Dried cellulose fiber

Abstract

Provided is a dried cellulose microfiber (fibrous cellulose) which has an extremely small proportion of coarse structures, has a uniform fiber diameter distribution, and is versatilely usable for various purposes. A dried cellulose fiber according to the present invention is characterized in that: the average fiber diameter is 0.3-3.0 μm; and the proportion of coarse structures, which is indicated by the proportion of the number of fibers having a wet fiber diameter of at least 20 μm as measured by optical automatic analysis after water dispersion, is at most 3.0%.

Description

Dried cellulose fiber

[0001] The present invention relates to a dried cellulose fiber body.

[0002] Cellulose is the most abundant carbohydrate on Earth and a naturally occurring, biodegradable resource. It is used in a variety of industrial fields, and with the recent establishment of the Sustainable Development Goals (SDGs), it is expected to become a sustainable resource with low environmental impact. Cellulose nanofibers and microfibers, which are produced by micronizing cellulose, have attracted attention as one of the ways to utilize cellulose. They have a wide range of applications, including fillers for resins and rubbers, substrates for transparent and optical materials in electronic products, filtration media, additives for food, paints, and cosmetics, substrates for packaging and gas barrier materials, and viscosity modifiers and dispersion stabilizers for various liquid products. Cellulose nanofibers and microfibers tailored to specific performance requirements are being explored. By combining the selection of raw cellulose species, physical micronization conditions, and chemical processing conditions, nanofibers and microfibers of cellulose materials with various shapes and properties can be obtained.

[0003] Cellulose nanofibers are generally obtained by pulp or other materials dispersed in water and are therefore primarily used in the form of aqueous dispersions. However, when adding aqueous dispersions to paints, cosmetics, resins, etc., the presence of large amounts of water limits the mixing process and the types of materials to which they can be added, and transportation requires significant costs and energy. For this reason, there is a demand for them to be provided in a dried form. However, cellulose nanofibers have very strong aggregating properties due to intermolecular forces, and they aggregate during drying, forming coarse structures that are bonded together by strong hydrogen bonds. The presence of these coarse structures significantly reduces the properties of cellulose nanofibers, but once these coarse structures have formed, it is extremely difficult to restore them to their pre-drying state.

[0004] In applications where nanofibers are not required, cellulose may be used in the form of cellulose microfibers with a reduced refining force. However, when the refining force is reduced, it is not possible to refining all of the original cellulose raw material, and coarse, unground portions remain. The unground portions are present in the aqueous dispersion before drying, and remain as coarse structures even after drying. On the other hand, when the refining force is increased to reduce the unground portions, some of the particles are refined to nanofibers, resulting in the formation of aggregated coarse structures during drying. Thus, even in the form of cellulose microfibers, it is extremely difficult to obtain a dried product with few coarse structures. The presence of coarse structures significantly reduces the properties of cellulose microfibers, just like nanofibers.

[0005] Methods reported to suppress aggregation during drying include adding surfactants, solvent substitution to remove water, and chemically modifying cellulose. For example, in Patent Document 1 below, a surfactant is added when drying cellulose fibers to obtain a dried body in which aggregation of cellulose molecules is suppressed. However, evaluation is limited to determining the presence of agglomerates with a maximum diameter of 200 μm or more after resin dispersion, and the presence or absence of smaller coarse structures is not evaluated. Although coarse structures appear to be few under SEM, evaluation of fiber diameter is limited to checking 50 fibers, and the overall fiber trend is not understood. Furthermore, the calculated value is the average fiber diameter, and the proportion of coarse structures is not discussed. Coarse, unground portions are already present in the aqueous dispersion before drying, and remain as coarse structures even when surfactants are added. Furthermore, surfactants are present in the materials to be kneaded using this dried body, which can have adverse effects such as foaming during kneading and loss of biodegradability.

[0006] In the following Patent Document 2, when drying cellulose fibers, softeners and plasticizers are added in amounts equivalent to or greater than the amount of cellulose in addition to surfactants to obtain a dried body in which aggregation of cellulose particles is suppressed. Adding a large amount of additives can suppress the strong aggregation effect of cellulose, but the proportion of coarse structures is not discussed. In the first place, coarse unground portions are already present in the aqueous dispersion before drying, and even if a large amount of additives is added, they remain as coarse structures.

[0007] In Patent Document 3 listed below, cellulose fibers are refined in the presence of an organic solvent, and then the organic solvent is removed to obtain a dried product, but the presence or absence of coarse materials due to aggregation is not discussed. Although the absence of water somewhat suppresses cellulose aggregation, aggregates derived from nanofibers are present, and coarse structures derived from the unpulverized portion are also present before drying, so the dried product contains coarse structures.

[0008] In Patent Document 4 below, sulfate ester groups are introduced into cellulose to suppress the strong cohesive force due to cellulose hydrogen bonds, thereby obtaining a cellulose fiber powder that is easily redispersible in water. While it is true that the generation of coarse structures is somewhat suppressed when the average fiber diameter is reduced to single-order nanofibers, as in Sample No. 1, nanofiber-derived aggregates are generated during drying. Furthermore, when the average fiber diameter is the same as that of microfibers, as in Sample No. 5, even if the cellulose is modified, coarse unpulverized portions remain due to the principle of micronization. In fact, the median diameter after aqueous dispersion is 50 μm or more, and coarse structures are generated.

[0009] Methods for obtaining cellulose microfibers without mechanical or chemical treatments have been reported, including dissolving cellulose in a solvent and performing wet spinning or electrospinning. Patent Document 5 (see below) reports obtaining cellulose microfibers by dissolving cellulose in a cuprammonium solution and performing tension spinning. While this method eliminates the generation of nanofibers or residual unground portions, if single yarns come into contact with each other in the spinning funnel before sufficient desolventization has occurred, coalescing fibers in which the fibers are tightly bonded together can occur. Furthermore, insufficient desolventization before copper removal with sulfuric acid can result in strong hydrogen bonds between the fibers during copper removal, resulting in coalescing fibers in which the fibers are tightly bonded together. These coalesced fibers are also a type of coarse structure, and depending on the application, can impair the inherent performance of the microfiber. Furthermore, for use in spun yarns, a certain degree of self-adhesion between single yarns is required. Therefore, the decopperization of the solvent is performed by scouring in a rectified sulfuric acid bath. This method fixes the structure without releasing the tension generated during spinning, resulting in excessive orientation of the cellulose molecular chains in the resulting fibers, making them prone to fibrillation. When easily fibrillated yarn is used as a filler, nanofibers are generated during mixing, and coarse particles are generated due to aggregation. In addition, oil is added to prevent adhesion during drying, which has adverse effects such as a loss of biodegradability.

[0010] In Patent Document 6 listed below, a nonwoven fabric made of cellulose microfibers is obtained by lowering the cellulose concentration dissolved in a cuprammonium solution, performing tension spinning under flow, and layering the fibers on a net. Even with this method, coalesced fibers occur due to contact between the single fibers before the solvent removal has progressed sufficiently in the funnel, or due to the formation of strong hydrogen bonds between the fibers during copper removal.

[0011] In Patent Document 7 below, cellulose is dissolved in a cuprammonium solution and electrospinned to obtain uniform cellulose microfibers with a fiber diameter CV value of 11-30% and few particulate portions with a diameter of 3.0 μm or more. This method does not generate nanofibers, and cellulose orientation hardly occurs, making it difficult to fibrillate. Therefore, even when used as a filler, the generation of nanofibers during kneading can be suppressed. However, electrospinning makes it difficult to fine-tune the coagulation rate, and the cellulose may reach the collector before sufficient desolventization has occurred, resulting in the generation of coalesced fibers in which the fibers are firmly bonded to each other. Furthermore, the particulate portions in the sheet are 500 pieces / mm ２ The fiber obtained by electrospinning has a low cellulose orientation and low crystallinity, making it prone to tangling, and during drying, it aggregates strongly, resulting in the formation of coarse structures. Other problems include low productivity.

[0012] JP 2020-125563 A JP 2013-133363 A JP 2012-224960 A JP 2023-66710 A JP 2010-216061 A International Publication No. 2020 / 226181 JP 2008-266828 A

[0013] In view of the above-mentioned state of the art, the problem to be solved by the present invention is to provide a dried cellulose microfiber (fibrous cellulose) that has an extremely small proportion of coarse structures, a uniform fiber diameter distribution, and can be used for a variety of general purposes.

[0014] As a result of extensive research and experiments to solve the above problems, the inventors have discovered that the causes of the formation of coarse structures in the dried product are the aggregation of nanofiberized cellulose, the remaining unpulverized parts of the raw cellulose, and strong adhesion between fibers.The inventors have unexpectedly discovered that by controlling these to a certain level or less, it is possible to obtain a dried product of cellulose microfibers (fibrous cellulose aggregates) with few coarse structures and a uniform fiber diameter without adding chemical substances or modifying the cellulose, and have completed the present invention.

[0015] That is, the present invention provides the following: [1] A dried cellulose fiber having an average fiber diameter of 0.3 μm or more and 3.0 μm or less, and a proportion of coarse structures of 3.0% or less, as indexed by the proportion of fibers having a wet fiber diameter of 20 μm or more measured by optical automatic analysis after aqueous dispersion. [2] The dried cellulose fiber according to [1] above, having an average wet fiber length of 1,000 μm or less. [3] The dried cellulose fiber according to [1] or [2] above, having an average wet fiber diameter of 1.0 μm or more and less than 10.0 μm after aqueous dispersion. [4] The dried cellulose fiber according to any of [1] to [3] above, having a coefficient of variation of fiber diameter of 1.00 or less. [5] The dried cellulose fiber according to any of [1] to [4] above, having an aggregation constant of fiber diameter of 5.0 or less after aqueous dispersion. [6] The dried cellulose fiber according to any of [1] to [5] above, wherein the crystalline structure of the cellulose constituting the cellulose fibers is type II. [7] The dried cellulose fiber according to any one of [1] to [6], wherein the proportion of additives extracted into water, ethanol, and hexane is less than 10% by weight. [8] The dried cellulose fiber according to any one of [1] to [7], wherein the crystallinity of the cellulose constituting the cellulose fiber is 30% or more and 90% or less. [9] The dried cellulose fiber according to any one of [1] to [8], wherein the degree of substitution of hydroxyl groups of the cellulose constituting the cellulose fiber is 0.3 or less.

[0016] The dried cellulose fiber material according to the present invention is a dried material of uniform cellulose microfibers (fibrous cellulose aggregates) with few coarse structures, without the addition of chemical substances or modification of cellulose, and therefore can be used for a variety of general purposes, such as additives for paints and cosmetics, fillers for reinforcing resins and rubbers, and substrates for porous bodies such as filters. Among these, it is particularly expected to function as an additive for paints and cosmetics.

[0017] 1 is a SEM image of the dried cellulose fiber body obtained in Example 1. FIG. 2 is a SEM image of the dried cellulose fiber body obtained in Comparative Example 1. FIG. 3 is a SEM image of the dried cellulose fiber body obtained in Comparative Example 6. FIG. 4 is a SEM image of the dried cellulose fiber body obtained in Comparative Example 7. FIG. 5 is a SEM image of the dried cellulose fiber body obtained in Comparative Example 8. FIG. 6 is an optical microscope image of a paint containing the dried cellulose fiber body of Example 1. FIG. 7 is an optical microscope image of a paint containing the dried cellulose fiber body of Comparative Example 1.

[0018]

[0013] In one embodiment of the present invention, there is provided a dried cellulose fiber material having an average fiber diameter of 0.3 μm to 3.0 μm, and a proportion of coarse structures of 3.0% or less, as indicated by the proportion of fibers having a wet fiber diameter of 20 μm or more as measured by automatic optical analysis after dispersion in water.

[0019] [Cellulose fiber] In this specification, the term "cellulose fiber" refers to a structure (aggregate) made of fibrous cellulose. The method for producing the same is not particularly limited, and examples include a method of pulverizing a cellulose raw material using physical force, a method of pulverizing using chemical force, and a method of dissolving the cellulose raw material in a solvent and forming it into fibers. A combination of these methods is also possible. In order to reduce the undisintegrated portion of the raw material that causes coarse structures, a method of dissolving the cellulose raw material in a solvent and forming it into fibers is preferred. Dissolving the cellulose raw material in a solvent also makes it possible to remove small foreign matter by filtration or centrifugation.

[0020] In this specification, the term "fiber diameter" of cellulose fibers refers to a value measured when the cellulose is in a dry state, and "average fiber diameter" refers to a number average value. There are various methods for measuring the diameter in a dry state, and a method can be selected depending on the fiber diameter. As described below, in the examples, measurements were made using an electron microscope.

[0021] The cellulose fibers of this embodiment are evaluated not only in a dry state but also in a wet state after dispersion in water. Evaluation in a dry state using an electron microscope or the like is limited in the number of fibers that can be measured, and while evaluation of the average value is somewhat accurate, it is difficult to evaluate the distribution or the proportion of coarse structures. Results evaluated in a wet state are distinguished from results evaluated in a dry state by expressing the fiber diameter as the "wet fiber diameter," the average fiber diameter as the "wet average fiber diameter," the fiber length as the "wet fiber length," and the average fiber length as the "wet average fiber length." Note that, because cellulose fibers swell in water, the "wet fiber diameter" is larger than the "fiber diameter."

[0022] There are various methods for evaluating the fiber diameter in a wet state, but a microscope, an optical automatic analysis method, etc. are preferred because they can distinguish between fiber diameter and fiber length, and can be selected depending on the fiber diameter. As will be described later, in the examples, the optical automatic analysis method was used for measurement.

[0023] The average fiber diameter of the cellulose fibers of this embodiment is 0.3 μm or more and 3.0 μm or less. If the average fiber diameter is 0.3 μm or more, aggregation due to hydrogen bonding during drying can be suppressed. The average fiber diameter is preferably 0.4 μm or more, more preferably 0.5 μm or more, and even more preferably 0.7 μm or more. On the other hand, if the average fiber diameter is 3.0 μm or less, the effect of adding the cellulose fibers to paints, cosmetics, etc. can be fully exerted, and the number of fibers per unit weight increases, thereby enhancing the effect of adding the cellulose fibers as a filler and increasing the specific surface area when formed into a porous body. The average fiber diameter is preferably 2.5 μm or less, more preferably 2.0 μm or less, even more preferably 1.5 μm or less, and most preferably 1.2 μm or less.

[0024] In this specification, the term "cellulose nanofiber" refers to a cellulose fiber having a fiber diameter of less than 0.1 μm, and the term "cellulose microfiber" refers to a cellulose fiber having a fiber diameter of 0.1 μm or more and less than 9.7 μm.

[0025] As used herein, the term "coarse structures" refers to structures whose maximum width on the short side perpendicular to the long side is 20.0 μm or more in a wet state after redispersion in water, as described below. Coarse structures include aggregates of structures smaller than 20.0 μm, such as nanofiber agglomerations and coalesced fibers in which fibers are firmly bonded together, as well as cellulose masses such as structures originally larger than 20.0 μm, such as unground portions of cellulose raw materials and thick fibers. The reason for using measurement results in a wet state after dispersion in water is that evaluation in a dry state cannot eliminate the effects of fiber entanglement and static adhesion, making it impossible to distinguish from strongly bonded coarse structures. Furthermore, measurement in a wet state makes it easier to increase the number of measurements and enables accurate evaluation of distribution. Furthermore, because dried cellulose fibers are primarily added to liquid substances, this measurement method is closer to the actual situation. Although it depends on the preparation method and crystalline structure of the cellulose fiber, by reducing the number of structures of 20.0 μm or more in the wet state, it is possible to reduce the number of structures of approximately 10.0 μm or more in the dry state.

[0026] The dispersion of dried cellulose fibers in water is carried out by applying a shear force sufficient to disintegrate the dried cellulose fibers without further pulverizing them. This is because redispersion using a device for pulverizing cellulose, as described below, will further pulverize the cellulose fibers, resulting in a shape and properties that differ from those they had before redispersion. Methods for dispersing in water include, but are not limited to, manually shaking the dried cellulose fibers in water, shaking in a shaking tank, disintegrating with a mixer or homogenizer, or dispersing with ultrasonic waves. Dispersion using ultrasonic waves is preferred from the viewpoint of efficiently disintegrating the entangled fibers and dispersing them while maintaining the shape and properties of the dried cellulose fibers.

[0027] The "proportion of coarse structures" of the dried cellulose fiber material of this embodiment, which is indexed by the proportion of fibers having a wet fiber diameter of 20 μm or more as measured by automatic optical analysis after dispersion in water, is 3.0% or less. If the proportion of coarse structures is 3.0% or less, effects such as suppression of clumping and aggregation when blended as an additive in paints or cosmetics, reduction of defects when used as a filler, reduction of defects when used as a base material for porous bodies, and improvement of functionality by increasing the number of fibers can be expected. The "proportion of coarse structures" is preferably 2.0% or less, more preferably 1.0% or less. A smaller proportion of coarse structures is preferable, but from the viewpoint of production efficiency, 0.1% or more is preferable.

[0028] The dried cellulose fiber material of this embodiment has a volatile component ratio of 15% by weight or less. Volatile components refer to substances with a boiling point of 150°C or less, such as water, organic solvents such as ethanol and hexane. If the volatile component ratio is 15% by weight or less, the dried cellulose fiber material can be used without impairing its function as a dried material. The volatile component ratio is preferably 12% by weight or less, more preferably 10% by weight or less, and particularly preferably 7% by weight or less. Furthermore, from the viewpoint of production efficiency, a ratio of 1% by weight or more is preferable.

[0029] The "wet average fiber length" of the dried cellulose fiber material of this embodiment can be adjusted to any length, but is preferably 1,000 μm or less in order to prevent entanglement of fibers and the generation of coarse structures. When using a method in which cellulose is dissolved in a solvent and formed into a fibrous form, it can be obtained in the form of very long fibers, or it can be obtained in the form of short fibers by cutting the obtained long fibers. When using short fibers, the fiber length and wet fiber length can be selected depending on the application. In this specification, the terms "average fiber length" and "wet average fiber length" refer to the length-weighted average fiber length. The "wet average fiber length" is preferably 500 μm or less, more preferably 300 μm or less, and particularly preferably 100 μm or less. A smaller "wet average fiber length" is preferable, but a value of 20 μm or more is preferred in terms of production efficiency.

[0030] The "coefficient of variation of fiber diameter" of the dried cellulose fiber material of this embodiment is preferably 1.00 or less. Fibers with a uniform fiber diameter, with a coefficient of variation of fiber diameter of 1.00 or less, are less susceptible to performance degradation due to breakage of thin fibers and defects due to thick fibers, and are more likely to exhibit desired performance in a variety of applications. In addition, the fiber length can be made uniform when shortened by physical impact. The "coefficient of variation of fiber diameter" is preferably 0.80 or less, more preferably 0.70 or less, and particularly preferably 0.60 or less. A smaller "coefficient of variation of fiber diameter" is preferable, but from the viewpoint of production efficiency, a value of 0.10 or more is preferred.

[0031] The "wet average fiber diameter" of the dried cellulose fiber material of this embodiment is preferably 1.0 μm or more and 10.0 μm or less. By making the wet average fiber diameter 1.0 μm or more, it is possible to suppress aggregation due to entanglement during stirring and aggregation due to hydrogen bonding during drying. The "wet average fiber diameter" is preferably 1.1 μm or more, and particularly preferably 1.2 μm or more. On the other hand, if the "wet average fiber diameter" is 10.0 μm or less, the cellulose fiber can be sufficiently added to paints, cosmetics, etc., and the number of fibers per unit weight is increased, thereby improving the effect as an added filler, and increasing the specific surface area when made into a porous material. The "wet average fiber diameter" is preferably 8.3 μm or less, more preferably 6.7 μm or less, even more preferably 5.0 μm or less, and particularly preferably 3.3 μm or less.

[0032] In this specification, the term "aggregation constant" is an index representing the degree of aggregation of a dried cellulose fiber material and is defined by the following formula: Aggregation constant = (wet average fiber diameter / average fiber diameter) A dried cellulose fiber material containing a large amount of nanofibers with a fiber diameter of less than 0.1 μm appears to have dispersed fibers when observed locally using an SEM, but in reality, some of the fibers are unground or form a network structure through hydrogen bonds. The wet fiber diameter of such cellulose fibers tends to be large. Furthermore, cellulose fibers that appear to have independent fibers overlapping with an SEM but actually have a large amount of coalesced fibers in which the fibers are firmly bonded to each other also tend to have a large wet fiber diameter.

[0033] The "aggregation constant" of the dried cellulose fiber material of this embodiment is preferably 5.0 or less. By setting the aggregation constant to 5.0 or less, effects such as suppression of clumping and aggregation when added to paints or cosmetics, reduction of defects when used as a filler, reduction of defects when used as a base material for porous bodies, and improvement of functionality by increasing the number of fibers can be expected. The "aggregation constant" is more preferably 4.5 or less, even more preferably 4.0 or less, and particularly preferably 3.5 or less. A smaller "aggregation constant" is preferable, but from the viewpoint of production efficiency, it is preferably 1.0 or more, more preferably 1.5 or more.

[0034] The "coefficient of variation of wet fiber diameter" of the dried cellulose fiber material of this embodiment is preferably 1.00 or less. Fibers with a uniform fiber diameter, with a coefficient of variation of wet fiber diameter of 1.00 or less, are less susceptible to performance degradation due to breakage of thin fibers and defects due to thick fibers, and are more likely to exhibit desired performance in a variety of applications. Furthermore, the fiber length can be made uniform when shortening the fibers by physical impact. The "coefficient of variation of wet fiber diameter" is preferably 0.90 or less, more preferably 0.80 or less, even more preferably 0.70 or less, particularly preferably 0.60 or less, and most preferably 0.50 or less. A smaller "coefficient of variation of wet fiber diameter" is preferable, but from the viewpoint of production efficiency, a value of 0.20 or more is preferred.

[0035] The "crystal structure (crystal form)" of the dried cellulose fiber material of this embodiment is not particularly limited, and various cellulose types, such as type I, type II, type III, and type IV, can be used. When adjusting the fiber length, a type II crystal structure is preferred because it allows the fibers to be cut with a weak force and tends to be uniform.

[0036] The "proportion of additives extractable into water, ethanol, and hexane" of the dried cellulose fiber material of this embodiment is preferably less than 10% by weight. Additives such as surfactants, plasticizers, and dispersants can be added to suppress aggregation during drying, but adding large amounts can have adverse effects such as foaming during blending and loss of biodegradability. The "proportion of additives extractable into water, ethanol, and hexane" is preferably 5% by weight or less, more preferably 2% by weight or less, and even more preferably 1% by weight or less. Most preferably, the dried cellulose fiber material does not contain any "additives extractable into water, ethanol, and hexane."

[0037] The "crystallinity" of the dried cellulose fiber material of this embodiment is not particularly limited, but is preferably 30% or more and 90% or less. A crystallinity of a certain level or higher can reduce aggregation during drying and increase the strength of the cellulose fibers, so the crystallinity is preferably 30% or more, more preferably 40% or more, and particularly preferably 45% or more. On the other hand, a crystallinity of a certain level or lower has the effects of improving dispersibility when used as an additive or filler, and increasing the strength of the substrate by bonding at the intersections of fibers when used as a substrate for a porous body. Therefore, the crystallinity is preferably 90% or less, more preferably 80% or less, and even more preferably 70% or less. Several methods for calculating the crystallinity of cellulose have been proposed, but in this embodiment, the crystallinity is calculated using the Segal method.

[0038] The cross-sectional shape of the dried cellulose fiber material of this embodiment is not particularly limited, and fibers of various cross-sectional shapes, such as round, irregular, and irregular, can be used, and the surface may be fibrillated. A round cross-section is preferred in terms of the texture when kneaded as an additive, the reinforcing effect when used as a filler, and the repair effect when the porous material is used as a filter. Furthermore, when using cellulose fibers as a filler, it is preferable that the surface be fibrillated to prevent fiber shedding.

[0039] The dried cellulose fiber body of this embodiment can be made uniform in both fiber diameter and fiber length, and by making both uniform, the effects of using fibers in various applications can be efficiently achieved. The "coefficient of variation of wet fiber diameter" and the "coefficient of variation of wet fiber length" are both preferably 1.00 or less. Preferably, both are 0.90 or less, more preferably both are 0.80 or less, and particularly preferably both are 0.70 or less. The smaller the "coefficient of variation of wet fiber length" and the "coefficient of variation of wet fiber length," the better, but from the viewpoint of production efficiency, they are preferably 0.05 or more.

[0040] The dried cellulose fiber of this embodiment can be converted into a cellulose derivative by reacting some of its hydroxyl groups, and any substituent can be introduced depending on the required function. To maintain the biodegradability of cellulose, a substituent that does not hydrophobize cellulose is preferred, and the degree of substitution is preferably 0.30 or less, more preferably 0.20 or less, and particularly preferably 0.01 or less. To maintain the biodegradation rate of cellulose and improve the environmental impact after decomposition, it is most preferable to use cellulose as is without derivatization.

[0041] [Method for producing cellulose fibers] An example of a method for producing cellulose fibers according to the present invention will be described below. This embodiment is not limited to this method. The cellulose raw material is not particularly limited, and various raw materials can be selected. Examples include wood pulp, non-wood pulp, cotton-based pulp such as cotton and cotton linter, cellulose from sea squirts and seaweed, recovered pulp, recycled cellulose, etc. It is also possible to use a mixture of multiple materials. Among these, cotton and cotton linter pulp are preferred due to their high purity, and cotton linter is particularly preferred.

[0042] The solvent for dissolving cellulose is not particularly limited, and various known solvents can be selected. Examples include cuprammonium solution, viscose solution, acid or alkali of a specific concentration, aqueous solution of inorganic salt such as zinc chloride, N-methylmorpholine N-oxide, and various ionic liquids. Among these, cuprammonium is preferred as the solvent from the viewpoints of adjusting the coagulation rate, facilitating the thinning of the fiber diameter, and enabling adjustment of fibrillation.

[0043] The method for dissolving cellulose in a solvent and forming it into a fiber is not particularly limited, and various spinning methods can be used. Examples include flow-down tension spinning, in which a solution is extruded from a spinning nozzle with many holes (orifices) and coagulated into a funnel together with a coagulation liquid for desolvation; air-gap spinning, in which the solution is once discharged into the air and stretched, and then coagulated with a coagulation liquid; dry spinning, in which stretching and desolvation are performed in air; melt-blown spinning, in which stretching and coagulation are performed using a high-speed airflow; and electrospinning, in which a solution is charged and thinned and accumulated by electrical repulsion. Among these, flow-down tension spinning, air-gap spinning, and melt-blown spinning, which can control the orientation of cellulose, are preferred. In particular, flow-down tension spinning, which can uniformly produce fiber diameters, is less likely to break even when the fiber diameter is thin, and can suppress strong adhesion between fibers, is preferred.

[0044] The following describes an example in which cellulose is dissolved in a cuprammonium solution and subjected to tension spinning. The concentration of cellulose dissolved in the cuprammonium solution can be selected as desired. Although it depends on the degree of polymerization of the cellulose to be dissolved, a dissolution concentration of 1 to 20 wt % is preferred. Furthermore, after dissolving the cellulose, it is preferable to remove foreign matter and undissolved materials by filtration or centrifugation. A concentration of a certain level or higher increases the viscosity of the stock solution, suppressing thread breakage in the funnel. This results in uniform drawing and reduces the likelihood of single threads coming into contact with other fibers before sufficient solvent removal occurs, resulting in a uniform fiber diameter. Furthermore, the single thread strength of the resulting fiber is improved. The concentration of cellulose dissolved in the cuprammonium solution is more preferably 2 wt % or higher, even more preferably 3 wt % or higher, and particularly preferably 4 wt % or higher. Furthermore, a concentration of a certain level or lower facilitates drawing, thereby enabling a thinner fiber diameter, and filtration or centrifugation of the stock solution facilitates removal of foreign matter. The concentration of cellulose dissolved in the cuprammonium solution is more preferably 9% by weight or less, even more preferably 8% by weight or less, and particularly preferably 7% by weight or less. The ammonia concentration is preferably 15% by weight or less, more preferably 12% by weight or less, and even more preferably 10% by weight or less. Within this range, uneven coagulation is unlikely to occur, and strong bonding of the fibers to each other can be suppressed.

[0045] Any shape of nozzle can be used for ejecting the dissolving solution. The number of holes is preferably 10 to 2000. If the number of holes is 2000 or less, the occurrence of internal and external differences in coagulation can be kept within a certain range, resulting in a uniform fiber diameter. Furthermore, by setting the flow rate of the coagulating solution at a certain level or less, the flow in the funnel can be rectified, preventing fibers from contacting each other before sufficient desolvation has occurred. The number of holes is more preferably 1500 or less. From the viewpoint of productivity, the number of holes is more preferably 50 or more, and even more preferably 100 or more. Furthermore, the hole diameter is preferably 0.10 to 0.50 mm. A hole diameter of 0.10 mm or more ensures high productivity, sufficient stretching, and high strength of the cellulose fibers. Furthermore, the ejection speed of the dissolving solution can be kept below a certain level, preventing the dissolving solution from swaying in the coagulating solution and preventing fibers from contacting each other before sufficient desolvation has occurred. The hole diameter is more preferably 0.12 mm or more, and even more preferably 0.15 mm or more. On the other hand, if the hole (hole) diameter is 0.50 mm or less, the solution discharged from the spinneret can be kept at a certain distance, preventing contact between fibers before sufficient desolventization has occurred. Furthermore, excessive stretching can be suppressed, making the cellulose fibers less likely to fibrillate and preventing the generation of thin fibers when shortening the fibers by physical impact or when kneading. The hole (hole) diameter is more preferably 0.40 mm or less, and even more preferably 0.30 mm or less. Furthermore, the distance between adjacent holes (holes) from end to end is preferably 0.60 to 2.00 mm. If the distance between holes (holes) is 0.60 mm or more, contact between fibers can be prevented even if the dissolving solution sways slightly in the coagulating solution. Furthermore, since the spinneret has holes (holes) arranged concentrically or in multiple rows, the dissolving solution discharged from the outermost holes (holes) comes into contact with fresh coagulating solution, but the dissolving solution discharged from the inner holes (holes) may experience uneven coagulation. If the distance between the holes (pores) is 0.60 mm or more, the coagulation liquid can be easily diffused into the inner layer, and uneven coagulation of the dissolving liquid discharged from the holes (pores) in the inner layer can be suppressed, and as a result, contact between the fibers before sufficient desolvation can be suppressed. In other words, the formation of coarse structures can be suppressed.The distance between the holes (perforations) is more preferably 0.80 mm or more, and even more preferably 1.0 mm or more.

[0046] Any type of coagulation liquid can be used as long as it can be deammonified, but from the viewpoints of economy and safety, it is preferable to use water with an adjusted temperature. To obtain cellulose fibers with a small average fiber diameter, it is necessary to perform drawing at a low spinning temperature. However, the solution discharged from the holes in the inner layer may not be fully coagulated. If copper is removed under these conditions, strong hydrogen bonds will be formed between the fibers. For this reason, it is preferable to first perform drawing and coagulation at a lower spinning temperature, and then complete the coagulation of the solution discharged from the holes in the inner layer at a higher spinning temperature.

[0047] After deammoniation in the funnel and fiber formation, copper removal can be performed by any method. Examples include pouring acid into the funnel, discharging the blue yarn into an acid bath after it leaves the funnel, redirecting the blue yarn after it leaves the funnel and immersing it in an acid bath, dripping acid onto the blue yarn, collecting the blue yarn on a net and showering it with acid, and collecting the blue yarn in a tank and adding acid batchwise. Decoppering under low tension on the yarn makes the cellulose fibers less likely to fibrillate, suppressing the formation of nanofibers that cause coarse aggregates. This also facilitates acid renewal between the fibers, improving copper removal efficiency and resulting in cellulose fibers with less residual copper and sulfuric acid. In the cellulose fiber production method of this embodiment, we used a method in which the blue yarn leaves the funnel and is then collected on a net to release the tension before showering it with sulfuric acid, and a method in which the blue yarn leaves the funnel and is then redirected before showering it with sulfuric acid to suppress tension increases. On the other hand, when deammoniation is performed using a method that does not sufficiently stretch the cellulose, such as electrospinning, applying a certain level of tension during copper removal can increase the strength of the resulting cellulose fibers.

[0048] By increasing the concentration or temperature of the acid used or by extending the treatment time, it is possible to promote hydrolysis and reduce the degree of polymerization of cellulose. By reducing the degree of polymerization, the cellulose becomes more easily disintegrated during the fiber shortening process described below, and the fiber length can be further reduced. The degree of reduction in the degree of polymerization can be controlled by adjusting the acid concentration, temperature, and treatment time.

[0049] After copper removal or hydrolysis using acid, any method can be selected to remove the acid used. Since the fibers are thin, it is difficult for the liquid between the fibers to be renewed when removing the acid, so a method that facilitates liquid renewal is preferred. In the cellulose fiber manufacturing method of this embodiment, sulfuric acid was removed by repeated washing with warm water. It is preferable that the cellulose fibers have low residual copper and sulfuric acid. If these levels are high, it can have effects such as fiber aggregation and discoloration during drying, reduced strength during storage, and a slower biodegradability rate.

[0050] Any drying method can be selected to obtain dried cellulose fibers. For example, air drying, heat drying, reduced-pressure drying, freeze drying, solvent substitution drying, supercritical drying, or a combination of these may be used. An oil or surfactant may be added during drying to suppress aggregation. From the viewpoints of production efficiency and cost, air drying, heat drying, and reduced-pressure drying are particularly preferred. Furthermore, in order to suppress cellulose aggregation during drying, it is preferable to sandwich the cellulose fibers between wire mesh or metal plates to suppress shrinkage of the cellulose fibers when water evaporates. Furthermore, drying while the fibers are uniform in length can suppress entanglement and aggregation of the fibers, making them easier to disentangle during the fiber shortening process.

[0051] The fiber diameter of the cellulose fibers can be adjusted as desired by adjusting the concentration of dissolved cellulose, the nozzle diameter for discharging the dissolving solution, the funnel shape, and the stretching ratio, which is determined by combining the temperature, composition, and flow rate of the coagulation solution. Various methods can be used to adjust the fiber length of the cellulose fibers depending on the required fiber length, including cutting the fibers with a blade and applying a physical impact to the fibers to shorten them. The fiber length adjustment can be performed in either a wet or dry state, or a combination of these methods. To particularly suppress the generation of coarse structures, it is preferable to shorten the fiber length to 1,000 μm or less in a wet state, then dry the fibers, and then apply a physical impact in a dry state to loosen them, or apply a physical impact to further shorten them. Applying a further physical impact after drying can disperse and pulverize even the small amount of aggregates generated during drying, further reducing the proportion of coarse structures.

[0052] Examples of methods for cutting cellulose fibers with a blade include rotary cutters, guillotine cutters, cutter mills, etc. These methods may be performed on cellulose fibers in a dry state, a wet state, or a state suspended in water or an organic solvent. Furthermore, the cellulose fibers may be subjected to a chemical treatment or heat treatment beforehand. While cutting methods using a blade tend to produce a uniform fiber length, there is a limit to the length that can be shortened.

[0053] Examples of fiber shortening methods using physical impact include mixers, homogenizers, ball mills, bead mills, beaters, disc refiners, grinders, high-pressure homogenizers, airflow pulverizers, pin mills, and jet mills. While fiber shortening methods using physical impact can shorten fiber length, they also tend to result in a certain distribution of fiber lengths in the resulting fibers. In particular, when natural cellulose is refined using physical force, a distribution of fiber diameters occurs, resulting in a mixture of thin, easily cuttable portions and thick, difficult-to-cut portions, which tends to result in a distribution of fiber lengths. On the other hand, the present dried cellulose fiber has a uniform fiber diameter, which can somewhat suppress the spread of the fiber length distribution. Furthermore, when shortening fibers using physical impact, rather than immediately shortening long fibers, cutting the fibers to a certain length with a blade before applying physical impact can further suppress the spread of the fiber length distribution. Furthermore, after fiber shortening, filtration or classification can be performed using a filter to remove fibers that are too short or fibers that remain long.

[0054] When shortening fibers by physical impact, if the impact is too strong, the fibers will fibrillate or break, resulting in a broad fiber diameter distribution. In other words, the purpose of applying physical impact to fibers is to shorten the fiber length or to separate fibers that are weakly associated with each other, not to reduce the fiber diameter. To achieve extremely short fiber lengths, it is preferable to combine the physical impact with techniques such as suppressing cellulose orientation or lowering the degree of polymerization of cellulose by acid treatment, as necessary, to suppress fiber fibrillation. Furthermore, even when physical impact is used, fiber shortening methods using impact forces, such as pin mills and collision-type jet mills, are preferred. Fiber shortening methods using frictional or shear forces, such as bead mills and grinders, tend to reduce not only fiber length but also fiber diameter, resulting in the generation of nanofibers. Furthermore, entanglement of fibers is likely to occur during the grinding process, leading to the generation of coarse structures. Fiber shortening methods using impact forces can shorten only fiber length, suppress the generation of nanofibers, and also suppress entanglement of fibers. In particular, since cellulose is soft and does not easily self-destruct, methods such as a wall collision jet mill or pin mill, in which samples are collided against a hard substance, are preferable to a counter collision jet mill, in which samples are collided against each other.

[0055] [Uses of Cellulose Fibers] The dried cellulose fiber material of the present embodiment can be used as an additive to be added to paints and cosmetics, a filler to be added to resins and rubbers, a porous material that can be used in filters, adsorbents, sound absorbing materials, heat insulating materials, coating agents, artificial leather substrates, anti-settling agents for liquid products, etc. The dried cellulose fiber material containing few nanofibers and coarse structures can be uniformly dispersed as an additive or filler, and a uniform porous material can be obtained without adding a surfactant or making the cellulose hydrophobic.

[0056] Below, an example of using the dried cellulose fibers of this embodiment as an additive is described. Of course, the present invention is not limited thereto. When the dried cellulose fibers of this embodiment are used as an additive, there are no particular limitations on the objects to which they are mixed, and they can be incorporated into a variety of products, including paints, cosmetics, foods, and resins. Below, an example of mixing the dried cellulose fibers into paint as an additive is described. In the following examples, the dried cellulose fibers and paint were mixed using the simplest method, and the effect of adding the dried cellulose fibers on the application performance of the paint was evaluated.

[0057] The method for mixing the dried cellulose fiber material and the coating material is not particularly limited, and various known methods can be used for mixing. Examples include shaking by hand, shaking in a shaking tank, stirring while applying shear with a mixer or homogenizer, and stirring with a propeller stirrer or a planetary stirrer. Homogenizers and planetary stirrers are preferred in that they allow a uniform mixture to be obtained using a simple method. Furthermore, after stirring, it is preferable to perform degassing to remove air bubbles. The degassing method is not particularly limited, and various methods can be used, such as leaving the mixture to stand, heating, vacuum degassing, and centrifugal degassing. Centrifugal degassing is preferred in that it allows for simple and efficient degassing.

[0058] The cellulose fiber of this embodiment is biodegradable and decomposes in compost, soil, and the ocean. Biodegradation in the ocean is particularly difficult due to the small number of microorganisms and the low temperature rise, so in order to increase the decomposition rate in the ocean, it is preferable that the cellulose is not hydrophobically modified. Furthermore, in consideration of the stability of the decomposition products, it is preferable not to add surfactants or additives for various applications.

[0059] The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to these examples. All operations, unless otherwise specified, were carried out in an environment of a temperature of 24°C and a relative humidity of 55% RH.

[0060] [Average Fiber Diameter] Ethanol was added to the dried cellulose fibers, and the mixture was dispersed for 30 seconds at a frequency of 20 kHz using an ultrasonic disperser (UH-150, manufactured by SMT Corporation). The cellulose fibers were then precipitated by centrifugation and resuspended in ethanol three times to replace the solvent with ethanol. The cellulose fibers were then precipitated by centrifugation and resuspended in t-butanol three times to replace the solvent with t-butanol. After solvent replacement, the mixture was freeze-dried and vapor-deposited with osmium to prepare an observation sample. This sample was observed under an electron microscope at a magnification of 1,000x, 10,000x, or 30,000x, depending on the fiber diameter. Specifically, two diagonal lines were drawn on the observed image, and a straight line was arbitrarily drawn passing through the intersection of the diagonal lines. The widths of 25 fibers intersecting with this straight line were visually measured. The same measurement was performed at a different observation location and magnified at a different location, observing a total of four locations. The average value was calculated from the widths of a total of 100 fibers, and the average fiber diameter was determined. However, if two or more fibers are found attached when visually measuring the fiber width and there is no separated area within the field of view of the image, they are considered to have been attached during the sample preparation stage, and the width of each fiber is measured as separate fibers.

[0061] [Coefficient of variation (CV) of fiber diameter] The standard deviation of the measured values ​​of 100 fibers in the measurement of the average fiber diameter is calculated using the following formula: Coefficient of variation of fiber diameter = {(standard deviation of fiber diameter) ÷ (average fiber diameter)}.

[0062] [Proportion of Nanofibers] The proportion of nanofibers (%) is calculated from the measured values ​​of 100 fibers in the average fiber diameter measurement using the following formula: Proportion of nanofibers (%) = {(number of fibers with a fiber diameter of less than 0.10 μm) ÷ 100} × 100.

[0063] [Wet Average Fiber Diameter] Optical automatic analysis was used to measure the wet average fiber diameter, using a Valmet FS5 Fiber Image Analyzer manufactured by Valmet. However, accurate measurements are not possible for fibers with diameters that are too small due to resolution limitations. Cellulose fibers with an average fiber diameter of 0.3 μm or less measured using an electron microscope are listed as reference values. Pure water and dried cellulose fibers were placed in a plastic beaker so that the solids concentration of the dried cellulose fibers was 0.005 to 0.0005% by weight, and the mixture was dispersed in a bath-type ultrasonic disperser for 1 minute. The beaker was then placed in the Valmet FS5, and measurements were performed with the various parameters set as follows: Minimum fiber length [mm]: 0 Maximum fiber length [mm]: 7.6 Minimum fiber width [μm]: 0 Maximum fiber width [μm]: 200 Fiber width calculation: 1. All fiber images per analysis: 2000 Analysis mode: 1. Normal image noise level [%]: 3 The fiber width (unit: μm) of the obtained measurement result is used as the wet average fiber diameter. However, when the wet average fiber diameter cannot be measured by optical automatic analysis within the range of 0.005 to 0.0005% by weight of the solids concentration of cellulose fiber relative to pure water, the solids concentration is increased by 0.005% by weight from 0.005% by weight, and the wet average fiber diameter is measured at each concentration of 0.010%, 0.015%, 0.020%, and 0.025% by weight.

[0064] [Coefficient of variation of wet fiber diameter] The standard deviation of wet fiber diameter was calculated from the measurement results of Valmet FS5, and the coefficient of variation of wet fiber diameter was determined using the following formula: Coefficient of variation of wet fiber diameter = {(standard deviation of wet fiber diameter) ÷ (average wet fiber diameter)}.

[0065] [Wet Average Fiber Length] From the measurement results of Valmet FS5, the length-weighted average fiber length Lc(l) (unit: mm) is used to calculate the wet average fiber length according to the following formula: Wet average fiber length (µm) = Lc(l) × 1000.

[0066] [Coefficient of Variation of Wet Fiber Length] The standard deviation of the length-weighted average fiber length was calculated from the measurement results of Valmet FS5, and the coefficient of variation of the wet fiber length was calculated according to the following formula: Coefficient of Variation of Wet Fiber Length = {(Standard deviation of Wet Fiber Length) ÷ (Average Wet Fiber Length)}.

[0067] [Percentage of Coarse Structures (%)] The percentage of coarse structures (%) is the sum of the percentages of fibers with a fiber width of 20 μm or more from the measurement results using the Valmet FS5. However, the fiber count must be 1,000 or more.

[0068] [Volatile Components] 1.0 g of dried cellulose fiber is weighed using a moisture content meter (Shimadzu Corporation, MOC63u) and measurement is performed at a set temperature of 150° C. in automatic stop mode (AUTO mode). The end point is the point at which the weight change within 30 seconds is within 0.05 wt %, and the obtained value (wt %) is the proportion of volatile components.

[0069] [Agglomeration Constant of Fiber Diameter] The agglomeration constant of fiber diameter is calculated using the following formula: Agglomeration Constant of Fiber Diameter=(Wet Average Fiber Diameter)÷(Average Fiber Diameter).

[0070] [Proportion of Additives] 1.0 g of dried cellulose fiber was weighed into a 50 mL container, 30 g of ethanol was added, and the mixture was dispersed for 1 minute using a bath-type ultrasonic disperser. After centrifugation, the supernatant was removed. 30 g of ethanol was added to the container again, and the same extraction process was performed. Next, the ethanol was replaced with n-hexane, and the same extraction process was performed twice. In addition, the n-hexane was replaced with pure water, and the same extraction process was performed twice to obtain cellulose fiber after aqueous extraction. The weight of the cellulose fiber after aqueous extraction was measured, and then the moisture content of the cellulose fiber after aqueous extraction was measured using a moisture meter (set temperature 150°C, automatic stop mode). The proportion of additives was calculated using the following formula: Proportion of additives (%) = {(weight before extraction) - (weight of cellulose fiber after aqueous extraction) x (100 - moisture content of cellulose fiber after aqueous extraction) ÷ 100 - (weight before extraction) x (proportion of volatile components) ÷ 100} x 100.

[0071] [Crystallization degree] The dried cellulose fiber material is measured using an X-ray diffractometer (RINT2200 manufactured by Rigaku), and the crystallinity (%) is calculated from the obtained intensity curve using the following equation proposed by Seagal et al.: Crystallinity (%) = {(diffraction intensity of 200 plane) - (diffraction intensity of amorphous part)} ÷ (diffraction intensity of 200 plane) × 100.

[0072] [Degree of substitution of carboxymethyl group] The degree of substitution of carboxymethyl group was measured using a nuclear magnetic resonance analyzer (AVANCE II 400, manufactured by Bruker). Ｈ - Nuclear magnetic resonance spectrum is measured under the following conditions: Measurement solvent: 11 wt% heavy caustic soda solution (prepared from heavy water and heavy caustic soda) Cellulose C1 proton appearance position: 4.13 ppm Methylene position proton appearance position: 3.37 ppm, 3.65 ppm From the spectrum, the integral ratio of the proton bonded to C1 of the cellulose skeleton and the proton at the methylene position of the carboxymethyl group is read to calculate the degree of substitution.

[0073] [Substitution Degree of Other Derivatives] In the same manner as in the confirmation of the carboxymethyl group, the substitution degree is calculated by comparing the integrated value of the appearance position of each substituent with the appearance position of the C1 proton.

[0074] [Blending with paint] A paint (water-based multi-purpose, white, manufactured by Dai Nippon Toryo Co., Ltd.) and dried cellulose fiber were placed in a kneading vessel so that the cellulose fiber concentration was 0.5% by weight, and the mixture was stirred for 5 minutes using a planetary stirrer. After that, the mixture was degassed for 5 minutes to obtain a blended sample.

[0075] [Evaluation 1 of Paint Formulation Samples] Using an applicator set to a thickness of 200 μm, a paint formulation sample is applied to a glass plate measuring 50 mm in length and 10 mm in width. After air drying for at least 2 hours, the sample is observed at five random locations using an optical microscope (magnification 100x) and evaluated for the presence of coarse structures with short sides of 10 μm or more according to the following evaluation criteria: "○": No coarse structures are present in any of the five locations. "△": Coarse structures are present in 1 to 4 of the five locations. "×": Coarse structures are present in all of the five locations.

[0076] [Evaluation 2 of Paint Formulation Samples] A paint formulation sample is applied with a finger onto a glass plate measuring 50 mm in length and 10 mm in width. The presence or absence of agglomerates (coarse structures) and spreading during application are evaluated by the sense of touch when tracing with a finger and by visual inspection according to the following evaluation criteria. (Evaluation criteria) "○": No agglomerates are detected with the finger or by visual inspection, and the coating can be easily spread. "△": Agglomerates are detected with the finger or by visual inspection in 1-2 places, but the agglomerates disperse and become less noticeable when the coating is spread. "×": Agglomerates are detected with the finger or by visual inspection in 3 or more places, and the agglomerates do not disappear even when the coating is spread.

[0077] [Marine biodegradability evaluation] The obtained dried cellulose fiber was placed in seawater according to ASTM D6691, and the amount of oxygen consumed during cellulose decomposition was measured for 28 days to evaluate the decomposition rate. At the same time, the decomposition rate of the reference substance, microcrystalline cellulose, was also evaluated in parallel. If the decomposition rate after 28 days was 90% or more compared to the reference substance, it was determined to have sufficiently rapid marine biodegradability. Evaluations were performed in duplicate for both the cellulose fiber and the reference substance, and the average value was used as the result.

[0078] [Preparation of Dried Cellulose Fibers] [Example 1] Cotton linter pulp was dissolved in a cuprammonium solution to prepare a cuprammonium cellulose solution with a cellulose concentration of 5.0 wt%, copper concentration of 1.8 wt%, and ammonia concentration of 6.5 wt%. The solution was then filtered through a sintered filter with an average pore size of 5 μm to remove impurities. The cuprammonium cellulose solution was discharged into warm water at 20°C through a spinneret with a 0.3 mm hole diameter, 1050 holes, and a hole spacing of 1.1 mm. The solution was stretched and deammoniated using a down-flow tension spinning method to produce a blue yarn. The blue yarn and the warm water were collected in a semicircular inclined trough located 20 cm below the funnel outlet while 50°C warm water was flowing through the trough, and the blue yarn and the warm water were separated by pouring the water into a plastic net. The blue yarn was thoroughly decoppered by showering with 10 wt% sulfuric acid, followed by a shower of pure water to thoroughly wash off the sulfuric acid, yielding a continuous wet cellulose fiber. The resulting continuous cellulose fibers were diluted with pure water to prepare an aqueous suspension with a cellulose concentration of 1.0 wt%, and 500 ml was placed in a mixer (Extreme Mill, MX-1200XT, manufactured by AS ONE Corporation) and processed for 5 minutes to prepare an aqueous dispersion of shortened cellulose fibers. The resulting aqueous dispersion was concentrated to a moisture content of approximately 90 wt%, then sandwiched thinly between metal plates, placed in a thermostatic dryer, and dried at 110 ° C for 3 hours to obtain a sheet-like dried product. The dried sheet was then disintegrated in a mill (High Speed ​​Mill, manufactured by Labnect) and further shortened using a pin mill (ZM300, manufactured by Retsch) at a rotation speed of 23,000 rpm and a screen diameter of 0.08 mm to obtain the desired dried cellulose fiber product.

[0079] Example 2 A dried cellulose fiber material was obtained in the same manner as in Example 1, except that the crushing conditions of the pin mill were a rotation speed of 18,000 rpm and a screen diameter of 0.5 mm.

[0080] Example 3 A dried cellulose fiber material was obtained in the same manner as in Example 1, except that the cellulose fiber was not crushed using a pin mill.

[0081] Example 4 A cuprammonium cellulose solution similar to that in Example 1 was prepared. A spinning nozzle with a hole diameter of 0.3 mm, 1,050 holes, and a hole spacing of 1.1 mm was used. The blue yarn emerging from the funnel outlet was deflected, hot water at 50°C was poured over it, and the tension of the blue yarn was reduced by adjusting the speed with a nip roller. 10 wt. % sulfuric acid was dripped onto the blue yarn running laterally through a nozzle to remove copper. The yarn was then immersed in a bath of countercurrent pure water to wash off the sulfuric acid, yielding cellulose fibers wound around a skein. The resulting skein-wound cellulose fibers were cut using a rotary cutter with multiple cutting blades arranged radially at 1 mm intervals to prepare shortened cellulose fibers. The resulting shortened cellulose fibers were sandwiched thinly between metal plates, placed in a constant-temperature dryer, and dried at 110°C for 3 hours to obtain a dried sheet. The dried sheet was disintegrated using a mill (High-Speed ​​Mill, manufactured by LabNect) to obtain a dried cellulose product.

[0082] [Example 5] A dried cellulose fiber body was obtained in the same manner as in Example 1, except that the cellulose concentration of the cuprammonium cellulose solution was 4.0 wt %, the ammonia concentration was 7.0%, and the crushing conditions of the pin mill were a rotation speed of 18,000 rpm and a screen diameter of 0.5 mm.

[0083] [Example 6] A dried cellulose fiber body was obtained in the same manner as in Example 1, except that the cellulose concentration of the cuprammonium cellulose solution was 7.0 wt %, the ammonia concentration was 9.0%, and the crushing conditions of the pin mill were a rotation speed of 18,000 rpm and a screen diameter of 0.5 mm.

[0084] [Example 7] A dried cellulose fiber body was obtained in the same manner as in Example 1, except that the cellulose concentration of the cuprammonium cellulose solution was 8.5 wt %, the ammonia concentration was 9.0%, and the crushing conditions of the pin mill were a rotation speed of 18,000 rpm and a screen diameter of 0.5 mm.

[0085] [Example 8] A wet continuous cellulose fiber prepared in the same manner as in Example 1 was immersed in sulfuric acid with a concentration of 10.0 wt % at 60°C for 30 minutes, then thoroughly washed with water and subjected to a hydrolysis treatment, and a dried cellulose fiber body was obtained in the same manner as in Example 1, except that the crushing conditions of the pin mill were a rotation speed of 18,000 rpm and a screen diameter of 0.5 mm.

[0086] [Example 9] A dried cellulose fiber body was obtained in the same manner as in Example 1, except that a surfactant (Pellicer L-30, manufactured by Asahi Kasei Finechem Co., Ltd.) was added to an aqueous dispersion of shortened cellulose fibers prepared in the same manner as in Example 1 in an amount of 100% by weight based on the cellulose fibers.

[0087] Example 10: A dried cellulose fiber was obtained in the same manner as in Example 1, except that an aqueous dispersion of shortened cellulose fibers prepared in the same manner as in Example 1 was subjected to a carboxymethylation treatment. The carboxymethylation treatment was carried out as follows: 100 mL of an aqueous dispersion with a cellulose concentration of 1.0 wt % was placed in a reaction vessel, and 8.8 g of ethanol and 1.6 g of sodium hydroxide were added, followed by stirring at 35°C for 30 minutes. Next, after draining the reagents from the reaction vessel, an ethanol aqueous solution containing sodium monochloroacetate (3.0 g of water, 9.6 g of ethanol, 0.8 g of sodium monochloroacetate) was added and stirred at 45°C for 3 hours. The resulting mixture was then immersed in an ethanol aqueous solution containing hydrochloric acid (0.6 g of hydrochloric acid, 6.0 g of water, 6.0 g of ethanol) and stirred for 1 hour. After washing once with 15 g of a 70 wt % ethanol aqueous solution and once with 15 g of a 90 wt % ethanol aqueous solution, the mixture was subjected to alcohol substitution twice with 15 g of 100 wt % ethanol. After drying, a calcium hydroxide-containing ethanol aqueous solution (0.18 g of calcium hydroxide, 10 g of water, and 10 g of ethanol) was added and stirred at 35° C. for 1 hour. The mixture was then washed once with 15 g of a 70 wt % ethanol aqueous solution and once with 15 g of a 90 wt % ethanol aqueous solution, and subjected to alcohol substitution twice with 100 wt % ethanol. The mixture was then dried to obtain a carboxymethylated dried cellulose fiber.

[0088] [Example 11] An aqueous dispersion of shortened cellulose fibers prepared in the same manner as in Example 1 was subjected to a high-pressure homogenizer (Starburst MINI, manufactured by Sugino Machine Co., Ltd.) at a pressure of 245 MPa to perform micronization twice, and then dried in a freeze dryer for 24 hours to obtain a dried cellulose fiber body.

[0089] [Comparative Example 1] With reference to Patent Document 1, a surfactant (CRS-75, manufactured by Sakamoto Pharmaceutical Industry Co., Ltd.) was added to an aqueous dispersion of ordinary cellulose microfibers (Cerish KY100G, manufactured by Daicel Miraize Co., Ltd.) so that the amount was 10% by weight based on the cellulose, and after thorough stirring, the mixture was dried at 80°C for 4 hours while stirring using a high-speed mixer (LS10, manufactured by Earth Technica Co., Ltd.), and then crushed in a pin mill at 18,000 rpm with a screen diameter of 0.5 mm to obtain a dried cellulose fiber product.

[0090] Comparative Example 2 A dried cellulose fiber body was obtained in the same manner as in Comparative Example 1, except that a softener (naphthenic process oil manufactured by Japan Sun Oil Co., Ltd.) was added in an amount of 100% by weight based on the cellulose in addition to the surfactant.

[0091] [Comparative Example 3] With reference to Patent Document 3, an aqueous dispersion of ordinary cellulose microfibers (Cerish KY100G, manufactured by Daicel Miraize Co., Ltd.) was replaced with isopropyl alcohol three times to obtain an isopropyl alcohol slurry, which was then subjected to two micronization treatments under an operating pressure of 100 MPa using a high-pressure homogenizer (NS015H, manufactured by Nia Sorobi Co., Ltd.) to concentrate the cellulose to approximately 10% by weight. The resulting mixture was then sandwiched between metal plates and dried in a constant temperature dryer at 110°C for 3 hours. The resulting dried sheet was then pulverized in a mill to obtain a dried cellulose fiber body.

[0092] [Comparative Example 4] With reference to Patent Document 4, 150 g of dimethyl sulfoxide, 16.5 g of acetic anhydride, 3.35 g of sulfuric acid, and 5.0 g of cotton linter pulp (SOUTHERN CELLULOSE, GRADE 872) were added to a 500 mL reaction vessel, and the mixture was stirred for 120 minutes at room temperature of 24.5 ° C., followed by the addition of 250 mL of pure water to stop the reaction. After neutralizing the sulfuric acid by adding a 2.0 wt% aqueous sodium hydroxide solution, the mixture was centrifuged at 15,000 rpm for 30 minutes using a centrifuge (HIMAC, CR22N) and the supernatant was removed. Furthermore, the mixture was washed with a 50 wt% aqueous ethanol solution, centrifuged under the same centrifugation conditions, and the operation of removing the supernatant was repeated three times. The resulting dispersion was stirred for 3 minutes using a mixer (AS ONE Corporation, Extreme Mill, MX-1200XT) to obtain a uniform dispersion. This was freeze-dried in a freeze dryer (Tokyo Rikakikai Co., Ltd., FDS-2000) to obtain a dried cellulose fiber material.

[0093] Comparative Example 5 A dried cellulose fiber material was obtained in the same manner as in Comparative Example 4, except that the reaction time of the cotton linter was changed to 30 minutes.

[0094] [Comparative Example 6] With reference to Patent Document 5, a cellulose cuprammonium solution with a cellulose concentration of 8.0 wt%, a copper concentration of 2.9 wt%, and an ammonia concentration of 7.0 wt% was prepared. A spinneret with an outlet orifice having a hole diameter of 0.6 mm, 2,430 holes, and a hole spacing of 1.4 mm was used for down-flow tension spinning to form a blue yarn. The blue yarn emerging from the funnel outlet was redirected and regenerated with sulfuric acid in a scouring bath in the same direction as the yarn. Thereafter, the sulfuric acid was washed off on a net as in Example 1. The cellulose fibers were shortened in a mixer to prepare shortened cellulose fibers, which were then sandwiched between metal plates, dried, and crushed in a pin mill (18,000 rpm, screen diameter 0.50 mm) to obtain dried cellulose fibers. The cellulose fibers obtained by this method had strongly bonded monomers, and using a 0.08 mm diameter screen in a pin mill caused clogging and made it impossible to crush them.

[0095] [Comparative Example 7] With reference to Patent Document 6, a hole diameter of 0.07 mm, the number of holes was 2430, and the distance between holes was 0.67 mm (hole density 180.9 holes / cm ２The cuprammonium cellulose solution was discharged into warm water at 20°C through a spinneret having an outlet hole (equivalent to 1.5 mm), and stretched and deammoniated by a flow-down tension spinning method to produce a blue yarn. The sulfuric acid was washed off on the net in the same manner as in Example 1, except that the blue yarn and spinning water were collected in a plastic net. The short cellulose fibers were prepared by fiber shortening in a mixer, and then sandwiched between metal plates and dried. The fibers were then crushed in a pin mill (18,000 rpm, screen diameter 0.50 mm) to obtain dried cellulose fibers. Note that the cellulose fibers obtained by this method had strong bonds between the monomers, and when a 0.08 mm diameter screen was used in the pin mill, fiber clogging occurred and the fibers could not be crushed. Furthermore, yarn swaying was observed in the funnel during spinning.

[0096] [Comparative Example 8] With reference to Patent Document 7, a cellulose cuprammonium solution containing 3.5 wt. %, 1.3 wt. %, 18.0 wt. %, 0.5 wt. % PEG 500,000, and 0.1 wt. % polyoxyethylene lauryl ether was prepared, and the solution was filtered through a sintered filter with an average pore size of 5 μm to remove foreign matter. Electrospinning was performed using a metal nozzle with an inner diameter of 0.41 mm, followed by decoppering with sulfuric acid to separate the cellulose fibers from the collector. The sulfuric acid, PEG, and surfactant were washed with pure water, and the cellulose fibers were shortened in a mixer in the same manner as in Example 1 to prepare shortened cellulose fibers. The fibers were then sandwiched between metal plates, dried, and crushed in a pin mill (18,000 rpm, screen diameter 0.50 mm) to obtain dried cellulose fibers.

[0097] Comparative Example 9 General cellulose nanofibers (BiNFi-sIMa, manufactured by Sugino Machine Ltd.) were dried in a freeze dryer for 24 hours and then crushed in a pin mill (18,000 rpm, screen diameter 0.50 mm) to obtain dried cellulose fibers.

[0098] Comparative Example 10 A dried cellulose fiber material was obtained in the same manner as in Comparative Example 9, except that no additional crushing with a pin mill was performed.

[0099] [Comparative Example 11] A dried cellulose fiber body was obtained in the same manner as in Example 1, except that the cellulose concentration of the cuprammonium cellulose solution was 10.0 wt %, the ammonia concentration was 6.5%, and the temperature of the hot water used during spinning was 30°C.

[0100] Comparative Example 12 A dried cellulose fiber material was obtained in the same manner as in Example 4, except that the pitch of the rotary cutter blades was set to 1.5 mm.

[0101] [Evaluation of Cellulose Fibers] SEM observation and optical automatic analysis were carried out on the dried cellulose fiber bodies of Examples 1 to 11 and Comparative Examples 1 to 12. X-ray diffraction measurements were carried out on Examples 1, 5, 6, 7, 8, Comparative Examples 1, 6, and 8. SEM images of Example 1, Comparative Example 1, Comparative Example 6, Comparative Example 7, and Comparative Example 8 are shown in Figures 1, 2, 3, 4, and 5, respectively. A summary of the evaluation results is shown in Table 1 below.

[0102]

[0103] As can be seen from Figures 1 to 5 and Table 1, the dried cellulose fibers obtained in Examples 1 to 11 were microfibers in terms of average fiber diameter, but because the proportion of nanofibers was low, they were highly stable fibers with a small fiber diameter aggregation constant, a low proportion of coarse structures, and a small coefficient of variation in fiber diameter. Furthermore, as in Examples 1 and 5 to 7, the fiber diameter could be freely controlled by adjusting the spinning conditions, and as in Examples 1 to 4, the fiber length could be freely controlled while maintaining the fiber diameter by adjusting the micronization conditions. In addition, as in Example 8, acid hydrolysis treatment can shorten the fiber length or increase the crystallinity. Furthermore, as in Examples 9 and 10, adding additives or modifying the cellulose can also produce dried cellulose fibers with few coarse structures. Furthermore, when no pulverization treatment was performed after drying, as in Example 11, the formation of coarse structures was suppressed due to the extremely low proportion of nanofibers, but the proportion of coarse structures increased compared to Example 1. The formation of coarse structures can be significantly suppressed by performing a pulverization treatment after drying.

[0104] In contrast, the fibers of Comparative Examples 6 to 8 and Comparative Examples 11 and 12 obtained using the same wet spinning method but with an existing spinning method contain a certain amount of coarse structures. In the dried cellulose fiber bodies of Comparative Examples 6 and 7, the cellulose fibers are firmly bonded to each other immediately after spinning, and even after a crushing treatment, a certain amount of coarse structures remain. In the dried cellulose fiber body of Comparative Example 8, it is impossible to determine whether the fibers are firmly bonded to each other by SEM, and only a very small amount of particulate portions with a diameter of 3 μm or more are visible within the field of view. However, evaluation by optical automatic analysis reveals the presence of a certain amount of coarse structures. In the dried cellulose fiber body of Comparative Example 11, the proportion of coarse structures is high due to the large fiber diameter. In the dried cellulose fiber body of Comparative Example 12, the long fiber length results in increased entanglement of the fibers, resulting in the formation of coarse structures. Furthermore, in fibers obtained by micronizing natural cellulose through physical treatment, as in Comparative Examples 1 to 3, a certain amount of coarse structures remains in the dried cellulose fiber body, even when a surfactant is added or dried using an organic solvent. When natural cellulose is refined to nanofibers by chemical treatment as in Comparative Example 4, the occurrence of coarse structures is suppressed, but when it is microfibers as in Comparative Example 5, a large amount of coarse structures remains in the dried product. Furthermore, even when general cellulose nanofibers are used as in Comparative Examples 9 and 10, the nanofibers aggregate and coarse structures are generated when the drying operation is performed. Even when a pulverization treatment is performed after drying, there is a limit to the reduction of coarse structures.

[0105] [Evaluation of performance when added to paint] Performance evaluation was carried out on samples in which the dried cellulose fibers obtained in Examples 1 to 11 and Comparative Examples 1 to 12 were added to paint. Optical microscope images of the paint formulations of Example 1 and Comparative Example 1 are shown in Figures 6 and 7, respectively. The evaluation results are also shown in Table 2 below.

[0106]

[0107] As can be seen from Figures 6, 7, and Table 2, when the dried cellulose fiber of Example 1 was added to a paint, a clean paint sample was obtained without coarse structures. In contrast, the fibers of Comparative Examples 6 to 8 and Comparative Examples 11 and 12, which were obtained using the same wet spinning method but an existing spinning method, contained a certain amount of coarse structures, and when added to a paint, significant agglomerates of coarse structures were formed. The presence of agglomerates significantly reduces the smoothness of the paint after application. Furthermore, even in the fibers of Comparative Examples 1 to 5 and Comparative Examples 9 and 10, which were obtained by micronizing natural cellulose through physical and chemical treatments, a certain amount of coarse structures was present, and thus agglomerates of coarse structures were formed when added to a paint.

[0108] [Evaluation of Marine Biodegradability] Marine biodegradability was evaluated using the cellulose fiber obtained in Example 1. The decomposition rate after 28 days was 93% compared to the reference substance, confirming that biodegradation in the ocean was sufficiently rapid.

[0109] The dried cellulose fiber material according to the present invention is a uniform cellulose microfiber (a fibrous cellulose aggregate) with few coarse structures, which is obtained without the addition of chemical substances or modification of cellulose, and therefore can be used for a variety of general purposes, such as an additive for paints and cosmetics, a filler for reinforcing resins and rubber, and a substrate for porous bodies such as filters. The dried cellulose fiber material according to the present invention can also be suitably used as a material for porous bodies such as adsorbents, sound-absorbing materials, and heat insulating materials, a substrate for transparent materials and optical materials for electronic products, an additive for foods, paints, and cosmetics, a substrate for packaging materials and gas barrier materials, a viscosity modifier or dispersion stabilizer for liquid products, a substrate for artificial leather, and apparel applications in which synthetic fibers and cellulose fibers are used.

Claims

1. A dried cellulose fiber body having an average fiber diameter of 0.3 μm or more and 3.0 μm or less, and having a proportion of coarse structures of 3.0% or less, as indicated by the proportion of fibers having a wet fiber diameter of 20 μm or more measured by optical automatic analysis after dispersion in water.

2. The dried cellulose fiber material according to claim 1, having a wet average fiber length of 1000 μm or less.

3. A dried cellulose fiber material according to claim 1 or 2, having an average wet fiber diameter after dispersion in water of 1.0 μm or more and less than 10.0 μm.

4. A dried cellulose fiber material according to claim 1 or 2, wherein the coefficient of variation of the fiber diameter is 1.00 or less.

5. The dried cellulose fiber material according to claim 1 or 2, wherein the aggregation constant of the fiber diameter after dispersion in water is 5.0 or less.

6. A dried cellulose fiber according to claim 1 or 2, wherein the crystalline structure of the cellulose constituting the cellulose fiber is type II.

7. The dried cellulose fiber material according to claim 1 or 2, wherein the proportion of the additive extracted into water, ethanol and hexane is less than 10% by weight.

8. A dried cellulose fiber material according to claim 1 or 2, wherein the crystallinity of the cellulose constituting the cellulose fiber is 30% or more and 90% or less.

9. The dried cellulose fiber material according to claim 1 or 2, wherein the degree of substitution of hydroxyl groups in the cellulose constituting the cellulose fiber is 0.3 or less.

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