Rubber composition, vulcanizate, molded body, and transmission belt

Abstract

A rubber composition comprising a chloroprene rubber that contains a sulfur-modified chloroprene rubber, the rubber composition having a heat of fusion ΔH, as obtained by a predetermined method, of 8.50 J / g or less.

Description

Rubber composition, vulcanizate, molded article, and transmission belt

[0001] The present invention relates to a rubber composition, a vulcanizate, a molded article, and a transmission belt.

[0002] Chloroprene rubber is excellent in mechanical strength, chemical resistance, heat resistance, cold resistance, oil resistance, etc., and is therefore used as a material for industrial transmission belts, vibration-proof rubber, hoses, wipers, seal parts, etc.

[0003] In Patent Document 1, with the problem of providing a chloroprene-based rubber composition capable of producing a vulcanized rubber that achieves both low-temperature characteristics and mechanical strength, when the total amount of the rubber component is 100 parts by weight, it contains 50 parts by weight or more of chloroprene rubber, and also contains 5 parts by weight or more of a saturated aliphatic carboxylic acid represented by a predetermined formula and having a molecular weight of 300 or more. A rubber composition is described.

[0004] Japanese Patent Application Laid-Open No. 2013-249409

[0005] The present invention provides a rubber composition with improved cold resistance after vulcanization.

[0006] The inventors of the present invention have found that in a rubber composition containing chloroprene rubber containing sulfur-modified chloroprene rubber, by setting the heat of fusion ΔH obtained by a predetermined method within a predetermined range, the cold resistance of the rubber composition after vulcanization can be improved, and the present invention has been completed.

[0007] That is, according to the present invention, the following rubber composition, vulcanizate, molded article, and transmission belt are provided.

[0008] [1] A rubber composition comprising chloroprene rubber containing sulfur-modified chloroprene rubber, wherein the heat of fusion ΔH obtained by the following method is 8.50 J / g or less. (Method) A vulcanized sheet is prepared by press vulcanizing the rubber composition in accordance with JIS K 6299:2012 under conditions of vulcanization temperature of 170°C, pressure of 10 MPa, and vulcanization time of 10 minutes. A test piece with a mass of 8 mg is cut out from the vulcanized sheet, and differential scanning calorimetry is performed in accordance with JIS K 7122:2012. Pretreatment is performed under conditions of heating from 25°C to 80°C at a heating rate of 10°C / min in a nitrogen atmosphere and holding for 10 minutes, cooling to 0°C at a cooling rate of 10°C / min and holding for 24 hours, and cooling to -60°C at a cooling rate of 10°C / min. Next, the measurement is performed under the conditions of holding at -60°C for 5 minutes and raising the temperature to 80°C at a heating rate of 10°C / min. The heat of fusion ΔH is determined from the DSC curve obtained in the above measurement. [2] The rubber composition according to [1], further comprising carbon black. [3] The rubber composition according to [2], wherein the amount of iodine adsorbed by the carbon black, as measured in accordance with JIS K 6217-1:2018, is 15 mg / g or more and 130 mg / g or less. [4] The rubber composition according to [2] or [3], wherein the carbon black content per 100 parts by mass of the chloroprene rubber is 15 parts by mass or more and 80 parts by mass or less. [5] The rubber composition according to any one of [1] to [4], further comprising an antioxidant. [6] The rubber composition according to [5], wherein the antioxidant comprises an amine-based antioxidant. [7] The rubber composition according to [5] or [6], wherein the content of the antioxidant per 100 parts by mass of the chloroprene rubber is 0.2 parts by mass or more and 8 parts by mass or less. [8] The rubber composition according to any one of [1] to [7], wherein the CS obtained by the following method is less than 51%. (Method) The rubber composition is press-vulcanized in accordance with JIS K 6299:2012 under conditions of vulcanization temperature of 170°C, pressure of 10 MPa, and vulcanization time of 15 minutes to produce a large test piece (cylindrical, diameter 29.0 ± 0.5 mm, thickness 12.5 ± 0.5 mm). Then, in accordance with JIS K 6262:2013, a compression set test is performed under conditions of compression ratio: 25%, test temperature: 0°C, and test time: 72 hours to determine the compression set CS.[9] A vulcanized rubber composition according to any one of [1] to [8].

[10] A molded article containing the vulcanized article according to [9].

[11] A power transmission belt containing the molded article according to

[10] .

[0009] According to the present invention, it is possible to provide a rubber composition with improved cold resistance after vulcanization.

[0010] Embodiments of the present invention will be described below. In this specification, unless otherwise specified, "A to B" indicating a numerical range means A or greater and B or less.

[0011] (Rubber Composition) The rubber composition of this embodiment contains chloroprene rubber containing sulfur-modified chloroprene rubber. The rubber composition of this embodiment has a heat of fusion ΔH of 8.50 J / g or less obtained by the following method.

[0012] (Method) A vulcanized sheet is prepared by press vulcanizing a rubber composition at a vulcanization temperature of 170°C, a pressure of 10 MPa, and a vulcanization time of 10 minutes, in accordance with JIS K 6299:2012. A test piece with a mass of 8 mg is cut from the vulcanized sheet, and differential scanning calorimetry is performed in accordance with JIS K 7122:2012. Pretreatment is performed under the following conditions: heating from 25°C to 80°C at a heating rate of 10°C / min in a nitrogen atmosphere and holding for 10 minutes, cooling to 0°C at a cooling rate of 10°C / min and holding for 24 hours, and cooling to -60°C at a cooling rate of 10°C / min. Then, the measurement is performed under the conditions of holding at -60°C for 5 minutes and heating to 80°C at a heating rate of 10°C / min. The heat of fusion ΔH is determined from the DSC curve obtained from the measurement.

[0013] In this embodiment, ΔH is 8.50 J / g or less, preferably 8.20 J / g or less, more preferably 8.00 J / g or less, even more preferably 7.80 J / g or less, even more preferably 7.60 J / g or less, even more preferably 7.50 J / g or less, and even more preferably 7.40 J / g or less. ΔH may be 0.10 J / g or more, 0.50 J / g or more, 1.00 J / g or more, 2.00 J / g or more, 3.00 J / g or more, or 3.50 J / g or more. That is, ΔH is 8.50 J / g or less, preferably 0.10 J / g or more and 8.20 J / g or less, more preferably 0.50 J / g or more and 8.00 J / g or less, even more preferably 1.00 J / g or more and 7.80 J / g or less, even more preferably 2.00 J / g or more and 7.60 J / g or less, even more preferably 3.00 J / g or more and 7.50 J / g or less, and even more preferably 3.50 J / g or more and 7.40 J / g or less.

[0014] Our investigations have revealed that by keeping ΔH below the above upper limit, the cold resistance of the rubber composition after vulcanization can be improved.

[0015] Although the mechanism by which such effects are obtained is not entirely clear, the inventors' research suggests the following: Chloroprene rubber loses its rubber elasticity as it hardens due to crystallization at low temperatures. ΔH is an index indicating the proportion of the crystalline phase formed at low temperatures. By keeping ΔH below the above upper limit, crystallization of chloroprene rubber at low temperatures can be suppressed, and the cold resistance of the rubber composition after vulcanization can be improved.

[0016] In this embodiment, by using chloroprene rubber with a relatively slow crystallization rate as the chloroprene rubber contained in the rubber composition, ΔH tends to be reduced.

[0017] (Chloroprene rubber) The chloroprene rubber of this embodiment contains sulfur-modified chloroprene rubber. Sulfur-modified chloroprene rubber is, for example, chloroprene rubber having an end group represented by the following general formula (1): -S ｘ ~S-C(=S)-NR ２(1) In general formula (1), R independently represents an alkyl group or an aryl group, and x represents an integer of 1 or more.

[0018] The sulfur bonds in sulfur-modified chloroprene rubber have relatively low bond energy, and when these sulfur bonds are broken, the viscosity of the rubber composition decreases. By including sulfur-modified chloroprene rubber in the chloroprene rubber mixture, the viscosity of the rubber composition decreases more easily during mixing, improving the processability of the rubber composition.

[0019] In general formula (1), R independently represents either an alkyl group or an aryl group. Preferably, R contains an alkyl group having 1 to 10 carbon atoms or an aryl group having 6 to 15 carbon atoms. This improves the low exothermic properties of the rubber composition after vulcanization.

[0020] The Mooney viscosity ML(1+4) 100°C of chloroprene rubber, measured in accordance with JIS K 6300-1:2013, is preferably 10 to 90, more preferably 20 to 80, even more preferably 30 to 70, even more preferably 35 to 60, and even more preferably 40 to 55. By setting the Mooney viscosity ML(1+4) 100°C of chloroprene rubber within the above range, the processability of the rubber composition can be improved.

[0021] (Carbon Black) The rubber composition of this embodiment preferably further contains carbon black. This improves the mechanical properties of the rubber composition after vulcanization, such as weather resistance, abrasion resistance, and dynamic fatigue resistance.

[0022] The amount of iodine adsorbed by carbon black, as measured in accordance with JIS K 6217-1:2018, is preferably 15 mg / g to 130 mg / g, more preferably 20 mg / g to 100 mg / g, and even more preferably 30 mg / g to 90 mg / g. By setting the amount of iodine adsorbed by carbon black within the above range, the balance of performance characteristics such as low heat generation, abrasion resistance, scorch resistance, and tensile strength of the rubber composition after vulcanization can be improved.

[0023] In accordance with JIS K 6217-6:2019, the average particle size of carbon black, calculated from the mass distribution density curve of the aggregate mass distribution, is preferably 20 nm to 60 nm, more preferably 22 nm to 56 nm, even more preferably 24 nm to 52 nm, and still more preferably 26 nm to 48 nm. By setting the average particle size of carbon black within the above range, the balance of performance characteristics such as low heat generation, abrasion resistance, scorch resistance, and tensile strength of the rubber composition after vulcanization can be improved.

[0024] The amount of DBP (dibutyl phthalate) absorbed by carbon black, as measured in accordance with JIS K 6217-4:2017, is preferably 80 mL / 100 g to 150 mL / 100 g, more preferably 90 mL / 100 g to 130 mL / 100 g, and even more preferably 100 mL / 100 g to 120 mL / 100 g. By setting the DBP absorption amount of carbon black within the above range, the balance of performance such as low heat generation, abrasion resistance, and scorch resistance of the rubber composition after vulcanization can be improved.

[0025] The carbon black preferably includes at least one selected from the group consisting of FEF carbon black and HAF carbon black. This improves the mechanical properties of the rubber composition after vulcanization, such as weather resistance, abrasion resistance, and dynamic fatigue resistance. FEF carbon black refers to carbon black designated as N550 in ASTM D 1765. HAF carbon black refers to carbon black designated as N330 in ASTM D 1765.

[0026] The carbon black content per 100 parts by mass of chloroprene rubber is preferably 15 parts by mass or more and 80 parts by mass or less, more preferably 20 parts by mass or more and 70 parts by mass or less, even more preferably 25 parts by mass or more and 60 parts by mass or less, and even more preferably 30 parts by mass or more and 55 parts by mass or less. By setting the carbon black content per 100 parts by mass of chloroprene rubber within the above range, the mechanical properties of the rubber composition after vulcanization, such as weather resistance, abrasion resistance, and dynamic fatigue resistance, can be improved.

[0027] (Fillers) The rubber composition of this embodiment may contain fillers other than carbon black, such as silica, clay, talc, and calcium carbonate, and may also contain silica.

[0028] (Anti-aging agent) The rubber composition of this embodiment preferably further contains an anti-aging agent. This suppresses the deterioration of the rubber composition after vulcanization.

[0029] The anti-aging agent may include, for example, amine-based anti-aging agents such as 4,4'-bis(α,α-dimethylbenzyl)diphenylamine, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, p-(p-toluenesulfonylamide)diphenylamine, and N-isopropyl-N'-p-phenylenediamine; phenol-based anti-aging agents such as 2,2'-methylenebis(4-methyl-6-tert-butylphenol) and 2,2'-methylenebis(4-ethyl-6-tert-butylphenol); sulfur-based anti-aging agents such as 2-mercaptobenzimidazole and 2-mercaptomethylbenzimidazole; and phosphorus-based anti-aging agents such as tris(nonylphenyl)phosphite.

[0030] The antioxidant preferably comprises at least one selected from the group consisting of amine-based antioxidants and sulfur-based antioxidants, more preferably comprising an amine-based antioxidant, and even more preferably comprising at least one selected from the group consisting of 4,4'-bis(α,α-dimethylbenzyl)diphenylamine and N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, and even more preferably comprising 4,4'-bis(α,α-dimethylbenzyl)diphenylamine. This suppresses deterioration of the rubber composition after vulcanization.

[0031] Anti-aging agents do not need to contain sulfur-based anti-aging agents.

[0032] The content of the antioxidant per 100 parts by mass of chloroprene rubber is preferably 0.2 parts by mass or more and 8 parts by mass or less, more preferably 0.4 parts by mass or more and 6 parts by mass or less, even more preferably 0.6 parts by mass or more and 4 parts by mass or less, even more preferably 0.8 parts by mass or more and 3 parts by mass or less, and even more preferably 1 part by mass or more and 2 parts by mass or less. By setting the content of the antioxidant per 100 parts by mass of chloroprene rubber within the above range, it is possible to improve the mechanical properties while suppressing deterioration of the rubber composition after vulcanization.

[0033] The content of the amine-based antioxidant per 100 parts by mass of chloroprene rubber is preferably 0.2 parts by mass or more and 8 parts by mass or less, more preferably 0.4 parts by mass or more and 6 parts by mass or less, even more preferably 0.6 parts by mass or more and 4 parts by mass or less, even more preferably 0.8 parts by mass or more and 3 parts by mass or less, and even more preferably 1 part by mass or more and 2 parts by mass or less. By setting the content of the amine-based antioxidant per 100 parts by mass of chloroprene rubber within the above range, it is possible to improve the mechanical properties while suppressing deterioration of the rubber composition after vulcanization.

[0034] (Vulcanizing agent and vulcanization accelerator) The rubber composition of this embodiment preferably further comprises a vulcanizing agent. This allows the rubber composition to be properly vulcanized.

[0035] The vulcanizing agent may, for example, contain a metal oxide, and may contain at least one selected from the group consisting of zinc oxide, magnesium oxide, lead oxide, trilead tetroxide, iron oxide, titanium dioxide, calcium oxide, and hydrotalcite, or it may contain at least one selected from the group consisting of zinc oxide and magnesium oxide.

[0036] The amount of vulcanizing agent per 100 parts by mass of chloroprene rubber is preferably 1 part by mass or more and 20 parts by mass or less, more preferably 2 parts by mass or more and 15 parts by mass or less, and even more preferably 3 parts by mass or more and 10 parts by mass or less. By setting the amount of vulcanizing agent per 100 parts by mass of chloroprene rubber within the above range, the rubber composition can be appropriately vulcanized, and the mechanical properties of the rubber composition after vulcanization can be improved.

[0037] The rubber composition of this embodiment may not contain a vulcanization accelerator such as a thiourea-based vulcanization accelerator, a thiazole-based vulcanization accelerator, a thiuram-based vulcanization accelerator, and a guanidine-based vulcanization accelerator. Since sulfur-modified chloroprene rubber easily crosslinks, there are cases where the rubber composition can be appropriately vulcanized even without a vulcanization accelerator.

[0038] (Processing Aid) The rubber composition of this embodiment preferably contains a processing aid. Thereby, the processability of the rubber composition can be improved.

[0039] The processing aid preferably contains at least one selected from the group consisting of fatty acids such as stearic acid, fatty acid metal salts such as zinc stearate, and fatty acid amides such as stearic acid amide, and more preferably contains at least one selected from the group consisting of fatty acids and fatty acid metal salts. Thereby, the processability of the rubber composition can be improved.

[0040] When the vulcanizing agent contains a metal oxide, the processing aid preferably contains a fatty acid, and more preferably contains stearic acid. Thereby, the processability of the rubber composition can be improved.

[0041] The content of the processing aid with respect to 100 parts by mass of chloroprene rubber is preferably 0.1 part by mass or more and 5 parts by mass or less, more preferably 0.2 part by mass or more and 4 parts by mass or less, and still more preferably 0.3 part by mass or more and 3 parts by mass or less. By setting the content of the processing aid with respect to 100 parts by mass of chloroprene rubber within the above range, the processability of the rubber composition and the balance of the mechanical properties of the rubber composition after vulcanization can be improved.

[0042] (Plasticizer) The rubber composition of this embodiment preferably further contains a plasticizer. Thereby, the processability of the rubber composition can be improved.

[0043] The plasticizer may contain, for example, at least one selected from the group consisting of ester-based plasticizers such as aliphatic dibasic acid-based plasticizers and epoxy-based plasticizers; and process oils such as aromatic process oils, naphthenic process oils, and paraffinic process oils.

[0044] The plasticizer may contain an aliphatic dibasic acid-based plasticizer, and may contain at least one selected from the group consisting of dioctyl sebacate, dioctyl azelate, dioctyl adipate, diisononyl adipate, and diisodecyl adipate, and may contain dioctyl sebacate.

[0045] The plasticizer may contain a process oil, and may contain at least one selected from the group consisting of aromatic process oils and naphthenic process oils.

[0046] The content of the plasticizer relative to 100 parts by mass of chloroprene rubber is preferably 3 parts by mass or more and 30 parts by mass or less, more preferably 4 parts by mass or more and 25 parts by mass or less, still more preferably 5 parts by mass or more and 20 parts by mass or less. By setting the content of the plasticizer relative to 100 parts by mass of chloroprene rubber within the above range, the processability of the rubber composition and the balance of the mechanical properties after vulcanization of the rubber composition can be improved.

[0047] (Physical properties of the rubber composition) The CS obtained by the following method may be less than 51%, may be 10% or more and 50% or less, may be 15% or more and 45% or less, may be 20% or more and 40% or less, or may be 25% or more and 35% or less.

[0048] (Method) In accordance with JIS K 6299:2012, the rubber composition is press-vulcanized under the conditions of a vulcanization temperature of 170 ° C, a pressure of 10 MPa, and a vulcanization time of 15 minutes to produce a large test piece (cylindrical, diameter 29.0 ± 0.5 mm, thickness 12.5 ± 0.5 mm). Next, in accordance with JIS K 6262:2013, a compression set test is performed under the conditions of a compression ratio: 25%, a test temperature: 0 ° C, and a test time: 72 hours, and the compression set CS is determined.

[0049] (Use of the rubber composition) The rubber composition of this embodiment is preferably used for a transmission belt.

[0050] (Method for producing the rubber composition) The rubber composition of this embodiment can be obtained, for example, by kneading the above-mentioned respective components using a kneading device such as a Banbury mixer.

[0051] According to the inventors' studies, the following mixing conditions make it easier to obtain a rubber composition with a small ΔH.

[0052] The method for producing the rubber composition of this embodiment preferably includes a first kneading step of kneading chloroprene rubber, and a second kneading step of kneading the kneaded chloroprene rubber with raw materials other than chloroprene rubber.

[0053] In the first kneading step, it is preferable to use a Banbury mixer as the kneading device. The kneading temperature in the first kneading step is preferably 30°C to 120°C, more preferably 40°C to 100°C, and even more preferably 50°C to 90°C. The kneading speed in the first kneading step is preferably 40 rpm to 60 rpm, more preferably 45 rpm to 55 rpm. The kneading time in the first kneading step is preferably 30 seconds to 120 seconds, more preferably 45 seconds to 90 seconds.

[0054] In the second kneading step, it is preferable to use a Banbury mixer as the kneading device. The kneading temperature in the second kneading step is preferably 80°C to 160°C, more preferably 90°C to 140°C, and even more preferably 100°C to 130°C. The kneading speed in the second kneading step is preferably 40 rpm to 60 rpm, more preferably 45 rpm to 55 rpm. The kneading time in the second kneading step is preferably 120 seconds to 420 seconds, more preferably 180 seconds to 300 seconds.

[0055] (Vulcanized product) The vulcanized product of this embodiment is a vulcanized product of the rubber composition of this embodiment. The vulcanized product of this embodiment is obtained by vulcanizing the rubber composition of this embodiment. The vulcanization temperature may be 140°C or more and 220°C or less, or 150°C or more and 180°C or less. The pressure during vulcanization may be 3.5 MPa or more and 20 MPa or 5 MPa or more and 15 MPa or less. The vulcanization time may be 1 minute or more and 60 minutes or 3 minutes or more and 30 minutes or less.

[0056] (Molded body and transmission belt) The molded body of this embodiment includes the vulcanized product of this embodiment. The transmission belt of this embodiment includes the molded body of this embodiment.

[0057] Although embodiments of the present invention have been described above, these are merely examples, and various other configurations can be adopted. Furthermore, the present invention is not limited to the embodiments described above, and modifications, improvements, etc., within the scope that can achieve the objectives of the present invention are included in the present invention.

[0058] The present invention will be described in detail below with reference to the examples. However, the present invention is not limited in any way to the descriptions in these examples.

[0059] (1) Preparation of rubber composition (Examples 1-4 and Comparative Example 1) Using a 1.7 L Banbury mixer (MS-1.7 Internal Mixer, manufactured by Minami Senju Seisakusho Co., Ltd.), chloroprene rubber was kneaded for 1 minute at 30°C under water cooling and 50 rpm. Then, each raw material other than chloroprene rubber was added and kneaded for 3 minutes. After that, the tank was cleaned and kneaded for 1 minute. The discharge temperature of the kneaded material in each example and comparative example is shown in Table 1. Next, using an 8-inch open roll, the mixture was kneaded at 30°C under water cooling and cut 6 times (3 times on each side) to obtain the rubber composition. The amount of each raw material added is shown in Table 1.

[0060] Details of each ingredient are shown below.

[0061] (Chloroprene rubber (CR)) Sulfur-modified CR1: Mooney viscosity 45, slow crystallization rate Sulfur-modified CR2: Mooney viscosity 52, slow crystallization rate Sulfur-modified CR3: Mooney viscosity 43, slow crystallization rate Sulfur-modified CR4: Mooney viscosity 51, moderate crystallization rate

[0062] Mooney viscosity is defined as Mooney viscosity ML(1+4) at 100°C, measured in accordance with JIS K 6300-1:2013.

[0063] Furthermore, the crystallization rate of chloroprene rubber was evaluated as follows. First, the polymer density of chloroprene rubber was measured in accordance with Method A specified in JIS K 6268:1998 to identify the crystalline and amorphous regions. Then, the ratio of the mass of the crystalline region to the total mass of the chloroprene rubber was determined as the degree of crystallinity (%) of the chloroprene rubber. Next, based on the obtained degree of crystallinity value, the crystallization rate was evaluated in four stages: "fast," "moderate," "slow," and "very slow." It should be noted that there is a correlation between the degree of crystallinity and the crystallization rate; the higher the degree of crystallinity, the faster the crystallization rate.

[0064] (Other raw materials) FEF carbon black: Asahi Carbon Co., Ltd. "Asahi #60UG", average particle size 43 nm, iodine adsorption capacity 40 mg / g, DBP absorption capacity 110 mL / 100 g HAF carbon black: Asahi Carbon Co., Ltd. "Asahi #70G", average particle size 28 nm, iodine adsorption capacity 80 mg / g, DBP absorption capacity 101 mL / 100 g Anti-aging agent: 4,4'-bis(α,α-dimethylbenzyl)diphenylamine, Ouchi Shinko Chemical Industry Co., Ltd. "Nocrac CD" Aromatic process oil: Fuji Kosan Co., Ltd. "Aromax 3" Naphthenic process oil: Idemitsu Kosan Co., Ltd. "Diana Process Oil NP-24" Dioctyl sebacate (DOS): Shin Nippon Rika Co., Ltd. "Sensor DOS" Zinc oxide: Sakai Chemical Industry Co., Ltd. "Zinc Oxide Type 2" Magnesium oxide: Kyowa Chemical Industry Co., Ltd. "Kyowa Mag 150" Stearic acid: Kao Corporation "Lunaq S-70V"

[0065] The average particle size of carbon black is calculated from the mass distribution density curve of the aggregate mass distribution in accordance with JIS K 6217-6:2019. The amount of iodine adsorbed by carbon black is measured in accordance with JIS K 6217-1:2018. The amount of DBP absorbed by carbon black is measured in accordance with JIS K 6217-4:2017.

[0066] (2) Evaluation of the rubber composition (Differential scanning calorimetry) In accordance with JIS K 6299:2012, 70 g of the rubber composition was press-vulcanized using a heating and cooling press molding machine (KMI Co., Ltd. "100 TON 500 square 3-stage compression molding machine") at a vulcanization temperature of 170°C, a pressure of 10 MPa, and a vulcanization time of 10 minutes to produce a vulcanized sheet. A test piece with a mass of 8 mg was cut from the vulcanized sheet, and differential scanning calorimetry was performed using a differential scanning calorimetry device (PerkinElmer Corporation "Diamond DSC") in accordance with JIS K 7122:2012. Pretreatment was performed under the following conditions: under a nitrogen atmosphere, the temperature was raised from 25°C to 80°C at a heating rate of 10°C / min and held for 10 minutes, then cooled to 0°C at a cooling rate of 10°C / min and held for 24 hours, and finally cooled to -60°C at a cooling rate of 10°C / min. Subsequently, measurements were taken under the following conditions: held at -60°C for 5 minutes, and then heated to 80°C at a heating rate of 10°C / min. The heat of fusion ΔH was determined from the DSC curve obtained from the measurements.

[0067] (Compression Set Test) In accordance with JIS K 6299:2012, 14 g of rubber composition was press-vulcanized using a heating and cooling press molding machine (KMI Co., Ltd. "100 TON 500 Square 3-Stage Compression Molding Machine") at a vulcanization temperature of 170°C, a pressure of 10 MPa, and a vulcanization time of 15 minutes to produce a large test specimen (cylindrical, diameter 29.0 ± 0.5 mm, thickness 12.5 ± 0.5 mm). Next, in accordance with JIS K 6262:2013, a compression set test was performed at a compressibility of 25%, a test temperature of 0°C, and a test time of 72 hours to determine the compression set CS. Note that compression set is an indicator of cold resistance.

[0068] Table 1 shows the evaluation results of the rubber compositions in each example and comparative example.

[0069]

[0070] This application claims priority based on Japanese Patent Application No. 2025-017522, filed on 5 February 2025, and incorporates all of its disclosures herein.

Claims

1. A rubber composition containing chloroprene rubber containing sulfur-modified chloroprene rubber, wherein the heat of fusion ΔH obtained by the following method is 8.50 J / g or less. (Method) A vulcanized sheet is prepared by press vulcanizing the rubber composition in accordance with JIS K 6299:2012 under conditions of vulcanization temperature of 170°C, pressure of 10 MPa, and vulcanization time of 10 minutes. A test piece with a mass of 8 mg is cut out from the vulcanized sheet, and differential scanning calorimetry is performed in accordance with JIS K 7122:2012. Pretreatment is performed under conditions of heating from 25°C to 80°C at a heating rate of 10°C / min in a nitrogen atmosphere and holding for 10 minutes, cooling to 0°C at a cooling rate of 10°C / min and holding for 24 hours, and cooling to -60°C at a cooling rate of 10°C / min. Next, the temperature is maintained at -60°C for 5 minutes, and then the temperature is raised to 80°C at a heating rate of 10°C / min for measurement. The heat of fusion ΔH is determined from the DSC curve obtained from the above measurement.

2. The rubber composition according to claim 1, further comprising carbon black.

3. The rubber composition according to claim 2, wherein the amount of iodine adsorbed by the carbon black, as measured in accordance with JIS K 6217-1:2018, is 15 mg / g or more and 130 mg / g or less.

4. The rubber composition according to claim 2 or 3, wherein the carbon black content is 15 parts by mass or more and 80 parts by mass or less per 100 parts by mass of chloroprene rubber.

5. The rubber composition according to any one of claims 1 to 3, further comprising an anti-aging agent.

6. The rubber composition according to claim 5, wherein the anti-aging agent comprises an amine-based anti-aging agent.

7. The rubber composition according to claim 5, wherein the content of the anti-aging agent is 0.2 parts by mass or more and 8 parts by mass or less per 100 parts by mass of the chloroprene rubber.

8. The rubber composition according to any one of claims 1 to 3, wherein the CS obtained by the method described below is less than 51%. (Method) The rubber composition is press-vulcanized in accordance with JIS K 6299:2012 under conditions of vulcanization temperature of 170°C, pressure of 10 MPa, and vulcanization time of 15 minutes to produce a large test piece (cylindrical, diameter 29.0 ± 0.5 mm, thickness 12.5 ± 0.5 mm). Then, in accordance with JIS K 6262:2013, a compression set test is performed under conditions of compression ratio: 25%, test temperature: 0°C, and test time: 72 hours to determine the compression set CS.

9. A vulcanized product of a rubber composition according to any one of claims 1 to 3.

10. A molded article comprising the vulcanized product described in claim 9.

11. A transmission belt comprising the molded body described in claim 10.

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