Patent Description:
<CIT> describes a tire having a tread composed of a rubber composition comprising an ethylene-propylene-styrene copolymer.

<CIT> relates to a tyre, the tread of which comprises an elastomer composition comprising: from <NUM> to <NUM> phr of a polybutadiene (BR) elastomer; from <NUM> to <NUM> phr of a saturated thermoplastic styrene (TPS) elastomer; and a reinforcing filler.

<CIT> relates to a rubber composition for treads including, based on <NUM>% by mass of a rubber component, <NUM>-<NUM>% by mass of BR and <NUM>-<NUM>% by mass of SBR; and includes, per <NUM> parts by mass of the rubber component, <NUM>-<NUM> parts by mass of a silica having a nitrogen adsorption specific surface area of <NUM>-<NUM><NUM>/g, and <NUM>-<NUM> parts by mass of a carbon black having a nitrogen adsorption specific surface area of <NUM>-<NUM><NUM>/g, wherein a ratio of a total content of silica to a total content of SBR is <NUM>-<NUM>, and a ratio of a total content of carbon black to a total content of BR is <NUM>-<NUM>.

<CIT> relates to a rubber composition comprising a predetermined amount of at least one selected from the group consisting of stearic acid, a metal salt of saturated fatty acid and a specific mold release agent, a specific resin, silica and a silane coupling agent, based on a diene rubber component, where an amount of an unreacted silane coupling agent in the rubber composition before vulcanization is within a predetermined range.

As in the rubber composition described in <CIT>, there is still a problem that when a blending amount of natural rubber in a rubber composition is increased for improving chipping resistance, the wet grip performance decreases.

An object of the present invention is to provide a vulcanized rubber composition (hereinafter also simply referred to as rubber composition) for a tread which is good in the balance between its wet grip performance and its chipping resistance.

The present invention has been achieved by the finding that when a rubber composition for a tread comprises a rubber component comprising prescribed amounts of butadiene rubber and styrene butadiene rubber, ethylene-propylene-styrene copolymer, and a filler comprising silica and prescribed amount of specified carbon black and has an elongation at break in a prescribed range, its wet grip performance and chipping resistance are improved in a balanced manner.

According to the present invention, a rubber composition for a tread which is good in the balance between its wet grip performance and its chipping resistance and a tire having a tread composed of the rubber composition can be provided.

The rubber composition for a tire of one embodiment of the present invention is characterized by comprising a rubber component comprising prescribed amounts of butadiene rubber and styrene butadiene rubber, an ethylene-propylene-styrene copolymer, and a filler comprising silica and prescribed amount of specified carbon black and having an elongation at break in a prescribed range. It should be noted that when "to" is used herein to indicate a numerical range, the range includes the values of the both ends.

The term "normal rim" used herein refers to a rim specified for each tire by a standard for tires in the standard system including the standard, for example, a standard rim for JATMA, "Design Rim" for TRA, and "Measuring Rim" for ETRTO.

The "normal internal pressure" herein is air pressure which the standard specifies for each tire, for example, the highest air pressure for JATMA, the maximum value found in the table "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" for TRA, "INFLATION PRESSURE" for ETRTO.

The present invention is defined by the appended claims and consists of the first to the third embodiments, which will be described in the following.

The first embodiment in line with claims <NUM> to <NUM> relates to:.

The rubber component used in the first embodiment comprises styrene butadiene rubber (SBR) and butadiene rubber (BR). Isoprene rubber such as natural rubber also may be blended to the extent that the effect of the present invention is not impaired. Also, the rubber component can be a rubber component consisting only of SBR and BR or can be a rubber component consisting only of isoprene rubber, SBR, and BR.

The SBR, which may be oil extended or non-oil extended, is not particularly limited, and examples thereof include a solution-polymerized SBR (S-SBR), an emulsion-polymerized SBR (E-SBR), a modified SBRs thereof (modified S-SBR, modified E-SBR). Examples of the modified SBRs include terminal-modified and/or main chain-modified SBR, and a modified SBR coupled with a tin or silicon compound (such as a condensate, one having a branch structure, etc.). Among these, E-SBR is preferred because it provides good chipping resistance.

Examples of S-SBRs usable in the first embodiment include S-SBRs manufactured by JSR Corporation, Sumitomo Chemical Company, Limited, Ube Industries, Ltd. , Asahi Kasei Corporation, ZEON CORPORATION, etc..

A styrene content of the SBR is, in terms of grip performance and rubber strength, preferably not less than <NUM>% by mass, more preferably not less than <NUM>% by mass, further preferably not less than <NUM>% by mass. Also, in terms of fuel efficiency, the styrene content of the SBR is preferably not more than <NUM>% by mass, more preferably not more than <NUM>% by mass, further preferably not more than <NUM>% by mass. It should be noted that herein the styrene content of the SBR is calculated in accordance with <NUM>H-NMR measurement.

A content of the SBR in the rubber component is, in terms of chipping resistance, not less than <NUM>% by mass, preferably not less than <NUM>% by mass, more preferably not less than <NUM>% by mass. Also, the content of the SBR in the rubber component is, in terms of abrasion resistance, not more than <NUM>% by mass, preferably not more than <NUM>% by mass, more preferably not more than <NUM>% by mass, further preferably not more than <NUM>% by mass.

BR is not particularly limited, and any of those which are common in a tire industry can be used, such as a BR having a content of cis-<NUM>,<NUM> bond of less than <NUM>% (low cis BR), a BR having a content of cis-<NUM>,<NUM> bond of <NUM>% or more (high cis BR), a rare-earth butadiene rubber (rare-earth BR) synthesized using a rare-earth element catalyst, a BR comprising syndiotactic polybutadiene crystals (SPB-containing BR), a modified BR (high cis modified BR, low cis modified BR) and the like. Among these BRs, a high cis BR is preferred because it provides good abrasion resistance.

Examples of high-cis BRs include BR1220 available from ZEON CORPORATION, BR130B, BR150B and BR150L available from Ube Industries, Ltd. , BR730 available from JSR Corporation and the like. When the rubber component comprises high cis BR, its low temperature characteristics and abrasion resistance can be enhanced. Examples of rare-earth BRs include BUNA-CB25 available from Lanxess K. and the like.

A cis-<NUM>,<NUM> bond content of (cis content) of the BR is, in terms of durability and abrasion resistance, preferably not less than <NUM>% by mass, more preferably not less than <NUM>% by mass, more preferably not less than <NUM>% by mass. When the BR has a larger cis content, a polymer chain is arranged regularly, and the interaction between the polymers becomes stronger, which improves the rubber strength. Therefore, the chipping resistance of the rubber composition is expected to be improved.

A content of the BR in the rubber component is not less than <NUM>% by mass, preferably not less than <NUM>% by mass, more preferably not less than <NUM>% by mass. When it is less than <NUM>% by mass, the effect of the present invention tends to be insufficient. Also, the content of the BR is not more than <NUM>% by mass, preferably not more than <NUM>% by mass, more preferably not more than <NUM>% by mass. When it is more than <NUM>% by mass, there is a tendency that chipping resistance decreases and block breakage becomes liable to occur.

Isoprene rubber which is commonly employed in a tire industry, such as isoprene rubber (IR) and natural rubber can be used. The natural rubber includes a modified natural rubber such as an epoxidized natural rubber (ENR), a hydrogenated natural rubber (HNR), a deproteinized natural rubber (DPNR) or a high purity natural rubber (UPNR) and the like, as well as un-modified natural rubber (NR). These rubbers may be used alone or in combination with at least two thereof.

NR is not particularly limited, and any of those which are common in a tire industry, such as SIR20, RSS#<NUM>, TSR20, can be used.

When the rubber component comprises the isoprene rubber, the content thereof in the rubber component is, in terms of chipping resistance, preferably not less than <NUM>% by mass, more preferably not less than <NUM>% by mass, further preferably not less than <NUM>% by mass. Also, in terms of wet grip performance, the content of the isoprene rubber is preferably not more than <NUM>% by mass, more preferably not more than <NUM>% by mass, further preferably not more than <NUM>% by mass.

As a rubber component according to the first embodiment, a rubber component other than the above-mentioned isoprene rubber, SBR, and BR may be contained. As the other rubber components, a crosslinkable rubber component which is commonly employed in a rubber industry can be used, and examples thereof include diene rubber such as a styrene-isoprene-butadiene copolymer (SIBR), a chloroprene rubber (CR) and an acrylonitrile-butadiene rubber (NBR), and butyl rubbers. These other rubber components may be used alone or in combination with at least two thereof.

An ethylene-propylene-styrene copolymer is a copolymer of ethylene, propylene, and styrene, wherein the ethylene phase and the propylene phase are compatible with a rubber component and the styrene phase is compatible with a filler. In the rubber composition according to the first embodiment, when a rubber component comprising SBR and BR, the copolymer, and a filler comprising silica are used together, the interaction between the polymer and the filler is improved. Accordingly, the dispersibility of the filler in the rubber component is improved, and the filler disperses uniformly; therefore, wet grip performance and chipping resistance are expected to be improved in a well-balanced manner.

The ethylene content of the ethylene-propylene-styrene copolymer is, in terms of fuel efficiency, preferably not less than <NUM>% by mass, more preferably not less than <NUM>% by mass, further preferably not less than <NUM>% by mass. Also, in terms of wet grip performance, it is preferably not more than <NUM>% by mass, more preferably not more than <NUM>% by mass, further preferably not more than <NUM>% by mass.

The propylene content of the ethylene-propylene-styrene copolymer is, in terms of wet grip performance, preferably not less than <NUM>% by mass, more preferably not less than <NUM>% by mass, further preferably not less than <NUM>% by mass. Also, in terms of fuel efficiency, it is preferably not more than <NUM>% by mass, more preferably not more than <NUM>% by mass, further preferably not more than <NUM>% by mass.

The styrene content of the ethylene-propylene-styrene copolymer is, in terms of chipping resistance, preferably not less than <NUM>% by mass, more preferably not less than <NUM>% by mass, further preferably not less than <NUM>% by mass. Also, in terms of the fuel efficiency, it is preferably not more than <NUM>% by mass, more preferably not more than <NUM>% by mass, further preferably not more than <NUM>% by mass.

Examples of such ethylene-propylene-styrene copolymers include PROMIX400 available from Flow Polymers Inc.

A content of the ethylene-propylene-styrene copolymer based on <NUM> parts by mass of the rubber component is not less than <NUM> parts by mass, preferably not less than <NUM> parts by mass, more preferably not less than <NUM> parts by mass. When the content of the ethylene-propylene-styrene copolymer is less than <NUM> parts by mass, chipping resistance tends to be decreased. Also, the content of the ethylene-propylene-styrene copolymer is not more than <NUM> parts by mass, preferably not more than <NUM> parts by mass, more preferably not more than <NUM> parts by mass, further preferably not more than <NUM> parts by mass. When the content of the ethylene-propylene-styrene copolymer is more than <NUM> parts by mass, wet grip performance tends to be decreased.

The filler used in the first embodiment is characterized by comprising silica and carbon black as essential components. Silica is preferably used with a silane coupling agent.

Silica is not limited particularly, and any of those which are common in a tire industry, such as silica prepared by a dry method (anhydrous silica) and silica prepared by a wet method (hydrous silica), can be used. Among those, hydrous silica prepared by a wet method is preferred because it contains a lot of silanol groups. Those kinds of silica may be used alone or in combination with at least two thereof.

A nitrogen adsorption specific surface area (N<NUM>SA) of the silica is, in terms of elongation at break, preferably not less than <NUM><NUM>/g, more preferably not less than <NUM><NUM>/g, further preferably not less than <NUM><NUM>/g, particularly preferably not less than <NUM><NUM>/g. Also, the N<NUM>SA of the silica is, in terms of fuel efficiency and processability, preferably not more than <NUM><NUM>/g, more preferably not more than <NUM><NUM>/g, further preferably not more than <NUM><NUM>/g. When the above-mentioned small particle-sized silica is dispersed in the neighborhoods of boundaries of each phase of isoprene rubber, BR and SBR, the contact area of the rubber component and the silica increases; therefore, chipping resistance is expected to be improved. It should be noted that herein the N<NUM>SA of the silica is a value measured by the BET method in accordance with ASTM D3037-<NUM>.

A content of silica based on <NUM> parts by mass of the rubber component is, in terms of wet grip performance, preferably not less than <NUM> parts by mass, more preferably not less than <NUM> parts by mass, further preferably not less than <NUM> parts by mass, particularly preferably not less than <NUM> parts by mass. Also, in terms of dispersibility and processability of the silica, the content of silica is preferably not more than <NUM> parts by mass, more preferably not more than <NUM> parts by mass, further preferably not more than <NUM> parts by mass, particularly preferably not more than <NUM> parts by mass.

Silica is preferably used in combination with a silane coupling agent. The silane coupling agent is not limited particularly, and any of those which are conventionally employed with silica in a rubber industry can be used. Examples of such silane coupling agents include: sulfide-based silane coupling agents such as bis(<NUM>-triethoxysilylpropyl) disulfide and bis(<NUM>-triethoxysilylpropyl) tetrasulfide; mercapto-based silane coupling agents such as <NUM>-mercaptopropyltrimethoxysilane and NXT-Z100, NXT-Z45, NXT and the like manufactured and sold by Momentive Performance Materials; vinyl-based silane coupling agents such as vinyltriethoxysilane, vinyl trimethoxysilane; amino-based silane coupling agents such as <NUM>-aminopropyltriethoxysilane, <NUM>-aminopropyltrimethoxysilane, and <NUM>-(<NUM>-aminoethyl)aminopropyltriethoxysilane; glycidoxy-based silane coupling agents such as γ-glycidoxypropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane; nitro-based silane coupling agents such as <NUM>-nitropropyltrimethoxysilane and <NUM>-nitroproropyltriethoxysilane; and chloro-based silane coupling agents such as <NUM>-chloropropyltrimethoxysilane and <NUM>-chloropropyltriethoxysilane. These silane coupling agent may be used alone or in combination with at least two thereof.

When the rubber composition comprises the silane coupling agent, the content thereof is preferably not less than <NUM> parts by mass, more preferably not less than <NUM> parts by mass, further preferably not less than <NUM> parts by mass based on <NUM> parts by mass of the rubber component. Also, the content of the silane coupling agent is preferably not more than <NUM> parts by mass, more preferably not more than <NUM> parts by mass, further preferably not more than <NUM> parts by mass based on <NUM> parts by mass of the rubber component.

Fillers other than silica and carbon black may be used as a filler. Such a filler is not limited particularly, and, for example, fillers commonly used in a rubber industry such as aluminum hydroxide, alumina (aluminum oxide), calcium carbonate, talc, clay, can be used. These fillers can be used alone or in combination with at least two thereof. Specifically, the filler comprises silica and carbon black, and a filler consisting only of silica and carbon black is preferable.

Carbon black which is commonly employed for rubber can be suitably used. Examples of carbon black include furnace black, acetylene black, thermal black, channel black, graphite, and specifically, N110, N115, N120, N125, N134, N135, N219, N220, N231, N234, N293, N299, N326, N330, N339, N343, N347, N351, N356, N358, N375, N539, N550, N582, N630, N642, N650, N660, N683, N754, N762, N765, N772, N774, N787, N907, N908, N990, N991 and the like can be used suitably. These kinds of carbon black may be used alone or in combination with at least two thereof.

A nitrogen adsorption specific surface area (N<NUM>SA) of the carbon black is not less than <NUM><NUM>/g, preferably not less than <NUM><NUM>/g, further preferably not less than <NUM><NUM>/g, particularly preferably not less than <NUM><NUM>/g. Although the upper limit of the N<NUM>SA of the carbon black is not specifically limited, the N<NUM>SA of the carbon black is, in terms of fuel efficiency, dispersibility and processability, preferably not more than <NUM><NUM>/g, more preferably not more than <NUM><NUM>/g, further preferably not more than <NUM><NUM>/g, particularly preferably not more than <NUM><NUM>/g. When the above-mentioned small particle-sized carbon black is dispersed in the neighborhoods of boundaries of each phase of isoprene rubber, BR and SBR, the contact area of the rubber component and the silica increases; therefore, chipping resistance is expected to be improved. It should be noted that, herein, the N<NUM>SA of the carbon black is a value measured according to JIS K <NUM>-<NUM> "Carbon black for rubber industry - Fundamental characteristics - Part <NUM>: Determination of specific surface area - Nitrogen adsorption methods - Single-point procedures".

An average primary particle size of the carbon black is, preferably not more than <NUM>, preferably not more than <NUM>, further preferably not more than <NUM>. Also, the average primary particle size of the carbon black is, in terms of processability, preferably not less than <NUM>, more preferably not less than <NUM>, further preferably not less than <NUM>. When the carbon black has a particle size within the above range, it becomes easier to maintain a good balance between wet grip performance and chipping resistance. It should be noted that herein the average primary particle size of the carbon black can be obtained by calculating the average of <NUM> or more primary particles observed with a transmission electron microscope in a field.

The rubber composition comprises the carbon black and the content thereof based on <NUM> parts by mass of the rubber component is, in terms of the weather resistance and reinforcing performance, not less than <NUM> parts by mass, preferably not less than <NUM> parts by mass, further preferably not less than <NUM> parts by mass, particularly preferably not less than <NUM> parts by mass. Regarding the upper limit of the content of the carbon black, the content of the carbon black is, in terms of the fuel efficiency and processability, <NUM> parts by mass, preferably not more than <NUM> parts by mass, further preferably not more than <NUM> parts by mass, particularly preferably not more than <NUM> parts by mass.

A content of the whole filler based on <NUM> parts by mass of the rubber component is, in terms of the effectiveness of the present invention, not less than <NUM> parts by mass, preferably not less than <NUM> parts by mass, more preferably not less than more preferably not less than <NUM> parts by mass. Also, in terms of dispersibility and processability of the filler, the content of the whole filler is preferably not more than <NUM> parts by mass, more preferably not more than <NUM> parts by mass, further preferably not more than <NUM> parts by mass, particularly preferably not more than <NUM> parts by mass.

A content of silica in the filler is, in terms of wet grip performance, preferably not less than <NUM>% by mass, more preferably not less than <NUM>% by mass, further preferably not less than <NUM>% by mass, particularly preferably not less than <NUM>% by mass. Also, in terms of weather resistance and reinforcing performance, preferably not more than <NUM>% by mass, more preferably not more than <NUM>% by mass, further preferably not more than <NUM>% by mass.

The rubber composition according to the first embodiment can suitably comprise, in addition to the above-mentioned rubber component, ethylene-propylene-styrene copolymer, and filler, a compounding agent and an additive which are conventionally employed in a tire industry, such as oil, wax, an antioxidant, stearic acid, zinc oxide, a vulcanizing agent, a vulcanization accelerator.

When the rubber composition comprises an oil, a content thereof is, for securing good abrasion resistance, preferably not more than <NUM> parts by mass, more preferably not more than <NUM> parts by mass, further preferably not more than <NUM> parts by mass based on <NUM> parts by mass of the rubber component. Also, in terms of processability, the content of the oil is preferably not less than <NUM> parts by mass, more preferably not less than <NUM> parts by mass, further preferably not less than <NUM> parts by mass. It should be noted that herein the content of the oil includes the oil content of oil-extended rubber.

When the rubber composition comprises a wax, a content thereof is, in terms of weather resistance of the rubber, preferably not less than <NUM> parts by mass, more preferably not less than <NUM> parts by mass based on <NUM> parts by mass of the rubber component. Also, for preventing whitening of a tire due to blooming, the content of the wax is preferably not more than <NUM> parts by mass, more preferably not more than <NUM> parts by mass.

An antioxidant is not limited particularly, and any of those which are employed in a rubber industry, such as quinoline-based, quinone-based, phenol-based, and phenylenediamine-based antioxidants, can be used.

When the rubber composition comprises an antioxidant, a content thereof is, in terms of ozone crack resistance of the rubber composition, preferably not less than <NUM> parts by mass, more preferably not less than <NUM> parts by mass based on <NUM> parts by mass of the rubber component. Also, in terms of abrasion resistance and grip performance, the content of the antioxidant is preferably not more than <NUM> parts by mass, more preferably not more than <NUM> parts by mass.

When the rubber composition comprises a stearic acid, a content thereof is, in terms of processability, preferably not less than <NUM> parts by mass, more preferably not less than <NUM> parts by mass based on <NUM> parts by mass of the rubber component. Also, the content of the stearic acid is, in terms of vulcanization rate, preferably not more than <NUM> parts by mass, more preferably not more than <NUM> parts by mass.

When the rubber composition comprises a zinc oxide, a content thereof is, in terms of the processability, preferably not less than <NUM> parts by mass, more preferably not less than <NUM> parts by mass based on <NUM> parts by mass of the rubber component. Also, in terms of abrasion resistance, the content of the zinc oxide is preferably not more than <NUM> parts by mass, more preferably not more than <NUM> parts by mass.

As a vulcanizing agent, sulfur is suitably used. Sulfur such as powdered sulfur, oil-treated sulfur, precipitated sulfur, colloidal sulfur, insoluble sulfur, highly dispersible sulfur can be used.

When sulfur is contained as a vulcanizing agent, the content thereof is, for assuring a sufficient vulcanization reaction and obtaining good grip performance and abrasion resistance, preferably not less than <NUM> parts by mass, more preferably not less than <NUM> parts by mass based on <NUM> parts by mass of the rubber component. Also, in terms of prevention of degradation, the content of the sulfur is preferably not more than <NUM> parts by mass, more preferably not more than <NUM> parts by mass.

Examples of vulcanizing agents other than sulfur include a vulcanizing agent containing a sulfur atom such as TACKIROL V200 manufactured by Taoka Chemical Co. , DURALINK HTS (<NUM>,<NUM>-hexamethylene-sodium dithiosulfate dehydrate) manufactured by Flexsys, KA9188 (<NUM>,<NUM>-bis(N,N'-dibenzylthiocarbamoyldithio)hexane) manufactured by LANXESS K. , and an organic peroxide such as a dicumyl peroxide.

Examples of vulcanization accelerators include sulfenamide-, thiazole-, thiuram-, thiourea-, guanidine-, dithiocarbamate-, aldehyde amine- or aldehyde ammonia-, imidazoline- and xanthate-based vulcanization accelerators. These vulcanization accelerators may be used alone or in combination with at least two or more thereof. Among these, sulfenamide-based vulcanization accelerators, thiazole-based vulcanization accelerators and guanidine-based vulcanization accelerators are preferable, and sulfenamide-based vulcanization accelerators are more preferable.

Examples of sulfenamide-based vulcanization accelerators include N-t-butyl-<NUM>-benzothiazolylsulfenamide (TBBS), N-cyclohexyl-<NUM>-benzothiazolylsulfenamide (CBS), N,N'-dicyclohexyl-<NUM>-benzothiazolylsulfenamide (DCBS). Among these, N-t-butyl-<NUM>-benzothiazolylsulfenamide (TBBS) and N-cyclohexyl-<NUM>-benzothiazolylsulfenamide (CBS) are preferred.

When the rubber composition comprises a vulcanization accelerator, a content thereof is, in terms of vulcanization acceleration, preferably not less than <NUM> parts by mass, more preferably not less than <NUM> parts by mass based on <NUM> parts by mass of the rubber component. Also, in terms of the processability, the content of the vulcanization accelerator is preferably not more than <NUM> parts by mass, more preferably not more than <NUM> parts by mass.

The rubber composition according to the first embodiment can be manufactured by a known method, for example, in which components other than a vulcanizing agent and a vulcanization accelerator are kneaded with a known kneading machine which is employed in a general rubber industry, such as, Banbury mixer, a kneader, or an open roll, then a vulcanizing agent and a vulcanization accelerator are added thereto, and vulcanizing the kneaded product.

The rubber composition according to the first embodiment is preferably used for a tread of a tire because it provides good chipping resistance.

An elongation at break (elongation at cut) EB (%) of the rubber composition according to the first embodiment is not less than <NUM>%, preferably not less than <NUM>%, more preferably not less than <NUM>%. It should be noted that herein the elongation at break is measured by conducting a tensile test of a No. <NUM> dumbell test piece made of the vulcanized rubber composition, in the atmosphere of <NUM> according to JIS K <NUM>: <NUM> "Vulcanized Rubber and Thermoplastic Rubber-Method of Obtaining Tensile Characteristics".

A tire comprising the rubber composition according to the first embodiment can be manufactured by a conventional method with the above-mentioned rubber composition. Specifically, the above-mentioned rubber composition, in which the above-mentioned compounding agent is blended to the rubber components as needed, is extruded into a shape of, for example, a tread, laminated with other tire members on a tire molding machine, and then molded by a conventional method to form an unvulcanized tire. A tire can be manufactured by heating and pressurizing this unvulcanized tire in a vulcanizer.

The second embodiment encompassed by claims <NUM> to <NUM> relates to:.

The rubber component used in the second embodiment comprises styrene butadiene rubber (SBR) and butadiene rubber (BR). Isoprene rubber such as natural rubber and other rubber components also may be blended to the extent that the effect of the present invention is not impaired. As SBR, BR, isoprene rubber, and other rubber component, the same rubber component as in the first embodiment can be used suitably in the same aspect.

A content of SBR of the rubber component is, in terms of chipping resistance, not less than <NUM>% by mass, preferably not less than <NUM>% by mass, more preferably not less than <NUM>% by mass. Also, in terms of abrasion resistance, it is not more than <NUM>% by mass, preferably not more than <NUM>% by mass, more preferably not more than <NUM>% by mass.

A content of BR in the rubber component is not less than <NUM>% by mass, preferably not less than <NUM>% by mass, more preferably not less than <NUM>% by mass. When it is less than <NUM>% by mass, the effect of the present invention tends to be insufficient. Also, the content of the BR is not more than <NUM>% by mass, preferably not more than <NUM>% by mass, more preferably not more than <NUM>% by mass. When it is more than <NUM>% by mass, there is a tendency that chipping resistance decreases and block breakage becomes liable to occur.

When the rubber component comprises the isoprene rubber, the content thereof in the rubber component is, in terms of chipping resistance, preferably not less than <NUM>% by mass, more preferably not less than <NUM>% by mass, further preferably not less than <NUM>% by mass. Also, the content of the isoprene rubber in the rubber component is not more than <NUM>% by mass, preferably not more than <NUM>% by mass, more preferably not more than <NUM>% by mass. When the content of the isoprene rubber in the rubber component is more than <NUM>% by mass, wet grip performance tends to be decreased.

The rubber composition according to the second embodiment comprises an ethylene-propylene-styrene copolymer. As an ethylene-propylene-styrene copolymer, the same rubber composition as in the first embodiment can be used suitably in the same aspect.

The filler used in the second embodiment is characterized by comprising silica and carbon black as essential components. Also, it is preferable to use the silica with a silane coupling agent. Further, other fillers may be used as a filler. As silica, a silane coupling agent, carbon black, and other fillers, the same rubber composition as in the first embodiment can be used suitably in the same aspect.

A content of silica is, in terms of wet grip performance, preferably not less than <NUM> parts by mass, more preferably not less than <NUM> parts by mass, further preferably not less than <NUM> parts by mass, particularly preferably not less than <NUM> parts by mass based on <NUM> parts by mass of the rubber component. Also, in terms of dispersibility and processability of the silica, the content of silica is preferably not more than <NUM> parts by mass, more preferably not more than <NUM> parts by mass, further preferably not more than <NUM> parts by mass.

When the rubber composition comprises a silane coupling agent, a content thereof is, preferably not less than <NUM> parts by mass, more preferably not less than <NUM> parts by mass, further preferably not less than <NUM> parts by mass based on <NUM> parts by mass of the rubber component. Also, the content of the silane coupling agent based on <NUM> parts by mass of the rubber component is, preferably not more than <NUM> parts by mass, more preferably not more than <NUM> parts by mass, further preferably not more than <NUM> parts by mass.

The rubber composition comprises a carbon black and the content thereof is, in terms of weather resistance and reinforcing performance, not less than <NUM> parts by mass, preferably not less than <NUM> parts by mass, further preferably not less than <NUM> parts by mass, particularly preferably not less than <NUM> parts by mass based on <NUM> parts by mass of the rubber component. Regarding the upper limit of the content of the carbon black, the content of the carbon black is, in terms of fuel efficiency and processability, not more than <NUM> parts by mass, preferably not more than <NUM> parts by mass, further preferably not more than <NUM> parts by mass.

A content of the whole filler based on <NUM> parts by mass of the rubber component is, in terms of the effectiveness of the present invention, not less than <NUM> parts by mass, preferably not less than <NUM> parts by mass, more preferably not less than <NUM> parts by mass. Also, in terms of dispersibility and processability of the filler, the content of the whole filler is preferably not more than <NUM> parts by mass, more preferably not more than <NUM> parts by mass, further preferably not more than <NUM> parts by mass, particularly preferably not more than <NUM> parts by mass.

A content of silica in the filler is, in terms of wet grip performance, preferably not less than <NUM>% by mass, more preferably not less than <NUM>% by mass, further preferably not less than <NUM>% by mass, particularly preferably not less than <NUM>% by mass. Also, in terms of weather resistance and reinforcing performance, the content of silica in the filer is preferably not more than <NUM>% by mass, more preferably not more than <NUM>% by mass, further preferably not more than <NUM>% by mass, particularly preferably not more than <NUM>% by mass.

The rubber composition according to the second embodiment can suitably comprise, in addition to the above-mentioned rubber component, ethylene-propylene-styrene copolymer, and filler, a compounding agent and an additive conventionally employed in the tire industry, such as oil, wax, antioxidant, stearic acid, zinc oxide, vulcanizing agent, vulcanization accelerator, as needed. As the above-mentioned compounding agent and additive, the same rubber composition as in the first embodiment can be used suitably in the same aspect.

The rubber composition according to the second embodiment can be manufactured by a known method, for example, in which components other than a vulcanizing agent and a vulcanization accelerator are kneaded with a known kneading machine which is employed in a general rubber industry, such as, Banbury mixer, a kneader, or an open roll, then a vulcanizing agent and a vulcanization accelerator are added thereto, and vulcanizing the kneaded product.

The rubber composition according to the second embodiment is preferably used for a tread of a tire because it provides good chipping resistance.

An elongation at break (elongation at cut) EB (%) of the rubber composition according to the second embodiment is not less than <NUM>%, preferably not less than <NUM>%, more preferably not less than <NUM>%. The elongation at break used herein is measured by conducting a tensile test of a No. <NUM> dumbell test piece made of the vulcanized rubber composition, in the atmosphere of <NUM> according to JIS K <NUM>: <NUM> "Vulcanized Rubber and Thermoplastic Rubber-Method of Obtaining Tensile Characteristics".

A tire comprising the rubber composition according to the second embodiment can be manufactured by a conventional method with the above-mentioned rubber composition. Specifically, the above-mentioned rubber composition, in which the above-mentioned compounding agent is blended to the rubber components as needed, is extruded into a shape of, for example, a tread, laminated with other tire members on a tire molding machine, and molded by a conventional method to form an unvulcanized tire. A tire can be manufactured by heating and pressurizing this unvulcanized tire in a vulcanizer.

The third embodiment encompassed by claims <NUM> to <NUM> relates to:.

The rubber component used in the third embodiment comprises styrene butadiene rubber (SBR) and butadiene rubber (BR). Isoprene rubber such as natural rubber and other rubber components also may be blended to the extent that the effect of the present invention is not impaired. As SBR, BR, isoprene rubber, and other rubber components, the same rubber component as in the first embodiment can be used suitably in the same aspect.

A content of SBR in the rubber component is, in terms of chipping resistance, not less than <NUM>% by mass, preferably not less than <NUM>% by mass, more preferably not less than <NUM>% by mass. Also, in terms of abrasion resistance, it is not more than <NUM>% by mass, preferably not more than <NUM>% by mass, more preferably not more than <NUM>% by mass.

The rubber composition according to the third embodiment comprises an ethylene-propylene-styrene copolymer. As the ethylene-propylene-styrene copolymer, the same rubber composition as in the first invention can be used suitably in the same aspect.

The filler used in the third embodiment is characterized by comprising silica and carbon black as essential components. Also, it is preferable to use the silica with a silane coupling agent. Further, other fillers may be used as a filler. As silica, a silane coupling agent, carbon black, and other fillers, the same rubber composition as in the first embodiment can be used suitably in the same aspect.

When the rubber composition comprises a silane coupling agent, a content thereof is preferably not less than <NUM> parts by mass, more preferably not less than <NUM> parts by mass, further preferably not less than <NUM> parts by mass based on <NUM> parts by mass of the rubber component. Also, the content of the silane coupling agent based on <NUM> parts by mass of the rubber component is, preferably not more than <NUM> parts by mass, more preferably not more than <NUM> parts by mass, further preferably not more than <NUM> parts by mass.

The rubber composition comprises a carbon black and a content thereof is, in terms of weather resistance and reinforcing performance, not less than <NUM> parts by mass, preferably not less than <NUM> parts by mass, further preferably not less than <NUM> parts by mass based on <NUM> parts by mass of the rubber component. Also, in terms of wet grip performance, the content of the carbon black is not more than <NUM> parts by mass, preferably not more than <NUM> parts by mass, further preferably not more than <NUM> parts by mass.

A content of the whole filler based on <NUM> parts by mass of the rubber component is, in terms of the effectiveness of the present invention, not less than <NUM> parts by mass, preferably not less than <NUM> parts by mass, more preferably not less than <NUM> parts by mass. Also, in terms of dispersibility and processability of the filler, A content of the whole filler is preferably not more than <NUM> parts by mass, more preferably not more than <NUM> parts by mass, further preferably not more than <NUM> parts by mass, particularly preferably not more than <NUM> parts by mass.

A content of silica in the filler is, in terms of wet grip performance, preferably not less than <NUM>% by mass, more preferably not less than <NUM>% by mass, further preferably not less than <NUM>% by mass, particularly preferably not less than <NUM>% by mass. Also, in terms of weather resistance and reinforcing performance, the content of silica in the filler is preferably not more than <NUM>% by mass, more preferably not more than <NUM>% by mass, further preferably not more than <NUM>% by mass.

The rubber composition according to the third embodiment can suitably comprise, in addition to the above-mentioned rubber component, ethylene-propylene-styrene copolymer, and filler, a compounding agent and an additive conventionally employed in the tire industry, such as oil, wax, antioxidant, stearic acid, zinc oxide, vulcanizing agent, vulcanization accelerator, as needed. As the above-mentioned compounding agent and additive, the same rubber composition as in the second embodiment can be used suitably in the same aspect.

The rubber composition according to the third embodiment can be manufactured by a known method, for example, in which components other than a vulcanizing agent and a vulcanization accelerator are kneaded with a known kneading machine which is employed in a general rubber industry, such as, Banbury mixer, a kneader, or an open roll, then a vulcanizing agent and a vulcanization accelerator are added thereto, and vulcanizing the kneaded product.

The rubber composition according to the third embodiment is preferably used for a tread of a tire because it provides good chipping resistance.

An elongation at break (elongation at cut) EB (%) of the rubber composition according to the third embodiment is not less than <NUM>%, preferably not less than <NUM>%, more preferably not less than <NUM>%. It should be noted that herein the elongation at break is measured by conducting a tensile test of a No. <NUM> dumbell test piece made of the vulcanized rubber composition, in the atmosphere of <NUM> according to JIS K <NUM>: <NUM> "Vulcanized Rubber and Thermoplastic Rubber-Method of Obtaining Tensile Characteristics".

A tire comprising the rubber composition according to the third embodiment can be manufactured by a conventional method with the above-mentioned rubber composition. Specifically, the above-mentioned rubber composition, in which the above-mentioned compounding agent is blended to the rubber components as needed, is extruded into a shape of, for example, a tread, laminated with other tire members on a tire molding machine, and molded by a conventional method to form an unvulcanized tire. A tire can be manufactured by heating and pressurizing this unvulcanized tire in a vulcanizer.

The present invention will be described in detail with reference to Examples, which however shall not be construed as limiting the invention thereto.

The chemicals used in Examples and Comparative Examples will be described.

According to the compounding formulations shown in Tables <NUM> to <NUM>, using a <NUM> sealed Banbury mixer, all of the chemicals except sulfur and a vulcanization accelerator were kneaded for five minutes until it reaches a discharge temperature of <NUM> to obtain a kneaded product. Then, the obtained kneaded product was kneaded again (remilled) at a discharge temperature of <NUM> for four minutes by the Banbury mixer. Then, sulfur and a vulcanization accelerator were added to the obtained kneaded product, and kneaded for <NUM> minutes until it reaches <NUM> using a biaxial open roll to obtain an unvulcanized rubber composition. The obtained unvulcanized rubber composition was press-vulcanized at <NUM> for <NUM> minutes to manufacture a test rubber composition.

Further, the obtained unvulcanized rubber composition was extruded and molded into the shape of a tire tread by an extruder equipped with a base having a predetermined shape, laminated with other tire members to form an unvulcanized tire, and then press-vulcanized to manufacture and prepare a test tire (12R22. <NUM>, a tire for a truck and a bus).

The obtained test rubber composition and test tires were subjected to the following evaluation. Evaluation results are shown in Table <NUM>.

Elongation at break (elongation at cut) EB (%) is measured by conducting a tensile test of No. <NUM> dumbell test pieces made of each of the vulcanized rubber compositions, in the atmosphere of <NUM> according to JIS K <NUM>: <NUM> "Vulcanized Rubber and Thermoplastic Rubber-Method of Obtaining Tensile Characteristics". A larger elongation at break indicates a better chipping resistance of a rubber composition.

Each of the test tires were mounted onto all the wheels of a vehicle (a Japanese, FF type 2000cc car), and braking distances were calculated on a wet asphalt road from an initial speed of <NUM>/h. The results are shown by an index, wherein the index values of the reference comparative examples (Comparative Example <NUM> in Table <NUM> and <NUM> and Comparative Example <NUM> in Table <NUM> and <NUM>) were set to <NUM>. A larger index value indicates a better wet grip performance. The index values were calculated by the following formula. It should be noted that the target performance value is <NUM> or higher, preferably <NUM> or higher, more preferably <NUM> or higher.

After being mounted to normal rims and filled with air to the normal internal pressure, each of the test tires were installed to a vehicle, and the vehicle was driven on a rough road at a speed of <NUM>/h for four hours. After driving, the circumferential lengths of all of the cracks occurred on the tire surface were measured and the longest circumferential lengths of each tire were obtained. The results are shown by index values, wherein the index values of the reference comparative examples (Comparative Example <NUM> in Table <NUM> and <NUM> and Comparative Example <NUM> in Table <NUM> and <NUM>) were set to <NUM>. A larger index value indicates a better chipping resistance. The index values were calculated by the following formula. It should be noted that the target performance value is <NUM> or higher, preferably <NUM> or higher, more preferably <NUM> or higher.

The target performance value of the total performance of the wet grip performance and chipping resistance (an average value of the wet grip performance index value and the chipping resistance index value) is <NUM> or higher, preferably <NUM> or higher, more preferably <NUM> or higher.

From the results of Table <NUM> to Table <NUM>, it can be seen that the rubber compositions for a tread of the present invention comprising a rubber component which comprises prescribed amounts of butadiene rubber and styrene butadiene rubber, an ethylene-propylene-styrene copolymer, and a filler comprising silica and having an elongation at break in a prescribed range has improved wet grip performance and chipping resistance in a balanced manner.

Claim 1:
A vulcanized rubber composition for a tread comprising:
<NUM> to <NUM> parts by mass of an ethylene-propylene-styrene copolymer and <NUM> parts by mass or more of a filler
based on <NUM> parts by mass of a rubber component comprising <NUM> to <NUM>% by mass of a butadiene rubber and <NUM> to <NUM>% by mass of a styrene butadiene rubber,
wherein the filler comprises silica and <NUM> to <NUM> parts by mass of a carbon black having a nitrogen adsorption specific surface area of not less than <NUM><NUM>/g, and
wherein an elongation at break of the vulcanized rubber composition is not less than <NUM>% in the atmosphere of <NUM> according to JIS K <NUM>: <NUM>.