Patent Description:
Tires require a variety of properties. In recent years, it has become desirable to improve especially abrasion resistance, wet grip performance, and bleed resistance in a well-balanced manner.

The present invention aims to solve the above problem and provide a tire with excellent overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance.

The present invention relates to a tire, including a tread including a rubber composition that contains at least one rubber component and a branched conjugated diene polymer,.

The tire according to the present invention includes a tread including a rubber composition that contains a rubber component and a branched conjugated diene polymer, wherein the branched conjugated diene polymer is a copolymer at least containing a branched conjugated diene, butadiene, and styrene as structural units, and the tire satisfies relationships (<NUM>) and (<NUM>). Thus, the present invention can provide a tire with excellent overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance.

The tire of the present invention includes a tread including a rubber composition that contains a rubber component and a branched conjugated diene polymer. The branched conjugated diene polymer is a copolymer at least containing a branched conjugated diene, butadiene, and styrene as structural units, and the tire satisfies the relationship (<NUM>): <NUM> < A ≤ <NUM> and the relationship (<NUM>): A × S ≤ <NUM>.

The mechanism for the above-mentioned advantageous effect is not clear, but it is believed to be as follows.

A branched conjugated diene polymer at least containing a branched conjugated diene, butadiene, and styrene as structural units may provide improved compatibility with rubber components such as polybutadiene rubbers and styrene-butadiene rubbers. Particularly, when the branched conjugated diene polymer is controlled to satisfy the relationship (<NUM>): <NUM> < A ≤ <NUM>, i.e., to adjust the weight average molecular weight of the branched conjugated diene polymer within a predetermined range, the single-domain phases may be decreased to further improve the compatibility. With the decrease of the single-domain phases, the bleeding of the branched conjugated diene polymer may be reduced, resulting in improved bleed resistance and abrasion resistance.

The branched conjugated diene polymer containing styrene as a structural unit may achieve high Tg, thereby improving wet grip performance.

When the value of A and the groove area ratio S (%) are controlled to satisfy the relationship (<NUM>): A × S ≤ <NUM>, i.e., to increase the ground contact surface of the tread (or reduce the groove area ratio) while reducing the weight average molecular weight of the branched conjugated diene polymer, grip performance may be ensured and at the same time wear may be reduced, resulting in improved wet grip performance and abrasion resistance.

The above-described mechanism is believed to improve overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance.

As discussed above, the tire can solve the problem (purpose) of improving overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance by being formulated to satisfy the relationship (<NUM>): <NUM> < A ≤ <NUM> and the relationship (<NUM>): A × S ≤ <NUM>. In other words, these relationships do not define the problem (purpose), and the problem herein is to improve overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance. In order to solve this problem, the tire has been formulated to satisfy these parameters.

Here, relationships (<NUM>) and (<NUM>) can be controlled to be satisfied by appropriately selecting the weight average molecular weight of the branched conjugated diene polymer used and the groove area ratio of the ground contact surface of the tread.

The tire includes a tread including a rubber composition that contains a rubber component and a branched conjugated diene polymer.

The rubber composition contains one or more rubber components.

The weight average molecular weight of the rubber components is preferably <NUM>,<NUM> or more, more preferably <NUM>,<NUM> or more, still more preferably <NUM>,<NUM> or more, but it is preferably <NUM>,<NUM>,<NUM> or less, more preferably <NUM>,<NUM>,<NUM> or less, still more preferably <NUM>,<NUM>,<NUM> or less. When the weight average molecular weight is within the range indicated above, the advantageous effect tends to be better achieved.

Herein, the weight average molecular weight (Mw) of the rubber components can be determined by gel permeation chromatography (GPC) (GPC-<NUM> series available from Tosoh Corporation, detector: differential refractometer, column: TSKgel SuperMultipore HZ-M available from Tosoh Corporation) and calibrated with polystyrene standards.

Any rubber component may be used, including those known in the tire field. Examples include diene rubbers such as isoprene-based rubbers, polybutadiene rubbers (BR), styrene-butadiene rubbers (SBR), acrylonitrile-butadiene rubbers (NBR), chloroprene rubbers (CR), butyl rubbers (IIR), and styrene-isoprene-butadiene copolymer rubbers (SIBR). These may be used alone or in combinations of two or more. To better achieve the advantageous effect, it is preferred to use any of BR, SBR, and isoprene-based rubbers, among others. It is more preferred to use either of BR or SBR, and it is still more preferred to use both BR and SBR.

The mechanism for such an advantageous effect achieved by using at least one of isoprene-based rubbers, polybutadiene rubbers, and styrene-butadiene rubbers is not clear, but it is believed that these rubbers provide good abrasion resistance and good wet grip performance, and at the same time the combination of these rubbers with the branched conjugated diene polymer can have a larger effect in preventing bleeding, thereby resulting in improved overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance.

Any BR may be used, and examples include those commonly used in the tire industry, including: high cis BR such as BR1220 available from Zeon Corporation, BR150B available from Ube Corporation, and BR1280 available from LG Chem; BR containing <NUM>,<NUM>-syndiotactic polybutadiene crystals (SPB) such as VCR412 and VCR617 both available from Ube Corporation; and polybutadiene rubbers synthesized using rare earth catalysts (rare earth-catalyzed BR). These may be used alone or in combinations of two or more. Hydrogenated polybutadiene polymers (hydrogenated BR) are also usable as the BR.

The cis content of the BR is preferably <NUM>% by mass or higher, more preferably <NUM>% by mass or higher, still more preferably <NUM>% by mass or higher, further preferably <NUM>% by mass or higher, but it is preferably <NUM>% by mass or lower, more preferably <NUM>% by mass or lower, still more preferably <NUM>% by mass or lower. When the cis content is within the range indicated above, the advantageous effect tends to be better achieved.

Here, the cis content of the BR can be measured by infrared absorption spectrometry.

When one type of BR is used, the cis content of the BR refers to the cis content of the one BR, while when multiple types of BR are used, it refers to the average cis content.

The average cis content of the BR can be calculated using the equation: {Σ(amount of each BR × cis content of the each BR)}/amount of total BR. For example, when <NUM>% by mass of rubber components include <NUM>% by mass of BR having a cis content of <NUM>% by mass and <NUM>% by mass of BR having a cis content of <NUM>% by mass, the average cis content of the BR is <NUM>% by mass (= (<NUM> × <NUM> + <NUM> × <NUM>)/(<NUM> + <NUM>)).

The amount of BR, if present, based on <NUM>% by mass of the rubber component content in the rubber composition is preferably <NUM>% by mass or more, more preferably <NUM>% by mass or more, still more preferably <NUM>% by mass or more, but it is preferably <NUM>% by mass or less, more preferably <NUM>% by mass or less, still more preferably <NUM>% by mass or less. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

Any SBR may be used, and examples include emulsion-polymerized styrene-butadiene rubbers (E-SBR) and solution-polymerized styrene-butadiene rubbers (S-SBR). Usable commercial products are available from Sumitomo Chemical Co. , JSR Corporation, Asahi Kasei Corporation, Zeon Corporation, etc. Hydrogenated styrene-butadiene copolymers (hydrogenated SBR) may also be used as the SBR.

The styrene content of the SBR is preferably <NUM>% by mass or higher, more preferably <NUM>% by mass or higher, still more preferably <NUM>% by mass or higher, further preferably <NUM>% by mass or higher, but it is preferably <NUM>% by mass or lower, more preferably <NUM>% by mass or lower, still more preferably <NUM>% by mass or lower. When the styrene content is within the range indicated above, the advantageous effect tends to be better achieved.

Herein, the styrene content of the SBR can be determined by <NUM>H-NMR analysis.

Particularly for use in summer tires, the styrene content of the SBR is preferably <NUM>% by mass or higher, more preferably <NUM>% by mass or higher, still more preferably <NUM>% by mass or higher, but it is preferably <NUM>% by mass or lower, more preferably <NUM>% by mass or lower, still more preferably <NUM>% by mass or lower. When the styrene content is within the range indicated above, the advantageous effect tends to be better achieved.

Particularly for use in winter tires, the styrene content of the SBR is preferably <NUM>% by mass or higher, more preferably <NUM>% by mass or higher, still more preferably <NUM> by mass or higher, but it is preferably <NUM>% by mass or lower, more preferably <NUM>% by mass or lower, still more preferably <NUM>% by mass or lower. When the styrene content is within the range indicated above, the advantageous effect tends to be better achieved.

The vinyl content of the SBR is preferably <NUM>% by mass or higher, more preferably <NUM>% by mass or higher, still more preferably <NUM>% by mass or higher, but it is preferably <NUM>% by mass or lower, more preferably <NUM>% by mass or lower, still more preferably <NUM>% by mass or lower. When the vinyl content is within the range indicated above, the advantageous effect tends to be better achieved.

Herein, the vinyl content (<NUM>,<NUM>-butadiene unit content) of the SBR can be measured by infrared absorption spectrometry.

Here, when one type of SBR is used, the styrene content of the SBR refers to the styrene content of the one SBR, while when multiple types of SBR are used, it refers to the average styrene content.

The average styrene content of the SBR can be calculated using the equation: {Σ(amount of each SBR × styrene content of the each SBR)}/amount of total SBR. For example, when <NUM>% by mass of rubber components include <NUM>% by mass of SBR having a styrene content of <NUM>% by mass and <NUM>% by mass of SBR having a styrene content of <NUM>% by mass, the average styrene content of the SBR is <NUM>% by mass (= (<NUM> × <NUM> + <NUM> × <NUM>) / (<NUM> + <NUM>)).

Also, for example, when rubber components include <NUM>% by mass of SBR (A) (styrene content <NUM>% by mass, Mw <NUM>,<NUM>), <NUM>% by mass of SBR (B) (styrene content <NUM>% by mass, Mw <NUM>,<NUM>), <NUM>% by mass of NR, and <NUM>% by mass of BR, first, the content ratio of SBR (A) and SBR (B) are <NUM> (= <NUM>/(<NUM> + <NUM>)) and <NUM> (= <NUM>/(<NUM> + <NUM>)), respectively, and the average molecular weight of the SBR is <NUM>,<NUM> (= <NUM> × <NUM>,<NUM> + <NUM> × <NUM>,<NUM>).

Then, the styrene content of the SBR is <NUM>% by mass (= {<NUM>/(<NUM> + <NUM>) × <NUM> × <NUM>,<NUM> + <NUM>/(<NUM> + <NUM>) × <NUM> × <NUM>,<NUM>}/<NUM>,<NUM>).

The vinyl content of the SBR refers to the vinyl bond content (unit: % by mass) based on the total mass of the butadiene moieties in the SBR taken as <NUM>. The sum of the vinyl content (% by mass), the cis content (% by mass), and the trans content (% by mass) equals <NUM> (% by mass). When one type of SBR is used, the viny content of the SBR refers to the vinyl content of the one SBR, while when multiple types of SBR are used, it refers to the average vinyl content.

The average vinyl content of the SBR can be calculated using the equation: Σ{amount of each SBR × (<NUM> (% by mass) - styrene content (% by mass) of the each SBR) × vinyl content (% by mass) of the each SBR}/Σ{amount of each SBR × (<NUM> (% by mass) - styrene content (% by mass) of the each SBR)}. For example, when <NUM> parts by mass of rubber components include <NUM> parts by mass of SBR having a styrene content of <NUM>% by mass and a vinyl content of <NUM>% by mass, <NUM> parts by mass of SBR having a styrene content of <NUM>% by mass and a vinyl content of <NUM>% by mass, and the remaining <NUM> parts by mass of a rubber component other than SBR, the average vinyl content of the SBR is <NUM>% by mass (= {<NUM> × (<NUM> (% by mass) - <NUM> (% by mass)) × <NUM> (% by mass) + <NUM> × (<NUM> (% by mass) - <NUM> (% by mass)) × <NUM> (% by mass)}/{<NUM> × (<NUM> (% by mass) - <NUM> (% by mass)) + <NUM> × (<NUM> (% by mass) - <NUM> (% by mass))}.

The amount of SBR, if present, based on <NUM>% by mass of the rubber component content in the rubber composition is preferably <NUM>% by mass or more, more preferably <NUM>% by mass or more, still more preferably <NUM>% by mass or more, particularly preferably <NUM>% by mass or more, but it is preferably <NUM>% by mass or less, more preferably <NUM>% by mass or less, still more preferably <NUM>% by mass or less. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

The combined amount of BR and SBR based on <NUM>% by mass of the rubber component content in the rubber composition is preferably <NUM>% by mass or more, more preferably <NUM>% by mass or more, still more preferably <NUM>% by mass or more, and may be <NUM>% by mass. When the combined amount is within the range indicated above, the advantageous effect tends to be better achieved.

Particularly for use in summer tires, the combined amount of BR and SBR based on <NUM>% by mass of the rubber component content in the rubber composition is preferably <NUM>% by mass or more, more preferably <NUM>% by mass or more, still more preferably <NUM>% by mass or more, and may be <NUM> by mass. When the combined amount is within the range indicated above, the advantageous effect tends to be better achieved.

Particularly for use in winter tires, the combined amount of BR and SBR based on <NUM>% by mass of the rubber component content in the rubber composition is preferably <NUM>% by mass or more, more preferably <NUM>% by mass or more, still more preferably <NUM>% by mass or more, and may be <NUM>% by mass. When the combined amount is within the range indicated above, the advantageous effect tends to be better achieved.

Examples of isoprene-based rubbers include natural rubbers (NR), polyisoprene rubbers (IR), refined NR, modified NR, and modified IR. Examples of NR include those commonly used in the tire industry such as SIR20, RSS#<NUM>, and TSR20. Any IR may be used, including for example those commonly used in the tire industry such as IR2200. Examples of refined NR include deproteinized natural rubbers (DPNR) and highly purified natural rubbers (UPNR). Examples of modified NR include epoxidized natural rubbers (ENR), hydrogenated natural rubbers (HNR), and grafted natural rubbers. Examples of modified IR include epoxidized polyisoprene rubbers, hydrogenated polyisoprene rubbers, and grafted polyisoprene rubbers. These may be used alone or in combinations of two or more. NR is preferred among these.

The amount of isoprene-based rubbers, if present, based on <NUM>% by mass of the rubber component content in the rubber composition is preferably <NUM>% by mass or more, more preferably <NUM>% by mass or more, still more preferably <NUM>% by mass or more, but it is preferably <NUM>% by mass or less, more preferably <NUM>% by mass or less, still more preferably <NUM>% by mass or less. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

The rubber components may include extended rubbers that have been extended with plasticizer components such as oils, resins, or liquid polymers. These may be used alone or in combinations of two or more. Examples of the plasticizers used in the extended rubbers include those described later. The plasticizer content of the extended rubbers is not limited, but it is usually about <NUM> to <NUM> parts by mass per <NUM> parts by mass of the rubber solid content.

The rubber components may be modified to introduce therein a functional group interactive with filler such as silica.

Examples of the functional group include a silicon-containing group (-SiR<NUM> where each R is the same or different and represents a hydrogen atom, a hydroxy group, a hydrocarbon group, an alkoxy group, or the like), an amino group, an amide group, an isocyanate group, an imino group, an imidazole group, a urea group, an ether group, a carbonyl group, an oxycarbonyl group, a mercapto group, a sulfide group, a disulfide group, a sulfonyl group, a sulfinyl group, a thiocarbonyl group, an ammonium group, an imide group, a hydrazo group, an azo group, a diazo group, a carboxy group, a nitrile group, a pyridyl group, an alkoxy group, a hydroxy group, an oxy group, and an epoxy group, each of which may be substituted. Preferred among these is a silicon-containing group. More preferred is - SiR<NUM> where each R is the same or different and represents a hydrogen atom, a hydroxy group, a hydrocarbon group (preferably a C1-C6 hydrocarbon group, more preferably a C1-C6 alkyl group), or an alkoxy group (preferably a C1-C6 alkoxy group), and at least one R is a hydroxy group.

Specific examples of the compound (modifier) used to introduce the functional group include <NUM>-dimethylaminoethyltrimethoxysilane, <NUM>-dimethylaminopropyltrimethoxysilane, <NUM>-dimethylaminoethyltriethoxysilane, <NUM>-dimethylaminopropyltriethoxysilane, <NUM>-diethylaminoethyltrimethoxysilane, <NUM>-diethylaminopropyltrimethoxysilane, <NUM>-diethylaminoethyltriethoxysilane, and <NUM>-diethylaminopropyltriethoxysilane.

The rubber composition contains a branched conjugated diene polymer at least containing a branched conjugated diene, butadiene, and styrene as structural units. One or two or more branched conjugated diene polymers may be used.

The term "branched conjugated diene" in the present invention refers to a compound having a branched conjugated diene structure in which double bonds are separated by one single bond. Moreover, the compound may or may not contain other double bonds.

Examples of the branched conjugated diene used to form the branched conjugated diene polymer include <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-butadiene, <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-pentadiene, <NUM>-methyl-<NUM>,<NUM>-pentadiene, farnesene, and myrcene. To better achieve the advantageous effect, farnesene and myrcene are preferred among these, with farnesene being more preferred. One or two or more branched conjugated dienes may be used.

The mechanism for such an advantageous effect achieved by using farnesene is not clear, but it is believed that the use of farnesene may further improve the compatibility with rubbers to provide abrasion resistance and wet grip performance and also to reduce bleeding, thereby resulting in improved overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance.

Several isomers of farnesene exist, such as α-farnesene ((3E,7E)-<NUM>,<NUM>,<NUM>-trimethyl-<NUM>,<NUM>,<NUM>,<NUM>-dodecatetraene) and β-farnesene (<NUM>,<NUM>-dimethyl-<NUM>-methylene-<NUM>,<NUM>,<NUM>-dodecatriene). Preferred among these is (E)-β-farnesene (<NUM>,<NUM>-dimethyl-<NUM>-methylene-<NUM>,<NUM>,<NUM>-dodecatriene) having the following structure:
<CHM>.

The term "myrcene" includes both α-myrcene (<NUM>-methyl-<NUM>-methyleneocta-<NUM>,<NUM>-diene) and β-myrcene. Preferred among these is β-myrcene (<NUM>-methyl-<NUM>-methyleneocta-<NUM>,<NUM>-diene) having the following structure:
<CHM>.

The butadiene used to form the branched conjugated diene polymer is desirably <NUM>,<NUM>-butadiene.

Here, the branched conjugated diene polymer may be incorporated in place of a conventionally-used plasticizer such as an oil.

The branched conjugated diene polymer may be any copolymer that at least contains a branched conjugated diene, butadiene, and styrene as structural units. To better achieve the advantageous effect, it is preferably a copolymer at least containing farnesene, butadiene, and styrene as structural units or a copolymer at least containing myrcene, butadiene, and styrene as structural units, more preferably a copolymer of farnesene, butadiene, and styrene or a copolymer of myrcene, butadiene, and styrene.

The weight average molecular weight (Mw) of the branched conjugated diene polymer is preferably <NUM> or more, more preferably <NUM> or more, still more preferably <NUM> or more, further preferably <NUM> or more, particularly preferably <NUM> or more, but it is preferably <NUM> or less, more preferably <NUM> or less, still more preferably <NUM> or less, further preferably <NUM> or less, further preferably <NUM> or less, further preferably <NUM> or less. When the Mw is within the range indicated above, the advantageous effect tends to be better achieved.

The mechanism for such an advantageous effect achieved by adjusting the Mw within a predetermined range is not clear, but it is believed that the Mw within a predetermined range may ensure the compatibility with rubbers to ensure good abrasion resistance and good wet grip performance and also to sufficiently reduce bleeding, thereby resulting in improved overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance.

Herein, the weight average molecular weight (Mw) of the branched conjugated diene polymer can be determined by gel permeation chromatography (GPC) (GPC device "HLC-<NUM>" available from Tosoh Corporation, detector: differential refractometer, column: TSKgel Super HZM-M available from Tosoh Corporation) and calibrated with polystyrene standards.

The amount of branched conjugated dienes based on <NUM>% by mass of the monomer components in the branched conjugated diene polymer (the branched conjugated diene unit content based on <NUM>% by mass of the branched conjugated diene polymer) is preferably <NUM>% by mass or higher, more preferably <NUM>% by mass or higher, still more preferably <NUM>% by mass or higher, but it is preferably <NUM>% by mass or lower, more preferably <NUM>% by mass or lower, still more preferably <NUM>% by mass or lower. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

The amount of butadiene based on <NUM>% by mass of the monomer components in the branched conjugated diene polymer (the butadiene unit content based on <NUM>% by mass of the branched conjugated diene polymer) is preferably <NUM>% by mass or higher, more preferably <NUM>% by mass or higher, still more preferably <NUM>% by mass or higher, further preferably <NUM>% by mass or higher, further preferably <NUM>% by mass or higher, but it is preferably <NUM>% by mass or lower, more preferably <NUM>% by mass or lower, still more preferably <NUM>% by mass or lower, further preferably <NUM>% by mass or lower. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

The amount of styrene based on <NUM>% by mass of the monomer components in the branched conjugated diene polymer (the styrene unit content based on <NUM>% by mass of the branched conjugated diene polymer) is preferably <NUM>% by mass or higher, more preferably <NUM>% by mass or higher, still more preferably <NUM>% by mass or higher, further preferably <NUM>% by mass or higher, but it is preferably <NUM>% by mass or lower, more preferably <NUM>% by mass or lower, still more preferably <NUM>% by mass or lower, further preferably <NUM>% by mass or lower, further preferably <NUM>% by mass or lower. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

The amount of butadiene (the butadiene unit content (% by mass) based on <NUM>% by mass of the branched conjugated diene polymer) and the amount of styrene (the styrene unit content (% by mass) based on <NUM>% by mass of the branched conjugated diene polymer) each based on <NUM>% by mass of the monomer components in the branched conjugated diene polymer desirably satisfy the following relationship: <MAT>.

The lower limit is preferably <NUM> or more, more preferably <NUM> or more, still more preferably <NUM> or more, particularly preferably <NUM> or more, while the upper limit is preferably <NUM> or less, more preferably <NUM> or less, still more preferably <NUM> or less, further preferably <NUM> or less, further preferably <NUM> or less. When the limits are within the respective ranges indicated above, the advantageous effect tends to be better achieved.

The mechanism for such an advantageous effect achieved by adjusting the value of "<NUM> × butadiene unit content + <NUM> × styrene unit content" within a predetermined range is not clear, but it is believed that in this case, the compatibility with rubber components may be improved to improve abrasion resistance, and the single-domain phases may be decreased to improve bleed resistance; further, the styrene may achieve high Tg to improve wet grip performance, thereby resulting in improved overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance.

The branched conjugated diene polymer can be synthesized by known techniques. For example, it may be synthesized by anionic polymerization in which a sufficiently nitrogen-purged pressure-resistant vessel may be charged with hexane, a branched conjugated diene, butadiene, styrene, and sec-butyllithium, and optionally other vinyl monomers, and then the mixture may be increased in temperature and stirred for several hours to obtain a polymerized solution, which may then be quenched and vacuum dried to obtain a branched conjugated diene polymer.

The polymerization procedure in the preparation of the branched conjugated diene polymer is not limited. For example, all monomers may be randomly copolymerized at once. Alternatively, a specific monomer (e.g., only a branched conjugated diene monomer, only a butadiene monomer, or only a styrene monomer) may previously be polymerized and then the remaining monomers may be added and copolymerized. Alternatively, each specific monomer may previously be polymerized and then the resulting polymers may be block-copolymerized.

The amount of the branched conjugated diene polymers per <NUM> parts by mass of the rubber component content in the rubber composition is preferably <NUM> parts by mass or more, more preferably <NUM> parts by mass or more, still more preferably <NUM> parts by mass or more, but it is preferably <NUM> parts by mass or less, more preferably <NUM> parts by mass or less, still more preferably <NUM> parts by mass or less. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

To better achieve the advantageous effect, the rubber composition desirably contains a filler.

The amount of fillers per <NUM> parts by mass of the rubber component content in the rubber composition is preferably <NUM> parts by mass or more, more preferably <NUM> parts by mass or more, still more preferably <NUM> parts by mass or more, particularly preferably <NUM> parts by mass or more, but it is preferably <NUM> parts by mass or less, more preferably <NUM> parts by mass or less, still more preferably <NUM> parts by mass or less, particularly preferably <NUM> parts by mass or less. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

The mechanism for such an advantageous effect achieved by adjusting the amount of fillers within a predetermined range is not clear, but it is believed that in this case, sufficient reinforcement of the rubber may be achieved to provide abrasion resistance and wet grip performance, thereby resulting in improved overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance.

The filler used may be a material known in the rubber field. Examples include inorganic fillers such as silica, carbon black, calcium carbonate, talc, alumina, clay, aluminum hydroxide, aluminum oxide, and mica; bio char; and recycled carbon black. To better achieve the advantageous effect, carbon black and silica are preferred among these.

The mechanism for such an advantageous effect achieved by using silica and carbon black is not clear, but it is believed that the combination of these fillers may achieve sufficient reinforcement of the rubber to provide abrasion resistance and wet grip performance, thereby resulting in improved overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance.

Non-limiting examples of carbon black include N134, N110, N220, N234, N219, N339, N330, N326, N351, N550, and N762. Usable commercial products are available from Asahi Carbon Co. , Cabot Japan K. , Tokai Carbon Co. , Mitsubishi Chemical Corporation, Lion Corporation, NIPPON STEEL Carbon Co. , Columbia Carbon, etc. In addition to the carbon black made from mineral oils, etc., carbon black made from biomass materials such as lignin is also usable. These may be used alone or in combinations of two or more.

The nitrogen adsorption specific surface area (N<NUM>SA) of the carbon black is preferably <NUM><NUM>/g or more, more preferably <NUM><NUM>/g or more, still more preferably <NUM><NUM>/g or more, but it is preferably <NUM><NUM>/g or less, more preferably <NUM><NUM>/g or less, still more preferably <NUM><NUM>/g or less, further preferably <NUM><NUM>/g or less. When the N<NUM>SA is within the range indicated above, the advantageous effect tends to be better achieved.

Here, the nitrogen adsorption specific surface area of the carbon black can be determined in accordance with JIS K <NUM>-<NUM>:<NUM>.

The amount of carbon black per <NUM> parts by mass of the rubber component content in the rubber composition is preferably <NUM> parts by mass or more, more preferably <NUM> parts by mass or more, still more preferably <NUM> parts by mass or more, but it is preferably <NUM> parts by mass or less, more preferably <NUM> parts by mass or less, still more preferably <NUM> parts by mass or less. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

Examples of silica include dry silica (anhydrous silica) and wet silica (hydrous silica). Wet silica is preferred among these because it contains a large number of silanol groups. Usable commercial products are available from Degussa, Rhodia, Tosoh Silica Corporation, Solvay Japan, Tokuyama Corporation, etc. Also usable is silica made from biomass materials such as rice husks. These may be used alone or in combinations of two or more.

The nitrogen adsorption specific surface area (N<NUM>SA) of the silica is preferably <NUM><NUM>/g or more, more preferably <NUM><NUM>/g or more, still more preferably <NUM><NUM>/g or more, particularly preferably <NUM><NUM>/g or more. Moreover, the upper limit of the N<NUM>SA of the silica is not limited, and it is preferably <NUM><NUM>/g or less, more preferably <NUM><NUM>/g or less, still more preferably <NUM><NUM>/g or less. When the N<NUM>SA is within the range indicated above, the advantageous effect tends to be better achieved. Here, when multiple types of silica are used, the N<NUM>SA of the total silica used is desirably within the range indicated above. Moreover, the N<NUM>SA of each of the multiple types of silica blended is desirably within the range indicated above.

Here, the N<NUM>SA of the silica is measured by a BET method in accordance with ASTM D3037-<NUM>.

The amount of silica per <NUM> parts by mass of the rubber component content in the rubber composition is preferably <NUM> parts by mass or more, more preferably <NUM> parts by mass or more, still more preferably <NUM> parts by mass or more, particularly preferably <NUM> parts by mass or more, but it is preferably <NUM> parts by mass or less, more preferably <NUM> parts by mass or less, still more preferably <NUM> parts by mass or less, particularly preferably <NUM> parts by mass or less. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

The rubber composition containing silica preferably further contains a silane coupling agent.

Any silane coupling agent may be used, including those known in the rubber field. Examples include sulfide silane coupling agents such as bis(<NUM>-triethoxysilylpropyl)tetrasulfide, bis(<NUM>-triethoxysilylethyl)tetrasulfide, bis(<NUM>-triethoxysilylbutyl)tetrasulfide, bis(<NUM>-trimethoxysilylpropyl)tetrasulfide, bis(<NUM>-trimethoxysilylethyl)tetrasulfide, bis(<NUM>-triethoxysilylethyl)trisulfide, bis(<NUM>-trimethoxysilylbutyl)trisulfide, bis(<NUM>-triethoxysilylpropyl)disulfide, bis(<NUM>-triethoxysilylethyl)disulfide, bis(<NUM>-triethoxysilylbutyl)disulfide, bis(<NUM>-trimethoxysilylpropyl)disulfide, bis(<NUM>-trimethoxysilylethyl)disulfide, bis(<NUM>-trimethoxysilylbutyl)disulfide, <NUM>-trimethoxysilylpropyl-N,N-dimethylthiocarbamoyltetrasulfide, <NUM>-triethoxysilylethyl-N,N-dimethylthiocarbamoyltetrasulfide, and <NUM>-triethoxysilylpropyl methacrylate monosulfide; mercapto silane coupling agents such as <NUM>-mercaptopropyltrimethoxysilane, <NUM>-mercaptoethyltriethoxysilane, and NXT and NXT-Z both available from Momentive; vinyl silane coupling agents such as vinyltriethoxysilane and vinyltrimethoxysilane; amino silane coupling agents such as <NUM>-aminopropyltriethoxysilane and <NUM>-aminopropyltrimethoxysilane; glycidoxy silane coupling agents such as γ-glycidoxypropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane; nitro silane coupling agents such as <NUM>-nitropropyltrimethoxysilane and <NUM>-nitropropyltriethoxysilane; and chloro silane coupling agents such as <NUM>-chloropropyltrimethoxysilane and <NUM>-chloropropyltriethoxysilane. Sulfide silane coupling agents and mercapto silane coupling agents are preferred among these, with mercapto silane coupling agents being more preferred. Here, usable commercial products are available from Degussa, Momentive, Shin-Etsu Silicone, Tokyo Chemical Industry Co. , Dow Corning Toray Co. , etc. These may be used alone or in combinations of two or more.

The mechanism for such an advantageous effect achieved by using a mercapto silane coupling agent is not clear, but it is believed that in this case, the rubber composition containing silica may achieve a sufficient reinforcement effect to provide abrasion resistance and wet grip performance, thereby resulting in improved overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance.

Usable mercapto silane coupling agents include compounds having a mercapto group and compounds in which a mercapto group is protected by a protecting group (for example, compounds represented by Formula (S1) below).

Examples of particularly suitable mercapto silane coupling agents include silane coupling agents represented by Formula (S1) below and silane coupling agents containing linking units A and B represented by Formulas (I) and (II), respectively, below.

In Formula (S1), R<NUM> represents a monovalent group selected from -Cl, -Br, -OR<NUM>, -O(O=)CR<NUM>, -ON=CR<NUM>R<NUM>, -NR<NUM>R<NUM>, and -(OSiR<NUM>R<NUM>)h(OSiR<NUM>R<NUM>R<NUM>) wherein R<NUM>, R<NUM>, and R<NUM> may be the same or different and each represent a hydrogen atom or a C1-C18 monovalent hydrocarbon group, and h is <NUM> to <NUM> on average; R<NUM> represents R<NUM>, a hydrogen atom, or a C1-C18 monovalent hydrocarbon group; R<NUM> represents the group: -[O(R<NUM>O)j]-wherein R<NUM> represents a C1-C18 alkylene group, and j represents an integer of <NUM> to <NUM>; R<NUM> represents a C1-C18 divalent hydrocarbon group; R<NUM> represents a C1-C18 monovalent hydrocarbon group; and x, y, and z are numbers satisfying the following relationships: x + y + 2z = <NUM>, <NUM> ≤ x ≤ <NUM>, <NUM> ≤ y ≤ <NUM>, and <NUM> ≤ z ≤ <NUM>. <CHM>
<CHM>.

In Formulas (I) and (II), v represents an integer of <NUM> or more; w represents an integer of <NUM> or more; R<NUM> represents a hydrogen atom, a halogen atom, a branched or unbranched C1-C30 alkyl group, a branched or unbranched C2-C30 alkenyl group, a branched or unbranched C2-C30 alkynyl group, or the alkyl group in which a terminal hydrogen atom is replaced with a hydroxy or carboxyl group; R<NUM> represents a branched or unbranched C1-C30 alkylene group, a branched or unbranched C2-C30 alkenylene group, or a branched or unbranched C2-C30 alkynylene group; and R<NUM> and R<NUM> may together form a ring structure.

Preferably, R<NUM>, R<NUM>, R<NUM>, and R<NUM> in Formula (S1) are each independently selected from the group consisting of C1-C18 linear, cyclic, or branched alkyl, alkenyl, aryl, and aralkyl groups. When R<NUM> is a C1-C18 monovalent hydrocarbon group, it is preferably selected from the group consisting of linear, cyclic, or branched alkyl, alkenyl, aryl, and aralkyl groups. R<NUM> is preferably a linear, cyclic, or branched alkylene group, particularly preferably a linear alkylene group. Examples of R<NUM> include C1-C18 alkylene groups, C2-C18 alkenylene groups, C5-C18 cycloalkylene groups, C6-C18 cycloalkylalkylene groups, C6-C18 arylene groups, and C7-C18 aralkylene groups. The alkylene and alkenylene groups may be either linear or branched. The cycloalkylene, cycloalkylalkylene, arylene, and aralkylene groups may each have a functional group such as a lower alkyl group on the ring. R<NUM> is preferably a C1-C6 alkylene group, particularly preferably a linear alkylene group such as a methylene group, an ethylene group, a trimethylene group, a tetramethylene group, a pentamethylene group, or a hexamethylene group.

Specific examples of R<NUM>, R<NUM>, R<NUM>, R<NUM>, and R<NUM> in Formula (S1) include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, pentyl, hexyl, octyl, decyl, dodecyl, cyclopentyl, cyclohexyl, vinyl, propenyl, allyl, hexenyl, octenyl, cyclopentenyl, cyclohexenyl, phenyl, tolyl, xylyl, naphthyl, benzyl, phenethyl, and naphthylmethyl groups.

For R<NUM> in Formula (S1), examples of the linear alkylene group include methylene, ethylene, n-propylene, n-butylene, and hexylene groups, and examples of the branched alkylene group include isopropylene, isobutylene, and <NUM>-methylpropylene groups.

Specific examples of the silane coupling agents of Formula (S1) include <NUM>-hexanoylthiopropyltriethoxysilane, <NUM>-octanoylthiopropyltriethoxysilane, <NUM>-decanoylthiopropyltriethoxysilane, <NUM>-lauroylthiopropyltriethoxysilane, <NUM>-hexanoylthioethyltriethoxysilane, <NUM>-octanoylthioethyltriethoxysilane, <NUM>-decanoylthioethyltriethoxysilane, <NUM>-lauroylthioethyltriethoxysilane, <NUM>-hexanoylthiopropyltrimethoxysilane, <NUM>-octanoylthiopropyltrimethoxysilane, <NUM>-decanoylthiopropyltrimethoxysilane, <NUM>-lauroylthiopropyltrimethoxysilane, <NUM>-hexanoylthioethyltrimethoxysilane, <NUM>-octanoylthioethyltrimethoxysilane, <NUM>-decanoylthioethyltrimethoxysilane, and <NUM>-lauroylthioethyltrimethoxysilane. These may be used alone or in combinations of two or more. Particularly preferred among these is <NUM>-octanoylthiopropyltriethoxysilane.

The linking unit A content of the silane coupling agents containing the linking units A and B of Formulas (I) and (II) is preferably <NUM> mol% or higher, more preferably <NUM> mol% or higher, but it is preferably <NUM> mol% or lower, more preferably <NUM> mol% or lower. On the other hand, the linking unit B content is preferably <NUM> mol% or higher, more preferably <NUM> mol% or higher, still more preferably <NUM> mol% or higher, but it is preferably <NUM> mol% or lower, more preferably <NUM> mol% or lower, still more preferably <NUM> mol% or lower. Moreover, the combined content of linking units A and B is preferably <NUM> mol% or higher, more preferably <NUM> mol% or higher, particularly preferably <NUM> mol%.

Here, the linking unit A or B content refers to the amount including the linking unit A or B present at the end of the silane coupling agent, if any. When the linking unit A or B is present at the end of the silane coupling agent, its form is not limited as long as it forms a unit corresponding to Formula (I) or (II) representing the linking unit A or B.

For R<NUM> in Formulas (I) and (II), examples of the halogen atom include chlorine, bromine, and fluorine; examples of the branched or unbranched C1-C30 alkyl group include methyl and ethyl groups; examples of the branched or unbranched C2-C30 alkenyl group include vinyl and <NUM>-propenyl groups; and examples of the branched or unbranched C2-C30 alkynyl group include ethynyl and propynyl groups.

For R<NUM> in Formulas (I) and (II), examples of the branched or unbranched C1-C30 alkylene group include ethylene and propylene groups; examples of the branched or unbranched C2-C30 alkenylene group include vinylene and <NUM>-propenylene groups; and examples of the branched or unbranched C2-C30 alkynylene group include ethynylene and propynylene groups.

In the silane coupling agents containing the linking units A and B of Formulas (I) and (II), the total number of repetitions (v + w) consisting of the sum of the number of repetitions (v) of the linking unit A and the number of repetitions (w) of the linking unit B is preferably in the range of <NUM> to <NUM>.

The amount of silane coupling agents per <NUM> parts by mass of the silica content in the rubber composition is preferably <NUM> parts by mass or more, more preferably <NUM> parts by mass or more. The amount is also preferably <NUM> parts by mass or less, more preferably <NUM> parts by mass or less, still more preferably <NUM> parts by mass or less. Here, a similar range is also desirable for the amount of mercapto silane coupling agents.

The rubber composition may contain a plasticizer.

Here, the term "plasticizer" refers to a material that can impart plasticity to rubber components. Examples include liquid plasticizers (plasticizers which are liquid at room temperature (<NUM>)), resins (resins which are solid at room temperature (<NUM>)), and ester plasticizers.

In the present invention, the above-described "branched conjugated diene polymer" is included in the term "plasticizer" when it is a material that can impart plasticity to rubber components. In this case, the amount of plasticizers (total plasticizer content) refers to the combined amount of such branched conjugated diene polymers (with plasticity) and other plasticizers.

The amount of plasticizers (total plasticizer content) per <NUM> parts by mass of the rubber component content in the rubber composition is preferably <NUM> parts by mass or more, more preferably <NUM> parts by mass or more, still more preferably <NUM> parts by mass or more, particularly preferably <NUM> parts by mass or more, but it is preferably <NUM> parts by mass or less, more preferably <NUM> parts by mass or less, still more preferably <NUM> parts by mass or less. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

Here, the amount of plasticizers includes the amount of plasticizer components used in the extended rubbers, if used.

Non-limiting examples of the liquid plasticizers (plasticizers which are liquid at room temperature (<NUM>)) include oils and liquid polymers (e.g., liquid resins, liquid diene polymers). To better achieve the advantageous effect, oils, liquid resins, and liquid diene polymers are desirable among these. These may be used alone or in combinations of two or more.

The mechanism for such an advantageous effect achieved by using an oil, a liquid resin, or a liquid diene polymer is not clear, but it is believed that such a plasticizer may provide good grip performance, thereby resulting in improved overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance.

The amount of liquid plasticizers per <NUM> parts by mass of the rubber component content in the rubber composition is preferably <NUM> parts by mass or more, more preferably <NUM> parts by mass or more, still more preferably <NUM> parts by mass or more, but it is preferably <NUM> parts by mass or less, more preferably <NUM> parts by mass or less, still more preferably <NUM> parts by mass or less. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

Here, the amount of liquid plasticizers includes the amount of oils contained in the extended rubbers, if used.

When the branched conjugated diene polymer used corresponds to a plasticizer which is liquid at room temperature (<NUM>), the amount of liquid plasticizers refers to the combined amount of such branched conjugated diene polymers and other liquid plasticizers.

Examples of oils include process oils, plant oils, and mixtures thereof. Examples of the process oils include paraffinic process oils, aromatic process oils, and naphthenic process oils. Examples of the plant oils include castor oil, cotton seed oil, linseed oil, rapeseed oil, soybean oil, palm oil, coconut oil, peanut oil, rosin, pine oil, pine tar, tall oil, corn oil, rice oil, safflower oil, sesame oil, olive oil, sunflower oil, palm kernel oil, camellia oil, jojoba oil, macadamia nut oil, and tung oil. Process oils, such as paraffinic process oils, aromatic process oils, and naphthenic process oils, and plant oils are preferred among these. Here, in view of life cycle assessment, oils that have been used as lubricating oils in mixers for mixing rubber, engines, or other applications, waste cooking oils, and other used oils may appropriately be used as the process oils or plant oils.

Examples of liquid resins include terpene resins (including terpene phenol resins and aromatic modified terpene resins), rosin resins, styrene resins, C5 resins, C9 resins, C5/C9 resins, dicyclopentadiene (DCPD) resins, coumarone-indene resins (including resins based on coumarone or indene alone), phenol resins, olefin resins, polyurethane resins, and acrylic resins. Hydrogenated products of these resins are also usable.

Examples of liquid diene polymers include liquid styrene-butadiene copolymers (liquid SBR), liquid polybutadiene polymers (liquid BR), liquid polyisoprene polymers (liquid IR), liquid styrene-isoprene copolymers (liquid SIR), liquid styrene-butadiene-styrene block copolymers (liquid SBS block polymers), and liquid styrene-isoprene-styrene block copolymers (liquid SIS block polymers), all of which are liquid at <NUM>. The chain end or backbone of these polymers may be modified with a polar group. Moreover, hydrogenated products of these polymers are also usable.

Examples of the aforementioned resins (resins which are solid at room temperature (<NUM>)) include aromatic vinyl polymers, coumarone-indene resins, coumarone resins, indene resins, phenol resins, rosin resins, petroleum resins, terpene resins, p-t-butylphenol acetylene resins, and acrylic resins, all of which are solid at room temperature (<NUM>). These resins may also be hydrogenated. These may be used alone or in combinations of two or more.

The amount of the aforementioned resins per <NUM> parts by mass of the rubber component content in the rubber composition is preferably <NUM> parts by mass or more, more preferably <NUM> parts by mass or more, still more preferably <NUM> parts by mass or more, but it is preferably <NUM> parts by mass or less, more preferably <NUM> parts by mass or less, still more preferably <NUM> parts by mass or less. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

The mechanism for such an advantageous effect achieved by adjusting the amount of the aforementioned resins within a predetermined range is not clear, but it is believed that in this case, the compatibility with rubbers may be improved to provide abrasion resistance and wet grip performance and also to reduce bleeding, thereby resulting in improved overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance.

The softening point of the aforementioned resins is preferably <NUM> or higher, more preferably <NUM> or higher, still more preferably <NUM> or higher, further preferably <NUM> or higher. The upper limit is preferably <NUM> or lower, more preferably <NUM> or lower, still more preferably <NUM> or lower. When the softening point is within the range indicated above, the advantageous effect tends to be better achieved.

The softening point of the aforementioned resins is determined as set forth in JIS K <NUM>-<NUM>:<NUM> using a ring and ball softening point measuring apparatus, and the temperature at which the ball drops down is defined as the softening point.

Here, the softening point of the aforementioned resins is usually within about <NUM> ± <NUM> of the glass transition temperature of the resin components.

The aromatic vinyl polymers refer to polymers containing aromatic vinyl monomers as structural units, such as styrene resins, etc. Examples of the styrene resins include resins produced by polymerization of α-methylstyrene and/or styrene. Specific examples include styrene homopolymers (styrene resins), α-methylstyrene homopolymers (α-methylstyrene resins), copolymers of α-methylstyrene and styrene, and copolymers of styrene and other monomers.

The coumarone-indene resins refer to resins containing coumarone and indene as the main monomer components constituting the skeleton (backbone) of the resins. Examples of monomer components which may be contained in the skeleton in addition to coumarone and indene include styrene, α-methylstyrene, methylindene, and vinyltoluene.

The coumarone resins refer to resins containing coumarone as the main monomer component constituting the skeleton (backbone) of the resins.

The indene resins refer to resins containing indene as the main monomer component constituting the skeleton (backbone) of the resins.

Examples of the phenol resins include known polymers produced by reacting phenol with an aldehyde such as formaldehyde, acetaldehyde, or furfural in the presence of an acid or alkali catalyst. Preferred among these are those produced by reaction in the presence of an acid catalyst, such as novolac phenol resins.

Examples of the rosin resins include rosin resins typified by natural rosins, polymerized rosins, modified rosins, and esterified compounds thereof, and hydrogenated products thereof.

Examples of the petroleum resins include C5 resins, C9 resins, C5/C9 resins, dicyclopentadiene (DCPD) resins, and hydrogenated products of these resins. DCPD resins or hydrogenated DCPD resins are preferred among these.

The terpene resins refer to polymers containing terpenes as structural units. Examples include polyterpene resins produced by polymerization of terpene compounds, and aromatic modified terpene resins produced by polymerization of terpene compounds and aromatic compounds. Examples of usable aromatic modified terpene resins include terpene-phenol resins made from terpene compounds and phenolic compounds, terpene-styrene resins made from terpene compounds and styrene compounds, and terpene-phenol-styrene resins made from terpene compounds, phenolic compounds, and styrene compounds. Here, examples of the terpene compounds include α-pinene and β-pinene; examples of the phenolic compounds include phenol and bisphenol A; and examples of the aromatic compounds include styrene compounds such as styrene and α-methylstyrene.

Examples of the p-t-butylphenol acetylene resins include solid resins obtained by condensation reaction of p-t-butylphenol and acetylene.

The acrylic resins refer to polymers containing acrylic monomers as structural units. Examples include styrene acrylic resins such as those which contain carboxy groups and are produced by copolymerization of aromatic vinyl monomer components and acrylic monomer components. Solvent-free, carboxy group-containing styrene acrylic resins can be suitably used among these.

Among the above-mentioned resins, the rubber composition desirably contains at least one resin selected from the group consisting of styrene resins, coumarone-indene resins, terpene resins, p-t-butylphenol acetylene resins, acrylic resins, dicyclopentadiene resins, C5 resins, C9 resins, and C5/C9 resins in order to better achieve the advantageous effect.

The mechanism for such an advantageous effect achieved by using any of these resins is not clear, but it is believed that these resins may provide good grip performance, thereby resulting in improved overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance.

The ester plasticizers may be any compound that is liquid at room temperature (<NUM>) and has an ester group. Examples include phthalic acid derivatives, long-chain fatty acid derivatives, phosphoric acid derivatives, sebacic acid derivatives, and adipic acid derivatives. These may be used alone or in combinations of two or more. Phosphoric acid derivatives, sebacic acid derivatives, and adipic acid derivatives are preferred among these, with sebacic acid derivatives being more preferred.

Non-limiting examples of the phthalic acid derivatives include phthalates such as di-<NUM>-ethylhexyl phthalate (DOP) and diisodecyl phthalate (DIDP). Non-limiting examples of the long-chain fatty acid derivatives include long-chain fatty acid glycerol esters. Non-limiting examples of the phosphoric acid derivatives include phosphates such as tris(<NUM>-ethylhexyl) phosphate (TOP) and tributyl phosphate (TBP). Non-limiting examples of the sebacic acid derivatives include sebacates such as di(<NUM>-ethylhexyl) sebacate (DOS) and diisooctyl sebacate (DIOS). Non-limiting examples of the adipic acid derivatives include adipates such as di(<NUM>-ethylhexyl) adipate (DOA) and diisooctyl adipate (DIOA).

Phosphates, sebacates, and adipates are preferred among these, with sebacates being more preferred. Moreover, TOP, DOS, and DOA are preferred as specific compounds, with DOS being more preferred.

The glass transition temperature (Tg) of the ester plasticizers is preferably -<NUM> or higher, more preferably -<NUM> or higher, still more preferably -<NUM> or higher, but it is preferably -<NUM> or lower, more preferably -<NUM> or lower, still more preferably -<NUM> or lower. When the Tg is within the range indicated above, the advantageous effect tends to be more suitably achieved.

Herein, the glass transition temperature is measured in accordance with JIS K <NUM> using a differential scanning calorimeter (Q200) available from TA Instruments Japan at a rate of temperature increase of <NUM>/min.

The amount of ester plasticizers per <NUM> parts by mass of the rubber component content is preferably <NUM> to <NUM> parts by mass.

For example, the plasticizers may be commercially available from Idemitsu Kosan Co. , Sankyo Yuka Kogyo K. , ENEOS Corporation, Olisoy, H&R, Hokoku Corporation, Showa Shell Sekiyu K. , Fuji Kosan Co. , The Nisshin Oillio Group. , Maruzen Petrochemical Co. , Sumitomo Bakelite Co. , Yasuhara Chemical Co. , Tosoh Corporation, Rutgers Chemicals, BASF, Arizona Chemical, Nitto Chemical Co. , Nippon Shokubai Co. , Arakawa Chemical Industries, Ltd. , Taoka Chemical Co. , DAIHACHI CHEMICAL INDUSTRY CO.

The rubber composition may contain a processing aid.

Examples of the processing aid include metal salts (compounds obtained by replacing the hydrogen atoms of acids by metal ions), fatty acid amides, amide esters, and fatty acid esters. These may be used alone or in combinations of two or more. Metal salts and fatty acid amides are preferred among these, with metal salts being more preferred.

Examples of the metals used in the metal salts include alkali metals such as potassium and sodium, and alkaline earth metals such as calcium and barium. Magnesium, zinc, nickel, molybdenum, etc. are also usable. Alkali metals are preferred among these.

Examples of the acids used in the metal salts include fatty acids such as lauric acid, myristic acid, and palmitic acid. Boric acid, carbonic acid, hydrochloric acid, nitric acid, sulfuric acid, etc. are also usable.

The processing aids may be commercially available from KISHIDA CHEMICAL Co. , KENEI Pharmaceutical Co. , Struktol, Performance Additives, etc..

The amount of processing aids per <NUM> parts by mass of the rubber component content in the rubber composition is preferably <NUM> part by mass or more, more preferably <NUM> parts by mass or more, still more preferably <NUM> parts by mass or more, but it is preferably <NUM> parts by mass or less, more preferably <NUM> parts by mass or less, still more preferably <NUM> parts by mass or less. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

The rubber composition may contain an antioxidant.

Examples of the antioxidant include naphthylamine antioxidants such as phenyl-α-naphthylamine; diphenylamine antioxidants such as octylated diphenylamine and <NUM>,<NUM>'-bis(α,α'-dimethylbenzyl)diphenylamine; p-phenylenediamine antioxidants such as N-isopropyl-N'-phenyl-p-phenylenediamine, N-(<NUM>,<NUM>-dimethylbutyl)-N'-phenyl-p-phenylenediamine, and N,N'-di-<NUM>-naphthyl-p-phenylenediamine; quinoline antioxidants such as polymerized <NUM>,<NUM>,<NUM>-trimethyl-<NUM>,<NUM>-dihydroquinoline; monophenolic antioxidants such as <NUM>,<NUM>-di-t-butyl-<NUM>-methylphenol and styrenated phenol; and bis-, tris-, or polyphenolic antioxidants such as tetrakis[methylene-<NUM>-(<NUM>',<NUM>'-di-t-butyl-<NUM>'-hydroxyphenyl)propionate]methane. Usable commercial products are available from Seiko Chemical Co. , Sumitomo Chemical Co. , Ouchi Shinko Chemical Industrial Co. , Flexsys, etc. These may be used alone or in combinations of two or more.

The amount of antioxidants per <NUM> parts by mass of the rubber component content in the rubber composition is preferably <NUM> parts by mass or more, more preferably <NUM> parts by mass or more, still more preferably <NUM> parts by mass or more, but it is preferably <NUM> parts by mass or less, more preferably <NUM> parts by mass or less, still more preferably <NUM> parts by mass or less. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

The rubber composition may contain a wax.

Any wax may be used, and examples include petroleum waxes such as paraffin waxes and microcrystalline waxes; naturally-occurring waxes such as plant waxes and animal waxes; and synthetic waxes such as polymers of ethylene, propylene, or other similar monomers. Usable commercial products are available from Ouchi Shinko Chemical Industrial Co. , Nippon Seiro Co. , Seiko Chemical Co. , etc. These may be used alone or in combinations of two or more.

The amount of waxes per <NUM> parts by mass of the rubber component content in the rubber composition is preferably <NUM> parts by mass or more, more preferably <NUM> parts by mass or more, still more preferably <NUM> parts by mass or more, but it is preferably <NUM> parts by mass or less, more preferably <NUM> parts by mass or less, still more preferably <NUM> parts by mass or less. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

Conventional stearic acid may be used. Usable commercial products are available from NOF Corporation, Kao Corporation, FUJIFILM Wako Pure Chemical Corporation, Chiba Fatty Acid Co. , etc. These may be used alone or in combinations of two or more.

The amount of stearic acid per <NUM> parts by mass of the rubber component content in the rubber composition is preferably <NUM> parts by mass or more, more preferably <NUM> parts by mass or more, still more preferably <NUM> parts by mass or more, but it is preferably <NUM> parts by mass or less, more preferably <NUM> parts by mass or less. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

Examples of the sulfur include those commonly used as crosslinking agents in the rubber industry, such as powdered sulfur, precipitated sulfur, colloidal sulfur, insoluble sulfur, highly dispersible sulfur, and soluble sulfur. Usable commercial products are available from Tsurumi Chemical Industry Co. , Karuizawa sulfur Co. , Shikoku Chemicals Corporation, Flexsys, Nippon Kanryu Industry Co. , Hosoi Chemical Industry Co. , etc. These may be used alone or in combinations of two or more.

The amount of sulfur per <NUM> parts by mass of the rubber component content in the rubber composition is preferably <NUM> parts by mass or more, more preferably <NUM> parts by mass or more, still more preferably <NUM> parts by mass or more, but it is preferably <NUM> parts by mass or less, more preferably <NUM> parts by mass or less, still more preferably <NUM> parts by mass or less. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

The rubber composition may contain a vulcanization accelerator.

Examples of the vulcanization accelerator include thiazole vulcanization accelerators such as <NUM>-mercaptobenzothiazole and di-<NUM>-benzothiazolyl disulfide; thiuram vulcanization accelerators such as tetramethylthiuram disulfide (TMTD) and tetrakis(<NUM>-ethylhexyl)thiuram disulfide (TOT-N); sulfenamide vulcanization accelerators such as N-cyclohexyl-<NUM>-benzothiazylsulfenamide (CBS), N-tert-butyl-<NUM>-benzothiazolylsulfenamide (TBBS), N-oxyethylene-<NUM>-benzothiazole sulfenamide, and N,N'-diisopropyl-<NUM>-benzothiazole sulfenamide; and guanidine vulcanization accelerators such as diphenylguanidine, diorthotolylguanidine, and orthotolylbiguanidine. Usable commercial products are available from Sumitomo Chemical Co. , Ouchi Shinko Chemical Industrial Co. , etc. These may be used alone or in combinations of two or more.

The amount of vulcanization accelerators per <NUM> parts by mass of the rubber component content in the rubber composition is preferably <NUM> parts by mass or more, more preferably <NUM> parts by mass or more, still more preferably <NUM> parts by mass or more, but it is preferably <NUM> parts by mass or less, more preferably <NUM> parts by mass or less, still more preferably <NUM> parts by mass or less. When the amount is within the range indicated above, the advantageous effect tends to be better achieved.

In addition to the above-mentioned components, the rubber composition may further contain additives commonly used in the tire industry, such as organic peroxides. The amounts of these additives are each preferably <NUM> to <NUM> parts by mass per <NUM> parts by mass of the rubber component content.

The rubber composition may be prepared by known methods. For example, it may be prepared by kneading the above-mentioned components using a rubber kneading machine such as an open roll mill or a Banbury mixer and then vulcanizing the kneaded mixture.

The kneading conditions are as follows. In a base kneading step of kneading additives other than vulcanizing agents and vulcanization accelerators, the kneading temperature is usually <NUM> to <NUM>, preferably <NUM> to <NUM>, and the kneading time is usually <NUM> seconds to <NUM> minutes, preferably <NUM> minute to <NUM> minutes. In a final kneading step of kneading vulcanizing agents and vulcanization accelerators, the kneading temperature is usually <NUM> or lower, preferably from room temperature to <NUM>. Then, the composition obtained after kneading vulcanizing agents and vulcanization accelerators is usually vulcanized by press vulcanization, for example. The vulcanization temperature is usually <NUM> to <NUM>, preferably <NUM> to <NUM>.

The rubber composition can be used in a tread of a tire.

The present invention may be applied to a tire such as a pneumatic tire or a non-pneumatic tire, preferably a pneumatic tire. In particular, the tire may be suitably used as a summer tire, winter tire (e.g., studless tire, cold weather tire, snow tire, studded tire), or all-season tire, particularly a summer tire or winter tire. Moreover, the tire may be used as a tire for passenger cars, large passenger cars, large SUVs, heavy duty vehicles such as trucks and buses, light trucks, or motorcycles, or as a racing tire (high performance tire), etc. Among these, the tire is desirably used as a tire for passenger cars.

The tire can be produced from the above-described rubber composition by usual methods. For example, an unvulcanized rubber composition containing various materials may be extruded into the shape of a tread and then formed together with other tire components on a tire building machine in a usual manner to build an unvulcanized tire, which may then be heated and pressurized in a vulcanizer to produce a tire.

The tire of the present invention includes a tread including the rubber composition that contains a rubber component and a branched conjugated diene polymer, and satisfies the following relationship (<NUM>): <MAT> wherein A is defined by A = <NUM>/√α + <NUM> wherein α represents the weight average molecular weight of the branched conjugated diene polymer.

The lower limit of A is preferably <NUM> or more, more preferably <NUM> or more, still more preferably <NUM> or more, further preferably <NUM> or more, further preferably <NUM> or more, further preferably <NUM> or more, further preferably <NUM> or more, further preferably <NUM> or more, further preferably <NUM> or more, while the upper limit is preferably <NUM> or less, more preferably <NUM> or less, still more preferably <NUM> or less, further preferably <NUM> or less, further preferably <NUM> or less. When A is within the range indicated above, the advantageous effect tends to be better achieved.

The mechanism for such an advantageous effect achieved by adjusting A within a predetermined range, particularly <NUM> ≤ A ≤ <NUM>, is not clear, but it is believed that in this case, the compatibility with rubbers may be improved such that the single-domain phases may be decreased to improve abrasion resistance, and the decrease of the single-domain phases may reduce bleeding of the branched conjugated diene polymer and improve bleed resistance, thereby resulting in improved overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance.

The tire includes a tread including the rubber composition and satisfies the following relationship (<NUM>): <MAT> wherein A is defined by A = <NUM>/√α + <NUM> wherein α represents the weight average molecular weight of the branched conjugated diene polymer; and S represents the groove area ratio (%) of the ground contact surface of the tread.

The upper limit of the value of A × S is preferably <NUM> or less, more preferably <NUM> or less, still more preferably <NUM> or less, further preferably <NUM> or less, further preferably <NUM> or less, further preferably <NUM> or less, further preferably <NUM> or less, particularly preferably <NUM> or less. The lower limit is not limited, but it is preferably <NUM> or more, more preferably <NUM> or more, still more preferably <NUM> or more, further preferably <NUM> or more, further preferably <NUM> or more, further preferably <NUM> or more, further preferably <NUM> or more. When the value is within the range indicated above, the advantageous effect tends to be better achieved.

The mechanism for such an advantageous effect achieved by adjusting the value of A × S to a predetermined value or less, particularly A × S ≤ <NUM>, is not clear, but it is believed that in this case, sufficient grip performance may be ensured and at the same time wear may be reduced, resulting in improved wet grip performance and abrasion resistance and thus in improved overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance.

In the tire, the groove area ratio S (%) of the ground contact surface of the tread is preferably <NUM>% or less.

The groove area ratio S is preferably <NUM>% or less, more preferably <NUM>% or less, still more preferably <NUM>% or less, further preferably <NUM>% or less, further preferably <NUM>% or less. The lower limit of the groove area ratio is preferably <NUM>% or more, more preferably <NUM>% or more, still more preferably <NUM>% or more, further preferably <NUM>% or more, further preferably <NUM>% or more, further preferably <NUM>% or more. When the groove area ratio S is within the range indicated above, the advantageous effect tends to be better achieved.

The mechanism for such an advantageous effect achieved by adjusting S within a predetermined range, particularly <NUM>% ≤ S ≤ <NUM>%, is not clear, but it is believed that in this case, sufficient grip performance may be ensured and at the same time wear may be reduced, resulting in improved wet grip performance and abrasion resistance and thus in improved overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance.

A branched conjugated diene polymer at least containing a branched conjugated diene, butadiene, and styrene as structural units may provide improved compatibility with rubber components such as polybutadiene rubbers and styrene-butadiene rubbers. Particularly, when the branched conjugated diene polymer is controlled to satisfy the relationship (<NUM>): <NUM> < A ≤ <NUM>, i.e., to adjust the weight average molecular weight of the branched conjugated diene polymer within a predetermined range, the single-domain phases may be decreased to improve abrasion resistance. With the decrease of the single-domain phases, the bleeding of the branched conjugated diene polymer may also be reduced, resulting in improved bleed resistance.

Here, the groove area ratio S (negative ratio) of the ground contact surface of the tread refers to the ratio of the total groove area within the ground contact surface relative to the total area of the ground contact surface and is determined as described below.

Herein, when the tire is a pneumatic tire, the groove area ratio S is calculated from the contact patch of the tire under conditions including a normal rim, a normal internal pressure, and a normal load. When the tire is a non-pneumatic tire, the groove area ratio S can be similarly determined without the need of a normal internal pressure.

The term "normal rim" refers to a rim specified for each tire by the standard in a standard system including standards according to which tires are provided, and may be, for example, the standard rim in JATMA, "design rim" in TRA, or "measuring rim" in ETRTO.

The term "normal internal pressure" refers to an air pressure specified for each tire by the standard and may be the maximum air pressure in JATMA, the maximum value shown in Table "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" in TRA, or "inflation pressure" in ETRTO.

The term "normal load" refers to a load specified for each tire by the standard and may be the maximum load capacity in JATMA, the maximum value shown in Table "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" in TRA, or "load capacity" in ETRTO.

The contact patch may be determined by mounting the tire on a normal rim, applying a normal internal pressure to the tire, and allowing the tire to stand at <NUM> for <NUM> hours, followed by applying black ink to the tread surface of the tire and pressing the tread surface against a cardboard at a normal load (camber angle: <NUM>°) for transfer to the cardboard.

The transfer may be performed on the tire in five positions, each rotated by <NUM>° in the circumferential direction. Namely, five contact patches may be determined.

The contour points of each of the five contact patches are smoothly connected to draw a figure, the area of which is defined as the total area, and the transferred area as a whole constitutes the ground contact area. The average of the results of the five positions is determined and then the groove area ratio S (%) is calculated by the equation: [<NUM> - {Average of areas of five contact patches transferred to cardboard (parts with black ink)}/{Average of five total areas transferred to cardboard (figures obtained from contour points)}] × <NUM> (%).

Here, the average length or area is the simple average of the five values.

The thickness G (mm) of the tread of the tire is preferably <NUM> or more, more preferably <NUM> or more, still more preferably <NUM> or more, further preferably <NUM> or more. The upper limit is not limited, but it is preferably <NUM> or less, more preferably <NUM> or less, still more preferably <NUM> or less, particularly preferably <NUM> or less. When the thickness is within the range indicated above, the advantageous effect tends to be better achieved.

The mechanism for such an advantageous effect is not clear, but it is believed that adjusting the thickness of the tread within a predetermined range may improve rigidity to improve abrasion resistance and wet grip performance, and such a thickness may also improve bleed resistance, thereby resulting in improved overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance.

The thickness G of the tread in the present invention refers to the thickness of the tread measured on the equator in a cross-section of the tread taken in the tire radial direction. Specifically, the thickness G of the tread is the distance from the tread surface to the interface of the belt-reinforcing layer, belt layer, carcass layer, or other reinforcing layer containing a fiber material, which is outermost with respect to the tire. Here, when the tread has a groove on the tire equator, it is the linear distance from the intersection of the equator and a straight line connecting the outermost edges of the groove with respect to the tire radial direction.

<FIG> is a meridional cross-sectional view showing a part of a tire <NUM> according to an embodiment of the present invention. It should be noted that the tire of the present invention is not limited to the embodiments described below.

In <FIG>, the vertical direction corresponds to the radial direction of the tire (hereinafter, also referred to simply as tire radial direction or radial direction), the horizontal direction corresponds to the axial direction of the tire (hereinafter, also referred to simply as tire axial direction or axial direction), and the direction perpendicular to the paper corresponds to the circumferential direction of the tire (hereinafter, also referred to simply as tire circumferential direction or circumferential direction). The present tire <NUM> has a shape that is horizontally substantially symmetrical about the center line CL in <FIG>. The center line CL is also referred to as a tread center line and represents the equator EQ of the tire <NUM>.

The present tire <NUM> includes a tread <NUM>, a sidewall <NUM>, a bead <NUM>, a carcass <NUM>, and a belt <NUM>. The present tire <NUM> is a tubeless tire.

The tread portion <NUM> includes a tread face <NUM>. The tread face <NUM> has a radially outwardly convex shape in a cross-section taken in the meridional direction of the tire <NUM>. The tread face <NUM> will contact the road surface. The present tread face <NUM> has a plurality of circumferentially extending grooves <NUM> carved therein. The grooves <NUM> define a tread pattern. The external part of the tread <NUM> with respect to the tire axial direction (tire width direction) is referred to as a shoulder portion <NUM>. The sidewall <NUM> extends substantially inwardly in the radial direction from the end of the tread <NUM>. The sidewall <NUM> is formed of a crosslinked rubber or the like.

As shown in <FIG>, the bead <NUM> is located radially substantially inward from the sidewall <NUM>. The bead <NUM> includes a core <NUM> and an apex <NUM> radially outwardly extending from the core <NUM>. The core <NUM> has a ring shape along the circumferential direction of the tire. The core <NUM> is formed of a wound inextensible wire. Typically, a steel wire is used in the core <NUM>. The apex <NUM> is radially outwardly tapered. The apex <NUM> is formed of a very hard crosslinked rubber or the like.

In the present embodiment, the carcass <NUM> is formed of a carcass ply <NUM>. The carcass ply <NUM> extends between the opposite beads <NUM> along the inner sides of the tread <NUM> and the sidewalls <NUM>. The carcass ply <NUM> is folded around the core <NUM> from the inside to the outside in the tire axial direction. Though not shown, the carcass ply <NUM> is formed of a parallel array of cords and a topping rubber. The absolute value of the angle of each cord relative to the equator EQ (CL) is usually <NUM>° to <NUM>°. In other words, the present carcass <NUM> has a radial structure.

In the present embodiment, the belt <NUM> is located radially outward of the carcass <NUM>. The belt <NUM> is stacked on the carcass <NUM>. The belt <NUM> reinforces the carcass <NUM>. The belt <NUM> may be formed of an inner layer belt <NUM> and an outer layer belt <NUM>. In the present embodiment, the widths of the belts <NUM> and <NUM> are different from each other.

Though not shown, each of the inner layer belt <NUM> and the outer layer belt <NUM> is usually formed of a parallel array of cords and a topping rubber. Each cord is desirably inclined with respect to the equator EQ. Desirably, the cord of the inner layer belt <NUM> is inclined in a direction opposite to the inclination of the cord of the outer layer belt <NUM>.

Though not shown, an embodiment may also be used in which a band is stacked on the outer side of the belt <NUM> with respect to the tire radial direction. The band is wider than the belt <NUM>. The band may be formed of a cord and a topping rubber. The cord is spirally wound. The belt is restrained by the cord, so that the belt <NUM> is inhibited from lifting. The cord is desirably formed of organic fibers. Preferred examples of the organic fibers include nylon fibers, polyester fibers, rayon fibers, polyethylene naphthalate fibers, and aramid fibers.

Though not shown, an embodiment may also be used in which an edge band is provided outward of the belt <NUM> in the tire radial direction and near the widthwise end (edge portion) of the belt <NUM>. Like the band, the edge band may be formed of a cord and a topping rubber. An exemplary edge band may be stacked on the upper surface of a step <NUM> of the wider inner layer belt <NUM>. In an exemplary embodiment, the cord in the edge band is inclined in the same direction as the direction of the cord in the narrower outer layer belt <NUM> and is biased relative to the cord in the wider inner layer belt <NUM>.

Though not shown, an embodiment may also be used in which a cushion rubber layer is stacked on the carcass <NUM> near the widthwise end of the belt <NUM>. In an exemplary embodiment, the cushion layer is formed of a soft crosslinked rubber. The cushion layer absorbs stress at the belt edge.

<FIG> shows a cross-section of the tread <NUM> of the present tire <NUM> taken along a plane including the tire axis. In <FIG> and <FIG>, a crown center <NUM> located at the equator EQ corresponds to "the equator in a cross-section of the tread <NUM> taken in the tire radial direction".

In the tire <NUM>, the thickness G of the tread <NUM> (the thickness G of the tread <NUM> measured on the equator in a cross-section of the tread <NUM> taken in the tire radial direction) refers to the distance from the tread face <NUM> (which corresponds to a straight line connecting the outermost edges of the groove <NUM> with respect to the tire radial direction as the tire <NUM> has the groove on the tire equator) to the interface of the outer layer belt <NUM>, which is outermost with respect to the tire, as measured on the equator in a cross-section taken along a plane including the tire axis.

Moreover, in the tire <NUM>, the tread <NUM> includes a tread rubber composition which contains a rubber component and a branched conjugated diene polymer, and the branched conjugated diene polymer is a copolymer at least containing a branched conjugated diene, butadiene, and styrene as structural units and satisfies the relationship (<NUM>): <NUM> < A ≤ <NUM> wherein A is defined by A = <NUM>/√α + <NUM> wherein α represents the weight average molecular weight of the branched conjugated diene polymer. Further, A and the groove area ratio S (%) of the ground contact surface of the tread <NUM> satisfy the relationship (<NUM>): A × S ≤ <NUM>.

Examples (working examples) which are considered preferable to implement the present invention are described below although he scope of the invention is not limited to the examples.

Tires produced with the chemicals listed below according to the formulations varied as shown in Table <NUM> were examined. The results calculated according to the below-described evaluations are shown in Table <NUM>.

The following materials are used in the production examples described below.

The weight average molecular weight of each polymer is determined by gel permeation chromatography (GPC) relative to polystyrene standards. The measurement device and conditions used are as follows.

A dried and nitrogen-purged pressure-resistant vessel is charged with <NUM> of cyclohexane as a solvent and <NUM> of sec-butyllithium (<NUM>% by mass solution in cyclohexane) as an initiator, and the temperature is raised to <NUM>. Thereafter, <NUM> of tetrahydrofuran is added, and <NUM> of a previously prepared mixture of butadiene and β-farnesene (<NUM> of butadiene and <NUM> of β-farnesene are mixed in a cylinder) is added at <NUM>/min, followed by polymerization for one hour.

The thus prepared polymerization reaction solution is treated with methanol and washed with water.

After the washing, water is separated from the polymerization reaction solution, followed by drying at <NUM> for <NUM> hours to obtain copolymer <NUM> having the physical properties shown in Table <NUM>.

A dried and nitrogen-purged pressure-resistant vessel is charged with <NUM> of cyclohexane as a solvent and <NUM> of sec-butyllithium (<NUM>% by mass solution in cyclohexane) as an initiator, and the temperature is raised to <NUM>. Thereafter, <NUM> of tetrahydrofuran is added, and <NUM> of a previously prepared mixture of styrene, butadiene, and β-farnesene (<NUM> of styrene, <NUM> of butadiene, and <NUM> of β-farnesene are mixed in a cylinder) is added at <NUM>/min, followed by polymerization for two hours.

A dried and nitrogen-purged pressure-resistant vessel is charged with <NUM> of cyclohexane as a solvent and <NUM> of sec-butyllithium (<NUM>% by mass solution in cyclohexane) as an initiator, and the temperature is raised to <NUM>. Thereafter, <NUM> of tetrahydrofuran is added, and <NUM> of a previously prepared mixture of styrene, butadiene, and β-farnesene (<NUM> of styrene, <NUM> of butadiene, and <NUM> of β-farnesene are mixed in a cylinder) is added at <NUM>/min, followed by polymerization for one hour.

Copolymer <NUM> having the physical properties shown in Table <NUM> is produced as in Production Example <NUM>, except that the amount of sec-butyllithium (<NUM>% by mass solution in cyclohexane) is changed to <NUM>.

Copolymer <NUM> having the physical properties shown in Table <NUM> is produced as in Production Example <NUM>, except that the previously prepared mixture of styrene, butadiene, and β-farnesene is changed to a mixture of <NUM> of styrene, <NUM> of butadiene, and <NUM> of β-farnesene.

The chemicals other than sulfur and vulcanization accelerators in the formulation amounts shown in Table <NUM> or <NUM> are kneaded in a <NUM> Banbury mixer (Kobe Steel, Ltd. ) for four minutes at <NUM> to give a kneaded mixture.

Then, sulfur and a vulcanization accelerator are added to the kneaded mixture, and they are kneaded in an open roll mill at <NUM> for four minutes to give an unvulcanized polymer composition.

The unvulcanized polymer composition is formed into the shape of a tread and assembled with other tire components on a tire building machine to build an unvulcanized tire. Subsequently, the unvulcanized tire is vulcanized at <NUM> for <NUM> minutes to produce a test tire (size: <NUM>/65R15, specification: shown in Table <NUM> or <NUM>).

The test tires of each example are mounted on a car, and the car run for <NUM>,<NUM>. Then, the groove depth in the tread portion is measured to calculate the abrasion loss of the tread portion, which is expressed as an index relative to the abrasion loss of Comparative Example <NUM> taken as <NUM>. A higher index indicates a lower abrasion loss and thus better abrasion resistance.

The test tires of each example are mounted on a car. The braking distance of the car with an initial speed of <NUM>/h on a wet asphalt road is determined and expressed as an index relative to the breaking distance of Comparative Example <NUM> taken as <NUM>. A higher index indicates a shorter braking distance and thus better wet grip performance.

Rubber specimens are cut out of the tread in each test tire, and the amount of matter extractable with acetone from each specimen is measured by a method for measuring the acetone extractable content in accordance with JIS K <NUM>. The acetone extractable content of each rubber specimen is expressed as an index relative to the acetone extractable content of Comparative Example <NUM> as standard. A higher index indicates a lower acetone extractable content and thus better bleed resistance. Acetone extractable content (% by mass) = ((Mass of sample before extraction) - (Mass of sample after extraction))/(Mass of sample before extraction) × <NUM>.

The average of the abrasion resistance (index), the wet grip performance (index), and the acetone extractable content (index) is evaluated as the overall performance in terms of abrasion resistance, wet grip performance, and bleed resistance. A higher index indicates better overall performance.

Claim 1:
A tire, comprising a tread comprising a rubber composition that comprises at least one rubber component and a branched conjugated diene polymer,
the branched conjugated diene polymer being a copolymer at least containing a branched conjugated diene, butadiene, and styrene as structural units,
the tire satisfying the following relationships (<NUM>) and (<NUM>): <MAT> and <MAT> wherein A is defined by A = <NUM>/√α + <NUM> wherein α represents a weight average molecular weight of the branched conjugated diene polymer; and S represents a groove area ratio (%) of a ground contact surface of the tread, and wherein α and S are measured according to the description.