Rubber composition and vulcanized rubber

Abstract

To provide a chloroprene rubber composition having excellent durability.SOLUTION: Disclosed is a rubber composition in which 0.01 to 0.90 pts.wt. of cellulose nanofiber is included based on 100 pts.wt. of mercaptan-modified chloroprene rubber. The increase width of 100% tensile stress (M100) of a vulcanized sheet of the rubber composition is 0.9 MPa / pts.wt. or more based on the addition amount of cellulose nanofiber, and this composition has durability of double or more compared to that of one to which cellulose nanofiber is not added.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a rubber composition containing chloroprene rubber. [Background technology]

[0002] Chloroprene rubber has a good balance of physical properties among various synthetic rubbers and is therefore used in a wide range of applications, such as belts, hoses, boots, air springs, wetsuits, pull cloth, vibration-proof rubber, adhesives, etc. Chloroprene rubber comes in a variety of types: general-purpose mercaptan-modified, sulfur-modified chloroprene with excellent dynamic properties, and xanthogen-modified chloroprene with excellent mechanical properties, and different types are used depending on the application.

[0003] In recent years, the demand for higher performance has led to a demand for improved durability in a variety of applications. The durability of rubber materials can be evaluated using methods such as flex crack tests, flexometer tests, and tensile fatigue tests, and these can usually be improved by adjusting the amount of reinforcing materials such as carbon black and silica, the amount of plasticizer added, and the molding temperature and molding method. However, there are limitations to these formulations because the hardness of the target molded product is fixed.

[0004] To address this issue, fibrous reinforcing materials have been proposed, including tires containing nano-order cellulose fibers (see, for example, Patent Document 1). However, hydrophilic cellulose has poor dispersibility compared to hydrophobic rubber, resulting in a low reinforcing effect. Furthermore, the hydroxyl groups of cellulose aggregate through hydrogen bonding, impairing durability. To address this issue, tires have been proposed in which nano-order cellulose, a dispersant for dispersing it, and a silane coupling agent for fixing it are blended with natural rubber latex (see, for example, Patent Documents 2 and 3). However, these methods require additional chemicals, such as dispersants, to disperse the rubber and cellulose, resulting in increased costs. Another proposed conveyor belt exhibits excellent durability, strength, and abrasion resistance, achieved by directly kneading and dispersing an aqueous dispersion of mechanically defibrated nano-order cellulose with polymers and compounding ingredients (1 to 25 parts by weight of cellulose per 100 parts by weight of rubber component) and vulcanizing the resulting rubber composition (see, for example, Patent Document 4). However, while nano-order cellulose is effective in applications such as conveyor belts, where heat generation characteristics during use are important, blending nano-order cellulose in amounts of 1 to 25 parts by weight generally results in reduced durability and abrasion resistance. Furthermore, direct kneading of nano-order cellulose aqueous dispersions results in aggregation of hydroxyl groups of cellulose through hydrogen bonding, making it difficult to achieve excellent durability. Therefore, a chloroprene rubber composition has been proposed in which nano-order cellulose aqueous dispersions are mixed with chloroprene latex in an amount of 1 to 7 parts by weight of cellulose per 100 parts by weight of rubber component to produce a cellulose nanofiber-dispersed rubber latex mixture. Water is removed from the cellulose nanofiber-dispersed rubber latex mixture, and the resulting polymer is kneaded with compounding ingredients. This rubber composition is then vulcanized to produce a rubber composition exhibiting excellent tensile stress at low strain. (See, for example, Patent Document 5.) However, it is generally known that an increase in low-strain tensile stress generally leads to a decrease in durability, and an increase in low-strain tensile stress and durability are inversely related. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-206864 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-191197 [Patent Document 3] Japanese Patent Application Laid-Open No. 2009-191198 [Patent Document 4] Japanese Patent Publication No. 2020-7156 [Patent Document 5] Japanese Patent Application Publication No. 2019-104896 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in view of this problem, and an object of the present invention is to provide a mercaptan-modified chloroprene rubber composition having excellent durability. [Means for solving the problem]

[0007] The present inventors have conducted extensive research to solve the above-mentioned problems and have found that a vulcanizate obtained by vulcanizing a rubber composition containing mercaptan-modified chloroprene rubber and cellulose nanofibers exhibits excellent durability. Specifically, the present invention provides the following aspects [1] to [5]. [1] A rubber composition comprising 0.01 to 0.90 parts by weight of cellulose nanofibers per 100 parts by weight of mercaptan-modified chloroprene rubber, wherein the increase in 100% tensile stress (M100) of a vulcanized sheet obtained by vulcanizing the rubber composition is 0.9 MPa / part by weight or more relative to the amount of cellulose nanofiber added, and the rubber composition exhibits durability at least twice that of a rubber composition without added cellulose nanofibers.

[0008] The vulcanized sheets were prepared by adjusting the amount of carbon black added to the compound to achieve a uniform hardness (Hs) as specified in JIS K6253 (2012). The increase in M100 was calculated by subtracting the M100 value of the vulcanized sheet without cellulose nanofiber from the M100 value of the vulcanized sheet with cellulose nanofiber, and dividing the result by the amount of cellulose nanofiber contained. Durability was evaluated by applying tensile fatigue at a constant amplitude according to JIS K6270 (2018), and counting the number of repetitions required for the test specimen to break. [2] The rubber composition according to [1], wherein the cellulose nanofibers have an average fiber diameter of 10 to 300 nm, an average fiber length of 0.3 to 200 μm, a lignin content of 20% by weight or less, and the hydroxymethyl groups of the cellulose are not modified with a carboxylic acid or a carboxylate. [3] The rubber composition according to [1] or [2], wherein the chloroprene rubber contains 3 to 7% by weight of a carboxylic acid or an alkali metal salt of a carboxylic acid. [4] The rubber composition according to any one of [1] to [3], wherein the cellulose nanofibers are unmodified and defibrated only by mechanical treatment. [5] A vulcanized rubber which is a vulcanizate of the rubber composition according to any one of [1] to [4]. [Effects of the Invention]

[0009] By using the rubber composition of the present invention, it is possible to obtain at low cost a vulcanized rubber having excellent durability as measured by a tensile fatigue test. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention will be described in detail below.

[0011] One embodiment of the rubber composition of the present invention is a rubber composition containing 0.01 to 0.90 parts by weight of cellulose nanofibers per 100 parts by weight of mercaptan-modified chloroprene rubber, and the increase in 100% tensile stress (M100) of a vulcanized sheet obtained by vulcanizing the rubber composition is 0.9 MPa / part by weight or more relative to the amount of cellulose nanofiber added, and the rubber composition exhibits durability at least twice that of a rubber composition without added cellulose nanofibers.

[0012] The vulcanized sheets were prepared by adjusting the amount of carbon black added to the compound to achieve a uniform hardness (Hs) as specified in JIS K6253 (2012). The increase in M100 was calculated by subtracting the M100 value of the vulcanized sheet without cellulose nanofiber from the M100 value of the vulcanized sheet with cellulose nanofiber, and dividing the result by the amount of cellulose nanofiber contained. Durability was evaluated by applying tensile fatigue at a constant amplitude according to JIS K6270 (2018), and counting the number of repetitions required for the test specimen to break.

[0013] Chloroprene rubber is a rubber obtained by polymerizing chloroprene or chloroprene and a monomer copolymerizable therewith. Chloroprene rubber includes mercaptan-modified chloroprene rubber, sulfur-modified chloroprene rubber with excellent dynamic properties, and xanthogen-modified chloroprene rubber with excellent mechanical properties. Sulfur-modified chloroprene rubber is characterized by excellent resistance to flex cracking, while mercaptan-modified chloroprene rubber is characterized by superior tensile fatigue resistance. In the present invention, the durability measured by tensile fatigue tests can be improved by incorporating cellulose nanofibers, but mercaptan-modified chloroprene rubber is used, which can provide even greater effects.

[0014] Chloroprene rubber can be obtained by emulsion polymerization of chloroprene or chloroprene and a monomer copolymerizable therewith.

[0015] Examples of monomers copolymerizable with chloroprene include, in addition to sulfur, 2,3-dichloro-1,3-butadiene, 2-cyano-1,3-butadiene, 1-chloro-1,3-butadiene, 1,3-butadiene, styrene, acrylonitrile, methyl methacrylate, methacrylic acid, and acrylic acid. One or more of these can be used in combination, and are used appropriately depending on the required physical properties.

[0016] Although the amount of copolymerizable monomer is not particularly limited, it is generally 30 parts by weight or less per 100 parts by weight of chloroprene rubber so as not to impair the properties of the chloroprene polymer. In particular, for sulfur, the amount is preferably 3 parts by weight or less, more preferably 1 part by weight or less, per 100 parts by weight of chloroprene monomer to achieve a good balance between heat resistance and physical properties.

[0017] The chloroprene rubber preferably contains 3 to 7% by weight of carboxylic acid or alkali metal salt of carboxylic acid. When the content is 3% by weight or more, excellent emulsion stability is achieved during chloroprene polymerization, and when the content is 7% by weight or less, excellent processability and stable quality of the resulting rubber product can be obtained.

[0018] In emulsion polymerization of chloroprene rubber, for example, the above-mentioned monomers are mixed with an emulsifier, water, a polymerization initiator, a chain transfer agent, other stabilizers, etc., and polymerization is carried out at a predetermined temperature, and a polymerization terminator is added to terminate the polymerization at a predetermined polymerization conversion rate.

[0019] Examples of emulsifiers include alkali metal salts of carboxylic acids and alkali metal salts of sulfonic acids, such as alkali metal salts of rosin acid, alkali metal salts of alkylbenzenesulfonic acids, alkali metal salts of fatty acids, alkali metal salts of alkenylsuccinic acids, alkali metal salts of polycarboxylic acids, and nonionic emulsifiers such as polyoxyethylene alkyl ethers, as well as water-soluble polymer compounds. Examples of alkali metal salts include lithium, sodium, potassium, and cesium. These may be used alone or in combination with two or more other types. However, from the viewpoints of polymerization stability, cohesion during drying, and rubber performance, it is preferable to use an alkali metal salt of carboxylic acid, and in particular, it is preferable to use an alkali metal salt of rosin acid, and more preferably, it is preferable to use a potassium salt of rosin acid.

[0020] The amount of emulsifier is not particularly limited, but considering the stability of the chloroprene rubber latex obtained after polymerization, it is preferably 3 to 10 parts by weight per 100 parts by weight of chloroprene rubber, and the amount of alkali metal salt of carboxylic acid is preferably 3 to 8 parts by weight, more preferably 5 to 7 parts by weight.

[0021] In order to maintain the stability of the polymerization solution, it is preferable to adjust the pH to 11 or higher using a pH adjuster. Below this, the alkali metal salt of carboxylic acid becomes acidic, reducing the stability of the latex. Examples of pH adjusters include basic compounds such as sodium hydroxide, potassium hydroxide, sodium phosphate, potassium phosphate, triethylamine, diethylamine, triethanolamine, diethanolamine, ethanolamine, and ammonia, and any of these can be used alone or in combination.

[0022] As the emulsion polymerization initiator, known free radical substances can be used, for example, peroxides such as potassium persulfate and ammonium persulfate, hydrogen peroxide, inorganic or organic peroxides such as tertiary butyl hydroperoxide, etc. These may be used alone or in combination with reducing substances such as thiosulfates, thiosulfites, hydrosulfites, organic amines, etc. in a redox system.

[0023] Examples of chain transfer agents include molecular weight regulators such as alkyl mercaptans, halogenated hydrocarbons, alkyl xanthogen disulfides, alkyl xanthogen polysulfides, and sulfur, and n-dodecyl mercaptan is preferred for mercaptan modification from the viewpoint of workability. The amount of chain transfer agent is not particularly limited as long as it is an amount used in general radical polymerization for molecular weight adjustment, but it is preferably 0.01 to 1.0 wt% relative to 100 wt% of the monomer mixture other than the chain transfer agent in order to achieve the desired molecular weight and toluene-insoluble content of the resulting polymer and to achieve the desired flexibility and good mechanical properties of the vulcanized rubber obtained by dip molding.

[0024] The polymerization temperature is not particularly limited, but is preferably in the range of 10 to 50°C.

[0025] The time when the polymerization is to be completed is not particularly limited, but a monomer conversion rate of 60% or more can ensure the production amount, and a monomer conversion rate of 95% or less prevents the polymerization time from becoming too long, so a range of 60 to 95% is preferred in terms of productivity.

[0026] The polymerization terminator is not particularly limited as long as it is a commonly used terminator, and examples thereof include phenothiazine, 2,6-t-butyl-4-methylphenol, and hydroxylamine.

[0027] In the rubber composition of the present invention, the content of the cellulose nanofiber is 0.01 to 0.90 parts by weight relative to 100 parts by weight of the chloroprene rubber. By setting the amount of the cellulose nanofiber to 0.01 to 0.90 parts by weight, an improvement effect in durability can be obtained.

[0028] Furthermore, the rubber composition according to one embodiment of the present invention can be obtained by mixing an aqueous dispersion of cellulose nanofibers with chloroprene latex to prepare a cellulose nanofiber-dispersed rubber latex mixture, and then removing water from the cellulose nanofiber-dispersed rubber latex mixture.

[0029] The chloroprene latex is not particularly limited as long as it is chloroprene rubber emulsified and dispersed in water with an emulsifier, and examples thereof include an emulsion obtained by emulsion polymerization of an unsaturated monomer copolymerizable with a chloroprene monomer, and an emulsion obtained by dissolving chloroprene rubber in an organic solvent such as toluene and then mixing it with water and an emulsifier.

[0030] Cellulose nanofibers are cellulose fibers contained in wood that have been defibrated to an average fiber diameter of several nanometers to several tens of nanometers. Wood, the raw material for cellulose nanofibers, contains lignin. Because cellulose nanofibers containing a large amount of lignin impair the stability of chloroprene latex, the lignin content is preferably 20% by weight or less. It is preferably 10% by weight or less, and more preferably 5% by weight or less. Cellulose defibration can be achieved primarily by mechanical treatment, or by chemical treatment to impart various functional groups and then mechanical treatment in combination to achieve even finer single-nanometer fiber diameters. In the present invention, in order to improve the dispersion of cellulose nanofibers in rubber and achieve excellent durability, it is preferable to use cellulose nanofibers in which the surface tension of a 1% by weight aqueous dispersion of cellulose nanofibers is 60 mN / m or less. Examples of such cellulose nanofibers include cellulose nanofibers that have been defibrated solely by mechanical treatment without chemical treatment and have amphiphilic properties. By not chemically treating cellulose nanofibers and by not containing carboxylates or carboxylic acids, the cellulose nanofibers are better dispersed in the rubber, improving the durability of the resulting vulcanized rubber. Therefore, it is preferable to use cellulose that does not contain carboxylates or carboxylic acids. Amphiphilicity refers to the cellulose nanofibers possessing both hydrophilic and hydrophobic moieties, which have a high affinity for water. This can be achieved by high-speed head-on collision of aqueous suspension samples, as described in Patent Publication No. 5419120. Amphiphilicity enhances the affinity between cellulose and hydrophobic rubber, significantly improving durability with a smaller blend volume. The surface tension of pure water is typically approximately 72 mN / m, but the greater the hydrophobicity, the lower the surface tension. A cellulose nanofiber aqueous dispersion with a surface tension of 60 mN / m or less at a 1 wt% concentration is considered to be amphiphilic and have high affinity with rubber.

[0031] In the rubber composition according to one embodiment of the present invention, it is preferable to use cellulose nanofibers obtained by mechanical processing, which have an average fiber diameter of 10 to 300 nm and an average fiber length of 0.3 to 200 μm, and in which the hydroxymethyl groups of the cellulose have not been modified with a carboxylic acid or a carboxylate salt. By setting the average fiber diameter to 10 nm or more, the viscosity of the cellulose nanofiber-dispersed rubber latex can be suppressed, maintaining workability in the rubber production process. Furthermore, by setting the average fiber diameter to 300 nm or less, the viscosity increase of the cellulose nanofiber-containing chloroprene rubber composition can be suppressed, resulting in excellent molding processability. An average fiber diameter of 10 to 100 nm is more preferable. Meanwhile, cellulose nanofibers with an average fiber length of 0.3 μm or more are excellent in improving the tensile stress of cellulose nanofiber-containing rubber vulcanizates, and by setting the average fiber length to 200 μm or less, the viscosity increase of the cellulose nanofiber-containing rubber composition can be suppressed, resulting in excellent molding processability. An average fiber length of 0.5 to 100 μm is more preferable. Furthermore, cellulose nanofibers that have been hydrophobicized by chemical modification are significantly more expensive than unmodified ones, so unmodified ones are preferred.

[0032] There are no particular limitations on the method for mixing the chloroprene rubber latex and the aqueous dispersion of cellulose nanofibers, and the mixture can be obtained by using a propeller-type stirring device, a homomixer, a high-pressure homogenizer, or the like, and mixing the chloroprene latex and the aqueous dispersion of cellulose nanofibers until they appear uniform (no lumps, etc.).

[0033] Methods for removing water (drying methods) from cellulose nanofiber-dispersed rubber latex mixtures include heat drying, coagulation with acid or salt, and freeze drying. However, coagulation leaves emulsifiers, coagulation liquid, and water inside the rubber. Therefore, the most efficient and easiest method for drying is to precipitate the rubber by freezing (freeze coagulation), wash off excess emulsifiers, etc., and then dry it with hot air.

[0034] In the method of freeze-coagulating and drying a cellulose nanofiber-dispersed rubber latex mixture, the viscosity of the cellulose nanofiber-dispersed rubber latex mixture is preferably 1500 mPa s or less. A viscosity of 1500 mPa s or less is preferable because it allows for easy handling, and a viscosity of 1000 mPa s or less but 30 mPa s or more is even more preferable.

[0035] Furthermore, by making the solids content of the cellulose nanofiber-dispersed rubber latex mixture 20% by weight or more, the strength of the rubber increases and handling in continuous production improves; from the viewpoint of the balance with viscosity, it is preferable that the solids content be 20% by weight or more and 40% by weight or less, and even more preferably 25% by weight or more and 35% by weight or less.

[0036] The rubber composition of one embodiment of the present invention exhibits an increase in 100% tensile stress (M100) of a vulcanized sheet of 0.9 MPa / part by weight or more relative to the amount of cellulose nanofiber added, and exhibits durability at least twice that of a rubber sheet containing no cellulose nanofiber added.

[0037] Chloroprene rubber vulcanizates containing amphiphilic cellulose nanofibers have a high affinity between hydrophobic rubber and cellulose, resulting in a greater increase in 100% tensile stress (M100) of the vulcanized sheet obtained by vulcanizing the rubber composition, and a significant improvement in durability can be achieved with a smaller amount of addition. When the increase in 100% tensile stress (M100) of the vulcanized sheet obtained by vulcanizing the rubber composition is 0.9 MPa / part by weight or more relative to the amount of cellulose nanofiber added, this indicates that the cellulose nanofiber has sufficient affinity with the rubber. A vulcanized sheet obtained by vulcanizing a rubber composition in which the rubber and cellulose nanofiber have sufficient affinity exhibits more than twice the durability of a sheet without the addition of cellulose nanofiber, demonstrating excellent durability.

[0038] The vulcanized rubber of one embodiment of the present invention can be obtained by blending and kneading various compounding ingredients in the same manner as for ordinary chloroprene rubber, and then vulcanizing the rubber composition at an appropriate vulcanization temperature until the rubber is approximately 90% cured at the maximum torque of the test material. The resulting vulcanized rubber exhibits superior durability compared to chloroprene rubber vulcanizates of the same hardness that do not contain cellulose nanofibers or chloroprene rubber vulcanizates that contain cellulose nanofibers that do not have amphiphilic properties, making it suitable for a variety of applications. In particular, chloroprene rubber vulcanizates containing amphiphilic cellulose nanofibers exhibit excellent durability and also exhibit excellent tensile stress at low strain, with significant improvements in 100% tensile stress. This makes them suitable for applications requiring particularly high durability, such as vibration isolators, wipers, CVJ boots, ball joint boots, and foams. [Example]

[0039] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0040] <Preparation of chloroprene latex> A monomer mixture of 100 parts by weight of chloroprene and 0.2 parts by weight of mercaptan was mixed with an aqueous emulsion containing 4.0 parts by weight of potassium salt of rosin acid, 0.3 parts by weight of the sodium salt of a condensate of naphthalenesulfonic acid and formaldehyde, 0.3 parts by weight of sulfate of oleic acid, 0.2 parts by weight of sodium hydroxide, and 100 parts by weight of water. The mixture was emulsified by stirring, and a polymerization catalyst consisting of 0.1 parts by weight of potassium persulfate, 0.01 parts by weight of sodium anthraquinone-β-sulfonate, and 20 parts by weight of water was added at a constant rate via a pump to carry out polymerization. The polymerization was continued with the addition of the polymerization catalyst until a polymerization conversion rate of 70% was reached, at which point the polymerization was terminated by the addition of a polymerization terminator consisting of 0.01 parts by weight of phenothiazine, 5.0 parts by weight of chloroprene, and 0.5 parts by weight of water. Subsequently, unreacted chloroprene was removed and recovered by steam stripping under reduced pressure, yielding a mercaptan-modified chloroprene rubber latex.

[0041] <Preparation of chloroprene (rubber composition) containing cellulose nanofibers> A predetermined amount of cellulose nanofiber aqueous dispersion was added to mercaptan-modified chloroprene rubber latex and mixed at 2,000 rpm for 10 minutes using an auto homomixer (PRIMIX, manufactured by Primix Corporation). The polymer was then precipitated by freeze-coagulation and dried.

[0042] <Creating a compound> 100 parts by weight of cellulose nanofiber-containing mercaptan-modified chloroprene rubber were mixed with the weight parts shown in Table 1 of carbon black (Seast SO manufactured by Tokai Carbon Co., Ltd.), 4 parts by weight of magnesium oxide (manufactured by Kyowa Chemical Industry Co., Ltd.), 1.5 parts by weight of stearic acid (manufactured by Shin-Nihon Rika Co., Ltd.), 5 parts by weight of zinc oxide (manufactured by Sakai Chemical Industry Co., Ltd.), 2 parts by weight of Nonflex OD-3 (manufactured by Seiko Chemical Co., Ltd.), 0.35 parts by weight of Nocrac DP (manufactured by Ouchi Shinko Chemical Co., Ltd.), and 5 parts by weight of DOA (manufactured by Daihachi Chemical Industry Co., Ltd.) in a kneader mixer. After cooling, 1.0 part by weight of Sancerer #22C (manufactured by Sanshin Chemical Co., Ltd.) and 0.5 parts by weight of Noccela TT-P (manufactured by Ouchi Shinko Chemical Co., Ltd.) were mixed in an open roll mixer to obtain a cellulose nanofiber-containing chloroprene rubber compound.

[0043] <Creating vulcanized products> The obtained cellulose nanofiber-containing mercaptan-modified chloroprene rubber compound was press-vulcanized at 160°C for 15 minutes to prepare a vulcanized sheet and a test piece for durability testing. <Hardness measurement of vulcanized products> The hardness of the obtained vulcanized sheet was evaluated according to JIS K6253 (2012). Type A durometer was selected.

[0044] <Measurement of 100% tensile stress increase of vulcanized material> The 100% tensile stress (M100) of the obtained vulcanized sheet was evaluated according to JIS-K-6251 (2012 edition) at a tension speed of 500 mm / min at 23°C. The M100 value of the obtained vulcanized sheet containing cellulose nanofiber was subtracted from the M100 value of the vulcanized sheet not containing cellulose nanofiber, and the result was divided by the amount of cellulose nanofiber contained to calculate the M100 value.

[0045] <Durability measurement of vulcanized products> The obtained durability test specimens were subjected to repeated tensile deformation at a constant amplitude according to JIS K6270 (2018), and were evaluated based on the number of repetitions required for the specimen to break. The durability values ​​in Table 1 are expressed as a ratio, with the value for Comparative Example 1 being 100, and a larger value indicates better durability.

[0046] Example 1 A water dispersion of cellulose nanofibers produced by mechanical defibration (manufactured by Chuetsu Pulp Co., Ltd., grade: S-1, solid content: 3% by weight, average fiber diameter: approximately 50 nm, average fiber length: approximately 100 μm, lignin content 1% or less, unmodified, amphiphilic) was mixed with 100 parts by weight of mercaptan-modified chloroprene rubber and stirred for 10 minutes using the method described above to obtain a cellulose nanofiber-dispersed rubber latex mixture. The amount of cellulose nanofiber mixed was such that the cellulose content was 0.2 parts by weight per 100 parts by weight of the solid rubber component.

[0047] This chloroprene rubber composition containing cellulose nanofibers was used according to the above-mentioned methods to obtain a cellulose nanofiber-containing mercaptan-modified chloroprene rubber compound, a vulcanized sheet, and a durability test specimen, and hardness and 100% tensile stress increase measurement test and durability test were carried out on the vulcanized product. The results are shown in Table 1. Table 1 shows that the hardness was 70, the increase in M100 per part by weight of cellulose nanofiber was 1.0 MPa / part by weight, and the durability was 248, which were good results.

[0048] Example 2 A cellulose nanofiber-containing mercaptan-modified chloroprene rubber compound, vulcanized sheet, and durability test specimen were obtained in the same manner as in Example 1, except that the amount of cellulose nanofiber mixed was adjusted so that the cellulose nanofiber content was 0.4 parts by weight per 100 parts by weight of the solid rubber component, and the amount of carbon black was adjusted to obtain the same hardness as in Example 1. Hardness, 100% tensile stress increase measurement test of the vulcanized product, and durability test were performed. The results are shown in Table 1. The hardness was 70, the increase in M100 per part by weight of cellulose nanofiber was 1.8 MPa / part by weight, and the durability was 218, all of which were good results.

[0049] Example 3 A cellulose nanofiber-containing mercaptan-modified chloroprene rubber compound, vulcanized sheet, and durability test specimen were obtained in the same manner as in Example 1, except that the amount of cellulose nanofiber mixed was adjusted so that the cellulose nanofiber content was 0.8 parts by weight per 100 parts by weight of the solid rubber component, and the amount of carbon black was adjusted to obtain the same hardness as in Example 1. Hardness and 100% tensile stress increase measurement tests and durability tests of the vulcanized material were conducted. The results are shown in Table 1. The hardness was 71, the increase in M100 per part by weight of cellulose nanofiber was 1.1 MPa / part by weight, and the durability was 275, all of which were good results.

[0050] Comparative Example 1 A mercaptan-modified chloroprene rubber compound, vulcanized sheet, and durability test specimen were obtained and subjected to hardness and durability tests in the same manner as in Example 1, except that cellulose nanofibers were not mixed and the amount of carbon black was adjusted to obtain the same hardness as in Example 1. The results are shown in Table 1. The hardness was 70 and the durability was 100, indicating insufficient durability.

[0051] Comparative Example 2 A cellulose nanofiber-containing mercaptan-modified chloroprene rubber compound, vulcanized sheet, and durability test specimen were obtained in the same manner as in Example 1, except that the amount of cellulose nanofiber mixed was adjusted so that the cellulose nanofiber content was 2.0 parts by weight per 100 parts by weight of the solid rubber component, and the amount of carbon black was adjusted to obtain the same hardness as in Example 1. Hardness and 100% tensile stress increase measurement tests and durability tests of the vulcanized product were conducted. The results are shown in Table 1. The hardness was 71, the increase in M100 per part by weight of cellulose nanofiber was 1.0 MPa / part by weight, and the durability was 112. Although durability improved slightly, the improvement effect was insufficient.

[0052] Comparative Example 3 A sulfur-modified chloroprene rubber compound was prepared in the same manner as in Comparative Example 1, except that a sulfur-modified chloroprene rubber was used instead of the mercaptan-modified chloroprene rubber, and 1.0 part by weight of Sancerer #22C (manufactured by Sanshin Chemical Co., Ltd.) and 0.5 part by weight of Noccela TT-P (manufactured by Ouchi Shinko Chemical Co., Ltd.) were not mixed in the open roll after mixing the masterbatch. The obtained sulfur-modified chloroprene rubber compound was press-vulcanized at 150°C for 30 minutes to prepare a vulcanized sheet and a test piece for durability testing. Hardness and durability tests were conducted in the same manner as in Example 1. The results are shown in Table 1. The hardness was 71 and the durability was 42, indicating that the durability was inferior to that of the mercaptan-modified chloroprene rubber.

[0053] Comparative Example 4 A cellulose nanofiber-containing sulfur-modified chloroprene rubber compound, vulcanized sheet, and durability test specimen were obtained in the same manner as in Comparative Example 3, except that the amount of cellulose nanofiber mixed was adjusted so that the cellulose nanofiber content was 0.2 parts by weight per 100 parts by weight of the solid rubber component, and the amount of carbon black was adjusted to obtain the same hardness as in Comparative Example 3. Hardness and 100% tensile stress increase measurement tests and durability tests of the vulcanized product were conducted. The results are shown in Table 1. The hardness was 70, the increase in M100 per part by weight of cellulose nanofiber was 0.5 MPa / part by weight, and the durability was 46. Although the durability was slightly improved from Comparative Example 3, the improvement effect was insufficient.

[0054]

Table 1

Claims

1. A rubber composition comprising 0.01 to 0.8 parts by weight of cellulose nanofibers per 100 parts by weight of mercaptan-modified chloroprene rubber, wherein the increase in 100% tensile stress (M100) of a vulcanized sheet obtained by vulcanizing the rubber composition is 0.9 MPa / part by weight or more relative to the amount of cellulose nanofibers added, and the rubber composition exhibits durability at least twice that of a rubber composition to which no cellulose nanofibers are added. The vulcanized sheets were prepared by adjusting the amount of carbon black added to the compound to standardize the hardness (Hs) as specified in JIS K6253 (2012). The increase in M100 was calculated by subtracting the M100 value of the vulcanized sheet containing no cellulose nanofiber from the M100 value of the vulcanized sheet containing cellulose nanofiber, and dividing the result by the amount of cellulose nanofiber contained. Durability was evaluated according to JIS K6270 (2018), where a constant amplitude tensile fatigue test was applied to the test specimen, and the number of repetitions required for the specimen to break was counted.

2. The rubber composition according to claim 1, wherein the cellulose nanofibers have an average fiber diameter of 10 to 300 nm, an average fiber length of 0.3 to 200 μm, a lignin content of 20% by weight or less, and a hydroxymethyl group of the cellulose is not modified with a carboxylic acid or a carboxylate.

3. 3. The rubber composition according to claim 1, wherein the chloroprene rubber contains 3 to 7% by weight of a carboxylic acid or an alkali metal salt of a carboxylic acid.

4. The rubber composition according to any one of claims 1 to 3, wherein the cellulose nanofibers are unmodified and defibrated only by mechanical treatment.

5. A vulcanized rubber which is a vulcanizate of the rubber composition according to any one of claims 1 to 4.

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