Cellulose fibers, aqueous dispersion of cellulose fibers, rubber composite material and method for producing same, rubber compound, and crosslinked rubber composition

Abstract

The present invention relates to providing: cellulose fibers which, when added to a rubber component, achieve an excellent modulus improvement effect in a low elongation region in a crosslinked rubber composition obtained after crosslinking, and which can maintain a satisfactory tensile elongation of the crosslinked rubber composition; an aqueous dispersion of said cellulose fibers (an aqueous cellulose fiber dispersion); a rubber composite material containing a rubber component and said cellulose fibers; a rubber compound containing said rubber composite material and a crosslinking agent; a crosslinked rubber composition obtained by crosslinking said rubber compound; and a method for producing the rubber composite material.　When the cellulose fibers according to the present invention are subjected to centrifugal separation as an aqueous dispersion having a solid content concentration of 0.2 mass% under the conditions to obtain a precipitate and a supernatant liquid, the precipitate includes coarse cellulose fibers and has an irregular freeness of 950 mL, and solid matter in the supernatant liquid includes fine fibrous cellulose having fiber diameters of 100 nm or less.　Conditions: Centrifugal separation at 12,000 G for 10 minutes

Description

Cellulose fibers, aqueous dispersions of cellulose fibers, rubber composites and methods for producing the same, rubber compounds, and crosslinked rubber compositions.

[0001] This invention relates to cellulose fibers, aqueous dispersions of cellulose fibers, rubber composites and methods for producing the same, rubber compounds, and crosslinked rubber compositions.

[0002] In recent years, materials made from renewable natural fibers have attracted attention due to the need for alternatives to petroleum resources and growing environmental awareness.

[0003] To improve the reinforcing properties of rubber products, fillers are added, and the use of fibrous cellulose as such a filler is being considered. Reference 1 discloses a rubber composite comprising a rubber component, long-fiber biomass nanofibers, and short-fiber biomass nanofibers, with the aim of providing a rubber composite that has a good initial modulus of elasticity and, when formed into a sheet, has a small difference in tensile properties between the grain direction and the non-grain direction perpendicular thereto, wherein the viscosity-average degree of polymerization of the long-fiber biomass nanofibers is greater than that of the short-fiber biomass nanofibers, the difference between the viscosity-average degree of polymerization of the long-fiber biomass nanofibers and the viscosity-average degree of polymerization of the short-fiber biomass nanofibers is 400 to 1400, the viscosity-average degree of polymerization of the long-fiber biomass nanofibers is 600 to 1500, the viscosity-average degree of polymerization of the short-fiber biomass nanofibers is 100 to 500, and the long-fiber biomass nanofibers and the short-fiber biomass nanofibers are mechanically defibrated biomass nanofibers, and are one of chitin nanofibers, chitosan nanofibers, and cellulose nanofibers.

[0004] Patent No. 7346525

[0005] Through the inventors' research, it has been found that depending on the type of fiber used as a filler, fiber aggregates may occur in rubber products, and in this case, the tensile elongation tends to decrease. The present invention aims to provide cellulose fibers, an aqueous dispersion of said cellulose fibers (cellulose fiber aqueous dispersion), a rubber component, and a rubber composite material containing said cellulose fibers, a rubber compound containing said rubber composite material and a crosslinking agent, and a crosslinked rubber composition obtained by crosslinking said rubber compound, which, when added to a rubber component, exhibits excellent modulus improvement in the low elongation region in the crosslinked rubber composition obtained after crosslinking, and which can sufficiently maintain the tensile elongation of the crosslinked rubber composition. Furthermore, the present invention aims to provide a method for producing said rubber composite material.

[0006] The inventors have found that the above problems can be solved by cellulose fibers containing coarse cellulose fibers having a specific irregular freeness and fine fibrous cellulose having a specific fiber width. The present invention relates to the following <1> to <15>. <1> Cellulose fiber wherein, when a precipitate and supernatant are obtained by centrifugation under the following conditions as an aqueous dispersion with a solid content of 0.2% by mass, the precipitate contains coarse cellulose fibers, the irregular freeness of the precipitate is 950 mL or less, and the solid content in the supernatant contains fine fibrous cellulose with a fiber width of 100 nm or less. Condition: Centrifugation at 12,000 G for 10 minutes <2> Cellulose fiber according to <1>, wherein the dry mass ratio of the precipitate to the solid content in the supernatant (precipitate:solid content in supernatant) is 90:10 to 10:90. <3> Cellulose fiber according to <1> or <2>, wherein the irregular freeness of the precipitate is 400 mL or less. <4> A cellulose fiber according to any one of <1> to <3>, wherein the precipitate contains unmodified coarse cellulose fibers. <5> A cellulose fiber according to any one of <1> to <4>, wherein the fine fibrous cellulose with a fiber width of 100 nm or less contains at least one selected from the group consisting of phosphorus oxoacid groups and groups derived from phosphorus oxoacid groups. <6> A cellulose fiber according to any one of <1> to <5> that is a rubber modifier. <7> A cellulose fiber aqueous dispersion containing the cellulose fiber according to any one of <1> to <6>. <8> A rubber composite material containing a rubber component and the cellulose fiber according to any one of <1> to <6>. <9> A rubber composite material according to <8>, wherein the content of cellulose fibers in the rubber composite material is 5 parts by mass or more and 50 parts by mass or less per 100 parts by mass of the rubber component. <10> A rubber composite material according to <8> or <9>, wherein the rubber component contains diene rubber. <11> A rubber compound containing the rubber composite material and crosslinking agent described in any one of <8> to <10>. <12> A crosslinked rubber composition obtained by crosslinking the rubber compound described in <11>.<13> The crosslinked rubber composition according to <12>, wherein the ratio (σ25B / σ25A) of the 25% modulus (σ25B) of the crosslinked rubber composition according to <12>, measured according to JIS K 6251:2017, to the 25% modulus (σ25A) of the crosslinked rubber composition obtained by crosslinking a rubber compound containing a rubber component and a crosslinking agent, measured according to JIS K 6251:2017, is 2.0 or more and 40.0 or less. <14> The crosslinked rubber composition according to <12> or <13>, wherein the ratio (σ50B / σ50A) of the 50% modulus (σ50A) of the crosslinked rubber composition according to <12> or <13>, measured according to JIS K 6251:2017, to the 50% modulus (σ50A) of the crosslinked rubber composition according to <12> or <13>, measured according to JIS K 6251:2017, is 2.0 or more and 35.0 or less. <15> A method for producing a rubber composite, comprising the following steps (I) and (II). (I) A cellulose fiber aqueous dispersion with a solid content of 0.2% by mass is centrifuged under the following conditions to obtain a precipitate and supernatant, wherein the precipitate contains coarse cellulose fibers, the irregular freeness of the precipitate is 950 mL or less, and the solid content in the supernatant contains fine fibrous cellulose with a fiber width of 100 nm or less. Mixing process conditions to obtain mixture A by mixing a cellulose fiber aqueous dispersion with rubber latex: centrifugation at 12,000 G for 10 minutes. (II) A solidification process to obtain a solid rubber composite from mixture A.

[0007] According to the present invention, a cellulose fiber that, when added to a rubber component, exhibits excellent modulus improvement in the low elongation region in the crosslinked rubber composition obtained after crosslinking, and also maintains sufficient tensile elongation of the crosslinked rubber composition, an aqueous dispersion of the cellulose fiber (cellulose fiber aqueous dispersion), a rubber component, and a rubber composite material containing the cellulose fiber, a rubber compound containing the rubber composite material and a crosslinking agent, and a crosslinked rubber composition obtained by crosslinking the rubber compound are provided. Furthermore, according to the present invention, a method for producing the rubber composite material is provided.

[0008] This is a schematic side view of a double drum dryer, an example of a heated cylindrical dryer. This is a schematic side view of a double drum dryer, an example of a heated cylindrical dryer. This is a graph showing the relationship between the amount of NaOH added to a slurry containing fibrous cellulose with phosphorus oxoacid groups and pH.

[0009] [Cellulose Fibers] The cellulose fibers of this embodiment, when centrifuged as an aqueous dispersion with a solid content concentration of 0.2% by mass under the following conditions to obtain a precipitate and supernatant, contain coarse cellulose fibers, have an irregular freeness of 950 mL or less, and contain fine fibrous cellulose with a fiber width of 100 nm or less in the supernatant. Conditions: Centrifugation at 12,000 G for 10 minutes. The cellulose fibers of this embodiment are obtained by mixing coarse cellulose fibers and fine fibrous cellulose. By adding the cellulose fibers to the rubber component, the crosslinked rubber composition obtained after crosslinking exhibits excellent modulus improvement in the low elongation region, and can also sufficiently maintain the tensile elongation of the crosslinked rubber composition. The detailed mechanism by which the above effects are obtained is unknown, but some of it is thought to be as follows. Cellulose nanofibers with a fiber width of 1,000 nm or less are used as a modifier for the rubber component (see, for example, Japanese Patent Application Publication No. 2024-120444), and the crosslinked rubber composition can be reinforced by adding cellulose nanofibers. However, while the addition of cellulose nanofibers maintains sufficient tensile elongation of the crosslinked rubber composition, the modulus improvement effect in the low elongation region is insufficient. On the other hand, when using pulp fibers as they are, or when using pulp fibers with a low degree of beating, although a modulus improvement effect in the low elongation region can be obtained, when the aqueous dispersion of pulp fibers is mixed with rubber latex containing rubber components, agglomerations of pulp can form in the mixture, and agglomerations can also occur in the crosslinked rubber composition, resulting in a decrease in tensile elongation. This is thought to be because the agglomerations trigger fracture. In this invention, we have found that by using cellulose fibers containing specific coarse cellulose fibers and specific fine fibrous cellulose (cellulose nanofibers), it is possible to maintain sufficient tensile elongation of the crosslinked rubber composition while obtaining a modulus improvement effect in the low elongation region of the pulp fibers.Specifically, when a cellulose fiber aqueous dispersion with a solid content of 0.2% by mass is centrifuged at 12,000 G for 10 minutes, coarse cellulose fibers with an irregular freeness of 950 mL or less of precipitate, and fine fibrous cellulose with a fiber width of 100 nm or less contained in the solid content of the supernatant liquid are combined to form a cellulose fiber aqueous dispersion, which is then mixed with a rubber component. As a result, the coarse cellulose fiber dispersion effect of the fine fibrous cellulose with a fiber width of 100 nm or less in the rubber component is exhibited at a high level. Consequently, a uniform rubber composite material can be obtained by mixing the cellulose fibers and the rubber component. Furthermore, in the crosslinked rubber composition obtained after crosslinking, the generation of aggregates and the decrease in tensile elongation are suppressed, and an excellent modulus improvement effect in the low elongation region is observed, leading to the completion of the present invention. It should be noted that the mechanism by which the effects of the present invention are obtained is not limited to the above.

[0010] Embodiments of the present invention will be described below in detail. The following descriptions of constituent elements may be based on representative embodiments or specific examples, but the present invention is not limited to such embodiments. In this specification, numerical ranges expressed using "~" mean a range that includes the numbers written before and after "~" as the lower and upper limits. When numerical ranges are described in steps, the upper and lower limits of each numerical range can be arbitrarily combined. Cellulose fibers, aqueous dispersions of cellulose fibers, rubber composites, rubber compounds, and crosslinked rubber compositions, as well as each component and raw materials used in the method for producing rubber composites, may be used individually or in combination of two or more types.

[0011] [Precipitate] The precipitate contains coarse cellulose fibers, which are pulp fibers, and the irregular freeness of the precipitate is 950 mL or less. In a crosslinked rubber composition, from the viewpoint of suppressing the generation of aggregates, obtaining sufficient tensile elongation, and furthermore, having an excellent effect in improving modulus in the low elongation region, the irregular freeness of the precipitate is preferably 100 mL or more and 900 mL or less, more preferably 750 mL or less, even more preferably 600 mL or less, even more preferably 450 mL or less, even more preferably 400 mL or less, and more preferably 200 mL or more, and even more preferably 260 mL or more. Here, irregular freeness refers to the freeness (filtration rate) measured in the Canadian standard filtration rate method specified in JIS P 8121-2:2012, by changing the amount of pulp sampled from 3 g to 0.3 g and changing the screen plate used in the JIS standard to an 80 mesh wire. The irregular freeness of the precipitate can be adjusted by the type of fiber raw material used and the conditions of the beating process, such as the beating time. Specifically, increasing the beating time tends to decrease the irregular freeness.

[0012] The length-weighted average fiber length (Ll) of the precipitate is preferably 0.40 mm to 2.5 mm, more preferably 0.45 mm or more, even more preferably 0.50 mm or more, more preferably 2.2 mm or less, even more preferably 1.8 mm or less, even more preferably 1.5 mm or less, even more preferably 1.0 mm or less, and even more preferably 0.8 mm or less, from the viewpoint of suppressing the generation of aggregates in the crosslinked rubber composition, obtaining sufficient tensile elongation, and furthermore, having an excellent effect on improving the modulus in the low elongation region. The length-weighted average fiber length (Ll) of the precipitate can be adjusted by the type of pulp used, the conditions of the beating process in the beating process, for example, the beating time. Specifically, when using softwood pulp as the raw material, the length-weighted average fiber length (Ll) tends to be longer compared to when using hardwood pulp, under the same beating treatment. When using both softwood pulp and hardwood pulp, a higher proportion of softwood pulp tends to result in a longer length-weighted average fiber length (Ll). Furthermore, increasing the beating time tends to decrease the length-weighted average fiber length (Ll). The length-weighted average fiber length (Ll) of the precipitate is measured by the method described in the examples.

[0013] <Coarse Cellulose Fibers> From the viewpoint of having excellent dispersibility in rubber composites, suppressing the generation of aggregates in crosslinked rubber compositions, obtaining sufficient tensile elongation, and further having an excellent effect in improving modulus in the low elongation region, the irregular freeness of the coarse cellulose fibers used in this embodiment is preferably 950 mL or less and 50 mL or more, more preferably 900 mL or less, even more preferably 750 mL or less, even more preferably 600 mL or less, even more preferably 450 mL or less, even more preferably 300 mL or less, even more preferably 250 mL or less, and more preferably 100 mL or more, even more preferably 200 mL or more. Note that the precipitate may contain not only coarse cellulose fibers but also fine fibrous cellulose, so the preferred range for the irregular freeness of pulp fibers and the preferred range for the irregular freeness of the precipitate are not the same. When a small amount of fine fibrous cellulose with a fiber width of 100 nm or less is mixed in the precipitate, the fine fibrous cellulose passes through the mesh, so the number of objects to be measured decreases, and as a result, the irregular freeness of the precipitate tends to increase. The irregular freeness of coarse cellulose fibers can be adjusted by the fiber raw material and the conditions of the beating process, such as the beating time. Specifically, increasing the beating time tends to decrease the irregular freeness.

[0014] The degree of polymerization of coarse cellulose fibers in a crosslinked rubber composition is preferably 400 to 1,200, more preferably 600 or more, even more preferably 700 or more, even more preferably 800 or more, even more preferably 850 or more, and more preferably 1,100 or less, even more preferably 1,000 or less, and even more preferably 950 or less, which is desirable for suppressing the formation of aggregates, obtaining sufficient tensile elongation, and furthermore for having an excellent effect in improving the modulus in the low elongation region. The degree of polymerization is also called the "viscosity-average degree of polymerization" and is the degree of polymerization calculated from the intrinsic viscosity obtained from the viscosity measured according to Tappi T230. Specifically, the viscosity (η1) measured by dissolving the coarse cellulose fibers to be measured in an aqueous copper ethylenediamine solution, and the blank viscosity (η0) measured with only the aqueous copper ethylenediamine solution are measured, and then the specific viscosity (ηsp) and intrinsic viscosity ([η]) are measured according to the following formula. ηsp = (η1 / η0) - 1 [η] = ηsp / (c(1 + 0.28 × ηsp)) Here, c in the formula represents the concentration (g / mL) of coarse cellulose fibers at the time of viscosity measurement. Furthermore, the degree of polymerization (DP) is calculated from the following formula: DP = 1.75 × [η] This degree of polymerization is the average degree of polymerization measured by the viscosity method and is sometimes called the "viscosity-average degree of polymerization".

[0015] The degree of polymerization of coarse cellulose fibers is adjusted by whether or not a low-polymerization treatment is performed, the treatment conditions, etc. Examples of low-polymerization treatments include the ozone treatment process, enzyme treatment process, hypochlorous acid treatment process, and subcritical water treatment process described in paragraphs 0085 to 0093 of Japanese Patent Application Publication No. 2021-175798.

[0016] The length-weighted average fiber length (Ll) of coarse cellulose fibers in a crosslinked rubber composition is preferably 0.40 mm to 2.5 mm, more preferably 0.45 mm or more, even more preferably 0.50 mm or more, more preferably 2.2 mm or less, even more preferably 1.8 mm or less, even more preferably 1.5 mm or less, even more preferably 1.0 mm or less, and even more preferably 0.80 mm or less. However, since the precipitate may contain not only pulp fibers but also fine fibrous cellulose, the preferred range for the length-weighted average fiber length (Ll) of pulp fibers and the preferred range for the length-weighted average fiber length (Ll) of the precipitate are not the same. This is because the precipitate may contain a small amount of fine fibrous cellulose having a length-weighted average fiber length (Ll) different from that of the coarse cellulose fibers. The length-weighted average fiber length (Ll) of coarse cellulose fibers can be adjusted by the type of pulp used and the conditions of the beating process, such as the beating time. Specifically, when softwood pulp is used as the raw material, the length-weighted average fiber length (Ll) tends to be longer than when hardwood pulp is used as the raw material, under the same beating treatment. When softwood pulp and hardwood pulp are used in combination, a higher proportion of softwood pulp tends to result in a longer length-weighted average fiber length (Ll). Furthermore, increasing the beating time tends to decrease the length-weighted average fiber length (Ll). The length-weighted average fiber length (Ll) of pulp fibers is measured by the method described in the examples.

[0017] Coarse cellulose fibers may be modified or unmodified, but in a crosslinked rubber composition, it is preferable that they be unmodified from the viewpoint of suppressing the generation of aggregates, obtaining sufficient tensile elongation, and furthermore, having an excellent effect in improving modulus in the low elongation region. Examples of modification of coarse cellulose fibers include modification by introducing ionic substituents, and the ionic substituents may include either or both an anionic group and a cationic group, with an anionic group being preferred. Furthermore, it is preferable that the ionic substituent is introduced into the coarse cellulose fiber via an ester bond or an ether bond, and more preferably via an ester bond. In this case, it is preferable that the ester bond is formed by the dehydration condensation of the coarse cellulose fiber and the compound that becomes the ionic substituent. Examples of anionic groups as ionic groups include phosphorus oxoacid group or substituents derived from phosphorus oxoacid group (sometimes simply referred to as phosphorus oxoacid group), carboxyl group or substituents derived from carboxyl group (sometimes simply referred to as carboxyl group), sulfur oxoacid group or substituents derived from sulfur oxoacid group (sometimes simply referred to as sulfur oxoacid group), xantate group or substituents derived from xantate group (sometimes simply referred to as xantate group), phosphonone group or substituents derived from phosphonone group, phosphine group or substituents derived from phosphine group, sulfone group or substituents derived from sulfone group, carboxyalkyl group, etc. In particular, the anionic group is preferably at least one selected from the group consisting of phosphorus oxoacid group, substituents derived from phosphorus oxoacid group, carboxyl group, sulfur oxoacid group, substituents derived from sulfur oxoacid group, carboxymethyl group, carboxyethyl group, and sulfone group, and more preferably at least one selected from the group consisting of phosphorus oxoacid group, substituents derived from phosphorus oxoacid group, carboxyl group, sulfur oxoacid group, and substituents derived from sulfur oxoacid group.In other words, the anionic group is more preferably selected from the group consisting of phosphorus oxoacid groups (phosphorus oxoacid groups or substituents derived from phosphorus oxoacid groups), sulfur oxoacid groups (sulfur oxoacid groups or substituents derived from sulfur oxoacid groups), and carboxyl groups, and is even more preferably a phosphorus oxoacid group. Examples of cationic groups as ionic groups include ammonium groups, phosphonium groups, sulfonium groups, etc., and is preferably an ammonium group. For details of ionic groups, methods of introduction, and preferred ranges of the amount of ionic groups introduced, refer to paragraphs 0038 to 0114 of Japanese Patent Application Publication No. 2022-104299.

[0018] Coarse cellulose fibers are obtained by beating the fiber raw material. As mentioned above, if the coarse cellulose fibers are modified coarse cellulose fibers, for example, coarse cellulose fibers to which ionic substituents have been introduced, the ionic substituents may be introduced before or after the beating treatment, and are not particularly limited.

[0019] (Fiber Raw Materials) The fiber raw materials that are the raw materials for coarse cellulose fibers are fiber raw materials containing cellulose, and are not particularly limited in terms of fiber raw materials, but examples include pulps such as wood pulp, non-wood pulp, and deinked pulp. Wood pulps are not particularly limited in terms of types, but examples include chemical pulps such as hardwood kraft pulp (LBKP), softwood kraft pulp (NBKP), sulfite pulp (SP), dissolved pulp (DP), soda pulp (AP), unbleached kraft pulp (UKP), and oxygen-bleached kraft pulp (OKP), semi-chemical pulps such as semi-chemical pulp (SCP) and chemigroundwood pulp (CGP), and mechanical pulps such as crushed wood pulp (GP) and thermomechanical pulp (TMP, BCTMP). Non-wood pulps are not particularly limited in terms of types, but examples include cotton pulps such as cotton linters and cotton lint, and non-wood pulps such as hemp, straw, bamboo, and bagasse. The deinked pulp is not particularly limited, but examples include deinked pulp made from recycled paper. Among the above pulps, wood pulp and deinked pulp are preferred from the viewpoint of availability, wood pulp is more preferred, it is even more preferred to use at least softwood pulp, it is even more preferred that 50% by mass or more is softwood pulp, and it is even more preferred that 80% by mass or more is softwood pulp. Furthermore, among wood pulps, chemical pulp is more preferred from the viewpoint of having a high cellulose ratio and a high yield of cellulose fibers in the beating process, and from the viewpoint of having less decomposition of cellulose in the pulp and being able to obtain cellulose fibers with the desired fiber length, and kraft pulp is even more preferred.

[0020] In the beating process (beating process), a dispersion (slurry) of the fiber raw material is prepared, and the fiber raw material in the slurry is beaten. In the beating process, it is preferable to dilute the fiber raw material with a dispersion medium to make it into a slurry. As the dispersion medium, one or more selected from water and organic solvents such as polar organic solvents can be used. The polar organic solvent is not particularly limited, but for example, alcohols, polyhydric alcohols, ketones, ethers, esters, aprotic polar solvents, etc. are preferred. Examples of alcohols include methanol, ethanol, isopropanol, n-butanol, isobutyl alcohol, etc. Examples of polyhydric alcohols include ethylene glycol, propylene glycol, glycerin, etc. Examples of ketones include acetone, methyl ethyl ketone (MEK), etc. Examples of ethers include diethyl ether, tetrahydrofuran, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-butyl ether, propylene glycol monomethyl ether, etc. Examples of esters include ethyl acetate, butyl acetate, etc. Examples of aprotic polar solvents include dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), and N-methyl-2-pyrrolizinone (NMP). Among these, the dispersion medium preferably contains at least water, more preferably 50% by mass or more of the dispersion medium is water, more preferably 80% by mass or more, even more preferably 90% by mass or more, and 100% by mass or less, and may even be 100% by mass.

[0021] In the beating process, the concentration of the fiber raw material in the dispersion is not particularly limited, but from the viewpoint of uniformity of the beating process and ease of removal of the dispersion medium when forming the rubber composition, it is preferably 1.0% by mass or more and 35% by mass or less, more preferably 2.0% by mass or more, even more preferably 3.0% by mass or more, more preferably 15% by mass or less, even more preferably 10% by mass or less, even more preferably 8.0% by mass or less, even more preferably 7.0% by mass or less, even more preferably 6.0% by mass or less, and even more preferably 5.0% by mass or less. The beating process method is not particularly limited, but can be carried out using a beating machine such as a double disc refiner, single disc refiner, conical refiner, or deluxe refiner.

[0022] The beating time is not particularly limited and can be adjusted as appropriate to obtain the desired irregular freeness, degree of polymerization, and length-weighted average fiber length (Ll). The preferred beating time varies depending on the slurry concentration, the type of fiber raw material used, the type of beating machine and clearance, etc., but for example, it is preferably 30 minutes to 24 hours, more preferably 1 hour or more, even more preferably 2 hours or more, even more preferably 2.5 hours or more, and more preferably 12 hours or less, even more preferably 6 hours or less, even more preferably 5 hours or less, even more preferably 4.5 hours or less, and even more preferably 4 hours or less. When using a disc refiner as the beating machine, it is preferable to reduce the clearance as much as possible in order to efficiently proceed with beating.

[0023] [Supernatant] The solid content in the supernatant contains fine fibrous cellulose with a fiber width of 100 nm or less (hereinafter also simply referred to as "fine fibrous cellulose").

[0024] <Fine Fibrous Cellulose> The fiber width of the fine fibrous cellulose used in the cellulose fibers of this embodiment is 100 nm or less, preferably 50 nm or less, more preferably 20 nm or less, even more preferably 10 nm or less, and preferably 2 nm or more, from the viewpoint of uniformly dispersing the cellulose fibers in the rubber composite material. The fiber width of the fine fibrous cellulose contained in the solids in the supernatant is 100 nm or less, preferably 50 nm or less, more preferably 20 nm or less, and preferably 2 nm or more, from the viewpoint of uniformly dispersing the cellulose fibers in the rubber composite material. It is preferable that 75% or more of the total number of fibrous cellulose fibers contained in the solids in the supernatant material are within the above range, more preferably 80% or more are within the above range, and even more preferably 85% or more are within the above range, with an upper limit of 100% or less.

[0025] The average fiber width of the fine fibrous cellulose used in the cellulose fibers of this embodiment is preferably 100 nm or less, more preferably 50 nm or less, even more preferably 20 nm or less, even more preferably 10 nm or less, and preferably 2 nm or more, from the viewpoint of uniformly dispersing the cellulose fibers in the rubber composite material. The fine fibrous cellulose is, for example, monofilamentous cellulose.

[0026] The fiber width of fine fibrous cellulose is measured, for example, using an atomic force microscope as follows: First, an aqueous suspension of fine fibrous cellulose with a concentration of 0.0005% by mass or more and 0.005% by mass or less is prepared, and this suspension is cast onto mica to prepare a sample for atomic force microscopy (AFM) observation. Next, the observation range is set to 1 μm × 1 μm, 2 μm × 2 μm, 5 μm × 5 μm, 10 μm × 10 μm, or 20 μm × 20 μm depending on the width of the fiber to be observed, and observation is performed. (1) Draw a straight line X at an arbitrary location in the observation image. (2) Draw a straight line Y perpendicular to the said line in the same image. (3) Create two or more sets of straight lines X and Y different from (1) and (2) in the same image. Read the width of the fiber that intersects with the straight lines X and Y and where the fibers do not overlap. By doing this, the width of at least 50 fibers is read. Then, the average value of the measured fiber width is taken as the number-average fiber width of the microfiber cellulose.

[0027] The fiber length of the fine fibrous cellulose is not particularly limited, but is preferably 0.1 μm to 1,000 μm, more preferably 0.1 μm to 800 μm, and even more preferably 0.1 μm to 600 μm. By keeping the fiber length within the above range, the destruction of the crystalline region of the fine fibrous cellulose can be suppressed. The fiber length of the fine fibrous cellulose can be determined, for example, by image analysis using an atomic force microscope (AFM).

[0028] It is preferable that the fine fibrous cellulose has a type I crystalline structure. The presence of a type I crystalline structure in the fine fibrous cellulose can be identified in the diffraction profile obtained from a wide-angle X-ray diffraction photograph using graphite-monochromatized CuKα (λ = 1.5418 Å). Specifically, it can be identified by the presence of typical peaks at two locations: around 2θ = 14° to 17° and around 2θ = 22° to 23°. The proportion of type I crystalline structure in the fine fibrous cellulose is, for example, preferably 30% or more, more preferably 40% or more, and even more preferably 50% or more. The degree of crystallinity can be determined by measuring the X-ray diffraction profile and analyzing its pattern using conventional methods (Seagal et al., Textile Research Journal, Vol. 29, p. 786, 1959).

[0029] The axial ratio (fiber length / fiber width) of the fine fibrous cellulose is not particularly limited, but is preferably 50 to 10,000, more preferably 100 to 1,000. Setting the axial ratio above the lower limit makes it easier to form rubber composite materials containing fine fibrous cellulose. Setting the axial ratio below the upper limit is preferable in that it makes handling, such as dilution, easier when handling the fine fibrous cellulose as a dispersion.

[0030] The degree of polymerization of fine fibrous cellulose in a crosslinked rubber composition is preferably 200 to 900, more preferably 350 or more, even more preferably 500 or more, and even more preferably 750 or less, and even more preferably 600 or less, from the viewpoint of suppressing the generation of aggregates, obtaining sufficient tensile elongation, and furthermore, having an excellent effect in improving the modulus in the low elongation region. From the viewpoint of ease of manufacture, the degree of polymerization is preferably 750 or less, and even more preferably 600 or less. Fine fibrous cellulose can be reduced in degree of polymerization by a low-polymerization treatment such as an ozone treatment process, an enzyme treatment process, a hypochlorous acid treatment process, a subcritical water treatment process, or an irradiation treatment process. The degree of polymerization of fine fibrous cellulose is a value calculated from the viscosity measured according to Tappi T230, and can be measured in the same way as the degree of polymerization of coarse cellulose fibers.

[0031] In this embodiment, the microfiber cellulose has both crystalline and amorphous regions. Microfiber cellulose having both crystalline and amorphous regions and having an axial ratio within the above range can be realized by the method for producing microfiber cellulose described later.

[0032] The fine fibrous cellulose preferably has at least one of ionic and nonionic groups. From the viewpoint of improving the dispersibility of fibers in the dispersion medium and increasing the defibration efficiency in the defibration process, it is more preferable for the fine fibrous cellulose to have ionic groups. The ionic group may include, for example, either an anionic group or a cationic group, or both. The nonionic group may include, for example, an alkyl group and an acyl group. In this embodiment, it is particularly preferable to have an anionic group as the ionic group. It is preferable to have an ionic group, preferably an anionic group, at least during the defibration process, and the ionic group may be removed after the defibration process. Furthermore, the fine fibrous cellulose does not necessarily have to undergo a process to introduce ionic groups.

[0033] Examples of anionic groups include phosphorus oxoacid groups or substituents derived from phosphorus oxoacid groups (sometimes simply referred to as phosphorus oxoacid groups), carboxyl groups or substituents derived from carboxyl groups (sometimes simply referred to as carboxyl groups), sulfur oxoacid groups or substituents derived from sulfur oxoacid groups (sometimes simply referred to as sulfur oxoacid groups), xantate groups or substituents derived from xantate groups (sometimes simply referred to as xantate groups), phosphonone groups or substituents derived from phosphonone groups, phosphine groups or substituents derived from phosphine groups, sulfone groups or substituents derived from sulfone groups, carboxyalkyl groups, and the like. In particular, the anionic group preferably includes at least one selected from the group consisting of phosphorus oxoacid group, substituents derived from phosphorus oxoacid group, carboxyl group, sulfur oxoacid group, substituents derived from sulfur oxoacid group, carboxymethyl group, carboxyethyl group, and sulfone group; more preferably includes at least one selected from the group consisting of phosphorus oxoacid group, substituents derived from phosphorus oxoacid group, carboxyl group, sulfur oxoacid group, and substituents derived from sulfur oxoacid group; and even more preferably includes at least one selected from the group consisting of phosphorus oxoacid group and groups derived from phosphorus oxoacid group. By introducing a phosphorus oxoacid group as the anionic group, for example, the dispersibility of the fine fibrous cellulose can be further improved even under alkaline or acidic conditions, and as a result, a crosslinked rubber composition with excellent tensile properties can be easily obtained.

[0034] A phosphorus oxoacid group or a substituent derived from a phosphorus oxoacid group is, for example, a substituent represented by the following formula (1). Multiple substituents represented by the following formula (1) may be introduced into each microfiber cellulose. In this case, the multiple substituents represented by the following formula (1) may be the same or different.

[0035] In equation (1), a, b, and n are natural numbers, and m is any number (where a = b × m). Of the n α and α', at least one is O － And the rest are R or OR. Note that all of each α and α' are O －It may be so. All of the n αs may be the same or each may be different. β ｂ＋ is a monovalent or higher-valent cation composed of an organic or inorganic substance.

[0036] Each R is a hydrogen atom, a saturated - straight-chain hydrocarbon group, a saturated - branched-chain hydrocarbon group, a saturated - cyclic hydrocarbon group, an unsaturated - straight-chain hydrocarbon group, an unsaturated - branched-chain hydrocarbon group, an unsaturated - cyclic hydrocarbon group, an aromatic group, or a derivative group thereof. Also, in formula (1), n is preferably 1.

[0037] Examples of the saturated - straight-chain hydrocarbon group include a methyl group, an ethyl group, an n - propyl group, or an n - butyl group, etc., but are not particularly limited. Examples of the saturated - branched-chain hydrocarbon group include an i - propyl group, or a t - butyl group, etc., but are not particularly limited. Examples of the saturated - cyclic hydrocarbon group include a cyclopentyl group, or a cyclohexyl group, etc., but are not particularly limited. Examples of the unsaturated - straight-chain hydrocarbon group include a vinyl group, or an allyl group, etc., but are not particularly limited. Examples of the unsaturated - branched-chain hydrocarbon group include an i - propenyl group, or a 3 - butenyl group, etc., but are not particularly limited. Examples of the unsaturated - cyclic hydrocarbon group include a cyclopentenyl group, a cyclohexenyl group, etc., but are not particularly limited. Examples of the aromatic group include a phenyl group, or a naphthyl group, etc., but are not particularly limited.

[0038] Also, as the derivative group in R, with respect to the main chain or side chain of the above various hydrocarbon groups, a carboxy group, a carboxylate group (-COO －Examples of the functional group in which at least one selected from functional groups such as a hydroxy group, an amino group, and an ammonium group is added or substituted are not particularly limited. The number of carbon atoms constituting the main chain of R is not particularly limited, but is preferably 20 or less, more preferably 10 or less. By setting the number of carbon atoms constituting the main chain of R within the above range, the molecular weight of the phosphono acid group can be set within an appropriate range, penetration into the fiber raw material can be facilitated, and the yield of microfibrillar cellulose can also be increased. When there are a plurality of Rs in formula (1) or when a plurality of substituents represented by formula (1) are introduced into the microfibrillar cellulose, the plurality of Rs may be the same or different from each other.

[0039] β ｂ＋ is a cation of one or more valences composed of an organic or inorganic substance. Examples of the cation of one or more valences composed of an organic substance include organic onium ions. Examples of the organic onium ion include, for example, organic ammonium ions and organic onium ions. Examples of the organic ammonium ion include, for example, aliphatic ammonium ions and aromatic ammonium ions, and examples of the organic onium ion include, for example, aliphatic phosphonium ions and aromatic phosphonium ions. Examples of the cation of one or more valences composed of an inorganic substance include ions of alkali metals such as sodium, potassium, or lithium, ions of metals belonging to any of Groups 2 to 14 of the periodic table, hydrogen ions, ammonium ions, and the like. In formula (1), when there are a plurality of β ｂ＋ or when a plurality of substituents represented by formula (1) are introduced into the microfibrillar cellulose, the plurality of β ｂ＋ may be the same or different from each other.

[0040] More specifically, examples of the phosphono acid group or a substituent derived from the phosphono acid group include a phosphoric acid group (—PO ３ H ２ ), a salt of a phosphoric acid group, a phosphorous acid group (phosphonic acid group) (—PO ２ H ２Examples include phosphate groups and salts of phosphonic acid groups. Furthermore, substituents derived from phosphate groups or phosphate groups may be groups formed by condensation of phosphate groups (e.g., pyrophosphate groups), groups formed by condensation of phosphonic acid (e.g., polyphosphonic acid groups), phosphate ester groups (e.g., monomethyl phosphate groups, polyoxyethylene alkyl phosphate groups), alkylphosphonic acid groups (e.g., methylphosphonic acid groups), etc.

[0041] Furthermore, the sulfur oxoacid group (sulfur oxoacid group or substituent derived from a sulfur oxoacid group) is, for example, a substituent represented by the following formula (2). Multiple substituents represented by the following formula (2) may be introduced into each microfiber cellulose. In this case, the substituents represented by the following formula (2) that are introduced may be the same or different.

[0042]

[0043] In equation (2), b and n are natural numbers, p is 0 or 1, and m is any number (where 1 = b × m). Note that if n is 2 or greater, the multiple p values ​​may be the same number or different numbers. In equation (2), β ｂ＋ β is a cation with one or more valencies composed of organic or inorganic substances. Examples of cations with one or more valencies composed of organic substances include organic onium ions. Examples of organic onium ions include organic ammonium ions and organic onium ions. Examples of organic ammonium ions include aliphatic ammonium ions and aromatic ammonium ions, and examples of organic onium ions include aliphatic phosphonium ions and aromatic phosphonium ions. Examples of cations with one or more valencies composed of inorganic substances include alkali metal ions such as sodium, potassium, or lithium, metal ions belonging to any of groups 2 to 14 of the periodic table, hydrogen ions, ammonium ions, etc. Note that when multiple substituents represented by formula (2) are introduced into fine fibrous cellulose, multiple β ｂ＋ These may be the same or different.

[0044] The amount of anionic groups introduced into the fine fibrous cellulose is preferably 0.10 mmol / g to 5.20 mmol / g per 1 g (mass) of fine fibrous cellulose, more preferably 0.20 mmol / g or more, even more preferably 0.50 mmol / g or more, even more preferably 1.00 mmol / g or more, and more preferably 3.65 mmol / g or less, and even more preferably 3.00 mmol / g or less. Here, the denominator in the unit mmol / g is the amount of the counterion of the anionic group which is a hydrogen ion (H ＋ This indicates the mass of the fine fibrous cellulose when the following conditions are met. By keeping the amount of anionic groups introduced within the above range, it is possible to facilitate the refinement of the fiber raw material and improve the stability of the fine fibrous cellulose.

[0045] The amount of anionic groups introduced into fine fibrous cellulose can be measured after the fibrous cellulose has been subjected to micronization treatment, for example, by a neutralization titration method. In the neutralization titration method, the amount introduced is determined by measuring the change in pH while adding an alkali such as an aqueous sodium hydroxide solution to the slurry containing the obtained fine fibrous cellulose.

[0046] Figure 3 is a graph showing the relationship between the amount of NaOH added to a slurry containing fine fibrous cellulose with phosphorus oxoacid groups and pH. The amount of phosphorus oxoacid groups introduced into the fine fibrous cellulose can be measured, for example, as follows: First, deionized water is added to the target fine fibrous cellulose to prepare a slurry with a solid content of 0.2% by mass. This slurry is processed four times at a pressure of 200 MPa using a wet atomizing device (Starburst, manufactured by Sugino Machine Co., Ltd.) to obtain a fine fibrous cellulose dispersion (slurry) containing fine fibrous cellulose. Then, the fine fibrous cellulose dispersion is treated with a strongly acidic ion exchange resin. Next, the change in pH is observed while adding an aqueous sodium hydroxide solution to obtain a titration curve as shown in the upper part of Figure 3. In the titration curve shown in the upper part of Figure 3, the measured pH is plotted against the amount of alkali added, and in the titration curve shown in the lower part of Figure 3, the increment (derivative value) (1 / mol) of pH with respect to the amount of alkali added is plotted. In this neutralization titration, two points are observed in the curve plotting the measured pH against the amount of alkali added, where the increment (the derivative of pH with respect to the amount of alkali added) is maximum. Of these, the first maximum increment obtained after starting to add alkali is called the first endpoint, and the next maximum increment obtained is called the second endpoint. The amount of alkali required from the start of titration to the first endpoint is equal to the amount of first dissociated acid from the fine fibrous cellulose contained in the slurry used for titration. The amount of alkali required from the first endpoint to the second endpoint is equal to the amount of second dissociated acid from the fine fibrous cellulose contained in the slurry used for titration. The amount of alkali required from the start of titration to the second endpoint is equal to the total amount of dissociated acid from the fine fibrous cellulose contained in the slurry used for titration. The value obtained by dividing the amount of alkali required from the start of titration to the first endpoint by the solid content (g) of the slurry being titrated is the amount of phosphorus oxoacid group introduced (mol / g). Note that when simply referring to the amount of phosphorus oxoacid group introduced (or the amount of phosphorus oxoacid group), it refers to the amount of the first dissociated acid. In Figure 3, the region from the start of titration to the first endpoint is called the first region, and the region from the first endpoint to the second endpoint is called the second region.For example, if the phosphorus oxoacid group is a phosphate group and this phosphate group undergoes condensation, the amount of weakly acidic group in the phosphorus oxoacid group (also referred to as the amount of the second dissociated acid in this specification) appears to decrease, and the amount of alkali required in the second region becomes less than the amount of alkali required in the first region. On the other hand, the amount of strongly acidic group in the phosphorus oxoacid group (also referred to as the amount of the first dissociated acid in this specification) is equal to the amount of phosphorus atoms, regardless of whether condensation occurs or not. Also, if the phosphorus oxoacid group is a phosphorous acid group, there is no weakly acidic group in the phosphorus oxoacid group, so the amount of alkali required in the second region becomes less, or in some cases, the amount of alkali required in the second region becomes zero. In this case, there is only one point on the titration curve where the pH increment is maximum.

[0047] The amount of phosphorus oxoacid groups introduced (mol / g) mentioned above represents the amount of phosphorus oxoacid groups present in the acid-type microfibrous cellulose, since the denominator represents the mass of acid-type microfibrous cellulose (hereinafter referred to as phosphorus oxoacid group amount (acid type)). On the other hand, if the counterion of the phosphorus oxoacid group is substituted with an arbitrary cation C such that it has an equivalent charge, the amount of phosphorus oxoacid groups present in the microfibrous cellulose with cation C as the counterion can be determined by converting the denominator to the mass of microfibrous cellulose with cation C as the counterion (hereinafter referred to as phosphorus oxoacid group amount (C type)). That is, it is calculated using the following formula. Phosphorus oxoacid group amount (C type) = Phosphorus oxoacid group amount (acid type) / {1 + (W - 1) × P / 1000} P [mol / g]: Total amount of anions derived from phosphorus oxoacid groups in the fine fibrous cellulose (total amount of dissociated acids from phosphorus oxoacid groups) W: Formula weight per unit of cation C (for example, Na is 23, Al is 9)

[0048] The amount of carboxyl groups (mol / g), sulfur oxoacid groups (mol / g), and sulfone groups (mol / g) introduced into the fine fibrous cellulose can be determined, for example, by the methods described in Japanese Patent Publication No. 2022-90648 and Japanese Patent Publication No. 2024-145179.

[0049] To obtain fine fibrous cellulose with anionic groups introduced as described above, it is preferable to have an anionic group introduction step, a washing step, an alkali treatment step (neutralization step), and a defibration treatment step in this order, and an acid treatment step may be included instead of the washing step, or in addition to the washing step. Examples of anionic group introduction steps include a phosphorus oxo acid group introduction step, a carboxyl group introduction step, a sulfur oxo acid group introduction step, a xantate group introduction step, a phosphone group or phosphine group introduction step, and a sulfone group introduction step. Each of these will be described below.

[0050] <Fiber Raw Materials> Fiber raw materials are fiber raw materials containing cellulose, and are not particularly limited, but examples include wood pulp, non-wood pulp, and deinked pulp. Wood pulp is not particularly limited, but examples include chemical pulps such as hardwood kraft pulp (LBKP), softwood kraft pulp (NBKP), sulfite pulp (SP), dissolved pulp (DP), soda pulp (AP), unbleached kraft pulp (UKP), and oxygen-bleached kraft pulp (OKP); semi-chemical pulps such as semi-chemical pulp (SCP) and chemical groundwood pulp (CGP); and mechanical pulps such as crushed wood pulp (GP) and thermomechanical pulp (TMP, BCTMP). Non-wood pulp is not particularly limited, but examples include cotton pulps such as cotton linters and cotton lint, and non-wood pulps such as hemp, straw, bamboo, and bagasse. While there are no particular limitations on the deinked pulp, examples include deinked pulp made from recycled paper. Among the above pulps, wood pulp and deinked pulp are preferred from the viewpoint of ease of availability. Among wood pulps, chemical pulp is more preferred from the viewpoint of having a high cellulose ratio and a high yield of fine fibrous cellulose during defibration, and from the viewpoint of obtaining long-fiber fine fibrous cellulose with a large axial ratio due to minimal cellulose decomposition in the pulp. Kraft pulp and sulfite pulp are even more preferred, and softwood kraft pulp is even more preferred. It should be noted that using long-fiber fine fibrous cellulose with a large axial ratio tends to increase viscosity.

[0051] <Phosphorus Oxoate Group Introduction Process> The phosphorus oxoate group introduction process involves reacting a cellulose-containing fiber raw material with at least one compound (hereinafter also referred to as "Compound A") selected from compounds capable of introducing phosphorus oxoate groups by reacting with the hydroxyl groups present in the cellulose-containing fiber raw material. This process yields fine fibrous cellulose having phosphorus oxoate groups.

[0052] In the phosphorus oxoacid group introduction step according to this embodiment, the reaction of the cellulose-containing fiber raw material with compound A may be carried out in the presence of at least one selected from urea and its derivatives (hereinafter also referred to as "compound B"). Alternatively, the reaction of the cellulose-containing fiber raw material with compound A may be carried out in the absence of compound B.

[0053] One example of a method for reacting compound A with a fiber raw material in the presence of compound B is to mix compound A and compound B with the fiber raw material in a dry, wet, or slurry state. Of these, it is preferable to use a fiber raw material in a dry or wet state, and particularly preferable to use a fiber raw material in a dry state, due to the high uniformity of the reaction. The form of the fiber raw material is not particularly limited, but for example, it is preferably in the form of cotton or a thin sheet. Compounds A and B can be added to the fiber raw material in the form of powder, a solution dissolved in a solvent, or after being heated above the melting point and melted. Of these, it is preferable to add them in the form of a solution dissolved in a solvent, particularly an aqueous solution, due to the high uniformity of the reaction. Compounds A and B may be added to the fiber raw material simultaneously, separately, or as a mixture. The method of adding compounds A and B is not particularly limited, but if compounds A and B are in solution form, the fiber raw material may be immersed in the solution and then removed, or the solution may be added dropwise to the fiber raw material. Alternatively, the required amounts of compound A and compound B may be added to the fiber raw material, or excess amounts of compound A and compound B may be added to the fiber raw material, and then the excess compound A and compound B may be removed by pressing or filtration.

[0054] Compound A used in this embodiment may be any compound having a phosphorus atom and capable of forming an ester bond with cellulose, and is not particularly limited to, but includes phosphoric acid or its salts, phosphorous acid or its salts, dehydrated condensed phosphoric acid or its salts, and phosphoric anhydride (phosphorus pentoxide). As phosphoric acid, various purities can be used, for example, 100% phosphoric acid (orthophosphoric acid) or 85% phosphoric acid can be used. As phosphorous acid, 99% phosphorous acid (phosphonic acid) can be used. Dehydrated condensed phosphoric acid is obtained by condensing two or more molecules of phosphoric acid through a dehydration reaction, and examples include pyrophosphoric acid and polyphosphoric acid. Phosphates, phosphites, and dehydrated condensed phosphates include lithium salts, sodium salts, potassium salts, and ammonium salts of phosphoric acid, phosphorous acid, or dehydrated condensed phosphoric acid, and these can be neutralized to various degrees. Of these, phosphoric acid, sodium phosphoric acid, potassium phosphoric acid, ammonium phosphoric acid, or phosphorous acid, sodium phosphorous acid, potassium phosphorous acid, or ammonium phosphorous acid are preferred, with ammonium dihydrogen phosphate being more preferred, from the viewpoint of high efficiency in introducing phosphate groups, ease of improving defibration efficiency in the defibration process described later, low cost, and ease of industrial application.

[0055] The amount of compound A added to the fiber raw material is not particularly limited, but for example, when the amount of compound A added is converted to the amount of phosphorus atoms, the amount of phosphorus atoms added per 100 parts by mass of fiber raw material (oven-dry mass) is preferably 0.5 parts by mass or more and 100 parts by mass or less, more preferably 1 part by mass or more and 50 parts by mass or less, and even more preferably 2 parts by mass or more and 30 parts by mass or less. By keeping the amount of phosphorus atoms added to the fiber raw material within the above range, the yield of fine fibrous cellulose can be further improved. On the other hand, by keeping the amount of phosphorus atoms added to the fiber raw material below the above upper limit, a balance can be struck between the effect of improving yield and cost.

[0056] As described above, compound B used in this embodiment is at least one selected from urea and its derivatives. Examples of compound B include urea, biuret, 1-phenylurea, 1-benzylurea, 1-methylurea, and 1-ethylurea. From the viewpoint of improving the uniformity of the reaction, it is preferable to use compound B as an aqueous solution. Furthermore, from the viewpoint of further improving the uniformity of the reaction, it is preferable to use an aqueous solution in which both compound A and compound B are dissolved.

[0057] The amount of compound B added per 100 parts by mass of fiber raw material (oven-dry mass) is not particularly limited, but for example, it is preferably 1 part by mass or more and 500 parts by mass or less, more preferably 10 parts by mass or more and 400 parts by mass or less, and even more preferably 100 parts by mass or more and 350 parts by mass or less.

[0058] In the reaction of cellulose-containing fiber raw materials with compound A, in addition to compound B, other substances such as amides or amines may be included in the reaction system. Examples of amides include formamide, dimethylformamide, acetamide, and dimethylacetamide. Examples of amines include methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, and hexamethylenediamine. Among these, triethylamine is known to act as a particularly good reaction catalyst.

[0059] In the phosphorus oxoacid group introduction step, it is preferable to add or mix compound A or the like to the fiber raw material and then subject the fiber raw material to heat treatment. The heat treatment temperature is preferably selected to efficiently introduce phosphorus oxoacid groups while suppressing thermal decomposition and hydrolysis reactions of the fibers. For example, the heat treatment temperature is preferably 50°C to 300°C, more preferably 100°C to 250°C, and even more preferably 130°C to 200°C. Furthermore, various heat transfer devices can be used for the heat treatment, such as agitation dryers, rotary dryers, disc dryers, roll-type heaters, plate-type heaters, fluidized bed dryers, band-type dryers, filtration dryers, vibrating fluidized bed dryers, airflow dryers, vacuum dryers, infrared heaters, far-infrared heaters, microwave heaters, and high-frequency dryers.

[0060] In this embodiment, the heat treatment can be performed by, for example, adding compound A to a thin sheet-like fiber raw material by impregnation or other methods, and then heating it, or by kneading or stirring the fiber raw material and compound A with a kneader or the like while heating. This makes it possible to suppress uneven concentration of compound A in the fiber raw material and to introduce phosphorus oxoacid groups more uniformly to the surface of the fibrous cellulose contained in the fiber raw material. This is thought to be because, as water molecules move to the surface of the fiber raw material during drying, dissolved compound A is attracted to the water molecules by surface tension and similarly moves to the surface of the fiber raw material (i.e., uneven concentration of compound A is created), and this can be suppressed.

[0061] Furthermore, the heating device used for the heat treatment is preferably one that can constantly discharge moisture, such as the moisture held in the slurry and the moisture generated by the dehydration condensation (phosphate esterification) reaction between compound A and hydroxyl groups contained in cellulose in the fiber raw material, from the device system. Examples of such heating devices include ovens with a forced-air system. By constantly discharging moisture from the device system, it is possible to suppress the hydrolysis reaction of phosphate ester bonds, which is the reverse reaction of phosphate esterification, as well as the acid hydrolysis of sugar chains in the fibers. As a result, it becomes possible to obtain fine fibrous cellulose with a high axial ratio.

[0062] The heating time is preferably 1 second to 300 minutes, more preferably 1 second to 1,000 seconds, and even more preferably 10 seconds to 800 seconds, starting from when the moisture has been substantially removed from the fiber raw material. In this embodiment, by setting the heating temperature and heating time within an appropriate range, the amount of phosphorus oxoacid group introduced can be kept within a preferred range.

[0063] The phosphorus oxoacid group introduction process only needs to be performed at least once, but it can also be repeated two or more times. By performing the phosphorus oxoacid group introduction process two or more times, a large number of phosphorus oxoacid groups can be introduced into the fiber raw material.

[0064] The amount of phosphorus oxoacid groups introduced into the fiber raw material is preferably 0.10 mmol / g or more and 5.20 mmol / g or less per 1 g (mass) of fibrous cellulose, more preferably 0.20 mmol / g or more, even more preferably 0.50 mmol / g or more, even more preferably 1.00 mmol / g or more, and more preferably 3.65 mmol / g or less, and even more preferably 3.00 mmol / g or less. By keeping the amount of phosphorus oxoacid groups introduced within the above range, the refinement of cellulose fibers in the defibration process can be facilitated, and the stability of the fine fibrous cellulose can be enhanced.

[0065] The steps of introducing a carboxyl group, a sulfur oxoacid group, a xantate group, a phosphone group or phosphine group, and a sulfone group can be carried out by referring to, for example, the methods described in Japanese Patent Publication No. 2022-90648 and Japanese Patent Publication No. 2024-145179.

[0066] <Washing Process> In the process of obtaining fine fibrous cellulose having anionic groups, a washing process may be performed on the anionic group-introduced fibers as needed. The washing process is carried out, for example, by washing the anionic group-introduced fibers with water or an organic solvent. Furthermore, the washing process may be performed after each of the processes described later, and the number of washes performed in each washing process is not particularly limited.

[0067] <Alkali Treatment Step> In the process of obtaining fine fibrous cellulose having anionic groups, an alkali treatment step may be provided between the anionic group introduction step and the defibration treatment step. The alkali treatment method is not particularly limited, but one example is immersing the anionic group-introduced fibers in an alkaline solution.

[0068] The alkali compound contained in the alkaline solution is not particularly limited and may be an inorganic alkali compound or an organic alkali compound. In this embodiment, for its high versatility, it is preferable to use sodium hydroxide or potassium hydroxide as the alkali compound, and more preferable to use sodium hydroxide. The solvent contained in the alkaline solution may be either water or an organic solvent. Among these, the solvent contained in the alkaline solution is preferably water or a polar solvent including a polar organic solvent such as alcohol, and more preferably an aqueous solvent containing at least water. As the alkaline solution, for its high versatility, it is preferable to use an aqueous solution of sodium hydroxide or an aqueous solution of potassium hydroxide, and more preferably an aqueous solution of sodium hydroxide.

[0069] The temperature of the alkaline solution in the alkaline treatment step is not particularly limited, but for example, it is preferably 5°C to 80°C, more preferably 10°C to 60°C. The immersion time of the anionic group-introduced fiber in the alkaline solution in the alkaline treatment step is not particularly limited, but for example, it is preferably 5 minutes to 30 minutes, more preferably 10 minutes to 20 minutes. The amount of alkaline solution used in the alkaline treatment is not particularly limited, but for example, it is preferably 100 parts by mass to 100,000 parts by mass, more preferably 1,000 parts by mass to 10,000 parts by mass, per 100 parts by mass of the absolute dry mass of the anionic group-introduced fiber.

[0070] To reduce the amount of alkaline solution used in the alkaline treatment process, the anionic group-introduced fibers may be washed with water or an organic solvent after the anionic group introduction process and before the alkaline treatment process. After the alkaline treatment process and before the defibration process, it is preferable to wash the alkaline-treated anionic group-introduced fibers with water or an organic solvent to improve handling.

[0071] <Fibration Process> By defibrating the fiber raw material or anionic group-introduced fiber in the defibration process, fine fibrous cellulose is obtained. Defibration is also called micronization. In the defibration process, for example, a defibration processing device can be used. The defibration processing device is not particularly limited, but for example, a high-speed defibrator, grinder (stone mill type grinder), high-pressure homogenizer or ultra-high-pressure homogenizer, high-pressure impact grinder, ball mill, bead mill, disc type refiner, conical refiner, twin-screw kneader, vibrating mill, homomixer under high-speed rotation, ultrasonic disperser, or beater can be used. Among the above defibration processing devices, it is more preferable to use a high-speed defibrator, high-pressure homogenizer, or ultra-high-pressure homogenizer, which have less influence from the grinding media and less risk of contamination.

[0072] In the defibration process, it is preferable to dilute, for example, the fiber raw material or the anionic group-introduced fiber with a dispersion medium to form a slurry. As the dispersion medium, one or more types of organic solvents selected from water and polar organic solvents can be used. The polar organic solvent is not particularly limited, but examples of preferred solvents include alcohols, polyhydric alcohols, ketones, ethers, esters, and aprotic polar solvents. Examples of alcohols include methanol, ethanol, isopropanol, n-butanol, and isobutyl alcohol. Examples of polyhydric alcohols include ethylene glycol, propylene glycol, and glycerin. Examples of ketones include acetone and methyl ethyl ketone (MEK). Examples of ethers include diethyl ether, tetrahydrofuran, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-butyl ether, and propylene glycol monomethyl ether. Examples of esters include ethyl acetate and butyl acetate. Examples of aprotic polar solvents include dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), and N-methyl-2-pyrrolizinone (NMP).

[0073] The solid content concentration of the fine fibrous cellulose during the defibration process can be set as appropriate. Furthermore, the slurry obtained by dispersing the anionic group-introduced fibers in a dispersion medium may contain solid components other than the anionic group-introduced fibers, such as hydrogen-bonding urea.

[0074] In this embodiment, the cellulose fibers are dispersed as a 0.2% by mass aqueous dispersion. After centrifugation at 12,000 G for 10 minutes and drying at 105°C, the dry mass ratio of the precipitate after centrifugation to the solid content in the supernatant is preferably 90:10 to 10:90, more preferably 85:15 to 15:85, and even more preferably 80:20 to 20:80, from the viewpoint of suppressing the generation of aggregates in the crosslinked rubber composition, obtaining sufficient tensile elongation, and furthermore, having an excellent effect on improving the modulus in the low elongation region. The content of coarse cellulose fibers in the precipitate is preferably 70% by mass or more, more preferably 75% by mass or more, even more preferably 80% by mass or more, and 100% by mass or less. The content of fine fibrous cellulose in the solid content of the supernatant is preferably 80% by mass or more, more preferably 85% by mass or more, even more preferably 90% by mass or more, and 100% by mass or less.

[0075] The cellulose fibers of this embodiment function as a reinforcing filler for the rubber component, modifying it. When the cellulose fibers of this embodiment are added to the rubber component to form a rubber composite, a crosslinking agent is added to the rubber composite to form a rubber compound, and the rubber compound is crosslinked to form a crosslinked rubber composition, the cellulose fibers exhibit an effect of improving the modulus in the low elongation region, thereby imparting sufficient tensile elongation to the crosslinked rubber composition.

[0076] [Cellulose Fiber Aqueous Dispersion] This embodiment also relates to a cellulose fiber aqueous dispersion (cellulose fiber-containing slurry) obtained by dispersing the cellulose fibers described above in a solvent (dispersion medium) containing water. The cellulose fiber aqueous dispersion is, for example, a rubber modifier used for addition to rubber components.

[0077] The cellulose fiber content in the cellulose fiber aqueous dispersion is preferably 0.3% to 7.0% by mass, more preferably 0.5% or more by mass, even more preferably 1.0% or more by mass, even more preferably 6.5% or less by mass, even more preferably 6.0% or less by mass, and even more preferably 5.5% or less by mass, based on the total mass of the cellulose fiber aqueous dispersion. A fibrous cellulose content above the lower limit is preferable because it allows for a smaller amount of aqueous dispersion to be added and is suitable for storage and transportation, while a content below the upper limit is preferable because it offers excellent ease of manufacture and maintains an appropriate viscosity.

[0078] The fibrous cellulose aqueous dispersion may contain a solvent containing water and other additives in addition to the fibrous cellulose of this embodiment. Examples of solvents other than water include organic solvents such as polar organic solvents. The polar organic solvent is not particularly limited, but preferred examples include alcohols, polyhydric alcohols, ketones, ethers, esters, and aprotic polar solvents. Examples of alcohols include methanol, ethanol, isopropanol, n-butanol, and isobutyl alcohol. Examples of polyhydric alcohols include ethylene glycol, propylene glycol, and glycerin. Examples of ketones include acetone and methyl ethyl ketone (MEK). Examples of ethers include diethyl ether, tetrahydrofuran, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-butyl ether, and propylene glycol monomethyl ether. Examples of esters include ethyl acetate and butyl acetate. Examples of aprotic polar solvents include dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), and N-methyl-2-pyrrolizinone (NMP). Among these, the dispersion medium preferably contains at least water, more preferably 50% by mass or more of the dispersion medium is water, even more preferably 80% by mass or more, even more preferably 90% by mass or more, and even more preferably 100% by mass. Other additives include, for example, defoamers, lubricants, ultraviolet absorbers, dyes, pigments, stabilizers, surfactants, and preservatives (e.g., phenoxyethanol).

[0079] A cellulose fiber aqueous dispersion can be obtained, for example, by mixing an aqueous suspension of coarse cellulose fibers (coarse cellulose fiber suspension) with an aqueous dispersion of fine fibrous cellulose (fine fibrous cellulose dispersion). When mixing the coarse cellulose fiber suspension and the fine fibrous cellulose dispersion, the mixing ratio of coarse cellulose fibers in the coarse cellulose fiber suspension to fine fibrous cellulose in the fine fibrous cellulose dispersion (coarse cellulose fibers:fine fibrous cellulose (mass ratio)) is preferably 90:10 to 10:90, more preferably 85:15 to 15:85, from the viewpoint of suppressing the generation of aggregates in the crosslinked rubber composition, obtaining sufficient tensile elongation, and furthermore, providing an excellent effect in improving modulus in the low elongation region. Similarly, the mass ratio of coarse cellulose fibers to fine fibrous cellulose in the cellulose fibers (coarse cellulose fibers:fine fibrous cellulose) is preferably 90:10 to 10:90, more preferably 85:15 to 15:85. As described above, it is preferable to obtain a cellulose fiber aqueous dispersion by mixing a coarse cellulose fiber suspension with a fine fibrous cellulose dispersion, such that the ratio of the precipitate to the solid content in the supernatant after centrifuging the cellulose fibers of this embodiment as a 0.2% by mass aqueous dispersion and drying at 12,000 G for 10 minutes and 105°C is preferably 90:10 to 10:90, more preferably 85:15 to 15:85, and even more preferably 80:20 to 20:80.

[0080] [Rubber Composite Material] The rubber composite material of this embodiment contains rubber components and cellulose fibers of this embodiment. The rubber composite material of this embodiment exhibits excellent dispersibility of cellulose fibers.

[0081] [Rubber Component] The rubber composite material of this embodiment contains a rubber component. As the rubber component, for example, natural rubber (NR) or synthetic rubber can be used. Examples of synthetic rubbers include styrene-butadiene rubber (SBR), nitrile rubber (NBR), chloroprene rubber (CR), ethylene propylene rubber (EPDM), butyl rubber (IIR), chlorobutyl rubber (CIIR), acrylic rubber (ACM), silicone rubber (Q), fluororubber (FKM), butadiene rubber (BR), epoxidized butadiene rubber (EBR), epichlorohydrin rubber (CO,CEO), urethane rubber (U), and polysulfide rubber (T). Examples of nitrile rubbers include modified nitrile rubbers such as hydrogenated nitrile rubber, carboxyl-modified nitrile rubber, silicone-modified nitrile rubber, maleic acid-modified nitrile rubber, and hydroxyl-modified nitrile rubber, or hydrogenated versions thereof; and acrylonitrile-butadiene-isoprene copolymers in which a portion of butadiene is replaced with isoprene. Hydrogenated nitrile rubber (H-NBR) is sometimes called hydrogenated nitrile rubber or hydrogenated acrylonitrile-butadiene rubber. Hydrogenated nitrile rubber can be obtained by hydrogenating the double bonds contained in nitrile rubber. Examples of natural rubber include natural rubber (NR), epoxidized natural rubber (ENR), modified natural rubber such as methyl methacrylate (MMA) graft polymerized natural rubber, hydrogenated natural rubber, and deproteinized natural rubber. These rubber components may be pre-crosslinked raw materials without a crosslinked structure, or they may have a crosslinked structure.

[0082] Among these, the rubber component preferably contains diene rubber, more preferably contains at least one selected from natural rubber, nitrile rubber, butadiene rubber, and styrene-butadiene rubber, and even more preferably contains natural rubber. The rubber component may also be a pre-crosslinking raw material. The rubber component is preferably the latex of these rubber components. By using the latex of the rubber component, the dispersibility of the cellulose fibers in this embodiment is improved, and a high modulus improvement effect is more easily obtained when it is made into a crosslinked rubber composition. When using the latex of the rubber component, a solid rubber component may be further mixed in. The solid rubber component may be added when kneading the rubber composition or when preparing the rubber compound, but it is preferable to add it when preparing the rubber compound.

[0083] In the rubber composite of this embodiment, the content of cellulose fibers of this embodiment per 100 parts by mass of rubber component is preferably 5 parts by mass or more and 50 parts by mass or less, more preferably 8 parts by mass or more, even more preferably 11 parts by mass or more, even more preferably 14 parts by mass or more, even more preferably 18 parts by mass or more, and more preferably 45 parts by mass or less, even more preferably 40 parts by mass or less, and even more preferably 35 parts by mass or less. Note that the content of cellulose fibers of this embodiment per 100 parts by mass of rubber component in the rubber composite of this embodiment may differ from the content of cellulose fibers per 100 parts by mass of rubber component when forming a crosslinked rubber composition. Specifically, the content of cellulose fibers may be increased in the rubber composite, and the content of cellulose fibers relative to the rubber component may be adjusted by adding rubber components when preparing the rubber compound described later.

[0084] The rubber composite material of this embodiment may also contain other components such as zinc oxide, vulcanization accelerators, antioxidants, and reinforcing agents, and may be added at any stage of the manufacturing process.

[0085] <Method for Manufacturing Rubber Composite Material> The method for manufacturing the rubber composite material involves preparing a mixed solution containing rubber components and cellulose fibers of this embodiment, and then removing the solvent from the mixed solution to obtain the rubber composite material. In the above manufacturing method, first, a mixed solution containing rubber components and cellulose fibers of this embodiment is prepared. That is, the method for manufacturing the rubber composite material of this embodiment includes the following steps (I) and (II). (I) A cellulose fiber aqueous dispersion with a solid content of 0.2% by mass is centrifuged under the following conditions to obtain a precipitate and supernatant, wherein the precipitate contains coarse cellulose fibers, the irregular freeness of the precipitate is 950 mL or less, and the solid content in the supernatant contains fine fibrous cellulose with a fiber width of 100 nm or less. A cellulose fiber aqueous dispersion is mixed with rubber latex to obtain mixed solution A. Mixing step conditions: Centrifugation at 12,000 G for 10 minutes. (II) A solidification step to obtain a solid rubber composite material from mixed solution A.

[0086] Specifically, step (I) is a mixing step in which the aqueous dispersion of cellulose fibers of this embodiment and rubber latex are mixed to obtain a mixture A. Mixing can be carried out using known devices such as a disperser, a three-one motor, a Creamix, a homomixer, a homogenizer, or a propeller agitator (e.g., a tornado agitator). The mixing temperature is not limited, but room temperature (20-30°C) is preferred. The mixing time can also be adjusted as appropriate. When preparing mixture A, thoroughly mixing the rubber component and cellulose fibers tends to improve the physical properties after crosslinking.

[0087] The rubber latex is preferably a dispersion of rubber components in an aqueous medium. The aqueous medium mainly consists of water, and the water content relative to the total aqueous medium is preferably 50% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and even more preferably 95% by mass or more, with no particular upper limit, but it is 100% by mass or less. In addition to water, known organic solvents and the like may be included to an extent that does not impair the effects of this disclosure.

[0088] The solid content concentration (by mass) of mixture A is, for example, 0.5% by mass or more and 50% by mass or less. By keeping the solid content concentration of the mixture within the above range, the amount of energy required to remove solvents such as water can be reduced, and furthermore, aggregation of cellulose fibers in the resulting rubber composite material becomes less likely, resulting in excellent kneadability of the resulting rubber composite material and superior properties of the crosslinked rubber composition.

[0089] The solid content concentration of the dispersion of mixture A can be calculated using the following formula (I) from the mass of the dried product obtained by drying a predetermined amount of the dispersion in a 105°C oven until a constant weight is reached, and from the mass of the dispersion of the mixture used for drying: Solid content concentration of mixture (mass%) = Mass of dried product [g] / Mass of mixture used for drying [g] × 100 (I)

[0090] The process for obtaining a rubber composite is a solidification step, as shown in step (II) above, in which a solid rubber composite is obtained from the obtained mixed liquid A. Preferably, the solidification step involves heating and drying the obtained mixed liquid A to obtain a solid rubber composite. However, the solidification step is not limited to this; solidification may also be achieved by coagulation through the addition of an acid. Common acids used for coagulation include sulfuric acid, hydrochloric acid, formic acid, and acetic acid. The temperature for coagulation is preferably 10 to 40°C. During coagulation, it is preferable to adjust the pH of mixed liquid A to 3 to 5, and more preferably to 3 to 4. Furthermore, a flocculant may be added to control the state of coagulation (size of the coagulated aggregated particles). Cationic polymers can be used as flocculants.

[0091] The heating and drying method is not particularly limited, and examples include coating the mixture onto a substrate or drying with a drum dryer. In the coating method, for example, a mixture A containing rubber components and cellulose fibers of this embodiment is coated onto the substrate. Furthermore, by using a coating apparatus and a long substrate, sheets can be produced continuously. The material of the substrate used in the coating method is not particularly limited, but a material with high wettability to the mixture A is preferable as it can suppress shrinkage of the sheet during drying, but it is preferable to select a material that allows the sheet formed after drying to be easily peeled off. Among these, resin films or plates, or metal films or plates are preferred, but are not particularly limited. For example, resin films or plates such as acrylic, polyethylene terephthalate, vinyl chloride, polystyrene, polypropylene, polycarbonate, and polyvinylidene chloride, metal films or plates of aluminum, zinc, copper, and iron, and those whose surfaces have been oxidized, stainless steel films or plates, brass films or plates, etc. can be used. In the coating method, if the viscosity of the mixed liquid A is low and it spreads on the substrate, a damming frame may be fixed to the substrate to obtain a sheet of a predetermined thickness and basis weight. Alternatively, a substrate with a bat-shaped damming frame may be used. The damming frame is not particularly limited, but it is preferable to select one that allows the edges of the sheet that adhere after drying to be easily peeled off. From this viewpoint, a molded resin plate or metal plate is more preferable. In this embodiment, for example, resin plates such as acrylic plates, polyethylene terephthalate plates, vinyl chloride plates, polystyrene plates, polypropylene plates, polycarbonate plates, and polyvinylidene chloride plates, as well as metal plates such as aluminum plates, zinc plates, copper plates, and iron plates, and molded stainless steel plates, brass plates, etc., can be used. The coating machine for coating the mixed liquid A onto the substrate is not particularly limited, but for example, a roll coater, gravure coater, die coater, curtain coater, air doctor coater, etc. can be used. Die coaters, curtain coaters, and spray coaters are preferred because they allow for a more uniform sheet thickness.

[0092] The temperature of the mixed liquid A and the ambient temperature when coating the substrate with the mixed liquid A (hereinafter, the temperature of the mixed liquid and the ambient temperature are collectively referred to as the "coating temperature") are not particularly limited, but are preferably 5°C to 80°C, more preferably 10°C to 60°C, even more preferably 15°C to 50°C, and even more preferably 20°C to 50°C. If the coating temperature is above the lower limit, the dispersion of the mixture can be coated more easily. If the coating temperature is below the upper limit, the volatilization of the dispersion medium during coating can be suppressed. The drying equipment used is not particularly limited, but examples include explosion-proof dryers. In the coating process, it is preferable to coat the mixed liquid A onto the substrate so that the finished basis weight and thickness of the sheet are within the preferred range described above.

[0093] The mixed liquid A may be heated and dried in a heated cylindrical dryer to obtain a rubber composite material. Figure 1 is a schematic side cross-sectional view of a double drum dryer 10, which is an example of a heated cylindrical dryer, and Figure 2 is a schematic side view of the double drum dryer 10. First, a heat transfer medium (generally steam) is introduced into the rotating cylinder 1 (drum), and the mixed liquid A is introduced into the feed section 2. The introduced mixed liquid A adheres to the surface of the heated cylinder 1 as a sheet-like mixture 3, and is quickly heated and dried. As the cylinder 1 rotates, the sheet-like mixture 3 is scraped off by a fixed scraper 4, and a rubber composite material 5 (heat-dried product) can be obtained.

[0094] The lower limit of the surface temperature of the heated cylindrical dryer is preferably 80°C or higher, and more preferably 90°C or higher. The upper limit of the surface temperature is preferably 250°C or lower, and more preferably 200°C or lower. The surface temperature of the heated cylindrical dryer is the temperature of the cylindrical surface in contact with the mixed liquid A. The surface temperature of the heated cylindrical dryer may be, for example, 80 to 250°C, 90 to 200°C, or 95 to 180°C. By keeping the surface temperature of the heated cylindrical dryer within the above range, it is possible to obtain a rubber composite material with good production efficiency and excellent physical properties after crosslinking.

[0095] The lower limit of the heating and drying time in a heated cylindrical dryer is preferably 2 seconds or more, and more preferably 4 seconds or more. The upper limit of the heating and drying time is preferably 1,800 seconds or less, and more preferably 600 seconds or less. The heating and drying time [sec] in a heated cylindrical dryer refers to the time [sec] that the mixture is in contact with the surface of the cylinder, and the heating and drying time may be, for example, 2 to 1,800 seconds, 4 to 600 seconds, or 6 to 400 seconds. By having the heating and drying time within the above range, a dried product that is neither under-dried nor over-dried is obtained, and the product exhibits excellent peelability from the drum.

[0096] The lower limit of the cylindrical width w of the heated cylindrical dryer is preferably 0.1 m or more, and more preferably 0.2 m or more. The upper limit of the cylindrical width w is not particularly limited, but is usually 7 m or less. The cylindrical width w may be, for example, 0.1 to 7 m, or 0.2 to 5 m. By having the cylindrical width w within the above range, a dried product with a uniform thickness can be obtained.

[0097] The lower limit of the cylindrical rotation speed v of the heated cylindrical dryer is preferably 0.001 [m / sec] or higher, and more preferably 0.002 [m / sec] or higher. The upper limit of the cylindrical rotation speed v is not particularly limited, but is usually 35 [m / sec] or lower. The cylindrical rotation speed v may be, for example, 0.001 to 3.5 [m / sec] or 0.002 to 0.2 [m / sec]. By having the cylindrical rotation speed v within the above range, the time the mixture is in contact with the heated drum can be set to an appropriate range, and a dried product with excellent kneadability and physical properties after crosslinking can be obtained.

[0098] The heated cylindrical dryer used in the method for manufacturing rubber composite materials can be any conduction-type dryer that introduces a heat transfer medium into the inside of a cylinder and heats and dries the mixture by bringing it into contact with the heated surface of the cylinder. For example, drum dryers such as double drum dryers, single drum dryers, and twin drum dryers may be used, and cylinder dryers, Yankee dryers, etc., can also be used. Among these, from the viewpoint of heating and drying efficiency, the heated cylindrical dryer is preferably a double drum dryer or a cylinder dryer.

[0099] [Rubber Compound] In this embodiment, "rubber compound" refers to a composition obtained by kneading rubber composite materials before crosslinking. The rubber compound preferably contains rubber components and cellulose fibers, as well as at least a crosslinking agent. In addition to the crosslinking agent, it may also contain additives usable in the field of rubber, such as zinc oxide, vulcanization accelerators, fillers, softeners, fatty acids, antioxidants, deconjugates, colorants, pH adjusters, and curing resins. Furthermore, solid rubber may be added in addition to the rubber components when preparing the rubber composite material. When adding solid rubber, the solid rubber may be the same as those exemplified as rubber components, but instead of adding it in latex form, solid rubber should be added. Examples of crosslinking agents include sulfur-based crosslinking agents (e.g., sulfur such as powdered sulfur, sulfurous salt, precipitated sulfur, colloidal sulfur, surface-treated sulfur, and insoluble sulfur; sulfur-containing compounds such as disulfide amines, polymer polysulfides, sulfur olefin adducts, sulfur chloride, and sulfur dichloride; insoluble polymer sulfur, etc.), peroxide-based crosslinking agents (e.g., dicumyl peroxide, dichlorobenzoyl peroxide, benzoyl peroxide, 1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane, n-butyl-4,4-bis(t-butylperoxy)valerate, di-t-butylperoxy-diisopropylbenzene, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane), and quinoid-based crosslinking agents (e.g., p-quinone dioxime, p,p'-dibenzoylquinone dioxime). Among these, sulfur-based crosslinking agents or peroxide-based crosslinking agents are preferred. The amount of crosslinking agent added is not particularly limited, but is preferably 0.1 parts by mass or more and 10 parts by mass or less, more preferably 0.5 parts by mass or more, even more preferably 1.0 part by mass or more, and even more preferably 5 parts by mass or less, and even more preferably 3 parts by mass or less, per 100 parts by mass of rubber component.

[0100] Furthermore, a crosslinking accelerator that promotes crosslinking by the crosslinking agent may be added, and when using a sulfur-based crosslinking agent, it is preferable to add a crosslinking accelerator (vulcanization accelerator). When using a sulfur-based crosslinking agent (vulcanization agent) as the crosslinking agent, it is preferable to use a sulfenamide-based vulcanization accelerator, a guanidine-based vulcanization accelerator, a thiazole-based vulcanization accelerator, a thiram-based vulcanization accelerator, a dithiocarbamate-based vulcanization accelerator, etc., as the crosslinking accelerator (vulcanization accelerator).

[0101] Examples of fillers include carbon black and silica. There are no particular restrictions on the amount of filler added, but it is preferably 10 to 150 parts by mass, and more preferably 20 to 100 parts by mass, per 100 parts by mass of rubber component.

[0102] Examples of softening agents include aromatic oils, paraffin oil, naphthenic oil, vegetable oils other than castor oil, low PCA oils such as MES, TDAE, SRAE, and heavy naphthenic oil. Suitable low PCA oils include various plant-derived oils that can be harvested from vegetables, nuts, and seeds. Examples of plant-derived oils include soybean oil, sunflower oil, safflower oil, corn oil, linseed oil, cottonseed oil, rapeseed oil, cashew oil, sesame oil, camellia oil, jojoba oil, macadamia nut oil, coconut oil, and palm oil. The amount of softening agent added is preferably 1 to 35 parts by mass, more preferably 5 parts by mass or more, even more preferably 10 parts by mass or more, and even more preferably 30 parts by mass or less, and even more preferably 25 parts by mass or less, per 100 parts by mass of rubber component.

[0103] Examples of fatty acids include stearic acid, palmitic acid, arachidic acid, oleic acid, linoleic acid, and arachidonic acid. Among these, stearic acid is preferred. There are no particular restrictions on the amount of fatty acid added, but it is preferably 0.1 parts by mass to 5 parts by mass, and more preferably 1 part by mass to 4 parts by mass, per 100 parts by mass of rubber component.

[0104] Examples of anti-aging agents include N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), 2,2,4-trimethyl-1,2-dihydroquinoline polymer (TMDQ), 6-ethoxy-2,2,4-trimethyl-1,2-dihydroquinoline (AW), N,N'-diphenyl-p-phenylenediamine (DPPD), and 2-mercaptobenzimidazole (MBI). There are no particular restrictions on the amount of anti-aging agent added, but preferably it is 0.1 parts by mass to 5 parts by mass, more preferably 1 part by mass to 3 parts by mass, per 100 parts by mass of the rubber component. The total content of anti-aging agents is preferably 0.2 parts by mass to 10 parts by mass, more preferably 1 part by mass to 5 parts by mass, per 100 parts by mass of the rubber component.

[0105] There are no particular restrictions on the amount of zinc oxide (zinc oxide) added, but it is preferably 1 to 10 parts by mass, and more preferably 1.5 to 8 parts by mass, per 100 parts by mass of the rubber component.

[0106] [Method for Manufacturing Rubber Compound] The method for manufacturing the rubber compound is not particularly limited, but it is preferable to have at least a kneading step of kneading the rubber composite material of this embodiment. In the kneading step, it is preferable to knead the rubber composite material of this embodiment first, and then add at least a crosslinking agent and knead it. In addition to the crosslinking agent, the additives mentioned above may also be added. Furthermore, in the kneading step, in addition to the rubber components contained in the rubber composite material, solid rubber may also be added. In this embodiment, in the rubber compound and the crosslinked rubber composition described later, the content of cellulose fibers of this embodiment per 100 parts by mass of rubber components is preferably 5 parts by mass or more and 50 parts by mass or less, more preferably 8 parts by mass or more, even more preferably 11 parts by mass or more, even more preferably 14 parts by mass or more, even more preferably 18 parts by mass or more, and more preferably 45 parts by mass or less, even more preferably 40 parts by mass or less, and even more preferably 35 parts by mass or less.

[0107] The method for manufacturing the rubber compound is not particularly limited, but it is preferable to include a step of kneading the rubber composite material of this embodiment. By kneading, the dispersibility of the crosslinking agent and other additives added thereafter is improved. The kneading can be done by conventional methods, but it may also be done by low-temperature kneading using a roll machine such as an open roll, or by high-temperature kneading using a Banbury mixer or the like. Among these, low-temperature kneading is preferable from the viewpoint of the dispersibility of the cellulose fibers of this embodiment in the rubber composite material. The temperature for low-temperature kneading is preferably 15°C to 70°C, more preferably 20°C to 65°C, and even more preferably 25°C to 60°C. The temperature for high-temperature kneading is preferably 80°C to 200°C. Furthermore, the rubber compound is manufactured by adding additives such as a crosslinking agent to the kneaded rubber composite material and kneading them together.

[0108] Mixing is the process of uniformly dispersing crosslinking agents and other compounding agents in a rubber composite (preferably a pre-mixed rubber composite). Mixing may be carried out in known manner, for example, using a Banbury mixer, kneader, or open roll. Examples of crosslinking agents include sulfur and peroxides. Examples of other compounding agents include the additives mentioned above and solid rubber.

[0109] [Crosslinked Rubber Composition and Method for Producing the Same] The crosslinked rubber composition of this embodiment is obtained by crosslinking a rubber compound containing a crosslinking agent. Alternatively, molding may be performed during the crosslinking process. That is, the method for producing the crosslinked rubber composition includes a step of crosslinking a rubber compound containing a rubber composite material and a crosslinking agent, and preferably includes a crosslinking and molding step. The method for producing the crosslinked rubber composition of the present invention may further include a step of molding the rubber compound obtained by the kneading step into the desired shape (molding step). Molding in this molding step can be carried out by various molding methods using an extrusion molding machine, calender roll, press, injection molding machine, transfer molding machine, hollow molding machine, foam molding machine, etc. The method should be appropriately selected according to the shape, application, and molding method of the final product. The kneading step and the molding step may be performed separately or continuously.

[0110] Regarding crosslinking, there are no particular restrictions on temperature as long as the conditions are suitable for the crosslinking reaction to proceed. Generally, a crosslinked rubber composition is obtained by heating an uncrosslinked rubber compound obtained by kneading to crosslink it (also called vulcanization if sulfur is included). The heating temperature is preferably 140°C or higher, preferably 200°C or lower, and more preferably 180°C or lower. Therefore, the heating temperature is preferably around 140 to 200°C, and more preferably around 140 to 180°C. For crosslinking, vulcanization equipment such as mold vulcanization, can vulcanization, or continuous vulcanization can be used.

[0111] In this embodiment, when the 25% modulus (σ25) of the crosslinked rubber composition of the formulation described in the example is denoted as σ25B, and the 25% modulus (σ25) of the crosslinked rubber composition obtained by crosslinking the rubber compound of this embodiment excluding reinforcing materials such as cellulose fibers and carbon black is denoted as σ25A, then σ25B / σ25A is preferably 2.0 to 40.0, more preferably 3.0 or more, even more preferably 4.0 or more, even more preferably 5.0 or more, even more preferably 5.5 or more, even more preferably 6.0 or more, and more preferably 35.0 or less, and even more preferably 30.0 or less. From the viewpoint of ease of manufacture, it is preferable that σ25B / σ25A is below the above upper limit. In this embodiment, when the 50% modulus (σ50) of the crosslinked rubber composition of the formulation described in the example is denoted as σ50B, and the 50% modulus (σ50) of the crosslinked rubber composition obtained by crosslinking the rubber compound of this embodiment excluding reinforcing materials such as cellulose fibers and carbon black is denoted as σ50A, then σ50B / σ50A is preferably 2.0 to 35.0, more preferably 3.0 or higher, even more preferably 4.0 or higher, even more preferably 5.0 or higher, even more preferably 5.5 or higher, even more preferably 6.0 or higher, and preferably 35.0 or lower, more preferably 30.0 or lower, and even more preferably 25.0 or lower. From the viewpoint of ease of manufacturing, it is preferable that σ50B / σ50A is below the above upper limit. When σ25B / σ25A and σ50B / σ50A are within the above range, it is preferable because it provides an excellent effect in improving the modulus in the low elongation region.

[0112] <Applications> The crosslinked rubber composition of this embodiment can be used for a variety of applications and is not particularly limited. For example, it is suitably used in sealing materials, hoses, shoe soles, tire components, vibration-damping rubber, etc. Among these, the crosslinked rubber composition is preferably used in tires.

[0113] The features of the present invention will be further described below with reference to examples and comparative examples. The materials, amounts used, proportions, processing content, and processing procedures shown in the following examples can be modified as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be interpreted as being limited by the following specific examples.

[0114] <Manufacturing Example 1> As a fiber raw material, softwood kraft pulp (solid content 90% by mass, basis weight 950 g / m²) ２ A sheet-like pulp with a Canadian standard filtration efficiency (CSF) of 800 mL (measured according to JIS P 8121-2:2012) was used. Deionized water was added to this raw material pulp to prepare a slurry with a solid content of 4.0% by mass. This slurry was beaten for 3 hours in a double disc refiner (manufactured by Mitsubishi Heavy Industries BELOIT) to obtain a coarse cellulose fiber suspension 1.

[0115] <Production Example 2> A coarse cellulose fiber suspension 2 was obtained in the same manner as in Production Example 1, except that the beating treatment time was 1 hour.

[0116] <Production Example 3> A coarse cellulose fiber suspension 3 was obtained in the same manner as in Production Example 1, except that the beating treatment time was 4.5 hours.

[0117] <Manufacturing Example 4> [Phosphorus Oxo-Oxidation Treatment] As raw material pulp, softwood kraft pulp manufactured by Oji Paper Co., Ltd. (solids content 93% by mass, basis weight 245 g / m²) ２A sheet-like pulp with a Canadian standard filtration efficiency (CSF) of 700 mL (measured according to JIS P 8121-2:2012) was used. This raw material pulp was subjected to phosphorus oxooxidation treatment as follows: First, a mixed aqueous solution of ammonium dihydrogen phosphate and urea was added to 100 parts by mass (oven-dry mass) of the raw material pulp to adjust the mixture to 45 parts by mass of ammonium dihydrogen phosphate, 120 parts by mass of urea, and 150 parts by mass of water to obtain chemically impregnated pulp. Next, the obtained chemically impregnated pulp was heated in a hot air dryer at 165°C for 250 seconds to introduce phosphate groups into the cellulose in the pulp and obtain phosphorylated pulp.

[0118] Next, the obtained phosphorylated pulp was subjected to a washing treatment. The washing treatment was carried out by repeatedly adding 10 L of deionized water to 100 g (dry mass) of phosphorylated pulp to obtain a pulp dispersion, stirring the mixture to ensure uniform dispersion of the pulp, and then filtering and dewatering it. The washing was terminated when the electrical conductivity of the filtrate was 100 μS / cm or less. Next, the washed phosphorylated pulp was subjected to a neutralization treatment as follows. First, the washed phosphorylated pulp was diluted with 10 L of deionized water, and then a 1N sodium hydroxide aqueous solution was gradually added while stirring to obtain a phosphorylated pulp slurry with a pH of 12 to 13. Next, the phosphorylated pulp slurry was dewatered and washed to obtain phosphorylated pulp that had undergone neutralization treatment.

[0119] The obtained phosphorylated pulp was subjected to infrared absorption spectroscopy using FT-IR. The result was 1230 cm⁻¹. －１ Absorption based on the P=O of phosphate groups was observed in the vicinity, confirming that phosphate groups were added to the pulp. Furthermore, when the obtained phosphorylated pulp was analyzed using an X-ray diffractometer, typical peaks were observed at two locations: around 2θ = 14° to 17° and around 2θ = 22° to 23°, confirming the presence of type I cellulose crystals. The amount of phosphate groups (amount of first dissociated acid) measured by the measurement method described later was 1.45 mmol / g. The total amount of dissociated acid was 2.45 mmol / g.

[0120] Deionized water was added to the obtained phosphorylated pulp to prepare a slurry with a solid content of 2.2% by mass. This slurry was processed twice at a pressure of 200 MPa using a wet atomizing device (Starburst, manufactured by Sugino Machine Co., Ltd.) to obtain a fine fibrous cellulose dispersion A containing phosphorylated fine fibrous cellulose. Furthermore, observation of the phosphorylated fine fibrous cellulose contained in the fine fibrous cellulose dispersion A using an atomic force microscope revealed that it contained phosphorylated fine fibrous cellulose with a fiber width of 3 to 5 nm.

[0121] X-ray diffraction confirmed that this phosphorylated microfibrous cellulose maintained its type I cellulose crystal structure. The amount of phosphate groups (first dissociated acid, strongly acidic group) measured by the method described in [Measurement of Phosphorus Oxoacid Group Amount] below was 1.45 mmol / g. The total amount of dissociated acid was 2.45 mmol / g.

[0122] (Measurement of Phosphorus Oxoacid Groups) In measuring the amount of phosphorus oxoacid groups (phosphate groups or phosphite groups) in fine fibrous cellulose, first, ion-exchanged water was added to the target fine fibrous cellulose to prepare a slurry with a solid content of 0.2% by mass. The obtained slurry was then treated with an ion exchange resin, and the amount was measured by titration using an alkali. The treatment with the ion exchange resin was performed by adding 1 / 10 the volume of strongly acidic ion exchange resin (Amberjet 1024; Organo Corporation, conditioned) to the slurry, shaking for 1 hour, and then pouring it onto a mesh with a mesh opening of 90 μm to separate the resin from the slurry (fine fibrous cellulose dispersion). The titration using an alkali was performed by adding 10 μL of 0.1 N sodium hydroxide aqueous solution to the slurry after treatment with the ion exchange resin at intervals of 5 seconds, and measuring the change in the pH value of the slurry. Nitrogen gas was blown into the slurry from 15 minutes before the start of the titration. In this neutralization titration, two points are observed in the curve plotting the measured pH against the amount of alkali added, where the increment (the derivative of pH with respect to the amount of alkali added) is maximum. Of these, the first increment maximum obtained after starting to add alkali is called the first endpoint, and the next increment maximum is called the second endpoint (Figure 3). The amount of alkali required from the start of titration to the first endpoint is equal to the amount of the first dissociated acid in the slurry used for titration. Also, the amount of alkali required from the start of titration to the second endpoint is equal to the total amount of dissociated acid in the slurry used for titration. Note that the amount of phosphorus oxoacid groups (amount of first dissociated acid) (mol / g) was defined as the amount of alkali (mol / g) required from the start of titration to the first endpoint divided by the solid content (g) in the slurry being titrated. Furthermore, the total amount of dissociated acid (mol / g) was defined as the amount of alkali (mol / g) required from the start of the titration to the second endpoint divided by the solid content (g) in the slurry being titrated.

[0123] <Example 1> [Step (I): Mixing Step] A fine fibrous cellulose dispersion A obtained in Production Example 4 was placed in a container, and ion-exchanged water was added so that the solid content concentration of the fine fibrous cellulose was diluted to 1.0% by mass. A tornado stirrer (general-purpose constrained stirrer, PM-202, manufactured by AS ONE Corporation) was used as a stirrer, and a 6-inch diameter stirring blade was attached, and the mixture was stirred at 1,000 rpm for 5 minutes to obtain a diluted fine fibrous cellulose dispersion. To this dispersion, the coarse cellulose fiber suspension 1 obtained in Production Example 1 was added so that the solid content of the coarse cellulose fibers was 100 parts by mass per 100 parts by mass of fine fibrous cellulose in the dispersion. A tornado stirrer was used as a stirrer, and a 6-inch diameter stirring blade was attached, and the mixture was stirred at 1,000 rpm for 5 minutes to obtain a cellulose fiber aqueous dispersion (a). To 100 parts by mass of cellulose fibers in a cellulose fiber aqueous dispersion (a), an aqueous dispersion of natural rubber latex (ULACOL, manufactured by Resitex Co., Ltd., ammonia content 0.1%) with a solid content concentration of 61% by mass was added so that the rubber component amounted to 500 parts by mass. A tornado agitator was used as the agitator, fitted with a 6-inch diameter agitator blade, and the mixture was stirred at 1,000 rpm for 5 minutes to obtain a cellulose fiber / natural rubber latex mixture.

[0124] [Process (II): Solidification Process] Mixture A was spread onto a tray coated with Teflon® so that the thickness after drying would be 0.05 mm, and dried in a dryer at 40°C for 48 hours to obtain a sheet-like rubber composite material.

[0125] [Preparation of Rubber Compound] Using an open roll mill (6-inch double roll mill, manufactured by Daihan Co., Ltd.), the rubber composite material was kneaded for 10 minutes at a rotation speed of 26 / 30 rpm without heating. Next, 2 parts by mass of a crosslinking agent (Parkmill D, manufactured by NOF Corporation), 3 parts by mass of two types of zinc oxide (manufactured by Seido Chemical Industry Co., Ltd.), and 1.5 parts by mass each of antioxidants (ANTAGE MB, ANTAGE RD, manufactured by Kawaguchi Chemical Industry Co., Ltd.) were added to 100 parts by mass of the rubber component, and kneaded for 6 minutes to obtain a rubber compound with a thickness of approximately 2 mm or more.

[0126] [Crosslinking Process] The obtained rubber compound was placed in a mold and pressed and heated at 165°C for 20 minutes to produce a sheet of crosslinked rubber composition with a thickness of 2 mm.

[0127] <Example 2> The amount of coarse cellulose fiber suspension 1 added was set to an amount such that the mass ratio of coarse cellulose fibers to fine fibrous cellulose (coarse cellulose fibers: fine fibrous cellulose) was 66.7:33.3 to obtain a cellulose fiber aqueous dispersion (b). The same procedure as in Example 1 was carried out, except that a natural rubber latex aqueous dispersion (ULACOL, manufactured by Resitex Co., Ltd., ammonia content 0.1%) with a solid content concentration of 61% by mass was added to 100 parts by mass of the cellulose fiber aqueous dispersion (b) so that the solid content of the rubber component was 333.3 parts by mass, to obtain a sheet of crosslinked rubber composition.

[0128] <Example 3> The amount of coarse cellulose fiber suspension 1 added was set to an amount such that the mass ratio of coarse cellulose fibers to fine fibrous cellulose (coarse cellulose fibers: fine fibrous cellulose) was 80:20 to obtain a cellulose fiber aqueous dispersion (c). A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 1, except that a natural rubber latex aqueous dispersion (ULACOL, manufactured by Resitex Co., Ltd., ammonia content 0.1%) with a solid content concentration of 61% by mass was added to 100 parts by mass of the cellulose fiber aqueous dispersion (c) so that the solid content of the rubber component was 400 parts by mass.

[0129] <Example 4> A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 2, except that an aqueous dispersion of natural rubber latex (ULACOL, manufactured by Resitex Co., Ltd., ammonia content 0.1%) with a solid content concentration of 61% by mass was added to 100 parts by mass of the cellulose fiber aqueous dispersion (b) so that the solid content of the rubber component became 666.7 parts by mass.

[0130] <Example 5> The amount of coarse cellulose fiber suspension 1 added was set to an amount such that the mass ratio of coarse cellulose fibers to fine fibrous cellulose (coarse cellulose fibers: fine fibrous cellulose) was 20:80 to obtain a cellulose fiber aqueous dispersion (d). A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 1, except that a natural rubber latex aqueous dispersion (ULACOL, manufactured by Resitex Co., Ltd., ammonia content 0.1%) with a solid content concentration of 61% by mass was added to 100 parts by mass of the solid content of this cellulose fiber aqueous dispersion (d) so that the solid content of the rubber component was 400 parts by mass.

[0131] <Example 6> A sheet-like rubber composite material was obtained in the same manner as in Example 3. Using an open roll mill (6-inch double roll mill, manufactured by Daihan Co., Ltd.), the rubber composite material was kneaded for 10 minutes at a rotation speed of 26 / 30 rpm without heating. Next, the kneaded rubber composite material was removed from the open roll mill. Using the open roll mill, natural rubber (RSS#3, manufactured by Koshigaya Rubber Industry Co., Ltd.) was kneaded for 10 minutes at a rotation speed of 26 / 30 rpm without heating. Next, 125 parts by mass of the kneaded rubber composite material was added to 100 parts by mass of the kneaded natural rubber and kneaded for 10 minutes. Next, 2 parts by mass of a crosslinking agent (Parkmill D, manufactured by NOF Corporation), 3 parts by mass of two types of zinc oxide (manufactured by Seido Chemical Industry Co., Ltd.), and 1.5 parts by mass each of antioxidants (ANTAGE MB, ANTAGE RD, manufactured by Kawaguchi Chemical Industry Co., Ltd.) were added to 100 parts by mass of rubber component, and the mixture was kneaded for 6 minutes to obtain a rubber compound with a thickness of approximately 2 mm or more. The obtained rubber compound was placed in a mold, and the same process as in Example 3 was carried out to produce a sheet of crosslinked rubber composition.

[0132] <Example 7> The same procedure as in Example 1 was performed, except that coarse cellulose fiber suspension 2 was used instead of coarse cellulose fiber suspension 1, to obtain a sheet of crosslinked rubber composition.

[0133] <Example 8> The same procedure as in Example 1 was performed, except that coarse cellulose fiber suspension 3 was used instead of coarse cellulose fiber suspension 1, to obtain a sheet of crosslinked rubber composition.

[0134] <Example 9> In the [Preparation of Compound Sheet] of Example 1, a rubber compound was obtained in the same manner as in Example 1, except that the additives were 1.5 parts by mass of sulfur, 2 parts by mass of two types of zinc oxide (manufactured by Seido Chemical Industry Co., Ltd.), 2 parts by mass of an antioxidant (ANTAGE RD, manufactured by Kawaguchi Chemical Industry Co., Ltd.), 2 parts by mass of stearic acid, and 2 parts by mass of a vulcanization accelerator (NOXELLER MSA-G, manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) per 100 parts by mass of rubber component. In the [Crosslinking Process], a sheet of crosslinked rubber composition with a thickness of 2 mm was prepared by press heating at 150°C for 10 minutes.

[0135] <Example 10> Except for the [Preparation of Compound Sheet] and [Crosslinking Process] being carried out in the same manner as in Example 9, the same process as in Example 2 was performed to obtain a sheet of crosslinked rubber composition.

[0136] <Example 11> Except for the [Preparation of Compound Sheet] and [Crosslinking Process] being carried out in the same manner as in Example 9, the same process as in Example 3 was performed to obtain a sheet of crosslinked rubber composition.

[0137] <Example 12> Except for the [Preparation of Compound Sheet] and [Crosslinking Process] being carried out in the same manner as in Example 9, the same process as in Example 4 was performed to obtain a sheet of crosslinked rubber composition.

[0138] <Example 13> Except for the [Preparation of Compound Sheet] and [Crosslinking Process] being carried out in the same manner as in Example 9, the same process as in Example 5 was performed to obtain a sheet of crosslinked rubber composition.

[0139] <Example 14> Except for the [Preparation of Compound Sheet] and [Crosslinking Process] being carried out in the same manner as in Example 9, the same process as in Example 6 was performed to obtain a sheet of crosslinked rubber composition.

[0140] <Example 15> Except for the [Preparation of Compound Sheet] and [Crosslinking Process] being carried out in the same manner as in Example 9, the same process as in Example 7 was performed to obtain a sheet of crosslinked rubber composition.

[0141] <Example 16> Except for the [Preparation of Compound Sheet] and [Crosslinking Process] being carried out in the same manner as in Example 9, the same process as in Example 8 was performed to obtain a sheet of crosslinked rubber composition.

[0142] <Comparative Example 1> In a container, 100 parts by mass of the coarse cellulose fiber suspension 1 obtained in Production Example 1 was mixed with an aqueous dispersion of natural rubber latex (ULACOL, manufactured by Resitex Co., Ltd., ammonia content 0.1%) having a solid content concentration of 61% by mass, so that the rubber component amounted to 500 parts by mass. A tornado agitator was used as the agitator, fitted with a 6-inch diameter agitator blade, and stirred at 1,000 rpm for 5 minutes to obtain a coarse cellulose fiber / natural rubber latex mixture. Except for using this mixture, the same process as in Example 1 was carried out to obtain a sheet of crosslinked rubber composition.

[0143] <Comparative Example 2> A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Comparative Example 1, except that an aqueous dispersion of natural rubber latex was added so that the rubber component amounted to 1,000 parts by mass.

[0144] <Comparative Example 3> The same procedure as in Comparative Example 1 was performed to obtain a sheet of crosslinked rubber composition, except that a fine fibrous cellulose dispersion A was used instead of the coarse cellulose fiber suspension 1.

[0145] <Comparative Example 4> The same procedure as in Comparative Example 2 was performed to obtain a sheet of crosslinked rubber composition, except that a fine fibrous cellulose dispersion A was used instead of the coarse cellulose fiber suspension 1.

[0146] <Comparative Example 5> Except for the [Preparation of Compound Sheet] and [Crosslinking Process] being carried out in the same manner as in Example 9, the same process as in Comparative Example 1 was performed to obtain a sheet of crosslinked rubber composition.

[0147] <Comparative Example 6> Except for the [Preparation of Compound Sheet] and [Crosslinking Process] being carried out in the same manner as in Example 9, the same process as in Comparative Example 2 was performed to obtain a sheet of crosslinked rubber composition.

[0148] <Comparative Example 7> Except for the [Preparation of Compound Sheet] and [Crosslinking Process] being carried out in the same manner as in Example 9, the same process as in Comparative Example 3 was performed to obtain a sheet of crosslinked rubber composition.

[0149] <Comparative Example 8> Except for the [Preparation of Compound Sheet] and [Crosslinking Process] being carried out in the same manner as in Example 9, the same process as in Comparative Example 4 was performed to obtain a sheet of crosslinked rubber composition.

[0150] <Reference Example 1> Natural rubber latex (ULACOL, manufactured by Resitex Co., Ltd., ammonia content 0.1%) was spread on a Teflon-coated tray and dried in a dryer at 40°C for 48 hours to obtain a sheet-like natural rubber composition. A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 1, except that this natural rubber composition was used.

[0151] <Reference Example 2> Natural rubber latex (ULACOL, manufactured by Resitex Co., Ltd., ammonia content 0.1%) was spread on a tray with a Teflon coating on its surface and dried in a dryer at 40°C for 48 hours to obtain a sheet-like natural rubber composition. Except for using this natural rubber composition, the same procedure as in Example 9 was followed to obtain a sheet of crosslinked rubber composition.

[0152] [Separation of Coarse Cellulose Fibers and Fine Cellulose in Cellulose Fiber Aqueous Dispersions] For Examples 1 to 16 and Comparative Examples 1 to 8, ion-exchanged water was added to each cellulose fiber aqueous dispersion to dilute it to a solid content of 0.2% by mass. After dilution, the samples were processed at 12,000 G for 10 minutes using a refrigerated high-speed centrifuge (H2000B, manufactured by Kokusan Co., Ltd.). The supernatant was collected as a dispersion containing fine cellulose, and the precipitate as coarse cellulose fibers, and these were used as samples for measuring fiber width and irregular freeness.

[0153] [Dry mass ratio of precipitate and solids in supernatant] For Examples 1 to 16 and Comparative Examples 1 to 8, ion-exchanged water was added to each cellulose fiber aqueous dispersion to dilute it so that the solid content was 0.2% by mass. 200 g of the diluted aqueous dispersion was processed at 12,000 G for 10 minutes using a refrigerated high-speed centrifuge (H2000B, manufactured by Kokusan Co., Ltd.), and the solids in the supernatant and precipitate were collected. Both were dried in a constant-temperature dryer at 105°C to an oven-dry state, and their dry mass ratio was determined.

[0154] [Irregular Freeness] Irregular freeness was measured by preparing 0.03% by mass solutions of coarse cellulose fiber suspension, fine fibrous cellulose dispersion, and precipitate using deionized water. The pulp concentration was changed from 0.3% by mass to 0.03% by mass using the Canadian standard filtration method specified in JIS P 8121:1995, and the JIS standard screen plate was changed to an 80-mesh wire.

[0155] [Measurement of Degree of Polymerization] The degree of polymerization of coarse cellulose fibers or fine fibrous cellulose was calculated from the viscosity measured according to Tappi T230. Specifically, the viscosity (η1) measured by dissolving the coarse cellulose fibers or fine fibrous cellulose to be measured in a 1 mol / L copper ethylenediamine aqueous solution, and the blank viscosity (η0) measured using only the above copper ethylenediamine aqueous solution were measured. Then, the specific viscosity (ηsp) and intrinsic viscosity ([η]) were calculated according to the following formulas: ηsp = (η1 / η0) - 1 [η] = ηsp / (c(1 + 0.28 × ηsp)) Here, c in the formula represents the concentration of coarse cellulose fibers or fine fibrous cellulose at the time of viscosity measurement. Furthermore, the degree of polymerization (DP) was calculated from the following formula: DP = 1.75 × [η]

[0156] [Length-weighted average fiber length (Ll)] The length-weighted average fiber length (Ll) is obtained by the method specified in JIS P 8226-2:2011. The length-weighted average fiber length (Ll) of precipitates and coarse cellulose fibers can be measured using Valmet's FS5.

[0157] [Fiber width] The fiber width of 50 randomly selected microfiber cellulose fibers contained in the solids of the supernatant liquid after centrifugation was measured using an atomic force microscope. It was confirmed that more than 90% of the measured microfiber cellulose fibers were in the range of 3 to 20 nm in fiber width.

[0158] [Evaluation of Aggregates] The sheets of the crosslinked rubber compositions obtained in Examples 1 to 16 and Comparative Examples 1 to 8 were visually inspected for the presence or absence of aggregates and evaluated as follows (A to C). A: No visible aggregates. B: Aggregates are observed, but they are small in size and uniformly distributed within the sheet. C: Aggregates are observed, are large in size, and are unevenly distributed within the sheet.

[0159] [Tensile Test] For the sheets of crosslinked rubber compositions obtained in Examples 1 to 16, Comparative Examples 1 to 8, and Reference Examples 1 to 2, test specimens punched out in the shape of dumbbells (Dumbbell No. 6) as described in JIS K 6251:2017 were subjected to tensile testing using an Autograph tensile testing machine (manufactured by Shimadzu Corporation) at 23±2℃, a gauge length of 20 mm, and a tensile speed of 500 mm / min, in accordance with JIS K 6251:2017. The 25% modulus (σ25 (MPa)), 50% modulus (σ50 (MPa)), and tensile elongation were measured.

[0160]

[0161] As shown in Table 1, the crosslinked rubber compositions obtained in the examples had fewer cellulose fiber aggregates, exhibited excellent improvement in modulus (σ25 and σ50) in the low-elongation region compared to rubber alone, and possessed sufficient tensile elongation. On the other hand, when coarse cellulose fibers were used and fine fibrous cellulose was not used (Comparative Examples 1, 2, 5, and 6), there were many aggregates and the tensile elongation was small. Furthermore, when fine fibrous cellulose was used instead of coarse cellulose fibers (Comparative Examples 3, 4, 7, and 8), a sufficient improvement in modulus (σ25 and σ50) in the low-elongation region was not obtained.

[0162] 10...Double drum dryer, 1...Cylindrical, 2...Feed section, 3...Sheet-like mixture, 4...Scraper, 5...Rubber composite material

Claims

1. When a 0.2% by mass aqueous dispersion is centrifuged under the following conditions to obtain a precipitate and supernatant, the precipitate contains coarse cellulose fibers, the irregular freeness of the precipitate is 950 mL or less, and the solid content in the supernatant contains fine fibrous cellulose with a fiber width of 100 nm or less. Conditions: Centrifugation at 12,000 G for 10 minutes.

2. The cellulose fiber according to claim 1, wherein the dry mass ratio of the precipitate to the solid content in the supernatant (precipitate:solid content in supernatant) is 90:10 to 10:

90.

3. The cellulose fiber according to claim 1 or 2, wherein the anomalous freeness of the precipitate is 400 mL or less.

4. The cellulose fiber according to any one of claims 1 to 3, wherein the precipitate contains unmodified coarse cellulose fibers.

5. The cellulose fiber according to any one of claims 1 to 4, wherein the fine fibrous cellulose has a fiber width of 100 nm or less and comprises fine fibrous cellulose having at least one selected from the group consisting of phosphorus oxoacid groups and groups derived from phosphorus oxoacid groups.

6. A cellulose fiber according to any one of claims 1 to 5, which is a rubber modifier.

7. A cellulose fiber aqueous dispersion containing the cellulose fiber described in any one of claims 1 to 6.

8. A rubber composite material containing a rubber component and cellulose fibers according to any one of claims 1 to 6.

9. The rubber composite material according to claim 8, wherein the content of cellulose fibers in the rubber composite material is 5 parts by mass or more and 50 parts by mass or less per 100 parts by mass of rubber component.

10. The rubber composite material according to claim 8 or 9, wherein the rubber component includes a diene rubber.

11. A rubber compound containing the rubber composite material and crosslinking agent described in any one of claims 8 to 10.

12. A crosslinked rubber composition obtained by crosslinking the rubber compound described in claim 11.

13. The crosslinked rubber composition according to claim 12, wherein the ratio (σ25B / σ25A) of the 25% modulus (σ25B) of the crosslinked rubber composition according to claim 12, measured according to JIS K 6251:2017, to the 25% modulus (σ25A) of the crosslinked rubber composition obtained by crosslinking a rubber compound containing a rubber component and a crosslinking agent, measured according to JIS K 6251:2017, is 2.0 or more and 40.0 or less.

14. The crosslinked rubber composition according to claim 12 or 13, wherein the ratio (σ50B / σ50A) of the 50% modulus (σ50A) of the crosslinked rubber composition according to claim 12 or 13, measured according to JIS K 6251:2017, to the 50% modulus (σ50A) of the crosslinked rubber composition according to claim 12 or 13, measured according to JIS K 6251:2017, is 2.0 or more and 35.0 or less.

15. A method for producing a rubber composite, comprising the following steps (I) and (II): (I) A cellulose fiber aqueous dispersion with a solid content of 0.2% by mass is centrifuged under the following conditions to obtain a precipitate and a supernatant, wherein the precipitate contains coarse cellulose fibers, the irregular freeness of the precipitate is 950 mL or less, and the solid content in the supernatant contains fine fibrous cellulose with a fiber width of 100 nm or less. Mixing step to obtain a mixture A by mixing a cellulose fiber aqueous dispersion with rubber latex: centrifugation at 12,000 G for 10 minutes. (II) A solidification step to obtain a solid rubber composite from mixture A.

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