JP2006249230A - Rubber composition for tread and pneumatic tire using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a rubber composition for tread, which exhibits excellent grip performance in a wide temperature range when used in a tire tread. <P>SOLUTION: The rubber composition for tread is obtained by mixing 100 parts by mass of a rubber component (A) with 5-50 parts by mass of a softener (B) having -50°C to 50°C softening point, 5-50 parts by mass of a resin (C) having ≥70°C softening point and/or 5-50 parts by mass of a plasticizer (D) having ≤-50°C freezing point. The softener (B) is a high-vinyl polybutadiene having ≥50% vinyl bond content. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a rubber composition for a tread and a pneumatic tire using the same, and more particularly to a rubber composition for a tread that can exhibit excellent grip performance over a wide temperature range when used in a tire.

  The most important product requirement for high-performance passenger car tires is to ensure high grip performance on dry road surfaces, especially braking performance. In order to ensure this braking performance, it is necessary to improve the hysteresis loss of the rubber composition used for the tread. Various techniques have been conventionally developed as techniques for improving the hysteresis loss of the rubber composition.

  For example, a rubber composition in which a styrene-butadiene copolymer rubber (SBR) having a high glass transition point (Tg), a high styrene content and a high molecular weight is used as a rubber component, and a large amount of a filler and a softening agent are added thereto. A technique using an object as a tread is known.

  By the way, as a softener, from the viewpoint of cost and good compatibility with a styrene-butadiene copolymer rubber, conventionally, an aromatic oil has been most often used. However, as long as only an aromatic oil is used as a softener. However, there is a limit to the improvement of hysteresis loss of the rubber composition, and the required performance for recent tires could not be sufficiently satisfied. Therefore, secondary use of various petroleum resins has been performed on the aromatic oil.

  For example, among petroleum resins, especially coumarone indene resin has good compatibility with styrene-butadiene copolymer rubber, and therefore can impart good fracture characteristics to the rubber composition, and even at high temperatures. Since high hysteresis loss characteristics can be maintained, it is widely used.

  However, even when the resin / aromatic oil combination system as described above is used, there is a concern about loss of low-temperature characteristics as contradictory performance. In particular, since the entire tread rubber is cured under low temperature conditions in winter, there is a risk that the vibration riding comfort and braking performance may be insufficient. In order to cope with such problems, it has been generally performed to use a styrene-butadiene copolymer rubber having a glass transition point lower than that of the conventional one or to reduce the blending amount of a resin having a high softening point.

As another technique, a styrene having a specific styrene content - butadiene copolymer of the polymer rubber, C ₉ aromatic resin petroleum resin and the softening point of the main component is less than 40 ° C. coumarone-indene resin (See, for example, Patent Document 1), specific cetyltrimethylammonium bromide specific surface area (CTAB) and dibutyl phthalate oil absorption (for styrene-butadiene copolymer rubber having a specific styrene content) DBP) and a carbon black having an alkylphenol-based resin (for example, see Patent Document 2), a co-weight of C ₅ fraction obtained by thermal decomposition of naphtha with diene rubber and styrene or vinyltoluene A method of blending a coalesced resin (see, for example, Patent Document 3), a styrene-butadiene copolymer having a specific structure Coalescence of the rubber and polyisoprene rubber, a method of blending a specific low freezing point plasticizers (e.g., see Patent Document 4) it is known, or the like.

  Furthermore, a method of blending a styrene-butadiene copolymer having a specific hydrogenation rate and a natural resin / synthetic resin with a styrene-butadiene copolymer rubber having a specific styrene content and vinyl bond content (for example, And a method of blending a styrene-butadiene copolymer rubber with a plasticizer having a freezing point of −25 ° C. or less and a resin having a softening point of 50 to 150 ° C. (for example, Patent Document 6). And the like) are also known.

  However, even with these methods, the grip performance cannot be sufficiently improved over a wide temperature range, and there is still room for improvement.

JP-A-5-214170 JP-A-6-200078 Japanese Patent Laid-Open No. 9-328577 Japanese Patent Laid-Open No. 10-273560 JP 2003-253051 A JP 2004-137463 A

  Under such circumstances, the present inventor examined various blends and used a low melting point oil component together with a styrene-butadiene copolymer rubber having a high glass transition point and a resin component having a high softening point. By doing so, it has been found that both hysteresis loss characteristics at high temperatures and low temperature characteristics can be achieved. However, even with this method, there is still room for improvement.

  Then, the objective of this invention is providing the rubber composition for treads which can exhibit the outstanding grip performance over a wide temperature range, when solving the problem of the said prior art and using it for the tread of a tire. . Another object of the present invention is to provide a pneumatic tire using such a tread rubber composition and having excellent grip performance.

  As a result of intensive studies to achieve the above object, the present inventor, in addition to a softening agent having a softening point in a specific range, a resin having a softening point above a certain level and / or a freezing point below a certain level as another softening component. It has been found that by using a rubber composition containing a plasticizer having a tire for a tire tread, the grip performance of the tire can be improved in a wide temperature range, and the present invention has been completed.

  That is, the rubber composition for a tread of the present invention has a softening point of 5 to 50 parts by weight of a softening point of -50 ° C. to 50 ° C. and a softening point of 70 parts per 100 parts by weight of the rubber component (A). 5 to 50 parts by mass of a resin (C) having a temperature of not lower than 5 ° C. and / or 5 to 50 parts by mass of a plasticizer (D) having a freezing point of −50 ° C. or lower. The softener (B) has a vinyl bond content. Is characterized by being a high vinyl polybutadiene of 50% or more.

  In the rubber composition for a tread of the present invention, the softening agent (B) is preferably a liquid high vinyl polybutadiene having a vinyl bond content of 50% or more and a molecular weight of 3000 to 30000. Here, the liquid state means a liquid state at room temperature (25 ° C.).

In a preferred example of the rubber composition for a tread of the present invention, the resin (C) is an aromatic petroleum resin obtained by (co) polymerizing a phenol resin and / or a C ₉ fraction.

  In another preferred embodiment of the rubber composition for a tread of the present invention, the plasticizer (D) is selected from the group consisting of a phthalic acid derivative, a long chain fatty acid derivative, a phosphoric acid derivative, a sebacic acid derivative and an adipic acid derivative. At least one kind.

  The pneumatic tire according to the present invention is characterized in that the tread rubber composition is used as a tread rubber.

  According to the present invention, the rubber composition has a softening agent (B) having a softening point in a specific range, a resin (C) having a softening point above a certain level, and / or a plasticizer (D) having a freezing point below a certain level. Can be provided to provide a rubber composition for a tread that can exhibit excellent grip performance in a tire over a wide temperature range. Moreover, the pneumatic tire which has the outstanding grip performance over the wide temperature range using this rubber composition for treads can be provided.

  The present invention is described in detail below. The rubber composition for a tread of the present invention has 5 to 50 parts by mass of a softening agent (B) having a softening point of −50 ° C. to 50 ° C. and a softening point of 70 ° C. or more with respect to 100 parts by mass of the rubber component (A). 5 to 50 parts by mass of the resin (C) and / or 5 to 50 parts by mass of a plasticizer (D) having a freezing point of −50 ° C. or less. The softener (B) has a vinyl bond content of 50. % Of high vinyl polybutadiene.

  The rubber composition for a tread of the present invention comprises a softening agent (B) having a moderate (-50 ° C to 50 ° C) softening point, a resin (C) having a high softening point (70 ° C or higher) and / or a low It is characterized in that it is used in combination with a plasticizer (D) having a freezing point (-50 ° C. or lower), that is, in combination with two or more softening components having different molecular structures. It is not essential to use the same composition resin having different softening points or freezing points, but it is an essential condition to use a combination of softening components having different structural factors and softening points or freezing points. As described above, when a plurality of softening components are used in combination, the reason why the grip performance in the low temperature region and the grip performance in the high temperature region can be compatible is that the rubber composition in which a plurality of softening components having different structural elements are used in combination has a modulus of elasticity. It is mentioned that the temperature characteristic is in a certain range over a wide range. That is, the grip performance in the high temperature region can be secured by a component having a high softening point (high glass transition point), and the grip performance in a low temperature region can be secured by a component having a low freezing point. It is.

  However, when the softening components used have the same structure, the component having a low freezing point and the component having a high softening point are completely compatible, and the temperature characteristic of the rubber composition is a single one having an intermediate softening point or freezing point. Since it becomes equivalent to the case where a component is used, it is impossible to achieve both grip performance in a low temperature region and grip performance in a high temperature region. Even if only two types of softening components are used, the effect of improving the desired grip performance can be obtained. However, when three types of softening components are used, the effect of improving the grip performance is greater. Thus, by using a combination of a plurality of softening components, the temperature characteristics of the tread rubber composition can be broadened, and the tire grip performance can be improved over a wide temperature range.

  The softening components used in the rubber composition for treads of the present invention must have different structures. It is also necessary to ensure sufficient compatibility with the rubber component (A) used at the same time. When the compatibility with the rubber component (A) is remarkably poor, the softening component serves as a fracture nucleus, and the fracture characteristics and wear resistance of the rubber composition are significantly degraded. Accordingly, as the softening component, those conventionally used as a softening agent for rubber compositions are preferable. In addition, when softening components having different structures and different molecular weights or softening components having different freezing points are used, even if softening components having different softening points or freezing points are used in combination, the softening components are easily compatible with each other. Therefore, the result is substantially the same as when a single component having an intermediate softening point or freezing point is used. Therefore, in order to limit the compatibility between the softening components, it is preferable to use softening components having different functional groups as molecular structures.

  The rubber composition for a tread of the present invention needs to contain a softening agent (B) having a softening point of -50 ° C to 50 ° C. When the softening point of the softener (B) used exceeds 50 ° C, the difference from the resin (C) having a softening point described later of 70 ° C or higher is insufficient, and the softening agent (B) used is softened. When the point is below -50 ° C, the difference from the plasticizer (D) with a freezing point below -50 ° C is insufficient, and in any case, excellent grip performance is exhibited over a wide temperature range. It becomes difficult to let you.

  The softening agent (B) needs to be a high vinyl polybutadiene having a vinyl bond content of 50% or more, and is preferably a liquid high vinyl polybutadiene having a vinyl bond content of 50% or more and a molecular weight of 3000 to 30000. .

  The amount of the softening agent (B) is in the range of 5 to 50 parts by mass with respect to 100 parts by mass of the rubber component (A) described later. When the blending amount of the softening agent (B) is less than 5 parts by mass, it is difficult to obtain a sufficient hysteresis loss in an intermediate temperature range. On the other hand, when it exceeds 50 parts by mass, the rubber composition and the tire using the rubber composition Destructive properties are reduced.

  The rubber composition for a tread of the present invention needs to contain at least one of a resin (C) having a softening point of 70 ° C or higher and a plasticizer (D) having a freezing point of -50 ° C or lower, and the softening point is 70 ° C or higher. The resin (C) and the plasticizer (D) having a freezing point of −50 ° C. or lower are preferably included.

The resin (C) needs to have a softening point of 70 ° C. or higher, and is preferably 100 ° C. or higher. When the softening point of the resin (C) used is less than 70 ° C., the rubber composition for tread cannot sufficiently secure the hysteresis loss at high temperature, and the tire can exhibit excellent grip performance in the high temperature region. It becomes difficult. As the resin (C), specifically, a phenolic resin and / or a C ₉ fraction (co) polymerizing an aromatic petroleum resin obtained by. Among these, phenolic resins are preferable. Among phenolic resins, alkyl-substituted phenol resins and long-chain alkyl-modified phenol resins are preferable. These phenolic resins are characterized by high compatibility with styrene-butadiene copolymer rubber and relatively high melting point.

  Examples of the alkyl-substituted phenol resin include pt-butylphenol / formaldehyde resin, pt-butylphenol / acetaldehyde resin, pt-octylphenol / formaldehyde resin, and styrene-modified phenol / formaldehyde resin. Examples of the alkyl-modified phenol resin include terpene-modified phenol resins.

The aromatic petroleum resin obtained by (co) polymerizing the C ₉ fraction is generally an aromatic fraction having 9 carbon atoms mainly containing vinyltoluene and indene obtained by thermal decomposition of naphtha. It is a resin obtained by polymerization. Here, other examples of the C ₉ fraction include styrene homologues such as α-methylstyrene and β-methylstyrene. The aromatic petroleum resin may contain a coumarone unit or the like. As a trade name of the above-mentioned aromatic petroleum resin, Neo Nippon Petrochemical's neopolymer and the like can be mentioned.

  When the rubber composition for treads of this invention contains resin (C), the compounding quantity of this resin (C) is the range of 5-50 mass parts with respect to 100 mass parts of rubber components (A) mentioned later. If the compounding amount of the resin (C) is less than 5 parts by mass, it is difficult to obtain a sufficient hysteresis loss in the high temperature region. On the other hand, if it exceeds 50 parts by mass, the fracture characteristics of the rubber composition and the tire using the rubber composition are degraded. To do.

  The plasticizer (D) needs to have a freezing point of -50 ° C or lower. When the freezing point of the plasticizer (D) used exceeds -50 ° C, it is difficult to obtain good temperature characteristics because the difference in freezing point from the softener (B) is insufficient. Also, the plasticizer (D) having a freezing point of -50 ° C or lower lowers the elastic modulus of the rubber composition at low temperatures, increases the real contact area between the tread and the road surface, and improves the adhesive frictional force. The grip performance in the area can be greatly improved. As the plasticizer (D), ester plasticizers are preferable, and specifically, phthalic acid derivatives, long chain fatty acid derivatives, phosphoric acid derivatives, sebacic acid derivatives, and adipic acid derivatives are preferable. Some hydrocarbon-based plasticizers have a freezing point of −50 ° C. or lower, but the rubber composition for a tread of the present invention preferably includes a plurality of softening components having different structures as described above. For this reason, it is preferable to use those having different structures such as ester plasticizers.

  Examples of the phthalic acid derivatives include phthalic acid esters such as di-2-ethylhexyl phthalate (DOP) and diisodecyl phthalate (DIDP). Examples of the long chain fatty acid derivatives include long chain fatty acid glycerin esters. Examples of the acid derivative include phosphate esters such as tri (2-ethylhexyl) phosphate (TOP) and tributyl phosphate (TBP). Examples of the sebacic acid derivative include di (2-ethylhexyl) sebacate (DOS), diiso Examples include sebacic acid esters such as octyl sebacate (DIOS), and examples of the adipic acid derivatives include adipic acid esters such as di (2-ethylhexyl) adipate (DOA) and diisooctyl adipate (DIOA). These plasticizers may be used alone or in combination of two or more.

  When the rubber composition for treads of this invention contains a plasticizer (D), the compounding quantity of this plasticizer (D) is the range of 5-50 mass parts with respect to 100 mass parts of rubber components (A) mentioned later. is there. When the compounding amount of the plasticizer (D) is less than 5 parts by mass, it is difficult to sufficiently improve the grip performance in a low temperature region, while when it exceeds 50 parts by mass, the rubber composition and the fracture characteristics of a tire using the rubber composition. Decreases.

  The rubber component (A) used in the rubber composition for a tread of the present invention is not particularly limited, but a general styrene-butadiene copolymer rubber (SBR), styrene-isoprene-butadiene rubber (SIBR). In addition, natural rubber (NR), polybutadiene rubber (BR), polyisoprene rubber (IR), and the like can be used. In consideration of the use of the softener (B), resin (C), and plasticizer (D), those having a high molecular weight such as those used in general oil-extended rubbers are preferable, and furthermore, ultrahigh molecular weight rubbers. By using the components, good fracture performance can be obtained. The rubber component may be a single type or a blend of two or more types.

  The rubber composition for a tread of the present invention usually contains a filler. The filler is not particularly limited, and carbon black and various inorganic fillers can be used. These fillers may be used alone or in combination of two or more. The blending amount of the filler is preferably in the range of 30 to 200 parts by mass with respect to 100 parts by mass of the rubber component (A), and in the range of 50 to 150 parts by mass from the viewpoint of reinforcing properties and the effect of improving various physical properties thereby. Is more preferable. If the blending amount of the filler is less than 30 parts by mass, sufficient grip performance of the tire cannot be ensured, and if it exceeds 200 parts by mass, the processability of the rubber composition tends to decrease.

The carbon black is not particularly limited, and grades such as SRF, GPF, FEF, HAF, ISAF, and SAF can be used. The iodine adsorption amount (IA) is 60 mg / g or more, and dibutyl phthalate (DBP). ) Carbon black having an oil absorption of 80 mL / 100 g or more is preferred. By using carbon black, the improvement effect of the grip performance and fracture resistance of the tire can be improved. In order to further improve the wear resistance of the tire, it is particularly preferable to use HAF, ISAF, or SAF grades. Moreover, the external surface area by the cetyltrimethylammonium bromide (CTAB) adsorption method is in the range of 130 to 200 m ² / g, and the 24M4DBP oil absorption is preferably 80 mL / 100 g or more. The 24M4 DBP oil absorption is the DBP oil absorption after four times of compression at a pressure of 24,000 psi.

Examples of the inorganic filler include silica, aluminum monohydrate (Al ₂ O ₃ .H ₂ O), aluminum hydroxide [Al (OH) ₃ ] such as gibbsite, bayerite, and alumina (Al ₂ O _3). ), Aluminum carbonate [Al ₂ (CO ₃ ) ₃ ], magnesium hydroxide [Mg (OH) ₂ ], magnesium oxide (MgO), magnesium carbonate (MgCO ₃ ), talc (3MgO · 4SiO ₂ · H ₂ O), Attapulgite (5MgO · 8SiO ₂ · 9H ₂ O), titanium white (TiO ₂ ), titanium black (TiO _2n-1 ), calcium oxide (CaO), calcium hydroxide [Ca (OH) ₂ ], aluminum magnesium oxide (MgO · Al ₂ O _3), clay _{(Al 2 O 3 · 2SiO 2} ), kaolin _{(Al 2 O 3 · 2SiO 2} · 2H 2 O), pyrophyllite _{(Al 2 O 3 · 4SiO 2} · H 2 ), Bentonite _{(Al 2 O 3 · 4SiO 2} · 2H 2 O), aluminum silicate _{(Al 2 SiO 5, Al 4} · 3SiO 4 · 5H 2 O etc.), magnesium silicate (Mg ₂ SiO _4, MgSiO ₃ etc. ), Calcium silicate (Ca ₂ SiO ₄ etc.), aluminum calcium silicate (Al ₂ O ₃ · CaO · 2SiO ₂ etc.), magnesium calcium silicate (CaMgSiO ₄ ), calcium carbonate (CaCO ₃ ), zirconium oxide (ZrO) ₂ ), zirconium hydroxide [ZrO (OH) ₂ .nH ₂ O], zirconium carbonate [Zr (CO ₃ ) ₂ ], various zeolites, feldspar, mica, montmorillonite, etc., among which silica is preferable.

The silica is not particularly limited, and examples include wet silica (hydrous silicic acid), dry silica (anhydrous silicic acid), calcium silicate, aluminum silicate, and the like. Wet silica is preferred in terms of the most remarkable effect of achieving both grip properties and low rolling resistance. Further, the silica is preferably the specific surface area measured by the nitrogen adsorption method is in the range of 80~300m ² / g, and still more preferably in the range of 100~220m ² / g. When the specific surface area of silica is 80 m ² / g or more, sufficient reinforcement is exhibited, and when it is 300 m ² / g or less, workability can be prevented from being lowered. Usually, fine powdered silicic acid or hydrous silicic acid used as a white reinforcing filler for rubber is used. As the silica, specifically, "Nipsil" [manufactured by Nippon Silica Industrial Co., Ltd.] having a specific surface area of about 200 meters ² / g, "Zeosil 1115MP" having a specific surface area of about 117m ² / g [Rhodia] etc. Commercial products can be used.

  In addition to the rubber component (A), the softener (B), the resin (C), the plasticizer (D), and the filler, the rubber composition for a tread of the present invention is a compound usually used in the rubber industry. An agent such as an anti-aging agent, a vulcanizing agent, a vulcanization aid, a vulcanization accelerator, a scorch preventing agent, and the like can be appropriately selected and blended within a range that does not impair the object of the present invention. As these compounding agents, commercially available products can be suitably used. In addition, the said rubber composition mix | blends the softening agent (B), resin (C) and / or a plasticizer (D), and the various compounding agents selected suitably as needed to the rubber component (A). Then, it can be produced by kneading, heating, extruding or the like.

  The pneumatic tire of the present invention is characterized by using the above-described tread rubber composition as a tread rubber, and is particularly suitable as a tire for a high-performance passenger car. Since the pneumatic tire of the present invention uses the rubber composition as a tread rubber, the grip performance is particularly excellent in a wide temperature range. The pneumatic tire of the present invention is not particularly limited except that the above rubber composition is used for the tread, and can be produced according to a conventional method. Further, as the gas filled in the pneumatic tire, an inert gas such as nitrogen, argon, helium, or the like can be used in addition to normal or air whose oxygen partial pressure is adjusted.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples.

  According to the formulation shown in Table 1, a rubber composition for tire treads was prepared using a Banbury mixer. Next, the hysteresis loss characteristic was evaluated by the following method with respect to the obtained rubber composition for treads. The results are shown in Table 1.

(1) Hysteresis loss characteristics For vulcanized rubbers obtained by vulcanizing each rubber composition, using a rheometrics viscoelasticity measuring tester at a dynamic strain of 1%, 50 ° C The tan δ was measured, and the index was expressed with the tan δ of the rubber composition of Comparative Example 1 as 100. The larger the index value, the larger the tan δ and the better the grip performance.

  Next, using the tread rubber composition as a tread rubber, a JY tire (competition tire) having a size of 225 / 40R18 was prototyped, and grip characteristics of the obtained tire were evaluated by the following method. . The results are shown in Table 1.

(2) Actual vehicle grip characteristics The actual vehicle equipped with the tire was actually run on a dry circuit, and the dry grip performance of the tire was evaluated. Specifically, the reciprocal of the average value of the lap times from the 10th lap to the 20th week was obtained, and the index was displayed with the tire of Comparative Example 1 as 100. The larger the index value, the smaller the average value of the lap time, and the better the grip performance.

* 1 Emulsion polymerization SBR, styrene content = 45 mass%, vinyl bond content = 18%, oil-extended with 37.5 mass parts aroma oil for 100 mass parts rubber component.
* 2 External surface area by CTAB adsorption method = 148m ² / g, 24M4DBP oil absorption = 102mL / 100g.
* 3 Made by Nippon Oil Corporation, “Aromax 1000”, pour point = 12.5 ℃.
* 4 Daihachi Chemical Co., Ltd., Di (2-ethylhexyl) sebacate (DOS), Freezing point = -65 ° C or less.
* 5 Manufactured by Daihachi Chemical Co., Ltd., tri (2-ethylhexyl) phosphate (TOP), freezing point = -70 ℃ or less
* 6 The C.P. Hall, Adipic acid diisooctyl ester (DIOA), freezing point = -60 ° C.
* 7 Made by BASF, "Cholecin", softening point = 110-130 ° C.
* 8 Made by Hitachi Chemical Co., Ltd., phenol aldehyde resin, “HITanol 1501”, softening point = 80 ~ 105 ℃.
* 9 C ₉ fraction (co) polymer obtained by aromatic petroleum resin, Nippon Petrochemicals Co., Ltd., "Neo Polymer 140", softening point = 145 ° C..
* 10 C ₉ fraction (co) polymerization to obtained aromatic petroleum resin, Nippon Petrochemicals Co., Ltd., "Neo polymer 170S", softening point = 160 ° C..
* 11 Sartomer, “Ricon 100”, softening point = −24.3 ° C., vinyl bond content = 63%, molecular weight = 6290.
* 12 Sartomer, “Ricon 131”, softening point = −70 ° C., vinyl bond content = 16.5%, molecular weight = 9717.
* 13 N- (1,3-Dimethylbutyl) -N'-phenyl-p-phenylenediamine.
* 14 1,3-Diphenylguanidine.
* 15 Dibenzothiazyl disulfide.

  From Table 1, the rubber composition of the Example which used softener (B), resin (C), and plasticizer (D) together, and the tire which applied this rubber composition to the tread, softener (B), resin Due to the synergistic effect of the combined use of (C) and the plasticizer (D), the hysteresis loss and the actual vehicle grip performance are greatly improved as compared with the rubber composition and tire of the comparative example not containing the softener (B). I understand.

Claims (5)

5 to 50 parts by mass of softening agent (B) having a softening point of -50 ° C to 50 ° C and 5 to 50 parts by mass of resin (C) having a softening point of 70 ° C or higher with respect to 100 parts by mass of the rubber component (A) And / or 5 to 50 parts by mass of a plasticizer (D) having a freezing point of −50 ° C. or lower,
A rubber composition for a tread, wherein the softening agent (B) is a high vinyl polybutadiene having a vinyl bond content of 50% or more.   The rubber composition for a tread according to claim 1, wherein the high vinyl polybutadiene is a liquid high vinyl polybutadiene having a molecular weight of 3000 to 30000. The rubber composition for a tread according to claim 1, wherein the resin (C) is an aromatic petroleum resin obtained by (co) polymerizing a phenol resin and / or a C ₉ fraction.   The plasticizer (D) is at least one selected from the group consisting of phthalic acid derivatives, long chain fatty acid derivatives, phosphoric acid derivatives, sebacic acid derivatives and adipic acid derivatives. Rubber composition for treads.   A pneumatic tire using the tread rubber composition according to claim 1 as a tread rubber.