Cellulose fiber for rubber modification, cellulose fiber aqueous-dispersion for rubber modification, rubber composition and method for producing the same, rubber compound, and crosslinked rubber composition

Abstract

The present invention pertains to: cellulose fibers for rubber modification which, when added to a rubber component, achieve an excellent modulus improvement effect particularly in a low elongation region, in a crosslinked rubber composition obtained after crosslinking; an aqueous dispersion of cellulose fibers for rubber modification; a rubber composition containing the cellulose fibers for rubber modification and a rubber component; a method for producing the rubber composition; a rubber compound including the rubber composition; and a crosslinked rubber composition obtained by crosslinking the rubber compound.　This cellulose fiber for rubber modification contains coarse cellulose fibers having an irregular freeness of 850 mL or less and a length-weighted average fiber length of 0.4 mm or more.

Description

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

[0001] The present invention relates to cellulose fibers for rubber modification, aqueous dispersions of cellulose fibers for rubber modification, rubber compositions 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 as an alternative to petroleum resources and due to growing environmental awareness. Among natural fibers, fibrous cellulose with a fiber diameter of 10 μm to 50 μm, especially fibrous cellulose derived from wood (such as pulp), has been widely used, mainly in paper products.

[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 masterbatch containing a polyolefin resin, cellulose fibers, and a silane coupling agent, with the aim of providing a masterbatch that improves the dispersibility of cellulose fibers in rubber and further enables high adhesion strength at the interface between the fibers and the rubber.

[0004] International Publication No. 2013 / 176452

[0005] Reference 1 discloses that dispersibility in a rubber composition can be improved by using polyolefin fibers and a silane coupling agent in addition to cellulose fibers. However, there is a need for cellulose fibers for rubber modification that can provide excellent reinforcing properties without the addition of other components. The present invention aims to provide cellulose fibers for rubber modification, which, when added to a rubber component, exhibit excellent modulus improvement effects, particularly in the low elongation region, in the crosslinked rubber composition obtained after crosslinking, as well as an aqueous dispersion of cellulose fibers for rubber modification, a rubber composition containing the cellulose fibers for rubber modification and a rubber component, and a method for producing the same. Furthermore, the invention aims to provide a rubber compound using the rubber composition and a crosslinked rubber composition obtained by crosslinking the rubber compound.

[0006] The present inventors have found that the above problems can be solved by a cellulose for rubber modification containing coarse cellulose fibers having specific irregular freeness and average fiber length. The present invention relates to the following <1> to <16>. <1> A cellulose fiber for rubber modification containing coarse cellulose fibers having an irregular freeness of 850 mL or less and a length-weighted average fiber length of 0.4 mm or more. <2> The cellulose fiber for rubber modification according to <1>, wherein the coarse cellulose fibers are unmodified cellulose fibers. <3> The cellulose fiber for rubber modification according to <1>, wherein when the cellulose fiber for rubber modification is centrifuged as an aqueous dispersion with a solid content of 0.2% by mass under the following conditions to obtain a precipitate and a supernatant, the precipitate contains coarse cellulose fibers, the irregular freeness of the precipitate is 850 mL or less, and the length-weighted average fiber length of the precipitate is 0.4 mm or more. Conditions: Centrifugation at 12,000 G for 10 minutes <4> Cellulose fiber for rubber modification according to <3>, wherein the precipitate contains unmodified coarse cellulose fibers. <5> Cellulose fiber for rubber modification according to <3> or <4>, wherein the supernatant contains solids, and the solids contain fine fibrous cellulose with a fiber width of 100 nm or less. <6> Cellulose fiber for rubber modification according to <5>, wherein the fine fibrous cellulose with a fiber width of 100 nm or less contains fine fibrous cellulose having at least one selected from the group consisting of phosphorus oxoacid groups and groups derived from phosphorus oxoacid groups. <7> Aqueous dispersion of cellulose fiber for rubber modification containing the cellulose fiber for rubber modification according to any one of <1> to <6>. <8> A rubber composition comprising the cellulose fiber for rubber modification according to <1> or <2> and a rubber component. <9> A rubber composition comprising the cellulose fiber for rubber modification according to any one of <3> to <6> and a rubber component. <10> The rubber composition according to <8> or <9>, wherein the content of cellulose fibers for rubber modification per 100 parts by mass of rubber components in the rubber composition is 1.0 part by mass or more and 50 parts by mass or less. <11> The rubber composition according to <9>, wherein the content of precipitate per 100 parts by mass of rubber components in the rubber composition is 1.0 part by mass or more and less than 10.0 parts by mass. <12> The rubber composition according to any one of <8> to <11>, wherein the rubber component includes diene rubber.<13> A rubber compound containing the rubber composition described in any one of <8> to <12> and a crosslinking agent. <14> A crosslinked rubber composition obtained by crosslinking the rubber compound described in <13>. <15> The crosslinked rubber composition described in <14>, which is a tire. <16> A method for producing a rubber composition, comprising the following steps (I) and (II): (I) A mixing step of mixing the cellulose fiber aqueous dispersion for rubber modification described in <7> with rubber latex to obtain a mixed liquid A. (II) A solidification step of obtaining a solid rubber composition from the mixed liquid A.

[0007] According to the present invention, a cellulose fiber for rubber modification, an aqueous dispersion of cellulose fiber for rubber modification, and a rubber composition containing the cellulose fiber for rubber modification and a rubber component are provided, which, when added to a rubber component, exhibit particularly excellent modulus improvement effects in the crosslinked rubber composition obtained after crosslinking, especially in the low elongation region. Furthermore, a rubber compound using the rubber composition and a crosslinked rubber composition obtained by crosslinking the rubber compound are also 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 fine fibrous cellulose with phosphorus oxoacid groups and pH.

[0009] [Cellulose Fibers for Rubber Modification] The cellulose fibers for rubber modification of this embodiment (hereinafter also referred to as "cellulose fibers of this embodiment") contain coarse cellulose fibers as pulp fibers, having an irregular freeness of 850 mL or less and an average fiber length of 0.4 mm or more (hereinafter also simply referred to as "coarse cellulose fibers"). The cellulose fibers for rubber modification of this embodiment may contain coarse cellulose fibers and also contain fine fibrous cellulose with a fiber width of 100 nm or less (hereinafter also simply referred to as "fine fibrous cellulose") as described later. The total content of coarse cellulose fibers and fine fibrous cellulose in the cellulose fibers for rubber modification is preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 98% by mass or more, and 100% by mass or less.

[0010] The cellulose fibers of this embodiment exhibit excellent dispersibility in a rubber composition containing the cellulose fibers and rubber components, and also exhibit excellent reinforcing effects in a crosslinked rubber composition obtained by crosslinking the rubber composition and a rubber compound containing a crosslinking agent, particularly in improving modulus in the low elongation region. 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 components (see, for example, Japanese Patent Application Publication No. 2024-120444), and it is possible to reinforce the crosslinked rubber composition by adding cellulose nanofibers. However, cellulose nanofibers consume a lot of power during manufacturing, increasing costs, so it is necessary to limit the amount added. Furthermore, there was a problem that the modulus improvement effect in the low elongation region could not be sufficiently obtained in the crosslinked rubber composition obtained by adding cellulose nanofibers. On the other hand, when pulp fibers are used as is, when an aqueous dispersion of pulp fibers is mixed with rubber latex containing rubber components, the pulp fibers precipitate in the mixture, resulting in a problem in obtaining a uniform rubber composition. The cellulose fibers of this embodiment contain coarse cellulose fibers in which the irregular freeness is below a specific value and the average fiber length is above a specific value. This results in excellent dispersibility in the mixed liquid, yielding a uniform rubber composition, and also exhibits a superior modulus improvement effect in the low elongation region. This discovery led to the completion of the present invention. However, the mechanism by which the effects of the present invention are obtained is not limited to the above. As will be described later, in some embodiments of the cellulose fibers of this embodiment, the generation of aggregates in the crosslinked rubber composition can be suppressed, and in particular, the modulus improvement effect in the low elongation region is excellent, resulting in a crosslinked rubber composition with sufficient tensile stress and tensile elongation.

[0011] 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 for rubber modification, aqueous dispersions of cellulose fibers for rubber modification, rubber compositions, rubber compounds, and crosslinked rubber compositions, as well as each component and raw materials used in each of the manufacturing methods thereof, may be used individually or in combination of two or more types.

[0012] The irregular freeness of the coarse cellulose fibers contained in the cellulose fibers of this embodiment is 850 mL or less. From the viewpoint of excellent dispersibility in the rubber composition, suppression of agglomeration in the crosslinked rubber composition, obtaining sufficient tensile stress and tensile elongation, and further excellent modulus improvement effect in the low elongation region, the irregular freeness is, for example, 50 mL or more and 850 mL or less, preferably 800 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, even more preferably 300 mL or less, even more preferably 250 mL or less, and preferably 100 mL or more, more preferably 150 mL or more. When the irregular freeness is below the above upper limit, the occurrence of precipitation and separation is suppressed when the aqueous dispersion of the cellulose fibers and the rubber latex are mixed, and the dispersibility of the cellulose fibers in the resulting rubber composition is excellent, which is preferable. Furthermore, if the irregular freeness is above the above lower limit, it is preferable because it provides excellent modulus improvement in the low elongation region and suppresses power consumption during manufacturing. Here, irregular freeness refers to the freeness (filtration efficiency) measured in the Canadian standard filtration efficiency method specified in JIS P 8121-2:2012, by changing the pulp sample amount from 3g to 0.3g and changing the screen plate used in the JIS standard to an 80-mesh wire. The irregular freeness of coarse cellulose fibers contained in cellulose fibers can be adjusted by the type of pulp used and the conditions of the beating process in the beating process, for example, the beating time. Specifically, increasing the beating time tends to decrease the irregular freeness.

[0013] The length-weighted average fiber length (Ll) of the coarse cellulose fibers contained in the cellulose fibers of this embodiment is 0.4 mm or more. The length-weighted average fiber length is preferably 0.45 mm or more and 2.5 mm or less, more preferably 0.5 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 1.2 mm or less, and even more preferably 1.0 mm or less. The number-average fiber length (Ln) of the coarse cellulose fibers contained in the cellulose fibers of this embodiment is preferably 0.3 mm or more and 2.0 mm or less, more preferably 0.35 mm or more, even more preferably 0.4 mm or more, more preferably 1.8 mm or less, even more preferably 1.5 mm or less, even more preferably 1.2 mm or less, even more preferably 1.0 mm or less, and even more preferably 0.8 mm or less. In this embodiment, it is preferable that the number-average fiber length and length-weighted average fiber length of the coarse cellulose fibers contained in the cellulose fibers are below the above upper limit, as this suppresses the occurrence of precipitation and separation when the aqueous dispersion of the cellulose fibers is mixed with rubber latex, resulting in excellent dispersibility of the cellulose fibers in the resulting rubber composition. Furthermore, it is preferable that the number-average fiber length and length-weighted average fiber length are within the above range, as this provides excellent modulus improvement in the low elongation region and reduces power consumption during manufacturing. The length-weighted average fiber length and number-average fiber length of the cellulose fibers 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 softwood pulp is used as the raw material pulp, the length-weighted average fiber length and number-average fiber length tend to be longer when the same beating process is performed compared to when hardwood pulp is used as the raw material pulp, and when softwood pulp and hardwood pulp are used in combination, the length-weighted average fiber length and number-average fiber length tend to be longer when the proportion of softwood pulp is high. Furthermore, increasing the beating time tends to decrease the length-weighted average fiber length and the number-average fiber length. The length-weighted average fiber length and the number-average fiber length of cellulose fibers are measured by the method described in the examples.

[0014] The average fiber width of the coarse cellulose fibers contained in the cellulose fibers of this embodiment is preferably 5 μm to 50 μm, more preferably 7.5 μm or more, even more preferably 10 μm or more, and even more preferably 40 μm or less, and even more preferably 30 μm or less. When the average fiber width of the coarse cellulose fibers contained in the cellulose fibers is within the above range, precipitation and separation are suppressed when the aqueous dispersion of cellulose fibers and rubber latex are mixed, and the dispersibility of cellulose fibers in the resulting rubber composition is excellent, which is preferable. Furthermore, when the average fiber width is within the above range, the modulus improvement effect in the low elongation region is excellent, and the power consumption during manufacturing is suppressed, which is also preferable. The average fiber width of the coarse cellulose fibers contained in the cellulose fibers can be adjusted by the conditions of the beating process in the beating process, for example, the beating time. Specifically, increasing the beating time tends to decrease the average fiber width. The average fiber width of the coarse cellulose fibers contained in the cellulose fibers can be measured by the method described in the examples.

[0015] The degree of polymerization of the coarse cellulose fibers contained in the cellulose fibers of this embodiment is preferably 400 to 1,200. From the viewpoint of obtaining cellulose fibers that have excellent dispersibility in the rubber composition, suppress the generation of aggregates, obtain sufficient tensile stress and tensile elongation, and furthermore have an excellent modulus improvement effect in the low elongation region in the crosslinked rubber composition, the degree of polymerization is 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. 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 using only the aqueous copper ethylenediamine solution, are measured. Then, the specific viscosity (ηsp) and intrinsic viscosity ([η]) are 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 (g / mL) of the 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] The degree of polymerization of the cellulose fibers is adjusted by whether or not a low-degree-of-polymerization treatment is performed and by the treatment conditions. Examples of low-degree-of-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 coarse cellulose fibers contained in the cellulose fibers of this embodiment may be modified or unmodified, but from the viewpoint of simplicity of the manufacturing process, it is preferable that they be unmodified. Examples of modification of the coarse cellulose fibers include modification by introducing ionic substituents, and the ionic substituents may include, for example, either an anionic group or a cationic group, or both, with an anionic group being preferred. Furthermore, the ionic substituent is preferably a group introduced into the coarse cellulose fibers via an ester bond or an ether bond, and more preferably a group introduced via an ester bond. In this case, the ester bond is preferably formed by the dehydration condensation of the coarse cellulose fiber and the compound that will become 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 a phosphorus oxoacid group (a phosphorus oxoacid group or a substituent derived from a phosphorus oxoacid group), a sulfur oxoacid group (a sulfur oxoacid group or a substituent derived from a sulfur oxoacid group), and a carboxyl group, and is even more preferably a phosphorus oxoacid group.Examples of cationic groups used as ionic groups include ammonium groups, phosphonium groups, sulfonium groups, etc., with ammonium groups being preferred. For details on ionic groups, methods of introduction, and preferred ranges for the amount of ionic groups introduced, please refer to paragraphs 0038 to 0114 of Japanese Patent Application Publication No. 2022-104299.

[0017] The coarse cellulose fibers contained in the cellulose fibers of this embodiment 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.

[0018] <<Fiber Raw Materials>> The fiber raw materials that are the raw materials for the coarse cellulose fibers contained in the cellulose fibers of this embodiment are fiber raw materials containing cellulose, and are not particularly limited as fiber raw materials, but examples include pulps such as wood pulp, non-wood pulp, and deinked pulp. As for wood pulp, there are not particularly limited examples, but examples include chemical pulps such as bleached hardwood kraft pulp (LBKP), unbleached hardwood kraft pulp (LUKP), bleached softwood kraft pulp (NBKP), unbleached softwood kraft pulp (NUKP), 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 chemiground wood pulp (CGP), and mechanical pulps such as crushed wood pulp (GP) and thermomechanical pulp (TMP, BCTMP). Non-wood pulps are 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. Deinking pulps are not particularly limited, but examples include deinking pulps made from recycled paper. Among the above pulps, wood pulp and deinking pulp are preferred from the viewpoint of availability, wood pulp is more preferred, softwood pulp is even more preferred, softwood pulp is even more preferred, softwood pulp is even more preferred, and softwood pulp is even more preferred. Among wood pulps, chemical pulp is more preferred, and kraft pulp is even 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 a desired fiber length.

[0019] In the beating process (beating process), an aqueous 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 even more preferably 100% by mass.

[0020] 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.

[0021] The beating time is not particularly limited and can be adjusted as appropriate to obtain the desired irregular freeness and average fiber length. The suitable 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, and even more preferably 5 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.

[0022] In this embodiment, the cellulose fibers for rubber modification are preferably prepared as an aqueous dispersion with a solid content of 0.2% by mass, and when centrifuged at 12,000 G for 10 minutes to obtain a precipitate and supernatant, the precipitate contains coarse cellulose fibers, the irregular freeness of the precipitate is 850 mL or less, and the length-weighted average fiber length of the precipitate is 0.4 mm or more. Hereinafter, the cellulose fibers of this embodiment may be referred to as "cellulose fibers N". Cellulose fibers N may consist only of coarse cellulose fibers, but it is preferable that it is a mixture of coarse cellulose fibers and fine fibrous cellulose with a fiber width of 100 nm or less. The coarse cellulose fibers contained in the precipitate may be modified or unmodified, but from the viewpoint of ease of production of the cellulose for rubber modification and the crosslinked rubber composition, it is preferable that they be unmodified.

[0023] [Precipitate] The precipitate contains coarse cellulose fibers, and the irregular freeness of the precipitate is 850 mL or less. In a crosslinked rubber composition, from the viewpoint of suppressing the generation of aggregates, obtaining sufficient tensile stress and tensile elongation, and furthermore, having an excellent effect on improving modulus in the low elongation region, the irregular freeness is preferably 100 mL to 750 mL, 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, even more preferably 260 mL or more, even more preferably 320 mL or more, and even more preferably 350 mL or more. The irregular freeness is as described above. 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 coarse cellulose fibers and the preferred range for the irregular freeness of the precipitate are not the same. When a small amount of fine fibrous cellulose 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 tends to increase. 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.

[0024] The length-weighted average fiber length (Ll) of the precipitate is 0.4 mm or more in the crosslinked rubber composition, from the viewpoint of suppressing the generation of aggregates, obtaining sufficient tensile stress and tensile elongation, and furthermore, providing an excellent effect in improving modulus in the low elongation region. However, since the precipitate may contain not only coarse cellulose fibers but also fine fibrous cellulose, the preferred range for the length-weighted average fiber length (Ll) of coarse cellulose 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 of the precipitate can be adjusted by the type of pulp used and the conditions of the beating process, such as the beating time. Specifically, when using softwood pulp as the raw material, the length-weighted average fiber length of the precipitate 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 of the precipitate. Furthermore, increasing the beating time tends to decrease the length-weighted average fiber length of the precipitate. The length-weighted average fiber length of the precipitate is measured by the method described in the examples.

[0025] [Supernatant] The supernatant preferably contains fine fibrous cellulose with a fiber width of 100 nm or less (hereinafter also simply referred to as "fine fibrous cellulose").

[0026] <Fine Fibrous Cellulose> The fiber width of the fine fibrous cellulose used in the rubber modifier cellulose fibers of this embodiment is preferably 1 nm to 100 nm, more preferably 50 nm or less, even more preferably 20 nm or less, even more preferably 10 nm or less, and more preferably 2 nm or more, from the viewpoint of uniformly dispersing the cellulose fibers in the rubber composition. The fiber width of the fine fibrous cellulose contained in the solids in the supernatant is preferably 100 nm or less, more preferably 2 nm to 50 nm, and more preferably 20 nm or less, from the viewpoint of uniformly dispersing the cellulose fibers in the rubber composition. It is preferable that 75% or more of the total number of fibrous cellulose fibers contained in the solids in the supernatant fall within the above range, more preferably 80% or more fall within the above range, and even more preferably 85% or more fall within the above range, with an upper limit of 100%.

[0027] The average fiber width of the fine fibrous cellulose used in the cellulose fibers of this embodiment is preferably 1 nm to 100 nm, more preferably 50 nm or less, even more preferably 20 nm or less, even more preferably 10 nm or less, and even more preferably 2 nm or more, from the viewpoint of uniformly dispersing the cellulose fibers in the rubber composition and suppressing the solubility of cellulose molecules in water. The fine fibrous cellulose is, for example, monofilamentous cellulose.

[0028] The fiber width of fine fibrous cellulose is measured, for example, using an atomic force microscope as follows: First, an aqueous dispersion 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 dispersion is cast onto mica to create 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.

[0029] 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).

[0030] 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).

[0031] 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 to or above the lower limit makes it easier to form a rubber composition containing fine fibrous cellulose. Setting the axial ratio to or below the upper limit is preferable in that it makes handling easier, such as dilution, when handling the fine fibrous cellulose as an aqueous dispersion.

[0032] 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, more preferably 750 or less, and even more preferably 600 or less, from the viewpoint of suppressing the generation of aggregates, obtaining sufficient tensile stress and tensile elongation, and furthermore, having an excellent effect on improving modulus in the low elongation region, as well as ease of manufacture. 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.

[0033] 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.

[0034] 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.

[0035] Examples of the anionic group include a phosphooxo acid group or a group derived from a phosphooxo acid group (sometimes simply referred to as a phosphooxo acid group), a carboxy group or a group derived from a carboxy group (sometimes simply referred to as a carboxy group), a sulfur oxo acid group or a group derived from a sulfur oxo acid group (sometimes simply referred to as a sulfur oxo acid group), a xanthate group or a group derived from a xanthate group (sometimes simply referred to as a xanthate group), a phosphon group or a group derived from a phosphon group, a phosphine group or a group derived from a phosphine group, a sulfone group or a group derived from a sulfone group, a carboxyalkyl group, and the like. Among these, the anionic group preferably contains at least one selected from the group consisting of a phosphooxo acid group, a group derived from a phosphooxo acid group, a carboxy group, a sulfur oxo acid group, a group derived from a sulfur oxo acid group, a carboxymethyl group, a carboxyethyl group, and a sulfone group, more preferably contains at least one selected from the group consisting of a phosphooxo acid group, a group derived from a phosphooxo acid group, a carboxy group, a sulfur oxo acid group, and a group derived from a sulfur oxo acid group, and even more preferably contains at least one selected from the group consisting of a phosphooxo acid group and a group derived from a phosphooxo acid group. By introducing a phosphooxo acid group as the anionic group, for example, the dispersibility of microfibrillated cellulose can be further enhanced even under alkaline conditions or acidic conditions, and as a result, a crosslinked rubber composition excellent in tensile physical properties can be easily obtained.

[0036] The phosphooxo acid group or the group derived from a phosphooxo acid group is a substituent represented by, for example, the following formula (1). A plurality of substituents represented by the following formula (1) may be introduced into each microfibrillated cellulose. In this case, the plurality of substituents represented by the following formula (1) introduced may be the same or different from each other.

[0037] In formula (1), a, b, and n are natural numbers, and m is an arbitrary number (however, a = b × m). At least one of the n αs and α's is O <\\(0000092\\)>, <\\(0000090\\)>, <\\(000009\\)1>, <\\(0000089\\)>, <\\(0000001\\)>, <\\(0000002\\)>, <\\(0000088\\)>[ and the rest are R or OR. Note that all of the αs and α's may be O － and it doesn't matter. The n αs may all be the same or different from each other. βｂ＋ is a monovalent or higher cation composed of organic or inorganic substances.

[0038] 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. In formula (1), n is preferably 1.

[0039] 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.

[0040] Examples of the derivative group in R include at least one functional group selected from functional groups such as a carboxy group, a carboxylate group (-COO － ), a hydroxy group, an amino group, and an ammonium group, etc., which are added or substituted to the main chain or side chain of the above various hydrocarbon groups, but are not particularly limited. Also, 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, making it easier to penetrate into the fiber raw material and increasing the yield of microfibrillar cellulose. When there are multiple Rs in formula (1) or when multiple types of substituents represented by formula (1) are introduced into the microfibrillar cellulose, the multiple Rs may be the same or different from each other.

[0041] β ｂ＋ is a cation of one or more valences composed of organic or inorganic substances. Examples of the cation of one or more valences 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 the cation of one or more valences composed of inorganic substances 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, etc. In the formula (1), β ｂ＋ When there are a plurality of β present or when a plurality of substituents represented by the formula (1) are introduced into the microfibrillar cellulose, the plurality of β ｂ＋ may be the same or different from each other.

[0042] More specifically, examples of the phosphooxo acid group or a group derived from the phosphooxo acid group include a phosphoric acid group (—PO ３ H ２ ), a salt of the phosphoric acid group, a phosphorous acid group (phosphonic acid group) (—PO ２ H ２ ), and a salt of the phosphorous acid group (phosphonic acid group). Further, the substituent derived from the phosphooxo acid group or the phosphooxo acid group may be a group in which phosphoric acid groups are condensed (e.g., a pyrophosphoric acid group), a group in which phosphonic acids are condensed (e.g., a polyphosphonic acid group), a phosphoric acid ester group (e.g., a monomethyl phosphoric acid group, a polyoxyethylene alkyl phosphoric acid group), an alkyl phosphonic acid group (e.g., a methyl phosphonic acid group), etc.

[0043] Further, the sulfur oxo acid group (sulfur oxo acid group or a group derived from the sulfur oxo acid group) is, for example, a substituent represented by the following formula (2). A plurality of substituents represented by the following formula (2) may be introduced into each microfibrillar cellulose. In this case, the plurality of substituents represented by the following formula (2) introduced may be the same or different from each other.

[0044]

[0045] 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.

[0046] The amount of anionic groups introduced into the fine fibrous cellulose is preferably 0.10 mmol / g or more and 5.20 mmol / g or less 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 in which the counterion of the anionic group 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.

[0047] 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 a slurry containing the obtained fine fibrous cellulose.

[0048] 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 to the fine fibrous cellulose can be measured, for example, as follows: First, ion-exchanged 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 aqueous dispersion (slurry) containing fine fibrous cellulose. Then, the fine fibrous cellulose aqueous 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.

[0049] 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)

[0050] 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.

[0051] 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.

[0052] <<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. 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. 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.

[0053] (Phosphorus oxoacid group introduction step) The phosphorus oxoacid group introduction step is a step in which at least one compound (hereinafter also referred to as "compound A") selected from compounds that can introduce phosphorus oxoacid groups by reacting with the hydroxyl groups present in the cellulose-containing fiber raw material is reacted with the cellulose-containing fiber raw material. This step yields fine fibrous cellulose having phosphorus oxoacid groups.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] (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 by washing the anionic group-introduced fibers with water or an organic solvent, for example. The washing process may also be performed after each of the processes described later, and the number of washes performed in each washing process is not particularly limited.

[0069] (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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] (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.

[0074] 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).

[0075] 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.

[0076] If the supernatant after centrifugation contains solids, the ratio of the precipitate to the solids 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 formation of aggregates in the crosslinked rubber composition, obtaining sufficient tensile stress and tensile elongation, and furthermore, providing an excellent effect in 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. If the supernatant contains solids, the content of fine fibrous cellulose in the solids 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.

[0077] The cellulose fibers for rubber modification in this embodiment function as a reinforcing filler for rubber components, thereby modifying them. When the cellulose fibers for rubber modification in this embodiment are added to a rubber composition and crosslinked to form a crosslinked rubber composition, they exhibit a modulus improvement effect. In particular, the cellulose fibers for rubber modification in this embodiment also exhibit an excellent modulus improvement effect in the low elongation region.

[0078] [Aqueous Dispersion of Cellulose Fibers for Rubber Modification] This embodiment also relates to an aqueous dispersion of cellulose fibers for rubber modification (a slurry containing cellulose fibers for rubber modification) obtained by dispersing the above-mentioned cellulose fibers for rubber modification in a solvent (dispersion medium) containing water. The aqueous dispersion of cellulose fibers is a rubber modifier used, for example, to be added to rubber components. As will be described later, the aqueous dispersion of cellulose fibers for rubber modification may contain solvents other than water.

[0079] A cellulose fiber aqueous dispersion can be obtained, for example, by using a coarse cellulose fiber aqueous dispersion (coarse cellulose fiber aqueous dispersion) as the cellulose fiber aqueous dispersion, or by mixing a coarse cellulose fiber aqueous dispersion with a fine fibrous cellulose aqueous dispersion (fine fibrous cellulose aqueous dispersion liquid). A cellulose fiber N aqueous dispersion is preferably obtained by mixing a coarse cellulose fiber aqueous dispersion liquid and a fine fibrous cellulose aqueous dispersion. When mixing a coarse cellulose fiber aqueous dispersion with a fine fibrous cellulose aqueous dispersion, the mixing ratio of coarse cellulose fibers in the coarse cellulose fiber aqueous dispersion to fine fibrous cellulose in the fine fibrous cellulose aqueous 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 stress and 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 cellulose fiber N (coarse cellulose fibers: fine fibrous cellulose) is preferably 90:10 to 10:90, and more preferably 85:15 to 15:85.

[0080] The content of cellulose fibers for rubber modification in the aqueous dispersion of cellulose fibers for rubber modification is preferably 0.1% by mass or more and 8.0% by mass or less, more preferably 0.3% by mass or more, even more preferably 0.5% by mass or more, even more preferably 1.0% by mass or more, even more preferably 1.5% by mass or more, even more preferably 2.0% by mass or more, even more preferably 3.0% by mass or more, and more preferably 7.0% by mass or less, even more preferably 6.5% by mass or less, even more preferably 6.0% by mass or less, even more preferably 5.5% by mass or less, and even more preferably 5.0% by mass or less. When the content of cellulose fibers for rubber modification is above the lower limit, the amount of aqueous dispersion to be added is small and it is suitable for storage and transportation, so it is preferable, and when it is below the upper limit, it is preferable because it is easy to manufacture and the viscosity is within an appropriate range.

[0081] The aqueous dispersion of cellulose fibers for rubber modification may contain a solvent containing water and other additives in addition to the cellulose fibers of this embodiment. Examples of solvents other than water include organic solvents such as polar organic solvents, and one or more types may 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, 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, 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 even more preferably 100% by mass. Other additives include, for example, defoamers, lubricants, ultraviolet absorbers, dyes, pigments, stabilizers, surfactants, and preservatives (e.g., phenoxyethanol).

[0082] [Rubber Composition] The rubber composition of this embodiment contains the cellulose fibers of this embodiment and a rubber component. The rubber composition of this embodiment exhibits excellent dispersibility of cellulose fibers.

[0083] [Rubber Components] The rubber composition of this embodiment contains rubber components. As rubber components, 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 those that have been hydrogenated; 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 used individually or in mixtures of two or more. Furthermore, these rubber components may be pre-crosslinked raw materials without a crosslinked structure, or they may have a crosslinked structure.

[0084] Among these, the rubber component is preferably a diene rubber, preferably at least one selected from natural rubber, nitrile rubber, butadiene rubber, and styrene-butadiene rubber, more preferably at least one selected from natural rubber, nitrile rubber, and styrene-butadiene rubber, and even more preferably at least one selected from natural rubber, carboxylated nitrile rubber, and styrene-butadiene 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.

[0085] In the rubber composition of this embodiment, the content of cellulose fibers of this embodiment per 100 parts by mass of rubber component is preferably 1.0 part by mass or more and 200 parts by mass or less, more preferably 2.0 parts by mass or more, even more preferably 5.0 parts by mass or more, even more preferably 10 parts by mass or more, even more preferably 15 parts by mass or more, and even more preferably 150 parts by mass or less, even more preferably 100 parts by mass or less, even more preferably 75 parts by mass or less, even more preferably 50 parts by mass or less, even more preferably 40 parts by mass or less, and even more preferably 30 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 composition of this embodiment may differ from the content of cellulose fibers per 100 parts by mass of rubber component when making a crosslinked rubber composition. Specifically, the content of cellulose fibers may be high in the rubber composition, 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.

[0086] In the rubber composition of this embodiment, the content of cellulose fibers N 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 N per 100 parts by mass of rubber component in the composition of this embodiment may differ from the content of cellulose fibers N per 100 parts by mass of rubber component when making a crosslinked rubber composition. Specifically, the content of cellulose fibers N may be high in the rubber composition, and the content of cellulose fibers N per 100 parts by mass of rubber component may be adjusted by adding rubber component when preparing the rubber compound described later.

[0087] From the viewpoint of excellent dispersibility in the rubber composition, suppression of agglomeration in the crosslinked rubber composition, obtaining sufficient tensile stress and tensile elongation, and furthermore, excellent improvement of modulus in the low elongation region, it is preferable that the content of precipitate in the rubber composition is 1.0 part by mass or more and less than 10.0 parts by mass per 100 parts by mass of rubber component.

[0088] In one embodiment of the rubber composition of this embodiment, the content of cellulose fiber N per 100 parts by mass of rubber component in the rubber composition is 1.0 part by mass or more and 50 parts by mass or less, and when the cellulose fiber N is dispersed as an aqueous dispersion with a solid content of 0.2% by mass and centrifuged at 12,000 G for 10 minutes to obtain a precipitate and supernatant, the precipitate contains coarse cellulose fibers, the irregular freeness of the precipitate is 850 mL or less, and the length-weighted average fiber length of the precipitate is 0.4 mm or more, and the content of the precipitate per 100 parts by mass of rubber component in the rubber composition is 1.0 part by mass or more and less than 10.0 parts by mass. Hereinafter, this embodiment of the rubber composition will also be referred to as "rubber composition N".

[0089] The rubber composition N of this embodiment also includes a form in which the supernatant obtained by centrifugation under the above conditions does not contain any solids. By using the rubber composition N of this embodiment, a crosslinked rubber composition can be obtained that is particularly excellent in improving modulus in the low elongation region and has sufficient tensile stress and tensile elongation. The detailed mechanism by which the above effects are obtained is unknown, but it is thought that some of it is as follows. When pulp fibers with a low degree of beating are used as coarse cellulose fibers, when an aqueous dispersion of pulp fibers with a low degree of beating is mixed with rubber latex containing rubber components, although an improvement in modulus in the low elongation region is obtained, agglomerates of pulp are formed in the mixed liquid, and agglomerates may also be formed in the crosslinked rubber composition, in which case it has been found that the tensile stress and tensile elongation decrease. This is thought to be because the agglomerates trigger fracture. In this invention, when a cellulose fiber aqueous dispersion with a solid content of 0.2% by mass is centrifuged at 12,000 G for 10 minutes to obtain a precipitate and supernatant, the precipitate contains coarse cellulose fibers, the irregular freeness of the precipitate is 850 mL or less, and the length-weighted average fiber length of the precipitate is 0.4 mm or more. When a specific amount of such cellulose fibers is mixed with the rubber component as a cellulose fiber aqueous dispersion, it is believed that they will be uniformly dispersed in the mixture. As a result, a uniform rubber composition N can be obtained by mixing the cellulose fibers and the rubber component, and in the crosslinked rubber composition obtained after crosslinking, the generation of aggregates is suppressed, and the modulus improvement effect in the tensile elongation and low elongation regions is considered to be excellent.

[0090] The rubber composition 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.

[0091] [Method for Manufacturing the Rubber Composition] The method for manufacturing the rubber composition is not particularly limited, but it is preferable to manufacture the rubber composition by preparing a mixed solution containing at least a rubber component and the cellulose fibers of this embodiment, and then removing the solvent from the mixed solution. In the above manufacturing method, it is preferable to first prepare a mixed solution containing a rubber component and the cellulose fibers of this embodiment. That is, the method for manufacturing the rubber composition of this embodiment preferably includes the following steps (I) and (II): (I) A mixing step of mixing an aqueous dispersion containing the cellulose fibers for rubber modification of this embodiment (aqueous dispersion of cellulose fibers for rubber modification) with rubber latex to obtain a mixed solution A. (II) A solidification step of obtaining a solid rubber composition from the mixed solution A.

[0092] Specifically, step (I) is a mixing step in which the aqueous dispersion containing cellulose fibers of this embodiment is mixed with rubber latex 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 with the cellulose fibers for rubber modification tends to improve the physical properties after crosslinking.

[0093] 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.

[0094] The solid content concentration (by mass) of mixed solution A is preferably 0.5% by mass or more and 50% by mass or less, more preferably 1% by mass or more, even more preferably 3% by mass or more, and more preferably 40% by mass or less. By having the solid content concentration of the mixed solution within the above range, the amount of energy required to remove solvents such as water can be reduced, and furthermore, aggregation of cellulose fibers is less likely to occur in the resulting rubber composition, resulting in excellent kneadability of the resulting rubber composition and excellent properties of the crosslinked rubber composition.

[0095] 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)

[0096] The rubber composition is obtained in a solidification step, as shown in step (II) above, from the obtained mixed liquid A. In the solidification step, it is preferable to obtain the rubber composition by heating and drying the obtained mixed liquid A. However, the solidification step is not limited to this, and solidification may also be achieved by coagulation by adding an acid. Common acids 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 and the like can be used as flocculants.

[0097] 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 or Teflon® processed, 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 those whose surfaces have been oxidized, 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 particularly preferred because they allow for a more uniform thickness of the sheet.

[0098] 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 particularly 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.

[0099] The mixed liquid A may be heated and dried in a heated cylindrical dryer to obtain a rubber composition. 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 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 composition 5 (heat-dried product) can be obtained.

[0100] 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 refers to the temperature of the cylindrical surface in contact with the mixed liquid. 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 composition with good production efficiency and excellent physical properties after crosslinking.

[0101] 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.

[0102] 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.

[0103] 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.

[0104] The heated cylindrical dryer used in the method for manufacturing rubber compositions 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, as well as cylinder dryers, Yankee dryers, etc. Among these, from the viewpoint of heating and drying efficiency, the heated cylindrical dryer is preferably a double drum dryer or a cylinder dryer.

[0105] [Rubber Compound] In this embodiment, "rubber compound" refers to a composition obtained by kneading a rubber composition and in its pre-crosslinking state. Hereinafter, the rubber compound obtained using rubber composition N may be referred to as "rubber compound N". The rubber compound preferably contains at least a crosslinking agent, and in addition to the crosslinking agent, it may also contain additives that can be used as additives 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 newly added in addition to the rubber components when preparing the rubber composition. When newly 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 less, and preferably 5 parts by mass or less, and more preferably 3 parts by mass or less, per 100 parts by mass of rubber component.

[0106] Furthermore, the crosslinking accelerator may be included, which has the effect of promoting crosslinking by the crosslinking agent. When a sulfur-based crosslinking agent is used, it is preferable to include a crosslinking accelerator (vulcanization accelerator). When a sulfur-based crosslinking agent (vulcanization agent) is used 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).

[0107] Examples of the fillers include carbon black and silica. These fillers may be used individually or in combination of two or more. There are no particular restrictions on the content of the fillers, but preferably it is 10 to 150 parts by mass, and more preferably 20 to 100 parts by mass, per 100 parts by mass of the rubber component.

[0108] Examples of the softening agent 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 content of the softening agent is preferably 1 part by mass or more and 35 parts by mass or less, 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 the rubber component.

[0109] Examples of the aforementioned fatty acids include stearic acid, palmitic acid, arachidic acid, oleic acid, linoleic acid, and arachidonic acid. Among these, stearic acid is preferred. The content of the aforementioned fatty acids is not particularly limited, but is preferably 0.1 parts by mass or more and 5 parts by mass or less, and more preferably 1 part by mass or more and 4 parts by mass or less, per 100 parts by mass of the rubber component.

[0110] Examples of the aforementioned antioxidants 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). The antioxidant may be used alone or in combination of two or more. There are no particular restrictions on the content of the antioxidants, but each is preferably 0.1 parts by mass or more and 5 parts by mass, more preferably 1 part by mass or more and 3 parts by mass, per 100 parts by mass of the rubber component. The total content of the antioxidants is preferably 0.2 parts by mass or more and 10 parts by mass, more preferably 1 part by mass or more and 5 parts by mass, per 100 parts by mass of the rubber component.

[0111] The content of the zinc oxide (zinc oxide) is not particularly limited, but 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.

[0112] [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 composition of this embodiment. In the kneading step, it is preferable to knead the rubber composition 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 composition, 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 0.1 parts by mass or more and 50 parts by mass or less, more preferably 1 part by mass or more, even more preferably 3 parts by mass or more, and more preferably 40 parts by mass or less, and even more preferably 30 parts by mass or less, from the viewpoint of improving the physical properties of the crosslinked rubber composition.

[0113] The method for producing the rubber compound is not particularly limited, but it is preferable to include a step of kneading the rubber composition of this embodiment. Kneading improves the dispersibility of the crosslinking agent and other additives that are subsequently added. 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 composition. 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 produced by adding additives such as a crosslinking agent to the kneaded rubber composition and kneading them together.

[0114] Mixing is the process of uniformly dispersing a crosslinking agent and other compounding agents in a rubber composition (preferably a kneaded rubber composition). 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.

[0115] [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 the rubber composition and a rubber compound containing a crosslinking agent, and preferably includes a step of crosslinking and molding. The method for producing the crosslinked rubber composition of the present invention may further include a step of molding the rubber compound obtained in 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.

[0116] 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.

[0117] In this embodiment, when the 25% modulus (σ25) of the crosslinked rubber composition of the formulation described in the example is A1, and the 25% modulus (σ25) of the crosslinked rubber composition obtained by crosslinking the rubber compound excluding the cellulose fibers, fine fibrous cellulose, carbon black, and other reinforcing materials of this embodiment is B1, then A1 / B1 is preferably 2.0 or higher, more preferably 3.0 or higher, even more preferably 4.0 or higher, even more preferably 6.5 or higher, even more preferably 8.5 or higher, and even more preferably 10.0 or higher, and there is no particular upper limit, but from the viewpoint of ease of manufacture, it is preferably 50 or lower, more preferably 40 or lower, and even more preferably 30 or lower. In this embodiment, when the 50% modulus (σ50) of the crosslinked rubber composition of the formulation described in the example is A2, and the 50% modulus (σ50) of the crosslinked rubber composition obtained by crosslinking the rubber compound excluding the cellulose fibers, fine fibrous cellulose, and carbon black of this embodiment is B2, then A2 / B2 is preferably 2.0 or higher, more preferably 3.0 or higher, even more preferably 4.0 or higher, even more preferably 6.0 or higher, even more preferably 7.5 or higher, and even more preferably 9.5 or higher, and there is no particular upper limit, but from the viewpoint of ease of manufacture, it is preferably 50 or lower, more preferably 40 or lower, and even more preferably 20 or lower. When A1 / B1 and A2 / B2 are within the above range, it is preferable because it has an excellent effect in improving the modulus in the low elongation region.

[0118] In this embodiment, when the 100% modulus (σ100) of the crosslinked rubber composition of the formulation described in the example is A3, and the 100% modulus (σ100) of the crosslinked rubber composition obtained by crosslinking the rubber compound excluding the cellulose fibers, fine fibrous cellulose, and carbon black of this embodiment is B3, then A3 / B3 is preferably 1.5 or more, more preferably 3.0 or more, and even more preferably 4.5 or more. The upper limit is not particularly limited, but from the viewpoint of ease of manufacture, it is preferably 30 or less, more preferably 20 or less, and even more preferably 15 or less. In other words, the cellulose fibers of this embodiment are particularly excellent in improving the modulus when used in a crosslinked rubber composition in the low elongation region, and it is preferable that they also have a modulus improvement effect outside the low elongation region.

[0119] In this embodiment, in particular, when the 25% modulus (σ25) of the crosslinked rubber composition N of the formulation described in the example is denoted as σ25B, and the 25% modulus (σ25) of the crosslinked rubber composition obtained by crosslinking a rubber compound excluding reinforcing materials such as cellulose fibers and carbon black is denoted as σ25A, then σ25B / σ25A is preferably 2.0 or more, more preferably 2.5 or more, even more preferably 3.0 or more, and even more preferably 3.5 or more, and there is no particular upper limit, but from the viewpoint of ease of manufacture, it is preferably 25.0 or less, more preferably 20.0 or less, and even more preferably 15.0 or less.

[0120] In this embodiment, in particular, when the 50% modulus (σ50) of the crosslinked rubber composition N of the formulation described in the example is denoted as σ50B, and the 50% modulus (σ50) of the crosslinked rubber composition obtained by crosslinking a rubber compound excluding reinforcing materials such as cellulose fibers and carbon black is denoted as σ50A, then σ50B / σ50A is preferably 2.0 or more, more preferably 2.5 or more, even more preferably 3.0 or more, and even more preferably 3.5 or more, and there is no particular upper limit, but from the viewpoint of ease of manufacture, it is preferably 25.0 or less, more preferably 20.0 or less, and even more preferably 15.0 or less.

[0121] In this embodiment, in particular, when the 100% modulus (σ100) of the crosslinked rubber composition N of the formulation described in the example is denoted as σ100B, and the 100% modulus (σ100) of the crosslinked rubber composition obtained by crosslinking a rubber compound excluding reinforcing materials such as cellulose fibers and carbon black is denoted as σ100A, then σ100B / σ100A is preferably 1.5 or more, more preferably 2.0 or more, even more preferably 2.5 or more, and even more preferably 3.0 or more, and there is no particular upper limit, but from the viewpoint of ease of manufacture, it is preferably 17.0 or less, more preferably 14.0 or less, and even more preferably 11.0 or less.

[0122] In this embodiment, in particular, when the 200% modulus (σ200) of the crosslinked rubber composition N of the formulation described in the example is denoted as σ200B, and the 200% modulus (σ200) of the crosslinked rubber composition obtained by crosslinking a rubber compound excluding reinforcing materials such as cellulose fibers and carbon black is denoted as σ200A, then σ200B / σ200A is preferably 1.5 or more, more preferably 2.0 or more, even more preferably 2.5 or more, and even more preferably 3.0 or more, and there is no particular upper limit, but from the viewpoint of ease of manufacture, it is preferably 16.0 or less, more preferably 13.0 or less, and even more preferably 10.0 or less.

[0123] When the above values ​​of σ25B / σ25A, σ50B / σ50A, σ100B / σ100A, and σ200B / σ200A are within the above range, it is preferable because it provides an excellent modulus improvement effect.

[0124] <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.

[0125] 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.

[0126] <Manufacturing Example 1-1> 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 (cellulose fiber aqueous dispersion 6) with a solid content concentration 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 (beaten pulp) aqueous dispersion 1.

[0127] <Production Example 1-2> A coarse cellulose fiber (beaten pulp) aqueous dispersion 2 was obtained in the same manner as in Production Example 1-1, except that the beating time was set to 4 hours.

[0128] <Production Example 1-3> A coarse cellulose fiber (beaten pulp) aqueous dispersion 3 was obtained in the same manner as in Production Example 1-1, except that the beating time was set to 4.5 hours.

[0129] <Production Example 1-4> A coarse cellulose fiber (beaten pulp) aqueous dispersion 4 was obtained in the same manner as in Production Example 1-1, except that the beating time was set to 1 hour.

[0130] <Production Example 1-5> A coarse cellulose fiber (beaten pulp) aqueous dispersion 5 was obtained in the same manner as in Production Example 1-1, except that the beating time was set to 2 hours.

[0131] <Manufacturing Example 1-6> [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.

[0132] 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 aqueous dispersion, stirring the dispersion to ensure uniform pulp distribution, 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.

[0133] 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.

[0134] Deionized water was added to the obtained phosphorylated pulp to prepare a slurry with a solid content concentration 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 aqueous dispersion (dispersion A) containing phosphorylated fine fibrous cellulose. The fiber width of the phosphorylated fine fibrous cellulose was measured using a transmission electron microscope and found to be 3–5 nm.

[0135] 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.

[0136] (Measurement of Phosphorus Oxoacid Groups) In measuring the amount of phosphorus oxoacid groups (phosphate groups or phosphite groups) in cellulose fibers, first, ion-exchanged water was added to the target cellulose fibers to prepare a slurry with a solid content concentration of 0.2% by mass. The obtained cellulose fiber aqueous dispersion was treated with an ion exchange resin, and then 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 above cellulose fiber aqueous dispersion, shaking for 1 hour, and then pouring it onto a mesh with a mesh opening of 90 μm to separate the resin and slurry. The titration using an alkali was performed by adding 10 μL of 0.1 N sodium hydroxide aqueous solution to the cellulose fiber-containing 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.

[0137] <Example 1-1> [Step (I): Mixing Step] In a container, 100 parts by mass of the solid content of the coarse cellulose fiber (beaten pulp) aqueous dispersion 1 obtained in Production Example 1-1 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 solid content of the rubber component was 500 parts by mass. A tornado agitator was used as the agitator, and a 6-inch diameter agitator blade was attached, and the mixture was stirred at 1,000 rpm for 5 minutes to obtain a coarse cellulose fiber (beaten pulp) / natural rubber latex mixture (mixture A1-1).

[0138] [Process (II): Drying Process (Solidification Process)] The obtained coarse cellulose fiber (beaten pulp) / natural rubber latex mixture (mixture A1-1) was spread on a Teflon-coated tray 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 coarse cellulose fiber (beaten pulp) / natural rubber composition.

[0139] [Preparation of Rubber Compound] The obtained coarse cellulose fiber (beaten pulp) / natural rubber composition was kneaded for 10 minutes at a rotation speed of 26 / 30 rpm without heating using an open roll mill (6-inch double roll mill, manufactured by Daihan Co., Ltd.). Then, 2 parts by mass of a crosslinking agent (Parkmill D, manufactured by NOF Corporation), 3 parts by mass of vulcanization accelerators (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.

[0140] [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.

[0141] <Example 1-2> The same procedure as in Example 1-1 was performed to obtain a sheet of crosslinked rubber composition, except that coarse cellulose fiber (beaten pulp) aqueous dispersion 2 was used instead of coarse cellulose fiber (beaten pulp) aqueous dispersion 1.

[0142] <Example 1-3> The same procedure as in Example 1-1 was performed to obtain a sheet of crosslinked rubber composition, except that a coarse cellulose fiber (beaten pulp) aqueous dispersion 3 was used instead of coarse cellulose fiber (beaten pulp) aqueous dispersion 1.

[0143] <Example 1-4> A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 1-1, except that an aqueous dispersion of natural rubber latex was added so that the solid content of the rubber component was 1,000 parts by mass.

[0144] <Example 1-5> A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 1-1, except that an aqueous dispersion of natural rubber latex was added so that the solid content of the rubber component was 2,000 parts by mass.

[0145] <Example 1-6> A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 1-1, except that an aqueous dispersion of natural rubber latex was added so that the solid content of the rubber component was 4,000 parts by mass.

[0146] <Example 1-7> In the [Preparation of Compound Sheet] of Example 1-1, a rubber compound was obtained in the same manner as in Example 1-1, except that the additives were 1.5 parts by mass of sulfur, 2 parts by mass of vulcanization accelerator, 2 parts by mass of antioxidant (ANTAGE RD), 2 parts by mass of stearic acid, and 2 parts by mass of vulcanization accelerator 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.

[0147] <Example 1-8> The same procedure as in Example 1-7 was performed to obtain a sheet of crosslinked rubber composition, except that coarse cellulose fiber (beaten pulp) aqueous dispersion 2 was used instead of coarse cellulose fiber (beaten pulp) aqueous dispersion 1.

[0148] <Example 1-9> The same procedure as in Example 1-7 was followed to obtain a sheet of crosslinked rubber composition, except that coarse cellulose fiber (beaten pulp) aqueous dispersion 3 was used instead of coarse cellulose fiber (beaten pulp) aqueous dispersion 1.

[0149] <Example 1-10> A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 1-7, except that an aqueous dispersion of natural rubber latex was added so that the solid content of the rubber component was 1,000 parts by mass.

[0150] <Example 1-11> A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 1-7, except that an aqueous dispersion of natural rubber latex was added so that the solid content of the rubber component was 2,000 parts by mass.

[0151] <Example 1-12> Except that HA-LATEX (manufactured by Resitex Co., Ltd., solids content: 61.5%, ammonia content: 0.7%) was used instead of ULACOL as the aqueous dispersion of natural rubber latex, the same procedure as in Example 1-7 was carried out to obtain a sheet of crosslinked rubber composition.

[0152] <Example 1-13> A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 1-7, except that LA-LATEX (manufactured by Resitex Co., Ltd., solids content: 61.5%, ammonia content: less than 0.3%) was used instead of ULACOL as the aqueous dispersion of natural rubber latex.

[0153] <Example 1-14> A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 1-7, except that MG-10 (manufactured by Resitex Co., Ltd., solids content: 54.0%, modified natural rubber (methyl methacrylate (MMA) graft polymer), ammonia content 0.5%) was used instead of ULACOL as the aqueous dispersion of natural rubber latex.

[0154] <Example 1-15> A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 1-1, except that a coarse cellulose fiber (beaten pulp) aqueous dispersion 4 was used instead of coarse cellulose fiber (beaten pulp) aqueous dispersion 1.

[0155] <Example 1-16> A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 1-1, except that a coarse cellulose fiber (beaten pulp) aqueous dispersion 5 was used instead of coarse cellulose fiber (beaten pulp) aqueous dispersion 1.

[0156] <Comparative Example 1-1> The process up to the mixing step was carried out in the same manner as in Example 1-1, except that a pulp aqueous dispersion (coarse cellulose fiber aqueous dispersion 6) before beating was used instead of the coarse cellulose fiber (beaten pulp) aqueous dispersion 1. In Comparative Example 1-1, the pulp separated and settled in the mixed liquid during the mixing step, making it impossible to carry out the steps from the drying step onward.

[0157] <Comparative Example 1-2> A fine fibrous cellulose aqueous dispersion (dispersion A) obtained in Production Example 1-6 was placed in a container so that the solid content of the fine fibrous cellulose was 100 parts by mass. Then, 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 agitator (general-purpose constrained agitator, PM-202, manufactured by AS ONE Corporation) was used as the agitator, and a 6-inch diameter agitator blade was attached. The mixture was stirred at 1,000 rpm for 5 minutes to obtain a diluted fine fibrous cellulose aqueous dispersion. The same procedure as in Example 1-1 was performed, except that the fine fibrous cellulose aqueous dispersion was used instead of the cellulose fiber (beaten pulp) aqueous dispersion 1, to obtain a sheet of crosslinked rubber composition.

[0158] <Comparative Example 1-3> A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 1-4, except that a fine fibrous cellulose aqueous dispersion (dispersion A) was used instead of coarse cellulose fiber (beaten pulp) aqueous dispersion 1.

[0159] <Comparative Example 1-4> A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 1-5, except that a fine fibrous cellulose aqueous dispersion (dispersion A) was used instead of coarse cellulose fiber (beaten pulp) aqueous dispersion 1.

[0160] <Comparative Example 1-5> A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 1-7, except that a fine fibrous cellulose aqueous dispersion (dispersion A) was used instead of coarse cellulose fiber (beaten pulp) aqueous dispersion 1.

[0161] <Comparative Example 1-6> A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 1-10, except that a fine fibrous cellulose aqueous dispersion (dispersion A) was used instead of coarse cellulose fiber (beaten pulp) aqueous dispersion 1.

[0162] <Comparative Example 1-7> A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 1-11, except that a fine fibrous cellulose aqueous dispersion (dispersion A) was used instead of coarse cellulose fiber (beaten pulp) aqueous dispersion 1.

[0163] <Reference Example 1-1> Natural rubber latex (ULACOL) 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 of natural rubber. A sheet of cross-linked rubber was obtained by performing the same procedure as in Example 1-1, except that this natural rubber was used.

[0164] <Reference Example 1-2> Natural rubber latex (ULACOL) 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 of natural rubber. A sheet of crosslinked rubber was obtained by performing the same procedure as in Example 1-7, except that this natural rubber was used.

[0165] <Reference Example 1-3> Except for using HA-LATEX instead of ULACOL as the natural rubber latex, the same processing as in Reference Example 1-2 was carried out to obtain a cross-linked rubber sheet.

[0166] <Reference Example 1-4> Except for using LA-LATEX instead of ULACOL as the natural rubber latex, the same processing as in Reference Example 1-2 was carried out to obtain a cross-linked rubber sheet.

[0167] <Reference Example 1-5> Except for using MG-10 instead of ULACOL as the natural rubber latex, the same processing as in Reference Example 1-2 was carried out to obtain a cross-linked rubber sheet.

[0168] [Separation and Sedimentation During Mixing] For Examples 1-1 to 1-16 and Comparative Examples 1-1 to 1-7, the state of the mixed liquid after standing for 30 minutes after mixing and the state of the dried product after the drying process were observed and evaluated as follows: A: Coarse cellulose fibers (beaten pulp), unbeaten pulp, or fine fibrous cellulose did not separate or precipitate in the mixed liquid, maintaining a uniform mixed liquid state, and no non-uniform areas or cellulose fibers were visible in the dried composition. B: Coarse cellulose fibers (beaten pulp), unbeaten pulp, or fine fibrous cellulose did not separate or precipitate in the mixed liquid, maintaining a uniform mixed liquid state. In addition, in the dried composition, non-uniform areas in the distribution of coarse cellulose fibers (beaten pulp), unbeaten pulp, or fine fibrous cellulose, and cellulose fibers were visible, but the layers of coarse cellulose fibers (beaten pulp), unbeaten pulp, or fine fibrous cellulose and the rubber layer were not clearly separated. C: Coarse cellulose fibers (beaten pulp), unbeaten pulp, or fine fibrous cellulose are partially separated and precipitated in the mixture, resulting in a non-uniform mixture. Furthermore, in the dried composition, there are areas where the distribution of coarse cellulose fibers (beaten pulp), unbeaten pulp, or fine fibrous cellulose is non-uniform, and cellulose fibers are visible to the naked eye, but the layers of coarse cellulose fibers (beaten pulp), unbeaten pulp, or fine fibrous cellulose and the rubber layer are not clearly separated. D: Coarse cellulose fibers (beaten pulp), unbeaten pulp, or fine fibrous cellulose are separated and precipitated in the mixture, resulting in a non-uniform mixture. Furthermore, in the dried composition, the layers of coarse cellulose fibers (beaten pulp), unbeaten pulp, or fine fibrous cellulose and the rubber layer are clearly separated.

[0169] [Irregular Freeness] Irregular freeness was measured by preparing a 0.03% by mass solution of a coarse cellulose fiber (beaten pulp) aqueous dispersion, a pulp aqueous dispersion before beating, or a fine fibrous cellulose aqueous dispersion, and measuring it using the Canadian standard filtration method specified in JIS P 8121-2:2012, changing the pulp concentration from 0.3% by mass to 0.03% by mass, and changing the JIS standard screen plate to an 80-mesh wire.

[0170] [Length-average fiber length (Ll)] The length-weighted average fiber length (Ll) and the number-average fiber length (Ln) are obtained by the method specified in JIS P 8226-2:2011. The length-average fiber length (Ll) of cellulose fibers (beaten pulp), unbeaten pulp, or fine fibrous cellulose can be measured using Valmet's FS5.

[0171] [Fiber width] The fiber width of 50 randomly selected microfiber cellulose samples was measured using an atomic force microscope. It was confirmed that more than 90% of the measured microfiber cellulose samples were in the range of 3 to 5 nm in fiber width.

[0172] [Tensile Test] For the crosslinked rubber compositions or sheets of crosslinked rubber obtained in Examples 1-1 to 1-16, Comparative Examples 1-2 to 1-7, and Reference Examples 1-1 to 1-5, test specimens punched out in the shape of a dumbbell (Dumbbell No. 6) as described in JIS K 6251:2017 were subjected to tensile testing using a Tensilon tensile testing machine (manufactured by A&D Co., Ltd.) at 23±2℃, a gauge length of 20 mm, and a tensile speed of 500 mm / min, in accordance with JIS K 6251:2017, and the 25% modulus (σ25 (MPa)) and 50% modulus (σ50 (MPa)) were measured. In addition, for the crosslinked rubber compositions or sheets of crosslinked rubber obtained in Examples 1-1 and 1-7, Comparative Examples 1-2 and 1-5, and Reference Examples 1-1 and 1-2, the 100% modulus (σ100 (MPa)) was measured in the same manner as above. Furthermore, for each modulus, the modulus improvement rate was calculated according to the following formula: Modulus Improvement Rate = Modulus in each example and comparative example / Modulus in the corresponding reference example (modulus in the case of rubber alone) The corresponding reference examples are as follows: Examples 1-1 to 1-6, 1-15, 1-16, Comparative Examples 1-2 to 3: Reference Example 1-1 Examples 1-7 to 1-11, Comparative Examples 1-4 to 1-7: Reference Example 1-2 Example 1-12: Reference Example 1-3 Example 1-13: Reference Example 1-4 Example 1-14: Reference Example 1-5

[0173] [Energy Consumption] Energy consumption was evaluated as follows based on the amount of electricity required to perform the beating and defibration processes from the pulp state: A: Energy consumption is less than 1.0 kWh / kg. B: Energy consumption is 1.0 kWh / kg or more and less than 10 kWh / kg. C: Energy consumption is 10 kWh / kg or more.

[0174]

[0175]

[0176] The results from Tables 1-1 and 1-2 show that the crosslinked rubber compositions obtained in the examples exhibit excellent improvement in modulus (σ25 and σ50) in the low-elongation region compared to plain rubber (crosslinked rubber). Furthermore, even outside the low-elongation region, they exhibit excellent improvement in modulus (σ100). On the other hand, in Comparative Example 1-1, where pulp before beating was added, precipitation occurred when mixed with the latex rubber component, making further evaluation impossible. In addition, in Comparative Examples 1-2 to 1-7, where phosphorylated cellulose nanofibers were added, a sufficient improvement in modulus was not obtained in the low-elongation region.

[0177] <Production Example 2-1> A coarse cellulose fiber aqueous dispersion was obtained in the same manner as the coarse cellulose fiber (beaten pulp) aqueous dispersion 1 of Production Example 1-1.

[0178] <Production Example 2-2> A fine fibrous cellulose aqueous dispersion containing phosphorylated fine fibrous cellulose was obtained in the same manner as in Production Example 1-6. Furthermore, when the phosphorylated fine fibrous cellulose contained in the fine fibrous cellulose aqueous dispersion was observed using an atomic microscope, it was found to contain phosphorylated fine fibrous cellulose with a fiber width of 3-5 nm.

[0179] 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] above was 1.45 mmol / g. The total amount of dissociated acid was 2.45 mmol / g.

[0180] <Example 2-1> [Step (I): Mixing Step] A mixture of coarse cellulose fibers and natural rubber latex (mixture A2-1) was obtained in the same manner as in Example 1-1, except that a mixture of 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 a container with a solid content of 100 parts by mass of the coarse cellulose fiber aqueous dispersion obtained in Production Example 2-1, so that the rubber component amounted to 2,000 parts by mass.

[0181] [Step (II): Solidification Step] A sheet-like rubber composition was obtained in the same manner as in Example 1-1, except that mixed solution A2-1 was used.

[0182] [Preparation of Rubber Compound] The rubber composition was kneaded in the same manner as in Example 1-1. Then, 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 the mixture was kneaded for 6 minutes to obtain a rubber compound with a thickness of approximately 2 mm or more.

[0183] [Crosslinking Process] A sheet of crosslinked rubber composition with a thickness of 2 mm was prepared in the same manner as in Example 1-1, except that the obtained rubber compound was used.

[0184] <Example 2-2> [Preparation process for cellulose fiber aqueous dispersion] The fine fibrous cellulose aqueous dispersion obtained in Production Example 2-2 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 aqueous dispersion. To this aqueous dispersion, the coarse cellulose fiber aqueous dispersion obtained in Production Example 2-1 was added so that the solid content of the coarse cellulose fiber aqueous dispersion was 400 parts by mass for every 100 parts by mass of fine fibrous cellulose in the aqueous 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 cellulose fiber aqueous dispersion (a).

[0185] [Step (I): Mixing Step] To 100 parts by mass of cellulose fibers, including coarse cellulose fibers and fine fibrous cellulose, obtained in the [Preparation Step of Cellulose Fiber Aqueous Dispersion], 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 solid content of the rubber component was 400 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 cellulose fiber / natural rubber latex mixture (mixture A2-2).

[0186] [Step (II): Solidification Step] A sheet-like rubber composition was obtained in the same manner as in Example 1-1, except that mixed solution A2-2 was used.

[0187] [Preparation of Rubber Compound] Using an open roll mill (6-inch double roll mill, manufactured by Daihan Co., Ltd.), the rubber composition was kneaded at a rotation speed of 26 / 30 rpm for 10 minutes without heating. Next, the kneaded rubber composition was removed from the open roll mill, and using the open roll mill, natural rubber (RSS#3, manufactured by Koshigaya Rubber Industry Co., Ltd.) was kneaded at a rotation speed of 26 / 30 rpm for 10 minutes without heating. Then, the kneaded rubber composition was added to 100 parts by mass of the kneaded natural rubber in an amount of 41.67 parts by mass, 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 components, and the mixture was kneaded for 6 minutes to obtain a rubber compound with a thickness of approximately 2 mm or more.

[0188] [Crosslinking Process] A sheet of crosslinked rubber composition with a thickness of 2 mm was prepared in the same manner as in Example 1-1, except that the obtained rubber compound was used.

[0189] <Example 2-3> The amount of coarse cellulose fiber aqueous dispersion added was set to 200 parts by mass of coarse cellulose fiber per 100 parts by mass of fine fibrous cellulose to obtain cellulose fiber aqueous dispersion (b). A sheet-like rubber composition was obtained by performing the same procedure as in Example 2-1, 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 this cellulose fiber aqueous dispersion (b) so that the rubber component amounted to 666.7 parts by mass. [Preparation of rubber compound] A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 2-2, except that 115 parts by mass of the kneaded rubber composition was added to 100 parts by mass of kneaded natural rubber and kneaded for 10 minutes.

[0190] <Example 2-4> The amount of coarse cellulose fiber aqueous dispersion added was 25 parts by mass per 100 parts by mass of fine fibrous cellulose to obtain a cellulose fiber aqueous dispersion (c). A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 2-1, 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 (c) so that the solid content of the rubber component became 400 parts by mass.

[0191] <Example 2-5> In the [Preparation of Compound Sheet] of Example 2-1, a rubber compound was obtained in the same manner as in Example 2-1, 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.

[0192] <Example 2-6> Except that the [Preparation of Compound Sheet] and [Crosslinking Process] were carried out in the same manner as in Example 2-5, a sheet of crosslinked rubber composition was obtained by performing the same process as in Example 2-2.

[0193] <Example 2-7> Except for the [Preparation of Compound Sheet] and [Crosslinking Process] being carried out in the same manner as in Example 2-5, the same process as in Example 2-3 was performed to obtain a sheet of crosslinked rubber composition.

[0194] <Example 2-8> Except that the [Preparation of Compound Sheet] and [Crosslinking Process] were carried out in the same manner as in Example 2-5, a sheet of crosslinked rubber composition was obtained by performing the same process as in Example 2-4.

[0195] <Example 2-9> In a container, 100 parts by mass of a solid content aqueous dispersion of coarse cellulose fibers obtained in Production Example 2-1 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 solid content of the rubber component was 1,000 parts by mass. A tornado stirrer was used as the agitator, fitted with a 6-inch diameter stirring 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 2-1 was carried out to obtain a sheet of crosslinked rubber composition.

[0196] <Example 2-10> A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 2-9, except that an aqueous dispersion of natural rubber latex was added so that the rubber component amounted to 500 parts by mass.

[0197] <Example 2-11> Except for the [Preparation of Compound Sheet] and [Crosslinking Process] being carried out in the same manner as in Example 2-5, the same process as in Example 2-9 was performed to obtain a sheet of crosslinked rubber composition.

[0198] <Example 2-12> Except for the [Preparation of Compound Sheet] and [Crosslinking Process] being carried out in the same manner as in Example 2-5, the same process as in Example 2-10 was performed to obtain a sheet of crosslinked rubber composition.

[0199] <Reference Example 2-1> A sheet of crosslinked rubber composition was obtained by performing the same procedure as in Example 2-9, except that a fine fibrous cellulose aqueous dispersion was used instead of a coarse cellulose fiber aqueous dispersion, and a natural rubber latex aqueous dispersion was added in such a manner that the rubber component amounted to 2,000 parts by mass per 100 parts by mass of fine fibrous cellulose.

[0200] <Reference Example 2-2> Except that the [Preparation of Compound Sheet] and [Crosslinking Process] were carried out in the same manner as in Example 2-5, a sheet of crosslinked rubber composition was obtained by performing the same process as in Reference Example 2-1.

[0201] <Reference Example 2-3> 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 of natural rubber. A sheet of crosslinked rubber was obtained by performing the same procedure as in Example 2-1, except that this natural rubber was used.

[0202] <Reference Example 2-4> 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 of natural rubber. A sheet of crosslinked rubber was obtained by performing the same procedure as in Example 2-5, except that this natural rubber was used.

[0203] [Separation of coarse cellulose fibers and fine fibrous cellulose in aqueous cellulose fiber dispersions] For Examples 2-2 to 2-4 and 2-6 to 2-8, ion-exchanged water was added to each aqueous cellulose fiber dispersion to dilute it to a solid content of 0.2 parts 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.), and the solid content and precipitate in the supernatant were collected and used as samples for measuring irregular freeness, average fiber length, and fiber width.

[0204] [Mass ratio of precipitate to solids in supernatant] For Examples 2-2 to 2-4 and 2-6 to 2-8, 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 and precipitate in the supernatant were collected. Each was dried in a constant-temperature dryer at 105°C to a completely dry state, and their ratios were determined.

[0205] [Irregular Freeness] The irregular freeness of the coarse cellulose fiber aqueous dispersion, the fine fibrous cellulose aqueous dispersion, and the precipitate was measured in the same manner as in Example 1.

[0206] [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, and 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.

[0207] [Average fiber length (Ll)] The average fiber length (Ll) of the coarse cellulose fibers and precipitate was measured in the same manner as in Example 1.

[0208] [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.

[0209] [Evaluation of Aggregates] The sheets of crosslinked rubber compositions obtained in Examples 2-1 to 2-12 and Reference Examples 2-1 to 2-2 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.

[0210] [Tensile Test] For the crosslinked rubber compositions obtained in Examples 2-1 to 2-12 and Reference Examples 2-1 to 2-2, and the crosslinked rubber sheets obtained in Reference Examples 2-3 to 2-4, the 25% modulus (σ25 (MPa)), 50% modulus (σ50 (MPa)), 100% modulus (σ100 (MPa)), 200% modulus (σ200 (MPa)), tensile elongation, and tensile stress were measured in the same manner as described above.

[0211]

[0212] In Table 2-1, the "-" next to "σ200" in Examples 2-9 to 2-12 means that measurement was not possible because the material fractured at an elongation of less than 200%.

[0213] The results in Table 2-1 show that the crosslinked rubber compositions obtained in the examples have fewer aggregates of cellulose fibers, exhibit excellent improvement in modulus compared to rubber alone, particularly in the modulus in the low elongation region (σ25 and σ50), and satisfy the requirements of "cellulose fiber N" and "the content of precipitate per 100 parts by mass of rubber components in the rubber composition is 1.0 part by mass or more and less than 10.0 parts by mass" (Examples 2-1 to 2-8).

[0214] 10...Double drum dryer, 1...Cylinder, 2...Feed section, 3...Sheet-like mixture, 4...Scraper, 5...Rubber composition

Claims

1. Cellulose fibers for rubber modification containing coarse cellulose fibers having an irregular freeness of 850 mL or less and a length-weighted average fiber length of 0.4 mm or more.

2. The cellulose fiber for rubber modification according to claim 1, wherein the coarse cellulose fiber is an unmodified cellulose fiber.

3. When cellulose fibers for rubber modification are dispersed as an aqueous dispersion with a solid content of 0.2% by mass and centrifuged under the following conditions to obtain a precipitate and supernatant, the precipitate contains coarse cellulose fibers, the irregular freeness of the precipitate is 850 mL or less, and the length-weighted average fiber length of the precipitate is 0.4 mm or more, according to claim 1 or 2. Conditions: Centrifugation at 12,000 G for 10 minutes 4. The cellulose fiber for rubber modification according to claim 3, wherein the precipitate contains unmodified coarse cellulose fibers.

5. The cellulose fiber for rubber modification according to claim 3 or 4, wherein the supernatant liquid contains solid components, and the solid components contain fine fibrous cellulose with a fiber width of 100 nm or less.

6. The cellulose fiber for rubber modification according to claim 5, 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.

7. Aqueous dispersion of cellulose fibers for rubber modification, containing the cellulose fibers for rubber modification described in any one of claims 1 to 6.

8. A rubber composition comprising cellulose fibers for rubber modification according to claim 1 or 2 and a rubber component.

9. A rubber composition comprising cellulose fibers for rubber modification according to any one of claims 3 to 6 and a rubber component.

10. The rubber composition according to claim 8 or 9, wherein the content of cellulose fibers for rubber modification is 1.0 part by mass or more and 50 parts by mass or less per 100 parts by mass of rubber components in the rubber composition.

11. The rubber composition according to claim 9, wherein the content of precipitate per 100 parts by mass of rubber component in the rubber composition is 1.0 part by mass or more and less than 10.0 parts by mass.

12. The rubber composition according to any one of claims 8 to 11, wherein the rubber component includes a diene rubber.

13. A rubber compound containing the rubber composition according to any one of claims 8 to 12 and a crosslinking agent.

14. A crosslinked rubber composition obtained by crosslinking the rubber compound described in claim 13.

15. The crosslinked rubber composition according to claim 14, which is a tire.

16. A method for producing a rubber composition comprising the following steps (I) and (II): (I) A mixing step of mixing the cellulose fiber aqueous dispersion for rubber modification described in claim 7 with rubber latex to obtain a mixed liquid A; (II) A solidification step of obtaining a solid rubber composition from the mixed liquid A.

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