Method for producing cellulose nanofiber-added wet masterbatch and method for producing rubber composition

Abstract

To provide a method for producing a cellulose nanofiber-added wet masterbatch whereby, in production of the cellulose nanofiber-added wet masterbatch, when a coagulate of a rubber / cellulose nanofiber composite is dehydrated under application of shear force, it is possible to prevent aggregation of hydrophilic cellulose nanofibers dispersed in a hydrophobic rubber matrix even with use of a screw press, thereby ensuring inherent performance of the cellulose nanofiber-added wet masterbatch.SOLUTION: This method for producing a cellulose nanofiber-added wet masterbatch involves acid-coagulating a mixed solution of a cellulose nanofiber aqueous dispersion and a natural rubber latex to obtain a rubber coagulate, dehydrating the rubber coagulate by applying shear force, and subsequently drying, wherein the mixed solution contains a modified natural rubber latex having an epoxy group and / or a glycidyl group.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a wet masterbatch containing rubber latex and cellulose nanofibers, and a method for producing a rubber composition using the cellulose nanofiber-added wet masterbatch. [Background technology]

[0002] Conventionally, the wet masterbatch method has been used to improve the dispersibility of rubber reinforcing fillers such as carbon black and silica (see Patent Document 1). The wet masterbatch method involves mixing a dispersion, in which a rubber reinforcing material is previously dispersed in a dispersion medium, with rubber latex in a liquid phase, and then adding a coagulant such as acid to obtain a coagulated product, which is then dehydrated and dried to produce a rubber masterbatch. Compared to the dry masterbatch method, in which a filler and rubber are mixed in a solid phase, the wet masterbatch method provides superior filler dispersibility and produces a rubber composition with excellent physical properties such as rubber reinforcement and processability.

[0003] In recent years, there has been an increasing number of attempts to use cellulose nanofibers, a lightweight and strong biomass material, as a reinforcing material for rubber. Cellulose nanofibers can be produced by preparation methods such as biological synthesis, but in general, pulp is mechanically defibrated in water to obtain an aqueous dispersion (referred to as an "aqueous dispersion" in this invention). In some cases, chemical or enzymatic treatment may be performed before mechanical defibration.

[0004] When using the dry masterbatch method to obtain rubber / cellulose nanofiber composites, the cellulose nanofibers must be separated from the cellulose nanofiber aqueous dispersion, dried, and extracted. In this case, the cellulose nanofibers aggregate tightly together due to hydrogen bonds formed through the hydroxyl groups of the cellulose molecules, making them extremely difficult to disaggregate. To prevent this, measures such as protecting the hydroxyl groups and hydrophobizing the nanofibers are taken.

[0005] Meanwhile, the wet masterbatch method, which allows the use of aqueous cellulose nanofiber dispersions as they are in rubber composites, has been attracting attention. In this method, an aqueous cellulose nanofiber dispersion is mixed with rubber latex in the liquid phase and coagulated to obtain a rubber / cellulose nanofiber composite. The presence of rubber molecules between the cellulose nanofibers can suppress self-aggregation of the cellulose nanofibers during drying, and this method is expected to have the great advantage of improving the dispersibility of the cellulose nanofibers (see Patent Documents 2 and 3). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-75900 [Patent Document 2] Japanese Patent Application Publication No. 2019-163414 [Patent Document 3] Japanese Patent Publication No. 2020-55962 Summary of the Invention [Problem to be solved by the invention]

[0007] In wet masterbatches, water removal by shear compression is often used to dehydrate and dry the coagulated rubber / cellulose nanofiber composite. This is because it reduces energy costs compared to removing solvents by heating and is particularly efficient in continuous production.

[0008] However, when shear force is applied to the coagulated rubber / cellulose nanofiber composite, if the dispersed components in the coagulated product are incompatible with the rubber matrix, aggregation occurs between the dispersed components, and the expected performance may not be achieved. Here, the component incompatible with the rubber matrix corresponds to the hydrophilic cellulose nanofibers in the hydrophobic rubber matrix. One method for solving this problem is to dehydrate the coagulated material by pressing it with a filter press, a drum dryer, etc. However, this method is inferior in shear dehydration efficiency from the viewpoint of efficient continuous production. From the viewpoint of dehydration efficiency, it is preferable to use a screw press. However, if a screw press is used, the hydrophilic cellulose nanofibers dispersed in the hydrophobic rubber matrix will aggregate, and the inherent performance of the cellulose nanofiber-added wet masterbatch will not be obtained.

[0009] On the other hand, if a compatibilizer is used to firmly bond the rubber component and the dispersed component, it is expected that the aggregation of the dispersed component due to shear during dehydration can be suppressed. However, when a compatibilizer is used in the wet masterbatch method, it must be used as an aqueous dispersion, but some compatibilizers cannot be used as an aqueous dispersion due to their chemical structure or other reasons, and therefore cannot be used in the wet masterbatch method.

[0010] An object of the present invention is to provide a method for producing a cellulose nanofiber-added wet masterbatch that can suppress aggregation of hydrophilic cellulose nanofibers dispersed in a hydrophobic rubber matrix when a shear force is applied to a coagulum of a rubber / cellulose nanofiber composite to dehydrate it, even when a screw press is used, thereby allowing the inherent performance of the cellulose nanofiber-added wet masterbatch to be obtained.A further object of the present invention is to provide a method for producing a rubber composition using the cellulose nanofiber-added wet masterbatch produced by this method. [Means for solving the problem]

[0011] As a result of intensive research conducted by the present inventors to solve the above-mentioned problems, they found that in a method for producing a wet masterbatch in which a mixed liquid of an aqueous dispersion of cellulose nanofibers and natural rubber latex is acid-coagulated to obtain a rubber coagulum, the rubber coagulum is dehydrated by applying shear force to the mixed liquid, and then dried, by adding modified natural rubber latex having epoxy groups and / or glycidyl groups to the mixed liquid, the epoxy groups and / or glycidyl groups contained in the modified natural rubber latex react with hydroxyl groups in the cellulose nanofibers to increase the adhesive strength between the rubber and cellulose, thereby inhibiting coagulation of the cellulose nanofibers and solving the above-mentioned problems, and completed the present invention.

[0012] That is, the present invention is summarized as follows [1] to

[10] .

[0013] [1] A method for producing a wet masterbatch, comprising acid-coagulating a mixed liquid of an aqueous dispersion of cellulose nanofibers and natural rubber latex to obtain a rubber coagulum, applying shear force to the rubber coagulum to dehydrate it, and then drying it; A method for producing a cellulose nanofiber-added wet masterbatch, comprising adding a modified natural rubber latex having an epoxy group and / or a glycidyl group to the mixed liquid.

[0014] [2] The method for producing a cellulose nanofiber-added wet masterbatch according to [1], wherein the modified natural rubber latex is a modified natural rubber latex having an epoxy group and / or a modified natural rubber latex having a glycidyl group.

[0015] [3] The method for producing a cellulose nanofiber-added wet masterbatch according to [2], wherein the modified natural rubber latex is a modified natural rubber latex having an epoxy group.

[0016] [4] The method for producing a cellulose nanofiber-added wet masterbatch according to [2], wherein the modified natural rubber latex is a modified natural rubber latex having a glycidyl group.

[0017] [5] The method for producing a cellulose nanofiber-added wet masterbatch according to [2] or [3], wherein the modified natural rubber latex having epoxy groups is an epoxidized natural rubber latex.

[0018] [6] The method for producing a cellulose nanofiber-added wet masterbatch according to [2] or [4], wherein the modified natural rubber latex having a glycidyl group is a glycidyl methacrylated natural rubber latex.

[0019] [7] The method for producing a cellulose nanofiber-added wet masterbatch according to any one of [1] to [6], wherein the content of the modifying unit in the rubber component contained in the wet masterbatch is 5% by mass or more.

[0020] [8] The method for producing a cellulose nanofiber-added wet masterbatch according to any one of [1] to [7], wherein the mixed liquid is a mixed liquid of the cellulose nanofiber aqueous dispersion, natural rubber latex, and a carbon black dispersion.

[0021] [9] The method for producing a cellulose nanofiber-added wet masterbatch according to any one of [1] to [8], wherein the rubber coagulum is dehydrated by applying the shear force using a single-axis or multi-axis screw press.

[0022]

[10] A method for producing a rubber composition, comprising producing a cellulose nanofiber-added wet masterbatch by the method for producing a cellulose nanofiber-added wet masterbatch according to any one of [1] to [9], and vulcanizing the resulting cellulose nanofiber-added wet masterbatch to obtain a rubber composition.

[0023] As described below, inorganic fillers may be blended into the mixture of the cellulose nanofiber aqueous dispersion and rubber latex. Examples of inorganic fillers include inorganic additives commonly used as rubber additives, such as carbon black as described in [8] above, as well as silica. [Effects of the Invention]

[0024] According to the method for producing a cellulose nanofiber-added wet masterbatch of the present invention, when a coagulate of a rubber / cellulose nanofiber composite is dehydrated by applying shear force, aggregation of the hydrophilic cellulose nanofibers dispersed in the hydrophobic rubber matrix can be suppressed even when a screw press is used. Therefore, according to the present invention, a cellulose nanofiber-added wet masterbatch having the inherent performance of a cellulose nanofiber-added wet masterbatch can be efficiently produced in continuous production using a screw press with excellent dehydration efficiency, and the produced cellulose nanofiber-added wet masterbatch can be used to efficiently produce a rubber composition with excellent mechanical properties, etc. [Brief explanation of the drawings]

[0025] [Figure 1] 1 is a graph showing stress-strain curves obtained in tensile tests of the rubber compositions obtained in Examples, Comparative Examples, and Reference Examples. DETAILED DESCRIPTION OF THE INVENTION

[0026] The present invention will be described in detail below. The following explanation of the constituent elements is a representative example of an embodiment of the present invention, and the present invention is not limited to these contents.

[0027] [Method of manufacturing cellulose nanofiber-added wet masterbatch] The method for producing a cellulose nanofiber-added wet masterbatch of the present invention comprises acid-coagulating a mixed solution of a cellulose nanofiber aqueous dispersion and natural rubber latex to obtain a rubber coagulum, applying shear force to the rubber coagulum to dehydrate it, and then drying it, and is characterized in that the mixed solution contains modified natural rubber latex having epoxy groups and / or glycidyl groups.

[0028] [Cellulose nanofiber aqueous dispersion] The cellulose nanofiber aqueous dispersion used in the present invention is made from cellulose fibers prepared from various natural plant fibers, and is prepared by dispersing the raw material in water and subjecting it to chemical or mechanical treatment to adjust the average fiber diameter to approximately less than 1000 nm. One method for chemically treating raw materials is to add 2,2,6,6-tetramethylpiperidine-1-oxyl radical as a catalyst to cellulose fibers dispersed in water, adjust the pH to 10, add an aqueous solution of sodium hypochlorite and stir, then filter, wash, and further dilute with water. Examples of the mechanical treatment method include grinding cellulose fibers dispersed in water using a high-pressure homogenizer or a millstone method.

[0029] In the present invention, the "average fiber diameter" refers to a value obtained by randomly extracting 10 cellulose nanofibers from a scanning electron microscope (SEM) image, measuring the minor axis, and calculating the arithmetic mean.

[0030] The solids concentration (cellulose nanofiber concentration) of the cellulose nanofiber aqueous dispersion is preferably adjusted to 0.4% by mass to 1.0% by mass, and particularly 0.5% by mass to 0.7% by mass. If the solids concentration is below the upper limit, fluidity can be obtained to the extent that no problems occur during liquid phase mixing. On the other hand, if the solids concentration is above the lower limit, good production efficiency of the wet masterbatch can be maintained.

[0031] [Rubber coagulation] In the present invention, the rubber coagulum is obtained by acid coagulating a mixed solution of an aqueous cellulose nanofiber dispersion and natural rubber latex. From the viewpoint of the viscosity and dehydration efficiency of the rubber coagulum, the aqueous cellulose nanofiber dispersion is preferably mixed with rubber latex so that the amount of cellulose nanofibers is 1 to 40 parts by mass, and more preferably 5 to 30 parts by mass, per 100 parts by mass of the solids content of the rubber latex.

[0032] [Natural rubber latex] Natural rubber latex is a natural product of plants, and is preferably a natural rubber / water system in which the dispersion medium is water. Regarding natural rubber latex, concentrated latex, fresh latex known as field latex, etc. can be used without distinction. In addition to natural rubber latex, synthetic rubber latex can be used in the present invention, such as those produced by emulsion polymerization of styrene-butadiene rubber, polybutadiene rubber, polyisoprene rubber, nitrile rubber, and chloroprene rubber.

[0033] When producing a wet masterbatch, the natural rubber latex is preferably diluted to a solids concentration of 20% to 30% by mass from the viewpoints of the fluidity of the latex and the productivity of the wet masterbatch, where the solids correspond to the rubber component.

[0034] [Modified natural rubber latex] In the present invention, modified natural rubber latex is used as all or part of the natural rubber latex. It is preferable that the content of modified units in the rubber component of the wet masterbatch (hereinafter simply referred to as "modified unit content") is 5% by mass or more in order to obtain multi-point interactions between the hydroxyl groups in the cellulose nanofibers described below and the modified groups. In the present invention, the "modified unit," when expressed in terms of a modified natural rubber having epoxy groups, refers to epoxy groups introduced by modifying cis-1,4-bonded polyisoprene units in the original natural rubber, and the "modified unit content" refers to the amount of epoxy groups introduced by this modification, expressed in mass%. The same applies to glycidyl groups. When a glycidyl group-containing compound such as glycidyl methacrylate is used to introduce the glycidyl group, the content of the modifying group derived from this glycidyl group-containing compound is the modifying unit content.

[0035] From the viewpoint of multi-point interaction between the hydroxyl groups of the cellulose nanofibers and the modifying groups, the modified natural rubber latex used in the present invention preferably has a higher content of modifying units, and is preferably 5% by mass or more, particularly 6% by mass or more, and especially 8% by mass or more. On the other hand, from the viewpoint of maintaining the properties of natural rubber, the modified unit content is preferably 25% by mass or less, particularly 20% by mass or less. Therefore, in order to adjust the modified unit content, it may be preferable to use a mixture of modified natural rubber latex and unmodified natural rubber latex.

[0036] For example, when a modified natural rubber latex having epoxy groups is used alone as the modified natural rubber latex, the multi-point interaction between the hydroxyl groups and the epoxy groups of the cellulose nanofibers increases, which contributes to an improvement in the modulus of elasticity, in terms of tensile properties, of a rubber composition produced using the obtained wet masterbatch.

[0037] In the present invention, the epoxy group or glycidyl group introduced into the natural rubber latex acts as follows. The present inventors believe that the acid added for acid coagulation also acts as a catalyst for the reaction between the modified natural rubber latex having epoxy or glycidyl groups and the cellulose nanofibers. In other words, the negatively charged oxygen atoms of the epoxy groups in the modified natural rubber latex attract hydrogen ions, causing the methine carbon atoms of the epoxy groups to become positively charged. This then promotes a nucleophilic substitution reaction between these carbon atoms and the hydroxyl groups in the cellulose nanofibers. As a result, strong adhesion is formed between the epoxy groups in the modified natural rubber latex and the hydroxyl groups in the cellulose nanofibers through multipoint covalent bonds. Furthermore, even if the water in the reaction medium reacts with the methine carbon atom of the epoxy group instead of the hydroxyl groups of the cellulose molecules as a nucleophile to produce glycol in the modified natural rubber, multipoint interactions are achieved through hydrogen bonds with the hydroxyl groups of the cellulose molecules, resulting in strong adhesion between the modified natural rubber and the cellulose molecules. This occurs similarly with glycidyl groups. Such acids (H + An example of a reaction scheme between epoxy groups or glycidyl groups in the modified natural rubber latex and hydroxyl groups in the cellulose nanofibers in the presence of hydroxyl groups is as follows:

[0038] [ka]

[0039] The modified natural rubber latex used in the present invention is specifically a modified natural rubber latex having an epoxy group and a modified natural rubber latex having a glycidyl group.

[0040] [Modified natural rubber latex containing epoxy or glycidyl groups] The modified natural rubber latex containing epoxy or glycidyl groups used in this invention is a type of modified natural rubber that possesses physical properties and characteristics not found in natural rubber. For example, the introduction of epoxy or glycidyl groups increases polarity and improves oil resistance, making it suitable for applications such as hoses and shoe soles under special environments. Furthermore, because it is a naturally degradable material, it is recognized as a material with a low environmental impact.

[0041] Epoxidation of natural rubber can be carried out by introducing epoxy groups into the double bonds of cis-polyisoprene that constitutes natural rubber through an oxidation reaction using peroxide, as shown below. Known commercially available epoxidized natural rubber latex products include Horn 70 manufactured by Regtex Co., Ltd. Another method for introducing glycidyl groups is to graft polymerize glycidyl methacrylate onto natural rubber, as shown below. Commercially known glycidyl methacrylated natural rubber latexes include "EMG-50" and "EMG-51" manufactured by Resitex Co., Ltd.

[0042] [ka]

[0043] In the present invention, the modified natural rubber latex may be an epoxy group-modified natural rubber latex alone, a glycidyl group-modified natural rubber latex alone, or a mixture of the two.

[0044] [Filling material] In the present invention, from the viewpoint of improving the rubber reinforcement properties of the resulting cellulose nanofiber-added wet masterbatch, an inorganic filler such as carbon black or silica can be blended as a filler in the mixed liquid to be subjected to acid coagulation. In particular, it is preferable to add a carbon black dispersion as an inorganic filler, because the hydrophobic carbon black penetrates into the meshwork of the cellulose nanofibers to form a composite, which helps incorporate the hydrophilic cellulose nanofibers into the wet masterbatch.

[0045] Examples of carbon black that can be used include carbon blacks typically used in the rubber industry, such as SAF, ISAF, HAF, FEF, and GPF, as well as conductive carbon blacks such as acetylene black and ketjen black. The carbon black may be granulated carbon black, which is granulated in the typical rubber industry in consideration of its handleability, or ungranulated carbon black.

[0046] As the silica, for example, wet silica or dry silica can be used, and among them, it is preferable to use wet silica containing hydrated silicic acid as the main component.

[0047] Inorganic fillers such as carbon black and silica are preferably mixed as an aqueous dispersion with the cellulose nanofiber aqueous dispersion and rubber latex. Alternatively, these inorganic fillers may be mixed in advance with either the cellulose nanofiber aqueous dispersion or the rubber latex, and then mixed with the other.

[0048] When inorganic fillers such as carbon black and silica are added, the amount added is adjusted according to the purpose.

[0049] [Coagulant] In the present invention, a mixed liquid of an aqueous dispersion of cellulose nanofibers and natural rubber latex, to which the above-mentioned inorganic filler is optionally added, is acid-coagulated to obtain a rubber coagulum. The importance of adding an acid in the present invention is as described above. For acid coagulation, an acid such as formic acid or sulfuric acid can be used.

[0050] [Flocculant] In the present invention, a flocculant may be added for the purpose of controlling the state of acid coagulation (the size of the coagulated flocculated particles). As the flocculant, a cationic polymer flocculant or the like can be used.

[0051] [dehydration] In the present invention, the rubber coagulum is dehydrated by applying a shear force to it. As a means for applying the shear force, it is preferable to use a single-screw or multi-screw press, which has been conventionally used in the production of wet masterbatches.

[0052] [Drying] As the final step in producing a cellulose nanofiber-added wet masterbatch, a drying step is usually carried out to remove water, which is the dispersion medium. In the present invention, the drying method is not limited, and known methods can be used. For example, a hot air dryer, a vacuum dryer, a drum dryer, a band dryer, etc. can be used. In the case of hot air drying, the drying temperature is preferably 60°C to 100°C from the viewpoints of drying efficiency and preventing thermal degradation of the rubber. There are no restrictions on the moisture content in the cellulose nanofiber-added wet masterbatch produced by removing the dispersion medium through drying, but it is generally preferred that it be 2.0 mass % or less.

[0053] [Rubber composition] A rubber composition can be obtained by vulcanizing the cellulose nanofiber-added wet masterbatch obtained in the present invention according to a conventional method. The rubber composition obtained by the present invention has excellent mechanical properties, particularly excellent stress (elastic modulus) in the low strain region, due to the suppression of coagulation of the cellulose nanofibers in the cellulose nanofiber-added wet masterbatch that is subjected to vulcanization and the cellulose nanofibers being well dispersed in the rubber matrix. [Example]

[0054] The present invention will be described in more detail below using examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention.

[0055] [raw materials] The various raw materials used in the following Examples, Comparative Examples and Reference Examples are as follows:

[0056] <Raw materials for wet masterbatch manufacturing> Cellulose nanofiber aqueous dispersion: Exilva F01-L manufactured by Borregaard, solid content = 1.4% by mass Natural rubber latex: Resitex, solids concentration = 61.8% by mass, high ammonia type Modified natural rubber latex with epoxy groups (epoxidized natural rubber latex or epoxy-modified natural rubber latex): Horn 70 manufactured by Regitex Co., Ltd. Solid content = 34.5% by mass, epoxy-modified unit content = 64% by mass Modified natural rubber latex having glycidyl groups (glycidyl methacrylate modified natural rubber latex): EMG-51 manufactured by Resitex Co., Ltd. Solid content = 40.4% by mass, glycidyl methacrylate modified unit content = 33% by mass Carbon black N330: Mitsubishi Chemical Corporation Diablack H Formic acid: Asahi Chemical Industry Co., Ltd. Formic acid concentration = 40% Polyamine: Polyeight manufactured by Tosoh Corporation Sodium lauryl sulfate: Sodium lauryl sulfate manufactured by Kanto Chemical Co., Ltd.

[0057] <Raw materials for rubber composition manufacturing> Stearic acid: Powdered stearic acid manufactured by NOF Corporation Sulfur: Tsurumi Chemical Industry Co., Ltd. Zinc oxide: Sakai Chemical Industry Co., Ltd. Zinc oxide type 2 Vulcanization accelerator TBBS: Sancerer NS-G manufactured by Sanshin Chemical Industry Co., Ltd. Antioxidant 6PPD: Ozonone 6C manufactured by Seiko Chemical Co., Ltd.

[0058] [Production of rubber composition] <Examples 1 to 4, Comparative Example 1> The cellulose nanofiber aqueous dispersion and the carbon black aqueous dispersion were mixed so that the solid content of the resulting wet masterbatch would be the blend amounts shown in Tables 1 and 2, and then diluted with water so that the cellulose nanofiber solid content concentration was 0.55% by mass and the carbon black solid content concentration was 0.75% by mass.

[0059] The mixture was then stirred at room temperature for 5 minutes at 16,300 rpm using a Waring blender (LBC-15, manufactured by Osaka Chemical Co., Ltd.) to obtain a cellulose nanofiber and carbon black mixed slurry. Natural rubber latex or natural rubber latex and modified natural rubber latex diluted with water to 26% by mass was added to the resulting slurry so that the solids content of the resulting wet masterbatch would be the blending amounts shown in Tables 1 and 2. At the same time, 0.5 parts by mass of sodium lauryl sulfate (used as a 10% by mass aqueous solution) was added as a surfactant coagulation aid per 100 parts by mass of the rubber solids content, yielding a cellulose nanofiber, natural rubber latex, and carbon black mixed liquid.

[0060] To the above mixture, 0.5 parts by mass of polyamine (used as a 2.0 mass% aqueous solution) was added as a cationic polymer flocculant per 100 parts by mass of rubber solids, and water was further added to adjust the solids concentration of the mixture to 3.2 mass%. Subsequently, the temperature of the mixture was kept at 40°C, and formic acid was added while stirring, and the pH was adjusted to 3.5 to 4.0, followed by acid coagulation to obtain a wet masterbatch.

[0061] The wet masterbatch was then washed with water, the pH was adjusted to 4.5 to 5.0, and dehydrated using a single-screw press extruder (V-01, manufactured by Suehiro EPM Co., Ltd.) After dehydration, the wet masterbatch was dried at 70°C under reduced pressure to adjust the water content to 1.0% by mass or less.

[0062] The rubber compounding ingredients (vulcanizing agent, etc.) shown in Tables 1 and 2 were kneaded with the dried wet masterbatch using an open roll to obtain a rubber composition.

[0063] <Reference example> A rubber composition was obtained in the same manner as in Comparative Example 1, except that the dehydration step was carried out by natural filtration.

[0064] [Measurement of tensile stress] <Creating test specimens> The obtained rubber composition was heated at 145°C in a predetermined mold to produce a vulcanized rubber, and a tensile test was carried out using the test method described below. The heating time was measured using a vulcanization tester (rheometer: VR-3110) manufactured by Ueshima Seisakusho Co., Ltd., and was determined to be the time at which the torque of the rubber composition reached its maximum at 145°C.

[0065] <Tensile test> The tensile test was performed in accordance with JIS K6251 (2017). The relative value of the tensile stress for each example was calculated using an index where the value of the tensile stress at a given elongation in Example 1 was set to 100. The results are shown in Tables 1 and 2. The stress-strain curve obtained in this tensile test is shown in Figure 1. The larger the value, the higher and more favorable the tensile stress at a given elongation. In Tables 1 and 2, the "modification rate of modified natural rubber" refers to the content (mass %) of modified units in the total rubber component of the modified natural rubber latex and / or natural rubber latex used.

[0066] [Table 1]

[0067] [Table 2]

[0068] The following can be seen from Tables 1 and 2. Comparing Comparative Example 1, which underwent screw press dehydration, with Reference Example 1, which underwent natural filtration dehydration, it can be seen that screw press dehydration significantly reduced the tensile stress in a specified strain range (strain of 50 to 150%) in Comparative Example 1. This is thought to be because the shear stress applied to the wet masterbatch during screw press dehydration caused the hydrophilic cellulose nanofibers to aggregate, reducing their original performance.

[0069] On the other hand, when comparing Comparative Example 1, which did not use modified natural rubber latex, with Examples 1 to 4, which did use modified natural rubber latex, even after undergoing the same screw press dehydration, Examples 1 to 4 had higher tensile stress than Comparative Example 1. This is thought to be because the epoxy groups or glycidyl groups in the modified natural rubber reacted with the hydroxyl groups in the cellulose nanofiber molecules during the acid coagulation process, forming multi-point bonds via hydrogen bonds or covalent bonds, which suppressed aggregation of the cellulose nanofibers during dehydration shear.

Claims

1. A method for producing a wet masterbatch by acid-coagulating a mixed liquid of an aqueous dispersion of cellulose nanofibers and natural rubber latex to obtain a rubber coagulum, applying shear force to the rubber coagulum to dehydrate it, and then drying it, wherein the mixed liquid contains modified natural rubber latex having epoxy groups and / or glycidyl groups.

2. 2. The method for producing a cellulose nanofiber-added wet masterbatch according to claim 1, wherein the modified natural rubber latex is a modified natural rubber latex having an epoxy group and / or a modified natural rubber latex having a glycidyl group.

3. 3. The method for producing a cellulose nanofiber-added wet masterbatch according to claim 2, wherein the modified natural rubber latex is a modified natural rubber latex having an epoxy group.

4. 3. The method for producing a cellulose nanofiber-added wet masterbatch according to claim 2, wherein the modified natural rubber latex is a modified natural rubber latex having a glycidyl group.

5. 3. The method for producing a cellulose nanofiber-added wet masterbatch according to claim 2, wherein an epoxidized natural rubber latex is used as the modified natural rubber latex having epoxy groups.

6. 3. The method for producing a cellulose nanofiber-added wet masterbatch according to claim 2, wherein a glycidyl methacrylated natural rubber latex is used as the modified natural rubber latex having a glycidyl group.

7. The method for producing a cellulose nanofiber-added wet masterbatch according to claim 1, wherein the content of the modifying unit in the rubber component contained in the wet masterbatch is 5% by mass or more.

8. 2. The method for producing a cellulose nanofiber-added wet masterbatch according to claim 1, wherein the mixed liquid is a mixed liquid of the cellulose nanofiber aqueous dispersion, natural rubber latex, and a carbon black dispersion.

9. The method for producing a cellulose nanofiber-added wet masterbatch according to claim 1, wherein the rubber coagulum is dehydrated by applying the shear force using a single-axis or multi-axis screw press.

10. A method for producing a rubber composition, comprising producing a cellulose nanofiber-added wet masterbatch by the method for producing a cellulose nanofiber-added wet masterbatch according to any one of claims 1 to 9, and vulcanizing the resulting cellulose nanofiber-added wet masterbatch to obtain a rubber composition.

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