Rubber composition for tire tread, and tire

Abstract

To provide a rubber composition for a tire tread which has excellent on-ice braking performance and response performance on a frozen road surface.SOLUTION: The rubber composition for a tire tread according to an embodiment contains, based on 100 pts.mass of a diene rubber, 1-50 pts.mass of an aromatic resin containing an aromatic vinyl compound as a constituent monomer and being liquid at 23°C.SELECTED DRAWING: None

Description

Technical Field

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

Background Art

[0002] In rubber compositions used for tires, particularly the treads of studless tires, it is required to improve the grip performance (ice braking performance) on frozen road surfaces.

[0003] For example, Patent Document 1 discloses a rubber composition containing a liquid diene rubber and porous cellulose particles for the purpose of achieving both ice braking performance and grip performance (wet grip performance) on wet road surfaces. Patent Document 2 describes blending a terpene resin together with a liquid rubber and porous cellulose particles.

[0004] Further, Patent Document 3 describes blending a predetermined terpene resin and silica into highly purified natural rubber for the purpose of improving ice performance.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] In studless tires, in response to increasingly strict market requirements, not only ice braking performance but also good behavior response including suppression of wandering on frozen road surfaces, that is, response performance, is required.

[0007] Although Patent Documents 1 to 3 describe blending terpene resins and petroleum resins, they do not suggest blending specific aromatic liquid resins.

[0008] An embodiment of the present invention aims to provide a rubber composition for a tire tread having excellent ice braking performance and response performance on a frozen road surface, and a tire using the same.

Means for Solving the Problems

[0009] The present invention includes the embodiments shown below. [1] A rubber composition for a tire tread, comprising 1 to 50 parts by mass of an aromatic resin that is liquid at 23°C and contains an aromatic vinyl compound as a constituent monomer, based on 100 parts by mass of a diene rubber. [2] The rubber composition for a tire tread according to [1], wherein the glass transition temperature of the aromatic resin is -100°C or higher and 0°C or lower. [3] The rubber composition for a tire tread according to [1] or [2], wherein the weight average molecular weight of the aromatic resin is 500 to 30,000. [4] The rubber composition for a tire tread according to [1] or [2], wherein the weight average molecular weight of the aromatic resin is 500 to 2,000. [5] The rubber composition for a tire tread according to any one of [1] to [4], wherein the aromatic resin contains at least one selected from the group consisting of styrene, α-substituted styrene, ortho-substituted styrene, meta-substituted styrene, and para-substituted styrene as a constituent monomer. [6] The rubber composition for a tire tread according to [5], wherein the aromatic resin further contains (meth)acrylate as a constituent monomer. [7] The rubber composition for a tire tread according to any one of [1] to [6], further comprising 0.3 to 20 parts by mass of porous cellulose particles based on 100 parts by mass of the diene rubber. [8] The 100 parts by mass of the diene rubber contains 20 to 65 parts by mass of an isoprene rubber and 10 to 55 parts by mass of a butadiene rubber, and is the rubber composition for a tire tread according to any one of [1] to [7]. [9] A tire having a tread formed of the rubber composition for a tire tread according to any one of [1] to [8].

Effects of the Invention

[0010] According to an embodiment of the present invention, excellent ice braking performance can be achieved and the response performance on a frozen road surface can be improved.

Modes for Carrying Out the Invention

[0011] The rubber composition for a tire tread according to the present embodiment (hereinafter, also simply referred to as a rubber composition) contains a diene rubber and an aromatic resin that is liquid at 23°C and contains an aromatic vinyl compound as a constituent monomer. Thereby, both ice braking performance and response performance on a frozen road surface are achieved. The mechanism is not clear and is not intended to be limited thereby, but it is presumed that the use of an aromatic resin that is liquid at normal temperature provides an appropriate balance of tire flexibility and rigidity.

[0012] In the rubber composition according to the present embodiment, the diene rubber used as the rubber component refers to a rubber having a repeating unit corresponding to a diene monomer having a conjugated double bond, and the main chain of the polymer contains a carbon-carbon double bond. As the diene rubber, a solid diene rubber is usually used. In this specification, "solid" means having no fluidity at normal temperature of 23°C.

[0013] Specific examples of diene rubbers include isoprene rubbers such as natural rubber (NR) and synthetic isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), nitrile rubber (NBR), chloroprene rubber (CR), styrene-isoprene copolymer rubber, butadiene-isoprene copolymer rubber, styrene-isoprene-butadiene copolymer rubber, and the like. These diene rubbers conceptually include those with modified ends or main chains (e.g., end-modified SBR) and those modified to impart desired properties (e.g., modified NR) as necessary. Any one of these diene rubbers may be used, or two or more thereof may be used in combination.

[0014] As the diene rubber, at least one selected from the group consisting of isoprene rubber, butadiene rubber, and styrene-butadiene rubber is preferable, and more preferably isoprene rubber and / or butadiene rubber. In one embodiment, 100 parts by mass of the diene rubber preferably contains 20 to 65 parts by mass of isoprene rubber (i.e., NR and / or IR) and 10 to 55 parts by mass of butadiene rubber, more preferably 30 to 60 parts by mass of isoprene rubber and 30 to 55 parts by mass of butadiene rubber, and still more preferably 45 to 60 parts by mass of isoprene rubber and 40 to 55 parts by mass of butadiene rubber. Optionally, the balance rubber may include, for example, styrene-butadiene rubber.

[0015] The rubber composition according to this embodiment is blended with a liquid aromatic resin containing an aromatic vinyl compound as a constituent monomer. Such an aromatic resin is a thermoplastic resin. Here, a thermoplastic resin refers to a resin having the property of softening at a temperature equal to or higher than the glass transition temperature or melting point and solidifying when it becomes equal to or lower than the glass transition temperature or melting point. In this embodiment, a liquid resin that is liquid at room temperature is used. "Liquid at room temperature" means having fluidity at 23°C.

[0016] The aromatic resin contains an aromatic vinyl compound as a constituent monomer. Here, "containing as a constituent monomer" means using it as a raw material (monomer) for synthesizing the resin, and the resin has a structure derived therefrom. Examples of the aromatic vinyl compound include styrene, α-substituted styrene (e.g., α-methylstyrene, α-ethylstyrene), ortho-substituted styrene (e.g., o-methylstyrene, o-ethylstyrene, o-isopropylstyrene, o-tert-butylstyrene), meta-substituted styrene (e.g., m-methylstyrene, m-ethylstyrene, m-isopropylstyrene, m-tert-butylstyrene), and para-substituted styrene (e.g., p-methylstyrene, p-ethylstyrene, p-isopropylstyrene, p-tert-butylstyrene). Any one of these may be used, or two or more thereof may be used in combination.

[0017] The constituent monomer of the aromatic resin may be only an aromatic vinyl compound, or may be a copolymer of an aromatic vinyl compound and other monomers. The amount of the aromatic vinyl compound in 100% by mass of the constituent monomer may be 10% by mass or more, may be 10 to 60% by mass, or may be 15 to 40% by mass.

[0018] Examples of other monomers used together with the aromatic vinyl compound as the above constituent monomer include (meth)acrylate, fatty acid vinyl ester, indene, and the like. Among these, it is preferable to use (meth)acrylate and / or indene. The amount of (meth)acrylate and / or indene in 100% by mass of the constituent monomer is not particularly limited, and may be 40 to 90% by mass, or may be 60 to 85% by mass. Here, "(meth)acrylate" means acrylate and / or methacrylate.

[0019] Examples of the (meth)acrylate include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, lauryl (meth)acrylate, tetradecyl (meth)acrylate, hexadecyl acrylate, and stearyl (meth)acrylate. Any one of these may be used alone, or two or more of them may be used in combination. Among these, esters of (meth)acrylic acid and an alcohol having 10 or more carbon atoms are preferably used, and more preferably, esters of (meth)acrylic acid and an alcohol having 10 to 18 carbon atoms are used. Here, (meth)acrylic acid means acrylic acid and / or methacrylic acid.

[0020] In one embodiment, the aromatic resin is preferably a copolymer containing an aromatic vinyl compound and (meth)acrylate as constituent monomers, more preferably, 100% by mass of the constituent monomers contains 10 to 60% by mass of the aromatic vinyl compound and 40 to 90% by mass of (meth)acrylate, and still more preferably, contains 15 to 40% by mass of the aromatic vinyl compound and 60 to 85% by mass of (meth)acrylate.

[0021] In one embodiment, the aromatic resin is preferably a copolymer containing an aromatic vinyl compound, (meth)acrylate, and indene as constituent monomers, more preferably, 100% by mass of the constituent monomers contains 10 to 50% by mass of the aromatic vinyl compound, 40 to 80% by mass of (meth)acrylate, and 10 to 50% by mass of indene, and still more preferably, contains 15 to 30% by mass of the aromatic vinyl compound, 50 to 70% by mass of (meth)acrylate, and 15 to 30% by mass of indene.

[0022] The aromatic resin preferably has a glass transition temperature (Tg) of -100°C or higher and 0°C or lower. When the glass transition temperature is 0°C or lower, the effect of improving the braking performance on ice can be enhanced. When the glass transition temperature is -100°C or higher, the effect of improving the response performance on frozen road surfaces can be enhanced. The glass transition temperature of the aromatic resin is more preferably -90°C to -5°C, still more preferably -50°C to -10°C, and may also be -30°C to -10°C. The glass transition temperature can be determined by the measuring method described in the Examples section.

[0023] The aromatic resin preferably has a weight average molecular weight (Mw) of 500 to 30,000 (g / mol). When the weight average molecular weight is 30,000 or lower, the effects of improving the braking performance on ice and the response performance on frozen road surfaces can be enhanced. The weight average molecular weight of the aromatic resin is more preferably 20,000 or lower. From the perspective of further enhancing the balance between the braking performance on ice and the response performance on frozen road surfaces, it is preferable for the aromatic resin to have a low molecular weight. Specifically, the weight average molecular weight of the aromatic resin is preferably 500 to 2,000, and more preferably 600 to 1,500. The weight average molecular weight can be determined by the measuring method described in the Examples section.

[0024] The content of the aromatic resin is 1 to 50 parts by mass, preferably 3 to 40 parts by mass, more preferably 5 to 35 parts by mass, still more preferably 10 to 25 parts by mass, and still more preferably 10 to 20 parts by mass, based on 100 parts by mass of the diene rubber.

[0025] Preferably, porous cellulose particles are blended in the rubber composition according to this embodiment, and the braking performance on ice can be improved. The content of the porous cellulose particles is preferably 0.3 to 20 parts by mass, more preferably 0.5 to 15 parts by mass, and still more preferably 1 to 10 parts by mass, based on 100 parts by mass of the diene rubber.

[0026] The porosity of the porous cellulose particles is not particularly limited, and for example, it is preferably 75 to 95%, more preferably 85 to 95%. Here, the porosity of the porous cellulose particles can be determined from the following formula by measuring the volume of a sample of a certain mass (i.e., the porous cellulose particles) with a graduated cylinder to obtain the bulk density. Here, the true density of cellulose is 1.5. Porosity [%] = {1 - (bulk density of the sample [g / ml]) / (true density of the sample [g / ml])} × 100

[0027] The average particle diameter of the porous cellulose particles is not particularly limited, and for example, it is preferably 1000 μm or less, more preferably 100 to 800 μm, and even more preferably 200 to 800 μm.

[0028] The porous cellulose particles are preferably spherical particles with a ratio of major axis / minor axis of 1 to 2, more preferably a ratio of major axis / minor axis of 1 to 1.5, and even more preferably 1 to 1.2. By using particles having such a spherical structure, the dispersibility in the rubber composition is improved, and excellent ice braking performance is easily obtained.

[0029] The average particle diameter and the ratio of major axis / minor axis of the porous cellulose particles are determined as follows. That is, the porous cellulose particles are observed with a microscope to obtain an image, and using this image, the major axis and minor axis of the particles (when the major axis and minor axis are the same, the length in a certain axial direction and the length in the axial direction perpendicular to this) are measured for 100 particles, and the average value is calculated to obtain the average particle diameter. Also, the ratio of major axis / minor axis is obtained from the average value of the values obtained by dividing the major axis by the minor axis.

[0030] Such porous cellulose particles are commercially available, for example, from Rayon Co., Ltd. under the trademark "Viscol Pearl", and are also described in JP-A-2001-323095 and JP-A-2004-115284, and they can be preferably used. Specifically, it is preferable to use cellulose particles obtained by adding a porogen (a carbonate such as calcium carbonate) to an alkaline cellulose solution such as viscose and simultaneously carrying out the coagulation and regeneration of cellulose and the foaming by the porogen.

[0031] A reinforcing filler may be blended in the rubber composition according to this embodiment. The content of the reinforcing filler is not particularly limited, and for example, it may be 40 to 120 parts by mass, 50 to 100 parts by mass, or 50 to 80 parts by mass with respect to 100 parts by mass of the diene rubber.

[0032] Examples of the reinforcing filler include silica and / or carbon black. The silica is not particularly limited, and for example, wet silica such as wet precipitation method silica or wet gel method silica may be used. The content of the silica is not particularly limited, and for example, it is preferably 20 to 100 parts by mass, more preferably 30 to 80 parts by mass with respect to 100 parts by mass of the diene rubber.

[0033] The carbon black is not particularly limited, and various known varieties can be used. The content of the carbon black is not particularly limited, and for example, it may be 1 to 30 parts by mass, or 1 to 20 parts by mass with respect to 100 parts by mass of the diene rubber.

[0034] When silica is blended in the rubber composition, it is preferable to further contain a silane coupling agent. In that case, the content of the silane coupling agent is preferably 1 to 20 parts by mass, more preferably 2 to 15 parts by mass with respect to 100 parts by mass of the silica.

[0035] An oil may be blended in the rubber composition according to this embodiment. Examples of the oil include mineral oils such as paraffin oil and aroma oil. The content of the oil is not particularly limited, and may be 0 to 30 parts by mass, or may be 5 to 25 parts by mass with respect to 100 parts by mass of the diene rubber.

[0036] In addition to the above components, various additives generally used in rubber compositions, such as zinc oxide, stearic acid, wax, anti-aging agent, vulcanizing agent, vulcanization accelerator, etc., may be blended as optional components in the rubber composition according to this embodiment.

[0037] The content of zinc oxide is not particularly limited, and may be, for example, 0 to 10 parts by mass, or may be 0.5 to 5 parts by mass, or may be 1 to 4 parts by mass with respect to 100 parts by mass of the diene rubber.

[0038] The content of stearic acid is not particularly limited, and may be, for example, 0 to 10 parts by mass, or may be 0.5 to 5 parts by mass, or may be 1 to 4 parts by mass with respect to 100 parts by mass of the diene rubber.

[0039] The content of wax is not particularly limited, and may be, for example, 0 to 10 parts by mass, or may be 0.5 to 5 parts by mass, or may be 1 to 4 parts by mass with respect to 100 parts by mass of the diene rubber.

[0040] Examples of the anti-aging agent include various anti-aging agents such as amine-ketone type, aromatic secondary amine type, monophenol type, bisphenol type, and benzimidazole type, and any one or a combination of two or more thereof can be used. The content of the anti-aging agent is not particularly limited, and may be, for example, 0 to 10 parts by mass, or may be 1 to 5 parts by mass with respect to 100 parts by mass of the diene rubber.

[0041] Sulfur is preferably used as the vulcanizing agent. The content of the vulcanizing agent is not particularly limited, and may be 0.1 to 10 parts by mass, or may be 0.5 to 5 parts by mass, or may be 1 to 3 parts by mass with respect to 100 parts by mass of the diene rubber.

[0042] Examples of the vulcanization accelerator include various vulcanization accelerators such as sulfenamide-based, guanidine-based, thiuram-based, and thiazole-based accelerators, and any one of them can be used alone 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, more preferably 0.5 to 5 parts by mass, and may be 1 to 4 parts by mass with respect to 100 parts by mass of the diene rubber.

[0043] The rubber composition according to this embodiment can be prepared by kneading according to a conventional method using a mixer such as a Banbury mixer, a kneader, or a roll, which are commonly used. That is, for example, in the first mixing stage (non-pro kneading step), additives other than the vulcanizing agent and the vulcanization accelerator are added and mixed with the diene rubber together with the aromatic resin. Next, the vulcanizing agent and the vulcanization accelerator are added and mixed to the obtained mixture in the final mixing stage (pro kneading step). Thereby, an unvulcanized rubber composition can be prepared.

[0044] The rubber composition according to this embodiment can be applied to the treads of pneumatic tires of various applications and various sizes, such as passenger car tires, large tires for trucks and buses. Preferably, it is used as a rubber composition for the tread of a studless tire. The pneumatic tire can be manufactured by producing a tread member using a rubber extruder or the like using the rubber composition, molding an unvulcanized tire (green tire) in combination with other tire members, and then vulcanizing and molding at, for example, 140 to 180°C according to a conventional method. When applied to a studless tire having a cap base structure, the rubber composition according to this embodiment may be applied only to the cap tread on the ground contact side.

Examples

[0045] Examples are shown below, but the present invention is not limited to these examples.

[0046] [Synthesis of Resin] (Synthesis Example 1: Resin 1) A magnetic stirrer chip, a thermometer, and a reflux condenser were attached to a 5000 ml separable four-necked flask. 300 g of α-methylstyrene, 700 g of lauryl acrylate, and 2400 ml of methylcyclohexane were charged into the flask as a reaction mixture and stirred well. 9.0 g of boron trifluoride phenol complex and 90 g of toluene were placed in a dropping funnel, and the dropping funnel was attached to the flask. The uniformly dispersed reaction mixture was maintained at 1 - 3 °C using an alcohol bath cooled with dry ice, and the above catalyst was added dropwise thereto over 15 minutes to initiate the polymerization reaction. After the dropping of the catalyst was completed, the reaction mixture was polymerized for an additional 1 hour while maintaining it at 1 - 3 °C, and then a 0.5 N aqueous sodium hydroxide solution was added to the reaction mixture to stop the polymerization. The obtained reaction product was washed with 1000 ml of water, and then the solvent and unreacted monomer were distilled off under reduced pressure to obtain liquid resin 1. The obtained resin 1 was an aromatic thermoplastic resin in a liquid state at room temperature, with a glass transition temperature Tg of -18 °C, a number average molecular weight (Mn) of 470, and a weight average molecular weight (Mw) of 880.

[0047] (Synthesis Example 2: Resin 2) The monomers were 200 g of α-methylstyrene and 800 g of lauryl methacrylate, and liquid resin 2 was obtained in the same manner as in Synthesis Example 1. The obtained resin 2 was an aromatic thermoplastic resin in a liquid state at room temperature, with a glass transition temperature Tg of -90 °C, a number average molecular weight (Mn) of 590, and a weight average molecular weight (Mw) of 920.

[0048] (Synthesis Example 3: Resin 3) The monomers were 200 g of p-methylstyrene, 200 g of indene, and 600 g of lauryl methacrylate, and liquid resin 3 was obtained in the same manner as in Synthesis Example 1. The obtained resin 3 was an aromatic thermoplastic resin in a liquid state at room temperature, with a glass transition temperature Tg of -6 °C, a number average molecular weight (Mn) of 430, and a weight average molecular weight (Mw) of 750.

[0049] (Synthesis Example 4: Resin 4) A magnetic stirrer chip, a thermometer, and a reflux condenser were attached to a 5000 ml separable four-necked flask. 300 g of α-methylstyrene, 700 g of lauryl acrylate, 6.870 g of azobisisobutyronitrile, and 2400 mL of toluene were charged into the flask as a reaction mixture and stirred well. After nitrogen bubbling for 1 hour, the reaction solution was maintained at 70 °C for 24 hours. Reprecipitation was carried out by adding methanol to the reaction solution, and the obtained reaction product was washed with 1000 ml of water. Then, the solvent and unreacted monomers were distilled off under reduced pressure to obtain a liquid resin 4. The obtained resin 4 was an aromatic thermoplastic resin in a liquid state at room temperature, with a glass transition temperature Tg of -15 °C, a number average molecular weight (Mn) of 12,000, and a weight average molecular weight (Mw) of 20,000.

[0050] (Synthesis Example 5: Resin 5) The monomers were 800 g of lauryl acrylate and 200 g of lauryl methacrylate, and a liquid resin 4 was obtained in the same manner as in Example 1. The obtained resin 4 was an aliphatic thermoplastic resin in a liquid state at room temperature without an aromatic ring, with a glass transition temperature Tg of -80 °C, a number average molecular weight (Mn) of 500, and a weight average molecular weight (Mw) of 1,100.

[0051] [Method for Measuring Glass Transition Temperature] Measurement was carried out by differential scanning calorimetry (DSC) in accordance with JIS K7121:2012 at a heating rate of 20 °C / min (measurement temperature range: -150 °C to 80 °C).

[0052] [Method for Measuring Average Molecular Weight] The weight-average molecular weight (Mw) and number-average molecular weight (Mn) were determined in terms of polystyrene by measurement using gel permeation chromatography (GPC). Specifically, as the measurement sample, a solution prepared by dissolving 10 mg of the resin in 5 mL of tetrahydrofuran was used. After the sample was filtered, using "Nexera" manufactured by Shimadzu Corporation, the column (manufactured by Agilent Technologies, PLgel GUARD 5μm 50×7.5mm + PLgel 50Å 5μm 300×7.5mm + PLgel 100Å 5μm 300×7.5mm + PLgel 500Å 5μm 300×7.5mm) was passed through at a temperature of 40°C and a flow rate of 1.0 mL / min, detected with a differential refractive index detector, and the molecular weight was calculated in terms of polystyrene using commercially available standard polystyrene.

[0053] [Preparation of Rubber Composition] Using a laboratory mixer, according to the formulation (parts by mass) shown in Table 1 below, first, in the first mixing stage, other compounding agents except sulfur and vulcanization accelerators were added to the diene rubber and kneaded (discharge temperature = 160°C). Then, sulfur and vulcanization accelerators were added to the obtained kneaded product in the final mixing stage and kneaded (discharge temperature = 90°C) to prepare a rubber composition. The details of each component in Table 1 are as follows.

[0054] ·NR: RSS#3 ·BR: "BR730" manufactured by ENEOS MATERIALS Co., Ltd. · Porous cellulose particles: "Viscopearl Mini" manufactured by Renk Chemical Co., Ltd. (average particle diameter = 400μm, ratio of particle major axis / minor axis = 1.11, porosity = 87%) · Silica: "Nipsil AQ" manufactured by Tosoh Silica Corporation · Silane coupling agent: "Si-75" manufactured by Evonik · Carbon black: "Seast 7HM" manufactured by Tokai Carbon Co., Ltd.

[0055] · Zinc oxide: "Zinc Oxide Type 2" manufactured by Mitsui Mining & Smelting Co., Ltd. · Stearic acid: "Lunac S-20" manufactured by Kao Corporation · Wax: "OZOACE0355" manufactured by Nippon Seiro Co., Ltd. · Anti-aging agent: "Nocrack 6C" manufactured by Ouchi Shinsei Chemical Co., Ltd. · Oil: "Process P200" manufactured by ENEOS Co., Ltd. · Vulcanization accelerator 1: "Soxinol CZ" manufactured by Sumitomo Chemical Co., Ltd. · Vulcanization accelerator 2: "Noceller DZ-G" manufactured by Ouchi Shinsei Chemical Co., Ltd. · Sulfur: "Powder sulfur" manufactured by Tsurumi Chemical Industry Co., Ltd.

[0056] · Terpene resin: Solid, "SYLVATRAXX 4150" manufactured by Clayton · Resin 1: Liquid aromatic resin (Tg: -18°C, Mw: 880) · Resin 2: Liquid aromatic resin (Tg: -90°C, Mw: 920) · Resin 3: Liquid aromatic resin (Tg: -6°C, Mw: 750) · Resin 4: Liquid aromatic resin (Tg: -15°C, Mw: 20000) · Resin 5: Liquid aliphatic resin (Tg: -80°C, Mw: 1100) · Resin 6: Solid aromatic resin, C5 / C9 resin, "Petrotack 90" manufactured by Tosoh Corporation (Tg: 55°C, Mw: 1700) · Resin 7: Solid aromatic resin, styrene resin, "SYLVATRAXX 4401" manufactured by Clayton (Tg: 50°C, Mw: 1020)

[0057] A studless tire (tire size: 195 / 65R15) with the obtained rubber compositions applied to the tread was produced, and the ice braking performance and response performance on frozen road surfaces were evaluated. The evaluation method is as follows.

[0058] [Ice braking performance] The studless tire was mounted on a 2000cc 4WD vehicle, and the braking distance on ice was measured by operating the ABS from a speed of 40 km / h at an air temperature of -2°C to -6°C (average value of n = 10). The reciprocal of the measured braking distance was expressed as an index with the value of Comparative Example 1 set to 100. The larger the index, the shorter the braking distance and the better the ice braking performance.

[0059] [Response performance] The driver in charge of the sensory test drove on the test course on the icy road surface at an air temperature of -2°C to -6°C while paying attention to steering responsiveness, driving stability (wandering), etc., and sensory-evaluated (evaluated the feeling performance) the response performance. The results were shown in Table 1, with those better than Comparative Example 1 (the control) being +2, slightly better being +1, equivalent being ±0, slightly inferior being -1, and inferior being -2.

[0060]

Table 1

[0061] Comparative Example 1 is a control formulation with excellent ice braking performance containing a terpene resin in a solid state at normal temperature. In Examples 1 to 6 containing an aromatic resin in a liquid state at normal temperature instead of the terpene resin, the response performance could be improved while maintaining or improving the excellent ice braking performance compared to Comparative Example 1.

[0062] From the comparison of Examples 1, 4, and 5, the ice braking performance improved with an increase in the amount of the aromatic resin in a liquid state at normal temperature, but the balance between the ice braking performance and the response performance peaked at an addition amount of about 30 phr. Therefore, considering the balance, a more preferable addition amount is considered to be 10 to 20 phr. Regarding the molecular weight of the aromatic resin, in Example 6 using a high molecular weight resin 4, the response performance improved compared to Comparative Example 1, but considering the balance with the ice braking performance, better results were obtained with Examples 1 to 3 having a lower molecular weight.

[0063] On the other hand, in Comparative Example 2 using a liquid aliphatic resin without an aromatic ring instead of the terpene resin, although it was excellent in ice braking performance compared to Comparative Example 1, the response performance was clearly inferior. Also, in Comparative Examples 3 and 4 with the addition of a solid aromatic resin instead of the terpene resin, no improvement effect on the response performance was observed compared to Comparative Example 1.

[0064] In addition, for the various numerical ranges described in the specification, their upper limit values and lower limit values can be arbitrarily combined respectively, and all such combinations are regarded as being described in this specification as preferred numerical ranges. Also, the description of a numerical range of "X to Y" means X or more and Y or less.

Claims

1. A rubber composition for tire treads, comprising 1 to 50 parts by mass of an aromatic resin that is liquid at 23°C and contains an aromatic vinyl compound as a constituent monomer, per 100 parts by mass of a diene rubber.

2. The rubber composition for tire treads according to Claim 1, wherein the glass transition temperature of the aromatic resin is -100°C or higher and 0°C or lower.

3. The rubber composition for tire treads according to Claim 1, wherein the weight average molecular weight of the aromatic resin is 500 to 30,000.

4. The rubber composition for tire treads according to Claim 1, wherein the weight average molecular weight of the aromatic resin is 500 to 2,000.

5. The rubber composition for tire treads according to Claim 1, wherein the aromatic resin contains at least one selected from the group consisting of styrene, α-substituted styrene, ortho-substituted styrene, meta-substituted styrene, and para-substituted styrene as a constituent monomer.

6. The rubber composition for tire treads according to Claim 5, wherein the aromatic resin further contains (meth)acrylate as a constituent monomer.

7. The rubber composition for tire treads according to Claim 1, further comprising 0.3 to 20 parts by mass of porous cellulose particles per 100 parts by mass of the diene rubber.

8. The rubber composition for tire treads according to Claim 1, wherein 100 parts by mass of the diene rubber contains 20 to 65 parts by mass of an isoprene rubber and 10 to 55 parts by mass of a butadiene rubber.

9. A tire having a tread formed of the rubber composition for tire treads according to any one of Claims 1 to 8. ​

Images