JPH02132142A - Rubber composition for tire tread - Google Patents

Abstract

PURPOSE:To provide the subject rubber composition for tire tread excellent in gripping properties and composed of a rubber component containing a rubber consisting of a specified butadiene block and a specified styrene/butadiene block, carbon black and a petroleum-based softening agent. CONSTITUTION:To 100 pts.wt. rubber component composed of (A) 90-50 pts.-wt. block copolymer with >=20X10<4> weight-average molecular weight, 10-25wt.% total bonded styrene in the whole copolymer and 20-45wt.% amount of vinyl bond in the total butadiene units consisting of (A1) a butadiene block with -100--70 deg.C glass transition temperature (herein after represented as Tg) and <=12 deg.C range thereof and (A2) a styrene/butadiene copolymer block with 20-50wt.% bonded styrene, -20-+15 deg.C Tg and <=12 deg.C range thereof where the difference between Tg of (A1)and that of (A2) is >=60 deg.C and (B) the residual pts. wt. natural rubber or isoprene rubber, (C) 80-130 pts.wt. carbon black with >=100m<2>/g specific surface area by the nitrogen adsorption method and (D) 20-90pts.wt. petroleum-based softening agent with 0.9-0.98 viscosity-gravity constant are added to provide the objective rubber composition.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a rubber composition for tire treads, which has stable grip performance over a wide temperature range and also has excellent gripping power on snowy road surfaces.

[Conventional technology]

Conventionally, performance requirements for automobile tires include safety, economy, and ride comfort.

In recent years, especially with the development of expressway networks, there has been a strong desire for tires with improved cornering characteristics, maneuverability such as braking performance, and safety when vehicles are running at high speeds. As a measure to improve the dynamic performance of tires, especially the grip performance, it is important to increase the friction with the road surface by increasing the hysteresis loss of trend rubber.

That is, the tread surface that is in friction with the road surface is deformed at high speed due to minute irregularities on the road surface, and the greater the energy dissipation due to hysteresis loss that occurs during this periodic deformation process, the greater the frictional force becomes. Moreover, since the deformation on the friction surface is extremely fast, it is known that according to Williams-Landeruferry's temperature-time conversion, it depends on hysteresis loss, which is measured at a lower temperature than the temperature at which the tire is used. . In fact, there is a good correlation between tan δ (loss coefficient), which is a measure of hysteresis loss, and the tire friction coefficient, but in this case, tan δ, which is measured at a temperature 30 to 40 degrees Celsius lower than the temperature at which the tire is used, is involved. are doing. Conventionally, in order to increase the hysteresis loss of a rubber composition, a method of compounding a rubber with a high glass transition temperature (Tg) such as a styrene-butadiene copolymer rubber containing a high styrene content has been used.
This is because the tan δ of rubber has a peak near Tg{'j, so tan δ is in the temperature range that is involved in friction performance (corresponding to 0 ℃ if the tire running temperature is 30 ℃).
The aim is to utilize a high tan δ by bringing the peaks of .

Figure 3 shows emulsion polymerized styrene with different styrene contents.
This figure shows the tan δ temperature dependence of butadiene copolymer rubber. As the styrene content increases, the peak temperature of tan δ shifts to the high temperature side, and the jan δ value increases near O°C, which is the base of the peak of tan δ. However, at the same time, the temperature dependence of tan δ also increases around 0° C., and therefore the grip performance of the tire also changes significantly depending on the environmental temperature. Furthermore, the elastic modulus also changes rapidly corresponding to the peak of tan δ. That is, as the temperature decreases, the elastic modulus increases rapidly, making it impossible for the rubber to follow the unevenness of the road surface. Alternatively, in the case of an icy road surface, the rubber cannot be deformed at all, resulting in a problem in that maneuverability and braking performance are reduced.

On the other hand, if a polymer with a low Tg, such as high cis BR, is used, the Gri/Nob performance at low temperatures will improve, but
On the other hand, the tan δ decreases around 0° C., causing a contradiction in that the gripping ability is insufficient at high temperatures. Therefore, attempts have been made to harmonize the above-mentioned antinomy by blending different types of polymers called SBR/BR or by adding a large amount of small particle size carbon (D, F, Moore, ``The Fri
tion of Pneumatic Tires
, Elasevier ScienLific
Publisher+g Company: 197
5, lI. s. Patent No. 4,748,16
8, JP-A-62-12932, etc.).

However, such rubber exhibits intermediate properties of the blended polymers, so it cannot take full advantage of the inherent strengths of the constituent polymers, and has the disadvantage that neither high-temperature grip nor low-temperature grip is completely satisfactory. In addition, incorporating a large amount of small particle size carbon has problems in processability and increases heat generation.

As a measure against this trade-off, Japanese Unexamined Patent Application Publication No. 1986-
66733 and Japanese Patent Application Laid-Open No. 62-62840 disclose a technique of adding a low-temperature plasticizer to these blend polymers, but in this case, although the low-temperature grip is certainly improved, it does not improve at high temperatures. The rubber elastic modulus is significantly lowered, and the combination of handling stability at high temperatures and grip at low temperatures is not at a satisfactory level. It is clear that this cause depends on the viscoelastic properties of the polymer.

Therefore, in recent years, in order to control the temperature dependence of viscoelasticity,
SB produced mainly using organolithium-based initiators
R. ．． ．． Attempts have been made to modify BR.

By the way, there are various environments that cars encounter, and even when considering just the road surface temperature, there can be changes of several tens of degrees. In order to stabilize Grisop performance over such a wide temperature range,
It is ideal to use a rubber that has a relatively high tan δ peak temperature and little temperature change, that is, a broad tan δ peak. Prosoc polymerer is known as an attempt to broaden the tan δ peak.

For example, JP-A-57-102912, JP-A-57-102912,
109817 proposes a Prosoc polymer consisting of two types of SBR blocks with different amounts of vinyl bonds in the styrene and butadiene moieties. Although these block polymers do have a broadened tan δ peak compared to conventional polymers, they are not wide enough to cover the temperature range of several tens of degrees as mentioned above, and depending on how the two types of blocks are combined, In some cases, the peak temperature position shifts, and no improvement in grip performance can be expected.

In addition, ice skid resistance has been taken into consideration.
Lehel is not satisfied.

Furthermore, in Japanese Patent Application Laid-open No. 54-62248, butaji x
7 out of 7 (7) 1. A technology has been disclosed that improves grip performance and reduces transfer resistance by increasing the amount of 2-hinyl bonds, but these polymers have a high Tg and inevitably have poor grip at low temperatures. ability becomes inferior. The composition of the diblock copolymer is disclosed in JP-A-57-109817 and JP-A-57-1.
It is disclosed in Japanese Patent No. 08142. These have a styrene content of 20 to 50% by weight, a vinyl content of 40 to 75% by weight,
It is a copolymerization of high Tg sBR block and low Tg SBR block containing 10% by weight or less of styrene and 20 to 50% by weight of vinyl, and is said to improve wet grip performance and reduce rolling resistance. There is.

JP-A-57-102912 discloses that a diblock copolymer contains blocks containing 10 to 50% by weight of styrene and 20 to 50% by weight of vinyl, and 1 to 30% by weight of vinyl.
, 60 weight percent or more blocks are used, because they have different solubility parameters, each block exhibits a Tg corresponding to each block when unvulcanized, but they become compatible with each other during the vulcanization process. It has been shown that as a result, the peak of the tan δ - temperature curve becomes extremely broad.
0-192739, when block copolymerizing butadiene with a vinyl content of 60% by weight or less into an SBR block with a styrene content of 10 to 80% by weight and a vinyl content of 30 to 70% by weight, the TgO difference between the two is set to 30°C or more. By widening the distribution width of vinyl bonds and widening the temperature range of the transition region, it is possible to make the peak of tan δ broader, thereby improving the properties of wet skid, ice skid, transfer resistance, and fracture properties. It is said that this will lead to a reduction in the level of harassment. In JP-A No. 61-55135, the ratio of weight average molecular weight/number average molecular weight of the block copolymer disclosed in JP-A No. 60-192739 is 1.8 to 5.
．． It is said that by setting it to 0, the performance halance will be further improved. However, with these techniques, tanδ
Although the peak of tan δ becomes broad, at the same time, a decrease in the absolute value of tan δ itself is unavoidable, and even though it is possible to balance wet grip performance, handling stability, and ice snow racing performance, it is possible to achieve a very high level of each performance. I am still dissatisfied with the goal of achieving it in Rehel. Unexamined Japanese Patent Publication 1986-
Publication No. 231016 also discloses a block polymerer consisting of three SBR blocks, which has relatively good ice skid resistance and well-balanced wet grip performance. I haven't reached the point where I'm satisfied.

[Problem to be solved by the invention]

An object of the present invention is to provide a rubber composition for tire treads and throats that has stable grip performance over a wide temperature range and also has excellent grip on snowy and icy road surfaces. This composition is suitable for all-season type high-performance tires.

The present inventors conducted intensive studies to broaden the peak of tan δ in styrene-butadiene copolymer rubber and further increase the absolute value of tan δ in the temperature range involved in friction. Consisting of a styrene-butadiene copolymer block that has a specific content and glass transition temperature and a narrow Tg transition temperature width, and a butadiene block that has a glass transition temperature in a specific range and a narrow Tg transition temperature width. The present invention was achieved by discovering that a block copolymer can achieve the above object. The gist of the present invention is that the SB has a high 'rg and therefore has a high grip performance.
The goal is to combine R and BR, which has a low Tg, that is, excellent snow and ice performance, to create a single polymer that has only the advantages of both. The Tg of the Prosoc polymers constituting the coalescence are separated by 60°C or more, and the transition temperature width is 1
The present invention is motivated by the discovery that the temperature needs to be below 2°C.

[Means to solve the problem]

The present invention has a glass transition temperature of -100°C to -70°C,
Butadiene block with a transition temperature range of 12℃ or less (A)
and a styrene-butadiene copolymer block containing 20 to 50% by weight of bound styrene, a glass transition temperature of -20°C to +15°C, and a transition temperature range of 12°C or less (B)
The block (A) and the block (B
) is 60° C. or more, the copolymer as a whole contains 10 to 25% by weight of fully bound styrene, and the amount of vinyl bonds in all butadiene parts is 20 to 45% by weight,
Nitrogen specific surface area 1QQm/g or more based on a total of 100 parts by weight of rubber with 90 to 50 parts by weight of styrene-butadiene block copolymer rubber having a weight average molecular weight of 20 x 10' or more and the balance being natural rubber or isobrene rubber. 80 to 130 parts by weight of carbon plaque and 20 to 90 parts by weight of a petroleum softener with a viscosity specific gravity constant of 0.90 to 0.98 are blended, and the shear storage modulus at -30°C is 50.
The gist of the present invention is a rubber composition for a tire tread, which is characterized by having a pressure of 0 MPa or less.

This means will be explained in detail below.

(1) Rubber content.

Styrene-butadiene block copolymer rubber 90-50
parts by weight and the balance of natural rubber or isoprene rubber (IR) (
(10 to 50 parts by weight).

■ Styrene-butadiene block copolymer rubber is '
rg is -100℃~-70"C, transition temperature range is 12℃
The following butadiene block (A), and a styrene-butadiene copolymer block (B) containing 20 to 50% bound styrene, having a Tg of -20°C to +15°C, and a transition temperature range of 12°C or less. consists of a block (A
) and block (B), the difference in glass transition temperature is 60℃
above, the total bonded styrene of the copolymer as a whole is 1
0 to 25% by weight, the amount of vinyl bonds in the total butadiene part is 20
~45% by weight, with a weight average molecular weight of 20 x 10
'That's it.

The Tg of block (A) is -100°C to -70°C,
The Tg of the other block (B) is -20°C to +15
℃ is essential, and furthermore, the difference between them needs to be 60℃ or more. That is, the difference in Tg between each block is 60
Below ℃, each block becomes compatible with each other, and t
The anδ peak is not sufficiently broad. moreover,
Each transition temperature range must be 12°C or less.

This is because even if the Tg's are separated by 60°C or more, if the transition temperature range is 12°C or more, the two will partially dissolve, and the resulting tanδ-temperature curve will have properties intermediate between the two. This is not desirable because it indicates that

JP-A-60-192739, 61-55135
In the publication, T of each pro-noc of the diblock copolymer is
It has been pointed out that when the g difference is 30° C. or more and the transition temperature range is wide, the tan δ -temperature curve becomes broad. Such a Bronok copolymer has two 'rg points when unvulcanized, but when vulcanized they become compatible and become integrated, resulting in one broad peak. According to their opinion, if 'rg is unified, the advantages of each block as a single block cannot be utilized, and as a result, the physical properties will be the same as those of a blend of constituent polymers.

In other words, although it is improved in terms of physical property halance compared to the prior art SBR/B R polymer blend, it does not show exceptionally good physical properties. Therefore, in order to preserve the properties of the binary block polymer even after copolymerization, we thought that it would be possible to actively make the block polymers incompatible with each other, and after considering the polymer composition for that purpose, we found that: Under the conditions described above, it was possible to obtain improved binary prosodic polymers that retained the unique properties of each constituent polymer alone.

The Tg of block (+3) with Tg on the high temperature side is 20℃
It needs to be ~+15°C. This is because at temperatures below -20°C, tan δ in the temperature range that is involved in grip performance.
If it is not possible to increase the temperature and the temperature is higher than +15°C, the heat generation property deteriorates, which adversely affects the durability of the tire, and the snow and ice road surface grissop deteriorates irreparably.

The amount of styrene and 1.2 = vinyl amount to obtain such Tg, the styrene content of block (B) is 20
If the styrene content is less than 20% by weight, it will not be possible to obtain a copolymer with a Tg that meets the requirements, and conversely, if the styrene content is more than 50% by weight, a styrene block will not be obtained. Easy to wear, wear resistant,
Unfavorable in terms of breaking properties. The amount of vinyl is limited due to the relationship between Tg and the amount of styrene, but a desirable range is approximately 40 to 75% by weight.

In addition, the 'rg of Floosek (A) is -100℃~70℃
It must be. This is because if the temperature is higher than -70°C, the elastic modulus at low temperatures becomes high, and the snow and ice road surface grissop performance deteriorates. On the other hand, T'g lower than -100℃
There are practical problems in obtaining a butadiene block with . Vinyl content is determined by Tg, but is approximately 5~
It becomes 25% by weight. Further, if the difference in Tg between block (A) and block (B) is less than 60° C., they will be compatible with each other and will have one broad tan δ -temperature curve. The temperature dependence of the viscoelasticity of such a compatible copolymer is
For example, it is suitable for low rolling resistance tires, but the physical properties of the tread rubber for all-season high-performance tires, which is the object of the present invention, are such that the tan δ - temperature curve is not only broad but also trapezoidal. It is necessary that the absolute value of tan δ itself be large ("About the temperature dependence of hysteresis friction", Vol. 2, Journal of the Japan Rubber Association '88).
October issue). For this purpose, it is important that block (A) and block (B) be in a state of incompatibility or semi-compatibility. Unless the temperature difference in Tg is 60° C. or more, such a trapezoidal tan δ -temperature curve cannot be obtained.

The transition temperature width of both blocks (A) and (B) must be 12° C. or less. If it is larger than 12°C, even if the difference in Tg is 60°C or more, Pro and 2 will partially dissolve, and the tan δ - temperature curve is not desirable as an all-season high-performance tire. The weight average molecular weight needs to be 20 X 10' or more, preferably 100 XIO' or less, for processing and fracture properties.

The purpose of the binary copolymer block found in previously published patent publications is to achieve both low transfer resistance and friction.
In this respect, the present invention is different from the objective of the present invention, which is to achieve high performance in all seasons.

Therefore, the desired temperature dependence of viscoelasticity is also different, and as a result, the styrene-vinyl composition of each block or the bonding method must be significantly different from those of the conventional patents.

Therefore, the present invention differs from conventional inventions in the following points:
It can be said that the obtained polymer has sufficient novelty.

1. The amount of styrene constituting blocks (A) and (B),
The amount of vinyl is different.

2. The TgO difference between blocks (A) and (B) is 60°C or more.

3. The Tg transition temperature widths of blocks (A) and (B) are each 12°C or less.

In the present invention, the higher the molecular weight component, the more Proso(A)
It is even better to include more. This is because if the ratio of each block is constant regardless of the molecular weight, when the molecular chains of the high Tg block freeze at low temperatures, the movement of the molecular chains of the low Tg block is inhibited. This is because it becomes difficult to lower the elastic modulus at low temperatures.Furthermore, as the temperature rises and the molecular chains of the high-Tg side block begin to move, the temperature dependence of the overall mobility suddenly increases. As a result, tan δ decreases rapidly, making it difficult for the curve to become broad.At the same time, it is also possible to apply known techniques for modifying copolymers, such as terminal modification or coupling. This is desirable as long as it does not adversely affect the improvement of the temperature dependence of viscoelasticity, which is the goal of the present invention.

As a method for producing such a block copolymer,
For example, by adjusting the amounts of styrene and 1,3-butadiene in each of block (A) and block (B) to a predetermined ratio, using a polar compound such as ether as a dispersant in a hydrocarbon solvent, and using a lithium-based polymerization initiator. Styrene and butadiene may be copolymerized in the presence of a copolymer while controlling the polymerization temperature, ratio, amount charged, polymerization initiator, etc.

This method may be a batch method or a continuous polymerization method.

(2) Blending natural rubber or IR with the styrene-butadiene block copolymer rubber is important from a practical standpoint. This is because automobile tires are often used not only on paved roads but also on rough and uneven roads, and in such cases, blending natural rubber prevents tread damage caused by sudden external forces such as rolling and canting. This is because it can be reduced. For this purpose, a blending amount of 10 parts by weight or more is required. On the other hand, if the amount of natural rubber or IR is too large, styrene
The essential properties of butadiene block copolymer rubber are thin
This will cause the grip to deteriorate.
It is necessary to suppress the blending amount to 50 parts by weight or less.

(2) Carbon blank.

It has a nitrogen specific surface area of 100 m/g or more.

Specifically, for example, ISAF, SAF, etc. In addition, nitrogen specific surface area is 10On? If it is less than /g, the grip performance will be poor and the abrasion resistance will be insufficient.

(3) Petroleum-based softener.

It has a viscosity specific gravity constant of 0.90 to 0.98. For example, paraffin oil, which has a viscosity specific gravity constant of less than 0.90, has poor grip performance, and if it exceeds 0.498, the softening effect during mixing will be insufficient. . This petroleum softener is
Examples include aromatic aroma oils and highly aromatic aroma oils.

(4) The rubber composition of the present invention contains 80 to 130 parts by weight of the carbon black and 20 to 90 parts by weight of the petroleum softener to 100 parts by weight of the rubber.

In order to be put into practical use as a trend for automobile tires, it must have sufficient performance in terms of wear resistance and handling stability. In order to improve handling stability to a level suitable for high-performance tires, it is necessary to blend 80 parts by weight or more of small particle size carbon black with a nitrogen specific surface area of 10 On{/g or more. However, if it exceeds 130 parts by weight, the abrasion resistance and heat generation properties will be extremely poor, so the content must be lower than that. In addition, other characteristics of tires include ride comfort, noise, and braking performance, and in order to improve these properties, and also for processability during tire manufacturing, extender oil (petroleum-based softener) is used. ) must be added. If the viscosity specific gravity constant of the extender oil is less than 0.90, no improvement in braking performance will be observed, and aromatic extender oils of 0.90 to 0.98 are preferable. It is necessary to adjust the amount of extension oil blended appropriately depending on the amount of carbon black blended to adjust the elastic modulus of red rubber. However, if it is less than 20 parts by weight, the compounded rubber will not elongate, resulting in poor chipping and casting properties, and processability is also difficult, which is not preferable. 90 on the contrary
If the amount exceeds the weight part, the strength will decrease and the abrasion resistance will become extremely poor, making it difficult to put it into practical use.

The rubber composition of the present invention, which is formed in this way, is required to have a shear storage modulus of 500 MPa or less at -30°C. In general, rubbery substances are
It is in a glass state at temperatures below, and the elastic modulus is 1 at room temperature.
It becomes more than 00 times -F, making it brittle and breaking with the slightest strain. The temperature at this time is called the low-temperature embrittlement temperature, and is known as an indicator of the low-temperature performance of rubber materials. but,
When the elastic modulus changes slowly with temperature as in the rubber composition of the present invention, the embrittlement temperature cannot be simply estimated from 'rg. FIG. 1 shows various rubber compositions,
Low temperature embrittlement temperature and shear storage modulus (G
) is plotted. From this, it can be seen that the G value at the embrittlement temperature of all the samples exceeds 500 MPa. Therefore, if it is 500 MPa or less, it can be said that the embrittlement temperature is not exceeded. Furthermore, the reason why the shear storage modulus is set at -30°C is that tires are not normally used at temperatures lower than -30°C.

In addition, this shear modulus was determined using a dynamic torsion tester.
It is measured at a distortion of 0.5% and a frequency of 20 Hz.

Examples and comparative examples are shown below.

[Example, comparative example]

Diblock copolymers A-D having the structures shown in Table 1 were prepared. Furthermore, these copolymers and compounding agents were vulcanized in the proportions shown in Table 2. Numbers in the table are parts by weight unless otherwise specified. The conditions for vulcanization were 160°C x 20 minutes, and 21N
A thick rubber sheet was obtained. The physical properties of this sort were measured.

In addition, this measurement was performed by the following method. The amount of vinyl bonds in the butadiene moiety was determined by the Morello method, and the amount of vinyl bonds in the butadiene moiety was determined by the Hampton method using an infrared spectrometer. Tg and transition temperature range are T H E R ? manufactured by DuPont. l
A L A N A L Y Z E l? using
Measurement was carried out at a heating rate of 10° C./min, and the extrapolated starting temperature and the method shown in FIG. 2 were used to determine the temperature. Figure 2 shows the DS
It shows a C curve, in which the vertical axis shows the heat flow near the glass transition point. Moreover, the weight average molecular weight M. is GPC
It is expressed in terms of polystyrene. Tensile strength TI and elongation at break E. According to JISK6301. -30″
The shear modulus G (-30°C) at C and tan δ at O°C were measured using a dynamic viscoelasticity measuring device manufactured by RIIEOMETRICS, at a frequency of 2011z and a shear strain of 0. 5
Measured in %.

(Left space below the wooden page) Table 2 Milk {fJ styrene-butadiene rubber: Nippon Zeon Ni
pol 9520.

N-(1,3-dimethylbutyl)-N”-phenylp
-Phenylenediamine.

SBR (1), +21 oil extension (total of 37.5 weight system).

N-cyclohexyl-2-benzothiaprylsulfenamide.

Comparative Example 3.4 is an example of a blend of emulsion polymerized SBR and natural rubber. In Comparative Example 3, the low temperature performance is good, but ta
nδ (0°C) is low and wesoto performance is poor. On the contrary, in Comparative Example 4, tan δ is high but low temperature performance is poor. Also,
Comparative Examples 1 and 2 are styrene with a high total vinyl content.
An example of blending with natural rubber using butadiene copolymer rubber, tan δ (0°C) is sufficient, but
Low temperature performance is not possible. Compared to these, Examples 1 and 2 are
It has a good balance of both performances.

〔Effect of the invention〕

As explained above, the rubber composition of the present invention has stable Grisop performance over a wide temperature range compared to the conventional rubber composition, and also has excellent gripping power on snowy channel surfaces. It can be suitably used for the tread part of a tire, especially the tread part of a high-performance all-season tire.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between the low-temperature embrittlement temperature and shear modulus of a rubber compound, Figure 2 is an explanatory diagram showing the transition temperature range near the glass transition point of the DSC curve, and Figure 3 is an emulsion diagram that differs only in styrene content. FIG. 2 is an explanatory diagram showing a tan δ-temperature curve of polymerized SBR showing that the peak temperature is higher when the styrene content is higher.

Claims (1)

[Claims] Contains a butadiene block (A) with a glass transition temperature of -100°C to -70°C and a transition temperature width of 12°C or less, and 20 to 50% by weight of bound styrene, and a glass transition temperature of -20°C.
Styrene with a transition temperature range of 12℃ or less from ℃ to +15℃
butadiene copolymer block (B), the difference in glass transition temperature between the block (A) and the block (B) is 60°C or more, and the total bonded styrene content of the entire copolymer is 10 to 25% by weight. %, the amount of vinyl bonds in the total butadiene portion is 20 to 45% by weight, and the weight average molecular weight is 20%.
x 10^4 or more styrene-butadiene block copolymer rubber with a nitrogen specific surface area of 100 m^2/g or more for a total of 100 parts by weight of rubber with 90 to 50 parts by weight of styrene-butadiene block copolymer rubber with the balance being natural rubber or isoprene rubber. 80 to 130 parts by weight of certain carbon black, viscosity specific gravity constant 0.90 to 0.
-30 by blending 20 to 90 parts by weight of 98 petroleum softener.
A rubber composition for a tire tread, characterized in that the shear storage modulus at °C is 500 MPa or less.