently. For aqueous dispersions with a B-type viscosity exceeding 4000 cP, the dehydration properties of the resulting pulp or wetted sheet obtained by mixing this aqueous dispersion with microfibrillated cellulose or pulp are insufficient.

[0131] The B-type viscosity (solid concentration of 1% (w / w)) of the cellulose nanofiber dispersion was measured according to JIS-Z8803 (2011) "Method for measuring the viscosity of liquids". B-type viscosity is the resistance torque during stirring of the dispersion; a higher B-type viscosity means more energy is required for stirring. The measurement temperature for B-type viscosity was 25℃.

[0132] <Crystallization>

[0133] The crystallinity of the cellulose nanofibers is preferably 50% or more, more preferably 55% or more, and particularly preferably 60% or more. If the crystallinity is less than 50%, the strength and heat resistance of the molded article may be insufficient.

[0134] On the other hand, the crystallinity of the cellulose nanofibers is preferably 100% or less, more preferably 90% or less, and particularly preferably 85% or less. If the crystallinity of the cellulose nanofibers is within the above range, strength can be ensured during the production of wet sheets, molded articles, etc., from the cellulose fiber slurry.

[0135] The crystallinity of cellulose nanofibers can be arbitrarily adjusted, for example, by selecting the raw pulp, pretreatment, and micronization.

[0136] Crystallinity was measured by X-ray diffraction according to the "General Rules for X-ray Diffraction Analysis" in JIS-K0131 (1996). It should be noted that cellulose nanofibers have both amorphous and crystalline components; crystallinity refers to the proportion of crystalline components in the overall cellulose nanofiber.

[0137] <Water retention>

[0138] The water retention of cellulose nanofibers is, for example, 90–600%, preferably 200–500%, and more preferably 240–460%. If the water retention of cellulose nanofibers is below 90%, the dispersibility deteriorates, and the cellulose nanofibers may not mix with microfibrillated cellulose or pulp. If the water retention exceeds 600%, the prepared pulp lacks water permeability or drying properties.

[0139] The water retention of cellulose nanofibers can be adjusted arbitrarily, for example, by selecting the raw pulp, pretreatment, and defiberization.

[0140] The water retention of cellulose nanofibers was measured according to JAPAN TAPPI No.26 (2000).

[0141] The content (solid component concentration) of cellulose nanofibers in the moistened sheet is, for example, 0 to 39.6% by mass, preferably 10 to 38% by mass, and more preferably 12 to 36% by mass. Within this range, the cellulose nanofibers are adequately dispersed in the moistened sheet, which is therefore preferable. Furthermore, even when molding articles or the like from the moistened sheet, the cellulose nanofibers are adequately dispersed in the molded articles or the like, which is also preferable. If the content exceeds 39.6% by mass, the water permeability or drying properties become poor.

[0142] (Microfibrillated cellulose)

[0143] Next, the microfibrillated cellulose contained in the moist sheet will be described in detail. Microfibrillated cellulose has multiple hydrogen bonding sites of cellulose fibers, is dehydrating, and disperses when mixed with media such as water and organic solvents. Microfibrillated cellulose can be manufactured by defibrillating raw pulp, and is a fiber with an average fiber diameter larger than that of cellulose nanofibers.

[0144] Microfibrillated cellulose can be obtained, for example, by defibrillating (microrefining) plant-based pulp. The pulp used as raw material for microfibrillated cellulose can be selected from one or more of the following: wood pulp made from broadleaf or coniferous trees; non-wood pulp made from wheat straw, bagasse, cotton, hemp, or bast fibers; and waste paper pulp (DIP) made from waste tea paper, envelope paper, magazine paper, flyer paper, corrugated paper, white paper, imitation paper, old paper, recycled paper, and damaged paper. It should be noted that the above-mentioned raw materials can be in the form of pulverized matter, such as cellulose powder.

[0145] However, to minimize the introduction of impurities, wood pulp is preferred. As wood pulp, one or more types can be selected, such as hardwood kraft pulp (LKP), softwood kraft pulp (NKP), and mechanical pulp (TMP).

[0146] Broadleaf kraft pulp can be bleached broadleaf kraft pulp, unbleached broadleaf kraft pulp, or semi-bleached broadleaf kraft pulp. Similarly, coniferous kraft pulp can be bleached coniferous kraft pulp, unbleached coniferous kraft pulp, or semi-bleached coniferous kraft pulp.

[0147] As mechanical pulp, one or more of the following can be selected: stone groundwood pulp (SGP), pressurized mechanical pulp (PGW), refined groundwood pulp (RGP), chemi-groundwood pulp (CGP), thermo-groundwood pulp (TGP), groundwood pulp (GP), thermo-mechanical pulp (TMP), chemi-thermal-mechanical pulp (CTMP), refined thermo-mechanical pulp (RMP), bleached thermo-mechanical pulp (BTMP).

[0148] In the manufacture of wet sheets and molded articles, from the viewpoint of having the best high strength, chemical pulp is preferred, and LKP and NKP are even more preferred.

[0149] In the method of defibrating raw pulp into microfibrillated cellulose, the above-mentioned method of defibrating into cellulose nanofibers can be applied. However, the defibrating process into microfibrillated cellulose does not necessarily result in a smaller average fiber diameter as the defibrating process progresses towards cellulose nanofibers.

[0150] The microfibrillated cellulose obtained by defibrillation can be pre-dispersed in an aqueous medium to prepare a dispersion before being mixed with cellulose nanofibers or pulp. The aqueous medium is particularly preferably entirely water (aqueous dispersion). However, the aqueous medium can also be a portion of other liquids that are miscible with water. Examples of other liquids include lower alcohols with 3 or fewer carbon atoms.

[0151] The defibrillation of the raw pulp is preferably performed in such a way that the physical properties of the resulting microfibrillated cellulose are as desired or evaluated as shown below. The methods for measuring the various physical properties of microfibrillated cellulose are the same as those for cellulose nanofibers or pulp, unless otherwise specified.

[0152] <Average fiber diameter>

[0153] The average fiber diameter (average fiber width, the average diameter of a single fiber) of microfibrillated cellulose exceeds 100 nm, preferably 200 nm or more, and more preferably 300 nm or more. The upper limit for the average fiber diameter of microfibrillated cellulose is 10,000 nm or less, preferably 5,000 nm or less, and more preferably 3,000 nm or less. When the average fiber diameter of microfibrillated cellulose is less than 100 nm, the dehydration properties of the wetted sheet may decrease, making it difficult to distinguish from cellulose nanofibers; therefore, this should be avoided. When the average fiber diameter of microfibrillated cellulose exceeds the upper limit of 10,000 nm, the micronization of the cellulose fibers may be insufficient.

[0154] The average fiber diameter of microfibrillated cellulose can be adjusted, for example, by selecting raw pulp, pretreatment, and defibrillation.

[0155] The method for measuring the average fiber diameter of microfibrillated cellulose is as follows.

[0156] First, 100 ml of an aqueous dispersion of microfibrillated cellulose with a solid content of 0.01–0.1% by mass was filtered through a Teflon (registered trademark) membrane filter. The solution was then subjected to one solvent exchange with 100 ml of ethanol and three solvent exchanges with 20 ml of tert-butanol. Next, the solution was freeze-dried and coated with osmium to prepare a sample. This sample was then observed using a SEM (Self-Electron Microscopy) image at any magnification between 3000x and 30000x, depending on the width of the fibers. Specifically, two diagonal lines were drawn on the image, and three straight lines were arbitrarily drawn passing through the intersection of these diagonal lines. The width of a total of 100 fibers intersecting these three straight lines was then visually measured. The median diameter of the measured values ​​was then set as the average fiber diameter.

[0157] <Average fiber length>

[0158] The average fiber length (average length of a single fiber) of microfibrillated cellulose is, for example, 10–1000 μm, preferably 30–700 μm, and more preferably 50–500 μm. If the average fiber length is less than 30 μm, the water permeability and drying properties decrease, and the reinforcing effect of the manufactured wet sheets, molded articles, etc., may be reduced. If the average fiber length exceeds 1000 μm, the entanglement of cellulose fibers increases, thereby reducing dispersibility.

[0159] The average fiber length can be adjusted arbitrarily, for example, by selecting the raw pulp, pretreatment, and defiberization.

[0160] The average fiber length of microfibrillated cellulose is measured visually, similar to the method used for the average fiber diameter. The median length of the measurements is taken as the average fiber length.

[0161] Aspect Ratio

[0162] When manufacturing molded articles from wet sheets, it is preferable to maintain the ductility of the molded article to a certain extent while improving its strength. From this viewpoint, the lower limit of the aspect ratio of microfibrillated cellulose is 3 or more, preferably 5 or more, more preferably 10 or more, and the upper limit is 10,000 or less, preferably 7,000 or less, more preferably 5,000 or less. If the aspect ratio of microfibrillated cellulose is less than 3, then the microfibrillated cellulose cannot be expected to have fibrous properties. If the aspect ratio of microfibrillated cellulose exceeds 10,000, the prepared cellulose fiber slurry has high viscosity, and the manufacture of wet sheets may become difficult.

[0163] It should be noted that the aspect ratio is the value obtained by dividing the average fiber length of the microfibrillated cellulose by the average fiber width of the microfibrillated cellulose. The larger the aspect ratio, the more hooking points occur in the fiber, and therefore the stronger the reinforcing effect can be. However, on the other hand, it can be expected that with more hooking, the extensibility of the molded body, etc., will decrease.

[0164] <Simulated particle size distribution curve>

[0165] The simulated particle size distribution curve of microfibrillated cellulose preferably shows only one peak. With only one peak, the microfibrillated cellulose exhibits high uniformity in fiber length and diameter, readily forming a dense three-dimensional structure, resulting in superior physical properties of the manufactured molded articles. Furthermore, the cellulose fiber slurry demonstrates excellent drying and dehydration properties.

[0166] When microfibrillated cellulose exhibits a single peak in the simulated particle size distribution curve, it is particularly advantageous for the fiber length and / or fiber diameter deviations (dispersion) to be smaller, as this facilitates the formation of a three-dimensional network structure. Ideally, the full width at half maximum (WHM) of this peak should be 250 μm or less, preferably 200 μm or less, and particularly preferably 150 μm. If the WHM exceeds 250 μm, sufficient micronization of the cellulose fibers may not be achieved, and the molded body may lack a dense three-dimensional network structure, potentially leading to a reduction in physical properties. To achieve a WHM of 150 μm or less, methods such as increasing the number of micronization treatments can be employed.

[0167] Regarding the peak value of microfibrillated cellulose, for example, the lower limit is 1 μm or more, preferably 5 μm or more, and more preferably 10 μm or more. If the peak value is below 1 μm, there is a possibility that the fibers are excessively defibrinated, and the water permeability and drying properties of the wetted sheet or molded article become unsatisfactory.

[0168] Regarding the peak value of microfibrillated cellulose, for example, the upper limit is 110 μm or less, preferably 100 μm or less, and more preferably 90 μm or less. When the peak value exceeds 110 μm, there is a possibility of insufficient fiber defibrillation, resulting in poor uniformity of fiber diameter and fiber length.

[0169] The peak value in the simulated particle size distribution curve of microfibrillated cellulose is the value measured according to ISO-13320 (2009). As an example of the measurement, firstly, the volume-based particle size distribution of the aqueous dispersion of microfibrillated cellulose was investigated using a particle size distribution measuring device (laser diffraction / scattering particle size distribution measuring instrument from SEISHIN Corporation). Next, the median diameter of the microfibrillated cellulose was measured based on this distribution. This median diameter was taken as the peak value.

[0170] The peak value and median diameter of the simulated particle size distribution curve of microfibrillated cellulose can be adjusted, for example, by selecting the raw pulp, pretreatment, and defibrillation.

[0171] <Pulp Viscosity>

[0172] The pulp viscosity of the defibrinated microfibrillated cellulose is preferably 1 cP or higher, more preferably 2 cP or higher. If the pulp viscosity is less than 1 cP, it may not be able to sufficiently suppress the aggregation of microfibrillated cellulose.

[0173] <Crystallization>

[0174] The crystallinity of the microfibrillated cellulose is preferably 45% or more, more preferably 55% or more, and particularly preferably 60% or more. If the crystallinity is less than 45%, the strength and heat resistance of the molded article may be insufficient.

[0175] On the other hand, the crystallinity of microfibrillated cellulose is preferably 90% or less, more preferably 88% or less, and particularly preferably 86% or less. If the crystallinity of microfibrillated cellulose is within the above range, strength can be ensured during the manufacturing process of wet sheets, molded articles, etc., from cellulose fiber slurry.

[0176] The crystallinity of microfibrillated cellulose can be adjusted arbitrarily, for example, by selecting the raw pulp, pretreatment, and microfibrillation.

[0177] Crystallinity was measured by X-ray diffraction according to the "General Rules for X-ray Diffraction Analysis" in JIS-K0131 (1996). It should be noted that microfibrillated cellulose has both amorphous and crystalline components, and crystallinity refers to the proportion of crystalline components in the overall microfibrillated cellulose.

[0178] <Water retention>

[0179] The water retention of microfibrillated cellulose is, for example, 10–500%, preferably 50–450%, and more preferably 90–400%. If the water retention of microfibrillated cellulose is less than 10%, its dispersibility deteriorates, and the microfibrillated cellulose may not mix with cellulose nanofibers or pulp. If the water retention exceeds 500%, the prepared pulp lacks water permeability or drying properties.

[0180] The water retention of microfibrillated cellulose can be adjusted arbitrarily, for example, by selecting the raw pulp, pretreatment, and defibrillation.

[0181] The water retention of microfibrillated cellulose was measured according to JAPAN TAPPI No.26 (2000).

[0182] The fibrillation rate of microfibrillated cellulose is preferably 0.5% or more, more preferably 1.0% or more, and particularly preferably 1.5% or more. Furthermore, this fibrillation rate is preferably 10% or less, more preferably 9% or less, and particularly preferably 8% or less. If the fibrillation rate exceeds 10%, the contact area with water becomes too large, which may make dehydration difficult. On the other hand, if the fibrillation rate is less than 0.5%, there are fewer hydrogen bonds between the fibrils, which may prevent the formation of a rigid three-dimensional network structure.

[0183] The freeness of microfibrillated cellulose is preferably 200 ml or less, more preferably 150 ml or less, and particularly preferably 100 ml or less. If the freeness of microfibrillated cellulose exceeds 200 ml, the microfibrillated cellulose exceeds 10 μm, which is the upper limit of the average fiber diameter, and it may not be possible to obtain sufficient strength-related effects.

[0184] The degree of freeness of microfibrillated cellulose was measured according to JIS P8121-2 (2012).

[0185] <Mixing Rate>

[0186] The content (solid component concentration) of microfibrillated cellulose in the moistened sheet is, for example, 0 to 39.6% by mass, preferably 10 to 38% by mass, and more preferably 12 to 36% by mass. If the content falls within this range, the microfibrillated cellulose will be adequately dispersed in the moistened sheet, which is therefore preferable. Furthermore, even when molding articles or the like are made from the moistened sheet, the microfibrillated cellulose will be adequately dispersed in the molded articles or the like, which is also preferable. When the content exceeds 39.6% by mass, the water permeability and drying properties become poor.

[0187] (Moistened sheet)

[0188] The wetted sheet is manufactured from a cellulose fiber pulp containing at least one of cellulose nanofibers and microfibrillated cellulose, and paper pulp. The method for manufacturing the wetted sheet is described below.

[0189] The conventional CNF molded bodies disclosed in Japanese Patent Application No. 2017-190529 and other publications are planar sheets. When deforming these CNF molded bodies into desired 3D shapes, the following problems exist: It is difficult to perform the deformation process to transform the molded body into a 3D shape while it is dry. Therefore, a method is adopted that softens the CNF molded body by immersing it in water and deforms it while it is in a softened state. This method adds a step of re-drying the deformed CNF molded body, which leads to decreased productivity and increased manufacturing costs.

[0190] In contrast, the wetted sheet produced by this method is tangible and easily deformable, capable of temporarily maintaining the desired shape, making it useful as a material for molding various 3D shapes. The various physical properties of the wetted sheet are shown below.

[0191] The moisture content of the wet sheet can be 60% by mass or more, more preferably 63% by mass or more, and even more preferably 65% ​​by mass or more. When the moisture content is below 60% by mass, the softness of the wet sheet decreases, making it difficult to mold the manufactured article into the desired shape. There is no particular upper limit to the moisture content, but if it is below 90% by mass, the unevenness in thickness that is easily generated in the molded article during manufacturing will be suppressed, resulting in a molded article with homogeneous strength, which is preferred.

[0192] The moisture content of wet sheets can be measured according to JIS P 8203 (2010).

[0193] The thickness of the wetted sheet is preferably 0.5 mm or more, more preferably 0.8 mm or more, even more preferably 1 mm or more, and preferably 10 mm or less, more preferably 9 mm or less, and even more preferably 8 mm or less. When the thickness of the wetted sheet is less than 0.5 mm, the wetted sheet is prone to cracking. When the thickness of the wetted sheet exceeds 10 mm, the molded body manufactured by pressing and heating is prone to uneven thickness.

[0194] In addition, the wet sheet is preferably 0.5 mm or more and 10 mm or less, and has a moisture content of 60% by mass or more. Even if the thickness is 0.5 mm or more and 10 mm or less, if the moisture content is less than 60% by mass, it is difficult to flex or bend the wet sheet, and therefore it is difficult to form the molded body into a three-dimensional shape.

[0195] The thickness of the wet sheet can be measured according to JIS P 8118 (2014).

[0196] The thickness variation rate indicates the ease with which a wet sheet can be compressed in the thickness direction. The lower the value, the more difficult it is to compress. In other words, when a wet sheet is heated and pressurized to form a molded body, there is less thickness unevenness, and the less likely it is to break. The higher the thickness variation rate of the wet sheet, the easier it is to compress, and the easier it is to deform during heating and pressurization. Therefore, it is easier to observe fractures in the molded body.

[0197] The thickness change rate can be calculated based on the following formula (1).

[0198] [Equation (1)]

[0199] Thickness change rate = ((thickness of the wet sheet after applying a pressure of 100 kPa in the thickness direction for 1 second) - (thickness of the wet sheet after applying a pressure of 100 kPa in the thickness direction for 5 seconds)) ÷ (thickness of the wet sheet after applying a pressure of 100 kPa in the thickness direction for 1 second)

[0200] The thickness change rate can be adjusted appropriately, but it is preferably below 0.4, more preferably below 0.35, and even more preferably below 0.3. If such a thickness change rate is achieved, the thickness unevenness will be reduced, making it difficult to break.

[0201] The thickness change rate can be measured as follows: Cover both sides of the wet sheet with a film, apply a pressure of 100 kPa along the thickness direction for 1 second, and measure the thickness of the wet sheet. Then, apply a pressure of 100 kPa along the thickness direction for 5 seconds and measure the thickness of the wet sheet again. The measurement can be performed under atmospheric pressure and room temperature (5–30°C, especially 25°C, 1 atm).

[0202] <Mixing Ratio>

[0203] The mixing ratio of pulp to cellulose microfibers in the wet sheet is, for example, 1:99 to 50:50, preferably 5:95 to 30:70, and more preferably 10:90 to 20:80. Furthermore, the mixing ratio of cellulose nanofibers to microfibrillated cellulose can be, for example, 100:0 to 0:100, preferably 80:20 to 20:80, and more preferably 70:30 to 30:70.

[0204] <Water retention>

[0205] Regarding the wetted sheet, the water retention capacity when the wetted sheet is dispersed in an aqueous medium to prepare a dispersion with a concentration of 1.5% is preferably 250–4000 g / m³. 2 More preferably, it is 500–3000 g / m 2 When the water retention is below 250g / m³ 2 At that time, the wettability is insufficient, when it exceeds 4000g / m 2 At this time, the wet sheet may not be able to maintain its shape.

[0206] <Density>

[0207] Regarding the wetted sheet, the preferred density for forming the molded body is 0.8–1.5 g / m³ when the wetted sheet is dehydrated and dried under conditions of 1–50 MPa and 100–150 °C to achieve high density. 3 More preferably, it is 0.9–1.4 g / m 3 The preferred concentration is 1.0–1.3 g / m³. 3 If the density is below 0.8 g / m³ 3If the concentration exceeds 1.5 g / m³, it is prone to breakage during the molding process. 3 If so, it may be difficult to process.

[0208] The wet sheet may contain more than 0% by mass of cellulose microfibers, but preferably more than 10% by mass, more preferably more than 11% by mass, and even more preferably more than 12% by mass, so that the strength when the molded article is formed is further improved. In addition, there is no particular upper limit to the amount of cellulose microfibers contained in the wet sheet, but since it also contains pulp, it is preferably 39.6% by mass or less, more preferably 38% by mass or less, based on the solid content concentration.

[0209] (Manufacturing method of wet sheet)

[0210] Next, the manufacturing method of the wet sheet and the molded body will be described. The manufacturing method includes a preparation step 10 of preparing the slurry, a forming step 20 of forming the wet sheet, and a heating and pressurizing step 30 of heating and pressurizing the wet sheet simultaneously. These steps will be described in turn.

[0211] (Preparation process)

[0212] In the slurry preparation process 10, such as Figure 2 As shown, an example is obtained by mixing pulp P and cellulose microfibers (cellulose nanofibers C and / or microfibrillated cellulose M) in an aqueous medium W to obtain pulp S.

[0213] The solid content concentration of cellulose fibers (i.e., the combined amount of pulp P and cellulose microfibers) in the pulp is preferably 1.0 to 10.0% by mass, more preferably 1.2 to 7.0% by mass, and particularly preferably 1.4 to 5.0% by mass. If the solid content concentration of cellulose fibers is less than 1.0% by mass, the fluidity is high, and the concern about cellulose fiber outflow in the subsequent forming step 20 increases.

[0214] On the other hand, if the concentration of solid components of cellulose fibers (i.e., the total amount of pulp P and cellulose microfibers) in the pulp exceeds 10.0% by mass, the fluidity is significantly reduced and the processability is worsened. Therefore, for example, uneven thickness is easily generated in the process of manufacturing wet sheets, and it may be difficult to obtain homogeneous wet sheets.

[0215] The medium W (aqueous medium) is preferably entirely water. However, the aqueous medium W may also be some other liquids that are miscible with water. Examples of other liquids include lower alcohols with 3 or fewer carbon atoms, ketones with 5 or fewer carbon atoms, etc.

[0216] Regarding cellulose fiber pulp, it is preferable to adjust the pulp content appropriately to achieve a water retention of 250–4000 g / m³. 2 More preferably, it should be 500–3000 g / m 2 The higher the water retention capacity, the easier the slurry is to dehydrate. However, if the water retention capacity exceeds 4000 g / m³, it will be difficult to achieve the desired results. 2 If the dispersibility decreases, the resulting molded articles will be difficult to produce homogeneous structures. If the water retention is below 250 g / m³... 2 If the dehydration process is incomplete or takes too long, productivity will suffer.

[0217] The water retention of cellulose fiber slurry was measured according to TAPPI T 701pm-01 (2001). The measurement procedure is as follows: (1) Place a PCTE filter on filter paper (pre-measured dry weight) for water retention measurement. (2) Clamp the aforementioned (1) with a special clamp and insert the test sample (slurry). (3) Perform the measurement (treatment) under the measurement conditions described below. (4) Remove the PCTE filter from the filter paper and measure the weight of the filter paper. (5) Calculate the water retention using the following formula (2). The measurement conditions are: cellulose fiber slurry (concentration of 1.5% by mass, temperature of 30°C) is inserted into the water retention measurement device AA-GWR (manufactured by Kaltec Scientific), and the air pressure is 1.5 kgf / cm. 2 The measurement time is 30 seconds.

[0218] [Equation (2)]

[0219] Water retention (g / m 2 = (Weight of filter paper after dehydration - Dry weight of filter paper) × 1250

[0220] (Forming process)

[0221] For the adjusted cellulose fiber slurry, in forming step 20, the slurry is sandwiched between two opposing mesh sheets and dehydrated under pressure, forming a wet sheet. In forming step 20, refer to... Figure 1 To illustrate, inside the cylindrical mold frame 13 placed on the table, mesh sheets 12 are stacked sequentially from bottom to top, and slurry 11 is filled on top of them. Mesh sheets 14 are then covered from top to bottom on the filled slurry 11. It should be noted that if the mold frame 13 is a porous component, dehydration is promoted, which can shorten the time spent in the wet sheet formation process 20.

[0222] The slurry 11 is dehydrated under its own weight or under relatively weak pressure. Then, the pressure 19 applied to the slurry is increased in stages or continuously. In this process, the water in the slurry 11 flows out through the mesh sheets 12 and 14. The initial pressure 19 applied in this process is very weak, so the slurry 11 remains at a high viscosity, which can suppress the outflow of cellulose microfibers. On the other hand, if the concentration of the slurry 11 increases as dehydration progresses, the fluidity decreases, so even if a stronger pressure 9 is applied to the slurry 11, it is difficult for the cellulose microfibers to flow out.

[0223] In this forming step 20, a pressure of 2.5 kPa or less is preferably applied in the initial stage. If a pressure exceeding 2.5 kPa is applied in the initial stage, cellulose microfibers tend to flow out of the slurry 11. It should be noted that if a mesh sheet with a finer mesh is used, the outflow of cellulose microfibers can be suppressed even if a pressure exceeding 2.5 kPa is applied in the initial stage, but in this case, the overall dewatering efficiency may be reduced. The pressure in this initial stage can be substantially atmospheric pressure. Alternatively, it can be the pressure generated solely based on the weight of the mesh sheet 14.

[0224] After dehydration to a certain extent through initial pressurization, the pressure 19 can be increased. The pressure 19 is gradually increased, and can eventually be set to 50 kPa or more, preferably 100 kPa or more, and more preferably 200 kPa.

[0225] The slurry 11 is pressurized at a pressure of 50 kPa or higher for more than 10 minutes. Then, the mold frame 13 is removed to obtain a wet sheet.

[0226] (Heating and pressurizing process)

[0227] In the heating and pressurizing process 30, dehydration and drying are carried out under conditions of 1-50 MPa and 100-150℃ to achieve high density, thereby producing a molded body X.

[0228] The density of the molded body X obtained as described above is preferably 0.8 to 1.5 g / m³. 3 More preferably, it is 0.9–1.4 g / m 3 The preferred concentration is 1.0–1.3 g / m³. 3 If the density of molded part X is less than 0.8 g / m³ 3 The strength may be insufficient due to the reduction in hydrogen bond points.

[0229] The density of molded part X is the value measured according to JIS-P-8118:1998.

[0230] In the cellulose fiber slurry S, additives such as antioxidants, corrosion inhibitors, light stabilizers, ultraviolet absorbers, heat stabilizers, dispersants, defoamers, slurry control agents, and preservatives can be added as needed.

[0231] The wetted sheet produced in this method can be used as a material for 3D shaped molded bodies.

[0232] Example

[0233] Next, embodiments of the present invention will be described.

[0234] (1) First, a pulp containing LBKP cellulose nanofibers (LBKP, 97% by mass) and cellulose nanofibers (LBKP, 97% by mass) was prepared as cellulose fibers to obtain a cellulose fiber pulp with a solid content concentration of 3% by mass. The LBKP cellulose nanofibers were obtained by pre-refining the raw pulp (97% by mass) using a refiner and then defibrating it using a high-pressure homogenizer. The LBKP cellulose nanofibers were an aqueous dispersion with a concentration of 3% by mass based on solid content. The average fiber diameter of the obtained LBKP cellulose nanofibers was 30 nm, and the crystallinity was 75%. A concentrated mixture was obtained by centrifuging the following mixture at 8500 rpm for 10 minutes using a centrifuge (HITACHI, CR22N cooled centrifuge) to obtain a concentrated mixture, wherein the mixture was prepared by mixing the LBKP cellulose nanofiber aqueous dispersion and the pulp using a mixer. The solid content concentration of the LBKP cellulose nanofibers in the concentrated mixture was 5% by mass. The mixture obtained by adding LBKP cellulose nanofiber aqueous dispersion and dilution water to the concentrated mixture was stirred / deaerated at 2000 rpm for 3 minutes using an Awatori Rentaro mixer to obtain a slurry with a solid content concentration of 5% by mass.

[0235] (2) Apply the above-mentioned slurry (1) to a 300-mesh metal mesh (lower metal mesh), and cover the slurry with another 300-mesh metal mesh (upper metal mesh) from above, to make a laminate consisting of metal mesh, slurry and metal mesh.

[0236] (3) The slurry sandwiched between the upper and lower metal meshes is pressurized to obtain a wetted sheet. Here, the laminate is placed on a support table with the lower metal mesh below and the upper metal mesh above. The wetted sheet obtained by placing a 5 kg weight on the upper metal mesh for 10 seconds is taken as Test Example 1, the wetted sheet obtained by placing a 5 kg weight on the upper metal mesh for 5 minutes is taken as Test Example 2, and the wetted sheet obtained by applying a pressure of 0.41 MPa to the upper metal mesh in the direction of the lower metal mesh for 5 minutes is taken as Test Example 3. The wetted sheets (Test Examples 1-3) are respectively made into test pieces with a length of 10 cm, a width of 10 cm, and a thickness of 0.2 cm.

[0237] (4) For the test pieces of the wet sheets (Examples 1-3), both sides of the wet sheets were completely covered with a resin film with a thickness of 0.04 mm to form a coating. The coating was placed on a support table, and a pressure of 100 kPa was applied to the coating in the thickness direction for 1 second, and then the thickness was measured. Similarly, a pressure of 100 kPa was applied in the thickness direction for 5 seconds, and then the thickness was measured to determine the thickness change rate.

[0238] (5) In addition, for Experimental Examples 1 to 3, the concentration of solid components (mass%) of LBKP cellulose nanofibers was measured.

[0239] The results are shown in Table 1.

[0240] [Table 1]

[0241] Thickness change rate Solid content concentration (mass%) of LBKP cellulose nanofibers Experimental Example 1 0.5 7 Experimental Example 2 0.32 10 Experimental Example 3 0.01 25

[0242] (other)

[0243] Unless otherwise specified, the other tests and measurements, such as JIS and TAPPI, mentioned above are conducted at room temperature (especially 25°C) and atmospheric pressure (especially 1 atm).

[0244] Industrial availability

[0245] This invention can be used as a molded body for cellulose fibers and a method for manufacturing the same.

[0246] Label Explanation

[0247] 10: Modulation process;

[0248] 20: The process of forming a wet sheet;

[0249] 30: Heating and pressurizing process;

[0250] S: Slurry;

[0251] P: Pulp;

[0252] W: Medium such as water;

[0253] X: Molded body.

Claims

1. A method for manufacturing a molded body, characterized by comprising: a mixing step of mixing a pulp and cellulose nanofibers having an average fiber diameter of 100 nm or less to prepare a slurry; a forming step of dehydrating the slurry by pressing the slurry between two opposing net-shaped sheets to form a wet sheet; and a heating and pressing step of obtaining a molded body by heating and pressing the wet sheet. The cellulose fibers contained in the slurry in the mixing step are composed of the pulp and the cellulose nanofibers, and the solid content concentration of the cellulose fibers is 1.0 to 10.0 mass%. In the pressing of the slurry in the forming step, a pressure of 2.5 kPa or less is applied in an initial stage, and the pressure is increased after the slurry is dehydrated to a certain degree by the pressing in the initial stage. The mixing ratio of the pulp and the cellulose nanofibers in the wet sheet is 10:90 to 20:80, and the cellulose nanofibers are contained at a rate of 10 to 38 mass%. The aspect ratio of the cellulose nanofibers is 10 or more and 100,000 or less. The thickness change rate of the wet sheet calculated according to Formula 1 below is 0.4 or less. [Formula 1] Thickness change rate = ((thickness of the wet sheet after 1 second of pressure application of 100 kPa in the thickness direction) - (thickness of the wet sheet after 5 seconds of pressure application of 100 kPa in the thickness direction)) ÷ (thickness of the wet sheet after 1 second of pressure application of 100 kPa in the thickness direction).

2. The method for manufacturing a molded body according to claim 1, characterized in that the forming step is a step performed without heating. The forming step is a step performed without heating. ​ ​ ​ ​ ​ ​

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