Cellulose fibers and products using said cellulose fibers

Abstract

Provided are cellulose microfibers (fibrous cellulose) that can be used in a general-purpose manner for a variety of applications and that have a uniform fiber diameter distribution and a low proportion of both cellulose nanofibers and coarse structures, a composition containing said cellulose fibers, and a porous body composed of said cellulose fibers. The present invention relates to cellulose fibers in which the average fiber diameter in a dry state is 0.3-3.0 μm as observed using an electron microscope, the proportion of fibers having a fiber diameter of less than 0.1 μm is 5% or less, and the proportion of coarse structures as indicated by the proportion of fibers having a wet fiber diameter of 20 μm or greater measured through automatic optical analysis is 3.0% or less. The present invention also relates to a composition containing said cellulose fibers, and a porous body composed of said cellulose fibers.

Description

Technical Field

[0001] The present invention relates to cellulose fibers and products using such cellulose fibers.

Background Art

[0002] Cellulose is the most abundant carbohydrate on earth, a naturally derived and biodegradable resource, and is used in various industrial fields. Furthermore, in response to the recent formulation of SDGs (Sustainable Development Goals), it is expected as a sustainable resource with low environmental impact. As one method of utilizing cellulose, cellulose nanofibers obtained by refining cellulose have attracted attention. As its usage methods, there are various applications such as fillers for resins and rubbers, base materials for transparent materials and optical materials for electronic products, filter materials, additives for foods, paints, and cosmetics, base materials for packaging materials and gas barrier materials, viscosity adjusters and dispersion stabilizers for various liquid products, etc., and cellulose nanofibers corresponding to required performances are being studied. By combining the selection of cellulose types as raw materials, the selection of physical refining conditions, the selection of chemical treatment conditions, etc., nanofibers of cellulose materials with various shapes and characteristics can be obtained.

[0003] On the other hand, cellulose nanofibers have strong aggregation due to intermolecular forces, and not only the generation of agglomerates due to aggregation during storage and the generation of agglomerates due to strong hydrogen bonds during drying, but also the generation of agglomerates due to entanglement of fibers during stirring and kneading, etc. are likely to occur. Furthermore, problems such as the viscosity becoming too high and being difficult to handle when handling as a slurry may occur. Furthermore, strong mechanical treatment or treatment with chemicals is required for refinement, resulting in high costs. From these points, in applications where nanofibers are not necessary, they may be used in the form of microfibers with weakened refinement power.

[0004] However, if the micronization force is weakened, not all of the original cellulose raw material can be micronized, leaving unpulverized parts. If the micronization force is strengthened to reduce the unpulverized parts, some of it will turn into nanofibers, resulting in the formation of aggregates. Aggregates resulting from the aggregation of nanofibers, as well as undisintegrated portions that could not be miniaturized, are coarse structures that differ significantly from the fiber structure with a certain L / D ratio, and depending on the application, they can impair the original performance of the microfiber.

[0005] For example, in Patent Document 1 below, cellulose nanofibers are oxidized and then mixed with rubber latex to obtain a filler-rubber composite. Although the aim is to reduce coarse structures in the resulting rubber by increasing the viscosity of the latex, as described in the examples, a considerable amount of coarse structures larger than 10 μm in a dry state are present in the rubber masterbatch. This is presumed to be due to the generation of aggregates caused by the aggregation and entanglement of nanofibers.

[0006] In Patent Document 2 below, cellulose microfibers are obtained by wet grinding using a cellulose raw material with a low hemicellulose content. Although the fiber diameter is adjusted under the wet grinding conditions, the finening by grinding inevitably results in either nanofiber formation, retention of unground parts, or both, ultimately resulting in a considerable amount of coarse structures being included. The average fiber diameter is calculated as the numerical average of 50 fibers, but the presence of nanofibers that cause agglomeration, the presence of unground parts, etc., are not evaluated. To confirm the dispersibility of cellulose fibers in the resin, the presence or absence of coarse structures with a maximum diameter of 200 μm or more in the dry state is checked, but the presence or absence of coarse structures smaller than that is not evaluated.

[0007] In Patent Document 3 below, cellulose microfibers are obtained by pre-treating cellulose raw material with enzymes to decompose the amorphous region before micronization, thereby improving the homogeneity and dispersibility of the fibers. Although the fiber diameter, fiber length, and fibrillation rate are adjusted by the number of refiner treatments, even with this treatment method, it is unavoidable that either nanofiberization or the retention of unground parts, or both, will occur, and ultimately a considerable amount of coarse structures will be included.

[0008] To solve these problems, methods such as adding surfactants, removing water by solvent replacement and then drying, and chemically modifying cellulose have been reported. In Patent Document 4, similar to Patent Document 3, cellulose microfibers with a coefficient of variation of fiber diameter distribution of 1.1 or less are obtained by pre-treating the cellulose raw material with enzymes to decompose the amorphous region and then micronizing it. The maximum fiber diameter of CNF-B obtained by weakening the micronization treatment to obtain microfibers is 1.271 μm, and although no aggregates or undisintegrated parts were found among the 100 observed fibers, the minimum fiber diameter was 0.022 μm, indicating that a considerable amount of nanofiberized material was present. If stabilizers or surfactants are not added, such microfibers will develop aggregates due to nanofiber aggregation during storage and aggregates due to nanofiber entanglement during stirring, ultimately resulting in a considerable amount of coarse structures being present.

[0009] Patent Document 5 describes a composite material that uses nanofibers but lacks coarse structures larger than 1 μm. This is achieved by adding a cationic surfactant to cellulose nanofibers, removing water in the presence of a polyhydric alcohol, simultaneously hydrophobizing them with a silane coupling agent, and applying shear force within a rubber component. While this special method is necessary to achieve the filler's effect, the surfactant and silane coupling agent are not naturally derived, and their use further inhibits the biodegradability of cellulose. Moreover, it is not a versatile technology applicable to uses other than composites with rubber, and the cost is extremely high.

[0010] As a method for obtaining cellulose microfibers without mechanical or chemical treatment, methods have been reported that involve dissolving cellulose in a solvent and performing wet spinning or electrospinning. In Patent Document 6 below, cellulose microfibers are obtained by dissolving cellulose in a copper ammonia solution and performing flow tension spinning. With this method, the residue of nanofibers and unground parts is eliminated, but if the single filaments come into contact with each other before sufficient desolvation is performed in the spinning funnel, strongly bonded, coalesced fibers are formed. Furthermore, because desolvation is insufficient before copper removal with sulfuric acid, strong hydrogen bonds are formed between the fibers during copper removal, resulting in strongly bonded, coalesced fibers. These coalesced fibers are also a type of coarse structure, and depending on the application, they may impair the original performance of the microfibers. In addition, since it is necessary to give the single filaments a certain degree of self-adhesion for use in spun yarn, decoction of the solvent is performed in a sulfuric acid bath under rectified conditions. In this method, the structure is fixed without the tension generated during spinning being relaxed, so the resulting fibers have excessive orientation of cellulose molecular chains and are prone to fibrillation. When yarns that are prone to fibrillation are used as fillers, nanofibers are generated during the mixing process, leading to the formation of aggregates and clumps. In the following Patent Document 8, a nonwoven fabric made of cellulose microfibers is obtained by reducing the concentration of cellulose dissolved in a copper ammonia solution, performing flow tension spinning, and laminating the fibers on a net. However, even with this method, contact between single filaments occurs before sufficient desolvation takes place in the funnel, and strong hydrogen bonds are formed between fibers during copper removal, resulting in the generation of coalesced fibers.

[0011] In Patent Document 7 below, cellulose is dissolved in a copper ammonia 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. With this method, no unground portions remain, cellulose orientation hardly occurs, and fibrillation is unlikely, so even when used as a filler, the generation of nanofibers during mixing is unlikely. However, it is difficult to finely adjust the solidification rate in electrospinning, and the material may reach the collector before sufficient desolvation is achieved, resulting in the generation of strongly bonded, coalesced fibers. Furthermore, the particulate portion in the sheet is approximately 500 fibers / mm². 2 As shown below, a considerable amount of particulate matter exists when converted to units of weight. Furthermore, fibers obtained by electrospinning have problems such as low cellulose orientation, limiting their applications, strong aggregation during drying due to low crystallinity, and low productivity. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] Japanese Patent Publication No. 2020-55951 [Patent Document 2] Japanese Patent Publication No. 2020-125417 [Patent Document 3] Patent No. 6799565 [Patent Document 4] Patent No. 6845510 [Patent Document 5] Japanese Patent Publication No. 2021-14512 [Patent Document 6] Patent No. 5584445 [Patent Document 7] Patent No. 4871196 [Patent Document 8] Patent No. 7101879 [Overview of the Initiative] [Problems that the invention aims to solve]

[0013] In view of the aforementioned level of prior art, the problem that the present invention aims to solve is to provide cellulose microfibers (fibrous cellulose) that have a uniform fiber diameter distribution with low proportions of both cellulose nanofibers and coarse structures, and that can be used universally for various applications. [Means for solving the problem]

[0014] The inventors of this invention diligently studied and conducted numerous experiments to solve the above-mentioned problems, and as a result, discovered that the causes of coarse structures are the aggregation of nanofiberized cellulose, the retention of unground parts of the raw cellulose, and the strong adhesion between fibers. By controlling these factors to a certain level or less, they unexpectedly discovered that it is possible to obtain cellulose microfibers (fibrous cellulose aggregates) with uniform fiber diameters and fewer coarse structures without adding chemical substances or modifying the cellulose, thus completing the present invention.

[0015] In other words, the present invention is as follows: [1] Cellulose fiber having an average fiber diameter of 0.3 μm or more and 3.0 μm or less in a dry state as observed by electron microscope, a percentage of fibers with a fiber diameter of less than 0.1 μm of 5% or less, and a percentage of coarse structures, as indicated by the percentage of fibers with a wet fiber diameter of 20 μm or more as measured by automated optical analysis, of 3.0% or less. [2] The cellulose fiber according to [1] above, wherein the average wet fiber diameter is 1.0 μm or more and less than 10.0 μm. [3] The cellulose fiber according to [1] or [2] above, wherein the average wet fiber length is 3000 μm or less. [4] A cellulose fiber according to any one of [1] to [3] above, wherein the coefficient of variation of the fiber diameter is 1.00 or less. [5] A cellulose fiber according to any one of [1] to [4] above, wherein the agglomeration constant of the fiber diameter is 5.0 or less. [6] The cellulose fiber according to any one of the above [1] to [5], wherein the crystal structure of the cellulose constituting the cellulose fiber is type II. [7] The cellulose fiber according to any one of the above [1] to [6], wherein the crystallinity of the cellulose constituting the fiber is 30% or more and 90% or less. [8] A composition containing the cellulose fiber according to any one of the above [1] to [7]. [9] A porous body made of the cellulose fiber according to any one of the above [1] to [7].

Effect of the Invention

[0016] The cellulose fiber according to the present invention is a uniform cellulose microfiber (fibrous cellulose aggregate) with few coarse structures without adding chemical substances or modifying cellulose. Therefore, it can be widely used in various applications such as a filler for reinforcing resins and rubbers, a base material for porous bodies such as filters.

Brief Description of the Drawings

[0017] [Figure 1] It is a SEM image of the cellulose fiber obtained in Example 1. [Figure 2] It is a SEM image of the cellulose fiber obtained in Comparative Example 1. [Figure 3] It is a SEM image of the cellulose fiber obtained in Comparative Example 7. [Figure 4] It is a SEM image of the surface of a porous sheet prepared from the cellulose fiber of Example 1. [Figure 5] It is a SEM image of the surface of a porous sheet prepared from the cellulose fiber of Comparative Example 7. [Figure 6] It is a SEM-EDX image of the tensile fracture surface of a cellulose fiber-rubber composite in which the cellulose fiber of Example 1 is composite. [Figure 7] It is a SEM-EDX image of the tensile fracture surface of a cellulose fiber-rubber composite in which the cellulose fiber of Comparative Example 6 is composite. [Figure 8] It is a SEM image of the cellulose fiber obtained in Comparative Example 4. [Figure 9] This is an SEM image of the cellulose fiber obtained in Comparative Example 8. [Figure 10] This is an SEM image of the surface of a porous sheet prepared from cellulose fibers in Comparative Example 4. [Figure 11] These are SEM-EDX images of the tensile fracture surfaces of cellulose fiber-rubber composites, which are composites of cellulose fibers from Examples 1-3. [Modes for carrying out the invention]

[0018] Embodiments of the present invention will be described in detail below. One embodiment of the present invention is a cellulose fiber having an average fiber diameter of 0.3 μm or more and 3.0 μm or less in a dry state as observed by an electron microscope, a percentage of fibers with a fiber diameter of less than 0.1 μm of 5% or less, and a percentage of coarse structures, as indicated by the percentage of fibers with a wet fiber diameter of 20 μm or more as measured by automated optical analysis, of 3.0% or less.

[0019] [Cellulose fiber] In this specification, the term "cellulose fiber" refers to a structure (aggregate) made of fibrous cellulose, and the method of manufacturing it is not particularly limited. Examples include methods of micronizing cellulose raw material by physical force, methods of micronizing by chemical force, and methods of dissolving it in a solvent and forming it into fibers. It is also possible to combine these methods. In order to reduce cellulose nanofibers, which cause coarse structures, and the undissolved portion of the raw material, a method of dissolving the cellulose raw material in a solvent and forming it into fibers is preferred. Dissolving it in a solvent also makes it possible to remove minute foreign matter by filtration or centrifugation.

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

[0021] In this embodiment, cellulose fibers are evaluated not only in a dry state but also in a wet state. When evaluating in a dry state using an electron microscope or the like, it is difficult to distinguish between fibers that are originally aggregated and fibers that aggregate during drying. Furthermore, the results vary depending on the measurement location. Moreover, there is a limit to the number of fibers that can be measured in a dry state, and while there is some accuracy in evaluating the average value, it is difficult to evaluate the distribution. The results of evaluation in a wet state will be distinguished from the results of evaluation in a dry state by expressing the fiber diameter as "Wet fiber diameter" and the average fiber diameter as "Wet average fiber diameter". Note that since cellulose fibers swell in water, the "Wet fiber diameter" will be larger than the "fiber diameter".

[0022] There are various methods for evaluating fiber in a wet state, but microscopes, microscopes, and automated optical analysis methods are preferred because they can distinguish between fiber diameter and fiber length, and can be selected according to the fiber diameter. As will be described later, in the examples, measurements were taken using automated optical analysis methods.

[0023] In this embodiment, the average fiber diameter of the cellulose fibers is 0.3 μm or more and 3.0 μm or less. If the average fiber diameter is 0.3 μm or more, it is possible to suppress the aggregation of fibers due to intermolecular forces during storage, aggregation due to entanglement during stirring, and aggregation due to hydrogen bonding during drying. 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 number of fibers per unit weight increases, which has effects such as an increased effect as a filler and a larger specific surface area when it is made into a porous material. 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 1.0 μm or less.

[0024] In this embodiment, the "percentage of fibers with a fiber diameter of less than 0.1 μm" in the cellulose fibers is 5% or less. By limiting the amount of nanofibers with a fiber diameter of less than 0.1 μm to 5% or less, it is possible to suppress not only the aggregation of fibers during storage, aggregation due to entanglement during stirring, aggregation due to hydrogen bonding during drying, but also deterioration of handling properties due to increased viscosity. The "percentage of fibers with a fiber diameter of less than 0.1 μm" is preferably 4% or less, more preferably 3% or less, even more preferably 2% or less, particularly preferably 1% or less, and most preferably 0%.

[0025] In this specification, the term "cellulose nanofiber" refers to cellulose fibers with a fiber diameter of less than 0.1 μm. The term "cellulose microfiber" refers to cellulose fibers with a fiber diameter of 0.1 μm or more and less than 9.7 μm.

[0026] In this specification, the term "coarse structure" 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, as will be described later. Coarse structures include not only cellulose clumps such as aggregates of structures smaller than 20.0 μm, such as aggregates of nanofibers due to aggregation or entanglement of fibers, or coalesced fibers where fibers are strongly bonded together, as well as structures originally larger than 20.0 μm, such as residual unground parts of cellulose raw material or thick fibers, but also foreign matter that was contained in the cellulose raw material. The reason for using measurement results in a wet state is that evaluation in a dry state cannot distinguish whether aggregation occurred originally or during drying. Furthermore, measurement in a wet state makes it easier to increase the number of measurements and enables accurate evaluation of the distribution. Depending on the preparation method and crystal structure of the cellulose fibers, reducing the number of structures larger than 20.0 μm in a wet state can reduce the number of structures larger than approximately 10.0 μm in a dry state.

[0027] The "proportion of coarse structures" in the cellulose fibers of this embodiment, as described later, is 3.0% or less. If the coarse structures are 3.0% or less, effects such as 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, and even more preferably 0.5% 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 coefficient of variation of fiber diameter of the cellulose fibers in this embodiment is preferably 1.00 or less. Fibers with a uniform fiber diameter and a coefficient of variation of fiber diameter of 1.00 or less have less functional degradation due to the cutting of thin fibers and less defects due to thick fibers, making it easier to achieve desired performance in various applications, and also allowing for uniform fiber length when shortened by physical impact. The coefficient of variation of fiber diameter is preferably 0.70 or less, more preferably 0.50 or less, even more preferably 0.40 or less, particularly preferably 0.30 or less, and most preferably 0.20 or less. A smaller coefficient of variation of fiber diameter is preferable, but from the viewpoint of production efficiency, 0.05 or more is preferable.

[0029] In this embodiment, the "wet average fiber diameter" of the cellulose fibers, as described later, 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 the aggregation of fibers during storage, aggregation due to entanglement during stirring, and aggregation due to hydrogen bonding during drying. The "wet average fiber diameter" is preferably 1.3 μm or more, and particularly preferably 1.7 μm or more. On the other hand, if the "wet average fiber diameter" is 10.0 μm or less, the number of fibers per unit weight increases, which has effects such as an increased effect as a filler and an increased specific surface area when it is made into a porous body. 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.

[0030] In this specification, the term "aggregation constant" is an index representing the degree of aggregation of cellulose fibers, and is defined by the following formula. Aggregation constant = (Wet average fiber diameter ÷ Average fiber diameter) Cellulose fibers, which contain many nanofibers with a fiber diameter of less than 0.1 μm, may appear as dispersed fibers in localized observations such as those performed by SEM, but in reality, some fibers may be unground or form a network structure through hydrogen bonding. Such cellulose fibers tend to have a larger wet fiber diameter. Similarly, cellulose fibers that appear as overlapping independent fibers in SEM, but actually contain many strongly bonded, homogenized fibers, also tend to have a larger wet fiber diameter.

[0031] In this embodiment, the "aggregation constant" of the cellulose fiber diameter is preferably 5.0 or less. By setting the aggregation constant to 5.0 or less, it is possible to expect effects such as reduction of defects when used as a filler, reduction of defects when used as a substrate for porous materials, and improvement of functionality by increasing the number of fibers. The "aggregation constant" is more preferably 4.0 or less, even more preferably 3.0 or less, and particularly preferably 2.5 or less. A smaller "aggregation constant" is preferable, but from the viewpoint of production efficiency, it is preferably 1.0 or more, and more preferably 1.5 or more.

[0032] In this embodiment, the "coefficient of variation of wet fiber diameter" of the cellulose fiber is preferably 1.00 or less. Fibers with a uniform fiber diameter and a coefficient of variation of wet fiber diameter of 1.00 or less have less functional degradation due to the cutting of thin fibers and less defects due to thick fibers, making it easier to achieve the desired performance in various applications. In addition, the fiber length can be made uniform when shortened 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, 0.20 or more is preferable.

[0033] The "crystal structure (crystal form)" of the cellulose fibers in this embodiment is not particularly limited, and various types of cellulose, including Type I, Type II, Type III, and Type IV, can be used. Furthermore, the "degree of crystallinity" of the cellulose fibers according to the present invention is not particularly limited, and cellulose fibers of any degree of crystallinity can be used. When adjusting the fiber length in a wet state, a Type II crystal structure is preferred because it allows for easy and uniform cutting with minimal force.

[0034] The "crystallinity" of the cellulose fibers in this embodiment is preferably 30% to 90%. If the crystallinity is above a certain level, aggregation during drying can be reduced and the strength of the cellulose fibers can be increased, so the crystallinity is preferably 30% or higher, more preferably 40% or higher, and particularly preferably 50% or higher. On the other hand, if the crystallinity is below a certain level, the dispersibility when used as a filler is improved, and when used as a substrate for a porous body, the fibers bond at the intersections and the strength of the substrate is increased, so the crystallinity is preferably 90% or lower, more preferably 80% or lower, and even more preferably 70% or lower. Several methods have been proposed for calculating the crystallinity of cellulose, but in this embodiment, it refers to the crystallinity calculated by the Segal method.

[0035] The cross-sectional shape of the cellulose fibers in this embodiment is not particularly limited, and fibers with various cross-sectional shapes such as round, irregular, or amorphous can be used, and the surface may be fibrillated. A round cross-section is preferred due to its reinforcing effect when used as a filler and its repair effect when a porous material is used as a filter. Furthermore, when using cellulose fibers as a filler, it is preferable that the surface be fibrillated in order to suppress fiber shedding.

[0036] The "fiber length" and "wet fiber length" of the cellulose fibers in this embodiment can be adjusted to any length. When using a method of dissolving cellulose in a solvent and forming it into fibers, it is possible to obtain very long fibers, or to cut the obtained long fibers to obtain short fibers. When using the short fibers, the fiber length and wet fiber length can be selected according to the application. In this invention, "average fiber length" and "wet average fiber length" represent the length-weighted average fiber length.

[0037] When the cellulose fibers of this embodiment are used as filler, the "wet average fiber length" is preferably 20 μm or more and 3000 μm or less. If the wet average fiber length is above a certain level, the fibers are less likely to come loose even when stress is applied, and the reinforcing effect is enhanced, so it is more preferably 30 μm or more, even more preferably 40 μm or more, and particularly preferably 50 μm or more. On the other hand, if the wet average fiber length is below a certain level, the reinforcing effect is enhanced by increasing the number of fibers, and the generation of coarse structures due to fiber entanglement during kneading can also be suppressed, so it is more preferably 1000 μm or less, even more preferably 800 μm or less, even more preferably 600 μm or less, and particularly preferably 400 μm or less.

[0038] When the cellulose fibers of this embodiment are used as a substrate for a porous material, the "wet average fiber length" is preferably 50 μm or more and 3000 μm or less. If the wet average fiber length is above a certain level, the entanglement of the fibers is maintained even if drying shrinkage occurs during drying, thus maintaining the porous structure, so it is more preferably 100 μm or more, and even more preferably 200 μm or more. On the other hand, if the wet average fiber length is below a certain level, the entanglement of the fibers can be suppressed, so it is more preferably 2000 μm or less, even more preferably 1000 μm or less, and particularly preferably 800 μm or less.

[0039] The coefficient of variation of the wet fiber length of the cellulose fibers in this embodiment is preferably 1.00 or less. Fibers with a uniform fiber length and a coefficient of variation of wet fiber length of 1.00 or less can suppress the generation of aggregates due to fiber entanglement. As a result, they can exhibit the desired performance when used in various applications. The coefficient of variation of the wet fiber length is more preferably 0.90 or less, even more preferably 0.80 or less, particularly preferably 0.70 or less, and most preferably 0.60 or less. A smaller coefficient of variation of wet fiber length is preferable, but from the viewpoint of production efficiency, 0.05 or more is preferable.

[0040] The cellulose fibers of this embodiment can have both fiber diameter and fiber length made uniform, and by making both uniform, the effects of using the fibers can be efficiently demonstrated in various applications. Preferably, both the "coefficient of variation of wet fiber diameter" and the "coefficient of variation of wet fiber length" are 1.00 or less. Preferably, both are 0.90 or less, more preferably both are 0.80 or less, even more preferably both are 0.70 or less, particularly preferably both are 0.60 or less, and particularly preferably both are 0.50 or less. While it is preferable for both the "coefficient of variation of wet fiber length" and the "coefficient of variation of wet fiber length" to be smaller, from the viewpoint of production efficiency, 0.05 or more is preferable.

[0041] The cellulose fibers of this embodiment can also be converted into cellulose derivatives by reacting some of their hydroxyl groups, and any substituents can be introduced depending on the required function. In order to maintain the biodegradability of cellulose, substituents that do not cause hydrophobicity of cellulose are preferred, and the degree of substitution is preferably 0.30 or less, more preferably 0.20 or less, even more preferably 0.10 or less, and most preferably 0.05 or less. In order to maintain the biodegradation rate of cellulose and to have a good environmental impact after decomposition, it is preferable to use cellulose as is without derivatization.

[0042] The cellulose fibers of this embodiment can be used as an aqueous solution dispersed in water as needed, and surfactants, inorganic salts, water-soluble polymers, etc. may be added, or other liquids compatible with water may be added if necessary, or they may be dispersed in a non-aqueous solvent. Naturally, they may be mixed with other fibers, particles, fillers, and other solid components. When surfactants are added, anionic, amphoteric, or nonionic surfactants are preferred in terms of improving the biodegradability of cellulose and the environmental impact after decomposition, and the amount used is preferably 10% by weight or less. More preferably 3% by weight or less, even more preferably 1% by weight or less, and particularly preferably 0.1% by weight or less, and it is most preferable not to add any surfactants.

[0043] The cellulose fibers of this embodiment can be used in a dry solid state as needed. To suppress aggregation during drying, freeze-drying, solvent-substituted drying, solvent-substituted freeze-drying, supercritical drying, etc., may be performed. Surfactants, inorganic salts, oils, etc. may be added, or cellulose derivatives may be formed by utilizing the hydroxyl groups of cellulose.

[0044] [Method for manufacturing cellulose fibers] The following describes an example of a method for producing cellulose fibers according to the present invention. This embodiment is not limited to this production method. The cellulose raw material is not particularly limited, and a variety of 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, and recycled cellulose. It is also possible to use a mixture of several of these. Among these, cotton and cotton linter pulp are preferred due to their high purity, and cotton linter is particularly preferred.

[0045] The solvent used to dissolve cellulose is not particularly limited, and various known solvents can be selected. Examples include copper ammonia solution, viscose solution, acids or alkalis of specific concentrations, aqueous solutions of inorganic salts such as zinc chloride, N-methylmorpholine N-oxide, and various ionic liquids. Among these, copper ammonia is preferred as a solvent because it allows for adjustment of the solidification rate, facilitates thinning of fiber diameter, and allows for adjustment of fibrillation.

[0046] The method for dissolving cellulose in a solvent and forming it into fibers is not particularly limited, and various spinning methods can be used. For example, there is flow tension spinning, in which the solution is extruded from a spinning nozzle with many holes and flowed into a funnel together with a coagulation solution for solvent removal and coagulation; air gap spinning, in which the cellulose is extruded into the air and stretched before coagulation with a coagulation solution; dry spinning, in which stretching and solvent removal are performed in the air; melt blow spinning, in which stretching and coagulation are performed with a high-speed airflow; and electrospinning, in which a charge is applied to the solution and finer and aggregates through electrical repulsion. Among these, flow tension spinning, air gap spinning, and melt blow spinning, which allow control of the orientation of cellulose, are preferred, and flow tension spinning is particularly preferred because it allows for uniform fiber diameter, the fibers are less likely to break even when the fiber diameter is thin, and strong adhesion between fibers can be suppressed.

[0047] The following explanation uses the example of dissolving cellulose in a copper ammonia solution and performing flow-tension spinning. The concentration at which cellulose is dissolved in the copper ammonia solution can be arbitrarily selected. Depending on the degree of polymerization of the cellulose being dissolved, 1 to 20% by weight is preferred. Furthermore, after dissolving the cellulose, it is preferable to remove foreign matter and undissolved material by filter filtration or centrifugation. If the concentration is above a certain level, the viscosity of the raw solution will increase, suppressing thread breakage in the funnel, resulting in the effect of uniform stretching and making it less likely for the single filaments to come into contact with other fibers and bond strongly before sufficient desolvation is achieved, resulting in a uniform fiber diameter of the resulting fibers. The strength of the single filaments of the resulting fibers is also improved. The concentration of cellulose dissolved in the copper ammonia solution is more preferably 2% by weight or more, even more preferably 3% by weight or more, and particularly preferably 4% by weight or more. Furthermore, if the concentration is below a certain level, it becomes easier to stretch, allowing for a finer fiber diameter, and foreign matter can be easily removed by filtering or centrifuging the raw solution. The concentration of cellulose dissolved in the copper ammonia 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. Furthermore, the ammonia concentration is preferably 10% by weight or less, more preferably 9% by weight or less, and even more preferably 8% by weight or less. Within this range, uneven coagulation is less likely to occur, and strong bonding between fibers can be suppressed.

[0048] The nozzle for discharging the dissolving solution can be of any shape. The number of holes is preferably 10 to 2000. If the number of holes is 2000 or less, the difference in solidification between the inside and outside can be kept within a certain range, resulting in uniform fiber diameters. Furthermore, by keeping the flow rate of the solidifying solution below a certain level, the flow within the funnel can be straightened, preventing fibers from contacting each other before sufficient desolvation is achieved. The number of holes is more preferably 1500 or less, even more preferably 1000 or less, and particularly preferably 500 or less. From a productivity standpoint, the number of holes is more preferably 50 or more. Also, the hole diameter is preferably 0.05 to 0.50 mm. If the hole diameter is 0.05 mm or more, productivity is high, sufficient stretching can be achieved, and the strength of the cellulose fibers can be increased. Furthermore, the discharge speed of the dissolving solution can be kept below a certain level, suppressing the dissolving solution from sloshing around in the solidifying solution, and preventing fibers from contacting each other before sufficient desolvation is achieved. The hole diameter is more preferably 0.08 mm or larger, and even more preferably 0.10 mm or larger. On the other hand, if the hole diameter is 0.50 mm or smaller, the solution extruded from the spinneret can maintain a certain distance from each other, preventing the fibers from coming into contact with each other before sufficient desolvation is performed, and also preventing excessive stretching, making it less likely for the cellulose fibers to fibrillate, and suppressing the generation of thin fibers when shortened by physical impact or when kneading. The hole diameter is more preferably 0.40 mm or smaller, and even more preferably 0.30 mm or smaller. Furthermore, the distance between adjacent holes from end to end is preferably 0.60 to 2.00 mm. If the distance between holes is 0.60 mm or larger, it is possible to prevent the fibers from coming into contact with each other even if the dissolving solution is shaken somewhat in the coagulating solution. Furthermore, because the spindle has holes arranged in concentric circles or in multiple rows, the dissolving solution extruded from the outermost holes comes into contact with the fresh coagulating solution, but the dissolving solution extruded from the inner holes may result in uneven coagulation. If the distance between holes is 0.60 mm or more, the coagulating solution diffuses more easily into the inner layer, which can suppress uneven coagulation of the dissolving solution extruded from the inner holes, and as a result, it is possible to prevent the fibers from coming into contact with each other before sufficient desolvation has occurred.The distance between holes is more preferably 0.80 mm or more, and even more preferably 1.0 mm or more.

[0049] Any type of liquid capable of ammonia removal can be used as the coagulation solution, but from the standpoint of economy and safety, it is preferable to use water with adjusted temperature. In order to obtain cellulose fibers with a fine average fiber diameter, it is necessary to lower the spinning temperature and perform drawing, but the coagulation of the dissolved solution extruded from the pores of the inner layer may be insufficient. If copper removal is performed in this state, 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 dissolved solution extruded from the pores of the inner layer at a higher spinning temperature. The method for removing copper after forming fibers by removing ammonia in a funnel can be arbitrarily selected. For example, methods include adding acid to the funnel, discharging the material into an acid bath after it exits the funnel, redirecting the blue thread after it exits the funnel and immersing it in an acid bath, dropping acid onto the blue thread, showering it with acid after receiving it on a net, and adding acid in a batch after receiving it in a tank. By performing copper removal under low tension on the thread, the cellulose fibers are less likely to fibrillate. In addition, the acid between the fibers is more easily renewed, improving the copper removal efficiency and allowing for the production of cellulose fibers with less residual copper and sulfuric acid. In the cellulose fiber production method of this embodiment, a method was used in which the tension was relieved by receiving the material on a net after it exits the funnel and then showering it with sulfuric acid, and a method was used in which tension increase was suppressed by showering the redirected blue thread with sulfuric acid after it exits the funnel. On the other hand, when performing ammonia removal using methods in which the cellulose is not sufficiently stretched, such as electrospinning, the strength of the resulting cellulose fibers can be increased by performing copper removal under a certain amount of tension.

[0050] After removing copper using acid, the method for removing the acid can be arbitrarily selected. When removing the acid, since the fibers are fine, the liquid between the fibers is not easily renewed, so a method that facilitates liquid renewal is preferred. In the cellulose fiber manufacturing method of this embodiment, sulfuric acid was removed by repeatedly washing with hot water. Cellulose fibers should preferably have low levels of residual copper and sulfuric acid. High levels of these can lead to fiber aggregation during drying, decreased strength during storage, and a reduced biodegradation rate.

[0051] When drying cellulose fibers, the method can be arbitrarily selected. For example, heat drying, vacuum drying, air drying, freeze drying, solvent displacement drying, supercritical drying, or a combination thereof may be used. An oiling agent may be applied during drying to suppress aggregation.

[0052] The fiber diameter of cellulose fibers can be arbitrarily adjusted by factors such as the concentration of dissolved cellulose, the diameter of the nozzle that dispenses the solution, the funnel shape, and the stretching ratio determined by the combination of the temperature, composition, and flow rate of the coagulated solution. Various methods can be used to adjust the fiber length of cellulose fibers, depending on the required fiber length. These include cutting the fibers with a blade and shortening the fibers by applying physical impact.

[0053] Examples of methods for cutting cellulose fibers with a blade include rotary cutters, guillotine cutters, and cutter mills. The cellulose fibers can be cut in a dry state, a wet state, or suspended in water or an organic solvent. Prior chemical or heat treatments may also be performed. While cutting with a blade makes it easier to maintain a consistent fiber length, there is a limit to how short the fibers can be.

[0054] Examples of methods for shortening fibers by physical impact include mixers, homogenizers, ball mills, beaters, disc refiners, grinders, and high-pressure homogenizers. While methods for shortening fibers by physical impact can shorten fiber length, they tend to result in a uniform distribution of fiber length in the resulting fibers. In particular, when natural cellulose is refined by physical force, a distribution of fiber diameter occurs, resulting in a mixture of thin, easily cut parts and thick, difficult-to-cut parts, which easily leads to a distribution of fiber length. On the other hand, since the cellulose of this invention has a uniform fiber diameter, the spread of fiber length distribution can be suppressed to some extent. Furthermore, when shortening fibers by physical impact, instead of shortening long fibers directly, cutting them to a certain length beforehand with a blade and then applying physical impact can further suppress the spread of fiber length distribution. In addition, after shortening, filtration or classification can be performed to remove fibers that have become too short or fibers that remain too long. When shortening fibers by physical impact, applying too much impact can cause fibrillation or fracture of the fibers, resulting in a wider distribution of fiber diameter. In other words, the purpose of applying physical impact to fibers is to shorten the fiber length and to detach attached fibers that have weakly bonded together, not to reduce the fiber diameter. If it is desired to shorten the fiber length very short, it is preferable to combine this with other methods, such as suppressing cellulose orientation or lowering the degree of polymerization of cellulose, to suppress fiber fibrillation as needed.

[0055] [Uses of cellulose fiber] The cellulose fibers of this embodiment can be used as a filler added to resins and rubbers, as well as a porous material that can be used as a filter, adsorbent, sound-absorbing material, or heat-insulating material, as well as a coating agent, a base material for artificial leather, and an anti-settling agent for liquid products. Because the cellulose fibers have low levels of both nanofibers and coarse structures, it is possible to obtain a uniformly dispersed filler or a uniform porous material without adding surfactants or hydrophobizing the cellulose.

[0056] The following describes an example of using the cellulose fibers of this embodiment as a porous material. Of course, the present invention is not limited to these examples. When using the cellulose fibers of this embodiment to form a porous material, the cellulose fibers may be used alone, mixed with other cellulose fibers or other material fibers, or laminated with another porous material. The following describes an example of forming a porous material using cellulose fibers alone. When forming a porous material using cellulose fibers, no surfactants or pore size retainers were added, and the cellulose was not hydrophobicized. The sheet was formed by the simplest method of heat drying, and the effect of the cellulose fiber shape on the formation of the porous material was evaluated.

[0057] The method for forming a porous body from cellulose fibers is not particularly limited, and various known methods can be used. Examples include the airlaid method, spunlace method, needle punching method, papermaking method, chemical bonding method, and freeze-drying method. The papermaking method is preferred because it is a simple method that can produce a homogeneous porous body. When suspending cellulose fibers during papermaking, it is preferable that the liquid used is one that does not cause cellulose to aggregate. Water is preferred from the viewpoint of dispersibility, but from the viewpoint of drying efficiency and suppression of aggregation during drying, alcohols such as methanol, ethanol, isopropyl alcohol, and t-butanol, ketones such as acetone and methyl ethyl ketone, and ethers such as diethyl ether are also preferred. Of course, these liquids can also be mixed and used in any ratio.

[0058] The method for drying after papermaking is not particularly limited, but when the crystalline structure of the cellulose fibers is type II, it is preferable to dry at a temperature of 200°C or lower, more preferably 190°C or lower, and even more preferably 180°C or lower. In the following examples, the obtained cellulose fiber sheets were used to evaluate their performance as porous materials.

[0059] The following describes an example of using the cellulose fibers of this embodiment as a filler. Of course, this embodiment is not limited to these examples. When using the cellulose fibers of this embodiment as a filler, the material to be mixed with is not particularly limited and can be used as a filler for various resins and rubbers. The following describes an example in which a masterbatch is prepared by mixing natural rubber latex and cellulose fibers, kneading, and then vulcanization and hot-press molding are performed to obtain a composite of cellulose fibers and rubber. In the following examples, when compounding cellulose fibers and rubber, solvent substitution of the cellulose fibers, freeze-drying, addition of surfactants or polyhydric alcohols, hydrophobization of cellulose, and addition of other fillers were not performed, and the compounding was carried out using a simple method to evaluate the effect of the shape of the cellulose fibers on the compounding.

[0060] The cellulose fibers and natural rubber latex can be mixed by adding the cellulose fibers in a dry state or in a slurry state suspended in water or an organic solvent. Adding them in a slurry state is preferable for more uniform dispersion of the cellulose fibers. It is preferable to use a rotation-and-revolution type mixer for mixing the cellulose fibers and natural rubber latex. The resulting aqueous solution of natural rubber latex and cellulose fibers is dried to prepare a masterbatch.

[0061] The cellulose concentration in the masterbatch is preferably 1.0 to 20.0% by weight or more. Because cellulose is highly dispersible and does not easily form a network structure in the presence of water, if the cellulose concentration is low, cellulose fibers will settle during drying, resulting in poor uniformity of the resulting masterbatch. The lower limit of the cellulose concentration in the masterbatch is more preferably 1.5% by weight or more, and even more preferably 2.0% by weight or more. Known methods can be used to knead the obtained masterbatch. Examples include Banbury mixers and oven rolls. After kneading the masterbatch, stearic acid, zinc oxide, sulfur, and a vulcanization accelerator are added and kneading is performed further. Carbon black may be added as needed. Subsequently, vulcanization and molding are performed by hot pressing and cold pressing to obtain a composite of cellulose fibers and rubber. If the crystalline structure of the cellulose fibers is type II, it is preferable to perform the series of operations at a temperature of 200°C or lower, more preferably 190°C or lower, and even more preferably 180°C or lower. The obtained composite is punched out into a dumbbell shape and the composite is evaluated.

[0062] The cellulose fibers of this embodiment are biodegradable, decomposing in compost, soil, and ocean. Biodegradation in ocean is particularly difficult due to the low number of microorganisms and low temperatures; therefore, to accelerate the decomposition rate in ocean, it is preferable that the cellulose is not hydrophobically modified. Furthermore, considering the safety of the decomposed products, it is preferable not to add surfactants or additives depending on the application. [Examples]

[0063] 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. Furthermore, all operations not specifically described were performed under conditions of 23°C and 55% RH relative humidity. First, we will explain the various measurement methods used in the examples and comparative examples.

[0064] [Average fiber diameter] The solid content concentration of cellulose fibers in pure water is adjusted to 0.10-0.01% by weight. The cellulose fibers are then settled by centrifugation and resuspended in ethanol three times, replacing the solvent with ethanol. The cellulose fibers are then settled again by centrifugation and resuspended in t-butanol three times, replacing the solvent with t-butanol. After solvent replacement, the sample is freeze-dried and coated with osmium to prepare the observation sample. This sample is observed using an electron microscope at a magnification of 1000x, 10000x, or 30000x depending on the fiber diameter. Specifically, two diagonal lines are drawn on the observation image, and one straight line is drawn arbitrarily passing through the intersection of the diagonals. The width of 25 fibers that intersect this straight line is measured visually. The same measurement is performed by changing the observation location and magnifying another location, observing a total of four locations, and the average value is calculated from the width of a total of 100 fibers to obtain the average fiber diameter. However, if two or more fibers are attached when visually measuring the width of a fiber, and there is no separation within the field of view of the image, they should be considered to have been attached during the sample preparation stage, and the width of each fiber should be measured separately.

[0065] [Coefficient of variation (CV) of fiber diameter] The standard deviation of the 100 measurements taken to measure the average fiber diameter is calculated using the following formula: Coefficient of variation of fiber diameter = {(standard deviation of fiber diameter) ÷ (average fiber diameter)} It is calculated by [method].

[0066] [Percentage of nanofibers] From the measurements of 100 fibers taken by measuring the average fiber diameter, the following formula is used: Nanofiber percentage (%) = {(Number of fibers with a diameter of less than 0.10 μm) ÷ 100} × 100 The percentage of nanofibers is determined by this method.

[0067] [Wet average fiber diameter] For measuring the average wet fiber diameter, an automated optical analysis system is employed, using the Valmet FS5 Fiber Image Analyzer from Valmet. However, due to resolution limitations, accurate measurements cannot be obtained for fibers that are too thin. Cellulose fibers with an average fiber diameter of 0.3 μm or less, as measured by electron microscopy, are listed as reference values. The solid content concentration of cellulose fibers in pure water is automatically adjusted to a range of 0.005 to 0.0005% by weight using "Valmet FS5". 300cc of this mixture is placed in a plastic beaker, and the beaker is dispersed in a bath-type ultrasonic disperser for 1 minute. After that, it is placed in the Valmet FS5, and the various parameters are set as follows for measurement. Low fiber length [mm]: 0 Maximum fiber length [mm]: 7.6 Minimum fiber width [μm]: 0 Maximum fiber width [μm]: 20 Fiber width calculation:1.All fibers Images per analysis: 2000 Analysis mode: 1.normal Image noise level [%]: 3 The obtained measurement result, Fiber width (in μm), is adopted as the average wet fiber diameter. However, if the average wet fiber diameter cannot be measured by automated optical analysis within the range of 0.005 to 0.0005 wt% of solid content of cellulose fibers relative to pure water, the average wet fiber diameter will be measured by increasing the solid content concentration from 0.005 wt% in increments of 0.005 wt% to 0.010 wt%, 0.015 wt%, 0.020 wt%, and 0.025 wt%. If the concentration is too high to measure, an automatic adjustment will be performed, and the value that remains constant regardless of the concentration will be taken as the average wet fiber diameter.

[0068] [Coefficient of variation of wet fiber diameter] From the Valmet FS5 measurement results, the standard deviation of the wet fiber diameter is calculated using the following formula: Coefficient of variation of wet fiber diameter = {(Standard deviation of wet fiber diameter) ÷ (Average wet fiber diameter)} The coefficient of variation of the wet fiber diameter is determined by this method.

[0069] [Wet average fiber length] From the measurement results of Valmet FS5, using the length-weighted average fiber length Lc(l) (unit: mm), the following formula is used: Wet average fiber length (μm)=Lc(l)×1000 The average wet fiber length is calculated using this method. However, if the average wet fiber length cannot be measured by automated optical analysis within the range of 0.005 to 0.0005 wt% of solid content of cellulose fibers relative to pure water, the average wet fiber length will be measured by increasing the solid content concentration from 0.005 wt% in increments of 0.005 wt% to 0.010 wt%, 0.015 wt%, 0.020 wt%, and 0.025 wt%. If the concentration is too high to measure, an automatic adjustment will be performed, and the value that remains constant regardless of the concentration will be taken as the average wet fiber length.

[0070] [Coefficient of variation of wet fiber length] From the measurement results of Valmet FS5, the standard deviation of the length-weighted average fiber length is calculated using the following formula: Coefficient of variation of wet fiber length = {(Standard deviation of wet fiber length) ÷ (Average wet fiber length)} The coefficient of variation of the wet fiber length is calculated using this method.

[0071] [Percentage of bulky structures (%)] Based on the measurement results from Valmet FS5, using the number of measured fibers (in units of fibers), the following formula is used: Percentage of coarse structures (%) = {(Number of fibers with a wet fiber diameter of 20 μm or more) ÷ (Fiber count)} × 100 The percentage of bulky structures is calculated based on this. However, the number of items measured must be 1000 or more.

[0072] [Aggregation constant of fiber diameter] The following formula: The aggregation constant of fiber diameter = (Wet average fiber diameter) ÷ (Average fiber diameter) The aggregation constant of the fiber diameter is calculated using this method.

[0073] [Crystallization] The dried cellulose fibers obtained by measuring the average fiber diameter were measured using an X-ray diffractometer (Rigaku RINT2200), and the following equation proposed by Seagal et al. was derived from the resulting intensity curve: Crystallinity (%) = {(diffraction intensity of 200 planes) - (diffraction intensity of amorphous region)} ÷ (diffraction intensity of 200 planes) × 100 The degree of crystallinity (%) is calculated using this method.

[0074] [Preparation of porous materials] Prepare an aqueous suspension with a cellulose fiber concentration of 0.10 wt% relative to pure water. Place a cellulose filter paper in a Buchner funnel and reduce the pressure to 0.01 MPa while increasing the cellulose content to 40 g / m². 2 The suspension is filtered to remove water, and the resulting laminated sheet of cellulose fibers and filter paper is dried at 110°C for 1 hour. After drying, the cellulose fiber layer is peeled off from the filter paper to obtain a cellulose fiber sheet.

[0075] [Seat appearance evaluation] A portion of the obtained cellulose fiber sheet is cut out and coated with osmium to create an observation sample. This sample is then observed using an electron microscope at 200x magnification. Three locations are magnified using an electron microscope, and the porous structure of the surface is evaluated according to the following criteria to determine if it has been damaged. "×": The holes are crushed in almost all places. "△": Some holes are crushed. "〇": Holes are maintained in almost all locations.

[0076] Furthermore, the following criteria will be used to evaluate whether there are any coarse structures larger than 10 μm in a dry state on the surface. "×": All three images contain coarse structures. "△": One or two out of the three images contain a large structure. "〇": No large structures are present in any of the three images.

[0077] [Measurement of air permeability resistance] The obtained cellulose fiber sheets are measured for air permeability resistance per 10 μm of film thickness (unit: seconds / 100 cc) in accordance with JIS-P8117. Sheets that do not allow air to pass through are marked as "unmeasurable".

[0078] [Preparation of Cellulose Fiber-Rubber Composites] A water suspension is prepared with a cellulose fiber concentration of 5.0% by weight relative to pure water. This suspension is then added to natural rubber latex with a solid content of 60.0% by weight, in a weight ratio of natural rubber to cellulose fiber of 20:1. The resulting mixture is stirred in a rotary-orbital agitator, and the resulting wet masterbatch is spread thinly on a Teflon® tray. It is dried at 50°C for 48 hours, and then dried in a vacuum dryer for 3 hours to obtain the masterbatch. The obtained masterbatch is pre-mixed using a laboplast mill at 50°C and 20 rpm for 5 minutes. Then, 0.5 g of stearic acid, 6.0 g of zinc oxide, 3.5 g of sulfur, and 0.7 g of vulcanization accelerator are added per 100 g of masterbatch, and the mixture is further kneaded at 50°C and 40 rpm for 10 minutes to obtain a mass-like composite. The obtained composite is then hot-pressed using a heated hydraulic press at 150°C and 30 MPa for 8 minutes, and then cool-pressed at 20°C and 10 MPa for 5 minutes to obtain a sheet-shaped cellulose fiber-rubber composite with a thickness of approximately 2 mm.

[0079] [Tensile test] The obtained cellulose fiber-rubber composite is punched into a dumbbell shape, and the 100% stress (in MPa), tensile strength (in MPa), and elongation at break (in %) are measured according to JIS-K6215. A rubber composition without cellulose fibers is prepared as a comparison blank, and its 100% stress, tensile strength, and elongation at break are set to 1.00 for comparison. All tests are performed five times, and the average value is used as the test result.

[0080] [Evaluation of fracture surfaces of coarse structures] After a tensile test, the fracture surface is collected, coated with osmium, and prepared as an observation sample. This sample is observed at 500x magnification using SEM-EDX, and carbon and oxygen atoms are mapped. The following evaluation criteria are used to assess whether there are any coarse structures larger than 10 μm in a dry state on the fracture surface. "×": All 5 images contain a large structure. "△": 1 to 4 out of 5 images contain a large structure. "〇": No large structures are present in any of the five images.

[0081] [Marine Biodegradability Assessment] The obtained cellulose fibers are placed in seawater according to ASTM D6691 guidelines, and the amount of oxygen consumed during cellulose decomposition is measured for 28 days to evaluate the decomposition rate. At the same time, the decomposition rate of microcrystalline cellulose, a reference material, is evaluated in parallel. If the decomposition rate is 90% or more compared to the reference material after 28 days, it is judged to have sufficiently rapid marine biodegradation. Both the cellulose fibers and the reference material are evaluated with n=2, and the average value is adopted as the result.

[0082] [Preparation of cellulose fibers] [Example 1] Cotton linter pulp was dissolved in a copper ammonia solution to prepare a copper ammonia cellulose solution with a cellulose concentration of 5.0% by weight, a copper concentration of 1.8% by weight, and an ammonia concentration of 5.5% by weight. This solution was filtered through a sintered filter with an average pore size of 5 μm to remove impurities. As a spinning nozzle, a copper ammonia cellulose solution was discharged into 20°C hot water from a spinneret with a hole diameter of 0.3 mm, 180 holes, and an inter-hole distance of 1.1 mm. The solution was then stretched and deammed using a downward tension spinning method to produce blue yarn. The blue yarn and hot water were collected in a semicircular inclined trough placed 20 cm below the funnel outlet while flowing 50°C hot water, and then poured into a plastic net to separate the blue yarn from the hot water. 10% by weight sulfuric acid was then poured onto the blue yarn using a shower to thoroughly remove copper, and then the sulfuric acid was thoroughly washed off by showering with pure water to obtain continuous cellulose fibers in a moist state. The obtained continuous cellulose fibers were diluted in pure water to prepare an aqueous suspension with a cellulose concentration of 1.0% by weight. 500 ml of this suspension was placed in a mixer (AS ONE Extreme Mill, MX-1200XT) and processed for 5 minutes to prepare short-fiber cellulose fibers.

[0083] [Example 2] Shortened cellulose fibers were prepared in the same manner as in Example 1, except that the ammonia concentration in the copper ammonia cellulose solution was set to 8.0% by weight.

[0084] [Example 3] Shortened cellulose fibers were prepared in the same manner as in Example 2, except that the cellulose concentration in the copper ammonia cellulose solution was 4.0% by weight and the copper concentration was 1.4% by weight.

[0085] [Example 4] Shortened cellulose fibers were prepared in the same manner as in Example 1, except that the ammonia concentration in the copper ammonia cellulose solution was set to 4.5% by weight.

[0086] [Example 5] Shortened cellulose fibers were prepared in the same manner as in Example 4, except that the cellulose concentration in the copper ammonia cellulose solution was 7.0% by weight and the copper concentration was 2.5% by weight.

[0087] [Example 6] Using the shortened cellulose fibers obtained in Example 1, five micronization processes were performed using a high-pressure homogenizer (NS015H, manufactured by Nia Solovi) at an operating pressure of 100 MPa to prepare shortened cellulose fibers.

[0088] [Example 7] The shortened cellulose fibers obtained in Example 1 were suspended in 10% by weight sulfuric acid to a cellulose concentration of 0.1% by weight, heated to 70°C, stirred with a magnetic stirrer for 30 minutes, and the sulfuric acid was washed off with pure water to obtain cellulose fibers that had undergone easy fibrillation treatment. The obtained cellulose fibers were treated with a high-pressure homogenizer in the same manner as in Example 7 to prepare shortened cellulose fibers.

[0089] [Example 8] The cellulose concentration of the copper ammonia cellulose solution was set to 7.0% by weight and the copper concentration to 2.5% by weight. The solution was stirred overnight under an oxygen atmosphere to reduce the degree of polymerization of the cellulose, and continuous cellulose fibers were prepared in the same manner as in Example 1. Subsequently, the fibers were processed using a high-pressure homogenizer in the same manner as in Example 6 to prepare short cellulose fibers.

[0090] [Example 9] A spinning nozzle with a hole diameter of 0.3 mm, 800 holes, and a hole spacing of 1.1 mm was used. The blue yarn coming out of the funnel outlet was redirected, 50°C hot water was applied, and the tension of the blue yarn was reduced by adjusting the speed with a nip roller. Copper removal was performed by dropping 10% by weight sulfuric acid onto the blue yarn, which was running horizontally, through the nozzle. After that, the sulfuric acid was washed off by immersing the yarn in a bath with pure water flowing countercurrently, and cellulose fibers wound on a skein were obtained. The obtained skein-wound cellulose fibers were cut using a rotary cutter with numerous cutting blades arranged radially at 1 mm intervals to prepare short cellulose fibers. Furthermore, touching the blue thread that had deviated and was running horizontally with a fingertip revealed that the surface of the thread was smooth, suggesting that the individual threads were not strongly bonded together.

[0091] [Example 10] Using the short-fiber cellulose fibers obtained in Example 9, short-fiber cellulose fibers were prepared by treating them with a high-pressure homogenizer in the same manner as in Example 6.

[0092] [Example 11] Using the shortened cellulose fibers obtained in Example 2, five micronization processes were performed using a high-pressure homogenizer (NS015H, manufactured by Nia Solovi) under an operating pressure of 50 MPa to prepare shortened cellulose fibers.

[0093] [Comparative Example 1] Based on Patent Document 6, a cellulose-copper-ammonia solution was prepared with a cellulose concentration of 8.0% by weight, a copper concentration of 2.9% by weight, and an ammonia concentration of 7.0% by weight. Using a spinneret with an extrusion hole having a hole diameter of 0.6 mm, 2430 holes, and an inter-hole distance of 1.4 mm, flow-tension spinning was performed to form blue yarn. The blue yarn coming out of the funnel outlet was redirected, and regeneration was performed with sulfuric acid in a scouring bath with a straight flow in the same direction as the yarn. Thereafter, the sulfuric acid was washed off on a net as in Example 1, and the yarn was shortened using a mixer to prepare shortened cellulose fibers. Furthermore, when the blue threads that had deviated and were running horizontally were touched with a fingertip, they had a rough texture, unlike in Example 9, suggesting that the individual threads were strongly bonded together.

[0094] [Comparative Example 2] Using the cellulose fibers obtained in Comparative Example 1, short-fiber cellulose fibers were prepared by treating them with a high-pressure homogenizer in the same manner as in Example 6.

[0095] [Comparative Example 3] Shortened cellulose fibers were prepared in the same manner as in Comparative Example 1, except that the cellulose concentration in the copper ammonia cellulose solution was 10.0% by weight and the copper concentration was 3.6% by weight.

[0096] [Comparative Example 4] Based on Patent Document 7, a cellulose-copper-ammonia solution was prepared with a cellulose concentration of 3.5% by weight, a copper concentration of 1.3% by weight, an ammonia concentration of 18.0% by weight, a PEG500,000 concentration of 0.5% by weight, and a polyoxyethylene lauryl ether concentration of 0.1% by weight. The solution was filtered through a sintered filter with an average pore size of 5 μm to remove impurities. Electrospinning was performed using a metal nozzle with an inner diameter of 0.41 mm, copper removal was performed with sulfuric acid to separate the cellulose fibers from the collector, and the sulfuric acid, PEG, and surfactant were washed off with pure water. Shortened cellulose fibers were then prepared using a mixer in the same manner as in Example 1.

[0097] [Comparative Example 5] We used general cellulose nanofibers (BiNFi-s WFo-100 biomass nanofibers manufactured by Sugino Machine Co., Ltd.).

[0098] [Comparative Example 6] Cotton linter pulp was used as the cellulose raw material. Before micronization, it was treated with an alkaline solution of caustic soda at pH=13. Five micronization processes were then performed using a high-pressure homogenizer (NS015H, manufactured by Nia Solovi) at an operating pressure of 100 MPa to prepare short-fiber cellulose fibers.

[0099] [Comparative Example 7] Shortened cellulose fibers were prepared in the same manner as in Comparative Example 6, except that the operating pressure of the high-pressure homogenizer was set to 50 MPa.

[0100] [Comparative Example 8] Based on Patent Document 8, a copper ammonia cellulose solution was extruded into 20°C warm water from a spinneret having an extrusion hole with a hole diameter of 0.07 mm, 2430 holes, and an inter-hole distance of 0.67 mm (corresponding to a hole density of 180.9 holes / cm2). Shortened cellulose fibers were prepared in the same manner as in Example 1, except that the blue yarn was obtained by the flow tension spinning method, and the blue yarn and spinning water were received in a plastic net. Observation of the situation inside the funnel suggested that the yarns were in contact with each other due to the movement of the yarn, and that the fibers were coalescing together.

[0101] [Evaluation of cellulose fibers] SEM observation and automated optical analysis were performed on cellulose fibers from Examples 1-11 and Comparative Examples 1-8. For Comparative Example 5, 99% of the fibers measured in the dry state had a diameter of 0.1 μm or less, while 1.4% were coarse structures measured in the wet state with a wet fiber diameter of 20 μm or more. Although this total exceeds 100%, this is because coarse structures may not be included in the measurement field when magnifying only a small portion with an electron microscope. Furthermore, fibers that appear independent in magnified images may be connected in areas not visible in the image. Since the average fiber diameter of Comparative Example 5 was less than 0.3 μm, the results of the automated optical analysis are indicated with an asterisk (*) in the upper right corner as reference values. X-ray diffraction measurements were performed on Examples 1, 6, Comparative Example 1, and Comparative Examples 4-7. SEM images of Examples 1, 1, 4, 7, and 8 are shown in Figures 1, 2, 8, 3, and 9, respectively. A summary of the evaluation results is shown in Table 1 below.

[0102] [Table 1]

[0103] As can be seen from Figures 1-3, 8, 9 and Table 1, the cellulose fibers obtained in Examples 1-10 are microfibers in terms of average fiber diameter, but because the proportion of nanofibers is small, they are highly stable fibers with a small cohesive constant for fiber diameter. Furthermore, they are uniform fibers with a small coefficient of variation for fiber diameter due to the small proportion of coarse structures. In addition, as in Examples 1-5, the fiber diameter can be freely controlled by adjusting the spinning conditions, and as in Examples 6-10, the fiber length can be freely controlled while maintaining the fiber diameter by adjusting the refinement conditions. In contrast, even when using the same wet spinning method, the fibers of Comparative Examples 1, 4, and 8 obtained by existing spinning methods contain a certain amount of coarse structures. In the cellulose fibers of Comparative Example 1, the cellulose fibers are strongly bonded to each other immediately after spinning, and as in Comparative Example 2, even after crushing treatment, a certain amount of coarse structures remains, and nanofibers are generated through fibrillation. The cellulose fibers of Comparative Example 8 are also strongly bonded to each other immediately after spinning, similar to Comparative Example 1. Furthermore, in the cellulose fibers of Comparative Example 4, it is not possible to determine whether the fibers are strongly bonded to each other from SEM observations, and only a very small amount of particulate matter with a diameter of 3 μm or more is visible in the field of view, but when evaluated by automated optical analysis, a certain amount of coarse structures is present. In addition, as in Comparative Examples 5 to 7, in fibers in which natural cellulose has been refined by physical or chemical treatment, a certain amount of nanofiberized parts and coarse structures are present.

[0104] [Performance evaluation as a filter substrate] Using the cellulose fibers obtained in Examples 1-6 and Comparative Examples 4-7, a basis weight of 40 g / m² was used. 2 Cellulose fiber sheets were prepared and evaluated as filter substrates. SEM images of the cellulose fiber sheets obtained from Example 1, Comparative Example 4, and Comparative Example 7 are shown in Figures 4, 10, and 5, respectively. A summary of the evaluation results is shown in the following figure.

[0105] [Table 2]

[0106] As can be seen from Figures 4 and 5, the cellulose fibers of Example 1 maintain the gaps between fibers even after drying by heat drying, resulting in a uniform porous sheet. In contrast, the cellulose fibers of Comparative Example 7 contain nanofibers and have shorter fiber lengths, and shrinkage during drying fills the gaps between fibers, resulting in a sheet that does not have a uniform porous structure. Furthermore, as can be seen from Table 2, in the sheets prepared from cellulose fibers with a high proportion of nanofibers obtained in Comparative Examples 5 and 6, the gaps between fibers become so filled that the air permeability resistance cannot be measured, and the sheets cannot be considered to have a porous structure. Also, in the cellulose fibers of Comparative Examples 5 and 6, which have short fiber lengths, the entanglement between fibers undone during drying, making it easier for the gaps between fibers to fill. As can be seen from Figure 10, the sheets prepared from cellulose fibers of Comparative Examples 4 and 7, which contain many coarse structures, contain coarse structures in the sheets, making them prone to defects such as pinholes and reduced strength.

[0107] [Performance evaluation of rubber compositions as fillers 1] Cellulose fiber-rubber composites were prepared using the cellulose fibers obtained in Examples 1-8, 11, and Comparative Examples 4-7, and evaluated for their filler properties. SEM-EDX images of the fracture surfaces of Example 1 and Comparative Example 6 are shown in Figures 6 and 7, respectively. A summary of the evaluation results is shown in Table 3 below.

[0108] [Table 3]

[0109] As is clear from Figures 6 and 7, the fracture surface of the cellulose fiber-rubber composite obtained from the cellulose fibers of Example 1 shows good dispersion of cellulose fibers, whereas the fracture surface of the composite obtained from cellulose fibers containing coarse structures in Comparative Example 6 shows scattered cellulose clumps. Furthermore, as is clear from Table 3, the cellulose fiber-rubber composites obtained from the cellulose fibers of Examples 1-8 and 11 show no decrease in elongation at break and improved tensile strength, and because they have a constant fiber length, they can also improve 100% stress, just like compounding agents made from type I cellulose. On the other hand, the composites obtained from cellulose fibers containing coarse structures, as in Comparative Examples 4-7, tend to show a decrease in elongation at break and a decrease in tensile strength.

[0110] [Performance evaluation of rubber compositions as fillers 2] Using the cellulose fibers obtained in Example 1, cellulose fiber-rubber composites were prepared by varying the amount of cellulose fibers relative to the rubber to 3% and 10% by weight, and their performance as fillers was evaluated. SEM-EDX images of the fracture surfaces of Examples 1-3 are shown in Figure 11. A summary of the evaluation results is shown in Table 4 below.

[0111] [Table 4]

[0112] As is clear from Figure 11, the cellulose fibers of Example 1 can maintain good dispersion even when the amount of cellulose fibers added is increased. Furthermore, as is clear from Table 4, the tensile strength is maintained even when 10% by weight of cellulose fibers is added, and the stress can be improved by 100% depending on the amount added.

[0113] [Evaluation of marine biodegradability] The cellulose fibers obtained in Example 1 were used to evaluate their biodegradability in the ocean. After 28 days, the degree of decomposition was 93% compared to the reference material, confirming that biodegradation in the ocean is sufficiently rapid. [Industrial applicability]

[0114] The cellulose fibers according to the present invention are uniform cellulose microfibers (fibrous cellulose aggregates) with few coarse structures, without the addition of chemical substances or modification of cellulose, and can therefore be used versatility in various applications such as fillers for reinforcing resins and rubbers, and base materials for porous materials such as filters. Furthermore, the cellulose fibers of the present invention can be suitably used as materials for porous materials such as adsorbents, sound absorbers, and heat insulating materials, base materials for transparent materials and optical materials such as electronic products, additives for food, paints, and cosmetics, base materials for packaging materials and gas barrier materials, viscosity modifiers and dispersion stabilizers for liquid products, base materials for artificial leather, and apparel applications where synthetic fibers or cellulose fibers are used.

Claims

1. Cellulose fiber having an average fiber diameter of 0.3 μm or more and 3.0 μm or less in a dry state as observed by electron microscopy, a percentage of fibers with a fiber diameter of less than 0.1 μm of 5% or less, and a percentage of coarse structures, as indicated by the percentage of fibers with a wet fiber diameter of 20 μm or more as measured by automated optical analysis, of 3.0% or less.

2. The cellulose fiber according to claim 1, wherein the average wet fiber diameter is 1.0 μm or more and less than 10.0 μm.

3. The cellulose fiber according to claim 1 or 2, wherein the average wet fiber length is 3000 μm or less.

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

5. Cellulose fiber according to claim 1 or 2, wherein the agglomeration constant of the fiber diameter is 5.0 or less.

6. The cellulose fiber according to claim 1 or 2, wherein the cellulose crystal structure constituting the cellulose fiber is type II.

7. The cellulose fiber according to claim 1 or 2, wherein the degree of crystallinity of the cellulose constituting the cellulose fiber is 30% or more and 90% or less.

8. A composition comprising cellulose fibers according to claim 1 or 2.

9. A porous body made of cellulose fibers according to claim 1 or 2.

Images