Rubber composition for tire treads, and pneumatic tire using the same.

Abstract

To provide a rubber composition for a tire tread, capable of giving a tire excellent in wet grip performance at a low temperature and wet grip performance at a high temperature.SOLUTION: The rubber composition for the tire tread contains 80-140 pts.mass of silica, 15-50 pts.mass of a thermoplastic resin having a softening point of 40°C or higher, and 5-25 pts.mass of an oil with respect to 100 pts.mass of a diene rubber containing (a) 30-50 pts.mass of an unmodified styrene-butadiene rubber having a glass transition point of -35°C or higher, (b) 25-35 pts.mass of a modified styrene-butadiene rubber having a glass transition point of -30°C or higher, and (c) 25-35 pts.mass of a modified styrene-butadiene rubber having a glass transition point of -50°C or lower.SELECTED DRAWING: None

Description

Technical Field

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[0005]

[0001] The present invention relates to a rubber composition for a tire tread and a pneumatic tire using the same.

Background Art

[0002] In rubber compositions used for tires, it is required to improve the grip performance (wet grip performance) on wet road surfaces.

[0003] Regarding such problems, Patent Document 1 describes using styrene-butadiene rubber having a glass transition point of -30 to 0°C as a rubber component. Further, Patent Documents 2 and 3 describe using a resin having a high softening point to improve the grip performance. However, there was room for improvement in the wet grip performance at low temperatures.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0005] In view of the above points, an object of the present invention is to provide a rubber composition that can obtain a pneumatic tire excellent in wet grip performance at low temperatures (5°C) and high temperatures (25°C).

Means for Solving the Problems

[0006] The present invention includes the following embodiments. [1] A rubber composition for tire treads comprising 100 parts by mass of a diene rubber containing (a) 30 to 50 parts by mass of unmodified styrene-butadiene rubber having a glass transition temperature of -35°C or higher, (b) 25 to 35 parts by mass of modified styrene-butadiene rubber having a glass transition temperature of -30°C or higher, and (c) 25 to 35 parts by mass of modified styrene-butadiene rubber having a glass transition temperature of -50°C or lower, with 80 to 140 parts by mass of silica, 15 to 50 parts by mass of a thermoplastic resin having a softening point of 40°C or higher, and 5 to 25 parts by mass of oil. [2] The tire tread rubber composition according to [1], wherein the difference in styrene content between modified styrene-butadiene rubber (b) and modified styrene-butadiene rubber (c) is 5 to 30% by mass. [3] The rubber composition for tire treads according to [1] or [2], wherein the thermoplastic resin is a styrene-based resin or a terpene-based resin. [4] A rubber composition for tire treads according to any one of [1] to [3], wherein the storage modulus at -5°C is 200 MPa or less. [5] A rubber composition for tire treads according to any one of [1] to [4], comprising 5 to 20 parts by mass of protected mercaptosilane per 100 parts by mass of silica. A pneumatic tire having a tread made of the rubber composition described in any one of items [6][1] to [5]. [Effects of the Invention]

[0007] The rubber composition for tire treads of the present invention provides a pneumatic tire with excellent wet grip performance at low temperatures (5°C) and high temperatures (25°C). [Modes for carrying out the invention]

[0008] The following describes in detail matters related to the implementation of the present invention.

[0009] The rubber composition for tire treads according to this embodiment contains 30 to 50 parts by mass of (a) unmodified styrene-butadiene rubber (SBR) having a glass transition temperature (Tg) of -35°C or higher, 25 to 35 parts by mass of modified styrene-butadiene rubber (SBR) having a glass transition temperature (Tg) of -30°C or higher, and (c) 25 to 35 parts by mass of modified styrene-butadiene rubber (SBR) having a glass transition temperature (Tg) of -50°C or lower. Per 100 parts by mass of diene rubber, the composition contains 80 to 140 parts by mass of silica, 15 to 50 parts by mass of thermoplastic resin having a softening point of 40°C or higher, and 5 to 25 parts by mass of oil. In this specification, the glass transition temperature is a value measured by differential scanning calorimetry (DSC) in accordance with JIS K7121 at a heating rate of 20°C / min (measurement temperature range: -150°C to 150°C). The softening point is a value measured using the ring-ball method in accordance with JIS K2207.

[0010] In this specification, "modified" rubber components refer to those having functional groups that react with silica, while "unmodified" rubber components refer to those that do not have functional groups that react with silica. Examples of functional groups that react with silica include hydroxyl groups, amino groups, carboxyl groups, alkoxy groups, alkoxysilyl groups, and epoxy groups. These functional groups may be introduced at the molecular ends or within the molecular chain.

[0011] The ratio of unmodified SBR (a) to modified SBR (total of components (b) and (c)) in diene rubber (unmodified SBR / modified SBR) is not particularly limited, but is preferably 0.5 to 0.9 by mass ratio, and more preferably 0.6 to 0.8.

[0012] The content ratio of component (b) to component (c) in the modified SBR (component (b) / component (c)) is not particularly limited, but is preferably 0.7 to 1.4 by mass ratio, and more preferably 0.8 to 1.2.

[0013] The diene rubber may contain components other than those listed in (a) to (c) above, but it is preferable that it consists of those components. Examples of diene rubbers other than those listed in (a) to (c) above include natural rubber (NR), synthetic isoprene rubber (IR), butadiene rubber (BR), nitrile rubber (NBR), chloroprene rubber (CR), butyl rubber (IIR), styrene-isoprene copolymer rubber, butadiene-isoprene copolymer rubber, and styrene-isoprene-butadiene copolymer rubber.

[0014] The difference in styrene content between modified SBR(b) and modified SBR(c) is preferably 5 to 30% by mass, and more preferably 5 to 20% by mass. When the difference in styrene content is within the above range, excellent wet grip performance is easily obtained.

[0015] The rubber composition according to this embodiment contains silica as a reinforcing filler. The silica is not particularly limited, and for example, wet silica such as wet sedimentation silica or wet gel silica may be used.

[0016] The silica content is 80 to 140 parts by mass per 100 parts by mass of diene rubber, preferably 100 to 140 parts by mass, and more preferably 110 to 130 parts by mass.

[0017] As a reinforcing filler, carbon black may be used in combination with silica. The content of the reinforcing filler (total amount of silica and carbon black) is not particularly limited and may be 80 to 170 parts by mass, 100 to 160 parts by mass, or 110 to 150 parts by mass per 100 parts by mass of diene rubber. The content of carbon black is also not particularly limited and may be 1 to 30 parts by mass or 5 to 20 parts by mass per 100 parts by mass of diene rubber.

[0018] The rubber composition according to this embodiment preferably contains a silane coupling agent. In that case, the content of the silane coupling agent is preferably 5 to 20 parts by mass, more preferably 5 to 15 parts by mass, based on 100 parts by mass of silica. Also, the content of the silane coupling agent is preferably 4 to 30 parts by mass, more preferably 5 to 20 parts by mass, based on 100 parts by mass of the diene rubber.

[0019] Examples of the silane coupling agent include sulfide silanes such as bis(3-triethoxysilylpropyl)tetrasulfide (e.g., "Si69" manufactured by Degussa), bis(3-triethoxysilylpropyl)disulfide (e.g., "Si75" manufactured by Degussa), bis(2-triethoxysilylethyl)tetrasulfide, bis(4-triethoxysilylbutyl)disulfide, bis(3-trimethoxysilylpropyl)tetrasulfide, bis(2-trimethoxysilylethyl)disulfide; mercapto silanes such as γ-mercaptopropyltrimethoxysilane, γ-mercaptopropyltriethoxysilane, mercaptopropylmethyldimethoxysilane, mercaptopropyldimethylmethoxysilane, mercaptoethyltriethoxysilane; and protected mercapto silanes such as 3-octanoylthio-1-propyltriethoxysilane, 3-propionylthiopropyltrimethoxysilane. Among these, protected mercapto silane is preferred.

[0020] The rubber composition according to this embodiment contains a thermoplastic resin having a softening point of 40°C or higher, and the softening point is preferably 40 to 160°C. By containing a thermoplastic resin having a softening point of 40°C or higher, excellent wet grip performance can be easily obtained.

[0021] The content of the thermoplastic resin is 15 to 50 parts by mass, preferably 20 to 40 parts by mass, based on 100 parts by mass of the diene rubber. When the content of the thermoplastic resin is within the above range, excellent wet grip performance can be easily obtained.

[0022] Examples of the thermoplastic resin include styrenic resins, terpene resins, petroleum hydrocarbon resins, rosin resins, etc. Among these, styrenic resins and terpene resins are preferred.

[0023] The styrenic resin may be a resin containing styrene and / or α-methylstyrene as a constituent monomer, and examples thereof include a homopolymer obtained by polymerizing styrene or α-methylstyrene alone, a copolymer obtained by copolymerizing styrene and α-methylstyrene, and a copolymer of styrene and / or α-methylstyrene and other monomers. Examples of the other monomers include terpene compounds (terpene-based monomers) such as α-pinene, β-pinene, dipentene, limonene, myrcene, allo-ocimene, ocimene, α-felandrene, α-terpinene, γ-terpinene, terpinolene, 1,8-cineole, 1,4-cineole, α-terpineol, β-terpineol, γ-terpineol, etc., and non-conjugated olefins such as ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, etc. These may be used alone or in combination of two or more.

[0024] Examples of the terpene resin include terpene resins such as α-pinene polymer, β-pinene polymer, dipentene polymer, etc., and modified terpene resins obtained by modifying these terpene resins (such as phenol modification, aromatic modification, hydrocarbon modification, etc.) (for example, terpene phenol-based resins, styrene-modified terpene resins, aromatic-modified terpene resins, etc.).

[0025] Examples of petroleum-based hydrocarbon resins include C5 aliphatic hydrocarbon resins, C9 aromatic hydrocarbon resins, and C5 / C9 aliphatic / aromatic copolymer hydrocarbon resins. Aliphatic hydrocarbon resins are obtained by cationic polymerization of unsaturated monomers such as isoprene and cyclopentadiene, which are petroleum fractions (C5 fractions) with 4 to 5 carbon atoms, and may be partially hydrogenated. Aromatic hydrocarbon resins are obtained by cationic polymerization of monomers such as vinyltoluene, alkylstyrene, and indene, which are petroleum fractions (C9 fractions) with 8 to 10 carbon atoms, and may be partially hydrogenated. Aliphatic / aromatic copolymer hydrocarbon resins are obtained by copolymerizing the above C5 fraction and C9 fraction by cationic polymerization, and may be partially hydrogenated.

[0026] Examples of rosin-based resins include raw rosins such as gum rosin, wood rosin, and tall oil rosin, as well as rosins such as disproportionates of raw rosins and polymerized rosins, esterified rosins (rosin ester resins), phenol-modified rosins, unsaturated acid (such as maleic acid)-modified rosins, and formylated rosins obtained by reduction treatment of rosins.

[0027] As the oil, various oils commonly used in rubber compositions can be used. For example, at least one mineral oil selected from the group consisting of mineral oils, namely paraffin oil, naphthenic oil, and aromatic oil may be used. The oil content is 5 to 25 parts by mass, preferably 10 to 25 parts by mass, per 100 parts by mass of diene rubber.

[0028] In addition to the components mentioned above, the rubber composition according to this embodiment may contain various additives commonly used in rubber compositions, such as zinc oxide, stearic acid, antioxidants, waxes, vulcanizing agents, and vulcanization accelerators.

[0029] Sulfur is preferably used as the vulcanizing agent. The content of the vulcanizing agent is not particularly limited, but is preferably 0.1 to 10 parts by mass, and more preferably 0.5 to 5 parts by mass, per 100 parts by mass of the rubber component. Examples of the vulcanization accelerator include sulfenamide-based, thiuram-based, thiazole-based, and guanidine-based vulcanization accelerators, which can be used individually or in combination of two or more. The content of the vulcanization accelerator is not particularly limited, but is preferably 0.1 to 7 parts by mass, and more preferably 0.5 to 5 parts by mass, per 100 parts by mass of the rubber component.

[0030] The storage modulus of the rubber composition according to this embodiment is preferably 200 MPa or less at -5°C, from the viewpoint of obtaining excellent wet grip performance. Here, "storage modulus at -5°C" refers to the value of the storage modulus measured under the conditions of -5°C, frequency of 10 Hz, dynamic strain of 0.2%, and static strain of 10%.

[0031] The rubber composition according to this embodiment can be prepared by kneading in accordance with conventional methods using a commonly used mixer such as a Banbury mixer, kneader, or roll. That is, for example, in the first mixing stage, other additives excluding the vulcanizing agent and vulcanization accelerator can be added and mixed to the diene rubber, and then, in the final mixing stage, the vulcanizing agent and vulcanization accelerator can be added and mixed to the resulting mixture to prepare the rubber composition.

[0032] The rubber composition obtained in this way can be applied to the treads of pneumatic tires of various uses and sizes, such as passenger car tires and large tires for trucks and buses. Specifically, the rubber composition can be molded into a predetermined shape by conventional methods, for example by extrusion, and combined with other parts to produce a green tire. After that, a pneumatic tire can be manufactured by vulcanizing the green tire at, for example, 140 to 180°C. [Examples]

[0033] The following are examples of the present invention, but the present invention is not limited to these examples.

[0034] Using a lab mixer, according to the formulation (parts by mass) shown in Table 1 below, first, in the first mixing stage, the diene rubber component was mixed with all compounding agents except sulfur and vulcanization accelerator (discharge temperature = 160°C). Next, in the final mixing stage, sulfur and vulcanization accelerator were added to the resulting mixture and mixed (discharge temperature = 90°C) to prepare the rubber composition. Details of each component in Table 1 are as follows.

[0035] • Unmodified SBR1: JSR Corporation "SL563", Tg = -35℃ • Unmodified SBR2: JSR Corporation's "SBR1502", Tg = -52℃ Modified SBR1: JSR Corporation "HPR850", Tg = -24℃, Styrene content = 27% by mass Modified SBR2: JSR Corporation "HPR840", Tg = -60℃, Styrene content = 10% by mass • Carbon Black: "Seast KH" manufactured by Tokai Carbon Co., Ltd. • Silica: "Nip Seal AQ" manufactured by Tosoh Silica Co., Ltd. • Silane coupling agent 1: Evonik "Si75", sulfidesilane • Silane coupling agent 2: Momentive's "NXT", protected mercaptosilane • Oil: ENEOS Corporation's "Process NC140" • Thermoplastic resin 1: Styrene resin, Kraton's "SYLVATRAXX 4401", softening point = 85°C • Thermoplastic resin 2: C5 / C9 resin, "Petrotac 90" manufactured by Tosoh Corporation, softening point = 100°C • Thermoplastic resin 3: Terpene resin, Kraton's "SYLVATRAXX 4150", softening point = 115°C • Zinc oxide: "Zinc Oxide Type 2" manufactured by Mitsui Mining & Smelting Co., Ltd. • Stearic acid: "Lunaq S-20" manufactured by Kao Corporation • Wax: "OZOACE0355" manufactured by Nippon Seiro Co., Ltd. • Anti-aging agent 1: "Nocrac 6C" manufactured by Ouchi Shinko Chemical Co., Ltd. • Anti-aging agent 2: "Antage RD" manufactured by Kawaguchi Chemical Industry Co., Ltd. • Vulcanization accelerator 1: "Noxellar D" manufactured by Ouchi Shinko Chemical Co., Ltd. • Vulcanization accelerator 2: "Soxinol CZ" manufactured by Sumitomo Chemical Co., Ltd. • Sulfur: Powdered sulfur manufactured by Tsurumi Chemical Industries, Ltd.

[0036] For each rubber composition obtained, test specimens of a predetermined shape were prepared by vulcanization at 160°C for 20 minutes, and dynamic viscoelasticity tests were performed at -5°C. Furthermore, pneumatic tires were prepared using each rubber composition, and their wet grip performance was evaluated. The measurement methods are as follows.

[0037] • Storage modulus at -5°C (E'(-5°C)): The storage modulus was measured using a viscoelasticity measuring instrument manufactured by GABO Corporation under the conditions of -5°C, frequency of 10 Hz, dynamic strain of 0.2%, and static strain of 10%.

[0038] • Wet grip performance (5°C): Four 215 / 45ZR17 test radial tires, each fitted with the rubber composition obtained above as the tread, were mounted on a vehicle. Under conditions of 5°C ambient temperature, the vehicle was driven at 100 km / h on a road surface with a water depth of 1 mm, and the ABS was activated. The braking distance was measured, and the reciprocal of the measured value was calculated. The results of Comparative Example 1 were evaluated on an index with a value of 100, and a higher index indicates better wet braking performance.

[0039] • Wet grip performance (25°C): Four 215 / 45ZR17 test radial tires, each fitted with the rubber composition obtained above as the tread, were mounted on a vehicle. Under conditions of 25°C ambient temperature, the vehicle was driven at 100 km / h on a road surface with a water depth of 1 mm, and the ABS was activated. The braking distance was measured, and the reciprocal of the measured value was calculated. The results of Comparative Example 1 were evaluated on an index with 100, and a higher index indicates better wet braking performance.

[0040] [Table 1]

[0041] The results are shown in Table 1. Examples 1 to 7 showed superior wet grip performance at low temperatures and high temperatures compared to Comparative Example 1.

[0042] Comparative Examples 2-5 were examples where the SBR content was outside the specified range. Comparative Examples 2, 3, and 5 had inferior wet grip performance at high temperatures compared to Comparative Example 1, while Comparative Example 4 had inferior wet grip performance at low temperatures compared to Comparative Example 1.

[0043] Comparative Example 6 was an example where the oil content exceeded the upper limit, resulting in inferior wet grip performance at high temperatures. [Industrial applicability]

[0044] The rubber composition of the present invention can be used in rubber compositions for various tires, such as those for passenger cars, light trucks, and buses.

Claims

1. (a) 30 to 50 parts by mass of unmodified styrene-butadiene rubber having a glass transition temperature of -35°C or higher, (b) 25 to 35 parts by mass of modified styrene-butadiene rubber having a glass transition temperature of -30°C or higher, (c) 100 parts by mass of diene rubber containing 25 to 35 parts by mass of modified styrene-butadiene rubber having a glass transition temperature of -50°C or lower, 80 to 140 parts by mass of silica, 15 to 50 parts by mass of a thermoplastic resin having a softening point of 40°C or higher, A rubber composition for tire treads containing 5 to 25 parts by mass of oil.

2. The tire tread rubber composition according to claim 1, wherein the difference in styrene content between the modified styrene-butadiene rubber (b) and the modified styrene-butadiene rubber (c) is 5 to 30% by mass.

3. The rubber composition for tire treads according to claim 1 or 2, wherein the thermoplastic resin is a styrene-based resin or a terpene-based resin.

4. The rubber composition for tire treads according to claim 1 or 2, wherein the storage modulus at -5°C is 200 MPa or less.

5. The rubber composition for tire treads according to claim 1 or 2, comprising 5 to 20 parts by mass of protected mercaptosilane per 100 parts by mass of silica.

6. A pneumatic tire comprising a tread made of the rubber composition described in claim 1 or 2.

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