JP2010248444A - Rubber composition for tire tread and pneumatic tire - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve gripping performance without impairing wear resistance and processability. <P>SOLUTION: A rubber composition for a tire tread is provided, which contains, with respect to 100 parts by weight of a rubber component, 20 to 100 parts by weight of silica and 1 to 30 parts by weight of polymer gel comprising hydroxy-modified and crosslinked diene-based polymer particles having a glass transition temperature of over 0 degree to 80 degree centigrade, and further contains 2 to 25 wt.% of a silane coupling agent with respect to the silica and 10 to 100 wt.% of an organic silane compound expressed by formula (1):SiR<SP>1</SP><SB>n</SB>(OR<SP>2</SP>)<SB>4-n</SB>with respect to the silane coupling agent. In the formula, R<SP>1</SP>represents one of 5-20 alkyl, alkenyl, cycloalkenyl or aromatic hydrocarbon groups; R<SP>2</SP>represents a 1-3C alkyl group; and n represents an integer of 1 to 3. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a rubber composition for a tire tread and a pneumatic tire having a tread using the rubber composition.

  Conventionally, in pneumatic tires, in order to improve grip performance, particularly wet grip performance on wet road surfaces, for example, a method of increasing the blending amount of filler and oil, or a polymer having a high glass transition point as a rubber component The technique used is used.

  However, the method of increasing the blending amount of the filler and oil deteriorates the heat generation and wear resistance. Moreover, since the viscosity in an unvulcanized state is increased and the dispersibility of the filler is deteriorated, the workability is reduced, for example, the number of times of mixing is increased in order to obtain sufficient performance. On the other hand, in the method using a polymer having a high glass transition point, exothermic property and wear resistance are deteriorated, and steering stability on a dry road surface is deteriorated due to deterioration of temperature dependency.

  In order to improve grip performance on wet road surfaces, it is known to use silica as a filler. However, when silica is used, workability is greatly inferior, such as an increase in the number of mixing times and a decrease in extrusion speed. As described above, with the conventional wet grip improving method, it is difficult to improve wet grip while suppressing deterioration of workability and wear resistance.

  By the way, in the following Patent Document 1, in order to improve wet grip properties and low fuel consumption, it is modified with a compound having a hydroxyl group and has a glass transition point of −100 to −65 ° C. and a toluene swelling index Qi of 1 to 15. In addition, it has been proposed to incorporate crosslinked rubber particles (polymer gel) having an average particle diameter of 5 to 2000 nm. However, the polymer gel easily aggregates in the rubber component, and wear resistance is impaired by the aggregation. For this reason, in the above-mentioned document, the combination with a modified diene rubber whose end is modified with a functional group is limited, and the selection of the rubber component is limited.

  In Patent Document 2 below, solution-polymerized styrene butadiene rubber, silica, carbon black, and rubber gel are blended for improving slip resistance and rolling resistance on wet road surfaces, and further improving wear resistance. A rubber composition containing a sulfur compound as a filler activator is disclosed.

  On the other hand, Patent Document 3 below discloses that an organic silane compound is blended with silica in a rubber composition constituting a tire tread in order to improve ice performance without reducing wear resistance. However, this document only discloses that the dispersibility of silica is improved by the interaction between the organosilane compound and silica.

JP 2008-169314 A Special table 2004-530760 gazette JP 2007-277437 A

  As described above, in the conventional rubber composition for tire treads, it has been known that a polymer gel is blended, but a polymer gel having a high glass transition point as in the present invention is used in combination with an organosilane compound. It has not been known, and it has not been known that grip performance can be advantageously improved without impairing wear resistance and workability. When a polymer gel having a high glass transition point is used, although grip performance can be improved, it tends to be inferior in wear resistance, and it is required to solve this problem.

  The present invention has been made in view of the above points, and by using a specific polymer gel and an organosilane compound in combination, a tire capable of improving grip performance without impairing wear resistance and workability. It aims at providing the rubber composition for treads, and the pneumatic tire using this rubber composition.

  The rubber composition for a tire tread according to the present invention is crosslinked with 20 to 100 parts by weight of silica with respect to 100 parts by weight of a rubber component made of a diene rubber and having a hydroxyl modification and a glass transition point of more than 0 ° C. and 80 ° C. or less. 1 to 30 parts by weight of the polymer gel, which is a diene polymer particle, and 2 to 25% by weight of a silane coupling agent with respect to the silica, and an organic silane represented by the following general formula (1) The compound is contained in an amount of 10 to 100% by weight based on the silane coupling agent.

SiR ¹ _n (OR ² ) _4-n (1)
In Formula (1), R ¹ is any of an alkyl group having 5 to 20 carbon atoms, an alkenyl group, a cycloalkenyl group, and an aromatic hydrocarbon group, R ² is an alkyl group having 1 to 3 carbon atoms, and n is It is an integer of 1-3.

  The pneumatic tire according to the present invention has a tread formed of the rubber composition.

  According to the present invention, by using a polymer gel having a specific glass transition point having a hydroxyl group in combination with an organosilane compound, the dispersibility of the polymer gel can be improved, and the wear resistance and workability are impaired. In addition, the grip performance can be improved.

  Hereinafter, matters related to the implementation of the present invention will be described in detail.

  The rubber composition according to the present invention comprises (A) a rubber component comprising a diene rubber, (B) a polymer gel, (C) silica, (D) a silane coupling agent, and (E) an organosilane compound. It contains.

  Examples of the diene rubber of the component (A) include natural rubber (NR), polyisoprene rubber (IR), styrene-butadiene rubber (SBR), polybutadiene rubber (BR), styrene-isoprene rubber, and butadiene-isoprene rubber. Styrene-isoprene-butadiene copolymer rubber, nitrile rubber, and the like. These may be used alone or in combination of two or more. Among the above, styrene-butadiene rubber, polybutadiene rubber, and natural rubber are preferable, and particularly those containing styrene-butadiene rubber as a main component, that is, styrene-butadiene rubber alone or 50% by weight or more of styrene-butadiene rubber. A blend of less than 50% by weight of diene rubber is preferred.

  In a more preferred embodiment, the diene rubber of component (A) is a copolymer rubber obtained by copolymerization of 1,3-butadiene and styrene using an organolithium compound as an initiator, that is, solution polymerization SBR. Thus, a glass transition point (Tg) of −40 ° C. or higher (Tg ≧ −40 ° C.) is used as a main component. Accordingly, as the rubber component (A), a solution polymerization SBR having a Tg of −40 ° C. or more, or a solution polymerization SBR having a Tg of −40 ° C. or more is 50% by weight or more, and another diene rubber is 50% by weight. % Or less. The other diene rubber includes the above-mentioned various diene rubbers, and may be a styrene butadiene rubber having a Tg of less than −40 ° C. Thus, grip performance can be improved by using SBR with a high glass transition point. Although the upper limit of a glass transition point is not specifically limited, Usually, it is 0 degrees C or less. Here, the glass transition point is a value (temperature increase rate 20 ° C./min) measured using differential scanning calorimetry (DSC) in accordance with JIS K7121.

  The solution polymerization SBR can be produced by, for example, a known solution polymerization method using an inert organic solvent such as pentane, hexane, heptane, benzene, toluene, diethyl ether, and the organic lithium compound includes n -Alkyl lithium such as butyl lithium, alkylene dilithium such as 1,4-dilithium butane, phenyl lithium and the like. The solution polymerized SBR may have a copolymer chain terminal treated with a tin-based, silicon-based or alkoxysilane-based coupling agent, and the terminal or main chain is mutually linked with a silanol group of silica. It may be modified with a functional group having an action or chemical reactivity (for example, a hydroxyl group or an amino group).

  As the polymer gel of the component (B), crosslinked diene polymer particles (rubber particles) that are hydroxyl-modified and have a glass transition point (Tg) of more than 0 ° C. and 80 ° C. or less are used. By using such a polymer gel having a high glass transition point, it is possible to improve grip performance, particularly wet grip properties. In addition, the hydroxyl group present on the particle surface reacts with the organosilane compound, thereby improving the dispersibility of the polymer gel, thereby further improving wet grip properties while maintaining wear resistance and workability. it can.

  Moreover, since a polymer gel consists of a diene polymer, it has a double bond on the surface. Therefore, when the silane coupling agent reacts with the double bond, the polymer gel and silica can be bonded via the silane coupling agent. Moreover, the hydroxyl group (OH group) of a surface reacts with a silane coupling agent, and a polymer gel can couple | bond a polymer gel and the said rubber component through a silane coupling agent. Alternatively, the polymer gel can be cross-linked by the reaction of the silane coupling agent between the double bond of the polymer gel and the OH group. These can also improve the performance.

  Such a polymer gel can be produced by crosslinking a rubber dispersion. Examples of the rubber dispersion include rubber latex produced by emulsion polymerization, rubber dispersion obtained by emulsifying solution-polymerized rubber in water, and examples of the crosslinking agent include organic peroxides and organic azo compounds. And sulfur-based crosslinking agents. The rubber particles can also be crosslinked by copolymerization with a polyfunctional compound having a crosslinking action during the emulsion polymerization of the rubber. Specifically, methods disclosed in JP-A-10-204225, JP-T 2004-504465, JP-T 2004-506058, JP-T 2004-530760, and the like can be used.

  Examples of the diene polymer constituting the polymer gel include the various diene rubbers described above, and these can be used alone or in combination of two or more. Preferably, the main component is polybutadiene rubber or styrene butadiene rubber, and more preferably the main component is styrene butadiene rubber.

  As the polymer gel, those having a glass transition point (Tg) larger than 0 ° C. and 80 ° C. or less as described above (0 <Tg ≦ 80 ° C.) are used. Thus, grip performance can be improved by using a thing with a high glass transition point. If the glass transition point is 0 ° C. or less, the effect of improving the grip performance cannot be obtained. The glass transition point is more preferably 30 ° C. or higher, and further preferably 50 ° C. or higher. In addition, the glass transition point of a polymer gel can be adjusted with the kind of diene polymer used as a base, and its crosslinking degree. The glass transition point is a value (temperature increase rate 20 ° C./min) measured using differential scanning calorimetry (DSC) in accordance with JIS K7121.

  As the polymer gel, a hydroxyl-modified product modified with a compound having an OH group as described above is used. Examples of such compounds (modifiers) include hydroxyalkyl (such as hydroxybutyl acrylate or methacrylate, hydroxyethyl acrylate or methacrylate, hydroxypropyl acrylate or methacrylate, as described in JP-T-2004-506058. And (meth) acrylate.

  The polymer gel preferably has a toluene swelling index Qi of less than 16. The toluene swelling index is more preferably 1 to 15, and further preferably 3 to 8. When the toluene swelling index Qi is too large, the particles become soft, the reinforcing effect is lost, and the strength and wear resistance are impaired. The polymer gel preferably has a gel content of 94% by weight or more. When the gel content is smaller than this, the elastic modulus of the rubber particles tends to be lowered, and the rubber composition containing the rubber particles is also affected.

  Here, the toluene swelling index and the gel content are measured by swelling a polymer gel in toluene and then drying it. That is, 250 mg of polymer gel was swollen in 25 mL of toluene under shaking for 24 hours, centrifuged at 20000 rpm, weighed, then dried to a constant mass at 70 ° C., and then dried mass Weigh. The gel content is the weight ratio (%) of the polymer gel after drying to the polymer gel used. The toluene swelling index is obtained by Qi = (wet mass of gel) / (dry mass of gel).

  The particle size of the polymer gel is not particularly limited, but the average particle size (DVN value according to DIN 53 206) is preferably 10 to 200 nm, more preferably 20 to 100 nm. By using a polymer gel having such an average particle size, the processability and reinforcing effect of the rubber composition can be maintained.

  The blending amount of the polymer gel is 1 to 30 parts by weight, more preferably 3 to 15 parts by weight, and further preferably 5 to 10 parts by weight with respect to 100 parts by weight of the rubber component. When the blending amount of the polymer gel is too small, it becomes difficult to obtain the effect of improving the grip performance. Conversely, when the blending amount is too large, it is difficult to maintain the wear resistance.

  Examples of the silica of component (C) include wet silica, dry silica, colloidal silica, precipitated silica, and the like. It is particularly preferable to use wet silica containing hydrous silicic acid as a main component. Silica can be blended in an amount of 20 to 100 parts by weight with respect to 100 parts by weight of the rubber component, more preferably 40 to 90 parts by weight. When the blending amount of silica is less than 20 parts by weight, it is difficult to obtain a sufficient improvement in grip performance.

  In the rubber composition according to the present invention, the filler may be the above silica alone, or carbon black may be blended together with the silica. Carbon black is preferably blended in an amount of 0 to 100 parts by weight with respect to 100 parts by weight of the rubber component. Silica and carbon black are preferably blended in a total amount of 70 to 150 parts by weight. In the rubber composition of the present invention, other fillers such as titanium oxide, aluminum silicate, clay, and talc can be blended as the filler in addition to the silica and carbon black.

  The component (D) silane coupling agent is, for example, an organic silane having an organic moiety capable of reacting with a polymer such as sulfide, amino group, mercapto group, vinyl group, methacryl group, epoxy group, and halogen or alkoxy group. It is a compound, and various known silane coupling agents can be used. Preferably, a sulfide silane represented by the following general formula (2) or a protected mercaptosilane represented by the following general formula (3) is used.

_{(C 2 H 5 O) 3} Si-C y H 2y -S x -C y H 2y -Si (OC 2 H 5) 3 ... (2)
_{(C p H 2p + 1 O} ) 3 Si-C q H 2q -S-CO-C k H 2k + 1 ... (3)
In said formula (2), y is an integer of 1-9, Preferably it is 2-5, x is 1-4, Preferably it is 2-4. Specifically, x has a normal distribution, that is, it is generally marketed as a mixture of different numbers of sulfur chain bonds, and x represents an average value thereof. Specific examples of the preferred sulfide silane represented by the formula (2) include bis (3-triethoxysilylpropyl) tetrasulfide, bis (3-triethoxysilylpropyl) disulfide, and bis (2-triethoxysilylethyl) tetra. And sulfides.

  In said formula (3), p is an integer of 1-2, q is an integer of 1-5, k is an integer of 5-9. Examples of the protected mercaptosilane represented by the formula (3) include 3-octanoylthio-1-propyltriethoxysilane in which p = 2, q = 3, and k = 7, and p = 1, q = 3, k. Preferred examples include 3-propionylthiopropyltrimethoxysilane where = 2.

  The compounding amount of the silane coupling agent is 2 to 25% by weight with respect to silica, that is, 2 to 25 parts by weight with respect to 100 parts by weight of silica, in order to sufficiently exhibit the effects of the present invention described above. Preferably, it is 5 to 15% by weight.

As the organosilane compound of the component (E), a long-chain organosilane represented by the general formula (1) is used. Organic silane compound increases with the alkoxysilane moiety is reacted with the hydroxyl groups of the polymer gel surface, the presence of a hydrocarbon moiety of the long chain represented by R ^1, the dispersibility of the polymer gel in the rubber component Can be made.

Examples of the alkyl group having 5 to 20 carbon atoms in R ¹ include pentyl, isopentyl, secondary pentyl, neopentyl, t-pentyl, hexyl, secondary hexyl, heptyl, secondary heptyl, octyl, 2-ethylhexyl and secondary octyl. , Nonyl, secondary nonyl, decyl, secondary decyl, undecyl, secondary undecyl, dodecyl, secondary dodecyl, tridecyl, isotridecyl, secondary tridecyl, tetradecyl, secondary tetradecyl, pentadecyl, hexadecyl, secondary hexadecyl, heptadecyl, octadecyl , Nonadecyl, eicosyl, stearyl, icosyl, docosyl, tetracosyl, triacontyl, 2-butyloctyl, 2-butyldecyl, 2-hexyloctyl, 2-hexyldecyl, 2-octyldecyl, 2-hexyldecyl, 2-octyl Examples include tildedecyl, 2-decyltetradecyl, 2-dodecylhexadecyl, 2-hexadecyloctadecyl, 2-tetradecyloctadecyl group and the like.

Examples of the alkenyl group for R ¹ include pentenyl, isopentenyl, hexenyl, heptenyl, octenyl, nonenyl, decenyl, undecenyl, dodecenyl, tetradecenyl, oleyl and the like. Examples of the cycloalkenyl group include cyclopentenyl, cyclohexenyl, cycloheptenyl and the like. Examples of the aromatic hydrocarbon group include groups formed by removing one hydrogen atom from benzene, toluene, xylene, styrene, ethylbenzene and the like.

Among these, R ¹ is preferably an alkyl group having 5 to 20 carbon atoms, more preferably a linear structure having 6 or more carbon atoms, more preferably 8 or more carbon atoms, which forms a long-chain alkyl group. . R ¹ may contain two or more of the above groups. When the number of carbon atoms is less than 5, the interaction between R ¹ and the rubber component becomes insufficient, and the dispersibility of the polymer gel is not improved. On the other hand, when the number of carbon atoms exceeds 20, the influence on the physical properties of rubber, particularly the rubber hardness tends to decrease.

R ² is an alkyl group having 1 to 3 carbon atoms, that is, a methyl, ethyl, or propyl group. R ² is bonded to an oxygen atom in the formula to form an alkoxyl group, that is, R ² O is any one of a methoxy group, an ethoxy group, and a propoxy group.

  Specific examples of preferred organic silane compounds include hexyltrimethoxysilane, hexyltriethoxysilane, octyltrimethoxysilane, octyltriethoxysilane, decyltrimethoxysilane, decyltriethoxysilane, octadecyltrimethoxysilane, octadecyltriethoxysilane, Examples include hexadecylmethoxysilane, hexadecylethoxysilane, dioctyldimethoxysilane, dioctyldiethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, diphenyldimethoxysilane, and diphenyldiethoxysilane.

  The amount of the organic silane compound is preferably 10 to 100% by weight with respect to the silane coupling agent, that is, 10 to 100 parts by weight, more preferably 10 to 70 parts by weight with respect to 100 parts by weight of the silane coupling agent. % By weight, more preferably 30-60% by weight. If the blending amount is less than 10% by weight, the effect of improving the dispersibility of the polymer gel becomes insufficient. Conversely, if it exceeds 100 parts by weight, the coupling effect on silica by the silane coupling agent becomes insufficient.

  In addition to the components described above, the rubber composition according to the present invention includes a softener, a plasticizer, an antiaging agent, zinc white, stearic acid, a vulcanizing agent, a vulcanization accelerator, and the like in a rubber composition for tire treads. Various commonly used additives can be blended. The rubber composition can be prepared by kneading using a commonly used Banbury mixer, a mixer such as a roll or a kneader. The rubber composition may be vulcanized and molded at 140 to 200 ° C. according to a conventional method to form various pneumatic tires, particularly preferably tread rubber (rubber constituting the ground contact surface) of a riding tire. it can.

  Examples of the present invention will be described below, but the present invention is not limited to these examples.

  Using a Banbury mixer, a tire tread rubber composition was prepared according to the formulation shown in Table 1 below. Each component in Table 1 is as follows.

SBR1: LANXESS solution polymerization SBR “VSL5025-OHM” (glass transition point Tg = −15 ° C.),
SBR2: “SBR1502” manufactured by JSR Corporation (glass transition point Tg = −66 ° C.),
Carbon black: “Diamond Black N234” manufactured by Mitsubishi Chemical Corporation
・ Silica: “Nip Seal AQ” manufactured by Tosoh Silica Co., Ltd.
Silane coupling agent: bis- (3-triethoxysilylpropyl) disulfide, “Si-75” manufactured by Degussa,
・ Oil: “Process NC-140” manufactured by Japan Energy Co., Ltd.
Polymer gel 1: “Micromov 1P” manufactured by Rhein Chemie (polymer gel based on SBR, Tg = 65 ° C., toluene swelling index Qi = 7, gel content = 96 wt%, average particle size = 60 nm, hydroxyl group modification Product),
Polymer gel 2: “Micromov 30B” manufactured by Rhein Chemie (polymer gel based on BR, Tg = −80 ° C., toluene swelling index Qi = 5.9, gel content = 97 wt%, average particle size = 130 nm, Hydroxyl group modified product),
Organic silane compound 1: n-decyltrimethoxysilane, “KBM-3103C” manufactured by Shin-Etsu Chemical Co., Ltd.
Organic silane compound 2: n-hexyltriethoxysilane, “KBM-3063” manufactured by Shin-Etsu Chemical Co., Ltd.)
-Organosilane compound 3: Phenyltriethoxysilane, "KBE-103" manufactured by Shin-Etsu Chemical Co., Ltd.).

  In each rubber composition, 2 parts by weight of stearic acid ("Lunac S-20" manufactured by Kao Corporation) and zinc white ("Zinc Flower" manufactured by Mitsui Kinzoku Mining Co., Ltd.) are used as a common compound. 1 type)) 3 parts by weight, anti-aging agent (Sumitomo Chemical Co., Ltd. “Antigen 6C”) 2 parts by weight, wax (Ouchi Shinsei Chemical Co., Ltd. “Sannok N”) 2 parts by weight, vulcanization accelerator ( Sumitomo Chemical Co., Ltd. “Soxinol CZ”) 1.5 parts by weight and sulfur (Tsurumi Chemical Co., Ltd. “powder sulfur”) 2.1 parts by weight were blended.

  About each obtained rubber composition, workability, wet grip property, and abrasion resistance were evaluated. Each evaluation method is as follows.

Processability: Processability was evaluated based on the Mooney viscosity of unvulcanized rubber measured with a Mooney viscometer manufactured by Shimadzu Corporation. The test method was based on JIS K6300, and expressed as an index with the value of Comparative Example 1 as 100 for the reciprocal of Mooney viscosity. Larger values indicate lower viscosity and better processability.

-Wet grip property: Rebound resilience measured in accordance with JIS K6301 was evaluated as an index of wet grip property. Specifically, a test piece having a predetermined shape is prepared by vulcanization at 160 ° C. for 30 minutes, and the rebound elastic modulus is measured according to JIS K6301 using the obtained test piece, and the reciprocal of the rebound elastic modulus. Is expressed as an index with the value of Comparative Example 1 as 100. The larger the value, the lower the resilience modulus and the better the wet grip properties.

Abrasion resistance: vulcanized at 160 ° C. for 30 minutes to prepare a test piece of a predetermined shape, and using the obtained test piece, a load of 40 N, using a Lambourne abrasion tester in accordance with JIS K6264 The wear loss was measured under the conditions of a slip rate of 30% and a sand fall amount of 20 g / min, and the reciprocal of the wear loss was shown as an index with the value of Comparative Example 1 being 100. The higher the value, the better the wear resistance.

  The results are as shown in Table 1. Compared with Comparative Example 1 as a control, in Comparative Example 2 in which a polymer gel having a high Tg was blended, the wet grip property was improved, but the wear resistance was lowered. In Comparative Example 3 in which a low Tg polymer gel was blended, although the decrease in wear resistance was small, the wet grip property was worse than that in Comparative Example 1. On the other hand, in Comparative Example 4 in which the organosilane compound was blended, the wet grip property was hardly improved and the wear resistance was deteriorated. Further, in Comparative Example 5 in which the amount of the silane coupling agent was reduced with respect to Comparative Example 4, the deterioration of the wear resistance was suppressed, but the effect of improving the wet grip property was not obtained similarly. Furthermore, in Comparative Example 6, although the polymer gel and the organosilane compound were used in combination, the effect of improving the wet grip property was not recognized because the polymer gel had a low Tg.

  On the other hand, in Examples 1 to 7 in which a high Tg polymer gel and an organic silane compound are combined, the wet grip property is greatly improved without impairing the wear resistance, and the wet grip property is improved. A synergistic effect was observed. Further, the workability was also maintained or improved with respect to Comparative Example 1.

  The rubber composition for tire tread of the present invention can be used for various pneumatic tires such as passenger cars, light trucks, trucks and buses.

Claims (4)

For 100 parts by weight of rubber component made of diene rubber,
20 to 100 parts by weight of silica,
Containing 1 to 30 parts by weight of a polymer gel which is a crosslinked diene polymer particle which is hydroxyl-modified and has a glass transition point of more than 0 ° C. and 80 ° C. or less,
Containing a silane coupling agent in an amount of 2 to 25% by weight based on the silica;
An organic silane compound represented by the following general formula (1) is contained in an amount of 10 to 100% by weight based on the silane coupling agent.
SiR ¹ _n (OR ² ) _4-n (1)
(In the formula (1), R ¹ is any one of an alkyl group having 5 to 20 carbon atoms, an alkenyl group, a cycloalkenyl group, and an aromatic hydrocarbon group, R ² is an alkyl group having 1 to 3 carbon atoms, n Is an integer from 1 to 3.)   The rubber component is a copolymer rubber having a glass transition point of −40 ° C. or more obtained by copolymerization of 1,3-butadiene and styrene using an organolithium compound as an initiator, or the copolymer rubber. The rubber composition for a tire tread according to claim 1, comprising a blend of 50% by weight or more and 50% by weight or less of another diene rubber.   The rubber composition for a tire tread according to claim 1 or 2, wherein the polymer gel has a toluene swelling index Qi of less than 16 and a gel content of 94% by weight or more.   A pneumatic tire having a tread formed of the rubber composition according to claim 1.