Method for producing rubber composition

Abstract

The purpose of the present invention is to provide a method for producing a rubber composition, the method being capable of efficiently and sufficiently drying fine cellulose fibers and a rubber component, having little influence on physical properties of the obtained rubber composition, and preferably being capable of enhancing the physical properties. Specifically, the present invention provides a method for producing a rubber composition, the method comprising a drying step for drying a mixture containing fine cellulose fibers and a rubber component and having a solid concentration of 3-70 mass% by using a drum dryer at a temperature of 70-150°C.

Description

Method for producing rubber composition

[0001] The present invention relates to a method for producing a rubber composition.

[0002] Cellulose nanofibers and microfibrillated cellulose (hereinafter collectively referred to as "fine cellulose fibers") obtained by micronizing cellulose are fine fibers with diameters on the nano to micro order. As new materials with properties not found in ordinary pulp, such as high strength, high elasticity, and thixotropy, they are expected to be used in a variety of fields.

[0003] Generally, the existing method for drying a mixture of rubber latex and filler involves acid-coagulating the mixture, removing most of the water in a dehydrator, and then drying with hot air in a dryer. However, when the filler is fine cellulose fiber, the water-retaining properties of the fine cellulose fiber and carboxymethyl cellulose used in combination as needed inhibit acid coagulation, so a direct drying method without acid coagulation has been adopted. As a drying method, drying methods using various dryers have been used, but hot air drying using an oven is commonly used (e.g., Patent Document 1).

[0004] Japanese Patent Application Laid-Open No. 2021-98791

[0005] However, oven drying has some issues, such as the long drying time required, the limited volume that can be dried at one time, the fact that drying begins from the surface, and therefore uneven drying of the surface and interior of a large amount of material, and the deterioration of the physical properties of the resulting rubber composition when drying is performed at high temperatures.

[0006] Therefore, an object of the present invention is to provide a method for producing a rubber composition that can efficiently and sufficiently dry fine cellulose fibers and a rubber component, and that has little effect on the physical properties of the resulting rubber composition, or even enhances them.

[0007] The present invention provides the following: [1] A method for producing a rubber composition, comprising a drying step of drying a mixture containing fine cellulose fibers and a rubber component and having a solids concentration of 3 to 70% by mass at a temperature of 70 to 150°C using a drum dryer. [2] The method according to claim 1, wherein the fine cellulose fibers include chemically modified fine cellulose fibers. [3] The method according to [1] or [2], wherein the weight ratio of the content of the fine cellulose fibers to the content of the rubber component in the mixture is 0.1 to 50 phr. [4] The production method according to [1] or [2], wherein the drum dryer is a double-drum type drum dryer. [5] The production method according to [4], wherein the clearance between the drums constituting the double-drum type drum dryer is 0.07 mm to 1 mm. [6] The production method according to [1] or [2], wherein the drum dryer comprises a drum having a coating layer containing ceramic or cermet on its surface. [7] The production method according to [1] or [2], wherein the drum dryer comprises a doctor blade made of high-carbon steel. [8] The manufacturing method according to [1] or [2], further comprising a vulcanization step of vulcanizing the dried product obtained in the drying step. [9] A composition comprising fine cellulose fibers and a rubber component, wherein the difference ΔL value between the L1 value, which is the lightness L* defined by the L*a*b* color system of the composition immediately after production and one week after production, and the L2 value, which is the lightness L* of a composition similar to the composition except that it does not contain fine cellulose fibers, is 1 to 15.

[0008] According to the present invention, by carrying out drying using a drum dryer in the process of producing a rubber composition, it is possible to improve the drying efficiency of the mixture of fine cellulose fibers and rubber components, and to obtain a rubber composition that exhibits good physical properties even when a high drying temperature is used.

[0009] The present invention will be described in detail below. In the present invention, "to" includes the end values. That is, "X to Y" includes the values ​​X and Y at both ends.

[0010] [1. Raw Materials for Rubber Composition] The raw materials for the rubber composition include at least fine cellulose fibers and a rubber component. Each raw material will be described below.

[0011] [1.1 Microfibrillated Cellulose Fibers] Microfibrillated cellulose fibers are fine fibrous cellulose derived from cellulose raw materials. Examples of microfibrillated cellulose include those in which a dispersion (1 wt%) of microfibrillated cellulose fibers exhibits a light transmittance of 1 to 99% at an optical path length of 1 cm / 660 nm using a visible spectrophotometer (UV-1800, manufactured by Shimadzu Corporation). The average fiber diameter of the microfibrillated cellulose is not particularly limited, but is typically approximately 1 nm to 60 μm. Methods for producing microfibrillated cellulose fibers include a method of defibrating pulp and a method of chemically modifying the pulp before or after defibration (usually before defibration) as needed. Microfibrillated cellulose fibers with nano-order fiber diameters are called cellulose nanofibers (CNF), and microfibrillated cellulose fibers with micron-order fiber diameters are called cellulose microfibrils (MFC). The size of the microfibrillated cellulose fibers can be adjusted by the conditions of the microfibrillation treatment and chemical modification treatment.

[0012] [Example of Fine Cellulose Fiber: Cellulose Nanofiber (CNF)] In this specification, CNF refers to cellulose fiber prepared through a micronization process and having a fiber diameter on the nano-order (for example, less than 500 nm).

[0013] The average fiber diameter (length-weighted average fiber diameter) of the CNF is 500 nm or less, preferably 300 nm or less, more preferably 100 nm or less, and even more preferably 50 nm or less. The lower limit is not particularly limited, but is usually 1 nm or more, preferably 2 nm or more. Therefore, the average fiber diameter (length-weighted average fiber diameter) of the CNF is usually 1 to 500 nm or 2 to 500 nm, preferably 2 to 300 nm or 2 to 100 nm, more preferably 2 to 50 nm or 3 to 30 nm. The average fiber length (length-weighted average fiber length) is usually 5 μm or less, preferably 3 μm or less, 2 μm or less, or 1 μm or less. The lower limit is usually 50 nm or more, preferably 100 nm or more. The aspect ratio of the CNF is usually 10 or more, preferably 50 or more. The upper limit is not particularly limited, but is usually 1,000 or less.

[0014] In this specification, the average fiber diameter of fibrous cellulose fibers can be obtained from the results of observing each fiber using a fiber tester manufactured by ABB Corporation, a fractionator manufactured by Valmet, a scanning electron microscope (SEM), an atomic force microscope (AFM), or a transmission electron microscope (TEM), appropriately selected depending on the fiber diameter. When the fiber diameter distribution of the fibrous cellulose fibers is very broad (for example, MFC), the average fiber diameter can be measured by the following method. When 0.1 g of fibrous cellulose fibers is dispersed in 300 mL of water using a fiber tester manufactured by ABB Corporation and circulated for 5 minutes, if the number of fibers counted is 10,000 or more, the value measured by the fiber tester can be determined to be the average fiber diameter. On the other hand, when the number of fibers counted is less than 10,000, the value measured by AFM can be determined to be the fiber diameter. In this specification, the average fiber length can be measured using a fiber tester manufactured by ABB Corporation, a fractionator manufactured by Valmet, a scanning electron microscope (SEM), an atomic force microscope (AFM), or a transmission electron microscope (TEM). When measuring fiber length using a scanning electron microscope (SEM), an atomic force microscope (AFM), or a transmission electron microscope (TEM), the fiber length can be measured by analyzing 200 randomly selected fibers and calculating the average. The average aspect ratio of fine cellulose fibers can be calculated using the formula: average aspect ratio = average fiber length / average fiber diameter.

[0015] [Example of Fine Cellulose Fiber: Microfibrillated Cellulose (MFC)] In this specification, cellulose microfibrils (microfibrillated cellulose, MFC) refer to cellulose fibers prepared through a micronization process and having a fiber diameter of the micron order (e.g., 500 nm or more). MFC can exhibit higher water retention than undefibrated cellulose fibers and can exhibit improved yield compared to finely defibrated CNF.

[0016] The lower limit of the average fiber diameter of MFC is not particularly limited, but is usually 500 nm or more, preferably 1 μm or more, and more preferably 4 μm or more. On the other hand, even if the average fiber diameter is less than 500 nm, if 1,000 or more undefibrated fibers (e.g., fibers with a fiber diameter of 4 μm or more) are counted when the average fiber diameter is measured under the following conditions, it is MFC. The presence of undefibrated fibers can be confirmed by dispersing 0.1 g of microfibrillated cellulose fibers in 300 mL of water using an ABB fiber tester, circulating the water for 5 minutes, and confirming that the number of fibers counted is 1,000 or more. The upper limit of the average fiber diameter is preferably 60 μm or less, more preferably 40 μm or less, even more preferably 30 μm or less, and even more preferably 20 μm or less, but there is no particular limit. The average fiber length is usually 5 μm or more, 10 μm or more, 20 μm or more, or 40 μm or more, preferably 200 μm or more, 300 μm or more, or 400 μm or more, more preferably 500 μm or more, or 550 μm or more, and even more preferably 600 μm or more, 700 μm or more, or 800 μm or more. The upper limit is not particularly limited, but is usually 3,000 μm or less, preferably 2,500 μm or less, more preferably 2,000 μm or less, even more preferably 1,500 μm or less, 1,400 μm or less, or 1,300 μm. The aspect ratio of the MFC is preferably 3 or more, more preferably 5 or more, even more preferably 7 or more, and may be 10 or more, 20 or more, or 30 or more. The upper limit of the aspect ratio is not particularly limited, but is preferably 1,000 or less, more preferably 100 or less, and even more preferably 80 or less.

[0017] [Cellulose Raw Material] Fine cellulose fibers can be produced by defibrating a cellulose raw material. The cellulose raw material is not particularly limited as long as it contains cellulose, and examples thereof include plants (e.g., wood, bamboo, hemp, jute, kenaf, agricultural waste, cloth, pulp (e.g., unbleached softwood kraft pulp (NUKP), bleached softwood kraft pulp (NBKP), unbleached hardwood kraft pulp (LUKP), bleached hardwood kraft pulp (LBKP), bleached kraft pulp (BKP), unbleached softwood sulfite pulp (NUSP), bleached softwood sulfite pulp (NBSP), thermomechanical pulp (TMP), recycled pulp, waste paper, etc.), animals (e.g., ascidians), algae, microorganisms (e.g., acetic acid bacteria (Acetobacter)), microbial products, and the like. The cellulose raw material may be any one of these or a combination of two or more types. Preferably, the cellulose raw material is derived from a plant or a microorganism (e.g., cellulose fiber), and more preferably from a plant (e.g., cellulose fiber).

[0018] [Modification] The microfibrillated cellulose fibers may be modified or unmodified. Modified microfibrillated cellulose fibers refer to microfibrillated cellulose fibers (e.g., CNF, MFC) in which at least one of the three hydroxyl groups contained in the glucose unit has been chemically modified (hereinafter simply referred to as "modified"). The chemical modification treatment generates electronic repulsion between the microfibrils of the cellulose fibers, which allows sufficient pulverization during defibration, resulting in the production of microfibrillated cellulose fibers. Therefore, when composited with a rubber component, a sufficient reinforcing effect can be exhibited. From this perspective, modified cellulose fibers are preferred.

[0019] The number average fiber diameter of the cellulose raw material is not particularly limited, but is about 30 to 60 μm in the case of softwood kraft pulp, which is a common pulp, and about 10 to 30 μm in the case of hardwood kraft pulp. In the case of other pulps, those that have undergone general refinement have a diameter of about 50 μm. For example, when chips or the like that are refined into pieces several centimeters in size are subjected to mechanical processing using a disintegrator such as a refiner or beater, and it is preferable to adjust the diameter to about 50 μm.

[0020] Examples of the modification include oxidation, etherification, esterification such as phosphate esterification, silane coupling, fluorination, cationization, etc. Among these, oxidation (carboxylation), etherification, cationization, and esterification are preferred, and oxidation (carboxylation) is more preferred.

[0021] -Carboxylation (Oxidation)- Oxidized fine cellulose fibers typically have a structure in which at least one carbon atom bearing a primary hydroxyl group contained in a glucopyranose unit constituting the cellulose molecular chain (e.g., the carbon atom bearing a primary hydroxyl group at the C6 position) is oxidized. The amount of carboxyl groups in the oxidized cellulose fibers or the oxidized fine cellulose fibers is preferably 0.5 mmol / g or more or 0.6 mmol / g or more, more preferably 0.8 mmol / g or more, and even more preferably 1.0 mmol / g or more, based on the bone dry mass. The upper limit of this amount is preferably 3.0 mmol / g or less, more preferably 2.5 mmol / g or less, and even more preferably 2.0 mmol / g or less. The amount of carboxyl groups is preferably 0.5 to 3.0 mmol / g, more preferably 0.8 to 2.5 mmol / g, and even more preferably 1.0 to 2.0 mmol / g. The amount of carboxyl groups can be adjusted by controlling the conditions for oxidizing the cellulose raw material (e.g., the amount of oxidizing agent added, the reaction time). Furthermore, by controlling these conditions, the amount of carboxylate groups and aldehyde groups can also be adjusted.

[0022] The amount of carboxy groups can be calculated using the following procedure. 60 ml of a 0.5% by mass slurry (aqueous dispersion) of oxidized cellulose is prepared. 0.1 M aqueous hydrochloric acid is added to the prepared slurry to adjust the pH to 2.5. 0.05 N aqueous sodium hydroxide is then added dropwise, and the electrical conductivity is measured until the pH reaches 11. The amount of carboxy groups is calculated using the following formula from the amount of sodium hydroxide (a) consumed in the neutralization stage of a weak acid, where the change in electrical conductivity is gradual: Amount of carboxy groups [mmol / g oxidized cellulose] = a [ml] × 0.05 / mass of oxidized cellulose [g]

[0023] The oxidation method is not particularly limited, but an example is a method in which a cellulose raw material is oxidized in water using an oxidizing agent in the presence of an N-oxyl compound, a bromide, an iodide, or a mixture thereof. According to this method, the C6 position of the glucopyranose ring on the surface of cellulose is selectively oxidized to produce an aldehyde group, a carboxy group (—COOH), and a carboxylate group (—COO - The concentration of the cellulose raw material during the reaction is not particularly limited, but is preferably 5% by mass or less.

[0024] An N-oxyl compound refers to a compound capable of generating a nitroxy radical. Examples of nitroxyl radicals include 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) and its derivatives (e.g., 4-hydroxyTEMPO). Any compound that promotes the target oxidation reaction can be used as the N-oxyl compound.

[0025] The amount of N-oxyl compound used is not particularly limited as long as it is a catalytic amount capable of oxidizing the raw cellulose. For example, about 0.001 to 5 mmol, preferably 0.01 to 1 mmol, and more preferably 0.01 to 0.5 mmol can be used per 1 g of bone-dry cellulose raw material. Furthermore, about 0.02 to 0.5 mmol / L of the reaction system is preferable.

[0026] Bromides are compounds containing bromine, such as alkali metal bromides that can dissociate and ionize in water. Iodides are compounds containing iodine, such as alkali metal iodides. The amount of bromide or iodide used can be selected within a range that can promote the oxidation reaction. The total amount of bromide and iodide is, for example, preferably 0.1 to 100 mmol, more preferably 0.1 to 10 mmol, and even more preferably 0.5 to 5 mmol, per 1 g of bone-dry cellulose raw material.

[0027] Known oxidizing agents can be used, such as halogens, hypohalous acids, halous acids, perhalogen acids or their salts, halogen oxides, and peroxides. Among these, hypohalous acids or their salts are preferred because they are inexpensive and have a low environmental impact, hypochlorous acid or its salts are more preferred, and sodium hypochlorite is preferred. The appropriate amount of oxidizing agent used is, for example, preferably 0.5 to 500 mmol, more preferably 0.5 to 50 mmol, even more preferably 1 to 25 mmol, and even more preferably 3 to 10 mmol, per 1 g of bone-dry cellulose raw material. Furthermore, for example, 2 to 500 mol is preferred per 1 mol of the N-oxyl compound.

[0028] The oxidation process of cellulose raw materials proceeds efficiently even under relatively mild conditions. Therefore, the reaction temperature is preferably 4 to 40°C, and may also be about 15 to 30°C, i.e., room temperature. As the reaction proceeds, carboxyl groups are generated in the cellulose, resulting in a decrease in the pH of the reaction solution. To efficiently proceed with the oxidation reaction, it is preferable to add an alkaline solution such as an aqueous sodium hydroxide solution to maintain the pH of the reaction solution at about 8 to 12, or 10 to 11. Water is preferred as the reaction medium because it is easy to handle and is less likely to cause side reactions.

[0029] The reaction time in the oxidation reaction can be appropriately set depending on the degree of progress of the oxidation, and is usually from 0.5 to 6 hours, for example, from about 0.5 to 4 hours.

[0030] The oxidation reaction may be carried out in two stages. For example, the oxidized cellulose obtained by filtration after the completion of the first-stage reaction can be oxidized again under the same or different reaction conditions, thereby enabling efficient oxidation without reaction inhibition by sodium chloride produced as a by-product in the first-stage reaction.

[0031] Another example of a carboxylation (oxidation) method is a method in which cellulose raw materials are oxidized by contacting them with an ozone-containing gas (ozone oxidation). This oxidation reaction oxidizes at least the hydroxyl groups at positions 2 and 6 of the glucopyranose ring, and decomposes the cellulose chain. The ozone concentration in the ozone-containing gas is 50 to 250 g / m.３ is preferred, and 50 to 220 g / m ３ The amount of ozone added is preferably 0.1 to 30 parts by mass, and more preferably 5 to 30 parts by mass, based on 100 parts by mass of the solids content of the cellulose raw material. The ozone treatment temperature is preferably 0 to 50°C, and more preferably 20 to 50°C. The ozone treatment time is not particularly limited, but is about 1 to 360 minutes, and preferably about 30 to 360 minutes. When the ozone treatment conditions are within these ranges, excessive oxidation and decomposition of the cellulose raw material can be prevented, and a good yield of oxidized cellulose can be achieved.

[0032] After the ozone treatment, a further oxidation treatment may be carried out using an oxidizing agent. The oxidizing agent used in the further oxidation treatment is not particularly limited, but examples include chlorine compounds such as chlorine dioxide and sodium chlorite, oxygen, hydrogen peroxide, persulfuric acid, and peracetic acid. The further oxidation treatment may be carried out, for example, by dissolving the oxidizing agent in water or a polar organic solvent such as alcohol to prepare an oxidizing agent solution, and then immersing the oxidized cellulose in the solution.

[0033] -Acid-Form Oxidized Cellulose and Desalting- Oxidized cellulose contains carboxy groups as a result of oxidation, but it may contain more acid-form carboxy groups (-COOH) than salt-form carboxy groups (e.g., -COO-, -COONa), or it may contain more salt-form carboxy groups than acid-form carboxy groups. The amounts of salt-form carboxy groups and acid-form carboxy groups can be adjusted by desalting treatment. Salt-form carboxy groups can be converted to acid-form carboxy groups by desalting treatment. In this specification, oxidized cellulose (which has been desalted) is referred to as acid-form oxidized cellulose, and oxidized cellulose (which has not been subjected to the desalting treatment described below) is referred to as salt-form oxidized cellulose. Salt-form oxidized cellulose usually contains primarily salt-form carboxy groups. On the other hand, acid-form oxidized cellulose contains many acid-form carboxy groups, and the proportion of acid-form carboxy groups in the carboxy groups is preferably 40% or more, more preferably 60% or more, and even more preferably 85% or more. Acid-form oxidized cellulose, together with component C, can exhibit a superior reinforcing effect. The proportion of acid-type carboxy groups can be calculated by the following procedure. 1) First, 250 mL of an aqueous dispersion of acid-type oxidized cellulose before desalting is prepared, with a solids concentration of 0.1% by mass. 0.1 M aqueous hydrochloric acid is added to the prepared aqueous dispersion to adjust the pH to 2.5, and then 0.1 N aqueous sodium hydroxide is added and the electrical conductivity is measured until the pH reaches 11. The amount of acid-type carboxy groups and the amount of salt-type carboxy groups, i.e., the total amount of carboxy groups, is calculated using the amount of sodium hydroxide (a) consumed in the neutralization stage of a weak acid, where the change in electrical conductivity is gradual, using the following formula: Total amount of carboxy groups (mmol / g oxidized cellulose (salt type)) = a (ml) × 0.1 / mass (g) of oxidized cellulose (salt type) 2) 250 mL of an aqueous dispersion of desalted acid-type oxidized cellulose with a solids concentration of 0.1% by mass is prepared. 0.1 N aqueous sodium hydroxide is added to the prepared aqueous dispersion, and the electrical conductivity is measured until the pH reaches 11.The amount of acid-type carboxy groups is calculated using the following formula from the amount of sodium hydroxide (b) consumed in the neutralization stage of a weak acid where the change in electrical conductivity is gradual: Amount of acid-type carboxy groups (mmol / g acid-type oxidized cellulose) = b (ml) × 0.1 / mass of acid-type oxidized cellulose (g) 3) The proportion of acid-type carboxy groups is calculated using the calculated total amount of carboxy groups and the amount of acid-type carboxy groups: Proportion of acid-type carboxy groups (%) = (amount of acid-type carboxy groups / total amount of carboxy groups) × 100.

[0034] Desalting can be carried out after oxidation, either before or after defibration (before or after step (2)). However, it is usually carried out after oxidation, and preferably before step (2). Desalting is usually carried out by substituting salts (e.g., sodium salts) contained in the salt-form oxidized cellulose with protons. Examples of desalting methods include adjusting the system to an acidic state and contacting the oxidized cellulose with a cation exchange resin. When adjusting the system to an acidic state, the pH of the system is preferably adjusted to 2 to 6, more preferably 2 to 5, and even more preferably 2.3 to 5. To adjust the system to an acidic state, an acid (e.g., inorganic acids such as sulfuric acid, hydrochloric acid, nitric acid, sulfurous acid, nitrous acid, and phosphoric acid; organic acids such as acetic acid, lactic acid, oxalic acid, citric acid, and formic acid) is usually used. After the addition of the acid, a washing treatment may be carried out as appropriate. As the cation exchange resin, either a strongly acidic or a weakly acidic ion exchange resin can be used as long as the counter ion is H+. The ratio of the oxidized cellulose and the cation exchange resin when they are contacted is not particularly limited, and can be appropriately determined by a person skilled in the art from the viewpoint of efficient proton substitution. After contact, the cation exchange resin can be recovered by a conventional method such as suction filtration.

[0035] -Etherification- Examples of etherification include carboxyalkylation, methylation, ethylation, cyanoethylation, hydroxyethylation, hydroxypropylation, ethylhydroxyethylation, and hydroxypropylmethylation. Carboxyalkylation is preferred, and carboxymethylation is more preferred.

[0036] Carboxyalkylated cellulose fibers typically have a structure in which at least one of the carbon atoms constituting the cellulose molecular chain (for example, the carbon atom bearing a primary hydroxyl group at the C6 position constituting the glucopyranose unit) is carboxymethylated.

[0037] The degree of carboxyalkyl substitution (DS, preferably the degree of carboxymethyl substitution) per anhydroglucose unit of the carboxyalkylated cellulose is preferably 0.01 or more, 0.02 or more, or 0.05 or more, more preferably 0.10 or more, even more preferably 0.15 or more, even more preferably 0.20 or more, and particularly preferably 0.25 or more. This ensures a degree of substitution sufficient to obtain the effects of chemical modification. The upper limit of the degree of substitution is preferably 0.50 or less, more preferably 0.45 or less, 0.40 or less, or 0.35 or less. This makes it difficult for the cellulose fibers to dissolve in water, allowing the fiber form to be maintained in water. Therefore, the degree of carboxyalkyl substitution is preferably 0.01 to 0.50, more preferably 0.01 to 0.45, even more preferably 0.02 to 0.40, 0.10 to 0.35, or 0.15 to 0.30.

[0038] The degree of carboxyalkyl substitution, for example, the degree of carboxymethyl substitution, can be measured by the following method. Approximately 2.0 g of carboxymethylated cellulose (bone dry) is weighed out and placed in a 300 mL Erlenmeyer flask with a ground stopper. 100 mL of a solution of 1,000 mL of methanol and 100 mL of concentrated nitric acid is added, and the mixture is shaken for 3 hours to convert the salt-form carboxymethylated cellulose (hereinafter also referred to as "salt-form carboxymethylated cellulose") to the acid-form carboxymethylated cellulose (hereinafter also referred to as "acid-form carboxymethylated cellulose"). 1.5 to 2.0 g of acid-form carboxymethylated cellulose (bone dry) is weighed out and placed in a 300 mL Erlenmeyer flask with a ground stopper. The acid-form carboxymethylated cellulose is wetted with 15 mL of 80% methanol, 100 mL of 0.1 N NaOH is added, and the mixture is shaken at room temperature for 3 hours. Using phenolphthalein as an indicator, excess NaOH is back-titrated with 0.1N H2SO4, and the degree of carboxymethyl substitution (DS) can be calculated by the following formula: A = [(100 x F - (0.1N H2SO4 (mL)) x F') x 0.1] / (bone-dry mass (g) of acid-type carboxymethylated cellulose) DS = 0.162 x A / (1 - 0.058 x A) A: amount (mL) of 1N NaOH required to neutralize 1 g of acid-type carboxymethylated cellulose F': factor of 0.1N H2SO4 F: factor of 0.1N NaOH

[0039] The degree of carboxyalkyl substitution can be adjusted by controlling the reaction conditions such as the amount of carboxyalkylating agent to be reacted, the amount of mercerizing agent, and the composition ratio of water to organic solvent.

[0040] As a method of carboxyalkylation, for example, a method in which a cellulose raw material as a starting material (bottom raw material) is mercerized and then etherified can be mentioned. Carboxymethylation will be explained below as an example.

[0041] Carboxymethylated cellulose can be produced by using unmodified cellulose fibers (cellulose raw material: e.g., pulp) as the starting material, subjecting them to a mercerization treatment with a mercerizing agent, followed by an etherification reaction. This reaction is typically carried out in the presence of a solvent. Examples of the solvent include water and lower alcohols (e.g., methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, and tertiary butanol), either singly or in combination. When a lower alcohol is mixed, the mixing ratio of the lower alcohol is preferably 60 to 95% by mass. The amount of solvent is approximately three times the amount of the cellulose raw material, in mass terms. The upper limit of this amount is not particularly limited, but is preferably 20 times or less. The amount of solvent is preferably 3 to 20 times the amount of the cellulose raw material, in mass terms.

[0042] Examples of mercerizing agents include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide. The amount of mercerizing agent used is, in molar terms, preferably 0.5 times or more, more preferably 1.0 times or more, and even more preferably 1.5 times or more per anhydroglucose residue of the starting material. The upper limit of this amount is usually 20 times or less, preferably 10 times or less, and more preferably 5 times or less. The amount of mercerizing agent used is, in molar terms, preferably 0.5 to 20 times, more preferably 1.0 to 10 times, and even more preferably 1.5 to 5 times.

[0043] The reaction temperature for mercerization is usually 0°C or higher, preferably 10°C or higher. The upper limit is usually 70°C or lower, preferably 60°C or lower. The reaction temperature is usually 0 to 70°C, preferably 10 to 60°C. The reaction time for mercerization is usually 15 minutes or longer, preferably 30 minutes or longer. The upper limit is usually 8 hours or shorter, preferably 7 hours or shorter. The reaction time is usually 15 minutes to 8 hours, preferably 30 minutes to 7 hours.

[0044] The etherification reaction is usually carried out by adding a carboxymethylating agent to the reaction system after mercerization. Examples of the carboxymethylating agent include sodium monochloroacetate. The amount of the carboxymethylating agent added is, in molar terms, preferably 0.05 times or more, more preferably 0.5 times or more, and even more preferably 0.8 times or more, per glucose residue of the cellulose raw material. The upper limit of this amount is usually 10.0 times or less, preferably 5 times or less, and more preferably 3 times or less. The amount of the carboxymethylating agent added is, in molar terms, preferably 0.05 to 10.0 times, more preferably 0.5 to 5 times, and even more preferably 0.8 to 3 times.

[0045] The reaction temperature is usually 30°C or higher, preferably 40°C or higher. The upper limit is usually 90°C or lower, preferably 80°C or lower. The reaction temperature is usually 30 to 90°C, preferably 40 to 80°C. The reaction time is usually 30 minutes or longer, preferably 1 hour or longer. The upper limit is usually 10 hours or shorter, preferably 4 hours or shorter. The reaction time is usually 30 minutes to 10 hours, preferably 1 hour to 4 hours. During the carboxymethylation reaction, the reaction solution may be stirred as needed.

[0046] -Differences from carboxymethyl cellulose- Carboxyalkylated cellulose fibers typically maintain at least a portion of their fibrous shape when dispersed in water. Carboxyalkylated cellulose fibers are distinguished from carboxymethyl cellulose, a type of water-soluble polymer that dissolves in water and imparts viscosity. When an aqueous dispersion of carboxyalkylated cellulose fibers is observed under an electron microscope, fibrous substances can be observed. On the other hand, when an aqueous dispersion of carboxymethyl cellulose, a type of water-soluble polymer, is observed, fibrous substances are usually not observed. Furthermore, when anion-modified cellulose fibers are measured by X-ray diffraction, peaks of cellulose type I crystals can be observed, but when carboxymethyl cellulose powder, a water-soluble polymer, is similarly measured, cellulose type I crystals are usually not observed.

[0047] -Acid-Type Carboxyalkylated Cellulose and Desalting- Carboxyalkylated cellulose may contain more acid-type carboxy groups than salt-type carboxy groups, or may contain more salt-type carboxy groups than acid-type carboxy groups. The amount of salt-type carboxy groups and acid-type carboxy groups can be adjusted by desalting treatment. Desalting treatment can convert salt-type carboxy groups to acid-type carboxy groups. In this specification, carboxyalkylated cellulose (which has undergone desalting) is referred to as acid-type carboxyalkylated cellulose, and carboxyalkylated cellulose (which has not undergone the desalting treatment described below) is referred to as salt-type carboxyalkylated cellulose. Salt-type carboxyalkylated cellulose usually contains primarily salt-type carboxy groups (—COO−). On the other hand, acid-type carboxyalkylated cellulose contains many acid-type carboxy groups, and the ratio of the amount of acid-type carboxy groups to the amount of carboxy groups in the acid-type carboxyalkylated cellulose is preferably 40% or more, more preferably 60% or more, and even more preferably 85% or more. The method for calculating the amount of acid-type carboxy groups is as described above.

[0048] The desalting is usually carried out after carboxyalkylation, preferably after etherification and before fibrillation. For example, the desalting method may be a method of contacting the carboxyalkylated cellulose with a cation exchange resin. The cation exchange resin has a counter ion of H + As long as the cation exchange resin is strong acidic or weak acidic, either a strong acidic or a weak acidic ion exchange resin can be used. The ratio of the carboxyalkylated cellulose and the cation exchange resin when contacting them is not particularly limited, and can be appropriately determined by a person skilled in the art from the viewpoint of efficient proton substitution. For example, the ratio can be adjusted so that the pH of the aqueous dispersion after addition of the cation exchange resin to the carboxyalkylated cellulose aqueous dispersion is preferably 2 to 6, more preferably 2 to 5. The cation exchange resin after contact can be recovered by a conventional method such as suction filtration.

[0049] Esterification (Phosphorylation) A first example of an esterified cellulose fiber is a phosphorylated cellulose fiber. Phosphorylated cellulose usually has a structure in which at least one of the carbon atoms constituting the cellulose molecular chain (for example, the carbon atom bearing a primary hydroxyl group at the C6 position constituting the glucopyranose unit) is phosphorylated.

[0050] The amount of ionic substituents introduced into the phosphated cellulose fibers (ionic substituent amount, phosphorus oxo acid substituent amount) may be 0.10 mmol / g or more, preferably 0.20 mmol / g or more, more preferably 0.30 mmol / g or more, even more preferably 0.40 mmol / g or more, even more preferably 0.50 mmol / g or more, even more preferably 0.60 mmol / g or more, and particularly preferably 0.70 mmol / g or more, per gram (mass) of phosphated cellulose fibers. The amount of ionic substituents introduced into the phosphated cellulose fibers may be 1.50 mmol / g or less, preferably 1.35 mmol / g or less, more preferably 1.20 mmol / g or less, and even more preferably 1.10 mmol / g or less, per gram (mass) of cellulose fibers. The amount of ionic substituents introduced into the phosphated cellulose fibers may be 1.00 mmol / g or less, more preferably 0.95 mmol / g or less, per gram (mass) of phosphated cellulose fibers. Here, the denominator in the unit mmol / g is the value obtained when the counter ion of the ionic substituent is a hydrogen ion (H ＋ The amount of phosphorus oxo acid substituents can be measured by the following method.

[0051] The amount of phosphorus oxoacid groups in the fine cellulose fibers can be measured by treating a cellulose fiber-containing slurry prepared by diluting a fine cellulose fiber dispersion containing the target fine cellulose fibers with ion-exchanged water to a content of 0.2 mass%, followed by titration with an alkali.

[0052] The treatment with ion exchange resin was carried out by adding 1 / 10 by volume of a strongly acidic ion exchange resin (Amberjet 1024; Organo Corporation, conditioned) to the above cellulose fiber-containing slurry, shaking for 1 hour, and then pouring it onto a mesh with 90 μm openings to separate the resin from the slurry.

[0053] In addition, the alkali titration was performed by adding 10 μL of 0.1 N sodium hydroxide aqueous solution every 5 seconds to the cellulose fiber-containing slurry after treatment with the ion exchange resin, while measuring the change in the pH value of the slurry. Nitrogen gas was blown into the slurry 15 minutes before the start of the titration. In this neutralization titration, two maximum points of increment (differential value of pH with respect to the amount of alkali added) were observed on the curve plotting the measured pH against the amount of alkali added. Of these, the maximum point of increment obtained first after starting the addition of alkali is called the first endpoint, and the maximum point of increment obtained next is called the second endpoint. The amount of alkali required from the start of the titration to the first endpoint is equal to the amount of first dissociated acid in the slurry used in the titration. Furthermore, the amount of alkali required from the start of the titration to the second endpoint is equal to the total amount of dissociated acid in the slurry used in the titration. The amount of alkali (mmol) required from the start of titration to the first endpoint was divided by the solid content (g) in the slurry to be titrated to determine the amount of phosphorus oxoacid groups (mmol / g). The presence or absence of phosphate groups was confirmed by measuring the infrared absorption spectrum, and the absorption due to phosphate groups (1230 cm －１ This may be done by checking the area around the

[0054] The amount of phosphate groups can be adjusted by controlling the reaction conditions such as the amount of the compound having a phosphate group added and the amount of the basic compound added as needed.

[0055] Examples of phosphorylation methods include a method of reacting unmodified cellulose fibers with a compound having a phosphate group (phosphorylation). Examples of phosphate esterification methods include a method of mixing a powder or aqueous solution of a compound having a phosphate group with a cellulosic raw material (e.g., a suspension (solids concentration of about 0.1 to 10% by mass)), and a method of adding an aqueous solution of a compound having a phosphate group to an aqueous dispersion of the cellulosic raw material, with the latter being preferred. This can increase the uniformity of the reaction and the efficiency of esterification. The pH of the aqueous solution of the compound having a phosphate group is preferably 7 or less from the viewpoint of increasing the efficiency of introduction of the phosphate group, and more preferably 3 to 7 from the viewpoint of suppressing hydrolysis.

[0056] Examples of compounds having a phosphate group include phosphoric acid, polyphosphoric acid, phosphorous acid, phosphonic acid, polyphosphonic acid, esters and salts thereof, etc. These compounds are low-cost and easy to handle, and can be used to introduce phosphate groups into cellulose, thereby improving defibration efficiency. Specific examples of compounds having a phosphate group include phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium pyrophosphate, sodium metaphosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, tripotassium phosphate, potassium pyrophosphate, potassium metaphosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, ammonium pyrophosphate, and ammonium metaphosphate. One or more compounds having a phosphate group can be used in combination. The amount of the compound having a phosphate group added to the cellulose raw material is preferably 0.1 to 500 parts by mass, more preferably 1 to 400 parts by mass, and even more preferably 2 to 200 parts by mass, in terms of elemental phosphorus, per 100 parts by mass of the solids content of the cellulose raw material. This allows for efficient yields commensurate with the amount of compound having a phosphate group used. The reaction temperature is preferably 0 to 95°C, more preferably 30 to 90°C. The reaction time is not particularly limited, but is usually about 1 to 600 minutes, preferably 30 to 480 minutes. When the esterification reaction conditions are within any of these ranges, it is possible to prevent the cellulose from being excessively esterified and becoming more soluble, thereby improving the yield of phosphated cellulose. When reacting a compound having a phosphate group, a basic compound (e.g., a compound having an amino group that exhibits basicity, such as urea, methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, or hexamethylenediamine) may be further added to the reaction system.

[0057] The suspension obtained after esterification is preferably dehydrated as needed, and then subjected to a heat treatment after dehydration. This can suppress hydrolysis of the cellulose raw material. The heating temperature is preferably 100 to 170°C. During the heat treatment, while water is still present, it is more preferable to heat at 130°C or lower (preferably 110°C or lower), and after removing the water, to heat at 100 to 170°C. After boiling, a washing treatment such as washing with cold water and / or a neutralization treatment is preferably performed. This allows for efficient defibration. Washing can be performed by adding water and then dehydrating (e.g., filtration), and may be repeated two or more times. Washing is preferably performed until the electrical conductivity of the filtrate decreases. For example, it can be performed until the electrical conductivity is preferably 200 or lower, more preferably 150 or lower, and even more preferably 120 or lower. Furthermore, after washing, a neutralization treatment may be performed as needed. The neutralization treatment can be performed by adding an alkali (e.g., sodium hydroxide), for example. Washing may be performed again after neutralization.

[0058] - Esterification (Phosphite Esterification) - A second example of a method for producing an esterified cellulose fiber is a phosphite cellulose fiber. Phosphite cellulose fibers usually have a structure in which at least one of the carbon atoms constituting the cellulose molecular chain (for example, the carbon atom bearing a primary hydroxyl group at the C6 position constituting the glucopyranose unit) is phosphorous-substituted.

[0059] The degree of substitution of phosphite groups per glucose unit in phosphite-esterified cellulose fibers (hereinafter simply referred to as "phosphite group substitution degree") is preferably 0.001 to 0.60. This facilitates electrical repulsion between cellulose molecules, facilitating nanofibrillation. The degree of substitution of phosphite groups can be measured using the same method as for measuring the degree of phosphate group substitution. The degree of substitution of phosphite groups can be adjusted by controlling reaction conditions such as the amount of phosphorous acid or its salt added, and the amount of alkali metal ion-containing material, urea, or its derivative added if necessary.

[0060] As a method for phosphite esterification, for example, a method in which phosphorous acid or a metal salt thereof (preferably sodium hydrogen phosphite) is reacted with unmodified cellulose fibers to introduce an ester group of phosphorous acid can be mentioned.

[0061] Examples of phosphorous acid and its metal salts include phosphorous acid compounds such as phosphorous acid, sodium hydrogen phosphite, ammonium hydrogen phosphite, potassium hydrogen phosphite, sodium dihydrogen phosphite, sodium phosphite, lithium phosphite, potassium phosphite, magnesium phosphite, calcium phosphite, triethyl phosphite, triphenyl phosphite, and pyrophosphorous acid, as well as combinations of two or more selected from these, with sodium hydrogen phosphite being preferred. This allows alkali metal ions to be introduced into the cellulose fibers. The amount of phosphorous acid or its metal salts added is preferably 1 to 10,000 g, more preferably 100 to 5,000 g, and even more preferably 300 to 1,500 g per kg of unmodified cellulose fibers. In addition to phosphorous acid and its metal salts, an alkali metal ion-containing substance (e.g., hydroxide, metal sulfate, metal nitrate, metal chloride, metal phosphate, or metal carbonate) may also be added to the reaction system.

[0062] Urea or a derivative thereof may also be added to the reaction system. This allows carbamate groups to be introduced into the cellulose fibers. Examples of urea and urea derivatives include urea, thiourea, biuret, phenylurea, benzylurea, dimethylurea, diethylurea, tetramethylurea, and combinations of two or more selected from these, with urea being preferred. The amount of urea and urea derivative added is preferably 0.01 to 100 mol, more preferably 0.2 to 20 mol, and even more preferably 0.5 to 10 mol per mol of phosphorous acid or its metal salt.

[0063] The reaction temperature is preferably 100 to 200°C, more preferably 100 to 180°C, and even more preferably 100 to 170°C. During the heat treatment, while water is present, it is more preferable to heat at 130°C or lower (preferably 110°C or lower), and after removing the water, to heat treat at 100 to 170°C. The reaction time is usually about 10 to 180 minutes, more preferably 30 to 120 minutes. The phosphite-esterified cellulose fiber is preferably washed prior to defibration. The degree of substitution of phosphite groups per glucose unit is preferably 0.01 or higher but less than 0.23.

[0064] Esterification (sulfate esterification)—A third example of a method for producing an esterified cellulose fiber is a sulfated cellulose fiber. Sulfated cellulose fibers usually have a structure in which at least one of the carbon atoms constituting the cellulose molecular chain (for example, the carbon atom bearing a primary hydroxyl group at C6 constituting the glucopyranose unit) is phosphorylated.

[0065] The amount of sulfate groups per glucose unit in sulfated cellulose fibers (hereinafter simply referred to as "sulfate group amount") is preferably 0.42 to 9.9 mmol / g, more preferably 0.5 mmol / g to 2.0 mmol / g. Introducing sulfate groups into the cellulose raw material causes electrical repulsion between cellulose molecules. Therefore, sulfated cellulose with sulfate groups introduced therein can be easily defibrated to nano-levels. When the amount of sulfate groups is 0.42 mmol / g or more, sufficient nano-defibration can be achieved due to the electrical repulsion between cellulose molecules. On the other hand, when the amount is 9.9 mmol / g or less, swelling or dissolution can be suppressed, preventing a situation in which fine cellulose fibers cannot be obtained. For efficient defibration, it is preferable to wash the sulfated cellulose obtained above.

[0066] The amount of sulfate groups per glucose unit can be measured by the following method. An aqueous dispersion of sulfated cellulose fibers is solvent-substituted with ethanol and then t-butanol, and then freeze-dried. 15 ml of ethanol and 5 ml of water are added to 200 mg of the obtained sample, and the mixture is stirred for 30 minutes. 10 ml of a 0.5 N aqueous sodium hydroxide solution is then added, and the mixture is stirred at 70°C for 30 minutes, and then at 30°C for a further 24 hours. Phenolphthalein is then added as an indicator, and the mixture is titrated with hydrochloric acid, and the amount of sulfate groups per glucose unit is calculated using the following formula: Amount of sulfate groups [mmol / g sample] = (5 - (0.1 × titration amount of hydrochloric acid [ml] × 2)) / 0.2

[0067] The amount of sulfate groups can be adjusted by controlling the reaction conditions such as the amount of sulfate compound added to the reaction mixture.

[0068] An example of a method for sulfate esterification is a method in which unmodified cellulose fibers are reacted with a sulfate compound to introduce sulfate groups derived from the sulfate compound into cellulose to produce sulfated cellulose. Examples of sulfate compounds include sulfuric acid, sulfamic acid, chlorosulfonic acid, sulfur trioxide, and esters or salts thereof. Among these, sulfamic acid is preferred because it has low cellulose solubility and low acidity.

[0069] For example, when sulfamic acid is used as the sulfate compound, the amount of sulfamic acid used can be appropriately adjusted taking into consideration the amount of anionic groups introduced into the cellulose chain, and is preferably 0.01 to 50 mol, more preferably 0.1 to 3.0 mol, per mol of glucose unit in the cellulose molecule.

[0070] - Salt type / acid type - Esterified cellulose may contain more acid type carboxy groups than salt type carboxy groups, or may contain more salt type carboxy groups than acid type carboxy groups. Among esterified celluloses, those that have not been subjected to desalting treatment and those that have been subjected to desalting treatment are called salt type esterified cellulose and acid type esterified cellulose, respectively. Salt type esterified cellulose mainly contains salt type carboxy groups. Acid type esterified cellulose is presumed to have a superior reinforcing effect due to the combination with component C. The counter cations of the salt type carboxy groups and the method for preparing them are as explained in the explanation of oxidized cellulose.

[0071] -Cationization- Cationized cellulose typically has a structure in which at least one of the carbon atoms constituting the cellulose molecular chain (for example, the carbon atom bearing a primary hydroxyl group at the C6 position constituting a glucopyranose unit) is cationized, and typically contains a cation such as ammonium, phosphonium, or sulfonium, or a group containing such a cation, in the molecule. Introducing cationic substituents into cellulose causes electrical repulsion between cellulose molecules. Therefore, cellulose into which cationic substituents have been introduced can be easily defibrated.

[0072] The degree of cationic substitution per glucose unit in cationized cellulose is preferably 0.02 or more. This allows cellulose fibers to be easily defibrated. The upper limit is preferably 0.50 or less. This allows cellulose to be prevented from swelling or dissolving. The degree of cationic substitution can be adjusted by the reaction conditions, such as the amount of cationizing agent added to be reacted and the composition ratio of water or alcohol having 1 to 4 carbon atoms.

[0073] The degree of cationic substitution per glucose unit can be measured by the following method. After drying the cationized cellulose fiber, the nitrogen content is measured using a total nitrogen analyzer (TN-10 manufactured by Mitsubishi Chemical Corporation), and the degree of cationic substitution (the average number of moles of substituents per mole of anhydroglucose unit) is calculated by the following formula: degree of cationic substitution=(162×N) / (1−151.6×N), where N is the nitrogen content.

[0074] Examples of cationization methods include reacting unmodified cellulose fibers or carboxylated cellulose fibers with a cationizing agent (e.g., glycidyltrimethylammonium chloride, 3-chloro-2-hydroxypropyltrialkylammonium hydride, or a halohydrin form thereof) and an alkali metal hydroxide catalyst (e.g., sodium hydroxide, potassium hydroxide) in the presence of water and / or an alcohol having 1 to 4 carbon atoms. By using any of the cationizing agents listed above, it is possible to obtain cationized cellulose having a group containing a quaternary ammonium group. The cationization reaction is usually carried out in the presence of water or an alcohol.

[0075] The amount of the cationizing agent is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, relative to 100 parts by mass of the cellulose raw material. The upper limit of the amount is usually 800 parts by mass or less, preferably 500 parts by mass or less.

[0076] Examples of catalysts that may be used as needed during cationization include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide. The amount of catalyst is preferably 0.5 parts by mass or more, and more preferably 1 part by mass or more, per 100 parts by mass of the cellulose raw material. The upper limit of this amount is usually 7 parts by mass or less, and preferably 3 parts by mass or less. In order to efficiently perform defibration after cationization, it is preferable to wash the cation-modified cellulose raw material obtained above.

[0077] - Base-type cationized cellulose fibers - After cationization, the cationized cellulose fibers are preferably converted into base-type cationized cellulose or base-type cationized fine cellulose fibers by desalting. By desalting, the salt in the cationized cellulose can be converted into a base. In this specification, cationized (fine) cellulose fibers that have been desalted are referred to as base-type cationized (fine) cellulose fibers or cationized (fine) cellulose fibers (base type). Furthermore, cationized cellulose and cationized fine cellulose fibers that have not been desalted are referred to as salt-type cationized (fine) cellulose fibers or cationized (fine) cellulose fibers (salt type). Desalting may be carried out either before defibration (cationized cellulose) or after defibration (cationized fine cellulose fibers), which will be described later. Desalting is carried out to remove salts (for example, Cl) contained in the cationized cellulose (salt type) and the cationized fine cellulose fibers (salt type). - The desalting method after cationization is, for example, a method in which the cationized cellulose or the cationized fine cellulose fiber is brought into contact with an anion exchange resin. The anion exchange resin is a resin in which the counter ion is OH. － As long as the anion exchange resin is strong-basic or weak-basic, both can be used. The ratio of the two when contacting the modified cellulose with the anion exchange resin is not particularly limited, and a person skilled in the art can appropriately set it from the viewpoint of efficiently carrying out cation substitution. As an example, the ratio can be adjusted so that the pH of the aqueous dispersion after addition of the anion exchange resin to the cationized microfibrillated cellulose fiber dispersion is preferably 8 to 13, more preferably 9 to 13. The anion exchange resin after contact can be recovered by a conventional method such as suction filtration.

[0078] [Refining (defibration, fibrillation)] Refining is usually carried out by mechanical treatment. The mechanical treatment is usually carried out in a wet manner (i.e., in the form of an aqueous dispersion of cellulose fibers). Examples of equipment used for the mechanical treatment include refiners (e.g., disc type, conical type, cylinder type), high-speed defibrators, shear-type agitators, colloid mills, high-pressure jet dispersers, beaters, PFI mills, kneaders, dispersers, high-speed disintegrators (top finers), high-pressure or ultra-high-pressure homogenizers, grinders (stone-type grinders), ball mills, vibration mills, bead mills, single-screw, twin-screw, or multi-screw kneaders / extruders, homomixers under high-speed rotation, refiners, defibrators, friction grinders, high-shear defibrators, etc. Examples of suitable mechanical treatments include devices capable of applying a mechanical defibration force, such as a high-speed disintegrator, a disperger, a homogenizer (e.g., a microfluidizer), and a cavitation jet device. Devices capable of applying a wet defibration force are preferred, and high-speed disintegrators, refiners, and ultra-high-pressure homogenizers are more preferred, but are not particularly limited thereto. For efficient defibration, devices that can apply a pressure of preferably 50 MPa or more, more preferably 100 MPa or more, and further preferably 140 MPa or more to the aqueous dispersion and apply a strong shear force, or cavitation jet devices that can efficiently defibrate at a pressure of about 7 MPa are preferred. The mechanical treatment may be carried out using two or more devices. For example, prior to the defibration and dispersion treatment using a high-pressure homogenizer, a preliminary treatment using a mixing, stirring, emulsifying, and dispersing device such as a high-speed shear mixer may be carried out as necessary. The number of treatments (passes) using the device may be one or two or more, and two or more is preferred.

[0079] When defibration is performed by a wet method, a dispersion of cellulose fibers is usually prepared. The solvent in the dispersion may be any solvent capable of dispersing cellulose, such as water, an organic solvent (e.g., a hydrophilic organic solvent such as methanol), or a mixture thereof. Water is preferred because the cellulose raw material is hydrophilic. The solids concentration of the modified cellulose in the dispersion is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, even more preferably 0.7% by mass or more, and even more preferably 1.0% by mass or more. The upper limit of the concentration is preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 8% by mass or less. During mechanical treatment, pH adjustment may be performed as necessary.

[0080] Prior to preparing a dispersion for defibration, pretreatment such as dry pulverization (e.g., pulverization after drying) or hydrophobicity impartation may be performed. Examples of equipment used for dry pulverization include, but are not limited to, impact mills such as hammer mills and pin mills, media mills such as ball mills and tower mills, and jet mills. Furthermore, post-treatment may be performed after defibration. Examples of post-treatment include, but are not limited to, drying (e.g., freeze drying, spray drying, tray drying, drum drying, belt drying, thin spreading on a glass plate or the like and drying, fluidized bed drying, microwave drying, heated fan-type reduced pressure drying, and reduced pressure (degassing) drying), redispersion in water (the dispersion device is not limited), and pulverization (e.g., pulverization using equipment such as a cutter mill, hammer mill, pin mill, or jet mill). Hydrophobicity can be imparted using a cationic additive.

[0081] [Form of Microfibrillated Cellulose Fibers] The microfibrillated cellulose fibers may be in the form of an aqueous dispersion, or may be a dry solid or a wet solid of the dispersion. Methods for preparing the dry solid or the wet solid include, for example, drying (e.g., freeze drying, spray drying, tray drying, drum drying, belt drying, a method of thinly spreading on a glass plate or the like and drying, fluidized bed drying, microwave drying, and heated fan-type reduced pressure drying).

[0082] [Physical Properties of Microfibrillated Cellulose Fibers] - Viscosity - It is preferable that the viscosity of the aqueous dispersion of microfibrillated cellulose fibers is low. This allows the material to be easily handled despite being fibrillated. For example, the Brookfield viscosity (25°C, 60 rpm) of an aqueous dispersion of microfibrillated cellulose fibers having a solids content of 1.0% by mass is usually 6,000 mPa·s or less or 5,000 mPa·s or less, preferably 3,500 mPa·s or less, and more preferably 2,300 mPa·s or less. The lower limit is preferably 10 mPa·s or more, more preferably 20 mPa·s or more, and even more preferably 50 mPa·s or more, 100 mPa or more, 500 mPa or more, 1,000 mPa or more, or 1,500 mPa or more. Furthermore, for example, the Brookfield viscosity (25°C, 6 rpm) of an aqueous dispersion with a solids content of 1% by mass is usually 25,000 mPa s or less or 20,000 mPa s or less, preferably 18,000 mPa s or less, and more preferably 15,000 mPa s or less. The lower limit is preferably 100 mPa s or more, more preferably 500 mPa s or more, and even more preferably 1,000 mPa s or more, 2,00 mPa or more, 3,000 mPa or more, 4,000 mPa or more, or 5,000 mPa or more. The Brookfield viscosity can be measured using a Brookfield viscometer (for example, manufactured by Eiko Seiki Co., Ltd.).

[0083] -Transparency- The transparency of a CNF aqueous dispersion with a solid content of 1.0 mass% is usually 40% or more, preferably 50% or more, and more preferably 60% or more. There is no particular upper limit, as long as it is 100% or less. The transparency of an MFC aqueous dispersion with a solid content of 1.0 mass% is usually 1% or more, preferably 5% or more. The upper limit is 50% or less. Transparency can be measured as the transmittance of 660 nm light using a visible light photometer.

[0084] -Crystallization degree of cellulose type I- The crystallinity of cellulose type I in fine cellulose fibers is usually 50% or more, preferably 60% or more. There is no particular upper limit, but in reality it is thought to be around 90%. The crystallinity of cellulose can be controlled by the degree of chemical modification. The crystallinity of cellulose type I can be calculated by measuring and comparing the intensities of the (200) peak around 22.6° and the valley between (200) and (110) (around 18.5°) in X-ray diffraction measurement.

[0085] -Water retention capacity- When the fine cellulose fibers are cellulose microfibrils, the water retention capacity is preferably 10 or more, more preferably 15 or more, even more preferably 20 or more, and even more preferably 30 or more. The upper limit is thought to be about 200 or less in practice, but is not particularly limited. The water retention capacity corresponds to the mass of water in the sediment relative to the mass of the solid content of the fibers in the sediment, and is the ratio of the water content to the solid content in the sediment gel, measured and calculated by centrifuging a 0.3 mass% aqueous dispersion of the fibers at 25,000 G. That is, it is calculated using the following formula: Water retention capacity = (B + C - 0.003 x A) / (0.003 x A - C) A: Mass of an aqueous dispersion of cellulose microfibrils with a solid content concentration of 0.3 mass% B: Mass of the sediment separated after centrifuging the aqueous dispersion of mass A at 30°C and 25,000 G for 30 minutes C: Mass of the solid content in the aqueous phase separated after the centrifugation

[0086] The higher the water retention capacity value, the stronger the fiber's ability to retain water. Water retention capacity can be measured or calculated for fibers that have undergone fibrillation, but it is usually not possible to measure it for fibers that have not undergone fibrillation or defibration, or for cellulose nanofibers that have been defibrated to single microfibrils. When cellulose fibers that have not been fibrillated or defibrated are centrifuged under the above-mentioned conditions, a dense sediment cannot be formed, making it difficult to separate the sediment from the aqueous phase. When cellulose nanofibers are centrifuged under the above-mentioned conditions, there is usually very little sedimentation.

[0087] -Fibrillation rate- When the fine cellulose fibers are cellulose microfibrils, the fibrillation rate (Fibrillation %) is preferably 1.0% or more, more preferably 1.2% or more, and even more preferably 1.5% or more. This allows confirmation that fibrillation is sufficient. The fibrillation rate can be adjusted depending on the type of cellulosic raw material used. The fibrillation rate can be determined using an image analysis type fiber analyzer such as a fractionator manufactured by Valmet Co., Ltd.

[0088] -Electrical Conductivity- The electrical conductivity of an aqueous dispersion of fine cellulose fibers (solid content concentration 1.0% by mass) is preferably 500 mS / m or less, more preferably 300 mS / m or less, even more preferably 200 mS / m or less, even more preferably 100 mS / m or less, and particularly preferably 70 mS / m or less. The lower limit is preferably 5 mS / m or more, more preferably 10 mS / m or more. The electrical conductivity can be measured by preparing 200 g of an aqueous dispersion of fine cellulose fibers having a solid content concentration of 1.0% by mass and using an electrical conductivity meter (ES-71 model manufactured by HORIBA).

[0089] - Degree of polymerization - The degree of polymerization of the fine cellulose fibers, as measured by a viscosity method using a copper ethylenediamine solution, is preferably in the range of 250 to 1000. It is more preferably 300 to 900, and even more preferably 350 to 800. Within such a range, the viscosity does not become too high when the fine cellulose fibers are mixed with a rubber component, making them easy to disperse, and as a rubber reinforcing material, the fine cellulose fibers can form a network structure in the rubber, thereby maintaining sufficient strength, which is an advantage.

[0090] The degree of polymerization by the viscosity method using a copper ethylenediamine solution can be calculated by the following method: In the case of TEMPO-oxidized fine cellulose fibers, reduction treatment is first carried out. A 1% aqueous dispersion of fine cellulose fibers is added with NaBH ４The solution is added at 10 wt% to the fine cellulose fibers, adjusted to pH 10 with NaOH, and then stirred for 4 hours for a reduction treatment. Ethanol is then added and centrifuged, after which the supernatant is discarded and ethanol is added again, followed by stirring and centrifugation. This process is repeated three times to recover the fine cellulose fibers, yielding reduced, TEMPO-oxidized fine cellulose fibers. The fine cellulose fibers are freeze-dried and dissolved in 0.5 M copper ethylenediamine solution 1 to form solution 2. The viscosities of solutions 1 and 2 are measured using a capillary viscometer (Cannon-Fenske viscometer). The viscosity of solution 2 is defined as η and the viscosity of solution 1 as η0, and the intrinsic viscosity [η] of the anion-modified pulp is calculated using the following formula: Intrinsic viscosity [η] = (η / η0) / {c(1 + 0.28 × η / η0)} (c is the concentration of the fine cellulose fibers (g / dL)). Furthermore, the degree of polymerization (DP) is calculated using the following formula: Degree of polymerization DP=intrinsic viscosity [η] / (5.7×10-3)

[0091] The fine cellulose fibers may be of one type, or may be a combination of two or more types of fine cellulose fibers, which may be different in cellulose raw material, chemically modified or not, and of different types.

[0092] [1.2 Rubber Component] The rubber component is a component containing natural rubber (NR) or synthetic rubber. Examples of synthetic rubber include isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), chloroprene rubber (CR), acrylonitrile butadiene rubber (NBR), butyl rubber (IIR), ethylene propylene rubber (EPM), ethylene propylene diene rubber (EPDM), chlorosulfonated polyethylene (CSM), acrylic rubber (ACM), fluororubber (FKM), epichlorohydrin rubber (CO, ECO), urethane rubber (U), silicone rubber (Q), halogenated butyl rubber, and polysulfide rubber, but are not particularly limited thereto. Furthermore, thermoplastic elastomers such as polystyrene-based thermoplastic elastomers, polypropylene-based thermoplastic elastomers, polydiene-based thermoplastic elastomers, chlorine-based thermoplastic elastomers, and engineering plastic-based elastomers can also be used. The rubber component is preferably natural rubber (NR) or acrylonitrile butadiene rubber (NBR).

[0093] [Form of Rubber Component] The rubber component is usually provided in the form of a latex for producing a rubber composition. The solid content of the latex is usually 30% or more, preferably 40% or more, and more preferably 50% or more. The upper limit is usually 80% or less, but is not particularly limited.

[0094] The rubber component may be one type alone or a combination of two or more types.

[0095] [1.3 Other Raw Materials] Raw materials other than the fine cellulose fibers and the rubber component may be used as raw materials for the rubber composition. Examples of the other raw materials include a dispersant, a solvent, and a rubber additive.

[0096] [Dispersant] By using a dispersant together with fine cellulose fibers, for example, when the fine cellulose fibers are in a dried state, the dispersibility can be improved. Examples of dispersants include water-soluble polymers and surfactants, and water-soluble polymers are preferred. When the fine cellulose fibers are chemically modified cellulose fibers, the water-soluble polymer covers the low charge density areas on the surface, suppresses the formation of hydrogen bonds, and prevents the fine cellulose fibers from aggregating together during drying.

[0097] -Water-soluble polymer- It is believed that water-soluble polymers penetrate between the fibers of fine cellulose fibers and increase the distance between the fibers, thereby improving redispersibility after drying. Examples of water-soluble polymers include cellulose derivatives (carboxymethylcellulose, methylcellulose, hydroxypropylcellulose, ethylcellulose), xanthan gum, xyloglucan, dextrin, dextran, carrageenan, locust bean gum, alginic acid, alginates, pullulan, starch, potato starch, arrowroot flour, modified starch (cationized starch, phosphorylated starch, phosphate cross-linked starch, phosphate monoesterified phosphate cross-linked starch, hydroxypropyl starch, hydroxypropylated phosphate cross-linked starch, acetylated adipate cross-linked starch, acetylated phosphate cross-linked starch, acetylated oxidized starch, starch sodium octenyl succinate, starch acetate, oxidized starch), corn starch, gum arabic, locust bean gum, gellan gum, polydextrose, pectin, chitin, and water-soluble chitin. Examples of suitable water-soluble polymers include chitosan, casein, albumin, soy protein lysate, peptone, polyvinyl alcohol, polyacrylamide, sodium polyacrylate, polyvinylpyrrolidone, polyvinyl acetate, polyamino acids, polylactic acid, polymalic acid, polyglycerin, rosin-based sizing agents, petroleum resin-based sizing agents, urea resin, melamine resin, epoxy resin, polyamide resin, polyamide-polyamine resin, polyethyleneimine, polyamine, vegetable gum, polyethylene oxide, hydrophilic crosslinked polymers, polyacrylates, starch-polyacrylic acid copolymers, tamarind gum, guar gum, and colloidal silica, as well as mixtures of one or more thereof. Cellulose derivatives are preferred because of their high affinity with chemically modified fine cellulose fibers, and carboxymethylcellulose or a salt thereof is more preferred. The water-soluble polymer may be one type or a combination of two or more types.

[0098] -Surfactant- Examples of surfactants include, but are not limited to, nonionic surfactants such as fatty acid salts, higher alkyl sulfates, alkylbenzene sulfonates, higher alcohols, alkylphenols, and alkylene oxide adducts of fatty acids, anionic surfactants, cationic surfactants, amphoteric surfactants, organic solvents, proteins, enzymes, natural polymers, synthetic polymers, etc. The surfactant may be one type or a combination of two or more types.

[0099] [Solvent] Examples of solvents include aqueous solvents, which can improve the dispersibility of the mixture of fine cellulose fibers and rubber components. Examples of aqueous solvents include water, water-soluble organic solvents, and mixtures thereof. Water is preferred because fine cellulose fibers are hydrophilic. Use of water ensures a good dispersion state during dispersion. The aqueous solvent may contain a non-water-soluble organic solvent to the extent that the effect of the invention is not impaired.

[0100] The water-soluble organic solvent may be any organic solvent that can be dissolved in water, and examples thereof include lower alcohols having 1 to 4 carbon atoms (e.g., methanol, ethanol, 2-propanol, butanol), glycerin, acetone, methyl ethyl ketone, 1,4-dioxane, N-methyl-2-pyrrolidone, tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, acetonitrile, and combinations thereof. Among these, lower alcohols having 1 to 4 carbon atoms are preferred, with methanol, ethanol, and 2-propanol being more preferred, and from the viewpoints of safety and availability, methanol and ethanol are even more preferred, with ethanol being even more preferred.

[0101] The amount of the water-soluble organic solvent in the mixed solvent is not particularly limited, but is preferably 10% by mass or more, more preferably 50% by mass or more, and even more preferably 70% by mass or more. The upper limit is preferably 95% by mass or less, and more preferably 90% by mass or less.

[0102] [1.4 Rubber Additives] Examples of rubber additives include additives used for crosslinking rubber components. Crosslinking is generally carried out using a vulcanization system that combines a crosslinking agent (vulcanizing agent) such as sulfur or a sulfur-donating compound (e.g., sulfur halide) with various general-purpose vulcanization accelerators such as sulfenamide-based and thiuram-based compounds. The crosslinking agent is not limited to sulfur-based vulcanizing agents. Examples of crosslinking agents other than vulcanizing agents include organic peroxides, quinone dioximes, organic polyamine compounds, alkylphenol resins having methylol groups; compounds containing crosslinking groups such as isocyanate groups, carbodiimide groups, oxazoline groups, aziridine groups, and epoxy groups, polyfunctional cations, and compounds containing polyvalent metals. Examples of organic peroxides that can be used include commonly used compounds such as tert-butyl peroxide, dicumyl peroxide, tert-butylcumyl peroxide, 1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne-3, 1,3-di(tert-butylperoxyisopropyl)benzene, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, tert-butylperoxybenzoate, tert-butylperoxyisopropyl carbonate, and n-butyl-4,4-di(tert-butylperoxy)valerate. In the case of organic peroxide crosslinking, it is preferable to use a polyfunctional unsaturated compound, for example, triallyl isocyanurate, triallyl cyanurate, triallyl trimellitate, trimethylolpropane trimethacrylate, or N,N'-m-phenylene bismaleimide in combination.

[0103] The content of the crosslinking agent is preferably 1.0 part by mass or more, more preferably 1.5 parts by mass or more, and even more preferably 1.7 parts by mass or more, per 100 parts by mass of the rubber component. The upper limit is preferably 10 parts by mass or less, more preferably 7 parts by mass or less, and even more preferably 5 parts by mass or less.

[0104] Examples of vulcanization accelerators include N-t-butyl-2-benzothiazole sulfenamide and N-oxydiethylene-2-benzothiazolylsulfenamide. The content of the vulcanization accelerator is preferably 0.1 parts by mass or more, more preferably 0.3 parts by mass or more, and even more preferably 0.4 parts by mass or more, per 100 parts by mass of the rubber component. The upper limit is preferably 5 parts by mass or less, more preferably 3 parts by mass or less, and even more preferably 2 parts by mass or less.

[0105] The rubber additives are not limited to the above examples, and examples thereof include vulcanization accelerators (e.g., zinc oxide, stearic acid), surfactants (cationic surfactants, anionic surfactants, nonionic surfactants, amphoteric surfactants), reinforcing agents (e.g., carbon black, silica), silane coupling agents, hydrophobizing agents, oils, curing resins, waxes, antioxidants, and colorants. The content of each agent is not particularly limited.

[0106] [1.5 Physical Properties of Rubber Composition] [Lightness Difference ΔL] The rubber composition preferably has a lightness difference ΔL value of 1 to 15. This can result in less discoloration and better strength. The ΔL value is the difference between the L1 value, which is the lightness L* of the rubber composition, and the L2 value, which is the lightness L* of a composition similar to the composition except that it does not contain fine cellulose fibers (a 100% rubber composition with the same composition but containing the same weight of rubber instead of the fine cellulose fibers). The lightness difference is a value based on the L*a*b* color system. That is, in the L*a*b* color system, lightness is represented by L* and chromaticity, which indicates hue and saturation, is represented by a*b*. When the colors of two components are represented by (L1, a1, b1) and (L2, a2, b2) in the L*a*b* color system, the lightness difference ΔL is a value defined by the following formula (1): Brightness difference: ΔL=|L1-L2|...Formula (1)

[0107] The L1 and L2 values ​​are preferably measured in an unvulcanized state between immediately after production and one week after production. The composition at the time of measurement is a dry product, and its water content is preferably 8% by mass or less, more preferably 5% by mass or less.

[0108] [2. Method for producing rubber composition] The method for producing a rubber composition includes a drying step in which a mixture containing fine cellulose fibers and a rubber component is dried at a temperature of 70 to 150°C using a drum dryer. [2.1 Mixture] The mixture to be dried contains fine cellulose fibers and a rubber component. The fine cellulose fibers and the rubber component are as described above.

[0109] [Mass ratio of fine cellulose fibers to rubber component in the mixture] The mass ratio of the fine cellulose fibers to 100 parts by mass of the rubber component in the mixture (ratio of the solid contents of each) is preferably 0.1 phr or more, more preferably 1 phr or more, and even more preferably 5 phr or more. This allows the effect of improving tensile strength to be fully exhibited. The upper limit is preferably 50 phr or less, more preferably 40 phr or less, and even more preferably 30 phr or less. This allows processability in the manufacturing process to be maintained, and also improves the dispersibility of the fine cellulose fibers in the rubber.

[0110] [Solids Concentration of Mixture] The solids concentration of the mixture is typically 3% by mass or more, preferably 4% by mass or more, more preferably 5% by mass or more, 6% by mass or more, or 7% by mass or more. This allows a thick film to be formed using a drum dryer, thereby reducing the heat transmitted to the fine cellulose fibers, thereby suppressing the progression of thermal denaturation and preventing a decrease in dispersibility after drying. The upper limit is typically 70% by mass or more, preferably 60% by mass or less, more preferably 50% by mass or less, 40% by mass or less, or 30% by mass or less. This suppresses an increase in the viscosity of the mixture and allows the mixture to be smoothly fed to the drum dryer. Therefore, the solids concentration is typically 3 to 70% by mass, preferably 4 to 60% by mass, more preferably 5 to 50% by mass, 6 to 40% by mass, or 7 to 30% by mass. The solids concentration of the mixture can be adjusted by the solids concentration of the fine cellulose fibers in a dispersion, the solids concentration of the rubber component in a latex, and the amount of solvent added as needed.

[0111] [Method for Producing the Mixture] The method for producing the mixture is not particularly limited, but examples include a method in which fine cellulose fibers, a rubber component, and other raw materials (e.g., a solvent) used as needed are added and stirred. The fine cellulose fibers are preferably used as a dispersion (e.g., an aqueous dispersion), and the dispersion may contain a dispersant as needed. The rubber component is preferably added as a rubber latex. Stirring may be performed using equipment such as a homodisper or a supermixer.

[0112] [2.2 Drying] In the drying step, the mixture is dried using a drum dryer.

[0113] [Drum Dryer] A drum dryer is a dryer equipped with a rotatable drum (cylinder) and a doctor blade (knife, scraper knife) that can scrape off the material on the drum to be dried as the drum rotates. The drum can be heated by introducing a heat medium (e.g., steam) into the drum (e.g., made of metal). A sample (liquid) to be treated is continuously supplied to the preheated, rotating drum surface, and the sample is adhered to the drum surface in the form of a thin film. During rotation, the water in the sample evaporates and condenses, drying the sample, which can then be scraped off the drum surface with a doctor blade to obtain a dried material. The drum dryer may further be equipped with a feeder that supplies the sample (liquid) to the drum surface.

[0114] -Types of Drum Dryers- Drum dryers are classified into double drum (inner rotating), twin drum (outer rotating), and single drum types depending on the number of drums. However, drum dryers consisting of multiple (usually two) drums, such as double drum and twin drum types, are preferred. In twin drum and double drum types, two drums are arranged closely, parallel, and horizontally. The sample to be treated is supplied to a liquid reservoir formed above the closest point between the two drums. The feeder may be either a pendulum type or a liquid supply pipe equipped with liquid supply nozzles at regular intervals. Double drum type dryers are preferred because the thin film thickness can be adjusted by adjusting the drum spacing (clearance: the distance between the closest points). Single drum type drum dryers are classified into dip type, spray type, splash type, upper roll type (single stage, multi-stage), side roll type, and lower roll type depending on the liquid supply method, and any of these can be used.

[0115] The drum dryer may be either a normal pressure type (drying treatment under normal pressure) or a vacuum type (drying treatment under vacuum or reduced pressure), with the normal pressure type being preferred in terms of ease of operation and manufacturing. When a vacuum drum dryer is used, the pressure condition is usually 50 kPa or less, preferably 30 kPa or less, and more preferably 10 kPa or less. There is no particular lower limit, and it is sufficient as long as it is 0 kPa or more.

[0116] - Drum Surface Material - The surface material of the drum provided in the drum dryer is not particularly limited, but preferably has a coating. Examples include metal plating such as chrome plating and ceramic coating, with ceramic coating being preferred. A drum with a ceramic coating formed on its surface has excellent peelability of the dried product after drum drying of the mixture containing the fine cellulose fibers and the rubber component, and the dried product can be efficiently scraped off without the need to press the doctor blade hard against the drum. Therefore, using a drum with a ceramic coating can suppress frictional heat generated between the knife and the drum, suppress thermal denaturation of the resulting dried product of fine cellulose fibers, and improve mechanical properties.

[0117] From the viewpoint of uniformity of the coating, thermal spraying is preferred as a method for forming a ceramic coating on the surface of a drum. In this specification, thermal spraying refers to a method in which a spray material is heated to a molten or softened state and sprayed onto the surface of a substrate to form a thermal spray coating. The thermal spraying method is not particularly limited, and examples include flame spraying, arc spraying, and plasma spraying. From the viewpoint of workability, plasma spraying is preferred.

[0118] Examples of thermal spray materials include metals, alloys, ceramics, plastics, etc., and it is preferable to use ceramic-containing materials because of their wear resistance and high strength. Examples of ceramics include tungsten carbide and chromium carbide, and tungsten carbide (WC) is preferred from the viewpoint of peelability of the dried body. When using a thermal spray material containing ceramic, it is preferable to use a metal binder in combination. A coating formed by a thermal spray material containing ceramic and a metal binder is a so-called cermet coating. For example, the metal binder is preferably at least one selected from chromium, nickel, and cobalt, more preferably at least one selected from chromium and nickel, and even more preferably one containing both chromium and nickel.

[0119] - Drum Size - The diameter (φ) of the circle at each end of the drum is usually 100 mm to 2,000 mm, preferably 120 mm to 1,800 mm, and more preferably 130 mm to 1,500 mm. The drum face length (w) is usually 150 mm to 4,000 mm, preferably 180 mm to 3,500 mm, and more preferably 200 mm to 3,000 mm. The total drum length (L: length including the shaft) is usually 300 mm to 5,000 mm, preferably 350 mm to 4,500 mm, and more preferably 400 mm to 4,000 mm.

[0120] - Clearance (drum spacing) - In the case of a drum dryer consisting of multiple drums, the clearance is usually 0.07 mm or more, preferably 0.1 mm or more, and more preferably 0.12 mm or more. This makes it possible to avoid a deterioration in mechanical properties. The upper limit is usually 1 mm or less, preferably 0.8 mm or less, and more preferably 0.6 mm or less. This can make scraping after drying easier.

[0121] -Rotation Speed- The rotation speed of the drum is usually 0.5 rpm or more, preferably 0.75 rpm or more. The upper limit is usually 3.5 rpm or less, preferably 2.5 rpm or less. The peripheral speed is usually 0.25 m / min or more, preferably 0.38 m / min or more. The upper limit is usually 1.8 m / min or less, preferably 1.3 m / min or less. The peripheral speed can be calculated from the drum diameter and rotation speed using the following formula: peripheral speed (m / min) = diameter (m) × π × rotation speed (rpm).

[0122] [Thickness of thin film during drum drying] The thickness of the thin film formed on the drum surface is usually 5 μm or more, preferably 50 μm or more. This can avoid deterioration of mechanical properties. The upper limit is preferably 500 μm or less, more preferably 250 μm or less. This can make it easier to scrape off the thin film after drying.

[0123] -Doctor Blade Material- Examples of materials for the doctor blade include high-carbon steel, stainless steel (SUS), polyether ether ketone (PEEK) resin, carbon, and phosphor bronze, and can be appropriately selected depending on the material of the drum surface. The peelability of the dried material varies depending on the material of the drum surface, and the force required for scraping may also be affected. For example, when the drum surface is made of a ceramic spray coating, a doctor blade made of high-carbon steel is preferred from the viewpoint of protecting the coating on the drum surface and / or the durability of the doctor blade. Examples of high-carbon steel include SK-1 to SK-7 (carbon atom content 0.6 to 1.5%). Of these, SK-4 to SK-6 (carbon atom content 0.7 to 1.0%) are preferred, and SK-5 (0.8 to 0.9%) is more preferred.

[0124] [Drying Temperature] The temperature conditions during drying are usually 70°C or higher, preferably 80°C or higher, more preferably 90°C or higher, 100°C or higher, or 110°C or higher. This allows the drying process to proceed efficiently. The upper limit is usually 150°C or lower, preferably 140°C or lower, and more preferably 135°C or lower. This can suppress cellulose denaturation due to heat and prevent a decrease in the mechanical properties of the resulting dried product after molding. Therefore, a temperature of usually 70 to 150°C, preferably 80 to 140°C, more preferably 90 to 135°C, 100 to 135°C, or 110 to 135°C allows the drying process to proceed efficiently and prevents a decrease in mechanical properties. In this specification, the temperature during drying means the temperature of the drum surface of a drum dryer.

[0125] The mixed liquid may be at room temperature when supplied to the drum dryer, or may be preheated. When heated, the heating temperature is, for example, 40° C. or higher, 50° C. or higher, 60° C. or higher, or 65° C. or higher. There is no particular upper limit, and the temperature may be 70° C. or lower.

[0126] After drying by drum drying, molding may be carried out as necessary. Molding can regulate the shape of the rubber composition and improve workability. Molding can be carried out using a device such as an open roll.

[0127] The moisture content of the dried product after the drying step is usually 8% by mass or less, preferably 5% by mass or less. This range improves workability in subsequent mixing steps (e.g., mastication and kneading). There is no particular lower limit, and the moisture content should be 0% by mass (absolutely dry) or more. The dried product can be used as a masterbatch, an intermediate in rubber production. Alternatively, it can be used as a rubber product by immediately performing the following treatment.

[0128] [2.4 Manufacturing Process of Rubber Product] When a rubber product is to be obtained as a final product from the dried product (masterbatch), it is preferable that the above-mentioned rubber additive components used as needed are added to the masterbatch and mixed (for example, masticated and kneaded).

[0129] The temperature during mixing (e.g., mastication and kneading) may be about room temperature (e.g., about 15 to 30°C), or may be heated to a high temperature to the extent that the rubber component does not undergo a crosslinking reaction. For example, it is 140°C or lower, more preferably 120°C or lower. The lower limit is usually 35°C or higher, preferably 40°C or higher. Therefore, the heating temperature is preferably about 35 to 140°C, more preferably about 40 to 120°C. Mixing can be carried out using an apparatus such as a Banbury mixer, a kneader, or an open roll.

[0130] After mixing, the mixture may be molded, if necessary. Examples of molding equipment include mold molding, injection molding, extrusion molding, blow molding, and foam molding, and may be appropriately selected depending on the shape, application, and molding method of the final product.

[0131] In the mixing step, it is preferable to heat the mixture after mixing, preferably after molding. When the rubber composition contains a crosslinking agent (preferably a crosslinking agent and a vulcanization accelerator), a crosslinking (vulcanization) treatment is performed by heating. Even when the rubber composition does not contain a crosslinking agent or a vulcanization accelerator, adding them before heating can achieve the same effect. The heating temperature is preferably 150°C or higher, with the upper limit being preferably 200°C or lower, and more preferably 180°C or lower. Therefore, a temperature of about 150 to 200°C is preferable, and about 150 to 180°C is more preferable. Examples of heating devices include vulcanization devices such as mold vulcanizers, can vulcanizers, and continuous vulcanizers.

[0132] Before the coagulated material is made into a final product, it may be subjected to a finishing treatment as necessary, such as polishing, surface treatment, lip finishing, lip cutting, chlorination, etc., and only one of these treatments may be performed, or two or more of them may be combined.

[0133] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In addition, unless the method for measuring / calculating each value in each example is specifically stated, it was measured / calculated by the method described in the specification.

[0134] (Production Example 1) (Production of Carboxylated (TEMPO-oxidized) CNF) 5 g (bone dry) of bleached, unbeaten kraft pulp derived from softwood (brightness 85%) was added to 500 mL of an aqueous solution containing 39 mg of TEMPO (Sigma-Aldrich) and 514 mg of sodium bromide, and the mixture was stirred until the pulp was uniformly dispersed. An aqueous solution of sodium hypochlorite was added to the reaction system to a concentration of 5.5 mmol / g to initiate the oxidation reaction. The pH of the system decreased during the reaction, but was adjusted to pH 10 by sequential addition of 3 M aqueous sodium hydroxide. The reaction was terminated when the sodium hypochlorite was consumed and the pH of the system no longer changed. The reaction mixture was acidified with hydrochloric acid, filtered through a glass filter to separate the pulp, and thoroughly washed with water to obtain oxidized pulp (hereinafter sometimes referred to as "carboxylated cellulose," "carboxylated pulp," or "TEMPO-oxidized pulp"). The pulp yield was 90%, the time required for the oxidation reaction was 90 minutes, and the carboxyl group content was 1.45 mmol / g. The oxidized pulp (solid content 3.1%) obtained in the above process was adjusted to 3.0% (w / v) with water and treated three times with an ultra-high-pressure homogenizer (20°C, 150 MPa) to obtain a TEMPO-oxidized CNF dispersion. The resulting TEMPO-oxidized CNF had an average fiber diameter of 3 nm and an aspect ratio of 150. The physical properties of the TEMPO-oxidized CNF dispersion are shown in Table 1.

[0135] (Production Example 2) (Production of Carboxylated (TEMPO-oxidized) MFC1) The oxidized pulp (solids content 3.1%) obtained in the process of Production Example 1 was adjusted to 3.0% (w / v) with water and treated once with an ultra-high-pressure homogenizer (20°C, 150 MPa) to obtain a TEMPO-oxidized MFC dispersion. The average fiber diameter of the obtained TEMPO-oxidized MFC1 measured by AFM was 3 nm. The physical properties of the TEMPO-oxidized MFC1 dispersion are shown in Table 1. (Production Example 3) (Production of Carboxylated (TEMPO-oxidized) MFC2) The oxidized pulp (solids content 3.1%) obtained in the process of Production Example 1 was adjusted to 1.27% (w / v) with water and subjected to six passes of cavitation treatment using a cavitation jet device at an upstream pressure of 14 MPa and a downstream pressure of 0.5 MPa to obtain a TEMPO-oxidized MFC2 dispersion. The average fiber diameter of the resulting TEMPO-oxidized MFC2 measured by AFM was 3 nm. The physical properties of the TEMPO-oxidized MFC2 dispersion are shown in Table 1. (Production Example 4) (Production of Carboxylated (TEMPO-oxidized) MFC3) The oxidized pulp (solids content 3.1%) obtained in the process of Production Example 1 was adjusted with water to a concentration of 1.08% (w / v). 3,000 kg of this slurry was beaten using a monoflow double-disc refiner (AWN20 manufactured by Aikawa Iron Works Co., Ltd., plate: blade width (X1): 0.8 mm, groove width (Y1): 1.3 mm, blade width (X2): 0.6 mm, groove width (Y2): 1.0 mm) with a clearance of 0.4 mm or less and a circulation rate of 75.5% for 45 minutes to obtain TEMPO-oxidized pulp, MFC3. The average fiber diameter of the obtained TEMPO oxidized MFC3 measured with a fiber tester was 22.2 μm.

[0136]

[0137] (Method for measuring physical properties) (Carboxy group amount) 60 mL of a 0.5 mass% slurry (aqueous dispersion) of TEMPO-oxidized CNF was prepared, and 0.1 M aqueous hydrochloric acid solution was added to adjust the pH to 2.5. After that, 0.05 N aqueous sodium hydroxide solution was added dropwise, and the electrical conductivity was measured until the pH reached 11. The amount of carboxyl group was calculated using the following formula (1) from the amount of sodium hydroxide (a) consumed in the neutralization stage of weak acid, where the change in electrical conductivity was gradual: Carboxy group amount [mmol / g TEMPO-oxidized cellulose] = a [mL] × 0.05 / mass of TEMPO-oxidized cellulose [g] (1)

[0138] (Average Fiber Diameter, Average Fiber Length, Aspect Ratio) The average fiber diameter and average fiber length of CNF were analyzed using an atomic force microscope (AFM) for 200 randomly selected fibers. The aspect ratio was calculated using the following formula: Aspect Ratio = Average Fiber Length / Average Fiber Diameter (Average Fiber Diameter of MFC) The average fiber diameter of MFC was measured as follows. 0.1 g of fine cellulose fibers was dispersed in 300 mL of water using an ABB fiber tester, and the fiber count measured after circulating for 5 minutes was 10,000 or more (e.g., in the above-mentioned Production Example 4). The value measured by the fiber tester was taken as the average fiber diameter. On the other hand, when the count was less than 10,000 (e.g., in the above-mentioned Production Examples 2 and 3), the value measured by AFM using the above-mentioned method was taken as the average fiber diameter.

[0139] (Viscosity) A CNF dispersion with a solid content of 1.0 mass% was prepared (after fibrillation (e.g., defibration), the dispersion was left to stand for at least one day, and then stirred with a homodisper (e.g., 3000 rpm, 5 min)). Using a B-type viscometer (manufactured by Eiko Seiki Co., Ltd.), the viscosity after 3 minutes at a rotation speed of 60 rpm and after 3 minutes at a rotation speed of 6 rpm was measured at 25°C.

[0140] (Transparency) For a CNF dispersion with a solids concentration of 1.0%, the transparency (transmittance of 660 nm light) was measured using a visible light photometer ASV11D (manufactured by AS ONE Corporation). (Lightness difference ΔL) A 3 cm square, 2 mm thick test piece was cut out from the dried rubber immediately after passing through the rolls, and the L value, expressed in the L*a*b* system, was measured three times using a spectrophotometer (Spectroeye, manufactured by Sakata Ink Corporation), and the average value was calculated to determine the L1 value. The L*a*b* system values ​​(L value, a value, and b value) were determined by a method conforming to JIS Z 8781-4:2013 "Colorimetry - Part 4: CIE 1976 L*a*b* color space". Meanwhile, the L value of a rubber sample not containing fine cellulose immediately after passing through the rolls was measured in the same manner, and the L2 value was determined. The ΔL of each sample was calculated by calculating the difference between the L1 value and the L2 value (the above formula (1)).

[0141] Examples 1 and 2: The TEMPO-oxidized CNF produced in Production Example 1 was diluted with pure water to a concentration of 2 wt %, yielding a 2 wt % CNF dispersion. This dispersion and NR latex (NR-Lx: ULACOL, manufactured by Regitex Corporation, solids content 60%) were mixed in a Super Mixer (manufactured by Kawata Corporation, SMV-20Ba) so that the solids content of CNF was 20 parts (20 phr) per 100 parts by mass of NR-Lx solids (solids content of the mixture was 10.3%). After drying, the resulting mixture was dried in a drum at atmospheric pressure. The drum drying conditions are shown in Table 2.

[0142] Example 3: The TEMPO-oxidized MFC1 produced in Production Example 2 was diluted with pure water to a concentration of 2 wt %, yielding a 2 wt % MFC dispersion. This dispersion and NR latex (NR-Lx: ULACOL, manufactured by Regitex Corporation, solids content 60%) were mixed in a Super Mixer (manufactured by Kawata Corporation, SMV-20Ba) so that the solids content of MFC was 20 parts (20 phr) per 100 parts by mass of NR-Lx solids (solids content of the mixture was 10.3%). After drying, the resulting mixture was dried in a drum at atmospheric pressure. The drum drying conditions were the same as those in Example 2.

[0143] Example 4: The TEMPO-oxidized MFC1 produced in Production Example 3 was diluted with pure water to a concentration of 1 wt %, yielding a 1 wt % MMFC dispersion. This dispersion and NR latex (NR-Lx: ULACOL, manufactured by Regitex Corporation, solids content 60%) were mixed in a Super Mixer (manufactured by Kawata Corporation, SMV-20Ba) so that the solids content of the MFC was 5 parts (5 phr) per 100 parts by mass of the NR-Lx solids (solids content of the mixture was 15.9%). After drying, the resulting mixture was dried in a drum at atmospheric pressure. The drum drying conditions were the same as those in Example 2.

[0144] Example 5 A dried product was obtained in the same manner as in Example 4, except that TEMPO-oxidized MFC2 produced in Production Example 3 was used instead of TEMPO-oxidized MFC1.

[0145] Example 6 A dried product was obtained in the same manner as in Example 4, except that TEMPO-oxidized MFC3 produced in Production Example 4 was used instead of TEMPO-oxidized MFC1.

[0146]

[0147] [Notes to Table 2] *Prepared by plasma spraying tungsten carbide and a nickel-chromium binder onto the drum surface to form a ceramic spray coating containing tungsten carbide.

[0148] The resulting dry product was passed through an open roll (Kansai Roll Co., Ltd.) at room temperature 3 to 5 times until it was visually uniform, and then molded to obtain a dried rubber product. This dried product (water content: approximately 4% or less (some were 0.8% or less)) was mixed with 0.5 parts of stearic acid (Fujifilm Wako Pure Chemical Industries Co., Ltd.), 6.0 parts of zinc oxide (Fujifilm Wako Pure Chemical Industries Co., Ltd.), 3.5 parts of sulfur (Fujifilm Wako Pure Chemical Industries Co., Ltd.), and 0.7 parts of a vulcanization accelerator (N-oxydiethylene-2-benzothiazolylsulfenamide: Noccela (MSA-G), Ouchi Shinko Chemical Industry Co., Ltd.) per 100 parts of rubber solids. The rubber was kneaded (open roll: Kansai Roll Co., Ltd.) at 40°C for approximately 20 minutes and vulcanized (press crosslinked at 150°C for 9 minutes). The resulting rubber composition was evaluated for dynamic viscoelasticity (DMA), static tensile properties, and hardness (Tables 3 and 4).

[0149] Comparative Example 1 The same procedure as in Example 1 was carried out, except that oven drying was carried out under the following conditions instead of drum drying. 200 g of the TEMPO-oxidized CNF / NR-Lx mixed solution was placed in a 25 cm x 35 cm x 5 cm Teflon (registered trademark) tray and dried at 70°C for approximately 16 hours. 200 g of each Teflon (registered trademark) tray was cast, and the dryer was operated with 14 stages x 2 trays = 28 trays per batch. The moisture content after oven drying was approximately 0%.

[0150] (Evaluation Method) (Dynamic Viscoelasticity (DMA)) Dynamic properties were measured in accordance with JIS K6394:2017 "Vulcanized rubber and thermoplastic rubber - Determination of dynamic properties - General guidelines." That is, using a dynamic viscoelasticity measuring device (Hitachi High-Tech Science, DMA7100), the loss tangent (tan δ) at 60°C and the storage modulus (E') at 23°C were measured under normal temperature control (temperature range: 18-110°C), measurement mode: tension, and DMA frequency: 10 Hz.

[0151] (Static Tensile Properties) Tensile strength, 50%, 100% and 300% tensile stresses (M50, M100, M300) and elongation at break were measured according to JIS K6251 "Vulcanized rubber and thermoplastic rubber - Determination of tensile properties".

[0152] (Drying Efficiency) The drying efficiency (g / h (solid content)) was calculated using the following formula (2) for the drum drying examples (Examples 1 to 4) and the following formula (3) for the oven drying example (Comparative Example 1).

[0153] (Hardness) The hardness was measured using a manual durometer stand (GS-615, manufactured by Teclock Corporation).

[0154] (Vulcanization Time (Tc90)) Tc90 was measured using a rubber vulcanization tester (MDRH2030 manufactured by M&K Co.).

[0155]

[0156] [Footnotes for Table 3] *The storage modulus E', tan δ and hardness of the Example and Comparative Example 1 are shown relative to the respective measured values ​​of the Reference Example (natural rubber) taken as 100.

[0157]

[0158] [Footnotes for Table 4] *The tensile test results and Tc90 for the Example and Comparative Example 1 are shown relative to the respective measured values ​​for the Reference Example (natural rubber) taken as 100.

[0159] Examples 1 and 2, which were drum-dried, had better drying efficiency and significantly higher storage modulus E' than Comparative Example 1, which was oven-dried. Repeated trials of the Example and Comparative Example were conducted, and E' values ​​for the drum-dried and oven-dried products were measured. A significant difference (t-test) was then performed, revealing a significant difference between the two (t(9) = 3.59, P value = 0.006, significance level = 0.05). The difference in E' and hardness is thought to be due to the difference in thermal history between oven drying and drum drying. It is presumed that high-temperature, short-time drum drying is superior to low-temperature, long-time oven drying. Furthermore, while slight discoloration was observed in Comparative Example 1, there was almost no discoloration in Examples 1 and 2 for the unvulcanized rubber after drum drying. Furthermore, the drum-dried products showed less discoloration after vulcanization.

[0160] Example 7 Drum drying was carried out under the same conditions as in Example 2, except that the drum rotation speed was changed to the speed shown in Table 5 (Tables 5 and 6).

[0161] (Method for Evaluating Solid Content Concentration) The weight of the dried sample after drying overnight in an oven at 70°C was regarded as the bone dry weight, and the solid content concentration was calculated according to the following formula (4).

[0162]

[0163]

[0164] Example 3 was excellent in both dynamic viscoelasticity and static tensile properties, had high drying efficiency, and was able to efficiently reduce residual moisture.

Claims

1. A method for producing a rubber composition, comprising a drying step of drying a mixture containing fine cellulose fibers and a rubber component and having a solid content concentration of 3 to 70 mass % at a temperature of 70 to 150°C using a drum dryer.

2. The method according to claim 1 , wherein the fine cellulose fibers include chemically modified fine cellulose fibers.

3. 3. The method according to claim 1, wherein the weight ratio of the fine cellulose fibers to the rubber component in the mixture is 0.1 to 50 phr.

4. The method according to claim 1 or 2, wherein the drum dryer is a double-drum type drum dryer.

5. The method according to claim 4, wherein the clearance between the drums constituting the double-drum type drum dryer is 0.07 mm to 1 mm.

6. The method according to claim 1 or 2, wherein the drum dryer includes a drum having a coating layer containing ceramic or cermet on the surface thereof.

7. The method of claim 1 or 2, wherein the drum dryer is equipped with a doctor blade made of high carbon steel.

8. The method according to claim 1 or 2, further comprising a vulcanization step of vulcanizing the dried product obtained in the drying step.

9. A composition containing fine cellulose fibers and a rubber component, A composition, wherein the difference ΔL between the L1 value, which is the lightness L* of the composition defined by the L*a*b* color system immediately after production and one week after production, and the L2 value, which is the lightness L* of a composition similar to the composition except that it does not contain fine cellulose fibers, is 1 to 15.