Rubber composition and vulcanized rubber

Abstract

To provide a chloroprene rubber composition excellent in durability.SOLUTION: A rubber composition comprises 0.01 to 0.40 pts. weight of a cellulose nanofiber relative to 100 pts. weight of the chloroprene rubber. A vulcanized sheet made of the rubber composition vulcanized has a 100% tensile stress (M100) increase rate relative to the amount of the cellulose nanofiber added of 0.9 MPa / pts. weight or more, and exhibits 5 times or more durability in comparison with those without addition of the cellulose nanofiber.SELECTED DRAWING: None

Description

Technical Field

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

Background Art

[0002] Chloroprene rubber has a good balance of various physical properties among various synthetic rubbers and is used in a wide range of applications. For example, it is used in belts, hoses, boots, air springs, wet suits, tow cloths, adhesives, etc. Chloroprene rubber includes general-purpose mercaptan-modified and sulfur-modified chloroprene with excellent dynamic properties. The latter is more durable, but due to the recent demand for higher performance, further improvement in durability is required.

[0003] Generally, durability can be improved by adjusting the blending amounts of reinforcing materials such as carbon black and silica and plasticizers, as well as the molding temperature and molding method. However, since the hardness of the target molded product is determined, there are limitations to these formulations. Also, even if durability can be improved, there is a problem that compression set and abrasion resistance deteriorate.

[0004] Therefore, fiber-shaped reinforcing materials have been proposed, and tires containing nano-order cellulose fibers have been proposed. (For example, see Patent Document 1.) However, for hydrophobic rubber, hydrophilic cellulose has poor dispersibility, resulting in a low reinforcing effect. Furthermore, the hydroxyl groups of cellulose aggregate through hydrogen bonding, impairing durability. As a countermeasure, tires have been proposed in which nano-order cellulose, a dispersant for dispersing it, and a silane coupling agent for fixing it are blended into natural rubber latex. (For example, see Patent Documents 2 and 3.) However, these methods require a separate chemical for dispersing rubber and cellulose, such as a dispersant, increasing costs. Also, a conveyor belt having excellent durability, strength, and abrasion resistance has been proposed by vulcanizing a rubber composition obtained by directly kneading and dispersing an aqueous dispersion of mechanically defibrated nano-order cellulose in an amount of 1 to 25 parts by weight based on 100 parts by weight of the rubber component together with a polymer and compounding agents. (For example, see Patent Document 4.) However, while effective for applications where the heat generation characteristics of the material during use, such as in a conveyor belt, are important, generally, when the amount of nano-order cellulose is blended in an amount of 1 to 25 parts by weight, durability and abrasion resistance decrease. Also, in the method of directly kneading an aqueous dispersion of nano-order cellulose, the hydroxyl groups of cellulose aggregate through hydrogen bonding, and excellent durability cannot be obtained. Therefore, an aqueous dispersion of nano-order cellulose is mixed in chloroprene latex in an amount of 1 to 7 parts by weight based on 100 parts by weight of the rubber component to prepare a cellulose nanofiber-dispersed rubber latex mixture, and a chloroprene rubber composition showing excellent tensile stress at low strain is proposed by vulcanizing a rubber composition obtained by kneading the polymer obtained by removing water from the cellulose nanofiber-dispersed rubber latex mixture together with compounding agents. (For example, see Patent Document 5.) However, it is generally known that durability deteriorates when the tensile stress at low strain increases, and there is an inverse relationship between the increase in tensile stress at low strain and durability.

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 Application Laid-Open No. 2020-7156 [Patent Document 5] Japanese Patent Application Laid-Open No. 2019-104896 [Summary of the Invention] [Problems to be Solved by the Invention]

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

[0007] Under such circumstances, the present inventors have earnestly studied to solve the above problems, and have found that a vulcanized product obtained by vulcanizing a rubber composition containing chloroprene rubber and cellulose nanofibers exhibits excellent durability. That is, each aspect of the present invention is as follows [1] to [5]. [1] A rubber composition containing 0.01 to 0.40 parts by weight of cellulose nanofibers with respect to 100 parts by weight of chloroprene rubber, wherein the increase in the 100% tensile stress (M100) of the vulcanized sheet obtained by vulcanizing the rubber composition is 0.9 MPa / part by weight or more with respect to the addition amount of cellulose nanofibers, and the durability is 5 times or more that of the composition without adding cellulose nanofibers. A rubber composition characterized by the above.

[0008] In addition, the vulcanized sheet used was prepared by adjusting the amount of carbon black added to the compound in order to unify the hardness (Hs) described in JIS K6253 (2012). Also, the increase in M100 was calculated by subtracting the value of M100 of the vulcanized sheet without cellulose nanofibers from the value of M100 of the vulcanized sheet containing cellulose nanofibers and then dividing by the amount of cellulose nanofibers contained. Further, the durability was evaluated according to JIS K6260 (2017) by repeatedly applying bending deformation to a test piece with a pre-cut notch and counting the number of bends when a crack starting from the notch reached 10 mm. [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 group of cellulose is 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 and is modified with sulfur. [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 characterized by being a vulcanizate of the rubber composition according to any one of [1] to [4]. [Advantages of the Invention]

[0009] By using the rubber composition of the present invention, a vulcanized rubber having excellent durability can be obtained at low cost. [Embodiments for Carrying Out the Invention]

[0010] Hereinafter, the present invention will be described in detail.

[0011] A rubber composition according to one embodiment of the present invention is a rubber composition containing 0.01 to 0.40 parts by weight of cellulose nanofibers with respect to 100 parts by weight of chloroprene rubber, wherein the increase in the 100% tensile stress (M100) of a vulcanized sheet obtained by vulcanizing the rubber composition is 0.9 MPa / part by weight or more with respect to the addition amount of cellulose nanofibers, and the durability is 5 times or more that of a composition without the addition of cellulose nanofibers.

[0012] Note that, in order to unify the hardness (Hs) described in JIS K6253 (2012), a vulcanized sheet prepared by adjusting the addition amount of carbon black in the compound was used. Also, the increase in M100 is calculated by subtracting the value of M100 of a vulcanized sheet not containing cellulose nanofibers from the value of M100 of a vulcanized sheet containing cellulose nanofibers and then dividing by the amount of cellulose nanofibers contained. Further, the durability was evaluated according to JIS K6260 (2017) by repeatedly applying bending deformation to a test piece with a notch in advance, and counting the number of bends when a crack starting from the notch reached 10 mm.

[0013] Chloroprene rubber is rubber obtained by polymerizing chloroprene or chloroprene and a monomer copolymerizable therewith. Chloroprene rubber includes mercaptan-modified chloroprene rubber and sulfur-modified chloroprene rubber having excellent dynamic properties, and the latter has characteristics of being more excellent in durability. In the present invention, durability can be improved by compounding cellulose nanofibers, but sulfur-modified chloroprene rubber, which can obtain more effects, is preferable.

[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, acrylic acid, etc. One or more of these can be used in combination, and they can be used as appropriate according to the required physical properties.

[0016] The amount of the copolymerizable monomer is not particularly limited, but generally 30 parts by weight or less is used per 100 parts by weight of chloroprene rubber so as not to impair the properties of the chloroprene polymer. Particularly with respect to sulfur, 3 parts by weight or less, more preferably 1 part by weight or less, is preferable per 100 parts by weight of the chloroprene monomer so as to achieve a good balance between heat resistance and physical properties.

[0017] Chloroprene rubber preferably contains 3 to 7% by weight of a carboxylic acid or an alkali metal salt of a carboxylic acid. By setting it at 3% by weight or more, the emulsion stability during chloroprene polymerization is excellent, and by setting it at 7% by weight or less, the processability is excellent, resulting in a rubber product with stable quality.

[0018] In the emulsion polymerization of chloroprene rubber, for example, there is a method in which the above 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 at a predetermined polymerization conversion rate to terminate the polymerization.

[0019] Examples of the emulsifier include alkali metal salts of carboxylic acids and alkali metal salts of sulfonic acids. For example, alkali metal salts of rosin acid, alkali metal salts of alkylbenzene sulfonic acid, alkali metal salts of fatty acids, alkali metal salts of alkenyl succinic acid, alkali metal salts of polycarboxylic acid, nonionic emulsifiers such as polyoxyethylene alkyl ether, and water-soluble polymer compounds can be mentioned. Examples of the alkali metal salt include lithium, sodium, potassium, cesium, etc. These may be used alone or in combination of two or more. However, from the viewpoints of polymerization stability, cohesiveness during drying, and rubber performance, it is preferable to contain an alkali metal salt of carboxylic acid, and among them, it is more preferable to contain an alkali metal salt of rosin acid, and further preferably to contain a potassium salt of rosin acid.

[0020] The amount of the emulsifier is not particularly limited. However, considering the stability of the chloroprene rubber latex obtained after polymerization, 3 to 10 parts by weight is preferable with respect to 100 parts by weight of chloroprene rubber. Among them, the alkali metal salt of carboxylic acid is preferably 3 to 8 parts by weight, and more preferably contains 5 to 7 parts by weight. In order to maintain the stability of the polymerization solution, it is preferable to adjust the pH to 11 or more with a pH adjuster. If it is less than this, the alkali metal salt of carboxylic acid is acidified, and the stability of the latex decreases. Examples of the pH adjuster include basic compounds such as sodium hydroxide, potassium hydroxide, sodium phosphate, potassium phosphate, triethylamine, diethylamine, triethanolamine, diethanolamine, ethanolamine, ammonia, etc. Any one or more of them can be used alone or in combination.

[0021] As the initiator for emulsion polymerization, known free radical substances such as peroxides such as potassium persulfate and ammonium persulfate, inorganic or organic peroxides such as hydrogen peroxide and tertiary butyl hydroperoxide can be used. Further, these may be used alone or in a redox system in combination with a reducing substance such as thiosulfate, thiosulfite, hydrosulfite, organic amine, etc.

[0022] The coincidence temperature is not particularly limited, but a range of 10 to 50 °C is preferred.

[0023] The polymerization end time is not particularly limited, but by setting the conversion rate of the monomer to 60% or more, the production amount can be ensured. Also, by setting it to 95% or less, the polymerization time does not become too long. Therefore, a range of 60 to 95% is preferred in terms of productivity.

[0024] The polymerization terminator is not particularly limited as long as it is a commonly used terminator. For example, phenothiazine, 2,6-t-butyl-4-methylphenol, hydroxylamine, etc. can be used.

[0025] In the case of sulfur-modified chloroprene copolymerized with sulfur, subsequently, a peptizer and a peptization aid can be added to the polymerized latex to perform peptization until an appropriate Mooney viscosity is obtained. Examples of the peptizer include thiuram compounds such as tetraethylthiuram disulfide, tetrabutylthiuram disulfide, and tetraoctylthiuram disulfide, and they can be added in an emulsified state using the above-mentioned emulsifier, etc. Examples of the peptization reaction initiator include thiocarbamic acid compounds such as sodium dibutyldithiocarbamate and ammonium dimethyldithiocarbamate.

[0026] The peptization temperature can be in the range of 5 to 60 °C, preferably in the range of 20 to 50 °C. Generally, the higher the peptization temperature, the faster the peptization reaction.

[0027] The Mooney viscosity at which the peptization reaction is terminated is not particularly limited as long as it satisfies the high elastic stress of the present invention, but considering the kneading workability, 20 to 80 is preferred.

[0028] In the rubber composition of the present invention, the content of cellulose nanofibers is 0.01 to 0.40 parts by weight with respect to 100 parts by weight of chloroprene rubber. By setting the amount of cellulose nanofibers to 0.01 parts by weight or more and 0.40 parts by weight or less, the effect of improving durability can be obtained.

[0029] Further, the rubber composition according to one aspect of the present invention can be obtained by mixing an aqueous dispersion of cellulose nanofibers with chloroprene latex to prepare a mixed liquid of cellulose nanofiber-dispersed rubber latex, and then removing water from the mixed liquid of cellulose nanofiber-dispersed rubber latex.

[0030] The chloroprene latex is not particularly limited as long as chloroprene rubber is emulsified and dispersed in water by an emulsifier, and examples thereof include an emulsion obtained by emulsion polymerization of an unsaturated monomer copolymerizable with chloroprene monomer, and an emulsion obtained by dissolving chloroprene rubber in an organic solvent such as toluene and then mixing with water and an emulsifier.

[0031] Cellulose nanofibers are obtained by defibrating cellulose fibers contained in wood to an average fiber diameter of several to several tens of nanometers. Although lignin is contained in wood, which is the raw material of cellulose nanofibers, cellulose nanofibers containing a large amount of lignin impair the stability of chloroprene latex. Therefore, the lignin content is preferably 20% by weight or less, preferably 10% by weight or less, and more preferably 5% by weight or less. The defibrillation treatment of cellulose is mainly by mechanical treatment, and various functional groups are imparted by chemical treatment, and mechanical treatment is used in combination to carry out the treatment to a finer single nanometer level. In the present invention, in order to improve the dispersion state of cellulose nanofibers in rubber and obtain excellent durability, it is preferable to use cellulose nanofibers having a surface tension of 60 mN / m or less in a 1% by weight aqueous dispersion of cellulose nanofibers. Such cellulose nanofibers include those defibrated only by mechanical treatment without chemical treatment and having amphiphilicity. By not subjecting cellulose nanofibers to chemical treatment and not having carboxylate and carboxylic acid, the dispersion state of cellulose nanofibers in rubber becomes good, and the durability of the obtained vulcanized rubber is improved. Therefore, it is preferable to use cellulose that does not contain carboxylate and carboxylic acid. Amphiphilicity means that cellulose nanofibers have both a hydrophilic part with a large affinity for water and a hydrophobic part with a small affinity for water. As described in Japanese Patent No. 5419120, it can be obtained by causing a water suspension sample to collide head-on at high speed. By having amphiphilicity, the affinity between hydrophobic rubber and cellulose increases, and a large improvement effect on durability can be obtained with a smaller mixing amount. Generally, the surface tension of pure water is about 72 mN / m, but the surface tension decreases as the hydrophobicity increases. If the surface tension of the aqueous dispersion of cellulose nanofibers is 60 mN / m or less at a concentration of 1% by weight, it has amphiphilicity and the affinity with rubber increases.

[0032] In the rubber composition which is one aspect of the present invention, it is preferable to use cellulose nanofibers having an average fiber diameter of 10 to 300 nm and an average fiber length of 0.3 to 200 μm obtained by mechanical treatment, and in which the hydroxymethyl group of cellulose is not modified with a carboxylic acid or carboxylate. By setting the average fiber diameter to 10 nm or more, the viscosity of the cellulose nanofiber-dispersed rubber latex can be suppressed, and the workability in the rubber manufacturing process can be maintained. Also, by setting the average fiber diameter to 300 nm or less, an increase in the viscosity of the chloroprene rubber composition containing cellulose nanofibers is suppressed, and the moldability is improved. The average fiber diameter is more preferably 10 to 100 nm. On the other hand, cellulose nanofibers having an average fiber length of 0.3 μm or more are excellent in the effect of improving the tensile stress of the vulcanized rubber containing cellulose nanofibers. By setting the average fiber length to 200 μm or less, an increase in the viscosity of the rubber composition containing cellulose nanofibers can be suppressed, and the moldability is excellent. The average fiber length is more preferably 0.5 to 100 μm. Also, since cellulose nanofibers hydrophobized by chemical modification are very expensive compared to unmodified ones, it is preferable that they are unmodified.

[0033] There is no particular limitation on the method of mixing the chloroprene rubber latex and the aqueous dispersion of cellulose nanofibers. A propeller-type stirrer, a homomixer, a high-pressure homogenizer, etc. can be used, and it can be obtained by mixing until the chloroprene latex and the aqueous dispersion of cellulose nanofibers are uniform in appearance (no lumps, etc.).

[0034] As a method for removing water (drying method) from the cellulose nanofiber-dispersed rubber latex mixture, there are heat drying, aggregation with an acid or salt, and freeze drying. However, when aggregated, an emulsifier, a coagulant, and moisture remain inside the rubber. Therefore, the method of precipitating (freezing and solidifying) the rubber by freezing, washing away excess emulsifier, etc. with water, and then drying with hot air is the most efficient and easy to dry and is preferable.

[0035] In the method of freeze-solidifying and drying from a cellulose nanofiber-dispersed rubber latex mixture, the viscosity of the cellulose nanofiber-dispersed rubber latex mixture is preferably 1500 mPa·s or less. By setting the viscosity to 1500 mPa·s or less, handling is good and preferable, and more preferably it is 1000 mPa·s or less and 30 mPa·s or more.

[0036] Also, by setting the solid content of the cellulose nanofiber-dispersed rubber latex mixture to 20% by weight or more, the strength of the rubber increases, the handling in continuous production improves, and from the balance with the viscosity, 20% by weight or more and 40% by weight or less, and more preferably 25% by weight or more and 35% by weight or less are preferable.

[0037] The rubber composition which is one aspect of the present invention is characterized in that the increase width of the 100% tensile stress (M100) of the vulcanized vulcanized sheet is 0.9 MPa / part by weight or more with respect to the addition amount of the cellulose nanofiber, and shows durability 5 times or more that of the one without adding the cellulose nanofiber.

[0038] In the chloroprene rubber vulcanizate having cellulose nanofibers having amphiphilicity, since the affinity between the hydrophobic rubber and cellulose becomes high, the increase width of the 100% tensile (M100) of the vulcanized sheet of the rubber composition increases, and a large improvement effect on durability can be obtained with a smaller mixing amount. When the increase width of the 100% tensile stress (M100) of the vulcanized sheet of the rubber composition is 0.9 MPa / part by weight or more with respect to the addition amount of the cellulose nanofiber, it indicates that the cellulose nanofiber is sufficiently compatible with the rubber. The vulcanized sheet of the rubber composition in which the rubber and the cellulose nanofiber are sufficiently compatible shows durability 5 times or more that of the one without adding the cellulose nanofiber, indicating excellent durability.

[0039] The vulcanized rubber, which is one aspect of the present invention, is obtained by compounding and kneading various compounding agents in the same manner as ordinary chloroprene rubber, and vulcanizing the above rubber composition at an appropriate vulcanization temperature until it is cured to about 90% of the maximum torque of the test material. The obtained vulcanized rubber exhibits excellent durability compared to a chloroprene rubber vulcanizate having the same hardness and not containing cellulose nanofibers, or a chloroprene rubber vulcanizate containing cellulose nanofibers having no amphiphilicity, and thus can be used in various applications. In particular, a chloroprene rubber vulcanizate having cellulose nanofibers with amphiphilicity exhibits excellent durability, excellent tensile stress at low strain, and also has the characteristic that the 100% tensile stress is greatly improved. Therefore, it is particularly suitable for anti-vibration rubbers, wipers, CVJ boots, ball joint boots, and foams, etc., which require particularly high durability.

Examples

[0040] Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited only to these examples.

[0041] <Preparation of chloroprene latex> As a monomer mixture, 0.3 parts by weight of sulfur was added to 100 parts by weight of chloroprene, and 4.0 parts by weight of the potassium salt of rosin acid, 0.5 parts by weight of the sodium salt of the condensate of naphthalenesulfonic acid and formaldehyde, 0.05 parts by weight of sodium hydroxide and 1.0 parts by weight of sodium orthophosphate, and 100 parts by weight of water were mixed and stirred in an emulsifying aqueous solution to be emulsified. To this, a polymerization catalyst composed of 1.0 part by weight of potassium persulfate, 0.01 part by weight of sodium anthraquinone-β-sulfonate, and 30 parts by weight of water was added at a constant rate by a pump to carry out polymerization. The polymerization was carried out by adding the polymerization catalyst until the polymerization conversion rate reached 70%. Here, 0.01 part by weight of thiodiphenylamine, 4-t-butylcatechol, 0.05 part by weight of 2,2'-methylenebis-4-methyl-6-t-butylphenol, 0.1 part by weight of diethylhydroxylamine, 0.05 part by weight of sodium lauryl sulfate, 5.0 parts by weight of chloroprene, and 1.0 part by weight of water were added as a polymerization terminator to terminate the polymerization. Subsequently, an emulsion of a toluene solution of 2 parts by weight of tetraethylthiuram disulfide emulsified with potassium rosin acid and 0.3 part by weight of sodium dibutyldithiocarbamate were added, and peptization was carried out at 40 °C until the Mooney viscosity reached 60, and then unreacted chloroprene was removed and recovered by steam stripping under reduced pressure to obtain a chloroprene rubber latex.

[0042] <Preparation of Chloroprene (Rubber Composition) Containing Cellulose Nanofibers> A predetermined amount of an aqueous dispersion of cellulose nanofibers was added to the chloroprene rubber latex, and after mixing at 2,000 rpm for 10 minutes with an autohomomixer (manufactured by Primix Corporation: PRIMIX), the polymer was precipitated by freeze coagulation and dried.

[0043] <Preparation of Compound> To 100 parts by weight of the chloroprene rubber containing cellulose nanofibers, carbon black (Seast SO manufactured by Tokai Carbon Co., Ltd.) in the parts by weight shown in Table 1, 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 Nippon Rika Co., Ltd.), 5 parts by weight of zinc oxide (manufactured by Sakai Chemical Co., Ltd.), 2 parts by weight of Nonflex OD-3 (manufactured by Seiko Chemical Co., Ltd.), 0.35 parts by weight of Nocrack DP (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.), and 5 parts by weight of DOA (manufactured by Daihachi Chemical Industry Co., Ltd.) were mixed in a kneader to obtain a chloroprene rubber compound containing cellulose nanofibers.

[0044] <Preparation of Vulcanizate> The obtained chloroprene rubber compound containing cellulose nanofibers was press-vulcanized at 150 °C for 30 minutes to prepare a vulcanized sheet and test pieces for durability tests. <Hardness Measurement of Vulcanizate> The hardness of the obtained vulcanized sheet was evaluated according to JIS K6253 (2012). Type A was selected for the durometer.

[0045] <Measurement of 100% Tensile Stress Increase of Vulcanizate> The 100% tensile stress (M100) of the obtained vulcanized sheet was evaluated according to JIS-K-6251 (2012 edition) under the conditions of a tensile speed of 500 mm / min and 23 °C. It was calculated by subtracting the value of M100 of the vulcanized sheet not containing cellulose nanofibers from the value of M100 of the vulcanized sheet containing the obtained cellulose nanofibers and then dividing by the amount of cellulose nanofibers contained.

[0046] <Durability Measurement of Vulcanizate> The obtained test pieces for durability tests were evaluated according to JIS K6260 (2017). Repeated bending deformation was applied to the pre-cut test pieces, and the growth rate of cracks starting from the cuts was evaluated. The values in the table are expressed as a ratio with the number of bending times when the crack reached 10 mm with the value of Comparative Example 1 set as 100, indicating that the larger the numerical value, the better the durability.

[0047] Example 1 Based on 100 parts by weight of chloroprene rubber, an aqueous dispersion of cellulose nanofibers (manufactured by Nakakoshi Pulp Co., Ltd., grade: S-1, solid content concentration: 3% by weight, average fiber diameter: about 50 nm, average fiber length: about 100 μm, lignin content 1% or less, unmodified, amphiphilic) was mixed and stirred by the above method for 10 minutes to obtain a cellulose nanofiber-dispersed rubber latex mixture. The mixing amount of the cellulose nanofibers was set to an amount such that the cellulose content was 0.05 parts by weight with respect to 100 parts by weight of the rubber component in solid form. S-1 is an amphiphilic cellulose nanofiber produced by mechanical fibrillation means, and the surface tension at 1 wt% of the aqueous dispersion of cellulose nanofibers was 55 mN / m.

[0048] The chloroprene rubber composition containing this cellulose nanofiber was used to obtain a cellulose nanofiber-containing chloroprene rubber compound and a vulcanized sheet according to the above method, and hardness and 100% tensile stress increase width measurement tests and durability tests of the vulcanizate were carried out. The results are shown in Table 1. From Table 1, the hardness was 69, the M100 increase width per 1 part by weight of cellulose nanofiber was 2.0 MPa / part by weight, and the durability was 3467, which were good results.

[0049] Example 2 The mixing amount of the cellulose nanofibers was set to an amount such that the cellulose nanofiber content was 0.2 parts by weight with respect to 100 parts by weight of the rubber component in solid form, and a cellulose nanofiber-containing chloroprene rubber compound, a vulcanized sheet, test pieces for durability tests, test pieces for abrasion tests, and test pieces for compression set tests were obtained in the same manner as in Example 1 except that the amount of carbon black was adjusted to have the same hardness as in Example 1, and hardness and durability tests were carried out. The results are shown in Table 1. The hardness was 69, the M100 increase width per 1 part by weight of cellulose nanofiber was 1.0 MPa / part by weight, and the durability was 639, which were good results.

[0050] Comparative Example 1 A chloroprene rubber compound, a vulcanized sheet, and a test piece for a durability test were obtained in the same manner as in Example 1, except that the amount of carbon black was adjusted so that the hardness was the same as in Example 1 without mixing cellulose nanofibers. A hardness measurement test, a 100% tensile stress increase measurement test of the vulcanizate, and a durability test were conducted. The results are shown in Table 1. The hardness was 69 and the durability was 100, indicating insufficient durability.

[0051] Comparative Example 2 The mixing amount of cellulose nanofibers was adjusted so that the content of cellulose nanofibers was 2.0 parts by weight based on 100 parts by weight of the rubber component in the solid part. A cellulose nanofiber-containing chloroprene rubber compound, a vulcanized sheet, a test piece for a durability test, a test piece for a wear test, and a test piece for a compression set test were obtained in the same manner as in Example 1, except that the amount of carbon black was adjusted so that the hardness was the same as in Example 1. A hardness test and a durability test were conducted. The results are shown in Table 1. The hardness was 69, the increase in M100 per 1 part by weight of cellulose nanofibers was 1.0 MPa / part by weight, and the durability was 37, indicating insufficient durability.

[0052] Comparative Example 3 The cellulose nanofibers used were those manufactured by Daicel Finechem Ltd.: KY-100G (solid content concentration: 10 wt%, average fiber diameter: about 0.01 nm to 1 μm, lignin content: 20% or less, unmodified). The mixing amount of cellulose nanofibers was adjusted so that the content of cellulose nanofibers was 2.5 parts by weight based on 100 parts by weight of the rubber component in the solid part. A cellulose nanofiber-containing chloroprene rubber compound, a vulcanized sheet, a test piece for a durability test, a test piece for a wear test, and a test piece for a compression set test were obtained in the same manner as in Example 1, except that the amount of carbon black was adjusted so that the hardness was about the same as in Example 1. KY-100G is a hydrophilic cellulose nanofiber manufactured by mechanical fibrillation means, and the surface tension at 1 wt% of the aqueous dispersion of cellulose nanofibers was 70 mN / m. The results are shown in Table 1. The hardness was 68, the increase in M100 per 1 part by weight of cellulose nanofibers was 0.7 MPa / part by weight, and the durability was 100, indicating insufficient durability.

[0053]

Table 1

Claims

**Claim 1**: A rubber composition containing 0.01 to 0.40 parts by weight of cellulose nanofibers per 100 parts by weight of chloroprene rubber, wherein the chloroprene rubber contains 3 to 7% by weight of a carboxylic acid or an alkali metal salt of a carboxylic acid and is a chloroprene rubber modified with sulfur, the cellulose nanofibers are unmodified and defibrated only by mechanical treatment, have amphiphilicity, 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 group of cellulose is not modified with a carboxylic acid or a carboxylate. The increase in the 100% tensile stress (M100) of the vulcanized sheet of the rubber composition is 0.9 MPa / part by weight or more with respect to the addition amount of the cellulose nanofibers, and the durability is 5 times or more that of the composition without the addition of cellulose nanofibers. A rubber composition characterized by the above. Note that the vulcanized sheet was prepared by adjusting the addition amount of carbon black in the compound to unify the hardness (Hs) described in JIS K6253 (2012). Also, the increase in M100 is calculated by subtracting the value of M100 of the vulcanized sheet without cellulose nanofibers from the value of M100 of the vulcanized sheet containing cellulose nanofibers and dividing by the amount of cellulose nanofibers contained. Further, the durability was evaluated according to JIS K6260 (2017) by repeatedly applying bending deformation to a test piece with a cut made in advance and counting the number of bends when the crack starting from the cut reached 10 mm. **Claim 2** A vulcanized rubber characterized by being a vulcanizate of the rubber composition according to Claim 1.

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